Patent Publication Number: US-8542862-B2

Title: Flat acoustic transducer and method for driving the same

Description:
TECHNICAL FIELD 
     The present invention relates to a flat acoustic transducer and a method for driving the same. 
     BACKGROUND ART 
     As conventional flat acoustic transducer (flat speaker), there are known ones obtained by fitting a plurality of permanent magnets to a base plane of a flat yoke such that the magnets adjacent to each other have opposite polarities, and arraying a plurality of spiral coils on a flat vibrating membrane facing the permanent magnets (see Patent Documents 1, 2). By application of an electrical signal to the coils, the coils receive magnetic force from magnetic pole faces of the permanent magnets, to vibrate above the permanent magnets. 
     In these flat acoustic transducers, the upper faces (magnetic pole faces) of the plurality of permanent magnets are configured flush with one another, and the magnetic pole face and the coil are spaced with a predetermined interval. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         [Patent Document 1] Japanese Laid-open patent publication No. 2001-333493 
         [Patent Document 2] Japanese Laid-open patent publication No. 2008-141570 
       
    
     DISCLOSURE OF THE INVENTION 
     In a flat speaker, when a current is applied to a coil so as to vibrate a vibrating membrane, the coil also vibrates along with the vibrating membrane. An amplitude distance of the vibration, for example, reaches the order of 1.0 mm at the maximum. 
     At this time, since heights of the upper faces of the plurality of permanent magnets arrayed on a yoke are made uniform, a degree of action of magnetic force upon the coil in the case of the coil being located at the lowest point of vibration is different from that in the case of the coil being located at the highest point. Herein, since magnetic force that acts upon the coil decreases in inverse proportion to the square of the distance between the magnetic pole face of the permanent magnet and the coil, when a current applied to the coil is constant, driving force that occurs on the vibrating membrane fluctuates in accordance with the position of the vibrating coil. As a consequence, there has occurred a problem in that a sound emitted from the flat speaker is distorted, to significantly impair reproducibility of an original sound. 
     The present invention was made in view of the above problem, and is to provide a flat acoustic transducer capable of faithfully reproducing an original sound, and a method for driving the flat acoustic transducer. 
     According to the present invention, there is provided a flat acoustic transducer comprising: a permanent magnet and a magnetic member which are arranged adjacent to each other with a predetermined interval; a flat vibrating membrane provided facing the permanent magnet and the magnetic member; and at least one coil fixed to the vibrating membrane, wherein an electrical signal is applied to the coil, to obtain vibration force on the vibrating membrane by way of a magnetic flux generated between a magnetic pole face of the permanent magnet and the magnetic member, a step is provided between the magnetic pole face and the upper face of the magnetic member, and at least part of a winding of the coil at the time of no application of the electrical signal is arranged inside the step. 
     Herein, in a static magnetic field generated by the permanent magnet, a magnetic flux density is maximal in an area from the magnetic pole face of the permanent magnet toward the upper face as an edge line of the adjacently arranged magnetic member. Therefore, providing the step between the magnetic pole face of the permanent magnet and the upper face of the magnetic member leads to formation of an area with a maximal magnetic flux density inside such a step. Accordingly, arranging the coil at the time of no application of an electrical signal inside the step can make equalize the magnetic force received by the coil in the case of downward vibration of the vibrating membrane and that in the case of upward vibration thereof. 
     Further, in the flat acoustic transducer of the present invention, as a more specific aspect, at least part of the winding of the coil at the time of no application of the electrical signal may be arranged at such a height position that a density of the magnetic flux component parallel to a coil face of the coil may be maximal. 
     Further, in the flat acoustic transducer of the present invention, as a more specific aspect, at least part of the winding of the coil at the time of no application of the electrical signal may be arranged at a middle height position between the magnetic pole face and the upper face, and above a line segment connecting respective proximal edges of the magnetic pole face and the upper face. 
     Further, in the flat acoustic transducer of the present invention, as a more specific aspect, the magnetic member may be another permanent magnet with a magnetic pole face having a polarity inverted from that of the adjacent permanent magnet. 
     Further, in the flat acoustic transducer of the present invention, as a more specific aspect, the coil may be provided projecting from the vibrating membrane toward the permanent magnet or the magnetic member. 
     Further, in the flat acoustic transducer of the present invention, as a more specific aspect, a winding axis of the coil may coincide with a central axis of the magnetic pole face or the upper face. 
     Further, in the flat acoustic transducer of the present invention, as a more specific aspect, a yoke may be further provided which is made of a magnetic material and provided with a step for mounting of the permanent magnet or the magnetic member. 
     Further, in the flat acoustic transducer of the present invention, as a more specific aspect, the yoke may have extending side wall sections on sides with respect to an arraying direction of the permanent magnets and the magnetic members. 
     Further, in the flat acoustic transducer of the present invention, as a more specific aspect, a respective plurality of the permanent magnets and magnetic members may be repeatedly arranged by pattern in a one-dimensional direction or two-dimensional directions. 
     Further, in the flat acoustic transducer of the present invention, as a more specific aspect, at least either the permanent magnet or the magnetic member may form a ring shape, while the permanent magnet and the magnetic member may be concentrically arranged. 
     According to the present invention, there is provided a method for driving a flat acoustic transducer that has a flat vibrating membrane fixed with at least one coil applied with an electrical signal, wherein a static magnetic field is generated such that a density of a magnetic flux component parallel to a coil face of the coil changes in a vibrating direction of the vibrating membrane, and the electrical signal is applied to the coil arranged at such a position that the density of the magnetic flux component is maximal, to vibrate the vibrating membrane. 
     It is to be noted that a variety of constitutional elements of the present invention are not necessarily present in an individual and independent manner, but a plurality of constitutional elements may be formed as one member, one constitutional element may be formed of a plurality of components, a certain constitutional element may be part of another constitutional element, or part of a certain constitutional element may overlap with part of another constitutional element in some other way. 
     Further, although the vertical direction is defined in the present invention, this definition was made for the sake of convenience in simply describing a relative relation of the constitutional elements of the present invention, and does not restrict a direction at the time of manufacturing or use in the case of implementing the present invention. 
     According to the flat acoustic transducer and the method for driving the same in the present invention, the magnetic force received from the permanent magnet in the case of downward vibration of the coil located at the center of vibration and the magnetic force in the case of upward vibration of the coil are made equal, thereby allowing faithful reproduction of an original sound regardless of the vibrating position of the coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages will be more apparent from the following description of preferred embodiments and the following drawings associated therewith. 
         FIG. 1  is a top perspective view showing a flat acoustic transducer according to a first embodiment. 
         FIG. 2A  is a sectional view of II-II of  FIG. 1 ,  FIG. 2B  is an expanded view of a broken-line area X of  FIG. 2A , and  FIG. 2C  is an explanatory view of an action of the flat acoustic transducer. 
         FIG. 3A  is a side view of a yoke, and  FIG. 3B  is a side view showing a modification of the yoke. 
         FIG. 4  is a top perspective view showing a flat acoustic transducer according to a second embodiment. 
         FIG. 5  is a top perspective view showing a flat acoustic transducer according to a third embodiment. 
         FIG. 6  is a top perspective view showing a flat acoustic transducer according to a modification of the third embodiment. 
         FIG. 7A  is a vertical sectional view of a flat acoustic transducer according to a fourth embodiment, and  FIG. 7B  is an expanded view of a broken-line area Y of  FIG. 7A . 
         FIG. 8  is an exploded perspective view of a flat acoustic transducer of a fifth embodiment. 
         FIG. 9  is a sectional view of IX-IX in  FIG. 8 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     In the following, embodiments of the present invention will be described based upon the drawings. It is to be noted that in every drawing, the same constitutional element is provided with the same numeral, and description thereof will not be repeated. 
     First Embodiment 
       FIG. 1  is a top perspective view showing a flat acoustic transducer  100  according to a first embodiment of the present invention. In the drawing, a vibrating membrane  30  in the state of being fixed to a yoke  60  is indicated by chain double-dashed lines, while a state of the under face side of the vibrating membrane  30  is indicated by solid lines. 
       FIG. 2A  is a sectional view of II-II of  FIG. 1 .  FIG. 2B  is an expanded view of a broken-line area X of  FIG. 2A .  FIG. 2C  is an explanatory view of an action of the flat acoustic transducer  100  in the present embodiment. Further, a coil  40  shown in each drawing of  FIG. 2  represents a position at the time of no application of an electrical signal. 
     First, a brief overview of the flat acoustic transducer  100  of the present embodiment will be described. 
     The flat acoustic transducer  100  includes a permanent magnet  10  and a magnetic member  20  which are arranged adjacent to each other with a predetermined interval, a flat vibrating membrane  30  provided facing the permanent magnet  10  and the magnetic member  20 , and a coil  40  fixed to the vibrating membrane  30 , and an electrical signal is applied to the coil  40 , to obtain vibration force F (see  FIG. 2B ) on the vibrating membrane  30  by way of a magnetic flux Φ generated between a magnetic pole face  12  of the permanent magnet  10  and the magnetic member  20 . 
     In the flat acoustic transducer  100 , a step  50  is provided between the magnetic pole face  12  and the upper face  22  of the magnetic member  20 , while at least part of a winding  42  of the coil  40  at the time of no application of an electrical signal is arranged inside the step  50 . 
     Next, the flat acoustic transducer  100  of the present embodiment will be described in detail. 
     The magnetic member  20  for use in the present invention is a member made of a magnetic body, for which a permanent magnet as a magnetized magnetic body or a non-magnetized magnetic body may be used. 
     Out of these, another permanent magnet with a magnetic pole face having a polarity inverted from the magnetic pole face  12  of the adjacent permanent magnet  10  is used as the magnetic member  20  in the present embodiment. That is, the upper face  22  of the magnetic member  20  is a magnetic pole face with its polarity being the north pole or the south pole, which is inverted from the polarity of the magnetic pole face  12  of the permanent magnet  10 . 
     Hereinafter, the permanent magnet  10  is referred to as a first magnet, and the magnetic member  20  as a second magnet, and those will be described, being provided with a common numeral. 
     The flat acoustic transducer  100  of the present embodiment further includes the yoke  60 , which is made of a magnetic material and provided with a step  62  for mounting of the first magnet (permanent magnet)  10  or the second magnet (magnetic member)  20 . 
       FIG. 1  shows one, as the yoke  60 , with the step  62  formed projecting from a base plane  64  corresponding to the upper face of the drawing. However, a concavity/convexity of the step  62  may be reversed, to form the step  62  depressing from the base plane  64 . 
     The first magnet  10  is mounted on the base plane  64  of the yoke  60 . The second magnet  20  is then mounted on the step  62  of the yoke  60 . With the yoke  60  being made of a magnetic material, the first magnet  10  and the second magnet  20  can be attached to the yoke  60  by magnetic force so as to be mounted thereon. The first magnet  10  and the second magnet  20  may be fixed to the yoke  60  by bonding, using a bonding unit such as an adhesive, or may be subjected to both attachment by magnetic force and fixation by bonding. 
     The magnetic pole face  12  of the first magnet  10  and the upper face (magnetic pole face)  22  of the second magnet  20  are mounted on the yoke  60  with polarities of those magnets inverted from each other. 
     It is to be noted that, when the first magnet  10  is referred to as “magnetic pole face  12 ” in regard to the first magnet  10  without a particular explanation, it means the magnetic pole face on the upper face side thereof. 
     The first magnet  10  and the second magnet  20  of the present embodiment are formed in the same shape and dimensions. 
     Thereby, the second magnet  20  mounted on the step  62  is located at a higher position than the first magnet  10  mounted on the base plane  64 . The upper face  22  of the second magnet  20  is located at a higher position than the magnetic pole face  12  on the upper face side of the first magnet  10  by a height of the projection (a distance L 3  shown in  FIG. 2B ) of the step  62 . 
     It should be noted that in the present embodiment, the vertical direction and high/low are defined with the base plane  64  of the yoke  60  taken as a reference. This does not necessarily coincide with the vertical direction of the gravity. 
     The step  62  of the yoke  60  is provided for the purpose of forming a height difference between the magnetic pole face  12  of the first magnet  10  and the upper face (magnetic pole face)  22  of the second magnet  20 . Therefore, in the case of making the height difference between the first magnet  10  and the second magnet  20 , the step  62  is not required, allowing the yoke  60  to be formed in flat plate shape. In other words, forming the step  62  in convexo-concave shape on the yoke  60  allows the first magnet  10  and the second magnet  20  to have the same dimensions, thereby contributing to reduction in number of components. 
     It should be noted that in the case of using as the magnetic member  20  a non-magnetized magnetic body instead of the permanent magnet as in the present embodiment, the yoke  60  and the magnetic member  20  may be configured to be separate members, or may be integrally configured. In the case of integrally constituting the yoke  60  with the magnetic member  20 , a projecting section corresponding to the magnetic member  20  is formed by projection from the base plane  64  between the discretely arranged permanent magnets  10 . 
     On the base plane  64  of the yoke  60 , a plurality of steps  62  is formed with predetermined intervals. 
     In the flat acoustic transducer  100  of the present embodiment, a respective plurality of first magnets (permanent magnets)  10  and second magnets (magnetic members)  20  are repeatedly arranged by pattern in a one-dimensional direction. As shown in  FIG. 2C , the first magnet  10  and the second magnet  20  are arranged with an interval from each other in a repeating direction (horizontal direction of the drawing). 
     In the flat acoustic transducer  100  of the present embodiment, the interval between the first magnet (permanent magnet)  10  and the second magnet (magnetic member)  20  adjacent thereto means a distance between these magnets in regard to an in-plane direction of the base plane  64  (horizontal direction of  FIG. 2 ). 
     Further, in the present embodiment, the interval between the first magnet  10  and the second magnet  20  is made the same with respect to each repeated pattern. However, as described later, the interval between the magnets in the vicinity of the center of the base plane  64  may be made different from the interval between the magnets in the vicinity of the periphery. 
     Further, as for the height of the step  50  between the magnetic pole face  12  of the first magnet  10  and the upper face  22  of the second magnet  20 , the height with respect to each pair of the adjacent magnets may be made common or different. 
     The yoke  60  is provided with erecting walls  66  erecting upward from the base plane  64  at both ends of the repeating direction (longitudinal direction) of the first magnet  10  and the second magnet  20 . The vibrating membrane  30  is vibratorily fitted to an upper end face  67  of the erecting wall  66 . 
     The vibrating membrane  30  is formed of a thin flexible sheet made of a polymeric material such as polyimide, polyethylene terephthalate (PET), or liquid crystal polymer. Further, the above is not restrictive, but a non-magnetized metal plate, such as aluminum, may also be used. Especially in the case of using the non-magnetized metal plate, with the plate being lightweight and having suitable hardness, it is possible to obtain an advantage of further improving reproducibility of an original sound. 
     The coil  40  is formed on one face or both faces of the vibrating membrane  30 . A wire material and a winding pattern for the coil  40  of the present embodiment are not particularly restricted so long as the coil  40  is one receiving a magnetic flux Φ from the first magnet  10  and the second magnet  20  at the time of application of an electrical signal to receive magnetic force in a perpendicular direction to the plane of the vibrating membrane  30 . The electrical signal in the present embodiment means an input signal for vibrating the vibrating membrane  30  to make a vocal output. 
     Preferable examples of the coil  40  may include a winding coil formed by winding wires and a patterning coil (film coil) formed by making a metal material coat on or adhere to a flexible substrate. In the case of the winding coil, it may be a cored coil or an air-core coil. 
     In the present embodiment, the wires and the pattern constituting the coil  40  is collectively referred to as a winding. 
     Further, the wire-winding pattern of the coil  40  is also not particularly restricted, but may be one including a line area extending in a direction across an orientation of the magnetic flux Φ generated between the first magnet  10  and the second magnet  20 . A specific wire-winding pattern may be wire-winding in multiple layers with the same diameter, wire-winding in spiral shape with varied winding diameters within the same layer, wire-winding not by turning-around but meandering of wires, or wire-winding by combination of these. 
     In the case of the coil  40  being a winding coil, since a sectional area of the wire can be made large as compared with the patterning coil, it is possible to make a resistance component low, so as to obtain a high output of the flat acoustic transducer  100 . 
     Meanwhile, in the case of the coil  40  being the patterning coil, since a weight of the coil can be held small, the vibrating membrane  30  is excellent in responsiveness to vibration, and the whole of the flat acoustic transducer  100  can be reduced in weight. 
     As the coil  40  of the present embodiment, a wire-wound air-core coil is used. As shown in each drawing of  FIGS. 2A to 2C , a plurality of wires is wound both in a winding diameter direction and a winding thickness direction. 
     In the present embodiment, a plurality of coils  40  is provided as mutually spaced from one another in the repeating direction of the first magnet (permanent magnet)  10  and the second magnet (magnetic member)  20 . The plurality of coils  40  is electrically connected to one another. 
     In the flat acoustic transducer  100  of the present embodiment, the number of turns and a winding thickness of the winding  42  is common with respect to each coil  40 . However, as described later, the number of turns and the winding thickness of the coil  40  arranged in the vicinity of the center of the vibrating membrane  30  and the number of turns and the winding thickness of the coil  40  arranged in the vicinity of the periphery may be made different from one another. 
     The coil  40  is arranged in an area corresponding to at least either the first magnet  10  or the second magnet  20  in the plane of the vibrating membrane  30 . In other words, the coil  40  is formed surrounding at least part of the area facing the first magnet  10  or the second magnet  20 . 
     It should be noted that, although the coil  40  is provided only on one side (lower face side) of the principal face of the vibrating membrane  30  in the present embodiment, an additional coil  40  may be arranged as laminated on the opposite-side principal face of the vibrating membrane  30  or inside the film thickness of the vibrating membrane  30 . 
     The coil  40  of the present embodiment is provided projecting from the vibrating membrane  30  toward the first magnet (permanent magnet)  10  or the second magnet (magnetic member)  20 . 
     A winding axis AX of the coil  40  coincides with a central axis of the magnetic pole face  12  of the first magnet (permanent magnet)  10  or the upper face (magnetic pole face)  22  of the second magnet (magnetic member)  20 . 
     More specifically, in the flat acoustic transducer  100  of the present embodiment, the winding axis AX of the coil  40  is made to coincide with the first magnet  10  located at a lower position than the second magnet  20 . 
     An inner diameter of the coil  40  of the present embodiment is smaller than outer dimensions of the magnetic pole face  12  of the first magnet  10 , while an outer diameter of the coil  40  is larger than outer dimensions of the magnetic pole face  12  of the first magnet  10 . 
     Herein, the outer diameter of the coil  40  is smaller than a distance from the center of the first magnet  10  to the second magnet  20 . In other words, the winding  42  on the outermost periphery of the coil  40  is present in an inner area of a gap V between the first magnet  10  and the second magnet  20 . This prevents the vibrating coil  40  from interfering with the second magnet  20 . It is to be noted that in the present embodiment, the winding  42  may mean each wire-wound turn. 
     As shown in  FIG. 2B , a distance L 1  between the magnetic pole face  12  and a proximal edge  24  of the upper face  22  of the second magnet  20  is shorter than a distance L 2  between the magnetic pole face  12  and the base plane  64  of the yoke  60 . Further, as shown in  FIG. 1 , the upper-face side magnetic pole face  12  of the first magnet  10  is the north pole. 
     Accordingly, within a vertical cross section (see  FIGS. 2B ,  2 C) of a static magnetic field H generated by the magnetic pole face  12  of the first magnet  10 , a density of a magnetic flux Φ is maximal on or slightly above a line segment L connecting a peripheral edge  18  of the magnetic pole face  12  and the proximal edge  24  of the upper face  22 , both being edges adjacent to each other. Then a component in a horizontal direction of the density of the magnetic flux, namely, a component in a winding-diameter direction of the coil  40  (horizontal direction of the drawing), is also maximal in a position almost on the line segment L. 
     Then, at least part of winding  42  of the coil  40  at the time of no application of an electrical signal is arranged at such a height position that the density of the magnetic flux component parallel to a coil face  44  of the coil  40  is maximal in the magnetic flux Φ. 
     Thereby, magnetic force received by the winding  42  from the first magnet  10  and the second magnet  20  is maximal at the center (antinode) of vibration. 
       FIG. 2C  shows a horizontal component (B ∥ ) and a vertical component (B⊥) of the magnetic flux Φ generated on the line segment L. The horizontal component B ∥  is a magnetic flux component that coincides with the coil face  44  as a wire-wound face of the winding  42 , and the vertical component B⊥ is a magnetic flux component that coincides with the winding axis AX of the winding  42 . That is, the horizontal component B ∥  and the vertical component B⊥ are vector components of the magnetic flux Φ. The horizontal component B ∥  is then orthogonal to an electrical signal flowing in the winding  42 . 
     Therefore, the winding  42 , having a center of vibration inside the magnetic flux Φ where the horizontal component B ∥  is maximal, receives reduced magnetic force both in the case of upward movement of the coil  40  from the center of the vibration and in the case of downward movement of the coil  40  from the center of the vibration. 
     For this reason, driving force received by the vibrating membrane  30  in the case of the coil  40  reaching a bottom dead center of an amplitude is made equal to that in the case of the coil  40  reaching a top dead center, thereby improving reproducibility of an original sound of the flat acoustic transducer  100 , especially reproducibility in the case of the coil  40  vibrating upward. 
     Further, at least part of the winding  42  of the coil  40  at the time of no application of an electrical signal is arranged at a middle height position of between the magnetic pole face  12  and the upper face  22 , and above the line segment L connecting respective proximal edges (the peripheral edge  18  and the proximal edge  24 ) of the magnetic pole face  12  and the upper face  22 . 
     More specifically, at the time of no application of an electrical signal, a winding  42   a  in the coil  40 , which corresponds to the center of the winding thickness direction and the outermost periphery (see  FIG. 2B ), is preferably located on or above the line segment L. Further, at the time of no application of an electrical signal, part of the winding of the coil  40  is preferably present below the line segment L, and another part thereof is preferably present above the line segment L. 
     Regarding  FIGS. 2B and 2C , in the static magnetic field H generated by the first magnet  10  and the second magnet  20 , more specifically, a magnetic flux density above the line segment L connecting the peripheral edge  18  of the magnetic pole face  12  and the proximal edge  24  of the upper face  22  is higher than a magnetic flux density below the line segment L. This is because the permanent magnet generally generates the stronger static magnetic field H on the outside in the axial direction thereof than on the magnetic pole faces of both sides thereof. 
     Therefore, at least part of the winding of the coil  40  may be arranged to have a height of the center of vibration slightly above the line segment L. This can make equal magnetic force received at the top dead center of the amplitude and magnetic force received at the bottom dead center in regard to the whole of the coil  40 . 
     The bottom dead center of vibration of the coil  40  is above the magnetic pole face  12  of the first magnet  10 , and the winding  42  of the coil  40  does not interfere with the magnetic pole face  12 . That is, the lower end position of vibration of the coil  40  is present above the upper face of one present at a lower position out of the first magnet (permanent magnet)  10  and the second magnet (magnetic member)  20 . The coil  40  then vibrates inside, and in a space above, the step  50 . 
     In the vibrating membrane  30 , a mount section  32  made of a non-magnetized material is provided projecting on the lower face side. The coil  40  is mounted on the mount section  32 . The mount section  32  may be integrally provided with the vibrating membrane  30 , or may be made in plate shape with a predetermined thickness and joined to the lower face side of the vibrating membrane  30 . Further, part of the mount section  32  in plate shape may be provided erecting vertically to the vibrating membrane  30 , so as to serve as a bobbin section for wire-winding of the winding  42  of the coil  40 . That is, a columnar section corresponding to the bobbin section and a plate section formed in flanged shape on the upper end of the columnar section may constitute the mount section  32 . 
     The mount section  32  is a spacer for ensuring a distance in the thickness direction between the vibrating membrane  30  and the coil  40 . Providing such amount section  32  adjusts the distance between the magnetic pole face  12  of the first magnet  10  and the coil  40  to the predetermined one, while preventing the vibrating membrane  30  that vibrates and the second magnet  20  from interfering with each other. 
     Actions and effects of the flat acoustic transducer  100  of the present embodiment are described. 
     In the flat acoustic transducer  100  of the present embodiment, with the step  50  provided between the first magnet  10  and the second magnet  20 , the area where the magnetic flux density generated by the permanent magnet is maximal, namely the line segment L connecting the respective proximal edges is oblique to a normal of the magnetic pole face of the permanent magnet. On such a line segment L, the magnetic flux density of the horizontal component B ∥  of the magnetic flux Φ is maximal. Therefore, in the flat acoustic transducer  100  of the present embodiment in which the winding  42  of the coil  40  is arranged on the line segment L, the winding  42  and the magnetic pole face do not interfere with each other, to maximize magnetic force received by the winding  42  at the center of vibration. This substantially symmetrizes magnetic force received by the coil  40 , regardless of the vibrating direction of the coil  40 , so as to improve reproducibility of an original sound of the flat acoustic transducer  100 . 
     Further, as shown in  FIG. 1 , the flat acoustic transducer  100  configured by arranging the first magnet  10  and the second magnet  20  in a row can be reduced in width dimension. For this reason, for example, an application to a space-restricted spatial domain such as a frame section of a flat-screen television is possible. 
     Further, the coil  40  of the present embodiment is provided projecting from the vibrating membrane  30  toward the first magnet  10 . Hence the coil  40  makes use of the inside of the step  50  as a vibration space, and the first magnet  10  and the second magnet  20  are prevented from interfering with the vibrating membrane  30 , whereby it is possible to obtain the flat acoustic transducer  100  which is thinned in total. 
     Herein, a brief overview of a method for driving the flat acoustic transducer  100  of the present embodiment (which may hereinafter be referred to as present method) will be described. 
     The present method relates to the method for driving the flat acoustic transducer  100  with the vibrating membrane  30  fixed with the coil  40  to which an electrical signal is applied. 
     The present method is to generate the static magnetic field H in which the density of the magnetic flux component (horizontal component B ∥ ) parallel to the coil face  44  of the coil  40  changes in the vibrating direction of the vibrating membrane  30 , while applying an electrical signal to the coil  40  arranged at such a position that the magnetic flux density is maximal, to vibrate the vibrating membrane  30 . 
     According to the present method, magnetic force received by the coil  40  from the static magnetic field H by application of the electrical signal is maximal in the position where the coil  40  is arranged. Therefore, in whichever vibrating direction the coil  40  moves from such an arranged position, driving force to be given to the vibrating membrane  30  is symmetrized, thereby reducing distortion of a sound in the flat acoustic transducer  100 , to improve reproducibility of an original sound. 
     It should be noted that a variety of modifications are permitted in regard to the present embodiment. 
       FIG. 3A  is a side view of the yoke  60  of the present embodiment shown in each drawing of  FIGS. 2A to 2C . The yoke  60  of the present embodiment is provided with the erecting walls  66  for fixing the vibrating membrane  30  (not shown in the drawing) to both ends of the longitudinal direction (horizontal direction in  FIG. 3A ). 
     On the other hand,  FIG. 3B  is a side view showing a modification of the yoke  60 . The yoke  60  according to the modification is not provided with the erecting wall, but formed totally flat except for the step  62 . The yoke  60  of the present modification may be mounted on a frame  70 . The frame  70  is made of a magnetic material or a non-magnetic material, and includes a flat bottom face  72  mounted with the yoke  60 , and erecting walls  74  provided erecting at both ends of the longitudinal direction of the bottom face  72 . The edge of the vibrating membrane  30  can be fitted to a top end face  76  of the erecting wall  74 . Further, the frame  70  may be provided with a circuit section (not shown in the drawing) for supplying an electrical signal to the coil. Forming such a frame  70  separately from the yoke  60  can prevent workability from being impaired at the time of positioning the first magnet and the second magnet with high accuracy. 
     Second Embodiment 
       FIG. 4  is a top perspective view showing the flat acoustic transducer  100  according to the present embodiment. However, the vibrating membrane and the coils are not shown in the drawing. 
     The yoke  60  of the present embodiment has side wall sections  68  extending lateral to the arraying direction of the first magnets (permanent magnets)  10  and the second magnets (magnetic members)  20 . 
     The side wall sections  68  are connected with the erecting walls  66  provided at both ends of the arraying direction of the yoke  60 , to surround the yoke  60 . 
     The side wall section  68  is made of a magnetic material of the same kind as or different one from that for the yoke  60 . Thereby, a magnetic circuit is also formed in the vertical direction to the arraying direction of the first magnets  10  and the second magnets  20  among in-plane directions of the vibrating membrane, whereby a magnetic field passing through the coil is strengthened, while being made totally uniform. Therefore, according to the flat acoustic transducer  100  of the present embodiment, output efficiency is high as compared with the first embodiment, thereby allowing stable reproduction of an original sound. It is to be noted that the erecting walls  66 ,  74  and the side wall section  68  may be combined with one another, or separately provided. 
     Third Embodiment 
       FIG. 5  is a top perspective view showing the flat acoustic transducer  100  of the present embodiment. In the drawing, as in  FIG. 1 , the vibrating membrane  30  and the yoke  60  are spaced from each other, to show the state of the lower face side of the vibrating membrane  30 . 
     In the flat acoustic transducer  100  of the present embodiment, a respective plurality of first magnets (permanent magnets)  10  and the second magnets (magnetic members)  20  are repeatedly arranged by pattern in two-dimensional directions. 
     That is, in the flat acoustic transducer  100  of the present embodiment, the first magnet  10  and the second magnet  20  are arranged in lattice shape or in zigzag shape. 
     As in the present embodiment, the flat acoustic transducer  100  where the first magnets  10  and the second magnets  20  are configured to be on a plurality of columns can be increased in width dimension. Therefore, the flat acoustic transducer  100  of the present embodiment is preferred as an application for a large-sized flat acoustic transducer, such as the case of use for a movie theater or a hall, or the case of changing a wall face of a house itself into a speaker. 
     Further,  FIG. 6  shows a modification of the present embodiment. The vibrating membrane  30  is not shown in the drawing. 
     In the present modification, the side wall section  68  made of the magnetic material is formed all around the yoke  60  on which the first magnets (permanent magnets)  10  and the second magnets (magnetic members)  20  are repeatedly arranged by pattern in two-dimensional directions. Thereby, the first magnet  10  and the second magnet  20  arranged on the outermost periphery form a magnetic circuit with the side wall section  68 , thereby allowing improvement in, as well as in-plane uniformization of, driving power of the vibrating membrane  30 . 
     Fourth Embodiment 
       FIG. 7A  is a vertical sectional view, cut along the longitudinal direction, of the flat acoustic transducer  100  of the present embodiment.  FIG. 7B  is an expanded view of a broken-line area Y of  FIG. 7A . 
     In the present embodiment, the winding axis AX of the coil  40  coincides with the central axis of the upper face  22  of the second magnet (magnetic member)  20 , while at least part of the winding  42  is provided surrounding the periphery of the second magnet  20 . 
     The coil  40  is formed using an air-core coil similarly to the first embodiment and having an inner diameter larger than that of outer dimensions of the second magnet  20 . 
     When the coil  40  vibrates in the vertical direction of the drawing along with the vibrating membrane  30 , the upper face  22  of the second magnet  20  moves inside the air core of the coil  40  in non-contact with the winding  42 . 
     Thereby, the second magnet  20  of the present embodiment acts as the core of the coil  40 . Therefore, magnetic force received from the first magnet  10  and the second magnet  20  increases as compared with that in the first embodiment, thereby to improve driving force of the vibrating membrane  30 . 
     It should be noted that as the coil  40 , in the case of using a patterning coil with a thickness smaller than a half amplitude, the mount section  32  (see  FIG. 2 ) to be provided between the coil  40  and the vibrating membrane  30  may be formed. That is, the ring-like mount section  32 , provided with a larger hollow section than outer dimensions of the upper face  22  of the second magnet  20 , is mounted between the coil  40  and the vibrating membrane  30 , whereby it is possible to prevent the vibrating membrane  30  and the upper face  22  from interfering with each other, while arranging the coil  40  as the patterning coil at a position lower than the upper face  22 . 
     Fifth Embodiment 
       FIG. 8  is an exploded perspective view of the flat acoustic transducer  100  according to the present embodiment. 
     In the flat acoustic transducer  100  of the present embodiment, at least either the first magnet (permanent magnet)  10  or the second magnet (magnetic member)  20  forms a ring shape. The first magnet (permanent magnet)  10  and the second magnet (magnetic member)  20  are concentrically arranged. 
     Herein, as the ring shape, either an annular shape or a rectangular shape may be selected. 
     More specifically, the first magnet  10  of the present embodiment is formed by combination of a core magnet  14  in cylindrical shape with the smallest outer diameter and a magnet  16  in annular shape with the largest outer diameter. The second magnet  20  has outer dimensions of the middle between the core magnet  14  and the ring magnet  16 , to form a ring shape. The core magnet  14 , the second magnet  20 , and the ring magnet  16  are concentrically arrayed in this order from the inside. These magnets are spaced from each other in respective diametrical directions with a predetermined interval. 
       FIG. 9  is a sectional view of IX-IX of  FIG. 8 , showing a vertical sectional view, cut along the diametrical direction, of the flat acoustic transducer  100  of the present embodiment. 
     A height dimension of the second magnet  20  is larger than a height dimension of the first magnet  10  (the core magnet  14  and the ring magnet  16 ). The height dimensions of the core magnet  14  and the ring magnet  16  may coincide with or differ from each other. 
     These magnets are mounted on the yoke  60  made of a magnetic body forming the shape of a flat disk. 
     Further, the yoke  60  is fitted to the frame  70  in bottomed cylindrical shape. The erecting wall  74  is provided on the periphery of the bottom face  72  of the frame  70 . 
     The vibrating membrane  30  of the present embodiment forms a disk shape. A peripheral edge of the vibrating membrane  30  is fixed to the top end face  76  of the erecting wall  74 . 
     Out of the upper and lower principal faces of the vibrating membrane  30 , the coil  40  is fitted to the lower face which faces the yoke  60 . For the coil  40  of the present embodiment, the annular first coil  46  and the second coil  47 , which are concentrically arranged, are used in combination. The first coil  46  and the second coil  47  are provided projecting downward from the vibrating membrane  30 . At this time, the mount section  32  may, as necessary, be put between at least either the first coil  46  or the second coil  47  and the vibrating membrane  30  (see  FIGS. 2B ,  2 C). In order to prevent the mount section  32  used in the present embodiment from interfering with the upper face  22  of the second magnet  20  at the time of vibration of the mount section  32  along with the vibrating membrane  30 , the mount section  32  may be formed in ring shape in line with the shape of the coil  40  (the first coil  46 , the second coil  47 ), and an area corresponding to the inner diameter of the coil  40  may be formed in concave shape. 
     As shown in  FIG. 9 , the first coil  46  is formed in an upper area of a gap V 1  between the core magnet  14  and the second magnet  20 , and the second coil  47  is formed in an upper area of a gap V 2  between the second magnet  20  and the ring magnet  16 . Then, at least part of the windings  42  of the first coil  46  and the second coil  47  are arranged at height positions lower than the upper face  22  of the second magnet  20  and higher than the upper faces of the core magnet  14  and the ring magnet  16 . 
     In such a state, an electrical signal is applied to the first coil  46  and the second coil  47 , and the first coil  46  thereby receives magnetic force from a static magnetic field generated between the core magnet  14  and the second magnet  20 . Further, the second coil  47  receives magnetic force from a static magnetic field generated between the ring magnet  16  and the second magnet  20 . 
     Further, also in the flat acoustic transducer  100  of the present embodiment, when the coil  40  (first coil  46 , second coil  47 ) moves upward or downward from the middle of vibration, magnetic force received from the first magnet  10  and the second magnet  20  is symmetrized. 
     It should be noted that in the present embodiment, a non-magnetized magnetic body may be used for the core magnet  14  and the ring magnet  16  (first magnet  10 ) among the core magnet  14 , the second magnet  20 , and the ring magnet  16  which are concentrically arranged. Accordingly, since a plurality of coils (the first coil  46  and the second coil  47 ) can be driven by use of only one second magnet  20  as the permanent magnet, an advantage in terms of cost can be obtained. 
     In each of the above embodiments, the interval between the first magnet  10  and the second magnet  20 , and the height of the step  50 , are made common with respect to each set of the adjacent magnets. Further, the winding thickness and the number of turns are made common among the plurality of coils  40 . However, the present invention is not restricted to this, but a variety of changes can be made. 
     For example, any one or more elements of the interval between the magnets, the height of the step  50 , and the winding thickness and the number of turns of the winding  42  may be made different with respect to each in-plane area of the flat acoustic transducer  100 , so that an amplitude generated in the vicinity of the center of the vibrating membrane  30  is substantially equal to an amplitude generated in the vicinity of the periphery when respective electrical signals are applied to the plurality of coils  40 . 
     Specifically, in the first embodiment shown in  FIG. 1 , the interval between the first magnet  10  and the second magnet  20  in the vicinity of the periphery in the longitudinal direction of the base plane  64  may be made smaller than the interval in the vicinity of the center. Further, in the fifth embodiment shown in  FIG. 9 , the gap V 2  between the ring magnet  16  and the second magnet  20  may be made smaller than the gap V 1  between the core magnet  14  and the second magnet  20 . Moreover, the number of turns of the coil  40  arranged in the vicinity of the periphery of the vibrating membrane  30  may be made larger than the number of turns of the coil  40  arranged in the vicinity of the center. Thereby, when a common electrical signal is applied to the plurality of coils  40 , magnetic force received by the vibrating membrane  30  in the vicinity of the periphery is stronger than magnetic force received in the vicinity of the center. Accordingly, in the case of fixing the peripheral edge of the vibrating membrane  30  to the yoke  60  or the frame  70  (see  FIG. 3A  or  3 B), an amplitude of the vicinity of the periphery which is close to such a fixed section and inferior in vibrating properties can be made substantially equal to an amplitude of the vicinity of the center which is superior in vibrating properties. 
     According to the flat acoustic transducer  100 , the vibrating membrane  30  can perform reciprocating vibration in the perpendicular direction to the plane while being held flat, so as to obtain a vocal output with high directivity. 
     This application claims benefit of priority based upon Japanese Published patent application 2008-312656 filed on. Dec. 8, 2008, the contents of which are incorporated herein by reference in their entirety.