Measurement of the backscatter from two pressure-release spheres using a near-field measurement system

Abstract

The backscattering of sound by two pressure-release spheres was studied using a near-field measurement system (NFMS) based upon the near-field calibration array (NFCA) concept developed by Trott and Van Buren. The NFMS consists of a planar near-field transmit array (NFTA) consisting of 34 real-shaded line elements comprised of 26 hydrophones and a synthetic cylindrical aperture near-field receive array (SA-NFRA) consisting of a single line element comprised of 26 hydrophones. The line elements of the NFTA were shaded to obtain a volume of plane wave uniformity of 1 m3 in front of the NFTA. The synthetic aperture was formed by moving the single line element with a precision positioning system. The total acoustic pressure field was measured by the SA-NFRA. Since the incident pressure and the scattered overlapped in the time domain, background subtraction was used to obtain the scattered pressure. Far-field scattered beam patterns for the SA-NFRA were formed using the NFCA concept to extrapolate near-field measurements to the far field. Three configurations of pressure-release spheres were studied: a single sphere, two spheres arranged perpendicularly to incident field, and two spheres arranged parallel to the incident field. Experimental and numerical model results will be presented as well as NFTA uniformity measurements.