ASTM C384-04(2011)
(Test Method)Standard Test Method for Impedance and Absorption of Acoustical Materials by Impedance Tube Method
Standard Test Method for Impedance and Absorption of Acoustical Materials by Impedance Tube Method
SIGNIFICANCE AND USE
The acoustical impedance properties of a sound absorptive material are related to its physical properties, such as airflow resistance, porosity, elasticity, and density. As such, the measurements described in this test method are useful in basic research and product development of sound absorptive materials.
Normal incidence sound absorption coefficients are more useful than random incidence coefficients in certain situations. They are used, for example, to predict the effect of placing material in a small enclosed space, such as inside a machine.
Estimates of the random incidence or statistical absorption coefficients for materials can be obtained from normal incidence impedance data. For materials that are locally reacting, that is, without sound propagation inside the material parallel to its surface, statistical absorption coefficients can be estimated from specific normal acoustic impedance values using an expression derived by London (1).5 Locally reacting materials include those with high internal losses parallel with the surface such as porous or fibrous materials of high density or materials that are backed by partitioned cavities such as a honeycomb core. Formulas for estimating random incidence sound absorption properties for both locally and bulk-reacting materials, as well as for multilayer systems with and without air spaces have also been developed (2).
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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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: C384 − 04(Reapproved 2011)
Standard Test Method for
Impedance and Absorption of Acoustical Materials by
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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope C634. In particular, the terms “impedance ratio,” “normal
incidence sound absorption coefficient,” and “specific normal
1.1 This test method covers the use of an impedance tube,
acoustic impedance,” appearing in the title and elsewhere in
alternatively called a standing wave apparatus, for the mea-
this test method refer to the following, respectively:
surement of impedance ratios and the normal incidence sound
absorption coefficients of acoustical materials. 3.2 Definitions:
3.2.1 impedance ratio, z/ρc ≡ r/ρc+jx/ρc;
1.2 This standard does not purport to address all of the
[dimensionless]—the ratio of the specific normal acoustic
safety concerns, if any, associated with its use. It is the
impedance at a surface to the characteristic impedance of the
responsibility of the user of this standard to establish appro-
medium. The real and imaginary components are called,
priate safety and health practices and determine the applica-
respectively, resistance ratio and reactance ratio. C634
bility of regulatory limitations prior to use.
1.3 The values stated in SI units are to be regarded as 3.2.2 normal incidence sound absorption coeffıcient, α ;
n
standard. No other units of measurement are included in this [dimensionless]—of a surface, at a specified frequency, the
standard. fraction of the perpendicularly incident sound power absorbed
or otherwise not reflected. C634
2. Referenced Documents
3.2.3 specific normal acoustic impedance, z≡r+jx;
-2 -1
2.1 ASTM Standards:
[ML T ]; mks rayl (Pa s/m)—at a surface, the complex
C423TestMethodforSoundAbsorptionandSoundAbsorp-
quotient obtained when the sound pressure averaged over the
tion Coefficients by the Reverberation Room Method
surface is divided by the component of the particle velocity
C634Terminology Relating to Building and Environmental
normal to the surface. The real and imaginary components of
Acoustics
thespecificnormalacousticimpedancearecalled,respectively,
E548Guide for General Criteria Used for Evaluating Labo-
specific normal acoustic resistance and specific normal acous-
ratory Competence (Withdrawn 2002)
tic reactance. C634
2.2 ANSI Standards:
S1.6Preferred Frequencies and Band Numbers forAcousti- 4. Summary of Test Method
cal Measurements
4.1 A plane wave traveling in one direction down a tube is
reflectedbackbythetestspecimentoproduceastandingwave
3. Terminology
that can be explored with a microphone.The normal incidence
3.1 The acoustical terminology used in this test method is
sound absorption coefficient, α , is determined from the stand-
n
intended to be consistent with the definitions in Terminology
ingwaveratioatthefaceofthetestspecimen.Todeterminethe
impedance ratio, z/ρc, a measurement of the position of the
standing wave with reference to the face of the specimen is
ThistestmethodisunderthejurisdictionofASTMCommitteeE33onBuilding
needed.
and Environmental Acoustics and is the direct responsibility of Subcommittee
E33.01 on Sound Absorption.
4.2 The normal incidence absorption coefficient and imped-
Current edition approved Nov. 1, 2011. Published December 2011. Originally
ance ratio are functions of frequency. Measurements are made
approved in 1956. Last previous edition approved in 2004 as C384–04. DOI:
withpuretonesatanumberoffrequencieschosen,unlessthere
10.1520/C0384-04R11.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
arecompellingreasonstodootherwise,fromthosespecifiedin
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ANSI S1.6.
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
The last approved version of this historical standard is referenced on 5. Significance and Use
www.astm.org.
5.1 The acoustical impedance properties of a sound absorp-
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. tive material are related to its physical properties, such as
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C384 − 04 (2011)
airflowresistance,porosity,elasticity,anddensity.Assuch,the It is best to work well below these limits whether the tube is
measurements described in this test method are useful in basic circularorrectangular.Atfrequenciesabovetheselimits,cross
research and product development of sound absorptive mate- modesmaydevelopandtheincidentandreflectedwavesinthe
rials. tubearenotlikelytobeplanewaves.Ifsoundwithafrequency
below the limiting value enters the tube as a non-plane wave,
5.2 Normal incidence sound absorption coefficients are
itwillbecomeaplanewaveaftertravelingashortdistance.For
more useful than random incidence coefficients in certain
this reason, no measurement should be made closer than one
situations. They are used, for example, to predict the effect of
tube diameter to the source end of the tube.
placing material in a small enclosed space, such as inside a
6.1.1.3 Length—The length of the tube is also related to the
machine.
frequencies at which measurements are made. The tube must
5.3 Estimates of the random incidence or statistical absorp-
belongenoughtocontainthatpartofthestandingwavepattern
tion coefficients for materials can be obtained from normal
needed for measurement. That is, it must be long enough to
incidence impedance data. For materials that are locally
containatleastoneandpreferablytwosoundpressureminima.
reacting, that is, without sound propagation inside the material
Toensurethatatleasttwominimacanbeobservedinthetube,
parallel to its surface, statistical absorption coefficients can be
its length should be such that:
estimated from specific normal acoustic impedance values
5 f.0.75 c/ l 2 d (3)
~ !
using an expression derived by London (1). Locally reacting
materials include those with high internal losses parallel with
where:
the surface such as porous or fibrous materials of high density
l = length of tube, m.
or materials that are backed by partitioned cavities such as a
If, for example, the tube is1min length and 0.1 m in
honeycomb core. Formulas for estimating random incidence
diameter and the speed of sound is 343 m/s, the frequency
sound absorption properties for both locally and bulk-reacting
should exceed 286 Hz if two sound pressure minima are to be
materials, as well as for multilayer systems with and without
observed.
air spaces have also been developed (2).
6.1.2 Test Specimen Holder—The specimen holder, a de-
tachable extension of the tube, must make an airtight fit with
6. Apparatus
the end of the tube opposite the sound source. Provision must
6.1 The apparatus is essentially a tube with a test specimen
be made for containing the specimen with its face in a known
at one end and a loudspeaker at the other.Aprobe microphone
position. The interior cross-sectional shape of the specimen
that can be moved along the length of the tube is used to
holder must be the same as the tube itself. Provision must be
explore the standing wave in the tube. The signal from the
made for backing the specimen with a metal backing plate that
microphone is filtered, amplified, and recorded.
forms a seal with the interior of the specimen holder. A
6.1.1 Tube:
recommended backing is a solid steel plate with a thickness of
6.1.1.1 Construction—The tube may be made of metal,
not less than 2 cm. The sample holder may be constructed in
plastic, portland cement, or other suitable material that has
such a way that a variable depth air space can be provided
inherently low sound absorption properties. Its interior cross
between the back of the test specimen and the surface of the
section may be circular or rectangular but must be uniform
metal backing plate. Provision must be made for substituting
from end to end. The tube must be straight and its inside
the metal backing plate for the specimen for calibration
surfacemustbesmooth,nonporousandfreeofdusttokeepthe
purposes.
sound attenuation with distance low. The interior of the tube
6.1.3 Sound Source:
may be sealed with paint, epoxy, or other coating material to
6.1.3.1 Kind and Placement—The sound source may be a
ensure low sound absorption of the interior surface. The tube
loudspeaker or a horn-driver coupled to a short exponential
wallsmustbemassiveandrigidenoughsothatthepropagation
horn. The source may face directly into the tube or, to avoid
of sound energy through them by vibration is negligible.
interference with the probe microphone, it may be placed to
6.1.1.2 Diameter—For circular tubes, the upper limit (3) of
oneside.Sincethesourcediametermaybelargerthanthetube
frequency is:
diameter,itisbesttomountthesourceinanenclosuretowhich
f,0.586 c/d (1)
the tube is connected.
6.1.3.2 Precautions—Precautions should be taken to avoid
where:
direct transmission of vibration from the sound source to the
f = frequency, Hz,
probe microphone where it enters the tube or to the tube itself.
c = speed of sound in the tube, m/s, and
Such vibrational transmission will be evidenced by a smaller
d = diameter of tube, m.
standing wave ratio (higher normal incidence sound absorp-
For rectangular tubes, with d used as a symbol for the larger
tion) than would be expected for the material under test.
cross section dimension, the upper limit is:
Vibration isolation material, such as polymeric foam, may be
f,0.500 c/d (2)
placed between the sound source and tube or the microphone
probe, or both, to minimize this effect. Interaction between the
soundfieldwithinthetubeandtheloudspeakerdiaphragmmay
cause the frequency response of the loudspeaker to be nonlin-
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard. ear. Although this has no effect on measurement accuracy, it
C384 − 04 (2011)
does require awkward changes in amplifier gain settings when 6.1.9 Monitoring Oscilloscope—While not required for any
switching between test frequencies. This effect can be mini- actual measurement purpose, it is recommended that an oscil-
mized by lining the interior of the tube near the sound source loscope be used to monitor both the voltage driving the sound
with a porous, absorbent material. source and the output of the amplifier. Observing the oscillo-
scope trace is useful in locating the exact position of pressure
6.1.4 Microphone—If the microphone is small enough, it
minima within the tube as well as in detecting distortion,
maybeplacedinsidetheimpedancetubeconnectedtoarodor
excess noise, and other possible problems in the voltage
otherdevicethatcanbeusedtomoveitalongthelengthofthe
signals.
tube. If the microphone is placed within the tube, the total
cross-sectional area of the microphone and microphone sup-
7. Sampling
ports shall be less than 5% of the total cross-sectional area of
thetube.Inmostapplications,themicrophoneisontheoutside
7.1 At least three specimens, preferably more if the sample
connected to a hollow probe tube that is inserted through the is not uniform, should be cut from the sample for the test.
source end of the apparatus and is aligned with the central axis
Whenthesamplehasasurfacethatisnotuniform(forexample
ofthetube.Inprinciple,thesensingelementofthemicrophone a fissured acoustical tile), each specimen should be chosen to
orofthemicrophoneprobemaybepositionedanywherewithin
include, in proper proportion, the different kinds of surfaces
thetubecross-sectionalarea.Inpractice,themicrophoneorthe
existing in the larger sample.
end of the probe tube must be supported by a spider or other
8. Test Specimen Preparation and Mounting
device to maintain its position on the central axis of the
impedance tube or at a constant distance from the central axis.
8.1 The measured impedance properties can be strongly
6.1.5 Microphone Position Indicator—Ascale shall be pro-
influenced by the specimen mounting conditions. Therefore,
vided to measure the position of the microphone with respect
the following guidelines for the preparation and mounting of
to the specimen face. It is not necessary that zero on the scale
specimens are provided.
correspondtothepositionofthespecimenface.Theresolution
8.2 The specimen must have the same shape and area as the
of this scale should be such that microphone position can be
tubecrosssection,neithermorenorless.Thespecimenmustfit
measured to the nearest 1.0 mm or, if a vernier is used, to the
snugly into the specimen holder, fitting not so tightly that it
nearest 0.1 mm.
bulgesinthecenter,norsolooselythatthereisaspacebetween
6.1.6 Test Signal:
its edge and the holder. Movement of the specimen as a whole
6.1.6.1 Frequency—The test signal shall be provided by a
and spaces between the specimen perimeter and sample holder
sinewaveoscillatorgeneratingapuretonechosenfromthelist
can result in anomalous values of normal incidence sound
of preferred band center frequencies listed in ANSI S1.6. The
absorption. Specimen movement can be minimized by the use
test frequency shall be controlled to within 61% during the
ofthin,double-sidedadhesivetapeappliedbetweenthebackof
course of a measurement. If a digital frequency synthesizer is
the specimen and the metal backing plate. Spaces at the
used,thetestsignalmaybeassumedtoagreewiththesetpoint
specimen perimeter can be sealed with petroleum jelly.
within the required 61%.
8.3 The specimen must have a relatively flat surface since
6.1.6.2 Frequency Counter—It may be necessary, and is
the reflected wave from a very uneven surface may not have
usually advisable, to measure the frequency of the signal with
become a plane wave at the position of the first minimum. If
an electronic counter rather than to rely on the calibration and
the specimen is an anechoic wedge, or an array of wedges,
indicated setting of the frequency generator. Frequency should
refer to Annex A1.
be indicated to the nearest 1 Hz.
8.4 When the specimen has a very uneven back, a layer of
6.1.7 Output-Measuring Equipment:
putty-like material should be placed between it and the metal
6.1.7.1 Filter—The microphone output should be filtered to
backing plate to seal the back of the specimen and to add
remove any harmonics and to reduce the adverse effect of
enough thickness to make the back of the specimen parallel to
ambientnoise.Thefilterwidthmus
...
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