Philip L. Marston
Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814
Ray methods describe guided-wave scattering contributions based on local coupling processes. To estimate the effective width of coupling regions for launching and radiation of sound by leaky guided waves of phase velocity c[sub l], Fresnel volume concepts of Y. A. Kravtsov and others [see, e.g., P. L. Marston, Physical Acoustics (Academic, New York, 1992), Vol. 21, pp. 1--234] were generalized. First, the usual coupling condition on the angle of incidence (theta)[sub l] for a sphere or cylinder, sin (theta)[sub l]=c/c[sub l], is derived by Fermat's principle. Then the angular shift (Delta)(theta)[sub l] is derived for a (pi) radian phase deviation from the Fermat ray. For a far-field source or observer, |(Delta)(theta)[sub l]|(approximately equal to)[radical 2(pi)/(ka cos (theta)[inf l])[radical is a measure of the ray width. Fermat's principle is also examined for a prompt backward wave mechanism [D. H. Hughes, Ph.D. thesis, Washington State Univ. (1992)] for the high-frequency scattering enhancement by shells. [Work supported by ONR.]