Shaping Sound Frequency Spectra of Axial Flow Fans

An analytical description of the sound pressure radiated aerodynamically by axial flow engine cooling fans is developed using semiempirical methods. Mathematical manipulation and digital computer techniques are employed to yield complete time and frequency sound pressure spectrums and overall sound pressure levels (unweighted and A-weighted).

The mathematical tools are used to investigate the sound pressure characteristics of fans constructed with nonuniform blade spacing configurations. The theoretical analysis, which was verified experimentally, shows that the distribution of sound pressure in the frequency spectrum can be accurately predicted as a function of fan rotational frequency and blade spacing configuration. Fans with blade spacing configurations possessing a high degree of nonuniformity are shown to produce sound frequency spectrums which contain many discrete frequency components with similar magnitudes but with no outstanding discrete frequency components.

A method is proposed for selecting a fan with an optimum blade spacing configuration such that the discrete frequency sound pressure components generated by the fan do not strongly reinforce discrete frequency sound pressure components inherent in the immediate environment external to the fan.

Air delivery characteristics of axial flow fans are not analyzed in this study.