US source of Panzerholtz?

bdp24, Corian is very difficult to get if you are not a licensed installer but there are other brands available that are essentially the same stuff. Corian will actually stain over time and is not as hard as Quartz solid surface material which will never stain. I made my sub woofers out of Corian layered with glass microspheres in epoxy (System 1). Layered with MDF as Phoenix did should work well also. If you edge the MDF with a hardwood like walnut you can get nice cosmetic results. Quartz is harder and heavier than Panzerholtz but more brittle. You can not use only a hardwood for the middle layer as it will expand and contract with humidity and in time the whole assembly will fall apart. I have also made dipole speakers this way with excellent results. Quartz really requires diamond tooling, Corion will cut and shape with carbide.

@bdp24-
I don't know the properties for slate vs granite;  look on-line for the parameters or get some pieces and test it.  The links I provided on the DIY site to Bryan's blog pages might have some insight as well.

Agreed that constrained layer damping works by converting vibration to heat, but don’t agree that it is more effective when both layers are less stiff.

A counterexample to the ASC assertion is industry-leading Quietrock 545 acoustic drywall, with an embedded layer of sheet steel. Works very well in my listening room. This is consistent with my understanding, that what matters is speed of sound in the material. Adjacent materials should have very different sos. I have read that panzerholz sos and aluminum sos are similar (6000 m/s), but have not tested it, though I keep meaning to.

OH - now I think I see what ASC is talking about. Perhaps they are talking about vibration THROUGH the wall material, whereas what we are interested in is vibration ALONG the plinth material.

All the same, I wouldn’t want my walls flexing like a 10 cent woofer.

I have no personal experience with Corian, but found the following comments when considering a plinth for a SP-10 Mk 2.

First, apparently Corian includes flakes of aluminum in the compound.  But there is no specification for size or quantity of those flakes, so the resonance characteristic can vary from one brand to another.

Second, it is mentioned in the Direct Drive site and is not recommended for plinths, too lifeless or laid back sonically.

Your mileage may vary.