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Ferrofluid Centered Voice Coil Speaker

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Ferrofluid Centered Voice Coil Speaker (19) & (11) EP 1 819 194 A2 (12) EUROPEAN PATENT APPLICATION (43) Date of publication: (51) Int Cl.: 15.08.2007 Bulletin 2007/33 H04R 9/04 (2006.01) (21) Application number: 07002539.0 (22) Date of filing: 06.02.2007 (84) Designated Contracting States: (72) Inventors: AT BE BG CH CY CZ DE DK EE ES FI FR GB GR • Tsuda, Shiro HU IE IS IT LI LT LU LV MC NL PL PT RO SE SI Asahi SK TR Chiba, 289-2515 (JP) Designated Extension States: • Rosensweig, Ronald E. AL BA HR MK YU Summit, NJ 07901 (US) (30) Priority: 14.02.2006 US 766831 P (74) Representative: von Hellfeld, Axel Wuesthoff & Wuesthoff (71) Applicant: FERROTEC CORPORATION Patent- und Rechtsanwälte Tokyo 104-0031 (JP) Schweigerstrasse 2 81541 München (DE) (54) Ferrofluid centered voice coil speaker (57) An audio speaker has a driver unit (20) having where the voice coil is movably mounted in the annular a support frame (22) with a central portion (24) forming gap, and a magnetic fluid (80) disposed in the annular a magnetic structure (26) defining an annular gap (28) gap only in a space between one side of the voice coil around a central magnetic post 40), a vibration system and a surface of the annular gap having a higher mag- 60) having a diaphragm (62) and a voice coil (64), the netic flux density. voice coil is attached to one side of the diaphragm where the vibration system is fixed to the support frame and EP 1 819 194 A2 Printed by Jouve, 75001 PARIS (FR) 1 EP 1 819 194 A2 2 Description signal without scraping against the metal pieces, e.g. pole piece and front plate, forming the gap. [0001] This application claims the benefit of US Provi- [0007] In a 1980 AES publication by Bottenberg et al., sional Patent Application No. 60/766,831, filed February titled "The Dependence of Loudspeaker Design Param- 14, 2006. 5 eters on the Properties of Magnetic Fluids," which was presented at the 61 st Convention of the Audio Engineer- BACKGROUND OF THE INVENTION ing Society, November 3-6, 1978, the authors derived a mathematical expression for the ferrofluid radial force for 1. Field of the Invention a 1 inch dome tweeter having the gap on both sides of 10 the coil filled with a ferrofluid. Many tweeter manufactur- [0002] The present invention relates to audio speak- ers now take advantage of the centering force of ferrofluid ers. Particularly, the present invention relates to audio by filling the gap on both sides of the coil and thus re- speakers that utilize liquid suspension mechanisms for quiring no mechanical centering device, i.e. a spider. the voice coils incorporated into audio speakers. [0008] Unfortunately, the use of ferrofluid in the air gap 15 causes other problems. The air gap is connected to a 2. Description of the Prior Art cavity between the speaker magnet and various compo- nents of the magnetic structure. Ferrofluid in the air gap [0003] Audio speakers produce audible sounds by dis- acts like an O-ring seal between the voice coil and the placing air by way of the movement of a diaphragm. The magnetic structure. The cavity is essentially sealed off diaphragm is attached to and moves under the control 20 from the air space in front of the front plate. As the voice of a voice coil, through which electric currents associated coil moves or as the temperature rises, it tends to elevate with the sounds to be reproduced are driven. The voice the air pressure within this cavity. If the pressure builds coil is disposed in an annular air gap of a magnetic struc- up to a point where it exceeds the pressure capacity of ture. The magnetic structure includes a permanent mag- the ferrofluid O-ring seal, the air bursts through the seal net that provides radial flux in the air gap. Current through 25 and relatively large amounts of the ferrofluid may then the coil interacts with this radial flux to provide axial forces be blown or flow out of the gap. One solution to this prob- on the coil and causes displacement of the coil and the lem was addressed by U.S. Patent No. 5,335,287. attached diaphragm. [0009] U.S. Patent No. 5,335,287 (1994, Athanas) dis- [0004] The alignment of the voice coil in the air gap is closes a loudspeaker with a viscous magnetic fluid sus- crucial to the performance of an audio speaker. Any30 pension for the voice coil rather than the corrugated disk scraping of the voice coil against the sides of the air gap suspension that is conventionally used. Specially de- causes humming and distortion, which affects sound signed vent passages are formed in the magnet assem- quality. Additionally, scraping creates undesirable bly in order to prevent internal pressure from either build- stresses on the suspension system as well as removal ing up or creating sub- atmospheric conditions that could of the insulation from the coil windings. This leads to early 35 cause the magnetic fluid to be blown out of the magnetic speaker failure. Speaker manufacturers commonly em- gap. The patent also discloses that the mechanical ploy a flexible, fibrous element called a spider to align centering device can be eliminated even in more de- the voice coil in the air gap. The inclusion of a spider manding high excursion woofers by utilizing ferrofluids requires additional space in the speaker, which is not of high magnetization values such as 600-800 Gauss available in audio speakers such as tweeters and very 40 filling the gap on both sides of the voice coil. small full range speakers. [0010] Other problems that arise when using ferrofluid [0005] For over 25 years, air gaps in some audio in the air gap around the voice coil involve equalization speakers have been filled with magnetic fluids (also of the amount of ferrofluid on both sides of the voice coil called ferrofluids). These ferrofluids offer significant per- (because filing the entire gap can be problematic) and formance advantages in tweeters, midrangers, woofers, 45 equalization of the air pressure under the dust cap of the compression drivers, and automotive speakers. Ferroflu- voice coil. To address these problems, a plurality of vent ids are used for a wide variety of reasons such as damp- openings were incorporated in the voice coil adjacent the ing of the voice coil, heat transfer, reduction in harmonic dome end of the voice coil such as is disclosed in U.S. distortion, lubrication, and centering of the voice coil in Patent No. 4,414,437. the air gap. 50 [0011] U.S. Patent No. 4,414,437 (1983, Trauernicht [0006] A unique aspect of ferrofluid is its ability to exert et al.) discloses a moving coil dynamic transducer. The a radial force on the voice coil, which has come to be electromagnetic transducer (voice coil) includes a mem- known as the centering force. The magnitude of this force ber producing a magnetic field and presenting an air gap depends on the magnetization of the ferrofluid and the traversed by the magnetic field, a mass of magnetic fluid magnetic flux density in the air gap of the speaker. The 55 extending across the air gap, and a moving coil mounted higher the magnetization and flux density, the greater the on a moving coil carrier supported for movement through radial force. The radial force keeps the voice coil properly the air gap. The moving coil carrier is provided with at aligned as it vibrates in the gap in response to an audio least one passage located to communicate with the mag- 2 3 EP 1 819 194 A2 4 netic liquid during at least part of the movement of the the pole piece that is situated in the lower magnetic field coil carrier through the air gap. The passage permits flow unless the gap portions are separated. The tubular form of magnetic liquid from one side to the other of the carrier of the voice coil of the present invention is structured to in the direction of the air gap. This provides an equaliza- maintain gap portion separation and prevent magnetic tion of the magnetic liquid on each side of the voice coil. 5 fluid migration. [0012] Therefore, what is needed is an audio speaker [0017] In one embodiment, the magnetic structure of that uses ferrofluid for centering a voice coil without the the driver unit is an assembly that includes a base plate need for ferrofluid equalizing vent openings. What is also with a central magnetic post, an annular permanent mag- needed is an audio speaker that uses ferrofluid for center- net and an annular top plate. The annular permanent ing a voice coil without the need for ferrofluid equalizing 10 magnet is sandwiched between the base plate and the vent openings or the use of a spider. annular top plate. The base plate and the central mag- netic post may be made from a single piece of magnetic SUMMARY OF THE INVENTION material or may be two or more pieces integrally joined together to form an inverted, T-shaped pole piece. The [0013] It is an object of the present invention to provide 15 annular permanent magnet is axially polarized where one an audio speaker that uses ferrofluid for centering a voice face of the magnet has one polarity and the other face coil without the need for ferrofluid equalizing vent open- has the opposite polarity. ings. It is another object of the present invention to pro- [0018] The central magnetic post extends from the vide an audio speaker that uses ferrofluid for centering base plate through the central space formed within the a voice coil without the need for ferrofluid equalizing vent 20 annular magnet and the annular top plate forming a cen- openings or the use of a mechanical centering device trally located pole piece.
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