Patent Publication Number: US-11659323-B2

Title: Systems and methods of user localization

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 16/948,609, filed Sep. 24, 2020, which is a continuation of U.S. application Ser. No. 16/353,774, filed Mar. 14, 2019, now U.S. Pat. No. 10,791,396, which is a continuation of U.S. application Ser. No. 16/149,992, filed on Oct. 2, 2018, now U.S. Pat. No. 10,277,981, which are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure is related to consumer goods and, more particularly, to methods, systems, products, features, services, and other elements directed to media playback or some aspect thereof. 
     BACKGROUND 
     Options for accessing and listening to digital audio in an out-loud setting were limited until in 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, 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.  1 A  is a partial cutaway view of an environment having a media playback system configured in accordance with aspects of the disclosed technology. 
         FIG.  1 B  is a schematic diagram of the media playback system of  FIG.  1 A  and one or more networks. 
         FIG.  1 C  is a block diagram of a playback device. 
         FIG.  1 D  is a block diagram of a playback device. 
         FIG.  1 E  is a block diagram of a network microphone device. 
         FIG.  1 F  is a block diagram of a network microphone device. 
         FIG.  1 G  is a block diagram of a playback device. 
         FIG.  1 H  is a partially schematic diagram of a control device. 
         FIGS.  1   -I through  1 L are schematic diagrams of corresponding media playback system zones. 
         FIG.  1 M  is a schematic diagram of media playback system areas. 
         FIG.  2 A  is a front isometric view of a playback device configured in accordance with aspects of the disclosed technology. 
         FIG.  2 B  is a front isometric view of the playback device of  FIG.  3 A  without a grille. 
         FIG.  2 C  is an exploded view of the playback device of  FIG.  2 A . 
         FIG.  3 A  is a front view of a network microphone device configured in accordance with aspects of the disclosed technology. 
         FIG.  3 B  is a side isometric view of the network microphone device of  FIG.  3 A . 
         FIG.  3 C  is an exploded view of the network microphone device of  FIGS.  3 A and  3 B . 
         FIG.  3 D  is an enlarged view of a portion of  FIG.  3 B . 
         FIG.  3 E  is a block diagram of the network microphone device of  FIGS.  3 A- 3 D   
         FIG.  3 F  is a schematic diagram of an example voice input. 
         FIG.  4    is a plan view of a playback environment. 
         FIG.  5    is a block diagram of a playback device. 
         FIG.  6    is a flow diagram representing a method for selecting a characteristic of audio reproduction based on a determined location of a person. 
         FIG.  7    is a graph of data output by a multi-stage noise-shaping (MASH) modulator. 
         FIG.  8    is a schematic diagram depicting two microphones receiving reflections of a sound signal. 
         FIG.  9    is a flow diagram representing a method for configuring a playback device. 
         FIG.  10    is a schematic representation of a data structure for storing calibration data. 
         FIG.  11    is a plan view of a playback environment. 
     
    
    
     The drawings are for the purpose of illustrating example embodiments, but those of ordinary skill in the art will understand that the technology disclosed herein is not limited to the arrangements and/or instrumentality shown in the drawings. 
     DETAILED DESCRIPTION 
     I. Overview 
     Embodiments described herein relate to selection of audio reproduction characteristics by a playback device based a location of a person. For example, it may be beneficial to adjust audio reproduction based on the location of a listener so that the audio experience is improved for that location. However, it can be difficult for a playback system to determine a location of a person within the playback environment. 
     In some embodiments, for example, a playback device transmits a first sound signal including a predetermined waveform. The playback device receives a second sound signal including at least one reflection of the first sound signal. The second sound signal is processed to determine a location of a person relative to the playback device, and a characteristic of audio reproduction by the playback device is selected, based on the determined location of the person. 
     Thus, a person can be located using sound signals and audio reproduction adjusted accordingly to provide an improved audio experience. 
     In some embodiments, a playback device comprises a transducer configured to generate audio signals; a microphone; and a processing system. The transducer is arranged to transmit a first sound signal comprising a predetermined waveform. The microphone is arranged to receive a second sound signal comprising at least one reflection of the first ultrasound signal. The processing system is arranged to: determine a location of a person relative to the playback device based on the second sound signal; and set a characteristic of audio reproduction by the playback device based on the determined location of the person. 
     In some embodiment, a non-transitory computer readable medium comprises computer program instructions which, when executed by a processing system, instruct the processing system to: cause an electroacoustic transducer in a playback device to transmit a first sound signal comprising a predetermined waveform; cause a microphone in the playback device to receive a second sound signal comprising at least one reflection of the first sound signal; process the second sound signal to determine a location of a person relative to the playback device; and set a characteristic of audio reproduction by the playback device based on the determined location of the person. 
     While some examples described herein may refer to functions performed by given actors such as “users,” “listeners,” and/or other entities, it should be understood that this is for purposes of explanation only. The claims should not be interpreted to require action by any such example actor unless explicitly required by the language of the claims themselves. 
     In the Figures, identical reference numbers identify generally similar, and/or identical, elements. To facilitate the discussion of any particular element, the most significant digit or digits of a reference number refers to the Figure in which that element is first introduced. For example, element  110   a  is first introduced and discussed with reference to  FIG.  1 A . Many of the details, dimensions, angles and other features shown in the Figures are merely illustrative of particular embodiments of the disclosed technology. Accordingly, other embodiments can have other details, dimensions, angles and features without departing from the spirit or scope of the disclosure. In addition, those of ordinary skill in the art will appreciate that further embodiments of the various disclosed technologies can be practiced without several of the details described below. 
     II. Suitable Operating Environment 
       FIG.  1 A  is a partial cutaway view of a media playback system  100  distributed in an environment  101  (e.g., a house). The media playback system  100  comprises one or more playback devices  110  (identified individually as playback devices  110   a - n ), one or more network microphone devices (“NMDs”),  120  (identified individually as NMDs  120   a - c ), and one or more control devices  130  (identified individually as control devices  130   a  and  130   b ). 
     As used herein the term “playback device” can generally refer to a network device configured to receive, process, and output data of a media playback system. For example, a playback device can be a network device that receives and processes audio content. In some embodiments, a playback device includes one or more transducers or speakers powered by one or more amplifiers. In other embodiments, however, a playback device includes one of (or neither of) the speaker and the amplifier. For instance, a playback device can comprise one or more amplifiers configured to drive one or more speakers external to the playback device via a corresponding wire or cable. 
     Moreover, as used herein the term NMD (i.e., a “network microphone device”) can generally refer to a network device that is configured for audio detection. In some embodiments, an NMD is a stand-alone device configured primarily for audio detection. In other embodiments, an NMD is incorporated into a playback device (or vice versa). 
     The term “control device” can generally refer to a network device configured to perform functions relevant to facilitating user access, control, and/or configuration of the media playback system  100 . 
     Each of the playback devices  110  is configured to receive audio signals or data from one or more media sources (e.g., one or more remote servers, one or more local devices) and play back the received audio signals or data as sound. The one or more NMDs  120  are configured to receive spoken word commands, and the one or more control devices  130  are configured to receive user input. In response to the received spoken word commands and/or user input, the media playback system  100  can play back audio via one or more of the playback devices  110 . In certain embodiments, the playback devices  110  are configured to commence playback of media content in response to a trigger. For instance, one or more of the playback devices  110  can be configured to play back a morning playlist upon detection of an associated trigger condition (e.g., presence of a user in a kitchen, detection of a coffee machine operation). In some embodiments, for example, the media playback system  100  is configured to play back audio from a first playback device (e.g., the playback device  100   a ) in synchrony with a second playback device (e.g., the playback device  100   b ). Interactions between the playback devices  110 , NMDs  120 , and/or control devices  130  of the media playback system  100  configured in accordance with the various embodiments of the disclosure are described in greater detail below with respect to  FIGS.  1 B- 1 M . 
     In the illustrated embodiment of  FIG.  1 A , the environment  101  comprises a household having several rooms, spaces, and/or playback zones, including (clockwise from upper left) a master bathroom  101   a , a master bedroom  101   b , 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 media playback system  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. 
     The media playback system  100  can comprise one or more playback zones, some of which may correspond to the rooms in the environment  101 . The media playback system  100  can be established with one or more playback zones, after which additional zones may be added, or removed to form, for example, the configuration shown in  FIG.  1 A . Each zone may be given a name according to a different room or space such as the office  101   e , master bathroom  101   a , master bedroom  101   b , the second bedroom  101   c , kitchen  101   h , dining room  101   g , living room  101   f , and/or the balcony  101   i . In some aspects, a single playback zone may include multiple rooms or spaces. In certain aspects, a single room or space may include multiple playback zones. 
     In the illustrated embodiment of  FIG.  1 A , the master bathroom  101   a , the second bedroom  101   c , the office  101   e , the living room  101   f , the dining room  101   g , the kitchen  101   h , and the outdoor patio  101   i  each include one playback device  110 , and the master bedroom  101   b  and the den  101   d  include a plurality of playback devices  110 . In the master bedroom  101   b , the playback devices  110   l  and  110   m  may be configured, for example, to play back audio content in synchrony as individual ones of playback devices  110 , as a bonded playback zone, as a consolidated playback device, and/or any combination thereof. Similarly, in the den  101   d , the playback devices  110   h - j  can be configured, for instance, to play back audio content in synchrony as individual ones of playback devices  110 , as one or more bonded playback devices, and/or as one or more consolidated playback devices. Additional details regarding bonded and consolidated playback devices are described below with respect to  FIGS.  1 B,  1 E, and  1 I- 1 M . 
     In some aspects, one or more of the playback zones in the environment  101  may each be playing different audio content. For instance, a user may be grilling on the patio  101   i  and listening to hip hop music being played by the playback device  110   c  while another user is preparing food in the kitchen  101   h  and listening to classical music played by the playback device  110   b . 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  101   e  listening to the playback device  110   f  playing back the same hip hop music being played back by playback device  110   c  on the patio  101   i . In some aspects, the playback devices  110   c  and  110   f  play back the hip hop music in synchrony such that the user perceives that the audio content is being played seamlessly (or at least substantially seamlessly) while moving between different playback zones. Additional details regarding audio playback synchronization among playback devices and/or zones can be found, for example, in U.S. Pat. No. 8,234,395 entitled, “System and method for synchronizing operations among a plurality of independently clocked digital data processing devices,” which is incorporated herein by reference in its entirety. 
     a. Suitable Media Playback System 
       FIG.  1 B  is a schematic diagram of the media playback system  100  and a cloud network  102 . For ease of illustration, certain devices of the media playback system  100  and the cloud network  102  are omitted from  FIG.  1 B . One or more communication links  103  (referred to hereinafter as “the links  103 ”) communicatively couple the media playback system  100  and the cloud network  102 . 
     The links  103  can comprise, for example, one or more wired networks, one or more wireless networks, one or more wide area networks (WAN), one or more local area networks (LAN), one or more personal area networks (PAN), one or more telecommunication networks (e.g., one or more Global System for Mobiles (GSM) networks, Code Division Multiple Access (CDMA) networks, Long-Term Evolution (LTE) networks, 5G communication network networks, and/or other suitable data transmission protocol networks), etc. The cloud network  102  is configured to deliver media content (e.g., audio content, video content, photographs, social media content) to the media playback system  100  in response to a request transmitted from the media playback system  100  via the links  103 . In some embodiments, the cloud network  102  is further configured to receive data (e.g. voice input data) from the media playback system  100  and correspondingly transmit commands and/or media content to the media playback system  100 . 
     The cloud network  102  comprises computing devices  106  (identified separately as a first computing device  106   a , a second computing device  106   b , and a third computing device  106   c ). The computing devices  106  can comprise individual computers or servers, such as, for example, a media streaming service server storing audio and/or other media content, a voice service server, a social media server, a media playback system control server, etc. In some embodiments, one or more of the computing devices  106  comprise modules of a single computer or server. In certain embodiments, one or more of the computing devices  106  comprise one or more modules, computers, and/or servers. Moreover, while the cloud network  102  is described above in the context of a single cloud network, in some embodiments the cloud network  102  comprises a plurality of cloud networks comprising communicatively coupled computing devices. Furthermore, while the cloud network  102  is shown in  FIG.  1 B  as having three of the computing devices  106 , in some embodiments, the cloud network  102  comprises fewer (or more) than three computing devices  106 . 
     The media playback system  100  is configured to receive media content from the networks  102  via the links  103 . The received media content can comprise, for example, a Uniform Resource Identifier (URI) and/or a Uniform Resource Locator (URL). For instance, in some examples, the media playback system  100  can stream, download, or otherwise obtain data from a URI or a URL corresponding to the received media content. A network  104  communicatively couples the links  103  and at least a portion of the devices (e.g., one or more of the playback devices  110 , NMDs  120 , and/or control devices  130 ) of the media playback system  100 . The network  104  can include, for example, a wireless network (e.g., a WiFi network, a Bluetooth, a Z-Wave network, a ZigBee, and/or other suitable wireless communication protocol network) and/or a wired network (e.g., a network comprising Ethernet, Universal Serial Bus (USB), and/or another suitable wired communication). As those of ordinary skill in the art will appreciate, as used herein, “WiFi” can refer to several different communication protocols including, for example, Institute of Electrical and Electronics Engineers (IEEE) 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.11ad, 802.11af, 802.11ah, 802.11ai, 802.11aj, 802.11aq, 802.11ax, 802.11ay, 802.15, etc. transmitted at 2.4 Gigahertz (GHz), 5 GHz, and/or another suitable frequency. 
     In some embodiments, the network  104  comprises a dedicated communication network that the media playback system  100  uses to transmit messages between individual devices and/or to transmit media content to and from media content sources (e.g., one or more of the computing devices  106 ). In certain embodiments, the network  104  is configured to be accessible only to devices in the media playback system  100 , thereby reducing interference and competition with other household devices. In other embodiments, however, the network  104  comprises an existing household communication network (e.g., a household WiFi network). In some embodiments, the links  103  and the network  104  comprise one or more of the same networks. In some aspects, for example, the links  103  and the network  104  comprise a telecommunication network (e.g., an LTE network, a 5G network). Moreover, in some embodiments, the media playback system  100  is implemented without the network  104 , and devices comprising the media playback system  100  can communicate with each other, for example, via one or more direct connections, PANs, telecommunication networks, and/or other suitable communication links. 
     In some embodiments, audio content sources may be regularly added or removed from the media playback system  100 . In some embodiments, for example, the media playback system  100  performs an indexing of media items when one or more media content sources are updated, added to, and/or removed from the media playback system  100 . The media playback system  100  can scan identifiable media items in some or all folders and/or directories accessible to the playback devices  110 , and generate or update a media content database comprising metadata (e.g., title, artist, album, track length) and other associated information (e.g., URIs, URLs) for each identifiable media item found. In some embodiments, for example, the media content database is stored on one or more of the playback devices  110 , network microphone devices  120 , and/or control devices  130 . 
     In the illustrated embodiment of  FIG.  1 B , the playback devices  110   l  and  110   m  comprise a group  107   a . The playback devices  110   l  and  110   m  can be positioned in different rooms in a household and be grouped together in the group  107   a  on a temporary or permanent basis based on user input received at the control device  130   a  and/or another control device  130  in the media playback system  100 . When arranged in the group  107   a , the playback devices  110   l  and  110   m  can be configured to play back the same or similar audio content in synchrony from one or more audio content sources. In certain embodiments, for example, the group  107   a  comprises a bonded zone in which the playback devices  110   l  and  110   m  comprise left audio and right audio channels, respectively, of multi-channel audio content, thereby producing or enhancing a stereo effect of the audio content. In some embodiments, the group  107   a  includes additional playback devices  110 . In other embodiments, however, the media playback system  100  omits the group  107   a  and/or other grouped arrangements of the playback devices  110 . Additional details regarding groups and other arrangements of playback devices are described in further detail below with respect to  FIGS.  1   -I through  1 -M. 
     The media playback system  100  includes the NMDs  120   a  and  120   b , each comprising one or more microphones configured to receive voice utterances from a user. In the illustrated embodiment of  FIG.  1 B , the NMD  120   a  is a standalone device and the NMD  120   b  is integrated into the playback device  110   n . The NMD  120   a , for example, is configured to receive voice input  121  from a user  123 . In some embodiments, the NMD  120   a  transmits data associated with the received voice input  121  to a voice assistant service (VAS) configured to (i) process the received voice input data and (ii) transmit a corresponding command to the media playback system  100 . In some aspects, for example, the computing device  106   c  comprises one or more modules and/or servers of a VAS (e.g., a VAS operated by one or more of SONOS®, AMAZON®, GOOGLE® APPLE®, MICROSOFT®). The computing device  106   c  can receive the voice input data from the NMD  120   a  via the network  104  and the links  103 . In response to receiving the voice input data, the computing device  106   c  processes the voice input data (i.e., “Play Hey Jude by The Beatles”), and determines that the processed voice input includes a command to play a song (e.g., “Hey Jude”). The computing device  106   c  accordingly transmits commands to the media playback system  100  to play back “Hey Jude” by the Beatles from a suitable media service (e.g., via one or more of the computing devices  106 ) on one or more of the playback devices  110 . 
     b. Suitable Playback Devices 
       FIG.  1 C  is a block diagram of the playback device  110   a  comprising an input/output  111 . The input/output  111  can include an analog I/O  111   a  (e.g., one or more wires, cables, and/or other suitable communication links configured to carry analog signals) and/or a digital I/O  111   b  (e.g., one or more wires, cables, or other suitable communication links configured to carry digital signals). In some embodiments, the analog I/O  111   a  is an audio line-in input connection comprising, for example, an auto-detecting 3.5 mm audio line-in connection. In some embodiments, the digital I/O  111   b  comprises a Sony/Philips Digital Interface Format (S/PDIF) communication interface and/or cable and/or a Toshiba Link (TOSLINK) cable. In some embodiments, the digital I/O  111   b  comprises a High-Definition Multimedia Interface (HDMI) interface and/or cable. In some embodiments, the digital I/O  111   b  includes one or more wireless communication links comprising, for example, a radio frequency (RF), infrared, WiFi, Bluetooth, or another suitable communication protocol. In certain embodiments, the analog I/O  111   a  and the digital I/O  111   b  comprise interfaces (e.g., ports, plugs, jacks) configured to receive connectors of cables transmitting analog and digital signals, respectively, without necessarily including cables. 
     The playback device  110   a , for example, can receive media content (e.g., audio content comprising music and/or other sounds) from a local audio source  105  via the input/output  111  (e.g., a cable, a wire, a PAN, a Bluetooth connection, an ad hoc wired or wireless communication network, and/or another suitable communication link). The local audio source  105  can comprise, for example, a mobile device (e.g., a smartphone, a tablet, a laptop computer) or another suitable audio component (e.g., a television, a desktop computer, an amplifier, a phonograph, a Blu-ray player, a memory storing digital media files). In some aspects, the local audio source  105  includes local music libraries on a smartphone, a computer, a networked-attached storage (NAS), and/or another suitable device configured to store media files. In certain embodiments, one or more of the playback devices  110 , NMDs  120 , and/or control devices  130  comprise the local audio source  105 . In other embodiments, however, the media playback system omits the local audio source  105  altogether. In some embodiments, the playback device  110   a  does not include an input/output  111  and receives all audio content via the network  104 . 
     The playback device  110   a  further comprises electronics  112 , a user interface  113  (e.g., one or more buttons, knobs, dials, touch-sensitive surfaces, displays, touchscreens), and one or more transducers  114  (referred to hereinafter as “the transducers  114 ”). The electronics  112  is configured to receive audio from an audio source (e.g., the local audio source  105 ) via the input/output  111 , one or more of the computing devices  106   a - c  via the network  104  ( FIG.  1 B )), amplify the received audio, and output the amplified audio for playback via one or more of the transducers  114 . In some embodiments, the playback device  110   a  optionally includes one or more microphones  115  (e.g., a single microphone, a plurality of microphones, a microphone array) (hereinafter referred to as “the microphones  115 ”). In certain embodiments, for example, the playback device  110   a  having one or more of the optional microphones  115  can operate as an NMD configured to receive voice input from a user and correspondingly perform one or more operations based on the received voice input. 
     In the illustrated embodiment of  FIG.  1 C , the electronics  112  comprise one or more processors  112   a  (referred to hereinafter as “the processors  112   a ”), memory  112   b , software components  112   c , a network interface  112   d , one or more audio processing components  112   g  (referred to hereinafter as “the audio components  112   g ”), one or more audio amplifiers  112   h  (referred to hereinafter as “the amplifiers  112   h ”), and power  112   i  (e.g., one or more power supplies, power cables, power receptacles, batteries, induction coils, Power-over Ethernet (POE) interfaces, and/or other suitable sources of electric power). In some embodiments, the electronics  112  optionally include one or more other components  112   j  (e.g., one or more sensors, video displays, touchscreens, battery charging bases). 
     The processors  112   a  can comprise clock-driven computing component(s) configured to process data, and the memory  112   b  can comprise a computer-readable medium (e.g., a tangible, non-transitory computer-readable medium, data storage loaded with one or more of the software components  112   c ) configured to store instructions for performing various operations and/or functions. The processors  112   a  are configured to execute the instructions stored on the memory  112   b  to perform one or more of the operations. The operations can include, for example, causing the playback device  110   a  to retrieve audio data from an audio source (e.g., one or more of the computing devices  106   a - c  ( FIG.  1 B )), and/or another one of the playback devices  110 . In some embodiments, the operations further include causing the playback device  110   a  to send audio data to another one of the playback devices  110   a  and/or another device (e.g., one of the NMDs  120 ). Certain embodiments include operations causing the playback device  110   a  to pair with another of the one or more playback devices  110  to enable a multi-channel audio environment (e.g., a stereo pair, a bonded zone). 
     The processors  112   a  can be further configured to perform operations causing the playback device  110   a  to synchronize playback of audio content with another of the one or more playback devices  110 . As those of ordinary skill in the art will appreciate, during synchronous playback of audio content on a plurality of playback devices, a listener will preferably be unable to perceive time-delay differences between playback of the audio content by the playback device  110   a  and the other one or more other playback devices  110 . Additional details regarding audio playback synchronization among playback devices can be found, for example, in U.S. Pat. No. 8,234,395, which was incorporated by reference above. 
     In some embodiments, the memory  112   b  is further configured to store data associated with the playback device  110   a , such as one or more zones and/or zone groups of which the playback device  110   a  is a member, audio sources accessible to the playback device  110   a , and/or a playback queue that the playback device  110   a  (and/or another of the one or more playback devices) can be associated with. The stored data can comprise one or more state variables that are periodically updated and used to describe a state of the playback device  110   a . The memory  112   b  can also include data associated with a state of one or more of the other devices (e.g., the playback devices  110 , NMDs  120 , control devices  130 ) of the media playback system  100 . In some aspects, for example, the state data is shared during predetermined intervals of time (e.g., every 5 seconds, every 10 seconds, every 60 seconds) among at least a portion of the devices of the media playback system  100 , so that one or more of the devices have the most recent data associated with the media playback system  100 . 
     The network interface  112   d  is configured to facilitate a transmission of data between the playback device  110   a  and one or more other devices on a data network such as, for example, the links  103  and/or the network  104  ( FIG.  1 B ). The network interface  112   d  is configured to transmit and receive data corresponding to media content (e.g., audio content, video content, text, photographs) and other signals (e.g., non-transitory signals) comprising digital packet data including an Internet Protocol (IP)-based source address and/or an IP-based destination address. The network interface  112   d  can parse the digital packet data such that the electronics  112  properly receives and processes the data destined for the playback device  110   a.    
     In the illustrated embodiment of  FIG.  1 C , the network interface  112   d  comprises one or more wireless interfaces  112   e  (referred to hereinafter as “the wireless interface  112   e ”). The wireless interface  112   e  (e.g., a suitable interface comprising one or more antennae) can be configured to wirelessly communicate with one or more other devices (e.g., one or more of the other playback devices  110 , NMDs  120 , and/or control devices  130 ) that are communicatively coupled to the network  104  ( FIG.  1 B ) in accordance with a suitable wireless communication protocol (e.g., WiFi, Bluetooth, LTE). In some embodiments, the network interface  112   d  optionally includes a wired interface  112   f  (e.g., an interface or receptacle configured to receive a network cable such as an Ethernet, a USB-A, USB-C, and/or Thunderbolt cable) configured to communicate over a wired connection with other devices in accordance with a suitable wired communication protocol. In certain embodiments, the network interface  112   d  includes the wired interface  112   f  and excludes the wireless interface  112   e . In some embodiments, the electronics  112  excludes the network interface  112   d  altogether and transmits and receives media content and/or other data via another communication path (e.g., the input/output  111 ). 
     The audio components  112   g  are configured to process and/or filter data comprising media content received by the electronics  112  (e.g., via the input/output  111  and/or the network interface  112   d ) to produce output audio signals. In some embodiments, the audio processing components  112   g  comprise, for example, one or more digital-to-analog converters (DAC), audio preprocessing components, audio enhancement components, a digital signal processors (DSPs), and/or other suitable audio processing components, modules, circuits, etc. In certain embodiments, one or more of the audio processing components  112   g  can comprise one or more subcomponents of the processors  112   a . In some embodiments, the electronics  112  omits the audio processing components  112   g . In some aspects, for example, the processors  112   a  execute instructions stored on the memory  112   b  to perform audio processing operations to produce the output audio signals. 
     The amplifiers  112   h  are configured to receive and amplify the audio output signals produced by the audio processing components  112   g  and/or the processors  112   a . The amplifiers  112   h  can comprise electronic devices and/or components configured to amplify audio signals to levels sufficient for driving one or more of the transducers  114 . In some embodiments, for example, the amplifiers  112   h  include one or more switching or class-D power amplifiers. In other embodiments, however, the amplifiers include one or more other types of power amplifiers (e.g., linear gain power amplifiers, class-A amplifiers, class-B amplifiers, class-AB amplifiers, class-C amplifiers, class-D amplifiers, class-E amplifiers, class-F amplifiers, class-G and/or class H amplifiers, and/or another suitable type of power amplifier). In certain embodiments, the amplifiers  112   h  comprise a suitable combination of two or more of the foregoing types of power amplifiers. Moreover, in some embodiments, individual ones of the amplifiers  112   h  correspond to individual ones of the transducers  114 . In other embodiments, however, the electronics  112  includes a single one of the amplifiers  112   h  configured to output amplified audio signals to a plurality of the transducers  114 . In some other embodiments, the electronics  112  omits the amplifiers  112   h.    
     The transducers  114  (e.g., one or more speakers and/or speaker drivers) receive the amplified audio signals from the amplifier  112   h  and render or output the amplified audio signals as sound (e.g., audible sound waves having a frequency between about 20 Hertz (Hz) and 20 kilohertz (kHz)). In some embodiments, the transducers  114  can comprise a single transducer. In other embodiments, however, the transducers  114  comprise a plurality of audio transducers. In some embodiments, the transducers  114  comprise more than one type of transducer. For example, the transducers  114  can include one or more low frequency transducers (e.g., subwoofers, woofers), mid-range frequency transducers (e.g., mid-range transducers, mid-woofers), and one or more high frequency transducers (e.g., one or more tweeters). As used herein, “low frequency” can generally refer to audible frequencies below about 500 Hz, “mid-range frequency” can generally refer to audible frequencies between about 500 Hz and about 2 kHz, and “high frequency” can generally refer to audible frequencies above 2 kHz. In certain embodiments, however, one or more of the transducers  114  comprise transducers that do not adhere to the foregoing frequency ranges. For example, one of the transducers  114  may comprise a mid-woofer transducer configured to output sound at frequencies between about 200 Hz and about 5 kHz. 
     By way of illustration, SONOS, Inc. presently offers (or has offered) for sale certain playback devices including, for example, a “SONOS ONE,” “PLAY:1,” “PLAY:3,” “PLAY:5,” “PLAYBAR,” “PLAYBASE,” “CONNECT:AMP,” “CONNECT,” and “SUB.” Other suitable playback devices may additionally or alternatively be used to implement the playback devices of example embodiments disclosed herein. Additionally, one of ordinary skilled in the art will appreciate that a playback device is not limited to the examples described herein or to SONOS product offerings. In some embodiments, for example, one or more playback devices  110  comprises wired or wireless headphones (e.g., over-the-ear headphones, on-ear headphones, in-ear earphones). In other embodiments, one or more of the playback devices  110  comprise a docking station and/or an interface configured to interact with a docking station for personal mobile media playback devices. In certain embodiments, 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. In some embodiments, a playback device omits a user interface and/or one or more transducers. For example,  FIG.  1 D  is a block diagram of a playback device  110   p  comprising the input/output  111  and electronics  112  without the user interface  113  or transducers  114 . 
       FIG.  1 E  is a block diagram of a bonded playback device  110   q  comprising the playback device  110   a  ( FIG.  1 C ) sonically bonded with the playback device  110   i  (e.g., a subwoofer) ( FIG.  1 A ). In the illustrated embodiment, the playback devices  110   a  and  110   i  are separate ones of the playback devices  110  housed in separate enclosures. In some embodiments, however, the bonded playback device  110   q  comprises a single enclosure housing both the playback devices  110   a  and  110   i . The bonded playback device  110   q  can be configured to process and reproduce sound differently than an unbonded playback device (e.g., the playback device  110   a  of  FIG.  1 C ) and/or paired or bonded playback devices (e.g., the playback devices  110   l  and  110   m  of  FIG.  1 B ). In some embodiments, for example, the playback device  110   a  is full-range playback device configured to render low frequency, mid-range frequency, and high frequency audio content, and the playback device  110   i  is a subwoofer configured to render low frequency audio content. In some aspects, the playback device  110   a , when bonded with the first playback device, is configured to render only the mid-range and high frequency components of a particular audio content, while the playback device  110   i  renders the low frequency component of the particular audio content. In some embodiments, the bonded playback device  110   q  includes additional playback devices and/or another bonded playback device. Additional playback device embodiments are described in further detail below with respect to  FIGS.  2 A- 3 D . 
     c. Suitable Network Microphone Devices (NMDs) 
       FIG.  1 F  is a block diagram of the NMD  120   a  ( FIGS.  1 A and  1 B ). The NMD  120   a  includes one or more voice processing components  124  (hereinafter “the voice components  124 ”) and several components described with respect to the playback device  110   a  ( FIG.  1 C ) including the processors  112   a , the memory  112   b , and the microphones  115 . The NMD  120   a  optionally comprises other components also included in the playback device  110   a  ( FIG.  1 C ), such as the user interface  113  and/or the transducers  114 . In some embodiments, the NMD  120   a  is configured as a media playback device (e.g., one or more of the playback devices  110 ), and further includes, for example, one or more of the audio components  112   g  ( FIG.  1 C ), the amplifiers  114 , and/or other playback device components. In certain embodiments, the NMD  120   a  comprises an Internet of Things (IoT) device such as, for example, a thermostat, alarm panel, fire and/or smoke detector, etc. In some embodiments, the NMD  120   a  comprises the microphones  115 , the voice processing  124 , and only a portion of the components of the electronics  112  described above with respect to  FIG.  1 B . In some aspects, for example, the NMD  120   a  includes the processor  112   a  and the memory  112   b  ( FIG.  1 B ), while omitting one or more other components of the electronics  112 . In some embodiments, the NMD  120   a  includes additional components (e.g., one or more sensors, cameras, thermometers, barometers, hygrometers). 
     In some embodiments, an NMD can be integrated into a playback device.  FIG.  1 G  is a block diagram of a playback device  110   r  comprising an NMD  120   d . The playback device  110   r  can comprise many or all of the components of the playback device  110   a  and further include the microphones  115  and voice processing  124  ( FIG.  1 F ). The playback device  110   r  optionally includes an integrated control device  130   c . The control device  130   c  can comprise, for example, a user interface (e.g., the user interface  113  of  FIG.  1 B ) configured to receive user input (e.g., touch input, voice input) without a separate control device. In other embodiments, however, the playback device  110   r  receives commands from another control device (e.g., the control device  130   a  of  FIG.  1 B ). Additional NMD embodiments are described in further detail below with respect to  FIGS.  3 A- 3 F . 
     Referring again to  FIG.  1 F , the microphones  115  are configured to acquire, capture, and/or receive sound from an environment (e.g., the environment  101  of  FIG.  1 A ) and/or a room in which the NMD  120   a  is positioned. The received sound can include, for example, vocal utterances, audio played back by the NMD  120   a  and/or another playback device, background voices, ambient sounds, etc. The microphones  115  convert the received sound into electrical signals to produce microphone data. The voice processing  124  receives and analyzes the microphone data to determine whether a voice input is present in the microphone data. The voice input can comprise, for example, an activation word followed by an utterance including a user request. As those of ordinary skill in the art will appreciate, an activation word is a word or other audio cue that signifying a user voice input. For instance, in querying the AMAZON® VAS, a user might speak the activation word “Alexa.” Other examples include “Ok, Google” for invoking the GOOGLE® VAS and “Hey, Siri” for invoking the APPLE® VAS. 
     After detecting the activation word, voice processing  124  monitors the microphone data for an accompanying user request in the voice input. The user request may include, for example, a command to control a third-party device, such as a thermostat (e.g., NEST® thermostat), an illumination device (e.g., a PHILIPS HUE® lighting device), or a media playback device (e.g., a Sonos® playback device). For example, a user might speak the activation word “Alexa” followed by the utterance “set the thermostat to 68 degrees” to set a temperature in a home (e.g., the environment  101  of  FIG.  1 A ). The user might speak the same activation word followed by the utterance “turn on the living room” to turn on illumination devices in a living room area of the home. The user may similarly speak an activation word followed by a request to play a particular song, an album, or a playlist of music on a playback device in the home. Additional description regarding receiving and processing voice input data can be found in further detail below with respect to  FIGS.  3 A- 3 F . 
     d. Suitable Control Devices 
       FIG.  1 H  is a partially schematic diagram of the control device  130   a  ( FIGS.  1 A and  1 B ). As used herein, the term “control device” can be used interchangeably with “controller” or “control system.” Among other features, the control device  130   a  is configured to receive user input related to the media playback system  100  and, in response, cause one or more devices in the media playback system  100  to perform an action(s) or operation(s) corresponding to the user input. In the illustrated embodiment, the control device  130   a  comprises a smartphone (e.g., an iPhone™, an Android phone) on which media playback system controller application software is installed. In some embodiments, the control device  130   a  comprises, for example, a tablet (e.g., an iPad™), a computer (e.g., a laptop computer, a desktop computer), and/or another suitable device (e.g., a television, an automobile audio head unit, an IoT device). In certain embodiments, the control device  130   a  comprises a dedicated controller for the media playback system  100 . In other embodiments, as described above with respect to  FIG.  1 G , the control device  130   a  is integrated into another device in the media playback system  100  (e.g., one more of the playback devices  110 , NMDs  120 , and/or other suitable devices configured to communicate over a network). 
     The control device  130   a  includes electronics  132 , a user interface  133 , one or more speakers  134 , and one or more microphones  135 . The electronics  132  comprise one or more processors  132   a  (referred to hereinafter as “the processors  132   a ”), a memory  132   b , software components  132   c , and a network interface  132   d . The processor  132   a  can be configured to perform functions relevant to facilitating user access, control, and configuration of the media playback system  100 . The memory  132   b  can comprise data storage that can be loaded with one or more of the software components executable by the processor  302  to perform those functions. The software components  132   c  can comprise applications and/or other executable software configured to facilitate control of the media playback system  100 . The memory  112   b  can be configured to store, for example, the software components  132   c , media playback system controller application software, and/or other data associated with the media playback system  100  and the user. 
     The network interface  132   d  is configured to facilitate network communications between the control device  130   a  and one or more other devices in the media playback system  100 , and/or one or more remote devices. In some embodiments, the network interface  132  is configured to operate according to one or more suitable communication industry standards (e.g., infrared, radio, wired standards including IEEE 802.3, wireless standards including IEEE 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac, 802.15, 4G, LTE). The network interface  132   d  can be configured, for example, to transmit data to and/or receive data from the playback devices  110 , the NMDs  120 , other ones of the control devices  130 , one of the computing devices  106  of  FIG.  1 B , devices comprising one or more other media playback systems, etc. The transmitted and/or received data can include, for example, playback device control commands, state variables, playback zone and/or zone group configurations. For instance, based on user input received at the user interface  133 , the network interface  132   d  can transmit a playback device control command (e.g., volume control, audio playback control, audio content selection) from the control device  304  to one or more of the playback devices  100 . The network interface  132   d  can also transmit and/or receive configuration changes such as, for example, adding/removing one or more playback devices  100  to/from a zone, adding/removing one or more zones to/from a zone group, forming a bonded or consolidated player, separating one or more playback devices from a bonded or consolidated player, among others. Additional description of zones and groups can be found below with respect to  FIGS.  1   -I through  1 M. 
     The user interface  133  is configured to receive user input and can facilitate control of the media playback system  100 . The user interface  133  includes media content art  133   a  (e.g., album art, lyrics, videos), a playback status indicator  133   b  (e.g., an elapsed and/or remaining time indicator), media content information region  133   c , a playback control region  133   d , and a zone indicator  133   e . The media content information region  133   c  can include a display of relevant information (e.g., title, artist, album, genre, release year) about media content currently playing and/or media content in a queue or playlist. The playback control region  133   d  can include selectable (e.g., via touch input and/or via a cursor or another suitable selector) icons to cause one or more playback devices in a selected playback zone or zone group to perform playback actions such as, for example, 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  133   d  may also include selectable icons to modify equalization settings, playback volume, and/or other suitable playback actions. In the illustrated embodiment, the user interface  133  comprises a display presented on a touch screen interface of a smartphone (e.g., an iPhone™, an Android phone). In some embodiments, however, 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 one or more speakers  134  (e.g., one or more transducers) can be configured to output sound to the user of the control device  130   a . In some embodiments, the one or more speakers comprise individual transducers configured to correspondingly output low frequencies, mid-range frequencies, and/or high frequencies. In some aspects, for example, the control device  130   a  is configured as a playback device (e.g., one of the playback devices  110 ). Similarly, in some embodiments the control device  130   a  is configured as an NMD (e.g., one of the NMDs  120 ), receiving voice commands and other sounds via the one or more microphones  135 . 
     The one or more microphones  135  can comprise, for example, one or more condenser microphones, electret condenser microphones, dynamic microphones, and/or other suitable types of microphones or transducers. In some embodiments, two or more of the microphones  135  are arranged to capture location information of an audio source (e.g., voice, audible sound) and/or configured to facilitate filtering of background noise. Moreover, in certain embodiments, the control device  130   a  is configured to operate as playback device and an NMD. In other embodiments, however, the control device  130   a  omits the one or more speakers  134  and/or the one or more microphones  135 . For instance, the control device  130   a  may comprise a device (e.g., a thermostat, an IoT device, a network device) comprising a portion of the electronics  132  and the user interface  133  (e.g., a touch screen) without any speakers or microphones. 
     e. Suitable Playback Device Configurations 
       FIGS.  1   -I through  1 M show example configurations of playback devices in zones and zone groups. Referring first to  FIG.  1 M , in one example, a single playback device may belong to a zone. For example, the playback device  110   g  in the second bedroom  101   c  ( FIG.  1 A ) may belong to Zone C. 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  110   l  (e.g., a left playback device) can be bonded to the playback device  110   m  (e.g., a right playback device) 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  110   h  (e.g., a front playback device) may be merged with the playback device  110   i  (e.g., a subwoofer), and the playback devices  110   j  and  110   k  (e.g., left and right surround speakers, respectively) to form a single Zone D. In another example, the playback devices  110   g  and  110   h  can be merged to form a merged group or a zone group  108   b . The merged playback devices  110   g  and  110   h  may not be specifically assigned different playback responsibilities. That is, the merged playback devices  110   h  and  110   i  may, aside from playing audio content in synchrony, each play audio content as they would if they were not merged. 
     Each zone in the media playback system  100  may be provided for control as a single user interface (UI) entity. For example, Zone A may be provided as a single entity named Master Bathroom. Zone B may be provided as a single entity named Master Bedroom. Zone C may be provided as a single entity named Second Bedroom. 
     Playback devices that are bonded may have different playback responsibilities, such as responsibilities for certain audio channels. For example, as shown in  FIG.  1   -I, the playback devices  110   l  and  110   m  may be bonded so as to produce or enhance a stereo effect of audio content. In this example, the playback device  110   l  may be configured to play a left channel audio component, while the playback device  110   k  may be configured to play a right channel audio component. In some implementations, such stereo bonding may be referred to as “pairing.” 
     Additionally, bonded playback devices may have additional and/or different respective speaker drivers. As shown in  FIG.  1 J , the playback device  110   h  named Front may be bonded with the playback device  110   i  named SUB. The Front device  110   h  can be configured to render a range of mid to high frequencies and the SUB device  110   i  can be configured render low frequencies. When unbonded, however, the Front device  110   h  can be configured render a full range of frequencies. As another example,  FIG.  1 K  shows the Front and SUB devices  110   h  and  110   i  further bonded with Left and Right playback devices  110   j  and  110   k , respectively. In some implementations, the Right and Left devices  110   j  and  102   k  can be configured to form surround or “satellite” channels of a home theater system. The bonded playback devices  110   h ,  110   i ,  110   j , and  110   k  may form a single Zone D ( FIG.  1 M ). 
     Playback devices that are merged may not have assigned playback responsibilities, and may each render the full range of audio content the 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, the playback devices  110   a  and  110   n  of the master bathroom have the single UI entity of Zone A. In one embodiment, the playback devices  110   a  and  110   n  may each output the full range of audio content each respective playback devices  110   a  and  110   n  are capable of, in synchrony. 
     In some embodiments, an NMD is bonded or merged with another device so as to form a zone. For example, the NMD  120   b  may be bonded with the playback device  110   e , which together form Zone F, named Living Room. In other embodiments, a stand-alone network microphone device may be in a zone by itself. In other embodiments, however, a stand-alone network microphone device may not be associated with a zone. Additional details regarding associating network microphone devices 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. 
     Zones of individual, bonded, and/or merged devices may be grouped to form a zone group. For example, referring to  FIG.  1 M , Zone A may be grouped with Zone B to form a zone group  108   a  that includes the two zones. Similarly, Zone G may be grouped with Zone H to form the zone group  108   b . 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. Playback devices may be dynamically grouped and ungrouped to form new or different groups that synchronously play back audio content. 
     In various implementations, the zones in an environment may be the default name of a zone within the group or a combination of the names of the zones within a zone group. For example, Zone Group  108   b  can have be assigned a name such as “Dining+Kitchen”, as shown in  FIG.  1 M . In some embodiments, a zone group may be given a unique name selected by a user. 
     Certain data may be stored in a memory of a playback device (e.g., the memory  112   c  of  FIG.  1 C ) 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 may also include the data associated with the state of the other devices of the media system, and shared from time to time among the devices so that one or more of the devices have the most recent data associated with the system. 
     In some embodiments, the memory 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 “a 1 ” to identify playback device(s) of a zone, a second type “b 1 ” to identify playback device(s) that may be bonded in the zone, and a third type “c 1 ” to identify a zone group to which the zone may belong. As a related example, identifiers associated with the second bedroom  101   c  may indicate that the playback device is the only playback device of the Zone C and not in a zone group. Identifiers associated with the Den may indicate that the Den is not grouped with other zones but includes bonded playback devices  110   h - 110   k . Identifiers associated with the Dining Room may indicate that the Dining Room is part of the Dining+Kitchen zone group  108   b  and that devices  110   b  and  110   d  are grouped ( FIG.  1 L ). Identifiers associated with the Kitchen may indicate the same or similar information by virtue of the Kitchen being part of the Dining+Kitchen zone group  108   b . Other example zone variables and identifiers are described below. 
     In yet another example, the media playback system  100  may variables or identifiers representing other associations of zones and zone groups, such as identifiers associated with Areas, as shown in  FIG.  1 M . An area may involve a cluster of zone groups and/or zones not within a zone group. For instance,  FIG.  1 M  shows an Upper Area  109   a  including Zones A-D, and a Lower Area  109   b  including Zones E-I. 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 another aspect, this 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 media playback system  100  may not implement Areas, in which case the system may not store variables associated with Areas. 
     III. Example Systems and Devices 
       FIG.  2 A  is a front isometric view of a playback device  210  configured in accordance with aspects of the disclosed technology.  FIG.  2 B  is a front isometric view of the playback device  210  without a grille  216   e .  FIG.  2 C  is an exploded view of the playback device  210 . Referring to  FIGS.  2 A- 2 C  together, the playback device  210  comprises a housing  216  that includes an upper portion  216   a , a right or first side portion  216   b , a lower portion  216   c , a left or second side portion  216   d , the grille  216   e , and a rear portion  216   f  A plurality of fasteners  216   g  (e.g., one or more screws, rivets, clips) attaches a frame  216   h  to the housing  216 . A cavity  216   j  ( FIG.  2 C ) in the housing  216  is configured to receive the frame  216   h  and electronics  212 . The frame  216   h  is configured to carry a plurality of transducers  214  (identified individually in  FIG.  2 B  as transducers  214   a - f ). The electronics  212  (e.g., the electronics  112  of  FIG.  1 C ) is configured to receive audio content from an audio source and send electrical signals corresponding to the audio content to the transducers  214  for playback. 
     The transducers  214  are configured to receive the electrical signals from the electronics  112 , and further configured to convert the received electrical signals into audible sound during playback. For instance, the transducers  214   a - c  (e.g., tweeters) can be configured to output high frequency sound (e.g., sound waves having a frequency greater than about 2 kHz). The transducers  214   d - f  (e.g., mid-woofers, woofers, midrange speakers) can be configured output sound at frequencies lower than the transducers  214   a - c  (e.g., sound waves having a frequency lower than about 2 kHz). In some embodiments, the playback device  210  includes a number of transducers different than those illustrated in  FIGS.  2 A- 2 C . For example, as described in further detail below with respect to  FIGS.  3 A- 3 C , the playback device  210  can include fewer than six transducers (e.g., one, two, three). In other embodiments, however, the playback device  210  includes more than six transducers (e.g., nine, ten). Moreover, in some embodiments, all or a portion of the transducers  214  are configured to operate as a phased array to desirably adjust (e.g., narrow or widen) a radiation pattern of the transducers  214 , thereby altering a user&#39;s perception of the sound emitted from the playback device  210 . 
     In the illustrated embodiment of  FIGS.  2 A- 2 C , a filter  216   i  is axially aligned with the transducer  214   b . The filter  216   i  can be configured to desirably attenuate a predetermined range of frequencies that the transducer  214   b  outputs to improve sound quality and a perceived sound stage output collectively by the transducers  214 . In some embodiments, however, the playback device  210  omits the filter  216   i . In other embodiments, the playback device  210  includes one or more additional filters aligned with the transducers  214   b  and/or at least another of the transducers  214 . 
       FIGS.  3 A and  3 B  are front and right isometric side views, respectively, of an NMD  320  configured in accordance with embodiments of the disclosed technology.  FIG.  3 C  is an exploded view of the NMD  320 .  FIG.  3 D  is an enlarged view of a portion of  FIG.  3 B  including a user interface  313  of the NMD  320 . Referring first to  FIGS.  3 A- 3 C , the NMD  320  includes a housing  316  comprising an upper portion  316   a , a lower portion  316   b  and an intermediate portion  316   c  (e.g., a grille). A plurality of ports, holes or apertures  316   d  in the upper portion  316   a  allow sound to pass through to one or more microphones  315  ( FIG.  3 C ) positioned within the housing  316 . The one or more microphones  315  are configured to received sound via the apertures  316   d  and produce electrical signals based on the received sound. In the illustrated embodiment, a frame  316   e  ( FIG.  3 C ) of the housing  316  surrounds cavities  316   f  and  316   g  configured to house, respectively, a first transducer  314   a  (e.g., a tweeter) and a second transducer  314   b  (e.g., a mid-woofer, a midrange speaker, a woofer). In other embodiments, however, the NMD  320  includes a single transducer, or more than two (e.g., two, five, six) transducers. In certain embodiments, the NMD  320  omits the transducers  314   a  and  314   b  altogether. 
     Electronics  312  ( FIG.  3 C ) includes components configured to drive the transducers  314   a  and  314   b , and further configured to analyze audio data corresponding to the electrical signals produced by the one or more microphones  315 . In some embodiments, for example, the electronics  312  comprises many or all of the components of the electronics  112  described above with respect to  FIG.  1 C . In certain embodiments, the electronics  312  includes components described above with respect to  FIG.  1 F  such as, for example, the one or more processors  112   a , the memory  112   b , the software components  112   c , the network interface  112   d , etc. In some embodiments, the electronics  312  includes additional suitable components (e.g., proximity or other sensors). 
     Referring to  FIG.  3 D , the user interface  313  includes a plurality of control surfaces (e.g., buttons, knobs, capacitive surfaces) including a first control surface  313   a  (e.g., a previous control), a second control surface  313   b  (e.g., a next control), and a third control surface  313   c  (e.g., a play and/or pause control). A fourth control surface  313   d  is configured to receive touch input corresponding to activation and deactivation of the one or microphones  315 . A first indicator  313   e  (e.g., one or more light emitting diodes (LEDs) or another suitable illuminator) can be configured to illuminate only when the one or more microphones  315  are activated. A second indicator  313   f  (e.g., one or more LEDs) can be configured to remain solid during normal operation and to blink or otherwise change from solid to indicate a detection of voice activity. In some embodiments, the user interface  313  includes additional or fewer control surfaces and illuminators. In one embodiment, for example, the user interface  313  includes the first indicator  313   e , omitting the second indicator  313   f  Moreover, in certain embodiments, the NMD  320  comprises a playback device and a control device, and the user interface  313  comprises the user interface of the control device. 
     Referring to  FIGS.  3 A- 3 D  together, the NMD  320  is configured to receive voice commands from one or more adjacent users via the one or more microphones  315 . As described above with respect to  FIG.  1 B , the one or more microphones  315  can acquire, capture, or record sound in a vicinity (e.g., a region within 10 m or less of the NMD  320 ) and transmit electrical signals corresponding to the recorded sound to the electronics  312 . The electronics  312  can process the electrical signals and can analyze the resulting audio data to determine a presence of one or more voice commands (e.g., one or more activation words). In some embodiments, for example, after detection of one or more suitable voice commands, the NMD  320  is configured to transmit a portion of the recorded audio data to another device and/or a remote server (e.g., one or more of the computing devices  106  of  FIG.  1 B ) for further analysis. The remote server can analyze the audio data, determine an appropriate action based on the voice command, and transmit a message to the NMD  320  to perform the appropriate action. For instance, a user may speak “Sonos, play Michael Jackson.” The NMD  320  can, via the one or more microphones  315 , record the user&#39;s voice utterance, determine the presence of a voice command, and transmit the audio data having the voice command to a remote server (e.g., one or more of the remote computing devices  106  of  FIG.  1 B , one or more servers of a VAS and/or another suitable service). The remote server can analyze the audio data and determine an action corresponding to the command. The remote server can then transmit a command to the NMD  320  to perform the determined action (e.g., play back audio content related to Michael Jackson). The NMD  320  can receive the command and play back the audio content related to Michael Jackson from a media content source. As described above with respect to  FIG.  1 B , suitable content sources can include a device or storage communicatively coupled to the NMD  320  via a LAN (e.g., the network  104  of  FIG.  1 B ), a remote server (e.g., one or more of the remote computing devices  106  of  FIG.  1 B ), etc. In certain embodiments, however, the NMD  320  determines and/or performs one or more actions corresponding to the one or more voice commands without intervention or involvement of an external device, computer, or server. 
       FIG.  3 E  is a functional block diagram showing additional features of the NMD  320  in accordance with aspects of the disclosure. The NMD  320  includes components configured to facilitate voice command capture including voice activity detector component(s)  312   k , beam former components  312   l , acoustic echo cancellation (AEC) and/or self-sound suppression components  312   m , activation word detector components  312   n , and voice/speech conversion components  312   o  (e.g., voice-to-text and text-to-voice). In the illustrated embodiment of  FIG.  3 E , the foregoing components  312   k - 312   o  are shown as separate components. In some embodiments, however, one or more of the components  312   k - 312   o  are subcomponents of the processors  112   a.    
     The beamforming and self-sound suppression components  312   l  and  312   m  are configured to detect an audio signal and determine aspects of voice input represented in the detected audio signal, such as the direction, amplitude, frequency spectrum, etc. The voice activity detector activity components  312   k  are operably coupled with the beamforming and AEC components  312   l  and  312   m  and are configured to determine a direction and/or directions from which voice activity is likely to have occurred in the detected audio signal. Potential speech directions can be identified by monitoring metrics which 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, which is measure of spectral structure. As those of ordinary skill in the art will appreciate, speech typically has a lower entropy than most common background noise. The activation word detector components  312   n  are configured to monitor and analyze received audio to determine if any activation words (e.g., wake words) are present in the received audio. The activation word detector components  312   n  may analyze the received audio using an activation word detection algorithm. If the activation word detector  312   n  detects an activation word, the NMD  320  may process voice input contained in the received audio. Example activation word detection algorithms accept audio as input and provide an indication of whether an activation word is present in the audio. Many first- and third-party activation 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 activation words. In some embodiments, the activation word detector  312   n  runs multiple activation word detection algorithms on the received audio simultaneously (or substantially simultaneously). As noted above, different voice services (e.g. AMAZON&#39;s ALEXA®, APPLE&#39;s SIRI®, or MICROSOFT&#39;s CORTANA®) can each use a different activation word for invoking their respective voice service. To support multiple services, the activation word detector  312   n  may run the received audio through the activation word detection algorithm for each supported voice service in parallel. 
     The speech/text conversion components  312   o  may facilitate processing by converting speech in the voice input to text. In some embodiments, the electronics  312  can include voice recognition software that is trained to a particular user or a particular set of users associated with a household. Such voice recognition software may implement voice-processing algorithms that are tuned to specific voice profile(s). Tuning to specific voice profiles may require less computationally intensive algorithms than traditional voice activity services, which typically sample from a broad base of users and diverse requests that are not targeted to media playback systems. 
       FIG.  3 F  is a schematic diagram of an example voice input  328  captured by the NMD  320  in accordance with aspects of the disclosure. The voice input  328  can include an activation word portion  328   a  and a voice utterance portion  328   b . In some embodiments, the activation word  557   a  can be a known activation word, such as “Alexa,” which is associated with AMAZON&#39;s ALEXA®. In other embodiments, however, the voice input  328  may not include an activation word. In some embodiments, a network microphone device may output an audible and/or visible response upon detection of the activation word portion  328   a . In addition or alternately, an NMB may output an audible and/or visible response after processing a voice input and/or a series of voice inputs. 
     The voice utterance portion  328   b  may include, for example, one or more spoken commands (identified individually as a first command  328   c  and a second command  328   e ) and one or more spoken keywords (identified individually as a first keyword  328   d  and a second keyword  328   f ). In one example, the first command  328   c  can be a command to play music, such as a specific song, album, playlist, etc. In this example, the keywords may be one or words identifying one or more zones in which the music is to be played, such as the Living Room and the Dining Room shown in  FIG.  1 A . In some examples, the voice utterance portion  328   b  can include other information, such as detected pauses (e.g., periods of non-speech) between words spoken by a user, as shown in  FIG.  3 F . The pauses may demarcate the locations of separate commands, keywords, or other information spoke by the user within the voice utterance portion  328   b.    
     In some embodiments, the media playback system  100  is configured to temporarily reduce the volume of audio content that it is playing while detecting the activation word portion  557   a . The media playback system  100  may restore the volume after processing the voice input  328 , as shown in  FIG.  3 F . 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. 
     IV. Playback Characteristics Based on Listener Location 
       FIG.  4    shows a representation of a playback environment  401  occupied by a listener  403 . The playback environment  401  in this example is a living room  110   f  ( FIG.  1 A ). Other examples may be applied to any other environment in which playback devices are installed. For clarity, the playback environment  401  is shown as being substantially rectangular, but it will be appreciated that other playback environments may have different sized and/or shapes, and may contain any number of additional features, for example furniture and/or doorways, which may affect the acoustic properties of the playback environment. Other examples of playback environments include interiors of vehicles and/or commercial settings, as discussed above. A playback environment may correspond to a playback zone in a playback system such as that described above with reference to  FIG.  1 A , though in other examples a playback environment may only be part of a playback zone, or alternatively may incorporate multiple playback zones. 
     The playback environment  401  contains a playback device  410  configured to perform audio reproduction from a media source. The playback device is substantially as described above with reference to  FIG.  1 C  but includes microphones. As shown in  FIG.  5   , the playback device  410  includes input/output  411 , electronics  412 , a user interface  413 , one or more transducers  414 , and one or more microphones  415 . In this example, the transducers  414  include a tweeter  414   a  that is configured to generate sound signals having a relatively high frequency (for example between about 2 kHz and about 22 kHz) and a mid-woofer  414   b  that is configured to generate low to mid-frequency acoustic waves (for example, acoustic waves having a frequency of between about 40 Hz and about 2 kHz). Other examples of playback devices configured to perform the methods described hereafter may include more or fewer transducers, and may include other types of transducer, for example a subwoofer, or may omit any of the above-mentioned types of transducer. Furthermore, the methods described hereafter may be performed by a bonded playback device as described above with reference to  FIG.  1 E . 
     The electronics  412  of the playback device  410  in this example include equivalent components to the electronics  112  of the playback device  110  described above with reference to  FIG.  1 C , and additionally include audio processing components for processing sound signals received by the one or more microphones  415 . It is noted that the methods described hereafter may be performed by an NMD incorporating a playback device, or by a playback device incorporating an NMD. For example, the NMD  320  ( FIG.  3 C ), which includes one or more transducers  314  that may be used for audio reproduction, may be configured to perform the methods described hereafter. 
     The playback device  410  is arranged to select a characteristic of audio reproduction based on a location of a person, for example listener  403 , with respect to the playback device  403 . Examples of characteristics that may be selected include a volume of audio reproduction or equalization levels for audio reproduction. Further examples of characteristics of audio reproduction will be described in detail hereafter. As will be apparent from these examples, selecting a characteristic of audio reproduction based on a location of a listener may result in an improved or enhanced listening experience for the listener, and/or further additional or improved aspects of user experience for the listener. 
       FIG.  6    is a flow diagram of a method  600  in which the playback device  410  selects a characteristic of audio reproduction based on a location of a person relative to the playback device  410 . The playback device  410  transmits, at S 610 , a first sound signal containing a predetermined waveform. In this example, the first sound signal is an up-chirp which is a sound signal having a frequency that increases with time in a predetermined manner. Other examples of sound signals that may be transmitted include a down-chirp (a sound signal having a frequency that decreases with time in a predetermined manner), constant-frequency pulse, or Frequency-Modulated Continuous Wave (FMCW) signals. In the example of  FIG.  4   , a first sound signal  405  is transmitted by the playback device  410  into the playback environment  401 . The first sound signal  405  is represented in  FIG.  4    as a ray. It will be appreciated that a ray is an idealized model of a sound wave, and corresponds to a direction of energy flow that is locally perpendicular to wave fronts of the sound wave. 
     In the present example, the first sound signal  405  is an ultrasonic sound signal generated by the tweeter  414   a . An ultrasonic sound signal has a frequency that is higher than the highest frequency of sound that is audible to an average person, for example higher than 20 kHz. It is noted that the highest audible sound varies significantly between people, so for the purpose of the present disclosure, an ultrasonic sound signal may be considered to have a frequency that remains above 20 kHz for the duration of the sound signal. As mentioned above, the tweeter  414   a  is configured to generate sound signals having a frequency of between about 2 kHz and 22 kHz, and therefore is configured to generate ultrasonic sound signals with a frequency of between about 20 kHz and 22 kHz. Transducers such as tweeters may be manufactured to be able to generate sound signals outside the audible range of human hearing in order to reduce detrimental effects associated with roll-off near the extremes of the audible hearing range. In the present example, the first sound signal  405  is an ultrasonic chirp with a duration of about 10 ms, with a frequency that rises in a predetermined manner within a range of between about 20 kHz and 22 kHz. In other examples, however, the tweeter  414  is configured to generate sound signals having frequencies greater than 22 kHz (e.g., sound signals having frequencies 25 kHz and higher). 
     Under standard temperature and pressure (STP) conditions, ultrasound waves in the frequency ranges discussed above typically have wavelengths less than about 2 cm. Due to the relatively short wavelength of ultrasonic sound waves relative to the size of many transducers (including tweeter  414   a ) and/or apertures through which sound signals are emitted, angular dispersion or angular spread of ultrasonic sound signals generated by such transducers is limited to a relatively narrow angle (e.g., less than about 10 degrees from normal, less than about 5 degrees from normal, or less than about 1 degree from normal). As a result, ultrasonic sound signals may be transmitted as a relatively narrow beam, in contrast to lower frequency sound signals for which diffractive effects cause wide-angle dispersion. This makes ultrasonic sound waves (as well as high-frequency audible sound waves, for example audible sound waves with a frequency greater than about 8 kHz, about 10 kHz, about 12 kHz, about 14 kHz, or about 18 kHz) suitable for use in the present method, for the reasons described hereafter. 
     Returning to the method of  FIG.  6   , the playback device  410  receives, at S 620 , a second sound signal using the one or more microphones  415 . The second sound signal contains at least one reflection of the first sound signal. In the example of  FIG.  4   , three reflections  407   a ,  407   b , and  407   c  (collectively referred to as reflections  407 ) of the first sound signal  405  are shown, each reflection represented by a ray. The first reflected ray  407   a  corresponds to a reflection of the first sound signal  403  in which the first sound signal  403  is normally incident on part of the listener  403 , and accordingly is reflected directly back to the playback device  410 . The second reflected ray  407   b  corresponds to a reflection of the first sound signal  403  in which the first sound signal  403  is obliquely incident on part of the listener  403 , and is reflected indirectly back to the playback device  410 , having first been reflected off the left hand wall of the playback environment  401 . The third reflected ray  407   c  also corresponds to a reflection of the first sound signal  403  in which the first sound signal  403  is reflected indirectly back to the playback device  410 , in this case having first been reflected off three walls of the playback environment  401 . In reality, the propagation of sound waves in the playback environment  410  will be complicated by, for example, diffractive effects, and many other reflections will be present in a reverberant environment such as the playback environment  410 . The reflected rays  407  are included for illustrative purposes only. 
     Although the number of reflections of the sound signal  405  is likely to be large, the relatively narrow beam angle of the first sound signal  405  reduces the likelihood that direct reflections of the first sound signal  405  from the back wall or side walls (shown in  FIG.  4    as the left, right, and lower walls) are present because the beam may be blocked before reaching them. In the specific arrangement shown in  FIG.  4   , the first reflected ray to reach the playback device  410  is the ray  407   a  reflected directly from the listener  403 . In other arrangements, rays reflected directly or indirectly from other objects of features within a playback environment may reach a playback device before any ray directly reflected from a person. In some arrangements, for example where a listener is not located within a path of a beam transmitted by a playback device, there may not be any significant direct reflection from the listener. 
     The second sound signal is processed, at S 630 , to determine a location of a person relative to the playback device  410 . In the present embodiment, audio processing components of the electronics  412  of the playback device  410  process the second sound signal. However, playback devices in other embodiments may alternatively be arranged to send data to a remote computer system for remote processing. In some of these examples, a playback device is arranged to transmit, using a network interface, data indicative of a second sound signal to a computing system for remote processing. The computing system is arranged to process the second sound signal and transmit, using the network interface, data indicative of the location of the person relative to the playback device. The playback device is arranged to receive the data indicative of the location of the person, thereby to determine the location of the person relative to the playback device. 
     A location of a person may be determined in a variety of ways. In some examples, a feature within a received second sound signal is identified as being a reflection of a transmitted signal by a person. An example of a feature within a received sound signal is a reflection of all or part of a predetermined waveform, or a peak in amplitude in the received signal. A reflection of a transmitted signal by a person may be direct or indirect, as discussed above with reference to  FIG.  4   . If a feature is identified as being a direct reflection of a transmitted signal by a person, a distance from the playback device to the person can be determined using echo-location or range-finding techniques, whereby the distance is determined using the equation d=ct/2, where d is the distance from the playback device to the determined location of the person, t is the time between transmission of a feature within the transmitted signal and receipt of a corresponding feature within the received signal, and c is the speed of sound in air (approximately 340 m/s under STP conditions). Identifying a feature as a direct reflection by a person may include comparing a received sound signal with a baseline signal received when no person is present in the playback environment, and identifying from the comparison one or more additional features in the sound signal received when a person is present in the playback zone. The first such feature to appear in the received signal may be determined to be a direct reflection by a person. For examples where echo-location techniques are employed to determine a location of a person, sound signals containing a predetermined FMCW (for example, a chirp) allow for improved distance resolution, as cross-correlation may be used to match the reflected signal with a reference signal containing the FMCW with a given temporal offset. 
     The baseline signal may be a signal transmitted at a time when no one is likely to be in the room, such as a time between midnight and 6 am, between lam and 5 am, or between 2 am and 4 am, for example at about 3 am, about 4 am, or about 5 am. The comparison may identify differences from the baseline signal, e.g. by subtracting the baseline signal from the received signal. Using a baseline signal can remove the effects of both the self-response of the playbacks device and the response from furniture and other items in the environment. In some examples, the baseline signal may be transmitted and measured periodically, for example once a day, once a week, or once a month to identify potential changes such as changes in the position of furniture and/or other items in the environment. The baseline signal may be established from a single measurement of from a plurality of measurements, e.g. by taking an average. 
       FIG.  7    shows an example of  1250  data samples generated by passing a sound signal received by a playback device through a multi-stage noise-shaping (MASH) modulator. Each of the discrete peaks in the data corresponds to an amplitude peak in the received second sound signal. In this example, the first peak  702  corresponds to a self-response of the playback device caused by a transducer of the playback device transmitting a first sound signal. The second peak  704  corresponds to a direct reflection of the first sound signal by a person (similar to the reflection  407   a  of  FIG.  4   ). The third peak  706  corresponds to an indirect reflection of the first sound signal by a person (similar to the reflection  407   b  of  FIG.  4   ). 
     In some examples, processing a second sound signal to determine a location of a person relative to a playback device includes disregarding a portion of the second sound signal corresponding to a self-response of the playback device. A playback device may have a significant self-response for at least a part of the frequency range generated by the playback device. A self-response may be, for example, a resonance of all or part of the playback device induced by a transducer of the playback device generating a sound signal, and/or internal reflections of a generated sound signal within the playback device. In some examples, the self-response may be the most prominent feature of a sound signal received by a microphone of the playback device. This is in contrast with devices solely or primarily designed to perform echo-location, for example sonar transceivers, which are designed to minimize self-response. Disregarding a portion of the received second sound signal corresponding to a self-response of the playback device may improve the accuracy with which a location of a person is determined, especially where determining a location of a person involves determining a correspondence between the received second sound signal and stored signal data, examples of which will be described in detail hereafter. Disregarding a portion of the received sound signal may include disregarding a portion of the signal having a predetermined duration. The predetermined duration for a given playback device will depend on the self-response of the given playback device, and may be different for different models of playback device. In some examples, the microphone is activated only after the portion of the second sound signal is expected to be received, resulting in the portion being disregarded automatically. 
     As discussed above with reference to  FIG.  1 C , some playback devices incorporate multiple microphones, for example as part of a microphone array. In such examples, signals received from two or more of the microphones may be processed to determine an angular component of a location from which a signal is reflected. In some examples, two microphones each receive a signal containing at least one reflection of a transmitted first sound signal. The two received signals are processed to determine a delay between the two received signals. For example, a cross-correlation may be determined between the two signals for a range of candidate delays, and the delay giving rise to a highest value of the cross-correlation is determined to be the delay between the two signals. Signals containing FMCWs are particularly suitable for such applications, as an FCMW can be arranged such that an autocorrelation of the FCMW (a cross correlation between the FCMW and a delayed copy of the FCMW) is uniquely maximized for a delay of zero. This is in contrast to signals having fixed frequencies, in which signal offsets corresponding to a multiple of a wavelength may give rise to further autocorrelation maxima. 
       FIG.  8    shows a top-down view in which a first sound signal transmitted by a playback device and containing a predetermined waveform is reflected from a person P in a playback environment. Two microphones  815   a  and  815   b , which have apertures separated by a distance d (as measured, in this example, between the centers of the apertures) each receive a respective sound signal containing a reflection of the transmitted sound signal by the person P. The first microphone  815   a  receives a sound signal corresponding to the reflected ray  802   a , and the second microphone  815   b  receives a sound signal corresponding to the reflected ray  802   b . A line BC passes through the center of the aperture of the second microphone  815   b , and is perpendicular to a line AB passing through the centers of the apertures of the microphones  815   a  and  815   b . An angle θ between the line BP (corresponding to the direction of the reflected ray  802   b ) and line BC is an angular component of the location of the person P from the playback device. In this example, the distance BP between the person and the second microphone  815   b  is much greater than the distance d between the microphones  815   a  and  815   b , and the angle θ is therefore approximately equal to an angular component of the location of the person P from any point on the line segment AC between the two microphones  815   a  and  815   b . In many examples, the distance between two microphones of a playback device is small (for example less than 10 mm or less than 5 mm), and therefore it is expected that the distance from the playback device to a listener will be much greater than the distance between the two microphones. 
     The difference in length between the line segment BP and the line segment AP is Δl, corresponding to the difference in path length between the rays  802   a  and  802   b . The difference Δl in path length is related to a delay Δt between the sound signals received by the two microphones  802   a  and  802   b  by the equation Δl=vΔt, where v is the speed of sound in air (approximately 340 m/s under STP conditions). The angular component θ of the location of the person P from the playback device is given by θ=arcsin(Δl/d)=arcsin(vΔt/d), where arcsin denotes the inverse of the sine function for angles θ within the interval −π/2&lt;θ&lt;π/2 (measured in radians). By processing the sound signals received by the microphones  815   a  and  815   b , for example by determining a cross-correlation to determine the delay Δt and then using the above equation to calculate θ, an angular component of the location P of the person may be determined. 
     In many situations, identifying a peak in signal amplitude as being caused by a direct reflection from a person is not straightforward, for example due to reflections from other objects in a reverberant environment. In such cases, range-finding techniques and/or delay analysis as described above may be less suitable for determining a location of a person with respect to a playback device. An alternative method of determining a location of a person involves storing location data or calibration data associating stored sound signal data received when a person is at known locations within the playback environment. An unknown location of a person is then determined to be the same as the known location associated with the stored signal, based on a correspondence between a received second sound signal and the stored sound signal. For example, a playback device may store location data associating each of a set of stored sound signals with a respective location of a person. Upon receiving a second sound signal containing one or more reflections of a transmitted first sound signal, a location of the person may be determined based on a best correspondence between the received second sound signal and one of the stored sound signals. A correspondence can be measured according to any suitable metric, and a location of a person may be determined only if a metric score for a received second sound signal is higher than a predetermined threshold value. 
     In some examples, configuration data is stored corresponding to a sound signal received when no person is present in the playback environment. If a best correspondence with a received second signal is determined to be with this configuration data, it is determined that no person is in the playback environment. This method avoids a location of a person being erroneously determined when no person is in the playback environment. Some examples may also apply the determination that a person is unlikely to be present in the playback environment to take appropriate action, such as ceasing or pausing audio reproduction by playback devices associated with the playback environment. 
       FIG.  9    is a flow diagram of a calibration method  900  in which a playback device (e.g., the playback device  410  of  FIGS.  4  and  5   ) is prepared for being used to determine a location of a person using stored location data. The playback device transmits, at S 910 , a third sound signal containing a predetermined waveform, when a person is at a known location relative to the playback device. In some examples, a user may specify a known location within a playback environment, for example using a control device (e.g., the control device  130   a  of  FIG.  1 H ) connected via a network to the playback device. In other examples, a user may be directed to position themselves in a predetermined known location, for example by providing an instruction to the user. Such an instruction may be audible, transmitted by a playback device or any other device in the playback system which is capable of audio reproduction. Such an instruction may be visual, for example using a display on the control device or other device in the playback system. Examples of known locations may include sitting on a particular chair, sitting in a particular position on a particular sofa, and standing in a doorway. Known locations may also be expressed in terms relative to a particular playback device or group of playback devices, such as “center” or “off-center”, “left” or “right”, and so on. It will be appreciated that if a person is determined to be sitting in the chair, the person may, in fact, be standing in front of the chair. In this way, each known location is associated with a region of the playback environment. 
     The playback device receives, at S 920 , a fourth sound signal containing at least one reflection of the third sound signal. Location data is stored, at S 930 , which includes data indicative of the fourth sound signal and further indicative of an association between the fourth sound signal and the known location of the person. In this example, the location data is stored in memory of the playback device itself. In some examples, location data may be sent to a remote computing system for storage, for example a computing system that is configured to process a second sound signal to determine a location of a person. In some examples, the location data may be sent to other devices on the playback network for storage, such as another playback device. 
     In the examples described above, a correspondence between two sound signals may be determined in any suitable manner. For example, a correspondence may be determined by comparing relative or absolute amplitudes and/or timings of a highest predetermined number of local peaks in the amplitude of the two sound signals. A metric score may then be determined in accordance with how closely matching the amplitudes and/or timings are, and/or the order in which the highest predetermined number of peaks occur in the signal. In another example, a correspondence may be determined by computing a correlation between the received second sound signal and each of the stored second sound signals. 
     In some examples in which location data is stored for determining a location of a person, a playback device may be moved between different positions or orientations within a playback environment, or between different playback environments (for example, between different rooms in a house). In such examples, the playback device may store location data for each of a set of orientations, where the location data stored for each sound signal is associated with an orientation of the playback device.  FIG.  10    shows an example of a hierarchical data structure for storing location data associated with different orientations of a playback device. In this example, location data is stored for eight different locations A-H of a person relative to the playback device. Locations A, B, and C are associated with a first orientation of the playback device. Locations D, E, and F are associated with a second orientation of the playback device, and locations G and H are associated with a third orientation of the playback device. In this example, orientation  1  and orientation  2  are different orientations of the playback device within the same room of a house (i.e. within the same playback environment). Orientation  3  is an orientation of the playback device within a different room of the house (i.e. within a different playback environment). Location data for each of the location includes signal data which is indicative of a received sound signal, and each set of stored signal data is associated with a respective location of a person relative to the playback device. 
     In an example in which location data is associated with an orientation of a playback device, for example as shown in  FIG.  10   , determining a location of a person relative to the playback device may include first determining an orientation of the playback device. 
     If, for example, the playback device is determined to be in orientation  1 , processing a received sound signal to determine the location of the person may include comparing the received sound signal with signal data  1 . 1 , signal data  1 . 2 , and signal data  1 . 3 , but not with signal data associated with orientation  2  or orientation  3 . This may increase the accuracy with which a location of a person may be located, because fewer sets of signal data are required to be compared with the received sound signal. If, for example, determining a correspondence between the received second sound signal and a stored candidate signal involves determining a best correspondence between the received second sound signal and each set of stored candidate signals, comparing the received signal with fewer stored candidate signals may result in fewer erroneous results. Furthermore, comparing with fewer candidate signals reduces the amount of processing that needs to be performed in order to determine the location of the person, resulting in faster determination of the location of the person. 
     Orientation can be determined in several ways. For example, an accelerometer within the device may be used to determine whether the device is positioned horizontally or vertically, a stored variable may be read to determine the orientation, or the orientation may be determined with reference to external devices, such as connection of the playback device to a particular docking station. In some examples, determining an orientation of a playback device may include receiving a user input, for example using a control device associated with the playback device. For example, a user may move the playback device from a known orientation in a kitchen (orientation  1 , for example) to a known orientation in a living room (orientation  2 , for example), and use the control device to inform the playback device (or a computing system performing processing on behalf of the playback device) that the playback device is now in orientation  2 . In other examples, determining an orientation of a playback device may include detecting a change in the orientation of the playback device from the received sound signal itself. For example, a playback device may transmit a sound signal containing a predetermined waveform, and receive a sound signal containing reflections of the transmitted sound signal. The playback device may determine that the received sound signal does not correspond to any of the stored signal data associated with orientation  1  (including signal data corresponding to orientation  1  when no person is present in the kitchen), and search for signal data associated with orientation  2  and orientation  3 . If a correspondence is determined between the received signal and signal data associated with orientation  2  (for example, signal data  2 . 2 ), the playback device is determined to be in orientation  2 . 
     In examples in which location data is associated with orientations of a playback device, calibration of the playback device may include a modification of the routine of  FIG.  9    in which the orientation of the playback device is determined before or after the third sound signal is transmitted, and storing the location data includes associating the fourth sound signal with the determined orientation of the playback device. The orientation of the playback device may be determined in any suitable manner, as discussed above. 
     Returning now to the method  600  of  FIG.  6   , a characteristic of audio reproduction by the playback device is selected at S 640 , based on the determined location of the person. As mentioned above, an example of a characteristic of audio reproduction is a volume of the audio reproduction. Other characteristics of audio reproduction may also be selected, for example those described in detail with reference to  FIG.  11    below. In the example of  FIG.  4   , the playback device  410  may determine that the listener  403  is located close to the playback device  410 , and accordingly may select a low volume for audio reproduction. At a later time, the playback device  410  may determine that the listener  403  is located further away from the playback device  410 , and may select a higher volume for audio reproduction. By periodically determining the location of the listener  413 , and adjusting the volume in this way, the apparent volume of the audio reproduction experienced by the listener  413  may be substantially constant as the listener moves throughout the playback environment. 
     More generally, a playback device having performed the method  600  of  FIG.  6    to determine a first location of a person, and selected a characteristic of audio production based on the determined first location, may at a later time transmit a third sound signal containing the predetermined waveform. The playback device may subsequently receive a fourth sound signal containing at least one reflection of the third sound signal. The playback device (or a computing system associated with the playback device) may process the fourth sound signal to determine a second location of a person, where the second location is different to the first location. The playback device may then adjust the characteristic of audio reproduction based on the determined second location of the person. In some examples, a playback device frequently transmits signals to determine a location of a person in this way, resulting in seemingly real-time adjustment of playback characteristics as a listener moves around a playback environment. A playback device may transmit signals for determining a location of a person at regular or irregular intervals, for example of at intervals greater than every 10 seconds, at intervals greater than 1 second, or at intervals greater than every 0.1 seconds. 
     In some examples, a playback device may perform audio reproduction from a media source simultaneously with transmitting a first sound signal containing a predetermined waveform. The playback device may subsequently receive a second sound signal containing at least one reflection of the first signal by a person, and process the received second signal to determine a location of the person relative to the playback device without interrupting the audio reproduction. In this way, characteristics of the audio reproduction may be selected and/or adjusted as the audio reproduction is performed, allowing the playback device to adapt to the location of the person. In such examples, the second sound signal received by the playback device may include reflections of audio being played back by the playback device. Such reflections will depend on the audio being played back, and therefore it is necessary to be able to distinguish reflections of the transmitted first sound signal from reflections of the audio being played back. This may be achieved, for example, by identifying the reflection of the predetermined waveform in the received second signal. In some examples, the first signal and the audio may be generated and transmitted simultaneously by the same transducer. In other examples, an additional transducer may be included for transmitting the first signal. 
     In some examples, a playback device may simultaneously perform audio reproduction and transmit an ultrasonic first sound signal containing a predetermined waveform for determining a location of a person. Processing the received second signal may then include passing the received second signal through a high-pass filter with a cutoff frequency just below the lowest frequency of the predetermined waveform (for example, around 20 kHz). This processing may substantially remove reflections of the audio being played back by the playback device, as the majority of the signal power in such audio is in the audible range below the lowest frequency of the predetermined waveform. Using ultrasonic signals to determine a location of a person may also cause little or no detrimental effect to the quality of audio reproduction, as the ultrasonic signal is substantially inaudible to listeners, or inaudible to a majority of listeners. 
     shows an example of a playback environment  110   l  occupied by a listener  1103 . The playback environment  110   l  contains four playback devices  1110   i ,  1110   j ,  1110   k ,  1110   h , collectively referred to as playback devices  1110 , that can be configured for home theater as described above with reference to  FIG.  1 K . In this example, the playback devices  1110  belong to the same playback zone. A first playback device  1110   h  comprises one or more microphones and is configured to determine a location of a person and to select a characteristic of audio reproduction in accordance with the methods described herein. The playback environment  110   l  also includes a sofa  1105 , a desk  1107 , a television  1109 , and a doorway  1111 . The playback devices  1110  are bonded, and are hence configured to synchronously perform audio reproduction from a media source, as described above with reference to  FIGS.  1 A- 1 M . In addition to determining a characteristic of audio production by the first playback device  1110   a , the determined location of a person, for example the listener  1103 , may be used to select a characteristic of audio production by one or more of the other playback devices  1110 . For example, the first playback device  1110   h  may determine, by processing reflected sound signals as described above, that the listener  1103  is located on the sofa  110   l , and configure the playback devices for home theater operation by selecting a particular audio channel or channels for audio reproduction by the first playback device  1110   h  The particular channel in this example may be a surround channel or a home cinema channel in which the first playback device  1110   h  is responsible for a particular part of the audio reproduction, and each of the other playback devices  1110   i ,  1110   j ,  1110   k  is responsible for a respective part of the audio reproduction. In this example, a respective surround channel is selected for each of the playback devices  1110   j ,  1110   k  and a Low Frequency Effects (LFE) channel is selected for the playback device  1110   i.    
     At a later time, after having been located at the sofa  1105 , the listener  1103  may walk to the desk  1107 . The first playback device determines, by processing reflected sound signals as described above, that the person is located at the desk  1107 . Accordingly, a different channel for audio reproduction by the first playback device may be selected. The different channel may be, for example, left and right stereo channels and the playback devices  1110  may be configured for stereo reproduction. Different channels may also be selected for audio playback by the other playback devices  1110 , for example left and right stereo channels may be selected for the playback devices  1110   j  or  1110   k  respectively. Alternatively, when the person is determined to be located at the desk  1107 , the playback devices  1110   j  and  1110   k  may be reconfigures so that they are not part of the playback zone and no audio reproduction is performed. It will be appreciated that the embodiment described with reference to  FIG.  11    is exemplary, and other arrangements of playback devices, and accordingly other configurations of audio channels are possible without departing from the scope of the invention. 
     A further example of a characteristic of audio reproduction that may be selected based on a determined location of a person is delay of audio reproduction by a first playback device relative to corresponding audio reproduction by another playback device bonded to the first playback device. In the embodiment of  FIG.  11   , the playback devices  1110   j ,  1110   k  may be configured to synchronously perform audio reproduction on respective surround sound channels. The listener  1103  may be determined to be located closer to the playback device  1110   j  than to the playback device  1110   k . The playback device may then select a respective delay of audio reproduction by the playback devices  1110   j  and  1110   k . By selecting a delay in this way, audio from different playback channels arrives at the listener  1103  substantially in phase, improving the listening experience for the listener  1103 . 
     In some examples, a bonding, pairing, or grouping of playback devices may be altered dynamically based on a determined location of a person. In the example of  FIG.  11   , if a person is determined to be located in or near the doorway  1111 , playback devices in the next room, which belong to a different playback zone to the playback devices  1110 , may be grouped with the playback devices  1110  so that the playback devices in the two neighboring rooms may perform audio reproduction synchronously. The person may therefore experience a substantially seamless transition as the person moves from the playback environment  1111  to the neighboring playback environment. Alternatively, bonding or pairing of playback devices within a single playback environment may be altered depending on a determined location of a person. In the example of  FIG.  11   , the playback devices  1110   h ,  1110   i ,  1110   j ,  1110   k , may only be mutually bonded to form a playback zone when a person is determined to be located on the sofa  1103 . Otherwise, the playback devices may each perform audio reproduction independently, or one or more subsets of the playback devices may form respective playback zones. 
     In some examples, a playback device arranged to determine a location of a person according to the methods described herein, may receive an audio signal including a spoken input, and may associate the spoken input with the determined location of the person. By associating a spoken input with a determined location of a person, the playback device (or a computing system processing the spoken input received by the playback device) may ascertain additional information relating to the spoken input. For example, the playback device may receive a spoken input from a person which may use a location-based context to interpret the input. This may be an explicit statement, an utterance such as “near me”, “where I am” or implicit in the command, such as an utterance to “turn down the volume” which may use a location to determine for which playback devices volume is turned down. Other examples are possible and in general a spoken input may be associated with a determined location of the person. The playback device may, for example, adjust a characteristic of audio reproduction in response to receiving the spoken input. Adjusting a characteristic of audio reproduction in response to receiving a spoken input may prevent unwanted adjustments of characteristics, for example if a person temporarily moves from the sofa  1105  of  FIG.  11    whilst the playback devices  1110  are configured in home cinema mode. Alternatively, the spoken input may include a voice command to control audio reproduction by the playback device. For example, a playback device may at a certain time be in an idle mode and not performing audio reproduction, and may receive an audio signal including a spoken input from a person saying, “turn the music on”. The playback device may associate the spoken input with a determined location of the person, and begin audio reproduction with characteristics selected based on the determined location of the person. In a further example, a playback device may receive an audio signal from a person saying, “turn the music down near me”. The playback device may reduce the volume of audio reproduction by one or more playback devices near the determined location of the person, but not reduce the volume of audio reproduction by playback devices not near the determined location of the person. 
     In some examples, a spoken input may include a voice command to control a further device, for example a further device other than a playback device. In response to receiving a spoken input, a playback device may send a control signal to the further device dependent on a determined location of a person. For example, a playback device arranged to determine a location of a person according to the methods described herein may be connected via a network to one or more further devices, such as a thermostat (e.g., NEST® thermostat), an illumination device (e.g., a PHILIPS HUE® lighting device), or a further media playback device (e.g., a Sonos® playback device). The playback device may receive an audio signal containing a spoken input including a voice command to control one of the further devices. 
     In one example, the listener  1103  in  FIG.  11    may initially be located on the sofa, watching a film on the television  1109 . The playback devices  1110  may be configured to perform audio reproduction in a home cinema mode, and lighting in the playback environment  110   l  may be dimmed. The listener  1103  may subsequently stop watching the film, and walk to the desk  1107  to do some work. The playback device  1110   a  may then determine that the listener  1103  is located at the desk  1107 . The playback device  1110   a  may receive a spoken input from the listener containing a voice command saying, “turn the lights on over here”. The playback device  1110   a  may associate the spoken input with the determined location of the person (at the desk  1107 ) and accordingly send a control signal to an illumination device controlling the lighting in the playback environment  110   l . The illumination device may, in response to receiving the control signal, turn on a light near the desk  1107 . The illumination device may further turn off the lights near the sofa, or alternatively may undim the lights near the sofa. 
     IV. Conclusion 
     The above discussions relating to playback devices, controller devices, playback zone configurations, and media content sources provide only some examples of operating environments within which functions and methods described below may be implemented. Other operating environments and configurations of media playback systems, playback devices, and network devices not explicitly described herein may also be applicable and suitable for implementation of the functions and methods. 
     The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. For example, multiple playback devices may be connected via a network within a smart home environment, each being configured to determine a location of a person within a respective portion of the environment, for example a particular room in the smart home, and to send control signals to devices within the respective portion. 
     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 ways) to implement such systems, methods, apparatus, and/or articles of manufacture. 
     Additionally, references herein to “embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one example embodiment of an invention. The appearances of this phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. As such, the embodiments described herein, explicitly and implicitly understood by one skilled in the art, can be combined with other embodiments. 
     The specification is presented largely in terms of illustrative environments, systems, procedures, steps, logic blocks, processing, and other symbolic representations that directly or indirectly resemble the operations of data processing devices coupled to networks. These process descriptions and representations are typically used by those skilled in the art to most effectively convey the substance of their work to others skilled in the art. Numerous specific details are set forth to provide a thorough understanding of the present disclosure. However, it is understood to those skilled in the art that certain embodiments of the present disclosure can be practiced without certain, specific details. In other instances, well known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the embodiments. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the foregoing 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.