Geoffkait
Nice to know that four course on vibration I took in university can be boiled down to one easy sentence Geoff. It is far more complex in study than what you have stated. It has everything to do with inertia.
"The heavy mass used in Vibraplane is employed to lower the resonant freq. of the "mass on spring" system and has nothing whatsoever to do with inertia, as you suggested. System resonant frequency is a (strict) function of the Sq. Root of spring rate over the mass."
What you talk of is a simple machine, a text book example not a real world application. What you talk of is a single degree of freedom spring and mass system. It does not particularly apply. Yes, additonal mass will lower the resonant frequency of the platform but is not the sole or most important reason for the mass. You could simply damp, clamp and change the resonant frequency with out the mass. You have completely ignored Newton and Kinetics all in one foul swoop. How about your transmission ratios, harmonic ground motion, forced periodic motion etc.?
"Thus, if vibraplane employed an even heavier mass, all things being equal, it would be an even better isolator. But not because of the reason you gave."
Beg to differ here sir. A larger mass requires larger surface areas and will not necessarily translate to better isolation. It introduces another set of issues. It is a large reason as to why it operates the way it does. Mass limits movement the isolators decouple from the source of the vibration. It is similar in operation to an inertia base under large rotating pieces of equipment, conversly though they are trying to limit the vibration transmitted into a building. Mass is key in keeping things in place and limiting both movement and transmission of vibration.
