Patent Publication Number: US-2022230635-A1

Title: VAS Toggle Based on Device Orientation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 120 to, and is a continuation of, U.S. non-provisional patent application Ser. No. 16/660,197, filed on Oct. 22, 2019, entitled “VAS Toggle Based On Device Orientation,” which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present technology relates to consumer goods and, more particularly, to methods, systems, products, features, services, and other elements directed to voice-assisted control of 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 2002, when SONOS, Inc. began development of a new type of playback system. Sonos then filed one of its first patent applications in 2003, entitled “Method for Synchronizing Audio Playback between Multiple Networked Devices,” and began offering its first media playback systems for sale in 2005. The Sonos Wireless Home Sound System enables people to experience music from many sources via one or more networked playback devices. Through a software control application installed on a controller (e.g., smartphone, tablet, computer, voice input device), one can play what she wants in any room having a networked playback device. Media content (e.g., songs, podcasts, video sound) can be streamed to playback devices such that each room with a playback device can play back corresponding different media content. In addition, rooms can be grouped together for synchronous playback of the same media content, and/or the same media content can be heard in all rooms synchronously. 
    
    
     
       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: 
       Features, aspects, and advantages of the presently disclosed technology may be better understood with regard to the following description, appended claims, and accompanying drawings, as listed below. A person skilled in the relevant art will understand that the features shown in the drawings are for purposes of illustrations, and variations, including different and/or additional features and arrangements thereof, are possible. 
         FIG. 1A  is a partial cutaway view of an environment having a media playback system configured in accordance with aspects of the disclosed technology. 
         FIG. 1B  is a schematic diagram of the media playback system of  FIG. 1A  and one or more networks. 
         FIG. 2A  is a functional block diagram of an example playback device. 
         FIG. 2B  is an isometric diagram of an example housing of the playback device of  FIG. 2A . 
         FIG. 2C  is a diagram of an example voice input. 
         FIG. 2D  is a graph depicting an example sound specimen in accordance with aspects of the disclosure. 
         FIGS. 3A, 3B, 3C, 3D and 3E  are diagrams showing example playback device configurations in accordance with aspects of the disclosure. 
         FIG. 4  is a functional block diagram of an example controller device in accordance with aspects of the disclosure. 
         FIGS. 5A and 5B  are controller interfaces in accordance with aspects of the disclosure. 
         FIG. 6  is a message flow diagram of a media playback system. 
         FIG. 7A  is a functional block diagram of an example network microphone device. 
         FIG. 7B  is an isometric diagram of the example network microphone device in a first orientation. 
         FIG. 7C  is an isometric diagram of the example network microphone device in a second orientation. 
         FIG. 7D  is an isometric diagram illustrating the example network microphone device transitioning from the first orientation to the second orientation. 
         FIG. 7E  is a functional block diagram of certain components of the example network microphone device in accordance with aspects of the disclosure. 
         FIG. 8A  is a schematic diagram illustrating the example network microphone device operating in a first mode while paired with an example network device. 
         FIG. 8B  is a schematic diagram illustrating the example network microphone device operating in a second mode while paired with the example network device. 
         FIG. 9  is a schematic diagram illustrating an example media playback system and cloud network in accordance with aspects of the disclosure. 
         FIGS. 10A, 10B, 10C, and 10D  show exemplary output of an example NMD configured in accordance with aspects of the disclosure. 
         FIG. 11  is a flow diagram of an example method to toggle a VAS based on orientation 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. 1A . 
     DETAILED DESCRIPTION 
     I. Overview 
     Example techniques described herein involve toggling voice input processing via a cloud-based voice assistant service (“VAS”). An example network microphone device (“NMD”) may enable or disable processing of voice inputs via a cloud-based voice assistant service based on the physical orientation of the NMD. While processing of voice inputs via the cloud-based VAS is disabled, the NMD may process voice inputs via a local natural language unit (NLU). 
     An NMD is a networked computing device that typically includes an arrangement of microphones, such as a microphone array, that is configured to detect sound present in the NMD&#39;s environment. NMDs may facilitate voice control of smart home devices, such as wireless audio playback devices, illumination devices, appliances, and home-automation devices (e.g., thermostats, door locks, etc.). NMDs may also be used to query a cloud-based VAS for information such as search queries, news, weather, and the like. 
     Some users are apprehensive of sending their voice data to a cloud-based VAS for privacy reasons. One possible advantage of a processing voice inputs via a local NLU is increased privacy. By processing voice utterances locally, a user may avoid transmitting voice recordings to the cloud (e.g., to servers of a voice assistant service). Further, in some implementations, the NMD may use a local area network to discover playback devices and/or smart devices connected to the network, which may avoid providing personal data relating to a user&#39;s home to the cloud. Also, the user&#39;s preferences and customizations may remain local to the NMD(s) in the household, perhaps only using the cloud as an optional backup. Other advantages are possible as well. 
     On the other hand, a cloud-based VAS is relatively more capable than a local NLU. In contrast to a NLU implemented in one or more cloud servers that is capable of recognizing a wide variety of voice inputs, practically, local NLUs are capable of recognizing a relatively smaller library of keywords (e.g., 10,000 words and phrases). Moreover, the cloud-based VAS may support additional features (such a querying for real-time information) relative to a local NLU. Moreover, the cloud-based VAS may integrate with other cloud-based services to provide voice control of those services. 
     Given these competing interests, a user may desire to selectively disable voice input processing via a cloud-based VAS (in favor of voice input processing via a local NLU). When voice input processing via the cloud-based VAS is disabled, the user has the benefit of increased privacy. Conversely, when voice input processing via the cloud-based VAS is enabled, the user may take advantage of the relatively more capable cloud-based VAS. 
     Example NMDs may selectively disable voice input processing via a cloud-based VAS using physical orientation of its housing. For instance, an example NMD may be implemented with a cylindrical-shaped housing (e.g., similar in shape to a hockey puck). The first and second ends of the housing may carry a first set of microphones and a second set of microphones, respectively. 
     When the cylindrical-shaped housing is placed on its first end (i.e., in a first orientation), the NMD disables voice input processing via a cloud-based VAS. Conversely, when the cylindrical-shaped housing is placed on its second end (i.e., in a second orientation), the NMD enables voice input processing via a cloud-based VAS. Disabling cloud-based processing using physical orientation may instill confidence in a user that their privacy is being protected, as the microphones associated with the cloud-based VAS are partially covered by whatever surface the housing of the network microphone device is resting upon. 
     Other example network microphone devices may implement different toggling techniques. For instance, the NMD may include a physical switch or other hardware control to toggle voice processing via the cloud-based VAS. Alternatively, a graphical user interface (GUI) or voice user interface (VUI) may be used to toggle voice processing via the cloud-based VAS. 
     Example NMDs that selectively disable voice input processing via a cloud-based VAS using physical orientation may include two or more sets of microphones. For instance, a first set of microphones may be utilized to capture audio in a first orientation while a second set of microphones is utilized in a second orientation. Continuing the puck-shaped housing example above, the housing may carry one or more first microphones near its first end and one or more second microphones in its second end. Then, when the housing is sitting on its first end, the NMD captures voice inputs using the one or more second microphones. Conversely, when the housing is sitting on its second end, the NMD captures voice inputs using the one or more first microphones. 
     Some cloud-based voice assistant services are triggered based on a wake word. In such examples, a voice input typically includes a wake word followed by an utterance comprising a user request. In practice, a wake word is typically a predetermined nonce word or phrase used to “wake up” an NMD and cause it to invoke a particular voice assistant service (“VAS”) to interpret the intent of voice input in detected sound. For example, a user might speak the wake word “Alexa” to invoke the AMAZON® VAS, “Ok, Google” to invoke the GOOGLE® VAS, or “Hey, Siri” to invoke the APPLE® VAS, among other examples. In practice, a wake word may also be referred to as, for example, an activation-, trigger-, wakeup-word or -phrase, and may take the form of any suitable word, combination of words (e.g., a particular phrase), and/or some other audio cue. 
     To identify whether sound detected by the NMD contains a voice input that includes a particular wake word, NMDs often utilize a wake-word engine, which is typically onboard the NMD. The wake-word engine may be configured to identify (i.e., “spot” or “detect”) a particular wake word in recorded audio using one or more identification algorithms. Such identification algorithms may include pattern recognition trained to detect the frequency and/or time domain patterns that speaking the wake word creates. This wake-word identification process is commonly referred to as “keyword spotting.” In practice, to help facilitate keyword 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 in the recorded audio. 
     When a wake-word engine detects a wake word in recorded audio, 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. With a VAS wake-word engine, these additional processes may include extracting detected-sound data from a buffer, among other possible additional processes, such as outputting an alert (e.g., an audible chime and/or a light indicator) indicating that a wake word has been identified. 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. 
     When a VAS receives detected-sound data, the VAS processes this data, which involves identifying the voice input and determining 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. 
     Within examples, local processing of a voice input may be trigged based on detection of one or more keywords in sound captured by the NMD. Example NMDs may include a local voice input engine to detect “local keywords” and generate events to process voice inputs when a local keyword is detected. These local keywords may take the form of a nonce keyword (e.g., “Hey, Sonos”) or a keyword that invokes a command (referred to herein as a “command keyword”). A command keyword is a word or phrase that functions as a command itself, rather than being a nonce word that merely triggers a wake word event. 
     As noted above, a detected local keyword event may cause one or more subsequent actions, such as local natural language processing of a voice input. For instance, when a local voice input engine detects a local keyword in recorded audio, the NMD may determine that a local keyword event has occurred and responsively process the voice input locally using a local NLU. Processing the input may involve the local NLU determining an intent from one or more keywords in the voice input. 
     In some implementations, voice input processing via the local NLU may remain enabled when the voice input processing via the cloud-based VAS is enabled. In such embodiments, a user may target the cloud-based VAS for processing a voice input by speaking a VAS wake word. The user may target the local NLU for processing of the voice input by speaking a local wake word or by speaking a voice command without a VAS wake word. Alternatively, the NMD may disable voice input processing via the local NLU when voice input processing via the cloud-based VAS is enabled. 
     As noted above, example techniques relate to toggling a cloud-based VAS between enabled and disabled modes. An example implementation involves a network microphone device including one or more first microphones, one or more second microphones, a network interface, one or more processors, and a housing carrying the one or more first microphones, the one or more second microphones, the network interface, the one or more processors, and data storage having stored therein instructions executable by the one or more processors. The network microphone device detects that the housing is in a first orientation. After detecting that the housing is in the first orientation, the device enables a first mode. Enabling the first mode includes (i) disabling voice input processing via a cloud-based voice assistant service and (ii) enabling voice input processing via a local natural language unit. While the first mode is enabled, the network microphone device (i) captures sound data associated with a first voice input via the one or more first microphones and (ii) detects, via a local natural language unit, that the first voice input comprises sound data matching one or more keywords from a local natural language unit library of the local natural language unit. The network microphone device determines, via the local natural language unit, an intent of the first voice input based on at least one of the one or more keywords and performs a first command according to the determined intent of the first voice input. The network microphone device may detects that the housing is in a second orientation different than the first orientation. After detecting that the housing is in the second orientation, the network microphone device enables the second mode. Enabling the second mode includes enabling voice input processing via the cloud-based voice assistant service. 
     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. 
     Moreover, some functions are described herein as being performed “based on” or “in response to” another element or function. “Based on” should be understood that one element or function is related to another function or element. “In response to” should be understood that one element or function is a necessary result of another function or element. For the sake of brevity, functions are generally described as being based on another function when a functional link exists; however, such disclosure should be understood as disclosing either type of functional relationship. 
     II. Example Operation Environment 
       FIGS. 1A and 1B  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. 1A , 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. 1A and 1B  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. 1B , 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. 1B ), a smart thermostat  110 , and a local computing device  105  ( FIG. 1A ). 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. 1B ) 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. 1B , the various playback, network microphone, and controller devices  102 ,  103 , and  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 network  111 , such as a LAN including a network router  109 . For example, the playback device  102   j  in the Den  101   d  ( FIG. 1A ), 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 NETWORK  111 . 
     As further shown in  FIG. 1B , 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. 1B , remote computing devices  106  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. 1B  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. 1B , 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 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. 1B , 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. 1A ). Some playback devices may be assigned names according to a zone or room, such as the playback devices  102   e ,  1021 ,  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. 1A ). 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. 1B , the NMDs  103  are configured to interact with the VAS  190  over a network via the network  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. 1A ) 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 MPS  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. 1A ) 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. 1A ) 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 No. 
     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 NETWORK  111  ( FIG. 1B ) may be eliminated and the single playback device  102  and/or the single NMD  103  may communicate directly with the remote computing devices  106 - 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. 2A  is a functional block diagram illustrating certain aspects of one of the playback devices  102  of the MPS  100  of  FIGS. 1A and 1B . 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. 2A , 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. 1A . 
     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. 2A  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. 2A , 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. 1B ), 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 . 
     As further shown in  FIG. 2A , 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. 2B  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. 2B , 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. 2B ) 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. 2A or 2B  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 MPS  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. 
       FIG. 2C  is a diagram of an example voice input  280  that may be processed by an NMD or an NMD-equipped playback device. The voice input  280  may include a keyword portion  280   a  and an utterance portion  280   b . The keyword portion  280   a  may include a wake word or a command keyword. In the case of a wake word, the keyword portion  280   a  corresponds to detected sound that caused a wake-word The utterance portion  280   b  corresponds to detected sound that potentially comprises a user request following the keyword portion  280   a . An utterance portion  280   b  can be processed to identify the presence of any words in detected-sound data by the NMD in response to the event caused by the keyword portion  280   a . In various implementations, an underlying intent can be determined based on the words in the utterance portion  280   b . In certain implementations, an underlying intent can also be based or at least partially based on certain words in the keyword portion  280   a , such as when keyword portion includes a command keyword. In any case, the words may correspond to one or more commands, as well as a certain command and certain keywords. A keyword in the voice utterance portion  280   b  may be, for example, a word identifying a particular device or group in the MPS  100 . For instance, in the illustrated example, the keywords in the voice utterance portion  280   b  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. 1A ). In some cases, the utterance portion  280   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. 2C . The pauses may demarcate the locations of separate commands, keywords, or other information spoke by the user within the utterance portion  280   b.    
     Based on certain command criteria, the NMD and/or a remote VAS may take actions as a result of identifying one or more commands in the voice input. 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. 
     In some implementations, the MPS  100  is configured to temporarily reduce the volume of audio content that it is playing upon detecting a certain keyword, such as a wake word, in the keyword portion  280   a . The MPS  100  may restore the volume after processing the voice input  280 . Such a process can be referred to as ducking, examples of which are disclosed in U.S. patent application Ser. No. 15/438,749, incorporated by reference herein in its entirety. 
       FIG. 2D  shows an example sound specimen. In this example, the sound specimen corresponds to the sound-data stream (e.g., one or more audio frames) associated with a spotted wake word or command keyword in the keyword portion  280   a  of  FIG. 2A . As illustrated, the example sound specimen comprises sound detected in an NMD&#39;s environment (i) immediately before a wake or command word was spoken, which may be referred to as a pre-roll portion (between times t 0  and t 1 ), (ii) while a wake or command 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 or command 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. In various implementations, aspects of the sound specimen can be evaluated according to an acoustic model which aims to map mels/spectral features to phonemes in a given language model for further processing. For example, automatic speech recognition (ASR) may include such mapping for command-keyword detection. Wake-word detection engines, by contrast, may be precisely tuned to identify a specific wake-word, and a downstream action of invoking a VAS (e.g., by targeting only nonce words in the voice input processed by the playback device). 
     ASR for command keyword detection may be tuned to accommodate a wide range of keywords (e.g., 5, 10, 100, 1,000, 10,000 keywords). Command keyword detection, in contrast to wake-word detection, may involve feeding ASR output to an onboard, local NLU which together with the ASR determine when command word events have occurred. In some implementations described below, the local NLU may determine an intent based on one or more other keywords in the ASR output produced by a particular voice input. In these or other implementations, a playback device may act on a detected command keyword event only when the playback devices determines that certain conditions have been met, such as environmental conditions (e.g., low background noise). 
     b. Example Playback Device Configurations 
       FIGS. 3A-3E  show example configurations of playback devices. Referring first to  FIG. 3A , in some example instances, a single playback device may belong to a zone. For example, the playback device  102   c  ( FIG. 1A ) 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. 1A ) named “Bed  1 ” in  FIG. 3A  may be bonded to the playback device  102   g  ( FIG. 1A ) named “Bed  2 ” in  FIG. 3A  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. 3A  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. 1A ) and the Bed  2  device may be the playback device  102   g  also in the master bedroom  101   h  ( FIG. 1A ). 
     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. 3B , 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. 3C , 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. 3D  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. 3A ). 
     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. 3E  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. 1A  is named “Closet” and forms Zone I in  FIG. 3A . 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. 3A , 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. 3A . 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. 3A . 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. 2A , 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 MPS  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. 1A , 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. 3A . An Area may involve a cluster of zone groups and/or zones not within a zone group. For instance,  FIG. 3A  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. 1A  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. 1B ) 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  is a functional block diagram illustrating certain aspects of a selected one of the controller devices  104  of the MPS  100  of  FIG. 1A . Such controller devices may also be referred to herein as a “control device” or “controller.” The controller device shown in  FIG. 4  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 , 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  540   a  and  540   b  shown in  FIGS. 5A and 5B . Referring to  FIGS. 5A and 5B  together, the controller interfaces  540   a  and  540   b  includes a playback control region  542 , a playback zone region  543 , a playback status region  544 , a playback queue region  546 , and a sources region  548 . 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 , 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  542  ( FIG. 5A ) 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  542  may also include selectable icons that, when selected, modify equalization settings and/or playback volume, among other possibilities. 
     The playback zone region  543  ( FIG. 5B ) may include representations of playback zones within the MPS  100 . The playback zones regions  543  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  543  ( FIG. 5B ) may be dynamically updated as playback zone or zone group configurations are modified. 
     The playback status region  544  ( FIG. 5A ) 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  543  and/or the playback status region  544 . 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  546  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. 5A and 5B , the graphical representations of audio content in the playback queue region  646  ( FIG. 5A ) 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  548  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. 1A , 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  548  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. 1A . 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. 
       FIG. 6  is a message flow diagram illustrating data exchanges between devices of the MPS  100 . At step  650   a , the MPS  100  receives an indication of selected media content (e.g., one or more songs, albums, playlists, podcasts, videos, stations) via the control device  104 . The selected media content can comprise, for example, media items stored locally on or more devices connected to the media playback system and/or media items stored on one or more media service servers (one or more of the remote computing devices  106  of  FIG. 1B ). In response to receiving the indication of the selected media content, the control device  104  transmits a message  651   a  to the playback device  102  ( FIGS. 1A-1B ) to add the selected media content to a playback queue on the playback device  102 . 
     At step  650   b , the playback device  102  receives the message  651   a  and adds the selected media content to the playback queue for play back. 
     At step  650   c , the control device  104  receives input corresponding to a command to play back the selected media content. In response to receiving the input corresponding to the command to play back the selected media content, the control device  104  transmits a message  651   b  to the playback device  102  causing the playback device  102  to play back the selected media content. In response to receiving the message  651   b , the playback device  102  transmits a message  651   c  to the computing device  106  requesting the selected media content. The computing device  106 , in response to receiving the message  651   c , transmits a message  651   d  comprising data (e.g., audio data, video data, a URL, a URI) corresponding to the requested media content. 
     At step  650   d , the playback device  102  receives the message  651   d  with the data corresponding to the requested media content and plays back the associated media content. 
     At step  650   e , the playback device  102  optionally causes one or more other devices to play back the selected media content. In one example, the playback device  102  is one of a bonded zone of two or more players. The playback device  102  can receive the selected media content and transmit all or a portion of the media content to other devices in the bonded zone. In another example, the playback device  102  is a coordinator of a group and is configured to transmit and receive timing information from one or more other devices in the group. The other one or more devices in the group can receive the selected media content from the computing device  106 , and begin playback of the selected media content in response to a message from the playback device  102  such that all of the devices in the group play back the selected media content in synchrony. 
     III. Example Network Microphone Device 
       FIG. 7A  is a functional block diagram illustrating certain aspects of an example network microphone device (NMD)  703 . Generally, the NMD  703  may be similar to the network microphone device(s)  103  illustrated in  FIGS. 1A and 1B . As shown, the NMD  703  includes various components, each of which is discussed in further detail below. Many of these components are similar to the playback device  102  of  FIG. 2A . In contrast to the NMD-equipped playback device of  FIG. 2A , however, the NMD  703  is not designed for audio content playback and therefore may exclude audio processing components  216 , amplifiers  217 , and/or speakers  218  or may include relatively less capable versions of these components. The various components of the NMD  703  may be operably coupled to one another via a system bus, communication network, or some other connection mechanism. 
     As shown, the NMD  703  includes at least one processor  712 , which may be a clock-driven computing component configured to process input data according to instructions stored in memory  713 . The memory  713  may be a tangible, non-transitory, computer-readable medium configured to store instructions that are executable by the processor  712 . For example, the memory  713  may be data storage that can be loaded with software code  714  that is executable by the processor  712  to achieve certain functions. 
     The at least one network interface  724  may take the form of one or more wireless interfaces  725  and/or one or more wired interfaces  726 . The wireless interface  725  may provide network interface functions for the NMD  703  to wirelessly communicate with other devices (e.g., playback device(s)  102 , other NMD(s)  103 , and/or controller device(s)  104 ) in accordance with a communication protocol (e.g., any wireless standard including IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4G mobile communication standard, and so on). The wired interface  726  may provide network interface functions for the NMD  703  to communicate over a wired connection with other devices in accordance with a communication protocol (e.g., IEEE 802.3). While the network interface  724  shown in  FIG. 7A  includes both wired and wireless interfaces, the playback device  102  may in various implementations include only wireless interface(s) or only wired interface(s). 
     As shown in  FIG. 7A , the NMD  703  also includes voice processing components  720  that are operably coupled to microphones  722 . The microphones  722  are configured to detect sound (i.e., acoustic waves) in the environment of the NMD  703 , which is then provided to the voice processing components  720 . More specifically, the microphones  722  are 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  720  to perform various functions based on the detected sound, as described in greater detail below. In one implementation, the microphones  722  are arranged as one or more arrays of microphones (e.g., an array of six microphones). In some implementations, the NMD  703  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 microphone). 
     In operation, similar to the voice-processing components  220  of the NMD-equipped playback device  102  the voice-processing components  720  are generally configured to detect and process sound received via the microphones  722 , identify potential voice input in the detected sound, and extract detected-sound data to enable processing of the voice input by a cloud-based VAS, such as the VAS  190  ( FIG. 1B ), or a local NLU. The voice processing components  720  may include one or more analog-to-digital converters, an acoustic echo canceller (“AEC”), a spatial processor, 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  720  may include or otherwise take the form of one or more DSPs or one or more modules of a DSP. In some implementations, one or more of the voice processing components  720  may be a subcomponent of the processor  712 . 
     The NMD  703  also includes one or more orientation sensors  723  configured to detect an orientation of the NMD  703 . The orientation sensor(s)  723  may include one or more accelerometers, one or more gyroscopes, and/or a magnetometer facilitate detecting an orientation of the NMD  703 . Various implementations may implement any suitable orientation detection techniques. 
     As further shown in  FIG. 2A , the NMD  703  also includes power components  727 . The power components  727  include at least an external power source interface  728 , which may be coupled to a power source (not shown) via a power cable or the like that physically connects the NMD  703  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  727  of the NMD  703  may additionally include an internal power source  729  (e.g., one or more batteries) configured to power the NMD  703  without a physical connection to an external power source. When equipped with the internal power source  729 , the NMD  703  may operate independent of an external power source. In some such implementations, the external power source interface  728  may be configured to facilitate charging the internal power source  729 . As discussed before, a NMD comprising an internal power source may be referred to herein as a “portable NMD.” On the other hand, a NMD that operates using an external power source may be referred to herein as a “stationary NMD,” although such a device may in fact be moved around a home or other environment (e.g., to be connected to different power outlets of a home or other building). 
     The NMD  703  further includes a user interface  740  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  740  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  740  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. 7B  shows an example housing  730  of the NMD  703  in a first orientation with a top portion  734   a  of the housing  730  oriented upwards. The top portion  734   a  of the housing  730  includes a user interface  740   a  carried on the top portion  734   a  of the housing  730 . The user interface  740   a  includes buttons  736   a - 736   c  for controlling audio playback, volume level, and other functions. The user interface  740   a  also includes a button  736   d  for toggling the microphones  722   a  to either an on state or an off state. 
     As further shown in  FIG. 7B , apertures are formed in the top portion  734   a  of the housing  730  through which one or more first microphones  722   a  receive sound in the environment of the NMD  703 . The microphones  722   a  may be arranged in various positions along and/or within the top portion  734   a  or other areas of the housing  730  so as to detect sound from one or more directions relative to the NMD  703 . 
       FIG. 7C  shows the example housing  730  of the NMD  703  in a second orientation with a bottom portion  734   b  of the housing  730  oriented upwards. Similar to the top portion  734   a , the bottom portion  734   b  of the housing  730  includes a user interface  740   b  carried on the bottom portion  734   a  of the housing  730 . The user interface  740   b  includes buttons  736   a ′- 736   c ′ for controlling audio playback, volume level, and other functions. The user interface  740   b  also includes a button  736   d ′ for toggling the microphones  722   b  to either an on state or an off state. 
     Similar to the top portion  734   a , apertures are formed in the bottom portion  734   b  of the housing  730  through which one or more second microphones  722   b  receive sound in the environment of the NMD  703 . The microphones  722   b  may be arranged in various positions along and/or within the bottom portion  734   b  or other areas of the housing  730  so as to detect sound from one or more directions relative to the NMD  703 . 
       FIG. 7D  illustrates the NMD  703  being re-oriented from the first orientation to the second orientation by flipping over the housing  730 . In operation, the orientation sensor(s)  723  detect that the housing  730  is in the first orientation or the second orientation. When the orientation sensor(s)  723  detect that the housing  730  is in the first orientation, the NMD  703  enables a first mode associated with local processing of voice inputs detected via the microphone(s)  722   a . Conversely, when the orientation sensor(s)  723  detect that the housing  730  is in the second orientation, the NMD  703  enables a second mode associated with cloud processing of voice inputs detected via the microphone(s)  722   b.    
     More particularly, in the first mode, voice input processing via cloud-based voice assistant services is disabled. Instead, voice inputs are processed locally on via a local natural language unit. Since voice inputs are not sent to any cloud-based VAS in the first mode, operation in the first mode may enhance user privacy. 
     In contrast, in the second mode, voice input processing via cloud-based voice assistant services is enabled. In this mode, voice inputs directed to a cloud-based VAS (e.g., via a VAS wake word) are send to the cloud-based VAS for processing. This second mode allows the user to take advantage of the relatively-greater capabilities of cloud-based voice assistant services relative to processing via a local NLU. At the same time, in some implementations, the local NLU remains enabled in the second mode, which allows users to direct certain voice inputs for local processing (e.g., via a local wake word). 
     In various examples, the top portion  734   a  and bottom portion of the  734   b  may be implemented using different colors, patterns, textures, or other visual differences. Visual differences between the top portion  734   a  and bottom portion of the  734   b  of the housing  730  may assist a user in determining whether the NMD  703  is operating in the first mode (with the top portion  734   a  facing upwards) or operating in the second mode (with the bottom portion  734   b  facing upwards), especially from across a room. 
     Within example implementations, enabling the first mode or the second mode may involve enabling or disabling the microphones  722 . In particular, while the NMD  730  is in the first orientation, the microphones  722   a  are enabled and the microphones  722   b  are disabled. Conversely, while the NMD  730  is operating in the second mode, the microphones  722   a  are disabled and the microphones  722   b  are disabled. This may prevent the microphones  722  on the bottom of the housing  730  (i.e., either the microphones  722   a  or  722   b , depending on the orientation) from receiving muffled or otherwise distorted audio. 
     Further, in example implementations, the NMD may include a control to toggle between the first mode and the second mode. For instance, the housing  730  of the NMD  703  may include a physical switch or other hardware control to toggle between the first mode and the second mode. Alternatively, a control on a graphical user interface on a control device (e.g., the controller interfaces  540  of the control device  104 ) or voice inputs to a voice user interface may be used to toggle between the first mode and the second mode. Such a control may be implemented in addition to a toggle based on device orientation or as an alternative to the toggle based on device orientation. 
       FIG. 7E  is a functional block diagram showing aspects of an NMD  703  configured in accordance with embodiments of the disclosure. As described in more detail below, the NMD  703  is configured to handle certain voice inputs locally while in a first mode (and possibly also in the second mode), without necessarily transmitting data representing the voice input to a VAS. The NMD  703  is also configured to process other voice inputs using a voice assistant service while the NMD  703  is in a second mode. 
     Referring to the  FIG. 7E , the NMD  703  includes voice capture components (“VCC”)  760 , a VAS wake-word engine  770   a , and a voice extractor  773 . The VAS wake-word engine  770   a  and the voice extractor  773  are operably coupled to the VCC  760 . The NMD  703   a  further a local voice input engine  771   a  operably coupled to the VCC  760 . 
     The NMD  703  further includes microphones  722   a  and  722   b  (referred to collectively as the microphones  722 ). The microphones  722  of the NMD  703   a  are configured to provide detected sound, S D , from the environment of the NMD  703  to the VCC  760 . 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 signals associated with respective channels  762   a  or  762   b  (referred to collectively as channels  762 ) that are fed to the VCC  760 . 
     Each channel  762  may correspond to a particular microphone  722 . 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. 7E , the VCC  760  includes an AEC  763 , a spatial processor  764 , and one or more buffers  768 . In operation, the AEC  763  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  764 . 
     The spatial processor  764  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  764  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  762  of the detected sound S D , as discussed above. As one possibility, the spatial processor  764  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  764  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  768 —one or more of which may be part of or separate from the memory  713  ( FIG. 7A )—capture data corresponding to the detected sound S D . More specifically, the one or more buffers  768  capture detected-sound data that was processed by the upstream AEC  764  and spatial processor  766 . 
     The network interface  724  may then provide this information to a remote server that may be associated with the MPS  100 . In one aspect, the information stored in the additional buffer  769  does not reveal the content of any speech but instead is indicative of certain unique features of the detected sound itself. In a related aspect, the information may be communicated between computing devices, such as the various computing devices of the MPS  100 , without necessarily implicating privacy concerns. In practice, the MPS  100  can use this information to adapt and fine-tune voice processing algorithms, including sensitivity tuning as discussed below. In some implementations the additional buffer may comprise or include functionality similar to lookback buffers disclosed, for example, in 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. 16/141,875, filed Sep. 25, 2018, titled “Voice Detection Optimization Based on Selected Voice Assistant Service”; and U.S. patent application Ser. No. 16/138,111, filed Sep. 21, 2018, titled “Voice Detection Optimization Using Sound Metadata,” which are incorporated herein by reference in their entireties. 
     In any event, the detected-sound data forms a digital representation (i.e., sound-data stream), S DS , of the sound detected by the microphones  720 . 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  768  for further processing by downstream components, such as the VAS wake-word engines  770  and the voice extractor  773  of the NMD  703 . 
     In some implementations, at least one buffer  768  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 in the at least one buffer  768  while older detected-sound data is overwritten when it falls outside of the window. For example, at least one buffer  768  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, downstream components of the NMD  703  may process the sound-data stream S DS . For instance, the VAS wake-word engines  770  are 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 . This process may be referred to as automatic speech recognition. The VAS wake-word engine  770   a  and command keyword engine  771   a  apply different identification algorithms corresponding to their respective wake words, and further generate different events based on detecting a wake word in the detected-sound S D . 
     Example wake word detection algorithms accept audio as input and provide an indication of whether a wake word is present in the audio. Many first- and third-party wake word detection algorithms are known and commercially available. For instance, operators of a voice service may make their algorithm available for use in third-party devices. Alternatively, an algorithm may be trained to detect certain wake-words. 
     For instance, when the VAS wake-word engine  770   a  detects a potential VAS wake word, the VAS work-word engine  770   a  provides an indication of a “VAS wake-word event” (also referred to as a “VAS wake-word trigger”). In the illustrated example of  FIG. 7A , the VAS wake-word engine  770   a  outputs a signal S VW  that indicates the occurrence of a VAS wake-word event to the voice extractor  773 . 
     In multi-VAS implementations, the NMD  703  may include a VAS selector  774  (shown in dashed lines) that is generally configured to direct extraction by the voice extractor  773  and transmission of the sound-data stream S DS  to the appropriate VAS when a given wake-word is identified by a particular wake-word engine (and a corresponding wake-word trigger), such as the VAS wake-word engine  770   a  and at least one additional VAS wake-word engine  770   b  (shown in dashed lines). In such implementations, the NMD  703  may include multiple, different VAS wake-word engines and/or voice extractors, each supported by a respective VAS. 
     Similar to the discussion above, each VAS wake-word engine  770  may be configured to receive as input the sound-data stream S DS  from the one or more buffers  768  and apply identification algorithms to cause a wake-word trigger for the appropriate VAS. Thus, as one example, the VAS wake-word engine  770   a  may be configured to identify the wake word “Alexa” and cause the NMD  703   a  to invoke the AMAZON VAS when “Alexa” is spotted. As another example, the wake-word engine  770   b  may be configured to identify the wake word “Ok, Google” and cause the NMD  520  to invoke the GOOGLE VAS when “Ok, Google” is spotted. In single-VAS implementations, the VAS selector  774  may be omitted. 
     In response to the VAS wake-word event (e.g., in response to the signal S VW  indicating the wake-word event), the voice extractor  773  is configured to receive and format (e.g., packetize) the sound-data stream S DS . For instance, the voice extractor  773  packetizes the frames of the sound-data stream S DS  into messages. The voice extractor  773  transmits or streams these messages, M V , that may contain voice input in real time or near real time to a remote VAS via the network interface  724 . 
     As noted above, in the first mode, voice input processing via cloud-based voice assistant services is disabled. In some examples, to disable the voice input processing via cloud-based voice assistant services, the NMD  703  physically or logically disables the VAS wake-word engine(s)  770 . For instance, the NMD  703  may physically or logically prevent the sound-data stream S DS  from the microphones  722   a  from reaching the VAS wake-word engine(s)  770  and/or voice extractor  773 . Alternatively, suppressing generation may involve the NMD  703  ceasing to feed the sound-data stream S DS  to the ASR  772 . Suppressing generation may involve gating, blocking or otherwise preventing output from the VAS wake-word engine(s)  770  from generating a local keyword event. 
     In the second mode, voice input processing via a cloud-based voice assistant service is enabled. The VAS is configured to process the sound-data stream S DS  contained in the messages M V  sent from the NMD  703 . More specifically, in the first mode, the NMD  703  is configured to identify a voice input  780   a  captured by the microphones  722   a  based on the sound-data stream S DS1 . In a second mode, the NMD  703  is configured to identify a voice input  780   a  captured by the microphones  772   b  based on the sound-data stream S DS2 . The voice inputs  780   a  and  780   b  are referred to collectively as a voice input  780  and the sound-data streams S DS1  and S DS2  are referred to collectively as a S DS . 
     As described in connection with  FIG. 2C , the voice input  780  may include a keyword portion and an utterance portion. The keyword portion may correspond to detected sound that causes a VAS wake-word event (i.e., a VAS wake word). Alternatively, the keyword portion may correspond to a local wake word or a command keyword, which may generate a local wake-word event. 
     For instance, when the voice input  780   b  includes a VAS wake word, the keyword portion corresponds to detected sound that causes the wake-word engine  770   a  to output the wake-word event signal S VW  to the voice extractor  773 . The utterance portion in this case corresponds to detected sound that potentially comprises a user request following the keyword portion. 
     When a VAS wake-word event occurs, the VAS may first process the keyword portion within the sound-data stream S DS  to verify the presence of a VAS wake word. In some instances, the VAS may determine that the keyword portion comprises a false wake word (e.g., the word “Election” when the word “Alexa” is the target VAS wake word). In such an occurrence, the VAS may send a response to the NMD  703  with an instruction for the NMD  703  to cease extraction of sound data, which causes the voice extractor  773  to cease further streaming of the detected-sound data to the VAS. The VAS wake-word engine  770   a  may resume or continue monitoring sound specimens until it spots another potential VAS wake word, leading to another VAS wake-word event. In some implementations, the VAS does not process or receive the keyword portion but instead processes only the utterance portion. 
     In any case, the VAS processes the utterance portion 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 one or more commands, as well as certain keywords. The keyword may be, for example, a word in the voice input identifying a particular device or group in the MPS  100 . For instance, in the illustrated example, the keyword 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. 1A ). 
     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 may include additional information such as detected pauses (e.g., periods of non-speech) between words spoken by a user, as shown in  FIG. 2C . The pauses may demarcate the locations of separate commands, keywords, or other information spoke by the user within the utterance portion. 
     After processing the voice input, the VAS may send a response to the MPS  100  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 MPS  100  to initiate playback on one or more of the playback devices  102 , control one or more of these playback devices  102  (e.g., raise/lower volume, group/ungroup devices, etc.), or turn on/off certain smart devices, among other actions. After receiving the response from the VAS, the wake-word engine  770   a  of the NMD  703  may resume or continue to monitor the sound-data stream S DS1  until it spots another potential wake-word, as discussed above. 
     In general, the one or more identification algorithms that a particular VAS wake-word engine, such as the VAS wake-word engine  770   a , applies are configured to analyze certain characteristics of the detected sound stream S DS  and compare those characteristics to corresponding characteristics of the particular VAS wake-word engine&#39;s one or more particular VAS wake words. For example, the wake-word engine  770   a  may apply one or more identification algorithms to spot spectral characteristics in the detected sound stream S DS  that match the spectral characteristics of the engine&#39;s one or more wake words, and thereby determine that the detected sound S D  comprises a voice input including a particular VAS wake word. 
     In some implementations, the one or more identification algorithms may be third-party identification algorithms (i.e., developed by a company other than the company that provides the NMD  703   a ). For instance, operators of a voice service (e.g., AMAZON) may make their respective algorithms (e.g., identification algorithms corresponding to AMAZON&#39;s ALEXA) available for use in third-party devices (e.g., the NMDs  103 ), which are then trained to identify one or more wake words for the particular voice assistant service. Additionally, or alternatively, the one or more identification algorithms may be first-party identification algorithms that are developed and trained to identify certain wake words that are not necessarily particular to a given voice service. Other possibilities also exist. 
     As noted above, the NMD  703   a  also includes a local voice input engine  771   a  in parallel with the VAS wake-word engine  770   a . Like the VAS wake-word engine  770   a , the local voice input keyword engine  771   a  may apply one or more identification algorithms corresponding to one or more wake words. A “local keyword event” is generated when a particular local keyword is identified in the detected-sound S D . Local keywords may take the form of a nonce wake word corresponding to local processing (e.g., “Hey Sonos”), which is different from the VAS wake words corresponding to respective voice assistant services. Local keywords may also take the form of command keywords. 
     In contrast to the nonce words typically as utilized as VAS wake words, command keywords function as both the activation word and the command itself. For instance, example command keywords may correspond to playback commands (e.g., “play,” “pause,” “skip,” etc.) as well as control commands (“turn on”), among other examples. Under appropriate conditions, based on detecting one of these command keywords, the NMD  703   a  performs the corresponding command. 
     The local voice input engine  771   a  can employ an automatic speech recognizer  772 . The ASR  772  is configured to output phonetic or phenomic representations, such as text corresponding to words, based on sound in the sound-data stream S DS  to text. For instance, the ASR  772  may transcribe spoken words represented in the sound-data stream S DS  to one or more strings representing the voice input  780  as text. The local voice input engine  771   a  can feed ASR output (labeled as S ASR ) to a local natural language unit (NLU)  779  that identifies particular keywords as being local keywords for invoking local-keyword events, as described below. 
     As noted above, in some example implementations, the NMD  703  is configured to perform natural language processing, which may be carried out using an onboard natural language processor, referred to herein as a natural language unit (NLU)  779 . The local NLU  779  is configured to analyze text output of the ASR  772  of the local voice input keyword engine  771   a  to spot (i.e., detect or identify) keywords in the voice input  780 . In  FIG. 7A , this output is illustrated as the signal S ASR . The local NLU  779  includes a library of keywords (i.e., words and phrases) corresponding to respective commands and/or parameters. 
     In one aspect, the library of the local NLU  779  includes local keywords, which, as noted above, may take the form of nonce keywords or command keywords. When the local NLU  779  identifies a local keyword in the signal S ASR , the local voice input engine  771   a  generates a local keyword event. If the identified local keyword is a command keyword, the NMD  703  performs a command corresponding to the command keyword in the signal S ASR , assuming that one or more conditions corresponding to that command keyword are satisfied. If the identified local keyword is a nonce keyword, the local NLU  779  attempts to identify a keyword or keywords corresponding to a command in the signal S ASR . 
     Further, the library of the local NLU  779  may also include keywords corresponding to parameters. The local NLU  779  may then determine an underlying intent from the matched keywords in the voice input  780 . For instance, if the local NLU matches the keywords “David Bowie” and “kitchen” in combination with a play command, the local NLU  779  may determine an intent of playing David Bowie in the Kitchen  101   h  on the playback device  102   i . In contrast to a processing of the voice input  780  by a cloud-based VAS, local processing of the voice input  780  by the local NLU  779  may be relatively less sophisticated, as the NLU  779  does not have access to the relatively greater processing capabilities and larger voice databases that a VAS generally has access to. 
     In some examples, the local NLU  779  may determine an intent with one or more slots, which correspond to respective keywords. For instance, referring back to the play David Bowie in the Kitchen example, when processing the voice input, the local NLU  779  may determine that an intent is to play music (e.g., intent=playMusic), while a first slot includes David Bowie as target content (e.g., slot1=DavidBowie) and a second slot includes the Kitchen  101   h  as the target playback device (e.g., slot2=kitchen). Here, the intent (to “playMusic”) is based on the command keyword and the slots are parameters modifying the intent to a particular target content and playback device. 
     Within examples, the local voice input engine  771   a  outputs a signal, S LW , that indicates the occurrence of a local keyword event to the local NLU  779 . In response to the local keyword event (e.g., in response to the signal S LW  indicating the command keyword event), the local NLU  779  is configured to receive and process the signal S ASR . In particular, the local NLU  779  looks at the words within the signal S ASR  to find keywords that match keywords in the library of the local NLU  779 . 
     Some error in performing local automatic speech recognition is expected. Within examples, the ASR  772  may generate a confidence score when transcribing spoken words to text, which indicates how closely the spoken words in the voice input  780  matches the sound patterns for that word. In some implementations, generating a local keyword event is based on the confidence score for a given local keyword. For instance, the local voice input engine  771   a  may generate a command keyword event when the confidence score for a given sound exceeds a given threshold value (e.g., 0.5 on a scale of 0-1, indicating that the given sound is more likely than not the local keyword). Conversely, when the confidence score for a given sound is at or below the given threshold value, the command keyword engine  771   a  does not generate the local keyword event. 
     Similarly, some error in performing keyword matching is expected. Within examples, the local NLU may generate a confidence score when determining an intent, which indicates how closely the transcribed words in the signal S ASR  match the corresponding keywords in the library of the local NLU. In some implementations, performing an operation according to a determined intent is based on the confidence score for keywords matched in the signal S ASR . For instance, the NMD  703  may perform an operation according to a determined intent when the confidence score for a given sound exceeds a given threshold value (e.g., 0.5 on a scale of 0-1, indicating that the given sound is more likely than not the command keyword). Conversely, when the confidence score for a given intent is at or below the given threshold value, the NMD  703  does not perform the operation according to the determined intent. 
     As noted above, in some implementations, a phrase may be used as a local keyword, which provides additional syllables to match (or not match). For instance, the phrase “Hey, Sonos” has more syllables than “Sonos,” which provides additional sound patterns to match to words. As another example, the phrase “play me some music” has more syllables than “play,” which provides additional sound patterns to match to words. Accordingly, local keywords that are phrases may generally be less prone to false wake words. 
     In example implementations, the NMD  703  generates a local keyword event based on a local keyword taking the form of a command keyword (and performs a command corresponding to the detected command keyword) only when certain conditions corresponding to a detected command keyword are met. These conditions are intended to lower the prevalence of false positive command keyword events. For instance, after detecting the command keyword “skip,” the NMD  703   a  generates a command keyword event (and skips to the next track) only when certain playback conditions indicating that a skip should be performed are met. These playback conditions may include, for example, (i) a first condition that a media item is being played back, (ii) a second condition that a queue is active, and (iii) a third condition that the queue includes a media item subsequent to the media item being played back. If any of these conditions are not satisfied, the command keyword event is not generated (and no skip is performed). 
     The NMD  703  may include one or more state machine(s)  775  to facilitate determining whether the appropriate conditions are met. An example state machine  775   a  transitions between a first state and a second state based on whether one or more conditions corresponding to the detected command keyword are met. In particular, for a given command keyword corresponding to a particular command requiring one or more particular conditions, the state machine  775   a  transitions into a first state when one or more particular conditions are satisfied and transitions into a second state when at least one condition of the one or more particular conditions is not satisfied. 
     Within example implementations, the command conditions are based on states indicated in state variables. As noted above, the devices of the MPS  100  may store state variables describing the state of the respective device. For instance, the playback devices  102  may store state variables indicating the state of the playback devices  102 , such as the audio content currently playing (or paused), the volume levels, network connection status, and the like). These state variables are updated (e.g., periodically, or based on an event (i.e., when a state in a state variable changes)) and the state variables further can be shared among the devices of the MPS  100 , including the NMD  703 . 
     Similarly, the NMD  703  may maintain these state variables (either by virtue of being implemented in a playback device or as a stand-alone NMD). The state machine(s)  775  monitor the states indicated in these state variables, and determines whether the states indicated in the appropriate state variables indicate that the command condition(s) are satisfied. Based on these determinations, the state machines  775  transition between the first state and the second state, as described above. 
     In some implementations, the local voice input engine  771  is disabled unless certain conditions have been met via the state machines  775 . For example, the first state and the second state of the state machine  775   a  may operate as enable/disable toggles to the local voice input engine  771   a . In particular, while a state machine  775   a  corresponding to a particular command keyword is in the first state, the state machine  775   a  enables the local voice input engine  771   a  of the particular command keyword. Conversely, while the state machine  775   a  corresponding to the particular command keyword is in the second state, the state machine  775   a  disables the local voice input engine  771   a  of the particular command keyword. Accordingly, the disabled local voice input engine  771   a  ceases analyzing the sound-data stream S DS . 
     In such cases when at least one command condition is not satisfied, the NMD  703  may suppress generation of local keyword events when the local voice input engine  771   a  detects a local keyword. Suppressing generation may involve gating, blocking or otherwise preventing output from the local voice input engine  771   a  from generating a local keyword event. Alternatively, suppressing generation may involve the NMD  703  ceasing to feed the sound-data stream S DS  to the ASR  772 . Such suppression prevents a command corresponding to the detected local keyword from being performed when at least one command condition is not satisfied. In such embodiments, the local voice input engine  771   a  may continue analyzing the sound-data stream S DS  while the state machine  775   a  is in the first state, but command keyword events are disabled. 
     Other example conditions may be based on the output of a voice activity detector (“VAD”)  765 . The VAD  765  is configured to detect the presence (or lack thereof) of voice activity in the sound-data stream S DS . In particular, the VAD  765  may analyze frames corresponding to the pre-roll portion of the voice input  780  ( FIG. 2D ) with one or more voice detection algorithms to determine whether voice activity was present in the environment in certain time windows prior to a keyword portion of the voice input  780 . 
     The VAD  765  may utilize any suitable voice activity detection algorithms. Example voice detection algorithms involve determining whether a given frame includes one or more features or qualities that correspond to voice activity, and further determining whether those features or qualities diverge from noise to a given extent (e.g., if a value exceeds a threshold for a given frame). Some example voice detection algorithms involve filtering or otherwise reducing noise in the frames prior to identifying the features or qualities. 
     In some examples, the VAD  765  may determine whether voice activity is present in the environment based on one or more metrics. For example, the VAD  765  can be configured distinguish between frames that include voice activity and frames that don&#39;t include voice activity. The frames that the VAD determines have voice activity may be caused by speech regardless of whether it near- or far-field. In this example and others, the VAD  765  may determine a count of frames in the pre-roll portion of the voice input  780  that indicate voice activity. If this count exceeds a threshold percentage or number of frames, the VAD  765  may be configured to output a signal or set a state variable indicating that voice activity is present in the environment. Other metrics may be used as well in addition to, or as an alternative to, such a count. 
     The presence of voice activity in an environment may indicate that a voice input is being directed to the NMD  703 . Accordingly, when the VAD  765  indicates that voice activity is not present in the environment (perhaps as indicated by a state variable set by the VAD  765 ) this may be configured as one of the command conditions for the local keywords. When this condition is met (i.e., the VAD  765  indicates that voice activity is present in the environment), the state machine  775   a  will transition to the first state to enable performing commands based on local keywords, so long as any other conditions for a particular local keyword are satisfied. 
     Further, in some implementations, the NMD  703  may include a noise classifier  766 . The noise classifier  766  is configured to determine sound metadata (frequency response, signal levels, etc.) and identify signatures in the sound metadata corresponding to various noise sources. The noise classifier  766  may include a neural network or other mathematical model configured to identify different types of noise in detected sound data or metadata. One classification of noise may be speech (e.g., far-field speech). Another classification, may be a specific type of speech, such as background speech, and example of which is described in greater detail with reference to  FIG. 8 . Background speech may be differentiated from other types of voice-like activity, such as more general voice activity (e.g., cadence, pauses, or other characteristics) of voice-like activity detected by the VAD  765 . 
     For example, analyzing the sound metadata can include comparing one or more features of the sound metadata with known noise reference values or a sample population data with known noise. For example, any features of the sound metadata such as signal levels, frequency response spectra, etc. can be compared with noise reference values or values collected and averaged over a sample population. In some examples, analyzing the sound metadata includes projecting the frequency response spectrum onto an eigenspace corresponding to aggregated frequency response spectra from a population of NMDs. Further, projecting the frequency response spectrum onto an eigenspace can be performed as a pre-processing step to facilitate downstream classification. 
     In various embodiments, any number of different techniques for classification of noise using the sound metadata can be used, for example machine learning using decision trees, or Bayesian classifiers, neural networks, or any other classification techniques. Alternatively or additionally, various clustering techniques may be used, for example K-Means clustering, mean-shift clustering, expectation-maximization clustering, or any other suitable clustering technique. Techniques to classify noise may include one or more techniques disclosed in U.S. application Ser. No. 16/227,308 filed Dec. 20, 2018, and titled “Optimization of Network Microphone Devices Using Noise Classification,” which is herein incorporated by reference in its entirety. 
     In some implementations, the additional buffer  769  (shown in dashed lines) may store information (e.g., metadata or the like) regarding the detected sound S D  that was processed by the upstream AEC  763  and spatial processor  764 . This additional buffer  769  may be referred to as a “sound metadata buffer.” Examples of such sound metadata include: (1) frequency response data, (2) echo return loss enhancement measures, (3) voice direction measures; (4) arbitration statistics; and/or (5) speech spectral data. In example implementations, the noise classifier  766  may analyze the sound metadata in the buffer  769  to classify noise in the detected sound S D . 
     As noted above, one classification of sound may be background speech, such as speech indicative of far-field speech and/or speech indicative of a conversation not involving the NMD  703 . The noise classifier  766  may output a signal and/or set a state variable indicating that background speech is present in the environment. The presence of voice activity (i.e., speech) in the pre-roll portion of the voice input  780  indicates that the voice input  780  might not be directed to the NMD  703 , but instead be conversational speech within the environment. For instance, a household member might speak something like “our kids should have a play date soon” without intending to direct the command keyword “play” to the NMD  703 . 
     Further, when the noise classifier indicates that background speech is present is present in the environment, this condition may disable the voice input engine  771   a . In some implementations, the condition of background speech being absent in the environment (perhaps as indicated by a state variable set by the noise classifier  766 ) is configured as one of the command conditions for the command keywords. Accordingly, the state machine  775   a  will not transition to the first state when the noise classifier  766  indicates that background speech is present in the environment. 
     Further, the noise classifier  766  may determine whether background speech is present in the environment based on one or more metrics. For example, the noise classifier  766  may determine a count of frames in the pre-roll portion of the voice input  780  that indicate background speech. If this count exceeds a threshold percentage or number of frames, the noise classifier  766  may be configured to output the signal or set the state variable indicating that background speech is present in the environment. Other metrics may be used as well in addition to, or as an alternative to, such a count. 
     Within example implementations, the NMD  703   a  may support a plurality of local keywords. To facilitate such support, the local voice input engine  771   a  may implement multiple identification algorithms corresponding to respective local keywords. Alternatively, the NMD  703   a  may implement additional local voice input engines  771   b  configured to identify respective local keywords. Yet further, the library of the local NLU  779  may include a plurality of local keywords and be configured to search for text patterns corresponding to these command keywords in the signal S ASR . 
     Further, local keywords may require different conditions. For instance, the conditions for “skip” may be different than the conditions for “play” as “skip” may require that the condition that a media item is being played back and play may require the opposite condition that a media item is not being played back. To facilitate these respective conditions, the NMD  703   a  may implement respective state machines  775  corresponding to each local keyword. Alternatively, the NMD  703  may implement a state machine  775  having respective states for each command keyword. Other examples are possible as well. 
     Further techniques related to conditioning of local keyword events and VAS wake word events are described in in U.S. application Ser. No. 16/439,009 filed Jun. 12, 2019, and titled “Network Microphone Device With Command Keyword Conditioning,” which is herein incorporated by reference in its entirety. 
     Referring still to  FIG. 7E , in example embodiments, the VAS wake-word engine  770   a  and the local voice input engine  771   a  may take a variety of forms. For example, the VAS wake-word engine  770   a  and the local voice input engine  771   a  may take the form of one or more modules that are stored in memory of the NMD  703  (e.g., the memory  713  of  FIG. 7A ). As another example, the VAS wake-word engine  770   a  and the local voice input engine  771   a  may take the form of a general-purposes or special-purpose processor, or modules thereof. In this respect, multiple engines  770  and  771  may be part of the same component of the NMD  703  or each engine  770  and  771  may take the form of a component that is dedicated for the particular wake-word engine. Other possibilities also exist. 
     In some implementations, in the second mode, voice input processing via a cloud-based VAS and local voice input processing are concurrently enabled. A user may speak a local keyword to invoke local processing of a voice input  780   b  via the local voice input engine  771   a . Notably, even in the second mode, the NMD  703  may forego sending any data representing the detected sound S D  (e.g., the messages M V ) to a VAS when processing a voice input  780   b  including a local keyword. Rather, the voice input  780   b  is processed locally using the local voice input engine  771   a . Accordingly, speaking a voice input  780   b  (with a local keyword) to the NMD  703  may provide increased privacy relative to other NMDs that process all voice inputs using a VAS. 
     As indicated above, some keywords in the library of the local NLU  779  correspond to parameters. These parameters may define to perform the command corresponding to a detected command keyword. When keywords are recognized in the voice input  780 , the command corresponding to the detected command keyword is performed according to parameters corresponding to the detected keywords. 
     For instance, an example voice input  780  may be “play music at low volume” with “play” being the command keyword portion (corresponding to a playback command) and “music at low volume” being the voice utterance portion. When analyzing this voice input  780 , the NLU  779  may recognize that “low volume” is a keyword in its library corresponding to a parameter representing a certain (low) volume level. Accordingly, the NLU  779  may determine an intent to play at this lower volume level. Then, when performing the playback command corresponding to “play,” this command is performed according to the parameter representing a certain volume level. 
     In a second example, another example voice input  780  may be “play my favorites in the Kitchen” with “play” again being the command keyword portion (corresponding to a playback command) and “my favorites in the Kitchen” as the voice utterance portion. When analyzing this voice input  780 , the NLU  779  may recognize that “favorites” and “Kitchen” match keywords in its library. In particular, “favorites” corresponds to a first parameter representing particular audio content (i.e., a particular playlist that includes a user&#39;s favorite audio tracks) while “Kitchen” corresponds to a second parameter representing a target for the playback command (i.e., the kitchen  101   h  zone. Accordingly, the NLU  779  may determine an intent to play this particular playlist in the kitchen  101   h  zone. 
     In a third example, a further example voice input  780  may be “volume up” with “volume” being the command keyword portion (corresponding to a volume adjustment command) and “up” being the voice utterance portion. When analyzing this voice input  780 , the NLU  779  may recognize that “up” is a keyword in its library corresponding to a parameter representing a certain volume increase (e.g., a 10 point increase on a 100 point volume scale). Accordingly, the NLU  779  may determine an intent to increase volume. Then, when performing the volume adjustment command corresponding to “volume,” this command is performed according to the parameter representing the certain volume increase. 
     Other example voice inputs may relate to smart device commands. For instance, an example voice input  780  may be “turn on patio lights” with “turn on” being the command keyword portion (corresponding to a power on command) and “patio lights” being the voice utterance portion. When analyzing this voice input  780 , the NLU  779  may recognize that “patio” is a keyword in its library corresponding to a first parameter representing a target for the smart device command (i.e., the patio  101   i  zone) and “lights” is a keyword in its library corresponding to a second parameter representing certain class of smart device (i.e., smart illumination devices, or “smart lights”) in the patio  101   i  zone. Accordingly, the NLU  779  may determine an intent to turn on smart lights associated with the patio  101   i  zone. As another example, another example voice input  780  may be “set temperature to 75” with “set temperature” being the command keyword portion (corresponding to a thermostat adjustment command) and “to 75” being the voice utterance portion. When analyzing this voice input  780 , the NLU  779  may recognize that “to 75” is a keyword in its library corresponding to a parameter representing a setting for the thermostat adjustment command. Accordingly, the NLU  779  may determine an intent to set a smart thermostat to 75 degrees. 
     Within examples, certain command keywords are functionally linked to a subset of the keywords within the library of the local NLU  779 , which may hasten analysis. For instance, the command keyword “skip” may be functionality linked to the keywords “forward” and “backward” and their cognates. Accordingly, when the command keyword “skip” is detected in a given voice input  780 , analyzing the voice utterance portion of that voice input  780  with the local NLU  779  may involve determining whether the voice input  780  includes any keywords that match these functionally linked keywords (rather than determining whether the voice input  780  includes any keywords that match any keyword in the library of the local NLU  779 ). Since vastly fewer keywords are checked, this analysis is relatively quicker than a full search of the library. By contrast, a nonce VAS wake word such as “Alexa” provides no indication as to the scope of the accompanying voice input. 
     Some commands may require one or more parameters, as such the command keyword alone does not provide enough information to perform the corresponding command. For example, the command keyword “volume” might require a parameter to specify a volume increase or decrease, as the intent of “volume” of volume alone is unclear. As another example, the command keyword “group” may require two or more parameters identifying the target devices to group. 
     Accordingly, in some example implementations, when a given command keyword is detected in the voice input  780  by the command keyword engine  771   a , the local NLU  779  may determine whether the voice input  780  includes keywords matching keywords in the library corresponding to the required parameters. If the voice input  780  does include keywords matching the required parameters, the NMD  703   a  proceeds to perform the command (corresponding to the given command keyword) according to the parameters specified by the keywords. 
     However, if the voice input  780  does include keywords matching the required parameters for the command, the NMD  703   a  may prompt the user to provide the parameters. For instance, in a first example, the NMD  703   a  may play an audible prompt such as “I&#39;ve heard a command, but I need more information” or “Can I help you with something?” Alternatively, the NMD  703   a  may send a prompt to a user&#39;s personal device via a control application (e.g., the software components  132   c  of the control device(s)  104 ). 
     In further examples, the NMD  703   a  may play an audible prompt customized to the detected command keyword. For instance, after detect a command keyword corresponding to a volume adjustment command (e.g., “volume”), the audible prompt may include a more specific request such as “Do you want to adjust the volume up or down?” As another example, for a grouping command corresponding to the command keyword “group,” the audible prompt may be “Which devices do you want to group?” Supporting such specific audible prompts may be made practicable by supporting a relatively limited number of command keywords (e.g., less than 100), but other implementations may support more command keywords with the trade-off of requiring additional memory and processing capability. 
     Within additional examples, when a voice utterance portion does not include keywords corresponding to one or more required parameters, the NMD  703   a  may perform the corresponding command according to one or more default parameters. For instance, if a playback command does not include keywords indicating target playback devices  102  for playback, the NMD  703   a  may default to playback on the NMD  703   a  itself (e.g., if the NMD  703   a  is implemented within a playback device  102 ) or to playback on one or more associated playback devices  102  (e.g., playback devices  102  in the same room or zone as the NMD  703   a ). Further, in some examples, the user may configure default parameters using a graphical user interface (e.g., user interface  430 ) or voice user interface. For example, if a grouping command does not specify the playback devices  102  to group, the NMD  703   a  may default to instructing two or more pre-configured default playback devices  102  to form a synchrony group. Default parameters may be stored in data storage and accessed when the NMD  703   a  determines that keywords exclude certain parameters. Other examples are possible as well. 
     In some implementations, while in the second mode, the NMD  703   a  sends the voice input  780  to a VAS when the local NLU  779  is unable to process the voice input  780  (e.g., when the local NLU is unable to find matches to keywords in the library, or when the local NLU  779  has a low confidence score as to intent). In an example, to trigger sending the voice input  780 , the NMD  703   a  may generate a bridging event, which causes the voice extractor  773  to process the sound-data stream S D , as discussed above. That is, the NMD  703   a  generates a bridging event to trigger the voice extractor  773  without a VAS wake-word being detected by the VAS wake-word engine  770   a  (instead based on a command keyword in the voice input  780 , as well as the NLU  779  being unable to process the voice input  780 ). 
     Before sending the voice input  780  to the VAS (e.g., via the messages M V ), the NMD  703   a  may obtain confirmation from the user that the user acquiesces to the voice input  780  being sent to the VAS. For instance, the NMD  703   a  may play an audible prompt to send the voice input to a default or otherwise configured VAS, such as “I&#39;m sorry, I didn&#39;t understand that. May I ask Alexa?” In another example, the NMD  703   a  may play an audible prompt using a VAS voice (i.e., a voice that is known to most users as being associated with a particular VAS), such as “Can I help you with something?” In such examples, generation of the bridging event (and trigging of the voice extractor  773 ) is contingent on a second affirmative voice input  780  from the user. 
     Within certain example implementations, while in the first mode, the local NLU  779  may process the signal S ASR  without necessarily a local keyword event being generated by the command keyword engine  771   a  (i.e., directly). That is, the automatic speech recognition  772  may be configured to perform automatic speech recognition on the sound-data stream S D , which the local NLU  779  processes for matching keywords without requiring a local keyword event. If keywords in the voice input  780  are found to match keywords corresponding to a command (possibly with one or more keywords corresponding to one or more parameters), the NMD  703   a  performs the command according to the one or more parameters. 
     Further, in such examples, the local NLU  779  may process the signal S ASR  directly only when certain conditions are met. In particular, in some embodiments, the local NLU  779  processes the signal S ASR  only when the state machine  775   a  is in the first state. The certain conditions may include a condition corresponding to no background speech in the environment. An indication of whether background speech is present in the environment may come from the noise classifier  766 . As noted above, the noise classifier  766  may be configured to output a signal or set a state variable indicating that far-field speech is present in the environment. Further, another condition may correspond to voice activity in the environment. The VAD  765  may be configured to output a signal or set a state variable indicating that voice activity is present in the environment. The prevalence of false positive detection of commands with a direct processing approach may be mitigated using the conditions determined by the state machine  775   a.    
     In example implementations, the NMD  703  is paired with one or more smart devices.  FIG. 8A  illustrates an example pairing arrangement between the NMD  703  and a smart device  802 , which includes an integrated playback device and smart illumination device. By pairing the NMD  703  with the smart device(s), voice commands to control the smart device(s) may be directed to the NMD  703  to control the smart device(s) without necessarily including a keyword identifying the smart device(s) in the voice command. For instance, commands such as “play back Better Oblivion Community Center” and “turn on lights” are received by the NMD  703 , but carried out on the smart device  802  without necessarily identifying the smart device  802  by name, room, zone, or the like. On the other hand, a user may still direct inputs to other smart devices in the MPS  100  by referencing the name, room, zone, group, area, etc. that the smart device is associated with. 
     Within examples, a user may configure the pairing arrangement using a graphical user interface or voice user interface. For instance, the user may use a GUI on a application of a control device  104  to configure the pairing arrangement. Alternatively, the user may speak a voice command such as “Please pair with the Ikea® lamp” or “Please pair with the Sonos® Play: 1 ” to configure the pairing relationship. The NMD  703  may store data representing the pairing arrangement in one or more state variables, which may be referenced when identifying a device to carry out a voice command. 
     In the illustrative example of  FIG. 8A , the NMD  703  is operating in the second mode. That is, the NMD  703  is in the second orientation and has enabled the second mode. Voice processing via the cloud-based voice assistant service(s) is enabled. The NMD  703  has established a local network connection via the LAN  111  to the smart device  802 , as well as an Internet-based connection to the VAS  190  via the network  107  ( FIG. 1B ). Likewise, the smart device  802  has established a local network connection via the LAN  111  to the NMD  703 , as well as an Internet-based connection to the VAS  190  via the network  107  ( FIG. 1B ). 
     Further, in the exemplary pairing relationship of  FIG. 8A , the smart device  802  may play back audio responses to voice inputs. As noted above, the NMD  703  may, in some examples, exclude audio playback components typically present in a playback device (e.g., audio processing components  216 , amplifiers  217 , and/or speakers  218 ) or may include relatively less capable versions of these components. By pairing the NMD  703  to a playback device, the playback device may provide playback functions to complement the NMD, including playback of audio responses to voice inputs captured by the NMD  703  and playback of audio content initiated via voice command to the NMD  703 . 
     For instance, while in the second mode, the user may speak the voice input “Alexa, what is the weather,” which is captured by the microphones  722   b  ( FIG. 7C ) of the NMD  703 . The NMD  703  transmits data representing this voice input to the servers  106   a  of the VAS  190 . The servers  106   a  process this voice input and provide data representing a spoken response. In some implementations, the smart device  802  receives this data directly from the computing devices  106   a  of the VAS  190  via the networks  107  and the LAN  111 . Alternatively, the NMD  703  may receive the data from the VAS  190 , but send the data to the smart device  802 . In either case, the playback device  802  plays back the spoken response. 
     As noted above, in the second mode, voice input processing via the VAS  190  and voice input processing via the local voice input engine  771   a  may be concurrently enabled. In an example, a user may speak the voice input “Alexa, play ‘Hey Jude’ by the Beatles and turn on the Ikea lamps.” Here, “Alexa” is an example of a VAS wake word and “Ikea” is an example of a local keyword. Accordingly, such an input may generate both a VAS wake work event and a local keyword event on the NMD  703 . 
       FIG. 8B  again shows the exemplary pairing relationship between the NMD  703  and the smart device  802 . In this  FIG. 8B  example, the NMD  703  is operating in the first mode, so voice input processing via the VAS  190  is disabled. This state is represented by the broken lines between the LAN  111  to the networks  107 . While in the first mode, the NMD  703  may receive voice inputs including commands to control the smart device  802 . The NMD  703  may process such voice inputs via the local voice input engine  771   a  and transmit instructions to carry out the commands to the smart device  802  via the LAN  111 . 
     In some examples, the library of the local NLU  779  is partially customized to the individual user(s). In a first aspect, the library may be customized to the devices that are within the household of the NMD (e.g., the household within the environment  101  ( FIG. 1A )). For instance, the library of the local NLU may include keywords corresponding to the names of the devices within the household, such as the zone names of the playback devices  102  in the MPS  100 . In a second aspect, the library may be customized to the users of the devices within the household. For example, the library of the local NLU  779  may include keywords corresponding to names or other identifiers of a user&#39;s preferred playlists, artists, albums, and the like. Then, the user may refer to these names or identifiers when directing voice inputs to the command keyword engine  771   a  and the local NLU  779 . 
     Within example implementations, the NMD  703   a  may populate the library of the local NLU  779  locally within the network  111  ( FIG. 1B ). As noted above, the NMD  703   a  may maintain or have access to state variables indicating the respective states of devices connected to the network  111  (e.g., the playback devices  104 ). These state variables may include names of the various devices. For instance, the kitchen  101   h  may include the playback device  101   b , which are assigned the zone name “Kitchen.” The NMD  703   a  may read these names from the state variables and include them in the library of the local NLU  779  by training the local NLU  779  to recognize them as keywords. The keyword entry for a given name may then be associated with the corresponding device in an associated parameter (e.g., by an identifier of the device, such as a MAC address or IP address). The NMD  703   a  can then use the parameters to customize control commands and direct the commands to a particular device. 
     In further examples, the NMD  703   a  may populate the library by discovering devices connected to the network  111 . For instance, the NMD  703   a  may transmit discovery requests via the network  111  according to a protocol configured for device discovery, such as universal plug-and-play (UPnP) or zero-configuration networking. Devices on the network  111  may then respond to the discovery requests and exchange data representing the device names, identifiers, addresses and the like to facilitate communication and control via the network  111 . The NMD  703  may read these names from the exchanged messages and include them in the library of the local NLU  779  by training the local NLU  779  to recognize them as keywords. 
     In further examples, the NMD  703   a  may populate the library using the cloud. To illustrate,  FIG. 9  is a schematic diagram of the MPS  100  and a cloud network  902 . The cloud network  902  includes cloud servers  906 , identified separately as media playback system control servers  906   a , streaming audio service servers  906   b , and IOT cloud servers  906   c . The streaming audio service servers  906   b  may represent cloud servers of different streaming audio services. Similarly, the IOT cloud servers  906   c  may represent cloud servers corresponding to different cloud services supporting smart devices  990  in the MPS  100 . Smart devices  990  include smart illumination devices, smart thermostats, smart plugs, security cameras, doorbells, and the like. 
     Within examples, a user may link an account of the MPS  100  to an account of a IOT service. For instance, an IOT manufacturer (such as IKEA®) may operate a cloud-based IOT service to facilitate cloud-based control of their IOT products using smartphone app, website portal, and the like. In connection with such linking, keywords associated with the cloud-based service and the IOT devices may be populated in the library of the local NLU  779 . For instance, the library may be populated with a nonce keyword (e.g., “Hey Ikea”). Further, the library may be populated with names of various IOT devices, keyword commands for controlling the IOT devices, and keywords corresponding to parameters for the commands. 
     One or more communication links  903   a ,  903   b , and  903   c  (referred to hereinafter as “the links  903 ”) communicatively couple the MPS  100  and the cloud servers  906 . The links  903  can include one or more wired networks and one or more wireless networks (e.g., the Internet). Further, similar to the network  111  ( FIG. 1B ), a network  911  communicatively couples the links  903  and at least a portion of the devices (e.g., one or more of the playback devices  102 , NMDs  103  and  703   a , control devices  104 , and/or smart devices  990 ) of the MPS  100 . 
     In some implementations, the media playback system control servers  906   a  facilitate populating the library of local NLU  779  with the NMD(s)  703   a  (representing one or more of the NMD  703   a  ( FIG. 7A ) within the MPS  100 ). In an example, the media playback system control servers  906   a  may receive data representing a request to populate the library of a local NLU  779  from the NMD  703 . Based on this request, the media playback system control servers  906   a  may communicate with the streaming audio service servers  906   b  and/or IOT cloud servers  906   c  to obtain keywords specific to the user. 
     In some examples, the media playback system control servers  906   a  may utilize user accounts and/or user profiles in obtaining keywords specific to the user. As noted above, a user of the MPS  100  may set-up a user profile to define settings and other information within the MPS  100 . The user profile may then in turn be registered with user accounts of one or more streaming audio services to facilitate streaming audio from such services to the playback devices  102  of the MPS  100 . 
     Through use of these registered streaming audio services, the streaming audio service servers  906   b  may collect data indicating a user&#39;s saved or preferred playlists, artists, albums, tracks, and the like, either via usage history or via user input (e.g., via a user input designating a media item as saved or a favorite). This data may be stored in a database on the streaming audio service servers  906   b  to facilitate providing certain features of the streaming audio service to the user, such as custom playlists, recommendations, and similar features. Under appropriate conditions (e.g., after receiving user permission), the streaming audio service servers  906   b  may share this data with the media playback system control servers  906   a  over the links  903   b.    
     Accordingly, within examples, the media playback system control servers  906   a  may maintain or have access to data indicating a user&#39;s saved or preferred playlists, artists, albums, tracks, genres, and the like. If a user has registered their user profile with multiple streaming audio services, the saved data may include saved playlists, artists, albums, tracks, and the like from two or more streaming audio services. Further, the media playback system control servers  906   a  may develop a more complete understanding of the user&#39;s preferred playlists, artists, albums, tracks, and the like by aggregating data from the two or more streaming audio services, as compared with a streaming audio service that only has access to data generated through use of its own service. 
     Moreover, in some implementations, in addition to the data shared from the streaming audio service servers  906   b , the media playback system control servers  906   a  may collect usage data from the MPS  100  over the links  903   a , after receiving user permission. This may include data indicating a user&#39;s saved or preferred media items on a zone basis. Different types of music may be preferred in different rooms. For instance, a user may prefer upbeat music in the Kitchen  101   h  and more mellow music to assist with focus in the Office  101   e.    
     Using the data indicating a user&#39;s saved or preferred playlists, artists, albums, tracks, and the like, the media playback system control servers  906   a  may identify names of playlists, artists, albums, tracks, and the like that the user is likely to refer to when providing playback commands to the NMDs  703  via voice input. Data representing these names can then be transmitted via the links  903   a  and the network  904  to the NMDs  703   a  and then added to the library of the local NLU  779  as keywords. For instance, the media playback system control servers  906   a  may send instructions to the NMD  703  to include certain names as keywords in the library of the local NLU  779 . Alternatively, the NMD  703  (or another device of the MPS  100 ) may identify names of playlists, artists, albums, tracks, and the like that the user is likely to refer to when providing playback commands to the NMD  703  via voice input and then include these names in the library of the local NLU  779 . 
     Due to such customization, similar voice inputs may result in different operations being performed when the voice input is processed by the local NLU  779  as compared with processing by a VAS. For instance, a first voice input of “Alexa, play me my favorites in the Office” may trigger a VAS wake-word event, as it includes a VAS wake word (“Alexa”). A second voice input of “Play me my favorites in the Office” may trigger a command keyword, as it includes a command keyword (“play”). Accordingly, the first voice input is sent by the NMD  703   a  to the VAS, while the second voice input is processed by the local NLU  779 . 
     While these voice inputs are nearly identical, they may cause different operations. In particular, the VAS may, to the best of its ability, determine a first playlist of audio tracks to add to a queue of the playback device  102   f  in the office  101   e . Similarly, the local NLU  779  may recognize keywords “favorites” and “kitchen” in the second voice input. Accordingly, the NMD  703   a  performs the voice command of “play” with parameters of &lt;favorites playlist&gt; and &lt;kitchen  101   h  zone&gt;, which causes a second playlist of audio tracks to be added to the queue of the playback device  102   f  in the office  101   e . However, the second playlist of audio tracks may include a more complete and/or more accurate collection of the user&#39;s favorite audio tracks, as the second playlist of audio tracks may draw on data indicating a user&#39;s saved or preferred playlists, artists, albums, and tracks from multiple streaming audio services, and/or the usage data collected by the media playback system control servers  906   a . In contrast, the VAS may draw on its relatively limited conception of the user&#39;s saved or preferred playlists, artists, albums, and tracks when determining the first playlist. 
     To illustrate,  FIG. 11  shows a table  1100  illustrating the respective contents of a first and second playlist determined based on similar voice inputs, but processed differently. In particular, the first playlist is determined by a VAS while the second playlist is determined by the NMD  703   a  (perhaps in conjunction with the media playback system control servers  906   a ). As shown, while both playlists purport to include a user&#39;s favorites, the two playlists include audio content from dissimilar artists and genres. In particular, the second playlist is configured according to usage of the playback device  102   f  in the Office  101   e  and also the user&#39;s interactions with multiple streaming audio services, while the first playlist is based on the multiple user&#39;s interactions with the VAS. As a result, the second playlist is more attuned to the types of music that the user prefers to listen to in the office  101   e  (e.g., indie rock and folk) while the first playlist is more representative of the interactions with the VAS as a whole. 
     A household may include multiple users. Two or more users may configure their own respective user profiles with the MPS  100 . Each user profile may have its own user accounts of one or more streaming audio services associated with the respective user profile. Further, the media playback system control servers  906   a  may maintain or have access to data indicating each user&#39;s saved or preferred playlists, artists, albums, tracks, genres, and the like, which may be associated with the user profile of that user. 
     In various examples, names corresponding to user profiles may be populated in the library of the local NLU  779 . This may facilitate referring to a particular user&#39;s saved or preferred playlists, artists, albums, tracks, or genres. For instance, when a voice input of “Play Anne&#39;s favorites on the patio” is processed by the local NLU  779 , the local NLU  779  may determine that “Anne” matches a stored keyword corresponding to a particular user. Then, when performing the playback command corresponding to that voice input, the NMD  703   a  adds a playlist of that particular user&#39;s favorite audio tracks to the queue of the playback device  102   c  in the patio  101   i.    
     In some cases, a voice input might not include a keyword corresponding to a particular user, but multiple user profiles are configured with the MPS  100 . In some cases, the NMD  703   a  may determine the user profile to use in performing a command using voice recognition. Alternatively, the NMD  703   a  may default to a certain user profile. Further, the NMD  703   a  may use preferences from the multiple user profiles when performing a command corresponding to a voice input that did not identify a particular user profile. For instance, the NMD  703   a  may determine a favorites playlist including preferred or saved audio tracks from each user profile registered with the MPS  100 . 
     The IOT cloud servers  906   c  may be configured to provide supporting cloud services to the smart devices  990 . The smart devices  990  may include various “smart” internet-connected devices, such as lights, thermostats, cameras, security systems, appliances, and the like. For instance, an IOT cloud server  906   c  may provide a cloud service supporting a smart thermostat, which allows a user to control the smart thermostat over the internet via a smartphone app or website. 
     Accordingly, within examples, the IOT cloud servers  906   c  may maintain or have access to data associated with a user&#39;s smart devices  990 , such as device names, settings, and configuration. Under appropriate conditions (e.g., after receiving user permission), the IOT cloud servers  906   c  may share this data with the media playback system control servers  906   a  and/or the NMD  703   a  via the links  903   c . For instance, the IOT cloud servers  906   c  that provide the smart thermostat cloud service may provide data representing such keywords to the NMD  703   a , which facilitates populating the library of the local NLU  779  with keywords corresponding to the temperature. 
     Yet further, in some cases, the IOT cloud servers  906   c  may also provide keywords specific to control of their corresponding smart devices  990 . For instance, the IOT cloud server  906   c  that provides the cloud service supporting the smart thermostat may provide a set of keywords corresponding to voice control of a thermostat, such as “temperature,” “warmer,” or “cooler,” among other examples. Data representing such keywords may be sent to the NMDs  703   a  over the links  903  and the network  904  from the IOT cloud servers  906   c.    
     As noted above, some households may include more than NMD  703 . In example implementations, two or more NMDs  703  may synchronize or otherwise update the libraries of their respective local NLU  779 . For instance, a first NMD  703   a  and a second NMD  703   b  may share data representing the libraries of their respective local NLU  779 , possibly using a network (e.g., the network  904 ). Such sharing may facilitate the NMDs  703   a  being able to respond to voice input similarly, among other possible benefits. 
     In some embodiments, one or more of the components described above can operate in conjunction with the microphones  720  to detect and store a user&#39;s voice profile, which may be associated with a user account of the MPS  100 . In some embodiments, 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, such as those described in previously-referenced U.S. patent application Ser. No. 15/438,749. 
     In some embodiments, one or more of the components described above can operate in conjunction with the microphones  720  to determine the location of a user in the home environment and/or relative to a location of one or more of the NMDs  103 . Techniques for determining the location or proximity of a user may include one or more techniques disclosed in previously-referenced U.S. patent application Ser. No. 15/438,749, U.S. Pat. No. 9,084,058 filed Dec. 29, 2011, and titled “Sound Field Calibration Using Listener Localization,” and U.S. Pat. No. 8,965,033 filed Aug. 31, 2012, and titled “Acoustic Optimization.” Each of these applications is herein incorporated by reference in its entirety. 
       FIGS. 10A, 10B, 10C, and 10D  show exemplary input and output from the NMD  703  configured in accordance with aspects of the disclosure. 
       FIG. 10A  illustrates a first scenario in which a wake-word engine of the NMD  703  is configured to detect four local keywords (“play”, “stop”, “resume”, “turn on”). The local NLU  779  ( FIG. 7E ) is disabled. In this scenario, the user has spoken the voice input “turn on” to the NMD  703 , which triggers a new recognition of one of the local keywords (e.g., a command keyword event corresponding to turn on). 
     Yet further, the VAD  765  and noise classifier  766  ( FIG. 7E ) have analyzed 150 frames of a pre-roll portion of the voice input. As shown, the VAD  765  has detected voice in 140 frames of the  150  pre-roll frames, which indicates that a voice input may be present in the detected sound. Further, the noise classifier  766  has detected ambient noise in 11 frames, background speech in 127 frames, and fan noise in 12 frames. In this example, the noise classifier  766  is classifying the predominant noise source in each frame. This indicates the presence of background speech. As a result, the NMD has determined not to trigger on the detected local keyword “turn on.” 
       FIG. 10B  illustrates a second scenario in which the local voice input engine  771   a  of the NMD  703  is configured to detect a local keyword (“play”) as well as two cognates of that command keyword (“play something” and “play me a song”). The local NLU  779  is disabled. In this second scenario, the user has spoken the voice input “play something” to the NMD  703 , which triggers a new recognition of one of the local keywords (e.g., a command keyword event). 
     Yet further, the VAD  765  and noise classifier  766  have analyzed 150 frames of a pre-roll portion of the voice input. As shown, the VAD  765  has detected voice in 87 frames of the  150  pre-roll frames, which indicates that a voice input may be present in the detected sound. Further, the noise classifier  766  has detected ambient noise in 18 frames, background speech in 8 frames, and fan noise in 124 frames. This indicates that background speech is not present. Given the foregoing, the NMD  703  has determined to trigger on the detected local keyword “play.” 
       FIG. 10C  illustrates a third scenario in which the local voice input engine  771   a  of the NMD  703  is configured to detect three local keywords (“play”, “stop”, and “resume”). The local NLU  779  is enabled. In this third scenario, the user has spoken the voice input “play Beatles in the Kitchen” to the NMD  703 , which triggers a new recognition of one of the local keywords (e.g., a command keyword event corresponding to play). 
     As shown, the ASR  772  has transcribed the voice input as “play beet les in the kitchen.” Some error in performing ASR is expected (e.g., “beet les”). Here, the local NLU  779  has matched the keyword “beet les” to “The Beatles” in the local NLU library, which sets up this artist as a content parameter to the play command. Further, the local NLU  779  has also matched the keyword “kitchen” to “kitchen” in the local NLU library, which sets up the kitchen zone as a target parameter to the play command. The local NLU produced a confidence score of 0.63428231948273443 associated with the intent determination. 
     Here as well, the VAD  765  and noise classifier  766  have analyzed 150 frames of a pre-roll portion of the voice input. As shown, the noise classifier  766  has detected ambient noise in 142 frames, background speech in 8 frames, and fan noise in 0 frames. This indicates that background speech is not present. The VAD  765  has detected voice in 112 frames of the  150  pre-roll frames, which indicates that a voice input may be present in the detected sound. Here, the NMD  703  has determined to trigger on the detected command keyword “play.” 
       FIG. 10D  illustrates a fourth scenario in which the local voice input engine  771   a  of the NMD is not configured to spot any local keywords. Rather, the local voice input engine  771   a  will perform ASR and pass the output of the ASR to the local NLU  779 . The local NLU  779  is enabled and configured to detect keywords corresponding to both commands and parameters. In the fourth scenario, the user has spoken the voice input “play some music in the Office” to the NMD  703 . 
     As shown, the ASR  772  has transcribed the voice input as “lay some music in the office.” Here, the local NLU  779  has matched the keyword “lay” to “play” in the local NLU library, which corresponds to a playback command. Further, the local NLU  779  has also matched the keyword “office” to “office” in the local NLU library, which sets up the office  101   e  zone as a target parameter to the play command. The local NLU  779  produced a confidence score of 0.14620494842529297 associated with the keyword matching. In some examples, this low confidence score may cause the NMD to not accept the voice input (e.g., if this confidence score is below a threshold, such as 0.5). 
     IV. Example VAS Toggle Techniques 
       FIG. 11  is a flow diagram showing an example method  1100  to toggle voice input processing based on device orientation. The method  1100  may be performed by a networked microphone device, such as the NMD  703  ( FIG. 7A ). Alternatively, the method  1100  may be performed by any suitable device or by a system of devices, such as the playback devices  103 , NMDs  103 , control devices  104 , computing devices  105 , computing devices  106 , and/or NMD  703 . 
     At block  1102 , the method  1100  involves detecting that the housing is in a first orientation. For instance, one or more orientation sensors (e.g., the orientation sensor(s)  723  ( FIG. 7A )) may generate data indicative of the orientation of the NMD  703 . The NMD  703  may detect that the housing  730  is in a first orientation ( FIG. 7B ). 
     In some implementations, the NMD  703  is configured to generate events when the orientation of the NMD  703  changes. Such events may trigger mode changes in the NMD  703 . For instance, when the housing  730  is switched from a second orientation ( FIG. 7C ) to a second orientation ( FIG. 7B ), the orientation sensors  723  may generate data indicative of acceleration of the NMD  703 . The NMD  703  may determine that this data indicates that the housing  730  is in the first orientation and generate an event indicating this orientation. In some examples, the orientation state of the NMD  703  is stored in one or more state variables, which can be referenced to determine the current orientation of the NMD  703 . 
     At block  1104 , the method  1100  involves enabling a first mode. Enabling the first mode involves disabling voice input processing via a cloud-based voice assistant service and enabling voice input processing via a local natural language unit, such as the NLU  779  ( FIG. 7E ). Enabling the first mode may further involve enabling one or more first microphones (e.g., the microphones  722   a  ( FIG. 7B )) and/or disabling one or more second microphones (e.g., the microphones  722   b  ( FIG. 7C )). 
     In some examples, the NMD  703  enables the first mode after detecting that the housing is in a first orientation. As noted above, detecting that the housing is in a first orientation may involve detecting an event. For example, the NMD  703  may enable the first mode based on a particular event being generated where the particular event corresponds to a change in orientation from the second orientation to the first orientation. 
     The first mode may remain enabled while the housing is in the first orientation. In some examples, while in the first mode, the NMD  703  may directly (e.g., via orientation sensor(s)  723 ) or indirectly (e.g., via the one or more state variables) determine whether the NMD  703  is still in the first orientation. If the NMD  703  determines that the NMD  703  is no longer in the first orientation, the NMD  703  may switch modes. 
     At block  1106 , the method  1100  involves receiving a voice input. Receiving a voice input may involve capturing sound data associated with a first voice input  780   a  via the one or more first microphones  722   a  ( FIG. 7E ). Receiving the voice input may further involve detecting, via a local natural language unit  779 , that the first voice input comprises sound data matching one or more keywords from a local natural language unit library of the local natural language unit  779 . For instance, local natural language unit  779  may determine that the voice input includes one or more local keywords that generate a local keyword event, such as a nonce local keyword and/or a command keyword, as well as one or more additional keywords that correspond to parameters of the voice command. 
     At block  1108 , the method  1100  involves determining, via the local natural language unit, an intent of the first voice input based on at least one of the one or more keywords. For instance, the NLU  779  may determine that the voice input includes a particular command keyword (e.g., turn on) and one or more keywords corresponding to parameters (e.g., the lights) and determine an intent of turning on the lights on a paired smart device  802  ( FIG. 8B ). 
     At block  1110 , the method  1100  involves performing a first command according to the determined intent of the first voice input. Performing the first command may involve sending instructions to one or more network devices over a network to perform one or more operations according to the first command, similar to the message exchange illustrated in  FIG. 6 . For instance, the NMD  703  may transmit an instruction over the LAN  111  to the smart device  802  to toggle the lights or to play back audio content. 
     Within examples, the target network devices to perform the first command may be explicitly or implicitly defined. For example, the target smart devices may be explicitly defined by reference in the voice input  780  to the name(s) of one or more smart devices (e.g., by reference to a room, zone or zone group name). Alternatively, the voice input might not include any reference to the name(s) of one or more smart devices and instead may implicitly refer to smart device(s) paired with the NMD  703 . Playback devices  102  associated with the NMD  703   a  may include a playback device implementing the NMD  703   a , as illustrated by the playback device  102   d  implementing the NMD  103   d  ( FIG. 1B )) or playback devices configured to be associated (e.g., where the playback devices  102  are in the same room or area as the NMD  703   a ). 
     Further, performing the first command may involve sending instructions to one or more remote computing devices. For example, the NMD  703  may transmit requests to the computing devices  106  of the MCS  192  to stream one or more audio tracks to the smart device  902  ( FIG. 8B ). Alternatively, the instructions may be provided internally (e.g., over a local bus or other interconnection system) to one or more software or hardware components (e.g., the electronics  112  of the playback device  102 ). 
     Yet further, transmitting instructions may involve both local and cloud based operations. For instance, the NMD  703  may transmit instructions locally over the LAN  111  to the smart device  802  to add one or more audio tracks to the playback queue over the LAN  111 . Then, the smart device  802  may transmit a request to the computing devices  106  of the MCS  192  to stream one or more audio tracks to the smart device  802  for playback over the networks  107 . Other examples are possible as well. 
     At block  1112 , the method  1100  involves detecting that the housing is in a second orientation different than the first orientation. For instance, one or more orientation sensors (e.g., the orientation sensor(s)  723  ( FIG. 7A )) may generate data indicative of the orientation of the NMD  703 . The NMD  703  may detect that the housing  730  is in a first orientation ( FIG. 7B ). 
     As noted above, in some implementations, the NMD  703  is configured to generate events when the orientation of the NMD  703  changes. Such events may trigger mode changes in the NMD  703 . For instance, when the housing  730  is switched from the first orientation to the second orientation ( FIG. 7D ), the orientation sensors  723  may generate data indicative of acceleration of the NMD  703 . The NMD  703  may determine that this data indicates that the housing  730  is in the second orientation and generate an event indicating this orientation. 
     At block  1114 , the method  1100  involves enabling a second mode. Enabling the second mode involves enabling voice input processing via a cloud-based voice assistant service. In some implementations, enabling the second mode also includes disabling voice input processing via a local natural language unit, such as the NLU  779  ( FIG. 7E ). Alternatively, voice input processing via the local natural language unit may remain enabled in the second mode. Enabling the second mode may further involve enabling one or more second microphones (e.g., the microphones  722   b  ( FIG. 7C )) and/or disabling one or more first microphones (e.g., the microphones  722   a  ( FIG. 7B )). 
     In some examples, the NMD  703  enables the second mode after detecting that the housing is in a second orientation. As noted above, detecting that the housing is in a first orientation may involve detecting an event. For example, the NMD  703  may enable the second mode based on a particular event being generated where the particular event corresponds to a change in orientation from the second orientation to the first orientation. Other examples are possible as well. 
     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. 
     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. 
     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 to be performed by a network microphone device including one or more first microphones, one or more second microphones, a network interface, one or more processors, and a housing carrying the one or more first microphones, the one or more second microphones, the network interface, the one or more processors, and data storage having stored therein instructions executable by the one or more processors. The network microphone device detects that the housing is in a first orientation. After detecting that the housing is in the first orientation, the device enables a first mode. Enabling the first mode includes (i) disabling voice input processing via a cloud-based voice assistant service and (ii) enabling voice input processing via a local natural language unit. While the first mode is enabled, the network microphone device (i) captures sound data associated with a first voice input via the one or more first microphones and (ii) detects, via a local natural language unit, that the first voice input comprises sound data matching one or more keywords from a local natural language unit library of the local natural language unit. The network microphone device determines, via the local natural language unit, an intent of the first voice input based on at least one of the one or more keywords and performs a first command according to the determined intent of the first voice input. The network microphone device may detects that the housing is in a second orientation different than the first orientation. After detecting that the housing is in the second orientation, the network microphone device enables the second mode. Enabling the second mode includes enabling voice input processing via the cloud-based voice assistant service. 
     Example 2: The method of Example 1, wherein enabling the first mode further comprises disabling the one or more second microphones. 
     Example 3: The method of any of Examples 1 and 2, wherein enabling the second mode further comprises at least one of: (a) disabling the one or more first microphones or (b) disabling voice input processing via the local natural language unit. 
     Example 4: The method of any of Examples 1-3, further comprising pairing the NMD to a network device and wherein performing the first command comprises transmitting an instruction over a local area network to the network device. 
     Example 5: The method of any of Examples 4, wherein the network device comprises a smart illumination device, and wherein the first command is a command to toggle the smart illumination device on or off. 
     Example 6: The method of any of Example 4, wherein the functions further comprise pairing the NMD to a playback device separate from the network device, wherein the playback device is configured to process playback commands transmitted to the playback device from one or more remote computing devices of the cloud-based voice-assistant service. 
     Example 7: The method of any of Examples 1-6, further comprising while the second mode is enabled, (i) detecting a sound data stream associated with a second voice input; (ii) detecting a wake-word in the second sound data stream; and (iii) after detecting the wake-word, transmitting the second sound data stream to one or more remote computing devices of the cloud-based voice-assistant service. 
     Example 8: The method of any of Examples 1-8, wherein the network microphone device further comprises one or more sensors carried in the housing wherein detecting that the housing is in a second orientation different than the first orientation comprises detecting, via the one or sensors, sensor data indicating that the housing has been re-oriented from the first orientation to the second orientation. 
     Example 9: A tangible, non-transitory, computer-readable medium having instructions stored thereon that are executable by one or more processors to cause a playback device to perform the method of any one of Examples 1-8. 
     Example 10: A playback device comprising a speaker, a network interface, one or more microphones configured to detect sound, one or more processors, and a tangible, non-tangible computer-readable medium having instructions stored thereon that are executable by the one or more processors to cause the playback device to perform the method of any of Examples 1-8.