US2963557A - Magnetic structure - Google Patents

Magnetic structure Download PDF

Info

Publication number
US2963557A
US2963557A US433634A US43363454A US2963557A US 2963557 A US2963557 A US 2963557A US 433634 A US433634 A US 433634A US 43363454 A US43363454 A US 43363454A US 2963557 A US2963557 A US 2963557A
Authority
US
United States
Prior art keywords
magnets
members
air gap
magnetic
magnetic structure
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US433634A
Inventor
Leslie J Anderson
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
RCA Corp
Original Assignee
RCA Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by RCA Corp filed Critical RCA Corp
Priority to US433634A priority Critical patent/US2963557A/en
Application granted granted Critical
Publication of US2963557A publication Critical patent/US2963557A/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04RLOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R9/00Transducers of moving-coil, moving-strip, or moving-wire type
    • H04R9/02Details
    • H04R9/04Construction, mounting, or centering of coil
    • H04R9/046Construction
    • H04R9/047Construction in which the windings of the moving coil lay in the same plane
    • H04R9/048Construction in which the windings of the moving coil lay in the same plane of the ribbon type

Definitions

  • This invention relates to magnetic circuits, and more particularly to magnetic structures associated with ribbon type microphones.
  • the space between the magnets within the magnetic structure and the ribbon may be as little as .002 of an inch. It is important, therefore, that magnets not be so large as to lessen this air gap, since an interference with the free movement of the ribbon may result. It is also important that the air gap between the magnets and the ribbon not be too great, as would be the case if the magnets are too small, since the sensitivity of the microphone resulting from the structure would be greatly impaired. For example, if the space between the ribbon and the magnet is .005 of an inch instead of .002 of an inch, the sensitivity of the microphone would be reduced approximately 50 percent.
  • a small efficient magnetic circuit for a ribbon type microphone is provided, without involving critical tolerances of the parts.
  • a pair of L or U-shaped members of substantially the same size is provided in a magnetic structure, each member having a magnet suitably attached thereto. These members provide the yoke pieces for the magnets and are mutually movable so as to provide accurate control of the air gap between the magnets.
  • the assembly of the magnetic structure is accomplished by inserting a gauge of proper size in the air gap between the two magnets and overlapping portions of the U or L-shaped members. When the proper spacing has been provided, the overlapped L or U members are clamped in place. The members are then soldered, or otherwise suitably fixed to each other, thereby maintaining the desired air gap between the magnets within the magnetic structure.
  • Figure 1 is a view, in front elevation, of a magnetic icestructure utilizing a pair of L-shaped members in accordance with the present invention
  • Figure l-a is a top plan view of the construction shown in Figure 1;
  • Figure 1-12 is a view, in side elevation, of the magnetic structure shown in Figure 1;
  • Figure 2 is a view, in front elevation, of a magnetic structure utilizing a pair of U-shaped members in accordance with the present invention.
  • Figure 3 is an elevational view, partly in section, of a ribbon type microphone utilizing a pair of U-shaped members in accordance with the present invention.
  • a pair of magnets 10 and 12 are soldered or otherwise suitably attached to a pair of L-shaped magnetic members 14 and 16, respectively.
  • the L-shaped members provide the yoke assembly for the magnetic structure.
  • the L-shaped members may be slightly tapered, as indicated to improve the efliciency of the magnetic circuit.
  • the magnets have pole ends adapted to define an air gap and may also be tapered at these pole ends towards the air gap, as indicated, to improve the efiiciency of the magnetic circuit.
  • a ribbon conductor may be suitably mounted and supported in the air gap between the magnets as shown in the structure of Figure 3. In carrying out the invention, a gauge of the proper size may be inserted in the air gap.
  • the L-shaped members being mutually slidable, are moved until the proper spacing of the air gap is attained.
  • the members are then clamped in place and solder is applied to points 18 and 20, or other suitably holding means employed, to maintain the magnets in a desired spaced relationship.
  • the magnetic structure shown comprises a pair of magnets 22 and 24 suitably attached to a pair of U-shaped magnetic members 26 and 28.
  • a gauge shown here as two circular rods 30 and 31 are inserted in the air gap between the magnets.
  • the members When the desired spacing is attained by moving the members 26 and 28 until the magnets come into physical relationship with the gauge, the members may be clamped in place. Heat and solder may'then be applied to points 32, 34, 36 and 38 to maintain the magnets in a fixed spaced relationship.
  • a ribbon type microphone comprises a ribbon conductor 40 suitably disposed between a pair of magnets 42 and 44.
  • the magnets are suitably at ached to a pair of U-shaped magnetic members 46 and 48.
  • the ribbon is electrically connected to a transformer 50 through the leads 52, 54 and 56.
  • the leads 52 and 56 are provided, in a known manner for balance.
  • the electrical output of the transformer is coupled to a utilization circuit (not shown) through a cable 58.
  • the U-shaped members are fixed to each other by means of solder 60 and 62, or by any other suitable means.
  • the assembly of the magnetic structure of this ribbon type microphone is in accordance with the assembly previously described in connection with Figures l and 2.
  • the entire microphone is suitably fixed to a base 64 and has a cover, or perforated screen, 66.
  • the magnets 42 and 44 first are suitably attached to the intermediate portions 68 and 70, respectively, of the U-shaped members, or yoke pieces 46 and 48, respec tively.
  • the magnets are disposed on the intermediate portions of the members between the top and bottom portions of the U-shaped members, with the top and bottom portions extending outwardly further from the intermediate portions than the magnets.
  • the increased width of the top and bottom portions of the members permit an overlapping of the top and bottom portions of one member with top and bottom portions of the other member, while still maintaining an air gap between the magnets.
  • suitable means are employed to maintain the air gap between the magnets by physically attaching the members to each other.
  • the present invention provides a pair of mutually movable yoke pieces whereby accurate control of the air gap without critical tolerances on the parts is attained.
  • the magnetic structure involved in this invention is relatively simple and inexpensive to construct.
  • the cost of constructing a ribbon microphone, where an accurate control of the air gap between the magnets .is necessary, is considerably reduced when the magnetic structure, embodying the present invention, is used.
  • a magnetic structure comprising .a pair of U-shaped elements, each of said elements having top, bottom and intermediate portions, -a pair of permanent magnets, means for attaching a diiterent one of said magnets to each of said intermediate portions of said .elements, said top and bottom portions of each of said elements extending in the same direction beyond said magnets, said elements being disposed with the top and bottom portions of one of said elements overlapping the top and bottom portions of the other of said elements and with said magnets opposed to each other to define an air gap therebetween, and means for securing said elements in fixed relationship with each other whereby the dimensions of said air gap between said magnets are fixed.
  • a ribbon type microphone having a magnetic structure comprising a pair of U-shaped members each having a pair of legs, a pair of permanent magnets each secured to a difierent one of said members between the legs thereof, said members being disposed with portions of one of said members overlapping portions of the other of said members and to provide an air gap between said magnets, means for securing said members in fixed relationship, and a ribbon conductor disposed within said air gap between said magnets.

Landscapes

  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Signal Processing (AREA)
  • Audible-Bandwidth Dynamoelectric Transducers Other Than Pickups (AREA)

Description

Dec. 6, 1960 L. J. ANDERSON MAGNETIC STRUCTURE Filed June 1, 1954 V if! rlllllllllllL I 'INVENTOR. Lr-sur- J. Amexsnnf A TTORNE 1' United States Patent MAGNETIC STRUCTURE Leslie J. Anderson, Moorestown, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed (lune 1, 19 54, Ser. No. 433,634
I 2 Claims. (Cl. 179--115.5)
This invention relates to magnetic circuits, and more particularly to magnetic structures associated with ribbon type microphones.
In constructing many types of ribbon microphones, it is often necessary to have proper spacing between magnetic members. A small variation in the dimensions of the magnets used often results in inefiiciency in the microphone. In the construction of such ribbon types of microphones, critical tolerances of the parts are, therefore, often involved.
In many types of magnetic structures associated with ribbons in ribbon microphones, the space between the magnets within the magnetic structure and the ribbon may be as little as .002 of an inch. It is important, therefore, that magnets not be so large as to lessen this air gap, since an interference with the free movement of the ribbon may result. It is also important that the air gap between the magnets and the ribbon not be too great, as would be the case if the magnets are too small, since the sensitivity of the microphone resulting from the structure would be greatly impaired. For example, if the space between the ribbon and the magnet is .005 of an inch instead of .002 of an inch, the sensitivity of the microphone would be reduced approximately 50 percent.
In accordance with the present invention, a small efficient magnetic circuit for a ribbon type microphone is provided, without involving critical tolerances of the parts. A pair of L or U-shaped members of substantially the same size is provided in a magnetic structure, each member having a magnet suitably attached thereto. These members provide the yoke pieces for the magnets and are mutually movable so as to provide accurate control of the air gap between the magnets. The assembly of the magnetic structure is accomplished by inserting a gauge of proper size in the air gap between the two magnets and overlapping portions of the U or L-shaped members. When the proper spacing has been provided, the overlapped L or U members are clamped in place. The members are then soldered, or otherwise suitably fixed to each other, thereby maintaining the desired air gap between the magnets within the magnetic structure.
It is an object of this invention to provide a small, eflicient magnetic circuit, which does not involve critical tolerances of the parts utilized in the circuit.
It is a further object of this invention to provide a small, efiicient magnetic structure for use in a ribbon type microphone.
It is a still further object of this invention to provide a method of manufacturing a magnetic structure, which does not involve critical tolerances of the parts included in the structure.
Other objects and advantages of the present invention will become apparent and suggest themselves to those skilled in the art to which the invention relates, from a reading of the following specification in conjunction with the accompanying drawings, in which:
Figure 1 is a view, in front elevation, of a magnetic icestructure utilizing a pair of L-shaped members in accordance with the present invention;
Figure l-a is a top plan view of the construction shown in Figure 1;
Figure 1-12 is a view, in side elevation, of the magnetic structure shown in Figure 1;
Figure 2 is a view, in front elevation, of a magnetic structure utilizing a pair of U-shaped members in accordance with the present invention; and
Figure 3 is an elevational view, partly in section, of a ribbon type microphone utilizing a pair of U-shaped members in accordance with the present invention.
Referring to Figures 1, 1a and 1-12, a pair of magnets 10 and 12 are soldered or otherwise suitably attached to a pair of L-shaped magnetic members 14 and 16, respectively. The L-shaped members provide the yoke assembly for the magnetic structure. The L-shaped members may be slightly tapered, as indicated to improve the efliciency of the magnetic circuit. The magnets have pole ends adapted to define an air gap and may also be tapered at these pole ends towards the air gap, as indicated, to improve the efiiciency of the magnetic circuit. A ribbon conductor may be suitably mounted and supported in the air gap between the magnets as shown in the structure of Figure 3. In carrying out the invention, a gauge of the proper size may be inserted in the air gap. The L-shaped members, being mutually slidable, are moved until the proper spacing of the air gap is attained. The members are then clamped in place and solder is applied to points 18 and 20, or other suitably holding means employed, to maintain the magnets in a desired spaced relationship.
Referring to Figure 2, the magnetic structure shown comprises a pair of magnets 22 and 24 suitably attached to a pair of U-shaped magnetic members 26 and 28. In assembly the structure, a gauge, shown here as two circular rods 30 and 31 are inserted in the air gap between the magnets. When the desired spacing is attained by moving the members 26 and 28 until the magnets come into physical relationship with the gauge, the members may be clamped in place. Heat and solder may'then be applied to points 32, 34, 36 and 38 to maintain the magnets in a fixed spaced relationship.
Referring now to Figure 3, a ribbon type microphone comprises a ribbon conductor 40 suitably disposed between a pair of magnets 42 and 44. The magnets are suitably at ached to a pair of U-shaped magnetic members 46 and 48. The ribbon is electrically connected to a transformer 50 through the leads 52, 54 and 56. The leads 52 and 56 are provided, in a known manner for balance. The electrical output of the transformer is coupled to a utilization circuit (not shown) through a cable 58. The U-shaped members are fixed to each other by means of solder 60 and 62, or by any other suitable means. The assembly of the magnetic structure of this ribbon type microphone is in accordance with the assembly previously described in connection with Figures l and 2. The entire microphone is suitably fixed to a base 64 and has a cover, or perforated screen, 66.
In assembly of the magnetic structure of Figure 3, the magnets 42 and 44 first are suitably attached to the intermediate portions 68 and 70, respectively, of the U-shaped members, or yoke pieces 46 and 48, respec tively. The magnets are disposed on the intermediate portions of the members between the top and bottom portions of the U-shaped members, with the top and bottom portions extending outwardly further from the intermediate portions than the magnets. The increased width of the top and bottom portions of the members permit an overlapping of the top and bottom portions of one member with top and bottom portions of the other member, while still maintaining an air gap between the magnets. When the proper size of the air gap is attained, by the insertion of a gauge between the magnets, for example, suitable means are employed to maintain the air gap between the magnets by physically attaching the members to each other.
It is seen that the present invention .provides a pair of mutually movable yoke pieces whereby accurate control of the air gap without critical tolerances on the parts is attained.
The magnetic structure involved in this invention is relatively simple and inexpensive to construct. The cost of constructing a ribbon microphone, where an accurate control of the air gap between the magnets .is necessary, is considerably reduced when the magnetic structure, embodying the present invention, is used.
What is claimed is:
1. A magnetic structure comprising .a pair of U-shaped elements, each of said elements having top, bottom and intermediate portions, -a pair of permanent magnets, means for attaching a diiterent one of said magnets to each of said intermediate portions of said .elements, said top and bottom portions of each of said elements extending in the same direction beyond said magnets, said elements being disposed with the top and bottom portions of one of said elements overlapping the top and bottom portions of the other of said elements and with said magnets opposed to each other to define an air gap therebetween, and means for securing said elements in fixed relationship with each other whereby the dimensions of said air gap between said magnets are fixed.
2. A ribbon type microphone having a magnetic structure comprising a pair of U-shaped members each having a pair of legs, a pair of permanent magnets each secured to a difierent one of said members between the legs thereof, said members being disposed with portions of one of said members overlapping portions of the other of said members and to provide an air gap between said magnets, means for securing said members in fixed relationship, and a ribbon conductor disposed within said air gap between said magnets.
References Cited in the file of this patent UNITED STATES PATENTS 1,748,993 Purdy Mar. 4, 1930 1,886,816 Jensen Nov. 8, 1932 1,941,476 Jensen Jan. 2, 1934 1,941,477 Jensen Jan. 2, 1934 1,995,025 Ellmore Mar. 19, 1935 2,020,211 Quam Nov. 5, 1935 2,020,212 Quam Nov. 5, 1935 2,046,515 Jensen July 7, 1936 2,233,886 Cowley et al. Mar. 4, 1941 2,307,205 Ewald et al. Jan. 5, 1943 2,501,615 Pugh Mar. 21, 1950 2,539,671 Olson Jan. 30, .1951 2,699,474 Olsonet al. Ian. 11, .1955 2,832,932 Baermann Apr. 29, 11958 FOREIGN PATENTS 155,495 Australia Dec. 13, 1951 168,557 Austria June 25, 1951 905,046 France Nov. 22, 1945 OTHER REFERENCES Magnetic Tape and Head Alignment Nomenclature by N. M. Haynes, published by Audio Engineering, June 1949, pages 22 and 23.
US433634A 1954-06-01 1954-06-01 Magnetic structure Expired - Lifetime US2963557A (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
US433634A US2963557A (en) 1954-06-01 1954-06-01 Magnetic structure

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US433634A US2963557A (en) 1954-06-01 1954-06-01 Magnetic structure

Publications (1)

Publication Number Publication Date
US2963557A true US2963557A (en) 1960-12-06

Family

ID=23720924

Family Applications (1)

Application Number Title Priority Date Filing Date
US433634A Expired - Lifetime US2963557A (en) 1954-06-01 1954-06-01 Magnetic structure

Country Status (1)

Country Link
US (1) US2963557A (en)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6434252B1 (en) * 1999-09-20 2002-08-13 Royer Labs Ribbon microphone
US20070223773A1 (en) * 2004-10-21 2007-09-27 Tripp Hugh A Methods for forming and using thin film ribbon microphone elements and the like
US20090116670A1 (en) * 2007-11-06 2009-05-07 Kabushiki Kaisha Audio-Technica Ribbon microphone and ribbon microphone unit
US20090141914A1 (en) * 2007-11-29 2009-06-04 Kabushiki Kaisha Audio-Technica Ribbon microphone and ribbon microphone unit
US9877110B2 (en) 2014-12-29 2018-01-23 Michael Patrick Timmins Ribbon support system for electrodynamic microphone

Citations (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1748993A (en) * 1926-10-19 1930-03-04 Western Electric Co Electrical coil and method of manufacturing it
US1886816A (en) * 1931-07-25 1932-11-08 Jensen Radio Mfg Co Magnetic sound reproducer
US1941476A (en) * 1930-06-27 1934-01-02 Jensen Radio Mfg Company Method of making sound reproducers
US1941477A (en) * 1931-07-25 1934-01-02 Jensen Radio Mfg Company Method of making magnetic sound reproducers
US1995025A (en) * 1931-07-22 1935-03-19 Utah Radio Products Company Dynamic speaker unit
US2020212A (en) * 1935-09-06 1935-11-05 James P Quam Method of making dynamic speakers
US2020211A (en) * 1934-10-29 1935-11-05 James P Quam Loud speaker
US2046515A (en) * 1930-06-27 1936-07-07 Jensen Radio Mfg Company Sound reproducer
US2233886A (en) * 1938-05-05 1941-03-04 Bell Telephone Labor Inc Acoustic device
US2307205A (en) * 1939-11-16 1943-01-05 Westinghouse Electric & Mfg Co Magnet system
FR905046A (en) * 1943-06-15 1945-11-22 Deutsche Edelstahlwerke Ag Process for manufacturing permanent magnet systems and devices manufactured by this process
US2501615A (en) * 1946-03-07 1950-03-21 Western Electric Co Method of forming magnetic field patterns
US2539671A (en) * 1946-02-28 1951-01-30 Rca Corp Directional microphone
AT168557B (en) * 1948-05-20 1951-06-25 Optische Anstalt C P Goerz Ges Magnet system with adjustable pole yokes, especially for moving coil measuring devices
US2699474A (en) * 1950-12-29 1955-01-11 Rca Corp Velocity microphone
US2832932A (en) * 1951-08-07 1958-04-29 Baermann Max Magnet arrangement for the production of eddy currents

Patent Citations (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1748993A (en) * 1926-10-19 1930-03-04 Western Electric Co Electrical coil and method of manufacturing it
US1941476A (en) * 1930-06-27 1934-01-02 Jensen Radio Mfg Company Method of making sound reproducers
US2046515A (en) * 1930-06-27 1936-07-07 Jensen Radio Mfg Company Sound reproducer
US1995025A (en) * 1931-07-22 1935-03-19 Utah Radio Products Company Dynamic speaker unit
US1886816A (en) * 1931-07-25 1932-11-08 Jensen Radio Mfg Co Magnetic sound reproducer
US1941477A (en) * 1931-07-25 1934-01-02 Jensen Radio Mfg Company Method of making magnetic sound reproducers
US2020211A (en) * 1934-10-29 1935-11-05 James P Quam Loud speaker
US2020212A (en) * 1935-09-06 1935-11-05 James P Quam Method of making dynamic speakers
US2233886A (en) * 1938-05-05 1941-03-04 Bell Telephone Labor Inc Acoustic device
US2307205A (en) * 1939-11-16 1943-01-05 Westinghouse Electric & Mfg Co Magnet system
FR905046A (en) * 1943-06-15 1945-11-22 Deutsche Edelstahlwerke Ag Process for manufacturing permanent magnet systems and devices manufactured by this process
US2539671A (en) * 1946-02-28 1951-01-30 Rca Corp Directional microphone
US2501615A (en) * 1946-03-07 1950-03-21 Western Electric Co Method of forming magnetic field patterns
AT168557B (en) * 1948-05-20 1951-06-25 Optische Anstalt C P Goerz Ges Magnet system with adjustable pole yokes, especially for moving coil measuring devices
US2699474A (en) * 1950-12-29 1955-01-11 Rca Corp Velocity microphone
US2832932A (en) * 1951-08-07 1958-04-29 Baermann Max Magnet arrangement for the production of eddy currents

Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6434252B1 (en) * 1999-09-20 2002-08-13 Royer Labs Ribbon microphone
US8218795B2 (en) 2004-10-21 2012-07-10 Shure Incorporated Methods for forming and using thin film ribbon microphone elements and the like
US7894619B2 (en) * 2004-10-21 2011-02-22 Shure Incorporated Acoustic ribbon transducer arrangements
US20080152186A1 (en) * 2004-10-21 2008-06-26 Crowley Robert J Composite acoustic transducers
US20070223773A1 (en) * 2004-10-21 2007-09-27 Tripp Hugh A Methods for forming and using thin film ribbon microphone elements and the like
US20070274555A1 (en) * 2004-10-21 2007-11-29 Crowley Robert J Acoustic ribbon transducer arrangements
US7900337B2 (en) 2004-10-21 2011-03-08 Shure Incorporated Method of making composite acoustic transducers
CN101431711A (en) * 2007-11-06 2009-05-13 欧力天工股份有限公司 Ribbon microphone and ribbon microphone unit
US20090116670A1 (en) * 2007-11-06 2009-05-07 Kabushiki Kaisha Audio-Technica Ribbon microphone and ribbon microphone unit
US8275156B2 (en) * 2007-11-06 2012-09-25 Kabushiki Kaisha Audio-Technica Ribbon microphone and ribbon microphone unit
CN101431711B (en) * 2007-11-06 2014-01-22 欧力天工股份有限公司 Ribbon microphone and ribbon microphone unit
US20090141914A1 (en) * 2007-11-29 2009-06-04 Kabushiki Kaisha Audio-Technica Ribbon microphone and ribbon microphone unit
US8275157B2 (en) * 2007-11-29 2012-09-25 Kabushiki Kaisha Audio-Technica Ribbon microphone and ribbon microphone unit
CN101448190B (en) * 2007-11-29 2013-07-24 欧力天工股份有限公司 Ribbon microphone and ribbon microphone unit
US9877110B2 (en) 2014-12-29 2018-01-23 Michael Patrick Timmins Ribbon support system for electrodynamic microphone

Similar Documents

Publication Publication Date Title
US2165307A (en) Means for translating magnetic variations into electric variations
US2963557A (en) Magnetic structure
US2034872A (en) Phonograph reproducer
US2677875A (en) Method of assembling electromagnetic horns
US3022377A (en) Sound reproducing system
US2233621A (en) Pole-piece assembly for telegraphones
GB1045807A (en) Loudspeaker
US3302143A (en) Reed relay assembly having improved mounting means
US2993104A (en) Electromagnetic relay
US2463786A (en) Electromagnetic mechanism for bone conduction receivers, etc.
US2831939A (en) Electromechanical relay
US2241105A (en) Sound translating device
US2877319A (en) Electromechanical vibrator
US3182232A (en) Electromagnetic relays
US3142734A (en) Magnetic structure for reed relays
US2800620A (en) Television receiver mechanical subassembly
US2393901A (en) Magnetic signal device
US2147137A (en) Electroacoustical apparatus
US2890289A (en) Ribbon-type magnetic armature transducer
US2396135A (en) Electromagnetic device
US2226934A (en) Acoustic device
US2113364A (en) Electroacoustical apparatus
US3439130A (en) Central armature telephone receiver
US2666826A (en) Magnetic relay with special armature mounting means
US3234339A (en) Speaker