# Patent

## Title
Systems and methods for adaptive noise cancellation by biasing anti-noise level

## Abstract

A processing circuit may comprise an adaptive filter having a response generating an anti-noise signal from a reference microphone signal, a secondary path estimate filter modeling an electro-acoustic path of a source audio signal, a biasing portion that generates a scaled anti-noise signal by applying a scaling factor and the response of the secondary path estimate filter to the anti-noise signal, and a coefficient control block that shapes the response of the adaptive filter in conformity with the reference microphone signal and a modified playback corrected error signal by adapting the response of the adaptive filter to minimize ambient audio sounds in the error microphone signal, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal and the modified playback corrected error signal is based on a difference between the playback corrected error signal and the scaled anti-noise signal.

## Background
RELATED APPLICATION
The present disclosure claims priority to U.S. Provisional Patent Application Ser. No. 61/812,842, filed Apr. 17, 2013, which is incorporated by reference herein in its entirety.


FIELD OF DISCLOSURE
The present disclosure relates in general to adaptive noise cancellation in connection with an acoustic transducer, and more particularly, to detection and cancellation of ambient noise present in the vicinity of the acoustic transducer, including biasing an anti-noise level for anti-noise generated by adaptive noise cancellation.
BACKGROUND
Wireless telephones, such as mobile/cellular telephones, cordless telephones, and other consumer audio devices, such as mp3 players, are in widespread use. Performance of such devices with respect to intelligibility can be improved by providing noise canceling using a microphone to measure ambient acoustic events and then using signal processing to insert an anti-noise signal into the output of the device to cancel the ambient acoustic events.
Because the acoustic environment around personal audio devices, such as wireless telephones, can change dramatically, depending on the sources of noise that are present and the position of the device itself, it is desirable to adapt the noise canceling to take into account such environmental changes. For example, many adaptive noise cancelling systems utilize an error microphone for sensing acoustic pressure proximate to an output of an electro-acoustic transducer (e.g., a loudspeaker) and generating an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer. When the transducer is close to a listener's ear, the error microphone signal may approximate the actual acoustic pressure at a listener's eardrum (a location known as a drum reference point). However, because of the distance between the drum reference point and the location of the error microphone (known as the error reference point) the error microphone signal is only an approximation and not a perfect indication of acoustic pressure at the drum reference point. Thus, because noise cancellation attempts to reduce ambient audio sounds present in the error microphone signal, performance of a noise cancellation system may be the greatest when the distance between the drum reference point and the error reference point is small. As the distance increases (e.g., transducer held against the ear at a lower pressure), the performance of the noise cancellation system may degrade, partly because the gain of the transfer function from the error reference point to the drum reference point decreases with such increased distance. This degradation is not accounted for in traditional adaptive noise cancellation systems.
SUMMARY
In accordance with the teachings of the present disclosure, the disadvantages and problems associated with existing approaches to adaptive noise cancellation may be reduced or eliminated.
In accordance with embodiments of the present disclosure, a personal audio device may include a personal audio device housing, a transducer, a reference microphone, an error microphone, and a processing circuit. The transducer may be mounted on the housing and may be configured to reproduce an audio signal including both a source audio signal 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. The reference microphone may be mounted on the housing and may be configured to provide a reference microphone signal indicative of the ambient audio sounds. The error microphone may be mounted on the housing in proximity to the transducer and may be configured to provide an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer. The processing circuit may comprise an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal, a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal having a response that generates a secondary path estimate from the source audio, a biasing portion that generates a scaled anti-noise signal by applying a scaling factor and the response of the secondary path estimate filter to the anti-noise signal, and a coefficient control block that shapes the response of the adaptive filter in conformity with the reference microphone signal and a modified playback corrected error signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the error microphone signal, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal and the modified playback corrected error signal is based on a difference between the playback corrected error signal and the scaled anti-noise signal.
In accordance with these and other embodiments of the present disclosure, a method for canceling ambient audio sounds in the proximity of a transducer of a personal audio device may include measuring ambient audio sounds with a reference microphone to produce a reference microphone signal. The method may also include measuring an output of the transducer and the ambient audio sounds at the transducer with an error microphone to produce an error microphone signal. The method may additionally include generating a source audio signal for playback to a listener. The method may also include adaptively generating an anti-noise signal, from a result of the measuring with the reference microphone, countering the effects of ambient audio sounds at an acoustic output of the transducer by adapting a response of an adaptive filter that filters the reference microphone signal in conformity with the reference microphone signal and a modified playback corrected error signal to minimize the ambient audio sounds in the error microphone. The method may also include generating a secondary path estimate from the source audio signal by filtering the source audio signal with a secondary path estimate filter modeling an electro-acoustic path of the source audio signal. The method may additionally include generating a scaled anti-noise signal by applying a scaling factor and the response of the secondary path estimate filter to the anti-noise signal. The method may further include combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer. The playback corrected error may be based on a difference between the error microphone signal and the source audio signal and the modified playback corrected error signal may be based on a difference between the playback corrected error signal and the scaled anti-noise signal.
In accordance with these and other embodiments of the present disclosure, an integrated circuit for implementing at least a portion of a personal audio device may include an output, a reference microphone input, an error microphone input, and a processing circuit. The output may be configured to provide a signal to a transducer including both a source audio signal for playback to a listener and an anti-noise signal for countering the effect of ambient audio sounds in an acoustic output of the transducer. The reference microphone input may be configured to receive a reference microphone signal indicative of the ambient audio sounds. The error microphone input may be configured to receive an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer. The processing circuit may comprise an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal, a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal having a response that generates a secondary path estimate from the source audio, a biasing portion that generates a scaled anti-noise signal by applying a scaling factor and the response of the secondary path estimate filter to the anti-noise signal, and a coefficient control block that shapes the response of the adaptive filter in conformity with the reference microphone signal and a modified playback corrected error signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the error microphone signal, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal and the modified playback corrected error signal is based on a difference between the playback corrected error signal and the scaled anti-noise signal.
Technical advantages of the present disclosure may be readily apparent to one skilled in the art from the figures, description and claims included herein. The objects and advantages of the embodiments will be realized and achieved at least by the elements, features, and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are examples and explanatory and are not restrictive of the claims set forth in this disclosure.



BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:
FIG. 1 is an illustration of an example wireless mobile telephone, in accordance with embodiments of the present disclosure;
FIG. 2 is a block diagram of selected circuits within the wireless telephone depicted in FIG. 1, in accordance with embodiments of the present disclosure; and
FIG. 3 is a block diagram depicting selected signal processing circuits and functional blocks within an example adaptive noise canceling (ANC) circuit of a coder-decoder (CODEC) integrated circuit of FIG. 3, in accordance with embodiments of the present disclosure.



DETAILED DESCRIPTION
The present disclosure encompasses noise canceling techniques and circuits that can be implemented in a personal audio device, such as a wireless telephone. The personal audio device includes an ANC circuit that may measure the ambient acoustic environment and generate a signal that is injected in the speaker (or other transducer) output to cancel ambient acoustic events. A reference microphone may be provided to measure the ambient acoustic environment and an error microphone may be included for controlling the adaptation of the anti-noise signal to cancel the ambient audio sounds and for correcting for the electro-acoustic path from the output of the processing circuit through the transducer.
Referring now to FIG. 1, a wireless telephone 10 as illustrated in accordance with embodiments of the present disclosure is shown in proximity to a human ear 5. Wireless telephone 10 is an example of a device in which techniques in accordance with embodiments of the invention may be employed, but it is understood that not all of the elements or configurations embodied in illustrated wireless telephone 10, or in the circuits depicted in subsequent illustrations, are required in order to practice the inventions recited in the claims. Wireless telephone 10 may include a transducer such as speaker SPKR that reproduces distant speech received by wireless telephone 10, along with other local audio events such as ringtones, stored audio program material, injection of near-end speech (i.e., the speech of the user of wireless telephone 10) to provide a balanced conversational perception, and other audio that requires reproduction by wireless telephone 10, such as sources from webpages or other network communications received by wireless telephone 10 and audio indications such as a low battery indication and other system event notifications. A near-speech microphone NS may be provided to capture near-end speech, which is transmitted from wireless telephone 10 to the other conversation participant(s).
Wireless telephone 10 may include ANC circuits and features that inject an anti-noise signal into speaker SPKR to improve intelligibility of the distant speech and other audio reproduced by speaker SPKR. A reference microphone R may be provided for measuring the ambient acoustic environment, and may be positioned away from the typical position of a user's mouth, so that the near-end speech may be minimized in the signal produced by reference microphone R. Another microphone, error microphone E, may be provided in order to further improve the ANC operation by providing a measure of the ambient audio combined with the audio reproduced by speaker SPKR close to ear 5 at an error microphone reference position ERP, when wireless telephone 10 is in close proximity to ear 5. In different embodiments, additional reference and/or error microphones may be employed. Circuit 14 within wireless telephone 10 may include an audio CODEC integrated circuit (IC) 20 that receives the signals from reference microphone R, near-speech microphone NS, and error microphone E, and interfaces with other integrated circuits such as a radio-frequency (RF) integrated circuit 12 having a wireless telephone transceiver. In some embodiments of the disclosure, the circuits and techniques disclosed herein may be incorporated in a single integrated circuit that includes control circuits and other functionality for implementing the entirety of the personal audio device, such as an MP3 player-on-a-chip integrated circuit. In these and other embodiments, the circuits and techniques disclosed herein may be implemented partially or fully in software and/or firmware embodied in computer-readable media and executable by a controller or other processing device.
In general, ANC techniques of the present disclosure measure ambient acoustic events (as opposed to the output of speaker SPKR and/or the near-end speech) impinging on reference microphone R, and by also measuring the same ambient acoustic events impinging on error microphone E, ANC processing circuits of wireless telephone 10 adapt an anti-noise signal generated from the output of reference microphone R to have a characteristic that minimizes the amplitude of the ambient acoustic events at error microphone E (e.g., at error microphone reference position ERP). Because acoustic path P(z) extends from reference microphone R to error microphone E, ANC circuits are effectively estimating acoustic path P(z) while removing effects of an electro-acoustic path S(z) that represents the response of the audio output circuits of CODEC IC 20 and the acoustic/electric transfer function of speaker SPKR including the coupling between speaker SPKR and error microphone E in the particular acoustic environment, which may be affected by the proximity and structure of ear 5 and other physical objects and human head structures that may be in proximity to wireless telephone 10, when wireless telephone 10 is not firmly pressed to ear 5. Because the listener of wireless telephone actually hears the output of speaker SPKR at a drum reference point DRP, differences between the error microphone reference signal produced by error microphone E and what is actually heard by the listener are shaped at least by the response of the ear canal, as well as a spatial distance between error microphone reference position ERP and drum reference position DRP.
While the illustrated wireless telephone 10 includes a two-microphone ANC system with a third near-speech microphone NS, some aspects of the present invention may be practiced in a system that does not include separate error and reference microphones, or a wireless telephone that uses near-speech microphone NS to perform the function of the reference microphone R. Also, in personal audio devices designed only for audio playback, near-speech microphone NS will generally not be included, and the near-speech signal paths in the circuits described in further detail below may be omitted, without changing the scope of the disclosure.
Referring now to FIG. 2, selected circuits within wireless telephone 10 are shown in a block diagram. CODEC IC 20 may include an analog-to-digital converter (ADC) 21A for receiving the reference microphone signal and generating a digital representation ref of the reference microphone signal, an ADC 21B for receiving the error microphone signal and generating a digital representation err of the error microphone signal, and an ADC 21C for receiving the near speech microphone signal and generating a digital representation ns of the near speech microphone signal. CODEC IC 20 may generate an output for driving speaker SPKR from an amplifier A1, which may amplify the output of a digital-to-analog converter (DAC) 23 that receives the output of a combiner 26. Combiner 26 may combine audio signals is from internal audio sources 24, the anti-noise signal generated by ANC circuit 30, which by convention has the same polarity as the noise in reference microphone signal ref and is therefore subtracted by combiner 26, and a portion of near speech microphone signal ns so that the user of wireless telephone 10 may hear his or her own voice in proper relation to downlink speech ds, which may be received from radio frequency (RF) integrated circuit 22 and may also be combined by combiner 26. Near speech microphone signal ns may also be provided to RF integrated circuit 22 and may be transmitted as uplink speech to the service provider via antenna ANT.
Referring now to FIG. 3, details of ANC circuit 30 are shown in accordance with embodiments of the present disclosure. Adaptive filter 32 may receive reference microphone signal ref and under ideal circumstances, may adapt its transfer function W(z) to be P(z)/S(z) to generate the anti-noise signal, which may be provided to an output combiner that combines the anti-noise signal with the audio to be reproduced by the transducer, as exemplified by combiner 26 of FIG. 2. The coefficients of adaptive filter 32 may be controlled by a W coefficient control block 31 that uses a correlation of signals to determine the response of adaptive filter 32, which generally minimizes the error, in a least-mean squares sense, between those components of reference microphone signal ref present in error microphone signal err. The signals compared by W coefficient control block 31 may be the reference microphone signal ref as shaped by a copy of an estimate of the response of path S(z) provided by filter 34B and a modified playback corrected error based at least in part on error microphone signal err. The modified playback corrected error may be generated as described in greater detail below.
By transforming reference microphone signal ref with a copy of the estimate of the response of path S(z), response SECOPY(z), and minimizing the difference between the resultant signal and error microphone signal err, adaptive filter 32 may adapt to the desired response of P(z)/S(z). In addition to error microphone signal err, the signal compared to the output of filter 34B by W coefficient control block 31 may include an inverted amount of downlink audio signal ds and/or internal audio signal ia that has been processed by filter response SE(z), of which response SECOPY(z) is a copy. By injecting an inverted amount of downlink audio signal ds and/or internal audio signal ia, adaptive filter 32 may be prevented from adapting to the relatively large amount of downlink audio and/or internal audio signal present in error microphone signal err. However, by transforming that inverted copy of downlink audio signal ds and/or internal audio signal ia with the estimate of the response of path S(z), the downlink audio and/or internal audio that is removed from error microphone signal err should match the expected version of downlink audio signal ds and/or internal audio signal ia reproduced at error microphone signal err, because the electrical and acoustical path of S(z) is the path taken by downlink audio signal ds and/or internal audio signal ia to arrive at error microphone E. Filter 34B may not be an adaptive filter, per se, but may have an adjustable response that is tuned to match the response of adaptive filter 34A, so that the response of filter 34B tracks the adapting of adaptive filter 34A.
To implement the above, adaptive filter 34A may have coefficients controlled by SE coefficient control block 33, which may compare a source audio signal (e.g., downlink audio signal ds and/or internal audio signal ia) and a playback corrected error, wherein the playback corrected error is equal to error microphone signal err after removal of the source audio signal (as filtered by adaptive filter 34A to represent the expected playback audio delivered to error microphone E) by a combiner 36. SE coefficient control block 33 may correlate the actual source audio signal with the components of the source audio signal that are present in error microphone signal err. Adaptive filter 34A may thereby be adapted to generate a secondary estimate signal from the source audio signal, that when subtracted from error microphone signal err to generate the playback corrected error, includes the content of error microphone signal err that is not due to the source audio signal.
The modified playback corrected error may be communicated to W coefficient control block 31 and compared with the filtered reference microphone signal ref, wherein the modified playback corrected error is equal to the playback corrected error after removal (e.g., by combiner 38) of a scaled anti-noise signal generated by a biasing portion comprising gain element 46 and filter 34C. Filter 34C may not be an adaptive filter, per se, but may have an adjustable response that is tuned to match the response of adaptive filter 34A, so that the response of filter 34C tracks the adapting of adaptive filter 34A. Gain element 46 may apply a multiplicative scaling factor and filter 34C may apply the response SECOPY(z) (which is a copy of SE(z)) to the anti-noise signal generated by filter 32 in order to generate the scaled anti-noise signal. Accordingly, a gain G of ANC system 30 (where the gain G is the ratio of the anti-noise signal generated by a typical ANC system without gain element 46 and filter 34C to the anti-noise generated by filter 32 of ANC circuit 30 depicted in FIG. 3) may be varied by modifying the scaling factor of gain element 46, without other parts of ANC circuit 30 compensating and nullifying the change in gain G as it is varied. The relationship between the gain G of filter 32 and the scaling factor k of gain element 46 may be given by the equation:

G=1/(1−k)

In order to compensate for changes in a distance between error microphone reference point ERP and drum reference point DRP, ANC circuit 30 may vary the scaling factor, and therefore the gain G, based on an estimation or other indication of the distance between error microphone reference point ERP and drum reference point DRP. Such distance may be estimated in any suitable manner, for example by detecting a pressure of wireless telephone 10 against a listener's ear 5 as described in U.S. patent application Ser. No. 13/844,602 filed Mar. 15, 2013, entitled “Monitoring of Speaker Impedance to Detect Pressure Applied Between Mobile Device in Ear,” and/or as described in U.S. patent application Ser. No. 13/310,380 filed Dec. 2, 2011, entitled “Ear-Coupling Detection and Adjustment of Adaptive Response in Noise-Cancelling in Personal Audio Devices,” in which distance may be estimated and/or pressure may be determined by analyzing the response SE(z) of filter 34A due to the fact that in certain personal audio devices, the response SE(z) may vary in amplitude within certain frequencies (e.g., less than 1-2 kilohertz) based on the pressure between speaker SPKR and the listener's ear 5, and thus, by examining amplitude of SE(z) at such frequencies, a pressure and/or distance may be estimated.
This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the exemplary embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the exemplary embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, or component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative.
All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are construed as being without limitation to such specifically recited examples and conditions. Although embodiments of the present inventions have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the disclosure.

## Claims

1. A personal audio device comprising:
a personal audio device housing;
a transducer coupled to the personal audio device housing for reproducing an audio signal including both a source audio signal 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;
a reference microphone coupled to the personal audio device housing for providing a reference microphone signal indicative of the ambient audio sounds;
an error microphone coupled to the personal audio device housing in proximity to the transducer for providing an error microphone signal indicative of the acoustic output of the transducer and the ambient audio sounds at the transducer; and
a processing circuit comprising:
an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal;
a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal and have a response that generates a secondary path estimate from the source audio signal;
a biasing portion that generates a scaled anti-noise signal by applying a scaling factor and the response of the secondary path estimate filter to the anti-noise signal; and
a coefficient control block that shapes the response of the adaptive filter in conformity with the reference microphone signal and a modified playback corrected error signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the error microphone signal, wherein a playback corrected error signal is based on a difference between the error microphone signal and the secondary path estimate and the modified playback corrected error signal is based on a difference between the playback corrected error signal and the scaled anti-noise signal.

2. The personal audio device of claim 1, wherein the scaling factor has a value between 0 and 1.

3. The personal audio device of claim 1, wherein the scaling factor defines a gain, wherein the gain is the ratio of the anti-noise signal that would be generated by filter without the biasing portion to the anti-noise generated by filter with the biasing portion.

4. The personal audio device of claim 1, wherein the value of the scaling factor is a function of a distance between the personal audio device and a portion of the listener.

5. The personal audio device of claim 4, wherein the distance is an estimated distance between the transducer and the listener's eardrum.

6. The personal audio device of claim 4, wherein:
the secondary path estimate filter is an adaptive filter, and the processing circuit further implements a secondary coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error signal by adapting the response of the secondary path estimate filter to minimize the playback corrected error signal; and
the distance is determined based on the response of the secondary path estimate filter.

7. The personal audio device of claim 1, wherein the value of the scaling factor is a function of a pressure applied to the personal audio device by the listener.

8. The personal audio device of claim 7, wherein the pressure is a pressure applied between the personal audio device and the listener's ear.

9. The personal audio device of claim 7, wherein:
the secondary path estimate filter is adaptive, and the processing circuit further implements a secondary coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error signal by adapting the response of the secondary path estimate filter to minimize the playback corrected error signal; and
the pressure is determined based on the response of the secondary path estimate filter.

10. A method for canceling ambient audio sounds in the proximity of a transducer of a personal audio device, the method comprising:
receiving a reference microphone signal indicative of the ambient audio sounds;
receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer; and
generating a source audio signal for playback to a listener;
adaptively generating an anti-noise signal for countering effects of the ambient audio sounds at an acoustic output of the transducer by adapting a response of an adaptive filter that filters the reference microphone signal in conformity with the reference microphone signal and a modified playback corrected error signal to minimize the ambient audio sounds in the error microphone signal;
generating a secondary path estimate from the source audio signal by filtering the source audio signal with a secondary path estimate filter modeling an electro-acoustic path of the source audio signal;
generating a scaled anti-noise signal by applying a scaling factor and a response of the secondary path estimate filter to the anti-noise signal; and
combining the anti-noise signal with a source audio signal to generate an audio signal provided to the transducer;
wherein a playback corrected error signal is based on a difference between the error microphone signal and the secondary path estimate and the modified playback corrected error signal is based on a difference between the playback corrected error signal and the scaled anti-noise signal.

11. The method of claim 10, wherein the scaling factor has a value between 0 and 1.

12. The method of claim 10, wherein the scaling factor defines a gain, wherein the gain is the ratio of the anti-noise signal that would be generated by filter without the biasing portion to the anti-noise generated by filter with the biasing portion.

13. The method of claim 10, wherein the value of the scaling factor is a function of a distance between the personal audio device and a portion of the listener.

14. The method of claim 13, wherein the distance is an estimated distance between the transducer and the listener's eardrum.

15. The method of claim 13, further comprising:
shaping the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error signal by adapting the response of the secondary path estimate filter to minimize the playback corrected error signal; and
determining the distance based on the response of the secondary path estimate filter.

16. The method of claim 10, wherein the value of the scaling factor is a function of a pressure applied to the personal audio device by the listener.

17. The method of claim 16, wherein the pressure is a pressure applied between the personal audio device and the listener's ear.

18. The method of claim 16, further comprising:
shaping the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error signal by adapting the response of the secondary path estimate filter to minimize the playback corrected error signal; and
determining the pressure based on the response of the secondary path estimate filter.

19. 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 a source audio signal for playback to a listener and an anti-noise signal for countering the effect 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 comprising:
an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal;
a secondary path estimate filter configured to model an electro-acoustic path of the source audio signal and have a response that generates a secondary path estimate from the source audio signal;
a biasing portion that generates a scaled anti-noise signal by applying a scaling factor and the response of the secondary path estimate filter to the anti-noise signal; and
a coefficient control block that shapes the response of the adaptive filter in conformity with the reference microphone signal and a modified playback corrected error signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the error microphone signal, wherein a playback corrected error signal is based on a difference between the error microphone signal and the secondary path estimate and the modified playback corrected error signal is based on a difference between the playback corrected error signal and the scaled anti-noise signal.

20. The integrated circuit of claim 19, wherein the scaling factor has a value between 0 and 1.

21. The integrated circuit of claim 19, wherein the scaling factor defines a gain, wherein the gain is the ratio of the anti-noise signal that would be generated by filter without the biasing portion to the anti-noise generated by filter with the biasing portion.

22. The integrated circuit of claim 19, wherein the scaling factor is a function of a distance between the personal audio device and a portion of the listener.

23. The integrated circuit of claim 22, wherein the distance is an estimated distance between the transducer and the listener's eardrum.

24. The integrated circuit of claim 22, wherein:
the secondary path estimate filter is an adaptive filter, and the processing circuit further implements a secondary coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error signal by adapting the response of the secondary path estimate filter to minimize the playback corrected error signal; and
the distance is determined based on the response of the secondary path estimate filter.

25. The integrated circuit of claim 19, wherein the scaling factor is a function of a pressure applied to the personal audio device by the listener.

26. The integrated circuit of claim 25, wherein the pressure is a pressure applied between the personal audio device and the listener's ear.

27. The integrated circuit of claim 25, wherein:
the secondary path estimate filter is adaptive, and the processing circuit further implements a secondary coefficient control block that shapes the response of the secondary path estimate filter in conformity with the source audio signal and the playback corrected error signal by adapting the response of the secondary path estimate filter to minimize the playback corrected error signal; and
the pressure is determined based on the response of the secondary path estimate filter.