An integrated circuit for implementing at least a portion of a personal audio device, comprising: an output for providing 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 for receiving a reference microphone signal (ref) indicative of the ambient audio sounds; an error microphone input for receiving an error microphone signal (err) indicative of the acoustic output of the transducer (SPKR) and the ambient audio sounds at the transducer (SPKR); and a processing circuit for implementing 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 at the error microphone; the response of the adaptive filter is further adjusted independent of the adapting by combining injected noise with the reference microphone signal (ref) and the processing circuit is further configured to implement a copy of the adaptive filter to receive the injected noise so that the response of the copy of the adaptive filter is controlled by the adaptive filter adapting to cancel a combination of the ambient audio sounds and the injected noise; and the processing circuit is further configured to control the response of the adaptive filter with the coefficients adapted in the copy of the adaptive filter, whereby the injected noise is not present in the anti-noise signal; characterized in that the processing circuit is configured to add the output of the copy of the adaptive filter only once every N samples, wherein N is an integer number, or to recompute and store the output of the copy of the adaptive filter in an error buffer responsive to a change in the response of the adaptive filter or at periodic intervals, wherein the frequency of the periodic intervals is less than the sample rate of filter. an output for providing 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 for receiving a reference microphone signal (ref) indicative of the ambient audio sounds; an error microphone input for receiving an error microphone signal (err) indicative of the acoustic output of the transducer (SPKR) and the ambient audio sounds at the transducer (SPKR); and a processing circuit for implementing 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 at the error microphone; the response of the adaptive filter is further adjusted independent of the adapting by combining injected noise with the reference microphone signal (ref) and the processing circuit is further configured to implement a copy of the adaptive filter to receive the injected noise so that the response of the copy of the adaptive filter is controlled by the adaptive filter adapting to cancel a combination of the ambient audio sounds and the injected noise; and the processing circuit is further configured to control the response of the adaptive filter with the coefficients adapted in the copy of the adaptive filter, whereby the injected noise is not present in the anti-noise signal; 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 at the error microphone; the response of the adaptive filter is further adjusted independent of the adapting by combining injected noise with the reference microphone signal (ref) and the processing circuit is further configured to implement a copy of the adaptive filter to receive the injected noise so that the response of the copy of the adaptive filter is controlled by the adaptive filter adapting to cancel a combination of the ambient audio sounds and the injected noise; and the processing circuit is further configured to control the response of the adaptive filter with the coefficients adapted in the copy of the adaptive filter, whereby the injected noise is not present in the anti-noise signal; characterized in that the processing circuit is configured to add the output of the copy of the adaptive filter only once every N samples, wherein N is an integer number, or to recompute and store the output of the copy of the adaptive filter in an error buffer responsive to a change in the response of the adaptive filter or at periodic intervals, wherein the frequency of the periodic intervals is less than the sample rate of filter.