Abstract
A multi-channel active control system has been applied to the reduction of free-field tonal noise from a small axial cooling fan. The experimental apparatus consists of an aluminum enclosure which houses the fan, an infrared detector-emitter pair which serves as a reference sensor, loudspeakers, microphones, and appropriate filters and amplifiers. The research fan and loudspeakers have been modeled theoretically as point sources to derive a mathematical expression for radiated power. The minimization of this power has served to guide the number and location of control sources, as well as to discover potential error microphone locations in the extreme near-field. Experiments with these microphone positions have shown that there are predictable near-field locations which consistently lead to significant reductions in the global mean-squared pressure for the first four harmonics of the blade passage frequency. For example, a four channel configuration with the error microphones placed appropriately results in global mean-squared pressure reductions on the order of 10 dB or more for the targeted frequencies.
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