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Diffusing 1st reflections of speakers that measure great on and off-axis - instead of absorbing

Kvalsvoll

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It was discussed but there wasn't much agreement with you pov. ;)
Well, I am in support of @Bjorn 's view. This is not to be disrespectful of research others have done, research that in many ways have contributed to a better understanding of how sound works. But then we move on, and learn more, using what has been proven as a foundation.
 

March Audio

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Well, I am in support of @Bjorn 's view. This is not to be disrespectful of research others have done, research that in many ways have contributed to a better understanding of how sound works. But then we move on, and learn more, using what has been proven as a foundation.
Well I was specifically referring to his refusal to accept the points about RT(x) and his assertion it doesn't exist, or is of no relevance in small rooms. I will go with @j_j and @amirm on this one ;) as discussed earlier in the thread.
 
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March Audio

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In post #193, talking about the construction of Harman listening rooms: .. "The inner walls and ceiling of the double-wall IAC shell are made of heavy gauge steel panels separated 10 cm and filled with fiberglass. The inner surfaces are perforated with 2.34-mm openings to provide substantial sound absorption inside the room.

@Bjorn had some comments on this, in post #195.
Well I'm afraid he is wrong, it does work in small rooms. That does not mean you dont need additional specific treatments, but still.....

It's a long established and widely used technique.
 
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Bjorn

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Well I'm afraid he is wrong, it does work in small rooms. That does not mean you may need additional specific treatments, but still.....

It's a long established and widely used technique.
Please don't make straw-man argument but stick to what's being said.

I've never said a perforated panel doesn't work or absorb at all. But I said it didn't work well and especially in regards to attenuating high gain specular reflections in a broadway matter in small rooms. Of course it will absorb. But with what absorption coefficient at what frequencies when placed in small rooms? This is the matter here.

The fact that acoustic treatment absorbs differently in large vs small rooms proves there are different acoustic properties in these spaces. There is no isotropic soundfield in small rooms, hence the absorption is very different from a large room. This is well understood in circles where there are expertise in small room acoustics. Measurements would also reveal this easily.

Floyd Toole, which you seem to put confident in, also writes about this which I showed earlier in this thread. Below are some quotes from him.
These rooms and those in which the recordings are reproduced are fundamentally different from live performance spaces, being smaller and acoustically far more absorbent—even large cinemas.
When reverberation times measure around or below 0.5 s, one has to acknowledge that this is not reverberation in the original “diffuse field” sense; it is something else.
Understandably, these thoughts and practices have drifted into the domain of sound reproduction but we seem to have lost focus on what the numbers mean in the small, dead, rooms in which we live, listen, and work. We continue to talk about r e v e r b e r a t i o n times and invoke notions of critical distance, etc. The properties of acoustical materials are measured as if they were to be placed in diffuse sound fields: randomincidence absorption coefficient for example. These are mismatched concepts and this is where problems arise.
 

March Audio

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.

Sound absorption from perforated panels in small rooms don't work very well
.
Like previously mentioned, perforated panels doesn't work very we in small rooms .
Please don't make straw-man argument but stick to what's being said.

I've never said a perforated panel doesn't work or absorb at all.
And I didn't say you did.

No strawman argument, just picking you up on the things you actually said. You even said it twice. Neither time was it correct.
 

March Audio

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But with what absorption coefficient at what frequencies when placed in small rooms? This is the matter here.

The fact that acoustic treatment absorbs differently in large vs small rooms proves there are different acoustic properties in these spaces. There is no isotropic soundfield in small rooms, hence the absorption is very different from a large room. This is well understood in circles where there are expertise in small room acoustics. Measurements would also reveal this easily.

Floyd Toole, which you seem to put confident in, also writes about this which I showed earlier in this thread. Below are some quotes from him.
Erm that's why you model and design with appropriate products and verify with measurement.

I'm not going to rehash the earlier conversation in the thread. Of course there are differences between small and large spaces, but there is no requirement for a completely isotropic field. Note the previous comments about CD; separating modal content plus @j_j comments about stationary measurements and perceptually diffuse V mathematically diffuse.

The issue here is your repeated sweeping blanket statements and assertions which are misleading at best.
 
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Bjorn

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And I didn't say you did.

No strawman argument, just picking you up on the things you actually said. You even said it twice. Neither time was it correct.
You need to read it in the content it's being said. Here it was mentioned in regards to attenuating specular reflections in small rooms. Something that perforated panels don't do well because the absorption coefficient of the panels is low used in small rooms. Even absorption over the frequencies down to Schroeder with perforated panels simply isn't possible in these spaces.

Again, we can't translate random indicence measurement of products to small rooms. They are only valid for very large rooms and the result is a very different in small rooms.
 

March Audio

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You need to read it in the content it's being said. Here it was mentioned in regards to attenuating specular reflections in small rooms. Something that perforated panels don't do well because the absorption coefficient of the panels is low used in small rooms. Even absorption over the frequencies down to Schroeder with perforated panels simply isn't possible in these spaces.

Again, we can't translate random indicence measurement of products to small rooms. They are only valid for very large rooms and the result is a very different in small rooms.
I did. You said they don't work well and added especially for specular.

No the panels work in small rooms. I have worked in an acoustics dept of engineering company. Modelled, implemented and measured.

Note I also said other measures/treatments may be required.
 
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Bjorn

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I did. You said they don't work well and added especially for specular.

No the panels work fine in small rooms. I have worked in an acoustics dept of engineering company. Modelled, implemented and measured.
Then I challenge you to post some measurements that shows that perforated panels absorb well and in an even matter down to the Schroeder frequency in a small room in order to disprove me.
 

March Audio

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Then I challenge you to post some measurements that shows that perforated panels absorb well and in an even matter down to the Schroeder frequency in a small room in order to disprove me.
I don't need to. It's a widely used proven method. I know how it works in small machine rooms, an example you will find everywhere. (even though they don't work :facepalm:)

If you want to go on a wild goose chase with your own personal definition "doesn't work well" that's up to you. I'm certainly not going to waste my time.

Do you actually think there was no design specification, review and verification in the Harman room? You judge when all the relevant information is in not in the public domain.

By virtue of the fact that this was stated: "The average Tm value for the MLL is about 0.23 s, which is slightly below the calculated ITU and EBU optimal value of 0.29 s. However, the curve falls within the minimum recommended value, and is quite uniform with frequency, only rising slightly below 125 Hz." shows the treatment works. Does that one treatment do everything that might be required? Maybe not, but I have no doubt it wasnt intended to.

I note the text about the Harman room mentions IAC. "double-wall IAC shell". I wonder if they are referring to the company IAC who specialise in acoustic control. I have had experience with them as my background is in aero engine test and measurement.

https://www.google.com/url?sa=t&sou...FjAAegQIAxAB&usg=AOvVaw0IKMkbdw1Nv5j30wnI3_oK
 
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Duke

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Hi Bjorn, pardon me if these are stupid questions but sometimes I miss and/or misinterpret things that matter.

Generally I would recommend attenuating early arriving reflections and diffuse late arriving ones. The room dimension and size decides what's possible besides speaker directivity.
How long after the first-arrival sound would you consider the "late" reflections to begin? Or is "late" in this context a function of the reflection path lengths in the specific room?

In this graph (lifted from one of your posts) it looks to my untrained eye like the Reflection-Free Zone in between the direct sound and the onset of diffuse reflections is about ten milliseconds. Am I reading it right?

ETC after treatment with diffusion.jpg


Do you know what kind of room this graph was taken in? Studio, home audio, etc... ?

The goal is to have a lateral late arriving diffuse tail which to a large degree emulates the best concert halls and yields spaciousness and envelopment without obscuring the recorded signal. You achieve both accuracy with a high degree of clarity, intelligibility, localization and correct tonality combined with a spacious and enveloping experience.
This makes a lot of sense to me.
 
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Duke

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For those of you have an interest, you can read a short write up about reverberation time here:
https://www.lydogakustikk.no/sma-rom-og-etterklangstid/
I have a question about something in that paper:

"Since in small rooms, there is no Dc, no well mixed sound field, hence, no reverberation but merely a series of early reflected energy, the measurement of RT60 becomes meaningless in such environments."

My understanding apparently has gaps and/or misconceptions. Can you explain to me why there is no Dc ("critical distance", I presume) in a small room? (I think I understand the rest of that statement, and that I have been using the term "reverberation" in a way which is not technically correct since small rooms always have discrete reflections.)

Thanks!
 

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I have a question about something in that paper:

"Since in small rooms, there is no Dc, no well mixed sound field, hence, no reverberation but merely a series of early reflected energy, the measurement of RT60 becomes meaningless in such environments."

My understanding apparently has gaps and/or misconceptions. Can you explain to me why there is no Dc ("critical distance", I presume) in a small room? (I think I understand the rest of that statement, and that I have been using the term "reverberation" in a way which is not technically correct since small rooms always have discrete reflections.)

Thanks!

The distinction between direct, relected, and diffuse is rather more complex, perceptually, than the 1960-era attempts to address this kind of issue mostly from the acoustical side. In addition, the "Schroeder Frequency" issue has been shown to hinge on analysis length windows as well as on power summation issues.

The hard-line "large" vs. "small" is simply silly, there is a continuum, and the breakpoints of the continuum are different for perception and linear independence, and even more different when stationarity over time is investigated.
 

March Audio

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I have a question about something in that paper:

"Since in small rooms, there is no Dc, no well mixed sound field, hence, no reverberation but merely a series of early reflected energy, the measurement of RT60 becomes meaningless in such environments."

My understanding apparently has gaps and/or misconceptions. Can you explain to me why there is no Dc ("critical distance", I presume) in a small room? (I think I understand the rest of that statement, and that I have been using the term "reverberation" in a way which is not technically correct since small rooms always have discrete reflections.)

Thanks!
Your understanding is probably better than you think. ;)

Have a read of JJs and Amirs posts (including #52 on DC) in the first 3 pages of the thread.

Bjorns interpretation is, how shall I put it, somewhat over zealous.
 
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Duke

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The distinction between direct, reflected, and diffuse is rather more complex, perceptually, than the 1960-era attempts to address this kind of issue mostly from the acoustical side.
Seems to me "direct" is pretty straightforward, and it seems to me that, along at least one continuum, reflections can range from specular to diffuse. I welcome correction if I'm mistaken.

The hard-line "large" vs. "small" is simply silly, there is a continuum, and the breakpoints of the continuum are different for perception and linear independence, and even more different when stationarity over time is investigated.
In the size rooms we see in home audio, what are the relevant perceptual breakpoints?

Have a read of JJs and Amirs posts (including #52 on DC) in the first 3 pages of the thread.

Bjorns interpretation is, how shall I put it, somewhat over zealous.
Nothing against J_J, but I have a hard time following him, and regarding room acoustics he's already told me "dead is my preference." Dead is not my preference so at some point, even if I am able to follow J_J, our paths will probably diverge unless he changes my mind. (I'm not challenging him to do so, but if it happens, it happens.)

Here is a splicing together of Bjorn's thoughts from two different posts:

"The clarity, detail or "resolution" is brought to a completely another level when early arriving specular reflections are strongly attenuated... The goal is to have a lateral late arriving diffuse tail which... yields spaciousness and envelopment without obscuring the recorded signal. You achieve both accuracy with a high degree of clarity, intelligibility, localization and correct tonality combined with a spacious and enveloping experience."

This is where I think he and I have some common ground. I'm pretty sure he knows more than I do so I hope to learn from him, even if I'm still stumbling over the meaning of terms here and there.
 

Bjorn

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Hi Bjorn, pardon me if these are stupid questions but sometimes I miss and/or misinterpret things that matter.



How long after the first-arrival sound would you consider the "late" reflections to begin? Or is "late" in this context a function of the reflection path lengths in the specific room?

In this graph (lifted from one of your posts) it looks to my untrained eye like the Reflection-Free Zone in between the direct sound and the onset of diffuse reflections is about ten milliseconds. Am I reading it right?

View attachment 76177

Do you know what kind of room this graph was taken in? Studio, home audio, etc... ?
This makes a lot of sense to me.
There isn't to my knowledge an objective definition of early and later reflections. But normally we're talking about early reflections till 10-12 ms area and late after that.

The graph is from Peter D'Antonio's book but I don't think it says from what specific room. Below is also the before impulse.
Impulse response.jpg


The RFZ or ISD (initial signal delay) gap of that graph is around 18 ms. Where you see the sudden rise in level, is where you have the termination of the ISD gap. How long should the ISD gap be? In a control room and with a LEDE/RFZ design the ISD gap needs to be longer than ITD (intial time delay) of the recorded room. It also needs to fall in the Haas zone. The idea is to hear no control room reflections until after you have significantly heard the reflections within the recording.
Take note the LEDE/RFZ design is something else than a "reflection free zone" and was a development of the LEDE design.

In a listening room, the ISD gap will depend on the room geometry and placement. But ideally, a ISD gap of 20-25 ms would be ideal because it enables one to hear much of the recorded signal, it's within the Haas zone and similar to the best concert halls in the world. In many small rooms, such a long ISD gap isn't possible though and has to be shortened. You basically do what you can do from the type of room you have. But a shorter ISD gap will not be as spacious and lively of one with a longer one besides not giving the ability to hear as much of the recording room when that's appropriate (many modern recordings are done in tiny rooms).

There's a lot that could be said about this. For example the termination of the ISD gap is very important. The level will decide how much liveliness of space you achieve and how well you can cover earlier arriving audible reflections. While treatment will attenuate specular reflections, it's common to still have some minor ones that's audible. A strong termination of the ISD gap tricks the brain to overlook these besides giving a boost of energy/liveliness which is very pleasing. The purpose of the ISD termination is also to remove the localization cues of the later arriving energies and to reinforce the localization cues of the direct energy. There's quite a bit going on here and how it relates to each other.
 
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Crazy question.

What if you deliberately added into your main signal a 6 ms delayed signal...to purposefully overwhelm inadvertent room reflections with intentional fake ones?
 

dasdoing

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There isn't to my knowledge an objective definition of early and later reflections. But normally we're talking about early reflections till 10-12 ms area and late after that.

The graph is from Peter D'Antonio's book but I don't think it says from what specific room. Below is also the before impulse.
View attachment 76250

The RFZ or ISD (initial signal delay) gap of that graph is around 18 ms. Where you see the sudden rise in level, is where you have the termination of the ISD gap. How long should the ISD gap be? In a control room and with a LEDE/RFZ design the ISD gap needs to be longer than ITD (intial time delay) of the recorded room. It also needs to fall in the Haas zone. The idea is to hear no control room reflections until after you have significantly heard the reflections within the recording.
Take note the LEDE/RFZ design is something else than a "reflection free zone" and was a development of the LEDE design.

In a listening room, the ISD gap will depend on the room geometry and placement. But ideally, a ISD gap of 20-25 ms would be ideal because it enables one to hear much of the recorded signal, it's within the Haas zone and similar to the best concert halls in the world. In many small rooms, such a long ISD gap isn't possible though and has to be shortened. You basically do what you can do from the type of room you have. But a shorter ISD gap will not be as spacious and lively of one with a longer one besides not giving the ability to hear as much of the recording room when that's appropriate (many modern recordings are done in tiny rooms).

There's a lot that could be said about this. For example the termination of the ISD gap is very important. The level will decide how much liveliness of space you achieve and how well you can cover earlier arriving audible reflections. While treatment will attenuate specular reflections, it's common to still have some minor ones that's audible. A strong termination of the ISD gap tricks the brain to overlook these besides giving a boost of energy/liveliness which is very pleasing. The purpose of the ISD termination is also to remove the localization cues of the later arriving energies and to reinforce the localization cues of the direct energy. There's quite a bit going on here and how it relates to each other.
It says small room, but the room can't be that small. 20ms require 6,86m of adicional(!) travel time. So for the backwall as an easy example you have to have this 6,86m of travel behind your listening position*. in smaller rooms you have no choice but creating a dead-ish room
*yea, you can difuse the backwall, but this also requires distance

personaly I never understood the problem with dead-ish rooms. you get used to them very quick and the sound is very clean. but my rooms never had carpet, so they never have been that dead
 

Kvalsvoll

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Actually, it is possible to define direct - reflected - diffuse. And in the answer lies the clue to understand the difference between small and large rooms:

Direct is easy - sound only from the speaker, it has pressure amplitude and direction (Exact properties depends on the radiator).
Reflected is sound reflected 1 or more times from a surface, sound that has pressure amplitude and still has direction.
Diffuse sound is sound that has reflected from several surfaces, mixed together so the resulting sound field has pressure amplitude, but no direction.

There is a difference between discrete reflections and diffuse reverb in that the former actually has direction, the latter has no direction. And for a diffuse field to develop, it requires a space large enough to create enough reflections from different angles at different time.

Since the diffuse sound has no direction, it does not affect the direct sound in the same way that reflections with direction does. Then we also understand why it is necessary to attenuate reflections in a small room.
 
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