US3881074A - Electro-acoustic transducer - Google Patents

Electro-acoustic transducer Download PDF

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Publication number
US3881074A
US3881074A US232142A US23214272A US3881074A US 3881074 A US3881074 A US 3881074A US 232142 A US232142 A US 232142A US 23214272 A US23214272 A US 23214272A US 3881074 A US3881074 A US 3881074A
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United States
Prior art keywords
pole piece
magnet
voice coil
electro
yoke
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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
US232142A
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English (en)
Inventor
Sinichiro Kawamura
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Hitachi Ltd
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Hitachi Ltd
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Publication date
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Publication of US3881074A publication Critical patent/US3881074A/en
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; ELECTRIC HEARING AIDS; PUBLIC ADDRESS SYSTEMS
    • H04R9/00—Transducers of moving-coil, moving-strip, or moving-wire type
    • H04R9/02—Details
    • H04R9/025—Magnetic circuit

Definitions

  • ABSTRACT An electro-acoustic transducer in which a voice coil having an axial length larger than the thickness of a yoke is disposed in the magnetic flux gap defined between a pole piece and the yoke, and an audio signal is applied to the voice coil for causing vibrations of the voice coil.
  • a shortcircuit ring is disposed around the pole piece for cancelling the portion of a.c. magnetic flux which emanates from the voice coil and circulates through the magnetic circuit, so as to reduce second harmonic distortions appearing in the acoustic output signal.
  • This invention relates to electro-acoustic transducers commonly known as loudspeakers and more particularly to an electro-acoustic transducer preferably used as a high fidelity loudspeaker which generates a large acoustic output in response to the application of a large input signal.
  • An electro-acoustic transducer which is connected to a radio receiver or an audio amplifier for converting an electrical audio signal into an acoustic signal is required to operate satisfactorily with or accept a large input signal and to generate a large acoustic output in response to such input. Further. it is desirable that such an electro-acoustic transducer generates an acoustic output which is free from distortions over the entire range of the reproduced frequency band.
  • the sound pressure of an acoustic output signal obtained by the vibration of the diaphragm of an electroacoustic transducer is proportional to the acceleration imparted to the diaphragm during vibration, and the diaphragm and voice coil vibrate with an increased amplitude when a large input signal is applied. Therefore, in order that the electro-acoustic transducer can operate satisfactorily in response to the application of a large input signal.
  • the diaphragm and voice coil must be such that these elements are free from any breakdown even when vibrated with a large amplitude.
  • the diaphragm is capable of vibrating with a large amplitude when the shape and material thereof are suitably selected so that its edge portion can make a sufficient expanding and contracting movement.
  • the voice coil can be adapted to deal with vibrations of large amplitude by selecting its axial length to be sufficiently greater than the axial length of the magnetic flux gap defined between the magnet and the yoke so that it may not escape or move out of the magnetic flux gap even when subjected to vibrations of large amplitude.
  • Another object of the present invention is to provide an electro-acoustic transducer which employs a voice coil having a large axial length and whose acoustic output is substantially free from second harmonic distortions.
  • a further object of the present invention is to provide an electro-acoustic transducer in which a ferrite magnet is used in the magnetic circuit. which employs a voice coil having a large axial length and whose acoustic output is substantially free from second harmonic distortions.
  • the electro-acoustic transducer according to the present invention is featured by the fact that it comprises a pole piece. a yoke disposed in the vicinity of the upper end portion of the pole piece and having a magnetic pole portion facing the pole piece while defining an annular magnetic flux gap therebetween. a magnet magnetically coupled to the pole piece and the yoke. a voice coil inserted in the magnetic flux gap. the voice coil having a large axial length compared with the thickness of the magnetic pole portion of the yoke. a diaphragm operatively connected to the voice coil. and a short-circuit ring of metal material such as copper or aluminum disposed around the pole piece so as to cancel the portion of a.c. magnetic flux emanating from the voice coil and circulating through the magnetic circuit including the yoke. the magnet and the pole piece thereby preventing undesirable appearance of second harmonic distortions in the acoustic output signal.
  • FIG. 1 is an axial section of an embodiment of the electro-acoustic transducer according to the present invention
  • FIG. 2 is an axial section of driving parts of the electro-acoustic transducer, illustrating the flow of a.c. magnetic flux emanating from the voice coil;
  • FIG. 3 is an enlarged perspective view of one form of the short-circuit ring.
  • FIG. 4 is an enlarged perspective view of another form of the short-circuit ring.
  • a generally frusto-conical frame I is provided with a flange portion 2 along the outer periphery of one of the open ends, and a magnetic circuit 3 is mounted on the other end having a central opening.
  • the magnetic circuit 3 is composed of an annular magnet 4 having a central opening. an annular first yoke bonded or otherwise fixed to the upper surface of the magnet 4 and having a central circular magnetic pole portion 16, a pole piece 8 inserted in the central opening of the magnet 4 opposite to the magnetic pole portion 16, and a second yoke 9 bonded or otherwise fixed to the lower surface of the magnet 4.
  • the outer peripheral surface of the pole piece 8 is suitably spaced from the inner wall surface of the central opening of the first yoke 5 to define a magnetic flux gap 6 therebetween.
  • the mag net 4 is made of ferrite. but it may be made of an aluminum-nickel-cobalt alloy.
  • a generally frusto-conical diaphragm 7 is corrugated at the outer peripheral edge portion at one end thereof as shown by 10, and this corrugated edge portion 10 is bonded or otherwise fixed to an annular sheet of cardboard 11 secured to the flange portion 2 of the frame 1.
  • a voice coil bobbin 12 is inserted in the magnetic :flux gap 6 in the magnetic circuit 3 and is bonded or otherwise fixed at its upper end to the other end of the diaphragm 7. Further. the diaphragm 7 is connected to a spider 13 which is secured to the frame 1.
  • the corrugated edge portion 10 of the diaphragm 7 and the spider 13 are flexibly pliable so that the diaphragm 7 can vibrate freely.
  • the voice coil bobbin 12 is in the form of an elongated cylinder extending downwardly beyond the lower end of the magnetic flux gap 6 defined between the pole piece 8 and the first yoke 5, and a voice coil 14 is wound around the bobbin 12.
  • the voice coil 14 has a large axial length so that it extends downwardly beyond the lower end of the magnetic flux gap 6.
  • the axial length of the voice coil 14 is sufficiently large compared with the thickness of the first yoke 5 so that the voice coil 14 may not escape or move out of the magnetic flux gap 6 even when a large input signal is supplied thereto.
  • a short-circuit ring 15 of cylindrical shape made of metal material such as aluminum or copper is disposed in the central opening of the magnet 4 and is bonded or otherwise secured to the first and second yokes 5 and 9.
  • the voice coil 14 disposed in the magnetic flux gap 6 defined between the first yoke 5 and the pole piece 8 has an axial length I which is sufficiently large compared with the length L of the magnetic flux gap 6, hence the thickness of the magnetic pole portion 16 of the first yoke 5.
  • an a.c. magnetic flux emanates from the voice coil 14. This a.c.
  • magnetic magnetic flux portion 20 which circulates around the voice coil 14 and another magnetic flux portion 21 which passes through the first yoke 5, magnet 4, second yoke 9 and pole piece 8 to circulate through the magnetic circuit 3.
  • the magnetic flux portion 21 circulating through the magnetic circuit 3 appears when the axial length l of the voice coil 14 is greater than the length L of the magnetic flux gap 6, and it increases with the increase in the axial length of the voice coil 14. Since this magnetic flux portion 21 passes through the interior of the first and second yokes 5 and 9 and the pole piece 8 which are biased by a dc.
  • the magnetic flux portion 21 is distorted due to the non-linear properties of the first yoke 5, pole piece 8 and second yoke 9 thereby producing second harmonic distortions in the audio signal current supplied to the voice coil 14.
  • the second harmonic distortions appear more in the audio signal current when the magnet 4 is made of ferrite than when the magnet 4 is made of an aluminum-nickel-cobalt alloy for the two reasons described below. In the first place, the appearance of such distortions depends upon the degree of saturation of the magnetic paths in the magnetic circuit. The experiment made by the inventor has proved that less second harmonic distortions appear when the density of magnetic flux passing through the first and second yokes 5 and 9 is sufficiently high to an extent that these yokes 5 and 9 are nearly saturated. However, the operating d.c.
  • magnetic flux density for the magnet 4 when made of an aluminum-nickel-cobalt alloy is of the order of 10,000 gauss, whereas that for the magnet 4 when made of ferrite is of the order of 3,000 gauss.
  • the magnetic flux density for the first and second yokes 5 and 9 engaging the magnet 4 is naturally equal to the operating d.c. magnetic flux density for the magnet 4 although its value differs depending on the material of the magnet 4.
  • the yokes 5 and 9 are not saturated and the second harmonic distortions appear in a greater degree.
  • the a.c. reluctance of the magnetic circuit is low when the magnet 4 is made of ferrite compared with the a.c. reluctance of the mag- -netic circuit when the magnet 4 is made of the aluminum-nickel-cobalt alloy.
  • more a.c. magnetic flux flows through the magnetic circuit when the magnet 4 is made of ferrite.
  • the ferrite magnet and the magnet of aluminum-nickel-cobalt alloy used in the magnetic circuit have different shapes depending on their magnetic properties.
  • the ferrite magnet is shaped to have a large surface area and a small thickness
  • the magnet of aluminum-nickel-cobalt alloy is shaped to have a small surface area and a large thick-
  • the magnetic reluctance Rm of the magnetic circuit against an a.c. magnetic field is given by the following equation:
  • the reversible permeability is approximately equal to 1 since the ferrite magnet has a small thickness and a large surface area.
  • the magnet of aluminum-nickel-cobalt alloy is used in the magnetic .circuit, the reversible permeability is approximately equal to 4 since this magnet has a small surface area and a large thickness.
  • the magnetic. reluctance Rm in the former case is lower than the magnetic reluctance Rm in the latter case, and an increased amount of a.c. magnetic flux circulates through the magnetic circuit resulting in an increase in the second harmonic distortions.
  • the density B of the a.c. magnetic flux flowing through the magnetic circuit is proportional to the current flowing through the voice coil and is given by the following equation:
  • Tli'eh force F acting upon the voice coil is given by the following equation:
  • the increase in the axial length of the voice coil results in the appearance of second harmonic distortions due to the fact that the ac. magnetic flux portion 21 emanating from the voice coil flows through the magnetic circuit. Therefore, the ac. magnetic flux portion 21 circulating through the magnetic circuit may be suitably cancelled in order to prevent undesirable appearance of the second harmonic distortions.
  • the short-circuit ring of metal material such as aluminum or copper is disposed in the space between the first and second yokes 5 and 9 as seen in FlG. 1 in order to cancel this undesirable magnetic flux portion 21.
  • an ac. magnetic flux portion circulating around the voice coil 14 and another a.c. magnetic flux portion passing through the first yoke 5, magnet 4, second yoke 9 and pole piece 8 to circulate through the magnetic circuit 3 emanate from the voice coil 14 in response to the supply of an audio signal current to the voice coil 14 as previously described.
  • the ac. magnetic flux portion 21 flows across the short-circuit ring .15 thereby inducing a counter electromotive force in the ring 15.
  • the short-circuit ring 15 is equivalent to a coil having a single turn and the counter electromotive force induced in the ring 15 causes a current to flow therein thereby producing a magnetic flux.
  • the magnetic flux emanating from the short-circuit ring 15 flows in a direction opposite to the direction of flow of the magnetic flux portion 21 which emanates from the voice coil 14 and circulates through the magnetic circuit.
  • this magnetic flux cancels the ac. magnetic flux portion 21 emanating from the voice coil 14 and circulating through the magnetic circuit.
  • FlGS. 3 and 4 show preferred forms of the shortcircuit ring 15 disposed around the pole piece 8 adja- Cent to the magnet 4.
  • the short-circuit ring 15 shown in FIG. 3 is formed in a cylindrical shape from a plate of metal such as aluminum or copper.
  • This short-circuit ring 15 is so sized that its inner diameter is larger than the outer diameter of the pole piece 8 and its outer diameter is smaller than the diameter of the central opening of the magnet 4.
  • This short-circuit ring 15 can be easily made by cutting an elongated pipe of metal such as aluminum or copper into predetermined lengths.
  • the short-circuit ring 15 shown in FIG. 4 is made by coiling a large-diameter wire of metal such as aluminum or copper over a plurality of turns and connecting the starting end 41 of the coil convolutions to the terminating end 42.
  • the present invention provides a useful electroacoustic transducer in which, in spite of the fact that a voice coil having a large axial length is employed. an unnecessary a.c. magnetic flux emanating from the voice coil can be effectively cancelled to prevent occurrence of undesirable distortions. Therefore. the electro-acoustic transducer can deliver a large acoustic output in response to a large input signal and undesirable distortions appearing in the acoustic output signal can be minimized.
  • An electro-acoustic transducer comprising a pole piece, a yoke having a magnetic pole portion facing said pole piece while defining an annular magnetic flux gap therebetween a magnet magnetically coupled to said pole piece and said yoke, a voice coil inserted in said magnetic flux gap, said voice coil having a large axial length compared with the thickness of said mag netic pole portion of said yoke.
  • an annular conductive short-circuit ring disposed concentrically around said pole piece between said magnet and said pole piece. and a diaphragm operatively connected to said voice coil, whereby the magnetic coupling between said magnet and said pole piece caused by the large axial length voice coil is cancelled by said short-circuit ring.
  • An electro-acoustic transducer comprising:
  • annular ferrite magnet having an opening formed in a central portion of said annular magnet
  • first annular yoke bonded to one surface of said ferrite magnet and having a circular magnetic pole in a central annular opening of said first yoke
  • a voice coil bobbin operatively disposed in said magnetic flux gap between said first yoke and said pole piece:
  • a diaphragm operatively connected to said voice coil bobbin.
  • An electro-acoustic transducer as claimed in claim 7. in which said short-circuit ring disposed around said pole piece is made by winding a metal wire into a coil form and connecting the starting and terminating ends of the wire forming the coil convolutions.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Signal Processing (AREA)
  • Audible-Bandwidth Dynamoelectric Transducers Other Than Pickups (AREA)
US232142A 1971-03-10 1972-03-06 Electro-acoustic transducer Expired - Lifetime US3881074A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
JP1971015019U JPS542647Y2 (fr) 1971-03-10 1971-03-10

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US3881074A true US3881074A (en) 1975-04-29

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JP (1) JPS542647Y2 (fr)
CA (1) CA982687A (fr)

Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4295011A (en) * 1979-09-11 1981-10-13 Epicure Products Inc. Linear excursion-constant inductance loudspeaker
US4421956A (en) * 1981-09-29 1983-12-20 Peavey Electronics Corp. Loud speaker with minimized magnetic leakage
US4628154A (en) * 1981-12-24 1986-12-09 Kort Eckehard K Annular gap magnet system, particularly for low frequency loudspeakers
EP0208907A1 (fr) * 1985-07-17 1987-01-21 WILLI STUDER AG Fabrik für elektronische Apparate Système magnétique pour haut-parleur dynamique
US5381483A (en) * 1993-04-05 1995-01-10 Commonwealth Of Puerto Rico Minimal inductance electrodynamic transducer
US5455867A (en) * 1992-11-18 1995-10-03 Matsushita Electric Industrial Co., Ltd. Speaker apparatus and television receiver using the same
US5748760A (en) * 1995-04-18 1998-05-05 Harman International Industries, Inc. Dual coil drive with multipurpose housing
WO1999048329A1 (fr) * 1998-03-19 1999-09-23 Jbl Incorporated Anneaux de mise en court-circuit pour circuits d'attaque de haut-parleurs a deux bobines et a deux entrefers
WO2002037644A3 (fr) * 2000-10-25 2003-03-27 Harman Int Ind Moteur electromagnetique a anneau de stabilisation de flux, extremites de saturation et radiateur
US20030190052A1 (en) * 1998-03-19 2003-10-09 Jbl Incorporated Shorting rings in dual-coil dual-gap loudspeaker drivers
US20040042631A1 (en) * 2002-08-28 2004-03-04 Mineba Co., Ltd. Magnetic circuit for speaker with short-circuiting ring
US20040208337A1 (en) * 1999-10-19 2004-10-21 Sagem Sa Permanent magnet actuator with electric excitation coil, especially loudspeaker and mobile telephone
US10280348B2 (en) 2017-01-31 2019-05-07 Prc-Desoto International, Inc. Low density aerospace compositions and sealants
US20220141592A1 (en) * 2019-02-28 2022-05-05 Purifi Aps Loudspeaker motor with improved linearity
US20220232325A1 (en) * 2018-10-15 2022-07-21 Suzhou Sonavox Electronics Co., Ltd. Magnetic circuit of loudspeaker and loudspeaker
US12441049B2 (en) 2019-02-11 2025-10-14 Ppg Industries Ohio, Inc. 3D printing of seal caps

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1953542A (en) * 1929-08-06 1934-04-03 Magnavox Co Sound reproducing device
US3079472A (en) * 1960-10-06 1963-02-26 Rca Corp Magnetic field structure for transducers and method of constructing same
US3413579A (en) * 1966-03-14 1968-11-26 Westinghouse Electric Corp Magnetic field assembly for electro-mechanical transducers

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1953542A (en) * 1929-08-06 1934-04-03 Magnavox Co Sound reproducing device
US3079472A (en) * 1960-10-06 1963-02-26 Rca Corp Magnetic field structure for transducers and method of constructing same
US3413579A (en) * 1966-03-14 1968-11-26 Westinghouse Electric Corp Magnetic field assembly for electro-mechanical transducers

Cited By (32)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4295011A (en) * 1979-09-11 1981-10-13 Epicure Products Inc. Linear excursion-constant inductance loudspeaker
US4421956A (en) * 1981-09-29 1983-12-20 Peavey Electronics Corp. Loud speaker with minimized magnetic leakage
US4628154A (en) * 1981-12-24 1986-12-09 Kort Eckehard K Annular gap magnet system, particularly for low frequency loudspeakers
EP0208907A1 (fr) * 1985-07-17 1987-01-21 WILLI STUDER AG Fabrik für elektronische Apparate Système magnétique pour haut-parleur dynamique
US4980921A (en) * 1985-07-17 1990-12-25 Willi Studer Ag Magnetic system for dynamic loudspeaker
US5455867A (en) * 1992-11-18 1995-10-03 Matsushita Electric Industrial Co., Ltd. Speaker apparatus and television receiver using the same
EP0748116A1 (fr) * 1992-11-18 1996-12-11 Matsushita Electric Industrial Co., Ltd. Haut-parleur pour récepteur de télévision
US5381483A (en) * 1993-04-05 1995-01-10 Commonwealth Of Puerto Rico Minimal inductance electrodynamic transducer
US5748760A (en) * 1995-04-18 1998-05-05 Harman International Industries, Inc. Dual coil drive with multipurpose housing
US6768806B1 (en) 1998-03-19 2004-07-27 Harman International Industries, Incorporated Shorting rings in dual-coil dual-gap loudspeaker drivers
WO1999048329A1 (fr) * 1998-03-19 1999-09-23 Jbl Incorporated Anneaux de mise en court-circuit pour circuits d'attaque de haut-parleurs a deux bobines et a deux entrefers
US6847726B2 (en) * 1998-03-19 2005-01-25 Harman International Industries, Incorporated Shorting rings in dual-coil dual-gap loudspeaker drivers
US20030190052A1 (en) * 1998-03-19 2003-10-09 Jbl Incorporated Shorting rings in dual-coil dual-gap loudspeaker drivers
US20040208337A1 (en) * 1999-10-19 2004-10-21 Sagem Sa Permanent magnet actuator with electric excitation coil, especially loudspeaker and mobile telephone
US6901150B1 (en) * 1999-10-19 2005-05-31 Sagem, Sa Permanent magnet actuator with electric excitation coil, especially loudspeaker and mobile telephone
US7012345B2 (en) 2000-10-25 2006-03-14 Harman International Industries, Inc. Electromagnetic motor with flux stabilization ring, saturation tips, and radiator
US6774510B1 (en) 2000-10-25 2004-08-10 Harman International Industries, Inc. Electromagnetic motor with flux stabilization ring, saturation tips, and radiator
US7057314B2 (en) 2000-10-25 2006-06-06 Harman International Industries, Inc. Electromagnetic motor system capable of removing heat away from its magnetic gap
US20040239193A1 (en) * 2000-10-25 2004-12-02 Jerry Moro Electromagnetic motor with flux stabilization ring, saturation tips, and radiator
WO2002037644A3 (fr) * 2000-10-25 2003-03-27 Harman Int Ind Moteur electromagnetique a anneau de stabilisation de flux, extremites de saturation et radiateur
US20050179326A1 (en) * 2000-10-25 2005-08-18 Harman International Industries Incorporated Electromagnetic motor with flux stabilization ring, saturation tips, and radiator
US7031489B2 (en) * 2002-08-28 2006-04-18 Minebea Co., Ltd. Magnetic circuit for speaker with short-circuiting ring
EP1395082A3 (fr) * 2002-08-28 2004-06-16 Minebea Co., Ltd. Circuit magnétique pour haut-parleur avec bague de court-circuit
US20040042631A1 (en) * 2002-08-28 2004-03-04 Mineba Co., Ltd. Magnetic circuit for speaker with short-circuiting ring
US10280348B2 (en) 2017-01-31 2019-05-07 Prc-Desoto International, Inc. Low density aerospace compositions and sealants
US11066584B2 (en) 2017-01-31 2021-07-20 Prc-Desoto International, Inc. Low density aerospace compositions and sealants
US11760909B2 (en) 2017-01-31 2023-09-19 Prc-Desoto International, Inc. Low density aerospace compositions and sealants
US20220232325A1 (en) * 2018-10-15 2022-07-21 Suzhou Sonavox Electronics Co., Ltd. Magnetic circuit of loudspeaker and loudspeaker
US11962987B2 (en) * 2018-10-15 2024-04-16 Suzhou Sonavox Electronics Co., Ltd. Magnetic circuit of loudspeaker and loudspeaker
US12441049B2 (en) 2019-02-11 2025-10-14 Ppg Industries Ohio, Inc. 3D printing of seal caps
US20220141592A1 (en) * 2019-02-28 2022-05-05 Purifi Aps Loudspeaker motor with improved linearity
US11956612B2 (en) * 2019-02-28 2024-04-09 Purifi Aps Loudspeaker motor with improved linearity

Also Published As

Publication number Publication date
CA982687A (en) 1976-01-27
JPS4712923U (fr) 1972-10-16
JPS542647Y2 (fr) 1979-02-05

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