70 Lecture 4 
+40 
{e) 
RESPONSE RATIO (db) 
-40 
-80 
0.1 | 10 100 1000 
Fig. 4.2, Response ratio of a mount on a masslike foundation (schematic). 
of the applied force, i.e., some isolation is already present. This effect vanishes 
as the foundation becomes increasingly rigid. 
In practice, foundations would have both spring- and masslike properties, 
with one or many internal resonances. Thus a foundation with one resonance 
gives a response-ratio curve (Fig. 4.4) which is essentially a combination of 
those of Figs. 4.2 and 4.3, but it has a new peak corresponding to the foundation 
resonance. Above this the curve shows a shift to the right compared with that 
for a rigid foundation, as in Fig. 4.2, and the loss in isolation is again (M + M)/M;. 
With a multiresonant foundation such as a metal beam, the response ratio is 
as in Fig. 4.5, showing now a progressive shift to the right. 
A word of warning about the interpretation of such curves may be in order. 
In general, apart from the basic mounting resonance, they will show pairs of 
adjacent troughs and peaks, representing the positions of the foundation reso- 
nance before and after inserting a mount. Thus the existence of such a peak, 
i.e., a region of poor isolation, is not necessarily an indictment of the mount's 
Performance, but simply indicates that a shift of a resonant frequency has oc- 
curred which may or may not be deleterious depending on whether the peak 
moves closer to or further from any strong components in the excitation spec- 
