Measurement of Acoustic Impedances of Surfaces in Water

An analytical and experimental investigation of the practicability of a water-filled acoustic impedance tube is described. It is shown that by proper choice of tube material, and of diameter and wall thickness with respect to wave-length, a satisfactory tube for measuring the impedance of underwater acoustic materials can be built. The phase velocity of sound in the tube is nearly constant in the useful frequency range but less than the velocity of sound in free water, and the variation in sound pressure along a radius of the tube is frequency dependent. Both rigorous and approximate formulae are given for the phase velocity and radial pressure dependence. These formulae are checked experimentally.