The calculation model of the farfield tonal noise of a subsonic fan was studied in this paper in order to find the main sources of tonal noise. A full model and a simplified model, which neglects the acceleration noise, were brought out. Experiment was done to verify the full model and good fitness were found between the calculated result and the measured result. Numerical experiments were then done to compare among those two models and the Gutin formula, which calculates the acceleration noise of steady force. Results reveal that at small Mach number the simplified model, which takes only the fluctuating force into account, approximates exactly with the full model. Because the noise calculated by Gutin formula is very small, it is concluded that the tonal noise of a subsonic fan with the Mach number less than 0.2 is caused mainly by the fluctuating force. It gives a guideline to cut fan noise down.

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