Proximity effect is a physical phenomenon that causes directional microphones (cardioid, supercardioid, hypercardioid, bidirectional) to boost low frequencies as the sound source gets closer. At 2 inches from the mic, bass frequencies can be boosted by 6–16 dB compared to measuring at 3 feet.
Radio broadcasters and voice-over artists deliberately use proximity effect to add warmth and depth to their voice. Positioning the mic 2–4 inches away and speaking directly into it produces the rich, full sound often described as 'broadcast quality' or 'announcer voice'.
For music recording, proximity effect can be a problem or a tool. Recording acoustic guitar too close causes muddy, bass-heavy sound. Recording a bass cabinet close uses the effect to add low-end weight.
Omnidirectional microphones do not exhibit proximity effect — their response remains flat regardless of distance. If you need consistent tonality at varying distances, an omni pattern is more predictable.
Most audio interfaces and many microphones include a high-pass filter (low-cut filter) specifically to counter proximity effect when needed.