Theoretical Analysis of Compressor Noise

A theory is presented for the discrete‐frequency sound radiated by axial‐flow fans and compressors. The theory is based on the noise radiation from the fluctuating forces on either a rotor or a stator stage due to interactions with upstream components. The model used is a cascade of point forces, one on each blade, radiating into free air. It appears that, provided that the correct phase relations are retained, this model may be expected to give accurate predictions of farfield noise even for long, hard‐wall duct cases. Both over‐all power and directionality curves for the sound radiation are presented, and methods are given for calculating the fluctuating forces acting from the wake geometry. Once wake geometry is defined, the theory enables one to perform calculations of the noise observed at any point. Preliminary agreement with experiment is demonstrated. Virtually all significant compressor design parameters can be included in the theory, and therefore it appears that the theory could be used in tradeoff studies to minimize noise at the preliminary design stage of an engine.