Patent Publication Number: US-RE48233-E

Title: Passive directional acoustic radiating

Description:
More than one reissue application has been filed for U.S. Pat. No. 8,447,055, both this application and U.S. application Ser. No. 14/675,034, now U.S. Pat. No. RE46,811. This application is a reissue application of U.S. application Ser. No. 12/854,982, now U.S. Pat. No. 8,447,055, which is a continuation in part of U.S. application Ser. No. 12/114,261, now U.S. Pat. No. 8,351,630. This application is also a continuation reissue application of U.S. application Ser. No. 14/675,034, now U.S. Pat. No. RE46,811, which is a reissue of U.S. Pat. No. 8,447,055.  
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of, and claims priority of, U.S. patent application Ser. No. 12/114,261, published as U.S. Published Pat. App. 2009-0274329 A1, entitled “Passive Directional Acoustic Radiating”, filed May 2, 2008 by Ickler, et al. 
    
    
     BACKGROUND 
     This specification describes an audio system for a television employing directional audio devices. 
     SUMMARY 
     In one aspect an audio system includes at least a left channel, a right channel, and a center channel. The audio system includes a crossover network for separating the left channel, the right channel, and the center channel into low frequency content, midrange frequency content, and high frequency content; an omnidirectional acoustical device for radiating acoustic energy corresponding to the low frequency content of the combined left channel, right channel, and center channel; a first directional array for radiating acoustic energy, comprising signal processing circuitry and more than one acoustic driver, for radiating acoustic energy corresponding to the midrange content of one of the left channel and right channel signal so that more acoustic energy corresponding to the midrange content of one of the left channel signal and the right channel signal is radiated laterally than in other directions; and a first passive directional device, for radiating acoustic energy corresponding to the high frequency content of the one of the left channel and right channel signal so that more acoustic energy corresponding to the high frequency content of the one of the left channel signal and the right channel signal is radiated laterally than in other directions. The audio system may include a second directional array for radiating acoustic energy, comprising signal processing circuitry and more than one acoustic driver for radiating acoustic energy corresponding to the midrange content of the other of the left channel and right channel so that more acoustic energy corresponding to high frequency content of the other of the left channel and right channel signal is radiated laterally than in other directions; and a second passive directional device, for radiating acoustic energy corresponding to the midrange content of the other of the left channel and right channel so that more acoustic energy corresponding to high frequency content of the other of the left channel and right channel signal is radiated laterally than in other directions. The first directional array, the second directional array, the first passive directional device and the second passive directional device may be mounted in a common enclosure. The common enclosure may be a television cabinet. The first directional array and the second directional array may include at least one common driver. The audio system of may further include a third directional array for radiating acoustic energy, comprising signal processing circuitry and more than one acoustic driver for radiating acoustic energy corresponding to the midrange content of the center channel so that more acoustic energy corresponding to the center channel signal is radiated in a direction substantially orthogonal to the direction of greater radiation of the first directional array and the direction of greater radiation of the second directional array. The audio system may further include a non-directional high frequency acoustical device for radiating the high frequency content of the center channel. The non-directional high frequency device and the third directional array may positioned in a television on vertically opposite sides of a television screen. At least two of the first directional array, the second directional array, and the third directional array may include at least one acoustic driver in common. The direction substantially orthogonal to the direction of greater radiation of the first directional array and the direction of greater radiation of the second directional array is substantially upward. The direction substantially orthogonal to the direction of greater radiation of the first directional array and the direction of greater radiation of the second directional array may be substantially toward an intended listening area. The omnidirectional device may include a waveguide. The waveguide may be mounted in a television cabinet. At least two of the first directional array, the second directional array, and the third directional array include more than one acoustic driver in common. The first directional array, the second directional array, and the third directional array may include more than one acoustic driver in common. The audio system may be mounted in a television cabinet. The omnidirectional acoustical device, the first directional array, the second directional array, the third directional array, the first passive directional device, and the second passive directional device each have an exit through which acoustic energy is radiated to the environment, and none of the exits may be in a front face of the television cabinet. The first passive directional device may include a slotted pipe type passive directional acoustic device comprising an acoustic driver, acoustically coupled to a pipe to radiate acoustic energy into the pipe. The pipe may include an elongated opening along at least a portion of the length of the pipe; and acoustically resistive material in the opening through which pressure waves are radiated to the environment. The pressure waves characterized by a volume velocity. The pipe, the opening, and the acoustically resistive material may be configured so that the volume velocity is substantially constant along the length of the pipe. 
     In another aspect, a method for operating an audio system comprising at least a left channel, a right channel, and a center channel, includes radiating omnidirectionally acoustic energy corresponding to the low frequency content of the combined left channel, right channel, and center channel; radiating directionally, from a first directional array comprising signal processing circuitry and more than one acoustic driver, acoustic energy corresponding to the midrange content of the left channel so that more acoustic energy corresponding to the left channel signal is radiated leftwardly than in other directions; radiating directionally, from a second directional array comprising signal processing circuitry and more than one acoustic driver, acoustic energy corresponding to the midrange content of the right channel so that more acoustic energy corresponding to the right channel signal is radiated rightwardly than in other directions; radiating directionally, from a third directional array comprising signal processing circuitry and more than one acoustic driver, acoustic energy corresponding to the midrange content of the center channel so that more acoustic energy corresponding to the center channel signal is radiated in a direction substantially orthogonal to the direction of greater radiation of the first directional array and the direction of greater radiation of the second directional array; radiating directionally, from a first passive directional device, acoustic energy corresponding to the high frequency content of the left channel so that more acoustic energy is radiated leftwardly than other directions; and radiating directionally, from a second passive directional device, acoustic energy corresponding to the high frequency content of the right channel so that more acoustic energy is radiated rightwardly than other directions. The method may further include radiating non-directionally the high the high frequency content of the center channel. Radiating non-directionally the high frequency content of the center channel may include radiating from a vertically opposite side of a television screen from the radiating directionally of the midrange content of the center channel. The radiating omnidirectionally acoustic energy corresponding to the low frequency content of the combined left channel, right channel, and center channel may include radiating from a waveguide. 2.2.1. The radiating omnidirectionally may include radiating from a waveguide is mounted in a television cabinet. The directionally radiating in a direction substantially orthogonal to the direction of greater radiation of the first directional array and the direction of greater radiation of the second directional array may include radiating substantially upward. The directionally radiating in a direction substantially orthogonal to the direction of greater radiation of the first directional array and the direction of greater radiation of the second directional array may include radiating substantially toward an intended listening area. The radiating directionally from a first directional array, the radiating directionally from a second directional array, the radiating directionally from a third directional array, the radiating directionally from a first passive directional device and the radiating directionally from a second passive directional device may include radiating from a television cabinet. The radiating directionally from a first directional array, the radiating directionally from a second directional array, the radiating directionally from a third directional array, the radiating directionally from a first passive directional device and the radiating directionally from a second passive directional device may include radiating from one of a side, a bottom, or a top of a television cabinet. 
     In another aspect, an audio system for a television may include a television cabinet; a slotted pipe type passive directional acoustic device that includes an acoustic driver, acoustically coupled to a pipe to radiate acoustic energy into the pipe. The pipe may include an elongated opening along at least a portion of the length of the pipe; and acoustically resistive material in the opening through which pressure waves are radiated to the environment. The pressure waves may be characterized by a volume velocity. The pipe, the opening, and the acoustically resistive material may be configured so that the volume velocity is substantially constant along the length of the pipe. The passive directional acoustic device may be mounted in the television cabinet to directionally radiate sound waves laterally from the television cabinet. the pipe may be at least one of bent or curved. The opening may be at least one of bent or curved along its length. The opening may be in a face that is bent or curved. The television cabinet may be tapered backwardly, and the passive directional acoustic device may be mounted so that a curved or bent wall of the slotted pipe type passive directional acoustic device is substantially parallel to the back and a side wall of the television cabinet. The opening may include two sections, a first section in a top face of the pipe and a second section in a side face of the pipe. The audio system for a television of claim  10 . 0 , wherein the acoustic apparatus may be for radiating the high frequency content of a left channel or a right channel laterally from the television. The passive directional acoustic device may be for radiating the left channel or right channel content above 2 kHz. The audio system may further include a directional array for radiating midrange frequency content of the left channel or right channel laterally from the television. The audio system may further include a waveguide structure for radiating bass frequency content of the left channel or right channel; the other of the left channel or right channel; and a center channel. The cross sectional area of the pipe may decrease along the length of the pipe. The audio system may further include The audio system may further include a second slotted pipe type passive directional acoustic device comprising a second acoustic driver, acoustically coupled to a pipe to radiate acoustic energy into the pipe. The second pipe may include an elongated opening along at least a portion of the length of the pipe; and acoustically resistive material in the opening through which pressure waves are radiated to the environment. The pressure waves may be characterized by a volume velocity. The pipe, the opening, and the acoustically resistive material may be configured so that the volume velocity is substantially constant along the length of the pipe. The first passive directional acoustic device may be mounted in the television cabinet to directionally radiate sound waves laterally leftward from the television cabinet and the second passive radiator may be mounted in the television cabinet to directionally radiate sound waves laterally rightward from the television cabinet. 
     Other features, objects, and advantages will become apparent from the following detailed description, when read in connection with the following drawing, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIGS. 1A .  1 C, and  1 E are top diagrammatic views of an audio module mounted in a television; 
         FIGS. 1B and 1D  are front diagrammatic views of the audio module mounted in a television; 
         FIG. 2  is a front diagrammatic view of the audio module, showing the location of the center channel speakers; 
         FIG. 3A  is a block diagram of an audio system; 
         FIG. 3B  is a block diagram showing an alternate configuration of some of the elements of the audio system of  FIG. 3A ; 
         FIG. 4A  is a diagrammatic view of a low frequency device of the audio system; 
         FIG. 4B  is an isometric drawing of an actual implementation of the audio system; 
         FIG. 5  is a diagrammatic view of the audio module; 
         FIGS. 6A-6D  are diagrammatic views of the elements of the audio module used as directional arrays; 
         FIGS. 7A and 7B  are diagrammatic views of a passive directional acoustic device; 
         FIG. 7C  is an isometric view of an actual implementation or the passive directional device of  FIGS. 7A and 7B ; and 
         FIG. 8  is a diagrammatic view of a passive directional audio device, mounted in a television. 
     
    
    
     DETAILED DESCRIPTION 
     Though the elements of several views of the drawing may be shown and described as discrete elements in a block diagram and may be referred to as “circuitry”, unless otherwise indicated, the elements may be implemented as one of, or a combination of, analog circuitry, digital circuitry, or one or more microprocessors executing software instructions. The software instructions may include digital signal processing (DSP) instructions. Operations may be performed by analog circuitry or by a microprocessor executing software that performs the mathematical or logical equivalent to the analog operation. Unless otherwise indicated, signal lines may be implemented as discrete analog or digital signal lines, as a single discrete digital signal line with appropriate signal processing to process separate streams of audio signals, or as elements of a wireless communication system. Some of the processes may be described in block diagrams. The activities that are performed in each block may be performed by one element or by a plurality of elements, and may be separated in time. The elements that perform the activities of a block may be physically separated. One element may perform the activities of more than one block. Unless otherwise indicated, audio signals or video signals or both may be encoded and transmitted in either digital or analog form; conventional digital-to-analog or analog-to-digital converters may not be shown in the figures. For simplicity of wording “radiating acoustic energy corresponding to the audio signals in channel x” will be referred to as “radiating channel x.” “Directional arrays”, as used herein, refers to arrays that use a combination of signal processing and geometry, placement, and configuration of more than one acoustic driver to cause the radiation to be greater in some directions than in other directions. Directional arrays include interference arrays, such as described in U.S. Pat. No. 5,870,484 and U.S. Pat. No. 5,809,153. “Passive directional device”, as used herein, refers to devices that do not use any signal processing, but rather use only mechanical or physical arrangements or devices to cause the radiation of wavelengths that are large (for example 2×) relative to the diameter of the radiating elements to be greater in some directions than in others. Passive directional devices could include acoustic lenses, horns, dipole radiators, or slotted pipe type directional devices shown below and in  FIGS. 7A-7C  and described in the corresponding portions of the specification. 
       FIG. 1A  shows a diagrammatic view of an audio module  10 . The audio module  10  may be associated with, or built into, a television  12 . The audio module radiates acoustic signals of some frequency ranges corresponding to a audio system including at least a left channel, a right channel, and a center channel. 
     The left channel midrange (L M ) frequency sound is radiated by a directional array so that more acoustic energy is radiated laterally leftward relative to a listening area than in other directions as indicated. The right channel midrange (R M ) frequency sound is radiated by a directional array so that more acoustic energy is radiated laterally rightward than in other directions as indicated. 
     The left channel high (L H ) frequency sound is radiated by a passive directional device so that more acoustic energy is radiated laterally leftward than in other directions as indicated. The right channel high (R H ) frequency sound is radiated by a passive directional device so that more acoustic energy is radiated laterally rightward than in other directions as indicated. 
     Radiating the left and right channels directionally laterally causes more of radiation experienced by the listener to be indirect radiation than direct radiation or radiation of the left and right channels toward the listening area. Causing more of the radiation to be indirect radiation results in a more spacious acoustic image and permits the radiation of the left and right channels from a device in the lateral middle of the listening area. 
       FIGS. 1B-1E  show different implementations of the radiation pattern of the center channel. 
     In  FIGS. 1B and 1C , the center channel midrange (C M ) frequency sound is radiated by a directional array so that more energy is radiated in a direction substantially orthogonal to the directions of maximum radiation of the left and right channel midrange frequency sound than is radiated in other directions. The center channel high (C H ) frequency sound is radiated directionally by a passive directional device so that more energy is radiated in a direction substantially orthogonal to the directions of maximum radiation of the left and right channel midrange frequency sound than is radiated in other directions. In  FIG. 1B , the direction of maximum radiation of the center channel midrange frequency sound and the high frequency sound is upward relative to the listening area. In  FIG. 1C , the direction of maximum radiation the center channel midrange frequency sound and the high frequency sound is toward the listening area. In other implementations, the direction of maximum radiation of the center channel midrange frequency and the high frequency could be substantially downward. The direction of maximum radiation of the center channel midrange frequency sound and the direction of maximum radiation of the center channel high frequency sound do not need to be the same direction; for example, the center channel midrange frequency sound could be radiated substantially upwardly, and the center channel high frequency sound could be radiated substantially toward the listening area. The low frequency device, which will be described below, may be mounted in a television cabinet  46 . 
     In  FIGS. 1D and 1E , the center channel midrange frequency sound is radiated by a directional array so that more energy is radiated in a direction substantially orthogonal to the directions of maximum radiation of the left and right channel midrange frequency sound than is radiated in other directions. The center channel high frequency sound is radiated substantially omnidirectionally. In  FIG. 1D , the direction of maximum radiation the center channel midrange frequency is upward relative to the listening area. In  FIG. 1E , the direction of maximum radiation the center channel midrange frequency sound is toward the listening area. 
     When implemented in a television, the center channel high frequency acoustical device may be vertically on the opposite side of the television screen from the center channel directional array to cause the acoustic image to be vertically centered on the television screen. For example, as shown in  FIG. 2 , if the center channel directional array  44  is above the television screen  52 , the center channel high frequency acoustical device  45  may be positioned below the television screen. 
       FIG. 3A  is a block diagram showing some signal processing elements of the audio module  10  of  FIGS. 1A-1E . The signal processing elements of  FIG. 3A  are parts of a three-way crossover system that separates the input channel into three frequency bands (hereinafter referred to as a bass frequency band, a midrange frequency band, and a high frequency band), none of which are substantially encompassed by any of the other frequency bands. The signal processing elements of  FIG. 3A  processes and radiates the three frequency bands differently. 
     The left channel signal L, the right channel signal R, and the center channel signal C are combined at signal summer  29  and low pass filtered by low pass filter  24  to provide a combined low frequency signal. The combined low frequency signal is radiated by a low frequency radiation device  26 , such as a woofer or another acoustic device including low frequency augmentation elements such as ports, waveguides, or passive radiators. Alternatively, the left channel signal, the right channel signal, and the center channel signal may be low pass filtered, then combined before being radiated by the low frequency radiation device, as shown in  FIG. 3B . 
     In  FIG. 3A , the left channel signal is band pass filtered by band pass filter  28  and radiated directionally by left channel array  30 . The left channel signal is high pass filtered by high pass filter  32  and radiated directionally (as indicated by the arrow extending from element  34 ) by passive directional device  34 . 
     The right channel signal is band pass filtered by band pass filter  28  and radiated directionally by right channel array  38  as shown in  FIGS. 1A-1E . The right channel signal is high pass filtered by high pass filter  32  and radiated directionally by passive directional device  42 . 
     The center channel signal is band pass filtered by band pass filter  28  and radiated directionally by center channel array  44  as shown in  FIGS. 1B-1E . The center channel signal is high pass filtered by high pass filter  32  and radiated directionally by a high frequency acoustical device  45  (which, as stated above may be directional or omnidirectional, as indicated by the dotted line arrow extending from element  45 ). 
     In one implementation, the break frequency of low pass filter  24  is 250 Hz, the pass band for band pass filter  28  is 250 Hz to 2.5 k Hz, and the break frequency for high pass filter  32  is 2 kHz. 
     In one implementation, the low frequency device  26  of  FIG. 3A  includes a waveguide structure as described in U.S. Published Pat. App. 2009-0214066 A1, incorporated herein by reference in its entirety. The waveguide structure is shown diagrammatically in  FIG. 4A . An actual implementation of the low frequency device of  FIG. 4A  is shown in  FIG. 4B . Reference numbers in  FIG. 4B  correspond to like numbered elements of  FIG. 4A . The low frequency device may include a waveguide  412  driven by six 2.25 inch acoustic drivers  410 A- 410 D mounted near the closed end  411  of the waveguide. There are acoustic volumes  422 A and  422 B acoustically coupled to the waveguide at the locations  434 A and  434 B along the waveguide. The cross sectional area of the waveguide increases at the open end  418 . The implementation of  FIG. 4B  has one dimension that is small relative to the other two dimensions and can be conveniently enclosed in a flat panel wide screen television cabinet, such as the cabinet  46  of the television  12 . 
     Directional arrays  30 ,  38 , and  44  are shown diagrammatically in  FIG. 3A  as having two acoustic drivers. In actual implementations, they may have more than two acoustic drivers and may share common acoustic drivers. In one implementation, the left directional array  30 , the right directional array  38 , and the center directional array  44  are implemented as a multi-element directional array such as is described in U.S. patent application Ser. No. 12/716,309 filed Mar. 3, 2010 by Berardi, et al., incorporated herein by reference in its entirety. 
       FIG. 5  shows an acoustic module that is suitable for the left channel array  30 , the right channel array  38  of  FIG. 3A , and the center channel array  44  (all shown in  FIG. 3A ). An audio module  212  includes a plurality, in this embodiment seven, of acoustic drivers  218 - 1 - 218 - 7 . One of the acoustic drivers  218 - 4  is positioned near the lateral center of the module, near the top of the audio module. Three acoustic drivers  218 - 1 - 218 - 3  are positioned near the left extremity  220  of the audio module and are closely and non-uniformly spaced, so that distance l 1 ≠l 2 , l 2 ≠l 3 , l 1 ≠3. Additionally, the spacing may be arranged so that l 1 &lt;l 2 &lt;l 3 . Similarly, distance l 6 ≠l 5 , l 5 ≠l 4 , l 6 ≠4. Additionally, the spacing may be arranged so that l 6 &lt;l 5 &lt;l 4 . In one implementation, l 1 =l 6 =55 mm, l 2 =l 5 =110 mm, and l 3 =l 4 =255 mm. The left channel array  30 , the right channel array  38 , and the center channel array  44  of  FIG. 3A  each include subsets of the seven acoustic drivers  218 - 1 - 218 - 7 . 
     The directional radiation patterns of the midrange frequency bands of  FIGS. 1A-1E  are accomplished by interference type directional arrays consisting of subsets of the acoustic drivers  218 - 1 - 218 - 7 . Interference type directional arrays are discussed in U.S. Pat. No. 5,870,484 and U.S. Pat. No. 5,809,153. At frequencies at which the individual acoustic drivers radiate substantially omnidirectionally (for example frequencies with corresponding wavelengths that are more than twice the diameter of the radiating surface of the acoustic drivers), radiation from each of the acoustic drivers interferes destructively or non-destructively with radiation from each of the other acoustic drivers. The combined effect of the destructive and non-destructive interference is that the radiation is some directions is significantly less, for example, −14 dB, relative to the maximum radiation in any direction. The directions at which the radiation is significantly less than the maximum radiation in any direction may be referred to as “null directions”. Causing more radiation experienced by a listener to be indirect radiation is accomplished by causing the direction between the audio module and the listener to be a null direction and so that more radiation is directed laterally relative to the listener. 
       FIG. 6A  shows a diagrammatic view of audio module  212 , showing the configuration of directional arrays of the audio module. The audio module is used to radiate the channels of a multi-channel audio signal source  222 . Typically, a multi-channel audio signal source for use with a television has at least a left (L), right (R), and Center (C) channel. In  FIG. 6A , the left channel array  30  includes acoustic drivers  218 - 1 ,  218 - 2 ,  218 - 3 ,  218 - 4 , and  218 - 5 . The acoustic drivers  218 - 1 - 218 - 5  are coupled to the left channel signal source  238  by signal processing circuitry  224 - 1 - 224 - 5 , respectively that apply signal processing represented by transfer function H 1L (z)-H 5L (z), respectively. The effect of the transfer functions H 1L (z)-H 5L (z) on the left channel audio signal may include one or more of phase shift, time delay, polarity inversion, and others. Transfer functions H 1L (z)-H 5L (z) are typically implemented as digital filters, but may be implemented with equivalent analog devices. 
     In operation, the left channel signal L, as modified by the transfer functions H 1L (z)-H 5L (z) is transduced to acoustic energy by the acoustic drivers  218 - 1 - 218 - 5 . The radiation from the acoustic drivers interferes destructively and non-destructively to result in a desired directional radiation pattern. To achieve a spacious stereo image, the left array  232  directs radiation laterally toward the left boundary of the room as indicated by arrow  213  and cancels radiation toward the listener. The use of digital filters to apply transfer functions to create directional interference arrays is described, for example, in Boone, et al., Design of a Highly Directional Endfire Loudspeaker Array, J. Audio Eng. Soc., Vol 57. The concept is also discussed with regard to microphones van der Wal et al., Design of Logarithmically Spaced Constant Directivity-Directivity Transducer Arrays, J. Audio Eng. Soc., Vol. 44, No. 6, June 1996 (also discussed with regard to loudspeakers), and in Ward, et al., Theory and design of broadband sensor arrays with frequency invariant far-field beam patterns, J. Acoust. Soc. Am. 97 (2), February 1995. Mathematically, directional microphone array concepts may generally be applied to loudspeakers. 
     Similarly, in  FIG. 6B , the right channel array  38  includes acoustic drivers  218 - 3 ,  218 - 4 ,  218 - 5 ,  218 - 6 , and  218 - 7 . The acoustic drivers  218 - 3 - 218 - 7  are coupled to the right channel signal source  240  and to signal processing circuitry  224 - 3 - 224 - 7 , respectively that apply signal processing represented by transfer function H 3R (z)-H 7R (z), respectively. The effect of the transfer functions H 3R (z)-H 7R (z) may include one or more of phase shift, time delay, polarity inversion, and others. Transfer functions H 3R (z)-H 7R (z) are typically implemented as digital filters, but may be implemented with equivalent analog devices. 
     In operation, the right channel signal R, as modified by the transfer functions H 3R (z)-H 7R (z) is transduced to acoustic energy by the acoustic drivers  218 - 3 - 218 - 7 . The radiation from the acoustic drivers interferes destructively and non-destructively to result in a desired directional radiation pattern. To achieve a spacious stereo image, the right array  234  directs radiation laterally toward the right boundary of the room as indicated by arrow  215  and cancels radiation toward the listener. 
     In  FIG. 6C , the center channel array  44  includes acoustic drivers  218 - 2 ,  218 - 3 ,  218 - 4 ,  218 - 5 , and  218 - 6 . The acoustic drivers  218 - 2 - 218 - 6  are coupled to the center channel signal source  242  by signal processing circuitry  224 - 2 - 224 - 6 , respectively that apply signal processing represented by transfer function H 2C (z)-H 6C (z), respectively. The effect of the transfer functions H 2C (z)-H 6C (z) may include one or more of phase shift, time delay, polarity inversion, and others. Transfer functions H 2C (z)-H 6C (z) are typically implemented as digital filters, but may be implemented with equivalent analog devices. 
     In operation, the center channel signal C, as modified by the transfer functions H 2C (z)-H 6C (z) is transduced to acoustic energy by the acoustic drivers  218 - 2 - 218 - 6 . The radiation from the acoustic drivers interferes destructively and non-destructively to result in a desired directional radiation pattern. 
     An alternative configuration for the center channel array  44  is shown in  FIG. 6D , in which the center channel array  44  includes acoustic drivers  218 - 1 ,  218 - 3 ,  218 - 4 ,  218 - 5 , and  218 - 7 . The acoustic drivers  218 - 1 ,  218 - 3 - 218 - 5 , and  218 - 7  are coupled to the center channel signal source  242  by signal processing circuitry  224 - 1 ,  224 - 3 - 224 - 5 , and  224 - 7 , respectively that apply signal processing represented by transfer function H 1C (z), H 3C (z)-H 5C (z), and H 7C (z), respectively. The effect of the transfer functions H 1C (z), H 3C (z)-H 5C (z)), and H 7C (z), may include one or more of phase shift, time delay, polarity inversion, and others. Transfer functions H 1C (z), H 3C (z)-H 5C (z)), and H 7C (z) are typically implemented as digital filters, but may be implemented with equivalent analog devices. 
     In operation, the center channel signal C, as modified by the transfer functions H 1C (z), H 3C (z)-H 5C (z)), and H 7C (z) is transduced to acoustic energy by the acoustic drivers  218 - 1 ,  218 - 3 - 218 - 5 , and  218 - 7 . The radiation from the acoustic drivers interferes destructively and non-destructively to result in a desired directional radiation pattern. 
     The center channel array  44  of  FIGS. 6C and 6D  may direct radiation upward, as indicated by arrow  217  and in some implementations slightly backward and cancels radiation toward the listener, or in other implementations may direct radiation toward the listening area. 
     Other types of directional array are appropriate for use as directional arrays  30 ,  38 , and  44 . For example, each of the arrays may have as few as two acoustic drivers, without any acoustic drivers shared by arrays. 
     In one implementation, the left passive directional device  34  and the right passive directional device  42  of  FIG. 3A  are implemented as shown diagrammatically in  FIGS. 7A and 7B  with an actual example (without the acoustic driver) in  FIG. 7C . The passive directional devices of  FIGS. 7A and 7B  operate according to the principles described in U.S. Published Pat. App. 2009-0274329 A1, incorporated herein by reference in its entirety. 
     The passive directional device  310  of  FIGS. 7A and 7B  includes a rectangular pipe  316  with an acoustic driver  314  mounted in one end. The pipe tapers from the end in which the acoustic driver  314  is mounted to the other end so that the cross-sectional area at the other end is substantially zero. A lengthwise slot  318  that runs substantially the length of the pipe is covered with acoustically resistive material  320 , such as unsintered stainless steel wire cloth, 165×800 plain twill Dutch weave. The dimensions and characteristics of the pipe, the slot, and the acoustically resistive material are set so that the volume velocity is substantially constant along the length of the pipe. 
     In the actual implementation of  FIG. 7C , one lengthwise section  354  of the rectangular pipe is bent at a 45 degree angle to a second section  352 . The slot  318  of  FIG. 7A  is divided into two sections, one section  318 A of the slot in the side face  356  of first section  354  of the pipe and a second section of the slot  318 B in the top face  358  in the second section  352  of the pipe. 
     The implementation of the slotted pipe type directional loudspeaker of  FIG. 7B  is particularly advantageous in some situations.  FIG. 8  shows a curved or bent slotted pipe type directional radiator  110  in a television cabinet  112 . The dotted lines represent the side and back of the television cabinet  112 , viewed from the top. For cosmetic or other reasons, the back of the cabinet is tapered inwardly, so that the back of the cabinet is narrower than the front. A slotted pipe type directional radiator is positioned in the cabinet so that the curve or bend generally follows the tapering of the cabinet, or in other words so that the curved or slanted wall of the slotted pipe type directional radiator is substantially parallel with the back and side of the television cabinet. The directional radiator may radiate through an opening in the side of the cabinet, which may, for example, be a louvered opening. The direction of strongest radiation of the directional loudspeaker is generally sideward and slightly forward as indicated by arrow  62 , which is desirable for use as passive directional devices such as devices  32  and  42  of  FIG. 3A . 
     Other types of passive directional devices may be appropriate for passive directional devices  32  and  42 , for example, horns, lenses or the like. 
     Using passive directional devices for high frequencies is advantageous because it provides desired directionality without requiring directional arrays. Designing directional arrays that work effectively at the short wavelengths corresponding to high frequencies is difficult. At frequencies with corresponding wavelengths that approach the diameter of the radiating elements, the radiating elements themselves may become directional. 
     Numerous uses of and departures from the specific apparatus and techniques disclosed herein may be made without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features disclosed herein and limited only by the spirit and scope of the appended claims.