Abstract: An active noise and vibration control system is constructed such that the residual signal from the residual sensor is fed back into the controller and used to generate the probe signal. Measurements of the residual signal are used to create a related signal, which has the same magnitude spectrum as the residual signal, but which is phase-uncorrelated with the residual signal. This latter signal is filtered by a shaping filter and attenuated to produce the desired probe signal. The characteristics of the shaping filter and the attenuator are chosen such that when the probe signal is filtered by the plant transfer function, its contribution to the magnitude spectrum of the residual signal is uniformly below the measured magnitude spectrum of the residual by a prescribed amount (for example, 6 dB) over the entire involved frequency range. The probe signal is then used to obtain a current estimate of the plant transfer function.