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

[wgb113]

Would it be wrong to simply suggest to the OP that it's a matter of preference when it comes to 1st reflections in their room as to whether the diffuse/absorb/do nothing?

 

[Bjorn]

Would it be wrong to simply suggest to the OP that it's a matter of preference when it comes to 1st reflections in their room as to whether the diffuse/absorb/do nothing?

It think that's valid. It highly depends on areas like:
- Distance to surfaces
- How the rest of the room is treated (or not)
- Directivity of speakers
- How the speaker is equalized and whether you can EQ them yourself or not afterwards
- What music material you listen to
- The quality of the treatment (also depended on budget)

That being said, you don't find many who have treated broadband that go back to no treatment. Also, a benefit of such treatment is also tightening up part of the upper low frequencies as well. Who doesn't like a tigther and more definable bass?

This is an area I don't think short listening studies tell the whole truth by the way. What's most impressive right away isn't necessarily what's preferred over time in my experience. That's part of the reason I prefer studies on accuracy. "Preference" isn't very straightforward.

 

[Igor Kirkwood]

This is why we have targets of 0.2 to 0.5 for "RT60" for our listening spaces. If RT60 had no value, we could not make such suggestions.

For Masterings Studios we have (the bad ??) habit to mesure and speak about "RT 60"
Here 2 curves of "RT 60" of my 2 last studios, same loudspeakers
-55 m3 RT 60 of 0,34
-130 m3 RT 60 of 0,2

Notice best results, even in stéreo, in the studio of 130 m3

 

[j_j]

My "pedantry" in the area is actually the belief the community of small room acoustics and was established decades ago.

Modern instrumentation, measurement, and understanding have long, long since made your "belief" obsolete. It is telling that you use the word "belief"rather than 'understand' or the like. This explains your almost religious advocacy for an obsolete idea.

Even back in the 1980's Manfred (Schroeder), Jim Flanagan, Jont Allen, and Joe Hall were questioning the idea of orthogonality above the "Schroeder Frequency", just for a vaguely related point. I know, I was there. Measurement wasn't quite up to it then (memory size was the limit, not mathematical understanding, by the way), but is trivially done in Matlab (no you can't have my code, it's proprietary) nowadays.

I also know at this point that the separation between perceptually diffuse and mathematically diffuse is now understood, and well established, but you will need to measure at least two points in a room to fully establish that. Therein lies the weakness in small rooms with any substantial "life" in them. If you're talking about listening rooms, perceptual issues rule. If you're talking about feedback in a PA system, then analytic diffusion is more important.

Sometimes, you know, the other guy actually knows what he's talking about. You need to actually consider that before you utter truly offensive professional accusations. You still haven't retracted them, you know, so they are still standing in public view, and I regard every reader who sees them as being harm to me. So what do you plan to do about that, now?

Those of you who wonder why I'm so damn prickly about this haven't walked in my shoes for the last 30 years, being told the same old, obsolete crap over and over and over. It wasn't a big problem at BTL, because people were there to figure things out (they might want to win the argument as well, but the overall goal was to figure out how things worked), but substantially in the AES, to some extent in the IEEE, and horribly so in ASA, people simply wanted to play "king of the hill". Then I moved elsewhere (Amir knows where I mean) and ran into an entire community of managers who only wanted to win arguments and embiggen their head count, and into privately owned companies where both egos and panic ruled the roost.

So I've had some interesting experiences. But eventually, it all has to work. Physics works. I say that for a reason.

 

[j_j]

This is an area I don't think short listening studies tell the whole truth by the way. What's most impressive right away isn't necessarily what's preferred over time in my experience. That's part of the reason I prefer studies on accuracy. "Preference" isn't very straightforward.

You mean "difference evaluation" perhaps?

In 2 channel stereo, the only question is "which illusion do you prefer", in any case. So preference is preference and never the mane shall tweet.

 

[j_j]

Would it be wrong to simply suggest to the OP that it's a matter of preference when it comes to 1st reflections in their room as to whether the diffuse/absorb/do nothing?

No, it would not be. There are some things that must be done (clobber terrible modes for instance) but then the question is "what does the listener prefer".

 

[Kal Rubinson]

Physics works. I say that for a reason.

Is there any choice?

 

[amirm]

Small rooms that are treated according to RTx measurements end up most of the time becoming encredible poor and needs to be fixed later.

Who does this kind of thing in small rooms? RT60 in all cases, good and bad, is used as a secondary or even tertiary metric. Primary metrics are frequency response, reflections (treat or not), etc. RT60, or frankly someone's ear, is used to determine if the room is too dead or too live. That's all.

 

[amirm]

If true reverberation time existed in small rooms, that would imply that thin absorption would work very well.

Only if you ignored other facts that you want such absorption to be broadband in which case 4 inch becomes minimum.

It is true that for a single number value, RT60 at 500 Hz is used by that is a convention and again part of a rough metric.

 

[j_j]

Is there any choice?

Well, not according to Bell's Theorem ( Note, kind of a joke, kind of not. )

 

[Bjorn]

Who does this kind of thing in small rooms? RT60 in all cases, good and bad, is used as a secondary or even tertiary metric. Primary metrics are frequency response, reflections (treat or not), etc. RT60, or frankly someone's ear, is used to determine if the room is too dead or too live. That's all.

It's commonly done by those who believe RT60 exist in a small room. Typically large room acousician who don't understand small room acoustics is completely different and they wrongly use large room acoustics methods to small rooms. It happens often.

I true reveberartion time existed in small room one could also rely on reverberation measurements of absorbents. That was my point with thin absorptiont. A well designed absorbent that is 2" thick measures with an absorption coefficient of 0.8 to 250-270 Hz area. In other words sufficient absorption to the schroeder frequency in most small rooms. But still it doesn't in real life, because the conditions for reverberation simply doesn't exist in small rooms.

Like pointed out in the link about small room: 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.

 

[amirm]

Like pointed out in the link about small room: 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.

I corrected you on this very thing three years ago: https://www.audiosciencereview.com/.../lets-discuss-room-correction.1448/post-38324

-----------------------
Please read the post that I wrote that invalidates all of this. Dc does exist. And by definition, anything past transition frequencies is random (enough). That is what Schroeder frequency is: the point where enough modes pile up on top of each other as to create the equiv. of randomness/lack of distinct nodes.

All of this confusion was created online by misreading Davis' and Dr. Toole's texts. And by repeating it enough times, it seemed to be reality to some. But it just isn't true. Please see all of these points addressed here: http://audiosciencereview.com/forum...large-small-rooms-a-matter-of-statistics.569/

This is for example the Dc (Critical Distance where reflections equal to power to direct sound) of my living room:

The solid black line and marker at 1.0 is the (relative) critical distance. As you see my room very closely follows that. Response no longer get by power of two as I go past that, showing that reflections are dominating.

Here is another from AES/ASA paper measuring small rooms and their Dc:

As you see they all have critical distances despite the rooms being acoustically "small."
--------------------

You are quoting Davis' ancient text without understanding the context under which he made that statement.

 

[Bjorn]

Only if you ignored other facts that you want such absorption to be broadband in which case 4 inch becomes minimum.

It is true that for a single number value, RT60 at 500 Hz is used by that is a convention and again part of a rough metric.

In a large room with true reverberation you can rely on random incidence measurements of acoustic treatment. If true reverberation existed in small rooms, you could do the same there. But that's not the case. Absorbents function very differently in small rooms compared to large rooms. Something that we see when measuring the rooms as well. The reason is because reverberation time doesn't exist in small rooms.

The small acoustical space is dominated by focused specular reflections. The days of shotgun application of treatment which leads to dead rooms with no control of specular reflections should have been over a long time ago. Which is what you get by following RTx measurements. We have better tools.

There's no misunderstanding here. Fundamental point: modal decay rates are not reverberation. Reverberation is “the time in seconds that it takes a diffuse sound field, well beyond a real critical distance, to lower in level by 60 dB when the sound source is silenced.” Modal decay rates are dB-per-second (dB/s) rate of decay for a specific modal frequency.

 

[Bjorn]

While I doesn't necessarily agree with everything Toole writes about acoustics, he also seem to understand that reverberation doesn't exist in small rooms and points it out.
From the following paper: http://www.aes.org/e-lib/browse.cfm?elib=17338

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.

We continue to talk about reverberation 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.

There's something wrong with the measurements of the Dc. I need to look at it closer later.

 

[March Audio]

There's no misunderstanding here. Fundamental point: modal decay rates are not reverberation. Reverberation is “the time in seconds that it takes a diffuse sound field, well beyond a real critical distance, to lower in level by 60 dB when the sound source is silenced.” Modal decay rates are dB-per-second (dB/s) rate of decay for a specific modal frequency.

Oh yes there is. You keep misunderstanding or ignoring the fact that multiple posters have specifically differentiated modal content. You seem to be under the impression that people are just taking a Rt single value at a spot frequency, or at modal frequencies and basing their treatments on this.

Amir has covered Dc.

 

[Igor Kirkwood]

Who does this kind of thing in small rooms? RT60 in all cases, good and bad, is used as a secondary or even tertiary metric. Primary metrics are frequency response, reflections (treat or not), etc. RT60, or frankly someone's ear, is used to determine if the room is too dead or too live. That's all.

Of course the frequency response of the room is the more important .
Firsts reflexions so gating : (2 ms/5ms/10ms/20ms/50ms/100ms)

For the RT 60 I prefer write "RT 60" even when used in smalls rooms, but I don't fight for that ! !
But for the quality of the sound of listening room I olso prefer "RT 60" of 0,2.
Notice what is important, especially in small rooms is the "RT 60" dont fall in treeble frequencies . Its gives bad and dead sound

 

[j_j]

Absorbents function very differently in small rooms compared to large rooms. Something that we see when measuring the rooms as well.

The physics surrounding the absorber are exactly the same. One reason that someone might make this mistake is that they have not measured T20 or T60 as a function of frequency. That, of course, also is the clear answer to understanding modes, as well as the clear understanding of where and when transition happens, first perceptually, and then analytically. This is also a mistake people make when they insist that spacing absorption makes no difference to absorption, and don't realize that one must look at T60 as a function of frequency, as opposed to energy envelope or at one frequency. Of course, you've finally made it clear that your entire mistake is one of semantics, you insist that everyone use your obsolete definition of reverberation, rather than understanding that we are all talking about are impulse response issues.

You have, still, not responded to the issues of stationarity of impulse response Perhaps thinking about that might help you realize you have an oversimplified view of the subject.

As to critical distance, well, you avoid the whole issue of that also being a function of frequency. Again, you refuse to deal with the fact that any modern treatment addresses everything as a function of frequency, not one simplified number.

 

[j_j]

Oh yes there is. You keep misunderstanding or ignoring the fact that multiple posters have specifically differentiated modal content. You seem to be under the impression that people are just taking a Rt single value at a spot frequency, or at modal frequencies and basing their treatments on this.

Amir has covered Dc.

And, of course, modal decay as a function of frequency is nothing more or less than measuring T60 as a function of frequency. Did you notice how his phrasing very carefully disguised that?

 

[j_j]

Of course the frequency response of the room is the more important .
Firsts reflexions so gating : (2 ms/5ms/10ms/20ms/50ms/100ms)

For the RT 60 I prefer write "RT 60" even when used in smalls rooms, but I don't fight for that ! !
But for the quality of the sound of listening room I olso prefer "RT 60" of 0,2.
Notice what is important, especially in small rooms is the "RT 60" dont fall in treeble frequencies . Its gives bad and dead sound

You might look at the perceptual discussion in the slide deck that bjorne defamed. That has some clear comments about direct and reverberant sound in regard to perception. You are absolutely correct that things can go very wrong with some combinations of speaker polar patterns and particular absorber and diffusor layouts.

 

[Bjorn]

Here is another from AES/ASA paper measuring small rooms and their Dc:

As you see they all have critical distances despite the rooms being acoustically "small."
--------------------

A gather of data doesn't prove anything. You need to consider the conditions. From the paper you posted the graph:

The new method uses measurements made within the critical distance; it does not require that the reverberant energy be uniform in the room.

They are overlooking the preconditions.