I guess I'm not sure when I measure how far a sub is from a back/side wall, if I measure from the front of the sub. When you say centerline, are you saying where the horizontal and vertical lines cross on the front of the sub?
The back wall is 15ft wide. The screen is 10ft wide and is centered dead nuts in the middle. This gives me 2.5ft left over on the right/left sides.
So, in graph 1, the sub was right below the left side of the screen. The left side of the cabinet on the sub was lined up with the left side of the screen.
In graph 2, I moved the sub exactly in the middle of the back wall, so if your measuring from the center of the driver, it would be 7.5 ft from the left/right walls.
In both cases, there was about 6-8" of room behind the sub cabinet to the back wall.
In regards to the ICF forms, the back wall and left side wall would have this product behind the drywall.
Another thing I'm sure you saw in the pictures, is that I have a stage built below the speakers. It is built out of 2x8's and the top is covered with padding/carpeting.
