The heated air passing over an exhaust fan stresses the fan s bearings much more than the 25 C air flowing over an intake fan. NMB s Fan Warranty Statement and Fan Life Derating Curve. Most designs require minimal fan noise to satisfy users demands for quiet systems. Compounding this challenge is the desire for smaller enclosures and higher system performance, both of which increase the need for airflow which, in turn, increases noise. Audible noise originates from several sources, some of which can be controlled by the enclosure designer, whereas the fan manufacturer designs others in.
Aerodynamic noise is a result of turbulence caused by the fan blades.
The predominant component of this noise is at the blade-passing frequency, the frequency at which blades pass the struts that connect the motor to the fan s outer casing. This relationship might seem to imply that the motor speed, number of blades, and number of struts should be minimized. Unfortunately, the fan requires a certain speed and number of blades to produce the required airflow. Also, the fan needs a certain number of struts to provide rigidity without which mechanical vibration contributes to noise. Mechanical noise can be caused by bearings or unbalanced rotating elements that cause vibration.