The 3:1 rule prevents phase problems when using multiple mics on separate sources

When two microphones each capture a different source (e.g., voice and object) and their signals are mixed, phase discrepancies arise if the mics are too close together relative to their distance from their own source. The 3:1 rule states that the distance between the two microphones should be at least three times the distance each mic is from its own source. This separation ensures that a sound from source A reaching mic B has decayed sufficiently that the resulting phase relationship with mic A’s signal causes only minimal comb filtering. Violating the rule produces frequency-specific cancellations that weaken the sound or create unnatural coloration, especially noticeable in mono.

Examples

Mic 1 is 30 cm from a speaker’s mouth. Mic 2 (on the object being described) must be at least 90 cm from Mic 1. For stereo field recording with two mics aimed at different sounds, apply the same ratio to avoid inter-mic phase issues.

Assessment

A recordist places one mic 20 cm from a narrator and another 40 cm away on a prop. Is the 3:1 rule satisfied? What artifacts might appear in mono playback if it is not?