THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

606 20. Vibration and Vibration ControlFrom the Equation set (20.44) it can be deduced that when the natural frequencyof the vibrating absorber is tuned to the vibrational forcing frequency, the motionof the principal mass m 1 is ideally zero and the spring force of the absorber isat all times equal and opposite to the applied force F 0 . It follows that no netforce is transmitted to the supporting structure. Because the natural frequencymust be tuned to the vibration forcing frequency, the absorber is customarily usedfor constant speed machinery. Even though this isolation feature is useful over abroad band of frequencies, absorption is useful principally for very narrow-bandor single-frequency control.Active SystemsActive isolation and absorption systems normally incorporate a feedback control,which may be electromagnetic, electronic, fluidic, pneumatic, mechanical, or acombination thereof. Because the cost of such systems is quite high, they are usedmainly in precision instruments such as electronic microscopes, lasers, stabilizedplatforms, and other devices where a high degree of isolation is required. Activecontrol is also applied where a high degree of isolation is required, for example, inthe case of an air-conditioning system that is mounted on the top floor of a buildingabove work areas and offices. Because the cost of computerization is falling continuously,active control is becoming a more viable option in many applications(including automotive suspensions), particularly for isolating very low-frequencyvibration (