Shear material property determination from underwater acoustic panel tests

Abstract

A technique is presented for determining, from underwater acoustic panel tests, passive-material shear sound speed and attenuation as functions of frequency and temperature. The frequency band of 10–200 kHz was used for development, but the applicability of the techniques described is not limited to that range. The method uses measured values of oblique-incidence insertion loss (and the corresponding change in phase) and can also take advantage of oblique-incidence echo-reduction data. Initial values for the longitudinal properties are determined by least-squares fitting of a causal material model (incorporated into a layer model) to normal-incidence insertion-loss and phase-change data. Shear properties are determined similarly, using a small number of oblique incidence angles. The method assumes a single-layer panel. A revised method of performing echo-reduction measurements that permits its accurate determination at oblique angles of incidence is also described. The angle of specular reflection is not equal to the rotation angle in the standard echo-reduction measurement geometry, and it is important to properly account for this. Consequently at a given angle of panel rotation it is necessary to use one value of the incidence angle for evaluating echo-reduction data, but another value of the incidence angle for evaluating insertion-loss data.