US2454425A - Magnetic translating device - Google Patents

Magnetic translating device Download PDF

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US2454425A
US2454425A US515463A US51546343A US2454425A US 2454425 A US2454425 A US 2454425A US 515463 A US515463 A US 515463A US 51546343 A US51546343 A US 51546343A US 2454425 A US2454425 A US 2454425A
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magnetic
variable
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portions
pole faces
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Benjamin B Bauer
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Shure Inc
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Shure Brothers Inc
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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04RLOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R11/00Transducers of moving-armature or moving-core type
    • H04R11/08Gramophone pick-ups using a stylus; Recorders using a stylus
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04RLOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R11/00Transducers of moving-armature or moving-core type

Description

Nov. 23, 1948. B. B. BAUER MAGNETIC TRANSLATING DEVICE 2 She'ets-Sheet 1 Original Filed Dec. 24, 1941 Nov. 23, 1948. B. B. BAUER 2,454,425
MAGNETIC THANSLATING DEVICE Original Filed Dec. 24, 1941 2 Sheets-Sheet 2 III] 935 I W2 84 420. 236' 22% I f 1 l E; O ss= D5337 fivezzz orx jezy'a zzzzkz jazzej {7 QZQMK M 4 7 357 011 currents.
Patented Nov. 23, 1948 MAGNETIC TRAN SLATING DEVICE Benjamin B. Bauer, Chicago, Ill., assignor, by
mesne assignments, to Shure Brothers, Incorporated, Chicago, Ill., a corporation of Illinois Continuation of application Serial No. 424,282, December 24, 1941. This application December 23, 1943, Serial No. 515,463
17 Claims. 1
The present invention relates to electromagnetic translating devices of the type having a movable armature and has particular application to devices involving voice or other variable In such an electromagnetic translating device, the translating action fundamentally involves the variable reaction between a substantially constant and variable magnetic fields. The constant field may be obtained by electromagnets or more commonly by permanent magnets. The variable field may be obtained by variable currents acting on an electromagnet or by mechanical motion of an armature, depending upon the direction in which translation occurs. I
Both the variable and constant fields have magnetic circuits and for maximum efficiency have conflicting requirements. Thus for high efficiency, the constant fie'ld magnetic circuit must have certain portions, notably the pole pieces defining air gaps, operate at high fiux densities. The variable magnetic fields are generally weak in comparison to the constant field and high efficiency demands that the magnetic circuit have high permeability. The art has gen erally provided common Paths for a substantial portion of the magnetic circuit of each system particularly at those portions of the constant field circuit operating at high flux densities.
Since iron or other ferromagnetic material operating at high flux densities has low permeability it is clear that .the efiiciency of the variable field part of the translating device and hencethe translating device itself suffered.
In general, the art has provided constructions which have aggravated the above problem. Thus the disposition of an armature between the arms of a U-shaped magnet is common, the armature generally extending back from the poles toward the central part of the magnet. Such a construction not only promotes flux leakage but at the same time the flux density in the armature tends to reduce its permeability and thus renders it less efilcient, for use with weak variable magnetic fields. i
This invention contemplates the provision of minimum paths common to the constant and variable magnetic field circuit-s. Since translation involves the interaction between such two fields, complete segregation is impossible. .However, this invention contemplates reducing the common paths substantially to the air gap portions of the system, including of course any intervening armature. In general this is accomplished by providing two substantially separate 2 ferromagnetic circuits which interlink at the air gaps and are otherwise substantially complete in themselves.
Referring now to the drawings:
Figure 1 is a diagrammatic isometric illustration to show the magnetic circuits;
Fig. 1a shows a modified coil construction for Fig. 1;
Fig. 2 is a view in a diagrammatic form similar to Fig. 1 but showing a modification;
Fig. 3 is a plan view of a translating device;
Fig. 4 is a section on line 4-4 of Fig. 3;
Figs. 5 and 6 are details of the armature bearing magnetic circuit with coils shown diagrammatically; and
Fig. 7 is a section on line 1-1 of Fig. 4 with the voice current coil omitted and showing in more or less diagrammatic form some magnetic lines of force.
Referring first to Fig. 1 a permanent magnet l0 having poles II and I2 is provided. This magnet is preferably of any alloy such as A1nico," having high coercive force and generating a relatively intense field. Pole pieces I4 and I! are disposed on poles H and I2 01' the magnet and are shaped to provide pole faces i6 and i1. Pole face l3 may be shaped to provide a pair of recesses l3 and I3 and leave three projecting parts 20, 2i and 22. Similarly pole face I! has recesses 24 and 25 with projecting parts 26, 21 and 28. For economy in manufacture, pole pieces I! and 15 may be substantially alike. Projections 2i and 21 in this figure are spaced apart to form an air gap 33 therebetween in which translation may occur. The remaining pairs of projections 20 and 28; 22 and 23 are kept separated by non-magnetic spacers 32 and 33.
Tightly clamped between the non-magnetic spacers is a generally E-shaped member 35 of soft steel or other ferromagnetic material. Member 35 has end arms 36 and 31 with an intermediate arm 33 all connected by a body portion 39. The precise shape of the various arms is unimportant. End arms 36 and 31 are tightly gripped between spacers 32 and 33. Center arm 38 extends into air gap 30 and has sufiicient elasticity to vibrate therein. Normally, the three arms are symmetrical with respect to the median plane between the opposed pole faces. It is preferred to have end arms 36 and 31 about equal in width and each have a width about half that of the center arm or armature 33.
Surrounding armature 38 is a voice current coil 40 and within this coil armature 33 is free portion 38 to anchor member 35 and restrict vibration to armature 38.
The general paths of the lines of magnetic force are suggested by heavy lines for the constant field and light lines for the variable field. Thus, as a rule, parts 43, 44, 45 and 48 bordering on the recesses have low fielddensity from magnet I0. By providing recesses, the reluctance between opposing pole piece surIaces is increased. Obviously these recesses may be made as large or small as desired and may even be omitted. The clamped ends of arms 38 and 31 between the pole pieces will normally provide a substantial reduction in reluctance tor the permanent field flux so that even without recesses, there would be a tendency for the field todivide somewhat as shown. The variable flux goes from the ends of arms 38 and 31 through spacers 32 and 33 to parts 43, 44, 45 and 46 and thence to central projections 2| and 21 through air gap 30 and armature 38.
Movement of the armature in the air gap either causes or results from (depending on the direction of translation) the variable reaction between two magnetic fields. The main field has part of the lines of force passing through the armature magnetic circuit and during translation the flux distribution in the gaps is altered. Paths of comparatively low reluctance are provided for the main flux between the pole pieces.
'The variable flux path through member 38 has low reluctance, generally avoids highly saturated parts and thus has high permeability throughout most of the path. Furthermore, member 38 has most of its bulk out of the space enclosed by the main field structure and thus leakage is reduced to a minimum.
Armature 38 may have an apertured tip 38 for receiving or transmitting vibratory motion. Thus a diaphragm or stylus or other means may be operatively connected thereto.
It is evident that the secondary magnetic circult, here the E-shaped member including the ar-= mature furnishes a substantially separate com= plete path for the variable or what may be termed the alternating flux. Except for the gaps and the armature tip in air gap 30, the two circuits are quite separate.
The disposition of member 33 outside of the space enclosed by the main magnetic field structure exposes member 3&3 and voice coil 60 to stray fields, unless suitable shielding is provided. In order to eliminate any hum or noise due to stray fields, coil 30 may be replaced by two cells Ml and 30" so connected as to be magnetically opposed, and disposed on parts 39' and 33" of body part- 33, as shown in Fig. la. For voice currents, the two coils are the equivalent of single coil 40 since the opposed fields go into center leg 38. However, any stray fields particularly along the direction of arms 36 and 3? will result in equal and opposite potentials. Thus hum may be easily eliminated.
Referring now to Fig. 2. a modified form is shown wherein member I35 has central arm I38 tightly clamped between non-magnetic spacers I32 and I33 disposed between parts Hi and I2! of pole pieces III and H2. Arms I35 and I3! operate in air gaps I30 and I30 between pole parts I20, I26 as one pair and I22 and I28 as the other pair. Instead of one coil for voice currents, a pair of coils I40 and I40 are disposed over arms I36 and I31, the arms being free to vibrate therein. The coils are connected as shown so that corresponding coil ends have opposite magnetic polarity. A torsion rod I50 is suitably anchored to central arm I38 so that as arms I38 and I31 vibrate, rod I50 is vibrated torsionally. A stylus I5I may be carried by rod I50 for phonograph recording or reproduction. If desired a diaphragm may be actuated. It is understood that torsion rod I50 may be supported in bearings. This construction has hum buck properties similar to the modified construction of Fig. 1a..
The coils may be connected so that the current flow is in the same direction in both and will result in arms I35 and I3'I being urged in the same direction. Obviously only one coil may be disposed over central arm I38 in that case.
Referring now to Figs. 3 to 7 inclusive, a magnet 2I0 having pole pieces 2I4 and 2I5 is retained in assembled relation by non-magnetic bolt 20f. Bolt 202 extends through both pole pieces to engage a massive base 203. Pole pieces 2I4 and 2I5 have pole faces 2I8 and 2I1. The center parts. HI and 221 of the pole faces define an air gap 230 while the end parts 220, 228, 222 and 228 are spaced apart by non-magnetic spacers 232 and 233. Gripped between non-magnetic spacers 232 and 233 is a member 235 of a soft steel. A voice current coil 240 wound on a suitable bobbin is disposed around arm 238 within arms 238 and 231 and may be secured to base 203 in a manner to permit free vibration of arm 238. Body portion 238 is bolted at I to a part of base 203. Tip 248 of arm 238 has a non-magnetic rod 25! secured. thereto and this rod extends down through an aperture 282 in pole piece 2I5 to the apex of a diaphragm 263 suitably carried by base 203. An insulating block 255 carried by bolts 2M may support voice coil terminals 255.
In order to protect against stray fields, a hum buclr coil 2% is provided. This coil is preterably disposed to enclose arms 233 and 23l and voice current coil 240. The hum bucl: coil 240' may have relatively few turns, as compared to cell 240}. The two coils 240 and 240' are connected in series and the polarity is such that the magnetic fields of the two coils oppose each other. This is shown diagrammatically inFig. 6.
Tips 233' and 23'! function to reduce the air gaps extending toward arm 238 and thus reduce the reluctance of the variable field path throughout member 235 and arm 238; Since these tips have air gaps (in the main circuit) on opposite sides transverse to the planes of the arms; '1. e. toward pole faces 2I6 and 2" the reluctance of the main field path is kept at a predetermined value and controlled.
Thus referring to Fig. 7, the reluctance of the magnetic circuit containing the armature is low and the lines of force may be imagined as drawn. The reluctance of the main magnetic circuit must be considered generally along lines connecting the main pole pieces; i. e. extending up and down Fig. 7 and is maintained at a desirable value all the time.
Thus as the armature goes down in Fig. 7, the variable flux density is increased below the armature and decreased above the armature. As the armature goes up the reverse occurs. The reluctance for the constant field remains more or less constant during armature travel. When used to translate armature movements into currents, the permanent magnet supplies both the constant and variable fields. It is understood that the variable fields may be either due to a variation in field strength or variation in path or both.
Thus in the various forms of the invention, the constant and variable fields have their magnetic circuits with minimum common paths.
This application is a continuation of my 00-- pending application Serial No. 424,282, filed December 24, 1941, now abandoned.
I claim:
1. In a translating device, the combination of a constant flux magnetic member having opposed pole faces spaced apart to form a space therebetween, a variable flux magnetic member having a plurality of end portions extending in'a generally similar direction and an intermediate portion connecting said end portions together, said intermediate portion being disposed outside of the volume encompassed by said constant flux magnetic member and said end portions being disposed in said space and in adjacent spaced relation to said pole faces to form with said constant flux magnetic member one or more variable flux ferromagnetic circuits which are substantially closed except for the spaces between said end portions and said pole faces, one of said end portions in said ferromagnetic circuit or in each of said ferromagnetic circuits being vibratable in said space to form an armature, means for substantially fixing the other of said end portions in said ferromagnetic circuit or each of sa d ferromagnetic circuits in position in said space out of ferromagnetic contact with said pole faces, and a winding surrounding a portion of said variable flux magnetic member.
2. A construction as claimed in claim 1, in which the pole faces of the constant flux magnetic member are in substantially parallel spaced relation to each other and the end portions of the variable flux magnetic member project into the space between said pole faces and at least one of the pole faces has projecting surfaces at the portions thereof opposed to said end portions, whereby the magnetic reluctance between said pole faces is lower at said projecting portions than at the spaces between said portions.
3. A construction as claimed in claim 1, in which the space between the pole faces of the constant flux magnetic member i-sthroughout of substantially uniform length in the direction of the flux path at said space and the end portion or portions of the variable flux magnetic member define the paths of low magnetic reluctance between said pole faces.
4. A construction as claimed in claim 1, in which the end portions of the variable flux magnetic member are thin and flat and, in the at rest position of the vibratable end portion, all of said end portions are located in substantially the same plane.
5. A construction as claimed in claim 1, in which the pole faces of the constant flux magnetic member are in substantially parallel spaced relation to each other and the end portions of the variable flux magnetic member project into the space between said pole faces and, in the at rest position of the vibratable end portion or portions, are arranged to lie in a plane substantially midway between and parallel to said pole faces.
6. A construction as claimed in claim 1, in which a coil is wound about a portion of the variable flux magnetic member disposed outside the volume encompassed by the constant flux magnetic member.
7. A construction as claimed in claim 1, in which a voice coil and a hum buck coil are wound about a portion of the variable flux magnetic -member with said body portion disposed outside of the volume encompassed by said constant flux magnetic member and the free ends of said arms disposed in said space and in adjacent spaced relation to and out of ferromagnetic contact with said pole faces to form with said constant flux magnetic member one or more variable flux ferromagnetic circuits which are substantially closed except for the spaces between said arm ends and said pole faces, at least one of said arms in said ferromagnetic circuit or in each of said ferromagnetic circuits being. vibratable in said' space, and a winding surrounding a portion of said variable flux member.
9. In a translating device, the combination of a constant flux magnetic member having a pair of opposed pole faces spaced apart to form a single space therebetween, a variable flux magnetic member having a body portion and a plurality of spaced apart substantially fiat arms lying in substantially the same plane and extending from said body portion in a generally similar direction, means for supporting said variable flux magnetic member with said body portion disposed outside of the volume encompassed by said constant flux magnetic member and the free ends of said arms disposed in said space and in adjacent spaced relation to and out of ferromagnetic contact with said pole faces to form with said constant flux magnetic member one or more variable flux ferromagnetic circuits which are substantially closed except for the spaces between said arm ends and said pole faces, at least one of said arms being fixed in said space and the remainder of said arms being vibratable in said space, and a winding surrounding a portion of said variable flux member.
10. In a translating device, the combination of a constant flux magnetic member having opposed pole faces separated to provide a space therebetween, a variable flux magnetic member having a main body portion disposed outside the volume encompassed by said constant flux magnetic member and a plurality of end portions disposed in said space, means for maintaining said end portions in spaced relation to and out of ferromagnetic contact with said pole faces, whereby said variable flux magnetic member forms with said constant flux magnetic member one or more variable flux ferromagnetic circuits which are substantially closed except for the spaces between said end portions and said pole faces, at least one end portion of said ferromagnetic member being vibratablc to form an armature, and a winding surrounding a portion of said variable flux magnetic member.
1 11. In a translating device. the combination of a constant flux magnetic member having opposed pole faces separated to provide a space therebetween, a variable iiux magnetic member having a main body portion disposed outside the volume encompassed by said constant flux magnetic member and a plurality of end portions disposed in said space, means for maintaining said end portions in spaced relation to and out of terromagnetic contact with said pole faces, whereby said variable ilux magnetic member forms with said constant flux magnetic member one or more variable flux ferromagnetic circuits which are substantially closed except for the spaces between said end portions and said pole faces, at least one end portion of said ferromagnetic member being vibratable to form an armature; and a winding surrounding a portion oi. said variable flux magnetic member, the arrangement being such that the variable flux path in the constant flux magnetic member is shorter than the variable flux path in the variable flux magnetic member.
. 12. In a translating device, the combination of a constant flux magnetic member having opposed pole faces separated to provide a space-therebetween, a variable flux magnetic member havin a main body portion disposed outside the volume encompassed by said constant flux magnetic member and a plurality of end portions disposed in said space, means for maintainingsaid end portions in spaced relation to and out of ferromagnetic contact with said pole faces, whereby said variable flux magnetic member forms with E said constant flux magnetic member one or more variable flux ferromagnetic circuits which are substantially closed except for the spaces between said end portions and said pole faces, at least one end portion of said ferromagnetic member being vibrat'able to form an armature, and a windin surrounding a portion of said variable flux magnetic member, the arrangement being such that the constant flux path within the variable flux magnetic member is substantially confined to said end portions of said variable flux magnetic member.
13. In a translating device, the combination of a constant flux magnetic member having opposed pole faces spaced apart to form a space therebetween, a substantially U-shaped, variable flux magnetic member having the end portions thereof disposed in said space and in adjacent spaced relation to said pole faces and the intermediate portion thereof disposed outside of the volume encompassed by said magnetic field member, one
of said end portions being vibratable in said space, means for fixing the other of said end portions in 8- said space and out of ferromagnetic contact with said pole faces, and a winding surrounding a portion 'of said variable flux magnetic member.
14. In a translating device, the combination of a constant flux magnetic member having oppo ed pole faces spaced apart to form a space therebetween, a substantially E-shaped variable flux magnetic member having a body portion and spaced apart arms extending from said body portion, said body portion being disposed outside of the volume encompassed by said constant fiux magnetic member and the end portions of said arms being disposed in said space and in adjacent spaced relation to said pole faces to form with said constant flux magnetic member two variable flux ferromagnetic circuits which are substantially closed except for the spaces between said arm end portions and said pole faces, one of said arms in each of said ferromagnetic circuits being vibratable in said space, means for fixing the end portion of the other of said arms in each of said ferromagnetic circuits in said space and out of ferromagnetic contact with said pole faces, and a winding surrounding a portion of said variable flux magnetic member.
15. A construction as claimed in claim 14, in which the middle arm of the E-shaped magnetic member is vibratable and the outer arms are substantially fixed in position.
16. A construction as claimed in claim 14, in which the middle arm of the E-shaped magnetic member is substantially fixed in position and the outer arms are vibratable'.
17. A construction as claimed in claim 14, in which the middle arm of the E-shaped magnetic member has awidth substantially equal to the sum of the widths of the outer arms thereof.
1 BENJAMIN B. BAUER.
REFERENCES CITED The following references are of record in the file of this patent:
-UNITED STATES PATENTS Number Name Date 1,463,831 Miller Aug. 7, 1923 1,573,739 ONeill Feb. 16, 1926 2,108,275 Vermeulen et a1. Feb. 15, 1938 2,163,161 Wadsworth June 20, 1939 2,215,782 Ellis Sept. 24, 1940 FOREIGN PATENTS Number Country Date 505,122 Germany Aug. 14, 1930 698,263 France Nov. 17, 1930
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Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2545859A (en) * 1949-01-31 1951-03-20 Pickering Associates Inc Magnetic sound reproducer having two styli
US2588327A (en) * 1948-01-31 1952-03-04 Dictaphone Corp Recorder head
US2686470A (en) * 1952-04-16 1954-08-17 Florez Company Inc De Hammer impelling means for high-speed printers
US2692918A (en) * 1952-07-09 1954-10-26 Samuel I Berger Magnetic sound powered telephone
DE1044169B (en) * 1954-10-22 1958-11-20 Elektronik Ges Mit Beschraenkt Magnetic-electric acoustic transducer
US2912522A (en) * 1954-06-14 1959-11-10 Industrial Res Prod Inc Electro-mechanical transducing device
DE1079115B (en) * 1957-12-23 1960-04-07 Krone Kg Four-pole magnet system for electroacoustic apparatus
DE1086748B (en) * 1955-10-26 1960-08-11 Ind Res Products Inc Sound recording or playback device
US3002057A (en) * 1956-03-29 1961-09-26 Vigren Sten Daniel Polarized electromagnetic device
DE1119336B (en) * 1956-03-06 1961-12-14 Krone Kg Drive system for electroacoustic converters
US3029387A (en) * 1953-09-03 1962-04-10 Time Inc Electro-mechanical device
US3111563A (en) * 1960-05-05 1963-11-19 Industrial Res Prod Inc Electro-mechanical transducer
DE1158115B (en) * 1961-10-07 1963-11-28 Sennheiser Electronic Miniature microphone
DE1175745B (en) * 1960-05-05 1964-08-13 Ind Res Products Inc Electromagnetic converter
DE1222982B (en) * 1963-07-29 1966-08-18 Ind Res Products Inc Electromagnetic miniature transducer, especially for hearing aids
US3436568A (en) * 1966-04-19 1969-04-01 Indian Telephone Ind Ltd Electromagnetic vibratory devices

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US1463831A (en) * 1921-05-05 1923-08-07 Western Electric Co Electromagnetic device
US1573739A (en) * 1922-10-02 1926-02-16 O'neill John Hugh Telephonic device
DE505122C (en) * 1927-09-28 1930-08-14 Siemens Schuckertwerke Akt Ges Resonance relay with a loop-shaped spring that is excited to vibrate by an electromagnet
FR698263A (en) * 1930-07-01 1931-01-29 Const D App S Et Specialites E Advanced training in electromagnetic vibrators
US2108275A (en) * 1934-05-17 1938-02-15 Philips Nv Electromagnetic device and method of making same
US2163161A (en) * 1937-05-20 1939-06-20 Control Instr Co Inc Magnetic unit
US2215782A (en) * 1938-08-31 1940-09-24 Control Instr Co Inc Telephonic unit

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1463831A (en) * 1921-05-05 1923-08-07 Western Electric Co Electromagnetic device
US1573739A (en) * 1922-10-02 1926-02-16 O'neill John Hugh Telephonic device
DE505122C (en) * 1927-09-28 1930-08-14 Siemens Schuckertwerke Akt Ges Resonance relay with a loop-shaped spring that is excited to vibrate by an electromagnet
FR698263A (en) * 1930-07-01 1931-01-29 Const D App S Et Specialites E Advanced training in electromagnetic vibrators
US2108275A (en) * 1934-05-17 1938-02-15 Philips Nv Electromagnetic device and method of making same
US2163161A (en) * 1937-05-20 1939-06-20 Control Instr Co Inc Magnetic unit
US2215782A (en) * 1938-08-31 1940-09-24 Control Instr Co Inc Telephonic unit

Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2588327A (en) * 1948-01-31 1952-03-04 Dictaphone Corp Recorder head
US2545859A (en) * 1949-01-31 1951-03-20 Pickering Associates Inc Magnetic sound reproducer having two styli
US2686470A (en) * 1952-04-16 1954-08-17 Florez Company Inc De Hammer impelling means for high-speed printers
US2692918A (en) * 1952-07-09 1954-10-26 Samuel I Berger Magnetic sound powered telephone
US3029387A (en) * 1953-09-03 1962-04-10 Time Inc Electro-mechanical device
US2912522A (en) * 1954-06-14 1959-11-10 Industrial Res Prod Inc Electro-mechanical transducing device
DE1044169B (en) * 1954-10-22 1958-11-20 Elektronik Ges Mit Beschraenkt Magnetic-electric acoustic transducer
DE1086748B (en) * 1955-10-26 1960-08-11 Ind Res Products Inc Sound recording or playback device
DE1119336B (en) * 1956-03-06 1961-12-14 Krone Kg Drive system for electroacoustic converters
US3002057A (en) * 1956-03-29 1961-09-26 Vigren Sten Daniel Polarized electromagnetic device
DE1079115B (en) * 1957-12-23 1960-04-07 Krone Kg Four-pole magnet system for electroacoustic apparatus
US3111563A (en) * 1960-05-05 1963-11-19 Industrial Res Prod Inc Electro-mechanical transducer
DE1175745B (en) * 1960-05-05 1964-08-13 Ind Res Products Inc Electromagnetic converter
DE1158115B (en) * 1961-10-07 1963-11-28 Sennheiser Electronic Miniature microphone
DE1222982B (en) * 1963-07-29 1966-08-18 Ind Res Products Inc Electromagnetic miniature transducer, especially for hearing aids
US3436568A (en) * 1966-04-19 1969-04-01 Indian Telephone Ind Ltd Electromagnetic vibratory devices

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