The proximity effect refers to a change in the frequency response of a microphone that produces an emphasis on lower frequencies, causing the source -- such as a singer's voice, instrument or amp -- to sound deeper and fuller. Over the years, performers and singers have taken advantage of the effect in various ways, using it as a part of their tool bag instead of treating it as an unwanted issue. Not all microphones are affected though: The proximity effect is a characteristic of directional microphones, such as those with cardioid polar patterns, as opposed to omni-directional mics.
Proximity effect can either be a blessing or a curse, depending on how it is used. A singer can get a deep, earthy sound by singing very close, then change to a more penetrating sound by singing louder while moving the microphone away. This kind of creative use takes some practice but can be very effective. On the other hand, singing at the same volume (with no special effects) and moving the microphone in and out will create problems with tonal balance, not to mention changes in overall mic level. Some singers also like to work very close at all times to fatten up an ordinarily light voice.
Proximity effect can be used effectively to cut feedback in a sound reinforcement situation. If the performer works very close to the mic, and doesn't need the extra bass, an equalizer can be used to turn down the channel's bass response. This makes the microphone less sensitive to feedback at low frequencies, since it is now less sensitive to any low-frequency signal arriving from more than a foot away. Better yet, this equalization technique will also help reduce the effect of any handling noise.
Check out the video below for closer look at the proximity effect:
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Have a few tips of your own on using the proximity effect?