Near-field calibration arrays for acoustic wavefield determination

The use of near-field calibration array (NFCA) to determine the far-field properties of acoustic receivers and projectors by measurements made in their near-field is described. When used as a projector to evaluate receivers, the NFCA produces a nearly uniform plane wave over a large volume in its near-field and over a large frequency range. When used as a receiver to evaluate projectors, the NFCA becomes a plane-wave filter for acoustic radiation (or target scattering) originating from within the plane-wave volume. The basis of the NFCA is a reciprocity principle. Numerical implementation of this principle provides the complex weighting factors to be used as shading coefficients for the individual elements in the NFCA. The original Trott NFCAs, given by W.J. Trott (see J. Acoust. Soc. Am., vol.36, p.1557-68, Aug. 1964), were planar. Subsequent extensions of the concept to a cylindrical NFCA suitable for determining the azimuthal far-field pattern and to a spherical NFCA that allows a determination of the entire three-dimensional far-field pattern are described