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Author Topic: 3 mic (LCR) distance  (Read 620 times)

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Offline Chanher

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3 mic (LCR) distance
« on: October 19, 2022, 04:05:20 PM »
What kind of distances between mics can you guys recommend for a 3 mic mix?

-it will be a LCR (left, center, right) on a stereo bar. (2) Line Audio cm4’s (wide) cardioid on the outside and a single at853 hyper as the center channel.

-I’m really starting to prefer PAS method and I will be doing this with the outside pair. The on-axis response, when pointing your mics forward more, is much more preferable to my ears than leaving the mics pointed outward in a standard ORTF and getting more off-axis sound hitting your capsules. I understand the more you point (directional) mics forward, you need to increase the distance of the mics to maintain stereo qualities.

I have exchanged an email and polite request with Scott from SRS (Shapeways) about taking his swivel ORTF CM4 bar:

https://www.ebay.com/itm/133336035095

and simply making it wider. Then potentially adding a clip dead center to hold the at853 hyper if he can. If he can’t I plan on finagling my own clip.

Knowing this, any recommendations on what the new width should be? If ORTF is 17cm and NOS is 30cm maybe somewhere in the middle at 25cm? Or perhaps just a NOS bar at 30cm? I hope to be running 3 mics most of the time, but in the rare occasion I only run the CM4’s, would 30cm be too far apart for a stereo pair even if they’re wide cardioids and I’m pointing them forward more?

I wish I could experiment myself but I don’t have my cm4’s yet and I wouldn’t mind having my mount ready by the time they arrive. Thanks again in advance for everyone who has indulged my hairbrained ideas as of late, I certainly welcome any critiques of my (probably) flawed logic so far.
« Last Edit: October 19, 2022, 04:07:53 PM by Chanher »
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Offline EmRR

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Re: 3 mic (LCR) distance
« Reply #1 on: October 19, 2022, 04:55:07 PM »
No clear answer.  If you haven't played with the Neumann Recording Tools app or the Shoeps Image Assistant, they'll help visualize what happens with at least the LR pair. 
Mics: DPA 4060 w/MPS 6030 PSU/DAD6001/DAD4099, Neumann KM 131, Oktava MK 012, Sennheiser MKH 105, MKH 20, MKH 30, MKH 40, MKH 800 TWIN
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Offline Chanher

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Re: 3 mic (LCR) distance
« Reply #2 on: October 19, 2022, 05:47:42 PM »
No clear answer.  If you haven't played with the Neumann Recording Tools app or the Shoeps Image Assistant, they'll help visualize what happens with at least the LR pair.

oooh very cool, i didn’t know those resources existed. I will check those out after work, thanks.

I know there’s a reference table of some kind that shows the recommended distances with angle change (or something like that)) but I can’t find it and I don’t know if TS search is broken but my search results are off and it doesn’t allow the “3 mic mix” search. I’ll figure it out.
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Offline EmRR

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Re: 3 mic (LCR) distance
« Reply #3 on: October 19, 2022, 06:07:53 PM »
No clear answer.  If you haven't played with the Neumann Recording Tools app or the Shoeps Image Assistant, they'll help visualize what happens with at least the LR pair.

oooh very cool, i didn’t know those resources existed. I will check those out after work, thanks.

I know there’s a reference table of some kind that shows the recommended distances with angle change (or something like that)) but I can’t find it and I don’t know if TS search is broken but my search results are off and it doesn’t allow the “3 mic mix” search. I’ll figure it out.

http://www.sengpielaudio.com/Visualization-AB60-E.htm
Mics: DPA 4060 w/MPS 6030 PSU/DAD6001/DAD4099, Neumann KM 131, Oktava MK 012, Sennheiser MKH 105, MKH 20, MKH 30, MKH 40, MKH 800 TWIN
Recorders: Zoom F8n, Sony MZ-R50

Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #4 on: October 19, 2022, 07:11:15 PM »
Great question.  The TL;DR is to go as wide as you are able to in practical terms. Two feet is reasonable if you can push the center mic forward somewhat in front of the other two.   The wider you can go the less the center one needs to be pushed forward, which is helpful in multiple ways.


If an extended answer is welcome, read on..

I understand the more you point (directional) mics forward, you need to increase the distance of the mics to maintain stereo qualities.
^
This angle / spacing relationship (in combination with directional pattern) is the core stereo relationship between a stereo pair of mics.. and by extension it applies to arrays of more than two microphones  as well. 

Very generally, a minimum of twice the spacing appropriate for an L/R pair is the starting point for L/C/R triplet spacing.  That kind of makes sense as that makes the distance between any two adjacent microphones the same as it was before.  Problem is the angle between those two adjacent mics is halved, so ideally more spacing is required to compensate for the narrower angle.  The other thing driving the need for increased spacing is the angle between mics that you need to achieve PAS.

Quote
I’m really starting to prefer PAS method and I will be doing this with the outside pair. The on-axis response, when pointing your mics forward more, is much more preferable to my ears than leaving the mics pointed outward in a standard ORTF and getting more off-axis sound hitting your capsules.
^
This, in my experience is spot on.  Problem is that from a typical recording position, PAS typically equates to a rather narrow angle between mics, which ideally benefits from increased spacing between them to compensate. Because of that things get wide fast.  Check out the >>Improved PAS table<<, also linked in my signature line.  It's useful as an easy way to determine the optimal spacing between a pair of microphones when Pointed At Stacks, based on the PAS angle from the recording position and mic pattern.  Yet that's two mics, not three.  Three takes more spacing, but it illustrates the trend.

The Neumann Recording Tools app and Schoeps Image Assistant EmRR mentions are useful tools and work for analyzing a stereo pair of mics. These tools are primarily about predicting image width and stereo image position accuracy, which while nice to have is not the most critical thing to a good recording.  But its a good start.  These apps can be a bit wonky and tricky to interpret, so don't feel like mastering or fully understanding them is necessary. The Image Assistant [http://ima.schoeps.de/] is able to analyze a 3-mic array in addition to a stereo pair (not sure about the Neumann app).  To do that it will be easiest to start with the Decca tree preset and modify the mic patterns to emulate those of your mics, then modify mic angle (labeled as Epsilon, equating to half the total angle between L/R mics), horizontal spacing, and center forward spacing to reasonable values while watching the graph display. Read the total image width (the recording angle within which imaging should be accurate) across the bottom, where the diagonal lines curve back to horizontal at lower left and upper right.  Best optimization will form a straight diagonal line running from lower left to upper right across the display, meeting up across the center without too much variation. Play with it and try to get the two diagonals to line up (usually takes adjusting forward spacing as well as L/R spacing). 

Other resources:
I've attached a couple PDFs.  The first covers 3-microphone configurations, except using omnis instead of subcards.  You can use a bit less spacing with subcards in place of the omnis if you angle them.  The second covers 4-microphone configurations using an X/Y or M/S pair in the center in place of the single mic.  Consider doing that if you can.  Page two shows the most compact version, which works in challenging rooms and replaces the omnis with supercardioids.  That can work with subcards too.  Space them a bit more if you can, but don't worry too much if you can't. (These are new revised versions of the old OMT booklet linked in my signature which needs updating)

Michael William's MMAD (Multi Microphone Array Design) website. He's the "Stereo Zoom" author, and much of the stuff above is based on his work. Start here: https://www.mmad.info/MAD/Ch_n_cov.htm.  Similar to the apps, that website all about achieving seamless imaging between microphone pairs, in this case  the multiple mic pairs of an array.  It's a hyperlinked decision matrix which links to printable PDFs showing different microphone array setups.  The 3-mic configuration section was never completed, but the 5 mic section works and starts with the L/C/R front portion, so just choose any configuration angle for the rear-facing mics, and it won't matter if you choose Front Sound Stage Coverage or Surround Sound.  It only uses a single mic pattern for all mic positions however, so choose either cardioid or hypocardioid (subcard).  Then compare choose between approximate PAS angles.  It can be useful to see these known-good arrangements even if you can't achieve them simply as reference points.
« Last Edit: October 19, 2022, 07:19:43 PM by Gutbucket »
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Offline Chanher

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Re: 3 mic (LCR) distance
« Reply #5 on: October 19, 2022, 08:15:20 PM »
The extended answer is welcome. :)

I have my homework tonight! It appears my initial idea for an LCR is off, at least in terms of stereo imaging, but I need to read more. I have recorded a couple LCR’s in the last year and I simply enjoyed them, especially the blues trio where I ran the mics onstage. Perhaps this means I need to do some field research and go try more 3 mic mixes with my current gear.
« Last Edit: October 19, 2022, 10:07:21 PM by Chanher »
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Re: 3 mic (LCR) distance
« Reply #6 on: October 19, 2022, 10:29:12 PM »
here is a 1 minute sample of a blues trio i recorded last march in a small theater:

https://www.dropbox.com/s/zb2ajplhisfmlfb/mic%20onstage%20blues%20trio%20sample%202448.flac?dl=0

3 mics LCR onstage, stage lip.
(2) C4 cards ~25cm apart, around 40 degrees
(1) at853rx hyper center

no editing except adding gain to the C4 stereo file (I added a bit more to the right channel to help the mix sound better to my ears).
I did NOT position the center mic forward more (so sound is hitting it first) as I didn't know about the potential benefits at that time. I plan to experiment with this, maybe run it ahead for one set and on an even plane for another.

Is that enough stereo imaging? Yes, for me. But probably because of the onstage setup of the trio. 3 instruments all aligned at the back of the stage about 15-20ft from my mics will do that. You can probably guess the location of the players.

If I'm in a noisy club and I'm pointed at the PA in the back of the room, I question if this setup would make that much more of a difference. The sweet spot up front could potentially catch some stage action. I'll try it out and report my findings.
« Last Edit: October 19, 2022, 11:29:16 PM by Chanher »
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Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #7 on: October 20, 2022, 09:10:42 AM »
My work headphones broke yesterday, but will try and give your sample a listen later.  L/C/R works great on stage.  And in that scenario you can often angle the L/R mics wider and push the center mic forward, both of which reduce the need for as much spacing between L/R.

Here's an example of such an on stage setup for a jazz trio-


Based on this MMAD setup-


Similar MMAD setup using cardioids instead of supercards (note the wider angle between mics)
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Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #8 on: October 20, 2022, 09:48:48 AM »
To run a L/C/R without much spacing, ideally the L/R mics should be highly directional and angled widely.  That works up close or on stage, but not so much from a distance.  From an audience recording locations the L/C/R triangle grows increasingly wider and less deep.  That trend continues the farther back the recording position is and as the angle between L&R becomes narrower.  From far back the L/R spread is wide and the forward center spacing minimal.

I've found no better way to deal with a crappy room from an audience position than the "tough room' setup on page 2 of the OMT4 PDF, which uses four channels of directional mics pointed at stacks.

Advantages other than imaging-
I think for tapers the primary advantage of using three mics instead of two is increased flexibility over balance.  It gives you level control over three inputs for L/R balance, center/sides  balance, Left/Center and Right/Center balances.  Much easier to even things out as needed to get a good balanced sound.  Beyond that, the center can be EQ'd differently, which in combination with level control is a super powerful tool.  If open to EQ'ing, try reducing the low frequency content in the center mic channel and EQing higher up for good clarity and presence.  Or try it the opposite way by doing that to the L/R pair.  You can sort of play one off the other to some degree, with lots of options to play around with.  You can do more and push this kind of thing further than you could with a stereo pair because everything gets blended together in a balanced way.

Secondly, the wide spacings or wide mic angles provide a better portrayal of the diffuse reverberent part of the recording - the part that conveys the "sounds like you are there in the room" sensation - while the center mic anchors the middle so there is no hole in the middle problem.
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Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<< (note: This is a 1st draft, now several years old and in need of revision!  Stay tuned)

Offline Chanher

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Re: 3 mic (LCR) distance
« Reply #9 on: October 20, 2022, 09:51:15 PM »
I think for tapers the primary advantage of using three mics instead of two is increased flexibility over balance.  It gives you level control over three inputs for L/R balance, center/sides  balance, Left/Center and Right/Center balances.  Much easier to even things out as needed to get a good balanced sound.  Beyond that, the center can be EQ'd differently, which in combination with level control is a super powerful tool.

This. There’s an extra channnel on my recorder so why not.

I want to experiment with how far in front I like the center mic. I always assumed having your mics on an even plane in relation to the sound source was a given. Apparently the small time delay difference that comes from moving a centered microphone forward has desireable results. I must test this theory.
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Offline DSatz

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Re: 3 mic (LCR) distance
« Reply #10 on: October 21, 2022, 04:40:57 AM »
Theory? We don't need no steenkin' theory ...

The three-mike idea comes from the old Bell Labs concept of stereophonic sound in which there would be as many loudspeakers in the playback system as there had been microphones used in the pickup arrangement. When record companies such as Decca, RCA got hold of it for LP recording, home stereo setups were already defined as having two loudspeakers. The center track was thus a utility track, often used for solo elements which could be EQ'ed and balanced (as a whole) in post-production against what the L and R microphones were picking up (as a whole). Think spot miking the soloist in an instrumental concerto or an opera aria--or in a recording of a complete opera, all the singers as a group in relation to the orchestra, where the singers were placed away from the orchestra during the session, so that their closer pickup in the C track would overwhelm their accidental pickup in L and R. This relieved the producer and engineers from having to set those balances during the live session, where they would then become unalterable.

Many Decca recordings had the center microphone placed forward of the line between L and R, but it's a myth that there is one definitive "Decca Tree" geometry; they tried many different arrangements over the years, never settling on any one of them, and often using baffles between/among the microphones to increase the separateness of their pickup from one another. However, in no event did they follow Gutbucket's idea that L and R should be spaced at all widely apart. The entire European conception of spaced-omni miking for stereo is based on a smaller distance between L and R than has ever been traditional in the United States; there are incoherencies in the stereo image (ambiguous / unstable localization of sound sources) that occur with widely-spaced omnis that their ears can't stand (and having learned to hear them myself, neither can I).

RCA, on the other hand, did space their L and R microphones apart more widely, with C deployed either in the true center or else closer to the soloist(s) depending on how the track was intended to be used--as a spot mike, or as a general fill-in. The engineers always ran the center track as hot as possible below the overload limits of the tape, then in the 3->2 mixdown, (a) if C was spot-miked soloists of course the levels were adjusted by ear and from section to section of the music, while (b) if it was a general center fill, then its level was always reduced about 6 dB on average so that it added clarity and anchored the stereo image but didn't collapse everything into mono, as it would do if mixed in at anything like equal gain to L and R.

The Mercury "Living Presence" recordings on the third hand so to speak, if I'm not mistaken, came closer to the idea of three microphones all in a row. The Fines (husband and wife team who recorded the series) didn't always use the same mike for all three positions (actually, six since for safety they ran two separate sets of mikes into two separate three-track recorders), but that was mainly because the mike that they wanted to use for all six positions had already been discontinued before they started recording in stereo, and it was difficult to scrounge up six of them--it took them a few years to do so, but from then on they used that one type of microphone--the Schoeps M 201 in its omni setting--for all their stereo recordings. That's a roughly diffuse-field-equalized pressure transducer with a collar around the diaphragm (apart from when the mike was set to its cardioid position, but they never did that).

Decca and RCA also used microphones with rather special characteristics for these recordings: small-diaphragm pressure transducers (single-diaphragm omnis) where the capsule was embedded in a 40 cm sphere, which gives the microphone added sensitivity in the "presence" region in the front, and causes the on-axis rise in the treble to start at somewhat lower frequencies than it otherwise would for a small diaphragm construction. (The collar around the omni capsule in the Schoeps M 201 had a somewhat similar effect.) Their preferred mike was the Neumann M 50 although both labels also used the M 49 at times--a weird substitution since the capsule construction and sound are quite different. It may have been some situations in which the M 50 wasn't available, or some odd acoustic in some recording venues that required them to use the M 49s as directional microphones, an option which the M 50 didn't offer.

In addition to the three main microphones, Decca and RCA used spot mikes sometimes, and/or "outriggers" (additional, quite widely-spaced L and R microphones whose signals were mixed in to the L and R tape channels at lower levels), but Mercury as far as I'm aware never did so. In all cases there was a separate control room setup with loudspeaker monitors, and test takes were always made at the start of the session, then played back and listened to, so that adjustments could be made before starting the "real" recording.

--best regards
« Last Edit: October 21, 2022, 04:53:39 AM by DSatz »
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Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #11 on: October 21, 2022, 10:42:26 AM »
Good historical references, thanks.  Past informs the present.

To clarify, when I suggest the L/R pair of an L/C/R microphone array be "spaced widely apart", that's relative to the "near-spaced" arrangements such as ORTF, NOS, (DIN), etc. concert tapers commonly use.  Relative to the old RCA and Mercury recordings these spacing are not anywhere close to as wide.  The methodology is quite different. 

Backing up a bit- As I see it, the most common problem concert tapers encounter when attempting to introduce additional microphone channels (that is to say, using 3 or four microphones on the same microphone stand, with the intention of mixing them all down to two channel stereo) is placing too many microphones in close proximity / not angled far enough apart from each other to avoid problems.  And this is exactly what I read as being asked about in the original post in this thread.  The well known two-channel near-spaced stereo microphone pair configurations represent optimized combinations of spacing, pattern, and angle.
 Introducing a third channel throws that optimization out of whack and an entirely different combination of spacing, pattern, and angle is required.  Simply adding an additional center microphone channel to a standard two-channel near-spaced stereo pair configuration may seem attractive in practical setup terms, yet is in my opinion not the way to go. 

There are essentially two good choices:
1) Stick with two channels and abandon the idea of using additional microphones.  This is arguably "correct", certainly more simple to implement and is likely the most appropriate choice for most folks.  Use any extra channels for recording the soundboard feed, spot mics.. or to record a second stereo pair arrangement for comparison purposes, choosing one or the other, yet not with the primary intention of mixing them together.

2) Re-arrange things so that a microphone array which includes additional microphone channels it setup to work correctly without problems.  This is what I've been playing around with for years and what I'm attempting to help Chanher with in his exploration of this path.  I've found it engaging and worthwhile for a number of reasons, including the pragmatic aspect of gaining some degree of increased control in situations where tapers otherwise have very little of it, and also in terms of the quality of the resulting recordings when it is done right and it all comes together correctly.
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Offline Gutbucket

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Re: 3 mic (LCR) distance
« Reply #12 on: October 21, 2022, 11:29:49 AM »
I have played around in the past with wider "in a line" spacings somewhat more similar to what Mercury  "Living Presence" was using, perhaps RCA, some of Owsley Stanley's Old and In the Way recordings, and more recently described by David Griesinger.  Mostly at the stage-lip, using three, four, or five relatively wide-spaced omnis arranged in a line across the front of the stage.  It worked and was fun to do but isn't really practical to do regularly.  Good practical balance control, good resulting recordings, and a fun taper option for some things, yet very different from what most tapers will want to do and what we are discussing here.

A better reference for the spacings I'm suggesting is the more modern 3-channel arrays dating back not 6 decades but more like 2, intended for L/C/R speaker playback (think surround system playback, excluding the surround speakers.. or not).  This is were I initially started with this exploration of recording more than two microphone channels and how best to arrange things for that approach, and I was somewhat surprised and pleased that what worked well for multichannel playback translated extremely well to two-channel stereo playback as well while providing the very welcome increase in practical flexibility mentioned previously.

Am I right or wrong?  Do I care?
The suggestions I'm making here are based on my empirical explorations of this stuff, informed by review of historical methods of recording, the psychoacoustics of human hearing, lots of experimentation and listening, and working within practical constraints of the situations in which tapers find themselves doing our thing.  In doing so I've tried to determine what the underlying core relationships are that seem to be most important, and I try to describe things in those terms on this forum rather than simply saying, "just set things up this way".  It works really well for me, and I'd like to give back to the community by helping other tapers who are interested in exploring recording in similar ways.

That said, my recommendation to most folks remains to stick with two channels!
Fair enough?
« Last Edit: October 21, 2022, 12:56:43 PM by Gutbucket »
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Re: 3 mic (LCR) distance
« Reply #13 on: October 21, 2022, 11:56:18 AM »
Specifically on the thing about forward spacing of the center microphone- Don't worry about it too much. 

Achieving sufficient spacing/angle between adjacent mics is the main thing (something typically near-spaced-like is about right, more as the angle between adjacent microphones grows narrower).  The primary reason for pushing the center mic forward is a way to achieve that if/when you can't otherwise get the L/R pair spaced widely enough, which is to say about twice the spacing of the standard near-spaced two-channel configs). I'd rather have the L/R pair wide enough that the center doesn't need to be forward by much, but having it forward a little bit is good.

^This is not primarily about imaging. Those imaging apps have the center microphone pushed forward to get the image distribution to link up nicely across the center, and playing around with adjusting the forward spacing parameter in them appears to produce major implications.  Don't worry about that too much. Technically accurate source position imaging is nice to achieve, yet is well down on the hierarchy of what is most important.  You'll get plenty of good sounding width and imaging regardless of center-channel forward spacing.

Here's a reasonable practical take on optimizing it- assuming you can achieve sufficient L/R spacing to begin with, push the center microphone forward just enough that the Left-Center microphone pair is perpendicular to an imaginary line pointing at the left edge of the ensemble or left PA stack, and the Center-Right microphone pair is perpendicular to an imaginary line pointing at the right edge of the ensemble or right PA stack.  How far forward that is will be determined by how far back the recording position is.
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Offline fireonshakedwnstreet

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Re: 3 mic (LCR) distance
« Reply #14 on: October 21, 2022, 01:22:27 PM »
Great discussion, as always. I am typically running an OMT8 with 4ft spread for omnis, but that is with 3 other pairs of directional mics in between them. 2ft should be plenty for two cards and a hyper. The Smallrig 15mm camera rails make a great modular mic bar with the appropriate hardware. Not too obtrusive, light, and sturdy. I have my most directional pair, short shotguns, on a separate bar out front maybe 6 inches or so. I am just trying to get as much direct signal as possible at the show, and I think in that regard the additional directional mics do make a difference. 
Mics: AT 3031; AT 853Rx (c, o); CA-14(c); Naiant X-R (h); Studio Projects C4 (c, o, h); Nak 300/Tascam PE-125/JVC M510 (cp-1, cp-2, cp-3, JVC M510 superdirectional caps)
Recorders: Tascam DR-680 MkII; Tascam DR-70D; Tascam DR-5; Tascam DR-2D
Pres: Edirol UA-5 (Oade PMod & WMod); Marantz PMD661 (OCM); Naiant MidBox

 

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