ASTM C384-98
(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:C384–98
Standard Test Method for
Impedance and Absorption of Acoustical Materials by the
1
Impedance Tube Method
This standard is issued under the fixed designation C384; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (ε) indicates an editorial change since the last revision or reapproval.
1. Scope 3.2 impedance ratio, z/ρc ≡ r/ρc + jx/ρc; [dimensionless]—
theratioofthespecificnormalacousticimpedanceatasurface
1.1 This test method covers the use of an impedance tube,
to the characteristic impedance of the medium. The real and
alternatively called a standing wave apparatus, for the mea-
imaginarycomponentsarecalled,respectively, resistance ratio
surement of impedance ratios and the normal incidence sound
and reactance ratio. C634
absorption coefficients of acoustical materials.
3.3 normal incidence sound absorption coeffıcient, α ;
1.2 This standard does not purport to address all of the n
[dimensionless]—of a surface, at a specified frequency, the
safety concerns, if any, associated with its use. It is the
fraction of the perpendicularly incident sound power absorbed
responsibility of the user of this standard to establish appro-
or otherwise not reflected. C634
priate safety and health practices and determine the applica-
3.4 specific normal acoustic impedance, z ≡ r + jx;
bility of regulatory limitations prior to use.
−2 −1
[ML T ]; mks rayl (Pa·s/m)—at a surface, the complex
1.3 The values stated in SI units are to be regarded as the
quotient obtained when the sound pressure averaged over the
standard.
surface is divided by the component of the particle velocity
2. Referenced Documents
normal to the surface. The real and imaginary components of
2.1 ASTM Standards: thespecificnormalacousticimpedancearecalled,respectively,
specific normal acoustic resistance and specific normal acous-
C423 Test Method for Sound Absorption and Sound Ab-
2
sorption Coefficients by the Reverberation Room Method tic reactance. C634
2
C634 Terminology Relating to Environmental Acoustics
4. Summary of Test Method
E548 Guide for General Criteria Used for Evaluating
3
4.1 A plane wave traveling in one direction down a tube is
Laboratory Competence
reflectedbackbythetestspecimentoproduceastandingwave
2.2 ANSI Standards:
that can be explored with a microphone.The normal incidence
S1.6 Preferred Frequencies and Band Numbers for Acous-
4
sound absorption coefficient, α , is determined from the stand-
n
tical Measurements
ingwaveratioatthefaceofthetestspecimen.Todeterminethe
3. Terminology
impedance ratio, z/ρc, a measurement of the position of the
standing wave with reference to the face of the specimen is
3.1 The acoustical terminology used in this test method is
intended to be consistent with the definitions in Terminology needed.
4.2 The absorption coefficient and impedance ratio are
C634.Inparticular,theterms“impedanceratio,”“absorption,”
and “impedance,” appearing in the title and elsewhere in this functions of frequency. Measurements are made with pure
tones at a number of frequencies chosen, unless there are
test method refer to the following, respectively:
compelling reasons to do otherwise, from those specified in
ANSI S1.6.
1
This test method is under the jurisdiction of ASTM Committee E-33 on 5. Significance and Use
EnvironmentalAcousticsandisthedirectresponsibilityofSubcommitteeE33.01on
5.1 The acoustical impedance properties of a sound absorp-
Sound Absorption.
tive material are related to its physical properties, such as
Current edition approved July 10, 1998. Published March 1999. Originally
published as C384–56T. Last previous edition C384–95.
airflow resistance, porosity, elasticity, and density.As such, the
2
Annual Book of ASTM Standards, Vol 04.06.
measurements described in this test method are useful in basic
3
Annual Book of ASTM Standards, Vol 14.02.
4 research and product development of sound absorptive mate-
Available from American National Standards Institute, 11 W. 42nd St., 13th
Floor, New York, NY 10036. rials.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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C384
5.2 Normal incidence sound absorption coefficients are this reason, no measurement should be made closer than one
more useful than random incidence coefficients in certain tube diameter to the source end of the tube.
situations. They are used, for example, to predict the effect of
6.1.1.3 Length—The length of the tube is also related to the
placing material in a small enclosed space, such as inside a
frequencies at which measurements are made. The tube must
machine.
belongenoughtoconta
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