Frequency with which a system left to itself oscillates after an external excitation (damped natural oscillation). For a 2nd order oscillatory delay element (e.g. spring-mass system, hydraulically fixed actuator) the following relationships are obtained:
![2b1c9f204ad0f1252b96ee169ddb078920726774 eigenfrequenz_f___e_01.gif](/fileadmin/smc/files/2b1c9f204ad0f1252b96ee169ddb078920726774.gif)
![960a882f60d8b2fbbb6ab0bb692afe5d47f15836 eigenfrequenz_f___e_02.gif](/fileadmin/smc/files/960a882f60d8b2fbbb6ab0bb692afe5d47f15836.gif)
with f o = characteristic frequency.
D = damping
On weakly damped systems the natural frequency and the resonant frequency are approximately equal to the characteristic frequency f o.