Best type of metal for turntable platform?

foot fall is when you walk up to the turntable and it starts skipping because the floor is bouncing under you.

A well isolated turntable will not be bothered by anything. This requires a suspension. If you really want to isolate an unsuspended turntable you get a MinusK platform. They are the best. But , if you get a properly suspended turntable you can put it on a foldable card table and it will not care. Examples are the SME turntables, Sota turntables, The Avid Actus,
The Kuzma Ref 2, Basis turntables, Dohmann turntables, Oracle turntables and probably a few I do not know about. You can put almost any turntable on a MinusK platform and get excellent isolation.

Don’t go crazy, but dekay has a reasonable idea. When actually leveling a floor, you have to know what your are doing, and do it fractions at a time, for a variety of reasons. All you want to do is reinforce, stabilize, and provide support at the joists along the area where your turntable is. Thus, if you use a jack, I would not raise the area more than a 1/4”, put a couple 2x4’s, or perhaps get some treated 4x4’s, stand them in place (on a 12x12 concrete block as a ‘footer’) and lower the joists back down upon the vertical 4x4 with the jack. Pretty easy actually, but depending how much crawl height you have, can be cramped, so scope everything out, precut the lumber, etc. There is no real need to make full height pilasters out of block for what you are doing and trying to achieve, that would be overkill.

The theory of more stuff.

Vibration isolation in audio is a subject surrounded in mystery half truths and any number of wild theories. As an engineering exercise, the explanation is quite straight foreword and may be explained by the “Theory of more stuff”.

Take a surface, be it the floor or a table, on which your hi fi component is placed and it is desired to reduce the vibration from the support to the equipment. The way this is done is to put “some stuff” between the equipment and the supporting surface. There are three possible outcomes.

1 The vibration in the equipment is more than the vibration in the support.
This is not possible as if it were; the energy crisis would be solved! More
out than what is put in. Free power forever! Unfortunately, this scenario
contradicts the first and second laws of thermodynamics, so is not
possible.

2 The vibration in the supported equipment will be the same as in the case of no stuff. The chances of this are one in a million because something has been changed… it may be the same, but that is extremely unlikely, therefore, the only possibility is,

3 The vibration will be attenuated, to a greater or lesser degree, and this is the case.

There are many products out there that do in fact attenuate vibration. Be it spikes on glass, wood and slate, aluminium spikes in cups, ball bearings in cups, solid plates separated by compliant sheets, lead, Bluetack, sand, marble, concrete, the list is endless. It is also known that multiple combinations of the above produce better results because there is more stuff. E.g. multiple platforms stacked really high.

The engineering approach is to get the best result in the simplest manner by optimizing the “stuff” and way back about two centuries ago the Victorian engineers came up with the solution…. the spring! The spring may be anything “springy”, from elastic, rubber, coiled steel, straight steel, air-bladders to flexible wooden strips. As long as it has sufficient spring or compliance, when optimised with an appropriate mass, a mechanical low pass filter is realised.

The ideal is to have the resonant frequency as low as is possible, ideally around 2Hz in both the horizontal and vertical planes and with a damping ratio of about 0.16. This will give an attenuation of about 25dB at 10 Hz increasing at 20dB per decade above. This will ensure excellent isolation for the deleterious audio system vibrations which are from 5Hz to 500Hz.

 

if you can brace the floor, your solution at the table will be greatly minimized.

+1 @bkeske 

Good to see you taking the advice. Install a concrete plinth directly beneath the turntable for maximum stability. I ended up in the concrete basement such was the improvement in sound quality over suspended floors, live walls, and all manner of vibrations.