DIY Quadratic Root Diffusor

Rob Babcock · « **on:** 9 Jun 2006, 05:37 am »

I've been meaning to post this for some time- a pretty good looking recipe for a DIY QRD. I have been thinking of trying this out for a long time; maybe once I get a few tools here.

Folsom · « **Reply #1 on:** 9 Jun 2006, 08:27 am »

Perhaps you should get the Foobar2000 up first

That is probably the heaviest diffuser I have seen. I would feel uncomfortable mounting it just on the studs in my wall.

samplesj · « **Reply #2 on:** 9 Jun 2006, 12:58 pm »

I think just a couple wouldn't be a big deal at all, think about bookshelves and such. Wall studs are pretty strong. I've got 5 5' shelves on both sides of my "office". I'm using 2 standard strips with 6 screws in each and they are fine. Actually the only issue we had was that my wife piled vertically the big and heavy too tall books all in the same spot and ripped the screws out when it was first getting loaded. Luckily my Maggies weren't down there yet.

I almost built those. It is a low cost project. Its not much in material costs at all. Its just the time. I was a little concerened about the weight, buy I was thinkg about a 4'x6' patch behind each speaker. It also looked like painting could take forever. I had decided to leave them raw and just put something acoustically transparent in front, but even that was going to take a while. In the end I just had too many other projects and bought Skylines (which don't weight much of anything).

ctviggen · « **Reply #3 on:** 9 Jun 2006, 04:58 pm »

How do you like the Skylines? You could also build these with polystyrene (which is what theSkylines use).

samplesj · « **Reply #4 on:** 9 Jun 2006, 05:41 pm »

If you had access to material so you could make them out of stryo then the DIY route is a no-brainer (no painting, quick assembly, and lightweight).

Over all I'm happy with the Skylines. I've still not had enough free time to have build something myself (either QRD like Rob's link and the skylines or even a quicker to build Poly). I've not done any real tests with and without using RTF and my mic, but they seem to make focus/imaging a little sharper/clearer and they smoothed out a bit of sharpness/brightness in the highs. I can't tell that I lost any of the bigness/ambience from the dipoles (why I didn't use absorption instead).

Nothing against the DIY route. If I had more time I would have definately used Rob's link (much cheaper), but either way I'm very glad I did get QRD style diffusion behind the speakers.

ctviggen · « **Reply #5 on:** 9 Jun 2006, 09:19 pm »

I've been wanting to try diffusion (I have quite a bit of absorption) for a while. I just haven't had the time to make anything, and most diffusion products can be a bit expensive. I'm planning on trying out some Linkwitz Orion speakers (open baffle), so I may have to go the diffusion route, as recommended by Linkwitz, although some people still say that absorption works. I'll be interested to compare the Orions with and and without absorption to my VMPS RM40s. I'm guessing that it's going to be an "apples or oranges" kind of test.

JLM · « **Reply #6 on:** 10 Jun 2006, 11:15 am »

Thanks for the link Rob, looks like a good project for me as I've recently picked up 6 GIK 244 panels and was looking for something specifically for first reflections on the side walls that was about that size. Looks cheaper/easier that other DIY options I've found, although I might add a frame around it just to dress it up a bit.

Do you know what range of frequencies they would be effective for?

Rob Babcock · « **Reply #7 on:** 11 Jun 2006, 04:03 am »

Not for certain. Obviously you'd expect them to create good diffusion at much lower frequencies than the lightweight plastic or foam ones (eg Skylines). But how much lower I can't say.

samplesj · « **Reply #8 on:** 12 Jun 2006, 02:50 am »

Range of diffusion in a QRD is based on the depth of the well (low) and number of wells (high). "The well width is about a half wavelength at the shortest wavelength to be scattered' (Master Handbook of Acoustics - Everest page 294). Its also pointed out in the BBC paper references by the DIY site. Neither source talks about material having any impact at all.

According to the formulas linked to on the DIY site the DIY QRD is good from 900-4.5k. (19cm depth and 3.75 cm width X 12 colums). In case you are curious here is what the formula ended up with. 45cm wide with 19.11 cm deep with 3.75 cm square colums. That yields 0, 4.78, 9.56, 14.33, 19.11, which matches pretty close to the DIY receipe in non-metric measurements.

Rob Babcock · « **Reply #9 on:** 12 Jun 2006, 03:42 am »

Yes, I also have that book. I believe the thinking is that regardless of the size of the diffusor at low wavelengths sound would penetrate flimsy material and pass thru as opposed to being scattered by it. Admittedly that's sorta theoretical on our part but it's fairly consistant with the literature from Aurelex.

samplesj · « **Reply #10 on:** 12 Jun 2006, 01:29 pm »

But does it matter if at frequencies lower than they are designed to "properly" diffuse they allow the waves to pass through? Do we care how well it bounces or not when it won't be bouncing the sound "randomly"? Sure Auralex and RPG both publish specs for diffusion lower than the QRD boundary, but do we really understand how much audible benefit they provide below it?

While the PME site talks about hollow plastic being only good to 800hz and wood going lower, using the exact same calculator they did the wood plans are only performing as a QRD to 900hz. So does the added depth wood can diffuse matter then?

The deepest QRD style Auralex diffusors (Q-Fusors because T-Fusors aren't QRD style) are only 3" deep. Regardless of material that in itself will keep them from diffusing very low. If it were laid out as a standard QRD it would have a lower limit of ~1500hz.

Skylines are only 7" deep so they bottom as QRDs at ~1000hz. Does the solid stryofoam like material hurt their performance at that range?

Even if the QRD limit were lower do we really care for diffusors? None of these QRD style diffusors are meant to replace bass treatment. These are more imaging/focusing tools. I've still got 8 2'x4' rigid fiberglass panels 4"-6" thick on the walls and the entire ceiling cavity stuffed with fluffy 12+" thick for bass treatment.

Like I and Ctviggen both mentioned these are very popular in the dipole (open baffle is intrinsically dipole) camp because they help keep the rear wave from confusing the main signal (by arriving too early), but still keep the added ambience it provides (which absorption would kill).

Ethan Winer · « **Reply #11 on:** 12 Jun 2006, 05:12 pm »

Jeremy,

> But does it matter if at frequencies lower than they are designed to "properly" diffuse they allow the waves to pass through? <

It depends on what's behind the diffusor. If you have a row of bass traps on the rear wall, putting a heavy rigid wooden diffusor there, in front of the bass traps, will prevent the traps from working effectively.

> Even if the QRD limit were lower do we really care for diffusors? None of these QRD style diffusors are meant to replace bass treatment. <

That's a great question and great point (respectively). In most cases absorption does as good a job as diffusion if not a better job. The original purpose of diffusion was to break up flutter echo and comb filtering, but without adding yet more absorption to the room. For a professional recording studio control room that's 25 by 40 feet this is not a bad approach. But in rooms the size you'll find in most homes, you want to reduce most of the excess ambience. Especially in a home theater setting where all of the ambience is embedded in the sound track.

This is not to say that diffusion is never useful in smaller rooms. A good diffusor can add a nice sense of spaciousness. Then again, absorption does this too. If you think about it, a surface that absorbs 100 percent is acoustically equal to that surface being infinitely far away. Either way the sound goes toward the surface and never returns. It don't get no bigger than that!

--Ethan

Rob Babcock · « **Reply #12 on:** 12 Jun 2006, 07:32 pm »

Hey, I don't claim to be an expert!

To me the issue isn't the material so much as the fact that Skylines are about $150/box vs $15 worth of lumber! They may not do any better job at low frequencies or they might, but I'd be pretty surprised if they didn't work just as well. For a lot less money.