Performance of an acoustic sonde design
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The potential of multi-frequency acoustic technology for quantifying fish and plankton is widely recognized. At a U.S. GLOBEC workshop in 1991, one of the considered problems was measuring macrozooplankton and micronekton. It was concluded that in order to survey animals in the size range 0.5-5 cm, it would be expedient to use at least three and perhaps as many as 8-10 frequencies in the range 38-420 kHz. Here, a specific design is evaluated: ten more or less standard, approximately logarithmically spaced frequencies are chosen. Each frequency represents the resonant condition of a circular piston transducer with 10-deg beamwidth. The performance of this system is assessed through the maximum detection range of single targets, assuming transmission near the cavitation limit but consistent with dynamic strength and heat generation in ceramic elements, isotropic ambient noise, and detection threshold of 20 dB. Target strength is treated as a parameter, with investigated values from -130 to -50 dB. Performance assessment for a volumetric distribution of scatterers is similarly treated.