A Comparison of Two and Four Microphone Standing Wave Tube Procedures for Estimating the Normal Incidence Absorption Coefficient

摘要

A new ASTM standard has been adopted for characterizing acoustical materials in a tube.This new standard is ASTM E2611-09,Measurement of Normal Incidence Sound Transmission of Materials Based on the Transfer Matrix Method.This test method describes the use of a tube, four microphones,and a digital frequency analysis system for the measurement of normal incidence transmission loss and other important acoustical properties of materials by determination of the sample’s acoustic transfer matrix.This four microphone test method is similar to Test Method E-1050-08,which utilizes a two microphone technique to calculate acoustical material properties,in that it also uses a tube with a sound source connected to one end and a test sample mounted in the tube.For transmission loss,four microphones,at two locations on each side of the sample,are mounted so that the diaphragms are flush with the inside surface of the tube perimeter.Plane waves propagate in the tube and a broadband signal is used as the noise source.At each frequency the resulting standing wave pattern is decomposed into forward-and backward-traveling components by measuring sound pressure simultaneously at the four locations and examining their relative amplitudes and phases.The acoustic transfer matrix is calculated from the pressure and particle velocity of the traveling waves evaluated on either side of the specimen.The transmission loss,as well as several other acoustical properties of the material,including the normal incidence sound absorption coefficient and the reflection coefficient for a hard backed termination,can then be calculated once the transfer matrix elements are known.In this paper the two and four microphone methods for calculating the reflection coefficient and the related normal incidence sound absorption coefficient will be compared with each other and with the results predicted using finite element software.