Sound scattering and its cancellation by an elastic spherical shell in free space and near a free surface

Sound scattering by an elastic spherical shell is analysed using linear acoustics and linear structural dynamics. It is suggested to utilize the shell's structural dynamics to reduce or even eliminate the scattered sound field, thus making it practically acoustically invisible. This can be achieved using a prescribed external pressure distribution acting on the shell's wall. Exact analytical solutions are found for that external pressure distribution, eliminating the scattered wave when the sphere is in free space or near a free surface and is subject to an incoming planar monochromatic sound wave. The latter is assumed to propagate in a direction perpendicular to the free surface (if it exists). The case of a few pressure-actuators acting on the shell's wall is also modelled and an optimal solution which reduces the sound scattering by these actuators is found. An aluminium shell of 1 m radius and 5 mm thickness, situated in fresh water is analysed for sound frequencies of up to 10 kHz. The scattered wave fields are presented as well as the external pressure distributions that eliminate these scattered sound field, i.e. achieving acoustic cloaking. Significant reduction in the scattered wave energy and the target strength of more than 10 dB are also realized using a few pressure-actuators as long as the distance between the actuators is no more than three times the incident wave length for the investigated cases.