Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R9/00—Transducers of moving-coil, moving-strip, or moving-wire type
  • H04R9/02—Details
  • H04R9/025—Magnetic circuit
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R2209/00—Details of transducers of the moving-coil, moving-strip, or moving-wire type covered by H04R9/00 but not provided for in any of its subgroups
  • H04R2209/021—Reduction of eddy currents in the magnetic circuit of electrodynamic loudspeaker transducer
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R7/00—Diaphragms for electromechanical transducers; Cones
  • H04R7/02—Diaphragms for electromechanical transducers; Cones characterised by the construction
  • H04R7/04—Plane diaphragms
  • H04R7/045—Plane diaphragms using the distributed mode principle, i.e. whereby the acoustic radiation is emanated from uniformly distributed free bending wave vibration induced in a stiff panel and not from pistonic motion

Definitions

• the invention relates to a magnet assembly, in particular a magnet assembly for an actuator such as a loudspeaker transducer.
• FIG. 1 This shows a permanent magnet 3 sandwiched between first 5 and second 7 pole pieces.
• the first pole piece 5 is in the form of a disc having an outer periphery forming an inner pole tip 9.
• the second pole piece 7 is in the form of a cup having an outer pole tip 11.
• Inner and outer pole tips define a gap 13 therebetween in which a coil 15 moves.
• the view shown is in section; the whole magnet assembly is circularly symmetrical about the axis 17.
• the coil is a ring that is provided in the gap 13 which has the form of an annulus .
• the force exerted on the coil is given by the product of the flux density B, and current density J.
• the current density J and hence the force, will fall at high frequencies, due to the rise in inductance of the coil.
• it is possible to model and plot the flux associated with this current flowing in the coil If you plot the field around the coil in air, then the field looks like that shown in Figure 3.
• the axis of symmetry is shown as a vertical line to the left of the figure.
• the field lines are smooth and there is no distortion of the flux lines.
• the curved line which bounds the model, represents an absorbing boundary whose purpose is to terminate the mathematical model, ensuring a finite model size .
• the skin depth (S) for a conductor is given by; where ⁇ is relat permeability ⁇ is the electrical conductivity
• o is the angular frequency ( . ⁇ - frequency) .
• JP-3201797 describes an example of this technique .
• a magnet assembly comprising inner and outer pole tips defining a gap therebetween for mounting a voice coil in a predetermined axial direction, the pole tips being magnetically permeable, wherein at least one of the pole tips is electrically conductive in a plane substantially orthogonal to the predetermined axial direction in a region extending into the body of the pole tips from the edge of the pole tips adjoining the gap.
• the gap is in the form of an annulus .
• the inductance can make up a significant, even dominant part of the impedance of a conventional loudspeaker coil at frequencies around 1-10 KHz or higher.
• the inductance Since the inductance contributes j ⁇ L to the impedance, the contribution to the impedance rises with frequency. When driven at a constant voltage, the current in the coil accordingly falls at higher frequencies which likewise reduces the force on the coil. The inductance therefore causes a reduction in sound output at high frequencies.
• the magnet assembly according to the invention reduces the inductance at higher frequencies.
• the electrical conductivity of the pole tips causes the tips to be shielded from a coil placed in the annular gap by eddy currents. Accordingly, the coil inductance at these frequencies is reduced, and the sound pressure level output by a loudspeaker using such a magnet assembly can accordingly be more uniform with frequency.
• both pole tips are electrically conductive in the said plane.
• the pole tips may be near to magnetic saturation which also helps reduce the inductance of the coil slightly, especially at low frequencies.
• the pole tips may accordingly have a magnetisation that is at lest 75%, preferably 90% of the magnetisation of the saturation.
• the pole tips can be formed by a laminate stacked perpendicularly to the plane.
• the laminate can be a laminate of a ferromagnetic material and a high conductivity material, such as steel and copper respectively. This achieves the increase in conductivity in the said plane to encourage eddy currents, without impeding excessively the flow of magnetic flux through the magnet assembly.
• the pole tips may be formed from steel with slots extending into the pole tips from the edge adjoining the annular gap. A copper shroud can then be swaged onto the tips to fill the slots. It will be understood that eddy currents caused by a magnetic field produced by a coil are substantially perpendicular to the axis of the coil in the plane of the coil. Accordingly, it is not necessary that the electrical conductivity be high in the axial direction, although it is not excluded. It can however be more difficult to arrange for the magnetic permeability to be high as well as to arrange for the electrical conductivity to be high in all directions.
• the pole tips may be formed from a sintered mixture of steel and copper.
• the magnet assembly according to the invention may minimise the increase in permeability that the coil sees, and at the same time improve the conductivity of the pole tips, to encourage the eddy currents.
• the object is to couple the flux lines into the core, and to reduce the effect of eddy currents, which prevent these flux lines penetrating into the core. In this case the opposite effect is intended; coupling to the core might undesirably increase the inductance .
• a transducer having a magnet assembly as discussed above and a coil positioned in the gap.
• the coil may comprise an annular former with electrically conductive wire wound outside it.
• the invention may also provide a loudspeaker having a magnet assembly as discussed above, a diaphragm and a voice coil rigidly attached to the diaphragm and positioned in the annular gap in the magnet assembly.
• the loudspeaker may be a distributed mode loudspeaker as described in WO 97/09842.
• Figure 1 shows a cross section through a traditional magnet assembly
• Figure 2 shows the flux in the magnet assembly of Figure 1
• Figure 3 shows the flux around a coil in air
• Figure 4 shows the flux around a coil in a conventional magnet assembly at 100Hz
• Figure 5 shows the flux around a coil in a conventional magnet assembly at lOOOOHz
• Figure 6 shows the skin depth of steel as a function of frequency
• Figure 7 shows the flux lines in a magnet assembly having a copper shroud around the pole tips
• Figure 8 shows the magnet assembly according to the invention
• Figure 9 shows the flux lines in a magnet assembly according to the invention
• Figure 10 shows the flux lines from the coil at 100Hz in a magnet assembly according to the invention
• Figure 11 shows the flux lines from the coil at 1000Hz in a magnet assembly according to the invention
• Figure 12 shows the flux lines from the coil at lOOOOHz in a magnet assembly according to the invention
• FIG. 13 shows another embodiment of the invention.
• Figure 14 shows a further embodiment of the invention.
• the magnet assembly according to a specific embodiment of the invention shown in Figure 8 comprises two pole pieces 5,7, one being a circular steel cup 7 which surrounds the other pole piece, a steel disk 5.
• a permanent magnet 3 separates the pole pieces. It should be noted that the Figures show a cross section extending outwards from the centre of the assembly, the assembly is in fact circular when viewed from above .
• the outer rim of the pole and the inner rim of the pole have inner 9 and outer 11 pole tips.
• Each pole tip is a lamination of alternate layers of steel 19 and copper 21 to allow the DC flux to pass through, from inner pole piece
• a coil 15 having a metallic wire 23 wound on a former 25 is provided in the annular gap 13 between the pole pieces.
• the coil is fixed to a diaphragm 27 of a loudspeaker to drive the loudspeaker.
• the magnet assembly is mounted on the diaphragm by resilient supports 28.
• the diaphragm may be a distributed mode panel and the loudspeaker a distributed mode loudspeaker, such as described in WO98/09842.
• the flux from the coil itself is shown in Figure 10.
• the flux lines pass through the thin steel plates 19, the interleaved copper plates 21, having much higher conductivity, short the eddy currents.
• the steel plates are thin enough to be the order of the skin depth at high frequencies, so that the frequency rises, eddy currents cannot form and the copper interleave shorts them out anyway.
• the steel plates are 0.2mm in thickness, and the copper plates are 0.1 mm in thickness .
• Fig.13 illustrates an alternative embodiment in which the pole tips 9,11 are not a laminate, but a sintered mixture of steel and copper.
• Fig.14 illustrates a pole tip as used in a further alternative embodiment in which the pole tips are steel formed with slots 31 extending in a direction perpendicular to the axis of the coil. A copper piece 33 is then swaged onto the opposed face of the pole pieces to fill the slots.
• any suitable magnetically permeable material may be used for the pole tips.
• the conductive pieces need not be copper, but other conductive materials may also be used, such as silver or aluminium. Any suitable shape of the pole tips and pieces may also be used.
• one pole piece may be a U- shaped piece having two ends, one pole tip being on each end.
• An inner pole piece having a pole tip facing each pole tip of the U-shaped pole piece across two gaps may be provided; defining two gaps.
• the coil may surround the inner pole piece and pass through both gaps.

Landscapes

• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Audible-Bandwidth Dynamoelectric Transducers Other Than Pickups (AREA)
• Electrostatic, Electromagnetic, Magneto- Strictive, And Variable-Resistance Transducers (AREA)
• Diaphragms For Electromechanical Transducers (AREA)

Applications Claiming Priority (3)

Publications (1)

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Family Applications (1)

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Family Cites Families (5)

• 1999
  • 1999-07-20 GB GBGB9916926.0A patent/GB9916926D0/en not_active Ceased
• 2000
  • 2000-07-19 WO PCT/GB2000/002706 patent/WO2001006523A2/en not_active Application Discontinuation
  • 2000-07-19 EP EP00946095A patent/EP1205088A2/de not_active Withdrawn
  • 2000-07-19 AU AU59982/00A patent/AU5998200A/en not_active Abandoned
  • 2000-07-19 JP JP2001511697A patent/JP2003505959A/ja active Pending
  • 2000-07-20 TW TW89114529A patent/TW483008B/zh active

Non-Patent Citations (1)

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