Noise Level

• The most audible component of any computer is its fans. CPU fans are rated in decibels based on the noise they produce at maximum speed. Most CPU fans are rated between 20 and 40 decibels. There are a number of factors that influence the amount of noise a CPU fan makes, including speed, design and size. Smaller fans designed to operate at high speeds are usually louder than larger fans that run at lower speeds. The number of fan blades on the fan also impacts how loud it is -- generally more fan blades generate more noise.
  
  

Air Flow

• One of the most important characteristics of any CPU fan is how much air it can move per minute. Fans that move larger quantities of air are more effective at pulling heat away from the heat sink and the CPU. The air flow of a fan is measured in cubic feet per minute or CFM. Larger CPU fans tend to have greater maximum CFM ratings at slower speeds.
  
  

Size and Compatibility

• The size of a CPU fan is based on the diameter of its fan blades measured in millimeters. CPU fans are available in sizes ranging from 40 to 140mm. The size of a CPU fan and the type of processor it is compatible with are the two primary features determining compatibility for any CPU fan. Each CPU fan comes equipped with mounting hardware that is only compatible with the fan mounts for motherboards that support specific processor sockets. Many CPU fans are compatible with more than one type of socket by design or include an adapter kit.
  
  

Design

• There are two basic types of CPU fan that have specific advantages. Most CPU fans are based on the ball bearing design. This design is heat resistant and works well in a variety of applications but is generally louder than the alternative. CPU fans using sleeve bearings are marginally quieter than their bearing-based counterparts but are more prone to sudden mechanical failures and are vulnerable to heat stress.