How to isolate turntable from footstep shake or vibration

Even while the Oracle turnable that I use has a built-in springs suspension by design there is a low or even sub-low frequency boom every time someone walks in a room. This becomes really bad with the subwoofer’s volume set high as the low frequency footsteps make straight to subwoofer where they are amplified shaking everything around. It seems the cartridge is picking up the footsteps very efficiently as even a lightest foot down becomes audioable. What can be done to attempt to isolate the turntable from the low frequency vibrations? Interesting, that the lower the volume of the subwoofer, the less the footstep shake is evident and with the subwoofer turned off it is a barely a problem at all. 
Usually it’s a very bad idea to “suspend” on springs a turntable that already has a built in suspension. So I am surprised that a few recommended adding a sprung shelf. The two suspensions will inevitably fight each other which could end up with positive feedback of vibration. Remember the Verrazano Narrows bridge collapse.
At any rate Lewm is correct. Putting a suspended turntable on springs will probably have an adverse affect. The problem here is the Oracle's springs are not isolating the sub chassis from footfall. Either because they are tuned to the wrong frequency or they are poorly dampened and unstable. The only solution other than changing turntables would be a wall shelf if the wall does not shake. 
The Tacoma Narrows bridge collapsed twice. Neither time due to being suspended. Both times due to wind vortex. Wind blowing through the Narrows creates a low pressure on one side, causing the bridge to first vibrate up and down and then twist slightly. The twist creates low pressure on the other side. The twisting of the bridge increases until failure.  

If you go looking for answers to problems it helps to find those who actually have the answers. In cases like this they tend to be engineers. Springs do work great, you got a good tip to go read the turntable spring thread. Springs however have the problem of resonance. Without damping they can sometimes create as many problems as they solve. A lot of things are like that. See for example all the ideas above.  

So springs are a big part of the answer. But they need to be damped. The trick is to engineer just the right amount of damping. Townshend Pods are designed to do exactly that.   

My rack is on a suspended wood floor. It is massive, because mass itself is one means of controlling vibration. Mine uses a combination of concrete, sand, and granite, with carbon fiber. But even this is not enough. Springs were a big improvement, and Townshend Pods the best improvement of all. What exactly will be best in your particular situation depends a lot on your particular situation. I can tell you though you can stomp around all you want in my room the woofers move only to the music.

There are many possible solutions to your problem. They all start with first understanding exactly what the problem really is: vibration control. Pods are the easy fix. But there is a lot more you can do to go even beyond that, if you want.