Two-microphone nearfield acoustical holography

Abstract

In conventional Nearfield acoustical holography (NAH), a large number of measurements is required to cover the entire source area. In addition, when the complete source comprises several incoherent sub-sources, the sound field must be decomposed into coherent partial fields before projection. The latter operation is made possible by using a fixed array of reference microphones and computing the reference cross-spectral matrix and the transfer functions between the reference microphones and the field microphones. Here it is shown that by combining the transfer functions from the reference to the field microphones obtained from one measurement with reference cross spectra measured in another, the sound field radiated any combination of different level (fixed-directionality) subsources can be visualized from measurements made with only enough reference microphones to represent the incoherent sources. The implementation of the procedure will be illustrated using an array of two loudspeakers, each fed by an independent white noise source. Comparisons are made of directly measured partial fields and those generated using only the reference information. By using this procedure, it is possible to avoid repetitive measurements when visualizing the sound fields radiated by different combinations of subsources having different levels, so long as the directionality of each subsource is fixed.