Patent Publication Number: US-8542854-B2

Title: Virtual surround for loudspeakers with increased constant directivity

Description:
BACKGROUND OF THE INVENTION 
     In traditional surround sound systems, a listener places 5 or more speakers at various positions around a listening position (sometimes also referred to as a listening area) to create an immersive sound experience for a listener. Each of the speakers in the system typically receives its own audio signal from an audio source, and consequently, the listener typically must wire each of the speakers to the audio source. The speakers in the audio system then produce sound that converges at the listening position to properly create a surround sound experience for the listener. 
     Virtual surround is a surround sound technique that can make sound appear to come from locations other than the location of the actual speakers in order to create a surround sound experience for a listener. As a result, virtual surround sound systems typically use fewer speakers than traditional surround sound systems, and the speakers in a virtual surround sound system are usually located in front of the listener. Virtual surround sound systems are thus more practical for a variety of different setups, such as with a personal computing system or a television. 
     Virtual surround sound widens the soundscape beyond the physical location of the speakers used to produce the sound, and is based on how humans localize sound. Humans localize sound using three methods: 1) Interaural Intensity Difference (IID), 2) Interaural Time Difference (ITD), and 3) Spectrally, with the Head Related Transfer Function (HRTF). Interaural Intensity Difference occurs when a sound is louder at one ear than at the other ear. This can occur when the sound source is closer to one of the ears. Similarly, Interaural Time Difference occurs when the sound reaches one ear before it reaches the other ear because the sound source is closer to one of the ears. This can cause a difference in time and therefore a difference in phase between the ears. A Head Related Transfer Function refers to the unique spectral shaping of sound as it reflects off of the pinna (outer ear), head, and shoulders of the listener. The spectral shaping can vary depending on the location of the sound source. Additionally, the spectral shaping can vary depending on the particular listener. 
     Virtual surround sound may employ one or more different techniques to create the impression on a listener that sound is coming from a location other than the location of the speakers based on one or more of the three above methods. For example, dipole beamforming is one method for creating virtual surround using IID. Dipole pairs of transducers can be used to artificially increase the difference in sound level between the ears. The transducers in a dipole pair are driven out of phase with each other in order to create a null for certain frequencies or channels, and a delay is used to steer the radial direction of the null. The result is that sound for certain frequencies or channels is more intense at one ear of the listener compared to the other ear, and the listener is left with the impression that the sound is originating from a location other than the actual location of the transducers producing the sound. 
     The transducers used in a dipole beamforming array are generally chosen for their dispersion characteristics in the targeted array frequency range. For example, woofers have good efficiency and near omni-directional radiation at lower frequencies. Woofers thus are a good choice for a lower frequency array. At higher frequencies, woofers start to beam and have less consistent directionality. This phenomenon is related to the size of the transducer relative to the wavelength of sound that it produces. In contrast, tweeters are physically smaller and thus have better dispersion for higher frequencies with smaller wavelengths. Therefore, tweeters are a good choice for a high frequency array. However, higher frequencies can be difficult to properly implement with a dipole beamforming array. Ideally, the distance between the center of the transducers used to form a dipole pair in a dipole beamforming system should be separated by a quarter-wavelength; the relative wavelength in this case is the center frequency for which the dipole pair is optimized. Since higher frequencies have smaller wavelengths, it may not always be physically possible to place tweeters (or other transducers) close enough together for an optimized dipole beamforming system. The array is optimized over approximately 4 octaves: 2 octaves above and below the center frequency. Above this frequency range, the distance between the transducers can become large relative to the wavelength of sound being produced, and more nulls are created as the frequency increases. The implication of this is that the sound at one ear may no longer be louder than at the other ear, and the virtual surround effect is lost. Below this range, the efficiency of sound production can decrease as sound from the out of phase transducers cancels. 
     Accordingly, it would be desirable to have a better virtual surround system that produces constant directivity across a wide range of frequencies in a small system that is useful for a variety of different setups. A number of different techniques are known in the art for creating virtual surround sound. For example, U.S. Application Pub. No. 2006/0072773 entitled “Dipole and monopole surround sound speaker system,” U.S. Application Pub. No. 2009/0060237 entitled “Array Speaker System,” U.S. Application Pub. No. 2008/0273721 entitled “Method for spatially processing multichannel signals, processing module, and virtual surround-sound systems,” and U.S. Application Pub. No. 2003/0021423 entitled “System for transitioning from stereo to simulated surround sound” all show different virtual surround systems. However, each of these systems could be improved to have more constant directivity across a wider range of frequencies. 
     BRIEF SUMMARY OF THE INVENTION 
     Various embodiments provide virtual surround with only 1 or 2 enclosures that can be placed in front of the listener. These embodiments also have substantially constant directivity across a range of frequencies. Various embodiments accomplish this by combining techniques that can be effective at different frequency ranges. For example, some embodiments combine dipole beamforming with pointing transducers to the side (i.e., away from the listening area). Pointing a transducer to the side provides directionality due to transducer beaming at higher frequencies. Additional directionality from “shading” can occur when the sound is shaded by the edge of the speaker box. Sound from the side firing transducers that is reflected off nearby objects or walls can also increase the sense of spaciousness, listener envelopment, and the apparent source width. 
     One embodiment of the invention is directed to a speaker system comprising at least one speaker enclosure, a first array of horizontally displaced transducers, mounted in the speaker enclosure, and at least a second array of horizontally displaced transducers. The speaker system further comprises a speaker input port and a controller operatively coupled with the speaker input port. The controller is configured to provide signals to the transducers such that the first and second arrays of horizontally displaced transducers are tuned to different center frequencies. The controller is further configured to provide signals to the transducers to cause dipole beamforming. 
     Another embodiment of the invention is directed to a speaker system comprising at least one speaker enclosure having a front face, a plurality of transducers mounted in the speaker enclosure and at least two of said transducers forming a horizontally displaced array. The speaker system further comprises a speaker input port and a controller operatively coupled with the speaker input port. The controller is configured to provide high frequency signals to a high frequency transducer, wherein the high frequency transducer is a side firing transducer. The high frequency transducer is positioned in the enclosure so that the center line of a sound beam emitted from the enclosure is at an angle to the front face of the enclosure. The controller is further configured to provide lower frequency signals to the transducers forming the horizontally displace array to cause dipole beamforming. 
     Another embodiment of the invention is directed to a speaker system comprising at least one speaker enclosure having a front face, a speaker input port, and a controller operatively coupled with the speaker input port. The controller is configured to provide low frequency signals to a low frequency transducer. The controller is configured to provide high frequency signals to a high frequency transducer and not to the low frequency transducer. The high frequency transducer is positioned in the enclosure so that the center line of a sound beam emitted from the enclosure is at an angle to the front face of the enclosure. The high frequency transducer is further positioned to take advantage of shading caused by at least one of a baffle, a waveguide, a lens, or a side of the speaker enclosure. The controller is further configured to provide signals to the transducers to cause dipole beamforming. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an exemplary embodiment of a virtual surround sound system. 
         FIGS. 1B-F  show exemplary signal processing diagrams for the embodiment illustrated in  FIG. 1A . 
         FIGS. 2A-2D  show an exemplary embodiment of a virtual surround sound system. 
         FIGS. 2E-J  show exemplary signal processing diagrams for the embodiment illustrated in  FIGS. 2A-2D . 
         FIG. 3A  shows an exemplary embodiment of a virtual surround sound system. 
         FIGS. 3B-G  show exemplary signal processing diagrams for the embodiment illustrated in  FIG. 3A . 
         FIG. 4  shows a block diagram of an exemplary system according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various embodiments use combinations of different methods for creating virtual surround. Some of the methods used in various embodiments include: dipole beamforming, multi-stage arrays, transducer directionality, and enclosure shading. In general, each of these methods may operate over a specific frequency band in various embodiments. The use of multiple methods to create virtual sound can increase the virtual sound effect and better maintain sound quality compared to the use of a single method for creating virtual surround. Each method used to create virtual surround can be optimized for a specific system configuration based on factors such as physical locations of the transducers, directionality of the transducers, the size and shape of the enclosure, and the input signal configuration. Various embodiments allow for an intensity difference to be created for a listener across a wide range of frequencies in order to produce constant directionality. 
     As used herein, a “transducer” can refer to a device that converts electrical signals from an electrical source into sound for a listener. As used herein, the term “driver” may be used interchangeability with transducer. 
     As used herein, “dipole beamforming” can refer to a method for creating virtual surround sound based on Interaural Intensity Difference (IID). More specifically, a system that uses dipole beamforming may have one or more dipole pairs of transducers that can be used to artificially increase the difference in sound level between the ears of a listener. The transducers in a dipole pair can be driven out of phase with each other to create a null for certain frequencies or channels. A delay can be used to steer the radial direction of the null created by the transducers. Dipole beamforming may also be referred to as crosstalk cancellation. 
     As used herein, the transducer “region of operation” is the frequency region where a transducer operates at a high enough level to contribute to the overall sound. It is a combination of the audio frequencies sent to the driver using filtering and the dispersion characteristics of the driver itself. 
     As used herein, “transducer directionality,” also called “driver beaming,” can refer to the change in the sound polar radiation pattern from the transducer over its operating frequency range. In the lower frequency end of the operating range, the sound is radiated more uniformly in all directions. For higher frequencies, the sound intensity is generally stronger on-axis, or directly in front of the transducer, than it is off-axis. Additionally, at the higher end of the frequency operating range, there can be “lobing,” where the sound intensity varies from high to low depending on the polar degree. Lobing is generally avoided because it is by definition, inconstant directivity. However, transducer directionality can be used to an advantage for virtual surround when it is used to increase the sound level at one ear relative to the other. This effect is enhanced when used with enclosure shading. 
     As used herein, “enclosure shading” can refer to the use of a speaker enclosure to “shade” a sound. Shading can also be caused by use of a baffle, a waveguide, or a lens. As with transducer directionality, this effect is frequency dependant. At lower frequencies, the shading effect is less. The wavelengths are longer and the sound wraps around the enclosure. At higher frequencies, the shading is increased. This effect is also dependant upon the size of the enclosure, where smaller enclosures do not shade as low in frequency as larger enclosures. As described in the next paragraph, this effect can be combined with transducer directionality for a better virtual surround effect. 
     To maintain the IID to higher frequency regions with more constant directivity, enclosure shading and transducer beaming are used instead of dipole beamforming. Enclosure shading and transducer beaming are ways of using the inherent directionality of objects to create the IID. When a transducer(s) is placed on the side of a speaker, the low frequency sound will bend around the enclosure and reach the listener. At higher frequencies, the enclosure begins to “shade” the sound such that higher frequencies are directed more to the side. The transducer beaming will further focus the sound. Transducer beaming occurs above the enclosure shading frequency. These two effects create a gradient in the sound field were the sound is louder at one ear than at the other ear. 
     Enclosure shading may occur above the enclosure transition frequency, F_et. F_et=(0.6*c)/(2*π*R_e), where “c” is the speed of sound in meters per second and “R_e” is the effective radius of the enclosure section that is shading the side firing transducer, given in meters. The enclosure transition frequency is expressed in Hertz, or cycles/second. Similarly, the transducer beaming may occur above transducer transition frequency, F_tt, F_tt=(0.6*c)/(2*π*R_t), where “c” is the speed of sound in meters per second and “R_t” is the effective radius of the transducer, given in meters. To allow for optimization of system components, the frequency region of transition for enclosure shading and transducer beaming shall be banded by +/− one octave, which translates to ½ transition frequency to 2 times the transition frequency. 
     In addition to multi-staged dipole beamforming arrays, enclosure shading, and transducer beaming, other effects that are used to create virtual surround and widen the listening soundscape are driving the surround channels out of phase, and using the side firing transducers in conjunction with front firing transducers to maximize the width of the speaker, while maintaining full audio bandwidth at the listening position. 
     The operating frequency range for constant directivity of the dipole beamforming array is limited by the physical center to center distance between the transducers. At the higher frequencies, dipole beamforming does not produce a good virtual surround experience because the IID is inconstant. The radiation from the transducers interfere producing irregular “lobing,” which is inconstant in directivity. A more stable IID with more constant directivity can be created by using a single side firing transducers and tuning both the transducer directionality and enclosure shading at the higher frequencies. Thus the difference in sound levels at each ear can be maintained and “lobing” can be minimized. A side-firing transducer also increases the reflected energy of the sound. The reflected sound can enhance the sensation of spaciousness, listener envelopment, and the apparent source width. 
     The center frequency of a dipole array is determined by the distance between the centers of the transducers used to form a dipole pair. This distance corresponds to one quarter wavelength. The center frequency, f_c, is given by the formula f_c=c/(4d), where “c” is the speed of sound and “d” is the center to center distance between the dipole array transducers. 
     As used herein, “multi-stage arrays” can refer to the use of different transducers and virtual surround IID generation across for different frequencies. A multi-stage dipole beamforming array has transducer pairs that are optimized for different frequency ranges. The various transducers in a multi-stage array can be configured to produce different frequencies of sound in order to create a better surround sound effect for a listener. In some embodiments, the array may comprise one or more dipole pairs that use dipole beamforming to create virtual surround sound. Such a dipole pair is typically optimized for a four octave bandwidth. Below two octaves, the efficiency of the array may be greatly reduced due to the cancellation of sound. Above two octaves, spatial interference may cause multiple unwanted nulls. Multiple nulls reduce the virtual surround effect and lead to inconstant directivity, which additionally can reduce the sound quality. In a dipole beamforming setup, the center frequency of an optimized band for a dipole pair generally occurs at the frequency corresponding to the quarter-wavelength of the transducer separation. For more constant directivity, multiple transducer arrays can be optimized to cover different frequency bands. Some frequency bands may use dipole beamforming to create virtual surround, while other frequency bands may rely on transducer directionality or enclosure shading to create a virtual surround effect. 
     As used herein, “controller” refers to a digital signal processor or analog circuitry that processes sound content from an audio source. The controller may be operatively coupled between a speaker input port and one or more transducers. Alternatively, or in addition, processing of sound content can be carried out by software or firmware on a computer readable medium on a computer (e.g., personal computer, laptop computer, portable music player, personal digital assistant (PDA), phone, etc.) and then multichannel content used as input into a speaker. 
     As used herein, “computer readable medium” for containing computer code or instructions, or portions of computer code or instructions, can include any appropriate media known or used in the art, including storage media and communication media, such as but not limited to volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage and/or transmission of information such as computer readable instructions, data structures, program modules, or other data, including RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disk (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, data signals, data transmissions, or any other medium which can be used to store or transmit the desired information and which can be accessed by the computer. Based on the disclosure and teaching provided herein, a person of ordinary skill in the art will appreciate other ways and/or methods to implement the various embodiments. 
     As used herein, “listening area” or “listening position” refers to the intended position of a listener or the area around a listener in a surround sound system or a virtual surround sound system. This area or position is used in the design of the surround sound system to create a good surround sound experience for a listener. 
       FIG. 4  shows an exemplary virtual surround sound system according to some embodiments of the invention.  FIG. 4  shows a speaker  400  with transducers  401 ,  402 ,  403  and  404 . An optional controller  405  for virtual surround sound processing may be operatively coupled between a speaker input port  406  and one or more transducers  401 - 404 . Transducers  402  and  403  may make up a first array, and transducers  401  and  404  may make up a second array. 
       FIG. 4  further shows a host  450  with an audio source  451  (e.g., disk, MP3, stream, 5.1 or 7.1 channel content, stereo content, etc.), a processor  452 , and a computer readable medium (CRM)  453 . Virtual surround processing can be done at the host (e.g., by software or firmware on CRM  453 ) alternatively or in addition to processing at optional controller  405 . The speaker  400  may be operatively coupled to the host  450  via a wired or wireless connection  407 . The signal may be amplified after processing and before it is sent to transducers  401 - 404 . 
     The speaker  400  may comprise any combination of the above described components. For example, the speaker  400  may include the audio source  451 , controller  405 , amplification of the signal, and the transducers  401 - 404 . In the alternative, just the processor  405 , amplification, and the transducers  401 - 404  may be in the speaker. In another alternative, only amplification and transducers may be in the speaker. In yet another alternative, only the transducers  401 - 404  may be in the speaker. 
     Sound Bar Embodiment 
     According to one embodiment, multiple transducers are placed within a single enclosure. Some of the transducers are pointed straight ahead toward a listening position, while some of the transducers are pointed to the side, away from the listening position.  FIG. 1A  illustrates an example such an embodiment in the form of a sound bar. According to some embodiments, a sound bar can be configured so that it can attach to a computer monitor that is in front of the listening position. 
     In the embodiment illustrated in  FIG. 1A , two transducers,  103  and  104 , are pointed straight ahead toward a listening area while two transducers,  101  and  102 , are pointed to the side.  FIG. 1A  illustrates this arrangement of transducers from a top-down perspective. Side transducers  101  and  102  can be used to take advantage of directionality and shading. Five channels of sound can be used in the embodiment illustrated in  FIG. 1A : left  110 , right  120 , center  130 , left surround  140 , and right surround  150 . According to some embodiments, a separate subwoofer may also be used in the system to help improve the generation of low frequency sound. 
     In the embodiment illustrated in  FIG. 1A , a two stage dipole beamforming array is used with transducer directionality and enclosure shading for enhanced virtual surround with more constant directivity. The two stage array can be broken up into low and medium frequency arrays. These arrays are used to create virtual surround sound effectively at their respective frequencies. According to one embodiment, low frequencies are considered to be frequencies up to 1 khz, medium frequencies are considered to be frequencies between 1 khz and 4 khz, and high frequencies are considered to be frequencies greater than 4 khz. The low and medium frequencies may use dipole beamforming to create virtual surround sound, while the high frequencies may rely on directionality and enclosure shading to create virtual surround. A low frequency array can be created using side transducers  101  and  102 , a medium frequency array can be creating using front transducers  103  and  104 , and the side transducers  101  and  102  can use high frequency directionality and enclosure shading. More details on how these sound arrays are created are given below. 
     Referring to  FIG. 1A , four separate transducers are shown in the enclosure  100 : left firing  101 , right firing  102 , left front  103 , and right front  104 . Each of the transducers may be full range transducers capable of producing frequencies ranging from 200 hz to 20 khz. The left firing  101  and right firing  102  transducers, which can be used for low frequency dipole beamforming, may be spaced apart by roughly the quarter wavelength of the center of the frequency range outputted by the array. According to one embodiment, the spacing between left firing  101  and right firing  102  is 20 cm as measured from the center of the transducers. Thus, according to this embodiment, the wavelength of the center frequency for this dipole pair is 80 cm. 80 cm roughly corresponds to a frequency of 400 hz. Similarly, the left front  103  and right front  104  transducers may be placed approximately 3-4 cm apart. This spacing leads to a wavelength of approximately 16-20 cm, or around 2 khz for the center frequency. 
       FIGS. 1B-1F  show the signal processing used to implement a three-stage array according to one embodiment. As with the embodiment shown in  FIG. 1A , five channels of sound from an audio source can be processed: left  110 , right  120 , center  130 , left surround  140 , and right surround  150 . These channels can be sent to various embodiments from an audio source using well-known means. 
       FIG. 1B  shows the signal processing for the left channel  110 . The audio signals from the left channel  110  are sent to the left firing  101  transducer, and to the left front  103  transducer. 
       FIG. 1C  shows the signal processing for the right channel  120 . The audio signals from the right channel  120  are sent to the right firing  102  transducer, and to the right front  104  transducer. 
       FIG. 1D  shows the signal processing for the center channel  130 . The audio signals from the center channel  130  are sent to the left front  103  and the right front  104  transducers. 
       FIG. 1E  shows the signal processing for the left surround channel  140 . As illustrated in  FIG. 1E , the left surround channel  140  has its signal broken up into a low frequency range (&lt;1 khz) by a low pass filter  141 , a medium frequency range (between 1 khz and 4 khz) by a combination of a low pass filter  144  and a high pass filter  142 , and a high frequency range (&gt;4 kz) by a high pass filter  143 . 
     The high frequencies from the left surround channel  140 , after passing through high pass filter  143 , are then sent to the left firing  101  transducer. 
     The medium frequencies from the left surround  140  channel, after passing through high pass filter  142  and low pass filter  144 , are then further split. The medium frequency signal from the left surround  140  channel is sent to the left front  103  transducer. The medium frequency signal from the left surround  140  channel is also inverted by an inverter  147  and sent to the right front  104  transducer after a 0.023 millisecond (ms) delay  148 . The time delay can be tuned for listening position. 
     The low frequencies from the left surround  140  channel, after passing through low pass filter  141 , are also further split. The low frequency signal from the left surround  140  channel is sent to the left firing  101  transducer. The low frequency signal from the left surround  140  channel is also inverted by an inverter  145  and sent to the right firing  102  transducer after a 0.113 ms delay  146 . The time delay can be tuned for desired listening position. 
       FIG. 1F  shows the signal processing for the right surround channel  150 . Similar to the left surround channel  140 , the right surround channel  150  has its signal broken up into a low frequency range (&lt;1 khz) by a low pass filter  151 , a medium frequency range (between 1 khz and 4 khz) by a combination of a low pass filter  154  and a high pass filter  152 , and a high frequency range (&gt;4 khz) by a high pass filter  153 . However, one difference between the left surround channel  140  and the right surround channel  150  is that the right surround channel has its signal inverted by an inverter  159  before the signal is divided by frequency. Alternately, the left surround channel could be inverted instead of the right surround channel. The condition is that they are out of phase with each other. 
     The inverted high frequencies from the right surround channel  150 , after passing through high pass filter  153 , are then sent to the right firing  102  transducer. 
     The inverted medium frequencies from the right surround  150  channel, after passing through high pass filter  152  and low pass filter  154 , are then further split. The inverted medium frequency signal from the right surround  150  channel is sent to the right front  104  transducer. The inverted medium frequency signal from the right surround  150  channel is also inverted again by an inverter  157  and sent to the left front  103  transducer after a 0.023 ms delay  158 . The time delay can be tuned for listening position. 
     The inverted low frequencies from the right surround  150  channel, after passing through low pass filter  151 , are also further split. The inverted low frequency signal from the right surround  150  channel is sent to the right firing  102  transducer. The inverted low frequency signal from the right surround  150  channel is also again inverted by an inverter  155  and sent to the left firing  101  transducer after a 0.113 ms sample delay  156 . The time delay can be tuned for listening position. 
     As can be seen from the above signal processing diagrams, a low frequency array is created using the two side firing transducers. The low frequencies from the left surround channel  140  are sent to the left firing  101  transducer and the right firing  102  transducer, with the signal to the right firing  102  transducer inverted and delayed so as to create a virtual surround sound effect using dipole beamforming. This can create the impression to a listener in the listening area that the left surround channel  140  is being created from a speaker to the far left of the listener. The low frequencies from the right surround channel are first inverted and then sent to the left firing  101  transducer and the right firing  102  transducer. The signal to the left firing  101  transducer are inverted and delayed so as to create a virtual surround using dipole beamforming. As a result, the listener is given the impression that the right surround channel  140  is being created from a speaker to the far right of the listener. 
     A medium frequency array is created from the left and right firing channels  140  and  150  using the two transducers in the front of the enclosure  103  and  104 . The medium frequency array, by inverting and delaying signals as described above, uses dipole beamforming to create a virtual surround sound for those frequencies. 
     High frequency IID is created using the two side firing transducers  101  and  102 . The high frequencies may not create their virtual surround through the use of dipole beamforming in the same way that the low and medium frequencies may do. Rather, the high frequencies rely on the directionality of the sound from left firing  101  and right firing  102  to create virtual surround using the transducer directionality and the shading of the enclosure. This is used for the surround channel content. The side-firing transducer also increases the reflected energy, which enhances the sensation of spaciousness and apparent source width. 
     Stand Embodiment 
     According to one embodiment, multiple transducers are placed within a single enclosure. Some of the transducers are pointed straight ahead toward a listening area, while some of the transducers are pointed to the side.  FIG. 2A  illustrates an example such an embodiment in the form of a stand speaker. Five channels of sound can be used in the embodiment illustrated in  FIG. 2A : left  320 , right  340 , center  330 , left surround  360 , and right surround  370 . Various embodiments may also include a subwoofer  310  that is separate from the stand. Various embodiments may include a separate subwoofer channel  350  for the subwoofer  310 . 
     In the embodiment illustrated in  FIGS. 2A-2D , five full-range transducers are shown. According to some embodiments, each of the transducers may be 2″ drivers. Note that the drawings shown in  FIGS. 2A-D  are not shown to scale. In the embodiment illustrated in  FIGS. 2A-D , three transducers are pointed straight at the listening area, while two transducers are pointed to the side to take advantage of directionality and shading. As will be explained in more detail below, the side transducers can be used to create the surround channels. Additionally, a subwoofer that is separate from the stand speaker shown in  FIG. 2A  can be used to produce the lowest frequencies. 
       FIG. 2A  shows the front view of a stand  300  embodiment. In this view, the left  301 , center  302 , and right  303  transducers are clearly visible. According to one embodiment, the height  300 A of the front is 12.5 cm. According to one embodiment, the distance from the edge of the stand  300  to the center of the left transducer  301  is 4.25 cm (as represented as  300 B in  FIG. 2A ). According to one embodiment, the width  300 C of the stand is 36.5 cm. According to one embodiment, the width of the back edge  300 D is 11 cm. The two back edges of the stand rise up at an angle relative to the front of the stand and contain the side firing transducers. The below diagrams show this shape in more detail. 
       FIG. 2B  shows the right side view of a stand  300  embodiment. In the view shown in  FIG. 2B , the right firing  305  transducer is clearly shown. If a left view was shown, the view would look similar to  FIG. 2B  with the left firing  304  transducer. According to one embodiment, the depth  300 F of the stand  300  is 15 cm. According to one embodiment, the height  300 E of the back edge is 16 cm. According to one embodiment, the edge  300 G of the stand above the side firing transducer is 11.5 cm. According to one embodiment, edge  300 F is 2 cm. 
       FIG. 2B  shows a subwoofer  310  that can be used with some embodiments. The subwoofer may have its own channel for audio signals. 
       FIG. 2D  shows the left and right view of an embodiment of the stand. In  FIG. 2D , the left firing  304  and right firing  305  can be seen in relation to the right  303  and left  301  transducers. 
       FIGS. 2E-2J  show the signal processing used to implement a virtual surround effect according to one embodiment. As with the embodiment shown in  FIG. 2A , five channels of sound can be used with various embodiments: left  320 , right  340 , center  330 , left surround  360 , and right surround  370 . Various embodiments may include a separate subwoofer channel  350  for the subwoofer  310 . These channels can be sent to various embodiments from an audio source using well-known means. 
       FIG. 2E  shows the signal processing for the left channel  320 . The signal from the left channel  320  is sent to the left transducer  301 . 
       FIG. 2F  shows the signal processing for the center channel  330 . The signal from the center channel  330  is sent to the center transducer  302 . 
       FIG. 2G  shows the signal processing for the right channel  340 . The signal from the right channel  340  is sent to the right transducer  303 . 
       FIG. 2H  shows the signal processing for the subwoofer channel  350  according to some embodiments. The signal from the subwoofer channel  350  is sent to the subwoofer  310 . 
       FIG. 2I  shows the signal processing for the left surround channel  360 . The signal from the left surround channel  360  is split between the left firing transducer  304  and the right firing transducer  305 . The left surround channel  360  is sent directly to the left firing transducer  304  without any filtering, inversion, or other operation. For the right firing transducer  305 , the left surround channel  360  is sent through a low pass filter  361  and a delay module  362  before the signal is sent to the right firing transducer  305 . 
       FIG. 2J  shows the signal processing for the right surround channel  370 . The signal from the right surround channel  370  is split between the left firing transducer  304  and the right firing transducer  305 . The right surround channel  370  is sent directly to the right firing transducer  305  without any filtering, inversion, or other operation. For the left firing transducer  304 , the right surround channel  370  is sent through a low pass filter  371  and a delay module  372  before the signal is sent to the left firing transducer  304 . 
     In the embodiment shown in  FIGS. 2A-J , virtual surround can be created from the side firing transducers. Shading by enclosure and the natural beaming of the transducers help to create the virtual surround effect for a listener in the listening area. 
     Two Speaker Embodiment 
     According to another embodiment, two speakers are used to create virtual surround sound.  FIG. 3A  illustrates an example of a two speaker embodiment. According to some embodiments, a left  530 , left surround  540 , right  550 , and right surround channel  560  are used in the speaker system as shown in  FIGS. 3B-3E . As shown in  FIG. 3F , the center channel  570  can be mixed to the left  571  and right  572  channels prior to virtual surround processing. The left and right sides are mirror images of each other, so only the left side will be explained in detail. For example, if left signal  530  is shown being routed to transducer  525  on left speaker  520 , then the corresponding right signal  550  would be transmitted from transducer  515  on the right speaker  510 . 
     In many dipole beamforming setups, getting the transducers close together in order to optimize the canceling effect is a problem. As mentioned previously, the quarter-wavelength rule dictates the optimum distance between the centers of transducers of a dipole pair for canceling certain frequencies. For a high frequency dipole pair, this lends itself to closely spaced small drivers. Additionally, dipole beamforming at low frequencies may cause some sound to cancel. Thus, the low frequencies may need to be more efficient in this region and may need to be boosted to create a better surround sound experience. In various two speaker embodiments, these problems are addressed by having dipole arrays of different sized drivers optimized for lower and higher frequencies and by having an additional set of drivers to boost low frequencies. 
     In the embodiment shown in  FIG. 3A , two separate speakers are shown  510  and  520 . Each speaker is comprised of two dipole beamforming arrays. The array pairs in left speaker  520  are transducers  521  and  522 , and transducers  525  and  526 . Similarly, the array pairs in right speaker  510  are transducers  511  and  512 , and transducers  515  and  516 . The transducer array between  521  and  522  of the left enclosure and  511 , and  512  of the right enclosure provide low frequency dipole beam-forming while transducer pairs  525 / 526  and  516 / 515  provide high frequency dipole beam-forming for the left and right speakers, respectively. Some embodiments may uses a subwoofer  580  in a separate enclosure to further reinforce the low frequency sounds. 
     According to one embodiment, transducers  511  and  512  are a low frequency woofer array. Similarly,  521  and  522  are also a low frequency woofer array. Transducers  515 - 516  and  525 - 526  are high frequency tweeter arrays. According to one embodiment, the high frequency tweeter array is centered at 2.5 KHz. According to one embodiment, the low frequency woofer arrays are centered at 800 Hz. 
     If the transducers are centered on the frequencies listed above, the quarter-wavelength spacing rule may dictate the desirable separation of the transducers. According to one embodiment, transducer pairs  521  and  522  are separated by 11 cm between their centers. Similarly,  511  and  512  are separated by 11 cm between their centers. Transducers  525  and  526  are separated by 3.4 cm between their centers according to one embodiment. Similarly, transducers  516  and  515  are separated by 3.4 cm between their centers according to one embodiment. 
       FIGS. 3B and 3C  illustrate the signal processing according to one embodiment. As previously mentioned, while  FIGS. 3B and 3C  show the left-sided channels, the right-sided channel shown in  FIGS. 3D and 3E  would simply be the mirror image of what is presented in  FIGS. 3B and 3C . However, one difference between the left surround channel  540  and the right surround channel  560  is that the left surround channel has its signal inverted by an inverter  543  before the signal is divided by frequency. Alternately, the right surround channel could be inverted instead of the right surround channel. The condition is that they are out of phase with each other. 
     As with the embodiment shown in  FIG. 3A , four channels of sound can be used with various embodiments: a left  530 , left surround  540 , right, and right surround. Various embodiments may use more channels, such as by including a center channel  570  or a subwoofer channel  580 , and various embodiments may use fewer channels, such as only a left channel and a right channel. The center channel  570  input can be mixed into the left and right channel prior to surround processing. Additionally, the left and right channel may be processed as surround channels to widen the stereo image. These channels can be sent to various embodiments from an audio source using well-known means. 
       FIG. 3B  shows the signal processing for the left channel  530 . The signal from the left channel  530  is split into its high frequency and low frequency components. The high frequency signal can be sent to the tweeter dipole pair  525  and  526 . The low frequency signal can be sent to the left woofers  521 ,  522 . 
       FIG. 3C  shows the signal processing for the left surround channel  540 . The left surround channel is inverted by an inverter  543  and then split into its high frequency and low frequency components. For this implementation with the left surround channel inverted, the right surround channel would be non-inverted. Additionally, this can be reversed such that the right surround channel is inverted with the left surround channel non-inverted. The high frequency component of the surround channel, after passing through a high pass filter, is sent to transducer  525 . The high frequency component is also sent through a delay module  544  and inverted again  545  before being sent to transducer  526 . According to some embodiments, the delay module  544  might introduce a 0.045 ms delay to the signal, where the delay is tuned to correspond to the desired listening position. The low frequency component, after passing through a low pass filter is sent to transducer  521 . The low frequency component is also sent through a delay module  546  and inverted again  547  before being sent to transducer  522 . According to some embodiments, the delay module  546  might introduce a 0.181 ms delay to the signal, where the delay is tuned to correspond to the desired listening position. 
     Alternative embodiments could apply dipole beamforming to the left signal and right signal in addition to the left surround and right surround signals. Various embodiments may use the left and right outputs from a computer or television without the use of any center or surround channels. The left and right outputs may be processed like surround channels to achieve a wider stereo image. 
     According to various embodiments, one channel of surround is inverted. 
     Having described and illustrated the principles of various embodiments of the invention, it will be apparent to one skilled in the art that embodiments can be modified in arrangement and detail without departing from such principles. Many of the examples given above are meant to be illustrative and not limited to the precise details given. One or more features from any embodiment may be combined with one or more features of any other embodiment without departing from the scope of the invention. In view of the many possible embodiments to which the principles may be put, it should be recognized that the detailed embodiment is illustrative only and should not be taken as limiting the scope of the invention. 
     Any of the software components or functions described in this application, may be implemented as software code to be executed by a processor using any suitable computer language such as, for example, Assembly, C, or C++ using, for example, conventional or object-oriented techniques. The software code may be stored as a series of instructions, or commands on a computer readable medium, such as a random access memory (RAM), a read only memory (ROM), a magnetic medium such as a hard-drive or a floppy disk, flash drive, or an optical medium such as a CD-ROM. Any such computer readable medium may reside on or within a single computational apparatus, and may be present on or within different computational apparatuses within a system or network. 
     A recitation of “a”, “an” or “the” is intended to mean “one or more” unless specifically indicated to the contrary. 
     All patents, patent applications, publications, and descriptions mentioned above are herein incorporated by reference in their entirety for all purposes. None is admitted to be prior art.