PVF2 velocity hydrophones

In underwater acoustical low-frequency applications a hydrophone with intrinsic directivity is often needed. 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 particle 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 be greater than 1 mm), make possible a compromise between the opposite requirements that a high sensitivity demands a thick polymer and for a high mechanical flexibility of the membrane (to obtain the lowest possible resonance frequency) a thin bimorph is needed. 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. The same hydrophone can be connected so as to be also pressure sensitive: Intensity measurements might become possible with only one sensor.