US 3777078 A
A linkage bar, having a rigid central portion and flexible end portions, links a pivoted armature to an acoustical diaphragm in an electroacoustic transducer. The armature carries an extension, normal to the plane of the armature, to which one end of the linkage is spot welded. The other end of the linkage bar is attached to the diaphragm. Longitudinal motion is thus coupled between the armature and the diaphragm while associated lateral motion caused by the pivoting of the armature is substantially absorbed by the flexible end portions. The spot welding of the linkage to the armature is an economical production process which provides reliable joints of consistant quality.
Description (OCR text may contain errors)
United States Patent [191 Boutros-Attia et a1.
Quebec; Kenneth Edwin Nixon, Ottawa, Ontario, all of Canada  Assignee: Bell Canada Northern Electric Research Limited, Ottawa, Ontario,
Canada 22 Filed: Jan. 14, 1972 21 Appl. No.: 217,926
 US. Cl. ..179/114 A, 179/] 15A  Int. Cl H04r 11/00  Field of Search 179/114 A, 115 A,
 References Cited UNITED STATES PATENTS 3,454,912 7/1969 Morrison 179/114 A 7/1946 Caughey et a] H 179/114 A 12/1949 Sweger 335/231 Primary Examiner-Kathleen H. Claffy Assistant ExaminerThomas L. Kunder Attorney-John E. Mowle [5 7 ABSTRACT A linkage bar, having a rigid central portion and flexible end portions, links a pivoted annature to an acoustical diaphragm in an electroacoustic transducer. The armature carries an extension, normal to the plane of the armature, to which one end of the linkage is spot welded. The other end of the linkage bar is attached to the diaphragm. Longitudinal motion is thus coupled between the armature and the diaphragm while associated lateral motion caused by the pivoting of the armature is substantially absorbed by the flexible end portions. The spot welding of the linkage to the armature is an economical production process which provides reliable joints of consistant quality.
3 Claims, 7 Drawing Figures PATENTEUDEC 4 1975" saw REF 4 PATENTEI] DEC 4 1975 SHEET 3 BF 4 PATENTED DEC 4 I975 SHEET & 0F 4 E on LIIHIIII lllllllll Illllllll HIIHH lllllll lllllllll lllllllll lllHHll IHIIIHI IIIHIIII IIIHIIII IIIIIIIH lllllll lllllllll lllllllll\l\llllll LINKAGE ARRANGEMENT IN PIVOTING ARMATURE TRANSDUCER The present invention relates to a linkage arrangement for coupling motion between a diaphragm and a pivoting armature in an electroacoustic transducer.
Electroacoustic transducers used as receivers in headsets, telephones and the like, generally have a somewhat common form. These receivers typically have a housing within which a drive meansis fixed and coupled to an acoustic diaphragm at one end of the housing. The diaphragm is so placed that it is able to be closely coupled with the air volume in a users ear cavity. An advantage of this arrangement is that the closely coupled condition provides high coupling efi'iciency. A disadvantage lies in that an air leak around the periphery of the users ear into the coupling volume causes a drastic reduction in coupling efficiency at the lower frequencies.
During prolonged telephone usage, the users ear often becomes quite sore from the extended application of sufficient pressure to obviate air leaks. Telephone manufacturing companies have recognized this problem. As transducers having better efficiency were developed, the receiver portion of the telephone was altered to provide a larger coupling air volume between the users eardrum and the receiver diaphragm. The larger coupling volume renders the receiver less sensitive to coupling conditions less than ideal. A given air leakage or gap around the earpiece affects the larger coupling volume by a lesser degree than it would affect a smaller coupling volume. Thus there is relatively less degradation in the acoustical quality perceived.
The above solution to maintaining acoustic quality is satisfactory regarding current telephones. However, lightweight headsets and future electronic telephones, proportioned for aesthetic appearance, are incompatible with current transducer design. Since such apparatus will be of reduced dimensions, they will not lend themselves readily to accommodating a significant ear cup.
A transducer design, currently in extensive use, and commercially known as a U1 receiver, is described in Canadian Pat. No. 496,187 issued Sept. 15, 1953 to Edward E. Mott. This transducer is a ring armature type having a circular diaphragm. This transducer, used as a receiver, inherently has a number of disadvantages. It is relatively heavy and large, particularly in diameter, as compared to a balanced armature receiver of similar efficiency and acoustical characteristics. The shape and sheer bulk of the ring armature and magnet assembly makes the receiver relatively expensive. In addition, tolerances are somewhat critical in order to maintain consistant receiver efficiency and dynamic range. It does carry one advantage in that there is sufficient space in the central portion of the receiver for the diaphragm to be deeply dished such that an increased coupling volume is available. This provides an acceptable acoustical quality over a range of coupling conditons.
An alternative to the above discussed ring armature receiver is a transducer of the pivoted or balanced armature type. Prior pivoted armature receiver designs provided a relatively small coupling volume. The receiver diaphragm cannot be significantly dished, as in the ring armature receiver, because the driver mechanism effectively occupies most of the space behind the diaphragm. Additionally, the coupling volume is further reduced with any reduction of receiver diameter. Receivers of smaller diameter are required in order to be dimensionally compatible with future telephone design trends. Thus the requirement of acoustical quality and aesthetic appearance are in direct contradiction of each other.
Current pivoted armature receiver designs also have problems in the linkage between the armature and the diaphragm. The US. Pat. No. 3,454,912 which issued on July 9, 1969 to L.A. Morrison comprehensively discusses these problems with illustrations in FIGS. 4A and 4B. In summary, prior linkages were either too rigid, causing distortion of the diaphragm; too flexible, causing distortion of the drive linkage; or too prone to wear between moving parts.
During the development of a balanced armature receiver for use in future electronic telephone sets and as a replacement for the present ring armature type receiver, it was noticed that a dip or notch was typically present in the receiver's frequency response in the range of between 2,800 Hz to 3,300 Hz. In this particular receiver the linkage between the armature and the diaphragm was a shaped extension of the body of the armature. The problem here seemed to be that of too rigid a linkage between the armature and the diaphragm. An analysis of the phenomena showed that this dip or notch probably occurs when the diaphragm is not moving purely as a piston, since the driving force (f) has a small component which is not along the longitudinal axis of the diaphragm. This is true if the linkage between armature and diaphragm is rigid and has a rigid connection at each end. In this case, the driving force (f) is perpendicular to the plane defined by connecting a mechanical driving point on the diaphragm and the axis around which the armature pivots. A component F sine 0, where 0 is the resulting angle between the longitudinal axis and the force F is responsible for the reduction of sound output in a limited frequency range. This component induces resonant antiphase modes of vibration of different areas of the diaphragm.
Various acceptably workable linkages which absorb or redirect the component F sine 6 are available. One particularly functional linkage is described by Morrison. However, the fabrication and assembly costs, related to producing Morrison s linkage in a transducer assembly, are prohibitive, if realistic pricing is to induce a sufficient market demand to support mass production.
Morrisons linkage basically consists of a tubular drive rod flattened at its ends, one end being cemented to the diaphragm, the other end being soldered to one end of the balanced armature. In a production assembly, the soldering of the drive rod to the armature is unattractive. It is of necessity a hand operation requiring a skilled craftsman. At best the resulting solder joints are of non-uniform quality and of unpredictable lifetime reliability. If too little solder is applied to the joint, the joint instead of the flattened end will flex and fatigue during operation of the receiver. Eventually, the joint will break. If the joint has too much solder applied, the flattened end of the drive rod is excessively soldering in addition to a requirement for clamped alignment until the cement has hardened. This, in itself, requires a number of jigs and fixtures in addition to a degree of craftsmanship. The cost of this joining process is prohibitive when providing joints between the balanced armature and the drive rod. As with a solder joint, the quantity of cement applied is critical. Cementing, however, is quite acceptable for later joining the drive rod to the diaphragm. At this point in the assembly, alignment and positioning is inherent in the structure of the transducer thus far assembled. Further, the cement is restricted to a non-critical area of the drive rod by the presence of the diaphragm.
Applying spot welding techniques in joining Morrisons linkage to the armature is impractical. The available surface area relative to weld area, required for strength, is insufficient to dissipate the heat required to effect a conventional spot weld. Thus any attempt to effectively spot weld Morrisons drive rod to the armature tends to destroy the end of the drive rod.
The present invention provides an economical and reliable electroacoustic transducer having an inverted balanced armature and diaphragm assembly. By placing the diaphragm in the end of the transducer opposite the sound emission orifice in a housing, a relatively large coupling volume is provided. The frequency response characteristic, in a loosely coupled condition, is thus improved. Another feature in the transducer is that of an improved linkage between the balanced armature and the diaphragm. The linkage includes a linkage bar and an armature extension. The armature extension is normal to the armature. This extension provides a relatively large flat surface suitable for spot welding. The linkage bar is a thin strip of highly elastic metal with a central strengthening formation, such as flanges. Thus a light, centrally rigid linkage bar having flexible end portions is provided. This linkage bar is able to transmit the driving force, between the armature and the diaphragm, along the longitudinal axis of the diaphragm, the related lateral component of the driving force being absorbed by flexing of the end portions. The surface area of one end of the linkage bar and of the armature extension provide sufficient area for heat dissipation during spot welding of the one end of the linkage bar to the armature extension. In production, spot welding provides joints of consistant high quality at relatively low cost.
The present invention is a linkage between a pivoting armature and an acoustical diaphragm in an electroacoustic transducer. The linkage comprises an armature extension normal to the armature and approximately co-planar with the longitudinal axis of the diaphragm. A linkage bar, having a reinforcing formation along its central portion, to provide a relatively rigid central portion and two flexible end portions, is spot welded at one end, to the armature extension and attached at the other end to the diaphragm. Thus, the linkage bar couples longitudinal motion between the armature and the diaphragm. Associated lateral motion, induced by the pivoting action of the armature, is substantially absorbed by the flexible end portions of the linkage bar.
The present invention will be more fully understood with reference to the following description of an example embodiment incorporating the invention and with reference to the accompanying drawings, in which:
FIG. 1 is an exploded assembly view of a balanced armature transducer in accordance with the present invention;
FIG. 2 is an after-assembly cross-sectional view of the balanced armature transducer illustrated in FIG. 1;
FIG. 3a is a side view of the armature and linkage bar assembly in FIGS. 1 and 2;
FIG. 3b is a bottom plan view of the armature and linkage bar assembly in FIG. 3a;
FIG. 30 is an end view of the armature and linkage bar assembly in FIG. 3a;
FIG. 4a graphically illustrates the relative frequency responses of various receivers when closely coupled to an artificial ear;
FIG. 4b graphically illustrates the relative frequency responses of an inverted" balanced armature receiver and a conventional balanced armature receiver when loosely coupled to an artificial ear.
In FIGS. 1 and 2, the chamber of the transducer consists of an aluminum housing 1, in a hollow cylindrical form having a side wall 6. The side wall 6 terminates at a shoulder 3 adjacent an annular ridge 4 which encompasses a face portion 5 having orifices 2 therein. A thin acoustically transparent plastic membrane 60 lies across the face portion 5, resting on the ridge 4.
A retaining ring 40 fits into the housing 1 with its lower surface adjacent the shoulder 3, such that the edge of the plastic membrane 60 is held therebetween. The upper surface of the retaining ring 40 carries four clearance notches 41, three of the notches being visible in FIG. 1, and a rectangular protrudance 42. A circular frame 50 rests upon the upper surface of the retaining ring 40. A vertical notch 52, in the side of the circular frame 50, cooperates with the rectangular protrudance 42 to contain a pair of electrical leads 61. A rim 53, forming the upper edge of the circular frame 50, defines the outer circumference of a depressed clamping surface 56. Adjacent the inner circumference of the clamping surface 56, the frame extends laterally defining an annular retaining flange 54, having a wedge shaped cross-section and terminating at an orifice defined by an inner rim 55. An electromechanical transducing unit resides within the frame 50 fixed to the lower edge of the frame 50 and retained against the flange 54. The assembly rests upon the retaining ring 40, with portions of the unit 70 occupying space in the clearance notches 41.
An acoustical diaphragm 10, having a conical portion 12, bordered by a fiat peripheral rim ll, rests upon the clamping surface 56. The rim 11 lies between the clamping surface 56 and a clamping ring 17. An attachment hole 13 lies in the center portion of the conical portion 12. A by-pass hole 14 is also provided within the area of the conical portion 12.
The transducing unit 70 includes lower and upper pole pieces 71 and 73 having a cylindrical permanent magnet clamped therebetween. A pair of lower pole faces 72 and a pair of upper pole faces 74, extending toward each other on U-shaped extensions of the lower and upper pole pieces 71 and 73 respectively, are spaced closely adjacent to provide a pair of gaps between the pole faces 72 and 74. In one embodiment, the permanent magnet 75 is ALNICO V* (Trademark) and suitable pole pieces 71 and 73 are fabricated from PERMALLOY* (Trademark). A bobbin 28 having a coil 29 wound thereon and connected to the electrical leads 61 resides within the U-shaped extension of the pole pieces 71 and '73. The central portion of the bobbin is hollow to accommodate an armature 20. The armature 20, later discussed in detail with reference to FIGS. 3a, 3b and 3c, is an integral structure. The armature 20 includes an armature body 21, the end portions of which reside within the gaps between the pole faces 72 and 74. The armature body 21 is pivotable on a pair of torsion bars 22 each terminating at one of a pair of arms 24L along each edge of the armature body 21. One end of the armature body 21 carries an armature extension 23 normal to the armature 20. Shims 25 fit over the arms 24 whereupon the assembly is clamped into position between the pole faces '72 and 74. The shims 25 are of a non-magnetic material such as brass and serve to maintain the required gaps between the armature body 21 and the pole faces 72 and '74.
A linkage bar 100 carries a pair of reinforcing flanges 102 on either edge of the central portion of the bar 100. One end 101 of the linkage bar 100 is spot welded to the armature extension 23. The other end 103 of'the linkage bar 100 extends through the attachment hole 13 in the diaphragm and is cemented thereto with a bead of epoxy resin. A non-magnetic elastic material such as a copper beryllium alloy is suitable for fabrication of the linkage bar 100.
A backplate 30, of thermoplastic material, basically consists of a rim 35 and a central raised portion 36. A notch 31 in the rim 35 cooperates with the vertical notch 52 to route the leads 61 to a pair of terminals 63. The terminals 63 are fixed in and extend through the raised portion 36. The terminals 63 each include a furl 64. The leads 61 are each drawn through a furl 64. The backplate 30 is placed upon the clamping ring 17. A shoulder 32 adjacent the under surface of the rim 35, centers the backplate 30 on the clamping ring 17, with the under surface of the rim 35 resting upon the clamping ring 17. The upper edge of the side wall 6 is crimped inward to bear upon the upper surface of the rim 35. Thus, the whole assembly is rigidlyheld together. The leads 61 are lightly tensioned to remove any slack and then soldered within the furls 64. Screws 65 on the terminals 63 provide for attachment of external leads. An opening 38, in the raised portion 36, receives an acoustical resistance disc 62 which is positioned by an inner shoulder 39 in the opening 38. In one embodiment the acoustical resistance disc 62 is of sintered stainless steel. To fix the disc 62 in place it is heated and then pressed into the opening 38. The thermoplastic backplate material, in contact with the heated disc 62 flows into the surface of the sintered stainless steel disc 62 thereby fixing the disc 62 in position. The interior of the transducer is thus sealed by the acoustical resistance 62 and the plastic membrane 60.
Referring to FIGS. 3a, 3b and 3c, the armature is an integral structure and includes a body portion 21 linked to a pair of arms 24 by a pair of torsion bars 22. An armature extension 23 carried at one end of the armature body 21 is normal to the plane of the armature. A linkage bar 100, having flexible end portions 101 and 103, serves to link the armature 20 to the diaphragm 10 illustrated in FIGS. 1 and 2. One end 101 of a linkage bar 100 is spot welded to the armature extension. The linkage bar includes a pair of reinforcing flanges 102 centrally located along the edges of the linkage bar about equidistant from the upper surface of the armature 20 and a pair of shoulders 104. A portion of the other end 103 of the linkage bar 100 extends beyond the pair of shoulders 104 and provides for cementing of the diaphragm to the linkage bar 100. Near the end of the armature body 21, adjacent the linkage bar 100, a lightening hole 26 is provided to improve the balance of the mass of the armature 20 and the linkage bar about the axis of the torsion bars 22. The final assembly of the armature 20 and the linkage bar 100 includes a pair of U-shaped non-magnetic shims 25 which fit over the arms 24. The shims serve to provide gaps between the pole faces 72 and 74 as illustrated in FIG. 2.
In this embodiment the armature 20 is fabricated from sheet stock material having a high magnetic permeability, such as PERMALLOY, of a thickness dimension of between 0.0140 and 0.0150 inches. The shims are formed from brass stock sheet of about 0.005 inches thickness thus providing about .005 inches of gap between the lower and upper sides of the end portions of the armature body 21 and the pole faces 72 and 74 respectively. The linkage bar 100 is formed from a copper berillium alloy sheet stock of about 0.005 inches in thickness. One advantage in this embodiment is that the spot welding process, used to join the linkage bar 100 to the armature extension 23, is economical and easily controlled to provide spot weld joints of constant quality. The area of the spot weld is relatively small as compared to the surrounding area of the one end 101 of the linkage bar 100 and the armature extension 23. The surrounding area acts advantageously as a heat sink during the spot welding process.
The operating principle of balanced armature transducers is well known. Thus, in the following description of the operation of the example embodiment illustrated in the FIGS. 1 to 3, only the advantageous characteristics with respect to prior art devices will be elaborated upon.
In the art of telephony, consistant quality, a high degree of lifetime reliability and economy of manufacture are of prime importance. Consequently, in practical transducer design, somewhat less than the highest obtainable efficiency is tolerated in consideration of the above requirements. The receiver herein described provides a unique balance between the above requirements. It displays an improved frequency response, as compared to other balanced armature receivers operating under similar conditions, while maintaining a minor advantage in efficiency over the previously discussed ring armature receiver.
In operation of the transducer as a receiver, fluctuating electrical signals are applied to the coil 29. The resulting coil current induces fluctuation in the magnetic flux between the respective pole faces 72 and 74 thus causing sympathetic pivoting armature motion in a well known manner. The motion of the armature 20 is transmitted to the linkage bar 100 via the armature extension 23 and the spot weld at the one end 101. This motion includes desirable longitudinal motion and undesirable associated lateral motion induced by the pivoting armature motion. The longitudinal motion is transmitted by the linkage bar 100 to the diaphragm 10. Most of the lateral motion is absorbed by flexing of the linkage bar 100 in the area of the one end 101 where it is clear of contact with the armature extension 23. The remainder of the lateral motion is absorbed in the area of the other end 103 between the shoulders 104 and the upper ends of the reinforcing flanges 102.
Sound waves, caused by motion of the diaphragm 10 in unison with the longitudinal motion of the armature 100, are transmitted through the atmosphere in the chamber 1, through the acoustically transparent membrane 60 and out through the orifices 2. The by-pass hole 14 modifies the frequency response of the receiver to provide at least a 6 decibel per octave roll off in the portion of the frequency response below about 425 Hz. This roll off is approximately inverse to the transmission characteristics of typical telephone line and thus prevents the receiver from having a boomy sound, when used as a telephone receiver.
The chamber 1 in combination with backplate 30 and the acoustical resistance 62 contain a volume which is effectively partitioned by the diaphragm 10. There is thus defined a coupling cavity between the diaphragm l and the orifice 2 within which the balanced annature drive assembly is fixed. 0n the other side of the di aphragm a damping cavity is defined by the diaphragm l0 and the backplate 30 carrying the acoustical resistance 62.
In this embodiment the volume of the damping cavity is about 1 cubic centimeter and the volume of the coupling cavity is about 3.4 cubic centimeters. The volume of the coupling cavity is sufficient to lower the impedance of the receiver such that variations in acoustical coupling between the diaphragm 10 and a users ear have a minimized effect upon the frequency response characteristic. The damping cavity provides damping to compensate for the natural resonate frequency of the diaphragm and armature assembly so that the transducer exhibits a relatively smooth frequency response.
In typical prior art pivoted armature receivers the volume of the coupling cavity is smaller than that of the damping cavity. In most practical embodiments of the present inverted balanced armature receiver for use in telephones, the volume of the coupling cavity is at least 3 times greater than that of the damping cavity. The relatively large volume of the coupling cavity effectively provides a transducer having a relatively low acoustic source impedance. Thus the frequency response characteristics, over a range of physical coupling conditions with a users ear, tend to be significantly more constant that with prior pivoted armature receivers.
The frequency response characteristics of an inverted balanced armature receiver, suitable for use in a telephone, as compared to the frequency response characteristics of some other receivers, are seen with reference to FIGS. 4a and 4b. In graphically plotting the curves in FIGS. 4a and 4b, an artificial ear Type 4153 manufactured by Bruel and Kjaer of Naerum Denmark was used. In each of the figures the horizontal axis is in Hertz and the vertical axis is in decibels.
In FIG. 4a, curve A represents the frequency response of the example embodiment inverted balanced armature receiver. Curve B, an excursion or notch on the curve A, represents the typical effect of using a rigid linkage in the example embodiment. This effect is generally well known and would be expected to occur in any pivoted armature transducer having a rigid linkage. In a receiver as generally discussed above but with a rigid linkage the notch would occur between about 2,800 Hz and 3,000 Hz. However, this will vary somewhat with relation to the physical dimensions of various pivoted armature transducers. The use of a linkage arrangement in accordance with the present invention completely eliminates this notch and provides a very reliable linkage of economical manufacture.
Curve C represents the frequency response of a typical ring armature receiver extensively used in telephones. Curve C provides a comparison between the frequency response of the example embodiment and the typical ring armature receiver. In comparison of the curves A and C, there is little difference in the overall acoustical quality of the receivers. The overall frequency response characteristic of the example embodiment makes it most attractive as an economical replacement for the popular ring armature receiver.
In FIG. 4b the frequency response of the example embodiment, represented by curve D, and of a conventional balanced armature receiver, represented by curve E, are provided. In this case however, a 0.12 inch air gap between each receiver and the artificial ear was maintained to simulate a typical users ear loosely coupled with the receiver. In the area between about 1,000 Hz and 2,800 Hz, there is a vast improvement in the response of the inverted balanced armature receiver, as compared to the conventional balanced armature receiver. In using the conventional receiver, one notices a predominant tinny or thin quality of sound in the loosely coupled condition. In using the inverted receiver, one notices a drop in volume between closely and loosely coupled conditions however, the sound quality is relatively constant and pleasing.
Although the example embodiment of the receiver has been described with respect primarily to telephone receiver applications, it will be understood by those skilled in the art that the principles of construction of the linkage assembly herein disclosed are similarly extendible to all electroacoustic transducers of the pivoting armature type. The principles of construction of inverted" diaphragm transducers are extendible to any transducer having a diaphragm which may be required to operate under variable coupling conditions.
What is claimed is:
1. In an electroacoustic transducer having a pivoting armature and an acoustical diaphragm rigidly fixed along its periphery with a central portion of the diaphragm being longitudinally displaceable, a linkage means, linking the central portion of the diaphragm to the armature, the linkage means comprising:
an armature extension normal to the armature, said extension having a relatively large flat surface area approximately co-planar with the longitudinal axis of the diaphragm and extending away from the diaphragm;
a linkage bar having a substantially rectangular crosssection, a reinforcing protrusion, extending out of the plane of the longer dimension of the rectangular cross-section along the central portion of the linkage bar to provide a relatively rigid central portion and flexible end portions, one flexible end portion of the linkage bar attached to the central portion of the diaphragm the other flexible end portion of the linkage bar spot-welded to the armature extension, so that the joining length of the linkage bar between the diaphragm and the armature extension is longer than the distance between the diaphragm and the armature, the whole so constructed and arranged that longitudinal motion is coupled between the diaphragm and the armature by the linkage bar while associated lateral motion induced by the pivoting action of the armature is substantially absorbed by the flexible end portions of the linkage bar in the areas between the diaphragm and the reinforcing formation and between the spot weld and the reinforcing formation.
2. An electroacoustictransducer comprising:
an acoustical diaphragm, a central portion of the diaphragm being longitudinally displaceable;
an electromechanical transducer assembly having a pair of magnetic pole pieces each having a U- shaped portion;
an armature in the electromechanical transducer assembly, the armature being an integral structure having a body portion having two broad end portions tapering to a narrower central portion, one end portion having a lightening hole therein, and carrying a narrow projection in the plane of the armature, a pair of torsion bars extending laterally from opposite edges of the narrower central portion in the pivot axis of the body portion, a pair of support arms each terminating one of the pair of torsion bars, the support arms being normal to the torsion bars and extending about the length of the body portion, one adjacent each edge of the body portion, an armature extension carried by said narrow projection normal to the body of the armature, the extension having a relatively large flat surface area approximately co-planar with the longitudinal axis of the diaphragm and extending away from the diaphragm; the assembly being so constructed and arranged that the ends of the arms in combination with spacers are clamped between U-shaped portions of the pole pieces, the spacers providing air gaps between the U shaped portions and the ends of the armature body to permit pivoting of the armature body upon the torsion bars;
a linkage bar having a reinforcing formation along its central portion to provide a relatively rigid central portion and flexible end portions of substantially rectangular cross-section, one flexible end portion of the linkage bar attached to the central portion of the diaphragm, the other flexible end portion of the linkage bar spot-welded to the armature extension so that the length of the linkage bar between the diaphragm and the armature extension is longer than the distance between the diaphragm and the armature; the whole so constructed and arranged that longitudinal motion is coupled between the diaphragm and the armature by the linkage bar while associated lateral motion induced by the pivoting action of the armature is substantially absorbed by flexing of the flexible end portions of the linkage bar in the areas between the diaphragm and the reinforcing formation and between the spot weld and the reinforcing formation, the longer length of the linkage bar reducing the flexing requirement of the flexible end portions.
3. In an electroacoustic transducer having a pivoting armature and an acoustical diaphragm being longitudinally displaceable, a linkage means, linking the central portion of the diaphragm to the armature, the linkage means comprising:
an armature extension normal to the armature, said extension having a relatively large flat surface area approximately co-planar with the longitudinal axis of the diaphragm and extending away from the diaphragm;
a linkage bar having a pair of parallel reinforcing flanges along opposite edges of the central portion of the linkage bar, to provide a relatively rigid central portion and flexible end portions, one flexible end portion of the linkage bar attached to the central portion of the diaphragm, the other flexible end portion of the linkage bar spot-welded to the armature extension, so that the joining length of the linkage bar between the diaphragm and the armature extension is longer than the distance between the diaphragm and the armature, the whole so constructed and arranged that longitudinal motion is coupled between the diaphragm and the armature by the linkage bar while associated lateral motion induced by the pivoting action of the armature is substantially absorbed by the flexible end portions of the linkage bar in the areas between the diaphragm and the reinforcing formation and between the spot weld and the reinforcing formation.