Settling time : The time , following a disturbance, that is required for the transient response to fade out. The time it takes for the response of an underdamped component to remain within a small band above and below the 100% change line. Damping ratio : A number that determines the type of damping of a second-order system. The response is underdamped if < 1 , critically damped if = 1 , and overdamped if > 1. Natural frequency (designated byωd ): The frequency of oscillation of an underdamped second-order system. Rise time : The time required for the output to go from a small percentage of change to a large percentage of change, after a step change in input. Unless otherwise specified, the change is from 10% to 90%.