The shadow behind a sphere immersed in water-measured, estimated, and computed

Acoustical pressure distribution behind a steel sphere 5 mm in diameter was measured in water for an incident plane wave with a 2.4-MHz frequency (ka=8 pi ). Measurements were carried out for continuous and pulse waves at various distance behind the sphere by means of a PVDF membrane hydrophone. The shadow range determined by measurements behind the steel sphere was about 15% shorter than estimated by means of formulae deduced earlier for a rigid sphere. The distributions of the acoustical pressure when measured and computed show almost the same shape. The obtained experimental results to a great extent agree with the theory for the rigid sphere elaborated for ka=4 pi to 200 pi . Therefore, these formulae can be used as the first estimation of the shadow range behind an elastic steel sphere. The details of the measurement technique and possible reasons of differences between experiment and theory are discussed.