Acoustic Horn

Abstract

An acoustic horn having a plurality of spaced longitudinal ribs extending into the horn for reducing the cross-sectional area of the throat thereof to provide a broadband impedance transformation with minimum phase cancellation between an acoustic transducer element and an acoustic transmission medium.

Description

BACKGROUND

1. Field of Invention

This invention relates generally to acoustic transducers, and more particularly to acoustic transducers employing a horn to provide an impedance transformation between a transducer element and an acoustic transmission medium.

2. Prior Art

Compression horn type transducers of the prior art comprise a vibrating diaphragm and a tapered horn to provide an impedance transformation between the diaphragm and the air or other acoustic transmission medium. A compression chamber is generally employed between the diaphragm and horn to transfer acoustic energy therebetween. To minimize the distortion produced by the transducer, the area of the diaphragm must be relatively large, and is usually larger than the cross-sectional area of the throat of the horn. This size difference causes acoustic waves from the center of the diaphragm to reach the throat of the horn before vibrations from the periphery of the diaphragm which must travel across the compression chamber before reaching the horn. This causes the acoustic vibrations from the various portions of the diaphragm to have phase differences between them. These phase differences can result in phase cancellations at particular frequencies, thereby limiting the high frequency response of the compression horn transducer.

Several techniques for reducing the phase errors caused by the various path lengths are known. One such system employs a plurality of horns, each horn receiving energy from a different portion of the diaphragm. Another such system uses a phase correcting plug having a plurality of holes, radial slots or annular rings therein for equalizing the path length between the various portions of the diaphragm and the horn.

Whereas these techniques provide a way to reduce the phase cancellation effects in a compression horn, multiple horn structures are relatively complex, and phase cancelling plugs must be built to precision tolerances. Both techniques result in increased complexity and transducer manufacturing cost.

SUMMARY

It is an object of the present invention to provide a horn for an acoustic transducer which minimizes phase cancellations.

It is another object of this invention to provide a horn for an acoustic transducer that does not require a phase correction plug.

Yet another object of this invention is to provide a broadband low distortion acoustic transducer.

It is a further object of this invention to provide a high quality acoustic transducer that is simple and relatively inexpensive to manufacture.

A still further object of this invention is to provide a horn having a substantially constant path length between various areas of the diaphragm and mouth of the horn that can be fabricated as a single assembly.

In accordance with a preferred embodiment of the invention, a horn having longitudinal ribs therein for reducing the cross-sectional area of the horn in the region of the throat thereof is molded from plastic or formed from any suitable material. The ribs reduce the cross-sectional area of the horn in the throat area to provide the horn effect, and the resultant spaces between the ribs provide passages wherein all points on the driver diaphragm are approximately the same distance away from the throat, thereby substantially minimizing phase cancellation. In addition, the sound waves from the diaphgram travel in a straight line to the mouth of the horn, thereby eliminating the losses caused by the contoured path of prior art phase correcting devices. Finally, the horn can be molded in a single assembly very inexpensively using current plastic molding techniques.

DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a cross-sectional view of a compression horn according to the prior art, and is included to illustrate the problem of phase cancellation in compression horns;

FIG. 2 is a cross-sectional view of a similar horn utilizing a phase correction plug according to the prior art;

FIG. 3 is a cross-sectional view of a horn according to the prior art utilizing another type of phase correcting plug;

FIG. 4 is a cross-sectional view of a horn according to the invention which provides a substantially constant path length between the diaphragm of the transducer and the mouth of the horn;

FIGS. 4a, 4b and 4c are cross-sectional views taken along line A--A, B--B and C--C of FIG. 4, respectively;

FIG. 5 is an end view of one embodiment of a horn according to the invention having a center plug to provide for more accurate control of the cross-sectional area of the passageway between the longitudinal ribs;

FIG. 6 is a cross-sectional view of the horn of FIG. 5 taken along the section line D--D of FIG. 5;

FIG. 7 is a cross-sectional view of the horn shown in FIGS. 5 and 6 taken along the section line E--E of FIG. 6; and

FIG. 8 is an end view of the throat end of the horn shown in FIG. 5.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a diagram of a basic compression horn according to the prior art. A driver 10, which may be a magnetic or piezoelectric transducer element, or other acoustic translating means, is coupled to a cone or diaphragm 14 by means of a suitable coupling element 12. The cone 14 forms one wall of a compression chamber 16 to which a horn 18 having a mouth 20 and a throat 22 is attached.

In operation, the driver 10 applies acoustic vibrations to the cone 14 via the coupling means 12. The cone 14 serves as an impedance matching device between the throat 22 of the horn and the driver 10. Some degree of impedance matching is necessary, particularly for piezoelectric drivers which provide high force and low displacement, in order to achieve proper coupling of energy to the throat of the horn. In order to minimize distortion, the cone should be made as large as possible, and should be stiff enough not to break up into resonant modes within the operating frequency range. The cone 14, which moves in response to the force applied thereto by driver 10, causes the air in the compression chamber 16 to be compressed in accordance with the force applied by the driver 10, and the pressure variations caused thereby are applied to the throat of the horn 22. The pressure wave in the throat 22 of the horn 18 is a high pressure, low volume wave which is transformed by the horn 18 to a low pressure, high volume wave which appears at the mouth 20 of the horn. In this manner the horn provides impedance transformation between the compression chamber and air or transmission medium.

Horns of the above described type work relatively well at low frequencies, but at higher frequencies, the distance D between the throat 22 of the horn and the edge of the cone 14 causes the pressure waves generated by different areas of the cone 18 to reach the throat 22 of the horn 18 at different times with respect to each other, thereby causing phase cancellations. These phase cancellations manifest themselves as periodic minima in the frequency response curve of the transducer and seriously limit the high frequency operation of the horn.

Referring to FIG. 2, there is shown a horn similar to the horn of FIG. 1 having a phase correction plug therein to reduce the phase cancellation problem. The transducer of FIG. 2 is similar to the transducer of FIG. 1 and has a driver 30, a coupling means 32, a cone or diaphragm 34, a compression chamber 36 and a horn 38 having a throat 42 and a mouth 40. The aforementioned components of the transducer are similar to analogous components of the transducer of FIG. 1. In the transducer of FIG. 2, a phase correction plug 44 is interposed between the compression chamber 36 and the throat 42 of horn 38. The phase correction plug includes a series of passages 46 between the compression chamber 36 and the throat 42. The passages are designed to provide a substantially constant path length between the compression chamber 36 and throat 42 to cause the pressure waves from various portions of the diaphragm 34 to arrive at the throat 42 at approximately the same time, thereby minimizing phase cancellations. Although this technique provides some phase cancellation correction, the correction is not complete because the length of the passages are not identical, and the plug may have an adverse effect on other performance characteristics of the horn. Furthermore, the plug must be made to close tolerance specifications and the distance between the cone and the plug is critical.

Referring to FIG. 3, there is shown a cross-sectional view of a horn utilizing another type of phase correction device. In this embodiment, a driver 50 drives a cone 54 which in turn applies energy to a horn 58 through an annular port 62. A phase correcting plug 64 is mounted within the horn 58 and forms a second horn 68. In this type of horn, the larger horn 58 is driven primarily by the edge of cone 54 and the smaller horn 68 receives its energy primarily from the center of the cone 54 through passageways 66. The distances between the throat of each horn and its respective portion of the cone 54 are approximately the same and phase cancellations are minimized. However, this scheme suffers from the same cost and complexity limitations as the scheme shown in FIG. 2.

Referring to FIG. 4, there is shown a cross-sectional view of a transducer employing a horn 78 having a plurality of ribs 84 for reducing the cross-sectional area of the horn, particularly in the throat area thereof, according to the invention. In this embodiment, a piezoelectric driver 70, which is supported by a support member 71, drives a cone or diaphragm 74 which couples acoustic energy to a compression chamber 76. The diaphragm 74 is attached to a frame member for support and forms one wall of the compression chamber 76. The horn 78, which also has a throat 82 and a mouth 80, is coupled to receive acoustic energy from the compression chamber 76. The throat end of the horn 78 has a smaller diameter than the diameter of the frame in this embodiment, however it should be noted that the throat end of the horn and the frame may have the same diameter and still fall within the scope of the invention.

In order for a horn to perform the function of providing an impedance transformation between a transducer element and air or other transmission medium, it is necessary for the cross-sectional area of the horn to vary along the longitudinal dimension of the horn. Generally the throat of a horn has a smaller area than the mouth. The conventional techniques of reducing the area at the throat by reducing the throat diameter have caused the problems described previously in this specification.

In the horn 78 of the instant invention, the area at the throat is reduced by the plurality of ribs 84 mounted within a tubular member which forms the horn. In this embodiment, the ribs have a nonuniform cross-sectional area and are positioned with the ends having the larger cross-sectional area toward the throat of the horn. The ribs 84 reduce the throat area of the horn, but allow the diameter of the throat area of the horn to remain substantially similar to the diameter of the diaphragm or cone 74. This effectively spreads the throat area 82 over the surface of the diaphragm 74 to provide a substantially constant distance, or path length, between each portion of the diaphragm area and the mouth of the horn to minimize phase cancellations within the operating range of the horn.

FIG. 4a shows the cross-sectional area of the throat 82 of the horn 78 at point A--A. In this embodiment, a circular diaphragm is used to drive a circular horn, however any cross-sectional shape including square or rectangular shapes may be used. The diameter of the throat area is substantially similar to the diameter of the diaphragm 74, and six triangular ribs 84 extend radially into the throat area to provide the star shaped reduced throat area 82. The area of the throat 82 may be made similar to the area of throats 22 and 42 of the horns of FIGS. 1 and 2, respectively, to provide an impedance transformation similar to the transformation provided by horns of the prior art.

FIG. 4b shows the cross-sectional area of the horn at a point B--B approximately midway between the throat and the mouth thereof. In this embodiment, the diameter of the horn in FIG. 4b is approximately the same as the diameter of the horn at FIG. 4a. However, the area within the horn in FIG. 4b is substantially larger than the area of the throat 82 due to a reduction in the cross-sectional area of the ribs 84 at point B--B.

FIG. 4c shows a cross-section of the mouth 80 at point C--C. The ribs 84 shown in FIG. 4c have a substantially reduced cross-sectional area and the area of the mouth of the horn is determined primarily by the diameter thereof.

The taper of the ribs can be linear or non-linear depending on the type of horn configuration desired. The taper of the ribs can be chosen to provide a linearly tapered horn, a hyperbolic horn, an exponential horn or any horn taper obtainable with conventional horns. Note also that the ribs need not extend the entire longitudinal length of the horn, but may end before reaching the mouth 80 of the horn, beyond which point the cross-sectional area of the horn can be increased by simply increasing the size of the horn. In addition, all of the ribs need not be tapered, as in the case of rectangular horns wherein it may be desirable to taper some ribs and not others.

Referring to FIG. 6, there is shown another embodiment of the horn according to the invention. For purposes of clarity, no transducer has been shown in FIGS. 5-8, and only the horn has been shown, however, it should be understood that the horn shown in FIGS. 5-8 may be driven by any suitable transducer, such as, for example, the piezoelectric transducer shown in FIG. 4. The horn shown in FIG. 6 includes a throat end 90 and a mouth end 92. A plurality of ribs 94 are employed to reduce the crosssectional area of the horn, particularly in the throat area. A central plug 96 is positioned along a central axis of the horn and is supported by the ribs 94. The plug 96 provides a means for more accurately controlling the cross-sectional area of the passages between the ribs, particularly in the throat area.

Referring to FIG. 5, which shows an end view of the horn looking into the mouth portion thereof, it can be seen that the central plug 96 is supported by the ribs 94, the central plug 96 thereby plugging the center portion of the throat area to define a series of passages 98 between the ribs 94 and the plug 96. Because the passages 98 are shorter along the radial dimension than the individual arms of the star shaped cross-sectional area 82 shown in FIG. 4a, for a given cross-sectional area, the width of each of the passages 98 is greater than the width of each of the radial arms of the star shaped cross-sectional area, thereby making the throat area more independent of width tolerances and providing better control of the throat area. Since the throat area is controlled by the thickness of the ribs 94 and by the diameter of the central plug 96, the size of the ribs 94 and the central plug 96 can be chosen to provide an optimum configuration for the passages 98 in the throat area.

As can be seen from FIG. 6, the combination of the ribs 94 and the central plug 96 define a smoothly increasing cross-sectional area between the throat portion and the mouth portion of the horn. There is no abrupt change in cross-sectional area at the point of separation 100 between the rib 94 and the central plug 96. The diameter of the central plug 96 shown in FIG. 6 is substantially constant along its longitudinal dimension, however, the diameter may be varied along with the cross-sectional area of the ribs to provide any desired taper such as, for example, maintaining the cross-sectional area of the slots 98 within the throat area 90 substantially constant.

Referring to FIGS. 7 and 8, it can be seen that the ribs 94 and the central plug 96 are of solid construction and molded integrally with the horn. This method provides economical and simple construction of the horn according to the invention. In an alternate embodiment, the central portion 96 and the ribs 94 may be of hollow construction, similar to the construction of the horn of FIG. 4 to provide increased lightness while maintaining simplicity of construction.

Although many particular configurations of a horn according to the instant invention are possible, it should be noted that any horn utilizing a plurality of ribs to reduce the size of the throat area while maintaining a substantially constant path length between the transducer diaphragm and the mouth or throat of the horn fall within the scope of the invention. Furthermore the horn of the instant invention may be used with a translating element for either reproducing or picking up acoustic waves from air, water or other acoustic transmission medium.

In summary, the horn of the instant invention provides a simple, inexpensive way to produce a horn type transducer having a minimum of phase cancellations. This is achieved by a structure wherein all points on the diaphragm are substantially the same distance away from the mouth and throat, and wherein the sound from the diaphragm travels in a substantially straight line to the mouth of the horn, thereby further extending the high frequency response of the transducer.

Claims

I claim:

1. A horn for providing an impedance transformation between an acoustic translating means and an acoustic transmission medium comprising: an elongated tube member having a throat end connectible to said acoustic translating means and a mouth end for transferring acoustic energy between said horn and said acoustic transmission medium, said tube member having a plurality of elongated ribs positioned longitudinally within the interior of said tube member, each rib extending from an interior surface of said tube member into the space enclosed thereby to reduce the cross-sectional area, thereof, adjacent ones of said ribs being spaced substantially an equal distance from each other within said tube member and defining therebetween longitudinally extending passages having substantially equal lengths between said throat and mouth end to provide maximum high frequency response and to minimize phase cancellations within the operating range of said horn.

2. A horn as recited in claim 1 further including a plug positioned along a central axis thereof, said plug being supported at the throat end by said ribs.

3. A horn as recited in claim 1 wherein at least one of said ribs is tapered and positioned with the end of the rib having the larger cross-sectional area toward the throat end of said tube member.

4. A horn as recited in claim 1 wherein said ribs are integral to said tube member.

5. A horn type acoustic transducer including in combination; an acoustic transducer element, a frame member, a diaphragm attached to said frame member to define a compression chamber having an opening therein, said diaphgram forming a wall of said compression chamber, means coupling said transducer element and said diaphragm for transferring vibrational energy therebetween, and a horn having a mouth end and a throat end, said throat end communicating with the opening in said chamber, wherein said horn has spaced longitudinal ribs on the interior surface thereof for reducing the cross-sectional area of the throat, adjacent ones of said longitudinal ribs being spaced substantially an equal distance from each other and defining a plurality of similar longitudinally extending passages to effectively spread the area of said throat over the surface of said diaphragm to provide substantially equal path lengths for said passages between said diaphragm and said mouth whereby phase cancellations within the operating range of said horn are minimized.

6. A transducer as recited in claim 5 further including a plug supported by said ribs, said plug and said ribs defining a plurality of passages therebetween, each of said passages having a cross-sectional area which increases smoothly toward the mouth of the horn along a longitudinal axis thereof.

7. A transducer as recited in claim 5 wherein each of said passages is substantially straight.

8. A transducer as recited in claim 6 wherein said acoustic transducer element is a piezoelectric transducer element.

9. A transducer as recited in claim 6 wherein said acoustic transducer element is a magnetic transducer element.

10. An acoustic transformation device including in combination; a frame member, a diaphragm connectible to an acoustic transducer element to be driven thereby attached to said frame member to define a chamber having an opening therein, said diaphragm forming a wall of said chamber, and a horn having a mouth end and a throat end, said throat end communicating with the opening in said chamber, wherein said horn has spaced longitudinal ribs on the interior surface thereof for reducing the cross-sectional area of the throat, adjacent ones of said longitudinal ribs being spaced substantially an equal distance from each other and defining a plurality of similar longitudinally extending passages to effectively spread the area of said throat over the surface of said diaphragm to provide substantially equal path lengths for said passages between said diaphragm and said mouth whereby phase cancellations within the operating range of said horn are minimized.

11. An acoustic transformation device as recited in claim 10 further including an elongated plug positioned longitudinally within said horn and supported at the throat end by said ribs, said plug co-operating with said ribs to define said passages.

12. An acoustic impedance transformation device as recited in claim 10 wherein each of said passages is substantially straight.

13. An acoustic impedance transformation device as recited in claim 12 wherein at least one of said ribs is tapered and positioned with the end of the rib having the larger cross-sectional area positioned toward the throat of said horn.

14. An acoustic impedance transformation device as recited in claim 12 wherein said ribs are integral to said horn.

15. An acoustic impedance transformation device as recited in claim 14 wherein said horn has a substantially circular cross-section and wherein said ribs extend radially into the horn from the interior surface thereof.

16. An acoustic impedance transformation device as recited in claim 15 wherein said diaphragm is circular and wherein the diameter of said throat is substantially similar to the diameter of said diaphragm.