Patent Publication Number: US-10762915-B2

Title: Systems and methods of detecting speech activity of headphone user

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 121 as a division of U.S. patent application Ser. No. 15/463,259, titled SYSTEMS AND METHODS OF DETECTING SPEECH ACTIVITY OF HEADPHONE USER, filed Mar. 20, 2017, which is incorporated by reference herein in its entirety for all purposes. 
    
    
     BACKGROUND 
     Headphone systems are used in numerous environments and for various purposes, examples of which include entertainment purposes such as gaming or listening to music, productive purposes such as phone calls, and professional purposes such as aviation communications or sound studio monitoring, to name a few. Different environments and purposes may have different requirements for fidelity, noise isolation, noise reduction, voice pick-up, and the like. In some environments or in some applications it may be desirable to detect when the user of the headphones or headset is actively speaking. 
     SUMMARY OF THE INVENTION 
     Aspects and examples are directed to headphone systems and methods that detect voice activity of a user. The systems and methods detect when a user is actively speaking, while ignoring audible sounds that are not due to the user speaking, such as other speakers or background noise. Detection of voice activity by the user may be beneficially applied to further functions or operational characteristics. For example, detecting voice activity by the user may be used to cue an audio recording, to cue a voice recognition system, activate a virtual personal assistant (VPA), trigger automatic gain control (AGC), acoustic echo processing or cancellation, noise suppression, sidetone gain adjustment, or other voice operated switch (VOX) applications. Aspects and examples disclosed herein may improve headphone use and reduce false-triggering by noise or other people talking by targeting voice activity detection of the wearer of the headphones. 
     According to one aspect, a headphone system is provided and includes a left and right earpiece, a left microphone is coupled to the left earpiece to receive a left acoustic signal and to provide a left signal derived from the left acoustic signal, a right microphone is coupled to the right earpiece to receive a right acoustic signal and to provide a right signal derived from the right acoustic signal, and a detection circuit is coupled to the left microphone and the right microphone and is configured to compare a principal signal to a reference signal, the principal signal derived from a sum of the left signal and the right signal and the reference signal derived from a difference between the left signal and the right signal, and to selectively indicate that the user is speaking based at least in part upon the comparison. 
     In some examples the detection circuit is configured to indicate the user is speaking when the principal signal exceeds the reference signal by a threshold. In some examples the detection circuit is configured to compare the principal signal to the reference signal by comparing a power content of each of the principal signal and the reference signal. 
     According to some examples the principal signal and the reference signal are each band filtered. 
     In certain examples at least one of the left microphone and the right microphone comprises a plurality of microphones and the respective left signal or right signal is derived from the plurality of microphones, at least in part, as a combination of outputs from one or more of the plurality of microphones. 
     Some examples further include a rear microphone coupled to either earpiece and positioned to receive a rear acoustic signal, the rear acoustic signal being toward the rear of the user&#39;s head relative to either or both of the left acoustic signal and the right acoustic signal, and the detection circuit is further configured to compare a rear signal derived from the rear microphone to at least one of the left signal and the right signal to generate a rear comparison, and to selectively indicate that the user is speaking further based upon the rear comparison. In further examples the detection circuit may indicate the user is speaking when the principal signal exceeds the reference signal by a first threshold and the at least one of the left signal and the right signal exceeds the rear signal by a second threshold. 
     According to another aspect, a headphone system is provided and includes an earpiece, a front microphone coupled to the earpiece to receive a first acoustic signal, a rear microphone coupled to the earpiece to receive a second acoustic signal, the second acoustic signal being toward the rear of a user&#39;s head relative to the first acoustic signal, and a detection circuit coupled to the front and rear microphones and configured to compare a front signal derived from the front microphone to a rear signal derived from the rear microphone, and to selectively indicate that the user is speaking based at least in part upon the comparison. 
     In some examples the detection circuit is configured to indicate the user is speaking when the front signal exceeds the rear signal by a threshold. In some examples the detection circuit is configured to compare the front signal to the rear signal by comparing a power content of each of the front signal and the rear signal. 
     In certain examples the front and rear signals are band filtered. 
     According to some examples the front microphone comprises a plurality of microphones and the front signal is derived from the plurality of microphones, at least in part, as a combination of outputs from one or more of the plurality of microphones. 
     Some examples include a second earpiece, a second front microphone coupled to the second earpiece to receive a third acoustic signal, and a second rear microphone coupled to the second earpiece to receive a fourth acoustic signal, the fourth acoustic signal being toward the rear of the user&#39;s head relative to the third acoustic signal. In these examples the detection circuit is further configured to perform a second comparison comprising comparing a second front signal derived from the second front microphone to a second rear signal derived from the second rear microphone, and to selectively indicate that the user is speaking based at least in part upon the first comparison and the second comparison. 
     Some examples include a second earpiece and a third microphone coupled to the second earpiece to receive a third acoustic signal and provide a third signal, and the detection circuit is further configured to combine the third signal with a selected signal, the selected signal being one of the front signal and the rear signal, determine a difference between the third signal and the selected signal, perform a second comparison comprising comparing the combined signal to the determined signal, and selectively indicate that the user is speaking based at least in part upon the second comparison. 
     According to another aspect, a method of determining that a headphone user is speaking is provided and includes receiving a first signal derived from a first microphone, receiving a second signal derived from a second microphone, providing a principal signal derived from a sum of the first signal and the second signal, providing a reference signal derived from a difference between the first signal and the second signal, comparing the principal signal to the reference signal, and selectively indicating that a user is speaking based at least in part upon the comparison. 
     In some examples, comparing the principal signal to the reference signal comprises comparing whether the principal signal exceeds the reference signal by a threshold. In some examples, comparing the principal signal to the reference signal comprises comparing a power content of each of the principal signal and the reference signal. 
     Some examples include filtering at least one of the first signal, the second signal, the principal signal, and the reference signal. 
     In certain examples the first signal is derived from a plurality of first microphones at least in part as a combination of outputs from one or more of the plurality of first microphones. 
     Some examples further include receiving a third signal derived from a third microphone, comparing the third signal to at least one of the first signal and the second signal to generate a second comparison, and selectively indicating that the user is speaking based at least in part upon the second comparison. 
     Still other aspects, examples, and advantages of these exemplary aspects and examples are discussed in detail below. Examples disclosed herein may be combined with other examples in any manner consistent with at least one of the principles disclosed herein, and references to “an example,” “some examples,” “an alternate example,” “various examples,” “one example” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one example. The appearances of such terms herein are not necessarily all referring to the same example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of at least one example are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide illustration and a further understanding of the various aspects and examples, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of the invention. In the figures, identical or nearly identical components illustrated in various figures may be represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures: 
         FIG. 1  is a perspective view of a headphone set; 
         FIG. 2  is a left-side view of a headphone set; 
         FIG. 3  is a flow chart of an example method to compare signal energy to detect voice activity; 
         FIG. 4  is a flow chart of another example method to compare signal energy to detect voice activity; 
         FIG. 5  is a schematic diagram of an example system to detect voice activity; 
         FIG. 6  is a schematic diagram of another example system to detect voice activity; and 
         FIG. 7  is a schematic diagram of another example system to detect voice activity. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present disclosure are directed to headphone systems and methods that detect voice activity by the user (e.g., wearer) of a headphone set. Such detection may enhance voice activated features or functions available as part of the headphone set or other associated equipment, such as a cellular telephone or audio processing system. Examples disclosed herein may be coupled to, or placed in connection with, other systems, through wired or wireless means, or may be independent of any other systems or equipment. 
     The headphone systems disclosed herein may include, in some examples, aviation headsets, telephone headsets, media headphones, and network gaming headphones, or any combination of these or others. Throughout this disclosure the terms “headset,” “headphone,” and “headphone set” are used interchangeably, and no distinction is meant to be made by the use of one term over another unless the context clearly indicates otherwise. Additionally, aspects and examples in accord with those disclosed herein, in some circumstances, may be applied to earphone form factors (e.g., in-ear transducers, earbuds), and are therefore also contemplated by the terms “headset,” “headphone,” and “headphone set.” Advantages of some examples include low power consumption while monitoring for user voice activity, high accuracy of detecting the user&#39;s voice, and rejection of voice activity of others. 
     Examples disclosed herein may be combined with other examples in any manner consistent with at least one of the principles disclosed herein, and references to “an example,” “some examples,” “an alternate example,” “various examples,” “one example” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described may be included in at least one example. The appearances of such terms herein are not necessarily all referring to the same example. 
     It is to be appreciated that examples of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other examples and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. Any references to front and back, left and right, top and bottom, upper and lower, and vertical and horizontal are intended for convenience of description, not to limit the present systems and methods or their components to any one positional or spatial orientation. 
       FIG. 1  illustrates one example of a headphone set. The headphones  100  include two earpieces, e.g., a right earcup  102  and a left earcup  104 , coupled to a right yoke assembly  108  and a left yoke assembly  110 , respectively, and intercoupled by a headband  106 . The right earcup  102  and left earcup  104  include a right circumaural cushion  112  and a left circumaural cushion  114 , respectively. Visible on the left earcup  104  is a left interior surface  116 . While the example headphones  100  are shown with earpieces having circumaural cushions to fit around or over the ear of a user, in other examples cushions may sit on the ear, or may include earbud portions that protrude into a portion of a user&#39;s ear canal, or may include alternate physical arrangements. As discussed in more detail below, each of the earcups  102 ,  104  include one or more microphones, such as one or more front microphones, one or more rear microphones, and/or one or more interior microphones. Although the example headphones  100  illustrated in  FIG. 1  include two earpieces, some examples may include only a single earpiece for use on one side of the head only. Additionally, although the example headphones  100  illustrated in  FIG. 1  include a headband  106 , other examples may include different support structures to maintain one or more earpieces (e.g., earcups, in-ear structures, etc.) in proximity to a user&#39;s ear, e.g., an earbud may include a shape and/or materials configured to hold the earbud within a portion of a user&#39;s ear. 
       FIG. 1  and  FIG. 2  illustrate multiple example placements of microphones, any one or more of which may be included in certain examples.  FIG. 1  illustrates an interior microphone  120  in the interior of the left earcup  104 . In some examples, an interior microphone may additionally or alternatively be included in the interior of the right earcup  102 , either earcup may have multiple interior microphones, or neither earcup may have an interior microphone.  FIG. 2  illustrates the headphones  100  from the left side and shows details of the left earcup  104  including a pair of front microphones  202 , which may be nearer a front edge  204  of the earcup, and a rear microphone  206 , which may be nearer a rear edge  208  of the earcup. The right earcup  102  may additionally or alternatively have a similar arrangement of front and rear microphones, though in examples the two earcups may have a differing arrangement in number or placement of microphones. Additionally, various examples may have more or fewer front microphones  202  and may have more, fewer, or no rear microphones  206 . While the reference numerals  120 ,  202 , and  206  are used to refer to one or more microphones, the visual element illustrated in the figures may, in some examples, represent an acoustic port wherein acoustic signals enter to ultimately reach the microphones  120 ,  202 ,  206 , which may be internal and not physically visible from the exterior. In examples, one or more of the microphones  120 ,  202 ,  206  may be immediately adjacent to the interior of an acoustic port, or may be removed from an acoustic port by a distance, and may include an acoustic waveguide between an acoustic port and an associated microphone. 
     Various microphone signals will be processed in various ways to detect whether a user of the headphones  100 , i.e., a person wearing the headphones, is actively speaking. Detection of a user speaking will sometime be referred to as voice activity detection (VAD). As used herein, the terms “voice,” “speech,” “talk,” and variations thereof are used interchangeably and without regard for whether such speech involves use of the vocal folds. 
     Examples disclosed herein to detect user voice activity may operate or rely on various principles of the environment, acoustics, vocal characteristics, and unique aspects of use, e.g., an earpiece worn or placed on each side of the head of a user whose voice activity is to be detected. For example, in a headset environment, a user&#39;s voice generally originates at a point symmetric to the left and right sides of the headset and will arrive at both a right front microphone and a left front microphone with substantially the same amplitude at substantially the same time and substantially the same phase, whereas background noise and vocalizations of other people will tend to be asymmetrical between the left and right, having variation in amplitude, phase, and time. Additionally, a user&#39;s voice originates in a near-field of the headphones and will arrive at a front microphone with more acoustic energy than it will arrive at a rear microphone. Background noise and vocalizations of other people originating farther away may tend to arrive with substantially the same acoustic energy at front and rear microphones. Further, background noise and vocalizations from people that originate farther away than the user&#39;s mouth will generally cause acoustic energy received at any of the microphones to be at a particular level, and the acoustic energy level will increase when the user&#39;s voice activity is added to these other acoustic signals. Accordingly, a user&#39;s voice activity will cause an increase in average acoustic energy at any of the microphones, which may be beneficially used to apply a threshold to voice activity detection. Various spectral characteristics can also play a beneficial role in detecting a user&#39;s voice activity. 
       FIG. 3  illustrates a method  300  of processing microphone signals to detect a likelihood that a headphone user is actively speaking. The example method  300  shown in  FIG. 3  relies on processing and comparing characteristics of binaural, i.e., left and right, signals. As discussed above, left and right vocal signals due to the user&#39;s voice are substantially symmetric with each other and may be substantially identical due to the substantially equidistant position of left and right microphones from the user&#39;s mouth. The method of  FIG. 3  processes a left signal  302  and a right signal  304  by adding them together to provide a principal signal  306 . The method of  FIG. 3  also processes the left signal  302  and the right signal  304  by subtracting them to provide a reference signal  308 . The left and right signals  302 ,  304  are each provided by, and received from, microphones on the left and right sides of the headphones, respectively, and may come from multiple microphones on each side. For example, a left side may have one microphone or may have multiple microphones, as discussed above, and the left signal  302  may be provided by a single microphone on the left side or may be a combination of signals from multiple microphones on the left side. In the case of multiple microphones on the left side, the left signal  302  may be provided from a steered beam formed by processing the multiple microphones, e.g., as a phased array, or may be a simple combination (e.g., addition) of signals from the multiple microphones, or may be provided through other signal processing. Similarly, the right signal  304  may be provided by a single microphone, a combination of multiple microphones, or an array of microphones, all on the right side. 
     As discussed above, the left signal  302  and the right signal  304  are added together to provide a principal signal  306 , and the right signal  304  is subtracted from the left signal  302  to provide a reference signal  308 . Alternatively the left signal  302  may instead be subtracted from the right signal  304  to provide the reference signal  308 . If the user of the headphones is talking, the user&#39;s voice will be substantially equal in both the left signal  302  and the right signal  304 . Accordingly, the left signal  302  and the right signal  304  constructively combine in the principal signal  306 . In the reference signal  308 , however, the user&#39;s voice may substantially cancel itself out in the subtraction, i.e., destructively interferes with itself. Accordingly, when the user is talking, the principal signal  306  will include a user voice component with approximately double the signal energy of either of the left signal  302  or the right signal  304  individually; while the reference signal  308  will have substantially no component from the user&#39;s voice. This allows a comparison of the principal signal  306  and the reference signal  308  to provide an indication whether the user is talking. 
     Components of the left signal  302  and the right signal  304  that are not associated with the user&#39;s voice are unlikely to be symmetric between the left and right sides and will tend neither to reinforce nor interfere with each other, whether added or subtracted. In this manner, the principal signal  306  and the reference signal  308  will have approximately the same signal energy for components that are not associated with the user&#39;s voice. For example, signal components from surrounding noise, other talkers at a distance, and other talkers not equidistant from the left and right sides, even if nearby, will be of substantially the same signal energy in the principal signal  306  and the reference signal  308 . Substantially, the reference signal  308  provides a reference of the surrounding acoustic energy not including the user&#39;s voice, whereas the principal signal  306  provides the same components of surrounding acoustic energy but also includes the user&#39;s voice when the user is talking. Accordingly, if the principal signal  306  has sufficiently more signal energy than the reference signal  308 , it may be concluded that the user is talking. 
     With continued reference to  FIG. 3 , each of the principal signal  306  and the reference signal  308  are processed through a smoothing algorithm  310 . The smoothing algorithm  310  may take many forms, or may be absent altogether in some examples, and the details of the smoothing algorithm  310  shown in  FIG. 3  merely represent one example of a smoothing algorithm. The example smoothing algorithm  310  of  FIG. 3  generates a slowly-changing indicator of average energy/power content of an input signal, e.g., the principal signal  306  or the reference signal  308 . At least one benefit of a smoothing algorithm is to prevent sudden changes in the acoustic environment from causing an erroneous indication that the user is talking. The smoothing algorithm  310  processes the signals to measure a power of each signal, at block  312 , and calculates a decaying weighted average of each signal&#39;s power measurements over time, at block  318 . The weighted average of current and previous power measurements may be based upon some characteristic value, e.g., an alpha value or time constant, selected at block  316 , that impacts the weighting, and the selection of the alpha value may be dependent upon whether the current power measure is increasing or decreasing, determined at block  314 . The smoothing algorithm  310  acting upon each of the principal signal  306  and the reference signal  308  provides a principal power signal  320  and a reference power signal  322 , respectively. 
     In certain examples, the principal signal  306  may be directly compared to the reference signal  308 , and if the principal signal  306  has larger amplitude a conclusion is made that the user is talking. In other examples, the principal power signal  320  and the reference power signal  322  are compared, and a determination that the user is talking is made if the principal power signal  320  has larger amplitude. In certain examples, a threshold is applied to require a minimum signal differential, to provide a confidence level that the user is in fact talking. In the example method  300  shown in  FIG. 3 , a threshold is applied by multiplying the reference power signal  322  by a threshold value at block  324 . For example, a certain confidence level may be had that the user is talking if the principal power signal  320  is at least 8% higher than the reference power signal  322 , and in such case the reference power signal  322  may be multiplied by 1.08 at block  324  to provide a threshold power signal  326 . The principal power signal  320  is then compared to the threshold power signal  326  at block  328 . If the principal power signal  320  is higher than the threshold power signal  326  it is determined that the user is talking, otherwise it is determined that the user is not talking. Various confidence levels may be selected via selection of a threshold value. For example, in various examples, a threshold value may include any value in a range of 2% to 30%, i.e., various examples test whether the principal power signal  320  is greater than the reference power signal  322  by, e.g., 2% to 30%, which may be achieved by multipliers of, e.g., from 1.02 to 1.30, applied to the reference power signal  322  at block  324  to provide the threshold power signal  326  to the comparison at block  328 . 
     In other examples, the smoothed principal signal  320  may be multiplied by a threshold value (e.g., less than unity) rather than, or in addition to, the reference power signal  322  being multiplied by a threshold value. In certain examples, a comparison between a principal signal and a reference signal in accord with any of the principal and reference signals discussed above may be achieved by taking a ratio of the principal signal to the reference signal, and the ratio may be compared to a threshold, e.g., unity, 1.08, or any of a range of values such as from 1.02 to 1.30, or otherwise. The example method  300  of  FIG. 3 , however, which multiplies one of the signals by a threshold value prior to a direct comparison, may require less computational power or fewer processing resources than would a method that calculates a ratio and compares the ratio to a fractional threshold. 
     In certain examples, a method of processing microphone signals to detect a likelihood that a headphone user is actively speaking, such as the example method  300 , may include band filtering or sub-band processing. For example, the left and right signals  302 ,  304  may be filtered to remove frequency components not part of a typical voice or vocal tract range, prior to processing by, e.g., the example method  300 . Further, the left and right signals  302 ,  304  may be separated into frequency sub-bands, and one or more of the frequency sub-bands may be separately processed by, e.g., the example method  300 . Either of filtering or sub-band processing, or a combination of the two, may decrease the likelihood of a false positive caused by extraneous sounds not associated with the user&#39;s voice. However, either of filtering or sub-band processing may require additional circuit components at additional cost, and/or may require additional computational power or processing resources, therefore consuming more energy from a power source, e.g., a battery. In certain examples, filtering may provide a good compromise between accuracy and power consumption. 
     The method  300  of  FIG. 3  discussed above is an example method of detecting a user&#39;s voice activity based on processing and comparison of binaural, i.e., left and right, input signals. An additional method in accord with aspects and examples disclosed herein to detect a user&#39;s voice activity involves a front signal and a rear signal. An example method  400  is illustrated with reference to  FIG. 4 . The example method  400  receives a front signal  402  and a rear signal  404  and compares their relative weighted average power to determine whether a user is speaking. 
     When a user wearing headphones speaks, acoustic energy from the user&#39;s voice will reach a front microphone (on either side, e.g., the left earcup or the right earcup) with greater intensity than it reaches a rear microphone. Many factors influence the difference in acoustic intensity reaching the front microphone versus the rear microphone. For example, the rear microphone is farther away from the user&#39;s mouth, and both microphones are located in a near-field region of the user&#39;s voice, causing distance variation to have significant effect as the acoustic intensity decays proportional to distance cubed. An acoustic shadow is also created by the user&#39;s head and the existence of the earcup and yoke assembly, which further contribute to a lower acoustic intensity arriving at the rear microphone. Acoustic energy from background noise and from other talkers will tend to have substantially the same acoustic intensity arriving at the front and rear microphones, and therefore a difference in signal energy between the front and rear may be used to detect that a user is speaking. The example method  400  accordingly processes and compares the energy in the front signal  402  to the energy in the rear signal  404  in a similar manner to how the example method  300  processes and compares a principal signal  306  and a reference signal  308 . 
     The front and rear signals  402 ,  404  are each provided by, and received from, front and rear microphones, respectively, on a single side of the headphones, e.g., either the left earcup or the right earcup. For example, a left front signal  402  may come from either front microphone  202  as shown in  FIG. 2  (which is a left side view), or may be a combination of outputs from multiple left-side front microphones, or there may be only a single left front microphone. A left rear signal  404  may come from the rear microphone  206  shown in  FIG. 2 , or a combination (as discussed above) of rear microphones (not shown). 
     Each of the front signal  402  and the rear signal  404  may be processed by a smoothing algorithm  310 , as discussed above, to provide a front power signal  420  and a rear power signal  422 , respectively. The rear power signal  422  may optionally be multiplied by a threshold at block  424 , similar to the threshold applied at block  324  in the example method  300  discussed above, to provide a threshold power signal  426 . The front power signal  420  is compared to the threshold power signal  426  at block  428 , and if the front power signal  420  is greater than the threshold power signal  426 , the method  400  determines that the user is speaking; otherwise the method  400  determines that the user is not speaking. Certain examples may include variations or absence of the smoothing algorithm  310 , as discussed above with respect to the example method  300 , and certain examples may include differing approaches to making a comparison, e.g., by calculating a ratio or by application of threshold, similar to such variations discussed above with respect to the example method  300 . 
     While reference has been made to a number of power signals, e.g., principal and reference power signals  320 ,  322  and front and rear power signals  420 ,  422 , the signals provided for comparison in the example methods of  FIGS. 3-4  may be measures of power, energy, amplitude, or other measurable indicators of signal strength suitable for making comparisons as described or otherwise drawing conclusions as to the user vocal content of the various signals. 
     One or more of the above described methods, in various examples and combinations, may be used to detect that a headphone user is actively talking, e.g., to provide voice activity detection. Any of the methods described may be implemented with varying levels of reliability based on, e.g., microphone quality, microphone placement, acoustic ports, headphone frame design, threshold values, selection of smoothing algorithms, weighting factors, window sizes, etc., as well as other criteria that may accommodate varying applications and operational parameters. Any example of the methods described above may be sufficient to adequately detect a user&#39;s voice activity for certain applications. Improved detection may be achieved, however, by a combination of methods, such as examples of those described above, to incorporate concurrence and/or confidence level among multiple methods or approaches. 
     One example of a combinatorial system  500  for user voice activity detection is illustrated by the block diagram of  FIG. 5 . The example system  500  of  FIG. 5  includes front and rear microphones on each of a left and right side of a headphone set. The microphones provide a left front signal  502 , a right front signal  504 , a left rear signal  506  and a right rear signal  508 . As discussed above, any of the microphones may be a set of multiple microphones whose output signals may be combined in various ways. The left front signal  502  and right front signal  504  may be processed by a binaural detector  510  implementing an example of the binaural detection method exemplified by the method  300  above to produce a binary output  512  indicating user voice activity or not. The left front signal  502  and the left rear signal  506  may be processed by a first front-to-rear detector  520  implementing an example of the front-to-rear detection method exemplified by the method  400  above to produce a binary output  522  indicating user voice activity or not. Similarly, the right front signal  504  and right rear signal  508  may be processed by a second front-to-rear detector  530  implementing an example of front-to-rear detection (exemplified by the method  400  above) to produce a binary output  532  indicating user voice activity or not. 
     Any of the binary outputs  512 ,  522 , or  532  may reliably indicate user voice activity, but they may be further combined by logic  540  to provide a more reliable combined output  550  to indicate detection of user voice activity. In the example system  500  of  FIG. 5 , the logic  540  is shown as an AND logic that requires all three binary outputs  512 ,  522 , and  532  to indicate user voice activity to provide a combined output  550  that indicates user voice activity. Other examples may include different combinatorial logic  540 . For example, in certain examples the combined output  550  may require only two of the three binary outputs  512 ,  522 , and  532  to indicate user voice activity to provide a combined output  550  that indicates user voice activity. In other examples, one of the binary outputs  512 ,  522 ,  532  may have precedence over the other two, i.e., unless the other two agree in a specified result. In examples, there may be differing number or types of detectors (e.g., detectors  510 ,  520 ,  530 ) and there may be more or fewer binary outputs based upon the number and type of detectors included. 
     For example,  FIG. 6  illustrates a combinatorial system  600  similar to that of system  500  but including a different combinatorial logic  640 . In the example system  600 , the combinatorial logic  640  includes AND logic  642  to indicate user voice activity if both the left and right front-to-rear detectors  620 ,  630  indicate user voice activity, and OR logic  644  to provide an overall combined output  650  to indicate user voice activity if either the binaural detector  610  or the combination of left and right front-to-rear detectors  620 ,  630  indicate user voice activity. 
     Additional types of detectors include at least a threshold detector and an interior sound detector. A threshold detector may detect a general threshold sound level, and may provide a binary output to indicate that the general sound level in the vicinity of the headphones is high enough that a user may be talking. Alternately, a threshold detector may indicate that the general sound level has increased recently such that a user may be talking. The binary output of a threshold detector, or any detector disclosed herein, may be taken as an additional input to a combined output  550 , or may be used as an enable signal to other detectors. Accordingly, various detectors could remain in an off state or consume lower power so long as a certain detector, e.g., a threshold detector, or combination of detectors, indicates no user voice activity. 
     An interior sound detector may detect sound levels inside one or both earcups, such as from one or more interior microphones  120  (see  FIG. 1 ) positioned in the interior of an earcup. An interior microphone is especially robust against wind noise and is also robust against other sounds because the interior microphone may be physically isolated from the exterior of the headphones. The signal level of an interior microphone may be monitored to determine if a user is speaking. When a user speaks, the signal at the interior microphone increases due to acoustic conduction through bones, nasal cavity, etc., and the signal level at the interior microphone may be measured and compared to a threshold to determine if a user&#39;s voice is present, or to confirm (e.g., enhanced confidence level) determination of voice activity by other detectors. 
     As discussed above, filtering or sub-band processing may also enhance the operation of a voice activity detection system in accord with aspects and examples described herein. In one example, microphone signals may be filtered to be band-limited to a portion of the spectrum for which a user&#39;s head creates a substantial head shadow, i.e., frequencies that will have a significant front-to-rear differential for sounds coming from in front or behind, and a significant left-to-right differential for sounds coming from the side. In certain examples, one or more of the various microphone signals is band-pass filtered to include a frequency band substantially from about 800 Hertz to 2,000 Hertz prior to processing by one or more of the various detectors described herein. 
       FIG. 7  illustrates an example of a system  700  incorporating multiple examples of the various detection methods and combinatorial logic discussed above. In the example system  700  there are one or more front, rear, and interior microphones  702  in each of the left and right earcups of a headphone set. Signals from any of the microphones  702  may be processed by a filter  704  to, e.g., remove non-vocal frequency bands, or to limit a frequency range expected to have substantial differentials as discussed above. A threshold detector  706  may monitor any one or more of the microphones  702  and enable any of the detectors  710 ,  720 ,  730 , and/or  740 , when there is sufficient sound level, or change in sound level, that indicate a user may be speaking. As discussed above, a threshold detector may conserve energy because the detectors  710 ,  720 ,  730 , and/or  740  may remain off whenever the sound environment exhibits characteristics, e.g., lacking spectral content or too quiet, that a user is likely not talking. The binaural detector  710  may be any example of binaural detectors as discussed above, or variations thereof, and the left and right front-to-rear detectors  720 ,  730 , may be any example of front-to-rear detectors as discussed above, or variations thereof. The example system  700  also includes an interior detector  740  that compares one or more signals from one or more of the interior microphones  702  to a threshold level to indicate a likelihood that the user is speaking. Binary outputs from each of the detectors  710 ,  720 ,  730 , and  740 , are provided to a combinatorial logic  750  to provide a combined output  760 . It is to be understood that the example system  700  of  FIG. 7  is meant to be merely illustrative of an example of a system that incorporates many of the aspects and examples of the systems and methods disclosed herein, and is not presented as a primary or preferred example. Multiple variations of combinatorial logic, number and types of microphones, number and types of detectors, threshold values, filters, etc. are contemplated by examples in accord with systems and methods disclosed herein. 
     It is to be understood that any of the functions of methods  300 ,  400 , or similar, and any components of the systems  500 ,  600 ,  700 , or similar, may be implemented or carried out in a digital signal processor (DSP), a microprocessor, a logic controller, logic circuits, and the like, or any combination of these, and may include analog circuit components and/or other components with respect to any particular implementation. Functions and components disclosed herein may operate in the digital domain and certain examples include analog-to-digital (ADC) conversion of analog signals generated by microphones, despite the lack of illustration of ADC&#39;s in the various figures. Any suitable hardware and/or software, including firmware and the like, may be configured to carry out or implement components of the aspects and examples disclosed herein, and various implementations of aspects and examples may include components and/or functionality in addition to those disclosed. 
     Having described above several aspects of at least one example, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.