I'm making building plans for a box in box studio. First of all I want to make the floor float with this: dead link or this: http://www.isover.nl/producten/producten-alfabet/sonefloor_304/ (sorry its in dutch, but as I understand it, all that matters is a good mass-spring ratio and it's less important what you actually use to make it float) On top of that I want to make a skeleton with wood, use plywood to strengthen the floor, use rockwool for absorption and use concrete for mass (both for sound isolation and for a good mass-spring ratio) Since concrete is more difficult and more expensive then sand I thought why not use sand. It has great sound isolation qualities and it's much easier to use. But after reading the thread above I don't understand the problem with it. If the top layer would be a layer of plywood and then sand for example, what makes it different then using concrete... Layering the floor would then go like this for example (from bottom to top): - concrete (already existing in the room I'm using - sylomer, sonofloor (or neoprene) - plywood (with wooden skeleton attached to put rockwool in place) - rockwool - plywood (with 2nd wooden skeleton attached for sand) - sand - end floor like laminate resting on skeleton In this way you can make the proper mass with sand, still have a MAM layering and still have a complete detached (floating) floor. Thanks in advance!