Isolation of the resonant component in acoustic scattering from fluid-loaded elastic spherical shells

Abstract

The theory of resonance scattering [L. Flax, L. R. Dragonette, and H. Überall, J. Acoust. Soc. Am. 63, 723 (1978)], recently developed for acoustic wave scattering from elastic objects and elastic-wave scattering from cavities, is applied to the problem of sound scattering from fluid-filled elastic spherical shells in a fluid, in particular air-filled aluminum shells in water. The eigenvibrations of the shell appear as resonances in the scattering amplitude of each normal mode; these are superimposed on a smooth background which if the shell is made thinner, converts from the amplitude of a rigid to that of a soft sphere. We have isolated the resonant component from the non-resonant background by using an “intermediate background” formalism in order to exhibit the pure resonances. The sequence of resonance peaks in successive partial waves may be interpreted as due to circumferential waves (Regge poles) which resonate when n + 12 wavelengths span the sperical-shell circumference; they undergo a phase jump of π/2 when passing the north and south pole of the shell which explains the 12. [J. G., A. N., and H. U. supported by the Office of Naval Research, Code 421.]