1、Designation: C 384 04Standard Test Method forImpedance and Absorption of Acoustical Materials byImpedance Tube Method1This standard is issued under the fixed designation C 384; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the ye
2、ar of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 apparatus, for the me
3、a-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 appro-priate safety
4、 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 as thestandard.2. Referenced Documents2.1 ASTM Standards:2C 423 Test Method for Sound Absorption and Sound Ab-sorption Coefficients by the Reverberation
5、Room MethodC 634 Terminology Relating to Environmental AcousticsE 548 Guide for General Criteria Used for EvaluatingLaboratory Competence32.2 ANSI Standards:S1.6 Preferred Frequencies and Band Numbers for Acous-tical Measurements43. Terminology3.1 The acoustical terminology used in this test method
6、isintended to be consistent with the definitions in TerminologyC 634. In particular, the terms “impedance ratio,” “normalincidence sound absorption coefficient,” and “specific normalacoustic impedance,” appearing in the title and elsewhere inthis test method refer to the following, respectively:3.2
7、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,respectively, resistance ratio and reactance ratio. C 6343.2.2 normal incidence
8、 sound absorption coeffcient, an;dimensionlessof a surface, at a specified frequency, thefraction of the perpendicularly incident sound power absorbedor otherwise not reflected. C 6343.2.3 specific normal acoustic impedance, z r+jx;ML-2T-1; mks rayl (Pa s/m)at a surface, the complexquotient obtained
9、 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, respectively,specific normal acoustic resistance and specific normal acous-tic reactance
10、. C 6344. 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 absorption coefficient, an, is determined from thestanding wave ratio at the face
11、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-ance ratio are functions of frequency. Measurements are madewith pure tones at
12、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 to its physical properties, such asairflow resistance, porosity, elasticity,
13、 and density. As such, the1This test method is under the jurisdiction of ASTM Committee E33 onEnvironmental Acoustics and is the direct responsibility of Subcommittee E33.01 onSound Absorption.Current edition approved April 1, 2004. Published May 2004. Originallyapproved in 1956. Last previous editi
14、on approved in 2003 as C 384 03.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Withdrawn.4Available from Am
15、erican National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036.1Copyright 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 o
16、f sound absorptive mate-rials.5.2 Normal incidence sound absorption coefficients 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
17、 incidence or statistical absorp-tion coefficients for materials can be obtained 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 specif
18、ic normal acoustic impedance valuesusing an expression derived by London (1).5Locally 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. Fo
19、rmulas for estimating random incidencesound absorption properties for both locally 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 loudspeake
20、r at the other. A probe microphonethat can be moved along the length of the tube 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 suitabl
21、e material that hasinherently low sound absorption properties. Its interior crosssection 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 i
22、nterior of the tubemay be sealed with paint, epoxy, or other coating material toensure 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 u
23、pper limit (3) offrequency is:f , 0.586 c/d (1)where:f = frequency, Hz,c = speed 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
24、whether the tube iscircular or rectangular. At frequencies above these limits, crossmodes 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 af
25、ter traveling a short distance. Forthis reason, no measurement should be made closer 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 stan
26、ding wave patternneeded for measurement. That is, it must be long enough tocontain 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
27、 tube is1minlength and 0.1 m indiameter and the speed of sound is 343 m/s, the frequencyshould 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 t
28、he sound source. Provision mustbe made for containing the specimen with its face in 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 inter
29、ior of the specimen holder. Arecommended backing is a solid steel plate with a thickness 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 m
30、ust be made for substitutingthe metal backing plate for the specimen for calibrationpurposes.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
31、 the probe microphone, it may be placed toone side. Since the source diameter may 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 sourc
32、e to theprobe microphone where it enters the tube or to the tube itself.Such vibrational 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,
33、 may beplaced between the sound source and tube or the microphoneprobe, or both, to 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 a
34、ccuracy, itdoes require awkward changes in amplifier gain settings when5The boldface numbers in parentheses refer to the list of references at the end ofthis standard.C384042switching between test frequencies. This effect can be mini-mized by lining the interior of the tube near the sound sourcewith
35、 a porous, absorbent material.6.1.4 MicrophoneIf the microphone is small enough, 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 micr
36、ophone and microphone sup-ports shall be less than 5 % of the total cross-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 prin
37、ciple, the sensing element of the microphoneor of the microphone probe may 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 tub
38、e or at a constant distance from the central axis.6.1.5 Microphone Position 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
39、 scale should be such that microphone position can bemeasured to the nearest 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 frequen
40、cies listed in ANSI S1.6. Thetest frequency shall be controlled to within 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 a
41、dvisable, to measure the frequency of the signal withan electronic counter 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
42、toremove any harmonics and to reduce the adverse effect ofambient noise. The 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 amplifi
43、ed signal maybe read and recorded as a voltage or as a sound pressure level(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
44、pressure measure-ment system be calibrated to a known, reference soundpressure 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
45、the tube to within 62C.6.1.9 Monitoring OscilloscopeWhile not required for 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 e
46、xact position of pressureminima within the tube as well as in detecting distortion,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 su
47、rface that is not uniform (for examplea fissured acoustical tile), each specimen 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
48、the specimen mounting conditions. Therefore,the following guidelines for the 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 tigh
49、tly 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 adhesive tape applied between the back ofthe specimen and the metal backing plate. Spaces at thespecimen perimeter can be sealed with petroleum jelly.8.3 The specimen must have a relatively flat surface sincethe reflected wave fro
