Best type of metal for turntable platform?

Personally, my choices wouldn’t include a metal plinth at all. Instead, ordered by decreasing preference: heavy marble or other stone slab; a leather bag filled with buckshot; concrete; glass; thick acrylic or Lucan, HDPE, or a couple layers of heavy rubber, like lSorbothane sheeting. 

Butcher block acoustics makes affordable and good looking isolation platform made of wood, maple or walnut. Been using them for years to good effect. Can be bought with or without metal threaded inserts for the feet of your choice, which they also sell. My latest in maple replaces an existing wood shelf that came with my turntable wall mount, so no inserts and no feet required, as the isolation rubber feet are integrated into the turntable wall mount. The wood is very hard and so I can do without the tip toe discs on the turntable. Wood is a far more "dead" material than metal. Not sure why one would consider metal. Between the added butcher block, the dual plinth design of the turntable, the TT tip toe feet, and the fact the shelf is bolted to a concrete outside wall which faces the back of house, and away from "possible" street/traffic induced vibrations etc....not sure I can better my set up much. Plus the shelf is up high, eye level when standing....well above speakers. I tend not to worry about so called seismic vibrations, unless there is a 6 on the richter scale. 😁

Townsend is the clear winner. I’ve been saying this for almost 2 years now.

I have a vpi classic 3. It’s extremely heavy. The plinth is made out of steel. I would find out what type because even if you tap hard on the player there is no skipping. You can even tap it hard with your hand. Truly amazing. It weighs 65lbs. 

@deanshias,

As others have said , Townsend knows what he talks about. But, let me if I can expound upon what he said. If you view Figure 7 of this Fundamentals of Vibration (newport.com) , when your designing a vibration isolator, the resonant frequency needs to be less than the Hz you are trying to isolate. At resonance (and all vibration isolators have one), any vibration is amplified and this is called the transmissibility. The damping factor reduces the transmissibility peak, but reduces how quickly (the slope) the vibration is reduced after the resonant frequency. So when Townsend say a damping factor of 0.16 - that is near optimum. Townsend may very well have a curve that looks like Figure 7; and each design is limited to a range of supported weights.

Because of the low frequency (foot fall) you are trying to isolate, which has a lot energy associated with it you have an engineering problem that the number of solutions is pretty limited. You can dismiss as you wish; and spend all the $$$ you wish; but your up against some very hard truths. Good Luck.