Design and evaluation of underwater hydrophones with self-noise suppressing structures

Underwater hydrophones are mounted on a vibrating surface in common applications and their performance is usually degraded by the interference of various external noises. This paper describes the design and evaluation of a self-noise suppressing underwater hydrophone without sacrificing their performance as a receiver of external acoustic signals. The hydrophone under consideration is an array sensor composed of several tonpilz transducers. To increase the insensitivity to external noises, first, effects of transfer paths of the external noises on the performance of the hydrophone is checked through computer simulation with finite-element methods (FEM). Second, several geometrical variations are imposed on the original structure of a hydrophone. The variations include additional air pockets and acoustic walls that work as acoustic shields or scatterers of the noises. Results show that the transverse noise induced by the outside water flow is isolated most effectively when a thin compliant (damping) layer combined with two air pockets is inserted into the circumference of the hydrophone nose. The noise level is reduced to about 40% of that of the original structure. When the transducer array is surrounded by two thin sandwich compliant layers with an acoustic wall the noise level is as low as 25% of that of the original model. Further, the optimum acoustic impedence of the compliant layer is estimated to be either less than 1 Mrayl or more than 4 Mrayl. The structure developed in this paper allows the hydrophone to be mounted on vibrating surfaces with a better signal-to-noise ratio.