@dmk_calgary, one layer of ASC WallDamp is all that is required, or even desired. You want it’s two sides to be in direct contact with both surfaces it is damping. ASC says WD need cover only a portion of the two surfaces; I think they told me something like 20-25%. I’m planning on trying 50% with Baltic Birch, with a 4" space between all the 4" squares.
Just yesterday I picked up a box of 18" square x 1/2" thick granite floor tiles (each weighs 17lbs.), and was surprised and disappointed by how "ringy" are they when struck. I know, just don’t strike them ;-). If you are going to use granite, thicker would undoubtedly be better. But 3" thick granite is pretty heavy! 1/2" may be fairly stiff---which is of course good, but apparently has a fairly high "Q" factor---which is not. I have some smaller pieces of granite, and they are much more non-resonant than are these 18" square floor tiles.
ASC told me WallDamp won’t be as effective on granite as it is on any wood product, due to the granite’s stiffness. Constrained-layer damping works by converting mechanical vibration into heat, thereby deceasing resonance---the sound a physical body makes when vibrating. Walls move just as do drumheads! That’s of particular relevance when the supported component is itself a device which measures mechanical motion, as does a turntable/tonearm/cartridge of an LP groove. You want to isolate as much as possible any such component from outside vibrations, at all frequencies.
