1、Designation: E1050 12Standard Test Method forImpedance and Absorption of Acoustical Materials Using aTube, Two Microphones and a Digital Frequency AnalysisSystem1This standard is issued under the fixed designation E1050; the number immediately following the designation indicates the year oforiginal
2、adoption or, in the case of 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,two microphone
3、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 is given in AnnexA1.1
4、.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.4 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 a
5、ppropriatesafety and health practices and determine the applicability ofregulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2C384 Test Method for Impedance andAbsorption ofAcous-tical Materials by Impedance Tube MethodC634 Terminology Relating to Building and EnvironmentalA
6、cousticsE548 Guide for General Criteria Used for Evaluating Labo-ratory Competence32.2 ISO Standards:ISO 10534-1 AcousticsDetermination of Sound Absorp-tion Coefficient and Impedance or AdmittancePart 1:Impedance Tube Method4ISO 105342 AcousticsDetermination of Sound Absorp-tion Coefficient and Impe
7、dance in Impedance TubesPart2: Transfer-Function Method42.3 ANSI Standards:4ANSI/ASA S1.11 Octave-Band and Fractional-Octave-Band Analog and Digital Filters3. Terminology3.1 DefinitionsThe acoustical terminology used in thistest method is intended to be consistent with the definitions inTerminology
8、C634.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 notstrictly rigorous when the initial conditions have a non-zero value. Theterm “frequenc
9、y response function” arises from more general Fouriertransform theory (1).5This test method shall retain the use of the formerterm although not technically correct. Users should be aware that modernFFT analyzers may employ the latter terminology.3.2 Symbols: The following symbols are used in Section
10、 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 microphone locations 1 and 2, respectively.3.2.5 G12cross power spectrum of the acoustic
11、 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.3.2.8 HI,HIIcalibration transfer functions for the micro-phones in the standard and swit
12、ched configurations, respec-tively.1This 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 approved April 1, 2012. Published July 2012. Originallyapproved in 198
13、5. Last previous edition approved in 2010 as E1050 - 10. DOI:10.1520/E1050-12.2For 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 on
14、the 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.ansi.org.5The boldface numbers in parentheses refer to the list of ref
15、erences at the end ofthis test method.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.9 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
16、is com-plex where k = k8 -jk9.k8 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 ra
17、tio.3.2.14 Rcomplex acoustic reflection 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.3.2.18 z/rcnormal specific acoustic impedance ratio.3.2.19 anormal incidence sound absor
18、ption 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 variable X for illustrativepurposes
19、, 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 quantity prior to correction
20、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 C384 in thatit also uses an impedance tube with a sound source connectedto one end and the test sample mounted at the other end. Themeasurement technique
21、s 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 intoforward- and backward-trav
22、eling 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 themeasured transfer function.4.2 T
23、he 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 frequency range may beobtained b
24、y 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 C384.5. Significance and Use5.1 This test method can be applied to measure soundabsorption coefficients of absorptive materials at no
25、rmal 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 coefficients can bequite useful
26、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 largesamples for accurate random-i
27、ncidence measurements in areverberation room. Estimates of the random incidence absorp-tion coefficients can be obtained from normal impedance datafor locally-reacting materials (2).NOTE 2The classification, “locally-reacting” includes fibrous materi-als having high internal losses. Formulas have be
28、en developed forconverting sound absorption properties from normal incidence to randomincidence, for both locally-reacting and bulk-reacing materials (3).5.4 Measurements described in this test method can bemade with high precision, but these measurements may bemisleading. Uncertainties of greater m
29、agnitude 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 end and a sound source at the other.Microphone ports are mou
30、nted 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 with a constant dimension from end-to-end. The tube shall be s
31、traight 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.NOTE 3The tube can be constructed from materials including metal,plastic, cement, or wood. It m
32、ay be necessary to seal the interior wallswith a smooth coating in order to maintain low sound attenuation for planewaves.6.2.2 Working Frequency RangeThe working frequencyrange is:fl, f , fu(1)where:f = operating frequency, hertz,fl= lower working frequency of the tube, hertz, andfu= upper working
33、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 isrecommended that the microphone spacing exceed one percentof the wavelength corresponding to the lower frequency ofinterest.E1050 1226.2.2.2 The upper
34、 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 wave propaga-tion, the upper frequency limit (4) is defined as follows:fu, Kc/ dord, Kc/ fu(2)where:fu= upper frequency limit, hertz,c =
35、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 is defined as 0.500. Extremeaspect rations greater than 2:1 or less than 1:2 should beavoided. A square cross-section is recommended.6.
36、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 sufficiently long as planewaves are fu
37、lly 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 subsideat a distance equivalent
38、 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-ments of 6.4.3, 6.5.3, and 6.
39、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 to excessive deflectionmay be
40、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 acoustic measurements.6.3 Test
41、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 placing a heavy backing plate
42、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 holdermust make an airtight fit with
43、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 recom-mended for sealing.6.3.3 In
44、tegral 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 sealant is recommended forsealing.
45、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 a rigid, clear material, such
46、as 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 frontsurface.6.3.6 Backing Pla
47、teThe 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 PlacementThe sound sources shoul
48、d havea uniform power response over the frequency range of interest.It may either be coaxial with the main tube or joined to theFIG. 1 Sound Source ConfigurationsE1050 123main tube by means of a transition having a straight, tapered,or exponential section (see Fig. 1).6.4.2 IsolationThe sound source
49、 and transition 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 TerminationResonances 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
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