Reduction of the Response to Vibration of Structures Possessing Finite Mechanical Impedance

Expressions have been derived from which the isolation provided by a simple mounting system when supported by any nonrigid foundation may be determined, if the mechanical impedance of the foundation is known. As an example, the mechanical impedance of a damped beam at its midpoint has been employed to simulate the behavior of such a foundation. Knowledge of the dynamic mechanical properties of natural rubber and a high-damping synthetic rubber has enabled the behavior of antivibration mountings to be described realistically. If the ratio of the mass of the mounted item to the mass of the foundation is large, the isolation afforded by a mounting is found to be much less than that predicted by its transmissibility curve, which relates to an ideally rigid foundation. It is shown, however, that the isolation provided by a mounting can be increased significantly at high frequencies if an additional mass is employed to load the foundation, the greatest isolation then being provided by a low-damping rubber such as natural rubber. Damping of the foundation is found to have little influence upon the over all level of the isolation afforded by the mounting system.