Best type of metal for turntable platform?

Hello,

I remember audiophile discuss we had during  the vinyl  " golden age" from middle of seventies to eighties"
We had discovered the " sound" of the turntable highlighted by the audio litterature ( Ivor tieffenbrun Linn sondek)

In general two approach have been acknoledged for the structure design

The ratio of mass
- plinth -platters-tonearm-cartrige tracktability( high or low)
A)
High mass plinth 
High mass platter
High mass tonearm
Low cartridge tracktability

In this configuration the transmission of the energy of vibration is balanced
B)

Low mass plinth
Low mass suspended (or not) platters
Low mass tonearm
Low tracktability cartrige

In that case, the ratio of mass structure between  the elements is also balanced

So, what ever the platter structure is the " bell"  effect must be be reduced keeping in mind that each component have specific " sound" never neutral..

I own two vintage turntables  whitch are complying  perfectly  these rules as example
- Thorens TD 124 II 
( injected aluminium & cast iron)
- sme 3012/2 & DL103)

-Walker Cj 55
Wood and suspended platter in tufnol ( the most neutral backelit phenolic component to be used in aircraft for years)
Colin walker was engineer who used to work for castle engineering in the seventies and launched the linn sondek and Ariston RD11s in uk, Usa and australia inthe seventies...

Sorry fpr this backward looking story !
It is just my experience to be shared...

Raymond

+1 +1 +1 noromance

You sure got it correct.
Concrete is the answer to vibration and indeed marble and stone, and plenty of it.

OP has a suspended floor above crawl space and bare earth below. Great start.

Rolls Royce solution:  Take up the floor, dig out a bit if required, lay new DPC, in-fill with concrete screed, lower floor level to get more ceiling height if desired.  According to taste, re-lay boards except where equipment and speakers are to stand.  Or lay carpet.
OK there is a cost, but it will be worth it.

Use marble or stone blockwork for equipment supports, standing on spikes (PLEASE NOT SPRINGS - BOINNGGGGG...GGG...GGG!).  All you equipment will be rock steady, no vibration.  It is mass-loaded to Planet Earth.  Infinitely mass-loaded as I have said.  OK, not quite infinite but the mass of the earth is quite large.

This is absolutely excellent for turntables and speakers, good for CD players and amps benefit too.  There is no ground-borne vibration (except in California) and the mass damps away all air-borne waves.

I have implemented this construction in my sound engineered basement listening room.  Improvement to SQ is much bigger than many component changes.

Target wall mount TT stand fastened to a load bearing wall.  $150 and you are done.  

Best to fasten 3/4 plywood to the wall studs for the Target TT platform. Gives more positioning options and more peace-of-mind.

I would use a run-of the-mill Technics turntable as a plinth... completely immune to footfall.

What's the next problem?

BTW, there are plenty of round things that are designed to vibrate and resonate...look at bells, cymbals and drums. 

Going square is probably fine. Just decouple it and add mass (that doesn't have a very uniform grain structure). How about good old concrete?