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Relationships between spaced omnis and center mic

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I've read several old threads on here about spaced omnis, with some discussions as well about running a mic (or pair) in the center.  The threads have been very interesting, though sadly I'm unable to keep up with the more technical discussions.

My question concerns the relationship between the omnis and the center mic.  I gather that they should all be on the same horizontal plane (right?), and that it's generally okay for the center mic (in my case a cardioid) to be further forward than the omnis.  There must be a point where the center mic is too far forward, though, right?  How far is too far, though, and does it depend on the distance the omnis are spread? 

Yes to most of your questions.  You're on the right track in your thinking.  The answer ranges between all mics in a single horizontal line, to the center mic being moved well forward of the omnis (with compensating delay applied to it).  Usually having the center just slightly but not overly far out in front (without any compensating delay) works well for both practical and acoustical reasons, which I'll try to outline in more detail later.

The good news is there is a lot of leeway without screwing things up.  Much more than there is with simple 2-channel stereo configurations.

After posting a bunch of stuff explaining why later in the thread, here's a short synopses of my practical take away- 

For section recording using only a pair of omnis alone, I want an spacing of about 3'.  Less isn't as good in terms of imaging, ambiance, bass, and openness.  More spacing might work but risks a hole-in-the middle. 

If using a center mic I want an omnis spacing of 3' or more.  Less isn't as good in terms of imaging, ambiance, bass, and openness, and risks imaging and comb-filtering complications more than just a pair of omnis alone.  It could be advantageous for other reasons though.  More spacing between omnis can be beneficial and is less problematic because the center mic fills the hole in the middle, but I still wouldn't go crazy wide or the center is becomes separate and "spotlighted".

If the omnis can't be spaced far enough apart, it can help to move the center mic well far forward (like 6' or so) and delay it to compensate for that distance, but that's not often practical.  Alternatively one could move the center mic it straight up or straight down, without requiring any delay, but that might not be very practical either.  Either approach gets the three mics far enough apart to minimize problems of having them too close to each other.  You don't want the three mics too close together.

By using a coincident center pair in the center instead of a single mic, I optimize things and get the best of envelopment and imaging at the same time.  Like a single center mic, I ideally want an omni spacing of 3' or more for the same reasons.  Yet because I can then adjust the image width of the center coincident pair, even wider spacings between omnis no longer present a problem.  I can go double-wide or even more with the omnis without potential problems, giving me a wide diffuse ambience which stays out of the way of the sharp directional imaging stuff in the center provided by the coincident pair.  I also gain control over image blend between the center and sides of the playback image by varying the width from the coincident stereo pair afterwards.


--- Quote from: Gutbucket on June 19, 2017, 07:17:20 PM ---The answer ranges between all mics in a single horizontal line, to the center mic being moved well forward of the omnis (with compensating delay applied to it)
--- End quote ---

How do I figure out how much compensating delay to apply to the center mic? 

in our experience the center mic can be 3-6 inches forward of the omnis with no delay needed. We haven't done a config where the centers are more than that so I can't answer the how far is far enough to require delay. Lee can answer that I'm sure.
edited to add two photos for you-
First one is set up at home with three mics, you can see the center is about 3-4 inches ahead of the sides. (it is also a 1-2 inches out of axis in the horizontal plane, which we've tried to have the correct adapters to eliminate that distance- that said, I do not think it has affected the sound quality)
Second is our set up at DeadnCo 2016. (pink dead rats not our rig!)

That's a cool setup rocksuitcase.  What are the outer mics?  Looks like maybe AKG bodies, but capsules that are at a right angle?  Pardon my ignorance.

I'd be curious to hear a sample of one of your recordings...are there any on LMA?


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