[jk@home]

I'm looking to purchase or build some QRD diffuser panels that will go behind a pair of Magnepan speakers that are 5' off the front wall, 2 feet of the side walls. Basically need (and can fit) a 2'w x 4'h or 6'h panel behind each one. They will be mounted on closet doors. I've been told by other Maggie owners that the QRD-1D variety is preferred over the 2D style in this case. Correct?

Any preference of foam vs wood construction? These are hollow core doors on double hinges, so weight is a concern here.

Also I've notice that different diffusers are rated at certain frequency ranges, some start and cut off a little lower or higher than others. Is there a optimum range for my app? I will have 6" OC703 bass trap panels in the corners of the room, also have some 4" absorbers of the same to place where needed.

And how about stepped diffusers like below? Can they work just as well?

http://arqen.com/sound-diffusers/

Thanks, John

[stevenkelby]

Hi John,

I assume you know about QRDude?

http://www.subwoofer-builder.com/qrdude.htm

I have OB speakers and built diffusers to go on the wall right behind them.

I chose foam as it's lighter, cheaper, easier to work with and gives the same results from what I read on Gearslutz.

I didn't use fins to simplify construction, apparently it's a very minor difference.

1D is better in that application as it scatters sound horizontally. A 2D spreads the sound up and down into the ceiling and floor too which might not be ideal in that use.

The width of the wells/blocks determines how high it will diffuse to. I used 50mm wide which only works up to 3440 Hz. 25mm would go up to 6880 but is twice as many pieces, twice the work.

I felt that 3440 was plenty high enough, that's well out of the midrange which is what matters most IMO. Higher frequencies are all over the place anyway.

How deep the diffuser works to is determined mainly by the depth of the wells/blocks but also slightly by the period width and whether it's side shifted.

I chose 250mm deep as that fits my room well and goes down to 332.

Mine are 1450mm wide.

Hope that helps!

Steve.

[jk@home]

Thanks Steve. Yes, I am just starting to dive into the QRDude site, lots of good info there. My doors would probably appreciate foam over wood, how is the best way to cut it for a project like this?

[stevenkelby]

No worries, I'm no expert but have been reading a lot about diffusion the last couple of months.

I bought the foam (EPS) in big sheets, 50mm thick, 1 meter wide (the height of the diffusers I built) and 2.5 meters long so I could get it home from the foam factory in my wagon. I got 4 of them, so 10 meters total. To make 2 traps, each 1.45 meters wide, 1 meter tall and 250mm deep, I needed about 8.5 meters from memory, so I have plenty spare. Cost was less than $100.

I sliced it up on my bandsaw, worked great and easy to do, leaving nice clean straight edges but a little messy. Then again, isn't anything involving a bandsaw a little messy? I just vacuum well afterwards.

A table saw would work fine too but I think the best option is a hot wire cutter. I already had a bandsaw or else I would have made a hot wire cutter.

I know some guys have sprayed or brushed the finished foam diffusers with a thin mortar and that is probably the best way but I just sprayed them with black paint for now. From what I read there is only a little to be gained by finishing the blocks with a hard surface like mortar or strips of wood. The design parameters are largely arbitrary anyway so a bit of absorption or lower efficiency is no big deal to me as long as it still works more or less as intended, and there is certainly a large audible improvement with them in place. I will take pics when I have them finished nicely, the wood frames are ugly currently.

I didn't measure or calculate precisely what I needed them to do, just wanted a diffuser there and these seemed like a good compromise.

Best,

Steve.

[stevenkelby]

PS, I don't know about the stepped diffusers, I assumed they would be less effective but would start a thread here to ask the experts in such things

http://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/

The Argen guy posts there too.

[jk@home]

I have a table saw, but not a band saw, so I will check into the hot wire cutter. One coating that I recently purchased to use to paint some diy subwoofers is DuraTex, which is sold to the pro audio/roadie market as a thick cabinet repair paint. Still have plenty left, so could try some on a piece of spare foam.

http://store.acrytech.com/Speaker-Cabinet-Coatings/

[stevenkelby]

Sounds good as long as it doesn't melt the foam. Technically they should have a hard surface but it didn't seem the end of the world from what I've read, and heard.

I would like to compare mine with ones built the same but with hard surfaces, and fins between the wells. I'm not sure you can measure the effectiveness of a diffuser easily though anyway.

[ptmconsulting]

I built these to go behind my Apogees. No reason to think they wouldn't work just as well for Maggie's. A bunch of different sized dowels, some pine board, a few screws, stain and paint. Cost me all of $100 and some time, and they work wonderfully. They are basically a DIY copy of Shakti diffusers.

[PDR]

Hey Stephen.....glad to hear your enjoying the diffuser!

I would second the styrofoam method....really easy, and made a big improvement.

<----You can get a little fancy with the paint if you want...

[stevenkelby]

Thanks Perry, couldn't have done it without your help and advice

Mine do not look as nice as yours! They will be nice enough when finished out though. Meanwhile, they sure do the job well.

[jk@home]

Hey Stephen.....glad to hear your enjoying the diffuser!

I would second the styrofoam method....really easy, and made a big improvement.

<----You can get a little fancy with the paint if you want...

Very nice looking!