Reduction of discrete frequency noise in small, subsonic axial-flow fans

The discrete frequency noise radiated from representative tyes of axial-flow fans used in electronic equipment is studied in detail. Narow-band analysis of the discrete frequency noise radiated by these types of fans has been conducted in an anechoic environment. The farfield sound pressure level and radiated directivity of the discrete frequency noise are presented. The influence of operating condition on the radiated sound is determined. The discrete frequency noise dominates the characteristic acoustic spectra at high flow coefficients. The radiated directivity of the discrete frequency noise is generally uniform. Based on the data collected and on the theoretical aspects presented, the principal source of the discrete frequency noise is identified for each of the fans as the interaction of the fan rotor with a circumferentially distorted, nonuniform flow. A collection of prototype design modifications is developed for each representative fan to reduce the radiated discrete frequency noise by improving the uniformity of the local flow field. A modified prototype configuration is developed for each representative fan which is evaluated acoustically and compared to the original baseline data.