Abstract
The planar nearfield calibration array (NFCA) has previously been used to determine the farfield radiation of underwater transducers from nearfield measurements. We consider here a cylindrical NFCA constructed from a single line of suitably shaded reciprocal hydrophones. The transducer to be calibrated is rotated within a volume V near the line about an axis parallel to the line. The response, beth amplitude and phase, of the line is measured at N angular positions equally spaced over the 360°. In this way a virtual cylindrical array of N lines is created. The azimuthal farfield pattern of the transducer is then obtained by computer processing the N responses together with a set of complex shading coefficients. The coefficients are designed so that the entire virtual array, if driven as a source, would produce a plane wave over the volume V. Experimental results were obtained for a large piston transducer. The computed farfield patterns are in good agreement with directly obtained farfield patterns.
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