Patent Publication Number: US-11399255-B2

Title: Adjusting the beam pattern of a speaker array based on the location of one or more listeners

Description:
RELATED MATTERS 
     This patent application is a continuation of pending U.S. application Ser. No. 16/030,736, filed Jul. 9, 2018, which is a continuation of U.S. application Ser. No. 14/771,475, filed Aug. 28, 2015 (now issued as U.S. Pat. No. 10,021,506), which is a National Phase filing under 35 U.S.C. § 371 of International Application No. PCT/US2014/020433, filed Mar. 4, 2014, which claims the benefit of the earlier filing date of U.S. Provisional Application No. 61/773,078, filed Mar. 5, 2013, and these applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     An audio device detects the distance of a listener from a speaker array and adjusts the directivity index of a beam pattern output by the speaker array to maintain a constant direct-to-reverberant sound energy ratio. Other embodiments are also described. 
     BACKGROUND 
     Speaker arrays may be variably driven to form numerous different beam patterns. The generated beam patterns can be controlled and altered to change the direction and region over which sound is radiated. Using this property of speaker arrays allows some acoustic parameters to be controlled. One such parameter is the direct-to-reverberant acoustic energy ratio. This ratio describes how much sound a listener receives directly from a speaker array compared to how much sound reaches the listener via reflections off walls and other reflecting objects in a room. For example, if a beam pattern generated by a speaker array is narrow and pointed at a listener, the direct-to-reverberant ratio will be large since the listener is receiving a large amount of direct energy and a comparatively smaller amount of reflected energy. Alternatively, if a beam pattern generated by the speaker array is wide, the direct-to-reverberant ratio is smaller as the listener is receiving comparatively more sound reflected off surfaces and objects. 
     SUMMARY 
     Loudspeaker arrays may emit both direct sound energy and an indirect or reverberant sound energy at a listener in a room or listening area. The direct sound energy is received directly from transducers in the speaker array while reverberant sound energy reflects off walls or surfaces in the room before arriving at the listener. As the listener moves closer to the speaker array, the direct-to-reverberant sound energy level increases as the propagation distance for the direct sounds is noticeably decreased while the propagation distance for the reverberant sounds is relatively unchanged or only slightly increased. 
     An embodiment of the invention is a directivity adjustment device that maintains a constant direct-to-reverberant ratio based on the detected location of the listener in relation to the speaker array. The directivity adjustment device may include a distance estimator, a directivity compensator, and an array processor. The distance estimator detects the distance between the speaker array and the listener. For example, the distance estimator may use (1) a user input device; (2) a microphone; (3) infrared sensors; and/or (4) a camera to determine the distance between the speaker array and the listener. Based on this detected distance, the directivity compensator calculates a directivity index from a beam produced by the speaker array that maintains a predefined direct-to-reverberant sound energy ratio. The direct-to-reverberant ratio may be preset by a manufacturer or designer of the directivity adjustment device and may be variable based on the content of sound program content played. The array processor receives the calculated directivity index and processes each channel of a piece of sound program content to produce a set of audio signals that drive one or more of the transducers in the speaker array to generate a beam pattern with the calculated directivity index. By maintaining a constant direct-to-reverberant directivity ratio, the directivity adjustment device improves the consistency and quality of sound perceived by 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. 1  shows a beam adjustment system that adjusts the width of a generated sound pattern based on the location of one or more listeners in a room or listening area according to one embodiment. 
         FIG. 2A  shows one loudspeaker array with multiple transducers housed in a single cabinet according to one embodiment. 
         FIG. 2B  shows another loudspeaker array with multiple transducers housed in a single cabinet according to another embodiment. 
         FIG. 3  shows a functional unit block diagram and some constituent hardware components of a directivity adjustment device according to one embodiment. 
         FIGS. 4A and 4B  shows the listener located at various distances from the loudspeaker array. 
         FIG. 5  shows an example set of sound patterns with different directivity indexes that may be generated by the speaker array. 
     
    
    
     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. 1  shows a beam adjustment system  1  that adjusts the width of a generated sound pattern emitted by a speaker array  4  based on the location of one or more listeners  2  in a room or listening area  3 . Each element of the beam adjustment system  1  will be described by way of example below. 
     The beam adjustment system  1  includes one or more speaker arrays  4  for outputting sound into the room or listening area  3 .  FIG. 2A  shows one speaker array  4  with multiple transducers  5  housed in a single cabinet  6 . In this example, the speaker array  4  has 32 distinct transducers  5  evenly aligned in eight rows and four columns within the cabinet  5 . In other embodiments, different numbers of transducers  5  may be used with uniform or non-uniform spacing. For instance, as shown in  FIG. 2B, 10  transducers  5  may be aligned in a single row in the cabinet  6  to form a sound-bar style speaker array  4 . Although shown as aligned is a flat plane or straight line, the transducers  5  may be aligned in a curved fashion along an arc. 
     The transducers  5  may be any combination of full-range drivers, mid-range drivers, subwoofers, woofers, and tweeters. Each of the transducers  5  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;  5  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 a source (e.g., a signal processor, a computer, and an audio receiver). Although described herein as having multiple transducers  5  housed in a single cabinet  6 , in other embodiments the speaker arrays  4  may include a single transducer  5  housed in the cabinet  6 . In these embodiments, the speaker array  4  is a standalone loudspeaker. 
     Each transducer  5  may be individually and separately driven to produce sound in response to separate and discrete audio signals. By allowing the transducers  5  in the speaker arrays  4  to be individually and separately driven according to different parameters and settings (including delays and energy levels), the speaker arrays  4  may produce numerous directivity patterns to simulate or better represent respective channels of sound program content played to the listener  2 . For example, beam patterns of different widths and directivities may be emitted by the speaker arrays  4  based on the location of the listener  2  in relation to the speaker arrays  4 . 
     As shown in  FIGS. 2A and 2B , the speaker arrays  4  may include wires or conduit  7  for connecting to a directivity adjustment device  8 . For example, each speaker array  4  may include two wiring points and the directivity adjustment device  8  may include complementary wiring points. The wiring points may be binding posts or spring clips on the back of the speaker arrays  4  and the directivity adjustment device  8 , respectively. The wires  7  are separately wrapped around or are otherwise coupled to respective wiring points to electrically couple the speaker arrays  4  to the directivity adjustment device  8 . 
     In other embodiments, the speaker arrays  4  are coupled to the directivity adjustment device  8  using wireless protocols such that the arrays  4  and the directivity adjustment device  8  are not physically joined but maintain a radio-frequency connection. For example, the speaker arrays  4  may include a WiFi receiver for receiving audio signals from a corresponding WiFi transmitter in the directivity adjustment device  8 . In some embodiments, the speaker arrays  4  may include integrated amplifiers for driving the transducers  5  using the wireless audio signals received from the directivity adjustment device  8 . 
     Although shown as including two speaker arrays  4 , the audio system  1  may include any number of speaker arrays  4  that are coupled to the directivity adjustment device  8  through wireless or wired connections. For example, the audio system  1  may include six speaker arrays  4  that represent a front left channel, a front center channel, a front right channel, a rear right surround channel, a rear left surround channel, and a low frequency channel (e.g., a subwoofer). Hereinafter, the beam adjustment system  1  will be described as including a single speaker array  4 . However, as described above, it is understood that the system  1  may include multiple speaker arrays  4 . 
       FIG. 3  shows a functional unit block diagram and some constituent hardware components of the directivity adjustment device  8  according to one embodiment. The components shown in  FIG. 3  are representative of elements included in the directivity adjustment device  8  and should not be construed as precluding other components. Each element of  FIG. 3  will be described by way of example below. 
     The directivity adjustment device  8  may include multiple inputs  10  for receiving one or more channels of sound program content using electrical, radio, or optical signals from one or more external audio sources  9 . The inputs  10  may be a set of digital inputs  10 A and  10 B and analog inputs  10 C and  10 D, including a set of physical connectors located on an exposed surface of the directivity adjustment device  8 . For example, the inputs  10  may include a High-Definition Multimedia Interface (HDMI) input, an optical digital input (Toslink), a coaxial digital input, and a phono input. In one embodiment, the directivity adjustment device  8  receives audio signals through a wireless connection with an external audio source  9 . In this embodiment, the inputs  10  include a wireless adapter for communicating with the external audio source  9  using wireless protocols. For example, the wireless adapter may be capable of communicating using Bluetooth, IEEE 802.11x, cellular Global System for Mobile Communications (GSM), cellular Code division multiple access (CDMA), or Long Term Evolution (LTE). 
     As shown in  FIG. 1 , the external audio source  9  may include a laptop computer. In other embodiments, the external audio source  9  may be any device capable of transmitting one or more channels of sound program content to the directivity adjustment device  8  over a wireless or wired connection. For example, the external audio source  9  may include a desktop computer, a portable communications device (e.g., a mobile phone or tablet computer), a streaming Internet music server, a digital-video-disc player, a Blu-ray Disc™ player, a compact-disc player, or any other similar audio output device. 
     In one embodiment, the external audio source  9  and the directivity adjustment device  8  are integrated in one indivisible unit. In this embodiment, the loudspeaker arrays  4  may also be integrated into the same unit. For example, the external audio source  9  and the directivity adjustment device  8  may be in one computing unit with loudspeaker arrays  4  integrated in left and right sides of the unit. 
     Returning to the directivity adjustment device  8 , general signal flow from the inputs  10  will now be described. Looking first at the digital inputs  10 A and  10 B, upon receiving a digital audio signal through the input  10 A and/or  10 B, the directivity adjustment device  8  uses a decoder  11 A and/or  11 B to decode the electrical, optical, or radio signals into a set of audio channels representing sound program content. For example, the decoder  11 A may receive a single signal containing six audio channels (e.g., a 5.1 signal) and decode the signal into six audio channels. The decoder  11 A may be capable of decoding an audio signal encoded using any codec or technique, including Advanced Audio Coding (AAC), MPEG Audio Layer II, MPEG Audio Layer III, and Free Lossless Audio Codec (FLAC). 
     Turning to the analog inputs  10 C and  10 D, each analog signal received by analog inputs  10 C and  10 D represents a single audio channel of the sound program content. Accordingly, multiple analog inputs  10 C and  10 D may be needed to receive each channel of a piece of sound program content. The audio channels may be digitized by respective analog-to-digital converters  12 A and  12 B to form digital audio channels. 
     The digital audio channels from each of the decoders  11 A and  11 B and the analog-to-digital converters  12 A and  12 B are output to the multiplexer  13 . The multiplexer  13  selectively outputs a set of audio channels based on a control signal  14 . The control signal  14  may be received from a control circuit or processor in the directivity adjustment device  8  or from an external device. For example, a control circuit controlling a mode of operation of the directivity adjustment device  8  may output the control signal  14  to the multiplexer  13  for selectively outputting a set of digital audio channels. 
     The multiplexer  13  feeds the selected digital audio channels to an array processor  15 . The channels output by the multiplexer  13  are processed by the array processor  15  to produce a set of processed audio channels. The processing may operate in both the time and frequency domains using transforms such as the Fast Fourier Transform (FFT). The array processor  15  may be a special purpose processor such as application-specific integrated circuit (ASICs), a general purpose microprocessor, a field-programmable gate array (FPGA), a digital signal controller, or a set of hardware logic structures (e.g., filters, arithmetic logic units, and dedicated state machines). The array processor  15  generates a set of signals for driving the transducers  5  in the speaker array  4  based on inputs from a distance estimator  16  and/or a directivity compensator  17 . 
     The distance estimator  16  determines the distance of one or more human listeners  2  from the speaker array  4 .  FIG. 4A  shows the listener  2  located a distance r A  away from a speaker array  4  in the room  3 . The distance estimator  16  determines the distance r A  as the listener  2  moves around the room  3  and while sound is being emitted by the speaker arrays  4 . Although described in relation to a single listener, the distance estimator  16  may determine the distance r A  of multiple listeners  2  in the room  3 . 
     The distance estimator  16  may use any device or algorithm for determining the distance r. In one embodiment, a user input device  18  is coupled to the distance estimator  16  for assisting in determining the distance r. The user input device  18  allows the listener  2  to periodically enter the distance r he/she is from the speaker array  4 . For example, while watching a movie the listener  2  may initially be seated on a couch six feet from the speaker array  4 . The listener  2  may enter this distance of six feet into the distance estimator  16  using the user input device  18 . Midway through the movie, the listener  2  may decide to move to a table ten feet from the speaker array  4 . Based on this movement, the listener  2  may enter this new distance r A  into the distance estimator  16  using the user input device  18 . The user input device  18  may be a wired or wireless keyboard, a mobile device, or any other similar device that allows the listener  2  to enter a distance into the distance estimator  16 . In one embodiment, the entered value is a non-numeric or a relative value. For example, the listener  2  may indicate that they are far from or close to the speaker array  4  without indicating a specific distance. 
     In another embodiment, a microphone  19  may be coupled to the distance estimator  16  for assisting in determining the distance r. In this embodiment, the microphone  19  is located with the listener  2  or proximate to the listener  2 . The directivity adjustment device  8  drives the speaker arrays  4  to emit a set of test sounds that are sensed by the microphone  19  and fed to the distance estimator  16  for processing. The distance estimator  16  determines the propagation delay of the test sounds as they travel from the speaker array  4  to the microphone  19  based on the sensed sounds. The propagation delay may thereafter be used to determine the distance r A  from the speaker array  4  to the listener  2 . 
     The microphone  19  may be coupled to the distance estimator  16  using a wired or wireless connection. In one embodiment, the microphone  19  is integrated in a mobile device (e.g., a mobile phone) and the sensed sounds are transmitted to the distance estimator  16  using one or more wireless protocols (e.g., Bluetooth and IEEE 802.11x). The microphone  19  may be any type of acoustic-to-electric transducer or sensor, including a MicroElectrical-Mechanical System (MEMS) microphone, a piezoelectric microphone, an electret condenser microphone, or a dynamic microphone. The microphone  19  may provide a range of polar patterns, such as cardioid, omnidirectional, and figure-eight. In one embodiment, the polar pattern of the microphone  19  may vary continuously over time. Although shown and described as a single microphone  19 , in one embodiment, multiple microphones or microphone arrays may be used for detecting sounds in the room  3 . 
     In another embodiment, a camera  20  may be coupled to the distance estimator  16  for assisting in determining the distance r. The camera  20  may be a video camera or still-image camera that is pointed in the same direction as the speaker array  4  into the room  3 . The camera  20  records a video or set of still images of the area in front of the speaker array  4 . Based on these recordings, the camera  20  alone or in conjunction with the distance estimator  16  tracks the face or other body parts of the listener  2 . The distance estimator  16  may determine the distance r A  from the speaker array  4  to the listener  2  based on this face/body tracking. In one embodiment, the camera  20  tracks features of the listener  2  periodically while the speaker array  4  outputs sound program content such that the distance r A  may be updated and remains accurate. For example, the camera  20  may track the listener  2  continuously while a song is being played through the speaker array  4 . 
     The camera  20  may be coupled to the distance estimator  16  using a wired or wireless connection. In one embodiment, the camera  20  is integrated in a mobile device (e.g., a mobile phone) and the recorded videos or still images are transmitted to the distance estimator  16  using one or more wireless protocols (e.g., Bluetooth and IEEE 802.11x). Although shown and described as a single camera  20 , in one embodiment, multiple cameras may be used for face/body tracking. 
     In still another embodiment, one or more infrared (IR) sensors  21  are coupled to the distance estimator  16 . The IR sensors  21  capture IR light radiating from objects in the area in front of the speaker array  4 . Based on these sensed IR readings, the distance estimator  16  may determine the distance r A  from the speaker array  4  to the listener  2 . In one embodiment, the IR sensors  21  periodically operate while the speaker array  4  outputs sound such that the distance r A  may be updated and remains accurate. For example, the IR sensors  21  may track the listener  2  continuously while a song is being played through the speaker array  4 . 
     The infrared sensors  21  may be coupled to the distance estimator  16  using a wired or wireless connection. In one embodiment, the infrared sensors  21  are integrated in a mobile device (e.g., a mobile phone) and the sensed infrared light readings are transmitted to the distance estimator  16  using one or more wireless protocols (e.g., Bluetooth and IEEE 802.11x). 
     Although described above in relation to a single listener  2 , in one embodiment the distance estimator  16  may determine the distance r A  between multiple listeners  2  and the speaker array  4 . In this embodiment, an average distance r A  between the listeners  2  and the speaker array  4  is used to adjust sound emitted by the speaker array  4 . 
     Using any combination of techniques described above, the distance estimator  16  calculates and feeds the distance r to the directivity compensator  17  for processing. The directivity compensator  17  computes a beam pattern that maintains a constant direct-to-reverberant sound ratio.  FIGS. 4A and 4B  demonstrate the changes to the direct-to-reverberant sound ratio relative to the listener  2  as the distance r increases. 
     In  FIG. 4A , the listener  2  is a distance r A  from the speaker array  4 . In this example situation, the listener  2  is receiving a direct sound energy level D A  from the speaker array  4  and an indirect or reverberant sound energy level R A  from the speaker array  4  after the original sound has reflected off surfaces in the room  3 . The distance r A  may be viewed as the propagation distance for the direct sounds while the distance g A  may be viewed as the propagation distance for the reverberant sounds. In one embodiment, the direct sound energy D A  may be calculated as 
             1     r   2           
while the reverberant sound energy R A  may be calculated as
 
                 100   ⁢      ⁢           ⁢     T   60       VDI     ,         
where T 60  is the reverberation time in the room, V is the functional volume of the room, and DI is the directivity index of a sound pattern emitted by the speaker array  4  at the listener  2 . In this example, since the direct sounds have a shorter distance to travel to the listener  2  than the reverberant sounds (i.e., shorter propagation distance), the direct sound energy level D A  is greater than the reverberant sound energy level R A .
 
     As the listener  2  moves farther from the speaker array  4  to generate a larger propagation distance r B  as shown in  FIG. 4B , the direct sound energy D B  has time to spread out before arriving at the listener  2 . This increased propagation distance r B  results in D B  being noticeably less than D A . In contrast, as the listener  2  moves farther from the speaker array  4  the propagation distance g B  only slightly increases from the original distance g A . This minor change in reverberant propagation distance results in a marginal decrease in reverberant energy from R A  to R B . The reverberant field as shown in  FIGS. 4A and 4B  is merely illustrative. In some embodiments, the reverberant field may be made up of hundreds of reflections such that when the listener  2  moves farther away from the speaker array  4  (e.g., the source) the listener  2  is moving farther from the first reflections (as shown in  FIGS. 4A and 4B ) but the listener  2  might actually be moving closer to other reflections (e.g., reflections off of the back wall) such that overall the reverberant energy is not noticeably affected by the listener  2 &#39;s location in the room  3 . 
     As can be seen in  FIGS. 4A and 4B  and described above, as the listener  2  moves away from the speaker array  4 , the direct-to-reverberant energy ratio decreases since the propagation distance of the reflected sound waves only slightly increases while the propagation distance of the direct sound waves increases relatively more. To compensate for this ratio change, the directivity index DI of a sound pattern emitted by the speaker array  4  may be changed to maintain a constant ratio of direct-to-reverberant sound energy based on the distance r. For example, if a beam pattern generated by a speaker array is narrow and pointed at a listener, the direct-to-reverberant ratio will be large since the listener is receiving a large amount of direct energy and a comparatively smaller amount of reflected energy. Alternatively, if a beam pattern generated by the speaker array is wide, the direct-to-reverberant ratio is smaller as the listener is receiving comparatively more sound reflected off surfaces and objects. Altering the directivity index DI of a sound pattern emitted by the speaker array  4  may increase or decrease the amount of direct and reverberant sound emitted toward the listener  2 . This change in direct and reverberant sound consequently alters the direct-to-reverberant energy ratio. 
     As noted above, each of the transducers in the speaker array  4  may be separately driven according to different parameters and settings (including delays and energy levels). By independently driving each of the transducers  5 , the directivity adjustment device  8  may produce a wide variety of directivity patterns with different directivity indexes DI to maintain a constant direct-to-reverberant energy ratio.  FIG. 5  shows an example set of sound patterns with different directivity indexes. The leftmost pattern is omnidirectional and corresponds to a low directivity index DI, the middle pattern is slightly more directed at the listener  2  and corresponds to a larger directivity index DI, and the rightmost pattern is highly directed at the listener  2  and corresponds to the largest directivity index DI. The described set of sound patterns is purely illustrative and in other embodiments other sound patterns may be generated by the directivity adjustment device  8  and emitted by the speaker array  4 . 
     In one embodiment, the directivity compensator  17  may calculate a directivity pattern with an associated directivity index DI that maintains a predefined direct-to-reverberant energy ratio. The predefined direct-to-reverberant energy ratio may be preset during manufacture of the directivity adjustment device  8 . For example, a direct-to-reverberant energy ratio of 2:1 may be preset by a manufacturer or designer of the directivity adjustment device  8 . In this example, the directivity compensator  17  calculates a directivity index DI that maintains the 2:1 ratio between direct-to-reverberant energy in view of the detected distance r between the listener  2  and the speaker array  4 . 
     Upon calculation of a directivity index DI, the directivity compensator  17  feeds this value to the array processor  15 . As noted above, the directivity compensator  17  may continually calculate directivity indexes DI for each channel of the sound program content played by the directivity adjustment device  8  as the listener  2  moves around the room  3 . The audio channels output by the multiplexer  13  are processed by the array processor  15  to produce a set of audio signals that drive one or more of the transducers  5  to produce a beam pattern with the calculated directivity index DI. The processing may operate in both the time and frequency domains using transforms such as the Fast Fourier Transform (FFT). 
     In one embodiment, the array processor  15  decides which transducers  5  in the loudspeaker array  4  output one or more segments of audio based on the calculated directivity index DI received from the directivity compensator  17 . In this embodiment, the array processor  15  may also determine delay and energy settings used to output the segments through the selected transducers  5 . The selection and control of a set of transducers  5 , delays, and energy levels allows the segment to be output according to the calculated directivity index DI that maintains the preset direct-to-reverberant energy ratio. 
     As shown in  FIG. 3 , the processed segment of the sound program content is passed from the array processor  15  to the one or more digital-to-analog converters  22  to produce one or more distinct analog signals. The analog signals produced by the digital-to-analog converters  22  are fed to the power amplifiers  23  to drive selected transducers  5  of the loudspeaker array  4 . 
     In one example situation, the listener  2  may be seated on a couch across from a speaker array  4 . The directivity adjustment device  8  may be playing an instrumental musical piece through the speaker array  4 . In this situation, the directivity adjustment device  8  may seek to maintain a 1:1 direct-to-reverberant energy ratio. Upon commencement of the musical piece, the distance estimator  16  detects that the listener  2  is six feet from the speaker array  4  using the camera  20 . To maintain a 1:1 direct-to-reverberant energy ratio based on this distance, the directivity compensator  17  calculates that the speaker array  4  must output a beam pattern with a directivity index DI of four decibels. The array processor  15  is fed the calculated directivity index DI and processes the musical piece to output a beam pattern of four decibels. Several minutes later, the distance estimator  16 , with assistance from the camera  20 , detects that the listener  2  is now seated four feet from the speaker array  4 . In response, the directivity compensator  17  calculates that the speaker array  4  must output a beam pattern with a directivity index DI of two decibels to maintain a 1:1 direct-to-reverberant energy ratio. The array processor  15  is fed the updated directivity index and processes the musical piece to output a beam pattern of two decibels. After another several minutes has passed, the distance estimator  16 , with assistance from the camera  20 , detects that the listener  2  is now seated ten feet from the speaker array  4 . In response, the directivity compensator  17  calculates that the speaker array  4  must output a beam pattern with a directivity index DI of eight decibels to maintain a 1:1 direct-to-reverberant energy ratio. The array processor  15  is fed the updated directivity index and processes the musical piece to output a beam pattern of eight decibels. As described in the above example situation, the directivity adjustment device  8  maintains the predefined direct-to-reverberant energy ratio regardless of the location of the listener  2  by adjusting the directivity index DI of a beam pattern emitted by the speaker array  4 . 
     In one embodiment, different direct-to-reverberant energy ratios are preset in the directivity adjustment device  8  corresponding to the content of the audio played by the directivity adjustment device  8 . For example, speech content in a movie may have a higher desired direct-to-reverberant energy ratio in comparison to background music in the movie. Below is an example table of content dependent direct-to-reverberant energy ratios. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                   
                 Direct-to-Reverberant Energy 
               
               
                   
                 Content Type 
                 Ratio 
               
               
                   
                   
               
             
            
               
                   
                 Foreground 
                 4:1 
               
               
                   
                 Dialogue/Speech 
                   
               
               
                   
                 Background 
                 3:1 
               
               
                   
                 Dialogue/Speech 
                   
               
               
                   
                 Sound Effects 
                 2:1 
               
               
                   
                 Background Music 
                 1:1 
               
               
                   
                   
               
            
           
         
       
     
     The directivity compensator  17  may simultaneously calculate separate beam patterns with associated directivity indexes DI that maintain corresponding direct-to-reverberant ratio for segments of audio in separate streams or channels. For example, sound program content for a movie may have multiple streams or channels of audio. Each channel may include distinct features or types of audio. For instance, the movie may include five channels of audio corresponding to a front left channel, a front center channel, a front right channel, a rear right surround, and a rear left surround. In this example, the front center channel may contain foreground speech, the front left and right channels may contain background music, and the rear left and right surround channels may contain sound effects. Using the example direct-to-reverberant energy ratios shown in the above table, the directivity compensator  17  may maintain a direct-to-reverberant ratio of 4:1 for the front center channel, a 1:1 direct-to-reverberant ratio for the front left and right channels, and a 2:1 direct-to-reverberant ratio for the rear left and right surround channels. As described above, the direct-to-reverberant ratios would be maintained for each channel by calculating beam patterns with directivity indexes DI that compensate for the changing distance r of the listener  2  from the speaker array  4 . 
     In one embodiment, the sound pressure P apparent to the listener  2  at a distance r from the speaker array  4  may be defined as: 
     
       
         
           
             
               P 
               2 
             
             = 
             
               Q 
               ⁡ 
               
                 [ 
                 
                   
                     1 
                     
                       r 
                       2 
                     
                   
                   + 
                   
                     
                       1 
                       ⁢ 
                       00 
                       ⁢ 
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         T 
                         
                           6 
                           ⁢ 
                           0 
                         
                       
                     
                     VDI 
                   
                 
                 ] 
               
             
           
         
       
     
     Where Q is the sound power level (e.g., volume) of a sound signal produced by the directivity adjustment device  8  to drive the speaker array  4 , T 60  is the reverberation time in the room, V is the functional volume of the room, and DI is the directivity index of the sound pattern emitted by the speaker array  4 . In one embodiment, the directivity adjustment device  8  maintains a constant sound pressure P as the distance r changes by adjusting the sound power level Q and/or the directivity index DI of a beam pattern emitted by the speaker array  4 . 
     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 which 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.