Noise source analysis and control for two axial-flow cooling fans in series

Abstract

The noise signature of two identical small axial-flow cooling fans in series was analyzed and a technique of synchronous averaging with time-base stretching was used to decompose the raw sound signals into various noise source components. Acoustic directivity measurements were conducted for the case of distorted inlet flow caused by a flat plate covering half of the circular inlet flow passage. The effects of the flat plate and the axial distance between the flat plate and the fan inlet were studied as well. It is found that the inlet flow distortion increases the total sound pressure level (SPL), random and rotary noise of the upstream fan a lot but it barely affects the rotary noise of the downstream fan. A flow straightener consisting of many small hexagons is mounted at the inlet of the upstream fan to reduce the adverse effect of the inlet flow distortion. The total SPL is reduced by 1.2 dB on average.