Patent Publication Number: US-2023164491-A1

Title: Speaker

Description:
RELATED APPLICATION 
     The present application claims priority to Japanese Patent Application Number 2021-051732, filed Mar. 25, 2021, the entirety of which is hereby incorporated by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a speaker that is capable of detecting, with a magnetic sensor, the operation of a vibrating unit including a diaphragm and a bobbin. 
     2. Description of the Related Art 
     Speakers for acoustic systems according to the related art are configured only to perform processing which involves simply receiving an audio signal output from an amplifier and reproducing sound pressure. That is, since the speakers are not configured to perform a control operation in accordance with an audio signal, the resulting sound tends to be distorted and the sound quality tends to vary. Additionally, when the amplitude of a diaphragm is excessively large, the diaphragm or a damper may be damaged. 
     As a solution to the problems described above, JP 57-184397 A discloses a speaker system that is configured to perform feedback control by detecting the movement of a diaphragm with a magnetic sensor. 
     The speaker system includes a Hall element serving as a magnetic sensor. At a position opposite a voice coil, the Hall element is supported by a plate constituting a magnetic circuit unit. An effective magnetic flux density inside a gap in the magnetic circuit unit is detected by the Hall element, and the detection signal is amplified and sent as feedback to a power amplifier. When a driving current applied from the power amplifier to the voice coil causes a bobbin to vibrate together with the voice coil, the effective magnetic flux density in the gap is changed by current flowing in the voice coil and counter-electromotive force generated in the voice coil. The change in the effective magnetic flux density is detected by the Hall element and sent as feedback to the power amplifier, so that distortion in the driving current applied to the voice coil is corrected. 
     In the feedback control performed in the speaker system disclosed in JP 57-184397 A, the Hall element smaller than an optical detector element and a coil is used as a detector element. This prevents an excessive increase in the size of the speaker and prevents an increase in power consumption. With the technique in which the Hall element detects a change in the effective magnetic flux density inside the gap in the magnetic circuit unit, however, the movement of the voice coil and the bobbin cannot be directly detected. This makes it difficult to highly precisely correct sound distortion and variation in sound quality. 
     The speaker system disclosed in JP 57-184397 A has a structure in which the Hall element is embedded in a surface of the plate facing the voice coil. The Hall element has a complex installation structure and cannot be assembled efficiently. 
     The present disclosure has been made to solve the problems of the related art described above. An object of the present disclosure is to provide a speaker that is capable of highly precisely detecting vibration of a vibrating unit using a moving magnet and a magnetic sensor, and is structured to allow the moving magnet to be easily attached to a movable unit. 
     SUMMARY 
     A speaker according to an aspect of the present disclosure includes a vibrating unit and a drive supporting unit. The vibrating unit includes a diaphragm supported by a frame, a bobbin configured to vibrate with the diaphragm, and a voice coil disposed on the bobbin. The drive supporting unit includes the frame and a magnetic circuit unit configured to apply a magnetic flux to the voice coil. The speaker includes a detecting unit including a moving magnet and a magnetic sensor. A magnet support made of a non-magnetic material is attached to the bobbin having a circular cylindrical shape. The magnet support extends along at least part of the bobbin in a circumferential direction. The moving magnet is secured to the magnet support, and the magnetic sensor is secured to the drive supporting unit. 
     In the speaker according to the aspect of the present disclosure, the magnet support preferably has an annular shape extending along an entire circumference of the bobbin. 
     In the speaker according to the aspect of the present disclosure, a balancing mass is preferably secured to the magnet support. The balancing mass is configured to eliminate imbalance in mass caused by securing the moving magnet to the magnet support. 
     In the speaker according to the aspect of the present disclosure, the magnet support is attached to extend along a front edge of the bobbin. The front edge is on a side of the bobbin opposite the voice coil. 
     In the speaker according to the aspect of the present disclosure, a phase plug supported by the magnetic circuit unit is provided in a region surrounded by the bobbin. An inner periphery of the magnet support can serve as a positioning portion that determines a relative position with respect to the phase plug. 
     In the speaker according to the aspect of the present disclosure, the magnet support may be secured to an outer periphery of the bobbin in a middle part of the bobbin in a front-back direction. 
     For example, a damper member may be provided between the frame and the bobbin, and the magnet support may be attached to extend along a joint between the bobbin and the damper member. 
     Alternatively, the magnet support may be attached to extend along a joint between the bobbin and the diaphragm. 
     In the speaker according to the aspect of the present disclosure, a direction of a magnetic field applied from the magnetic circuit unit to the magnetic sensor preferably crosses a direction of a magnetic field applied from the moving magnet to the magnetic sensor. A detection output based on a change in a direction of a composite vector of the two magnetic fields is preferably obtained from the magnetic sensor. 
     In the speaker according to the aspect of the present disclosure, the magnetic sensor detects a magnetic flux from the moving magnet disposed on the bobbin, which is part of the vibrating unit. The movement of the vibrating unit can thus be directly detected. This enables highly precise feedback control for correcting the operation of the vibrating unit. The magnetic sensor provides a detection output based on a change in the direction of the composite vector of the magnetic field of a leakage magnetic flux from the magnetic circuit unit and the magnetic field from the moving magnet. The movement of the vibrating unit can thus be detected with high precision, regardless of the intensity of the driving magnetic flux generated from the magnetic circuit unit. 
     The magnet support made of a non-magnetic material and extending along at least part of the circular cylindrical bobbin in the circumferential direction is attached to the bobbin, and the moving magnet is secured to the magnet support. The moving magnet can thus be accurately positioned and secured to the bobbin formed of a thin material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a sectional perspective view of a speaker according to a first embodiment of the present invention taken along a plane parallel to the X-Z plane; 
         FIG.  2    is a half sectional view of the speaker according to the first embodiment taken along a plane parallel to the X-Z plane; 
         FIG.  3    is a half sectional view of a speaker according to a second embodiment of the present invention taken along a plane parallel to the X-Z plane; 
         FIG.  4    is an explanatory diagram illustrating an operation of assembling the speaker according to the second embodiment, the diagram being a partial enlarged half sectional view taken along a plane parallel to the Y-Z plane; 
         FIG.  5 A  and  FIG.  5 B  illustrate a bobbin, a magnet support, and a moving magnet of the speaker according to the first and second embodiments,  FIG.  5 A  being an exploded perspective view as viewed from above and  FIG.  5 B  being an exploded perspective view as viewed from below; 
         FIG.  6    is a half sectional view of a speaker according to a third embodiment of the present invention taken along a plane parallel to the X-Z plane; 
         FIG.  7    is an enlarged sectional view of a part of  FIG.  6   ; 
         FIG.  8    is a partial enlarged sectional view of a speaker according to a fourth embodiment of the present invention taken along a plane parallel to the X-Z plane; and 
         FIG.  9    is an exploded perspective view of a bobbin, a magnet support, and a moving magnet of the speaker according to the third and fourth embodiments, as viewed from above. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG.  1    and  FIG.  2    illustrate a speaker  1  according to a first embodiment of the present invention. In the speaker  1 , a Z 1 -Z 2  direction is a front-back direction, a Z 1  direction is a forward and sound output direction, and a Z 2  direction is a backward direction.  FIG.  1    and  FIG.  2    show a central axis O extending in the front-back direction (Z 1 -Z 2  direction). A main part of the speaker  1  has a substantially rotationally symmetrical structure centered on the central axis O.  FIG.  1    shows an X axis and a Y axis orthogonal to each other in a plane orthogonal to the central axis O. The X axis coincides with the direction of a magnetic field H 1  in a driving magnetic flux F 1  formed by a magnetic circuit unit  10 , and the Y axis coincides with the direction of a magnetic field H 2  in a moving magnetic flux component F 2  formed by a moving magnet  21 . The magnetic field H 1  and the magnetic field H 2  are to be detected by a magnetic sensor  22 . 
     The speaker  1  illustrated in  FIG.  1    and  FIG.  2    includes a frame  2 . The frame  2  is formed of a non-magnetic material or a magnetic material. The frame  2  has a tapered shape with a diameter that gradually increases toward the front (in the Z 1  direction). The magnetic circuit unit  10  is secured to the back (Z 2  side) of the frame  2 , for example, by bonding or with screws. The frame  2  and the magnetic circuit unit  10  constitute a drive supporting unit. 
     The magnetic circuit unit  10  includes an annular driving magnet  11  centered on the central axis O, an annular counter yoke  12  joined to the front of the driving magnet  11 , and a back yoke  13  joined to the back of the driving magnet  11 . A center yoke  14  is formed integrally with the back yoke  13 . The center yoke  14  is disposed inside the driving magnet  11  and the counter yoke  12  and formed to protrude forward (in the Z 1  direction) from the back yoke  13 . The center yoke  14  may be formed independent of the back yoke  13  and joined to the back yoke  13 . The center yoke  14  has a hole  15  passing therethrough in the front-back direction (Z 1 -Z 2  direction). The counter yoke  12 , the back yoke  13 , and the center yoke  14  are formed of a magnetic material, that is, a magnetic metal material. 
     The center yoke  14  is a circular columnar member. The outer periphery of the center yoke  14  and the inner periphery of the counter yoke  12  have a magnetic gap G formed therebetween. The magnetic gap G is along the circumference centered on the central axis O. In the magnetic circuit unit  10 , the driving magnetic flux F 1  generated from the driving magnet  11  travels from the counter yoke  12  across the magnetic gap G and moves along the center yoke  14  and the back yoke  13 . 
     A diaphragm  3  is disposed inside a space forward of the frame  2 . The diaphragm  3  has a conical shape. A front edge  2   a  of the frame  2  and an outer edge  3   a  of the diaphragm  3  are joined to each other, with an elastically deformable edge member  4  therebetween. The front edge  2   a  is secured with an adhesive to the edge member  4 , and the outer edge  3   a  is also secured with an adhesive to the edge member  4 . The frame  2  internally has, in its middle part, an inner fixing portion  2   b.  An outer edge  5   a  of an elastically deformable damper member  5  having a corrugated cross-section is secured with an adhesive to the inner fixing portion  2   b  of the frame  2 . 
     A bobbin  6  is disposed inside the frame  2 . The bobbin  6  is a circular cylindrical member centered on the central axis O. An inner edge  3   b  of the diaphragm  3  is secured with an adhesive to an outer periphery of the bobbin  6 , and an inner edge  5   b  of the damper member  5  is also secured with an adhesive to the outer periphery of the bobbin  6 . A dome-shaped cap  8  that bulges forward is disposed in the center of the diaphragm  3 . The cap  8  covers a front opening of the bobbin  6 . An edge portion  8   a  of the cap  8  is secured with an adhesive to the front surface of the diaphragm  3 . 
     A voice coil  7  is disposed on an outer periphery of a rear portion  6   b  (see  FIG.  5 B ) of the bobbin  6  facing backward (in the Z 2  direction). A coated wire constituting the voice coil  7  is wound a predetermined number of turns around the outer periphery of the bobbin  6 . The voice coil  7  is disposed inside the magnetic gap G in the magnetic circuit unit  10 . The magnetic circuit unit  10  and the voice coil  7  constitute a magnetic driving unit. 
     The diaphragm  3  and the bobbin  6  are supported by elastic deformation of the edge member  4  and the damper member  5  in such a way as to freely vibrate in the front-back direction (Z 1 -Z 2  direction) with respect to the frame  2  (or with respect to the drive supporting unit). The diaphragm  3 , the cap  8 , the bobbin  6 , and the voice coil  7  constitute a vibrating unit that vibrates in the front-back direction with respect to the drive supporting unit including the frame  2 . 
     The speaker  1  includes a detecting unit (vibration detecting unit)  20  that detects vibration of a movable unit. The detecting unit  20  is constituted by the moving magnet  21  on the movable unit and the magnetic sensor  22  on the drive supporting unit. The moving magnet  21  is secured to a magnet support  23 . The bobbin  6 , the magnet support  23 , and the moving magnet  21  are shown separately in  FIG.  5 A  and  FIG.  5 B . The magnet support  23  is an annular member extending continuously over an angular range of  360  degrees and formed of a non-magnetic material, such as a synthetic resin material. The magnet support  23  is attached to a front edge  6   a  of the bobbin  6  facing forward (in the Z 1  direction). That is, the magnet support  23  is attached to the front edge  6   a  of the bobbin  6  opposite the rear portion  6   b  having the voice coil  7  thereon. 
     The front edge  6   a  of the bobbin  6  has a plurality of positioning recesses  6   c  evenly spaced apart in the circumferential direction. As illustrated in  FIG.  5 B , the back surface of the annular magnet support  23  has arc-shaped grooves  23   a  at intervals, and adjacent ones of the grooves  23   a  are provided with a positioning protrusion  23   b.  That is, the grooves  23   a  and the positioning protrusions  23   b  are alternately arranged along the circumferential direction. The front edge  6   a  is inserted into the grooves  23   a,  the positioning protrusions  23   b  are inserted into the respective positioning recesses  6   c,  and the bobbin  6  and the magnet support  23  are secured with an adhesive. The front edge  6   a  of the bobbin  6  is fitted into the grooves  23   a  in the magnet support  23  to form a first fit, and the positioning protrusions  23   b  of the magnet support  23  are fitted into the positioning recesses  6   c  of the bobbin  6  to form a second fit, so that these fits serve as a positioning structure. The positioning structure allows positioning of the magnet support  23  and the bobbin  6  without causing their relative movement in the radial direction, the front-back direction, and the circumferential direction. 
     As illustrated in  FIG.  5 A , the magnet support  23  has a magnet retaining recess  23   c  formed in the front surface thereof. The magnet retaining recess  23   c  is a recessed portion that opens forward and outward in the radial direction. The moving magnet  21  is retained in, and secured with an adhesive to, the magnet retaining recess  23   c.  The magnet support  23  has at least one balancing-mass retaining recess  23   d  formed in the front surface thereof. A balancing mass  24  (see  FIG.  1   ) is retained in, and secured with an adhesive to, the balancing-mass retaining recess  23   d.  The balancing mass  24  is designed to eliminate imbalance in mass on the bobbin  6  caused by mounting the moving magnet  21 . The magnet support  23  is provided with at least one balancing mass  24 . The balancing mass  24  is preferably spaced  180  degrees apart from the moving magnet  21  about the central axis O. It is more preferable, as illustrated in  FIG.  5 A , that there be a total of three balancing masses  24  (balancing-mass retaining recesses  23   d ), including two additional balancing masses  24  spaced 90 degrees apart from the moving magnet  21  about the central axis O. For example, the balancing masses  24  are made of metal or synthetic resin, and the balancing masses  24  each preferably have the same or substantially the same mass as the moving magnet  21 . Note that it is also possible to form balancing masses  24  integrally with the magnet support  23 , for example, by insert molding, without creating the balancing-mass retaining recesses  23   d.    
     In the embodiments illustrated in  FIG.  5 A  and  FIG.  5 B , the magnet support  23  is an annular member extending continuously over an angular range of 360 degrees. However, the magnet support  23  may be formed to extend along at least part of the bobbin  6  in the circumferential direction. For example, the magnet support  23  may be divided into arc-shaped portions, which are individually secured to the bobbin  6 . In this case, for example, the arc-shaped portions include one having a length corresponding to a predetermined angle and formed with the magnet retaining recess  23   c  at the center, and another having a length corresponding to a predetermined angle and formed with the balancing-mass retaining recess  23   d  at the center. 
     As illustrated in  FIG.  1    and  FIG.  2   , the magnetic sensor  22  constituting the detecting unit  20  is disposed in an interior space of the bobbin  6 . A base  27  is bonded and secured to a front face  14   a  of the center yoke  14 . The base  27  is a block-shaped member formed of a non-magnetic material, such as synthetic resin. A wiring board  28  is secured to the front surface of the base  27 , and the magnetic sensor  22  is mounted on the front surface of the wiring board  28 . The wiring board  28  also serves as a base. These bases (i.e., the base  27  and the wiring board  28 ) allow the magnetic sensor  22  to be disposed forward of and at a distance from the front face  14   a  of the center yoke  14  and, at the same time, close to the moving magnet  21 . A distribution cable  25  electrically connected to the magnetic sensor  22  is connected to the wiring board  28 . The distribution cable  25  passes through a hole in the center of the base  27  and the hole  15  in the center yoke  14  and extends outward from the back of the magnetic circuit unit  10 . 
       FIG.  1    and  FIG.  2    illustrate a cross-section of the speaker  1  taken along a plane parallel to the X-Z plane containing the central axis O. The center of the moving magnet  21  and the center of the magnetic sensor  22  are in the same cross-section containing the central axis O. In the driving magnetic flux F 1  travelling inside the magnetic circuit unit  10 , a leakage magnetic flux emerging forward of the magnetic circuit unit  10  contains a component acting on the magnetic sensor  22  in the radial direction (X direction). As illustrated in  FIG.  5 A  and  FIG.  5 B , magnetized end faces  21   a  and  21   b  of the moving magnet  21  are oriented in the direction tangential to the bobbin  6  (in the direction parallel to the Y direction). If the end face  21   a  is north-polarized and the end face  21   b  is south-polarized, the moving magnetic flux component F 2  indicated by an arrow in  FIG.  5 A  and  FIG.  5 B  acts on the back (Z 2  side) of the moving magnet  21 . The moving magnetic flux component F 2  refers to a component of the magnetic flux generated by the moving magnet  21  and acts in the direction substantially tangential to the bobbin  6  (in the direction substantially parallel to the Y direction). 
     The magnetic sensor  22  is capable of detecting a change in the direction of a magnetic field, which is a vector quantity, in a plane orthogonal to the central axis O and passing through the center of the magnetic sensor  22  (in a plane parallel to the X-Y plane). The leakage magnetic flux of the driving magnetic flux F 1  generated by the magnetic circuit unit  10  acts on the magnetic sensor  22  in the radial direction (X direction). In  FIG.  1   , the magnetic field (or vector quantity) acting on the magnetic sensor  22  in the X direction on the basis of the leakage magnetic flux of the driving magnetic flux F 1  is denoted by H 1 . The moving magnetic flux component F 2  generated by the moving magnet  21  acts on the magnetic sensor  22  in the Y direction. In  FIG.  1   , the magnetic field (or vector quantity) acting on the magnetic sensor  22  in the Y direction on the basis of the moving magnetic flux component F 2  is denoted by H 2 . A detection output corresponding to a change in the direction of a detection magnetic field Hd, which is a composite vector of the magnetic field H 1  and the magnetic field H 2 , is obtained from the magnetic sensor  22 . Since the relative position of the magnetic sensor  22  and the magnetic circuit unit  10  does not change, there is virtually no change in the intensity of the magnetic field H 1  acting on the magnetic sensor  22 . On the other hand, since the distance between the moving magnet  21  and the magnetic sensor  22  changes as the movable unit vibrates in the front-back direction (Z 1 -Z 2  direction), the intensity of the magnetic field H 2  detected by the magnetic sensor  22  changes. Accordingly, the direction θ of the detection magnetic field Hd (composite vector), or the angle θ of the detection magnetic field Hd in a plane orthogonal to the central axis O, changes as the movable unit vibrates. 
     The magnetic sensor  22  includes at least one magnetoresistive element. The magnetoresistive element is a giant magnetoresistive (GMR) element or a tunneling magnetoresistive (TMR) element including a pinned magnetic layer and a free magnetic layer. The direction of magnetization of the pinned magnetic layer is fixed whereas the direction of the magnetic field in the free magnetic layer follows a change in the direction of the detection magnetic field Hd. An electrical resistance value thus changes in accordance with a change in the relative angle of the fixed magnetic field in the pinned magnetic layer and the magnetization of the free magnetic layer. A change in the angle θ of the vector of the detection magnetic field Hd can be detected in accordance with a change in this electrical resistance value. Alternatively, two Hall elements may be used as the magnetic sensor  22  to detect a change in the direction θ of the detection magnetic field Hd. In this case, the two Hall elements are arranged in such a way that the detection directions cross each other (preferably orthogonal to each other) in a plane orthogonal to the central axis O. Then, one of the Hall elements detects the intensity of the magnetic field H 1  and the other Hall element detects the intensity of the magnetic field H 2 , so that a detection output corresponding to a change in the direction of the vector of the detection magnetic field Hd can be obtained. 
     A sound output operation of the speaker  1  will now be described. 
     In the sound output operation, a driving current is applied to the voice coil  7  on the basis of an audio signal output from an audio amplifier. Since the driving magnetic flux F 1  generated from the magnetic circuit unit  10  travels across the voice coil  7 , an electromagnetic force excited by the driving magnetic flux F 1  and the driving current causes the vibrating unit including the bobbin  6  and the diaphragm  3  to vibrate in the front-back direction. This generates sound pressure corresponding to the frequency of the driving current, and enables sound to be output toward the front. 
     A control unit connected to the speaker  1  performs feedback control on the basis of a detection output from the magnetic sensor  22 . By obtaining, from the magnetic sensor  22 , a detection output based on a change in the angle θ of the detection magnetic field Hd in a plane, the control unit can identify the position of the vibrating unit including the diaphragm  3  in the front-back direction and can also identify the change in this position. For example, the control unit determines an ideal position of the vibrating unit in the front-back direction achieved by application of an audio signal and a change in this ideal position, and also determines an actual position of the vibrating unit and a change in this actual position from the detection output from the magnetic sensor  22 . The control unit then calculates the amount of deviation of the actual position and its change from the ideal position and its change. If the amount of deviation exceeds a threshold, a correction signal (offset signal) for correcting the deviation is generated. The correction signal is superimposed on the driving signal (voice current) applied to the voice coil  7 . The feedback control thus corrects distortion and deviation of sound output from the speaker  1 , and prevents excessive vibration of the diaphragm  3  in the front-back direction. 
     The magnetic sensor  22  detects a change in the angle of the detection magnetic field Hd, which is a composite vector quantity of the leakage magnetic flux of the driving magnetic flux F  1  generated by the magnetic circuit unit  10  and the moving magnetic flux component F 2  generated by the moving magnet  21 . That is, the magnetic sensor  22  obtains a detection output using a detection component of the leakage magnetic flux of the driving magnetic flux F 1  from the magnetic circuit unit  10 . Therefore, the leakage magnetic flux of the driving magnetic flux F 1  does not obstruct the detection of the position of the movable unit, and does not cause noise. Feedback control can thus be always performed with high precision and sensitivity. 
     The speaker  1  illustrated in  FIG.  1    and  FIG.  2    can be assembled by the following process. The driving magnet  11  is magnetized in advance, and the counter yoke  12  and the back yoke  13  are joined to the magnetized driving magnet  11  to form the magnetic circuit unit  10 . Alternatively, a ferromagnetic body to be formed into the driving magnet  11  may be joined to the counter yoke  12  and the back yoke  13  before being magnetized to form the driving magnet  11 . It is also possible that after a ferromagnetic body is joined to the counter yoke  12  and the back yoke  13 , the counter yoke  12  is further joined to the frame  2  to form the drive supporting unit before the ferromagnetic body is magnetized to form the driving magnet  11 . The moving magnet  21  is secured to the magnet support  23  after being independently magnetized. Alternatively, after a ferromagnetic body to be formed into the moving magnet  21  is secured to the magnet support  23 , the ferromagnetic body may be magnetized to form the moving magnet  21 . As described, the driving magnet  11  and the moving magnet  21  are magnetized in processes independent of each other. Therefore, the magnetizing process for one magnet has no influence on the magnetized state of the other magnet. 
     In the process of assembling the speaker  1 , the magnetic circuit unit  10  and the frame  2  are joined together, and the wiring board  28  and the magnetic sensor  22  are mounted in place to form the drive supporting unit. With a jig, the bobbin  6  having the voice coil  7  wound therearound is inserted into and positioned inside the magnetic gap G in the magnetic circuit unit  10 . Then, the outer edge  5   a  of the damper member  5  is bonded to the frame  2 , and the inner edge  5   b  of the damper member  5  is bonded to the outer periphery of the bobbin  6 . Additionally, the outer edge  3   a  of the diaphragm  3  is bonded to the frame  2  and the inner edge  3   b  of the diaphragm  3  is bonded to the outer periphery of the bobbin  6 . After completion of this basic structure of the speaker  1 , the magnet support  23  having the moving magnet  21  and the balancing mass  24  secured thereto is mounted onto the front edge  6   a  of the bobbin  6  protruding forward from the center of the diaphragm  3 . The bobbin  6  is then bonded and secured to the magnet support  23 . This is followed by bonding the edge portion  8   a  of the cap  8  to the front surface of the center part of the diaphragm  3 . In this assembling process, the magnet support  23  is secured to the front edge  6   a  of the bobbin  6  protruding forward of the diaphragm  3  after completion of the basic structure of the speaker  1 . The moving magnet  21  can therefore be easily positioned and attached to the bobbin  6 . 
     In the process of assembling the speaker  1 , the magnet support  23  may be secured to the bobbin  6  in advance. In this case, for example, after the bobbin  6  having the magnet support  23  secured thereto is inserted into and positioned inside the magnetic gap G in the magnetic circuit unit  10  using a jig, the damper member  5  and the diaphragm  3  are attached to the bobbin  6 . 
     In the speaker  1  illustrated in  FIG.  1    and  FIG.  2   , the moving magnet  21  is positioned at the magnet support  23 , which is positioned and secured to the front edge  6   a  of the bobbin  6 . Therefore, the relative position of the moving magnet  21  and the magnetic sensor  22  can be determined without variation. It is thus possible to make the direction and intensity of the moving magnetic flux component F 2  acting from the moving magnet  21  on the magnetic sensor  22  constant, and minimize variation in feedback control in each product. 
     In the speaker  1 , the moving magnet  21  is attached to the front edge  6   a  of the bobbin  6  forward (on the Z 1  side) of the damper member  5 . Therefore, even when the amplitude of the vibrating unit in the front-back direction increases and the bobbin  6  moves backward (in the Z 2  direction) significantly, the moving magnet  21  does not become very close to the magnetic circuit unit  10 . Since the moving magnet  21  is prevented from being magnetically attracted to the counter yoke  12  of the magnetic circuit unit  10 , it is unlikely that the magnetism of the moving magnet  21  will be reduced. 
     The magnet support  23  is shaped to extend along the circumference of the bobbin  6 . The bobbin  6 , which is thus reinforced by the magnet support  23 , is less likely to be deformed, and it is less likely that the circular cylindrical shape of the bobbin  6  and the voice coil  7  will be distorted. Also, since the balancing mass  24  for balancing with the moving magnet  21  is secured to the magnet support  23 , imbalance in the mass of the movable unit can be corrected. It is also possible to reduce occurrence of rolling which is caused, for example, by displacement of the center of gravity during operation of the movable unit. 
       FIG.  3    illustrates a speaker  101  according to a second embodiment of the present invention,  FIG.  6    illustrates a speaker  201  according to a third embodiment of the present invention, and  FIG.  8    illustrates a speaker  301  according to a fourth embodiment of the present invention. In the second, third, and fourth embodiments, the same component parts as those of the speaker  1  according to the first embodiment illustrated in  FIG.  1    and  FIG.  2    are denoted by the same reference numerals and detailed description thereof will be omitted. 
     In the speaker  101  of the second embodiment illustrated in  FIG.  3   , a phase plug  31  is secured forward of the base  27  that supports the magnetic sensor  22 . The speaker  101  does not include the cap  8 . When viewed from the front, the magnet support  23  is exposed on the outer periphery of the phase plug  31 . The inner periphery of the magnet support  23  serves as a positioning portion (positioning face)  23   e  for determining the relative position with respect to the phase plug  31 . 
     In the process of assembling the speaker  101 , as illustrated in  FIG.  4   , a positioning jig  35  having a circular cylindrical shape is installed inside the bobbin  6  at some point in time. The magnet support  23  and the phase plug  31  are mounted in place, with an outer periphery  35   a  and an inner periphery  35   b  of the positioning jig  35  being in firm contact with the positioning portion  23   e  (inner periphery) of the magnet support  23  and the outer periphery  31   a  of the phase plug  31 , respectively. This process enables the center of the bobbin  6  to coincide with the center of the phase plug  31 . Also, after the assembly, a uniform gap can be created between the inner periphery of the magnet support  23  and the outer periphery  31   a  of the phase plug  31  over the entire circumference of the phase plug  31 . This makes it less likely that the bobbin  6  will hit the phase plug  31  when the bobbin  6  vibrates back and forth in the sound output operation. 
     In the speaker  201  of the third embodiment illustrated in  FIG.  6    and  FIG.  7   , a magnet support  123  on the outer periphery of the bobbin  6  is located in a middle part of the bobbin  6  in the front-back direction (Z 1 -Z 2  direction). As illustrated in  FIG.  9   , the magnet support  123  is composed of two separate pieces, each of which is formed of a non-magnetic material, such as a synthetic resin material, and has a positioning protrusion  123   a  integrally formed on the inner periphery thereof. The bobbin  6  has positioning holes  6   e,  which are each open at a position corresponding to the positioning protrusion  123   a.  The positioning protrusions  123   a  and the positioning holes  6   e  constitute a positioning structure for positioning the bobbin  6  and the magnet support  123 . After the positioning protrusions  123   a  are inserted into the respective positioning holes  6   e,  the two separate pieces of the magnet support  123  are bonded to the outer periphery of the bobbin  6 . Then, the two separate pieces of the magnet support  123  are bonded and joined together to form the magnet support  123  having an annular shape that extends over the entire circumference of the bobbin  6 . The two separate pieces of the magnet support  123  may be spaced apart in the circumferential direction and bonded in a discontinuous state to the outer periphery of the bobbin  6 . 
     As illustrated in  FIG.  6   ,  FIG.  7   , and  FIG.  9   , the magnet support  123  has a magnet retaining recess  123   c,  in which the moving magnet  21  is positioned and secured. The magnet support  123  preferably has a balancing-mass retaining recess which allows the balancing mass  24  to be positioned and secured therein. 
       FIG.  7    is an enlarged sectional view of a joint between the bobbin  6  and the magnet support  123  and a joint (i) between the bobbin  6  and the damper member  5 . A tapered surface  123   b  is formed at the front of the inner periphery of the magnet support  123 , and a gap portion  42  that gradually widens toward the front is formed between the outer periphery of the bobbin  6  and the tapered surface  123   b.  At the joint (i), the inner edge  5   b  of the damper member  5  is bonded with an adhesive  41  to the outer periphery of the bobbin  6 . The magnet support  123  is disposed immediately behind the joint (i), and a part of the inner edge  5   b  of the damper member  5  is positioned in the gap portion  42  between the bobbin  6  and the magnet support  123 . By positioning the part of the inner edge  5   b  in the gap portion  42 , positioning of the inner edge  5   b  of the damper member  5  in the rearward direction (Z 2  direction) can be performed, and the inner edge  5   b  can be bonded to the bobbin  6  after the positioning. 
     As illustrated in  FIG.  6   , in the speaker  201  of the third embodiment, a base  26  formed of a non-magnetic material is secured to the frame  2  in a region outside the bobbin  6 . The wiring board  28  is secured to the front surface of the base  26 , and the magnetic sensor  22  is mounted on the front surface of the wiring board  28 . The magnetic sensor  22  of the speaker  201  has the same function of detecting magnetic fields as, for example, the speaker  1  of the first embodiment illustrated in  FIG.  1   . 
     In the speaker  201  of the third embodiment illustrated in  FIG.  6    and  FIG.  7   , the magnet support  123  having an annular shape reinforces the middle part of the bobbin  6  in the front-back direction. This makes the bobbin  6  less likely to be distorted and makes it easier to maintain the circular cylindrical shape of the bobbin  6 . Also, since the joint (i) between the bobbin  6  and the damper member  5  is reinforced or protected by the magnet support  123 , the joint (i) is less likely to be damaged. The moving magnet  21  is retained by the magnet support  123 . Therefore, when the bobbin  6  moves backward (in the Z 2  direction) significantly, even if the magnet support  123  hits the magnetic circuit unit  10 , the moving magnet  21  is prevented from directly hitting the magnetic circuit unit  10 . 
     In the speaker  301  of the fourth embodiment illustrated in  FIG.  8   , the magnet support  123  having an annular shape is disposed forward of the damper member  5  and immediately behind a joint (ii) between the bobbin  6  and the diaphragm  3 . In the joint (ii), the outer periphery of the bobbin  6  is secured with an adhesive  43  to the inner edge  3   b  of the diaphragm  3 . A gap portion  44  is formed between the tapered surface  123   b  of the magnet support  123  and the outer periphery of the bobbin  6 . A part of the inner edge  3   b  of the diaphragm  3  is in the gap portion  44 , and the joint (ii) is protected by the magnet support  123 . Before the inner edge  3   b  of the diaphragm  3  is bonded to the bobbin  6 , positioning of the diaphragm  3  in the rearward direction (Z 2  direction) can be performed. 
     The speaker  301  has the same structure as the speaker  201  of the third embodiment illustrated in  FIG.  6   , except the position of the magnet support  123 . In the speaker  301 , the magnet support  123  is disposed forward of the damper member  5 . Therefore, even when the bobbin  6  moves backward (in the Z 2  direction) significantly, the moving magnet  21  is not magnetically attracted to the magnetic circuit unit  10  and the magnetized state of the moving magnet  21  is not changed. It is unlikely that the moving magnet  21  will be damaged by colliding with the magnetic circuit unit  10 . 
     While there has been illustrated and described what is at present contemplated to be preferred embodiments of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the invention. In addition, many modifications may be made to adapt a particular situation to the teachings of the invention without departing from the central scope thereof. Therefore, it is intended that this invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.