1、Designation: E 1050 98 (Reapproved 2006)Standard Test Method forImpedance and Absorption of Acoustical Materials Using ATube, Two Microphones and A Digital Frequency AnalysisSystem1This standard is issued under the fixed designation E 1050; the number immediately following the designation indicates
2、the year oforiginal adoption or, in the case of revision, the year 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
3、 tube,two microphone locations, and a digital frequency analysissystem for the determination of normal incidence soundabsorption coefficients and normal specific acoustic impedanceratios of materials.1.2 Laboratory AccreditationA procedure for accreditinga laboratory for performing this test method
4、is given in AnnexA1.1.3 This standard does not purport to address the safetyconcerns, if any, associated with its use. It is the responsibilityof the use of this standard to consult and establish appropriatesafety and health practices and determine the applicability ofregulatory limitations prior to
5、 use.2. Referenced Documents2.1 ASTM Standards:C 384 Test Method for Impedance and Absorption ofAcoustical Materials by Impedance Tube MethodC 634 Terminology Relating to Environmental AcousticsE 548 Guide for General Criteria Used for EvaluatingLaboratory Competence22.2 ISO Standards:ISO 10534-1 Ac
6、ousticsDetermination of Sound Absorp-tion Coefficient and Impedance or AdmittancePart 1:Impedance Tube Method3ISO 105342 AcousticsDetermination of Sound Absorp-tion Coefficient and Impedance in Impedance TubesPart2: Transfer-Function Method33. Terminology3.1 DefinitionsThe acoustical terminology use
7、d in thistest method is intended to be consistent with the definitions inTerminology C 634.NOTE 1Historical literature regarding the measurement of normalincidence absorption coefficients referred to “transfer function” measure-ments; however, the term arises from Laplace transform theory and is not
8、strictly rigorous when the initial conditions have a non-zero value. Theterm “frequency response function” arises from more general Fouriertransform theory (1).4This test method shall retain the use of the formerterm although not technically correct. Users should be aware that modernFFT analyzers ma
9、y employ the latter terminology.3.2 Symbols: The following symbols are used in Section 8(Procedure):3.2.1 brcnormal specific acoustics susceptance ratio.3.2.2 cspeed of sound, m/s.3.2.3 grcnormal specific acoustic conductance ratio.3.2.4 G11,G22auto power spectra of the acoustic pressuresignal at mi
10、crophone locations 1 and 2, respectively.3.2.5 G12cross power spectrum of the acoustic pressuresignals at microphones locations 1 and 2.3.2.6 Htransfer function of the two microphone signalscorrected for microphone response mismatch.3.2.7 Hmeasured transfer function of the two micro-phone signals.3.
11、2.8 HI,HIIcalibration transfer functions for the micro-phones in the standard and switched configurations, respec-tively.3.2.9 Hccomplex microphone calibration factor.3.2.10 jequals=1.3.2.11 kequal 2pf/c; wave number, m-1.3.2.11.1 DiscussionIn general the wave number is com-plex where k = k8 -jk9.k8
12、 is the real component, 2pf/c and k9is the imaginary component of the wave number, also referredto as the attenuation constant, Nepers-m-1.3.2.12 ldistance from the test sample to the centre of thenearest microphone, m.3.2.13 r/rcnormal specific acoustic resistance ratio.3.2.14 Rcomplex acoustic ref
13、lection coefficient.3.2.15 scentre-to-center spacing between microphones,m.3.2.16 x/rcnormal specific acoustic reactance ratio.3.2.17 yrcnormal specific acoustic admittance ration.1This test method is under the jurisdiction of ASTM Committee E33 onEnvironmentalAcoustics and is the direct responsibil
14、ity of Subcommittee E33.01 onAbsorption.Current edition approved Sept. 1, 2006. Published September 2006. Originallyapproved in 1985. Last previous edition approved in 2000 as E 1050 - 00.2Withdrawn.3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY
15、10036, http:/www.ansi.org.4The boldface numbers in parentheses refer to the list of references at the end ofthis test method.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.18 z/rcnormal specific acoustic impedance ratio.3.2.19 a
16、normal incidence sound absorption coefficient.3.2.20 fphase of the complex transfer function, radians.3.2.21 fRphase of the complex acoustic reflection coef-ficient, radians.3.2.22 rdensity of air, kg/m3.3.3 Subscripts, Superscripts, and Other NotationThe fol-lowing symbols, which employ the variabl
17、e X for illustrativepurposes, are used in Section 8:3.3.1 Xccalibration.3.3.2 Xiimaginary part of a complex quantity.3.3.3 Xrreal part of a complex quantity.3.3.4 XI,XIIcalibration quantities measured with micro-phones placed in the standard and switched configurations,respectively.3.3.5 Xmeasured q
18、uantity prior to correction for ampli-tude and phase mismatch.3.3.6 |X|magnitude of a complex quantity.4. Summary of Test Method4.1 This test method is similar to Test Method C 384 in thatit also uses an impedance tube with a sound source connectedto one end and the test sample mounted at the other
19、end. Themeasurement techniques for the two methods are fundamen-tally different, however. In this test method, plane waves aregenerated in the tube using a broad band signal from a noisesource rather than a discrete sinusoid from an oscillator. Thedecomposition of the stationary sound wave pattern i
20、ntoforward- and backward-traveling components is achieved bymeasuring sound pressures simultaneously at two spacedlocations in the tubes side wall. Calculations of the normal-incidence absorption coefficients for the acoustical material areperformed by processing an array of complex data from themea
21、sured transfer function.4.2 The quantities are determined as functions of frequencywith a resolution determined by the sampling rate of a digitalfrequency analysis system. The usable frequency range de-pends on the diameter of the tube and the spacing between themicrophone positions. An extended fre
22、quency range may beobtained by using tubes with various diameters and micro-phones spacings.4.3 This test method is intended to provide a much fastermeasurement technique than that of Test Method C 384.5. Significance and Use5.1 This test method can be applied to measure soundabsorption coefficients
23、 of absorptive materials at normal inci-dence, that is, 0. It also can be used to determine specificimpedance and admittance ratios. The properties measuredwith this test method are useful in basic research and productdevelopment of sound absorptive materials.5.2 Normal incidence sound absorption co
24、efficients can bequite useful in certain situations where the material is placedwithin a small acoustical cavity close to a sound source, forexample a closely-fitted machine enclosure.5.3 This test method allows one to compare relative valuesof sound absorption when it is impractical to procure larg
25、esamples for accurate random-incidence measurements in areverberation room. Estimates of the random incidence absorp-tion coefficients can be obtained from normal impedance datafor locally-reacting materials (2).55.4 Measurements described in this test method can bemade with high precision, but thes
26、e measurements may bemisleading. Uncertainties of greater magnitude than those fromthe measurements may occur from other sources. Care shouldbe exercised to sample nonuniform materials adequately (see11.1).6. Apparatus6.1 The apparatus is a hallow cylinder, or tube, with a testsample holder at one e
27、nd and a sound source at the other.Microphone ports are mounted at two or more locations alongthe wall of the tube. A two channel digital frequency analysissystem is used for data acquisition and processing.6.2 Tube:6.2.1 ConstructionThe interior section of the tube may becircular or rectangular wit
28、h a constant dimension from end-to-end. The tube shall be straight and its inside surface shall besmooth, nonporous, and free of dust to maintain low soundattenuation. The tube construction shall be massive so soundtransmission through the tube wall is negligible.66.2.2 Working Frequency RangeThe wo
29、rking frequencyrange is:fl, f , fu(1)where:f = operating frequency, hertz,fl= lower working frequency of the tube, hertz, andfu= upper working frequency of the tube, hertz.6.2.2.1 The lower frequency limit depends on the spacing ofthe microphones and the accuracy of the analysis system. It isrecomme
30、nded that the microphone spacing exceed one percentof the wavelength corresponding to the lower frequency ofinterest.6.2.2.2 The upper frequency limit, fu, and the correspondingwavelength, lu, depends on the diameter of the tube and uponthe speed of sound.6.2.3 DiameterIn order to maintain plane wav
31、e propaga-tion, the upper frequency limit (4) is defined as follows:fu, Kc/ dord, Kc/ fu(2)where:fu= upper frequency limit, hertz,c = speed of sound in the tube, m/s,d = diameter of the tube, m, andK = 0.586.6.2.3.1 For rectangular tubes, d is defined as the largestsection dimension the tube and K i
32、s defined as 0.500. Extremeaspect rations greater than 2:1 or less than 1:2 should beavoided. A square cross-section is recommended.5The classification, “locally-reacting” includes fibrous materials having highinternal losses. Formulas have been developed for converting sound absorptionproperties fr
33、om normal incidence to random incidence, for both locally-reacting andbulk-reacing materials (3).6The tube can be constructed from materials including metal, plastic, cement, orwood. It may be necessary to seal the interior walls with a smooth coating in orderto maintain low sound attenuation for pl
34、ane waves.E 1050 98 (2006)26.2.3.2 It is best to conduct the plane wave measurementswell within these frequency limits in order to avoid cross-modes that occur at higher frequencies when the acousticalwave length approaches the sectional dimension of the tube.6.2.4 LengthThe tube should be sufficien
35、tly long as planewaves are fully developed before reaching the microphonesand test specimen. A minimum of three tube diameters must beallowed between sound source and the nearest microphone.The sound source may generate nonplane waves along withdesired plane waves. The nonplane waves usually will su
36、bsideat a distance equivalent to three tube diameters from thesource. If measurements are conducted over a wide frequencyrange, it may be desirable to use a tube which providesmultiple microphone spacings or to employ separate tubes. Theoverall tube length also must be chosen to satisfy the require-
37、ments of 6.4.3, 6.5.3, and 6.5.4.6.2.5 Tube VentingSome tube designs are such that, dur-ing during installation or removal of the test specimen, largetemporary pressure variation may be generated. This mayinduce microphone diaphragm deflection. The potential fordamage to a microphone diaphragm due t
38、o excessive deflectionmay be reduced including a pressure relief opening in the tube.This may be accomplished by drilling a small hole, 1 to 2 mmthrough the wall of the tube. It is recommended to locate thetube vent near the sound source, away from microphonelocations, and to seal the vent during ac
39、oustic measurements.6.3 Test Specimen Holder:6.3.1 General FeaturesThe specimen holder may eitherbe integrated with the impedance tube or may be a separate,detachable extension of the tube. Provision must be made formounting the specimen with its face in a known position alongthe tube axis and for p
40、lacing a heavy backing plate behind thespecimen. For some measurements it may be desirable tomaintain an airspace of known dimensions between the speci-men and the backing plate. One such arrangement may be tosimulate a suspended ceiling tile.6.3.2 Detachable HolderAs a detachable unit, the holdermu
41、st make an airtight fit with the end of the tube opposite thesound source. The holder must conform with the interior shapeand dimensions of the main part of the impedance tube. Theconnecting joint must be finished carefully and the use of asealant, such as petroleum jelly or silicone grease, is reco
42、m-mended for sealing.6.3.3 Integral HolderIf the sample holder is in an integralpart of the impedance tube, it is recommended to make theinstallation section of the tube accessible for mounting of thespecimen by a removable cover. The mating surfaces must befinished carefully, and the use of a seala
43、nt is recommended forsealing.6.3.4 Circular HolderFor circular tubes, it is recom-mended to make the specimen accessible from both the frontand back end of the sample holder. It is possible then to checkthe position and flatness of the front surface and back position.Holders may be constructed from
44、a rigid, clear material, suchas acrylic, to facilitate inspection.6.3.5 Rectangular HolderWith rectangular tubes, it isrecommended to install the specimen from the side, making itpossible to check the fitting and the position of the specimen inthe tube and to check the position and flatness of the f
45、rontsurface.6.3.6 Backing PlateThe backing plate of the sampleholder shall be rigid and shall be fixed tightly to the tube sinceit serves to provide a sound-reflective termination in manymeasurements. A metal plate having a minimum thickness of20 mm is recommended.6.4 Sound Source:6.4.1 Kind and Pla
46、cementThe sound sources should havea uniform power response over the frequency range of interest.It may either be coaxial with the main tube or joined to themain tube by means of a transition having a straight, tapered,or exponential section (see Fig. 1).6.4.2 IsolationThe sound source and transitio
47、n shall besealed and isolated from the tube to minimize structure-bornesound excitation of the impedance tube. If a direct radiatorloudspeaker is utilized, it shall be contained in a sound-isolating enclosure in order to avoid airborne flanking trans-mission to the microphones (see Fig. 1).6.4.3 Ter
48、minationResonances of the air column in theimpedance tube may arise if the mechanical impedance of theloudspeaker membrane or diaphragm is high. In this case, it isrecommended to apply a porous absorber coating or lininginside either the impedance tube near the loudspeaker or insidethe sound transit
49、ion. Alternatively, the locations describesabove may be filled lightly with a low density absorbingmaterial.FIG. 1 Sound Source ConfigurationsE 1050 98 (2006)36.4.4 EqualizationWhen an absorptive medium is placednear the sound source as described in 6.4.3, significant soundenergy will be lost at higher frequencies. An electronicequalizer may be required to shape and sound spectra measuredat the microphone positions so that they are relatively flat. Thiswill minimize the loss of signal-to-noise capability at highfrequencies.6.5
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