Patent Publication Number: US-11664023-B2

Title: Voice detection by multiple devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 120 to, and is a continuation of, U.S. non-provisional patent application Ser. No. 16/416,752, filed on May 20, 2019, entitled “Voice Detection By Multiple Devices,” which is incorporated herein by reference in its entirety. 
     U.S. non-provisional patent application Ser. No. 16/416,752 claims priority under 35 U.S.C. § 120 to, and is a continuation of, U.S. non-provisional patent application Ser. No. 16/214,666, filed on Dec. 10, 2018, entitled “Voice Detection By Multiple Devices,” and issued as U.S. Pat. No. 10,297,256 on May 21, 2019, which is incorporated herein by reference in its entirety. 
     U.S. non-provisional patent application Ser. No. 16/214,666 claims priority under 35 U.S.C. § 120 to, and is a continuation of, U.S. non-provisional patent application Ser. No. 15/211,748, filed on Jul. 15, 2016, entitled “Voice Detection By Multiple Devices,” and issued as U.S. Pat. No. 10,152,969 on Dec. 11, 2018, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure is related to consumer goods and, more particularly, to methods, systems, products, features, services, and other elements directed to media playback or some aspect thereof. 
     BACKGROUND 
     Options for accessing and listening to digital audio in an out-loud setting were limited until in 2003, when SONOS, Inc. filed for one of its first patent applications, entitled “Method for Synchronizing Audio Playback between Multiple Networked Devices,” and began offering a media playback system for sale in 2005. The Sonos Wireless HiFi System enables people to experience music from many sources via one or more networked playback devices. Through a software control application installed on a smartphone, tablet, or computer, one can play what he or she wants in any room that has a networked playback device. Additionally, using the controller, for example, different songs can be streamed to each room with a playback device, rooms can be grouped together for synchronous playback, or the same song can be heard in all rooms synchronously. 
     Given the ever growing interest in digital media, there continues to be a need to develop consumer-accessible technologies to further enhance the listening experience. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, aspects, and advantages of the presently disclosed technology may be better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG.  1    shows an example media playback system configuration in which certain embodiments may be practiced; 
         FIG.  2    shows a functional block diagram of an example playback device; 
         FIG.  3    shows a functional block diagram of an example control device; 
         FIG.  4    shows an example controller interface; 
         FIG.  5    shows an example plurality of network devices; 
         FIG.  6    shows a functional block diagram of an example network microphone device; 
         FIG.  7    shows a technique according to example embodiments. 
     
    
    
     The drawings are for the purpose of illustrating example embodiments, but it is understood that the inventions are not limited to the arrangements and instrumentality shown in the drawings. 
     DETAILED DESCRIPTION 
     I. Overview 
     Listening to media content out loud can be a social activity that involves family, friends, and guests. Media content may include, for instance, talk radio, books, audio from television, music stored on a local drive, music from media sources (e.g., Pandora® Radio, Spotify®, Slacker®, Radio, Google Play™, iTunes Radio), and other audible material. In a household, for example, people may play music out loud at parties and other social gatherings. In such an environment, people may wish to play the music in one listening zone or multiple listening zones simultaneously, such that the music in each listening zone may be synchronized, without audible echoes or glitches. 
     Such an experience may be enriched when voice commands are used to control an audio playback device or system, among other devices in a household (e.g., lights). For example, a user may wish to change the audio content, playlist, or listening zone, add a music track to a playlist or playback queue, or change a playback setting (e.g., play, pause, next track, previous track, playback volume, and EQ settings, among others) using various voice commands. Some example audio playback devices or systems may include a microphone to detect such voice commands. 
     In some cases, listening to media content out loud is an individual experience. For example, an individual may play music out loud for themselves in the morning before work, during a workout, in the evening during dinner, or at other times throughout the day at home or at work. For these individual experiences, the individual may choose to limit the playback of audio content to a single listening zone or area. Such an experience may be enriched when an individual can use a voice command to choose a listening zone, audio content, and playback settings, among other settings. 
     In some instances, networked microphone devices (NMDs) may be used to control a household. An NMD may be, for example, a SONOS® playback device, server, or system capable of receiving voice input via a microphone. Additionally, an NMD may be a device other than a SONOS® playback device, server, or system (e.g., AMAZON® ECHO®, APPLE® IPHONE®) capable of receiving voice inputs via a microphone. U.S. application Ser. No. 15/098,867 entitled, “Default Playback Device Designation,” which is hereby incorporated by reference, provides examples of voice-enabled household architectures. 
     In some conventional approaches, a single NMD may independently receive or process voice inputs or commands. Indeed, some commercially-available devices contemplate the presence of a single NMD. Accordingly, in a situation where multiple such conventional NMDs are present in a single location, the conventional NMDs might react independently to a single voice input. In such a household in which multiple NMDs are present, coordinating and identifying a set of NMDs from which to determine a voice command from the voice recordings of respective NMDs may provide an improved voice recognition technique that enhances user experience. 
     In some examples, NMDs in a given household may each have one or more microphones to record voice inputs or commands from a user. A computing device may receive a set of respective voice recordings from one or more NMDs and process the voice recordings locally at the computing device or remotely on another device that is connected to the computing device by way of one or more networks. For example, the computing device may communicate with a networked microphone system server, one or more NMDs, playback devices, and/or another computing device to receive or process voice recordings. In some embodiments, the computing device, media playback system server and/or networked microphone system server may be cloud-based server systems. In other embodiments, the computing device itself may be an NMD, playback device, or any other device or server described herein. 
     The computing device may identify, among the set of voice recordings received from multiple NMDs, which voice recordings to process to determine a given voice command. For instance, in some embodiments, the voice input from any NMD that registers the voice input is processed. Alternatively, the computing device may identify a subset of the recordings. This subset might include any NMD that registered a given voice command at or above a given threshold (e.g., a threshold sound pressure level). As another example, this subset might include voice inputs from a pre-defined number of NMDs (e.g., the three NMDs registering the voice command at or above a given threshold). Other examples are possible as well. 
     For example, the computing devices may select from among multiple recorded voice inputs based on pre-determined rules. To illustrate, where the NMDs are playback devices, a subset of voice inputs may be selected for processing based on zone configurations of the playback devices. For instance, recordings of a given command from multiple playback devices joined together as a bonded pair (e.g., a stereo pair or surround sound configuration) may be processed together. In some cases, recordings from devices outside the bonded zone may be ignored. As another example, recordings of a given command from multiple zones that are grouped together (as a zone group) may be processed together. Further examples are contemplated as well. 
     The computing device may cause the identified subset of voice recordings to be analyzed to determine the given voice command. In other words, voice recordings of multiple NMDs may be processed to determine a single voice command. Processing a particular subset of voice recordings may improve accuracy in refining and processing the voice recordings, which in turn may enable a higher-quality speech-to-text conversion of voice commands. More particularly, refining the identified recordings may prevent duplicate, redundant, or separate processing of the same voice recordings (or same portions of a voice recording). In further instances, identifying a subset of voice recordings may reduce processing time in determining a given voice command, perhaps by avoiding duplicate, redundant, or separate processing of the same voice recordings. Examples are described further herein. 
     NMDs may continuously record or start recording in response to a trigger, among other examples. For instance, a given NMD might continuously record ambient noise but might only provide its recording to the computing device (to possibly be included in processing) if the given NMD (1) itself is woken up by a wake-up word or voice input, or (2) receives an instruction from another device to provide the recording to the computing device. In such implementations, processing of recordings of the given NMD may be triggered, despite the given NMD not necessarily registering a far-field voice input itself (e.g., by registering a wake-up word or voice input). 
     While some examples described herein may refer to functions performed by given actors such as “users” and/or other entities, it should be understood that this is for purposes of explanation only. The claims should not be interpreted to require action by any such example actor unless explicitly required by the language of the claims themselves. It will be understood by one of ordinary skill in the art that this disclosure includes numerous other embodiments. Moreover, the examples described herein may extend to a multitude of embodiments formed by combining the example features in any suitable manner. 
     II. Example Operating Environment 
       FIG.  1    shows an example configuration of a media playback system  100  in which one or more embodiments disclosed herein may be practiced or implemented. The media playback system  100  as shown is associated with an example home environment having several rooms and spaces, such as for example, a master bedroom, an office, a dining room, and a living room. As shown in the example of  FIG.  1   , the media playback system  100  includes playback devices  102 - 124 , control devices  126  and  128 , and a wired or wireless network router  130 . 
     Further discussions relating to the different components of the example media playback system  100  and how the different components may interact to provide a user with a media experience may be found in the following sections. While discussions herein may generally refer to the example media playback system  100 , technologies described herein are not limited to applications within, among other things, the home environment as shown in  FIG.  1   . For instance, the technologies described herein may be useful in environments where multi-zone audio may be desired, such as, for example, a commercial setting like a restaurant, mall or airport, a vehicle like a sports utility vehicle (SUV), bus or car, a ship or boat, an airplane, and so on. 
     a. Example Playback Devices 
       FIG.  2    shows a functional block diagram of an example playback device  200  that may be configured to be one or more of the playback devices  102 - 124  of the media playback system  100  of  FIG.  1   . The playback device  200  may include a processor  202 , software components  204 , memory  206 , audio processing components  208 , audio amplifier(s)  210 , speaker(s)  212 , a network interface  214  including wireless interface(s)  216  and wired interface(s)  218 , and microphone(s)  220 . In one case, the playback device  200  may not include the speaker(s)  212 , but rather a speaker interface for connecting the playback device  200  to external speakers. In another case, the playback device  200  may include neither the speaker(s)  212  nor the audio amplifier(s)  210 , but rather an audio interface for connecting the playback device  200  to an external audio amplifier or audio-visual receiver. 
     In one example, the processor  202  may be a clock-driven computing component configured to process input data according to instructions stored in the memory  206 . The memory  206  may be a tangible computer-readable medium configured to store instructions executable by the processor  202 . For instance, the memory  206  may be data storage that can be loaded with one or more of the software components  204  executable by the processor  202  to achieve certain functions. In one example, the functions may involve the playback device  200  retrieving audio data from an audio source or another playback device. In another example, the functions may involve the playback device  200  sending audio data to another device or playback device on a network. In yet another example, the functions may involve pairing of the playback device  200  with one or more playback devices to create a multi-channel audio environment. 
     Certain functions may involve the playback device  200  synchronizing playback of audio content with one or more other playback devices. During synchronous playback, a listener will preferably not be able to perceive time-delay differences between playback of the audio content by the playback device  200  and the one or more other playback devices. U.S. Pat. No. 8,234,395 entitled, “System and method for synchronizing operations among a plurality of independently clocked digital data processing devices,” which is hereby incorporated by reference, provides in more detail some examples for audio playback synchronization among playback devices. 
     The memory  206  may further be configured to store data associated with the playback device  200 , such as one or more zones and/or zone groups the playback device  200  is a part of, audio sources accessible by the playback device  200 , or a playback queue that the playback device  200  (or some other playback device) may be associated with. The data may be stored as one or more state variables that are periodically updated and used to describe the state of the playback device  200 . The memory  206  may also include the data associated with the state of the other devices of the media system, and shared from time to time among the devices so that one or more of the devices have the most recent data associated with the system. Other embodiments are also possible. 
     The audio processing components  208  may include one or more digital-to-analog converters (DAC), an audio preprocessing component, an audio enhancement component or a digital signal processor (DSP), and so on. In one embodiment, one or more of the audio processing components  208  may be a subcomponent of the processor  202 . In one example, audio content may be processed and/or intentionally altered by the audio processing components  208  to produce audio signals. The produced audio signals may then be provided to the audio amplifier(s)  210  for amplification and playback through speaker(s)  212 . Particularly, the audio amplifier(s)  210  may include devices configured to amplify audio signals to a level for driving one or more of the speakers  212 . The speaker(s)  212  may include an individual transducer (e.g., a “driver”) or a complete speaker system involving an enclosure with one or more drivers. A particular driver of the speaker(s)  212  may include, for example, a subwoofer (e.g., for low frequencies), a mid-range driver (e.g., for middle frequencies), and/or a tweeter (e.g., for high frequencies). In some cases, each transducer in the one or more speakers  212  may be driven by an individual corresponding audio amplifier of the audio amplifier(s)  210 . In addition to producing analog signals for playback by the playback device  200 , the audio processing components  208  may be configured to process audio content to be sent to one or more other playback devices for playback. 
     Audio content to be processed and/or played back by the playback device  200  may be received from an external source, such as via an audio line-in input connection (e.g., an auto-detecting 3.5 mm audio line-in connection) or the network interface  214 . 
     The network interface  214  may be configured to facilitate a data flow between the playback device  200  and one or more other devices on a data network. As such, the playback device  200  may be configured to receive audio content over the data network from one or more other playback devices in communication with the playback device  200 , network devices within a local area network, or audio content sources over a wide area network such as the Internet. In one example, the audio content and other signals transmitted and received by the playback device  200  may be transmitted in the form of digital packet data containing an Internet Protocol (IP)-based source address and IP-based destination addresses. In such a case, the network interface  214  may be configured to parse the digital packet data such that the data destined for the playback device  200  is properly received and processed by the playback device  200 . 
     As shown, the network interface  214  may include wireless interface(s)  216  and wired interface(s)  218 . The wireless interface(s)  216  may provide network interface functions for the playback device  200  to wirelessly communicate with other devices (e.g., other playback device(s), speaker(s), receiver(s), network device(s), control device(s) within a data network the playback device  200  is associated with) in accordance with a communication protocol (e.g., any wireless standard including IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4G mobile communication standard, and so on). The wired interface(s)  218  may provide network interface functions for the playback device  200  to communicate over a wired connection with other devices in accordance with a communication protocol (e.g., IEEE 802.3). While the network interface  214  shown in  FIG.  2    includes both wireless interface(s)  216  and wired interface(s)  218 , the network interface  214  may in some embodiments include only wireless interface(s) or only wired interface(s). 
     The microphone(s)  220  may be arranged to detect sound in the environment of the playback device  200 . For instance, the microphone(s) may be mounted on an exterior wall of a housing of the playback device. The microphone(s) may be any type of microphone now known or later developed such as a condenser microphone, electret condenser microphone, or a dynamic microphone. The microphone(s) may be sensitive to a portion of the frequency range of the speaker(s)  220 . One or more of the speaker(s)  220  may operate in reverse as the microphone(s)  220 . In some aspects, the playback device  200  might not include the microphone(s)  220 . 
     In one example, the playback device  200  and one other playback device may be paired to play two separate audio components of audio content. For instance, playback device  200  may be configured to play a left channel audio component, while the other playback device may be configured to play a right channel audio component, thereby producing or enhancing a stereo effect of the audio content. The paired playback devices (also referred to as “bonded playback devices”) may further play audio content in synchrony with other playback devices. 
     In another example, the playback device  200  may be sonically consolidated with one or more other playback devices to form a single, consolidated playback device. A consolidated playback device may be configured to process and reproduce sound differently than an unconsolidated playback device or playback devices that are paired, because a consolidated playback device may have additional speaker drivers through which audio content may be rendered. For instance, if the playback device  200  is a playback device designed to render low frequency range audio content (i.e. a subwoofer), the playback device  200  may be consolidated with a playback device designed to render full frequency range audio content. In such a case, the full frequency range playback device, when consolidated with the low frequency playback device  200 , may be configured to render only the mid and high frequency components of audio content, while the low frequency range playback device  200  renders the low frequency component of the audio content. The consolidated playback device may further be paired with a single playback device or yet another consolidated playback device. 
     By way of illustration, SONOS, Inc. presently offers (or has offered) for sale certain playback devices including a “PLAY:1,” “PLAY:3,” “PLAY:5,” “PLAYBAR,” “CONNECT:AMP,” “CONNECT,” and “SUB.” Any other past, present, and/or future playback devices may additionally or alternatively be used to implement the playback devices of example embodiments disclosed herein. Additionally, it is understood that a playback device is not limited to the example illustrated in  FIG.  2    or to the SONOS product offerings. For example, a playback device may include a wired or wireless headphone. In another example, a playback device may include or interact with a docking station for personal mobile media playback devices. In yet another example, a playback device may be integral to another device or component such as a television, a lighting fixture, or some other device for indoor or outdoor use. 
     b. Example Playback Zone Configurations 
     Referring back to the media playback system  100  of  FIG.  1   , the environment may have one or more playback zones, each with one or more playback devices. The media playback system  100  may be established with one or more playback zones, after which one or more zones may be added, or removed to arrive at the example configuration shown in  FIG.  1   . Each zone may be given a name according to a different room or space such as an office, bathroom, master bedroom, bedroom, kitchen, dining room, living room, and/or balcony. In one case, a single playback zone may include multiple rooms or spaces. In another case, a single room or space may include multiple playback zones. 
     As shown in  FIG.  1   , the balcony, dining room, kitchen, bathroom, office, and bedroom zones each have one playback device, while the living room and master bedroom zones each have multiple playback devices. In the living room zone, playback devices  104 ,  106 ,  108 , and  110  may be configured to play audio content in synchrony as individual playback devices, as one or more bonded playback devices, as one or more consolidated playback devices, or any combination thereof. Similarly, in the case of the master bedroom, playback devices  122  and  124  may be configured to play audio content in synchrony as individual playback devices, as a bonded playback device, or as a consolidated playback device. 
     In one example, one or more playback zones in the environment of  FIG.  1    may each be playing different audio content. For instance, the user may be grilling in the balcony zone and listening to hip hop music being played by the playback device  102  while another user may be preparing food in the kitchen zone and listening to classical music being played by the playback device  114 . In another example, a playback zone may play the same audio content in synchrony with another playback zone. For instance, the user may be in the office zone where the playback device  118  is playing the same rock music that is being playing by playback device  102  in the balcony zone. In such a case, playback devices  102  and  118  may be playing the rock music in synchrony such that the user may seamlessly (or at least substantially seamlessly) enjoy the audio content that is being played out-loud while moving between different playback zones. Synchronization among playback zones may be achieved in a manner similar to that of synchronization among playback devices, as described in previously referenced U.S. Pat. No. 8,234,395. 
     As suggested above, the zone configurations of the media playback system  100  may be dynamically modified, and in some embodiments, the media playback system  100  supports numerous configurations. For instance, if a user physically moves one or more playback devices to or from a zone, the media playback system  100  may be reconfigured to accommodate the change(s). For instance, if the user physically moves the playback device  102  from the balcony zone to the office zone, the office zone may now include both the playback device  118  and the playback device  102 . The playback device  102  may be paired or grouped with the office zone and/or renamed if so desired via a control device such as the control devices  126  and  128 . On the other hand, if the one or more playback devices are moved to a particular area in the home environment that is not already a playback zone, a new playback zone may be created for the particular area. 
     Further, different playback zones of the media playback system  100  may be dynamically combined into zone groups or split up into individual playback zones. For instance, the dining room zone and the kitchen zone  114  may be combined into a zone group for a dinner party such that playback devices  112  and  114  may render audio content in synchrony. On the other hand, the living room zone may be split into a television zone including playback device  104 , and a listening zone including playback devices  106 ,  108 , and  110 , if the user wishes to listen to music in the living room space while another user wishes to watch television. 
     c. Example Control Devices 
       FIG.  3    shows a functional block diagram of an example control device  300  that may be configured to be one or both of the control devices  126  and  128  of the media playback system  100 . As shown, the control device  300  may include a processor  302 , memory  304 , a network interface  306 , a user interface  308 , microphone(s)  310 , and software components  312 . In one example, the control device  300  may be a dedicated controller for the media playback system  100 . In another example, the control device  300  may be a network device on which media playback system controller application software may be installed, such as for example, an iPhone™, iPad™ or any other smart phone, tablet or network device (e.g., a networked computer such as a PC or Mac™). 
     The processor  302  may be configured to perform functions relevant to facilitating user access, control, and configuration of the media playback system  100 . The memory  304  may be data storage that can be loaded with one or more of the software components executable by the processor  302  to perform those functions. The memory  304  may also be configured to store the media playback system controller application software and other data associated with the media playback system  100  and the user. 
     In one example, the network interface  306  may be based on an industry standard (e.g., infrared, radio, wired standards including IEEE 802.3, wireless standards including IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4G mobile communication standard, and so on). The network interface  306  may provide a means for the control device  300  to communicate with other devices in the media playback system  100 . In one example, data and information (e.g., such as a state variable) may be communicated between control device  300  and other devices via the network interface  306 . For instance, playback zone and zone group configurations in the media playback system  100  may be received by the control device  300  from a playback device or another network device, or transmitted by the control device  300  to another playback device or network device via the network interface  306 . In some cases, the other network device may be another control device. 
     Playback device control commands such as volume control and audio playback control may also be communicated from the control device  300  to a playback device via the network interface  306 . As suggested above, changes to configurations of the media playback system  100  may also be performed by a user using the control device  300 . The configuration changes may include adding/removing one or more playback devices to/from a zone, adding/removing one or more zones to/from a zone group, forming a bonded or consolidated player, separating one or more playback devices from a bonded or consolidated player, among others. Accordingly, the control device  300  may sometimes be referred to as a controller, whether the control device  300  is a dedicated controller or a network device on which media playback system controller application software is installed. 
     Control device  300  may include microphone(s)  310 . Microphone(s)  310  may be arranged to detect sound in the environment of the control device  300 . Microphone(s)  310  may be any type of microphone now known or later developed such as a condenser microphone, electret condenser microphone, or a dynamic microphone. The microphone(s) may be sensitive to a portion of a frequency range. Two or more microphones  310  may be arranged to capture location information of an audio source (e.g., voice, audible sound) and/or to assist in filtering background noise. 
     The user interface  308  of the control device  300  may be configured to facilitate user access and control of the media playback system  100 , by providing a controller interface such as the controller interface  400  shown in  FIG.  4   . The controller interface  400  includes a playback control region  410 , a playback zone region  420 , a playback status region  430 , a playback queue region  440 , and an audio content sources region  450 . The user interface  400  as shown is just one example of a user interface that may be provided on a network device such as the control device  300  of  FIG.  3    (and/or the control devices  126  and  128  of  FIG.  1   ) and accessed by users to control a media playback system such as the media playback system  100 . Other user interfaces of varying formats, styles, and interactive sequences may alternatively be implemented on one or more network devices to provide comparable control access to a media playback system. 
     The playback control region  410  may include selectable (e.g., by way of touch or by using a cursor) icons to cause playback devices in a selected playback zone or zone group to play or pause, fast forward, rewind, skip to next, skip to previous, enter/exit shuffle mode, enter/exit repeat mode, enter/exit cross fade mode. The playback control region  410  may also include selectable icons to modify equalization settings, and playback volume, among other possibilities. 
     The playback zone region  420  may include representations of playback zones within the media playback system  100 . In some embodiments, the graphical representations of playback zones may be selectable to bring up additional selectable icons to manage or configure the playback zones in the media playback system, such as a creation of bonded zones, creation of zone groups, separation of zone groups, and renaming of zone groups, among other possibilities. 
     For example, as shown, a “group” icon may be provided within each of the graphical representations of playback zones. The “group” icon provided within a graphical representation of a particular zone may be selectable to bring up options to select one or more other zones in the media playback system to be grouped with the particular zone. Once grouped, playback devices in the zones that have been grouped with the particular zone will be configured to play audio content in synchrony with the playback device(s) in the particular zone. Analogously, a “group” icon may be provided within a graphical representation of a zone group. In this case, the “group” icon may be selectable to bring up options to deselect one or more zones in the zone group to be removed from the zone group. Other interactions and implementations for grouping and ungrouping zones via a user interface such as the user interface  400  are also possible. The representations of playback zones in the playback zone region  420  may be dynamically updated as playback zone or zone group configurations are modified. 
     The playback status region  430  may include graphical representations of audio content that is presently being played, previously played, or scheduled to play next in the selected playback zone or zone group. The selected playback zone or zone group may be visually distinguished on the user interface, such as within the playback zone region  420  and/or the playback status region  430 . The graphical representations may include track title, artist name, album name, album year, track length, and other relevant information that may be useful for the user to know when controlling the media playback system via the user interface  400 . 
     The playback queue region  440  may include graphical representations of audio content in a playback queue associated with the selected playback zone or zone group. In some embodiments, each playback zone or zone group may be associated with a playback queue containing information corresponding to zero or more audio items for playback by the playback zone or zone group. For instance, each audio item in the playback queue may comprise a uniform resource identifier (URI), a uniform resource locator (URL) or some other identifier that may be used by a playback device in the playback zone or zone group to find and/or retrieve the audio item from a local audio content source or a networked audio content source, possibly for playback by the playback device. 
     In one example, a playlist may be added to a playback queue, in which case information corresponding to each audio item in the playlist may be added to the playback queue. In another example, audio items in a playback queue may be saved as a playlist. In a further example, a playback queue may be empty, or populated but “not in use” when the playback zone or zone group is playing continuously streaming audio content, such as Internet radio that may continue to play until otherwise stopped, rather than discrete audio items that have playback durations. In an alternative embodiment, a playback queue can include Internet radio and/or other streaming audio content items and be “in use” when the playback zone or zone group is playing those items. Other examples are also possible. 
     When playback zones or zone groups are “grouped” or “ungrouped,” playback queues associated with the affected playback zones or zone groups may be cleared or re-associated. For example, if a first playback zone including a first playback queue is grouped with a second playback zone including a second playback queue, the established zone group may have an associated playback queue that is initially empty, that contains audio items from the first playback queue (such as if the second playback zone was added to the first playback zone), that contains audio items from the second playback queue (such as if the first playback zone was added to the second playback zone), or a combination of audio items from both the first and second playback queues. Subsequently, if the established zone group is ungrouped, the resulting first playback zone may be re-associated with the previous first playback queue, or be associated with a new playback queue that is empty or contains audio items from the playback queue associated with the established zone group before the established zone group was ungrouped. Similarly, the resulting second playback zone may be re-associated with the previous second playback queue, or be associated with a new playback queue that is empty, or contains audio items from the playback queue associated with the established zone group before the established zone group was ungrouped. Other examples are also possible. 
     Referring back to the user interface  400  of  FIG.  4   , the graphical representations of audio content in the playback queue region  440  may include track titles, artist names, track lengths, and other relevant information associated with the audio content in the playback queue. In one example, graphical representations of audio content may be selectable to bring up additional selectable icons to manage and/or manipulate the playback queue and/or audio content represented in the playback queue. For instance, a represented audio content may be removed from the playback queue, moved to a different position within the playback queue, or selected to be played immediately, or after any currently playing audio content, among other possibilities. A playback queue associated with a playback zone or zone group may be stored in a memory on one or more playback devices in the playback zone or zone group, on a playback device that is not in the playback zone or zone group, and/or some other designated device. 
     The audio content sources region  450  may include graphical representations of selectable audio content sources from which audio content may be retrieved and played by the selected playback zone or zone group. Discussions pertaining to audio content sources may be found in the following section. 
     d. Example Audio Content Sources 
     As indicated previously, one or more playback devices in a zone or zone group may be configured to retrieve for playback audio content (e.g. according to a corresponding URI or URL for the audio content) from a variety of available audio content sources. In one example, audio content may be retrieved by a playback device directly from a corresponding audio content source (e.g., a line-in connection). In another example, audio content may be provided to a playback device over a network via one or more other playback devices or network devices. 
     Example audio content sources may include a memory of one or more playback devices in a media playback system such as the media playback system  100  of  FIG.  1   , local music libraries on one or more network devices (such as a control device, a network-enabled personal computer, or a networked-attached storage (NAS), for example), streaming audio services providing audio content via the Internet (e.g., the cloud), or audio sources connected to the media playback system via a line-in input connection on a playback device or network devise, among other possibilities. 
     In some embodiments, audio content sources may be regularly added or removed from a media playback system such as the media playback system  100  of  FIG.  1   . In one example, an indexing of audio items may be performed whenever one or more audio content sources are added, removed or updated. Indexing of audio items may involve scanning for identifiable audio items in all folders/directory shared over a network accessible by playback devices in the media playback system, and generating or updating an audio content database containing metadata (e.g., title, artist, album, track length, among others) and other associated information, such as a URI or URL for each identifiable audio item found. Other examples for managing and maintaining audio content sources may also be possible. 
     The above discussions relating to playback devices, controller devices, playback zone configurations, and media content sources provide only some examples of operating environments within which functions and methods described below may be implemented. Other operating environments and configurations of media playback systems, playback devices, and network devices not explicitly described herein may also be applicable and suitable for implementation of the functions and methods. 
     e. Example Plurality of Networked Devices 
       FIG.  5    shows an example plurality of devices  500  that may be configured to provide an audio playback experience based on voice control. One having ordinary skill in the art will appreciate that the devices shown in  FIG.  5    are for illustrative purposes only, and variations including different and/or additional devices may be possible. As shown, the plurality of devices  500  includes computing devices  504 ,  506 , and  508 ; network microphone devices (NMDs)  512 ,  514 , and  516 ; playback devices (PBDs)  532 ,  534 ,  536 , and  538 ; and a controller device (CR)  522 . 
     Each of the plurality of devices  500  may be network-capable devices that can establish communication with one or more other devices in the plurality of devices according to one or more network protocols, such as NFC, Bluetooth, Ethernet, and IEEE 802.11, among other examples, over one or more types of networks, such as wide area networks (WAN), local area networks (LAN), and personal area networks (PAN), among other possibilities. 
     As shown, the computing devices  504 ,  506 , and  508  may be part of a cloud network  502 . The cloud network  502  may include additional computing devices. In one example, the computing devices  504 ,  506 , and  508  may be different servers. In another example, two or more of the computing devices  504 ,  506 , and  508  may be modules of a single server. Analogously, each of the computing device  504 ,  506 , and  508  may include one or more modules or servers. For ease of illustration purposes herein, each of the computing devices  504 ,  506 , and  508  may be configured to perform particular functions within the cloud network  502 . For instance, computing device  508  may be a source of audio content for a streaming music service. 
     As shown, the computing device  504  may be configured to interface with NMDs  512 ,  514 , and  516  via communication path  542 . NMDs  512 ,  514 , and  516  may be components of one or more “Smart Home” systems. In one case, NMDs  512 ,  514 , and  516  may be physically distributed throughout a household, similar to the distribution of devices shown in  FIG.  1   . In another case, two or more of the NMDs  512 ,  514 , and  516  may be physically positioned within relative close proximity of one another. Communication path  542  may comprise one or more types of networks, such as a WAN including the Internet, LAN, and/or PAN, among other possibilities. 
     In one example, one or more of the NMDs  512 ,  514 , and  516  may be devices configured primarily for audio detection. In another example, one or more of the NMDs  512 ,  514 , and  516  may be components of devices having various primary utilities. For instance, as discussed above in connection to  FIGS.  2  and  3   , one or more of NMDs  512 ,  514 , and  516  may be the microphone(s)  220  of playback device  200  or the microphone(s)  310  of network device  300 . Further, in some cases, one or more of NMDs  512 ,  514 , and  516  may be the playback device  200  or network device  300 . In an example, one or more of NMDs  512 ,  514 , and/or  516  may include multiple microphones arranged in a microphone array. 
     As shown, the computing device  506  may be configured to interface with CR  522  and PBDs  532 ,  534 ,  536 , and  538  via communication path  544 . In one example, CR  522  may be a network device such as the network device  200  of  FIG.  2   . Accordingly, CR  522  may be configured to provide the controller interface  400  of  FIG.  4   . Similarly, PBDs  532 ,  534 ,  536 , and  538  may be playback devices such as the playback device  300  of  FIG.  3   . As such, PBDs  532 ,  534 ,  536 , and  538  may be physically distributed throughout a household as shown in  FIG.  1   . For illustration purposes, PBDs  536  and  538  may be part of a bonded zone  530 , while PBDs  532  and  534  may be part of their own respective zones. As described above, the PBDs  532 ,  534 ,  536 , and  538  may be dynamically bonded, grouped, unbonded, and ungrouped. Communication path  544  may comprise one or more types of networks, such as a WAN including the Internet, LAN, and/or PAN, among other possibilities. 
     In one example, as with NMDs  512 ,  514 , and  516 , CR  522  and PBDs  532 ,  534 ,  536 , and  538  may also be components of one or more “Smart Home” systems. In one case, PBDs  532 ,  534 ,  536 , and  538  may be distributed throughout the same household as the NMDs  512 ,  514 , and  516 . Further, as suggested above, one or more of PBDs  532 ,  534 ,  536 , and  538  may be one or more of NMDs  512 ,  514 , and  516 . 
     The NMDs  512 ,  514 , and  516  may be part of a local area network, and the communication path  542  may include an access point that links the local area network of the NMDs  512 ,  514 , and  516  to the computing device  504  over a WAN (communication path not shown). Likewise, each of the NMDs  512 ,  514 , and  516  may communicate with each other via such an access point. 
     Similarly, CR  522  and PBDs  532 ,  534 ,  536 , and  538  may be part of a local area network and/or a local playback network as discussed in previous sections, and the communication path  544  may include an access point that links the local area network and/or local playback network of CR  522  and PBDs  532 ,  534 ,  536 , and  538  to the computing device  506  over a WAN. As such, each of the CR  522  and PBDs  532 ,  534 ,  536 , and  538  may also communicate with each over such an access point. 
     In one example, a single access point may include communication paths  542  and  544 . In an example, each of the NMDs  512 ,  514 , and  516 , CR  522 , and PBDs  532 ,  534 ,  536 , and  538  may access the cloud network  502  via the same access point for a household. 
     As shown in  FIG.  5   , each of the NMDs  512 ,  514 , and  516 , CR  522 , and PBDs  532 ,  534 ,  536 , and  538  may also directly communicate with one or more of the other devices via communication means  546 . Communication means  546  as described herein may involve one or more forms of communication between the devices, according to one or more network protocols, over one or more types of networks, and/or may involve communication via one or more other network devices. For instance, communication means  546  may include one or more of for example, Bluetooth™ (IEEE 802.15), NFC, Wireless direct, and/or Proprietary wireless, among other possibilities. 
     In one example, CR  522  may communicate with NMD  512  over Bluetooth™, and communicate with PBD  534  over another local area network. In another example, NMD  514  may communicate with CR  522  over another local area network, and communicate with PBD  536  over Bluetooth. In a further example, each of the PBDs  532 ,  534 ,  536 , and  538  may communicate with each other according to a spanning tree protocol over a local playback network, while each communicating with CR  522  over a local area network, different from the local playback network. Other examples are also possible. 
     In some cases, communication means between the NMDs  512 ,  514 , and  516 , CR  522 , and PBDs  532 ,  534 ,  536 , and  538  may change depending on types of communication between the devices, network conditions, and/or latency demands. For instance, communication means  546  may be used when NMD  516  is first introduced to the household with the PBDs  532 ,  534 ,  536 , and  538 . In one case, the NMD  516  may transmit identification information corresponding to the NMD  516  to PBD  538  via NFC, and PBD  538  may in response, transmit local area network information to NMD  516  via NFC (or some other form of communication). However, once NMD  516  has been configured within the household, communication means between NMD  516  and PBD  538  may change. For instance, NMD  516  may subsequently communicate with PBD  538  via communication path  542 , the cloud network  502 , and communication path  544 . In another example, the NMDs and PBDs may never communicate via local communications means  546 . In a further example, the NMDs and PBDs may communicate primarily via local communications means  546 . Other examples are also possible. 
     In an illustrative example, NMDs  512 ,  514 , and  516  may be configured to receive voice inputs to control PBDs  532 ,  534 ,  536 , and  538 . The available control commands may include any media playback system controls previously discussed, such as playback volume control, playback transport controls, music source selection, and grouping, among other possibilities. In one instance, NMD  512  may receive a voice input to control one or more of the PBDs  532 ,  534 ,  536 , and  538 . In response to receiving the voice input, NMD  512  may transmit via communication path  542 , the voice input to computing device  504  for processing. In one example, the computing device  504  may convert the voice input to an equivalent text command, and parse the text command to identify a command. Computing device  504  may then subsequently transmit the text command to the computing device  506 . In another example, the computing device  504  may convert the voice input to an equivalent text command, and then subsequently transmit the text command to the computing device  506 . The computing device  506  may then parse the text command to identify one or more playback commands. 
     For instance, if the text command is “Play ‘Track 1’ by ‘Artist 1’ from ‘Streaming Service 1’ in ‘Zone 1’,” The computing device  506  may identify (i) a URL for “Track 1” by “Artist 1” available from “Streaming Service 1,” and (ii) at least one playback device in “Zone 1.” In this example, the URL for “Track 1” by “Artist 1” from “Streaming Service 1” may be a URL pointing to computing device  508 , and “Zone 1” may be the bonded zone  530 . As such, upon identifying the URL and one or both of PBDs  536  and  538 , the computing device  506  may transmit via communication path  544  to one or both of PBDs  536  and  538 , the identified URL for playback. One or both of PBDs  536  and  538  may responsively retrieve audio content from the computing device  508  according to the received URL, and begin playing “Track 1” by “Artist 1” from “Streaming Service 1.” 
     One having ordinary skill in the art will appreciate that the above is just one illustrative example, and that other implementations are also possible. In one case, operations performed by one or more of the plurality of devices  500 , as described above, may be performed by one or more other devices in the plurality of device  500 . For instance, the conversion from voice input to the text command may be alternatively, partially, or wholly performed by another device or devices, such as NMD  512 , computing device  506 , PBD  536 , and/or PBD  538 . Analogously, the identification of the URL may be alternatively, partially, or wholly performed by another device or devices, such as NMD  512 , computing device  504 , PBD  536 , and/or PBD  538 . 
     f. Example Network Microphone Device 
       FIG.  6    shows a function block diagram of an example network microphone device  600  that may be configured to be one or more of NMDs  512 ,  514 , and  516  of  FIG.  5   . As shown, the network microphone device  600  includes a processor  602 , memory  604 , a microphone array  606 , a network interface  608 , a user interface  610 , software components  612 , and speaker(s)  614 . One having ordinary skill in the art will appreciate that other network microphone device configurations and arrangements are also possible. For instance, network microphone devices may alternatively exclude the speaker(s)  614  or have a single microphone instead of microphone array  606 . 
     The processor  602  may include one or more processors and/or controllers, which may take the form of a general or special-purpose processor or controller. For instance, the processing unit  602  may include microprocessors, microcontrollers, application-specific integrated circuits, digital signal processors, and the like. The memory  604  may be data storage that can be loaded with one or more of the software components executable by the processor  602  to perform those functions. Accordingly, memory  604  may comprise one or more non-transitory computer-readable storage mediums, examples of which may include volatile storage mediums such as random access memory, registers, cache, etc. and non-volatile storage mediums such as read-only memory, a hard-disk drive, a solid-state drive, flash memory, and/or an optical-storage device, among other possibilities. 
     The microphone array  606  may be a plurality of microphones arranged to detect sound in the environment of the network microphone device  600 . Microphone array  606  may include any type of microphone now known or later developed such as a condenser microphone, electret condenser microphone, or a dynamic microphone, among other possibilities. In one example, the microphone array may be arranged to detect audio from one or more directions relative to the network microphone device. The microphone array  606  may be sensitive to a portion of a frequency range. In one example, a first subset of the microphone array  606  may be sensitive to a first frequency range, while a second subset of the microphone array may be sensitive to a second frequency range. The microphone array  606  may further be arranged to capture location information of an audio source (e.g., voice, audible sound) and/or to assist in filtering background noise. Notably, in some embodiments the microphone array may consist of only a single microphone, rather than a plurality of microphones. 
     The network interface  608  may be configured to facilitate wireless and/or wired communication between various network devices, such as, in reference to  FIG.  5   , CR  522 , PBDs  532 - 538 , computing device  504 - 508  in cloud network  502 , and other network microphone devices, among other possibilities. As such, network interface  608  may take any suitable form for carrying out these functions, examples of which may include an Ethernet interface, a serial bus interface (e.g., FireWire, USB 2.0, etc.), a chipset and antenna adapted to facilitate wireless communication, and/or any other interface that provides for wired and/or wireless communication. In one example, the network interface  608  may be based on an industry standard (e.g., infrared, radio, wired standards including IEEE 802.3, wireless standards including IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4G mobile communication standard, and so on). 
     The user interface  610  of the network microphone device  600  may be configured to facilitate user interactions with the network microphone device. In one example, the user interface  608  may include one or more of physical buttons, graphical interfaces provided on touch sensitive screen(s) and/or surface(s), among other possibilities, for a user to directly provide input to the network microphone device  600 . The user interface  610  may further include one or more of lights and the speaker(s)  614  to provide visual and/or audio feedback to a user. In one example, the network microphone device  600  may further be configured to playback audio content via the speaker(s)  614 . 
     III. Example Systems and Methods for Voice Detection by Multiple NMDs 
     As discussed above, in some examples, a computing device may facilitate and coordinate voice recordings of multiple NMDs to determine a voice command. Example voice commands may include commands to modify any of the media playback system controls or playback settings. Playback settings may include, for example, playback volume, playback transport controls, music source selection, and grouping, among other possibilities. Other voice commands may include operations to adjust television control or play settings, mobile phone device settings, or illumination devices, among other device operations. As more household devices become “smart” (e.g., by incorporating a network interface), voice commands may be used to control these household devices. 
     Generally, it should be understood that one or more functions described herein may be performed by the computing device individually or in combination with the media playback system server, networked microphone system server, PBDs  532 - 538 , NMDs  512 - 516 , CR  522 , or any other devices described herein. Alternatively, the computing device itself may be the media playback system server, networked microphone system server, one of the PBDs  532 - 538 , one of the NMDs  512 - 516 , CR  522 , or any other device described herein. 
     Implementation  700  shown in  FIG.  7    presents an embodiment of example techniques described herein. Implementation  700  can be implemented within an operating environment including or involving, for example, the media playback system  100  of  FIG.  1   , one or more playback devices  200  of  FIG.  2   , one or more control devices  300  of  FIG.  3   , the user interface of  FIG.  4   , and/or the configuration shown in  FIG.  5   . Implementation  700  may include one or more operations, functions, or actions as illustrated by one or more of blocks  702 - 706 . Although the blocks are illustrated in 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 implementation  700  and other processes and methods disclosed herein, the flowchart shows functionality and operation of one possible implementation of some 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 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 non-transitory computer readable medium, for example, such as tangible, non-transitory 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 media 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 implementation  700  and other processes and methods disclosed herein, each block in  FIG.  7    may represent circuitry that is wired to perform the specific logical functions in the process. 
     a. Receive Set of Voice Recordings 
     At block  702 , implementation  700  involves receiving a set of voice recordings. For instance, a computing device, such as computing device  506 , may receive a set of voice recordings from one or more NMDs. In some embodiments, a given NMD may have one or more microphones to record voice inputs or commands from a user. For example, one or more NMDs located in or near the living room of a household may record a voice input from a user located in the living room. Additionally, the computing device itself may operate as an NMD and include one or more microphones to record voice input inputs or commands. 
     In some instances, the computing device may receive voice recordings via a network interface of the computing device, perhaps in addition to receiving voice recordings via a microphone of the computing device. For example, the computing device may communicate and receive voice recordings from the media playback system server, networked microphone system server, PBDs  532 - 538 , NMDs  512 - 516 , CR  522 , or any other devices described herein. In some embodiments, the media playback system server and/or networked microphone system server may be cloud-based server systems. The processing NMD may receive voice recordings from any one or a combination of these devices and/or servers. 
     An NMD may be continuously recording ambient noise (e.g., listening for voice inputs) via one or more microphones. The continuous recording may be stored in a ring or circular buffer, wherein the recording may be discarded unless the recording is needed for processing and determining a given voice command. The buffer may be stored locally and/or remotely via any of the devices or servers described herein. 
     In other embodiments, some NMDs might not continuously record ambient noise. Rather, in some instances, one or more NMDs may receive a voice input or indication that instructs the one or more NMDs to “wake up” and start recording voice inputs or commands. For example, the computing device  506  may receive a voice input and, in certain situations described herein, send an indication to one or more NMDs to start recording. In other examples, one or more NMDs may receive a specific “wake-up word” (e.g., “hey Sonos”, “Siri”, “Alexa”) that triggers the one or more NMDs to start recording or listen for a voice command. 
     An NMD may send its voice recording to the computing device upon detecting a voice command or upon being instructed to send its recordings, among other options. For instance, an NMD may transmit a voice recording of a given voice command after registering that voice command (e.g., by registering a voice command preceded by a wake-up word). Alternatively, another NMD or other device may register a voice command and instruct the NMD to transmit recent recordings to the computing device. 
     In further examples, the computing device may receive only some of the voice recordings from multiple NMDs. The selected voice recording may be provided to the computing device based on various criteria described further herein. For instance, a given NMD might provide its recording to the computing device if the recording meets certain criteria (e.g., that the voice recording was registered at or above a threshold sound pressure level). As another example, another device may register a voice command and, if the registered voice command satisfies certain criteria, may instruct the NMD to transmit recent recordings corresponding to the voice command to the computing device. 
     Within examples, the voice recordings from multiple NMDs may be refined, processed, and/or combined into a single voice input before the computing device receives the voice recordings. By way of example, the media playback system server may receive voice recordings from one or more NMDs, such as  512 - 516 . In some embodiments, PBDs  532 - 538  may be configured as NMDs, and the media playback system server may receive voice recordings from PBDs  532 - 538 . The media playback system may refine, process, and/or combine the voice recordings into a single voice input and send the single voice input to the computing device for further processing. Other examples involving the combination of devices and/or servers described herein are possible. 
     In some examples, the computing device may receive voice recordings from multiple NMDs at different sound pressure levels. For example, a first NMD that is proximate to a user may recorded the user&#39;s voice command at a higher sound pressure level voice recording of the user&#39;s voice command relative to a second NMD that is further away from the user. As another example, a user might not be stationary when providing the voice command (e.g., a user might moving from the living room to the bedroom). In such instances, a first NMD may have recorded a higher sound-pressure-level voice recording of a first portion of a user&#39;s voice command and a second NMD may have recorded a higher sound-pressure-level recording of a second portion of a user&#39;s voice command, as caused by the movement of the user while speaking the voice command. 
     In some cases, multiple NMDs may have recorded identical portions of a user&#39;s voice input. For example, a first and second NMD may be proximate to each other and may have each been listening for a voice input. In other cases, multiple NMDs may have recorded different portions of a user&#39;s voice input (e.g., the content of the recordings might not overlap at all or might overlap to some degree). By way of example, the second NMD might not have been initially listening for a voice input or a user might not have been nearby (or may have moved to another location while providing a voice input). After the first NMD detects a first portion of a voice command, the first NMD may instruct the second NMD to start recording, which may cause the second NMD to detect a second portion of the voice command. Alternatively, as noted above, the second NMD may be continuously recording, and the computing device may instruct the second NMD to send the portion of its recordings corresponding to the voice command to the computing device. 
     As noted above, the computing device may itself operate as an NMD. In some cases, the computing device may register a voice command and perhaps instruct other NMDs to record the voice command. For instance, the computing device may record at least a first portion of a given voice command via one or more microphones of the computing device, and cause one or more NMDs to record at least a second portion of the given voice command. 
     In some implementations, a given NMD may continuously record ambient noise but might only provide its recording to the computing device if the given NMD (1) itself is woken up by a wake-up word or voice input, or (2) receives an instruction from another NMD, device, or system to provide the recording to the computing device. For example, a first NMD may be continuously recording and may, in some instances, record at least a portion of a given voice command received from a user. A second NMD may cause the first NMD to send the voice recording to the computing device via the network interface. In such examples, processing of a particular portion of continuous recordings by a given NMD may be triggered, despite the given NMD not necessarily registering a far-field voice input itself. 
     To illustrate, referring back to  FIG.  1   , a user may speak a particular voice command while walking from the Master Bedroom zone to the Bathroom zone. Playback device  122  (and/or playback device  124 ), operating as NMDs, may register the user&#39;s voice as a voice command by way of a wake-up word spoken while the user was in the Master Bedroom. Playback device  122  may record a first portion of the particular voice command. Playback devices  116 ,  118 , and  120  may be instructed to send a portion of their recordings corresponding to the time period when playback device  122  detected the voice command. Given that the user moved to the Bathroom zone in this example, playback device  116  may detect a second portion of the particular voice command and send this second portion to a computing device for processing with the recording of the first portion of the particular voice command made by playback device  122 . In this example, a recording made by playback device  116  is processed, despite playback device  116  not registering the voice command itself. 
     Additionally or alternatively, a given NMD that may be recording ambient noise and/or at least a portion of a given voice command may receive an instruction from another NMD, device, or system to stop recording. Such embodiments may prevent duplicate or separate processing of the same voice input and may provide faster processing of voice recordings to determine the given voice command. 
     In some implementations, various NMDs may be configured to operate jointly, which may influence which recordings are transmitted to the computing device. For instance, some NMDs may include playback device functionality (or vice versa). As noted above, playback devices may form various groupings (e.g., bonded zones or zone groups, among other examples). When an NMD that is in a group detects a voice command, that NMD may instruct other NMDs in the group to transmit their recordings corresponding to the voice command to the computing device. 
     For instance, a computing device may receive a first voice recording corresponding to at least a first portion of the given voice command from a first NMD (e.g., a PBD configured as an NMD). While (or after) the computing device receives the first voice recording, the computing device may determine that the first NMD and a second NMD are paired devices (or bonded playback devices) that typically play media content in synchrony with other playback devices. Based on determining that the first and second NMDs are paired devices, the computing device may cause the second NMD to record and provide a second voice recording corresponding to at least a second portion of the given voice command. In some instances in which the second NMD was continuously recording, the computing device may cause the second NMD to provide the voice recording to the computing device. Alternatively, the computing device may cause the second NMD to stop recording to prevent duplicate processing of the same voice input. 
     In further examples, the computing device may receive a first voice recording corresponding to at least a first portion of the given voice command from a first NMD (e.g., a PBD configured as an NMD). While (or after) the computing device receives the first voice recording, the computing device may determine that the first NMD and one or more other NMDs are part of a zone group that typically play media content in synchrony within the playback zone. In some instances, the computing device may determine that the first NMD and one or more other NMDs are part of a zone scene (e.g., playback devices that are located on a first floor of a household, or playback devices that are grouped at 5 pm on weekdays). Based on such determination, the computing device may cause the one or more other NMDs to record and provide a second voice recording corresponding to at least a second portion of the given voice command. In some instances in which the one or more other NMDs were continuously recording, the computing device may cause the one or more other NMDs to provide the voice recording to the computing device. Alternatively, the computing device may cause the one or more other NMDs to stop recording to prevent duplicate processing of the same voice input. 
     In some embodiments, a user may define a voice input or command identifying a set of NMDs (e.g., PBDs configured as NMDs) that may be used together as bonded devices, playback zones, and/or zone scenes to record a portion of a given voice command. In such embodiments, the computing device may receive a user-defined command identifying a set of NMDs that are grouped together as a bonded pair, playback zone, or zone scene. Accordingly, the computing device may cause one or more NMDs that are grouped together to record and/or provide a portion of the given voice command. For example, a user may define “downstairs” as part of a given voice command that identifies a set of NMDs in the basement of a household as a zone scene. The computing device may cause one or more of the identified NMDs that are part of the basement zone scene to record the user&#39;s voice input corresponding to the given voice command. 
     In other embodiments, the computing device may cause a set of NMDs that are grouped together as a bonded pair, playback zone, or zone scene to record and/or provide a given voice command when a pre-defined condition is triggered. For example, the computing device may cause a set of NMDs that are part of a zone group to record and/or provide at least a portion of a given voice command only when the user&#39;s command is for playback purposes to for instance, watch a movie, or control one or more playback settings (e.g., play or pause a song, play the next or previous song, adjust volume, etc). 
     In further embodiments, the computing device may learn that a set of NMDs are commonly grouped together as a zone scene to operate jointly (e.g., to play media content in synchrony). Such learning may be based on the configuration history of the NMDs. For instance, an example configuration history may indicate that the set of NMDs have been grouped together on more than a threshold number of instances. As noted above, to illustrate, such a zone scene might include NMDs that are located on a given floor of a house, NMDs that are in listening proximity of one another, NMDs that are commonly grouped together at a particular time (e.g., party mode on weekends) or other scenes. Accordingly, in response to receiving a portion of a given voice command from a first NMD in a particular zone scene, the computing device may cause one or more other NMDs that have been commonly grouped together with the first NMD to record and/or provide at least a portion of the given voice command to the computing device. 
     In still further embodiments, the computing device may receive a first voice recording of at least a portion of a given voice command from a first NMD, and determine the orientation or direction of the given voice command relative to the first NMD (e.g., relative direction in which a user faces while recording the voice command). Based on the direction of the given voice command relative to the first NMD, the computing device (or any other device/server) may cause a second NMD to listen and record a second voice recording that represents at least a portion of the given voice command. 
     To illustrate, referring back to  FIG.  1   , playback device  104  may detect a voice command coming from the direction of the hallway between the bathroom and office zones. Given the direction of the voice command, playback device  116  and/or playback device  118  may be instructed to record and/or transmit recordings corresponding to the voice command to the computing device. 
     As a further example, referring still to  FIG.  1   , a user may move from the living room towards the master bedroom. As the user moves, a playback device in the living room (e.g., playback device  104 ) may send an indication to a second NMD in the bedroom that the second NMD may start recording and/or preprocessing in anticipation of the user&#39;s voice command or input. For example, the first NMD may send an indication to the second NMD to start filtering noise outside the fundamental frequency band for human voice (e.g., 85-255 Hz). In other instances, one or more microphones of the second NMD may be steered toward the direction in which the user faces (e.g., entry of the bedroom). Such example embodiments may be accomplished based on determining the orientation or direction of the given voice command relative to the NMDs as described further herein. 
     Within examples, the media playback system, computing device, and/or NMD receiving a voice command may acknowledge the identity of the particular user providing the voice command to disambiguate from other voice inputs (e.g., other speakers, television, etc). In some instances, the media playback system, computing device, and/or NMD may identify the particular user based on user profiles or voice configuration settings stored in the media playback system and/or one or more combinations of devices described herein. User profiles information may be shared between any of the devices via a network interface. Example user profiles may include voice characteristics that include the tone or frequency of the particular user&#39;s voice, age, gender, and user history, among other information identifying the particular user. 
     In example operations, referring back to  FIG.  1   , a user may move from the living room towards the master bedroom while providing a voice command. As the user moves, a playback device in the living room (e.g., playback device  104 ) may identify the user based on the user&#39;s profile and send an indication to a second NMD in the bedroom that the second NMD may start recording and/or preprocessing in anticipation of the user&#39;s voice command or input. Knowing the identity of the user, the playback device  104  in the living room and the second NMD in the bedroom may disambiguate voice inputs from a television nearby that may interfere with recordings of the user&#39;s voice command. 
     In other instances, the media playback system, computing device, and/or NMD may identify the particular user in response to determining the voice characteristic of the user while receiving a portion of a user&#39;s voice input at a particular location. By way of example, referring back to  FIG.  1   , a male user may be in the master bedroom that may have one or more NMDs (e.g., playback device  122 ,  124 ). While the male user provides a voice command or input near or within the master bedroom, the one or more NMDs may acknowledge the identity of the user and determine the voice characteristic (e.g., frequency or tone of male user&#39;s voice) of the user. As the user moves away from the master bedroom, the one or more NMDs in the master bedroom may send the voice characteristic to other NMDs located in other living spaces nearby (e.g., bathroom, living room, etc) and instruct the other NMDs to actively listen for voice inputs matching the voice characteristic of the user to disambiguate voice inputs from other sources (e.g., television, female user, etc). 
     In further instances, an NMD at a particular location may receive a voice command or input that may trigger a time period or window for the NMD or any one or more other NMDs to actively listen for additional voice inputs or commands. In some examples, a wake-up word or phrase (e.g., Hey Sonos) may trigger a time period or window for one or more NMDs to actively listen for additional voice inputs or commands. In other examples, one or more NMDs receiving at least a portion of a voice input may trigger the time period or window for one or more other NMDs to actively listen for additional voice inputs or commands. Within examples, one or more NMDs receiving at least a portion of a voice input may acknowledge the identity of the particular user and trigger the time period or window for one or more other NMDs to actively listen for additional voice inputs or commands from the particular user. 
     In some implementations, the time period or window may expire after a certain duration of time (e.g., one minute after one or more NMDs receive an initial voice input). In other implementations, a user may specify the time period or window for one or more NMDs to receive additional voice inputs or commands. In particular, one or more NMDs may receive a voice command (e.g., “let&#39;s queue up some songs for a minute”) that specifies a time period or window (e.g., one minute) for one or more NMDs to actively listen for additional voice inputs (e.g., voice inputs to add songs to a playback queue). In further implementations, one or more NMDs may close or key off the time period or window for receiving additional voice inputs before such time period or window expires. U.S. application Ser. No. 15/131,776 entitled, “Action based on User ID,” which is hereby incorporated by reference describes further examples. 
     In some embodiments, orientation or direction may be determined based on frequency response of the voice inputs or commands. Generally, an NMD that a user is facing while recording a voice input or command may have a larger high-frequency component than an NMD that the user is not facing. Analysis of such components may indicate to the computing device directionality of a voice command. For instance, given (1) data representing the frequency responses of the respective microphones of multiple NMDs and (2) separate time-aligned recordings of the voice inputs by multiple NMDs, a computing device may normalize the frequency response (e.g., 35 Hz-22.05 kHz) of the respective recordings of the voice inputs with respect to the low frequency band. For instance, the frequency response of the voice inputs recorded from a first NMD may be normalized with a second NMD with respect to the fundamental frequency band for human voice (e.g., 85-255 Hz). The high-frequency components of the normalized responses of the voice inputs may then be compared to determine the direction in which the user is facing while recording the voice command. 
     In other embodiments, orientation or direction of a voice input may be determined by using the variance in the known polar responses of two or more microphones of an NMD. The variance may help determine the angle of the user (or voice input) relative to an NMD, perpendicular to the plane of the two or more microphones. The angle of the user relative to an NMD may help more precisely locate the direction in which the user is facing while recording the voice input and may add an additional level of confidence that the voice input was received from a certain direction or orientation. Such angles may be identified by measuring the polar responses of the voice inputs at each microphone simultaneously and matching the variance with the known polar responses. 
     In further embodiments, the angle of the user relative to an NMD may be determined by measuring the delay across two or more microphones with a known distance between them. Further examples may include visual imaging, measuring the relative magnitude across two or more microphones or NMDs, Bluetooth proximity detection between an NMD and another computing device, such as a mobile phone, or monitoring WiFi Received Signal Strength Indication (RSSI) to detect user proximity and/or location. 
     b. Identify Subset of Voice Recordings 
     At block  704 , implementation  700  involves identifying a subset of voice recordings. For instance, the computing device (e.g., computing device  506 ) may identify, among the set of voice recordings, a subset of voice recordings from which to determine a given voice command. Alternatively, the computing device may use all of the voice recordings received from respective NMDs to determine a given voice command. 
     In some instances, identifying the subset of voice recordings may include a device other than the computing device (e.g., the media playback server) locally or remotely (via a network interface) determining the subset of voice recordings and providing the subset to the computing device. Some implementations may involve one or more combinations of devices or servers other than the computing device determining the subset of voice recordings. 
     In some embodiments, the computing device may identify a subset of voice recordings by comparing the received voice recordings from a set of NMDs with a threshold sound pressure level or threshold volume level. The threshold, for example, may be an absolute threshold such as magnitude or a relative threshold that may be normalized according to the highest-magnitude of the voice recordings. In some embodiments, the computing device may identify the voice recordings (or NMDs) that exceed the threshold level as the subset of voice recordings (or NMDs) to determine the given voice command. In other embodiments, the computing device may identify a predetermined number of NMDs (e.g., three NMDs) that recorded at least a portion of the given voice command at the highest sound pressure levels to determine the given voice command. Identifying the subset of voice recordings in such manner may ensure greater accuracy in refining and processing the voice recordings and enable a higher-quality speech to text conversion to determine a given voice command. 
     In other embodiments, the computing device may identify the voice recordings of NMDs based on various rules or criteria. For instance, voice recordings from NMDs that are grouped together as bonded pairs, playback zones, and/or zone scenes may be identified as the subset of voice recordings from which to determine the given voice command. 
     Identifying the subset of voice recordings in such manner may be useful when a voice command is invoked for playback purposes. For example, NMDs in the living room and kitchen of a household may record at least a portion of a user&#39;s voice command. The computing device may identify the voice recordings of NMDs in the living room as the subset of voice recordings from which to determine the given voice command. In other examples, a set of NMDs in the living room may record at least a portion of a user&#39;s voice command. The computing device may identify the voice recordings of a pair of NMDs in the living room that are bonded together as the subset of voice recordings from which to determine the given voice command. 
     In further examples, a set of NMDs on the first and second floor of a household may record at least a portion of a user&#39;s voice command. The computing device may identify the voice recordings of the NMDs on the first floor as the subset of voice recordings from which to determine the given voice command. In some instances, as described above, the computing device may identify the voice recordings of NMDs that have been commonly grouped together as a zone scene on a threshold number of instances as the subset of voice recordings from which to determine the given voice command. 
     In other embodiments, the computing device may identify two or more voice recordings of NMDs that are acoustically coupled as the subset of voice recordings from which to determine the given voice command. In some instances, the computing device may cause an NMD to determine whether it is acoustically coupled to one or more other NMDs. For example, the computing device may cause a first NMD to play or output a test tone (or any other audio content) and may cause a second NMD to detect the tone via one or more microphones of the second NMD. The computing device may compare the magnitude of the detected test tone with a threshold sound pressure level to determine whether the first and second NMD are acoustically coupled. In other examples, the first NMD may be playing audio content and the second NMD may register the audio content via one or more microphones. The computing device may compare the magnitude of the registered audio content with a threshold sound pressure level to determine whether the first and second NMD are acoustically coupled. Based on the acoustic coupling of two more NMDs, the computing device may identify such NMDs as the subset from which to determine the given voice command. 
     c. Cause Identified Subset of Voice Recordings to be Analyzed to Determine Given Voice Command 
     At block  706 , implementation  700  involves causing the identified subset of voice recordings to be analyzed to determine the given voice command. For instance, the computing device, such as computing device  506 , may cause a subset of voice recordings to be analyzed to determine the given voice command. 
     In some cases, the computing device may analyze the subset of voice recordings itself. Alternatively, any one or a combination of the devices or servers described herein may cause the identified subset of voice recordings to be analyzed to determine the given voice command, which may be facilitated by one or more networks connecting the devices (e.g., connection means  546 ). 
     In some examples, the computing device may cause the identified subset of voice recordings to be analyzed by communicating with an NMD. The computing device may send the identified recordings to the NMD, and the NMD may determine and execute the voice command. Within examples, the computing device may determine the given voice command from the identified subset of voice recordings and send the determined voice command to the NMD to execute the voice command. Other examples involving one or a combination of the devices or servers described herein are possible. 
     In some embodiments, the computing device may cause the identified subset of voice recordings to be analyzed to determine the given voice command by processing and refining the identified subset of voice recordings. Alternatively, the computing device may process the identified subset of voice recordings without refinement. Refining the identified recordings, however, may enable a higher-quality speech to text conversion. In some implementations, the subset of voice recordings may be refined before identifying the subset of voice recordings. For example, computing device  506  may receive a set of voice recordings and refine the voice recordings locally or remotely before identifying the subset of voice recordings. In other examples, the set of voice recordings may be refined by one or more combination of devices or servers before the computing device (e.g., computing device  506 ) receives the set of voice recordings. The computing device may identify a subset of voice recordings from the received set of refined voice recordings. 
     The computing device may time-align the identified subset of voice recordings. Time aligning the voice recordings may prevent processing of redundant or duplicate portions of the given voice command. For instance, a first NMD and a second NMD may have recorded at least a portion of a given voice command at a given time. The first NMD may have recorded some overlapping portion at a given time in which the second NMDs may have been recording. Accordingly, the computing device may time-align the voice recordings from the first and second NMDs and refine or strip out the overlapping portions of the voice recordings according to various criteria described herein. 
     In some examples, the computing device may take the magnitude-weighted average of the identified subset of voice recordings. A respective portion of a voice recording with a higher magnitude (e.g., higher sound pressure level) may be given a higher weight and may be more likely to be processed to determine the given voice command. 
     In other examples, the computing device may combine the recordings by determining an average of the identified subset of voice recordings. Such an average may be weighted according to microphone quality and/or quantity of respective NMDs. Some NMDs may have multiple microphones and some NMDs may have better quality microphones than other NMDs. Accordingly, the computing device may determine the magnitude-weighted average of the identified subset of voice recordings based on such factors. 
     The computing device may “chop” the identified subset of voice recordings and splice them together. For example, at noted above, a user may be moving from the living room to the bedroom of a household while providing a given voice command. Based on determining the relative orientation or direction of the identified subset of voice recordings of respective NMDs, the computing device may use the identified subset of voice recordings from the living room as the first portion from which to determine the given voice command and the identified subset of voice recordings from the bedroom as the second portion from which to determine the given voice command. 
     IV. Conclusion 
     The description above discloses, among other things, various example systems, methods, apparatus, and articles of manufacture including, among other components, firmware and/or software executed on hardware. It is understood that such examples are merely illustrative and should not be considered as limiting. For example, it is contemplated that any or all of the firmware, hardware, and/or software aspects or components can be embodied exclusively in hardware, exclusively in software, exclusively in firmware, or in any combination of hardware, software, and/or firmware. Accordingly, the examples provided are not the only way(s) to implement such systems, methods, apparatus, and/or articles of manufacture. 
     Additionally, references herein to “embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one example embodiment of an invention. The appearances of this phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. As such, the embodiments described herein, explicitly and implicitly understood by one skilled in the art, can be combined with other embodiments. 
     The specification is presented largely in terms of illustrative environments, systems, procedures, steps, logic blocks, processing, and other symbolic representations that directly or indirectly resemble the operations of data processing devices coupled to networks. These process descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. Numerous specific details are set forth to provide a thorough understanding of the present disclosure. However, it is understood to those skilled in the art that certain embodiments of the present disclosure can be practiced without certain, specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the embodiments. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the forgoing description of embodiments. 
     When any of the appended claims are read to cover a purely software and/or firmware implementation, at least one of the elements in at least one example is hereby expressly defined to include a tangible, non-transitory medium such as a memory, DVD, CD, Blu-ray, and so on, storing the software and/or firmware.