Hey Rich, thanks for the note. Baby is officially here, so we're in recovery mode now.
Steve,
Actually, I think it is better if your supports go front to back for what you're trying to do with dispersing sound through the riser, but to really accomplish what you have in mind, you'd need to cut holes in the front as well as registers (or ports, whatever you want to call them) in the back, especially near the corners.
In my case, I get the job done with the horizontal cross beams and the front ports (as opposed to drilling them all the way through all cross beams) as all of my cross beams are floating. i.e. the cross beams only go across the top level (1 2x10 high, hanging on joist hangers, not 2 high resting on the floor).
So the riser consists of two large cavities that run front to back divided only by the center beams which run front to back and do sit two high to provide structural support in the middle of the riser. The sound can pass freely front to back in each cavity and be dispersed through the insulation in each side. Don't get me wrong, sound in the back of the room still is not good as the primary position where the bass is tuned to a flat response, but it is quite a bit better this way than without the ports cut.
