Proximity effect is an increase in low frequency response when the microphone is close to the sound source, and often effects directional microphones more than omnidirectional. The low frequency response of a microphone is increased dramatically when the microphone is closer than two feet away from the sound source, and can boost the response by 16dB or more.
How Microphone Proximity Effects Gain
The low frequency “gain” occurs because the overall sound pressure level increases as the microphone gets closer to the sound source, but the highs are more likely to by canceled out, resulting in a perceived boost in lower frequencies. This has the potential to cause preamp overload, creating distortion. When used correctly, proximity effect can, however, be used to the advantage of the user to augment their dynamic range.
If the user is inexperienced with proximity effect, they tend to “eat” the microphone causing the distortion mentioned earlier. This can be corrected with simple high pass filter.
(In the Image, you can see that the closer the microphone gets to the sound source. there is more gain in low frequencies.)
-By John Redick