Liquid-filled focusing cylinders and the caustic merging transition

Abstract

Thin spherical shells filled with a liquid having a sound speed significantly less than the speed of sound in water are widely used for targets having enhanced backscattering. This enhancement has been analyzed using quantitative ray theory [P. L. Marston, Phys. Acoust. 21, 1–234 (1992)]. Backscattering enhancements are investigated here for a liquid-filled flat-ended thin fiberglass circular cylindrical shell in water. In addition to partially focused enhanced backscattering for near-broadside illumination, this target exhibits enhancements for specific ranges of tilt angles associated with caustic merging transitions previously demonstrated for blunt plastic cylinders [F. J. Blonigen and P. L. Marston, J. Acoust. Soc. Am. 107, 689–698 (2000)]. The refractive properties for the tilted cylinder depend on the effective refractive index N that is a function of the tilt angle and the actual acoustic refractive index of 1.68 (relative to water) for the enclosed liquid. Backscattering enhancements are visible from the tilted cylinder associated with two- and four-chord transmitted internally reflected rays when the tilts are such that N is close to 2 and is close to 4, respectively, as predicted by ray theory. The wave-number-radius product was as small as 16. [Work supported by the Office of Naval Research.]