Comparison of equal-area cylindrical and circular piston transducers
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Design of sondes for in situ measurement of zooplankton or other scatterers requires choosing among alternative transducer geometries. This contribution addresses the problem of choosing between cylindrical and circular piston transducers by comparing the performance of the two according to the principle that the acoustically active areas be equal. Computations are performed with the actual dimensions of six fabricated cylindrical transducers, whose beam patterns have been measured by the manufacturer at a total of eleven frequencies spanning the range 27-710 kHz. Nominal power levels assigned to the cylindrical transducers are also used for both transducer types. Comparison of theoretically computed beam patterns with measurement gives confidence in the radiation model, which is used to compute the directivity index and on-axis sensitivity loss due to curvature of the cylindrical transducers, referred to as the curvature loss. Under identical conditions of excitation, isotropic ambient noise, and detection threshold of 20 dB, the active sonar equation is exercised to estimate the maximum detection range of both single targets and multiple targets distributed throughout the sampling volume. In every single case, the performance of the equal-area circular piston is superior to that of the corresponding cylindrical transducer. This is directly attributable to differences in directivity index and curvature loss. Other, pragmatic considerations argue for the choice of the circular piston transducer over the cylindrical transducer. Three problems requiring future treatment are identified.