In particular, closed-loop ANR systems are prone to oscillation due to excessive phase shift of the feedback signal caused by the time delay from the distance between the microphone and the transducer and by the inherent delays of the microphone and transducer. The phase shift from the first cause is minimized by placing the microphone as close as possible to the transducer. The second cause of phase shift is handled by applying a low pass filter to adjust the phase and high frequency content of the feedback signal, which, of course, also limits the operational range of the ANR to low frequencies. A high pass filter prevents oscillation due to violent movements of the listener's head. The phase margin at the feedback cutoff frequency should be 30 degrees or more.
Digital filters are more versatile and more easily configured than analog filters.
Digital filters for noise cancellation are called adaptive filters and can correct for both phase and amplitude errors. Figure 8 is an example of a NCH using adaptive noise reduction. A reference microphone at the top of the headband receives a noise signal. The adaptive filter attempts to predict the noise inside the earcup by passing the signal through a transfer function that models the headphone system. The inverse of the predicted noise is added to the desired audio signal and then sent to the headphone transducer.