re suspended turntables...most of the inbuilt isolation system focuses on separating the turntable base from the suspended part of the plinth that carries the platter and tonearm...placing something like an inner tube underneath can upset the levelling of the turntable due to uneven weight distribution on a compliant platform...the suspension may not work as well as intended if the springs disconnecting the subchassis are not bouncing as vertically as possible...
furthermore...adding a 'springy' compliant base such as an inflated tube under a 'sprung' subchassis design may counteract the damping effect of the suspension and actually extend the time it takes to absorb and dampen external vibrations...
sorry for my non scientific explanation...its a bit like adding one spring to another...you create an energy feedback loop that takes longer to stop...so in effect you can end up adding vibration instead of removing it...
imagine bouncing a car suspension while it is on hard ground...the shock absorbers and suspension spring absorbs the shock in minimal time...place the car on a trampoline and bounce it again..and both car and trampoline takes longer to settle...
http://forum.ecoustics.com/bbs/messages/1/68488.html
Your non-scientific explanation is, with all due respect, but not too surprisingly, not what I would call accurate.
You don't need to add an inner tube underneath a board for sprung subchassis type turntables such as a Linn-Sondek, Systemdek, Michell Gyro or Gyrodec, Avid, etc. Their spring suspension systems perform the function of the inner tube in acting as a seismic filter. The inner tube setup is for rigid plinth type turntables such Rega, Music Hall, Pro-Ject, entry level Clearaudio, etc; in this context, it performs the same function as a spring-loaded TT, but not by acting as a spring. The key thing to understand here is the the inner tube and the springs on suspended TTs are two different ways of performing the same FUNCTION, which is ISOLATION. This is NOT the same as saying that the inner tube is working like a spring. It is actually working more like a damper, whose function is to damp oscillations. This is why the air pressure in the inner tube should be quite low, so that it does NOT act like a spring, but as a damper (like what we mistakenly refer to as a shock absorber).
The function of the inner tube is not to act as a spring, but as a filter for subsonic seismic resonances. The subsonic seismic resonances that this "air bearing" damps are low in frequency, 3-12 Hz, small in amplitude and therefore, difficult for us to sense physically. But they are there, and isolating not only the TT, but the preamp, amp, CD player, etc from them produces big gains in the quality of your sound system. You will also gain futher improvements in this direction if you place all your components on "roller blocks" of the type I describe, and show photos of, in early in this thread. In this application, the roller blocks act as filters for horizontal and rotational subsonic seismic resonances, and the air bearing acts as a filter for vertical resonances. These resonances acts as forces and your system, inducing vibration, which then induces noise into in your electrical circuits, increasing your noise floor, and decreasing S/N. Remember that forces are vector quantitites, and therefore, have magnitude AND direction. The function of these "filters" is to work to attenuate these resonances with respect to direction as well amplitude (the magnitude dimension of a force vector). This makes the functional response of your system more robust to the effects of noise factors in the manner espoused by Toguchi, et. al. in his pioneering work in Robust Design and Robust Design DOE.
