Piezoelectric polymer velocity hydrophone

In underwater acoustical low frequencies applications, a hydrophone with intrinsic directivity is often needed. Velocity-sensitive hydrophones are not the only devices having this property: so have vector-sensitive devices as displacement, acceleration or gradient-pressure hydrophones. Poled polyvinylidene fluoride (PVF2) bimorphs are suitable for design of velocity hydrophones. Because of its softness and mechanical flexibility, a PVF2 bimorph, associated with an inertial structure, follows water particles movement and so the output current from the hydrophone is proportional to the particle velocity of the incident wave. Moreover, the PVF2 bimorphs manufactured in our laboratories,whose thickness may reach 1 mm, make possible a compromise between two opposite requirements: a high sensitivity which needs a thick polymer, a high mechanical flexibility of the membrane to get the lowest possible resonance frequency, that requires a thin bimorph. The study of a low frequency particle velocity calibration system, made possible the achievement of small lightweight PVF2 velocity hydrophones, showing an intrinsic directivity below 100 Hz.