How to isolate turntable from footstep shake or vibration

A pair of heavy duty shelf brackets I found at a local hardware store, attached to a couple of studs with four lag bolts and a shelf made from a few pieces of pre-finished bamboo flooring did wonders for my Linn LP 12. An outside wall is best for this implementation. This proved to be a splendid, economical solution.

There is no need to spend silly money on various other fix-it schemes. The brackets and lag bolts cost about $15.00. I already had the bamboo. A small piece of 3/8" or 1/2" plywood likely would work just as well.

But, of course YMMV, to use the cliche.’

they should have maximum dimensional and warp stability relative to the platter and ideally be free of resonance: Brass, acrylic blocks or panzerwood are way preferable to mdf or papp.

Off topic, but the arm board should be composed of the same material as the plinth and ideal be the plinth itself, so that any vibration present at the bearing of the platter will be in the same plane at the base of the arm, such that the arm simply can't pick it up. This does assume a rigid and dead plinth...

Mass does not solve everything. There is loads of environmental rumble that will transmit through anything. When it gets bad enough buildings fall over. Church bells weight tons and they ring like crazy. The only way to isolate a turntable is to suspend it. 

I guess that depends on what is meant by 'suspend'. My platforms are isomerically isolated from my equipment stand- does that count?

I had same situation in 2nd story of our house. It was built in 1929 and we still have the original hardwood flooring. The best approach is wall a mounted turntable platform. You can get them on Amazon. I've also read people here having success with Sorbothane feet from this site,  Turntable Phonograph Vinyl Record Player lsolation Feet – Mnpctech

antigrunge2, Pretty sure you agree with me completely: Listen and learn. The point I was making is not that any old thing will make a fine arm board. The point is anyone can try these things very simply and quickly - listen and learn - as long as they set aside the urge to make it look good. 

Many times guys agonize over how to do something, spend hours debating this and that, when in far less time they could just go and do it and find out. Listen and learn. 

This is by the way very similar to what professionals do in designing or improving a component. They build a breadboard, basically just a board with room for all the parts. This makes it real easy to swap out parts and compare. They can even compare things like the same part in a different place, or oriented differently, all kinds of things. Only once they figure this out then they package it nice and neat. They listen and learn. Only at the very end do they make it look good. Same thing. 

The thread title suggests footfall issues while using a spring suspended subchassis type of turntable. But the text of the opening post suggests an issue with a subwoofer which makes a loud "boom" that seems to be triggered by footsteps.
This to me suggests a combination of issues. But the "boom" noise coming out of your subwoofer suggests, to me, acoustic feedback. Is it possible that the turntable and subwoofer are in close proximity to one another? If so the simplest solution might be to move either the turntable or the subwoofer further apart. Or aim the subwoofer differently just to see if this helps to eliminate the "boom"...
Otherwise the issue of footfall and spring suspension turntables has been covered within this thread and many others more or less completely over the past 60 or 70 years ever since the AR-XA turntable was introduced in 1961..

I do have some experience dealing with spring suspended turntables, having owned a few Thorens models including the TD150, TD160, TD126, etc. And those will respond well to being parked on a wall rack. The idea being to decouple the turntable from the floor so that the seismic vibration of a footfall on a light flooring system does not reach it. Or there is always the idea of putting your turntable up on a true lab quality isolation shelf such as a Minus-K, or Vibraplane or Herzan. Any of these will decouple the turntable from the floor....for a price. But the wall mounted shelf would be the least expensive way.

And this would, no doubt, be of benefit even to non suspended turntable designs.

Still, based on the opening post I’m leaning toward this being a problem related to acoustic feedback which can be alleviated by moving either the turntable or the subwoofer.
-Steve

https://www.theanalogdept.com/what’s_under_your_table.htm