Designing acoustic transmitting-receiving system for volume backscatteringmeasurement of zooplankton

A basic designing method of the acoustic transmitting-receiving system for measuring or utilizing volume backscattering of zooplankton is developed, a pplying the method established for the quantitative echo sounder design. Th e primary principle is to make the signal-to-noise ratio in measuring volum e backscattering strength of zooplankton as high as possible by selecting a ppropriate parameters such as frequency, beam width, etc. The scattering pr operty of zooplankton applied is Johnson's high-pass liquid sphere model. T he frequency characteristics of the signal-to-noise ratio are derived combi ning the scattering model and several characteristics of sound transmission and reception. A universal diagram for basic design of transmitting-receiv ing system, which shows all relationships in the observation range, the pla nkton size, the signal-to-noise ratio, the frequency, the beam width and th e diameter of transducer, is made. Suitable range of the frequency, maximum depth range, etc, can be easily found at a glance from this diagram. The m ethod is experimentally examined using the field data obtained by a Doppler current meter. The results show that the experimentally observed signal st rength is nearly equal to the predicted one. The signal-to-noise ratio is f urther discussed using the noise level observed during the experiment.