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 the ambient audio sounds at the transducer (SPKR); and a processing circuit configured to implement an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal (ref) to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit is further configured to shape the response of the adaptive filter in conformity with the error microphone signal (err) and the reference microphone signal (ref) by adapting the response of the adaptive filter to minimize the ambient audio sounds at an error microphone, characterised in that the response of the adaptive filter is further adjusted independent of the reference microphone signal (ref) by adding injected noise (n(z)) to an input to a coefficient control block of the adaptive filter so that the response of the adaptive filter is biased by the coefficient control block adapting to attenuate frequencies present in the injected noise, whereby the response of the adaptive filter is reduced in frequency regions in a frequency range of the injected noise to constrain the adaptive filter to alter the adapting of the adaptive filter to the ambient audio sounds. 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 the ambient audio sounds at the transducer (SPKR); and a processing circuit configured to implement an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal (ref) to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit is further configured to shape the response of the adaptive filter in conformity with the error microphone signal (err) and the reference microphone signal (ref) by adapting the response of the adaptive filter to minimize the ambient audio sounds at an error microphone, characterised in that the response of the adaptive filter is further adjusted independent of the reference microphone signal (ref) by adding injected noise (n(z)) to an input to a coefficient control block of the adaptive filter so that the response of the adaptive filter is biased by the coefficient control block adapting to attenuate frequencies present in the injected noise, whereby the response of the adaptive filter is reduced in frequency regions in a frequency range of the injected noise to constrain the adaptive filter to alter the adapting of the adaptive filter to the ambient audio sounds.