How to isolate turntable from footstep shake or vibration

@esputnix, 

If you check with Oracle you will see where they have added a damping system to their sprung suspension -  Parts & Upgrades | oracle (oracle-audio.com)  MkVI Micro Vibration Stabilizer System (MVSS) upgrade; its a significant modification.  

What is likely happening is that the sub-woofer and foot-fall frequencies  are exciting a table spring resonance.  At resonance, an undamped system - in this case your floor - can amplify the vibration/displacement many times - often well over 20 dB.  So, every foot step causes the floor to effectively bounce causing the stand to bounce with it and the undamped table springs really bounce.  As has been suggested:

-a rumble/subsonic filter 'may' help because it may isolate the frequency that the subwoofer is producing that is exciting the floor from foot fall; example  Line Level Crossovers (hlabs.com).  There are pro/cons to these, but they are cheap and there are other types available - just search.

-modifying the table for the Oracle MVSS 'may' help because it now adds damping to the Oracle suspension that it currently does not have.  If it does not fix the problem, it may still benefit the table once the subwoofer/foot-fall/floor issue is resolved - but this is a significant mod that requires machining the base.

-mounting the table on the wall 'will' work; it isolates the table from the floor.

-adding a large mass under the table 'may' work.  

-adding subwoofer isolation feet or pad 'may' work because it may isolate the subwoofer from the floor enough so that the resonance is reduced or eliminated (shifts/isolates the problem frequency enough); and these are not expensive.  Examples are  Amazon.com: SVS SoundPath Subwoofer Isolation System, 4-Pack: Home Audio & Theater;  Amazon.com: IsoAcoustics Iso-200Sub Subwoofer Isolation Stand (Single) (New Model): Electronics;  Amazon.com: Auralex Acoustics SubDude-II Subwoofer Acoustic Isolation Platform, 1.75" x 15" x 15", v2: Musical Instruments.

-as has been stated - adding secondary springs under the table, is not generally recommended.  The phase relationship of the two spring systems can lead to very unpredictable results with resonances at different frequencies that can make the situation worse.  

Good Luck,

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@cleeds , that is right, flat to 20 hz at 1 meter. There is no full range loudspeaker I know of capable of reproducing the bottom two octaves in your average room at realistic levels. You have to be able to have separate control of that part of the spectrum. Your average room requires at least two 15" or four 12" subwoofer drivers to do the job. Room control and a lot of power help. To get the sensations of a live performance you have to be able to boost the low end 5 to 10 dB  sloping up from 100 Hz  to 18 Hz. Smaller drivers have to take overly long excursions to do this and distortion levels are much higher. 
At any rate, adding a bad subwoofer system is worse than no subwoofer at all so I can understand why many people do not.

mijostyn

@cleeds , that is right, flat to 20 hz at 1 meter. There is no full range loudspeaker I know of capable of reproducing the bottom two octaves in your average room at realistic levels ...

You obviously never heard an Infinity IRS Beta system. Essentially flat in my room to below 20 hZ. No Band-Aid "rumble filter" or heroic digital DSP required.

@cleeds, not a tone arm resonance problem at all. It is set perfectly at 8 Hz (horizontal. vertical is a little higher 9-10 Hz.) The cartridge is reading irregularities in the records surface. No record is absolutely flat. Unfiltered the subwoofers will gladly try to reproduce this. They are not projecting any "sound" into the room. They are no where near large enough to produce those frequencies but, if there is an electrical signal at 8 Hz they will gladly flap around at 8 Hz adding distortion to the sound you want to hear. In my case each subwoofer has 1800 Class AB watts behind it that will amplify right down to DC. Filtering all this garbage below 18 Hz cleans up the signal very audibly. You would have absolutely no problem hearing it.