Polypropylene as a Turntable Base Material

PP seems like an intriguing possibility for a TT base material. But I have some questions/comments about a few of the statements in your threadhead. Maybe you don't know the answers any more than I do and are only repeating what this boat maker says, but...

What is a "natural harmonic", as inherent to a particular material? Maybe I'm ignorant, but this sounds like more of a metaphysical than scientific concept to me. Two objects made from the same material can have very different fundamental resonance frequencies, depending on their form and size.

Also, who defined the "problem hearing range" -- what does that mean, and why? The human ear is sensitive in the 1-3KHz range, but it's also very sensitive in the range occupied by the human speaking voice, which is lower.

And I don't understand the relevence of the point about lower frequencies "requiring" more energy to sound equally loud as higher frequencies. Concerning resonance, if playing a record produces energy at frequencies at which the TT resonates, then it seems to me that's what will happen, as dictated by the amplitude of the exciting frequency and the Q vs. damping of the resonant system. I guess that ideally nothing about the turntable would resonate within the audioband, but that isn't achievable, and for something the size and rigidity of a TT base, I don't think a fundamental resonance as low as 50Hz is possible. If you could do it, the main advantage it seems to me would be that the RIAA curve demphasizes lower frequencies cut on the record (and boosts higher ones), but I don't know how the curve actually intersects with the realistic range of resonances present in TT's.

As I stated above, I got the info in the second paragraph of the threadhead from the WWW, from the website of someone who uses PP materials to make boats quiet from stucture-borne vibrations. Here's the link:
http://www.nida-core.com/french/nidaprod_honeyinfo_rigid.htm
I agree that this is not proof of anything, but was looking for comments from others who may have experimented with PP in the past. Since one of the functions of a TT base is to isolate the stylus from structure-borne vibrations, I thought that maybe there was some relevance there.
As a Ph.D. scientist myself, I realize that there is a lot of bogus and misleading info on the web, that's why I'm reaching out to others who may have gone before.

I think what I'll do is use the PP as a base without cones, so that it just lays flat on the shelf, with the motor attached to it. Then I'll spike the subchassis (which contains the platter and tonearm) around the platter recess and simply lower it onto the top of the PP base so that the motor pulley is in the correct position relative to the platter, and the spikes are in position to drain away bearing and belt-borne vibrations before they get to the tonearm mounting. In this way, I won't need to worry about the PP warping or flowing (since its glass transition is below room temperature this may be a real issue). The PP should still perform it's function of damping structural vibrations in this layout.

Hi Ait,

This topic was one of the most contentious ones during the late Winter and early Spring of 2000 - when the original Teres project was its most active. In December, 1999 through January, 2000, the original Teres-5 worked through the basic parts design, after which point we frozen the design and invited the "public" into the project. It was at this point that peoples' attention turned to the design of their bases - having some 5 months' time before the delivery of their parts.

There was a split into two main camps, which someone (I think it was Ken Schei) designated as the "stiffies" and the "woodies". Of course, those of our friends from the British Isles would contend that a woodie is a stiffie, but I digress ...

The argument espoused by the "stiffies" was that although the frequency of a rigid piece of metal is quite high, and centered in the most sensitive area of human hearing, that this problem is negated by our good ol' friend the RIAA curve which attenuates high frequencies. They further argued that these higher frequencies are lower in amplitude and are easier to damp. Their approach in general centered on working from a perspective of damping a ringing substance and letting the RIAA equalization deal with the rest.

The "woodies" favored damping over rigidity, and for the most part they didn't work much at stiffening their bases. Their philosophy centered on using a material which could not easily be excited. The argument against soft, lossy bases has centered around its poor dimensional stability (both thermal and mechanical), combined with the additional challenge of draining vibrations away from the bearing due to the dissimilar materials interface (e.g. brass bearing --> wood base). The more similar the material interface is, the more vibration is transmitted and the less it is reflected.

We all know the different directions taken by both the DIY-ers as well as by Teres / Galibier / Redpoint, and it doesn't bear repeating here.

Because yours is a DIY project, I would advocate a flexible architecture which would allow you to experiment - adding a thin sheet of aluminum (ca 1/8" to 1/4" thick) to the top surface. If you go modular, you can experiment to your heart's content. You may not be in a position to generalize your knowledge and your discoveries beyond the context of your own turntable, but the good news is that it will be your design and will suit both your listening tastes as well as your system.

Cheers,
Thom @ Galibier

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...