Scattering of ultrasonic waves by oblate spheroidal voids of high aspect ratios

The scattering of ultrasonic waves by oblate spheroidal voids of very high aspect ratios (a:b=1:20) is analyzed using the T-matrix method. By using an iterative Gram-Schmidt orthogonalization procedure, and by enforcing the unitarity and the symmetry properties of the T matrix at each iteration, numerical results are presented for normalized frequencies k_pb up to 15.0. Aspect ratios up to 1:20 and such high frequencies were previously not amenable to computations using the conventional T-matrix procedure. Scattering by smooth as well as rough voids having periodically corrugated surfaces is studied. All computations were carried out on a DEC VAX 11/730 minicomputer.