http://patft.uspto.gov

U.S. Pat. No. 4,928,263 to Armstrong et al. discloses an accelerometer-based hydrophone using piezoelectric ceramic as the active material. The hydrophone comprises orthogonally arranged accelerometers that share an inertial mass and housed in a pressure vessel. 

U.S. Pat. No. 6,370,084 to Cray discloses an acoustic vector sensor comprising an acoustics sensing tri-axial accelerometer, a body of syntactic foam encasing the accelerometer, a body of viscoelastic rubber encasing the body of syntactic foam, and a rigid plastic coating encasing the body of viscoelastic rubber. The viscoelastic body is acoustically transparent and isolates the accelerometer and syntactic foam sphere from the mount, and insulates the accelerometer from structure-borne flexural vibrations from supporting structures near the sensor. 

U.S. Pat. No. 6,697,302 to Cray et al. discloses an underwater acoustic sensor that measures up to seven quantities of acoustic field at a collocated point. The quantities measured by the acoustic receiver sensor are acoustic pressure, three orthogonal components of acoustic particle acceleration, and three spatial gradients of the acceleration vector. These quantities are then combined to provide improved directivity of the acoustic receiver sensor. 

Patent Application #20050034519 is directed to an acoustic vector sensor, specifically an underwater acoustic vector sensor. The acoustic vector sensor contains three piezoelectric sensors orthogonally mounted inside a rigid housing, where each of the piezoelectric sensors measures acoustic energy from one of the three different, orthogonal, axial directions (X, Y, and Z). The piezoelectric sensor contains a proof mass, a base, and a piezoelectric crystal sandwiched therebetween. The bonding surfaces of the proof mass and the base are preferably castellated; and the piezoelectric crystal is preferably a shear mode (d15) relaxor single crystal.