US3800102A

US3800102A - Electrostatic transducer and method and means for making same

Info

Publication number: US3800102A
Application number: US00157191A
Authority: US
Inventors: A Janszen
Original assignee: ELECTROSTATIC RES CORP
Current assignee: ELECTROSTATIC RES CORP
Priority date: 1971-06-28
Filing date: 1971-06-28
Publication date: 1974-03-26
Anticipated expiration: 1991-03-26

Abstract

An electrostatic loudspeaker unit of the push-pull type employing grids of spaced insulated wires disposed on each side of a flexible diaphragm. The electrode grids are in the form of a flat spiral so as readily to accommodate effects of variations in temperature without causing the wires to move out of their designed plane. The electrode grids are preformed using a special tool, a part of which is utilized to mount the spiral electrode in the support frame with a high degree of precision.

Description

United States Patent [191 J anszen [451 Mar. 26, 1974 ELECTROSTATIC TRANSDUCER AND METHOD AND MEANS FOR MAKING SAME [75] Inventor: Arthur A. Janszen, Belmont, Mass.

[73] Assignee: Electrostatic Research Corporation,

Salem, Mass.

22 Filed: June 28,1971

21 Appl.No.: 157,191

[52] US. Cl 179/111 R [51] Int. Cl H04r 19/00 [58] Field of Search 179/111 R [56] References Cited UNITED STATES PATENTS 1,708,846 4/1929 Nagel 179/111 R 2,000,437 5/1935 Colvin 179/111 R FOREIGN PATENTS OR APPLICATIONS 611,249 3/1935 Germany 179/111 R 808,593 7/1951 Germany 179/111 R Primary Examiner-Ralph D. Blakeslee Attorney, Agent, or FirmKenway, Jenney & Hildreth 5 7 ABSTRACT An electrostatic loudspeaker unit of the push-pull type employing grids of spaced insulated wires disposed on each side of a flexible diaphragm. The electrode grids are in the form of a fiat spiral so as readily to accommodate effects of variations in temperature without causing the wires to move out of their designed plane. The electrode grids are preformed using a special tool, a part of which is utilized to mount the spiral electrode in the support frame with a high degree of preci- 7 Claims, 8 Drawing Figures mimwmzsm 3800402 SHEET 1 or 3,

INVENTOR ARTHUR A. JANSZEN ATTORNEYS ELECTROSTATIC TRANSDUCER AND METHOD AND MEANS FOR MAKING SAME BACKGROUND OF THE INVENTION In the present inventors prior US. Pat. Nos. 2,631,196 and 2,896,025, there are disclosed electrostatic loudspeakers characterized by fixed electrodes in the form of grids of insulated wires disposed on opposed sides of a flexible diaphragm to provide a pushpull transducer. The second patent is especially concerned with a construction that permits the use of closely-spaced wires arranged in straight, parallel relation while avoiding the effects of thermal expansion and contraction that would normally cause the wires to move out of their desired precisely-determined flat plane. In particular, the method and structure taught by the patent requires winding carefully pre-straightened insulated wire onto a pair of support frames having a multiplicity of accurately formed notches to receive the wires. Subsequent steps involve cutting the insulating sheath on each wire, soldering the exposed portion of each wire to a cross-bar, cementing the insulating sheath of each wire to the frame at each notch region, and finally cutting the wires along lines inwardly of the frame edge. The wires, being anchored at the bus bar end only, are free to slide endwise within their sheaths and thus prevent buckling that would otherwise result from differential changes in dimensions of wire and support with variations in temperature.

It is evident that this procedure, when carefully carried out, insures that the wires are not caused by thermal expansion to distort or bend out of the precise plane so essential to proper operation of the electrostatic transducer. It is difficult, however, to manufacture the support frames so that the slots are of the proper depth to provide the desired spacing of the wires from the diaphragm when the frames are assembled, and exceptional care is needed in stripping and soldering the many wires that make up the grids, as well as in properly securing the sheaths at the numerous crossing points of the supports.

SUMMARY OF THE INVENTION The present invention is concerned with improvements in the construction and configuration of electrostatic loudspeakers, and especially the improvements and advantages that result from a novel electrode configuration that avoids the difficulties associated with the use of electrode grids employing straight parallel wires to form a rectilinear electrode configuration.

A principal feature of the invention is the provision of flat spiral wire grid electrodes, each portion of which is a segment of an are, formed by winding a continuous insulated wire into a flat spiral. The provision of flat spiral electrodes permits the diaphragm to be clamped both at its center and ,at a circular outer margin, as well as at an intermediate concentric ring if desired, so that the diaphragm may flex in one or more annular bands or regions. The use of electrodes formed out of wire wound in a fiat spiral eliminates the problems arising out of differential thermal expansion and contraction, as the curvature of the wires between supports permits any relative motion between wire and frame motion due to thermal effects to take place in the plane of the spiral, without altering the critical spacing between electrode and diaphragm.

Another feature of the invention resides in a novel technique, and special tool for carrying out the same, by which the flat spiral grid electrodes are accurately formed and mounted on the support frames in a manner which insures that the grids are secured in extremely precise spatial relation to the diaphragm plane. As a further feature, the securing of the insulated wires to the frames at each of the support points is readily carried out during the mounting procedure.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a support frame for use in an electrostatic transducer of the type in which the diaphragm and the electrodes are arranged in two annular bands or regions, showing portions of the spiral electrode mounted on the support.

FIG. 2 is an enlarged sectional view of an assembled unit, taken from the center outwardly on a line corresponding to the line 22 of FIG. 1, showing the positioning of the spiral electrodes within their support frames.

FIG. 3 is a sectional detail view of a portion of an alternative form of electrostatic transducer embodying the invention, wherein the diaphragm is clamped only at its center and at a circular outer rim region.

FIG. 4 is an exploded view of one form of special winding tool, suitable forming spiral electrodes having two annular bands.

FIG. 5 is a view of the tool on a somewhat larger scale than FIG. 4, showing in greater detail the faces of the inner and outer circular members of the winding tool, (the two elements shown separated at the right in FIG. 4) with portions of the windings shown in the starting, cross-over and finishing regions.

FIG. 6 is an enlarged sectional view of the winding tool with the parts in operative relation for forming the inner portion of a spiral electrode having spaced annular regions.

FIG. 7 is a corresponding view with the tool components in position for carrying out the forming of the outer region of the spiral electrode.

F IG. 8 is a view of another form of electrostatic transducer employing spiral electrodes, wherein the electrode support region consists of an apertured or perforated structure.

DESCRIPTION OF THE INVENTION 1. The Transducer The electrostatic transducers of the present invention resemble in some respects those of my prior US. Pat. No. 2,896,025. A very thin flexible diaphragm having a conductive coating of high resistivity serves as the sound-radiating element as well as one electrode. The diaphragm is mounted between support frames which carry the acoustically-transparent wire grids that serve as the fixed electrodes, energized in a conventional manner by electrical signals from a push-pull source. A high voltage bias potential applied between the diaphragm and the grid electrodes maintains the requisite high voltage across the air gaps between diaphragm and fixed electrodes, as disclosed in said patent.

According to the present invention, the support frames illustrated in FIGS. 1 and 2 are distinguished by their provision of circular clamping means for the diaphragm 12. A central disk region 14 and a coplanar outer ring 16 on each frame provide the circular center and outer clamping surfaces between which the diaphragm is secured. Outwardly of the outer clamp ring 16, the margins 22 of the frames may be square or, if desired, of other configuration, depending on mounting requirements or other considerations.

The support frames, which may advantageously be molded from dimensionally stable insulating material, are formed with a plurality of radially disposed integral ribs 24 on which the spiral electrodes are mounted. For large-size low frequency radiators, and for units such as high frequency radiators where it is desired to divide the radiating surface of the diaphragm into a plurality of annular bands, the frames preferably also include a ring 26 intermediate the central disk 14 and the outer clamping ring 16. This permits a greater number of ribs to be disposed between the intermediate ring 26 and the outer ring 16 than within the annular region inwardly of the intermediate ring, in order that the arcuate distances between the support ribs may be more nearly the same over the entire electrode region.

For low frequency radiators the intermediate ring 26 will not project above the surfaces of the ribs 24 in order that the diaphragm may flex in a single annular band, as illustrated in FIG. 3. For tweeter units, where the diaphragm is advantageously divided into two annular radiating bands, the surface of the rib 26 extends into the plane of the central support 14 and outer rim 16 so as to provide an intermediate circular clamping region for the diaphragm, as illustrated in FIG. 2. In such event, it is desirable at the intersections between radial ribs 24 and intermediate rim 26 to provide holes 34 adapted to receive staking pins 36 which secure the frames together in the intermediate ring region when the frames are subsequently assembled with the diaphragm clamped between.

The fixed electrodes, like those of my prior patent, are preferably formed of single strand copper wire having a dielectric insulating sheath of vinyl chloride composition. A wire size of No. 28 A.W.G. is generally satisfactory for tweeters and head phone units; a somewhat heavier gauge is desirable for the larger low frequency radiators. The wire is wound into a flat spiral of uniformly spaced turns so as to occupy the open annular region or regions between the center and the outer rim of each support frame. By the provision of these flat spiral wire grid electrodes, it becomes practical for the first time, it is believed, to employ what are in effect circular wire grid electrodes, in conjunction with circular flexing regions of the radiating diaphragm. Not only are such flat spiral electrodes free of any tendency for the wires to distort out of the flat plane by reason of differential thermal expansion, but also such configuration permits the electrodes to be formed out of a single continuous wire that eliminates the necessity for multiple soldered connections to a common bus.

A further feature, which not only simplifies the construction but also is responsible for the high degree of precision with which the wire grid electrodes are mounted in a plane parallel to and accurately spaced from the diaphragm, results from the fact that the radial support ribs 24 are not slotted or notched to receive the insulated sheaths of the wires. In the construction shown in my prior US. Pat. No. 2,896,025, the depth of the slots determines the ultimate spacing of the wires from the diaphragm, and the requisite accuracy is difficult to provide.

In my present invention, not only are the ribs 24 not required to be slotted, but the surfaces or edges of the ribs adjacent the wires are not required to engage the insulating sheaths on the wires to determine their supported position. The frames are preferably so designed that, as best illustrated in FIGS. 2 and 3, the perpendicular distance between the plane of the rib edges and the plane of the

diaphragm support surfaces

14 and 16 is slightly greater than the distance from the diaphragm to a plane tangent to the insulating sheaths on the side away from the diaphragm, for the desired spacing of the conductive wires from the diaphragm in the completed transducer. The desired relatively critical spacing between electrode wires and the diaphragm is effectively attained by the procedures to be described wherein the wires are positioned with their insulating sheaths in spaced relation to the ribs 24 through the use of an intervening hardenable material 40 within which the spiral electrode grids are embedded solely with reference to their distance from the plane of .the diaphragm support

surfaces

14 and 16 and without regard to the resulting spacing or variations in spacing of the insulated wires from the adjacent edges of the ribs of the support frame.

2. The Electrode-Forming Tool The electrode grids are fabricated within a special winding tool having spaced walls extending outwardly from a central hub to define a narrow slot or guideway in which the insulated wire can be wound in a manner to produce the desired flat spiral having uniform spacing between turns. The winding tool to be described is adapted for making the dual spiral windings of the type shown in FIGS. 1 and 2, in which the diaphragm is supported by an intermediate rim so that the electrode grid is disposed in inner and outer annular bands. By a simple modification, a tool for forming a continuous, single spiral winding may readily be provided.

The winding tool shown in FIGS. 4-7 employs circular members having cooperating coaxial connecting means by which the members may be assembled to form an accurately-defined narrow slot within which the spiral winding can take place. Upon completion of the winding operation, the tool can be disassembled and part of the tool, to which the wound electrode has been caused to adhere, then utilized to transfer the spiral electrode to the support frame and to position it within the frame in precise relationship to the plane of the clamping surfaces between which the diaphragm is supported in the completed transducer.

In the exploded view of FIG. 4, the circular member at the left, indicated generally at 52, provides one wall of the guideway and serves also as the transfer means, while

members

54 and 56, shown at the right, cooperate to define the other wall of the winding slot with the inner member 54 being operative during the formation of the winding for the inner annular region, while the member 56 having the outer annular face is utilized during the winding of the outer annular portion of the spiral electrode.

As best shown in FIGS. 6 and 7, the guideway face of tool member 52 has coplanar inner and outer

annular surfaces

62 and 64, separated by a circular channel 66 whose dimensions and radial position correspond to those of the intermediate annular ring 26 of the support frame. In spaced opposing relation to the inner annular face 62 of member 52 is the guideway surface 72 of member 54, while the outer annular face 74 of member 56 opposes surface 64 of member 52. The annular surface 74 terminates at its inner edge in a projecting annular shoulder 76 which registers with the channel 66 in member 52. The inner member 54 is located inwardly of rim 76 within a circular recess 82 formed in member 56. The inner member 54 is provided with a short projecting hub 84 adapted to fit within a coaxial recess 86 of corresponding diameter at the center of member 52 to insure precise coaxial registration of

members

52 and 54. Bolt 88 extends through member 52 into a threaded recess in the center of member 54 to secure the parts during the winding operation.

The outer member 56 is mounted on the inner member 54 in a manner which permits precise axial adjustment, for a purpose to be described. The hub 92 of the member 56 is bored to receive the shaft 94 of the inner flanged member 54. A bolt 96 extends through the outer end of hub 92 into threaded engagement with the end of shaft 94, while a compression spring 98 urges the outer member 56 in an axial direction away from inner member 54 to establish a relative position determined by the adjustment of bolt 96. Thus rotation of the bolt by a suitable wrench permits accurate axial adjustments to be made to position the outer member 56 in first one and then the other of the positions shown in the enlarged views of FIGS. 6 and 7. One or more pins 102 extend into registering apertures 104 in the central regions of

members

52, 54 and 56 to insure that the parts can be assembled in only a single predetermined angular relationship about the common center.

3. Winding the Spiral Electrode In preparation for the operation of winding the spiral electrode within the special tool it is necessary, before the tool is assembled, to place a suitable coating on the

surfaces

62 and 64 of the member 52, to which the completed winding may be caused to adhere in order to facilitate the transfer of the wound spiral to its proper position on the support frame. A thin coating of a wax having a melting point within the range of 100C to 200C has been found suitable. The free end of the insulated wire 112 is then inserted from the coated face of the disk out through a passage extending through member 52 approximately one sheath diameter outwardly of the edge of the circular recess 86 that receives the end 84 of member 54. The end of the wire is then secured to a suitable anchor point such as indicated at 114 in FIG. 3.

In addition, a spacer strand 118 of round plastic material of a diameter substantially that of the sheath is anchored in the member 54 by running the free end through a passage at the base of the guideway face 72, continuing through an aligned passage in member 56, and forming a knot 120. The strand can then be pulled back until the knot seats against the outside wall of member 56.

The disk 52 is now assembled with

members

54 and 56, using clamping bolt 88 to draw the

members

52 and 54 into firm engagement at the hub region. The adjusting bolt 96 at the right hand end of the assembly is now turned until

members

54 and 56 are in the relation shown in FIG. 6 with the surface 72 approximately flush with the side of rim 76. In this position, the rim does not obstruct access to the winding passage defined by the opposing faces 62 and 72 of disk 52 and member 54 respectively.

It is important to note that the spacing between these

surfaces

62 and 72 is greater than the diameter of the insulating sheath on the wire. This is to permit the winding to be carried out in a manner which results in the turns of the wire being disposed toward the surface 62 of member 52 while the turns of the spacing material are offset in the other direction so as to rest against the opposite surface 72 of member 54. This oblique or staggered relation of wires and spacer strand not only provides the desired spacing between turns of the flat spiral, but also prevents jamming of the spacer strand between turns of the conductor, which would interfere with the subsequent separation of the spacer strand from the wound spiral, upon disassembly of the tool.

By way of example, with a conductor diameter (outside diameter of sheath) of approximately 0.023 inch and a similar diameter of the spacer strand, a slot width or spacing between guideway surfaces of the order of 0.038 inch is desirable. The resulting space between turns of the spiral is approximately 0.015 inch.

It has been found that the conductor and the spacing strand may be wound in the desired offset relation by leading the wire, which has been suitably straightened, close to the left hand side of the slot, as viewed in FIG. 6, while the spacer strand is fed in close to the right hand edge. To aid in the proper formation of the turns at the beginning of the winding, the region of the projecting hub of member 54 inwardly of the circular end 84 is formed with a spiral ramp 124 ending close to the starting point of the winding, as shown in FIGS. 4 and 5. This causes the paired conductor and spacer strand during the first rotation of the tool to rise gradually to provide a smooth transition as the second turn is laid over the starting turns of conductor and spacer. Note also the previously-described radial offset in the positions of lead-in holes for conductor and spacer, shown in enlarged detail in FIG. 5, which establishes the initial relationship of wire and spacing strand.

To carry out the winding, the tool is mounted by its hub 92 in a power-driven rotary support by which the tool may be rotated to cause the winding to proceed until the space between

surfaces

62 and 72 has been substantially filled. The driving means is now stopped and the tool slowly rotated until the conductor and the spacer strand, still in their superposed relation, are seen to be aligned with a cross-over slot 126 extending obliquely through the projecting rim 76.

Once the cross-over slot has been brought into alignment with the conductor and spacer strand, the adjusting bolt 96 is rotated to move member 56 toward member 52 until the rim 76 seats in the recess 66 and thereby establish the desired winding space between

outer surfaces

64 and 74 on

member

52 and 56 respectively. The winding of the outer region of the grid may now proceed in the same manner as with the forming of the inner annular region. Advantageously the radially outer surface of rib 76 in the region approaching the exit end of the cross-over slot 126 may have a rising ramp region 130 (see FIG. '4) to facilitate a smooth transition at the commencement of the second turn, just as results from the ramp 124 for the first turn of the inner winding.

When the required number of turns have been wound to fill the winding space outwardly of rim 76 to the margin of surface 64, the tool rotation is discontinued and the conductor brought out through a peripheral oblique slot 132 in member 52 and suitably anchored to retain the conductor under adequate tension, after which the conductor may be out free beyond the anchor point.

4. The Transfer Procedure To carry out the mounting of the spiral electrode on the support frame, it is necessary that the wound electrode be retained on the surface of member 52 when the tool is disassembled. If the thin wax coating applied to the surface of the tool member 52 preparatory to the winding is relatively soft and sticky, the wires will have been caused to adhere to the surface of member 52 during the carrying out of the winding operation. If the wax coating is of a somewhat harder composition, the wax may be softened by passing a current through the wire and thereby cause the wax to bond where it is in contact with the insulating sheath. The securing bolt 88 is then loosened and removed, which permits transfer member 52 to be separated from the

parts

54 and 56 of the winding tool. Due to the staggered relation of conductor and spacer strand, the strand 118 freely releases from between the turns of the conductor spiral, leaving the spiral electrode adhering to the face of member 52.

A support frame is prepared by applying a hardenable material 40, such as an epoxy cement, to the top surfaces of the radial ribs 24 of the support frame. The member 52, now serving as a carrier, is then assembled with the frame so that the spiral grids are in opposed facing relation to the cement-coated support ribs. The frame may be provided in its central region 14 with apertures 134 which register with the guide pins 102 in member 52 to aid in aligning the frame and transfer member in proper relation during assembly so that the grids are properly centered with the cross-over region of the winding located to enter the cross-over slot 136 provided for the purpose in the intermediate rim 26 and the outer end of the winding passing out through slot 138 in the outside rim 16.

The frame and the member 52 are then clamped together with the central region 14 of the frame seated in the central recess 86 in the member 52, the intermediate rim 26 seated in the channel 66, and the outer rim l6 seated on the marginal surface 140 of the member 52. By this engagement of circular surfaces on tool and electrode support, the electrode grids are caused to become embedded in the still-soft cement 40 the precise distance that provides the desired design spacing of the electrode grids from the surface of the diaphragm. As the support ribs have preferably been fabricated with their edge surfaces disposed slightly further from the plane of the diaphragm support surfaces than would provide the desired position of the conductors if the insulating sheaths rested directly on the ribs, the hardenable cement bond, utilized in conjunction with the positioning of the transfer tool solely by reference to the surfaces on which the diaphragm is supported, provides the accomodation which eliminates the need for close tolerances in rib edge position and insures positive and precise mounting of the grids on their support frames. The assembly is then allowed to set for sufficient time to permit the cement to harden. Thereafter, by a heating current through the electrode, the wax may be softened to release the wires from the transfer tool and permit removal of the frame from the tool 52.

The final steps of applying a tensioned diaphragm, suitably coated with a slightly conductive coating, to one of the frames, and applying the other clamping frame are all taught in my prior US. Pat. No. 2,896,025. The support frames are provided with holes 142 located in the marginal region of the frame outwardly of the outer rim 16 to receive terminals 144 for connection to the outer end of the wire electrode and to the conductive coating of the diaphragm substantially as shown in that patent.

As has previously been indicated, electrostatic transducers formed in accordance with my invention to serve as low frequency radiators will generally employ a diaphragm that is supported only at its center and at an outer circular rim, without intermediate support. Thus the intermediate rib 26, desirable for providing intermediate support for the radial ribs 24, will not project above the surface of ribs 24, but will be as shown in FIG. 3, with the spiral electrode having a continuous uniform spacing of turns from center frame support 14 to outer rim 16. In this sectional detail, it will be seen that the cement 40 is disposed both on the radial ribs 24 and on the intermediate rib 26 so as to provide bonding support to all turns of the spiral winding whenever the wires are in proximity to a rib edge.

For winding the electrodes for low frequency radiators, and also for small-diameter radiators suitable for electrostatic headphones, the winding tool may not need provision for successive adjustments of the winding slot spacing, such as have been described and shown'where the winding has to accomodate an intermediate support rib for the diaphragm. For a single continuous spiral, the tool member 52 will omit recess 66, leaving

surfaces

62 and 64 to form a single continuous surface.

Members

54 and 56 can be combined in a single tool having no projecting rim 126 and with surface 74 coplanar and continuous with surface 72.

For small-size transducers, for example units especially suitable for use in electrostatic headphones, the modified form of support frame illustrated in FIG. 8 may be employed. In this construction, instead of a plurality of radial ribs where edges are coated with a hardenable cement or adhesive to receive the wires, as shown in FIGS. 1, 2 and 3, the region 148 of the frame intermediate the center 150 and the margin 152 may simply be a perforate or apertured area, across which narrow bands 154 of cement are laid radially to receive and support the spirally-wound electrode. The openings 156 which provide the acoustic transparency for the support may be readily formed during the molding of the frame, and can be circular or of other shape, as well as of uniform or of varying size.

It will be apparent from the foregoing that a new form of electrostatic transducer has been provided wherein the fixed electrodes comprise grids of spaced insulated wires disposed in curvilinear rather than rectilinear arrangement, thereby insuring that differential thermal expansion does not force the wires out of their design plane. By achieving the curvilinear arrangement of wires through forming the electrode in a flat spiral of spaced turns disposed in an annular band, the diaphragm may advantageously be supported at its center and at a circular rim close to the outer boundary of the electrode.

The invention likewise comprises, as a further feature, a novel tool and technique for conveniently and precisely forming the spiral electrode and for accurately mounting it in its support frame, so as to greatly simplify the manufacturing procedures for electrostatic transducers of various sizes and types while insuring that the requisite tolerances will be consistently attained.

I claim:

1. An electrostatic transducer comprising a flexible diaphragm having a conductive coating of high resistivity, a pair of frames each having a circular rim between which the margin of the diaphragm is clamped and a circular center region coplanar with the rim between which the center of the diaphragm is clamped, and conductive electrodes mounted on the frames on each side of the diaphragm intermediate the rim and the center region in closely-spaced planar relation to the diaphragm, the frames having electrode support means intermediate said rim and center region, each electrode comprising a continuous wire having an insulating sheath, disposed in a flat spiral having spaced turns, the electrode support means being spaced from the diaphragm a greater distance than the distance from the diaphragm to a plane tangent to the insulating sheaths of the wires on their sides remote from the diaphragm, and bonding material intermediate the sheaths and the support means.

2. An electrostatic transducer according to claim 1 wherein the bonding material is disposed in radial strips between center region and the rim.

3. An electrostatic transducer according to claim 1 wherein the support means for the electrodes comprise spaced radially disposed members intermediate the central and rim portions of the frames and the bonding material is intermediate the radially disposed members and adjacent portions of the insulating sheaths.

4. An electrostatic transducer according to claim 1 wherein the electrode support means comprises a web having a multiplicity of small closely-spaced apertures and the bonding material is disposed in radial strips on the web.

5. An electrostatic transducer according to claim 1 wherein each frame has an intermediate ring portion between the central region and the outer circular rim.

6. An electrostatic transducer according to claim 1 wherein a plurality of radial supports extend between central disc and intermediate ring, and between the intermediate ring and the outer rim and the bonding material is intermediate the radial supports and adjacent portions of the insulating sheaths.

7. An electrostatic transducer comprising a flexible diaphragm having a conductive coating of high resistivity, a pair of frames each having a circular rim between which the margin of the diaphragm is clamped and a circular center region coplanar with the rim between which the center of the diaphragm is clamped, and conductive electrodes mounted on the frames on each side of the diaphragm intermediate the rim and the center region in closely-spaced planar relation to the diaphragm, the frames having electrode support means intermediate said rim and center region, each electrode comprising a continuous wire disposed in a flat spiral having spaced turns, the electrode support means being spaced from the diaphragm a greater distance than the distance from the diaphragm to a plane tangent to the insulating sheaths of the wires on their sides remote from the diaphragm, and bonding material intermediate the sheaths and the support means.

Claims

1. An electrostatic transducer comprising a flexible diaphragm having a conductive coating of high resistivity, a pair of frames each having a circular rim between which the margin of the diaphragm is clamped and a circular center region coplanar with the rim between which the center of the diaphragm is clamped, and conductive electrodes mounted on the frames on each side of the diaphragm intermediate the rim and the center region in closelyspaced planar relation to the diaphragm, the frames having electrode support means intermediate said rim and center region, each electrode comprising a continuous wire having an insulating sheath, disposed in a flat spiral having spaced turns, the electrode support means being spaced from the diaphragm a greater distance than the distance from the diaphragm to a plane tangent to the insulating sheaths of the wires on their sides remote from the diaphragm, and bonding material intermediate the sheaths and the support means.