Patent Publication Number: US-9900723-B1

Title: Multi-channel loudspeaker matching using variable directivity

Description:
RELATED MATTERS 
     This application claims the benefit of the earlier filing date of U.S. provisional application No. 62/004,111, filed May 28, 2014. 
    
    
     FIELD 
     An audio device adjusts beam patterns used by two or more loudspeakers in an audio system to achieve a preferred direct-to-reverberant ratio of sound produced by each loudspeaker at a listening position. Accordingly, each loudspeaker may be assigned a beam pattern that achieves the preferred direct-to-reverberant ratio at the listening position to maintain a consistency for sound in the system. Other embodiments are also described. 
     BACKGROUND 
     The optimal reproduction of multichannel audio content (e.g., stereo audio, 5.1 channel audio, 7.1 channel audio) imposes restrictions on loudspeaker placement relative to a listening position. For instance, some audio systems recommend preferred angles and distances between loudspeakers to achieve optimal performance. These measures ensure that the spatial imaging produced by loudspeakers is in line with the intent during a mixing phase. 
     However, in a practical situation it is not always possible (e.g., room layout constraints) or desired (e.g., aesthetical preferences) to place loudspeakers at their recommended distances and angles. To compensate for non-ideal placement, some surround sound receivers implement a gain and delay compensation technique. This technique aims at ensuring that the sounds from all loudspeakers reach a listening position at the same time and level. More advanced systems also offer the possibility to compensate for timbral differences between loudspeakers by including an equalization system. However, even when time, level and spectrum are equal at a listening position, some audible differences remain, which are the result of inconsistent direct-to-reverberant ratios from sound produced by each loudspeaker. 
     The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. 
     SUMMARY 
     An audio system is disclosed that includes an audio source and two or more speaker arrays. The speaker arrays may be configured to generate one or more different beam patterns. For example, the speaker arrays may be capable of producing omnidirectional, cardioid, second order, and fourth order beam patterns based on signals received from the audio source. Each of the beam patterns generated by the speaker arrays may generate separate direct-to-reverberant ratios at the location of a listener. The direct-to-reverberant ratio may be defined as the ratio of sound energy received directly from a speaker array (e.g., sound energy received at the location of the listener without reflection) to sound energy received indirectly from the speaker array (e.g., sound energy received at the location of the listener after reflection in a listening area). The direct-to-reverberant ratio may be dependent on several factors, including the directivity index of a beam pattern, the distance between a speaker array and the listener, room size and absorption. 
     In one embodiment, the audio system may determine a preferred direct-to-reverberant ratio. This preferred direct-to-reverberant ratio may be used by two or more speaker arrays in the audio system to produce sound for a listener. For example, the audio system may select beam patterns for each of the speaker arrays based on the distance between each speaker array and the listener. These beam patterns may be selected such that the direct-to-reverberant ratio at the location of a listener for sound produced by each of the speaker arrays is equal or within a predefined threshold to the preferred direct-to-reverberant ratio. By matching direct-to-reverberant ratios for sound produced by multiple speaker arrays, the audio system described herein ensures a more consistent listening experience for the listener. 
     The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment of the invention in this disclosure are not necessarily to the same embodiment, and they mean at least one. 
         FIG. 1A  shows a view of an audio system with two speaker arrays according to one embodiment. 
         FIG. 1B  shows a view of an audio system with four speaker arrays according to one embodiment. 
         FIG. 2A  shows a component diagram of an example audio source according to one embodiment. 
         FIG. 2B  shows a component diagram of a speaker array according to one embodiment. 
         FIG. 3A  shows a side view of one speaker array according to one embodiment. 
         FIG. 3B  shows an overhead, cutaway view of a speaker array according to one embodiment. 
         FIG. 4  shows a set of beam patterns that may be produced by the speaker arrays according to one embodiment. 
         FIG. 5  shows a method for driving one or more speaker arrays to generate sound with similar or identical direct-to-reverberant ratios at the location of the listener according to one embodiment. 
         FIG. 6  shows sound produced by multiple speaker arrays sensed by a listening device according to one embodiment. 
         FIG. 7  shows a chart of direct-to-reverberant ratios for a set of beam pattern types in relation to distances between the speaker arrays and a listener according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Several embodiments are described with reference to the appended drawings are now explained. While numerous details are set forth, it is understood that some embodiments of the invention may be practiced without these details. In other instances, well-known circuits, structures, and techniques have not been shown in detail so as not to obscure the understanding of this description. 
       FIG. 1A  shows a view of an audio system  100  within a listening area  101 . The audio system  100  may include an audio source  103  and a set of speaker arrays  105 . The audio source  103  may be coupled to the speaker arrays  105  to drive individual transducers  109  in the speaker array  105  to emit various sound beam patterns for the listener  107 . In one embodiment, the speaker arrays  105  may be configured to generate audio beam patterns that represent individual channels for one or more pieces of sound program content. Playback of these pieces of sound program content may be aimed at the listener  107  within the listening area  101 . For example, the speaker arrays  105  may generate and direct beam patterns that represent front left, front right, and front center channels for a first piece of sound program content to the listener  107 . In one embodiment, the audio source  103  and/or the speaker arrays  105  may be driven to maintain a similar or identical direct-to-reverberant ratio for sound produced by each of the speaker arrays  105  at the location of the listener  107 . The techniques for driving these speaker arrays  105  to maintain this similar/identical direct-to-reverberant ratio will be described in greater detail below. 
     As shown in  FIG. 1A , the listening area  101  is a room or another enclosed space. For example, the listening area  101  may be a room in a house, a theatre, etc. In each embodiment, the speaker arrays  105  may be placed in the listening area  101  to produce sound that will be perceived by the listener  107 . 
       FIG. 2A  shows a component diagram of an example audio source  103  according to one embodiment. As shown in  FIG. 1A , the audio source  103  is a television; however, the audio source  103  may be any electronic device that is capable of transmitting audio content to the speaker arrays  105  such that the speaker arrays  105  may output sound into the listening area  101 . For example, in other embodiments the audio source  103  may be a desktop computer, a laptop computer, a tablet computer, a home theater receiver, a set-top box, a personal video player, a DVD player, a Blu-ray player, a gaming system, and/or a mobile device (e.g., a smartphone). Although shown in  FIG. 1A  with a single audio source  103 , in some embodiments the audio system  100  may include multiple audio sources  103  that are coupled to the speaker arrays  105  to output sound corresponding to separate pieces of sound program content. 
     As shown in  FIG. 2A , the audio source  103  may include a hardware processor  201  and/or a memory unit  203 . The processor  201  and the memory unit  203  are generically used here to refer to any suitable combination of programmable data processing components and data storage that conduct the operations needed to implement the various functions and operations of the audio source  103 . The processor  201  may be an applications processor typically found in a smart phone, while the memory unit  203  may refer to microelectronic, non-volatile random access memory. An operating system may be stored in the memory unit  203  along with application programs specific to the various functions of the audio source  103 , which are to be run or executed by the processor  201  to perform the various functions of the audio source  103 . For example, a rendering strategy unit  209  may be stored in the memory unit  203 . As will be described in greater detail below, the rendering strategy unit  209  may be used to generate beam attributes for each channel of one or more pieces of sound program content to be played by the speaker arrays  105  in the listening area  101 . For instance, the beam attributes may include beam types for sound beams produced by each of the speaker arrays  105  (e.g., omnidirectional, cardioid, second order, and fourth order). 
     In one embodiment, the audio source  103  may include one or more audio inputs  205  for receiving audio signals from external and/or remote devices. For example, the audio source  103  may receive audio signals from a streaming media service and/or a remote server. The audio signals may represent one or more channels of a piece of sound program content (e.g., a musical composition or an audio track for a movie). For example, a single signal corresponding to a single channel of a piece of multichannel sound program content may be received by an input  205  of the audio source  103 . In another example, a single signal may correspond to multiple channels of a piece of sound program content, which are multiplexed onto the single signal. 
     In one embodiment, the audio source  103  may include a digital audio input  205 A that receives digital audio signals from an external device and/or a remote device. For example, the audio input  205 A may be a TOSLINK connector or a digital wireless interface (e.g., a wireless local area network (WLAN) adapter or a Bluetooth receiver). In one embodiment, the audio source  103  may include an analog audio input  205 B that receives analog audio signals from an external device. For example, the audio input  205 B may be a binding post, a Fahnestock clip, or a phono plug that is designed to receive and/or utilize a wire or conduit and a corresponding analog signal from an external device. 
     Although described as receiving pieces of sound program content from an external or remote source, in some embodiments pieces of sound program content may be stored locally on the audio source  103 . For example, one or more pieces of sound program content may be stored within the memory unit  203 . 
     In one embodiment, the audio source  103  may include an interface  207  for communicating with the speaker arrays  105  and/or other devices (e.g., remote audio/video streaming services). The interface  207  may utilize wired mediums (e.g., conduit or wire) to communicate with the speaker arrays  105 . In another embodiment, the interface  207  may communicate with the speaker arrays  105  through a wireless connection as shown in  FIG. 1A  and  FIG. 1B . For example, the network interface  207  may utilize one or more wireless protocols and standards for communicating with the speaker arrays  105 , including the IEEE 802.11 suite of standards, cellular Global System for Mobile Communications (GSM) standards, cellular Code Division Multiple Access (CDMA) standards, Long Term Evolution (LTE) standards, and/or Bluetooth standards. 
       FIG. 2B  shows a component diagram of a speaker array  105  according to one embodiment. As shown in  FIG. 2B , the speaker array  105  may receive audio signals corresponding to audio channels from the audio source  103  through a corresponding interface  213 . These audio signals may be used to drive one or more transducers  109  in the speaker arrays  105 . As with the interface  207 , the interface  213  may utilize wired protocols and standards and/or one or more wireless protocols and standards, including the IEEE 802.11 suite of standards, cellular Global System for Mobile Communications (GSM) standards, cellular Code Division Multiple Access (CDMA) standards, Long Term Evolution (LTE) standards, and/or Bluetooth standards. In some embodiments, the speaker array  105  may include digital-to-analog converters  217 , power amplifiers  211 , delay circuits  214 , and beamformers  215  for driving transducers  109  in the speaker arrays  105 . The digital-to-analog converters  217 , power amplifiers  211 , delay circuits  214 , and beamformers  215  may be formed/implemented using any set of hardware circuitry and/or software components. For example, the beamformers  215  may be comprised of a set of finite impulse response (FIR) filters and/or one or more other filters that control the relative magnitudes and phases between the transducers. 
     Although described and shown as being separate from the audio source  103 , in some embodiments, one or more components of the audio source  103  may be integrated within the speaker arrays  105 . For example, one or more of the speaker arrays  105  may include the hardware processor  201 , the memory unit  203 , and the one or more audio inputs  205 . In this example, a single speaker array  105  may be designated as a master speaker array  105 . This master speaker array  105  may distribute sound program content and/or control signals (e.g., data describing beam pattern types) to each of the other speaker arrays  105  in the audio system  100 . 
       FIG. 3A  shows a side view of one of the speaker arrays  105  according to one embodiment. As shown in  FIG. 3A , the speaker arrays  105  may house multiple transducers  109  in a curved cabinet  111 . As shown, the cabinet  111  is cylindrical; however, in other embodiments the cabinet  111  may be in any shape, including a polyhedron, a frustum, a cone, a pyramid, a triangular prism, a hexagonal prism, or a sphere. 
       FIG. 3B  shows an overhead, cutaway view of a speaker array  105  according to one embodiment. As shown in  FIGS. 3A and 3B , the transducers  109  in the speaker array  105  encircle the cabinet  111  such that the transducers  109  cover the curved face of the cabinet  111 . The transducers  109  may be any combination of full-range drivers, mid-range drivers, subwoofers, woofers, and tweeters. Each of the transducers  109  may use a lightweight diaphragm, or cone, connected to a rigid basket, or frame, via a flexible suspension that constrains a coil of wire (e.g., a voice coil) to move axially through a cylindrical magnetic gap. When an electrical audio signal is applied to the voice coil, a magnetic field is created by the electric current in the voice coil, making it a variable electromagnet. The coil and the transducers&#39;  109  magnetic system interact, generating a mechanical force that causes the coil (and thus, the attached cone) to move back and forth, thereby reproducing sound under the control of the applied electrical audio signal coming from an audio source, such as the audio source  103 . Although electromagnetic dynamic loudspeaker drivers are described for use as the transducers  109 , those skilled in the art will recognize that other types of loudspeaker drivers, such as piezoelectric, planar electromagnetic and electrostatic drivers are possible. 
     Each transducer  109  may be individually and separately driven to produce sound in response to separate and discrete audio signals received from an audio source  103 . By allowing the transducers  109  in the speaker arrays  105  to be individually and separately driven according to different parameters and settings (including delays and energy levels), the speaker arrays  105  may produce numerous directivity/beam patterns that accurately represent each channel of a piece of sound program content output by the audio source  103 . For example, in one embodiment, the speaker arrays  105  may individually or collectively produce omnidirectional, cardioid, second order, and fourth order beam patterns.  FIG. 4  shows a set of beam patterns that may be produced by the speaker arrays  105 . As shown, the directivity index of the beam patterns in  FIG. 4  increase from left to right. 
     Although shown in  FIG. 1A  as including two speaker arrays  105 , in other embodiments a different number of speaker arrays  105  may be used. For example, as shown in  FIG. 1B  four speaker arrays  105  may be used within the listening area  101 . Further, although described as similar or identical styles of speaker arrays  105 , in some embodiments the speaker arrays  105  in the audio system  100  may have different sizes, different shapes, different numbers of transducers, and/or different manufacturers. 
     Further, as noted above, although the speaker arrays  105  shown in the  FIGS. 1A, 1B, 3A, and 3B  are shown with a cylindrical cabinet  111  and uniformly spaced transducers  109 , in other embodiments, the speaker arrays  105  may be differently sized and transducers  109  may be differently arranged within the cabinet  111 . Accordingly, the style of the speaker arrays  105  shown and described herein is merely illustrative and in other embodiments, different types and styles of speaker arrays  105  may be used. 
     Turning now to  FIG. 5 , a method  500  for driving one or more speaker arrays  105  to generate sound with similar or identical direct-to-reverberant ratios at the location of the listener  107  will be discussed. Each operation of the method  500  may be performed by one or more components of the audio source  103  and/or the speaker arrays  105 . For example, one or more of the operations of the method  500  may be performed by the rendering strategy unit  209  of the audio source  103 . 
     As noted above, in one embodiment, one or more components of the audio source  103  may be integrated within one or more speaker arrays  105 . For example, one of the speaker arrays  105  may be designated as a master speaker array  105 . In this embodiment, the operations of the method  500  may be solely or primarily performed by this master speaker array  105  and data generated by the master speaker array  105  may be distributed to other speaker arrays  105 . 
     Although the operations of the method  500  are described and shown in a particular order, in other embodiments, the operations may be performed in a different order. For example, in some embodiments, two or more operations of the method  500  may be performed concurrently or during overlapping time periods. 
     In one embodiment, the method  500  may commence at operation  501  with the determination of one or more characteristics describing each of the speaker arrays  105 . For example, operation  501  may determine the direct-to-reverberant ratio experienced at the location of the listener  107  from sound produced by each speaker array  105 . The direct-to-reverberant ratio may be defined as the ratio of sound energy received directly from a speaker array  105  (e.g., sound energy received at the location of the listener  107  without reflection) to sound energy received indirectly from the speaker array  105  (e.g., sound energy received at the location of the listener  107  after reflection in the listening area  101 ). The direct-to reverberant ratio may be quantified by Equation 1 shown below: 
     
       
         
           
             
               
                 
                   
                     Direct 
                     ⁢ 
                     
                       - 
                     
                     ⁢ 
                     To 
                     ⁢ 
                     
                       - 
                     
                     ⁢ 
                     Reverberant 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Ratio 
                   
                   = 
                   
                     
                       
                         DI 
                         ⁡ 
                         
                           ( 
                           f 
                           ) 
                         
                       
                       × 
                       V 
                     
                     
                       100 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                       × 
                       
                         r 
                         2 
                       
                       × 
                       
                         
                           T 
                           60 
                         
                         ⁡ 
                         
                           ( 
                           f 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     In this equation, T 60  (f) is the reverberation time in the listening area  101  at the frequency f, V is the functional volume of the listening area  101 , DI(f) is the directivity index of a beam pattern emitted by the speaker array  105  at the frequency f, and r is the distance from the speaker array  105  to the listener  107 . 
     In one embodiment, operation  501  may be performed by emitting a set of test sounds by one or more of the speaker arrays  105  using different beam pattern types. For example, in the audio system  100  shown in  FIG. 1A , the speaker arrays  105 A and  105 B may be driven with separate test signals and with multiple different beam pattern types. For instance, speaker arrays  105 A and  105 B may be each sequentially driven with omnidirectional, cardioid, second order, and fourth order beam patterns using a set of test signals. As shown in  FIG. 6 , sounds from each of the speaker arrays  105  and for each of the beam patterns may be sensed by a listening device  601 . The listening device  601  may be any device that is capable of detecting sounds produced by the speaker arrays  105 . For example, the listening device  601  may be a mobile device (e.g., a cellular telephone), a laptop computer, a desktop computer, a tablet computer, a personal digital assistant, or any other similar device that is capable of sensing sound. The listening device  601  may include one or more microphones for detecting sound, a processor and memory unit that are similar to the processor  201  and memory unit  203  of the audio source  103 , and/or an interface similar to the interface  207  for communicating with the audio source  103  and/or the speaker arrays  105 . As noted above, in one embodiment, the listening device  601  may include multiple microphones that operate independently or as one more microphone arrays to detect sound from each of the speaker arrays  105 . 
     In one embodiment, the listening device  601  may be placed proximate to the listener  107  such that the listening device  601  may sense sounds produced by the speaker arrays  105  as they would be heard/sensed by the listener  107 . For example, in one embodiment, the listening device  601  may be held near an ear of the listener  107  while operation  501  is being performed. The sounds sensed by the listening device  601  may be analyzed at operation  501  to determine the direct-to-reverberant ratio for each beam pattern produced by each of the speaker arrays  105 . For example, operation  501  may compare the level of early sound energy detected for a particular speaker array  105  and beam pattern combination to later sound energy detected for the particular speaker array  105  and beam pattern combination. In this embodiment, since the beam patterns are focused on the listener  107 , direct sound will arrive sooner than indirect sound, which must take a longer route to the listener  107  as a result of reflection off walls and other surfaces/objects in the listening area  101 . Accordingly, the sensed early energy may represent direct sound energy while energy levels of sound later in time may represent reverberant sound energy. 
     Table 1 below shows a set of direct energy levels, reverberant energy levels, and direct-to-reverberant ratios that may be detected at the location of the listener  107  based on a set of directivity patterns produced by the speaker array  105 A. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Beam Pattern 
                 Direct Energy 
                 Reverberant 
                 Direct-to- 
               
               
                 Type 
                 Level 
                 Energy Level 
                 Reverberant Ratio 
               
               
                   
               
             
            
               
                 Omni-Directional 
                 6 dB 
                 15 dB 
                 −9 dB 
               
               
                 Cardioid 
                 8 dB 
                 12.5 dB   
                 −4.5 dB   
               
               
                 Second Order 
                 8.5 dB   
                 11.5 dB   
                 −3 dB 
               
               
                 Fourth Order 
                 8.5 dB   
                 11 dB 
                 −2.5 dB   
               
               
                   
               
            
           
         
       
     
     Table 2 below shows a set of direct energy levels, reverberant energy levels, and direct-to-reverberant ratios that may be detected at the location of the listener  107  based on a set of directivity patterns produced by the speaker array  105 B. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Beam Pattern 
                 Direct Energy 
                 Reverberant 
                 Direct-to- 
               
               
                 Type 
                 Level 
                 Energy Level 
                 Reverberant Ratio 
               
               
                   
               
             
            
               
                 Omni-Directional 
                 3.5 dB 
                 15 dB 
                 −11.5 dB  
               
               
                 Cardioid 
                 5.5 dB 
                 12.5 dB   
                   −7 dB 
               
               
                 Second Order 
                   6 dB 
                 11.5 dB   
                 −5.5 dB 
               
               
                 Fourth Order 
                 6.5 dB 
                 11 dB 
                 −4.5 dB 
               
               
                   
               
            
           
         
       
     
     As shown in Tables 1 and 2, the direct-to-reverberant ratios between each of the speaker arrays  105 A and  105 B and for each corresponding beam pattern vary. The variance may be attributed to various factors, including differences in distances between each of the speaker arrays  105 A and  105 B and the listener  107 , the different types or arrangement/orientation of transducers  109  used in each of the speaker arrays  105 A and  105 B, and/or other similar factors. These direct-to-reverberant ratios for each different type of beam pattern and each speaker array  105  may be used to select beam patterns for each of the speaker arrays  105 A and  105 B as will be described in greater detail below. 
     Although operation  501  is described above in relation to measurement of particular test sounds, in another embodiment, direct-to-reverberant ratios for multiple beam patterns emitted by the speaker arrays  105 A and  105 B may be estimated based on the reverberation time of the listening area  101  (e.g., T 60 ) and/or the distance between each of the speaker arrays  105  and the listener  107 . The reverberation time T 60  is defined as the time required for the level of sound to drop by 60 dB in the listening area  1 . In one embodiment, the listening device  601  is used to measure the reverberation time T 60  in the listening area  101 . The reverberation time T 60  does not need to be measured at a particular location in the listening area  101  (e.g., the location of the listener  107 ) or with any particular beam pattern. The reverberation time T 60  is a property of the listening area  101  and a function of frequency. 
     The reverberation time T 60  may be measured using various processes and techniques. In one embodiment, an interrupted noise technique may be used to measure the reverberation time T 60 . In this technique, wide band noise is played and stopped abruptly. With a microphone (e.g., the listening device  601 ) and an amplifier connected to a set of constant percentage bandwidth filters such as octave band filters, followed by a set of ac-to-dc converters, which may be average or rms detectors, the decay time from the initial level down to −60 dB is measured. It may be difficult to achieve a full 60 dB of decay, and in some embodiments extrapolation from 20 dB or 30 dB of decay may be used. In one embodiment, the measurement may begin after the first 5 dB of decay. 
     In one embodiment, a transfer function measurement may be used to measure the reverberation time T 60 . In this technique, a stimulus-response system in which a test signal, such as a linear or log sine chirp, a maximum length stimulus signal, or other noise like signal, is measured simultaneously in what is being sent and what is being measured with a microphone (e.g., the listening device  601 ). The quotient of these two signals is the transfer function. In one embodiment, this transfer function may be made a function of frequency and time and thus is able to make high resolution measurements. The reverberation time T 60  may be derived from the transfer function. Accuracy may be improved by repeating the measurement sequentially from each of the speaker arrays  105  and each of multiple microphone locations (e.g., locations of the listening device  601 ) in the listening area  101 . 
     In another embodiment, the reverberation time T 60  may be estimated based on typical room characteristics dynamics. For example, the audio source  103  and/or the speaker arrays  105  may receive an estimated reverberation time T 60  from an external device through the interface  107 . 
     In one embodiment, the distance between each of the speaker arrays  105  and the listener  107  may be calculated at operation  501 . For example, the distances r A  and r B  may be estimated using various techniques. In one embodiment, the distances r A  and r B  may be determined using 1) a set of test sounds and the listening device  601  through the calculation of propagation delays, 2) a video/still image camera of the listening device  601 , which captures the speaker arrays  105  and estimates the distances r A  and r B  based on these captured videos/images, and/or 3) inputs from the listener  107 . 
     Based on the calculated reverberation time T 60  and/or the distances r A  and r B , operation  501  may estimate the direct-to-reverberant ratios for a set of beam patterns. For example,  FIG. 7  shows a chart of direct-to-reverberant ratios for a set of beam pattern types in relation to distances between the speaker arrays  105 A and  105 B and the listener  107 . In one embodiment, the values in the chart shown in  FIG. 7  may be retrieved based on the calculated reverberation time T 60 . For example, the values in the chart of  FIG. 7  may represent expected direct-to-reverberant ratios based on known distances between a speaker array  105  and a location (e.g., the location of the listener  107 ) and characteristics of the listening area  101  (e.g., the calculated reverberation time T 60 ). This chart may be retrieved from a local data source (e.g., the memory unit  203 ) or a remote data source that is retrievable using the interface  207  based on the calculated reverberation time T 60 . 
     In one embodiment, the direct-to-reverberant ratios shown in  FIG. 7  may be calculated using Equation 1 listed above, based on the directivity indexes of each beam pattern, the calculated reverberation time T 60 , and the distances r A  and r B . 
     Accordingly, as described above operation  501  may determine characteristics of the speaker arrays  105 , including the direct-to-reverberant ratio experienced at the location of the listener  107  from sound produced by each speaker array  105  using a variety of beam patterns. In one embodiment, the listener  107  may select which technique to use based on a set of user manipulated preferences. 
     Following operation  501 , operation  503  may determine a preferred direct-to-reverberant ratio. The preferred direct-to-reverberant ratio describes the amount of direct sound energy in relation to the reverberant sound energy experienced by the listener  107 . In one embodiment, the preferred direct-to-reverberant ratio may be preset by the audio system  100 . For example, the manufacturer of the audio source  103  and/or the speaker arrays  105  may indicate a preferred direct-to-reverberant ratio. In another embodiment, the preferred direct-to-reverberant ratio may be relative to the content being played. For example, speech/dialogue may be associated with a high preferred direct-to-reverberant ratio while music may be associated with a comparatively lower preferred direct-to-reverberant ratio. In still another embodiment, the listener  107  may indicate a preference for a preferred direct-to-reverberant ratio through a set of user manipulated preferences. 
     In yet another embodiment, operation  503  may select the direct-to-reverberant ratio of one of the speaker arrays  105  as the preferred direct-to-reverberant ratio. For example, the speaker array  105 A, which is at a distance of three meters from the listener  107  (e.g., r A  is three meters), may be currently emitting a cardioid beam pattern directed at the listener  107 . Based on the chart in  FIG. 7 , the direct-to-reverberant ratio at the location of the listener  107  would be approximately −4.5 dB based on sound produced from the speaker array  105 A. In this example, the preferred direct-to-reverberant ratio would be set to −4.5 dB. 
     In one embodiment, multiple preferred direct-to-reverberant ratios may be determined at operation  503 . For example, separate preferred direct-to-reverberant ratios may be calculated for separate types of content (e.g., speech/dialogue, music and effects, etc.). In this embodiment, beam patterns corresponding to a first content type may be associated with a first preferred direct-to-reverberant ratio while beam patterns corresponding to a second content type may be associated with a second preferred direct-to-reverberant ratio. For instance, in the audio system  100  configuration shown in  FIG. 1B , the speaker arrays  105 A and  105 B may emit front left and front right beam patterns, respectively, that include dialogue for a movie. In contrast, the speaker arrays  105 C and  105 D may emit left surround and right surround beam patterns respectively, that include music and effects for the movie. In this example, the front left and front right beam patterns may be associated with a preferred direct-to-reverberant ratio of 2.0 dB while the left surround and right surround beam patterns speaker arrays  105  may be associated with a preferred direct-to-reverberant ratio of −3.0 dB. 
     Following the selection of the preferred direct-to-reverberant ratio (or ratios) at operation  503 , operation  505  may select a beam pattern for each of the speaker arrays  105  such that the preferred direct-to-reverberant ratio at the listener  107  is achieved by each of the speaker arrays  105 . For example, when the preferred direct-to-reverberant ratio is determined at operation  503  to be −4.5 dB and the distances r A  and r B  are determined at operation  501  to be three meters and four meters, respectively, operation  505  may select a cardioid beam pattern for the speaker array  105 A and a fourth order beam pattern for the speaker array  105 B based on the chart shown in  FIG. 7 . In particular, as shown in  FIG. 7 , a cardioid beam pattern at a distance of three meters (i.e., the distance r A ) produces a direct-to-reverberant ratio of approximately −4.5 dB while a fourth order beam pattern at a distance of four meters (i.e., the distance r B ) produces a direct-to-reverberant ratio of approximately −4.5 dB. Accordingly, a cardioid beam pattern assigned to the speaker array  105 A and a fourth order beam pattern assigned to the speaker array  105 B will produce an identical direct-to-reverberant ratio for sound produced by each of the arrays  105 A and  105 B at the location of the listener  107 . 
     In some embodiments, a single speaker array  105  may emit multiple beam patterns corresponding to different channels and/or different types of audio content (e.g., speech/dialogue, music and effects, etc.). In this embodiment, a single speaker array  105  may emit beams to produce separate direct-to-reverberant ratios for each of the channels and/or types of audio content. For example, the speaker array  105 A may produce a first beam corresponding to dialogue and a second beam corresponding to music for a piece of sound program content. In this embodiment, preferred direct-to-reverberant ratios may be separately assigned at operation  503  for each of dialogue and music components for the piece of sound program content. Based on these separate preferred direct-to-reverberant ratios, operation  505  may select different beam patterns such that each corresponding preferred direct-to-reverberant ratio is achieved at the location of the listener  107 . 
     Although described above as selecting beam patterns that exactly achieve a preferred direct-to-reverberant ratio, in some embodiments beam patterns may be selected at operation  505  that produce a direct-to-reverberant ratio within a predefined threshold of a preferred direct-to-reverberant ratio. For example, the threshold may be 10% such that a beam pattern is selected that produces sound with a direct-to-reverberant ratio at the location of the listener  107  within 10% of a preferred direct-to-reverberant ratio. In other embodiments, a larger threshold may be used (e.g., 1%-25%). 
     Following selection of beam patterns at operation  505 , operation  507  may drive each of the speaker arrays  105  using the selected beam patterns. For example, a left audio channel may be used to drive the speaker array  105 A to produce a cardioid beam pattern while a right audio channel may be used to drive the speaker array  105 B to produce a fourth order beam pattern. In one embodiment, the speaker arrays  105  may use one or more of the digital-to-analog converters  217 , power amplifiers  211 , delay circuits  214 , and beamformers  215  for driving transducers  109  to produce the selected beam patterns at operation  507 . As noted above, the digital-to-analog converters  217 , power amplifiers  211 , delay circuits  214 , and beamformers  215  may be formed/implemented using any set of hardware circuitry and/or software components. For example, the beamformers  215  may be comprised of a set of finite impulse response (FIR) filters and/or one or more other filters. 
     In one embodiment, operation  507  may adjust drive settings for one or more of the speaker arrays  105  to ensure the level at the location of the listener  107  from each of the speaker arrays  105  is the same. For instance, in the example provided above in relation to Table 1 and Table 2, the level at the location of the listener  107  based on sound from the speaker array  105 A may be 1.5 dB higher than sound from the speaker array  105 B. This level difference may be based on a variety of factors, including the distance between the speaker arrays  105 A and  105 B and the location of the listener  107 . In this example, to ensure that the sound level from each of the speaker arrays  105  is the same, operation  507  may apply a 1.5 dB gain to audio signals used to drive the speaker array  105 B such that the level of sound at the location of the speaker arrays  105 A and  105 B is the same. Accordingly, based on this adjustment/application of gain at operation  507  and the selection of beam patterns at operation  505 , both the direct-to-reverberant ratio and the level of sound from each of the speaker arrays  105 A and  105 B at the location of the listener  107  may be identical. 
     In one embodiment, the beam patterns selected at operation  505  may be transmitted to each corresponding speaker array  105 . Accordingly, each of the speaker arrays  105  may receive a selected beam pattern and generate a set of delays and gain values for corresponding transducers  109  such that the selected beam patterns are generated. In other embodiments, the delays, gain values, and other parameters for generating the selected beam patterns may be calculated by the audio source  103  and/or another device and transferred to the speaker arrays  105 . 
     As described above, the method  500  may drive separate speaker arrays  105  to produce sound at the location of the listener  107  with identical or nearly identical direct-to-reverberant ratios. In particular, the direct-to-reverberant ratio perceived by the listener  107  based on sound produced by the speaker array  105 A may be identical or nearly identical to the direct-to-reverberant ratio perceived by the listener  107  based on sound produced by the speaker array  105 B. By matching direct-to-reverberant ratios for sound produced by multiple speaker arrays  105 , the method  500  ensures a more consistent listening experience for the listener  107 . In some embodiments, time of arrival, level of sound, and spectrum matching may also be applied to sound produced by multiple speaker arrays  105 . 
     In one embodiment, the method  500  may be run during configuration of the audio system  100 . For example, following installation and setup of the audio system  100  in the listening area  101 , the method  500  may be performed. The method  500  may be subsequently performed each time one or more of the speaker arrays  105  and/or the listener  107  moves. 
     Although described in relation to a single listener  107 , in other embodiments, the method  500  and the audio system  100  may be similarly applied to multiple listeners  107 . For example, in embodiments in which separate beam patterns are generated for separate listeners  107 , each set of beam patterns for each set of listeners  107  may be associated with a preferred direct-to-reverberant ratio. Accordingly, each listener  107  may receive sound from corresponding beam patterns such that separate preferred direct-to-reverberant ratios are maintained for each of the listeners  107 . In another embodiment, a constant direct-to-reverberant ratio may be maintained for multiple listeners  107  based on individualized beams. For example, an average direct-to-reverberant ratio may be generated by beams across multiple locations/listeners  107  based on sound heard from each of the listeners  107  from each beam. 
     As explained above, an embodiment of the invention may be an article of manufacture in which a machine-readable medium (such as microelectronic memory) has stored thereon instructions that program one or more data processing components (generically referred to here as a “processor”) to perform the operations described above. In other embodiments, some of these operations might be performed by specific hardware components that contain hardwired logic (e.g., dedicated digital filter blocks and state machines). Those operations might alternatively be performed by any combination of programmed data processing components and fixed hardwired circuit components. 
     While certain embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that the invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those of ordinary skill in the art. The description is thus to be regarded as illustrative instead of limiting.