It all comes down to what you are trying to absorb or dissipate...
The spring constant and the mass, will define the frequency of a systems resonance - so anything with give - you need to make sure that the native resonance will be "out of harms way" (beyond audible frequencies...)
And damping - different materials will damp at different frequencies, and they will refract vibration at different rates and frequencies, resulting in situations were some of it is damped, and some of the energy is emitted at a different frequency - which then has to be tackled seperately!
My solution with a unsprung solid base turntable and a sprung vintage wooden room floor...
Layer 1 - 5 large sorbothane pucks (act as dampers/absorbers at certain frequencies, and springs at others)
Layer 2 - large very massive/heavy concrete paver 60cm x 60cm
Layer 3 - more Sorbothane under feet of rack sitting on paver
Layer 4 - Ikea "Lack" tables stacked to make a rack - paper honeycomb structure tends to damp/absorb high frequency vibrations - it is very light, and needs mass, which is provided by roughly 50kg of equipment (AVR, power amps, phono stage, PC, Power conditioner)
Layer 5 - firm silicone rubber feet replacing the original TT feet, 5 of them pressing against the resonant cheap plywood underside of the TT
Layer 6 - Internal to TT plinth, between plywood underside and MDF/Roswood veneer top - plasticine stuffing and plasticine in "toilet rolls" cut to size, and compressed between plinth and underside - plasticine is providing both mass, springy damping, and it is an excellent absorbant damping material.
Getting there was a saga that took months, experimenting with a wide variety of materials and devices - and using the needle placed on the (immobile) platter to detect vibrations and feedback... mag Lev feet were tried, various materials and sizes of silicone and sorbothan (and other) feet, pucks and domes were tried - and each was measured.
End result was a massive improvement for that specific turntable.
Having said that, my other turntable, with a sprung isolating base, saw only a very marginal improvement... but it wasn't suffering from issues in the first place! (horses for courses)
