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Discussion Starter #1
During my theater construction, I've accumulated a large pile of scrap wood, mainly from trimming 2x4 studs down to size. Today I looked down at the pile, and it reminded me of the RPG skyline diffusor panels as outlined in the article about WSR's Holosonic Theatre in the August 2001 issue of Widescreen Review.


So I got to thinking. If I glued all those small pieces to a 4' x 8' sheet of thin plywood or sheetrock, then I'd have my own skyline diffusor panel to attach to the ceiling.


I like the idea, but I'm trying to decide the best placement. Should I mount the entire sheet to the ceiling between the screen and seating area, or DIRECTLY over the seating area? Or should I cut the panel into smaller pieces and mount them on the side walls?


Or is this a goofy idea (I've had many of those) that's unlikely to be of any sonic benefit? Besides the skyline panel(s), my theater won't have much of anything in the way of diffusion except for shelving on the lower rear wall.


I'd be grateful for any opinions/suggestions.
 

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filmnut,


For some good ideas on DIY diffusors you may want to go to

www.audioasylum.com


Click on the Search function. Type in Skyline diffusors, then select Tweakers asylum in the drop menu. Click go.


You will get 70 hits. See #69 before you leave. Lots of recipes and good discussion.


Also see here --
http://snkby.50megs.com/whats_new.html


Photo of DIY diffusor a la Skyline. Also "snkby" moniker talks about his pictured DIY diffusor.


Good Luck,
 

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Discussion Starter #3
Thanks for the links, bok! I didn't know about the audioasylum site....I can see I'll be spending a few evenings there.


Most of the diy skylines that they talk about are made using cut lengths of pvc pipe or pipe insulation, but several posts also mention that wood blocks provide the best diffusion.


I've decided to make several 2' x 2' panels and try various placements rather than make one big 4' x 8' panel.
 

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If you go to the Patent Office's site and do a little searching you can find the actual patent for the Skyline diffusors. That patent tells you exactly how to calculate the ideal layout.


Mike
 

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I browsed through the patent documentation, but didn't find any drawings or specifications. Nor did I find the word "skyline" anywhere in the text. It mentioned curved surfaces several times, which the skyline diffusor is not. Besides, I'm sure the construction specifications are more involved than I would be interested in. I want this to be an easy project.
 

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Filmnut,


The ceiling needs to be lively for the surround to work. The DVD is recoded with this expectation. Most ceilings are left as drywall because of this. Of course you're talking diffusion with your devices, not absorbtion. I'm not sure how a diffusor would affect the acoustic result.


This is a distinctly different scenario from a two channel environment where there are soooo many surfaces treated.


The one area on the ceiling that deserves consideration to cover, possibly even with absorbtive material, is the ceiling area in the front where the "first reflections" from the front channels would come from. That has always appeared to be a candidate for absorbtion.


Anyone have any thoughts on that?


Ted
 

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Absolutely right about the need for absorption at the first reflection points, but my understanding is that it's only needed for boundary surfaces that are close to the point source (the speaker). Usually, that means the wall behind the speaker and the side walls just in front of the speaker. The ceiling is usually a lot farther away (perhaps 5 or 6 feet from the speaker), so those reflections are delayed long enough that they don't muddy the sound. Or so the theory goes, as I understand it.


My main reason for wanting diffusion (not absorption) on the ceiling is simply to break up the large planar surface and scatter those reflections more uniformly throughout the room.
 

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I think you mean, more randomly...uniformity is not what a good diffusor will do.


In multi-channel, you need absorption at at least ear level and below through out the entire room. Whether or not it is required for boundary effects depends on speaker dispersion patterns and location. With proper speaker design, ceiling reflections are not often a problem.
 

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Quote:
Originally posted by filmnut
I browsed through the patent documentation, but didn't find any drawings or specifications. Nor did I find the word "skyline" anywhere in the text. It mentioned curved surfaces several times, which the skyline diffusor is not. Besides, I'm sure the construction specifications are more involved than I would be interested in. I want this to be an easy project.
I just looked over United States Patent 5,401,921 again, and it seems quite complete to me. Right down to the FORTRAN code to calculate the various depths (heights).


Mike
 

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OK, my Fortran's a little rusty, but I'll check again......


Oh, lordy. It's rocket science. I still like my styrofoam coffee cup idea. If I cut the coffee cups to prime number lengths, adjusted by their subprime cofactors N-1 sub h, and take the 5th root of cosine x squared sub A multiplied by its relative co-prime subfactor integral b cubed over h-sine a squared, then...
 

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Quote:
Originally posted by filmnut
If I cut the coffee cups to prime number lengths, adjusted by their subprime cofactors N-1 sub h, and take the 5th root of cosine x squared sub A multiplied by its relative co-prime subfactor integral b cubed over h-sine a squared, then...
By George, I think he's got it!
 

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I've made one a 2x2 panel of skyline using 1inch MDF. When RPG talks about 156 distinct phase blocks I beleive what they have done is taken a 24 inch panel less the width of a ceiling grid and divided it by 13. 13 x 13 = 169 - 13 for repeating patterns of block heights and you come up with 156 distinct phase blocks.
 

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Discussion Starter #14
I would think building a mold, mixing and pouring plaster, etc. would be harder than just gluing random-sized wood blocks to a sheet of plywood in a random manner. I already have the wood blocks ready. But that's an interesting idea.


Dennis, if you're reading this, do you know if marshmallows might make a good abfusor material? I was thinking of using them instead of wood blocks.
 

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Discussion Starter #16
Yeesh! 552 blocks in a 2 x 2 panel? I'm not interested in doing anything that ambitious. I don't wanna measure or calculate anything. I just wanna glue 'em and stick 'em.
 

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Someone in another post said:

=====

It took me 8 hours to put together one 2x2 panel. It has 552 distinct phase blocks and weighs 32 lbs. If you want to do very many of them, pouring is definitely easier.

========


This forum sure makes it hard to reply to an individual post, so sorry about not crediting this poster. Anyway, that was my idea, i.e. work really hard on the mould, then cast away. Now the trick may be how do you connect to this plaster diffuser. Would you place L-hooks in the plaster similar to the way you put bolts in cement to attach boards later.


What I envision in wall/ceiling treatments is something like the Dolby Labs theatre. The walls had wood beams running up them periodically. Between these beams was an acoustically transparent cloth, like a grill cloth, probably on a frame. Behind the cloth there was either a diffuser or absorber. Now Dolby had motors to select the type of material behind the cloth, which would be serious overkill for home use. However, you could put absorbers in the right area to kill the first reflection, then diffusers elsewhere. If the absorbers and diffusers are hidden behind this cloth, they could look pretty ugly. As ugly as a plaster casting!
 

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I think any diffuser that is visually distracting will be a problem. I've looked at those photos with serious abfusors along the walls and can't contemplate watching a movie with so many distractions.


I don't think you want to drill holes in a plaster item after the fact. You could use masonry bits and lead anchors, but something mounted in the plaster before it cures would be much stronger.


If you look at the plaster diffuser that RPG sells, it is made out of fiberglass reinforced plaster, so some thought needs to go into making such a device regarding strength.


The plaster diffuser is relatively cheap (compared to other RPG products). One idea is to simply buy an official plaster diffuser, then make your mould (or is it mold?) from the official item.


Regarding the color of the plaster unit, you could add a tint to the plaster. This would probably be better than painting them afterwards since the painted surface would be highly reflective. Even if you use the grill cloth approach, you probably want to tint the plaster to be the same color as the cloth, or perhaps make them a dark gray, so they don't show through the cloth.
 
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