Acoustic Physics: The Inverse Square Law
In free air, acoustic sound waves expand spherically from a point source. Because the surface area of a sphere grows with the square of the radius ((4pi r^2)), acoustic energy is diluted over a rapidly expanding wavefront according to the Inverse Square Law:
Every time you double the distance between a sound source and a microphone, the sound pressure level drops by exactly 6.02 dB. Conversely, cutting the distance in half boosts the acoustic level by 6 dB.
The 3:1 Distance Rule for Multi-Microphone Bleed
When tracking multiple instruments or vocalists simultaneously in the same room (e.g. acoustic guitar and vocals, or a live rhythm section), sound from Instrument A inevitably leaks into Microphone B. Mixing both signals causes destructive comb filtering (a hollow, phasy sound caused by time delay phase cancellations).
The 3:1 Rule states: If Mic 1 is placed 1 unit of distance from Source 1, any adjacent Mic 2 must be positioned at least 3 units of distance away from Source 1. This ensures that leakage is attenuated by at least 9.5 dB, reducing comb filter notches to an imperceptible level.
Directional Proximity Effect
Directional microphones (Cardioid, Supercardioid, Ribbon) are pressure-gradient transducers whose diaphragms react to the pressure difference between front and rear ports. Within a few inches of a source (the acoustic near-field), spherical wavefront curvature creates a massive low-frequency boost (often +6 to +12 dB below 100 Hz). Omnidirectional microphones are pure pressure transducers and exhibit zero proximity effect.