The primary purpose of this study is to evaluate and better understand the
isolation properties of semiactive suspensions. Specifically, this study wi
ll answer the question regarding why semiactive dampers are able to isolate
at frequencies well below; those for passive dampers, even though they do
not add any energy to the system. A single suspension model is used to deri
ve and analytically evaluate the transmissibility properties of passive and
semiactive dampers. The results show that for semiactive dampers, the freq
uency range of isolation and the transmissibility amplitude are functions o
f xi, the damping ratio. In contrast, the isolation frequency range for pas
sive dampers is completely independent of xi. Furthermore, the results show
that for sufficiently large xi, semiactive dampers are able to provide iso
lation at all frequencies. This feature is useful for many applications, pa
rticularly for sensitive machinery that cannot tolerate any overshoot in po
wer-up or power-down, and yet must have good isolation during normal operat
ion. Passive suspensions offer only one of the two aspects, whereas semiact
ive suspensions offer both.