Mic Placement Calculator - The 3-to-1 Rule for Multi-Mic Setups

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The 3-to-1 rule, explained

When two microphones record in the same room, each one hears both sources, not just its own. The unwanted sound is called bleed, and when bleed mixes with the direct sound at a slightly different time, the two copies interfere: some frequencies add up and others cancel, producing the thin, hollow sound engineers call comb filtering. The 3-to-1 rule is the classic defence. It says the distance between the microphones should be at least three times the distance from each microphone to its own source. The calculator above checks your spacing against this rule, estimates the bleed level, and works out the time delay between the microphones.

Why three times? Sound level falls with distance by the inverse-square law: doubling the distance drops the level by about 6 decibels. If the second microphone sits three times further from a source than the first microphone sits from its own source, the bleed arrives roughly 9 to 10 decibels quieter than the direct sound, which is usually quiet enough that the interference stays subtle rather than destructive. The rule is a guideline, not physics: it describes a spacing where bleed stops dominating, and rooms, sources and microphone patterns all shift the exact point. But as a starting position, it has survived decades of studio practice because it works.

How to use the calculator

Enter the distance from a microphone to its own source, for example 0.3 metres for a close vocal or guitar.
Enter the distance between the two microphones.
Enter the distance from the second source to the first microphone, for the bleed estimate.
Read the verdict: pass means your spacing meets the rule, fail means the microphones need to move further apart.

A worked example

Suppose two singers stand 0.6 metres apart, each with a microphone 0.3 metres from their mouth. The rule demands microphone spacing of at least 0.9 metres, but the singers’ spacing only allows 0.6 metres, so the setup fails the rule. The calculator flags this honestly. The fixes are practical, not theoretical: move the microphones closer to each mouth, say 0.2 metres, which drops the required spacing to 0.6 metres and passes the rule, or angle the microphones so each one’s rejection null points at the other singer, or accept the bleed and mix the two voices as a single blend rather than trying to process them separately. Knowing which fix applies is the whole value of the calculation.

The bleed estimate uses the same inverse-square idea. If the second singer is 1.2 metres from the first microphone while the microphones are 0.6 metres apart, the calculator compares those distances and reports the bleed level in decibels. It also reports the time delay across the microphone spacing at the speed of sound, 343 metres per second: sound takes about 1.75 milliseconds to travel 0.6 metres, and that delay is what sets the comb filter’s notch frequencies. None of this replaces listening, but it tells you where to listen: if the rule fails badly, expect phase problems and check the mix in mono, where comb filtering is most obvious.

Beyond the rule

Some setups deliberately break the 3-to-1 rule and sound wonderful. Spaced-pair stereo recording of a piano or a drum kit uses microphones whose bleed is the point: the blend of direct and room sound is the stereo image. Coincident pairs, where two capsules sit almost on top of each other, break the rule completely yet avoid comb filtering, because the time delay between them is near zero and the level differences alone create the image. The rule applies to the case it was written for, separate sources with separate microphones, and recognising when you are outside that case is part of using the tool well.

Common mistakes

Measuring from the wrong point: distances are from the capsule, not the microphone body or the stand.
Fixing phase problems with equalisation instead of placement: moving a microphone an inch often beats an hour of EQ.
Checking phase only in stereo: collapse the mix to mono regularly, because mono exposes comb filtering that stereo width can hide.
Assuming the rule guarantees isolation: it reduces bleed to a manageable level, it never removes it, and noisy sources still need separation, gobos, or separate takes.

Who uses this calculation

Live engineers use it when placing microphones on a crowded stage, podcast producers use it when two hosts share a table, and choir recordists use it when spacing microphones across a section. The rule takes ten seconds to check and can save a recording that no amount of mixing can rescue afterwards, because phase damage baked into a take stays in the take. Check before you press record, not after.

Mic Placement Calculator – The 3-to-1 Rule for Multi-Mic Setups