LECTURE 4 
THE IMPROVEMENT OF VIBRATION ISOLATION 
G.G. Parfitt 
Imperial College of Science and Technology 
London, England 
4.1. INTRODUCTION 
The noise and vibration generated by a ship's engines and machinery will 
to a greater or lesser degree be transmitted to its hull plating by air-borne and, 
Particularly, structure-borne paths and be radiated into the surrounding sea, 
thus helping to disclose the ship's presence to passive listening devices. Ef- 
fective isolation of machinery vibration from its supporting structure may thus 
be of especial importance in naval applications. The present paper examines 
the elementary principles of vibration isolation and discusses various potential 
means of improving on the simplest form of spring isolator. This discussion is 
based mainly on work done at Imperial College and the University of Michigan 
by Dr. J.C. Snowdon. 
The simplest idealized resilient isolation system may be depicted as in 
Fig. 4.1a by a simple spring of stiffness (force-to-displacement ratio) S sup- 
porting a machine, represented by a simple mass M, above an infinitely rigid 
foundation. The mass is supposed free to move only in the vertical direction, 
under the action of an applied force F. A force F, is transmitted to the foundation, 
and the ratio F,/F is termed the transmissibility T of the system, or in decibels 
T = 20log (1) 
Here, 1/T or -T in decibels is a measure of the isolation. The system has a 
natural resonance frequency w 9/27 given by 
@6 -3 (2) 
If the spring stiffness S$ is linear and frequency-independent, the downward 
deflection d of the spring under the weight Mg of the supported body is given by 
Sd= Ms, whence 
-§ (3) 
