Proximity effect occurs when a sound source is close to a bidirectional or cardioid microphone causing an increase in bass. Because proximity effect is caused by the use of ports, Omni microphones are not affected.
With a cardioid or bidirectional microphone, the diaphragm moves when the pressure in front differs from the pressure behind it. The pressure on the back of the microphone changes slightly later than the pressure on the front due to the longer length the sound waves must travel. It is also due to the fact that the back of the diaphragm is further from the source. Omni microphones are unaffected by the proximity effect because the diaphragm is enclosed.
Vocalists often use the proximity effect to obtain a warmer tone in their voice. However, when using an instrument such as an acoustic guitar, it can change the sound of the instrument completely. The added bass can also make it harder to understand words. The recommended distance is at least 6 inches between the microphone and the sound source to reduce proximity effects.
Depending upon how the microphone is designed, the proximity effect can cause a boost of up to 16 decibels. The closer the source is to the microphone, the greater the increase.