OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT

NATIONAL DEFENSE RESEARCH COMMITTEE

The Design and Construction of

Anechoic Sound Chambers

Report of October 15Z 1945

O.S.R.D. NO. 4190

ELECTRO-ACOUSTIC LABORATORY

CRUFT BUILDING-HARVARD UNIVERSITY

CAMBRIDGE 38, MASSACHUSETTS

ELECTRO-ACOUSTIC LABORATORY

Cruft Building

Harvard University

Cambridge jQ, Massachusetts

INITIAL MAILING- LIST

THE DESIGN AND CONSTRUCTION OF ANECHOIC SOUND CHAMBERS

October 15, I9U5 - OSRD No. U190

0.S.R.D«

Dr. Irvin Stewart, Executive Secretary

N.D.R.C. (Section 17.5)

Dr.. Harvey Fletcher, Section Chief

Dr. P. M. Morse, Project Supervisor

Dr. C. T. Morgan, Technical Aide

Section Members:

Dr. Hallowell Davis

Dr. F. A. Firestone

Prof. V. 0. Knudsen

Dr. S. S. Stevens

NAVY DEPARTMENT

Bureau of Snips, Shipbuilding Division

Att: Capt. J. B. Duval

ORI, Planning Division, EXOS

ORI, Naval Research Laboratory

Att: Director

Att: Weiant Wathen-Dunn

U.S. Navy Yard, Washington, D. C.

Naval Ordnance Laboratory, Washington

Navy Yard

Att: C. F. Langford

Naval Air Material Center,

Naval Air Experimental Station,

Aeronautical Radio and Radar Lab

WAR DEPARTMENT

ATSC, Engineering Division

Att: Mi’. J. E. Wilson. TSERR2B5

Att: Dr. 0. R. Rogers, TSEAL-2E

Squier Signal Laboratory,

Special Equipment Branch

Att: SPSGS-SSE

BRITISH

Advisory Comm, on Army Tele. Instrumente

Att: Mr. W. G. Radley

Signals Res. and Dev. Establishment

Att: Chief Superintendent

S.J. Gray, Director of Signals and

Radar Research

AUSTRALIAN

Radio-Physics Laboratory

Research Laboratories, PostmasterGeneral's Department

National Standards Laboratory

CANADIAN

RCAF, Otológica! Laboratory

Att: Flight Lt. R„ W. White

National Research Council

U.S. Department of Commerce

Bureau of Standards

Att: Dr. R. K. Cook

OSRD Report No. 4190

Copy No.

OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT

NATIONAL DEFENSE RESEARCH COMMITTEE

DIVISION 17 SECTION 5

Contract OEMsr-658

THE DESIGN AND CONSTRUCTION OF ANECHOIC SOUND CHAMBERS

(Relevant to Service Control Nos. NA-108 and. AC-9)

OSRD Report No. 4190

October 15, 19^5

ELECTRO-ACOUSTIC LABORATORY

Cruft Building

Harvard. University

Cambridge, Massachusetts
SUBMITTED BY

L. L. Beranek

H. P. Sleeper, Jr.

E. E. Moots

APPROVED FOR THE CONTRACTOR

L. L. Beranek, Director

Electro-Acoustic Laboratory

Cruft Building

Harvard University

APPROVED FOR HDRC

Harvey Fletcher

Chief, Section 17.5

P. M. Morse

Project Supervisor

iv
THE DESIGN AND CONSTRUCTION OF ANECHOIC CHAMBERS

Table of Contents

Page No.

I• Introduction 1

II. Summary 5

A. Historical Background 5

B. Rank order of Sound Absorbing Structures (according

to performance) 6

1. Harvard Long Wedge 6

2. Exponential Pyramid 8

5. Exponential Wedge 8

4. Long Pyramid 8

5. Short Pyramid 8

6. Medium Linear Wedge 8

7. Short Linear Wedge 8

8. Blanket Layers 9

9. Sheet Layers 9

C. Generalized Wedge Specifications 14

D. Constructional Features of the Chambers 18

E. Calibration of the Completed AnechoicChambers 18

III. Design of the Chamber Lining 22

A. Fundamental Considerations 22

B. Types of Structures Tested 25

C. Comparative Data 24

1. Sheet Layer Structures 25

2. Blanket Layer Structures 26

5. Stuffed Pyramidal Structures 27

4. Semi-Rigid Pyramidal Structures 28

5. Linear Wedge Structures 28

6. Exponential Wedge and Pyramidal Structures 51

v

Page No.

D. Generalized. Data for the Design of LinearWedge

Structures 51

IV. Calibration of the Anechoic Chambers 52

A. Method of Calibration 52

B. Apparatus 52

C. Procedure 55

D. Data for Largest Anechoic Chamber 54

E. Data for Electro-AcousticLaboratory Anechoic

Chamber No. 2 60

F. Data for Psycho-Acoustic Laboratory Anechoic

Chamber 60

V. Construction of the Large Anechoic Chamber and Control

Rooms 66

A. Site 66

B. Sound Chamber 66

C. Additional Featuresof theAcoustical Laboratory 69

1. Control Rooms 69

2. Service Apparatus Room 69

5. Sound Lock and Suspended Track 71

4. Storage Room, Supply Entrance and Corridor

to the Cruft Laboratory 72

D. Installation of Acoustical Treatment 72

1. Main Support for the Wedges 72

2. Auxiliary Chain Supports 78

5. Installation of Wedges on the Floor 78

E. Interior AcousticalDoors 78

VI. Fabrication of the Acoustical Wedges 84

VII. Constructional Features of the Small Electro-Acoustic

and Psycho-Acoustic Anechoic Chambers 92

Appendix I Absorption Measurement 97

Appendix II Flow Resistance Measurement 99

vii
INTERIOR VIEW

OF LARGE ANECHOIC CHAMBER

FIG. I

vili
THE DESIGN AND CONSTRUCTION OF ANECHOIC* SOUND CHAMBERS

I. INTRODUCTION

During World. War II, the National Defense Research Committee

sponsored, a program of acoustical research at the Cruft Laboratory,

Harvard. University, dealing with many problems of acoustic transmission and. the development of acoustical instruments. In order to

perform the more complex of these problems as efficiently as possible,

it was found necessary to construct acoustical chambers in which

echoes were reduced to very low values.

One of these chambers, having internal dimensions before addition

of acoustical treatment of 58 x 50 x 58 feet (See Fig. 1), was designed to permit the investigation of large directional devices which,

in normal use, would receive sound from sources located at a distance

of several hundred feet in free space. In this room a source of sound

located approximately 20 to 30 feet from the device under test will

establish a reasonably plane sound wave over an area of approxlmat.ely

20 square feet. Similar freedom from reflecting surfaces is to be

found only in open space a thousand or more feet above the earth, and

it was in order to solve a sound transmission problem under such circumstances that the chamber was built. Moreover, the chamber is

valuable because the research can proceed independent of weather conditions .

Two smaller chambers having dimensions before treatment of approximately 11 x 15 x 10 feet and 12 x 20 x 12 feet were designed to

assist in the development of microphones, handsets, earphone cushions

and in the studies of the acoustical properties of gas and oxygen

masks.

In all, the specific problems for which these anechoic chambers

have been used are as follows:

1. Determination of the response and directional characteristics of acoustical locating devices, microphones

and loudspeakers.

+ No adjectival word exists in the English language meaning ’’free from

echo.” In an attempt to avoid the customary expressions, "dead roam,"

"highly absorbent sound chamber," "absorptive chamber," "free-fleld

roam," etc., the word anechoic was devised. The word is made up of

the Greek prefix an- meaning not or without, the Greek word echo

meaning echo and the adjectival suffix -ic, meaning characterized

by. We prefer to pronounce it e_oh6 _io fa * Ik).
2. Development of loudspeakers and. intense noise sources.

5. Measurement of the response and. directional characteristics of outdoor announcing systems.

1|-. Precise free-field calibration of microphones.

5. Tests and calibrations of microphones, loudspeakers,

voice tubes and other electro-acoustical equipment.

6. Performance of auditory measurements dealing with

the determination of the threshold of hearing, the 

loudness and pitch of sounds, and the precision of

■binaural localization.

The first part of this report is a lengthy summary of the research

program. Following the summary is a section devoted to a chronological

review of the experiments which led to the development of the acoustical

lining that was finally selected for the largest chamber. In Section IV

the behaviors of the finished chambers are compared with the behaviors

of the more important of other anechoic chambers known to be in existence.

Section V deals with the construction of the building in which the largest

of the three chambers is housed, while Section VI describes the methods

of fabrication of the wedge lining. Finally, certain constructional features of the smaller chambers are discussed.

-2-
II. SUMMARY

A. Historical Background.

Before discussing the previous literature, a brief description of

the apparatus used for obtaining the experimental data is given.

To measure the sound absorbing characteristics of the structures,

a tube method using principles described in the literature^ was employed. The structure to be tested is placed at one end of a steel

tube 15 feet long and 8 inches square, shown in Fig. 59 of Appendix I.

A loudspeaker is located at the opposite end of the tube and produces

a sound wave which travels down the tube and is reflected from the

structure under test. If the absorption is not perfect, then the reflected wave will combine with the incident wave to form a standing

wave pattern. This pattern is examined with the aid of a moving microphone and the maximum and minimum sound pressures are determined and

used for calculation of the percentage sound reflection. The tube

measurements cover the frequency range between 50 and 1500 ops. Other

data were taken in a reverberation chamber to determine the percentage

reflection in the frequency range between 1500 and 4000 cps. For frequencies above 4000 cps, it is believed that the acoustic Impedance at

the surface of the material should be made as near 42 pc units as

possible based on the assumption that because of the thickness of the

material, no reflection will occur from the rear surface. The degree

to which this requirement at high frequencies is met can be determined

from calculations based on a knowledge of the specific flow resistance

of the material.

In expressing the performance of the acoustical structure under

test, it has been decided not to adopt the conventional quantity,

percentage sound energy absorption, as an index of the absorbing efficiency. The percentage sound energy absorption is defined as 100

times the, ratio of the sound energy absorbed by the structure to the 

sound energy incident upon it. Instead it was decided to plot the percentage sound pressure reflection, which is defined as 100 times the 

ratio of reflected sound pressure to the incident sound pressure for

sound normally incident on the structure. This choice of ordinate is

desirable because the region between 99 and 100 per cent energy absorption

corresponds to 10 to 0 per cent pressure reflection respectively. Thus

using the latter scale, one obtains a more sensitive indication of differences among highly absorbent structures.

+

H. J. Sabine, ’’Notes on Acoustic Impedance Measurement,” Jour.Acous.

Soc.Am., 14, 145 (1942)

-5-
In this report, th© percentage sound pressure reflection appears

as the left hand ordinate of all the pertinent graphs and is plotted

linearly with 0 per cent reflection at the bottom. See the left hand

ordinate for Structure A, page 10 as an example. Analytically, the 

percentage pressure reflection R, is related to the percentage energy

absorption A, by the expression R = 10 / 100 - A. For convenience in

interpretation, the percentage energy absorption is indicated on the 

right hand ordinate scale on the graph, and as can be seen, is not

linear with respect to R.

Historically, two essentially different types of sound absorbing

structures have been described in published papers. The earlier of the 

two papers was presented by E. H. Bedell^ the Bell Telephone Laboratories in 1956. He described a treatment consisting of ten sheets of

muslin and six sheets of flannel stretched and subspaced -within a

depth of 18 1/2 inches from the wall. The acoustical behavior of thia

structure is given on page 10, Structure B as interpreted from Bell

Laboratories' data. A diagram of the structure and a list of component

parts accompanies the graph. Further details on the properties of the

materials are given in Table I on page 24. The reflection coefficient

is seen to decrease from a value of 25^ at 100 cps to a value of 15^ at

150 cps and averaging approximately 16^ from 120 to 500 cps. Above

1000 cps the reflection coefficient is 3^ or less.

The second paper was published by E. Meyer, G. Ruchmann and

A. Schoch^"*- of Berlin in 1940 and the approach used is of a radical ly

different nature. Their anechoic chamber was lined with thousandr of

acoustical stalagmites and stalactites projecting from the wal1 r ,

ceiling and floor. These acoustical units consisted of long pyramidshaped muslin bags stuffed with loose rock wool to a density of about

12 Lbs/ft< Sagging was prevented by placing a wooden lath in the 

center of the structure. The base of the pyramid consisted of a triangle of plywood board covering slightly more than one half of the 

area and so shaped that it received the lath to form a rigid unit whlnh

could be hung on hooks at the wall. The remaining area of the base was

covered with perforated cardboard to hold the rock wool in place. The

pyramid was 1 meter long and 15 centimeters square at the bottom and

Included a prismatic base section 15 centimeters high whl ch butted

tightly against the neighboring pyramids. Thirty-two thousand of these

+ - -------------------------------—--- -------------------

E. H. Bedell, "Some Data on a Room Designed, for Free-Field. Measurements,” Jour .A cous. Soo .Am., 8, 118 (1956)

+ Meyer, Buchmann and. Schoch, "A Novel, Highly Effective Sound.-Absorbing

Arrangement and. the Construction of a Dead. Roam,” Akustische Zelts.,

5, 552 (1940). Review in English by Young and. Schuck, Jour.Acous.Soc.

Am., 15, 191 (1941).

-4-
units were used to line the chamber. The walls, but not the floor and.

ceiling, were built of hollow tile blocks which effectively formed, an

additional air space behind, the pyramids.

Data were not given in the Berlin paper on the experimental behavior

of a pyramid of the exact dimensions which they used in their chamber.

However, based on the data which they did present and on data which we

took on a structure made as similar to theirs as possible, the reflection

characteristic is shown on page 10 structure C for comparison with the

sheet layer structure previously described. For this pyramid, the reflection coefficient is 5^ or less for frequencies between 150 and 2000

cps and rises to 10^ at about 115 cps.

It is obvious that the behavior of this pyramid structure is

superior to that of the sheet layer structure. However, it is both difficult and expensive to construct, and it does not absorb well at frequencies below 150 cps. According to Meyer, it took eleven worlmen

¡1/2 months to stuff and install the 52,000 pyramids In the Berlin

chamber, the dimensions of which were 50 x 55 x 50 feet. If an eighthour day were adhered to, this should represent about 9500 man hours of

labor, while the wall area covered was 82^ of that of the Harvard Chamber.

By direct proportion installation of the Berlin pyramids in the Harvard

Chamber would represent 11,600 man hours of work. (About 8000 hours were

required for the equivalent work on the Harvard Chamber using the Fiberglas wedges).

Because of the expense involved, it was decided that unless more

efficient manufacturing techniques could be developed, this method of

treatment with its higher acoustical efficiency would have to be abandoned. On the other hand, the immediate alternative of stretching a

large number of sheets of cloth close together over large floor, ceiling

and wall areas, contained many inherent difficulties of installation.

Hence, an extensive research program was undertaken for the purpose of

developing a structure which more nearly meets all requirements.

Since completion of the Harvard Chamber, a paper has been published by H. F. Olson of the RCA Laboratories, giving data on a type

of structure made up of blankets of Ozite mounted perpendicular to the 

side walls and consuming a total depth of 8 feet. No absorption tube

measurements have been made on this structure at Harvard, nor were any

data of this type given in the RCA paper. Data were given, however,

on the properties of the room as a whole, and these are included in a

later section of this report dealing with the calibration of completed

anechoic chambers.

+ H. F. Olson, "Acoustic Laboratory in the New RCA Laboratories,"

Jour.Acous.Soc.Am., 15, 98 (19^5)

-5-
B. Rank Order of Sound. Absorbing Structures according to Performance

The graphs on pages 10 to 15 show the pressure reflection and.

energy absorption coefficients for eleven acoustical structures investigated. as part of the general program; each of which was the best designed,

for its particular type of configuration. With the exception of the

comparisons on page 10, the structures are ranked in order of their

acoustical merit. The curves on page 10 are included to show a comparison between the Harvard long wedge structure and the two structures

previously described in the literature.

On all the graphs the percentage pressure reflection is plotted

as a function of frequency over a range extending from approxlmat.ely

50 to 1500 cps. On the right hand ordinate of each of the graphs, the

two small black lines near 100^ absorption indicate values of 99.5 and

99*9# energy absorption.

Our research has shown in general that the acoustical properties of

a given material are correlated best with its specific flow resistance,

i.e., the flow resistance per unit thickness. A method of measurement

of the resistance to air flow by a material is given in detail in

Appendix II. The apparatus required is sufficiently simple so that

flow resistance tests may easily be made on production samples of material for a check on uniformity.

Unfortunately, the P.F. Fiberglas board used in the experimental

development of absorbing units did not have a constant specific flow

resistance for a given nominal density. The flow resistance character,

istics of P.F. Fiberglas vary with the average fiber size, the orientation of the fibers and the amount of phenol-formaldehyde binder used in

the semi-rigid board. In Fig. 5, page 17 the range of flow resistance

characteristics usually encountered for the Fiberglas material is given

as a function of measured volume density.

The materials used in the experimental structures diagrammad here

are designated only by their nominal densities. The average flow resistance and volume density associated with these nominal densities are

contained in Table I, page 24. In the early experiments, P.F. board

stock having a thickness of two inches was used for all semi-rigid board

structures. For the mass production of long wedge structures the P.F.

stock used was four inches in thickness.

!• NDRC-Harvard Long Wedge Structure, No, A, page 10

This long wedge structure was the best of the wedge structures

developed and the one adopted for use in the large anechoic chamber -

The overall length of the structure is approximately 56 5/4 Inches

excluding the 1 inch of cork insulation cemented to the rigid wall

backing. The main body of the wedge is fabricated from phenol-

-6-
formaldehyde impregnated. Flberglas (P.F. Flberglas), with a nominal

density of 2.5 lbs/ft^. At the base of the wedge are two 1 inch

blankets of P.F. Flberglas with a nominal density of 5»25 lbs/ft5

and a 7 inch square frame of wood. The entire wedge is covered

with a thin muslin bag which stretches around and is stapled to the 

wooden base. The pressure reflection for an average production

wedge is approximately 5^ in the frequency region above 70 cps,

increasing to only 10^ at 70 ops. The spread in performance obtained due to variations among wedges produced on a production

basis is given in Fig. 2. The center curve on this graph is the

average. Actually, the small wiggles in the curves shift about

along the frequency scale more than is indicated here. These curves

represent the reflection coefficients obtained for typical "bad,”

"average" and "good" wedges measured.

HARVARD 57" WEDGE STRUCTURE

FIG. 2

Six inches of the eight inch square base of the wedge are in

the form of a prismatic section and the frame at the base is made

7 inches square so that on Installation adjacent wedges are

compressed tightly together. The 1 inch of cork against the rigid

wall backing is used in the anechoic chamber for heat Insulation

and to prevent sweating of the walls in cold weather. In the laboratory experimental tube, this rigid backing wall is a 1/4 inch steel

plate, while in the building it is 1 foot or more of reinforced concrete .

The P.F. Fiberglas is manufactured in various densities in the 

form of semi-rigid boards 2 feet wide, 4 feet long and in various

thicknesses up to 4 inches. The components of the wedges were cut

from these boards by band saws and put together in such a way as to

form a wedge unit with an 8 inch square base. Complete details of

construction are given in Section VI.

-7-
2. Exponential Pyramidal Structure, No. 62, page 11

An attempt was made to see if a convex exponential pyramid

would be appreciably better than a linear pyramid. The behavior

of this structure approached that of the linear wedge except for

a rise in reflection to 7# at 125 ops. It is possible that more

research on the proper flow resistance characteristics and the

proper rate of taper would yield a better structure for a given

length than the linear wedge. However, the expense of the fabrication, the waste of material and the lack of more time restricted

further research in this direction.

5. Exponential Wedge Structure, No. 65, page 11

The best convex exponential wedge developed from P.E. Fiberglas

was 55 inches long and the pressure reflection averaged about 5#

above 90 ops, which is somewhat greater than that for the 1 inear

shaped wedge.

4. Long Linear Pyramidal Structure, No. 42, page 11

The best of the linear pyramidal structures yielded the percentage pressure reflections shown on the graph. The valurr

averaged about 5# above 150 cps except for a rise to 5# at approximately 275 ops. The percentage pressure reflection increased to

10# at 100 cps.

5» Short Pyramidal Structure, No.. 41, page 12

At one stage of the development, it appeared that a structure

containing a short pyramid backed by a substantial air space might

be satisfactory for adoption. The best short pyramid tested had

a length of 55 inches and was made from P.F. Fiberglas having a

nominal density of 5*25 Ibs/ft^. It was backed by 1 inch of 2.5 

lbs/ft5 p.F. blanket and an air space of 12 inches. The curve obtained is similar in shape to that of the long pyrami dal structure

except for the rise in reflection to 11# at 200 cps.

6. Medium Length Linear Wedge Structure, No. 59, page 12

A linear wedge of 21 inches length was developed for use as a

lining of medium depth for our second anechoic chamber (11 x 15 x 1.0

feet). A cutoff frequency of 150 cps, i.e., the frequency at which

R reaches 10#, was obtained for a total depth of 25 inches.

7« Short Linear Wedge Structure, No. 60, page 12

A third chamber (12 x 20 x 12 feet) was lined with short wedges

to allow for more working space. Those wedges were very similar to

that shown for Structure No. 60. The lack of low frequency absorption

-8-
was compensated, for to a small degree by making the room asymmetrical in shape. This high density structure has a cutoff frequency

of about 250 cps for a depth of 15 inches.

8. Blanket Layer Structure, No. 24, page 1^

The best blanket layer structure developed yielded a pressure

reflection of about 12^ above 90 cps and rose to 20^ at 70 cps.

The total depth, the different materials and the spacings in the

structure are shown in the sketch.

9. Sheet Layer Structure, No. 7, page 15

Measurements on this sheet layer structure were made at the 

Cruft Laboratory, and the results compare well with the data obtained at the Bell Laboratories. Fifteen sheets of cloth, nine of

muslin and six of flannel, are mounted with varying spacing from

the rigid wall backing. The average pressure reflection above 1J0

cps is about 16#, with considerable deviation from the average.

Below 150 cps there is a rapid increase in reflection. Data were

taken on the structure with the cloth sheets stretched and with

them loose. The curve plotted is for stretched sheets and the

performance was found to be superior to that for the unstretched

sheets.

A survey of our data leads to the conclusions that a linear

wedge structure is the most practical structure tested. The pressure reflection coefficient for it is lower than that for a linear

pyramid or any type of blanket or sheet layer structure, and it is

not difficult to fabricate and mount. Structures with an exponential

convex taper, while showing promise, were believed to be so difficult and wasteful in terms of material to manufacture, that a

highly refined design was not developed. The use of P.F. Flberglas

in the fabrication of wedges makes them practical for installation

in chambers of any size.

-9-
HARVARD

LINEAR WEDGE STRUCTURE

STRUCTURE NO. A

E.H.BEDELL-BELL TELEPHONE LABORATORIES

SHEET LAYER STRUCTURE

STRUCTURE NO. B

MEYER, BUCHMANN, AND SCHOCH-BERLIN

PYRAMIDAL STRUCTURE

STRUCTURE NO. C

-10-
EXPONENTIAL PYRAMIDAL STRUCTURE

STRUCTURE NO. 62

EXPONENTIAL WEDGE STRUCTURE

STRUCTURE NO. 63

LONG LINEAR PYRAMIDAL STRUCTURE

STRUCTURE NO. 42

-11-
SHORT PYRAMIDAL STRUCTURE

STRUCTURE NO. 41

MEDIUM LINEAR WEDGE STRUCTURE

STRUCTURE NO. 59

SHORT LINEAR WEDGE STRUCTURE

STRUCTURE NO. 60

12
BLANKET LAYER STRUCTURE

STRUCTURE NO. 24

SHEET LAYER STRUCTURE

STRUCTURE NO. 7

-13-
C. Generalized. Wedge Specifications

Each of the three types of wedge structures used in the anechoic

chambers at Harvard was hand tailored to achieve optimum absorption over

a wide frequency range. In particular, the dimensions of the lining

finally adopted for the large anechoic chamber were not finally fixed

until the five carloads of material were on the siding in Cambridge.

The finishing touches, based on measurements of the material as actually

received, consisted of adding two layers of 1 inch Fiberglas blanket

at the base of the wedges and changing the depth of the air space by

several Inches in order to remove a small peak in reflection which occurred in the vicinity of 180 cps.

If the optimal physical characteristics of the material of the

wedge were known, and if they could be closely enough controlled in

production, there would be no need for final adjustments in design

based upon what actually arrives in the carload shipment. With this

thought in mind, we subsequently entered into a research program designed to yield the optimal dimensions of linear wedges for various

overall dimensions or desired cutoff-frequencies, and to determine the 

flow resistance which should be specified to the manufacturer in order

to yield a uniform absorption coefficient over the entire frequency

range. Over 500 wedge structures with different dimensions and flow

resistance characteristics were subsequently constructed and their

pressure reflection coefficients measured in the 8 inch square tube.

The dimensional design specifications for the P.F. Fiberglas wedge

structures are summarized as a function of desired cutoff frequency in

Fig. 3« The cutoff frequency is defined as that frequency at which the

'’-.pressure reflection rises to 10^ of the pressure in a normally incident

-•sound wave. This corresponds to the frequency at which the absorption

sound energy drops to 99^ or at which there is a sound reduction of

-SD-’db a single reflection. The independent variables were taken as

taper length, base length and air space, and the dependent variables

. are -then the total depth and the base depth. The base depth is specifled-because experimentally this dimension is fairly critical, whereas

a'chajige in the ratio of base length to air space is not quite so important. No layers of blanket were added at the base. A range of lower

cutoff frequencies between 50 and 400 cps was covered.

It must be emphasized that these design characteristics hold only

for wedges sliced diagonally (See Section VI) from Fiberglas board

manufactured in thicknesses of two or four inches and with the bases

of adjacent wedges compressed approximately 3^ to prevent 1 eakage of

sound at the interfaces between them. Other methods of formi ng the

wedges may lead to different results.

In addition to its dimensions, the behavior of a wedge atrncture

is a function of its flow resistance characteristic. The optimal val nea

-14-
DIMENSIONAL SPECIFICATIONS FOR

PF. FIBERGLAS WEDGE STRUCTURES

FIG. 3

-15-
FLOW RESISTANCE SPECIFICATIONS FOR

P.F. FIBERGLAS WEDGE STRUCTURES

FIG. 4

-16-
FLOW RESISTANCE CHARACTERISTICS

OF P.F. FIBERGLAS BOARD

FIG. 5

-17-
of specific flow resistance are given as a function of the desired,

cutoff frequency in Fig. 4 together with the range of flow resistance

over which there is no appreciable change in wedge behavior (i.e., the 

reflection remains below 5^)» This range corresponds to about +20^ of

the recommended resistance. The range of flow resistance measured on

production samples with a number of different densities Is given in

Fig. 5» The flow resistance of P.F. Flberglas for production wedges

must be controlled in order to obtain reasonably uniform results. The 

allowable limits of variation should not exceed those of Fig. 4.

D. Constructional Features of the Chambers

Every effort was made in the construction of the large anechoic

chamber to reduce noise from the outside as much as possible and to

keep to a minimum the reflecting surfaces inside the room. To this

purpose a suspended track was installed in the room which can be made

into a solid working surface by rolling in the smel 1 flat cars shown

in Fig. 1. But, when highly accurate measurements are being made, the 

cars can be removed from the room, leaving only a few square feet of

track to act as reflecting surfaces. Equipment used during tests can

be hung from the overhead. The side walls are made of 12 inches of

poured concrete. The floor plan of the building (Fig. 6) shows that

there is only one entrance to the room and this is guarded by three

sets of doors.

The working space is 29 x 41 x 29 feet after treatment. The air

space, shown behind the wedges, is filled with an egg crate structure

made from 1/2 inch Celotex wall board and having 16 inch square cells to

prevent standing waves from forming at low frequencies. This insures that

there will be a minimum variation of absorption coefficient with angle of

incidence of the sound. The knife edges of the wedges are successively

alternated to prevent possible diffraction grating effects. A total of

nineteen thousand wedges was required to cover the interior. The manner

in which they were installed on the six surfaces of the room is shown

in Fig. J.

In the two smaller chambers, the general Installation plan is the 

same except that grills cover the floor Just above the edges of the

wedges. Special doors for preventing the transmission of sound from

outside into the chamber were installed at the entrances.

E. Calibration of the Completed Anechoic Chambers

The degree to which the anechoic chambers approach conditions which

would exist in quiet air at a point free from all reflecting surfaces was

tested by checking the Inverse square law in the chamber. The decrease

in sound pressure of a signal produced by a sound source was measured as

a function of distance over a frequency range of 50 to 10,000 ops. The

pressure level in decibels is plotted vs distance from the source on

-18-
ANECHOIC CHAMBER SECTION

ELECTRO-ACOUSTIC LABORATORY

HARVARD UNIVERSITY

FIG. 6
METHOD OF INSTALLING WEDGES

FIG. 7

semi-logarithmic paper for each of the frequencies investigated.. The

observed, rate of decrease in level may be compared with the theoretical

slope of 6 db for each doubling in distance. A sample of these data can

be seen in Figs. 11 to 15, page 55 to 56 of Section IV.

When the removable platform of cars is not in the large anechoic

chamber, the data taken along a diagonal axis of the chamber ten feet

above the tracks show that for frequencies from 70 to 10,000 ops, the

inverse square law holds to within +0.5 db up to 10 feet from the source;

to within +0.5 db up to 20 feet from the source; to within +1.0 db up

to JO feet from*the source; and to within +1.5 db up to 40 feet from the 

source. Beyond UO feet there are evidences of standing waves as the

microphone comes in proximity to the walls. A detailed plot of the

measured maximum deviations as a function of frequency are presented in

Fig. 1U, page 57 of Section IV. The introduction of the removable cars

into the chamber produced marked sound pressure variations at frequencies

above 1500 cps. Less favorable results were obtained when the measurements were made at a few feet above and parallel to the tracks.

-20-
A test was run parallel to and. at a distance of 2 1/2 feet out from

one of the -walls. In this case, a slightly anomalous increased, rate of

attenuation was found, to occur "below 200 cps. This rate of increase was

much less marked, than the anomalous increase which was discovered, at all

frequencies in the Meyer chamber in Berlin. It is "believed, that the apparently "better characteristics of the Harvard, chamber are due to the

more nearly optimum impedance match "between the acoustic impedance of the

wedges and. the specific acoustic resistance of air. This satisfactoriness

of impedance match can "be attributed to two factors, first, the low flow

resistance of the P.P. Fiberglas, secondly, to the gradual and linear increase in quantity of absorbing material as one passes from unobstructed

air down to the base of the absorbing structure, and finally, to the

constancy of the absorption as a function of angle of incidence brought

about at the low frequencies by the addition of the egg-crate structure

behind the wedges.

Similar data taken in the smaller chambers are given in Section IV.

-21-
III. DESIGN OF THE CHAMBER LINING

A. Fundamental Considerations

Basic considerations involved in the design of an anechoic chamber

have been described in the literature. 01son+ publishes a formula giving

the ratio of generally reflected to direct sound in an enclosure as follows:

Er /Ep = (16 itr2 /Sa) (1 - a)

Er = energy density of reflected sound

Ep = energy density of direct sound

r = distance of the detector from a uniformly radicating

(point) source in cm.

o

S = surface area of the room in cm

a = average energy absorption coefficient of the bounding

surface

From this formula it is seen that in order to reduce to a minimum

the generally reflected sound, the surface area must be large and the 

absorption coefficient of the lining must approach unity. The effect of

the reflected sound on the measurements is also a function of the separation of the source and the detector as is seen by the term r in the

equation. While it is possible to make measurements very near a source

in certain cases, It is not possible to do so if a plane wave is required

over a large area for proper operation of the instrument under test.

In designing an anechoic chamber, a first consideration should be

to make it as large as possible. The large size not only reduces the

Er /Ed ratio to a minimum, but it permits greater working space. A

second consideration is that of providing on all surfaces of the room a

lining which will be maximally effective in absorbing the incident sound.

It is the purpose of the investigation described in this part of the report to determine by acoustical measurements a type of acoustical lining

which,

(a) provides maximum and uniform percentage

sound energy absorption as a function of

frequency over a wide frequency range,

(b) can be manufactured and installed by mass

production techniques, and

+ H. F. Olson, "Elements of Acoustical Engineering," (D. Van Nostrand

Company, New York, 19^0) p. 5O5

-22-
(c) is macle from a commercially available,

reasonably priced, material.

Meeting these requirements within the limits of time, money and.

wartime controls on materials that existed, in late 19^2, required, the

setting up of a vigorous testing program. The first moves were, of

course, to see whether any of the existing methods of treatment could

be adapted to our requirements. This line of thought finally led us

through a series of tests involving the measuring of over five hundred

acoustic structures. On the pages immediately following will be found

samplings of these data covering the absorbing characteristics of

several variations of each of the six categories of structures which

were included in the series. These data are presented in chronological

form for the most part and are followed by a brief description of data

taken after the large chamber was completed. Those data taken following

the completion of the chamber were for the purpose of determining generalized specifications of the dimensions and acoustical properties of

wedges for a variety of cutoff frequencies or overall allowable lengths.

About five hundred additional structures were included in this series,

of which all were of the linear wedge shaped type.

The apparatus used for the measurements has been briefly outlined

in the Summary and is described in more detail in Appendix I. As was

mentioned in the Summary, the characteristics of the acoustical structures

are described by the percentage pressure reflection R, defined as being

100 times the ratio of reflected to normally incident sound pressure.

R is related to the more faml liar normal incidence energy absorption

coefficient, a, by the expression R = 100 / 1 - a and is plotted as

the left hand ordinate of all graphs. For purposes of comparison, the

percentage energy absorption A, equal to 100^, is plotted as the right

hand ordinate of all graphs. Thus a pressure reflection (R) of 10^

corresponds to an energy absorption (a ) of 99^, while R = corresponds

to A = 99 «9^»

B. Types of Structures Tested •

As has already been mentioned, data were taken during the course

of the investigation on six categories of structures as follows:

1. Sheet layer structures

2. Blanket layer structures

3* Stuffed pyramidal structures

4. Semi-rigid pyramidal structures

5. Wedge-shaped structures

6. Exponentially tapered pyramidal and wedge structures

Of the 500 different structures investigated only data on a selection

are presented here. In many cases, only the best design with respect to

a given variable is shown. The rigid-walled tube used for the measurements

was 15 feet long and 8 inches square.

-23-
C, Comparative Data

The physical characteristics of the materials used in the absorptive

structures are given in Table I. At the time most of these measurements

were made it was not realized that for a given density of material, substantially different acoustical performance would be obtained when different samples were selected. Later it was learned that in general the 

acoustical properties of a given material correlate fairly well with the 

specific flow resistance of the sample. The flow resistance is a function

not only of the density and type of material but also of the statistical

distribution of the fibers comprising it. We then went back and measured

flow resistance on those samples that were still available and the average

values are tabulated in Table I. It should also be noted that the densities

quoted with each graph and diagram on the following pages are the nominal

densities. The average density is given in Table I and differs by 10^ or

more in some cases.

A method of measuring resistance to air flow by the material is

described in Appendix II. The apparatus is so simple to build and use

that it is within the reach of any laboratory to gauge the adherence of

any particular material to specifications.

TABLE I

Physical Characteristics of Materials

Used in Absorptive Structures

(Continued on next page)

Cloth Sheets

Material Surface Density

lb/ft2

Flow Resistance

__-5

gm cm c sec

Structure Comments

Muslin

Flannel

0.019

0.035

6

15

BTL

Sheet

Flameproofed

Muslin

Flannel

O.O25

O.O35

6

15

'Harvard'

Sheet ,

Flameproofed

Muslin

Fiberglas

0.016

0.01U

1

2

Harvard'

Wedge

Flameproofed

Fireproof

-2U-
Table I continued.

Blankets and. P.F. Board.

Material Thickness

inches

Nominal

Volume Density

lb/ft^

Specific Flow Resistance

gm cm"^ sec“^ per inch

Nominal Average Average

Akoustikos Feit 0.9 9.0 9.0 72

Fiberglas Wool 2.0 1.5 1.5 9

P.F. Fiberglas 1.0 5.25 5.5

P.F. Fiberglas 2.0 2.5 5.0 12

P.F. Fiberglas 2.0 5.25 5.5 17

P.F. Fiberglas 2.0 6.0 7.0 43

P.F. Fiberglas 4.0 2.5 2.75 7.5

Detailed, data in the form of graphs for each of several variations

within each major category follow in the next six sub-sections.

1. Sheet Layer Structures

Structures 1 to 7 on pages 52 and. 55 were tested, to see if any

simple change in the Bell Telephone Laboratories’ sheet layer

structure would, result in a lowered, pressure reflection characteristic. A sketch of each structure appears beside the corresponding

graph of R vs f.

Special attention should be directed toward structures 7 and 1.

It is seen that they are alike in configuration, but that the reflection characteristics are substantially different. The difference between the two structures as measured is that the layers of

cloth were stretched in the case of No. 7, while they were merely

held in place in the other case. This difference in reflecting

properties clearly indicates the necessity for a complicated and

precise installation of the materials if the results are to be as

good as shown for structure No. 7»

The arrangement used to hold the layers of sheets in place

during test is shown in Fig. 61, Appendix I. It consisted of a

family of wooden frames with inside dimensions equal to those of

the rigid walled tube, namely 8 inches square and with outside

dimensions of 12 inches on a side. The frames varied in thickness

from 1/2 inch to 2 inches. The squares of cloth were tacked to the 

faces of the frame and were separated by the desired amount through

the choice of suitable frames.

Inspection of the data for sheet layer structures shows that

Increasing the overall length extends the region of low reflection

to a lower frequency. The data on structures 4, 5 and 6 were taken

-25-
to see if uniformly high absorption over a wide frequency range can

be accomplished, by a succession of layers of material whose flow

resistance increases as the wall is approached.. Apparently a more

careful design of the characteristics and spacing of the sheets

would be necessary to bring about a satisfactorily low percentage

reflection. The uniformly poor results and the large number of

individual sheets which would be required in the chamber led to the 

abandonment of this type of structure.

2. Blanket Layer Structures

The next set of measurements was planned to yield a low pressure reflection coefficient down to frequencies of the order of

80 or 90 cps using a minimum number of layered elements. The data

on some of the more significant types of structures, l.e.,

structures 8 to 24, are shown on pages 54 to 58.

The materials selected for these structures were Johns-Manvilie

Akoustikos Felt, Crown Bairfelt, Flberglas Wool and Seaman Paper Co.

Kapok Kwilko. The flow resistances and other pertinent characteristics of these materials are summarized in Table I.

Structure No. 8 was designed so that each successive element

approaching the wall had a higher flow resistance. It was believed

that the muslin layers on the front would absorb the higher frequency

sounds and that each successive blanket would be effective in ahsorbing sounds of lower frequency, say in the region around 500 to

500 cps. This arrangement seems to provide satisfactory absorption

in the vicinity of 1000 cps but the reflection below 400 cps is quite

large.

Structure No. 10 becomes quite good by comparison with. Structure

No. 8 because of the greater overall length and because the quantity

of material is not excessive. If too much acoustical material is

used, the pressure reflection coefficients are significantly increased.

A comparison between structures 9 and 10 serves to demonstrate thi s.

A series of structures starting with No. 11 and extending through

No. 20 were investigated to determine the optimum spacing of the 

heavier blankets to produce the most uniform pressure reflection

characteristic. The three muslin sheets were not used in part of

the tests because they did not contribute significantly to the absorption below 700 cps. The best structures of thia group seem to

be Nos. 19 and 20.

Beginning with structure No. 21, muslin sheets were again introduced to see what their effect would be upon the absorption

characteristic. In structure 21, one of the Flberglas wool sheets 

was replaced with five sheets of muslin. In structure 22, further

Improvement was obtained by changing the spacing of the heavier

-26-
blankets and reducing the number of muslin sheets to three. Still

in further improvement was obtained in structure 25 by rearranging the

spacing and adding another layer of Akoustikos Felt. A slightly

better characteristic is obtained if still another layer of

Akoustikos Felt is added in a position near the wall. This structure is shown as No. 24.

18 In summary, the research on blanket layer structures shows

that a fairly uniform pressure reflection coefficient extending

from 80 cps to frequencies above 1000 cps can be obtained if the

structure is deep enough and if the proper number, spacing and kind

of acoustical blankets are used. The absolute value of the percentage pressure reflection is, on the average, about 11^. The

best structure tested consists of seven elements as compared with

sixteen for the best sheet layer structure, and a substantial improvement in acoustical behavior is obtained. Part of this improvement, of course, is due to the overall depth of 51 1/2 inches

shown for structure 24.

5. Stuffed Pyramidal Structures

Trial construction in a small-sized room was undertaken about

this time to see if a suitable method of installing blanket layer

structures in the large chamber which we were contemplating building

could be found. Because of the extreme size in plan of the projected

chamber (58 x 50 feet), innumerable difficulties presented themselves

in the devising of a method for covering the celling or floor. So

discouraging were the model tests that it was decided that an effort

should be made to see if pyramidal absorbing units similar to those

of Meyer could be developed which would be simple to manufacture and

install.

Following this decision, measurements were made on many pyrami dal 

units filled with varying densities of rock wool insulation, Flberglas

wool, loose kapok and cotton. Also, the performance as a function of

the length, air space and construction of the pyramidal units was to

be investigated. A representative set of curves for stuffed units

■ are shown for structures 25 to 5^ inclusive.

It is seen from the data for Noe. 25 to 54 that the pressure

reflection characteristic for all of the stuffed pyramidal structures is excellent in the frequency region above 500 cps. At the

lower frequencies, the best absorption characteristics are obtained

for long units mounted against the wall or for shorter units mounted

with an air space between the base and the wall. The measurements

show that even at low frequencies the lowest reflection is obtained

for pyramids filled with low density material. One such example can 

be seen in structure 55 which was filled with 1 1/2 lbs/ft^ of loose

kapok. For this particular structure, the pressure reflection remains

below 10^ for frequencies between 100 and 400 cps find below 4^ for

frequencies above 400 cps.

-27-
In spite of the superior performance of structure 55, it was

not practical to manufacture these units. The problem of filling a

muslin bag with loose kapok to the proper uniform density on a mass

production basis seemed insurmountable. This difficulty will be

appreciated even further when it is realized that flow resistance

is a radical function of density for kapok.

4. Semi-Rigid Pyramidal Structures

Discussions then followed with Mr. 0. C. Eckel, the local agent

of the Owens-Corning Fiberglas Company, and it was learned that it

was possible to fabricate any shape of structure easily from a

semi-rigid Fiberglas board, now commercially available. These Fiberglas boards, made of glass fibers impregnated with a phenol formal -

dehyde binding solution are manufactured in 4 foot lengths, various

widths and can be made to any thickness up to 4 inches in several

different volume densities. Samples of this semi-rigid Flberg!as

board which consists of glass fiber impregnated with phenol formal -

dehyde, were obtained and made up into pyramids of different dimensions and densities. Data on a variety of constructions and spacings are shown for structures 55 to 44 inclusive on pages 4-1 to 45.

Inspection of the curves shows that the reflection characteristics

are satisfactory. The best results were obtained for structures

56 and 42. For these two structures the pressure reflection was

approximately 5# in the frequency range extending from I50 to above

1000 cps and increased to 10# at 100 cps. The overall length of the 

two structures varied between 45 inches and 48 inches.

5« Wedge-shaped Structures

It was at this point that it was discovered on re-reading the

paper of Meyer, et al., that they had obtained some data on wedgeshaped structures as well as on pyramidal ones. For some reason,

however, they decided that there was no substantial improvement in

the performance of a wedge over that of a pyramid, although their

data do show a difference. The next measurements at the Cruft

Laboratory were made to establish conclusively the relative merits

of each type of structure, i.e., pyramidal and wedge. Structures

45 to 60 (linear wedge-shaped structures) were fabricated from P.F.

Fiberglas board having various volume densities. Almost all of the 

measurements were run with 1 inch of cork cemented against the rigid

wall backing. The cork was installed in the chamber as heat insulation.

Inspection of the data shows that the lowest frequency at which

small percentage pressure reflections are obtained is almost entirely a function of the overall length of the structures, the 

length being measured from the knife edge of the wedge to the rigid

backing wall. In fact, the lowest frequency at which good absorp-

-28-
tion is obtained, is that for which the length of the wedge equals

approximately one-fourth of a wave length. A prismatic section

should, be provided, at the base of each wedge to prevent leakage

around the edges. The uniformity of the pressure reflection curve

over the frequency range is determined by the length of the prismatic section, the depth of the air space and the flow resistance

of the material. It appears for these particular lots of material

that placing a single layer of Fiberglas P.F. blanket having a

density slightly greater than that of the wedge at the base of the

wedge aids in producing a more uniform frequency characteristic.

It will be seen later that by optimal design a layer or layers of

blanket material at th * base of the wedge can be eliminated.

The wedge structure finally adopted using the material as received from the Owens-Corning Fiberglas Corporation is shown

mounted in different ways in the form of structures 5^ to 58 inclusive. It is seen that the best reflection characteristic for 

this particular wedge was obtained when the wedge was spaced out

from the wall by a distance of 10 3/4 inches and when its base

contained 2 inches of 5«25 lbs/ft^ of P.F. Fiberglas blanket.

The density of the main body of the wedge was 2.5 lbs/ft*. Data

on the flow resistance are given in Table I, page 24. The pressure reflection characteristic is uniform and of the order of 3^

over the frequency range from 70 to well above 1000 cps, decreasing

to 10^ at 60 cps, which defines the cutoff frequency.

The air space at the base of the wedge was found to be not

overly critical, that is, it can be specified to + 1 inch from the

value shown. The cork cemented to the outer wall"heed not be

covered with asbestos cement as was believed to be necessary at one

time. The muslin bag In which the P.F. Fiberglas wedge material is

encased is of very light density, i.e., 40 x 44 threads per Inch,

but still heavy enough to contribute to the support of the structure. Its flow resistance is very low, being 1 acoustic ohm.

During the construction of the chamber measurements were made

on the wedges as they were manufactured, to determine uniformity.

Unfortunately, the production samples did not appear to be quite as

good as was Structure 58» Presumably this was due to irregularities

in the Fiberglas board from which the wedges were cut. The average

and range of data for the production samples were shown in Fig. 2

of the summary.

At frequencies between 1500 and 4000 cps, measurements were

made in the reverberation chamber of the Johns-Manville Company,

and it was found that the absorption characteristic remains excellent

out to high frequencies because the muslin covering and the Fiberglas

have low flow resistance. The real test of the magnitude of absorption at high frequencies is to be found in Section IV of this report

-29-
in which measurements of the deviation from the inverse square law

are described. There is no indication that the absorption at

10,000 cps is any less than that obtained between 5000 and 10,000

cps.

The design of the adopted wedge structure was completed at

almost the same time that the concrete chamber was nearing completion.

This meant that if the construction of the chamber were not to be

held up, research must stop. Hence, many interesting factors were

left uninvestigated, although generalized specifications covering

the optimal dimensions and flow resistance of P.F. Flberglas wedges

were Investigated later.

It is known that further improvement in the absorbing properties

of the wedge structure will result if longer wedges are used. Investigation of the graphs shown in Fig. 5 of the summary shows that

for absorptions down as low as 40 cps, a wedge structure would need

to have a total length of 94 Inches measured from its tip to the

rigid backing wall. The graphs of Fig. 4 show that for this length

the optimum flow resistance R of the material would need to be 4.8

acoustic ohm per inch, which corresponds approximately to a Flberglas board density of about 1.7 Ibs/ft^. This is a very low density

and results in wedges which are frail, and hence, might be difficult

to manufacture and install.

At the time the materials were purchased for the construction

of the wedges for the Harvard Chamber, P.F. Flberglas boards were

being made with a maximum length of 4 feet. This meant that the 

maximum length of wedge which we could use was about 44 Inches, excluding the blankets at the base. In the case longer wedges are

desired, some difficulties may be encountered in that because the

lower density material is more flexible, more then two points of

support will be required when mounting the wedge in the chamber.

Also, unless Flberglas is made in larger boards, it may be necessary

to build the wedge in more sections which will increase the cost of

construction. A further factor involved is that stuffing of the

wedge into the muslin bag requires that the acoustical materi al be

rigid enough to stand a certain amount of hand! 1ng. However, a

possible way of eliminating the use of muslin bags has been suggested by the Bell Telephone Laboratories (See Section VI). All of

the difficulties Just named were taken into consideration in the

design of the Harvard chamber. As will be discussed later, other

methods of covering and supporting the wedge are about to be tried

by other laboratories and perhaps these particular obstacles will no

longer exist to 11ml t extending the use of wedges to lower frequencies .

Data for th.© h©st two shorter wedges are shown for structures

59 and 60. These were adopted respectively for use in the small

anechoic chambers at the Electro- and Psycho-Acoustic Laboratories

of Harvard.

-50-
6. Exponential Tapered. Pyramidal and. Wedge Structures

Additional data were taken on exponential wedge and pyramidal

structures to see if a linear rate of taper was optimum. Structures

No. 61 to 64 on page 48 are shown as examples. The pressure reflection curve for a 55 inch long convex exponential pyramid with

a nominal density of 5*25 Ibs/ft^ (Structure 61) is shown. Its

behavior is seen to be comparable to that of some of the poorer

wedge structures. The optimum exponential pyramid developed was

greater in length than No. 61 and of lower density. It is shown

as Structure No. 62. The results are «imllar to those obtained

for the best wedge structure. Further Improvement might have been

effected with more research, but the difficulty of and the waste

of material involved in the manufacture of such a shaped pyramidal

wedge made it seem unsuitable for adoption.

A similar investigation was made of exponential tapered wedge

structures. Those which performed best are shown as structures

Nos. 65 and 64, page 48. In those cases, the performance is not

so good as that of the linear-tapered wedge. It was decided,

therefore, that the linear wedge structure already described should

be adopted for use in the Harvard chambers.

D. Generalized Data for the Design of Linear Wedge Structures

In endeavoring to determine the optimum shape and material for use 

in a wedge structure in order to obtain low reflection over a wide frequency range, one is confronted by a great number of variables which

must be considered, the more important of which are:

1. Type of material

2. Flow resistance of the material

5. Base area

4. Taper length (Lt )

5. Base length (12)

6. Air space at the base (lj)

If, for a variety of materials, we expected to achieve the best

values for each of these variables, we would end up by performing many

thousands of experiments. Believing that a broad approach was out of the

question, we concentrated on certain Umi ted aspects of the problem.

In the first place, we limited our studies to the use of Fiberglas boards

impregnated with phenol formaldehyde. Further, we tested only wedges

made from boards having a thickness of 4 inches and in a few cases, 2

inches, and which were manufactured by the procedures described in

Section VI of this report. Next, we restricted the studies to a consideration of wedges having square bases, 64 square inches in area.

This particular area was determined by the size of measuring apparatus

available and by the fact that two 4-inch thick Fiberglas boards form

an 8-inch wide base without waste. (Continued on page 49).

-51-
SHEET LAYER STRUCTURES

STRUCTURE NO. I

STRUCTURE NO. 2

STRUCTURE NO. 3

STRUCTURE NO. 4

-52-
SHEET LAYER STRUCTURES

(continued)

STRUCTURE NO. 5

STRUCTURE NO. 6

STRUCTURE NO. 7

-35-
BLANKET LAYER STRUCTURES

STRUCTURE NO. 8

STRUCTURE NO. 9

STRUCTURE NO. IO

STRUCTURE NO. 11

-5^-
BLANKET LAYER STRUCTURES

(continued)

STRUCTURE NO. 12

STRUCTURE NO. 13

STRUCTURE NO. 14

STRUCTURE NO. 15

-55-
BLANKET LAYER STRUCTURES

(continued)

STRUCTURE NO. 16

STRUCTURE NO. 17

STRUCTURE NO. 18

STRUCTURE NO. 19

-36-
BLANKET LAYER STRUCTURES

(continued)

STRUCTURE NO. 20

STRUCTURE NO. 21

STRUCTURE NO. 22

STRUCTURE NO. 23

-37-
BLANKET LAYER STRUCTURES

(continued)

STRUCTURE NO. 24

-58-
PYRAMIDAL STRUCTURES

STRUCTURE NO. 25

STRUCTURE NO. 26

STRUCTURE NO. 27

STRUCTURE NO. 28

-59-
PYRAMIDAL STRUCTURES

(continued)

STRUCTURE NO. 29

STRUCTURE NO. 30

STRUCTURE NO. 31

STRUCTURE NO. 32

-4o-
PYRAMIDAL STRUCTURES

(continued)

STRUCTURE NO. 33

STRUCTURE NO. 34

STRUCTURE NO. 35

STRUCTURE NO. 36

-Ul-
PYRAMIDAL STRUCTURES

(continued)

STRUCTURE NO. 37

STRUCTURE NO. 38

STRUCTURE NO. 39

STRUCTURE NO. 40

-42-
PYRAMIDAL STRUCTURES

(continued)

STRUCTURE NO. 41

STRUCTURE NO. 42

STRUCTURE NO. 43

STRUCTURE NO. 44

-43-
LINEAR WEDGE STRUCTURES

STRUCTURE NO. 45

STRUCTURE NO. 46

STRUCTURE NO. 47

STRUCTURE NO. 48

-U-
LINEAR WEDGE STRUCTURES

(continued)

STRUCTURE NO. 49

STRUCTURE NO. 50

STRUCTURE NO. 51

STRUCTURE NO. 52

-US-
LINEAR WEDGE STRUCTURES

(continued)

STRUCTURE NO. 53

STRUCTURE NO. 54

STRUCTURE NO. 55

STRUCTURE NO. 56

-1|6-
LINEAR WEDGE STRUCTURES

(continued)

STRUCTURE NO. 57

STRUCTURE NO. 58

STRUCTURE NO. 59

STRUCTURE NO. 60

A7-
SPECIAL STRUCTURES

EXPONENTIAL PYRAMIDAL STRUCTURES

STRUCTURE NO. 61

STRUCTURE NO. 62

EXPONENTIAL WEDGE STRUCTURES

STRUCTURE NO. 63

STRUCTURE NO. 64

-48-
(Continued, from page 51)

We then have left four variables to be investigated, on a detailed,

basis, namely, flow resistance, taper length, base length and air space

depth. A variety of densities of Fiberglas board were obtained from

the manufacturer and served to give a wide choice of flow resistances

to choose from. These boards were then formed into approximately 500

wedges and data on each were obtained using the tube method of measuring the reflection R at normal incidence as a function of frequency.

A typical set of data are shown in Table II and the actual graphs

corresponding to three of the cases are given in Fig. 8. In this table

Li and L5 were varied while Lg was maintained constant. In the fourth

column is tabulated the cutoff frequency obtained for each case, i. e.,

the frequency at which the reflection rises to 10^. The numbers in the 

fifth column indicate the maximum value which the pressure reflection

coefficient R achieves above the cutoff frequency, and in the sixth

column the integrated average value of R lying between the cutoff frequency and 500 cps is given. For the particular flow resistance and

base length of the wedges tested in Table II, it appears that the

optimum dimensions are Li = 15, Lg = 5 smd L5 = 2 inches respectively.

A great many such tabulations were made and the averaged results are

presented in Figs. 5 and U on pages 15 and 16 of the summary.

The flow resistance values given by the solid line in Fig. 4 are

not overly critical, and can on the average vary by +20^. This is fortunate, because the flow resistance values for Fiberglas boards vary

widely in production for different lots with the same nominal density

and thickness. Data were given in Fig. 5, page 17 of the summary showing

the magnitude and variation of flow resistance as a function of volume

density, when P.F. Fiberglas with a given average density is compressed.

This flow resistance characteristic will not vary much for samples from

a given board or for boards in the same lot, but samples from boards in

different lots may vary considerably. If the fiber distribution in the

-49-
TABLE II

Determination of Li and L5. Rj_ = 55 ohms/inch P.F. Fiberglas

L1

l 2 L5 fc Max. R (#)

above fc

Average R (^)

Below 500 ops

7 3 0 545 7 4.8

7 5 1 500 6 4.0

7 5 2 290 9 7

9 4 0 240 9 5

9 4 1 280 9 5.4

9 4 2 325 9 7

15 5 1 187 5 4

15 5 2 178 4.5 3.5

15 3 3 175 6.5 4

16 3 1 165 8 5

16 3 2 145 7 4.2

16 3 3 137 8 5.3

P.F. board could be kept very uniform, the flow resistance characteristic*

R/T vs p, would be independent of the nominal density and thickness of the

board. In practice, most of the lots fall in the area marked "average,”

while occasionally there are deviations as large as those indicated by

the limits in Fig. 5» It is this variation from lot to lot whi ch necessitates a careful check on flow resistance choracteriRtlcr for production wedges.

In all of the measurements of flow resistance, samples having a

diameter of 2 inches were used, cut perpendicularly through the 4 x 24 x 48

inch boards parallel to the 4 inch dimension. A range of densities covering the values 2.5 to 9 Ibs/ft^ was investigated. The spread in flow resistance is a function of the average fiber diameter, the orientation of

the fibers and the amount of phenol formaldehyde which happened to accumulate in that particular area of board. The manufacturer feels that each

of these three variables is difficult to control in production and that

the specification of flow resistance can never be made to too close tolerances. However, R1 for a given lot can be maintained fairly constant.

R, meaning flow resistance in acoustic ohms, should not be confused

with R, meaning percentage pressure reflection. The dual use of R

is customary in acoustic literature.

-50-
It is our feeling that with more refinement production can and. should he

controlled in such a way as to yield desired flow resistance characteristics .

-51-
IV. CALIBRATION OF THE ANECHOIC CHAMBERS

A. Method, of Calibration

At least three methods might be used for calibrating an anechoic

sound chamber to determine the degree to which it approaches infinite

free space. The first and most logical of these is to determine the

decrease in pressure obtained as an observer moves away from a source

of sound. If there are no reflections then the energy in a sound wave

ought to decrease according to the inverse square law out to distances

as great as the dimensions of the chamber will allow, i.e., the pressure decreases inversely with distance. A second method of evaluating

the chamber would be to use a highly directional microphone and to

measure the ratio of the sound which travels directly to the microphone

from the source to the sound which arrives at the microphone from al 1

other directions. If there are any reflections from the walla, sound

waves should arrive from directions other than that of the source and

can be measured accordingly. A third method of measurement would be

to produce a very short pulse of sound and to measure the echo occurring

at a later time due to reflection from the walls. Presumably the 

microphone and source of sound could be separated in such a way that, a

substantial time delay would exist between the directly received pul r r

and the reflected pulse. Because of the lack of time, the first and

most simple of these tests was all that was performed in the Harvard

Chamber.

B. Apparatus

A block diagram of the apparatus used for performing the measurement of sound pressure as a function of distance from the source is

shown in Fig. 9» In the frequency range between 60 and 5000 ops a W.E.

-52-
6J0-A dynamic microphone was used, to measure the sound, pressure. Above

that range, a W.E. 6^0-AA condenser microphone was used. The data were

taken first by turning on the oscillator and adjusting the gain of

amplifier No. 1 with the switch in position 1 until a fairly high but

undistorted sound level was produced. Then the switch was thrown in

position No. 2 and the gain of amplifier No. 2 was adjusted until a convenient meter reading on the voltmeter V-2 was obtained with approximately JO db of attenuation set into the attenuator. As the microphone

was moved away from the source, the magnitude of the decrease in sound

level was measured by readjusting the attenuator to give the same meter

reading with the switch in either position. The difference between any

particular attenuator setting and any previous one was a measure of the

decrease of sound as a function of change in microphone position. A

carefully calibrated attenuator was used and the precision of a measurement was believed to be + 0.1 db.

Four loudspeakers were employed over the frequency range. At the 

very low frequencies, a large folded horn was used. When the wave length

of radiated sound became of the order of a lateral dimension of the horn,

a smaller horn was substituted. This was done to avoid using a loudspeaker so directional that no reflections would be obtained from the 

side walls. Actually, however, the best data were obtained when a very

small source was used. Apparently this happened because with a very

small source, there was little opportunity for reflections to occur

from the body of the horn itself.

In the experiments, the source was suspended from one of the overhead monorails. The microphone was suspended from an overhead traveling

crane which was marked off in one-foot intervals. At each setting of the

microphone, a series of attenuator readings were obtained as a function of

frequency. The data were then plotted on semi-logarithmic paper with

distance as an abscissa and decibels’ decrease in sound level as an ordinate. A separate plot was prepared for each frequency.

C. Procedure

Inverse square law measurements were made for various axes of the

roam. In one case, the source of sound was mounted at one comer of the

room and the microphone moved in 1-foot intervals diagonally across the

room. These data were taken with the microphone U feet and 10 feet above

the tracks. Data were also taken perpendicular to the side walls along

the two central axes. Finally, one set of data was taken parallel to and

2 1/2 feet out from a side wall along a horizontal line.

It was believed that the data would show which part of the room was

the more desirable to use and would indicate to what degree the tracks

were responsible for deviations from inverse square law in any part of

the frequency range. Data were also taken with the carts in the room as

well as out. They served to show the disturbing effect of added equipment

in the room.

-55-
The room was temperature controlled., and. the humidity was not permitted. to rise above 60^. Throughout the measurements, the temperature

was recorded, and. in the frequency region above 5000 ops, the humidity

was checked with a sling psychrometer. At the higher frequencies, it

was necessary to correct the data for absorption in the air according to

the curves shown in Fig. 10.

D. Data for Largest Anechoic Chamber

Typical graphs obtained in the frequency region between 60 and 10,000

cps for the case of the carts removed from the room with the mi crophone

moving diagonally across the room and 10 feet above the tracks are shown

in Figs. 11 to 13. The maximum deviation observed at distances up to 10,

20, 50 and 40 feet respectively as a function on frequency are shown In

Fig. 14.

A summary of the data taken in all the frequency regions Investigated

is shown plotted in the form of distribution curves in Fig. 15. The 

standard deviation of each of the curves is indicated on the graphs. Two

lines are drawn on each graph to delineate the maximum expected deviation

from Inverse square law. A few points fall outside these lines and it is

believed that disregarding 19 out of a total of 571 points is Justified on

the premises that errors in the measuring equipment were not reduced below

0.1 of a decibel, and also it was difficult to measure accurately the distances between the microphone and loudspeaker thus introducing the possibility

54-
-55-
of another 0.1 db of error. In summary, it is seen from these data that

in the frequency region between 70 and 10,000 ops, the deviations from

inverse square law are « + 0.J db in the region from U feet to 10 feet;

+ 0.5 db in the region 10 feet to 20 feet; + 1.0 db in the region 20 feet

to JO feet; and + 1.5 db in the region JO feet to 40 feet. In the frequency

region above J000 cps, a slight tendency was noticed for the points to fall

off less rapidly than does the inverse square law. This systematic and

progressive deviation from the inverse square law approached a maximum of

0.J of a decibel at 10,000 cps and became progressively less as the frequency was lowered.

Dataware shown in Table III for measurements made U feet above the

tracks along the long axis of the room with and without the cars in place.

It is seen that even without the cars in the room, the deviations from

inverse square law are higher in that case. With the cars in place, the

deviations from inverse square law are quite pronounced.

It is of interest to compare the data taken in this large anechoic

chamber with those published for the two other chambers mentioned in the

historical summary at the beginning of this report, which are substantially

of the same size. Reference should be made to Figs. 16 and 1? to see these

data. It is not clearly stated in the different authors’ papers how their

-56-
HARVARD ANECHOIC CHAMBER

57“ PE FIBERGLAS WEDGE LINING

FIG 14

-57-
DECIBEL DEVIATIONS FROM INVERSE SQUARE LAW

70 to 10,000 CPS

FIG. 15

curves were obtained.. They do not state whether their data represent the 

maxi mum observed deviation or whether statistical curves were plotted and

the standard deviation determined. It can be seen, however, that over the

entire frequency range the large Harvard anechoic chamber compares well

with those previously built.

-58-
-59-
TABLE III

Deviations in Decibels from Inverse Square Law

Longitudinal Axis — 4 feet above tracks

Frequency Cars below 10 ft 10-20 ft 20-50 ft

1000 out + 0.5 + 1.0 + 1.5

in + 1.0 + 1.0 + 5-0

4000 out + 0.1 + 0.5 + 1.0

in + 0.2 + 1.0 + 1«5

5000 out + 0.2 + 0.7 + 1.5

in + 0.5 + + 2.5

10,000 out + 0.7 + 0.8 + 2.0

in + 1.0 + 2.0 + 1.5

E. Data for Electro-Acoustic Laboratory Anechoic Chamber No« 2

Inverse square law data were obtained on the small anechoic chamber

in the Cruft Laboratory Building using essentially the same apparatus as

was used for the large chamber. The chamber is rectangular and its dimensions before the 25-inch wedge structure was installed were 11 x 15 x 10

feet, leaving after treatment, a working space of about 7 x 11 x 6 feet.

This size of room is somewhat crowded for any other than small apparatus

measurements. Plots of the data taken along the longitudinal and transverse axes of the room are shown in Figs. 18 and 19« In Rummary, these

curves indicate that in the frequency range from 500 to 5,000 ops the

deviations from Inverse square law are + 0.5 db out to 5 feet and + 1.0

db out to 10 feet. For frequencies between 5,000 and 10,000 cps, these

values are + 1.0 db and + 2.0 db respectively. The greater deviations

at the higher frequencies are caused by the presence of the iron grills

and the supporting steel structure which cover about one half the total

area of the floor.

It is interesting to compare these data with those reported in 1956

by Bedell of the Bell Telephone Laboratories for their small chamber.

Their published results are shown in Fig. 20 and the deviations from inverse square law for their chamber between 500 and 5,000 cps are + 5 db

out to 5 feet and + 5 out to 10 feet.

F. Data for Psycho-Acoustic Laboratory Anechoic Chamber

The chamber at the Psycho-Acoustic Laboratory is of an odd shape,

(See Fig. 5&, page 9M, and before the wedges were added it was lined

-60-
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-61-
-62-
with a 2-inch layer of rock wool spaced, out 2 inches from the side

walls.

The "sq.uared.-off” dimensions of the roam were about 12 x 20 x 12

feet before treatment, leaving a sizeable working space after the addition of the 15-inch wedge structure of 10 x 18 x 10 feet approxl matAly.

Inverse square law data were taken before the wedges were added

and are presented in Fig. 21. In the frequency range of 400 to 5,000

cps the deviations are of the order of +5 db out to 5 feet; + 5 db out

to 10 feet; and + 10 db out to 15 feet. Addition of the short" wedges,

however, reduced"these values to the order of + 2 db, + 2 db and _+ 5 db

respectively, as is shown in Fig. 22.

Because of the short length and the high density of the wedges, it

is believed that this roam would not be nearly as satisfactory if the

side walls did not form oblique angles with each other. The presence of

a grill completely covering the floor contributes to reflections at frequencies above 5,000 cps.

-65-
-64-
65
V. CONSTRUCTION OF THE LARGE ANECHOIC CHAMBER AND CONTROL ROOMS

A. Site

The large anechoic chamber at Harvard, was constructed, on a site conveniently located adjacent to the Cruft Laboratory Building. Salient

features contributing to the choice of this location included: generally

low ambient noise levels due to the absence of heavy vehicular traffic in

the vicinity; the comparative absence of subways and other sources of

earth-conducted noises; convenience in receiving supplies and equipment;

and accessibility to the Jefferson and Cruft Physical Laboratories. A

photograph of its location on Oxford Street is given in Fig. 25.

To assist in a physical study of the foundation conditions, a 4-foot

test well was driven to a depth of 50 feet on the site. The test well revealed that below a depth of 6 or 7 feet there were layers of sand with

coarse to fine gravel and an occasional small boulder which alternated

with layers of blue clay.

Ground water was observed approximately 17 feet below the ground

surface - an elevation which is safely below the footings and floor of

the structure. Upon investigation, it was found that only once during

the last twelve years had the ground water been as high as the proposed

floor elevation of the new chamber. Accordingly, any heavy waterproofing

of surfaces and other costly means of protection against ground water under

pressure were regarded as unwarranted. Considerable care, however, was

given throughout to the construction of tight Joints and to the production

of a well-vibrated and dense concrete.

B. Sound Chamber

The anechoic chamber proper constitutes the central feature of the

acoustical laboratory, (See Fig. 24). This chamber is rectangular in

shape and is built of concrete carefully proportioned and adequately reinforced with steel.

The floor is 12 inches thick and rests on a sand and gravel foundation mechanically compacted In place. At the edges, the floor slab rests

on the footings and extends to within one-half inch of the inside wall

surfaces of the chamber. This space is calked with asphaltic filler to

form a tight Joint which will maintain a seal during variations in temperature and/or displacements caused by relative settling of walls and floor.

The floor surface was floated with a small gradient to drain into a sump

placed at the south end of the chamber.

Approximately one third of the chamber volume lies below ground

level. The corresponding portion of the walls is 2 feet in thickness

and was poured continuously and mechanically vibrated.

-66-
EXTERIOR VIEW OF LARGE ANECHOIC CHAMBER

FIG. 23

SECTIONAL VIEW OF LARGE ANECHOIC CHAMBER

FIG. 24

67-
To help reduce transmitted ground waves, a vertical fill of fine

cinders 4 feet thick and extending from the footings to the ground surface was imbedded in a trench dug around the chamber Just outside of

the walls.

Above the ground level the walls were 12 inches in thickness. At the

top they were built to Join continuously with a 9-Inch horizontal roofing

slab which was built in one piece and entirely covered the chamber.

The inside measurements of the reinforced concrete chamber are

50 feet 4 inches long by 58 feet 4 inches wide by 58 feet high. There

are no columns or structural supports inside the chamber. The walls are

stiffened on the outside by eight pilasters properly spaced and built

integrally with the walls. At the top, opposite pilasters on the sidewalls Join continuously with long horizontal girders 50 Inches x 60 inches

in section which extended over and support the roof slab. The girders

also are used as purlins to support the timbers of a low-pitched roof

which forms the outside top covering of the chamber.

A 6-inch layer of cinder concrete was spread over the 9-inch ceiling

slab of concrete as additional sound Insulation.

Nine inserts of short pieces of 2 1/2 inch gas pipe were placed

along three rows in the celling, through which drop cords can be lowered

for lighting the chamber or through which cables might be dropped to add

other structural elements. Three incandescent lamps along the central

axis of the room are used. To facilitate a replacement of the lamps, extra

drop cord was coiled above each gas pipe and is paid out in lowering the 

light until it can be reached from within the chamber. a scuttle opening

50 Inches square in the form of an inverted frustum was constructed at the 

center of the celling slab. The scuttle was filled with a removable concrete plug.

Fir plywood 5/8 inch thick was used throughout to build forme for

the concrete. The initial cost of plywood was comparatively high but in

the long run it proved to be economical because of the large size of the

sheets (U feet x 8 feet) which were relatively easy to erect, and which

were reset several times for progressive concrete pourings.

Forms for the walls of the chamber consisted of inside and outside

layers of plywood, each backed by 4 inches x 4 Inches fir vertical ribs

frequently spaced and rigidly held from spreading by numerous tie anchors

of alloyed steel. The forms were braced laterally with additional timbers

to Insure true walls when under the pressure of green concrete.

A platform of plywood, well supported by wooden Joists and stringers

was built as a form to carry the fresh 9-inch concrete cell 1ng slab end

other loads, including reinforcing steel and roof girders. Struts resting

on the ground floor rose to support this plywood ceiling structure. Two

-68=
rows of these struts, 50 inches apart, and. of 6 inches by 6 inches 

timbers, set at intervals of 6 feet in a row, were placed, under each

of the two girder forms. The rest of the celling was supported, by other

struts or columns 4 inches by 6 inches, spaced. 6 feet each way. All

verticals were scabbed, and. braced, horizontally and. diagonally with 1

inch by 6 inch rough fir boards securely nailed..

Spool anchors to support the framework for the acoustical treatment

set flush with the concrete, were drilled, and. threaded, for inserting

5/8 inch bolts. These anchors were placed in the walls and ceiling at

intervals of 4 feet each way. Additional rows of anchors were set in

the ceiling to hold three small monorails.

Anchors of two other types - knees and lead sheaved bolts - (See

Figs. 5^ and 51), were set in the concrete along the comers to hold

cables which supported diagonally four ways a two-rail track extending

the length of the chamber at approximately 12 feet above the floor. The 

track was properly aligned and rigidly held by means of turnbuckles to

adjust the cable tension (See Fig. 25).

A 1-inch layer of sheet cork was cemented with hot asphalt to the

walls and floor of the chamber to provide for moisture and thermal

insulation.

The foundations for the wooden section of the acoustical laboratory

including the floor of the heating and ventilating room, were built entirely separate from the anechoic chamber. This provision was for the 

purpose of holding to a minimum the transfer of mechanical vibrations

from external sources.

C. Additional Features of the Acoustical Laboratory

In addition to the anechoic chamber itself, attention may be called

to several other features (See Fig. 26).

Control Rooms There are three research or control rooms whose primary

function is to service the anechoic chamber during experimentation (Rooms 101, 102 and 105). The rooms are equipped with

benches with built-in drawers, various electrical sources, including 110

volt direct, 110 and 220 volt alternating current, and 1000 cps standard

frequency. Wires from each of the rooms may be run into the chamber via

rectangular ducts with removable covers built into the floor. These ducts

connect with the anechoic chamber through nine 2 1/2 inch diameter conduits

which are set in the concrete at each side of the main entrance. Electrical

cables, telephones, air, etc., leading from the chamber to any control room

may be threaded through the tubes and laid along the floor duct as far as

necessary.

Service Apparatus Room (Room 106) In this room is set the air conditioning unit which controls the air supplied to the

-69-
VIEW FROM INSIDE OF ANECHOIC CHAMBER

SHOWING ENTRY-WAY AND SUSPENDED TRACK

FIG. 25

FLOOR PLAN OF ANECHOIC CHAMBER

FIG. 26

-70-
anechoic chamber. Outside air is filtered and properly humidified, then

heated or cooled, depending on the season of the year.

The conditioned air enters the chamber through a removable plug in

the acoustical treatment and is directed towards the center of the chamber.

During the periods of experimentation in the chamber, the intake and discharge ducts of the conditioner, which have been hinged and made removable,

are disconnected and rolled away from the chamber, after which an acoustical

plug made up of wedges is set in place and a heavy door framed in the concrete wall is closed and sealed. Air is withdrawn from the chamber through

a tapered rectangular duct with air intakes at the floor level behind the

acoustical treatment. A vertical closed channel was formed in the concrete

wall and extended up to the floor level of the service room and thence to

an exhaust fan of the air conditioning unit. From here all of the air or

any portion of it may be recirculated and the complementary part may be

discharged outside of the building.

Sound Lock and Suspended Track An acoustical trap or sound lock 8 x 10

x 9 feet was built at the entrance to

the anechoic chamber. The walls, ceiling and floor of this lock were hollow

or shell constructed and filled with a densely packed acoustical felt. Two

different pairs of doors, one pair closing each end of the lock, were built

of wood and sheets of "transite” with interior spaces packed with soundproofing material (See Fig. 27). They were fitted with ball-thrust hinges,

FIG. 27

carrying caster wheels, double

astrakals, three heavy level

compression clamps for each

pair of doors, and a complete

set of sponge rubber calking

strips which operate in compression when the doors are

closed. The sound lock and

doors were built integrally

with the concrete chamber, and

to minimize the transfer of

vibration, they were carefully

separated by a small, continuous

air space from other sections of

the building. A 4-foot track

with two 2x2 inch angles for 

rails starts at the south entrance to the acoustical laboratory and proceeds at floor level

along the corridor of the wooden

annex, then passes through the

sound lock and continues to the

north end of the anechoic chamber. The elevation of the track

is approximately 12 feet above

-71-
the concrete floor. Seven steel-wheeled., framed, and. mesh-covered, carts

coupled, together may he pushed, along the track into the chamber. They

serve dually to carry apparatus and to act as a floor.

Storage Room, Supply Entrance and A storage room of 16 x 28 feet is

Corridor to the Cruft Laboratory provided as a part of the wooden

building which adjoins the anechoic

chamber (Room 104). It furnishes a space to stack the seven steel cars

when not in use, and for packing, unpacking and storing instruments and

equipment. It also serves as working space for emergency Jobs. A supply

entrance is located at one side of the building where the tracks end.

The acoustical building connects with the Cruft Laboratory by means of

an enclosed wooden runway 6 feet wide and 12 feet long, equipped with

double doors at each end and heating and lighting services.

D. Installation of Acoustical Treatment

Main Support A wooden framework 10 1/2 inches deep measured from the 

for the cork surfacing to the base frames of the wedges was built

Wedges to give mechanical support to the acoustical treatment of

the anechoic chamber. This framework was held in place

by 5/8 inch steel studbolts, 11 1/2 inches long, threaded on both ends

and screwed into spool anchors placed at intervals of 4 feet in the concrete walls and celling (See Fig. 28).

Wood blocks, 5 1A * 5 5A x 10 inches long, and 5 5/8 x 1 5/8 inches 

horizontal nail girths were drilled edgewise and pushed over the bolts; 

nuts were then screwed against the girths which in turn held the blocks

tightly against the cork-covered wall. Next, 1/2 inch Celotex wallboard

was cut in strips 8 Inches wide, notched and fitted together in egg crate

fashion to form cells approximately 16 x 16 x 8 inches deep. This crate

assembly was made to fit snugly between adjacent horizontal girths, to

which it was securely nailed after the outer edges had been placed flush

with the corresponding edges of the nail girths (See Fig. 29). This arrangement provided approximately a 7/8 inch space between the Inner

edges of the egg crate cells and the cork surfacing for ventilation purposes. Strips of furring 1 1/2 inches wide and 1 j/h Inches deep were

nailed to the horizontal girths at intervals of 7 5/4 inches on centers.

The girths are located 4 feet apart (See Figs. 28 to 50).

Before the installation of the wedges, strips of 1 inch felting 45

inches wide, l.e., the overall wedge length, were fastened to the nal1

strips and run as an outside border completely around the cel ling and

lower wall surfaces. Vertical strips of the same width were placed similarly in each corner of the chamber. These strips of felt served to reduce reflections from the base surfaces which would otherwise occur in

the regions of overlap (See Fig:. 51).

The acoustical wedges were supported by attaching their bases directly

to the nail strips. By rabbettlng the base frames on two opposite edges,

-72-
SAMPLE. CONSTRUCTION OF

WEDGE SUPPORTING FRAMEWORK

FIG. 28

-73-
FIG. 29

FIG. 30

-7^-
FIG. 31

FIG. 32

-75-
each wedge could be installed by setting one 2 1/4 inch round-headed

screw on the leading rabbetted edge and driving it into the supporting

nail strip immediately underneath. The opposite or following rabbetted

edge was anchored by slipping it under the screwed edge of the wedge

last set, thus forming a simple but rigidly interlocking system of

bases, easy to install (See Figs. 55 and 5M• All wedges as erected

in the chamber were snugly placed with their prism-shaped bases compressed in good contact with each other. The first wedges inata]led

were on the side walls, Just below the ceiling (See Fig. 52). Following that, the entire ceiling was covered with the exception of the 45

inch border previously treated with the 1 inch of felting. It was

found to be time-saving in setting the wedges to install four or five

rows at a time in a direction at right angles to the nail strips. The

screws, one to a wedge, were driven with automatic screw drivers. The

completed ceiling is shown in Fig. 56. The wood struts erected earlier

to support the concrete roof slab were shortened to hold a new floor

at a lower elevation, which was used as a scaffolding by the workmen

while Installing the wedges on the ceiling. The production of a diffraction grating effect by the walls was prevented by turning the

dihedral of adjoining wedges through 90 degrees. Twied lately after

the ceiling treatment was finished, all scaffolding, timbers and 1 umber

were removed from the chamber. Portable staging with adjustable platform brackets was used as scaffolding when installing acoustical wedges

on the wall surfaces (See Figs. 57 and 58). To prevent sagging of the

WOOD BASE FRAME FOR MOUNTING WEDGES

FIG. 33

-76-
FIG. 34

FIG. 35

-77-
wedges, a second point of support was provided "by means of a ladder of

chain and wire. A good picture of this structure can be seen in Fig. 55,

and a discussion follows shortly. When the floor level was approached,

the installation of wedges on the side walls was stopped 45 inches from

the floor. A vertical border of 1 inch felting lined the bottom part of

the walls and wedges were installed on the floor next to the felt strip

below the horizontal wedges. (See Fig. 59). Vertical edges were similarly constructed. Wedges starting any horizontal course were set against

the 45 inch vertical felted strip and continued away from the corner

until tenninated at a place 45 inches from the next corner following.

Here as above, at right angles to those Just preceding, wedges were set

against the felt and continued along the next wall; thus the course proceeded horizontally around the chamber.

Auxiliary Chain All wedges installed on the walls of the chamber were

Supports supported at the middle sections to avoid sagging (See

Fig. 55) • Lengths of sash chains were suspended from

reinforced nail strips at the ceiling and allowed to hang to the floor

every 7 5/4 Inches on either side of the vertical tiers of wedges. Individually, each wedge was lifted and held horizontally in position by

placing under it a short piece of No. 9 wire, the ends of which had been

pushed through appropriate links of the nearby chains. To keep the wires

from slipping out of the chains, one end of each one was first given a U

bend and then when the wire was in position under the wedge, this end was

closed tightly with pliers to form an eye around a half-link of the sash

chain. After three years, a slight sagging of the tips of those wedges

whose dihedrals are placed horizontally, can be noticed. It is possible

that some further support should have been arranged for these.

Installation of Wedges The Installation of wedges over the floor of the

on the Floor chamber was quickly accomplished. During this

operation, staging was reduced to a minimum,

since the crew of workmen, while setting the wedges, were able to stand

on a platform of boards resting on the nail strips. The work was further

simplified by fastening the wedges with 5-d box nails in place of the

usual screws. In completing the floor, wedges were filled in from different directions until there remained under the track at the end nearest

the door, only a small area approximately 51 x 51 inches in size. To fill

this opening, a square plug of sixteen wedges was framed together and

lowered by ropes into the space provided. If at any time it should be

found necessary, for repairs or adjustments, to remove some of the floor

wedges, as a first step the 51 x 51 inch plug could be hoisted. A man

would then be able to stand In the space vacated, and by means of a small

D-handled shovel or other tool, could readily pry loose the 5-d nails and

remove the wedges without damage. Also, the wedges could readily be replaced in their original position.

E. Interior Acoustical Doors

After the acoustical wedges were placed on the wall, ceiling and

-78-
-79-
FIG. 38

FIG. 39

-80-
METAL FRAMEWORK OF DEEP BASKET DOORS

FIG. 40

-81-
floor surfaces of the anechoic chamber, there still remained to be treated

the space 8 feet wide by 9 feet high immediately Inside of the entry way.

A pair of doors, each 9 feet high by 4 feet wide and 45 inches deep

were constructed for this entrance (See Fig. 40). The depth was determined

from the overall length of an acoustical wedge, in order to maintain an

interior treatment of uniform thickness. The doors were built of a strong,

lightweight framework of inch gas pipe, well braced with smaller pipe

sizes against skewing and sagging. The Joints were welded. This door has

sagged slightly in three years and presumably should have been built heavier.

The adjacent surfaces of the two doors when in a closed position are

cylindrical in shape. The radii were designed to accomplish a quick separation without slipping and rubbing of surfaces as the doors opened.

Several brackets equal in length to the depth of the acoustical treatment are welded to the main door frames and extend into the chamber. Three

hinges are fastened to the inner ends of these brackets and the doors are

hung from them. When closed, the inner surfaces of the doors form a part

of the chamber's boundary and give continuity to the acoustical lining.

The doors when opened, swing through arcs of 180° each way, and when

in this position, completely clear the full width of the passageway (See

Fig. 41).

FIG. 41

82-
Nail strips to hold, the rabbetted wedge bases were set against the 

gas pipe frames. Wedges and acoustical treatment were installed in the

usual manner to fill the spaces within the basket doors, so as to make

each door a compact acoustical unit. It was necessary to tailor a

number of wedges to fit the cylindrical surfaces which are nearly in

contact with each other when the doors are closed.

-83-
VI. FABRICATION OF TBE ACOUSTICAL WEDGES

The acoustical wedges were fabricated, from P.F. Fiberglas board,

shipped, in blocks having dimensions 4 x 24 x 48 inches. These blocks

were sawed, into the shape of a wedge using a standard, band saw and

were covered with a muslin bag as shown in the accompanying figures.

The first step in the process of manufacture is shown in Fig. 42.

The Fiberglas board was sawed into three units each having the dimensions

4 x 8 x 48 inches. The second process of manufacture is shown in Figs.

45 and 44 where the 8 inch wide unit was sawed diagonally to form the 

two halves of the wedge. On completion of this operation, the two

halves were placed back to back as shown in Fig. 45»

The next step in the assembly was to place two light gauge metal

plates on the two sides of the wedge. These plates were held in place

by two small wooden clamps as shown in Fig. ^6. The following operation

was to draw the muslin bag down over the outside of the structure as

shown in Figs. 47 and 48. A workman then removed the metal plates

("shoe horns") from inside the muslin bag as shown in Fig. 49.

The muslin-covered wedges were then placed in a holding Jig as

shown In Fig. 5$» The two acoustical blankets, having dimensions

1x8x8 Inches were next inserted into the base of each of the wedges

as shown in Fig. 51» The workman then put the wooden frame in place

as shown in Fig. 52, and stapled the muslin bag to the inside of the 

wooden frame at approximately four places (See Fig. 53).

The final operation in the assembly of the wedge was to insert a

number of additional staples into the frame at the base of the wedge

using an automatic stapling gun as shown in Fig. 54. After completion

the wedges were then placed in the original cartons, having dimensions

18 x 24 x 48 inches (See Fig. 55) and- trucked to the anechoic chamber

where they were installed immediately following their manufacture.

To manufacture these wedges, a small factory was formed under the

supervision of Mr. 0. C. Eckel of the Mundet Cork Company of Cambridge,

which supplied the Fiberglas and by working two shifts of twelve people

each, it was possible to fabricate approximately 1000 wedges per day.

By actual count, 5785 man (and woman) hours of labor were expended in

producing 20,000 wedges, or 5*28 wedges per man hour. In addition,

800 man hours were consumed in producing the wooden frames for the base

and 250 in sewing the muslin bags. The total Job added up to 4.15

wedges per man hour. Based on 1942 prices, the 50,000 yards of flameproofed muslin cost $5,475, while the Fiberglas cost about $6,500 delivered.

-84-
FIG. 42

FIG. 43

-85-
FIG. 44

FIG. 45

-86-
FIG. 47 FIG. 46

FIG. 48 FIG. 49

-87-
FIG. 50

FIG. 51

FIG. 52

-88-
FIG. 53

FIG. 54

-89-
FIG. 55

* Fabrication and Installation Experience

With the experience

which the Electro-Acoustic

Laboratory has accumulated

from the fabrication and

installation of wedge linings for three anechoic

chambers, it is possible to

evaluate new suggestions

for design variations.

At least two other

laboratories are planning

the construction of large

anechoic chambers using

P.F. Fiberglas wedges. One

of these, the Bell Telephone Laboratories, expects to cut the wedges

so that they will have an

8 x 24 inch base and to

mount them in groups of

threes, i.e., forming a

square block, 2 feet on

a side. Each of these

three wedges is to be encased in a metal cage made of hardware cloth, and

cut to fit snugly over each 24 x 8 inch wedge. The hardware cloth will

probably be spot welded to a metal base which will in turn be bolted to

metal stringers in the chamber. The 24 x 24 inch blocks will be alternately

rotated to prevent possible diffraction grating effects.

The second chamber is being built by the Naval Ordnance Laboratory in

Washington, D.C. Their plans call for a similar arrangement of three

8 x 24 inch wedges, except they expect to preserve the muslin rtoverlng

over each of the wedges. The bases of these square wedge panels are to

be inserted a distance of 7 inches directly into a 24 inch square egg crate

cell and cemented to the side partitions of the cell to leave a rear sir

space of about 4 inches. Since rigid base frames are no longer needed as

the major means of support for the wedges, it should be necessary to use

only thin cardboard stiffeners at the base of the wedge to support and

stretch the muslin bag covers, which would be glued together. They have

also spoken of replacing the muslin covers by a thin light Fiberglas bonded

mat which has a low flow resistance and desirable fireproof characteristics.

They hope to eliminate the chain ladder support one-half way down the wedge

by stiffening the wedge structures with wooden laths Inserted down the

center and extending part way to the tip. This will necessitate a rigid

connection between the laths and the base of the wedge panel.

-90-
The plans of these two laboratories will fully meet the acoustical

requirements imposed, by our experience. The BTL design looks unusually

simple to build and has the advantage of being basically non-inflammable.

To obtain an efficient structure of greater depth than 44 inches

and a cutoff frequency below 80 cps, it is necessary to use P.F.

Fiberglas board with a specific flow resistance of less than 10 ohms

per inch and of greater length than 48 inches. It is possible that

the manufacturer would produce such boards on request. Trimming ”shims”

of P.F. Fiberglas blanket placed at the base of the wedges may be used

to overcame the effects on absorption produced by any non-uniformity in

production material. However, these last-minute modifications should

not be necessary if the production is carefully controlled by means of

flow resistance measurements.

-91-
VII. CONSTRUCTIONAL FEATURES OF THE SMALL ELECTRO-ACOUSTIC AND PSYCHOACOUSTIC ANECHOIC CHAMBERS

The two smaller anechoic chambers were built in the two laboratories

in order that experiments, where the source and. receiver might be brought

very near to each other, could, be carried, on simultaneously with experiments requiring the use of the large chamber. The one in the ElectroAcoustic Laboratory was built in a small rectangular room formerly used,

by Drs. D. Griffin, R. Galambos and. G. W. Pierce to study the ability of

bats to ”fly blind" using ultra-sonic waves as their "radar." The treatment in the room at the time we took it over consisted of a 2-inch layer

of rock wool spaced out 2 inches from the wall. This treatment was not

removed. The room was 11 x 15 i 10 feet in size, and it was decided to

use as deep an acoustical treatment on the wall a as could be managed. The 

treatment finally selected consisted of a 21-inch wedge, which when installed foimed a structure approximately 25 inches deep, this leaving a

usable working space of about 7 x 11 x 6 feet. Adjacent to the room is

a control room which fortunately is large because it is not unusual for 

several experimenters to have their apparatus set up at one time -

The wedges were built without wooden frames at the bottoms and were

fastened against 1x2 inch furring strips by means of staples ejected

with considerable force from an automatic stapling machine - No second

point of support other than staples at the base was required, although

with buffeting, some of them tend to work loose in time. The wedges would

remain more secure if a flat perforated square of cardboard had been included in the base inside the muslin and if the staples had been driven

through the cardboard. The floor of the room Is covered with a removable

grill work which is supported Just above the dihedral edges of the wedges

by a steel framework. About one half of the total area is left uncovered.

The details of this support are shown in Fig. 56 and a general photograph

of the room can be seen in Fig. 57* The outer entrance is closed by a

suitable "ice-box" door.

A floor plan of the Psycho-Acoustic Laboratory chamber is shown in

Fig. 58A and a photograph in Fig. 58B. This chamber was also lined with

2 Inches of rock wool batts backed by a 2-inch air space. The batts were

enclosed in a chicken-netting type of wire screen and it seemed easiest

to fasten the wedges to their surfaces by meana of safety pins. The entire floor of this room is covered with a steel mesh and because a depth

of only 15 Inches is consumed by the treatment the room is much more

usable than is the one previously described.

-92-
STEEL FRAMEWORK USED IN ELECTRO-ACOUSTIC LABORATORY

ANECHOIC CHAMBER FOR SUPPORTING FLOOR OF REMOVABLE GRILL

FIG. 56

INTERIOR VIEW OF ELECTRO-ACOUSTIC

LABORATORY ANECHOIC CHAMBER

FIG. 57

-95-
58A

58 B

FLOOR PLAN AND INTERIOR VIEW OF

PSYCHO-ACOUSTIC LABORATORY ANECHOIC CHAMBER

-9^-
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-95-

APPENDIX I

The tube method of measuring absorption coefficients has been described in many papers in the literature and the reader is referred to

a summary article -written by Bale J. Sabine, “Notes on Impedance Measurement," Jour. Acous. Soc. Am., 14, 143 (1942) for greater detail than is

given here.

The tube used in this laboratory is made of steel with a loudspeaker

mounted at one end and a termination to receive the sample at the other

end (See Pig. 59). The overall length of the main portion of the tube

is 12 feet and "It is 8 Inches square. A rigid steel plate mounted on a

lead screw and contained in a square section the same size as the main

tube is mounted on the end of the wooden section in which the sample is

contained. This steel plate acts as a rigid wall which can be moved to

provide for more or less air space at the base of a sample. The electrical

circuit is shown in Fig. 60. For the cloth layer structures, a special

sample holder was devised as shown in Fig. 61. As seen in the figure, it

consisted of a great many wooden frames having various thicknesses corresponding to the separations of the sheets or blankets desired. The cloths

were stretched across the faces of the frames and were tacked in place on

the outside edges. Long bolts extended through the frames and were drawn

tight to prevent air leaks.

After the sample is mounted in place, the loudspeaker is turned on.

If perfect absorption does not take place at the sample the incident sound

wave will be partially reflected back toward the source, thus setting up

a standing wave pattern in the tube. A traveling microphone in the tube

controlled by a wire and pul 1 ey system is used to explore this pattern.

The electrical output from the microphone is measured at the points of

minimum and maximum pressure nearest the sample-. The ratio, N, of the maximum and minimum sound pressure is then obtained. The percentage pressure

reflection R at normal incidence is then given simply by the formula

R - JLl A

N + 1

-97-
FIG. 61

A limitation of the tube method,

of measurement is that the range of

frequencies over which accurate measurements can be made is restricted

by the cross-sectional dimensions of

the tube. For an 8-inch square cross

section, the pressure reflection coefficient can be measured only out

to 1500 cps with fair accuracy. At

higher frequencies, measurements are

not possible because of transverse

resonances in the tube.

-98-
APPENDIX II

Measurement of Flow Resistance of Acoustical Materials

In determining the flow resistance of acoustical materials, the

quantities to be measured, are:

p = pressure differential between the two faces

of the sample, in dyne/cm^

v = velocity of linear flow of air through the

sample, in cm/sec produced by the pressure

differential p

The velocity v is the most conveniently determined by measuring the

volume flow v/t, in cm^/sec, per unit area A, in cm^, of sample;

v = v/At cm/sec

The flow resistance R is then

R = p/v = pAt/v gm cm"2 sec“^

in the c.g.s. system of units.

The essential equipment required is therefore;

1. A means of mounting the sample to be tested.

2. A means of drawing air at a uniform rate through

the sample.

3. A gauge for measuring the pressure drop across

the sample.

4. A gauge, or other means, for measuring the rate

of volume flow of air through the sample.

The equipment used in this laboratory is shown schematically in Fig. 62

and is described below in detail.

1. Sample Holder The sample holder consists of a brass tube, A,

in Fig. 62, Fig. 63 and Fig. 64, of 2 1/8 inches

inside diameter, about 10 inches long and circular in cross section.

Samples are cut with a circular punch, as shown in Fig. 65, which may be

used with a hammar or a drill press, or by hand. The inside diameter of

the punch is about 2 5/52 inches so that the sample is cut slightly

larger than the I.D. of the mounting tube in order to insure a snug fit

against the wall of the tube.

A thin sleeve, B, in Fig. 62, is attached to the inside of the mounting tube about 1 inch from its lower end. This supports a loose circular

-99-
FLOW-RESISTANCE APPARATUS

FIG. 62

-100-
FIG. 63

-101-
FIG. 64

-102-
screen of 1/4 inch mesh, C, in Fig. 62 and. Fig. 64, against which the

lower face of the sample rests. This screen is left loose to facilitate

removal of samples from the tube after testing. It should, he mounted, at

least 1 inch from the lower end. of the tube in order to equalize the

pressure over the lower face of the sample.

Compression of the sample is achieved by means of another 1/4 inch

mesh screen attached to a thin movable sleeve, D, in Fig. 62 and Fig. 64.

This screen and sleeve may be advanced along the tube by means of the

screw, E, which runs through a yoke, F, attached by a bayonet lock to

the top of the tube. A graduated scale, G-, indicates the thickness to

which the sample is compressed. Obviously, the yokes attaching the screw

to the movable sleeve and to the top of the tube should be as open as

possible, so as not to Interfere significantly with the air flow through

the tube.

The sample holder slips on a base, H, which has two short tubes

mounted in it to allow attachment of rubber tubes for the pressure gauge

and for drawing air through the sample. The sample holder may be removed

easily from the base to permit introduction or removal of the test sample.

It is Important that the sample holder be sealed to the base during measurements to prevent leakage of air Into the chamber below the sample.

Stopcock grease or rubber cement may be used as a sealing agent'.

For measurements of the flow resistance of trim cloths a sample

holder as shown at Q, Fig. 62 and in Fig. 66, has been used. If the 

cloth being measured is of very low flow resistance, several layers may

be mounted as indicated, with spaces between them. The average flow resistance per layer is then the total resistance divided by the number of

layers . (This holder need not be square in cross section).

2. Air Supply Any of a number of systems which will supply up to

about 0.5 liter of air per second at pressures up

to about 5 inches of water may be used.. The arrangement shown in Fig. 62

is simple and reliable, involving only standard plumbing supplies, and it

has been used for all flow resistance measurements in this laboratory.

A cylindrical ’’range boiler” type of tank, J, in Fig. 62 is suspended near the ceiling. Piping connected to the bottom side of the tank

permits filling it with water from the mains by opening valve K. (A

water-level gauge on the tank is imperative, to aid in avoiding overflow).

With valve E closed, water may be drained from the tank by opening valve

L until a desired rate of flow Is obtained. As the water flows out, air

is drawn into the top of the tank to replace it, through pipe M, and this

air is drawn through the sample. The fineness of control of the rate at

vhl ch air is drawn through the sample is almost entirely dependent on the

fineness of adjustment of valve L. A regular 1/2 inch gate valve has been

found satisfactory.

-105-
FIG. 65

FIG. 66

-104-
If a sample is in place when the tank is being filled., it is necessary

to disconnect the rubber tubing from the sample holder, so as to avoid,

disturbing the sample by reversed, air flow, and. particularly, to avoid,

blowing the fluid, out of the pressure gauge.

5. Pressure Gauge The pressure gauge used, in this laboratory,

N, is Figs. 62 and 63, is the Ellison Inclined Draft Gauge, (Ellison

Draft Gauge Company, Chicago, Illinois), which is a slant type manometer,

giving pressure in inches of water. Two sizes have been used, with full

scale readings of 1.0 inch of water and 5.0 Inches of water, respectively.

The 1.0 inch scale type suffices for the general range of aircraft acoustical materials.

4. Flow meter The flow meter used in this laboratory for measurement of cm5/sec of air flow through the sample is a Type 12HCB Rotameter

made by Fischer and Porter, Hatboro, Pennsylvania. It is shown at P in

Fig. 63. This Instrument, placed in the line between the sample holder

and the water tank, gives the rate of flow of air directly in am?/sec.

It has a scale range of 10 to 220 cm5/sec, which is generally adequate.

Other types of flow or gas meters might also be used.

If a flow meter Is not available, a simple method of measuring rate

of flow is suggested, as follows: For practical purposes, the volume of

air flowing through the sample and into the tank during a given time is

equal to the volume of water flowing out of the tank. Hence, if the water

flowing from the tank over a known period of time is caught in a graduate

And measured, the rate of flow In cm?/sec may be determined. A stopwatch

for measuring the time interval is useful in this procedure. The water is

turned on and allowed to run until the pressure drop across the sample has

reached equilibrium; then the graduate is quickly inserted under the stream

and at the same time the stopwatch is started. When several hundred am?

of water have been collected, the graduate is removed quickly from the

stream, and the watch stopped. At any given pressure, several measurements

should be made, and the results averaged to insure accuracy.

Detailed Procedure for Flow Resistance Measurement:

When the flow resistance, R, of a material with a given volume

density, p, and thickness, T, is to be determined, the steps outlined

below are followed. The method assumes that the thickness of the sample

and Its density are fairly uniform.

1. Cut circular samples from the material such

that the diameter, D, in cm is slightly larger

than that of the sample holder.

2. The flow resistance is given by

1? AP

-105-
where A = area of sample in cm

p = pressure across sample in

dyne/cm2

= 2490 times the number of

inches of water read, on

the manometer

(v/t) = volume velocity of the water

in cc/sec as read, hy the flow

meter

5. Measure the flow resistance of several samples.

Then determine the flow resistance per cm (or

inch) of thickness by dividing by the thickness

of the sample T in cm (or inches). Finally,

determine the arithmetic average of the resistances for unit thickness.

For materials of less uniform density and thickness, a more complex procedure may be followed:

1. Measure the average volume density p

in of a large sample of material.

This figure will be needed later.

2. Cut a number of circular samples from

the material as before, each with a

diameter D in cm. Samples should be

chosen which range in thickness from

2 to 6 cm, by splitting if necessary.

5. Measure the weight G in grams of each

of the several samples at whatever

thickness each is. Then calculate the 

surface density S = g /a in gm/cm^ for each.

4. Measure the flow resistance R as a function of thickness T for the several

samples with the plunger E, Fig. 64.

Then plot values of R vs T for each

individual sample separately on loglog paper and draw the best straight

line through the points. From these

graphs determine R1? the flow resistance

for 1 cm thickness, for each of the

samples. Next, on log-log paper, plot

R^ vs S, using the one datum point for

each sample and draw the best straight

line through the points. Note that now

S is Identically equal to the volume

-IO6-
density at one cm thickness which we

shall call p^. Hence, we shall label

, the abscissa of this graph p^.

5. Using the value of p determined, in (1)

above, find a value of R^ from the graph

Just completed. This value of R-^ should

be approximately the average flow resistance per cm thickness for the large

sample.

6. If the sample is a blanket several cm

thick, the total flow resistance will

be equal to the value of R^, Just

determined times the thickness.

General Comments

Flow resistance is often a function of velocity of air flow through

the sample. It is usually found desirable to make measurements at two or

three different rates of air flow and to extrapolate the results to zero

air flow. However, for many types of acoustical material the variation in

resistance as a function of air flow is decidedly secord order. Little

variation in resistance was found for the Fiberglas samples discussed in

this report. However, the rate of air flow may influence markedly the

value of flow resistance measured for certain trim cloths. Hence, in

testing trim cloths, it is suggested that the measurements be extended to

take this into account.

It is of prime importance that there be no air leaks in the system.

If air enters at any point other than through the sample, the measured

results will be incorrect.

When inserting the sample into the holder, a rod should be used to

tamp gently the edges so that the entire lower surface is in contact with

the screen. When the plunger is inserted the entire upper surface should

also be in contact with the upper screen. If these precautions are not

taken, the thickness as read on Scale G of Fig. 64 will be incorrect.

After opening Valve L, Fig. 65, sufficient time should be allowed

for the pressure and air flow to come to equilibrium before making measurements .

If the pressure differential, p, is measured in inches of water, it

must be multiplied by the conversion factor 2490 to give p in dyne/cm^.

If the area of the sample A is measured in square inches, it must be

multiplied by 6.45 to convert to cm . To convert from volume density in

gm/cc, multiply by 62.4 to obtain the answer in lb/ft3.

-IO?-
Pressure differentials of less than 0.5 inch of water should, he

preferably used, since the sound, pressures encountered, in practice are

selclom greater than 1000 dyne/cm .

-108-