What about constrained layer damping (CLD).... From reading here I've seen that mentioned with low mass plinths, where it's probably more important. If I was building a heavy mass plinth, would layers of different materials add anything?
If I was building one myself out of wood (because I have a saw!!
here's some questions:
1. birch plywood seems to be a popular choice.... anything better? MDF? Layers of different materials?
2. Does the type of glue matter? Somewhere else I read the type of glue can matter (animal glue vs. normal wood glue) -- it's structure when dried can make a difference... fact or fiction? It's hard for a novice to tell what's splitting hairs and what really matters!
3. How "tall" should it be (how many layers)? Does this matter? Most pictures seem to show something like 3". Is anything gained by 4"? 5"?
5. is there a mass or weight I'm aiming for? 10 lbs? 20 lbs? 80 lbs? A plinth maker on the Bay has ones from Baltic Birch that weigh 8-10 pounds but wood can be removed for lead shot or sand.
Since I haven't seen any detailed plans for a TD-124 (dimensions of the openings in individual layers) this may be something I don't want to get into (although I do have a friend with a complete wood shop) and should just buy one!!!
Constrained layer damping is simple to build, but difficult to design properly, so that the system will actually work as desired.
The fundamental principle behind it is that the middle of a beam's cross section is in shear, while the top and bottom areas are in tension/compression. By laminating the beam, we can choose to put a material with high energy loss (hysteresis) under shear in that center. Then, as the beam flexes downward in the middle, the top skin goes into compression, the bottom skin into tension, and the central layer is in shear. (Shear means that the top of the layer is being pulled one way, and the bottom of the layer is being pulled in the opposite direction.) The small shear deformation of the central layer is repeated, back and forth, when the beam vibrates up and down, generating heat, and dissipating energy.
Constrained layer damping, (CLD from here on), when well designed, is an elegant way to build damping into a structure, since it uses a minimum amount of damping material which is placed in the area that it will be most effective.
If you have a thin beam or plate that is obviously vibrating a lot (enough to see or hear), then there's an easy way to use CLD to help. Bonding a thin sheet of neoprene to a thin sheet of aluminum or fiberglass, and bonding that to the part of the beam or plate that is moving (not just the over center, extend it to cover at least a third of span), will create a CLD system that should be quite effective. The skin of aluminum or fiberglass acts as the new outer surface of our CLD beam, the rubber is the center. The glue has to be strong/rigid enough that it doesn't creep, and will drive the shear into the rubber. The rubber has to be thin enough that it the shear will be concentrated enough to dissipate enough heat to damp the system.
That obviously leaves a lot design decisions, so let's say the rubber should be 1/8", the aluminum/fiberglass could be .025" to .063". I think that the strongest of the 3M spray adhesives (90?) should do the job.
Note that by ADDING the rubber/skin, we have made the existing structure stiffer as well as more damp. That will tend to reduce the amplitude of the vibration as well.
So, back to your plinth. I wouldn't pursue CLD, since it's going to take a lot of analysis to figure out where and how much. And the plinth likely won't be flexing enough to make it viable, anyway. If you're going to laminate it from birch ply, use woodworking glue, like Titebond. Then, maybe add a damping layer between the plinth and the turntable frame. That will kill some of the transmitted vibration.
It would be good to design the plinth with cavities that could be filled w/ sand or shot. Or you could put sections of steel rod/bar in there.
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