nce you have calculated the cooling requirements of your system and installed the corresponding number and strength of fans, you will discover an unavoidable compromise that must be made: The more airflow the fans generate, the noisier they are. Studies show that the level of noise emitted by a fan is directly proportional to the amount of airflow it creates.
At least this is true for fans of the same basic design. The fan’s construction impacts its noise output greatly, and different makes of fan produce very different amounts of noise.
Measuring noise
Noise emission is measured in dB (decibel). Standard fans used in component systems typically have a noise output of 30 dB or less. Such fans are rated as “quiet.” 30 dB corresponds to a whisper in open air.
“Quiet” does not mean completely inaudible, however. 30 dB fans will produce a low background “whisper” or hum. Whether that is enough to bother you depends on your own sensitivity to this type of noise, as well as what you are using your system for. If the fan is cooling your home theater system, then this low-level noise is most likely drowned out by booming movie sounds. On the other hand, if you are trying to enjoy Classical music at an audiophile level, then the low hum of a fan may ruin the experience.
Reducing noise
A fan’s noise output depends on its operating speed (RPM) and the amount of friction among its moving parts. The speed-dependent portion of the noise is the characteristic hum of fan blades slicing the air thousands of times every minute. This effect is unavoidable, but it can be reduced by using larger fans. Fan blades with a larger diameter move the same quantity of air at a lower speed.
The amount of friction depends on the fan’s construction. Generally speaking, cheaper fans – made of inferior materials and constructed with less precision – tend to produce more noise. On the other end of the price spectrum there are special low-friction fan designs that keep noise emission to a minimum.
Standard ball-bearing fans tend to be noisier than sleeve design fans, but ball bearing fans have a 50% longer lifespan and withstand higher temperatures.
A cost-effective method for keeping noise emission down is to add a thermostat control to the fan. Instead of operating at their full capacity, temperature-controlled fans run only at the speed that is needed at any given moment. On average they run more slowly and produce less noise. The variable speed also helps extend the fan’s lifespan. Generally speaking, the faster a fan runs, the faster it wears out.