A micro-machined Tonpilz hydrophone for audible frequency sounds

We report a micro-machined Tonpilz hydrophone that uses the piezoelectric thickness mode and is suitable for the audible frequency range (i.e., 20Hz-20kHz). The structure of the sensor is based on the structure of a conventional Tonpilz transducer; however, two different design approaches were investigated to enhance sensitivity and to withstand the high-pressure underwater environment. To improve the sensitivity, the area ratio of the head mass to the piezoelectric body is several hundreds to one, which amplifies the acoustic pressure input of the transduction body. To improve the resistance to the hydrostatic pressure in the deep-water environment, the structure uses a castor oil backing to balance the hydrostatic pressure at the front surface of the acoustic sensing structure. We developed a lumped element model to investigate the design parameters for miniaturized hydrophones. The hydrophone was then fabricated using micromachining, and tested by comparing the device with a commercially available hydrophone in an anechoic bath to assess the feasibility of its use as an effective underwater sound receiver.