Poly Diffuser effectiveness?

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Postby Eric.Desart » Fri Jan 18, 2008 8:24 am

Bob wrote:BTW, in home theatre 80hz is the THX crossover point for main speakers to subwoofers. i.e. the point where THX felt that localization became difficult. Some claim they can still tell at 80hz, but the lower below that the fewer that claim as far as I know.


Bob,

Thanks for your post and included link. I personally didn't want to reenter it anymore, until knowing the why, but for me it's OK.
I could reenter it here in the Studiotips files directory as well.
I did not enter new data on the net, and I used it in a transformative educational manner, respecting the original name of designer and studio.
Section 107 of the Copyright Act of 1976. Limitations on exclusive rights: Fair use

This 80 Hz is rather common for better systems. (Genelec has settings for 80 and 120 Hz I believe).
But since these low frequencies can be rather disturbing via the mains (modal bahavior, interference) there is a plus and a minus involved in shifting this crossover higher.  The higher the easier to control acoustics with treatment, even at the possible (not necessary) expense of low frequent localization info.
.
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Postby Trip- » Sat Jan 19, 2008 8:02 am

Love reading posts like that... Lotsa teachers - feels like there's a test tomorrow on this subject :)

A practical addition,
in my previous listening room, when I was playing only Left speaker - signals of around 110-120hz, the percieved "direction" or space from which the signal was coming from was back-right. And when both speakers were turned on, "direction" was changed to 'kinda' right-front. This is unlike feeling the whole room resonating modally from any place at the LP. Room had untreated back corners, modal points etc... Scott, in that situation I believe the level of modal excitment of the back corner was higher than the speaker's, although this is lower than half of 300hz - OR it was the phase shift that made me 'think' the back corner is louder and thus a direction source.

on the topic,
I wanna make polys already!!!! Just for the spatiousness, the studio feels sucked out a bit.
Symetrical on both walls, six on each wall - 90 angled - square panels.
The only question for you guys would be, 50x50cm poly would diffuse what range? I'm sure it will work on the 2-5khz, which is what I want for the feeling, but just to know what's the boundaries?
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Postby BIG8 » Sat Jan 19, 2008 10:42 am

Trip- wrote:Love reading posts like that... Lotsa teachers - feels like there's a test tomorrow on this subject :)


you're welcome!  :8

let me know if you want to read more, I'll take care of that!   :lol:

Cheers,
Jean
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Postby avare » Sat Jan 19, 2008 2:05 pm

Trip- wrote: 90 angled - square panels.
The only question for you guys would be, 50x50cm poly would diffuse what range?


The general guideline is down to the frequency where the depth of the diffuser equals the wavelength.  For 50cm 90º diffusers the depth 14.6cm giving a theoretical low frequency of 2 kHz.

Andre

[edit] corrected http error
Last edited by avare on Sun Jan 20, 2008 1:21 pm, edited 1 time in total.
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Postby Ido » Sun Jan 20, 2008 10:34 am

Bob wrote:......b) small front wall absorbers (not the entire front wall)
....


Bob, any mention of why the differentiation bewteen full coverage and non-full coverage of the front wall?
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Postby Bob » Mon Jan 21, 2008 3:45 am

Ido.
Hmm. Google books doesn't have the one page you want.
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Postby Bob » Mon Jan 21, 2008 3:51 am

Ido
Not really.
It's more about killing all reflections except side wall 100%, but only the reflections (not the entire front wall, not the entire ceiling, not the entire floor).
And then reducing the side wall reflections to increase spaciousness.
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Postby Terry Montlick » Mon Jan 21, 2008 4:04 pm

Bob wrote:Master Handbook Of Acoustics 4th, pg 409 to 414 have some interesting ideas on this.
The graph Figure 19-5 relates "reflection delay ms" against "reflection level dB" giving spaciousness vs image effects.
The above ITU rule represents a single point on this graph -- for different delays, the dB amount should be different.
Table 19-2 brings in distance dB drops (no absorber, no diffuser, just distance).
And the text says:
a) kill the reflections from ceiling and floor
b) small front wall absorbers (not the entire front wall)
c) start with thick absorbers of minimum surface area at the left and right walls, then make them thinner and thinner until the desired spaciousness is achieved. Thinner would include: velour, cloth, carpet.

Hey Bob,

I don't have the 4th edition of this book (1st instead! :D), so I haven't seen the graph. Is there an original reference for this? Most of Everest's stuff is taken from other sources.

Thanks,
Terry
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Postby Bob » Tue Jan 22, 2008 3:52 am

Ido and Terry:

For "Master Handbook of Acoustics, pages 409 to 414", just click on the book.

Image
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Postby Ido » Tue Jan 22, 2008 12:16 pm

Bob wrote:Ido and Terry:

For "Master Handbook of Acoustics, pages 409 to 414", just click on the book.

Image


thanks as always Bob.

to my way of thinking it's less an accurate thing and a more fluid thing (regarding areas of coverage and their locations), but never mind.
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Postby Terry Montlick » Tue Jan 22, 2008 3:01 pm

Bob wrote:Ido and Terry:

For "Master Handbook of Acoustics, pages 409 to 414", just click on the book.

Cool! You are a wonder, Bob. :)
I thought such a diagram was most likely from Olive and Toole's 1988-89 paper. It is a classic!

Regards,
Terry
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Postby andrebrito » Sat Jun 07, 2008 11:39 am

QRDs have advantages over polys and the opposite is also true.

I would rather discuss where each should be applied other than A is better than B

1 - would you use polys on stages ( I would not, I have only used QRD) ?

2 - would you use QRDs as first reflectors (I would not, have only used poly!) ?

etc etc...
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Postby bert stoltenborg » Sat Jun 07, 2008 11:45 am

What is the advantage of a QRD over a poly?
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Postby andrebrito » Sat Jun 07, 2008 12:49 pm

Advantages of the QRD

A QRD 1D provides temporal diffusion (there's a paper regarding that matter done by Trevor Cox, I can't seem to find it)

Advantages of a poly

better spatial diffusion

Not sure about the poly's absorption...  but the QRD and other grating diffusers have some absorption which maybe inconvenient for large room acoustics, depending on the desirable reverberation time and bass ratio of the room. So you need to be careful on the amount of diffusers used to create spatiality  and avoid echoes and at the same time get the required deacy of sound for the room's purpose
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Postby andrebrito » Sat Jun 07, 2008 12:51 pm

I found this link


http://www.acousticsfirst.com/articles/ ... fusion.htm

It has a chart regarding the absorption of a poly... it has an higher amount of absorption compared to a QRD. That might be ideal for a music studio or speech purpose room, but not for a music room where the bass ratio should be at least 1.2

Can anyone confirm this data regarding poly's absorption?
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Postby Eric.Desart » Sat Jun 07, 2008 1:53 pm

Andre,

The absorption of a poly is completely dependent on the construction. Hence you cannot just take an arbitrary curve of a poly interpreting this as some representative information.

A free hanging poly which is stiff and heavy will absorp very little (also depending on relation with a  backing wall or not).

Hence there is no general answer to your question.

Polys are used in labs exact for their low absorption. It shouldn't be possible to use Schroeder like stuff as substitute.
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Postby bert stoltenborg » Sat Jun 07, 2008 2:47 pm

A qrd in fact makes a mess of a reflection.
I think the common theory is that a reflection should have a linear freq response.
At least in smaller listening environments.
A 'normal' poly is fairly linear. An array of poly's is another story, of course.
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Postby andrebrito » Sat Jun 07, 2008 10:53 pm

Eric:

That's why I have asked if someone could confirm this since I could not find any official data regarding this matter. The only thing I have comparing QRD to poly is regarding diffusion, and basically is the Farina and the Trevor Cox paper.

Bert:

Yes that is expected , it creates a kind of comb filtering,. but it does not make a reflection in terms of time and the poly does... this is what Trevor Cox considered to be temporal diffusion I believe. He has a paper on this comparing the QRD and the poly
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Postby Eric.Desart » Sat Jun 07, 2008 11:43 pm

andrebrito wrote:Eric:

That's why I have asked if someone could confirm this since I could not find any official data regarding this matter. The only thing I have comparing QRD to poly is regarding diffusion, and basically is the Farina and the Trevor Cox paper.


Andre,

It's not clear if my reply answered your question or gave you the impression I diverted my answer from your question.

With a QRD absorption and diffusion go (partly) hand in hand. The design needed to get diffusion by definition will show absorption.
With polys it isn't.  The poly principle can perfect without any significant absorption. If you build a poly as an intrinsic part of a concrete wall it will not absorp (matter of speech).  If you build it like a bend panel trap, a mass-spring system, a lightweight panel it will absorb.  But you're not speaking about absorption related to the poly diffusion principle but to a construction triggering other phenomena.
Therefore you can't make a comparison between the absorption of a poly as a diffuser and whatever else, because the absorption isn't related to the poly diffusing principle.
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Postby andrebrito » Sun Jun 08, 2008 2:00 pm

Eric,

I actually was only thinking about polys made like a bend trap panel and not polys that are part of the architecture of the building, made from heavy weighted materials.  

Bert and others,

I have found the paper from T. Cox regarding QRD vs. polys... here it goes.
Attachments
diffusers trevor cox.pdf
QRD vs polys by Trevor Cox
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