Abstract:
An improved throat ( 2 ) for transmitting acoustic energy from a source driver unit ( 7 ) to a feeder section ( 3 ) of a directivity controlling acoustic horn is disclosed. The throat ( 2 ) comprises: a circular throat entrance ( 2   i ) connectable to the source driver unit ( 7 ); a rectangular throat exit ( 2   e ) connectable to or integral with the feeder section ( 3 ); and a circular cross-section to rectangular cross-section transition portion ( 2   a ) extending between the throat entrance ( 2   i ) and the throat exit ( 2   e ). The throat ( 2 ) is shaped such that its profiles initially diverge from an axis longitudinal to the throat ( 14 ) at the same angle in a direction from the throat entrance ( 2   i ) towards the throat exit ( 2   e ). Such a throat, when fitted in an appropriate acoustic horn with a source driver unit having a taper matching the aforesaid profile angles, provides a smooth transition for sound waves propagating out from the source driver unit into the horn.

Description:
FIELD OF THE INVENTION 
   The field of the invention relates to acoustic horns, and more particularly to acoustic horns providing substantially uniform polar frequency-response plots in both the horizontal and vertical directions. 
   BACKGROUND TO THE INVENTION 
   An acoustic horn is a structure which utilises outwardly flaring rigid walls to provide an expanding passage for acoustic energy between a throat entrance and a mouth exit. The acoustic horn is stimulated by a source driver unit which produces acoustic energy, and the acoustic horn then modifies and controls the acoustic energy. 
   The audio industry has spent many decades on the design of acoustic horns with defined areas of coverage. For instance, 90° in a horizontal plane by 40° in a vertical plane, or 60° by 40°, and so on. Generically they are called constant directivity horns. 
   A constant directivity acoustic horn generally comprises a throat entrance and a mouth exit joined by continuous rigid walls. A throat section extends away from the throat entrance and then extends to a feeder section which is rectangular in transverse cross-sectional shape. Acoustical energy is coupled thereto from a source driver unit connected to the throat entrance. The feeder section has an expanding transverse area formed by a first pair of walls which diverge outwardly from each other, and a second pair of walls which are substantially parallel and joined to the first pair. 
   The mouth exit of the horn has a rectangular configuration and is formed by a bell section having walls which diverge outwardly from the end of the feeder section, there being a first pair of diverging walls, and a second pair of diverging walls which join with the first pair of walls of the bell section along the edges to form an integral unit. The walls of the bell section may be flared outwardly an additional amount at a transverse plane immediately adjacent to the mouth to provide improved control of the radiation of acoustic energy. 
   In general the divergence angle between the first pair of walls and between the second pair of walls of the bell section determines the dispersion angle of the acoustical energy. A feature of this geometry is that the side profile view and top profile view angles and the dimensions of the mouth can be varied independently in order to obtain specified outcomes. 
   Many shapes of constant directivity horns have been evolved over the years to try to achieve a more uniform coverage. Initial attempts were by Olsen with multi-cellular horns, Klipsch (U.S. Pat. No. 2,537,141) with radial sectorial, Keele (U.S. Pat. No. 4,071,112) with the concept of outer flanges, Henricksen et al (U.S. Pat. No. 4,187,926) with a design “in reverse” (Manta Ray), Keele again (U.S. Pat. No. 4,308,932) with profiles specified by a formula, Gunness (U.S. Pat. No. 4,685,532) with throat vanes (pseudo horns). Most of these shapes (e.g. the Manta Ray) which have evolved to meet the need for uniform coverage (directivity control) have other disadvantages, for example, an irregular on-axis frequency response. 
   It is an object of the present invention to provide an improved constant directivity horn and/or horn component. 
   It is a further object of the present invention to provide a horn and/or horn component that provides improved directivity control in the high frequency ranges. 
   SUMMARY OF INVENTION 
   According to the invention there is provided a throat for transmitting acoustic energy from a source driver unit to a feeder section of a directivity controlling acoustic horn, the throat comprising: 
   circular throat entrance connectable to the source driver unit, the throat entrance having a diameter; 
   a rectangular throat exit connectable to or integral with the feeder section, the throat exit defined by a pair of parallel long sides and a pair of parallel short sides, the short sides having a length less than or equal to the diameter of the throat entrance; and 
   circular cross-section to rectangular cross-section transition portion extending between the throat entrance and the throat exit, the transition portion having an internal surface, 
   wherein a pair of opposite profiles of the internal surface of the throat, lying within a first plane that bisects the throat entrance and perpendicularly bisects the long side of the throat exit, initially diverge in a direction from the throat entrance towards the throat exit. 
   Preferably each said profile initially diverges at substantially the same angle with respect to an axis longitudinal to the throat. 
   Preferably said profiles of the throat converge to a neck having a width less that the diameter of the entrance to the throat, thereby improving the dispersion of high frequency acoustic energy. 
   Preferably the throat is shaped such that its profiles, through substantially all cross-sections longitudinal to the throat, initially diverge from the longitudinal axis of the throat in a direction from the throat entrance towards the throat exit. 
   Preferably all of the initial angles of divergence match. 
   According to a second aspect of the invention there is provided a throat for transmitting acoustic energy from a source driver unit to a feeder section of a directivity controlling acoustic horn, the throat comprising: 
   a circular throat entrance connectable to the source driver unit, the throat entrance having a diameter; 
   a rectangular throat exit connectable to or integral with the feeder section, the throat exit defined by a pair of parallel long sides and a pair of parallel short sides; and 
   a circular cross-section to rectangular cross-section transition portion extending between the throat entrance and the throat exit, the transition portion having an internal surface, 
   wherein the throat is shaped such that its profiles, through substantially all cross-sections longitudinal to the throat, initially diverge from an axis longitudinal to the throat at the same angle in a direction from the throat entrance towards the throat exit. 
   According to a third aspect of the invention there is provided a directivity controlling acoustic horn assembly comprising: 
   a source driver unit having a divergent frusto-conical portion terminating in a circular exit for transmission of acoustic energy; 
   a throat having: a circular entrance; a rectangular exit; and a circular cross-section to rectangular cross-section transition portion extending between the throat entrance and the throat exit, the circular entrance matching the circular exit of the source driver and the rectangular exit having a height and a width; 
   a feeder section having a first end and a second end, the first end connected to the exit of the throat; and 
   a bell section having an entrance and terminating in an open mouth, the entrance of the bell section connected to or integral with the second end of the feeder section, 
   wherein opposite profiles of the throat, lying within a first plane that bisects the throat entrance and perpendicularly bisects the long side of the throat exit, substantially match the angle of the frusto-conical portion at the exit to the source driver unit thereby providing a smooth transition for sound waves propagating from the source driver unit into the throat. 
   Preferably the height of the rectangular throat exit is less than the diameter of the throat entrance. 
   Preferably said profiles of the throat converge to a neck having a height less that the diameter of the entrance to the throat, thereby improving the dispersion of high frequency acoustic energy. 
   Preferably the throat is shaped such that its profiles, through substantially all cross-sections longitudinal to the throat, substantially match the angle of the frusto-conical portion at the exit to the source driver unit thereby providing a smooth transition for sound waves propagating from the source driver unit into the throat. 
   Specific embodiments of the invention will now be described in some further detail with reference to and as illustrated in the accompanying figures. These embodiments are illustrative, and are not meant to be restrictive of the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front view of a generic constant directivity acoustic horn. 
       FIG. 2   a  is a vertical profile cross-sectional view of the acoustic horn in  FIG. 1 . 
       FIG. 2   b  is a horizontal profile cross-sectional view of the acoustic horn of  FIG. 1 . 
       FIG. 3  is a cross-sectional view of a typical source driver unit (a “compression driver”). 
       FIG. 4   a  is a vertical profile cross-sectional view of a constant directivity “angular” acoustic horn. 
       FIG. 4   b  is a horizontal profile cross-sectional view of the “angular horn” of  FIG. 4   a.    
       FIG. 5   a  is a vertical profile cross-sectional view of a constant directivity “curvy” acoustic horn. 
       FIG. 5   b  is a horizontal profile cross-sectional view of the “curvy” acoustic horn of  FIG. 5   a.    
       FIG. 6   a  is a vertical profile cross-sectional view of a throat of a constant directivity acoustic horn with the source driver unit of  FIG. 4  attached. 
       FIG. 6   b  is a horizontal profile cross-sectional view of the throat and source driver shown in  FIG. 6   a.    
       FIG. 6  is a perspective view of the throat shown in  FIGS. 6   a  and  6   b.    
       FIG. 7  is a perspective view of a throat according to a first embodiment of the invention. 
       FIG. 7   a  is vertical profile cross-sectional view of the throat of  FIG. 7 . 
       FIG. 7   b  is a horizontal profile cross-sectional view of the throat of  FIGS. 7 and 7   a.    
       FIG. 8   a  is a vertical profile cross-sectional view of a throat according to a second embodiment of the invention. 
       FIG. 8   b  is a horizontal profile cross-sectional view of throat of  FIG. 8   a.    
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 ,  2   a  and  2   b , a generic prior art constant directivity acoustic horn is shown. The acoustic horn comprises a throat  2  having a circular entrance  2   i  and a rectangular exit  2   e , a feeder section  3  having an expanding rectangular cross-section ending at a plane indicated by the line  4  and a bell section  5  that terminates in a open mouth  6 . The divergent profile of the first pair of walls  3   a , which is determined by the specified beam angle is clearly shown in  FIG. 2   a.  The second pair of walls  5   b  of the bell section  5  are shown in  FIG. 2   b.    
   In  FIG. 1  front view, the throat  2 , the mouth  6 , and the location of the feeder section wall  3   b  is shown. 
   A typical source driver unit  7  is shown as  FIG. 3 . It is known as a compression driver, and is an electromagnetic converter of electrical energy to acoustical energy. Acoustical energy is generated by movement of the diaphragm  7   c , which is moved by a coil of wire  7   e  immersed in the magnetic field of the magnet structure  7   m . The diaphragm assembly is mounted in a circular frame  7   f . The acoustical energy (sound) radiated from the concave side of the diaphragm is guided by a series of concentric tapered cylinders called phase plugs  7   d  into the throat  7   t  of the unit. The driver throat  7   t  is frusto-conical in shape and has an exit angle shown as  7   i . Acoustical energy is also radiated from the convex side of the diaphragm  7   c , but is confined by the cover  7   a . The surface  7   h  is the mounting surface which attaches to a flange on the horn. 
   Further prior art constant directivity acoustic horns are shown in  FIGS. 4   a  to  5   b . In general they have the same features referred to already. The source driver unit is attached to the flange  1 , and passes acoustic energy into the throat entrance  2   i.  Note that throat entrance  2   i  is usually round in transverse shape to provide a better match to the circular shape of the source driver unit. The acoustic energy then passes through a short section of transition  2   a  from round to rectangular and through the feeder section  3  into the bell section  5 . The acoustic energy is guided in the side view plane by profile  3   a  and  5   a  and in the top view plane by profiles  5   a  and  5   b , depending on whether the acoustic horn has an “angular” or “curvy” appearance. 
   Enlarged views of the source driver  7  and throat  2  are shown in  FIGS. 6   a  and  6   b.    
   Referring to the vertical profile cross-sectional view of  FIG. 6   a , the source driver unit  7  is attached to the flange  1 , and passes acoustic energy into the throat entrance  2   i  of the acoustic horn and through the round to rectangular transition region  2   a . The feeder section  3  is shown, as is the profile of the first set of walls or wall portions  3   a . The exit taper angle  7   i  on the throat of the source driver unit  7  shows a discontinuity at  10   a  compared to the profile of the first set of walls  3   a.    
   Referring to the horizontal profile cross-sectional view of  FIG. 6   b , it can be seen that the exit taper angle  7   i  on the throat of the source driver unit  7  also shows a discontinuity at  10   d  compared to the profile of the second set of walls or wall portions  3   b.    
   The discontinuities at  10   a  and  10   d  referred to above create disturbances in the sound waves as they pass through the throat entrance into the throat at shorter wavelengths, in particular where the wavelengths are less than the diameter of the throat entrance. In the horizontal profile, illustrated in  FIG. 6   b , the discontinuity is particularly apparent with tangent lines  15   d  and  15   d ′ converging in a direction towards the throat exit  2   e . While this convergence is convenient given that generally the diameter of the throat entrance  2   i  is greater than the length of the short sides of the rectangular throat exit  2   e , the inventor has observed that it creates acoustic disturbances. The conveyance towards the throat exit  2   e  is also illustrated in  FIG. 6 . 
   Referring now to  FIG. 7 , a first embodiment of the invention is shown. It can be seen that a pair of opposite profiles of the internal surface of the throat  2 , lying within a plane indicated in dotted outline and marked  7   b — 7   b — 7   b — 7   b , initially diverge in a direction from the throat entrance towards the throat exit. This divergence, clearly illustrated by tangent lines  15   a  and  15   a ′ in  FIG. 7   b  is in marked contrast to the convergence shown by tangent lines  15   d  and  15   d ′ in  FIG. 6   b.    
     FIGS. 7   a  and  7   b  show cross-sectional views of the first embodiment of the invention at planes  7   a — 7   a — 7   a — 7   a  and  7   b — 7   b — 7   b — 7   b  (shown in  FIG. 7 ). Referring to the vertical profile cross-sectional view of  FIG. 7   a , the source driver unit  7  is attached to the flange  1  and passes acoustic energy into the throat entrance  2   i  and through the round to rectangular transition region  2   a  into the feeder region  3 . The profile of the first pair of walls or wall portions  3   a  has an angle of commencement  11   a  which matches the exit angle  7   i  of the driver source unit. The profile smoothly changes through  11   b  to that desired for the beam angle  3   a . The acoustic energy then passes into the feeder region  3 , where the second pair of walls are substantially parallel. 
   Referring to the horizontal profile cross-sectional view of  FIG. 7   b , it can be seen that the profile of the second pair of walls or wall portions  3   b  also has an angle of commencement  11   d  which matches the exit angle  7   i  of the source driver unit. The profile then smoothly changes through  11   e  and  11   f  to that of  3   b.    
   A feature of this change is that the appropriate transverse area is maintained while the shape of its transverse section smoothly changes from circular to elliptical to rectangular. That is, the cross-sectional area growth rate down the throat  2  towards the feeder section  3  can be made according to a desired formula. The acoustic energy then passes into the feeder region  3 , where the second pair of walls is substantially parallel and the first pair of walls diverge. 
     FIGS. 7   a  and  7   b  show opposite profiles in vertical and horizontal profiles respectively. In this preferred embodiment of the invention, the throat is shaped such that its profiles through substantially all cross sections longitudinal to the throat (not just the vertical and horizontal cross-sections) substantially match the angle  7   i  of the frusto-cronical portion at the exit to the source driver unit  7  thereby providing a smooth transition for sound waves propagating from the source driver unit  7  into the throat  2 . 
   A second embodiment of the invention is shown in  FIGS. 8   a  and  8   b . Referring to the vertical profile cross-sectional view of  FIG. 8   a  the source driver unit  7  is again attached to the flange  1  and passes acoustic energy into the throat entrance  2   i  and through the round to rectangular transition region  2   a  into the feeder region  3 . Again, the profile of the first pair of walls or wall portions  3   a  has an angle of commencement  11   a  which matches the exit angle  7   i  of the driver source unit. The profile smoothly changes through  11   b  and  11   c  to that desired  3   a  for the beam angle. The acoustic energy then passes into the feeder region  3 , where the second pair of walls are substantially parallel. 
   With this embodiment of the invention, the profile converges/narrows to a neck having a height/width  11   c , a length smaller than the exit size of the source driver unit  7 , giving a better dispersion of high frequency acoustic energy into the acoustic horn. 
   Referring to the horizontal profile cross-sectional view of  FIG. 8   b , it can be seen that the profile of the second pair of walls or wall portions  3   b  also has an angle of commencement  11   d  which matches the exit angle  7   i  of the source driver unit. The profile then smoothly changes through  11   e  and  11   f  to that of  3   b.    
   Again, a feature of this change is that the appropriate transverse area is maintained while the shape of its transverse section smoothly changes from circular to elliptical to rectangular. That is, the cross-sectional area growth rate down the throat  2  towards the feeder section  3  can be made according to a desired formula. The acoustic energy then passes into the feeder region  3 , where the second pair of walls are substantially parallel and the first pair of walls diverge. 
   With the embodiments described above, directivity control is improved particularly in the high frequency ranges where wavelengths are less than the diameter of the throat entrance. 
   While the present invention has been described in terms of preferred embodiments in order to facilitate better understanding of the invention, it should be appreciated that various modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications within its scope.