I haven't seen many builds using QRD diffusion panels so I thought I'd document my build process in a little more detail. In a nutshell I'm treating the majority of the upper side and rear walls with DIY QRD panels that will be hidden behind the wall fabric. Because the the different treatment depths as well as design reasons I have a total of 25 panels to build! So I needed to create an efficient build process.
The panels consist of 1", 2" and 4" depths. Specifically they are all 2D design in 1" N7, 2" N17, 4" N7 & N11 configurations. The 1" and 2" are all 32" tall and the 4" versions are 38" tall. They will all be mounted with vertical fins. Since all my diffusers have zero depth wells at the ends I decided to make each of the panels symmetric with half a well at each side. Technically when they are mounted I should cut off 1/2" from the extreme left panel and fit to the extreme right. I'm not sure how significant this detail is but I'll probably do it right else my OCD will get the better of me ;-)
For further reading this is an excellent reference: http://www.subwoofer-builder.com/qrd.htm
and includes some cool design software called QRDude http://www.subwoofer-builder.com/qrdude.htm
After some thought I decided to build the panels with hollow wells to keep the weight under control and reduce the quantity of material required. The first step was working out the materials list. This was the first eye opener on the magnitude of this undertaking.
For the 25 panels I will need:
226 well bottoms
50 routed top/bottom plates
25 back panels
... in a myriad of sizes...
After drawing out some cut sheets I needed:
7 sheets of 3'x7' 1/8" MDF (fins)
2 sheets of 4'x8' 1/4" MDF (well bottoms)
1 sheet of 4x8' 1/2" MDF (sides)
1/2 sheet of 4'x8' 3/4" MDF (top/bottom)
2 sheets of 4'x8' 1/8" ply (backing)
To ensure accuracy of the panel and particularly the well depth I started by creating a template for each of the 4 QRD sizes. Step one was to mark up the fin and well positions and figure out the depth of each well.
Then, using scrap wood from around the shop I cut furring strips to the depth of each well and attached to the template.
As I build the panel the front side will be facing the template so a couple of the wells needed special treatment -- no well bottoms in the full depth wells and 1/8" bottoms in a couple of wells that were 1 7/8" depth (could not fit 1/4" stock).
With the template moved out of the way I started cutting. After considerable cutting I cut some more. Then spent hours cutting followed by lunch and more cutting. Then after an 8 hour ordeal I was done.
Seriously, I did give a lot of thought to how I was going to cut 600 panels efficiently and so invested in a few tricks. Firstly I laminated as many layers together as I could with low-tack double sided tape as well as some painters blue tape. This allowed me to cut 7 pieces of the 1/8" stock in one go.
The top and bottom panels have slots to accurately accept the fins and this was made by using a 5/32" blade (not the thin kurf blades that are popular these days) to 1/4" deep on three quarter sheet of MDF. After all the slots were cut I ripped the panels to the three sizes, used a router to channel the ends and then chopped to size on the table saw or chop saw.
The old adage of "measure twice, cut once" is doubly true when you are cutting 7 sheets at a time. Luckily I only made a slight mistake of initially cutting 200 well bottoms 1/2" too long. Better long than short! The solution was to load up the sled on my table saw and cut them down to size ninety at a time!
So, after 8 hours at the table saw, this is the little pile I was able to create.
The moment of truth -- a dry fitting of one of the QRD designs. I was very careful to make accurate cuts, because in a design like this an error will compound. I'm happy to report everything fitted perfectly (the 1/64" undersized well bottoms was a really good idea).
Tomorrow I'll start gluing these up 4 at a time and filling the back side with expanding foam... stay tuned!