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The construction and evaluation of a hydrophone based on porous piezoelectric ceramics with high d h g h figure of merit ͑FOM͒ is described. It has been shown that, in order to improve the hydrophone signal-to-noise ratio, a piezoelectric material with a high FOM should be employed. A porous piezoelectric material has been prepared by mixing calcined lead zirconate titanate ͑PZT͒ powder with fine particle starch powder. Square plate samples have been cold pressed from this material, which were then heated to eliminate the organic component, sintered, electroded, and poled in a high electric field. An optimum pore volume fraction of approximately 40% has been selected in order to obtain materials with high piezoelectric coefficients and reasonably good mechanical resistance. For this composition a hydrostatic figure of merit of approximately 10 Ϫ11 m 2 /N has been obtained that is a few orders of magnitude higher than traditional piezoceramics. Square plate elements were assembled in a planar hydrophone which was made watertight with polyurethane resin. The hydrophone was characterized by different measurements performed in a water tank, by using a pulsed sound technique. Results on acoustical sensitivity measurements, directivity, equivalent noise pressure level, and sensitivity variation with pressure are presented and discussed.

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