M. F. Werby
Code 7181, Naval Res. Lab., Stennis Space Center, MS 39529
The pioneering paper on resonant acoustical scattering from elastic solids by Flax et al. [J. Acoust. Soc. Am. 63, 723--731 (1978)] (FDU) formed the basis for resonance scattering theory; proposing the decomposition of the elastic response into resonance and nonresonance ``background'' components. The resonance component was equivalent to the Breit--Wigner form from quantum mechanics, in which Flax and Uberall were trained. The ``background'' for elastic solids was that of a rigid scatterer. In a series of papers, FDU introduced many resonance classification schemes based on actual physical mechanisms. The FDU development bore much fruit in target classification physics in over a decade of lively research. Flax, also interested in elongated targets and elastic shells, initiated theoretical studies involving Waterman's T-matrix method. Here, some recent work stemming from Flax's interests stimulated by our early interaction with him is reported. The best backgrounds for generally shaped evacuated and fluid-filled elastic shells are discussed; and their use to interpret complicated resonance phenomena are outlined. Also demonstrated is that the FDU theory can be derived formally from an abstract mathematical structure. [Work sponsored by the Naval Research Laboratory 6.1 Program Element 0601153N.]