The acoustic scattering by a submerged, spherical shell. II: The high-frequency region and the thickness quasiresonance

Abstract

This article presents a fundamentally oriented analysis of the complex-ka plane pole structure of the S matrix for spherical shells in the thickness range 0.02a⩽h⩽0.10a (a is the outer radius of the sphere) for the high-frequency region (100<ka<1000); the Poisson’s ratio for these shells is fixed at σ=0.34 (aluminum). Surprisingly, this analysis reveals that the most dominant feature in the backscattered form function of a thin spherical shell is a single prominent high-frequency peak. The underlying modal structure and accompanying regions of negative group velocity are related to this feature, which is interpreted as a thickness “quasiresonance.” In particular, the significance of stationary values of the group velocity to the relative amplitude of modal excitation is emphasized.