The proximity effect in audio is an increase in low frequency response as the sound source moves closer to the mic. You can think of the proximity effect as a “distortion” caused by the use of ports to create directional polar pickup patterns, so omni-directional mics are not affected. To me, the proximity effect is more like distortion-free EQ. Depending on the mic design, proximity effect may easily result in a boost of up to 16dB, usually focused at about 100Hz or below.
Singers tend to like the proximity effect, as it fattens up their voice. When I recorded Michael Jackson singing the lead vocal on “Earth Song,” I had him sing as close as possible to my Neumann M-49—about two inches or less, from the mic’s front grille. The resultant proximity effect is quite dramatic.
Incidentally, in case you’ve wondered how a mic acquires directivity, a microphone is constructed with a diaphragm whose mechanical movement is converted to electrical signals (via a magnetic coil, for example). The movement of the diaphragm is a function of the air pressure difference across the diaphragm arising from incident sound waves. In a directional microphone, sound reflected from surfaces behind the diaphragm is permitted to be incident on the rear side of the diaphragm.
As the sound reaching the rear of the diaphragm travels slightly farther than the sound at the front, it’s slightly out of phase. The greater this phase difference, the greater the pressure difference and the greater the diaphragm movement.
As the sound source moves off the diaphragm axis, this phase difference decreases due to decreasing path length difference. This is what gives directional microphones their directivity.
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