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 combiner (26, 46B) adapted to combine a source audio signal containing the source audio and the anti-noise signal to provide an output signal for reproduction by the transducer (SPKR); a reference microphone input adapted to receive a reference microphone signal indicative of the ambient audio sounds; an error microphone input adapted to receive an error microphone signal indicative of the acoustic output of the transducer (SPKR) and the ambient audio sounds at the transducer (SPKR); a controllable noise source adapted to provide a noise signal; a source audio detector for determining whether source audio of sufficient amplitude is present in the source audio signal; and a processing circuit configured to generate the anti-noise signal from the reference signal to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error signal and the reference microphone signal, wherein the processing circuit implements a secondary path adaptive filter (34A, 55A-B) having a secondary path response that shapes the source audio and a second combiner (36, 46C) that removes the shaped source audio signal from the error microphone signal to provide the error signal, and wherein the processing circuit, in response to the source audio detector determining that source audio of sufficient amplitude is not present in the source audio signal, is configured to selectively inject noise from the noise generator into the secondary path adaptive filter (34A, 55A-B), and to further inject the noise into the first combiner (26, 46B) in place of or in combination with the source audio to cause the secondary path adaptive filter (34A, 55A-B) to continue to adapt when the source audio is absent or has reduced amplitude, and wherein the processing circuit is further configured to control the controllable noise source in conformity with an output of the secondary path adaptive filter (34A, 55A-B), characterised by the source audio detector having an input coupled to the source audio signal. 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 combiner (26, 46B) adapted to combine a source audio signal containing the source audio and the anti-noise signal to provide an output signal for reproduction by the transducer (SPKR); a reference microphone input adapted to receive a reference microphone signal indicative of the ambient audio sounds; an error microphone input adapted to receive an error microphone signal indicative of the acoustic output of the transducer (SPKR) and the ambient audio sounds at the transducer (SPKR); a controllable noise source adapted to provide a noise signal; a source audio detector for determining whether source audio of sufficient amplitude is present in the source audio signal; and a processing circuit configured to generate the anti-noise signal from the reference signal to reduce the presence of the ambient audio sounds heard by the listener in conformity with an error signal and the reference microphone signal, wherein the processing circuit implements a secondary path adaptive filter (34A, 55A-B) having a secondary path response that shapes the source audio and a second combiner (36, 46C) that removes the shaped source audio signal from the error microphone signal to provide the error signal, and wherein the processing circuit, in response to the source audio detector determining that source audio of sufficient amplitude is not present in the source audio signal, is configured to selectively inject noise from the noise generator into the secondary path adaptive filter (34A, 55A-B), and to further inject the noise into the first combiner (26, 46B) in place of or in combination with the source audio to cause the secondary path adaptive filter (34A, 55A-B) to continue to adapt when the source audio is absent or has reduced amplitude, and wherein the processing circuit is further configured to control the controllable noise source in conformity with an output of the secondary path adaptive filter (34A, 55A-B), characterised by the source audio detector having an input coupled to the source audio signal.