How dose your plinth sound?

Bill, you pose a good question. There are some significant misconceptions about the resonant properties of different materials. Acrylic is NOT non-resonant at all. It does in fact have a significant low frequency resonance. When you tap on it there is a "bong" resonance that often is thought of as being dead. This is actually good and bad news. A low frequency resonance is much more benign and even can be a little euphonic, sounding warm. But it clearly does obscure detail and adds coloration. The bad news is that a low frequency resonance is nearly impossible to control. Adding damping helps little. The only way to effectively control a low frequency resonance is to add rigidity. Mass loading can easily make things worse.

Very hard, dense woods like cocobolo have a pronounced high frequency resonance. When you tap it is "pings". In spite of the resonance it sounds surprisingly good as is. Can't say that I understand why. Unlike acrylic a little lead completely eliminates the resonance. Tap on a lead loaded cocobolo base and it is really dead, as opposed to the "bong" that you get from acrylic. As TWL so generously has pointed out, the sound of a wood/lead base is significantly better.

I should also point out that I have tried lead loading an acrylic base. It does help a small amount. However, with a wood base adding the lead is a major transformation.

Metals, like wood are rigid with a high frequency resonance that is likewise easy to control. But for reasons unknown to me they don't sound all that good. I have tried using very well damped aluminum and steel. Steel usually sounded better than aluminum but neither were very satisfying. Clearly there are important factors other than simple resonance that are not understood, at least not by me.

I mostly agree with Edle's point that a heavier base is better. Just adding more mass is not a good idea. But if you add both mass and rigidity at the same time the results are very good. The upcoming Teres 340 has a 80 lb. base and does sound considerably better. However, it adds a lot of extra rigidity along with the mass.
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Chris

Why not use ironwood? Very hard to work with but can be possibly better than cocobolo.

I rebuilt the plinth for my Well Tempered Classic out of Granite. 3 layers of 1.25 inch granite slabs that allowed me to preserve the WTC layered look totaling 3.75 inches thick and custom made (3) 2 inch diameter cone legs totaling at 132 pounds. It's dead quiet.

This forum encourages me to make a plinth out of solid slap 2x9x21 inch cocobolo. This plinth beats the VPI HW19 MK III badly. Then I added 15 lbs of lead to the plinth. This even provided tighter more defined bass, extended highs, more present mids, better focused image, darker back ground. I also used cork footing to reduce the sound echo from cocobolo material.

I would think that the best plinths would be be a high mass design composed of multiple materials with each having different vibration dampening characteristics. I don't know this for a fact, but such constrained layer designs work very well in loudspeaker cabinets and isolation shelves.