Okay, one more post and I'll call it quits as I'm sure most of this is way deeper than most here care to go. But happy to discuss if anyone wants to do so.
My previous post covers things you can do if you can't space a pair of omnis as far as you'd like, and relates to either straight 2-omni recording or using an additional microphone or pair in the center between omnis. But what about a situation where you have the omnis spaced exactly as far apart as you want them for recording using the omni pair alone, but then want to add a center microphone to that?
As mentioned previously, in most cases we want to greatly increase the omni spacing when introducing a third mic in the center- often doubling the spacing in general simplified terms. There are several reasons for that- reducing combfiltering from three mics in close proximity, improving the imaging across three mics instead of two, etc. But there is another way of getting the spacing we need without moving the omnis. We can space the center mic sufficiently far away from the omni pair. In other words, we can move the center mic away from the omnis instead of increasing the spacing between the Left and Right omnis, in order to get the omnis far enough away from the center mic.
One way to do that is to mount the center mic directly between the omnis as before, but put it much higher or much lower than the omnis. Imagine a tall 15' mic stand with the forward facing center mic at the very top and the spaced omnis on a bar clamped to the same stand something like 7' or 8' up. Together, the three microphone locations form an upward pointing triangle. That arrangement keeps the timing relationship more or less unchanged for direct sounds arriving from the horizontal plane. Sound from directly in front reaches all three mics with about the same timing relationships as if all three were in a single line. But the diffuse reverberant sound arriving from all directions has a significant time of arrival difference between the center mic and omnis because the arrival time difference serves to decorrelate the signal between the center and omnis, reducing comb-filtering issues and providing some of that wide-spaced omni openness and ambient sheen. I recently suggested this as one option which avoids having to do any post filtering in
Edtyre's thread- Suggestions for a center channel omni to run into my MixPre3. Basically it's the same idea applied there, even though we were mostly talking about adding a single omni to a standard near-spaced directional mic configuration rather than a center directional mic to a spaced omni pair. Alternately one can simply mount all three mics relatively close together and low pass the omni channel so that it only provides low bass below where the cardioids begin to roll off, extending the low bass response without comb filtering and imaging problems since it will not interact with the near-spaced cardioids at higher frequencies.
Another way to physically space the center mic away from the omnis is to leave all the mics in the horizontal plane, but move the center microphone far enough forward of the omni pair to sufficiently decorrelate it from them. To keep the imaging correct we need to introduce a compensating delay the center mic so that it's signal will be time-aligned with the omnis for sounds arriving from in front (approximately 1.1ms for each foot of distance). Sounds from other directions will arrive with different timing relationships between the three mics. Sound from in back will arrive with a delay equal to that of twice the forward spacing of the center mic (corresponding to it's distance in front + the delay compensating for that same distance for sound arriving from the opposite direction). The obvious problem with this is the setup- you probably need a separate mic stand for the forward center mic.
That can actually be done in such a way that the SRA imaging between each omni and the center mic is seamless and can be made to match that of the two omnis alone, eliminating the inherent imaging conflicts which otherwise would occur between three mics all pointed in the same direction. Michael Williams extends the Stereo Zoom (2channel) to Mulitchannel Microphone Array Design (3 to 7 channels) to special multi-channel arrays which can be used to record both simultaneously which he dubs "Magic Arrays". His AES papers are available at his website-
http://www.mmad.info/. Below is a photo of an 8-channel
Magic Array from Jerry Bruck's Posthorn site. Obviously that's a specialty multi-channel recording array, but if you were to eliminate all but the 3 forward-most microphones, notice that the center mic pair are more or less in a NOS or DIN configuration with the center microphone moved forward by a significant distance.
Edit to insert this note of clarification on the photo below- These may appear to be wide-Left/Right-spaced omnis plus six other mics but are not. A 3-mic main output like what we are discussing as produced from this array would consist of just the forward most extended mic and the two angled mics closest to it. In the photo those microphones are the three closest to the top of the frame (center mic top-most + the Left/Right pair immediately beneath it). Please ignore the other five microphones for the sake of this discussion. The diagrams below are more clear on this- the center microphone being well forward of the left/right pair (closer to the top of the diagram). With respect to the diagrams, simply ignore the two rear facing (bottom most) microphones.Below are William's setup examples for a few different 5-channel Magic Array using omnis, cardioids, and supercarioids. Each array can be used to provide mono (center forward facing mic only), 2-channel stereo (90degrees/24.5cm forward facing pair only), quad-stereo (all four central mics in an IRT-cross-like configuration), or 5-channel audio at the same time. Although not intended this way I also find that of the MMAD arrays I've used, they all seem to fold down to 2-channel stereo really well. I extracted the setup diagram images below from this paper on his site-
Magic Arrays - Multichannel Microphone Array Design applied to Multi-format Compatibility As you look at these examples, simply ignore the two rear-facing microphones and you have a 3-microphone array. The first example below is then a spaced omni pair arranged to achieve a 90 degree SRA, with a completely optional center omni added to it which in imaging terms at least isn't going to mess up the main 2-channel omni pair whether you choose to use much of the center mic or not. Substitute a cardioid for the center omni and you have very taper optimized spaced omni pair with a directional center mic. Notice that in the examples using cardioids and supercardioids, the left/right pairs are pretty much in basic microphone configurations everyone here is familiar with- close to NOS and ORTF for the cardioids, and 110 degree X/Y for the supercards.
This sort of brings my thread-jack full circle, with reasoning for why one might want to put a center mic far forward of a pair of omnis- It allows you to use standard stereo setups for the left/right pair. Another way of thinking about a far forward center mic is as a sort of as a spot mic, or like matrixing a SBD feed with your AUD pair. In booth of those cases you often need to delay the spot or SBD to align it with the AUD source, and after doing that everything meshes nicely without comb filtering conflicts because the spot or SBD
are placed far enough away from the main AUD pair.