Scattering of impulse signal by elastic shells with internal structures.

Abstract

Sound scattering from an elastic shell can be significantly affected by structures loaded inside the shell. A major effect of internal loading has been shown to be the conversion of subsonic flexural waves in the shell into strongly radiating acoustic waves. In the frequency domain, this leads to a deeply scalloped scattering form function with the empty-shell result as its ‘‘mean value.’’ Since flexural waves in the shell are much slower than supersonic compressional leaky waves that dominate the scattering process for an empty shell, the conversion of flexural waves into sound by internal structural loading is well separated in time from other processes such as the initial ‘‘bubble-like’’ reflection and the phase-matched leaky wave re-radiation. Also, because of the dispersive nature of the flexural waves, the internal loading effect in time domain can be clearly identified; they distinguish themselves in the scattered field by a series of wave packets whose amplitudes decay due to both acoustic damping and dispersion. This is in contrast to the other two main components in the scattered time signal, one being the initial reflection that is basically the inverse of the incident signal and the other being the leaky wave contribution consisting of decaying pulses. All these are clearly demonstrated in this paper, which examines the scattering of impulse incidence from elastic shells with internal structures. The response of the loaded shell is also examined, which further reveals the interactions between the acoustic field, the shell, and its internals. [Work supported by ONR.]