Reduction of fan noise emission by enclosure modification.

In the present study, a structural modification approach to the reduction of 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 low frequencies when the source is acoustically compact. When a fan is mounted to an enclosure, however, the sound radiation pattern becomes monopole-like since the interaction between the sound fields on the two sides of the fan is prevented by the enclosure: thus the fan can radiate sound more efficiently than when operating in free-space. In addition, it is possible for the source level to be amplified by coupling with interior resonances of the enclosure. The radiation enhancement caused by both effects can be suppressed by introducing an acoustical path that allows the sound fields on both sides of the fan to interact with each other while also changing the resonance frequencies of, and providing damping, for the acoustical cavity, thus reducing the radiation efficiency of the fan towards its free-space value. The effect of the latter type of modification is demonstrated here by using a consumer electronics device equipped with two fans. The sound fields corresponding to various enclosure modifications were visualized by using near-field acoustical holography, and the modifications' effects are quantified in terms of the radiated sound power.