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 that implements 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 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 in the error microphone signal (err), wherein a signal derived from the error microphone signal (err) is filtered to weight one or more particular frequency regions within the response of the adaptive filter to weight a frequency content of the signal derived from the error microphone signal by increasing or decreasing a gain applied to the signal in the one or more particular frequency regions relative to gain applied to other frequency regions within the response of the adaptive filter, and wherein the signal is provided to a coefficient control of the adaptive filter to shape the amplitude response of the adaptive filter to, respective to and in conformity with the increasing or decreasing of the gain applied to the signal in the one or more particular frequency regions, increase or decrease the degree to which the anti-noise signal cancels the ambient audio sounds in the one or more particular frequency regions relative to the degree to which the anti-noise signal cancels the ambient audio sounds in the other frequency regions; characterised in that said filtering of said signal derived from the error microphone signal (err) is configured to compensate for multipath nulls in a frequency response of an external acoustic path between the reference microphone and the error microphone. 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 that implements 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 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 in the error microphone signal (err), wherein a signal derived from the error microphone signal (err) is filtered to weight one or more particular frequency regions within the response of the adaptive filter to weight a frequency content of the signal derived from the error microphone signal by increasing or decreasing a gain applied to the signal in the one or more particular frequency regions relative to gain applied to other frequency regions within the response of the adaptive filter, and wherein the signal is provided to a coefficient control of the adaptive filter to shape the amplitude response of the adaptive filter to, respective to and in conformity with the increasing or decreasing of the gain applied to the signal in the one or more particular frequency regions, increase or decrease the degree to which the anti-noise signal cancels the ambient audio sounds in the one or more particular frequency regions relative to the degree to which the anti-noise signal cancels the ambient audio sounds in the other frequency regions; characterised in that said filtering of said signal derived from the error microphone signal (err) is configured to compensate for multipath nulls in a frequency response of an external acoustic path between the reference microphone and the error microphone.