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Author Topic: Why Blumlein sounds more spacious than other coincident or near-so arrays  (Read 31629 times)

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Offline boojum

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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #15 on: October 16, 2008, 01:34:41 AM »
I help myself to "visualize" what the sound is doing by thinking of it as light, and the walls running from black felt to mirrors.  That way I can more easily understand where the sound is going as I am more used to visualizing light and how it plays than I am a sound.  YMMV.

I hope to soon build some sort of mic "thingy" to make a Schoeps DMS array again.  It also allows infinite fiddling with mic placement and type, from omni to figure 8; from two mics to five mics and with varying angles.   All pretty sweet, and controllable in post.  This is the charm for me.  I am not sure that the SDMS would be as forgiving of mic placement as the SF, though.  Hopefully I will get off my dead ass soon and do more than just talk about this.

This is an interesting thread.  GB, thanks for pumping up the tech and intellectual level a notch or two.      8)
Nov schmoz kapop.

Offline Gutbucket

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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #16 on: October 16, 2008, 10:44:06 AM »
Here's another paper about reverberation and image solidity.  It basically recommends 90 degree separation for figure-8 and 120 degrees for cardioid, based on the most equal distribution of reverberation across the soundstage.
http://www.audiosignal.co.uk/Resources/Stabilising_stereo_images_USL.pdf

Thanks for the link Hugh, I'll give that a read. I've enjoyed reading about your experince with the tetramic and hope to read more.

Thanks to you too boojum, I think you might enjoy using one of these: Head Visor- Mic array & analysis device with video output to 'see' sound.
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Offline BayTaynt3d

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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #17 on: October 21, 2008, 12:17:36 AM »
Suppose I have a narrow room, and I put an ORTF or XY pair about halfway back.  I often feel that I'm getting "more wall" than I'm getting sound from the band. 

I usually go with a narrower angle and wider spacing in that situation, heh...
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Offline Gutbucket

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As I mentioned previously in this thread, most of the above considers only sound on the horizontal plane. Linkwitz only looks at the horizontal plane as I recall, yet reverberant sound comes from all directions.  When considering the entire 3-D pickup of a stereo pair in a real room, that 48% mono reverberation figure for ORTF would be much higher.

This spring I ran across an excellent paper by Geoff Martin that he's made available on his website which goes further in depth: General Response Characteristics of Microphone Configurations. The enlightening parts for me were the sections with 3-D analysis of both individual mics and of stereo pairs.  In most cases, the farther the source of sound is above or below the horizontal plane of the mics, the more it is recorded as mono instead of stereo information, and that happens surprisingly rapidly.
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Offline Nick's Picks

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M-S and Blumlein share an important aspect that adds to their wonderful 3D sound., and that is the mixing of the OOP lobes of the F-o-8. 

Offline Gutbucket

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I think the primary contributor is the figure-8 pattern itself, specifically it's null plane, which reaches not only to either side of the the mic, but above and below it as well.  That vertical aspect of the null aligns with the maximum point at which sound is always recorded as mono information with any intensity based mic setup - directly above and below the mics.
« Last Edit: July 09, 2009, 10:46:40 AM by Gutbucket »
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
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)

Online DSatz

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The main reason for the very nice sense of spaciousness which can sometimes be obtained with Blumlein stereo (especially in a good room, and with microphones having adequate low-frequency response) is its very low level of correlation between channels at low frequencies. No other coincident or near-coincident setup has this characteristic to such an extent.

--best regards
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Offline Gutbucket

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..That vertical aspect of the null aligns with the maximum point at which sound is always recorded as mono information with any intensity based mic setup - directly above and below the mics.
The main reason for the very nice sense of spaciousness which can sometimes be obtained with Blumlein stereo (especially in a good room, and with microphones having adequate low-frequency response) is its very low level of correlation between channels at low frequencies. No other coincident or near-coincident setup has this characteristic to such an extent.

I always appreciate your insights David, thank you.  Would it be fair to say that the above two statements are basically talking about the same aspect, though specifying that the low frequencies are where the perceptual impact is most important?  High correlation between channels would seem to me to be basically equivalent to mono information.  I'm wondering if I'm missing any subtleties of definition here or just describing the same things more awkwardly.
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
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)

Online DSatz

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Mr. bucket, it's certainly true that if the signals in both channels of a recording were fully correlated, it would be mono. It's also true that coincident stereo recording techniques pick up sound from directly above or directly below the microphones in mono, to the degree to which the microphones are sensitive at that angle of incidence. If a Blumlein pair is arranged with its microphones' main axes in the horizontal plane that sensitivity will be near-zero, and that's a good thing e.g. in a room with a low ceiling.

But that's not what I'm talking about. Actually I'm not sure whether you were asking why the correlation tends to be lower with a Blumlein arrangement, or why the lower correlation of the Blumlein arrangement aids the sense of spaciousness. The former is just a matter of pattern geometry--each lobe of a figure-8 is narrower than the [front] lobe of any other first-order pattern. If you consider sound sources that are moving farther and farther away from the center line in either direction, the microphone pointing away from the sound source is losing sensitivity faster and the microphone pointing toward the sound source is gaining sensitivity faster with figure-8s than with any other first-order pattern.

Empirically, we impute spaciousness to representations of sound in which there are significant differences between left and right at low frequencies. (That's something I learned from David Griesinger; when I get a chance, I'll try to find which of his papers make this case the most clearly.) The ads for mono subwoofers have said for years that we don't localize accurately at low frequencies, and that's true enough--but it certainly doesn't mean that we get no audible benefit from having difference energy between the channels at low frequencies. If anything, it would often be desirable to place stereo (sub)woofers farther apart than the midrange and tweeter elements of a playback system.

Just as a side-note, this particular superiority of Blumlein over coincident cardioids is compounded when switchable-pattern microphones are used, since nearly all of them have dual-diaphragm capsules, and those can never be true cardioids at low frequencies; instead, they become wide cardioids. As a result the correlation between channels increases at low frequencies just where it needs to decrease; the pickup becomes more and more nearly mono. Back in the LP era that was a fine thing, since LPs with a lot of low-frequency difference energy can be hard for a needle to track without skipping, but this is not a concern that digital recordings need to bow down to.

--best regards
« Last Edit: July 24, 2009, 05:07:43 PM by DSatz »
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Offline illconditioned

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I always thought the reason was that Blumlein capture both front and back sounds, and the back sounds make the recording sound completely natural, hearing all sounds, as an omni mic does.  So you get the "complete" image of an omni mic, with the directionality of the Blumlein pattern.

Hmm.  What I don't like about coincident techniques is you throw away phase information.  I wonder if you could take Blumlein configuration and just split the mics by about 15cm, while leaving the directions the same.  This might give you the best of all worlds: directionality, phase information, and each mic hearing both front and back.

  Richard
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Offline Gutbucket

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Thanks Richard, you got me thinking through this again this morning. 

As you say, Blumlein array is equally sensitive to sounds arriving from any horizontal direction.  I agree that's one aspect that lends a certain naturalness of capture. That along with other non-stereo based perceptions of spaciousness which come into play may be what your are considering.  For example, in the monophonic case, an single omni will pickup reflected sounds arriving from all directions and those sounds give us information about the size and shape of the room the mic is placed in.  A single more directional mic attenuates those off-axis sounds and sounds less spacious in the monophonic sense, trading ambient reflection information for more direct pick-up.  There may be other aspects that also lend a sense of spaciousness as well, but what I'm mostly referring to here is a sense of stereophonic spaciousness.

Consider that two coincident cardioids angled 180 degrees apart are also equally sensitive to sounds arriving from any horizontal direction as a stereo pair, just like Blumlein.  So are two coincident omnis.  But the degree of perceived spaciousness decreases with each case.  It's the difference information between the two signals that lends that stereophonic sense of spaciousness.   In these coincident examples that difference information is comprised solely of intensity differences.

Once I space the mics apart I get time of arrival differences as well, depending on the direction of the sound source.  There is no reason you cannot space a pair of figure-8 mics, but in doing so you may want to adjust the angle between them at the same time.  It's the Michael Williams Stereo Zoom thing- the relationship of pattern, angle and spacing between mics.  If a Blumlein array has a SRA (the Stereo Recording Angle of pickup in which your source is located) of around +/- 40 degrees, then spacing the mics without changing the angle between them would narrow that SRA further.  To keep the same +/- 40 degree SRA while introducing some time difference information you'd narrow the angle between the mics as you space them further apart.  For your 15cm suggestion, that angle would be about 62 degrees (see the chart below). Continue on in that way and you'll eventually end up with two figure-8 mics spaced about 60cm apart facing directly forward and using only time differences for stereo pickup, acting like spaced omnis in that aspect.  Of course they'll sound different for a number of reasons, a big one being that they will reject all sounds arriving from the sides (and form above and below) which might be useful.  The Tony Faulkner array of two forward facing figure-8s is this type of setup, though he mentions a closer spacing between mics which would widen the SRA from the above example. Stereo Zoom is just one approach, but conceptually I find it helpful. One detail to consider is that for anything thing between the Blumlein and fully forward facing configurations, sounds arriving from the rear of the array will have contradictory time and intensity differences.  I don't know if that would actually make any difference or not in a real situation.

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Offline Gutbucket

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..Empirically, we impute spaciousness to representations of sound in which there are significant differences between left and right at low frequencies. (That's something I learned from David Griesinger; when I get a chance, I'll try to find which of his papers make this case the most clearly.)..

Seems many of David Griesinger's papers address spaciousness and low frequency differences, a common theme in his research. One in particular that I'm aware of is his Vancouver in 2005 AES paper: “Loudspeaker and listener positions for optimal low-frequency spatial reproduction in listening rooms” another is "General overview of spatial impression, envelopment, localization, and externalization"  Another is titled "Spaciousness and Localization in Listening Rooms - How to make Coincident Recordings Sound as Spacious as Spaced Microphone Arrays" which would seem to apply directly to this thread (it's been a while since I read these so I'm fuzzy on the exact content and which best applies most clearly)

He has generously made his papers available on his website. It's a virtual gold mine there.
« Last Edit: July 29, 2009, 01:23:01 PM by Gutbucket »
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
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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..The ads for mono subwoofers have said for years that we don't localize accurately at low frequencies, and that's true enough--but it certainly doesn't mean that we get no audible benefit from having difference energy between the channels at low frequencies. If anything, it would often be desirable to place stereo (sub)woofers farther apart than the midrange and tweeter elements of a playback system..

That concept taken to the logical conclusion leads to something like the diagram below, optimized for a single listening position. More information and photos on that at this web page on Optimal Source Distribution at the Institute of Sound and Vibration Research at the University of South Hampton, England.

[edit- I seem to have a hard time getting this image to display properly, you may need to click on the linked JPG below to view it]
« Last Edit: July 29, 2009, 01:32:31 PM by Gutbucket »
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Offline Gutbucket

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Following that tangent, I played around with a quick and dirty DIY crosstalk cancellation playback setup and ended up incorporating those ideas several years ago.  It was an interesting ear-opening experiment.  I sat on the floor with my back against the wall and a sheet of plywood held to my face as a baffle to block the cross-talk from each speaker to the opposite ear.  I was using small computer speakers which as small point sources worked better than my bigger 3-way driver stereo speakers.  To increase their piddly bass range I added my regular stereo speakers to either side, facing away into couch cushions as a simple low pass filter.

It was very interesting to hear the middle solidly anchored by the center placed speakers and the phantom image spreading out to the sides and deep into the room with stereo information.  Sort of stereo reversed.  I found that solid center with phantom width much more natural to listen to.  After all most sounds we record are primarily in front of us.  The stereo image width spread much farther than a typical stereo triangle, probably due to the strong cross-cancellation of the baffle. In effect it created a giant externalized set of headphones that eliminated all the headphone problems.  The fidelity was ultimately limited by the quality of the computer speakers and the practicality of the setup was obviously pretty much non-existent.  One listener at a time only, but it was fun and I learned a lot from it.

My amazingly understanding female partner literally fell to the floor laughing when she walked in the door and saw me doing this.  The only practical way to get away with this in the living room is to have the house torn apart when moving in or remodeling!  Luckily I was doing both.  ;)

Here's a little diagram I drew up to explain it to a friend-
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
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 notlance

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I've done some playing around ("experiments" is too good a word) with ORTF figure-8s, and everything in between.  Let me explain.  I have a couple of Sennheiser MKH800 Twin mics which I've used in an ORTF setup.  I record all four channels and thus I have control over the polar pattern in post.  So I can get anything from 17cm spaced omnis to ORTF figure-8s.  What I have heard so far is not definitive; the figure-8s do not sound much better (or even much different) than say, the cardioid pattern.  The main difference is the width of the stereo image, as may be expected.  That being said, most of the time I do pick the figure-8 pattern for the final mix.

Up to now I've tried both ORTF and coincident configurations, not wanting to confuse myself even more by introducing other mic spacings.

I still need to do more work to eliminate some variables in my listening sessions.  For example, I need to find out a way to change the polar patterns in real time without changing the overall sound level.  Maybe then I can come to some conclusion as to which mic spacing/angle/polar pattern I prefer.

 

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