1、Designation: C384 04 (Reapproved 2011)Standard Test Method forImpedance and Absorption of Acoustical Materials byImpedance Tube Method1This standard is issued under the fixed designation C384; the number immediately following the designation indicates the year oforiginal adoption or, in the case of
2、revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the use of an impedance tube,alternatively called a standing wave appara
3、tus, for the mea-surement of impedance ratios and the normal incidence soundabsorption coefficients of acoustical materials.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appr
4、o-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.2. Referenced Documents2.1 ASTM Standards:2C423 Test Method for Sou
5、nd Absorption and Sound Ab-sorption Coefficients by the Reverberation Room MethodC634 Terminology Relating to Building and EnvironmentalAcousticsE548 Guide for General Criteria Used for Evaluating Labo-ratory Competence32.2 ANSI Standards:S1.6 Preferred Frequencies and Band Numbers for Acous-tical M
6、easurements43. Terminology3.1 The acoustical terminology used in this test method isintended to be consistent with the definitions in TerminologyC634. In particular, the terms “impedance ratio,” “normalincidence sound absorption coefficient,” and “specific normalacoustic impedance,” appearing in the
7、 title and elsewhere inthis test method refer to the following, respectively:3.2 Definitions:3.2.1 impedance ratio, z/rc r/rc+jx/rc;dimensionlessthe ratio of the specific normal acousticimpedance at a surface to the characteristic impedance of themedium. The real and imaginary components are called,
8、respectively, resistance ratio and reactance ratio. C6343.2.2 normal incidence sound absorption coeffcient, an;dimensionlessof a surface, at a specified frequency, thefraction of the perpendicularly incident sound power absorbedor otherwise not reflected. C6343.2.3 specific normal acoustic impedance
9、, z r+jx;ML-2T-1; mks rayl (Pa s/m)at a surface, the complexquotient obtained when the sound pressure averaged over thesurface is divided by the component of the particle velocitynormal to the surface. The real and imaginary components ofthe specific normal acoustic impedance are called, respectivel
10、y,specific normal acoustic resistance and specific normal acous-tic reactance. C6344. Summary of Test Method4.1 A plane wave traveling in one direction down a tube isreflected back by the test specimen to produce a standing wavethat can be explored with a microphone. The normal incidencesound absorp
11、tion coefficient, an, is determined from thestanding wave ratio at the face of the test specimen. Todetermine the impedance ratio, z/rc, a measurement of theposition of the standing wave with reference to the face of thespecimen is needed.4.2 The normal incidence absorption coefficient and imped-anc
12、e ratio are functions of frequency. Measurements are madewith pure tones at a number of frequencies chosen, unless thereare compelling reasons to do otherwise, from those specified inANSI S1.6.5. Significance and Use5.1 The acoustical impedance properties of a sound absorp-tive material are related
13、to its physical properties, such asairflow resistance, porosity, elasticity, and density.As such, the1This test method is under the jurisdiction ofASTM Committee E33 on Buildingand Environmental Acoustics and is the direct responsibility of SubcommitteeE33.01 on Sound Absorption.Current edition appr
14、oved Nov. 1, 2011. Published December 2011. Originallyapproved in 1956. Last previous edition approved in 2004 as C384 04. DOI:10.1520/C0384-04R11.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandard
15、s volume information, refer to the standards Document Summary page onthe ASTM website.3Withdrawn. The last approved version of this historical standard is referencedon www.astm.org.4Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.a
16、nsi.org.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.measurements described in this test method are useful in basicresearch and product development of sound absorptive mate-rials.5.2 Normal incidence sound absorption coefficients
17、aremore useful than random incidence coefficients in certainsituations. They are used, for example, to predict the effect ofplacing material in a small enclosed space, such as inside amachine.5.3 Estimates of the random incidence or statistical absorp-tion coefficients for materials can be obtained
18、from normalincidence impedance data. For materials that are locallyreacting, that is, without sound propagation inside the materialparallel to its surface, statistical absorption coefficients can beestimated from specific normal acoustic impedance valuesusing an expression derived by London (1).5Loc
19、ally reactingmaterials include those with high internal losses parallel withthe surface such as porous or fibrous materials of high densityor materials that are backed by partitioned cavities such as ahoneycomb core. Formulas for estimating random incidencesound absorption properties for both locall
20、y and bulk-reactingmaterials, as well as for multilayer systems with and withoutair spaces have also been developed (2).6. Apparatus6.1 The apparatus is essentially a tube with a test specimenat one end and a loudspeaker at the other. A probe microphonethat can be moved along the length of the tube
21、is used toexplore the standing wave in the tube. The signal from themicrophone is filtered, amplified, and recorded.6.1.1 Tube:6.1.1.1 ConstructionThe tube may be made of metal,plastic, portland cement, or other suitable material that hasinherently low sound absorption properties. Its interior cross
22、section may be circular or rectangular but must be uniformfrom end to end. The tube must be straight and its insidesurface must be smooth, nonporous and free of dust to keep thesound attenuation with distance low. The interior of the tubemay be sealed with paint, epoxy, or other coating material toe
23、nsure low sound absorption of the interior surface. The tubewalls must be massive and rigid enough so that the propagationof sound energy through them by vibration is negligible.6.1.1.2 DiameterFor circular tubes, the upper limit (3) offrequency is:f , 0.586 c/d (1)where:f = frequency, Hz,c = speed
24、of sound in the tube, m/s, andd = diameter of tube, m.For rectangular tubes, with d used as a symbol for the largercross section dimension, the upper limit is:f , 0.500 c/d (2)It is best to work well below these limits whether the tube iscircular or rectangular.At frequencies above these limits, cro
25、ssmodes may develop and the incident and reflected waves in thetube are not likely to be plane waves. If sound with a frequencybelow the limiting value enters the tube as a non-plane wave,it will become a plane wave after traveling a short distance. Forthis reason, no measurement should be made clos
26、er than onetube diameter to the source end of the tube.6.1.1.3 LengthThe length of the tube is also related to thefrequencies at which measurements are made. The tube mustbe long enough to contain that part of the standing wave patternneeded for measurement. That is, it must be long enough tocontain
27、 at least one and preferably two sound pressure minima.To ensure that at least two minima can be observed in the tube,its length should be such that:f . 0.75 c/l 2 d! (3)where:l = length of tube, m.If, for example, the tube is1minlength and 0.1 m indiameter and the speed of sound is 343 m/s, the fre
28、quencyshould exceed 286 Hz if two sound pressure minima are to beobserved.6.1.2 Test Specimen HolderThe specimen holder, a de-tachable extension of the tube, must make an airtight fit withthe end of the tube opposite the sound source. Provision mustbe made for containing the specimen with its face i
29、n a knownposition. The interior cross-sectional shape of the specimenholder must be the same as the tube itself. Provision must bemade for backing the specimen with a metal backing plate thatforms a seal with the interior of the specimen holder. Arecommended backing is a solid steel plate with a thi
30、ckness ofnot less than 2 cm. The sample holder may be constructed insuch a way that a variable depth air space can be providedbetween the back of the test specimen and the surface of themetal backing plate. Provision must be made for substitutingthe metal backing plate for the specimen for calibrati
31、onpurposes.6.1.3 Sound Source:6.1.3.1 Kind and PlacementThe sound source may be aloudspeaker or a horn-driver coupled to a short exponentialhorn. The source may face directly into the tube or, to avoidinterference with the probe microphone, it may be placed toone side. Since the source diameter may
32、be larger than the tubediameter, it is best to mount the source in an enclosure to whichthe tube is connected.6.1.3.2 PrecautionsPrecautions should be taken to avoiddirect transmission of vibration from the sound source to theprobe microphone where it enters the tube or to the tube itself.Such vibra
33、tional transmission will be evidenced by a smallerstanding wave ratio (higher normal incidence sound absorp-tion) than would be expected for the material under test.Vibration isolation material, such as polymeric foam, may beplaced between the sound source and tube or the microphoneprobe, or both, t
34、o minimize this effect. Interaction between thesound field within the tube and the loudspeaker diaphragm maycause the frequency response of the loudspeaker to be nonlin-ear. Although this has no effect on measurement accuracy, itdoes require awkward changes in amplifier gain settings when5The boldfa
35、ce numbers in parentheses refer to the list of references at the end ofthis standard.C384 04 (2011)2switching between test frequencies. This effect can be mini-mized by lining the interior of the tube near the sound sourcewith a porous, absorbent material.6.1.4 MicrophoneIf the microphone is small e
36、nough, itmay be placed inside the impedance tube connected to a rod orother device that can be used to move it along the length of thetube. If the microphone is placed within the tube, the totalcross-sectional area of the microphone and microphone sup-ports shall be less than 5 % of the total cross-
37、sectional area ofthe tube. In most applications, the microphone is on the outsideconnected to a hollow probe tube that is inserted through thesource end of the apparatus and is aligned with the central axisof the tube. In principle, the sensing element of the microphoneor of the microphone probe may
38、 be positioned anywhere withinthe tube cross-sectional area. In practice, the microphone or theend of the probe tube must be supported by a spider or otherdevice to maintain its position on the central axis of theimpedance tube or at a constant distance from the central axis.6.1.5 Microphone Positio
39、n IndicatorA scale shall be pro-vided to measure the position of the microphone with respectto the specimen face. It is not necessary that zero on the scalecorrespond to the position of the specimen face. The resolutionof this scale should be such that microphone position can bemeasured to the neare
40、st 1.0 mm or, if a vernier is used, to thenearest 0.1 mm.6.1.6 Test Signal:6.1.6.1 FrequencyThe test signal shall be provided by asine wave oscillator generating a pure tone chosen from the listof preferred band center frequencies listed in ANSI S1.6. Thetest frequency shall be controlled to within
41、61 % during thecourse of a measurement. If a digital frequency synthesizer isused, the test signal may be assumed to agree with the set pointwithin the required 61%.6.1.6.2 Frequency CounterIt may be necessary, and isusually advisable, to measure the frequency of the signal withan electronic counter
42、 rather than to rely on the calibration andindicated setting of the frequency generator. Frequency shouldbe indicated to the nearest 1 Hz.6.1.7 Output-Measuring Equipment:6.1.7.1 FilterThe microphone output should be filtered toremove any harmonics and to reduce the adverse effect ofambient noise. T
43、he filter width must be no wider than one-thirdoctave, but a one-tenth octave or narrower filter bandwidth ispreferable.6.1.7.2 AmplifierThe signal-to-noise ratio of the measur-ing amplifier must be at least 50 dB. The amplified signal maybe read and recorded as a voltage or as a sound pressure leve
44、l(dB). It is presumed in Sections 9 and 10 of this test methodthat voltages rather than dB levels are being used. As onlypressure ratios are required for the computations in this testmethod, it is not necessary that the sound pressure measure-ment system be calibrated to a known, reference soundpres
45、sure level or to a known voltage.6.1.8 Temperature IndicatorA thermometer or other am-bient temperature sensing device shall be located in the vicinityof the impedance tube. This device should indicate air tem-perature inside the tube to within 62C.6.1.9 Monitoring OscilloscopeWhile not required for
46、 anyactual measurement purpose, it is recommended that an oscil-loscope be used to monitor both the voltage driving the soundsource and the output of the amplifier. Observing the oscillo-scope trace is useful in locating the exact position of pressureminima within the tube as well as in detecting di
47、stortion,excess noise, and other possible problems in the voltagesignals.7. Sampling7.1 At least three specimens, preferably more if the sampleis not uniform, should be cut from the sample for the test.When the sample has a surface that is not uniform (for examplea fissured acoustical tile), each sp
48、ecimen should be chosen toinclude, in proper proportion, the different kinds of surfacesexisting in the larger sample.8. Test Specimen Preparation and Mounting8.1 The measured impedance properties can be stronglyinfluenced by the specimen mounting conditions. Therefore,the following guidelines for t
49、he preparation and mounting ofspecimens are provided.8.2 The specimen must have the same shape and area as thetube cross section, neither more nor less. The specimen must fitsnugly into the specimen holder, fitting not so tightly that itbulges in the center, nor so loosely that there is a space betweenits edge and the holder. Movement of the specimen as a wholeand spaces between the specimen perimeter and sample holdercan result in anomalous values of normal incidence soundabsorption. Specimen movement can be minimized by the useof thin, double-sided a
