### ASA 127th Meeting M.I.T. 1994 June 6-10

## 5aUW13. Symmetry and axial focusing in backscattering by elastic objects
in water.

**Philip L. Marston
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*Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814
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Early calculations [L. Flax, J. Acoust. Soc. Am. 62, 1502--1503 (1977)] of
the exact partial-wave series of elastic objects in water for ka from 900 to
950 suggest that elastic contributions to backscattering are relatively more
important for spheres than for circular cylinders. The main distinction between
the two situations is the axial symmetry of the sphere that produces backwards
directed toroidal wave fronts associated with off-axis rays [P. L. Marston and
L. Flax, J. Acoust. Soc. Am. Suppl. 1 68, S81 (1980)]. In addition to
geometrical considerations and diffraction theory leading to a (ka)[sup 1/2]
factor for such focusing, early support for this hypothesis was provided by
calculations [L. Flax (personal communication, 1980)] for an aluminum sphere
with ka=938 showing the form function magnitude |f| as a function of the
backscattering angle (gamma)=(pi)-(theta). The enhancement of |f| is localized
to small values of (gamma) as expected from the theory of axial focusing. The
analysis of axial focusing [P. L. Marston, Physical Acoustics (Academic, New
York, 1992), Vol. 21, pp. 1--234] is not limited to such extremely large values
of ka and includes elastic shells and other symmetric objects. [Work supported
by ONR.]