Patent Publication Number: US-2021176548-A1

Title: Haptic Output System

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/191,373, filed Nov. 14, 2018, which is a nonprovisional patent application of and claims the benefit of U.S. Provisional Patent Application No. 62/736,354, Sep. 25, 2018, the disclosures of which are hereby incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The described embodiments relate generally to wearable electronic devices, and, more particularly, to wearable electronic devices that produce haptic outputs that can be felt by wearers of the electronic devices. 
     BACKGROUND 
     Wearable electronic devices are increasingly ubiquitous in modern society. For example, wireless audio devices (e.g., headphones, earbuds) are worn to provide convenient listening experiences for music and other audio. Head-mounted displays are worn to provide virtual or augmented reality environments to users for gaming, productivity, entertainment, and the like. Wrist-worn devices, such as smart watches, provide convenient access to various types of information and applications, including weather information, messaging applications, activity tracking applications, and the like. Some wearable devices, such as smart watches, may use haptic outputs to provide tactile alerts to the wearer, such as to indicate that a message has been received or that an activity goal has been reached. 
     SUMMARY 
     A method of providing a haptic output includes detecting a condition, determining if a head-mounted haptic accessory comprising an array of two or more haptic actuators is being worn by a user, determining an actuation pattern for the array of haptic actuators, and in response to detecting the condition and determining that the head-mounted haptic accessory is being worn by the user, initiating the actuation pattern to produce a directional haptic output that is configured to direct the user&#39;s attention along a direction. 
     The head-mounted haptic accessory may include a pair of earbuds, each earbud including an earbud body, a speaker positioned within the earbud body, and a haptic actuator positioned within the earbud body and configured to impart a haptic output to the user&#39;s ear. Detecting the condition may include detecting a presence of an audio source in an audio signal that is sent to the pair of earbuds. The method may further include determining a virtual position of the audio source relative to the user. Initiating the actuation pattern may include initiating a first haptic output at a first earbud of the pair of earbuds and subsequently initiating a second haptic output at a second earbud of the pair of earbuds. The directional haptic output may be configured to direct the user&#39;s attention toward the direction, which corresponds to the virtual position of the audio source. The audio signal may correspond to audio of a teleconference having multiple participants, the audio source may correspond to a participant of the multiple participants, and each respective participant of the multiple participants may have a distinct respective virtual position relative to the user. 
     The head-mounted haptic accessory may include an earbud including an earbud body and a haptic actuator positioned within the earbud body and comprising a movable mass, and initiating the actuation pattern may cause the haptic actuator to move the movable mass along an actuation direction that is configured to impart a reorientation force on the user. 
     Detecting the condition may include detecting a presence of an audio source in an audio signal that is sent to the pair of earbuds. The method may further include determining a virtual position of the audio source relative to the user, after initiating the actuation pattern, determining the user&#39;s orientation relative to the virtual position of the audio source, and increasing a volume of an audio output corresponding to the audio signal as the user&#39;s orientation becomes aligned with the virtual position of the audio source. 
     Detecting the condition may include detecting a notification associated with a graphical object. The graphical object may have a virtual position in a virtual environment being presented to the user, and the directional haptic output may be configured to direct the user&#39;s attention toward the direction, which corresponds to the virtual position of the graphical object. 
     Detecting the condition may include detecting an interactive object in a virtual environment being presented to the user. The interactive object may have a virtual position within the virtual environment, and the directional haptic output may be configured to direct the user&#39;s attention toward the direction, which corresponds to the virtual position of the interactive object. 
     An electronic system may include an earbud comprising an earbud body configured to be received at least partially within an ear of a user, a speaker positioned within the earbud body and configured to output sound into an ear canal of the user&#39;s ear, and a haptic actuator positioned within the earbud body and configured to impart a haptic output to the user&#39;s ear. The haptic actuator may be a linear resonant actuator having a linearly translatable mass that is configured to produce the haptic output. 
     The electronic system may further include a processor communicatively coupled with the haptic actuator and configured to detect a condition, determine an actuation pattern for the haptic actuator, and in response to detecting the condition, initiate the haptic output in accordance with the actuation pattern. The electronic system may further include a portable electronic device in wireless communication with the earbud, and the processor may be within the portable electronic device. 
     The electronic system may further include an additional earbud comprising an additional earbud body, an additional speaker positioned within the additional earbud body, and an additional haptic actuator positioned within the additional earbud body. The haptic actuator may include a mass configured to move along a horizontal direction when the earbud is worn in the user&#39;s ear, and the mass may be configured to produce an impulse that is perceptible as a force acting on the user&#39;s ear in a single direction. 
     A method of providing a haptic output may include detecting an audio feature in audio data, determining a characteristic frequency of the audio feature, causing a wearable electronic device to produce an audio output corresponding to the audio data and including the audio feature, and while the audio feature is being outputted, causing a haptic actuator of the wearable electronic device to produce a haptic output at a haptic frequency that corresponds to the characteristic frequency of the audio feature. The haptic frequency may be a harmonic or subharmonic of the characteristic frequency. The haptic output may be produced for an entire duration of the audio feature. 
     Detecting the audio feature may include detecting a triggering event in the audio data, and the triggering event may correspond to a rate of change of volume of the audio output that satisfies a threshold. Detecting the audio feature may include detecting audio content within a target frequency range. 
     The method may further include determining a variation in an audio characteristic of the audio feature and varying a haptic characteristic of the haptic output in accordance with the variation in the audio characteristic of the audio feature. The variation in the audio characteristic of the audio feature may be a variation in an amplitude of the audio feature, and varying a component of the haptic output in accordance with the variation in the audio characteristic of the audio feature may include varying an intensity of the haptic output in accordance with the variation in the amplitude. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIGS. 1A-1B  depict an example electronic system in use by a user. 
         FIGS. 2A-2B  depict an example head-mounted haptic accessory. 
         FIGS. 3A-3B  depict another example head-mounted haptic accessory. 
         FIGS. 4A-4B  depict another example head-mounted haptic accessory. 
         FIG. 5  depicts an example process for producing a haptic output. 
         FIG. 6A  depicts an example directional haptic output produced by a head-mounted haptic accessory. 
         FIG. 6B  depicts additional examples of directional haptic outputs produced by a head-mounted haptic accessory. 
         FIGS. 7A-7B  depict an additional example directional haptic output produced by a head-mounted haptic accessory. 
         FIG. 8  depicts an example haptic output scheme. 
         FIG. 9  depicts an example chart showing differences between various head-mounted haptic accessories. 
         FIGS. 10A-10B  depict participants in a teleconference. 
         FIG. 11  depicts participants in a teleconference. 
         FIGS. 12A-12B  depict a user engaged in a virtual-reality environment. 
         FIG. 13A  depicts an example audio feature in audio data. 
         FIG. 13B  depicts an example haptic output associated with the audio feature of  FIG. 13A . 
         FIGS. 14A-14B  depict a spatial arrangement of a user and two audio sources. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following description is not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     The embodiments herein are generally directed to wearable electronic devices that include haptic actuators, and more particularly, to haptic outputs that are coordinated with a position of a virtual object (which may correspond to or represent a person, an audio source, an instrument, a graphical object, etc.) relative to the wearer of the electronic device. The wearable electronic devices may include an array of haptic actuators (e.g., two or more haptic actuators) that can be actuated according to an actuation pattern in order to direct the wearer&#39;s attention in a particular direction. For example, an array of haptic actuators in contact with various locations on a wearer&#39;s head may be actuated in a pattern that produces a sensation having a distinct directional component. More particularly, the user may feel the pattern moving left or right. The user may then be motivated to turn his or her head or body in the direction indicated by the haptic pattern. 
     Indicating a direction via directional haptic outputs may be used to enhance various types of interactions with audio and/or visual content, and in particular to enhance interaction with content that has a real or virtual position relative to the wearer, and/or content that has a visual or audible component. For example, and as described in greater detail herein, directional haptic outputs may be used to direct a wearer&#39;s attention along a direction towards a virtual location of a participant in a multi-party telephone conference. As another example, a directional haptic output may be used to direct a user&#39;s attention towards the position of a graphical object in a virtual or augmented reality environment. 
     Haptic outputs provided via a wearable electronic device may also be used to enhance an experience of consuming audio or video content. For example, haptic outputs may be synchronized with certain audio features in a musical work or with audio or visual features of video content. In the context of music, the haptic outputs may be synchronized with notes from a certain instrument or notes having a certain prominence in the music. In some cases, the position of the wearer relative to a virtual position of an instrument may also affect the haptic output provided to the user. In the context of video, the haptic outputs may be synchronized with some visual and/or audio content of the video, such as by initiating a haptic output when an object appears to move towards or near the viewer. 
     These and other haptic outputs may be imparted to the user via various types of wearable devices. For example, a pair of earbuds, such as those that are conventionally used to provide audio to a user, may include haptic actuators that can produce haptic or tactile sensations to a user&#39;s ear. As used herein, the term ear may refer to any portion of an ear of a person, including the outer ear, middle ear, and/or inner ear. The outer ear of a person, which may include the auricle or pinna (e.g., the visible part of the ear that is external to a person&#39;s head) and the ear canal. Earbuds may reside at least partially in the ear canal, and may contact portions of the ear canal and/or the auricle of the ear. Accordingly, haptic actuators in earbuds may produce haptic or tactile sensations on the auricle and/or ear canal of a person&#39;s ear. 
     As another example, a pair of glasses may include haptic actuators (e.g., on the temple pieces and/or nose bridge). As yet another example, a headband, hat, or other head-worn object may include haptic actuators. In some cases, these wearable device(s) include an array of two or more haptic actuators, which may facilitate the production of directional haptic outputs by using different types of actuation patterns for the various actuators in the array. 
       FIGS. 1A-1B  illustrate right and left sides, respectively, of a user  100  using an electronic system  101 . The electronic system  101  may include a head-mounted haptic accessory  102  and a processing system  104 , and may define or be referred to as a haptic output system. For example, the head-mounted haptic accessory  102  and the portions of the processing system  104  that interact with the head-mounted haptic accessory  102  (or otherwise provide functionality relating to producing haptic outputs via the head-mounted haptic accessory  102 ) may define the haptic output system. 
     The head-mounted haptic accessory  102  is shown as a pair of earbuds that are configured to be positioned within an ear of the user  100 . The head-mounted haptic accessory  102  may include an array of two or more haptic actuators. For example, in the case of the earbuds shown in  FIGS. 1A-1B , each earbud may include a haptic actuator to define an array of two haptic actuators in contact with the user  100  (e.g., with the user&#39;s ears). In other embodiments, as described herein, the head-mounted haptic accessory may be another type of wearable, head-mounted device, such as over-ear or on-ear headphones, in-ear monitors, a pair of glasses, a headband, a hat, a head-mounted display, etc. In some cases, the head-mounted haptic accessory  102  may also include one or more speakers that produce audio outputs. 
     The electronic system  101  may include a processing system  104 , which may be a device that is separate from the head-mounted haptic accessory  102  (as shown in  FIG. 1A ), or it may be integrated with the head-mounted haptic accessory  102 . The processing system  104  is depicted in  FIG. 1A  as a portable electronic device, such as a mobile phone or smartphone, however, this merely represents one type or form factor for the processing system  104 . In other cases, the processing system  104  may be another type of portable electronic device, such as a tablet computer, a wearable electronic device (e.g., a smart watch, a head-mounted display), a notebook computer, or any other suitable portable electronic device. In some cases, the processing system  104  may be another type of electronic or computing device, such as a desktop computer, a gaming console, a voice-activated digital assistant, or any other suitable electronic device. The processing system  104  may perform various operations of the electronic system  101 , including for example determining whether a head-mounted haptic accessory  102  is being worn, determining when haptic outputs are to be produced via the head-mounted haptic accessory  102 , determining actuation patterns for the haptic actuators of the head-mounted haptic accessory  102 , and the like. The processing system  104  may also provide audio signals to the head-mounted haptic accessory  102  (such as where the head-mounted haptic accessory  102  is a pair of headphones or earbuds). Audio signals may be digital or analog, and may be processed by the processing system  104  and/or the head-mounted haptic accessory  102  to produce an audio output (e.g., audible sound). Audio signals may correspond to, include, or represent audio data from various different sources, such as teleconference voice data, an audio portion of a real-time video stream, an audio track of a recorded video, an audio recording (e.g., music, podcast, spoken word, etc.), or the like. The processing system  104  may also perform other operations of the electronic system  101  as described herein. 
       FIG. 2A  is a side view of a user  200  wearing a head-mounted haptic accessory that includes earbuds  202  each having a haptic actuator positioned within an earbud body.  FIG. 2B  is a schematic top view of the user  200 , illustrating how the earbuds  202  define an array of haptic actuation points  204  on the head of the user  200 . Because the earbuds  202  (or another pair of headphones or head-worn audio device) are positioned on or in the ear of the user  200 , the haptic actuation points are on opposite lateral sides of the user&#39;s head. 
       FIG. 3A  is a side view of a user  300  wearing a head-mounted haptic accessory embodied as a pair of glasses  302  that includes haptic actuators  303  positioned at various locations on the glasses  302 . For example, an actuator may be positioned on each temple piece, and another may be positioned on a nose bridge segment of the glasses  302 .  FIG. 3B  is a schematic top view of the user  300 , illustrating how the glasses  302 , and more particularly the actuators  303  of the glasses  302 , define an array of haptic actuation points  304  on the head of the user  300 . As shown in  FIG. 3B , two haptic actuation points are positioned on opposite lateral sides of the head, and one is positioned on the center of the head (e.g., on or near the bridge of the user&#39;s nose). In some cases, more or fewer haptic actuators may be included in the glasses  302 . For example, the actuator on the nose bridge segment may omitted. 
       FIG. 4A  is a side view of a user  400  wearing a head-mounted haptic accessory embodied as a headband  402  that includes haptic actuators  403  positioned at various locations along the headband  402 . For example, eight actuators  403  may be positioned at various locations around the headband  402 , though more or fewer actuators  403  are also contemplated.  FIG. 4B  is a schematic top view of the user  400 , illustrating how the headband  402 , and more particularly the actuators  403  of the headband  402 , define an array of haptic actuation points  404  on the head of the user  400 . As shown in  FIG. 4B , the actuation points  404  are positioned equidistantly around the circumference of the user&#39;s head, though this is merely one example arrangement. Further, while  FIGS. 4A-4B  illustrate the head-mounted haptic accessory as a headband, this embodiment may equally represent any head-worn clothing, device, or accessory that wraps around some or all of the user&#39;s head, including but not limited to hats, caps, head-mounted displays, hoods, visors, helmets, and the like. 
     The arrays of haptic actuators shown and described with respect to  FIGS. 2A-4B  illustrate examples in which the haptic actuators define a radial array of actuators that at least partially encircle or surround a user&#39;s head. The radial array configurations may help convey directionality to the user via the haptic outputs. For example, the haptic actuators of the various head-mounted haptic accessories may be initiated in accordance with an actuation pattern that is recognizable as indicating a particular direction to a user. Such directional haptic outputs can be used to direct a user&#39;s attention in a particular direction, such as towards a virtual position of a virtual audio source. By directing the user&#39;s attention in this way, the user may be subtly directed to move his or her head to face the position of the virtual audio source, which may increase engagement of the wearer with the audio source, especially where multiple audio sources (and thus multiple positions) are active. Additional details of example actuation patterns and particular use cases for producing the actuation patterns are described herein. 
       FIG. 5  is an example flow chart of a method  500  of operating an electronic system that produces directional haptic outputs, as described herein. At operation  502 , a condition is detected (e.g., by the electronic system  101 ). The condition may be any suitable condition that is a triggering event for initiating a haptic output (e.g., a directional haptic output) via a wearable haptic device (e.g., a head-mounted haptic accessory  102 ). For example, detecting the condition may include or correspond to detecting a presence of an audio source in an audio signal, where the audio source may be associated with a virtual position relative to the user. More particularly, as described in greater detail with respect to  FIGS. 10A-10B , if the user is engaged in a conference call with multiple participants, each participant may have an assigned virtual location relative to the user. In this case, detecting the condition may include detecting that one of the participants is speaking or otherwise producing audio. Detecting the condition may also include detecting whether a characteristic of a signal, including but not limited to a volume or amplitude of an audio output corresponding to an audio signal, has satisfied a threshold value. For example, in the context of a multi-party conference call, detecting the condition may include detecting that an audio output associated with one of the participants has satisfied a threshold value (e.g., a threshold volume). 
     As another example, detecting the condition may include or correspond to detecting a notification indicating that the user has received a message, or that a graphical object (or audio message) has been received or is otherwise available in a virtual environment. As yet another example, detecting the condition may include or correspond to detecting the presence of an interactive object or affordance in a virtual environment. As used herein, an interactive object may correspond to or be associated with a graphical object in a virtual environment and that a user can interact with in a manner beyond mere viewing. For example, a user may be able to select the interactive object, virtually manipulate the interactive object, provide inputs to the interactive object, or the like. As one specific example, where the virtual environment corresponds to a gaming application, an interactive object may be an item that the user may select and add to his or her inventory. As another specific example, where the virtual environment corresponds to a word processing application, the interactive object may be a selectable icon that controls a program setting of the application. 
     At operation  504 , it is determined whether a wearable haptic accessory is being worn by a user. For example, a processing system  104  may detect whether a head-mounted haptic accessory  102  is being worn by a user. In some cases, the head-mounted haptic accessory  102  may determine whether it is being worn by either sensing the presence of the user (using, for example, a proximity sensor), or by inferring from an orientation or motion of the head-mounted haptic accessory  102  that it is being worn (using, for example, an accelerometer or magnetometer or motion sensor). The head-mounted haptic accessory  102  may report to the processing system  104  whether it is or is not being worn. If the processing system  104  cannot communicate with a head-mounted haptic accessory, the processing system  104  may assume that no head-mounted haptic accessory is available. 
     If it is determined that a head-mounted haptic accessory is being worn by a user, a directional component for a haptic output may be determined at operation  506 . The directional component for the haptic output may correspond to a direction that a user must turn his or her head or body in order to be facing a desired position or location. For example, if a user is not facing a virtual position or location of an audio source, the directional component for the haptic output may be a direction that the user must turn his or her head or body in order to face the virtual position or location. In some cases, the determination of the directional component for the haptic output may be based at least in part on an orientation of the wearer of the head-mounted haptic accessory. Such information may be determined by the head-mounted haptic accessory, such as via sensors (e.g., accelerometers, magnetometers, gyroscopes, orientation sensors) incorporated with the head-mounted haptic accessory. Such information may be reported to the processing system  104 , which may then determine the directional component. Determining the directional component may also include determining an actuation pattern for an array of actuators on the head-mounted haptic accessory. For example, if the directional component indicates that the user needs to turn his or her head 30 degrees to the left, the pattern may cause the haptic actuators to fire in a sequence that moves across the user&#39;s body from right to left. 
     At operation  508 , in response to detecting the condition and determining the directional component (e.g., determining the actuation pattern), determining that the haptic accessory is being worn by the user, and determining the directional component for the haptic output, the haptic output may be produced. As described herein, this may include sending a signal to the haptic accessory that will cause the haptic accessory to produce the haptic output in accordance with the directional component. As described in greater detail herein, the haptic output may produce a sensation that has an identifiable directional component or that otherwise suggests a particular direction to a user. For example, a sequence of haptic outputs may travel around a user&#39;s head from left to right, indicating that the user should direct his or her orientation along that direction (e.g., to the right). As another example, a haptic output may produce a tugging or pulling sensation that suggests the direction that a user should move (e.g., rotate) his or her head. 
     In some cases, a signal defining or containing the actuation may be sent to the haptic accessory from the processing system. In other cases, data defining haptic patterns is stored in the haptic accessory, and the processing system sends a message (and optionally an identifier of a particular actuation pattern) to the haptic accessory that causes the haptic accessory to produce the haptic output. 
       FIG. 5  describes a general framework for the operation of an electronic system as described herein. It will be understood that certain operations described herein may correspond to operations explicitly described with respect to  FIG. 5 , while other operations may be included instead of or in addition to operations described with respect to  FIG. 5 . 
     As described above, haptic outputs delivered via a head-mounted haptic accessory may include a directional component or may otherwise be configured to direct the user&#39;s attention along a particular direction. In order to indicate a direction to a user, an actuation pattern or sequence may be used to produce a tactile sensation that suggests a particular direction to the wearer. Actuation patterns where haptic outputs are triggered or produced sequentially (e.g., at different times) may be referred to as a haptic sequence or actuation sequence. 
       FIGS. 6A-6B  are schematic top views of a user wearing various types of head-mounted haptic accessories, as well as example actuation patterns that may produce the intended tactile sensation.  FIG. 6A  illustrates a schematic top view of a user  600  having a head-mounted haptic accessory with two actuation points  602 - 1 ,  602 - 2 . The head-mounted haptic accessory may correspond to a pair of earbuds or other headphones that are worn on, in, or around the user&#39;s ears. Alternatively, the head-mounted haptic accessory may be any device that defines two haptic actuation points. 
       FIGS. 6A-6B  provide an example of how a haptic output may be configured to orient a user toward a virtual objet or direct the user&#39;s attention along a particular direction. For example, in order to produce a haptic output to direct the user  600  to turn to the right (indicated by arrow  604 ), the electronic system may initiate a haptic sequence  605  that causes an actuator associated with the first actuation point  602 - 1  to produce a haptic output  606  that decreases in intensity over a time span. (Arrow  610  in  FIG. 6A  indicates a time axis of the actuation sequence.) After, or optionally overlapping with, the first haptic output  606 , a haptic actuator associated with the second actuation point  602 - 2  may produce a haptic output  608  that increases in intensity over a time span. This haptic sequence may produce a tactile sensation that is indicative or suggestive of a right-hand direction, which may signal to the wearer that he or she should turn his or her head to the right. 
     The intensity of a haptic output may correspond to any suitable characteristic or combination of characteristics of a haptic output that contribute to the perceived intensity of the haptic output. For example, changing an intensity of a haptic output may be achieved by changing an amplitude of a vibration of the haptic actuator, by changing a frequency of a vibration of the haptic actuator, or a combination of these actions. In some cases, higher intensity haptic outputs may be associated with relatively higher amplitudes and relatively lower frequencies, whereas lower intensity haptic outputs may be associated with relatively lower amplitudes and relatively higher frequencies. 
       FIG. 6B  illustrates a schematic top view of a user  611  having a head-mounted haptic accessory with three actuation points  612 - 1 ,  612 - 2 , and  612 - 3 . The head-mounted haptic accessory may correspond to a pair of glasses (e.g., the glasses  302 ,  FIG. 3A ), a headband (e.g., the headband  402 ,  FIG. 4A ), or any other suitable head-mounted haptic accessory. 
     In order to produce a haptic output that is configured to direct the user&#39;s attention along a given direction, and more particularly to direct the user  611  to turn to the right (indicated by arrow  614 ), the electronic system may initiate an actuation sequence  615 . The actuation sequence  615  may cause an actuator associated with the first actuation point  612 - 1  to produce a first haptic output  616 , then cause an actuator associated with the second actuation point  612 - 2  to produce a second haptic output  618  , and then cause an actuator associated with the third actuation point  612 - 3  to produce a third haptic output  620 . (Arrow  622  in  FIG. 6A  indicates a time axis of the actuation sequence.) The actuation sequence  615  thus produces a series of haptic outputs that move along the user&#39;s head from left to right. This haptic sequence may produce a tactile sensation that is indicative or suggestive of a right-hand direction, which may signal to the wearer that he or she should turn his or her head to the right. As shown, the haptic outputs  616 ,  618 ,  620  do not overlap, though in some implementations they may overlap. 
       FIG. 6B  also illustrates another example actuation sequence  623  that may be used to direct the user to turn to the right. In particular, the electronic system may cause an actuator associated with the first actuation point  612 - 1  to produce a first haptic output  624  having a series of haptic outputs having changing (e.g., increasing) duration and/or period. The electronic system may then cause an actuator associated with the second actuation point  612 - 2  to produce a second haptic output  626  having a series of haptic outputs having changing (e.g., increasing) duration and/or period. The electronic system may then cause an actuator associated with the third actuation point  612 - 3  to produce a third haptic output  628  having a series of haptic outputs having changing (e.g., increasing) duration and/or period. As shown, the first, second, and third haptic outputs  624 ,  626 ,  628  may overlap, thus producing a tactile sensation that continuously transitions around the user&#39;s head from left to right. This haptic sequence may produce a tactile sensation that is indicative or suggestive of a right-hand direction, which may signal to the wearer that he or she should turn his or her head to the right. 
     The haptic outputs shown in  FIG. 6B  include square waves, though this is merely a representation of example haptic outputs and is not intended to limit the haptic outputs to any particular frequency, duration, amplitude, or the like. In some cases, the square waves of the haptic outputs may correspond to impulses, such as mass movements along a single direction. Thus, the haptic output  624 , for example, may be perceived as a series of taps having an increasing duration and occurring at an increasing time interval. In other cases, the square waves of the haptic outputs may correspond to a vibrational output having a duration represented by the length of the square wave. In such cases, the haptic output  624 , for example, may be perceived as a series of vibrational outputs having an increasing duration and occurring an at increasing time interval but maintaining a same frequency content. 
     Directional haptic outputs such as those described with respect to  FIGS. 6A-6B  may be used to direct a user&#39;s attention along a particular direction, such as towards a virtual position of a participant on a conference call, along a path dictated by a navigation application, or the like. In some cases, the haptic outputs are produced a set number of times (e.g., once, twice, etc.), regardless of whether or not the user changes his or her orientation. In other cases, the electronic system monitors the user after and/or during the haptic outputs to determine if the user has directed his or her attention along the target direction. In some cases, a haptic output will be repeated until the user has reoriented himself or herself to a target position and/or orientation, until a maximum limit of haptic outputs is reached (e.g., which may be two, three, four, or another number of haptic outputs). 
     As used herein, a haptic output may refer to individual haptic events of a single haptic actuator, or a combination of haptic outputs that are used together to convey information or a signal to a user. For example, a haptic output may correspond to a single impulse or tap produced by one haptic actuator (e.g., the haptic output  616 ,  FIG. 6B ), or a haptic output that is defined by or includes a haptic pattern (e.g., the actuation sequence  623 ,  FIG. 6B ). As used herein, a haptic output that includes a directional component or otherwise produces a tactile sensation that travels along a direction, or that appears to act in a single direction, may be referred to as a directional haptic output. 
       FIG. 7A  illustrates an example earbud  702  that may be part of a head-mounted haptic actuation accessory. The earbud  702  may include an earbud body  704  that is configured to be received at least partially within an ear of a user. As noted above, the earbud  702  may include a speaker positioned within the earbud body and configured to output sound into the user&#39;s ear. The earbud  702  may also include a haptic actuator  706  positioned within the earbud body and configured to impart a haptic output to the user&#39;s ear. More particularly, the haptic actuator  706  may be configured to impart the haptic output to the user&#39;s ear via the interface between the earbud body  704  and the portion of the user&#39;s ear canal that the earbud body  704  touches when the earbud  702  is positioned in the user&#39;s ear. The haptic actuator  706  may be any suitable type of haptic actuator, such as a linear resonant actuator, piezoelectric actuator, eccentric rotating mass actuator, force impact actuator, or the like. 
     The earbud  702  (and more particularly the haptic actuator  706 ) may be communicatively coupled with a processor, which may be onboard the earbud  702  or part of a processing system (e.g., the processing system  104 ,  FIG. 1A ). While  FIG. 7A  shows one earbud  702 , it will be understood that the earbud  702  may be one of a pair of earbuds that together form all or part of a head-mounted haptic accessory, and each earbud may have the same components and may be configured to provide the same functionalities (including the components and functionalities described above). 
     In some cases, the haptic actuator  706  may be configured to produce directional haptic outputs that do not require a pattern of multiple haptic outputs produced by an array of haptic actuators. For example, the haptic actuator  706 , which may be linear resonant actuator, may include a linearly translatable mass that is configured to move along an actuation direction that is substantially horizontal when the earbud is worn in the user&#39;s ear. This mass may be moved in a manner that produces a directional haptic output. More particularly, the mass may be accelerated along a single direction and then decelerated to produce an impact that acts in a single direction. The mass may then be moved back to a neutral position without producing a significant force in the opposite direction, thus producing a tugging or pushing sensation along a single direction. 
       FIG. 7B  illustrates a schematic top view of a user wearing earbuds as shown in  FIG. 7A , defining haptic actuation points  710 ,  711  (e.g., in the ear of the user).  FIG. 7B  illustrates how a haptic output from the haptic actuator  706  may produce a directional haptic output that is configured to direct the user to the right (as indicated by the arrow  712 ). In particular, the mass of the haptic actuator  706  may be moved in direction indicated by arrow  708  in  FIG. 7A  to produce an impulse acting along a horizontal direction. This may cause the earbud  702  to impart a reorientation force  714  on the user via the actuation point  710 , where the reorientation force  714  acts (or is perceived by the user to act) only in a single direction. The reorientation force  714  may in fact be perceived as a tap or tug on the user&#39;s ear in a direction that corresponds to the desired orientation change of the user. For example, the reorientation force may direct the user&#39;s attention to the left or to the right along a horizontal plane. 
     A directional haptic output as described with respect to  FIG. 7B  may be produced with only a single earbud and/or single haptic actuator. In some cases, however, the effect may be enhanced by using the other earbud (e.g., at the haptic actuation point  711 ) to produce a reorientation force  716  acting in the opposite direction as the force  714 . While this force may be produced along an opposite direction, it would indicate the same rotational or directional component as the force  714 , and thus would suggest the same type of reorientation motion to the user. The reorientation forces  714 ,  716  may be simultaneous, overlapping, or they may be produced at different times (e.g., non-overlapping). 
     The earbud(s) described with respect to  FIG. 7A  may be used to produce the haptic outputs described with respect to  FIG. 7B , or any other suitable type of haptic output. For example, the earbuds may be used to produce directional haptic outputs using the techniques described with respect to  FIGS. 6A-6B . 
     In some cases, in addition to or instead of directional outputs, a head-mounted haptic accessory may be used to produce non-directional haptic outputs. In some cases, a user may only be able to differentiate a limited number of different haptic outputs via their head. Accordingly, a haptic output scheme that includes a limited number of haptic outputs may be used with head-mounted haptic accessories.  FIG. 8  illustrates one example haptic output scheme  800 . The scheme may include three haptic syllables  802 - 1 - 802 - 3  that may be combined to produce larger haptic words  804 - 1 - 804 - 7  and  806 - 1 - 806 - 3 . The haptic syllables may include a low-intensity syllable  802 - 1 , a medium-intensity syllable  802 - 2 , and a high-intensity syllable  802 - 3 . The intensity of the syllable may correspond to any suitable property or combination of properties of a haptic output. For example, if all of the haptic syllables are vibrations of the same frequency, the intensity may correspond to the amplitude of the vibrations. Other combinations of haptic properties may also be used to create syllables of varying intensity. For example, lower frequencies may be used to produce the higher-intensity haptic syllables. Further, the haptic syllables  802  may have multiple different properties. For example, they each may have a unique frequency and a unique amplitude and a unique duration. 
     The haptic syllables  802  may also be combined to form haptic words  804 - 1 - 804 - 7  (each including two haptic syllables) and haptic words  806 - 1 - 806 - 3  (each including three haptic syllables). In some cases, each haptic syllable (whether used alone or in haptic words) may be produced by all haptic actuators of a head-mounted haptic accessory simultaneously. For example, when the haptic word  804 - 3  is produced by the headband  402  ( FIG. 4A ), all of the actuators  403  may simultaneously produce the low-intensity haptic syllable  802 - 1 , and subsequently all actuators may produce the high-intensity haptic syllable  802 - 3 . This may help differentiate the haptic words  804  and  806  from directional haptic outputs. (Directional haptic outputs as described above may also be considered part of the haptic output scheme  800 .) 
     In some cases, each haptic word or syllable may have a different meaning or be associated with a different message, alert, or other informational content. For example, different haptic words may be associated with different applications on a user&#39;s smartphone or computer. Thus, the user may be able to differentiate messages from an email application (which may always begin with a low-intensity syllable) from those from a calendar application (which may always begin with a high-intensity syllable). Other mappings are also possible. Moreover, in some cases only a subset of the syllables and words in the haptic output scheme  800  is used in any given implementation. 
     While the directional haptic outputs and the haptic output schemes described herein may all be suitable for use with a head-mounted haptic accessory, each head-mounted haptic accessory may produce slightly different sensations when its haptic actuator(s) are fired. Due to these differences, each type of head-mounted haptic accessory may be associated with a different haptic output scheme that is tailored to the particular properties and/or characteristics of that particular head-mounted haptic accessory.  FIG. 9  is a chart showing example differences in how haptics may be perceived when delivered via different types of head-mounted haptic accessories. For example,  FIG. 9  depicts the relative intrusiveness of haptic outputs provided by a pair of earbuds  902 , a headband  904 , and glasses  906 . For example, due to the positioning of the earbuds  902  directly in a user&#39;s ear, haptic outputs from the earbuds  902  may be relatively more intrusive than those produced by the headband  904  or the glasses  906 . As used herein, intrusiveness may refer to the subjective annoyance, irritation, distraction, or other negative impression of a haptic output. For example, an oscillation having a high amplitude and duration that is felt within a user&#39;s ear may be considered highly intrusive, whereas that same physical haptic output may be found to be less intrusive and potentially even too subtle when delivered via glasses. 
     Due to the differences in intrusiveness of haptic outputs, haptic schemes for the various head-mounted haptic accessories may have different properties.  FIG. 9 , for example, shows each head-mounted haptic accessory  902 ,  904 , and  906  using a different haptic scheme, with each scheme using haptic outputs with different durations. More particularly, the haptic accessory that may be considered to have the greatest intrusiveness may use haptic outputs of a shorter duration, while the haptic accessories with lower intrusiveness may use haptic outputs of a greater duration. This is merely one example property that may differ between various haptic schemes, and other properties and/or characteristics of the haptic outputs may also vary between the schemes to accommodate for the differences in the head-mounted haptic accessories. For example, each haptic scheme may use oscillations or outputs having different frequencies, amplitudes, actuation patterns or sequences, and the like. 
     In some cases, an electronic system as described herein may be used with different types of head-mounted haptic accessories. Accordingly, a processing system (e.g., the processing system  104 ) may determine what type of head-mounted haptic accessory is being worn or is otherwise in use, and select a particular haptic scheme based on the type of head-mounted haptic accessory. In some cases, the haptic schemes may be pre-defined and assigned to particular head-mounted haptic accessories. In other cases, a processing system may adjust a base haptic scheme based on the type of head-mounted haptic accessory in use. For example, the base scheme may correspond to haptic outputs of the shortest available duration. If earbuds are determined to be in use, the base haptic scheme may be used without modification. If the headband is in use, the base haptic scheme may be modified to have longer-duration haptic outputs. And if the glasses are determined to be in use, the base haptic scheme may be modified to have even longer-duration haptic outputs. Other modifications may be employed depending on the duration of the haptic outputs in the base scheme (e.g., the modifications may increase or decrease the durations of the haptic outputs in the base scheme, in accordance with the principles described herein and shown in  FIG. 9 ). 
     Various types of directional haptic outputs are described above. Directional haptic outputs may be configured to direct a user&#39;s attention along a direction. This functionality may be used in various different contexts and for various different purposes in order to enhance the user&#39;s experience. Several example use cases for directional haptic outputs are described herein with respect to  FIGS. 10A-10B and 12A-12B . It will be understood that these use cases are not exhaustive, and directional haptic outputs described herein may be used in other contexts and in conjunction with other applications, interactions, use cases, devices, and so forth. Moreover, while these use cases are shown using earbuds as the head-mounted haptic accessory, it will be understood that any other suitable head-mounted haptic accessory may be used instead of or in addition to the earbuds. 
       FIGS. 10A-10B  illustrate an example use case in which a directional haptic output is used to direct a user&#39;s attention to a particular audio source in the context of a teleconference. For example, a user  1000  may be participating in a teleconference with multiple participants,  1002 - 1 ,  1002 - 2 , and  1002 - 3  (collectively referred to as participants  1002 ). The teleconference may be facilitated via telecommunications devices and associated networks, communication protocols, and the like. 
     The user  1000  may receive teleconference audio (including audio originating from the participants  1002 ) via earbuds  1001 . The earbuds  1001  may be communicatively connected to another device (e.g., the processing system  104 ,  FIG. 1A ) that sends the audio to the earbuds  1001 , receives audio from the user  1000 , transmits the audio from the user  1000  to the participants  1002 , and generally facilitates communications with the participants  1002 . 
     The participants  1002  may each be assigned a respective virtual position relative to the user  1000  (e.g., a radial orientation relative to the user and/or the user&#39;s orientation and optionally a distance from the user), as represented by the arrangement of participants  1002  and the user  1000  in  FIGS. 10A-10B . When it is detected that one of the participants  1002 - 3  is speaking, the earbuds  1001  may produce a directional haptic output  1006  that is configured to direct the user&#39;s attention to the virtual position of the participant  1002 - 3  from which the audio is originating. For example, a directional haptic output as described herein may be produced via the earbuds  1001  to produce a directional sensation that will suggest that the user  1000  reorient his or her head or body to face the participant  1002 - 3  (e.g., a left-to-right sensation, indicated by arrow  1004 , or any other suitable haptic output that suggests a left-to-right reorientation).  FIG. 10B  illustrates the user  1000  after his or her orientation is aligned with the virtual position of the audio source (the participant  1002 - 3 ). 
     A system may determine the participant  1002  from which an audio source is originating (e.g., which participant is speaking or active) based on any suitable information or data. For example, in some cases, the participant  1002  to whom attention is directed may be the only participant who is speaking, or the first participant to begin speaking after a pause, or the participant who is speaking loudest, or the participant who has been addressed with a question, or the participant to whom other users or participants are already looking at. As one particular example of the last case, in a teleconference with four participants, if two participants direct their attention to a third participant (e.g., by looking in the direction of the third participant&#39;s virtual position), a directional haptic output may be provided to the fourth participant to direct his or her attention to the third participant (e.g., to the third participant&#39;s virtual position). 
     As shown, the haptic output  1006  is not active in  FIG. 10B . This may be due to the earbuds  1001  (or other device or sensor) determining that the user&#39;s orientation is aligned with the virtual position of the audio source. For example, in some cases the haptic output  1006  may continue (e.g., either continuously or repeatedly) until it is determined that the user is facing or oriented towards the desired position. In other cases, the haptic output  1006  is produced once or a set number of times, regardless of the user&#39;s orientation or change in orientation. The latter case may occur when position or orientation information is not available or is not being captured. 
     Haptic outputs may also be used in the context of a teleconference to indicate to the user that other participants have directed their attention to the user.  FIG. 11  illustrates an example teleconference that includes a user  1100  using a head-mounted haptic accessory  11101  (e.g., earbuds) and participants  1102 - 1 ,  1102 - 2 , and  1102 - 3  (collectively referred to as participants  1102 ). As indicated by the dashed arrows, all of the participants  1102  have directed their attention to the user. Determining when and whether the participants  1102  have directed their attention to the user may be performed in any suitable way. For example, the participants  1102  may be associated with sensors (which may be incorporated in a head-mounted haptic accessory) that can determine whether or not the participants  1102  are facing or otherwise oriented towards a virtual position associated with the user  1100 . Such sensor may include gaze detection sensors, accelerometers, proximity sensors, gyroscopes, motion sensors, or the like. In other examples, the participants  1102  may manually indicate that they are focused on the user  1100 , such as by clicking on a graphic representing the user  1100  in a graphical user interface associated with the teleconference. 
     A processing system associated with the user  1100  may detect or receive an indication that attention is focused on the user  1100  or that the user  1100  is expected to speak and, in response, initiate a haptic output  1106  via the head-mounted haptic accessory  1101 . In this case, the head-mounted haptic accessory may not have a directional component. 
     The use cases described with respect to  FIGS. 10A-11  may be used in conjunction with one another in a teleconference system or context. For example, the user  1100  and the participants  1102  (or a subset thereof) may each have a head-mounted haptic accessory and a system that can determine their orientation and/or focus. Directional haptic outputs may then be used to help direct attention to an active participant, and non-directional haptics may be used to indicate to the active participant that he or she is the focus of the other participants. These haptic outputs may all be provided via head-mounted haptic accessories and using haptic outputs as described herein. 
     Another context in which directional and other haptic outputs delivered via a head-mounted haptic accessory includes virtual- , augmented-, and/or mixed-reality environments. As used herein, the term virtual reality will be used to refer to virtual-reality, mixed-reality, and augmented-reality environments or contexts. In some cases, virtual-reality environments may be presented to a user via a head-mounted display, glasses, or other suitable viewing device(s). 
       FIGS. 12A-12B  illustrate an example use case in which directional haptic outputs are used to enhance a virtual-reality experience. A user  1200  may be wearing a head-mounted display (HMD)  1202 , which may be displaying to the user  1200  a graphical output representing a virtual environment  1201 . The user  1200  may also be wearing a head-mounted haptic accessory  1204 , shown in  FIGS. 12A-12B  as earbuds. 
     While the user is viewing the virtual environment  1201 , a notification may be received by the HMD (or any suitable processing system) indicating that a graphical object  1210  ( FIG. 12B ) is available to be viewed in the virtual environment  1201 . The graphical object  1210  may be out of the field of view of the user when the notification is received. For example, as shown in  FIG. 12B , the graphical object  1210  may have a virtual position that is to the right of the user&#39;s view of the virtual environment  1201 . Accordingly, the HMD (or any other suitable processing system) may direct the head-mounted haptic accessory  1204  to initiate a directional haptic output  1206  that is configured to orient the user towards the virtual position of the graphical object  1210  (e.g., to the right, as indicate by arrow  1208 ). As shown in  FIG. 12B , in response to the user  1200  moving his or her head in the direction indicated by the directional haptic output  1206 , the scene of the virtual environment  1201  may be shifted a corresponding distance and direction (e.g., a distance and/or direction that would be expected in response to the reorientation of the user&#39;s head). This shift may also bring the graphical object  1210  into the user&#39;s field of view, allowing the user  1200  to view and optionally interact with the graphical object  1210 . Directional haptic outputs may also or instead be used to direct users&#39; attention to other objects in a virtual environment, such as graphical objects with which a user can interact, sources of audio, or the like. 
     Head-mounted haptic accessories may also be used to enhance the experience of consuming audio and video content. For example, haptic outputs may be initiated in response to certain audio features in an audio stream, such as loud noises, significant musical notes or passages, sound effects, and the like. In the context of a video stream, haptic outputs may be initiated in response to visual features and/or corresponding audio features that accompany the visual features. For example, haptic outputs may be initiated in response to an object in a video moving in a manner that appears to be in proximity to the viewer. Directional haptic outputs may also be used in these contexts to enhance the listening and/or viewing experience. For example, different instruments in a musical work may be assigned different virtual positions relative to a user, and when the user moves relative to the instruments, the haptic output may change based on the relative position of the user to the various instruments. These and other examples of integrating haptic outputs with audio and/or video content are described with respect to  FIGS. 13A-14B . 
       FIGS. 13A-13B  depict an example feature identification technique that may be used to integrate haptic outputs with audio content.  FIG. 13A  illustrates a plot  1300  representing audio data  1302  (e.g., a portion of a musical track, podcast, video soundtrack, or the like). The audio data  1302  includes an audio feature  1304 . The audio feature  1304  may be an audibly distinct portion of the audio data  1302 . For example, the audio feature  1304  may be a portion of the audio data  1302  representing a distinctive or a relatively louder note or sound, such as a drum beat, cymbal crash, isolated guitar chord or note, or the like. In some cases, the audio feature  1304  may be determined by analyzing the audio data to identify portions of the audio data that satisfy a threshold condition. The threshold condition may be any suitable threshold condition, and different conditions may be used for different audio data. For example, a threshold condition used to identify audio features in musical work may be different from a threshold condition used to identify audio features in a soundtrack of a video. 
     In one example, the threshold condition may be based on the absolute volume or amplitude of the sound in the audio data. In this case, any sound at or above the absolute volume or amplitude threshold may be identified as an audio feature. In another example, the threshold condition may be based on a rate of change of volume or amplitude of the sound in the audio data. As yet another example, the threshold condition may be based on the frequency of the sound in the audio data. In this case, any sound above (or below) a certain frequency value, or a sound within a target frequency range (e.g., within a frequency range corresponding to a particular instrument), may be identified as an audio feature, and low-, high-, and/or band-pass filters may be used to identify the audio features. These or other threshold conditions may be combined to identify audio features. For example, the threshold condition may be any sound at or below a certain frequency and above a certain amplitude. Other threshold conditions are also contemplated. 
     In some cases, once an audio feature is identified, or as part of the process of identifying the audio feature, a triggering event of the audio feature may be detected. The triggering event may correspond to or indicate a time that audio feature begins. For example, detecting the triggering event may include determining that a rate of change of an amplitude of the audio signal and/or the audio output satisfies a threshold. This may correspond to the rapid increase in volume, relative to other sounds in the audio data, that accompanies the start of an aurally distinct sound, such as a drumbeat, a bass note, a guitar chord, a sung note, or the like. The triggering event of an audio feature may be used to signify the beginning of the audio feature, and may be used to determine when to initiate a haptic output that is coordinated with the audio feature. 
     A duration or end point of the audio feature may also be determined. For example, in some cases the end of the audio feature may correspond to a relative change in volume or amplitude of the audio data. In other cases, it may correspond to an elapsed time after the triggering event. Other techniques for identifying the end point may also be used. 
     Once the audio feature is detected, a characteristic frequency of the audio feature may be determined. The characteristic frequency may be the most prominent (e.g., loudest) frequency or an average frequency of the audio feature. For example, a singer singing an “A” note may produce an audio feature having a characteristic frequency of about 440 Hz. As another example, a bass drum may have a characteristic frequency of about 100 Hz. As yet another example, a guitar chord of A major may have a characteristic frequency of about 440 Hz (even though the chord may include other notes as well). 
     Once the characteristic frequency has been determined, a haptic output may be provided via a head-mounted haptic accessory, where the haptic output has a haptic frequency that is selected in accordance with the characteristic frequency of the audio feature. For example, the haptic frequency may be the same as the characteristic frequency, or the haptic frequency may be a complementary frequency to the characteristic frequency. 
     As used herein, a complementary frequency may correspond to a frequency that does not sound discordant when heard in conjunction with the audio feature. More particularly, if an audio feature has a characteristic frequency of 200 Hz, a haptic output having a haptic frequency of 190 Hz may sound grating or discordant. On the other hand, a haptic frequency of 200 Hz or 100 Hz (which may be the same note one octave away from the 200 Hz sound) may sound harmonious or may even be substantially or entirely masked by the audio feature. In some cases, the complementary frequency may be a harmonic of the characteristic frequency (e.g., 2, 3, 4, 5, 6, 7, or 8 times the characteristic frequency, or any other suitable harmonic) or a subharmonic of the characteristic frequency (e.g., ½, ⅓, ¼, ⅕, ⅙, 1/7, or ⅛ of the characteristic frequency, or any other suitable subharmonic). 
       FIG. 13B  illustrates a plot  1310  representing a haptic response of one or more haptic actuators of a head-mounted haptic accessory. The haptic response includes a haptic output  1312 , which is produced while the audio feature  1304  is being outputted. In some cases, the haptic output is provided for the full duration of the audio feature, for less than the full duration of the audio feature, or for any other suitable duration. In some cases, the haptic output is provided for a fixed duration after the triggering event of the audio feature (e.g., 0.1 seconds, 0.25 seconds, 0.5 seconds, 1.0 seconds, or any other suitable duration). The experience of hearing the audio feature  1304  while also feeling the haptic output  1312  may produce an enhanced listening experience. 
     While the haptic output  1312  is shown as a square output, this is merely for illustration, and the haptic output  1312  may have varying haptic content and/or characteristics. For example, the intensity of the haptic output  1312  (which may correspond to various combinations of frequency, amplitude, or other haptic characteristics) may vary as the haptic output  1312  is being produced. As one example, the intensity may taper continuously from a maximum initial value to zero (e.g., to termination of the haptic output). As another example, the intensity of the haptic output  1312  may vary in accordance with the amplitude of the audio feature (e.g., it may rise and fall in sync with the audio feature). As yet another example, the frequency of the haptic output  1312  may vary. More particularly, the frequency of the haptic output  1312  may vary in accordance with a variation in an audio characteristic of the audio feature (e.g., a varying frequency of the audio feature). In this way, an audible component of the haptic output  1312  may not detract from or be discordant with the audio feature, and may even enhance the sound or listening experience of the audio feature. 
     Identifying audio features in audio data, and associating haptic outputs with the audio features, may also be used for audio data that is associated with video content. For example, audio data associated with a video (such as a soundtrack or audio track for the video) may be analyzed to identify audio features that correspond to video content that may be enhanced by a haptic output. As one specific example, a video may include a scene where a ball is thrown towards the viewer, or in which a truck passes by the viewer, or another scene that includes or is associated with a distinctive sound. Processing the audio data and associating a haptic output in the manner described above may thus result in associating a haptic output with a particular scene or action in the video content. With respect to the examples above, this may result in the viewer feeling a haptic output (e.g., via a head-mounted haptic accessory) when the ball or the truck passes by the viewer. This may provide a sensation that mimics or is suggestive of the tactile or physical sensation that may be experienced when a ball or truck passes a person in real-life. Even if the sensation does not specifically mimic a real-world sensation, it may enhance the viewing experience due to the additional sensations from the haptic output. 
     Other features and aspects described above with respect to configuring a haptic output for audio content may also apply for video content. For example, the haptic output may be configured to have a complementary frequency to the characteristic frequency of the video&#39;s audio feature. Further, the intensity (or other haptic characteristic) of the haptic output may vary in accordance with a characteristic of the audio feature. For example, the intensity of the haptic output may increase along with an increase in the amplitude of the audio feature. 
     The processes and techniques described with respect to  FIGS. 13A-13B  may be performed by any suitable device or system. For example, a smartphone, media player, computer, tablet computer, or the like, may process audio data, select and/or configure a haptic output, send audio data to an audio device (e.g., earbuds) for playback, and initiate a haptic output via a head-mounted haptic accessory. The operations of analyzing audio data to identify audio features, select or configure haptic outputs, and to associate the haptic outputs with the audio features (among other possible operations) may be performed in real-time while the audio is being presented, or they may be performed ahead of time and resulting data may be stored for later playback. Further, a device or processing system that sends audio data to an audio device for playback may also send signals to any suitable head-mounted haptic accessory. For example, if a user is wearing earbuds with haptic actuators incorporated therein, a processing system (e.g., a smartphone or laptop computer) may send the audio and haptic data to the earbuds to facilitate playback of the audio and initiation of the haptic outputs. Where a separate audio device and head-mounted haptic accessory are being used, such as a pair of headphones and a separate haptic headband, the processing system may send the audio data to the headphones and send haptic data to the headband. 
     In addition to or instead of initiating a haptic output to correspond to an audio feature, haptic outputs may be varied based on the position or orientation of a user relative to a virtual location of an audio source.  FIGS. 14A-14B  illustrate one example in which audio sources may be associated with different virtual positions, and in which the relative location of the user to the various audio sources affects the particular haptic output that is produced. 
     In particular,  FIG. 14A  shows a user  1400  at a first position relative to a first audio source  1408  and a second audio source  1410 . As shown in  FIGS. 14A-14B , the first and second audio sources  1408 ,  1410  correspond to different musical instruments (e.g., a drum kit and a guitar, respectively). While they are described as being different audio sources, the sound associated with the first and second audio sources  1408 ,  1410  may be part of or contained within common audio data. For example, the first and second audio sources  1408 ,  1410  may correspond to different portions of a single audio track. As another example, the first and second audio sources  1408 ,  1410  may correspond to different audio tracks that are played simultaneously to produce a song. 
     In some cases, a single audio track may be processed to isolate or separate the audio sources  1408 ,  1410 . For example, sounds within a first frequency range (e.g., a frequency range characteristic of a drum set) may be established as the first audio source  1408 , and sounds within a second frequency range (e.g., a frequency range characteristic of a guitar) may be established as the second audio source  1410 . Other types of audio sources and/or techniques for identifying audio sources may also be used. 
     The multiple audio sources may be assigned virtual positions. For example, the first and second audio sources  1408 ,  1410  may be assigned positions that mimic or are similar to the spatial orientation of two musical instruments in a band. The user  1400  may also be assigned a virtual position.  FIG. 14A  shows the user  1400  at one example position relative to the first and second audio sources  1408 ,  1410  (e.g., the user  1400  is closer to the first audio source  1408  than the second audio source  1410 ). When the user  1400  moves in the real-world environment, the user&#39;s position relative to the virtual positons of the first and second audio sources  1408 ,  1410  may change. For example,  FIG. 14B  shows the user  1400  at another position relative to the first and second audio sources  1408 ,  1410  (e.g., the user  1400  is closer to the second audio source  1410  than the first audio source). Movements and/or translations of the user  1400  in the real-world environment may be determined by any suitable devices, systems, or sensors, including accelerometers, gyroscopes, cameras, imaging systems, proximity sensors, radar, LIDAR, three-dimensional laser scanning, image capture, or any other suitable devices, systems, or sensors. In some cases, instead of the user  1400  moving in real space, the user&#39;s position may be changed virtually. For example, the user  1400  may interact with a device to change his or her position relative to the first and second audio sources  1408 ,  1410 . 
     As noted above, haptic outputs that correspond to or are otherwise coordinated with the first and second audio sources  1408 ,  1410  may be outputted to the user  1400  via a head-worn haptic accessory (or any other suitable haptic accessory). For example, haptic outputs may be initiated in response to audio features from the first and second audio sources  1408 ,  1410 . Thus, for example, haptic outputs may be synchronized with the drumbeats, and other haptic outputs may be synchronized with guitar notes or chords. Techniques described above may be used to identify audio features in the first and second audio sources  1408 ,  1410  and to associate haptic outputs with those features. 
     Changes in the user&#39;s position relative to the first and second audio sources  1408 ,  1410  (based on the user  1400  moving in the real-world environment or based on a virtual position of the user being changed programmatically without a corresponding movement in the real-world environment) may result in changes in the haptic and/or audio outputs provided to the user. For example, as a user moves away from one audio source, the haptic outputs associated with that audio source may reduce in intensity.  FIGS. 14A-14B  illustrate such a phenomenon. In particular, in  FIG. 14A , the user  1400  is positioned relatively closer to the first audio source  1408  (depicted as a drum set) than the second audio source  1410 . A haptic output  1406  and optionally audio corresponding to the first and second audio sources  1408 ,  1410  may be provided via a head-mounted haptic accessory (depicted as earbuds). The haptic output  1406  may be associated with audio features from the first audio source  1408 . When the user  1400  moves further from the first audio source  1408 , either in the real-world environment or by changing his or her virtual position, as shown in  FIG. 14B , a different haptic output  1412  may be produced. As shown, the haptic output  1412  may be of a lower intensity than the haptic output  1406 , representing the increased distance from the first audio source  1408 . This may mimic or suggest a real-world experience of moving around relative to various different audio sources such as a drum set. In particular, a person may feel as well as hear the sound from the drum set. Accordingly, moving away from the drum set may attenuate or change the tactile sensations produced by the drum. This same type of experience may be provided by modifying haptic outputs based on the changes in relative position to an audio source. 
     While  FIGS. 14A-14B  illustrate an example in which multiple audio sources are used, the same techniques may be used for a single audio source. Also, where multiple audio sources are used, the particular haptic outputs provided to the user may include a mix of haptic outputs associated with the various audio sources. For example, the haptic outputs  1406  and  1412  in  FIGS. 14A-14B  may include a mix of haptic outputs that are associated with and/or triggered by the audio from both the first and second audio sources  1408 ,  1410 . In some cases, the haptic outputs associated with the audio sources are weighted based on the relative position of the user to the audio sources. For example, with respect to  FIGS. 14A-14B , the haptic output  1406  may predominantly include haptic outputs associated with the first audio source  1408 , due to the relative proximity of the user  1400  to the first audio source  1408 , while the haptic output  1412  may predominantly include haptic outputs associated with the second audio source  1410 , due to the relative proximity of the user  1400  to the first audio source  1410  in  FIG. 14B . 
     Further, because the audio sources  1408 ,  1410  are associated with virtual positions relative to the user, directional haptic outputs may be provided to direct the user&#39;s attention towards particular audio sources. For example, a directional haptic output may be used to direct the user&#39;s attention to an instrument that is about to perform a solo. When the user moves or reorients himself or herself based on the directional haptic output, aspects of the audio output may also change. For example, the volume of the instrument that the user has turned towards may be increased relative to other instruments. Other audio output manipulations based on changes in the user&#39;s position or orientation, as described above, may also be used. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. For example, while the methods or processes disclosed herein have been described and shown with reference to particular operations performed in a particular order, these operations may be combined, sub-divided, or re-ordered to form equivalent methods or processes without departing from the teachings of the present disclosure. Moreover, structures, features, components, materials, steps, processes, or the like, that are described herein with respect to one embodiment may be omitted from that embodiment or incorporated into other embodiments.