INDUSTRIAL NOISE CONTROL MANUAL

Vincent Salmon

James S. Mills

Andrew C. Petersen

Industrial Noise Services, Inc.

Palo Alto, California

Contract No. HSM 99-73-82

U. S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE

Public Health Service

Center for Disease Control

National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

June 1975
The contents of this report are reproduced herein as

received from the contractor except for minor changes.

The opinions, findings, and conclusions expressed are

those of the authors and not necessarily those of

NIOSH. Hention of company or product names is not

to be considered as an endorsement by NIOSH.

DHEW Publication No. (NIOSH) 75-183

NIOSH Project Officer: Roy M. Fleming

ii
BIBLIOGRAPHIC DATA 11. Keporl No. NIOSH-75-183

SHEET

~. Tide and Sublide

INDUSTRIAL NOISE CONTROL MANUAL

S. Report Dale

June 1975

6.

-

' •.Authorlsl •

Vincent Salmon, James S. M~lls, and Andrew C. Petersen

9. Performing Organizalion Name and Address

Industrial Noise Services, Inc.

Palo Alto, California

12. SpoDsoring Organization Name and Addless

National Institute for OCcupational Safety and Health

4676 Columbia Parkway

Cincinnati, Ohio 45226

1S. Supplemenla ry NOle s

8. Performing Organization Repl.

No.

10. Project/Task/Work Unit No.

11. ContraCt/GraDI No.

RSM-99~73-82

13. Type of Report & Period

Covered

14.

16. Abstracts This manual contains fundamental information to aid the user in understanding,

measuring, and controlling noise. It was written for persons having little or no experience in solving noise control problems, realir.ing that a large number of businesses are

not prepared to deal with their noise problems eff~tiuely. There are seven chapters in

the manual covering the following subjects: fundamental principals of sound, noise measurement, noise control techniques, noise control materials, case histories of successful

applications of noise control methods in actual industrial situations, how to choose a

qualified consultant, and references to additional pertinent literature. The manual

should be used as a guide to help the reader develop solutions to his particUlar noise

problems using proven methods.

17. Key WOlds and Documenl Analysis. 170. Descriplors

Industrial hygiene

Acoustic measurement

Noise reductionIndustrial plants

iNoise analyzers

Noise generators

~ound level meters

~cdustic insulation

~coustic screening

~ontrol equipment 17b. Identifiers/Open-Ended Terms

17e. COSATI Field 'Group 06/J

18. Availability Statemenl

Release unlimited

"OR'" NT'$-" .REV. 10 '" ENllORSlD IIY ANSI AND UNESCO.

19. Sccurit)'Class (This

R,,~~~t)1 .a.C::C::IFIr.n

20. Security C1"ss (This

Page

IIN'-1. A~<;I rrl'n

THIS FORM MAY III:: KHJI{OIlUCI::U

es

ABSTRACT

This manual contains fundamental information to aid the user

in understanding, measuring, and controlling noise. It was

written for persons having little or no experience in solving

noise control problems, realizing that a large number of

businesses are not prepared to deal with their noise problems

effectively. There are seven chapters in the manual covering

the following subjects: fundamental principles of sound,

noise measurement, noise control techniques, noise control

materials, case histories of successful applications of noise

control methods in actual industrial situations, how to choose

a qualified consultant, and references to additional pertinent

literature. The manual should be used as a guide to help the

reader develop solutions to his particular noise problems

using proven methods.

iii

FOREWORD

Following passage of the Occupational Safety and Health Act

of 1970, the United States Department of Labor established a

workplace noise exposure regulation which affects the majority

of businesses having excessively noisy environments. Many

companies may not be prepared to deal with their noise problems effectively but are trying to implement noise reduction

programs nonetheless. Company personnel can be faced with

uncertainty when attempting to choose a noise control method

or device, noise control materials, noise measuring instrumentation, or a qualified consultant. Industries need practical

noise control advice which draws upo~ the successes and failures

of previously tested methods in order to reduce the time and

expense involved in achieving an acceptable noise environment.

Accordingly, a contract was awarded for the development of a

manual which would provide essential information for understanding noise control techniques, including a large number

of noise control case histories fro~ actual industrial situations. Several excellent reference books have been available

for many years, but none have stressed case histories sufficiently. (However, during the term of this contract, at least

two noise control books were published which do contain useful

case histories; these are included in the list of references.)

It was decided that this manual, in particular, should present

sufficient practical information to provide the user with a

solid foundation for understanding, measuring, and controlling

noise. For readers desiring additional information, the

manual would give references which cover areas of interest

in greater detail.

It was recognized that noise control is not a new discipline,

and that many effective noise control programs have been

instituted by industry, both on an in-house basis and by hiring

qualified noise control consultants. A wide range of noise.

problems and control solutions needed to be compiled from many

industrial programs in order for this manual to benefit a

large number of companies. Each case history necessarily would

contain sufficient detail to guide the reader in using the

method to solve the same or a similar noise problem. A minimum of thirty varied case histories was specified to the contractor.

Although the coverage provided by the case histories documented

in the manual is not as broad as was anticipated, it is hoped

that subsequent publications of additional case histories can

v ~reced;ng page blank! - --~
be made available to eXfand this important chapter of the

manual. A free ezchanye of mutually be~eficial information can

ai~ many co~panies in solving their noise problems effectively.

In planning~or future supplemental sections to the case

history chapter of this manual, we invite all interested

IJill"ties to subF1i tease> histories of solutions to industrial

noise p~ohlems. Mo case tlistories will be published without

giving acknowledgement to each contributor who so desires;

and case histories will be revised, if necessary, to fit the

general format listed below. The time of publ~cation for any

supplement would, of course, be dependent upon the amount of

response to this invitation.

Case History Format:

A. Description of the process, machine, and noise problem.

B. Noise measurements made and discussion of findings.

C. Control approaches -- advantages and disadvantages.

D. Results in terms of noise reduction achieved and cost.

E. Pitfalls to avoid in implementing the control methods.

F. Figures -- noise data (e.g., octave band levels).

G. Sketches of area layout, machine-operator relationship,

construction details of noise control devices.

H. Photographs of machines before and after modifications

8 x 10 glossy preferred.

Case Histories can be sent to:

Noise Section/Physical Agents Branch

Division of Laboratories and Criteria Development

National Institute for Occupational Safety and Health

532 u.S. Post Office and Courthouse

Cincinnati, Ohio 45202

c >c

cc vi
CONTENTS

LIST OF ILLUSTRATIONS

LIST OF TABLES

CHAPTER 1 FUNDAMENTALS OF SOUND

Basic Principles

Definitions and Relationships

Hearing and Noise Level Weighted Scales

Combining Noise Levels

Noise Generation and Noise Fields

Sound Power

Summary of Symbols and Definitions

CHAPTER 2 MEASUREt~NT OF NOISE

Compliance Measurements

Basic Instruments, Use and Calibration

Dosimeters

Diagnostic Measurements

Sound Level Meters

Octave Band Filters

Fractional Octave-Band Analyzers

Photographic Equipment

Tape Recorders

Other Equipment

Making Readings

Relating to A-levels

Vibration

Ears and Mind

General Tips

Combining Decibels

Data Requirements

Follow-Up Measurements'

CHAPTER 3 NOISE CONTROL TECHNIQUES

Source

Distance or Relocation

Vibration Control or Isolation

Damping

Lagging

Air and Gas Flow Noise Reduction

Air Jet Flow

vii

xi

xv

1

1

2

5

8

10

13

14

15

15

15

19

19

19

24

24

24

25

26

27

27

28

28

29

29

30

32

33

33

34

34

36

37

38

39
Hydraulic Systems

Motor Air Noise

Enclosed Drives

Balancing

Noise Source Enclosures or Barriers

Path of Sound

Acoustical Shields and Barriers

Total Enclosure

Room Acoustics

Receiver

Administrative Controls

Personal Protective Equipment

Audiometry

CHAPTER 4 NOISE CONTROL MATERIALS

Absorption Materials

Transmission Loss Materials

Damping Materials

Vibration Isolators

Material Selection

Acoustic Considerations

Vibration Isolators

Silencers

Nonacoustic ~onsiderations of Materials

CHAPTER 5 CASE HISTORIES

3'9

42

42

42

43

43

43

46

57

60

60

61

62

63

63

65

66

67

67

67

71

72

72

75

Case History Data

Outline of Complete Procedure for Development

of Noise Control

1 -

2 -

3 -'

4

5 -

6

7

8

9 -

10

11 -

12

13 -

Steel Wire Fabric Machine

BOO-Ton Verson Blanking Press - Vibration

Isolation

Blood Plasma Centrifuge

Blanking Press Ram

Spinning Frame Air Noise

Barley Mill

Sheeter for Boxboard

Air Scrap Handling From Bobst Cutting

presses, Folding Carton Manufacturing

Jordans for Paper Mill for Boxboard

Folding Carton Packing Stations, Air

Hammer Noise Source

In-Line Gravure Printing and Cutting

Press Foldinq Cartons

Air Scrap Handling Ducts for Corrugated

Container Industry

Paper Machine, Wet End

viii

75

75

81

88

95

III

116

119

129

133

136

142

146

149

152
14 - Punch Press Noise

15 - Straight and Cut Machines

16 - Cut-Punch Press in Metal Fabricating . Plant

17 - Parts Conveying Chute

18 - Nail Making Machine

19 - Wood Planer

20 - Punch Press

21 - ~1aterials Handling, Air Motors

22 - Textile Braiding Machines

23 - Metal Cut-Off Saw

24 - Planer-Wood

25 - Punch Press Total Enclosure

26 - Dewatering Vacuum Pump, Paper Mill

27 - Steam Line Regulators

28 - Plastics Scrap Grinder

29 - Newspaper Printing Press

30 - Chemical Process Plants

CHAPTER 6 SELECTING AND USING A CONSULTANT

Knowing When a Consultant is Needed

Selection of a Consultant

Guideline Questions

The Proposal

Other Services

CHAPTER 7 ANNOTATED REFERENCES

ix

157

170

177

184

187

191

196

205

207

210

213

215

216

218

220

223

224

231

231

232

233

236

238

239

1.1

1.2

1.3

1.4

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

3.10

3.11

3.12

3.13

3.14

3.15

3.16

4.1

4.2

5.1.1

5.1. 2

5.1. 3

5.1. 4

5.1. 5

5.2.1

5.2.2

5.2.3

5.2.4

5.2.5

ILLUSTRATIONS

Frequency-wavelength chart

Response characteristics of weighting

scales and of ear at threshold

Method for combining decibel levels

Noise field around a single machine

Effect of reduced pump speed on noise level

Side branch muffler

Dissipative muffler

Source-barrier-receiver geometry

Split panels for services

Welded angle iron frame

Enclosure panels secured to frame by

quarter-turn fasteners

Vibration isolation and toe coving

Enclosure panel interior treatment

Door and hatch detail

Window detail

Cooling and ventilation system

Examples of partial and complete enclosures

Effect of enclosure sound leaks on potential

noise reduction

Example of isolating wall

Sound field behavior in a reverberant room

Transmission loss plateaus

The mass law relation between average sound

transmission loss and mass per unit area of

partition

Noise levels for 8-foot fabric machine,

1 meter from main drive gear

Noise levels for 8-foot fabric machine

operator station

Noise levels for 8-foot fabric machine

1 meter behind operator station in wire

spool area

Steel plate barrier with window

Lexan barrier in two sections

Vertical acceleration on pier, before and

after isolation

Horizontal acceleration on pier, before and

after isolation

Attenuation of vertical and horizontal

acceleration at pier by adding isolation

Quasi-peak levels 4 feet, from press foot,

before and after isolation

A-weighted quasi-peak levels 4 feet from

press, before and after isolation

3

6

9

12

40

41

41

45

47

48

48

49

50

51

51

52

53

54

56

58

69

70

83

84

85

86

87

90

91

92

93

95

xi

lPreceding page blank
5.3.1

5.3.2

5.3.3

5.3.4

5.3.5

5.3.6

5.3.7

5.3.8

5.3.9

5.3.10

5.3.11

5.3.12

5.3.13

5.4.1

5.4.2

5.4.3

5.5.1

5.6.1

5.6.2

5.6.3

5.6.4

5.6.5

5.7.1

5.7.2

5.7.3

5.8.1

5.9.1

5.9.2

5.9.3

Sketch of one centrifuge

Operator exposure in -5°C box room

Sketch of centrifuge showing locations

for close-in diagnostic readings

Centrifuge noise measured parallel-to

motor shaft

Centrifuge noise measured 15 cm directly

below pulley

Centrifuge noise measured perpendicular

to spout

Centrifuge noise measured between centrifuge

motor and pUlley belt

Centrifuge noise measured 2 cm from centrifuge

motor exhaust

Centrifuge noise measured at tub height

Operator's exposure in reconstituting room

Noise in reconstituting room, measured

2 cm from bottom edge of exhaust face of

refrigeration unit

Noise in reconstituting room, measured 2 cm

below rear intake face of refrigeration unit

Motor exhaust muffler

Method used to cover slots in blanking press

ram

Quasi-peak readings of blanking press before

noise control

Quasi-peak readings of blanking press after

ram ringing was contained

Air exhaust vent modification for spinning

machine noise control

Barley mill noise levels, measured betw~en

Moorspeed and Ross mills

Barley mill noise levels, measured at guard

rail of hay shredder

Barley mill noise levels, measured at control

operator chair

Floor plan of barley mill

Sectional view of barley mill

F1oorp1an of sheeter for boxboard

Sheeter drive box enclosure

Layout of sheeter and operators

Scrap handling system for cutting press

Noise levels 1 m from Jordan refiner

Noise levels inside enclosure of control room

for Jordans

Noise reduction lagging for Jordan shell

98

99

100

101

102

103

104

105

106

107

108

109

'110

113

114

115

118

124

125

126

127

128

131

131

132

135

139

140

141

xii
5.10.1

5.10.2

5.11.1

5.11.2

5.12.1

5.13.1

5.13.2

5.13.3

5.14.1

5.14.2

5.14.3

5.14.4

5.14.5

5.14.6

5.14.7

5.14.8

5.14.9

5.14.10

5.14.11

5.15.1

5.15.2

5.15.3

5.15.4

5.15.5

5.16.1

5.16.2

5.16.3

Air hammer stripper and packer line

Barrier wall theory

Top view of in-line gravure-cut press

with sound barrier curtain

Side view of in-line gravure-cut press

with sound barrier curtain

Noise levels in scrap duct for corrugated

box industry, before and after covering

Noi~e levels at wet end aisle of paper

machine

Paper mill--wet end

Noise levels inside control booth at wet

end of paper machine

Layout of punch press room

Ambient noise level with furnace and punch

presses off

Ambient noise level with furnace on, punch

presses off

Ambient noise level with furnace on, two

Summit, two Bliss, and one Benchmaster

presses in operation

operator's exposure at punch I: stamping

stock

Operator's exposure at punch I: stamping

without stock

Operator's exposure at punch VII

Operator's exposure at punch VII,

operating without work piece

Operator's exposure at punch VII,

operating without stock and without

air exhaust

Design of nozzle

Sketch of Plexiglas barrier

Straight and cut machine: close-in

measure near west side of clutch cutter

mechanism

Straight and cut machine: close-in

measure near east side of clutch cutter

mechanism

Straight and cut machine: operator's

near field exposure

Straight and cut machine: operator's

far field exposure

Barrier wall for straight and cut machine

Cut-punch press, operator's exposure

Cut-punch press, close-in diagnostic data,

14 crn from gears

Cut-punch press, close-in diagnostic data,

5 cm from the dog-flywheel interface

xiii

144

145

148

148

151

154

155

156

160

161

162

163

164

165

166

167

168

169

169

172

173

174

175

176

179

180

181
5.16.4

5.16.5

5.17.1

5.17:2

5.18.1

5.18.2

5.19.1

5.19.2

5.19.3

5.20.1

5.20.2

5.20.3

5.20.4

5.20.5

5.21.1

5.22.1

5.23.1

5.26.1

5.27.1

5.28.1

5.28.2

5.30.1

5.30.2

5.30.3

5.30.4

Cut-punch press, close-in diagnostic data,

5 cm from piston-collar impact on air

cylinder

Sketch of hanging barrier for cut-punch

press

Chute for conveying cartridge cases

Noise spectra measured 3 feet from chute

Nail making machine: operator position

noise levels

Nail making machine: operator position

noise levels after vibration isolators

installed

Noise levels for wood planer -

straight cutter

Noise levels for wood planer with helical

cutter

Effect of board length on noise from

wood planer

Comparison of punch press noise levels

with air ejection and with mechanical

ejection

Comparison of punch press noise levels

with and without a shield between operator

and air ejection noise

Comparison of punch press noise levels:

standard die versus slanted die

Noise levels 30 feet from bench press area,

before and after sound absorption treatment

Noise 30 inches from punch press, before

and after test cardboard enclosure

Effect of muffler on air exhaust from hoist

Textile braiding machine: comparison of

noise levels from steel carriers and from

polyurethane carriers

Metal cut-off saw: operator's exposure,

before and after enclosure of saw

Octave-band analysis of paper mill

vacuum pump noise

Main valve plug with throttling vanes to

reduce noise in steam line regulator

Plastics grinder; range of noise levels, dBA

Plastics grinder; range of noise levels

after soundproofing, dBA

Noise reduction achieved by reducing fan

speed, using increased blade pitch to

offset decrease in speed

Compressor discharge noise reduction achieved

by lagging expansion joint

Noise reduction achieved by redesigning pump

by-pass loop

Noise reduction achieved by adding silencer

to air blower intake

xiv

182

183

185

186

189

190

193

194

195

200

201

202

203

204

206

209

212

217

219

221

222

225

226

227

228
TABLES

1.1 Corrections from flat levels to A-levels 7

1.2 Method for approximating decibel combinations 9

1.3 Example of level combining 10

2.1 Sound level meters and accessories available 21

3.1 Required static deflection for common

industrial speeds or forcing frequencies 35

3.2 A-level noise reduction by use of heavier pipe 42

4.1 Sound absorption coefficients of common

acoustic materials 64

4.2 Sound absorption coefficients of common

construction materials 64

4.3 Transmission loss of common materials 68

5.30.1 Sources of noise and methods of noise

reduction 229

xv

Chapter 1

FUNDAMENTALS OF SOUND

The need for noise control is certainly not newi the ancient

Romans complained about chariot noise. But as the industrial revolution progressed, noise has become more intense and more persistent. Machinery to do specific jobs was designed, manufactured and

sold to its users without much, if any, concern for noise.

Recently, both State and Federal Governments have become active in

dealing with environmental pollution of all kinds, of which noise

is a part. Thus, there is a new emphasis on reduction of noise at

its source.

When faced with the problem of high or unacceptable noise

levels, you must be able to make a choice of the many different

ways of achieving reduction of these levels. In some cases you

will be able to apply more or less standard solutions that can be

found in the literature about your particular industry or machine.

However, the general problem of noise reduction is not so simple.

Rather than applying a standard recipe, you must learn and apply

the first principles of sound and vibration to the problem and

design the noise reduction means yourself. This chapter is designed

to introduce you to these basic principles so that you can apply

them.

Basic Principles

Sound is transmitted through any elastic medium (gas, solid,

or liquid) and propagates as wave motion. A convenient way to

visualize wave motion is to use a long spring, such as a "Slinky"

toy. If such a spring is stretched vertically and the top is

quickly lowered, a motion down the length of the spring will result.

The bunching together and spreading apart of the individual spring

elements is a wavelike motion. These motions help us to visualize

and thus understand wave motion.

The visual wave motion of the spring corresponds to the

invisible wave motion of sound waves. Actually, sound propagates

by elastic interactions among atomic or molecular components of

the medium through which it travels. This means that the speed

of sound depends on the mass of the molecules (density) and on

their elastic reactions (pressure).
To discuss the properties of wave motion, let us return to the

coil spring example. Again, stretch the spring vertically and then

suddenly push down on the top coils, but this time continue to

repeat this motion. You will see groupings of compressed coils

traveling down the spring. Between these compressed coil groupings

will be groupings whose coils are expanded farther apart than usual.

Of course, the coils do not move down; they stay in one average

position and oscillate about it. What moves in the wave motion is

the grouping of spring elements. This motion of .the grouping of

elements travels with a definite speed. If you change the rate of

up and down motion of your hand holding the top of the spring, the

speed of the groups will stay the same, but the spacing between

groups will change. This spacing is called the wavelength.

The wave speed in the spring remains constant while the wavelength of the groupings changes. The same situation occurs with

sound in the air. Airborne sound travels at a speed (c) of 344 meters/

second at 20°C (which is 1127 feet/second at 6SoF). The wave

motion in air involves particles (atoms and molecules) that vibrate

because of the sound energy present. The number of complete cycles

per second executed by these particles is called the frequency of

the wave. In our spring analogy, we noticed that we could change

the frequency or wavelength, the sound speed remaining unchanged.

Definitions and Relationships

We are basically interested in four parameters descriptive of

sound waves in air: sound speed, frequency, wavelength, and amplitude. The sound speed depends on the temperature of the medium,

while amplitude depends on both temperature and density.

In many noise control problems you will be concerned with both

frequency and wavelength. In modern terminology, the unit of frequency is cycles per second or hertz, abbreviated Hz. The symbol

for frequency is f.

Wavelength is the distance between similar configurations or

groupings of molecules in the sound wave. The Greek letter lambda

( t.) is the symbol for wavelength. From the coil spring experiment

it can be noted that the spacing between groups decreased as the

frequency of starting motion was increased. In mo~e precise form,

we relate sound speed, frequency, and wavelength by the formula

c = ft.. (1.1)

Figure 1.1 shows th~s relation for sound in air at 6SoF. At 1000 Hz

the wavelength is about 1.1 ft, 13.5 in., or 34 cm. In Chapter 3

we shall discuss how these concepts are used in nois~ contr~l technology.

2
20 50 100 200 500 1000 2000 5000 10,000

c at 20° C, 68 0 F

=344 meters per sec = "29 feet per sec

=13,540 inches per sec = 769.8 miles per hour

I

>,= clf =wavelength

f =frequency

c =velocity of sound

. 1 .....__......_ ......_"_......._"_.......l....l..l....-__......._ ....... ---'~.....................__......._ _"___""_........lo..oI.........

10

ill '.0 l----+-------II-----+----t-----"IiiL---i----+-~-__+----_+_--____t

ill

ll.

Z

I .5

"

Z

W

-l

W

>

« .2

:'!:

en

a:

ill.5. J 1-----+-----f---..llIlr--+----+_~----+---+_--__+-.....3Illr_-_+_--__t

ll.'

:2'

a:

o

en

ill I 2.0 I-----+-----f----_+_--"""'-

U

Z

, 00. 0 r- ---r--....,.........,.....,...,.....,...."I"""I"'.,......-~...,..-_r_...,.""""'T"""'T"_r"I""T"',._--_r_-.,......"""'T".....,.....,....,....,_n

50.0 ~--+--:'---+----+---f---3IliI~-+---+----t----+-----t

, O. 0 1-----+----.:~-f----_+___'IliIr_-+_-----+---+_--'''"-_+----_+~-__t

20.0 1----.3Il.::-'-----+---3\E---t----+-----t-.....3Ill~+_---+----__t_--_t

FREQUENCY - Hz

FIGURE 1.1 - Frequency - wavelength chart

Airborne sound is typically what we are involved with in

industrial noise problems, and the two parameters of most concern

are frequency and amplitude. The amplitude, or magnitude, of a

noise cannot be described easily without an explanation of some of

the basic physics of sound. The perception of amplitude, called

loudness, and the measurement of amplitude are related to this

understanding.

,,"

Sound has traditionally been measured by observing its effect on

something. Early methods used diaphragms and direct recorders

that were displaced by sound waves alone, much as is the eardrum.

Scientists were able to calculate the amount of sound pressure

3
required to produce displacements. However, simple and reliable

quantitative measurements were not possible until stable microphones and amplifiers were developed. The term "sound pressure"

describes the alternating pressure above and below atmospheric

pressure. The magnitude of the pressure fluctuations can be

sensed by noting that a fluctuation of only 0.1% of atmospheric

pressure represents an intolerably loud sound. Mathematical

descriptions of these pressure fluctuations are not as important

here as are the concepts of the physical motion and the definitions

of sound pressure, sound power, and sound pressure level. The

relationships between these terms is somewhat complicated, but

the following discussion will provide an introduction.

The equation relating sound power to sound pressure (for

plane and spherical waves) is

W = p2 sip c (1. 2)

where W is the sound power, p is the sound pressure, S is the

surface area associated with the measured sound pressure, p is

the greek letter "rho" used for density of the medium, and c is

the speed of sound. For noise control in which spherical waves

are encountered, S = 41T'r 2 , whereTT= 3.1416 and r is the distance

from source to measurement point.

The sound pressure, p, could be measured to indicate the

magnitude of a sound, but the range of values between a high and

low sound pressure could easily have a ratio of a million-to-one.

To eliminate the problem of this wide numerical range, "sound

pressure level" (Lp) is used. The logarithmic relationship

between sound pressure level and sound pressure is defined by the

equation.

(1. 3)

where Po is a reference pressure. The reference pressure has

been chosen to be a value which is approximately the threshold of

hearing at 1.000 cycles per second (0.0002 dyne/cm2 , 20 micrometer/

m2 , or 20 microPa). The unit of sound pressure level is the

"decibel" (dB), so that if p = Po' then Lp = 0 dB. From the

definition of reference pressure, it follows that a person with

excellent hearing could hear sounds levels at least as low as 0 dB.

Experiments have developed the approximate relation between

loudness and sound level. For levels around that of conversational

speech, the sensation change that is described as "twice as loud"

corresponds to a change in level. of to 10 dB. Thus, sound at

60 dB would feel about twice as loud as sound at 50 dB; and at

90 dB, half as loud as 100 dB.

The sensation of pitch is well-correlated with frequency.

The musical interval called the "octave" is recognized by most people.

It corresponds to a frequency ratio of two to one. Thus,middle C,

at 262 Hz, is an octave above 131 Hz and an octave below 524 Hz.

Often you will need to estimate the frequency of such machinery

4
sounds as those from fan blades and gear teeth. Remember that you

need the number of events per second, ~hereas rpm is usually given.

For example, if a large air-moving centrifugal fan has 24 blades

and operates at 1165 rpm, the fan blade passage frequency is (1165)

(24)/60 = 466 Hz. This tone should be audible to the ear. With

practice, you can learn to listen for such definite tones in the

presence of other components of the noise.

Quality refers to that aspect of sound that enables us to

distinguish between two similar sounds. The physical equivalent

of quality is the character of the spectrum. The spectrum is an

objective and precise numerical statement of how much of the total

sound exists in all frequency regions, from low to high. Spectra

are measured with electrical filters that pick out and pass only

the frequency regions for which they are designed. The one in most

common use by the noise control engineer is the octave band filter,

which is discussed in Chapter 2. With this, the resulting spectrum gives the sound levels in each octave band, usually 63 through

8000 Hz.

Hearing and Noise Level Weighted Scales

The whole human hearing system, which we refer to as the ear,

can receive and interpret a wide variety of sounds. When sound of

only one frequency is present, and the sound pressure variation

with time is sinusoidal, the sound is said to be a pure tone. Complex sounds, such as the usual noise spectrum, may be considered

to be made up of a multiplicity of random phase pure tones.

We have seen that the ear is capable of sensing very small

amounts of sound energy. Figure 1.2 shows the relationship between

excellent hearing and the response of the weighting circuits in

standard sound level meters.

Th~ combination of noise levels and the exposure time to such

levels is called noise exposure. "(Noise exposure calculation is

explained in Chapter 2.) The spectrum of noise is also important,

as can be seen from examination of Figure 1.2. The ear is much

more sensitive to components with frequencies above 1000 HZ, and

is much more easily damaged by noise of high intensity at, say,

3000 Hz than at 300 Hz. Instead of dealing with the many frequencies in the hearing range, it would be convenient to be able to use

a single number descriptive of the noise spectrum character, to simplify the measurement and evaluation of noise.

Botsford, Young, and others have successfully demonstrated the

value of this single number approach. It turns out that A-scale

readings of noise show reasonably acceptable correlation with the

potential for causing hearing loss. At the same time, some simple

indicator of spectrum character is needed. Botsford takes the

difference between the A-weighted or A-scale noise and the C-weighted or C-scale noise levels measured on a standard sound level meter.

Reexamination of Figure 1.2 shows that the C-weighted noise includes more of the lower frequencies than does the A-weighted.

Thus, if we consider the following three possibilities, we can see

what is meant about quality of noise.

5
co

"'0 10 -10 >- I I- w(/) 0 0 >co _"'0

Z !::::I 0

c.. ·10 10 (/) a:: ~<t (/)

w sound level

a:: 20 (/) w

W ·20 meter scales WW

>:I: > ·30 30 -lI- I-u..

<t ear "response" at threshold <to ....J

W ·40 40

a:: a::

·50 50

100 1000 10,000

FREQUENCY - Hz

FIGURE 1.2 - Response characteristics of weighting scales and of ear at threshold

(L - LA) > 0 Low-frequency quality C

(L - LA) = 0 Medium-frequency quality C

(L - LA) < 0 High-frequency quality C

For a fixed C-level, noise in the last two cases will induce more

hearing loss than low-frequency quality noise. It must be noted

that consideration of the actual level is left out of this spectrum

quality parameter and must be considered first before quality judgements are made. As you get deeper into noise level measurements

and analyses, you will find that this kind of single number description aids you when you need a quick, or on the spot, analysis of

noise.

The evolution of the use of A-scale noise levels, LA' has

basically been the result of experimental evidence on the relative

hearing loss from constant-level sounds of different frequencies.

'High-frequency sounds cause much more hearing loss than low. The

similarity to hearing sensitivity was shown earlier in Figure 1.2.

Thus, A-weighted sound has become the common legal description for

6
,

sound lev~ls, because it incorporates a good single approximation

to the weightings that show good correlation with such effects as

loudness, speech interference and annoyance, as well as hearing

loss. The American National Standard for sound level meters states

that, unless otherwise indicated, sound level means LA' in units of

dBA. To convert flat response readings to A levels, apply the weighting factors at octave band center frequencies shown in Table 1.1.

TABLE 1.1 - Corrections from flat levels to A-levels

Octave Band

Center dB Frequency

(Hz) (Change)

31.5 -39.5

63 -26

125 -16

250 -8.5

500 -3.0

1000 0

2000 +1.0

4000 +1.0

8000 -1.0

16000 -6.5

So far we have talked about noise as complex sounds or noise

spectra and characterized the quality of noise, but we have not

actually defined noise. The standard usage or accepted definition is that noise is unwanted sound. This is more a psychological than a physical definition. The sound of a racing car engine

running may not be noise to you but to your neighbor it may be.

7
In a legal sense, noise is whatever sound may be subjected to

regulation because of undesirable effects. Usually, sound level

and duration are the predominant criteria. Effects of sounds or

noise include not only hearing loss, but also annoyance, as well

as speech and sleep interference.

Combining Noise Levels

A frequent calculation in noise control engineering is combining

levels. Suppose we have two levels and we are interested in finding out what our soun~ level meter would read if the two sounds

were simultaneous. We make the basic assumption that the noises

are random; that is, only weak pure tones are present. The formula for calculating the combined level, L

c ' given Ll and L2 , is

L

c = Ll

+ 10 log (10(L2 - Ll)/10 + 1) (1.4)

A practical example would be to have two closely spaced noise

sources and to stand, say, about halfway between them. Measure

the levels with a sound level meter, first with one on and the

other off, then vice versa. Combination of the levels should give

you the level if they were both running. This combination of sound

levels of random noise requires adding the power or energy of each

source and using the decibel or logarithmic transformation to get

what is read on a sound level meter.

Some peculiarities in the measurement of sound and the use of

decibels are worth mentioning. Doubling of the energy raises the

level by 3 dB. For example, two equal random noises of 40 dB each

combine to a 43 dB total. Two equal 90 dB random noises combine

to 93 dB. The combination process uses the physical fact that the

square of the sound pressure is usually closely proportional to the

sound energy.

The chart of Figure 1.3 enables this combination to be performed without the formula.

For rough work, the scheme of Table 1.2 is easy to remember

and to use; its results are usually good to 1 dB. To remember

the break points at which 3, 2, or 1 dB is to be added to the larger level, note that the break point differences of 1, 4, and 9 turn

out to be the squares of the first three integers. The exact values of these break points are 1.1, 3.8, and 9.1 dB.

As an example of the use of Table 1.2 and Figure 1.3, consider

the problem of calculating the noise from several identical machines that are at different distances from the point at which the

combined level is needed. Suppose that the noise from each machine

8
4 5 6 7 8 9 10 11 12 13 14

NUMERICAL DIFFERENCE

BETWEEN TOTAL AND SMALLER LEVELS - dB

FIGURE 7.3 - Method for combining decibel levels·

NUMERICAL 01 FFERENCE BETWEEN

TWO LEVELS BEING ADDED - dB

-J o

ct

5 3

Z

wm

W-c

:!:,

~-J

WW2J.----H.I m>

WW

U-J

Za: Ww

a: c)' Wa: u. ct

!:!:-J

00

-JZ

ct ct , U

a:

W

::2 Ol.-_"'----l'--....._....L.._......._..-._.l.-_~___L .;.J

3

* From Handbook of Noise Measurement, General Radio Co., Concord, MA.

TABLE 1.2 - Method for approximating decibel combinations

Difference Add to Larger Scale

(dB)

0 3.0

1 2.6

2 2.1

3 1.8

4 i 1.4

5 1.2

6 1.0

7 0.8

8 0.6

9 0.5

10 0.4

11 0.3

12 0.2

9
operating alone is given by the levels in the first line of Table

1.3. In the second line We have arranged these, for convenience

only, in order of increasing level. Successive lines below show

the steps of combining levels in pairs by the rules of Table 1.2.

The result is 98 dB. By rigorous calculation, the correct value

is 97.5 dB.

TABLE 1.3 - Example of levef combining

Original levels

Rearranged

85

84

92 90 84

85 87 90

93

92

87

93

Lr-I

84 & 85 =88 88

I . 88 & 87 =91 91

I . 91 & 90 = 94 94

• 92 & 93 :.=96 96

I

94 & 96 =98

Noise Generation and Noise Fields

98

When investigating noise it is often convenient to consider

separately the source, path, and receiver components of the problem. At the source we are concerned with the forces that cause

surfaces to vibrate or air to generate turbulence, or both, and

cause sound to be radiated. Along the path the sound spreads out

and may be absorbed. The receiver is (for our problems) the human

ear, with attention to the noise exposure experienced.

It is important to be acquainted with how the noise is generated and radiated. Such knowledge can sometimes permit the introduction of simple modifications to machines to reduce the amount

of sound that is radiated. The two chief items about which information is usually sought are the exciting force and the size,

10
nature, and orientation of radiating surfaces.

tion forces are from unbalanced rotational and

sources, impact aerodynamic sources, friction,

(AC transformers), hydrodynamic sources (water

cavitation.

The usual excitareciprocating

magnetic sources

hammer), and liquid

Sound is radiated into the air by such surfaces as machine

frames, panels, and guards; unrestrained stock and workpieces;

such stressed parts as saws and diesiand friction-excited surfaces such as brakes. Fan blades leave a wake of alternating

eddies behind. Steam and air venting makes noise by shear against

the surrounding relatively still air; the sound power is typically

proportional to the gas velocity to about the fifth power.

When sound spreads out from a small source with no major reflecting surfaces nearby, the sound level along the path decreases

with distance. If the level at distance r is L , then at r the

1 1 · 0 0 eve 1S

(1. 5)

The term "inverse square law field" is given to the region in which

equation (l.5) holds. Conversely, if we are reasonably certain

that inverse square law conditions pertain, and we know the level at

one position, then by (l.5) we can calculate the level at any other

position in this region, along the same radius from the source at

which the given level was measured. However, if the source is directional, then it will be necessary to measure at many angular

values to get a complete description of the sound field.

As we have seen, the magnitude of the loudness sensation depends on the sound level at the ear. However, a complication develops when sound in a room, such as in a factory space, is to be calculated. At a sufficient distance from the source of noise, we

will hear not only the direct sound, but also all the sound that is

reflected from the room surfaces. The behavior of sound level with

distance from a noisy machine is sketched in Figure 1.4.

There are three relatively distinct regions to which we shall

refer. Close to the machine, or in the near field, levels will

depend very much on near-by noise spots, such that levels

may remain constant or may decrease. In the second region we have

inverse square law behavior, where the noise from the machine is

much greater than that reflected from the walls and other room

surfaces. In the third region, the reflected sound predominates,

and the level is relatively constant. Typically, because we are

not dealing with ideal point noise sources, the falloff of noise

level with distance does not behave in a smooth way. Differences

in the way the noise falls off with distance away from the source

will indicate to some degree the kind of noise control to be used.

11
The critical distance (rc ) concept is useful. Critical distance

is ,that at which the noise level changes from inverse square behavior and becomes relativ~ly constant (reverberant level) as shown

in Figure 1.4. (For applications see Chapter 3.)

3 reverberant

___________ l__

70

60

2 inverse

square

50

ID

"0

1 near

...J 40 ill

>

ill

...J

0 3C z

::J

0

CJ)

20

10

3-dB change point

rc r4

LOG DISTANCE - r

FIGURE 7.4 - Noise field around a single machine. Critical distance is rc'

The junction between regions 2 and 3 (Figure 1.4) is at the

critical distance, r c . Beyond that distance, increased absorption

on the room surfaces will reduce the reverberant level but not the

level in the inverse square law level of region 2. The importance

of this for noise control engineering is that if a worker is in

region 2 (and most are), acoustical treatment of the room surfaces

will not reduce the noise he receives from his own machine. The

chief exception occurs when there are many machines, close together in a low-ceiling room. If a factory room has average absorption coefficient a and has n identical floor mounted machines

that emit noise equally in all directions, the critical distance is

r = 0.20 / S a In

c (1. 6)

12
where S is the total area of the walls, floor, and ceiling and is

measured in the same units as r c . Note that additional absorption

(increasing a) will· reduce only those levels in zone 3, beyond

the critical distance, r c . Thus, the ambient room background levels

can be reduced by increasing the acoustical absorption at room surfaces.

Sound Power

For calculating the reverberant sound field in region 3, it is

necessary to know the total sound power emitted by the machines or

a standard source in a room. Sound power cannot be measured directly but must be calculated from measurements made 'under closely controlled conditions, usually in the laboratory. One procedure uses

a nonreflecting (anechoic) environment: another uses a highly reflecting room for the machine in question. It is usual to state, not

the sound power, but the sound power level. This is defined by

(1. 7)

where W is the sound power in watt, and Wo is the reference power

of 10- 12 watt, and corresponds to Po or 20 micronewton/m2 reference

sound pressure.

Except for absorption in the air, the radiated sound power is

independent of distance from the machine. However, the intensity

(power per unit area) does change inversely as the square of the

distance away from a point source. The area over which intensity is

measured is easily related to the shape of the measurement surface

and the distance away from the source.

For calculating noise levels in the open, away from all reflecting surfaces, the sound pressure levels at various distances can

be calculated by equation (1.5). However, in a room, and in factory

workshops 'in particular, it is first necessary to start with sound

power. Also, sound power characterizes the total noise from the

machine and is useful in comparing different machines on a noise

basis. In terms of hearing damage, however, it is the A-weighted

sound pressure level that is needed and to which other measures

must eventually be converted.

At our reference power Wo = 10-12 watt, the sound pressure

level measured 0.28 meter away from this ideal point source would

be zero with no reflecting surfaces.

The general relationship for a point source well away from

any reflecting surfaces, emitting sound power level Lw is:

Lp = Lw - 10 log S = Lw - 20 log r - 11

13

(1. 8)
(1. 9)

where r is in meters and S is in square meters: if r is in feet,

replace the numeric 11 by 0.7. The constants are the result of

the units used. Now for point sources this can be an extremely

useful relationship. If the source is on a large flat surface

such as a floor in middle of large room with no reflectors other.

than the surface, the relation is

L = L - 20 log r - 8

p w

where r is in meters.

In the future, possibly the sound power (W) of a noise source

will be the single more important parameter to know when analyzing

the sound level in a room, especially with noise-rated machines.

After some additional reading in references such as those listed

in Chapter 7, you will be able to use equation (1.9) to aid in

writing purchase specifications. Combination of the level you

read on your sound level meter before a new machine is installed

with the one you predict using equation (1.9), plus information on

the acoustics of the room, will tell you whether the new noise

level will be acceptable. If several machines are present, decibels

may be combined by the rules discussed earlier to give the total

level.

Summary of Symbols and Definitions

Some of the definitions used in sound measurements aresummarized below.

W = Sound power

p = Sound pressure

p = Density of medium in which sound is being propagated

c = Speed of sound in medium

S = Area over which sound pressure is measured

r = Distance away from source

W

o

-12 = Reference sound power, 10 watt

= Reference sound pressure, 20 micronewton/meter 2

= Radius of reference sphere = 1 meter

Pc = 410 rayls, where p is the sea level density of air and c is

corresponding sound speed at 20°C.

14
Chapter 2

MEASUREMENT OF NOISE

In the usual industrial noise situation, there will be two

types of measurements:

(1) Compliance measurements, which are made in

accordance with some relatively precise set

of instructions, usually based on laws or

regulations.

(2) Diagnostic measurements, which aid in engineering

control of noise by locating specific noise

sources and determining their magnitudes, and

by indicating the types of sontrols needed,

their locations, and the amount of reduction

sought.

The two types will be explored in detail in this chapter.

Compliance Measurements

Compliance measurements are made with some relatively precise

set of instructions, usually based on laws and regulations. The

purpose is usually to determine the extent of compliance with the

limits set forth in the laws or regulations. Thus, in a n6ise exposure compliance survey for industrial noise, the data will be the

slow A-weighted levels measured at the ear location of the workers,

together with the times spent at the levels encountered. From these

data, the daily noise dose is calculated by means specified in the

regulations.

Basic Instruments, Use and Calibration. The chief instrument

for noise measurements is the sound level meter (SLM) , which should

be a Type 1 or 2, made in accordance with American National Standard

S1.4 (1971), "Specification for Sound Level Meters." The Type 2

instrument has broader tolerances on performance than the Type 1 and

is acceptable under the OSHA Occupational Noise Exposure regulations. It is usually less bulky, lighter, and less expensive than

the Type 1 SLM. The SLM consists of an omnidirectional microphone,

a calibrated attenuator, a stabilized amplifier, an indicating

meter, and weighting networks. The networks provide the three common sound level meter filters: A, Band C, and can be set to

either Fast or Slow response. More details on types of meters and

their manufacturers are given in the discussion of diagnostic measurements later in this chapter.

15
When the noise levels are known to change very little throughout the working day, a simple SLM reading suffices. However, the

reading must be taken properly. The standard procedure is to locate the microphone at the ear position concerned, but with the

worker at least three feet away. This is the so-called free field

measurement that is specified as preferred in American National

Standard Sl.13-1971, "Methods for the Measurement of Sound Pressure

Levels." For a general standing position, the preferred microphone

height is 1.5 m (about 5 ft); for a seated worker, 1.1 m (about

3.5 ft).

When it is necessary to make sound measurements that will

withstand scrutiny in the courts, several criteria are important:

(1) The data should be obtained by a qualified and disinterested individual, to avoid charges of bias. .

An acoustical consultant of known accomplishment,

preferably also a registered professional engineer

is appropriate.

(2) The instruments and measurement procedures used

should conform fully with the applicable American

National Standards.

(3) Instruments should be calibrated before and after

each significant set of readings. If the calibration is out of tolerance, readings back to

the previous calibration must be repeated.

(4) The calibration should be traceable to the National

Bureau of Standards.

Obtaining reliable data depends on periodic calibration of

the instruments. The preferred calibrators deliver an acoustical

signal of known frequency and sound pressure level. Some calibrators provide a variety of signals of different frequencies and

levels. To ensure that the calibrators are correct*, it is best

to buy two and reserve one (as a comparison instrument) which have

been certified as to sound level by means traceable to the

National Bureau of Standards.

The manufacturer's instructions for holding the SLM should be

followed. The practice in the United States is to calibrate the

microphone for a correct reading ~~0n the sound grazes the diaphragm

* For moderate use, annual calibration of the calibrator is often

satisfactory.

16
of the microphone. Thus, the axis of the microphone is at a

90-degree angle to the direction the sound is travelling. Instruments made in Europe (such as the B&K) are calibrated to be correct

when the microphone is aimed at the source. However, these instruments can be used with a random incidence corrector fitting, which

effectively converts them to the u.s. type of use.

For both types the observer, microphone, and sound source must

be at right angles to the direction of the sound. To be absolutely

sure there is no i~terference from the body of the observer, position the microphone at least a meter away from the observer. With

steady noise conditions, possible interference can be determined

easily: with the microphone at a fixed position, decrease the distance between the observer and microphone to less than 1 m. The

position at which a significant change in reading occurs indicates

(for that specific situation only) how far away the observer need

be.

For most industrial situations, a reading on the slow A-scale

is specified for compliance measurements. Despite the averaging

properties of the "Slow" setting, industrial noise is often variable enough that reading the meter becomes a problem.

As a general matter, it is best to first explore A-levels in

the region of interest before obtaining the final sound level for

compliance measurements. Sometimes directional effects can change

the reading a few dB in a few feet. One example is a noise source

that is partially shielded by a machine structure, with the operator in and out of the acoustical shadow. Several readings may be

needed to completely delineate the noise in the range of positions

used by the worker in question.

In sampling noise for obtaining statistical data using the

"Fast" setting on the meter, you should allow no less than one-half

second between observations; in using the "Slow" setting on the

meter, an interval of at least two seconds is recommended. Machinery that operates at varying speeds or cycles of operation produces

noise that fluctuates ln level. Cyclical noises are those that

change repetitively in excess of 5 dB. A suggested sampling method

is to take readings, with the SLM set to slow response, every 15

seconds for a period of 3 to 5 minutes from which an average value

can be calculated.

It is often necessary to record a single reading that is the

average over a short period. The following rules may be used.

(1) If the difference between average minima and

average maxima is less than 6 dB, use the average

of these two extremes.

17
(2) If the difference is greater than 6 dB, use

the reading 3 dB below the average maxima.

(3) It is ~lways preferable to record the range

of readings if over 6 dB, plus comments on

probable cause. Typical causes include machine

cycling and very low frequency pulsation from

air handling equipment.

When an operator performs various duties during the working day,

he can be exposed to varied permissible noise exposures specified

by the Occupational Safety and Health Administration (OSHA). These

can be used to determine the total daily noise dose, as follows:

When the daily noise exposure is composed of two or more noise

exposures of different levels, their combined effect is the sum

of the fractions, at all levels, of the actual time (hr) divided by

permissible time. The sum of these fractions must be equal to or

less than 1.0 for a permitted daily noise dose.

Permissible noise exposures are:

Hours/Day

8

6

4

3

2

dBA

90

92

95

97

100

Hours/Day

l~

1

or less

dBA

102

105

110

115 maximum

Exposure to 'steady noise of 115 dBA is the maximum allowed. Exposure to impulsive or impact noise should not exceed 140 dB peak.

This cannot be measured on the basic sound level meter and requires

more expensive special meters.

Some general precautions should be observed when using the

sound level meter. Wind or air currents can distort readings (a

wind screen attachment will minimize this problem and also protect

the microphone in dirty industrial environments). Vibration of the

meter, supported or on a tripod, can distort readings. Excess room

humidity or temperature can also be a problem, as well as magnetic

distortion of the meter from adjacent power equipment. Barriers or

walls can obstruct sound and reduce measurements of a SLM, or, by

reflection, increase measurements. Background noise should be at

least 10 dB below the total measured level. (The instruction booklet with the meter gives corrections to readings for level differences below 10 dB.)

To avoid dropping the meter when it is hand held, keep the

safety cord that comes with most meters wrapped around your wrist.

18
Many other nonacoustical data are required to support the

noise dose information. Such data include plant location and

product, pertinent personnel and their positions in the organization (especially important if they are to be witnesses), persons

present during measurements, time span of measurements, room layout and dimensions, sketches of machines, descriptions of machines,

production data, the daily time machines are producing, worker and

measurement locations and photographs.

Dosimeters. If the employee (such as a maintenance or repair

person) moves through differing noise situations, a dosimeter is a

useful instrument for measuring noise. At the present time, there

is no completed American National Standard covering the performance of dosimeters, although these instruments have been available

for at least two years. Calibration is essential to dosimeters,

and the manufacturer's instructions should be followed until standards for use are available. Any newly received dosimeter should

be left on continuously for at least eight hours and calibrated

periodically to determine if there are any shifts in reading.

Some observation of the employee wearing the dosimeter may be

needed to attest to normal activity. By deliberately favoring high

or low noise level positions, the employee can influence the indicated

dose upward or downward. Readout should'not be available to the

employee directly from the instrument, lest the employee be encouraged to play with the instrument. The dosimeter should contain

simple provisions (as circuit board replacement) for changes anticpated in OSHA regulations.

Diagnostic Measurements

Diagnostic measurements are made to aid engineering control

of noise by locating specific noise sources and their magnitudes

and by indicating the types of controls needed, their locations,

and the amount of reduction sought. Such diagnostic measurements

are not always straightforward, and their success depends mostly

on the capabilities and experience of the person making the measurements. In a very real sense, diagnostic measurements of noise

constitute a sort of detective activity, in which clues must be

properly read, hunches followed up, and data cross-checked.

No single detailed method can be specified, for each situation is unique. However, workable general procedures and the

equipment used are described below.

Sound Level Meters. Diagnostic measurements can be made with

the Type 1 or Type 2 sound level meters that are limited to A, B,

and C scales (B is not normally used) and "Fast" or "Slow" settings. The "Slow" setting is the basis for OSHA compliance

regulations.

19
To the basic sound l~vel meter, special attachments are

available, including:

Windscreen

Earphone coupler

Vibration pick-up and adaptor

Tripod

Extension cable for microphone.

This equipment package can be used to solve many noise source

problems for which specific data based on an octave band analysis

are not required. For example, absorption of a material is

frequency dependent, but using the absorption data at 500 Hz

will generally result in a satisfactory result because the reduced

absorption at lower frequencies is offset by the lesser importance

of these frequencies in an A-scale reading. Although these readings

may result in using slightly better materials than the absolute

minimum, in most cases this cost difference on a single installation

is small. So the limitations of measurements with only A- and Cscale are of degree only. Many acceptable solutions have been

developed without full octave band data. Many of the case histories

included in Chapter 5 used only these basic data.

Table 2.1 lists the"suppliers and model numbers of several

sound level meters. The table also provides data on presence andl

or availability of A, B, and 0 scales, "Fast" or "Slow" meter

movements, calibrator, windscreen, and microphone extension cord;

decibel range (for Type 2); and availability of an octave band

analyzer, accelerometer, and amplifier.

Following Table 2.1 is a list of suppliers who state explicitly that their sound level meters meet at least Type 2 specifications under American National Standard Sl.4-l97l. These specifications" are somewhat more rigorous than those of IEC 123, which are

sometimes acceptable. If you are not sure that an instrument

thus qualifies, any reputable manufacturer can supply the backup

data.

20
TABLE 2.1 - Sound level mete rs and accessories available

Scales Speed Capabilities Accessories

Name, Model A B C F S Peak dB Ranqe Cal WS EXI OBA Acc AnlDl

ACOS SLM 3 (1) X X X 50-130 X X X X X

Advanced Acoustical Research 101A X X X X X 40-140 X X X

Amstro SL-1 (1) X X X X X 40-125 X X X

Bausch & Lomb X X X X X 40-140 X

Bendi x 300 X X X X X 55-140 X

B & K 2205 X X X X X 32-140 X X

B & K 2209 (2) X X X X X X 6-140 X X X X X - Cast Ie CS 16A X X 60-130 X (3)

Castle CS17A X X X X 20-140 X

Columbia SPL-103 X X X X X 40-140 X -- Dahlberg GS-9510 X X X X X 20-130 X X X

Dawe 1400G (1) X X X X X 24-140 X (3) X X X X

Digital Acoustics DA100 (2) X X X X X X 14-154 (4)

Edmont 60-510 X X 60-120 X

Electro-Sonic 2010 X X X X X 40-140

General Radio 1565B X X X X X 40-140 X X X X

General Radio 1933 (2) X X X X X X X X X (4) X

Mamash S716 X X X X X 18-140 X X

Mechanalysi s 308 X X X X X 45-140 X X X

Metroson i cs dB-201 X X X X X 40-140 X X

M-S-A X X X X X 32-140 X X

Peekel GRB (1) X X X X X 20-150 (4) X

Pulsar X X X

Quest X X 60-120 X

RAC SPL-103 X X X X X 40-140 X

Rhode & Schwartz ELT (2) X X X X 55-120 X X X

Rhode & Schwartz E ZGA (2) X X X X X X 20-160 X X X X X

Scott 452 X X X X X 40-140 X X

Tracor SPL-103 X X X X X 40-140 X

Tri pi ell 375 X X X X X 40-140 X X

Unico SLP-21 (1) X X X ? 25-130 X X

F last speed

S = slow speed

Cal = Cal ibrat or Notes:

WS = wind screen

Ext = extension cord for microphone (1) Conforms to standard IEC 123, slightly less rigorous than ANSI

Peak = true peak reading capabil ity S1.4-1971, Type 2.

OBA = octave band analy zer (2) Conforms to standard ANSI S1.4-1971, Type 1, precision.

Acc = accelerometer (3) Calibrator is of falling ball type.

Ampl = amplifier for extending low level range (4) Octave band analyzer Integral with instrument.

21
METER AND MANUFACTURER/SUPPLIER ADDRESSES

ACOS: Cowl Industries Ltd., Milwaukee, WI

Advanced Acoustical Research Corp., Sawmill River Road, Elmsford,

'NY 10523

Amstro Corp., 120 Clinton Road, Dept. D, Fairfield, NJ 07006

Bausch & Lomb, Rochester, NY 14602 (as Columbia)

Bendix: National Environmental Instruments, Inc., P.O. Box 590,

Warwick, RI 02888

B&K Instruments, Inc., 5111 W 164 Stree~ Cleveland, OH 44142

Castle Associates: R-D Eck, Inc., 12 Dale Street, Waltham,

MA 02154

Columbia Research Laboratories, Inc., MacDade Blvd. & Bullens

Lane, Woodlyn, PA 19094

Dahlberg Electronics, Inc., P.O. Box 549, Minneapolis, MN 55440

Dawe Instruments Ltd., Concord Road, Western Avenue, London W3,

England

Digital Acoustics, Inc., 1415 E McFadden, 'Santa Ana, CA 92705

Edmont-Wilson, Coshocton, OH 43812

Electro-Sonic: Monotherm Insulation Systems, Inc., 551 So.

Yosemite Ave., Oakland, CA 95361

General Radio Co., 300 Baker Avenue, Concord, MA 01742

Mamash Applied Science Laboratories Ltd., Ramat Aviv, Tel Aviv,

Israel

Mechanalysis: IRD-Mechanalysis, Inc., 6150 Huntley Road,

Columbus, OH 43229

Metrosonics, Inc., Box 18090, Rochester, NY 14618

Mine Safety Appliances Co., 201 N Braddock Ave., Pittsburgh,

PA 15208 (as B&K 2205)

Peekel Laboratorium voor Electronica N.V., Alblasstraat 1,

Rotterdam, The Netherlands

22
Pulsar Instrument Co., 650 So. Vaqueros Street, Sunnyvale,

CA 94086

Quest Electronics, 510 Worthington Street, Oconomowoc,

WI 53066

RAC: Research Appliance Co., Rt. 8, Allison Park, PA 15101

(as Columbia)-

Rhode & Schwartz, Muehldorfstr. 15, 8000 Muenchen 80, West

Germany

Scott Instrument Laboratories, 533 Main Street, Acton,

MA 01720

Simpson Electric Co., 5200 W. Kinzie Street, Chicago, IL 60644

(as General Radio)

Tracor Medical Instrument Division, 6500 Tracor Lane, Austin,

TX 78721 (as Columbia)

Triplett Corp., Bluffton, OR 45817

Unico: National Environmental Instruments, Inc., Fall River,

MA 02720

23
Octave Band Filters. This equipment is normally an attachment to a sound level meter. It measures the sound pressure

level in each octave band, normally from 63 Hz to 8000 Hz. Separate octave band analyzers often include weighted band readings

of A and C (slow and fast) plus peak readings. This instrument

can spot the band in which a maximum occurs. This aids in determining the noise source, as for example in verifying that the

blade-pass frequency of a fan is in the band with maximum level.

With the data from the octave band analysis, barriers and enclosures may be designed in detail, using the prediction of the required attenuation as a function of band frequency .. By investigating the source in the machine having the same frequency as in

the band maxima found at the operator position, you can determine

the exact reason for the noise level and possibly change it by

redesign, local barrier or enclosure, damping, or other methods,

which will be discussed in Chapter 3.

Fractional Octave-Band (Proportional-Band) Analyzers. Depending upon the degree of exactness required, proportional-band

analyzers can be obtained for 1/3 octave or 1/10 octave. This

fine resolution would find critical use where machine redesign is

contemplated and all specific noise sources need to be identified

to lead to corrective design to reduce noise. This is normally

not the problem on existing equipment, for which noise can

usually be controlled by barriers, enclosures, isolators, damping,

and so on. Exceptions occur when sources of single frequency

noise may be close together in frequency, and fine resolution may

be required ..The usual example is that of isolating the contributions from gearing systems, where several gear tooth passage

frequencies (and harmonies) may be close together.

Photographic Equipment. The role of photography in documenting the situation cannot be over-estimated. You will need a

single lens reflex camera, a 28 or 35 mm close-focusing wide angle

lens, and an electronic flash, preferably with automatic control

and remote sensor. The flash should be \vide angle. Be careful

with the flash gun if explosive atmospheres may be present; you

may not be allowed to use it. If at all possible, include a scale

of length in all photos. Use Tri-X or similar high speed film.

If you cannot use flash, a tripod may be needed for time exposure

photographs. An essential part of the photographic effort is documenting the acoustical and production ronditions for each shot.

If you have a plan qr sketch of the workroom, establish a dimension grid and refer to that in your notes on each shot. The first

exposure in each roll should be a close-up of a sheet of paper on

which you indicate the necessary identification information.

Start with establishing shots that show the entire area and the

relations of the machines to each other. Take pictures from several angles. Then corne in closer to show the whole machine, again

at several angles. For every microphone position at which sound

24
level data are obtained, photograph the setup. Use close-ups for

added clarity. When noise sources are identified or even suspected,

photograph their locations from several angles, showing the relations to other parts of the machine.

The photographs will be useful for documenting the measurements, for revealing machine features that must be considered when

noise control means are designed, for showing worker positions, for

identifying competing noise sources, and for possible evidence in

any legal procedures.

Tape Recorders. A suitable battery-operated magnetic tape

recorder can be useq to obtain a noise sample for repeated analysis

in the laboratory. A major difficulty is that tape recorders are

adjusted for optimum operation with speech or music signals. Industrial noise signals may be different enough in peak-to-average

level characteristics and in spectrum shape that the full performance of the tape recorder cannot be realized. Competent advice

should be sought before purchasing a tape recorder for noise. Some

instrument manufacturers are now supplying such tape recorders.

A convenient combination is a precision (Type 1) SLM and a

two-channel (stereo) tape recorder. The SLM serves as the iriput

device to the tape recorder. By proper setting of the controls,

and with a knowledge of the peak levels present, it is possible to

make tape recordings from which most of the original information

may be recovered. There is one exception: for complex noise signals, the original and playback signal waveforms will usually not

be the same. Hence, do not expect a tape recorder (unless specially adjusted or equalized) to provide the same peak-to-average

level information that was in the original noise signal.

To recover actual levels from the tape record, you must record

an acoustic calibration signal of known sound level. This signal

is conveniently supplied by the acoustic calibrator used on the

microphone of the SLM. Use it to calibrate the SLM, and connect

the latter to the signal channel of the tape recorder. The tape

recorder should have a meter that indicates recording level. By

a series of qualifying measurements on the tape recorder, you

should have established for each octave band the level above the

top end of the meter scale at which distortion becomes excessive,

and the level below th~bottom end of the meter scale at which internal noise limits the signal that can be recorded. The distortion limit is often considered to be the level at which the playback is 1 dB below that expected on a linear basis.

While the calibration signal is being recorded, use the other

stereo channel for voice recording of notebook information: data,

time, job number, location, description of setup, description of

calibration signal, and most importantly, settings of all controls

25
on SLM and tape recorder, together with the time, and a brief

description of the content of the run.

On playback, the scheme for recovering the absolute levels,

depends on that fact that the output voltage from the SLM depends

only on the reading of its meter and not on the settings of its

attenuator(s). For example, suppose that the calibration siqnal

sound pressure level is 114 dB and that the meter scale is 0 to

10 dB. Then the SLM range switch will be set at 110 dB, and the

meter movement will read 4 dB. The output voltage from the SLM

due to the calibration signal is then recorded. Next, suppose

you tape a noise that fluctuates between 85 and 95 dEC (not dBA)

Then you set the SLM range at 90 and note this setting on the

voice channel. On playback into a suitable system (which may be

the same SU1) with a decibel output scale, you set the controls

so that the calibrate signal reads 4 dB on the output meter.

Then when the unknown noise signal is played back, zero dB

on the output meter corresponds to the range setting on the SLM

when the signal was recorded "flat", that is, with the dBC or

preferably "linear" setting. Then suppose on playback you wish

to insert an A-weighting filter; if the SLM is being used as the

playback indicator, set it to A-scale. If the output meter reads

2, then the level was 92 dBA (original recording with the SLM

range at 90).

If the original signal has large low-frequency content, such

as noise from process air blowers, it may all too easily overload

the tape recorder. For such signals, recording can be done with

the A (or B) scales to remove some of the excessive low-frequency

noise. Thus, on playback using a "flat" system, the total level

will be in dBA (or dBB). Octave band levels can be then approximated by adding the weighting for band center. However, if the

spectrum slopes rapidly, additional and more uncertain corrections

are needed.

Another use for a tape recorder is as an electronic notebook.

In this use, all data, descriptions, comments, and suggestions for

noise control can be placed directly on tape. Use a cassette

recorder, preferably one taking standard cassettes. It should also

have an automatic volume control and an automatic voice switch to

initiate recording simply by talking. Set recording level low so

that you must speak loudly when the microphone is directly in front

of your mouth. This procedure will increase the signal to noise

ratio so that you can make understandable tapes even when the noise

is in excess of 105 dBA.

Other Equipment. For diagnostic measurements you will need,

as described above, a standard Type 1 SLM,·preferably with peakreading capability; octave band analyzer; calibrator; heavy-duty

26
tripod strong enough to hold the meter; extension cord; lightweight

tripod to hold microphone on extension cord end and also to serve

as a wand for probe measurements; accelerometer and integrator network; tape recorder; voice microphone for tape recorder; and a

camera, flash, and tripod.

Other equipment that you will find useful in diagnostic measurements includes a'time and motion study stop watch calibrated in

hundredths of,a minute (not seconds) for measuring exposure time; a

steel tape calibrated in both inches and centimeters; an optical

range finder (to 150 feet) for getting dimensions (es~ecially ceiling heights) when building or workroom plans are unobtainable; an

inclinometer for getting heights of stacks beyond the range of the

rangefinder; a hand tally counter for counting repeated operations,

such as a punch press; and a vibrating reed tachometer for obtaining rotational rates of fans, etc~

Making readings. At the worker's ear, obtain the peak sound

level, the A and C levels, and the octave band sound pressure

levels from (at the most) 31.5 Hz to 16,000 Hz (usually 63 through

8000 Hz suffices). Be sure the same machines are operating throughout the measurement period. The A-level weights each octave band

level to make the total consistent with the judgement of the human

ear. Conversion of flat response to A-level was discussed in

detail in Chapter 1 (Table 1.1).

Relating to A-levels. A useful technique is to identify the

A-weighted band that contributes most to the A-level and set the

octave band analyzer for the frequency of this maximum band. Then

with the probe microphone, locate the area on the machine responsible for the maximum contribution in this band.

If a peak reading SLM is available, it can be used to advantage when you suspect that impacts or high frequency noise bursts

are important components of the noise. Maximum peak level permitted by current regulations is 140 dB. For many industrial

noises, the peak sound level minus the A-level is in the range of

14 to 16 dB. For sounds like chains rattling against metal panels,

this difference may exceed 30 dB. Candidate sources of impact

noise include metal-to-metal contact, rapid linear acceleration of

parts, short duration air noise bursts, and metal parts traveling

down metal chutes. The peak-to-dBA difference should increase as

the impact noise source is approached.

A useful field technique in diagnostic measurements is to

observe the difference between the A-level and the maximum Aweighted octave band. The greater the difference, the more likely

it is that several A-weighted octave bands of approximately the

same level are contributing to the total A-level. If the difference is small, it is likely that pure tones are causing the level

in one A-weighted octave band to be much greater than others.

27 .
When the C-level is greater than the A-level, there are

strong low frequency components, but when A is greater than C,

the energy is mainly in the high frequency bands.

For making close to the source readings, many sound level

meters come with an extension microphone cable that can be used

to probe into those portions of the machine at which candidate

noise sources are located. Sources include surfaces excited, for

example, by rotary unbalance, vibrating panels, metal-to-metal impact, vibration of unrestrained workpieces, rapid acceleration of

machine parts, friction, worn gears, poor bearings, hydraulic

systems, air and stearn discharge, and materials handling noise

(chutes, conveyors). The action needed to reduce the A-level of

noise can bp selected in part by a knowledge of the frequency

region that contributes most to the A-level. You will rarely

find that the bands centered at 250 Hz and below contribute significantly to the A-levels of noise (see Table 1.1 in Chapter 1)

For most industrial noises, the maximum A-weighted contribution

is in the 500 to 2000 Hz rpgion.

Vibration. Vibration measurements can often be used to

verify the connection between received sound and its source. Candidate locations are thin machine structural panels and belt

guards. However, there is no reliable connection between amount

of vibration and A-level. Nevertheless, in more difficult diagnostic problems, vibration measurements can prove useful. If the

spectrum of the average alternating acceleration of a surface is

similar to that of the noise around it, it is likely that most of

the noise below about 400 Hz carne from that surface. Above 400 HZ,

average surface velocity is a good indicator. Some sound level

meters have vibration attachments based on accelerator input. With

some meters, a resistor/capacitor integrating network is available

so that acceleration, velocity, or displacement may be read from

the SLf1. There is as yet no standard means of calculating noise

levels from vibration measurements, especially at higher frequencies. Use the mean square reading over a surface, rather than a

single reading. .

Occasionally vibration requires analysis with more sophisticated instruments, such as fixed narrow band analyzers, or 1/2,

1/3, or 1/10 octave analyzers. Such instruments are usually used

in the laboratory, but field use is possible.

Ears and Mind. Trained ears play an important role in diagnostic measurements. They serve as spectrum analyzers and directional indicators. Masking of the observers own speech can also

provide rough indications of sound level, once a personal calibration has been developed. The observer, must be listening critically during all measurements to detect when an unusual noise

event may have occurred. This event is often a change in operation

of a machine and may require a new set of data on that condition.

28
The high-frequency directionality of the ears can be greatly

enhanced by cupping the hands behind the ears. On turning the

head, it will be easy to localize the noise sources in the plane

of rotation. By doing this at a couple of positions around the

machine, you can often home in on a suspected source location.

Because most of the A-level is ordinarily due to higher frequency

noise, this localization by ear is very helpful.

Another clue to source location is the shielding of noise by

a part of the machine. If the ear or a probe microphone is swept

past the suspected region, there will be a sudden drop in level

(particularly in high frequency octave bands) as the source is

eclipsed.

General Tips. Diagnostic measurements are directed toward

noise from specific machines, not the noise from many sources that

is received by a worker in a compliance measurement situation.

Ideally, only the machine in question should be operating to avoid

contamination of the measurements by noise from other sources.

The machine may then be put through its several operating modes:

idling, full production, or with some functions selectively disabled. These modes of operation aid in identification of noise

sources that may be close together or have noise levels nearly

identical.

This ideal arrangement is usually impossible, and measurements

must ordinarily be made during normal production. However, if the

noise is relatively continuous and does not contain audible pure

tones, it is possible to calculate for each octave band the level

due to the machine alone. The only requirement is that at some

time during the measurements, the machine in question must be off.

This often occurs, owing to jams, maintenancei rest breaks, or

change of work. Measure the noise from all other sources, as well

as the noise with all machines operating. If there are small

changes between the two noise level measurements, it is possible

to infer the noise level (in each octave band) from the machine as

if it alone were operating. The principle is simple: the total

noise is the sum of the two contributions: machine and everything

else. However, since decibel levels do not add directly, calculation is needed. The scheme works best when the difference of

the two levels is over 2 dB.

Combining Decibels. Decibel levels cannot be added directly.

If there are two equal noise sources, the combined noise level is

3 dB greater. If the two noise sources are unequal, a smaller number of decibels on a graduated scale of difference is added, as

discussed in Chapter 1 (see Table 1.2 and Figure 1.3).

This same method can be used in combining octave band values

two at a time to get overall spectrum (flat) values, or if octave

29
bands are A-weighted, to obtain overall A-values (approximately

what the ear hears) .

Thus, when data are available for levels from several machines, as explained above, and the decibel decrease is recorded

when the machine under study is shut down, you can determine the

decibel decrease to be subtracted from the reading with all machines on to give the contribution from the machine under study.

In the following list, a is decibel decrease when the machine under

study is shut down, and-b is the number of decibels to be subtracted from the reading with all machines on to give the contribution

from the machine under study.

a b a b a· b

0-:-5 9-:6 4-:-5 1-:9 8-:5 0-:-7

1.0 6.9 5.0 1.7 9.0 0.6

1.5 5.3 5.5 1.5 9.5 0.5

2.0 4.3 6.0 1.3 10.0 0.5

2.5 3.6 6.5 1.1 11.0 0.4

3.0 3.0 7.0 1.0 11. 5 0.4

3.5 2.6 7.5 0.9 11. 5 0.3

4.0 2.2 8.0 0.7 12.0 0.3

Final levels should be rounded off to the nearest decibel.

Decibels can also be combined mathematically by using the

formula in constructing the above table:

L2 = Ll + 10 log (lOB - 1) (2.1)

where B = (L - Ll)/lO c

L = Combined level, background plus machine level

c

Ll = Background level (all other machines)

L2 = Machine level

Data Requirements. Depending on the noise source and the

possible methods of noise reduction contemplated, some or all of

the following data may be required:

(1) Sound pressure levels, A and C, at location of interest

(operator position) for compliance measurements plus

peak.

(2) Octave band analysis made I ft from the source, or at

any distance for which noise from that source

predominates.

30
(3) Octave band analysis made I meter from a surface

that envelopes the source. Use in sound power

calculations.

(4) A and C at critical distance: the point where

there is a 3 dB increase when approaching the

noise source, above the generally constant level

remote from the source (only that source operating) .

(5) Background noise level: A and C, and octave band.

(6) Noise source probe readings in near field, comparing

fast and slow readings related to machine operation

if possible. Use for diagnostic measurements.

(7) Sketch of area, machine, noise source, and operator

location. Augment with photographs if required for

later analysis.

(8) Vibration measurements to determine acceleration

levels of vibrating surfaces (in some cases velocity

and displacement) plus description of vibrating region.

For isolation requirements, obtain weight on footings

and lowest forcing frequency (rpm of motor, drive, and

other vibration contributors) .

(9) General data, such as type and description of machine,

type of stock and product, production rate, requirements for monitoring and access, use of automation,

locations with respect to other machines, dimensions

of enveloping surface, times of measurements. Also

information on all personnel concerned.

(10) Plot data on octave band form (Codex graph paper

No. 31460 or equivalent) .

The basic aim of this documentation is to record as clear an understanding of the noise sources as possible for detailed analysis

later. The documentation should be such as to be clearly under~

stood by others and to serve as reminders of the problem details

for final corrective recommendations.

Several things about the noise can be learned from the basic

sound level readings, A and C, and the octave band data (Items

(1) through (3) above):

C greater than A indicates low frequency components predominate

A greater than C indicates high frequency components

predominate

31
Peak minus A indicates impact when greater than 14-16 dB

A minus maximum A-weighted band of 6 dB indicates possibility

of 4 high A-weighted octave bands

A minus maximum A-weighted band of 5 dB indicates possibility

of 3 high octave bands

A minus maximum A-weighted band of 3 dB indicates possibility

of 2 high octave bands

A minus maximum A-weighted band of close to zero indicates

strong pure tones

= L + 10 log S

P

is the sound pressure level,

position, and

, L

w

where L

p

S is the area in m

2 at each

The data about an enveloping surface can be used later if they are

found to be an advantage in estimating sound power. Sound power is

estimated by placing an imaginary surface one meter away from a

surface that envelopes the machine and touches it at all essential

areas. Measure the noise in each octave band one meter away from

this imaginary surface where the noise (A-weighted, for example)

changes by about 5 dBA or more. The enveloping surface is quickly

defined by drawing a box around the machine with a chalk mark on

the floor. By moving the microphone over a surface one meter away

from the enveloping surface, you can locate each-measurement spot.

If this is done carefully, the value of the sound power can be obtained. The sound power level associated with a given surface is

(2.2)

-12 LW is the sound power level in dB re 10 watt.

Since equation (2.2) must be used for the l~vel of each octave band

for the total surface area, you must use the combination rules developed in Chapter 1 to obtain the total power for each octave band

and then the total A- or C-weighted power. Sound power information

will be used to determine expected sound pressure levels if room

acoustics are changed, which will be discussed in Chapter 3.

Follow-Up Measurements. After the changes have been made to

reduce the noise level (to be discussed in Chapter 3) I a followup series of mpasurements should be mdor to record the attenuation

achieved. These measurements arp usu~l ly made at the operator

position, and revised noise dose is calculated to determine if the

results are now in compliance ~ith current regulations. This gives

a permanent file record covering the original noise level and noise

dose calculations, method of noise reduction or insertion loss* of

noise attenuator, and final noise level and noise dose calculations.

Such data are useful, should a similar problem arise in the future.

* Insertion loss is defined as the reduction in level at a measurement point after noise control means have been introduced.

32
Chapter 3

NOISE CONTROL TECHNIQUES

The objectionable noise sources have been identified and measured as outlined in Chapter 2, Measurement of Noise. In noise control analysis the next approach is to consider where the most effective and economical control of the noise can be obtained: at the

source, the path, or the receiver, and in what order, if more than

one treatment is needed.

Source

A full knowledge of the operation or process is needed to answer t~e basic questions: Can the machine or operation be eliminated or replaced by a quieter operation with equal or better efficiency (weld for rivet, etc.)? Can the present noisy machine be

replaced economically by newer equipment designed with lower noise

levels (perhaps as well for increased production)? Can the specific noise source be corrected by minor design changes (avoiding .

metal-to-metal contact by use of plastic bumpers, reolacing noisy

drives by quieter types, use of improved gears, improved maintenancei etc.)?

Can the specific machine elements causing noise be corrected

by a local source approach rather than considering the entire machine as a noise source? (Later data covering the specific designs

for barriers, enclosures, damping vibrations, vibration isolation,

lagging of vibrating surfaces, mufflers and silencers for air and

gas flows, reduced air velocities of free jets, etc., may be considered for the individual noise producing elements of the total

operation.) Can the noisy machine elements be moved (such as

pumps, fans, air compressors that service the basic machine but

need not be an integral part)? Can the machine parts be vibration

isolated to reduce airborne noise from vibrating panels or guards?

The major constraints in changes of noise sources will be

stated including interference with the productivity of the machine

or the operator, and the relative costs of alternative choices

developed. Some of the specific noise reduction methods that can

be applied at the noise source are:

1. Distance or relocation

2. Vibration control or isolation

3. Damping of vibrating surfaces

4. Lagging of vibrating surfaces

33
5. Air (and gas) flow and jet noise reduction by

design, mufflers

6. Hydraulic systems, acoustic filters, surge tanks

7. Motor air cooling mufflers'

8. Mechanical drive enclosures

9. Balancing of rotating equipment

10. Noise source enclosures

Distance or Relocation. Can a noise source be moved further

away from the operator? Distance reduces noise level from a point

source by 6 dB per each doubling of distance in a free field. In

a closed space such as a factory work room, reverberant sound will

eventually determine when no further reduction by distance may be

obtained. The distance at which this occurs is called the critical

distance (see Chapter 1). Relocation may apply to machine service

units such as pumps, fans, drives, hydraulic systems, and air and

stearn flows that may be relatively easily moved and do not need

constant attention.

Vibration Control or Isolation. Vibration control or isolation can be investigated on two bases: isolate a vibration source,

such as a motor-pump assembly that is part of a machine, or vibration isolate the entire machine. A separate noise level test with

the component on and off would help to determine if the component

is a major contributor to the total noise.

Vibration control is necessary for quiet operation, and equipment mountings should be reviewed and changed from solid mountings

to vibration isolating mountings of springs, rubber, cork, felt, or

fiberglass as the situati~n requires. Vibration causes flat panel

surfaces of machines, guards, and building structures to radiate

noise. The excitation can come from rotating equipment, vibrators,

tumblers, or shakers, and the introduction of isolation between the

machine and such surfaces is often the complete solution to some

noise problems.

Vibration isolators are commercially available. They are selected by specifying the weight supported, the deflection required,

and the lowest vibratory frequency of the unit to be isolated.

They are made from, elastomers (rubber in compression and shear,

ribbed rubber) ; other compressible materials (cork); fibrous mats

(felt, fiberglass); and steel springs.

Basic {solation requires a knowledge of the lowest forcing

frequency (f) of the machine to be isolated, as related to the

34
natural frequency or the isolator (fn ) plus machine, and the

weight on the footing to be isolated. The transmissibility of

vibratory energy is greatest (and is to be avoided) when the ratio

of f/fn = 1. Isolation begins above f/fn = The isolator plus

machine resonance frequency f n is determined from

f = 3.13 where d is the static deflection of isolator n

under load in inches. This relation holds only when the deflection

is strictly proportional to the load (linear systems).

The steps for selecting spring isolators are (Bell 1973), B.4:

(1) Establish that part of the total weight that is on the

footing in question.

(2) Determine lowest forcing deflection required for degree

of transmission percent required (see Tabl~ 3.1); 5% is

normally adequate.

(4) Choose a suitable isolator that will sustain the load

and have the proper deflection. Isolator manufacturers

often list spring constants (lb/in. deflection).

(5) Ensure that deflection is uniform for each footing.

(6) For more complex isolation problems, submit the data

from vibration tests to isolator manufacturers for more

exact specifications.

TABLE 3.1 - Required static deflection (inches) for common industrial speeds or forcing

frequencies (base is assumed immovabl e)

Speed Vibration Transmission (percent)

rpm freq. 0.5% 1.0% 5.0% 10% 25%

3600 60 0.55 0.27 0.06 0.03 0.01

2400 40 1.2 0.62 0.13 0.07 0.03

1800 30 2.2 1: 1 0.27 0.12 0.05

1600 27 2.8 1.4 0.29 0.15 0.07

1400 23 3.6 1.8 0.38 0.20 0.09

1200 20 4.9 2.5 0.52 0.27 0.12

1000 17 7.1 3.6 0.74 0.39 0.18

800 13 5.6 1.2 0.61 0.28

600 10 2.1 1.1 0.49

400 7 4.6 2.4 1.1

35
The selection of isolator pads follows the same general

method, using the data from the suppliers as to the recommended

grade, material, and thickness. However, many pads are highly

nonlinear and cannot be selected directly on the above basis.

A pump-motor set mounted on a common platform furnishes a

typical isolation problem, and also one that has a simple solution: use four properly selected vibration isolators.

Other vibration problems can be more complex, and the suppliers of isolating devices should be consulted. Give them the

machine weight, operating frequencies, weight distribution on

footings, and test measurements covering acceleration, velocity,

and displacement at machine footings and at other points on machine to aid in determining the isolator requirements. For example, machines with low forcing frequencies may require a heavy

concrete inertia block, generally 1.5 to 2.0 times the weight of

3upported equipment. In addition the inertia block and entire

support structure could rest on spring isolators.

Another problem is a machine on a rather limber floor. Such

designs and specifications call for special expertise. Complex

vibration in more than one plane also requires specialized assistance from the suppliers or a qualified consultant. Under optimum

conditions, the reduction in noise level (in dBA) should range

from 2 dB for a machine with no vibrating panels, mounted on a

very heavy inertia block, to perhaps as much as 14 dB for a heavy

machine on a second story and limber floor.

Damping. Damping is the reduction of vibration and resultant,

noise by adding a layer or layers of vibration-absorbing material

to either side of the vibrating surface. For example, metal parts

striking a sheet metal pan cause noise that can be damped with a

wide variety of materials including roofing paper, sheet lead,

damping compounds, special tapes, acoustic lagging, and other commercially available materials to fit the problem.

Surface acceleration readings made in the diagnostic measurements can point out problem surface areas that are efficient noise

radiators. Power radiated as low-frequency sound from a vibrating

surface can be estimated from acceleration readings:

W (watt) = 5.3 x 10- 2 g2 s2 (3.1)

g = Average (root mean square over surface)

acceleration in g - units

2 S = Area of surface, m

36
( 3 • 2)

The free field sound pressure level from a point source can

be determined from sound power level from the formula:

L = L - 20log r - 0.7

p W (r = distance from source in ft) .

This relation does not hold close to a machine with many noise

sources, or at a sufficient distance in a workroom where reverberant sound predominates.

The dominant frequency, f D, can be estimated from acceleration

and velocity:

f = a/27Tv D a = v = f =

acceleration (m/sec 2 )

velocity (m/sec)

frequency (Hz)

( 3. 3)

Lagging. Lagging of pipes or other surfaces radiating noise

is a combination of a resilient member, noise absorption, and

shielding. It usually consists of 1 to 4 inches of resilient acoustic sound absorber covered with a layer or layers of heavy shielding such as sheet lead, lead filled vinyl, or other metal protective coverings.

Some typical constructions and attenuations are given below

(in dB) :

500 Hz 1000 Hz 2000 Hz

I-in. Molded fiberglass and

aluminum foil covering 1.5 4.8 13.8

I-in. Molded fiberglass and

lead impregnated vinyl 5.0 12.0 24.0

2-in. Molded fiberglass and

lead impregnated vinyl 4.5 13.5 26.0

Notes: 1. Fiberglass is 4 lb/cu ft. Lead vinyl is 0.87 lb/sq ft.

2. Note low attenuation at 500 Hz, less at lower frequencies.

3. Good seal at all joints is critical.

4. In special cases two layers of 2-in. fiberglass plus

lead impregnated vinyl between layers plus a cover

layer of lead impregnated vinyl would increase attenuation.

5. Sheet lead of same weight/area would also satisfy.

6. Source: Dear, 1972, P.15.

37
6L =

P =

8 =

a =

Air and Gas Flow Noise Reduction. To reduce the noise of air

dU': gas flow and discharge, silencers are used: absorptive silencers contain porous materials to absorb and reduce noise; reactive

silencers depend on reflection (impedance mismatch) of the sound

waves as the basic noise reduction method. A combination of both

types is used in some designs, with various configurations.

Commercially available silencers are specified by the insertion

loss (by octave bands) and by other specifications such as velocity

of flow, temperature, and allowable pressure drop. Large industrial

silencers are also known as snubbers and are sometimes combined with

spark arrestors. There is a great deal of art in silencer design.

One unsolved problem with dissipative silencers is fouling of the

absorbent by particulate matter. In general, under such conditions,

do not use· a dissipative silencer.

Fans and blowers when near or part of an operation can be a

major noise source. Fan types used are propeller, axial, and centrifugal. Blades on centrifugal fans may be radial, forward curved,

or backward curved, but backward curved blades are the quietest.

The resulting air noise is a combination of b~ade-pass frequency and

harmonic peaks plus broadband aerodynamic noise and turbulence.

Reduced speeds will reduce noise, and replacement with lower

noise level fans such as backward curved blade types can be considered. If this is not practical or economical, the air flow noise

can be reduced by commercial or custom-made silencers. Custom-made

silencers that can be constructed in maintenance shops include

acoustical labyrinths, parallel baffle silencers, acoustic lined

plenums, acoustic lined ducts, and acoustic lined bends ..

If duct walls are lined with an absorbent with absorption

coefficient, a, the dB reduction obtained is given by

6 L = 12. 6 paL 4/8 (3.4)

Change in sound pressure level

Perimeter of duct, inches

Area of duct, square inches

Coefficient of absorption (note that this is

frequency dependent, so octave band data will

be used to determine required insertion loss

and length of duct to be treated.

Plenum chambers can also be lined with sound absorbing material. An approximate relation for the reduction in level is:

( 3 • 5)

38
Discharge area of blower

a = Coefficient of absorption of liner

S = Area treated on plenum walls

p

Sb =

An absorbent lined bend should add about 5 dB attenuation,

with length of treatment at about five times duct width. Commercial silencers are available for greate~ attenuation to fit any

fan or duct size, and suppliers can give insertion loss at each

octave band under varied conditions of flow. Note that noise

travels both upstream and downstream, and silencers may thus be

needed on both intake and delivery sides of the fan.

Air Jet Flow. In industrial applications where high velocity

air is discharged, shear-induced boundary layer turbulence can be

a serious noise source. Normally, maximum levels from this aerodynamic noise occur in the 2000, 4000, and 8000 octave bands. If

velocity can be reduced, the noise level is reduced. Since air

noise power (from several mechanisms) varies approximately as the

5th power of velocity, noise level reduction can be estimated from:

6L ( 3 • 6)

Vl = Present high velocity

V2 = Reduced velocity

A free jet impinging on a surface increases the noise level up to

7 dB.

Low noise level jets are available commercially that use a

divider at the exit to change a single jet stream into multiple

smaller jet streams. These devices should be tested .to see if they

can do the job required. Some hand air guns use a version of bypass jet engines, in which a mantle of slower moving air is aspirated over the jet. Jet noise reductions of as much as 10 dB can

be realized.

If the jet in question is from a vent, it could be moved to

be vented outside the plant or it could be muffled.

Air operated tools can be equipped with commercially available

exhaust air mufflers.

Hydraulic Systems (Miller, 1973), P.13. Hydraulic systems, as

part of a machine or operation, can have specific noise sources.

Motor-pump assembly contributes most to the overall noise pattern,

being vibrated both by overall vibration and by the blade tooth or

piston pass frequency plus harmonics. Noise levels depend on type

of pump: gear pumps are more noisy than screw pumps, piston pumps

are in between. Noise levels of most pumps increase with speed.

Some screw pumps are available that, with a quieted electric motor

produce about 81 dBA at 3 ft.

39
Fluid line noise sources are often sharp bends, flow restrictions, and undersize sections. These configurations cause cavitation and turbulence, causing vibration and noise. The major factors that affect fluid flow and possible noise are pipe diameter, 0,

fluid velocity, V, fluid density, p, and coefficient of viscosity,~.

These parameters are correlated by the Reynolds number, NR:

N ( 3. 7) R = OVP/~

A Reynolds number of less than 1000 will usually guarantee quiet

laminar flow. In general a pipe size large enough to ensure a fluid

velocity of less than 10 fps should be used for water-like liquids.

.Vibration isolation of the pump-motor unit will attenuate noise

from the vibration of the floor. Other noise reduction means include substitution of a larger pump at reduced speed (Figure 3.1) i

addition of hydraulic mufflers, which may be tuned reactive, untuned reactive, or dissipative. Tuned or side branch mufflers may

be used for limited frequency band attenuation for constant speed

1800 2700 3600

100

.---- "," I

" constant power

.....-..,.. CD .."".--

"0 90

....J ", --'constant di spl acement

w

>

" W

....J

W(f)

0

z 80

PU~C:> SPEED - RPM

FIGURE 3.1 - Effect of reduced pump speed on noise level

40
pumps (Figure 3.2). Dissipative mufflers, shown in Figure 3.3,

may be used where the diameter d is given by

d = 0.014 Q ( 3 • 8)

where d = Inner pipe diameter, in.

Q = Flow rate, gal/min

;\/4,3'\/4, 5;\/4

f

length

----T#--1

FIGURE 3.2 - Side branch muffler

11------

,

o

~----' l_

FIGURE 3.3- Dissipative muffler

Chamber diameter, D, and length, I, are determined from:

f = 0.282 cd/IV

c

where c = Velocity of sound, fps

f = Cut-off frequency, Hz

c

V - 0.78S-D21

( 3 . 9 )

Both types of mufflers could be used in the same system for effectiveness over a wider frequency range. Less attenuation is achieved

if each is used independently for the specific frequency.

41
If the pipe size for avoiding turbulent flow is not economical, other means of attenuation must be used, such as storing

energy elastically with a flexible hose or surge chamber, use of

elastic spacers, wrapping and supporting of the line, use of isolation type hangers, or increase in pipe wall thickness. Noise

reductions for several pipe wall thicknesses are shown in Table

3.2.

TABLE 3.2 - A-level noise reduction by use of heavier pipe

Noise reduction, dB

Pipe

Diameter Pipe Grades

(inches)

XS80' 120 160' • XXS

-

2 6 8 12 16

4 7 10 13 16

6 8 12 15 18

8 9 14 18 19

10 9 15 19

12 10 16 20

• Extra heavy

•• Double extra heavy

Fluid lines can also be lagged (see discussion of lagging above) .

Motor Air Noise. Motor air noise, if found to be a problem,

can be reduced by acoustic line air flm..' c:hambers which are

available for some motors. Totally enclosed fan cooled (TEFC)

motors with integral quieting are also being manufactured. Less

than 80 dBA at 5 ft is claimed for motors up to 5000 HP.

Enclosed Drives. Vibrating surfaces of a drive enclosure are

noise sources. Damping these surfaces reduces radiated noise. If

the drive enclosure is a steel box structure, the inside can be

lined with a combination absorption layer (such as open cell polyurethane foam) with a damping layer backing. Such materials must

be oil-resistant. If there are air vent openings for cooling,

noise traps can be added. (For further details see Case History 7

in Chapter 5.) Quieter drives can also be considered, such as substituting belts for gears, "silent" chains for roller chains, and

so on. Precision made and carefully aligned gears are always

quieter than other types.

Balancing. It is always preferable to reduce noise at the

source by avoiding the generation of noise. Unbalanced rotating

42
parts cause surfaces to vibrate and generate noise. All significant rotating parts should be balanced in situ, that is, in the

machine at its final position. This is-Particularly true for large

fans, where there may be unbalanced aerodynamic forces on the

blades. Rotating mechanisms that drive reciprocating devices can

use counterbalance weights.

Noise Source Enclosures or Barriers. These will be discussed

in greater detail in treating the noise path in the next section

and are noted here to call attention to the fact that the technique

can be used close to localized noise sources. Examples of such

sources are drives, pumps, gears, stearn and air vents, punch operations (die area), or other localized metal-to-metal contact areas

of a machine.

Path of Sound

After as much noise as 'possible has been reduced at each

source, the next step is to reduce noise along the path from the

source to the receiver (operator). The sound may be blocked by ~coustical shields, barrier walls, partial enclosures, or total

en~los0res. All these techniques depend on interposing in the path

a material, called an isolator, whose wave transmitting properties

are as different as possible from those of the path. In air paths,

such materials are ideally solid, nonporous, and limp. For liquid

paths, a stream of gas bubbles is a good isolator. Along a solid,

path, resilient vibration isolators perform the same function,

weakening the path. Our discussion will be in terms of the most

important path, in air.

Acoustical Shields and Barriers. An acoustical shield is an

approximately square piece of material, usually (for visual access)

safety glass or clear plastic (polycarbonate or polymethylmethacrylate), placed between worker and noise source. Such a shield is

effective only if its smaller dimension is at least three times the

wavelength contributing most to the noise exposure received. Thus,

shields of reasonable size are effective only against high frequency

sound, such as from concentrated air or gas jets. Examples include

air ejection systems in punch presses, plasma guns, air guns, and

metal spray guns. Improperly designed shields can interfere with

access. However, they can be combined with automatic safety mechanisms to serve a mechanical safety function as well. A shield must

be carefully engiueered to the job if maximum noise reduction is

to be obtained with minimum interference with manual access. For

high frequency noise, reductions up to 8 dB can be obtained.

Shields must be vibration-isolated from the machine.

An acoustical barrier is a much larger shield, typically free

standing on the floor and usually much wider than high. Visual

43
access can be provided by viewing ports of safety glass or clear

plastic. For both barriers and shields, the sound arrives at the

operator around the edges as well as through the material of the

barrier. Because the former path most often limits the attenuation that can be obtained, the transmission loss of the barrier

material need not be large. Indeed, for the best reasonable barrier, a reduction of 15 to 20 dB in A-level is about the maximum

possible. (For a method of barrier attenuation estimating, see

Case History 6 in Chapter 5.) Thus, at best the barrier (welldamped) need be no heavier than 0.5 Ib per sq ft, if the A-weighted

spectrum peaks at 1000 Hz; less may suffice if the maximum occurs

at higher frequencies.

For best results, at least the machine side of a useful barrier should be covered with an acoustical absorbent, preferably

oil resistant and cleanable. The barrier material can be plywood;

it ordinarily needs no added damping, and 1/2 inch will usually be

thick enough for a 10-dB reduction. If steel is used, damping

must be added to control resonances. Handles and sometimes casters

can be provided on the barrier for ease of moving. If the barrier

is hinged (for stability), the joints can be severe noise leaks.

These' leaks can be controlled by a resilient strip of 1/8-inch-thick

Neoprene over the gap. Use a width of at least three inches, placed

on the concave side of the bend. Fasten by one long edge, with the

other free to slide as the hinge bends.

Machine guards for mechanical protection can often be replaced

by acoustical guards, covered on the inside by absorbent materials.

These guards constitute a type of partial enclosure over the noise

source. If visual access is needed, safety glass or clear plastic

panels can be used. However, the acoustical guard will have to be

more complete than the mechanical, and must be vibration-isolated

from the machine. Much engineering must go into fitting the guard

so as to cover all the noise leaks. The importance of the leaks Cdn

be seen from an example. If the guard is designed for a reduction

of 15 dB and has a total area of 6 sq ft, then a crack around it

that is about 1/4-inch wide will pass as much sound as the guard

material. In most acoustical guards, the leaks will limit the noise

reduction possible; the chief task of the noise control engineer is

to design for minimum leaks.

When a barrier is wrapped around a machine, with its top more

or less open, it becomes a partial enclosure. Such an enclosure can

be effective in reducing noise to workers nearby. How~ver, the noise

escapes through the top and contributes to the reverberant sound in

the workroom. In addition, specular (mirrorlike) reflection from

the ceiling can contribute reflected-path levels that can become obvious when the direct path is reduced by the enclosure, as shown in

Figure 3.4.

44
7'"

/'"

/'"

/ /

'"

/

/

/

/

/

/

/

/

~----.

barri er

source

machine

ceiling

ray grazing top of barrier

'"

'"'" ray reflected from ceiling

"'-

'"

'"

Jl

recei ver

floor

FIGURE 3.4- Source-barrier-receiver geometry. The angle into acoustic shadow should be greater

than 30° lor at least 70-dB attenuat ion. Cei ling reflection can offset barrier attenuat ion if ceiling

height is less than 7.5 times distance from source to receiver.

These spill-over noise effects can be reduced by covering the

inside of the enclosure with absorbent. Also," suspended absorbents

may be placed over the openings to reduce the escaping noise. If

all other machines in the workroom are quieted, then the ceiling

reflection may become apparent. Such reflections are usually specular, and the patch of ceiling at which the reflection takes place

can be located geometrically on building plans. Absorbent placed

on the ceiling at this location will effectively reduce the reflected

sound received.

Partial and total enclosures will usually need access for incoming material, product, scrap removal, operator, maintenance person, and vision. Doors, windows, and hatches will handle most

access problems, but the usual precautions about avoiding leaks hold

strongly at these openings. Hinged or sliding doors can use a gasket for a seal. A convenient material is the closed-cell foamed

elastomer weather stripping sold with a pressure-sensitive adhesive.

Special acoustical gaskets, designed specifically for sealing leaks,

are also available. For less stringent sealing, the magnetic strip

gaskets used on refrigerator doors supply both seal and positive

closure. Hatches can be dogged down by quarterturn latches.

45
Windows for visual access may need added internal illumination to

make visual monitoring easy and positive. Heat buildup should be

no problem with an open top in a partial enclosure. Noise reduction also removes acoustic signals that some workers use in evaluating the performance of a machine. Hence, if the reduction is

great enough, acoustic cues may have to be separately supplied.

This is easily done by a rugged microphone (at the site where the

essential information is generated), feeding a small loudspeaker

at the worker position.

Openings for workpiece, product, and scrap flow can permit

noise to escape. Such openings should be in the form of tunnels

lined with absorbent material. The length of the tunnel determines

the amount of noise attenuation obtainable. In the design, theabsorbent can be selected for maximum effect on the noise spectrum

at that opening. Use of lined tunnels should be accompanied by

some degree of automation.

Total Enclosure. The next step of noise control, a total

enclosure, has the same problems (and solutions) as the partial

enclosure. The chief added problem is that of heat buildup. This

problem is easily handled by adding a ventilating blower, together

with silencers for both supply and exhaust air. Some internal ducting may be needed if there are heat-sensitive components in the

machine, but these ducts can also selectively supply cooling air

and remove hot air. The minimum flow rate of cooling air, Q (in

cfm) depends on W, the watts of heat generated, and on ~T, the

temperature rise permitted (degree F). For air cooling at sea

level, Q = 1.76 W/~T. More flow is needed at higher altitudes.

There is no doubt that the total enclosure will require a

change in work habits, and as such will usually be resisted by all

concerned. The shock of the change can be eased if the people

most involved--the workers and the foremen--are provided an opportunity to enter into the design discussions. Enclosures can also

force consideration of modernizing equipment, say, for automatic

feed by conveyor, so that less personal attention to the machine

is required. Such automation may also offset the difficulties that

arise from less free access to the machine. In most instances the

noi~e control engineer will have little difficulty with the acoustical aspects of enclosure design. The chief job is to ensure an

industrially viable design, taking account of the requirements for

access, minimum change in productivity, and minimum installed cost.

To meet these requirements, the noise control engineer must work

closely with the industrial, plant, and process engineer; with

foremen and workers; with maintenance crews; and with management.

As a general matter, enclosures must not touch any part of the

machine and should be vibration-isolated from the floor. Nevertheless, the enclosure must be pierced for such services as electricity,

46
air, steam, water, oil, or hydraulic power. These services can be

regrouped, together with mechanical controls, to a convenient location where the enclosure panel can be split. A resilient acoustical seal can then be made from two ring-shaped pieces of liB-inch

(or heavier) Neoprene. See Figure 3.5. Slot each piece at the

pipe or conduit and overlap the two pieces with the slots facing

away from each other. Seal the straight edges with strips of Neoprene or similar oil-resistant, heavy, and resilient material .

semicircular cut-outs

for services

split paneis

./

/~ -

./ ./~ sealing strip; glue line

./ 1/8 X 3 inches Neoprene

"~

split washers of 1/8 inch Neoprene,

3 inches wide on radius

FIGURE 3.5 - Split panels for services

For mechanical controls operating through an arc-shaped hole

in a panel, the seal can be of abutted, multiple strips of Neoprene.

The control lever should be as thin as possible. Better ·yet, replace it by a servo control operated from the outside.

Many of the features of a convenient enclosure design are

illustrated in Figures 3.6 through 3.13. The general design is

based on panels secured (by quarter-turn captive screws) to an angle

iron frame (Figures 3.6 and 3.7). Thus, rapid access is provided

for all types of servicing of the machine. This type of enclosure

is to be as close as possible to the machine. Up to 20 dB of noise

reduction is usually easily obtained. The angle iron frame can be

of bolted sections, to permit quick and complete disassembly and

removal.

47
FIGURE 3.6 - Welded angle iron frame. This frame can be welded in segr:;ents

that are bolted together.

enclosure panel

gasket of closed cell toamed elastomer

edge of angle iron frame

.1-4-::------- quarter-turn captive screw fastener: space

about every 6 to 9 inches. Wing screw

fasteners can be used if rapid access is

needed wi thout tools.

FIGURE 3.7 - Enclosure panels secured to frame by quarter-turn fasteners.

48
Machine vibration may still create a problem by vibrating

the floor, which then acts as a resonant sounding board to vibrate the enclosure. This problem is easily handled by vibration-isolating mounts, using steel springs, or elastomers in shear

(Figure 3.8). Special care in design is needed if the exciting

force is of short duration but is repeated, as in a punch press.

Not all vibration-isolator suppliers recognize the need for careful selection of isolators in this special repeated impact situation. Be sure that you have enough data on the machines and the

isolators to ensure an effective design. You will need data on

three time scales: (1) duration of the impact, (2) time between

impacts: and (3) the minimum period of oscillation of machine on

a suggested isolator.

.. enclosure panel

toe caving, bent 1/8 X 9 inches cold-rolled

steel; 5 inches deep, 4 inches high

vibration isolating mount, elastomer in shear;

static deflection at least 1/4 inch

strip seal to floor, 118 X 3 inches Neoprene

clearance about 1/2 inch

FIGURE 3.8 - Vibration isolation and toe coving

In any machine, the time comes when major repairs are due:

additions or changes may also be called for. The enclosure design

suggested in Figures 3.5 through 3.13 affordS some flexibility in

this regard. The panels can be made separately and fastened in

place with a gasket material (such as weatherstripping) to close

off chance leaks. If the panel material is a metal, its resonances

can be distributed more uniformly in frequency if the panel is

49
enclosure panel; usual I y steel

damping sheet; can also be spray, trowel on

absorbent: seal raw edges with paint.

main surface to be oil-resistant and

cleanable

FIGURE 3.9 - Enclosure panel interior treatment

50
angle iron stiffener for door frame ...

hinge spacer _..

enclosure panel _

hinge .....

door panel -.

gasket of closed cell foamed elastomer

(weatherstri ppi ng)

door panel stiffener of 1/8 X 1 inch __-.

cold-rolled steel

FIGURE 3.10 - Door and hatch detail. Interior of doors and hatches have same acoustical treatment as

enclosure panel. Secure doors by vibration resistant latches or by quarter-turn fasteners. Doors and

hatches must make airtight seal to enclosure panel.

enclosure panel

hold down clamp; use quarter-turn screws

I--....L.~ bead of silicone rubber to make resilient

gasket. use silicone oil on window to

get quick release for servicing window

panel frame/stiffener of 1/8 X 1 inch

cold-rolled steel

glazing: Plexiglas, Lucite, Lexan or safety glass

FIGURE 3.11 - Window detail

51
reinforced by bolted-on angle iron (bolting adds more damping than

welding). The stiffeners should be placed so as to divide the

panel into smaller areas, no two of which should be the same size

and shape. Frames for doors, windows, and hatches can also be

used as stiffeners.

3 foot exhaust si Iencer

....------at least 4 feet j

....------ 3 foot inlet silencer

flexible collar connector_--.--;----r

vibration isolating

mounts

collection duct

""",---centrifugal blower

FIGURE 3.12 - Cooling and ventilation system. Three-foot silencers are satisfactory

up to 15-dB isolation. .

Windows pose a special problem because they are a weak spot

acoustically. Generally, if more than 20-dB .reduction is needed,

double glazing must be used. The inside layer should be safety

glass, because it must withstand cleaning to remove oil, grease,

and dirt. All panes should be set into soft elastomer gaskets.

Room temperature-setting silicone rubbers are useful here. The

visual access that windows provide should be carefully thought out

in terms of the information the operator needs. Glareless lighting of the components to be monitored is helpful. In extreme

cases, closed circuit video monitoring can be used.

A special adaptation of the total enclosure for the machine

is a total enclosure for the operator when this is the more practical or economical approach. Such enclosures may require air

52
parti al enclosure parti al enc losure

complete enclosure

FIGURE 3.13 - Examples of partial and complete enclosures

53
intake and exhaust fans, with noise traps, lighting, heating, or

in some cases, air conditioning. As in machine enclosures, some

inside absorpt.ion is recommended, such as an acoustic tile ceiling,

and special care must be taken in window and door design to avoid

leaks. See Figure 3.14 for effect of leaks.

90

% opening

70

0.0

50

40

0 30 0.1

w

N

...J

« 20 1.0

w 2.0 a:

m 5.0 "0

Z

0 10 10.0

f=

«

:::l 7 20.0

z

w

30.0 5 «.

w 4 40.0 (/)

0 3 50.0 z

2

o 10 20 30 40

TRANSMISSION LOSS POTENTIAL

50 60

FIGURE 3.14 - Effect of enclosure sound leaks on potential noise reduction

54
The most important information required for the acoustical

design of enclosures is the transmission loss, Lt. This quantity

represents, in decibels, the reduction of sound power in going

through an isolating wall. It is measured by a well-defined and

accepted standard. The transmission loss varies considerably with

frequency, and the term sound transmission class (5TC) is sometimes

used. This is a single number indication of average transmission

loss. However, it applies specifically to speech sounds and acoustical privacy requirements. It should be used with caution in industrial situations, where sound spectra differ from those of

speech and where reduction of A-level is the requirement. If a

single number is needed, use the transmission loss at the band an

octave below that for which the maximum A-weighted octave band

level occurs at the operator position.

The noise reduction, Rn , is the difference in level at operator and in enclosure after installation of the enclosure. It depends on several factors: the average transmission loss of the

enclosure surfaces, the amount of absorption in the enclosure, and

to some extent, the acoustics of the work room. The worker is

usually close enough to the machine that room acoustics effects are

not important. If the average absorption coefficient of the interior of the enclosure is at least 25% at the band an octave below

that for the maximum A-weighted octave band level, then approximately Rn = Lt. A more useful measure of the effect of the enclosure is the insertion loss. This is the reduction of level at the

operator position before and after introduction of the enclosure.

Its calculation requires knowledge of total noise power from the

machine, its directional characteristics, the total absorption within the enclosure, the transmission loss of the enclosure, the sha22

of the enclosure facing the worker, and the acoustics of the workroom.

A problem occurs in calculating the net transmission loss when

the enclosure has panels, doors, hatches, windows, silencers, and

leaks, each with its own area 5i and associated transmission loss Li.

However, a formula which can be used is

-L /10 L t == 1010g 5 - 1010g L: 5 i 10 i (3.10)

This amounts to adding up all the sound power that escapes and

dividing by the total area. As an example consider a machine control room in a corner that has ceiling-high walls that separate it

completely from the rest of the shop, where the level is 100 dBA.

The design of the wall is shown in Figure 3.15. It is desired to

compare the performance of single and double glazed windows at a

midrange octave band. We assume that there is negligible leakage

through the roof and that all leaks have been well sealed.

55
24 feet wide

concrete block areas

3 feet high

glass

door

3 feet

wide

glass

5 feet high

glass area

door and frame

concrete block

total

105 sq 1t

24 sq ft

135 sq ft

264 sq ft

FIGURE 3.15 - Example of isolating wall

The calculations can conveniently be put in the form shown

below. Values of S· lO-Li/lO

10-Li/lO Single J.

S. L. DJuble

l l Glazing Glazing

Single Glazing 105 31 7.94 x 10-4 0.0834

IX:>uble Glazing 105 45 3.16 x 10-5 0.00332

IX:>or 24 31 7.94 x 10-4 0.0191 0.0191

Concrete Block 135 45 3.16 x 10-5 0.00427 0.00427

Sums 264 0.1068 0.0267

10 log 264 = 24.2

Lt (single)= 24.2 - 10 log (0.1068) = 34 dB

Lt (double) = 24.2 - 10 log (0.0267) = 40 dB

56
For the particular octave band for which the above was calculated, the control room levels would be 66 dB for single glazing

and 60 dB for double. Note how the single glazed window is the

weak element, while with double glazing the door is the chief source

of sound entering. Such calculations reveal where improvement may

be needed, and how much.

Room Acoustics

In addition to the direct path from the noise source to the

receiver (operator), there is also the sound reflected repeatedly

from walls, ceiling, and other surfaces, such as equipment. The

study of the sound fields generated by such reflections is termed

room acoustics.

The sound level from a point source, with rio reflecting surfaces nearby, changes at the rate of 6 dB drop per doubling of distance from the source. In a normal factory space with acoustically

hard walls, floors, ceilings, partitions, and machines, sound is

multiply reflected, and an operator may receive this reflected

sound from essentially all directions. The result is a reverberant

buildup of noise. This means that as the distance from the machine

increases, the noiBe level will diminish for awhile but a point

will be reached where little or no reduction is achieved. Beyond

this distance is the reverberant field, and the distance at which

the change-over occurs is called the critical distance, r c ' The

reverberant field is the only portion of the total noise that is

affected by changes in room acoustics, For most industrial situations the operator is inside the critical distance. Thus, the noise

that he receives is affected but little by adding absorption to the

walls or ceiling of the work room. This absorption will affect the

sound level experienced by those who walk through, at some distance

from the machines.

The total absorption of a room is the sum of the products of

each room area, S, and its associated absorption coefficient, a.

The room constant, R, is a value based on the summation of all

areas and is:

R = IS a/ (1 - Ci) = Sal (l - a ) (3.11)

Here Q is the average absorption coefficient (I Sa/ S). As the

value of R is increased in a room, the reverberant field sound level

is reduced. Figure 3.16 shows relative sound level versus distance

from the acoustic center of a noise source, for several values of R.

If we had a single source of noise in a room, which factories

normally do not have, the relationship of Lp (sound pressure level),

Lw (sound power level), and room constant, R (distance from the

57
o.----~r__---......-----..,...._---__r---_r-----......---~

100

200

1000

2000

Sa , R = -:--=-- 1 - 6:

I

I

free field

I

I -301----------+-----+----t------i-'~------=="""""'o;;;;:::_-l--5000

I

I

-51--------.......~+_-----_t_----+_---+_-----_+----__1

-251----------+-------+-----t--"~--"'l~-----_+_----j

w

a:

=>en

en

W

a: -15l-----------+------"'WlIl~--"""""-=+-----+--------I--------i

a..

o

z

=>

o

en

-201--------I-----T----.:"CT---+:::::!!!~-lIiiiiiiiiiiiiiii~~5:.:0~0.;;;;;;;l f-

<X:

...J

W

a:

...J

W

>

-10r---------t-~~,,~r_--=~'!!!!IIt_-lliiiiiiiiiiiiiii*-;;;;;;;;;;;;;;;;;;;;;;;;;;~~R~:=~5~0~;;;;;;;;~

64 128

-35 .......----:--=----~:___-----":""'"""---~:___--~---...&.---'------' 0.5 1.0 2.0 4.0 8.0 32

DISTANCE FROM ACOUSTIC CENTER - FT

FIGURE 3.16,- Sound field behavior in a reverberant room

58
noise source, r, is in feet and area, S, is in square feet) is:

L = L + 10 log ((Q/4~r2)+(4/R))+10.3 p W

Here Q is the directivity of the source. For rand S in m and

omit the 10.3.

(3.12)

2

m ,

From this relation, it is seen that the inverse square law and

reverberant fields are equal when

2 Q/4~r = 4/R

c

where the critical distance is

(3.13 )

(3.14)

where ais the average absorption coefficient. For the ordinary factory workroom, ain middle frequencies is usually between .05 and 0.1.

For machines near a floor, the directivity factor Q = 2. Then approximately

r == 0,2 I Sa

c

If the room has N identical machines,

(3.15 )

r == 0.2 /Sa/N (3.16) c

Note that because ais usually an increasing function of frequency,

the critical distance increases somewhat with frequency.

If a rough estimate of an overall r c is all that is needed, this

can be accomplished by using nothing moxe than your ears. Focus

your attention on a dominant, easily identified noise source. Walk

away from the source machine until the level appears relatively constant to your ears. Then walk rapidly toward the source. Stop

when you are first aware of an increase in level. Repeat until you

have localized the point at which the direct sound begins to predominate, at the critical distance position. Measure from there to the

source.

As an example, consider a workroom 80 ft x 120 ft with a 25-ft

ceiling, having 10 identical automatic screw machines. Suppose that

the average absorption coefficient, at the maximum A-weighted octave

band level, is 0.15. Then with a total surface area of 29,200 sq ft,

r == 0.2

c [29,200(0.15/l0)J = 4.2 ft (3.17)

Workers on screw machines will be within 4 ft, which is just at this

critical distance. Hence they will receive about equal amounts of

direct and reverberant sound.

59
If the noise level at a distance from the source requires

attenuation, absorption must be added to the room in the form of

absorptive materials on ceiling and walls, or by hanging absorbent

baffles, which are commercially available. The amount of attenuation needed, L, can be related to room: constant R by

(3.18)

where Rl and R2 are before and after values, respectively.

We again repeat the caution: adding room absorption will normally

note reduce the direct noise from the source, which is what the operator receives. Indeed, adding absorption increases the critical

distance, so the operator can be still further from his machine and

still receive the major amount of noise from it. In the example

above, adding 100% absorption would decrease the received level

only 3 dB.

Receiver

If noise source corrections and noise path corrections do not

reduce the noise to acceptable limits or before these corrections

have been applied, attention should be directed toward the receiver.

This may be in the form of administrative controls or personal protective equipment or both.

Administrative Controls. Administrative controls can in some

cases be developed to limit the duration of exposure to noise levels

above 90 dBA to the permissible noise exposure time specified in the

Section 1910.95 of OSHA (Occupational Safety and Health Administration) regulations:

Hours/Day dBA Hours/Day dBA -- -- 8 90 l~ 102

6 92 1 105

4 95 110

3 97 l:I or

2 100 less 115 max.

For combinations of exposures, the sum of the actual daily exposure

time divided by the allowed exposure time for each time period must

equal or be less than 1.0 to be in compliance regarding noise levels

and should be equal or less than 0.5 to reduce the need for wearing

ear protectors. The sum of these ratios is called the daily noise

dose. Administrative controls include the following approaches:

(1) Workers at 90 dBA are not exposed to higher levels.

(2) Workers at higher levels are removed from the noise after

the above time limits and spend the balance of the day in

lower than 85 dBA areas.

(3) Work time is divided between more operators, that is, split

shifts are arranged between high noise and low noise areas.

60
(4) When less than full-time operation of a machine is

required, time is split into partial days instead

of an occasional full day operation.

(5) Exposure is reduced by shift scheduling to reduce

number of exposed employees and number requiring

protective equipment.

Personal Protective, Equipment. Present (October 1974) Department of Labor "Guidelines, to Occupational Noise Standards"

(Bulletin 334), say: "The use of personal protective equipment is

considered by the Department to be an interim measure while engineering and administrative controls are being perfected. There·

will be very few cases in which the use of this equipment will be

acceptable as a permanent solution to noise problems."

The Departrnentof Labor Bulletin 334 Guidelines cover the

following major points on protective equipment:

(1) Only approved ear protectors that have been tested in

accordance with ANSI Standard Z24.22-l957, should be

used.

(2) Five dB less than stated attenuation of equipment

should be allowed because test data were obtained under

ideal conditions that are not normal in day-to-day operation.

(3) Ear muffs and ear plugs should be fitted and supplied

through a properly trained person who can educate the

workers in the use and maintenance of the muffs and

plugs.

(4) Wax impregnated cotton and fine glass wool are acceptable, but cotton stuffed in the ears has very little

value and is not acceptable.

In addition, it is our experience that ear plugs should be

inspected periodically to ensure that they have not deteriorated

and that the workers have not cut off the ends for greater comfort

(but no protection) .

Selling the use of protective equipment to employees and supervisors will usually require that an educational and promotional program precede the required use of ear protection. There should be

continual follow-up by supervisors to see that the program is accepted and that ear protection is worn where needed. This program

can be aided by signs in areas where protective equipment is mandatory. Supervisors should be aware that if a plug or muff is uncomfortable, it may not be worn. Other difficulties are with long

61
hair, which can hide a plug that has deliberately been loosely inserted; plugs may be cut off and ear muff bands weakened to reduce

pressure. Tight .muffs over broad-temple glasses can restrict blood

flow. Devices are available to ease this last problem.

Audiometry. Audiometric tests should be made of all individuals working regularly in areas in which the noise level is above

85 dBA. This is an excellent way to determine how successful engineering or other controls have been. Several recommendations for

an audiometric program include:

(1) Test facilities and procedures are to meet established

standards of ANSI S3.1-1960 and 3.6-1969 (or latest)

(2) Tests are to be made as described in ANSI standard

S3.1-1960 and 3.6~l969, and by a person certified

as an aUQ10metrlc technician by a course of training in

accordance with Intersociety guidelines.

(3) Audiometers used are to meet specifications ln ANSI

standard S3.6-1969 or latest and shall have a certificate of at least yearly calibration. They are to

be subjected to a biological check at least once a

month; a log of checks and calibrations is to be

maintained.

62
Chapter 4

NOISE CONTROL MATERIALS

This chapter describes the four types of materials most often

used in noise control: absorbers and isolators for airborne sound,

and vibration isolators and damping materials for controlling vibration solid-borne sound. Guidelines for selecting materials are

also given, based on acoustic and nonacoustic considerations.

By far the best location for noise control is as close to the

source as possible. Once the sound is airborne, it is difficult

and expensive to control. Control at the receiver by ear plugs,

ear muffs, or control booths is not usually included in the meaning

of engineering control, which commonly refers to machine-oriented

efforts at the source or along the early path.

Absorption Materials

With absorption, sound in air is changed into heat. Suitable

materials are usually fibrous, lightweight, and porous. The fibers

should be relatively rigid. If a cellular material is used, the

cells must intercommunicate. Foams should be reticulated to the

proper degree.

Examples of absorbents include acoustical ceiling tile, clothing, loss-type mufflers, and foamed elastomers. Physically the flow

resistance of fibrous materials is the most important characteristic.

For optimum results, the flow resistance must usually be increased

as the thickness of the absorbent decreases, to maintain peak absorption.

The flow resistance can be sensed rather crudely by attempting

to blow through the material. Comparison with an accepted material

of the same thickness provides a personal calibration. The effectiveness of an absorbent is measured by the absorption coefficient.

Ideally, this is the fraction of the sound energy flowing toward the

absorbent that enters the material and is not reflected; thus" a

perfect absorbent would soak up all the sound incident on it.

Industrially useful absorbents have coefficients above 60% in the

frequency range from 500 Hz and up.

Absorbents on room surfaces reduce the amount of reverberant

sound. The relative reduction of such sound levels by equal amounts

of any two absorbents is given approximately by 10 times the logarithm of the ratio of their absorption coefficients. Table 4.1 shows

average absorption coefficients of various absorbents, and for comparison Table 4.2 gives absorption coefficients of relatively nOnabsorbent construction materials plus those for some special materials.

63
TABLE 4.1 - Sound absorption coefficients of common acoustic materials

Frequency Hz

Materials· 125 250 500 1000 2000 4000

Fibrous glass

(typically 4 Ib / cu ft) hard backing

1 inch thick o.oi 0.23 0.48 0.83 0.88 0.80

2 inches thick 0.20 0.55 0.89 0.97 0.83 0.79

4 inches thick 0.39 0.91 0.99 0.97 0.94 0.89

Polyurethane foam (open cell)

1/4-inch thick 0.05 0.07 0.10 0.20 0.45 0.81

1/2-inch thick 0.05 0.12 0.25 0.57 0.89 0.98

1 inch thick 0.14 0.30 0.63 0.91 0.98 0.91

2 inches thick 0.35 0.51 0.82 0.98 0.97 0.95

Hairfelt

1/2-inch thick 0.05 0.07 0.29 0.63 0.83 0.87

1 inch thick 0.06 0.31 0.80 0.88 0.87 0.87

• For specific grades see manufacturer's data;note that the term NCR, when used, is a single

term rating that is the arithmetic average of the absorption coefficients at 250, 500, 1000,

and 2000 Hz.

TABLE 4.2 - Sound absorption coefficients of common construction materials

Frequency Hz

Material 125 250 500 100C 2000 4000

Brick

Unglazed 0.03 0.03 0.03 0.04 0.04 0.05

Painted 0.01 0.01 0.02 0.02 0.02 0.02

Concrete block, pai nted 0.10 0.05 0.06 0.07 0.09 0.08

Concrete 0.01 0.01 0.015 0.02 0.02 0.02

Wood 0.15 0.11 0.10 0.07 0.06 0.07

Glass 0.35 0.25 0.18 0.12 0.08 0.04

Gypsum board 0.29 0.10 0.05 0.04 0.07 0.09

Plywood 0.28 0.22 0.17 0.09 0.10 0.11

Soundblox concrete block

Type A (slotted), £3 inch 0.62 0.84 0.36 0.43 0.27 0.50

Type 8, 6 inch 0.31 0.97 0.56 0.47 0.51 0.53

Carpet 0.02 0.06 0.14 0.37 0.60 0.65

64
The absorption coefficient depends not only on the material

but also on what is in front and back of it. Most coefficients

are stated for an unobscured front, but with a rigid impervious

backing spaced various distances away from the material. Noise

control engineers use designations of the Acoustical and Insulating

Materials Association to describe the material mountings:

(1) Cemented to backing with about 1/8 in. air

space

(2) Spaced 3/4 in. away by furring strips

(4) Laid directly on surface - very little air

space

(7) Suspended 16 in. from the backing

When the mounting is not specified, usually it is No. (1) or (4).

Materials may have special facings. For resistance to grease

and water that would clog pores, a thin plastic film covering is

often used. Such films, as well as perforated hardboard facings,

tend to produce a maximum in the mid-frequency absorption coefficient.

Some thin construction materials, notably plywood, can show increased

low frequency absorption by panel resonance, if they are not securely

fastened down.

The standard reverberation room method of measurement of absorption coefficient (ASTM C423-66, or latest version) essentially subjects the absorbent to sound from all angles. Data on absorption coefficient cannot be regarded as useful and meaningful unless they

have been obtained in this standard fashion.

Often a single-number statement of average absorption is needed.

Not always useful to the noise control engineer, but at least standardized in presentation is the noise reduction coefficient (NRC).

This is the average, to the nearest, 0.05, of the coefficients at 250,

500, 1000, and 2000 Hz.

Transmission Loss Materials

The noise isolation properties of materials are stated in terms

of transmission loss. As with absorption, the concept of energy

flow is used; here it is the energy transmitted through the material,

relative to that flowing toward it. Transmission loss is 10 log

(incident)/(transmitted energy), and usually increases with frequency

at the rate of 6 dB per doubling of frequency. Measurement is difficult, and only a handful of laboratories in this country are qualified to make the standard measurement (ASTM E90-61T, or latest revision). Data on the transmission loss of materials appearing in

advertising literature cannot be regarded as meaningful unless

they have been determined in this standard manner.

65
As a result of the search for a single number to indicate the

average full transmission 1(35, the concept of sound transmission

class (STC) was developed. It is useful specifically in assessing

the degree to which intelligible speech is prevented from being

transmitted through a wall. Because the intelligibility of speech

depends chiefly on the higher frequency sounds, the STC is arrived

at by requiring greater loss at higher frequencies than at low.

The STC should be used with caution in industrial work, however,

where the noise spectrum can be much different from that of speech.

The noise control engineer will need the transmission loss in each

octave band for the proper application of isolating materials.

Isolating materials are effective because the material is as

different as possible from air, on a wave-transmitting basis. In

many situations where a large transmission loss is needed, the materials can be installed in several layers, rather than one thick one.

The chief requirement is that there be minimum mechanical connection

between layers, other than the air space. Large increases in transmission loss are possible by subdividing the total mass.

Damping Materials

Damping materials are used to reduce resonance effects in solids. Essentially, damping materials are absorbents for solid-borne

sound and convert it into heat.

DampiLg materials are used in many applications. If a machine

panel (such as a belt guard) is subjected to vibration, it will

radiate sound strongly at its resonant frequencies. Damping the

panels or guards can thus reduce this radiated sound. In another

application, parts that fall into (and are carried along) metal

chutes can excite the chute panels by repeated impact. Damping

materials along the chute surfaces will reduce the noise; these

materials must also be selected with heat resistance and mechanical

integrity in mind. Damped stock tubes are available for quieting

screw machine operation. Panels for isolating enclosures can transmit large amounts of sound in the critical frequency region. Damping can greatly reduce the severity of the decrease of transmission

loss in that region.

There are two types of damping materials: homogeneous and

constrained layer. A homogeneous material is applied in a relatively thick cast, sprayed or troweled layer, depending on the thickness and type of metal to be damped. A constrained layer material

consists of a thin layer of the actual damping (lossy) material,

with a backing of thin metal or stiff plastic. The mechanical action

is one of making the damping layer much more effective than if it

were homogeneous. Constrained layer damping materials can be purchased as an adhesive/metal foil tape combination, where the adhesive

is selected for its energy loss properties as well as its adhesion.

These damping tapes are especially useful on thin panels (1/16-in.

steel or less).

66
Vibration Isolators

Vibration isolators act on the same principle as isolators for

airborne sound: introducing into the transmission path a material

whose wave-transmitting properties are as different as possible from

the medium carrying the wave. For vibration in solids, such materials are spring-like. Examples include resilient elastomer and

metal springs, elastomer pads, and in extreme cases, air springs.

The weaker the spring, usually the greater the isolation.

Solid rubber or rubber-fabric pads are not too effective, because the displacement is small and is not proportional to the load.

This latter effect, called nonlinearity, is best described by a

curve showing load on the isolator to produce a given deflection.

Some manufacturers provide these data.

If an isolator is too weak vertically, it may not be laterally

stable. There are available side-restrained metal spring isolators

to avoid this difficulty. In extreme cases it may be necessary to

use many isolators, all acting along lines that pass through the

center of gravity of the machine. This use of mat~rials is most

effective when the vibration situation is reversed, i.e. when a

delicate mechanism is to be protected from external shock and vibration. '

The proper amount of damping is needed with vibration isolation in many applications. Steel springs alone are highly undamped;

if they rest on elastomer pads, there is much improvement.

Material Selection

The most commonly used materials for control of noise in industry are isolators and absorbers for airborne sound and vibration

isolators and dampers for solid-borne sound. Selection of materials

is governed by many factors other than acoustical, and installed

cost becomes less important than possible interference with production. Here we first discuss the acoustic factors and then consider

the others.

Acoustic Considerations. Isolators for airborne sound have

many configurations, ranglng from a simple shield to a barrier to a

total enclosure. The most important acoustical parameter is the

weight per unit area: the heavier the barrier, the more isolation

is obtained. A characteristic of most isolating materials, such as

sheet steel, glass, or solid wood panels, is that they are resonant.

At the frequencies for which the various types of resonances occur

(and particularly in the coincidence region), the isolation afforded

becomes much smaller. At frequencies above the resonance, the isolation again increases.

67
An idealized and smoothed way of describing the above behavior

is given by Beranek in Noise and Vibration Control (B.5). Isolation, expressed (in dB of transmission loss) as the ratio of energy

transmitted to energy incident, first increases with frequency at

the rate of 6 dB per octave. At the coincidence region, a plateau

is reached for which the average transmission loss is practically

constant (see Table 4.3). The frequency extent of this plateau depends on the damping, or deadness, of the panel material. Thus,

lead has a short plateau and steel a long one. Above the plateau,

the isolation again increases. Damping can be added to reduce the

plateau length.

TABLE 4.3 - Transmission loss of common materials

Frequency Hz

Material Ity'sq It 125 250 500 1000 2000 4000 8000

Lead

1/32-inch thick 2 22 24 29 33 40 43 49

1/64-inch thick 1 19 20 24 27 33 39 43

Plywood

3/4-inch thick 2 24 22 27 28 25 27 35

1/4-inch thick 0.7 17 15 20 24 28 27 25

Lead vinyl 0.5 11 12 15 20 26 32 37

Lead vinyl 1.0 15 17 21 28 33 37 43

Steel

18-gauge 2.0 15 19 31 32 35 48 53

16-gauge 2.5 21 30 34 37 40 47 52

Sheet metal (viscoelastic laminate-core) 2 15 25 28 32 39 42 47

Plexiglas

1/4-inch thick 1.45 16 17 22 28 33 35 35

1/2-inch thick 2.9 21 23 26 32 32 37 37

1-inch thick 5.8 25 2B 32 32 34 46 46

Glass

1/8-inch thick 1.5 11 17 23 25 26 27 28

1/4-inch thick 3 17 23 25 27 28 29 30

Double glass

1/4 x 1/2 x 1/4-inch 23 24 24 27 2B 30 36

1/4 x 6 x 1/4-inch 25 28 31 37 40 43 47

5/B-inch Gypsum

On 2 x 2-i nch stud 23 28 33 43 50 49 50

On staggered stud 26 35 42 52 57 55 57

Concrete, 'I-inch thick 48 ,

29 35 37 43 44 50 55

Concrete block, 6-inch 36 33 34 35 38 46 52 55

Panels of 16-gauge steel, 'I-inch

absorbent, 20-gauge stee I 25 35 43 48 52 55 56

68
Figure 4.1 shows the plateau average height and width for some

common single panel isolating materials. Note that the variable is

the~product of frequency and weight/area (abbreviated as fw and

given in Hz-lb/ft2 ). To select material and weight/area, first

decide how much noise reduction is needed. A good approximation is

obtained by measuring the octave band spectrum and A~level at the

ear position of the worker closest to the machine. If this position

is indefinite, measure at 1 meter (3.28 feet) from the major noiseproducing surface of the machine. Then the minimum noise reduction,

NR, needed is given by:

where LA

level of

85 dBA.

related

margin,

NR = LA - Lc

is the equivalent 8-hour level, and Lc is

90 or 85 dBA, as appropriate. It is wise

The transmission loss (TL) for industrial

to the noise reduction by TL = NR - 6. To

it is wise to use TL = NR.

( 4. 1)

the criterion

to plan for

situations is

afford design

60

50

m""0

Cf)

Cf) 40 0

-l

Z

0

Cf) 30 Cf)

::2:

Cf)

Z

«

II 20

flO

I I I I I I I I I I I I I 1IIIfII" I I I I I I I

/ ~~ lead (59)

,o~c7 ~~ ;/,0

\0<::- . ~~ steel (40) ~' concrete (12) <::-~ "0~ brick (11l , ue;/ plaster (9) I . oeP' .<::-u' aluminum (14) glass (13)

'"

/ plywood (3)

V ,

103 104 lOb

FREQUENCY-WEIGHT/UNIT AREA (fw) [~z-Ib/ft 2 J

FIGU RE 4,1 - Transmission loss pIa teaus. Ends of plateaus occur at the folIo wing fw values:

plywood 500, 2600; glass 1200, 13,000; aluminum 1400, 25,400; plaster 1400, 11,200; brick 3500,

12,250; concrete 4000, 18,000; steel 4500,49,500; lead 30,000, 120,000. Numbers in ( ) are

weights in Ib/~ ftPe;-;nchoTthickness.·:~'--'~J

.. '. ,:~'. '. '. ·,-jj·;j~~t!

69
50

5 ----

The next question is: In what octave band is the maximum noise

reduction needed? To estimate this, modify the measured octave band

levels by subtracting from each its midband A-weighting. Locate the

octave band in which the maximum A-weighted level occurs. Use the

designating frequency of the octave band below this in calculating

the weight.

Now use Figure 4.1. Choose a material whose plateau height is

greater than TL required. If the plateau is too low, added damping

will be needed to avoid extreme coincidence effects. From the curve,

find the value of the fw (frequency X weight/area) product corresponding to the TL needed. Divide this fw product by the designating

frequency of the octave band below the maximum in the A-weighted octave band spectrum. This gives the minimum weight per unit area for

the panel material. See Table 4.3 for the TL of some common materials and Figure 4.2 for a comparison of various materials and wall

designs (from Harris 1957, p. 20-6), B.7.

WEIGHT IN KG PER SQUARE METER

S 10 20 50 100 200 60~-----r---...,..--F'---,-""",-"--,--,r-T~-----"~~-"",---,:",~==:--,,,--""':;":"',,,

651--------r-------1--

.D 451----

o

Z

40 - -- -..

en

en

o-l ~5 -'--.----------

z

o

en

en

:2

en

Z

«

a:

......

I

I -- ----

I O~I------:-----:~-~-~--:~~7-!S~9......\0:------...... 20:----J.~0--4..l.0--5.i..0-6J.0-7J...0-S";'0-9J.O....lIOO

WEIGHT IN POUNDS PER SQUARE FOOT

Source: C.M. Harris, ed., Handbook 0 1 Noise Control, McGraw-Hili, New York, 1957.

FIGURE 4.2 - The mass law relation between average sound transmission loss and mass per unit

area of partition

r---- 10 Reproduced from

l~_~~-_ ,~~_::allable co

p

_

y

. )

70
If the panel material is resonant (long plateau), it must be

damped to reduce plateau length. For irregular panels, the damping

material can be sprayed on or troweled on. As a convenient guide,

several popular materials are satisfactory when the dry thickness_ of

the damping treatment is about twice that of base metal, usually

steel. Manufacturers will furnish recommended treatment thickness.

Design recommendations based on this material selection are

covered in more detail in Chapter 3.

Many damping materials are quite temperature-dependent. Be

sure that the material selected has optimum performance for the

temperature range in which it will be used. Most manufacturers can

supply the necessary engineering information for making an optimum

selection. Damping materials in sheet form for gluing to a vibrating surface can be supplied with a pressure-sensitive adhesive.

The interior of enclosures must be sound absorbent to prevent

reflective buildup of sound within the enclosure. Often the func-

. tions of absorption and damping can be combined in a single composite

material. The absorbent is usually a foamed polyurethane; the minimum thickness of the foam should be 1 inch. Special oil and waterimpervious film coverings can be specified. This covering protects

the material from becoming liquid soaked. However, deposits on the

film must be removed periodically to prevent loss of high frequency.

absorption. Use a mild warm detergent solution, applied with a

cloth. Wipe carefully to prevent damage to the film.

Absorbents protected by a film still have exposed edges. These

may be sealed by a latex paint that anchors itself to the pores of

the absorbent and closes the edges. An alternative is a thin film

that is adhesively fastened over the edges. By choice of thickness

of air space between the absorbent and the mounting surface, the

shape of the absorption-frequency curve can be controlled. This

can be matched to the need for maximum noise reduction in frequency

regions determined by the noise measurements.

Vibration Isolators. The selection of vibration isolators is

based on placement of the resonance frequency of both machine mass

and isolator spring constant. All of these should be well below the

operating frequency. As a general rule, a machine on a heavy rigid

foundation is well isolated when the resonance frequency is less

than 1/5 the lowest operating frequency. The latter is usually that

of rotary unbalance in the slowest rotating part. If the machine is

on a lightweight floor or is hung from a springy roof, the ratio

should preferably be less than 1/5. Vibrating pipes or suspended

fans can be in this category. Steel spring pipe hangers, supplied

with damping elastomer seat, are the usual pipe hanger materials

selected. For many machines on a solid foundation, a crossribbed

rubber pad often suffices. Manufacturer's literature can be helpful in making an optimum selection, provided that the spring constant and maximum deflection is supplied.

71
A special problem arises with punch presses or other sources of

periodic impulse noise. Here the problem is to reduce the transmission of both the shock and the ensuing vibration. The optimum

choice of vibration isolator is governed by the relation among three

time intervals. First is tl' the effective duration of the exciting

shock. Second is t2' the period (time for one cycle) of motion resulting from resonance between mass of the machine and the effective

stiffness of the vibration/shock isolator. Third is t3' the interval between repetitions of the impulse. The isolator should be

chosen so that tl < t2 < t3 to obtain near-optimum results. The value

of tl for a punch press or shear is approximately the time between

contact of the tool with the workpiece and the completion of the

cutting action. This can be determined from machine parameters; direct measurement with an oscilloscope gives more reliable data.

Vibration isolation can reduce A-levels from 2 to 15 dB. The

lesser amounts are obtained when the base is solid and heavy, as for

a foundation set directly on the eart~. See Chapter 3 for more detailed discussion of vibration isolation.

Silencers. Silencers or mufflers can be considered as sections of a duct or pipe that have been acoustically designed and

treated or shaped specifically to reduce the noise transmission.

The noise may originate from a machine, or be flow-generated by

gas or liquid flow.

Silencers for airborne sound are best selected by reference to

manufacturers' literature. The usual silencers are lined with special absorbents. The reductions available are tabulated for the

octave bands for several silencer lengths. Knowing the reduction

desired, you can choose readily. Silencers are often available in

several pressure drop categories. Final selection requires some

expertise in air-handling system design, and a mechanical engineer

should be consulted in critical situations. The degree of silencing also depends on the amount of steady flow and on whether the

sound is traveling upstream or down. Some manufacturers supply

such data.

Nonacoustic Considerations of Materials. Selection may also

require consideratiori of nonacoustic factors, which often control

the final choice. These factors may be broadly classified as environmental and regulatory. Environmental factors include:

- Moisture, water spray, water immersion

- Oil, grease, dirt

Vibration

- Temperature

- Erosion by fluid flow.

72
Regulatory factors include:

- Lead-bearing material forbidden near food processinq

lines.

- Restrictions on materials that may be in contact with

.foods being processed - glass, monel or stainless steel

permitted.

- Requirements for material not to be damaged by disinfecting.

Firebreak requirements on ducts, pipe runs, shafts.

- Flamespread rate limits on absorbents.

- Fire-endurance limits on absorbents.

Restrictions on shedding of fibers in air by

absorbents.

Elimination of uninspectable spaces in which vermin

may hide.

- Requirements for secure anchoring of heavy equipment.

Restrictions on hole sizes in machine guards (holes

can reduce radiated noise of vibrating sheets).

A good example of the influence of these factors is seen in

the selection of absorbents for use inside machine enclosures.

It is typical of ordinary maintenance practice to over-lubricate

rather than to install or service oil or grease seals. Hence it

is common to· find oil and grease deposits on machines, often with

dirt, metal chips, and other debris. Such deposits greatly degrade

the perforMance of absorptive coatings, which are porous materials

that easily wick oil and water. However, absorbent materials are

now available with a thin imperforate skin or film covering of

Mylar, Saran or Tedlar, which prevents fluid wicking. Nevertheless,

the sheer weight of grease deposits will degrade higher frequency

performance even without wicking; fire hazard will also be increased.

Hence the film must be strong enough that the deposits can be

cleaned off with a cloth wet with warm detergent, plus mild rubbing.

Such maintenance will be necessary with machine enclosures lined

with absorbent. The "time between cleanings can be greatly lengthened if oil and grease seals are installed or if deflecting shields

are used on severe oil spray, such as comes from impacting parts in

a punch press.

73
Curtain types of isolating materials are convenient for constructing an enclosure rapidly. The usual material is lead-loaded

vinyl plastic. Where leaded materials cannot be used, as in some

stages of food processing, a barium-loaded type is available. Monel

and stainless steel are the only common metals usually permitted in

contact with food. Fibrous absorbents in shop-made silencers and

mufflers can be eroded by high speed gas flow, say, above 30 mls

(100 fps). The fibers may pose a health hazard and also can interfere with machine operations. The situation is worsened if vibration is present, as this tends to break and shake out small fibers.

The material used should have some bonding agent to hold fibers

securely in place. In addition, the absorbent can be covered with

wire screen or perforated metal. If the latter is used, the ratio

of open to total area should be over 0.5. The effective absorption

will be decreased if lesser open areas are used. Foamed absorbents

shed much less than fibrous types, but all need sealing of raw

edges by a film-making paint or by a thin plastic cover.

Since most damping materials are very temperature sensitive,

the operating temperature must be known before making an informed

selection. For unusual conditions, consult the manufacturer.

Often if a large lot of material is to be purchased, it can be

specially formulated for the conditions specified.

Fire resistance is often required by building codes. Absorbents are available for several degrees of resistance. With suitable materials, fire breaks are sometimes unnecessary in isolating

walls that are filled with absorbent. Since local building codes

may not be applicable to structures that can be described as a

part of the machine, prudent language must be used in describing

the function of the enclosure.

A most important nonacoustical factor in the selection of

noise control materials is net cost. The noise control engineer

must always be aware of this and should design so that labor plus

materials cost is minimized. A part of the net cost is also the

loss in production while a machine is being treated, so time to

restore production must be considered. Ease of maintenance must

also guide the selection. Achieving a viable design means that

material selection cannot be accomplished on a purely acoustical

basis.

74
Chapter 5

CASE HISTORIES

The case histories presented here should be of some use to

production and safety engineers, health personnel and other factory

personnel who are not specialists in noise control. The case histories are examples of engineering tasks that have been completed by

professional noise control engineers. Collected here are actual

cases on various industrial devices. These devices were typically

machines utilized in a production process and in some cases had been

cited by safety officials for unsafe high noise levels.

The case histories presented here were chosen from some of

those where the amount of noise reduction actually achieved was

measured. Such engineering results, even if not directly applicable

to your situation, may, illustrate general principles that will point

the way to a successful result in your problems.

Each case history was selected because the work stations associated with it were in violation of the recommended noise exposure

levels in all but two or three examples, where uncomfortable noise

levels affected productivity even though they were lower than the

limits in the OSHA noise regulations.

Case, History Data

Ideally, case histories should be complete in all details.

However, the prime duty of a practicing noise control engineer is

to solve his client's problems at minimum net long term cost. In

this the experience of the noise control engineer is invaluable in

knowing when information is unnecessary or has been handled verbally.

Indeed, the client may already have done part of the work.

In the following outline, there is described the whole process

of accomplishing noise control that is viable in both engineering

and economic senses. The outline will also serve as a check list

to guide you in learning and applying the principles of noise control engineering that have been discussed earlier. The case histories that follow contain the essential data for the simpler problems and somewhat more for the complicated ones.

Outline of Complete Procedure for Development of Noise Control

1. Plant data

- SIC classification of industry

- Location, address; division

- Product or process,

75
2. Problem definition

- Compliance plan

Compliance measurements, daily noise dose

- Diagnostic measurements and source locations

- Design of experimental noise control

- Design of final noise control

- Supervision of construction, installations

- Post-installation checkout, performance evaluation

- Oral briefings

- Preparation of technical paper

3. Machine data

3.1. Identification

- Make, model, serial number, factory number

- Appearance (drawing or photo); identification

of significant parts, functions

- Layout drawing of workroom, all machines shown

- Location of aisles, vertical clearances; service lines; conveyors; hazard-posted areas

3.2. Operating data

- Functions of machine; relation to others

- Type of input: gage, size, shape of stock

- Type of output: shape, size

- Type of scrap: how collected

- General product flow with respect to other

machines

- Use of automation: conveyors, robots

- Services and ratings: electrical, air, water,

fuel, steam, hydraulic, internal combustion

engine, vibrator

- Production rate (maximum)

- Down time: jams, breakdowns; repair, maintenance, set-up; reload, idling; operator at

rest room, meals

- Constraints on operation: access, both physical and visual; for worker, input stock, output product and scrap; access for repair,

maintenance, set-up, reload; safety, union regulations, sanitation, special materials for

food industries, rodent control; operator need

for aural cues; limits on capital and operating expenses

76
Special machine features: noise control

features already installed; use of vibration isolators; use of air; evidence of

over-lubrication

4. Noise situation data

4.1. General observations (ear)

- Noise high or low pitched

Directional location by cupping hands behind

ears

- Presence of pure tones

- Level constant, varying slowly or with much

impact noise

- Feeling of vibration in floor

- Workers communicating byword or sign

- Use by workers of aural cues in detecting

and evaluating machine performance, jams

4.2. Measuring equipment

- Name, make, model, SiN

- Calibration data: when; traceable to NBS

- Check list for diagnostic acoustical measurements: SLM, octave band analyzer, 1/2-in.

and l-in. microphones, tripod, extension

cord for microphone, windscreens, calibrator

and adaptors; accelerometer; control box for

acceleration, velocity, displacement; stroboscope; vibrating reed tachometer - Check list for optional equipment for diagnostic acoustical measurements: two channel

tape recorder, connecting cords, microphone

for voice channel, blank reel, AC cord,

charger; range finder, measuring roller,

steel tape (inches and centimeters); flashlight; pressure-sensitive labels; camera

with wide-angle lens, flash; spare batteries

for all equipment (alkaline only); ear muffs,

safety glasses, safety shoes, hard hat, paper

towels, handsoap; pliers, diagonal cutters,

screwdrivers; circuit tester

4.3. Acoustical measurements

- A, C, peak and octave band readings

- Measure at ear positions of worker, worker

absent, with all machines going, then with

machine in question off

77
- Run machine at different speeds to

locate resonances

- Run with portions of machine selectively

disabled

- Measure rpm's with stroboscope, vibrating reed tachometer

- Measure at suspected noise sources on

machine; photograph the set-ups; locate

microphone precisely

- Set to octave band for which A-weighted

spectrum at ear of worker maximizes.

Probe around machine to locate sources

- Locate around machine an imaginary box

that touches all major surfaces; record

the dimensions; at I m away from box, obtain levels for calculating total sound

power, using procedures described by Diehl,

in Machinery Acoustics, reference B.G

- On slow A-scale obtain contours of equal

level around machine, others off; locate

paths of workers among contours. Repeat

with all machines on

4.4. Vibration measurements

- A, C, peak and octave band readings

- Probe over the surface (pickup coupled so

it is not rattling) for acceleration and

velocity

- Calculate root mean square values over

surface

- Observe if maximum A-weighted band for rms

acceleration or velocity coincides with

maximum A-weighted band for airborne noise

- Run machine at different speeds to locate

resonant excitation of vibration

- Selectively disable parts of machine to

locate exciting sources

4.5. Auxiliary data

- Data per (3.2)

Unusual conditions: breakdowns; machine

with bad bearing, gears, loose parts

- Tape recordings of noise situations that

are short-lived or non-repetitive, together

with calibration signal; also useful for

later narrow-band analysis, judging rpm,

pure tones .

78
- photographs of all pertinent parts of

machine, from establishing shots to closeups of name plate

- Names, position and possibly addresses of

operating, supervisorial and management

personnel concerned

- Time of entry to plant, time spent at

each machine, time left plant

5. Development of noise control

5.1. Preliminary report

- Data, raw and reduced; evaluation, interpretation

- Preliminary noise control recommendations,

taking full account of const~aints in (3.2)

above

- Preliminary estimate of noise reduction

expected

- Preliminary estimate of capitalized installed

cost

- Preliminary estimate of possible change in

productivity and change in piece part cost

- Recommendations on use of automation

- Conference to discuss implications of report

5.2. Development of revised recommendations

- Remeasure as needed

- Re-estimate noise reduction, costs

- Prepare recommended experimental program if

problem sufficiently unusual

- Prepare sketches showing acoustically essential

features of the noise control devices; if required, prepare drawings

- Recommend special materials; provide alternate

suppliers

- Estimate construction, installation costs

5.3. Installation, use

- Monitor construction and installation for

adherence to acoustical specifications

- Introduce corrective measures for improperly

installed devices

- Evaluate emergency alternate materials

- Measure installed performance; correct

deficiencies

79
- Measure daily noise dose to applicable

workers

6. New work

- Recommend improvements if similar noise control is

to be applied to other machines of the same class

- Recommend action on problems remaining

- Provide briefings on results to technical and

management people

- Prepare paper for publication

- Help prepare formal compliance reports

80
CASE HISTORY #1 - STEEL WIRE FABRIC MACHINE

Description of the Problem

This 8-foot fabric machine man~factures wire netting spaced at

6-inch spacings, starting with individual wires from large spools

that run through the length of the machine. A perpendicular wire,

known as staywire, is fed across at 6-inch intervals and spot welded

at each intersection. This staywire is then cut off at the lefthand side of the machine. The long wires are then moved through the

machine another 6 inches, and the staywire operation is repeated.

This machine produces 6 x 6 inch #8 or #10 wire netting, which is

used as concrete reinforcement in the home building industry. Machine is made by Keystone Steel and Wire Company.

Measurement and Analysis

At the operator position, the noise level was found to be 99

dBA and 102 dBC, indicating low frequency components. This kind of

noise (LC - LA = 3) is very unpleasant.

Daily noise dose was found to be 2.5, where the acceptable level

is 1.0.

Criteria were established to reduce the noise exposure to 1.0 or

less, which is equivalent level to 90 dBA or less.

The octave band measurements made at the main drive gear (Figure

5.1.1), at the operator station (Figure 5.1.2), at the wire spool

area (Figure 5.1.3) showed that noise sources included (1) general

mechanical noises due to needed maintenance, (2) the wire wrapper, a

ratchet-action machine operated from main drive gears and found to

cause 10aa-Hz peak noise, and (3) mechanical sources within the machines, which could lend themselves to isolation.

Control Approaches Considered

In addition to the direct noise corrections implied above, another solution could have been to construct a noise shelter for the

operator. This solution was dropped in favor of working on specific noise sources. A program was established to:

(1) Overhaul the machine: replace bearings, reduce metal-tometal banging, replace worn gears, and so on.

(2) Replace ratchet type drive on wire wrapper with chain

drive. (This device pulled the long wires through the

fabric machine.)

81
(3) Add steel plates (10 1b/ft2 ) to the frame of the machine.

These plates were welded to the frame to block direct air

path noise to the operator from gears. The machine frame

casting had many openings, which were covered by these

steel plates, as shown in Figure 5.1.4.

Noise Reduction Achieved

Noise at the operator station was reduced from 99 dBA to 93 dBA

(93 dBC). With this reduction, an additional source was noted and

determined to be the staywire lifter arms. These were covered with

a 3/8-inch-thick piece of Lexan (see Figure 5.1.5) for the full

length of the operator position, hinged so that it could be easily

removed for maintenance.

Operator station levels were reduced to 89 dBA and were therefore in compliance with current standards.

A shelter could have solved the problem, but where possible,

attack on direct noise is recommended. When major noise sources are

reduced, the contribution of other noise sources can be better determined and corrected. By redesigning the wrapper device from a

ratchet to a chain drive, the production rate was increased by 50%.

Costs

Costs were mainly internal plant labor for machine overhaul,

plus the cost of the steel barrier plates welded to the frame (estimated at less than $100), plus the cost of the piece of Lexan at

$5.00/sq ft, or about $50 plus installation labor, in plant.

Pitfalls

The major pitfall in this kind of approach is moving too fast.

Testing each technique under actual conditions is far better than

moving rapidly into failure. This solution required approximately

2-years-Iapsed time from beginning to end.

82
OCTAVE PASS BANDS IN HERTZ

120

90 180 355 710 1..00 - 2800 - S600 - 1'200

110 _dBC

_dBA

(l)

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50 5

63 125 250 500 1000 2000 4000 8000

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.1.1 - Noise levels for 8-foot fabric machine, 1 meter from main drive gear

83
OCTAVE PASS BANDS IN HERlZ

120 180 355 710 UOO - 2800 - 5600 - 11200

I

I

60

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63 125 250 500

5

1000 2000

5

8000

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.1.2 - Noise levels for a-foot fabric machine operator station

84
OCTAVE PASS BANDS IN HER12

120

90 180 35S 710 l«lO - 2800 - 5600 - 11200

110

-dBC

___ dBA

A weighted

I =1= -+-

50 63 121 250 500 I c~o 2CCO .000 8000

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60

CD

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FIGURE 5.1.3 - Noise levels for 8-foot fabric machine, 1 meter behind operator station

in wire spool area

85
FIGURE 5.1.4 - Steel plate barrier with window (stop sign hung on it).

86
FIGURE 5.1.5 - Lexan barrier in two sections; slides up 'or access.

87
CASE HISTORY #2 - aOO-TON VERSON BLANKING PRESS -

VIBRATION ISOLATION

Description of the Problem

The SOO-ton Verson press is a massive unit weighing about

275,000 pounds, mounted on four footings set on heavy concrete

piers. Production on this press was automobile chassis steel sections of 1/4-inch steel about 10 inches wide and a to 10 feet long.

Normal operating speed was 30/min. Steel stock was fed to the

press from a reel. Noise levels were about 120 dB impact, 105 dB

quasi-peak, and 94.5 dBA at operator location, which was about four

feet in front of the press.

Control Approach Chosen

As a starting point to the total solution of the noise problem,

it was decided to vibration isolate the press and determine the

attenuation gained before working on other noise sources, which are

not part of this case history.

Measurement and Analysis

The press was operated in a single shot mode. Hence quasi-peak

readings for each octave band were more meaningful for ear effect

than rms readings (slow A-scale). The peak value is the maximum

level reached by the noise, whereas quasi-peak is a continuously indicating measure of the average (over 600 msec) of the high levels

reached just before the time of indication and is thus lower than

the actual peak but is greater than slow A-scale values.

Vibration data were recorded for the support foundation, floor

near press, adjacent building column, and press structure at the

press feet, before and after installation of the isolators.

Isolators

From the data supplied on strokes per minute and press weight,

the isolators were specified to be Vibration Dynamics Corporation

(of Lagrange, Illinois) series BFM micro/level isolators, under the

press feet. No price lists are available because each isolation

problem is specifically engineered and quoted. Cost was about

$2,000 for the isolators, and installation by in-plant labor was

probably about $1,000.

Noise Reduction and Vibration Reduction Achieved

Adding isolating pads reduced the vertical acceleration at the

pier by 9.5 dB, as shown in Figure 5.2.1. Most of the reduction

88
occurred in the 2, 4, and 8 kHz bands. The vertical foot-to-pier

acceleration reduction was 30 dB.

In Figure 5.2.2 is shown the horizontal acceleration at the

pier. Adding isolation effected a 12 dB reduction in acceleration.

The horizontal foot-to-pier acceleration was reduced 36 dB by the

isolating pads.

The attenuation afforded by the pads is shown in Figure 5.2.3,

for both vertical and horizontal motion of the pier. Note that it

is the vertical motion that is responsible for most of the sound

radiated by the floor.

Figures 5.2.4 and 5.2.5 are comparisons of the sound level

readings at 4 ft before and after isolation (quasi-peak readings,

single shot operation). The calculated dBA level in Figure 5.2.5

shows a reduction of 6.5 dBA in airborne noise.

Isolators reduced vibration in support foundation, floor, building, column, and press structure. It has been found that a primary

cause of background, or ambient, noise is the vibration in the building structure, which is presumed to be caused by the anchor bolt

after-shock.

Calculation here shows that there was a 105-dBA quasi-peak

sound level before isolation and a 98.5-dBA level after isolation.

With a relationship of about 10 dB quasi-peak to rms, a reduction in

level from 94.5 dBA to 88 dBA at operator location has been made.

Additional presses will add their own noise and will increase levels

to above 90 dBA. Other operational noise sources in the press must

be controlled separately.

Pitfalls and Rewards

The major pitfall of this approach is that airborne sound level

reduction due to vibration isolation is almost impossible to predict.

However, a serious noise control program in such operations must include isolation devices for all presses.

A reward is that the.die life and maintenance of such machines

is significantly increased for presses that are vibration isolated.

Isolators improve operation and maintenance by reducing failures of

anchor bolts, foundation failure, or breaking of press feet.

89
OCTAVE PASS BANDS IN HERTZ

90 180 355 710 1<00 - 2800 - S600 - 11200

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90
OCTAVE PASS BANDS IN HERTZ

ISO 355 710 1.0(10 - 'Soo - 5600 - 11200

o

total

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FIGURE 5.2.2 - Horizontal acceleration on pier, before and after isolation

91
OCTAVE PASS BANDS IN HERTZ

90 180 355 710 ,.00 - 2800 - 5600 - 11200

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FIGURE 5.2.3 - Attenuation of vertical and horizontal accelerations at pier by adding isolation

92
OCTAVE PASS BANDS IN HERTZ

180 355 710 1400 - 2800 - 5l>OO - 11200

100

60

I

-r

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FIGURE 5.2.4 - Quasi-peak levels 4 feet from press foot before and after isolation

93
OCTAVE PASS BANDS IN HERTZ

180 355 710 10600 - 2800 - S600 - 11200

105

98.5

4000 8000

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F IQURE 5.2.5 - A-weighted quas i-peak levels 4 feet from press foot before and after isolation

94
CASE HISTORY #3 - BLOOD PLASMA CENTRIFUGE

Description of the Problem

This plasma production room has two parallel banks of centrifuges, 15 to a bank, plus refrigeration units. A sketch of one

centrifuge is shown in Figure 5.3.1. Centrifuge spinning frequency

is 13,000 rpm (217 Hz). Though centrifuges appeared to be the major

noise source, refrigeration units were also considered. The same

refrigeration units without centrifuges are used in a separate reconstituting room.

Measurements and Analysis

Operator level was 97 dBA with one bank in operation; 100 dBA

was predicted with both banks operating. Octave band readings of

operator levels are shown in Figure 5.3.2.

Often close-in measurements of a complex noise source will aid

in determining specific areas requiring correction. With this

thought in mind, we made close-in diagnostic readings at the locations shown in Figure 5.3.3. Octave band readings for each of these

locations are shown in Figures 5.3.4. through 5.3.9.

See Figure 5.3.3 for sketch of centrifuge showing locations of

close diagnostic readings and octave band charts, from 31.5 to

8000 Hz. Note from these the following facts:

(1) From Figure 5.3.2, operator position, note that the

major noise contribution to the A-reading is in the

500-Hz and 1000-Hz bands.

(2) All other locations show similar maximum bands when

close readings are made of the various suspect items

of this assembly, Figures 5.3.4 through 5.3.9.

(3) Highest levels with similar bands are noted for Figure

5.3.8, close to motor exhaust, showing that the motor

exhaust is the major noise source.

(4) From these octave band readings, the spectral content

of the overall noise can be gleaned. Of course, it

is the A-weighted level at the operator's work station

(dotted line Figure 5.3.2) that is OSHA regulated and

needs to be reduced, since the A-level measurement

characterizes the noise much as the human ear responds

to it. Figure 5.3.2 shows that the largest contribution

to the A-readings are from the 500- and 1000-Hz bands,

95
so our noise control diagnosis and correction

efforts should concentrate on these bands.

(5) The centrifuge spinning frequency of 217 Hz from

the original data adds to the weak 250-Hz level,

but as can be seen from Figure 5.3.2, is not a

large contributor to existing A-levels and can be

safely ignored.

Data from the reconstituting room are shown in Figures 5.3.10

through 5.3.12. This room has no centrifuge, so the noise source is

predominantly the refrigeration unit, which has a maximum at 500 Hz

(though only at 82 dBA) , but does confirm that this is a possible

noise source for the SOD-Hz band in the centrifuge plus compressor

room.

Control Approaches Considered

We conclude from the above data that the maximum band at 1000 Hz

is a major contributor. The same maximum occurs on Figure 5.3.8

(close to the motor exhaust) and in Figure 5.3.4, where the noise is

presumed to be reflected from the highly reflective metal pulley

guard surfaces. These data confirm that the major noise source to

be attenuated is the motor exhaust.

The results of our measurements indicated an unacceptable exposure when the operator was exposed for 4 hours to both centrifuge

banks (Figure 5.3.2 predicted level at 100 dBA). Under these conditions, the operator's daily dose was 2.0, and thus exceeded the

acceptable exposure of 1.0 as specified by OSHA noise regulations.

There are three general locations for controlling noise: at

the source, along the transmission path, or at the receiver. Two

could be used in this case. A properly designed and constructed

muffler or partial enclosure for the centrifuge motor exhaust would

provide the necessary source control, using materials and geometric

configuration that have effective attenuation in the 1000 to 4000 Hz

bands. Such a control measure would provide an expected 3 to 7 dBA

attenuation.

Noise control measures can also be used along the path of transmission. The paths of airborne noise transmission were from direct

and reverberant fields such as walls, floor, and ceiling, which

supply very little noise absorption.

For direct field reduction, barriers with the proper transmission loss, dimensions, and orientation may be used. The reverberant field can be controlled by the addition of absorbent materials.

These combined measures would reduce operator exposure 5 to 15 dBA.

96
Control Approach Chosen

Although the above solution would satisfy the noise control

requirements, other solution possibilities should be discussed with

company representatives as they are able to specify important operating, maintenance, and production constraints that may limit the

type of noise control means.

In this case, after discussion, two of the possible control

methods were eliminated. The hard and impervious walls had to remain because of the need for daily sterilization by high pressure

steam and water hosing, and hand scrubbing. A porous absorbent surface would provide areas for bacterial growth and would not withstand the rigorous daily cleaning. A barrier to shield the operator from the centrifuge motor noise was eliminated from consideration because it would block the proper flow of refrigerated air.

A small warming of the plasma would produce an unusable product.

It was decided to try the motor exhaust muffler and see how much

noise reduction could be achieved.

The muffler was designed with a stainless steel outer skin,

lined with acoustical absorbent spaced 2 inches from the inside of

the steel shell, using small blocks. The 2-inch air space allows

absorption since it reflects from the inner steel surface and back

through the absorbent. The muffler has one 90-degree bend, as

shown in the sketch in Figure 5.3.13. In-house shop cost was estimated at $300.

Noise Reduction Achieved

Noise at the operator's position was reduced from 97 dBA to

92 dBA, satisfactory for a four-hour employee. The motors of both

centrifuges must be treated if they operate together.

Pitfalls

An airtight seal at the junction of muffler and motor is very

important; we used resilient caulking compound as sealants. An

air leak of area no more than ten percent of the muffler cross section would pass as much noise as if the muffler were not there at

all. An intake muffler would probably also help, but noise from

other sources would then become prominent.

Effectiveness of noise control is reduced if path of vibration

transmission is not held to minimum. Accordingly, it was very important to use as few absorbent spacers as possible. In so doing,

the steel skin vibration was kept small. The spacers can be made

of damping material to reduce resonant vibration of the skin.

97
-5° box

product tub

centrifuge

'--

- - - -- - - - - --- ---- - - --- - --- --------- --

FIGURE 5.3.1 - Sketch of one centrifuge (front view)

98
OCTAVE PASS BANDS IN HERTZ

90 180 JS5 710 1400 - '800 - 5600 - 11:200

100

predicted level

two centri fuge

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FREQUENCY IN HERTZ

FIGURE 5.3.2 - Operator exposure in -5° C box room (below clock, 3cm from clock wall)

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100
OCTAVE PASS BANDS IN HERTZ

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FREQUENCY IN HERTZ

10,000

FIGURE 5.3.4 .. Centrifugenoisemeasu:edparallef to motor shaft (2 cm from pulley belt guard; between

#25 guard and motor exhaust of adjacent centrifuge)

~-------

C Reproduced trom

[ ,\ best available COpy· : :

G - ~-- --.,-;;

101
OCTAVE PASS BANDS IN HERTZ

100

90 180 355 710 1~OO - 2800 - 5600 - 11200

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FIGURE 5.3.5 - Centrifuge noise measured 15cm directly below pulley

102
OCTAVE PASS BANDS IN HERTZ

180 355 710 1'00 - 2800 - 5600 - 11200

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FIGURE 5.3.6 - Centrifuge noise measured perpendicular to spout (3 cm away, facing clock)

103
OCTAVE PASS BANDS IN HERTZ

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FREQUENCY IN HERTZ

FIGURE 5.3.7 - Centrifuge noise measured between centrifuge motor and pulley belt (normal

to shaft 10 cm away)

104
OCTAVE PASS BANDS IN HERTZ

180 355 710 l~oo - 2800 - 5600 - 11200

dOOO 8000

,

i

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FIGURE 5.3.8 - Centrifuge noise measured 2 cm from centrifuge motor exhaust

105
OCTAVE PASS BANDS IN HERTZ

100

180 355 710 1400 - 2800 - 5600 - 11200

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FREQUENCY IN HERTZ

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FIGURE 5.3.9 - Centrifuge noise measured at tub height (2 em from front tub edge)

106
OCTAVE PASS BANDS IN HERTZ

., - 90 180 355 710 1400 - 2800 - 5600 - 11200

100

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100 1000 10,000

FREQUENCY IN HERTZ

FIGURE 5.3.10 - Operator's exposure in reconstituting room (center of room, ear level of a standing

worker to the right of tank # 24)

107
90

OCTAVE PASS BANDS IN HERTZ

180 3SS 710 TAoo - 2800 - 5600 - 11200

100

CD

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100 1000 10,000

FREQUENCY IN HERTZ

FIGURE 5.3.11 - Noise in reconstituting room, measured 2 cm from bottom edge of exhaust face

of refrigeration unit

108
OCTAVE PASS BANDS IN HERTZ

180 J~5 710 1.00 7800 - '600 - 11700

100

90

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100 1000 10,000

FREQUENCY IN HERTZ

FIGURE 5.3.12 - Noise in reconstituting room, measured 2 em below rear intake face of

refrigeration unit

109
~I

Cross section of muffler

showing absorbent, bend,

and dimensions. Absorbent

is 2 in. from surfaces.

18 inches

6 inches

12 inches

.. __ 12 inches Exterior shape

and dimensions

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FIGURE 5.3.13 - Motor exhaust muffler

110
CASE HISTORY #4 - BLANKING PRESS RAM

Description of the Problem

In forming operations, large blanking presses are used. The

ram, which is like a connecting rod in a reciprocating engine, is

hollow. The forming die runs in grooves on the side of the press,

like a piston in the cylinder of a reciprocating engine, and completely closes off the end of the hollow ram. There are slots in

the ram that are used normally when the press is used in blanking

operation to extricate the work from the die. This is similar to

removal of a cookie from a cookiecutter. These slots are in the

side of the ram (see Figure 5.4.1). When the press is being used

in the forming mode, these slots are not required, and when the

die "snaps through", it cuts off the work. 'This gives rise to

high noise levels.

Measurement and Analysis

In a vibration isolated forming press, operator position noise

levels were LA = 94 dBA, LC = 100 dBC in the "slow" reading position.

An octave band measurement disclosed that levels in the 250-, 500-,

1000-, and 2000-Hz bands were much higher than in other bands. This

ringing noise, which had a maximum near 2 kHz, was easily discernible by ear. By careful listening, it was determined that the

source was radiation from the slots in the ram.

The technical conclusion was that the ram hollow interior plus

slots was essentially behaving as a shock-excited Helmholtz resonator. A Helmholtz resonator is a closed volume of air connected by

a tube, which will resonate at various frequencies (like blowing

across a glass jug opening).

Control Approaches Considered

The one approach that would obviously work would be to fill the

cavity in the ram with rubber-like material. Another approach would

be to simply plug the slots and keep the noise inside the ram. The

second approach was chosen because it was easy to try and certainly

not expensive to test. It would also allow the machine to be easily

reconverted to a blanking operation.

Control Approach Chosen

The ram slots were each covered with a plywood plate sealed

with a Neoprene gasket, as shown in Figure 5.4.1. Weatherstripping

(nonhardening sealant) was used to prevent small ·leaks. These control measures were easily installed.

111
Noise Reduction Achieved

The first attempt was satisfactory and achieved a 6-dBA reduction of quasi-peak A-weighted level from 99 to 93 dBA OP. See '

Figures 5.4.2 and 5.4.3. Applying this to the observed slow A

reading of 94 dBA yields the observed 88 dBA.

This case history demonstrates both the simplicity (the solution) and the complexity (the resonator) of noise control. It also

demonstrates a more subtle feature: simple solutions are worth

trying if there is a good physical reason for them.

Pitfalls

The obvious pitfall here would be to apply this solution to a

press that had not first been vibration isolated. If the press were

on other than piers isolated from the building, or had sheet metal

guards, one would probably not have been able to measure any improvement. Filling the ram cavity would have been another pitfall. It

would have accomplished the noise reduction, but would have prevented

easy reconversion of the press to blanking operation.

112
PRESS FRAME

ram

through bol t

SLOT

NOTE:

use Neoprene

gasket both

sides; seal

washers and joints

with weatherstripping

cement

weather stri pp ing

,cement

ram

"-

plate (typical)

gasket

FIGURE 5.4.1 - Method used to cover slots in blanking press ram

113
OCTAVE PASS BANDS IN HERTZ

120 90 1eo 355 710 '.400 7800 - 5600 - 11200

110

4-dBA~ QP

JI 'f

-+-----"-.,.--.,--,-r----r-----

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100 1000

FREQUENCY IN HERTZ

10.000

FIGURE 5.4.2 - Quasi-peak readings of blanking press before nOise control

114
OCTAVE PASS BANDS IN HERTZ

120 90 180 355 710 1400 - 2800 - 5600 - 11200

110

10,000

dBA QP

- _ Quasi-peak

1000

FREQUENCY IN HERTZ

!--I .

A weighted

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FIGURE 5.4.3 - Quasi-peak readings Of blanking press after ram ringing was contained

115
CASE HISTORY #5 - SPINNING FRAME AIR NOISE

Description of the Problem

Cord manufacturers use a machine called a spinning frame to

convert yarn to cord.. In the process of spinning this yarn into

cord or thread, lint or small pieces of yarn fall away. At each

spinning station along the frame, air suction removes this lint

by a system that works essentially like a vacuum cleaner. This

system requires a rather large air moving system for each spinning

frame, and the noise created from these air moving systems causes

the ambient noise levels to range from 88 to 93 dBA at the work

stations throughout this system. This unit was a Whitins Model

M-2.

Measurements and Analysis

Measurements were made with a Type 2 sound level meter. At

about 2 cm from the air exhaust of the lint scavenger system, the

levels were: LA = 100 dBA, LC = 100 dBC. The. major noise source

was unquestionably the air escaping from the lint removal system,

as was verified by the fact that LA = Le. This problem is cornmon

in high velocity air systems.

Control Approaches Considered

The obvious solution to a problem of this nature is to use a

muffler or an acoustical isolator. However, a more fundamental

approach considered was to slow the escaping air at the scavenger

exhaust. This could be accomplished by simply giving the exhaust

vent a bigger open area, as shown in Figure 5.5.1. The velocity

of the escaping air was estimated to be 115 ft/sec (the fan moved

1800 cfm through an area of about 37 in. 2 ). To simply open the

fan cover was not practical, since the air must be directed upward.

The reason this control approach is a good one to consider can

be best summarized in the following relationship:

5 X = 10 10910 (Vo/Vn )

where X is the reduction in decibels

V

o

is the original air velocity

V is t~e new air velocity. n

This equation is widely used by noise control engineers to estimate

the relative noise reduction if air stream slowdown is possible.

116
Our design increased the area through which the air exhausted

by a factor of 10: from 0.26 ft 2 to 2.6 ft 2 . Because the flow is

practically incompressible, Vo/V = 10- 1 and X = 50 dB reduction.

However, the net noise reductlonnwill ordinarily be less because

other noise sources are still present. A useful rule of thumb is

to expect a useful reduction of at most 10 dB if a major source is

completely removed. The chief exception to this is the intense

and often high frequency pure-tone single source, such as a whistle,

stearn vent, or automatic 'control valve.

Noise Reduction Achieved

The noise was measured, with all but one fan cover unchanged;

it was LA = 93 dBA and LC = 94 dBC, a reduction of 7 dB in A-level.

It is thought that this reduction fairly well represents the background level without this fan running.

Pitfalls

The most common pitfall in a treatment of this kind is to attempt to do a makeshift or sloppy final job. Care must be taken

for the final result to be effective. A professional metal shop can

fabricate the device shown in Figure 5.5.1 much more easily and,

certainly in quantity~ much less expensively than it could be fabricated in your plant. The rubber gaskets and sealant are both important to the overall effectiveness of the job.

This kind of noise control is certainly straightforward and is

not expensive. The cursory results reported here make it well worthwhile. The plant for which this was designed will no longer find it

necessary to require hearing protection for its workers.

117
NOTE:

1 - for final assembly, seal all gaskets with

nonharden i ng weatherstri ppi ng cement

2 - make certain existing fan cover is strong

enough to support modification

vent - muffler, to be designed and

attached only if required

new air exhaust stack, directs

air upward to vent - muffler- --1~--_.....

exi sting fan cover, modified to

accept new air exhaust stack

FIGURE 5.5.1 - Air exhaust vent modification for spinning machine noise control

118
CASE HISTORY #6 - BARLEY MILL

Description of the Problem

Excessive noise levels existed around Moorspeed and Ross barley

mills (rolls S-inch-diameter, 15-inch-long), a hay shredder, and a

control operator's chair in a cattle feed grinding mill. The objective was to reduce the noise level at the operator's position

for OSHA compliance.

Measurements and Analysis

A and C level and octave band measurements were made between

the Moorspeed and the Ross mills (Figure 5.6.1) and at the hay

shredder with both mills in normal continuous operation (Figure

5.6.2). With LC - LA = 9 dB, excessive low frequency noise levels

were predicted. These were confirmed by octave band measurements.

Octave band measurements at the control operator's chair are shown

in Figure 5.6.3. A sketch of the room showing the relative locations of the equipment is shown in Figure 5.6.4.

Control Approaches Considered

Roller crushing actions produced high noise levels, and correction by machine redesign was believed to be too costly a method

for solving this problem. When the source is too difficult or

uneconomical to attempt to correct, working on the noise path will

often result in a more economical solution. Therefore, a partial

enclosure, open at the top, was chosen.

Control Approach Chosen

Although walls can be of solid construction with a minimum of

access doors, in this case access was needed for adjustment,

maintenance, repair, and roll replacement. For roll replacement, a

forklift truck entry was required. For ease of quick access, a

fixed barrier wall was discarded in favor of a lead-vinyl curtain

wall extending, if required, up to the 17-ft height of the roof

-support beams. All three noise sources could be enclosed by two

curtain walls enc19sing the corner of the building as shown in

Figure 5.6.4. The curtains run on rails for easy sliding back and

are held together by Velcro closures.

Figure 5.6.1 shows that, if the sound levels from 250 Hz up

are reduced by at least 14 dB, the resulting A-level readings would

be less than 90 dBA for compliance outside the curtain walls.

119
Barrier wall attenuation is limited in three ways: (1) direct

transmission loss in each octave band, (2) noise over the wall, and

(3) room absorption, noise-source side.

(1) Direct transmission loss (TL):

vinyl fiberglas curtains, 0.75 Ib!sq ft,

literature gave the transmission loss in

lows:

The manufacturer of leadwas chosen. Manufacturer's

each octave band as fol125 Hz

TL = 11 dB

250 Hz

16 dB

500 Hz

20 dB

1000 Hz

26 dB

2000 Hz

31 dB

It is seen that the transmission loss is not a limiting factor.

(2) Noise over wall: Barrier wall attenuation can be estimated from data in Noise and Vibration Control, Leo L. Beranek,

page 178, using the dimensions from Figure 5.6.4 and from the sectional view in Figure 5.6.5.

N = ?(A + B - D) = f (16.6 + 16.6 - 18)

N = 30.4 (Fresnel number) -A125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz

Ix = 9.6 ft 4.8 2.4 1.2 0.6

N = 3. 2 6. 3 12.6 25.2 50.4

Attenuation (dB) 14 16 18 20 20

(from graph page 178, Beranek) . In practical situations, the attenuation is limited to about 20 dB.

By a rough first approximation procedure we can obtain an estimate of the reduction afforded by the curtain walls. In the listing

below we start with the worst-case octave band levels of Figure

5.6.2 and then list the transmission loss and barrier effects just

calculated. Subtracting the minimum of these two reduction mechanisms yields a tentative spectrum of the resulting sound in the room.

After A-weighting and combining levels, the predicted reduced room

level is 84 dBA.

120
Octave bands 125 250 500 1000 2000

Noise sour-c-e-----:;-l-;:;0:-::6:-----:;-l-;:::0~1------;:;:9-;:::8:----9=7-----:9"""0-

Direct TL 11 16 20 26 31

Over wall 14 16 18 20 20

Reduced levels 95 85 80 79 70

A-weighting -16 -9 -3 0 1

A-weighted 79 76 77 77 71

A-level 84 dBA

For visual access the enclosure can have 10 x 20-inch plastic

windows placed to order; use only minimum number. The curtains

should be long enough to drag a bit on the floor to reduce leaks.

Some rerouting of power, steam, and air lines may be required.

The approximate 1973 costs were: $4.00/ft 2 for curtains made

to order with grommets; Velcro fasteners, $3.00/ft; track, $1.50/ft;

rollers (one per grommet), about $2.50 each; windows, $25.00 each;

total cost about $4,000.

The preceding simplified treatment neglects an important fact:

we have not gotten rid of the noise, but have merely redistributed

it. Thus, the total sound power from the machines escapes from the

topless enclosure and spreads throughout the room. Close to the

curtains, there should be some reduction, but very little farther

away. Absorption is required to actually reduce .the sound power.

This was next considered.

(3) Absorption, noise-source side of wall: When noise sources

are confined to a space with less absorption than before, they may

build up higher levels due to reverberation. The sound barrier

curtain material can be obtained with sections of sound absorbent on

the inside, to counteract this effect. In the barley mill, however,

this was not recommended as the porous open material could easily

become dust clogged. Shortly after this noise control job was completed, absorbents covered with a plastic film became available.

At the time the recommendation was for an easily installed and maintained material, Owens-Corning Fiberglas Noise Stop Baffles. These

are 23 x 48 x 1.5 inches and comprise an absorbent board wr~pped in

a washable, noncombustible plastic film; each baffle is supp~ed

with two wires through the 23-inch dimension. These wires terrn~nate

in hooks; to install, stretch wires three feet on center, parallel~­

to the line joining the two mills and about flush with the top of

the enclosure rails. The baffles are available through local acoustical material suppliers. The cost is about $lO.OO/baffle.

The enclosure developed by the curtain walls is in effect a

separate small room, and the noise reduction can be estimated from

the relationship of total absorption before and after adding the

sound absorption panels. This relationship is

121
where:

dB Attenuation = 10 log a 2/a1

a 2 is new total absorption

a l is original absorption

(from reference: Harris Handbook of Noise Control, pages 18-19).

Original absorption, a:

Area x Coeff. = Abs. (sabins)

Long wall 32' x 17' x 2 x 0.02 22

End wall

Roof

17' X 17' x 2 x 0.02

17' X 20' x 1 x 0.02

11

7

Absorption by adding 100 panels 2 x 4 feet:

100 X 2' x 4' x 2 sides x 0.8 (average A-weighted

absorp. coeff. of

panel) = 1280

1320

Original absorption

New total absorption

dB attenuation = 10 log :~ = 10 log 1~~0 = 15.2 dB

Resultant level = measured level - reduction = 86 dBA.

40

Noise Reduction Achieved

The measured final noise level was 87 dBA, giving a 7 dBA reduction. This was 3 dBA lower than the maximum desired level, and was

the result of paying careful attention to elimination of leaks.

The room formed by the curtain did not realize such.a reduction, but

since these machines required no attention while running, the noise

exposure of personnel was significantly reduced below D = 1. The

major remaining path is reflection from the ceiling.

~itfalls

Barrier walls of various heights can often be used between a

noise source and a machine operator. A major pitfall is that, in a

122
room with a high level of reverberant noise, the partial barrier

will be short-circuited by the reflected noises from walls, ceilings,

and other surfaces. In such cases, attenuation based on the partial

wall theory will not be obtained, often resulting in no attenuation

at all in highly reverberant rooms. Curtain walls must be kept

closed to get attenuation. Sound absorbing units must be kept clean

to be efficient.

Even in a semireverberant room, a reduced barrier height can be

used. In this case a 7-ft barrier should ideally reduce the level

to 89 dBA at the receiving location. However, since the semireverberant conditions will introduce more reflected sound with the lower

barrier, the high wall used in this case history is recommended

because the added absorption within the barrier area has, in effect,

made a separate small room and created the condition on which the

barrier wall theory was based.

123
OCTAVE PASS BANDS IN HERTZ

120 90 ISO 355 710 1400 - 2BOO - 5600 - 11200

_ peak

r- I ±

I I =r:: "*=1= 150 500 IC')() 2000 4000 SOOO

i I I I I I

I 2 5

100 1000 10,000

50

60

110

_ dBC

m"0 100

-l _dBA

w

>

W

-l

W

a: 90 I :J

U? 1- U?

w I

a:

a. - I

0 - =I A weighted Z 80 :J

0

U?

0

z

« /

m

w 70 >

«

I0

0

FREQUENCY IN HERTZ

FIGURE 5.6.1 - Barley mill noise levels, measured between Moorspeed and Ross mills (4m out,

1 m up, both mi lis on)

124
OCTAVE PASS BANDS IN HERTZ

120 180 355 710 1./00 - 2BOO - 5600 - 11200

_peak

110

m

:' IQG

...J

UJ

>

UJ

...J

UJ

a: 90 :l

C/)

C/)

UJ

a:

0..

o

80

o

C/)

o

z

«

m

UJ 70 >

«

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o

60

50

I

I

5

63

I

I

I

125

A weighted

250 I1000 2WO

-r

_dBA

8000

5

100 1000

FREQUENCY IN HERTZ

10,000

FIGURE 5.6.2 - Barley mill noise levels, measured at guard rail of hay shredder (2 m opposite

blower, both mills on)

125
OCTAVE PASS BANDS IN HERTZ

120 180 3SS 710 1.400 - 2800 - 5600 - 11200

110

_ peak

r

CD _dBC

"0 100

-i

w

>

W

..J _dBA

w

a:

::> 90 (J)

(J) .~- W _J _

a:

a..

0

z

::> 80

0

(J) -j0 A weighted Z

<{

CD / ±

w /

> 70

<{

I-

()

0 I

I

60

I

I

-i:

63 125 250 500 lenD 20CQ 4000 8000

50 S

100 1000 10,000

FREQUENCY IN HERTZ

FIGURE 5.6.3 - Barley mill noise levels, measured at control operator chair (ear level, operator absent)

126
door

roof 17' at walls, 26' at rt.

70' ....1

--,,.....--_-----------------r----------or--------,

~. ~\ ' • IMoorspeed Ross

\ lead - vinyl curtain~ test 7

2,,\:3' 17'l~ _°r:~ __h~:'~:'_ g' \ _ _ _ 32' _ door -f

;' .

I operator

42' I

1//~~--

FIGURE 5.6.4 - Floor plan of barley mill

127
FIGURE 5.6.5 - Sectional view of barley mill

128
CASE HISTORY #7 - SHEETER FOR BOXBOARD

Description of the Problem

The sheeter, starting from large rolls of boxboard about 6 feet

in diameter, cuts the web to length using a rotary knife that can be

adjusted to rotary speed, and therefore sheet length, by means of

variable speed drive (Reeves Drive). The cut sheets are delivered

to pallet. The speed is about 700 ft/min.

Measurements and Analysis

At the operator control station near the sheeter (see Figure

5.7.1), the noise level was found to be 93 dBA. Close-in probe readings at the variable speed drive were high, indicating that the

drive is a major noise source. Readings were as follows:

96 dBA close to front drive guard, in aisle.

98 dBA close to front drive guard, in aisle 3.

105-107 dBA close to front drive vent openings.

The drive box enclosure was a steel shell 6 ft high, 3.5 ft wide,

and 3.5 ft deep, having two vent openings in the side for natural

air cooling (see Figure 5.7.2).

Other operator locations that were far from the drive were

checked:

90 dBA operator at delivery

88 dBA operator at rollstand in feed (see Figure 5.7.3

for general layout.

From the close-in readings, the drive was determined to be the

major noise source and not the roll unwind stands, rotary cutter, or

delivery belts to finished pallet of boxboard.

Control Approach Chosen

To reduce the drive noise within the steel box enclosure, it

was decided to line the interior walls with an acoustic absorbing

polyurethane foam with a layer of 0.017-inch-thick sheet lead to

provide damping of the steel surface panels. To reduce the noise

corning out of the air vents, an acoustic traps was designed to absorb the noise at vent but allow full normal air circulation. This

acoustic trap is shown in Figure 5.7.2.

129
Noise Reduction Achieved

The level at the operator control panel near the drive unit

was found to be 89 dBA, reduced from 93 dBA. In addition, some

reduction was obtained in other operator positions:

86-87 dBA from 90 dBA operator at delivery.

86 dBA from 88 dBA operator at roll stand.

Close readings at vents were reduced to 94 dBA from 105 dBA; this

is not an operator position.

Sound absorbing polyurethane foam with a lead septum designed

for combined damping and absorption is available from various suppliers at less than $4/ft2 ; material cost was about $400, and inhouse labor to glue in place and fabricate a holder for the sound

trap was about another $400; total cost was about $800.

The polyurethane sound absorbing material shows a 0.90 absorption coefficient at 1000 Hz and above. Although the absorption

coefficient at lower frequencies is less, the effect of increasing

A-weighting with decreasing frequency makes the loss of absorption

at lower frequencies less important.

Pitfalls

Without close-in reading to locate the drive unit as the

major noise source, we could have jumped to the conclusion that

the entire sheeter, including the drive unit, must be installed

in an acoustic enclosure, thereby spending a great deal more money

for the solution.

This kind of noise reduction is typically not as satisfactory

as one would like. The major problem that can arise is the existence of other direct sound paths from the knives to the operator.

Another pitfall for sheeters is the knife design. Some of

the older models have straight knives instead of an angular striking

or cutting edge. Straight knife sheeters will probably require an

acoustic absorbing lined metal or wood hood over the knife assembly

and perhaps under the knife assembly.

130
.....l~==::;~-'-.:o.!:r:k~f=- _

sheet web

(cut)

vents (see FIG. C 7.2)

variable speed drive

I •

I I 6 it high X 3.5 ft X 3.5 ft

L ...J in steel cabinet

FIGURE 5.7.1 - Floorplan of sheeter·for boxboard

noise

/barriers

,_4--~~-f,

acoustic

foam rubber

3.5' .~~I

II I I

I vent I

6'

I I

vent

,~ -w"

FIGURE 5.7.2 - Sheeter drive box enclosure

131
delive,y ope,a,,,,!

rotary knife

rr

l drive

I ,

L

1 ........-1

..J

FIGURE 5.7.3 - Layout of sheeler and operators

web

sheeter --_~r----,

/

roll operator

~';

roll

132
CASE HISTORY #8 - AIR SCRAP HANDLING FROM BOBST CUTTING PRESSES,

FOLDING CARTON MANUFACTURING

Description of the Problem

In the folding carton industry, printed sheets are cut on Bobst

and similar cutting presses, equipped with automatic strippers for

removal of waste material between cartons. When the press is operated and is in good mechanical adjustment, there is no serious noise

problem. Often, however, noise from the scrap disposal system results in levels above 90 dBA on the pressman platform.

This popular scrap disposal system (see Figure 5.8.1) used a

horizontal air vane conveyor to move the scrap from under the stripping station to the intake of a centrifugal fan that pushes the scrap

to a baler or to bins at a baler in a remote location.

The noise problem arises from the pieces of paper scrap striking

the sides of the intake conveyor under the press stripper, the sides

of the intake hood to the fan, and the fan and outlet ducts. These

all contributed noise that resulted in noise levels of over 90 dBA at

the pressman station~ Depending on amount of scrap and size of

pieces, the noise level reached 95 dBA each stroke of the press,

normally making the noise almost continuous.

Measurement and Analysis

In this type of problem, it was not considered necessary to

make octave band measurements when simple direct sound level readings would tell the story of the obvious problem before and the

results after damping. Octave band levels aid in determination of

the noise source, but in this case the noise source was known and

before and after levels could be expressed in dBA.

Control Approach Chosen

The sheet metal of the stripper intake, fan intake from horizontal air vane, the fan, and outlet ducts were all damped (and

transmission loss improved) by gluing a layer of lead sheeting to

the outside surfaces, using a resin glue recommended by supplier of

sheeting. Sheeting used was 1/32-inch thick, 2 Ib/sq ft.

Other sheet damping materials that are on the market could have

been used as effectively, as discussed below.
Noise Reduction Achieved

The damping of the sheet metal reduced the noise level at the

pressman platform to 88 to 90 dBA.

The concept of using sheet lead to damp the sheet metal ducts

came from supplier literature citing successful sheet metal damping

on ducts and fans and other surfaces. (Cost is about $0.90/ft 2 .)

For less damping, a 1 Ib/ft2 material may be used·at $0.46/ft2 .

For minimum damping, stiff roofing felt may do. For even greater

damping, there are many products on the market in sheet form and

tape form. Suppliers can be consulted on specific problems~ prices

range from $1.50 to $3.50/ft2 ..

For very high vibration- and noise levels, a further duct treatment step would be lagging, which is a spring-absorber-mass combination of 1 to 3 inches of resilient acoustic absorbing material

(Fiberglas or polyurethane) with a heavy cover sound barrier of

sheet lead or lead-filled vinyl sheeting over the entire surface.

See Case History #9 for an example of this method.

134
top

view

5 hp motor ___ vertical discharge duct

... ./8inch diameter

fan damped to 10 feet from floor

"1' .... ) ,. !J

intake duct 7C~I I

I I

: I

- - I I - - - - I I

out load stripper I I press feed in load

I I

: I

III II II

pressman platform

side

view

-stripper-- -------

=out load- press feed - in load

pressman platform

floor __.......tL--.l._--I__-,""~-r--_-.J...LL.. -.l. ...L.._~"""_"""

air - veyor

FIGURE 5.8.1 - Scrap handling system for cutting press

135
CASE HISTORY #9 - JORDANS FOR PAPER MILL FOR BOXBOARD

Description of the Problem

In a boxboard waste paper mill, Jordans are used to refine the

waterborne pulp. A Jordan consists of a conical shell and rotating

conical plug, both with steel blades on their mating surfaces. The

refining action can be adjusted by decreasing the spacing of the revolving cone to the stationary cone within the limits of power

available from the drive motor. In this case, drive motor power is

500 H.P.

The action of the Jordan refiner results in high noise levels,

as shown in Figure 5.9.1. With maximum refining power the noise

level was found to be 97 dBA at the aisle (102 dBA one foot from

Jordan surface) during start-up with only one Jordan in operation.

As normal operation required six of the eight Jordans in operation

with varied power settings, depending on degree of refining required,

the operating noise level in the aisle was often at about 100 dBA.

Although the paper maker did not have to be in the area continuously

during the shift, he was required to make periodic adjustments, which

took more time than the maximum exposure_ of 2 hours at 100 dBA permitted by current regulations. Some noise reduction was therefore

required.

Measurement and Analysis

It was determined from vibration measurements that the major

vibration surfaces were the conical shell, end cover plate, and outlet box. Shell acceleration measurements were made with a hand-held

pickup attachment to the sound level meter to determine the major

vibration surfaces as summarized below:

Areas selected for treatment

Shell surface, large end

Shell surface, rib

Flange face

Shell surface, center

Shell surface, rib

Shell surface, small end

Outlet box

Areas not treated

Bearing support

Overhead piping

Drive motor and bell (1)

Drive motor and bell (2)

136

Vibration

(g)

15.9

20

10

15.9

11.2

5

7.9

2.5

1

0.9

1.4
A-level noise readings only were obtained because octave band

data would not have added any information for these purposes.

Control Approaches Considered

Two possible solutions were considered: (1) complete enclosure and (2) partial lagging of Jordan shell. Total enclosure of

each Jordan was possible but was not considered as practical as

lagging because there were eight Jordans near each other, because

adjustment and maintenance would create additional problems, and

because the customary sound absorption material inside the enclosures would not be compatible with the wet conditions of this paper

mill. Partial lagging was chosen since it could be installed on

the major vibrating surfaces producing the noise. Separation of

lagging could be designed to allow for maintenance dismantling of

the front face and plug. The noise absorbing material of the lagging could be covered with layers of sheet lead, taped to form a

water-resistant surface that would withstand cleaning normal to

paper mills.

The lagging consisted of a resilient (and absorbing) layer of

3 inches of TIW fiberglass, covered by an impervious and heavy

layer of lead sheeting to serve as the mass element and to contain

the noise. In theory, a I-lb/sq ft lead sheeting, perfectly covering

the area, would give an attenuation of 24 dB at 500 Hz and 28 dB at

1000 Hz. From a practical standpoint of making a more impervious

continuous barrier fqr both noise and water from cleaning hoses,

two layers of lead were used, each layer taped for tightest joints

under the conditions of use.

Alternate materials could be considered: the absorbing layer

could be polyurethane foam, and the barrier layers could be leadfilled vinyl. However, the fiberglass is much more resilient, and

thus is preferred.

In another paper mill, a third method was chosen: enclosure

for the operators. As the time actually spent in making the adjustments during the shift was small, the daily noise exposure index would be at or below the required 1.0 if l~ to 2 hours exposure at 100 dBA were balanced with the remainder of the shift in

a protected area below 85-dBA levels.

Since this reduction would require only a minimum shelter for

15 dBA attenuation, a simple construction could be used consisting

of ~-inch plywood, two sides on a 2 x 4 inch framing, with one door

and a viewing window plus lighting. Due to paper mill conditions,

the enclosure was heated and air conditioned for worker comfort.

Record keeping could be done inside the enclosure.

137
Figure 5.9.2 shows the attenuation obtained inside the control

room; the noise level was reduced from 97 dBA outside (Figure 5.9.1)

to 77 dBA inside, well within compliance levels. Reducing the noise

levels further would require an enclosure with improved transmission

loss construction (such as concrete block).

Control Approach Chosen

A sketch was made following the lagging design parameters discussed above, requiring 3-inch Fiberglas grade TIW, plus two top

layers of 1/64-inch lead sheeting, all held to the surface by "sticklips" glued to the surface (normal method of acoustic application

in buildings). Each layer of the lead was waterproof taped to

protect the absorbing layer from water cleanup. Two layers were

used to get best waterproofing.

Shafts~ bearing boxes, and the drive motor were not lagged

because vibration readings did not justify the action, as noted

above.

The sketch shown in Figure 5.9.3 was used by an acoustic contractor for estimating and installation purposes. Cost was $600

per Jordan, averaged over all six. Total enclosure cost would have

been at least $4,000 for each Jordan, based on a quotation from an

enclosure supplier; thus lagging achieved a net savings of $3,400

per machine.

Noise Reduction Achieved

For a single Jordan paper-pulp refiner, the noise level at the

operator station was reduced by 11 dBA, from 97 dBA to 86 dBA. With

all eight Jordan refiners similarly lagged and running at normal

production, the aisle noise level ranged from 88 to 91 dBA. Even

for unusually high production in the fastest mode, these same levels

ranged from 92 to 94 dBA. This lagging produced acceptable daily

noise exposures of less than unity.

Pitfalls

Paper mill conditions should be studied carefully because the

noise absorbing materials cannot be exposed to the wet conditions

of a paper mill. Maintenance methods were reviewed before design,

leading to a separation of the lagging at the front flange so

that it could be removed for maintenance.

Cover plates requiring occasional removal were not lagged, as

noted in Figure 5~9.3. A disadvantage is the lack of smoothness

of this surface--it makes lagging look like upholstering!

138
___I

OCTAVE PASS BANDS IN HERTZ

180 355 710 1400 - 2800 - S600 - 11200

8000

_97 dBA

4000

-jI

I 63 125 250 500 IC:JO 20aO

I

I

60

CD

"l:l

I 100

..J

W

>

W

..J

W

a:

::J 90 en

W

en ,,"

a:

Q.

0 -"

Z

::J 80 0 ,-" A weighted en

0

I z

«

CD I I

W

> 70

« I

()

0

100 1000

FREQUENCY IN HERTZ

FIGURE 5.9.1 - Noise levels 1 meter from Jordan refiner

10000

139
OCTAVE PASS BANDS IN HERlZ

90 180 355 710 1<100 - 2800 - 5600 - 11200

CD

';' 100

--l

ill

>

ill

--l

ill

0: 90

:l

(f)

(f)

ill

0:

0..

o

80

o(f)

o

z

«

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W 70

>

«

fo

o

60

// A weighted

_77 dBA

63

100

12~ 250 ~~o

1000

FREQUENCY IN HERTZ

2000 _000 8000

10000

FIGURE 5.9.2 - Noise levels inside enclosure of control room for Jordans

140
separate front flange

c. o. clear

cover shaded area with 3-inch fiberglass TIW plus 2 plies of 1/64-inch

sheet lead. Tape for water resistant joint. Fasten stik-klips and caps.

studs clear

FIGURE 5.9.3 - Noise reduction lagging for Jordan shell

141
CASE HISTORY #10 - FOLDING CARTON PACKING STATIONS,

AIR HAMMER NOISE SOURCE

Description of the Problem

In the manufacture of folding cartons, the individual cartons

are cut, and the cut sheets are stacked by the cutting press on a

pallet. To deliver the multiple sheets from the press, the cartons

are held together with a nick or uncut portion. When stacked, the

individual cartons are separated by stripping with an air-driven

chisel which breaks the nicks and frees an entire stack. When no

additional operations are needed, these stacks are packed in cases

for shipment.

Air hammers/chisels produce noise that has not yet been eliminated by equipment manufacturers. Currently available air hammer

mufflers do not reduce the noise to an acceptable level. The air

hammer operator therefore must wear ear protection. The problem in

this case was to protect other workers (packers) from the air

hammer noise. A typical production air hammer stripping and packing set-up is shown in Figure 5.10.1.

The production sequence for this operation is for the stripper to air hammer a stack of cartons (precounted by the cutting

press) and place them on the conveyor at Point C. The packer, at

the end of the conveyor E, prepares the case, packs the stacks of

cartons, seals, labels, and stacks the finished pack on a delivery

skid. Two packers are required to handle the output from one stripper. The stripper is actually using the air hammer about 50% of

his time, with the balance of the time used in stacking or preparing

the load. Thus he can get some relief from continuous use of his

ear muffs by hanging them around his neck while not actually using

the hammer. It is easier to sell the use of ear muffs when needed

if the operator can get some relief when muffs are not needed.

Measurements

As the specific wavelength requirement is minimum in this problem; no octave band readings were made; all dat~ were based on A and

C scale readings from an acceptable Type 2 sound level meter.

Control Approach Chosen

It was decided to protect the packers from the air hammer stripping noise by using a barrier wall. The previous chapters described

the theory of barrier walls in detail, but a convenient rule of

thumb is that useful protection is afforded by the barrier wall

beyond 30 degrees into the acoustical shadow. Note that in Figure

142
5.10.1, the packers behind a wall 10-feet long and 6-feet high are

within this protected zone in both top view and side view of the

operation.

The barrier wall need be no better acoustically than the attenuation afforded around the sides and top of the wall. The wall was

therefore fabricated using a 2 x 4-inch frame faced on both sides by

~-inch plywood for a simple sturdy barrier wall.

If there had been any reason to reduce noise reflections from

the noise source side, this side could have been faced with sound

absorbing acoustic materials, but it was not needed in this case.

We used the rule of thumb of aiming for the packer to be well

within the 30 degree line from the acoustic shadow line. Other means

of estimating the attenuation of barrier walls are covered by Beranek

in Noise and Vibration Control, page 178, and illustrated in Figure

5.10.2. The attenuation calculated for this barrier wall ranges from

10 to 15 dB, depending on the wavelength. This agreed with the measured attenuation of 7 to 12 dB and the noise reduction from the 92

to 97 dBA range to about the 85 dBA average measured at the packer's

ear level.

Cost

The barrier costs were:

~-inch plywood, 2 sides, 5 sheets, 4 x 8 160 sq ft

2 x 4 inch framing 60 ft

In-plant labor

Approximate total

Pitfalls

$30.00

10.00

60.00

$100.00

In this installation, we were fortunate not to have low ceilings,

which would have established a serious sound reflection problem and

defeated the barrier wall. Barrier walls will not give good results

in a highly reverberant, low-ceilinged room. If there had been a low

ceiling, useful level reduction would still have been possible by

adding sound absorbing material at the reflecting portion on the

ceiling (about 12 feet over the barrier wall and the noise source).

The amount of attenuation gained is easily estimated by using the

r~tio of absorption of new material to that of the existing ceiling.

Ceiling reflection is a major pitfall of the use of barrier walls

indoors. The design of the wall alone is based on free field

conditions.

143
packer ear level

-~

barrier wall /

ceiling

c

B conveyor _ - --E

--

/

stripper

0 r~

barrier wall

packers

C

B conveyor- - -E

stripper

FIGURE 5.10.1 - Air hammer stripper and packer line

144
---- e-10feet

-------------

source

...-... 4A ,ee\ __ - --

O· .:.--

--- -- -- ---

barrier wall receiver

2 N=X(A+ B-D) 250 Hz 500 Hz 1000 Hz 2000 Hz

N::; ~(5.5+ 10-14) A 4.8 2.4 1.2 0.6

N=~ N 0.625 1.25 2.5 5

dB 10 11 13 15

Source: Beranek, Noise and Vibration Control, page 178.

FIGURE 5.10.2 - Barrier wall theory

145
CASE HISTORY #11 - IN-LINE GRAVURE PRINTING AND CUTTING

PRESS :CLDING CARTONS

Description of the Problem

In the manufacture of folding cartons, one method is to print

the cartons in a web, using multiple gravure color stations and

feeding the printed web into a reciprocation cutting press.

The reciprocation cutting press, using. a rule die, cuts the

cartons and delivers cut cartons to a delivery belt. The rotary

printing operation was not noisy, but the cutting press noise from

the cutting head was in the range of 93 to 95 dBA at the normal

operator position. The take-off operators were far enough from the

noise source, so noise at their station was below 90 dBA.

Control Approach Chosen

Figure 5.11.1 shows the operator location, control station,

cutting head, and carton delivery. To reduce the noise of the

cutter head at the operator position, we decided on a barrier wall.

As access to the unit for job changes and maintenance was important,

the barrier wall was specified to be the lead-filled vinyl sound

stopper curtain material available on a made-to-order basis and

designed to be portable.

The curtain unit ordered was 7 feet high and 8 feet long with

a viewlng port 10 x 20 inches, since the attenuation required for

OSHA compliance was only about 5 dBA minimum.

Noise Reduction Achieved

The operator location noise at the control console was reduced

from the 93-95 dB range to an 86-87 range. The operator performed

inspection and adjustment at the cutter head for a few hours daily,

as required, but was still within the time exposure limits.

Total cost, using a lead-filled vinyl curtain at about $4/ft2 ,

was about $300, including hanging fasteners, viewing window in curtain, and pipe supports.

Pitfalls

To get any attenuation from barrier walls, the receiver must

be located with respect to the noise source so as to be beyond 30

degrees into the acoustical shadow line, as a rule of thumb. Note

that in the top view, Figure 5.11.1, the pressman operator is just

within this line. In Figure 5.11.2, showing over-the-wall vertical

plane limitations of this same rule of thumb, the pressman operator

146
is well within this limiting area. The curtain met the objective

since only a small attenuation of about 5-6 dBA was required and

the actual real attenuation was 7-8 dBA. More attenuation would

require a larger curtain.

A design pitfall in barrier walls is that if room conditions

are too reverberant and the ceiling is too low, the barrier wall is

bypassed. Low ceiling reflections can be overcome by adding an absorbent to the reflecting area of the ceiling over the barrier

wall.

In Case History #10, a relatively permanent wood construction

wall was used. This case required a different treatment because

regular access was required to the cutter head between the console

and the press. The free standing, easily movable curtain wall

provided both protection during operation and easy access to the

press for set up.

147
8 feet long

-------ef -§t;......._-- noise source

'\?/ ..console 1- sound barrier curtain, 7 feet high,

pressman

drive

I

delivery cut head gravure printing - web ---------i

>300 ---- J

take-off \

Do

D°

skids

FIGURE 5.11.1 - Top view of in-line gravure-cut press with sound barrier curtain

barrier -_.......;......--

noi se source

____-=r _-_-_-~.,....~ressman ear level

FIGURE 5.11.2 - Side view of in-line gravure-cut press with sound barrier curtain

148
CASE HISTORY #12 - AIR SCPAP HANDLING DUCTS FOR

CORRUGATED CONTAINER INDUSTRY

Description of the Problem

In a corrugated box factory, slit side-trim is removed from

the conveyor by air. Trim blower fans with extra heavy blades cut

the trim while conveying it via ducts to bins and balers. The 12-

inch ducts are suspended from the ceiling about 10 feet from the

floor, crossing a 40-foot-long work room en route to the bins and

baler room.

Ear level noise levels were 93 dBA.

The trim is carried along in the ducts by the air, which normally moves about 6000 ft/min. The trim often strikes the duct

walls (mainly at bends), causing noise. Noise reduction was desired to improve worker communication for operations under ducts

and to meet the requirements for minimal OSHA compliance.

Measurement and Analysis

Octave band data were collected because this case required

not only a reduction of a few decibels to comply. with minimum regulations, but noise reduction for safety reasons, to control speech

interference. Octave band data are a truer measure of speech interference noise levels than a single number dBA reading. For the

minimum compliance data, the dBA reading would have been adequate.

Control Approach Chosen

The solution chosen was to wrap the problem duct locations

with 2 inches of mineral wool building insulation to furnish a

resilient and absorbing layer. Over this insulation were placed

two impervious layers of heavy tar paper, spirally wrapped with

50% overlap.

Noise Reduction Achieved

Noise was reduced considerably in the problem area: levels

changed from 93 dBA to 72 dBA, a reduction of 21 dB. To the ear,

the noise could hardly be heard above other noise. The octave band

comparison is shown in Figure 5.12.1. Although the standard materials used were very economical, special acoustic absorber pipe

coverings with lead-filled vinyl sheeting could also have been

used and may have given even more attenuation. This was not needed

here.

149
We do not have detailed costs for this case history. Howpver, since the materials are inexpensive, the major cost must

have been labor. The job was probably done for less than $200.

If less attenuation had been required for OSHA compliance

only, the sheet metal ducts could have been damped and transmission

loss improved by gluing 1/32-inch sheet lead to the duct outer surface. Comparison with the experience at other installations indicated that a 5-dB attenuation would probably have been attained.

(See Case History #8 for other methods.)

Pitfalls

Use the most economical methods to attain the attenuation required. The building insulation plus roofing paper used here is a

very economical solution.

Note that a large overlap was used; lack of overlap on any

wrapping will cause leaks and redu~e attenuation.

150
OCTAVE PASS BANDS IN HERlZ

90 180 355 710 1.-00 - 2800 - 5600 - "200

1020 ~OOO 8000

I 63 115 150 500 leGO

60 I----+---+----+---+----+---+_-

rn

""0

...J 100

w

>

W

.....J

w 93 dBA

a:

if) 90 if)

w

a: before -

0

0.. - - - - z

"

0 80 if)

0

z

<:

rnI

w 72 dBA

> 70 <:

fa

0

5

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.12.1 - Noise levels in scrap duct for corrugated box industry. before and after covering

151
CASE HISTORY #13 - PAPER MACHINE, WET END

Description of the Problem

Th~ major noise sources of the wet end of this paper machine

were the couch roll suction air movement, the pumps, and the

whipper roll. The whipper roll supplies a beating action on the

felt of the paper machine to provide continual web felt cleaning.

Measurement and Analysis

Noise in the aisle at the wet end is shown in the octave band

analysis of Figure 5.13.1. The level is 92 to 94 dBA in the operator aisle. Higher readings of more than 100 dBA were obtained

close to the couch roll. See Figure 5.13.2 for a sketch of the

area.

Paper machine manufacturers have developed a quieter couch

roll in which the suction holes in the drum are in a staggered

rather than a regular pattern. However, the replacement cost of a

couch roll is high, and it will probably be used only on mill expansion projects or new mill construction.

An alternative method to reduce the operator noise exposure

was by constructing a personnel booth to house the operator, plus

the operating controls, during most of the operating shift. The

wet end paper machine operator spent an hour or less making couch

roll adjustments during a typical operating day. If the balance of

each day were exposure in the mill operating aisle at 92 to 94 dBA,

the resulting exposure would be over the dose limits of unity.

However, if the operator spent the 1 hour at 100 dBA (couch roll

adjustments), 2 hours on general observations near machine at 92

dBA, and the balance of the shift in areas under 85 dBA, including

a personnel booth, his daily noise dose would be:

1 hour actual

2 hours allowed

(100 dBA)

+ 2 hours actual

6 hours allowed

(92 dBA)

= 5/6 = 0.83

Since this dose is less than 1.0, it is within normal exposure

compliance.

Control Approach Chosen

The recommendation for the wet end of the machine (couch roll

and whipper noise exposure) was to provide an operator enclosure

with operating controls and instruments, and with viewing windows

to observe machine operation.

152
Calculations indicated that the required 15 dB attenuation

could be attained with a simple structure consisting of 2 x 4-inch

framing with ~-inch plywood walls inside and out, plus one solid

door and two windows 3 x 5 foot each, double glazed. Ceiling and

upper half of walls were covered with acoustic tile to reduce reverberant noise. The room was provided with light, heat, and air conditioning for worker comfort. In-plant construction cost was $2,500.

Noise Reduction Achieved

Results achieved by the enclosure are shown in Figure 5.13.3.

Inside noise level was reduced to 75 dBA from 92 to 94 dBA outside.

Greater attenuation can be obtained by purchasing special

acoustic shelters or by using more elaborate (from acoustic standpoint) construction such as concrete block walls, double windows,

or interior sound absorption.

Pitfalls

Most of the difficulties to be avoided are non-acoustical.

It is essential that the operator has no interference with visual

monitoring of machine operation. This consid"eration fixes the booth

location and window placement.

153
OCTAVE PASS BANDS IN HERTZ

ISO 355 710 1400 - 2800 - 5600 - 11200

m"0

..J 100

w

>

W

..J 94 dBA

w

a:

::>

if) 90 if)

w

a:

a. /

0

z

::>

0 80 if)

0

Z

<t:

m A weighted

W ,-

>

<t: 70

l- I u

0 I

60

63 125 250 500 I coo 20~O

I

4000 8000

100 1000

FREQUENCY IN HERTZ

FIGURE 5.13.1 - Noise levels at wet end aisle of paper machine

10000

154
,..- ,

OOOOOOOO:control :

DDDGcBo~DDL----'

top hydropu Iper

control area

enclosed 10' X 14'

92 dB

bottom hydropulper

Ii Iler hydropulper

)94 dBA

major noi se source

,,/ 100 dBA

i couch roll 7 : WET END

dock

FIGURE 5.13.2 - Paper mill - wet end

155
OCTAVE PASS BANDS IN HERTZ

90 ISO 355 710 I~OO - 2800 - 5600 - 11200

SOOO

±--

,00 1COO

I

2000 ~OOO

2 5

125 250

A weighted

63 I

60

en

"0

I 100

..J

W

>

W

..J

W

a: 90 ::J

C/)

C/)

W

a:

0-

0

Z

::J 80

0

C/)

0

Z 75 dBA

«

enI

W 70 >

«

I0

0

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.13.3 - Noise levels inside control booth at wet end of paper machine

156
CASE HISTORY #14 - PUNCH PRESS NOISE

Description of the Problem

Punch presses in use in this shop were Summit, Bliss Diamond,

and Benchmaster. Within the room were four large presses and four

small punches. One of the Summit presses was chosen as representative of the large press group, and the Benchmaster was chosen as

representative of the small press group. The general room layout

is shown in Figure 5.14.1.

Measurements and Analysis

Octave band measurements were made of the ambient noise levels

when all the presses and the nearby furnaces were shut down. Readings were taken near the central supervisor's desk. The overall

A-weighted level was a very low 58 dBA, as shown in Figure 5.14.2.

This indicated that there were no other serious noise sources. Also

noted was the difference in noise level with and without the furnaces.

Figure 5.14.3 shows that, with the furnace on, the level increased

to 69 dBA, still quite low for most industrial noise levels. Thus,

the furnace was also eliminated as an irritant noise source.

Figure 5.14.4 is an octave band analysis taken from the center

desk with two Summit presses, two Bliss punches, and one Benchmaster

in operation. The level at the desk is 97 dBA, a definite overexposure condition.

The Summit punch, Location I in Figure 5.14.1, was chosen as

typical of a large press. Operator levels, shown in Figure 5.14.5

and 5.14.6, were 106 dBA during operating cycle and 90 dBA during

preparation (punch I off, reaching for a new sheet, etc.). At 106

dBA, the permitted exposure time is 0.87 hr. The octave band analysis showed that major noise contributions to the A-level came from

the 1000-Hz and higher bands.

Figure 5.14.6 shows the spectrum of noise from operation with

nothing in the die. Although a reduction was noted in the 500-Hz

band and a small reduction in the 250-Hz and 1000-Hz bands, the

2000-8000-Hz bands, which were main contributors to A-level noise,

remained the same as with the full operation. The 1000-8000-Hz

bands that were the chief contributors to the A-level were entirely

due to the effect of air exhaust noise from jets for removing parts

and pushing them into the collection chute. For these higher frequencies and short wavelengths, barriers are efficient. Close7in

diagnostic measurement~ were made behind the press, but no new

noise sources were noted except the directionality of some of the

air ejection noise.

157
The reduction sought was from 103 dBA to 86 dBA, with 90 dBA

acceptable. This required reductions of:

13 dB in 500-Hz band

20 dB in 1000-Hz band

26 dB in 2000-Hz band

28 dB in 4000-Hz band

31 dB in 8000-Hz band

For a separate study of a typical small press, the Benchrnaster

(punch VII) was chosen. The operator's position octave band analysis in Figure 5.14.7 shows somewhat less noise than the large press;

it has the same general configuration and air jet noise source.

Figure 5.14.8 shows the noise levels with no stock in the press, and

Figure 5.14.9, the press in punching operation with no stock and no

air ejection. Again data were very similar to those for the larger

press.

Control Approaches Considered

Our recommendations were: reduce air noise along path by

installing a barrier between noise source and operatot; and reduce

noise from air ejection at the source. The latter was considered

first.

Noise caused by high air velocity can be reduced by decreasing

the linear flow velocity by increasing the nozzle opening, for same

air mass flow. If the diameter of the nozzle is doubled, in a constant volume velocity system, flow velocity is reduced to one-fourth,

and noise level is reduced nearly 30 dB (noise of air jet varies

approximately as fifth power of velocity). However, thrust would

also be reduced, to one-fourth of original value. For proper ejection, the nozzle must be aimed more accurately and more efficiently

toward the target. If the distance from the nozzle to the target

were reduced 50%, a 30% velocity reduction would give the same

thrust. Experiments must be conducted to determine the maximum

thrust required for minimum noise.

A barrier between source and operator can add to the attenuation

obtained. The barrier should be box shaped around the die (with

far side and bottom missing). This barrier replaces the present

guard, and handles both mechanical and acoustical guard functions.

Materials suggested include ~-inch plywood, ~-inch Plexiglas, and

~-inch Lexan, made with airtight corner joints. Noise absorbent

material, Mylar faced for dirt and oil protection, was added inside

the box; it must be kept clean during normal operations.

158
Control Approaches Chosen

Based on suggested possible methods of nozzle construction, a

quiet nozzle cover was made. The design of this nozzle is shown

in Figure 5.14.10. AJr pressure, controlled by a reducing valve,

was reduced to the minimum to do the ejection job. (Low-noise air

jets are also available commercially.)

A sketch of the barrier is shown in Figure 5.14.11. To afford

visual access, the material chosen for the barrier was ~-inch Plexiglas. The three-sided barrier was locally designed, aiming to have

minimum leakage at bottom of barrier (toward the operator).

For absorption, I-inch acoustical (fully reticulated) polyurethane, with Mylar film covering for ease of cleaning, was glued to

the inside surface, leaving a mInimum uncovered portion for operator vi~wing of punch action.

Accurate costs were not available for this in-plant effort;

however, the materials were less than $100 and labor was estimated

at $250.

Noise Reduction Achieved

After experiments with reduced jet velocity and with the barriers described, t~e following noise levels were attained:

Large punch press reduced from 106 dBA to 85 dBA.

Small punch press reduced from 99.5 dBA to 82.5 dBA.

Pitfalls

The major pitfall for barriers will be to see that it is used.

Also when used, the bottom opening or noise leak toward the operator should be kept at a minimum. Another pitfall to continued

efficiency will be allowing the Mylar-covered noise absorbent to

become dirt and grease laden; periodic cleaning is needed.

A pitfall associated with air volume reduction is the tendency

of operators to increase pressure or remove the nozzle.

Attenuation will depend on the success of air velocity reduction in maintaining the needed thrust for ejection in conjunction

with noise reduction of barrier. Unless these experiments were

done with operator involvement, they may not accept the alterations.

If a mechanical method could be developed to replace air jet

part ejection, this would be the best alternative.

159
V Roussell

r---,

I I

I I

I

L... _ _ -.J

D

IV Bliss

D

III Bliss

II Diamond I Summit

V I

D

Benchmaster

center desk

VIII Summit

VII

D

Benchmaster

furnace

FIGURE 5.14.1 " Layout of punch press room

160
OCTAVE PASS BANDS IN HERTZ

80

Ld 70

>

W

....J

W

II

::>

60

w

II

0-

o

Z

::>

o

C/) 50

o

Z

-<

CO

W

>

40

U

o

,

,

,

I

I

I

A weighted

180 355 710

58 dBA

30

20

5

63 125

100

250

-1..._

+

500 IC"O

1

]O~O 4000

1000

FREQUENCY IN HERTZ

8(1C()

10000

FIGURE 5.14.2 - Ambient noise level with furnace and punch presses off (microphone 1.5 meter

above floor, directly above desk chair)

161
OCTAVE PASS BANDS IN HERTZ

90 180 355 710 1400 - 2BOO - 5600 - 11200

80

m

"

40

125 80GO

69 dBA

$

--I-

-t- f t-

--+ .

250 :JO 1['0 ?C')O '000

,

I -

I

I A weighted

I

I

63 20

30

...J

W

70

...J

W

a:

::>

(j)

(j)

w

a: 60

11.

o

Z

::>

o(j)

50

«

m

w

>

«

f-

()

o

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.14.3 - Ambient noise level with furnace on, punch presses off (microphone 1.5 meter

above floor. directly above desk chair)

162
OCTAVE PASS BANDS IN HERTZ

90 180 355 710 lAOQ - 2800 - 5600 - 11200

97 dBA

I

I

I

40

90

IT)

"'0

...J

UJ 80

>

UJ

...J

W

a:

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CI)

CI) 70 I UJ

a: I Q.

0 I z I ::J

0

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0

Z

«

IT) I'A weighted ::

w

> I

« 50 f-

<-)

0

30 63 125 250 500 20~J AOOQ 8000

5

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.14.4 - Ambient noise level with furnace on, two Summit, two Bliss, and one Benchmaster

presses in operation (microphone 1.5 meter above fI~or, directly above desk chair)

163
OCTAVE PASS BANDS IN HERTZ

lBO J55 710 1400 - 2BOO - 5600 - 11200

dB fast, I inear scale

110

CD dBA fast "0

dBA slow

-.J

w

>

w 100 -.J

W

a:

(j)

(j)

w.

a:

0.. 90

0

Z

0

(j)

0

Z

« 80

CD I

w

> I'A weighted' «

I0 I 0 70

60

50

I

I,

I

I 125

. I

-~-+

250 500

---.-

IC10 I

2CCO 4000 I

Booo

5 5

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.14.5 - Operator's exposure at punch I: stamping stock (air exhaust on, noise level

a t operator's ear)

164
· OCTAVE PASS BANDS IN HERTZ

180 355 710 1400 - 2BOO - 5600 - '1200

110

106 dBA

m"0

--I 100

w

>

W

--I

W ., a:

::J 90 -' (f) 90 dBA (f)

w punch I off a:

Q..

0 punch lion

Z

::J

0 60 (f)

0

z

«

m

w

>

« 70

f0

0

10000

5

1000

FREQUENCY IN HERTZ

I

I

I

I

I 63 125

100

60 t----+--.......---t---+----+---t----+----"1f-----+----t

FIGURE 5.14.6 - Operator's exposure at punch I: stamping without stock (air exhaust on, noise

level at operator's ear)

165
OCTAVE PASS BANDS IN HERTZ

90 180 355 710 10100 - 2800 - 5600 - 11200

100

--I

W 90

>

W

--I

W

a::

:J

(f)

[3 80

a::

0-

o

Z

:J

G

(f) 70

o

Z

<t:

OJ

W

>

60

u

o I

I

I

I

dBA level from punches I, III, and VII

dBA level with VII on only _-!-__--I~--_+--_ _+_

dBA level from I and III only

IA weighted .:.

50

40 63 , ~5

2

250 500

5

-tlCOO ~OCO 4000 8000

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.14.7 • Operator's exposure at punch VII

166
OCTAVE PASS BANDS IN HERTZ

11200 - 5600 - 2800 - OOןס1 710 355 180 90

dBA

,

I

I

I

I

I

I

I-----t-----i-l-'A-wei ghted -+----+---+----+----if------l

90

100

40 L--r-~__T'_:.,....T""T"'T'"""---12i.:.5--,.......;2:.i.50:.....,..-,.__~50~O-r-..,..,,....,:.:1O+-OO:....-_--...:2-+0C:.:..O_-.--...:.O~OO~__r.,;:.8+00..,O-r-----J

50

co

-0

-l

UJ

>

UJ

-l

UJ

a: 80

en

en

W

a:

0..

o

70

o

en

o

Z

<{

CO

UJ 60

>

<{

fU

o

5

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.14.8 - Operator's exposure at punch VII, operating without work piece

167
OCTAVE PASS BANDS IN HERTZ

180 J55 710 1400 '800 - 11'00

100 I---_t_--__t--~+_--_+---+_--_t_--__t---_;_--_;

90

dBA

/

/

I

...J

W

>

W

...J

W

a:

80 en

en

W

a:

Q.

o

Z

70 ~-~~~=::t:...--_=ji!~-+_-----.:+_----=:J:_----=~~~=---~ o

en

o

Z

<{

ro

w

> 60 I----+---HL-.----L----+----+---+-----+---+------i

<{

f-

()

o

I

50 1----+---.j~__+---+---_+---+----+-----4---+---~

I

, "--;-

40 L..-..,.....Lr-T'6J......"T'""'1......_ I

....' 5__....-_'..

50-r_.,....-...,501-0..........-.,....,.l-+r~_o '0+0_0_-r-_4.....0l-00""'"T""'"T--r8..... 000-rr-_......I

oS oS

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.14.9 - Operator's exposure at punch VII, operating without stock and without air exhaust

168
cap 118-inch diamater

r-"\

----; I

I .J-_

I I

I I

I

I

I

I

I r ___ .....J )

~-------f '--

'----f--- --_

/

1/2-inch

/1--

..... / I

,.--f---- I

I inlet

FIGURE 5.14.10 - Design of nozzle

out let

nch polyurethane foam and Mylar

Plexiglas

die

~'/4-;nCh

,... -,

0

V 1-i

I I

close fit

FIGURE 5.14.11 - Sketch of Plexiglas barrier

169
CASE HISTORY #15 - STRAIGHT AND CUT MACHINES

, . Description of the Problem

The straight and cut machine straightens heavy gauge wire in

an in-feed to cut-off unit set to cut repeat lengths, resulting in

noise levels of 92 dBA at operator position.

Measurement and Analysis

Figures 5.15.1 and 5.15.2 are close-in octave band analyses of

the diagnostic measurements made in front of the clutch mechanism.

In Figure 5.15.1, curve A shows peak cutting levels, and curve B is

the slow response of the same cutting levels (wide separation indicates impact noise). Curve C is idling, noncutting machine noise

level. The differences indicate dominance of the total spectrum by

the cutting noise. In Figure 5.15.2, curve D is the peak response

and curve E is the slow response. Curves D and E exceed curves A

and B, indicating some directionality of the cutting noise.

Figures 5.15.3 and 5.15.4 are octave band analyses made at the

operator position. Most of the operator time is represented by

Figure 5.15.3, with the cutting cycle noise level at 92 dBA (idling

cycle at only 83 dBA), indicating the dominant noise source of the

clutch cutter mechanism is the same form as in the close-in diagnostic measurements. A-weighted levels of these readings show that

the required attenuation is 9 dB at 1000 Hz, 7 dB at 2000, and 10

dB at 8000 Hz to reduce noise level at the operator position to

85 dBA.

Control Approaches Chosen

Having determined from management discussions that the machine

should not be changed or minor redesign attempted, we recommended

attenuation by a barrier wall that would block the sound path from

the cutting assembly to the operator.

Barrier materials for obtaining the required attenuation were

1/4-inch plywood, with 1/8- to 1/4-inch Plexiglas for viewing ports

where necessary. The barrier wall was extended 26 inches past the

extremities of the area encompassed by the cutter and was close to

the cutter, about 6 to 8 inches away. The barrier was hung in place,

supported by chain from overhead. In addition, an absorbent layer

was hooked to the barrier on both sides. To prevent clogging of

absorbent, the I-inch polyurethane foam absorbent was supplied with

Mylar facing. See Figure 5.15.5.

Normally, the noise absorbent for barriers is used only on the

machine noise source side. In this case, however, noise absorbent

170
was used on the operator side of the barrier as well to reduce

sound field build-up in the space between barriers. With the barrier close to the cutter, the operator would be within the safe

sound shadow area--the area beyond a line at least 30 degrees from

the edge of the acoustical shadow line.

As the barrier was built in-plant, no actual costs are available but material costs are estimated at about $100.

Noise Reduction Achieved

The cutting cycle noise levels at the operator location were

reduced from 92 dBA to 85 dBA, a 7-dBA reduction. Idle cyc,le noise

level was reduced from 83 dBA to 76 dBA.

Pitfalls

Barriers are easy to remove by the operator for many reasons,

real and imaginary, and use must be maintained by supervision. Location of an effective portable barrier must be standardized so that

the barrier is not by-passed. Barriers can be by-passed by noise

reflections from a low ceiling. If this problem had existed in this

case, a section of the ceiling above and about 4 feet on each side

of the barrier could have been treated with an absorbing material.

171
OCTAVE PASS BANDS IN HERTZ

130

120

90 180 J55 710 1400 - 2800 - 5600 - 11200

A peak cutting levels

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mechanism (1.2 meter above floor, 0.5 meter from cutter)

172
OCTAVE PASS BANDS IN HERlZ

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(1.2 meter above floor, 0.5 meter from cutter)

173
OCTAVE PASS BANDS IN HERTZ

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174
OCTAVE PASS BANDS IN HERTZ

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175
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176
CASE HISTORY #16 - CUT-PUNCH PRESS IN METAL

FABRICATING PLANT

Description of the Problem

This punch press had been modified to produce metal stampings

out to a predetermined size. This machine was the first stage of

a stamping operation in which the metal was sized and roughly shaped.

In two following stages, each part was finished.

Measurement and Analysis

Figure 5.16.1 gives the octave band analysis of the operator

exposure, which is 102 dBA while punching and 88 dBA during idling.

Figure 5.16.2 shows close-in octave band data for gear noise, illustrating the continuous nonpunching noise source in the gear mechanism.

Figure 5.16.3 shows close-in measurements of the dog and flywheel

noise. Figure 5.16.4 shows similar close-in measurements of noise

from piston-collar impact on the air cylinder.

A reduction of gear mechanism noise 7 dB at 500 HZ, 10 dB at

1000 HZ, and 9 dB at 2000 Hz would reduce the idling, nonpunching

level from 88 dBA to 81 dBA. A reduction of dog-flywheel and pistoncollar impact by 17 dB at 500 Hz, 21 dB at 1000 Hz, and 25 dB at

2000 Hz would reduce the punching level from 102 dBA to about 85 dBA.

Control Approaches Considered

As machine change was not practical, changes had to be made in

the noise transmission path to the operator on two of these noise

sources. The other source, piston-collar impact on the air cylinder,

was modified at the source by adding washers made from Unisorb Type D

pad between the piston stop and the collar to reduce metal-to-metal

impact noise.

Control Approach Chosen

The gear noise and dog-flywheel impact noises were attenuated

by constructing an extended barrier about these noise sources. To obtain the attenuation required, l-inch plywood was used. The enclosure was attached to the right side of the press (as the operator

looks at press) and extended upward to the top of the press, downward

to operator chest level, and outward several inches past the flywheel

guard. The top, bottom, and right-hand edges had a small 6-inch

extension at the barrier extending 90 degrees away from the operator,

as shown in Figure 5.16.5.

An absorbent was added to both sides of enclosure using Mylarcovered l-inch acoustical foam absorbent available from several

177
suppliers. The joint between the enclosure and the right-hand side

of the press was sealed to p:_event noise leakage; a 2-inch~wide

strip of closed cell foam weatherstripping was specified.

Normally, absorbing material is used only on the noise source

side of a barrier wall; however, if other noise sources might reflect from the barrier wall to the operator, absorbing materials on

the operator side will reduce this noise component.

Noise Reduction Achieved

Sound pressure levels during idling were reduced from 88 dBA

to 81 dBA. Punch operational sound pressure levels were reduced

from 102 dBA to 88 dBA, thus bringing the entire operation into

compliance.

Though not recorded, costs are estimated at less than $200 for

plywood, polyurethane foam, and the labor for attachment.

178
OCTAVE PASS BANDS IN HERTZ

110 180 355 710 10100 - 2800 - 5600 - 11200

punching dBA

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179
OCTAVE PASS BANDS IN HERTZ

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180
OCTAVE PASS BANDS IN HERTZ

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181
OCTAVE PASS BANDS IN HERTZ

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on air cylinder

182
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183
CASE HISTORY #17 - PARTS CONVEYING CHUTE

This case was taken from published data*, because of the importance of illustrating the method for other applications.

Description of the Problem

Chutes for conveying small parts can radiate much noise from

the impact of parts on the sheet metal of the chute. The noise

(for a given part) can be reduced by keeping to a minimum the distance the part must fall to the chute. For reducing the remaining

noise, the chute must be stiffened and damped.

Control Approach Chosen

Constrained layer damping is used, in which the treatment can

be placed on either the parts side or the underside of the chute.

If placed on the parts side, the metal layer should be wear-resistant to the impacting parts. In this example, 30-caliber cartridge

cases were carried in the chute shown in Figure 5.17.1. The bottom

of the chute was 14-gauge steel, which was lined with 0.035-inch

cardboard and then covered with a wear plate of 20-gauge galvanized

steel. Rubber deflector plates were positioned to funnel parts to

the center of the chute, so that they would not hit the untreated

sides of the chute.

Noise Reduction Achieved

Figure 5.17.2 shows the spectra measured 3 feet to one side of

the chute. The level has been reduced from 88 dBA to 78 dBA, a

decrease of 10 dB. Greater reduction could have been obtained if

multiple layers of thinner cardboard were used (in solid contact

with the cover sheet). Still better would be replacement of the

cardboard by commercially available damping materials specifically

formulated for constrained layer use.

Pitfalls

Much noise still comes out of the top of the conveyor. A cover

over it, lined with absorbent, should reduce the noise an additional

5 to 10 dB. Prior to any noise control effort, the relative amounts

of noise from top and bottom shGuld be determined. Ordinarily damping is always to be used.

* Cudworth, A. L., Field and Laboratory Example of

Industrial Noise Control. Noise ControlS (1): 39, 1959.

184
14 gauge steel chute

?O gauge galvanized steel

0.035 inch cardboard

FIGURE 5.17.1 - Chute for conveying cartrtdge cases

185
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FREQUENCY BAND - CYCLES PER SECOND

FIGURE 5.17.2 - Noise spectra measured 3 feet from chute

186
CASE HISTORY #18 - NAIL MAKING MACHINE*

Description of the Problem

A nail making machine was operating under conditions causing

severe impacts. The vibration was solidly transmitted to a weak

concrete floor, which radiated considerable noise. There were 10

machines, operating at 300 strokes per minute. Operator noise level

was 103.5 dBA.

Measurement and Analysis

An octave band analysis was made at the operator position and

is shown in Figure 5.18.1.

Control Approach Chosen

It was decided to use vibration isolating mounts to reduce

floor radiated noise. Because of the repeated shock situation, selection at the isolator followed these rules:

(1) The natural period of isolator plus machine should be

much greater than the shock pulse duration (10 msec) .

(2) The natural period of isolator plus machine should be

less than the time between pulses (200 msec).

Elastomer type isolators were used, having a static deflection

of 0.1 inch under machine load. This corresponds to a natural

period of 100 msec, thus fulfilling the design conditions.

Noise Reduction Achieved

Figure 5.18.2 shows octave band spectra at the operator's

position after all machines had been vibration isolated. The Alevels have been reduced about 8.5 dB to 95 dBA. This is still in

excess of permitted levels, and additional noise control is needed.

Pitfalls

To maintain the isolation, maintenance people should be warned

not to short-circuit the isolators by any solid connection from

machine to floor. This short-circuiting can also occur when dirt

and grease are allowed to build up around the pods.

* From Crocker, M. J., and J. F. Hamilton, Vibration Isolation for

Machine Noise Reduction. Sound and Vibration 5 (11): 30, 1971

187
Additional Noise Reduction

As a reduction to a noise level of 95 dBA is not considered

satisfactory for full day operator exposure, additional noise reduction could be obtained by the design of a barrier between the major

noise source in the machine and the operator. Depending on the

needs for vision through the barrier, plywood, lead-filled vinyl

curtain, or Plexiglas could be used. Such a barrier should yield a

reduction of 5 to 8 dBA at the operator position. (For calculated

design parameters, see Case History #6 and for rule of thumb parameters, see Case History #10.) This noise reduction should result

in compliance with 87 to 90 dBA.

Where there is a series of machines, additional reduction of

several decibels could be obtained by added room absorption, either

in the form of spray-on acoustic absorbent on ceilings and walls or

in the form of hanging absorbent baffles from the ceiling. (See

Chapter 3 on Room Acoustics.)

188
OCTAVE PASS BANDS IN HERTZ

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FIGURE 5.18.1 - Nail making machine: operator position noise levels

189
OCTAVE PASS BANDS IN HERlZ

oM 90 180 355 710 1400 - 2800 - 5600 - 11~ '----""'T""--""""'-- --,--....;...--r-----r---,-.....;.,,-...,----,-----,

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FIGURE 5,18.2 - Nail making machine: operator position noise levels after vibration isolators instal

190
CASE HISTORY #19 - WOOD PLANER*

Description of the Problem

Wood planers use a high-speed rotating cutter head to produce

lumber with a finished surface. Noise levels to the operator are

high.

There are apparently many noise sources for investigation:

(1) The board, excited by cutter knife impacts.

(2) The heavy structure under the cutter head, excited

by vibration transmitted through the board.

(3) Modulation of air flow by cutter knife chopping at

the chip collector air stream.

(4) Motor windage, hum.

(5) Dust collector blower, vibration noise.

(6) Machine surfaces excited by impacts.

Measurements and Analysis and Control

The octave band operator levels are shown in Figure 5.19.1.

Analysis resulted in the following possibilities for control of

planer noise:

(1) Restrain the board from vibrating. Feed belts on

both sides can be used with considerable backup

mass and pressure. This would require a radical

machine design change.

(2) Contact the board by means that add damping, to

reduce resonant vibration. If this is done as

an add-on, it must occur beyond the feed and

delivery ports of the planer. Thus it would be

helpful only for long lengths of board.

(3) Use a helical knife cutter head, which will also

reduce idling noise. A helix angle larger than is

commonly available would be desirable.

* Stewart, J. S., and F. D. Hart, Analysis and Control of Wood

Planer Noise. Sound and Vibration 6 (3): 24, 1972.

191
(4) Enclose the planer and board. This is a brute

force method that depends for its success on controlling the amount of sound that escapes from

the feed and delivery areas; most of the sound

contributing to A-level noise is between 500

and 5000 Hz. This increases the enclosure design

problem, because enclosures are more effective

at high frequencies. (See Case History #24.)

Noise Reduction Achieved

Results achieved by the helical knife cutter head are shown in

Figure 5.19.2: reduction from 106 dBA to 93 dBA. Figure 5.19.3

shows the operator noise level related to length of board planed,

comparing the helical knife cutter with the straight knife cutter.

The helical knife is by far the quieter.

Pitfalls

To meet OSHA operator levels for full day operation, the plant

would need to make further noise level reduction, perhaps by the

design of a total enclosure with an acoustic lined tunnel for the

in-feed and out-feed.'This should not be tried until it has indeed

been determined that the openings are the chief sources. In many

mills, however, the planer is not operated on a full-time basis,

thus allowing a higher noise level for the shorter time period that

an operator is present. At 93 dBA, 5.3 hours is permitted.

192
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193
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194
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FIGURE 5.19.3 - Effect of board length on noise from wood planer

195
CASE HISTORY #20 - PUNCH PRESS*

Description of the Problem

Punch presses constitute a most troublesome source of industrial noise, both because of their number and because of their high

noise output.

From various papers on the subject of punch presses, the following list of noise sources has been gathered. These may not all be

present on anyone press but are listed as a guide to specific press

noise source analysis.

(1) Shock excitation of the workpiece, machine guards, ,

floor and building.

(2) Gears, drive, bearings, and components, such as '

clutch and brake mechanism and drive shaft.

(3) Plunger resonance.

(4) Air ejection, air jet cleaning, and air cylinder

exhausts.

(5) Die design.

(6) Stripper plate design.

* American Industrial Hygiene Association, Industrial Noise Manual,

Detroit, 1966. Examples 11.C" 11.EE.

Allen, C. H., and R. C. Ison, A Practical Approach to Punch Press

Quieting, Noise Control Engg. 3 (1): 18, 1974.

Bruce, R. D., Noise Control of Metal Stamping Operations. Sound

and Vibration (11): 41, 1971.

Shinaishin, O. A" On Punch Press Diagnostics and Noise Control.

Proc. Inter-Noise 72: 243, 1972.

Shinaishin, O. A., Sources and Control of Noise in Punch Presses,

Proceedings Purdue University Conference on Reduction of Machine

Noise, May 1974, page 240.

Stewart, N. D., J. A. Daggerbart, and J. R. Bailey, Identification

and Reduction of Punch Press Noise. Proc. Inter-Noise 74: 225,

1974.
(7) Ejection of parts leaving press on chute or bin.

(8) Vibration of sheet metal being fed to the press.

(9) Start and stop of automatic feed to the press.

(10) Building acoustics.

Control Approaches Considered

Shock excitation of surrounding structures--This effect can be

minimized by properly designed vibration mounts for the entire press

to reduce excitation of floors, walls, and other equipment. As an

example of this see Case History #2.

Drives, etc.--Good maintenance can contribute to noise reduction. The noise of drive gears can be reduced by damping the gear

body, improving gear surface quality and tolerances, precision installation and bearings, better lubrication, and/or changing gear

material for a better damped material. On existing equipment many

of the above aids cannot be done at reasonable expense, but gear

drives are often enclosed in a box-like structure whose surfaces

radiate noise. These surfaces can be damped with off-the-shelf materials, or the drive unit, if space is available, can be enclosed,

fully or partially. Heat dissipation must be considered. Solid

metal or plastic guards can be changed to expanded metal or wire

mesh for less noise, or the guard surface can be vibration damped.

The entire guard, if solid, must be vibration isolated from the

vibrating machine.

Plunger resonance--If a hollow plunger or ram is a Helmholtz

resonant type of noise source, its noise radiation can often be

reduced by covering the hole in the plunger. See Case History #4.

Air ejection of punched parts--If possible, substitute mechanical ejection to eliminate a large noise source. One comparison,

shown in Figure 5.20.1, (AIHA 1966) resulted in an 8-dBA noise reduction. Multiple jet nozzles are also available for reduced noise.

Reduce to a minimum the air velocity used for ejection (since noise

level is related to velocity) by reducing the air pressure available. Achieve better efficiency of air jet by accurate setting and

aiming where needed.

Shield the area of punch-air ejection from the operator. An

example of the result of this method, in Figure 5.20.2, shows the

noise levels of a press with and without a 24 x 48 inch shield to

protect operator ~rom air ejection noise.

197
Die design--Changes in die design can reduce noise by attempting to design for spreading the punching action, slanting the

blanking punch or die, or other means of promoting consecutive

shear action instead of instant action. Shinaishin reported the

results of a slanted die as shown in Figure 5.20.3. Changes in die

materials can reduce noise. As presses produce sound energy from

vibration of metal plates upon impact, the velocity of impact can

be reduced by using hard rubber mounts (snubbers). Another possibility is laminated and more massive plate, reducing the size of

plate and radiating area.

A change of work stock material from steel to a lead-steel

composition has also reduced impact noise; Shinaishin reported a

14-dB reduction with this test method. Noise radiation can be

lessened by reducing plate area by cutting out any surface areas

that perform no function.

These comment~ emphasize that the tool engineer must now consider designing for noise reduction as well as for mechanical performance. Within such general framework outlined, any improvements

in noise level will come by experiment and testing results.

Stripper plates--Stripper plates in some dies contribute to

noise levels due to metal-to-metal contact, which could be changed

to plastic or elastomeric contact with better damping and reduced

noise.

Ejection of parts to chute or bin--Noise levels can be reduced

by damping metal chutes, using damping materials on the market or

making a constrained layer design. See Case History #17.

Vibration of sheet metal being fed to press--Noise levels can

be reduced by preventing vibration, such as by adding a hold-down

conveyor. The noise can also be constrained by using an acoustic

tunnel in-feed, or the operator can be shielded by properly designed

barriers.

Start and stop feed mechanisms--Noise can be reduced by redesign: substitute plastic contact areas where possible; enclose the

noise source partially; or add barriers between noise source and

operator.

Building acoustics--In a room with many noise sources, the operator may be in the reverberant field. Such noise can be reduced by

adding absorption. From Bruce, an example of use of absorption to

reduce noise in a press room is shown in Figure 5.20.4, 30 feet

from presses. Closer to presses, noise reduction would be less

with probably no more than 2 to 3 dB at the operator position.

The press area can also be enclosed or walled off from the rest of

the plant.

198
Use of Enclosures

Allen and Ison (1974), P.18, reported a partial enclosure of ram,

die, in-feed, and ejection on a 50-ton test press. A reduction

of 13 dBA was obtained for an enclosure; see Figure 5.20.5. The

model enclosure was made of cardboard, lb/sq ft, lined with

1 in. of polyurethane foam. Later a steel enclosure was installed,

for durability.

Total enclosures with opening via an acoustic tunnel may be

required and should follow recommended enclosure design as reviewed in Chapter 3.

Pitfalls

The remaining radiation came chiefly from the flywheel cover,

which was neither damped nor vibration isolated. Diagnostic measurements should indicate the relative contributions from each

source, so that the residual noise will be known.

199
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63 175 750 500

-l--

lena 7CDO .000

5

8000

100 1000

FREQUENCY IN HERTZ

10000

Source: AIHA, 1966.

FiGURE 5.20.1 - Comparison of punch press noise levels with air ejection and with mechanical ejection

200
OCTAVE PASS BANDS IN HERTZ

180 J55 710 l.«lO - 2800 - 5600 - "200

101 dBA

100 1----+-----1---+----+---+----+-----11----+----1

with shield

90 I---=+=----.:+---+--~--II!!!!!~~---~--+--89 dBA

...J

W

>

W

...J

W

§ 80 1----+----1--I-J,....-+----+---+---7'-t--...;;;;;:~~.........__----i (J)

(J)

w

a:

0-

o

Z

:J 701---+----+-----j~--+----+----+--'---+---+-----j o(J)

o

z

«

(D

I

W

>

«

IU

o

5

6J 125

2

250 I 500 le00 2000

I

4000 8000

100 1000

FREQUENCY IN HERTZ

10000

Source: AIHA, 1966.

FIGURE 5.20.2 - Comparison of punch press noise levels with and without a shield between operator

and air ejection noise

201
OCTAVE PASS BANDS IN HERTZ

180 355 710 uoo 2SOO - 5600 - 11200

8000

108 dBA

I

t

1 i -t

+-

63 115 150 ~oo Ier,O 2000 AOOO

III

"'C standard die

...J 100 1---_+_......._~-+_~rt_:l~----::IIllrl_-~~~r2"~_+_--~-- 100 dBA

w

>

W

...J

W

§ 90~---=t:l---\:~-I-~~-~=------i=----~~~~~--+:--_--.j en

en

W

cr

a..

o

z

:::l 80 1----II'----H~-_+---_t_--_+--~+_~.,......_+-t-~c.._1---_1 o

en

o

z

«

III

W

> 701----4----t---+----+---+----+-----+----+----i

«

()

o

100 1000

FREQUENCY IN HERTZ

10000

Source: Shinai shin, 1972.

FIGURE 5.20.3 - Comparison of punch press noise levels: standard die versus slanted die

202
OCTAVE PASS BANDS IN HERTZ

90 180 355 710 10100 - 2800 - S600 - 11200

(])

"C

100 1---4----+-------4---+----+----+---1---+----1

before sound absorption 94 dBA

after sound absorption

1

-l

W

>

W

-l

W

a:

90 1--i:---=~~.-.~~<,......1~=-=-±~;;:_=t--1t-M~3A-1 en

W

a:

0-

o

Z

:J 801-----+-----1--...,....-+----+---+-~lk__+_--___1f--~=__-+---__l o

en

o

z

«

(])

I

W

> 701-----+------4---+----+---+-----t-----If----t----t

«

lt)

o

63 125 250 500 lena 2000 ~ooo 8000

5

100 1000

FREQUENCY IN HERTZ

10000

Source: Bruce, 1971.

FIGURE 5.20.4 - Noise levels 30 feet from bench press area before and after sound absorption treatment

203
OCTAVE PASS BANDS IN HERTZ

90 180 710 1~ - 2100 - 5600 - 11200

110 1----t----+---t---t----+----+---;t---+----1

83 dBA

96 dBA

- r~~__ . normal operation

CD

"0

, 100 1-----+-----t---+----+---+----+---___1---+-----1

.....J

ill

>

ill

.....J

ill

§ 901-----+---+----+---+----+---+----:::....~;;;.....--+------1 C/)

C/)

ill

a:

0-

o

Z

::l 801----+-----+---+--#---+---t------+-----+---+-----1

o

C/)

o

Z

<t:

CD

>

W 701----+--~.-:::-...:..::=!'+o.:::-----+-----l-----+---t---+------I

<t: with test enclosure

le..:>

o

s

63 12S 250 SOO ICOO i

2000 _000 8000

S 2 S 1 -

60 I---+----+----l---+----+----+---l---+---~

100 1000

FREQUENCY IN HERTZ

10000

Source: Allen and Ison, 1974.

FIGURE 5.20.5 - Noise 30 inches from punch press before and after test cardboard enclosure

204
CASE HISTORY #21 - MATERIALS HANDLING, AIR MOTORS

Description of the Problem

Air operated motor hoists are a noise source in many industries that make extensive use of materials-handling systems.

Control Approach

As the noise source is the exhaust air, this exhaust can be

muffled by using off-the-shelf mufflers selected for the air

pressure and delivery of the exhaust.

Noise Reduction Achieved

A typical octave band analysis, before and after installation

of an exhaust muffler, is shown in Figure 5.21.1. Note the rising

spectrum that is characteristic of freely-escaping high pressure

gas. Another case showed the following A-weighted sound levels at

the floor for a I-ton air hoist:

up - no load

Up - 600 lb

Down - no load

Down - 600 lb

No Muffler

98

96

102

100

With Muffler

85

84

88

86

Air exhaust from other tools can be similarly muffled. Newer

designs include mufflers, which should be specified at purchase.

205
OCTAVE PASS BANDS IN HERTZ

90 180 3S5 710 lAOO - 7800 - 5600 - 11200

115 dBA

11 0 t--..:....-+---+----+---+----+---I----::~--+_--_l

CD

"0

-l 100

w after muff ler

>

w

-l

w

a: 90 (/)

(/)

w

a:

a..

0

z

80 0

(/) 81 dBA

0

Z

c:{

W

rnI

before muffler

"

> 70

c:{

lt)

0

60

63 125 250 500 1000 2000 8000

S 5 S

100 1000

FREQUENCY IN HERTZ

10000

Source: AFS-Foundry Environmental Control of Noise, 1972.

FIGURE 5.21.1 - Effect of muffler on air exhaust from hoist

206
CASE HISTORY #22 - TEXTILE BRAIDING MACHINES*

Description of the Problem

In braiding operations, a bobbin of thread is rotated on a

carrier base in a special slotted cam. This cam revolves as it

is rotated around the machine, with several other carriers and

cams. The carriers are thrown from one cam to another. With steel

carriers, the major source of the intense noise present is the resulting metal-to-metal impact. The manufacturer was willing to

consider machine modifications to reduce noise in the case history

reported here.

Measurement and Analysis and Control

In a laboratory study, the metal-to-metal contact was easily

identified as the chief noise source. It was recognized that a

carrier with inherent damping properties should reduce the noise.

Replacement of the carrier by a nonmetallic one was thus considered.

Of the several materials tried, the best combinations of strength,

light weight, and damping was an injection moldable polyurethane.

Noise Reduction Achieved

The carriers were installed in a l3-carrier braider operating

at a handle speed of 340 rpm. with the microphone 10 inches above

the top plate of the braider and 18 inches out, the levels shown

in Figure 5.22.1 were obtained. A reduction of 11 dB was obtained.

The above results were obtained in the laboratory. For an

in-plant test, a row of 84 braiders was converted to plastic carriers. The adjacent row was left with steel carriers; other ~ows

of braiders were operating. The microphone was 3 feet from the

centerline between the test rows, and 3 feet above the floor. The

noise levels for various combinations of machines is shown below

(an x indicates on).

Level, dBA 97

Steel test row x

Plastic test row x

All other x

97

x

x

90

x

x

85

x

Residual noise from the motor cooling system remained and lowered

the noise reduction to the 7 dB achieved in this production test.

* From Cudworth, A. L., and J. E. Stahl, Noise Control in the Textile

Industry. Proc. Inter-Noise 72: 177, 1972.

207
Pitfalls

Since this study, it has been round that the plastic carriers

are not strong enough for some operations requiring heavy yarn (or

wire). This suggests consideration of a composite carrier with a

steel core for strength and a cladding of heavy polyurethane for

damping. To our knowledge, this concept has not yet been tried.

This emphasizps the need for considering non-acoustical parameters

along with the acoustical.

208
OCTAVE PASS BANDS IN HERTZ

180 J55 710 l~OO - 2800 - !600 11200

94 dBA

CD

"'0

...J 80

w

...J

>

W - W

a:

:J 70 C/)

C/)

ill

a:

a..

0 polyurethane Z

=> 60 0

C/) til'

0

z

« ",'"

CDI

W

> 50 «

I-

()

0

63 125 250

-j500 le,oo I

7000 4000 aoao

5

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.22.1 - Textile braiding machine: comparison of noise levels from steel carriers and

from polyurethane carriers

209
CASE HISTORY #23 - METAL CUT-OFF SAW*

Description of the Problem

A common problem in industry is that of protecting workers from

noise produced by machines that the worker must guide or manipulate

directly. An example is a cut-off saw used on metal shapes. Noise

comes from two main vibrating sources: the saw blade itself and the

workpiece, which is largely unconstrained. The saw itself is actuated downward and into the work by a lever attached to the hinged

and counter-balanced (or spring-loaded) saw and motor.

The worker must visually monitor the cutting operation. Also,

and perhaps more importantly, the vibration and opposing force

,transmitted to him through the lever arm also furnish useful cues

on the progress Of the cutting operation. The problem is to reduce

the noise he receives, without undue interference with work flow,

with visibility, and with the use of the lever arm.

Control Approach Chosen

The solution was an enclosure covering the whole saw. Workpieces pass transversely through slots in the enclosure. Flaps of

lead-loaded vinyl close off the opening and reduce to a small

amount the unavoidable leakage area when a workpiece is present.

The front, above saw bed height, is closed by two doors whose'surface is mostly ~-inch clear plastic (polymethylmethacrylate). This

plastic provides very good vision. The doors close with a gap the

width of the control lever. Each door has a flap of lead-loaded

vinyl about 3 inches wide to close the gap. The lever pushes aside

t.he flaps only where it protrudes. Thus the leakage toward the worker is greatly reduced.

Noise Reduction Achieved

Figure 5.23.1 shows the spectra at the worker position before

and after the enclosure was installed. The decrease in A-level is

13 dB. The standard panels used in the enclosure are very much

better than indicated by the reduction measured, illustrating again

the importance of leaks in determining the performance of enclosures.

Pitfalls

Several features of the design could be improved. The ears of

the workers are very close to the leak at the door flaps. It should

be relatively simple to offset the saw feed lever to the right (for

* Handley, J. M., Noise--The Third Pollution. lAC Bulletin

6.0011.0, 1973.

210
the right-handed worker). This has several advantages: (l)it

places his right hand in a more comfortable position, (2) with

the door gap and flaps moved to the right, his vision is greatly

improved, and (3) the noise leak is moved further from his ears.

A nonacoustical improvement would be to have the doors slide open,

rather than open out, which can be a safety hazard.

211
OCTAVE PASS BANDS IN HERTZ

90 leo J55 710 1.we - 2800 - 5600 - 11200

97 dBA

90 r--~--:=~=~-~~=-+:-----::+-~;;;;;;;;;;;:;;;+:--I before enclosure

84 dBA

70 1-----4----+----+----+----+---+----1----+-----1

6J 125 250 500

--+-

1000 2000 4000 . eooo

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.23.1 - Metal cut-off saw: operator's exposure before and after enclosure of saw

212
CASE HISTORY #24 -PLANER-WOOD*

Description of the Problem

Wood planers in the forest products ind~stry produce noise

levels of 102-108 dBA at the operator (feeder) work stations. Noise

levels are 103 dBA at the grading station and trimmer and 95 dBA

elsewhere in the planing mill.

Control Approach Chosen

In the area cited in the article, enclosures were installed on

30 large planers. Out of the general program, the following specific guidelines for viable enclosures were developed by experience.

(1) Walls and roof should be from 8 to 10 feet high, using

staggered studs; thus keeping inside and outside wall independent

with separate sills and headers. Isolate wall structure from floor

with felt or mastic. Fill space between walls with rock wool or

fiberglass. Sheathing used was l~-inch tongue and groove or equivalent plywood. Additional acoustical board was used on upper twothirds of walls and ceilings for noise absorption. Removable wall

or roof sections should be installed as needed for major machine

repairs.

(2) Floors are usually adequate as constructed for a normal

planer installation, but if the planer is elevated on piers, the

enclosure walls should be extended to the main floor or acoustical

floor similar to the walls constructed between piers.

(3) Doors should be refrigeration type, with beveled or

stepped edges. They should open out, so that suction from blowers

keeps them closed. Doors or jams should be sealed with weatherstripping. Use heavy duty hinges. Alternatively, standard acoustical doors may be purchased.

(4) Windows should be as small as practical, using doubleglazed shatterproof or screened glass with an air space between.

(5 )

A tunnel

conveyor

the belt

Infeed and outfeed openings should be as small as possible.

type opening provides room for multiple baffles of old

belting or lead-filled vinyl to block the noise path. Slit

at intervals to accommodate various board widths, keeping

* From Pease, D. A., Forest Industries, March 1972.

213
the unused portion of the tunnel width blocked. The outfeed tunnel

should be at least as 16ng ai the longest boards fed through the

~laner so that noise caused by the vibrating board is confined

inside. Install funnel-shaped metal facing inside to guide the

stock into the tunnel opening.

(6) Opening for ducts and pipes should be just enough oversize to permit packing the annular space with insulation.

(7) Make-up air openings, to compensate for air exhausted by

the blower system of the planer, must be constructed as a silencer

to control noise leakage. Construct a chimney several feet high,

with baffles arranged inside so that incoming air must follow a

zig-zag path; baffles should be lined with acoustic material.

Another method is a smooth-wall chimney with a "weather cap" baffle

lined with acoustic material at the top.

Noise Reduction Achieved

The article states that noise levels were reduced to less than

90 dBA, to comply with OSHA regulations.

Pitfalls

At these high levels leaks are critical, and should be kept

to the minimum.

The absorbent must be covered by a plastic film to avoid fouling by the dust. In addition, because lumber is not always fed in

straight, the absorbent must be protected by a heavy galvanized

screen.

The feed tunnels should be long enough to hold the whole board,

or else there must be positive hold-down to prevent board vibration.

214
CASE HISTORY #25 - PUNCH PRESS TOTAL ENCLOSURE

Description of the Problem

Two Minster model P2~2000, 200-ton straightside presses were

running over 250 strokes per minute in stamping out laminations

for a particular motor model. The press is located in a metalconstruction building. Dies are changed often.

Noise level at the operator station was 104 dBA~ and the general plant level was 92 dBA.

Control Approach Chosen

Panels forming a total enclosure were constructed using:

1 layer absorbent polyurethane acoustical foam

1 layer 1/64-inch sheet lead

1 layer 3-inch fiberglass Trw blanket

1 layer fiberglass cloth to withstand industrial solvents.

The enclosure used was circular, 1?6-inch in diameter,- 16-feet

high, with top of domed construction. Access doors allow for maintenance, and there is a stock feed opening. Finished parts leave

the enclosure by means of two under-floor part guides. Supply lines

were rerouted under floor, using flexible conduits. A 3500 cfm

heat exhaust system with a silencer was added to each dome.

The operator is outside the enclosure except to change dies,

change feed reels, or make adjustments.

Noise Reduction Achieved

Total enclosures reduced noise level for operator to 83 dBA

and general plant level (with other equipment) to 87 dBA.

215
CASE HISTORY #26 - DEWATERING VACUUM PUMP, PAPER MILL*

Description of the Problem

In paper manufacturing, a fine-grained mineral slurry is

dewatered by a suction press and a vacuum pump. The mixture

of air and water discharges into a drain; slugs of water make

the discharge pulsate irregularly. Although no workers stay

long in the discharge area, the levels are high even at considerable distances from the discharge pipe. In one instance, the

pipe was inadvertently tuned to the pump pulse frequency, thus

compounding the problem.

Control Approach Chosen

The solution is a special combination muffler and water/air

separator called a snubber. Besides use of the snubber, the

line length should be changed to de tune the system.

Figure 5.26.1 shows the effect of adding a snubber and the

greater effect of two snubbers. The low frequency resonances

are greatly reduced by this direct scheme. A-levels close to

the discharge pipe were changed from 112 dBA without snubbers to

103 dBA with one, and 93 dBA with two snubbers. Some of the low

frequency reduction is probably due to the detuning effect of

the change in the acoustic length that the snubbers introduce.

* Young, R. L., Practical Examples of Industrial Noise Control.

Noise Control 4 (2): 11, 1958

216
120 ,.....--""'T""----r---r---...,..---"""T'"----,.---r---'"T"""---,

...J

UJ 110

>

UJ

...J

UJ

a:

::>

en

en

UJ

a:

a..

0 90 z

::>

0

en

0

Z 80

-<III

UJ

>

-<I- 70

()

0

60 L-__.....L. ......__...J...__.....L. .L....__-I-__......I -'-__......

20-

75

75-

150

150- 300- 600-

300 600 1,200

FREQUENCY BAND IN Hz

1,200-

2,400

2,400-

4,800

4,800-

10,000

FIGURE 5.26.1 - Octave-band analysis of paper mill vacuum pump noise

217
CASE HISTORY #27 - STEAM LINE REGULATORS*

Description of the Problem

Steam lines with regulators are used in many industries and

can be a problem noise source if they are in an area occupied by

employees.

Control Approach Chosen

The method used here, which can be used to regulate other gas

flows, was to modify the design of the main valve plug. The redesigned valve plug has throttling vanes, as shown in Figure 5.27.1,

to reduce the turbulence of the steam flowing through the space

between the regulator's main valve and its valve seat, which is

the noise source.

Noise Reduction Achieved

For a 2~-inch steam line handling 50,000 lb/hr through a

reduction of 555 to 100 psia, the redesigned valve reduced pipe

line noise from 97 dBA to 85 dBA.

* From Electrical World, January 1973.

218
FIGURE 5.27.1 - Main valve plug with throttling vanes to reduce nois':l in steam fine regulator

219
CASE HISTORY #28 - PLASTICS SCRAP GRINDER*

Description of the Problem

In the molding room, primary noise sources are from scrap

grinders and plastic granulators. The noise has increased during

the past few years because of the growth in number of grinders

and increasing toughness of the newer plastics.

Measurement and Analysis

Noise level maxima of 125 dBA in the initial grinding phase

have been. recorded, and 100 dBA are common. Figure 5.28.1 shows

the octave band analysis of an untreated grinder, giving the range

of readings for one load of 4 pounds of polycarbonate.

Control Approach

Although the optimum mechanical conditions of the plastics

scrap grinder, such as sharp blades, proper screen size, bladeto-screen clearance, and proper feeding procedures, help reduce

grinder noise on existing equipment, this alone could not bring

the unit within acceptable noise limits. Much of the noise came

from resonant excitation of metal panels.

A damping material was applied to all surfaces: hopper,

interiors of pedestals, stands, and covers. In general, a ~-inch

coating has been satisfactory for most grinders from bench models

to 18 x 3D-inch throat grinders.

Noise Reduction Achieved

The results of the soundproofing, shown in Figure 5.28.2

(for one load of 4 pounds of polycarbonate), bring noise levels

down to OSHA criteria, reducing the maximum level from 100 dBA

to a range of 88 dBA to 90 dBA.

* Morse, A. R., Plastic Technology, July 1968.

220
OCTAVE PASS BANDS IN HERlZ

90 180 355 710 1400 - 2800 - 5600 - 11200

11 0 I----+-----+---+----+----I----+---~I----+----I

CD

"0

100 100 dBA, max

...J

W

>

W

...J

W

a:

::> 90 (/)

(/)

w

a:

0-

0

Z

::> 80 0

" (/)

0

Z

<{

CDI

W

> 70

<{

I0

0

5

63 125 250 500

---t1000 2000 4000

5

8000

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5.28.1 - Plastics grinder,- range of noise levels, dBA

221
OCTAVE PASS BANDS IN HERlZ

ISO 355 710 1400 - 2800 - 5600 - 11200

110 I---+_---+----i---+----+---l----~--+_--_;

90 dBA

BB dBA

-

CD

"'C

I 100 t----t----f---+----+---t----t-----f---+----f

..J

W

>

W

...J

W

II

:J 90t----t----f---+----+---t----t-----1-- rn

rn

W

II

a..

o

z

is Bo~-4+--~---- ..--~---~----:3t---~--._011!1......--_4 rn

o

z

«

CDI

W

>

«

t-

()

o

63 12~ 250 ~oo 1000 2000 8000

2

100 1000

FREQUENCY IN HERTZ

10000

FIGURE 5. 2B.2 - Plastics grinder; range of noise levels after soundproofing, dBA

222
CASE HISTORY #29 - NEWSPAPER PRINTING PRESS*

Description of the Problem

This pressroom is equipped with five double 3 to 2 Hoe

folders and four double 2 to 1 Hoe folders with a complement

of 45 colormatic press units.

Control Approach Chosen

The following methods were used for noise reduction:

Enclosures for folders reduced noise from III to 101 dBA.

In the reel room, all openings in the floor or deck plates

between the press room level and the reel room were sealed and

isolated. The opening in the arch of the press was closed to the

smallest dimension that would still allow paper to feed through

to the unit. On the basis of dosimeter data, the noise exposure

was reduced to acceptable levels.

On the press room floor, an existing folder enclosure was

retained and improved. A control booth was constructed for noise

isolation. An S-foot wall was added on the press room floor as

noise barrier, plus a 4-foot panel at the top of the wall, angled

upward and toward press. Wall surfaces were lined with 2-inch

absorbent polyurethane. The S-foot wall was constructed of:

26 gauge metal, l/S-inch masonite, 3/4-inch air space, 3/S-inch

plywood, plus 26 gauge metal. The panel was 2-inch polyurethane,

1/2-inch plywood, and 26 gauge metal.

Sound traps were made at the tops of ladders at catwalk

level. No isolation of the stairs, from reel room to press room,

was necessary, ~s they are outside the press enclosures and not

affected by the high noise levels of the press. Wall panels are

easily removed for maintenance.

Pressmen going inside enclosure for adjustments on a shorttime basis wear ear protection.

Materials used for sound absorption were flame resistant

and approved by insurance inspection.

Noise Reduction Achieved

Noise levels were reduced to comply with OSHA standards.

* From Editor & Publisher, November 10, 1973.

223
CASE HISTORY #30 - CHEMICAL PROCESS PLANTS*

Description of the Problem

Existing chemical process plant noise reduction requires

source analysis to determine the method of noise reduction.

Methods

As a result of this study, a list of noise sources is shown

in Table 5.30.1, with recommended methods of noise reduction.

Some specific examples and the results obtained by each noise control method are cited in Figures 5.30.1 through 5.30.4. The attenuation attained is shown in each figure.

* From Judd, S. H., Noise Abatement in Process Plants, Chemical

Engineering, January 11, 1971.

.224
II I I

sound pressure level measured 3 feet

above fan, 3 feet out from rim

, "

" V 12,000 fee't per minute tip speed

- '4

- ......~<' ........... .......... '- I~ ....

...............-

.. -.... -.... 10, 000 feet per minute tip speed - ....

.. ~-

.....-

105

100

en

"0

--l 95

w

>

w 90 --l

W

a:

85 en

en

W

a: 80 11..

0

Z

75

0

en

70

65

20 -

75

75 -

150

150 -

300

300 -

600

600 -

1,200

1,200 -

2,400

2,400 -

4,800

4,800 -

10,000

OCTAVE BAND, Hz

FIGURE 5.30.1 - Noise reduction achieved by reducing fan speed, using increased blade pitch to offset

decrease in speed (measured 3 feet above fan, 3 feet out from rim)

225
I I I I I r

sound pressure level measured

1-foot from expansion joint -

J,

before I

-,

" 'U~~~ I

" ".... .- - , .......-'- after -

,,

OCTAVE BAND, Hz

120

'¥r~ fi rst stage expans ion .. [!) 115 :~1; joi nt on a two-stage "0 "::::

>

~~~-: centrifugal 9,600-rpm, 110

:;.~j~ W

.~\~ 6,000-scfm at 100 psig >

W 105

compressor

W

a: 100

:::>

en

en 95

w

a:

0.. 90

0

z 85 :::>

26 gauge galvanized steel

0

en 80

1/16-i nch lead

3-inch, ti-pound glass fiber 75

20 - 75-

75 150

300 -

600

1,200 -

2,400

4,800 -

10,000

FIGURE 5.30.2 - Compressor discharge noise reduction achieved by lagging expansion joint (measured

1 foot tram expansion joint)

226
IlPump III 'pump 1/2

inlet psi 35 580

outlet psi 580 840

8 volute impellers 4 2

rpm 3,600 3,600

gpm 2,100 2,100

new 10-inch loop

1,200 - 4,800 -

2,400 10,000

300 - 600

OCTAVE BAND, Hz

20 - 75-

75 150

, ~. I /. II 1. T

soun pressure level measured 10-lnches

from #2 pump discharge line -

j~

: '.. _origina: 8-inch ... "jump over" -

I -1Il~-"

". ~II" , ... -IIt.~ 'I :-"""'III

~. new lO-inch loop·

I I "'

70

"

115

110

CD

"0105

...J

w

> 100

ill

...J

w 95

c:;::

~90 (J)

w

er 85

0..

o 80 Z

=>

75

8-inch by

6-i nch taper

original 8-inch

"jump over"

pump #2

I

pump #1

diSCharge1

line

out

t

u

r

b

i

n

e

• in

FIGURE 5.30.3 - Noise reduction achieved by redesigning pump by-pass loop (measured 10 inches

from tl2 pump discharge line)

227
section through intake si lencer

on a 17-psig centrifugal blower

11 Olr--T""'"--,r--.....,---r-""'T"-~-..,...-'t""""'_

2,400 -

4,800

65,....--......._ ........._ ......._ ......._""'--_............-""

20 - 75 - 300 - 1,200 -

75 150 600 2,400

OCTAVE BAND, Hz

sound pressure level measured I

lD 105 ....-+--+--- 6 feet from air intake slot,

"0

-.J- 100 t-~a__-~-+_-~-+_-+_-,.......--,.......-___'II

ill

>

ill

-.J

ill

a: 851~-~r_+-_+...."",.1IIIF_-_+_-4.J'"'"""~~-~ en

ill

a: 80 t---+--+--+-----'+=..,.;;po..,...+--~r___f',...___4 (L

75

:::>

,~ 701--+--+---4----1

drain

!

115,000

scfm to

blower

FIGURE 5.30.4 - Noise reduction achieved by adding silencer to air blower intake (measured 6 feet

from air intake slot)

228
TABLE 5.30.1· Sources 01 noise and methods 01 noise reduction

Equipment Source of noi se Method of noi se reduction

Heaters Combustion at burners Acoustic plenum' (10 Bwg. plate)

Seal s around control rods and over

sight holes

Inspiration of premix air at burners Inspirating intake silencer

Draft fans Intake sil encer or acoustic p!enum

Ducts Lagging

Motors TEFC cooling air fan Intake si lencer

Undirectional fan

WP II cooiing system Absorbent duct liners

Mechanical and el ectrical Enclosure

Airfin coolers Fan Decrease rpm (increasing pitch)

Tip and hub seals

Increase number 01 blades"

Decrease static pressure drop"

Add more fin tubes"

Speed changer Belts in place of gears

Motors Quiet motor

Slower motor

Fan shroud Streamline airflow

Stiflenlng and damping (reducing vibration)

Compressors Discharge piping and expansion joint Inline silencer and/or lagging

Anti surge bypass Use quiet valves and enlarge and streamline piping"

Lag valves and piping

Inline silencers

Intake piping and suction drum Lagging

Air intake Silencer

Di scharge to air Si lencer

Timing gears (aXial) Enclosure (or constrained damping on case)

Silencers on discharge and lagging

Speed changers Enclosure (Or constrained damping on case)

Engines Exhaust Si lencer (mUffler)

Air intake Si lencer

Cooling Ian Enclose intake or di scharge Or both

Use quieter tan

Mi scellaneous Turbine steam di scharqe Si lencer

Air and steam vents Si lencer

Use quiet valve

Eductors Lagging

Piping Limit velocities

Avoid abrupt changes in size and direction

Lagging

Valves Limit pressure drops and velocities

Llmi t mass 1I0w

Use constant velocity or other quiet valve

Divide pressure drop

Size adequately for total flow

Si ze for control range

Pumps Enclosure

'If oil fired, provide for drainage of oil leaks and inspection .

•• Usually limited to replacement or new facilities.

229

Chapter 6

SELECTING AND USING A CONSULTANT

Knowing When a Consultant is Needed

Having read the previous chapters, you will already have a

good feeling for the situations that you can deal with on your own.

If you are still unsure of the solution or if preliminary measures

have proved unsatisfactory, it may be time to consider the use of a

consultant.

Situations where a consultant is needed arise when the machine

to be quieted is complex~ as when there are present many noise

sources of approximately equal strength. Locating the sources and

obtaining their relative noise strengths will need perhaps more sophisticated equipment and procedures than you may have. If you

find that the A-level at all points at a constant distance from themachine (but within the critical distance) covers a range of 5 dB

or less, this is likely to be the case.

You may also need a consultant if unusual situations exist. A

common one with belt-driven blowers, for example, is a slow but

considerable variation in level. Another is impact noise, as from

a punch press, where several events take place in rapid succession.

A narrow-band analysis of a tape recording is usually called for.

Inadvertent tuning of some part of the machine may lead to pure

tone ringing that is difficult to locate. For such situations,

using a consultant is often the most rapid way of getting results.

If you have installed noise control means that don't work, you

may (albeit reluctantly) have to use a consultant to retrieve the

situation. Although this may be a painful decision, it will usually occur but once. You should document the situation thoroughly

and use the consultant to supply information on what went wrong.

Sometimes you may be approaching a lawsuit situation, where

data must be obtained and presented (as an expert witness) by a

disinterested third party. Many consultants can provide this complete service.

Now that you have decided to obtain a consultant, how do you

proceed? You should first be warned that there is no legal bar to

anyone offering services as an acoustical consultant. Consequently,

it is up to you to avoid those who are unsuitable because of lack

of training or experience, as well as simple venality or greed.

231

iPreceding page blank
Selection of a Consultant

People billing themselves as consultants can be broadly classified according to whether or not they have a special interest in

recommending a particular acoustical product or solution. Both

types, properly used, have their special advantages and disadvantages.

In the former group of "special interest consultants", individuals vary in their backgrounds from product salesmen to professionals

who are quite capable in their line of business. This group, which

is most commonly indicated by the degree of their association with

manufacturing or retail sales of acoustical products, should be used

directly only if, by use of the techniques described in previous

chapters, you have satisfied yourself that their solution is applicable to your problem. In this case you have progressed to the point

where the "consulting" aspect consists mainly in soliciting proposals for design and installation. The main problem remaining is to

write your contract in such a way that you are guaranteed (to the

extent possible) of actually solving your problem in a cost effective manner. The advantage of using this group directly is that you

avoid ,consultant costs and pay for the product. In effect you are

acting as your own consultant.

The disadvantage in dealing with a product-oriented consultant

is that a costly mistake, out of proportion to the independent consultant's fees, is rendered more likely. Examples abound of cases

in which thousands of dollars were spent in implementing a particular solution, only to find that no good was done. (A common mistake is to utilize acoustical tile in situations where reverberant

noise is not the problem.) If there are any doubts in your mind as

to the proper method for solving your problem, then an "independent

consultant" (one free from ties to a particular line of products)

should be called in. Since this "independent consultant" is what

is usually meant by the word "consultant", it is this type of professional that will be discussed for the remainder of this chapter.

The word independent will be dropped.

In choosing a consultant a first step is to inquire of the two

organizations in the field that are interested in the qualifications

of their members. The most inclusive is the Institute of Noise Control Engineering (INCE), P. O. Box 1758, Poughkeepsie, New York

12601. This group will have as members only those who have passed

suitable examinations. Applicants must pass both the Engineer-inTraining examination given for registered professional engineers

and a special examination on noise control engineering that was

first given in 1974. There will be no grandfathering - the term

applied to certification purely on the basis of past activity in

the field. It is expected that this certification of noise control

232
engineers will receive approval by and support from the governmental agencies needing such engineers directly, or in the contract

work they support.

The second source of information on qualified acoustical consultants is the National Council of Acoustical Consultants (NCAC),

8811 Colesville Road, Suite 225, Silver Spring, Maryland 10910.

This group has a rigorous code of ethics ~equiring (as with all registered professional engineers) that no member be associated with

the sale of a product. Consequently some consultants, otherwise

well qualified, are not members because of this association. A

membership list is available. Not all the completely classified

consultants are yet members.

At the present writing, INCE certification is just beginning

and the NCAC list is not yet complete. Therefore the most common

recourse is to question the prospective consultant yourself. A

series of questions is given below. These questions are rather

completely presented here, and you may wish to ask only those that

are pertinent to your particular task.

Guideline Questions

Education

(1) What schools did you attend?

(2) What courses did you take bearing on acoustics?

(3) What degrees did you receive? When?

(4) In what special conferences, seminars, symposia, or

graduate courses in acoustics have you been involved,

either as a student or as an instructor?

Experience

(1) For how many years have you been professionally

active in acoustics?

(2) Please supply a list of recent clients that you have

served, preferably in my geographical area, and on

problems similar to those in which I am interested.

(3) What teaching or training have you done in acoustics,

and to what groups--university, industry, trade

associations, civic groups, engineers, symposia?

233
Status

(1) Are you now an independent consultant? For how

many years? Full time or part time?

(2) If part time:

(2.1) Who is your chief employer or in what other

business ventures are you involved?

(2.2) Is your employer aware and does he approve

of your part time activity as an acoustical

consultant?

(2.3) May we contact your employer concerning you?

(2.4) What restrictions does your employer place

on you as a part time acoustical consultant?

(3) Are you associated with the manufacture or sale of

a product that could create a conflict of interest

in your activities as an acoustical consultant?

Professional Affiliations

(1) Of what engineering or scientific societies or

associations are you a member? Representative

ones are the Acoustical Society of America, the

Institute of Noise Control Engineering, and the

National Council of Acoustical Consultants.

(2) What is your present grade of membership and length

of time in that grade, for each association?

(3) Have you been accorded any professional honors ln

these associations such as offices, committee

chairmanships, awards, or prizes?

(4) Are you a registered professional engineer? In

what states? In what disciplines?

(5) Of what professional engineer associations are

you or your firm a member?

(6) Of what trade associations, chambers of commerce,

or similar business groups are you or your firm a

member?

234
Special Capabilities

(1) In what areas of acoustics do you specialize?

Noise measurement and control

Architectural acoustics

Hearing conservation

Shock and vibration measurement and control

Nondestructive testing

Medical ultrasonics

Underwater acoustics

(2) What equipment do you have for conducting

acoustical measurements in the field? In the

laboratory?

(3) With what national standards do you comply in

conducting your acoustical measurements?

(4) Are you listed by any governmental or trade

association body as an acceptable or certified

acoustical test laboratory?

(5) What equipment do you have for the absolute

calibration of test apparatus?

(6) Can you serve as an expert witness, either for

your client or as a friend of the court? What

experience have you had?

Business Practices

(1) Please indicate your fee structure. Do you handle

this by hourly charges, estimates for total job,

retainer charges~ or all of these? (1974 hourly

fees vary from $30 to $60, depending on the experience and training of the consultant.)

(2) If you use a contract form, please supply a sample.

(3) In your charges, how do you treat such expenses as

travel, subsistence, shipping, report reproduction,

and computer time? Consultants usually charge to

you the time spent during travel for you on Monday

through Friday, 8:00 a.m. to 5:00 p.m. There may

be a charge for use of highly specialized and expensive equipment.

(4) What insurance and bonding do you have?

235
(5) Are you operating as an individual, partnership,

or corporation?

(6) What statements do you have in your contracts

covering commercial security, liability, patent

rights?

(7) What restriction is there on the use of your name

in our reports, in litigation, in advertisements?

(8) What is the character and extent of reports that

you prepare?

(9) What facilities do you have for producing design

shop drawings on devices that you may develop for

the specific purposes of a consulting task?

(10) Do you have branch offices? Where?

(11) What size is your staff? What are their qualifications? Who will be working on this project?

The Proposal

Once you have selected a consultant, you can arrange to obtain

his services in several ways. With most professional people, a

verbal commitment is sometimes all that is necessary. However, you

may wish to request a written proposal that spells out the steps to

be taken in the solution of your problem.

Often, in a larger job, proposals from several points of view

are evaluated and used as one of the bases for the final selection

of the consultant. In this case answers to pertinent questions in

the preceding section may be sought in the proposal rather than in

the interview. If so, evaluation of the proposal from this point of

view is self-evident from the above discussion. If the questions

you are interested in are not answered to your satisfaction, don't

hesitate to ask for further clarification. In the discussion below,

we are concerned with the section of the proposal that outlines the

consultant's approach to your problem.

Aside from background qualifications of the consultant, the

proposal should answer the questions:

(1) How much is the service going to cost? Smaller

jobs are often bid on an hourly basis, with a minimum commonly specified of one-half day's work, plus

direct expenses. Larger jobs are usually bid at

a fixed amount, based on the work steps described.

236
( 2) What is the consultant going to do?

this question may range all the way

agreement to study the problem to a

step-by-step plan to solve it.

The answer to

from a simple

comprehensive

(3) What will be the end result? The answer to this

question is all too often not clearly understood;

the result is usually a report that specifies the

consultant's recommendation. If you do not want

to pay for the preparation of a written report, and

a verbal one will do, specify this in advance. Since

recommendations often call for construction to be

carried out by others, whose work is not subject to

the consultant's control, results can usually not be

guaranteed. Rather, an estimate of the noise reduction to be attained is all that can be expected.

If the consultant is to provide drawings from which

the contractor will work, one must specify sketches

or finished drawings. Generally, sketches are sufficient. If special materials are required, the consultant should agree to specify alternative selections if possible. If you want a guaranteed result,

experimental work will usually be necessary.

In the case of a proposal to quiet machine noise, the proposal,

if detailed, will probably callout the following steps:

(1) Determine the daily noise dose, so that the amount

of reduction required is known.

(2) From diagnostic measurements, determine the location

and relative strength of the major noise sources on

the machine in question, all other competing noise

sources being more than 10 dB below the intended

noise.

(3) Design preliminary noise control means; discuss

design with production people for pos~ible interference with access to the machine.

(4) Prepare and submit final recommendations in a report,

with construction data.

(5) In a post-report conference, resolve any questions

or compromises; submit memorandum of conference.

(6) If experimental work is needed, it can be added

between (3) and (4) above.

237
Other Services

If you wish, the consultant can also, as additional services,

provide monitoring of construction to determine compliance with

specifications. The consultant can also make post-installation

measurements to confirm predictions and supply oral briefings as

needed.

By working with the consultant during his measurements, you

can learn a great deal about how"\:o handle the special situation

for which he has been retained. However, he brings to the job an

instrument that is most difficult to reproduce: ears trained to

listen and to guide the use of the physical instruments. It takes

much practice and not a little aptitude to achieve this condition.

This aspect of a consultant's expertise is most difficult to replace.

If the consultant is to serve as an expert witness for you,

you will find that he is not automatically on your side. Rather,

he is more like a friend of the court, devoted to bringing out the

facts he has developed, with careful separation of fact from expert opinion. Complete frankness is needed if you want to avoid

unpleasant surprises. For example, the consultant may be asked by

the opposing attorney for a copy of his report to you. Thus, this

report should be prepared with this eventuality in mind.

If the consultant is retained to develop a quieter machine

for you, there should be a meeting of minds on handling of patent

rights. Ordinarily the patent is assigned to the client, with perhaps a royalty arrangement for the inventor. .

For many situations the consultant will need photographs and

plans of machines and shop layout to facilitate his evaluation.

Permission to obtain these data can be handled in a manner consistent with your industrial security system. A qualified consultant will not have to be told to regard this materials as private,

not to be divulged to others without your prior consent. If you

regard him as the professionai person he is, your association can

be fruitful to all concerned.

238
Chapter 7

ANNOTATED REFERENCES

B.l American Industrial Hygiene Association. Industrial Noise

Manual, Second Edition. Detroit, MI, American Industrial

Hygiene Association, 1966. 171 p. Although ~he instrument

section is outdated, the described measurement techniques are

still applicable. Much data are given on ear plugs and muffs.

The chapter on engineering control is very practical; it is

copiously illustrated and describes many useful techniques.

A most valuable section on examples presents compact, illustrated case histories in which the noise reduction obtained

is given, usually with octave band spectra.

B.2 American Foundrymen's Society. Control of Noise, Third Edition. Des Plaines, IL, American Foundrymen's Society, 1972.

67 p. The engineering section of this manual was prepared

by an experienced consultant. It is written without equations, but with many charts, graphs, and tables. Although

the many examples are taken from foundry technology, the control techniques are quite general in application. There are

many compact case histories, together with data on the noise

reductions obtained. The point of view is very practical.

B.3 American Petroleum Institute. Guidelines to Noise. American

Petroleum Institute. Washington, DC. Medical Research Report EA 7301. 1973. III p. This commissioned report summarizes measurement instruments and procedures, explicit

noise reduction techniques, new plant design for low noise,

and source characteristics. The appendices have detailed

information on noise control materials, levels from machinery,

and addresses of suppliers of noise control materinls.

B.4 Bell, L. H. Fundamentals of Industrial Noise Control.

Trumbull, CT, Harmony Publications, 1973. 250 p. This practical book is written from the point of view of the practicing noise control engineer/consultant. A minimum amount of

mathematics is used; many examples and exercises are given.

The chapters on enclosures, fans, gears, silencers, and vibration control are quite useful. A feature of the book is the

compact case histories, for which photographs and drawings

amply describe the techniques used.

B.5 Beranek, L. L. (ed.). Noise and Vibra+ion Control. New York,

NY, McGraw-Hill, 1971. 630 p. This is the major modern reference source for the noise control engineer. The treatment

is often mathematical, but there are plenty of illustrative

worked-out problems. Especially useful are the treatments of

239
transmission loss of simple and complex panels; mufflers and

silencers; sound in rooms; vibration isolation; and sound

power measurement.

B.6 Diehl, G. M. Machinery Acoustics. New York, NY, John Wiley

& Sons, 1973. 204 p. The chief contribution of this book is

in a detailed description of practical techniques, backed by

analysis, for the in situ measurements required for calculating sound power. Every professional noise control engineer

should be aware of these techniques. The sections on noise

sources and reduction procedures have a great deal of directly

useful information, especially for enclosure design.

B.7 Harris, C~ M. (ed.). Handbook of Noise Control. New York, NY,

McGraw-Hill, 1957. 985 p. Although old, this is still the

fundamental reference handbook for the noise control engineer.

Of particular interest are these sections: 13, vibration isolation; 14, vibration damping; 21, acoustical filters and

mufflers; 23, gear noise; 24, bearing noise; 25, fan noise;

26, noise in water and stearn systems; 27, heating and ventilating system noise; and 30, electric motor and generator noise.

Of course, recent developments in acoustical materials and

measuring equipment are missing, but the fundamentals are unchanged.

B.8 King, A. J. The Measurement and Suppression of Noise. London,

Chapman and Hall, 1965. 180 p. This British book is devoted

chiefly to noise from electrical machinery. Much attention is

paid to the design of duct silencers.

B.9 Peterson, A.P.G., and E. E. Gross. Handbook of Noise Measurement. General Radio Co., 1972. 317 P. This is an excellent

source of data on measurement of sound pressure and calculation

of sound power levels. Valuable details are given on sound

analysis techniques, characteristics of many types of acoustical instruments, and a summary of noise reduction procedures.

An especially useful section covers precautions to be observed

to ensure that valid data are acquired.

B.IO Noise and Its Control. Pollution Engineering Magazine.

Reprint of articles, 1973. This very readable set of articles summarizes characteristics of machine noise sources and

noise control techniques. It will provide a general background to the problems.

B.ll Thumann, A., and R. K. Miller. Secrets of Noise Control.

Atlanta, FairmontPresss, 1974. 256 p. This book presents

much practical noise control information in graphs and tables,

with a minimum of mathematics. Especially useful are data on

cost estimating, a listing of suppliers of noise control products, means of source location, silencers, and check lists for

management of noise control. There are many useful workedout problems. A comprehensive list is supplied for all the

standard methods of measurement that a professional noise control engineer should use.

240
B.12 Yerges, L. F. Sound, Noise and Vibration Control. New York,

NY, 1969. Van Nostrand Reinhold, 1969. 203 p. This practical book has practically no mathematics and relies almost completely on tables, charts, and graphs for its data. The

author, an experienced acoustical ·consultant, provided a great

deal of directly useful information on materials selection,

noise characteristics of machinery, design of noise control

means, and translation of subjective reactions to noise ir~~0

causes and solutions.

Periodical References, Arranged by Topic

P.l Absorber, resonant

- Mikeska, E. E., and R. N. Lane. Measured Absorption Characteristics of Resonant Absorbers Employing Perforated Panel

Facings. J. Acoust. Soc. Am. 28: 987, September 1956.

Gives results for many differen~configurations, tuning from

100 to 800 Hz.

P.2 Air moving equipment - general

- American Society of Heating, Air-Conditioning and Refrigerating Engineers Handbook. Chapter 33, "Sound and Vibration,"

1970. The definitive treatment of air conditioninq noise.

- Diehl, G. M. Think Quiet.

of set of articles, 1971.

author's book on machinery

moving machinery.

Compressed Air Magazine. Reprint

This is a forerunner of the

noise. The emphasis is on air

- Kodaras, M. J.

trol. ~, (2):

rooms.

Suppression of Ventilating Noise. Noise Con42 (1958). Treatment of ducts, mechanical

- Sanders, G. Noise Control in Air-Handling Systems. Sound/

Vibration. 1 (2): 8, February 1967. Descriptive. Covers

concepts of noise control in air conditioning, cooling

towers, gas turbines, blowers, internal combustion engines,

jet engines.

P.3 Brewing

- Melling, T. H. Noise in the Brewing Industry--the Sources,

Its Control. Proc. Inter-Noise 72: 313 (1972). Describes

bottling and keg line noise. Few data on control.

P.4 Compressor

Diehl, G. M. Stationary and Portable Air Compressors. Proc.

Inter-Noise 72: 154 (1972). The chief noise sources are

discussed thoroughly, and a few noise control suggestions

are offered.

241
P.5 Cooling tower

- Dyer, I., and L. N. Miller. Cooling Tower Noise. Noise Control. 5 (3): 44, May 1959. Most of noise from fans. Estimation procedures given. No noise control suggestions.

- S~elbach, H., Jr., and F. M. Oran. What to do About CoolingTower Noise. Sound. 2 (5): 32, September-October 1963.

Describes characteristics of the noise, gives a detailed

estimating procedure, and describes effect of intake and

discharge silencers.

P.6 Damping

- Miles, R. C. Steel-Viscoelastic Composites. Sound/Vibration.

7 (7): 27, July 1973. Describes a panel of two layers of

steel bonded by a lossy adhesive. Excellent damping is obtained.

- Warnaka, G. E., et ale Structural Damping as a Technique for

Industrial Noise Control. Reprint from J. Am. Indus. Hyg.

Assn., January 1972. Comparison of noise radiated from damped

and undamped machine surfaces.

P.7 Diagnosis: locating .sources

- Baade, P. K. Identification of Noise Sources. Proc. InterNoise 72 Tutorials, p. 98 (i972). Valuable and practical information on techniques for locating noise sources.

- Gilbrech, D. A., and R. C. Binder. Portable Instrument for

Locating Noise Sources in Mechanical Equipment. J. Acous.

Soc. Am. 30: 842, September 1958. Two microphones correlated

by a multiplier allow the direction of a sound source to be

found.

- Hodgson, T. H., and C. E. Ebbing. Diagnostic Tests for Locating Noise Source. Proc. Noise-Con 73: 266 (1973). Describes use of critical listening, noise contours, close-in

measurements, sound power estimating, vibration measurements,

time domain event analysis, cross-correlation, and coherence

measurement. Many illustrations, references.

- Miller, T. D. Machine Noise Analysis and Reduction. Sound/_

Vibration. 1 (3): 8, March 1967. Uses rotational periodicity to locate sources in bearings, electrical equipment,

fans. Briefly considers control means.

- Porter, M. A., and J. Q. Delap. Discrete Source Identification

in the Presence of High Ambient Noise Levels. Proc. InterNoise 72: 409 (1972). Uses damped, absorbent-lined tube in

front of sound level meter to reduce pickup of undesired sound

for close-in measurements.

P.B Enclosures, panels

- Nordby, K. S. Attenuation of Non-Metallic Panels. Proc.

Inter-Noise 72: 127 (1972). For moderate amounts of transmission loss, light weight panels can suffice. Paper describes

characteristics of plastics, foams, aluminum, safety glass.

242
P.9 Fans

Carrier. Sound and the Centrifugal Fan. 1969. This engineeringguide provides alignment charts for estimating octave band

levels from centrifugal fans.

- Grahanl, J. B. Acoustical Considerations for the Selection of

Fans for New Facilities. Proc. Noise-Con 73: 343 (1973).

Gives data on effect on noise of fan type, variable inlet

vanes, blade angle, and operating point on fan characteristics.

P.IO Gas turbine

- Tatge, R. B.

Vibration. 7

curves, noise

cers.

Noise Control of Gas Turbine Power Plants. Sound/

(6): 23, June 1973. Defines NEMA noise limit

reduction for regenerator equipment, use of silenP.ll Gears

- Dunlap, T. A., and W. G. Halvorsen. Transmission Noise Reduction. SAE Paper 720735. September 1972. Discusses measurements of bending and torsional compliance, and noise reduction

by detuning drive train components, decoupling housing areas,

and housing damping.

- Route, W. D. Gear Design for Noise Reduction. SAE Paper 208E·

June 1960. Discusses influence of contact ratio, pre.ssure

angle, angles of recess and approach, dimension control of

mounting, and tooth errors.

P.12 General - Crocker, M. J. Noise Control Approaches. Proc. Inter-Noise

72 Tutorial, p. 17 (1972). Excellent summary of procedures.

- Cudworth, A. L. Field and Laboratory Examples of Industrial

Noise Control. Noise Control. 5 (1): 39, January 1959.

Case histories on ejector chutes~ tote boxes, chipping hammer,

air nozzles, and air compressors.

- Engstrom, J. R. Noise Reduction by Covers. Noise Control.

1 (2): 19, March 1955. Case histories on multislide punch

press, riveting machine.

- Engstrom, J. R. Riveting Hammer and Paper Shredder. Noise

Control. 2 (2): 18, March 1956. Case histories on riveting

hammer, paper shredder.

- Handley, J. M.

6.0011.0, 1973.

on cut-off saw,

steamboiler.

Noise--The Third Pollution. lAC Bulletin

Case histories from Industrial Acoustics

looms, punch press, power transformer, process

- Ihde, W. M. The Application of Engineering Noise Control

Measures. National Safety News. 107 (6): 85, June 1973.

This article provides an intensely practical summary of characteristics of a wide variety of noise sources and of procedures

for noise control. There are many illustrations and examples.

243
- Yerges, L. F. Methods of Noise Control for Machinery Already

Installed. Proc. Noise-Con 73: 376 (1973). Excellent and

compact summary of procedures, costs. Case histories on

grinders, automatic screw machines.

- Young, R. L. Practical Examples of Industrial Noise Control.

Noise Control. 4 (2): 11, March 1958. Case histories on

chain drives, pinch rollers, tumbling barrels, vacuum pumps,

supercharger test stand, boiler fabricating operations.

P.13 Hydraulic pumps

- Dean, R. C. Origins of Noise in Hydraulic Pumps. Proc.

Inter-Noise 72: 249 (1972.). Discusses chief noise sources in

lobe and lug type pumps, and how to interpret measurements and

inlet conditions.

- Miller, J. E. Silencing the Noisy Hydraulic System. Machine

Design: June 14, 1973, p. 138-143.

- Parker Hannifin Co. Reducing the Operatlng Noise of Industrial

Hydraulic Systems. 1972. Exhaustive and informative treatment,

with excellent list of noise reduction techniques.

P.14 Materials, composite - Cavanaugh, w. V. Composite Materials for Noise Reduction.

Proc. Inter-Noise 72 Tutorial, p. 59 (1972). Discussion of

many materials, advantages and shortcomings.

- Miller, R. K. Acoustical Materials for the Food Process Indus-

, try. Proc. Noise-Con 73: 519 (1973). GiveB noise levels,

sanitary constraints on absorption materials, and isolator

selection.

P.15 Pipes

- Dear, T. A. Noise Reduction Properties of Selected Pipe

Covering Configurations. Proc. Inter-Noise 72: 138 (1972).

Exhaustive study of many pipe lagging systems for reducing r~diating noise, under controlled conditions. Excellent

reference.

P.16 Product noise - Guinter, J. J. Action Plan for Product Noise Reduction.

Sound/Vibration. 6 (12): 18, December 1972. Outlines

procedures.

P.17 Pumps, centrifugal - Evans, L. M." Control of Vibration and Noise from

Pumps. Noise Control. 4 (1): 28, January 1958.

selection and proper operating point.

Centrifugal

Emphasizes

P.18 Punch press

- Allen, C. H., and R. C. Ison. A Practical Approach to Punch

Press Quieting. Noise Control Engineering: Vol. 3, No. 18, 1974

244
- Bruce, R. B. Noise Control of Metal Stamping Operations.

Sound/Vibration. 5 (11): 41, November 1971. Absorption,

partial and total enclosures are discussed. Control at the

source is also considered.

- Shinaishin, O. A. Punch Press Diagnostics and Noise Control.

Proc. Inter-Noise 72: 243 (1972). Suggests changes in die,

stock, snubbers, perforated plates, enclosures.

P.19 Refinery noise

- DeBiase, J. L. Criteria and Design Specifications for Plant

Noise Control. Sound/Vibration. 6 (9): 26, September 1972.

Good listing of sources, control procedures. Also where to

measure OSHA levels.

P.20 Screw machine

- Schweitzer, B. V. Silent Stock Tube for Automatic

Machines. Noise Control. 2 (2): 14, March 1956.

development use of stock'tubes with damping.

Screw

Describes

P.2l Silencers

- Sanders, G. J. Silencers: Their Design and Applications.

Sound/Vibration. 2 (2): 6, February 1968. Describes types,

and attenuation/frequency characteristics. Useful data on

materials resistant to high temperatures, flow velocities.

P.22 Sound power measurement

- Huebner, G. Sound Power Determination of Machines in situ.

Proc. Inter-Noise 72: 381 (1972). Discusses errors from

being in near field, from finite nUmber of measurements, from

measurement error, and from room reflections. Latter unimportant if area ratio (total absorption in room)/(area of

measuring surfaces) is greater than 10.

P.23 Stack noise - Hersh, A. S., et ale Refinery Auxiliary Stack Redesign to

Eliminate Induced Resonant Tone. Proc. Noise-Con 73: 441

(1973). Many small changes, including avoiding turbulence,

are described.

P.24 Valve noise - Heymann, F. J. Some Experiments Concerning Control Valve

Noise. Proc. Noise-Con 73: 382 (1973). Several plug and

seat configurations are described. Much useful data on getting data from which to predict noise from scale models.

P.25 Vibration and radiated power - Chan, C.M.P., and D. Anderson. Correlation of Machine Structure Surface Vibration and Radiated Noise. Proc. InterNoise 72: 261 (1972). Mean square velocity or acceleration

correlates well with sound power.

245
P.26 Vibration and shock isolation

- Miller, H. T., et all. Practical Design of Machinery Foundations for Vibration and Noise Control. Proc. Inter-Noise 72:

185 (1972). Discusses real life parameters that must be

considered.

P.27 Woodworking industry

- Christman, R. P., et al.

Products Industry. Noise

1956. Octave spectra for

Sound Pressure Levels in the Wood

Control. 2 (5): 33, September

many machines.

Additional specific references used in the case histories are

included as footnotes to the specific case.

246 '" u.s. GO'IIRIIIIDlT PRIIlJUlG OfFICE, 1911- 757 -057/6739