{'fraction_non_alphanumeric': 0.016435541859270673, 'fraction_numerical': 0.0, 'mean_word_length': 5.493333333333333, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Acoustical Visualization of a Refrigeration Compressor by Using Statistically Optimized Nearfield Acoustical Holography in Cylindrical Coordinates

This paper describes how nearfield acoustical holography (NAH) is a useful tool for visualizing noise sources and their associated sound fields because it allows sound fields that are measured on a surface to be projected throughout a three-dimensional space. However, in order to avoid spatial Fourier transform-related truncation effects, the measurement aperture (i.e., the hologram surface) must typically extend well beyond the source to a region where the sound pressure level drops to a level significantly lower than the peak level within the measurement aperture. If it is not possible to make measurements in the region where the sound pressure drops sufficiently, because of physical obstructions, for example, accurate back-projections cannot be performed by using NAH. Statistically optimized nearfield acoustic holography (SONAH) is based on a formulation similar to that of NAH. However, in SONAH, it is assumed that the pressure field on the projection surface can be represented by a linear combination of wave functions that represent the pressure on the hologram surface. That is, surface-to-surface projection of the sound field is performed by using wave functions without performing a spatial Fourier transform. Thus the requirement that the measurement surface extend beyond the source region is eliminated without compromising the accuracy of the procedure. The SONAH process may therefore be viewed as an alternative to the sound field extrapolation procedure. In this paper SONAH was reformulated in cylindrical coordinates and applied to the visualization of the sound field radiated by a refrigeration compressor. A comparison with results obtained by using conventional NAH in cylindrical coordinates suggests that the new procedure works well in realistic applications.