The resonance features present, and observable, in the form-functions of submerged, fluid-filled, elastic, thin shells, usually appear combined with smooth, undesirable, background components that tend to obscure them. In order to isolate these features in the frequency domain, at fixed angles of observation, (i.e., the “spectograms”), or in the angular domain, at selected frequencies (i.e., the “rosette” patterns), one usually attempts to suppress the background components by subtracting “suitable” background coefficients. Since the rigid/soft background coefficients of solid elastic bodies/cavities in solids yield unsatisfactory results for thin shells, various other background coefficients have been tried with various levels of success. We present here a more general scattering formulation, valid for many concentric shells, that permits the isolation of these resonance features without having to subtract any type of background coefficients. This resonance feature extraction is achieved either conceptually, or physically, filling the various spaces in between the various concentric shells, with the same fluid present in the outer ambient medium. The extraction of these echo-features is crucial for purposes of active classification by sonar.

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