The rotational noise is one of the main sources of the noise emitted from a fan. This type of noise has discrete frequency components, which is so harsh that many methods have been developed to reduce the noise. Studies on the rotational noise, however, have hardly been made to clarify the generation mechanism and find concrete methods to reduce the noise.

In order to suppress the discrete frequency components of the rotational noise, the spacing of the blades were changed and the comparison was made with a fan having equally spaced blades. The discrete frequency components were found to be suppressed in the unequally spaced fan and the noise tended to have a nature of whitenoise if the blade spacing and blade setting angles were properly chosen.

The instantaneous pressure change on the shroud was measured in order that the pressure change was related to the discrete frequency noise was closely related to the pressure changes on the blade surfaces. By expressing the pressure change due to the blade passing in terms of the blade spacing and decomposing it into Fourier series, the noise characteristics was analyzed. The prediction result for the rotational noise emitted by an equally spaced fan was presented and found to agree well with the experiment.

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