Originally Posted by Blue
Nick, this is incredibly helpful. I have a few questions, if you will bear with me:
(1) For a wall panel, why not just use 4" of insulation? Cost? How much extra does 4" insulation cost for the typical panel size? Do you recommend a 4" piece, or two 2" pieces (if there's a difference)?
(2) For the wall panel, is it better to use a frame that is flush with the wall, or should the frame of the panel have spacers? In other words, I could use 1x4s to build the frame, which actually are 3.5" deep, and then add 1 inch spacers in key spots so that most of the frame will not sit flush with the wall. Is that better or worse?
(3) For a corner base trap, how wide does the face of the 4" insulation panel need to be? Is there a cheat sheet of recommended dimensions?
Some quick answers:
(1) 4" of acoustical insulation costs quite a bit more than 2". We aren't talking standard household insulation, but rigid insulation like OC703, Roxul Rockboard, etc. 2" OC703 is about 40% less cost than 4" OC703 for example. So a single 4" piece is still cheaper than two 2" pieces stuck together, but you still get really close performance with a 2" and a 2" (or close) air gap without the extra money. If you have the extra dollars, then by all means go with 4" of rigid insulation. Does not matter if that is a single 4" thick piece or two 2" pieces.
(2) Not 100% sure that I follow here, but I think that I do. I used 1x4 material for my side wall panels (2" of OC703 and a 1.5" air gap) and 1x6 material for a wide back wall panel (4" of OC703 and a 1.5" air gap). They are butted right up against the wall and it has been discussed elsewhere here on AVS that the solid wood frames against the wall actually doesn't matter vs when people try to use something more porous, drill large holes into the sides, or make their frames 2" and put spacers for creating an air gap. The reason is that those wood frames are actually reflecting sound back away from the listening area, sort of like diffusing it, and many times they hit the wood panel, and reflect against the wall and away, so that sound energy is actually losing its strength with every bounce, and like I said it is going away from the listening area anyway. Spacers can certainly work too, I just didn't want anything "floating" on my wall, but it certainly works too.
People have asked me if you need an air gap that matches the thickness of the insulation, and the answer is a solid no. Any air gap will help to capture deeper frequencies. The thicker the material & air gap combination, the deeper you can absorb. Now there is a point of "failure" here. At some point, if your air gap is really big, then the sound wave can pass through the front, bounce off of the wall, and then completely miss going back through the insulation again. Sure you could make larger panels, but again the bigger the panel, and the larger the air gap, the more intrusive into the room they will be. I went with 1.5" air gaps because it was convenient. My rear panel being 5.5" thick is a pretty deep panel. I wouldn't want something like that on my side walls.
(3) I've heard of people just taking the stock 24" x 48" rigid insulation panel (4" thick of course) and just straddling the corner with the panel in a vertical orientation. By that I mean for an 8 foot ceiling, you would only use 2 sheets. Some people have turned things sideways and use 4 sheets for an 8 foot ceiling and you should be able to trap a lot deeper bass. A 4 foot wide sheet will come out in to your room a ways, but is a better acoustical configuration if you can swing the space and the cost being twice what the vertical method is.