Proximity effect (audio)

The proximity effect in audio is an increase in bass or low frequency response when a sound source is close to a microphone.

Proximity effect is a change in the frequency response of a directional pattern microphone that results in an emphasis on lower frequencies. It is caused by the use of ports to create directional polar pickup patterns, so omni-directional microphones do not exhibit the effect (it should be noted that this is not necessarily true of the "omni" pattern on multipattern condenser mics, which create the "omni" pattern by summing two back-to-back cardioid capsules, which may or may not share a common backplate.) Depending on the microphone design, proximity effect may result in a boost of up to 16 dB or more at lower frequencies, depending on the size of the microphone's diaphragm and the distance of the source. A ready (and common) example of proximity effect can be observed with cardioid dynamic vocal microphones (though it is not limited to this class of microphone) when the vocalist is very close to or even touching the mic with their lips. The effect is heard as a 'fattening up' of the voice. Many radio broadcast microphones are large diameter cardioid pickup pattern microphones, and radio announcers are often observed to employ proximity effect, adding a sense of gravitas and depth to the voice. Proximity effect is sometimes referred to as "bass tip-up."

To explain how the proximity effect arises in directional microphones, it is first necessary to briefly describe how a directional microphone works. 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 impinge on the rear side of the diaphragm. Since the sound reaching the rear of the diaphragm travels slightly farther than the sound at the front, it is 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 a directional microphone its directivity.

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