Application of theoretical modeling to multichannel active control of cooling fan noise

Multichannel active control has been applied to the global reduction of tonal noise from a cooling fan. In order to achieve consistent far-field attenuation of multiple harmonics of the blade passage frequency (BPF) of the fan, an analytical model has been applied to the control system in order to determine appropriate transducer configurations. The results of the modeling show that the additional global reduction possible by locating acoustically compact secondary sources coplanar with a compact primary source rapidly lessens as the number of symmetrically placed sources is increased beyond three. Furthermore, the model suggests that there are locations in the extreme near field of the sources that can be considered ideal for the minimization of far-field radiated power. Experiments carried out show that a four-channel control system is more effective than a two-channel system at achieving far-field attenuations, especially at the higher harmonics of the BPF for the fan tested. In addition, greater far-field mean-square pressure attenuations are achieved with the error microphones located along the calculated ideal regions than for nonideal placement.