Modal analysis of a shell's acoustic radiation in light and heavy fluids

A modal analysis study of acoustic radiation by a finite cylindrical shell immersed in light of heavy fluid is presented. Attention is focused on the explanation of radiation phenomena in both fluids. The concepts of radiation, structural and evenly damped modes are explained. These lead to clarification of the shell's vibro-acoustic behaviour in heavy fluid and to confirmation of the well known case of light fluid. Relationships between these new concepts and the more classical one of radiating and non-radiating modes are presented. It is shown that this previous classification of modes is insufficient to clarify the heavy fluid-shell coupling, especially in the critical frequency range. In this range the behaviour of a shell in water is very different from that of one in air, and is entirely controlled by modes which are overdamped by radiation. In particular, it is shown that structural damping is ineffective in reducing radiated power in the critical frequency range when the fluid is water.