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Calculating slat / slot width for a binary diffuser Diffusion Products
Old 6th September 2011
  #1
Gear Maniac
 

Calculating slat / slot width for a binary diffuser

I want to add some binary diffusion to some broadband absorbers on the side walls of my project studio. What I have in mind is a binary sequence slat / slot diffuser like the ones that boggy posted here, just smaller: https://www.gearslutz.com/board/6976716-post31.html

The absorbers I want to cover measure 60x100cm. I am trying to use Porous Absorber Calculator to get an idea of how wide the slats should be ideally. But something seems to be wrong with the Excel sheet. It simply can't be that absorption is so low, given that the open area is 50% and the absorption behind the slats 40cm???

Note: I know that the results from Porous Absorber Calculator can't be fully applied to my design, because PAC assumes that slat width and spacing is always the same. But in my design, the open area will also be roughly 50%, so I figured the results would be useful anyway.

I was thinking of using slats 3cm / 1.2" wide. Would this be ok for the area I want to cover? Should I attach the slats vertically or horizontally? What's the difference?
Attached Thumbnails
Calculating slat / slot width for a binary diffuser-slotted_panel.gif   Calculating slat / slot width for a binary diffuser-243070d1309599289-can-bamboo-wall-treatment-help-dead-room-big_pic10.jpg   Calculating slat / slot width for a binary diffuser-243071d1309599289-can-bamboo-wall-treatment-help-dead-room-big_pic6.jpg  
Old 7th September 2011
  #3
Gear Guru
Bounce

The PAC has no real relevance to the random nature of Binary Amplitude Diffusion Gratings. I may not have the terminology exactly right, but I am sure the meaning is clear.
If you want simple, take the Newell sequence of 345 and simply multiply to get your slat and gap sizes. Slat Gap Slat Gap .......
Bigger sizes will interfere with the sound at lower frequencies.
We could speak of this in terms of 'tone' without getting numerical.
Boggy uses more sophisticated sequences. John Brandt, Rod, and Jeff, and I think Wes, have lovely looking ones on their sites.
Overall any old random collection of nice looking wood will do the biz. If the biz is a bit of HF bounce back with no real danger of tuned issues or directional lobes etc.
DD
Old 7th September 2011
  #4
Gear Maniac
 

Ok, so what frequencies exactly are reflected by a slat of any given width? Frequencies with a wavelength equal or shorter than the slat width?

Regardless of my own reflecting device, isn't there something wrong with the Slotted Panel sheet in PAC? Why is absorption so low in the graph?
Old 7th September 2011
  #5
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Jens Eklund's Avatar
Quote:
Originally Posted by bermudaben View Post
Regardless of my own reflecting device, isn't there something wrong with the Slotted Panel sheet in PAC? Why is absorption so low in the graph?
The last time I used the PAC (I use "SoundFlow" and "Reflex" now), it did not compute slotted panels correctly.
Old 7th September 2011
  #6
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boggy's Avatar
Quote:
Originally Posted by bermudaben View Post
.........
I was thinking of using slats 3cm / 1.2" wide. Would this be ok for the area I want to cover?
Yes, it's ok. Use coin flipping to generate a longer sequence than MLS I provided: https://www.gearslutz.com/board/6890286-post6.html
Quote:
Originally Posted by bermudaben View Post
Should I attach the slats vertically or horizontally? What's the difference?
If I can, I always place slats perpendicular to direction of sound radiation from loudspeakers. This means that orientation of slats at side walls need to be vertical, for example. But, I find that acousticians usually have horizontally oriented slats at side walls, and I suppose that his reasons for this is only aesthetic. In this case we have some liveliness in room but not diffusion.

Calculation of "resonant frequency" of combined binary diffuser and wideband porous absorber isn't easy, but it's not needed really. Adding slats lowering working frequency of existing porous absorbers without slats, but because there are many combinations of slat/gap sizes, results are superposition of many theoretical working frequencies with fairly low Q factor. Combination of all this frequencies is again a wideband absorber for low frequencies which reflect mid and high frequencies in better way than ordinary porous absorber (we need this).

Also, we really don't need "100%" reflections of mid and high frequencies, so, binary diffusers may add enough reflected sound energy in room with fully absorptive treatment to satisfy AES/EBU/ITU recommendations for control rooms:

1. "Multichannel surround sound systems and operations", AES Technical Council, Document AESTD1001.1.01-10, New York.
2. “Methods for the subjective assessment of small impairments in audio systems including multichannel sound systems”, ITU-R Recommendation BS.1116 (rev. 1), ITU, Geneva, 1997.
3. W. Hoeg, L. Christensen, R. Walker, “Subjective assessment of audio quality– the means and methods within the EBU”, EBU Technical Review Winter 1997, pp. 40-43.
4. "Listening conditions for the assessment of sound programme material: monophonic and two–channel stereophonic", EBU Tech. 3276 – 2nd edition May 1998, European Broadcasting Union, Geneva, Switzerland

Adding a binary diffuser over porous absorbers is very "soft" change which cannot generate any "spectacular" behavior in the room, only some better absorption of LF, liveliness and diffusion sound field which did not exist before.
Old 7th September 2011
  #7
Gear Addict
 
Yeah, right...'s Avatar
 

Quote:
Originally Posted by DanDan View Post
Overall any old random collection of nice looking wood will do the biz. If the biz is a bit of HF bounce back with no real danger of tuned issues or directional lobes etc.
DD
Hmmm, a timely thread...

I'm about to do exactly the same thing to two 1.5m x 1.5m broadband side traps, for the same reason - to get a bit of HF bounce back. My room is quite small (3.5m x 4.5m) and is heavily "trapped."

From what you've said, am I correct in my understanding that to achieve a bit of HF bounce back, it doesn't really matter what the slat size or the gap between them is? I do assume that both traps should be identical, slat & gap-wise?

For example, would using 40mm x 25mm slats, both flat & on edge, plus a few, say 70 x 25's interspersed, with gaps of say 12 - 25 do "the biz?"

Thanks in advance, Mike
Old 7th September 2011
  #8
Gear Guru
Any Old

I reckon that would be fine Mike. Although lean towards the random part of 'any old random'.
I am using this homey language quite deliberately.
Personally I shy away from calling these things Diffusors or BADG etc.
They have a gentle tonal effect, but is is noticeable and welcome. Remember that an entire half of the surface has been made reflective.
Common sense and visual intuition would guide me somewhat. I wouldn't go with 2 foot wide slats and gaps a couple of feet from my ears, but these would be fine at some distance, say the back wall.

+1 to Boggy's entire post, I am bookmarking it. I hope the generosity shown in sharing such actual hard won information is appreciated.

DD
Old 7th September 2011
  #9
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Yeah, right...'s Avatar
 

Quote:
Originally Posted by DanDan View Post
I reckon that would be fine Mike. Although lean towards the random part of 'any old random'.
I am using this homey language quite deliberately.
Personally I shy away from calling these things Diffusors or BADG etc.
They have a gentle tonal effect, but is is noticeable and welcome. Remember that an entire half of the surface has been made reflective.
Common sense and visual intuition would guide me somewhat. I wouldn't go with 2 foot wide slats and gaps a couple of feet from my ears, but these would be fine at some distance, say the back wall.

+1 to Boggy's entire post, I am bookmarking it. I hope the generosity shown in sharing such actual hard won information is appreciated.

DD
Thanks for the clarification DD. Actually, Boggy's post pretty much answered my query, but he posted it while I was writing mine & I didn't see it in time.

Thanks again & best regards, Mike
Old 7th September 2011
  #10
Gear Maniac
 

do slats give better diffusion than space couplers?
Old 7th September 2011
  #11
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boggy's Avatar
Quote:
Originally Posted by Chris_Nicks View Post
do slats give better diffusion than space couplers?
Ordinary slat absorber with same slats and gaps is a periodic construction similar to space couplers.

Diffusers needs to have at least aperiodic structure to avoid lobbing in their radiation characteristic (that is common with periodic structures). Random number sequences are a way to have aperiodic structure in construction.

Binary diffuser construction is based on binary random number sequency (0-no slat,1-slat) that may be generated using algorithms like Maximum Length Sequence (pseudo-random), or coin flipping (random)... and this way lobbing is minimized, diffusion is much more uniform over working frequency range.

Binary diffuser have better functionality than both acoustic coupler and ordinary slat absorber in many ways.
Old 7th September 2011
  #12
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boggy's Avatar
Quote:
Originally Posted by Chris_Nicks View Post
do slats give better diffusion than space couplers?
Idea 1: you can make a 2D binary diffuser from a space coupler if you close half of openings using some random sequence already described.

Idea 2: You can make a 1D binary diffuser if you remove half of slats from uniform slat absorber usign some random sequence...

heh

EDIT: you can use even glasses to close openings of space coupler https://www.gearslutz.com/board/874061-post23.html
Old 7th September 2011
  #13
Gear Guru
Words

I prefer not to use the word diffusion for simple slat arrangements or space couplers. Or Polys for that matter. How about scattering?
DD
Old 7th September 2011
  #14
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
I prefer not to use the word diffusion for simple slat arrangements or space couplers. Or Polys for that matter. How about scattering?
DD
Yes.

Old 7th September 2011
  #15
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Glenn Kuras's Avatar
Quote:
Or Polys for that matter. How about scattering?
When something scatters it is diffused, but we can play this word game for hours.
Old 7th September 2011
  #16
Gear Addict
 

The word "diffusor" for acoustic elements was originally describing the poly diffusor, abit awkward going back on this isn`t it?
Old 7th September 2011
  #17
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Jens Eklund's Avatar
Quote:
Originally Posted by Glenn Kuras View Post
When something scatters it is diffused, ...
When discussing diffusers one normally talks not only about diffusion and scattering coefficients but also spatial and temporal diffusion.

Any object scatters energy more or less due to diffraction but a good diffuser needs to scatter the energy evenly in the plane or planes it operates in (1D or 2D) or it is not a diffuser. A simple pane panel can have a very high scattering coefficient for a certain angle of incident if angled slightly but it is naturally not a good diffuser since it just redirects the energy in another direction. Even a devise like a poly that at least if only one period, offers very good and even spread of energy, it offers little (or no, if single period) temporal diffusion so if one wants to be strict; it is not a good diffuser either.
Old 7th September 2011
  #18
Gear Addict
 

or one could be less strict and use several of them in an array(with different width/debth) ...voila, much better temporal diffusion.
Old 7th September 2011
  #19
Gear Guru
 
Glenn Kuras's Avatar
Quote:
Originally Posted by Jens Eklund View Post
When discussing diffusers one normally talks not only about diffusion and scattering coefficients but also spatial and temporal diffusion.

Any object scatters energy more or less due to diffraction but a good diffuser needs to scatter the energy evenly in the plane or planes it operates in (1D or 2D) or it is not a diffuser. A simple pane panel can have a very high scattering coefficient for a certain angle of incident if angled slightly but it is naturally not a good diffuser since it just redirects the energy in another direction. Even a devise like a poly that at least if only one period, offers very good and even spread of energy, it offers little (or no, if single period) temporal diffusion so if one wants to be strict; it is not a good diffuser either.
Wow I make a joke (I thought this was clear ) but you took the time to write all of that? What is your paypal email? I want to send you money for a drink. YOU NEED ONE!! lol
Old 7th September 2011
  #20
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Jens Eklund's Avatar
Quote:
Originally Posted by Glenn Kuras View Post
Wow I make a joke (I thought this was clear )
Not many people know the difference between scattering and diffusion. How many know that even if you have QRD N7s (for example), you need to put a few of them together in periods (side by side, not with a space between them) in order to get proper diffusion instead of just some "scattering" ...
Old 8th September 2011
  #21
Gear Maniac
 

thanks for the explanation boggy. And I still would prefer QRD:D
Old 8th September 2011
  #22
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avare's Avatar
 

Quote:
Originally Posted by boggy View Post
Idea 1: you can make a 2D binary diffuser from a space coupler if you close half of openings using some random sequence already described.

Idea 2: You can make a 1D binary diffuser if you remove half of slats from uniform slat absorber usign some random sequence...

heh

EDIT: you can use even glasses to close openings of space coupler https://www.gearslutz.com/board/874061-post23.html
+1, 2 and 3. Too many people are mislead by RPG's advertising to believe that only their style of diffusers are real diffusers.

Andre
Old 8th September 2011
  #23
Gear Maniac
 

Most people say qrd's minimum effective distance from listener is 10ft, is there a minimum with space couplers?
Old 8th September 2011
  #24
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Jens Eklund's Avatar
Quote:
Originally Posted by avare View Post
Too many people are mislead by RPG's advertising to believe that only their style of diffusers are real diffusers.

Andre
Sorry Andre but I think that’s a tad unfair.


RPG (Dr. Peter D’Antonio) is after all an authority when it comes to diffusers:

RPG Diffusor Systems
Acoustic absorbers and diffusers ... - Google Böcker
Old 8th September 2011
  #25
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Jens Eklund's Avatar
Quote:
Originally Posted by Chris_Nicks View Post
Most people say qrd's minimum effective distance from listener is 10ft
The general rule of thumb (assuming a position that receives scattered energy from the diffuser, not necessarily the case if 1D diffusers), the recommended distance for a QRD is about 3x the design frequency (wavelength) of the diffuser. Usually this does not matter much in control room design since the required ISD-gap often determines the distance to diffusing surfaces.
Old 8th September 2011
  #26
Gear Maniac
 

ohh.. So in a control room where I am only 8ft away from the back wall, what diffusor would you recommend to be placed on the back wall?
Old 8th September 2011
  #27
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Jens Eklund's Avatar
Quote:
Originally Posted by Chris_Nicks View Post
ohh.. So in a control room where I am only 8ft away from the back wall, what diffusor would you recommend to be placed on the back wall?
In order to answer that question one needs to know:


* Which design philosophy are you aiming for?

* If LEDE/RFZ, do you have a recording space connected to the control room and if so, what is the ISD of the recording space?

* What is the current ISD-gap of the control room (if any) and if manly due to the back wall, at what level is the termination now?


What I’m trying to say here is; don’t just throw in a diffuser without exploring the correct ways of implementing them. It might end up as a waste of money. There are many threads on this topic, I suggest that you read some of them.
Old 8th September 2011
  #28
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boggy's Avatar
Quote:
Originally Posted by Chris_Nicks View Post
ohh.. So in a control room where I am only 8ft away from the back wall, what diffusor would you recommend to be placed on the back wall?
I personally believe that QRD diffusers which works from 1kHz and up can give most of diffuse sound field needed for better mix translation in small rooms, then if you not sure, follow this. Otherwise you can build diffuser which works from 400Hz if you never approach more to your back wall, when you work.
Old 8th September 2011
  #29
Gear Maniac
 

Rectangular room.. LEDE... recording space is separate room adjacent to control room.. sorry for the noob question but.. how do you measure ISD?
Old 8th September 2011
  #30
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avare's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
Sorry Andre but I think that’s a tad unfair.
You are disagreeing with Boggy then, as I am agreeing with him.

Andre
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