US3258543A

US3258543A - Dynamic microphone

Info

Publication number: US3258543A
Application number: US227498A
Authority: US
Inventors: Jr Harold F Mosier
Original assignee: Electro Voice Inc
Current assignee: Bosch Security Systems Inc
Priority date: 1962-10-01
Filing date: 1962-10-01
Publication date: 1966-06-28
Anticipated expiration: 1983-06-28
Also published as: AT259044B; GB1055747A; US3434205A; DE1226645B

Description

June 28, 1966 H. F. MoslER, JR

DYNAMIC MICROPHONE 2 Sheets-Sheet. 1

Filed Oct, l, 1962 i www, ws da t 0 m fir @Me/.w Ehm? n Im @.5 0 t www ya z v0 9, ||||1. ww

June 28. 1966 H. F. MoslER, JR

DYNAMIC MICROPHONE 2 Sheets-Sheet 2 Filed 0oz, 1, 1962 ,Burmeister {infr/mer United States Patent O 3,258,543 DYNAMIC MICROPHONE Harold F. Mosier, Jr., Buchanan, Mich., assigner to Electro-Voice, Incorporated, Buchanan, Mich., a corporation of Indiana Filed Oct. 1, 1962, Ser. No. 227,498 22 Claims. (Cl. 179-1155) The present invention relates to electromagnetic transducers, and particularly to microphones and earphones of the moving coil type. The invention also relates to methods for making electroacoustical devices.

Moving coil type microphones and earphones are well known, and the textbook, Acoustical Engineering, by Harry F. Olson, D. Van Nostrand & Co., Princeton, New Jersey, 1957, describes and analyzes the operation of such microphones at pages 260 through 263. Such microphones and earphones generally employ a case which has an opening closed by a vibratal diaphragm. The vibratal diaphragm of the microphone or earphone carries a coil which is disposed within a gap in a magnetic circuit. In such a microphone, movement of the diaphragm, and hence the coil, referred to as a voice coil, generates an electric current in the voice coil, since it is cutting magnetic lines of force. In such an earphone, an alternating electric current ilowing through the coil vibrates the diaphragm.

Since the magnitude of the current generated in the voice coil of a microphone responsive to vibration of the diaphragm is proportional to the ilux density in the magnetic gap, the magnetic gap is made as small as practical, and the voice coil is carefully centered in the magnetic gap. The same considerations also apply to construction of earphones. It i-s one of the objects of the present invention to provide a structure for use in microphones and earphones which facilitates centering of the coil within the magnetic gap.

In a microphone, the voltage generated in the voice coil is .also proportional to the velocity of the voice coil Within the magnetic gap. As a result, the microphone will have a uniform sensitivity throughout its frequency range if the velocity of the voice coil is made independent of frequency. The velocity of the voice coil may be made independent of frequency if it is essentially resistance controlled rather than mass controlled.

Resistance control of the voice coil is conventionally obtained by damping the diaphragm which carries the voice coil. For this purpose, the flow of air behind the diaphragm is restricted by silk or felt resistance members, such as shown in Patent No. 3,014,099 to Fiala, entitled, Electroacoustic Transducer. It has often proven difficult to provide the proper material to produce the desired resistance for a particular microphone. Further, felt and silk resistance members are subject to deterioration.

Slots or air gaps have also been employed prior to the present invention to provide the desired resistance control of the diaphragm, as shown by the Fiala patent and Patent No. 2,773,130 to Olson, entitled, Acoustical Resistance for Pressure Type Microphones. It has been dicult in practice to control these slots or air gaps to provide the required damping.

It is therefore a further object of the present invention to provide an improved dynamic microphone structure which utilizes slots or air gaps to control a diaphragm, and to provide such a microphone structure which may be readily reproduced.

It is also an object of the present invention to provide a dynamic microphone structure which may be assembled more readily than prior microphone structures, is less costly to assemble, and possesses more uniform operating characteristics when produced under mass production techniques.

It is also an object of the present invention to provide 3,258,543 Patented June 28, 1966 an earphone structure which may be assembled more readily than prior earphone structures, is less costly to assemble, and which produces more uniform operating characteristics when produced by mass production techniques.

It is a further object of the present invention to provide a method of making electroacoustical devices which simplies assembly techniques, reduces costs, and increases reliability, particularly reducing the number of units rejected due to coils scraping Within the magnetic gaps of the units.

Further objects of the present invention will become apparent from a further consideration of this specication, particularly when viewed in the light of the drawings, in which:

FIGURE 1 is an exploded view of a microphone structure constructed according to the teachings of the present invention and illustrated in FIGURES 1 through 5;

FIGURE 2 is a sectional view of the microphone structure taken along the line 2-2 of FIGURE 3;

FIGURE 3 is a sectional view of the microphone structure taken along the line 3-3 of FIGURE 2;

FIGURE 4 is a sectional view of the microphone taken along the line 4-4 of FIGURE 3;

FIGURE 5 is a fragmentary sectional view of the microphone taken along the line 5-5 of FIGURE 4, the external casing being omitted;

FIGURE 6 is a rear elevational view (partly broken away and in section) of a modified microphone construction according to the present invention;

FIGURE 7 is a schematic electrical circuit diagram illustrating the mechanical network which constitutes the microphone structure in electrical symbols;

FIGURE 8 is a fragmentary sectional View of another embodiment of the present invention taken on a plane similar to that of FIGURE 5;

FIGURE 9 is a vertical sectional view of an earphone constructed according to the teachings of the present invention;

FIGURE 10 is a front elevational view of .the earphone of FIGURE 9;

FIGURE 11 is a rear elevational view of the earphone of FIGURES 9 and l0; and

FIGURE 12 is a vertical sectional 'View of a mold suitable for manufacturing the earphone of FIGURES 9 through 11.

In the embodiment illustrated in FIGURES 1 through ICC ` 5, the microphone structure has a diaphragm 10, which is mounted on a supporting ring 12 and carries a voice coil 14. The supporting ring 12 also mounts a cylindrical magnet 16 and a U-shaped magnetic pole piece 18. In addition, a pair of lugs 20 are supported on the surface of the supporting ring 12 opposite the diaphragm 10.

Referring to FIGURES 2 through 5, it will be seen that the supporting ring 12 has a ferromagnetic part 22 and a nonmagnetic part 24. The ferromagnetic part 22 is in the form of a circular disc, or plate, except for an indentation 26 at its periphery. The ferromagnetic part or disc 22 has a central cylindrical opening 28 for accommodating the magnet 16, and a plurality of apertures 30 disposed at the same distance from the center of the ferromagnetic disc 22 and spaced at equal intervals about the periphery of the disc, lalthough it is not required that the apertures be equally spaced or at a common distance from the center.

The nonmagnetic part 24 has a disc portion 32 disposed on one side of the ferromagnetic part or disc 22 and a ring portion 34 disposed on the other side of the disc 22. The disc portion 32 and the ring portion 34 are interconnected by rod portions 36 which extend through the apertures 30, and the disc portion 32, ring portion 34 and rod portions 36 are preferably constructed of an integral mass of the same material, such as plastic. The

disc portion 32 of the nonmagnetic part 24 is also provided With a central cylindrical opening 38 which is coaxial with the opening 28 in the ferromagnetic part or disc of the supporting ring 12. The diameter of the opening 38 is equal to the outer diameter of the magnet 16, but the disc portion 32 is provided with a coaxial recess 39 and a plurality of slots 44 which extend from the recess 39 parallel to the axis of the magnet 16, thus forming ribs 40. The magnet 16 is in abutment with the ribs 40 of the opening 38 and the cylindrical surface 42 of the magnet 16 and slots 44, thereby form arcuate spaced passages from the surface of the recess 39 through the disc portion 32, as illustrated in FIGURE 4. In the particular construction here described, the passages formed by the slots 44 are equally spaced about the opening 38 forming four slots 44, although it is to be understood that the slots 44 need not be equally spaced and may be more or fewer in number.

The ribs 40 have the double function of securing and centering the magnet 16 on the supporting ning 12 and providing the proper clearance for the slots 44. The slots provide a resistance and an inductive component for damping the diaphragm, as will be hereinafter explained.

The magnet 16 extends from the supporting ring 12 to the pole piece 18. The pole piece 18 has a pair of

legs

46 and 48 which extend from a cross member 50, the cross member 50 abutting the end of the magnet 16 opposite the supporting ring 12. The legs 46 and 43 extend into abutment with the ferromagnetic disc 22 which has a diameter greater than the diameter of the disc portion 32 of the nonmagnetic part 24 of the supporting ring 12. The

legs

46 and 48 abut the edges of the disc portion 32 and abut the ferromagnetic disc 22 at their ends. The magnet 16 extends into the opening 28 in the ferromagnetic disc 22, thus forming a circular gap 52 between the magnet surface 42 and the confronting opening 28 of the ferromagnetic disc 22. The voice coil 14 is translat ably disposed Within this gap 52 along the axis of the magnet 16. The magnetic circuit includes the magnet 16, the cross member 50,

legs

46 and 48, ferromagnetic disc 22, and the gap 52.

The diaphragm has .a circular perimeter with a diameter equal to the inner diameter of the ring portion 34 of the supporting ring 12. The diaphragm 10 has a flat ring-shaped penipheral portion 54 which is cemented on the confronting surface of the ferromagnetic disc 22, and is centered coaxially about the central axis of the

openings

28 and 38 by the inner perimeter of the ring portion 34 of the supporting ring 12. The diaphragm 10 has a plurality of utes 56 disposed in a circular band coaxially within a circular ilange portion 57 which extends from the peripheral flat portion 54. The band of flutes 56 and flange portion 57 space a circular central portion 58 of the diaphragm from the ferromagnetic disc 22. The circular central portion 53 of the diaphragm 10 is in the form of a dome protruding outwardly from the ferromagnetic disc 22, and the portion of the diaphragm within the circular flange portion 57, including the domed central portion 58 and utes 56, is free to vibrate relative to the disc 22. The coil 14 also is circular in form and mounted on the surface of the diaphragm 10 confronting the ferromagnetic disc 22. The coil 14 has a diameter selected to permit it to be freely translated within the gap 52. The coil 14 is cemented onto the diaphragm 10 about the perimeter of the domed central portion 58 thereof, and the ends of the coil 14 are brought out through apertures 60 in the diaphragm 10 at the interface between the domed central portion 58 and `the portion having the flutes 56.

The nonmagnetic part 24 of the supporting ring 12 has a flange 62 which fills the indentation 26 of the ferromagnetic part 22, thereby providing a completely circular periphery to the supporting ring 12. A groove 64 normal to the surface of the ferromagnetic disc 22 is disposed centrally of the flange 62 to permit the ends of the coil 14 to traverse from the face of the diaphragm 10 to the lugs 20 mounted on the nonmagnetic part 24 of the supporting ring 12 on the side of the ferromagnetic disc 22 opposite the diaphragm 10. The ring portion 34 of the supporting ring 12 is also provided with a gap 68 for the ends of the coil 14.

In the manufacture of a microphone structure as described in FIGURES l through 5, the nonmagnetic part 24 of the supporting ring 12 is molded as a unit on the ferromagnetic part 22 thereof, the ring portion 34, rod portions 36, and disc portion 32 being molded in a single operation. This same molding operation forms the ribs 40 on the opening 38. The microphone structure is assembled by inserting the cylindrical magnet into the opening 38 and the opening 28, and thereafter mount-ing the pole piece 18 in position. A layer of cement 70 may be disposed between the pole piece 18 and the end of the magnet 16, and cement layers 72 and '74 may be disposed between the ends of the

legs

46 and 48 of the pole piece 18 and the ferromagnetic disc 22.

The voice coil 14 is cemented on the surface of the diaphragm 10 prior to assembly, and thereafter the diaphragm 10 is trimmed or cut to have an outer diameter approximately equal to the inner diameter of the ring portion 34 of the supporting ring 12 prior to assembly and coaxial with the coil. Thereafter, it is only necessary to cement the flat peripheral portion 54 of the diaphragm on the surface of the ferromagnetic disc 22 within the ring portion 34 of the supporting ring 12 to automatically center the voice coil 14 in the gap 52.

In the construction of FIGURES l through 5, the pole piece 13 is in the form of `a U-shaped

strip having legs

46 and 48 and a cross member 50. In the construction of FIGURE 6, the pole piece 18A is in the form of a ferromagnetic -cup in which a cylindrical portion 46A replaces the

legs

46 and 48, and a flat bottom 50A replaces the cross member 50 of the first embodiment. Further, the flange 62 is provided with a pair of outwardly extending

tabs

76 and 78 which carry a pair of lugs 80 which replace the lugs 20 of the prior construction. In other respects, the embodiment of the invention illustrated in FIGURE 6 is the same as that illustrated in FIGURES l through 5.

It is of course intended that Ithe microphone structures here set forth shall be used in combination with a casing, designated 82 in FIGURE 3 and a wind screen 84 to form a complete microphone. Depending upon the type of microphone desired, the casing and wind screen may include other acoustical or mechanical elements.

FIGURE 7 illustrates in electrical symbols the mechanical circuit employed by the microphone structure. The acoustical force impressed upon the diaphragm 10 is indicated by the letter f. The inductance 86 represents the mass of the moving system of the microphone, the resistance symbol 33 represents the mechanical resistance of the moving system, and Ithe capacitor symbol 9) represents the compliance of the diaphragm and suspension of the microphone structure. The capacitor 92 represents the compliance of the chamber between the ferromagnetic disc 22 and the diaphragm. The other three symbols in FIGURE 7 represent the effect produced by the slots 44. More specifically, the inductance symbol 94 represents the equivalent mass of the slots, the resistance 92 represents the acoustical resistance of the slots 44, and the capacitor 98 represents the compliance of the chamber on the side of the supporting ring 12 opposite the diaphragm 10. By proper proportioning of these elements, the diaphragm may be damped to approach a resistance controlled microphone structure. Since the compliance of `the enclosed volume of air within the pole piece 46A or within the casing 82 enters into the damping of the diaphragm, these must be considered in designing the microphone.

In one particular construction, the outer diameter of the diaphragm 10 is approximately 1.06 inches and the diameter of the voice coil 14 is approximately 0.50 inch.

The diameter of the opening 38 is approximately 0.58 inch and the inner diameter of the recess 39 is approximately 0.535 rinch, the diameter of the arcuate surface of the slots 44 is approximately 0.528 inch, and the gap 52 has a width of approximately 0.0035 inch. The ferromagnetic disc 22 has a thickness of approximately 0.07 inch and is constructed of magnet iron. The ring portion 34 of the supporting ring 12 has an outer diameter of approximately 1.225 inches and an inner diameter of approximately 1.063 inches. The ring portion 34 extends from the ferromagnetic disc 22 a distance of approximately 0.03 inch, and the disc portion 32 has a thickness of approximately 0.108 inch. The material of the nonmagnetic portion 22 of the supporting ring 12 is polyethylene plastic. The diaphragm is also constructed of plastic and may be Mylar.

FIGURE 8 fragmentarily illustrates a dynamic microphone which is a modification of the microphone illustrated in FIGURES l through 5. Identical reference numerals have been used in FIGURE 8 for elements which are identical to those in FIGURES 1 through 5. In this embodiment of the invention, the ferromagnetic disc 22A has no apertures, such as the apertures 30 of the disc 22, for accommodating rod portions of the nonmagnetic part, but the disc 22A is in all other respects identical with the disc 22 `of the embodiment of FIGURES 1 through 5.

The nonmagnetic part 24A of FIGURE 8 has `a disc portion 32A with a larger diameter than the plate or disc 22A. A coaxial circular ange 90 extends from the perimeter of the disc portion 32A about the perimeter of the disc 22A, and a flat ring portion 92 extends from the flange 90 inwardly and in parallel abutment with the surface of the plate 22A opposite the disc portion 32A of the nonmagnetic part 24A. In this manner, the disc or plate 22A is clamped between the disc por-tion 32A of the nonmagnetic portion 24A Iand the ring portion 92. The inner periphery of the ring portion 92 is a cylindrical surface symmetrically disposed about the gap 52, and the outer perimeter of the diaphragm 10 abuts this surface and is -cemented on the surface of the disc 22A in the manner of the embodiment of FIGURES l through 5.

It is to be noted that a pair of apertures 94 extend through the disc portion 32A of the nonmagnetic part to accommodate the legs 46 of the pole piece in order to permit the legs to abut the disc of plate 22A and complete the magnetic circuit. Also, the nonmagnetic part may be plastic, as in the previous embodiments, and the nonmagnetic part may be molded on the magnetic part, as previously described.

FIGURES 9, 10 and 11 illustrate an earphone constructed according to the present invention. As illustrated, the earphone is intended to be positioned within a casing, but it is to be understood that the illustrated earphone is fully operative. As illustrated in the figures, a ring magnet 100 is disposed in abutment with a ferromagnetic disc 102 which has a circular central aperture 104 therein. A cylindrical pole piece 106 of ferromagnetic material is disposed coaxially within the ring magnet 100, and a generally circular ferromagnetic plate 108 is mounted adjacent to end of the pole piece 106 opposite the disc 102. The plate 108 has a plurality of apertures l112 therethrough disposed along a circular path coaxially disposed Iabout lthe opening 110 in the plate 108.

The entire assembly of magnet 100, disc 102, pole piece 106, and plate 108 are held together in a unitary assembly by a casing 114 of plastic material, such as nylon. The casing 114 has a flat bottom 116 disposed in abutment with the disc 102, a cylindrical wall 118 extending from the flat bottom in abutment with the exterior surface of the ring magnet 100, and a cylindrical sleeve 120 disposed between the ring magnet 100 and the pole piece 106 abutting these elements to form a spacer member therebetween. In addition, the casing has a ring portion 122 disposed in abutment with the surface of the plate 108 opposite the ring magnet and coaxial with the ring magnet 100. The ring portion 122 is connected to the other portions of the casing by posts 124 to form an integral plastic body. The plate 108 has a recess at the periphery thereof, and this recess contains a protrusion 126 of the casing, and two terminal lugs 128 are embedded within the protrusion of the casing 114.

A damping plug 130 is mounted within the inner cylindrical surface of the pole piece 106 and has a hollow sleeve 132 which abuts the pole piece 106 and may be cemented thereto. The plug 130 also has a cap 134 with an opening 136, and the opening 136 is covered by a layer of cloth 138.

A diaphragm 140 with a circular perimeter of approximately the same diameter as the inner diameter of the ring portion 122 of the casing 114 is mounted within the ring portion 122 on the plate 108. The diaphragm is similar in construction to the diaphragm 10 previously described, and has a peripheral portion 54, a flange 57, and a dome 58, but the fluted portion 56A between the dome 58 and the flange 57 has flutes disposed along radii of the diaphragm 140. In the embodiment of FIGURES 1 through 5, the flutes are disposed on axes tangential to the voice coil 14. In the construction of FIGURES 1 through 5 the flutes provide additional compliance to the diaphragm 10, which is desirable for a microphone application. The flutes of the diaphragm 140 form stiifening ribs and reduce the compliance of this diaphragm, which is desirable in an earphone construction.

It is to be noted that the voice coil 14 is also mounted on the diaphragm 140 in an identical manner to that illustrated in FIGURES l through 5 and disposed in a magnetic gap formed between the pole piece 106 and the plate 108. Also, the sleeve 120 of the casing 114 has a circular recess 142 extending from the plate 108 to provide adequate room for translation of the voice coil. The voice coil leads are brought out from the coil 14 between the diaphragm 140 and the plate 108 to the lugs 128 mounted in the protrusion 126.

FIGURE 12 illustrates a cavity mold for injection molding the casing 114 and simultaneously mounting the disc 102, magnet 100, pole piece 106 and plate 108. The mold has an upper part and a lower part 152 which together form a cavity 154 with the contour of the exterior surface of the casing 114. In addition, there is a passage 156 extending through the upper portion of the mold for injection of plastic into the cavity 154. The mold also has a cylindrical sleeve 158 extending upwardly from the lower portion of the mold centrally within the cavity, and a cylindrical boss 160 extending from the lower portion of the mold coaxially within the sleeve 158.

To utilize the mold, the plate 108 is placed against the lower portion of the mold with the opening 110 thereof disposed about the sleeve 158. Also, the pole piece 106 is mounted coaxially between the sleeve 158 and the boss 160 of the lower portion of the mold. Next, the ring magnet 100 and the disc 102 are positioned on the plate 108, and thereafter the upper portion of the mold is closed over the lower portion of the mold.

The upper portion of the mold is provided with four pins 162 which mate with four openings 164 in the disc 102. The pins are positioned to also abut the interior surface of the ring magnet 100. As a result, the pins 162 hold the ring magnet 100 and the disc 102 in xed position within the mold. Plastic, in liquid phase, may then be injected into the cavity under pressure through the passage 156 to form the casing 114 when solidified, and simultaneously assemble the magnet 100, disc 102, pole piece 106, and plate 108 in a single operation.

After removing the assembly from the mold, the plug 130 is cemented into the pole piece 106. Thereafter, the assembled diaphragm and voice coil are cemented into place as indicated in connection with the embodiment of FIGURES 1 through 5. The leads from the voice coil are also connected to the terminal plugs 128 before the diaphragm 140 is cemented in position.

It is well known that materials and dimensions other than those illustrated may also be employed. Further, those skilled in the art will readily devise many modifications of the present invention and applications of the present invention beyond those here set forth. It is therefore intended that the scope of the present invention be not limited by the foregoing disclosure, but rather only by the appended claims.

The invention claimed is:

1. An electroacoustical device comprising a support member including a ferromagnetic plate having a cylindrical opening and a nonmagnetic member mounted on the plate having a first portion on one side of the plate, la second portion on the opposite side of the plate and an interconnecting portion extending from the irst portion to the second portion, .the first portion having an orice with a central axis on the axis of the opening in the plate, and the second portion forming a rib extending about the opening in the plate iand having a wall confronting the opening, a magnetic circuit structure including the plate and a ferromagnetic member having a central axis disposed on the axis of the opening in the plate, said member abutting -the orifice in the first portion of the nonmagnetic member -and having a cylindrical portion with a smaller diameter than the opening in the plate disposed coaxially within the opening, a diaphragm having a perimeter with the contour of the wall of the rib of the second portion of the nonmagnetic member, said diaphragm being disposed within the rib and mounted on the support member, and a cylindrical voice coil mounted on the diaphragm and disposed within the opening between the plate and the cylindrical portion of the ferromagnetic member.

2. An electroacoustical device comprising the elements of claim 1 in combination with an air impermeable casing acoustically sealed on the support member and surrounding the magnetic circuit structure.

3. An electroacoustical device comprising the elements of claim 1 wherein the magnetic circuit structure includes a ferromagnetic yoke having a first portion abutting the end of the rod opposite the plate and a pair of leg por- 'tions extending from opposite sides of the rst portion and abutting the plate at opposite sides of the nonmagnetic member.

4. An electroacoustical device compris-ing the elements of claim 1 wherein the magnetic circuit structure includes a ferromagnetic cup having an opening at one end disposed about the rst portion of the nonmagnetic member in abutment with the plate, said cup having a closed end in abutment with the end of the rod opposite the plate.

5. An electroacoustical device comprising the elements of cl-aim 1 wherein the magnetic structure includes a ring magnet disposed coaxially about the member and having one pole thereof in abutment with the ferromagnetic plate, a second ferromagnetic plate abutting the other pole of the ring magnet and a portion of the member remote from the rst plate.

6. An electroacoustical device comprising the elements of claim 5 wherein .the member is provided with a channel extending therethrough, and a plug is disposed within the channel having a porous member to provide acoustical damping.

'7. An electroacoustical dev-ice comprising the elements of claim 5 in combination with a casing secured to the rst plate and extending about the ring magnet and member on the side of the plate remote from the diaphragm, said casing being of plastic material and integral with the nonmagnetic member.

8. An electroacoustical dev-ice comprising a magnetic circuit including a source of magnet-omotive force, a ferromagnetic plate having a flat surface and a cylindrical opening extending through the plate normal to said surface, a ferromagnetic rod having a cylindrical portion of smaller diameter than the opening disposed coaxially within the opening, a diaphragm having a at portion extending outwardly from a circular central portion, said flat portion being disposed with one surface thereof mounted on said surface of the plate and disposed coaxially withthe opening of the central portion, said diaphragm having a generally truncated conical flange portion extending from the inner side of the at portion coaxially with the opening at an angle to the at circular portion and away from the plate, the central portion of said diaphragm having an inner portion extend-ing inwardly from the flange portion and spaced from the plate, and a cylindrical voice coil mounted on the central por- -ti-on of the diaphragm coaxial with the opening and translatably disposed in the opening between the plate and the cylindrical portion of the rod.

9. An electroacoustical device comprising the combination of claim 8 wherein the flat surface of the ferromagnetic plate is provided with an outwardly extending rim and the outer edge of the flat portion of the diaphragm abuts the rim.

1t). An electroacoustical device comprising the combination of claim 8 wherein the flat portion of ythe diaphragm has a circular outer edge coaxially disposed about the voice coil, and the at surface of the ferromagnetic plate is provided with a circular rim extending coaxially about the opening of the plate, said rim having an inner surface disposed in abutment with the outer edge of the fiat portion of the diaphragm.

11. An electroacoustical device compris-ing the elements of claim S wherein lthe inner portion of the diaphragm comprises a circular dome centered on the axis of the opening in the plate, said dome having a diameter approximately equal to the diameter of the opening in the plate and extending away from the plate.

12. An electroacoustical device comprising the elements of claim 11 wherein Ithe inner portion of the diaphragm includes a second truncated conical portion between the dome and the ange portion, said second truncated conical portion sloping toward the surface of the plate.

13. An electroacoustical device comprising the elements of claim 8 in combination with a pair of lugs mounted on the plate and electrically insulated from each other, the diaphragm having an aperture therein adjacent to the voice coil and the voice coil having a pair of wire leads extend-ing through the aperture and electrically connected to the lugs.

14. An electroacoustical device comprising the elements of claim 12 wherein the voice coil is mounted on the surface of the dome adjacent to the outer edge of the dome.

15. An electroacoustical device comprising the elements of claim 12 wherein the second truncated conical portion of the inner portion of the diaphragm is provided with a plurality of futes which extend about the dome.

16. An electroacoustical device comprising the elements of claim 15 wherein each of the flutes has a straight ridge disposed tangentially of a circle centering on the center of the inner portion.

17. An electroacoustical device comprising the elements of claim 15 wherein each of the flutes has a straight ridge disposed on one of the radii of the flange portion.

18. A dynamic microphone comprising a support member including a ferromagnetic plate hav-ing a cylindrical opening and a nonmagnetic member constructed of plastic mounted on one side of the plate and having a cylindrical orifice coaxial with the opening, said nonmagnetic plastic member having a plurality of spaced ribs disposed parallel to the axis of the opening and extending inwardly from the orifice, said ribs terminating in surfaces disposed at a common distance from the axis of the orifice, a magnetic circuit structure including the plate and a cylindrical rod disposed within the orifice of the nonmagnetic member, said rod having an outer radius slightly greater than the distance between the surfaces of the ribs and the axis of the orifice and being wedged between the ribs of the nonmagnetic member and supported by said ribs, said rod having a portion disposed within the opening of the plate to form a circular gap with the plate, a ferromagnetic yoke extending from a portion of the rod remote from the plate and abutting the plate, a diaphragm mounted on the side of the plate opposite the nonmagnetic member, and a cylindrical voice coil disposed within the gap between the rod and the plate and mounted on the diaphragm.

19. A dynamic microphone comprising, in combination: a support ring including a circular ferromagnetic disc having a central circular opening and a plurality of apertures extending therethrough centered on a circle disposed coaxially about the opening and spaced from each other by equal distances, and a unitary nonmagnetic part mounted on the ferromagnetic disc having a ring portion disposed on one side of the disc confronting the coaxial axis of 4the apertures and having a cylindrical inner surface coaxial with the opening, a disc portion abutting the side of the disc opposite the ring portion with an outer surface disposed on the side of the apertures remote from the opening, said disc portion having a cylindrical orifice coaxial with the open-ing in the disc and of the same diameter as the diameter of the opening in the disc and a plurality of ribs extending inwardly from the cylindrical surface of the orifice parallel to the axis thereof and terminating in surfaces disposed at a common distance from the axis of the opening, and post portions extending through each of the apertures from the ring portion to the disc portion; a cylindrical magnet having a radius equal to 4the common distance between the surfaces of the ribs and the axis of the opening disposed within the opening in the disc and orifice in the disc portion, the ends of said magnet being on fiat planes normal to the central axis of the magnet and one end of said magnet being on the plane of the surface of the disc adjacent to the ring portion, thereby forming a gap between the magnet and d-isc; a ferromagnetic yoke having a flat portion abutting the flat end of the magnet opposite the disc and a portion extending from the flat portion of the yoke to abut the ferromagnetic disc; a diaphragm constructed of thin compliant material having a circular periphery with the diameter of the inner surface of the ring port-ion of the support ring, said diaphragm having a flat peripheral portion disposed within the ring portion of the support ring and abutting the surface of the ferromagnetic disc, said diaphragm also having a circular band of utes disposed within the flat portion and extending away from the disc and a circular dome extending away from the disc coaxially with the opening therein; and a cylindrical voice coil mounted on the diaphragm coaxial with the periphery of the diaphragm,

said voice coil having a diameter between the diameter of the opening in the disc and the diameter of the magnetic and being translatably disposed in the gap between the disc and the magnet.

20. A subassembly for an electroacoustical device compris-ing a ring magnet, a cylindrical ferromagnetic pole piece disposed within the ring magnet, a first ferromagnetic plate having a circular opening coaxially disposed about the pole piece forming a magnetic gap and extending to abut one pole of the ring magnet, a second plate abutting the other pole of 4the ring magnet and the pole piece remote from the gap, and a nonmagnetic casing having a sleeve disposed between the ring magnet and the pole piece, said casing being secured to the first plate and extending about the ring magnet and pole piece to secure the pole piece, ring magnet, first and second plates into an assembly.

21. A subassembly for an electroacoutical device cornprising the elements of claim 20 wherein the Pole p-iece has a channel extending therethrough, a plug disposed within the channel having a porous member to provide acoustical damping, and a diaphragm sealed on the first plate about the plug.

22. An electroacoustical device comprising the combination of claim 20 -in combination with a diaphragm having a flat portion with an inner circular region mounted on the first plate remote from the second plate, the perimeter of the circular region being coaxially disposed about the cylindrical pole piece, said diaphragm having a flange portion within the flat portion extending at an angle from the first plate, and said diaphragm having a central portion extending from the ange portion, and a cylindrical coil mounted on the central portion coaxial with the cylindrical pole piece and translatably disposed in the magnetic gap.l

References Cited by the Examiner UNITED STATES PATENTS 1,766,473 v 6/1930 Wente 179-115.5 2,442,791 6/ 1948 Wente 181-31 2,490,227 12/ 1949 Murkham 179-115.5 2,773,130 12/1956 Olson et al. 179-115.5 2,773,933 12/1956 Hawley 179-1 2,775,653 12/1956 Wurdel 179-115.5 2,862,069 11/1958 Marchard et al. 179-115.5 2,950,359 8/1960 Perry 179-115.5 3,033,945 5/1962 Villchur 179-115.5 3,061,690 10/ 1962 Swinehart 179-1 ROBERT H. ROSE, Primary Examiner. H, W. GARNI-ER, F. N, CARTEN, Assistant Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3,258,543 June Z8, 1966 Harold P. Moser, Jr.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, line 6, "opening of the central portion," should read opening, the central portion of line S6, "futes" should read flutes Signed and sealed this 16th day of December 1969.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

Claims

1. AN ELECTROACOUSTICAL DEVICE COMPRISING A SUPPORT MEMBER INCLUDING A FERROMAGNETIC PLATE HAVING A CYLINDRICAL OPENING AND A NONMAGNETIC MEMBER MOUNTED ON THE PLATE HAVING A FIRST PORTION ON ONE SIDE OF THE PLATE, A SECOND PORTION ON THE OPPOSITE SIDE OF THE PLATE AND AN INTERCONNECTING PORTION EXTENDING FROM THE FIRST PORTION TO THE SECOND PORTION, THE FIRST PORTION HAVING AN ORIFICE WITH A CENTRAL AXIS ON THE AXIS OF THE OPENING IN PLATE, AND THE SECOND PORTION FORMING A RIB EXTENDING ABOUT THE OPENING IN THE PLATE AND HAVING A WALL CONFRONT ING THE OPENING, A MAGNETIC CIRCUIT STRUCTURE INCLUDING THE PLATE AND A FERROMAGNETIC MEMBER HAVING A CENTRAL AXIS DISPOSED ON THE AXIS OF THE OPENING IN THE PLATE, SAID MEMBER ABUTTING THE ORIFICE IN THE FIRST PORTION OF THE NONMAGNETIC MEMBER AND HAVING A CYLINDRICAL PORTION WITH A SMALLER DIAMETER THAN THE OPENING, A DIAPHRAGM HAVING A COAXIALLY WITHIN THE OPENING, A DIAPHRAGM HAVING A PERIMETER WITH THE CONTOUR OF THE WALL OF THE RIB OF THE SECOND PORTION OF THE NONMAGNETIC MEMBER, SAID DIAPHRAGM BEING DISPOSED WITHIN THE RIB AND MOUNTED ON THE SUPPORT MEMBER, AND A CYLINDRICAL VOICE COIL MOUNTED ON THE DIAPHRAGM AND DISPOSED WITHIN THE OPENING BETWEEN THE PLATE AND THE CYLINDRICAL PORTION OF THE FERROMAGNETIC MEMBER.