where ς ς = c 2 k m is called the damping ratio of the system, ω ω n = k m is the natural angular frequency of the undamped system (when c =0) and ω ω d = ω ω n 1 − − ς ς 2 is the angular frequency when damping effect is taken into account (when c ≠ ≠ 0 ). If the impulse happens at t = τ instead of t =0, i.e. p ( t ) = δ δ ( t − − τ τ ) , the impulse response is