Polypropylene as a Turntable Base Material

I have a large block (19" x 16" x 21/2") of high density polypropylene (PP), which I plan to use to make a base for a turntable I'm building. It must weigh 30-40 lbs based on feel. I found the following on the web regarding PP:

Polypropylene's "natural harmonic" is at a very low frequency of 125 to 150 Hz. The normal "problem hearing range" is 1,000 to 3,000 Hz. Therefore, the material's natural harmonic is far below the "problem hearing range." The nature of sound is that the lower the frequency, the greater amount of energy is required for the sound to be heard. To quantify the difference, the amount of energy required for a 50-Hz noise to be noticed is 1 million times that required for a 3000-Hz noise.

Based on this information (from a manufacturer of boats who uses PP materials in construction), I think PP may be a very good material to use for a turntable base. It is a viscoelastic thermoplastic polymer, and should thus have good sound absorption properties. Has anyone here ever experimented with PP? I see a lot of acrylic turntable bases and platters, but none from PP. Why?
You shouldn't have any problem with noise transfer through the polypro, but it will change shape and size with changes in temperature and relative humidity. It also has a tendency to warp. It is also difficult to cut or mill accurately because of this.

You can minimize the warp with additional curing. Check with manufacturer or extruder. This material will kick butt in the midrange-bass area with a huge soundstage.

Acrylic is a joke compared to it. Turntable manufacturers can't be bothered with the time/expense in machining the material plus the stability under temperature is its main weakness.

Would you recommend bolting the subchassis directly to the PP Base, or suspending it above the base on springs? The motor will be isolated from the base in a separate mount.
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:
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.

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...
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!

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.

Thom @ Galibier
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...