Dr Huge Longjohns wrote:Sam, what do you mean by 'coincident' in this context?
Sam has answered, but just to colour things in a little more, if microphones are spaced apart -- even by just a few inches -- they will receive sound at different times because sound travels at a fixed speed through the air (roughly 1foot per ms, or a metre every 3.4ms)
If those spaced microphones are hearing the same sources, they will have similar signals, but the waveforms will be phase-shifted relative to each other because of the time delays involved.
If you then try and mix those signals together -- either because you want to mix the voice over a guitar, say, or because you're combining stereo into mono -- the phase-shifts will result in a comb-filter effect where some frequencies (and their harmonics) will add together and boost the output, whereas others will tend to cancel and reduce it. This typically creates a rather nasal-sounding colouration -- a bit like the sound of your voice if you cup your hands in front of your mouth.
The extent of the colouration -- how unpleasant or obvious it is (or not) depends on the relative levels of the mics in the mix, the amount of similar audio the mics are picking up (spill), the audio frequency range of the source material, and the spacing (and hence time delay/phase shift) between the mics.
However, this problem can be avoided completely by ensuring the mic capsules are as close together as physically possible, so that sounds from all the directions of interest arrive more or less simultaneously at all the capsules. This is called a coincident mic technique because the capsules are coincident in space. The physical construction of the capsules limits how close they can actually be, but for stereo arrays it's possible to get them aligned perfectly for all sounds in the horizontal plane (which is usually the one that matters). The SoundField mics are essentially coincident for sounds from all directions at all frequencies up to 12kHz or more...
All of the Blumlein XY and Mid-Side stereo mic arrays rely on this kind of configuration, as does the SoundField mic and its derivatives.
Hope that helps.