[William Sommerwerck]

OK, so you get substantially all unwanted acoustic energy
out of the vinyl and into the platter by matching the acoustic
impedance. But where does it then go?

It's absorbed by the plastic itself. I'm not claiming _all_ of it is
absorbed (or for that matter, that "all" the energy leaves the LP). Rather,
the platter provides a much bigger chunk of substance to dissipate the
energy than the LP by itself.

This effect is audible on a smaller scale simply by comparing "heavy" (180g)
LP pressings with regular pressings. Not surprisingly, they sound "deader",
less spacious and "lively".


I suspect the unwanted acoustic energy (assuming a longitudinal
wave) gets mostly reflected back at the platter/air boundary impedance
mismatch underneath the platter and then gets transmitted back to the
vinyl because of the excellent vinyl/platter impedance match.

All this could be proven (or disproven) with an LP having two sets of
grooves -- one unmodulated, the other with lots o' loud music. If both
grooves were played simultaneously, one could judge the damping effect of
the platter, mats, etc.

By the way, about 10 years ago a well-known Canadian manufacturer produced a
platterless LP player -- the disk was supported only at the center! The
designer (whose name I will not repeat) claimed that air made a better
impedance match to the LP than a metal or plastic turntable platter! Not
surprisingly, this product didn't last long. If nothing else, it failed to
provide a stable azimuth for the pickup.


The only hope is for the platter to be acoustically lossy.
Ideal materials aren't lossy, of course.

The Well-Tempered platter -- and some others -- have lead centers. Sort of a
plumbum Tootsie Pop.


But I have never looked up the acoustic loss coefficients of real
plastics so I don't know if this is a reasonable hope.

I haven't looked it up, either. But you can get an idea by tapping the
platter.