Conditions for precise measurement of in situ fish target strength (TS) are empirically studied and two indexes are introduced for this purpose. One is the number of fish in the effective reverberation volume which contributes echo formation at a certain instant and the other is the percentage of the multiple echoes which is derived from a residual of the single echo extraction. With the decrease of both indexes measured target strength approach a certain asymptotic value which is admitted as reliable from the past study. This shows the existence of some threshold values and below these threshold values TS measurement will be successful. The effectiveness of both indexes is confirmed by the data set obtained from one large same fish school in the eastern shelf of Bering sea during the intership calibration between Japanese and U.S. vessels on 15 and 16 August 1991.
Most transducers and arrays show directional beam patterns which are frequency dependent. In this paper, transducers with a constant beamwidth in one-dimensional plane over a wide frequency range are studied both theoretically and experimentally. Some test transducers, curved rectangular and curved Hamming types with various values of curvature, are constructed from a flexible piezoelectric rubber sheet. It is shown that the rectangular type maintains a fairly constant beamwidth but the pattern is not desirable because of the appearance of large ripples in the main lobe, while the Hamming type has no ripples and the directivity is almost constant at all frequencies above a cut-off frequency.
Porous Pb (Zr, Ti) O3 (P-PZT), characterizing a large g33 constant, low acoustic impedance (Zo) and low Q compared with conventional (nonporous) PZT, is applied to the transducer of the resonance frequency of 50kHz. Because the frequency is adjusted in the thickness mode, the resonance impedance of the P-PZT transducer becomes high. The impedance is lowered by the transformer composed in the transducer. The characteristics of the P-PZT transducer evaluated in water show a lower Q, wider frequency band width and shorter rise/fall time of the acoustic response than the non porous PZT transducer.