A 5- to 50-kHz synthetic cylindrical nearfield calibration array

Abstract

The direct measurement of farfield acoustic radiation from underwater transducers in a shore-based facility is often impossible because of the limited size of measurement tanks. The Trott nearfield calibration array (NFCA) concept solves this problem by allowing the farfield radiation to be determined from measurements made close to the transducer. We describe here the development of a 5- to 50-kHz synthetic cylindrical NFCA that is suitable for measurement tanks as small as about 2.5 m in each dimension and that can be operated at hydrostatic pressures up to 6.8 MPa. The NFCA is synthesized using a single line of 48 piezoelectric ceramic hydrophone elements spaced 2.54-cm apart center-to-center 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. We then obtain the azimuthal farfield pattern of the transducer by computer processing the angular responses together with a set of complex shading coefficients. We evaluated the line experimentally by calibrating a large piston transducer in a cylindrical measurement tank less than 2.5 m in diameter. The resulting computed farfield patterns are in excellent agreement with corresponding patterns obtained from direct farfield measurements made in a lake.