Jean, FYI, Raul Iregas had an SP10 mk II in a marble plinth and did not like the results. He "reverted" thereafter to a simple and not very heavy wood plinth (maybe made of Baltic Birch), and that is what he uses with the SP10. Two qualifiers are: (1) This was a Technics SP10, not a Lenco. The optimal plinth for a direct drive may be different from that for an idler, and (2) we don't know much about Raul's marble plinth except he did state that it was very heavy (100lbs?). Perhaps Raul would care to comment.
I've often wondered what is the point of the ping test in selecting materials for audio use? It seems to me that a good shelf or plinth material should be a material that is able to dissipate the vibrational energy put into it, as heat. There is a large body of knowledge in materials science as regards the ability of energy to pass from one material to another. (Some is reflected back and some is absorbed at each interface. When the energy pulse reaches the far boundary of the absorbing material, some of it is reflected back again toward the admitting surface.) So the question is how does the plinth or shelf material mate with the metal chassis of the turntable and then from there what happens to the energy that does get into the plinth. You could have the deadest substance on the planet, but if most of the energy from the tt chassis is reflected back at the interface OR if the mating is suboptimal in any way, the plinth is worthless. Maybe that's why marble failed in Raul's experiment. As for me, I am fairly satisfied that undamped granite sucks as shelf material, for another example. I would definitely not consider granite for a plinth.
I've often wondered what is the point of the ping test in selecting materials for audio use? It seems to me that a good shelf or plinth material should be a material that is able to dissipate the vibrational energy put into it, as heat. There is a large body of knowledge in materials science as regards the ability of energy to pass from one material to another. (Some is reflected back and some is absorbed at each interface. When the energy pulse reaches the far boundary of the absorbing material, some of it is reflected back again toward the admitting surface.) So the question is how does the plinth or shelf material mate with the metal chassis of the turntable and then from there what happens to the energy that does get into the plinth. You could have the deadest substance on the planet, but if most of the energy from the tt chassis is reflected back at the interface OR if the mating is suboptimal in any way, the plinth is worthless. Maybe that's why marble failed in Raul's experiment. As for me, I am fairly satisfied that undamped granite sucks as shelf material, for another example. I would definitely not consider granite for a plinth.