Acoustical Testing

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Technical Note (National Research Council of Canada. Division of Building Research), 1955-09-01

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Acoustical Testing

Northwood, T. D.

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DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

.li

_{'fEClHIN ][CAIL}

_NOTE

No.

202

NOT FOR PUBLICATION

PREPARED BY T.D. Northwood CHECKED BY

FOR INTERNAL USE

APPROVED BY N.B.H. PREPARED FOR SUBJECT General Distribution ｾ September 1955 Revised September 1960 Revised August 1962 Acoustical Testing

This note is intended as a guide to manufacturers of acoustical materials, architects, and others who may require the acoustical testing services of the Division

of Building Research. In the first section the common

problems of building acoustics are discussed, to

illus-trate the application of test results. Following this

the test procedures are described, with emphasis on details that should be considered in planning a test. ACOUSTICAL PROBLEMS IN BUILDING

(1) Noise Transmission Through Building Walls and Floors

The solution of the problem of noise transmission between portions of a building may be facilitated at the planning stage by locating noisy regions as far as possible from regions which should

be quiet. In this way the maximum amount of protection may be obtained

for the critical regions. It then remains to provide adequate sound

barriers where spatial separation is impossible or insufficient. A

general guide to the requirements for various adjacent occupancies is given in the Use and Occupancy section of the National Building Code.

The most common transmission problem is the separation of dwelling units in apartment bUildings and other multiple dwellings.

On this continent the usual criterion for the transmission loss of a

' 2 '

-made at nine standard frequencies, between 125 and ｌｾｯｯｯ cycles per

second. Various building recommendations and regulations specify

minimum transmission losses ranging from

45

to

55

decibels. The lower

figure is adequate for separating non-critical regions such as

kitchens, hallways and bathrooms; the higher figure is desirable when

one side of the partition is a bedroom or living room. Central Mortgage

and Housing Corporation require a minimum value of

45

decibels.

When walls or floors having a high transmission loss are used it is necessary to ensure that the value of the partition itself is not nullified by alternative transmission paths, such as doors, service openings, air ducts, and inferior adjacent walls or floors.

The above comments refer to airborne noise - i.e. noise that is transmitted through the air to a partition, setting it in

vibration. The vibrating partition then radiates into the adjacent

room. In addition to transmission of airborne noise there is the

special problem of impact or structure-borne noises such as footsteps, doors slamming and machinery vibrations, which begin as a vibration in

the structure itself. The mechanics of producing and transmitting such

noises vary so much with the type of source and structure that there is

not yet a uniform method of evaluating structures ｦｾ impact transmission.

However, it is possible to make comparative tests, on similar structures. It is expected that a standard test suitable for rating most building structures will shortly be' adopted.

(2) Control of Sound Within a Room

Rooms included under this topic may range from noisy factory areas, where the objective is to avoid damaging the occupants'

hearing, to study areas and libraries, where the ideal is SILENCE. Included are noise reduction problems, in which the object is to minimize unwanted sounds, and what will be called sound distribution problems, in which the object is to provide a proper distribution of wanted sounds, such as speech or music in auditoria lecture halls, etc.

In both types of problems it is frequently useful to employ acoustical materials, which are designed to absorb a large fraction

of the incident sound energy. Perhaps because of this there is a

tendency to confuse the two types of problems, which are in fact almost

exactly opposite. Without going into design details, it might be

worthwhile to stress the distinction. Noise reduction problems usually

require the liberal use of acoustical materials, particularly in the

vicinity of the noise source. Sound distribution problems require the

conservation of the wanted sound, and acoustical materials are used

only where necessary to prevent the sound being muddled by delayed echoes or excessive reverberation.

J

-The commonest criterion for acoustical materials is the sound absorption coefflcient, which 1s the ratio of absorbed to incident

sound energy. In ｧｮｾｲ｡ｬＬ the absorption coefficient varies with the

frequency and with the angle of incidence of the sound wave. For

purposes of comparing materials it is standard practice to determine the sound absorption coefficient at six standard fre0upncles and for

a random sound field (i.e. a sound field composod 0 ｜ﾷＮＺ｡ｶｾｳ arriving

at all angles). A si gle figure commonly used for straightforward

noise reduction problems is the Noise Reduction Coefficient which is

the ｡ｶ･ｲ｡ｾ･ of the absorption coefficients at the four middle frequencies,

rounded off to the nearest five per cent.

Other well-known measures of the acoustical properties of materials are the normal-incidence absorption coefficient and the

normal acoustic impedance. These quantities are easier to measure

than the random-incidence absorption coefficient, and they are of direct

value in the design and production of acoustical materials. They

are of limited value, however, in dealing with the common architectural uses of acoustical materials.

ACOUSTICAL TEST FACILITIES

(1) Sound Absorption Coefficient (for random sound field)

'*

l-teproduced bJi courtesy of tbE; Acoustical f'iaterials Association from

their Bulletin XX, 1960: "Sound Absorption 00efficients of Architectural

Acoustical Materials ".

Generally the method of mounting a material, inclUding any air-space existing behind it, affects its sound-absorbing properties.

Hence it is desirable to test it on mountings similar to those used

in practice. In the interests of uniformity, certain moUntings have'

become standard smong testing laboratories. These are shown in Appendix

A.

*

However tests are frequently undertkaen with special mountings.

The sound absorption coefficient of a material is measured by observing the effect of introducing a sample of the material into a

large bare room known as a reverberation chamber. The chamber is

designed to absorb very little sound, and hence has a very long reverberation time (defined as the time, after the source is turned

off, for the sound level to drop 60 decibels). A typical test sample

has the same order of absorption as the empty chamber and therefore

placing the sample in the chamber significantly reduces the reverberation

time. The absorption coefficient is calculated from reverberation times

measured with and without the sample in the chamber.

The standard sample area is

9

ft. by

8

fto, although these

dimensions may be varied slightly to accommodate whole units of the material under test.

4

-Some materials such as acoustical plasters are formed ｾ

situ in ordinary use. The acoustical properties of such materials

may depend critically on the method of preparing and applying them, and it is preferred that the client either prefabricate the sample

(possibly in several small panels) or supervise its preparation in

the laboratory.

A standard test comprises measurements of the sound absorption

coefficient at six frequencies: 125, 250, 500, 1000, 2000, and 4000

cycles per second. Test procedures follow ASTM C423-60T.

The ｾ･･ for one test (at six frequencies) is $100.00. In

addition, an installation charge may be made for samples requiring special preparation or special mounting arrangements (other than those shown in AppendiX A).

(2) Transmission Loss of Walls and Floors for Air-Borne Sound

Facilities are available for testing wall panels 10 ft. wide

by 8 ft. high and floor panels 8 ft. square. Doors of special wall

panels of smaller dimensions can also be tested, although an extra charge may be incurred in reducing the size of test opening.

The regular test includes measurements at eleven frequencies:

125, 175, 2509 350, 500, 700,1000, 1400, 20009 2800, and 4000 cycles

per secondo Test procedures follow A.S.T.M. Recommended Practice

No. E90-6lT.

The fee for one test (at eleven frequencies) is $100.00 plus

the cost of constructing or installing the test panel. The client

may? if he prefers, construct or install the sample himself. (3) Impedance Tube Tests

Facilities are available for normal-incidence absorption or impedance measurements on small samples of absorptive materials in

the frequency range 200 to 3000 cyoles per second. These are

re-commended particularly for comparisons of similar materials, for example for quality control, for developing improved materials or

for determining the effect of various paints" Two sizes may be

aocommodated:

(a) Samples

2*"

in diameter

(b) Samples 12" square (Note: These can be tested on their

standard mountings)

Test details suitable for a particular application are set up

in consultation with the client. There is no established fee, tests

5

-GENERAL PROCEDURE FOR ACOUSTICAL TESTS

The following notes will indicate the usual procedure for carrying out these tests:

1. Tests are carried out for the manufacturer of a material, or for any other interested party subject to the permission of the manufacturer if a particular product is in question;

2. Initial inquiries regarding tests should be addressed to: Division of Building Research,

National Research Council, Ottawa, Ontario.

3.

If it is possible to conduct the test the client will be requested

to make application on special';:forms provided, and instructions will be given for preparing and shipping the test samples;

4.

Test results are reported to the client only and are regarded as

confidential. It should be noted that the National Research Council

does not approve or endorse particular materials, but merely carries

out specified tests on samples submitted by the client. It is not

permissible, in advertisements, technical brochures, or other

publications, to refer to the National Research Council in connection with the test results without the consent of the Council.

-'.

APPENDIX A

TYPES OF MOUNTING

(Used in Conducting Sound Absorption Tests)

Reproduced with permission of the Acoustical Materials Association from their Bulletin XX:

"Sound Absorption Coefficients of Architectural

-,

TYPES OF MOUNTING

(Used in Conducting Sound Absorption Tests)

I.

Cemented to pla8ter board with ｾＢ air space. Con8idered equivalent to cementing to pla8ter or concrete ceiling.

3.

Attached to metal 8upports applied. to nominal I" x 3" wood furring.

t:l8O

'.: -0'" ｾｾ ".' '.. -•..｟ｾＧ •.ＺＧ［ＧＧＧｾＧＮ ",. :.•

.;:. ｾＧ｜ﾷＺｩ ＾ｾＧＢ {:,y' ,:...::: :i;';: ｾＮＺＺ

:'.-5.

Furred I", furring 24" O.c. I" Min-erai wool between furring.

. 7.

Mechanically mounted on Ｘｾ｣ｩ｡ｬ metal support8.

2.

Nailed to nominal 1"x 3" wood fur-ring 12" o.c. unle88 otherwise indicated.

4.

Laid directly on laboratory floqr.

6.

Attached to 24 gao sheet iron, sup-ported by metal angle8.

s.

Furred 2", furring 24" O.C. 2" Mineral wool between furring.