Abstract:
A method for manufacturing a stator ( 24 ) for an electrostatic loudspeaker in which at least a part of a structure ( 28 ) for forming the stator ( 24 ) is moulded from an electrically insulating material. This structure ( 28 ) may be a frame of the stator. To complete the stator ( 24 ), electrically conductive portions ( 30 ) are combined with the moulded structure to form a complete structure that includes an electrically conductive grid ( 29 ). The electrically conductive portions ( 30 ) may be a preformed grid ( 29 ). The frame ( 28 ) and the grid ( 29 ) may be press-fitted together. Alternatively the moulded structure may be electrically conductive, and electrically insulating portions may be combined with it to form a complete stator. Manufacture of electrostatic loudspeaker stators using a moulding process allows for relatively low cost production methods that can repeatedly achieve a required high degree of accuracy.

Description:
TECHNICAL FIELD 
     The present invention relates to electrostatic loudspeaker stators and their manufacture. 
     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 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™, impregnated or covered with a 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 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. 
     The stators should generate as uniform an electric field as possible, while still allowing for sound to pass through, that is, they need to be substantially acoustically transparent, and generally need to be very flat. They can therefore be difficult to manufacture because of the required degree of accuracy. 
     An object of the present invention is to provide methods for manufacturing a stator for an electrostatic loudspeaker which allow accurate, relatively low cost production. 
     DISCLOSURE OF THE INVENTION 
     According to a first aspect, the present invention provides a method for manufacturing a stator for an electrostatic loudspeaker including the steps of: 
     (i) moulding at least a part of a structure for forming the stator from an electrically insulating material, 
     (ii) combining electrically conductive portions with the at least part of the structure to form a complete structure that includes an electrically conductive grid and is suitable for use as a stator for an electrostatic loudspeaker. 
     The steps of moulding and combining may be performed simultaneously by providing the electrically conductive portions as a preformed grid (as hereinafter defined) and moulding the electrically insulating material around the preformed grid to provide a peripheral frame for supporting the preformed grid. In this manufacturing method, the preformed grid may be formed of steel rods which are joined together for example by welding. The electrically insulating material may be a plastics material, or an epoxy resin, or any insulating material with appropriate mechanical characteristics. 
     Alternatively the steps of moulding and combining may be performed sequentially by first moulding a grid structure including a peripheral frame from an electrically insulating material and then combining the electrically conducting portions with the grid structure by applying an electrically conductive layer to parts of the grid structure. The electrically conductive layer may be applied by electrodeposition (electrodeposition techniques for applying metallic coatings to non metallic objects are known in the art) or by a spraying, brushing or dipping process. The electrically conductive layer may be an organic or metallic based substance. An insulating coating is then preferably applied over the electrically conductive layer. 
     Another alternative for sequentially performing the steps of moulding and combining is to mould a frame such that it has attachment means for receiving a preformed electrically conductive grid, and to then combine the preformed grid with the frame by affixing the grid to the attachment means. In this alternative the attachment means may be grooves which may be sized such that the portions of the grid that engage therein will be an interference fit. In this method the preformed grid may be formed of steel rods which are joined together, for example by welding. Preferably the performed grid is coated with an insulating material prior to being affixed to the frame. Alternatively it may be covered by an insulating material after its fitting to the frame. The insulating material may be applied by a spraying, brushing or dipping process. 
     A second aspect of the present invention provides a method for manufacturing a stator for an electrostatic loudspeaker including the steps of: 
     (i) moulding at least a part of a structure for forming the stator from an electrically conducting material, 
     (ii) combining electrically insulating portions with the at least part of the structure to form a complete structure that includes an electrically conductive grid and is suitable for use as a stator for an electrostatic loudspeaker. 
     The moulding step for the second aspect of the invention may involve moulding a complete grid structure from an electrically conductive plastics material or a metal or other electrically conductive material, and the combining step may then involve applying an electrically insulating coating onto the complete grid structure. The electrically insulating coating may be a plastics or other insulating material and may be applied by a spraying, brushing or dipping process. 
     Alternatively the moulding and combining steps of the first and second aspects of the invention may be performed via a multi-stage moulding process using a single mould, wherein, for the method of the second aspect, conductive sections of the stator are moulded first using an appropriate mouldable material, and the insulating sections are moulded next using an appropriate but different mouldable material, where the respective mouldable materials have sufficiently different melting points to avoid remelt of the first formed sections. For the method of the first aspect, insulating sections are first moulded followed by moulding of the conductive sections. 
     The step of moulding in the above first and second aspects of the invention may be an injection moulding, casting or similar process. The insulating materials will generally be a plastics material that has sufficient strength and rigidity (without being brittle) to stably perform as a stator or a stator component such as the frame for a grid. A suitable plastics would be a polypropylene, for example a high density polypropylene (HDPP) or a polyvinylchloride (PVC), although many other insulating materials would be suitable. The materials should also be such that the manufactured stator has appropriate natural frequency of vibration qualities, for example its natural frequency of resonance should not interfere with the audio frequencies to be reproduced by the electrostatic loudspeaker. The electrically conducting material can include electrically conducting plastics or ceramics as well as metals. 
     The present invention also includes stators as such which may be manufactured by any of the above described methods. 
     The above described methods have several advantages, including ease of creation of complex shapes which can have advantages in terms of sound reproduction, ease of high volume production, lower unit cost of manufacture compared to traditional methods, high degree of accurate repeatability irrespective of the quantity of stators produced, high dimensional accuracy allowing consistency of sound reproduction between speakers. Additional advantages include the ability to manufacture composite structures using two or more different materials and the maintenance of close dimensional tolerances inherent in moulding processes allows reduction of stator to diaphragm air gaps, resulting in improved efficiency of sound reproduction. Also a wide range of possible shapes can be created to improve aesthetics and nodes can be incorporated in the design, eliminating the need for manual insertion. With some stator designs, a complete stator can be created in one manufacturing operation. The stators can be fully self supporting, that is, there is no need for an external support structure, and they can be formed with a high level of electrical insulation between the electrically conducting stator elements and the external environment without any separate interposing insulation. This provides a high degree of inherent electrical safety, that is, it is safe for human contact. 
     For a better understanding of the present invention, various 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. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  schematically illustrates a typical electrostatic loudspeaker system. 
         FIG. 2  is a schematic elevational front view of an electrostatic loudspeaker. 
         FIG. 3  is a cross sectional view on section line III-III of  FIG. 2 . 
         FIGS. 4A ,  4 B and  4 C illustrate an embodiment of a stator manufactured according to the first aspect of the invention. 
         FIGS. 5A ,  5 B and  5 C illustrate another embodiment of a stator manufactured according to the first aspect of the invention. 
         FIGS. 6A and 6B  are schematic cross sectional views illustrating a third embodiment of a stator manufactured according to the first aspect of the invention. 
         FIGS. 7A and 7B  are schematic cross sectional views illustrating an embodiment of an electrostatic loudspeaker construction employing a stator embodiment manufactured according to the second aspect of the invention. 
         FIGS. 8A and 8B  are schematic cross sectional views illustrating an embodiment of an electrostatic loudspeaker construction employing a stator embodiment manufactured according to the second aspect of the present invention. 
         FIG. 9A  is an illustration of portion of a structure for forming a stator which is moulded according to the first aspect of the invention. 
         FIG. 9B  is a cross sectional view (to a larger scale) of the  FIG. 9A  structure on section line IX-IX. 
     
    
    
     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. 
     The schematic electrostatic loudspeaker system of  FIG. 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 diaphragm  26 . Each stator  24  comprises an insulating peripheral frame  28  (see  FIGS. 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 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 . 
     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  FIG. 1 ). The resistor  25  is needed for constant charge and electrical safety. Circuit arrangements  22  other than as illustrated by  FIG. 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  FIG. 3 ). Electrical connection to the electrically conductive grids  32  of each stator  24  may be via a terminal  46  (see  FIG. 2 ). 
     The diaphragm  26  may be formed from a thin (for example 2-12 microns) film of a material such as Mylar™ 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 the frame  28  of one of the stators  24  for example by an adhesive. The attachment is such that it does not creep with time so that the tension in the diaphragm is maintained. 
     Electrostatic loudspeakers can be of enormously varied sizes and rectilinear shapes, for example small square shapes say 50 mm×50 mm to large rectangular shapes say 3000 mm×600 mm, or 3000 mm×1200 mm, or larger. 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 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. 
       FIG. 4A  schematically illustrates a stator  24  in elevation view,  FIG. 4B  is a schematic cross section (not to scale) on section line IV-IV of  FIG. 4A , and  FIG. 4C  shows a detail of the  FIG. 4B  representation. 
     With reference to  FIGS. 4A-C , a stator  24  is shown which is 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 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, 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 frame  28  (see  FIG. 4C  which illustrates only one end as indicated by the broken line circle on  FIG. 4B ). 
     The steel rods  29  may be about 2 mm 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. 
     Also, in the first and second aspects of the invention, 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. 
       FIG. 5A  schematically illustrates a stator in elevation view,  FIG. 5B  is a schematic cross section on section line IV-IV of  FIG. 5A , and  FIG. 5C  shows a detail of the  FIG. 5B  representation. 
     With reference to  FIGS. 5A-C , instead of a precoated preformed grid  32  being provided in a moulding die, a non-coated preformed grid  32  may be provided (shown by dashed lines in  FIG. 5A ). In this embodiment, the moulding, which is preferably injection moulding, provides the insulating frame  28  plus an insulating covering  50  for the stator elements  30 . Persons skilled in the art will routinely be able to construct suitable moulding dies for the embodiment of  FIGS. 5A-C . 
       FIGS. 6A and 6B  illustrate schematic cross sectional views of an electrostatic loudspeaker  20  wherein each of its stators  24  is manufactured by first moulding a complete grid structure  32  including its peripheral frame  28  from an electrically insulating plastics material and then combining the electrically conducting portions with the complete grid structure by applying an electrically conductive layer  54  to parts of the complete grid structure.  FIG. 6B  (I) illustrates that the electrically conductive layer  54  may cover the whole surface of each stator element  30  or alternatively, as indicated by  FIG. 6B  (II), only a surface portion  56  of each stator element  30  adjacent to the diaphragm  26 . The electrically conductive coating  54 ,  56  may be applied by spraying, dipping or brushing. Persons skilled in the art will routinely be able to construct suitable moulding dies for the embodiments of  FIGS. 6A and 6B . 
       FIGS. 7A and 7B  illustrate an alternative to  FIGS. 6A and 6B  wherein a stator  24  is manufactured by first 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 loudspeaker  20  may be assembled by including electrically insulating spacers  60  between the frames  28  (see  FIG. 7B ) 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 voltage  42  to it. The electrically insulating coating  58  may be applied by spraying, brushing or dipping. 
       FIGS. 8A and 8B  illustrate stators  24  in an electrostatic loudspeaker  20  formed via a multi-stage moulding process using a single mould wherein the frame  28  of a stator  24  is moulded first and the conductive stator elements  30  are moulded next, or vise versa. The respective materials of each moulding step will have sufficiently different melting points to avoid remelt of the first moulded material by the second moulded material, as will be known by persons skilled in the art. Such person will also routinely be able to design appropriate moulding dies for performing such a multi-stage moulding method. 
       FIGS. 9A and 9B  illustrate a portion of a structure  82  which is moulded from a plastics material according to the first step of the method of the first aspect of the invention. This structure  82  includes a peripheral stator frame  28  and intermediate cross struts  84  for imparting strength and rigidity to the stator. Each of the top  28   a  and bottom  28   b  portions of the frame  28  are formed to have a ledge  86 . The ledges  86  and the cross struts  84  are each formed with attachment means in the form of grooves  88  (best seen in  FIG. 9B ) which are aligned along the structure  82 . A preformed electrically conductive grid (not shown in  FIG. 9 , but which may be similar to the grid  32  illustrated in  FIG. 4A  without the intermediate bridging rods or wires  31 ) is, according to the second step of the method of the first aspect of the invention, combined with the structure  82  by force fitting (lightly) the grid  32  into the grooves  88 . That is, the grooves  88  are sized relative to the diameter of the stator elements  30  of a grid  32  such that the stator elements  30  will be an interference fit within the grooves  88 , and the grid  32  is then press fitted into the grooves  88 . The fitting together of the electrically conductive grid  32  and the frame  28  must not cause relative distortion between the grid and the frame. Preferably the electrically conductive grid  32  is coated with an electrically insulating material before it is combined with the structure  82  (which can alleviate any relative distortion between the grid and the frame that might otherwise occur), however it may be so coated after the two components are combined. The bottom surfaces  92  of the heightwise side edges of the frame  28  may include a longitudinal groove  90  (see  FIG. 96 ) for receiving a clamp that bridges two stators to hold the stators together in facing relationship with a diaphragm between them to form an electrostatic loudspeaker. Persons skilled in the art will routinely be able to manufacture moulding dies suitable for forming the  FIGS. 9A and 9B  embodiment. 
     As alternatives to the press fitting of the grid  32  into the grooves  88 , other fixation methods such as gluing, that is adhesively joining them (which can also alleviate any tendency towards relative deformation to occur between the grid and the frame), or other suitable fixation means, may be used. 
     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.