Turntable platforms

You could also consider a passive isolation platform such as those made by eg Thor Labs. These can be tuned to have around 0.5 Hz resonant frequency, and cost around $2,000+. 

They'd benefit from a solid floor but will still be sensitive to some degree to environmental ground noise, e.g. traffic. 

I've seen very good results with one of these, may take a bit of work to get it right, and whether it works better than a pendulum will depend on your situation. 

@geoffkait Springs, yes - but what design and how to damp?

Looks to me like Ligo is using coupled pendulums. Do those provide vertical isolation? Looks like that comes from the cables, and the intermediate masses?

The springs should not be damped. Damping hurts isolation effectiveness and it’s audible. You just place the springs under the component. Voila! 🤗 My springs provide vertical isolation, some rotational isolation. LIGO uses many types of isolation including dual-layer heavy mass-on-spring systems and sapphire thread suspension. LIGO pendulums provide isolation in many directions. My first design was a pendulum with very heavy suspended mass that isolated in 6 directions with Fr as low as 0.5 Hz (vertical 🔝).

Undamped springs will ring. If the frequency's low enough it may not be a problem, but the ideal is to damp critically. 

I can't see any downside if damping is an option. It may be impractical to damp a pendulum. 

6 directions? Don't you mean 3?

How did you achieve 0.5hz vertical? 

If you’re so smart why ask me?