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IS0
I N T E R NAT1 O N AL O R G A N I ZATl O N
FOR STAN DAR DI ZAT I O N
I SO RE C OM M EN DATl ON
R 354
MEASUREMENT OF ABSORPTION COEFFICIENTS
IN A REVERBERATION ROOM
1st EDITION
Decem ber 1 963
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The copyright of IS0 Recommendations and IS0 Standards
belongs to IS0 Member Bodies. Reproduction of these
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by the national standards organization of that country, being
a member of ISO.
For each individual country the only valid standard is the national standard of that country.
Printed in Switzerland
be obtained through the national standards organizations.
Also issued in French and Russian. Copies to
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BRIEF HISTORY
The IS0 Recommendation R 354, Measurement of Absorption Coefjcient in a Reverberation
Room, was drawn up by Technical Committee ISO/TC 43, Acoustics, the Secretariat of which is
held by the British Standards Institution (B.S.I.).
Work on this question by the Technical Committee began in 1955 and led, in 1961, to the
adoption of a Draft IS0 Recommendation.
In November 1961, this Draft IS0 Recommendation (No. 477) was circulated to all the
IS0 Member Bodies for enquiry. It was approved, subject to a few modifications of an editorial
nature, by the following Member Bodies:
Australia Germany New Zealand
Austria Greece Portugal
Belgium India Switzerland
Brazil Indonesia Sweden
Denmark Ir e 1 and United Kingdom
Finland Netherlands U.S.S.R.
Three Member Bodies opposed the approval of the Draft:
France, Japan, U.S.A.
The Draft IS0 Recommendation was then submitted by correspondence to the IS0 Council,
which decided, in December 1963, to accept it as an IS0 Recommendation.
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ISOlR 354 - 1963 (E)
IS0 Recommendation R 354
Decem ber 1963
MEASUREMENT OF ABSORPTION COEFFICIENTS
IN A REVERBERATION ROOM
INTRODUCTION
With a source of sound operating in an enclosed space, the level to which reverberant sound builds
up, and the subsequent decay of reverberant sound when the source is stopped, are governed by
the sound-absorbing characteristics of the boundary surfaces or objects in the room. In general,
the fraction of the incident sound power which is absorbed at a surface depends upon the angle of
incidence. In order to be able to assess the reverberation time of an auditorium or the noise re-
duction which would be effected by an absorbing treatment in e.g. an office or workshop, a know-
ledge is required of the absorbing characteristics of the surfaces, normally in the form of a suitable
average over all angles of incidence. Since the angular distribution of sound in actual enclosures
varies, it is convenient for the purpose of standardization to take as the basis the simplest angular
distribution, namely a uniform distribution, since there are no grounds for preferring special
directions. This is called the difiuse state.
The aim of measurements of the sound absorption of objects, or the absorption coefficients of
surfaces in a reverberation room, is to obtain the values appropriate to this condition. In view of
the difficulty of obtaining complete diffusion, absorption coefficients determined in a reverb-
eration room may not be so precise as absorption coefficients measured at specific angles of inci-
dence, especially at normal incidence by the tube method. The fact that, owing to diffraction at the
edges, the equivalent absorption area of a plane specimen is not directly proportional to its surface
area, also contributes to this lack of precision. Nevertheless, measurements under reverberant con-
ditions are necessary, because only in this way can one include the effects of practical mounting
conditions, which cannot be simulated in the tube method. Furthermore, it is the only way to
determine the absorption of separate objects.
The purpose of this IS0 Recommendation is to promote uniformity in the methods and condi-
tions of measurement of absorption coefficients in reverberation rooms, so that values determined
by different laboratories should agree as far as possible.
Again, it should be pointed out that to attain the above aims stricter diffuse conditions are required
than ordinarily exist in most auditoria, and certain other restrictions, e.g. the volume of the rever-
beration room, are necessary. As a consequence, the absorption coefficients determined by the
method described may not be directly applicable for design calculations in special cases.
It is hoped that these recommendations will not restrict in any way development of new techniques
for the measurement of sound absorption. In some respects, the present methods may fall short
of the declared aims, and these proposals will possibly be revised as improved methods come into
use.
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ISOlR 354 - 1963 (E)
1. SCOPE
This IS0 Recommendation describes how a reverberation room should be used to measure, under
specified conditions, the sound absorption coefficients of acoustical materials used as wall or
ceiling treatments, or the equivalent absorption area of separate objects, such as furniture, persons
or space absorbers. The general principle is that the specimen is introduced into the room and the
absorption added is computed from measurements of the reverberation time of the room (or the
decay rate of the reverberant sound) before and after the introduction of the specimen.
This IS0 Recommendation specifies certain features of the size and shape of the room, the size
and dispasition of the test specimei.,, the methods of measuring the reverberation time (or the
decay rate of the reverberant sound) and of computing the results, the frequencies to be used
and the manner in which the results should be stated.
2. EXPLANATION OF TERMS AND PRINCIPLE OF MEASUREMENTS
2.1 Reverberation. For the purpose of this IS0 Recommendation, the decay of the sound energy
in an enclosure after the source has stopped is called reverberation.
In most cases, for a given frequency or frequency band, this decay takes place approximately
exponentially with time after the first reflections. If this is so, the process of reverberation
may be characterized quantitatively in either of the two following manners :
(U) by the decay rate d, i.e. the rate of decay of the measured sound pressure level with time
in decibels per second (see clause 2.2 below),
(b) by the reverberation time T, i.e. the time in seconds required for the measured sound
pressure level to decrease by 60 dB.
60
T=-
d
2.2 The decay rate d is given by two additive parts:
(1) the first is due to the dissipation of sound energy during the propagation in air, and may
be characterized by the energy attenuation coefficient m
da = (10/ln 10) c - m = 4.34 ce m
(W
where c is the velocity of sound in air;
(2) the second depends on the absorption of sound energy at the partial reflections at the
boundaries of the room and the objects in it.
This part defines the equivalent absorption area A of the room, according to the following
formula :
db = [10/(4 In lo)] c A/V = 1.086 c - A/V
(2b)
where V is the volume of the room.
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IÇOIR 354 - 1963 (E)
2.2.1 The equivalent absorption area A may be evaluated from the measured decay rate or the
measured reverberation time, according to the equations :
d = da + db = 1.086 c (4 m + A/V) = 60/T
(2c)
d 55.3
giving A = (0.92 - - 4 m) V ou A= ( -4m)~
(24
C cT
If the equivalent absorption area of the empty reverberation room is increased by the
AA of objects brought into the room, AA may be evaluated
additional absorption area
from the difference of the reverberation decay rates dz after, and dl before, the introduction
of the objects, according to the formula:
V V1 1
A A = (d.92 - (d, - dl) = (55.3 ; (- - -)
C T2 TI
if the temperature and humidity, which influence the energy attenuation coefficient m,
have not been altered substantially between the two measurements. Otherwise, for high
frequencies, additional terms have to be taken into account:
Furthermore, it is assumed that the introduction of the additional objects does not alter
the equivalent absorption area of the empty room by an appreciable amount due to the
covering of part of the walls, floors, etc.
2.3 Absorption coefficient. In the case of a plane absorber mounted on the floor, wall or ceiling
of the room, AA may be divided by the surface area S of the specimen under test, and a
specific quantity, the absorption coefficient, may be so evaluated :
us = AA/S (4)
In the case of a poor absorber (e.g. plaster), it may be necessary to consider that AA is actually
the difference between the equivalent absorption area of the material under test and that of the
portion of wall or floor covered. The formula (4) has then to be amended to :
us = AAjS f us, (5)
where usl is the absorption coefficient of the wall or floor covered. It should be mentioned,
however, that this quantity can be measured only very crudely by the evaluation
of the decay rate dl or the reverberation time TI of the empty room (see Appen-
dix, clause A.2.3).
3. MEASUREMENT ARRANGEMENTS
3.1 Room
3.1.1 The volume of the reverberation room should be larger than 180 m3. It is further recom-
mended, in the case of new constructions, that the volume should be as close to 200 m3 as
possible (see Appendix, clause A.3.1.1).
3.1.2 The shape of the room should be such that
Imax < 1.9 V1/3,
where Zmax is the length of the greatest straight line which can fit within the boundary of
the room (e.g. in a rectangular room, it is the major diagonal).
3.1.3 The room should be such that the decaying sound field is sufficiently diffuse (see Appendix,
clause A.3.1.3). Natural frequencies in the low-frequency region should be as uniformly
spaced as possible.
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ISOIR 354 - 1963 (E)
3.1.4 The reverberation times of the empty room should exceed the values of:
5.0 5.0 5.0 4.5 3.5 2.0 seconds
at 125 250 500 1000 2000 4000 Hz(c/s)
(see Appendix, clauses A.3.1.1 and A.3.1.4).
Moreover, the curve of the reverbe
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