Though Vibration Isolation is directly related to the deflection (‘squash’, if you will) of the isolating medium or product, any deflection of the supporting or supported structures that occurs is either directly subtractable from the isolator deflection or significantly affects the natural frequency of the isolation medium or product.
Think about a guitar. Pluck a string and it gives off a specific envelope of frequencies or ‘tone’. Tighten the string and the frequency goes up. Loosen the string and the frequency goes down. If the bridge on that guitar is not rigid or the face of the guitar where it rests is ‘soft’, trying to generate a specific tone would be VERY challenging.
In most cases, the structure to which equipment is mounted is concrete, wood or steel. All of which will deflect, somewhat, under the gravitational load of the equipment, but it is nominal and can be calculated. On the other hand, many pieces of equipment come with a common support base, directly from the factory, that was meant to sit on a continuous structure, not to be point-loaded on vibration isolation mounts or pads. Hence, providing a secondary base of appropriate stiffness is critical, not only to ensure that the isolation media or products, appropriately isolate the equipment vibration getting to the structure, but also, to maintain equipment component alignments.
In some cases, equipment, by their very nature, may have imbalances, which cause higher amplitudes of vibration or vibrational energy is occurring above the center of gravity, creating significant eccentric vibration due to the overall geometry of the equipment. Invariably, probability of these issues arising is directly related to the size and energy requirements of the equipment. In order to address these situations it is critical to add mass, usually concrete, to the supporting base to create an ‘inertia base’ and appropriately increasing the load carrying capacity of the isolating pads or mounts. Inertia bases lower the center of gravity of the supported system and reduce the amplitude of vibrational energy to the vibration isolators … in both instances, significantly improving the efficiency of the vibration isolation system.