Abstract:

The modal structure of an empty nearly spherical metallic shell subject to fluid loading was examined using a novel form of EMAT (electromagnetic acoustic transducer) used previously for shells in air [B. T. Hefner and P. L. Marston, J. Acoust. Soc. Am. 99, 2594 (1996)]. It was found to be possible to magnetically excite and acoustically detect the high-Q torsional modes of the shell in water using this method. The quality factor, Q, was found to be approximately 1500. The presence of a bias field in addition to the oscillating field of the coil is needed to excite these modes. The bias field was introduced either by the magnetization of the 440C stainless-steel shell itself or by introducing an external field in the case of an unmagnetized shell. It was found that the lowest torsional mode could be excited by an oscillating field at 60 kHz (the mode frequency) and at 30 kHz (half the excited mechanical frequency). The modes were detected by a hydrophone placed near the shell, however, the mechanism for producing this acoustic signal is poorly understood because it depends on some lack of perfect symmetry for the shell. [Work supported by the Office of Naval Research.]