Bandlimiting anti-noise in personal audio devices having adaptive noise cancellation (ANC)

A method of canceling ambient audio sounds in proximity of a transducer of a personal audio device, the method comprising:

• first measuring ambient audio sounds with a reference microphone to produce a reference microphone signal;
• second measuring an output of the transducer and the ambient audio sounds at the transducer with an error microphone;
• adaptively generating an anti-noise signal from a result of the first measuring and the second measuring for countering effects of ambient audio sounds at an acoustic output of the transducer by adapting a response of an adaptive filter that filters an output of the reference microphone by adjusting coefficients of the adaptive filter that control the response of the adaptive filter in conformity with an output of the error microphone and the output of the reference microphone by adapting the response of the adaptive filter to minimize the ambient audio sounds at the error microphone;
• combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer;
• adjusting a response of the adaptive filter independent of the reference microphone signal and independent of the source audio signal by altering an input to the adjusting of the coefficients independent of the adaptively generating by combining injected noise with an input to a coefficient control block of the adaptive filter, in order to constrain the adaptive filter to alter the adapting of the adaptive filter to the ambient audio sounds by biasing the response of the adaptive filter 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;
• and
• providing a result of the combining to the transducer to generate the acoustic output.

An integrated circuit for implementing at least a portion of a personal audio device, comprising:

• an output for providing a signal to a transducer including both source audio for playback to a listener and an anti-noise signal for countering effects of ambient audio sounds in an acoustic output of the transducer;
• a reference microphone input for receiving a reference microphone signal indicative of the ambient audio sounds;
• an error microphone input for receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer;
• and
• a processing circuit that implements an adaptive filter having a response that generates the anti-noise signal from the reference microphone signal to reduce the presence of the ambient audio sounds heard by the listener, wherein the processing circuit shapes the response of the adaptive filter in conformity with the error microphone signal and the reference microphone signal by adapting the response of the adaptive filter to minimize the ambient audio sounds at the error microphone, wherein the response of the adaptive filter is further adjusted independent of the reference microphone signal and independent of the source audio by altering an input to a coefficient control block of the adaptive filter to constrain the adaptive filter to alter the adapting of the adaptive filter to the ambient audio sounds, and wherein the response of the adaptive filter is adjusted by combining injected noise with the input to the coefficient control block 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.