Miniature hydrophones from hollow ceramic spheres

Millimeter size microprobe hydrophones have been designed and fabricated from miniature piezoelectric hollow ceramic spheres for underwater applications such as mapping acoustic fields of projectors, and flow noise sensors for complex underwater structures. Green spheres are prepared from soft lead zirconate titanate powders using a coaxial nozzle slurry process. A compact hydrophone with a radially-poled sphere is investigated using inside and outside electrodes. Characterization of these hydrophones is done through measurement of hydrostatic piezoelectric charge coefficients, free field voltage sensitivities (FFVS) and directivity beam patterns. Results indicate a FFVS of around -215 dB re 1V//spl mu/Pa with a flat response between 5 kHz up to 90 kHz and omnidirectional properties from 5 kHz up to 70 kHz.