Near-field acoustical holography (NAH) is conventionally used to visualize sound fields in a three-dimensional space based on sound pressure measurements conducted on a surface close to a noise source. Traditional Fourier-based acoustical holography procedures require the use of a large microphone array setup to avoid spatial truncation effects: that means it is necessary that the array be large enough so that the sound pressure level measured at the edges of the array is significantly lower than that at the peak locations. To avoid the need to use large microphone arrays, a method referred to as statistically optimized near-field acoustical holography (SONAH), was proposed by Steiner and Hald, initially in planar coordinates. In that method, an extension of the measurement surface well beyond the actual source surface is not necessary since the projected sound field is calculated by using a transfer matrix defined in such a way such the propagating waves and evanescent waves are optimally represented. In the current work, the development of SONAH in cylindrical coordinates as formulated by Cho, Bolton, and Hald is first reviewed and is then applied to the observation of the sound field generated by a bladeless fan. It will be shown that the noise source locations can be clearly identified at significant frequencies by using this procedure, and that a knowledge of the source locations makes it possible to suggest effective noise control solutions targeted at specific frequencies.
SONAH, Nearfield acoustical holography, Cylindrical, Fan noise, Source identificationur
Acoustics and Noise Control
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