Scattering of tone bursts from spherical shells: Computations based on Fourier transform method

Abstract

Abstract : An algorithm to compute the transient scattering of sound by elastic spheres and shells has been developed and tested. Short tone bursts incident on elastic spherical shells under various conditions have been investigated. Previous experiment results for elastic spheres and shells have been confirmed. Some interesting physical aspects concerning the basics of the scattering mechanism have been detected. A mid-frequency enhancement has been examined for thin shells and compared with the ray model results. The ray model is found to closely approximate the enhanced backscattering for tone bursts associated with a subsonic wave on thin shells. A related filter-like effect of thin shells for the selected frequencies of the enhancement has been detected and quantitatively described and the impulse response has been computed. The subsonic wave associated with the mid-frequency enhancement is referred to as the Scholte Stoneley wave by some authors. The present computations can be of practical use and can be compared with observations or ray models of the scattering from shells under the same conditions if such observations or models become available later on. The scattering of tome bursts from shells was calculated when the carrier frequency is at or close to the frequency of resonance associated with a reverberation of longitudinal waves across the thickness of the shell. When the shell is a fluid, the results are easily interpreted in terms of an existing ray model. When the shell is elastic, they suggest the presence of a mechanism for prompt radiation noninvolving complete circumnavigation of the sphere by guided elastic waves.