A micro-machined Tonpilz hydrophone for audible frequency sounds

A micro-machined Tonpilz hydrophone based on the piezoelectric thickness mode was designed for audible frequency range of 20 Hz~20kHz. The structure of the sensor was motivated by conventional Tonpilz transducers, but two different design approaches were adopted to enhance the sensitivity and to endure the high hydrostatic pressure in deep-sea. For improved sensitivity, the area ratio of the head-mass and the piezoelectric body was designed to be several hundreds to one, which amplifies the input of the transduction body due to acoustic pressure. Since this approach is adopted in order to develop a miniaturized hydrophone manufactured by a batch process, the size of the piezoelectric transduction body becomes too small to generate the amount of charge enough for detecting the signal using the conventional pre-amplifier at low frequencies below 500 Hz. We have developed and validated the lumped parameter model and used it to identify the requirements for the pre-amplifier circuits and the available sensitivity at low frequencies and to search for the appropriate design for miniaturized hydrophone. Then, the designed hydrophone was fabricated by micro-machining and assembled with the custom-made pre-amplifier inside with caster-oil filled rubber housing. The evaluation of the hydrophone will be presented. [Research supported by MRCnD.]