Excitation and response of dynamic systems - Building Acoustics - page 51

(machine), isolator and structure 2 (foundation) respectively, we may express the
efficiency E as

′

v MMM ++

2 m f i

E == .

(2.43)

v MM +

2 m f

To make the isolator system efficient it is not sufficient to make the isolator “soft”, i.e.
choose one with a high mobility. It must also be high compared with the sum of the
motilities of the attached structures.

F

a) b)

F

v 1

Structure 1

Isolator

Structure 1

v 2 v ´ 2

Structure 2 Structure 2

Figure 2.12 Sketches for the calculation of the efficiency of a vibration isolating system. a) Structures coupled
through isolator. b) Structures in direct contact.

40

30

20

10

0

-10

-20

-30

-40

10

20 50

100

200 500

1000

Figure 2.13 The vibration isolation given by the efficiency on a logarithmic scale. The system is shown in
Figure 2.10 , where the mass m 1 represents the “machine” and the components with suffix 2 represents the
“foundation”. Solid curve - component values as in Figure 2.11. Dashed curve - m 2 = m 1 = 1.0 kg.

Frequency (Hz)