WO2014176657A1

WO2014176657A1 - Electrostatic loudspeaker, tools and methods for making same

Info

Publication number: WO2014176657A1
Application number: PCT/CA2013/000436
Authority: WO
Inventors: Murray Ronald Harman
Original assignee: Murray Ronald Harman
Priority date: 2013-05-01
Filing date: 2013-05-01
Publication date: 2014-11-06

Abstract

Traditionally, the edges of untreated, punched perforated sheet metal are coated with an insulative coating for use as stator panels for electrostatic loudspeakers; however, such panels have limitations on withstanding high voltage, which causes various performance impediments. Rounding the edges of perforations on the punched perforated sheet metal has been found to be effective in improving one or more of such performance limitations. Unfortunately, the process for rounding edges is time consuming and very expensive. The present invention provides an improved or alternative solution for rounding the edges of perforations on perforated sheet metals. It further provides tools and methods for producing multi-directional or omni-directional electrostatic speakers using such treated perforated metals.

Description

ELECTROSTATIC LOUDSPEAKER, TOOLS AND METHODS FOR MAKING

SAME

FIELD OF THE INVENTION

The present invention relates to electrostatic loudspeakers. Specifically, it relates to a method and apparatus for treating perforated sheet metal for electrostatic speakers and a method and apparatus for making electrostatic loudspeakers.

BACKGROUND OF THE INVENTION

The following is the list of references discussed in the present application:

U.S. Pat. No. 2,419,862 to G.F.Wales ("G.F.Wales")

U.S. Pat. No. 2,752,982 to R.A. Lalli ("R.A. Lalli")

U.S. Pat. No. 2,952,294 to G.E.Beverley et al. ("G.E.Beverley et al.")

U.S. Pat. No. 3,299,688 to L.R. Gray ("L.R. Gray")

U.S. Pat. No. 3,459,026 to J.W. Allen et al. ("J.W. Allen et al.")

U.S. Pat. No. 3,935,397 to West ("West")

U.S. Pat. No. 4,068,366 to Hillesheim ("Hillesheim")

U.S. Pat. No. 4,248,075 to Whitley ("Whitley")

U.S. Pat. No. 4,289,936 to Civitello

U.S. Pat. No. 4,430,784 to Brooks et al.

U.S. Pat. No. 4,463,055 to Hodges

U.S. Pat. No. 5,546,784 to Haas et al.

U.S. Pat. No. 5,816,093 to Takeuchi et al.

U.S. Pat. No. 5,851,334 to Shikata et al. ("Shikata '334")

U.S. Pat. No. 5,943,897 to Tsue et al.

U.S. Pat. No. 5,957,553 to Shikata et al. ('Shikata '553")

U.S. Pat. No. 6,018,970 to Ford et al.

U.S. Pat. No. 6,958,105 B2 to Herrmann et al.

U.S. Pat. No. 7,669,334 B2 to Bakhuis et al.

U.S. Pat. No. 7,972,129 B2 to O'Donoghue

U.S. Pat. No. 8,194,832 B2 to Harman Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of a common general knowledge in the field.

Punched perforated sheet metal has been traditionally used for stator panels in the fabrication of electrostatic loudspeakers (ESL). These punched perforated metal sheets have been widely accepted in the ESL industry as the most cost effective stator material; however, such stator panels have had some performance issues, including limitation in withstanding high voltage. These issues have driven the development over the last 50 years or more of other stator constructions. These include printed circuit boards with embedded conductors and routed slots for apertures as well as intricate space frame structures covered in insulated wires or insulated rods. These constructions have had little or no success.

It has also been observed that, although electrostatic loudspeakers have been considered by many audiophiles to be the premier transducer technology with the highest fidelity, electrostatic speakers have failed to gain wide acceptance due in part to limitations in sound dispersion. A listener effectively needs to be able to "see" their reflection in a large ESL membrane surface to properly hear the higher frequency sounds. Present flat and curved panel ESL art cannot be used to implement practical speaker systems with wide dispersion and natural point source radiation.

Various methods for making perforated sheet metal for purposes other than the making of electrostatic speakers have been proposed in several prior art references.

For example, G.F. Wales teaches a method of and apparatus for punching holes in sheet materials using punch and die means and does not disclose or discuss a method of rounding the sharp edges of the holes produced by said punching operation, as would be preferred in the manufacture of electrostatic loudspeaker stator panels.

R.A. Lalli discusses a device used to form metal sheets into complex shapes such as these used as covering materials for an airplane. A prescribed shape is imparted to a metal sheet as it is pulled down over a fixed hollow die.

West teaches an electrostatic loudspeaker element with an improved wire grid electrode assembly wherein stator panels are constructed by wrapping insulated electrical wire around flat supporting frames with cut-outs to allow sound to pass through. West teaches the construction of flat discrete loudspeaker elements and mentions assembling multiple flat elements into a larger faceted curved array to increase the angle of sound dispersion.

Hillesheim teaches a method and apparatus for producing openings in sheet metal without the lost waste material that is produced by a typical perforating punch press. The method in summary involves imparting a pattern of thinned regions or divots into a sheet with a roller die which can be pulled open during a subsequent stretching operation. Although Hillesheim describes a method of making an aperture there is no mention about edge quality or smoothness of the aperture edges. In fact Figure 8 shows a torn edge within the resulting apertures that would not be ideally suited for obtaining an even insulation coating thickness if said apertures were insulated in order to make a stator panel for an electrostatic loudspeaker.

Whitley teaches a method of rounding the edges of an aperture in sheet material by using a pair of opposed coining dies that impart a rounded edge through metal deformation. The application is for the manufacture of a metal circuit board substrate with improved high voltage insulation properties due to the more uniform coating thickness above the resulting rounding aperture edge. Whitely discusses methods to smooth a singular aperture. Due to the distortion of the material and resulting sheet to die run-out, it is doubtful that such a coining method could be applied in a practical manner on a mass scale with a line or area array of multiple dies to fabricate a practical sized ESL stator panel.

J.W. Allen et al. teaches an apparatus used for forming a cellular core panel from pre-slit sheet metal. In use, a flat sheet with apertures is modified to become a 3- dimensional form with increased section thickness. Such core panels are used in the manufacture of hollow stiffened panel assemblies such as a honeycomb type structure. Although Allen teaches a method of modifying an aperture in shaping the sheet into a core, no mention is made of modifying the edge radius by a means or method of smoothing said aperture edge.

Furthermore, none of these methods have been used for producing stator panels for electrostatic loudspeakers. Forming or curving stator panels for electrostatic speakers have not been considered previously, due to various complications that arise during the process of making such speakers. However, in various other technology areas, several methods for forming sheet metals or perforated sheet metals have been proposed.

For example, Brooks et al. gives a manufacturing process for forming orifices in a metal sheet suitable for use in an ink jet printing system. Said metal sheet has a number of grooves and openings constructed by the displacement of metal; however, Brooks does not discuss or require a method of rounding an aperture edge as taught in the present invention.

Takeuchi et al. teaches a method and tool for forming a tapered hole with a compound punch that contains a piercing punch to make the initial hole and a second tapered section that engages the hole to make an enlarged tapered section. It is possible in theory to replace the tapered section in the Takeuchi tool as denoted by "T" with a rounded edge to form a radius on the hole edge. It is not however practical to adapt commercial perforating presses and required die tooling in order to create a line array of dies the width of a practical sheet. It would also be extremely difficult to adapt the tooling of a typical perforating press to create a rounded edge on the second side of the sheet. With reference to FIG. 4 of Takeuchi, where sheet 3 contacts the striker plate 2, protrusions would have to be formed on the surface of the striker plate 2 which is typically ground flat. Another consideration is that the Takeuchi method of shaping a hole would undoubtedly generate significant accumulated distortion in a larger sheet and would become an issue if the percentage of the sheet occupied by holes were to become too high, such as in the case of perforated metal suitable for an ESL stator panel.

Tsue et al. teaches a method for making a hole in a plate with a shaped edge geometry using a shaped punch and die set. The edge shapes depicted include rectangular counter-bores, conical chamfers, spherical indentations and rounded edges on one side of a punched hole. Tsue does not describe a method of forming a smoothed edge on both sides of a hole. Such a punching method would result in significant sheet distortion similar to the method of Techeuchi et al., and thus, it would not be suitable for making perforated sheet metal for the purpose of constructing electrostatic loudspeakers. Shikata '334 describes a method of producing a casing for audiovisual equipment where a loudspeaker sound transmission hole area is integrated into the primary plastic casing in order to make the outer appearance more uniform. In the preferred embodiments, an array of fine holes is punched directly into the casing. Shikita '553 further describes audiovisual equipment and the method of producing a perforated casing as an extension to Shikita '334. However, these casings as proposed are not suitable for constructing electrostatic loudspeakers.

Furthermore, while various sheet metal forming techniques were proposed for various other applications, forming the perforated sheet metal to make, for example, curved panels, has not been considered previously. Previously, to create "curved" electrostatic loudspeakers, Civitello proposed construction of a flat triangular electrostatic loudspeaker element to be arranged in a geodesic skeletal structure so as to approximate curvature in a horizontal and vertical direction for improved dispersion of sound. The assembly as taught is in principal capable of approximating the radiation of sound from a point source. In practice the membrane surface should not be allowed to deviate more than one-quarter of a wavelength from the ideal wave- front shape, which is purported to be spherical in nature as generated by the structure as shown in FIG 4B of Civitello. This geometric requirement dictates that cells must be proportionately small in relation to the overall solid angle so as to enable uniform intensity reproduction of sound up to twenty- thousand Hertz over the projected solid angle. The subject of this current application teaches a fabrication method that can be used to form single perforated stator panels into large swept angles up to 90 degrees, 180 degrees and possibly 360 degrees, and that could also incorporate curvature about a second axis. Civitello would require a large number of triangular elements with a suitable support frame to produce, for example, spherical, ball or barrel shaped speakers.

Forming of sheet metal has been proposed in various references for other applications. For example, G. E. Beverley et al. presents a method of reducing friction between a rigid moving die and an engaged sample of sheet metal during deformation by introducing an intermediate layer of ice wherein a melting surface layer provides lubrication between the die and said metal surface. L.R. Gray teaches apparatus and method for stretch-forming sheet metal. What is of interest is that Gray mentions the use of a fixed engagement die comprised of discretely adjustable sections. The sheet is clamped at the edge margin with multiple moving clamps and pulled down over the engagement die to impart curvature into said sheet, including double curvature, which is referred to as a concavo-convex shape.

Haas et al. teaches an adjustable form die comprising a die engagement surface made up of a matrix of height settable pins, with the height of said pins being computer controllable by mechanical and motorized means. Although Hass teaches a die set designed to form complex surfaces, including multiple axes of curvature, the die surfaces are not movable in a radial direction. Although the Haas form die could likely be applied to deforming perforated sheets into useful stators panels, such use is not anticipated. As such the Haas form die is not optimized for producing deformed shapes subtending large swept angles such as described in the present invention.

On the other hand, Hodges teaches a method of applying a reflective metal film to a polymeric support carrier to create a laminate that can be substantially deformed and stretched to fit complex shapes. The film's intended use is the reflection of light and radio frequency waves, and it has a thickness of the order of 0.001 to 0.010 inch. Hodges discusses the use of vacuum pressure to deform the laminate onto a prescribed form or support surface, held in place with a pressure sensitive adhesive. The present invention also describes a method of utilizing air pressure, either positive or negative, to deform a membrane to form an electrostatic loudspeaker membrane not anticipated by Hodges.

Ford et al. describes a stretch-forming machine with servo-controlled curved jaws. The opposed clamping jaws are comprised of individually controllable segments. This allows the jaws to grip the opposing edges of a section of planar sheet metal to impart a shape to the clamped boundaries that best matches the shape of the form over which the sheet metal is to be stretched down over. Said form could include a double-axis curved shape similar to the ESL stator panel as described in the present invention. Ford describes in detail the complex mechanism required to provide the servo-controlled hydraulically moveable segmented clamping jaws but does not teach about the form over which the sheet metal is pulled. It is only the form as referred to by Ford that is of interest in the present invention. Herrmann et al. teaches a method to fabricate a fiber-reinforced composite structural component using a forming jig. A barrier film is held onto the forming jig using vacuum and various process steps including the application of fiber skin layers and resin injection. This application is carried out in order to create a desired composite shape after subsequent curing. Although the method and jig described by Herrmann could be used to create a double-curved shape similar to a stator, as described in this present invention, the method used to form the shape and the materials used are fundamentally different.

Bakhuis et al. teaches a system and method for forming a blade-section as used in the construction of a wind-turbine rotor assembly. The forming structure is shown as a non-moving curved surface.

O'Donoghue describes a compound tooling system for moulding applications, and although the system in its preferred embodiment could be used to create plastic parts of double-curvature, it is unlikely that O'Donoghue anticipated forming a stator panel or thin acoustic membrane for use in an electrostatic loudspeaker assembly as described in the present invention.

Harman teaches a stator panel having rounded edges to enhance the application of insulative coatings. Described in the body text of the application are material finishing methods used to dull the sharp edges of punched apertures in perforated metal sheet. These methods include controlled impact with shot media and the use of known commercial vibratory finishing media. Vibratory means of finishing are generally slow, with cycle times of the order of hours, and they rely on consumable finishing media that dulls over time and must be replenished. The present invention teaches a permanent roll- forming tooling operable with a cycle time of a few seconds and capable of forming consistent aperture edges.

A need exists for the production of a perforated sheet metal that would have a much higher voltage withstanding limitation than exists currently in the loudspeaker industry. There also exists a need for much wider sound dispersion by an electrostatic loudspeaker and the method and tools to construct the same. SUMMARY OF THE INVENTION

It is an object of the present invention to provide methods and apparatus useful in the manufacture of electrostatic loudspeaker (or "ESL") panel assemblies with improved dispersion of sound as compared to a traditional planer or a partially cylindrical ESL panel, which are presently used in the loudspeaker industry. The present invention presents improved or alternative methods and apparatus for the construction of ESL panel assemblies being shaped with curvatures in two directions (horizontal and vertical directions).

It is another object of the present invention to provide practical means to de-sharpen and insulate a piece of perforated sheet metal containing upwards of 50,000 hole edges.

It is another object of the present invention to provide a method(s) and apparatus for rounding the edges of the apertures within perforated metal sheet to facilitate fabrication of high voltage withstanding insulated ESL stator panels.

It is a further object of the present invention to present a practical or alternative method and apparatus to form stator panels and required acoustic membranes by means of a novel, radial expanding method that enables the construction of optimally aligned ESL panels with double-curved construction.

It is yet a further object of the present invention to provide improved high-voltage performance for ESL stator as well as tools to form the key components of curved ESL panels including double-curved shapes.

It is a further object of the present invention to provide a method(s) and apparatus for forming said stator panels with single curvature, and preferably with double-curvature or curvature in two distinct directions, in order to combine many individual ESL facets into a single assembly.

It is a yet further object of the present invention to provide a method(s) and apparatus to enable the forming of an acoustic membrane suitable for mounting onto a dielectric spacer panel affixed to a single or preferably double-curved stator.

According to one aspect of the present invention, it provides a method and apparatus to round the edges of the apertures in perforated metal material prior to the application of insulative coatings. In another aspect of the present invention, it provides a bar member or rod for roll- forming as compared to traditional hydraulic forming presses and is useful to form a smooth edge on an aperture where a small force of typically less than one pound can be applied remotely to the end of the rod. The rod, in accordance with the present invention, provides significant leverage due to its length as compared to the spacing of the forming surfaces. Secondly, only a small portion of the edge of the aperture is engaged by the forming surfaces of the rod. In punching methods as taught by Wales, Whitley, Takeuchi and Tsue, hydraulic presses are fitted with single-aperture die tooling, which engage the full perimeter of the aperture. The deformation is carried out over a very short stroke length of a hydraulic ram. Forces of the order of 500 to 2000 pounds per aperture are generally required to pierce sheet metal or subsequently engage die set features to form smoothed edges. As taught, the roll-forming rod requires approximately 1000 times less force to round an aperture edge than direct compression with hydraulic operated die tooling.

According to yet another aspect of the present invention, it provides a press with radially expanding contacting spars and provides a method and apparatus useful in stretch-forming an ESL stator panel with single or preferably double-curvature. The press is generally configured so as to accept a perforated sheet clamped along opposite edges. The press then engages the sheet forcibly by means of contact with multiple curved spars that move in an outward radial manner. By stopping the expansion of the spars at a desired radial position, it is thereby possible to form inner and outer curved ESL stator panels as would be required to manufacture a curved ESL panel assembly. It may also be preferable to utilize spars with differing contact radii or to impart differing rates of radial advancement. It may also be preferable for the contacting face of a spar to be made up of several flattened sections to create an approximated axis of curvature as disclosed by Harman. The term curved including cylindrical forms with a single axis of curvature and double-curved forms (for example, spherical forms or similar) having a second axis of curvature. One specific requirement in forming the stator panels is that the inner and outer stator panels have geometry suitable so as to be able to accurately nest within each other and, in doing so, provide uniform clamping of the dielectric spacers (generally in strip form) and proximally located acoustic membrane. Concerning prior art, Lalli, Beverley, Gray, Ford and Bakhuis teach devices useful for stretch-forming sheet metal over fixed forms that can include a double curved shape. With these apparatus, it would be necessary to create separate fixed forms for shaping a respective inner and outer stator panel, which would make it difficult to ensure accurate nesting of the layers of the ESL assembly over the entire surface area of a double-curved ESL assembly. A non-structural benefit of the present invention is that respective inner and outer panels are formed in a radially expansive manner, which can afford a common alignment of the holes between inner and outer stator panels. This formation gives uniform light transmission through the holes for a uniform visual appearance. Hass et al. teaches an adjustable form die that could conceivably be used to form double curved panels that could nest accurately if the die shape were so adjusted. However, Hass claims a self-adjusting die with a grid array of pins independently moveable along a single axis. As a result the Haas device would likely only be applicable in making ESL stator panels with small swept angles up to approximately 45 degrees in one or both directions.

In according to yet another aspect of the present invention, it provides a vacuum- forming method using air pressure to enable forming an acoustic membrane with double curvature as well as controllably affixing said membrane to a dielectric space panel affixed to a stator panel. No prior art was found relating to a method or apparatus useful in forming an acoustic membrane with single or double curvature. Hodges discussed a method of bonding reflective foil to a polymeric carrier useful in reflecting light and radio waves wherein the laminated carrier could be deformed into complex shapes using a vacuum deformation method and suitable support forms to impart and maintain a desired shape. Herrmann et al. discussed the use of a vacuum to pull various film layers onto an assembly support to enable the fabrication of an integral structural component made of a fibre reinforced composite material, for example a section of an aircraft fuselage. O'Donoghue teaches a method of using vacuum to form plastic substrate within a mould cavity. In the present invention, a thin acoustic membrane of the order of two to twenty microns thickness is affixed to a support frame. Said frame is then flexibly installed into a curved vacuum box so as to enable deformation of the acoustic membrane into a shape having a double curvature. In a typical application of the present invention, an excess of vacuum pressure would be applied within the vacuum box, so as to deform the acoustic membrane an additional amount so as to allow the positioning of said double-curved stator panel with affixed dielectric spacer panel such that the fragile membrane is not contacted in any detrimental way. A suitable adhesive is applied to the surface of the dielectric spacer panel so as to affix the acoustic membrane, either on contact or after a suitable curing cycle depending on the type of adhesive used. Next, a reduction in level of vacuum would allow said acoustic membrane to controllably collapse and make full contact with the corresponding surface of said dielectric spacer panel in order to enable permanently affixing the acoustic membrane by said adhesive to the dielectric spacer panel. Once the adhesive engages the acoustic membrane, the vacuum pressure is then released and the completed double-curved stator panel inner assembly is then removed by trimming excess membrane from the dielectric spacer panel.

It should be noted that the term vacuum refers to a negative relative change in pressure as used to deform a membrane as describe in the present invention. It would also be equally functional to mount said acoustic membrane supporting frame to an outside surface of a similar closed curved box and then apply a positive air pressure within said box to similarly deform the membrane in an outward direction. In such an implementation, the double curved stator would have to be pre-existing within the box and manipulated with remote mechanical means. The term box, here, is intended to denote an air-tight or near air tight closed shape of non-specific form, capable of sustaining an applied pressure difference with respect to the ambient environment surrounding said box.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an electrostatic loudspeaker panel assembly, shown in a flat planar form in accordance with the present invention.

FIG. 2 is an exploded view of the electrostatic loudspeaker panel assembly of FIG.

1.

FIG. 3a shows a section view of prior art depicting perforated sheet metal with sharp edges on the apertures with an overall insulative coating.

FIG. 3b shows a section view of prior art depicting perforated sheet material similar to FIG. 3a except that the perforated sheet material is thinner with larger apertures.

FIG. 4 shows a schematic representation of a chamfering operation in accordance with prior art wherein a pair of opposed conical dies impart chamfered edges on both sides of a sample of perforated sheet material.

FIG. 5 is a cross-section view of the sample of the prior art perforated sheet material shown in FIG. 4.

FIG. 6 is a perspective view of a plain rod of the present invention engaging a perforated sheet material in a manner herein referred to as roll-forming, wherein the method is used to form a chamfered edge on both sides of apertures.

FIG. 7 shows a rod with notch features engaging a sample of perforated sheet material in a manner herein referred as roll-forming, wherein the method is used to form an approximated round edge on both sides of apertures.

FIG. 8 shows a rod with smooth radius features engaging a sample of perforated sheet material in a manner herein referred as roll-forming, wherein the method is used to form a smooth radius edge on both sides of apertures.

FIG. 9 shows a cross-section of the perforated sample and a side view of a roll- forming bar member or rod with smooth radius features engaging the sample from FIG. 8.

FIG. 10 shows a cross-section with cut-away view of an insulated sample of perforated metal with aperture edges having been smoothed in accordance to the method shown in FIG. 8. FIG. 11 shows a drill with shaped edge engaging a sample of perforated sheet material in a manner herein referred to as machine forming, wherein the method is used to form an approximated round edge on an aperture.

FIG. 12 shows a partial view of a press or stretch- forming apparatus intended for stretch- forming perforated sheet material in a radially expanding manner.

FIG. 13a is a partial view of FIG. 12 with radially moveable spars shown in inward rest positions suitable for loading perforated sheet material.

FIG. 13b is a partial view of another press for stretch-forming perforated sheet material in a radially expanding manner, while the press is in retracted position.

FIG. 14 is a partial view of FIG. 13b with radially moveable spars shown in outward positions, representing engagement and stretch-forming of perforated sheet metal.

FIG. 15a shows a stator panel having double-curvature on a stretch- forming apparatus, formed in accordance with the method of the present invention.

FIG. 15b shows a 360 degree stretch-form apparatus for stretch-forming perforated sheet metal tube.

FIG. 16 shows a stator panel having double-curvature, formed in accordance with the method of the present invention with a dielectric spacer panel mounted.

FIG. 17 shows a support frame with mounted membrane, which will be bent in a curved shape so as to form a proximal sealing surface.

FIG. 18 shows the support frame and mounted membrane of FIG. 17 as bent in a curved manner and positioned adjacent to a cut-away view of a vacuum fixture.

FIG. 19 shows the curved support frame and mounted membrane positioned within the vacuum enclosure in a loading position according to FIG. 18, with the membrane being deflected by the application of a partial vacuum within the represented closed box.

FIG. 20 shows the stator panel and dielectric spacer panel as depicted in FIG. 16 located within the assembly as shown in FIG. 19 with the acoustic membrane positioned so to be in proximal location to and not in contact with the adjacent dielectric spacer.

FIG. 21 shows a perspective view of the double-curved stator panel made in accordance with a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a flat electrostatic loudspeaker panel 1 made in accordance with the present invention. The flat electrostatic loudspeaker panel 1 is hereafter referred to as an ESL panel.

FIG. 2 gives an exploded view of the ESL panel 1 of FIG. 1 showing five of the principal layers of a substantially flat push-pull ESL panel assembly. Included are inner and outer stator panels 4, 16. If an ESL panel were to be implemented with one axis cylindrical curvature or two axis double-curvature, the stator panel having the smaller average radius of curvature would be typically referred to as an inner stator panel. The term stator panel refers to insulated perforated sheet material with all exposed surfaces coated with a high-voltage withstanding insulative coating, such as a polyester or nylon powder-coated resin. Said sheet material, preferably metal, is covered with apertures sufficient to create an average of approximately 40 to 60 percent open area so as to enable unhindered transmission of sound through the stator panels. The assembly also includes inner and outer dielectric spacer panels 6, 14 containing a number of large openings 12. Although it may be desirable for the dielectric spacer panels to be constructed from a single sheet as shown in drawings, denoted as reference numerals 6, 14, (or 158 in FIG. 16), it is also quite acceptable to use individual strips of dielectric material similarly positioned and affixed so as to perform an identical function. Typically an insulative plastic material such as acrylic would be used for the dielectric spacer panels 6, 14 due to its excellent insulative properties. In the centre of the assembly, an acoustic membrane (or diaphragm) 8 of the order of 2 to 20 microns thickness with a high resistivity coating 10 on one or both sides of the membrane surface, covers the surface over all openings extensively 12. In operation, a high voltage bias is applied to the coating 10 on the membrane 8, with respect to the stator panels in order to electrically charge the capacitance of the assembly. In a push-pull ESL, an audio input signal is applied differentially with opposite phase audio signals being applied to the conductive core of each respective stator panel 4, 16. In order to maximize the acoustic output of the panel, it is necessary to operate an ESL panel assembly at the highest possible voltage bias in order to maximize electrostatic forces present within the ESL assembly. When operating an ESL, the maximum allowable high-voltage bias is generally limited by the onset of corona discharge that occurs when the dielectric strength of air of approximately 3 million Volts per meter is reached near any surface feature of a stator panel. This is particularly the case at aperture edges, if they are not smoothed prior to coating. A corona discharge generally precedes an arc discharge and acts as an audible warning that bias voltage should be reduced. In most ESL assemblies an actual arc discharge can result in damage to the stator panels. Corona generally precedes an arc discharge and also produces a characteristic purple glow that is visible in dark conditions.

FIG. 3a is a section view of prior art stator panel 18 for electrostatic loudspeakers with a centre core comprised of perforated sheet metal 20 approximately 1.5mm thick with apertures of in the order of 3 to 4 mm diameter, with an insulative coating 22 extensively covering all surfaces. As illustrated the edges of the apertures 24 are typically sharp, but in practice a perforated panel used in the fabrication of an ESL stator panel would generally be subjected to a de-burring process to smooth the edges of the apertures. It should be noted that most commercial methods used to de-burr sheet metal are unable to create a significantly rounded aperture edges due to the limitations of the process of removing metal within the bore of the individual apertures. Shown is an insulative coating that tends to flow into a rounded natural shape 25 as shown in section view. It can be seen that the sharp edges of the apertures 24 will typically have a reduced thickness of coating at the edge of an aperture 26 as compared to thickness of coating at the bore of an aperture 28. Such a sharp feature with reduced coating serves to limit the maximum voltage breakdown of the stator panel 18 and also leads to early onset of audible corona discharge due to the resultant concentration of the electrostatic field at or near sharp aperture edges.

FIG. 3b shows a section view of prior art stator panel 30 for an electrostatic loudspeaker of a commercial stator construction with a centre core 32 of approximately 0.8 mm thickness with apertures in the order of 4 mm diameter. When the size of the bridging section of metal 34 is reduced, the variation in coating thickness is also reduced and having a rounded edge becomes less important. The down side to increasing hole size and reducing thickness as shown in FIG. 3b is that the electrostatic field will tend to be concentrated at the surface 31 of the reduced bridging sections of metal 34, leading to an earlier onset of audible corona. A preferred solution for maximum acoustic output would be to preserve a larger bridging section of metal and also have rounded aperture edges for enabling an even coating thickness, which is the subject matter of the present invention.

FIG. 4 depicts a hydraulic press setup 36, in accordance with prior art, engaging a sample of perforated sheet metal 38 with a central reference axis 40, including a vertically movable set of forming dies 42. These forming dies 42are moveable along a common axis 43 with conical shaped tip features 44. When the forming dies 42 close on either side of an aperture 46 in a controlled manner either by preset force F or preset displacement, a deformed region 48 results on either side of the respective aperture, similar in shape to the tip 44 of the forming die 42. However, such method is not desirable as the press would displace the edge portions around the apertures 46 of the sheet metal 38 outwardly from the apertures 46, which may cause unpredictable expansion or deformation of the sheet metal 38.

FIG. 5 shows a cross section view of the perforated sheet metal 38 at the line 40 shown in FIG. 4. This shows the resultant shape of the bridging section 52 between adjacent apertures 53, made up of the deformed regions 48 on each side of an aperture which form a conical or chamfered shape similar to the tip feature 44 of the forming die 42 as shown in FIG. 4. However, the pressure applied through the die 42 on the perforated sheet metal would displace the edge portion of the aperture to cause some deformations 51 on the surface around the treated apertures.

FIG. 6 shows an arrangement 54 for chamfering the edges of an aperture in perforated sheet metal 56 using a plane rod 58 using a roll-forming method according to one aspect of the present invention. In accordance with the present invention, the rod 58 is an elongated bar member, which is substantially cylindrical, made of a suitable material, for example, titanium, steel (or their alloys etc.), placed within an aperture 60 of the perforated sheet metal 56 and positioned off axis of the aperture 60 until it contacts edges of the aperture 60 on both sides of the sheet metal 56. A controlled force 62 is then applied in an outward radial direction while the rod 58 is rotated in a circular direction 64 by an actuator or like (not shown). It may be preferable to allow the rod 58 to rotate freely about its own central axis to minimize relative velocity between the rod surface and the surface of the metal it is engaging upon within a given aperture. In doing so, a roll-forming action is realized when deformed metal is not (or is minimally) smeared or deposited from the perforated sheet metal 56 to the surface of the rod 58. A view of the sheet metal 56 if sectioned in the YZ plane of axis 68 along a central line 66 would have a nearly identical appearance to the aperture 54 as shown in the section view of the perforated sheet metal 38 in FIG 5. The actuator (not shown) may actuate more than one rod 58 at one time for chamfering the corresponding number of apertures in a section, array or column. While it is not shown in the figure, it would not be difficult to have the rod 58 contact and chamfer only one of the edges of the aperture 60 at a time, by, for example, having the rod 58 pass through the aperture, having one end of the rod 58 remain stationery or nearly stationery at a center of the aperture (a distance away from the opposite surface of the perforated sheet), and applying the controlled force 68 at or near the corresponding end of the rod 58 to only engage the edge on the corresponding surface. The controlled force 62 could also be applied not only to one end of the rod 58 as shown in the present figure, but could also be applied to both ends of the rod 58.

FIG. 7 shows another arrangement 70 for chamfering the edges of an aperture in a perforated sheet metal 78, which is similar to FIG. 6 except that the plain rod 58 has been replaced with a different style of a rod 72 having grooved features 74 that are intended to engage and align with the edges of the apertures 76. A controlled force 62 is applied in an outward radial direction while the rod 72 is rotating in a circular direction 64. As the result, a view of the sheet metal 78 if sectioned in the YZ plane of axis 68 along a central line 66, the chamfered edges of the apertures 76 would, for example, correspond to the groove feature 74. In this embodiment, the grooved feature 74 comprises straight lines on the longitudinal cross-section of the rod 72 to a bottom where the diameter of the rod 72 is the narrowest. By selecting the geometry of the grooved features 74 accordingly, it is possible to form an approximately rounded edge 78 comprised of a number of approximated surfaces (e.g. having additional bottoms in the groove features) that are smoother and more rounded than the simple chamfered edge 48 (as shown in Fig. 5) that would result from the apparatus and method shown in FIG. 6.

FIG. 8 shows yet another arrangement 80 for chamfering the edges of an aperture in a perforated sheet metal 88, which is almost identical to FIG. 6 and FIG. 7 except that plain rod 58 and grooved rod 72, respectively, have been replaced with a yet another rod 81 having smoothly rounded features 82 that are intended to engage and align with the edges 84 of the apertures 86 in the perforated metal sample 88. A controlled force 62 is applied in an outward radial direction while the rod 81 is rotating in a circular direction 64. As the result, a view of the sheet metal 88 if sectioned in the YZ plane of axis 68 along a central line 66, the chamfered edges of the apertures 86 would, for example, correspond to the groove features 82. By selecting the geometry of the rounded features 82 accordingly, the aperture edges 84 can be formed to be smooth, preferably rounded 90 and of comparable surface quality to that obtained with conventional metal removal type milling cutters. In another preferred embodiment of the present invention, according to FIG. 8, a force represented by an arrow 62, which leveraged forces in turn, would act on the edges. In a preferred embodiment of the present invention, the force represented by the arrow 62 would be of the order of 2 to 5 Newtons, and acts on the rod 81 of nominal 15cm length. The amount of the force to be applied by the arrow 62 may depend on various factors and can be applied with more force to round the edge by 1 revolution of the rod 81 (or 58 in FIG. 6, 72 in FIG. 7) around the aperture of the sheet metal. In a preferred embodiment of the present invention, a smooth continuously rounded edge can be realized with approximately 4 to 10 revolutions of the rod 81 around the aperture of the sheet metal, about the Z Axis 68. A comparable die forming method as shown in FIG. 4 (where the entire aperture edge 48 is formed in one short stroke operation of a hydraulic ram mechanism) can require a force F of the order of 2000 to 5000 Newtons. This is approximately 1000 times more applied force than required with the present invention.

FIG. 9 shows a section view 92 of the perforated sheet metal 88 as sectioned along a central line 66 in the YZ plane 68 from FIG. 8. The rod 81 having smoothly rounded upper and lower features 82 that engage and form the edges of the apertures 86 by plastic deformation of metal. Specifically the upper and lower smoothly rounded features 82 preferably include upper and lower concave surfaces 94 and adjacent upper and lower convex surfaces 96, 98. Convex surfaces 96, 98 are included to reform excess metal displaced by the engagement of the concave surface 94 so as to minimize any burr that may be formed inside the resulting bore of the smoothed apertures. A sectioning line 100 indicates that not the entire length of the rod 81 is shown in the drawings and that it may be of arbitrary length. FIG. 10 shows a cross-section view 102 of a sample of perforated metal 104 having ideally rounded edges 106 formed in accordance to this present invention and having an insulative coating 108 as is preferred for use in the construction of ESL stator panels. It is preferable to have an edge radius 106 that is approximately 25% or more of the thickness of the perforated material 104 to ensure a maximally uniform coating thickness near the aperture edges 110 where electrical field lines are at maximal intensity within an energized ESL panel assembly. It is also preferred to avoid any excess thickness of coating to minimize excess energy storage within the dielectric coating which detracts from useable acoustic output from the ESL for a given input audio signal input amplitude. Although it is preferred that the aperture edges be substantially rounded for maximal coverage by an insulative coating, other aperture edge geometries such as a chamfered or approximated round edge as taught in this present invention (or alternate edge shapes otherwise conceived), may be employed within the scope of this present invention as long as the resulting formed aperture geometry can be reasonably covered in an adequate manner by an insulative coating.

FIG. 11 shows a yet another arrangement 200 of the present invention for chamfering the edge of an aperture in a perforated sheet metal 206. A drill (or router) bit 201 having a cutting edge 202 is formed to engage with the aperture 212 for trimming or rounding the edge. According to one aspect of the present invention, the diameter of the drill bit 201 is slightly larger than that of the aperture, and the cutting edge 202 is configured and shaped to engage only the edge of the aperture so as to trim or round the edge of the aperture. Yet another aspect of the present invention is that the diameter of the drill bit 201 may be smaller than the diameter of the aperture of the perforated sheet metal (not shown), that the cutting edge 202 is configured and shaped to engage with a portion of the edge of the aperture and that the drill bit 201 is arranged to move along the edge of the aperture to chamfer (or round) the entire edge of the aperture.

FIG. 12 shows a perspective view of a stretch-forming apparatus 112 configured so as to stretch- form perforated sheet metal 114 in a radially expanding manner. The apparatus 112 includes clamping means 111 to rigidly fix the position of the opposing sides of sheet metal (represented by fixed vertical bars 116 and moveable bars 118) with threaded bolts 120 to represent a means of closure. The clamping means 111 can reasonably vary in form and can alternately be closed by hydraulic force without departing from the scope of the present invention. Contact with the sheet metal 114 is made with a number of spars 122, which extend radially outwardly from a retracted position to an extended position by an actuator 121. According to one aspect of the present invention, the actuator 121 comprises a ramp block 124 and a centre support pillar 126. The centre support pillar 126 provides a reactionary surface for the ramp block 124 to act against and a lower supporting plate 128 provides support for the apparatus 112. A similar upper supporting plate as well as a hydraulic working cylinder were not shown for clarity. If the perforated sheet metal 114 were in the form of a continuous cylinder and the spars were located uniformly around a complete circle, it would fabricate an omni-directional ESL with a cylindrical form having continuous dispersion of sound principally in a horizontal direction or with double curvature for increased dispersion of sound in a vertical direction.

FIG. 13a is a partial view of the stretch- forming apparatus 112 in a first (or retracted) position. Shown are an actuator or movable ramp assembly 121 comprising upper 124 and lower 132 ramp blocks, a centre support pillar 126 and a rigid connecting member 138. The upper ramp block 124 and lower ramp block 132 have a number of individual angled upper surfaces 134 and corresponding lower surfaces 136, respectively arranged in radial increments so as to controllably move a corresponding number of spars 122. Individual spars 122 are arranged in a radial manner and are guided with suitable linear bearings so as to move accurately inward or outward from a central axis without rotating when acted upon at angled interfaces 140 by the corresponding angled surfaces 134, 136 of the central movable ramp assembly as defined herein. Additional mechanical elements of said apparatus (such as the aforementioned linear bearings, return springs, main hydraulic cylinder and connecting linkages, fastening hardware and other items as would be known to someone with skill in the metal forming industry and knowledge of detailed stretch-forming press design) have not been included nor will they be discussed herein, as they do not form a critical part of the present invention.

FIG. 13b is a partial view of a stretch-forming apparatus 112b in accordance with another aspect of the present invention. The stretch-forming apparatus 112b comprises a movable ramp assembly 121b. The movable ramp assembly 121b comprises an upper 124b and lower 132b ramp block and a rigid connecting member 138 having a member, having number of individually angled upper surface 134a and 134b, and lower corresponding lower surfaces 136a and 136b. As it can be seen, the angle of the upper surface 134a and the corresponding lower surface 136a are different from the other upper surface 134b and the corresponding lower surface 136b, such that, when they engage with the corresponding angled interfaces 140, they provide a different rate, speed and amount of resulting movement between the spars 122a, 122b and the spars 122. This is done to improve yield, depending on the shape or curvature required.

FIG. 14 is another partial view of the stretch- forming apparatus 112b in a second (or extended) position, in which the movable ramp assembly 124b, 132b, 138 in a lowered position has been acted upon by a force 144 represented by an arrow. The ramp assembly 121b is moveable along the central pillar 126 and is constrained by linear bearing means that are not shown. The force 144 would typically be provided by hydraulic means such as a working cylinder and be of the order of 10 to 50 tonnes (more, or less) as required to form a sample of perforated sheet metal, for example, of approximately one-half metre dimension in accordance with the present invention. As the movable ramp assembly 121b is lowered the angled surfaces 134a, 134b, 136a and 136b engage the spars 122a, 122b and 122 at angled interfaces 140a, 140b causing the spars 122a, 122b and 122 to expand outwardly in a radial manner as indicated by movement arrows 146a and 146b. Other means of moving the individual spars 122a, 122b and 122 in radial outward directions in a controlled manner can also be envisioned (e.g. by means of individual hydraulic cylinders, lead-screw actuators, linear ball or roller bearings, etc.) without departing from the scope of the present invention. As indicated above, it may be preferable to move the individual spars at differing rates or speeds, distances, etc. For example, one or more of the outer most spars 122a, 122b near the clamps 116, 118 (not shown) may be moved at a slower rate to progressively engage the perforated sheet so as to minimize any tendency for the sheet to tear. In accordance with one aspect of the present invention, the working surfaces of the spars 148 are shown as being curved about a horizontal axis in order to impart a second direction of curvature to a one dimensional curved perforated metal panel after it is stretch-formed accordingly. It is not necessary that the working surface of the spars 148 be a continuous curve, and in some instances it is preferred for the working surface of the spars 148 to be made up of a number of smaller substantially flat conjoined surfaces which approximate a curve similar in overall shape to the curved surface as indicated 148.

The stretch-forming apparatus in accordance with the present invention can also be used to stretch-form cylinder-shaped ESL panels having one direction of approximated curvature without departing from the scope of the present invention. Other common methods than stretch-forming can be used to form sheet metal into multi-facet cylinders such as sheet metal folding with a manual or CNC press-break. It is however advantageous to maintain a continuous radial alignment of apertures between inner and outer stator panels for uniform sound transmission and visual appearance, which can be obtained with the stretch-forming method taught in this present invention. A cylinder shaped stretch-formed perforated sheet metal panel can be realized by using spars 122 and/or 122a, 122b, alternately fabricated to have working surfaces 148 that are vertically straight instead of curved as shown.

FIG. 15a shows the stretch-forming apparatus 112 (or 112b in FIG. 13b) in the second position with the spars 122 (or 122a, 122b in FIG. 13b) extended radially outwards as illustrated in FIG. 12 (or FIG. 13b) with their respective working surfaces 148 engaged with a perforated sheet metal 114 wherein the sheet 114 is being clamped by clamping means 111 by placing the ends of the sheet 114 between the fixed vertical bars 116 and moveable bars 118 and being fastened by the threaded bolts 120, and has been stretch- formed in accordance to the present invention. The working surfaces 148 of the spars 122 (or 122a, 122b in FIG. 13b) have imparted a second direction of curvature as defined by the shape of their respective working surfaces 148. If the working surfaces 148 of the spars 122 (or 122a, 122b in FIG. 13b) had been flat as opposed to curved then the resultant shape of the sheet 114 would have been a segmented cylindrical shape. An engagement region is illustrated by lines 152 where each spar 122 (or 122a, 122b in FIG. 13b) engages the sheet. In the embodiment of the apparatus 112 as shown, the spars 122 (or 122a, 122b in FIG. 13b) are shown spaced at even angular intervals; however even spacing is not essential in stretch-forming perforated metal used in making a stator panel. Uneven spacing may be beneficial in reducing the dominance of a single resonant frequency of an acoustic membrane if mounted thereon. FIG. 15b shows a perspective view of a 360 degree stretch- forming apparatus 212 for stretch-forming a perforated sheet metal tube 214. The 360 degree stretch-forming apparatus 212 has a lower supporting plate 228, a plurality of spars 222, surrounding an actuator 221 extending radially outward as shown by arrows 246 when actuated by the actuator 221, such that the spars 222 engage the inner surface of the perforated sheet metal tube 214 for stretch-forming. The spars 222 may be actuated at a suitable rate / speed and force, such that use of any fastener for fastening the perforated sheet metal tube 214 to the lower supporting plate 228 would not be required. In a preferred embodiment of the present invention, the actuator 221 has substantially similar features to those shown in FIGs. 13a, 13b and 14. For example, the actuator 221 has a ramp block 224 and a centre support pillar 226, and being acted upon by a force 244. The centre support pillar 226 provides a reactionary surface for the ramp block 224 to act against, and the lower supporting plate 228 provides support for the apparatus 212. For clarity, a similar upper supporting plate and a hydraulic working cylinder are not shown. In a preferred embodiment of the present invention, for forming a substantially circular cross- section, a sufficient number of the spars 222 may be arranged to cover the actuator 221 entirely as shown in Figure 15b and extended outwardly at the same speed, movement range and force. However, as would be readily understood by a person ordinary skilled in the pertinent art, the rate, movement range and force extended on each spar 222 can be different but may nonetheless achieve substantially similar results. In addition each spar 222 may be individually controlled for a certain circumstance(s). In this regard, the apparatus 212 may stretch form a portion or section of the tube 214, instead of the entire 360°. The perforated sheet metal tube 214 may be a seamless tube or tube of perforated sheet metal with ends secured appropriately for sufficient strength to sustain its connection through stretch-forming. The stretch-formed tube 214 may be used as the inner or outer stator panel for an ESL speaker. In a preferred embodiment of the present invention, the 360 degree stretch-forming apparatus 212 forms the outside stator panel for an ESL speaker, and the application of the membrane, spacer and inner stator panel would be done on a section by section basis. It is not necessary that the working surface of the spars 222 be a continuous curve, and in some instances it is preferred for the working surface of the spars 222 to be made up of a number of smaller substantially flat conjoined surfaces which approximate a curve similar in overall shape to the curved surface. The working surface of the spars 222 may further be flat through their longitudinal length. The spars 222 may be arranged, angled and/or shaped such that the apparatus 212 stretch- forms one opening of the tube 214 more widely than the other opening thereof. It is further to be noted that the apparatus 212 may be used to stretch form segmented cylinders for forming ESL speakers. Depending on the specific segment or section of the ESL, the profile and angles of the working surface of the spars 222 may be selected or adjusted accordingly.

It may be possible to form cross sectional shapes other than substantially circular shapes. For example, the rate, movement range, and/or force of each spar 222 may further be individually controlled for this purpose, or the number of spars 222 may be reduced to as few as two (2) for producing elongated rectangular or oval shapes by placing the spars 222, three (3) for forming triangular tube or like shapes, four (4) for forming a square or rectangular tube shape, etc.

FIG. 16 shows a part of an ESL panel assembly 154 in accordance with the present invention comprising two of the five principal layers as shown in FIG. 2, including a stator panel 156 and a dielectric spacer panel 158 affixed thereto with fastening means such as a permanent adhesive. The stator panel 156 as shown is taken from the resulting stretch-formed perforated sheet material (for example, the material 104 shown in FIG. 10), having been coated with a suitable insulative coating 108 as shown in FIG. 10. The assembly 154 will hereafter be referred to as an inner stator panel assembly. The assembly 154 as shown is comprised of an arbitrary number of sections 155 shown as six sections in the figure, each section 155 preferably located at a common radial distance 157 from a central axis 159.

FIG. 17 depicts a support frame 162 with an acoustic membrane 164 affixed to a main surface 166. An acoustic membrane 164 as shown is ideally mounted to the support frame 162 with an adhesive tape around its perimeter 168 having a sufficient hold so as to allow stretch-forming of the membrane 164 without movement of said perimeter 168. Vertical dashed visualization lines 170 represent a group of ink marks on surface of the membrane, merely in order to help visualize the relative positions of the surface of the membrane if it were to undergo deformation while mounted on said support frame 162. FIG. 18 illustrates an apparatus 172 for a stretch-forming process for shaping an acoustic membrane 164 in accordance with one aspect of the present invention. The support frame 162 (ideally flexible in at least one direction) and membrane 164 shown in FIG. 17 are shown in FIG. 18 in a predisposed curved shape. The support frame 162 can also be a curved rigid frame provided that application method for mounting the membrane 164 allows for curvature of the frame.

A sealable box 174 is shown to be of arbitrary cylindrical form with curved inner surfaces 176 suitable for maintaining the radius of curvature of the support frame 162. The sealable box 174 as illustrated has five principal surfaces, a top 178, a bottom 180, a first side 182, a second side 184 and an outer surface 186 shown as a partial cutaway. The sealable box 174 as presented can be made suitable for supporting air pressure when the support frame 162 with mounted acoustic membrane 164 are placed in tight contact with the sealable box 174, forming a sixth surface defined as an inner surface. The sealable box 174 also includes means for changing air pressure within the box, depicted as a tube 188 that passes through the box, which could be connected to a suitable vacuum pump (or positive pressure pump as required) to deflect the acoustic membrane 164.

FIG. 19 shows the membrane deflection apparatus 190 based on FIG. 18 with support frame 162 and acoustic membrane 164 as installed into the sealable box 174. Once installed, the support frame 162 and membrane 164 are conformed to the surface they are mounted on 176, which is generally, but not limited to, an arc preferably having a central axis 191. It is a general requirement for the surfaces of the sealable box to meet in closure with a tight enough seal to support a change in internal air pressure sufficient to deform the membrane. The term plastically deformed implies that if the deforming pressure were to be removed, a new shape would have been imparted to the membrane as compared to the original unstressed state. To do so, internal air pressure (not shown) sufficient to deform the membrane may be provided. In accordance with the present invention in its preferred embodiment, a partial vacuum is applied by the air pressure controller via tube 188 resulting in the acoustic membrane 164 being deflected in a radially expanding manner (or causing the acoustic membrane 164 to move from a first (or released) position to a second (or deflected) position) as represented by the now curved visualization lines 192. Various other means or methods for controlling air pressure would provide substantially similar or same results. For example, instead of controlling pressure by exhausting the air inside the sealed region via tube 188, the air may simply be compressed and maintained inside the sealed area. On the other hand, one may surround the membrane deflection apparatus 190 with pressurized air such that, without exhausting air from the sealed region, deformation of the membrane 164 can be achieved. Accordingly, a person ordinary skilled in the art would be able to easily replace it with various other means and devices for causing the membrane 164 to deflect and/or for modulating, regulating or influencing the air inside the sealed frame such that the membrane 164 is deflected or shaped to substantially correspond to a mounting surface of a dielectric spacer panel of the shaped stator panel.

It is also possible to remove outer surface 186 and to install in its place a support frame similar to 162 and a mounted acoustic membrane similar to 164 as shown in FIG. 17 but of a size suitable to be sealed and mounted in contact with surface 194. It is also possible to then install a rigid inner surface onto the inner surface 176 of the sealable box 174 as shown in FIG. 18. With such an alternate embodiment, positive air pressure instead of vacuum could used to deflect the acoustic membrane in a similar radially expanding manner.

FIG. 20 shows the inner stator panel assembly 154 of FIG. 16 co-located within the membrane deflection apparatus 190 of FIG. 19 to facilitate application of an acoustic membrane to the outer or mounting surface of the dielectric spacer panel 158. In accordance with the present invention, a membrane 164 placed on the support frame 162 is expanded in an outward radial manner to a loading position by changing air pressure within the box through the pipe 188 as indicated by the visualization lines 192 located on the surface of the membrane 164. In accordance with the present invention, the membrane is deflected in a similar shape to that of the surface of an inner stator panel assembly 154 so as to allow a deformed membrane to controllably collapse in a safe manner so as to make contact with the entire outer surface (or mounting surface) of the dielectric spacer panel 158 when vacuum pressure is reduced accordingly. It is preferable to have the membrane 164 sufficiently deformed in an outward radial manner so as not to make contact with any surface of the panel assembly 154 when it is positioned within membrane deflection apparatus 190. Having no contact with the acoustic membrane 164 is desirable, as the mounting surface of the dielectric spacer panel 158 is pre-prepared before this step with an adhesive such as an ultraviolet activated adhesive or a pressure sensitive applied film. If the membrane were to contact any portion of said adhesive it could become damaged or torn during any attempt to separate the membrane from adhesive. It is also preferred but not essential to co-locate the central axis 159 (shown in FIG. 16) of the inner stator panel assembly 154 with the central axis 191 of the membrane deflection apparatus 190. Co-location helps to ensure that a minimum initial deflection of the acoustic membrane is required to give the desired final result of uniform contact between the membrane dielectric spacer panel. Minimal initial deflection also requires that the lowest level of vacuum be applied to deflect the membrane to a suitable loading position thereby limiting the possibility of bursting the membrane.

FIG. 21 shows an inner stator panel assembly with an affixed acoustic membrane 200 formed in accordance with one aspect of the present invention. For the purpose of visualization, the surface of the membrane is shown with diagonal visualization lines 202 bounded by dashed perimeter lines 204. The acoustic membrane would typically be trimmed slightly smaller than the surface of the dielectric spacer panel due to the fact that the membrane in a typical ESL panel assembly must have a conductive coating on the surface. As such, it is preferable to cover the edge of the membrane with a suitable insulator, for example, Kapton ^®, Mylar ^® adhesive tape, or other equivalent. The assembly as shown up to this point includes layers similar in function to 4, 6 and 8 of FIG. 2. To complete a finished ESL assembly what remains is to add electrical contacting strips (to contact the conductive coating on the membrane), an outer dielectric spacer panel similar to 14 and finally an outer stator panel similar to 16.

Claims

1. An apparatus for chamfering the edges of an aperture in a perforated sheet metal, comprising

a) a bar member having first and second ends, and the diameter of the bar member being smaller than the diameter of the aperture, wherein the bar member engages at a first edge of the aperture on a first surface of the perforated sheet metal and at a second edge of the aperture on a second surface of the perforated sheet metal; and

b) an actuator for applying force to the bar member causing the bar member to rotate along and against said first and second edges of the aperture.

2. The apparatus as recited in claim 1 , wherein the bar member is cylindrical.

3. The apparatus as recited in claim 2, wherein the bar member is elongated.

4. The apparatus as recited in claim 3, wherein the force to the bar member is applied at or near at least one of the first end and the second end of the bar member for applying leveraged force at the first and second edges of the aperture.

5. The apparatus as recited in claim 4, wherein the bar member is arranged to freely rotate about its own axis while the bar member rotates along and against the first and second edges of the aperture.

6. The apparatus as recited in claim 4, wherein the bar member comprises a groove that engages with the edge of the aperture.

7. The apparatus as recited in claim 4, wherein the bar member comprises two grooves, being spaced longitudinally along the bar member at a distance where each groove engages with the corresponding edge of the aperture.

8. The apparatus as recited in claim 7, wherein the groove is concaved on the longitudinal cross-section of the bar member for rounding the corresponding edge.

9. The apparatus as recited in claim 8, wherein the groove further comprises convex lines on the longitudinal cross-section of the bar member, being adjacent to the concaved portion of the groove of the bar member.

10. The apparatus as recited in any one of claims 6 to 7, wherein the groove comprises straight lines on the longitudinal cross-section of the bar member to a first bottom where the diameter of the bar member is the narrowest.

11. The apparatus as recited in claim 10, wherein the groove further comprises one or more second bottoms having the same or wider diameter than at the first bottom.

12. The apparatus as recited in claim 1, wherein the applied force to the bar member is sufficient to chamfer the first and second edges of the aperture by rotating the bar member around and against the aperture in four to ten turns.

13. An apparatus for chamfering one of edges of an aperture in a perforated sheet metal for a stator panel of an electrostatic speaker, comprising

a) a bar member having the diameter of the bar member being smaller than the diameter of the aperture, wherein the bar member engages at one of a first edge of the aperture on a first surface of the perforated sheet metal and a second edge of the aperture on a second surface of the perforated sheet metal; and

b) an actuator for applying force to the bar member causing the bar member to rotate along and against said one of the first and second edges of the aperture.

14. The apparatus as recited in claim 13, wherein the bar member is substantially cylindrical.

15. The apparatus as recited in claim 14, wherein the bar member is elongated.

16. The apparatus as recited in claim 15, wherein the force to the bar member is applied at or near at least one end of the bar member for applying leveraged force at said one of the first and second edges of the aperture.

17. The apparatus as recited in claim 16, wherein the bar member is arranged to freely rotate about its own axis while the bar member rotates along and against said one of edges of the aperture.

18. The apparatus as recited in claim 16, wherein the bar member comprises a groove that engages with the edge of the aperture.

19. The apparatus as recited in claim 18, wherein the groove is concaved on the longitudinal cross-section of the bar member.

20. The apparatus as recited in claim 19, wherein the groove further comprises convex lines on the longitudinal cross-section of the bar member, being adjacent to the concaved portion of the groove of the bar member.

21. The apparatus as recited in claims 20, wherein the groove comprises straight lines on the longitudinal cross-section of the bar member to a first bottom where the diameter of the bar member is the narrowest.

22. The apparatus as recited in claim 21, wherein the groove further comprises one or more second bottoms having the same or wider diameter than at the first bottom.

23. The apparatus as recited in claim 13, wherein the applied force to the bar member is sufficient to chamfer the first and second edges of the aperture by rotating the bar member around and against the aperture in four to ten turns.

24. A method of chamfering the edges of an aperture in a perforated sheet metal, comprising:

a) preparing a bar member having first and second ends, and the diameter of the bar member being smaller than the diameter of the aperture;

b) engaging the bar member to at least one of the edges of the aperture of the perforated sheet metal; and

c) applying a force to cause the bar member to rotate along and against said at least one of the edges.

25. The method as recited in claim 24, wherein the bar member is cylindrical.

26. The method as recited in claim 25, wherein the bar member is elongated.

27. The method as recited in claim 26, wherein the force to the bar member is applied at or near at least one of the first and second ends of the bar member.

28. The method as recited in claim 27, wherein the bar member is arranged to freely rotate about its own axis while the bar member rotates along and against said at least one of the edges of the aperture of the perforated sheet metal.

29. The method as recited in claim 27, wherein the bar member comprises a groove that engages with the edge of the aperture.

30. The method as recited in claim 27, wherein the bar member comprises two grooves, being spaced longitudinally along the bar member at a distance where each groove engages with the corresponding edge of the aperture.

31. The method as recited in claim 30, wherein the groove is concaved on the longitudinal cross-section of the bar member.

32. The method as recited in claim 31, wherein the groove further comprises convex lines on the longitudinal cross-section of the bar member, being adjacent to the concaved portion of the groove of the bar member.

33. The method as recited in claim 29 or 30, wherein the groove comprises straight lines on the longitudinal cross-section of the bar member to a first bottom where the diameter of the bar member is the narrowest.

34. The method as recited in claim 33, wherein the groove further comprises one or more second bottoms having the same or wider diameter than the first bottom.

35. The method as recited in claim 24, wherein the applied force to the bar member is sufficient to cause a plastic deformation of the metal.

36. An apparatus for chamfering the edges of an aperture in a perforated sheet metal stator panel, comprising:

a drill bit comprising a cutting edge for chamfering the edge of the aperture.

37. A perforated sheet metal for an electrostatic loudspeaker, having the edges of apertures being chamfered by the method recited in any one of claims 24 to 35.

38. A perforated sheet metal for an electrostatic loudspeaker, having the edges of apertures being chamfered by the apparatus recited in any one of claims 1 to 23.

39. An apparatus for stretch-forming a perforated sheet metal for a stator panel of an electrostatic loudspeaker, comprising:

a) a base;

b) a plurality of spars that contacts one surface of the perforated sheet metal, said spars being radially and slidably connected to the base;

c) a fastener for holding both lateral ends of the perforated sheet metal during stretch-forming; and d) one or more actuators for applying forces to cause the plurality of spars to move radially outward for stretch-forming the perforated sheet metal.

40. The apparatus as recited in claim 39, wherein the apertures of the perforated sheet metal are chamfered.

41. The apparatus as recited in claim 39, wherein each of the plurality of spars comprises an individually angled interface, and said one or more actuators comprises a ramp block movable between first and second positions, said ramp block having corresponding individual angles to the individually angled interfaces of the spars, wherein moving from the first to the second conditions causes the plurality of the spars to move radially outward for stretch-forming the perforated sheet metal.

42. A method for stretch-forming a perforated sheet metal for a stator panel of an electrostatic loudspeaker, comprising:

a) securing the perforated sheet metal at the lateral sides thereof;

b) preparing a plurality of spars contacting one surface of the perforated sheet metal; and

c) actuating the plurality of spars moving radially outward at individually controlled speed and distance for stretch-forming the perforated sheet metal.

43. A shaped stator panel for an electrostatic loudspeaker made by the process recited in claim 42.

44. An apparatus for stretch-forming a diaphragm to a shaped stator panel for an electrostatic loudspeaker, comprising:

a) a frame for securing the diaphragm;

b) a sealable box having an opening for receiving the frame having the diaphragm being secured thereon to form a wall to create a sealed space inside the sealable box; and

c) an air pressure controller for causing the diaphragm to deflect from a first position to a second position to allow the shaped stator panel to be placed along and about the first position such that modulating the air inside the sealed frame causes the diaphragm to cover a mounting surface of a dielectric spacer panel of the shaped stator panel.

45. The apparatus as recited in claim 44, wherein the air pressure controller comprises an outlet for exhausting air out from the sealed space.

46. The apparatus as recited in claim 44, wherein the opening is shaped to cause the surface of the diaphragm of the wall to substantially correspond to and equal distance to the mounting surface of the dielectric spacer panel of the shaped stator panel.

47. The apparatus as recited in claim 45, wherein the frame is flexible.

48. A method of fitting a diaphragm to a shaped stator panel for an electrostatic loud speaker, comprising:

a) providing a sealable box having an opening;

b) securing the diaphragm to a frame, the frame substantially corresponds to the opening and the frame with the diaphragm forms a wall for the sealable box to create a sealed space inside the sealable box; and c) controlling an air pressure causing the diaphragm to deflect from a first position to a second position for allowing the shaped stator panel to be placed along and about the first position such that modulating the air inside the sealed frame causes the diaphragm to cover a mounting surface of a dielectric spacer panel of the shaped stator panel.

49. The method as recited in claim 48, wherein the opening is shaped to cause the surface of the diaphragm on the wall to substantially correspond to and be equal distance to the mounting surface of the shaped stator panel.

50. An electrostatic loudspeaker having shaped stator panels, and a diaphragm of which is fitted by the method recited in claim 48 or 49.

51. An apparatus for stretch-forming a perforated sheet metal for a stator panel of an electrostatic loudspeaker, comprising:

a) a base;

b) a plurality of spars that contacts the inner surface of the perforated sheet metal tube, said spars being radially and slidably connected to the base; and c) one or more actuators for applying forces to cause the plurality of spars to move radially outward for stretch-forming the perforated sheet metal.

52. The apparatus as recited in claim 51, wherein the tube is seamless.

53. The apparatus as recited in claim 51, wherein the tube is formed from a perforated sheet metal by welding the ends together.

54. A method for stretch-forming a perforated sheet metal cylinder for a stator panel of an electrostatic loudspeaker, comprising:

a) preparing a plurality of spars contacting the inner surface of the perforated sheet metal tube; and

b) actuating the plurality of spars moving radially outward at individually controlled speed and distance for stretch- forming the perforated sheet metal tube.

55. A shaped stator panel for an electrostatic loudspeaker made by the process recited in claim 54.