G. G. Parfitt 81 
40 
30 
20 
-20 
-40 
TRANSMISSIBILITY , T (db) 
-60 
-70 
SIMPLE MOUNTING 
COMPOUND MOUNTINC 
-90 
“100 
! 5 10 50 100 500 1000 
FREQUENCY (CPS) 
Fig. 4.12, Transmissibility of compound mounts using rubber springs [10]. 
(with the appropriate optimum damping value) can lead to a suppression of the 
main resonance comparable with that achieved by heavy damping of the main 
spring mount, but without the loss of isolation at high frequencies which the 
latter causes. Snowdon has also shown that a further improvement in per- 
formance can be obtained by replacing the viscously damped absorber spring 
by a viscoelastic material having a high and constant damping factor and a 
modulus which increases quite rapidly with frequency. Such a material is quite 
practical to realize. 
At frequencies well above resonance, the absorber mass remains almost 
stationary and the performance of the system becomes virtually independent 
of its presence. If the three cases considered in Fig. 4.14 has been compared 
on the basis of a common stiffness in the main spring instead of a common 
resonance frequency of the complete coupled system, then the system trans- 
