Abstract:
An electromagnetic transducer such as an audio speaker, having an air-return motor. The use of an air return geometry lacking motor components in the region outside the voice coil assembly permits the spider and cone to be coupled to the bobbin much lower, significantly reducing the thickness of the transducer. The use of both a radially-charged primary magnet and axially-charged concentrating magnets provides greatly increased magnetic flux in the voice coil region. The primary magnet may be a cylindrical magnet or it may include a plurality of flat magnet segments arranged in a polygon. The motor may be coupled to the frame by steel bolts which pass through holes in the spider, to reduce the reluctance of the magnetic circuit.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 11/194,258 entitled “Multi-Gap Air Return Motor for Electromagnetic Transducer” filed Aug. 1, 2005 by this inventor, which was (a) a continuation-in-part of U.S. patent application Ser. No. 11/105,779 entitled “Dual-Gap Transducer with Radially-Charged Magnet” filed Apr. 13, 2005 by this inventor, and (b) a continuation-in-part of U.S. patent application Ser. No. 11/114,737 entitled “Semi-Radially-Charged Conical Magnet for Electromagnetic Transducer” filed Apr. 25, 2005 by this inventor. All are commonly assigned to STEP Technologies Inc. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field of the Invention  
         [0003]     This invention relates generally to electromagnetic transducers such as audio loudspeakers, and more specifically to a transducer motor structure utilizing both radially and axially charged magnets to improve magnetic flux density and focusing, and allowing for a transducer with a reduced axial height.  
         [0004]     2. Background Art  
         [0005]     The terms “internal” and “external” generally refer to whether an electromagnetic transducer component, such as a magnet, yoke, plate, spider, diaphragm, etc. is located radially inside the transducer&#39;s voice coil assembly, or radially outside the voice coil assembly, respectively. The terms “lower” and “upper” generally refer to components with respect to their axial position within the transducer, with upper components being nearer the “front” or sound-producing end of the transducer where the diaphragm is located, and lower components being nearer the “back” or motor end of the transducer; no specific transducer orientation is implied by either term.  
         [0006]     Conventional electromagnetic transducers utilize motor structures which have yokes, magnets, or other fixed external components. Because these fixed external components would otherwise interfere with various moving external components, the transducer is made significantly deeper in the axial direction, with a greatly elongated bobbin, to provide clearance between the moving external components and the fixed external components.  
         [0007]      FIG. 1  illustrates a conventional electromagnetic transducer  10  having an external magnet geometry motor structure. The transducer includes a motor  12  coupled to a diaphragm assembly  14  by a frame  16 . The diaphragm assembly includes a diaphragm  18  which is coupled to the frame by an upper suspension component  20  such as a surround. The diaphragm is typically equipped with a dust cap  21  to seal its front side from its back side. A voice coil assembly includes a voice coil  22  wound onto the lower end of a bobbin  24 , with the upper end of the bobbin being coupled to the diaphragm. The upper end of the bobbin or the lower end of the diaphragm is also coupled to the frame by a lower suspension component  26  such as a spider.  
         [0008]     The motor includes a pole plate  28  which includes a pole piece  30  which extends internally within the voice coil assembly, and a back plate  32  which extends outwardly beyond the voice coil assembly. One or more axially charged external magnets  34  are magnetically coupled to the back plate, and an external top plate  36  is magnetically coupled to the magnets.  
         [0009]     The internal pole piece and the external top plate define a magnetic air gap  38  in which the magnetic flux is highly concentrated. The advantage of this conventional motor is that, other than the magnetic air gap, the motor provides a very-low-reluctance magnetic circuit path, in which the magnetic flux is conducted very efficiently.  
         [0010]     Because the voice coil assembly moves axially, there must be sufficient clearance between the lower suspension component and the uppermost fixed external motor component such as the top plate, or, in the example shown, the base plate of the frame which is coupled to the top plate. Otherwise, when the motor pulls the voice coil into the motor, the lower suspension component will strike the topmost external fixed component. This requires that the bobbin be elongated, with a significant space between the voice coil and the spider. The end result is that the transducer as a whole is made deeper (or “thicker”). Also, the increased distance between the lower end of the voice coil assembly and the spider reduces the suspension components&#39; ability to prevent rocking, and the voice coil assembly may rock and strike the motor, as a result of the inherent difficulty of trying to control the cantilevered mass of the voice coil winding.  
         [0011]      FIG. 2  illustrates a conventional electromagnetic transducer  40  having an internal magnet geometry motor structure including a motor  42  coupled to a diaphragm assembly  44  by a frame  46 . The motor includes an external yoke  48  such as a cup. An axially charged internal magnet  50  is magnetically coupled within the cup, and an internal top plate  52  is magnetically coupled to the magnet. The top plate and the yoke define a magnetic air gap  54 . The diaphragm assembly includes a voice coil  56  wound onto the lower end of a bobbin  58 . A lower suspension component  60  such as a spider is coupled to the frame, and is coupled to the bobbin sufficiently near the upper end that it does not strike the uppermost external component during the designed range of inward movement of the voice coil assembly.  
         [0012]     U.S. Pat. No. 6,865,282 “Loudspeaker Suspension for Achieving Very Long Excursion” to Rick Weisman illustrates an excellent transducer which uses an ingenious spring spider and slotted cup to reduce the transducer thickness for a given Xmax travel, while preventing the lower suspension component from striking the uppermost fixed external structure. Axial slots in the cup provide axial clearance, and the spring spider provides lower suspension in only those locations.  
         [0013]     U.S. Pat. Nos. 5,550,332 “Loudspeaker Assembly” and 5,701,657 “Method of Manufacturing a Repulsion Magnetic Circuit Type Loudspeaker” to Yoshio Sakamoto, and U.S. Pat. Nos. 5,590,210 “Loudspeaker Structure and Method of Assembling Loudspeaker” and 5,701,357 “Loudspeaker Structure with a Diffuser” to Shinta Matsuo and Yoshio Sakamoto illustrate transducers which avoid external fixed components altogether. In each, the motor consists of an internal top plate sandwiched between oppositely-charged magnets. These motors do not have a magnetic air “gap”, and do not have a low-reluctance magnetic circuit. Instead, they rely on high-reluctance leakage air paths for their magnetic flux return. The purpose of the oppositely-charged second magnet is to increase the magnetic flux at the outer perimeter of the top plate. Without a low reluctance return path in the circuit, a single magnet does not provide much flux to the voice coil, and the second magnet somewhat improves this.  
         [0014]     What is needed is an improved motor structure which does not require external motor components in positions where they would be struck by the lower suspension, and which provides improved magnetic flux density in a motor suitable for use in a thin transducer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  shows an electromagnetic transducer having an external magnet geometry motor according to the prior art.  
         [0016]      FIG. 2  shows an electromagnetic transducer having an internal magnet geometry motor according to the prior art.  
         [0017]      FIG. 3  shows a motor structure according to one embodiment of this invention.  
         [0018]      FIG. 4  shows an electromagnetic transducer using the motor structure of  FIG. 3 .  
         [0019]      FIG. 5  shows a motor structure according to another embodiment of this invention.  
         [0020]      FIG. 6  shows an electromagnetic transducer using the motor structure of  FIG. 5 .  
         [0021]      FIG. 7  shows a bolt having a flux carrying appendage.  
         [0022]      FIG. 8  shows an electromagnetic transducer using the bolts of  FIG. 7 .  
         [0023]      FIGS. 9 and 10  are computer model generated flux line diagrams for motors which are the same other than that the motor of  FIG. 10  uses a tapered focusing ring.  
         [0024]      FIGS. 11 and 12  are magnetic flux density charts for the computer model generated analysis of the motors of  FIGS. 9 and 10 , respectively.  
         [0025]      FIGS. 13 and 14  show an exploded view and a cutaway view, respectively, of an embodiment having a polygonal mating structure enabling the use of conventional, flat magnets. 
     
    
     DETAILED DESCRIPTION  
       [0026]     The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only.  
         [0027]      FIG. 3  illustrates a motor  60  according to one embodiment of this invention. The motor is built around a radially-charged primary magnet  62 . In some embodiments, as shown, the radially-charged magnet is disposed within and magnetically coupled to an inner surface of a steel focusing ring  64 .  
         [0028]     An axially-charged concentrating magnet  66  is disposed adjacent to one end or the other of the radially-charged magnet and focusing ring. The axially-charged magnet is oriented with its same pole facing the radially-charged magnet and focusing ring as the radially-charged magnet has facing the focusing ring. Optionally (but quite advantageously for increasing and concentrating the magnetic flux at the outer surface of the focusing ring, as well as for improving symmetry of the high flux density region at the outer surface of the focusing ring), another axially-charged concentrating magnet  68  is disposed adjacent the other end of the radially-charged magnet and focusing ring, and is oriented in the reverse of the first concentrating magnet, such that the focusing ring is surrounded on three sides (its two ends and its inner surface) by the same magnetic polarity.  
         [0029]     This creates a region  70  of high flux density, with the magnetic flux field extending substantially radially, in the area just beyond the outer surface of the focusing ring (or radially charged magnet, if there is no focusing ring). An underhung voice coil  72  is disposed within this region, and is wound onto a bobbin  74 . It is important that the voice coil be underhung, because the magnetic flux travels radially outward only in the immediate vicinity of the focusing ring. At positions axially beyond the focusing ring, the magnetic flux quickly turns axially and then travels radially inward as it returns to the other pole of the magnets.  
         [0030]     The motor may optionally also include a steel core  76  which can, depending upon the strengths of the magnets and the respective geometries of the motor components, lower the overall reluctance of the magnetic circuit.  
         [0031]      FIG. 4  illustrates an electromagnetic transducer  80  using the motor  60  of  FIG. 3 . The transducer includes a frame  82  to which the motor is coupled, a diaphragm or cone  84  coupled to the bobbin, an upper suspension component  86  such as a surround coupling the diaphragm to the frame, a dust cap  88  (of any suitable shape) coupled to seal the front of the diaphragm from the back of the diaphragm, and a lower suspension component  90  such as a spider coupling the bobbin (or the diaphragm) to the frame.  
         [0032]     The absence of any external motor components outside the voice coil enables the construction of a very thin transducer, as the spider can be coupled directly to the lower end of the bobbin. Coupling the spider at the lower end of the bobbin has the additional advantage of increasing the axial distance between the spider and the surround, improving their ability to prevent rocking of the voice coil assembly and thus preventing it from rubbing or striking the motor.  
         [0033]     The motor and frame may provide an axial vent  92  for depressurizing the motor. If the diaphragm is constructed in the inverted-V configuration shown, the portion of it between the dust cap and the bobbin may also be ventilated (as shown), as may the frame or basket.  
         [0034]     The basket may be formed of any suitable material, such as forged aluminum, stamped steel, injection molded plastic, or what have you.  
         [0035]      FIG. 5  illustrates a motor  91  according to another embodiment of this invention. It is similar to the motor of  FIG. 3 , except that it omits the optional steel core, and it uses a focusing ring  93  having an outer surface with ends which are tapered inward to provide a more uniform flux density over the axial distance of the voice coil region  95 .  
         [0036]      FIG. 6  illustrates an electromagnetic transducer  100  according to another embodiment of this invention, using the motor  91 . The transducer includes a frame  102  which may in some embodiments be made of stamped steel. The frame has a base plate  104  to which the lower end of the motor is coupled. The steel frame itself serves to gather flux for a reduced-reluctance return path to the lower concentrating magnet and the steel core.  
         [0037]     An upper retention plate  106  is coupled to the upper end of the motor. If the upper retention plate is e.g. stamped steel, it also serves to gather flux for a reduced-reluctance return path to the upper concentrating magnet and the steel core. Optionally, the retention plate may be shaped to mirror the shape of some portion of the frame near the rear of the motor, to provide a flux gathering member as equivalent as possible to the frame, to improve symmetry in the flux density of the two respective high-flux regions.  
         [0038]     The retention plate may serve to retain the motor and fasten it to the frame, with the addition of retention bolts  108 . The retention bolts extend through the frame and thread into the retention plate, or into nuts (as shown) on the upper side of the retention plate; alternatively, they could, of course, go the other direction. The spider  110  and cone  112  are adapted with a corresponding set of holes  114 ,  116  through which the retention bolts pass. A dust cap  118  is coupled to seal the diaphragm.  
         [0039]     The retention bolts may advantageously be made of steel, such that they provide an even greater reduction in the reluctance of the flux return paths to the magnets. As such, it is desirable to position the retention bolts as close as possible to the voice coil assembly, with a suitable safety margin to avoid strikes and rubbing. The number of retention bolts can be selected according to the needs of the particular application at hand; the more bolts there are, the more holes there will be through the cone and the spider, the weaker the cone and the spider will be, but the lower the reluctance of the return paths will be.  
         [0040]      FIG. 7  illustrates an improved bolt  120  for coupling a motor to a basket as taught above. The bolt includes a steel appendage  122  which extends generally in only one radial direction away from the axis of the bolt. The appendage may be generally flat, or it may be generally wedge shaped.  
         [0041]      FIG. 8  illustrates an electromagnetic transducer  130  in which the motor  91  is retained using a plurality of such bolts  120 . The cone  132  includes holes  134  which are sized and shaped such that the cone does not rub or strike the bolts, including the bolts&#39; appendages  122 . The spider  135  includes holes  136  which are also sized to permit the bolts and appendages to pass through the spider without contacting it. The spider&#39;s holes should be sized and located with the fact in mind that the spider will stretch and deform as the voice coil assembly moves.  
         [0042]     The appendages increase the flux gathering and flux carrying capacity of the bolt, further lowering the reluctance of the magnetic circuit. When designing to a particular reluctance goal, the use of such appendage bolts may enable the use of a reduced number of bolts versus conventional bolts, and, consequently, a reduced number of holes weakening the spider and the cone. The holes for the appendage bolts will necessarily be larger, but only in the radially outward direction, which will have a less damaging effect than if the same surface area of circular holes were placed close to the inner diameter of the spider and cone.  
         [0043]      FIG. 9  illustrates a computer model of the motor shown, using a flat focusing ring. The motor is modeled as an axisymmetric revolve about the axis (shown as a heavy dashed line).  
         [0044]      FIG. 10  illustrates a computer model of the motor shown, which is the same as the motor of  FIG. 9  except that it uses a tapered focusing ring.  
         [0045]      FIG. 11  illustrates an exemplary magnetic flux chart for the motor of  FIG. 9 . The Y axis indicates axial position in the high flux region just outside the focusing ring. The curve has undesirable spikes near the ends of the motor, where the flux density is extra high near the point where the outer corners of the focusing ring meet the inner corners of the concentrating magnets.  
         [0046]      FIG. 12  illustrates an exemplary magnetic flux chart for the motor of  FIG. 10 .  FIGS. 11 and 12  are not to the same scale. The high peaks have been eliminated, and the lowest part of the trough has actually been raised, such that the significantly flat active region in  FIG. 12  is at substantially the level shown by the vertical bold line in  FIG. 11 .  
         [0047]      FIG. 13  illustrates another embodiment of an air return motor  140 . Rather than the annular radially charged magnets used in previously described embodiments, this embodiment uses a plurality of flat magnet segments  146 . The magnet segments may have wedge-shaped abutting edges, as shown, or they may instead have conventional 90° edges. The tolerance of the thickness of flat magnets is very easily controlled during manufacturing, as compared to somewhat difficult-to-control ID and OD of annular magnets. Using the flat magnet segments may ease manufacturing and assembly, and may reduce BOM cost.  
         [0048]     The magnet segments are coupled to respective faces of a polygonal inner surface of a steel focusing ring  148 . The outer surface of the steel focusing ring is shaped to match the shape of the voice coil assembly  152 , which may be circular, as shown, or which may have another shape as dictated by the application at hand. The motor optionally includes an inner steel core  142  having a polygonal outer surface matching the number of magnet segments—six in the example shown.  
         [0049]     The motor includes at least one, and preferably two, axially charged magnets  144 ,  150  coupled at opposite ends of the motor.  
         [0050]      FIG. 14  shows a cross-section view of the motor  140  with the upper magnet ( 150 ) removed to permit visibility of the mating of the magnet segments  146 , focusing ring  148 , and inner core  142 .  
       CONCLUSION  
       [0051]     When one component is said to be “adjacent” another component, it should not be interpreted to mean that there is absolutely nothing between the two components, only that they are in the order indicated.  
         [0052]     The various features illustrated in the figures may be combined in many ways, and should not be interpreted as though limited to the specific embodiments in which they were explained and shown.  
         [0053]     The term “primary magnet” is not intended to imply anything about the strength of the radially-charged magnet relative to the strengths of the concentrating magnets, and is simply a name chosen for convenience.  
         [0054]     Optionally, the focusing ring and/or inner core could be formed as multiple segments. Or, optionally, the focusing ring and/or inner core could be formed in a C shape having a narrow slit that permits expansion of the focusing ring or compression of the inner core, to facilitate assembly.  
         [0055]     In some embodiments, the focusing ring may be omitted, with the outer surface of the radially-charged magnet (segments) itself defining the magnetic flux region in which the voice coil is disposed.  
         [0056]     Those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.