Polypropylene as a Turntable Base Material

Thom: That's an interesting point about the RIAA curve. When I mentioned it above, I was thinking from the standpoint of the curve as it's recorded on the record, and how that would impact excitation of the machine through the stylus/groove interface, instead of from the standpoint of how it's applied in the phonoamp. I guess those two factors would tend to offset each other to some degree in isolation, but factor in the re-equalized sound as produced by the speakers and introduced back into the loop through the air and the gear rack, and avoiding a lower-frequency resonance would seem to make more sense...

Thom, first of all, thanks. That was one of the most useful responses I've ever gotten on a discussion board.
I don't know why this didn't strike me earlier, but a few years ago I was involved in some research with a major US National Lab centering around using both active and passive acoustic spectroscopy to determine changes in the texture of materials being heated inside a sealed steel pressure vessel. One of our biggest challenges was acoustic impedance matching, since that determined the degree of penetration of the sound waves (and we were exploring everything from audible to ultrasound) through the vessel, into the materials of interest, back through the vessel, and to our sensor unit. There is actually quite a body of research in this area - I'm going to go back through my notes and references now, since the light that just went on in my head is telling me that proper impedance matching is the key to properly draining vibrations from where they shouldn't be.
The sheet of aluminum you suggested is one way of coupling the dissimilar materials - I'm thinking that the use of specific acoustical coupling materials (think about the gels the techs use when giving you an ultrasound exam to couple the metal probe to your skin) may produce a much better effect.
Thanks for the inspiration - this should be fun!

For those who would like to explore acoustic impedance matching in the design of their systems, here are some very useful tables of acoustic properties (Z is the acoustic impedance). Remember, the farther apart the impedances, the more acoustical energy is reflected instead of transmitted at the interface. Have fun!

http://www.ondacorp.com/tecref_acoustictable.html

This is great stuff! Thanks for the link Ait.

As you reflect on all of this, bear in mind that there are quite a few different ways to solve a problem. This will help you keep your sanity and enjoy the ride.

If you think about Frank Schröder's tonearm (a "woodie" if there every was one), he has to approach the problem entirely differently. To my understanding (never discussed this with Frank) his problem involves draining all of the energy before (or at least by the time) it reaches the end of the arm. There's not much of an energy drain through the bearing and I don't expect he'd want the vibes making a return trip to the headshell/cartridge.

Of course, from his point of view, this provides a degree of isolation from vibes traveling through the stand into the base and then into the tonearm.

Certainly, I love Frank's tonearms, but I approach a turntable design more from a coupling perspective - trying to eliminate as many "wiggles" as possible. One advantage (to my way of thinking) of a rigid base is that it better fixes the relationship between the tonearm and the bearing. We've already mentioned acoustic impedance matching, so I won't get into that again. This fixed relationship would seem to me to afford better speed stability at the micro level - which is perceived as an absence of FM distortion (way below the perception threshold of wow and flutter).

Of course, nothing comes for free in any physical design which is what makes this game so interesting ... watching solutions come literally out of left field.

Cheers,
Thom @ Galibier

Thom,
My sanity has long ago been compromised, when I think I'm "on to something" I'm borderline OCD (OK full blown OCD). That's one of the traits that has made me pretty successful in science, to the exclusion of some other things - there's certainly no free lunch.
Anyway, I hold no pretenses that what I'm thinking about has never been done before (a quick USPTO.gov search should confirm that), but I'm having fun thinking about multi-layer designs to drain away what we generate despite our best efforts and block what we don't want intruding from the environment. What makes it interesting is that Z is not just Z, but Z(f), so depending upon what frequencies you are interested in, the answers may be quite different. For example, impedance matching is critical in ultrasound work, but subwoofers don't much care what they shake to bits.
When I settle on something, I'll share, unless something else captures my pathology first...