Abstract:
A horn assembly for high frequency acoustic speakers. In an array of speakers, a spacing between adjacent speakers needs to be less than the wavelength of sound being emitted in order to combine effectively. For high frequency sound, a relatively small wavelength imposes a limitation on such a spacing. Such limitations are sometimes physically difficult to implement. A horn assembly increases the exit dimensions of the small speaker to larger desired dimensions by utilizing one or more plugs that divide a larger horn cavity into smaller horn cavities and creating similar pathlengths thereto. The similar pathlengths and the smaller horn cavities having desired dimensions allow the exiting sound to combine effectively. The overall dimensions of the exit portion of the horn assembly can be selected to match the dimensions of larger bass speakers, thus allowing improved arraying of the high frequency speakers with respect to other larger speakers.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/674,458 filed Feb. 13, 2007 which is a continuation of U.S. application Ser. No. 10/274,627, filed Oct. 18, 2002, (now U.S. Pat. No. 7,177,437), which claims the benefit of U.S. Provisional Application No. 60/345,279 filed Oct. 19, 2001 which are hereby incorporated in their entirety herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to sound technology in general and, in particular, relates to a speaker having a single driver element and multiple apertures in an array. 
         [0004]    2. Description of the Related Art 
         [0005]    Speakers convert electrical signals to sound waves that allow listeners to enjoy amplified sounds. One of the factors that determine the quality of the speaker-generated sound heard by the listener is the sound pressure level (SPL). The quality of the SPL generally depends, among other factors, on the size of the speaker relative to the distance between the speaker and the listener. Generally, a larger distance requires a larger speaker size. Obviously, there is a practical limit on how large a speaker can be made. For example, an overly large speaker may create difficulties in transporting or mounting. Furthermore, a correspondingly large driving element needed to drive such a large speaker may require an impractical amount of power. 
         [0006]    To circumvent such drawbacks, an array of smaller sized speakers can be used to achieve similar acoustic results. As is generally understood, sound waves from the individual smaller speakers may combine to yield a combined sound wave that behaves similar to that emanating from a single large speaker. 
         [0007]    Effective and coherent combination of sound waves may be achieved when certain wave related parameters are satisfied. One such requirement is that the individual waves emanating from the smaller speakers need to have a substantially fixed phase difference among themselves. When all of the smaller speakers in a linear arrangement are driven substantially in phase (substantially zero phase difference), a resulting combined wave propagates in a direction normal to a line defined by the speakers. A substantially fixed non-zero phase difference among the individual waves results in a combined wave that propagates at an angle with respect to the normal direction. In typical arrayed speaker applications, the individual smaller speakers are driven substantially in phase. 
         [0008]    Another requirement for a quality combined wave from the array of smaller speakers is that the spacing between the speakers need to have certain dimension relative to the wavelength of the sound waves. As a rule of thumb, it is generally accepted that the spacing between two neighboring speakers needs to be smaller than the wavelength of the sound wave in question. In some standards, the spacing requirement is tighter at half the wavelength. One reasons is that if the spacing is larger than the wavelength (or half the wavelength), the resulting combination of the waves suffers from poor directional properties, including unwanted side lobes of sound patterns away from the desired direction. 
         [0009]    The wavelength of a wave is determined as wave velocity divided by wave frequency. The wave velocity of sound in room temperature air is approximately 1130 ft/sec. For an exemplary low frequency audio sound having a frequency of 200 Hz, the corresponding wavelength is approximately 68″. Similarly, a midrange audio sound with a frequency of 2000 Hz, the corresponding wavelength is approximately 6.8″ For the low frequency audio sound, maintaining the spacing between the speakers less than the wavelengths under the exemplary 68″ is easily achieved. For the midrange audio sound, arranging the midrange speakers with spacing under the exemplary 6.8″, while more challenging than that of the low frequency case, is still achievable. 
         [0010]    For a high frequency audio sound with an exemplary frequency of 20000 Hz, the corresponding wavelength is approximately 0.68″. This relatively small wavelength poses a problem for spacing of the high frequency speakers, since the components of the speaker has physical limitations on how small they can be made. For example, the magnet assembly that drives the speaker cone needs to be of certain minimum size such that positioning two such speakers adjacent to each other yields a center-to-center spacing larger than the exemplary wavelength of 0.68″. Thus, the resulting high frequency sound emitted from such an array of high frequency speakers suffers from the aforementioned directionality problems. 
         [0011]    For the foregoing reasons, there is a continuing need for an improved system and method for transmitting a sound wave from a speaker or a plurality of speakers. In particular, there is a need for transmitting high frequency sound waves in a manner that allows increasing of the dimension of the transmitted wavefronts while mitigating the undesired effects that degrade the sound quality. 
       SUMMARY OF THE INVENTION 
       [0012]    The aforementioned needs are satisfied by one aspect of the invention relating to a speaker assembly comprising a sound source that produces a sound signal. The speaker assembly further comprises a housing having an input aperture and a plurality of output apertures that are aligned in a first direction. The housing is attached to the sound source so as to receive the sound signal at the input aperture. The housing defines a plurality of isolated paths having substantially equal path lengths that link the input aperture to the plurality of output apertures. The sound signal is divided into a plurality of sound signals that are distributed in the first direction by travel along the plurality of isolated paths. The plurality of sound signals emanate from the plurality of output apertures at substantially the same time so as to combine to form a substantially coherent combined sound signal that is expanded in the first direction. 
         [0013]    In one embodiment, the housing defines the plurality of isolated paths by one or more plugs having a first end biased towards the input aperture and a second end biased towards the output aperture. The first end of a given plug divides an existing path into two isolated paths and the second end of the given plug divides an existing output aperture into two smaller output apertures. The plug has a maximum width at a location between the first and second ends such that the isolated paths formed by the plug flare open into the output apertures. 
         [0014]    The amount of flare and the corresponding dimension of the output aperture are selected such that the curvature δ of the wavefronts emanating therefrom is less than a quarter of the wavelength of the sound signal. The curvature δ=(L/2)tan(φ/2) where L is the dimension of the output aperture and φ is the opening angle of the flare. In one embodiment, the plug has a diamond shape elongated along a line that joins the first and second ends. 
         [0015]    The aforementioned needs are satisfied by another aspect of the invention relating to a speaker assembly comprising a sound source that produces a first sound signal. The speaker assembly further comprises a horn assembly that receives the first sound signal and directs the first sound signal along a plurality of paths so as to expand the first sound signal into a plurality of sound signals that are distributed in at least a first direction. The horn assembly includes a plurality of flared apertures that are aligned in the first direction such that the plurality of sound signals emanate from the plurality of flared openings so as to produce a combined substantially coherent sound signal. 
         [0016]    In one embodiment, the plurality of paths comprise a plurality of isolated paths. In one embodiment, the horn assembly includes a housing having an output wall of a first length. The plurality of flared apertures are formed in the output wall such that each of the plurality of sound signals have a length that is less than the first length so that the overall curvature of the combined substantially coherent sound signal is reduced to thereby facilitate coherent combination with sound signals emanating from adjacent sound sources. 
         [0017]    In one embodiment, the horn assembly housing includes an input opening that receives the first sound signal from the sound source. The housing defines the plurality of paths, and the plurality of paths emanate outward from the input opening in a pattern where the outermost paths define first angle therebetween. The plurality of flared apertures are flared at an angle which is less than or equal to the first angle. The flare angle and the corresponding length of the sound signal are selected such that the curvature δ of the sound signal emanating therefrom is less than a quarter of the wavelength of the sound signal. The curvature δ=(L/2)tan(φ/2) where L corresponds to the length of the sound signal and φ is the flare angle. 
         [0018]    The plurality of paths and their corresponding flared apertures are defined by one or more plugs having a first end biased towards the sound source and a second end biased towards the flared apertures. The first end of a given plug divides an existing path into two paths and the second end of the given plug divides an existing flared aperture into two smaller flared apertures. The plug has a maximum width at a location between the first and second ends. In one embodiment, the plug has a diamond shape elongated along a line that joins the first and second ends. 
         [0019]    The aforementioned needs are satisfied by yet another aspect of the invention relating to a speaker assembly comprising a sound source that produces a sound signal The speaker assembly further comprises a housing having a first input aperture and a first output aperture. The housing is attached to the sound source such that the first input aperture is adjacent the sound source. The first output aperture is larger than the first input aperture along at least a first direction. The speaker assembly further comprises at least one plug positioned between the first input aperture and the first output aperture so as to define two or more smaller output apertures that are smaller than the first output aperture along at least the first direction. The first input aperture and the two or more smaller output apertures are linked by isolated paths having substantially equal path lengths such that the sound signal is divided into two or more sound signals that are distributed in the first direction by travel along the two or more isolated paths. The two or more sound signals emanate from the two or more smaller output apertures at substantially the same time so as to combine to form a substantially coherent combined sound signal that is expanded in the first direction. 
         [0020]    In one embodiment, the two or more isolated paths are flared adjacent the corresponding two or more smaller output apertures. The plug has a first end biased towards the first input aperture and a second end biased towards the first output aperture. The first end of a given plug divides an existing path into two isolated paths and the second end of the given plug divides an existing output aperture into two smaller output apertures. The plug has a maximum width at a location between the first and second ends so as to provide the flaring of the isolated paths adjacent their corresponding smaller output apertures. 
         [0021]    The amount of flare and the corresponding dimension of the smaller output aperture along the first direction are selected such that the curvature δ of the sound signals emanating therefrom is less than a quarter of the wavelength of the sound signal. The curvature δ=(L/2)tan(φ/2) where L is the dimension of the smaller output aperture and φ is the opening angle of the flare. In one embodiment, the plug has a diamond shape elongated along a line that joins the first and second ends. 
         [0022]    The aforementioned needs are satisfied by yet another aspect of the invention relating to an array of speakers comprising a plurality of low frequency speakers arranged along a first direction. The low frequency speakers have a first dimension along the first direction. The array further comprises a plurality of high frequency speakers arranged along the first direction. Each high frequency speaker comprises a driver coupled to a horn assembly having an input aperture that receives a sound signal from the driver, and a plurality of flared apertures that are aligned in the first direction. The input aperture is linked to the plurality of flared apertures by a plurality of paths that direct the sound signal therethrough so as to expand the sound signal into a plurality of sound signals that are distributed in the first direction. The plurality of sound signals emanating from the plurality of flared openings produce a substantially coherent combined sound signal. 
         [0023]    In one embodiment, each of the plurality of flared aperture is dimensioned such that the curvature δ of the sound signals emanating therefrom is less than a quarter of the wavelength of the sound signal. The curvature δ=(L/2)tan(φ/2) where L is the dimension of the flared aperture and φ is the opening angle of the flare along the first direction. In one embodiment, the sum of the first direction dimension of the plurality of the flared apertures is at least 80% of the first dimension. In one embodiment, the high frequency speakers are arranged along a vertical direction. In one embodiment, each high frequency speaker further comprises a horizontal flare attached to the plurality of flared openings, thereby controlling the horizontal dispersion of the emanating sound signals. 
         [0024]    The aforementioned needs are satisfied by yet another aspect of the invention relating to a speaker assembly comprising a sound source that produces a sound signal. The speaker assembly further comprises a housing that defines an input aperture and two or more flared horn cavities having exit apertures. Each flared horn cavity has an opening angle and each exit aperture has a length along a first direction. The input aperture is adjacent the sound source, and the exit apertures are aligned along a first direction. The input aperture is linked to the flared horn cavities by paths that are at least partially isolated from each other. The sound signal from the sound source is distributed to the flared horn cavities and exit through the exit apertures. The opening angles of the flared horn cavities and the lengths of the exit apertures are selected so as to approximate a segmented line source of sound. 
         [0025]    In one embodiment, each of the two or more flared horn cavities is dimensioned such that the curvature δ of sound wavefronts emanating therefrom is less than a quarter of the wavelength of the sound signal. The curvature δ=(L/2)tan(φ/2) where L is the length of the exit aperture and φ is the opening angle of the flared horn cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1A  illustrates a side view of one embodiment of a horn assembly that provides multiple acoustic paths to multiple exit apertures to allow expansion of a relatively small sound source to a larger dimensioned exit; 
           [0027]      FIG. 1B  illustrates a front view of the horn assembly of  FIG. 1A ; 
           [0028]      FIG. 2  illustrates a horn cavity geometry and its effects on the emitted sound wave; 
           [0029]      FIG. 3  illustrates an array of horn cavities stacked vertically; 
           [0030]      FIGS. 4A  and B illustrate some possible embodiments of a plug that is positioned within a larger horn cavity to produce two smaller horn cavities, thereby allowing desirable horn geometry to be obtained for effective combining of the emitted sound waves; 
           [0031]      FIGS. 5A  and B illustrate some possible embodiments of the horn assembly where the plugs are diamond shaped to yield straight walled horn cavities; 
           [0032]      FIG. 5C  illustrates one possible embodiment of the horn assembly where the plug has a curved profile to accommodate flared wall horn cavities; 
           [0033]      FIGS. 6A  and B illustrate some possible methods of arraying the enlarged exits provided by various embodiments of the horn assembly; and 
           [0034]      FIGS. 7A  and B illustrate one embodiment of the horn assembly having a horizontal flare at the horn exit thereby allowing control of the horizontal coverage of the emitted sound. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    Reference will now be made to the drawings wherein like numerals refer to like parts throughout. A multiple-aperture acoustic horn is an apparatus that provides multiple paths for a sound wave being emitted from a single speaker driver. The multiple paths can be advantageously configured to suit various application needs. A general operating principle is described in reference to  FIGS. 1-3 , and some of the various possible embodiments are described in reference to  FIGS. 4-6 . 
         [0036]      FIGS. 1A-C  illustrate one possible embodiment of a multiple-aperture acoustic apparatus  100  comprising a single speaker driver  102  attached to a horn assembly  104 . The horn assembly  104  comprises a first horn  106  that has a back end and a front end, and the back end defines a first input aperture  124  dimensioned to receive the sound waves being emitted by the speaker driver  102 . The first input aperture  124  may be a circular aperture to mate with a circular speaker driver. Alternatively, the first input aperture  124  may have any number of shapes and dimensions to mate efficiently with any number of speaker driver shapes. 
         [0037]    The first horn  106  also defines a first exit aperture  128  at the front end that is larger than the first input aperture  124 , thereby defining a horn shaped first cavity  114 . As shown in  FIG. 1A , a side sectional profile of the first cavity  114  generally opens up from the first input aperture  124  to the first exit aperture  128 . As shown in  FIG. 1B , a frontal view of the horn assembly  104  shows that in one embodiment, the first cavity  114  has a generally rectangular shape. It will be understood, however, that various other frontal shapes of the first cavity may be utilized without departing from the spirit of the invention. Various possible dimensions and materials that can be implemented for the first horn  106  are described below. 
         [0038]    The horn shape of the first cavity  114 , in absence of other structures described below, causes sound waves being emitted from the speaker driver  102  to generally cause the wavefronts to become rounded, thereby causing the sound waves&#39; directionality to spread out. If the speaker driver  102  pumps into the first input aperture  124  generally plane waves, the wavefronts become rounded due to the fact that wavefronts tend to be orthogonal to the boundaries. Thus, the degree of rounding of the wavefronts generally depend on the taper angle of the horn. 
         [0039]    As is described below, two or more horn assemblies may be stacked vertically. The manner in which the sound waves from such horn assemblies combine depends on factors such as the frequency of the sound waves, dimension of the exit aperture, and the pitch of the taper. In audio applications, a generally accepted rule is that a curvature (defined below) of the rounded wavefront needs to be less than approximately ¼ of the wavelength λ of the sound wave. One possible method determining the wavefront curvature is disclosed in an Acoustic Engineering Society convention paper titled “Line Arrays: Theory and Applications”, authored by Mark S. Ureda and presented in May, 2001. The derivation of the wavefront curvature in the Ureda paper is in context of segmented line sources, but the general principle also holds in context of the horn shaped source. 
         [0040]      FIG. 2  illustrates a generic horn shaped cavity and some corresponding geometry related parameters to put the wavefront curvature parameter in a proper context. A horn cavity  140  defined by flanking structures has an input aperture  142  and an exit aperture  144 . The exit aperture  144  has a dimension of L along a direction perpendicular to a center axis). The horn cavity  140  tapers in a opening manner from the input aperture  142  to the exit aperture  144  at an opening angle of φ (angle between the center axis and one tapered side). As previously described, a wavefront propagating through such a tapered cavity becomes rounded. Thus, as a wavefront  146  exits the exit aperture  144 , a distance from the face of the exit aperture  144  and the wavefront  146  along the center axis is defined as a wavefront curvature δ. As derived in the Ureda paper, the curvature δ may be expressed as 
         [0000]    
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     
                       ( 
                       
                         L 
                         2 
                       
                       ) 
                     
                      
                     
                       
                         tan 
                          
                         
                           ( 
                           
                             φ 
                             2 
                           
                           ) 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0041]    As seen in Equation 1, the curvature δ is proportional to the dimension L of exit aperture, and also increases with the opening angle φ within the range of 0 to 45 degrees. Thus, the parameters L and/or φ determine the limit on the effectively combinable wavelength (i.e., δ&lt;¼λ) of the signals emitted from the horn cavity  140 . 
         [0042]    Based on the rule δ&lt;¼λ, a minimum wavelength of effectively combinable sound wave can be expressed as 
         [0000]      λ min =4δ.  (2)
 
         [0000]    Alternatively, since frequency of sound is a more common parameter used in audio industry, and since frequency and wavelength is related in a simple inverse relationship, Equation 2 can be expressed as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       f 
                       
                         m 
                          
                         
                             
                         
                          
                         ax 
                       
                     
                     = 
                     
                       c 
                       
                         4 
                          
                         δ 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0043]    where c is the speed of sound and the curvature δ is determined from Equation 1. Thus, the geometry dependent parameters L and/or φ determine the maximum effectively combinable sound wave being emitted from a horn cavity. It will be understood that the frequency limit f max  relates to the effective combining of the sound waves emanating from two or more horn cavities arranged in a linear array to approximate a segmented line source, and not necessarily to the sound quality of the individual horn cavity by itself. 
         [0044]    In certain audio applications, it may be desirable to have the dimension L of the exit aperture conform to some selected value. For example, an ensemble of various speakers may form a plurality of vertical arrays, where each vertical array comprises either low frequency, mid-range, or high-frequency speakers (or horns extending therefrom). In one such configuration, a vertical stack of high-frequency speaker assemblies (speaker assembly comprising speaker driver and horn assembly, for example) may be interposed between two vertical stacks of bass speakers. For various reasons, it may be desirable to have the vertical dimension of the exit aperture of the high-frequency speaker assembly be similar to that of the bass speaker. One difficulty encountered in such a design is that bass speakers are generally relatively large, thus the corresponding value of L partially determines the upper frequency limit of the high-frequency speaker assembly. For example, if L is approximately 9″ (being positioned next to a 9″ diameter bass speaker) and the opening angle φ is approximately 10 degrees, then the curvature δ is approximately 0.4″, and the upper frequency limit f max  is approximately 8.6 KHz which is substantially below what is considered a high-frequency audio range. Thus while such a horn may function well by itself as a high frequency component, an array of such horns yields a degraded quality combined sound wave when the frequency exceeds the exemplary f max  of 8.6 KHz. 
         [0045]    In one aspect of the invention, various embodiments of horn assemblies comprise one or more wave dividing structure referred hereinafter as a “plug”. A plug, positioned in the horn cavity, is shaped so as to define additional smaller exit apertures, and also provide different paths for the sound waves from the input aperture to the smaller exit apertures. Thus, a given plug defines a new set of exit apertures, each having a smaller dimension than the original dimension L. As described below in greater detail, each of the exit apertures advantageously has dimensions and opening angle that yield a higher value for the frequency limit f max . 
         [0046]    Referring to  FIG. 1A , the horn assembly  104  comprises a first plug  110  positioned within the first horn cavity  114 , thereby defining, along with the first horn  106 , second horn cavities  116   a, b  having second input apertures  126   a, b  and second exit apertures  118   a, b . Furthermore, the first plug  110  and the first horn  106  define first conduits  108   a  and  108   b  that respectively connect the first input aperture  124  to the second input apertures  126   a  and  126   b . Thus, the sound wave originating from the first input aperture is split into two waves by the first plug  110 , and the two waves travel through their respective first conduits  108   a, b , through the second input apertures  126   a, b,  and into the second horn cavities  116   a, b.    
         [0047]    Preferably, the first plug  110  is dimensioned and positioned so as to be symmetric with respect to the axis of the first horn  106 . Then, each of the second exit apertures  118   a, b  has a vertical dimension that is approximately half of the vertical dimension of the first aperture  128 . Thus, for the aforementioned example where overall L=9″ and φ=10 degrees, each of the newly formed two smaller horn cavities have l=L/2 and φ=10 degrees, thereby yielding f max  of approximately 17 KHz (Equations 1-3). Such configuration of the horn assembly may be utilized for mid-range sound application if desired, or the exit apertures may be divided further, as described below, to achieve higher f max . 
         [0048]    As illustrated in  FIG. 1A , the horn assembly  104  further comprises second plugs  112   a  and  112   b  positioned respectively within the second horn cavities  116   a  and  116   b , thereby defining, along with the first horn  106  and the first plug  110 , third horn cavities  120   a - d  having third input apertures  130   a - d  and third exit apertures  132   a - d . Furthermore, the second plugs  112   a, b , the first plug  110  and the first horn  106  define second conduits  138   a - d  that respectively connect the second input apertures  126   a, b  to the third input apertures  130   a - d . Thus, the two sound waves passing through the second input apertures  126   a, b  are split into four waves by the second plugs  112   a, b , and the four waves travel through their respective second conduits  138   a - d , through the third input apertures  130   a - d , and into the third horn cavities  120   a - d.    
         [0049]    Preferably, the second plugs  112   a, b  are dimensioned and positioned so as to be symmetric with respect to the axes of their respective second horn cavities  116   a, b . Then, each of the third exit apertures  132   a - d  has a vertical dimension that is approximately quarter of the vertical dimension of the first aperture  128 . Thus, for the aforementioned example where the overall L=9″ and φ=10 degrees, each of the newly formed four smaller horn cavities have l=L/4 and φ=10 degrees, thereby yielding f max  of approximately 34 KHz (Equations 1-3) which is well above the audio high-frequency range. Such configuration of the horn assembly may be utilized for high-frequency sound application. 
         [0050]    It will be appreciated that additional plugs may be incorporated in a manner similar to that described above to yield, for example, eight smaller exit apertures. While such a configuration is not necessary for the exemplary horn assembly with L=9″ and φ=10 degrees, other larger sized horn assemblies may benefit from having eight or more smaller exit apertures. Furthermore, as the dimension L is divided with introduction of plug(s), the opening angles of the resulting horns may have opening angles different than that of their parent horn to achieve the desired result. For example, in the exemplary original configuration of L=9″ and φ=10 degrees, the plug(s) may be configured such that the resulting smaller horns have different opening angles (than 10 degrees—for example, greater than 10 degrees) while achieving the desired value for f max . 
         [0051]    As previously described, the plugs are shaped and positioned so as to be symmetric with respect to their respective horn cavities. As illustrated in  FIG. 1A , such symmetry results in different sound paths  122   a - d  having a substantially similar pathlength. Thus, the sound waves travelling via the sound paths  122   a - d  and exiting the exit apertures  132   a - d  are in phase with each other, and with other similar waves from other similar and stacked horn assemblies, thereby allowing substantially coherent combination of the waves. 
         [0052]    The plugs described above in reference to  FIG. 1  have a side cross sectional shape of a diamond to fit within the straight walled horn cavities (again, in cross sectional view). The diamond shape has a first pointed end proximate its corresponding input aperture, thereby allowing efficient splitting of the sound wave into two symmetric pathways. The diamond shape also has a second pointed end opposite from the first pointed end, thereby allowing a minimum vertical gap between adjacent exit apertures. 
         [0053]    In other embodiments, the horn cavity is not straight walled. A flared horn cavity is one such example. As described below in greater detail, a plug for such a cavity may have some curvatures on its “facets” to accommodate the flare. Thus it will be appreciated that the plug performing the aforementioned function may have different shapes and sizes without departing from the spirit of the invention. 
         [0054]      FIG. 3  now illustrates a stack of horn assemblies and the associated geometry parameters that affect how well the sound waves combine. As referred to in the “Description of the Related Art” section, the spacing between adjacent sound sources relative to the wavelength affects the how effectively the waves combine. In  FIG. 3 , a plurality of exit apertures  152  can be considered to be the sound sources. The source-to-source (center-to-center) distance is h, which, for the exemplary 9″ horn assembly with four exit apertures, is approximately 2.25″—substantially greater than the 0.68″ source spacing (for the 20 KHz sound) referred to in Related Art section. It should be understood that the exemplary 0.68″ spacing is for a circular wavefront (isotropic) being emitted from the source (a point source, for example). As described above, the sound wave emerging from the horn exit aperture is made to behave like a finite length line source, thereby allowing the substantial increase in the workable vertical dimension of the source 
         [0055]    Despite the fact that the vertical dimension of the source, and hence the center-center spacing of the sources can be increased substantially by the apparatus described herein, it is nevertheless advantageous to minimize gaps between the adjacent exit apertures. One reason is that the combining effects of the curved wavefronts degrade at greater distances. 
         [0056]    The exit apertures described above in reference to  FIGS. 1 and 3  are defined by the pointed (side view; an edge in front view) second ends of the diamond shaped plugs. Thus, gaps between the exit apertures within the same horn assembly is minimal. However, as shown in  FIG. 3 , a horn assembly  150  may comprise an outer housing  154  such that when stacked with another horn assembly  150 , the housings  154  may form a gap between the two end exit apertures. In  FIG. 3 , this vertical gap is depicted as being 2a in dimension. One possible method of quantifying the acceptable limit on the gap is disclosed in the Acoustic Engineering Society Preprint #5488 titled “Wavefront Sculpture Technology”, authored by Urban, Heil, and Bauman in 2001, where a ratio of the total source area to the total “vertical” area of 80% or greater is considered to be acceptable. The vertical area is simply a portion of the total area of the front face that is covered if the source (horn apertures in this case) extends vertically. Thus, the vertical area would not include the area covered by the side walls with thickness of b. 
         [0057]    As shown in  FIG. 3 , the total vertical area of the horn assembly  150  is w(2a+4h), while the total source area is 4wh. In one embodiment, the horn exit aperture has a height h of approximately 2.25″, and a width w of approximately 1″. Furthermore, the top and bottom housing thickness a is approximately ⅛″. Thus, the total source area is approximately 9 square inches and the total vertical area is approximately 9.25 square inches, yielding a ratio of approximately 97%, well above the acceptable limit. 
         [0058]      FIGS. 4A-B  now illustrate some common properties of the plugs described above in reference to  FIG. 1A , and those of other various embodiments described below.  FIG. 4A  illustrates a straight walled horn cavity  162  defined by first and second boundaries  164  and  166  that opens up from an input aperture  190  to an exit aperture  192 . Such boundaries may be part of a main horn ( 106  in  FIG. 1A , for example) or part of a larger plug. A plug  160  is positioned within the cavity  162  in a generally symmetric manner such that a longitudinal axis  170  of the plug  160  generally coincides with a longitudinal axis of the horn cavity  162 . 
         [0059]    In one embodiment, the plug  160  in side vertical cross section has a diamond shape, with a first end  172  and a second end  174  positioned along the longitudinal axis  170 . The diamond shaped plug  160  further comprises side vertices  176  and  178  that form the widest lateral dimension of the plug  160  between the first and second ends  172 ,  174 . The first end  172  and the side vertices  176 ,  178  are joined by interior edges  180 ,  182 , respectively. In a similar manner, the side vertices  176 ,  178  and the second end  174  are joined by exterior edges  184 ,  186 , respectively. The interior edges  180 ,  182  and the boundaries  164 ,  166  define conduits  206 ,  208 , respectively, from a location proximate the input aperture  190  to a location proximate the side vertices  176 ,  178 . The exterior edges  184 ,  186  and the boundaries  164 ,  166  define, respectively, two new horn cavities  198  and  200  having input apertures  194 ,  196  defined by the boundaries  164 ,  166  and the side vertices  176 ,  178 , and exit apertures  202 ,  204  defined by the boundaries  164 ,  166  and the second end  174  of the plug  160 . 
         [0060]    It will be appreciated that the shape of the diamond plug  160  as described above in reference to  FIG. 4A  can be varied in any number of ways to obtain any number of desired configuration of the plug  160  with respect to the horn cavity  162 . For example, the lateral dimension of the plug  160  at the side vertices  176 ,  178  can be increased or decreased to increase or decrease the dimensions of the conduits  206 ,  208  and the input apertures  194 ,  196 . Furthermore, the longitudinal location of the side vertices  176 ,  178  can also be varies to alter the general shape of the horn cavities  198 ,  200 . In one particular embodiment, the horn cavities created by the plug have a similar but scaled down horn profile as that of the original horn cavity. It will be appreciated, however, that the scaled down horn profiles do not have to have a similar profile as the original profile. 
         [0061]      FIG. 4B  illustrates another embodiment of a horn cavity, a flared horn cavity  212  defined by first and second curved boundaries  214  and  216  that opens up from an input aperture  240  to an exit aperture  242 . Such boundaries may be part of a main horn or part of a larger plug. A plug  210  is positioned within the cavity  212  in a generally symmetric manner such that a longitudinal axis  220  of the plug  210  generally coincides with a longitudinal axis of the horn cavity  212 . 
         [0062]    In one embodiment, the plug  210  in side vertical cross section has an at least partially curved double ended spear shape, with a first end  222  and a second end  224  positioned along the longitudinal axis  220 . The plug  210  further comprises widest lateral dimension location, indicated by a double ended arrow  226 , somewhere between the first and second ends  222 ,  224 . The first end  222  and both sides of the laterally widest location  226  are joined by interior edges  230 ,  232 , respectively. In a similar manner, both sides of the laterally widest location  226  and the second end  224  are joined by exterior edges  234 ,  236 , respectively. The interior edges  230 ,  232  and the boundaries  214 ,  216  define conduits  256 ,  258 , respectively, from a location proximate the input aperture  240  to a location proximate the laterally widest location  226 . The exterior edges  234 ,  236  and the boundaries  214 ,  216  define, respectively, two new horn cavities  248  and  250  having input apertures  244 ,  246  defined by the boundaries  214 ,  216  and the laterally widest location  226 , and exit apertures  252 ,  254  defined by the boundaries  214 ,  216  and the second end  224  of the plug  210 . 
         [0063]    It will be appreciated that the shape of the at least curved plug  210  as described above in reference to  FIG. 4B  can be varied in any number of ways to obtain any number of desired configuration of the plug  210  with respect to the horn cavity  212 . For example, the lateral dimension of the plug  210  at the laterally widest location  226  can be increased or decreased to increase or decrease the dimensions of the conduits  256 ,  258  and the input apertures  244 ,  246 . Furthermore, the longitudinal location of the laterally widest location  226  can also be varies to alter the general shape of the horn cavities  248 ,  250 . In one particular embodiment, the horn cavities created by the plug have a similar but scaled down horn profile as that of the original horn cavity. It will be appreciated, however, that the scaled down horn profiles do not have to have a similar profile as the original profile. 
         [0064]      FIGS. 5A-C  illustrate some possible embodiments of the horn assembly described above. In one embodiment, a horn assembly  270  comprises a plug  280  positioned with a cavity defined by a first horn  272 . An interior portion of the plug  280  and the cavity define first conduits  274  and  276 . An exterior portion of the plug  280  and the cavity define two smaller secondary cavities in which secondary plugs  282 ,  284  are positioned, thereby creating front end cavities  290   a - d.    
         [0065]    As seen in  FIG. 5A , the plug  280  and its corresponding cavity wall are dimensioned such that the conduits  274 ,  276  are directed at an angle that is larger than the opening angle of the end cavities  290   a - d . This feature is achieved by the plug  280  having side vertices positioned towards the interior portion of the cavity. In one embodiment, the horn assembly  270  has exterior dimensions of approximately 12″ (L)×9″ (H). 
         [0066]      FIG. 5B  illustrates another embodiment, a similar horn assembly  300  having a plug  310  positioned within a cavity defined by a first horn  302 . The plug  310  has side vertices that are located more towards its center (than that of the plug  280  in  FIG. 5A ), such that resulting conduits  304 ,  306  are oriented at a smaller angle than the angle of the conduits  274 ,  276  described above. In one embodiment, the horn assembly  300  has exterior dimensions of approximately 12.5″ (L)×8.2″ (H). 
         [0067]      FIG. 5C  illustrates yet embodiment, a flared horn assembly  330  having a first horn  332  that defines a flaring cavity  334 . Positioned within the cavity  334  is a horn  336  that yields two end horn cavities  340   a, b  in a manner described above in reference to  FIG. 4B . 
         [0068]    The exemplary profiles of the cavities and their corresponding plugs, described above in reference to  FIGS. 5A-C , show that the configuration horn assembly can be varied in a number of ways to accommodate the desired dimension. Similarly, the configuration can be varied to allow sound quality tuning to suit various applications. 
         [0069]      FIGS. 6A-B  illustrate some possible methods of using the horn assemblies described above.  FIG. 6A  illustrates a speaker array  350  comprising a stack  356  of high frequency horn assemblies  364  interposed between two stacks  352 ,  354  of bass speakers  360 . The vertical dimension of the horn assembly  364  may be selected to be similar to the vertical dimension of the bass speakers  360 . 
         [0070]    In one embodiment of the stack  356  illustrated in  FIG. 6A , each of the four high frequency horn assemblies  364  has an actively transmitting area that has a vertical dimension H horn  of approximately 9″. The array  350  has an overall height H array  of approximately 43.9″. Thus, the fraction (vertical) of actively transmitting area in such a configuration is approximately 4×9/43.9=0.82, which satisfies the previously described 80% rule. 
         [0071]      FIG. 6B  illustrates an ensemble  370  of flared horn assemblies  372  arranged in two possible configurations. Each of the horn assembly  372  defines a flared horn cavity, and a plug  374  is positioned therein in a similar manner to that described above in reference to  FIG. 5C . The horn assembly  372  has an angled exterior such that its exit end&#39;s dimension is greater than its speaker driver end&#39;s dimension. As such, the horn assemblies  372  can be arranged in a first exemplary configuration  376  wherein the front faces of the exit apertures are aligned in a same plane. Alternatively, the horn assemblies  372  can be arranged in a second exemplary configuration  380  wherein the angles sides of the adjacent horn assemblies engage each other, such that the front faces of the exit apertures fan out. The first configuration  376  generally offers more directionality of the sound emitted therefrom, and the fanned second configuration  380  offers more coverage, if desired. 
         [0072]      FIGS. 7A  and B illustrate one possible embodiment of a horn assembly  390  having a horizontal flare  392  attached to a vertically oriented exit apertures  394 . The horn assembly  390  without the horizontal flare  392  may be one of the horn assemblies described above. As previously described, the sound emanating from the exit apertures  394  (without the horizontal flare) generally has a cylindrical shaped wavefronts generally having a cross sectional shape of a half circle. Thus, such a cylindrical wave spreads in a range of approximately 180 degrees. While such spreading of the cylindrical wave covers a wide horizontal range, range is reduced because of the wide spreading. By placing the horizontal flare  392  in front of the exit apertures  394 , the horizontal spreading of the wavefronts may be controlled in an advantageous manner. For example, the horizontal flare  392  has an opening angle less than 180 degrees, thereby reducing the horizontal dispersion and extending the range of the waves. Thus, it will be appreciated that the opening angle of the horizontal flare  392  may be selected from a range of approximately zero to 180 degrees to control the horizontal coverage and the range as desired. 
         [0073]    The horn assembly  390  having the horizontal flare  392  may be used in conjunction with large bass speakers  400 , as shown in  FIGS. 7A  and B. Furthermore, such a combination high frequency horn assembly  390  and the bass speakers  400  may be stacked vertically in a manner similar to that described above in reference to  FIG. 6A . Alternatively, the horn assembly  390  may be operated by itself or arrayed with other horn assemblies (with or without the horizontal flares), without being proximate the bass speakers, without departing from the spirit of the invention. 
         [0074]    Various embodiments of the horn assembly described herein extend the dimension of the wavefront along the vertical direction. It will be understood that the vertical direction is only one possible preferred direction. The novel concept of increasing the output dimension of the horn assembly along a preferred direction by forming a plurality of apertures along the preferred direction is applicable with any choice of the preferred direction, including the horizontal direction. 
         [0075]    The vertically oriented horn assemblies disclosed herein comprise various plug structures that isolate the plurality of apertures and acoustic paths from each other vertically. These vertically isolated multiple apertures and paths are described above in reference to  FIGS. 1A-B ,  3 ,  5 A-C,  6 A-B, and  7 A-B. In one aspect of the invention, the multiple apertures and their corresponding paths being isolated along the preferred direction allows the plugs to be configured in a relatively simple manner. In particular, as exemplified in the side sectional view of one embodiment in  FIG. 1A , the plugs may be relatively simple slabs having appropriate side profiles. For example, the plugs  112   a, b  in  FIG. 1A  may be diamond shaped slabs, with the slab thickness being approximately same as the horizontal width of the multiple apertures thereby vertically isolating them from each other. Such a configuration allows, if desired, the horizontal dimension of the horn portion to be relatively thin, thereby providing more flexibility in design and implementation of the horn assembly. In certain embodiments, such as that shown in  FIG. 7B , the horn portion (other than the horizontal flare) of the assembly may be substantially narrower than the horizontal dimension of the driving element at the rear. In such applications, the depth of the horn assembly may be sufficiently large to allow the driving element from interfering with the adjacent bass speakers. Thus, if the horizontal flare is absent in the configuration of  FIG. 7B , the two flanking bass speakers may be brought closer together if desired. 
         [0076]    Various embodiments of the horn assembly described above utilize one or more plugs to allow advantageous increase in the exit dimension. The plugs and their corresponding horns can be constructed in a variety of ways using any of the acoustic materials. The material may include, by way of example, aluminum, polyvinyl chloride (PVC), glass filled nylon, urethane, or any number of acoustically favorable materials. These possible materials may be formed, by way of example, by machining, sand casting, injection molding, or any number of processes configured to form three dimensional objects. It will be appreciated that the various embodiments of the novel concepts described herein may be formed by one or more, or any combination of the aforementioned fabrication methods from one or more, or any combination of the aforementioned materials without departing from the spirit of the invention. 
         [0077]    Although the foregoing description has shown, described and pointed out the fundamental novel features of the invention, it will be understood that various omissions, substitutions, and changes in the form of the detail of the apparatus as illustrated as well as the uses thereof, may be made by those skilled in the art, without departing from the spirit of the invention. Consequently, the scope of the present invention should not be limited to the foregoing discussions, but should be defined by the appended claims.