Concerning the Noise
The major sources of acoustic noise come from the flow generated by impeller through housing, bearing system and electro-magnetic switching. These are explained in detailed below:
A. Flow Generated by Impeller Through Housing
Generally, the noise you hear from your fan is the sound from air flow. When a fan is running, the impeller is doing work, moving a mass of air from intake side to exhaust side. There are relative motions between air and blades, air and housing, air and the ribs that support the motor. These relative motions are usually not laminar flow (streamlined). That is, turbulences (or wind shear) are generated and vortices of different scales are formed. These vortices are shedding from the leading edges or the trailing edges of blades or ribs with dissimilar frequencies and energies. This is why you may feel differently when hearing the operation of fans of different design. You may correlate tone with frequency, and loudness with energy. If you have a fan with 7 blades when rotated at 4200 RPM, you may imagine a major frequency of noise at around 7 × 4200 ÷ 60(sec)= 490 Hz. Other frequencies of noise depend on the design of the fan.
B. Bearing System
Bearing system is the mechanism to hold the rotor (or say, impeller) to create an axis of rotation. The noise comes from the sliding motion between shaft and sleeve type of bearing, or the rolling (driven by the shaft) motions between ball and bearing races of ball bearing. Normally, you should not be able to distinguish the bearing noise from the noise of air flow (wind shear) by your ears. However, if the bearing system is not good enough, you may hear the bearing noise clearly when the fan is operated at low speed. It should be noted that, among the sources of noise, bearing noise is the only source that may change with time of operation. It is because that bearing will wear out gradually and therefore change its original characteristics. Therefore, the quality of bearing is very important.
C. Electro-Magnetic Switching
It is sometimes referred to as “buzzing”. The interaction between the magnet and motor core due to pole switching, and the internal switching of the induction IC are the two sources of the buzzing sound. It is not so susceptible as compared to the noise due to air flow. However, it does create certain level of annoying noise if the electro-magnetic design is not well taken care of
Another source of noise comes from the application, not the inherent noise of the fan. Remember that fan will not be used alone. It will be installed on a system. T he layout of the system may cause flow disturbances, and thus noise. The noise can be very sensitive to disturbances caused by card guides, brackets, capacitors, transformers, cables, finger guards, filter assemblies, walls or panels, inlet and outlet guards, etc. It is something that needs your experiment, combined with your sense and intuition, for determining fan selection and component placement for low noise operation.
Your job to do is to find a fan that results in the lowest noise level. Sometimes a larger fan with lower speed may be a good alternative for reducing the noise. If you have a problem finding a fan for low noise operation, probably you need to review the design (layout) of the system to avoid obstructions and compactness for smoothing the flow
Cooling fan noise is expressed in decibels (dBA). The dBA rating is determined directly by a sound level meter (microphone) in an anechoic chamber, equipped with a filtering system which de-emphasizes both the low and high frequency portions of the audible spectrum. This measurement is recorded at a distance of 1 meter from the intake side of the fan, which is running without resistance.
The below figure illustrates the setup: