Development of the underwater acoustic prism

Abstract

The acoustic prism (i.e., leaky wave antenna) has been experimentally demonstrated in air as a way to steer an emitted beam using only a single broadband acoustic source. The prism relies on a leaky, dispersive waveguide to provide a unique radiation angle for each narrowband frequency projected by the acoustic source. In air, the leakage occurs through a series of periodically spaced shunts in the waveguide. This study examines an acoustic prism design that is capable of operating underwater, where leakage occurs through the waveguide wall itself due to the much lower impedance contrast of the waveguide material in water to that in air. This results in a geometrically simpler design in the underwater case. However, shear wave effects must be considered in the design of the underwater acoustic prism. The waveguide wall is constructed out of a composite material to have a high impedance but a low shear modulus, which are both necessary conditions to decrease sidelobes in the radiated pressure field. Numerical results indicate that the acoustic prism design is capable of scanning a range of frequencies from broadside to forward endfire. Experimental realization of the underwater acoustic prism is also discussed. [Work sponsored by ONR.]