Single-ended planar-magnetic speaker Number:7,142,688 from the United States Patent and Trademark Office (PTO) owispatent

Title: Single-ended planar-magnetic speaker

Abstract: A planar-magnetic electro-acoustic transducer including a support structure and a magnetic structure carried by the support structure, the magnetic structure comprising a multiplicity of high-energy magnets configured so as to have shared loop field maxima and local loop field maxima, and a diaphragm carried by the support structure, comprising a plurality of conductors carried by and coupled to the diaphragm, said conductors being disposed in relation to local loop maxima and configured to exploit the energy of local loop maxima, as well as the energy of shared loop maxima in driving the diaphragm to produce an acoustic output; and the magnetic structure can be configured so that it includes magnet rows, and the transverse cross-sectional width of the magnets does not exceed their transverse cross-sectional height, and the distance between adjacent elongated magnet rows is greater than one half the width of either of the magnets of the adjacent magnet rows.

Patent Number: 7,142,688 Issued on 11/28/2006 to Croft, III,   et al.

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Inventors:	Croft, III; James J. (Poway, CA), Graebener; David (Carson City, NV)
Assignee:	American Technology Corporation (San Diego, unknown)
Appl. No.:	10/055,821
Filed:	January 22, 2002

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Current U.S. Class:	381/431 ; 381/176; 381/399
Current International Class:	H04R 25/00 (20060101)
Field of Search:	381/423,431,401,408,176-177,152,190-191,395-396,398-399

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References Cited [Referenced By]

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Primary Examiner: Ni; Suhan
Attorney, Agent or Firm: Thorpe North & Western LLP

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Parent Case Text

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This application claims priority of U.S. provisional application Ser. No. 60/263,480, filed Jan. 22, 2001, which is hereby incorporated herein by reference for the teachings consistent herewith, and this disclosure shall control in case of inconsistency.

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Claims

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The invention claimed is:

1. A planar-magnetic transducer comprising: at least one thin film vibratable diaphragm with a first surface side and a second surface side, including a predetermined active region, said predetermined active region including a predetermined conductive surface area for converting an input electrical signal into a corresponding acoustic output; primary magnetic structure including at least three elongated magnets placed adjacent and substantially parallel to each other with said magnets being of high energy and each having an energy product of greater than 25 mega Gauss Greased (MGO) which results in strong interaction between adjacent magnets; and a mounting support structure coupled to the primary magnetic structure and the diaphragm to capture the diaphragm, hold it in a predetermined state of tension and space it at a predetermined distance from the primary magnetic structure adjacent one of the surface sides of the diaphragm; said conductive surface area including elongate conductive paths running substantially parallel to said magnets; the mounting support structure, the at least three magnets of the primary magnetic structure, and the diaphragm having coordinated compositions and being cooperatively configured and positioned in predetermined spaced apart relationships wherein (i) the mounting support structure stabilizes the diaphragm in a static configuration at the predetermined tension which remains stable over and between extended periods of use, despite occurrence of dynamic conditions in response to extreme high energy forces driving the diaphragm to audio output, and (ii) the high energy magnetic forces interacting between the at least three magnets do not interfere with the predetermined tension of the diaphragm; said planar-magnetic transducer being operable as a single-ended planar-magnetic transducer.

2. A planar-magnetic transducer as set forth in claim 1 wherein the high energy magnets comprise neodymium.

3. A planar-magnetic transducer as set forth in claim 1 wherein the high energy magnets are neodymium magnets with an energy rating of at least 34 MGO.

4. A planar-magnetic transducer as set forth in claim 1, wherein the at least one thin film vibratable diaphragm includes a predetermined active region of less than 150 square inches, said predetermined active region including a predetermined conductive surface area for converting the input electrical signal into the corresponding acoustic output having an upper audio bandwidth extending down to a low range audio frequency.

5. A planar-magnetic transducer as set forth in claim 4 wherein said transducer diaphragm has a vibratable area and a fundamental resonant frequency representing approximately the lowest potential cutoff frequency of operation, the vibratable area and lowest cutoff frequency of operation of the planar-magnetic transducer falling into the unique range of Fr<(2000/ A) wherein (Fr) equals the fundamental resonant frequency of the transducer in Hertz and (A) equals the vibratable area of the transducer diaphragm in square inches.

6. A planar-magnetic transducer as set forth in claim 4 wherein: said transducer diaphragm has a vibratable area and a centered gap between the magnetic structure and the diaphragm measured at the center of the diaphragm, said transducer has a fundamental resonant frequency representing approximately the lowest potential cutoff frequency of operation, and the vibratable area and lowest cutoff frequency of operation of the planar-magnetic transducer are in the range of Fr<(1500/ A G) wherein (Fr) equals the fundamental resonant frequency of the transducer in Hertz and (A) equals the vibratable area of the transducer diaphragm in square inches and (G) equals the magnet to diaphragm gap measured in millimeters at the center of the transducer diaphragm.

7. A planar-magnetic transducer as set forth in claim 4 wherein: said transducer diaphragm has a vibratable area and a centered gap between the magnetic structure and the diaphragm measured at the center of the diaphragm, said transducer has a fundamental resonant frequency representing approximately the lowest potential cutoff frequency of operation, and the vibratable area and lowest cutoff frequency of operation of the planar-magnetic transducer are in the range of Fr<(1000/ A G) wherein (Fr) equals the fundamental resonant frequency of the transducer in Hertz and (A) equals the vibratable area of the transducer diaphragm in square inches and (G) equals the magnet to diaphragm gap measured in millimeters at the center of the transducer diaphragm.

8. A planar-magnetic transducer as set forth in claim 4 wherein: said transducer diaphragm has a vibratable area with a length and a width dimension wherein the width dimension is the lesser of the length and width dimensions, said transducer has a fundamental resonant frequency representing approximately the lowest potential cutoff frequency of operation, and the width of the vibratable area and lowest cutoff frequency of operation of the planar-magnetic transducer are in the range of Fr<(1000/W) wherein (Fr) equals the fundamental resonant frequency of the transducer in Hertz and (W) equals width dimension of the vibratable area of the transducer diaphragm in inches.

9. A planar-magnetic transducer as set forth in claim 4 wherein: said transducer diaphragm has a vibratable area with a width dimension less than a length dimension, the transducer further has a gap dimension between the magnetic structure and the diaphragm and said gap dimension measured at the center of the diaphragm, said transducer has a fundamental resonant frequency representing approximately the lowest potential cutoff frequency of operation, and the width of the vibratable area and lowest cutoff frequency of operation of the planar-magnetic transducer are in the range of Fr<(800/W)/G; wherein (PR) equals the fundamental resonant frequency of the transducer in Hertz and (W) equals width dimension of the vibratable area of the transducer diaphragm in inches and (G) equals the magnet to diaphragm gap measured in millimeters at the center of the transducer diaphragm.

10. A planar-magnetic transducer as set forth in claim 4 wherein the predetermined active diaphragm region has a total surface area of less than 100 square inches.

11. A planar-magnetic transducer as set forth in claim 4 wherein the predetermined active diaphragm region has a total surface area of less than 80 square inches.

12. A planar-magnetic transducer as set forth in claim 4 wherein the predetermined active diaphragm region has a total surface area of less than 60 square inches while having an operating resonant frequency of less than 400 Hz.

13. A planar-magnetic transducer as set forth in claim 12 having an operating resonant frequency of less than 300 Hz.

14. A planar-magnetic transducer as set forth in claim 4 wherein the predetermined active diaphragm region has a total surface area of less than 20 square inches while having an operating resonant frequency of less than 400 Hz.

15. A planar-magnetic transducer as set forth in claim 14 having an operating resonant frequency of less than 300 Hz.

16. A planar-magnetic transducer as set forth in claim 4 wherein the predetermined active diaphragm region has a total surface area of less than 9 square inches while having an operating resonant frequency of less than 900 Hz.

17. A planar-magnetic transducer as set forth in claim 4 further comprising a plurality of said transducers inter coupled as a line source of serially mounted transducers which form a loudspeaker taller than one transducer.

18. A planar-magnetic transducer as set forth in claim 1 further comprising at least one spacer structure positioned and abutting between at least two adjacent high energy magnets to eliminate the effect of magnetic attraction forces from potentially reducing the predetermined distance between at least two of the high energy magnets so the high energy magnetic forces do not interfere with the predetermined tension of the diaphragm.

19. A planar-magnetic transducer as set forth in claim 1 wherein the predetermined distance between at least two of the adjacent high energy magnets is at least seventy five thousandths of an inch.

20. A planar-magnetic transducer as set forth in claim 1 wherein the predetermined distance between at least two of the adjacent high energy magnets is at least ninety thousandths of an inch.

21. A planar-magnetic transducer as set forth in claim 1 wherein the predetermined distance between at least two of the adjacent high energy magnets is at least one hundred and fifty thousandths of an inch.

22. A planar-magnetic transducer as set forth in claim 1 wherein at least two of the adjacent high energy magnets have common dimensions and the predetermined distance therebetween is at least one half the width of one of the magnets.

23. A planar-magnetic transducer as set forth in claim 1, wherein the predetermined distance between the at least two high energy magnets is at least seventy percent of the width of one of the at least two adjacent magnets.

24. A planar-magnetic transducer as set forth in claim 1, wherein the predetermined distance between at least two of the adjacent high energy magnets is at least 100 percent of the width of one of the at least two adjacent magnets.

25. A planar-magnetic transducer as set forth in claim 1, wherein the mounting support stricture further includes forward support structure coupled to the mounting support structure and extending across and forward of the diaphragm to eliminate the effect of combined diaphragm tension forces and magnetic attraction forces from potentially reducing the predetermined distance between the adjacent magnets.

26. A planar-magnetic transducer as set forth in claim 1 further comprising: a rigid covering structure attached to the mounting support structure and having open areas and closed areas which substantially cover one of said first or second surface sides of the diaphragm, the primary magnetic structure being attached to the mounting support structure and mounted over the first surface side of the diaphragm, said covering structure open areas having acoustic transparency.

27. A planar-magnetic transducer as set forth in claim 26 wherein said rigid covering structure is ferrous composition and provides magnetic shielding.

28. A planar-magnetic transducer as set forth in claim 27 wherein said rigid covering structure braces the transducer against support structure flexing and very high magnetic forces caused by the adjacently mounted high energy magnets and supports the maintenance of predetermined diaphragm tension calibration.

29. A planar-magnetic transducer as set forth in claim 1 wherein a long term viscous material is applied along at least a portion of a periphery of the vibratable diaphragm and configured to provide damping properties to the diaphragm.

30. A planar-magnetic transducer as set forth in claim 29 wherein application of said viscous material is limited to an area outside of the conductive surface area but extends into the active region of the diaphragm.

31. A planar-magnetic transducer as set forth in claim 30 wherein application of said viscous material is limited to an area of the diaphragm outside and proximate to a last row of magnets on each side of the primary magnetic structure but extends into the active region of the diaphragm.

32. A planar-magnetic transducer as set forth in claim 31, wherein said viscous material is a solvent based polyurethane compound.

33. A planar-magnetic transducer as set forth in claim 1 wherein: said diaphragm has a central region and lateral regions that are a distance away from said central region, said primary magnetic structure has central region magnets and lateral magnets that are spaced away from said central region magnets, the predetermined spaced-apart relationship of the diaphragm from the magnets of the primary magnetic structure being greater at the central region of the diaphragm which is positioned over at least one central magnet than at the lateral diaphragm regions which are positioned over at least one lateral magnet.

34. A planar-magnetic transducer as set forth in claim 1 wherein at least a first of the transducers is optimized for higher frequencies and attached to at least a second of the transducers which is optimized to operate down to a lower frequency than that of said first transducer thereby forming a multi way loudspeaker, said multi way loudspeaker further including at least a high pass crossover filter for driving said first transducer.

35. A planar-magnetic transducer comprising: at least one thin film vibratable diaphragm with a first surface side and a second surface side, including a predetermined active region, said predetermined active region including a predetermined conductive surface area for converting an input electrical signal into a corresponding acoustic output; a magnetic structure including at least three elongated magnet rows placed adjacent and substantially parallel to each other with said magnets each being of high energy product greater than 25 mega Gauss Quested (MGO); and a mounting support structure coupled to the primary magnetic structure and the diaphragm to capture the diaphragm, hold it in a predetermined state of tension and space it at a predetermined distance from the primary magnetic structure adjacent one of the surface sides of the film diaphragm; said conductive surface area including elongate conductive paths running substantially in parallel with said magnets; the mounting support structure, the diaphragm and the at least three magnets of the primary magnetic structure having coordinated compositions and being cooperatively configured and positioned in predetermined spaced apart relationships wherein (i) the mounting support structure stabilizes the static and dynamic relationship between the diaphragm and the primary magnetic structure over and between extended periods of use and (ii) concurrently resists the high energy magnetic forces interacting between the at least three magnets which would otherwise interfere with the predetermined tension of the diaphragm; said planar-magnetic transducer being operable as a single-ended planar-magnetic transducer.

36. A planar-magnetic transducer as set forth in claim 35 wherein the high energy magnets comprise neodymium.

37. A planar-magnetic transducer as set forth in claim 35 wherein the high energy magnets are neodymium magnets with an energy rating of at least 34 MGO.

38. A planar-magnetic transducer comprising: at least one thin film vibratable diaphragm with a first surface side and a second surface side, including a predetermined active region, said predetermined active region including a predetermined conductive surface area for converting an input electrical signal into a corresponding acoustic output; a mounting support structure coupled to the primary magnetic structure and the diaphragm to capture the diaphragm, hold it in a predetermined state of tension and space it at a predetermined distance from the primary magnetic structure adjacent one of the surface sides of the film diaphragm; and primary magnetic structure including at least three high energy, elongated magnets placed adjacent and substantially parallel to each other with each magnet having an energy product of greater than 25 mega Gauss Oersteds (MGO); said conductive surface area including elongate conductive paths running substantially in parallel with said magnets; the mounting support structure, the diaphragm and the at least three magnets of the primary magnetic structure being cooperatively configured and positioned in predetermined spaced apart relationships; at least two of said high energy magnets being adjacently positioned in a predetermined spaced apart relationship wherein adjacent poles of the adjacent magnets have non shared, localized magnetic loops represented by local loop energy maxima in a plane of the diaphragm which are respectively greater than a shared energy maxima at a central position between the adjacent poles and extending along a shared magnetic loop of the respective adjacent poles in the plane of the diaphragm; said planar-magnetic transducer being operable as a single-ended planar-magnetic transducer.

39. A planar-magnetic transducer as set forth in claim 38, wherein the predetermined active region has a total surface area of less than 150 square inches, yet generates a high acoustic output having an upper audio bandwidth extending down to a low range audio frequency.

40. A planar-magnetic transducer as set forth in claim 38, further comprising a plurality of adjacently positioned high energy magnets having respective local loop energy maxima, wherein the majority of local loop energy maxima in the plane of the diaphragm have an average value which is greater than an average value of energy levels at the central positions in the plane of the diaphragm between corresponding adjacent poles of the adjacent magnets.

41. A planar-magnetic transducer as set forth in claim 38, wherein the shared energy maxima is no greater than 90 percent of the local loop energy maxima.

42. A planar-magnetic transducer as set forth in claim 38, wherein the shared energy is no greater than 80 percent of the local loop energy.

43. A planar-magnetic transducer as set forth in claim 38, wherein the shared energy is no greater than 75 percent of the local loop energy maxima.

44. A planar-magnetic transducer as set forth in claim 38, wherein a predetermined distance between the local loop energy maxima for adjacent magnets is approximately equal to a separation distance between the corresponding adjacent magnets.

45. A planar-magnetic transducer as set forth in claim 44, wherein the predetermined distance between the local loop energy maxima for adjacent magnets is at least seventy five thousandths of an inch.

46. A planar-magnetic transducer as set forth in claim 45, wherein the predetermined distance between the local loop energy maxima is at least ninety thousandths of an inch.

47. A planar-magnetic transducer as set forth in claim 38 wherein the predetermined distance between the local loop energy maxima is at least 100 percent of the width of the magnets.

48. A planar-magnetic transducer as set forth in claim 38 wherein the predetermined spaced apart relationship between any two of the at least three adjacent, high energy magnets is at least seventy five thousandths of an inch.

49. A planar-magnetic transducer as set forth in claim 38 wherein the predetermined spaced apart relationship between any two of the at least three adjacent, high energy magnets is at least ninety thousandths of an inch.

50. A planar-magnetic transducer as set forth in claim 38 wherein the predetermined spaced apart relationship between at least two of the at least three adjacent, high energy magnets is at least one hundred and fifty thousandths of an inch.

51. A planar-magnetic transducer as set forth in claim 38 wherein the at least three adjacent, high energy magnets have common dimensions and the predetermined spaced-apart relationship between at least two of said adjacent magnets is at least one half the width of one of the adjacent magnets.

52. A planar-magnetic transducer as set forth in claim 38, wherein the predetermined spaced-apart relationship between at least two of the at least three adjacent, high-energy magnets is at least seventy percent of the width of one of said adjacent magnets.

53. A planar-magnetic transducer as set forth in claim 38 wherein the predetermined spaced-apart relationship between at least two of the at least three adjacent, high-energy magnets is at least 100 percent of the width of one of said adjacent magnets.

54. A planar-magnetic transducer as set forth in claim 38 wherein the high energy magnets are neodymium magnets with an energy rating of at least 34 MGO.

55. A planar-magnetic transducer comprising: at least one thin film vibratable diaphragm with a first surface side and a second surface side, including a predetermined active region, said predetermined active region including a predetermined conductive surface area for converting an input electrical signal into a corresponding acoustic output; primary magnetic structure including at least three elongated magnets placed adjacent and substantially parallel to each other with at least one of said magnets being of high energy with each having an energy product of greater than 25 mega Gauss Greased (MGO); and a mounting support structure coupled to the primary magnetic structure and the diaphragm to capture the diaphragm, hold it in a predetermined state of tension and space it at a predetermined distance from the primary magnetic structure adjacent one surface side of the film diaphragm; said conductive surface area including elongate conductive paths running substantially in parallel with said magnets; any of the at least three adjacent magnets being oriented to be of opposite polarity orientation in relation to an adjacent magnet; said primary magnetic structure having at least three adjacent rows of side by side magnets with at least an outer two rows of the at least three rows of magnets providing less magnetic field strength through the conductive surface area of the diaphragm than provided through the conductive surface areas of the diaphragm by a center row of the magnets; said planar-magnetic transducer operating as a single-ended planar-magnetic transducer.

56. A planar-magnetic transducer as set forth in claim 55 including at least five adjacent rows of magnets with at least two outer rows of said five rows of magnets providing less magnetic field strength through the conductive surface area of the diaphragm than provided through the conductive surface area of the diaphragm by a center row of magnets.

57. A planar-magnetic transducer as set forth in claim 55 wherein the primary magnetic structure includes neodymium magnets with an energy rating of at least 34 MGO.

58. A planar-magnetic transducer as set forth in claim 55 wherein: said diaphragm has a central region and lateral regions that are a distance away from said central region, said primary magnetic structure has central region magnets and adjacent lateral magnets that are spaced away from said central region magnets, the predetermined spaced apart relationship of the diaphragm from the magnets of the primary magnetic structure being greater at a central region of the diaphragm over at least one central magnet than at the lateral regions over at least one lateral magnet.

59. A planar-magnetic transducer comprising: at least one thin film vibratable diaphragm with a first surface side and a second surface side, including a predetermined active region, said predetermined active region including predetermined, elongate conductive surface areas formed of a plurality of conductive elements for converting an input electrical signal into a corresponding acoustic output; a mounting support structure coupled to the primary magnetic structure and the diaphragm to capture the diaphragm, hold it in a predetermined state of tension and space it at a predetermined distance from the primary magnetic structure adjacent one of the surface sides of the film diaphragm; and primary magnetic structure including at least three high energy, elongated magnets placed adjacent and substantially parallel to each other with each magnet having an energy product of greater than 25 mega Gauss Oersteds (MGO); at least two of said high energy magnets being adjacently positioned in a predetermined spaced apart relationship wherein adjacent poles of the adjacent magnets have non shared, localized magnetic loops represented by local loop energy maxima as well as shared magnetic loops between the respective adjacent poles of the high energy magnets; said conductive surface area running substantially parallel to said magnets and more proximate to the local loops of the high energy magnets than to a center point of the shared magnetic loops between the adjacent magnets; said planar-magnetic transducer being operable as a single-ended planar-magnetic transducer.

60. A transducer as set forth in claim 59, wherein the conductive elements are substantially parallel to the elongated magnets and the conductive surface areas are most proximate to the respective local loop energy maxima associated with an adjacent magnet.

61. A planar-magnetic transducer as set forth in claim 59 wherein the high energy magnets are neodymium magnets with an energy rating of at least 34 MGO.

62. The transducer of claim 59, wherein the respective conductive surface areas are approximately centered over the local loops of adjacent high energy magnets.

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Description

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to improvements in fringe-field planar-magnetic speakers; and, more particularly, to fringe-field planar-magnetic speakers with single-ended primary magnetic circuits.

2. Background

Two general fields of loudspeaker design comprise (i) dynamic, cone devices and (II) electrostatic thin-film devices. A third, heretofore less exploited area of acoustic reproduction technology is that of thin-film, fringe-field, planar-magnetic speakers. This third area represents a bridging technology between these two previously recognized general areas of speaker design; combining a magnetic motor of an electro dynamic/cone transducer with a film-type diaphragm of a electrostatic device. However, it has not produced conventional planar-magnetic transducers, which, as a group, have achieved a significant level of market acceptance over the past 40-plus years of evolution. Indeed, planar-magnetic speakers currently comprise well under 1% of the total loudspeaker market. It is a field of acoustic technology that has remained exploratory, and embodied only in a limited number of relatively high-priced commercial products over this time period.

As with market acceptance of any speaker, competitive issues are usually controlling. In addition to providing performance and quality, a truly competitive speaker must be reasonable in price, practical in size and weight, and must be robust and reliable. Assuming that two different speakers provide comparable audio output, the deciding factors in realizing a successful market penetration will usually include price, convenience, and aesthetic appearance. Price is primarily a function of market factors, such as cost of materials and assembly, perceived desirability from the consumer's standpoint (as distinguished from actual quality and performance), demand for the product, and supply of the product. Convenience embodies considerations of adaptation of the product for how the speaker will be used, such as mobility, weight, size, and suitability for a customer-desired location of use. Finally, the aesthetic aspects of the speaker will also be of consumer interest; including considerations of appeal of the design, compatibility with decor, size, and simply its appearance in relation to the surroundings at the point of sale and at the location of use. If planar-magnetic speakers can be advanced so as to compare favorably with conventional electro dynamic and electrostatic speakers in these areas of consideration, further market penetration can be possible; as reasonable consumers should adopt the product that provides the most value for the purchase price paid.

With this background, a discussion of the relative successes and failures of conventional planar-magnetic speakers, and design goals and desired traits of operation will be given. It is interesting to note that the category of fringe-field, planar-magnetic speakers has evolved around two basic categories: a)"single-ended"; and, b) symmetrical "double-ended" designs, the later sometimes being called "push-pull," and both will be touched on as background for discussion of single-ended designs.

A conventional push-pull device is illustrated in FIG. 1. This structure is characterized by two magnetic arrays, 10 and 11, each supported by perforate substrates 14, 24; and positioned on opposite sides of a flexible diaphragm 12 which includes a conductive coil 13. The film is tensioned into a planar configuration. An audio signal is supplied to the coil 13, and a variable voltage thereby provided in the coil interacts with the otherwise fixed magnetic field between the magnet arrays 10 and 11. The diaphragm is displaced in accordance with variations in the audio signal, thereby generating a desired acoustic output. A representative example can be found in the disclosure of U.S. Pat. No. 4,156,801 issued to Whelk.

Because of a doubled-up, front/back magnet layout of prior push-pull planar-magnetic transducer structures, these double-ended systems have been generally regarded as more efficient, but as more complex to build. Also, they have certain performance limitations stemming from the formation of cavity resonances derived from the passage of sound waves through the cavities, or channels 16 formed by the spaces between the magnets of the arrays 10, 11, and acoustically radiating to the external environment through holes 15 in the substrates 14, 24. This can cause problems at certain frequencies, including giving rise to resonant peaks and band-limiting attenuation. In all fairness it must be said that single-ended designs are not immune from this problem; and particularly where the magnet spacing is close together, cavity resonances can occur in single-ended as well as double-ended designs.

Double-ended designs are also particularly sensitive to deformation from repulsive magnetic forces, which tend to deform the structures of such devices outward. This outward bowing draws the edges of the diaphragm closer together and alters the tension on the diaphragm. This can significantly degrade performance, to the point of rendering the speaker unusable.

As mentioned, a second category of planar-magnetic speakers comprises single-ended devices. With reference to FIG. 2, a typical conventional single-ended speaker configuration, having a flexible diaphragm 17 with a number of conductive elements 18, is set forth by way of example. The diaphragm is tensioned and supported by frame members (not shown) carried by a substrate 19 of the frame, and which frame members extend outward (upward in the figure) beyond the top of a single array of magnets 35 to position the diaphragm an offset distance away from the tops of the magnets to accommodate vibration of the diaphragm. The array provides a fixed magnetic field with respect to the coil conductors 18 disposed on the diaphragm. It will be apparent that the single array of magnets (typically of ceramic or rubberized ferrite composition) provides a much-reduced energy field, compared to the previously discussed push-pull device, assuming comparable magnets are used. Because of this and other reasons, previous single-ended devices of compact size have not provided performance that has been deemed acceptable for commercial applications.

Furthermore, conventional single-ended devices have had to be quite large to work effectively; and, even so, are less efficient than standard electrostatic and electro dynamic loudspeaker designs mentioned above. Small, or even average-sized single-ended planar-magnetic devices (compared to electro dynamic and electrostatic speakers) have not effectively participated in the loudspeaker market in the time since the introduction of planar magnetic speakers. Comparatively large devices, generally greater than 300 square inches, have been available to consumers in the speaker market; and these exhibit limited competitiveness. That is to say, they are on par with standard speakers in terms of acceptance, suitability to certain applications, cost, and performance. But again, the market penetration of planar-magnetic speakers is less than 1%, including both single-ended and push-pull devices. Prior single-ended planar-magnetic devices with such large diaphragm areas require correspondingly relatively large and expensive structures; and, such relatively larger speakers can be cumbersome to place in some environments. They have relatively low efficiencies as well, compared with electro dynamic and electrostatic speakers, requiring more powerful, and hence more expensive, amplifiers to provide adequate signal power to drive them.

At first impression, a single-ended device might appear to be simpler and cheaper to build than a double-ended design. The same amount of magnet material can be used by doubling the thickness of the magnets to correspond to the combined thickness of a double-ended array of magnets. Because magnets of a given material made twice as thick are cheaper installed than twice as many magnets half as thick (as in a double-ended device) there should be significant savings in a single-ended configuration. Furthermore, the structural complexity is significantly less with regard to single-ended designs, further adding to expected cost savings.

However, doubling the depth of the magnets from that of most designs does not achieve the expected design goal of providing twice the magnetic energy in the gap between the diaphragm and the array of magnets when using conventional ferrite magnets. Accordingly, the expectation for lower cost per a given performance level in the single-ended device has not been realized. Some attempts to improve the design of single-ended planar-magnetic devices have involved the use of relatively many more, and very closely spaced, magnets to provide sufficiently high magnet