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Designation: C 423 – 08a

Standard Test Method for

Sound Absorption and Sound Absorption Coefficients by
the Reverberation Room Method1
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 lastreapproval. A
superscript epsilon (´) 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
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 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 tomeasure 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 thisstandard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

S1.6 Preferred Frequencies, Frequency Levels, and Band
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
shallbe 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.
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
withnon-absorptive material and any paths to the back of the
specimen should be sealed. See Practices E 795 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 m2 (25 ft2). For the purpose of determining
the soundabsorption coefficient, a, 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-screenheight (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 theresult 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.

10. Procedure for Measuring Decay Rate...
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