80 Lecture 4 
mount, with some additional advantage appearing at very high frequencies. For 
substantial masses and a light foundation, the secondary resonance peak for a 
compound mounting appears on the response-ratio curve at a position only a 
little above that of the foundation with a simple mount and loading mass. The 
response-ratio curve for high secondary masses is rather less smooth at high 
frequencies than that for the simple mount with added mass, but, as indicated 
above, this disadvantage may be more apparent than real. 
Figure 4.13 represents a comparison due to Snowdon [9] of various methods 
of mounting on a light foundation beam of low damping, and employing an added 
mass of 0.2M. The "hevea" mount referred to is of a representative vulcanized 
natural rubber, while the "parallel" mount uses this rubber in parallel witha 
section of a high-damping polymer as discussed insection 4.3 above. On balance 
it appears that the simplest and broadly the most effective technique in these 
circumstances is to use the simple mounting with a substantial mass at its base 
and moderate damping to minimize the foundation and mount resonances. 
A fourth method of employing extra mass to minimize vibration is to attach 
this mass via a damped spring to the primary vibrating mass. If the added mass 
and spring are tuned to some resonance frequency of the primary system, then 
they absorb energy strongly at this frequency and so help to suppress the reso- 
nance of the primary system. The device is normally termed a dynamic absorber. 
Its behavior has been considered by a number of investigators, and Snowdon [12] 
in particular has recently examined the values of tuning and damping which give 
optimum suppression when various types of idealized viscoelastic media are 
used for the absorber spring. Figure 4.14 shows atypical set of transmissibility 
curves which he derived for the system shown in Fig. 4.11b. The secondary 
mass M, is attached via a spring with viscous damping to the spring-mounted 
main mass ™,. Curves are plotted for various values of mass ratio p = My/(M,+ M2). 
Thus, for example, »='% corresponds to M,;=M>2. It is seen that, owing to the 
frequency selectivity of the device, the use of relatively large absorber masses 
(a) (b) 
Fig. 4.11. (a) The compound mount and (b) the dynamic absorber. 
