Microphone setup and configurations-
Omnis-
In a basic sense, the ‘stereo-ness’ of true omnis depends only on differences in arrival time between microphones which creates phase/timing difference between channels. (Directional mics can do that too or not, depending on the mic setup). To do that, the microphones need enough space between them to produce significant timing differences, and that significant distance varies by frequency. When the mics are close together, the differences are minimal and the recording will essentially be mono for most of the frequency range, exhibiting some ‘stereo-ness’ only at rather high frequencies. The closer the microphones are together, the higher that mostly-mono frequency. As a rough guide, something like 12” or 15” is what I’d consider a minimal amount of spacing for an unbaffled omni pair. I usually want 3 or 4 feet between mics, sometimes more, like when I’m using other mics as well.
To get around that spacing requirement, we can place a baffle between the omnis, which produces level differences as the frequency rises, essentially making them increasingly directional at high frequencies. The recording will still be mostly mono at low frequencies due to the close proximity of the mics to each other, but the level differences as the frequency goes up creates sharper imaging than spaced omnis without a baffle. So one reason baffled omnis is a preferred way of recording ambiences is that it leverages the good attributes of omni mics while also getting some improved directional imaging that is more like angled cardioids.
With a baffle, the spacing between the omni mics can then be less (say just a couple inches up to 16” or so, I like more spacing if possible but that requires a bigger baffle to work), but you still get the benefits of omni mics, and a 360 degree pickup pattern that does a good job of recording the ambience of the entire space.
Cardioids-
Can be A-B spaced, but are usually near-spaced or coincident (meaning they are stacked to be as close as possible, pointed left/right, which is also called X,Y). Coincident x/y patterns have no timing information between channels, and the ‘stereo-ness’ of the recordings are entirely level difference based. That can make them easier to mix with other sources, more mono-compatible, and sharp imaging, but less “big, open and enveloping” sounding. The mono compatibility and level-difference-only stereo can benefit recording a close single source like someone talking, singing or playing guitar. And that’s primarily what X/Y mics on small recorders are designed for doing rather than recording ambiences.
Near-spaced microphone techniques depend on a small amount of spacing between the mics as well as the angle between mics and that introduces some timing differences above the frequency determined by distance of the spacing. Those techniques are often preferred around here as sounding more natural than coincident x/y setups, but still retain much of the imaging and focus-on-a-subject aspects of cardioid mics. Plus the setups are smaller than A-B spaced omnis or omnis with a baffle, so they are easier to use when recording music.
