Dear Lewm, while tonearm alignment in general is the most critical mechanic-related topic in the whole audio periphery, it is even more critical in the "outboard arm pod".
We like to think that if we do not "touch" the arm pod nor the tt by accident, the distance between the pivot and the spindle will stay constant if the arm pod has any decent mass.
Unfortunately this is not the case.
The "eigen"/resonance frequency of the two different masses in relation to each other will inevitably cause a movement between the tt and the "free" arm pod.
Or mechanical world isn't as quite and stable as it seems to the mere eye.
It is a bit like the nice example with old church windows.
Glass is a liquid not a solid. It just appears to be solid, because it is moving so slowly, that we can't see it.
The movements of arm pod (separate and not fixed to the solid ground) in relation to the turntable is ever so subtle ( at least in our awareness ..), but there is almost constant movement in microscopic dimensions.
Now the polished area of the stylus itself, as well as the most subtle modulations in the groove are already in the scale of 1/1000 of a millimeter.
You stepping on the ground while walking, a car passing by your house, the slamming of a door, a truck passing 3 blocks away from you - all this and much more is causing movement of your "free" arm pod relative to the turntable and as such a movement of the pivot in relation to the spindle.
Nothing apparent to ou eyes at first glance of course.
In addition, there is the chain of mechanical "detonations" by the tracking process of your stylus in the groove. It puts energy into tonearm, arm pod and turntable - all causing movement.
The energy of the tracking process is causing resonance and stimulating movement.
All this is subtle and you most likely won't notice anything.
With an electron microscope you would even see and could measure the movement. Not linear nor in a particular direction.
A "free" arm pod is what it is: - free ..... to move.
Forced by nature.
Cheers,
D.
