Boundary condition effects of porous material in absorption measurements: Comparison of two impedance tubes

Abstract

Porous materials are widely used for acoustical treatment and insulation of various environments. Its main feature is acoustic absorption, yielding the acoustical absorption coefficient over frequency. One of the ways to extract this parameter experimentally is with impedance tube (or Kundt's Tube) measurement via the transfer function method. However, the compression of the sample or the existence of gaps between the sample and the tube causes of uncertainty in the experiment. This research focuses upon the study of these effects. In order to analyze the influence of the boundary condition in the measurements, they were performed into two impedance tubes of slightly different internal diameters (both have the maximum frequency of analysis up to 6.4 kHz, approximately). Two different porous materials with samples of the same thickness were characterized using both tubes. In this situation, the procedure forces the influence of lateral leakage or compression of the samples against the tube to arise. Thus, this allows the analysis on the effect of the material structure in the acoustical absorption coefficient.