Acoustic scattering from simple and complex submerged objects

The amplitude of sound scattered from submerged elastic objects is dominated by resonance phenomena corresponding to the mechanical eigenvibrations of the object. It is often difficult to predict the resonance frequencies of bounded elastic targets using exact methods. It has been shown, however, that resonances arise due to phase matching of circumferential waves traveling over the object surface. This leads to a standing-wave picture for the resonances, and a phase matching condition can be established that can lead to accurate predictions of resonances for objects of simple or complex shapes. In order to use this method, it is necessary to obtain the phase velocity of the surface waves. This is illustrated for spheroids and cylinders with hemispherical endcaps. The water loading of the object is of importance since it leads, in addition to the elastic surface waves, to the appearance of a fluid-borne "Scholteu or "Stoneley" wave which interacts with the lowest-order elastic wave, giving rise (for thin shells) to a repulsion phenomenon in the phase-velocity dispersion curves of the corresponding waves analogous to that known for energy levels in atomic or molecular systems. Of further importance is the effect of internal attachments to shell-type objects which also leads to the interaction of different wave types. For multilayered structures, it is finally shown that one may isolate and identify the resonances arising from individual layers using the method of Ggrard.