Recalling Equation 4 from the Forced Vib Eqn Solution page, the phase shift is:
(1)
The plot at the right illustrates a phase shift for an underdamped system with a damping ratio of 0.7071. The maximum phase shift occurs when the forcing frequency equals the natural frequency.
A linear phase shift will occur when the system operates at frequencies less
than 40% of the resonant frequency (
< 0.40) for the case of an underdamped system.
