Acoustic modeling of fan noise generation and scattering in a modular duct system

Fan noise is an important noise source in computers. The noise spectrum of fans contains tonal noise, found at the so-called Blade Passing Frequency (BPF) and its higher harmonics, that plays an important role in the perceived sound quality. An acoustic resonator integrated in the duct of an in-duct axial fan causes an impedance change in the duct and, depending on the dimensions and location, the resonator acts as an acoustic mirror reflecting the noise back to the fan. By using a resonator on both the inlet and outlet side of the fan the emitted noise can be contained between the resonators thus reducing the noise radiated to the surroundings. In previous publications by the authors, a model was outlined describing viscothermal wave propagation in the duct and in different resonator geometries. A model of the complete resonator setup can be constructed by coupling the solutions for wave propagation in the different elements of the setup. Such a modular model can be used to determine the resonator dimensions and position to optimally reflect noise near the BPF. An important factor in this model is the element that describes noise generation and scattering of incident sound waves by the fan. The description of such an element was not yet available and is presented in this paper. Furthermore, an experimental setup is presented that was built to obtain the different model parameters and to experimentally validate the theory.