M. F. Werby
Naval Res. Lab., Code 7181, Stennis Space Center, MS
N. A. Sidorovskaia
Univ. of New Orleans, New Orleans, LA
In a pioneering work Talmant et al. [J. Acoust. Soc. Am. 86, 278 (1989)] established the presence of pseudo-Stonely resonances excited when acoustical signals scatter from shells at or near coincidence frequency. This notion was supported by the argument that Stoneley waves exist at the fluid--elastic interface of a plate when water is on one side and the other side is evacuated. It is known that nondispersive waterborne waves are excited at the fluid--water interface for that case in the frequency region around coincidence frequency. This coincided with the bounded shell case in which very narrow strong resonances corresponded with the waterborne waves and a broad envelope function corresponded with the onset of the flexural resonances which become manifest at coincidence frequency when the flexural waves become supersonic and thus radiated into the water. The envelope effect corresponds to an abrupt phase change of pressure at coincidence frequency. An analogous argument predicts the existence of pseudo-Scholte resonances. The analog for that case is a plate in which fluids of like properties exist on both sides. In that case waterborne waves exist over the entire frequency range. The implication is that if one scatters from such an object there should be a proliferation of waterborne waves and thus for closed shells an abundance of resonances associated with waterborne waves circumnavigating the shell should by present. Can the abundance of sharp peaks excited on fluid filled shells be explained in terms of this mechanism? This issue is examined and the question is answered. [Work sponsored by NRL and ONR.]