Abstract:
An array of paired bar magnets ( 200 ) provides a reinforced magnetic field ( 203 ) on a first side and a nearly canceled magnetic field ( 204 ) on a second side of each pair. The array may be a planar array ( 600 ) with a plurality of parallel, coplanar pairs ( 620 ). The array may provide air gaps between consecutive pairs, and within individual pairs, to provide improved transparency to sound. The array may be doubled ( 700 ), with the reinforced fields ( 713 ) of one half of the array opposing the reinforced fields ( 723 ) of the other half to produce a more intense field ( 730 ). In another configuration, the array may be doubled ( 800 ) with the nearly canceled fields of one pair facing the nearly canceled fields of the other, producing an array with reinforced fields ( 801 - 804 ) on four sides.

Description:
FIELD OF THE INVENTION 
     The present invention and embodiments thereof relates generally to arrays of permanent magnets and more specifically to arrays wherein the magnetic axes of the individual magnets are oriented to be strictly oblique to the axes of the array in cross-section. 
     BACKGROUND OF THE INVENTION 
     Typically, planar magnetic acoustic transducers use a flat, lightweight diaphragm suspended in a magnetic field, rather than a cone attached to a voice coil. The magnetic field is typically produced by a planar array of bar magnets, the bar magnets spaced apart regularly, but aligned parallel to each other, the poles of the bar magnets oriented to be perpendicular to the layer the magnets form. The diaphragm is suspended above the magnets, and substantial portions of the electrically conductive circuit pattern run parallel to individual bar magnets, as when current passes through these portions of the circuit, an induced magnetic field will react with the field produced by the magnets, causing the conductor, and the attached diaphragm, to be drawn to or away from the magnets. 
     However, there are drawbacks to the magnetic arrangement in the classic planar magnetic acoustic transducer design. In simple configurations, the magnetic field imposed by the array of bar magnets not only permeates the volume in which the diaphragm operates, but also imposes a like-intensity magnetic field on the opposite side of the array. In most applications this opposite side magnetic field is wasted. In a more sophisticated configuration, a stator is applied on the backside of the array, to contain and redirect the opposite side magnetic flux to bolster the field acting on the diaphragm. The added mass of the stator is a drawback to this configuration, as is increased impedance to the passage of sound due to the spaces between bar magnets being covered by the stator, even partially, as when the stator is perforated or consists of separated strips. 
     Prior art Halbach magnet arrays rely on a cyclical rotation in magnetic orientation from magnet to magnet, as shown in  FIG. 1 , wherein the magnetic axis of each magnet is 90-degrees further rotated than it&#39;s preceding neighbor. For example, in Halbach magnet array  100 , shown in cross-section, each consecutive magnet from left-to-right has a magnetic axis that is 90-degrees further counterclockwise than its predecessor, as the axis of magnet  102  is 90-degrees more counterclockwise than magnet  101 . The valued property that emerges from this arrangement in Halbach array  100 , is that rather than having a symmetrical field evenly distributed on either side of the array, the field  103  on one side is greater than it would be for an traditional array of alternating magnets (which would correspond to magnet  101  and every alternate magnet to either side, with the intervening magnets, e.g.,  102 , removed). The field  104  on the other side is nearly canceled. As with a stator, in a planar magnetic acoustic transducer application, Halbach arrays suffer from the added magnets (e.g.,  102 ) obstructing the passage of sound. While the property of an intensified field  103  on one side and substantially canceled field  104  on the opposite side makes a Halbach array more efficient in one way, the added weight corresponding to doubling the count of magnets and the obstruction to the passage of sound (even when some of the magnets are drilled through), reduce the effectiveness of Halbach arrays. 
     Additionally, Halbach arrays are difficult to assemble. The individual magnets are not in equilibrium when arranged as a Halbach array, and the array will collapse into a jumble if not glued or otherwise supported and braced. In some cases, particularly with individual magnets  101 ,  102  that are strong, manual assembly of the array can be fraught with pinched fingers and frequent starting over again. 
     A need exists for a magnetic array, having the property of an intensified field on one side and a nearly canceled field on the other, suitable for efficient use in planar magnetic and other acoustic transducers. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Embodiments of the present invention include a magnet array well suited for use in planar magnetic acoustic transducers. 
     It is an object of embodiments of the present invention to allow a planar magnetic transducer, used as a speaker, to develop more acoustic power than is possible with a particular amount of conductive material on a single diaphragm. 
     It is an object of embodiments of the present invention to provide a planar magnetic array whose individual magnets form elements that are easy to assemble and when assembled are in substantial mechanical equilibrium without glue. 
     It is a further object of embodiments of the present invention to provide lightweight planar magnetic elements allowing efficient passage of sound. 
     It is an object of embodiments of the present invention to provide processes of manufacture for the magnetic elements of these magnetic arrays. 
     The present embodiments of the invention satisfy these and other needs and provides further related advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aspects of embodiments of the present invention will be apparent upon consideration of the following detailed description taken in conjunction with the accompanying drawings, in which like referenced characters refer to like parts throughout, and in which: 
         FIG. 1  shows a cross section of a prior art Halbach planar magnetic array; 
         FIG. 2  is a cross-section of a single magnetic array element pair having an intensified magnetic field on one side and a canceled magnetic field on an opposite side; 
         FIG. 3  is a cross-section of the same array of  FIG. 2 , showing the placement of an acoustic transducer diaphragm and a plot of the magnetic field intensity for that placement; 
         FIG. 4  is a cross-section of bar magnet and the cuts made in one process of forming the magnetic elements of embodiments of the present invention; 
         FIG. 5  is a cross-section of a sheet magnet and the cuts made in another process of forming the magnetic elements of embodiments of the present invention; 
         FIG. 6A  is a cross-section showing a planar magnetic array and acoustic transducer diaphragm of embodiments of the present invention the magnetic array comprising multiple element pairs spaced to provide, acoustic transparency; 
         FIG. 6B  is a cross-section showing a similar planar magnetic array, but with an additional air gap between the bar magnets of each pair; 
         FIG. 7A  is a cross-section of a magnetic array comprising two of magnetic array element pairs of  FIG. 2 , arranged in opposition with an acoustic transducer diaphragm positioned between them; 
         FIG. 7B  is a cross-section of a magnetic array comprising multiple arrays such as in  FIG. 7A , addressing a larger diaphragm; 
         FIG. 8  is a cross-section of a magnetic array comprising two of the magnetic array element pairs of  FIG. 2 , closely packed, to provide an intensified field on all four sides; 
         FIG. 9  is a cross-section of the same array of  FIG. 8 , showing the placement of four acoustic transducer diaphragms; and 
         FIG. 10  shows the range of directions for the magnetic axes in a magnetic array element pair of embodiments of the present invention. 
     
    
    
     While embodiments of the invention will be described and disclosed in connection with certain preferred embodiments and procedures, they are not intended to limit the invention to those specific embodiments. Rather they are intended to cover all such alternative embodiments and modifications as fall within the spirit and scope of embodiments of the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 2 , a magnet array  200  comprises a pair of bar magnets  201 ,  202 , shown in cross-section. Each bar magnet  201 ,  202  is magnetized with a field orientation as shown by the corresponding internal arrow. The combination of the two bar magnets  201 ,  202  produces a reinforced magnetic field  203  on a first side of the array and a nearly-canceled field  204  on a second, opposite side of the array. In this configuration, the reinforced field  203  is much stronger as the field projected by either bar magnet  201 ,  202  operating alone. The smaller, nearly-canceled field  204  substantially limits the magnetic influence of the array  200  toward the second side. 
       FIG. 3  shows a plot of the magnetic flux density or magnetic induction B for the reinforced field  203  along the line segment A-A′. The plot shows that the magnetic induction B is most intense along A-A′ at the midpoint, and falls off toward either end. When used in a planar magnetic acoustic transducer, a diaphragm located at rest along A-A′ would have conductors running perpendicular to the drawing sheet (i.e., with currents flowing into and out of the page). As taught in U.S. Patent Application No. 61/892,431, filed Oct. 17, 2013, and entitled “Thin Film Circuit for Acoustic Transducer and Methods of Manufacture” by Colich, et al., such conductors can be made to have widths varying in proportion to B so that the current density in the conductor is inversely proportional to B, whereby the Lorentz force on each conductor is evenly distributed across the conductor and more generally, across the diaphragm, a configuration that is valuable to minimize distortion in an electro-acoustic transducer. 
       FIG. 4  shows one example bar magnet manufacturing process  400  for making the individual magnetic elements  201 ,  202  from  FIG. 2 , where a bar magnet blank  401 , shown in cross-section, is sliced by cuts (e.g.,  402 ) and/or grinding to remove waste elements  403 , which may not survive the process, and leave magnetic element  404 . The bulk material of bar magnet blank  401  is suitable for use as a permanent magnet, but typically is not initially magnetized, at least not strongly (i.e., forming operations may take it above its Curie point). In cases where the bulk material of bar magnet blank  401  exhibits magnetic anisotropy, that is the material has one or more preferred axes for magnetization, then the easy axis should be aligned (that is, parallel with) with the arrow as show. This would be the case, for example, with a neodymium rare-earth magnet created with the sintered magnet process, wherein application of a magnetic field and/or mechanical deformation is applied to align anisotropic crystalline grains prior to a liquid-phase sintering that produces blank  401 . After a grinding and/or cutting away of the waste elements (e.g.,  403 ) to achieve the final shape and orientation of magnetic element  404 , element  404  is typically electroplated or otherwise covered to protect the bulk material from corrosion, and the element  404  is magnetized in the direction of the arrow. Once magnetized, magnetic element  404  is suitable for inclusion in the magnet arrays of embodiments of the present invention, e.g.,  200 . 
       FIG. 5  shows a similar bar magnet manufacturing process  500  for making the individual magnetic elements, e.g.,  201  and  202 , wherein a bar magnet blank  501  is cut (e.g.,  502 ) to remove waste elements  503 ,  504  and to separate magnetic elements  504 , which may be further ground and coated (e.g., by electroplating or painting) and then magnetized. 
     In still another process for manufacturing the individual magnetic elements  201 ,  202 , a magnetic material as a powder or slurry may be formed into the final or near-final shape (e.g., having the square cross-section of elements such as  201 ,  202 ) and sintered, without need for subsequent cutting to reshape the cross-section. A magnetic field may be applied during sintering to induce anisotropic grains to at least partially align their easy axis with the field, the applied field running parallel to the with the arrow as shown (e.g., in the individual magnetic elements  201 ,  202  in  FIG. 2 , but not as shown together as magnet array  200 . Applying such a field during forming and/or sintering improves the final saturation magnetization along the axis parallel to the arrow as shown. The surfaces resulting from the sintering step may be smoothed by grinding and/or protected by coating, if desired. Once formed and cool (below their Curie temperature), the elements are magnetized along the indicated diagonal axis. One particular advantage of this process is that there is far less waste of magnetic material, since no large waste elements  403 ,  503  are cut away. 
       FIG. 6A  shows, in cross-section, a compound planar magnetic array  600  comprising multiple magnetic array pairs  620 , similar to  200 . The planar magnetic field  603  is suitable for interaction with an electro-acoustic transducer diaphragm located in plane  610 . The gaps between individual magnetic array pairs  620  allow transmission of sound. In some embodiments, the magnetic elements of each pair may be further shaped, e.g., by additional grinding during manufacturing process  400  or  500  before coating, so as to produce curved regions  621 . This reduces the diffraction effects on sound propagating between the magnetic array pairs  620 , as taught in U.S. Patent Application No. 61/892,417, filed Oct. 17, 2013, and entitled “Anti-Diffraction and Phase Correction Structure for Planar Magnetic Transducers” by Colich. In another embodiment, the individual magnetic elements of magnetic array pairs  620  can be manufactured through the process described above, in which the magnetic material as a powder or slurry is molded or die pressed into the final or near-final cross-sectional shape, as shown, where the curved regions  621  are created as the piece is formed and sintered. 
     In another embodiment, shown in  FIG. 6B , planar magnetic array  640 , comprises magnetic array pairs, e.g.,  642 , each of which encompasses an intra-pair air gap  644  between the individual bar magnets of the pair. As with array  600 , consecutive pairs are separated by an inter-pair air gap  645 . The intra-pair air gap  644  may be the same size as the inter-pair air gap  645 , i.e., so all the gaps are identical, or smaller. The intra-pair gaps  644  improve the transmission of sound through the planar magnetic array. Note that, as in  FIG. 6A , adjacent pairs, e.g.,  642  and  643 , have corresponding bar magnets of opposite magnetic orientation. An electro-acoustic diaphragm  650  is positioned in magnetic field  641  provided by the array  640 . 
       FIG. 7A  shows dual magnetic array  700  comprises two opposed magnet array pairs  710 ,  720 , each similar to  200  or  620 . The lower magnetic array pair  710  comprises bar magnets  711 ,  712 , shown in cross-section. Each bar magnet  711 ,  712  is magnetized with a field orientation as shown by the corresponding internal arrow. The combination of the two bar magnets  711 ,  712  produces a reinforced field  713  on a first side of the array and a nearly-canceled field  714  on a second, opposite side of the array  710 . Likewise, the upper magnetic array pair  720  comprises bar magnets  721 ,  722 , shown in cross-section. Each bar magnet  721 ,  722  is magnetized with a field orientation as shown by the corresponding internal arrow. The combination of the two bar magnets  721 ,  722  produces a reinforced field  723  on a first side of the array facing the lower magnetic array  710 , and a nearly-canceled field  724  on a second, opposite side of the array  720 . 
     Whereas in  FIG. 2 , the reinforced field  203  was roughly twice as strong as the field projected by either bar magnet  201 ,  202  operating alone, in the configuration of  FIG. 7 , the opposing reinforced fields  713 ,  723  combine to produce an combined field  730  roughly four times as strong as the field projected by any of the bar magnets  711 ,  712 ,  721 ,  722  operating alone. 
     In this configuration, placement of an electro-acoustic transducer diaphragm is along the plane of centerline  731 . 
     The dual magnet array  700  can be repeated according to the pattern of planar magnetic array  640  in  FIG. 6B , to accommodate wider diaphragms, as shown in  FIG. 7B  where extended dual magnetic array  740  comprises multiple arrays  742 , similar to  700 , alternating with mirrored arrays  743  in which each corresponding element having an opposite magnetization compared to arrays  742 . Array  740  is able to address a larger diaphragm along centerline  741 . 
       FIG. 8  shows another magnetic array  800  comprising two magnetic array pairs  810 ,  820 , each like  200 . The upper magnetic array pair  810  comprises bar magnets  801 ,  802 , and the lower magnetic array pair  820  is similarly constructed. Each of the bar magnets (e.g.,  801 ) is magnetized with a field orientation as shown by the corresponding internal arrow. Upper pair  810  produces reinforced field  811  and while lower pair  820  produces reinforced field  812 . However, the combination of the two pairs  810 ,  820  further produces reinforced side fields  813 ,  814 . The nearly-canceled field  204 , shown in  FIG. 2 , is present for each of pairs  810 ,  820 , but in the configuration of four-magnet array  800 , they are internal and provide a binding force to hold these individual magnets stably together. 
       FIG. 9  shows the use of four-magnet array  800  in an electro-acoustic transducer  900 , where diaphragms  901 - 904  are positioned in each corresponding magnetic field  811 - 814 . Such a configuration is useful as a microphone with particular sensitivity in four directions, which can be kept as four separate electrical signals representing sound from each of the four directional lobes. If desired, such signals can be separately delayed, filtered, and summed or differenced as needed to provide fewer signals, each representing an adjusted directional sensitivity. 
       FIG. 10  shows a cross-section of a single magnetic array element pair  1000  of embodiments of the present invention, the elements  1010 ,  1020  of the pair being substantially coplanar bar magnets (i.e., the two bar magnets are parallel), the pair  1000  having a central plane  1001 . The magnetic axis of the first bar magnet  1010  lying within limits  1013  (30° inclined to the plane  1001 ) and  1016  (60° inclined to the plane  1001 ), with 45° being the middle of this range. The magnetic axis of the second bar magnet  1020  lying within limits  1023  (30° inclined to the plane  1001 ) and  1026  (60° inclined to the plane  1001 ), likewise with 45° being the middle of this range. The two magnetic axes of the pair thus having a mutual angle of between 60° and 120° degrees, with 120° corresponding to both axes being close to the shallow 30° angle limits  1013 ,  1023  and 60° corresponding to both axes being close to the steeper 60° angle limits  1016 ,  1026 . With both axes near the 45° mid-range angle relative to the central plane  1001 , their mutual angle will be about 90°. The mid-range value of 45° with the mutual angle of about 90° is near optimal when there is no gap between bar magnets  1010  and  1020  and the magnets are square in cross-section. 
     Various additional modifications of the described embodiments of the invention specifically illustrated and discussed herein will be apparent to those skilled in the art, particularly in light of the teachings of embodiments of this invention. It is intended that embodiments of the invention cover all modifications and embodiments, which fall within the spirit and scope of embodiments of the invention. Thus, while preferred embodiments of the present invention have been disclosed, it will be appreciated that it is not limited thereto but may be otherwise embodied within the scope of the following claims.