Patent Publication Number: US-9894443-B2

Title: Loudspeaker and mobile device incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national stage application of PCT International Application No. PCT/JP2015/004193 filed on Aug. 21, 2015, which claims the benefit of foreign priority of Japanese patent application 2014-183938 filed on Sep. 10, 2014, the contents all of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an electrodynamic loudspeaker used in audio systems and to a mobile device equipped with the loudspeaker. 
     BACKGROUND ART 
     A conventional loudspeaker includes a frame, a diaphragm, a voice coil body, and a magnetic circuit. The diaphragm is connected at its outer peripheral end to the frame via an edge. The magnetic circuit has a magnetic gap and is connected to the frame. The voice coil body includes a bobbin and a voice coil wound near a first end of the bobbin. The bobbin is connected at one end to the diaphragm. The voice coil is located in the magnetic gap. 
     An example of a conventional technique related to the invention of the present application is shown in Patent Literature 1. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2011-35812 
     SUMMARY OF THE INVENTION 
     The loudspeaker includes a frame, a diaphragm, an edge, a magnetic circuit, and a voice coil body. 
     The frame has a cylindrical internal space. 
     The diaphragm is located in the internal space of the frame. 
     The edge connects the outer peripheral end of the diaphragm and the frame. 
     The magnetic circuit includes a magnet and a bottom plate. 
     The bottom plate includes an installation part and a plurality of arms. 
     The installation part is located on the first side of the magnet and is magnetically connected to the magnet. 
     The arms project from the installation part toward the outer periphery and are connected to the frame. 
     The magnetic circuit is located in the internal space of the frame and has a magnetic gap. 
     The voice coil body includes a cylindrical bobbin and a voice coil. 
     The voice coil is wound around at least part of the bobbin. 
     The voice coil body is located in the center of the diaphragm so that the voice coil can be located in the magnetic gap. 
     The mobile device of the present disclosure includes a body, a drive part mounted in the body, an amplifier mounted in the body, and the loudspeaker of the present disclosure receiving an output of the amplifier. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a loudspeaker according to a present exemplary embodiment. 
         FIG. 2  is a front view of the loudspeaker according to the present exemplary embodiment. 
         FIG. 3  is a sectional view taken along line  3 - 3  of  FIG. 2 . 
         FIG. 4  is a rear view of the loudspeaker according to the present exemplary embodiment. 
         FIG. 5  is a perspective view of a frame of the loudspeaker according to the present exemplary embodiment. 
         FIG. 6  is an enlarged sectional view of an essential part of a magnetic gap in the loudspeaker according to the present exemplary embodiment. 
         FIG. 7  is a sectional view of a bonding area where a bottom plate and a yoke are bonded together in the loudspeaker according to the present exemplary embodiment. 
         FIG. 8  is an enlarged view of a to-be-engaged portion of a frame in the loudspeaker according to the present exemplary embodiment. 
         FIG. 9  is an enlarged view of fins formed on arms in the loudspeaker according to the present exemplary embodiment. 
         FIG. 10  is an enlarged view of fins formed on an installation part in the loudspeaker according to the present exemplary embodiment. 
         FIG. 11  is a sectional view of another loudspeaker according to the present exemplary embodiment. 
         FIG. 12  is a sectional view of still another loudspeaker according to the present exemplary embodiment. 
         FIG. 13  is a rear view of the still another loudspeaker according to the present exemplary embodiment. 
         FIG. 14  is a conceptual view of a mobile device according to the present exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Loudspeakers require a diaphragm and a magnetic circuit located close to each other in order to have a thin profile. Some loudspeakers include two edges instead of a damper in order to support the diaphragm to the frame. Unless the frame and the magnetic circuit have appropriate shapes and are connected properly, loudspeakers have a thick profile. 
     Such loudspeakers are demanded to have a frame made of resin in terms of productivity and cost. The frame of the loudspeakers, however, has many bent portions and therefore requires an opening for releasing the sound pressure (back pressure) generated on the rear side of the diaphragm. As a result, loudspeakers have a complicated shape, making it sometimes difficult to have a resin frame in terms of productivity and strength. 
     Loudspeakers come in various shapes depending on the application. For example, in-car loudspeakers need to have a thin profile because they are placed in small spaces such as a dashboard, a door, or a roof. Thin loudspeaker  51  according to the present exemplary embodiment will now be described with reference to drawings.  FIG. 1  is a perspective view of loudspeaker  51 .  FIG. 2  is a front view of loudspeaker  51 .  FIG. 3  is a sectional view taken along line  3 - 3  of  FIG. 2 .  FIG. 4  is a rear view of loudspeaker  51 . The term “upward direction” represents the direction from magnetic circuit  61  toward diaphragm  53 , and the term “downward direction” represents the direction from diaphragm  53  toward magnetic circuit  61  in  FIG. 3 . In  FIG. 3 , the upward direction is toward the front side of diaphragm  53 , and the downward direction is toward the rear side of diaphragm  53 . In  FIG. 4 , arms  91  are composed of arms  91 A- 91 D. 
     Loudspeaker  51  includes frame  52 , diaphragm  53 , edge  54 , magnetic circuit  61 , and voice coil body  55 . 
     Frame  52  has cylindrical internal space  52 D. 
     Diaphragm  53  is located in internal space  52 D of frame  52 . 
     Edge  54  connects the outer peripheral end of diaphragm  53  and frame  52 . 
     Magnetic circuit  61  includes magnet  62  and bottom plate  64 . 
     Bottom plate  64  includes installation part  64 A and arms  91 . 
     Installation part  64 A is located on the first side of magnet  62  and is magnetically connected to magnet  62 . 
     Arms  91  project from installation part  64 A toward the outer periphery and are connected to frame  52 . 
     Magnetic circuit  61  is located in internal space  52 D of frame  52  and has magnetic gap  56 . 
     Voice coil body  55  includes cylindrical bobbin  55 A and voice coil  55 B. 
     Voice coil  55 B is wound around at least part of bobbin  55 A. 
     Voice coil body  55  is located in the center (thin part  53 A) of diaphragm  53 , so that voice coil  55 B can be located in magnetic gap  56 . 
     The above-described configuration allows loudspeaker  51  to have a thin profile. 
     Magnetic circuit  61  and frame  52  are connected together via arms  91  and therefore, each two adjacent arms  91  are separated by an opening. As a result, frame  52  does not have to have an opening for releasing the sound pressure generated on the rear side of diaphragm  53 , allowing frame  52  to have a simple shape. The sound pressure from the rear side of diaphragm  53  allows air to pass through the openings, thereby improving the effect of air-cooling magnetic circuit  61 . 
     Loudspeaker  51  will now be described in more detail.  FIG. 5  is a perspective view of frame  52  of loudspeaker  51 . As shown in  FIG. 5 , frame  52  includes base  52 A and attachment part  52 B. Base  52 A is a hollow cylinder, and in other words, has cylindrical internal space  52 D where diaphragm  53  and magnetic circuit  61  are located. Base  52 A is preferably cylindrical in the case that diaphragm  53  is circular when seen from the front. Frame  52  does not have to include attachment part  52 B, but preferably includes it for the following reason. Attachment part  52 B allows loudspeaker  51  to be attached to a baffle board (not shown) or other device. If attachment part  52 B has screw holes, loudspeaker  51  can be screwed to the baffle board. 
     Loudspeaker  51  preferably includes terminal  57 . The term “terminal  57 ” is used to collectively refer to positive terminal  57 A and negative terminal  57 B. Terminal  57  is fixed to the lateral side of base  52 A of frame  52 . 
     As shown in  FIG. 3 , diaphragm  53  has thin part  53 A and thick part  53 B formed around thin part  53 A. Thin part  53 A and thick part  53 B define recess  41 . Thick part  53 B is formed on the outer periphery of diaphragm  53 , and recess  41  is formed in the center of the rear side of diaphragm  53 . The upper surface of magnetic circuit  61  is located facing thin part  53 A. 
     Thin part  53 A is thinner than thick part  53 B, and the upper surface of magnetic circuit  61  is located facing thin part  53 A. As a result, the distance can be small between the upper surface of magnetic circuit  61  and the front side of diaphragm  53 , allowing the loudspeaker to have a thin profile. Furthermore, thick part  53 B formed on the outer periphery of diaphragm  53  increases the strength of diaphragm  53 . 
       FIG. 6  is an enlarged sectional view of an essential part of magnetic gap  56  of loudspeaker  51 . Voice coil body  55  includes cylindrical bobbin  55 A and voice coil  55 B wound around at least part of bobbin  55 A. The term “cylindrical” here includes rectangular-column shaped. One end of bobbin  55 A, or in other words, the first end (front side part) of bobbin  55 A, is connected to thin part  53 A of recess  41 . Voice coil  55 B is wound near the second end (rear side part) of bobbin  55 A and is located in magnetic gap  56 . When a drive signal is supplied to voice coil  55 B, diaphragm  53  vibrates according to the direction and magnitude of the drive signal. Diaphragm  53  is driven in the direction of the winding axis of voice coil  55 B (vertical direction in  FIG. 3 ) via bobbin  55 A. 
     The term “edge  54 ” is used to collectively refer to first edge  54 A and second edge  54 B. First edge  54 A and second edge  54 B connect the outer peripheral end of diaphragm  53  and frame  52 . Second edge  54 B is preferably opposite to first edge  54 A in the thickness direction of thick part  53 B. 
     Terminal  57  is located between first edge  54 A and second edge  54 B in the thickness direction of diaphragm  53  and is connected to connecting wire  58  (tinsel wire). Connecting wire  58  passes through a hole formed in thick part  53 B and is led out to the rear side of thin part  53 A of diaphragm  53 . The start and end points of the wire of voice coil  55 B are connected to connecting wire  58  on bobbin  55 A. The drive signal to drive loudspeaker  51  is supplied to voice coil  55 B through terminal  57  via connecting wire  58 . 
     Frame  52  preferably has affixing part  52 C. In this case, the outer peripheries of first and second edges  54 A and  54 B are affixed to affixing part  52 C. To achieve this, affixing part  52 C is formed near the end of the front side of base  52 A. Affixing part  52 C is formed on the inner peripheral surface of base  52 A in such a manner as to project toward the center of base  52 A. Frame  52  is preferably made of synthetic resin. This configuration allows attachment part  52 B and affixing part  52 C to be integrally formed with base  52 A, thereby having a high productivity of frame  52 . 
     Affixing part  52 C preferably has an upper surface to which the outer periphery of first edge  54 A is affixed and a lower surface to which second edge  54 B is affixed. As a result, first and second edges  54 A and  54 B can be properly connected to frame  52  and have a proper spacing between them, thereby reducing the distortion of loudspeaker  51 . 
     Affixing part  52 C is not limited the structure to affix both of first edge  54 A and second edge  54 B to the lateral side of diaphragm  53 . Alternatively, affixing part  52 C may be used to affix either one of first edge  54 A and second edge  54 B to the lateral side of diaphragm  53 . Further alternatively, either one of edges  54 A and  54 B may be directly affixed to the inner peripheral surface of base  52 A. 
     The outer peripheral end of diaphragm  53  is connected to frame  52  via first and second edges  54 A and  54 B. The direction of the vibration (amplitude) of diaphragm  53  (vertical direction in  FIG. 3 ) is perpendicular to the direction of the magnetic flux in magnetic gap  56 . Thus, diaphragm  53  is displaced in the direction of the winding axis of voice coil  55 B (vertical direction in  FIG. 3 ) by first and second edges  54 A and  54 B. 
     First edge  54 A is located on the front side of diaphragm  53  with respect to the center in the thickness direction of diaphragm  53 , and second edge  54 B is located on the rear side of diaphragm  53  with respect to the center in the thickness direction of diaphragm  53 . 
     First edge  54 A and second edge  54 B are preferably symmetrical with respect to the plane perpendicular to the direction of the winding axis of voice coil  55 B. This configuration allows diaphragm  53  to generate less rolling action, and to cause less distortion due to the asymmetry of the amplitude of diaphragm  53 . It is further preferable that first edge  54 A and second edge  54 B be symmetrical with respect to the center in the thickness direction of diaphragm  53 . 
     It is also preferable to connect first edge  54 A with the front side of diaphragm  53  and to connect second edge  54 B with the rear side of thick part  53 B. In this case, joint  72  is formed parallel with the front or rear side of diaphragm  53 . This configuration can provide a large distance between first edge  54 A and second edge  54 B, making diaphragm  53  have much less rolling action. Joint  72  joins diaphragm  53  and edge  54  (see  FIG. 2 ). 
     First edge  54 A and diaphragm  53  may alternatively be connected together at a location away from the front-side end on the lateral side of diaphragm  53  toward the rear side, allowing loudspeaker  51  shown in  FIG. 3  to have a thinner profile. 
     Diaphragm  53  is preferably made of foamed resin so as to allow diaphragm  53  to be lightweight. The front side of diaphragm  53  is preferably flat. 
     The rear side of thin part  53 A is flat, and in thin part  53 A, diaphragm  53  has a uniform thickness. 
     It is preferable that the boundary portion between thin part  53 A and thick part  53 B gradually increase in thickness toward the outer periphery of diaphragm  53 . More specifically, it is preferable to form inclined surface  53 D gradually increasing in thickness from thin part  53 A toward thick part  53 B so as to enhance the strength of diaphragm  53 . 
     Diaphragm  53  is preferably composed of core material made of foamed resin and a reinforcement layer formed on at least one of the front and rear sides of the core material. The reinforcement layer can be made of carbon or metal like aluminum or titanium. 
     Furthermore, the upper surface of magnetic circuit  61  is preferably located closer to the front side of diaphragm  53  than to the rear side of thick part  53 B as shown in  FIG. 3 . In short, it is preferable that part of the magnetic circuit be located in thin part  53 A so as to allow loudspeaker  51  to have a thin profile. 
     It is preferable to form tapered part  65 A on the lateral side of magnetic circuit  61 . In this case, inclined surface  53 D is preferably located facing tapered part  65 A so as to have the distance large enough to displace the amplitude of diaphragm  53  in the axial direction of the voice coil between diaphragm  53  and magnetic circuit  61 . This allows the upper surface of magnetic circuit  61  and the front side of diaphragm  53  to be located close to each other. 
     Magnetic circuit  61  is preferably of internal magnet type so as to allow thin part  53 A to have a small area, and hence, diaphragm  53  to have high strength. Magnetic circuit  61  of internal magnet type includes magnet  62 , top plate  63 , bottom plate  64 , and yoke  65 . Magnet  62  is columnar and is magnetized in the vertical thickness direction. Top plate  63  is planar and is placed on the upper surface of magnet  62 . Bottom plate  64  has installation part  64 A and arms  91 . Magnet  62  is placed on the upper surface of installation part  64 A. Bottom plate  64  is magnetically connected to magnet  62 . Yoke  65  extends upward from the outer peripheral end of the upper side of installation part  64 A, and is magnetically connected to magnet  62  via bottom plate  64 . Top plate  63 , installation part  64 A, and yoke  65  are made of magnetic material, such as hot-rolled steel plates (for example, SPHC and SPHD). 
       FIG. 7  is a sectional view of a bonding area where bottom plate  64  and yoke  65  are bonded together in loudspeaker  51 . It is preferable that bottom plate  64  and yoke  65  be bonded together with an adhesive. In addition, as shown in  FIG. 7 , it is possible to form recess  66  for holding the adhesive on the bonding surface between bottom plate  64  and yoke  65 . Recess  66  is only required to be formed on at least one of bottom plate  64  and yoke  65 . Recess  66  is composed of, for example, grooves of V-shaped cross section and can be formed by knurling the bonding surface of at least one of bottom plate  64  and yoke  65 . The adhesive held in recess  66  allows bottom plate  64  and yoke  65  to be tightly bonded together. 
     Magnet  62  magnetizes top plate  63  and yoke  65  with opposite polarities. The inner peripheral surface of yoke  65  is located facing the outer peripheral surface of top plate  63  so as to form magnetic gap  56  therebetween as shown in  FIG. 3 . In the case that top plate  63  is magnetized with the north pole and that yoke  65  is magnetized with the south pole, the magnetic flux in magnetic gap  56  flows from top plate  63  to yoke  65 . 
     It is preferable to form tapered part  65 A on the outer peripheral surface of yoke  65 . In other words, yoke  65  is preferably tapered from bottom plate  64  toward its tip. As a result, tapered part  65 A, which tapers from the lower surface toward the upper surface, is formed on the outer peripheral surface of magnetic circuit  61 . Tapered part  65 A may be linear, curved (such as arc-shaped), step-like, or of a combination of them. 
     Inclined surface  53 D is thicker than thin part  53 A, so that diaphragm  53  can be stronger and less deformed in the region having inclined surface  53 D. This prevents inclined surface  53 D from colliding with tapered part  65 A regardless of the small distance between them. As a result, diaphragm  53  can be made thick in inclined surface  53 D, and hence improved in strength. 
     Magnetic circuit  61  is preferably circular when seen from the front, and in this case, magnet  62  is preferably cylindrical. Top plate  63  is preferably disc-shaped, and in this case, diaphragm  53  is preferably either circular or oval when seen from the front. 
     Instead of being circular, magnetic circuit  61  may be oval or rectangular when seen from the front. In the case that magnetic circuit  61  is oval, bobbin  55 A and top plate  63  are preferably oval when seen from the front, and diaphragm  53  is preferably either oval or rectangular when seen from the front. Meanwhile, in the case that magnetic circuit  61  is rectangular, bobbin  55 A and diaphragm  53  are preferably rectangular when seen from the front. 
     Magnet  62  is preferably formed of a neodymium magnet because of its high energy product. This achieves a desired magnetic flux density in magnetic gap  56 , allowing magnet  62  and hence the loudspeaker to have a thin profile. 
     Arms  91  will now be described in detail. As shown in  FIG. 4 , bottom plate  64  includes installation part  64 A and a plurality of arms  91 . Arms  91  extend from installation part  64 A toward the outer periphery and are connected to frame  52 . With this configuration, magnetic circuit  61  is connected to frame  52  via arms  91 . 
     As shown in  FIG. 4 , arms  91  preferably have engaging portions  91 E at their respective tips. Engaging portions  91 E can be, for example, projections such as bumps.  FIG. 8  is an enlarged view of to-be-engaged portion  52 E of frame  52  in loudspeaker  51 . As shown in  FIG. 8 , it is preferable that base  52 A of frame  52  have to-be-engaged portions  52 E, which are recesses such as grooves or cutouts, for example. Engaging portions  91 E of arms  91  are engaged with to-be-engaged portions  52 E of frame  52 . 
     Engaging portions  91 E shown in  FIG. 4  project from the tips of arms  91  and have a smaller width than arms  91  have. Meanwhile, as shown in  FIG. 5 , to-be-engaged portions  52 E are holes formed at the bottom on the outer side (the side far from diaphragm  53 ) of frame  52 . As shown in  FIG. 8 , each to-be-engaged portion  52 E includes insertion slot  52 F and locking portion  52 G. Insertion slots  52 F are wider than engaging portions  91 E shown in  FIG. 4 . Locking portions  52 G are formed contiguous with and inside insertion slots  52 F. Locking portions  52 G are wider than insertion slots  52 F in the circumferential direction of base  52 A. 
     Engaging portions  91 E shown in  FIG. 4  are inserted into locking portions  52 G through insertion slots  52 F of to-be-engaged portions  52 E shown in  FIG. 8 . More specifically, engaging portions  91 E of all arms  91  shown in  FIG. 4  are inserted into locking portions  52 G of the respective to-be-engaged portions  52 E. After this, frame  52  is rotated along the circumference of base  52 A with respect to magnetic circuit  61  so that engaging portions  91 E are engaged with the respective to-be-engaged portions  52 E. After being engaged with each other, engaging portions  91 E (see  FIG. 4 ) and to-be-engaged portions  52 E (see  FIGS. 5 and 8 ) are preferably bonded together with an adhesive so as to prevent resonance. Thus, magnetic circuit  61  is connected to frame  52  via arms  91 . 
     In  FIG. 4 , bottom plate  64  includes four arms  91 A- 91 D extending in four directions perpendicular to each other. Each two adjacent arms  91  are separated by opening  92 . Openings  92  allow diaphragm  53  to vibrate without being disturbed by the sound pressure (back pressure) from its rear side. Openings  92  also prevent the magnetic flux in magnetic circuit  61  from leaking to arms  91 , thereby increasing the magnetic flux density in magnetic gap  56 . 
     Installation part  64 A and arms  91  are preferably integrally formed. This configuration makes it unnecessary to use a member for fixing magnetic circuit  61  to frame  52 , thereby making bottom plate  64  thin and low cost. Alternatively installation part  64 A and arms  91  may be formed separately and combined together. In this case, it is preferable that installation part  64 A be made of magnetic material, and that arms  91  be made of non-magnetic material. This configuration reduces magnetic field leakage to arms  91  and increases the magnetic flux density in magnetic gap  56 . 
       FIG. 9  is an enlarged view of fins  93  formed on arms  91  in loudspeaker  51 . As shown in  FIG. 9 , it is possible to provide fins  93  on the outer periphery of arms  91  so as to increase the surface area of arms  91 , thereby efficiently cooling magnetic circuit  61 . 
       FIG. 10  is an enlarged view of fins  94  formed on installation part  64 A in loudspeaker  51 . As shown in  FIG. 10 , bottom plate  64  may have fins  94  on the outer periphery of installation part  64 A. Fins  94  increase the surface area of bottom plate  64 , thereby efficiently cooling magnetic circuit  61 . 
     Bottom plate  64  can be a flat metal plate, so that fins  93  and  94  can be formed integrally with bottom plate  64  by sheet metal working, thereby having a high productivity of loudspeaker  51 . 
     Base  52 A of frame  52  is formed in cylindrical shape. Base  52 A has a simple shape because of not having bent portions or an opening for releasing the back pressure of the diaphragm. Thus, frame  52  of the present embodiment can have a simple shape. 
     Instead of being of the internal magnet type, magnetic circuit  61  may be of external magnet type or a combination of the internal and external magnet types.  FIG. 11  is a sectional view of another loudspeaker  251  according to the present exemplary embodiment. In loudspeaker  251 , the same components as in loudspeaker  51  are denoted by the same reference numerals, and the description thereof will be omitted. 
     Loudspeaker  251  includes magnetic circuit  81  of external magnet type instead of magnetic circuit  61  of internal magnet type. Magnetic circuit  81  includes ring-shaped magnet  82 , ring-shaped top plate  83 , and bottom plate  84 . Bottom plate  84  includes installation part  84 A and center pole  84 B. Center pole  84 B extends upward from the center of installation part  84 A. Magnet  82  is placed on the upper surface of installation part  84 A, which is located outside center pole  84 B. Top plate  83  is placed on the upper surface of magnet  82 . The outer peripheral surface of center pole  84 B is located facing the inner peripheral surface of top plate  83 , and magnetic gap  56  is formed therebetween. 
     In the case that magnetic circuit  81  of external magnet type is used, center pole  84 B is used as the yoke located facing top plate  83 . 
     Thus, magnetic circuit  81  is located facing thin part  53 A, so that loudspeaker  251  can have a thin profile even in the case that magnetic circuit  81  is of external magnet type. 
     It is preferable that tapered part  65 A be formed on the outer peripheral surface of top plate  83  and that tapered part  65 A be located facing inclined surface  53 D of recess  41 . Magnet  82  may be a bond magnet so as to be formed into a variety of shapes. Tapered part  65 A is also preferably formed on the outer peripheral surface of magnet  82 . Tapered part  65 A and inclined surface  53 D are preferably located facing each other. 
       FIG. 12  is a sectional view of still another loudspeaker  151  according to the present exemplary embodiment.  FIG. 13  is a rear view of yet another loudspeaker  151  according to the present exemplary embodiment. In loudspeaker  151 , the same components as in loudspeakers  51  and  251  are denoted by the same reference numerals, and the description thereof will be omitted. Loudspeaker  151  includes magnetic circuit  161  and pedestal  152  instead of magnetic circuit  61  of loudspeaker  51 . Pedestal  152  includes mounting part  152 A and arms  153 . 
     Magnetic circuit  161  includes magnet  62 , top plate  63 , and bottom plate  164 . Bottom plate  164 , which is a cylinder with a bottom, includes installation part  64 A and yoke  65 . Installation part  64 A and yoke  65  are preferably integrally formed. 
     Mounting part  152 A is formed in the center of pedestal  152 . Magnetic circuit  161  is placed on the upper surface of mounting part  152 A and is fixed to pedestal  152 . The fixation may be achieved by screwing, bonding, or other methods. 
     The magnetic saturation of bottom plate  164  is likely to occur under the outer peripheral surface of magnet  62 . For this reason, mounting part  152 A is preferably made of magnetic material, such as hot-rolled steel plates (for example, SPHC and SPHD). As a result, mounting part  152 A functions as a magnetic path in magnetic circuit  161 . With this configuration, the magnetoresistance is small in the region under the outer peripheral surface of magnet  62 , thereby achieving a low magnetic saturation of magnetic circuit  161  and a large magnetic flux density in magnetic gap  56 . 
     Pedestal  152  includes arms  153  extending from mounting part  152 A toward the outer periphery. The tips of arms  153  are fixed to frame  52  and, consequently, magnetic circuit  161  is fixed to frame  52 . Mounting part  152 A and arms  153  are preferably integrally formed, but may alternatively be formed separately and combined together. In this case, arms  153  are preferably made of non-magnetic material so as to prevent the magnetic flux in magnetic circuit  161  from leaking to arms  153 . 
     Pedestal  152  preferably includes a plurality of arms  153  arranged at regular intervals.  FIG. 13  shows six arms  153 A- 153 F extending radially from mounting part  152 A toward the outer periphery. Arms  153 A- 153 F are preferably arranged at 60-degree intervals around the center of mounting part  152 A. When magnetic circuit  161  is connected to frame  52  by arms  153 , each two adjacent arms  153  are separated by opening  92 . Openings  92  allow diaphragm  53  to vibrate without being disturbed by the sound pressure (back pressure) from its rear side. 
     Similar to arms  91 , arms  153  are preferably provided at their tips with engaging portions  91 E to be engaged with frame  52 . Frame  52  preferably has to-be-engaged portions  52 E to be engaged with engaging portions  91 E. Engaging portions  91 E can be, for example, projections such as bump. Note here that to-be-engaged portions  52 E can be, for example, recesses such as grooves and cutouts. The projections can be fitted into the recesses so that pedestal  152  can be fixed to frame  52 . 
     In addition, as shown in  FIG. 9 , arms  153  may have fins  93  on the outer periphery so as to increase at least one of the surface area and volume of arms  153 , thereby efficiently cooling magnetic circuit  161 . 
     Pedestal  152  can be a flat metal plate, so that fins  93  can be formed integrally with pedestal  152  by sheet metal working, thereby having a high productivity of the loudspeaker. Moreover, as shown in  FIG. 10 , installation part  64 A may have fins  94  on its outer periphery. 
       FIG. 14  is a conceptual view of mobile device  201  according to the present exemplary embodiment. Mobile device  201  is equipped with loudspeaker  500 . Loudspeaker  500  represents one of loudspeakers  51 ,  151 , and  251  according to the present exemplary embodiment. In  FIG. 14 , mobile device  201  is an automobile, but may be other vehicles such as ships, airplanes, trains, and motorcycles. 
     Mobile device  201  includes body  202 , drive part  203 , amplifier  211 , and loudspeaker  500 . Drive part  203 , amplifier  211 , and loudspeaker  500  are mounted in body  202 . Drive part  203  may include an engine, a motor, tires, a steering wheel, and other parts. The output of amplifier  211  is supplied to loudspeaker  500 . Amplifier  211  may include part of a car audio system, and additionally include a sound reproducing device. Also, amplifier  211  may include part of a car navigation system, and additionally include a display device. 
     Body  202  includes passenger space  202 A. Loudspeaker  500  is placed to deliver sound to passenger space  202 A. Body  202  may further include exterior part  202 B and interior part  202 C. Exterior part  202 B, which isolates passenger space  202 A from the outside, can be, for example, roof  202 D or door  202 E. Interior part  202 C is located between exterior part  202 B and passenger space  202 A. Loudspeaker  500  is placed between interior part  202 C and exterior part  202 B, but may alternatively be placed in a dashboard, a rear tray (not shown), or other places. 
     Loudspeaker  500  of the present exemplary embodiment has a thin profile and therefore can have a short distance between interior part  202 C and exterior part  202 B. In other words, loudspeaker  500  can be placed even if the distance is short between interior and exterior parts  202 C and  202 B. Thus, the use of loudspeaker  500  as mobile device  201  expands passenger space  202 A. 
     The above-described exemplary embodiment is an example to facilitate the understanding of the present invention and does not limit the invention thereto. The materials and shapes of the components of loudspeaker  500  described in the exemplary embodiment can be variously modified. 
     The present invention, without departing from the spirit thereof, may be modified or improved and may include equivalents thereof. 
     As described above, in the loudspeaker of the present disclosure, the magnetic circuit is located in the internal space of the frame. Furthermore, the plurality of arms is connected to the frame and project toward the outer periphery of the magnetic circuit, thereby allowing the loudspeaker to have a thin profile. In addition, the magnetic circuit is connected to the frame by the plurality of arms, and adjacent arms are separated by an opening. With this configuration, the frame does not need to have an opening for releasing the sound pressure generated on the rear side of the diaphragm, and hence, can have a simple shape. Furthermore, the sound pressure from the rear side of the diaphragm allows air to pass through the openings, thereby improving the effect of air-cooling the magnetic circuit. 
     INDUSTRIAL APPLICABILITY 
     The loudspeaker of the present disclosure has a thin profile and is therefore suitable for use especially in in-car audio systems and home audio systems.