TT on Symposium Super Segue - slate or maple underneath?

Tony, you can get WallDamp directly from ASC in Eugene Oregon, I believe. They (Acoustic Sciences Corp.) offers in in 4" squares, but also scrap pieces by the pound, trimmed pieces from end of rolls or installation jobs they do. I ordered some pieces years ago cut in the exact size (22" x 16") of the top (MDF) shelves of my Solid Steel racks, then bought extra top shelves to make doubled/constrained-layer damped shelves for the racks. I’m going to do it again, this time with Baltic Birch.

Speaking of Baltic Birch, I misspoke above when I said the plywood has each layer 180 degrees apposed in grain direction to it's two neighbors. That makes no sense! They are actually 90 degrees opposed, which of course greatly strengthens and stiffens the plywood, just what you want in a shelf or platform.

@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.