Acoustic radiation from arrays on coated prolate spheroids

Abstract

An important aspect in the design of conformal sonar arrays is the effect of an outer coating on the array response. The design of the coating is subject to the competing requirements of acoustic transparency, toughness for impacts, and flow noise rejection. The present study considers an array of rectangular pistons, conformal to a rigid prolate spheroidal baffle, covered by an outer coating. The coating is modeled as a fluid, i.e., it is assumed to have a vanishingly small value of the shear modulus, typical of many elastomers. Unlike a coating on a spherical or cylindrical baffle, where a uniform thickness coating can be defined by simply increasing the value of the radial coordinate, the corresponding operation in prolate spheroidal coordinates results in a confocal layer, i.e., a layer that is not of constant thickness. In this paper a new method is described that allows for a uniform thickness coating in prolate spheroidal geometry. This method employs the spheroidal addition theorem to satisfy the boundary conditions. Comparisons of the beamformed array response for a confocal coating versus a uniform thickness coating are given for a sample array with various coating properties. [Work supported by the NUWC ILIR Program.]