- blocking the transmission of vibration along a noise radiating structure by the
placement of a heavy mass on the structure close to the original source of the noise;
- isolating the vibration source from the noise radiating structure by physically
separating them (see Figure 10.10) or by using one or more of isolating elements (see
Figure 10.11) - see for example Figures 10.12 and 10.13 - and taking into account the
following factors ( Bies and Hansen, 1996)
the resonance frequency, f0 (Hz), associated with the stiffness of the isolating
spring (k Newtons/metre) and the mass which it is supporting (m kg) and given by
f , should be well below (less than half) the lowest frequency 0 [1 / (2)] k/m Hz
which is to be isolated (see Figure 10.14). The resonance frequency may also be
calculated by knowing how much the isolating spring compresses (d cm) under the
weight of the machine (static deflection); That is, f . 0 4.98/ d Hz
the excitation frequency, f (Hz), for a rotating machine mounted on an isolator is
generally equal to the rotational speed, expressed as revolutions per second.
the transmissibility, T, of an isolator is given by