Noise Control for Two Axial-flow Cooling Fans in Series

B1 - Signal processing for noise source location/characterization

Fan noise is a serious issue in electronic cooling applications. When pressure drop is high, two axial-flow fans are often used and the noise created by the two fans is complex. This experimental study analyzes the noise signature of two identical small axial-flow cooling fans in series (120 mm in diameter) in free field and the source distinction guides the efforts to control the overall noise radiation.
To study the discrete frequency noise, different rotational speeds are used for the two fans to distinguish the noise sources. The technique of time-base stretching synchronous averaging is used. For the broadband noise, the method of subtraction is used to estimate the contribution of each fan. In this method, the noises made by one and two fans are measured and the difference is considered to be the noise made by the downstream fan.
Acoustic directivity measurement and noise source analysis are conducted for two configurations. In the first, the inlet flow to the upstream fan is a free-field. The effect of a flow straightener is studied as well when it is placed between the two fans. In the second, the inlet flow is distorted by typical obstacles, a simplified flat plate covering one half of the inlet flow passage being studied.
With the knowledge derived from these diagnostic studies, one possible noise control measure is taken and evaluated. The same flow straightener is mounted at the inlet of the upstream fan in an attempt to reduce the negative effect of the inlet flow distortion caused by the flat plate. Averaged 2.5 dB SPL reduction is achieved for the six positions studied and the variations of contributions from each noise source component of each fan are analyzed.