Another consideration for noise generation is how many fans are in operation. Obviously, the more fans one uses, the more sound one will generate. However, the amount of sound pressure in dB does not increase with the addition of fans as one might think. This slide shows the equation to use for calculating sound pressure for multiple fans. Using an example of two fans, this results in an increase of only 3 dB. Therefore, an increase of only 3 dB results in a doubling of the perceptible sound. This fact is critical when selecting fans, since only a 3 dB variation has a large effect on the perceptible sound.