ASTM C384-03
(Test Method)Standard Test Method for Impedance and Absorption of Acoustical Materials by the Impedance Tube Method
Standard Test Method for Impedance and Absorption of Acoustical Materials by the Impedance Tube Method
SCOPE
1.1 This test method covers the use of an impedance tube, alternatively called a standing wave apparatus, for the measurement of impedance ratios and the normal incidence sound absorption coefficients of acoustical materials.
1.2 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.
1.3 The values stated in SI units are to be regarded as the standard.
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Designation: C 384 – 03
Standard Test Method for
Impedance and Absorption of Acoustical Materials by the
1
Impedance Tube Method
This standard is issued under the fixed designation C 384; 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.
634, they are not repeated here.
1. Scope
1.1 This test method covers the use of an impedance tube,
4. Summary of Test Method
alternatively called a standing wave apparatus, for the mea-
4.1 A plane wave traveling in one direction down a tube is
surement of impedance ratios and the normal incidence sound
reflected back by the test specimen to produce a standing wave
absorption coefficients of acoustical materials.
that can be explored with a microphone. The normal incidence
1.2 This standard does not purport to address all of the
sound absorption coefficient, a , is determined from the
n
safety concerns, if any, associated with its use. It is the
standing wave ratio at the face of the test specimen. To
responsibility of the user of this standard to establish appro-
determine the impedance ratio, z/rc, a measurement of the
priate safety and health practices and determine the applica-
position of the standing wave with reference to the face of the
bility of regulatory limitations prior to use.
specimen is needed.
1.3 The values stated in SI units are to be regarded as the
4.2 The absorption coefficient and impedance ratio are
standard.
functions of frequency. Measurements are made with pure
tones at a number of frequencies chosen, unless there are
2. Referenced Documents
compelling reasons to do otherwise, from those specified in
2.1 ASTM Standards:
ANSI S1.6.
C 423 Test Method for Sound Absorption and Sound Ab-
2
sorption Coefficients by the Reverberation Room Method
5. Significance and Use
2
C 634 Terminology Relating to Environmental Acoustics
5.1 The acoustical impedance properties of a sound absorp-
E 548 Guide for General Criteria Used for Evaluating
tive material are related to its physical properties, such as
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Laboratory Competence
airflow resistance, porosity, elasticity, and density. As such, the
2.2 ANSI Standards:
measurements described in this test method are useful in basic
S1.6 Preferred Frequencies and Band Numbers for Acous-
research and product development of sound absorptive mate-
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tical Measurements
rials.
5.2 Normal incidence sound absorption coefficients are
3. Terminology
more useful than random incidence coefficients in certain
3.1 The acoustical terminology used in this test method is
situations. They are used, for example, to predict the effect of
intended to be consistent with the definitions in Terminology
placing material in a small enclosed space, such as inside a
C 634. In particular, the terms “impedance ratio,” “absorption,”
machine.
and “impedance,” appearing in the title and elsewhere in this
5.3 Estimates of the random incidence or statistical absorp-
test method refer to the following, respectively: impedance
tion coefficients for materials can be obtained from normal
ratio, normal incidence sound absorption coefficient, and spe-
incidence impedance data. For materials that are locally
cific normal acoustic impedance, which are defined in Termi-
reacting, that is, without sound propagation inside the material
nology C 634.
parallel to its surface, statistical absorption coefficients can be
NOTE 1—Since the definitions for these terms appear in Terminology C
estimated from specific normal acoustic impedance values
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using an expression derived by London (1). Locally reacting
materials include those with high internal losses parallel with
1
This test method is under the jurisdiction of ASTM Committee E33 on
the surface such as porous or fibrous materials of high density
Environmental Acoustics and is the direct responsibility of Subcommittee E33.01 on
or materials that are backed by partitioned cavities such as a
Sound Absorption.
Current edition approved Sept. 10, 2003. Published September 2003. Originally
honeycomb core. Formulas for estimating random incidence
approved in 1956. Last previous edition approved in 1998 as C 384 – 98.
2
Annual Book of ASTM Standards, Vol 04.06.
3
Discontinued; see 2002 edition Annual Book of ASTM Standards, Vol 14.02.
4 5
Available from American National Standards Institute (ANSI), 25 W. 43rd St., The boldface numbers in parentheses refer to the list of references at the end of
4th Floor, New York, NY 10036. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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C384–03
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