P. L. Marston
N. H. Sun
Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814
Thin spherical shells filled with a liquid having a low speed of sound c[sub i] are known to have enhanced high-frequency backscattering that generally increases with ka and depends on the acoustics refractive index N=c/c[sub i]. Here, a is the radius of the sphere. For N between [radical 2] and 2 there is a backscattered two-chord ray with a nonzero impact parameter b. A physical-optics analysis [P. L. Marston and D. S. Langley, J. Acoust. Soc. Am. 73, 1464--1475 (1983)] is applicable except near N of [radical 2] and 2. The present analysis concerns the strong scattering case of N approaching 2 that corresponds to a vanishing value of b. The outgoing wave front is a surface of revolution W(approximately equal to)a[sub 4]s[sup 4]-a[sub 2]s[sup 2], where s is the distance from the optic axis and the coefficient a[sub 2] vanishes as N approaches 2. For N=2, a novel physical-optics analysis shows the amplitude contribution increases as (ka)[sup 1/2] and is proportional to a Pearcy--Fock function. That function is also used in scattering theory for bubbles [C. E. Dean and P. L. Marston, Appl. Opt. 30, 4764--4776 (1991)]. The analysis was confirmed by comparison with the partial-wave series for the case of a neutrally buoyant liquid neglecting any effects of the shell. The analysis shows that while N=2 has no major advantage, there are special aspects of the scattering. [Work supported by ONR.] [sup a)]Present address: EXP Group, Inc., 44063 Fremont Blvd., Fremont, CA 94538.