AU2006341770A1

AU2006341770A1 - An electrostatic loudspeaker

Info

Publication number: AU2006341770A1
Application number: AU2006341770A
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: AU2006341771A1; WO2007115348A1; AU2006341769A1; WO2007115350A1; WO2007115349A1

Description

WO 2007/115349 PCT/AU2006/001102 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 The diaphragm, which is supported by and between the frames of the stators, may be glued to one of the frames by a glue that does not creep over time (for example an ethyl cyanoacrylate i.e. a "super glue"). This creates a support edge about which the diaphragm bends or deflects as it vibrates. The bending 30 or deflection of the diaphragm about such an edge affects the operational lifespan of the diaphragm, that is, possible failure of the diaphragm at its edge support is a factor to take into account in determining an operational life span for an electrostatic loudspeaker. Such an edge support may also cause standing wave interferences in the diaphragm to occur.

WO 2007/115349 PCT/AU2006/001102 2 An object of the invention is to provide an electrostatic loudspeaker which has an increased life span. Another object is to provide an electrostatic loudspeaker which in one embodiment gives improved sound output by reducing standing wave interferences in the diaphragm. 5 According to the present invention, there is provided an electrostatic loudspeaker including: a first stator having a frame, a second stator having a frame and located opposite the first stator, and 10 an electrically conductive diaphragm between the first and second stators, wherein the diaphragm has edge regions which are supported by and between the frames of the first and second stators, wherein a material of lesser rigidity than that of the frames of the stators is 15 located to engage on both surfaces of the diaphragm next to where the diaphragm edge regions meet the frames of each stator to provide a cushioning transitional support zone for the diaphragm. Preferably the material of lesser rigidity is such that vibrations approaching an 20 edge region of the diaphragm are absorbed by the material to thereby substantially damp the vibrations. More preferably the material of lesser rigidity is such that reflection of the vibrations in the diaphragm are substantially eliminated. 25 The material of lesser rigidity may be a synthetic or natural rubber, for example a Neoprene (TM) or Sorbothane (TM), silicon, a resinous or similar material or felt. It may be provided as a cord that is suitably supported by the frames of the stators. Alternatively for viscous liquid or gel type materials, it may be applied 30 as a gel and allowed to cure. For a better understanding of the present invention, an embodiment 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 WO 2007/115349 PCT/AU2006/001102 3 drawn to scale, that is, the dimensions of the various components have been relatively varied for the purposes of clear illustration. Brief Description of Drawings 5 Figure 1 schematically illustrates a typical electrostatic loudspeaker system. Figure 2 is a schematic elevational front view of an electrostatic loudspeaker. 10 Figure 3 is a cross sectional view on section line Ill-Ill of Figure 2. Figures 4A, 4B and 4C illustrate an embodiment of a stator to assist description of a manufacturing method. 15 Figure 5 is a schematic cross sectional view of a portion of an electrostatic loudspeaker according to an embodiment of the invention having a diaphragm supported by a cushioning material. Detailed Description of Embodiments 20 In the drawings corresponding features or elements in the various figures are indicated by a common reference numeral for ease of understanding. The schematic electrostatic loudspeaker system of Figure 1 comprises an electrostatic loudspeaker 20 and circuitry 22 for driving the electrostatic 25 loudspeaker 20. The electrostatic loudspeaker 20 includes spaced apart first and second stators 24 between which is located an electrically conductive 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 30 "bars" 30 which are connected together electrically by at least end connections 34. The stators 24, because of the grid structure 32, are acoustically 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 WO 2007/115349 PCT/AU2006/001102 4 one of the stators, such that there is a small air gap 36 between the diaphragm 26 and each stator 24. The driving circuitry 22 includes a step up transformer 38 having input terminals 5 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). The resistor 25 is needed for constant charge and electrical safety. Circuit 10 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 between the frames 28 when they are clamped together (see Figure 3). 15 Electrical connection to the electrically conductive grids 32 of each stator 24 may be via a terminal 46 (see Figure 2). 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 20 (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 the frame 28 of one of the stators 24 for example by an adhesive such as "super glue". 25 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. Acoustic transparency of the grids 32 of the stators 24 is achieved by a suitable 30 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 that as high a ratio as 60% spacing and 40% stator element thickness gives excellent sound output results. This ratio may be reduced to 40% - 60% or values in between and still give effective sound output results.

WO 2007/115349 PCT/AU2006/001102 5 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 multiplicity of parallel steel rods 29 (to provide the stator elements 30) with 5 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 29 is then coated with an insulating material, for example nylon, by spraying, 10 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 resulting in the ends 48 of the precoated steel rods 29 being embedded in the 15 frame 28 (see Figure 4C). The material of the stator frame 28 may be a polypropylene, for example a high density polypropylene (HDPP) or a polyvinylchloride (PVC). It must have sufficient strength and rigidity (without being brittle) to stably perform as a frame of the stator. 20 The steel rods 29 may be about 2mm diameter and the preformed insulating 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. 25 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. 30 Figure 5 illustrates portion of an embodiment of an electrostatic loudspeaker 20 according to the invention in which the diaphragm 26 has edge regions 27 supported by the frames 28 of the stators 24, and in which a material 76 of lesser rigidity than the material of the frames 28 engages on both surfaces of the diaphragm 26 next to where the diaphragm edge regions 27 meet the side WO 2007/115349 PCT/AU2006/001102 6 faces 78 of the frames 28 of the stators 24. As shown in Figure 5, the side faces 78 of the frames 28 may include a recess or cut out 80 for receiving and supporting the material 76. The material 76 may be Neoprene (TM), Sorbothane (TM) or other synthetic rubber, a natural rubber, silicon, a resinous 5 or similar material or a felt, which is either provided as a cord, or in lengths to extend along portions of the sides of the stator frames 28, or as a flowable gel which is applied and then allowed to cure. The material 76 can extend continuously along both sides of the loudspeaker, or it may extend along separated portions of the sides. 10 The material 76 provides a cushioning transitional support zone for the diaphragm 26 in that it absorbs (to a certain extent, depending on the rigidity of the material 76) vibrations approaching the edge region 27 of the diaphragm 26. Thus there is a cushioning effect, the degree of which depends upon the rigidity 15 of the cushioning material 76, which material can be chosen to provide a desired degree of cushioning. Thus a quite soft material 76 may be chosen for an embodiment of the invention such that vibrations in the diaphragm 26 approaching an edge region 27 are absorbed so as to substantially damp the vibrations. In another embodiment, an even softer material 76 may be used 20 such that reflection of the vibrations in the diaphragm from the edge 27 are substantially eliminated thereby improving sound output by reducing standing wave interferences. In all embodiments, the cushioning effect provided by the material 76 reduces 25 the degree of bending or deflection of the diaphragm 26 about the support of its edge region 27 provided by the stator frames 28. This reduces the likelihood of failure of the diaphragm 26 at that support and thus improves the operational lifespan of the electrostatic loudspeaker 20. 30 In an example loudspeaker 20 having stators 24 with moulded plastic frames 28 and providing a bass response panel of 800 mm height and 230 mm width, a material 76 was incorporated around the panel as in Fig. 5. The material 76 was a 5mm square cross-section length of self adhesive expanded urethane foam (such material is sometimes used as a window sealer). This material WO 2007/115349 PCT/AU2006/001102 7 significantly absorbed the sound vibrations of the diaphragm 26 (which was a Mylar (TM) film of 3 microns thickness). It also provided a gain of 3-4 db from the panel while reducing secondary undesired frequencies. 5 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 having a frame, a second stator having a frame and located opposite the first stator, and an electrically conductive diaphragm between the first and second stators, wherein the diaphragm has edge regions which are supported by and between the frames of the first and second stators, wherein a material of lesser rigidity than that of the frames of the stators is located to engage on both surfaces of the diaphragm next to where the diaphragm edge regions meet the frames of each stator to provide a cushioning transitional support zone for the diaphragm.

2. An electrostatic loudspeaker as claimed in claim 1 wherein the material of lesser rigidity is such that vibrations approaching an edge region of the diaphragm are absorbed by the material to thereby substantially damp the vibrations.

3. An electrostatic loudspeaker as claimed in claim 1 or 2 wherein the material of lesser rigidity is such that reflection of the vibrations in the diaphragm are substantially eliminated.

4. An electrostatic loudspeaker as claimed in any one of claims 1 to 3 wherein the frame of each stator includes a recess within which the material of lesser rigidity is located.

5. An electrostatic loudspeaker as claimed in claim 4 wherein the recess in each frame extends between a side face of the frame adjacent the diaphragm and a face which supports an edge region of the diaphragm.

6. An electrostatic loudspeaker as claimed in claim 4 or 5 wherein the material of lesser rigidity is a preformed cord which is seated in each recess. WO 2007/115349 PCT/AU2006/001102 9

7. An electrostatic loudspeaker as claimed in claim 4 or 5 wherein the material of lesser rigidity is a cured viscous or gel material having cured in situ in each recess.

8. An electrostatic loudspeaker as claimed in any one of claims, 4, 5 or 7 wherein the material of lesser rigidity is in lengths, each length extending along a portion of the sides of the frames of the stators.

9. An electrostatic loudspeaker as claimed in anyone of claims 1 to 7 wherein the material of lesser rigidity is one of a synthetic or natural rubber, silicon, a resinous material and a felt.