Controlling the Tonal Characteristics of the Aerodynamic Noise Generated by Fan Rotors

Abstract

A technique has been developed for selecting circumferentially-unequal blade spacings that will reduce the tonal annoyance of the aerodynamic noise generated by the rotors of fans. The procedure has been successfully applied to a number of axial-flow fans. The predictions of harmonic spectra are based on an experimentally-determined sound-pressure wave shape for the individual blades of such fans. However, the technique can be extended to other types of tonal-annoyance problems provided that the particular relevant sound-pressure wave shape is determined. Unequal blade spacings that will produce a particular degree of “whiteness” in the noise with a minimum departure from equal spacing are tabulated for a range of blade numbers. Both unbalanced and naturally balanced blading arrangements are considered. A survey technique is also outlined for those applications in which it is important to reduce a particular frequency, or where blending with a given background noise must be achieved.