G. G. Parfitt 83 
protection against foundation resonances occurring well above the main resonance 
frequency of a spring-mounted equipment. However, the device might well be 
considered for use attached toa foundation beam at the base of the main mounting 
spring, or elsewhere. A detailed study of this case has not yet been made. 
4,5, SUMMARY 
A review has been given of the performance ofthe simple resilient anti- 
vibration mounting and of means of improving its performance, particularly 
when it is set upon a foundation which is nonrigid, by addition of damping or 
mass to the system. It appears that, by suitable choice of mount materials or 
arrangement, a fairly good compromise between low resonant peaks and good 
high-frequency isolation can be obtained. Damping of the foundation members 
may be useful but only at and near the frequencies of foundation resonance. 
The addition of substantial masses to the system as force barriers can be 
beneficial. Probably the simplest method to reduce the excitation of a foundation 
by this means is to attach the mass to the foundation at the base of the mount. 
The mass needs to be larger than that of the foundation to have a substantial 
effect. Although addition of large idle masses in a ship is not an appealing idea, 
it may occasionally be possible to use for the purpose the mass of other equip- 
ment which is already present. 
(db) 
BROKEN CURVES REFER TO THE 
SIMPLE MOUNTING SYSTEM. 
DAMPING OF THE VISCOUS TYPE 
TRANSMISSIBILITY 
mCON 02 05 | 2 5 iO 
FREQUENCY RATIO y 
Fig. 4.14. Transmissibility of the dynamic absorber with viscous damping [12]. 
