Hydrophone for Measuring Particle Velocity

Mathematical expressions are derived for the motion of a rigid, uniform sphere in an underwater sound field. The results show that at low frequencies a neutrally buoyant sphere of this type has a velocity equal to the particle velocity of the water at the same location when the sphere is removed. A velocity pickup may be mounted inside such a sphere to form a hydrophone sensitive to particle velocity. Two practical design applications are presented. A low-frequency hydrophone with a diameter of five inches was constructed having an essentially flat response between 15 and 700 cps with a sensitivity of 0.18 volt/cm/sec working into a 500-ohm load. A higher frequency design with a 2.5-inch diameter operated from 70 to 7000 cps with somewhat lower sensitivity and impedance level. Velocity hydrophones of this type are easy to build, easily calibrated, stable, and possess good directionality.