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PostPosted: Fri Jun 06, 2003 10:30 pm 
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OK, so here's this idea I have, and I wonder if you people think it will work.

The idea is based upon the oh-so-familiar slat resonator. But on the slats you mount long pieces of wood that have been cut length wise to a triangular (or quarter-circle) shape, as seen from the short end. This will make the resonators front diffusive AND it will make the slots much deeper, which should lower the resonant frequency of the resonator.

Damn, it's difficult to explain these things in a foreign language - I hope you get the idea.

Here's a pic, seen from above (assuming the slats are mounted vertically). The light brown area is the mineral wool behind the slats, the dark brown squares are the slats, and the triangles are the diffusive pieces of wood mounted to the slats.

Image

So whaddayathink?

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/Henrik

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PostPosted: Sat Jun 07, 2003 9:19 am 
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Hey - like all Swedes your English is excellent - wish I had a second language :):)

I'm not sure what effect the angled fronts would have on the action of the resonators themselves as the slat width is part of the formula.

Looks like a question for Barefoot again :):)

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john


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PostPosted: Sat Jun 07, 2003 11:51 am 
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John Sayers wrote:
I'm not sure what effect the angled fronts would have on the action of the resonators themselves as the slat width is part of the formula.



Sorry to jump in...I think Henrik was refering to those "angled fronts" (triangles) as diffusors...were you Henrik :? ?

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PostPosted: Sat Jun 07, 2003 3:17 pm 
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Yeah I understand that Sen - but I think he meant the whole thing to act as a diffusor as well as a slot resonator - correct??
Look it's going to act as both but to what degree I don't know. :)
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PostPosted: Mon Jun 09, 2003 7:53 am 
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Right, the idea for the added pieces of wood was that they'd serve as diffusors but also lower the resonant frequency of the resonator. According to the Excel file on the SAE site, this frequency is heavily affected by the depth of the slots. I guess you could also mount standard 2x4's, angled 90 degrees to the the front slats.

I guess nobody will know until we build one. But it seemed like an OK idea.

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PostPosted: Mon Jun 09, 2003 10:04 am 
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Henrik - in Guruland http://johnlsayers.com/Studio/index.htm I designed slots similar to what you are suggesting. I used 6" x 2" with the 2" being the front face thus giving a 6" slot depth. They are on the side walls of the drum booth to give high end diffusion and low-mid absorption.

I've attached a drawing

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PostPosted: Mon Jun 09, 2003 8:17 pm 
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Right, that's precisely the same principle I'm suggesting. So do you think the deeper slots do lower the resonant frequency of the Helmholz resonator?

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/Henrik

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PostPosted: Wed Jun 11, 2003 8:41 pm 
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Henrik, the way you've drawn your idea I would think that you'd have the equivalent of several concave "focused" reflectors, which could really raise havoc if aimed back to the mix position in a control room, or at a mic in a booth - I'd be more inclined to do what John did, with the 2x lumber on edge for deeper slots. The variable depths should act somewhat like a QRD as John mentioned... Steve

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PostPosted: Wed Jun 11, 2003 11:17 pm 
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knightfly wrote:
Henrik, the way you've drawn your idea I would think that you'd have the equivalent of several concave "focused" reflectors, which could really raise havoc if aimed back to the mix position in a control room, or at a mic in a booth - I'd be more inclined to do what John did, with the 2x lumber on edge for deeper slots. The variable depths should act somewhat like a QRD as John mentioned... Steve


Knightfly, thanks for this. I see what you mean by the "focused" reflectors the way I did my drawing. I suppose John's idea is better. Do you think the angles would work OK if you had wider slats, I mean like double the width compared to what I drew? Either way, it's absolutely nothing I'd place behind the monitoring position in a control room.

What is a QRD?
And do the deeper slots lower the resonant frequency according to the Excel file on the SAE site (The Helmholz calculator)? If so, John's approach would be a very efficient way of getting the resonator to absorb really low frequencies.

Cheers
/Henrik

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PostPosted: Thu Jun 12, 2003 4:18 am 
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Sorry, a QRD stands for Quadratic Residue Diffusor - looks sorta like John's pic, except the wells are sealed (it's ONLY a diffusor) and each well's depth is calculated from a prime number sequence - the well width is usually about half the wavelength of the highest frequency (shallowest) well. You've seen them in nearly every picture of every pro studio you've ever looked at -

Here's a fairly good description, but not very understandable pix -

http://www.suite101.com/mypage.cfm/unpluggedsound/8336

Here's a little better picture

http://www.acousticdiffusers.co.uk/diffusers/qrd734.htm

Diffusion is primarily only in a plane that is perpendicular to the wells - if the wells run vertically, diffusion is horizontal. That's why when you see these, usually there's a bank of them with some running horizontal but most running vertical... Steve

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PostPosted: Thu Jun 12, 2003 4:23 am 
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Sorry, a QRD stands for Quadratic Residue Diffusor - looks sorta like John's pic, except the wells are sealed (it's ONLY a diffusor) and each well's depth is calculated from a prime number sequence - the well width is usually about half the wavelength of the highest frequency (shallowest) well. You've seen them in nearly every picture of every pro studio you've ever looked at -

Here's a fairly good description, but not very understandable pix -

http://www.suite101.com/mypage.cfm/unpluggedsound/8336

Here's a little better picture

http://www.acousticdiffusers.co.uk/diffusers/qrd734.htm

Diffusion is primarily only in a plane that is perpendicular to the wells - if the wells run vertically, diffusion is horizontal. That's why when you see these, usually there's a bank of them with some running horizontal but most running vertical.

No matter how wide the elements in your slot idea, they would still be concave, so no. That's why when you see poly-cylindrical diffusors, they are bowed OUT into the room, not the other way 'round. You're looking for DIFFUSION, not CONCENTRATION -

Yes, deeper slots lowers the frequency... Steve

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PostPosted: Thu Jun 12, 2003 9:42 pm 
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OK, thanks.

knightfly wrote:
No matter how wide the elements in your slot idea, they would still be concave, so no. That's why when you see poly-cylindrical diffusors, they are bowed OUT into the room, not the other way 'round. You're looking for DIFFUSION, not CONCENTRATION -


I understand that, but I suppose even polycylindrical diffusors bowed out into the room should probably be placed at a certain distance from each other, right? I mean, if you place them side by side, you will get a certain amout of concentration in the area between each pair of diffusors.

If you look at my drawing, the wood parts attached to the slats might as well have been quarter-circles looking from the short end, instead of triangles. A pair of these quarter-circles would make a polycylindrical diffusor (with a slot in the middle).

See what I mean?

Cheers
/Henrik

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PostPosted: Fri Jun 13, 2003 8:44 am 
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Henrik,

Your idea will definitely work with respect to lowering the resonance frequency. If the wavelength of your target frequency has a similar size as the triangles (which it almost certainly will not) then the angled slat fronts will cause a very minor tuning deviation - but this really isn't a concern at all.

My only concern is that this design will probably not be a very good diffusor. The even periodicity and depth may actually cause reflection problems at wavelengths having integer relationships to the structure.

John's Guruland design looks outstanding. It would serve as both a diffusor and a true broadband resonator. As I've mentioned in other threads I don't think that varying the slat width, slot width, or cavity depth results in a true broadband absorber. Varying the slot depth, on the other hand, does.

John,

It's not clear from your drawing if there is a formal acoustic principle behind your choice of slat depths (thichnesses). If not, your design could be taken even further by making the slat depths correspond to those of a Primitive Root Diffusor (sort of a QDR without the thin partitions). The adjacent slats don't need to be the same depth for the slots of the resonator to work properly. Great design!

Thomas


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PostPosted: Fri Jun 13, 2003 10:42 am 
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Yeah - that's what I was sortta aiming at Thomas. I was trying to kill two birds with one stone as they say. It's definitely up for further development. I also put one of these on the rear wall of the control room.

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john


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PostPosted: Sat Jun 14, 2003 3:33 am 
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Thats a pretty cool idea. I calculated the sequence in excel using 17 as the prime root. The sequence is 16 long before it repeats.

2x6 lumber is 5-5/8" - so this would be the sequence in mm (124 = no cut)

0, 8, 31, 70, 124, 62, 16, 116, 101, 101, 116, 16, 62, 124, 70, 31, 8


John - can you add .xls as a permitted upload file extension?

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