A conventional vibration absorber is valid when the external excitation frequency falls into the neighborhood of a specific value. It is not feasible for vibration suppression applications subject to frequency-varying excitations. In this paper, an adaptive controller is proposed to learn the disturbance spectrum in real time so that output vibration suppression can be achieved regardless of the variations in the excitation frequency. The Lyapunov-like analysis is used to verify the closed-loop stability and ensure boundedness of internal signals. Simulation cases show that the proposed strategy can give satisfactory performance.