Control of directivity of PVDF hydrophones using anisotropic substrates

Development of a compact underwater sound intensity sensor based on the “two hydrophone” (or ‘p-p’) principle would be very useful for many underwater applications. Attempts to develop sound intensity probes based on the combination of particle velocity and pressure sensors have been reported by some authors but their suitability for use on a moving platform is unconvincing. Our previous work focused on using piezoelectric PVDF polymer films to construct such a p-p sound intensity sensor. This showed some promising results but difficulties arose in compensating for the directivity of the individual pressure sensor films. Ideally these should be omni-directional in order to accurately estimate the sound pressure gradient using the finite difference approximation. By using anisotropic backing materials it has been possible to control the directionality of the PVDF films up to frequencies as high as 50 kHz. The effect of various substrate anisotropies on the directionality is discussed.