Patent Publication Number: US-9894432-B2

Title: Modular acoustic horns and horn arrays

Description:
This application is a continuation of, and claims priority to, U.S. patent application Ser. No. 14/565,843 filed Dec. 10, 2014 (having the same title and inventors as the instant application), which is a continuation of, and claims priority to, U.S. patent application Ser. No. 12/898,947 filed Oct. 6, 2010 (having the same title and inventors as the instant application), which is a continuation-in-part of, and claims priority to, U.S. patent application Ser. No. 12/557,885 filed Sep. 11, 2009, by Ickler, et al. and titled “Automated Customization of Loudspeakers”, all of which are incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     This specification describes a modular horn type loudspeaker and horn loudspeaker arrays formed with modular horn type loudspeakers. 
     SUMMARY 
     In one aspect, an apparatus includes a first acoustic horn. The first acoustic horn includes a first acoustic module. The first acoustic module includes a first acoustic driver and a first acoustic duct, for conducting acoustic energy from the first acoustic driver. The first acoustic duct has a first opening through which acoustic energy is radiated. The first acoustic duct is characterized by a first centerline. The apparatus also includes a second acoustic module. The second module includes a second acoustic driver and a second acoustic duct, for conducting acoustic energy from the acoustic driver. The second acoustic duct has a second opening through which acoustic energy is radiated. The second acoustic duct is characterized by a second centerline. The first module and the second module are configured to be positioned and held in place so that the first and second openings are aligned to form a substantially continuous diffraction slot and so that the first and second centerlines are normal to an arc and intersect at a first one of a plurality of angles. The apparatus may include an additional plurality of acoustic modules. Each of the additional acoustic modules may include an acoustic driver and an acoustic duct. Each duct may include an opening through which acoustic energy is radiated. Each duct may be characterized by a centerline. Each of the additional plurality of acoustic modules may be configured to be positioned and held in place so that the opening of each of the additional plurality of acoustic modules is aligned with the openings of the others of the plurality of acoustic modules and with the openings of the first and second acoustic modules to form a substantially continuous diffraction slot. The first module, the second module, and the plurality of additional modules may be substantially identical. The additional plurality of acoustic modules may be configured to be positioned and held in place so that the centerlines of the additional plurality of modules intersect at the one angle of the plurality of angles. The first module and the second module may be substantially identical. The first module and the second module may be asymmetric about at least one axis, and wherein the first module may be oriented so that the first module is rotated 180 degrees about the axis relative to the second module. The plane of the first opening and the second opening may intersect at a first angle, and the apparatus may further includes a second acoustic horn. The second acoustic horn may include a third acoustic module. The third acoustic module may include a third acoustic driver and a third acoustic duct, for conducting acoustic energy from the third acoustic driver. The third acoustic duct may have a third opening through which acoustic energy is radiated. The third acoustic module may be characterized by a third centerline. The second acoustic horn may include a fourth acoustic module. The fourth acoustic module may include a fourth acoustic driver; and a fourth acoustic duct, for conducting acoustic energy from the acoustic driver. The fourth acoustic duct may have a fourth opening through which acoustic energy is radiated. The fourth acoustic duct may be characterized by a fourth centerline. The third module and the fourth module may be configured to be positioned and held in place so that the third and fourth openings are aligned to form a substantially continuous diffraction slot and so that the third centerline and the fourth centerline are normal to an arc and so that the third and fourth centerline intersect at a second angle, different from the first angle. The first acoustic horn and the second acoustic horn may be arranged so that the first horn diffraction slot and the second horn diffraction slot are aligned to form a combined diffraction slot with no gap substantially larger than the combined thickness of a top of one of the acoustic horns and the bottom of the other of the acoustic horns. The first module, the second module, the third module and the fourth module may be substantially identical. The first acoustic horn may further include a top and a bottom. The apparatus may be configured so that the top and bottom used when the centerlines intersect at the first of the plurality of angles is the same as when the centerlines intersect at another of the plurality of angles. 
     In another aspect, an apparatus includes a first acoustic horn. The first acoustic horn includes a first acoustic module. The first acoustic module includes a first acoustic driver; and a first acoustic duct, for conducting acoustic energy from the first acoustic driver. The first acoustic duct has a first elongated planar opening through which acoustic energy is radiated. The apparatus further includes a second acoustic module. The second acoustic module may include a second acoustic driver and a second acoustic duct, for conducting acoustic energy from the acoustic driver. The second acoustic duct may have a second elongated planar opening through which acoustic energy is radiated. The first module and the second module may be configured to be positioned so that the first and second elongated planar openings are aligned in the direction of elongation to form a substantially continuous diffraction slot and so that the plane of the first elongated planar opening intersect the plane of the second elongated planar opening at any one of a plurality of angles. The apparatus further includes a bracket to hold the acoustic modules in a desired position and orientation. The apparatus may further include an additional plurality of acoustic modules. Each of the additional acoustic modules may include an acoustic driver and an acoustic duct. Each duct may have an elongated planar opening through which acoustic energy is radiated. Each of the additional plurality of acoustic modules may be configured to be positioned so that the opening of each of the additional plurality of acoustic modules is aligned in the direction of elongation with the openings of the others of the plurality of acoustic modules and with the openings of the first and second acoustic modules to form a substantially continuous diffraction slot. The first module, the second module, and the plurality of additional modules may be substantially identical. The additional plurality of acoustic modules may be configured to be positioned so that the plane of the elongated opening intersects with the plane of the elongated opening of an adjacent acoustic module at the one of the plurality of angles. The first module and the second module may be substantially identical. The first module and the second module may be asymmetric about at least one axis and the first module may be oriented so that the first module is rotated 180 degrees about the axis relative to the second module. The plane of the first elongated planar opening and the plane of the second elongated planar opening may intersect at a first one of the plurality of angles. The apparatus may further include a second acoustic horn. The second acoustic horn may include a third acoustic module. The third acoustic module may include a third acoustic driver and a third acoustic duct, for conducting acoustic energy from the third acoustic driver. The third acoustic duct may have a third elongated planar opening through which acoustic energy is radiated. The apparatus may include a fourth acoustic module includes a fourth acoustic driver and a fourth acoustic duct, for conducting acoustic energy from the acoustic driver. The fourth acoustic duct may have a fourth elongated planar opening through which acoustic energy is radiated. The third module and the fourth module may be configured to be positioned so that the third and fourth openings are aligned in the direction of elongation to form a substantially continuous diffraction slot and so that the plane of the third elongated planar intersects the plane of the fourth elongated planar opening at a second one of the plurality of angles, different from the first one of the plurality of angles. The first acoustic horn and the second acoustic horn may be arranged so that the first horn diffraction slot and the second horn diffraction slot are aligned to form a combined diffraction slot with no gap substantially larger than the combined thickness of a top of one of the acoustic horns and the bottom of the other of the acoustic horns. The first module, the second module, the third module and the fourth module may be substantially identical. The apparatus may further include a top a bottom. The apparatus may be configured so that the top and the bottom used when the planes intersect at the one of the plurality of angles can be used when the planes intersect at a second one of the plurality of angles. 
     In another aspect, a method for forming loudspeaker arrays, includes providing at least two acoustic horns from a first plurality of acoustic horns each of the plurality of acoustic horns having a top having a planar top surface and a bottom having a planar bottom surface. The top and the bottom are characterized by a thickness. Each of the plurality of horns has a different vertical dispersion angle. Each horn includes a diffraction slot. The method further includes arranging the plurality so that a top surface of one acoustic horn is parallel to, and in planar contact with, the bottom surface of an adjacent acoustic horn. The horn diffraction slots are aligned to form an array diffraction slot with gaps not substantially larger than the combined thickness of the top of the one horn and the bottom of the adjacent acoustic horn. The providing may include forming a first of the acoustic horns from a first plurality of substantially identical acoustic modules. Each module may include an acoustic driver and an acoustic duct having an opening. Each acoustic duct may be characterized by a centerline. The forming may include arranging the first plurality of acoustic modules so that the centerlines are normal to a first arc and intersect at an angle and so that the openings are aligned to form the first acoustic horn diffraction slot. The method may further include forming a second of the acoustic horns from a second plurality of acoustic modules, substantially identical to the first plurality of acoustic modules. Each module may include an acoustic driver and an acoustic duct having an opening. Each acoustic duct may be characterized by a centerline. The forming may include arranging the second plurality of acoustic modules so that the centerlines are normal to a second arc and so that the openings are aligned to form the second acoustic horn diffraction slot. The forming of the first of the acoustic horns may further include arranging the first plurality of acoustic modules so that the centerlines intersect at a first one of a plurality of angles. The forming of the second of the acoustic horns may include arranging the second plurality of acoustic modules so that the centerlines intersect at a second one of the plurality of angles, different from the first one of the plurality of angles. 
     Other features, objects, and advantages will become apparent from the following detailed description, when read in connection with the following drawing, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  includes three diagrammatic plans views of an acoustic horn; 
         FIG. 2  is a diagrammatic oblique isometric view of an acoustic duct; 
         FIG. 3  includes two views of an acoustic horn array; 
         FIGS. 4-8A  are diagrammatic side views of acoustic horns and horn arrays, illustrating various aspects of the horns; 
         FIG. 8B  is a diagram of geometric elements for explaining aspects of the acoustic horn of  FIG. 8A ; 
         FIGS. 9 and 10  are diagrammatic side views of acoustic horn arrays; 
         FIG. 11  includes a top and side diagrammatic views of an acoustic horn; 
         FIGS. 12 and 13  are top diagrammatic views of an acoustic horn; 
         FIG. 14  is front oblique isometric view of an assembly including two acoustic modules; 
         FIG. 15  is an oblique isometric view of an acoustic module; 
         FIG. 16  is a front plan view of an assembly including six acoustic drivers and six acoustic ducts; 
         FIG. 17  is a back plan view of an assembly including six acoustic drivers and six acoustic ducts; 
         FIG. 18A-18E  are side plan views of an assembly including six acoustic modules; 
         FIGS. 19A and 19B  are oblique isometric views of an assembly including six acoustic modules; 
         FIG. 20  is a top plan view of an assembly including six acoustic modules and horn side walls; 
         FIG. 21  is a back oblique isometric view of an assembly including six acoustic modules and horn side walls; 
         FIG. 22  is an oblique isometric view of an acoustic horn; 
         FIG. 23  is an oblique isometric view of an assembly including some elements of an acoustic horn; and 
         FIG. 24  is an oblique isometric view of and assembly including some elements of an acoustic horn. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a horn type loudspeaker  10  for explaining some of the terms that are used in this specification. In the explanations that follow, a coordinate system will be used. The direction of intended radiation, indicated by arrow  28 , is along the Y-axis. The X-axis is horizontal relative to the loudspeaker in the orientation of  FIG. 1 , and perpendicular to the Y-axis, and the Z-axis is vertical and perpendicular to the plane defined by the Y-axis and the X-axis. “Forward” and “front” etc. will refer to a location or direction in the + direction along the Y-axis. “Backward”, “rear” and “behind” etc. will refer to a location or direction in the − direction along the Y-axis. “Leftward” and “Left”, etc. will refer to the − direction along the X-axis. “Rightward” and “Right”, etc. will refer to the + direction along the X-axis. “Above” or “upward” will refer to the + direction along the Z-axis and “below” or “downward” will refer to the − direction along the Z-axis. “Width” refers to the dimension along the X-axis, “height” refers to the dimension along the Z-axis, and “depth” refers to the dimension along the Y-axis. The axes are defined relative to the horn loudspeaker, regardless of the orientation of the horn loudspeaker in space. 
       FIG. 1  is a diagrammatic view of a horn loudspeaker  10 . A plurality, in this example four, of acoustic drivers  12  are acoustically coupled to the throat  13  of an acoustic horn  15  by acoustic ducts  16 . The duct outlet end (that is, the end of the duct that is acoustically coupled to the throat) may be mechanically coupled to the throat  13  directly. Alternatively, the outlet ends of the ducts may be combined into a manifold which is acoustically coupled to the throat  13 . The outlet ends of the ducts may be elongated. The elongated outlet openings of the acoustic ducts or the outlet of the manifold may be aligned in the direction of elongation at the throat to form a diffraction slot. The acoustic horn  15  includes horn side walls  18 A and  18 B and top and bottom walls  20 A and  20 B. In order to show details of the side walls  18 A and  18 B, top and bottom walls  20 A and  20 B are not shown in the top view. The side walls  18 A and  18 B flare outwardly. In some implementations, the walls may flare outwardly linearly. In other implementations, such as the implementation of  FIG. 1 , the side walls  18 A and  18 B can have two planar sections, a first planar section  21 A and  21 B flaring linearly outwardly at one rate and a second planar section  23 A and  23 B flaring outwardly linearly at a different rate. In other implementations, the horn walls make have a different geometry. For example, the walls may flare linearly or curve outwardly according to a continuous curve, such as an exponential curve or conic curve. Additionally, the side walls may flare out asymmetrically. The top and bottom walls  20 A and  20 B may be flared down and up, respectively, from the mouth  17  at an angle θ so that the vertical dispersion angle is 2θ. The horn may be partially enclosed in an enclosure  22 , shown in dotted line in the side view only. For reasons that will be described below, the top wall  24 A and the bottom wall  24 B may be non-parallel with each other and with the top and bottom  20 A and  20 B of the horn, respectively. The acoustic drivers  12  and the ducts  16  will be discussed in more detail below. The enclosure  22  may have side walls or a back wall, but they are not germane to this application and are not shown in the figures. 
     In operation, the acoustic drivers transduce electrical energy into acoustic energy, which is conducted to the acoustic horn. The acoustic energy enters the acoustic horn at the throat  13  and exits the horn at the mouth  17  in a controlled and predictable radiation pattern. 
       FIG. 2  is a diagrammatic view of an acoustic duct  16  for the purpose of explaining some terms used in the specification. The duct  16  may be characterized by a centerline  202  that passes through the geometric center of the duct opening and is perpendicular to the opening at the geometric center. In some implementations, the duct opening is substantially planar, so that the centerline  202  is perpendicular to the plane of the duct opening. In  FIG. 2 , the duct  16  is shown as straight and symmetric, but in an actual implementation, it may be curved and asymmetric about one or more axes. 
     It is desirable to use horns to radiate a full range of frequencies, including high frequencies, and to radiate the acoustic energy, particularly the high frequency acoustic energy, in a controlled and predictable radiation pattern. However, at high frequencies, with corresponding wavelengths that are less than the diameter of the acoustic driver, the individual acoustic drivers may exhibit radiation patterns that make it difficult to predict and control the radiation pattern of the horn loudspeaker. Using small diameter acoustic drivers is impractical, because radiating the sound pressure levels required of horn type loudspeakers would require a very large number of acoustic drivers. One frequently used element to radiate high amplitudes of high frequency acoustic energy is a diffraction slot. 
     In horn loudspeaker with a diffraction slot, the high frequency radiation is radiated by an acoustic driver and passes through an elongated diffraction slot, in some implementations via an intervening acoustic duct. The elongated slot may have, for example, a height of 34.3 cm (13.5 inches) and a width of, for example, 1.91 cm (0.75 inches), so the height is about 18 times the width. The diffraction slot diffracts the sound waves so that, in the horizontal direction, the sound waves behave as if they were radiated by an acoustic driver with a diameter of about the width of the diffraction slot, in this case 1.91 cm. A wavelength of 1.91 cm corresponds with a frequency of approximately 18 kHz. 
     To radiate high frequencies, horn type loudspeakers frequently use compression drivers and phase plugs. One suitable type of compression driver and phase plug arrangement is described in Wendell et. al. “Electroacoustic Transducing with Bridged Phase Plug”, U.S. patent application Ser. No. 12/490,463, incorporated herein by reference in its entirety. In one implementation, the acoustic driver has a dome size of 5.1 cm (2 inches) is enclosed in an enclosure with and outside diameter of, for example, 10.2 cm (four inches) and radiates into a phase plug with an exit diameter of 2.5 cm (1 inch). This combination of acoustic drivers, phase plugs, and diffraction slot dimensions permits the radiation of high amplitudes of high frequency acoustic energy with a practical number of acoustic drivers. 
     Horn type loudspeakers are often used in audio systems for large venues, such as large sports arenas or outdoor venues, where it is necessary to radiate acoustic energy over large distances to large areas. Frequently the total amount of acoustic energy that must be radiated is more than a single horn type loudspeaker can radiate. In addition, frequently the area to which sound is to be radiated is too large to practically radiate from a single horn loudspeaker. In such situations a plurality of horn type loudspeakers may be arrayed. One common arrangement is a “J” shaped configuration as shown in  FIG. 3 . The horn loudspeakers of an array may have a grille  130  covering the front of the horn for cosmetic purposes or to protect the horn from damage. In a “J” shaped arrangement, it is desirable for the individual horns to be arranged so that the diffraction slots are aligned. It is desirable to minimize the separation between the diffraction slots of adjacent horn loudspeakers in the array, or, in other words, to minimize the distance between the top end of the diffraction slot of one horn loudspeaker and the bottom end of the diffraction slot of the next horn loudspeaker above it in the array. 
     As best seen in  FIG. 5 , the top  24 A and bottom  24 B of the enclosure may be configured so that the height of the enclosure at the front  90  is greater that the height at the back  92  to permit the horns to be stacked at angle, as shown in  FIG. 4 . A typical angle φ (greatly exaggerated in  FIG. 5 ) is five degrees. For clarity, the acoustic drivers  12 , the acoustic ducts  16 , and the throat  13  are omitted in  FIG. 5  If the horns are stacked so that they are not angled (e.g. at the straight part of the “J”), the top of one horn may be non-coplanar with the bottom of the horn above, as shown in  FIG. 6 . If the plane of the bottom  24 B of the enclosure is non-parallel with the plane of the horn bottom  20 A, there is a gap  30  between the top edge of the diffraction slot  14 A of one horn loudspeaker and the bottom edge of the diffraction slot of the loudspeaker above in the array because the diffraction slot does not extend the entire height of the horn loudspeaker cabinet. Less commonly, the top and bottom are parallel. With this configuration, if the horns are stacked so that they are angled, as in  FIG. 7 , there is an undesirable gap  31  at the front of the array, between the top of one horn and the bottom of the horn above and an even wider gap between the bottom of one diffraction slot  14 A and the top of the diffraction slot  14 B of the horn loudspeaker underneath in the array. 
       FIG. 8A  shows another horn type loudspeaker arrangement in which the horn is configured so the acoustic paths from each acoustic driver to the combined diffraction slot are of equal length and so that centerlines  202  of the ducts are normal to an arc  204 . Arranging the ducts so that the centerlines  202  are in an arc permits the he top wall  20 A (of previous figures) and the bottom wall  20 B (of previous figures) of the horn to coincide with the top  24 A and bottom  24 B of the enclosure; for convenience, the top and bottom of the horn and the top and bottom of the enclosure will both be referred to by reference numbers  24 A and  24 B. When two horn loudspeakers according to  FIG. 8  are stacked, as in  FIG. 9 , the only significant gap in between the diffraction slots  14 A and  14 B is the thickness of the top wall of one horn loudspeaker and the bottom wall of the horn loudspeaker above. A typical thickness for the top wall and the bottom wall is 1.3 cm (0.5 inches) so that the gap is about 2.6 cm (1.0 inches). There may be other gaps equal to, for example, the thickness of the walls of the acoustic ducts  16  or of a manifold or of brackets or the like. The walls of acoustic ducts are typically about 3 mm (0.12 inches) thick, so the gaps are about 6 mm (0.24 inches). Gaps of less than an 1 cm generally do not affect the radiation pattern by a significant amount, so diffraction slot or diffraction slot section with gaps of less than 1 cm will be considered substantially continuous. To accommodate different horn loudspeaker array configurations, such as to form a “J” shaped horn array, with a continuous diffraction slot, it is desirable to have horn loudspeakers with a variety of vertical dispersion angles. For example, referring to  FIG. 10 , if it is desirable for the horns to be mounted at an angle α relative to each other, but the horns are only available with a vertical dispersion angle of φ, as in  FIG. 9 , an undesirable space between the horns and an undesirable gap in the diffraction slot will occur. Having horns with a variety of vertical dispersion angles permits the arrays to be formed without undesirable spaces between the horns and without undesirable gaps in the diffraction slot. For example, the angle φ of  FIG. 9  could be as small as five degrees or even zero degrees (so that the horn is rectangular when viewed from the side) or as large as thirty degrees or larger. The top and bottom may be flared at the same angle, so that the combined flare of the enclosure top  24 A and bottom  24 B is 2φ degrees. Since the top wall  20 A (of previous figures) and the bottom wall  20 B (of previous figures) of the horn are also the top  24 A and bottom  24 B of the enclosure, the combined flare of the top and bottom is the same as the vertical dispersion angle of the horn. Horns can be constructed so that any vertical dispersion can be provided, or the angle can be varied incrementally, for example in five or ten degree increments. 
       FIG. 8B  shows illustrates some features of the horn loudspeaker of  FIG. 8A . Lines  204 A- 204 D represent the ducts of four acoustic modules arranged to form a single continuous diffraction slot. Each of the ducts has a centerline  202 A- 202 D, respectively. The centerlines are normal to an arc that is a portion of circle  206 . The centerlines intersect at a point  208  at an angle μ. Line  210  from intersection point  208  to one end of the diffraction slot and line  212  form the intersection point  208  to the other end of the diffraction slot intersect at angle VD, which is the vertical dispersion angle of the horn loudspeaker. For clarity of illustration, an acoustic horn with four acoustic modules is shown, and the vertical dispersion angle VD is much larger than a typical dispersion angle. Lines  204 A- 204 D also represent the planes of the openings of the outlet ends of the acoustic ducts. The planes intersect at an angle P. Rearranging the ducts to change the vertical dispersion angle also causes the angle P to change. 
     A difficulty with horn loudspeakers according to  FIG. 8  with large vertical dispersion angles is that if the acoustic driver and acoustic duct assemblies are arranged so that the exits of the acoustic ducts are normal to an arc, the acoustic drivers and/or the acoustic ducts may overlap vertically. In that case, the acoustic ducts and the acoustic drivers may be displaced horizontally, as shown in  FIG. 11 . This allows the top and bottom walls  20 A and  20 B to coincide with the top and bottom walls  24 A and  24 B for larger vertical dispersion angles than are possible if the acoustic ducts and acoustic drivers are not displaced horizontally. 
     Using straight acoustic ducts extending in the Y-direction may cause the horn loudspeaker to have more depth than is desired. In that case, the acoustic ducts may be curved, as shown in  FIG. 12 . In some implementations, the curve may extend so far that one or more of the acoustic drivers may be partially or wholly forward of the throat  13 . In addition to decreasing the depth of the overall assembly, this has the advantage of moving the acoustic drivers to a location where there is more vertical room for them, allowing the use of drivers with larger outer diameters. 
     To provide more acoustic energy, more acoustic drivers can be added and the ducts merged at or before the horn throat. For example,  FIG. 13  shows a horn loudspeaker in which two acoustic drivers  12 A and  12 B are acoustically coupled to acoustic ducts  16 A and  16 B, respectively. The outlet end of acoustic ducts are merged at a position between the acoustic drivers and the throat  13 , so that combined acoustic energy radiated by acoustic drivers  12 A and  12 B is radiated into the horn through the diffraction slot in about the same vertical space that the acoustic energy from one acoustic driver is radiated into the horn through the diffraction slot in configurations such as  FIG. 1 . 
     The remainder of the figures show actual implementations of a horn loudspeaker incorporating elements of  FIGS. 1-13 . In the figures that follow, like reference numbers refer to corresponding elements in  FIGS. 1-13 . 
       FIG. 14  shows a first modular assembly  120 A including an acoustic driver  12 A and acoustic duct  16 A and a second modular assembly  120 B including an acoustic driver  12 B and acoustic duct  16 B. Modules  120 A and  120 B are asymmetric about the Y-Axis. The acoustic ducts are curved as in  FIG. 12 . The modular assembly  120 B is substantially identical to the modular assembly  120 A, but the second modular assembly  120 B is rotated 180 degrees about the Y-axis relative to the orientation of modular assembly  120 A. The opening at the outlet end of each of the ducts has a height of about 5.7 cm (2.25 inches) and a width of about 1.9 cm (0.75 inches). 
     The modular assemblies  120 A and  120 B are positioned so that the outlet ends are aligned in the direction of elongation and held in that position by attaching them to a mounting plate, or “keel”, most clearly seen in  FIGS. 16, 20, 21, and 23 . The combined dimension in the direction of elongation of the outlet end openings is about 2×5.7 cm=11.4 cm. Additional modular assemblies can be similarly aligned to form an acoustic assembly that can be acoustically coupled to the throat of a horn to form a horn loudspeaker. In one implementation, six modular assemblies are aligned in the manner shown in  FIG. 14 , with the outlet ends arranged as in  FIG. 8 . The combined dimension in the direction of elongation is then about 6×5.7 cm=34.2 cm while the width remains about 1.9 cm. The six modular assemblies can be mechanically and acoustically coupled to the throat of an acoustic horn to form a horn loudspeaker. The combined outlet end openings operate as a diffraction slot for the acoustic horn. The outlet ends of the acoustic ducts  120 A and  120 B may have vertical flanges  68 A and  68 B to facilitate mating with the horn wall and may have horizontal flanges  66 A and  66 B to facilitate mating with other acoustic ducts to form a diffraction slot, as will be described below. 
     A modular assembly such as modular assemblies  120 A and  120 B is advantageous because it enables providing horn loudspeakers with a wide range of horizontal and vertical dispersion angles with many of the parts being standard. The assemblies  120 A and  120 B including the acoustic driver  12 A and  12 B, respectively, and the acoustic duct  16 A and  16 B, respectively, are standard, as are the top wall  24 A and the bottom wall  24 B, and the bass modules  80 A and  80 B of  FIG. 24 , including bass enclosures  82 A and  82 B (of  FIG. 24 ) and woofer drivers  86  (of  FIG. 24 ). Only side walls  18 A and  18 B, keel  56  (most clearly seen in  FIGS. 16, 20, 21, and 23 ) and side bracket  57  (of  FIG. 24 ) vary from horn to horn. 
       FIG. 15  shows a modular assembly with mounting plates  112 A and  112 B, for two acoustic drivers (not shown in this view) in a configuration similar to the acoustic duct of  FIG. 13 . Modular assemblies such as shown in  FIG. 15  can be positioned in the same manner as modular assemblies  120 A and  120 B of  FIG. 14 . 
       FIGS. 16 and 17 , show a front view and a rear view, respectively, of an assembly of six acoustic drivers  12 A- 12 F and six acoustic ducts  16 A- 16 F. The outlets of the acoustic ducts  16 A- 16 F are aligned to form the diffraction slot  14 . The acoustic ducts are positioned by, and held in place by, the keel  56 . The keel  56  orients the outlets of the acoustic ducts normal to an arc and holds the acoustic modules in the desired position and orientation. Gaskets (not identified in this view) may be placed between the lower edge of one acoustic duct and the top edge of the acoustic duct below to prevent airflow leakage or airflow disturbances. 
       FIGS. 18A-18E  show side views of six modular assemblies  120 A- 120 F positioned to form an acoustic assembly  150  to mate with the throat of a horn to form a horn loudspeaker.  FIG. 18A  shows the orientation of the acoustic drivers and acoustic ducts assemblies with a vertical dispersion angle of five degrees; the curve of the arc is barely perceptible and there is moderate vertical overlap between the acoustic drivers  12 A- 12 F.  FIGS. 18B-18E  show the orientation of the acoustic driver and acoustic duct assemblies with vertical dispersion angles of 10 degrees, 20 degrees, 40 degrees, and 60 degrees, respectively. The curve of the arc becomes more pronounced and there is significant vertical overlap between the acoustic drivers  14 A- 14 F. 
       FIGS. 19A and 19B  show front oblique isometric views of an acoustic assembly similar to the acoustic assemblies of  FIGS. 18A-18E , with vertical dispersion angles of 5 degrees and 60 degrees, respectively.  FIGS. 19A and 19B  show how the openings at the outlet end of the acoustic ducts are aligned to form an arcuate diffraction slot  14 . In  FIG. 19A , the arc is barely perceptible, while in  FIG. 19B , the arc is more pronounced. 
       FIGS. 20 and 21  show a top view and an oblique back isometric view, respectively, of an acoustic driver and acoustic duct assembly according to  FIGS. 19A and 19B , with the horn side walls  18 A and  18 B. In this assembly, he horn side walls  18 A and  18 B are not planar and have some curvature, so a portion of the surface of the side walls is visible in the top view of  FIG. 19A . To show the side walls  18 A and  18 B, the top and bottom walls are omitted from this view. In the figures, the side walls  18 A and  18 B are shown as flaring symmetrically in the X-Y plane. In some implementations, the side walls may flare asymmetrically in the X-Y plane. Some of the acoustic drivers and some of the acoustic ducts are not visible in  FIG. 20 . 
       FIG. 22  shows an assembly including twelve acoustic drivers. In this view, six acoustic drivers  12 A- 12 F are visible, a seventh acoustic driver  12 G is partially obscured and the remaining five acoustic drivers are hidden in this view. In the implementation of  FIG. 22 , the twelve acoustic drivers are arranged in six pairs. Each pair of acoustic drivers are acoustically coupled to an acoustic duct  16 A- 16 F according to  FIGS. 13 and 15 . A portion of each of the acoustic drivers (for example acoustic driver  12 A) is forward of the diffraction slot which is positioned at the throat  13  of the horn. The horn of  FIG. 22  is formed according to U.S. patent application Ser. No. 12/557,885. A similar acoustic driver and acoustic duct arrangement can be implemented with a horn according to this specification. 
       FIG. 23  shows an oblique isometric front view of the assembly of  FIGS. 20 and 21  with the top and bottom enclosure walls  24 A and  24 B (which, as described above in the discussion of  FIG. 8  also are the top and bottom horn walls) angled to provide a 40 degree vertical dispersion angle. In  FIG. 23 , the curve of the front edge  70  of the keel  56  is visible. The top wall  24 A and the bottom wall  24 B may be mechanically fastened to the ends of keel  56 . The enclosure  22  has no sides or back, and the same parts can be used for the top wall  24 A and bottom wall  24 B regardless of the vertical dispersion angle. The horn side walls  18 A and  18 B may be held in place by mechanical fastening to the keel  56  and by inserting the top and bottom edges of the side walls into slots  74  in the top and bottom  24 A and  24 B. 
       FIG. 24  shows the assembly of  FIG. 23  with bass modules  80 A and  80 B. Bass modules  80 A and  80 B may includes a 25.4 cm (10 inch) nominal woofer driver  86  mounted in a bass enclosure  82  with a port  84 . The bass modules may be mechanically fastened to a side bracket  57  which may be mechanically fastened to the top wall  24 A and bottom wall  24 B. The assembly of  FIG. 23  enables providing horn loudspeakers with a wide range of vertical dispersion angle and horizontal dispersion angles with many parts that are standard for all vertical and horizontal dispersion angles and with a minimum of variation in the manufacturing process. For example, the top wall  24 A, the bottom wall  24 B, the acoustic drivers, acoustic ducts and the bass module may all be standard. Only the keel  56 , the side bracket  57 , and the horn side walls  18 A and  18 B need to be varied to vary the vertical dispersion angle. The horizontal dispersion angle can be varied by varying the orientation of the slots  74 . The assembly process for all horn loudspeakers, regardless of vertical or horizontal dispersion angle, is substantially identical. 
     Numerous uses of and departures from the specific apparatus and techniques disclosed herein may be made without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features disclosed herein and limited only by the spirit and scope of the appended claims.