The piezoelectric properties and the receiving characteristics of d31-zero Piezo-Rubber hydrophones are investigated. Piezo-Rubber is a piezoelectric composite material consisting of piezoelectric ceramic powder and Chloroprene rubber. The d31-zero Piezo-Rubber hydrophones consisting of lead titanate (PT) and lead zirconate-titanate (PZT) ceramic powder have almost flat frequency receiving characteristics. However, the d31≠zero Piezo-Rubber hydrophones consisting of PT and PZT have some variations in the receiving characteristics. The relationship between this variation and -2d31/d33 or hardness of Piezo-Rubber is discussed.
The LOFAR-Gram Which displays a result of spectrum analysis as time series is used to detect and classify target signals of the passive sonar. Signals-processing gain of frequency analysis is improved by narrowing the frequency bandwidth and by averaging (cumulatively processing) the results of frequency analysis in each observation time. However, conventional averaging in LOFAR-Gram cannot increase processing gain sufficiently. In this paper, processing gain for delay type cross-spectrum method is described. This method increases the processing gain. The effectiveness of this methods shown by the comparison with conventional auto-spectrum method.