ASTM C423-23

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: C423 − 22 C423 − 23
Standard Test Method for
Sound Absorption and Sound Absorption Coefficients by
the Reverberation Room Method
This standard is issued under the fixed designation C423; 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 (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. 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.
1.2 Field Measurements—Although this test method 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 method to measure the absorption of rooms in the field
is described in Test Method E2235.
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 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C634 Terminology Relating to Building and Environmental Acoustics
E795 Practices for Mounting Test Specimens During Sound Absorption Tests
E2235 Test Method for Determination of Decay Rates for Use in Sound Insulation Test Methods
2.2 ANSI Standards:
S1.6 Preferred Frequencies, Frequency Levels, and Band Numbers for Acoustical Measurements
S1.11 Specification for Octave-Band and Fractional-Octave-Band Analog and Digital Filters
This test method is under the jurisdiction of ASTM Committee E33 on Building and Environmental Acoustics and is the direct responsibility of Subcommittee E33.01
on Sound Absorption.
Current edition approved Feb. 1, 2022April 15, 2023. Published March 2022May 2023. Originally approved in 1958. Last previous edition approved in 20172022 as
C423 – 17.C423 – 22. DOI: 10.1520/C0423-22.10.1520/C0423-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
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S1.26 Method for the Calculation of the Absorption of Sound by the Atmosphere
S1.43 Specifications for Integrating-Averaging Sound Level Meters
2.3 IEC Standards
IEC 61672 Electroacoustics–Sound Level Meters–Part 1: Specifications
3. Terminology
3.1 Except as noted in 3.3, the terms and symbols used in this test method are defined in Terminology C634. The following
definition is not currently included in Terminology C634:
3.1.1 sound absorption average, SAA—a single number rating, the average, rounded off to the nearest 0.01, of the sound absorption
coefficients of a material for the twelve one-third octave bands from 200 through 2500 Hz, inclusive, measured according to this
test method.
3.1.1.1 Discussion—The sound absorption coefficients shall be rounded off to the nearest 0.01 before averaging. If the unrounded
average is an exact midpoint, round to the next higher multiple of 0.01. For example, report 0.625 as 0.63.
3.2 In previous versions of this test method a single number rating, called the noise reduction coefficient (NRC), was defined as
follows:
9Round the average of the sound absorption coefficients
for 250, 500, 1000, and 2000 Hz to the nearest multiple
of 0.05. If the unrounded average is an exact midpoint,
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
The noise reduction coefficient shall be reported in order to provide comparison with values reported in the past (see 12.1.3).
3.3 Definition of Term Specific to This Standard—The following term has the meaning noted for this test method only:
3.3.1 output interval, Δt, [T], s—of a real-time analyzer, the time between successive outputs; this time is not necessarily the same
as the integration time.
4. Summary of Test Method
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 turned on long enough (about the time for 20 dB decay in the test band
with the smallest decay rate) for the sound pressure level to reach a steady state. When the signal is turned off, the sound pressure
level will decrease and the decay rate in each frequency band may be determined by measuring the slope of a straight line fitted
to the sound pressure level of the average decay curve. The absorption of the room and its contents is calculated, based on the
assumptions that the incident sound field is diffuse before and during decay and that no additional energy enters the room during
decay, from the Sabine formula:
Vd
A 5 0.9210 (1)
c
where:
A = sound absorption, m ,
V = volume of reverberation room, m ,
c = speed of sound (calculated according to 11.13), m/s and
d = decay rate, dB/s,
NOTE 1—Previous editions of this test method, which included mixed units, included the in./lbs unit of sound absorption, the sabin (Sab). The number
of sabins is the value of A that would be derived from Eq 1 with the volume in ft and the speed of sound in ft/s. This unit finds frequent use in older
literature. One Sab of sound absorption is approximately equal to 0.0929 m of sound absorption.
These conditions must be fulfilled if the measurement is to have meaning. The sound absorption calculated according to Eq 1
is sometimes called the Sabine absorption.
4.1.2 In general, sound absorption is a function of frequency and measurements are made in a series of frequency bands.
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4.2 Measurement of a Sound Absorption Coeffıcient—The absorption of the reverberation room is measured as outlined in 4.1 both
before and after placing a specimen of material to be tested in the room. The increase in absorption divided by the area of the test
specimen is the dimensionless sound absorption coefficient.
4.3 Measurement of the Sound Absorption of an Object Such as an Offıce Screen, a Theater Chair, or a Space Absorber—The
absorption of the reverberation room is measured as outlined in 4.1 both before and after placing one or several identical objects
in the room. The increase in absorption divided by the number of objects is the absorption in square meters per object.
5. Significance and Use
5.1 Measurement of the sound absorption of a room is part of the procedure for other acoustical measurements, such as
determining the sound power level of a noise source or the sound transmission loss of a partition. It is also used in certain
calculations such as predicting the sound pressure level in a room when the sound power level of a noise source in the room is
known.
5.2 The sound absorption coefficient of a surface is a property of the material composing the surface. It is ideally defined as the
fraction of the randomly incident sound power absorbed by the surface, but in this test method it is operationally defined in 4.2.
The relationship between the theoretically defined and the operationally measured coefficients is under 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 method. When the test specimen is highly absorptive, these values may exceed
unity.
5.4 The coefficients measured by this test method should be used with caution because not only are the areas encountered in
practical usage usually larger than the test specimen, but also the sound field is rarely diffuse. In the laboratory, measurements must
be made under reproducible conditions, but in practical usage the conditions that determine the effective absorption are often
unpredictable. Regardless of the differences and the necessity for judgment, coefficients measured by this test method have been
used successfully by architects and consultants in the acoustical design of architectural spaces.
5.5 Field Measurements—When sound absorption measurements are made in a building in which the size and shape of the room
are not under the operator’s control, the approximation to a diffuse sound field is not likely to be very close. This matter should
be considered when assessing the accuracy of measurements made under field conditions. (See Test Method E2235 for a procedure
that can be used in the field with less sophisticated instrumentation.)
6. Interferences
6.1 Changes in temperature and relative humidity during the course of a measurement may have a large effect on the decay rate,
especially at high frequencies and at low relative humidities. The effects are described quantitatively in ANSI S1.26. These effects
of temperature and relative humidity changes shall be minimized as follows:
6.1.1 During all measurements of decay rate The average temperature shall be no less than 10 °C; Deviations from the average
temperature shall not exceed 5 °C. The average relative humidity in the room shall be no less than 40%. Deviations from the
average relative humidity shall not exceed 6 5% in the measured relative humidity value.
6.1.2 All decay rates in the 1000 Hz one-third octave band and above shall be adjusted by subtracting the decay rate due to air
absorption from the decay rate calculated according to 11.4. For these calculations, assume the values calculated for the mid-band
frequency apply to the complete one-third-octave band. The air absorption shall be calculated according to ANSI S1.26 using its
standard air absorption values at the center frequency of each one third octave band, respectively. Use Eq 2 below:
d 5 m’c (2)
air
Chrisler, V., “Dependence of Sound Absorption Upon the Area and Distribution of the Absorbent Material,” Journal of Research, National Bureau of Standards, Vol 13,
1934, p. 169: Northwood, T. D., Grisaru, M. T., and Medcof, M. A., “Absorption of Sound by a Strip of Absorptive Material in a Diffuse Sound Field,” Journal of the
Acoustical Society of America, Vol. 31, 1959, p. 595: and Northwood, T. D., “Absorption of Diffuse Sound by a Strip or Rectangular Patch of Absorptive Material,” Journal
of the Acoustical Society of America, Vol. 35, 1963, p. 1173.
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where:
d = decay rate due to sound absorption by the air, dB/s,
air
m’ = attenuation coefficient, dB/m, taken from ANSI S1.26, as described in 6.1.2.1, and
c = speed of sound, m/s, calculated according to 11.13.
6.1.2.1 The attenuation coefficients m’ shall be derived from the equations and calculation procedures of 5.1 – 5.3 and Annex B
of ANSI S1.26. Table 1 of ANSI S1.26 shall not be used.
7. Reverberation Room
7.1 Description—A reverberation room is a room designed so that the reverberant sound field closely approximates a diffuse sound
field both in the steady state, when the sound source is on, and during decay, after the sound source has stopped.
7.2 Construction:
7.2.1 The room is best constructed of massive masonry or concrete materials, but other materials, such as well-damped steel, may
be used. Lighter construction may be excessively absorptive, especially at frequencies below 200 Hz.
7.2.2 The average absorption coefficient of the room surfaces at each frequency, determined by dividing the absorption of the
empty room (measured according to Sections 10 and 11) by the area of the room surfaces, including both sides of the diffusers
(see 7.4), shall be less than or equal to 0.05 for the one-third octave bands centered at 250 through 2500 Hz, after allowance has
been made for atmospheric absorption according to ANSI S1.26. For the bands centered below 250 Hz, and above 2500 Hz, the
similarly determined coefficient shall be less than or equal to 0.10.
7.2.3 The room shall be isolated sufficiently to keep outside noises and structural vibrations from interfering with the
measurements.
3 3
7.3 Size and Shape—The volume of the room shall be no less than 125 m . It is recommended that the volume be 200 m or
greater. No two room dimensions shall be equal nor shall the ratio of the largest to the smallest dimension be greater than 2:1. (See
11.12 on calculating room volume.)
7.4 Sound Diffusion:
7.4.1 Means shall be taken to ensure an approximation to a diffuse sound field both before and during decay. Experience has shown
that a satisfactory approximation can be achieved with a number of sound-reflective panels hung or distributed with random
orientations about the volume of the room. It is strongly recommended that some of these panels be mounted on a rotating shaft
or otherwise kept moving, presenting, in effect, a room that continually changes its shape.
7.4.2 The goal is to achieve a rapid and continuous interchange of energy between the directions of sound propagation, thereby
increasing the probability that each surface area of the room is exposed to sound of the same intensity.
7.4.3 Laboratories are strongly encouraged to follow the procedures in Appendix X1 to determine the necessary area of diffusing
panels to maximize the measured absorption coefficients. If these procedures are followed, the data collected shall be preserved
and made available on request. If the procedures in Appendix X1 are not followed, the surface area of the diffusing elements in
the room (both faces) shall be at least 25 % of the surface area of the reverberation room. (See Note X1.1.)
7.4.4 The reverberation room shall be qualified according to Annex A3.
7.5 Background Noise—The level of the background noise in each measurement band, which includes both the ambient acoustical
noise in the reverberation room and the electrical noise in the measuring instruments, shall be at least 15 dB below the lowest level
used to calculate decay rate (see 11.3).
8. Instrumentation
8.1 Sound Source—The sound source shall be one or more loudspeaker systems in a configuration such that the test facility
satisfies the qualifications of Annex A3. With adequate diffusion, loudspeakers facing into the trihedral corners of the room will
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satisfy these requirements. The sound pressure level produced when the source is on and the sound in the reverberation room is
in the steady state shall be at least 45 dB above the background noise in each measurement band.
NOTE 2—The value of 45 dB is the minimum value required by this method. In fact, the steady state may need more than 45 dB above the background
noise to satisfy the requirements of 7.5 and 11.3.
8.2 Test Signal—The test signal shall be a band of random noise with a continuous spectrum covering the range over which
measurements are made. The frequency range of the measurements shall include the one-third octave bands with midband
frequencies, as defined in ANSI S1.6, from 100 Hz to 5000 Hz.
8.3 Microphones—The microphone or microphones used to measure decay rate shall be omnidirectional with a flat (6 2 dB within
any one-third octave band) random-incidence amplitude response over the range of frequencies and sound pressure levels used for
decay rate measurements.
8.4 Electronic Instrumentation—The electronic instruments used to measure sound pressure levels shall be functionally equivalent
to the instruments specified in 8.4.1 and 8.4.2.
8.4.1 Real-time Analyzer—Sound pressure level measurements shall be made with a one-third octave band real-time analyzer or
functional equivalent. The analyzer shall conform to or exceed the requirements of ANSI S1.43 or IEC 61672. The analyzer shall
be capable of measuring with an integration time of 50 ms or less and an output interval of 50 ms or less using either linear or
exponential averaging. Linear averaging is preferred. The filter response of the analyzer shall be class 1 or better according to ANSI
S1.11.
NOTE 3—The response of the real-time analyzer should be checked to determine the minimum decay rate that can be measured at a given integration time
setting by feeding a signal directly into the analyzer input and measuring the decay rate when the signal is turned off. The decay rate measured by this
check should be at least three times the decay rate measured during a sound absorption measurement.
8.4.2 Control and Storage Circuitry—Control and storage circuitry shall be provided to:
8.4.2.1 Turn the source on and off and start and stop the real-time analyzer as specified in Section 10, and
8.4.2.2 Store the levels measured during decays as required by Section 10.
9. Test Specimen
9.1 Floor, Wall, or Ceiling Specimens for Absorption Coeffıcient:
9.1.1 The specimen shall be a rectangular patch assembled from one or more pieces. An area of 6.69 m is customary and
recommended, in a shape 2.44 by 2.74 m . An area less than 5.57 m shall not be used, and extreme aspect ratios, such as long
narrow strips, shall be avoided.
2 2
NOTE 4—The un-rationalized SI units values of 6.69 m , 2.44 by 2.74 m, and 5.57 m were given as their equivalent inch-pound values of, respectively,
2 2
72 ft , 8 by 9 ft, and 60 ft in previous editions of this standard.
9.1.2 Mounting—Insofar as its acoustical properties are concerned, the specimen shall be mounted in a way that simulates actual
installation. The types of mountings most commonly used are specified in Practices E795. If a mounting fixture is used, it shall
be removed from the reverberation room during the empty room tests unless it can be shown that the mounting fixture does not
contribute to the empty room sound absorption.
2 2
NOTE 5—The un-rationalized SI units value of 2.32 m was given as its equivalent inch-pound value of 25 ft in previous editions of this standard.
9.1.3 Placement—The specimen may be placed on the floor of the reverberation room for convenience of measurement. It is best
to avoid symmetry: do not place the specimen in the exact center of the floor or with its sides parallel to the walls. When the
orientation of the specimen may affect its acoustical properties (if, for instance, the specimen is a curtain), provision shall be made
for mounting in the usual position. No part of the specimen shall be closer than 0.75 m to a reflective surface other than the one
backing it.
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9.1.4 Precautions—When testing ceiling materials it is important that sound be prevented from entering the specimen by any path
other than through the front surface. For this reason, the sides of the specimen should be covered tightly with non-absorptive
material and any paths to the back of the specimen should be sealed. See Practices E795 for methods to seal the edges of test
specimens.
9.2 Specimens that are Offıce Screens:
9.2.1 Size—For test purposes, an office screen shall have an overall area, measured on one side and including the frame, of not
less than 2.32 m . For the purpose of determining the sound absorption coefficient, α, the total area of the screen is the area of the
two sides. It does not include the area of the edges, that is, the product of the perimeter of the screen and its thickness. Should
the screens submitted for test be too small, two or more should be fitted together to make, in effect, a single screen. To prevent
extreme aspect ratios, the ratio of the screen or combined-screen height (including frame) to width (including frame) used to
calculate the total area shall be no greater than 2:1 and no less than 1:2.
9.2.2 Number of Screens—For a standard test the absorption of an office screen shall be measured with just one screen or a
combination of screens that are fitted together to make, in effect, a single screen (see 9.2.1) in the reverberation room. It is the result
of this measurement that is to be used when screens of different kinds are compared. However, if desired, two or more screens may
be tested at the same time provided all details of the arrangement are described in the report. The details shall include distances
from each other and the room boundaries, and the angles they make with each other.
9.2.3 Placement—The office screen shall be free-standing, at least 0.75 m away from the room boundaries and other reflective
surfaces except the floor, and not parallel to the walls.
9.2.4 For office screens that have different sound-absorptive constructions on either side of the central plane of the screen, see
Annex A2.
9.3 Specimens that are Detached Objects—The absorption of objects, such as space absorbers, theater chairs, or ceiling baffles,
is dependent on the number tested together and their distance from each other and from the room boundaries. Complete information
shall be given in the report.report (see Section 12).
9.3.1 When the specimen is a number of isolated objects, the minimum sample quantity shall be the number of objects necessary
to meet not less than 10.0 m of total exposed surface area for the entire array. This requirement does not apply to Type M and
Type K mountings defined by Practices E795. If accurate area calculations are not available for complex three-dimensional forms,
the total exposed area may be approximated from simplified shapes of nearest possible dimensions.
NOTE 6—A cuboid object suspended above the test surface has 6 exposed faces and its total exposed surface area is the sum of the areas of all 6 faces.
The total exposed surface area includes all surfaces regardless of acoustical properties.
9.4 Preconditioning—The test specimen shall be allowed to adjust to the temperature and humidity in the reverberation room
before tests are performed.
10. Procedure for Measuring Decay Rate
10.1 Microphone Positions:
10.1.1 If a fixed microphone or microphones are used, make measurements at five or more positions which are at least 1.5 m apart,
and at least 0.75 m from any surface of the test specimen.
10.1.2 If a moving microphone is used, the microphone path shall be at least 0.75 m from any surface of the room or test specimen.
The same limit shall apply to the distance from any fixed diffusing element (excluding edges). The length of the microphone path
shall be at least 7.5 m. Longer paths are preferred since they improve the precision of the measurements at low frequencies.
10.1.3 If moving or rotating diffusers are used, the period of the diffusers, the time between the beginning of successive decays
and the period of the motion of the microphone should be adjusted to spread out the points at which decays start, as much as
feasible, over the positions of the diffusers and the positions of the microphone.
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10.2 Number of Decays:
10.2.1 Measure at least 50 decays in each room condition (that is, in the empty room and in the room with the test specimen).
10.2.2 If stationary microphone positions are used, measure the same number of decays, at least 10 decays, at each microphone
position.
NOTE 7—It is no longer required (as it was in previous versions of this test method) not to use decays that deviate substantially from a straight line over
the measuring range when graphed on a logarithmic scale. Reverberation rooms that satisfy the requirements of Section 7 provide the best diffusion that
is practically achievable and, hence, are as likely as possible to be free from nonlinear decays.
10.3 Analyzer Settings:
10.3.1 If the real time analyzer has settings for both integration time and output interval, the integration time of the analyzer shall
be between 90 and 100 % of the output interval time.
10.3.2 The output interval shall be short enough to provide at least five measurement points that satisfy 11.3 in every measurement
band. Whenever conditions permit, the output interval shall be adjusted to provide at least ten measurement points that satisfy 11.3
in every measurement band.
10.3.3 The output shall include all of the one-third octave bands in the frequency range from 100 to 5000 Hz, inclusive, specified
by ANSI S1.6.
10.4 Measurement of Decay Rate:
10.4.1 Turn on the test signal until the sound pressure level in each measurement band is steady (see 4.1).
10.4.2 Turn off the test signal and start measuring sound pressure level in each measurement band either immediately or after a
delay in range of 100 to 300 ms (see Fig. 1). (Data collected before the first 100 to 300 ms have elapsed may be viewed or retained
for informational purposes, but these data are not used in the calculation of decay curves.)
NOTE 8—The delay time period in the range of 100 to 300 ms ensures that data collected for decay rate calculation include no distortions or transients
caused by turning off the test signal. Viewing the decays on an oscilloscope, computer screen or paper chart can help avoid a number of problems, such
as those related to transients.
10.4.3 Measure and store the sound pressure level in each measurement band every Δt seconds (see 3.3.1) until the level is about
32 dB below the steady state level (see 7.5).
10.4.4 Store the measured levels and repeat this procedure the number of times required by 10.2.
11. Calculations
11.1 In each measurement band, calculate the points in the average decay curve, defined as follows:
N
L 5 L (3)
~ !
i ij
(
N
j50
where:
i and j = integers,
(L ) = average of the sound pressure levels measured at the ith data point in each of N decays,
i
N = the number of decays, at least 50, and
L = the sound pressure level measured at the ith data point during the jth decay.
ij
11.2 In each measurement band, the first data point to be used to calculate the decay rate shall be the first data point for which
integration begins at least 100 to 300 ms after the test signal was turned off.
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FIG. 1 Schematic Example of a Decay Measurement—a) Starting the Real-Time Analyzer When the Source Stops: b) Starting the Real-
Time Analyzer 100 to 300 ms After the Source Stops
11.3 In each measurement band the number of data points in the average decay, M, shall be the maximum value of the index, i,
for which:
~L !2 ~L ! # 25dB (4)
1 i
where (L ) is the average of the first data points satisfying 11.2 (see Fig. 1).
11.4 Calculate the decay rate in every measurement band. In this test method, the operational definition for the decay rate is the
negative of the slope of the linear, first-order regression on the average decay curve of Eq 3. The expression for the decay rate is
shown below:
M M
d'5 M11 L 2 2 i L (5)
F ~ ! ~ ! ~ !G
2 ( i ( i
M~M 2 1!Δt
i51 i51
where d' is the decay rate, dB/s, and M is defined in 11.3.
11.4.1 Adjust the decay rate by subtracting the decay rate due to air absorption as noted in 6.1, thus:
d 5 d'2d (6)
air
11.5 The procedures of 11.1, 11.2, 11.3, and 11.4 may be used to calculate decay rates for each microphone position. In this case
the average of the decay rates in each measurement band over all microphone positions shall be used to calculate sound absorption.
11.6 The calculation of sound absorption of the reverberation room using the Sabine formula (Eq 1) is described in 4.1.1.
11.7 In every measurement band calculate the absorption added to the room by the test specimen as follows:
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A 5 A 2 A (7)
2 1
where:
A = absorption of the specimen, m ,
A = absorption of the empty reverberation room, m and
A = absorption of the room after the specimen has been installed, m .
11.8 For each test frequency, calculate the sound absorption coefficient of the test specimen and round to the nearest 0.01 as
follows:
α5 A 2 A /S1α (8)
~ !
...