Wideband acoustical holography

Patch Near-field Acoustical Holography (NAH) methods like SONAH and ESM are limited to relatively low frequencies where the average array inter-element spacing is less than half a wavelength, while beamforming provides useful resolution only at medium-to-high frequencies. With adequate array design, both methods can be used with the same array. But for holography to provide good low-frequency resolution, a small measurement distance must be used, while beamforming requires a larger distance to limit sidelobe issues. Wideband Holography was developed to overcome that practical conflict. Only a single measurement is needed at a relatively short distance and a single result is obtained covering the full frequency range. The underlying problem solved is that at high frequencies the microphone spacing is too large to meet the spatial sampling criterion, and thus there is no unique reconstruction of the sound field. A reconstruction must therefore have a built-in “preference” for specific forms of the sound field. Doing just a Least Squares solution will result in reconstructed sound fields with sound pressure equal to the measured pressure at the microphones, but very low elsewhere. By building in a preference for compact sources, a smoother form of the reconstructed sound field is enforced.