ASTM C423-99a
(Test Method)Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method
Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method
SCOPE
1.1 This test method covers the measurement of sound absorption in a reverberation room by measuring decay rate. Procedures for measuring the absorption of a room, the absorption of an object, such as an office screen, and the sound absorption coefficients of a specimen of sound absorptive material, such as acoustical ceiling tile, are described.
1.2Field Measurements—Although this test method primarily covers laboratory measurements, the test method described in 4.1 can be used for making field measurements of the absorption of rooms (see also 5.5). A non-standard method to measure the absorption of rooms in the field is described in Appendix X2.
1.3 This test method includes information on laboratory accreditation (see Annex A1), asymmetrical screens (see Annex A2), and reverberation room qualification (see Annex A3).
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: C 423 – 99a
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Sound Absorption and Sound Absorption Coefficients by
the Reverberation Room Method
This standard is issued under the fixed designation C 423; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the Department of Defense.
1. Scope S1.26 Method for the Calculation of the Absorption of
Sound by the Atmosphere
1.1 This test method covers the measurement of sound
absorption in a reverberation room by measuring decay rate.
3. Terminology
Procedures for measuring the absorption of a room, the
3.1 Except as noted in 3.3, the terms and symbols used in
absorption of an object, such as an office screen, and the sound
this test method are defined in Terminology C 634. The
absorption coefficients of a specimen of sound absorptive
following definition is not currently included in Terminology
material, such as acoustical ceiling tile, are described.
C 634:
1.2 Field Measurements—Although this test method prima-
3.1.1 sound absorption average, SAA—a single number
rily covers laboratory measurements, the test method described
rating, the average, rounded off to the nearest 0.01, of the
in 4.1 can be used for making field measurements of the
sound absorption coefficients of a material for the twelve
absorption of rooms (see also 5.5). A non-standard method to
one-third octave bands from 200 through 2500 Hz, inclusive,
measure the absorption of rooms in the field is described in
measured according to this test method.
Appendix X2.
3.1.1.1 Discussion—The sound absorption coefficients shall
1.3 This test method includes information on laboratory
be rounded off to the nearest 0.01 before averaging. If the
accreditation (see Annex A1), asymmetrical screens (see An-
unrounded average is an exact midpoint, round to the next
nex A2), and reverberation room qualification (see Annex A3).
higher multiple of 0.01. For example, report 0.625 as 0.63.
1.4 This standard does not purport to address all of the
3.2 In previous versions of this test method a single number
safety concerns, if any, associated with its use. It is the
rating, called the noise reduction coefficient (NRC), was
responsibility of the user of this standard to establish appro-
defined as follows:
priate safety and health practices and determine the applica-
9Round the average of the sound absorption coefficients
bility of regulatory limitations prior to use.
for 250, 500, 1000, and 2000 Hz to the nearest multiple
of 0.05. If the unrounded average is an exact midpoint,
2. Referenced Documents
round to the next higher multiple of 0.05. For example,
0.625 and 0.675 would be reported as 0.65 and 0.70, respectively.9
2.1 ASTM Standards:
C 634 Terminology Relating to Environmental Acoustics
The noise reduction coefficient shall be reported in order to
E 548 Practice for Generic Criteria for Use in Evaluation of
provide comparison with values reported in the past see
Testing and Inspection Agencies
12.1.3).
E 795 Practices for Mounting Test Specimens During
3.3 Definition of Term Specific to This Standard—The
Sound Absorption Tests
following term has the meaning noted for this test method only:
2.2 ANSI Standards:
3.3.1 output interval, Dt, [T], s—of a real-time analyzer, the
S1.6 Preferred Frequencies, Frequency Levels, and Band
time between successive outputs; this time is not necessarily
Numbers for Acoustical Measurements
the same as the integration time.
S1.11 Specification for Octave-Band and Fractional-
4. Summary of Test Method
Octave-Band Analog and Digital Filters
4.1 Measurement of the Sound Absorption of a Room:
4.1.1 A band of random noise is used as a test signal and
This test method is under the jurisdiction of ASTM Committee E-33 on
Environmental Acoustics and is the direct responsibility of Subcommittee E-33.01
turned on long enough (about the time for 20 dB decay in the
on Sound Absorption.
test band with the smallest decay rate) for the sound pressure
Current edition approved March 10, 1999. Published June 1999. Originally
level to reach a steady state. When the signal is turned off, the
published as C 423-58-T. Last previous edition C 423 – 99.
Annual Book of ASTM Standards, Vol 04.06. sound pressure level will decrease and the decay rate in each
Annual Book of ASTM Standards, Vol 14.02.
frequency band may be determined by measuring the slope of
Available from American National Standards Institute, 1430 Broadway, New
a straight line fitted to the sound pressure level of the average
York, NY 10018.
C 423
decay curve. The absorption of the room and its contents is values may exceed unity.
calculated, based on the assumptions that the incident sound
5.4 The coefficients measured by this test method should be
field is diffuse before and during decay and that no additional
used with caution because not only are the areas encountered in
energy enters the room during decay, from the Sabine formula:
practical usage usually larger than the test specimen, but also
the sound field is rarely diffuse. In the laboratory, measure-
Vd
A 5 0.9210 (1)
c ments must be made under reproducible conditions, but in
practical usage the conditions that determine the effective
where:
absorption are often unpredictable. Regardless of the differ-
A 5 sound absorption, m or Sab,
ences and the necessity for judgment, coefficients measured by
3 3
V 5 volume of reverberation room, m or ft ,
this test method have been used successfully by architects and
c 5 speed of sound (calculated according to 11.13), m/s or
consultants in the acoustical design of architectural spaces.
ft/s, and
5.5 Field Measurements—When sound absorption measure-
d 5 decay rate, dB/s,
ments are made in a building in which the size and shape of the
These conditions must be fulfilled if the measurement is to
room are not under the operator’s control, the approximation to
have meaning. The sound absorption calculated according to
a diffuse sound field is not likely to be very close. This matter
Eq 1 is sometimes called the Sabine absorption.
should be considered when assessing the accuracy of measure-
4.1.2 In general, sound absorption is a function of frequency
ments made under field conditions. (See Appendix X2 for a
and measurements are made in a series of frequency bands.
procedure that can be used in the field with less sophisticated
4.2 Measurement of a Sound Absorption Coeffıcient—The
instrumentation.)
absorption of the reverberation room is measured as outlined in
4.1 both before and after placing a specimen of material to be
6. Interferences
tested in the room. The increase in absorption divided by the
area of the test specimen is the dimensionless sound absorption
6.1 Changes in temperature and relative humidity during the
coefficient. In inch-pound units it is reported with the dimen-
course of a measurement may have a large effect on the decay
sionless “unit” sabin per square foot, Sab/ft .
rate, especially at high frequencies and at low relative humidi-
4.3 Measurement of the Sound Absorption of an Object Such
ties. The effects are described quantitatively in ANSI S1.26.
as an Offıce Screen, a Theater Chair, or a Space Absorber—
These effects due to temperature and relative humidity changes
The absorption of the reverberation room is measured as
can be accounted for by the procedure in 6.2.
outlined in 4.1 both before and after placing one or several
6.2 It is advisable to make measurements in the room when
identical objects in the room. The increase in absorption
it is empty and in the room when it contains the test specimen
divided by the number of objects is the absorption in square
under conditions of temperature and relative humidity so
meters per object or sabins per object.
nearly the same that the adjustments due to air absorption do
not differ significantly. In any case, the relative humidity in the
5. Significance and Use
room shall be greater than 40 % during the test. Unless the
5.1 Measurement of the sound absorption of a room is part
conditions given in Table 1 are satisfied, decay rates for the
of the procedure for other acoustical measurements, such as
measurements in the 1000 Hz one-third octave band and above
determining the sound power level of a noise source or the
in both the empty room and in the room containing the test
sound transmission loss of a partition. It is also used in certain
specimen shall be adjusted by subtracting the decay rate due to
calculations such as predicting the sound pressure level in a
air absorption, determined according to ANSI S1.26, from the
room when the sound power level of a noise source in the room
decay rate calculated according to 11.4 (see especially 11.4.1
is known.
and 11.4.2).
5.2 The sound absorption coefficient of a surface is a
property of the material composing the surface. It is ideally NOTE 1—The absorption coefficients given in Table 1 of the version of
ANSI S1.26 current when this test method was written are in units of
defined as the fraction of the randomly incident sound power
dB/km. The corresponding decay rates in dB/s are obtained by multiplying
absorbed by the surface, but in this test method it is operation-
by the speed of sound in m/s and dividing by 1000, as follows:
ally defined in 4.2. The relationship between the theoretically
defined and the operationally measured coefficients is under mc
d 5 (2)
air
continuing study.
5.3 Diffraction effects usually cause the apparent area of a
specimen to be greater than its geometrical area, thereby
increasing the coefficients measured according to this test TABLE 1 Requirements for the Temperature and the Relative
Humidity During Decay Rate Measurement When Adjustment Is
method. When the test specimen is highly absorptive, these
A
Not Made for Air Absorption
Relative Humidity Permitted Range of Permitted Range
Minimum Allowed
During Measure- Relative Humidity of Temperature (D
Chrisler, V., “Dependence of Sound Absorption Upon the Area and Distribution
Temperature, °C
ments, % (D RH), % T), °C
of the Absorbent Material,” Journal of Research, National Bureau of Standards, Vol
Through 60 3 3 10
13, 1934, p. 169: Northwood, T. D., Grisaru, M. T., and Medcof, M. A., “Absorption
Above 60 5 5 10
of Sound by a Strip of Absorptive Material in a Diffuse Sound Field,” Journal of the
A
Acoustical Society of America, Vol 31, 1959, p. 595: and Northwood, T. D.,
The permitted ranges in the second and third columns are the highest
“Absorption of Diffuse Sound by a Strip or Rectangular Patch of Absorptive
measured relative humidity or temperature minus the lowest measured relative
Material,” Journal of the Acoustical Society of America, Vol 35, 1963, p. 1173. humidity or temperature, respectively.
C 423
noise in the measuring instruments, shall be at least 15 dB
where:
d 5 decay rate due to sound absorption by the air, dB/s,
below the lowest level used to calculate decay rate (see 11.3).
air
m 5 absorption coefficient, dB/km, from Table 1 of ANSI S1.26,
and 8. Instrumentation
c 5 speed of sound, m/s, calculated according to 11.13.
8.1 Sound Source—The sound source shall be one or more
loudspeaker systems in a configuration such that the test
7. Reverberation Room
facility satisfies the qualifications of Annex A3. With adequate
7.1 Description—A reverberation room is a room designed
diffusion, loudspeakers facing into the trihedral corners of the
so that the reverberant sound field closely approximates a
room will satisfy these requirements. If more than one loud-
diffuse sound field both in the steady state, when the sound
speaker system is used and the systems are driven simulta-
source is on, and during decay, after the sound source has
neously, they shall be driven by incoherent signals. The sound
stopped.
pressure level produced when the source is on and the sound in
7.2 Construction:
the reverberation room is in the steady state shall be at least 45
7.2.1 The room is best constructed of massive masonry or
dB above the background noise in each measurement band.
concrete materials, but other materials, such as well-damped
8.2 Test Signal—The test signal shall be a band of random
steel, may be used. Lighter construction may be excessively
noise with a continuous spectrum covering the range over
absorptive, especially at frequencies below 200 Hz.
which measurements are made. The frequency range of the
7.2.2 The average absorption coefficient of the room sur-
measurements shall include the one-third octave bands with
faces at each frequency, determined by dividing the absorption
midband frequencies, as defined in ANSI S1.6, from 100 Hz to
of the empty room (measured according to Sections 10 and 11)
5000 Hz.
by the area of the room surfaces, including both sides of the
8.3 Microphones—The microphone or microphones used to
diffusers (see 7.4), shall be less than or equal to 0.05 after
measure decay rate shall be omnidirectional with a flat (61dB
allowance has been made for atmospheric absorption according
within any one-third octave band) random-incidence amplitude
to ANSI S1.26.
response over the range of frequencies and sound pressure
7.2.3 The room shall be isolated sufficiently to keep outside
levels used for decay rate measurements.
noises and structural vibrations from interfering with the
8.4 Electronic Instrumentation—The electronic instruments
measurements.
used to measure sound pressure levels shall be functionally
7.3 Size and Shape—The volume of the room shall be no
equivalent to the instruments specified in 8.4.1 and 8.4.2.
3 3
less than 125 m . It is recommended that the volume be 200 m
8.4.1 Real-time Analyzer—Sound pressure level measure-
or greater. No two room dimensions shall be equal nor shall the
ments shall be made with a one-third octave band real-time
ratio of the largest to the smallest dimension be greater than
analyzer or functional equivalent. The analyzer shall be ca-
2:1. (See 11.12 on calculating room volume.)
pable of measuring with an integration time of 50 ms or less
7.4 Sound Diffusion:
and an output interval of 50 ms or less using either linear or
7.4.1 Means shall be taken to ensure an approximation to a
exponential averaging. Linear averaging is preferred. The filter
diffuse sound field both before and during decay. Experience
response of the analyzer shall be Order 3 or better according
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