When are multiple side surrounds necessary? - Page 2 - AVS Forum
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post #31 of 43 Old 02-05-2013, 07:39 AM
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Originally Posted by J_P_A View Post

You may be right regarding the comb filtering. However, I think the phenomena that we are trying to replicate is much more complicated. The paper I linked to gives a good example of this. In a large concert hall, we hear a direct sound from the stage that is nearly perfectly correlated. The sound originates from the same spot and traveled the same distance to our ears on either side of our head. However, there is also sound reflecting from other surfaces in the concert hall, and some of those arrive at our ears a little later. The sounds arriving at the left ear have interacted with different stuff as well as traveling a different distance than the sounds arriving at the right ear. So both of these waves have a different "character" (or internal phase structure) than one another, and have a near 0 correlation. We've learned over the centuries to interpret this information and decide we are in a large reverberant space. Meaning if you were blindfolded and dropped in an concert hall, you would be able to identify that space as large and reverberant based on the sound signature alone.

the ITD will dictate the perceived size of the acoustical space one is in. the resultant reverberant sound-field in a concert hall becomes the effective noise floor.
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For our small listening rooms (relative to that concert hall) we don't have the benefit of of all that space to allow our sound to interact with stuff as well as be delayed, so we try to mimic that with our discrete sources. The original mixer will create a track for that single side surround by adding the sound effects that he/she likes, and by adding the appropriate amount of reverb to get the spaciousness that they want (I'm inferring that at this point). We want to essentially mix our own discrete channel, so we add a little delay to account for the path difference as well as add some reverb to further decorrelate our sounds. Again, we're trying to reproduce or predict that "character" or internal phase structure that would have occurred in the concert hall.

correct. a minimum volume is required to support a statistically random-incidence reverberant sound-field at a given wavelength and above.
in our small rooms, what little "reverberation" exists is typically at frequencies above our hearing range and below the ambient noise floor.

the "reverb" is merely an FX decay applied to the signal. that signal generated from the surround speaker is still emitted as a focused specular reflection within the bounded small room.

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Originally Posted by J_P_A View Post

It may very well boil down to comb filtering, but that paper suggests that decorrelation reduces the perception of comb filtering. It may be semantics, however.

BTW, I'm using the term reverb, but that may not be the right thing to call this randomization of the phase structure. I'm fairly confident that's what we're trying to emulate, though.

we use diffusers to break up the focused (sparse) specular reflections in the room that result in the polar lobing to create a dense array of reflections/lobing in order to get the "comb-filter" notches much more closely spaced together and dense.
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post #32 of 43 Old 02-05-2013, 08:24 AM - Thread Starter
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Originally Posted by localhost127 View Post

the ITD will dictate the perceived size of the acoustical space one is in. the resultant reverberant sound-field in a concert hall becomes the effective noise floor.
correct. a minimum volume is required to support a statistically random-incidence reverberant sound-field at a given wavelength and above.
in our small rooms, what little "reverberation" exists is typically at frequencies above our hearing range and below the ambient noise floor.

the "reverb" is merely an FX decay applied to the signal. that signal generated from the surround speaker is still emitted as a focused specular reflection within the bounded small room.
we use diffusers to break up the focused (sparse) specular reflections in the room that result in the polar lobing to create a dense array of reflections/lobing in order to get the "comb-filter" notches much more closely spaced together and dense.

ITD? I'm not familiar with that one.

My interpretation of the papers above is that the decorrelation attempts to artificially create the structure of the sound field that would be present in a large room. Are you saying that this is not possible using separate channels? Just making sure I'm interpreting your post correctly.
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just to be clear, reverb in the form of an FX is merely a decay. it is not the same behavior as reverb in terms of a reverberant sound-field. im sure you're aware, but the terms really do get mixed (no pun) and used interchangeably far too often (probably as a result of amp mfg'rs adding the "reverb knob" to represent decay FX on their products so many decades ago - the word has been dumbed down to mere slang).

and reproducing a recording or generation of a reverberant sound-field in your room does not imply the energy emitted from your speakers is magically diffuse/random-incidence - the FX signal generated is still at the mercy/physics of a small space (focused specular reflections).

I was not aware of that regarding reverb. That's good to know. Over the past few days I've moved away from using that term as I think decorrelation is a better fit. I'm still not clear on why a time delay will provide some decorrelation as suggested in some of these papers, though. Mathematically speaking, I don't think it should.

The paper posted by Dennis would indicate that proper processing of the separate channels results in a "perceived" larger space. I suppose there may be more going on there with regard to the test facility. I may need to go back and reread that.
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what you're referring to is polar lobing due to summation or superposition of two or more direct/indirect signals. comb-filters do not exist in the real world. 3D spatial polar lobing is the physical phenomenon of which results in an interference pattern manifested within the 2D frequency response referred to as "comb-filter".

the polar lobes and polar nulls are what physically exist, and the interference pattern will change based on your location in 3space with respect to the polar lobes/nulls, as well as wavelength and source spacing.

changing phase merely modifies the propagation of the 3d spatial polar lobing and thus the resultant location (in 3space) of the lobes and nulls.

IIRC, at least one of the sources above mentions that decorrelation reduces the "perceived" comb filtering (or lobing as you've pointed out). Is it possible there are some psychoacoustic phenomena going on there as well?

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post #33 of 43 Old 02-05-2013, 09:21 AM
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ITD ... initial time delay (sometimes called the initial time delay gap) Effectively the delta between the arrival of the direct sound and the first instance of the reflected sound. From that time delay, we perceive the size of a bounded space.

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post #34 of 43 Old 02-05-2013, 10:27 AM
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Originally Posted by J_P_A View Post

Quite helpful! Can this sort of calibration be done with REW, or is more sophisticated analysis equipment required?

REW - simply measure each set of surround speakers at a time for your baseline, then play the same signal through both sets and adjust your filters to minimize comb filtering. Once you've done that for a single location check results across the listening area.

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post #35 of 43 Old 02-05-2013, 10:34 AM - Thread Starter
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Quote:
Originally Posted by Dennis Erskine View Post

ITD ... initial time delay (sometimes called the initial time delay gap) Effectively the delta between the arrival of the direct sound and the first instance of the reflected sound. From that time delay, we perceive the size of a bounded space.

Nice. I'll add that to my list of home theater acronyms biggrin.gif
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REW - simply measure each set of surround speakers at a time for your baseline, then play the same signal through both sets and adjust your filters to minimize comb filtering. Once you've done that for a single location check results across the listening area.

It always sounds so easy when you put it like that smile.gif I've got a feeling it's like watching someone ride a skate board, though. It looks easy, but there's a lot a skill involved. It still sounds like something that would be interesting to play with.

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post #36 of 43 Old 02-05-2013, 10:37 AM
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correct. ITD per leo berenek; reserved for concert hall/auditorium acoustics. although schroeder used ITD to mean the interaural time difference (difference heard by two ears for same sound source). control rooms (small acoustical reproduction spaces) use ISD (inter-signal delay) - per davis.

and "initial time delay" is a bit comical, since one cannot delay time. inter signal delay - ok, we can delay signals wink.gif
there's a heyser joke in there somewhere. tongue.gif

edited: misread his post above.
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post #37 of 43 Old 02-05-2013, 12:27 PM
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...Schrödinger's cat got loose.
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since one cannot delay time.
Perhaps not in this context, but time can be delayed and, for a few, it is going on right now....I mean in a second from now... smile.gif Specifically those experiencing this slow down (relative to you and I) are Kevin Ford, Tom Marshburn, Chris Hadfield, Roman Romanenko, Oleg Novitiskiy and Evgeny Tarelkin.

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post #38 of 43 Old 02-05-2013, 12:50 PM - Thread Starter
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Well, I guess if Schrodinger's cat got out, that sort of answers the question, right?

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post #39 of 43 Old 02-05-2013, 01:11 PM
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Quote:
Originally Posted by Dennis Erskine View Post

...Schrödinger's cat got loose.
Perhaps not in this context, but time can be delayed and, for a few, it is going on right now....I mean in a second from now... smile.gif Specifically those experiencing this slow down (relative to you and I) are Kevin Ford, Tom Marshburn, Chris Hadfield, Roman Romanenko, Oleg Novitiskiy and Evgeny Tarelkin.

dr. manfred schroeder, not schrodinger (edit: oh i see what you did there)

it was a richard heyser joke (same goes for "time-alignment" - how do you "align time" - versus aligning "signals in time"). slang tongue.gif
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post #40 of 43 Old 02-05-2013, 01:54 PM
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I see ....
en.wikipedia.org/wiki/Schrödinger's_cat
http://www.lassp.cornell.edu/ardlouis/dissipative/Schrcat.html
http://physicsworld.com/cws/article/news/2000/jul/05/schrodingers-cat-comes-into-view
http://physics.about.com/od/quantumphysics/f/schroedcat.htm

Dr. Manfred Schroeder, while a physicist, is most known for his contributions to acoustics and computer graphics. I actually believe he had a dog (while at Bell Labs), not a cat and certainly not Schrödinger's famous cat.

The fault however is theirs for having such similar names and picking physics as a place to roost. Who knows, maybe somewhere within Quantum physics, string theory, or parallel universes they were, in fact, one and the same.

I do get the Heyser joke. I suspect George Carlin would have had a heyday with that.

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post #41 of 43 Old 02-05-2013, 02:21 PM
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post #42 of 43 Old 10-01-2013, 04:32 PM
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Quote:
Originally Posted by cybrsage View PostDue to the shape of my room, I went with two multiple side surrounds. Here is a picture to show why:



When you look at the right side of the picture, you will see a large "bump in" which makes the rear row 3 feet less wide than the front row. That bump in would block the sound from the side speaker on that wall if I did not use two of them. Also, I vastly prefer direct radiating sound vs the dispersed sound caused by bi/dipoles...but in my setup bi/dipoles would have the same problems as direct radiators have with that bump in.  (As a note, my camera has an issue where it angles straight lines if I do not hold the camera perfectly level - so the room looks more cramped than it actually is, etc).

Very nice room :)

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post #43 of 43 Old 10-04-2013, 09:32 AM
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Sadly his team not so nice this year... lol

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