Microphone distance and the room

The distance between a microphone and a source changes the recording more than almost any other choice. It decides how much room goes onto the recording, and with it the sense of space, clarity and depth.

Direct sound and room sound

A microphone receives two things:

• Direct sound from the source, which falls by 6 dB each time the distance doubles.
• Diffuse (reverberant) sound from the room, which has roughly the same level everywhere in the room.

At short distances the direct sound dominates and the recording is dry and detailed. Move away and the direct sound falls, while the reverberant level stays. At some distance, the critical distance, the two are equal. Beyond that the room dominates.

The critical distance depends on the room and the source

The critical distance grows with the room volume and shrinks with the reverberation time:

D_c = 0.057 · √(Q · V / RT60)

with V in cubic metres and RT60 in seconds. Q is the directivity of the source. A source that radiates mainly forwards, such as a trumpet or a singer, has a larger critical distance in that direction than an omnidirectional one. See the critical distance tool.

What a directional microphone changes

A directional microphone is less sensitive to the room, so for the same direct-to-room ratio it can be placed farther away. The factor, relative to an omni:

That is why many classical recordings use cardioid microphones and place them nearly twice as far away as the same sound would need with omnis. It also trades off against the character of the room sound: a directional microphone gives drier room sound.

Choosing a distance

1. Decide how much room the music needs. Chamber music in a good hall may want a lot. Close-miked pop wants almost none.
2. Estimate the critical distance of the room.
3. Place the main pair somewhere around or slightly beyond it for a balanced, natural result, and adjust with your ears.
4. Use spot microphones sparingly. Delay them to the main pair, see Spot microphones.

Practical notes

• Height matters as much as distance. The main pair is often placed high, 3 to 4 m above the floor, to see over the front rows and reduce the early floor reflection.
• Moving the pair backwards makes the ensemble cover a smaller angle, so a narrower recording angle is needed to still fill the loudspeaker base. See The recording angle, explained.

Draft: written by an AI assistant, awaiting technical review.