Development of broadband transducer using multilayer piezoelectric elements

A Langevin-type broadband transducer was developed using a multilayer piezoelectric actuator (hereafter referred to as a multilayer piezoelectric element). The frequency range of the broadband transducer was aimed to cover the frequency range from 38 to 120 kHz typically used for sensing fish and zooplankton. The 10-mm long multilayer piezoelectric elements whose resonance frequency is 138 kHz were chosen for covering the high-frequency region. Acrylic disks of 13-mm thick as the front and rear masses were used for realizing the resonance at the low-frequency region. Also the diameter of 11 cm was determined for realizing suitable beamwidths 5-20°. Thirty elements were set in circular-shaped arrays on the disk and the two acrylic disks were fastened with four bolts. The peaks of the transmitting sensitivity were measured at 46 and 135 kHz, and the values were 168.3 and 165.3 dB re 1 μPa/V, respectively. Although ripples were observed, useful band around 46 and 135 kHz were 26-65 kHz (39-kHz bandwidth) and 110-142 kHz (32-kHz bandwidth), respectively. The transmitting sensitivity function from 70 to 105 kHz was flat, but the sensitivity of 158.0 dB re 1 μPa/V was lower than those of the above frequency range. The peaks of the receiving sensitivity were measured at 28 and 135 kHz, and the values were -188.5 and -173.6 dB re 1 μPa/V, respectively. The receiving sensitivity function was not flatter than the transmitting sensitivity function and the sensitivity at 70 kHz of -207.9 dB re 1 V/μPa was considerably low. The measured beam patterns at 38, 70, and 120 kHz were almost the same as that of the 11-cm diameter circular piston source. The broadband acoustic system was constructed using commercially available equipments and the measurement of the target strength spectra of a 38.1-mm diameter tungsten carbide sphere using a 20-150 kHz linear-frequency-modulated signal. The measured target strength spectra of the 38.1-mm sphere we- - re in good agreement with the predictions except for the region of the low signal-to-noise ratio.