For this reason it is best to use the largest fan possible for a given application. RPM stands for revolutions per minute. RPMs are used to measure the speed that a fans blades rotate. The higher the RPMs a fan produces the more airflow it will create. However, higher RPMs also cause more noise.
Decibels are a measurement of sound levels.
The higher the decibels level the louder the sound. A whisper is about 23dB. Most high end fans are between 26 and 40dB's. There are approximately 25mm in 1 inch. Converting degrees Fahrenheit to degrees Celsius or vice versa requires a special formula. 3 pin connectors connect to fan headers directly on the motherboard.