An impedance measuring facility for the low-frequency evaluation of acoustic materials

A simple, compact facility has been constructed which measures the specific acoustic impedance directly. It is capable of evaluating the acoustic characteristics of materials for underwater applications as a continuous function of frequency from 100 Hz to 5 kHz. The facility consists of a short, thick-walled tube, a dual-sensor hydrophone, and its associated electronic instrumentation. The tube is a liquid-rifled standpipe with a sound source mounted at the top and a sample material at the bottom. The sample is terminated by a rigid boundary. The dual-sensor hydrophone is positioned inside the tube and its two outputs are processed to yield the acoustic impedance at a point in the fluid column. Acoustic characteristics of the sample are then obtained from these impedance data by a computer. Theories underlying the impedance measuring technique are presented, and the procedure for calculating the acoustic properties from the impedance data is described. Sound velocities of several samples evaluated agreed reasonably well with those obtained by the pulse technique.