Abstract:
A loudspeaker configuration capable of generating stereophonic sound separation from relatively loudspeaker drivers that are not necessarily widely separated. For example, such a loudspeaker configuration can be used to generate relatively good sound separation from a single chassis, such as a single speaker box, a boom box, a clock radio or the like. This can advantageously save space in many environments. One embodiment uses four mid-tweeter speaker drivers and a bass driver. The four mid-tweeter speaker drivers are arranged facing outward approximately 90 degrees apart from each other. A speaker driver facing the listener reproduces a mid channel signal, for example, L+R. A speaker driver facing away from the listener reproduces the same mid channel signal or a delayed version of the mid channel signal. A left facing speaker reproduces a side channel, for example, L−R. A right facing speaker reproduces a side channel, for example, R−L. The acoustic combination of the sound produced by the four speaker drivers creates a virtual left and right loudspeaker as experienced by the listener.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to audio speaker systems. In particular, the invention relates to relatively compact speaker systems capable of reproducing a stereophonic sound. 
     2. Description of the Related Art 
     Loudspeakers include electromechanical transducers that convert electrical signals into sound. A stereo system has separate signals for a left channel (L) and for a right channel (R). To reproduce stereophonic sound, a conventional stereo system layout typically has a loudspeaker to a listener&#39;s left reproducing the left channel (L), and a loudspeaker to the listener&#39;s right reproducing the right channel (R). 
     In addition, in many environments, the proper placement of loudspeakers can be difficult to achieve. For example, in a portable electronic device, the left loudspeaker and the right loudspeaker may be placed so close together that the resulting stereo separation is inadequate. In another example with separate left and right loudspeakers, space on a countertop or a desktop may be too limited for relatively good placement of the loudspeakers, and in both examples best fidelity is achieved at only one listening position, usually directly in front of and centered between the left and right loudspeakers. In addition, many people do not possess the expertise necessary to position separate loudspeakers for relatively good sound field reproduction. 
     SUMMARY OF THE DISCLOSURE 
     A relatively good stereophonic sound field can be reproduced from a relatively compact chassis such as a single speaker box, a boom box, a clock radio or the like. This can advantageously save space in many environments, such as indoors. One embodiment uses four mid-tweeter speaker drivers and an optional bass driver. The four mid-tweeter speaker drivers are arranged facing outward approximately 90 degrees apart from each other. A speaker driver that faces the listener reproduces a mid channel signal, for example, L+R. A speaker driver that faces away from the listener reproduces the same mid channel signal or a delayed version of the mid channel signal. A left-facing speaker reproduces a side channel, for example, L−R. A right-facing speaker reproduces a side channel, for example, R−L. The acoustic combination of the sound produced by the four speaker drivers creates a virtual left and right loudspeaker as experienced by the listener. Accordingly, sound is radiated in 360 degrees. For example, when used in an indoor room, sound reflected off of walls can result in spacious stereo sound being heard by the listener virtually anywhere in the room. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These drawings and the associated description herein are provided to illustrate specific embodiments of the invention and are not intended to be limiting. 
         FIG. 1  illustrates an exploded perspective view of a speaker system according to an embodiment of the invention. 
         FIG. 2  illustrates a perspective view of the speaker system viewed from the front, above, and right side. 
         FIG. 3  illustrates a perspective view of the speaker system viewed from the front, below, and right side. 
         FIG. 4  is a schematic diagram illustrating a circuit that can be used to process signals and drive speaker drivers of the speaker system. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Although particular embodiments are described herein, other embodiments of the invention, including embodiments that do not provide all of the benefits and features set forth herein, will be apparent to those of ordinary skill in the art. 
       FIG. 1  illustrates an exploded perspective view of a speaker system according to an embodiment of the invention. The illustrated speaker system includes a chassis  102 , which in the illustrated embodiment forms part of an enclosure. Stereo sound can be generated from the speaker system by itself, that is, without another speaker system to form a left and right pair. The enclosure of the illustrated speaker system also includes a first baffle  104 , a top  106 , an inner floor  108 , a woofer baffle  110 , and feet  112 . The chassis is configured to hold an array of speaker drivers. 
     Each of the speaker drivers of the array faces outwards. In the illustrated embodiment, the array includes a front-facing speaker driver  122 , a rear-facing speaker driver  124 , a left-facing speaker driver  126 , and a right-facing speaker driver  128 . Optionally, the speaker drivers  122 ,  124 ,  126 ,  128  are aligned in a common horizontal plane. 
     Optionally, each of the speaker drivers  122 ,  124 ,  126 ,  128  can be equidistant from a common point. The speaker drivers  122 ,  124 ,  126 ,  128  can accordingly be used with an enclosure having a circular cross-section, such as the illustrated cylinder shown. However, other shapes can be used, such as, a square cross-section box. In addition, it should be noted that the chassis can correspond to structures other than enclosures, such as, but not limited to, a frame that holds individual enclosures for the speaker drivers  122 ,  124 ,  126 ,  128 . Preferably, a single-chassis is used, as a single-chassis helps to ensure that the speaker drivers  122 ,  124 ,  126 ,  128  are properly arranged for their driving signals. However, more than one chassis can be used. In alternative embodiments, the speaker drivers  122 ,  124 ,  126 ,  128  are not equidistant from a common point, and other configurations of chassis can be used. Optionally, the speaker system can also include a woofer  132  to assist the reproduction of relatively low-frequency sounds. 
     One example of a schematic describing the signals provided to the speaker drivers  122 ,  124 ,  126 ,  128  is described in connection with  FIG. 4 . A conventional stereo system has a left channel (L) signal and a right channel (R) signal. In a conventional stereo system, a loudspeaker to the listener&#39;s left reproduces the left channel (L) signal and a loudspeaker to the listener&#39;s right reproduces the right channel (R). 
     The left channel (L) signal and the right channel (R) signal can be summed to form a mid channel (M) signal. The mid channel (M) signal can be generated in analog domain or digital domain. In one embodiment, summing is performed in analog domain to save cost. Examples of such summing circuits include resistive summing networks, op-amp summing circuits, such as a non-inverting summing amplifier, and the like. Other techniques can also be used to sum the left channel (L) signal and the right channel (R) signal, such as summing using dual voice coils, summing by wiring across differential output amplifiers, and the like. The particular technique used to generate the mid channel (M) signal is not critical, and it should be noted that some techniques apply before power amplification and some apply after power amplification. 
     A side channel (S) signal can be formed from the difference between the left channel (L) signal and the right channel (R) signal or vice-versa. The side channel (S) signal can also be generated in either the analog domain or the digital domain. In one embodiment, the side channel (S) signal is generated in analog domain. For example, an op-amp circuit known as a difference amplifier can be used to generate the side channel (S) signal. Other techniques can also be used generate the side channel (S) signal, such as, but not limited to, using dual voice coils, appropriate connection across differential output amplifiers, transformer circuits, and the like. The particular technique used to generate the side channel (S) signal is not critical, and it should be noted that some techniques apply before power amplification and some apply after power amplification. While the side channel (S) signal will be described in the context of the difference (L−R) between the left channel (L) signal and the right channel (R) signal, the opposite (R−L) can be used (with corresponding changes). The acoustic combination of an omnidirectional or a cardioid loudspeaker reproducing the mid channel (M) signal together with a dipole loudspeaker crossed 90 degrees reproducing the side channel (S) signal creates a virtual left and right loudspeaker. For improved performance, the acoustic centers of the omnidirectional or the cardioid loudspeaker configuration and the dipole loudspeaker configuration are preferably coincident. 
     One embodiment will now be described in greater detail. For the purposes of illustration, each speaker driver will be assumed to have one voice coil, a M-S processor  402  ( FIG. 4 ) generates the mid channel (M) signal and the side channel (S) signal before a power amplification stage, and the mid channel (M) signal and the side channel (S) signal are each individually available at outputs of power amplifiers. In the illustrated embodiment, the side channel (S) corresponds to the left minus right difference (L−R), but of course, the opposite can be used with corresponding changes. 
     The left-facing speaker driver  126  is coupled to the side channel (S) signal such that the left minus right difference signal (L−R signal)( FIG. 4 ) is reproduced by the left-facing speaker driver  126 . The right-facing speaker driver  128  is coupled to the side channel (S) such that the right minus left difference (R−L) is reproduced by the right-facing speaker driver  128 . In the illustrated embodiment of  FIG. 4 , this is accomplished by wiring an output of the power amplifier  404  ( FIG. 4 ) for the L−R signal in phase for the left-facing speaker driver  126  and out-of-phase for the right-facing speaker driver  128 . This configuration produces the sound field of the side channel (S) signal as a dipole sound source. 
     In one embodiment, the front-facing speaker driver  122  is coupled to the mid channel (M) signal, which is depicted in  FIG. 4  as the L+R signal. This mid channel (M) signal is amplified by a power amplifier  406  ( FIG. 4 ). The rear-facing speaker driver  124  can also be coupled to the mid channel (M) signal (in phase). This configuration produces the sound field of the mid channel (M) signal as an omnidirectional sound source relatively well. When the speaker drivers  122 ,  124 ,  126 ,  128  are relatively small, share a common horizontal plane, and are equidistant from a reference point, such as, but not limited to, a point in the center of a chassis, the acoustic centers of the dipole and the omnidirectional loudspeaker configurations are aligned and coincident. 
     In an alternative embodiment having a delay stage  408  ( FIG. 4 ) and an additional amplifier  410  ( FIG. 4 ), rather than driving the rear-facing speaker driver directly with the mid channel (M) signal, the rear-facing speaker driver  124  is driven by a delayed version of the mid channel (M) signal and connected out of phase with respect to the front-facing speaker driver  122 . In one embodiment, the phase change is accomplished by inverting the connections at the speaker terminals for the rear-facing speaker driver  124 . The delayed version is delayed so that sound emanating from the front-facing speaker driver  122  is aligned in time with the out-of-phase sound emanating from the rear-facing speaker driver  124  at the rear-facing speaker driver  124 , which sets up destructive interference. This configuration produces the sound field of a cardioid sound source relatively well. In one embodiment, the delay can be activated at the option of the listener to permit the listener to select the listening configuration. 
     When the speaker drivers  122 ,  124 ,  126 ,  128  are relatively small, share a common horizontal plane, and are equidistant from a reference point, such as, but not limited to, a point in the center of a chassis, with delay added to the rear-facing speaker driver  124 , the acoustic centers of the dipole and cardioid loudspeakers are aligned and coincident. The speaker drivers  122 ,  124 ,  126 ,  128  are preferably relatively small, such as from about 1 inch to 3 inches in diameter for cone-type speaker drivers, to better create the sound radiation pattern of the omnidirectional, cardioid, and dipole loudspeakers throughout the audible frequency range, particularly into the treble. Of course, other types of drivers can also be used. 
     With the speaker drivers  122 ,  124 ,  126 ,  128  spaced 90 degrees apart, sound is radiated in 360 degrees, creating good fidelity at virtually any listening position. Furthermore when used indoors, the 360-degree radiated sound reflects off of walls and can result in a spacious stereo sound being heard by the listener at essentially any position in the room. 
     Larger diameter speaker drivers will typically not permit the front-facing speaker driver  122  and the rear-facing speaker driver  124  to replicate a relatively good omnidirectional sound source or permit the left-facing speaker driver  126  and the right-facing speaker driver  128  to replicate a relatively good dipole sound source. For example, beyond about 3 inches in diameter, a speaker driver will typically be too directional at high frequency, thereby preventing the mid channel (L+R) and side channel (L−R and/or R−L) from the 4 speakers from mixing properly to replicate sound from virtual left and right loudspeakers. Preferably, the side channels are “pure” differences (L−R) or (R−L) as illustrated. However, a blend can be used. For example, the left-facing speaker driver  126  can be driven with a blend of the left (L) signal and the left minus right (L−R) side channel, and the right-facing speaker driver  128  can be driven with a blend of the right (R) signal and the right minus left (R−L) side channel. When blended, the signals can be blended such that the signal applied to the left-facing speaker driver  126  is (nL−R) and the signal applied to the right-facing speaker driver  128  is (nR−L) (not normalized). In one embodiment, the value of n is in a range from about 1 to about 3. 
     In the illustrated embodiment, each of the speaker drivers  122 ,  124 ,  126 ,  128  corresponds to a 2 inch mid-tweeter model 830970 available from Peerless. Other appropriate speaker drivers will be readily determined by one of ordinary skill in the art. When relatively small speaker drivers  122 ,  124 ,  126 ,  128  are used as suggested for a relatively good sound field, the woofer  132  can optionally be used to supplement low-frequency (bass) response. 
     A wide range of speaker drivers can be used for the woofer  132 . An appropriate woofer can be readily selected by one of ordinary skill in the art based on size, weight, and cost constraints. For example, 4 inch to 6 inch woofers can be effective, but other sizes will be applicable. The illustrated woofer  132  is mounted facing a bottom of the chassis  102 . This saves space and permits the speaker drivers  122 ,  124 ,  126 ,  128  of the array to be closer together. The woofer  132  can alternatively be mounted on a side, on top, or even in another chassis. 
     Preferably, the speaker drivers  122 ,  124 ,  126 ,  128  are evenly spaced across 360 degrees as viewed from above or below. For example, a square cross-section enclosure will typically ensure even spacing and equidistant spacing from a reference point between the speaker drivers  122 ,  124 ,  126 ,  128 . However, the spacing can be other than 90 degrees apart. For example, the spacings can be 90 degrees +/−10 degrees. In addition, while illustrated with each speaker driver pointing radially outward from a common reference point, the speaker drivers can also be pivoted slightly with respect to the outward direction. For example, the left-facing speaker driver  126  and the right-facing speaker driver  128  can be pivoted or tilted towards the listener if desired and still be generally facing outward. 
     Other mechanical components of the speaker system will now be described. The chassis  102 , the first baffle  104 , the top  106 , and the inner floor  108  form enclosures for the speaker drivers  122 ,  124 ,  126 ,  128  of the array. The chassis  102 , the inner floor  108 , and the woofer baffle  110  form an enclosure for the baffle  132 . While illustrated in the context of sealed enclosures, other types of enclosures, such as ported enclosures, can also be applicable. The feet  112  permit for air to flow below the woofer  132 . In the illustrated embodiment, the feet  112  are about 0.75 inches in height. The various mechanical components can be made out of a variety of materials, such as, but not limited to, wood, plastics, metals, or combinations thereof. 
       FIG. 2  illustrates a perspective view of the speaker system viewed from the front, above, and right side, with the components assembled. While the top  106  shown is featureless, the top  106  or other surfaces can have one or more of input connectors for an audio device, power connectors, displays, controls (such as selector switches, volume controls, etc.) and the like. One or more grilles covering the various speaker drivers are also not drawn for clarity, but can be included with the speaker system. In addition, other components such as players, power supplies, radio or satellite receivers, etc., can be integrated with the speaker system. 
       FIG. 3  illustrates a perspective view of the speaker system viewed from the front, below, and right side, with the components assembled. The woofer  132  is visible in this view. 
       FIG. 4  is a schematic diagram illustrating a circuit that can be used to process signals and drive speaker drivers of the speaker system. In the illustrated schematic, the M-S (mid-side) channel process is performed at a pre-amplifier level, but one of ordinary skill in the art will appreciate that other configurations are possible. 
     The left channel (L) signal and the right channel (R) signal are provided as inputs to the M-S processor  402 . For example, the left channel (L) and the right channel (R) can be provided from a CD player, satellite receiver, radio, digital audio player, such as an iPod or the like. The signals can be received through a wired connection or wirelessly. Other components, such as input selectors, volume controls, tone controls, or the like can also be provided. For clarity, these possible other features have been left off of  FIG. 4 . 
     The M-S processor  402  of the illustrated embodiment converts the left channel (L) signal and the right channel (R) signal into a mid channel signal and a side channel signal. The mid channel signal is represented by the L+R signal. The side channel(s) can be either or both of L−R or R−L, and is illustrated in  FIG. 4  as the L−R signal. 
     The L−R signal is provided as an input to an equalizer/filter  422 . The equalizer/filter  422  can correspond to a high-pass crossover network for the left-facing speaker driver  126  and the right-facing speaker driver  128 , and can include equalization. The equalizer/filter  422  can be optional. The equalized/filtered L−R signal is then amplified by the power amplifier  404 . With the L−R signal, the amplifier outputs are coupled in-phase for the left-facing speaker driver  126  and out-of-phase for the right-facing speaker driver  128 . If an R−L signal is used, the wiring described can be reversed at the speaker terminals. 
     The L+R signal can be equalized/filtered by an equalizer/filter  424 . The equalizer/filter  424  can be optional. In one embodiment, the L+R signal is then amplified by the power amplifier  406 , and is then provided in phase to both the front-facing speaker driver  122  and to the rear-facing speaker driver  124 . This approximates the sound field produced by an omnidirectional sound source relatively well. 
     In an alternative embodiment, the L+R signal (either with or without equalization/filtering) is amplified by the power amplifier  406  and provided in phase to the front-facing speaker driver  122 . The L+R signal (either with or without equalization/filtering) is also provided as an input to the delay stage  408  and then to the power amplifier  410 . The delay stage  408  can be implemented by, for example, a passive network. The delay stage  408  can also be implemented digitally with an analog-to-digital converter, a memory, and a digital-to-analog converter. The L+R signal is then provided not only delayed, but also out-of-phase to the rear-facing speaker driver  124 . With a delay set to match the propagation delay of sound from the front-facing speaker driver  122  to the rear-facing speaker driver  124 , the delay and inversion set up destructive interference at the rear-facing speaker. This approximates the sound field produced by a cardioid sound source relatively well. It should be noted that the phase can be inverted other ways, such as electronically before power amplification. 
     The schematic also illustrates an equalizer/filter  428  and a power amplifier  412  for the woofer  132 . The woofer  132  and corresponding circuits can be optional. In addition, any of the equalizer/filters can include gain adjustments for matching of levels from the various speaker drivers. 
     Various embodiments have been described above. Although described with reference to these specific embodiments, the descriptions are intended to be illustrative and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art.