In the present study, a structural modification approach to reducing the sound power radiated by an axial fan mounted to an enclosure is described. An axial fan operating in free-space exhibits a dipole-like radiation pattern at sufficiently low frequencies that the source is compact. When a fan is mounted to an enclosure, however, the sound radiation pattern becomes monopole-like since only one side of the fan is exposed to the exterior space; thus it radiates more efficiently than the same fan operating in free-space. Also, it is possible for the source level to be amplified by coupling with the interior resonances of the enclosure. The radiation enhancement can be suppressed by introducing a second “path” that allows the sound fields on both sides of the fan to interact with each other while also providing damping for the acoustical cavity, thus reducing the radiation efficiency of a fan towards its free-space value. Experimental results are presented here to demonstrate this effect. The sound fields radiated by two fans mounted to a consumer electronics enclosure were visualized by using near-field acoustical holography, and the baseline results were compared with those for various enclosure modifications. First, the top of the enclosure was replaced by an acoustically-transparent mylar sheet. Then two types of acoustical openings, i.e., a grilled port and a perforated panel, were introduced on the top of the original enclosure close to fan locations. A significant reduction of sound power resulted from the enclosure modifications, in particular at the fundamental and twice the blade passing frequencies, and a clearer conversion from a monopole to a dipole-like radiation pattern was observed when the size of the opening was made larger.
fan noise, nearfield acoustical holography, radiation efficiency
Acoustics and Noise Control
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