Microphone 3:1 Distance Calculator
When two or more microphones pick up the same sound source, combining their signals creates phase cancellation at frequencies where the path length difference equals half a wavelength. The 3:1 distance rule provides a practical guideline to minimise this comb filtering: the spacing between microphones must be at least three times the distance from each microphone to its intended source. Enter the source-to-mic distance and the calculator shows the minimum required spacing between microphones.
3:1 rule formula
Minimum mic spacing = 3 × source-to-mic distance
The minimum distance between any two microphones must be at least 3x the distance from either microphone to its sound source. For example, a microphone placed 0.3 m from a guitar requires any other open microphone to be at least 0.9 m away.
Practical placement examples
- Close mic at 5 cm: second mic must be at least 15 cm away
- Instrument mic at 30 cm: second mic at least 90 cm away
- Overhead drum mic at 60 cm: second overhead at least 1.8 m away
- Interview mic at 50 cm from speaker: second person mic at least 1.5 m away
Frequently asked questions
What is the 3:1 distance rule?
The 3:1 distance rule states that the distance between two microphones must be at least three times the distance from either microphone to the sound source. This minimizes comb filter phase cancellation when the two microphone signals are combined in mono.
Why 3:1 and not some other ratio?
At a 3:1 ratio, the level difference between the direct path and the indirect (bleed) path is approximately 10 dB. This 10 dB level difference reduces the audibility of phase cancellation to generally acceptable levels in most recording and live-sound contexts.
When does the 3:1 rule matter most?
The 3:1 rule is most critical when multiple open microphones are summed to mono (e.g. a live stage mix, broadcast, or studio tracking session). In stereo mixes, the pan positions partially separate the signals, reducing cancellation severity.
Does the rule apply to directional microphones?
Yes, but directional (cardioid, supercardioid) microphones have inherent rejection of off-axis sources, which helps reduce bleed. The 3:1 rule is a minimum guideline; greater distances always improve isolation.
What if I cannot achieve 3:1 spacing?
If physical constraints prevent 3:1 spacing, use directional microphones oriented away from each other, apply high-pass filters to remove low-frequency phase buildup, or use a polarity switch on one channel and check the mono sum. Phase rotation plugins can also help.
Official sources
- AES: Audio Engineering Society.
- OpenStax University Physics Vol. 1, Chapter 17: Sound.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.