How to Lower Fan Noise?
2016-04-13 cooltron
A growing number of Cooltron’s customers start to ask a lower quite fan for their computer, gaming machine case, and other high-end applications. As the need for low noise fans has become vital for many applications, we consider the knowledge of fan noise is essential for everyone to know. This month, we’d like to focus on the origin of fan noise and provide our readers the solution of how to lower the fan noise.

Origin of Fan Noise 
The derivation of noise can be from the air moving device (AMD) itself or it may even emerge from system disturbances. System disturbances are the biggest cause of fan noise. When a fan is designed for low noise operation, it is sensitive to inlet and outlet disturbances caused by such items as, brackets, capacitors, transformers, cables, finger guards, filter assemblies, walls or panels, etc.

Other contributions to fan noise include the shedding of the vortex, turbulence, system load, and construction of the AMD. Improper pitch angle can lead to shedding of the Vortex. Vortex shedding is a broadband noise created by air separation from the blade surface and the trailing edge. Speed is another major contributor to fan noise. If the speed of a fan is reduced by 20%, the decibel level will reduce by 5 dB.

The effects of a change in speed can be seen in the following:

dB1 = dB2 + 50log10 (rpm1/ rpm2)

Turbulence occurs in the air stream and is part of the cause of broad band noise. Disturbances to the inlet or outlet, due to real sharp edges and bends, will likely increase turbulence; therefore, increasing noise. As for system load, fans that run at peak efficiency tend to be quieter, but variations in the system load will also cause noise to change. Last but not least, poor construction of the fan will often times lead to structure vibration. For instance, bad design or mounting of the bearings will cause vibration, which in turn causes noise to increase. 

How to Lower Fan Noise?
System Impedance should be reduced to the lowest level possible so that the noise to airflow ratio remains low. The inlet and outlet ports of a cabinet can be between 60% and 80% of the total system impedance. This is too high for low-noise results. The impedance of the inlet and outlet could occupy a large fraction of the airflow potential. As a result, a faster and noisier fan will be required to provide the necessary cooling.

A rise in temperature may also lead to a reduction in noise. Since airflow has an inverse relationship with temperature, a rise in temperature brings a proportionate decrease in airflow. In any instance where even a small amount of the temperature limit can be slackened may aid in the reduction of noise. Even without temperature changes, selecting a fan size will help reduce noise. Large fans are quieter than smaller fans that yield the same airflow. 

One final approach to reduce noise is to isolate the fan from the cabinet. Because fans operate at a low frequency, and are light in weight, vibration isolators must be soft and flexible. This solution is favored in cases where the airflow of the AMD is at less than 20CFM and noise is predominantly from the cabinet.