Topic: Spaced Omnis - two schools of thought?

[voltronic]

Recently I've been reading more on spaced omni techniques, and I keep coming across conflicting information regarding minimum / maximum recommended spacing.  I realize that the "rules" may only be a starting point for experimentation, but I'm having a hard time understanding the two schools of thought here, and was hoping others might shed some light on this.

Some sources recommend 3ft/90cm as a minimum spread, but others, especially I'm finding European microphone manufacturers and engineers, prefer a range of only 40-80cm.  Michael Williams doesn't go beyond a 50 cm spread for a +/-50deg included angle on his SRA chart for omnis, and DPA's graphs don't go beyond 70 cm for the same recording angle.

I found this paragraph from DPA especially interesting, as it is the only explanation I've found for keeping the spacing relatively close (emphasis mine):

Since the stereo width of a recording is frequency-dependent, the deeper the tonal qualities you wish to reproduce in stereo, the wider your microphone spacing should be. Using a recommended microphone spacing of a quarter of the wavelength of the deepest tone, and taking into account the human ear's reduced ability to localize frequencies below 150Hz, leads to an optimal microphone spacing of between 40 and 60 cm. Smaller microphone spacings are often used close to sound-sources to prevent the sound image of a particular musical instrument from becoming "too wide" and unnatural. Spacings down to 17 to 20 cm are detectable by the human ear, as this distance is equivalent to the distance between the two ears themselves.

Realizing that the recording angle narrows with increasing distance from your source, could that be the factor that's missing from these recommendations?  Or is there something else at work to account for the different perspectives?

[capnhook]

I'm at 55cm spacing.

[aaronji]

I generally go about 66 cm if I am taping with a stand (that's the width of my bar) and more like a meter, give or take, if I am using my patented dual clamp system...

Personally, I find that the broader spacing makes for a better stereo image and is well suited for the tape of recording that many here do (and which I don't think DPA or Stereophonic Zoom are really taking into account).

[Tom McCreadie]

Some sources recommend 3ft/90cm as a minimum spread, but others, especially I'm finding European microphone manufacturers and engineers, prefer a range of only 40-80cm.  Michael Williams doesn't go beyond a 50 cm spread for a +/-50deg included angle on his SRA chart for omnis, and DPA's graphs don't go beyond 70 cm for the same recording angle.

I find that with 90cm or greater, you could drive a London doubledecker bus through the "hole in the middle" of the image. There's a gnawing, unsatisfactory lack of central solidity.
[Incidentally, the term "incliuded angle" generally refers to the physical angle between the splayed mic bodies of an array. The SRA can be thought of as the auditorium container sector into which the performers have to be shoehorned in order to fully utilize the spread between your playback speakers. If the players spill outside the SRA, their direct playback images start bunching up in the speakers..or even giving woozy phasey imaging.] 

[aaronji]

^ Bear in mind that a lot of people on this board are essentially recording PA systems, generally in mono, without a lot of sound directly from the instruments.  The hole isn't so noticeable in that context, I think, and either spacing or baffling is needed to get some "stereo-ness" in the recording.

[Bruce Watson]

Some sources recommend 3ft/90cm as a minimum spread, but others, especially I'm finding European microphone manufacturers and engineers, prefer a range of only 40-80cm.  Michael Williams doesn't go beyond a 50 cm spread for a +/-50deg included angle on his SRA chart for omnis, and DPA's graphs don't go beyond 70 cm for the same recording angle.

What I say below is about recording acoustical music. For example, an orchestra in a large hall, no amplification of any kind. For all other applications, I can't help much. Sorry.

What a lot of people find over time and lots of recording and listening, is that for two omnis, 67cm spacing is a practical limit. Any more spacing and you open up a hole in the middle. If you feel you need more spacing, outriggers become your friends. Or, like a Decca Tree, you add a mic in the middle of that "hole". Indeed, there's seems to have been somewhat of a split among the Decca engineers toward the end. Some where using a Decca Tree with outriggers, and others were using an AB pair spaced below 67cm and outriggers.

I'm sure the OP has searched through and read the posts from really fine engineers like Tony Faulkner over on GearSlutz. This 67cm figure seems to be an experimentally derived number that many engineers on both sides of the Atlantic come up with. I don't know the theoretical basis, but I'm sure there is one.

[voltronic]

Some sources recommend 3ft/90cm as a minimum spread, but others, especially I'm finding European microphone manufacturers and engineers, prefer a range of only 40-80cm.  Michael Williams doesn't go beyond a 50 cm spread for a +/-50deg included angle on his SRA chart for omnis, and DPA's graphs don't go beyond 70 cm for the same recording angle.

What I say below is about recording acoustical music. For example, an orchestra in a large hall, no amplification of any kind. For all other applications, I can't help much. Sorry.

What a lot of people find over time and lots of recording and listening, is that for two omnis, 67cm spacing is a practical limit. Any more spacing and you open up a hole in the middle. If you feel you need more spacing, outriggers become your friends. Or, like a Decca Tree, you add a mic in the middle of that "hole". Indeed, there's seems to have been somewhat of a split among the Decca engineers toward the end. Some where using a Decca Tree with outriggers, and others were using an AB pair spaced below 67cm and outriggers.

I'm sure the OP has searched through and read the posts from really fine engineers like Tony Faulkner over on GearSlutz. This 67cm figure seems to be an experimentally derived number that many engineers on both sides of the Atlantic come up with. I don't know the theoretical basis, but I'm sure there is one.

Thanks, Bruce.  I only record acoustic / classical also, as do the people I'll be giving advice to later this week.

Yes, I've read through many of the GS posts from Faulkner, Boojum, Jnorman, etc., and have recently been using Faulkner's 4-mic subcard / omni array with good results.  Even at that 67cm distance, I find that the omnis missing something when listening to just that pair on their own, but things immediately pop back into focus when I add the subcards back into the mix.

I've really enjoyed the recordings I've heard with just one omni pair at 40-50cm spacings, and even at that close distance there seems to be plenty of directional information being passed through.  Maybe that bears out the assertion from DPA that I quoted above.  Those small distances sound especially great to me on solo piano recordings.

[Gutbucket]

There are numerous situations in which a pair of omnis may be used for making a stereo recording, and those situations differ significantly.  Based on that oversimplification alone, there is never going to be consensus on any one particular omni-pair arrangement being "best".   To further complicate the matter, there is more than one kind of "stereo-ness".  Different techniques produce different qualities of "stereoness".  We make selections which are compromises between those different aspects, and sometimes we can mic separately for a couple different aspects and attempt combining them.

I can explain what I mean by that later, along with the basic stuff going on which is important to the discussion if you like, but let's jump to the more interesting conclusions.  Taperssection style of recording a band's live performance is a very, very, very different thing from recording individual musicians, and also a very different from recording an large unamplified acoustic ensemble like an orchestra.  What works for one does not necessarily apply to the other.

Some things which I think we tend to take for granted here, but which radically changes things with regards to a pair of spaced omnis:

1) Recording a PA reinforced performance from a position out in the room is a total odd-ball scenario in the recording world.  It only exists in live music taping!!!  It is the TS elephant in the room in most of the threads here.  It is one reason why super-wide spacings which would not work as a single pair for non-PA amplified acoustic ensemble can work very well and not play back with a huge hole in the middle..

Imagine I were to setup a band and PA, say a typical outdoor amphitheater setup with a mostly mono PA mix, but I put one mono PA stack in my back-yard and the other a mile away in the parking lot of the supermarket, and proceed to mic this PA with a very, very, very, widely spaced pair of omnidirectional mics, each placed an identical distance from it's respective PA stack.   The ambient sounds in each channel will be totally unrelated to each other.  All the environmental noise reaching each microphone will be uncorrelated.  It may as well be two completely different recordings made in different places at different times.  More than a hole in the middle, I've have two completely unrelated recordings.  However, even though the microphones are spaced a mile apart, on the resulting recording all sound which originates from the mono PA will will solidly fill the center of the playback image.  Mono PA stuff will be reproduced in the center from both speakers, and the random uncorrelated ambience out in either speaker, unrelated to each other.  The reverberation from the PA stacks will be related (since the same signal is being produced from each one, just a mile apart), but will be uncorrelated as each stack is illuminating a completely different environment, each environment providing it's own unique reverberation.

2) Inclusion of additional sources.  Mixing in another microphone or two in the array (Decca tree, outriggers, center pairs, or whatever), and/or mixing in spot microphones, and/or matrixing with a SBD completely changes the deal.  You no longer have a setup which is going to behave like a single spaced pair of omnis.  Completely different guidelines and concerns immediately apply.   A setup acts and interacts as a whole, not as isolated sums of it's individual parts.   Just as with mixing two near-spaced configurations or adding a center mic, or whatever - it's a mistake to approach it by thinking of it as "the near-spaced configuration I always use, plus this something else".  The "something else" completely changes the way a spaced pair of omnis work when used by themselves.

[voltronic]

^ I figured I'd see you here soon.   Yes, I get that TS-style recording is a very different / specialized thing.  I was surprised to see Shure specify 1m minimum distance in their literature though, and I doubt they are considering amplified concert taping.

Going back to the emphasized part of that quote from DPA, I remember reading somewhere a long time ago about the use of dual subwoofers in home theater systems, and there was a discussion that our ears/brains may be able to resolve directional information at lower frequencies than previously thought.  I dismissed it at the time because physics seemed to go against that idea - by that I mean our ears would have to be much farther apart to deal with the long wavelengths when you get down into the low bass register.

Regarding you Point #2, I think that gets right to the issue of things like the Faulkner subcard/omni setup Bruce and I were talking about, where you have to consider how the whole system works together.

[Gutbucket]

Going back to the emphasized part of that quote from DPA, I remember reading somewhere a long time ago about the use of dual subwoofers in home theater systems, and there was a discussion that our ears/brains may be able to resolve directional information at lower frequencies than previously thought.  I dismissed it at the time because physics seemed to go against that idea - by that I mean our ears would have to be much farther apart to deal with the long wavelengths when you get down into the low bass register.

This sort of points to aspects of "stereoness", that differ from left/right imaging.

That's probably referring to some of the work of David Griesinger (then working for Lexicon) who argues for introducing phase differences down at the lowest frequencies not for directional imaging, but rather as a way of externalizing the sensation of low bass with a more natural "out-of-head" sense of envelopment.  The Lexicon processors based on his research long included an option to for two subwoofers rather than one, intended to be placed to either side of the listening space, with a phase difference introduced between the two.  That phase difference is introduced by the processor, rather than relying on difference information existing on the recording itself.  Most recordings won't have stereo difference information down at the lowest octave(s), and that includes all analog turntable records, where is it deliberately mono-ized to keep the cartridge from jumping out of the groove.  Stereo difference information on an LP is encoded via vertical travel in the record groove, and significant low-frequency difference information will cause the needle to skip, jumping out of the groove while attempting to follow that vertical amplitude, like a snow skier at speed skiing the moguls.  Digital recording and playback doesn't have that playback limitation, yet there are other reasons for managing stereo width and "how-mono" the bass information is in different frequency ranges.

[Gutbucket]

A very wide omni spacing can achieve that aspect of "stereoness" and envelopment down low.  A wide spacing also decorrelates the hall ambience and reverberance down to a lower frequency, which makes things sound more "open and airy" and reproduces those sounds diffusely, throughout and even outside of the playback imaging space. That's a different kind of stereo thing than left/right imaging and doesn't relate to Williams SRA curves. It's not about accurate left/right imaging directionality or orchestra angle.

But more narrow omni spacings are usually going to have a more solid center, and often may be more accurate in terms in left/right direction imaging of the sound source locations as laid out on the stage, so the SRA curves then become applicable, at least for those imaging aspects.

(Always exceptions for TS taping, and here's another one-  "section recording" by the soundboard at an outside event, often places band/PA at a distance where the "orchestra angle' and appropriate SRA is only 40 or 45 degrees wide total.  So even in terms of following the William's curves for good L/R imaging using a single pair of omnis -and we need to extrapolate those curves, because as mentioned this scenario is a total oddball in the world of recording scenario which was never considered by the Stereo Zoom for typical studio or classical recording- we end up with Stereo Zoom suggested spacings of more than a meter.  That's a spacing of more than a meter not with the intention of optimizing those "wide spaced omni" stereo aspects, but rather optimizing the "near-spaced omni" stereo sense of accurate L/R imaging in terms of the a Stereo Zoom)

So if only using one pair of spaced omnis, determining the optimal spacing between them partly becomes a question of balancing the left/right directional stereo imaging aspects against other stereo aspects.

..Or another approach is to introduce more microphones, as a way of working around the compromise imposed by those contradictory things which are pulling the microphone spacing in opposite directions.

I can setup very wide spaced omnis which providing an open, lush ambience, envelopment and "stereoness" down low, and get good center balance and directional imaging by introducing other mics.  The "other  mic's" could be a third omni in the center.  That's the simplest arrangement - and it fixes the problem in two ways: First, the over-wide center 'hole' is plugged by introducing a center channel, which in 2ch stereo playback gets panned center, feeding both Left and Right channels and thus producing a strong mono component in the stereo signal, filling the hole; 2) There are now two stereo pairs instead of just one (actually 3 interacting pairs instead of just 1) and the spacing between the these pairs of omnis (L/C, and C/R) is now half of what it was for the former single L/R pair.  In that sense, adding a third omni to the center is equivalent to halving the previous L/R omni spacing, and by doing that it's not surprising that the "hole" in the middle problem is fixed simply due to that radically different spacing alone.  That in turn allows me to space the original L/R pair considerably wider without a hole-in-the-middle problem, and that gets me more of the other kinds of "stereoness" I want, which comes in addition to the improved left/right directional accuracy of three closer-spaced omnis.

A SBD feed counts as additional mics, and works similarly to a third omni in the center in the sense of "filling the hole", allowing a wider spacing without introducing problems.  Using a directional mic in the center instead of an omni is a bit more like a SBD feed because there will be more direct sound and less room reverberance in that center channel.  Using a coincident stereo pair in the center provides left/right directional information without phase-difference info that could complicate things when combining feeds (a coincident center pair is similar to the SBD feed in that it's L/R imaging is level-difference based without phase-differences, like much of the stereo informationpresent in a SBD feed- all pan-potted mono stuff, except maybe stereo verb, synth pads, stereo mic'd leslie cabs, etc).  Using a highly directional center mic as the mid in a M/S pair provides better isolated up-front center, plus control over L/R directional imaging in the center by bringing up in some Side, and that sharp imaging up-front stereo center can rests comfortably in the lush, wide-omni stereo bed.  Tight sharp accurate imaging "stereoness" from the M/S coincident center mic, plus lush open enveloping big deep and ambient "stereoness" from the wide omnis.  To my way of thinking, those are logical progressions in "adding a center mic" to a pair of wide omnis.

Deca tree is just a third center omni moved forward a bit.  Outriggers are just an additional pair of very wide omnis, added for the same "other-stereoness" reasons discussed above.

Every form of engineering is about juggling the imposed constraints to achieve a desired result.  Audio recording is no different.  Getting all the desirable sonic aspects well balanced is a big challenge using a single pair of mics.  That's no different if using an omni pair or a cardioid pair.  The addition of other mics in the main array, out-riggers, spot mics, section mics, SBD matrix or whatever, are all ways of working around the constraints imposed by the limitation to two microphones. 

And as soon as more than one stereo pair of microphones is introduced, the situation immediately turns into that "system-working-together" thing.  It's no longer an ORTF pair.  It's no longer a pair of spaced omnis and doesn't play by the same rules imposed upon recording using a pair of spaced omnis alone.  And we shouldn't unnecessarily constrain our thinking about using microphones in arrays by trying to conform to the compromises that work well when using a single pair of microphones.

[Editing done after posting a mess. Refresh, refresh view]

[justink]

i'm a big fan of spaced anything.

we've run "split stack" with cards before.  one mic on the left stack and one on the right stack.  came out amazing.  really picked up the stereo mix well and you hear one fan in one ear and not in the other too.  interesting listen.

if runnig ominis back by the board...  why not space them out as far as you can?  one mic/stand on the right side of the board, one on the right, both pointed at the respective stacks?  same for cards, hypers, whatever...

it's the best stereo config you could do, if you have the gear....

[Gutbucket]

..and you hear one fan in one ear and not in the other too.  interesting listen.

^
Yet another stereoness, and one that can be key for TS tapers with noisy audiences.  I've posted about this aspect previously, noticing it even with a 6' spread.  Nearby audience members tend to image further off to one side or the other, getting them out of the way of the music which spans the full middle.

if runnig ominis back by the board...  why not space them out as far as you can?  one mic/stand on the right side of the board, one on the right, both pointed at the respective stacks?  same for cards, hypers, whatever...

Not a bad choice.  Though more challenging to manage/block.  May get more susceptible to 'hole in the middle' if the distance to each stack isn't close enough to the same on each side, and maybe more susceptible to wind messing with the solidity of the center (wind 'phasing' is undulation of the air medium through which the sound waves are traveling, changing the path distance from speaker to mic slightly, similar to a 'chorus' phase effect).  If you have a complete soundboard patch representing the entire mix, then you don't even need to point the mics at the stacks.. in fact you're probably better off not doing so.  Since that stuff is already in the SBD feed you gain increased control over your SBD AUD matrix by excluding redundant pickup of the PA from the wide spaced mics.  I'd actually like to try wide spaced figure-8s in that scenario, one on either side of the board, oriented sideways with the nulls pointing at the center of the stage or each stacks.  That only works with a complete SBD though, or some other source acting as delivery mechanism for that info other than the wide-spaced pair. A typical near spaced pair, coincident pair or a single supercard or shotgun in the center would do nicely.

[voltronic]

OK, here's a situation where I'll be more like the "taper spacing" for omnis:  Even though I only do acoustic recording as I said, we're now into marching band season, and I plan to start using spaced omnis this year instead of cardiods.  For home games, I tend to be up on top of the press box for the best video angle.  If I wind up recording in that situation, my plan is to take two clamps with me and space my mics very wide on the rail, which is probably about 20ft wide or so. 

Of course, while I'm up there I can use my bearing compass app and find the exact angles, but if I had to estimate from memory it would be a recording angle of around 60deg total.  Popping that into the SRA tool tells me that 1 meter is the optimal distance, but I have a feeling that wider is going to be better because I'll be quite far away.

For away games and competitions, I'll be recording from just above field level, clamped onto the railing in front of the front rows of stands so that's probably a much better situation to deal with.  Note that crowd noise isn't a factor for the top-of-box placement - people are quiet for the bands, except when applause is (hopefully) warranted.

[Gutbucket]

Consider that the appropriate SRA for recording a marching band may or may not be equal to the Orchestra Angle into which the outer edges of the band's marching movements will fit.  I suspect a narrower SRA will be better, zooming into the action closer to center field a bit.  The outer limit edge of field stuff will then simply image over at one speaker or the other, with more band movement between speakers.  I suspect an SRA wide enough to cover the entire field would provide a more distant perspective with less dramatic, more centralized.

How much of the field do they actually use?

Do you have a way of recording three or four channels?  Would be interesting to do a 10 or 20ft omni split atop the press box with a single directional mic in the middle pointing at center field.. or a stereo pair in the middle instead of the single if you like (which also hedges the bet).

[Gutbucket]

I found this paragraph from DPA especially interesting, as it is the only explanation I've found for keeping the spacing relatively close (emphasis mine):

Since the stereo width of a recording is frequency-dependent, the deeper the tonal qualities you wish to reproduce in stereo, the wider your microphone spacing should be. Using a recommended microphone spacing of a quarter of the wavelength of the deepest tone, and taking into account the human ear's reduced ability to localize frequencies below 150Hz, leads to an optimal microphone spacing of between 40 and 60 cm. Smaller microphone spacings are often used close to sound-sources to prevent the sound image of a particular musical instrument from becoming "too wide" and unnatural. Spacings down to 17 to 20 cm are detectable by the human ear, as this distance is equivalent to the distance between the two ears themselves.

I'm not sure I'm onboard with the justification based on "taking into account the human ear's reduced ability to localize frequencies below 150Hz", party because I feel an appropriate spacing has as much to do with tone as it does with directional imaging.  That seems an attempt to combine to separate realms, a tonal one based on the quarter wavelength constraint, and an imaging one based on human directional listening acuity.

So another thing about omni spacing, which has perhaps as much to do with the overall sound of the configuration as it does with its "stereoness" is the effect of spacing on what I'll call the "contour" or "tone" of the low frequency response.  Shifting omni spacing while listening at the same time,  a change in bass "contour" is one of the primary things I hear, along with the sort of increasingly open "ambience" and "air" as spacing is increased.  I suppose it's the first ripples of the comb-filter cancellation/reinforcement shifting around, with the mic spacing playing against the frequency wavelengths in question.  The approximate 1 meter / 3ft spacing I use as a baseline starting point for a pair of omnis alone was arrived at primarily by listening to the tone and contour of the bass along with the "stereo openness" while listening on headphones and adjusting the spacing.  Although the spacing adjustment was also affecting SRA width, I consider that 'tuning' to have been more like making a sort of EQ adjustment, which I suspect would carry over to other scenarios more so than if I was simply tuning the SRA to adjust image width.  In that situation, things changed significantly and rapidly when shifting between narrow spacings of up to a couple feet, and seemed to open up and settle down frequency-wise at spacings of more than that but less than 5 feet or so.  There was plenty of bass information playing while doing that (it was in an amphitheater with a couple different band playing), so it may be more difficult to do by ear while adjusting spacing for a marching band.

[Bruce Watson]

A very wide omni spacing can achieve that aspect of "stereoness" and envelopment down low.  A wide spacing also decorrelates the hall ambience and reverberance down to a lower frequency, which makes things sound more "open and airy" and reproduces those sounds diffusely, throughout and even outside of the playback imaging space. That's a different kind of stereo thing than left/right imaging and doesn't relate to Williams SRA curves. It's not about accurate left/right imaging directionality or orchestra angle.

I'm thinking that this is the point in using an array such as the Faulkner / Boojum / JNorman, with outriggers. The former for the upper bass and higher, the latter for the low bass and to decorrelate the hall ambience and reflections. This is why, perhaps, Decca always used outriggers with the many variations of the Decca Tree. Maybe it's part of the reason anyway, IDK.

[voltronic]

Consider that the appropriate SRA for recording a marching band may or may not be equal to the Orchestra Angle into which the outer edges of the band's marching movements will fit.  I suspect a narrower SRA will be better, zooming into the action closer to center field a bit.  The outer limit edge of field stuff will then simply image over at one speaker or the other, with more band movement between speakers.  I suspect an SRA wide enough to cover the entire field would provide a more distant perspective with less dramatic, more centralized.

How much of the field do they actually use?

Do you have a way of recording three or four channels?  Would be interesting to do a 10 or 20ft omni split atop the press box with a single directional mic in the middle pointing at center field.. or a stereo pair in the middle instead of the single if you like (which also hedges the bet).

They generally use the area between the two 20-yard lines, so your suggestion of a narrower SRA makes sense.  Playing around with the Sengpiel tool, if I assume an orchestra angle of 60deg, it looks like somewhere between 60cm and 1m may be the ticket here.

I could do 3 or 4 channels, but at the present I only have 2 windscreens up to the task and 2 superclamps.  It's something I might consider down the road though.

[2manyrocks]

You could gaft tape your omnis to the rail and use some fuzzy fabric from the fabric store as a windscreen freeing up your clamps for a center card.  Or you can buy a $2.00 clamp from walmart or TSC if you have one and drill a 3/8 hole in it for a 3/8 bolt to make a useable mic clamp.  Or get the school welding shop to weld a 3/8 bolt on the clamp. 

I'd have reservations about using only omnis, but of course will be interested in your results.  How much boundary effect you get might depend on the distance from the railing to the wall of the press box. 

 

 

[voltronic]

You could gaft tape your omnis to the rail and use some fuzzy fabric from the fabric store as a windscreen freeing up your clamps for a center card.  Or you can buy a $2.00 clamp from walmart or TSC if you have one and drill a 3/8 hole in it for a 3/8 bolt to make a useable mic clamp.  Or get the school welding shop to weld a 3/8 bolt on the clamp. 

I'd have reservations about using only omnis, but of course will be interested in your results.  How much boundary effect you get might depend on the distance from the railing to the wall of the press box. 

 

 

Before I go into more channels, I really do want to try the omnis on their own at field level and also up on top of the press box.  My goal is also to keep this setup small and quickly deployed from my bag.  4 mics makes that more difficult.  I have two 6ft, rugged cables Darktrain made me for this purpose, and all my other cables are 25' snakes pairs which is just silly for this application.  I know that I can get good results from my CM3s up close; less way back on top of the box.  I want to experiment to see what I can get out of just the pair of omnis alone at different distances and widths before I decide to add anything else in.  These are all good ideas though, and I appreciate it.

The railing is on the roof of the press box, so no worries about boundary effect - there's nothing behind me but empty space.  In a couple weeks when I get up there, I'll try to take some pictures, and if I get a good capture I'll post it here on the marching band thread I started last year.

[MIQ]

Hi Voltronic,

I have been following this thread but haven't had a chance to chime in yet.  I wanted to add a little more info on the combing due to spaced mics.  I was looking at this last month and made a spreadsheet to study it.  Attached are some screen captures of the sheet I created that will illustrate the basic trade-offs. 

The fundamental idea is that the delay between to the two mic signals determines the freqs where the notches appear.  Two factors determine this delay, the distance between the mics and the angle of the sound source with respect to the microphones.  At any spacing, if the sound source is directly in front of the two mics (Theta = 90 degrees on my sheet), the arrival of the sound source at each of the mics will be the same and there will be no delay.  As you move further and further to one side or the other (off axis) the delay will get bigger and bigger since the sound will be arriving at one mic sooner than the other.  Obviously, the worst case angle is directly off to one side (Theta = 0 or 180 on my sheet).  As this delay gets bigger and bigger, the freq where the first notch occurs gets lower and lower in freq.  It is important to realize that this combing notch is at a different freq for all angles in front of the mics.  As Gutbucket mentioned, depending on where the low freq sounds are in relationship to the mics, there will be differences in the low freq contour created by the first notch.  Playing with the spacing allows you to change this bass contour, with larger spacing pushing this first notch lower and lower in freq.  Please take a look at the attached pics that illustrate these effects. 

When I created the sheet I limited the number of freq points where I graph the notches to keep the file size and calculations to a reasonable size.  Since I am not including every possibe freq in the chart, and the notches themselves are so narrow, the graph doesn't always "catch" the true minima of each notch.  The graph shows the basic location of the notches but may not give an accurate "depth" to each notch.  All the nothces should theoretically have the same amount of cut but my chart will not always show that.  It is an artifact of limiting the number of different freqs.

The attached show that as the angle of the sound source moves more off axis the freq of the first notch goes down in freq.

[MIQ]

In this set of pics I kept the sound source angle at 30 degrees and changed the spacing between mics.  Here you can see that the larger mic spacing will increase the delay between mics and again push the first notch down in freq. 

I started at 17cm (ear spacing) and went up to 70cm (beginning of "hole in the middle" imaging in many set ups). 

As others have noted, very wide spacings (3-5 ft) are often used not so much for accurate imaging location but to create very uncorrelated L/R signals even at very low freqs.  These wide spacing will push these notches even lower in freq.  Not that that is a huge deal but you may need to dial in the spacing so the notches do not lead to problems with the angles that the bass sound sources arrive from.

As far as the SRA and the visualizers (including the Stereo Mic Tools app I helped desing), these are all relying on the human psychoacoustic perception of sound localization.  This localization is based on the time and level differences that occur at your ears.  Since the signals recorded by both Omni mics will be the same level regardless of the sound source angle, all the imaging is created by the time differences.  Depending on who's psychoacoustic data you choose to work with (Williams/Simonsen, Sengpiel, Wittek/Theile, etc.) you will arrive at different SRAs based on the mic spacing.  Stereo Mic tools uses the Williams/Simonsen data for spaced mics to calculate the SRA but uses the Wittek/Theile data for calculating the RA_75%.  As has already been discussed, these SRA calclations will work for small mic spacing where the delays created fall within the narrow range used by our brain for determining source location but will fall apart at very large mic spacings.  Again, not a big deal since very wide spacings are used to create very uncorrelated signals even at low freq giving a sense of a very large ambient sound space, not neccessarily an accurate sound location.   

Hope this helps give a little clearer pic of signal delay created comb filtering. 

MIQ

[Gutbucket]

A couple important notes-

The comb-filtering seen above is what happens when the two signals are combined electrically.  In other words, mixed together to form a single monophonic signal.  This is the basis upon which 2-channel stereo recorded using spaced omnis is considered to be less than optimally mono-compatible.  Of course, in reality a recording made using spaced omnis is perfectly mono compatible by simply eliminating one channel and only using only the other-  "wa-la" we then have a single monophonic omnidirectional channel with zero comb-filtering, from a single microphone placed just slightly off the original center-line.

That clear and distinct comb filtering does not manifest in the same way when the recording is played back normally in 2-channel stereo.  It will not manifest at all with headphone listening since the signals don't mix at all until they are in the listener's brain.  With loudspeaker playback, the two signals mix only upon reaching the listeners ear(s), the sharp nulls go away, and the comb-interaction will be different at each place in the 3-dimensional space of the listening room, as well as being different for each frequency in question.  Regardless of the specific location of the listener's ears in the room relative to the speakers, there will be no sharply defined comb cancellation notches as seen in the graphs above, that only happens when the two signals are mixed electrically.  At anything above the first notch (which shifts around by both frequency and position as noted) the combing interactions become so complex they essentially become random.


There are going to be level differences in most widely spaced omni setups.  Any source which is close enough to the mics and not on the centerline is going to produce level differences as well as arrival time differences in the two channels. 

For spaced omnis out in the audience, the level differences will be mostly nearby audience sounds, and one reason I like wide-spaced omnis of 5' or 6' is that the sounds from nearby audience members will not be recorded with the same level in each channel unless they are directly in front or in back of the mics.  Because of that, those "unfortunately too close and unwanted" sounds image off to one side or the other, "getting out of the way" of the music imaging across the center between the two speakers.  It's still there, but becomes less distracting and offensive, easier to mentally separate from the music and ignore.

For wide-spaced omnis placed at the stage-lip or on-stage, the increased proximity of the various sound sources on stage to one or the other of the two microphones will produce level differences in addition to time of arrival differences.


Even quite wide-space omnis isn't going to decorrelate the lowest frequency information.  The spacing may be wide enough at those frequencies to produce some stereo difference phase information, but to be decorrelated, the signals need to have a difference of more than one full wavelength.  The first cancellation notch in the graphs appears at a frequency where the phase difference between the two signals is exactly half a wavelength apart (the phase difference = 180 degrees).

[MIQ]

Thanks Gut, those are all important notes.

While the graphs above do not apply directly to stereo playback at all spots in the room, the fundamentals are similar. The acoustic summing will cause combing based on the delay between the two channels arriving at the listening position in the room.  For listeners sitting in the center line of the playback system this delay between acoustic signals will be dominated by the delay between the recorded electrical signals however room reflections will also complicate the scene. There will be low freqs where these acoustic signals will combine destructively. Of course the combination will never be as destructive as the theoretical worst case (like the electrical combination).  Also, as you noted there will be so many reflections and complex interactions at high freqs that it becomes random and not clearly defined.   You can see how bunched up the combs get at high freqs. Due to the abundance of reflections at high freq this region of the room response becomes more "statistical" and is hard to predict or simulate.  You get a ton of reflections combining acoustically.

Miq

[voltronic]

More food for thought.  This has turned into a very interesting discussion.

MIQ, would you consider developing an Andriod version of your app?  I have been waiting for one to be released for quite some time, and it doesn't appear to be on the horizon.  I use a combination of these apps to get similar results, but yours is a much more efficient solution:

AR Bearing + Baseplate Compass
Protractor
Neumann Recording Tools

This Friday night, I plan to run my omnis for the first marching band show of the season, and I'll be on top of the press box as I mentioned earlier (estimated included angle of 60 deg).  My plan is to go with a 1-meter spacing for this first run, unless you all here think that's going to be too narrow a spacing / too wide an SRA.  If I get the opportunity to get up there again, (which may or may not happen) I'll try 2 meters for comparison.

For competitions through the season I'll be close, either on field level or in the first row of stands.  My starting point there will be 60cm (estimated included angle of 110-120 deg).  The ensemble balance is going to be somewhat worse because of the low height, but that will be balanced by the more optimal placement distance to the band.  A tall stand isn't possible for these events, nor would I risk it with the winds we tend to get.

[Gutbucket]

My plan is to go with a 1-meter spacing for this first run, unless you all here think that's going to be too narrow a spacing / too wide an SRA.  If I get the opportunity to get up there again, (which may or may not happen) I'll try 2 meters for comparison.

Sounds good to me.

A tall stand isn't possible for these events, nor would I risk it with the winds we tend to get.

You'll get some welcome bass reinforcement from having them lower to the ground instead of up high.  Just watch out for spectators in close proximally.

[MIQ]

More food for thought.  This has turned into a very interesting discussion.

MIQ, would you consider developing an Andriod version of your app?  I have been waiting for one to be released for quite some time, and it doesn't appear to be on the horizon.

Hi Voltronic

I get that question a lot. Unfortunately, the other half of the Stereo Mic Tools development team (Rob) is not set up to code for Android. He is strictly iOS for the forseable future. I was piecing together iPhone apps similar to you before we put together Stereo Mic Tools. 

I'm sorry we can't help you. How about buying a used iPod touch for cheap so you can run our app? 

MIQ

[Colin Liston]

i'm a big fan of spaced anything.

we've run "split stack" with cards before.  one mic on the left stack and one on the right stack.  came out amazing.  really picked up the stereo mix well and you hear one fan in one ear and not in the other too.  interesting listen.

if runnig ominis back by the board...  why not space them out as far as you can?  one mic/stand on the right side of the board, one on the right, both pointed at the respective stacks?  same for cards, hypers, whatever...

it's the best stereo config you could do, if you have the gear....

my split cards:  https://archive.org/details/dbt2005-03-02-dpa4023.flac16

[Gutbucket]

Colin, you did a similar straight split cards at the Suwannee a year or two back which I remember sounding quite nice.  Sounded very similar to my 6' split omnis from the same location there.  How big was that split? like 8' or so?

[DSatz]

A late entry to this thread: If the two schools of thought are ~3 feet apart vs. ~1 foot apart, times have really changed since I was first learning to record--and very much for the better as far as I'm concerned. If you look in any American-published books or magazine articles from the 1960s and 1970s about stereo recording, the "cookbook" formulas were exactly two: One was to take the width of the stage, divide it by three, and place one omni at each of the lines of demarcation (i.e. 1/3 of the way across and 2/3 of the way across respectively). The other was to take a pair of cardioids and aim them with the heads pointing together at a 90 degree angle.

Both are perfectly dreadful ways of recording music in stereo, and are especially regrettable because with the same microphones, it's so easy to do so, so much better.

But as far as omnis are concerned, the more enlightened texts used to distinguish between "small" A/B, which was the European style, and "large" A/B, which was the American style as described above. Back then, the two encampments were pretty much mutually exclusive. I remember being shocked that small A/B could work at all, since the 1/3 and 2/3 rule was the only approach I ever saw used; with anything less than that wide spacing, I couldn't believe that you would "get enough separation" for stereo until I tried it myself.

By the way, this included the fellow who used to record our high school band back in those years, as well as the Honors and State bands in Western Pennsylvania that I played in: a Mr. Renner who ran a company called "Century Recording Service" just on the other side of the Ohio state line. He later became better known as the chief engineer for Telarc Records. While I'm happy for the Grammy awards he received (and the fact that he mainly used Schoeps microphones), most of the famous Telarc orchestral recordings have a huge hole-in-the-middle problem; once it's pointed out, you can't not hear it.

[Gutbucket]

my split cards:  https://archive.org/details/dbt2005-03-02-dpa4023.flac16

Thanks Colin. That exemplifies the weird extenuating circumstance of recording live, PA-amplified music from out in the audience.  From the source description: "mics split 24' and hung from each side of the balcony approximately 4' from each stack".  Less extreme yet conceptually similar to the hypothetical situation described previously of one PA speaker and microphone a mile apart from the other, yet still producing a predominantly monophonic signal from the PA, common to both channels.

[voltronic]

A late entry to this thread: If the two schools of thought are ~3 feet apart vs. ~1 foot apart, times have really changed since I was first learning to record--and very much for the better as far as I'm concerned. If you look in any American-published books or magazine articles from the 1960s and 1970s about stereo recording, the "cookbook" formulas were exactly two: One was to take the width of the stage, divide it by three, and place one omni at each of the lines of demarcation (i.e. 1/3 of the way across and 2/3 of the way across respectively). The other was to take a pair of cardioids and aim them with the heads pointing together at a 90 degree angle.

Both are perfectly dreadful ways of recording music in stereo, and are especially regrettable because with the same microphones, it's so easy to do so, so much better.

But as far as omnis are concerned, the more enlightened texts used to distinguish between "small" A/B, which was the European style, and "large" A/B, which was the American style as described above. Back then, the two encampments were pretty much mutually exclusive. I remember being shocked that small A/B could work at all, since the 1/3 and 2/3 rule was the only approach I ever saw used; with anything less than that wide spacing, I couldn't believe that you would "get enough separation" for stereo until I tried it myself.

By the way, this included the fellow who used to record our high school band back in those years, as well as the Honors and State bands in Western Pennsylvania that I played in: a Mr. Renner who ran a company called "Century Recording Service" just on the other side of the Ohio state line. He later became better known as the chief engineer for Telarc Records. While I'm happy for the Grammy awards he received (and the fact that he mainly used Schoeps microphones), most of the famous Telarc orchestral recordings have a huge hole-in-the-middle problem; once it's pointed out, you can't not hear it.

My earliest education in recording technique also was having the classic 3-to-1 rule hammered in to me.  I think it works quite well for PA applications, such as when arranging hanging area mics for a choir or theater production.  The "small" A/B is something I've only come across recently, and it was so against everything I was taught that I also was shocked at how good it could sound.

Out here in Eastern Pennsylvania we still have the same problem with professional companies who are recording our marching band and drum corps shows.  Almost every single time, I see X/Y center, with omni flanks that are 30 yards (not feet) apart.  They cover the field I guess, but the imaging is, for lack of a better term, strange.  Here's an example of this, with my favorite pro corps:

https://www.youtube.com/watch?v=4xsVnAEwVDE

At 2:58 you can see the center array, but the mics appear to be pointing straight ahead?!?!

 

[Gutbucket]

^That's quite a production.

Hard to tell from the low res video, but looks like a near spaced pair in the center, and maybe a near-spaced pair at all three mic locations.  Regardless, I definitely hear a hole in the middle.

3-to-1 rule is different than the old 1/3-2/3's positions across the front of the orchestra omni spacing thing.  But that doesn't keep it from being commonly suggested as appropriate for a stereo pair of omnis, even though it's impossible to apply to stereo mic'ing.  Two microphones cannot be three units of measure apart from each other and one unit of measure from the same source, it's simply geometrically impossible.  I never thought about it before, but I suppose the 1:3 and 1/3-2/3 numbers are similar enough that the two 'rules' become prone to confusion even though they address entirely different things.


I've blathered way too much about arcane technical aspects in previous posts.  Pressed for a quick & dirty guideline of omni spacing for stereo recording, I'd probably sum up my approach like this:

1'-2' with some sort of baffle (disk, sphere, head, or body) in between
3' with nothing in between
6' with another mic in between
More than 6' may work well in certain situations common to TS for the reasons mentioned, but gets riskier, is probably at the point of diminishing returns, and may become harder to setup in a practical sense.

[voltronic]

I posted the results of the 1-meter omnis on the press box here: http://taperssection.com/index.php?topic=169429.msg2158669#msg2158669