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Blumlien array in reverberant spaces
Old 2 weeks ago
  #1
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Blumlien array in reverberant spaces

I would like to address one aspect of the Blumlein (also applicable to Blumlein MS) array, which I believe to be it's primary weakness for recordings made in reverberant spaces.

Generally, in a reverberant hall, a microphone array records the dry instrument primarily from the front of the array, and the natural reverb response primarily from the back of the array.

Because the rear stereo image is reversed from left to right in a Blumlein array, an instrument playing to the right of the stereo image has it's natural reverb response heard to the left of the stereo image, and vice versa. The further the instrument is placed to the right of the stereo image, the further to the left is heard it's natural reverberation, and vice versa.

This creates a particular distortion of the spacial perception of the hall for the listener. The effect is proportional to the width of the stereo image, the width of the hall, and the amount of natural reverb response in the hall.

Last edited by aracu; 2 weeks ago at 10:41 PM..
Old 2 weeks ago
  #2
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This is true, and I tend to use Blumlein arrays only in pretty dead acoustic spaces. If the natural verb of the place is tame and nice, Blumlein works a treat. If the verb in the space is wonky, I probably don't want to hear much of it anyway, and the directional mics come out.

D.
Old 2 weeks ago
  #3
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Quick pedant note: can we get Alan's name corrected, please? If not for anything else, just for searches?

Ta.

John
Old 2 weeks ago
  #4
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Hi aracu, the reverb coming into the rear of a Blumlein array in a good hall is usually well incoherent and de-correlated by the time it comes back to the mic having reflected abundantly from the rest of the hall. The channel discrimination should be very low.

Are you seeing real correlation and separation of this reverb signal, or are you getting a close stage out of phase nearer field reflection that might be interpreted as reverb but is another problem?

I should add that I use a Blumlein array in good halls for precisely this reason, ie that the rear lobe reverb is so beautifully picked up and mixed with the direct signal, one gets a very natural balance if the mic is in the right place.
Old 2 weeks ago
  #5
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Quote:
Originally Posted by David Spearritt View Post
Hi aracu, the reverb coming into the rear of a Blumlein array in a good hall is usually well incoherent and de-correlated by the time it comes back to the mic having reflected abundantly from the rest of the hall. The channel discrimination should be very low.

Are you seeing real correlation and separation of this reverb signal, or are you getting a close stage out of phase nearer field reflection that might be interpreted as reverb but is another problem?

I should add that I use a Blumlein array in good halls for precisely this reason, ie that the rear lobe reverb is so beautifully picked up and mixed with the direct signal, one gets a very natural balance if the mic is in the right place.
It's true that the effect I'm describing varies according to the reverb response of the particular hall (and how far away the array is from the source).

If the hall reverberates in such a way so that there are no obvious left-right cues in the reverb response, then the effect is not noticable. Keep in mind though, that a Blumlein array exaggerates localization. In this case, the localization of the dry sound is exaggerated on one (left or right) side of the stereo image, with an exaggeration of the localization of the reverb response on the opposite (left or right) side of the dry sound it is linked to.

It is most noticable, perhaps, in a recording in which an isolated group of instruments far to one (left or right) side of the stereo image play some loud notes followed by a musical rest, during which only the reverb tail is heard. In a composition in which there happens to be this sequence of events, but alternating, first with loud sounds on the right of the stereo image, then complete rest, then loud sounds to the left of the stereo image, followed by complete rest, the effect becomes even more obvious.

Addressing your question "Are you seeing real correlation and separation of this reverb signal, or are you getting a close stage out of phase nearer field reflection that might be interpreted as reverb but is another problem?", the answer is, I don't think so, because in a moment of complete silence of the dry instrument or section of instruments, we only hear the reverb tail and would not easily confuse it with the sound of the dry instrument.

Last edited by aracu; 2 weeks ago at 01:20 PM..
Old 2 weeks ago
  #6
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Quote:
Originally Posted by johnsound View Post
Quick pedant note: can we get Alan's name corrected, please? If not for anything else, just for searches?

Ta.

John
my apologies, looks like only the moderator can change the thread title (?)
Old 2 weeks ago
  #7
Quote:
Originally Posted by aracu View Post
Because the rear stereo image is reversed from left to right in a Blumlein array, an instrument playing to the right of the stereo image has it's natural reverb response heard to the left of the stereo image, and vice versa. The further the instrument is placed to the right of the stereo image, the further to the left is heard it's natural reverberation, and vice versa.

This creates a particular distortion of the spacial perception of the hall for the listener. The effect is proportional to the width of the stereo image, the width of the hall, and the amount of natural reverb response in the hall.
I've never noticed any problem before. The reverberate sound has little relationship to the direct sound so directional cues from the rear are not really a major concern. Echos from either side of the room are quite uniform regardless of the direction of the instrument on the stage. At least from my experience.
Old 2 weeks ago
  #8
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Originally Posted by rumleymusic View Post
I've never noticed any problem before. The reverberate sound has little relationship to the direct sound so directional cues from the rear are not really a major concern. Echos from either side of the room are quite uniform regardless of the direction of the instrument on the stage. At least from my experience.
Are you refering to recordings and your perception of reverb response in general, or recordings you make using a Blumlein array?
Old 2 weeks ago
  #9
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Decca used to reverse reverb fields from plates and rooms
I use this technique also , I love the spatialisation, comes as a bonus with fig 8 mid/side too
Roger
Old 2 weeks ago
  #10
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Old 2 weeks ago
  #11
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You can put a small absorption panel behind the blumlein array to cut down on the amount of imaging distortion. I have tried it in the past when a forum member suggested it.

Here is that thread: https://www.gearslutz.com/board/remo...ide-space.html

Edit: Keep in mind though that we were trying to solve a problem with a close reflection from a rear wall—this had nothing to do with reverb as the space had almost none.

Last edited by brhoward; 1 week ago at 04:28 PM..
Old 2 weeks ago
  #12
Quote:
Originally Posted by aracu View Post
Are you refering to recordings and your perception of reverb response in general, or recordings you make using a Blumlein array?
My own recordings.
Old 2 weeks ago
  #13
The same issue can be found in many arrays using dual-diaphgram microphones.

For XY (and MS) blumlein, this is the main reason you don't want the sound sources to expand beyond the 90* window. It will start to wrap around (as the reverb already does...).

Last edited by NorseHorse; 2 weeks ago at 12:21 PM..
Old 2 weeks ago
  #14
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I have never found a problem with ambience pick-up with Blumlein.

It always sounds natural.

I am using the AEA R88 and Neumann USM 69 for Blumlein pick up.
Old 2 weeks ago
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Originally Posted by Plush View Post
I have never found a problem with ambience pick-up with Blumlein.

It always sounds natural.

I am using the AEA R88 and Neumann USM 69 for Blumlein pick up.
I don't recall seeing a thread which analyzes the manner in which natural reverb is recorded in a Blumlein array. It is a process distinct from the way in which a person sitting in a hall hears a reverb response, and deserves at least a tiny amount of scientific interest.

it always sounds natural = it always sounds unnatural
Old 2 weeks ago
  #16
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Quote:
Originally Posted by aracu View Post
I don't recall seeing a thread which analyzes the manner in which natural reverb is recorded in a Blumlein array. It is a process distinct from the way in which a person sitting in a hall hears a reverb response, and deserves at least a tiny amount of scientific interest.

it always sounds natural = it always sounds unnatural
The only way the 'process' is going to be similar to a person sitting in a hall is if you use a dummy head like the one pictured in Plush's signature. All live recordings attempt to produce an impression of 'being there' but few are scientific recreations of the actual wave forms at a certain place in time. It is more a case of how pleasing the illusion is and well recorded Blumlein stereo is usually pretty pleasing to my ears.
Old 2 weeks ago
  #17
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I've never heard "imaged" reverb (directional and correlated) with Blumlein stereo.

The reverb is like any other--it is surrounding the stereo picture but usually not tied to it. It is more diffuse and non directional.

When you think about reverberation and how it forms in a room, one realizes that it is a sound that has bounced around the room and them into the ears. So, bouncing around, it has already lost its directionality.
Old 2 weeks ago
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Quote:
Originally Posted by Plush View Post
I've never heard "imaged" reverb (directional and correlated) with Blumlein stereo.

The reverb is like any other--it is surrounding the stereo picture but usually not tied to it. It is more diffuse and non directional.

When you think about reverberation and how it forms in a room, one realizes that it is a sound that has bounced around the room and them into the ears. So, bouncing around, it has already lost its directionality.
Reverb response is considerably less directional than a dry signal. We can also agree that in a Blumlein array, the rear stereo image is a complete reversal of stereo left - right imaging with exaggerated localization.

The result of how reverb response and the spatial perception of the hall is effected, is open to investigation, in each particular situation where a Blumlein recording is made.

Last edited by aracu; 2 weeks ago at 02:04 AM..
Old 2 weeks ago
  #19
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Quote:
Originally Posted by NorseHorse View Post
For XY blumlein, this is the main reason you don't want the sound sources to expand beyond the 90* window. It will start to wrap around (as the reverb already does...).
Not to muddy the waters, but is M/S "Blumlein" more forgiving of wrap around for sources outside of the 90 deg window, be they direct sources or reverb? I'm thinking that that the inverted signal from the duplicated and inverted side channel would cancel out anything that would otherwise be imaging as left-right flipped from the rear lobe of the side mic.

I have read many comments on this board that say M/S with two fig-8s, once decoded, gets you the same thing as X/Y Blumlein. I can see how that would be true for sound sources in front of the array (within the 90 deg window), but wouldn't anything outside of that 90 deg window be more phase-coherent and thus left-right accurate using the M/S version?
Old 2 weeks ago
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Originally Posted by aracu View Post
Reverb response is considerably less directional than a dry signal. We can also agree that in a Blumlein array, the rear stereo image is a complete reversal of stereo left - right imaging with exaggerated localization.

The result of how reverb response and the spatial perception of the hall is effected, is open to investigation, in each particular situation where a Blumlein recording is made.
Just idly conjecturing here, but could it be fair to say that Blumlein stereo treats the rear (reflected) input in an angle-selective way which is actually a distortion of reality ?

It takes a de-corellated, diffuse rear soundfield, as Plush has described it here, and imposes a spatially distinct character to it (by virtue of the dual fig 8 pickup) which gives it a 'definition characteristic' that diffuse, reflected sound in a hall returning to the mic doesn't have in reality ?
Old 2 weeks ago
  #21
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Quote:
Originally Posted by voltronic View Post
I can see how that would be true for sound sources in front of the array (within the 90 deg window), but wouldn't anything outside of that 90 deg window be more phase-coherent and thus left-right accurate using the M/S version?
No, they are identical.
Old 2 weeks ago
  #22
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Quote:
Originally Posted by studer58 View Post
a 'definition characteristic' that diffuse, reflected sound in a hall returning to the mic doesn't have in reality
Yes, it's both left-right reversed and localized within the boundaries of a particular geometry or vector space.

MS Blumlein alters the angles of the geometry when adding more mid or more side, similar to narrowing or widening the 90 degree angle of a Blumlein array.
Old 2 weeks ago
  #23
Yep. Thanks, David! XY v MS = the same.

Quote:
Originally Posted by Plush View Post
The reverb is like any other--it is surrounding the stereo picture but usually not tied to it. It is more diffuse and non directional.

... So, bouncing around, it has already lost its directionality.
Consider off-stage ensembles in the house. e.g. Belioz Requiem, Pines of Rome, Renaissance antiphony. Those make it obvious, but the sound coming to the back of your array always has an image.

It is more diffuse, but it is still directional. With Blumlein (and other arrays), the position of both direct and indirect sounds will be reversed, which can and should be considered a potential problem.
Old 2 weeks ago
  #24
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Yet, in practice, the results are often quite good with no complaint. I believe that the so-called "problem" is over thought. The supposed idea of the OP that there is a "problem" is mistakenly substituted for the actual recorded result which is good (with no complaint about reversed images or blurring.)

I do not agree with the OP who opens his post with the contention that reverb is primarily recorded by the rear capsules. Any ambience in the hall / church / cathedral is recorded by the front capsules as well.

So the reverb in the front and the back are somewhat matched because that is the sound of that space.
Old 2 weeks ago
  #25
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Quote:
Originally Posted by Plush View Post
I do not agree with the OP who opens his post with the contention that reverb is primarily recorded by the rear capsules.
There is one diaphragm in each fig 8 mic, sensing sound pressure from two different points, not front and rear capsules as you suggest (unless you mean a fig 8 made from two cardioids). The rear of a Blumlein array is more on-axis towards the hall, away from the ensemble, than the front.
Old 2 weeks ago
  #26
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Maybe here's what the OP was hearing, maybe? In a bad room, it seems intuitive that the wide open rear lobes of a Blumlein X-Y will simply "hear" more of that bad reverb. Maybe as simple as that?

D.
Old 2 weeks ago
  #27
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An example, comparing spaced omnis to MS Blumlein, to listen to on headphones, focusing attention on the localization of the bass singers and the localization of the reverb response of the bass singers. The difference is subtle.
Attached Files

sp.omnis.mp3 (372.6 KB, 277 views)

Bl.ms.mp3 (376.7 KB, 276 views)

Old 2 weeks ago
  #28
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Quote:
Originally Posted by aracu View Post
An example, comparing spaced omnis to MS Blumlein, to listen to on headphones, focusing attention on the localization of the bass singers and the localization of the reverb response of the bass singers. The difference is subtle.
The differences between these two tracks are enormous. Listening on headphones, the spaced omni track sounds like a natural choral spread. The Blumlein track sounds very strange. The sopranos have been pushed to the extreme left and basses now sound like they are next to my right ear. The alto and tenor sections have nearly disappeared in the Blumlein example as well.

EDIT: Listening again closely for the reverb of the bass section:

Omnis - Bass section is to the front-right, and their reverb seems to originate from the rear-left, but to be sort of all-enveloping, in the way Plush described.
Blumlein - Bass section is far right but very close, and their reverb sounds centered and mono.
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