The amount of vibration isolation depends on the deflection provided by the vibration Isolator. If the supporting surface is uneven the bases of different vibration isolators will be at different levels which means that their deflection will vary. This compromising effect is most significant for vibration isolators that provide limited deflection such as layer systems and rubber (particularly ‘compression’) isolators.
For example a variation of only 4mm in the supporting surface can increase the natural frequency of a 6mm deflection system by 73%, but this is reduced to 12% for a 20mm deflection system. The corresponding effect on the vibration isolator’s performance depends on the expected level of vibration isolation (the forcing frequency : natural frequency ratio). For a 6mm deflection system 73% increase in natural frequency can turn 75% isolation to slight amplification, whereas for the 20mm deflection system the 12% increase would still result in 65% isolation.