Kerry W. Commander
Raymond Lim
Coastal Systems Station, Panama City, FL 32407-7001
Theodore W. L. Huskey
Steven R. Baker
Naval Postgraduate School, Monterey, CA 93943-5000
Steven G. Kargl
Univ. of Washington, Seattle, WA 98105
An underwater acoustic scattering experiment was performed on three porous solid spheres of varying grain size to determine their acoustic form functions. The spheres were constructed from bonded glass beads, sized to yield permeabilities in the range of fine to medium grained sand. Measurements of required material and lattice parameters were determined on analogous cylindrical samples, including a measurement of the dry lattice moduli per the method described by Garrett [S. L. Garrett, J. Acoust. Soc. Am. 88, 210--221 (1990)]. Quantitative agreement between the acoustic measurements and predictions from a theoretical Biot model [S. G Kargl and R. Lim, J. Acoust. Soc. Am. 94, 1527--1550 (1993)] was found but only at the low-frequency end (<20 kHz) of the measurements and for the smaller grained spheres. Although the measured and predicted form functions were still qualitatively similar at higher frequencies, deviations due to inhomogeneities in the porous spheres became evident. For each discrete frequency measurement, the spheres were rotated 360 deg and variations in the backscattering strength noted. For values of ka where little variation with sphere orientation was obtained, there was good agreement with the theoretical model.