Microphone 3 to 1 rule calculator
Two microphones on the same source at different distances comb-filter each other. The three-to-one rule keeps the bleed far enough down that it stops mattering.
Hz
Minimum spacing under the 3:1 rule
0.900m
- Your spacing meets it
- 1
- Level difference of the bleed
- 9.54 dB
- Arrival time difference
- 1.749 ms
- First comb filter null
- 285.8 Hz
- Phase difference at your frequency
- 269.7 degrees
- Wavelength at that frequency
- 0.3430
Show the working
- Microphone to source distance = 0.3
- Spacing between microphones = 0.9
- Frequency of interest = 1000
- Speed of sound = 343
- Minimum spacing under the 3:1 rule = 0.3 * 3 = 0.9
- Your spacing meets it = 0.9 >= 0.9 ? 1 : 0 = 1
- path_difference = max(0.9 - 0.3, 0) = 0.6
- Arrival time difference = 343 > 0 ? 0.6 / 343 * 1000 : 0 = 1.749 27
- Level difference of the bleed = 0.3 > 0 ? 20 * log10(0.9/0.3) : 0 = 9.542 43
- First comb filter null = 1.749 27 > 0 ? 500 / 1.749 27 : 0 = 285.833
- Phase difference at your frequency = mod(360 * 1000 * 1.749 27/1000, 360) = 269.738
- Wavelength at that frequency = 1000 > 0 ? 343/1000 : 0 = 0.343
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- the 3:1 rule: microphone spacing at least three times the source distance
- that gives about 9.5 dB of level difference, enough that comb filtering falls below 1 dB
The rule assumes free field and omnidirectional pickup. Directional microphones aimed away from each other do better than it suggests.