Vibration Isolation Theory

Definition of common terms used:

Amplitude A m The magnitude of the displacement from mean position of a deflection caused by vibration.
Interference frequency f Hz Typically the rotational frequency of a machine
Frequency f0 Hz The number of vibrations in a freely oscillating system per unit of time (/second for Hz) f0  = 1/2π√ (Kdyn/m)
Mass M Kg The mass of the oscillating system
Spring force F N The force exerted on or from a spring (or AV mounting)
Deflection d m The deformation of a spring (AV Mount) from neutral position
Static spring stiffness Kstat N/m The force in Newtons to compress the spring or mounting by 1m
Dynamic spring stiffness Kdyn N/m Spring stiffness when an alternating force is applied
Tuning ratio Z - The ratio between Interference frequency (f) and natural frequency (f0)
Interference force Fs N The force transmitted to the base of an isolated machine
Impulse force Fi N The force transmitted to the base of a rigidly mounted machine
Level of isolation I - That part of the impulse force which is eliminated by the vibration isolation
Damping Coefficient c Ns/m The linear viscous damping coefficient
Critical damping ccr Ns/m The linear viscous damping at critical damping. i.e. no over oscillation after displacement
Damping factor D - The ration between c and ccr
Reduction R dB Isolation expressed in decibels  R=20log(1/B)
Deflection δstat Mm The static deflection for a spring = F/ Kstat


An essential difference between rubber and a steel spring is that the rubber material has an inherently high damping capacity built in.  This is particularly important for vibration isolation and shock absorption.  When deformed and released, a wide hysteresis curve is generated showing a loss of energy as illustrated.   


The lost energy is converted to heat which, in normal applications is readily dissipated.

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