Advantages of stereo matched pair?

[bradeasley]

With regards to recording acoustic instruments primarily in a traditional bluegrass context, what are the advantages to using a stereo matched pair of mics? In particular, I'm asking about a stereo pair of the same mic as opposed to a pair of different microphones.

Example (using Neumann models for reference):

How would a pair of KM84's sound as opposed to a KM84 and a U47/67/87 if all other variables were the same (mic position, etc.)? Why might the matched pair be preferable in this context?

[MontanaMatt]

I think it’s important when creating a stereo image, that one side not have an intrinsically different tonal quality….you don’t want asymmetry in sound, you would perhaps then perceive the audio image to not be centered or balanced…

[Mandolin Deep Cuts]

@Matt I’m no expert and am just getting into recording. I recently had a similar question as to the OP, and I found this video fun and instructive. I have no idea if it’s accurate but I do like the concept of using different mics to capture different sonic qualities of the same instrument. One mic is good at capturing this type of sound, and another is great at another thing. I had never heard your argument. I like it too. Haha

[BradKlein]

Perhaps an obvious point, but if you're mic-ing a single instrument - stereo will generally give you more of a sense of the room.

I hate it when instrument demos... from music stores... use stereo microphone set ups. When what you want to hear in that case is an accurate representation of the instrument.

[Bob Buckingham]

All mics are not created equal. I have two mics that are the same model, bought at the same time, but one is not a sensitive as the other. We use them for live sound and you can adjust to get a consistent sound but they aren't always exactly the same. These mics are large diaphragm AKG.

[keith.rogers]

The primary advantage, in my experience, is that when using the pair to capture (or induce) a wide stereo image, a “matched” pair avoids any kind of wandering of the placement of any instrument being captured.

I’m not sure exactly what kind of recording is being done “in a bluegrass context,” or what the setup, space and instrumentation is, so all of those those things would be a consideration.

[Tavy]

I've done it both ways (matched pair and very different mics) and have liked the effect of both approaches.

IMO different mics will necessarily need to be an "AB" image: in this case you're not constructing a stereo image as such, rather capturing two different aspects of the instrument and then blending the two to get the an effect you like. In this case I would typically mic different parts of the instrument with the 2 mics: say one on a soundhole, and one on the soundboard. You can still pan these left and right for a nice effect, but it's not a true stereo image. BTW AB micing absolutely will lead to phasing issues - even if you've checked the phase of the bass frequencies (and you do check the phase right?), there will always be some frequency which will be out of phase from time to time. This is not necessarily an issue though, and I do rather like AB micing.

The advantage of matched pairs is that you can place these next to each other in a true stereo pair (either XY or Mid-Side), get a true stereo image (possibly after suitable post-processing), and avoid the phasing issues of AB mic placements.

So... it all sort of depends what you want to do.

A couple of caveats:

* If the instrument is going to be panned to one side in the final mix, then starting with a stereo image is a waste of bandwidth as the stereo information will all get lost in the mix.
* A producer friend of mine, who has a pretty amazing collection of mics, will only ever record mono. But then he has a very good acoustic space and good mics to capture the whole sound of the instrument in the room.

I hate it when instrument demos... from music stores... use stereo microphone set ups. When what you want to hear in that case is an accurate representation of the instrument.

I hear you, and when I do demos they're mono just because it's easier and faster. But I doubt there is any such thing as "the true sound of the instrument". Mic placement has such a big effect when the mics are close, and the room has a big effect when they're not! I have worried over this one, but come to no really good conclusion as to what represents "best practice" here. Probably the best each store can do is be consistent, and not record in a room chock full of 300 instruments all humming in sympathy away in the background!!

[foldedpath]

I think it mainly depends on what type of stereo mic technique you're using, because there are different setups. If you're using a "coincident" technique like X/Y or a Blumlein pair, that's where having matched capsules is an advantage in having a stable, balanced stereo image.

It's one of the advantages of purpose-made stereo mics with two capsules built in and either locked to X/Y or more flexible for things like Blumelien or M-S technique: you know you're getting matched capsules. At least you will from a respected brand name.

If you're using A/B or spaced pair technique, the classic approach is to use matched pairs but you can get away with using different types for a certain effect. You'll just have to do a little work in matching volume levels and checking phase relationships.

For example, one thing I like on mandolin is placing two different mics in an A/B pair oriented vertically and aimed at the mandolin behind the picking hand: a Royer 121 ribbon mic aimed up below the lower F hole and a small diaphragm condenser aimed down at the upper F hole. Both mics pointing inward in a V arrangement. It's a sort of "best of both worlds" approach for those two different mics.

In the stereo field you can't hear that they were placed vertically; it sounds like a normal stereo recording. It's a way to capture the sweet spot on a small instrument like mandolin and avoid too much pick noise, compared to spacing the mics out horizontally for a spaced pair recording. Of course you can achieve the same thing with a coincident X/Y pair aimed behind the picking hand, finding a sweet spot. Experiment with mic placement! That, and good room acoustics, is the key to good recordings.

[JeffLearman]

MontanaMatt is correct: the primary purpose of a matched pair is to do either "XY" or "AB" stereo recording, so that you don't have a tonal imbalance between the left and right. (See https://en.wikipedia.org/wiki/Microp...#X-Y_technique for details on these two types.)

My go-to method for stereo miking is Mid-Side (called "M/S" in the Wikipedia article), which doesn't require a matched pair (I've never even used the same type mics.) I find several virtues for this setup:
* Mikes don't need to match
* It collapses to mono perfectly (it's the "mid" mic signal)
* I'm not sure why, but when miking a single instrument for recording, I don't have to sit as still when playing to avoid the image moving around. When close-miking with XY, I find I can hardly breathe without the image shifting.

While the article mentions using a figure-8 mic for Side, it's not necessary; a cardioid works fine too. A figure-8 will capture a more accurate image, but what I'm going for (when recording a single instrument) is a "nice" stereo image, which doesn't need to be accurate. When miking a show using mid-side, you'd want to use a figure 8 for the side mic, otherwise the guys on the opposite side of the stage would be attenuated. But when miking a single instrument in stereo just to get a nice (and real) image, that accuracy isn't needed.

In my DAW I use the handy free MDA-VST "image" plugin, a rock steady part of my kit for 20 years now. In addition to using it for MS->LR and back, you can also use it on any stereo track to reduce or increase the stereo width. (Perhaps the fact that I often reduce the stereo width on my M/S recordings is the reason why I don't have to sit stock-still when recording, but I haven't tried to verify that.)

There's a caveat to the "collapse to mono" advantage, though. Any time you use more than one mic, you have to consider the possibility of phase cancellation. With A/B this can be serious -- you test it by monitoring in mono after recording; if it sounds like shite, adjust and try again. When mic capsules are very close, as with XY, this is minimized. With the LDC mics I use for mid-side it could be more of an issue, but hasn't in practice. But here's the rub: rather than mixing to mono to check, you listen to just one side soloed, because that's where any phase cancellation will appear.

There's a flip side to this caveat, though. First, I hope we all understand why it's important for a mix to sound good in mono. For one thing, that just plain often happens due to streaming or whatever. For another, it happens when you're listening through a doorway. Finally, more of a general point, but IMHO if a mix doesn't sound good in mono, you goofed the mix and you'll have other issues that stereo is hiding. So, it's good to occasionally monitor in mono and make sure it sounds good.

That said, if there are phase cancellations with M/S, they only appear in one side. It's pretty rare (except for broken stereos) to hear just one side. Of course, you could be very close to one speaker and far from another. But testing this, I find that our brains are remarkably good at building a world out of the information they get. If I stop one side altogether and there is some phase cancellation, I hear it. But if I bleed just a tiny amount of that side back in, my mind no longer hears the tone difference of phase cancellation; instead it hears spatial imagery. I mean, like only 5% is enough, maybe even less. So unless you're sitting with your ear stuck to the left speaker, it's all good.

And as mentioned above, while two of the same model are usually pretty close, people that do this professionally generally get a matched pair. I used to have a pair of SM57's; they didn't really sound much alike. You could even hear the difference using just one at a time. Admittedly, two of the same model of Neumann's should be closer. I've done XY recordings of shows with two unmatched mikes of the same model and it worked fine for say a demo.

[Don Grieser]

If you are looking for a lower cost stereo pair, check out the CM4 mics by Line Audio. Made in Sweden by one person. Sold through No Hype Audio in Belgium. Dead flat frequency response unlike lots of low cost small diaphram condenser that have a big bump in the high end. NFI. There's usually a waiting list for them, but it's worth the trouble. It was easier to get them than it first seemed.

I mainly record in mono with a SE VR2 active ribbon mic. I bought the CM4s to try stereo but haven't gotten around to it yet. Lots of good ideas here. Thanks.

[bradeasley]

If you are looking for a lower cost stereo pair, check out the CM4 mics by Line Audio. Made in Sweden by one person. Sold through No Hype Audio in Belgium. Dead flat frequency response unlike lots of low cost small diaphram condenser that have a big bump in the high end. NFI. There's usually a waiting list for them, but it's worth the trouble. It was easier to get them than it first seemed.

I mainly record in mono with a SE VR2 active ribbon mic. I bought the CM4s to try stereo but haven't gotten around to it yet. Lots of good ideas here. Thanks.

I've heard great things about those mics. The more I'm learning the more I realize I might just stick with single mics. Seems like either way of using 2 mics on a source requires more effort to ensure the mix is good and not out of phase. I'm probably not at that point yet.