Hard for my mind to grasp how spaced mics aimed straight ahead (even omnis) less than 4', and when so close to the stage filed with players spread wide, wouldn't still emphasize the players right near the middle. I think that's one reason why I used ORTF/cards last time.
Although more
can be better, 3’ is plenty and is something of an A-B baseline starting point for me for 2-channel stereo. I’ll walk you through the mechanics of why it works-
Usually we want to fully fill the playback space between the speakers with the sound sources on stage, so that the musicians on the far left and right are reproduced from the left and right speakers and the musicians located between them are spread out evenly between the speakers.
Even though there are some signal level differences, A-B stereo is primarily based on the difference in arrival time of the same sound at each microphone. A timing difference of around ~1 millisecond is enough to pan a sound fully to one speaker or the other. In one millisecond, sound travels about 1 foot. With the microphones 1’ apart, we get that full 1ms timing difference only for a sounds arriving directly from one side of the other. Sounds arriving from any other angle will have a path length difference of less than 1 foot and a smaller time of arrival difference, which means all of those sounds will be reproduced somewhere between the two speakers. That means a one foot spacing produces a Stereo Recording Angle of 180 degrees. (approximately)
Stereo Recording Angle is a Stereo Zoom term which describes an imaginary window looking forward from the microphone perspective, framing the region which will be reproduced between the two speakers. All sounds arriving from within that window will be heard as coming from somewhere between the speakers and all sounds from outside that window will be fully panned over to one speaker or the other. So with an SRA of 180degrees, a musician fully to the left side of the microphones is heard from the left speaker and the one fully to the right from the right speaker. Those between them will be heard as coming from somewhere between the two speakers.
A perhaps unintuitive aspect of the Stereo Zoom is that the farther apart the microphones are, the narrower the SRA becomes. As the distance between microphones is increased, the 1ms timing threshold is reached at arrival angles closer to the centerline, outside that angle the timing difference will be greater than 1ms.
If the microphone position is moved farther away from the source, we typically want to compensate with a narrower SRA, so that the band will still fully fill the playback stage between the speakers. That requires a larger spacing between the mics. So usually, the farther away an omni pair is located from the source, the wider apart you want to space them. That’s the
zoom part of the Stereo Zoom.
With the mic stand position you have in that hall, the band is arrayed in almost a full half-circle around the microphones, which is why a 12” spacing that gets something like a super wide 180 degree SRA wouldn’t be unreasonable.
If the micophones are spaced too widely, on playback the sound will either come from the middle or will be clumped at one speaker or the other and not evenly distributed between them. People talk about a ‘hole in the middle’ on playback with over-wide mic spacings, but the sounds actually arriving from the middle (through the narrow SRA) will still sound centered, its just that there is less sound evenly distributed between the middle and the sides. You’ll just get a clump at the left speaker, a clump in the center, and a clump at the right without an even distribution between them.
Understanding that, I still like more spacing than the Stereo Zoom suggests for omnis because there are other things going on with this too.
