# Patent

## Title
Virtual microphone for adaptive noise cancellation in personal audio devices

## Abstract

A processing circuit may implement an adaptive filter having a response that generates an anti-noise signal from a reference microphone signal, one or more filters for modeling an electro-acoustic path of the anti-noise signal from a location of an error microphone to an eardrum of a listener and having a response that generates a filtered reference microphone signal from the reference microphone signal, one or more filters for modeling an acoustic path of ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized playback corrected error signal based on the error microphone signal, wherein the synthesized playback corrected error signal is indicative of ambient audio sounds present at the eardrum, and a coefficient control block that shapes the response of the adaptive filter in conformity with the filtered reference microphone signal and the synthesized playback corrected error signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the synthesized playback corrected error signal.

## Background
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 applying models of a human ear canal to estimate ambient audio sounds present at a listener's eardrum.
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 canceling 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 sum 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 microphone 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 at the error microphone reference point, the noise cancellation system may not cancel some noise present at the drum reference point.
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 coupled to the 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. The reference microphone may be coupled to the housing for providing a reference microphone signal indicative of the ambient audio sounds. The error microphone may be coupled to the 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. The processing circuit may implement an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal, one or more filters for modeling an electro-acoustic path of the anti-noise signal from a location of the error microphone to an eardrum of the listener and having a response that generates a filtered reference microphone signal from the reference microphone signal, one or more filters for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized playback corrected error signal based on the error microphone signal, wherein the synthesized playback corrected error signal is indicative of ambient audio sounds present at the eardrum, and a coefficient control block that shapes the response of the adaptive filter in conformity with the filtered reference microphone signal and the synthesized playback corrected error signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the synthesized playback corrected error signal.
In accordance with these and other embodiments of the present disclosure, a method for canceling ambient audio sounds in the proximity of a drum reference point of a user of a personal audio device may include receiving a reference microphone signal indicative of the ambient audio sounds. The method may also include receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer. The method may further include generating a source audio signal for playback to a listener. The method may additionally include generating a filtered reference microphone signal from the reference microphone signal by filtering the reference microphone signal by one or more filters for modeling an electro-acoustic path of the anti-noise signal from a location of the error microphone to an eardrum of the listener. The method may also include generating a synthesized playback corrected error signal based on the error microphone signal by filtering the error microphone signal by one or more filters for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum, wherein the synthesized playback corrected error signal is indicative of ambient audio sounds present at the eardrum. The method may further include adaptively generating an anti-noise signal from the reference microphone signal, countering the effects of ambient audio sounds at an acoustic output of the transducer, by adapting, in conformity with the filtered reference microphone signal and the synthesized playback corrected error signal, a response of an adaptive filter that filters an output of the reference microphone to minimize the ambient audio sounds in the error microphone signal. The method may additionally include combining the anti-noise signal with the source audio signal to generate an audio signal provided to the transducer.
In accordance with these and other embodiments of the present disclosure, an integrated circuit may include an output, a reference microphone input, an error microphone input, and a processing circuit. The output may be 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. The reference microphone input may be for receiving a reference microphone signal indicative of the ambient audio sounds. The error microphone input may be for receiving an error microphone signal indicative of the output of the transducer and the ambient audio sounds at the transducer. The processing circuit may implement an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal, one or more filters for modeling an electro-acoustic path of the anti-noise signal from a location of the error microphone to an eardrum of the listener and having a response that generates a filtered reference microphone signal from the reference microphone signal, one or more filters for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized playback corrected error signal based on the error microphone signal, wherein the synthesized playback corrected error signal is indicative of ambient audio sounds present at the eardrum, and a coefficient control block that shapes the response of the adaptive filter in conformity with the filtered reference microphone signal and the synthesized playback corrected error signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the synthesized playback corrected error 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;
FIG. 3A is a block diagram depicting selected signal processing circuits and functional blocks within an example active noise canceling (ANC) circuit of a coder-decoder (CODEC) integrated circuit of FIG. 2, in accordance with embodiments of the present disclosure; and
FIG. 3B is a block diagram depicting selected signal processing circuits and functional blocks within another example ANC circuit of a CODEC integrated circuit of FIG. 2, 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, earbud, or headphone. 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 and to a listener's ear or eardrum
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 invention 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. 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 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 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 disclosure 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. In addition, some aspects of the present disclosure may be practiced in a system that includes a plurality of reference microphones and/or a plurality of error microphones.
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. 3A, details of an example ANC circuit 30A are shown in accordance with embodiments of the present disclosure. Adaptive filter 32 may receive a filtered reference microphone signal filtered_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 the filtered reference microphone signal filtered_ref present in a synthesized playback corrected error signal PBCE DRP described in greater detail below. The signals compared by W coefficient control block 31 may be the reference microphone signal ref as shaped by a secondary ear canal path estimate filter 44A for modeling an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum and a copy of an estimate of the response of path S(z) provided by filter 34B (thus generating the filtered reference signal filtered_ref) and a synthesized playback corrected error signal (shown in FIG. 3A as “PBCE DRP”) based at least in part on error microphone signal err. By transforming reference microphone signal ref with an estimate of the response of the acoustic path of the anti-noise signal from the location of the error microphone to the eardrum, response HS(z), and a copy of the estimate of the response of path S(z), response SECOPY(z), an estimate of the cumulative electro-acoustical path of reference microphone signal ref from reference microphone R to the DRP is applied to reference microphone signal ref, thus balancing the inputs to W coefficient control block 31, and providing for robustness of adaptive filter 32. By minimizing the difference between the filtered reference signal and the synthesized playback corrected error signal, adaptive filter 32 may adapt to the desired response of P(z)/S(z).
To generate the synthesized playback corrected error signal, ANC circuit 30A may generate a playback corrected error at the ERP (shown in FIG. 3A as “PBCE ERP”) which comprises the error microphone signal combined (e.g., at combiner 36) with an inverted amount of source audio signal (e.g., downlink audio signal ds and/or internal audio signal ia) that has been processed by filter 34A having response SE(z), of which response SECOPY(z) is a copy. By injecting an inverted amount of source audio signal, adaptive filter 32 may be prevented from adapting to the relatively large amount of source audio signal present in error microphone signal err (and thus also present in the synthesized playback corrected error signal which is based at least in part on error microphone signal err) and by transforming that inverted copy of the source audio signal with the estimate of the response of path S(z), the source audio signal that is removed from error microphone signal err should match the expected version of the source audio signal reproduced at the ERP, because the electrical and acoustical path of S(z) is the path taken by the source audio signal to arrive at error microphone E.
ANC circuit 30A may also generate a synthesized error reference point anti-noise signal (shown in FIG. 3A as “SYNTHESIZED ANTI-NOISE AT ERP”) by shaping the anti-noise signal generated by filter 32 with filter 34C having a response SECOPY(z) which is a copy of response SE(z). Such synthesized error reference point anti-noise signal should match the expected version of the anti-noise signal reproduced at the ERP, because the electrical and acoustical path of S(z) is the path taken by the anti-noise signal to arrive at error microphone E.
The synthesized error reference point anti-noise signal may be combined (e.g., by combiner 38) with the playback corrected error at the ERP to generate a synthesized error reference point ambient signal (shown in FIG. 3A as “SYNTHESIZED AMBIENT AT ERP”) indicative of the ambient audio sounds present at the ERP. The synthesized error reference point ambient signal may be shaped by a primary ear canal path estimate filter 42 with a response HP(z) for modeling an acoustic path of the ambient audio sounds from the location of the error microphone E (the ERP) to the DRP, thus generating a synthesized drum reference point ambient signal indicative of the ambient audio sounds present at the DRP (shown in FIG. 3A as “SYNTHESIZED AMBIENT AT DRP”).
Furthermore, ANC circuit 30A may also generate a synthesized drum reference point anti-noise signal (shown in FIG. 3A as “SYNTHESIZED ANTI-NOISE AT DRP”) by shaping the synthesized error reference point anti-noise signal generated by filter 34C with a secondary ear canal path estimate filter 44B having a response HS(z) which may be a copy of the response of secondary ear canal path estimate filter 44A. Such synthesized drum reference point anti-noise signal should match the expected version of the anti-noise signal reproduced at the DRFP, because the electrical and acoustical path of HS(z) is the path taken by the synthesized error reference point anti-noise signal to arrive at the DRP.
The synthesized playback corrected error may be generated by subtracting (e.g., by combiner 39) the synthesized drum reference point anti-noise signal from the synthesized drum reference point ambient signal. The resulting synthesized playback corrected error may be indicative of the playback corrected error at the drum reference point.
Referring now to FIG. 3B, details of an example ANC circuit 30B are shown in accordance with embodiments of the present disclosure. Adaptive filter 32 may receive a filtered reference microphone signal filtered_ref indicative of the expected version of reference microphone signal ref reproduced at the DRP and under ideal circumstances, may adapt its transfer function W(z) to be P(z)/S(z) to generate a signal which, when further shaped by a canal path estimate filter 46B having a response HSOP(z) for modeling a ratio between a model of an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum (e.g., response HS(z) described in reference to ANC circuit 30A) and a model of the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum (e.g., response HP(z) described in reference to ANC circuit 30A), generates 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 the filtered reference microphone signal filtered_ref present in a synthesized playback corrected error signal PBCE DRP described in greater detail below. The signals compared by W coefficient control block 31 may be a synthesized playback corrected error signal (shown in FIG. 3B as “PBCE DRP”) based at least in part on error microphone signal err and the reference microphone signal ref as shaped by: (i) a canal path estimate filter 46A having a response HSOP(z) similar or identical to the response of canal path estimate filter 46B for modeling a ratio between a model of an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum (e.g., response HS(z) described in reference to ANC circuit 30A) and a model of the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum (e.g., response HP(z) described in reference to ANC circuit 30A); (ii) a primary ear canal path estimate filter 42A with a response HP(z) for modeling an acoustic path of the ambient audio sounds from the location of the error microphone E (the ERP) to the DRP; and (iii) a copy of an estimate of the response of path S(z) provided by filter 34B (thus generating the filtered reference signal filtered_ref). The cumulative effect of filters 46A, 42A, and 34B may be to balance the inputs to W coefficient control block 31, and providing for robustness of adaptive filter 32. By minimizing the difference between the filtered reference signal and the synthesized playback corrected error signal, adaptive filter 32 may adapt to the desired response of P(z)/S(z).
To generate the synthesized playback corrected error signal, ANC circuit 30B may generate a playback corrected error at the ERP (shown in FIG. 3A as “PBCE ERP”) which comprises the error microphone signal combined (e.g., at combiner 36) with an inverted amount of source audio signal (e.g., downlink audio signal ds and/or internal audio signal ia) that has been processed by filter 34A having response SE(z), of which response SECOPY(z) is a copy. By injecting an inverted amount of source audio signal, adaptive filter 32 may be prevented from adapting to the relatively large amount of source audio signal present in error microphone signal err (and thus also present in the synthesized playback corrected error signal which is based at least in part on error microphone signal err) and by transforming that inverted copy of the source audio signal with the estimate of the response of path S(z), the source audio signal that is removed from error microphone signal err should match the expected version of the source audio signal reproduced at the ERP, because the electrical and acoustical path of S(z) is the path taken by the source audio signal to arrive at error microphone E.
The playback corrected error may be shaped by a primary ear canal path estimate filter 42B with a response HP(z) for modeling an acoustic path of the ambient audio sounds from the location of the error microphone E (the ERP) to the DRP, thus generating the synthesized playback corrected error. The resulting synthesized playback corrected error may be indicative of the playback corrected error at the drum reference point.
In some embodiments of the ANC circuits 30A and 30B respectively depicted in FIGS. 3A and 3B, the responses SE(z) and SECOPY(z) may be adaptive. Accordingly, 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 the playback corrected error. 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. Filters 34B and 34C may not be adaptive filters, per se, but may have adjustable responses that are tuned to match the response of adaptive filter 34A, so that the responses of filters 34B and 34C track the adapting of adaptive filter 34A.
In some embodiments, the various responses HP(z), HS(z), and/or HSOP(z) for modeling acoustic paths of signals from the ERP to the DRP may be determined by offline modeling of a human ear canal.
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 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 housing for providing a reference microphone signal indicative of the ambient audio sounds;
an error microphone coupled to the 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 that implements:
an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal;
one or more filters for modeling an electro-acoustic path of the anti-noise signal from a location of the error microphone to an eardrum of the listener and having a response that generates a filtered reference microphone signal from the reference microphone signal;
one or more filters for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized playback corrected error signal based on the error microphone signal, wherein the synthesized playback corrected error signal is indicative of ambient audio sounds present at the eardrum; and
a coefficient control block that shapes the response of the adaptive filter in conformity with the filtered reference microphone signal and the synthesized playback corrected error signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the synthesized playback corrected error signal.

2. The personal audio device of claim 1, wherein the processing circuit further implements a secondary path estimate filter for modeling an electro-acoustic path of the source audio signal having a response that generates a secondary path estimate from the source audio signal.

3. The personal audio device of claim 2, wherein:
the one or more filters for modeling the electro-acoustic path of the anti-noise signal from the location of the error microphone to the eardrum of the listener comprise:
a first secondary ear canal path estimate filter for modeling an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum; and
a filter for modeling an electro-acoustic path of the reference microphone signal to the transducer; and

the one or more filters for modeling the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum comprise:
a filter for modeling an electro-acoustic path of the anti-noise signal to the transducer having a response that generates a synthesized error reference point anti-noise signal from the anti-noise signal;
a second secondary ear canal path estimate filter for modeling an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum having a response that generates a synthesized drum reference point anti-noise signal from the synthesized error reference point anti-noise signal; and
a primary ear canal path estimate filter for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized drum reference point ambient signal from the synthesized error reference point anti-noise signal and a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal; and

the synthesized playback corrected error is based on a difference between the synthesized drum reference point ambient signal and the synthesized drum reference point anti-noise signal.

4. The personal audio device of claim 3, wherein at least one of the filter for modeling the electro-acoustic path of the reference microphone signal to the transducer and the filter for modeling the electro-acoustic path of the anti-noise signal to the transducer has a response equal to the response of the secondary path estimate filter.

5. The personal audio device of claim 3, wherein the filter for modeling an electro-acoustic path of the reference microphone signal to the transducer and the filter for modeling an electro-acoustic path of the anti-noise signal to the transducer have the same response.

6. The personal audio device of claim 3, wherein the first secondary ear canal path estimate filter and the second secondary ear canal path estimate filter have the same response.

7. The personal audio device of claim 2, wherein:
the one or more filters for modeling the electro-acoustic path of the anti-noise signal from the location of the error microphone to the eardrum of the listener comprise:
a first primary ear canal path estimate filter for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum;
a first canal path estimate filter for modeling a ratio between a model of an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum and a model of the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum; and
a filter for modeling an electro-acoustic path of the reference microphone signal to the transducer;

the one or more filters for modeling the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum comprise a second ear canal path estimate filter for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized drum reference point ambient signal from a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal; and
wherein the processing circuit further implements a second canal path estimate filter for modeling the ratio between the model of an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum and the model of the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum, wherein the second canal path estimate filter and the adaptive filter are configured to together generate the anti-noise signal from the reference microphone signal.

8. The personal audio device of claim 7, wherein the filter for modeling the electro-acoustic path of the reference microphone signal to the transducer has a response equal to the response of the secondary path estimate filter.

9. The personal audio device of claim 7, wherein the first primary ear canal path estimate filter and the second primary ear canal path estimate filter have the same response.

10. The personal audio device of claim 7, wherein the first canal path estimate filter and the second canal path estimate filter have the same response.

11. The personal audio device of claim 2, 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 a playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal.

12. A method for canceling ambient audio sounds in the proximity of a drum reference point of a user 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 an output of a transducer and the ambient audio sounds at the transducer;
generating a source audio signal for playback to a listener;
generating a filtered reference microphone signal from the reference microphone signal by filtering the reference microphone signal by one or more filters for modeling an electro-acoustic path of the anti-noise signal from a location of the error microphone to an eardrum of the listener;
generating a synthesized playback corrected error signal based on the error microphone signal by filtering the error microphone signal by one or more filters for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum, wherein the synthesized playback corrected error signal is indicative of ambient audio sounds present at the eardrum;
adaptively generating an anti-noise signal from the reference microphone signal, countering the effects of ambient audio sounds at an acoustic output of the transducer, by adapting, in conformity with the filtered reference microphone signal and the synthesized playback corrected error signal, a response of an adaptive filter that filters an output of the reference microphone to minimize the ambient audio sounds in the error microphone signal; and
combining the anti-noise signal with the source audio signal to generate an audio signal provided to the transducer.

13. The method of claim 12, further comprising 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.

14. The method of claim 13, wherein:
the one or more filters for modeling the electro-acoustic path of the anti-noise signal from the location of the error microphone to the eardrum of the listener comprise:
a first secondary ear canal path estimate filter for modeling an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum; and
a filter for modeling an electro-acoustic path of the reference microphone signal to the transducer; and

the one or more filters for modeling the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum comprise:
a filter for modeling an electro-acoustic path of the anti-noise signal to the transducer having a response that generates a synthesized error reference point anti-noise signal from the anti-noise signal;
a second secondary ear canal path estimate filter for modeling an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum having a response that generates a synthesized drum reference point anti-noise signal from the synthesized error reference point anti-noise signal; and
a primary ear canal path estimate filter for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized drum reference point ambient signal from the synthesized error reference point anti-noise signal and a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal; and

the synthesized playback corrected error is based on a difference between the synthesized drum reference point ambient signal and the synthesized drum reference point anti-noise signal.

15. The method of claim 14, wherein at least one of the filter for modeling the electro-acoustic path of the reference microphone signal to the transducer and the filter for modeling the electro-acoustic path of the anti-noise signal to the transducer has a response equal to the response of the secondary path estimate filter.

16. The method of claim 14, wherein the filter for modeling an electro-acoustic path of the reference microphone signal to the transducer and the filter for modeling an electro-acoustic path of the anti-noise signal to the transducer have the same response.

17. The method of claim 14, wherein the first secondary ear canal path estimate filter and the second secondary ear canal path estimate filter have the same response.

18. The method of claim 13, wherein:
the one or more filters for modeling the electro-acoustic path of the anti-noise signal from the location of the error microphone to the eardrum of the listener comprise:
a first primary ear canal path estimate filter for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum;
a first canal path estimate filter for modeling a ratio between a model of an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum and a model of the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum; and
a filter for modeling an electro-acoustic path of the reference microphone signal to the transducer;

the one or more filters for modeling the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum comprise a second ear canal path estimate filter for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized drum reference point ambient signal from a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal; and
wherein the processing circuit further implements a second canal path estimate filter for modeling the ratio between the model of an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum and the model of the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum, wherein the second canal path estimate filter and the adaptive filter are configured to together generate the anti-noise signal from the reference microphone signal.

19. The method of claim 18, wherein the filter for modeling the electro-acoustic path of the reference microphone signal to the transducer has a response equal to the response of the secondary path estimate filter.

20. The method of claim 18, wherein the first primary ear canal path estimate filter and the second primary ear canal path estimate filter have the same response.

21. The method of claim 18, wherein the first canal path estimate filter and the second canal path estimate filter have the same response.

22. The method of claim 13, wherein the secondary path estimate filter is adaptive, and the response of the secondary path estimate filter is shaped in conformity with the source audio signal and a playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal.

23. An integrated circuit 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 that implements:
an adaptive filter having a response that generates an anti-noise signal from the reference microphone signal;
one or more filters for modeling an electro-acoustic path of the anti-noise signal from a location of the error microphone to an eardrum of the listener and having a response that generates a filtered reference microphone signal from the reference microphone signal;
one or more filters for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized playback corrected error signal based on the error microphone signal, wherein the synthesized playback corrected error signal is indicative of ambient audio sounds present at the eardrum; and
a coefficient control block that shapes the response of the adaptive filter in conformity with the filtered reference microphone signal and the synthesized playback corrected error signal by adapting the response of the adaptive filter to minimize the ambient audio sounds in the synthesized playback corrected error signal.

24. The integrated circuit of claim 23, wherein the processing circuit further implements a secondary path estimate filter for modeling an electro-acoustic path of the source audio signal having a response that generates a secondary path estimate from the source audio signal.

25. The integrated circuit of claim 24, wherein:
the one or more filters for modeling the electro-acoustic path of the anti-noise signal from the location of the error microphone to the eardrum of the listener comprise:
a first secondary ear canal path estimate filter for modeling an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum; and
a filter for modeling an electro-acoustic path of the reference microphone signal to the transducer; and

the one or more filters for modeling the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum comprise:
a filter for modeling an electro-acoustic path of the anti-noise signal to the transducer having a response that generates a synthesized error reference point anti-noise signal from the anti-noise signal;
a second secondary ear canal path estimate filter for modeling an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum having a response that generates a synthesized drum reference point anti-noise signal from the synthesized error reference point anti-noise signal; and
a primary ear canal path estimate filter for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized drum reference point ambient signal from the synthesized error reference point anti-noise signal and a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal; and

the synthesized playback corrected error is based on a difference between the synthesized drum reference point ambient signal and the synthesized drum reference point anti-noise signal.

26. The integrated circuit of claim 25, wherein at least one of the filter for modeling the electro-acoustic path of the reference microphone signal to the transducer and the filter for modeling the electro-acoustic path of the anti-noise signal to the transducer has a response equal to the response of the secondary path estimate filter.

27. The integrated circuit of claim 25, wherein the filter for modeling an electro-acoustic path of the reference microphone signal to the transducer and the filter for modeling an electro-acoustic path of the anti-noise signal to the transducer have the same response.

28. The integrated circuit of claim 25, wherein the first secondary ear canal path estimate filter and the second secondary ear canal path estimate filter have the same response.

29. The integrated circuit of claim 24, wherein:
the one or more filters for modeling the electro-acoustic path of the anti-noise signal from the location of the error microphone to the eardrum of the listener comprises:
a first primary ear canal path estimate filter for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum;
a first canal path estimate filter for modeling a ratio between a model of an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum and a model of the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum; and
a filter for modeling an electro-acoustic path of the reference microphone signal to the transducer;

the one or more filters for modeling the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum comprise a second ear canal path estimate filter for modeling an acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum and having a response that generates a synthesized drum reference point ambient signal from a playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal; and
wherein the processing circuit further implements a second canal path estimate filter for modeling the ratio between the model of an acoustic path of the anti-noise signal from the location of the error microphone to the eardrum and the model of the acoustic path of the ambient audio sounds from the location of the error microphone to the eardrum, wherein the second canal path estimate filter and the adaptive filter are configured to together generate the anti-noise signal from the reference microphone signal.

30. The integrated circuit of claim 29, wherein the filter for modeling the electro-acoustic path of the reference microphone signal to the transducer has a response equal to the response of the secondary path estimate filter.

31. The integrated circuit of claim 29, wherein the first primary ear canal path estimate filter and the second primary ear canal path estimate filter have the same response.

32. The integrated circuit of claim 29, wherein the first canal path estimate filter and the second canal path estimate filter have the same response.

33. The integrated circuit of claim 24, 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 a playback corrected error by adapting the response of the secondary path estimate filter to minimize the playback corrected error, wherein the playback corrected error is based on a difference between the error microphone signal and the source audio signal.