Array designs optimized for both low-frequency NAH and high-frequency Beamforming

Near-field Acoustical Holography (NAH) can perform source location with high spatial resolution even at low frequencies by measuring very close to the sound source and by reconstructing part of the evanescent near-field. But a measurement grid with less than half wavelength spacing is required, and the measurement area should cover the main radiating regions to avoid windowing effects. These requirements make the method impractical at higher frequencies, typically above 3-5 kHz. At those higher frequencies, however, Beamforming can provide good resolution with typically 40-90 measurement points, because it is possible to use irregular arrays at intermediate measurement distances. Various array designs exist that can provide good suppression of ghost images up to frequencies, where the average element spacing is much larger than half a wavelength. The problem to be addressed here is that it is not practical to have to use two different arrays to perform the two types of measurement. The desired solution is to take two measurements with the same array: One for NAH at a short measurement distance and one for Beamforming at a somewhat longer distance. The paper deals with the problems of how to design an array that has good performance for both of these two measurements and how to perform NAH processing on irregular array measurements. The combined measurement technique using only one array seems to offer a very efficient way of performing broadband noise source location.