Optimizing Circular Side-Resonators to Reduce Computer Fan Noise

One of the main noise sources in computers are the cooling fans. Tonal noise at the rotational frequency of the fan, the blade passing frequency (BPF) and its higher harmonics are important in fan noise. So called 'side-resonators' integrated in the duct of an in-duct axial fan cause an impedance change in the duct and, having proper dimensions and correct position, act as an acoustic mirror reflecting the noise back to the fan. As a result, noise emitted from the computer can be reduced. In this paper, a model describing viscothermal wave propagation in the duct and side-resonator is outlined. The model can be used to determine the resonator dimension and position to optimally reflect noise near the BPF. The developed model is compared with the models for prismatic tube and cylindrical resonators and the physical differences between these three resonators are explained. Preliminary results of a parameter study are presented. The results point out that the range and magnitude of the effective frequency band of a resonator setup can be successfully manipulated by changing the different dimensions of the setup.