How to isolate turntable from footstep shake or vibration

ghdprentice509 posts07-23-2021 3:18pmI had an AR which I could rap in the the top with my knuckle and it would not skip or have the sound transmitted to the speakers. It did what it was designed to do. When I walked by it would skip every time. The solution was simple, a wall rack… that is the solution for suspended floors.

mijostyn4,747 posts07-23-2021 3:34pmuser510, I have intimate experience with both the AR XA and I have owned 2 LP 12s. They are both potentially unstable and do not handle footfalls well just as ghdprentice indicates. A Sota you can rap on the surface and dance the jig on springy floor and it will not care. It is all about the self centering stability of the suspension that count.

good feedback and I don't mind being contradicted with factual evidence.  There definitely are other suspended subchassis designs which isolate well...and I think mjostyn's suggestion that the "hungie" type of suspension proves to be more stable than is the "sprungie".  Where the hungie tends to find center while the sprungie tends to never quite find it.  Although you can get close in those adjustment sessions but those conical coil springs always point their load in one direction, which needs to be steered...and the drive belt imposes a horizontal pull which will yank the lighter subchassis in a direction we don't want it to go.
Ime with a Td126-III i can say that this design suffers far less from footfall. Even though it is a "sprungie" and needs to be adjusted in order to obtain a vertical oscillation action, it does resist the pull of the belt far better with its larger mass.  And the cast chassis does indeed not flex. 

Btw, I can prove chassis flex on the TD150/TD160.  Having checked this using a dial indicator,to see that just the addition of the platter mass alone will cause the subchassis to flex.  Now if you put this into a dynamic situation where footfall ripples the suspended floor, which causes the floor supported audio rack to teeter just a little bit and that is enough to put the turntable in motion which excites the suspension components within and then we see even greater chassis flex due to the dynamics of the compression/extension action.  And that alters the distance between platter bearing center and pivot center of the tonearm, which skips the needle out of the groove.
https://www.theanalogdept.com/subchassis_rigidity.htm

-StevePs: have we solved the issues described by the OP yet?

What user510 said. In addition, altering the distance between the platter bearing center and the tonearm pivot must necessarily create a speed anomaly, as the belt must stretch or contract to accommodate changes in the distance between the motor pulley and the platter, where the motor is not mounted on the suspension.

I am sure there are turntables less susceptible to footfalls. But, the simplest solution is to remove the tt from the trampoline. 

Quote/ esputnix OP16 posts06-15-2021 2:42pmThank you for all your suggestions! I’ve double checked the suspension. And it appeared that the right tonearm side needed a stiffer spring as the aluminum housing was so low that it touched the base. It was simply sitting on the turnable instead of "floating in air". I have replaced the spring with the stiffer one and viola! The effect was instant. The footsteps rattle is gone. Even with the maximum subwoofer volume it is dead silent. It doesn’t even respond to me jumping in a room or shutting the door. It feels like I’ve got a free upgrade as it appears it sounds better!" /unquote

Above quote from page one. Aha...the solution was to correctly adjust the suspension of the TT by replacing the spring nearest the tonearm with a stronger one. Perhaps at one point the tonearm had been replaced with a heavier one...(speculation) Those Oracles apparently require a different technique to their adjustment than do the earlier designs based on the original AR.
And now he doesn’t have any footfall issues. Probably his floor isn’t as -flexy- as some are known to be...or perhaps the Oracle, when correctly adjusted, is less prone toward footfall.. I don’t know..... Still we don’t know exactly which design the Oracle in question is. A Delphi or Alexandria. Both seem to use a very similar subchassis.

agree with ghdprentice , the proper solution is to isolate the turntable from the "trampoline" via wall shelf....That is when you live on a trampoline floor, I know I have. Others on this list have very solid flooring entirely suitable for situating a turntable over...yet...among those that do.... many will insert an isolation device between said turntable and the floor mounted rack it sits on in order to improve overall sonic performance of the TT.

I think the idea is to isolate the turntable from its environment as much as is possible.-Steve

Mounting a turntable on the most solid rack or wall shelf might cure a footfall problem but it will not isolate a turntable. Only the mechanical filtering of a proper suspension will do that along with a proper dust cover or keeping the turntable in another room from the speakers.