Localized Maxwell stress excitation of low-frequency vibrational resonances of electrically conducting objects in water

When developing specialized sonar systems for structural modes, it can be helpful to have alternative methods of exciting low-frequency (LF) vibrational modes of objects to be studied. One non-contact approach for driving modes of electrically conducting solid objects is to excite eddy currents which cause oscillatory Maxwell stresses on objects [Hefner and Marston, J. Acoust. Soc. Am. 106: 3340–3347 (1999)]. In the present application, the Maxwell stress is localized using an appropriate coil with a long-narrow ferrite core mounted close to the object. Electric-current tone bursts in the coil cause stresses at twice the frequency of the current. Frequencies and locations are selected to drive modes of interest. This method has been used to excite LF flexural modes of objects of interest in lake-based experiments. The objects investigated include a solid cylinder, a spherical shell, and an aluminum unexploded ordnance model. In the experiments, the radiated sound is detected using an array of hydrophones. This approach is advantageous because it does not introduce a significant initial sound wave that can reverberate around the environment; only the excited object's sound emissions and ambient noise are recorded. Structural resonances can be detected in this way.