Building Acoustics

Excitation and response 51

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

2mfi
2mf

.

vMMM

E

vMM

′ ++

==

+

(2.43)

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.

Structure 2

Structure 1

F

Structure 2 v ́^2

Structure 1

Isolator

F

v 1

v 2

a) b)

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.

10 20 50 100 200 500 1000

Frequency (Hz)

-40

-30

-20

-10

0

10

20

30

40

20

⋅lg

E

(dB)

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.