MEASUREMENT OF THE CHARACTERISTIC IMPEDANCE AND PROPAGATION CONSTANT OF MATERIALS HAVING HIGH FLOW RESISTIVITY

A technique for the measurement of characteristic acoustical impedance and propagation constant of porous materials is presented. Samples are mounted in an impedance tube and both the surface impedance and a transfer function along the sample length are determined; characteristic impedance and propagation constant may be calculated using these quantities. For this implementation, a classical standing wave procedure is used for the determination of surface impedance. Measurements are obtained for frequencies between 100 and 4000 Hz. A special procedure that makes use of extension tubes, between sample and the fixed part of the impedance tube, is used to achieve the low-frequency results. The new technique yields results that are comparable in accuracy to the "two-cavity" technique described by Utsuno et al. [J. Acoust. Soc. Am. 86, 637-643 (1989)] for samples of low flow resistivity (17.2 and 43.6 cgsrayl cm-1), but substantially better for a sample of higher flow resistivity (380 cgs-rayl cm-1).