3-to-1 Microphone Distance Calculator
When two or more microphones pick up the same source and their signals are mixed, the small time difference between them causes comb filtering, a hollow, phasey colouration. The 3-to-1 rule keeps this inaudible: space the microphones at least three times as far apart as each one is from its source. This calculator takes the source-to-microphone distance and returns the minimum spacing between microphones, the bleed attenuation that spacing provides by the inverse-square law, and a pass or fail check against any spacing you propose. Use it whenever you set up multiple microphones on a stage, drum kit, or ensemble.
3-to-1 rule formula
Minimum spacing = ratio * source-to-mic distance
Bleed attenuation = 20 * log10(ratio) (dB)
Pass if: proposed spacing >= minimum spacing
With the standard ratio of three, the bleed attenuation is 20 times log10(3), about 9.54 decibels, which keeps comb filtering below the audible threshold for typical sources.
Worked example
A vocalist 0.3 metres from the first microphone: minimum spacing = 3 * 0.3 = 0.9 metres, giving 9.54 decibels of bleed attenuation. A proposed 1 metre spacing exceeds 0.9 metres, so it passes the 3-to-1 rule.
3-to-1 rule: frequently asked questions
What is the 3-to-1 microphone rule?
The 3-to-1 rule is a practical guideline for using more than one microphone on a source. It says that the distance between two microphones should be at least three times the distance from each microphone to its sound source. Following it keeps the level of one source in the second microphone low enough that comb-filter colouration when the signals are summed is not audible.
Why does the rule use a factor of three?
Three times the distance reduces the bleed signal by about 9 to 10 decibels by the inverse-square law (20 times log10(3) is about 9.5 decibels). At roughly 9 to 10 decibels down, the comb filtering produced when the two signals combine stays below the level most listeners notice. It is a rule of thumb, not an exact threshold.
What is the formula?
Minimum microphone spacing = 3 times the source-to-microphone distance. If a vocalist is 0.3 metres from the first microphone, a second microphone should be at least 0.9 metres away from the first. The calculator also reports the bleed attenuation the spacing achieves.
Does the rule guarantee no comb filtering?
No. The 3-to-1 rule reduces comb filtering to an inaudible level for typical sources; it does not eliminate it. Closely matched levels, reflective rooms, and stereo techniques can still produce audible interference. Treat the rule as a reliable starting point and verify by listening.
Official sources
- International Organization for Standardization: ISO 9613-1 sound propagation (inverse-square basis).
- U.S. National Institute of Standards and Technology: nist.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.