Some applications demand a high airflow while others require minimal noise. To meet these various requirements, fans are available in different speeds. Increases in the fan speed are positively correlated with higher airflow and noise. Note that this positive correlation applies to all brands of fans.


High Speed [HS]


High speed fans have a higher airflow but are noisier. By designing the impeller to spin faster, the fan delivers higher levels of air relative to other fans with the same case size. Consequently, increasing the fans speed generates more friction in the bearings which creates more noise. Fans categorized as HS or high speed are designed for applications that demand a high airflow; and the increase in noise level is acceptable.

Low Speed [LS]


Low speed fans are quieter but deliver less airflow. By designing the impeller to spin slower, the fan becomes relatively quiet to other fans with the same case size. Consequently, decreasing the fans speed also causes less air to be moved by the slower spinning blades. Fans categorized as LS or low speed are designed for applications that require minimal noise; and the lower airflow is acceptable.

             The noise a fan produces can come from various sources such as bearing friction or the vortex flow. The loudest of these noises is stated on the fans specification sheet in decibel units. A higher “A-weighted sound level” or dBA number means a noisier fan. However, this rating is just the starting point and should only be used as a reference. There are many components which can cause the fan to become louder over time. The primary factors of a fans noise level is the fans speed, bearing system, mounting direction, and its structural design.



      Sound Level Change      
 Human Perception
 2 to 3 dBA Barely Perceptible
 5 dBA  Readily Noticeable
10 dBA Doubling or Halving of Noise
 20 dBA Dramatic Change


Fan Speed


A faster spinning impeller delivers more air but causes a higher friction to be produced between the bearing and shaft. As a result, there is a positive correlation between airflow and noise. Keeping size and bearing constant, higher airflow fans tend to be noisier. The fans speed is determined by its RPM rating or rotations per minute number. A goal of many engineers is to reduce the fan’s RPM as much as possible without compromising its heat dissipation performance.

Bearing System


The noise a fan produces is also dependent on the bearing system it uses. Initially, the noise level from bearings is overshadowed by the fans rotor blade turbulences. As the fan ages, the bearings get louder and may become the primary source of noise. Sleeve bearing fans start off quieter but become louder at a faster rate due to its lubricant design. On the other hand, dual ball bearings start off slightly louder but will remain more consistent throughout its life. More Information.

Improper Mounting and Oil Loss


The sleeve bearing system consists of a sealed cylinder of lubricant. This oil based lubricant will eventually evaporate as the fan ages even if it wasn’t used. A lack of lubrication will increase the amount of friction between the bearing and shaft. As a result, the fan will become noisier if not refilled with oil. Improper mounting of sleeve fans will also accelerate the oils evaporation. For this reason, sleeve bearing fans are noisier if not mounted or stored horizontally.

Frame and Impeller Design


The design of the fan is largely responsible for how much air it delivers and the level of noise it will produce. However, a loud fan does not necessarily mean an inferior design. Many fans are used in applications that have strict high airflow or high static pressure requirements. Others are used in environments that are already very noisy. In these scenarios, the noise level of the fan can be ignored in order to maximizing heat dissipation.


     Sound Rating   
 Common Sounds
 0 dBA Threshold of Hearing
 10 dBA Breathing
 20 dBA  Rustling Leaves
 30 dBA  Whispering
 40 dBA Quiet Library
 50 dBA Refridgerator
 60 dBA  Air Conditioner