Abstract:
The lowest quadrupole mode of an empty nearly spherical metallic shell was excited by a novel form of EMAT (electromagnetic acoustic transducer) not having mechanical contact with the shell. An oscillating current is supplied to a coil located near the equator of the shell. The shell vibrates in response to the Maxwell stresses which are distributed over the shell's surface in a way especially favorable for excitation of the lowest quadrupole mode (a bending mode) which has a natural frequency near 64 kHz. The stresses are associated with the Lorentz forces on eddy currents around the equator. The mode is detected with a small microphone placed near the shell. In one version of the experiment a large dc bias magnetic field is superposed on a weaker oscillating field of frequency near 64 kHz. In another version of the experiment the bias field is removed and the frequency of the oscillating field is near 32 kHz (half of the excited mechanical frequency). The method may have applications to the inference of the contents or elastic properties of shells and to the investigation of the modal structure of open shells. [Work supported by the Office of Naval Research.]