Noise has no effect on a fan's cooling performance, but it makes a big difference to the people working nearby. Audible noise originates from several sources, some of which can be controlled by the enclosure designer. Others are a result of a fan manufacturer's design. Read below to find out more about the causes of fan noise and what you can do to minimize it.

Causes of Fan Noise

Noise emanating from axial fans is a function of many variables and causes:

• Whirl pooling: This is a broadband noise source generated by air separation from the blade surface and trailing edge. It can be partially controlled by good blade profile design, proper pitch angle and notched or serrated trailing blade edges.


• Turbulence: Also a broadband noise, generated within the airflow stream itself. Inlet and outlet disturbances, sharp edges and bends in the airflow will cause increased turbulence and noise.


• Speed: Speed of rotation is a major contributor to fan noise.


• Fan load: Noise varies with the system load; fans are generally quieter when operated near their peak efficiency.


• Structure vibration: This can be caused by the components and mechanism within the fan, such as residual unbalance, bearings, rotor to stator eccentricity, and by motor mounting. Cooling fans are basically motors and should be treated as such when mounted.


• System disturbance: System disturbances are the biggest cause of fan noise. When a fan is designed for low noise operation, it can be very sensitive to inlet and outlet disturbances caused by card guides, brackets, capacitors, transformers, cables, finger guards, filter assemblies, walls or panels, etc. Trial and error, combined with common sense and intuition, are often the best tools for determining fan selection and component placement for low noise operation.

How to Minimize Fan Noise

The following design actions will help produce a system with minimal fan noise:

• Reduce system impedance at the inlet and outlet ports. If a large part of the fan's flow potential is used up by the impedance of the inlet and outlet, a larger, faster, noisier fan will be required.


• Avoid obstructions to the airflow, especially in the critical inlet and outlet areas. When turbulent air enters the fan, noise is increased by as much as 10 dB, usually in a discrete tone form that is particularly annoying.


• Use a larger, slower fan rather than a faster, smaller version whenever possible. Often, this solution will produce less noise for the same airflow.


• Relax the temperature rise limits where possible, to reduce the airflow required. This will allow you to use a smaller, slower fan that produces less noise.


• Isolate the fan to avoid vibration transmission. Because fans operate at a low frequency and are light in weight, vibration isolators must be soft and flexible. Since the transmission is dependent on the system, trial and error is the best approach to a quiet system/fan interaction. In systems that require 20 CFM or less, cabinet vibration is the predominant source of noise, and isolation of the fan is the only practical solution.

Cooling fan noise is expressed in decibels (dBA). The dBA rating is determined directly by a sound level meter 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 source.