Coincident mic arrays sum to mono cleanly because the capsules share one point in space

Coincident mic techniques place two capsules at the same point in space, eliminating arrival-time differences between the microphones. Because there are no inter-mic time delays, the two channels sum without phase cancellation, giving excellent mono compatibility. X-Y is the simplest coincident array: two directional mics (usually cardioids) crossed at 90 degrees with the capsules stacked precisely above one another, producing a solid, mono-compatible image. The stereo image is created entirely by the mics’ directional pickup patterns, since there are no time-based or filtering cues. This is the defining trade-off of coincident recording: rock-solid mono behaviour, but a narrower, less spacious image than techniques that add arrival-time cues.

Examples

X-Y: two cardioid mics crossed at 90 degrees, capsules stacked vertically at one point. M-S is also a coincident array (cardioid + figure-8 as close together as possible). Sum either to mono in a DAW and note the near-total absence of comb filtering.

Assessment

Explain why X-Y recordings sum to mono cleanly while a widely spaced pair does not. What single physical property defines a coincident array?