Adequate, optimal, and
sucky are all relative positions on the non-linear continuum of personal preference.
Some run spaced omni splits of 10'-15' or more, some near-space them 10". I find 3' typically works well when spaced omnis are appropriate. Sometimes 4'-6' is better, but it's hard to say without knowing details so 3' is a safer bet.. usually close to optimal on my continuum. If using three omnis, I'd go with 3' between each, total spread of 6'.
A slightly more technical explanation:
The closer the mics, the wider the recording angle picking up direct arriving sounds and reproducing them as phantom sources between the speakers. If the mics are too close the recording angle is far wider than the apparent angle of the sound sources from your recording position, so the resulting recording sounds narrow and 'overly mono'. Too close and there is also insufficient decorelation between channels for the ambient, off-axis sound reaching the mics such as the reverb, crowd reaction and reflections. Decorelation of ambient sound in spaced omni recordings is often described by people as sounding 'big' and 'open'. Basically, if there is not enough spacing the sound picked up by each mic is too similar for good stereo effect and the results are lackluster and 'sucky'.
Too much spacing and there is not enough similarity between channels and more of the direct sounds appear bunched up around one speaker or the other on playback with little in the middle. So one needs to find a good balance. Generally PA reinforced music recordings made from the section are more forgiving of extra spacing between mics than non-PA supported recordings, partially because much of the FOH sound is mono and designed to project far to the back of the venue. When setup on the center-line, if the distance from left FOH stack to left mic is the same as right FOH stack to right mic, mono information on the PA will sound centered on playback, regardless of what would otherwise be extreme spacing between mics for other situations.
You might consider spacing the mics apart more the farther (Further
) your recording position is from the stage. That way, your auditiory recording angle is narrowed to match the narrower visual angle to the sources.
Besides, Phil bombs rock spaced omnis.