An integrated circuit for implementing at least a portion of a personal audio device, comprising: an output adapted to provide a signal to a transducer (SPKR) including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer (SPKR); a first microphone input adapted to receive a first microphone signal indicative of the ambient audio sounds from a first microphone; at least one second microphone input adapted to receive a second microphone signal indicative of the ambient audio sounds from at least one second microphone; and a processing circuit that implements an adaptive filter having a response that shapes the anti-noise signal, wherein the processing circuit implements a first signal level detector configured to detect a first amplitude of the first microphone signal to generate a first microphone level signal and at least one second signal level detector configured to detect at least one second amplitude of the at least one second microphone signal to generate at least one second microphone level signal, characterised in that: the processing circuit is further configured to compare (70, 74) the first microphone level signal and the at least one second microphone level signal, and wherein the processing circuit, in response to determining that differences between the first microphone level signal and the at least one second microphone level signal indicate that the first microphone is at least partially obstructed, is configured to take action (72, 76) to prevent the anti-noise signal from being generated erroneously. an output adapted to provide a signal to a transducer (SPKR) including both source audio for playback to a listener and an anti-noise signal for countering the effects of ambient audio sounds in an acoustic output of the transducer (SPKR); a first microphone input adapted to receive a first microphone signal indicative of the ambient audio sounds from a first microphone; at least one second microphone input adapted to receive a second microphone signal indicative of the ambient audio sounds from at least one second microphone; and a processing circuit that implements an adaptive filter having a response that shapes the anti-noise signal, wherein the processing circuit implements a first signal level detector configured to detect a first amplitude of the first microphone signal to generate a first microphone level signal and at least one second signal level detector configured to detect at least one second amplitude of the at least one second microphone signal to generate at least one second microphone level signal, characterised in that: the processing circuit is further configured to compare (70, 74) the first microphone level signal and the at least one second microphone level signal, and wherein the processing circuit, in response to determining that differences between the first microphone level signal and the at least one second microphone level signal indicate that the first microphone is at least partially obstructed, is configured to take action (72, 76) to prevent the anti-noise signal from being generated erroneously.