### ASA 124th Meeting New Orleans 1992 October

## 5pPA2. Liquid-filled spherical reflectors: The exceptional case of
refractive index approaching two.

**P. L. Marston
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N. H. Sun
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*Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814
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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.