Localized Maxwell-stress excitation of low-frequency modes of electrically conducting objects in water in reverberant environments

When testing some specialized sonar systems, it can be helpful to have alternative methods of exciting low-frequency (LF) modes of objects to be studied. This is especially the case in reverberant environments. One non-contact approach for driving modes of electrically conducting solid objects is to excite eddy currents which cause oscillatory Maxwell stresses on objects [B. T. Hefner and P. L. Marston, J. Acoust. Soc. Am. 106, 3340–3347 (1999)]. In the present application, the stress is localized using an appropriate coil-magnetic-core system 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 and compressional modes of objects of interest in freshwater-lake as well as laboratory-tank experiments. Some of the objects investigated include spherical and cylindrical shells, solid cylinders, and an aluminum UXO model. In the lake-based experiments, the radiated sound is detected using an array of hydrophones, though in the tank-based experiments it can be convenient to scan a hydrophone. Complications in the responses associated with environmental reverberation can be investigated. [Work supported by the U.S. Office of Naval Research.]