11.07.2015 Views

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

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

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

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598 20. Vibration and Vibration ControlFrom Equation (20.27)xF 0 /k = 12ξHence at resonance, the amplitude at resonance isx res =Static Deflection14.25 N/945 N/m0.276= 0.0546 m = 5.46 cmIn Figure 20.1 the deadweight W constitutes a static load that causes a deflectionδ st that is given byδ st = W k = mg(20.33)kwhere g represents the gravitational constant. Because the natural frequency of thesystem is found fromf n = 1√k(20.34)2π mwe can combine Equations (20.33) and (20.34) to obtain the relationship betweenstatic deflection and natural frequency as follows:Example Problem 5f n = 12π√ gδ st(20.35)A large machine weighs 875 N and the static deflection of the springs supportingthe machine is 0.83 cm. Find the undamped natural frequency.SolutionFrom Equation (20.35)√f n = 1 980.7 cm/s 2= 5.47 Hz2π 0.83 cm20.5 Vibration ControlTransmissibilityIn the case of a forcing function being harmonic in nature, two cases of vibrationtransmission can occur. One case occurs when force is transmitted to the supportingstructure, and the opposite case occurs when the motion of the supporting structure

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