Scan-based near-field acoustical holography on partially correlated fields: Theory and numerical experiments.

Abstract

Conventional near-field acoustical holography (NAH) requires a coherent field. For a coherent source, a scan-based approach can be used with one reference microphone to stitch the phase. For a noncoherent field, scan-based NAH can be performed if the virtual coherence technique is used. This technique uses multiple reference microphones to decompose the partial field into mutually uncorrelated partial fields, which are then processed by some NAH method. In particular, the statistically optimized near-field acoustical holography (SONAH) method is used with modified Tikhonov regularization. Numerical experiments are performed on a series of point sources with source strengths chosen to mimic the source characteristics of high-powered jets. The experiments are designed to aid in determining the proper number and location of reference microphones for doing NAH work on high-powered jet noise. [Work supported by Blue Ridge Research and Consulting and the Air Force Research Laboratory.]

Coherence length as a figure of merit in multireference near-field acoustical holography

“…2, one reference microphone per coherence length (RPL C ¼ 1) is considered sufficient to fully decompose the subsources. 13,18 Hence, RPL C , as defined in Sec. 2, is a useful figure of merit when source coherence properties or the number of independent subsources is ambiguous.…”

“…13 To obtain a spatial coherence measurement, a linear sensor array (collinear with the eventual location of the reference array) is placed near and along the length of an extended source. Coherence values, 14 c 2 z m ;z , between a given sensor at a location z m and all sensors along the array are calculated.…”