Application of Multilayer Piezoelectric Actuators to Broadband Backscattering Measurements of Aquatic Animals

To apply multilayer piezoelectric actuators to broadband backscattering measurements of aquatic animals, we built a Langevin-type transducer using 10-mm-long actuators. The useful frequency band was 20–150 kHz, and the main lobes of the beam patterns were almost identical to those of an ideal circular piston source with a diameter of 110 mm and the beamwidths of 21.2° at 38 kHz and 6.6° at 120 kHz. The transducer and commercially available equipment were integrated as an echo-sounding system. A linear frequency modulated signal (20–150 kHz) was employed as the transmitted signal. The system was calibrated using a 20.6-mm-diameter tungsten carbide sphere. The broadband backscattering measurement method was verified using a 38.1-mm-diameter tungsten carbide sphere. The backscattering measurements of three aquatic animals were conducted: a swimbladdered fish, a decapod shrimp, and a benthic gastropod. The target strength (TS) spectra specific to the species were obtained and those of the swimbladdered fish and the decapod shrimp were reasonably consistent with the predicted TS spectra based on theoretical scattering models. That of the benthic gastropod was different from those of the other two species. The Langevin-type transducer using the multilayer piezoelectric actuators is applicable to the broadband backscattering measurements of aquatic animals.