The multiport transducer developed by Dr. J. L. Butler is a novel low-frequency underwater acoustic source using free-flooded resonance cavity chambers with both the inside and outside driving surfaces made of cylindrical piezoceramics. In this paper, a lumped parameter circuit model and an equation for the transmitting voltage response of the multiport transducer are discussed. Simulation results from the developed circuit model in this paper strongly agree with Butler's measured results and better than with his circuit model. Some considerations for improving the response are also reported.

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The ratio between the speed of sound in animals and that of the surrounding water (the sound-speed contrast), particularly for fish lacking any gas-filled organs or for zooplankton, is an important parameter in a theoretical scattering model for estimating target strength (TS). Since the swimbladder of a juvenile southern bluefin tuna (SBT) is not developed or gas-filled, the accurate and precise measurement of the sound-speed inside the body is required for TS estimation by the scattering model. The sound-speed of juvenile SBT tissue was measured using a FURUNO medical bone density-measuring instrument, the CM-100, during acoustic surveys conducted in Western Australia. This instrument measures the ultrasound travel time in tissues (and converts sound-speed to bone density). Since it has been recognized that sound-speed in tissues changes as time progresses and that it varies between fresh and defrosted tissues, the measurement should be taken immediately after the fish is caught. The sound-speed of juvenile SBT tissue ranged from 1610 to 1655 m/s and showed a tendency to depend on body temperature. On the assumption that the muscle temperature of a living SBT was 5 degrees higher than the surrounding water, the sound-speed contrast ranged from 1.06 to 1.09.

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