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 reference microphone input adapted to receive a reference microphone signal (ref) indicative of the ambient audio sounds; an error microphone input adapted to receive an error microphone signal (err) indicative of the output of the transducer (SPKR); and a processing circuit that implements a first adaptive filter having a response that shapes the anti-noise signal to reduce the presence of the ambient audio sounds heard by the listener, wherein the first adaptive filter is adapted to filter the reference microphone signal (ref) to generate the anti-noise signal, wherein the processing circuit implements a secondary path adaptive filter having a secondary path response controlled by coefficients of a secondary path adaptive filter coefficient control, wherein the secondary path adaptive filter is adapted to shape the source audio, wherein the processing circuit implements a combiner that removes the shaped source audio from the error microphone signal (err) to provide an error signal indicative of the combined anti-noise signal and ambient audio sounds delivered to the listener, wherein the processing circuit is configured to adapt the response of the first adaptive filter to minimize the error signal, wherein the processing circuit is configured to determine a degree of coupling between the transducer (SPKR) and an ear of the 2. listener and wherein the processing circuit is configured to alter the adaptation of the response of the first adaptive filter in response to determining that the degree of coupling has changed; characterised in that the processing circuit is configured to: determine a value of an ear pressure indication computed from the coefficients of the secondary path adaptive filter coefficient control; compare said value to a predetermined threshold; and determine that the degree of coupling has changed in response to said value having crossed the predetermined threshold. 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 reference microphone input adapted to receive a reference microphone signal (ref) indicative of the ambient audio sounds; an error microphone input adapted to receive an error microphone signal (err) indicative of the output of the transducer (SPKR); and a processing circuit that implements a first adaptive filter having a response that shapes the anti-noise signal to reduce the presence of the ambient audio sounds heard by the listener, wherein the first adaptive filter is adapted to filter the reference microphone signal (ref) to generate the anti-noise signal, wherein the processing circuit implements a secondary path adaptive filter having a secondary path response controlled by coefficients of a secondary path adaptive filter coefficient control, wherein the secondary path adaptive filter is adapted to shape the source audio, wherein the processing circuit implements a combiner that removes the shaped source audio from the error microphone signal (err) to provide an error signal indicative of the combined anti-noise signal and ambient audio sounds delivered to the listener, wherein the processing circuit is configured to adapt the response of the first adaptive filter to minimize the error signal, wherein the processing circuit is configured to determine a degree of coupling between the transducer (SPKR) and an ear of the 2. listener and wherein the processing circuit is configured to alter the adaptation of the response of the first adaptive filter in response to determining that the degree of coupling has changed; determine a value of an ear pressure indication computed from the coefficients of the secondary path adaptive filter coefficient control; compare said value to a predetermined threshold; and determine that the degree of coupling has changed in response to said value having crossed the predetermined threshold.