An increase in bass or low frequency response when a sound source is close to a microphone. Proximity effect is distortion caused by the use of ports to create directional polar pickup patterns, so omni-directional mics are not affected. Depending on the mic design, proximity effect may easily result in a boost of up to 16 dB, usually focused below 100 Hz. Vocalists tend to like proximity effect since it fattens up their voice, but a constantly varying bass boost can wreak havoc on headroom and carefully set levels! Obviously, if a vocalist is “eating the mic” to get proximity effect, the Inverse Square Law (WFTD 6/12) tells us that the levels the mic sees are increasing dramatically as well – distortion can easily result, from either mic diaphragm breakup or electronic overload. (You may occasionally see proximity effect referred to as “bass tip-up”)