Reducing fan noise has been a major challenge for PC cooling fan design engineers these days, as consumers are becoming more critical about having quieter home and office space. To meet the challenge, fan engineers need to find the root cause of the noise, a complicated task that requires experience from aerodynamics, structural dynamics, electronics, and magnetics. In this paper we focus on two types of fan noise: prominent tone and modulation noise. Two examples were used to show the process of finding the root cause and the solutions. Vibration and acoustic measurement data were examined to find the clues to fix the problem. Frequency domain techniques were used to structure a clear picture of the fan dynamics. We found that structure resonances, when interacted with other resonances, cause undesirable modulation noise. We then manipulated the resonance modes to solve the noise problem.
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