# A Simple Acoustic Diffuser



## Prof.

I've never been particularly interested in adding diffusers/ reflectors to my theatre room..They all seem a bit complicated and require precise mathematical engineering..and if you're making them yourself, they take a lot of hours to build..
I always thought my sound was pretty good, so why would I want to add diffusers?

There are a number of styling's that all seem to do a good job, but I won't be going into the pros and cons of the different design aspects here..
There is another type of device called a "scatter plate" (developed by GIK acoustics I believe) which by all accounts provides some diffusion and reflection at the same time..These scatter plate panels are placed in front of your acoustic panels as an add on..Again, the holes in the scatter plates are mathematically calculated to provide a scattered dispersion of sound in the mid to upper frequencies..

I recently came across a video of a small recording studio in Melbourne that was adding acoustic treatments to the room..One of the things they added to the acoustic treatment was a series of small curved panels, placed over the top of the acoustic panels..These panels were made of thin aircraft ply and were quite flexible..and they were placed on wall and ceiling treatments..Some were positioned vertically and some horizontally..

So I thought that looks pretty easy to make, and started going further into the mechanics of the design..There doesn't seem to be any particular design parameters other than the depth of the curve in relation to the distance from the speakers for the first reflection point panels..
For example..If your L&R speakers are 3' from the centre of your first reflection point panel..then the depth of the curve is 3"..

Note.. If anyone who is more in the know about this type of panel and there is more to it than just the depth of the curve, please let me know as I'm no acoustic expert and just following what I read..

I tried to locate a supplier of the aircraft ply here and could only find a wholesale supply for a large quantity..
I did have some melamine faced 3mm. MDF on hand and decided to try that..It's fairly flexible to a point and to get a 3" depth curve took a bit of force, but I managed to get it in..You can use any firm flexible material for the diffusers..for example thin Acrylic sheet..

To locate the curved panels in front of the acoustic panels I just used 2 parallel strips of 19x19 mm. pine, screwed onto the face and at an appropriate spacing to provide the right curvature..You'll see this more clearly in the photos..

Initially I have just two panels on each of the first reflection point panels only..One vertical and one horizontal..They cover approx. half the height of the wall panels..

I didn't really expect to hear much difference in the sound, even though I had heard that they work very well..
My first impression was that voices could be pin pointed anywhere across the screen..very much like I used to get when I was using a baffle wall..
With a musical score..the orchestra just seemed to spread right out across the width of the room and beyond the sidewalls, with a very pleasant openness to overall sound..
All sounds just projected right off the screen, more than they had ever done before.(with the AT screen I always felt that the sound was more behind or level with the screen) and in one of the scenes where there was a sudden explosion, I literally jumped it was so much right in my face!
I still can't believe that such a small change could make so much difference!

I don't know if these improvements would be the same for any theatre, but definitely worth trying for so little expense and time to make..
My guess is that if you have a large theatre then you may possibly need larger panels or more of them..

Unfortunately the photos are not very good..My camera doesn't like taking photos in darkened rooms..but I think you can see the idea..








The buckling effect on the lower panel is only an illusion caused by the lighting from the downlight..
Also both panels are flat black, the lighting has made the bottom ones look grey..

Edit: I should also mention that I have a type of scatter plate fitted to the top sections of the second reflection point panels..This is basically a panel of peg board fitted between the insulation and the cloth covering..
It has helped to open up the surround sound giving a more spacious feel to the overall sound..


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## robbo266317

They look really simple and inexpensive. It certainly would be worth trying.


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## Erin H

Interesting, for sure. I'd definitely be interested in seeing if anyone can pull up some tech info on this method.


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## Erin H

wonder if there's any relation here to BAD panels:
http://www.silentsource.com/diffusors-rpg-bad-arc.html


hard for me to determine... stupid phone research. no patience.


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## Prof.

The BAD panels look more like the scatter plate panels with the multiple holes..
One thing I did notice is that some curved diffuser panels have the ends enclosed..I would have thought that having a sealed curved panel would give a drum effect, unless they were filled with insulation..but then why would you need to do that when you already have 2"- 4" of insulation in your wall panels!?..
That's what I like with just a simple curved panel..easy to make and fit and easy to remove..


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## Owen Bartley

Nice project, Prof., and I'm glad you were able to make some noticeable improvements in your setup! Isn't it great when you find a small gem like this? After reading quite a bit about acoustic treatments, I was thinking like you were that it seems like a lot of work to do right. But I guess you showed us that sometimes its worth just trying a good cheap idea, and it might work out great!


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## Prof.

Thanks OJ..

I'm all for trying something new if it's simple and cheap! :bigsmile:
Acoustic treatments has been one area that's made a significant difference to the sound in my room..and it just keeps getting better with new additions..


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## crabra

Hey Prof,
You may have to come down to Hindmarsh Is and give me a few tips....


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## Prof.

Hi Craig,

Wow you are close to home! I had no idea there was someone down here into HT! 
I was only on Hindmarsh Island the other day, down near Sugars Beach..
I'd be glad to help you anytime..Just PM me..


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## AudioMagic

These are polycylindrical diffusors, which have been used since the early 20th century. They're easy to make by cutting cylindrical concrete forms into 1/3 or 1/4 arcs along the length. The wavelength of the lowest frequency they'll diffuse is equal to the chord or back of the arc (the straight line between the two sides of the arc). The photos are what I did with polycylindricals for a sound editor.


Meanwhile, the scatter plates are one dimensional binary array diffusors, while the RPG product is two dimensional. Same math at work, just in two dimensions. They use a sequence of 0's and 1's, or absorptive and reflective spaces, to offer up some diffusion. In my experience, not as good as a polycylindrical, which is not as good as a quadratic residue diffusor. And they are not pure diffusors - they can also provide a lot of absorption. They're particularly handy when you are very limited in the depth of what you can put on the wall.

On a recent project we made these using aluminum sheet with the opening cut using a water jet cutter, glued to a sheet of 1 in. Owens-Corning 703. Sound expensive, but it was not.


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## Prof.

Very impressive! A little bit too big for my theatre though..


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## AudioMagic

True, and that is one of the drawbacks to polycylindricals. They need that kind of size to be truly effective. And that particular client cuts sound for motion picture and TV, so he needs the neutral sound that they bring. QRDs can be less massive and more effective, but they're more expensive and/or difficult to make.


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## Prof.

AudioMagic said:


> . QRDs can be less massive and more effective, but they're more expensive and/or difficult to make.


That's why I didn't go for QRD's..A bit too complex for my DIY skills..
As simple as my diffusers are, I'm very impressed with how they've widened the soundstage..


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## NBPk402

AudioMagic said:


> These are polycylindrical diffusors, which have been used since the early 20th century. They're easy to make by cutting cylindrical concrete forms into 1/3 or 1/4 arcs along the length. The wavelength of the lowest frequency they'll diffuse is equal to the chord or back of the arc (the straight line between the two sides of the arc). The photos are what I did with polycylindricals for a sound editor.
> 
> 
> Meanwhile, the scatter plates are one dimensional binary array diffusors, while the RPG product is two dimensional. Same math at work, just in two dimensions. They use a sequence of 0's and 1's, or absorptive and reflective spaces, to offer up some diffusion. In my experience, not as good as a polycylindrical, which is not as good as a quadratic residue diffusor. And they are not pure diffusors - they can also provide a lot of absorption. They're particularly handy when you are very limited in the depth of what you can put on the wall.
> 
> On a recent project we made these using aluminum sheet with the opening cut using a water jet cutter, glued to a sheet of 1 in. Owens-Corning 703. Sound expensive, but it was not.


Not to hijack the thread, but it would be nice to hear how these were made and about what they would cost to make (maybe you could start a thread so others could learn about this type too).


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## robbo266317

Prof. said:


> That's why I didn't go for QRD's..A bit too complex for my DIY skills..
> As simple as my diffusers are, I'm very impressed with how they've widened the soundstage..


Not to mention the potential WAF factor with large QRD's! :devil:
Unless it is "Your" room that some lucky people have. :whistling:


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## Prof.

It is "My" room!! :bigsmile: But those polycylindrical diffusers wood take up half my room!! :yikes:


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## noirx7

Well I have "my room" too, not sure I would sacrifice any more space unless it really improved the sound. Too many toys to store;four guitars, a charango computers...


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## yluko

The ones on the first post self made. how much did the cost per panel about?


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## Prof.

Cost per panel? $0!...It was all made from scrap timber..
To buy the material it would be about..
Sheet of 3mm. MDF or ply..$20.00
3.6M. 19x19 Pine...$5.00
Paint and screws...$10.00

Price / panel..$8.00 approx.


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## Tonto

So did you use the MDF because it's what you had? I'm wondering if using something like formica would work. I would think that would be more flexable & thus easier to work with.


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## Prof.

That's correct..You can use anything that can be curved and hold it's shape..
Formica would be a good alternative..


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## yluko

well i didn't know you could bend wood like that so easily, learn something new everyday. 
Thanks for sharing!


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## nathan_h

Anyone try luon? Seems like it bends easily, but might act like a drum.


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## Prof.

If you mean Luan ply, then it should work just as well and shouldn't be any more "drummy" than MDF..


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## nathan_h

Prof. said:


> If you mean Luan ply, then it should work just as well and shouldn't be any more "drummy" than MDF..


Yep you read my mind. 

I gotta think of a nice way to frame it....


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