AU2006341771A1

AU2006341771A1 - An electrostatic loudspeaker

Info

Publication number: AU2006341771A1
Application number: AU2006341771A
Authority: AU
Inventors: Robert Neil Mackinlay; Charles Corneles Van Dongen
Original assignee: Immersion Technology Property Ltd
Current assignee: Scirocco Energy PLC
Priority date: 2006-04-10
Filing date: 2006-08-04
Publication date: 2007-10-18
Also published as: WO2007115348A1; WO2007115350A1; AU2006341770A1; AU2006341769A1; WO2007115349A1

Description

WO 2007/115350 PCT/AU2006/001103 AN ELECTROSTATIC LOUDSPEAKER Technical Field The present invention relates to electrostatic loudspeakers. 5 Background A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that that document or matter was, in Australia, known or that the information that it contains was part of the common 10 general knowledge as at the priority date of any of the claims of the present application. Electrostatic loudspeakers use a thin flat diaphragm usually consisting of a plastic sheet, for example such as Mylar (TM), impregnated or covered with a 15 conductive material capable of holding an electric charge, for example such as graphite, located between two electrically conductive grids supported by frames, known as stators, with a small air gap between the diaphragm and stators. The diaphragm, by means of its conductive coating and an external high voltage which is applied to it, is held at a DC potential of several kilovolts with respect to 20 the stators. The stators are driven by the audio signal, the front and rear stators being driven in counterphase. As a result, an evenly distributed electrostatic field proportional to the audio signal is produced between both stators. This causes a force to be exerted on the charged diaphragm and its resulting movement drives the air on either side of it, providing an acoustic output. 25 Electrostatic loudspeakers are known to lack bass response (due to front to rear phase cancellation and difficulty in reproducing low frequencies with a vibrating taut film with a relatively short deflection range). Standing wave interferences in the diaphragm may also occur. An object of the present invention is to provide 30 an electrostatic loudspeaker which gives improved sound output by reducing standing wave interferences in the diaphragm.

WO 2007/115350 PCT/AU2006/001103 2 Disclosure of the Invention The present invention provides an electrostatic loudspeaker including: a first stator, a second stator spaced from the first stator, and an electrically 5 conductive diaphragm between the first and second stators, wherein a spacer is located between edges of the first and second stators and the electrically conductive diaphragm has edges that are attached to the spacer whereby the diaphragm is supported by the spacer, wherein the spacer, adjacent the attachment of the diaphragm, is shaped to 10 provide recesses having surfaces for redirecting sound vibrations reaching the edges of the diaphragm to substantially eliminate reflection of the sound vibrations in the diaphragm. The invention also provides an electrostatic loudspeaker including a first stator, 15 a second stator spaced from the first stator, and an electrically conductive diaphragm between the first and second stators, wherein the diaphragm has edges that are attached to one of the stators whereby the diaphragm is supported by that stator, wherein the stator to which the diaphragm is attached, adjacent the attachment of the diaphragm, is shaped to provide recesses 20 having surfaces for redirecting sound vibrations reaching the edges of the diaphragm to substantially eliminate reflection of sound vibrations in the diaphragm. Preferably the recesses provided by the spacer or stator have opposite side 25 walls that converge. For example the recesses may be such that a saw-tooth shaped edge adjacent the attachment of the diaphragm is provided. Such a saw-tooth shaped edge may have teeth that are uniformly shaped or non uniformly shaped. Alternatively the recesses may be such that the spacer or stator has a wavy shaped edge adjacent the attachment of the diaphragm. 30 Such a wavy shaped edge may be irregular or regular. Preferably each recess of substantially all the recesses has a depth that is greater than its opening facing the diaphragm edge.

WO 2007/115350 PCT/AU2006/001103 3 In an electrostatic loudspeaker according to embodiments of the invention, vibrations reaching an edge of the diaphragm are directed into the recesses where they lose their energy due to reflection effects. The recesses do not provide any faces parallel to the adjacent edge of the diaphragm and thus there 5 is no redirection of the vibrations back into the diaphragm, that is, reflection of the sound vibrations in the diaphragm at its edges is substantially eliminated. In embodiments of the invention, there is substantial elimination of standing waves across the diaphragm and thus reduced distortion of the output sound. 10 For a better understanding of the present invention, embodiments thereof will now be described, by way of non limiting example only, with reference to the accompanying drawings. The figures of the accompanying drawings are not drawn to scale, that is, the dimensions of the various components have been relatively varied for the purposes of clear illustration. 15 Brief Description of Drawings Figure 1 schematically illustrates a typical electrostatic loudspeaker system. Figure 2 is a schematic elevational front view of an electrostatic loudspeaker. 20 Figure 3 is a cross sectional view on section line Ill-111 of Figure 2. Figures 4A, 4B and 4C illustrate an embodiment of a stator to assist description of a manufacturing method. 25 Figures 5A and 5B are schematic cross sectional views illustrating an embodiment of an electrostatic loudspeaker construction. Figure 6A is a schematic front view of portion of an electrostatic loudspeaker 30 showing attachment of a diaphragm to a spacer with shaped edges according to an embodiment of the invention. Figure 6B is a schematic cross sectional view of the Figure 6A portion.

WO 2007/115350 PCT/AU2006/001103 4 Figures 6C and 6D illustrate edge configurations for a spacer of the Figure 6A and 6B embodiment. Figure 7 is a schematic cross sectional view of an edge portion of an 5 electrostatic loudspeaker according to another embodiment of the invention. Detailed Description of Embodiments In the drawings corresponding features or elements in the various figures are indicated by a common reference numeral for ease of understanding. 10 The schematic electrostatic loudspeaker system of Figure 1 comprises an electrostatic loudspeaker 20 and circuitry 22 for driving the electrostatic loudspeaker 20. The electrostatic loudspeaker 20 includes spaced apart first and second stators 24 between which is located an electrically conductive 15 diaphragm 26. Each stator 24 comprises an insulating peripheral frame 28 (see Figures 2 and 3) which supports a multiplicity of electrically conductive stator elements 30 forming a grid 32, that is a multiplicity of parallel rigid "rods" or "bars" 30 which are connected together electrically by at least end connections 34. The stators 24, because of the grid structure 32, are acoustically 20 transparent to audio sound output. The frames 28 of the stators 24 support the diaphragm 26, which is lightly tensioned across and attached to the frame 28 of one of the stators, such that there is a small air gap 36 between the diaphragm 26 and each stator 24. 25 The driving circuitry 22 includes a step up transformer 38 having input terminals 40 to which an audio signal is applied. Each stator 24 is connected to a respective end of the secondary winding of the step up audio transformer 38 and a high tension polarising voltage 42 is connected to the diaphragm 26 via a resistor 25 and a centre tap of the secondary winding (as shown in Figure 1). 30 The resistor 25 is needed for constant charge and electrical safety. Circuit arrangements 22 other than as illustrated by Figure 1 may be used. Electrical connection to the diaphragm 26 may be via a conductive strip 44 around the insulating frame 28 of one of the stators 24 and which is located WO 2007/115350 PCT/AU2006/001103 5 between the frames 28 when they are clamped together (see Figure 3). Electrical connection to the electrically conductive grids 32 of each stator 24 may be via a terminal 46 (see Figure 2). 5 The diaphragm 26 may be formed from a thin (for example 2-12 microns) film of a material such as Mylar (TM) or a biaxially oriented polyphenylene sulphide (PPS), to which a coating of a low conductivity substance (for example graphite) is applied to render it capable of holding an electric charge due to an applied high tension voltage 42. The diaphragm 26, suitability tensioned, is attached to 10 the frame 28 of one of the stators 24 for example by an adhesive. Electrostatic loudspeakers can be of enormously varied sizes and rectilinear shapes, for example small square shapes say 50mm x 50mm to large rectangular shapes say 3000mm x 600mm, or 3000mm x 1200mm, or larger. 15 Acoustic transparency of the grids 32 of the stators 24 is achieved by a suitable ratio of the spacing between the stator elements 30 to the thickness of the stator elements 30. It has been found with embodiments of the present invention disclosure that as high a ratio as 60% spacing and 40% stator element thickness gives excellent sound output results. This ratio may be 20 reduced to 40% - 60% or values in between and still give effective sound output results. With reference to Figures 4A-C, a stator 24 is shown which may be manufactured by first forming a grid 32 by mechanically joining together a 25 multiplicity of parallel steel rods 29 (to provide the stator elements 30) with cross-wise "end" steel rods 31 (to provide the end connections 34) by for example welding to provide electrical connection between the parallel rods 29. Other bridging rods or wires 31 may be welded across the parallel rods 29 to provide additional support to ensure rigidity of the grid 32. The grid of steel rods 30 29 is then coated with an insulating material, for example nylon, by spraying, dipping or brushing to provide a preformed precoated grid 32. The preformed precoated grid 32 is then placed into an injection moulding die and the insulating frame 28 is then injection moulded around the grid 32 WO 2007/115350 PCT/AU2006/001103 6 resulting in the ends 48 of the precoated steel rods 29 being embedded in the frame 28 (see Figure 4C). The steel rods 29 may be about 2mm diameter and the preformed insulating 5 coating thereon may be about 1 mm thick. The frame 28 may be moulded otherwise then by an injection moulding process, for example casting. Persons skilled in the art will routinely be able to construct suitable moulding dies for the moulding. 10 Also, the preformed grid may, instead of a grid as such, be provided by a mesh or an apertured plate, and the term "grid" is hereby defined as encompassing such alternatives. Figures 5A and 5B illustrate that a stator 24 may be manufactured by first 15 moulding a complete grid structure 32 including its peripheral frame 28 from an electrically conductive material and then combining electrically insulating portions with the structure 32-28 by applying an electrically insulating coating 58 onto at least the stator elements 30 of the complete grid structure 32. Where the insulating coating 58 does not cover the frame 28, the electrostatic 20 loudspeaker 20 may be assembled by including electrically insulating spacers 60 between the frames 28 (see Figure 5B) to ensure the diaphragm 26 is electrically insulated from the stators 24. One of the spacers 60 is used to support the diaphragm 26 and the other includes a conductive strip 44 for electrically contacting the diaphragm 26 to supply the high tension polarising 25 voltage 42 to it. The electrically insulating coating 58 may be applied by spraying, brushing or dipping. The spacers 60 which support the diaphragm 26, at their edges 61 adjacent the attachment of the diaphragm 26, may be shaped to provide recesses according to the invention, as will be described below in more detail with reference to Figures 6A - D and 7. 30 Figures 6A and B illustrate a portion of an embodiment of an electrostatic loudspeaker according to the invention that includes a first stator 24 and a second stator 24 with a diaphragm 26 between the stators 24. A spacer 70 is located between the edges of the stators 24, that is between their frames 28.

WO 2007/115350 PCT/AU2006/001103 7 The spacer 70 has edges 72 that are shaped to provide recesses 74. Edges of the diaphragm 26 are attached to the spacer 70, that is the spacer 70 supports the diaphragm 26 under tension. The surfaces of the recesses 74 in the edges 72 of spacer 70 are such that sound vibrations reaching the edges of the 5 diaphragm 26 are redirected to substantially eliminate reflection of sound vibrations in the diaphragm. Generally the recesses 74 will have opposite side walls that converge. For example the edge 72 could be saw-tooth shaped with the teeth either uniformly (see Figure 6C) or non uniformly shaped. Alternatively the recesses 74 provided by the spacer 70 may be such that the 10 spacer 70 has a wavy shaped edge 72 and the waves thereof may be either regular (see Figure 6A) or irregularly shaped. Each recess 74 of substantially all the recesses may have a depth that is greater than its opening facing the diaphragm edge (see Figure 6D). 15 Alternatively, instead of using a spacer 70, the edges of the frame 28 of a stator 24 to which the diaphragm 26 is attached may be shaped to provide the recesses. This alternative is illustrated in Figure 7 which shows one edge portion, in cross section, of an electrostatic loudspeaker 20 in which the stator frame 28a which supports the diaphragm 26, at its edge 80 adjacent the 20 attachment of the diaphragm 26, is shaped to provide recesses 82. The recesses 82 may have a wavy shape (for example similar to the recesses 74 illustrated by Fig 6A) or a saw - tooth shape (for example similar to the recesses 74 illustrated by Figures 6C and 6D). The recesses 82 may be uniformly or non uniformly shaped. 25 The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications, and/or additions which fall within the scope of the following claims.

Claims

1. An electrostatic loudspeaker including: a first stator, a second stator spaced from the first stator, and an electrically conductive diaphragm between the first and second stators, wherein the diaphragm has edges that are attached to one of the stators whereby the diaphragm is supported by that stator, wherein the stator to which the diaphragm is attached, adjacent the attachment of the diaphragm, is shaped to provide recesses having surfaces for redirecting sound vibrations reaching the edges of the diaphragm to substantially eliminate reflection of sound vibrations in the diaphragm.

2. An electrostatic loudspeaker including: a first stator, a second stator spaced from the first stator, and an electrically conductive diaphragm between the first and second stators, wherein a spacer is located between faces of the first and second stators and the electrically conductive diaphragm has edges that are attached to the spacer whereby the diaphragm is supported by the spacer, wherein the spacer, adjacent the attachment of the diaphragm, is shaped to provide recesses having surfaces for redirecting sound vibrations reaching the edges of the diaphragm to substantially eliminate reflection of the sound vibrations in the diaphragm.

3. An electrostatic loudspeaker as claimed in claim 1 or 2 wherein the recesses have opposite side walls that converge.

4. An electrostatic loudspeaker as claimed in claim 3 wherein the recesses are such that a saw-tooth shaped edge adjacent the attachment of the diaphragm is provided. WO 2007/115350 PCT/AU2006/001103 9

5. An electrostatic loudspeaker as claimed in claim 4 wherein the teeth of the saw-tooth shaped edge are uniformly shaped.

6. An electrostatic loudspeaker as claimed in claim 4 wherein the teeth of the saw-tooth shaped edge are non uniformly shaped.

7. An electrostatic loudspeaker as claimed in claim 1 or 2 wherein the recesses are such that a wavy shaped edge adjacent the attachment of the diaphragm is provided.

8. An electrostatic loudspeaker as claimed in claim 7 wherein the wavy shaped edge is regular or irregular.

9. An electrostatic loudspeaker as claimed in any one of claims 1 to 8 wherein each recess of substantially all the recesses has a depth that is greater than its opening facing the diaphragm edge.