Acoustic Nearfield of a Metallic Spherical Shell in Water

Abstract

Experimentally measured polar patterns of peak pressure and echo energy in the scattered acoustic nearfield of a thin-walled, water-filled aluminum sphere in water are presented. A pulse technique is employed. The range of r/a, the test distance in radii, is from 1 to 20. Patterns at two nondimensionalized frequencies, ka=18.7 and 56.1, are shown. A 1/r2 spreading loss in backscattered energy is obtained for r/a>5. For 1<r/a<5, the energy decays as 1/rβ, where the magnitude of β is less than 2 and is a function of r/a. Formation of the farfield lobe structure is traced through the nearfield polar patterns. A loss and reappearance of nonsurface-reflected components of the nearfield echo-pulse forms is observed; the loss of these components is related to the appearance of nulls in the nearfield lobar patterns. [Work done under contract with the U. S. Office of Naval Research, Code 463.]