Improving compactness for active noise control of a small axial cooling fan

An active noise control (ANC) system was previously developed by Gee and Sommerfeldt for the reduction of tonal noise radiated by small axial cooling fans, such as those found in desktop computers. That system had a 125×125-mm² footprint, composed of four small loudspeakers surrounding an 80×80-mm² axial cooling fan in a mock computer casing. A smaller system is described in this paper that has a footprint of 80×80 mm², which is the space allotted for the 80×80-mm² standard sized fan. Compared with the previous system, the current system employs a smaller fan running at a higher speed and smaller control speakers. It is demonstrated that the higher output noise levels and higher frequency tones produced by the smaller fan can be reduced by the current ANC system, such that the global control achieved by the smaller system is comparable or better than that achieved by the previous system for the targeted frequencies. It is also shown that control at the second and third harmonics of the blade passage frequency approach theoretical limits.