Suppression of side lobes can be accomplished by utilizing Dolph-Chebyshev amplitude shading. Research on shape shading is seldom found because of the great difficulty in cutting of PZT ceramics. Recently, a piezorubber sheet consisting of piezoelectric ceramic particles and chloroprene rubber has been developed. Since this material is quite soft compared with PZT ceramics, it is very useful for research in shape shading. In this paper, aperture functions, originally applied to the design of FIR digital filters, are shown to also be successfully applied for suppression of side lobes in hydrophones. Test hydrophones are constructed by shading a piezorubber sheet in the form of the aperture functions. It is clarified both experimentally and theoretically that some hydrophones such as Hamming and Hamming-like Kaiser windows suppress side lobes to less than -30dB.
The separation of a standing wave field in air into incident and reflected waves is possible with a wave separator, employing two-microphones and some delay units. This separation technique is called a two-microphone method. The two-microphone method is shown to also be successfully applied for water-bone sound waves. A new three-hydrophone method which does not need to consider the sound velocity factor is proposed. In this method, it is sufficient to measure only complex sound pressures at three points. Experimental results have verified the theoretical predictions on the separation of the standing wave in water.