AU2003246307B2 - Constant directivity acoustic horn - Google Patents

Description

Regulation 3.2 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Name of Applicant: Krix Loudspeakers Pty Ltd Actual Inventor: David John Murphy Address for Service: C/- MADDERNS, 1st Floor, 64 Hindmarsh Square, Adelaide, South Australia, Australia Invention title: CONSTANT DIRECTIVITY ACOUSTIC HORN Details of Associated Provisional Application No: 2002951421 dated 17th September 2002 The following statement is a full description of this invention, including the best method of performing it known to us.

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. 5 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 10 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 15 vertical plane, or 600 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 20 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 25 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 30 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 2 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 5 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. 10 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 (US 2,537,141) with radial sectorial, Keele (US 4,071,112) with the concept of outer flanges, Henricksen et al (US 4,187,926) with a design "in reverse" (Manta Ray), Keele again (US 4,308,932) with profiles specified by a formula, 15 Gunness (US 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. 20 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. 25 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: 30 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 3 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 a circular cross-section to rectangular cross-section transition portion 5 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 10 towards the throat exit. Preferably each said profile initially diverges at substantially the same angle with respect to an axis longitudinal to the throat. 15 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 20 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. 25 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; 30 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 4 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 5 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 10 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 15 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 20 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 25 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. 30 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 5 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 5 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 10 embodiments are illustrative, and are not meant to be restrictive of the scope of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a front view of a generic constant directivity acoustic horn. 15 Figure 2a is a vertical profile cross-sectional view of the acoustic horn in Figure 1. Figure 2b is a horizontal profile cross-sectional view of the acoustic horn of Figure 1. Figure 3 is a cross-sectional view of a typical source driver unit (a 20 "compression driver"). Figure 4a is a vertical profile cross-sectional view of a constant directivity "angular" acoustic horn. Figure 4b is a horizontal profile cross-sectional view of the "angular horn" of Figure 4a. 25 Figure 5a is a vertical profile cross-sectional view of a constant directivity "curvy" acoustic horn. Figure 5b is a horizontal profile cross-sectional view of the "curvy" acoustic horn of Figure 5a. Figure 6a is a vertical profile cross-sectional view of a throat of a constant 30 directivity acoustic horn with the source driver unit of Figure 4 attached. Figure 6b is a horizontal profile cross-sectional view of the throat and source driver shown in Figure 6a. 6 Figure 6 is a perspective view of the throat shown in Figures 6a and 6b. Figure 7 is a perspective view of a throat according to a first embodiment of the invention. Figure 7a is vertical profile cross-sectional view of the throat of Figure 7. 5 Figure 7b is a horizontal profile cross-sectional view of the throat of Figures 7 and 7a. Figure 8a is a vertical profile cross-sectional view of a throat according to a second embodiment of the invention. Figure 8b is a horizontal profile cross-sectional view of throat of Figure 8a. 10 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figures 1, 2a and 2b, a generic prior art constant directivity acoustic horn is shown. The acoustic horn comprises a throat 2 having a circular entrance 2i and a rectangular exit 2e, a feeder section 3 having an expanding 15 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 3a, which is determined by the specified beam angle is clearly shown in Figure 2a. The second pair of walls 5b of the bell section 5 are shown in Figure 2b. 20 In Figure 1 front view, the throat 2, the mouth 6, and the location of the feeder section wall 3b is shown. A typical source driver unit 7 is shown as Figure 3. It is known as a compression driver, and is an electromagnetic converter of electrical energy to 25 acoustical energy. Acoustical energy is generated by movement of the diaphragm 7c, which is moved by a coil of wire 7e immersed in the magnetic field of the magnet structure 7m. The diaphragm assembly is mounted in a circular frame 7f. The acoustical energy (sound) radiated from the concave side of the diaphragm is guided by a series of concentric tapered cylinders called phase plugs 7d into the throat 7t of 30 the unit. The driver throat 7t is frusto-conical in shape and has an exit angle shown as 7i. Acoustical energy is also radiated from the convex side of the diaphragm 7c, but is 7 confined by the cover 7a. The surface 7h is the mounting surface which attaches to a flange on the horn. Further prior art constant directivity acoustic horns are shown in Figures 4a to 5 5b. 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 2i. Note that throat entrance 2i 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 2a from round to rectangular and through the 10 feeder section 3 into the bell section 5. The acoustic energy is guided in the side view plane by profile 3a and 5a and in the top view plane by profiles 5a and 5b, 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 Figures 6a 15 and 6b. Referring to the vertical profile cross-sectional view of Figure 6a, the source driver unit 7 is attached to the flange 1, and passes acoustic energy into the throat entrance 2i of the acoustic horn and through the round to rectangular transition 20 region 2a. The feeder section 3 is shown, as is the profile of the first set of walls or wall portions 3a. The exit taper angle 7i on the throat of the source driver unit 7 shows a discontinuity at 10a compared to the profile of the first set of walls 3a. Referring to the horizontal profile cross-sectional view of Figure 6b, it can be 25 seen that the exit taper angle 7i on the throat of the source driver unit 7 also shows a discontinuity at 10d compared to the profile of the second set of walls or wall portions 3b. The discontinuities at 10a and 10d referred to above create disturbances in the 30 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 Figure 6b, the discontinuity is 8 particularly apparent with tangent lines 15d and 15d' converging in a direction towards the throat exit 2e. While this convergence is convenient given that generally the diameter of the throat entrance 2i is greater than the length of the short sides of the rectangular throat exit 2e, the inventor has observed that it creates acoustic 5 disturbances. The conveyance towards the throat exit 2e is also illustrated in Figure 6. Referring now to Figure 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 7b - 7b - 7b - 7b, initially 10 diverge in a direction from the throat entrance towards the throat exit. This divergence, clearly illustrated by tangent lines 15a and 15a' in figure 7b is in marked contrast to the convergence shown by tangent lines 15d and 15d' in Figure 6b. Figures 7a and 7b show cross-sectional views of the first embodiment of the 15 invention at planes 7a - 7a - 7a - 7a and 7b - 7b - 7b -7b (shown in Figure 7). Referring to the vertical profile cross-sectional view of Figure 7a, the source driver unit 7 is attached to the flange 1 and passes acoustic energy into the throat entrance 2i and through the round to rectangular transition region 2a into the feeder region 3. The profile of the first pair of walls or wall portions 3a has an angle of 20 commencement 11a which matches the exit angle 7i of the driver source unit. The profile smoothly changes through 11b to that desired for the beam angle 3a. The acoustic energy then passes into the feeder region 3, where the second pair of walls are substantially parallel. 25 Referring to the horizontal profile cross-sectional view of Figure 7b, it can be seen that the profile of the second pair of walls or wall portions 3b also has an angle of commencement 11d which matches the exit angle 7i of the source driver unit. The profile then smoothly changes through 11e and 11f to that of 3b. 30 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 9 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. 5 Figures 7a and 7b 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 7i of the frusto-cronical portion at the exit to the source driver unit 7 thereby providing a 10 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 Figures 8a and 8b. Referring to the vertical profile cross-sectional view of Figure 8a the source driver 15 unit 7 is again attached to the flange 1 and passes acoustic energy into the throat entrance 2i and through the round to rectangular transition region 2a into the feeder region 3. Again, the profile of the first pair of walls or wall portions 3a has an angle of commencement 11a which matches the exit angle 7i of the driver source unit. The profile smoothly changes through 11b and 11c to that desired 3a for the beam angle. 20 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 11c, a length smaller than the exit size of the source 25 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 Figure 8b, it can be seen that the profile of the second pair of walls or wall portions 3b also has an angle 30 of commencement 1ld which matches the exit angle 7i of the source driver unit. The profile then smoothly changes through 11e and 11f to that of 3b. 10 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. 5 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 10 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 15 principles of the invention. Therefore, the invention should be understood to include all such modifications within its scope. 11

Claims (14)

1. A throat for transmitting acoustic energy from a source driver unit to a feeder section of a directivity controlling acoustic horn, the throat comprising: 5 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, the short sides having a length less than the diameter of the throat entrance; and 10 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 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. 15

2. The throat as claimed in claim I wherein each said profile initially diverges at substantially the same angle with respect to an axis longitudinal to the throat.

3. The throat as claimed in claim 2 wherein said profiles of the throat converge to a neck having 20 a width less that the diameter of the entrance to the throat, thereby improving the dispersion of high frequency acoustic energy.

4. The throat as claimed in claim 3 wherein the throat is shaped such that its profiles, through substantially all cross-sections longitudinal to the throat, initially diverge from the longitudinal axis of 25 the throat in a direction from the throat entrance towards the throat exit.

5. The throat as claimed in claim 3 wherein 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 at the same angle in a direction from the throat entrance towards the throat exit. 30

6. 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; a rectangular throat exit connectable to or integral with the feeder section; and 35 a circular cross-section to rectangular cross-section transition portion extending between the throat entrance and the throat exit, wherein the throat is shaped such that its profiles, through substantially all cross-sections 12 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.

7. A directivity controlling acoustic horn assembly comprising: 5 a source driver unit having a divergent frusto-conical portion terminating in a circular exit for transmission of acoustic energy the circular exit having a first diameter; a throat having: a circular entrance having a second diameter substantially equal to the first diameter ; 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 aligned with and 10 abutting 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, 15 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 of the source driver unit thereby providing a smooth transition for sound waves propagating from the source driver unit into the throat. 20

8. The horn as claimed in claim 7 wherein the height of the rectangular throat exit is less than the diameter of the throat entrance.

9. The hom as claimed in claim 8 wherein 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 25 frequency acoustic energy.

10. The horn as claimed in claim 9 wherein 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 of the source driver unit thereby providing a smooth transition for sound 30 waves propagating from the source driver unit into the throat.

I. A throat for transmitting acoustic energy from a source driver unit to a feeder section of a directivity controlling acoustic horn, the throat substantially as hereinbefore described with reference to and as illustrated in Figures 7, 7a and 7b. 35

12. A throat for transmitting acoustic energy from a source driver unit to a feeder section of a directivity controlling acoustic horn, the throat substantially as hereinbefore described with reference to and as illustrated in Figures 8a and 8b. 5

13. A directivity controlling acoustic assembly as hereinbefore described with reference to and as illustrated in Figures 7, 7a and 7b.

14. A directivity controlling acoustic assembly as hereinbefore described with reference to and as illustrated in Figures 8a and 8b. 10