A computer-implemented method comprising: receiving a portion of a feedback signal of an active noise control, ANC, system , based on a feedback control and configured to output a signal of interest , the ANC system comprising an adaptive line enhancer (ALE) filter configured to detect a tonal signature in an enhanced output and to produce a measure of residual signal ; processing the portion of the feedback signal using the adaptive line enhancer (ALE) filter to detect the tonal signature; receiving the measure of residual signal from the adaptive line enhancer (ALE) filter; receiving a portion of the signal-of-interest to be output by an acoustic transducer ; computing a quantity indicative of a relative strength of the tonal signature in an output of the acoustic transducer of the ANC system using the detected tonal signature in the enhanced output; determining, by one or more processing devices, that the tonal signature represents an unstable condition based on said quantity satisfying a threshold condition; and responsive to determining that the tonal signature represents an unstable condition, generating one or more control signals configured to adjust one or more parameters of the ANC system, to mitigate the unstable condition; the computer-implemented method being characterized in that said quantity is computed as a ratio of (i) a first quantity indicative of the energy of the tonal signature, to (ii) a second quantity indicative of the energy of a combination of the residual signal from the adaptive line enhancer (ALE) filter and the portion of the signal-of-interest output by the acoustic transducer. receiving a portion of a feedback signal of an active noise control, ANC, system , based on a feedback control and configured to output a signal of interest , the ANC system comprising an adaptive line enhancer (ALE) filter configured to detect a tonal signature in an enhanced output and to produce a measure of residual signal ; processing the portion of the feedback signal using the adaptive line enhancer (ALE) filter to detect the tonal signature; receiving the measure of residual signal from the adaptive line enhancer (ALE) filter; receiving a portion of the signal-of-interest to be output by an acoustic transducer ; computing a quantity indicative of a relative strength of the tonal signature in an output of the acoustic transducer of the ANC system using the detected tonal signature in the enhanced output; determining, by one or more processing devices, that the tonal signature represents an unstable condition based on said quantity satisfying a threshold condition; and responsive to determining that the tonal signature represents an unstable condition, generating one or more control signals configured to adjust one or more parameters of the ANC system, to mitigate the unstable condition; the computer-implemented method being characterized in that said quantity is computed as a ratio of (i) a first quantity indicative of the energy of the tonal signature, to (ii) a second quantity indicative of the energy of a combination of the residual signal from the adaptive line enhancer (ALE) filter and the portion of the signal-of-interest output by the acoustic transducer. <EOS>