Resonance scattering by elastic cylinders and their experimental verification

We present an experimental technique that verifies the theoretical predictions of the resonance scattering theory (RST) which were obtained for fluid-loaded elastic objects of separable shapes. The rationale in RST permits the separation of the ‘‘backgrounds’’ or rigid-body portions of the scattering patterns, from the ‘‘resonance’’ portions which are due to the object’s penetrability or elastic composition. For structures of separable shape, this decomposition can be theoretically accomplished for the individual modes, even when these are coupled due to the fluid loading. For general, nonseparable shapes, the decomposition holds for the summed cross sections, rather than for the individual partial waves, since in this case, the modes cannot be individually isolated. In either situation, however, the experimental method described here achieves this crucial RST decomposition of the returns by sampling the pulses scattered by the underwater structure at two temporal locations by means of suitable time-delay circuitry. One location is within the steady-state regime of the forced elastic vibration and the other is in the later transient regime of the free vibration (i.e., the ‘‘ringing’’). For the example of solid and hollow elastic cylinders in water, the technique measures the locations of the many modal resonances predicted by the RST in wide spectral bands. It also determines the skewness, amplitude, and the area under each of the resonance shapes. This goes beyond the achievements of any other experimental configuration. The background-subtracted ‘‘spectrogram’’ thus obtained is the active signature of the object which serves to characterize it uniquely. Once the method has been shown to accurately verify the theoretical RST prediction, in cases where such predictions can be generated, the technique can be used for any complex shape for which predictions may be difficult or impossible to produce. Thus the validity of the RST principles is experimentally confirmed, and an experimental method for active classification of underwater objects was devised and implemented at the place of development of the RST.