Computation of Farfield Characteristics of Phased Portions of a Cylindrical Transducer from Nearfield Measurements

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Kirchoff's formula, with a simple approximation for the normal pressure gradient, was used to compute directivity patterns of two phased portions of a large cylindrical transducer from near-field-pressure measurements made over a cylindrical surface. In one case, 16 staves of the 48-stave transducer were phased to a plane and shaded to give 20-dB minor lobe suppression in the farfield. Nearfield-pressure amplitude and phase mensurements were made by receiving with the 16-stave array while transmitting with a line hydrophone that was rotated around the transducer. The transducer was mounted with the axis vertical, and the horizontal directivity pattern for the plane passing through the center of the transducer was computed. In the second case, several staves of the transducer were driven with phasing to produce a beam depression of about 30° Nearfield measurements were made by receiving with a probe hydrophone that was rotated around the transducer at several vertical levels. There is good agreement between the computed patterns and the equivalent measured ones in both cases. This is significant because one of the original restrictions on the gradient approximation in use was that the measurement surface closely fit the radiator. The approximation has been shown to be better than it was thought to be because in the present work the surfaces used were cylindrical and the radiators—due to phasing—were portions of a plane and a truncated cone.