Patent Publication Number: US-11646023-B2

Title: Devices, systems, and methods for distributed voice processing

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 16/271,560, filed Feb. 8, 2019, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present technology relates to consumer goods and, more particularly, to methods, systems, products, features, services, and other elements directed to voice-controllable media playback systems 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 a controller, for example, different songs can be streamed to each room that has 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 A  is a partial cutaway view of an environment having a media playback system configured in accordance with aspects of the disclosed technology. 
         FIG.  1 B  is a schematic diagram of the media playback system of  FIG.  1 A  and one or more networks; 
         FIG.  2 A  is a functional block diagram of an example playback device; 
         FIG.  2 B  is an isometric diagram of an example housing of the playback device of  FIG.  2 A ; 
         FIGS.  3 A- 3 E  are diagrams showing example playback device configurations in accordance with aspects of the disclosure; 
         FIG.  4 A  is a functional block diagram of an example controller device in accordance with aspects of the disclosure; 
         FIGS.  4 B and  4 C  are controller interfaces in accordance with aspects of the disclosure; 
         FIG.  5    is a functional block diagram of certain components of an example playback device in accordance with aspects of the disclosure; 
         FIG.  6 A  is a diagram of an example voice input; 
         FIG.  6 B  is a graph depicting an example sound specimen in accordance with aspects of the disclosure; 
         FIG.  7 A  is an example network configuration in accordance with aspects of the disclosure; 
         FIG.  7 B  is an example network configuration in accordance with aspects of the disclosure; 
         FIG.  7 C  is an example network configuration in accordance with aspects of the disclosure; 
         FIG.  7 D  is an example network configuration in accordance with aspects of the disclosure; 
         FIG.  7 E  is an example network configuration in accordance with aspects of the disclosure; 
         FIG.  8    is an example method in accordance with aspects of the disclosure; 
         FIG.  9    is an example method in accordance with aspects of the disclosure; 
         FIGS.  10 A and  10 B  are timelines of example voice inputs; 
         FIG.  11    is an example method in accordance with aspects of the disclosure; 
         FIG.  12    is an example network configuration in accordance with aspects of the disclosure. 
     
    
    
     The drawings are for purposes of illustrating example embodiments, but it should be understood that the inventions are not limited to the arrangements and instrumentality shown in the drawings. In the drawings, identical reference numbers identify at least generally similar elements. To facilitate the discussion of any particular element, the most significant digit or digits of any reference number refers to the Figure in which that element is first introduced. For example, element  103   a  is first introduced and discussed with reference to  FIG.  1 A . 
     DETAILED DESCRIPTION 
     I. Overview 
     Voice control can be beneficial in a “smart” home that includes smart appliances and devices that are connected to a communication network, such as wireless audio playback devices, illumination devices, and home-automation devices (e.g., thermostats, door locks, etc.). In some implementations, network microphone devices may be used to control smart home devices. 
     A network microphone device (“NMD”) is a networked computing device that typically includes an arrangement of microphones, such as a microphone array, that is configured to detect sounds present in the NMD&#39;s environment. The detected sound may include a person&#39;s speech mixed with background noise (e.g., music being output by a playback device or other ambient noise). In practice, an NMD typically filters detected sound to remove the background noise from the person&#39;s speech to facilitate identifying whether the speech contains a voice input indicative of voice control. If so, the NMD may act based on such a voice input. 
     An NMD often employs a wake-word engine, which is typically onboard the NMD, to identify whether sound detected by the NMD contains a voice input that includes a particular wake word. The wake-word engine is a type of voice-input identification engine that is configured to identify (i.e., “spot”) a particular keyword (e.g., a wake word) using one or more identification algorithms, using e.g., natural-language understanding (NLU), machine learning, and/or other suitable algorithms. In practice, to help facilitate wake-word spotting, the NMD may buffer sound detected by a microphone of the NMD and then use the wake-word engine to process that buffered sound to determine whether a wake word is present. 
     When a wake-word engine spots a wake word in detected sound, the NMD may determine that a wake-word event (i.e., a “wake-word trigger”) has occurred, which indicates that the NMD has detected sound that includes a potential voice input. The occurrence of the wake-word event typically causes the NMD to perform additional processes involving the detected sound. In some implementations, these additional processes may include outputting an alert (e.g., an audible chime and/or a light indicator) indicating that a wake word has been identified and extracting detected-sound data from a buffer, among other possible additional processes. Extracting the detected sound may include reading out and packaging a stream of the detected-sound according to a particular format and transmitting the packaged sound-data to an appropriate VAS for interpretation. 
     In turn, the VAS corresponding to the wake word that was identified by the wake-word engine receives the transmitted sound data from the NMD over a communication network. A VAS traditionally takes the form of a remote service implemented using one or more cloud servers configured to process voice inputs (e.g., AMAZON&#39;s ALEXA, APPLE&#39;s SIRI, MICROSOFT&#39;s CORTANA, GOOGLE&#39;S ASSISTANT, etc.). In some instances, certain components and functionality of the VAS may be distributed across local and remote devices. Additionally, or alternatively, a VAS may take the form of a local service implemented at an NMD or a media playback system comprising the NMD such that a voice input or certain types of voice input (e.g., rudimentary commands) are processed locally without intervention from a remote VAS. 
     In any case, when a VAS receives detected sound data, the VAS will typically process this data, which involves identifying the voice input and determining an intent of words captured in the voice input. The VAS may then provide a response back to the NMD with some instruction according to the determined intent. Based on that instruction, the NMD may cause one or more smart devices to perform an action. For example, in accordance with an instruction from a VAS, an NMD may cause a playback device to play a particular song or an illumination device to turn on/off, among other examples. In some cases, an NMD, or a media system with NMDs (e.g., a media playback system with NMD-equipped playback devices) may be configured to interact with multiple VASes. In practice, the NMD may select one VAS over another based on the particular wake word identified in the sound detected by the NMD. 
     In some implementations, a playback device that is configured to be part of a networked media playback system may include components and functionality of an NMD (i.e., the playback device is “NMD-equipped”). In this respect, such a playback device may include a microphone that is configured to detect sounds present in the playback device&#39;s environment, such as people speaking, audio being output by the playback device itself or another playback device that is nearby, or other ambient noises, and may also include components for buffering detected sound to facilitate wake-word identification. 
     Some NMD-equipped playback devices may include an internal power source (e.g., a rechargeable battery) that allows the playback device to operate without being physically connected to a wall electrical outlet or the like. In this regard, such a playback device may be referred to herein as a “portable playback device.” On the other hand, playback devices that are configured to rely on power from a wall electrical outlet or the like may be referred to herein as “stationary playback devices,” although such devices may in fact be moved around a home or other environment. In practice, a person might often take a portable playback device to and from a home or other environment in which one or more stationary playback devices remain. 
     In some cases, multiple voice services are configured for the NMD, or a system of NMDs (e.g., a media playback system of playback devices). One or more services can be configured during a set-up procedure, and additional voice services can be configured for the system later on. As such, the NMD acts as an interface with multiple voice services, perhaps alleviating a need to have an NMD from each of the voice services to interact with the respective voice services. Yet further, the NMD can operate in concert with service-specific NMDs present in a household to process a given voice command. 
     Where two or more voice services are configured for the NMD, a particular voice service can be invoked by utterance of a wake word corresponding to the particular voice service. For instance, in querying AMAZON, a user might speak the wake word “Alexa” followed by a voice command. Other examples include “Ok, Google” for querying GOOGLE and “Hey, Siri” for querying APPLE. 
     In some cases, a generic wake word can be used to indicate a voice input to an NMD. In some cases, this is a manufacturer-specific wake word rather than a wake word tied to any particular voice service (e.g., “Hey, Sonos” where the NMD is a SONOS playback device). Given such a wake word, the NMD can identify a particular voice service to process the request. For instance, if the voice input following the wake word is related to a particular type of command (e.g., music playback), then the voice input is sent to a particular voice service associated with that type of command (e.g. a streaming music service having voice command capabilities). 
     It can be difficult to manage the association between various playback devices with one or more corresponding VASes. For example, although a user may wish to utilize multiple VASes within her home, it may not be possible or preferable to associate a single playback device with more than one VAS. This may be due to the constraints of processing power and memory required to perform multiple wake word detection algorithms on a single device, or it may be due to restrictions imposed by one or more VASes. As a result, for any particular playback device, a user may be required to select only a single VAS to the exclusion of any other VASes. 
     In some instances, a playback device may be purchased with a pre-associated VAS. In such instances, a user may wish to replace the pre-associated VAS with a different VAS of the user&#39;s choosing. Additionally, some voice-enabled playback devices may be sold without any pre-associated VAS, in which cases a user may wish to manage the selection and association of a particular VAS with the playback device. 
     The systems and methods detailed herein address the above-mentioned challenges of managing associations between one or more playback devices and one or more VASes. In particular, systems and methods are provided for distributing wake word detection (and other voice processing functions) across multiple playback devices. As described in more detail below, in some instances the media playback system may include playback devices that are configured to detect different wake words and communicate with different VASes. For example, the media playback system may include a first playback device having a wake word engine associated with a first VAS (such as AMAZON&#39;s ALEXA) and configured to detect an associated first wake word (e.g., “Alexa”), and a second playback device having a second wake word engine associated with a second, different VAS (such as GOOGLE&#39;s ASSISTANT) and configured to detect a second, different wake word (e.g., “OK, Google”). In some aspects of the technology, the second playback device relies on sound detected by the first playback device for detecting the second wake word, thereby leveraging the existing voice processing capabilities (such as wake word detection) of the second playback device, even instances where the second playback device does not include any of its own microphones. Utilizing the wake word engine of the first playback device distributes the processing time and power associated with wake word detection, and thus frees up computational resources on both the first and second playback devices (as compared to a single playback device with two wake word engines). Moreover, distributed wake word detection may also allow a user to realize the benefits of multiple VASes, each of which may excel in different aspects, rather than requiring a user to limit her interactions to a single VAS to the exclusion of any others. 
     While some embodiments described herein may refer to functions performed by given actors, such as “users” and/or other entities, it should be understood that this description 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. 
     II. Example Operating Environment 
       FIGS.  1 A and  1 B  illustrate an example configuration of a media playback system  100  (or “MPS  100 ”) in which one or more embodiments disclosed herein may be implemented. Referring first to  FIG.  1 A , the MPS  100  as shown is associated with an example home environment having a plurality of rooms and spaces, which may be collectively referred to as a “home environment,” “smart home,” or “environment  101 .” The environment  101  comprises a household having several rooms, spaces, and/or playback zones, including a master bathroom  101   a , a master bedroom  101   b  (referred to herein as “Nick&#39;s Room”), a second bedroom  101   c , a family room or den  101   d , an office  101   e , a living room  101   f , a dining room  101   g , a kitchen  101   h , and an outdoor patio  101   i . While certain embodiments and examples are described below in the context of a home environment, the technologies described herein may be implemented in other types of environments. In some embodiments, for example, the MPS  100  can be implemented in one or more commercial settings (e.g., a restaurant, mall, airport, hotel, a retail or other store), one or more vehicles (e.g., a sports utility vehicle, bus, car, a ship, a boat, an airplane), multiple environments (e.g., a combination of home and vehicle environments), and/or another suitable environment where multi-zone audio may be desirable. 
     Within these rooms and spaces, the MPS  100  includes one or more computing devices. Referring to  FIGS.  1 A and  1 B  together, such computing devices can include playback devices  102  (identified individually as playback devices  102   a - 102   o ), network microphone devices  103  (identified individually as “NMDs”  103   a - 102   i ), and controller devices  104   a  and  104   b  (collectively “controller devices  104 ”). Referring to  FIG.  1 B , the home environment may include additional and/or other computing devices, including local network devices, such as one or more smart illumination devices  108  ( FIG.  1 B ), a smart thermostat  110 , and a local computing device  105  ( FIG.  1 A ). In embodiments described below, one or more of the various playback devices  102  may be configured as portable playback devices, while others may be configured as stationary playback devices. For example, the headphones  102   o  ( FIG.  1 B ) are a portable playback device, while the playback device  102   d  on the bookcase may be a stationary device. As another example, the playback device  102   c  on the Patio may be a battery-powered device, which may allow it to be transported to various areas within the environment  101 , and outside of the environment  101 , when it is not plugged in to a wall outlet or the like. 
     With reference still to  FIG.  1 B , the various playback, network microphone, and controller devices  102 - 104  and/or other network devices of the MPS  100  may be coupled to one another via point-to-point connections and/or over other connections, which may be wired and/or wireless, via a LAN  111  including a network router  109 . For example, the playback device  102   j  in the Den  101   d  ( FIG.  1 A ), which may be designated as the “Left” device, may have a point-to-point connection with the playback device  102   a , which is also in the Den  101   d  and may be designated as the “Right” device. In a related embodiment, the Left playback device  102   j  may communicate with other network devices, such as the playback device  102   b , which may be designated as the “Front” device, via a point-to-point connection and/or other connections via the LAN  111 . 
     As further shown in  FIG.  1 B , the MPS  100  may be coupled to one or more remote computing devices  106  via a wide area network (“WAN”)  107 . In some embodiments, each remote computing device  106  may take the form of one or more cloud servers. The remote computing devices  106  may be configured to interact with computing devices in the environment  101  in various ways. For example, the remote computing devices  106  may be configured to facilitate streaming and/or controlling playback of media content, such as audio, in the home environment  101 . 
     In some implementations, the various playback devices, NMDs, and/or controller devices  102 - 104  may be communicatively coupled to at least one remote computing device associated with a VAS and at least one remote computing device associated with a media content service (“MCS”). For instance, in the illustrated example of  FIG.  1 B , remote computing devices  106   a  are associated with a VAS  190  and remote computing devices  106   b  are associated with an MCS  192 . Although only a single VAS  190  and a single MCS  192  are shown in the example of  FIG.  1 B  for purposes of clarity, the MPS  100  may be coupled to multiple, different VASes and/or MCSes. In some implementations, VASes may be operated by one or more of AMAZON, GOOGLE, APPLE, MICROSOFT, SONOS or other voice assistant providers. In some implementations, MCSes may be operated by one or more of SPOTIFY, PANDORA, AMAZON MUSIC, or other media content services. 
     As further shown in  FIG.  1 B , the remote computing devices  106  further include remote computing device  106   c  configured to perform certain operations, such as remotely facilitating media playback functions, managing device and system status information, directing communications between the devices of the MPS  100  and one or multiple VASes and/or MCSes, among other operations. In one example, the remote computing devices  106   c  provide cloud servers for one or more SONOS Wireless HiFi Systems. 
     In various implementations, one or more of the playback devices  102  may take the form of or include an on-board (e.g., integrated) network microphone device. For example, the playback devices  102   a - e  include or are otherwise equipped with corresponding NMDs  103   a - e , respectively. A playback device that includes or is otherwise equipped with an NMD may be referred to herein interchangeably as a playback device or an NMD unless indicated otherwise in the description. In some cases, one or more of the NMDs  103  may be a stand-alone device. For example, the NMDs  103   f  and  103   g  may be stand-alone devices. A stand-alone NMD may omit components and/or functionality that is typically included in a playback device, such as a speaker or related electronics. For instance, in such cases, a stand-alone NMD may not produce audio output or may produce limited audio output (e.g., relatively low-quality audio output). 
     The various playback and network microphone devices  102  and  103  of the MPS  100  may each be associated with a unique name, which may be assigned to the respective devices by a user, such as during setup of one or more of these devices. For instance, as shown in the illustrated example of  FIG.  1 B , a user may assign the name “Bookcase” to playback device  102   d  because it is physically situated on a bookcase. Similarly, the NMD  103   f  may be assigned the named “Island” because it is physically situated on an island countertop in the Kitchen  101   h  ( FIG.  1 A ). Some playback devices may be assigned names according to a zone or room, such as the playback devices  102   e ,  102   l ,  102   m , and  102   n , which are named “Bedroom,” “Dining Room,” “Living Room,” and “Office,” respectively. Further, certain playback devices may have functionally descriptive names. For example, the playback devices  102   a  and  102   b  are assigned the names “Right” and “Front,” respectively, because these two devices are configured to provide specific audio channels during media playback in the zone of the Den  101   d  ( FIG.  1 A ). The playback device  102   c  in the Patio may be named portable because it is battery-powered and/or readily transportable to different areas of the environment  101 . Other naming conventions are possible. 
     As discussed above, an NMD may detect and process sound from its environment, such as sound that includes background noise mixed with speech spoken by a person in the NMD&#39;s vicinity. For example, as sounds are detected by the NMD in the environment, the NMD may process the detected sound to determine if the sound includes speech that contains voice input intended for the NMD and ultimately a particular VAS. For example, the NMD may identify whether speech includes a wake word associated with a particular VAS. 
     In the illustrated example of  FIG.  1 B , the NMDs  103  are configured to interact with the VAS  190  over a network via the LAN  111  and the router  109 . Interactions with the VAS  190  may be initiated, for example, when an NMD identifies in the detected sound a potential wake word. The identification causes a wake-word event, which in turn causes the NMD to begin transmitting detected-sound data to the VAS  190 . In some implementations, the various local network devices  102 - 105  ( FIG.  1 A ) and/or remote computing devices  106   c  of the MPS  100  may exchange various feedback, information, instructions, and/or related data with the remote computing devices associated with the selected VAS. Such exchanges may be related to or independent of transmitted messages containing voice inputs. In some embodiments, the remote computing device(s) and the media playback system  100  may exchange data via communication paths as described herein and/or using a metadata exchange channel as described in U.S. application Ser. No. 15/438,749 filed Feb. 21, 2017, and titled “Voice Control of a Media Playback System,” which is herein incorporated by reference in its entirety. 
     Upon receiving the stream of sound data, the VAS  190  determines if there is voice input in the streamed data from the NMD, and if so the VAS  190  will also determine an underlying intent in the voice input. The VAS  190  may next transmit a response back to the MPS  100 , which can include transmitting the response directly to the NMD that caused the wake-word event. The response is typically based on the intent that the VAS  190  determined was present in the voice input. As an example, in response to the VAS  190  receiving a voice input with an utterance to “Play Hey Jude by The Beatles,” the VAS  190  may determine that the underlying intent of the voice input is to initiate playback and further determine that intent of the voice input is to play the particular song “Hey Jude.” After these determinations, the VAS  190  may transmit a command to a particular MCS  192  to retrieve content (i.e., the song “Hey Jude”), and that MCS  192 , in turn, provides (e.g., streams) this content directly to the MPS  100  or indirectly via the VAS  190 . In some implementations, the VAS  190  may transmit to the MPS  100  a command that causes the MPS  100  itself to retrieve the content from the MCS  192 . 
     In certain implementations, NMDs may facilitate arbitration amongst one another when voice input is identified in speech detected by two or more NMDs located within proximity of one another. For example, the NMD-equipped playback device  102   d  in the environment  101  ( FIG.  1 A ) is in relatively close proximity to the NMD-equipped Living Room playback device  102   m , and both devices  102   d  and  102   m  may at least sometimes detect the same sound. In such cases, this may require arbitration as to which device is ultimately responsible for providing detected-sound data to the remote VAS. Examples of arbitrating between NMDs may be found, for example, in previously referenced U.S. application Ser. No. 15/438,749. 
     In certain implementations, an NMD may be assigned to, or otherwise associated with, a designated or default playback device that may not include an NMD. For example, the Island NMD  103   f  in the Kitchen  101   h  ( FIG.  1 A ) may be assigned to the Dining Room playback device  102   l , which is in relatively close proximity to the Island NMD  103   f . In practice, an NMD may direct an assigned playback device to play audio in response to a remote VAS receiving a voice input from the NMD to play the audio, which the NMD might have sent to the VAS in response to a user speaking a command to play a certain song, album, playlist, etc. Additional details regarding assigning NMDs and playback devices as designated or default devices may be found, for example, in previously referenced U.S. patent application Ser. No. 15/438,749. 
     Further aspects relating to the different components of the example MPS  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 MPS  100 , technologies described herein are not limited to applications within, among other things, the home environment described above. For instance, the technologies described herein may be useful in other home environment configurations comprising more or fewer of any of the playback, network microphone, and/or controller devices  102 - 104 . For example, the technologies herein may be utilized within an environment having a single playback device  102  and/or a single NMD  103 . In some examples of such cases, the LAN  111  ( FIG.  1 B ) may be eliminated and the single playback device  102  and/or the single NMD  103  may communicate directly with the remote computing devices  106   a - d . In some embodiments, a telecommunication network (e.g., an LTE network, a 5G network, etc.) may communicate with the various playback, network microphone, and/or controller devices  102 - 104  independent of a LAN. 
     a. Example Playback &amp; Network Microphone Devices 
       FIG.  2 A  is a functional block diagram illustrating certain aspects of one of the playback devices  102  of the MPS  100  of  FIGS.  1 A and  1 B . As shown, the playback device  102  includes various components, each of which is discussed in further detail below, and the various components of the playback device  102  may be operably coupled to one another via a system bus, communication network, or some other connection mechanism. In the illustrated example of  FIG.  2 A , the playback device  102  may be referred to as an “NMD-equipped” playback device because it includes components that support the functionality of an NMD, such as one of the NMDs  103  shown in  FIG.  1 A . 
     As shown, the playback device  102  includes at least one processor  212 , which may be a clock-driven computing component configured to process input data according to instructions stored in memory  213 . The memory  213  may be a tangible, non-transitory, computer-readable medium configured to store instructions that are executable by the processor  212 . For example, the memory  213  may be data storage that can be loaded with software code  214  that is executable by the processor  212  to achieve certain functions. 
     In one example, these functions may involve the playback device  102  retrieving audio data from an audio source, which may be another playback device. In another example, the functions may involve the playback device  102  sending audio data, detected-sound data (e.g., corresponding to a voice input), and/or other information to another device on a network via at least one network interface  224 . In yet another example, the functions may involve the playback device  102  causing one or more other playback devices to synchronously playback audio with the playback device  102 . In yet a further example, the functions may involve the playback device  102  facilitating being paired or otherwise bonded with one or more other playback devices to create a multi-channel audio environment. Numerous other example functions are possible, some of which are discussed below. 
     As just mentioned, certain functions may involve the playback device  102  synchronizing playback of audio content with one or more other playback devices. During synchronous playback, a listener may not perceive time-delay differences between playback of the audio content by the synchronized playback devices. U.S. Pat. No. 8,234,395 filed on Apr. 4, 2004, and titled “System and method for synchronizing operations among a plurality of independently clocked digital data processing devices,” which is hereby incorporated by reference in its entirety, provides in more detail some examples for audio playback synchronization among playback devices. 
     To facilitate audio playback, the playback device  102  includes audio processing components  216  that are generally configured to process audio prior to the playback device  102  rendering the audio. In this respect, the audio processing components  216  may include one or more digital-to-analog converters (“DAC”), one or more audio preprocessing components, one or more audio enhancement components, one or more digital signal processors (“DSPs”), and so on. In some implementations, one or more of the audio processing components  216  may be a subcomponent of the processor  212 . In operation, the audio processing components  216  receive analog and/or digital audio and process and/or otherwise intentionally alter the audio to produce audio signals for playback. 
     The produced audio signals may then be provided to one or more audio amplifiers  217  for amplification and playback through one or more speakers  218  operably coupled to the amplifiers  217 . The audio amplifiers  217  may include components configured to amplify audio signals to a level for driving one or more of the speakers  218 . 
     Each of the speakers  218  may include an individual transducer (e.g., a “driver”) or the speakers  218  may include a complete speaker system involving an enclosure with one or more drivers. A particular driver of a speaker  218  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, a transducer may be driven by an individual corresponding audio amplifier of the audio amplifiers  217 . In some implementations, a playback device may not include the speakers  218 , but instead may include a speaker interface for connecting the playback device to external speakers. In certain embodiments, a playback device may include neither the speakers  218  nor the audio amplifiers  217 , but instead may include an audio interface (not shown) for connecting the playback device to an external audio amplifier or audio-visual receiver. 
     In addition to producing audio signals for playback by the playback device  102 , the audio processing components  216  may be configured to process audio to be sent to one or more other playback devices, via the network interface  224 , for playback. In example scenarios, audio content to be processed and/or played back by the playback device  102  may be received from an external source, such as via an audio line-in interface (e.g., an auto-detecting 3.5 mm audio line-in connection) of the playback device  102  (not shown) or via the network interface  224 , as described below. 
     As shown, the at least one network interface  224 , may take the form of one or more wireless interfaces  225  and/or one or more wired interfaces  226 . A wireless interface may provide network interface functions for the playback device  102  to wirelessly communicate with other devices (e.g., other playback device(s), NMD(s), and/or controller device(s)) 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). A wired interface may provide network interface functions for the playback device  102  to communicate over a wired connection with other devices in accordance with a communication protocol (e.g., IEEE 802.3). While the network interface  224  shown in  FIG.  2 A  include both wired and wireless interfaces, the playback device  102  may in some implementations include only wireless interface(s) or only wired interface(s). 
     In general, the network interface  224  facilitates data flow between the playback device  102  and one or more other devices on a data network. For instance, the playback device  102  may be configured to receive audio content over the data network from one or more other playback devices, network devices within a LAN, and/or audio content sources over a WAN, such as the Internet. In one example, the audio content and other signals transmitted and received by the playback device  102  may be transmitted in the form of digital packet data comprising an Internet Protocol (IP)-based source address and IP-based destination addresses. In such a case, the network interface  224  may be configured to parse the digital packet data such that the data destined for the playback device  102  is properly received and processed by the playback device  102 . 
     As shown in  FIG.  2 A , the playback device  102  also includes voice processing components  220  that are operably coupled to one or more microphones  222 . The microphones  222  are configured to detect sound (i.e., acoustic waves) in the environment of the playback device  102 , which is then provided to the voice processing components  220 . More specifically, each microphone  222  is configured to detect sound and convert the sound into a digital or analog signal representative of the detected sound, which can then cause the voice processing component  220  to perform various functions based on the detected sound, as described in greater detail below. In one implementation, the microphones  222  are arranged as an array of microphones (e.g., an array of six microphones). In some implementations, the playback device  102  includes more than six microphones (e.g., eight microphones or twelve microphones) or fewer than six microphones (e.g., four microphones, two microphones, or a single microphones). 
     In operation, the voice-processing components  220  are generally configured to detect and process sound received via the microphones  222 , identify potential voice input in the detected sound, and extract detected-sound data to enable a VAS, such as the VAS  190  ( FIG.  1 B ), to process voice input identified in the detected-sound data. The voice processing components  220  may include one or more analog-to-digital converters, an acoustic echo canceller (“AEC”), a spatial processor (e.g., one or more multi-channel Wiener filters, one or more other filters, and/or one or more beam former components), one or more buffers (e.g., one or more circular buffers), one or more wake-word engines, one or more voice extractors, and/or one or more speech processing components (e.g., components configured to recognize a voice of a particular user or a particular set of users associated with a household), among other example voice processing components. In example implementations, the voice processing components  220  may include or otherwise take the form of one or more DSPs or one or more modules of a DSP. In this respect, certain voice processing components  220  may be configured with particular parameters (e.g., gain and/or spectral parameters) that may be modified or otherwise tuned to achieve particular functions. In some implementations, one or more of the voice processing components  220  may be a subcomponent of the processor  212 . 
     In some implementations, the voice-processing components  220  may detect and store a user&#39;s voice profile, which may be associated with a user account of the MPS  100 . For example, voice profiles may be stored as and/or compared to variables stored in a set of command information or data table. The voice profile may include aspects of the tone or frequency of a user&#39;s voice and/or other unique aspects of the user&#39;s voice, such as those described in previously-referenced U.S. patent application Ser. No. 15/438,749. 
     As further shown in  FIG.  2 A , the playback device  102  also includes power components  227 . The power components  227  include at least an external power source interface  228 , which may be coupled to a power source (not shown) via a power cable or the like that physically connects the playback device  102  to an electrical outlet or some other external power source. Other power components may include, for example, transformers, converters, and like components configured to format electrical power. 
     In some implementations, the power components  227  of the playback device  102  may additionally include an internal power source  229  (e.g., one or more batteries) configured to power the playback device  102  without a physical connection to an external power source. When equipped with the internal power source  229 , the playback device  102  may operate independent of an external power source. In some such implementations, the external power source interface  228  may be configured to facilitate charging the internal power source  229 . As discussed before, a playback device comprising an internal power source may be referred to herein as a “portable playback device.” On the other hand, a playback device that operates using an external power source may be referred to herein as a “stationary playback device,” although such a device may in fact be moved around a home or other environment. 
     The playback device  102  further includes a user interface  240  that may facilitate user interactions independent of or in conjunction with user interactions facilitated by one or more of the controller devices  104 . In various embodiments, the user interface  240  includes one or more physical buttons and/or supports graphical interfaces provided on touch sensitive screen(s) and/or surface(s), among other possibilities, for a user to directly provide input. The user interface  240  may further include one or more of lights (e.g., LEDs) and the speakers to provide visual and/or audio feedback to a user. 
     As an illustrative example,  FIG.  2 B  shows an example housing  230  of the playback device  102  that includes a user interface in the form of a control area  232  at a top portion  234  of the housing  230 . The control area  232  includes buttons  236   a - c  for controlling audio playback, volume level, and other functions. The control area  232  also includes a button  236   d  for toggling the microphones  222  to either an on state or an off state. 
     As further shown in  FIG.  2 B , the control area  232  is at least partially surrounded by apertures formed in the top portion  234  of the housing  230  through which the microphones  222  (not visible in  FIG.  2 B ) receive the sound in the environment of the playback device  102 . The microphones  222  may be arranged in various positions along and/or within the top portion  234  or other areas of the housing  230  so as to detect sound from one or more directions relative to the playback device  102 . 
     By way of illustration, SONOS, Inc. presently offers (or has offered) for sale certain playback devices that may implement certain of the embodiments disclosed herein, including a “PLAY:1,” “PLAY:3,” “PLAY:5,” “PLAYBAR,” “CONNECT:AMP,” “PLAYBASE,” “BEAM,” “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 should be understood that a playback device is not limited to the examples illustrated in  FIG.  2 A or  2 B  or to the SONOS product offerings. For example, a playback device may include, or otherwise take the form of, a wired or wireless headphone set, which may operate as a part of the media playback system  100  via a network interface or the like. 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 Device Configurations 
       FIGS.  3 A- 3 E  show example configurations of playback devices. Referring first to  FIG.  3 A , in some example instances, a single playback device may belong to a zone. For example, the playback device  102   c  ( FIG.  1 A ) on the Patio may belong to Zone A. In some implementations described below, multiple playback devices may be “bonded” to form a “bonded pair,” which together form a single zone. For example, the playback device  102   f  ( FIG.  1 A ) named “Bed 1” in  FIG.  3 A  may be bonded to the playback device  102   g  ( FIG.  1 A ) named “Bed 2” in  FIG.  3 A  to form Zone B. Bonded playback devices may have different playback responsibilities (e.g., channel responsibilities). In another implementation described below, multiple playback devices may be merged to form a single zone. For example, the playback device  102   d  named “Bookcase” may be merged with the playback device  102   m  named “Living Room” to form a single Zone C. The merged playback devices  102   d  and  102   m  may not be specifically assigned different playback responsibilities. That is, the merged playback devices  102   d  and  102   m  may, aside from playing audio content in synchrony, each play audio content as they would if they were not merged. 
     For purposes of control, each zone in the MPS  100  may be represented as a single user interface (“UI”) entity. For example, as displayed by the controller devices  104 , Zone A may be provided as a single entity named “Portable,” Zone B may be provided as a single entity named “Stereo,” and Zone C may be provided as a single entity named “Living Room.” 
     In various embodiments, a zone may take on the name of one of the playback devices belonging to the zone. For example, Zone C may take on the name of the Living Room device  102   m  (as shown). In another example, Zone C may instead take on the name of the Bookcase device  102   d . In a further example, Zone C may take on a name that is some combination of the Bookcase device  102   d  and Living Room device  102   m . The name that is chosen may be selected by a user via inputs at a controller device  104 . In some embodiments, a zone may be given a name that is different than the device(s) belonging to the zone. For example, Zone B in  FIG.  3 A  is named “Stereo” but none of the devices in Zone B have this name. In one aspect, Zone B is a single UI entity representing a single device named “Stereo,” composed of constituent devices “Bed 1” and “Bed 2.” In one implementation, the Bed 1 device may be playback device  102   f  in the master bedroom  101   h  ( FIG.  1 A ) and the Bed 2 device may be the playback device  102   g  also in the master bedroom  101   h  ( FIG.  1 A ). 
     As noted above, playback devices that are bonded may have different playback responsibilities, such as playback responsibilities for certain audio channels. For example, as shown in  FIG.  3 B , the Bed 1 and Bed 2 devices  102   f  and  102   g  may be bonded so as to produce or enhance a stereo effect of audio content. In this example, the Bed 1 playback device  102   f  may be configured to play a left channel audio component, while the Bed 2 playback device  102   g  may be configured to play a right channel audio component. In some implementations, such stereo bonding may be referred to as “pairing.” 
     Additionally, playback devices that are configured to be bonded may have additional and/or different respective speaker drivers. As shown in  FIG.  3 C , the playback device  102   b  named “Front” may be bonded with the playback device  102   k  named “SUB.” The Front device  102   b  may render a range of mid to high frequencies, and the SUB device  102   k  may render low frequencies as, for example, a subwoofer. When unbonded, the Front device  102   b  may be configured to render a full range of frequencies. As another example,  FIG.  3 D  shows the Front and SUB devices  102   b  and  102   k  further bonded with Right and Left playback devices  102   a  and  102   j , respectively. In some implementations, the Right and Left devices  102   a  and  102   j  may form surround or “satellite” channels of a home theater system. The bonded playback devices  102   a ,  102   b ,  102   j , and  102   k  may form a single Zone D ( FIG.  3 A ). 
     In some implementations, playback devices may also be “merged.” In contrast to certain bonded playback devices, playback devices that are merged may not have assigned playback responsibilities, but may each render the full range of audio content that each respective playback device is capable of. Nevertheless, merged devices may be represented as a single UI entity (i.e., a zone, as discussed above). For instance,  FIG.  3 E  shows the playback devices  102   d  and  102   m  in the Living Room merged, which would result in these devices being represented by the single UI entity of Zone C. In one embodiment, the playback devices  102   d  and  102   m  may playback audio in synchrony, during which each outputs the full range of audio content that each respective playback device  102   d  and  102   m  is capable of rendering. 
     In some embodiments, a stand-alone NMD may be in a zone by itself. For example, the NMD  103   h  from  FIG.  1 A  is named “Closet” and forms Zone I in  FIG.  3 A . An NMD may also be bonded or merged with another device so as to form a zone. For example, the NMD device  103   f  named “Island” may be bonded with the playback device  102   i  Kitchen, which together form Zone F, which is also named “Kitchen.” Additional details regarding assigning NMDs and playback devices as designated or default devices may be found, for example, in previously referenced U.S. patent application Ser. No. 15/438,749. In some embodiments, a stand-alone NMD may not be assigned to a zone. 
     Zones of individual, bonded, and/or merged devices may be arranged to form a set of playback devices that playback audio in synchrony. Such a set of playback devices may be referred to as a “group,” “zone group,” “synchrony group,” or “playback group.” In response to inputs provided via a controller device  104 , playback devices may be dynamically grouped and ungrouped to form new or different groups that synchronously play back audio content. For example, referring to  FIG.  3 A , Zone A may be grouped with Zone B to form a zone group that includes the playback devices of the two zones. As another example, Zone A may be grouped with one or more other Zones C-I. The Zones A-I may be grouped and ungrouped in numerous ways. For example, three, four, five, or more (e.g., all) of the Zones A-I may be grouped. When grouped, the zones of individual and/or bonded playback devices may play back audio in synchrony with one another, as described in previously referenced U.S. Pat. No. 8,234,395. Grouped and bonded devices are example types of associations between portable and stationary playback devices that may be caused in response to a trigger event, as discussed above and described in greater detail below. 
     In various implementations, the zones in an environment may be assigned a particular name, which may be the default name of a zone within a zone group or a combination of the names of the zones within a zone group, such as “Dining Room+Kitchen,” as shown in  FIG.  3 A . In some embodiments, a zone group may be given a unique name selected by a user, such as “Nick&#39;s Room,” as also shown in  FIG.  3 A . The name “Nick&#39;s Room” may be a name chosen by a user over a prior name for the zone group, such as the room name “Master Bedroom.” 
     Referring back to  FIG.  2 A , certain data may be stored in the memory  213  as one or more state variables that are periodically updated and used to describe the state of a playback zone, the playback device(s), and/or a zone group associated therewith. The memory  213  may also include the data associated with the state of the other devices of the media playback system  100 , which may be 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. 
     In some embodiments, the memory  213  of the playback device  102  may store instances of various variable types associated with the states. Variables instances may be stored with identifiers (e.g., tags) corresponding to type. For example, certain identifiers may be a first type “a1” to identify playback device(s) of a zone, a second type “b1” to identify playback device(s) that may be bonded in the zone, and a third type “c1” to identify a zone group to which the zone may belong. As a related example, in  FIG.  1 A , identifiers associated with the Patio may indicate that the Patio is the only playback device of a particular zone and not in a zone group. Identifiers associated with the Living Room may indicate that the Living Room is not grouped with other zones but includes bonded playback devices  102   a ,  102   b ,  102   j , and  102   k . Identifiers associated with the Dining Room may indicate that the Dining Room is part of Dining Room+Kitchen group and that devices  103   f  and  102   i  are bonded. Identifiers associated with the Kitchen may indicate the same or similar information by virtue of the Kitchen being part of the Dining Room+Kitchen zone group. Other example zone variables and identifiers are described below. 
     In yet another example, the MPS  100  may include variables or identifiers representing other associations of zones and zone groups, such as identifiers associated with Areas, as shown in  FIG.  3 A . An Area may involve a cluster of zone groups and/or zones not within a zone group. For instance,  FIG.  3 A  shows a first area named “First Area” and a second area named “Second Area.” The First Area includes zones and zone groups of the Patio, Den, Dining Room, Kitchen, and Bathroom. The Second Area includes zones and zone groups of the Bathroom, Nick&#39;s Room, Bedroom, and Living Room. In one aspect, an Area may be used to invoke a cluster of zone groups and/or zones that share one or more zones and/or zone groups of another cluster. In this respect, such an Area differs from a zone group, which does not share a zone with another zone group. Further examples of techniques for implementing Areas may be found, for example, in U.S. application Ser. No. 15/682,506 filed Aug. 21, 2017 and titled “Room Association Based on Name,” and U.S. Pat. No. 8,483,853 filed Sep. 11, 2007, and titled “Controlling and manipulating groupings in a multi-zone media system.” Each of these applications is incorporated herein by reference in its entirety. In some embodiments, the MPS  100  may not implement Areas, in which case the system may not store variables associated with Areas. 
     The memory  213  may be further configured to store other data. Such data may pertain to audio sources accessible by the playback device  102  or a playback queue that the playback device (or some other playback device(s)) may be associated with. In embodiments described below, the memory  213  is configured to store a set of command data for selecting a particular VAS when processing voice inputs. 
     During operation, one or more playback zones in the environment of  FIG.  1 A  may each be playing different audio content. For instance, the user may be grilling in the Patio zone and listening to hip hop music being played by the playback device  102   c , while another user may be preparing food in the Kitchen zone and listening to classical music being played by the playback device  102   i . 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  102   n  is playing the same hip-hop music that is being playing by playback device  102   c  in the Patio zone. In such a case, playback devices  102   c  and  102   n  may be playing the hip-hop 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 MPS  100  may be dynamically modified. As such, the MPS  100  may support numerous configurations. For example, if a user physically moves one or more playback devices to or from a zone, the MPS  100  may be reconfigured to accommodate the change(s). For instance, if the user physically moves the playback device  102   c  from the Patio zone to the Office zone, the Office zone may now include both the playback devices  102   c  and  102   n . In some cases, the user may pair or group the moved playback device  102   c  with the Office zone and/or rename the players in the Office zone using, for example, one of the controller devices  104  and/or voice input. As another example, if one or more playback devices  102  are moved to a particular space in the home environment that is not already a playback zone, the moved playback device(s) may be renamed or associated with a playback zone for the particular space. 
     Further, different playback zones of the MPS  100  may be dynamically combined into zone groups or split up into individual playback zones. For example, the Dining Room zone and the Kitchen zone may be combined into a zone group for a dinner party such that playback devices  102   i  and  102   l  may render audio content in synchrony. As another example, bonded playback devices in the Den zone may be split into (i) a television zone and (ii) a separate listening zone. The television zone may include the Front playback device  102   b . The listening zone may include the Right, Left, and SUB playback devices  102   a ,  102   j , and  102   k , which may be grouped, paired, or merged, as described above. Splitting the Den zone in such a manner may allow one user to listen to music in the listening zone in one area of the living room space, and another user to watch the television in another area of the living room space. In a related example, a user may utilize either of the NMD  103   a  or  103   b  ( FIG.  1 B ) to control the Den zone before it is separated into the television zone and the listening zone. Once separated, the listening zone may be controlled, for example, by a user in the vicinity of the NMD  103   a , and the television zone may be controlled, for example, by a user in the vicinity of the NMD  103   b . As described above, however, any of the NMDs  103  may be configured to control the various playback and other devices of the MPS  100 . 
     c. Example Controller Devices 
       FIG.  4 A  is a functional block diagram illustrating certain aspects of a selected one of the controller devices  104  of the MPS  100  of  FIG.  1 A . Such controller devices may also be referred to herein as a “control device” or “controller.” The controller device shown in  FIG.  4 A  may include components that are generally similar to certain components of the network devices described above, such as a processor  412 , memory  413  storing program software  414 , at least one network interface  424 , and one or more microphones  422 . In one example, a controller device may be a dedicated controller for the MPS  100 . In another example, a controller device 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 memory  413  of the controller device  104  may be configured to store controller application software and other data associated with the MPS  100  and/or a user of the system  100 . The memory  413  may be loaded with instructions in software  414  that are executable by the processor  412  to achieve certain functions, such as facilitating user access, control, and/or configuration of the MPS  100 . The controller device  104  is configured to communicate with other network devices via the network interface  424 , which may take the form of a wireless interface, as described above. 
     In one example, system information (e.g., such as a state variable) may be communicated between the controller device  104  and other devices via the network interface  424 . For instance, the controller device  104  may receive playback zone and zone group configurations in the MPS  100  from a playback device, an NMD, or another network device. Likewise, the controller device  104  may transmit such system information to a playback device or another network device via the network interface  424 . In some cases, the other network device may be another controller device. 
     The controller device  104  may also communicate playback device control commands, such as volume control and audio playback control, to a playback device via the network interface  424 . As suggested above, changes to configurations of the MPS  100  may also be performed by a user using the controller device  104 . 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 merged player, separating one or more playback devices from a bonded or merged player, among others. 
     As shown in  FIG.  4 A , the controller device  104  also includes a user interface  440  that is generally configured to facilitate user access and control of the MPS  100 . The user interface  440  may include a touch-screen display or other physical interface configured to provide various graphical controller interfaces, such as the controller interfaces  440   a  and  440   b  shown in  FIGS.  4 B and  4 C . Referring to  FIGS.  4 B and  4 C  together, the controller interfaces  440   a  and  440   b  includes a playback control region  442 , a playback zone region  443 , a playback status region  444 , a playback queue region  446 , and a sources region  448 . The user interface as shown is just one example of an interface that may be provided on a network device, such as the controller device shown in  FIG.  4 A , and accessed by users to control a media playback system, such as the MPS  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  442  ( FIG.  4 B ) may include selectable icons (e.g., by way of touch or by using a cursor) that, when selected, 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, etc. The playback control region  442  may also include selectable icons that, when selected, modify equalization settings and/or playback volume, among other possibilities. 
     The playback zone region  443  ( FIG.  4 C ) may include representations of playback zones within the MPS  100 . The playback zones regions  443  may also include a representation of zone groups, such as the Dining Room+Kitchen zone group, as shown. 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 MPS  100 , 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 MPS  100  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 are also possible. The representations of playback zones in the playback zone region  443  ( FIG.  4 C ) may be dynamically updated as playback zone or zone group configurations are modified. 
     The playback status region  444  ( FIG.  4 B ) 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 a controller interface, such as within the playback zone region  443  and/or the playback status region  444 . The graphical representations may include track title, artist name, album name, album year, track length, and/or other relevant information that may be useful for the user to know when controlling the MPS  100  via a controller interface. 
     The playback queue region  446  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 comprising 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, which may then be played back 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 streamed 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 may 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 may 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. 
     With reference still to  FIGS.  4 B and  4 C , the graphical representations of audio content in the playback queue region  446  ( FIG.  4 B ) may include track titles, artist names, track lengths, and/or 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. Playback of such a playback queue may involve one or more playback devices playing back media items of the queue, perhaps in sequential or random order. 
     The sources region  448  may include graphical representations of selectable audio content sources and/or selectable voice assistants associated with a corresponding VAS. The VASes may be selectively assigned. In some examples, multiple VASes, such as AMAZON&#39;s Alexa, MICROSOFT&#39;s Cortana, etc., may be invokable by the same NMD. In some embodiments, a user may assign a VAS exclusively to one or more NMDs. For example, a user may assign a first VAS to one or both of the NMDs  102   a  and  102   b  in the Living Room shown in  FIG.  1 A , and a second VAS to the NMD  103   f  in the Kitchen. Other examples are possible. 
     d. Example Audio Content Sources 
     The audio sources in the sources region  448  may be audio content sources from which audio content may be retrieved and played by the selected playback zone or zone group. 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., via 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. As described in greater detail below, in some embodiments audio content may be provided by one or more media content services. 
     Example audio content sources may include a memory of one or more playback devices in a media playback system such as the MPS  100  of  FIG.  1   , local music libraries on one or more network devices (e.g., a controller device, a network-enabled personal computer, or a networked-attached storage (“NAS”)), streaming audio services providing audio content via the Internet (e.g., cloud-based music services), or audio sources connected to the media playback system via a line-in input connection on a playback device or network device, among other possibilities. 
     In some embodiments, audio content sources may be added or removed from a media playback system such as the MPS  100  of  FIG.  1 A . 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/directories shared over a network accessible by playback devices in the media playback system and generating or updating an audio content database comprising 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. 
     e. Example Network Microphone Devices 
       FIG.  5    is a functional block diagram showing an NMD  503  configured in accordance with embodiments of the disclosure. The NMD  503 , for example, may be configured for use with the MPS  100  and may be in communication with any of the playback and/or network microphone devices described herein. As noted above, in some implementations an NMD may be standalone, while in other implementations be a playback device or a different device, such as smart household appliance (e.g., a smart washing machine, microwave, etc.). As shown in  FIG.  5   , The NMD  503  includes a voice processor  560 , a wake-word engine  570 , and at least one voice extractor  572 , each of which is operably coupled to the voice processor  560 . The NMD  503  may be NMD-equipped such that it includes the microphones  222  and the at least one network interface  224  described above. The NMD  503  may also include other components, such as audio amplifiers, etc., which are not shown in  FIG.  5    for purposes of clarity. 
     The microphones  222  of the NMD  503  are configured to provide detected sound, S D , from the environment of the NMD  503  to the voice processor  560 . The detected sound S D  may take the form of one or more analog or digital signals. In example implementations, the detected sound S D  may be composed of a plurality of signals associated with respective channels  562  that are fed to the voice processor  560 . Each channel  562  may provide all or a portion of the detected sound S D  to the voice processor  560 . 
     Each channel  562  may correspond to a particular microphone  222 . For example, an NMD having six microphones may have six corresponding channels. Each channel of the detected sound S D  may bear certain similarities to the other channels but may differ in certain regards, which may be due to the position of the given channel&#39;s corresponding microphone relative to the microphones of other channels. For example, one or more of the channels of the detected sound S D  may have a greater signal to noise ratio (“SNR”) of speech to background noise than other channels. 
     As further shown in  FIG.  5   , the voice processor  560  includes one or more voice capture components, such as an AEC  564 , a spatial processor  566 , and one or more buffers  568 . In operation, the AEC  564  receives the detected sound S D  and filters or otherwise processes the sound to suppress echoes and/or to otherwise improve the quality of the detected sound S D . That processed sound may then be passed to the spatial processor  566 . 
     The spatial processor  566  is typically configured to analyze the detected sound S D  and identify certain characteristics, such as a sound&#39;s amplitude (e.g., decibel level), frequency spectrum, directionality, etc. In one respect, the spatial processor  566  may help filter or suppress ambient noise in the detected sound S D  from potential user speech based on similarities and differences in the constituent channels  562  of the detected sound S D , as discussed above. As one possibility, the spatial processor  566  may monitor metrics that distinguish speech from other sounds. Such metrics can include, for example, energy within the speech band relative to background noise and entropy within the speech band—a measure of spectral structure—which is typically lower in speech than in most common background noise. In some implementations, the spatial processor  566  may be configured to determine a speech presence probability, examples of such functionality are disclosed in U.S. patent application Ser. No. 15/984,073, filed May 18, 2018, titled “Linear Filtering for Noise-Suppressed Speech Detection,” which is incorporated herein by reference in its entirety. 
     In operation, the one or more buffers  568 —one or more of which may be part of or separate from the memory  213  ( FIG.  2 A )—capture data corresponding to the detected sound S D . More specifically, the one or more buffers  568  capture detected-sound data that was processed by the upstream AEC  564  and spatial processor  566 . The detected-sound and/or any associated data may be referred to as a “sound specimen” when retained in at least one buffer  568 . A sound specimen may comprise, for example, (a) audio data or (b) audio data and metadata regarding the audio data. As an example, a first buffer may temporarily retain audio samples used for streaming audio data, as described below. A second buffer may temporarily retain metadata (e.g., spectral data, sound pressure-level, etc.) regarding the current audio samples in the first buffer, a certain number of audio samples captured prior to the current audio samples, and/or a certain number of audio samples captured after the current audio samples. In some implementations, this type of second buffer may be referred as a look-back buffer. Additional details describing buffers, including look-back buffers, and configurations of buffers with voice processors (e.g., spatial processors) may be found in, for example, U.S. patent application Ser. No. 15/989,715, filed May 25, 2018, titled “Determining and Adapting to Changes in Microphone Performance of Playback Devices,” U.S. patent application Ser. No. 16/138,111, filed Sep. 21, 2018, titled “Voice Detection Optimization Using Sound Metadata,” and U.S. patent application Ser. No. 16/141,875, filed Sep. 25, 2018, titled “Voice Detection Optimization Based on Selected Voice Assistant Service,” all of which are incorporated by reference herein in their entireties. 
     In general, the detected-sound data form a digital representation (i.e., sound-data stream), S DS , of the sound detected by the microphones  222 . In practice, the sound-data stream S DS  may take a variety of forms. As one possibility, the sound-data stream S DS  may be composed of frames, each of which may include one or more sound samples. The frames may be streamed (i.e., read out) from the one or more buffers  568  for further processing by downstream components, such as the wake-word engine  570  and the voice extractor  572  of the NMD  503 . 
     In some implementations, at least one buffer  568  captures detected-sound data utilizing a sliding window approach in which a given amount (i.e., a given window) of the most recently captured detected-sound data is retained as a sound specimen in the at least one buffer  568  while older detected-sound data are overwritten when they fall outside of the window. For example, at least one buffer  568  may temporarily retain 20 frames of a sound specimen at given time, discard the oldest frame after an expiration time, and then capture a new frame, which is added to the  19  prior frames of the sound specimen. 
     In practice, when the sound-data stream S DS  is composed of frames, the frames may take a variety of forms having a variety of characteristics. As one possibility, the frames may take the form of audio frames that have a certain resolution (e.g., 16 bits of resolution), which may be based on a sampling rate (e.g., 44,100 Hz). Additionally, or alternatively, the frames may include information corresponding to a given sound specimen that the frames define, such as metadata that indicates frequency response, power input level, SNR, microphone channel identification, and/or other information of the given sound specimen, among other examples. Thus, in some embodiments, a frame may include a portion of sound (e.g., one or more samples of a given sound specimen) and metadata regarding the portion of sound. In other embodiments, a frame may only include a portion of sound (e.g., one or more samples of a given sound specimen) or metadata regarding a portion of sound. 
     In any case, components of the NMD  503  downstream of the voice processor  560  may process the sound-data stream S DS . For instance, the wake-word engine  570  can be configured to apply one or more identification algorithms to the sound-data stream S DS  (e.g., streamed sound frames) to spot potential wake words in the detected-sound S D . Many first- and third-party wake word detection algorithms are known and commercially available. Different voice services (e.g. AMAZON&#39;s ALEXA, APPLE&#39;s SIRI, MICROSOFT&#39;s CORTANA, GOOGLE&#39;S ASSISTANT, etc.), for example, each use a different wake word for invoking their respective voice service, and some voice services make their algorithms available for use in third-party devices. In some embodiments, the wake-word engine  570  is configured to run multiple wake word detection algorithms on the received audio simultaneously (or substantially simultaneously). To support multiple voice services, the wake-word engine  570  may run the received sound-data stream S DS  through the wake word detection algorithm for each supported voice service in parallel. In such embodiments, the NMD  503  may include VAS selector components (not shown) configured to pass voice input to the appropriate voice assistant service. In other embodiments, the VAS selector components may be omitted, such as when each of the NMD&#39;s wake-word engine(s) are dedicated to the same VAS. 
     In any event, when a particular wake-word engine  570  spots a potential wake word, that wake-word engine can provide an indication of a “wake-word event” (also referred to as a “wake-word trigger”). The indication of the wake-word event, in turn, can cause the NMD to invoke the VAS associated with the triggered wake-word engine. 
     In the example shown in  FIG.  5   , a triggered wake-word engine  570  produces a signal S W , which causes the voice extractor  572  to initiate streaming of the sound-data stream S DS . More specifically, in response to the wake-word event (e.g., in response to a signal S W  from the wake-word engine  570  indicating the wake-word event), the voice extractor  572  is configured to receive and format (e.g., packetize) the sound-data stream S DS . For instance, the voice extractor  572  may packetize the frames of the sound-data stream S DS  into messages, M V , for relaying the sound-data to a VAS over a network. In the example shown in  FIG.  5   , the voice extractor  572  transmits or streams these messages in real time or near real time, to one or more remote computing devices associated with a VAS, such as the VAS  190  ( FIG.  1 B ), via the network interface  224 . 
     The VAS is configured to process the sound-data stream S DS  contained in the messages My sent from the NMD  503 . More specifically, the VAS is configured to identify any voice input based on the sound-data stream S DS  and/or data derived from the sound-data stream S DS . Referring to  FIG.  6 A , a voice input  680  may include a wake-word portion  680   a  and an utterance portion  680   b . The wake-word portion  680   a  corresponds to detected sound that caused the wake-word event. For instance, the wake-word portion  680   a  corresponds to detected sound that caused the wake-word engine  570  to provide an indication of a wake-word event to the voice extractor  572 . The utterance portion  680   b  corresponds to detected sound that potentially comprises a user request following the wake-word portion  680   a.    
     As an illustrative example,  FIG.  6 B  shows an example first sound specimen. In this example, the sound specimen corresponds to detected-sound data that is streamed, e.g., as part of the sound-data stream S DS . This detected-sound data can include audio frames associated with the spotted wake word  680   a  of  FIG.  6 A . As illustrated, the example first sound specimen comprises sound detected in the NMD  503 &#39;s ( FIG.  5   ) environment (i) immediately before a wake word was spoken, which may be referred to as a pre-roll portion (between times to and t 1 ), (ii) while the wake word was spoken, which may be referred to as a wake-meter portion (between times t 1  and t 2 ), and/or (iii) after the wake word was spoken, which may be referred to as a post-roll portion (between times t 2  and t 3 ). Other sound specimens are also possible. 
     Typically, the VAS may first process the wake-word portion  680   a  within the sound-data stream S DS  to verify the presence of the wake word. In some instances, the VAS may determine that the wake-word portion  680   a  comprises a false wake word (e.g., the word “Election” when the word “Alexa” is the target wake word). In such an occurrence, the VAS may send a response to the NMD  503  ( FIG.  5   ) with an indication for the NMD  503  to cease extraction of sound data, which may cause the voice extractor  572  to cease further streaming of the detected-sound data to the VAS. The wake-word engine  570  may resume or continue monitoring sound specimens until another potential wake word, leading to another wake-word event. In some implementations, the VAS may not process or receive the wake-word portion  680   a  but instead processes only the utterance portion  680   b.    
     In any case, the VAS processes the utterance portion  680   b  to identify the presence of any words in the detected-sound data and to determine an underlying intent from these words. The words may correspond to a certain command and certain keywords  684  (identified individually in  FIG.  6 A  as a first keyword  684   a  and a second keyword  684   b ). A keyword may be, for example, a word in the voice input  680  identifying a particular device or group in the MPS  100 . For instance, in the illustrated example, the keywords  684  may be one or more words identifying one or more zones in which the music is to be played, such as the Living Room and the Dining Room ( FIG.  1 A ). 
     To determine the intent of the words, the VAS is typically in communication with one or more databases associated with the VAS (not shown) and/or one or more databases (not shown) of the MPS  100 . Such databases may store various user data, analytics, catalogs, and other information for natural language processing and/or other processing. In some implementations, such databases may be updated for adaptive learning and feedback for a neural network based on voice-input processing. In some cases, the utterance portion  680   b  may include additional information, such as detected pauses (e.g., periods of non-speech) between words spoken by a user, as shown in  FIG.  6 A . The pauses may demarcate the locations of separate commands, keywords, or other information spoke by the user within the utterance portion  680   b.    
     Based on certain command criteria, the VAS may take actions as a result of identifying one or more commands in the voice input, such as the command  682 . Command criteria may be based on the inclusion of certain keywords within the voice input, among other possibilities. Additionally, or alternatively, command criteria for commands may involve identification of one or more control-state and/or zone-state variables in conjunction with identification of one or more particular commands. Control-state variables may include, for example, indicators identifying a level of volume, a queue associated with one or more devices, and playback state, such as whether devices are playing a queue, paused, etc. Zone-state variables may include, for example, indicators identifying which, if any, zone players are grouped. 
     After processing the voice input, the VAS may send a response S 1  to the NMD  503  via network interface  224  with an instruction to perform one or more actions based on an intent it determined from the voice input. For example, based on the voice input, the VAS may direct the NMD  503 , or the MPS  100  via the NMD  503 , to initiate playback on one or more of the playback devices  102 , control one or more of these devices (e.g., raise/lower volume, group/ungroup devices, etc.), turn on/off certain smart devices, among other actions. After receiving the response from the VAS, the wake-word engine  570  the NMD  503  may resume or continue to monitor the sound-data stream S DS  until it spots another potential wake-word, as discussed above. 
     The NMD  503  may be operatively coupled to playback components of a playback device, of which the NMD  503  may form a part in various embodiments. The playback components can include an audio interface  519 , an audio-output processor  515 , and speakers  218 . One, some, or all of the playback components may be on-board a playback device comprising the NMD  503 , or may be associated with a different playback device of MPS  100 . The network interface  224  may communicate a signal S 1  to the audio interface  519  based on the response from the VAS, and the audio interface  519  may transmit an audio signal A S  to the audio-output processor  515 . The audio-output processor  515 , for example, may comprise one or more of the audio processing components  216  discussed above with reference to  FIG.  2 A . Finally, the audio-output processor  515  transmits the processed audio signal A P  to the speakers  218  of a playback device for playback. The audio-output processor  515  may also transmit one or more reference signals REF to the AEC  564  based on the processed audio signal A P  to suppress echoed audio components from the audio content played back by a playback device that may otherwise be present in detected sound S D . 
     In some implementations, the NMD  503  may include one or more other voice-input identification engines (not shown), in addition to or in lieu of the one or more wake word engines  570 , that enable the NMD  503  to operate without the assistance of a remote VAS. As an example, such an engine may identify in detected sound certain commands (e.g., “play,” “pause,” “turn on,” etc.) and/or certain keywords or phrases, such as the unique name assigned to a given playback device (e.g., “Bookcase,” “Patio,” “Office,” etc.). In response to identifying one or more of these commands, keywords, and/or phrases, the NMD  503  may communicate a signal (not shown in  FIG.  5   ) that causes the audio processing components  216  ( FIG.  2 A ) to perform one or more actions. For instance, when a user says “Hey Sonos, stop the music in the office,” the NMD  503  may communicate a signal to the office playback device  102   n , either directly, or indirectly via one or more other devices of the MPS  100 , which causes the office device  102   n  to stop audio playback. Reducing or eliminating the need for assistance from a remote VAS may reduce latency that might otherwise occur when processing voice input remotely. In some cases, the identification algorithms employed may be configured to identify commands that are spoken without a preceding wake word. For instance, in the example above, the NMD  503  may employ an identification algorithm that triggers an event to stop the music in the office without the user first saying “Hey Sonos” or another wake word. 
     III. Example Systems and Methods for Distributed Voice Processing 
       FIGS.  7 A- 7 E  depict networked playback devices  702  (identified individually as a first playback device  702   a  and a second playback device  702   b ) configured to distribute voice processing in accordance with the present technology. The playback devices  702 , for example, may be part of a media playback system (such as MPS  100 ). In some embodiments, the playback devices  702  may be positioned in various areas of an environment (e.g., a household), such as in different rooms, or in the same room. For example, the first playback device  702   a  may be positioned in a first area, such as “Room 1” (as shown), and the second playback device may be positioned in a second area, such as within Room 1 or a different room (e.g., “Room 2”). As described in greater detail below, the playback devices  702  may be configured to share the workload of one or more voice processing functions, such as voice-input detection, including wake-word detection. Although the methods described below are with reference to two playback devices, the methods of the present technology include distribution of one or more voice processing functions (such as wake-word detection) across more than two playback devices (e.g., 3 playback devices, 4 playback devices, 8 playback devices, twenty playback devices, etc.). 
     As shown in  FIG.  7 A , each of the playback devices  702  may include components that are generally similar to components of the playback and network microphone devices described above. For example, the playback devices  702  may include playback components (not shown) such as an audio interface, an audio-output processor, speakers, etc. The playback devices  702  further include voice processing components that may be similar to some or all of the voice processing components of the NMD  503  described above with reference to  FIG.  5   . For example, the first and second playback devices  702   a  and  702   b  include respective first and second voice processors  760   a  and  760   b  (collectively “voice processors  760 ”), first and second wake word engines  770   a  and  770   b  (collectively “wake word engines  770 ”) associated with respective first and second VASes  790   a  and  790   b . The first and second playback devices  702   a  and  702   b  further include respective first and second network interfaces  724   a  and  724   b  (collectively “network interfaces”) configured to communicate with one another over local and/or wide area networks. The first and second network interfaces  724   a  and  724   b  may also be configured to communicate with other computing devices of the MPS  100  and/or one or more remote servers (such as those associated with a VAS) over local and/or wide area networks. 
     The first voice processor  760  of the first playback device  702   a  may include voice processing components, such as a first AEC  764   a , a first spatial processor  766 , and a first buffer  768   a . The components of the first voice processor  760   a  are configured to process and feed the detected sound to the first wake-word engine  770   a  (represented by arrow I(a)). The first wake-word engine  770   a  may be configured to detect a first wake word specific to the first VAS  790   a . For example, the first wake word engine  770   a  may be associated with AMAZON&#39;s ALEXA and be configured to run a corresponding wake word detection algorithm (e.g., configured to detect the wake word “Alexa” or other associated wake word). The first wake word engine  770   a  may be configured to detect only wake words associated with the first VAS  790   a  (such as the first wake word), and cannot detect wake words associated with a different VAS (such as a second VAS  790   b , described below). 
     In the example depicted in  FIG.  7 A , the second voice processor  760   b  includes a second buffer  768   b  and does not include an AEC and a spatial processor. Such a configuration may be beneficial, for example, as wake word engines associated with certain VASes, such as GOOGLE&#39;s ASSISTANT, may not require acoustic echo cancellation and/or spatial processing for wake word detection. In other embodiments, the second voice processor  760   b  may not include a buffer and/or may include an AEC, a spatial processor, and/or other voice processing components. In any event, the components of the second voice processor  760   b  are configured to process and feed detected sound data to the second voice processor  760   b  via the network interfaces  724  (represented by arrows I(b)-I(d)). The second playback device  702   b  and/or the second wake word engine  770   b  may be associated with the second VAS  790   b  and configured to detect a second wake word specific to the second VAS  790   b  that is different than the first wake word. For example, the second wake word engine  770   b  may be associated with GOOGLE&#39;s ASSISTANT and configured to run a corresponding wake word detection algorithm (e.g., configured to detect the wake word “OK, Google” or other associated wake word). Thus, in some aspects of the technology, the first and second wake word engines  770   a  and  770   b  are configured to detect different wake words associated with different VASes. 
     In one aspect, the first playback device  702   a  may be configured to be NMD-equipped in a manner similar to that described above with reference to NMD  503  ( FIG.  5   ). For example, the first playback device  702   a  includes a plurality of on-board microphones  722  (e.g., far field microphones) configured to detect sound. In the illustrated example, the first playback device  702   a  has six microphones  722  and six corresponding channels (labeled as “mic/ch. 1,” “mic/ch. 2,” etc.). In other embodiments, the first playback device  702   a  may have more or fewer than six microphones or channels. The sound detected by the microphones  722  may be processed by the first voice processor  760   a  and fed to the first wake-word engine  770   a  and the first network interface  724   a . In the example depicted in  FIG.  7 A , the first voice processor  760   a  transmits the processed detected sound from microphones 1-6 to the first wake word engine  770   a , and transmits the processed detected sound from microphones 5 and 6 to the first network interface  724   a  (for subsequent transmission to the second playback device  702   b , detailed below). 
     The second playback device  702   b  may also be configured to be NMD-equipped but in a different manner than that of the first playback device  702   a . In contrast to the first playback device  702   a , the second playback device  702   b  does not have any on-board microphones. Instead, the second playback device  702   b  is configured to receive and process sound detected by the microphones  722  of the first playback device  702   a  (via communication of the first and second network interfaces  724   a  and  724   b ). The second playback device  702   b  may receive the detected sound in the form of raw mic data or processed sound data (e.g., pre-processed by the first voice processor  760   a ). In the example shown in  FIG.  7 A , the second playback device  702   b  receives detected sound from a designated subset of the microphones  722  (e.g., microphones 5 and 6). In other embodiments, the second playback device  702   b  may receive detected sound from more or fewer microphones  722  of the first playback device  702   a  (e.g., 1 microphone, 4 microphones, all of the available microphones, etc.). 
     As noted above, the detected sound (from the first playback device  702   a ) is passed via the second network interface (represented by arrow I(d)) to the second voice processor  760   b  which processes and transmits the detected sound to the second wake word engine  770   b  (represented by arrow I(e)). The second wake word engine  770   b  then processes the detected sound for detection of the second wake word, which may occur before, after, or while the first wake word engine  770   a  processes the detected sound for the first wake word. As such, the first and second playback devices  702   a ,  702   b  are configured to monitor sound detected by the microphones  722  of the first playback device  702   a  for different wake words associated with different VASes which allows a user to realize the benefits of multiple VASes, each of which may excel in different aspects, rather than requiring a user to limit her interactions to a single VAS to the exclusion of any others. Moreover, the distribution of wake word detection across multiple playback devices of the system frees up computational resources (e.g., processing time and power) (as compared to a single playback device with two wake word engines). As such, the playback devices of the present technology may be configured to efficiently process detected sound, thereby enhancing the responsiveness and accuracy of the media playback system to a user&#39;s command. 
     In various embodiments, the data transmitted from the first playback device  702   a  to the second playback device  702   b  may comprise, for example, raw microphone data and/or processed sound data from one, some or all of the microphones (e.g., after being processed by one or more of the first AEC  764   a  and the first spatial processor  766   a ). Processing the data to be transmitted may include compressing the data prior to transmission. In some implementations, it may be beneficial to perform acoustic echo cancellation (via the first AEC  764   a ) with the reference signal(s) before transmitting the detected sound to reduce bandwidth. In some embodiments, the second AEC  764   b  may be bypassed or omitted from the second voice processor  760   b  in configurations in which acoustic cancellation is applied to sound data to be transmitted from the first playback device  702   a  to the second playback device  702   b . In additional or alternate embodiments, spatial processing may be carried out on the data to be transmitted to the second playback device  702   b , in which case the second spatial processor  766   b  may be bypassed or omitted from the second voice processor  760   b.    
     In the scenario depicted in  FIG.  7 A , a user has spoken a command (“[First wake word], turn on the lights”) that includes the first wake word and is intended to invoke the first VAS  790   a . The microphones  722  detect the sound associated with the command and pass the detected sound to the first voice processor  760   a  for processing by one or more of its components. The first voice processor  760   a  passes the detected sound data from microphones 1-6 to the first wake word engine  770   a , and passes the detected sound data from microphones 5 and 6 to the first network interface  724   b  for transmission to the second playback device  702   b  via the second network interface  724   b . The second network interface  724   b  feeds the detected sound data to the second voice processor  760   b , which may apply one or more voice processing techniques before sending to the second wake word engine  770   b  for detection of the second wake word. Because the command includes the first wake word, the first wake word engine  770   a  triggers the voice extractor (for example, voice extractor  572  in  FIG.  5   ) to stream messages (e.g., messages containing packetized frames of the detected sound to the first VAS  790   a ) via first network interface  724   a . As the command does not include the second wake word, the second wake word engine  770   b  does not trigger voice extraction to the second VAS  790   b . The first VAS  790   a  processes the packetized voice data and sends a response to the first network interface  724  with instructions for the first playback device  702   a  to perform the action requested by the user, i.e., to play back music by the Beatles. The first VAS  790   a  may also send the first playback device  702   a  a voice response for playback by the first playback device  702   a  to acknowledge to the user that the MPS  100  and/or first VAS  790   a  has processed the user&#39;s request. 
       FIG.  7 B  depicts the first and second playback devices  702   a ,  702   b  within the example environment of  FIG.  7 A , but in this example the user has spoken a command that includes the second wake word and is intended to invoke the second VAS  790   b . As shown in  FIG.  7 B , in such a scenario the second wake word engine  770   b  detects the second wake word in the detected sound and triggers the voice extractor (such as voice extractor  572  in  FIG.  5   , which may then extract sound data (e.g., packetizing frames of the detected sound into messages). In the example shown in  FIG.  7 B , the voice extractor extracts sound data to one or more remote computing devices associated with the second VAS  790   b  (e.g., via second network interface  724   b ). The remote computing device(s) associated with the second VAS  790   b  are configured to process the sound data associated with the detected sound and send a response to the second playback device  702   b  (e.g., via the second network interface  724   b ) that may include instructions for the first playback device  702   a , the second playback device  702   b , and/or another playback device(s) of the MPS  100  to perform an action or series of actions (or, in some instances, do nothing). For the example command provided in  FIG.  7 B  (“play the Beatles”), the second VAS  790   b  sends a message to the second playback device  702   b  with instructions for the first playback device  702   a  to play music by the Beatles. The second playback device  702   b  may then forward the instructions to the first playback device  702   a , and the first playback device  702  performs the action. The second VAS  790   b  may also send the second playback device  702   b  a voice response for playback by the first playback device  702   a  to acknowledge to the user that the MPS  100  and/or second VAS  790   b  has processed the user&#39;s request. As shown in  FIG.  7 B , the first playback device  702   a  may then play back the voice response (“okay”). 
     In some embodiments the second VAS  790   b  may be made aware of the first playback device  702   a , the relationship between the first and second playback devices  702   a ,  702   b , and/or the functional capabilities and/or limitations of each playback device (i.e., has/does not have a speaker/capable of playback, has/does not have a microphone/is NMD-equipped, etc.), and the response may include a message instructing the second playback device  702   b  to send instructions to the first playback device  702   a  that causes the first playback device  702   a  to do nothing or perform an action. Thus, even though the second playback device  702   b  is the playback device in direct communication with the second VAS  190   b , in some embodiments the second playback device  702   b  may not take any action other than to instruct the first playback device  702   a  to act. 
     In some embodiments, the second VAS  790   b  may not receive any information regarding which playback device was the originator of the detected sound and/or which playback device will be performing the action (i.e., the second VAS  790   b  is not aware of the first playback device  702   a ). In such embodiments, the second VAS  790   b  may send a message to the second playback device  702   b  with instructions to do nothing or perform an action, and the second playback device  702   b  may forward the message to the first playback device  702   a . The second playback device  702   b  may automatically forward the message, or may first process the message to decide whether the message should be transmitted to the first playback device  702   a.    
     As shown in the example flow diagram of  FIG.  7 C , the second playback device  702   b  may optionally include an identifier, such as a tag T, in the messages  783  containing the sound data transmitted to the second VAS  790   b  so that, when the second VAS  790   b  sends the response(s)  784 ,  785  containing the instructions for responding to the user&#39;s request, the instructions are identified to the second playback device  702   b  for playback by the first playback device  702   a . In some embodiments, the tag T is only meaningful to the second playback device  702   b  and the second VAS  790   b  passively includes the tag in the responses without being aware of its function or implication. In other embodiments, the tag T also indicates to the second VAS  790   b  that the first playback device  702   a  will be performing the requested action (or at least that the second playback device  702   b  is not performing the requested action). 
     Whether to be performed by the first playback device  702   a , the second playback device  702   b , or other playback device of the MPS  100 , the action may comprise playing back an audio response on the first and/or second playback device  702   a ,  702   b  (and/or other playback device of the MPS  100 ). For example, the audio response may be an acknowledgment of receipt of the command, such as instructions to play back a chime or a voice response (e.g., an audio file) to play back (such as “okay,” etc.). The audio response may additionally or alternately comprise a voice response with an answer to a question asked in the voice input (e.g., “53 degrees and raining” in response to “what is the weather?”) or a follow-up request for information (“did you mean the kitchen lights or the patio lights?”). 
     In some embodiments, the second VAS  790   b  may instruct the MPS  100  to download media content (e.g., music, podcasts, audio books, etc.) requested in the voice input to the first and/or second playback device  702   a ,  702   b . The second VAS  790   b  may provide instructions for the first and/or second VAS  190   b  to perform an action related to media content, such as increasing/decreasing the volume, starting or resuming playback of a media item, playing the next song in the queue, playing the previous song in the queue, stopping or pausing playback, grouping certain playback device(s) of the MPS  100  with other playback device(s) of the MPS  100 , transferring playback of a media item to a different playback device, and others. 
     The action may additionally or alternately include an action that does not directly implicate playback of audio content, such as instructions for the first and/or second playback device  702   a ,  702   b  (or other playback device of the MPS  100 ) to instruct or otherwise cause a smart home appliance to perform an action (such as instructing a smart light to turn on/off, instructing a smart lock to lock/unlock, etc.). Other non-auditory actions include setting a timer, adding an item to a shopping list, calling one of the user&#39;s contacts, etc. For all such non-auditory actions, the second playback device  702   b  may receive instructions to provide an audible acknowledgment (e.g., “okay,” a chime, etc.) of the command. 
     While the second VAS  790   b  is processing the detected sound, the first playback device  702   a  may continue monitoring detected sound for the first wake word and/or transmitting detected sound to the second playback device  702   b.    
     Referring again to  FIG.  7 B , in some embodiments the second playback device  702   b  and/or MPS  100  may temporarily disable (e.g., via a disable signal, as shown) the first wake word engine  770   a  while the second VAS  790   b  processes a voice input in which the second wake word was detected. Disabling the first wake word engine  770   b  may occur regardless of whether the first and second playback devices  702   a ,  702   b  share detected sound from any of the microphones  722  and/or are individually using some or all of the microphones  722 . It may be beneficial to disable the first wake word engine  770   a  while the second VAS  790   b  processes a voice input to suppress inadvertent detection of the first wake word and prevent potentially conflicting actions and/or output by the first and/or second playback devices  702   a ,  702   b . In some embodiments, once the second VAS  790   b  has completed processing of the voice input, the first wake word engine  770   a  may be re-enabled. Likewise, in some embodiments the first playback device  702   a  and/or the MPS  100  may temporarily disable the second wake word engine  770   b  when the first wake word engine  770   a  detects a wake word. Additionally or alternatively, the microphones assigned to the first or second playback device  702   a ,  702   b  may be temporarily disabled when the wake word engine of the other playback device detects its respective wake word. In some embodiments, disabling a wake-word engine may include allowing the wake-word engine to continue to monitor for wake-words but temporarily muting the audio input upstream from the spatial processor, such as by inserting zeroes in a digital input stream or silence at a low noise level such that wake-word is less or not capable of detecting wake-words while muted. 
       FIG.  7 D  depicts another configuration of the first and second playback devices  702   a ,  702   b  within the example environment in which the user has spoken the same command as in  FIG.  7 A  that invokes the first VAS  790   a  by using the first wake word. In contrast to the scenario described above with respect to  FIG.  7 A , the first voice processor  760   a  receives detected sound from a first subset of the microphones  722  (e.g., microphones 1-4), and the second playback device  702   b  receives detected sound from a second subset of the microphones  722  (e.g., microphones 5 and 6) different than the first subset of microphones. In such embodiments, the first and/or second subset of microphones may include any number of microphones less than the total number of microphones of the first playback device  702   a  (including a single microphone). In some aspects, certain ones of the microphones  722  are assigned exclusively to the first playback device  702   a  (for example, by one or both of the playback devices  702 , the MPS  100 , and/or another playback device of the MPS  100 ), and certain ones of the microphones  722  are assigned exclusively to the second playback device  702   b . In such embodiments, the first and second subsets of microphones have no microphones in common. In other embodiments, the first and second subsets of microphones may have at least one microphone in common. 
     In some embodiments, the MPS  100  and/or the first playback device  702   a  may include a microphone selector (not shown) that dynamically determines which, if any, of the microphones  722  are used for collecting signals for transfer to the second playback device  702   b . The microphone selector, for example, may utilize a lookback buffer to provide feedback to one or more remote computing devices of the MPS  100  for determining if, when, and/or which of the microphones  722  of the first playback device  702   a  can be shared with or assigned for exclusive use to the second playback device  702   b . Additional details regarding microphone selection and/or aggregation across multiple playback devices may be found in, for example, in previously referenced U.S. patent application Ser. Nos. 15/989,715; 16/138,111; and Ser. No. 16/141,875. 
     In these and other implementations, the spatial processor may implement linear filtering or related techniques for selectively disabling/enabling microphones in a way that is not constrained by traditional beamforming techniques. For example, traditional beamforming techniques typically require the number of microphone inputs for a beamformer to be fixed (e.g., to six known microphone channels) because these techniques rely on filtering algorithms that are not adaptive or not readily adaptive to an environment. Linear filtering and related techniques, by contrast, implement algorithms and filtering coefficients that can be adapted on the fly, such that, for example, additional or fewer microphone channels can be selectively routed to the respective voice processors  760   a ,  760   b  depending on the particular ambient noise in an environment, available processing power, etc. Additional examples of spatial processors and/or associated filters, such as multi-channel Wiener filters, for processing speech, reverberated speech, and noise signals, s(t), x(t), v(t), may be found in, for example, in previously referenced U.S. patent application Ser. No. 15/984,073 and U.S. patent Ser. No. 16/147,710, filed Sep. 29, 2018, titled “Linear Filtering for Noise-Suppressed Speech Detection Via Multiple Network Microphone Devices,” both of which are incorporated by reference herein in their entireties. 
       FIG.  7 E  depicts another configuration of the first and second playback devices  702   a ,  702   b  within the example environment in which the user has spoken the same command as in  FIG.  7 B  that invokes the second VAS  790   b  by using the second wake word. However, in  FIG.  7 E , the first playback device  702   a  sends the second playback device  702   b  reference data from the first AEC  764   a , (represented by arrow I(f)) as well as the raw mic data from designated ones of the microphones (e.g., microphones 5 and 6, represented by arrows (I(g) and I(h)). In such embodiments, the second voice processor  760   b  may include a second AEC  764   b  and a second spatial processor  766   b  in addition to the second buffer  768   b . The second AEC  764   b  and the second spatial processor  766   b  may have generally similar components and functions to respective first AEC  764   a  and first spatial processor  766   a . The second voice processor  766   b  may be configured to receive and process the reference data and detected sound data before sending the detected sound data to the second wake word engine  770   b  for detection of the second wake word. 
       FIGS.  8  and  9    show, respectively, methods  800  and  900  in accordance with embodiments of the present technology that can be implemented by a network microphone device, such as any of the PBDs (such as first and second PBD&#39;s  702   a  and  702   b ), NMDs, and/or controller devices disclosed and/or described herein, or any other voice-enabled device now known or later developed. 
     Referring to  FIG.  8   , method  800  begins at block  801 , which includes detecting sound via a microphone array of a first playback device. Next, the method  800  advances to block  802 , which includes analyzing, via a first wake-word engine of the first playback device, the detected sound from the first playback device. At block  803 , the method  800  includes transmitting data associated with the detected sound to a second playback device. In some example implementations, the second playback device is a local area network. At block  804 , the method  800  includes identifying that the detected sound contains either (i) a first wake word based on the analysis via the first wake-word engine or (ii) a second wake word based on the analysis via the second wake-word engine. Based on the identification, at block  805 , the method  800  includes transmitting sound data corresponding to the detected sound over a wide area network to a remote computing device associated with a particular voice assistant service. 
     Turning to  FIG.  9   , method  900  begins at block  901 , which includes detecting sound via a microphone array of a first playback device (such as PBD  702   a ). At block  902 , method  900  includes transmitting data associated with the detected sound from the first playback device to a second playback device (such as PBD  702   b ). In some aspects, the data is transmitted over a local area network. Method  900  further includes analyzing, via a wake word engine of the second playback device, the transmitted data associated with the detected sound for identification of a wake word, as shown at block  903 . Method  900  continues at block  904  with identifying that the detected sound contains the wake word based on the analysis via the wake word engine. Based on the identification, transmitting sound data corresponding to the detected sound from the second playback device to a remote computing device over a wide area network (block  905 ), where the remote computing device is associated with a particular voice assistant service. The method advances at block  906 , which includes receiving, via the second playback device, a response from the remote computing device, where the response is based on the detected sound. At block  907 , method  900  includes transmitting a message from the second playback device to the first playback device, where the message is based on the response from the remote computing device and includes instructions for the first playback device to perform an action. In some embodiments, the message is transmitted over a local area network. Method  900  further includes performing the action via the first playback device, as shown at block  908 . 
       FIGS.  10 A and  10 B  depict example timelines for voice inputs  1080   a  and  1080   b , respectively, in both of which the user makes two requests, each utilizing a different one of the first and second wake words and intended to invoke a different one of the first and second VASes  702   a ,  702   b  (e.g., “[First wake word], play the Beatles and [second wake word], turn on the lights”). For each of the voice inputs  1080   a  and  1080   b , the user speaks the first wake word at a first time t 1  and speaks the second wake word at a second time t 2 . In some embodiments, the MPS  100  may only allow concurrent voice processing if the voice input and/or the detected wake words fall within a predetermined time interval Δt. If both of the first and second wake words are detected within the time interval Δt (as is the case in  FIG.  10 A ), then concurrent processing of the associated voice input by the first and second VASes  790   a  and  790   b  is allowed to proceed. If the first and second wake words are detected outside of the time interval Δt (as is the case in  FIG.  10 B ) then concurrent voice processing is not allowed to proceed and one or both of the first and second playback devices  702   a ,  702   b  (or voice processing functions thereof) are temporarily disabled. For example, in  FIG.  10 B , the second wake word falls outside of the time interval Δt, and thus only the first playback device  702   a  is allowed to proceed with contacting the first VAS  790   a  while the second playback device  702   b  is disabled or otherwise prevented from communicating with the second VAS  790 . 
     When voice processing is allowed to proceed, each of the first and second VASes  790   a  and  790   b  may send a response to the corresponding first and second playback devices  702   a  and  702   b , which may include instructions to perform an action or to do nothing. The responses from the first and second VASes  790   a  and  790   b  may be transmitted at the same time or at different times, and may or may not be in the same order as the corresponding wake word detection. Likewise, performance of the action (if applicable) by the corresponding playback device may occur at the same time or at different times, and may or may not be in the same order as the corresponding wake word detection and/or receipt of response. 
     Whether performance of the actions by the first and second playback devices  702   a ,  702   b  occurs at least partially at the same time may depend on the nature of the actions to be performed. For example, in the illustrated embodiment, the action for the first playback device  702   a  is to output the requested media content, while the action for the second playback device  702   b  is to cause the smart lights to turn on. Turning on the lights does not require output of audio content by the second playback device  802   b , and thus the second playback device  702   b  may perform the action without interfering with the output of the media content by the first playback device  702   a . However, if the action does require playback of audio content (for example, the second playback device  702   b  may output a voice response of “okay” to acknowledge that the voice input has been processed), the first and second playback devices  702   a ,  702   b  may coordinate output of their respective audio contents. 
       FIG.  11    shows a method  1100  in accordance with embodiments of the present technology that can be implemented by a network microphone device, such as any of the PBDs (such as first and second PBD&#39;s  702   a  and  702   b ), NMDs, and/or controller devices disclosed and/or described herein, or any other voice-enabled device now known or later developed. Method  1100  begins at block  1101 , which includes detecting sound via a microphone array of a first playback device (such as first playback device  702   a ). The sound may comprise a first voice input including a first wake word. At block  1102 , the method  1100  includes detecting sound via the microphone array of the first playback device, wherein the sound comprises a second voice input including a second wake word. As indicated at blocks  1103 - 1105 : (a) if the second wake word is detected within a predetermined time interval t of detection of the first wake word, then voice processing is allowed to process with both the first and second playback devices; (b) if the second wake word is not detected within a predetermined time interval t of detection of the first wake word, then voice processing is disabled at the second playback device (or whichever device is associated with the wake word uttered second.) 
     Various embodiments of methods  800 ,  900 , and  1100  include one or more operations, functions, and actions illustrated by blocks  801 - 805 ,  901 - 908 , and  1101 - 1105 , respectively. Although the blocks are illustrated in sequential order, these blocks may also be performed in parallel, and/or in a different order than the order disclosed and described herein. Also, the various blocks may be combined into fewer blocks, divided into additional blocks, and/or removed based upon a desired implementation. 
     In addition, for the methods of  800 ,  900 , and  1100  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 one or more processors 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 media, 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 method  800  and other processes and methods disclosed herein, each block in  FIG.  8    may represent circuitry that is wired to perform the specific logical functions in the process. 
       FIG.  12    depicts another configuration of the first and second playback devices  702   a  and  702   b  within the example environment, except in  FIG.  12    the first and second wake word engines  770   a  and  770   b  are associated with a common VAS (such as first VAS  790   a ) even though the first and second wake word engines  770   a  and  770   b  are configured to detect different wake words. For example, the first wake word engine  770   a  may be configured to run a wake word detection algorithm for a wake word spoken with a Spanish accent, while the second wake word engine  770   b  may be configured to run a wake word detection algorithm for the same wake word but spoken with a French accent. In the foregoing example, both the first and second wake word engines  770   a  and  770   b  may be associated with the same VAS. In another aspect of the technology, the first wake word engine  770   a  may be configured to detect a first wake word associated with the VAS  990  (such as the first wake word) while the second wake word engine  970   b  may be configured to detect a wake word associated with the MPS  100  (e.g., “Hey Sonos”). 
     In addition or alternatively, the second wake-word engine  770   b  may be configured to detect sounds in addition to or in lieu of wake words in the voice stream received from the first playback device  702   a  over the network interface  724 . For example, the second wake-word engine  770   b  may be configured to run a local NLU engine to detect certain playback control commands, such as volume, grouping, playback/transport control, etc. In these and other embodiments, the second wake-word engine  770   b  can be configured to run other algorithms for event detection, such as listening for window breaks, fire alarms, breach of security events, etc. In some embodiments, the first playback device  702   a  may have limited processing resources (e.g., available system memory, power constraints, etc.) relative to the second playback device  702   b . As such, a playback device without sufficient resources to run microphone DSP, a wakeword engine, and an additional NLU/event-detection engine may offload NLU/event-detection engine to another playback device. As an example, the first playback device  702   a  may be a portable playback device, such as set of wireless headphones. In related embodiments, the second wake-word engine  770   b  may be able to detect wake-words more accurately than the first wake-word engine  770   a . In such instances, the second wake-word engine  770   b  may intervene if the first wake-word engine  770   a  failed to detect a certain wake-word and/or if the first wake-word engine  770   a  was triggered by a wake word that the second wake-word engine  770   b  determined to be a false positive. 
     Although the foregoing systems and methods for distributed wake word processing are described with respect to a configuration in which the second playback device  702   b  does not have any microphones, it will be appreciated that the systems and methods described herein may also be carried out using a second playback device  702   b  with onboard microphones. In such embodiments, the second playback device  702   b  may still receive and/or process transmitted data related to sound detected by one, some, or all of the microphones  722  of the first playback device  702   a , which may be in addition to or in lieu of sound detected by its own microphones. In some embodiments, the second voice processor  760   b  receives and/or processes sound data from one, some, or all of the first microphones  722   a  and one, some, or all of the second microphones. The second playback device  702   b  may have the same or a different number and/or configuration of microphones as the first playback device  702   a . The second voice processor  760   b  may still receive and/or process data related to the sound detected by the first microphones  722   a  even when the second playback device  702   b  is in the same room as the first playback device  702   a  or otherwise detecting sound generated by at least one of the same sources via its own microphones  722   b.    
     In some aspects of the technology, one, some, or all of the microphones of the second playback device  702   b  may be functionally disabled (for example, by one or both of the playback devices  702 , the MPS  100 , and/or another playback device of the MPS  100 ). One or more of the second microphones may be functionally disabled, for example, in response to the second voice processor  760   b  receiving data related to the sound from the microphones  722  of the first playback device  702   a.    
     EXAMPLES 
     The present technology is illustrated, for example, according to various aspects described below. Various examples of aspects of the present technology are described as numbered examples (1, 2, 3, etc.) for convenience. These are provided as examples and do not limit the present technology. It is noted that any of the dependent examples may be combined in any combination, and placed into a respective independent example. The other examples can be presented in a similar manner. 
     Example 1: A method comprising: detecting sound via a microphone array of a first playback device and analyzing, via a first wake-word engine of the first playback device, the detected sound; transmitting data associated with the detected sound from the first playback device to a second playback device over a local area network; analyzing, via a second wake-word engine of the second playback device, the transmitted data associated with the detected sound; identifying that the detected sound contains either (i) a first wake word based on the analysis via the first wake-word engine or (ii) a second wake word based on the analysis via the second wake-word engine; and based on the identification, transmitting sound data corresponding to the detected sound over a wide area network to a remote computing device associated with a particular voice assistant service. Example 2: The method of Example 1, wherein the sound data further contains a voice utterance and the method further comprises receiving, via one of the first playback device and the second playback device, at least one message from the remote computing device, where the message includes a playback command based on the voice utterance. The method may further include playing back, via at least one of the first playback device and the second playback device, audio content based on the playback command. Example 3: The method of Example 1 or Example 2, wherein the identifying comprises identifying the second wake word (i) based on the transmitted data associated with the detected sound and (ii) without detecting the sound via the second playback device. Example 4: The method of any one of Examples 1 to 3, wherein the microphone array comprises a plurality of individual microphones and the first playback device comprises a voice processor configured to receive portions of the detected sound from respective ones of the individual microphones. In such embodiments, the method may comprise processing, via the voice processor, one or more of the portions of the detected sound to produce the data associated with the detected sound that is transmitted to the second playback device. Example 5: The method of any one of Examples 1 to 4, further comprising processing the one or more portions of the detected sound comprises processing fewer than all of the portions of the detected sound. Example 6: The method of any one of Examples 1 to 5, further comprising spatially processing, via the voice processor, the detected sound based on one or more of the portions of the detected sound. In such embodiments, analyzing the detected sound via the first wake-word engine comprises analyzing the spatially processed detected sound. Example 7: The method of any one of Examples 1 to 6, further comprising (a) playing back, via the first playback device, audio content; and (b) producing, via the first playback device, at least one reference signal based on the audio content, where the data associated with the detected sound that is transmitted to the second playback device comprises data that is based on the at least one reference signal. 
     Example 8: A system comprising a first playback device and a second playback device. The first playback device may comprise one or more processors, a microphone array, and a first computer-readable medium storing instructions that, when executed by the one or more processors, cause the first device to perform first operations, the first operations comprising: detecting sound via the microphone array; analyzing, via a first wake-word engine of the first playback device, the detected sound; and transmitting data associated with the detected sound from the first playback device to a second playback device over a local area network. The second playback device may comprise one or more processors and a second computer-readable medium storing instructions that, when executed by the one or more processors, cause the second device to perform second operations, the second operations comprising: analyzing, via a second wake-word engine of the second playback device, the transmitted data associated with the detected sound; identifying that the detected sound contains a second wake word based on the analysis via the second wake-word engine; and based on the identification, transmitting sound data corresponding to the detected sound over a wide area network to a remote computing device associated with a particular voice assistant service. Example 9: the system of Example 8, wherein the sound data further contains a voice utterance and the second operations further comprise receiving at least one message from the remote computing device. The message may comprise a playback command that is based on the voice utterance. In such embodiments, the first operations may further comprise playing back audio content based on the playback command. Example 10: the system of Example 8 or Example 9, wherein identifying the second wake word is (i) based on the transmitted data associated with the detected sound and (ii) without detecting the sound via the second playback device. Example 11: the system of any one of Examples 8 to 10, wherein the microphone array comprises a plurality of individual microphones and the first playback device comprises a voice processor configured to receive portions of the detected sound from respective ones of the individual microphones. In such operations, the first operations may comprise processing, via the voice processor, one or more of the portions of the detected sound to produce the data associated with the detected sound that is transmitted to the second playback device. Example 12: the system of any one of Examples 8 to 11, wherein processing the one or more portions of the detected sound comprises processing fewer than all of the portions of the detected sound. Example 13: the system of any one of Examples 8 to 12, wherein the first operations further comprise spatially processing, via the voice processor, the detected sound based on one or more of the portions of the detected sound. In such embodiments, analyzing the detected sound via the first wake-word engine comprises analyzing the spatially processed detected sound. Example 14: the system of any one of Examples 8 to 13, wherein the first operations further comprise playing back, via the first playback device, audio content, and producing, via the first playback device, at least one reference signal based on the audio content. In such embodiments, the data associated with the detected sound that is transmitted to the second playback device comprises data that is based on the at least one reference signal. 
     Example 15: A plurality of non-transitory computer-readable media storing instructions for distributed wake-word detection, including a first computer-readable storage medium and a second computer-readable storage medium. The first computer-readable medium may store instructions that, when executed by one or more processors, cause the one or more processors to perform first operations. The first operations may comprise detecting sound via the microphone array; analyzing, via a first wake-word engine of the first playback device, the detected sound; and transmitting data associated with the detected sound from the first playback device to a second playback device over a local area network. The second computer-readable medium may store instructions that, when executed by one or more processors, cause the one or more processors to perform second operations. The second operations may comprise: analyzing, via a second wake-word engine of the second playback device, the transmitted data associated with the detected sound; identifying that the detected sound contains a second wake word based on the analysis via the second wake-word engine; and based on the identification, transmitting sound data corresponding to the detected sound over a wide area network to a remote computing device associated with a particular voice assistant service. Example 16: the plurality of non-transitory computer-readable media of Example 15, wherein the sound data further contains a voice utterance, and wherein (a) the second operations further comprise receiving at least one message from the remote computing device, wherein the message comprises a playback command, and wherein the playback command is based on the voice utterance; and (b) the first operations further comprise playing back audio content based on the playback command. Example 17: the plurality of non-transitory computer-readable media of Example 15 or Example 16, wherein identifying the second wake word is (i) based on the transmitted data associated with the detected sound and (ii) without detecting the sound via the second playback device. Example 18: the plurality of non-transitory computer-readable media of any one of Examples 15 to 17, wherein the microphone array comprises a plurality of individual microphones, the first playback device comprises a voice processor configured to receive portions of the detected sound from respective ones of the individual microphones, and the first operations comprise processing, via the voice processor, one or more of the portions of the detected sound to produce the data associated with the detected sound that is transmitted to the second playback device. Example 19: the plurality of non-transitory computer-readable media of any one of Examples 15 to 18, wherein processing the one or more portions of the detected sound comprises processing fewer than all of the portions of the detected sound. Example 20: the plurality of non-transitory computer-readable media of any one of Examples 15 to 19, wherein the first operations may further comprise spatially processing, via the voice processor, the detected sound based on one or more of the portions of the detected sound, and wherein analyzing the detected sound via the first wake-word engine comprises analyzing the spatially processed detected sound. 
     Example 21: A method comprising: detecting sound via a microphone array of a first playback device; transmitting data associated with the detected sound from the first playback device to a second playback device over a local area network; analyzing, via a wake word engine of the second playback device, the transmitted data associated with the detected sound for identification of a wake word; identifying that the detected sound contains the wake word based on the analysis via the wake word engine; based on the identification, transmitting sound data corresponding to the detected sound from the second playback device to a remote computing device over a wide area network, wherein the remote computing device is associated with a particular voice assistant service; receiving via the second playback device a response from the remote computing device, wherein the response is based on the detected sound; transmitting a message from the second playback device to the first playback device over the local area network, wherein the message is based on the response from the remote computing device and includes instructions to perform an action; and performing the action via the first playback device. Example 22: the method of Example 21, wherein the action is a first action and the method further comprises performing a second action via the second playback device, where the second action is based on the response from the remote computing device. Example 23: the method of Example 21 or Example 22, further comprising disabling a wake word engine of the first playback device in response to the identification of the wake word via the wake word engine of the second playback device. Example 24: the method of any one of Examples 21 to 23, further comprising enabling a wake word engine of the first playback device after the second playback device receives the response from the remote computing device. Example 25: the method of Example 24, wherein the wake word may be a second wake word, and the wake word engine of the first playback device is configured to detect a first wake word that is different than the second wake word. Example 26: the method of any one of Examples 21 to 25, wherein the first playback device is configured to communicate with the remote computing device associated with the particular voice assistant service. Example 27: the method of any one of Examples 21 to 26, wherein the remote computing device is a first remote computing device and the voice assistant service is a first voice assistant service, and the first playback device is configured to detect a wake word associated with a second voice assistant service different than the first voice assistant service. 
     Example 28: A first playback device comprising one or more processors and a computer-readable medium storing instructions that, when executed by the one or more processors, cause the first playback device to perform operations. The operations may comprise receiving, from a second playback device over a local area network, data associated with sound detected via a microphone array of the second playback device; analyzing, via a wake word engine of the first playback device, the data associated with the detected sound for identification of a wake word; identifying that the detected sound contains the wake word based on the analysis via the wake word engine; based on the identification, transmitting sound data corresponding to the detected sound to a remote computing device over a wide area network, wherein the remote computing device is associated with a particular voice assistant service; receiving a response from the remote computing device, wherein the response is based on the detected sound; and transmitting a message to the second playback device over the local area network, wherein the message is based on the response from the remote computing device and includes instructions for the second playback device to perform an action. Example 29: the first playback device of Example 28, wherein the action is a first action and the operations further comprise performing a second action via the first playback device, where the second action is based on the response from the remote computing device. Example 30: the first playback device of Example 28 or Example 29, wherein the operations may comprise disabling a wake word engine of the second playback device in response to the identification of the wake word via the wake word engine of the first playback device. Example 31: the first playback device of any one of Examples 28 to 30, wherein the operations of the first playback device may comprise enabling the wake word engine of the second playback device after the first playback device receives the response from the remote computing device. Example 32: the first playback device of any one of Examples 28 to 31, wherein the wake word is a first wake word and the wake word engine of the second playback device is configured to detect a second wake word that is different than the first wake word. Example 33: the first playback device of any one of Examples 27 to 32, wherein the first playback device is configured to communicate with the remote computing device associated with the particular voice assistant service. Example 34: the first playback device of any one of Examples 28 to 33, wherein the remote computing device is a first remote computing device and the voice assistant service is a first voice assistant service. In such embodiments, the second playback device may be configured to detect a wake word associated with a second voice assistant service different than the first voice assistant service. 
     Example 35: A system comprising a first playback device and a second playback device. The first playback device may comprise one or more processors, a microphone array, and a first computer-readable medium storing instructions that, when executed by the one or more processors, cause the first playback device to perform first operations. The first operations may comprise: detecting sound via the microphone array; transmitting data associated with the detected sound to a second playback device over a local area network. The second playback device may comprise one or more processors and a second computer-readable medium storing instructions that, when executed by the one or more processors, cause the second playback device to perform second operations. The second operations may comprise analyzing, via a wake word engine of the second playback device, the transmitted data associated with the detected sound from the first playback device for identification of a wake word; identifying that the detected sound contains the wake word based on the analysis via the wake word engine; based on the identification, transmitting sound data corresponding to the detected sound to a remote computing device over a wide area network, wherein the remote computing device is associated with a particular voice assistant service; receiving a response from the remote computing device, wherein the response is based on the detected sound; and transmitting a message to the first playback device over the local area network, wherein the message is based on the response from the remote computing device and includes instructions to perform an action. The first computer-readable medium of the first playback device may cause the first playback device to perform the action from the instructions received from the second playback device. Example 36: the system of Example 35, wherein the action is a first action and the second operations further comprise performing a second action via the second playback device, where the second action is based on the response from the remote computing device. Example 37: the system of Example 35 or Example 36, wherein the second operations may further comprise disabling a wake word engine of the first playback device in response to the identification of the wake word via the wake word engine of the second playback device. Example 38: the system of any one of Examples 35 to 37, wherein the second operations may further comprise enabling the wake word engine of the first playback device after the second playback device receives the response from the remote computing device. Example 39: the system of any one of Examples 35 to 38, wherein the first playback device may be configured to communicate with the remote computing device associated with the particular voice assistant service. Example 40: the system of any one of Examples 35 to 39, wherein the remote computing device is a first remote computing device and the voice assistant service is a first voice assistant service, and wherein the first playback device is configured to detect a wake word associated with a second voice assistant service different than the first voice assistant service. 
     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. 
     In addition to the examples described herein with respect to grouping and bonding playback devices, in some implementations multiple playback devices may be merged together. For example, a first playback device may be merged with a second playback device to form a single merged “device.” The merged playback devices and may not be specifically assigned different playback responsibilities. That is, the merged playback devices and may, aside from playing audio content in synchrony, each play audio content as they would if they were not merged. However, the merged devices may present to the media playback system and/or to the user as a single user interface (UI) entity for control. 
     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.