A method comprising: receiving an input signal captured by one or more first sensors, the input signal representative of an undesired acoustic noise in a region; processing the input signal, using one or more processing devices, to generate a cancellation signal; generating, based on the cancellation signal, an output signal for one or more acoustic transducers, the output signal configured to cause the acoustic transducers to cancel, at least in part, the undesired acoustic noise in the region; receiving a feedback signal captured by one or more second sensors in vicinity of the region, the feedback signal at least in part representative of residual acoustic noise in the region; determining a characteristic of the feedback signal; determining a characteristic of an estimate of the cancellation signal at said region, by combining an estimate of a transfer function from the one or more acoustic transducers to the one or more second sensors with the cancellation signal; determining a characteristic of a combination of the cancellation signal estimate and the feedback signal; and comparing one or more thresholds to a first ratio of (i) the characteristic of the combination of the cancellation signal estimate and the feedback signal and (ii) a combination of the characteristic of the feedback signal and the characteristic of the cancellation signal estimate, the comparison determining a convergence state; characterised in that: each one of the characteristic of the combination of the cancellation signal estimate and the feedback signal, the characteristic of the feedback signal, and the characteristic of the cancellation signal estimate is a power spectral density. receiving an input signal captured by one or more first sensors, the input signal representative of an undesired acoustic noise in a region; processing the input signal, using one or more processing devices, to generate a cancellation signal; generating, based on the cancellation signal, an output signal for one or more acoustic transducers, the output signal configured to cause the acoustic transducers to cancel, at least in part, the undesired acoustic noise in the region; receiving a feedback signal captured by one or more second sensors in vicinity of the region, the feedback signal at least in part representative of residual acoustic noise in the region; determining a characteristic of the feedback signal; determining a characteristic of an estimate of the cancellation signal at said region, by combining an estimate of a transfer function from the one or more acoustic transducers to the one or more second sensors with the cancellation signal; determining a characteristic of a combination of the cancellation signal estimate and the feedback signal; and comparing one or more thresholds to a first ratio of (i) the characteristic of the combination of the cancellation signal estimate and the feedback signal and (ii) a combination of the characteristic of the feedback signal and the characteristic of the cancellation signal estimate, the comparison determining a convergence state; characterised in that: each one of the characteristic of the combination of the cancellation signal estimate and the feedback signal, the characteristic of the feedback signal, and the characteristic of the cancellation signal estimate is a power spectral density.