Sound scattering from spheroidal shells in water

A numerical study is presented for plane-wave scattering from thin steel spheroidal shells in the frequency region corresponding to the onset of resonances. The authors allow for arbitrary incidence angles in order to analyze the possible aspect-angle dependence of resonances. Both monostatic angular distributions and backscattered echoes are examined as a function of the non-dimensionalized quantity kL/2 where the wave number k is 2π/lambda and L is the length of the spheroid. Shell-thickness is chosen to be very thin so as to isolate the physical mechanisms involved in the process. In the analysis and in the absence of a resonance, only ''background'' effects associated with soft-body scattering are observed (due to the very small thickness of the shell), while a characteristic coherent effect is observed in the resonance region. Analysis of the exact theoretical predictions leads to the conclusion that elastic resonances for these objects correspond to modal, vibrational, surface standing waves at discrete frequencies with a relatively fixed phase velocity