Acoustic Scattering Fromacoated Elastic Shell: Exact vs. Approximate Theory

The interaction of acoustic waves with fluid-loaded shell structures is an ongoing area of interest in underwater acoustics, ship construction and detection. The elasticity of the structure permits strong surfaceborne wave effects, which can be significant or dominant in large structures (many wavelengths in size). The significance of surface borne waves is compounded by low loss factors, enabling them to propagate large distances on the structure, including multiple circumnavigations, on e.g. cylindrical structures, which can lead to strong resonance response. Acoustic scattering from fluid-loaded thin elastic shells can be determined exactly for two canonical geometries only, i.e., circular cylindrical and spherical shells [1], both using normal mode expansions. Other methods must be employed for other geometries. However, the large size and the multiple wave effects makes exact numerical solution of realistic acoustic scattering problems untenable for the foreseeable future. Although exact computation remains prohibitive, there is a robust alternative technique that is based on asymptotic approximations: the ray method. Being based upon asymptotic approximations, the ray technique includes important physical effects such as energy conservation along rays and phase matching. The purpose of this paper is two-fold. First to develop and apply the ray technique to coated shells, comprising steel and an outer softer coating. The presence of the coating introduces significant damping and other effects which have not been previously considered. At the same time, the approximate coated shell theory is bench-marked against an exact code for cylindrical multi-layered structures, using the full elas-