Advantages of active over passive systems:
small size, inherently stiff, rapid recovery from shock (1-10 ms), no air required, no set up time,
and a performance better than the best passive system.
Passive damping systems work well at relatively high frequencies (>50Hz) but fail at low frequencies
where building vibrations occur. Passive systems have their resonance frequency there and amplify the
vibrations instead of damping it in that region.
Active vibration isolation systems employ inertial velocity in a feedback loop. A signal proportional
to inertial velocity is very similar to a viscous damping force, but with the very important difference that
the damping is relative to an inertial plane. So in contrast to the passive system, by increasing the damping
as in curve [C] and [D], the system becomes 'glued' to the inertial plane which by definition means the
system is isolated from the environment.
[more details in chapter "ACTIVE vs. PASSIVE"].