Detuning

• In the last section, for balancing rotors, we have noticed that the operating speed closer to the critical speed of the rotor would create resonance and consequently large amplitude of vibration of the system. Thus, it is always desirable to keep the natural frequency of the system away from the excitation frequency.
  
  
• The technique of change of system parameters like mass and stiffness to avoid resonance is known as Detunting.
  

• Example - let us look at the new design of the Tacoma narrows suspension bridge.

Referring to above figures showing, respectively, the original and redesigned decks, we notice that, in the modified design, the side trusses were lowered and tied at the bottom end by a horizontal truss. The original open section was replaced by a closed-box section. The new design is torsionally much stiffer than the original design, and as a result, the fundamental torsional natural frequency exceeds the excitation frequencies generated even by a very high wind speed. In this way, the bridge was detuned from the excitation frequencies.