ASTM_C_384_-_04_2016.pdf

Designation:C38404(Reapproved 2016)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 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 apparatus,for the mea-surement of impedance ratios and the normal incidence soundabsorption coefficients of acoustical materials.1.2 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.3 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 and health practices and determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2C423 Test Method for SoundAbsorption and SoundAbsorp-tion Coefficients by the Reverberation Room MethodC634 Terminology Relating to Building and EnvironmentalAcousticsE548 Guide for General Criteria Used for Evaluating Labo-ratory Competence(Withdrawn 2002)32.2 ANSI Standards:S1.6 Preferred Frequencies and Band Numbers for Acousti-cal Measurements43.Terminology3.1 The acoustical terminology used in this test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 title and elsewhere inthis test method refer to the following,respectively:3.2 Definitions:3.2.1 impedanceratio,z/cr/c+jx/c;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.C6343.2.2 normal incidence sound absorption coeffcient,n;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,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,respectively,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 absorption coefficient,n,is determined from the stand-ing wave ratio at the face of the test specimen.To determine theimpedance ratio,z/c,a measurement of the position of thestanding wave with reference to the face of the specimen isneeded.4.2 The normal incidence absorption coefficient and imped-ance 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 to its physical properties,such as1This 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 1,2016.Published April 2016.Originallyapproved in 1956.Last previous edition approved in 2011 as C384 04(2011).DOI:10.1520/C0384-04R16.2For 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.3The last approved version of this historical standard is referenced onwww.astm.org.4Available from American National Standards Institute(ANSI),25 W.43rd St.,4th Floor,New York,NY 10036,http:/www.ansi.org.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 airflow resistance,porosity,elasticity,and density.As such,themeasurements described in this test method are useful in basicresearch and product development of 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 incidence or statistical absorp-tion coefficients for materia