Best type of metal for turntable platform?

I would do some homework on damping properties of different materials. There is a name for it, which I forget. You can use two different materials with a damping material in between. Or look at the plinth of the VPI Prime Signature - looks like its aluminum in between two other materials. 

I to live in a very old house with bouncy floors.In my secondary system I have an old Sota on a walnut turntable platform that utilizes rubber balls. When someone walks into this room the stylus  skips. In my main system I have a VPI Classic 3 on a 4 inch thick maple butcher block platform with 4 rubber and cork anti vibration pads. I never have any skipping and can even rap on the rack without any issues. The vibration pads are the same ones that are used in the HVAC industry. These pads are very inexpensive and you will find that the same pads are being sold under some audiophile brand names priced at 10 times their actual cost!

As far as metal goes the one material that has been in use for machine tools for decades is a special grade of cast iron called Meehanite. It's known for it's superior damping quality's , very important when designing a machine tool. The last thing that a machine tool needs to do is vibrate or ring, it wouldn't work very well.
I do believe it has been used before in a TT plinth, not sure who it was though.
If your going to use aluminum, maybe check out cast tooling plate. Being cast instead of wrought it should offer better damping characteristics. It's offered in many thicknesses and comes in a machined finish on both sides. It's cut to size using large CNC circular saws.

BillWojo

They are $1500 but an incredible upgrade for a TT-Silent Running Audio. VR platform. I put one under mine and the improvement in sound was the biggest change in my system short of upgrading my speakers from $10K ones to $30K ones!
SRA has two separate companies that make vibration control devices for the military and medical. Submarines and CATs!
It is clear, with over 40 patents, SRA devices allow complex machines and components to function as they were designed.

@bjw54

I have an old Sota on a walnut turntable platform that utilizes rubber balls.

I’m a bit confused. What SOTA table do you have? And why do you have an internally suspended table (with springs, if that is what it is) on a separate base? That would seem to defeat the internal SOTA suspension system.

My SOTA sits atop my rack, all by itself, and I can walk, jump, yell at, and bang on my SOTA Sapphire TT’s plinth, and it is not effected, by anything. Perhaps a bomb going on within the room might.....might. 😁