Why the curved soffits? Why not, I say. I had a 12" trunk line in the upper left corner of the theater, and I had to do something about it. It's wrapped in insulation, and probably measured about 15-16" in diameter total. A rectangle or square soffit would have been huge, and besides, I've always enjoyed looking at curves... after all, very few straight lines exist naturally.
So, going with the curve idea, I thought that I could kill 2, 3, maybe even 4 birds with one stone:
1) I like curves.
2) I could incorporate diffusion into the design (see below).
3) By stuffing the soffit with fiberglass, I could cut way down on the air noise in the duct.
4) By building a mirror image on the right, I could create a deadvent air return: see here: http://www.avsforum.com/avs-vb/showt...1256990&page=4
post #104. I used 12" concrete former tubes from Home Depot to form the scaffolding in which I ran 10" flex duct. I had to strip the insulation off of the flex duct to fit it into the tubes, but it worked out well. Very quiet!
5) Maybe good, maybe bad: lots of fiberglass inside the soffits makes for some bass trapping. However, I don't know what frequencies are attenuated yet- I'll measure later.
Here's how I built them. I had two 1x10" oak rails on legs that I had left over from a previous project. I joined them together at their ends to form a long, narrow rectangle, whose ends I made coplanar using 2 levels as "winding sticks." That served as my jig for framing up the soffits. See next post, image #1 for a partial view of the jig.
I decided to build the soffits as three separate frames per side and to stagger them about 5 inches apart. I figured that one long, curved soffit would look a little blah, and, more importantly, would be harder to build
For the first curve, I freehanded a 3 inch wide ellipse "rib" on some MDF, using a technique like this: http://www.mathopenref.com/constellipse1.html
I then cut that out with a band saw and sanded down the rough spots. This was my template for the ribs that I made out of OSB: I attached this template to jigsawed roughcut OSB via three screws, and the ran it over a table router with a flush cut bit, such as this: http://www.woodcraft.com/Product/200...-CL-14-SH.aspx
After making about 9-10 of these ribs, I laid them out on the jig so that their convex sides pointed up. I joined them with roughly 2x1s that I cut down from 2x4s using deckmate screws. Once the frame was done, it looked sort of like an inverted boat hull.
Next, I covered the frame in plastic and placed upon it 1/4" drywall which I had soaked on one side with water, wet side down. The trick to bending drywall is to take your time! I came up with a pretty good, controlled way to do it: place levels on either edge, and slowly clamp them down until the drywall is flush with the frame. I let that sit overnight and then took it off to dry.
Then, I picked up the frame and hoisted it into position, tacking it down with clamps. Once I liked where it was, I thoroughly screwed it into place. I used a plum bob that I bought at home depot to check reference points on the floor, something like this: http://www.amazon.com/Stanley-47-969.../dp/B00009OYGP
The tough part about placing the frames on the wall was knowing what to use as a reference. If the room had been perfectly square, the it would have been easy, but since it wasn't, I had to go a bit by eye.
For the second frame rib template, I flexed a dowel to form a curve and then traced that onto MDF. For the third, I combined elements of both curves. The idea was to create three different diffusing surfaces, none of which was based on semicircles- I read somewhere that semicircles don't work as well for diffusion.