Would this be an effective way to 'soffit' mount speakers?

Do the speakers have to be mounted in a cavity, or can the be 'floated' on stands. The panel assy would be mounted in each corner of the room, fastened to the walls. The concrete base would sit on the floor with a sheet of rubber on the bottom. (please see pic below) Thanks, Tom

I don't think that is all that out of the ordinary, actually. However, that 1/2" isn't gonna cut it, IMO- that's still going to audibly resonate in the low end. I'd be thinking two sheets of 3/4" mdf glued together, at least.

Tom, Yes, I've suggested this sort of freestanding soffit in other threads. Like Jon wrote, I would also recommend building the panel more substantially. Additionally, I would move the speaker opening a little more towards the center vertically, and slightly off center horizontally. You want the speaker to be centered in order to maximize the low frequency extension of the baffle, but you also want to make the distances to the 4 panel edges dissimilar in order to minimize diffraction effects. Thomas