Vibration isolation may be achieved by means of resilient elements called mountings or isolators. The purpose of such an isolator is to reduce the transmitted dynamic forces and moments. Natural rubber based mountings through the years have proved to be extremely successful, realising a good compromise between stiffness, static load carrying capabilities, dynamic stiffening and internal damping. Application of isolators may however also have drawbacks such as undesired sound and vibrations causedby excitation of rigid body modes which can not always be avoided. A well-known example is the engine bounce phenomenon in cars which may compromise both comfort and drivability. In this case the excitation originates from undulations in the road surfaceor unbalance in the wheel-tire system.Hydraulic mountings seem to provide a solution to this problem and are widely applied in automotive industry. These mountings incorporate an hydraulic damper which is activated at large deformations only (by means of a so called "decoupler" device). Theyare claimed to efficiently damp the rigid body modes without any penalty in high frequency stiffness.
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