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Author Topic: Why Blumlein sounds more spacious than other coincident or near-so arrays  (Read 31630 times)

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Offline Gutbucket

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Blumlein is an odd child in that it seems the only coincident array that really captures space well to my ear.   Of course the imaging is entirely different, but in the spaciousness/ambience department, Blumlein sounds almost more akin to A-B spaced omnis than other coincident or near coincident patterns.   I found some Michael Williams AES papers recently and some new material on Stan Linkwitz's website that have helped me understand the technical reason for why it sounds that way to me. It appears that other than spaced omnis, Blumlein has the highest proportion of stereo reverberation vs mono reverb on playback - meaning the proportion of reverberation that is spread between the playback speakers and not reproduced as a mono source from one speaker or the other. Not to be confused with the direct to reverberant ratio which changes with different patterns and microphone proximity to the source. 

Stan Linkwitz's analysis is here. Of the entire 360degree ambient field, an ORTF arrangement reproduces 48% of that reverberant field as effectively mono, 'hard-panned' to the speaker positions (48% appears to originate from one speaker or the other on playback), where Blumlien reproduces just 30% of the reverberant field that way, the remaining 70% of the reverberation is stereo information spread between the speakers.

Here are his polar diagrams (pickup areas reproduced as mono reverberation are marked in green), for ORTF-

and for Blumlein-


Edit: Here's the significance of the polar plots- All the sound which arrived at the microphone position from within the angles highlighted in green will be reproduced effectively in only one speaker or the other. Reproduction of those regions will be bound to the speaker locations as opposed to emanating from somewhere between them.

Of the limited number of arrays he takes a look at, Linkwitz mentions that the rear lobe of surpercardioid pattern mics allows them to pick up more stereo reverberation than cardioids and extends this observation to figure-8's, noting the ORTF vs Blumlein stereo reverb percentages I quoted in bold above.  Yet he stops short of endorsing the Blumlein array because he is concerned about the folded and reversed imaging of the side and rear pickup quadrants. The supercardioid array he suggests mixes in an additional pair of spaced omnis for reverberant pick-up farther back in the hall. 

Interestingly, Williams suggests intentionally introducing those folded side and reversed rear quadrants for non-Blumlien, multi-channel recordings that are mixed to stereo in his recent 2005 (& 2007) paper below. At the end of the 2005 paper on acoustic cross-talk in multi-microphone arrays, he outlines a possible improvement on the 2 microphone to 2 channel stereo selection by mixing in the 2 additional rear channels in what he calls a twisted-quad mixing scheme that looks quite similar to a the rear-lobes of a Blumlien configuration from the reverberant pickup aspect, but avoids the phase reversed sides.  Doing so spreads the reverberation and reflections arriving at the mic array from each quadrant of the recording space between the two speakers instead of collapsing everything from the far sides into one speaker or the other, but also reverses the back image in a way similar to Blumlien. The 2007 paper goes on to explore mic array design for inter-format compatibility (using one array to simultaneously record for mono, 2 channel stereo, 3, 4, and 5-channel formats) and suggests using twisted-quad mixdown to improve on the 2 channel version.

Also consider that everything above only addresses the horizontal plane (though Williams goes farther in his 1991 paper below to analyze the full 3-D space) and although the mono/stereo reverb ratio of a Blumlein pair shouldn't change with elevation, there is still a sensitivity roll-off and null above and below the mics. An omni pair with perhaps similar mono/stereo reverb proportions would be equally sensitive to reverb, early reflections, etc from those quadrants.

Michael William's page with preprints of his papers and link to Stereo Zoom and Multi-Microphone Array Design tools. 
In particular-
1991 : 91st AES Convention in New York - Preprint 3155
« Early Reflections and Reverberant Field Distribution in Dual Microphone Stereophonic Sound Recording Systems »

2005 : 118th AES Convention in Barcelona: Preprint 6373
«The Whys and Wherefores of Acoustic Cross-talk in Multichannel Microphone Array Design »

2007 : 122nd AES Convention in Vienna - Preprint 7057
«Magic Arrays – Multichannel Microphone Array Design Applied to Microphone Arrays Generating Interformat Compatability»


Stan Likwitz's page on recording for stereo.
« Last Edit: June 10, 2013, 11:33:43 PM by Gutbucket »
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Offline Will_S

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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #1 on: September 30, 2008, 03:36:02 PM »
I think a similar argument could explain why I prefer M/S to cards XY, and why I often like hypers.  There too you have directional rear lobes pointed off at angles and should get more of the reverberent field in stereo.

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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #2 on: September 30, 2008, 03:47:35 PM »
I think that Blumlein sounds really nice in near field applications, but rarely have I ever heard a Blumlein in far field recording that did not have reverse polarity artifacts from the rear lobes that detract from it.

Since the front and rear lobes are combined in traditional Blumlein recordings, I find them to capture far too much of the room, versus direct signal in PA recording, but that is the only experience I have with them. I do think that today, with a Soundfield, one could make a Blumlein recording in B format and mix it where each lobe is represented by a separate channel, and when played back on a surround system, would replicate the physical space around the mic when the listener is seated in the center of the 4 speakers.

I can think of 2 examples of Blumlein that I experienced, one positive and one negative. In the spring of '94, I patched out of my friend Ronnie's Soundfield at the Phish Beacon run where he ran Blumlein. The bass slap on the back wall, as well as the crowd energy from behind made this a poor choice of patterns, IMO.

In contrast, that summer he ran at the GD RFK shows and in the open air, it was far more pleasing. However, you can still hear more reflections from the rear half of the stadium than an adjacent MK21 recording I have from the same shows.

Application is everything with mic patterns, as PA taping in the traditional sense usually benefits from not accurately capturing the space, at least indoors.
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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #3 on: September 30, 2008, 03:55:16 PM »
only time i've run blumlein was June of this year for WSP.  tape came out really nice and clear, but it did lack some low-end, which I had to add in post.  still an enjoyable tape, even if the high end is kinda of grating at times.
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Offline Gutbucket

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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #4 on: September 30, 2008, 04:42:22 PM »
I think that Blumlein sounds really nice in near field applications, but rarely have I ever heard a Blumlein in far field recording that did not have reverse polarity artifacts from the rear lobes that detract from it.

Since the front and rear lobes are combined in traditional Blumlein recordings, I find them to capture far too much of the room, versus direct signal in PA recording..
only time i've run blumlein was June of this year for WSP.  tape came out really nice and clear, but it did lack some low-end, which I had to add in post.  still an enjoyable tape, even if the high end is kinda of grating at times.

Good observations and I agree.

Yet I must make a clarification that the observations you both posted above concern different aspects of a Blumlien mic array than what I'm talking about above. They more concern when or when not to choose to use it. Blumlein can be tricky to use and very room dependant. Interestingly, some of the things you guys mention are cases where Blumlien is either similar to spaced omnis (no reduced rear sensitivity so you hear sound bouncing off the back wall) or more like the opposite choice (rolled off bass repose of the fig-8s vs flat omni response down to the lowest octave).  Another problem is that if using it up close it can be difficult to fit all the sound sources in the very narrow recording angle.

I have made some great outdoor Blumlien recordings from farther back were the narrow angle was an advantage, but the 'room' sounded great with nothing but trees surrounding an amphitheater for a really sweet airy ambience - no back wall for the sound to bounce back.  That's pretty much the same situation I would use spaced omnis for. I've run both spaced omnis and Blumlien there simultaneously which is part of what I'm basing my observations on. To my ear, the imaging, bass response and presence are the biggest differences between those recordings.

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Offline anechoic

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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #5 on: September 30, 2008, 09:50:52 PM »
excellent post and thanks for the AES papers!!
:)
I've been interested in trying Blumlein for field recording
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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #6 on: September 30, 2008, 10:21:20 PM »
I haven't tried to read the papers yet.

But I have often wondered at the following:  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.  The crazy thing is if I listen from back there (or just use hat mics), it actually sounds better than the cardioids.

My intuition is that the mics are basically pointing at the walls, so if the room is "bright", then I end up hearing a lot of reverberation.  Somehow I'm less bothered by the reverberation when I'm actually in the room.

Anyway, I think Williams is (somehow) trying to explain this effect.

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Offline Gutbucket

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Richard, I think you are intuitively on to it.  I've noticed the same thing.  I suspect that the reverberation off the walls is harder to 'hear around' and mentally decipher when it is what is described as mono reverberation above. When the reverberation off the walls is reproduced as stereo information the automatic mental brain processor can better decode the sound stream and hear more clearly 'though' that room sound so you can mentally concentrate on the direct sound.  A bit of the 'cocktail party effect' I guess. 

I've been playing with some surround recording and notice an extension of the same effect.  It becomes much easier to clearly hear whatever you want to concentrate on. In a way it seems that multi-channel recording and playback can 'tolerate' more ambience and reverb while still sounding clear and direct, much like 2 channel stereo can tolerate more ambient reverb than a mono recording could.

excellent post and thanks for the AES papers!!
:)
I've been interested in trying Blumlein for field recording

Thanks should go to Michael Williams who has made all of his papers freely accessible on his website.
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Offline Roger Gustavsson

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Offline Gutbucket

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Thanks Roger,
Those are good visual summaries of the angular distortions & totally decipherable even with my extremely limited German.  I like how they are grouped by the angle seen from the microphone perspective which make a lot of sense from a taping perspective since that's how we usually set up - most often picking a configuration to suit the angle imposed by a set distance to the stage, constrained by our position in the room vs the more ideal situation of having the freedom to move wherever we want first.

Richard,
I thought about it some more and it occurred to me that for wider mic angles, even though the Stereo Recording Angle is less, the level difference between channels outside that SRA is greater. I notice in the polar patterns above that the mono reverb areas are regions where the level difference between channels is greater than a certain threshold amount.  Linkwitz notes on his site that using a wider spacing and corresponding narrower angle between the mics for the same Stereo Recording Angle increases the stereo reverb proportion, which is in-line with that since the level difference between mics is less at any angle outside the SRA, by an increase in the time/phase difference.

That makes me wonder what the 'worst' configuration would be that maximizes the mono reverb proportion.  I suppose it would be cardioids angled 180 degrees apart, so that the null of one corresponds with the primary axis of the other and everything outside the narrow SRA to the front and back would exceed the level difference threshold.  That insight also helps me to understand why widely spaced omnis with a big hole in the middle and the playback image of sources pulled toward one speaker or the other still sound stereo reverberant - the phase/timing difference between channels draws the apparent source positions toward the speakers (which at first seemed similar to the mono-reverb situation to me) but the level difference between channels is not great enough to exceed that threshold.  But that's all just my suspicion at this point.  I need to go back and re-read to look for that threshold amount.
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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #10 on: October 05, 2008, 01:43:39 PM »
No discussion of Blumlein can be complete without mentioning the Kentucky Colonel's Appalachian Swing!




1st, yes that is David "Dawg" Grisman in a bolo necktie, white collared shirt and spiffy red jacket!

2nd, there was use of Blumlein and multiple players on an array  (a bit experimental) which literally spins in the mix, like a ride at Disneyland

3rd, I'm just kidding about Grisman, that's actually Roland White on the album....     ;D

4th, I'm not kidding about the Blumlein and dizzying sense of space and placement
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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #11 on: October 05, 2008, 01:50:09 PM »
No discussion of Blumlein can be complete without mentioning the Kentucky Colonel's Appalachian Swing!




1st, yes that is David "Dawg" Grisman in a bolo necktie, white collared shirt and spiffy red jacket!

2nd, there was use of Blumlein and multiple players on an array  (a bit experimental) which literally spins in the mix, like a ride at Disneyland

3rd, I'm just kidding about Grisman, that's actually Roland White on the album....     ;D

4th, I'm not kidding about the Blumlein and dizzying sense of space and placement

looks epic

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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #12 on: October 05, 2008, 08:24:06 PM »
looks epic

It's Bluegrass standards, played totally kickass, and no vocals.
No true bluegrass fan should be without it.
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Offline Gutbucket

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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #13 on: October 06, 2008, 09:58:59 AM »
^^^
I'm imagining the band dong the classic bluegrass, 'band around a single mic dance', lifting instruments up for solos and accents, sliding to the side to make room for the featured instrument, crowing up for harmonies - an amazing coordination of movement.  I always love seeing that both for the purity of the sound, the self-mixing of instrument dynamics and voices by a talented band as well as the visual entertainment - its a wonder that they can get in there and out of the way in time.  I saw a guest mando player get knocked on his ass once, hip-checked by the banjo player when he didn't get out of the way in time - quite a crowd pleaser, but both the banjo player and mando man were red faced.

If that's the case on that album, the dizzying flying instruments could have the same effect as a trip down the blue ridge parkway for a motion sick-prone kid in the back seat.  Quick pull over!

Thanks for the tip Bob, I'll have to search that one out.
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Re: Why Blumlein sounds more spacious than other coincident or near-so arrays
« Reply #14 on: October 15, 2008, 10:25:42 PM »
Interesting stuff.  Plenty of good reading in those links!  And that bluegrass record looks worth tracking down :-)

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

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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)
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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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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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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 »
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Offline 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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..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.
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Offline 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 »
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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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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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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-
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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.

Offline Gutbucket

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Interesting, thanks.  You've got the perfect tools to 'play around' with that.  So when keeping the spacing and mic angle constant and changing the pattern from cardioid to figure-8  does the resulting playback image appear to widen?
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Offline Gutbucket

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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.

..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.. 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.

Below are some visualizations of the spacing and angles above provided by a great website applet someone posted a link to on TS a while back.  It can be quite interesting to play around with.

You can either choose standard mic'ing setups in the drop-down on the right, or roll your own by selecting the mic pattern on the left and entering values on the top right for mic spacing, mic angle and orchestra angle which is the apparent width of the source viewed from the mic position. Orchestra angle determines the position of the colored lines in the arc in front of the mics.  The applet then calculates the SRA and the level and time differences between mics for full left/right sources.  Those values are displayed in the grayed boxes on the right and the SRA is also visually displayed in gray under the colored orchestra angle lines.  It also displays two other useful pieces of information: the angular distortion of the source during playback using a typical 60 degree stereo setup is shown by the spacing of the colored lines between the two speakers at the top.  Finally and importantly, the overall sensitivity to sounds in the horizontal plane around the stereo mic array is shown as a gray line surrounding the mics.  For Blumlein the gray line is a circle, indicating equal sensitivity in all horizontal directions.  As the angle between the mics is reduced (regardless of the spacing between them) the sensitivity to sounds originating from the sides of the array is reduced, giving the 62 degree angled example a bean shape.  Once the bi-directional mics are facing directly ahead, their nulls are fully aligned and the overall sensitivity of the array looks like a single bi-directional mic facing forward.

In order below is-
1) Bi-directional mics spaced 0cm, angled 90 degrees (standard Blumlein)
2) Bi-directional mics spaced 15cm, angled 90 degrees (same angle just spaced some in Richard's conjecture)
3) Bi-directional mics spaced 15cm, angled 62 degrees (notice the same SRA and source distribution as Bumlein, but with two differences- time difference information introduced by the spacing, and reduced sensitivity to the sides of the array)
4) Bi-directional mics spaced 77cm, angled 0 degrees (full time based spaced technique, needed a bit more than the 60cm mentioned previously to approximate the same SRA, note the increased angular distortion on playback and the full null for sources to the sides of the array)
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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I forgot about this.

I'm still interested in trying the third configuration picture above some time:


According to the applet it should produce an identical SRA as Blumlein coincident 8s with what appears to be the same imaging source distribution across the front (indicated by spacing of the colored lines), but introduces some phase infomation and reduces stereo pattern sensitivity towards the sides of the room.  The combined stereo pattern sensitivity is indicated by the black outline around the microphones in the diagram.  Notice that it changes from being a circle, which indicates equal sensitivity in all horizontal directions for Blumlein, to becoming elongated in the front-back axis, indicting increased sensitivity to the front and back and reduced sensitivity to either side.

..or supercards spaced ~8-3/4" and angled 90 degrees, which produces very similar in SRA and imaging data in the applet, but with a a combined stereo pattern which looks like a single forward facing cardioid with a rear null.  Trading more sideward for less rearward sensitivity:
« Last Edit: July 01, 2013, 06:06:19 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 macdaddy

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checking in, and i had missed this thread originally. nice bump, gb ;)
-macdaddy ++

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