FRESNEL ZONE EFFECTS IN THE SCATTERING OF SOUND BY CYLINDERS OF VARIOUS LENGTHS

The backscattering behavior of straight cylinders is examined whose lengths range from much less than the diameter of the first Fresnel zone of the source/receiver pair to much greater than the first Fresnel zone, with special emphasis on the complicated "transitional region," where the cylinders occupy a finite number of Fresnel zones (almost-equal-to 1-5). In general, the scattering characteristics of cylinders in this region can only be described numerically. The scattering is described by first adapting the deformed cylinder formulation [T. K. Stanton, J. Acoust. Soc. Am. 86, 691-705 (1989)] to the point-source/point-receiver combination. Numerically evaluating this expression showed the scattering characteristics to be dominated by Fresnel zone effects-oscillations in the backscatter versus length curve caused by constructive and destructive wave interferences due to phase shifts from contributions along the cylinder axis. An experiment was performed that involved measurement of backscatter versus cylinder length in the transitional region, and there is reasonable agreement between the results and the trend as predicted by the approximate theory.