Abstract:
A speaker apparatus for reproducing acoustic sounds includes a magnetic circuit unit having a substantially ring-shaped center plate of a magnetic material and a set of substantially annular magnets arranged on both sides of the center plate so that magnetic poles of repulsive polarities face each other with the center plate in-between. The magnetic circuit unit produces a magnetic field on each of an inner rim side and an outer rim side of the center plate. The speaker apparatus also includes a first vibrating system including a first vibrating plate arranged on the inner rim side of the center plate so as to be driven by the magnetic circuit unit, and a second vibrating system including a second vibrating plate arranged on the outer rim side of the center plate so as to be driven by the magnetic circuit unit. The first and second vibrating plates are substantially flat plate shaped and arranged so that their respective major surfaces lie on substantially the same horizontal plane. Since the first and second vibrating plates having respective different reproducing frequency ranges and are positioned on substantially the same horizontal plane, the reproduced acoustic sound may be improved in sound quality.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a speaker apparatus for transducing electrical signals into acoustic sound in which the apparatus can be reduced in thickness. 
     2. Description of the Related Art 
     There has hitherto been used a speaker apparatus in which the sound in the high frequency range and the sound in the mid to low frequency range are reproduced with respective different vibrating plates. As this speaker apparatus, there is proposed a coaxial two-way speaker apparatus in which a vibrating plate reproducing the sound of a high frequency range and another vibrating plate reproducing the sound of the mid to low frequency range are arranged superimposed with the centers in the vibrating direction coincident with each other. 
     As this type of speaker apparatus, one arranged as shown in FIG. 1 is proposed. A speaker apparatus  101 , shown in FIG. 1, includes a vibrating system for the high sound range  105  for generating the acoustic sound of a high sound range, a magnetic circuit for high sound range  106  for driving the vibrating system for the high sound range  105 , a vibrating system for the mid to low sound range  107  for generating the acoustic sound of a mid to low sound range, and a magnetic circuit for mid to low sound range  108  for driving the vibrating system for the vibrating system for the mid to low sound range  107 . 
     The vibrating system for the high sound range  105  is supported by a frame  109  and includes a dome-shaped vibrating plate  110 , a cylindrically-shaped bobbin  111 , carrying one end of the vibrating plate  110 , and a resilient edge  112  connected to the outer rim of the vibrating plate  110 . 
     The magnetic circuit for high sound range  106  includes a voice coil  114 , for vibrating the vibrating system for the high sound range  105 , a yoke  115  for forming a magnetic path, a magnet  106  for according the magnetic flux to the yoke  115 , and a top plate  117  provided adjacent to the magnet  116  for defining a magnetic gap. The voice coil  114  is provided on the outer periphery of the bobbin  111  of the vibrating system for the high sound range  105  within the magnetic gap. Both ends of the voice coil  114  are connected via a braided wire, not shown, to a connection terminal  110  provided on the outer rim of the frame  109 . The yoke  115  is formed of a magnetic material in a disc shape and has an upstanding columnar center pole  118 . This center pole  118  is inserted into the inside of the bobbin  111  of the vibrating system for the high sound range  105 . The magnet  116 , mounted on the yoke  115 , has a ring shape into which is inserted the center pole  118 . This magnet  116  has the yoke side and the top plate side magnetized to an S-pole and to an N-pole, respectively. The yoke  115  has its position adjacent to the S-pole side of the magnet  116  magnetized to an S-pole. The magnet  116 , mounted on the yoke  115 , has a ring shape, with a center opening into which is inserted the center pole  118 . The top plate  117 , magnetized to an N-pole, has a magnetic gap defined between its inner rim and the outer rim of the center pole  118 . 
     The magnetic circuit  106  is covered by a shield cover  119  for prohibiting the magnetic flux from leaking from the magnetic circuit  106  to outside, as shown in FIG.  1 . 
     Referring to FIG. 1, the vibrating system for the mid to low sound range  107  includes a substantially conically-shaped vibrating plate  120 , having a center through-hole, a cylindrically-shaped bobbin  121 , having its one end mounted in a through-hole in the vibrating plate  120 , a resilient edge  122 , connected to the outer rim of the vibrating plate  120 , and a resilient vibration-absorbing damper  123  mounted on the bobbin  121 . 
     Referring to FIG. 1, the magnetic circuit for mid to low sound range  108  includes a voice coil  124  for vibrating the vibrating system for the mid to low sound range  107 , a yoke  125  forming a magnetic path, a driving magnet  126  for according the magnetic flux to this yoke  125 , a top plate  127  provided adjacent to the driving magnet  126  for defining the magnetic gap and a cancelling magnet  128  for prohibiting the magnetic flux of the driving magnet  126  from leaking to outside of the magnetic circuit for mid to low sound range  108 . 
     The voice coil  124  is provided on the outer rim of the bobbin  121  of the vibrating system for the mid to low sound range  107  within the magnetic gap. Both ends of the voice coil  124  are connected to the connection terminal  110  provided on the outer rim of the frame  109  via a braided wire  130 . The yoke  125  is formed of a magnetic material in a disc shape and has an upstanding columnar center pole  131 . This center pole  131  is inserted into the inside of the bobbin  121  of the vibrating system for the mid to low sound range  107 . The driving magnet  126 , provided on the yoke  125 , has a ring shape with a center opening into which is inserted the center pole  131 . This driving magnet  126  has the yoke side and the top plate side magnetized to an S-pole and to an N-pole, respectively. The yoke  125  has its position adjacent to the S-pole side of the driving magnet  126  magnetized to an S-pole. The top plate  127 , mounted on the driving magnet  126 , has a ring shape into which is inserted the center pole  131 . The top plate  127 , magnetized to an N-pole, has a magnetic gap defined between its inner rim and the outer rim of the center pole  118 . The cancelling magnet  128  has a ring shape and is provided on the rear end of the yoke  125 . The cancelling magnet  128  has the yoke side and the rear end magnetized to the S-pole and to the N-pole, respectively. 
     This speaker apparatus  101  has a frame  109  supporting the vibrating system for the mid to low sound range  107 , as shown in FIG.  1 . This frame  109  is formed of a metallic material and substantially conically shaped and flared from the rear end towards the front end. The frame  109  has, on its front end side, a holder  132  for holding the vibrating system for the mid to low sound range  107 . To this holder  132  is secured the outer rim of the edge  122  of the vibrating system for the mid to low sound range  107  via a gasket, not shown. 
     In the above-described speaker apparatus  101 , if the current is fed to the voice coils  114 ,  124 , the voice coils  114 ,  124  are set into vibrations and, in keeping with the vibrations of the voice coils  114 ,  124 , the vibrating plates  110 ,  120  of the vibrating system for the high sound range  105  and the vibrating system for the mid to low sound range  107  are set into vibrations to produce the acoustic sound. 
     Referring to FIG. 2, a modified speaker apparatus  102 , having a vibrating system for the high sound range  135  and a magnetic circuit for high sound range  136  different from those of the above-described speaker apparatus  101 , is explained. Since the vibrating system for the mid to low sound range and the magnetic circuit for mid to low sound range of the speaker apparatus  102  are similar to the vibrating system for the mid to low sound range  107  and to the magnetic circuit for mid to low sound range  108  of the above-described speaker apparatus  101 , shown in FIG. 2, corresponding parts are depicted by the same reference numerals and are not explained specifically. 
     Referring to FIG. 2, the vibrating system for the high sound range  135  is supported by the magnetic circuit for mid to low sound range  108 , and includes a dome-shaped vibrating plate  140 , a cylindrically-shaped bobbin  141 , carrying one end of the vibrating plate  140 , and a resilient edge  142  connected to the outer rim of the vibrating plate  140 . 
     The magnetic circuit for high sound range  136  includes a voice coil  144 , for vibrating the vibrating system for the high sound range  135 , a bottomed tubular yoke  145  for forming a magnetic path, a magnet  146  for according the magnetic flux to the yoke  145 , and a top plate  147  provided adjacent to the magnet  146  for defining a magnetic gap. The voice coil  144  is provided on the outer periphery of the bobbin  141  of the vibrating system for the high sound range  135  within the magnetic gap. Both ends of the voice coil  144  are connected via a braided wire, not shown, to a connection terminal  110  provided on the outer rim of the frame  109 . The yoke  145  is formed of a magnetic material. This magnet  146  has the yoke side and the top plate side magnetized to an S-pole and to an N-pole, respectively. The yoke  145  has its position adjacent to the S-pole side of the magnet  146  magnetized to an S-pole. The top plate  147 , mounted on the magnet  146 , is formed of a magnetic material in a disc shape. This top plate  147 , delimiting a magnetic gap between its outer rim and the inner rim of the opening end of the yoke  145 , is magnetized to an N-pole. 
     In the above-described speaker apparatus  102 , if the current is fed to the voice coils  124 ,  144 , the voice coils  124 ,  144  are set into vibrations and, in keeping with the vibrations of the voice coils  124 ,  144 , the vibrating plates  140 ,  120  of the vibrating system for the high sound range  135  and the vibrating system for the mid to low sound range  107  are set into vibrations to produce the acoustic sound. 
     The above-described conventional speaker apparatus  101  has a drawback that the sound source for the high frequency range obstructs the reproduced sound of the sound source for the mid to low frequency range to affect reproduced sound pressure versus frequency characteristics. 
     Thus, it is in general critical to approach and match the mounting positions of the sound source for the high frequency range and the sound source for the mid to low frequency range to each other. However, this speaker apparatus  101  has a drawback that the positions of the voice coils  114 ,  124  constituting the respective sound sources are spaced apart in the fore-and-aft direction in the amplitude direction, as shown in FIG.  1 . 
     In the above-described speaker apparatus  102 , since the sound source for the high frequency range is separated from the amplitude plane of the vibrating plate  120  of the sound source for the mid to low frequency range, as shown in FIG. 2, the sound source for the high frequency range is prevented from affecting the reproduced sound pressure versus frequency characteristics. However, this speaker apparatus  102  is not desirable in that the sound sources are not positioned in the same plane relative to the playback sound pressure direction. 
     The above-described speaker apparatus  101 ,  102  suffer from the problem that the magnet of a larger magnetic power is used to reduce the size of the sound source for higher frequency range, thus raising the production cost. These speaker apparatus  101 ,  102  are inconvenient in that a magnetic circuit for the sound source for higher frequency range has to be provided separately to increase the number of component parts to detract from ease in assembling. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a novel speaker apparatus free from the above-mentioned problems inherent in the conventional electro-acoustic transducer. 
     It is another object of the present invention to provide a speaker apparatus that is able to reproduce the acoustic sound with high sound quality. 
     It is yet another object of the present invention to provide a speaker apparatus in which the structure is simplified and the apparatus itself can be reduced in thickness and size. 
     In one aspect, the present invention provides a speaker apparatus including a magnetic circuit unit having a substantially ring-shaped center plate of a magnetic material and a set of substantially annular magnets arranged on both sides of the center plate so that magnetic poles of repulsive polarities face each other with the center plate in-between. The magnetic circuit unit produces a magnetic field on each of an inner rim side and an outer rim side of the center plate. The speaker apparatus also includes a first vibrating system having a first vibrating plate which is arranged on the inner rim side of the center plate so as to be driven by the magnetic circuit unit, and a second vibrating system having a second vibrating plate which is arranged on the outer rim side of the center plate so as to be driven by the magnetic circuit unit. 
     The first and second vibrating systems may be of different reproducing frequency ranges. 
     In another aspect, the present invention provides a speaker apparatus including a magnetic circuit unit having a substantially ring-shaped center plate of a magnetic material and a set of substantially annular magnets arranged on both sides of the center plate so that magnetic poles of repulsive polarities face each other with the center plate in-between. The magnetic circuit unit produces a magnetic field on each of an inner rim side and an outer rim side of the center plate. The speaker apparatus also includes a first vibrating system including a first vibrating plate arranged on the inner rim side of the center plate so as to be driven by the magnetic circuit unit, and a second vibrating system including a second vibrating plate arranged on the outer rim side of the center plate so as to be driven by the magnetic circuit unit. The first and second vibrating plates are substantially flat plate shaped and arranged so that respective major surfaces thereof lie on substantially the same horizontal plane. 
     The first and second vibrating plates are arranged on the same axial line. 
     With the present speaker apparatus, since the first and second vibrating plates are positioned on substantially the same horizontal plane, the reproduced acoustic sound may be improved in sound quality. 
     Other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing a conventional speaker apparatus. 
     FIG. 2 is a cross-sectional view showing another conventional speaker apparatus. 
     FIG. 3 is a cross-sectional showing a speaker apparatus according to an embodiment of the present invention. 
     FIGS. 4A and 4B are cross-sectional views for illustrating the magnetic field of a magnetic circuit provided on the speaker apparatus of FIG.  3 . 
     FIGS. 5A and 5B are cross-sectional views for illustrating the magnetic field of a magnetic circuit provided on the conventional speaker apparatus. 
     FIGS. 6A and 6B are cross-sectional views showing a center plate constituting a magnetic circuit employed in the speaker apparatus embodying the present invention. 
     FIG. 7 is a cross-sectional showing a speaker apparatus according to another embodiment of the present invention. 
     FIG. 8 is a cross-sectional showing a speaker apparatus according to yet another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, preferred embodiments of the speaker apparatus according to the present invention will be explained in detail. 
     A speaker apparatus  1  according to the present invention includes a vibrating system for the high sound range  5  for generating the acoustic sound of a high sound range, a vibrating system for the mid to low sound range  6  and a magnetic circuit  7  for driving the vibrating system for the high sound range  5  and the vibrating system for the mid to low sound range  6 , as shown in FIG.  3 . This speaker apparatus  1  includes a frame  8  for supporting the vibrating system for the high sound range  5 , vibrating system for the mid to low sound range  6  and the magnetic circuit  7 , as shown in FIG.  3 . 
     The vibrating system for the high sound range  5  includes a substantially conically-shaped vibrating plate  10 , having a center through-hole, a cylindrically-shaped bobbin  11 , having its one end attached to the through-hole in the vibrating plate  10 , a resilient edge  12  connected to the outer periphery of the vibrating plate  10  and a dome-shaped cap  13  mounted on the vibrating plate  10  for closing its through-hole, as shown in FIG.  3 . 
     The vibrating system for the mid to low sound range  6  includes a substantially conically-shaped vibrating plate  15 , having a center through-hole, a cylindrically-shaped bobbin  16 , having its one end attached to the through-hole in the vibrating plate  15 , a resilient edge  17  connected to the outer periphery of the vibrating plate  15 , and a resilient vibration-absorbing damper  18 , mounted on the bobbin  16 , as shown in FIG.  3 . 
     The driving circuit  7  includes first and second voice coils  21 ,  22  for vibrating the vibrating plates  10 ,  15  of the vibrating system for the high sound range  5  and the vibrating system for the mid to low sound range  6 , respectively, a center plate  23  for constituting a magnetic path and a magnet set comprised of a first magnet  24  and a second magnet  25  for according the magnetic flux to the center plate  23 . 
     The first voice coil  21  is provided on the outer rim of the bobbin  11  of the vibrating system for the high sound range  5  and is arranged on the inner rim of the magnetic circuit  7 . Both ends of the first voice coil  21  are connected via a braided wire  27  to a connection terminal  28  provided on the outer rim of the frame  8 . The second voice coil  22  is provided on the outer rim of the bobbin  16  of the vibrating system for the mid to low sound range  6  and is arranged on the outer rim of the magnetic circuit  7 . Both ends of the second voice coil  22  are connected via braided wire  27  to the connection terminal  28  provided on the outer rim of the frame  8 . 
     The center plate  23  is formed of a magnetic material, such as hot forged steel plate, into a ring shape, and is provided on the inner rim of the bobbin  16  of the vibrating system for the mid to low sound range  6 . 
     The first and second magnets  24 ,  25  are anisotropic sintered magnets of rare earth elements, such as neodymium, and are each formed in a ring shape. Each of the first and second magnets  24 ,  25  has its outer diameter slightly smaller than the outer diameter of the center plate  23 , while having the inner diameter of the center opening thereof slightly larger than the inner diameter of the center opening of the center plate  23 . The first and second magnets  24 ,  25  are arranged with the center openings thereof coincident with the center opening of the center plate  23 , so that the outer rim and the inner rim of the center plate  23  are protruded from the outer rims and the center openings of the first and second magnets  24 ,  25 , respectively. The first and second magnets  24 ,  25  are arranged on both sides of the center plate  23  so that the magnetic poles of repulsive polarities face each other with the center plate  23  in-between. Specifically, each of the first and second magnets  24 ,  25  has the center plate side along the direction of thickness magnetized to the N-pole, while having the opposite side magnetized to an S-pole. Therefore, the magnetic circuit  7  forms a repellent magnetic field. 
     To the magnetic circuit  7 , constructed as described above, an outer rim of an edge  12  of the vibrating system for the high sound range  5  is fixedly mounted via an edge ring  26  provided on the first magnet  24 . On the outer rim and the inner rim of the magnetic circuit  7  are arranged a bobbin  16  of the vibrating system for the mid to low sound range  6  and the bobbin of the vibrating system for the high sound range  5 , respectively, as shown in FIG.  3 . 
     The frame  8 , formed of a metallic material, is substantially conically shaped so that it is flared from the rear end side towards the front end side, as shown in FIG.  3 . On the front end side of the frame  8 , there is formed a holder  29  for holding the vibrating system for the mid to low sound range  6 . To this holder  29  is secured the outer rim of an edge  17  of the vibrating system for the mid to low sound range  6  by a gasket, not shown. On the outer rim of the frame  8 , there is mounted a connection terminal  28  connected to the first and second voice coils  21 ,  22  via braided wire  27 . To this connection terminal  28  is connected a connection line of an external speech signal source, not shown. 
     With the above-described speaker apparatus  1 , if the driving current proportionate to the speech signals is fed to the first and second voice coils  21 ,  22  of the magnetic circuit  7 , the first and second voice coils  21 ,  22  are set into vibrations, in accordance with the Flemings left hand rule. The vibrating plates  10 ,  15  of the vibrating system for the high sound range  5  and the vibrating system for the mid to low sound range  6  are vibrated in keeping with the vibrations of the first and second voice coils  21 ,  22  to produce the acoustic sound. 
     The magnetic flux distribution of the magnetic circuit  7 , constructed as described above, is explained with reference to FIG. 4A, in which the ordinate and the abscissa denote the position in the direction of thickness of the magnetic circuit  7  parallel to the amplitude direction of the vibrating system for the high sound range  5  and the vibrating system for the mid to low sound range  6 , respectively. Also, in FIG. 4 b , the arrows indicate the direction of the magnetic lines of force. 
     On the inner and outer rims of the magnetic circuit  7 , there are formed magnetic fields of approximately equal magnetic flux densities, as shown in FIG. 4 b . This magnetic circuit  7  has the maximum magnetic flux density in the magnetic field at a position facing the outer rim of the center plate  23 , as shown in FIG.  4 A. The magnetic flux density of the magnetic circuit  7  is progressively smaller in a direction away from the center plate  23  in the direction of thickness, with the direction of the magnetic flux being reversed at a mid portion along the direction of thickness of the first and second magnets  24 ,  25 . 
     Also, in this magnetic circuit  7 , the magnetic flux in the reverse direction is progressively increased in a direction proceeding away from the center plate in the direction of thickness from the center in the direction of thickness of the first and second magnets  24 ,  25 , while being progressively smaller in the direction proceeding away from the ends of the first and second magnets  24 , 25 . The magnetic circuit  7  sets the vibrating system for the high sound range  5  and the vibrating system for the mid to low sound range  6 , arranged on the inner and outer rim sides, respectively. 
     A conventional magnetic circuit  200 , providing a repellant magnetic field, is explained with reference to FIGS. 5A and 5 b . The magnetic circuit  200  includes a disc-shaped center plate  201 , formed of a magnetic material, and a magnet set, namely a first magnet  202  and a second magnet  203 , arranged on both sides of the center plate  201  so that the magnetic poles of repulsive polarities will face each other with the center plate  201  in-between. Each of the first and second magnets  202 ,  203  has its center plate side along the direction of thickness thereof magnetized to an N-pole, while having its opposite side magnetized to an S-pole. 
     In the conventional magnetic circuit  200 , the magnetic field is formed only in the outer rim of the disc-shaped center plate  201 , as shown in FIG.  5 B. In the magnetic circuit  7  of the present invention, the magnetic field is formed on the inner and outer rims of the center plate  23 , as shown in FIG. 4B, and hence the vibrating system for the high sound range  5  and the vibrating system for the mid to low sound range  6 , arranged on the inner and outer rim sides of the center plate  23 , respectively, can be arranged on substantially the same plane. 
     Meanwhile, the speaker apparatus  1  according to the present invention may be provided with a magnetic circuit  37  configured as shown in FIGS. 6A and 6B. 
     This magnetic circuit  37  includes a ring-shaped center plate  38  of a magnetic material, and a magnet set, namely a first magnet  39  and a second magnet  40 , arranged on both sides of the center plate  23  so that the magnetic poles of repulsive polarities will face each other with the center plate  23  in-between. Each of the first and second magnets  39 ,  40  has its center plate side along the direction of thickness thereof magnetized to an N-pole, while having its opposite side magnetized to an S-pole. 
     On the inner rim of the center plate  38  provided on the magnetic circuit  37  is formed a chamfered portion  41  extending across both corners on both sides in the direction of thickness thereof. That is, the center plate  38  has the width of the inner peripheral surface in a direction parallel to the amplitude direction of the vibrating plate  10  smaller than the width of its outer peripheral surface. Thus, with the center plate  38  of the magnetic circuit  37 , the width of its inner peripheral surface in a direction parallel to the amplitude direction of the vibrating plate  10  is reduced, whereby the magnetic flux of the inner rim acting on the voice coil  21  for vibrating the vibrating system for the high sound range  5  not in need of the larger amplitude is concentrated at a mid portion in the thickness direction. It is noted that the inner rim of the center plate  38  need not only be chamfered since any other suitable configuration, such as arcuate cross-section, may be used if it permits concentration of the magnetic flux of the inner rim. 
     With the above-described speaker apparatus  1 , in which the magnetic circuit  7  includes a ring-shaped center plate  23  and the first and second magnets  24 ,  25 , arranged on both sides of the center plate  23  with the magnetic poles of repulsive polarities facing each other, the magnetic fields are formed on the inner and outer rims of the center plate  23 , so that the vibrating system for the high sound range and the vibrating system for the mid to low sound range can be arranged on the same plane with respect to the playback sound pressure direction. Thus, the present speaker apparatus  1  represents a coaxial two-way type speaker apparatus in which the sound source positions of the playback sound pressure of the high sound range accurately coincides with that of the playback sound pressure of the low sound range. 
     Also, with the speaker apparatus  1  according to the present invention, plural magnetic circuits are not necessitated, thus correspondingly simplifying the structure, so that the laborious assembling operation of combining two magnetic circuits as in the case of the above-mentioned conventional speaker apparatus  101 ,  102  is eliminated, thus improving the ease in assembling. 
     Although the above-described speaker apparatus is configured so that its vibrating system for the high sound range  5  has the cone-shaped vibrating plate  10 . Alternatively, the vibrating system for the high sound range may be provided with a dome-shaped vibrating plate  46 . In a speaker apparatus  2 , shown in FIG. 7, the parts other than a vibrating system for the high sound range  45  are similar to those of the above-described speaker apparatus  1  and hence depicted by the same reference numerals. Therefore, detailed description therefor is not made for simplicity. 
     Referring to FIG. 7, the vibrating system for the high sound range  45  provided on the speaker apparatus  2  is supported on the magnet  24  of the magnetic circuit  7 , and includes a dome-shaped vibrating plate  46 , a cylindrically-shaped bobbin  47 , carrying one end of the vibrating plate  46 , and a resilient edge  48  connected to the outer rim of the vibrating plate  46 . On the outer periphery of the bobbin  47  is placed the voice coil  21  of the magnetic circuit  7  so that the bobbin  47  is arranged on the inner rim of the magnetic circuit  7 . 
     In the above-described vibrating system for the high sound range  45 , the current is fed to the voice coil  21  of the magnetic circuit  7 , whereby the voice coil  21  is vibrated in keeping with the vibrations of the voice coil  21  to generate the acoustic sound. 
     In the above-described speaker apparatus  1 ,  2 , the so-called cone-shaped vibrating plates  10 ,  15  are used for the vibrating system for the high sound range  5  and the vibrating system for the mid to low sound range  6 , respectively. Another speaker apparatus  3  having a substantially planar vibrating plate is hereinafter explained. 
     This speaker apparatus  3  has a vibrating system for the high sound range  55 , a vibrating system for the mid to low sound range  56  and a magnetic circuit  57  for driving the vibrating system for the high sound range  55  and the vibrating system for the mid to low sound range  56 . 
     Referring to FIG. 8, the speaker apparatus  3  includes a frame  58  having the vibrating system for the high sound range  55 , the vibrating system for the mid to low sound range  56  and the magnetic circuit  57 . The vibrating system for the high sound range  55  includes a disc-shaped vibrating plate  60  and a resilient supporting member  62  for movably supporting the outer rim of the vibrating plate  60 . 
     The vibrating plate  60  is formed to a desired thickness by a lightweight flat plate of a honeycomb structure or a foamed structure having voids in the inside or in the surface, such as a foamed mica structure. This vibrating plate  60  is arranged on the inner rim side of the magnetic circuit  57 . The resilient supporting member  62  is formed of an elastic material to a substantially ring shape, as shown in FIG.  8 . The resilient supporting member  62  has its inner rim portion attached to the outer rim of the vibrating plate  60 , while having its outer rim attached to the inner rim of the magnetic circuit  57 . 
     Referring to FIG. 8, the vibrating system for the mid to low sound range  56  includes a ring-shaped vibrating plate  65 , having a center through-hole, a set of substantially ring-shaped resilient supporting members  66 ,  67  for movably supporting the outer rim of the vibrating plate  65 , and a cap  63  for movably supporting the inner rim of the vibrating plate  65 . The vibrating plate  65  is formed by a lightweight planar sheet member of a desired thickness having a honeycomb structure or a foamed structure presenting voids in the interior or in its surface, such as a foamed mica structure. Within the through-hole of the vibrating plate  65  is mounted a magnetic circuit  57 . The resilient supporting members  66 ,  67  are formed of an elastic material in concentric corrugated or rolled configurations, as shown in FIG.  8 . These resilient supporting members  66 ,  67  are provided line-symmetrically and parallel to each other with respect to the centerline along the direction of thickness of the vibrating plate  65 . Each of the resilient supporting members  66 ,  67  has its one end attached to both ends along the direction of thickness of the outer rim portions of the vibrating plate  65 , while having its opposite end mounted on a ring-shaped edge ring  76  provided on the frame  58 . The resilient supporting members  66 ,  67  support the vibrating plate  65  at a pre-set spacing from each other along the direction of thickness of the vibrating plate  65  to hold the voice coil provided in the through-hole of the vibrating plate  65  as later explained at an optimum position in the magnetic field, thus suppressing the rolling otherwise produced during large-amplitude vibrations of the vibrating plate  65 . The cap  63  has an inner rim and an outer rim and is attached at its outer rim to the inner rim of the vibrating plate  65 . Therefore, the cap  63  supports the inner rim of the vibrating plate  65  for movement along the amplitude direction. 
     Referring to FIG. 8, the magnetic circuit  57  is arranged in the through-hole of the vibrating plate  65  of the vibrating system for the mid to low sound range  56 , and includes first and second voice coils  71 ,  72  for setting the vibrating plates  60 ,  65  of the vibrating system for the high sound range  55  and the vibrating system for the mid to low sound range  56 , respectively, into vibrations, a center plate  73  constituting a magnetic path, and a set of magnets  74 ,  75  for according the magnetic flux to the center plate  73 . The first voice coil  71  is mounted on the outer peripheral surface of the vibrating plate  60  of the vibrating system for the high sound range  55  so that its centerline in the winding width direction is coincident with the centerline in the direction of thickness of the vibrating plate  60 . The winding width of the first voice coil  71  is selected to be not larger than the thickness of the center plate  73 . The second voice coil  72  is mounted on the inner peripheral surface of the through-hole of the vibrating plate  65  ofthe vibrating system for the mid to low sound range  56  so that its centerline in the winding width direction is coincident with the centerline in the direction of thickness of the vibrating plate  65 . The winding width of the second voice coil  72  is selected to be not larger than the thickness of the center plate  73 . The center plate  73  is formed in a disc shape from a soft magnetic material, such as hot forged steel plate. As the magnets  74 ,  75 , anisotropic rare earth sintered magnets, such as neodymium magnets, are used, and are formed to a disc shape having an outer diameter slightly smaller than the outer diameter of the center plate  73 . 
     The first and second magnets  74 ,  75  are arranged on both sides of the center plate  73  so that the magnetic poles of repulsive polarities face each other with the center plate  73  in-between, with both ends of the center plate  73  being protruded from the outer rims of the magnets, as shown in FIG.  8 . Although not shown, the center plate sides of the magnets  74 ,  75  are magnetized to an N-pole, with the opposite sides thereof being magnetized to an S-pole. 
     The magnetic circuit  57 , constructed as described above, is secured by having its inner rim attached to the outer rim of the resilient supporting member  62  of the vibrating system for the high sound range  55 , as shown in FIG.  8 . On the outer and inner rims of the magnetic circuit  57 , the second voice coil  72  for vibrating the vibrating system for the mid to low sound range  56  and the first voice coil  71  for vibrating the vibrating system for the high sound range  55  are arranged, respectively. 
     The frame  58  is formed of a metallic material to a substantially bottomed cylinder shape, as shown in FIG.  8 . On the front end side of the frame  58  is formed a holder  79  for holding the vibrating system for the mid to low sound range  56 . To this holder  79  is secured the outer rims of the resilient supporting members  66 ,  67  of the vibrating system for the mid to low sound range  56  via a ring-shaped edge ring  76 . On the outer rim of the frame  58  is mounted a connection terminal connected via a braided wire, not shown, to the first and second voice coils  71 ,  72 . To this connection terminal is connected a connection line to an external power source, not shown. 
     The speaker apparatus  3  includes a resilient supporting member  62  operating for prohibiting air in an enclosure from leaking via an interstice between the outer rim of the vibrating plate  60  and the inner rim of the magnetic circuit  57 . The cap  63  also operates for prohibiting air in the enclosure from leaking out via an interstice between the through-hole n the vibrating plate  65  and the outer rim of the magnetic circuit  57 . 
     With the above-described speaker apparatus  3 , if the driving current proportionate to the speech signals is fed to the first and second voice coils  71 ,  72  of the magnetic circuit  57 , the first and second voice coils  71 ,  72  are set into vibrations, in accordance with the Flemings left hand rule. The vibrating plates  60 ,  65  of the vibrating system for the high sound range  55  and the vibrating system for the mid to low sound range  56  are vibrated in keeping with the vibrations of the first and second voice coils  71 ,  72  to produce the acoustic sound. With the speaker apparatus  3 , having the flat plate shaped vibrating plates  60 ,  65 , the thicknesses of the vibrating system for the high sound range  55  and the vibrating system for the mid to low sound range  56  can be reduced further to enable the apparatus in its entirety to be reduced in thickness. 
     In the above-described respective speaker apparatus, the vibrating system for the high sound range and the vibrating system for the mid to low sound range are arranged on the inner and outer rim sides thereof, respectively. It is however possible to arrange the vibrating system for the high sound range and the vibrating system for the mid to low sound range in a reverse relation to each other. 
     Also, if the vibrating system for the high sound range and the vibrating system for the mid to low sound range are designed to reproduce different reproducing frequency ranges partially overlapped with each other, it is possible to reproduce the sound of high sound quality over a wide frequency range from the low to high ranges without fluctuations in the sound pressure level. 
     The present invention can be modified in a wide gamut without departing from the purport of the invention.