Patent Publication Number: US-9906886-B2

Title: Audio filters based on configuration

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of and priority as a continuation under 35 U.S.C. § 120 to U.S. patent application Ser. No. 14/329,501, filed on Jul. 11, 2014, entitled “SYSTEMS, METHODS, AND APPARATUS TO FILTER AUDIO”, which is a continuation of U.S. patent application Ser. No. 13/333,595, filed on Dec. 21, 2011 (now U.S. Pat. No. 8,811,630), entitled “SYSTEMS, METHODS, AND APPARATUS TO FILTER AUDIO”, the contents each of which are herein incorporated by reference in their entireties for all purposes. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure is related to consumer electronics and, more particularly, to systems, methods, and apparatus to filter audio. 
     BACKGROUND 
     Audio filter circuits and digital audio filtering are used to achieve desired effects by modifying an audio signal prior to outputting the audio signal via a speaker. Example uses include attenuating certain frequencies of the signal to reduce or eliminate undesired background noises or amplifying frequencies of the signal to emphasize particular elements of the audio signal. Such audio filtering, or equalization (EQ), can affect how the audio is perceived by a listener. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, aspects, and advantages of the presently disclosed technology are better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  shows an illustration of an example system in which embodiments of the methods and apparatus disclosed herein can be implemented; 
         FIG. 2A  shows an illustration of an example zone player having a built-in amplifier and speakers; 
         FIG. 2B  shows an illustration of an example zone player having a built-in amplifier and connected to external speakers; 
         FIG. 2C  shows an illustration of an example zone player connected to an A/V receiver and speakers; 
         FIG. 3  shows an illustration of an example controller; 
         FIG. 4  shows an internal functional block diagram of an example zone player; 
         FIG. 5  shows an internal functional block diagram of an example controller; 
         FIG. 6  shows an example pair of zone players having mid-tweeter-mid (MTM) speaker arrangements, including audio filters, and configured as a stereo pair; 
         FIG. 7  shows a block diagram of an example audio filter to implement the first mid-range filter of  FIG. 6 ; 
         FIG. 8  shows a block diagram of an example audio filter to implement the second mid-range filter of  FIG. 6 ; 
         FIG. 9  shows a block diagram of an example audio filter to implement the tweeter filters of  FIG. 6 ; 
         FIG. 10  shows a simplified frequency response of a known MTM speaker arrangement; 
         FIG. 11  shows a simplified frequency response of an example apparatus using the example audio filters of  FIGS. 7-9 ; and 
         FIG. 12  is a flowchart representative of example method to implement the example apparatus and/or the example audio filters of  FIGS. 4 and 6-8 . 
         FIG. 13  shows a block diagram of an example audio filter to implement the first and second mid-range filters of  FIG. 6 . 
     
    
    
     In addition, the drawings are for the purpose of illustrating example embodiments, but it is understood that the present disclosure is not limited to the arrangements and instrumentality shown in the drawings. 
     DETAILED DESCRIPTION 
     I. Overview 
     Example systems, methods, and apparatus to filter audio are disclosed herein. In some examples, an audio filtering circuit such as a digital audio processor filters audio to a mid-tweeter-mid (MTM) speaker configuration to reduce or eliminate a narrowing phenomenon normally experienced using such a configuration. Example systems, methods, and/or apparatus disclosed herein also provide MTM speaker configurations that can be used in different orientations (e.g., vertical and horizontal), where the filtering applied to the audio signals may be different based on the orientation of the speakers. The example systems, methods, and/or apparatus disclosed herein may be used in combination with a multiple-speaker configuration in which the speakers included in the configuration share one or more sources of audio information and play the audio in synchrony. 
     Some example systems, methods, and/or apparatus provide first filtering to an audio signal to be output from a first mid-range speaker of an MTM configuration and provide second filtering to an audio signal to be output from a second mid-range speaker of the MTM configuration to reduce or eliminate the narrowing effect and generally improve (e.g., objectively and subjectively) the audio consistency between different physical locations. Accordingly, such example systems, methods, and/or apparatus provide MTM speaker configurations that are highly flexible in terms of types of uses by providing improved audio output quality in different orientations. This flexibility allows for potentially increased aesthetics and flexibility in placing the speakers for different uses. 
     An example embodiment of apparatus implemented in accordance with the present disclosure includes first and second audio speakers having first audio characteristics, a third audio speaker having second audio characteristics, wherein the third speaker is positioned between the first and second audio speakers, a first audio filter to process an audio input signal to have a first frequency response including a first cutoff frequency, the first audio filter to output a first audio output signal to the first audio speaker, and a second audio filter to process the audio input signal to have a second frequency response to compensate for interference between the first and second frequency responses caused by a position of the first audio speaker relative to the second audio speaker. 
     In some embodiments, the first, second, and third audio speakers include a mid-tweeter-mid speaker arrangement, the second audio filter to have a second cutoff frequency greater than the first cutoff frequency when the mid-tweeter-mid speaker arrangement is oriented horizontally. Some example apparatus include a digital audio processor to implement the first and second audio filters. 
     In some embodiments, the first audio filter includes a low-frequency shelf filter, a bandpass shelf filter, and a low-pass filter having the first cutoff frequency. In some embodiments, the second audio filter includes a low-frequency shelf filter, a bandpass shelf filter, a low-pass filter having the second cutoff frequency, an allpass filter, and high-frequency shelf filter. In some such embodiments, the allpass filter applies a −360 degree phase shift to the audio input signal or to a filtered signal based on the audio input signal. Some embodiments of an apparatus also include an orientation detector to determine whether the first, second, and third speakers are in a first orientation or a second orientation, where the second audio filter is to apply the allpass filter and high-frequency shelf filter when the first, second, and third speakers are in the first orientation and is to omit the allpass filter and the high-frequency shelf filter when the first, second, and third speakers are in the second orientation. 
     In some embodiments, the apparatus further includes a network interface to receive at least one of the input audio signal, a synchronization signal associated with a multiple-device arrangement, or a configuration signal associated with a multiple-device arrangement. In some such embodiments, the second speaker is to be positioned on the outside when in a multiple-device configuration. 
     Another example apparatus implemented in accordance with the present disclosure includes first and second audio speakers having first audio characteristics, a third audio speaker having second audio characteristics, wherein the first, second, and third speakers are arranged in a mid-tweeter-mid (MTM) configuration, and a digital audio processor to filter an audio signal to cause first audio output from the first audio speaker to have a dominant frequency response in a first frequency range relative to the second audio output from the second audio speaker. 
     In some embodiments, the digital audio filter is to filter the audio signal to cause the first audio output to have a −360 degree lag relative to the second audio output. In some embodiments, the first frequency range is between a cutoff frequency of the second audio output and a crossover frequency. 
     In some embodiments, the first frequency range is based on a spacing between the first and second audio speakers. In some embodiments, the digital audio filter is to cause the first audio output and the second audio output to have a substantially equal frequency response when the first, second, and third speakers are in a second orientation. 
     In some embodiments, the apparatus further includes a network interface to synchronize the first and second audio output with another audio device. In some such embodiments, the network interface is to receive at least one of the audio signal or a user input. 
     An example method implemented in accordance with the disclosure includes applying a first mid-range filter of a mid-tweeter-mid speaker arrangement to a first audio signal, applying a second mid-range filter different from the first mid-range filter to a second audio signal to compensate for interference caused by a position of a first mid-range speaker in the mid-tweeter-mid speaker arrangement relative to a second mid-range speaker in the mid-tweeter-mid speaker arrangement, and outputting a filtered audio signal from the second mid-range filter via the second mid-range speaker. 
     In some embodiments, the method further includes determining a configuration of the mid-tweeter-mid speaker arrangement in a multiple-device arrangement, wherein applying the second mid-range filter is based on the configuration. In some such embodiments, applying the second mid-range filter includes providing an output audio signal filtered using the second mid-range filter to an outermost mid-range speaker of the mid-tweeter-mid speaker arrangement in the multiple-device arrangement. 
     In some embodiments, a second frequency response of the second mid-range filter has a higher cutoff frequency than a first frequency response of the first mid-range filter. 
     Although the following discloses example systems, methods, and apparatus including, among other components, firmware and/or software executed on hardware, it should be noted that such systems, methods, and/or apparatus are merely illustrative and should not be considered as limiting. For example, it is contemplated that any or all of these firmware, hardware, and/or software components could be embodied exclusively in hardware, exclusively in software, exclusively in firmware, or in any combination of hardware, software, and/or firmware. Accordingly, while the following describes example systems, methods, and/or apparatus, the examples provided are not the only way(s) to implement such systems, methods, and/or apparatus. 
     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 medium such as a memory, digital versatile disk (DVD), compact disc (CD), Blu-ray, and so on, storing the software and/or firmware. 
     These embodiments and many additional embodiments are described more below. Further, the detailed description 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. 
     Reference 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 the 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. 
     II. Example Environment 
     Referring now to the drawings, in which like numerals can refer to like parts throughout the figures,  FIG. 1  shows an example system configuration  100  in which one or more of the method and/or apparatus disclosed herein can be practiced or implemented. By way of illustration, the system configuration  100  represents a home with multiple zones. Each zone, for example, represents a different room or space, such as an office, bathroom, bedroom, kitchen, dining room, family room, home theater room, utility or laundry room, and patio. While not shown here, a single zone can cover more than one room or space. One or more of zone players  102 - 124  are shown in each respective zone. A zone player  102 - 124 , also referred to as a playback device, multimedia unit, speaker, and so on, provides audio, video, and/or audiovisual output. A controller  130  (e.g., shown in the kitchen for purposes of illustration) provides control to the system configuration  100 . The system configuration  100  illustrates an example whole house audio system, though it is understood that the technology described herein is not limited to its particular place of application or to an expansive system like a whole house audio system  100  of  FIG. 1 . 
       FIGS. 2A, 2B, and 2C  show example illustrations of zone players  200 - 204 . The zone players  200 - 204  of  FIGS. 2A, 2B, and 2C , respectively, can correspond to any of the zone players  102 - 124  of  FIG. 1 . While certain embodiments provide multiple zone players, an audio output can be generated using only a single zone player.  FIG. 2A  illustrates a zone player  200  including sound producing equipment  208  capable of generating sound or an audio output corresponding to a signal received (e.g., wirelessly and/or via a wired interface). The sound producing equipment  208  of the zone player  200  of  FIG. 2A  includes a built-in amplifier (not shown in this illustration) and speakers (e.g., a tweeter and two mid-range speakers). In certain embodiments, the zone player  200  of  FIG. 2A  can be configured to play stereophonic audio or monaural audio. In some embodiments, the zone player  200  of  FIG. 2A  can be configured as a component in a combination of zone players to play stereophonic audio, monaural audio, and/or surround audio. As described in greater detail below, in some embodiments, the example zone player  200  of  FIG. 2A  can also transmit a second signal to, for example, other zone player(s) in the same or different zone(s), speaker(s), receiver(s), and so on. Transmission of the second signal can be part of, for example, a system in which multiple zone players, speakers, receivers, and so on, form a network to, for example, present media content in a synchronization or distributed manner. 
     The example zone player  202  of  FIG. 2B  includes a built-in amplifier (not shown in this illustration) to power a set of detached speakers  210 . The speakers  210  of  FIG. 2B  can include, for example, any type of loudspeaker. The zone player  202  of  FIG. 2B  can communicate a signal corresponding to audio content to the detached speakers  210  via wired and/or wireless channels. Instead of receiving and generating audio content as in  FIG. 2A , the zone player  202  of  FIG. 2B  receives the audio content and transmits the same (e.g., after processing the received signal) to the detached speakers  210 . Similar to the example zone player  200  of  FIG. 2A , in some embodiments the zone player  202  can transmit a second signal to, for example, other zone player(s) in the same or different zone(s), speaker(s), receiver(s), and so on. 
     The example zone player  204  of  FIG. 2C  does not include an amplifier, but allows a receiver  214 , or another audio and/or video type device with built-in amplification, to connect to a data network  128  of  FIG. 1  and to play audio received over the data network  128  via the receiver  214  and a set of detached speakers  216 . In addition to the wired couplings shown in  FIG. 2C , the detached speakers  216  can receive audio content via a wireless communication channel between the detached speakers  216  and, for example, the zone player  204  and/or the receiver  214 . In some embodiments the zone player  202  can transmit a second signal to, for example, other zone player(s) in the same or different zone(s), speaker(s), receiver(s), and so on. 
     Example zone players include a “Sonos Play:3,” “ZonePlayer® 120,” and “ZonePlayer® 90,” which are offered by Sonos, Inc. of Santa Barbara, Calif. Any other past, present, and/or future zone players can additionally or alternatively be used to implement the zone players of example embodiments disclosed herein. A zone player can also be referred to herein as a playback device, and a zone player is not limited to the particular examples illustrated in  FIGS. 2A, 2B, and 2C . For example, a zone player can include a wired or wireless headphone. In other examples, a zone player might include a subwoofer. In yet other examples, a zone player can include a sound bar. In an example, a zone player can include or interact with a docking station for an Apple iPod™ or similar device. In some embodiments, a zone player can relay one or more signals received from, for example, a first zone player to another playback device. In some embodiments, a zone player can receive a first signal and generate an output corresponding to the first signal and, simultaneously or separately, can receive a second signal and transmit or relay the second signal to another zone player(s), speaker(s), receiver(s), and so on. Thus, an example zone player described herein can act as a playback device and, at the same time, operate as a hub in a network of zone players. In such instances, media content corresponding to the first signal can be different from the media content corresponding to the second signal. 
       FIG. 3  shows an example illustration of a wireless controller  300  in a docking station  302 . The controller  300  can correspond to the controlling device  130  of  FIG. 1 . The controller  300  is provided with a touch screen  304  that allows a user to interact with the controller  300 , for example, to retrieve and navigate a playlist of audio items, control operations of one or more zone players, and provide overall control of the system configuration  100 . In some examples, the wireless controller  300  may be used to group zone players into stereo and/or other multiple-device configurations. In certain embodiments, any number of controllers can be used to control the system configuration  100 . In certain embodiments, there can be a limit on the number of controllers that can control the system configuration  100 . The controllers might be wireless like wireless controller  300  or wired to the data network  128 . Furthermore, an application running on any network-enabled portable devices, such as an iPhone™, iPad™, Android™ powered phone, or any other smart phone or network-enabled device can be used as a controller by connecting to the data network  128 . An application running on a laptop or desktop PC or Mac can also be used as a controller. Example controllers include a “Sonos® Controller  200 ,” “Sonos Controller for iPhone,” “Sonos® Controller for iPad,” “Sonos® Controller for Android, “Sonos® Controller for Mac or PC,” which are offered by Sonos, Inc. of Santa Barbara, Calif. The flexibility of such an application and its ability to be ported to a new type of portable device is advantageous. 
     Referring back to the system configuration  100  of  FIG. 1 , a particular zone can contain one or more zone players. For example, the family room of  FIG. 1  contains two zone players  106  and  108 , while the kitchen is shown with one zone player  102 . Zones can be dynamically configured by positioning a zone player in a room or space and assigning via the controller  130  the zone player to a new or existing zone. As such, zones can be created, combined with another zone, removed, and given a specific name (e.g., “Kitchen”), if so programmed. The zone players  102  to  124  are coupled directly or indirectly to a data network, such as the data network  128  shown in  FIG. 1 . The data network  128  is represented by an octagon in the figure to stand out from other components shown in the figure. While the data network  128  is shown in a single location, it is understood that such a network can be distributed in and around the system configuration  100 . 
     Particularly, the data network  128  can be a wired network, a wireless network, or a combination of both. In some embodiments, one or more of the zone players  102 - 124  are wirelessly coupled to the data network  128  based on a proprietary mesh network. In some embodiments, one or more of the zone players  102 - 124  are wirelessly coupled to the data network  128  using a non-mesh topology. In some embodiments, one or more of the zone players  102 - 124  are coupled via a wire to the data network  128  using Ethernet or similar technology. In addition to the one or more zone players  102 - 124  connecting to the data network  128 , the data network  128  can further allow access to a wide area network, such as the Internet. 
     In certain embodiments, the data network  128  can be created by connecting any of the zone players  102 - 124 , or some other connecting device, to a broadband router. Other zone players  102 - 124  can then be added wired or wirelessly to the data network  128 . For example, a zone player (e.g., any of zone players  102 - 124 ) can be added to the system configuration  100  by simply pressing a button on the zone player itself, which enables a connection to be made to the data network  128 . The broadband router can be connected to an Internet Service Provider (ISP), for example. The broadband router can be used to form another data network within the system configuration  100 , which can be used in other applications (e.g., web surfing). The data network  128  can also be used in other applications, if so programmed. Further, in certain embodiments, the data network  128  is the same network used for other applications in the household. 
     In certain embodiments, each zone can play from the same audio source as another zone or each zone can play from a different audio source. For example, someone can be grilling on the patio and listening to jazz music via zone player  124 , while someone is preparing food in the kitchen and listening to classical music via zone player  102 . Further, someone can be in the office listening to the same jazz music via zone player  110  that is playing on the patio via zone player  124 . In some embodiments, the jazz music played via zone players  110  and  124  is played in synchrony. Synchronizing playback amongst zones allows for someone to pass through zones while seamlessly listening to the audio. Further, zones can be put into a “party mode” such that all associated zones will play audio in synchrony. 
     In certain embodiments, a zone contains two or more zone players. For example, the family room contains two zone players  106  and  108 , and the home theater room contains at least zone players  116 ,  118 , and  120 . A zone can be configured to contain as many zone players as desired, and for example, the home theater room might contain additional zone players to play audio from a 5.1 channel or greater audio source (e.g., a movie encoded with 5.1 or greater audio channels). If a zone contains two or more zone players, such as the two zone players  106  and  108  in the family room, then the two zone players  106  and  108  can be configured to play the same audio source in synchrony, or the two zone players  106  and  108  can be paired to play two separate sounds in left and right channels, for example. In other words, the stereo effects of a sound can be reproduced or enhanced through the two zone players  106  and  108 , one for the left sound and the other for the right sound. In certain embodiments, paired zone players can play audio in synchrony with other zone players. 
     In certain embodiments, three or more zone players can be configured to play various channels of audio that is encoded with three channels or more sound. For example, the home theater room shows zone players  116 ,  118 , and  120 . If the sound is encoded as 2.1 channel audio, then the zone player  116  can be configured to play left channel audio, the zone player  118  can be configured to play right channel audio, and the zone player  120  can be configured to play bass frequencies. Other configurations are possible and depend on the number of zone players and the type of audio. Further, a particular zone can be configured to play a 5.1 channel audio in one instance, such as when playing audio from a movie, and then dynamically switch to play stereo, such as when playing audio from a two channel source. 
     In certain embodiments, two or more zone players can be sonically consolidated to form a single, consolidated zone player. A consolidated zone player (though made up of multiple, separate devices) can be configured to process and reproduce sound differently than an unconsolidated zone player or zone players that are paired, because a consolidated zone player will have additional speaker drivers from which sound can be passed. The consolidated zone player can further be paired with a single zone player or yet another consolidated zone player. Each playback device of a consolidated playback device is preferably set in a consolidated mode. 
     According to some embodiments, one can continue to do any of: group, consolidate, and pair zone players, for example, until a desired configuration is complete. The actions of grouping, consolidation, and pairing are preferably performed through a control interface, such as using controller  130 , and not by physically connecting and re-connecting speaker wire, for example, to individual, discrete speakers to create different configurations. As such, certain embodiments described herein provide a more flexible and dynamic platform through which sound reproduction can be offered to the end-user. 
     Sources of audio content to be played by zone players  102 - 124  are numerous. Music from a personal library stored on a computer or networked-attached storage (NAS) can be accessed via the data network  128  and played. Internet radio stations, shows, and podcasts can be accessed via the data network  128 . Music services that let a user stream and download music and audio content can be accessed via the data network  128 . Further, music can be obtained from traditional sources, such as a turntable or CD player, via a line-in connection to a zone player, for example. Audio content can also be accessed through AirPlay™ wireless technology by Apple, Inc., for example. Audio content received from one or more sources can be shared amongst the zone players  102  to  124  via the data network  128  and/or the controller  130 . The above-disclosed sources of audio content are referred to herein as network-based audio information sources. However, network-based audio information sources are not limited thereto. 
     III. Example Playback Device 
     Referring now to  FIG. 4 , there is shown an example functional block diagram of a zone player  400  in accordance with an embodiment. The zone player  400  of  FIG. 4  includes a network interface  402 , a processor  408 , a memory  410 , an audio processing component  412 , a module  414 , an audio amplifier  416 , and a speaker unit  418  coupled to the audio amplifier  416 .  FIG. 2A  shows an example illustration of such a zone player. Other types of zone players can not include the speaker unit  418  (e.g., such as shown in  FIG. 2B ) or the audio amplifier  416  (e.g., such as shown in  FIG. 2C ). Further, it is contemplated that the zone player  400  can be integrated into another component. For example, the zone player  400  could be constructed as part of a lamp for indoor or outdoor use. 
     Referring back to  FIG. 4 , the network interface  402  facilitates a data flow between zone players and other devices on a data network (e.g., the data network  128  of  FIG. 1 ) and the zone player  400 . In some embodiments, the network interface  402  can manage the assembling of an audio source or file into smaller packets that are to be transmitted over the data network or reassembles received packets into the original source or file. In some embodiments, the network interface  402  can further handle the address part of each packet so that it gets to the right destination or intercepts packets destined for the zone player  400 . Accordingly, in certain embodiments, each of the packets includes an Internet Protocol (IP)-based source address as well as an IP-based destination address. 
     In some embodiments, the network interface  402  can include one or both of a wireless interface  404  and a wired interface  406 . The wireless interface  404 , also referred to as an RF interface, provides network interface functions for the zone player  400  to wirelessly communicate with other devices (e.g., other zone player(s), speaker(s), receiver(s), component(s) associated with the data network  128 , and so on) in accordance with a communication protocol (e.g., any of the wireless standards IEEE 802.11a, 802.11b, 802.11g, 802.11n, or 802.15). To receive wireless signals and to provide the wireless signals to the wireless interface  404  and to transmit wireless signals, the zone player  400  of  FIG. 4  includes one or more antennas  420 . The wired interface  406  provides network interface functions for the zone player  400  to communicate over a wire with other devices in accordance with a communication protocol (e.g., IEEE 802.3). In some embodiments, a zone player includes both of the interfaces  404  and  406 . In some embodiments, a zone player  400  includes only the wireless interface  404  or the wired interface  406 . 
     In some embodiments, the processor  408  is a clock-driven electronic device that is configured to process input data according to instructions stored in memory  410 . The memory  410  is data storage that can be loaded with one or more software modules  414 , which can be executed by the processor  408  to achieve certain tasks. In the illustrated embodiment, the memory  410  is a tangible machine readable medium storing instructions that can be executed by the processor  408 . In some embodiments, a task might be for the zone player  400  to retrieve audio data from another zone player or a device on a network. In some embodiments, a task might be for the zone player  400  to send audio data to another zone player or device on a network. In some embodiments, a task might be for the zone player  400  to synchronize playback of audio with one or more additional zone players. In some embodiments, a task might be to pair the zone player  400  with one or more zone players to create a multi-channel audio environment. Additional or alternative tasks can be achieved via the one or more software modules  414  and the processor  408 . 
     The audio processing component  412  can include one or more digital-to-analog converters (DAC), an audio preprocessing component, an audio enhancement component or a digital signal processor, and so on. In certain embodiments, the audio that is retrieved via the network interface  402  is processed and/or intentionally altered by the audio processing component  412 . In some examples, the audio processing component  412  filters audio signals differently for different speakers  418 . Further, the audio processing component  412  can produce analog audio signals. The processed analog audio signals are then provided to the audio amplifier  416  for play back through speakers  418 . In addition, the audio processing component  412  can include necessary circuitry to process analog or digital signals as inputs to play from zone player  400 , send to another zone player on a network, or both play and send to another zone player on the network. An example input includes a line-in connection (e.g., an auto-detecting 3.5 mm audio line-in connection). 
     The audio amplifier  416  is a device that amplifies audio signals to a level for driving one or more speakers  418 . The one or more speakers  418  can include an individual transducer (e.g., a “driver”) or a complete speaker system that includes an enclosure including one or more drivers. A particular driver can be a subwoofer (for low frequencies), a mid-range driver (middle frequencies), and a tweeter (high frequencies), for example. An enclosure can be sealed or ported, for example. 
     A zone player  400  can also be referred to herein as a playback device. An example playback device includes a Sonos® Play:3, which is manufactured by Sonos, Inc. of Santa Barbara, Calif. The Play:3 is an example zone player with a built-in amplifier and speakers. In particular, the Play:3 is a three-driver speaker system that includes a tweeter and two mid-range speakers (also referred to as drivers). When playing audio content via the Play:3, the left audio data of a track is output from the left mid-range speaker, the right audio data of a track is output from the right mid-range driver, and the tweeter outputs center or both left and right audio data for a track. Audio from Internet radio stations, online music and video services, downloaded music, analog audio inputs, television, DVD, and so on, can be played from a Sonos® Play:3. While the Play:3 is an example of a zone player with speakers, it is understood that a zone player with speakers is not limited to one with a certain number of speakers (e.g., three speakers as in the Play:3), but rather can contain one or more speakers. Further, a zone player can be part of another device, which might even serve a purpose different than audio (e.g., a lamp). 
     IV. Example Controller 
     Referring now to  FIG. 5 , there is shown an example controller  500 , which can correspond to the controlling device  130  in  FIG. 1 . The controller  500  can be used to facilitate the control of multi-media applications, automation and others in a system. In particular, the controller  500  is configured to facilitate a selection of a plurality of audio sources available on the network and enable control of one or more zone players (e.g., the zone players  102 - 124  in  FIG. 1 ) through a wireless network interface  508 . According to one embodiment, the wireless communications is based on an industry standard (e.g., infrared, radio, wireless standards IEEE 802.11a, 802.11b 802.11g, 802.11n, or 802.15). Further, when a particular audio is being accessed via the controller  500  or being played via a zone player, a picture (e.g., album art) or any other data, associated with the audio source can be transmitted from a zone player or other electronic device to the controller  500  for display. 
     The controller  500  is provided with a screen  502  and an input interface  514  that allows a user to interact with the controller  500 , for example, to navigate a playlist of many multimedia items and to control operations of one or more zone players. The screen  502  on the controller  500  can be a liquid crystal display (LCD) screen, for example. The screen  500  communicates with and is commanded by a screen driver  504  that is controlled by a microcontroller (e.g., a processor)  506 . The memory  510  can be loaded with one or more application modules  512  that can be executed by the microcontroller  506  with or without a user input via the user interface  514  to achieve certain tasks. In some embodiments, an application module  512  is configured to facilitate grouping a number of selected zone players into a zone group and synchronizing the zone players for audio play back. In some embodiments, an application module  512  is configured to control the audio sounds (e.g., volume) of the zone players in a zone group. In operation, when the microcontroller  506  executes one or more of the application modules  512 , the screen driver  504  generates control signals to drive the screen  502  to display an application specific user interface accordingly. 
     The controller  500  includes a network interface  508  that facilitates wireless communication with a zone player. In some embodiments, the commands such as volume control and audio playback synchronization are sent via the network interface  508 . In some embodiments, a saved zone group configuration is transmitted between a zone player and a controller via the network interface  508 . The controller  500  can control one or more zone players, such as  102 - 124  of  FIG. 1 . There can be more than one controller for a particular system. Further, a controller can be integrated into a zone player. 
     It should be noted that other network-enabled devices such as an iPhone®, iPad® or any other smart phone or network-enabled device (e.g., a networked computer such as a PC or Mac®) can also be used as a controller to interact or control zone players in a particular environment. In some embodiments, a software application or upgrade can be downloaded onto a network enabled device to perform the functions described herein. 
     In some embodiments, a user can create a zone group including at least two zone players from the controller  500 . The zone players in the zone group can play audio in a synchronized fashion, such that all of the zone players in the zone group play back an identical audio source or a list of identical audio sources in a synchronized manner such that no (or substantially no) audible delays or hiccups could be heard. Similarly, in some embodiments, when a user increases the audio volume of the group from the controller  500 , the signals or data of increasing the audio volume for the group are sent to one of the zone players and causes other zone players in the group to be increased together in volume. 
     A user via the controller  500  can group zone players into a zone group by activating a “Link Zones” or “Add Zone” soft button, or de-grouping a zone group by activating an “Unlink Zones” or “Drop Zone” button. For example, one mechanism for ‘joining’ zone players together for audio play back is to link a number of zone players together to form a group. To link a number of zone players together, a user can manually link each zone player or room one after the other. For example, assume that there is a multi-zone system that includes the following zones: Bathroom, Bedroom, Den, Dining Room, Family Room, and Foyer. 
     In some embodiments, a user can link any number of the six zone players, for example, by starting with a single zone and then manually linking each zone to that zone. 
     In some embodiments, a set of zones can be dynamically linked together using a command to create a zone scene or theme (subsequent to first creating the zone scene). For instance, a “Morning” zone scene command can link the Bedroom, Office, and Kitchen zones together in one action. Without this single command, the user would need to manually and individually link each zone. The single command might include a mouse click, a double mouse click, a button press, a gesture, or some other programmed action. Other kinds of zone scenes can be programmed. 
     In some embodiments, a zone scene can be triggered based on time (e.g., an alarm clock function). For instance, a zone scene can be set to apply at 8:00 am. The system can link appropriate zones automatically, set specific music to play, and then stop the music after a defined duration. Although any particular zone can be triggered to an “On” or “Off” state based on time, for example, a zone scene enables any zone(s) linked to the scene to play a predefined audio (e.g., a favorable song, a predefined playlist) at a specific time and/or for a specific duration. If, for any reason, the scheduled music failed to be played (e.g., an empty playlist, no connection to a share, failed Universal Plug and Play (UPnP), no Internet connection for an Internet Radio station, and so on), a backup buzzer can be programmed to sound. The buzzer can include a sound file that is stored in a zone player, for example. 
       FIG. 6  shows an example pair of zone players  602 ,  604  having mid-tweeter-mid (MTM) speaker arrangements, including audio filters, and configured as a stereo pair. Either of the example zone players  602 ,  604  may be implemented by the zone player  400  of  FIG. 4 . In particular, each of the example zone players  602 ,  604  includes an audio processing circuit (e.g., the audio processing circuit  412  of  FIG. 4 ) to implement a set of audio filters, and a plurality of speakers (e.g., the speakers  418  of  FIG. 4 ) to implement a corresponding set of speakers or audio reproduction devices. 
     An MTM speaker arrangement includes two mid-range speakers  606 ,  608 ,  610 ,  612  (e.g., speakers having a diameter between about 3.5 inches and 6.75 inches, operating in the approximate frequency range of 300-5000 Hz) and a high-range speaker, also known as a tweeter  614 ,  615  (e.g., 3.5 inch diameter or smaller, operating in the approximate frequency range of 2,000-20,000 Hz) per zone player  602 ,  604 . The example zone players  602 ,  604  of  FIG. 6  may be oriented horizontally, where the centers of the mid-range speakers are substantially level in a horizontal direction, and/or vertically, where the centers of the midrange speakers are substantially aligned in a vertical direction. The example mid-range speakers  606 ,  608  are spaced such that the centers of the speakers  606 ,  608  are approximately one-half wavelength of a selected center frequency (e.g., λ0/2) apart. An example center frequency λ0 that may be used to determine the spacing of the example pairs of mid-range speakers  606 ,  608  and  610 ,  612  is 1,000 Hz, which has a wavelength of about 344 millimeters (e.g., at sea level at about 20 degrees Celsius). The example tweeter  614  is positioned between the example mid-range speakers  606 ,  608 . 
     The example zone players  602 ,  604  include respective audio filters  616 ,  618 . As mentioned above, the example audio filters  616 ,  618  may be implemented using digital audio processing circuitry, such as a digital audio processor or other digital processing unit. The following example will be described with reference to the example audio filter  616  of FIG.  6 . However, the description of the audio filter  616  is also applicable to the example audio filter  618 . The operation of the example zone players  602 ,  604  as a stereo pair with respect to the audio filters  616 ,  618  is also discussed below. The example zone players  602 ,  604  may also be operated in other multiple-device arrangements. As used herein “multiple-device” refers to separate sets of speakers, such as multiple-MTM speaker arrangements, and not merely multiple speakers in the same device. 
     The example audio filter  616  of  FIG. 6  is an active filter, which filters the received audio information prior to amplification. The example audio filter  616  includes an audio preprocessing block  620 , a first mid-range filter  622 , a second mid-range filter  624 , and a tweeter filter  626 . The example audio preprocessing block  620  may include, for example, audio decoding to decompress and/or otherwise convert an audio information (e.g., an audio file) from a storage format (e.g., compressed) to audio information in a playback format. The audio pre-processing block  620  provides the audio information to the example filters  622 - 626  for processing. 
     Some MTM speaker arrangements experience a “narrowing” phenomenon, in which a listener positioned straight in front of the speakers will experience the audio differently than a listener positioned to the left or the right of the speakers. This phenomenon is particularly acute for audio frequencies around the center frequency f 0  (e.g., the frequency on which the spacing of the mid-range speakers is based). In some cases, certain frequencies are completely canceled out in some positions relative to the speaker. While this behavior may be desirable in some circumstances, such as when the MTM speakers are oriented vertically (which reduces reflections and/or echoes off of the ceilings and floors), such behavior may be undesirable in others, such as when the MTM speakers are oriented horizontally (which results in a limited range of positions in which the frequency response is consistent and the audio sounds substantially as intended). Unlike such known MTM speaker arrangements, the example zone players  602 ,  604  of  FIG. 6  reduce or eliminate the narrowing phenomenon and increase the angular audibility range (e.g., the range of angles measured from straight in front of the speaker) in which the frequency response is consistent and the sounds from the speakers are heard substantially as intended. 
     In the example of  FIG. 6 , the audio filter  616  processes the audio based on the configuration of the zone player  602  in the stereo pair. In particular, the zone player  602  is set up as the left speaker (when viewing from the front). Additionally, the zone player  602  is configured with a horizontal orientation. Thus, the zone player  602  is aware that the speaker  606  is the left mid-range speaker and the speaker  608  is the right mid-range speaker for the left zone player  602  of the stereo pair (when viewing from the front). Based on this configuration information, the example audio filter  616  applies a first filtering configuration (e.g., the first mid-range filter  620 ) to the left (e.g., outer) mid-range speaker  606  and applies a second filtering configuration (e.g., the second mid-range filter  624 ) to the right (e.g., inner) mid-range speaker  608 . 
     The example audio filter  618  also includes audio preprocessing  628 , a first mid-range filter  630 , a second mid-range filter  632 , and a tweeter filter  634 . Like the audio filter  616 , the audio filter  618  applies the different filtering configurations to the example mid-range filters  610 ,  612  based on configuration information for the zone player  604  (e.g., physical orientation, status as right/left speaker of a stereo pair, etc.). In the example of  FIG. 6 , the audio filter  618  applies the first mid-range filter  630  to the right (e.g., outer) mid-range speaker  612  and applies the second mid-range filter  632  to the left (e.g., inner) mid-range speaker  610 . The example audio filters  616 ,  618  result in the zone players  602 ,  604  steering audio and expanding the angular field of audibility relative to known MTM speakers. 
       FIG. 7  shows a block diagram of an example audio filter  700  to implement the first mid-range filters  622 ,  630  of  FIG. 6 . The example audio filter  700  of  FIG. 7  receives an audio signal and outputs a filtered audio signal to a mid-range speaker (e.g., an outer mid-range speaker, the mid-range speakers  606 ,  612  of  FIG. 6 ). 
     On receiving the input audio signal, a second-order low-frequency (LF) shelf filter  702  is applied. The example second-order LF shelf filter  702  has a cutoff frequency of 100 Hz, a quality factor (Q) of 0.707, and a gain of 4.5 dB, and increases the gain of the audio signal for frequencies below the cutoff frequency. The Q of a filter refers to the damping and/or resonance of a filter. The resulting signal  703  is then applied to a second-order bandpass shelf filter  704 , which has a center frequency of 1.9 kHz, a Q of 1.2, and a gain of −5.0 dB. The bandpass shelf filter  704  attenuates a band of frequencies centered on 1.9 kHz. The signal  705  from the bandpass shelf filter  704  is input to a fourth-order low-pass filter  706 , which has a cutoff frequency of 3.0 kHz and a Q of 0.5. The example low-pass filter  706  attenuates the signal for frequencies higher than 3.0 kHz. The signal  707  from the low-pass filter  706  is input to a second second-order bandpass shelf filter  708 . The second bandpass shelf filter  708  has a center frequency 12.0 kHz, a Q of 2.0, and a gain of −5.0 dB, and therefore attenuates frequencies around the frequency 12.0 kHz. 
     The resulting signal  709  is input to a second-order allpass filter  710 , which has a center frequency of 1.0 kHz and a Q of 0.707. The example allpass filter  710  does not substantially amplify or attenuate the signal, but applies a −360 degree phase shift to the signal  709  to generate a lagging signal  711 . The phase shift applied by the allpass filter  710  may change for frequencies above and below the center frequency of 1.0 kHz. The lagging signal  711  is applied to a second-order high-frequency (HF) shelf filter  712 , which has a cutoff frequency of 800 Hz, a Q of 0.707, and a gain of 8.0 dB. As explained below, the example HF shelf filter  712  provides amplification to the output of the example filter  700  to create a higher crossover frequency for the first mid-range filter  622  than for the second mid-range filter  624  of  FIG. 6 . 
     The example signal  713  from the HF shelf filter  712  is output to a dynamic bass boost filter  714  and a limiter  716 . The example filter  700  outputs the filtered signal  717  to an amplifier (e.g., the amplifier  416 ) and/or to a mid-range speaker (e.g., the mid-range speaker  606 ). 
     While the example filter blocks  702 - 716  are shown in the order illustrated in  FIG. 7 , the blocks  702 - 716  may be applied in different ways to achieve identical, similar, or different filtering. Additionally, the example center and/or cutoff frequencies, Q values, and/or gains applied by the example filter blocks  702 - 716  may be modified to change a frequency response of the example audio filter  700 . 
     In some examples, the second-order bandpass shelf filter  708 , the second-order allpass filter  710 , and/or the second-order HF shelf  712  blocks are omitted or skipped when a zone player  602 ,  604  is in a vertical orientation. In these examples, the audio filter  700  temporarily operates similar to the example second mid-range filters  624 ,  632  of  FIG. 6  and the MTM narrowing phenomenon may be observed from the zone player  602 ,  604 . In some examples, however, the mid-range filters  622 ,  624 ,  630 ,  632  are implemented using a different filter such as the example filter  1300  described below with reference to  FIG. 13 . The invocation of the example second-order bandpass shelf filter  708 , the second-order allpass filter  710 , and/or the second-order HF shelf  712  may be based on the configuration information (e.g., orientation, participation and/or position in a stereo pair, etc.) for the example zone player  602 ,  604 . 
       FIG. 8  shows a block diagram of an example audio filter  800  to implement the second mid-range filters  624 ,  632  of  FIG. 6 . The example audio filter  800  of  FIG. 8  receives an audio signal and outputs a filtered audio signal to an amplifier and/or to a mid-range speaker (e.g., the mid-range speakers  608 ,  614  of  FIG. 6 ). 
     On receiving the input audio signal (e.g., the same audio signal received by the example filter  700  of  FIG. 7  when the filters  700 ,  800  are implemented by the same zone player  602 ,  604 ), a second-order LF shelf filter  802  is applied. The example second-order LF shelf filter  702  has a cutoff frequency of 100 Hz, a Q of 0.707, and a gain of 4.5 dB, and increases the gain of the audio signal for frequencies below the cutoff frequency. Thus, the example LF shelf filter  802  performs substantially the same filtering as the LF shelf filter  702  of  FIG. 7 . In some examples, the LF shelf filters  702  and  802  may be combined, in which case the resulting signal is provided to the different first and second mid-range filters (e.g., the first and second mid-range filters  622 ,  624  of  FIG. 6 ) for further processing. 
     Continuing with the example of  FIG. 8 , the signal  803  from the LF shelf filter  802  is then applied to a second-order bandpass shelf filter  804 , which has a center frequency of 1.9 kHz, a Q of 1.2, and a gain of −5.0 dB. The bandpass shelf filter  804  attenuates a band of frequencies centered on 1.9 kHz. Thus, the example bandpass shelf filter  804  performs substantially the same filtering as the bandpass shelf filter  704  of  FIG. 7 . In some examples, the bandpass shelf filters  704  and  804  may be combined (e.g., additionally or alternatively to combining the LF shelf filters  702 ,  802 ), in which case the resulting signal from the combined bandpass shelf filter is provided to the different first and second mid-range filters (e.g., the first and second mid-range filters  622 ,  624  of  FIG. 6 ) for further processing. 
     Continuing with the example of  FIG. 8 , the signal  805  from the bandpass shelf filter  804  is input to a fourth-order low-pass filter  806 , which has a cutoff frequency of 1000 Hz and a Q of 0.5. The example low-pass filter  806  attenuates the signal for frequencies higher than 1000 Hz. The example signal  809  from the low-pass filter  806  is output to a dynamic bass boost filter  808  and a limiter  810 . The example filter  800  outputs the filtered signal  811  to an amplifier (e.g., the amplifier  416 ) and/or to a mid-range speaker (e.g., the mid-range speaker  608 ). 
       FIG. 9  shows a block diagram of an example audio filter  900  to implement the tweeter filters  626 ,  634  of  FIG. 6 . The example audio filter  900  of  FIG. 9  receives an audio signal and outputs a filtered audio signal to a tweeter (e.g., the tweeters  614 ,  615  of  FIG. 6 ). 
     On receiving the input audio signal (e.g., the same audio signal received by the example filters  700 ,  800  of  FIGS. 7 and 8 ), a fourth-order high-pass filter  902  is applied. The example high-pass filter  902  has a cutoff frequency of 3.0 kHz and a Q of 0.5, and attenuates the input audio signal for frequencies lower than 3.0 kHz. The signal  903  output from the high-pass filter  902  is input to a first second-order bandpass shelf filter  904 , which has a center frequency of 5.0 kHz, a Q of 2.0, and a gain of −5.0 dB. The first bandpass shelf filter  904  attenuates a band of frequencies centered on 5.0 kHz. The resulting signal  905  is input to a second second-order bandpass shelf filter  906 . The example second bandpass shelf filter  906  has a center frequency of 8.1 kHz, a Q of 1.0, and a gain of 4.0 dB. Thus, the example bandpass shelf filter  906  amplifies a band of frequencies centered on 8.1 kHz. 
     The example signal  907  from the low-pass filter  906  is output to a dynamic bass boost filter  908 , a 1-sample delay  910 , a signal amplifier (gain)  912  and a limiter  914  The example filter  900  outputs the filtered signal  917  to an amplifier (e.g., the amplifier  416 ) and/or to a mid-range speaker (e.g., the tweeter  614  of  FIG. 6 ). 
     Any or all of the example filters  700 ,  800 ,  900  of  FIGS. 7-9  may be implemented using one or more digital audio processors, discrete digital and/or audio components, and/or any other type of logic circuit and/or analog filters. 
       FIG. 10  shows a simplified frequency response  1000  of a known MTM speaker arrangement. The frequency response  1000  of  FIG. 10  includes a mid-range response  1002  (representative of the mid-range speakers) and a high-range frequency response  1004  (representative of the tweeter). At a crossover frequency f x , the mid-range response  1002  and the high-range response  1004  are substantially equal. The mid-range response  1002  dominates in audio frequencies below the crossover frequency f x  and the high-range response  1004  dominates in audio frequencies above the crossover frequency f x . As a result, the λ 0 /2 distance between the mid-range speakers causes destructive interference to occur for frequencies around the center frequency f 0  outside of an angular audibility range. The destructive interference causes substantial changes in the objective and subjective perceptions of the audio between different physical locations inside and outside of the angular audibility range. 
       FIG. 11  shows a simplified frequency response  1100  of the example apparatus (e.g., the zone player  602 ,  604  of  FIG. 6 ) using the example audio filters of  FIGS. 7-9 . The example zone player  602 ,  604  of  FIG. 6  reduces or avoids the destructive interference experienced by known MTM speaker arrangements, thereby increasing the angular audibility range and decreasing distortion of the audio between different physical locations. 
     The example frequency response  1100  includes a first mid-range frequency response  1102 , a second mid-range frequency response  1104 , and a high-range frequency response  1106 . The example first mid-range frequency response  1102  corresponds to the mid-range speaker(s) coupled to the first mid-range filters of  FIG. 6  (e.g., the mid-range speakers  606 ,  612 ). The example second mid-range frequency response  1104  corresponds to the mid-range speaker(s) coupled to the second mid-range filters of  FIG. 6  (e.g., the midrange speakers  608 ,  610 ). The example high-range frequency response  1106  corresponds to the tweeters  614 ,  615 . 
     As illustrated in  FIG. 11 , the mid-range frequency responses  1102 ,  1104  have different crossover frequencies. The cutoff frequency f c2  for the example second mid-range response  1104  is lower than the center frequency f 0  on which the mid-range speaker spacing is based. Therefore, at frequencies around the center frequency f 0 , the first mid-range speakers  606 ,  612  has the dominant response and the destructive interference is substantially reduced. As used herein, a dominant response refers to a first frequency response being substantially stronger (e.g., 3 dB greater or more) than a second frequency response. As a result, the frequencies around the center frequency f 0  have reduced or eliminated interference outside of the angular audibility range. The combination of responses  1102 ,  1104  and the response  1106  have a crossover frequency of f x1 , which in some examples is higher than the crossover frequency f x  of known MTM crossover frequencies. 
     The example frequency response  1100  of  FIG. 11  is achieved by filtering the signal to the first mid-range speakers  606 ,  612  using the fourth-order low-pass filter  706 , the allpass filter (e.g., allpass filter block  710  of  FIG. 7 ) and the HF shelf filter (e.g., the second-order HF shelf filter  712 ). The example low-pass filter  706  has an increased cutoff frequency relative to known MTM crossover filters and relative to the low-pass filter  806  used in the example second mid-range filter  800 . As a result, the attenuation of the 1000 Hz-3.0 kHz frequency range is reduced or avoided by the signal to the first mid-range speakers  606 ,  612  and applied to the signal to the second mid-range speakers  608 ,  610 . 
     The example HF shelf filter  712  of  FIG. 7  provides additional gain for the first mid-range speakers  606 ,  612  above the cutoff frequency f c2  of the response. Because the second mid-range speakers  608 ,  610  are substantially attenuated above the cutoff frequency f c2  (e.g., 1000 Hz-3.0 kHz), the first mid-range filter  700  includes the example HF shelf filter  712  to provide a more consistent response for the frequencies provided by the mid-range speakers  606 ,  612 . In contrast, if the HF shelf filter  712  was not present (or not used), the frequencies above the cutoff frequency f c2  would sound substantially attenuated in every physical location. 
     The use of allpass filter in a first signal and omission in a second signal causes the example zone players  602 ,  604  to steer the audio outward. As used here, outward refers to steering the audio from the left zone player  602  toward the left and steering the audio from the right zone player  604  toward the right. Steering the audio further decreases the narrowing effect compared to known MTM speaker arrangements. 
       FIG. 12  is a flowchart representative of example method  1200  to implement the example zone players and/or the example audio filters of  FIGS. 4 and 6-8 . The example method  1200  may be used to provide an MTM speaker arrangement having improved audio characteristics in a horizontal orientation. The example method  1200  may be performed by a logic circuit such as a microprocessor, a digital audio processor, and/or any other type of logic circuit. 
     The example method  1200  begins by determining (e.g., via an orientation detector, the accelerometer  422  of  FIG. 4 ) an orientation of an MTM speaker arrangement (e.g., the zone players  400 ,  602 ,  604  of  FIGS. 4 and 6 ) (block  1202 ). For example, the accelerometer  422  determines whether the zone player  400 ,  602 ,  604  is in a vertical arrangement or a horizontal arrangement. The example method  1200  also determines (e.g., via the network interface  402 , the processor  408 , and/or the memory  410  of  FIG. 4 ) a configuration of the zone player  400 ,  602 ,  604  in a multiple-device arrangement (block  1204 ). For example, the processor  408  may determine whether the zone player  400 ,  602 ,  604  is the left speaker or the right speaker of a stereo speaker arrangement. 
     The example method  1200  receives an audio signal to be played (block  1206 ). The example audio signal may be compressed and/or encoded audio from an audio source, or may be an uncompressed and/or decoded audio signal. The example method  1200  splits into two portions that may operate substantially simultaneously (e.g., via a multi-threading and/or multi-core processor) or sequentially. In a first branch, the zone player  400 ,  602 ,  604  determines whether it is in a horizontal orientation (block  1208 ). If the zone player  400 ,  602 ,  604  is in a horizontal orientation (block  1208 ), a first audio filter (e.g., the example audio processing circuit  412  of  FIG. 4 , the example audio filters  616 ,  618  via the first mid-range filter  622 ,  630 ,  700  of  FIGS. 6 and 7 ) is applied to the first audio signal (block  1210 ). For example, the audio processing circuit  412  may apply the first mid-range filter  700  of  FIG. 7  to the second audio signal. An example second audio filter (e.g., the example audio processing circuit  412  of  FIG. 4 , the example audio filters  616 ,  618  via the second mid-range filter  624 ,  632 ,  800  of  FIGS. 6 and 8 ) is applied to the second audio signal (block  1212 ). For example, the audio processing circuit  412  may apply the second mid-range filter  800  of  FIG. 8  to the second audio signal. 
     After applying the appropriate filters to the first and second audio signals (blocks  1208 - 1214 ), the example method  1200  applies a tweeter filter (e.g., the tweeter filter  626 ,  634  of  FIG. 6 ) to a third audio signal (block  1216 ). Based on the configuration (e.g., left/right speaker in a multiple-device arrangement) and/or the orientation (e.g., horizontal or nonhorizontal orientation), the example method  1200  outputs the first filtered audio signal to the first mid-range speaker (e.g., the first mid-range speakers  606 ,  612  of  FIG. 6 ), outputs the second filtered audio signal to the second mid-range speaker (e.g., the first mid-range speakers  608 ,  610 ), and outputs the third filtered audio signal to the tweeter speaker (e.g., the tweeter speakers  614 ,  615 ) (block  1218 ). Thus, when the example zone player  400 ,  602 ,  604  is in a horizontal orientation, the first and second mid-range speakers output audio signals that are filtered differently. For example, the frequency response of the first and second midrange speakers may be similar to the simplified frequency response illustrated in  FIG. 11 . 
     After applying the appropriate filters to the first and second audio signals (blocks  1208 - 1214 ), the example method  1200  applies a tweeter filter (e.g., the tweeter filter  626 ,  634  of  FIG. 6 ) to a third audio signal (block  1216 ). Based on the configuration (e.g., left/right speaker in a multiple-device arrangement) and/or the orientation (e.g., horizontal or nonhorizontal orientation), the example method  1200  outputs the first filtered audio signal to the first mid-range speaker (e.g., the first mid-range speakers  606 ,  612  of  FIG. 6 ), outputs the second filtered audio signal to the second mid-range speaker (e.g., the first mid-range speakers  608 ,  610 ), and outputs the third filtered audio signal to the tweeter speaker (e.g., the tweeter speakers  614 ,  616 ) (block  1218 ). Thus, when the example zone player  400 ,  602 ,  604  is in a horizontal orientation, the first and second mid-range speakers output audio signals that are filtered differently. For example, the frequency response of the first and second mid-range speakers may be similar to the simplified frequency response illustrated in  FIG. 11 . 
     The example method  1200  may then end and/or iterate to output additional audio signals. The example method  1200  may monitor the orientation and/or the configuration of the example zone player  400 ,  602 ,  604  to change the filtering as appropriate. 
       FIG. 13  shows a block diagram of an example audio filter  1300  to implement the first and second mid-range filters  622 ,  624 ,  630 ,  632  of  FIG. 6 . The example audio filter  1300  of  FIG. 13  receives an audio signal and outputs filtered audio signals to the mid-range speakers (e.g., an outer mid-range speaker, the mid-range speakers  606 ,  608 ,  610 ,  612  of  FIG. 6 ). In particular, the example  FIG. 13  may be used to filter audio signals when the MTM speakers  602 ,  604  of  FIG. 6  are in a vertical orientation. 
     On receiving the input audio signal, a second-order low-frequency (LF) shelf filter  1302  is applied. The example second-order LF shelf filter  1302  has a cutoff frequency of 100 Hz, a quality factor (Q) of 0.707, and a gain of 4.5 dB, and increases the gain of the audio signal for frequencies below the cutoff frequency. The Q of a filter refers to the damping and/or resonance of a filter. The resulting signal  1303  is then applied to a second-order bandpass shelf filter  1304 , which has a center frequency of 1.9 kHz, a Q of 1.2, and a gain of −5.0 dB. The bandpass shelf filter  1304  attenuates a band of frequencies centered on 1.9 kHz. The signal  1305  from the bandpass shelf filter  1304  is input to a fourth-order low-pass filter  706 , which has a cutoff frequency of 3.0 kHz and a Q of 0.5. The example low-pass filter  1306  attenuates the signal for frequencies higher than 3.0 kHz. The signal  1307  from the low-pass filter  1306  is input from the low-pass filter  1306  to a dynamic bass boost filter  1308  and a limiter  1310 . The example filter  1300  outputs the filtered signal  1311  to an amplifier (e.g., the amplifier  416 ) and/or to a mid-range speaker (e.g., the mid-range speaker  606 ). 
     While the example filter blocks  1302 - 1310  are shown in the order illustrated in  FIG. 13 , the blocks  1302 - 1310  may be applied in different ways to achieve identical, similar, or different filtering. Additionally, the example center and/or cutoff frequencies, Q values, and/or gains applied by the example filter blocks  1302 - 1310  may be modified to change a frequency response of the example audio filter  1300 . 
     In view of the foregoing, it should be apparent that disclosed example systems, methods, and apparatus can be used to provide an MTM speaker arrangement having improved audio quality and perception. Example systems, methods, and apparatus apply different filtering to audio signals for output by different mid-range speakers based on the orientation of the MTM speaker arrangement and/or the configuration of the speakers in a multiple-device arrangement. 
     Various inventions have been described in sufficient detail with a certain degree of particularity. It is understood to those skilled in the art that the present disclosure of embodiments has been made by way of examples only and that numerous changes in the arrangement and combination of parts can be resorted without departing from the spirit and scope of the present disclosure as claimed. While the embodiments discussed herein can appear to include some limitations as to the presentation of the information units, in terms of the format and arrangement, the embodiments have applicability well beyond such embodiment, which can be appreciated by those skilled in the art. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the forgoing description of embodiments.