Motional feedback speaker system with radially polarized magnet and underhung voice-coil
First Claim
1. A motional feedback loudspeaker system comprisinga dynamic loudspeaker, an amplifier, and a motional feedback means,said loudspeaker including a magnet structure having a hollow cylindrical radially-polarized magnet and an elongated cylindrical magnet gap coaxial with said magnet and of a predetermined axial length with the magnetic flux of said magnet extending radially through said magnet and then through said cylindrical magnet gap to provide in the cylindrical gap a substantially uniform radial magnetic field of substantially constant flux density therein throughout said axial length thereof,said loudspeaker further including a cylindrical voice-coil within said cylindrical magnet gap and coaxial therewith and having an axial length less than said magnet gap axial length so as to be underhung with respect to said gap and so as to remain immersed within said uniform radial magnetic field during large axial displacements of said voice-coil,said loudspeaker further including a speaker cone secured to said voice-coil and a pair of speaker terminals connected to said voice-coil,said amplifier having a feedback injection node,network means connecting said amplifier to said speaker terminals,said motional feedback means including signal generating means responsive to the motion of said speaker cone for generating a motional feedback signal,said signal generating means comprising means responsive to a back electromotive force induced by cutting said radial magnetic field by said voice-coil as the voice-coil moves axially within said cylindrical magnet gap, andmeans for injecting said feedback signal into said amplifier feedback injection node.
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Abstract
A motional feedback loudspeaker system wherein the loudspeaker distortion is reduced by a negative feedback signal derived from the back electromotive force induced by the motion of the voice-coil within the magnet field. It is essential that the feedback signal be a linear function of the loudspeaker cone motion, and this is achieved by locating the voice-coil within a uniform elongated cylindrical magnetic field of substantially constant flux density and having an axial length substantially greater than the axial length of the voice-coil. Therefore the voice-coil is underhung so as to remain immersed within the uniform field even at peak displacements of the voice-coil during maximum excursions of the speaker cone. The uniform elongated magnetic field is preferably provided by a cylindrical radially polarized magnet of neodymium. In order to sense the back electromotive force due to the voice-coil motion the speaker is located within a bridge network so that the magnitude of voltage imbalance across the bridge is proportional to the cone velocity. A difference amplifier senses this voltage from which is derived the feedback signal which is fed degeneratively into an early amplification stage. This negative feedback reduces the nonlinear distortion of the loudspeaker, and also improves the transient response and frequency response.
40 Citations
20 Claims
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1. A motional feedback loudspeaker system comprising
a dynamic loudspeaker, an amplifier, and a motional feedback means, said loudspeaker including a magnet structure having a hollow cylindrical radially-polarized magnet and an elongated cylindrical magnet gap coaxial with said magnet and of a predetermined axial length with the magnetic flux of said magnet extending radially through said magnet and then through said cylindrical magnet gap to provide in the cylindrical gap a substantially uniform radial magnetic field of substantially constant flux density therein throughout said axial length thereof, said loudspeaker further including a cylindrical voice-coil within said cylindrical magnet gap and coaxial therewith and having an axial length less than said magnet gap axial length so as to be underhung with respect to said gap and so as to remain immersed within said uniform radial magnetic field during large axial displacements of said voice-coil, said loudspeaker further including a speaker cone secured to said voice-coil and a pair of speaker terminals connected to said voice-coil, said amplifier having a feedback injection node, network means connecting said amplifier to said speaker terminals, said motional feedback means including signal generating means responsive to the motion of said speaker cone for generating a motional feedback signal, said signal generating means comprising means responsive to a back electromotive force induced by cutting said radial magnetic field by said voice-coil as the voice-coil moves axially within said cylindrical magnet gap, and means for injecting said feedback signal into said amplifier feedback injection node.
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7. A motional feedback loudspeaker system comprising
a dynamic loudspeaker, an amplifier drivingly connected to said loudspeaker, and motional feedback means connected to said loudspeaker and said amplifier, said loudspeaker having a cone and suspension means for suspending said cone for axial reciprocal motion of the cone between predetermined maximum limits, said loudspeaker further comprising means for generating a back electromotive force which is a substantially linear function of the cone velocity throughout said motion of the cone between said predetermined limits. said generating means including a magnet structure having a cylindrical magnet gap with a magnetic field therein, a cylindrical voice-coil coaxially suspended within said cylindrical magnet gap and secured to said speaker cone and having an axial length substantially less than the axial length of said magnet gap, and a pair of speaker terminals connected to said voice-coil, said voice-coil thereby being underhung with respect to the gap so as to remain entirely immersed within the magnetic field throughout axial reciprocal displacements of the speaker cone to said maximum limits, whereby a back electromotive force is induced by cutting of said magnetic field by said voice-coil as the voice-coil moves axially within said cylindrical magnet gap, said motional feedback means including network means connected between said amplifier and said speaker terminals for sensing said back electromotive force induced during the axial movement of the voice-coil, means for generating a feedback signal functionally related to said induced back electromotive force, said amplifier having a feedback injection node, and said motional feedback means further including a feedback network connected to said feedback injection node for injecting said feedback signal into said injection node.
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15. A motional feedback loudspeaker system comprising
a dynamic loudspeaker, an amplifier drivingly connected to said loudspeaker, and motional feedback means connected to said loudspeaker and said amplifier, said loudspeaker having a magnet structure including a cylindrical magnet and a cylindrical magnetic flux return element coaxial with said magnet, said magnet and said return element each having a cylindrical surface with each surface spaced radially from the other surface to provide therebetween a cylindrical magnet gap coaxial with said magnet and said return element, said cylindrical magnet gap extending axially from a forward end of the magnet to a rear end thereof so as to be coextensive in axial length with said magnet, said magnet being radially polarized throughout its axial length so as to generate a radial magnetic field having magnetic flux extending radially through said magnet and then radially from said magnet cylindrical surface and then radially through said cylindrical magnet gap to said cylindrical surface of the return element, said radial magnetic field being uniform throughout the axial length of the cylindrical magnet gap, said loudspeaker having a cone and suspension means for suspending said cone for axial reciprocal movement between predetermined maximum limits, a cylindrical voice-coil coaxially suspended within said cylindrical magnet gap and secured to said cone, said voice-coil having an axial length substantially less than the axial length of said cylindrical magnet gap so as to be underhung with respect to the gap and so as to remain entirely immersed within the uniform magnetic field throughout axial reciprocal displacements of the cone to said maximum limits, whereby as the voice-coil reciprocates axially within said magnet gap to cut said magnetic flux of uniform density there is induced in the voice-coil a back electromotive force which is a linear function of the instantaneous velocity of the voice-coil and cone, said motional feedback means including network means for sensing said back electromotive force induced during the reciprocal movement of the voice-coil, means for generating a feedback signal functionally related to said sensed back electromotive force, said amplifier having a feedback injection node, and said motional feedback means including a feedback network for injecting said feedback signal into said amplifier injection node.
Specification