Patent Publication Number: US-8989417-B1

Title: Method and system for implementing stereo audio using bone conduction transducers

Description:
BACKGROUND 
     Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
     Computing systems such as personal computers, laptop computers, tablet computers, cellular phones, and countless types of Internet-capable devices are prevalent in numerous aspects of modern life. Over time, the manner in which these devices are providing information to users is becoming more intelligent, more efficient, more intuitive, and/or less obtrusive. 
     The trend toward miniaturization of computing hardware, peripherals, as well as of sensors, detectors, and image and audio processors, among other technologies, has helped open up a field sometimes referred to as “wearable computing.” In the area of image and visual processing and production, in particular, it has become possible to consider wearable displays that place a very small image display element close enough to a wearer&#39;s (or user&#39;s) eye(s) such that the displayed image fills or nearly fills the field of view, and appears as a normal sized image, such as might be displayed on a traditional image display device. The relevant technology may be referred to as “near-eye displays.” 
     Near-eye displays are fundamental components of wearable displays, also sometimes called “head-mounted displays” or “head-mountable devices” (HMDs). A head-mounted display places a graphic display or displays close to one or both eyes of a wearer. To generate the images on a display, a computer processing system may be used. Such displays may occupy part or all of a wearer&#39;s field of view. Further, head-mounted displays may be as small as a pair of glasses or as large as a helmet. 
     SUMMARY 
     In one aspect, the present application describes a method. The method may comprise a wearable computing device receiving a first audio signal effective to cause the wearable computing device to provide a first sound to a first ear and at least a portion of the first sound to a second ear. The method may also comprise the wearable computing device receiving a second audio signal that is out of phase with the first audio signal and effective to substantially cancel at least a portion of the first audio signal, where the second audio signal is based on a transform applied by the wearable computing device to the first audio signal, the transform being based on one or more wearer-specific parameters. The method may further comprise, based on the first audio signal, the wearable computing device causing a first bone conduction transducer (BCT) coupled to the wearable computing device to vibrate so as to provide the first sound to the first ear and provide the portion of the first sound to the second ear. The method may still further comprise, based on the second audio signal, the wearable computing device causing a second BCT coupled to the wearable computing device to vibrate substantially simultaneous to the vibration of the first BCT so as to provide a second sound to the second ear, the second sound being effective to substantially cancel the portion of the first sound. 
     In another aspect, the present application describes a non-transitory computer readable medium having stored thereon executable instructions that, upon execution by a wearable computing device, cause the wearable computing device to perform functions. The functions may comprise receiving a first audio signal effective to cause the wearable computing device to provide a first sound to a first ear and at least a portion of the first sound to a second ear. The functions may also comprise receiving a second audio signal that is out of phase with the first audio signal and effective to substantially cancel at least a portion of the first audio signal, where the second audio signal is based on a transform applied by the wearable computing device to the first audio signal, the transform being based on one or more wearer-specific parameters. The functions may further comprise, based on the first audio signal, causing a first bone conduction transducer (BCT) coupled to the wearable computing device to vibrate so as to provide the first sound to the first ear and provide the portion of the first sound to the second ear. The functions may still further comprise, based on the second audio signal, causing a second BCT coupled to the wearable computing device to vibrate substantially simultaneous to the vibration of the first BCT so as to provide a second sound to the second ear, the second sound being effective to substantially cancel the portion of the first sound. 
     In yet another aspect, the present application describes a system. The system may comprise a head-mountable device (HMD) and at least one processor coupled to the HMD. The system may also comprise data storage comprising instructions executable by the at least one processor to cause the system to perform functions. The functions may comprise receiving a first audio signal effective to cause the HMD to provide a first sound to a first ear and at least a portion of the first sound to a second ear opposite the first ear. The functions may also comprise receiving a second audio signal that is about 180 degrees out of phase with the first audio signal and effective to substantially cancel at least a portion of the first audio signal, where the second audio signal is based on a transform applied by the HMD to the first audio signal, the transform being based on one or more wearer-specific parameters. The functions may further comprise, based on the first audio signal, causing at least one first bone conduction transducer (BCT) coupled to the HMD to vibrate so as to provide the first sound to the first ear and provide the portion of the first sound to the second ear. The functions may still further comprise, based on the second audio signal, causing at least one second BCT coupled to the HMD to vibrate substantially simultaneous to the vibration of the at least one first BCT so as to provide a second sound to the second ear, the second sound being effective to substantially cancel the portion of the first sound. 
     These as well as other aspects, advantages, and alternatives will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. Further, it should be understood that this summary and other descriptions and figures provided herein are intended to illustrative embodiments by way of example only and, as such, that numerous variations are possible. For instance, structural elements and process steps can be rearranged, combined, distributed, eliminated, or otherwise changed, while remaining within the scope of the embodiments as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  illustrates a wearable computing system according to at least some embodiments described herein. 
         FIG. 1B  illustrates an alternate view of the wearable computing system illustrated in  FIG. 1A . 
         FIG. 1C  illustrates another wearable computing system according to at least some embodiments described herein. 
         FIG. 1D  illustrates another wearable computing system according to at least some embodiments described herein. 
         FIGS. 1E-1G  are simplified illustrations of the wearable computing system shown in  FIG. 1D , being worn by a wearer. 
         FIG. 2  illustrates a schematic drawing of a computing device according to at least some embodiments described herein. 
         FIG. 3  is a flow chart of an example method according to at least some embodiments described herein. 
         FIG. 4  is a block diagram of a system for implementing the example method, in accordance with at least some embodiments described herein. 
         FIGS. 5A-5D  illustrate various configurations of a simplified system for measuring a transform, in accordance with at least some embodiments described herein. 
         FIG. 6  is a block diagram of a more detailed system for measuring a transform, in accordance with at least some embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Example methods and systems are described herein. It should be understood that the words “example” and “exemplary” are used herein to mean “serving as an example, instance, or illustration.” Any embodiment or feature described herein as being an “example” or “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or features. In the following detailed description, reference is made to the accompanying figures, which form a part thereof. In the figures, similar symbols typically identify similar components, unless context dictates otherwise. Other embodiments may be utilized, and other changes may be made, without departing from the scope of the subject matter presented herein. 
     The example embodiments described herein are not meant to be limiting. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     Bone conduction audio can be provided to a wearer of a wearable computing device, such as a head-mountable device (HMD), by vibrating the skull of the wearer and propagating bone-conducted sound through the bones and tissues of the wearer&#39;s head with low attenuation. However, due to this propagation through the wearer&#39;s head, when a bone-conducted signal is intended to be heard by the wearer&#39;s right ear only, part of that signal may also be heard by the wearer&#39;s left ear. Likewise, when a bone-conducted signal is intended to be heard by the wearer&#39;s left ear only, part of that signal may also be heard by the wearer&#39;s right ear. The parts of the intended signals that are heard by ears contralateral to the intended ears are known as crosstalk signals. Crosstalk signals may impede a wearer&#39;s ability to localize sound, which can make it difficult to implement stereophonic audio (e.g., binaural hearing, spatial hearing, lateralization, and the like) with bone conduction transducers (BCTs). 
     As such, disclosed herein is a method for a wearable computing device, such as an HMD, to cancel crosstalk between two different bone conduction audio channels. The HMD may receive a first audio signal effective to cause the HMD to provide a first sound to a first ear and at least a portion of the first sound (e.g., a crosstalk signal) to a second ear opposite the first ear. The HMD may then receive a second audio signal that is out of phase with the first audio signal and effective to substantially cancel at least a portion of the first audio signal. The first audio signal may be processed by a crosstalk cancellation processor coupled to the HMD, and the processing may involve a transform being applied to the first audio signal so as to generate the second audio signal. The transform may be based on one or more wearer-specific parameters because a given wearer&#39;s head may have unique properties unlike other wearer&#39;s heads. 
     Next, the HMD may cause a first BCT coupled to the HMD to vibrate based on the first audio signal. The first BCT may be located adjacent to one side of the wearer&#39;s head on the same side as a first ear of the wearer (e.g., located proximate to the first ear of the wearer), and the vibration may provide the first sound to the first ear and provide the portion of the first sound to the second ear of the wearer. Based on the second audio signal, the HMD may also cause a second BCT coupled to the HMD to vibrate substantially simultaneous to the vibration of the first BCT. The second BCT may be located adjacent to another side of the wearer&#39;s head on the same side as the second ear of the wearer (e.g., located proximate to the second ear of the wearer), and the vibration may provide a second sound to the second ear, the second sound being effective to cancel the portion of the first sound. 
     In some examples, the crosstalk signals that are received at a right and left ear of a given wearer during stereo bone conduction audio implementations may be based on in-head response functions (i.e., a matrix R, including the R XY  values, as shown in  FIGS. 4-5D ) that are based on the given wearer&#39;s tissue and bone composition and structure. The in-head response functions may be further based on other aspects of the wearer&#39;s head, such as head shape, head size, and tissue parameters (e.g., type, elasticity, damping), among others. Each R XY  value may represent a transfer function R from X transducer to Y cochlea. The transform (i.e., a matrix T, including the T XY  values, as shown in  FIG. 4 ) applied to the first audio signal at the crosstalk cancellation processor may be based on the in-head response functions. Each T XY  value may represent a transfer function T from X audio channel to Y transducer. In some examples, the in-head response functions may be measured prior to the method being performed so as to calibrate the HMD for the given wearer. In other examples, the in-head response functions may be predetermined based on an average of various in-head response functions of a population of wearers. 
     In some examples, the second audio signal may be about 180 degrees out of phase with the first audio signal, so as to cancel as much of the first audio signal as possible. 
     The method and examples described above may pertain to a cancellation of one of the two crosstalk signals. In practice, the same method and aspects may be applied to a cancellation of the other crosstalk signal. Specifically, a third audio signal may be received at the HMD effective to provide a third sound to the second ear and a portion of the third sound to the first ear. A crosstalk cancellation processor may then generate a fourth audio signal based on the third audio signal, the fourth audio signal effective to provide a fourth sound to the first ear of the wearer and cancel the portion of the third sound. In some examples, the first, second, third, and fourth sounds may be provided to the wearer substantially simultaneous to one another in order to better implement stereo bone conduction audio. 
     Systems and devices in which example embodiments may be implemented will now be described in greater detail. In general, an example system may be implemented in or may take the form of a wearable computing device. In some examples, a wearable computing device may take the form of or include an HMD, as noted above. Henceforth, “wearable computing device” and “HMD” may be used interchangeably. 
     An example system may also be implemented in or take the form of other devices, such as a mobile phone, tablet computer, laptop computer, and computing appliance, each configured with sensors, cameras, and the like arranged to capture/scan a user&#39;s eye, face, or record other biometric data. Further, an example system may take the form of non-transitory computer readable medium, which has program instructions stored thereon that are executable by at a processor to provide the functionality described herein. An example system may also take the form of a device such as a wearable computer or mobile phone, or a subsystem of such a device, which includes such a non-transitory computer readable medium having such program instructions stored thereon. 
     An HMD may generally be any display device that is capable of being worn on the head and places a display in front of one or both eyes of the wearer. An HMD may take various forms such as a helmet or eyeglasses. As such, references to “eyeglasses” or a “glasses-style” HMD should be understood to refer to an HMD that has a glasses-like frame so that it can be worn on the head. Further, example embodiments may be implemented by or in association with an HMD with a single display or with two displays, which may be referred to as a “monocular” HMD or a “binocular” HMD, respectively. 
       FIG. 1A  illustrates a wearable computing system according to at least some embodiments described herein. In  FIG. 1A , the wearable computing system takes the form of a head-mountable device (HMD)  102  (which may also be referred to as a head-mounted display). It should be understood, however, that example systems and devices may take the form of or be implemented within or in association with other types of devices, without departing from the scope of the invention. As illustrated in  FIG. 1A , the HMD  102  includes frame elements including lens-frames  104 ,  106  and a center frame support  108 , lens elements  110 ,  112 , and extending side-arms  114 ,  116 . The center frame support  108  and the extending side-arms  114 ,  116  are configured to secure the HMD  102  to a user&#39;s face via a user&#39;s nose and ears, respectively. 
     Each of the frame elements  104 ,  106 , and  108  and the extending side-arms  114 ,  116  may be formed of a solid structure of plastic and/or metal, or may be formed of a hollow structure of similar material so as to allow wiring and component interconnects to be internally routed through the HMD  102 . Other materials may be possible as well. 
     One or more of each of the lens elements  110 ,  112  may be formed of any material that can suitably display a projected image or graphic. Each of the lens elements  110 ,  112  may also be sufficiently transparent to allow a user to see through the lens element. Combining these two features of the lens elements may facilitate an augmented reality or heads-up display where the projected image or graphic is superimposed over a real-world view as perceived by the user through the lens elements. 
     The extending side-arms  114 ,  116  may each be projections that extend away from the lens-frames  104 ,  106 , respectively, and may be positioned behind a user&#39;s ears to secure the HMD  102  to the user. The extending side-arms  114 ,  116  may further secure the HMD  102  to the user by extending around a rear portion of the user&#39;s head. Additionally or alternatively, for example, the HMD  102  may connect to or be affixed within a head-mounted helmet structure. Other configurations for an HMD are also possible. 
     The HMD  102  may also include an on-board computing system  118 , an image capture device  120 , a sensor  122 , and a finger-operable touchpad  124 . The on-board computing system  118  is shown to be positioned on the extending side-arm  114  of the HMD  102 ; however, the on-board computing system  118  may be provided on other parts of the HMD  102  or may be positioned remote from the HMD  102  (e.g., the on-board computing system  118  could be wire- or wirelessly-connected to the HMD  102 ). The on-board computing system  118  may include a processor and memory, for example. The on-board computing system  118  may be configured to receive and analyze data from the image capture device  120  and the finger-operable touchpad  124  (and possibly from other sensory devices, user interfaces, or both) and generate images for output by the lens elements  110  and  112 . 
     The image capture device  120  may be, for example, a camera that is configured to capture still images and/or to capture video. In the illustrated configuration, image capture device  120  is positioned on the extending side-arm  114  of the HMD  102 ; however, the image capture device  120  may be provided on other parts of the HMD  102 . The image capture device  120  may be configured to capture images at various resolutions or at different frame rates. Many image capture devices with a small form-factor, such as the cameras used in mobile phones or webcams, for example, may be incorporated into an example of the HMD  102 . 
     Further, although  FIG. 1A  illustrates one image capture device  120 , more image capture device may be used, and each may be configured to capture the same view, or to capture different views. For example, the image capture device  120  may be forward facing to capture at least a portion of the real-world view perceived by the user. This forward facing image captured by the image capture device  120  may then be used to generate an augmented reality where computer generated images appear to interact with or overlay the real-world view perceived by the user. 
     The sensor  122  is shown on the extending side-arm  116  of the HMD  102 ; however, the sensor  122  may be positioned on other parts of the HMD  102 . For illustrative purposes, only one sensor  122  is shown. However, in an example embodiment, the HMD  102  may include multiple sensors. For example, an HMD  102  may include sensors  102  such as one or more gyroscopes, one or more accelerometers, one or more magnetometers, one or more light sensors, one or more infrared sensors, and/or one or more microphones. Other sensing devices may be included in addition or in the alternative to the sensors that are specifically identified herein. 
     The finger-operable touchpad  124  is shown on the extending side-arm  114  of the HMD  102 . However, the finger-operable touchpad  124  may be positioned on other parts of the HMD  102 . Also, more than one finger-operable touchpad may be present on the HMD  102 . The finger-operable touchpad  124  may be used by a user to input commands, and such inputs may take the form of a finger swipe along the touchpad, a finger tap on the touchpad, or the like. The finger-operable touchpad  124  may sense at least one of a pressure, position and/or a movement of one or more fingers via capacitive sensing, resistance sensing, or a surface acoustic wave process, among other possibilities. The finger-operable touchpad  124  may be capable of sensing movement of one or more fingers simultaneously, in addition to sensing movement in a direction parallel or planar to the pad surface, in a direction normal to the pad surface, or both, and may also be capable of sensing a level of pressure applied to the touchpad surface. In some embodiments, the finger-operable touchpad  124  may be formed of one or more translucent or transparent insulating layers and one or more translucent or transparent conducting layers. Edges of the finger-operable touchpad  124  may be formed to have a raised, indented, or roughened surface, so as to provide tactile feedback to a user when the user&#39;s finger reaches the edge, or other area, of the finger-operable touchpad  124 . If more than one finger-operable touchpad is present, each finger-operable touchpad may be operated independently, and may provide a different function. 
     In a further aspect, HMD  102  may be configured to receive user input in various ways, in addition or in the alternative to user input received via finger-operable touchpad  124 . For example, on-board computing system  118  may implement a speech-to-text process and utilize a syntax that maps certain spoken commands to certain actions. In addition, HMD  102  may include one or more microphones (or other types of input transducers) via which a wearer&#39;s speech may be captured. Configured as such, HMD  102  may be operable to detect spoken commands and carry out various computing functions that correspond to the spoken commands. 
     As another example, HMD  102  may interpret certain head-movements as user input. For example, when HMD  102  is worn, HMD  102  may use one or more gyroscopes and/or one or more accelerometers to detect head movement. The HMD  102  may then interpret certain head-movements as being user input, such as nodding, or looking up, down, left, or right. An HMD  102  could also pan or scroll through graphics in a display according to movement. Other types of actions may also be mapped to head movement. 
     As yet another example, HMD  102  may interpret certain gestures (e.g., by a wearer&#39;s hand or hands) as user input. For example, HMD  102  may capture hand movements by analyzing image data from image capture device  120 , and initiate actions that are defined as corresponding to certain hand movements. 
     As a further example, HMD  102  may interpret eye movement as user input. In particular, HMD  102  may include one or more inward-facing image capture devices and/or one or more other inward-facing sensors (not shown) that may be used to track eye movements and/or determine the direction of a wearer&#39;s gaze. As such, certain eye movements may be mapped to certain actions. For example, certain actions may be defined as corresponding to movement of the eye in a certain direction, a blink, and/or a wink, among other possibilities. 
     HMD  102  also includes a speaker  125  for generating audio output. In one example, the speaker could be in the form of a bone conduction speaker, also referred to as a bone conduction transducer (BCT). Speaker  125  may be, for example, a vibration transducer or an electroacoustic transducer that produces sound in response to an electrical audio signal input. The frame of HMD  102  may be designed such that when a user wears HMD  102 , the speaker  125  contacts the wearer. Alternatively, speaker  125  may be embedded within the frame of HMD  102  and positioned such that, when the HMD  102  is worn, speaker  125  vibrates a portion of the frame that contacts the wearer. In either case, HMD  102  may be configured to send an audio signal to speaker  125 , so that vibration of the speaker may be directly or indirectly transferred to the bone structure of the wearer. When the vibrations travel through the bone structure to the bones in the middle ear of the wearer, the wearer can interpret the vibrations provided by BCT  125  as sounds. 
     Various types of bone-conduction transducers (BCTs) may be implemented, depending upon the particular implementation. Generally, any component that is arranged to vibrate a part of a wearer&#39;s head adjacent to the HMD  102  may be incorporated as a vibration transducer. Yet further it should be understood that an HMD  102  may include a single BCT or multiple BCTs. In addition, the location(s) of BCT(s) on the HMD may vary, depending upon the implementation. For example, a BCT may be located proximate to a wearer&#39;s temple (as shown), behind the wearer&#39;s ear, proximate to the wearer&#39;s nose, and/or at any other location where the BCT can vibrate the wearer&#39;s bone structure. 
       FIG. 1B  illustrates an alternate view of the wearable computing device illustrated in  FIG. 1A . As shown in  FIG. 1B , the lens elements  110 ,  112  may act as display elements. The HMD  102  may include a first projector  128  coupled to an inside surface of the extending side-arm  116  and configured to project a display  130  onto an inside surface of the lens element  112 . Additionally or alternatively, a second projector  132  may be coupled to an inside surface of the extending side-arm  114  and configured to project a display  134  onto an inside surface of the lens element  110 . 
     The lens elements  110 ,  112  may act as a combiner in a light projection system and may include a coating that reflects the light projected onto them from the projectors  128 ,  132 . In some embodiments, a reflective coating may not be used (e.g., when the projectors  128 ,  132  are scanning laser devices). 
     In alternative embodiments, other types of display elements may also be used. For example, the lens elements  110 ,  112  themselves may include: a transparent or semi-transparent matrix display, such as an electroluminescent display or a liquid crystal display, one or more waveguides for delivering an image to the user&#39;s eyes, or other optical elements capable of delivering an in focus near-to-eye image to the user. A corresponding display driver may be disposed within the frame elements  104 ,  106  for driving such a matrix display. Alternatively or additionally, a laser or LED source and scanning system could be used to draw a raster display directly onto the retina of one or more of the user&#39;s eyes. Other possibilities exist as well. 
       FIG. 1C  illustrates another wearable computing system according to at least some embodiments described herein, which takes the form of an HMD  152 . The HMD  152  may include frame elements and side-arms such as those described with respect to  FIGS. 1A and 1B . The HMD  152  may additionally include an on-board computing system  154  and an image capture device  156 , such as those described with respect to  FIGS. 1A and 1B . The image capture device  156  is shown mounted on a frame of the HMD  152 . However, the image capture device  156  may be mounted at other positions as well. 
     As shown in  FIG. 1C , the HMD  152  may include a single display  158  which may be coupled to the device. The display  158  may be formed on one of the lens elements of the HMD  152 , such as a lens element described with respect to  FIGS. 1A and 1B , and may be configured to overlay computer-generated graphics in the user&#39;s view of the physical world. The display  158  is shown to be provided in a center of a lens of the HMD  152 , however, the display  158  may be provided in other positions, such as for example towards either the upper or lower portions of the wearer&#39;s field of view. The display  158  is controllable via the computing system  154  that is coupled to the display  158  via an optical waveguide  160 . 
       FIG. 1D  illustrates another wearable computing system according to at least some embodiments described herein, which takes the form of a monocular HMD  172 . The HMD  172  may include side-arms  173 , a center frame support  174 , and a bridge portion with nosepiece  175 . In the example shown in  FIG. 1D , the center frame support  174  connects the side-arms  173 . The HMD  172  does not include lens-frames containing lens elements. The HMD  172  may additionally include a component housing  176 , which may include an on-board computing system (not shown), an image capture device  178 , a button  179  for operating the image capture device  178  (and/or usable for other purposes), and a finger-operable touch pad  182  similar to that described with respect to  FIG. 1A . Component housing  176  may also include other electrical components and/or may be electrically connected to electrical components at other locations within or on the HMD. HMD  172  also includes a BCT  186 . In some embodiments, HMD  172  may include at least one other BCT as well, such as BCT  188  opposite BCT  186 . The BCTs may be piezoelectric BCTs (e.g., thin film piezoelectric BCTs) or other types of BCTs. 
     The HMD  172  may include a single display  180 , which may be coupled to one of the side-arms  173  via the component housing  176 . In an example embodiment, the display  180  may be a see-through display, which is made of glass and/or another transparent or translucent material, such that the wearer can see their environment through the display  180 . Further, the component housing  176  may include the light sources (not shown) for the display  180  and/or optical elements (not shown) to direct light from the light sources to the display  180 . As such, display  180  may include optical features that direct light that is generated by such light sources towards the wearer&#39;s eye, when HMD  172  is being worn. 
     In some embodiments, the HMD  172  may include one or more infrared proximity sensors or infrared trip sensors. Further, the one or more proximity sensors may be coupled to the HMD  172  at various locations, such as on the nosepiece  175  of the HMD  172 , so as to accurately detect when the HMD  172  is being properly worn by a wearer. For instance, an infrared trip sensor (or other type of sensor) may be operated between nose pads of the HMD  172  and configured to detect disruptions in an infrared beam produced between the nose pads. Still further, the one or more proximity sensors may be coupled to the side-arms  173 , center frame support  174 , or other location(s) and configured to detect whether the HMD  172  is being worn properly. The one or more proximity sensors may also be configured to detect other positions that the HMD  172  is being worn in, such as resting on top of a head of a wearer or resting around the wearer&#39;s neck. 
     In a further aspect, HMD  172  may include a sliding feature  184 , which may be used to adjust the length of the side-arms  173 . Thus, sliding feature  184  may be used to adjust the fit of HMD  172 . Further, an HMD may include other features that allow a wearer to adjust the fit of the HMD, without departing from the scope of the invention. 
       FIGS. 1E ,  1 F, and  1 G are simplified illustrations of the HMD  172  shown in  FIG. 1D , being worn by a wearer  190 . As shown in  FIG. 1F , when HMD  172  is worn, BCT  186  is arranged such that when HMD  172  is worn, BCT  186  is located behind the wearer&#39;s ear. As such, BCT  186  is not visible from the perspective shown in  FIG. 1E . However, HMD  172  may include other BCTs such that when HMD  172  is worn, the other BCTs may contact the wearer at the wearer&#39;s right and/or left temples, at a location proximate to one or both of the wearer&#39;s ears, and/or at other locations. 
     In the illustrated example, the display  180  may be arranged such that when HMD  172  is worn, display  180  is positioned in front of or proximate to a user&#39;s eye when the HMD  172  is worn by a user. For example, display  180  may be positioned below the center frame support and above the center of the wearer&#39;s eye, as shown in  FIG. 1E . Further, in the illustrated configuration, display  180  may be offset from the center of the wearer&#39;s eye (e.g., so that the center of display  180  is positioned to the right and above of the center of the wearer&#39;s eye, from the wearer&#39;s perspective). 
     Configured as shown in  FIGS. 1E ,  1 F, and  1 G, display  180  may be located in the periphery of the field of view of the wearer  190 , when HMD  172  is worn. Thus, as shown by  FIG. 1F , when the wearer  190  looks forward, the wearer  190  may see the display  180  with their peripheral vision. As a result, display  180  may be outside the central portion of the wearer&#39;s field of view when their eye is facing forward, as it commonly is for many day-to-day activities. Such positioning can facilitate unobstructed eye-to-eye conversations with others, as well as generally providing unobstructed viewing and perception of the world within the central portion of the wearer&#39;s field of view. Further, when the display  180  is located as shown, the wearer  190  may view the display  180  by, e.g., looking up with their eyes only (possibly without moving their head). This is illustrated as shown in  FIG. 1G , where the wearer has moved their eyes to look up and align their line of sight with display  180 . A wearer might also use the display by tilting their head down and aligning their eye with the display  180 . 
       FIG. 2  illustrates a schematic drawing of a computing device  210  according to at least some embodiments described herein. In an example embodiment, device  210  communicates using a communication link  220  (e.g., a wired or wireless connection) to a remote device  230 . The device  210  may be any type of device that can receive data and display information corresponding to or associated with the data. For example, the device  210  may be a heads-up display system, such as the head-mounted devices  102 ,  152 , or  172  described with reference to  FIGS. 1A to 1G . 
     Thus, the device  210  may include a display system  212  comprising a processor  214  and a display  216 . The display  210  may be, for example, an optical see-through display, an optical see-around display, or a video see-through display. The processor  214  may receive data from the remote device  230 , and configure the data for display on the display  216 . The processor  214  may be any type of processor, such as a micro-processor or a digital signal processor, for example. The processor  214  may also include other processors, such as a crosstalk cancellation processor (not shown), which may be implemented in accordance with at least one example embodiment described herein. 
     The device  210  may further include on-board data storage, such as memory  218  coupled to the processor  214 . The memory  218  may store software that can be accessed and executed by the processor  214 , for example. 
     The remote device  230  may be any type of computing device or transmitter including a laptop computer, a mobile telephone, or tablet computing device, etc., that is configured to transmit data to the device  210 . The remote device  230  and the device  210  may contain hardware to enable the communication link  220 , such as processors, transmitters, receivers, antennas, etc. 
     Further, remote device  230  may take the form of or be implemented in a computing system that is in communication with and configured to perform functions on behalf of client device, such as computing device  210 . Such a remote device  230  may receive data from another computing device  210  (e.g., an HMD  102 ,  152 , or  172  or a mobile phone), perform certain processing functions on behalf of the device  210 , and then send the resulting data back to device  210 . This functionality may be referred to as “cloud” computing. 
     In  FIG. 2 , the communication link  220  is illustrated as a wireless connection; however, wired connections may also be used. For example, the communication link  220  may be a wired serial bus such as a universal serial bus or a parallel bus. A wired connection may be a proprietary connection as well. The communication link  220  may also be a wireless connection using, e.g., short range wireless radio technology, communication protocols described in IEEE 802.11 (including any IEEE 802.11 revisions), Cellular technology (such as GSM, CDMA, UMTS, EV-DO, WiMAX, or LTE), or personal area network technology, among other possibilities. The remote device  230  may be accessible via the Internet and may include a computing cluster associated with a particular web service (e.g., social-networking, photo sharing, address book, etc.). 
       FIG. 3  is a flow chart of an example method  300 , according to at least some embodiments described herein. Method  300  may include one or more operations, functions, or actions as illustrated by one or more of blocks  302 - 308 . Although the blocks are illustrated in a sequential order, these blocks may also be performed in parallel, and/or in a different order than those described herein. Also, the various blocks may be combined into fewer blocks, divided into additional blocks, and/or removed based upon the desired implementation. 
     In addition, for the method  300  and other processes and methods disclosed herein, the block diagram shows functionality and operation of one possible implementation of present embodiments. In this regard, each block may represent a module, a segment, or a portion of program code, which includes one or more instructions executable by a processor or computing device for implementing specific logical functions or steps in the process. The program code may be stored on any type of computer readable medium, for example, such as a storage device including a disk or hard drive. The computer readable medium may include a non-transitory computer readable medium, for example, such as computer-readable media that stores data for short periods of time like register memory, processor cache and Random Access Memory (RAM). The computer readable medium may also include non-transitory media, such as secondary or persistent long term storage, like read only memory (ROM), optical or magnetic disks, compact-disc read only memory (CD-ROM), for example. The computer readable medium may also be any other volatile or non-volatile storage systems. The computer readable medium may be considered a computer readable storage medium, for example, or a tangible storage device. 
     In addition, for the method  300  and other processes disclosed herein, each block in  FIG. 3  may represent circuitry that is wired to perform the specific logical functions in the process. 
     For the sake of example, the method  300  will be described as implemented by an example head-mountable device (HMD), such as the HMDs illustrated in  FIGS. 1A-1G . It should be understood, however, that other computing devices, such as wearable computing devices (e.g., watches), or combinations of computing devices maybe configured to implement one or more steps of the method  300 . 
     At block  302 , the method  300  includes an HMD receiving a first audio signal effective to cause the HMD to provide a first sound to a first ear and at least a portion of the first sound to a second ear. The portion of the first sound that reaches the second ear (e.g., the inner ear of the second ear, bypassing the outer ear) may be a crosstalk sound resulting from a crosstalk signal, as opposed to the first (“direct” or “desired”) sound that reaches the first ear (e.g., the inner ear of the first ear, bypassing the outer ear) resulting from the first audio signal. For example, the first ear may be a right ear of a wearer of the HMD, and the first sound may be produced by a BCT and intended to be heard by the right ear. However, the portion of the first sound (e.g., the crosstalk sound) may be heard by the second ear (e.g., the left ear of the wearer) as well. 
     At block  304 , the method  300  includes the HMD receiving a second audio signal that is out of phase with the first audio signal and effective to substantially cancel at least a portion of the first audio signal. Namely, the second audio signal may be effective to produce a second sound (e.g., a “crosstalk-cancelling” sound). The second audio signal may be based on a transform applied by the HMD to the first audio signal, where the transform may be based on one or more wearer-specific parameters (e.g., unique properties of a given wearer&#39;s head and/or torso). The wearer-specific parameters may include wearer-specific mechanical-acoustical parameters based on a bone thickness of a skull of the wearer, a bone shape of the wearer, a tissue thickness of a head of the wearer, health of the given wearer&#39;s ears (e.g., outer ear, middle ear, inner ear, etc.), and/or other parameters of the wearer&#39;s head and/or torso described herein or not described herein. 
     In some examples, the second audio signal may be approximately 180 degrees out of phase with the first audio signal (i.e., antiphase). Further, the second audio signal may have approximately the same amplitude, or exactly the same amplitude, as the first audio signal. 
     At block  306 , the method  300  includes, based on the first audio signal, causing a first bone conduction transducer (BCT) coupled to the HMD to vibrate so as to provide the first sound to the first ear and provide the portion of the first sound to the second ear. The first BCT may contact the wearer at the back of the first ear or at another location such as a temple of the wearer on the same side of the wearer&#39;s head as the first ear. The first BCT may thus vibrate the wearer&#39;s skull and provide the direct sound to the inner ear of the first ear and provide the crosstalk sound to the inner ear of the second ear. 
     At block  308 , the method  300  includes, based on the second audio signal, causing a second BCT coupled to the HMD to vibrate substantially simultaneous to the vibration of the first BCT so as to provide a second sound to the second ear, the second sound being effective to substantially cancel the portion of the first sound. The second BCT may contact the wearer at the back of the second ear or at another location such as a temple of the wearer on the same side of the wearer&#39;s head as the second ear and contralateral to the first ear. The second BCT may vibrate the wearer&#39;s skull and provide the crosstalk-cancelling sound to the inner ear of the second ear to substantially cancel the crosstalk sound from the first BCT. 
     In some examples, the first BCT and the second BCT may vibrate at exactly the same time as one another. In other examples, the first BCT and the second BCT may vibrate at different times, with one BCT vibrating prior to the other BCT. 
     While in some examples, the crosstalk sound may be entirely cancelled by the crosstalk-cancelling sound, the crosstalk sound may not be entirely cancelled in other examples. Rather, the crosstalk sound may be at least partially cancelled by the crosstalk-cancelling sound. Other examples are also possible. 
     In some examples, a method similar to the aforementioned method  300  may be performed such that the second BCT provides another direct sound and the first BCT provides another crosstalk-cancelling sound to substantially cancel the crosstalk sound that results from the other direct sound. This similar method may be performed by the HMD or other device substantially simultaneous to the aforementioned method  300  being performed, so as to provide stereophonic sound (e.g., two or more audio channels/signals) to the wearer of the HMD. 
     The similar method can be performed in various ways. In some examples, the HMD may receive a third audio signal effective to cause HMD to provide a third sound to the second ear and at least a portion of the third sound to the first ear. The portion of the third sound that reaches the first ear (e.g., the inner ear of the first ear) may be another crosstalk sound resulting from another crosstalk signal, as opposed to the third sound (e.g., another “direct” or “desired” sound) that reaches the second ear (e.g., the inner ear of the second ear) resulting from the third audio signal. For instance, in line with the discussion above, the second ear may be the left ear of a wearer of the HMD, and the third sound may be produced by the second BCT and intended to be heard by the left ear. However, the portion of the third sound (e.g., the crosstalk sound) may be heard by the first ear (e.g., the right ear of the wearer) as well. 
     The HMD may then receive a fourth audio signal that is out of phase with the third audio signal and effective to substantially cancel at least a portion of the third audio signal, wherein the fourth audio signal is based on the transform applied by the HMD to the third audio signal. Namely, the fourth audio signal may be effective to produce a fourth sound (e.g., the other crosstalk-cancelling sound). The fourth audio signal may be based on the same transform as discussed above, applied by the HMD to the third audio signal. In some examples, however, the transform may be different than the transform discussed above. 
     In some examples, the fourth audio signal may be approximately 180 degrees out of phase with the third audio signal (i.e., antiphase). Further, the fourth audio signal may have approximately the same amplitude, or exactly the same amplitude, as the third audio signal. 
     In some examples, as noted above, a processor of the HMD (e.g., a crosstalk cancellation processor) may be calibrated for a given wearer so as to configure the processor to apply the transform to the third audio signal. 
     Based on the third audio signal, the HMD may then cause the second BCT to vibrate so as to provide the third sound to the second ear and provide the portion of the third sound to the first ear. Further, based on the fourth audio signal, the HMD may cause the first BCT to vibrate substantially simultaneous to the vibration of the second BCT so as to provide a fourth sound to the first ear, the fourth sound being effective to substantially cancel the portion of the third sound. 
     In some examples, the first audio signal and the fourth audio signal may comprise a first set of signals. Further, the second audio signal and the third audio signal may comprise a second set of signals. As such, the HMD may cause the first BCT to vibrate so as to provide the first sound and the fourth sound to the first ear based on the first set of signals, and the HMD may cause the second BCT to vibrate substantially simultaneous to the vibration of the first BCT so as to provide the second sound and the third sound to the second ear based on the second set of signals. In other words, each BCT may provide to the wearer a sound with two components: a direct sound and a crosstalk-cancelling sound effective to substantially cancel any crosstalk sound that may result from the vibration of the contralateral BCT. 
     While in some examples the method  300  (and the similar method) just described may be implemented using two BCTs, in other examples the method(s) can be implemented using more than two BCTs. 
       FIG. 4  is a block diagram of a system  400  for implementing the method described above, in accordance with at least some embodiments described herein. The system  400  may include original signals  402 , S L  and S R , which represent stereophonic audio signals that are intended to be heard by a left ear and a right ear of a wearer of an HMD, respectively. For example, in line with the discussion above, S L  and S R  may take the form of the first audio signal and the third audio signal, as noted above. 
     In some examples, the original signals  402  may be processed by a crosstalk cancellation processor  404  of the HMD to preemptively account for the crosstalk effect caused by the wearer&#39;s head. In other words, the crosstalk cancellation processor  404  may modify the original signals  402  to each include a component that is effective to substantially cancel any crosstalk signal from the opposite ear. Left and right BCTs  406  may then produce stereo sound based on the modified signals. For instance, as shown, the crosstalk cancellation processor  404  may apply response function T LR  to original signal S L  (e.g., the first audio signal, as noted above) in order to generate a crosstalk-cancelling signal (e.g., the second audio signal, as noted above) effective to cause the right BCT to produce a corresponding crosstalk-cancelling sound (e.g., the second sound, as noted above) simultaneous to the left BCT producing an original sound based on original signal S L  (e.g., the first sound, as noted above). 
     Likewise, as shown, the crosstalk cancellation processor  404  may apply response function T RL  to original signal S R  (e.g., the third audio signal, as noted above) in order to generate a crosstalk-cancelling signal (e.g., the fourth audio signal, as noted above) effective to cause the left BCT to produce a corresponding crosstalk-cancelling sound (e.g., the fourth sound, as noted above) simultaneous to the right BCT producing an original sound based on original signal S R  (e.g., the third sound, as noted above). 
     In other examples, prior to the HMD processing the original signals  402  with the crosstalk cancellation processor  404 , the HMD may apply a head-related transfer function (HRTF) to the original signals  402 , where the HRTF is associated with the wearer and based on the wearer-specific parameters. In some examples, the HRTF may comprise two transfer functions, each representative of the diffraction of an incoming sound waveform by a torso and a head of a particular wearer. The HRTF may be measured so as to be unique for the particular wearer of the HMD, or the HRTF may be predetermined based on an average of various measured HRTFs of a population of wearers. 
     In some examples, the original signals  402  and crosstalk-cancelling signals may then be transmitted to the wearer of the HMD via BCTs  406 , namely a left BCT and a right BCT with corresponding responses B L  and B R , respectively. The BCTs&#39;  406  responses may be represented by Equation 1. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             B 
                             L 
                           
                         
                       
                       
                         
                           
                             B 
                             R 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               T 
                               LL 
                             
                           
                           
                             
                               T 
                               RL 
                             
                           
                         
                         
                           
                             
                               T 
                               LR 
                             
                           
                           
                             
                               T 
                               RR 
                             
                           
                         
                       
                       ] 
                     
                     · 
                     
                       [ 
                       
                         
                           
                             
                               S 
                               L 
                             
                           
                         
                         
                           
                             
                               S 
                               R 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     As an example, in a monophonic scenario, such as when S R  is equal to zero, the response of the left BCT and the response of the right BCT may be represented by Equation 2 and Equation 3, respectively.
 
 B   L   =T   LL   *S   L   Equation (2)
 
 B   R   =T   LR   *S   L   Equation (3)
 
     Likewise, in another monophonic scenario when S L  is equal to zero, the response of the left BCT and the response of the right BCT may be represented by Equation 4 and Equation 5, respectively.
 
 B   L   =T   RL   *S   R   Equation (4)
 
 B   R   =T   RR   *S   R   Equation (5)
 
     On the other hand, in a stereophonic scenario, the response of the left BCT and the response of the right BCT may be represented by Equation 6 and Equation 7, respectively.
 
 B   L   =T   LL   *S   L   +T   RL   *S   R   Equation (6)
 
 B   R   =T   LR   *S   L   +T   RR   *S   R   Equation (7)
 
     After the BCTs  406  vibrate to produce stereo audio sound, the stereo audio sound travels through an in-head transmission path  408  before being heard at the wearer&#39;s left and right cochleae  410 . In general, the responses at a wearer&#39;s cochleae  410  may be represented by Equation 8. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             C 
                             L 
                           
                         
                       
                       
                         
                           
                             C 
                             R 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               R 
                               LL 
                             
                           
                           
                             
                               R 
                               RL 
                             
                           
                         
                         
                           
                             
                               R 
                               LR 
                             
                           
                           
                             
                               R 
                               RR 
                             
                           
                         
                       
                       ] 
                     
                     · 
                     
                       [ 
                       
                         
                           
                             
                               B 
                               L 
                             
                           
                         
                         
                           
                             
                               B 
                               R 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     8 
                     ) 
                   
                 
               
             
           
         
       
     
     As shown in Equation 8, the signals received at the wearer&#39;s left and right cochlea, C L  and C R , are determined by multiplying the BCT signals, B L  and B R , by an in-head response matrix. For the in-head response matrix, R LL  and R RR  represent the response of the direct paths from the left BCT to the left cochlea and from the right BCT to the right cochlea, respectively. Further, R LR  and R RL  represent the response of the crosstalk paths from the left BCT to the right cochlea and from the right BCT to the left cochlea, respectively. 
     As such, by the HMD&#39;s implementation of the crosstalk cancellation processor  404 , the responses at the wearer&#39;s cochleae  410  may be represented by Equation 9, which is a combination of Equation 1 and Equation 8. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             C 
                             L 
                           
                         
                       
                       
                         
                           
                             C 
                             R 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               R 
                               LL 
                             
                           
                           
                             
                               R 
                               RL 
                             
                           
                         
                         
                           
                             
                               R 
                               LR 
                             
                           
                           
                             
                               R 
                               RR 
                             
                           
                         
                       
                       ] 
                     
                     · 
                     
                       [ 
                       
                         
                           
                             
                               T 
                               LL 
                             
                           
                           
                             
                               T 
                               RL 
                             
                           
                         
                         
                           
                             
                               T 
                               LR 
                             
                           
                           
                             
                               T 
                               RR 
                             
                           
                         
                       
                       ] 
                     
                     · 
                     
                       [ 
                       
                         
                           
                             
                               S 
                               L 
                             
                           
                         
                         
                           
                             
                               S 
                               R 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     9 
                     ) 
                   
                 
               
             
           
         
       
     
     Further, in order to have the original signals  402  equal the stereo audio signals that reach the wearer&#39;s cochleae  410 , thereby providing the wearer with a stereo audio experience with substantially cancelled crosstalk from the in-head responses, R LR  and R RL , the transform {right arrow over (T)} can equal the inverse of the in-head response, as shown in Equation 10. 
     
       
         
           
             
               
                 
                   
                     T 
                     → 
                   
                   = 
                   
                     
                       
                         R 
                         → 
                       
                       
                         - 
                         1 
                       
                     
                     = 
                     
                       
                         ( 
                         
                           1 
                           
                             
                               
                                 R 
                                 LL 
                               
                               ⁢ 
                               
                                 R 
                                 RR 
                               
                             
                             - 
                             
                               
                                 R 
                                 RL 
                               
                               ⁢ 
                               
                                 R 
                                 LR 
                               
                             
                           
                         
                         ) 
                       
                       · 
                       
                         [ 
                         
                           
                             
                               
                                 R 
                                 RR 
                               
                             
                             
                               
                                 - 
                                 
                                   R 
                                   RL 
                                 
                               
                             
                           
                           
                             
                               
                                 - 
                                 
                                   R 
                                   LR 
                                 
                               
                             
                             
                               
                                 R 
                                 LL 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     10 
                     ) 
                   
                 
               
             
           
         
       
     
     It should be understood that for embodiments where the system  400  is implemented with more than two BCTs, the matrices noted above may be larger in accordance with the amount of BCTs present. 
       FIGS. 5A-5D  illustrate various configurations of a simplified system for measuring a transform, in accordance with at least some embodiments described herein. In particular, each of  FIGS. 5A-5D  illustrate a respective simplified system for measuring a given in-head response (R XY ) of the transform T described above (e.g., R from X transducer to Y cochlea). Further, each respective simplified system includes a wearer wearing an HMD such as the HMDs or other wearable computing devices described herein. 
       FIG. 5A  illustrates a simplified system for measuring in-head response R LL . To measure R LL , the HMD may transmit a first pure tone signal  500  to a left ear of the wearer (e.g., an outer and middle ear of the left ear) via a left output transducer  502  (e.g., a headphone or earphone) that is coupled to the HMD. The transmitting may be effective to provide an air-conducted pure tone sound to the left ear of the wearer. The amplitude and phase of the first pure tone signal  500  may be predetermined or determined by the wearer of the HMD. Further, in other examples, similar or different first and/or second pure tone signals may be used for measuring other R XY  values. For instance, different frequencies of the first and/or second pure tone signals may be used for each R XY  value. 
     The HMD may also transmit a second pure tone signal  500  to the left ear of the wearer. In some examples, the second pure tone signal  500  may have the same initial parameters as the first pure tone signal  500 . In other examples, the second pure tone signal  500  may have different initial parameters than the first pure tone signal  500 . The transmission of the second pure tone signal  500  may be effective to cause a left BCT  504 L to vibrate so as to provide a portion of a bone-conducted pure tone sound to the left ear of the wearer (e.g., the inner ear of the left ear) and another portion of the bone-conducted pure tone sound (e.g., crosstalk sound) to the right ear of the wearer (e.g., the inner ear of the right ear). Further, it should be understood that similar or different second pure tone signals may be used for measuring other R XY  values, including signals at varying frequencies. 
     Furthermore, substantially simultaneous to the HMD transmitting the first pure tone signal  500 , the HMD may transmit a noise signal  506  to the right ear of the wearer (e.g., an outer and middle ear of the right ear) via a right output transducer  508 . The noise signal  506  may be effective to provide a noise to the right ear of the wearer and substantially mask the other portion of the bone-conducted pure tone sound (due to the left ear being measured) so that the wearer can hear both the air-conducted pure tone sound and the portion of the bone-conducted pure tone sound at the left ear of the wearer without distraction by sound at the right ear of the wearer. In some examples, including each example shown in  FIGS. 5A-5D , the HMD may continuously transmit the noise signal  506 . For instance, the noise signal  506  may take the form of an mp3 or other sound clip repeatedly played by the HMD. In other examples, the HMD may begin transmitting the noise signal  506  within a given time interval before the HMD transmits the first pure tone signal  500 , and then the HMD may stop transmitting the noise signal  506  within a given time interval after the HMD stops transmitting the first pure tone signal  500 . In still other examples, the amplitude of the noise signal may be predetermined and may be the same (or different) for each in-head response measurement. Other examples are also possible. 
     Moreover, while the first and second pure tone signals  500  and the noise signal  506  are being transmitted to the wearer of the HMD, the wearer may adjust the phase and/or amplitude of the first pure tone signal  500  being transmitted by the left output transducer  502  via a phase/amplitude shifter  510  coupled to the HMD until no sound (or minimal sound) is perceived at the left ear of the wearer. For instance, the wearer may adjust the phase and/or amplitude of the first pure tone signal  500  until the air-conducted pure tone sound at least substantially masks the portion of the bone-conducted pure tone sound at the left ear of the wearer. Because each wearer&#39;s wearer-specific parameters are unique, the adjustments made to the phase and/or amplitude of the first pure tone signal  500  may be different for each wearer. In some scenarios, based on the adjustments, the air-conducted pure tone sound may be almost 180 degrees out of phase with the bone-conducted pure tone sound, yet other scenarios are also possible. In some examples, the adjustments may be made by the wearer via the finger-operable touch pad  182 , as shown in  FIG. 1D , or another input device. Based on the adjustments to the phase and amplitude of the first pure tone signal  500 , the HMD may determine R LL . 
     Each R XY  value may include a respective amplitude response and a respective phase response. In some examples, the HMD may determine the amplitude response directly from the phase/amplitude shifter  510 , and the HMD may determine the phase response by adding 180 degrees to the adjusted value of the phase of the first pure tone signal  500  that is outputted by the phase/amplitude shifter  510  received by the left (or right, in some examples) output transducer. In other examples, the HMD may include a microphone coupled proximate to the left ear for measuring R LL  and R RL  (or proximate to the right ear for measuring R RR  and R LR ). Other locations of the microphone are possible. Other examples are possible as well. 
       FIG. 5B  illustrates a simplified system for measuring in-head response R RL . To measure R RL , the HMD may transmit a first pure tone signal  500  to a left ear of the wearer via the left output transducer  502  that is coupled to the HMD. The transmitting may be effective to provide an air-conducted pure tone sound to the left ear of the wearer. 
     The HMD may also transmit a second pure tone signal  500  to the left ear of the wearer. The transmission of the second pure tone signal  500  may be effective to cause a right BCT  504 R to vibrate so as to provide a portion of a bone-conducted pure tone sound to the right ear of the wearer and another portion of the bone-conducted pure tone sound (e.g., crosstalk sound) to the left ear of the wearer. 
     Furthermore, substantially simultaneous to the HMD transmitting the first pure tone signal  500 , the HMD may transmit a noise signal  506  to the right ear of the wearer via a right output transducer  508 . The noise signal  506  may be effective to provide a noise to the right ear of the wearer and substantially mask the portion of the bone-conducted pure tone sound at the right ear (due to the left ear being measured) so that the wearer can hear both the air-conducted pure tone sound and the other portion of the bone-conducted pure tone sound at the left ear of the wearer without distraction by sound at the right ear of the wearer. 
     Moreover, while the first and second pure tone signals  500  and the noise signal  506  are being transmitted to the wearer of the HMD, the wearer may adjust the phase and/or amplitude of the first pure tone signal  500  being transmitted by the left output transducer  502  via a phase/amplitude shifter  510  coupled to the HMD until no sound (or minimal sound) is perceived at the left ear of the wearer. For instance, the wearer may adjust the phase and/or amplitude of the first pure tone signal  500  until the air-conducted pure tone sound at least substantially masks the other portion of the bone-conducted pure tone sound at the left ear of the wearer. Based on the adjustments to the phase and amplitude of the first pure tone signal  500 , the HMD may determine R (e.g., crosstalk). 
       FIG. 5C  illustrates a simplified system for measuring in-head response R LR . To measure R LR , the HMD may transmit a first pure tone signal  500  to a right ear of the wearer via the right output transducer  508  that is coupled to the HMD. The transmitting may be effective to provide an air-conducted pure tone sound to the right ear of the wearer. 
     The HMD may also transmit a second pure tone signal  500  to the left ear of the wearer. The transmission of the second pure tone signal  500  may be effective to cause a left BCT  504 L to vibrate so as to provide a portion of a bone-conducted pure tone sound to the left ear of the wearer and another portion of the bone-conducted pure tone sound (e.g., crosstalk sound) to the right ear of the wearer. 
     Furthermore, substantially simultaneous to the HMD transmitting the first pure tone signal  500 , the HMD may transmit a noise signal  506  to the left ear of the wearer via a left output transducer  502 . The noise signal  506  may be effective to provide a noise to the left ear of the wearer and substantially mask the portion of the bone-conducted pure tone sound at the left ear (due to the right ear being measured) so that the wearer can hear both the air-conducted pure tone sound and the other portion of the bone-conducted pure tone sound at the right ear of the wearer without distraction by sound at the left ear of the wearer. 
     Moreover, while the first and second pure tone signals  500  and the noise signal  506  are being transmitted to the wearer of the HMD, the wearer may adjust the phase and/or amplitude of the first pure tone signal  500  being transmitted by the right output transducer  508  via a phase/amplitude shifter  510  coupled to the HMD until no sound (or minimal sound) is perceived at the right ear of the wearer. For instance, the wearer may adjust the phase and/or amplitude of the first pure tone signal  500  until the air-conducted pure tone sound at least substantially masks the other portion of the bone-conducted pure tone sound at the right ear of the wearer. Based on the adjustments to the phase and amplitude of the first pure tone signal  500 , the HMD may determine R LR  (e.g., crosstalk). 
       FIG. 5D  illustrates a simplified system for measuring in-head response R RR . To measure R RR , the HMD may transmit a first pure tone signal  500  to a right ear of the wearer via the right output transducer  508  that is coupled to the HMD. The transmitting may be effective to provide an air-conducted pure tone sound to the right ear of the wearer. 
     The HMD may also transmit a second pure tone signal  500  to the right ear of the wearer. The transmission of the second pure tone signal  500  may be effective to cause a right BCT  504 R to vibrate so as to provide a portion of a bone-conducted pure tone sound to the right ear of the wearer and another portion of the bone-conducted pure tone sound (e.g., crosstalk sound) to the left ear of the wearer. 
     Furthermore, substantially simultaneous to the HMD transmitting the first pure tone signal  500 , the HMD may transmit a noise signal  506  to the left ear of the wearer via a left output transducer  502 . The noise signal  506  may be effective to provide a noise to the left ear of the wearer and substantially mask the portion of the bone-conducted pure tone sound at the left ear (due to the right ear being measured) so that the wearer can hear both the air-conducted pure tone sound and the portion of the bone-conducted pure tone sound at the right ear of the wearer without distraction by sound at the left ear of the wearer. 
     Moreover, while the first and second pure tone signals  500  and the noise signal  506  are being transmitted to the wearer of the HMD, the wearer may adjust the phase and/or amplitude of the first pure tone signal  500  being transmitted by the right output transducer  508  via a phase/amplitude shifter  510  coupled to the HMD until no sound (or minimal sound) is perceived at the right ear of the wearer. For instance, the wearer may adjust the phase and/or amplitude of the first pure tone signal  500  until the air-conducted pure tone sound at least substantially masks the portion of the bone-conducted pure tone sound at the right ear of the wearer. Based on the adjustments to the phase and amplitude of the first pure tone signal  500 , the HMD may determine R RR . 
       FIG. 6  is a block diagram of a more detailed system for measuring the transform {right arrow over (T)} described herein. For an HMD to measure a given in-head response value (R XY ), a pure tone signal  600  may be fed into both a bone conduction channel  602  and an air conduction channel  604  such that both a bone-conducted sound and an air-conducted sound are perceived by the wearer of the HMD at the wearer&#39;s cochlea  606 . Further, as noted above, the wearer may use an interface such as a phase and amplitude adjustor  608  coupled to the HMD to adjust the phase and amplitude of the pure tone signal  600  fed into the air conduction channel  604  such that the air-conducted sound substantially cancels the bone-conducted sound at the cochlea  606 . 
     The bone conduction channel  602  may include components such as a bone conduction digital amplifier  610 , a bone conduction analog amplifier  612 , a BCT  614  for converting the pure tone signal  600  into the bone-conducted sound, and the wearer&#39;s human skin and skull  616  (e.g., wearer-specific parameters). Each component of the bone conduction channel  602  may include a respective response, A BC-X , which can be measured by the HMD or may be predetermined (e.g., measured in a laboratory or factory). A BC-X  may be a vector transfer function that includes both a respective phase and a respective amplitude. 
     The air conduction channel  604  may include components such as the phase and amplitude adjustor  608 , an air conduction digital amplifier  618 , an air conduction analog amplifier  620 , an air conduction transducer  622 , such as a headphone or earphone, and an outer and middle ear  624  of the wearer. Each component of the air conduction channel  604  may include a respective response, A AC-X , which can be measured by the HMD or may be predetermined. A AC-X  maybe a vector transfer function that includes both a respective phase and a respective amplitude. 
     In the example system shown in  FIG. 6 , the response associated with the wearer&#39;s skin and skull  616 , A BC-H , may represent a given in-head response value, R XY . In some examples, each of the responses may be predetermined and may have known values except for A BC-H  (which is being measured) and A AC-U  (which is adjustable by the wearer). The response A AC-U  may then be adjusted until the air-conducted sound substantially cancels the bone-conducted sound (i.e., when the sum of all the responses of the system is equal to zero, as shown in Equation 11). The HMD can then determine A BC-H , as shown in Equation 12. A BC-H  may be a vector summation of the other responses and may include both a respective phase and a respective amplitude.
 
 A   AC-U   +A   AC-D   +A   AC-A   +A   AC-T   +A   AC-H   +A   BC-D   +A   BC-A   +A   BC-T   +A   BC-H =0  Equation (11)
 
 A   BC-H =−( A   AC-U   +A   AC-D   +A   AC-A   +A   AC-T   +A   AC-H   +A   BC-D   +A   BC-A   +A   BC-T )  Equation (12)
 
     In some examples, the measurement process as described with respect to  FIGS. 5A-6  may be applied multiple times for a given in-head response value. For instance, each measurement of the multiple measurements may be performed with a different pure tone signal frequency. Other examples are also possible. 
     In some examples, the transform can be calibrated/determined for each unique wearer of the HMD. In other examples, the transform may be an average of a plurality of transforms, each corresponding to a particular wearer. Other examples are also possible. 
     The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. 
     The above detailed description describes various features and functions of the disclosed systems, devices, and methods with reference to the accompanying figures. In the figures, similar symbols typically identify similar components, unless context dictates otherwise. The example embodiments described herein and in the figures are not meant to be limiting. Other embodiments can be utilized, and other changes can be made, without departing from the scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     With respect to any or all of the ladder diagrams, scenarios, and flow charts in the figures and as discussed herein, each block and/or communication may represent a processing of information and/or a transmission of information in accordance with example embodiments. Alternative embodiments are included within the scope of these example embodiments. In these alternative embodiments, for example, functions described as blocks, transmissions, communications, requests, responses, and/or messages may be executed out of order from that shown or discussed, including substantially concurrent or in reverse order, depending on the functionality involved. Further, more or fewer blocks and/or functions may be used with any of the ladder diagrams, scenarios, and flow charts discussed herein, and these ladder diagrams, scenarios, and flow charts may be combined with one another, in part or in whole. 
     A block that represents a processing of information may correspond to circuitry that can be configured to perform the specific logical functions of a herein-described method or technique. Alternatively or additionally, a block that represents a processing of information may correspond to a module, a segment, or a portion of program code (including related data). The program code may include one or more instructions executable by a processor for implementing specific logical functions or actions in the method or technique. The program code and/or related data may be stored on any type of computer readable medium such as a storage device including a disk or hard drive or other storage medium. 
     The computer readable medium may also include non-transitory computer readable media such as computer-readable media that stores data for short periods of time like register memory, processor cache, and random access memory (RAM). The computer readable media may also include non-transitory computer readable media that stores program code and/or data for longer periods of time, such as secondary or persistent long term storage, like read only memory (ROM), optical or magnetic disks, compact-disc read only memory (CD-ROM), for example. The computer readable media may also be any other volatile or non-volatile storage systems. A computer readable medium may be considered a computer readable storage medium, for example, or a tangible storage device. 
     Moreover, a block that represents one or more information transmissions may correspond to information transmissions between software and/or hardware modules in the same physical device. However, other information transmissions may be between software modules and/or hardware modules in different physical devices. 
     The particular arrangements shown in the figures should not be viewed as limiting. It should be understood that other embodiments can include more or less of each element shown in a given figure. Further, some of the illustrated elements can be combined or omitted. Yet further, an example embodiment can include elements that are not illustrated in the figures. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.