Modified two-thickness method for measurement of the acoustic properties of porous materials

Abstract

In this study, a modified two-thickness method to measure acoustic properties of porous materials using surface acoustic impedance is proposed and compared to those obtained by the four-microphone impedance tube and traditional two-thickness methods. In the two-thickness method, the complex propagation constant and characteristic impedance are estimated from the measured surface acoustic impedances, and the sample thicknesses should have a ratio of two. Through the proposed method, the complex characteristic impedance and wavenumber of a porous material are calculated under arbitrary sample thicknesses condition. The surface acoustic impedances for samples of different thicknesses are measured via the two-microphone impedance tube method to estimate their complex acoustic properties. Sensitivity analysis is performed to analyze the measurement uncertainty on the resulting values. The sensitivity depended on the sample thickness and mount edge constraint in the tube. Using the method proposed in this study, acoustic properties are measured accurately with small differences in thickness and surface acoustic impedance.