If you build your stage and fill it with sand (and the stage does not contact the walls), you have created a structure that, while not decoupled, does damp the kinetic energy transfer into the slab. As you suggest, a better method is to place a damping agent (such as a rubber mat) under the stage framing to further isolate the stage from the slab. Acoustik Mat is one such product that will work very nicely. (contact Ted at www.soundproofingcompany.com
). The product will NOT flatten out (total pounds and pounds per square inch are different). Because the impedance of a concrete slab is entirely different than that of a wood framed floor, this technique is important when you're building a room over a framed floor. The downside, however, is unless you plan for the static weight of the stage, it is very likely your floor system will not handle the weight.
You've got a concrete slab, you make a box and fill it with sand, which weighs more then your speaker. Sit the speaker on it and its de-coupled? I fail to see how this transfers energy into the room rather than into the slab and into the house?
Good question. The damping does not re-direct energy, it simply converts kinetic energy to heat reducing the transfer into the remainder of the home's structure. The high mass of the stage, and the positive attachment of the sub to that stage, is the mechanism which works to improve efficiency. When the driver moves forward, the cabinet wants to move backward. Preventing this backward motion is helpful. It takes mass to do that.
That doesn't mean we have overcome the equal but opposite reaction...it means we're doing a better job of managing what's going on.