The Reverberant Field: Why it matters, where the goal posts are

Blackmorec

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Feb 1, 2019
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#41
Here’s the thing with all this.
Single reflections in small rooms are fast and energetic because they have no distance to lose amplitude. However if the reflections match the direct sound spectrally they are summed and you get a single sound with a single location. But in a small OR large listening room, its not a single location because the sound is coming from 2 locations, 2 loudspeakers, so the brain has more work to do....it takes the sounds from both ears, detects early arriving reflections, sees they match spectrally so combines (sums) them and uses the summed amplitude difference to pinpoint location of the sound.

If those early arriving reflections are different in amplitude L&R.....which they would be in a non-symmetrical room, then the sums of the direct and reflected sounds are not going to be the same as those launched by the loudspeakers and therefore the imaging is going to change, based on different amplitude relationship between the 2 ears. That’s where your early reflections in a small room change the overall imaging.....

Referring to the passage about strong sidewall reflections causing poor imaging outside the speakers...again logic precludes this. If strong reflections were the cause for aberrant side images, they wouldn’t only be present with strongly left or right panned images that appear outside of the speaker boundary. They’d be present whenever the speaker reached a certain level. The speaker output is ALWAYS the same shape from a polar response point of view and strong reflections will always be present when a loudspeaker outputs sufficient sound energy. But if the images occur only for strongly left or right panned signal, its not only reflections that are causing the outside of speaker boundary images.
My system will produce these types of super wide images several times a listening session, depending on the recording. The images are sharp, focused and just as energetic as all other images....in other words they are a completely valid part of the signal being reproduced in exactly the same way as the rest of the images....no blurring, no veiling, no lack of focus....just perfect images of instruments or voices outside the speaker boundaries. Also these images do not have to have a great deal of energy. Many of them are subtle, whisper quiet or even just a feeling of air and ambience. So they are on the recording and are supposed to be played outside the speaker boundary. The context in which they play is simply too coherent to be any other way.

Finally lets consider listening room reflections vs recording venue reflections.

Because the speaker’s polar response and the room dimension don’t change, listening room reflections are going to arrive at the same time for every recording. In a small, uniform room, you shouldn’t hear them as they are combined with the direct sound under the ‘Law of first Wavefront’ . So, if you are listening in a small room and your imaging sounds 3 dimensional and highly focused, things are working well. When you play a recording you may or may not hear the recording venue’s reverberation, depending on the size of the studio or hall. For items recorded in a small studio with very short reverb times, the reverb of the recording will be treated by the brain in exactly the same way as the listening room‘s early reflections, namely combined with the direct sound (summed). Recordings done in a large hall or room, will have much later arriving reflections, which, as long as we‘re talking a really good recording, you’ll hear as spaciousness and reverberation....the air of the hall should have presence and some texture....that hall’s ID.
So can a small room sound spacious? Of course it can...it can play the large room‘s delayed first reflections and the hall’s spacial identity should be present. For studio recordings its often the case that sound engineers add reverb and panned decay Into the sound, creating an artificial large hall ambience. These recordings should also be highly ‘immersive‘ in a small room that’s been treated to remove multiple reflections.
In principal, if a small room is getting out of the way i.e only very short delay reflections with no multiple bouncing (so lots of diffusion) it should be able to sound like a large space, as its the recordings soundwaves and their arrival time that denotes ‘space’. In the best, most resolving systems that space has presence and texture so you can ‘hear’ you in a large venue, even when you’re actually sitting in a small room, with your eyes closed of course :cool:
 
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Duke LeJeune

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Jul 22, 2013
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#42
Blackmorec, thank you for your in-depth reply.

....That’s where your early reflections in a small room change the overall imaging.
Well at least we agree that early reflections in small rooms can change the imaging.

Referring to the passage about strong sidewall reflections causing poor imaging outside the speakers...again logic precludes this.
According to Toole, early sidewall reflections which are above the image-shift thresholds will broaden the apparent source width. Many listeners find this effect pleasing. Geddes finds that it degrades image precision. I agree with Geddes and suggest a wider-than-normal speaker spacing.

My system will produce these types of super wide images several times a listening session, depending on the recording.
I'm not talking about beyond-the-speakers image locations which are on some recordings; I'm talking about a loudspeaker/room interaction which would be present regardless of the recording.

In principal, if a small room is getting out of the way i.e only very short delay reflections with no multiple bouncing (so lots of diffusion) it should be able to sound like a large space, as its the recordings soundwaves and their arrival time that denotes ‘space’. In the best, most resolving systems that space has presence and texture so you can ‘hear’ you in a large venue, even when you’re actually sitting in a small room, with your eyes closed of course :cool:
Agreed.

In my experience diffusion of the early reflections is a very effective approach to "getting the room out of the way."

Avoiding those early reflections in the first place via radiation pattern control is another.

I use the latter because I prefer for the speakers not to be heavily dependent on room acoustics and/or room treatments in order to perform well. But I have heard magnificent sound from wide-pattern speakers in well-treated dedicated rooms.
 
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LL21

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Dec 26, 2010
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#44
Absolutely agree...i am definitely reading this one carefully. Thanks, Duke!

Awesome Duke, this is the type of stuff we can make good use of.
 
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Duke LeJeune

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Jul 22, 2013
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#45
Fascinating read (thread); I truly enjoy...as a casual reader/learner. :cool:
Absolutely agree...i am definitely reading this one carefully. Thanks, Duke!
I thank you both!

And I appreciate Blackmorec and KlausR making me work at this, regardless of whether or not we end up on the same page.
 

Blackmorec

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Feb 1, 2019
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#47
According to Toole, early sidewall reflections which are above the image-shift thresholds will broaden the apparent source width. Many listeners find this effect pleasing. Geddes finds that it degrades image precision. I agree with Geddes and suggest a wider-than-normal speaker spacing.
I'm not talking about beyond-the-speakers image locations which are on some recordings; I'm talking about a loudspeaker/room interaction which would be present regardless of the recording.
Let's say it is a room reflection. So what triggers whether I hear it or not? Because whatever triggers is is going to trigger it ever time, not just sometimes. When the music is panned hard left or right and loud enough to cause strong reflections Is that when it happens? If so you wouldn’t hear it with quiet passages but you would hear it during very loud passages from either or both speakers, regardless whether they’re panned or not. If its therefore volume dependent you’ll hear it everytime a loudspeaker reaches that amplitude, regardless whether its supposed to be there or not. But if when the speaker reaches high amplitude, you hear it sometimes but not others then its not the room and its the recording.
i can also promise you that there’s no image degradation associated with these wider than the speakers images..they are as focused and as intense or whisper quiet as the rest of the recording. Its probably just that the recording has a massive soundscape.
And i really enjoy this adult discussion. Makes a nice change...discussing and disagreeing without becoming adversaries. :)
 
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Duke LeJeune

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#48
It don't matter which page we all end up @ the end...it's the ambiance of the educative discussion you three have so far...enriching and very adult. [emphasis Duke's]
[irrelevant tangent] I can't resist telling this story.

Back when I was a dealer only - not yet a manufacturer - my wife's boss invited her employees and their spouses to dinner at a nice restaurant. We went around the table introducing ourselves and saying what our jobs were. My introduction went something like this:

"My name is Duke, and I'm Lori's husband. I sell expensive stereo equipment... "toys for grownups", one might say."

Then a little while later the boss's husband arrived, and this is how she introduced me to him:

"This is Lori's husband Duke. He sells adult toys."

[/tangent]
 
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Duke LeJeune

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Jul 22, 2013
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#49
Let's say it is a room reflection. So what triggers whether I hear it or not? Because whatever triggers is is going to trigger it ever time, not just sometimes. When the music is panned hard left or right and loud enough to cause strong reflections Is that when it happens?
I hadn't really thought about the distinction between a hard-panned image and an image that is actually supposed to be beyond the speakers, but I see now that I should have!

Anyway if it's just a room reflection, then my understanding is that whether or not it shifts the image depends mainly on its intensity... and I assume that would include its intensity relative to the direct sound, which would be greater with a wide-pattern speaker (assuming equivalent placement and toe-in).

i can also promise you that there’s no image degradation associated with these wider than the speakers images..they are as focused and as intense or whisper quiet as the rest of the recording. Its probably just that the recording has a massive soundscape.
Yes, I've heard these wider-than-the-speakers images too on relatively narrow-pattern speakers, and always just assumed it was intentionally recorded (and/or engineered) that way.

And i really enjoy this adult discussion. Makes a nice change...discussing and disagreeing without becoming adversaries. :)
Agreed!

Whereabout are you located, if you don't mind? I'm not far from Dallas.
 

Blackmorec

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Feb 1, 2019
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#50
I hadn't really thought about the distinction between a hard-panned image and an image that is actually supposed to be beyond the speakers, but I see now that I should have!

Anyway if it's just a room reflection, then my understanding is that whether or not it shifts the image depends mainly on its intensity... and I assume that would include its intensity relative to the direct sound, which would be greater with a wide-pattern speaker (assuming equivalent placement and toe-in).



Yes, I've heard these wider-than-the-speakers images too on relatively narrow-pattern speakers, and always just assumed it was intentionally recorded (and/or engineered) that way.



Agreed!

Whereabout are you located, if you don't mind? I'm not far from Dallas.
Cool! I like America and have a lot of friends there from my working life (A lot moved to Florida and one has a ranch in Montana).

I live in the North East of England and in Zurich Switzerland....at the moment we time share between the two. If home is where the hi-if is; North East England.
 

caesar

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May 31, 2010
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#51
All of the differences between the sound in a good room and the sound in a bad room are in the reverberant field.

All of the money spent on acoustic construction and treatment at home, in professional studios, and in concert halls is spent on the reverberant field.

Most of the sound that reaches your ears in most rooms is reverberant sound (nearfield and quasi-anechoic setups being the exceptions).

Done right in a home audio setting, the reverberant field enriches timbre, improves clarity, and enables the spatial information on the recording to come through, which (given a good recording) includes a sense of immersion in the original acoustic space (whether it be natural or synthetic or both).

Done wrong in a home audio setting, the reverberant field degrades timbre and clarity, superimposes a "small room signature" and/or a characteristic coloration onto every recording, and causes listening fatigue.

In this thread I'm going to look at the loudspeaker's role in getting the reverberant field right, and will leave acoustic treatments to those with expertise in that field.

In my next post, we'll try to get an idea of where the goal posts are by looking through the lens of one of the world's foremost experts on concert hall acoustics and psychoacoustics, Dr. David Griesinger.
Great stuff, Duke. Your knowledge, understandable communications style, and willingness to share make you a treasure to our hobby.
 
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caesar

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May 31, 2010
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#52
Duke, a question for you: Just to make sure I am not extrapolating / applying extra imagination to your points above, is the magical combination of MBL speakers with time delayed diffusion of SMT panels explained by your post(s)?

Thank you
 
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Duke LeJeune

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Jul 22, 2013
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#53
Great stuff, Duke. Your knowledge, understandable communications style, and willingness to share make you a treasure to our hobby.
Thank you very much, Caesar.


Duke, a question for you: Just to make sure I am not extrapolating / applying extra imagination to your points above, is the magical combination of MBL speakers with time delayed diffusion of SMT panels explained by your post(s)?
YES!!!

The general principles can be applied many different ways. Once you start thinking in terms of "Two Streams", you start seeing a correlation between that concept and what sounds good.

As applied to loudspeakers and rooms, the "Two Streams" paradigm assumes the speaker's off-axis energy is spectrally correct, or close to it. If the speaker's off-axis energy sounds significantly different from the direct sound then the emphasis in a good system set-up will probably include addressing that situation.
 
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caesar

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May 31, 2010
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#54
Thank you very much, Caesar.




YES!!!

The general principles can be applied many different ways. Once you start thinking in terms of "Two Streams", you start seeing a correlation between that concept and what sounds good.

As applied to loudspeakers and rooms, the "Two Streams" paradigm assumes the speaker's off-axis energy is spectrally correct, or close to it. If the speaker's off-axis energy sounds significantly different from the direct sound then the emphasis in a good system set-up will probably include addressing that situation.
Duke, thanks. I’m curious about another item:

For a large open space , SMT recommends to add about 3 of their diffusion panels , elevated off the floor and about 2 feet behind the listener’s head.

So, in effect, it’s a small make-shift , 72 inch wall about 2 feet behind the sweet spot.

The sound waves can escape on the sides of this “wall”, obviously, but whatever waves hit the panels behind the listener are fed back into “listening area”. It actually sounds much better this way. In effect, this small makeshift wall is a better value for the dollar than the vast majority of component upgrades.

Do you have any idea why it’s better and what may be going on? Thanks
 
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spiritofmusic

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Jun 13, 2013
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#55
Caesar, I have in effect 20'-25' of free space behind me. Are you suggesting a diffuser "false" wall be setup behind my head?
 

Duke LeJeune

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Jul 22, 2013
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#57
Duke, thanks. I’m curious about another item:

For a large open space , SMT recommends to add about 3 of their diffusion panels , elevated off the floor and about 2 feet behind the listener’s head.

So, in effect, it’s a small make-shift , 72 inch wall about 2 feet behind the sweet spot.

The sound waves can escape on the sides of this “wall”, obviously, but whatever waves hit the panels behind the listener are fed back into “listening area”. It actually sounds much better this way. In effect, this small makeshift wall is a better value for the dollar than the vast majority of component upgrades.

Do you have any idea why it’s better and what may be going on? Thanks
I have no experience with the SMT products but their Wing diffusors sure do look interesting. If I understand what I'm seeing, I think they spread the energy out in time as well as spatially. And presumably that allows them to work well when the path lengths are quite short.

(From what I understand, "specular" early reflections - those which are powerful and distinct - are the most likely to be detrimental. By diffusing the energy temporally as well as spatially, I think the end result with the Wing diffusors is about as far from "specular" as you can get, with only a small reduction in the total amount of energy. That being said, I welcome correction from anyone who actually knows... I'm really just winging it here...)

Regarding their suggestion of placing 3 of their diffusion panels about 2 feet behind your head if you have a very deep space behind the listening position, here is my guess: [edit: I now think Blackmorec and DaveC's thoughts on the subject are correct and mine below is incorrect]

By harvesting that reverberant energy before it has made the long round trip to a faraway rear wall, the level (loudness) of the energy is much better preserved, and so it is still strong enough to be beneficial. This in addition to whatever the panels are doing spatially and temporally, which I do not claim to have a good understanding of.
 
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Blackmorec

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Feb 1, 2019
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#58
Duke, thanks. I’m curious about another item:

For a large open space , SMT recommends to add about 3 of their diffusion panels , elevated off the floor and about 2 feet behind the listener’s head.

So, in effect, it’s a small make-shift , 72 inch wall about 2 feet behind the sweet spot.

The sound waves can escape on the sides of this “wall”, obviously, but whatever waves hit the panels behind the listener are fed back into “listening area”. It actually sounds much better this way. In effect, this small makeshift wall is a better value for the dollar than the vast majority of component upgrades.

Do you have any idea why it’s better and what may be going on? Thanks
Hey Caesar,

I’m going to take a guess at what’s going on.

With a large space behind the listening point, direct sound will pass the ears, travel to the back wall, reflect and travel back to the ears. That reflection is significantly delayed by the distance it travels so by the time it returns to the ears its no longer related at all to the direct sound and is purely a room artefact that interferes with following low level direct sounds....masking things like hall ambience and low level detail.

Putting a large diffusion panel 2 feet behind the listening position will have 2 simultaneous beneficial effects. First direct sound will pass the ears, hit the panel and the reflection will be diffused into a large number of different wavelengths each having a lot less energy than the original direct sound, essentially rendering the reflection harmless in terms of the direct sound, yet adding a level of airiness and space to the sound. Secondly and probably more important, the listener is sitting in the panel‘s ‘shadow’, so any reflections from the back wall area going to hit the back of the panel and reflect back to the wall rather than reaching the listener’s ears. So the listener is completely sheltered from late room reflections and is therefore able to hear pristine hall ambience and low level detail that would otherwise be partially masked
 
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DaveC

Industry Expert
Nov 16, 2014
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#59
Caesar, I have in effect 20'-25' of free space behind me. Are you suggesting a diffuser "false" wall be setup behind my head?
I would definitely recommend trying this, imo that much space behind you isn't ideal. Once the time delay gets too long and you hear it as an echo, it will cause problems. If you play an impulse, or find a track with a loud drum strike and otherwise quiet background, and can hear that as multiple sources (hear an echo) it's probably having a negative effect. I'd also measure overall decay times, look up rt60, the time it takes for direct sound to be 60 dB down. IMO there is some personal preference involved here but an overly long rt60 is definitely an issue.

I would think that diffusion behind the LP combined with absorption to reduce rt60 (if needed) may be a good thing for your space.
 

Duke LeJeune

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Jul 22, 2013
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#60
With a large space behind the listening point, direct sound will pass the ears, travel to the back wall, reflect and travel back to the ears. That reflection is significantly delayed by the distance it travels so by the time it returns to the ears its no longer related at all to the direct sound and is purely a room artefact that interferes with following low level direct sounds....masking things like hall ambience and low level detail.
I would definitely recommend trying this, imo that much space behind you isn't ideal. Once the time delay gets too long and you hear it as an echo, it will cause problems. If you play an impulse, or find a track with a loud drum strike and otherwise quiet background, and can hear that as multiple sources (hear an echo) it's probably having a negative effect. I'd also measure overall decay times, look up rt60, the time it takes for direct sound to be 60 dB down. IMO there is some personal preference involved here but an overly long rt60 is definitely an issue.
I think you guys are more right about this than I was.
 

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