I’m no mechanical engineer, but I do know that a spring is an isolator starting about an octave above it’s "effective" resonant frequency. That frequency is determined by the spring rate and the mass placed upon the spring, such as a pre-amp. Max Townshend explains the theory behind his Seismic Isolators, and demonstrates their effectiveness in a video viewable on You Tube. I would include a link to it if only I knew how---I’m somewhat of a luddite!
Below it’s resonant frequency a spring is a coupler, as are all objects (right, mechanical engineers?). That’s why getting as low a resonant frequency as possible is desirable---that provides the most isolation. Isolators made of rubber materials, as well as cones and spikes, have a much higher resonant frequency than does a well-designed spring system---Max has a chart on his Townshend Audio website showing the resonant frequency of rubber isolators, cones and spikes, and his Seismic products.
All this assumes one actually desires isolation; some designers, and audiophiles themselves, are instead interested in "draining" vibrations out of their components---the noise from turntable motors, amplifier transformers, spinning CD transports, etc. When the Mod Squad introduced the Tip Toe (the original cone isolator, as far as I know), they claimed it acts as a mechanic diode---a one-way "path" that evacuates vibrations entering it from above, and preventing vibrations entering and traveling up it from below. That claim is disputed by some mechanical engineers, who say that cones and spikes are in fact NOT mechanical diodes, NOT a one way street for vibration. That a cone or spike allows vibrations to enter it as readily from below as from above, providing isolation only above a fairly high frequency (10Hz or so resonance, 20Hz or so effective), acting as a coupler below, with vibrations traveling in both directions into and through it. As I said, I’m no mechanical engineer.
