Impedance tube measurements on porous media: The effects of air-gaps around the sample

Abstract

The effects, on the measured impedance, of air-gaps around the edge of a sound-absorbing test sample in an impedance tube are examined both theoretically and experimentally, for a rigid-framed anisotropic porous medium. The results show that media of high flow resistivity tend to be more susceptible to measurement errors incurred by air-gaps than those with low resistivities. Experimental and predicted data concerning the effects of air-gaps on the impedances of both isotropic and anisotropic materials are presented, and are in reasonable agreement. Errors in the bulk acoustic properties, infered from normal surface impedance data on two different thicknesses of an isotropic material against a rigid backing, are also examined in an illustrative numerical study.