Proximity effect and pressure-gradient microphones
Move a directional microphone close to a voice and the bass swells. This is the proximity effect, and it comes straight from the physics of how directional microphones work.
Why it happens
A directional capsule responds in part to the pressure gradient: the difference in sound pressure between its front and its back. Far from the source, a sound wave is nearly flat, and the difference between two points a centimetre apart is determined by the wave's frequency. At low frequencies, where the wavelength is long, that difference is small, so the capsule's design has to compensate with extra sensitivity at low frequencies to give a flat response.
Close to the source, the wave is strongly curved and the pressure falls quickly with distance. The difference between front and back is much larger than the flat-wave case, especially at low frequencies. The compensation is then too much, and the bass is boosted.
An omnidirectional microphone responds to pressure only and shows no proximity effect.
How large is it?
For the gradient part of the response, the boost relative to a distant source is approximately
boost = √(1 + 1 / (k · r)²), with k = 2π f / c
where r is the distance and c the speed of sound. For a pure figure-8:
| Distance | 50 Hz | 100 Hz | 200 Hz | 500 Hz | 1 kHz |
|---|---|---|---|---|---|
| 5 cm | +27 dB | +21 dB | +15 dB | +8 dB | +3 dB |
| 10 cm | +21 dB | +15 dB | +9 dB | +3 dB | +1 dB |
| 20 cm | +15 dB | +9 dB | +5 dB | +1 dB | 0 dB |
| 50 cm | +8 dB | +3 dB | +1 dB | 0 dB | 0 dB |
A cardioid is part omni and part figure-8, so the effect is smaller, and real microphones have a built-in bass roll-off that offsets part of it.
Using it
- For broadcasters and singers, working close adds warmth and fullness. It is the reason why many voices sound big on stage microphones.
- For recording, it can be a problem: mud in the low frequencies, uneven bass when the performer moves.
Taming it
- Increase the distance. Doubling it takes roughly 6 dB off at low frequencies.
- Use a high-pass filter set below the vocal range, for example 80 to 100 Hz, or the microphone's own low-cut switch.
- Use an omnidirectional microphone, or one with a pattern near omni.
- Choose a microphone designed to compensate, with more bass roll-off built in.
Why it matters for stereo
Pairs of figure-8 microphones (Blumlein, mid/side) show proximity effect more strongly than cardioids, so placing them close to a source colours the sound. Another good reason to use them at a distance.
Draft: written by an AI assistant, awaiting technical review.
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