For the purpose of fundamental consideration of the underwater random sound field, an approximate expression for the probability distribution function of sound power fluctuation radiated from a moving ship (which is very important as the sound source) is first derived, by paying special attention to the standard shape of sound pressure spectrum level of radiated noise. Next, by using the above theoretical result, a simplified probability expression is proposed as a statistical method of practical use. Finally, the validity and usefulness of the theoretical result is experimentally confirmed by applying it to data obtained by simulation experiment.

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Conventional piezoelectric ceramics, such as lead zirconate titanate (PZT), are hard materials (high Q materials). This hardness exhibits noticeable resonances and causes a narrow bandwidth. A flexible piezoelectric rubber is very promising piezoelectric material for wide band applications because of its low Q property. The transmitting and receiving sensitivities are measured over the frequency ranges 20-150 and 100-1000 kHz. The results of these measurements are compared with those obtained PZT, anisotropic (Pb, Ca) TiO₃ and P (VDF/TrFE). It is clarified that the piezoelectric rubber, (Pb, Ca) TiO₃ and P (VDF/TrFE) are very useful for underwater hydrophone applications over wider frequency ranges. In addition, a tone-burst response is tested. A damped ringing in a tone-burst pulse is measured-for piezoelectric rubber and (Pb, Ca) TiO₃ ceramics.

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