Experimental implementation of the RST predictions for elastic objects in water

Selected predictions of the Resonance Scattering Theory (RST) were experimentally investigated at NSWC's Hydroacoustic Measurements Facility during the summer of 1984. Elastic cylinders and cylindrical shells of various thicknesses and aspect ratios were placed in a water-filled redwood tank 30 ft in diameter and 20 ft deep, and were insonified by trains of 1-ms sinusoidal pulses. The frequencies were varied between 50 and 500 kHz. The signal from a hydrophone, located between the projector and the objects, was sampled at three judiciously chosen time-gated windows to yield the strengths of the incident wave, of the backscattered echo, and finally of the free-vibration target reradiation. Spectral recordings showed that the penetration of sound energy into the object is maximized when the input frequency matches one of the natural frequencies of the object. This is manifested in large dips in the spectral displays of the reflected echoes. The spectral display of the reradiation plot shows sharp rises at these resonance frequencies. For the chosen object shapes, the experimentally observed phenomena, resonant amplitude, bandwidth, and frequency, match well with the theoretical predictions [G. C. Gaunaurd and D. Brill, J. Acoust. Soc. Am. 75, 1680–1693 (1984)]. The observations, in general, show that: (a) each object returns a characteristic resonant signature, (b) this signature contains shape and composition information about the object, (c) agreement between theory and experiment exists for various simple shapes, and (d) a signal processing method for active classification using this signature is possible.