Patent Description:
Acoustic devices such as headphones can include active noise reduction (ANR) capabilities that block and constructively cancel at least portions of ambient noise from reaching the ear of a user. Therefore, ANR devices create an acoustic isolation effect, which isolates the user, at least in part, from the environment.

<CIT>, <CIT>, <NPL>, as well as <NPL>, are prior art references disclosing ANR devices having multiple microphones and some gain adjustment capabilities. IWAI ET AL propose a multichannel feedforward active noise control system combined with noise source separation by microphone arrays. A delay-and-sum, DS, beamformer and a generalized sidelobe canceller, GSC, are used in the noise source separation. The DS beamformer is a fixed beamformer composed of multichannel fixed filters, and delays each observed signal, with the fixed filters used to compensate for propagation delays.

The present invention relates to a method for implementation in an active noise reduction, ANR, device according to independent claim <NUM> and an active noise reduction, ANR, device according to independent claim <NUM>. Advantageous embodiments are set forth in the dependent claims.

Two or more of the features described in this disclosure, including those described in this summary section, may be combined without departing from the scope of the present invention as defined by the appended claims.

This document describes technology that uses multiple feedforward microphones in an Active Noise Reduction (ANR) system to improve ANR performance, noise performance, and reduce the likelihood of an unstable condition. When an ANR system is deployed, for example, in noise canceling headphones, certain unstable conditions can cause the headphones to generate an acoustic artifact (e.g., a loud noise) that is uncomfortable for the user. By providing multiple feedforward microphones in the ANR system, the technology described herein allows for the gain through each of the feedforward signal paths to be reduced relative to the situation where a single feedforward microphone is used. Because the gain through an individual signal path is lower, there is more headroom in the system, which results in fewer opportunities for clipping, and there is more margin to deal with an instability that may arise, for example, due to coupling between one of the feedforward microphones and the transducer. In addition, the individual gains of the multiple feedforward microphones can be assigned based on their likelihood of coupling, such that the total target gain is not compromised as compared to a single microphone case. For example, if one of the microphones is at a location where the microphone is susceptible to coupling to the driver (and by extension, susceptible to instability), a lower gain can be applied to that microphone to reduce the likelihood of coupling. However, the gain for another microphone can be adjusted accordingly such that the target total gain of the feedforward microphones is not reduced. In one example, a target gain of unity can be allocated between two feedforward paths such that a first microphone that is more susceptible to coupling has a gain of <NUM>, while a second microphone that is less susceptible to coupling has a gain of <NUM>. Thus, while the gains of the individual signal paths are reduced as compared to unity (e.g., to allow the ANR system to tolerate non-ideal microphone locations, such as microphone locations that are closer to the periphery of the ear-cup or near a port, where there may be greater coupling between the microphone and the transducer), the total feedforward gain is not compromised due a weighted distribution of the gain between the multiple feedforward paths. In some implementations, the weighting can also be done on a frequency-by-frequency basis such that the distributions of gains among two or more feedforward paths are different for different frequencies (or frequency ranges).

Active Noise Reduction (ANR) systems can be deployed in a wide array of acoustic devices to cancel or reduce unwanted or unpleasant noise. For example, ANR headphones can provide potentially immersive listening experiences by reducing the effects of ambient noise and sounds. The term headphone, as used herein, includes various types of such personal acoustic devices such as in-ear, around-ear or over-the-ear headphones, earphones, earbuds, and hearing aids. ANR systems can also be used in automotive or other transportation systems (e.g., in cars, trucks, buses, aircrafts, boats or other vehicles) to cancel or attenuate unwanted noise produced by, for example, mechanical vibrations or engine harmonics.

In some cases, an ANR system can include an electroacoustic or electromechanical system that can be configured to cancel at least some of the unwanted noise (often referred to as "primary noise") based on the principle of superposition. For example, the ANR system can identify an amplitude and phase of the primary noise and produce another signal (often referred to as an "anti-noise signal") of approximately equal amplitude and opposite phase. The anti-noise signal can then be combined with the primary noise such that both are substantially canceled at a desired location. The term substantially canceled, as used herein, may include reducing the "canceled" noise to a specified level or to within an acceptable tolerance, and does not require complete cancellation of all noise. ANR systems can be used in attenuating a wide range of noise signals, including, for example, broadband noise and/or low-frequency noise that may not be easily attenuated using passive noise control systems.

<FIG> shows an example of an ANR system <NUM> deployed in a headphone <NUM>. The headphone <NUM> includes an ear-cup <NUM> on each side, which fits on, around or over the ear of a user. The ear-cup <NUM> may include a layer <NUM> of soft material (e.g., soft foam) for a comfortable fit over the ear of the user. The ANR system <NUM> can include or otherwise be coupled with a feedforward sensor <NUM>, a feedback sensor <NUM>, and an acoustic transducer <NUM>. The feedforward sensor <NUM> may be a microphone or another acoustic sensor and may be disposed on or near the outside of the ear-cup <NUM> to detect ambient noise. The feedback sensor <NUM> may be a microphone or another acoustic sensor and may be deployed proximate to the user's ear canal and/or the transducer <NUM>. The transducer <NUM> can be an acoustic transducer that radiates audio signals from an audio source device (not shown) that the headphone <NUM> is connected to and/or other signals from the ANR system <NUM>. While <FIG> illustrates an example where the ANR system is deployed in an around-ear headphone, the ANR system could also be deployed in other form-factors, including in-ear headphones, on-ear headphones, or off-ear personal acoustic devices (e.g., devices that are designed to not contact a wearer's ears, but may be worn in the vicinity of the wearer's ears on the wearer's head or on body).

The ANR system <NUM> can be configured to process the signals detected by the feedforward sensor <NUM> and/or the feedback sensor <NUM> to produce an anti-noise signal that is provided to the transducer <NUM>. The ANR system <NUM> can be of various types. In some implementations, the ANR system <NUM> is based on feedforward noise cancellation, in which the primary noise is sensed by the feedforward sensor <NUM> before the noise reaches a secondary source such as the transducer <NUM>. In some implementations, the ANR system <NUM> can be based on feedback noise cancellation, where the ANR system <NUM> cancels the primary noise based on the residual noise detected by the feedback sensor <NUM> and without the benefit of the feedforward sensor <NUM>. In some implementations, both feedforward and feedback noise cancellation are used. The ANR system <NUM> can be configured to control noise in various frequency bands. In some implementations, the ANR system <NUM> can be configured to control broadband noise such as white noise. In some implementations, the ANR system <NUM> can be configured to control narrow band noise such as harmonic noise from a vehicle engine.

In some implementations, the ANR system <NUM> can include a configurable digital signal processor (DSP) and other circuitry for implementing various signal flow topologies and filter configurations. Examples of such DSPs are described in <CIT> and <CIT>. The various signal flow topologies can be implemented in the ANR system <NUM> to enable functionalities such as audio equalization, feedback noise cancellation, and feedforward noise cancellation, among others. For example, as shown in <FIG>, the signal flow topologies of the ANR system <NUM> can include a feedforward signal flow path <NUM> that drives the transducer <NUM> to generate an anti-noise signal (using, for example, a feedforward compensator <NUM>) to reduce the effects of a noise signal picked up by the feedforward sensor <NUM>. In another example, the signal flow topologies can include a feedback signal flow path <NUM> that drives the transducer <NUM> to generate an anti-noise signal (using, for example, a feedback compensator <NUM>) to reduce the effects of a noise signal picked up by the feedback sensor <NUM>. The signal flow topologies can also include an audio path <NUM> that includes circuitry (e.g., an equalizer <NUM>) for processing input audio signals <NUM> such as music or communication signals, for playback over the transducer <NUM>.

In some implementations, the headphone <NUM> can include a feature that may be referred to as "talk-through" or a "hear-through mode. " In such a mode, the feedforward sensor <NUM> or other detection means can be used to detect external sounds that the user might want to hear, and the ANR system <NUM> can be configured to pass such sounds through to be reproduced by the transducer <NUM>. In some cases, the sensor used for the talk-through feature can be a sensor, such as a microphone, that is separate from the feedforward sensor <NUM>. In some implementations, signals captured by multiple sensors can be used (e.g., using a beamforming process) to focus, for example, on the user's voice or another source of ambient sound. In some implementations, the headphone <NUM> can allow for multi-mode operations including a hear-through mode in which the ANR functionality may be switched off or at least reduced, over at least a range of frequencies (e.g., the voice band), to allow relatively wide-band ambient sounds to reach the user. In some implementations, the ANR system <NUM> can also be used to shape a frequency response of the signals passing through the headphones. For instance, the feedforward compensator <NUM> and/or the feedback compensator <NUM> may be used to change an acoustic experience of having an earbud blocking the ear canal to one where ambient sounds (e.g., the user's own voice) sound more natural to the user.

In some implementations, the ANR system <NUM> can allow a user to control the amount of ambient noise passed through the device while maintaining ANR functionalities, such as described in <CIT>. For example, to allow for intermediate target insertion gains between <NUM> and <NUM> and enable a user to control the amount of ambient noise passed through the device, the feedforward compensator <NUM> can include an ANR filter <NUM> and a pass-through filter <NUM> disposed in parallel, with the gain of the pass-through filter being adjustable by a factor C, as shown in <FIG>. The adjustable gain C may be implemented using a variable gain amplifier (VGA) disposed in the pass-through signal flow path of the feedforward compensator <NUM>.

In implementations where the headphone <NUM> includes a hear-through mode, some conditions can lead to the onset of an unstable condition. For example, if the output of the transducer <NUM> gets fed back to the feedforward sensor <NUM>, and the ANR system <NUM> passes the signal back to the transducer <NUM>, a fast-deteriorating unstable condition could occur, resulting in an objectionable sound emanating from the transducer <NUM>. This condition may be demonstrated, for example, by cupping a hand around a headphone to facilitate a feedback path between the transducer <NUM> and the feedforward sensor <NUM>. Such a feedback path may be established during use of the headphone, for example, if the user puts on a headgear (e.g., a head sock or winter hat) over the headphone <NUM>.

In some implementations, the unstable condition can also occur even where the headphone <NUM> does not include a hear-through mode. For example, the unstable condition could occur due to changes in the transfer function of a secondary path (e.g., an acoustic path between the feedback sensor <NUM> and the transducer <NUM>) of the ANR system <NUM>. This can happen, for example, if the acoustic path between the transducer <NUM> and the feedback sensor <NUM> is changed in size or shape. This condition may be demonstrated, for example, by blocking the opening (e.g., using a finger or palm) through which sound emanates out of the headphone <NUM>. In the case of a headphone having a nozzle with an acoustic passageway that acoustically couples a front cavity of an acoustic transducer to a user's ear canal, this condition may be referred to as a blocked-nozzle condition. This condition can result in practice, for example, during placement/removal of the headphone in the ear. This effect may be particularly observable in smaller headphones (e.g., in-ear earphones) or in-ear hearing aids, where the secondary path can change if the earphone or hearing-aid is moved while being worn. For example, moving an in-ear earphone or hearing aid can cause the volume of air in the corresponding secondary path to change, thereby causing the ANR system to be rendered unstable. In some cases, pressure fluctuations in the ambient air can also cause the ANR system to go unstable. For example, when the door or window of a vehicle (e.g., a bus door) is closed, an accompanying pressure change may cause an ANR system to become unstable. Another example of pressure fluctuations that can result in an unstable condition is a significant change in the ambient pressure of air relative to normal atmospheric pressures at sea level.

Unless an unstable condition is quickly detected and addressed, the unstable condition may cause the transducer <NUM> to produce acoustic artifacts (e.g., a loud audible noise), which may be uncomfortable for the wearer. The technology described herein uses multiple feedforward sensors, such as microphones, to improve ANR performance and reduce the likelihood of unstable conditions. In some implementations, when multiple feedforward sensors are used in the ANR system <NUM>, the gain through each of the feedforward paths can be lower as compared to the case where a single feedforward sensor is used. Accordingly, the compensators, filters, and other circuitry in any individual signal path can have a lower overall gain than in the situation where a single feedforward sensor is used. Further, because the gain of any individual signal path is lower than compared to the situation where a single sensor is used, there is more headroom in the system, which results in fewer opportunities for clipping, and provides more margin to prevent instabilities, for example, due to coupling between the feedforward sensors and the transducer. The term headroom, as used herein, refers to the difference between the signal-handling capabilities of an electrical component and the maximum level of the signal in the signal path, such as the feedforward signal path. The reduced gain applied to any individual signal path may also allow the ANR system to better tolerate non-ideal sensor locations, such as sensor locations that are closer to the periphery of the ear-cup <NUM> where the chances of coupling between the sensor and the transducer may be higher as compared to a sensor located at a distance farther away from the periphery of the ear-cup <NUM>.

<FIG> is a block diagram of an ANR system <NUM> according to an example not forming part of the claimed invention, having multiple feedforward sensors 402a, 402b,. , 402N disposed along the feedforward path <NUM>. Each of the feedforward sensors 402a, 402b,. , 402N may be an analog microphone, a digital microphone, or another acoustic sensor, and may be disposed on or near the outside of the ear-cup <NUM> to detect ambient noise. In some implementations, each of the feedforward sensors 402a, 402b,. , 402N may be positioned to detect ambient noise incident from a particular direction and/or to detect certain types or frequencies of ambient noise, such as a user's voice. The number of feedforward sensors included in the ANR system <NUM> can be as few as two sensors. In general, there is no upper bound to the number of feedforward sensors that can be included in the ANR system <NUM>. In some implementations, practical considerations, such as space and cost, may create an upper bound for the number of sensors included in the system. In some implementations, technological limitations of other circuitry in the feedforward path <NUM>, such as the compensator or the transducer, may create an upper bound for the number of sensors included in the system. Although the ANR system <NUM> is described in the context of deployment within the headphone <NUM>, the techniques described herein are equally applicable to ANR systems deployed in other contexts, such as automotive or other transportation systems.

The ambient noise signal produced by each of the feedforward sensors 402a, 402b,. , 402N in the ANR system <NUM> may be combined using a combination circuit <NUM>, such as a summing circuit. It should be understood that the combination circuit <NUM> can perform summation in either the digital or analog domain, and the location of the combination circuit <NUM> can vary along the feedforward signal path <NUM>. While not shown, it should also be understood that the feedforward signal path <NUM> may include additional circuitry such as an amplifier and analog-to-digital converter. The gain of the combined signal may be adjusted by a gain factor Gff using a variable gain amplifier (VGA) <NUM> or other amplification circuitry disposed in the feedforward path <NUM>. The gain factor Gff can be a reduced gain factor relative to a gain factor applied in an ANR system having a single feedforward sensor, as described in detail below. The feedforward compensator <NUM> can process the combined ambient noise signal to produce, for example, an anti-noise signal. In some implementations, the feedforward compensator <NUM> can include an ANR signal flow path disposed in parallel with a pass-through signal flow path to provide at least a portion of the ambient noise to a user, as described with reference to <FIG>. In some implementations, the VGA <NUM> may be included within the feedforward compensator <NUM>. The signal produced by the feedforward compensator <NUM> may be combined with other signals in the ANR system <NUM>, such as the signals from the feedback path <NUM> and/or the audio path <NUM>, and the resultant signal may be provided to the transducer <NUM>.

In some implementations, the gain factor Gff can be selected by the ANR system <NUM> based on the number of the feedforward sensors 402a, 402b,. , 402N present in the system. For example, if the ANR system <NUM> includes two feedforward sensors, the gain factor Gff can be reduced by up to <NUM>%, which in one example could be about <NUM> decibels (dB), relative to an ANR system having a single feedforward sensor. In other cases, if the ANR system <NUM> includes three feedforward sensors, the gain factor Gff can be reduced by up to <NUM>%, which in one example could be about <NUM>-<NUM> dB, relative to an ANR system having a single feedforward sensor. In still other cases, if the ANR system <NUM> includes four feedforward sensors, the gain factor Gff can be reduced by up to <NUM>%, which in one example could be about <NUM> dB relative to an ANR system having a single feedforward sensor.

In some cases, the ANR system <NUM> may adjust the gain factor Gff based on the intended application of the system, requirements of other parts of the system, or other practical considerations. For example, if the ANR system <NUM> includes two feedforward sensors, the gain factor Gff can be reduced by up to <NUM>% relative to an ANR system having a single feedforward sensor, as described above. However, the ANR system <NUM> may reduce the gain by some amount less than <NUM>% relative to an ANR system having a single feedforward sensor to accommodate, for example, signal-level requirements of the feedforward compensator <NUM>.

The lower overall gain reduces the chance that coupling between, for example, the transducer <NUM> and one or more of the feedforward sensors 402a, 402b,. , 402N will lead to an instability. This in turn allows for non-ideal placement of one or more of the feedforward sensors 402a, 402b,. , 402N (e.g., near a location of acoustic leakage that could lead to coupling with the driver, such as near the periphery of the ear-cup or near an acoustic port). Further, combining the ambient noise signals detected by the multiple feedforward sensors may produce a combined ambient noise signal that has a higher signal to noise ratio than an ambient noise signal from a single sensor. For example, when the random noise generated by each feedforward path is uncorrelated to every other feedforward path, the overall combined noise can be reduced by a certain amount (e.g., 3dB) per pair combination while obtaining a higher amount of total signal (e.g., 6dB) per pair combination. This increases the performance of the ANR system <NUM> by, for example, reducing the noise floor and providing a more reliable signal for processing to generate an anti-noise signal.

<FIG> depicts a block diagram of an ANR system <NUM> having multiple feedforward sensors 402a, 402b,. , 402N disposed along the feedforward signal path <NUM>. As shown in <FIG>, each feedforward sensor 402a, 402b,. , 402N can be coupled with a corresponding VGA 502a, 502b,. Each of the VGAs 502a, 502b,. , 502N can be configured to apply a respective gain factor Gff1, Gff2,. , GffN to the ambient noise signal produced by the corresponding feedforward sensor. For example, the VGA 502a can be coupled with the feedforward sensor 402a and can apply a gain factor Gff1 to the signal generated by the feedforward sensor 402a, and so on. This in turn allows for the gains of the different feedforward microphones to be adjusted separately such that microphones that are more susceptible to coupling with a driver has a lower gain as compared to another microphone that is less susceptible to coupling. Also, the total target gain can be distributed across the different microphones such that the total feedforward gain is at a target level. For example, a target gain of unity can be distributed between two feedforward microphones such that a first microphone that is more susceptible to coupling has a gain of <NUM>, while a second microphone that is less susceptible to coupling has a gain of <NUM>.

The signal output by each of the VGAs 502a, 502b,. , 502N is combined using the combination circuit <NUM> (e.g., a circuit including one or more adders). It should be understood that the combination circuit <NUM> can perform summation in either the digital or analog domain, and the location of the combination circuit <NUM> can vary along the feedforward signal path <NUM>. While not shown, it should also be understood that the feedforward signal path <NUM> may include additional circuitry such as an amplifier and analog-to-digital converter. The feedforward compensator <NUM> processes the combined signal to produce an anti-noise signal. In some implementations, the feedforward compensator <NUM> can include an ANR signal flow path disposed in parallel with a pass-through signal flow path to provide at least a portion of the ambient noise to a user, as described with reference to <FIG>. The signal produced by the feedforward compensator <NUM> may be combined with other signals in the ANR system <NUM>, such as the signals from the feedback path <NUM> and/or the audio path <NUM>, and the resultant signal may be provided to the transducer <NUM>. While <FIG> shows the VGAs <NUM> and the combination circuit <NUM> as separate entities from the feedforward compensator <NUM>, in some implementations, the VGAs <NUM> and the combination circuit <NUM> can be included as a part of the feedforward compensator <NUM>.

The individual gain applied by each of the VGAs 502a, 502b,. , 502N, is reduced relative to the gain applied in an ANR system having a single feedforward sensor. This in turn reduces the likelihood of an unstable condition in the system and increases ANR performance. The amount by which the gain is reduced is determined by the ANR system <NUM> based on the number of feedforward sensors present in the system (as described with reference to <FIG>) and/or other factors as described herein. Further, by providing a separate VGA 502a, 502b,. , 502N for each of the feedforward sensors 402a, 402b,. , 402N, the ANR system <NUM> can individually adjust the gain applied to the ambient noise signal produced by the respective feedforward sensor (e.g., through adjustments to Gff1, Gff2,. In doing so, the ANR system <NUM> can exert control over the individual ambient noise signals before they are combined and processed by the feedforward compensator <NUM>, without compromising on a target overall gain of the feedforward path.

Referring to <FIG>, in some implementations according to examples not forming part of the claimed invention, an ANR system <NUM> may include a separate compensator 602a, 602b,. , 602N for each of the feedforward sensors 402a, 402b,. , 402N, respectively. As shown in <FIG>, each compensator 602a, 602b,. , 602N may be coupled with a corresponding feedforward sensor 402a, 402b,. , 402N through the VGA 502a, 502b,. In some implementations, a separate compensator for each feedforward sensor <NUM> allows for separate frequency-dependent filtering and/or gain assignment for the different feedforward paths. For example, if a particular microphone is located near the periphery or port where a coupling to a highfrequency driver is likely, a digital filter can be disposed in the corresponding compensator Kff to reduce the likelihood of such coupling. Such a digital filter can be configured to filter out a portion of the frequency spectrum of the signal captured by the particular microphone to reduce the likelihood of the coupling. In some cases, if the sensors/microphones <NUM> are located far apart from each other on the ear cup or earpiece, the signals captured by the microphones may not be correlated with one another. In such cases, different frequencies can be weighted differently, by applying an individual Kff to each of the microphones.

In some implementations, each compensator 602a, 602b,. , 602N can include the corresponding VGA 502a, 502b,. Each compensator 602a, 602b,. , 602N may include one or more filters, controllers, or other circuitry to process the signal produced by the corresponding feedforward sensor to generate, for example, an anti-noise signal. In some implementations, each compensator 602a, 602b,. , 602N can include an ANR signal flow path disposed in parallel with a pass-through signal flow path to provide at least a portion of the ambient noise to a user, as described with reference to <FIG>. The signals output by each of the compensators 602a, 602b,. , 602N may be combined using the combination circuit <NUM>. It should be understood that the combination circuit <NUM> can perform summation in either the digital or analog domain, and the location of the combination circuit <NUM> can vary along the feedforward signal path <NUM>. While not shown, it should also be understood that the feedforward signal path <NUM> may include additional circuitry such as an amplifier and analog-to-digital converter. The resultant signal may be combined with other signals in the ANR system <NUM>, such as the signals from the feedback path <NUM> and/or the audio path <NUM>, and the resultant signal may be provided to the transducer <NUM>.

<FIG> is a flowchart of an example process for generating a drive signal in an ANR system having multiple acoustic sensors disposed in a signal path. At least a portion of the process <NUM> can be implemented using one or more processing devices such as DSPs described in <CIT> and <CIT>. Operations of the process <NUM> include receiving a first input signal representing audio captured by a first sensor disposed in a signal path of an ANR device (<NUM>). Operations of the process <NUM> also include receiving a second input signal representing audio captured by a second sensor disposed in the signal path of the ANR device (<NUM>). In some implementations, each of the first sensor and the second sensor include a microphone, such as a feedforward microphone of an ANR device. In some implementations, the ANR device can be an around-ear headphone such as the one described with reference to <FIG>. In some implementations, the ANR device can include, for example, in-ear headphones, on-ear headphones, open headphones, hearing aids, or other personal acoustic devices. In some implementations, the audio captured by the first sensor and/or the second sensor can be ambient noise associated with the ANR device. In some implementations, the signal path can be a feedforward signal path of the ANR device. In some implementations, the gain of the signal path can be reduced relative to an ANR signal path having only the first input signal, such as described with reference to <FIG>.

Operations of the process <NUM> further include processing, by at least one compensator and/or variable gain amplifier, the first input signal and the second input signal to generate a drive signal for an acoustic transducer of the ANR device (<NUM>). In some implementations, the at least one compensator can include a feedback compensator and/or a feedforward compensator, such as described with reference to <FIG>. In some implementations, the at least one compensator can include a compensator having an ANR signal flow path disposed in parallel with a pass-through signal flow path to provide at least a portion of the ambient noise to a user, as described with reference to <FIG>. In some implementations, the drive signal may be combined with one or more additional signals (e.g., a signal produced in an audio path of the ANR device) before being provided to the acoustic transducer. The audio output of the acoustic transducer may therefore represent a noise-reduced audio combined with audio representing the ambience as adjusted in accordance with user-preference.

In some implementations, the processing in step <NUM> includes combining the first input signal and the second input signal to generate a combined input signal, applying a gain to the combined input signal using an amplifier, and processing the output of the amplifier using the at least one compensator to generate the drive signal for the acoustic transducer, such as described with reference to <FIG>. In some implementations, the processing includes applying a first gain to the first input signal using a first amplifier, applying a second gain to the second signal using a second amplifier, combining the first input signal and the second input signal to generate a combined input signal, and processing the combined input signal using the at least one compensator to generate the drive signal for the acoustic transducer, such as described with reference to <FIG>. In some implementations, the processing includes processing the first input signal using a first variable gain amplifier and compensator to generate a first processed signal for the acoustic transducer of the ANR device, processing the second input signal using a second variable gain amplifier and compensator to generate a second processed signal for the acoustic transducer of the ANR device, and combining the first processed signal and the second processed signal to generate the drive signal for the acoustic transducer, such as described with reference to <FIG>. In each case, it should be understood that the variable gain amplifier(s) could be included within the respective compensators associated with the respective feedforward signal path.

While <FIG> depict particular example arrangements of components for implementing the technology described herein, other components and/or arrangements of components may be used without deviating from the scope of this disclosure. In some implementations, the arrangement of components along a feedforward path can include an analog microphone, an amplifier, an analog to digital converter (ADC), a digital adder (in case of multiple microphones), a VGA, and a feedforward compensator, in that order. This arrangement is similar to the arrangement of components depicted in <FIG> with the addition of an amplifier and an ADC between each microphone <NUM> and combination circuit <NUM> (which, in this example, includes a digital adder). In some implementations, the arrangement of components along a feedforward path can include an analog microphone, an analog adder (in case of multiple microphones), an ADC, a VGA, and a feedforward compensator. This arrangement is also similar to the arrangement of components depicted in <FIG> with the combination circuit <NUM> including an analog adder, and an ADC disposed between the combination circuit <NUM> and the VGA <NUM>. The arrangement of components can be selected based on target performance parameters. For example, in applications where limiting quantization noise is important, the latter arrangement can be selected because it introduces only a single noise source (an ADC) prior to the gain stage. However this can come at a cost of a dynamic range issue (because of the signals from all microphones passing through a single ADC), which in turn may cause clipping of signals captured by some of the microphones. On the other hand, if avoiding clipping is more important at the cost of potentially more quantization noise, the former arrangement (with an amplifier and an ADC disposed between each microphone <NUM> and combination circuit <NUM>) may be used.

<FIG> is block diagram of an example computer system <NUM> that can be used to perform operations described above. For example, any of the systems <NUM>, <NUM>, and <NUM>, as described above with reference to <FIG>, <FIG>, and <FIG>, respectively, can be implemented using at least portions of the computer system <NUM>. The system <NUM> includes a processor <NUM>, a memory <NUM>, a storage device <NUM>, and an input/output device <NUM>. Each of the components <NUM>, <NUM>, <NUM>, and <NUM> can be interconnected, for example, using a system bus <NUM>. The processor <NUM> is capable of processing instructions for execution within the system <NUM>. In one implementation, the processor <NUM> is a single-threaded processor. In another implementation, the processor <NUM> is a multi-threaded processor. The processor <NUM> is capable of processing instructions stored in the memory <NUM> or on the storage device <NUM>.

In one implementation, the input/output device <NUM> can include one or more network interface devices, e.g., an Ethernet card, a serial communication device, e.g., and RS-<NUM> port, and/or a wireless interface device, e.g., and <NUM> card. In another implementation, the input/output device can include driver devices configured to receive input data and send output data to other input/output devices, e.g., keyboard, printer and display devices <NUM>, and acoustic transducers/speakers <NUM>.

Alternatively or in addition, the program instructions can be encoded on an artificially generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, which is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus.

A computer program, which may also be referred to or described as a program, software, a software application, an app, a module, a software module, a script, or code, can be written in any form of programming language, including compiled or interpreted languages, or declarative or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment.

To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a light emitting diode (LED) or liquid crystal display (LCD) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer.

Other examples and applications not specifically described herein are also within the scope of the following claims. Elements of different implementations described herein may be combined to form other examples not specifically set forth above. Elements may be left out of the structures described herein without adversely affecting their operation. Furthermore, various separate elements may be combined into one or more individual elements to perform the functions described herein. deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a data communication network.

Claim 1:
A method (<NUM>) for implementation in an active noise reduction, ANR, device comprising a plurality of feedforward microphones, the method comprising:
receiving (<NUM>) a first input signal representing audio captured by a first feedforward microphone (402a) disposed in a feedforward signal path (<NUM>) of the active noise reduction (ANR) device (<NUM>);
receiving (<NUM>) a second input signal representing audio captured by a second feedforward microphone (402b) disposed in the feedforward signal path of the ANR device; and
processing (<NUM>), by at least one compensator (<NUM>), the first input signal and the second input signal to generate a drive signal for an acoustic transducer (<NUM>) of the ANR device,
wherein, prior to said processing by the at least one compensator, a gain is applied to the feedforward signal path, the gain being reduced based on the number of feedforward microphones of said plurality and being at least 3dB less relative to an ANR signal path having a single sensor,
the method further comprising:
applying, using a first amplifier (502a), a first gain to the first input signal;
applying, using a second amplifier (502b), a second gain to the second input signal;
combining the first input signal and the second input signal to generate a combined input signal; and
filtering, by the at least one compensator (<NUM>), the combined input signal to generate the drive signal for the acoustic transducer,
wherein the first and second gains are adjusted separately such that the one of the first and second feedforward microphones that is more susceptible to coupling with the acoustic transducer has a lower gain as compared to the other one of the first and second feedforward microphones that is less susceptible to coupling.