Cooling Fan Noise - Why It's Necessary

Cooling fan noise has no effect on cooling, but it is very important to the system end user.

Most designs require minimal fan noise to satisfy users’ demands for quiet systems. Compounding this challenge is the desire for smaller enclosures and higher system performance, both of which increase the need for airflow which, in turn, increases noise. It’s a necessary byproduct of a system that’s so crucial, we wouldn’t have the advancements in technology we have today if we couldn’t deal with a little fan noise.

Audible noise originates from several sources. Aerodynamic noise is a result of turbulence caused by the fan blades. The frequency and magnitude of the noise generated by a cooling fan increases with rotational speed. Mechanical noise can be caused by bearings, or unbalanced rotating elements that can cause vibration. If this vibration is at a frequency that matches any resonant frequencies of the enclosure it can be amplified to an intolerable or even destructive level. The motor can contribute to the noise, but this is a very small part of the total noise generated by the cooling system.

All these noise components are inherent in the design of the fan and are almost entirely out of the enclosure designer’s control. Efforts to significantly reduce noise levels take away from the effectiveness of a fan and could render a fan useless.

However, there are some details to which the enclosure designer should pay attention to minimize noise:

Check for obstacles in front and behind the fan. If they exist, move the fan farther away.
In general, obstacles on the intake side of the fan negatively impact the noise level more than on the exhaust side.
Lower the RPM of the fan operating speed (this action also lowers airflow)
Mount the fan on an interior surface on the enclosure rather than on an exterior surface.

If you’re curious about how fan manufacturers use various methods of measuring cooling fan noise, discover how NMB specifically measures fan acoustic noise.