Holographic visualization of multi-component sources by using reference measurements only

When conventional scanning, nearfield acoustical holography (NAH) is applied to a stationary source, a large number of measurements must usually be made both to cover the entire source area and to avoid truncation of the sound field. 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 both the reference cross-spectral matrix and the transfer functions between the reference microphones and the field microphones. Here it is noted 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 fields radiated by any combination of different relative level (but fixed-directionality) sub-sources can be visualized from measurements made with only as many reference microphones as there are incoherent sources. The implementation of the procedure is illustrated here both by simulation and experimentally by using a composite source consisting of two loudspeakers, each fed by an independent white noise source. The procedure has been verified by comparing directly measured partial fields and those generated based upon the reference information. By using this procedure, it is possible to avoid repetitive measurements when visualizing the sound fields radiated by different combinations of sub-sources having different relative levels, so long as the directionality of each sub-source is fixed.