4-77
Limitations
•  Poor at low frequencies
•
•  Adds size, weight and cost
•
•  Ineffective for acoustic
    noise
Region of
Amplification
1
0.2
1
Forcing Freq./Resonant Freq.
Text Box: Transmissibility
Transmissibility
Region of
Isolation
Vibration Isolation
   A simple vibration isolation system is itself a resonant structure.  It can be effective at high frequencies (along one direction), but it amplifies the vibration at, and below its resonant frequency.  Moreover, the isolation system’s dimensions must accommodate large displacements at low frequencies and high accelerations.
   For sinusoidal vibration, the vibration displacement d = dosin 2ft, and the acceleration     a = -do(2f)2 sin 2ft, where do is the peak displacement and f is the vibration frequency.  Therefore, do = 0.50 G/f2 meters, peak-to-peak, where G is the acceleration in units of g. For example, the peak-to-peak displacement at 1 Hz and 1 g is 0.5 meters.
   Acoustic noise can be especially troublesome in certain applications.  For example, when an extremely low noise oscillator was required in an aircraft radar application, after the system designers built a three-level vibration isolation system to isolate the oscillator from the vibration of the aircraft, they discovered that the isolation system failed to deliver the expected phase noise of the oscillator because the isolation system failed to deal with the acoustic noise in the aircraft; i.e., the isolation system was effective in isolating the oscillator from the vibrations of the airframe, but it was ineffective in blocking the intense sound waves that impinged on the oscillator.


J. R. Vig, C. Audoin, L. S. Cutler, M. M. Driscoll, E. P. EerNisse, R. L. Filler, R. M. Garvey, W. L. Riley, R. C. Smythe, and R. D. Weglein, "Acceleration, Vibration and Shock Effects - IEEE Standards Project P1193," Proc. 1992 IEEE Frequency Control Symposium, 763-781, 1992; also, The Effects of Acceleration on Precision Frequency Sources, U. S. Army Laboratory Command Research and Development Technical Report SLCET-TR-91-3, March 1991, AD-A235470.