How to isolate turntable from footstep shake or vibration

MC, You might be correct about the cause of the bridge collapse. I mentioned it more for the sake of humor than anything else, and my guess is or was that the resonant frequency of the bridge suspension(s) and the wind velocity had something to do with it. But I certainly did not back up my remark by actually researching the cause. If you look at the famous video of the bridge shaking itself to death, it appears that the suspensions, one on each side of the bridge expanse, are resonating at different frequencies, which causes the road bed to be twisting and tilting so crazily, which eventually led to disaster. I dunno.
Anyway, we all agree that putting a turntable with a built in spring suspension on top of a spring-suspended shelf is usually not a good idea, unless the doer has a good understanding of the physics and the proper instruments to determine what might work.  This is not a knock on spring suspensions per se.

Matter of fact that is exactly what I recommend doing, putting the spring suspended table here on top of a Townshend Podium or Platform. Don't know which Oracle we're talking about here and I'm not familiar with their suspensions either. But all we really need to know is the one we have here isn't isolating from people walking around. A Podium definitely will accomplish that.  

The only real questions are how bad does the OP want to eliminate this, and what is he willing to do? A massive rack like mine will just about do it, and it is very inexpensive to make. But it is incredibly massive (well over 500lbs) and a time consuming DIY project. Townshend Podiums will definitely do it, and improve all aspects of sound quality as well, but does cost a lot more especially as it will have to be pretty big, those Oracle have a big footprint!   

The most cost-effective answer will be if the Oracle feet can be replaced with Pods. Some turntables like Basis this is easy and the way to go. But Oracle half the turntable is about how it looks, so I don't know.....

It is picking up the footfalls because it is amplifying the octave that the turntable suspension system is moving at and affecting so you will need to wall mount the table and use an appropriate shelf to wall mount it with there are some very good wall shelf choices.

@esputnix: Is your Oracle the Delphi model? If so, does it have the Brooks Berdan mod?

Brooks was an expert at turntable suspensions, bringing with him to hi-fi the knowledge he had acquired working in the field of race car design. He was an Oracle dealer in the 1980’s, and realized the mass of the Delphi’s floating sub-chassis was not evenly distributed around the bearing. As a result of that mass imbalance, when the sub-chassis was pushed down upon and then let free, the s-c would not rebound straight up and down, but would instead cant to one side and then the other. He devised a simple solution to the problem: add a certain amount of mass (in the form of a thick disc of stainless steel) at a specific location on the underside of the sub-chassis. With the added mass, the sub-chassis behaved in a perfectly pistonic manner.

Brooks’ mod was written up in TAS (I believe), and lots of Delphi owners sent their tables to Brooks to have the mod installed. Brooks is no longer with us, but his son Brian is (his shop in Pasadena, California is named Audio Elements), and he may still be offering his Dad’s mod. Oracle themselves eventually incorporated Brooks’ mod into the Delphi.

As others have stated, adding springs under a table having a spring suspension is a very bad idea (imagine a car having two sets of springs for each wheel!), even the superior Seismic Pod from Townshend Audio. Years ago Townshend made his Seismic Sink isolation platforms using an internal inner tube as the means of creating isolation, but I don’t know if their use was approved of for tables with spring suspensions. Max Townshend incorporated the Seismic Sink into the Mk.3 incarnation of his Rock turntable, the Seismic Pod into the Mk.7 version.

One way to consider going is to install Townshend Seismic Pods under not the base of the Oracle, but in place of the stock springs, with the Pods resting on the shelf, not the Oracle’s base. Will that solve your footfall problem? I don’t know. Shoot Max T an email.

Putting a table on a wall-mounted shelf has for years been the most commonly employed solution to your problem, an "outer" wall if at all possible.

Another possibility is installing a set of "roller bearings" under the Oracle’s base. That design is a ball bearing sitting in a shallow "cup" (made of aluminum), and is available from a couple of companies (most famously Symposium Acoustics). Roller bearings provide isolation in the lateral plane, but zero in the vertical. Some people have used roller bearings in conjunction with inner-tube type isolators, with great success.

The resonant frequency of the system needs to be lower than the operating frequency for isolation to occur. Most isolation systems have a resonant frequency in the audio range. The only systems I know of, which are also non-powered, which the resonant frequency is below audio frequencies are all based on the "Minus K" platform. Most "Minus K" platforms are under 4hz resonant frequency.