Abstract
In acoustic measurement tanks it is often impossible to directly determine the farfield radiation properties of underwater transducers because of limited space. The Trott nearfield calibration array (NFCA) concept solves the problem by allowing the farfield properties to be obtained by measurements made close to the transducer. We describe here the development of a 5–50 kHz synthetic cylindrical NFCA that is suitable for medium size measurement tanks such as the pressure tank at the Underwater Sound Reference Detachment of the Naval Research Laboratory. The NFCA is synthesized using a single line of 48 piezoelectric ceramic hydrophone elements spaced 2.54 cm apart and amplitude shaded in a special way. The transducer to be calibrated is driven electrically and rotated near the line about an axis parallel to the line. The response of the line is measured at equally spaced angular positions, thereby creating a virtual cylindrical array surrounding the transducer. The azimuthal farfield pattern of the transducer is obtained by computer processing the responses together with a set of complex shading coefficients. We evaluated the line experimentally by calibrating a large piston transducer. The resulting computed farfield patterus are in excellent agreement with corresponding patterns obtained from direct farfield measurements made in a lake. [Work supported by NAVSEA 63R.]
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