The causes of noise from cooling fans

The core sources of noise from cooling fans are mainly divided into three categories: aerodynamic noise, mechanical noise, and electromagnetic noise. Among them, aerodynamic noise and mechanical noise account for more than 90%. 

1、 Pneumatic noise (primary source)

Generated by the interaction between fan blades and air, when the fan rotates at high speed, this interaction triggers a series of airflow changes, resulting in noise.
Vortex noise: During the process of cutting air, the blade surface and edges will form airflow vortices. These vortices are not stable, and when they break, they produce high-frequency noise, which is usually manifested as sharp sounds. For example, when the computer host is running at high speed, if the blade design of the cooling fan is not reasonable, it is easy to produce obvious eddy current noise.
Turbulent noise: When the airflow becomes turbulent at the inlet and outlet, or when the airflow hits the casing, heat sink, and other components of the fan, it will produce mid to low frequency noise, which sounds like a whistling sound. For example, in some poorly ventilated computer cases, the airflow at the fan inlet is obstructed, which can easily cause turbulent noise.
Wind cutting noise: The faster the speed of the fan blades, the higher the frequency at which they cut the air, resulting in sharper and more piercing noise. For example, when the fan suddenly increases from low speed to high speed, the wind cutting noise will significantly increase.

2、 Mechanical noise

Friction and vibration originating from the internal structure of the fan can cause additional noise during operation.
Bearing noise: Bearings are one of the core components of fans, and the noise generated by different types of bearings varies significantly.
Oil bearing: During initial use, oil bearing can maintain relative quietness. However, as the usage time increases, the lubricating oil will gradually evaporate, and the bearings will wear out, resulting in a "rustling sound".
Ball bearings: Ball bearings operate through friction between steel balls and raceways, and their noise is slightly higher than that of oil containing bearings. But it has a longer lifespan and stable noise, and there will be no later deterioration.
Magnetic levitation bearings: Magnetic levitation bearings use advanced non-contact friction technology to suspend the rotor through magnetic field force, thereby eliminating mechanical friction and resulting in the lowest noise, making them a high-end noise reduction solution.
Vibration noise: Unbalanced motor rotor, eccentric fan blades, or poorly secured fan during installation can all cause the fan to vibrate as a whole. When this vibration resonates with the device casing, low-frequency buzzing will occur. For example, if the fan is not tightened with screws during installation, it is prone to vibration noise.

3、 Electromagnetic noise (with the smallest proportion)

Caused by electromagnetic force fluctuations inside the motor, it usually manifests as a low-frequency "buzzing sound". This type of noise is common in low-quality DC fans because the motor winding and drive circuit of the low-quality fan are designed.