NBSIR 75-653

EPA 550/8-76-002

Measurement Methodology and

Supporting Documentation

for Portable Air Compressor

Noise

Curtis I. Holmer

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

January 1975

Final Report

Prepared for U. S. Environmental Protection Agency

Office of Noise Abatement and Control

Washington, D. C. 20460

NBSIR 75-653

EPA 550/8-76-002

MEASUREMENT METHODOLOGY AND

SUPPORTING DOCUMENTATION

FOR PORTABLE AIR COMPRESSOR

NOISE

Curtis I. Homer

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

January 1975

Final Report

Prepared for U.S. Environmental Protection Agency

Office of Noise Abatement and Control

Washington, D. C. 20460

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

Summary 1

1. Background 2

1.1. Source Description 2

1.2. Parties Affected by Noise 3

1.3. Present Accepted Noise Measurement Methodologies 3

2. Rationale for Methodology 5

2.1. Introduction 5 2.2. Determination of Soimd Power 5 2.3. Use of Weighted Sound Levels 7

a. Limitations of Weighted Sound Levels 7

b. Representative Noise Spectra of Portable Air

Compressors , . . 8

c. Use of A-Weighted Level 8

2.k. Discussion of Accuracy and Precision 17

3. Reviev of Methodology l8

k. Portable Air Compressor Methodology 22

k.l. Ptirpose and Applicability 22

U.2. References 22

h.3' Measurement Uncertainty 22

h.k. Acoustic Environment 22

a. General Requirements 22

b. Criteria for the Adequacy of the Environment 23

c. Ambient Noise 23

d. Other Ambient Factors 23

Instrumentation 23

a. General Requirements 23

b. Additional Instriimentation Requirements 2h

h.6. Installation and Operation of the Source 2h

a. Source Operation 2h

b. Source Monitoring 2h

U.7 Measurement Procedure 25

a. Microphone Positions 25
h.Q, Information to be Recorded 25

h.^. Calculation ProcediJires 31

5 . Acknowledgement 36

6. Bibliography 37

Appendix A. Procedure for Predicting A-¥eighted Soimd Level from

A-Weighted Soiuid Power 39
Summary

This report presents recommendations and. supporting rationale on a measurement methodology for portable air compressors. The methodology is

believed to he general enough to allow determination of noise emission

from other outdoor stationary noise sources. The methodology provides for

the determination of A-veighted sound power level or the equivalent

weighted sound pressure level at a reference distance. A-weighted level

is used because of its strong, positive correlation with community

response to noise from internal combustion engine noise. It is

recommended, however, that the spectra associated with the regulated

source be monitored in some manner to insure that the spectra remain

similar to those for which A-weighted sound level retains good correlation

with community response. The methodology uses weighted sound level

measurements at eight positions on a curved surface surrounding the source

at a distance of one metre from the surface of the machine. Data recorded

at these positions are used to calculate the average A-weighted sound

pressure level of the machine on the measurement surface. This is

combined with the area of the measurement surface to give the sound power

level of the machine. From this value, a rating sound pressure at a rating distance may be calculated by subtracting a constant value.

While a specific methodology is proposed which is believed to be

consistent with achievable accuracy and precision of field measurements

conducted for regulatory purposes, we believe that the flexibility

inherent in regiilation by use of sound power level should be exploited by

permitting the use, for quality control purposes, of any measurement

methodology which provides estimates of the sound power level which has

demonstrable precision and accuracy consistent with the difference between

the actual level radiated by the source and the regulated level.

Procedures which permit the rapid estimation of A-weighted sound

level are presented in Appendix A. These are applicable for estimation of

A-weighted sound level in a variety of circumstances when the sound power

or equivalent sound pressirre level at a reference distance is known.

Key Words: Acoustics; air compressor; internal combustion engine; noise;

sound power level; sound pressure level.
1 . BACKGROUND

The construction site is an omni-present fixture of present life in

U. S. cities, and represents a significant source of noise to occupants

and residents in nearby areas. While the noise arises from many sources vithin the site, the U. S. Environmental Protection Agency has identified

portable air compressors as a major noise source under authority of

Section 6 of the Noise Control Act of 19T2. At the request of EPA, the

National B\areau of Standards, with the cooperation of industry, has

derived a test procedure applicable to portable air compressors. In this

report ve describe this measurement methodology and review the rationale

behind the decisions which led to its development.

' ' 1.1 Source Description

Portable air compressors are self-powered devices which produce

compressed air for the operation of other equipment. They are portable in

the sense that they are mounted on trailers or truck beds, to facilitate

moving from site to site. The machine consists of a driver, i.e., a gasoline or dies el engine (or possibly an electric motor) , and a driven

element which is a reciprocating, or rotary vane or rotating screw pump.

Currently, units are available which may be described as standard and

quieted (the latter category generally implys some degree of noise control

treatment added to an otherwise standard unit). Also included is an air

receiving tank, an automatic regulation system which matches air output to

demand, and suitable performance indicating gauges such as air pressure

and engine speed. The output automatic control system may be of an on-off

or continuously variable type.. In the former case, depending on air

pressure demand from the receiving tank, the engine will be operated

either at idle or rated (i.e., governed) speed. In the continuously

variable case, engine speed is varied through a control loop to match

compressor output to air demand. Because of the relatively fine gradation

in model size (there are approximately twenty-five model sizes to span the

output range from 85-2000 cubic feet per minute (cfm)) compressors tend to

be selected to closely match the demand requirements for the tools which

they power, and are probably operated very close to rated output nearly

all of the time when they are in use. In physical size, the units vary

from about O.Tmxl.Omxl.Sm with a 30 horsepower engine to

approximately 3mx3mx6m with a 65O horsepower engine. Within the

range of compressors manufactured, there are logical divisions, for

example, as a function of driver type and pump type. There is a less

clearcut division on size. Virtually all gasoline powered units have a rated capacity below 250 cfm (^2^+ m /s), while all diesel units are larger

than 125 cfm (213 m /s) capacity. No similar categories for pump type

occur, since-j^all three types are used throughout the range of sizes

produced, [l]—

— Numbers in brackets indicate literature references at the end of this

report

.

2
The predominent noise sources associated with the compressor operation are those associated vith the operation of internal combustion engines

(i.e., engine intake, exhaust and casing radiation plus cooling system fan)

and also radiation from the compressor casing and air receiving system and

associated valving. No single routine operation leads to the production

of noise from warning devices or from pressure relief valves.

There does not appear to he justification for fundamental variations

in the measurement methodology to accommodate the ahove mentioned machine

parameters.

There is no typical use pattern established for portable air

compressors since the requirements and site descriptions vary substantially

from construction job to construction job. The site may vary from a large

controlled area (such as a site for a major building or power plant), to

a city street where pavement is being removed. In a typical urban site,

the compressor is frequently located on or near the street where passersby and nearby building occupants may be expected to receive substantial

exposure.

1.2 Parties Affected by Noise

The two principal groups affected by compressor noise are construction

workers on the job site, and the siirrounding community. The nature of

the effects on construction workers range from task interference and

annoyance to possible hearing loss. Potential hearing loss, task interference,

commimication interference, and annoyance will constitute the most significant

effects in the community. Construction workers presimably will be protected

by occupational health and safety regulations, with the possible exception of

small construction firms which do not engage in interstate commerce. The

proportion of exposed workers in this latter category is unknown. For these

reasons the principal concern in this methodology is to equitably assess the

noise emission which contributes to community noise levels.

1.3 Present Accepted Noise Measurement Methodologies

The Compressed Air and Gas Institute and its European counterpart

( CAGI/PNEUROP) have promulgated a test code for the measurement of

noise from pneumatic equipment [ 2 ] , including portable air compressors,

which is now nationally and internationally accepted[3]. This code

consists of determining the average octave band and A-weighted sound

pressure level at ten positions near the ground plane (1.2-1.5 meters

above the ground) at 1 and 7 meters from the surfaces of the machine.

Current standardization activity includes proposed modifications to these

standards which incorporate sound power level determinations[^]

.

The data resulting from use of the CAGI/PNEUROP methodology are not

equitable measures for characterizing the noise radiated to the community

for two major reasons. The first problem is that the sound pressure level

(SPL) measured at a fixed distance from the surface of a machine is not

3
simply proportional to the level at another distance from a machine, since

the proportionality constant depends on machine size. Consider, for

example, tvo machines ("whose sizes represent the extremes mentioned earlier)

"With the same SPL at 1 meter from the surface. At 30 meters from the center

of the machines the SPL from the larger machine vill be about 5 dE higher;

while for the same SPL at 7 meters, the difference in SPL at 30 meters will

be about 2 dB with the larger machine again higher. Thus it is clear that

without some allowance for machine size, the present methodology will not

produce an unbiased estimate of noise in the community. A second problem

arises because the CAGI/PNEUROP code provides for no measurement above

the umit, so that noise radiated to upper stories of nearby buildings

would not be controlled. Both of these problems have been recognized by

the industry which is one reason for the present work on modification of

the international standards.

Both of the above problems may be rectified by a change of determined

quantity from average sound pressure level to radiated sound power or sound power level. The proposed methodology is based on this latter

concept

.

Present methods for determining sound power involve the calculation

of average sound pressiore level from measurements on large radiu3 hemispherical

surface, centered on the source which is placed on a hard reflecting plane[5],

combined with an added constant which is proportional to the logarithm of

the surface area of the hemisphere. An alternate procedure[6] involves

the determination of average sound pressure over a surface surrounding a device at a \miform distance from the surface of that device, times a constant which is equal to the area of that meas"urement surface. The

former method involves extensive test site requirements which make the

procedure very difficult to use directly as a quality control or field

enforcement test. The latter test has good possibilities for such use because it does imply the possibility of a measurement at short distances

from the machine with associated easing of test site and background noise

requirements. The major problem associated with this latter test is the

relatively lower experience with use of the test by the acoustics community,

which creates difficulties in estimating, a priorij how many measurement

positions are required to achieve a given level of precision, or what

acciiracy may be expected. [7]
2. RATIONALE FOR METHODOLOGY

2.1. Introduction

Section k of this report consists of a detailed description of the

measurement methodology for characterizing the noise from portable air

compressors with respect to the resulting noise exposure in the

community. In the following paragraphs of this section we review the

information which led to the proposal of this methodology, and which

in fact constitutes the rationale for the decisions made during its

preparation. The remainder of this introduction will provide a brief

overview of the methodology, while subsequent paragraphs will address

the significant decisions concerning the use of sound power and weighted

sound level to describe the source, the questions of accuracy and precision

in the measurement, and finally a detailed discussion of the important

paragraphs within the methodology itself.

The recommended methodology involves the determination of the A-weighted

sound power output of the source. The sound power output is estimated

from the average of eight measurements of sound pressure level (SPL) on

a measurement surface which is one metre from the surface of the source,

with the source placed on a hard reflecting plane.

2.2. Determination of Sound Power

The sound power output of a device is a quantity which characterizes

the power radiated by that device in a manner which is, to first order,

independent of the surrounding environment. There is a strong analogy

between thermal and acoustical problems which may assist in visualizing

this situation. In both cases power is used to characterize energy output

per xmit time. Temperature is the variable in the thermal case which

corresponds to sound pressure in the acoustic case. Both variables are measures of the intensity of the radiation from the source at a given

position. In either case, the value variable is directly related to the

source strength and is also influenced by the environment. As energy

propagates away from the source, it may be thought of as spreading over

a surface of equal distance from the source. In the case of a small

so\irce, for example, which radiates in all directions, placed on a nonabsorbing surface, the power radiated from that source is distributed

over a hemispherical surface.

The acoustic power W radiated by a simple source such as a pulsating

sphere is determined from the equation[5]

W ^ <p^> S/pc (1)

2 where p is the density of air, c the velocity of sound m air, <p > is

the space agd time average sound pressure, and S is the area of the surface

on which <p > is determined. This equation is accurate within one decibel

for complex sources when the effective radius of the measurement surface

5
is greater than (l20/f meters) where f is the center frequency of the

frequency band of greatest contribution to the weighted sound level. It

should be noted that this equation appears to produce an upper bound estimate

of the sound power determined by other standard procedures under typical

conditions [ 8 ]

.

Since the area of the hemispherical surface is proportional to the

square of the radius, the intensity (i.e., power per unit area) is

inversely proportional to the square of the distance from the source.

Thus if the distance from the center of the source is doubled, the

intensity is reduced by a factor of four. Since a 3 dB reduction in sound

pressure level (SPL) corresponds to a halving of sound pressure, a doubling of distance in this case will produce a 6 dB reduction in SPL.

The total power radiated may be determined from the mean square sound

pressure averaged over a measurement surface, the area of that measurement

surface, and the characteristic impedance of the ambient air.

The sound radiated from a device of complex shape spreads out on a wavefront which is always at a uniform distance from the surface of the

source. Thus, in principle, the determination of sound power can be made

on any measiurement surface which is a uniform distance from the source

surface. At large distances from any source mounted near a reflecting

plane, the wavefront surface becomes nearly hemispherical in shape, and a hemispherical measurement surface is appropriate. Thus the determination

of so\ind power output may be made on any hemisphere of any arbitrary

radius greater than some appropriate minimum radius — subject to other

limitations of a practical nature which affect the accuracy or precision

of the measurement. The major practical limitations are imposed by

background noise, atmospheric propagation effects, the necessity of

testing over reflecting plane which is larger than the test hemisphere and

accuracy of microphone positioning. The first three factors all become

increasingly important as distance increases, and thus minimizing the

measurement radius is desirable. The last factor is more significant for

smaller radii , and thus also suggests a minimum radius

.

The foregoing information suggests that, in principle, it does not

matter whether or not the measurements are made on a hemispherical surface

whose radius is several source dimensions, or on a surface surrounding the

source at a fixed, relatively small distance as long as the effective

radius is greater than the minimum specified for complex sources. The

same information is obtainable using either surface, but the number of

measurements required to obtain information to the same degree of accuracy

may differ. This is due to the fact that close to a complex soiirce (i.e., one composed of a number of independent subsources), the sound level is a stronger function of the contribution of the nearest subsource, and thus

additional measurement positions may be required to appropriately include

the contributions from each subsource. The results of recent research[8]

suggests, however, that for compressor noise the number of measurement

positions required is nearly the same and is independent of the distance

from the source (outside a region very close to the source).

6
2.3. Use of Weighted Sound Levels

a. Limitations of Weighted Sound Levels

A-veighted soirnd level has been identified by many researchers as

a simple, useful measurement which correlates with human response to noise

in a number of situations. It has been noted that A-weighted levels show

fair correlation with speech interference and annoyance for a wide variety

of sources, and, in particular, A-weighted levels are one of the better

availaMe descriptors of community response to internal combustion engine

noise.— These considerations strongly indicate that A-weighted level is an appropriate measure for present air compressors . It should be noted,

however, that two significant factors which contribute to this fortuitous

situation are that a) compressors which are currently manufactured do not

radiate dominant discrete tones at mid and high frequencies and b) the

spectra of energy from presently manufactured compressors are "regular"

in the sense that they do not contain large amounts of low frequency

(i.e., below 125 Hz) energy. We believe it is appropriate that the

methodology be a suitable measure of community response not only for

compressors as presently manufactured, but also for compressor designs

which can reasonably be anticipated to result in response to noise emission

regulation.

The lack of discrete tones at mid and high frequencies from present

compressors is attributed to the fact that it is physically difficult (with

the exception of poorly designed compressor air handling systems) to produce such sounds with present compressors internal combustion engines and

cooling fans with a relatively small number of blades. The only anticipated mechanism for changing this situation would be the future use by the

industry of other types of engines for driving the compressor, such as the

gas turbine, which may generate strong high frequency tones. This is

considered to be a reasonable possibility in view of the rapidly developing technology of small gas turbines. In this event, it is conceivable

that a more complex descriptor such as perceived noise level may be

required in order that the measured value will correlate well with

community response. [9]

The other major question which must be addressed is the frequency

spectra of noise from compressors which is to be expected when a internal

combustion engine powered compressor is quieted to meet an A-weighted noise

regulation. It should be noted that the A-weighting puts very little

emphasis on low frequency soiinds, and low frequency sounds are not reduced

as much, and may in fact be increased, by noise control treatments which are designed to control high frequency sound. Thus there are both physical

and economic grounds for expecting low frequency noise from portable air

compressors to at best remain the same and potentially increase under the

influence of an A-weighted noise regulation.

— It was noted earlier in this report that the engine driving the compressor

is the principle noise source on presently manufactured compressors.

7
Given the possilDility that low frequency noise from compressors

could increase, vhat might be the resulting response from the community?

One major potential source for community disturbance is the vibration

induced in structvires by low frequency sound— . Another source of

complaint has been identified by one author[lO] as "disturbance or unease"

in the presence of noise which peaks in the 31.5 Hz octave band or lower

frequencies. In particular he identifies "the indadequacy of dB(A) as a predictor of adverse reaction to sound with most of their energy at low

frequencies," which he attributes to the fact that "dB(A) was originally

designed for use at fairly low intensities and makes no allowance for the

rapid growth of loudness with intensity which occurs in this [low frequency] region." The spectra identified as disturbing exhibited a difference between C- and A-weighted levels of 22 dB or more, and substantially exceeded the threshold of hearing (^^.60 dB SPL) in the 31.5 Hz

octave band. A typical range of difference between C- and A-weighted

level for presently manufactured portable air compressors is 5-13 dB.

If we assume that A-weighted noise from compressors is reduced by 10 dB,

and assuming C-weighted noise, which is determined by low frequency com- ponents of the spectrm, remains unchanged, we find that the C-minus Aweighted difference will be in the range 15-23 dB, thus creating a potential

problem.

Should the need for reduction of the low frequency noise emission

from a compressor be explicitly identified, then engineers can design to

incorporate low frequency noise control treatments. The principle source of

possible increase in low frequency noise is associated with the large surface

area enclosure (large with respect to the size of the source) which is needed

to contain the noise radiated by the engine and compressor. If the enclosure

is directly connected to the vibrating surface of the compressor, it will

produce a new, far more efficient, path for radiation of low frequency

energy. This problem can be reduced or avoided by judicious selection

of attachment points for the enclosure.

The proposed methodology incorporates no measure of this low frequency

noise. It is recommended that some appropriate mechanism be created which

permits the detection of significant increases in radiated noise in this

frequency region, so that this may be regulated if it occurs. The justification for this approach is that the lack of current community response

data for this low frequency noise problem makes it extremely difficult to

define a suitable measure, or an appropriate limit, even though we expect,

based on limited experience that A-weighted level alone will not completely

describe the community response to signals containing very high proportions

of low frequency energy. Additional research on community response to low

frequency noise is clearly needed in order to provide a basis for regulating

sources with these characteristics.

— Several acoustical consultants (G. W. Kamperman, Kamperman and Assoc.,

R. M. Hoover, Bolt Beranek and Newman, In. , F. M. Kessler, Dames and

Moore, Inc.) have observed that sound levels on the order of 75-80

dB in the 31.5 Hz octave band are sufficient to rattle windows and

dishes in wood frame housing.

8
b. Representative Noise Spectra of Portable Air Compressors

Figiires 1-8 depict 1/3-octave band sound pressure level spectra for

rotary vane, rotary screw, and reciprocating compressors vith diesel and

gas power plants, in standard and silenced configurations [11 ] . All

spectra represent either single position or average levels at a seven

metre distance from the surface of the machine in question. The data

indicate, within the limitations of the analysis bandwidth, the lack of

high frequency pure tones from compressors. Of the seven spectra, only

the data for the 750 cfm, silenced, rotary screw compressor exhibits

potential high frequency tones. In this case it was verified that the

signal at l600 Hz represented a narrow band noise radiated by the muffler

shell, and not a tone per se. The spectra are representative in the sense

that they include a diversity of machine types and manufacturers, but

should not be considered as typical or indicative of the sound levels

produced by any given type or model of compressor, or of the limitations

of noise control treatments of machines. Additional spectral data may be

found in reference 8, which further confirms these generalizations.

The presence of significant, apparently-discrete-frequency energy at

low frequencies is attributed primarily to exhaust discharge at the firing

frequency and its first few harmonics for the case of relatively minimal

exhaust silencing.

An inverse A-weighting curve is superimposed on each spectrum in

such a fashion as to identify the important spectral components for that

weighting. The most important portions of the spectra are those which are within five to ten decibels of the weighting curve as plotted. It should

also be noted that noise in the low frequency region below 100 Hz is such

that mid frequency noise would have to decrease by 15 dB or more for this

group of compressors before the low frequency noise would significantly

affect the A-weighted sound level, indicating lack of positive response of

the A-weighting in this low frequency region. Alternatively, this low

frequency noise could increase by as much as 15 dB in some cases in the

present situation without affecting A-weighted so\md level.

c. Use of A-Weighted Level

One major correlative study on the use of various physical measiores

to describe human response to noise has identified the A-weighted sound

level as "...the only measure [of motor vehicle noise] having high correlation with subjective reaction that can also be read directly on a commercially available meter having standardized performance" [12 ] . This

conclusion is not at variance with other published studies which have

considered noise produced by vehicles. However there has been little

study of stationery sources. The lack of more general studies of

community response to noise is unfortunate from at least two points of

view. The first difficulty results from the fact that the above mentioned

study was specifically concerned with the driveby situation (both uniform

speed and accelerating) which is a transient event, while compressor noise

is a steady state event. The second cause for concern is the change in

spectrum shape (already discussed in this section) which may result when

an A-weighted criterion is applied. Neither of these questions can be

resolved from presently available information in the open literature.

9
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1—1—

SO-200-2.

1 ' 0-3

1

1X>-iiX)-430-8(

1—^ rWX)-700-I.

1 1 L_ »0-3'

1JO-S

1—

»-d30-«

1—H-T- 100 1000

Frequency, Hz

10000

Figure 6. 330 cfm Silenced, (engine unidentified), Reciprocating

Compressor (T meter Measurement ) [11 ]

15
BAND NO.

16 17 18 19 iO 7^ 72 73 74 2S 76 tf 38 W 30 31 32 33 34 M y 3? M 39 40 41 48

03 O

CM

(U

100 '

pq

i

o

o

I

EH

So

7^

5e

-40— 30—«— 80-100- )25-l<0- 200-250-3 H 1 |

l I 1 I t

' ' \ —

T

e5

5-^00-S'JO- 630-300- '00-125- 140- 200-250-3?5-400-500-*30-800-|00-I25- —I—I—rW-r-,-i

' ' 1-— —i

i i' i I i T "

100 1000 10000

Frequency, Hz

Figiare 7- 750 cfm. Silenced, Diesel Powered, Rotary Screw Compressor

(7 meter Measurement) [11 ]

.

16
We conclude at this point that A-weighted sound level is an adequate

predictor of community noise for "regular spectra" of internal combustion

engine noise, but that some additional measure of low frequency noise

levels may "be required in the future in order to adequately protect public

welfare. C-weighted level could provide this latter measure, since it

is also the only single scale which is available on standardized instruments which adequately reflects sound levels in this low frequency region.

The lack of" suitable correlation of community response with C-weighted

level alone [12] is the reason that measurement of C-weighted level is not

included at this time. Further research is required in order to quantify

community response to low frequency noise and evaluate the validity of

vario\is objective measures of this noise.

2.k. Discussion of Accuracy and Precision

The proposed methodology is intended to provide a measure of

A-weighted sound power radiated to the far field. The estimated precision

(one standard deviation) of the measurement methodology using eight

measiirement positions is jy..? decibels with respect to A-weighted power of

the same source determined under laboratory conditions utilizing a large

number of measurement positions and laboratory grade equipment [ 8 ] . The

principal sources of inacciiracy of the field measurement are attributed

to:

(a) instrument tolerances

(b) sound field sampling error.

The instrument tolerances for American National Standard S1.U-19T1

(Specification for Sound Level Meters) Type 1 precision instruments, as required by the measurement methodology, are no more than +1 dB for measurement of A-weighted sound level at any given frequency from 50 to UOOO

Hz. This uncertainty, coupled with the +_0.5 dB uncertainty for the

calibrator, produced the principal soiorces of instrument error in the

measurement, as well as the principal soiirce of imprecision when different instrumentation systems are used.

If the measurement methodology uses five measurement positions, the overall measurement imprecision is expected to increase to about 2.h dB.

The overall estimated standard error (one standard deviation) of

the recommended methodology (i.e., reproducibility of measirrement

using different observers, test sites and instrumentation, but sources with the same sound power output) is estimated (on the basis of

limited experience with similar measurements, assuming normal distributions

of all variables and ignoring truncation error) to be on the order of 2 dB

with 95% confidence[8] . (That is, 6Q% of the measurements on a given

source are expected to be within +2 dB of each other with a 95% level

of confidence in the value 2. Similarly 95% of the measurements will

lie within +k dB.) If the number of sample points for sound pressure

level measurement is reduced to five measurement positions, the estimate

of standard error of the methodology will increase to 2.7 dB (one standard

deviation). (That is to say 6Q% of the measurements will lie within +2.7 dB

with 95% confidence.)

IT
3. REVIEW OF METHODOLOGY

In the following paragraphs, we will discuss individual sections of

the methodology in the order of their appearance in Section h, and identify

the rationale behind detailed decisions that have not been discussed under

previous more general headings

.

a. Acoustic Environment . Requirements for acoustic environment for

the measurements are derived from presently accepted national and

international measurement standards [2, 3, 5, 6]. The requirements for outdoor

measurements are based on reasonable considerations of contributions from

reflections in the context of the accuracy requirements outlined earlier.

No indoor measurement qualification procedure is provided, since procedures

presently being considered have serious defects, and no new procedure could

be developed within the time constraints of this program. Such a procedure

would be highly desirable, since indoor testing would eliminate weather

restrictions. The outdoor site, used for tests by NBS[8], was checked using

one proposed indoor site qualification procedure[6 ]

, which resulted in a calculated correction of more than 2.5 dB when in fact none was required.

Because of such problems, we believe considerable further study is

required before indoor testing for regulatory purposes with an environmental correction, can be considered. Since the facilities

required to test indoors without a correction are much more expensive than

is believed to be justifiable, this matter has not been pursued further.

(The acoustic treatment alone, for one such test facility, suitable to

frequencies as low as say 50 Hz, is estimated to cost in the region of

$250k-500k, which is more than the estimated cost of research needed to

fully clarify the correction problem.

)

b. Instrumentation . The instrument specifications are derived from a minimum required performance to achieve desired accuracy. More sophisticated

or faster systems involving multiple microphone arrays and/or computerized

data reduction may also be implemented within this specification.

c. Source Operation . These requirements are derived from present

standards for these devices, [2, 3] and are necessary to assure a reproducible

measurement.

d. Measurement Procedures . The measurement procedure described here

is adapted, with very minor improvements, from the currently proposed ISO

Draft International Standard[6]. Based on our experimental data[8], this

methodology offers the maximum precision and minimum bias for total measurement effort expended. Reducing the number of measurement positions from

eight (as used here) to five as suggested for some survey class measurements

significantly increases the estimated 95% confidence interval for the

measurements from on the order of dB to +_5.5 dJB (two standard

deviations).

The modifications include small changes in the heights of the lower

microphone positions to bring the microphone height closer to the center of

the area it is intended to sample, thereby reducing the potential of correla18
tion between the measiirement points. The specification of the height of the

upper microphone positions are changed to bring them onto the measurement

surface

.

No vertical traverse of the microphone at a measurement position has

been incorporated, since it is believed, based on the experimental data, that

the interference pattern arising from ground reflections is not as significant

at small measurement distances from the source. At large distances from the

source, such traversing is believed to be necessary to maintain the same level

of precision with a given number of measurement positions.

The precision estimates for the methodology are based on the

assumption that the variation in the sound level with position on the

measurement surface will be comparable in magnitude or less than that for

the sample group of .compressors . The requirement for additional

measurement positions has thus been added for cases where the variance is

large in order to maintain the precision of the measurement. This

requirement for additional positions is only expected to be significant

for some large compressors, however, none of the seventeen compressors

measured in a previous study[8] exceeded this requirement.

e. Information to be Recorded . The data included as line items in

the tables are believed to be those which are both necessary and

sufficient to insure reproducibility of a given measurement at another

time within the above precision limits. Note that the tables are provided

as a guide only, and are not in any way intended to be specific recording

formats

.

f . Calculation Procedures . The calculation procedures described

are straightforward applications of the intended measurement. The

included corrections for barometric press\ire and temperature are not

required for measurement on a standard day at altitudes below 1 km, as defined by the pressure and temperature range in section U.9, but are included to anticipate the potential variations implicit in application of

the methodology throughout the world. The reference conditions are those

which produce an acoustic impedance in air (pc) equal to hOO MKS Rayls +_ 10% (+0.5 dB).

The expression used in the equation for computation of measurement

surface area is exact. The following approximate expression, given in a draft ISO Standard, may be found to be computationally more convenient

than the equation used

S' = i;(ab + ac + be) (a + b + c)

(a + b + c + 2r)

where a, b, c and r are as defined in Table I. The expression gives a value which is always larger than the true value, by an amount which

varies from 0 at r = 0, up to +15^ (0.6 dB) at large r. The error for r - 1 meter eind a, b, c comparable with typical compressor dimensions is on the order of +Q% (0.3 dB)

.

In view of the number of questions that have been raised concerning

the potential difficiilties with calculating soimd power level, the following example is included.

19
Example computation of sound power, using the proposed methodology (Data

taken from test 9 of reference 11).

A compressor is measured on a surface of one metre distance from the

source.

.. , The physical size of the compressor is 3.66 m long by 1.82 m wide by

2.lU m high. Test conditions are 25°C, 755 mm Hg. The coordinates of the

measurement positions are as follows (r = Im)

a =

^2 + r) = 2.8 m, - = l.i+ m

b = (-^2 + r) = 1.9 m

c = (h + r) = 3.1 m

|-(a + b -r/2) = 2.1 m

|-(a + r/2) = 1.7 m (less than £/2)

|-(b + c/2) = 1.7 m (less than b)

The measured A-weighted sound level at the eight measurement positions were

found to be (rounded to the nearest half dB)

Position |_1 2 3 U 5 6 7 8

Source Sound

level (dB) 79-5 78,5 76.5

Background Sound

level 50 51 ^8

Mean square

^ pressure (pascals ) .036 .028 .018

(from Table IV)

Since the spread of the source data is less than 9 dB, only eight positions

were required according to the methodology.

The background noise was found to be negligible (more than 10 dB below

the measured levels). The sound levels were converted to mean square pressure

using Table IV. In using Table IV, all the data were noted to be in the range

7^ to 80 dB. The upper portion of Table IV s^ows that the mean square pressure

should be in the range 0.01 to 0.0^ (pascals) . The lower (expanded) scale

permits easy, accTirate interpolation to find values more precisely.

If equation 1— had been used to calculate the values (very easily

handled with a pocket calculator that includes log and power (x ) functions),

the calculation would go as follows:

Eqtiation numbers refer to the equations given in Section k.9

Calculation Procedures.

^1 =

^1

=

78.5

57

.028

79.5

^9

.036

77.5

35

.023

76.5

51

.018

79.5

51

.036

20
p(l) = io(T9.5/lO)-9.1. . ioT-95-9.U ^

^^-1.1.5

= 0.035^813389

with similar simple calculations for the remaining values.

Averaging (adding up and dividing by eight) gives:

The measurement surface area is calculated from equation (3) as follows

.

The sound power is computed from equation (U). Since temperature and barometric pressure are within the acceptable range, the value of C is unity and

rounded to the nearest dB. This number is expected to be within +^.0 dB of

the true sound power with 95^ confidence. An independent, more acciirate

measurement gave equals 98 •? dB.

If the compressor rating were in terms of average sound level at ten

metres from the center of the source, this could be computed using equation (6) .

w = 2.5 (i)(6t.o6)(o.028)

= U.69 milliwatts

The sound power level from equation (5) is

L = 10 Log {h.69 + 90)

= 6.1k + 90

= 97 dB

L (r) = 97 - 8 - 20 Log 10

P =97-8-20

= 69 dB.
h. PORTABLE AIR COMPRESSOR MEASUREMENT METHODOLOGY

k.l. Purpose and Applicability

This measurement methodology defines an acceptable procedure by which

the noise from portable air compressors may be measured to ascertain whether

the noise emitted is in compliance with the noise emission standards promulgated by the U. S. Environmental Protection Agency pursuant to Section 6 of

the Noise Control Act of 1972 (Public Law 92-57^). The method is applicable

to air compressors of a portable type intended for outdoor use. The measurement method provides for the determination of the A-weighted sound power

output of the device in the free-field over a reflecting plane based on A-weighted sound level (SPL) measurements determined on a measurement

surface one metre from the surface of the machine.

\ k.2. References

The following standards are referenced within this methodology.

a. American National Standard Sl.U-1971 "Specifications for

SoTond Level Meters".

' "b. International Electrotechnical Commission Publication 179}

"Precision Sound Level Meters".

c. American National Standard SI. 2-1962 "The Physical Measurement

of Sound".

d. American National Standard SI.I-I966 "Specifications for Octave

Half Octave and One-third Octave Filters".

' ;

" 'ev American National Standard SI. 13-1971 "Methods for the Measurement of Sound Pressure Levels".

. " ^.3. Measurement Uncertainty

The achievable uncertainty for a measurement of A-weighted soumd power

level of a portable air compressor according to this methodology using

commercially available instrumentation meeting the requirements of Section

i|.5 is estimated to be a precision of 2 dB (one standard deviation with

95% level of confidence) with negligible bias.

h.k. Acoustic Environment —

. ' - a. General Requirements

The test site shall be such that the compressor radiates soiind into a free field over a reflecting plane. This condition may be considered fulfilled

if the test site consists of an open space free of large reflecting surfaces

within the distances specified below from any microphone position or equipment location (See U.2(b)). The minimum measiirement area shall consist of a

22
circular, flat, hard surface which extends from the source at least 1 meter

beyond the most distant measurement point (see Section U.5.2, Microphone

Positions )

.

b. Criteria for Adequacy of the Environment

1. Reflecting plane . The reflecting plane shall be flat (+0.1 meters),

and of smooth concrete or sealed asphalt, or other hard material. Materials

other than concrete or sealed asphalt shall have normal incidence absorption

coefficient of less than 0.06 over the frequency range 20 Hz to 10 kHz.

2. Outdoor measurement . No large reflecting surface such as a signboard, building, hillside, trees, etc. shall be located vithin 10 meters of

a microphone position or source location.

c. Ambient Noise

It is strongly preferred that the backgroiind ambient noise at the test

site shall be more than ten decibels below the A-weighted soiind levels of

the unit under test, especially if the noise is fluctuating. In no case shall

a site be used in "which the maximum background ambient noise is within four

decibels of the levels to be measured. The background ambient noise shall be

recorded before the start of the test. In the event that the average ambient

background noise is within ten decibels of the measured sound levels, the

ambient backgroiond noise after the test shall be recorded, and the smaller of

the before and after levels shall be Included in the calculation of sound

power

.

See also Section U.6.(a) for ambient noise from sources other than the

compressor during compressor operation.

d. Other Ambient Factors

The temperat\are and barometric pressure at the time and place of the

test shall be recorded for reference purposes. No measurements shall be

taken when the wind speed exceeds 5 m/s (l2 mph). (See paragraph U.5(a)

for windscreen requirements.)

Instrumentation

a. General Requirements

The measurement system used shall conform to the requirements of

American National Standard SI. 13 "Methods for the Measurement of Sound

Pressure Levels", Section 5, Field Method, except that the so\md level meter

or equivalent instrumentation shall meet the frequency response requirements

of a Type 1 meter as stated in American National Standard SI.U-I97I "Specifications for Sound Level Meters" over the frequency range 50 Hz - 10,000 Hz.

A windscreen shall be used when the average wind noise is within 20 dB of

the levels to be measured. The windscreen shall not affect the measured

23
A-weighted sound levels from the noise source in excess of +_0.5 dB.

may be experimentally evaluated by comparing the measured A-veighted

level with and without the windscreen, at a position close enough to source that wind noise is not a factor.

, - b . Additional Instrumentation Requirements

a) A sound level calibrator accurate within +_0.5 dB.

b) An anemometer or other device for measurement of ambient

wind speed and direction acciirate within 10% at 5 m/sec.

c) An engine speed indicator, accurate within +2%.

d) An air pressure guage, accurate within +_5^.

e) A thermometer for measurement of ambient temperature accurate

within +1°C.

• f) A barometer for measurement of ambient pressure accurate within

+1%.

"" -'' U.6. Installation and Operation of the Source

- a. Source Operation

The machine under test shall be operated continuously at design f\ill

speed with the compressor on load, delivering its rated airflow and pressure.

The compressed air discharge should be piped clear of the test area or fed

into an effective silencer. The air discharge line shall be provided with

a throttling device remote from the compressor such that no significant

pressiire drop need be maintained at the compressor air distribution valve(s).

The sound level from the compressed air discharge shall be at least 10 dB.

below the machine sound level at all microphone positions. All cooling

air vents in the engine/compressor enclosure shall be full open during all

sound level measurements. Service doors that should be closed during normal

operation (at any and all ambient temperatures) shall be closed during all

sound level measurements.

All sound level measurements reported shall be obtained after the

machine is operating at normal engine temperature.

- - b. Source Monitoring

The compressor speed and air discharge pressure shall be monitored and

recorded at the beginning and at the completion of the sound level tests

.

This

sound

the

2k
h.J. Measurement Procedure

a. Microphone Positions

The measurements are made at eight positions on a measurement surface

which is a uniform distance from the reference surface. The reference

s\irface is the smallest hypothetical rectangular box of dimensions L x W x with top surface parallel to the reflecting plane which encloses the source

The minimxim number of measurement positions shall he eight, including one near the center of each of the four sides of the source and four above the

top of the source near the corners of the measurement surface. The

coordinates of these positions are given in Table I. The measurement

positions are shown in Figure h.l. Note the height for positions 5 to 8 is

determined by the requirement that the distance from the closest point on the reference surface to the measurement point is equal to the measurement

distance r.

Accuracy of microphone positioning with respect to the reference surface shall be r + 0.1 m. Acciaracy of microphone positioning on the measure

ment surface shall be within 0.2 r of the position defined by the three

coordinates of Table I.

The minimum allowable value for r to be used shall be 0.75 m. The

preferred value of r is 1.0 m. The maximum allowable value for r shall be

that value which is consistent with background noise level and test site

dimension requirements given in Section h.h.

The microphone shall be oriented with respect to the measurement

surface, in the manner which it is calibrated for optimum flat frequency

response assuming that the direction of sound field propagation at each

measurement location is perpendicular to the measurement surface at the

measiirement position. Observers shall be at least one meter away from the

microphone, and r + 1 meters away from the measurement surface.

The time average A-weighted soimd level using the "slow" response mode

of the sound level meter shall be determined to a precision of +0.5 dB at

each measurement location with the compressor operating and the compressor

off.

If the range of the eight sound-level values exceeds 8 dB for the

operating compressor condition, additional sound level data shall be

recorded at eight additional positions. The coordinates of the additional

positions shall be as given in Table lA.

h.Q Information to be Recorded

The maximum steady observed A-weighted sound level using the slow

response characteristic, shall be recorded for each measurement location to

the nearest 0.5 dB.— The average A-weighted background noise levels shall

be recorded at all locations after the measurement, if within ten decibels

of the measured value.

If levels are resolved to the nearest 0.1 dB, then levels in the range

X.8 to y.2, shall be recorded as y.O; values in the range x.3 to x.7

shall be recorded as x.5 (y = x + l).

25
Measurement surface Reference parallel piped

x9

2 14

_|_X (S X--

14 15 T 13 16

Measurement surface Reference parallel piped

O Key measurement position

X Secondary (additional)

^ measurement position

Figure h.l. Location of measurement positions on the measurement surface.

2t
TABLE I

Microphone Position Coordinates

Source reference surface dimensions are L, W, H.

L W a=2+r, b=2+r, c=H+r

1 r ^1 ~ "2^^ * - ' if is greater than H then take h^ = H

1 r L

a^ = —(a + —), if a^ is greater than — then take a^ = L/2

b = ^(t) +•!•), if h is greater than b then take b = b

Position Number ' _ X Y

0

z

'-'p — —~"

'

1

] Dist. from

reference surface

1 a \ r

2 0 b h r

3 0 h

r

i

i ^

i

0 -b r

1 5 \ greater than H r

i 6

! \

greater than H r

T -\ greater than H r

8

* - . — .„ ^1 -\ greater than H r

Origin for the

the geometric

coordinate system is

center of the soiarce.

the point on the ground plane under

2T
Position Number

9

10

11

12

13

li^

15

16

TABLE lA

Coordinates of Additional Positions

X Y Z

-a.

a^

-a.,

-a.

a.

0

0

b

b

-b

-b

greater than H

greater than H

greater than H

greater than H

Distance from

reference surface

r

r

r

r

r

r

a„ = ^(a

"I")

» if greater than a, then take a^ = a

28
TABLE II

EXAMPLE DATA FORM FOR MEASUEEMENT OF SOUND FROM PORTABLE AIR COMPRESSORS

Manufacturers Test Report No. Date of Test

SUBJECT

Model: Manufacturer: Serial No:

Rated Speed & Capacity:

Description:

Dimensions (2.,w,h)

TEST CONDITIONS:

Manufacturers Test Site Identification

Test Radius (r) for Measurement: meters

Distance from Observer to Microphone: .meters

Operating Speed as Tested: (Beginning) rpm

(End of test) rpm

Reflecting Plane Composition:

Ambient Temperature: ^°C Ambient Maximum Wind Speed During Sound Level Measurements: _km/h

Wind Direction (see Sketch Following Page):

Atmospheric pressure: mmHg

Remarks

:

INSTRUMENTATION:

Microphone: Ser. No.

Sound Level Meter: Ser. No.

Calibrator: Ser. No.

Other: Ser. No.

SOUND LEVEL DATA

A-weighted Sound Level (dB re 2 x 10 pascal)

Position

1 2 1 3 h 5 6 7 8

Full

Load

Condition

Pos 1-8 t

j Pos 9-l6

\ Background

(compressor

off)

Before

(pos 1:;:8)

I

(

t

-i After

(pos 1-8)

i

Tested by Date

29
TABLE II (Continued)

Sketch: Indicate source orientation, wind direction.

Computed Measurement Location Coordinates '

'

a = (L/2 + r) meters observed value of height for

positions 5-8 meters

b = (W/2 + r) meters observed value of height for

positions 13-l6 meters

c = (H + r) meters

1 r ! h^ = {-^is. + b - —) ) meters (Not greater than H)

1 r a^ = (2"(a- "•" 2')) meters (Not greater than L/2)

^1 ~ ^2"^^

l"^ ^ meters (Not greater than b)

a = (•^•(a + ^) ) meters (Not greater than a)
The additional data identified in Table II shall be recorded.

A calibration history of all equipment used, meeting the requirements

of American National Standard SI. 13-1971, Section 5 shall be maintained.

h.9- Calculation Procedures

The average A-weighted sound level on the measurement surface, and the

A-veighted sound power level shall be computed in a manner similar to that

shown in Table III. Use of the specific format shown is not required.

The steps of the computation of sound power level are as follows

.

(1) Determine the corrected sound pressure (or pressure level).

(2) Compute the average mean square press\ire for the measurement

surface

.

(3) Compute the area of the measurement surface.

(h) Compute the sound power for the measurement surface from the

product of the mean square pressure and the area of the surface.

(5) Compute the sound power level from the logarithm of the soimd

power.

The equations which may be used for this calculation are shown below.

(1) Corrected sound pressure.

The corrected mean square sound pressure at a measurement position may

be calculated using the following equation.

2

where p(i) is the mean square pressure at position i (pascals )

L (i) is the sound level of the source with background noise at ^ position i (dB)

L' (i) is the sound pressure level of the background noise at posi-

^. tion i (dB).

(Table IV may be used to convert pressure level to squared pressure.)

If the background noise sound level at a position is more than ten

decibels below the source sound level then the correction will be negligible.

31
( 2 ) Average sound pressure.

The average mean square sound pressure (P) may be calculated from

the equation

, p ^

(P(l) + P(2) + ... +P(n))

( 3) Measurement surface area.

The measurement surface area is calculated using the equation:

. . ... S = 2 LH + 2 WH + LW + TTr (2H + L + W) + 2TTr^

where S is the area of the measurement surface (square meters)

L,¥,H are the source reference surface length, width and height

respectively (metres)

r ' is the distance from the source to the measurement surface

(metres)

{h) Sound power. v i*;; :-:.'^(yr 6;,^f';n =tdv

The sound power is computed from the equation

W=2.5xCxSxP

avg

where W is the sound power (milliwatts)

1+00 •"'•-'^i'^^-

C is the value of . .lO i- .u vtot^oi

pc '

-

where p is the density of air at the time of the test (kg/m ) and c is

the speed of sound in the air at the time of the test (m/s). (See

note below.

)

S is the measurement surface area from eq. 3.

P is the average sound pressure from eq, 2.

Note: If the test temperature is in the range -15°C to hO°C and the barometric

pressure at the test site is in the range 680 - T80 mm Hg the value for C shall

be taken as 1.

'

. . Outside this range the following expression shall be used for C.

^LO , T^ ^ .

. ^ B 586

'

Where B is the barometric pressure (mm Hg)

T is the air temperature (degrees Celsius)

32
(5) Sound power level.

The sound power level is calculated from

L = 10 Log, ^ W + 90 w °10

—12

L is the sound power level (dB re 10 watt)

w

W is the sound power (mW) , from eq. k.

The sound power level shall be rounded to the nearest dB.

( 6) Average sound level.

The average sound level at any distance from the center of the source over a reflecting plane (with no nearby vertical reflecting surfaces) may be

computed from:

L (R) = L - 8 - 20 LogR dB

p w

°

R Is the distance from the center of the source (meters).

33
-

• TABLE III

EXAMPLE FORM FOR SOUND POWER CALCULATION FOR NEAR FIELD MEASUREMENTS

Manufacturers Test No. Test Date

Machine Identification

Test source dimensions

Test radius (r)

Test surface area S = (2LH + 2 WH + LW + + Trr (2H + L + W) + 2 Trr ) Barometric Pressure (B) . >. Temperature (T)

Number of measurement positions

L

¥

H

m

m

m

m

m

jnmHg

C° n Average A-weighted Sound Pressure

-Position A-weighted SPL

(dB)

A-weighted

^ (Sound Pressure)

Corrected ^

1

(Sound Pressure)

(l) Source

On

(2) Source

Off

(3) Source

On

{k) Source

Off

(3) - ih)

1

2

3

1+

5

6

7

8 _

8.

9.

10.

Sum of (Sound Pressiore) position 9-l6 (Enclose tahle)

Sum of (Sound Pressure) (sum of table plus Line 8)

_2

P^ = Average Soiind Pressure (line 5 divided by n)

3k
TABLE IV

CONVERSION OF WEIGHTED SOUND PRESSURE LEVEL TO SQUARED SOUND PRESSURE (pascal

;

A solution of the equation: P =10 (pascal)^

avg

PRESSURE

LEVEL (dB)

100 Jh

:: 2

1

90 O.k

0.2

I 0.1

01+

02

01

80

1-

}

TO k.io .ooh)

1r

6o ^ h.io - 2

t 1

•i

-k

SQUARED

PRESSURE,

(pascal)

( .OOOii)

50 -^I+.IO ^ ( .OOOOii)

EXPANDED SCALE (TYPICAL FOR ALL 10 dB INTERVALS)

PRESSURE LEVEL

23i+56T89 t- H 1- 10

-I h .5 ,8 1 1.25 1.5 2 2.5 3 1+

SQUARED PRESSURE

35
5 . ACKNOWLEDGEMENT

The author recognizes the significant contributions made hy members

of the NBS Task Group which assisted in the formulation of earlier drafts

of the measurement methodology and indicated some of the significant

problems in need of investigation in the experimental program. Members of

the task group included:

NAME : p ORGANIZATION

, . Worthington CEI , Inc

.

f Ingersoll-Rand Co.

i^ Atlas Copco , Inc . '4-09 Schramm Inc.

i ' Gardner-Denver Co.

Dames & Moore

Gordon Smith Co.

Mr. Harry Baker

Mr. George M. Diehl

Mr. R. S. Geney

Mr. L. T. Hayes

Mr. F. Wm. Heckenkamp

Mr. F. M. Kessler

Mr. Richard Ostwald

36
6 . BIBLIOGRAPHY

[I] Kearney, A. T., Inc., A study to determine the economic impact of

noise emission standards in the construction equipment industry —

Draft portable air compressors report, Contract No. 68-01-15^2 (U.S.

Environmental Protection Agency, Washington, D. C, December 1973).

[2] CAGI-PNEUROP Test Code for the Measurement of Soimd from Pneumatic

Equipment (Compressed Air and Gas Institute, New York, New York,

1969) (Also American National Standard S5.I).

[3] American National Standard Test Code for the Measurement of Sound

from Pneumatic Equipment, S5. 1-1971 (American National Standards

Institute, New York, New York, 1971); International Organization for

Standardization Recommendation for the Measurement of Airhorne Noise

Emitted by Compressor/Primemover Units Intended for Outdoor Use,

R215I-I972 (American National Standards Institute, New York, New

York, 1972).

[h] Work by International Organization for Standardization Technical

Committee h3 (Acoustics)/ Subcommittee 1 (Noise) /Working Group 9

(Pneumatic Equipment).

[5] American National Standard Method for the Physical Measurement of

Soiond, SI. 2-1962 (revised 1971) (American National Standards

Institute, New York, New York, 1971).

[6] International Organization for Standardization Draft Engineering

Method for the Determination of Sound Power Levels of Noise Soiirces

for Free- Field Conditions over a Reflecting Plane, DIS-37^^

(American National Standards Institute, New York, New York).

[7] For example: Hubner, G. , Analysis of errors in measuring machine

noise \mder free-field conditions, J. Acous . Soc . Am. 5^( U) , 967-977

(1973).

[8] Holmer, C. I., Procedures for Estimating Sound Power from

Measurements of Sound Pressure, to be published as an NBS report,

IR-652/EPA 550/8-76-001.

[9] International Organization for Standardization Recommendation for

Acoustics, Assessment of Noise with Respect to Community Response,

RI996-I97I (American National Standards Institute, New York, New

York, 1971).

[10] Tempest, W. , Loudness and annoyance due to low frequency soiand,

Acustica 29., 205-209 (1973).

[II] Private communication, W. Patterson (Bolt Beranek and Newman, Inc.,

Cambridge, Massachusetts) to C. I. Holmer (National Bureau of

Standards, Washington, D. C), April 197^. Data were recorded

during the preparation of reference [13]. Also from data supplied

by members of the compressor noise task force.

37
[12] Galloway, ¥. J., et al. , Highway noise — Measurement, simulation and

mixed reaction. National Cooperative Highway Research Report No. 78

(NAS-NRC Highway Research Board, Washington, D. C, I969).

[13] Patterson, W.N. , Ely, R. , and Huggins , G., Portable air compressor

noise measurement. Report No. 2795a (U. S. Environmental Protection

agency, Washington, D.C. , March 197^)-

"1.

'

38
APPENDIX A

PROCEDURE FOR PREDICTING A-WEIGHTED SOUND LEVEL FROM A-WEIGHTED SOUND POWER

A.I. Introduction

This appendix is intended as a guide for users to estimate the A-weighted

sound level to be expected from a portable air compressor in practical

situations under rated operating conditions. The following applications

are considered: (l) estimating the A-weighted sound level at various distances from a portable air compressor operating in a relatively open area,

(2) estimating the A-weighted sound level at various distances from a portable air compressor operating in a confined space, such as in an alley

or on a street between tall buildings, and (3) estimating the effective

A-veighted soiind level reduction due to introducing a "noise shield" or barrier vail between the portable air compressor and the observer. This

appendix is only concerned "with the relative A-weighted somd level contribution from one specific source. To determine the total A-weighted sound level

from multiple sound sources, refer to American National Standard SI.2-I962.

A. II. Sound Level Decrease With Distance

• The following procedure is to be used for estimating the A-weighted

so\ind level from the geometric center of a portable air compressor, operating

\mder rated conditions more than ten meters (33 feet) away. For this situation it is assumed that the area including both the portable air compressor

and the observer is relatively free of large sound reflecting surfaces (except

the ground), such as buildings or berms , and there is a direct line of sight

from the observer to the air compressor.

From the A-weighted sound power level of the source (A) , determine A ^dB the corresponding A-weighted sound level for the reference distance -28 dB

of 10 meters (B), by subtracting 28 dB from (A). B_ dB

Measure the direct distance from the geometric center of the air

compressor to the observer (in meters) Cj m

Using Nomogram No. 1 read the corresponding sound level correction Dj ^dB

Subtract the value fo\md in Step D from that found in Step B. X: ^dB

The final result above assumes no excess sound attenuation due to atmospheric conditions which may affect the actual A-weighted sound level at

distances greater than about 100 meters (330 feet).

39
NOMOGRAM NO. 1

A-WEIGHTED SOUND LEVEL DECREASE WITH DISTANCE

Distance From

Center of Air

Compressor in

Meters

c D

1000-T-40

-39

-38

-37

-36

-35

500- WWW -34

-33

-32

-31

-30

-29

-28

-27

200- -26

-25

-24

-23

-22

-21

100- JU w w -20

-19

-18

-17

-16

-15

-14

-13

-12

-11

30- -10

-9

-8

-7

20- -6

-5

-4

-3

-2

-1

10-^0

D: A-weighted Sound

Level Reduction

(dB)

ho
NOMOGRAM NO. 2

APPROXIMATE SOUND LEVEL ATTENUATION USING A BARRIER

P = PATH LENGTH DIFFERENCE) = A' + B' - C (Meters)

PATH LENGTH

DIFFERENCE (METERS)

(MULTIPLY BY 3.3

TO OBTAIN P IN FEET)

0

.03^

.1

.2

.3

• 5

2.0

l+.O

6.0

5

7

9 - 11

13

t 15

IT

1.0

G. A-WEIGHTED SOUND

LEVEL DECREASE DUE

TO BARRIER ATTENUATION

(dB).

19

21

23

25

PRACTICAL LIMIT OF

BARRIER ATTENUATION

hi
A.m. Sound Level Reduction From Barriers

The following procediire can be used for estimating the minimum

attenuation due to a barrier, or the height of a barrier to be constructed, or the distance the portable air compressor or observer must be from a barrier to

achieve a specified A-weighted sound level reduction. (The A-weighted

spectrum of the operating compressor is assumed to have maximum energy near or above 500 Hz for this noise reduction estimating procedure.) The length of

the barrier is assumed to be at least tvo times the barrier height or two

times the largest compressor (source) dimension, whichever is greater.

A. To determine the attenuation for a given barrier height and compressor

location, we must determine the effective path length for sound propagation

over the barrier.

The effective height of the barrier is the height of the top of the

barrier above the line of sight from the top of the compressor to the

observer. The barrier changes the sound level by blocking the sound and

forcing the sound to "bend around" the top of the barrier. The amoimt of

bending is measured by the "path length difference" (P) between the direct

path (if there was no barrier) and the distance along the shortest path over

the top of the barrier. If the compressor and the observer have approximately

the same elevation, then a rough estimate of the path length difference is

calculated by:

Measuring the height of the barrier above the top of the compressor H jn

Determining the longer of the distance from the barrier to the

compressor or barrier to the observer > R m

Calculate P = H x H/R .

,^ " .

- P m

If the compressor and observer are on significantly different elevations,

then the simplest method is to make a scale sketch of a section showing the

top of the compressor, top of the barrier and the observer, and measuring the

dimensions A', B', and C indicated in the nomogram. P is simply computed from

these dimensions.

With the value of P, the nomogram is entered to find the A-weighted level

difference (G). The sound level with the barrier is then

A-weighted sound level at observer = X - G = dB -

Note that the barrier provides some attenuation even if it just meets the

line of sight (but this attenuation reduces rapidly toward zero within a short

distance above the barrier)

.

B. To determine the height of a barrier to be constructed to achieve a given

A-weighted sound level reduction:

Measure the compressor to barrier distance in meters or the observer to barrier distance and enter whichever is

larger m

h2
Note the desired. A-weighted reduction dB

Note the path length difference indicated for the desired

level reduction P m

Enter the line of sight distance C m

Estimate the effective height of the barrier Heff C X P/2 Heff m

Estimate the elevation of the line of site (H, ) above the

ground at the proposed barrier site. (Use scale sEetch if

necessary) H

es- m

The total height "of the barrier is H

eff

+ H

Is

m

This is the total barrier height necessary to achieve the desired

"A-veighted reduction" assuming no other large sound reflecting surfaces in

the vicinity of the compressor.

barrier is midway between the compressor and observer. To check the result,

perform the calculation described in A above.

The above procedure for predicting noise reduction effectiveness of a barrier placed near a compressor assumes that both the compressor and the

observer are in a relatively open, level area. In many practical situations

this is not the case and the effectiveness of the barrier will be somewhat

reduced because of sound reflecting off of other surfaces. Also, if the Aweighted sound level reaching the observer before installation of a barrier is

determined by low frequencies (such as the fundamental firing rate of the

engine) due to an inadequate or poorly maintained compressor engine muffler

the barrier will provide less A-weighted reduction.

A . IV . Summary

This appendix is a simplified application guide for estimating noise

level from sound power rated portable air compressors. The guide can prove

very useful for determining whether the sound level of the compressor is in

compliance with local or state noise control regulations. Alternately, the

g\iide may be used to determine what techniques are available to the user of a portable air compressor for reducing the compressor sound level exposure at

specific locations of interest.

The value of H is the approximate maximum height required if the

USCOMM-NBS-DC
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. iniHLK A l lON OH RIvPOR I' NO,

NBSIR 75-653

2. Gov't Accession

No.

3. Recipient's Accession No. EPA 550/8-76-002

4. TITLl'; AND SUHTII IJ-; Measurement Methodology and Supporting Documentation for

Portable Air Compressor Noise

5. Publication Date Jan. 1975

6. Performing Organization Code

7. AUTHOR(S) ^ ^ ^ „ . Curtis I. Holmer

{y^^-j^rny^g_^^n. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

^ . ' DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2130152

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Joint NBS and Environmental Protection Agency (EPA)

Office of Noise Abatement and Control

Washington, D. C. 20460

13. Type of Report & Period _ . Cov-ercd Final report

Feb. 1974 -Dec. 1974

14. Sponsoring Agency Cod<-

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-worcl or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) This report presents recommendations and supporting

rationale on a measurement methodology for portable air compressors. The methodology

provides for the determination of A-weighted sound power level or the equivalent weighted

sound pressure level at a reference distance. A-weighted level is used because of its

correlation with community response to noise from internal combustion engine noise. It

is recommended, however, that the spectra associated with the regulated source be monitored in some manner to insure that the spectra remain similar to those for which

^-weighted sound level retains good correlation with community response. The methodology

uses weighted sound level measurements at eight positions on a curved surface surrounding the source at a distance of one metre from the surface of the machine. Data recorded

at these positions are used to calculate the average weighted sound pressure level of the

nachine on the measurement surface. This is combined with the area of the measurement

surface. This is combined with the area of the measurement surface to give the sound

jower level of the machine. From this value, a rating sound pressure at a rating distance may be calculated by subtracting a constant value. Procedures which permit the

rapid estimation of A-weighted sound level are included. These are applicable for estimation of A-weighted sound level in a variety of circumstances when the sound power

jr equivalent sound pressure level at a reference distance is known.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Acoustics; air compressor; internal combustion engine; noise; sound power level;

sound pressure level.

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I

' For Official Distribution. Do Not Release to NTIS

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