Magnetic excitation and acoustical detection of torsional and quasi-flexural modes of spherical shells in water

A simple electromagnetic acoustic wave transducer (EMAT) based on a wire co il was used to excite the resonant modes of a stainless steel spherical she ll without direct mechanical contact. The coupling produced by this EMAT wa s examined first for shells in air and then for shells immersed in water to examine the effects of fluid loading on the shell's spectrum. It was found that the torsional modes were excited using this method and these modes ra diated sound into the surrounding water contrary to expectations. This exci tation is shown to depend on the presence of a permanent magnetization in t he shell itself or on the presence of a static external field applied at ri ght angles to the axis of the coil. Possible mechanisms for the excitation and the acoustic radiation of the torsional modes are considered. The excit ation of quasi-flexural shell modes is also discussed for shells in air and in water. The shell responds at the oscillation frequency of the applied f ield and at twice the frequency. Some potential applications of this method of measuring modes are noted. (C) 1999 Acoustical Society of America. [S00 01-4966(99)07012-5].