Patent Publication Number: US-10334367-B2

Title: Electroacoustic transducer

Description:
TECHNICAL FIELD 
     The present invention relates to an electroacoustic transducer, and more particularly to a speaker device such as an earphone and a headphone worn on user&#39;s ears and head, or a large-sized speaker, and further to improvement of an electroacoustic transducer usable as a microphone. 
     DESCRIPTION OF RELATED ART 
     Conventionally, as a speaker device to be worn on the user&#39;s ear, for example, as shown in  FIG. 9 , the following configuration is well-known: one end surface of a cylindrical drive magnet  5  is fixed in a cup-shaped yoke  3  placed in a case body  1 , and a thin diaphragm  7  facing the other end surface of the drive magnet  5  with a space provided therebetween is fixed to a tip of the yoke  3 , and a cylindrical voice coil  9  fixed to the diaphragm  7  is inserted into an outer circumference of the drive magnet  5  with a small space provided therebetween. 
     The case body  1  is composed of a funnel shaped base portion  1   a  and a front cover  1   b  covering a tip of the base portion  1   a  (the right side in  FIG. 9 ). A flexible ear tip (earpat, earpiece)  13  is fitted around the outer circumference of a sound tube  11  protruding from the front cover  1   b.    
     Reference numeral  15  in  FIG. 9  is a cable led to the outside, and there is a knot  15   a  of the cable in the base portion  1   a.    
     In this speaker device, a driving unit  17  for vibrating the diaphragm  7  is formed by the drive magnet  5  and the voice coil  9 , and by applying a voice signal from the outside to the voice coil  9  using the cable  15 , the diaphragm  7  is caused to vibrate and output sound, and the output sound is propagated from the sound tube  11  on the front face of the diaphragm  7  to the outside. 
     The speaker device as an actual product is configured, for example, in an ear canal insertion type earphone device, and is used in such a way that the case body  1  is inserted into an ear conchal cavity  25  surrounded by a user&#39;s tragus  19 , antitragus  21 , and concha  23 , so that the diaphragm  7  approaches the concha  23 , and an ear tip  13  is elastically abutted on an inner wall of an ear canal  27  extending from the conchal cavity  25  to an ear drum (not shown). 
     As an actual product, there is a coaxial type in which a central axis of the sound tube  11  is aligned with a central axis of the diaphragm  7  as shown in  FIG. 9  described above, and there is a noncoaxial type in which the central axis of the sound tube  11  is obliquely set with respect to the central axis of the diaphragm  7  although not shown.  FIG. 9  shows a state in which the earphone device is worn on the left car. 
     A known example of the earphone is disclosed in Japanese Patent Laid-Open Publication No. 2010-283643 (Patent Document 1), and a known example of the speaker device is disclosed in Japanese Patent Laid-Open Publication No. 11-168799 (Patent Document 2). 
     PRIOR ART DOCUMENT 
     Patent Document 
     [Patent Document 1] Japanese Patent Laid-Open Publication No. 2010-283643 
     [Patent Document 2] Japanese Patent Laid-Open Publication No. 11-168799 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, as shown in  FIG. 10A , the abovementioned speaker device is configured, for example so that a both-end lead wire  9   a  is led to an outside space from one place of a root of the voice coil  9 . In this configuration, it is found that when the voice coil  9  is displaced and vibrated by applying a voice signal to the both-end lead wire  9   a , there is the following effect on a sound reproduction. 
     That is, in the configuration shown in  FIG. 10A , at a lead-out position P 1  of the both-end lead wire  9   a  of the voice coil  9  and a diagonal position P 2  diagonal thereto, the diaphragm  7  is liable to be more largely displaced at the diagonal position P 2  than at the lead-out position P 1  of the both-end lead wire  9   a , mainly due to presence or absence of a load of the both-end lead wire  9   a  added on the diaphragm  7 . 
     It is found that this tends to affect a sound reproduction, and there is a room for improvement in order to reproduce high-quality sound in a wide frequency band. 
     Further, as shown in  FIG. 10B , there is a configuration in which the both-end lead wire  9   a  is led from one position P 3  of the voice coil  9  to an outside position P 4  of the diaphragm  7 , and the both-end lead wire  9   a  is fastened to a circumferential region (corrugation edge part) of the diaphragm  7  up to the outside position P 4 , using a flexible adhesive agent (not shown). Although not shown, there is also a configuration in which the both-end lead wire is led out from two positions of the voice coil. 
     Whether the place where the both-end lead wire  9   a  is led out from the voice coil  9  is one place or two places, a node of a rolling motion of the diaphragm  7  is more liable to occur at one point in a case of one place, and on a line connecting two places in a case of two places. 
     This causes an increase of a distortion factor, or deterioration of a transient response characteristic, and further an abnormal vibration such as buzziness at the time of an excessive input of the voice signal. Therefore, it is also found that there is a room for improvement in order to reproduce high-quality sound in a wide frequency band. 
     Therefore, a new configuration is found by inventor of the present invention, as a result of earnest study on various configurations in pursuit of further higher quality. Thus, the present invention is completed. 
     In order to solve the above problem, the present invention is provided, and an object of the present invention is to provide an electroacoustic transducer having a wide frequency band and excellent transient response characteristics, and capable of reproducing high-quality sound particularly in a low frequency range. 
     Means for Solving the Problem 
     SUMMARY OF THE INVENTION 
     In order to solve the above problem, according to a first configuration of the electroacoustic transducer of the present invention, there is provided an electroacoustic transducer, including: 
     a cylindrical yoke; 
     a drive magnet placed coaxially with the yoke so as to circumferentially face the yoke, and forming a magnetic circuit between the drive magnet and the cylindrical tip end portion of the yoke; 
     a thin diaphragm fixed at the cylindrical tip end portion of the yoke so as to face end surfaces of the yoke and the drive magnet with a space provided therebetween and having a central region and an outer circumferential region surrounding the central region, and 
     a voice coil formed into a cylindrical shape with a thin insulating wire wound thereon and fixed to yoke-side one surface of the diaphragm so as to surround the central region, and inserted into a ring-shaped space between the drive magnet and the yoke, and configured to cause the diaphragm to vibrate by a voice signal applied to the both-end lead wire. 
     wherein the diaphragm is formed, with the central region having rigidity so as to be less flexible compared with the outer circumferential region, and the voice coil has the both-end lead wire led out to the outside through the central region of the diaphragm. 
     According to the first configuration of the electroacoustic transducer of the present invention, the both-end lead wire extending from the voice coil is directly fastened to the diaphragm at least in the central region. 
     According to the first configuration of the electroacoustic transducer of the present invention, the both-end lead wire is fastened to a recessed portion formed in the central region of the diaphragm, and led out to the outside. 
     According to the first configuration of the electroacoustic transducer of the present invention, the both-end lead wire is led out to the outside through a small hole formed on the recessed portion of the diaphragm so as to penetrate therethrough. 
     According to a second configuration of the electroacoustic transducer of the present invention, a first speaker unit and a second speaker unit are placed so as to be coaxially stacked. 
     The first speaker unit includes: 
     a cylindrical first yoke; 
     a first drive magnet placed coaxially with the first yoke so as to circumferentially face the first yoke, and forming a magnetic circuit between the first drive magnet and the cylindrical tip end portion of the first yoke: 
     a thin common diaphragm fixed at the cylindrical tip end portion of the first yoke so as to face end surfaces of the first yoke and the first drive magnet with a space provided therebetween and having a central region and an outer circumferential region surrounding the central region; and 
     a first voice coil formed into a cylindrical shape with a thin insulating wire wound thereon and fixed to the first yoke-side one surface of the common diaphragm so as to surround the central region, and inserted into a ring-shaped space between the first drive magnet and the first yoke, and configured to cause the common diaphragm to vibrate by a voice signal applied to the both-end lead wire, 
     wherein the common diaphragm is formed, with the central region having rigidity so as to be less flexible compared with the outer circumferential region, and the first voice coil has the both-end lead wire led out to the outside through the central region of the common diaphragm. 
     Further, the second speaker unit includes: 
     the common diaphragm in common; 
     a cylindrical second yoke placed coaxially with the first yoke, interposing the common diaphragm; 
     a second drive magnet placed to face the common diaphragm and coaxially with the second yoke so as to circumferentially face the second yoke, and forming a magnetic circuit between the second drive magnet and the cylindrical tip end portion of the second yoke on the common diaphragm side; and 
     a second voice coil formed into a cylindrical shape with a thin insulating wire wound thereon and fixed to the second yoke-side one surface of the common diaphragm so as to surround the central region, and inserted into a ring-shaped space between the second drive magnet and the second yoke, and configured to cause the common diaphragm to vibrate in the same direction as the first voice coil by a voice signal which has a phase opposite to that of the voice signal to the first voice coil and which is applied to the both-end lead wire, 
     wherein the second voice coil has the both-end lead wire led out to the outside through the central region of the common diaphragm. 
     In addition, according to a third configuration of the electroacoustic transducer of the present invention, the first speaker unit and the second speaker unit are placed so as to be coaxially stacked. 
     The first speaker unit includes: 
     a cylindrical first yoke: 
     a first drive magnet placed coaxially with the first yoke so as to circumferentially face the first yoke, and forming a magnetic circuit between the first drive magnet and the cylindrical tip end portion of the first yoke: 
     a thin first diaphragm fixed at the cylindrical tip end portion of the first yoke so as to face end surfaces of the first yoke and the first drive magnet with a space provided therebetween and having a first central region and an outer circumferential region surrounding the first central region; and 
     a first voice coil formed into a cylindrical shape with a thin insulating wire wound thereon and fixed to the first yoke-side one surface of the first diaphragm so as to surround the first central region, and inserted into a ring-shaped space between the first drive magnet and the first yoke, so that the first diaphragm is caused to vibrate by a voice signal applied to the both-end lead wire, 
     wherein the first diaphragm is formed, with the central region having rigidity so as to be less flexible compared with the first outer circumferential region, and the first voice coil has the both-end lead wire led out to the outside through the first central region of the first diaphragm. 
     Further, the second speaker unit includes: 
     a cylindrical second yoke placed coaxially with the first yoke interposing the first diaphragm; 
     a second drive magnet placed to face the first diaphragm and coaxially with the second yoke so as to circumferentially face the second yoke, and forming a magnetic circuit between the second drive magnet and the cylindrical tip end portion of the second yoke, 
     a second thin diaphragm placed at the cylindrical tip end portion of the second yoke and fixed so as to face end surfaces of the second yoke the second drive magnet with a space provided therebetween and having a second central region and an outer circumferential region surrounding the second central region; and 
     a second voice coil formed into a cylindrical shape with a thin insulating wire wound thereon and fixed to the second yoke-side one surface of the second diaphragm so as to surround the second central region, and inserted into a ring-shaped space between the second drive magnet and the second yoke, and configured to cause the second diaphragm to vibrate in the same direction as the first diaphragm by a voice signal which has a phase opposite to that of the voice signal applied to the first voice coil and which is applied to the both-end lead wire, 
     wherein the second diaphragm is formed, with the central region having rigidity so as to be less flexible compared with the second outer circumferential region, and the second voice coil has the both-end lead wire led out to the outside through the second central region of the second diaphragm. 
     According to the second and third configurations of the electroacoustic transducer of the present invention, the both-end lead wire from the first voice coil is directly fastened to the central region of the diaphragm to which at least the first voice coil is fixed, and the both-end lead wire from the second voice coil is directly fastened to the central region of the diaphragm to which at least the second voice coil is fixed. 
     According to the second and third configurations of the electroacoustic transducer of the present invention, the both-end lead wire from the first voice coil is directly fastened to a recessed portion formed in the central region of the diaphragm to which the first voice coil is fixed and led out to the outside, and the both-end lead wire from the second voice coil is directly fastened to a recessed portion formed in the central region of the diaphragm to which the second voice coil is fixed and led out to the outside. 
     According to the first to third configurations of the electroacoustic transducer of the present invention, in the central region of the diaphragm, a bent portion for suppressing flexure is formed on the diaphragm itself, or a reinforcing layer is superimposed on the diaphragm, and the central region has a great rigidity so as to be less flexible compared with the outer circumferential region. 
     Advantage of the Invention 
     According to the first configuration of the electroacoustic transducer of the present invention, the diaphragm has a central region having a rigidity so as to be less flexible compared with the outer circumferential region, and the both-end lead wire of the voice coil is led to the outside through the central region. Therefore, a rolling motion of the diaphragm is prevented compared with a conventional both-end lead wire lead-out configuration, and a smooth piston motion of the diaphragm is secured. As a result, it is possible to obtain excellent transient response, low distortion rate, and a wide frequency band, and a high-quality sound can be reproduced particularly in a low frequency range. 
     According to the first configuration of the electroacoustic transducer of the present invention, the both-end lead wire from the voice coil is directly fastened to the diaphragm at least in the central region. Therefore, a reliable holding state of the both-end lead wire is secured, and it is easy to stabilize the characteristics, and manufacturing is facilitated. 
     According to the first configuration of the electroacoustic transducer of the present invention, the both-end lead wire is fastened to the recessed portion formed in the central region of the diaphragm and led out to the outside. Therefore, it is easy to hold an adhesive agent or the like in the center region, a fastening work of the both-end lead wire is simplified, and a manufacturing efficiency is further improved. 
     According to the first configuration of the electroacoustic transducer of the present invention, the both-end lead wire is led out to the outside through a small hole formed on the recessed portion of the diaphragm so as to penetrate therethrough. Therefore, the both-end lead wire can be led out from either side of the diaphragm and a lead-out position becomes constant, and it is easy to stabilize and maintain frequency characteristics. 
     According to the second configuration of the electroacoustic transducer of the present invention, the diaphragm has the central region that exhibits rigidity that is less flexible than the outer circumferential region, and the first and second speaker units are configured so that the both-end lead wires of the first and second voice coils are led out to the outside through the central region of each of the common diaphragms, and the common diaphragms are caused to vibrate in the same direction by the first and second voice coils. Therefore, in a configuration having a common diaphragm and coaxially stacking the first and second speaker units, a wide frequency band and excellent transient response characteristics are exhibited while securing a loud sound output, and a high-quality sound can be reproduced particularly in a low frequency range. 
     According to the third configuration of the electroacoustic transducer of the present invention, the first and second speaker units have first and second diaphragms having the central region exhibiting rigidity which is less flexible as compared with the outer circumferential region, and the both-end lead wire of the first voice coil for vibrating the first diaphragm is led out to the outside through the first central region of the first diaphragm, and the both-end lead wire of the second voice coil for vibrating the second diaphragm is led out to the outside through the second central region of the second diaphragm, and the first and second diaphragms are vibrated in the same direction by the first and second voice coils. Therefore, in the electroacoustic transducer having the first and second speaker units which are coaxially stacked on each other, a wide frequency band and excellent transient response characteristics are exhibited while securing a loud sound output, and a high-quality sound can be reproduced particularly in a low frequency range. 
     According to the second and third configurations of the electroacoustic transducer of the present invention, the both-end lead wire from the first voice coil is fastened to the central region of the diaphragm, and the both-end lead wire from the second voice coil is fastened to the central region of the diaphragm. Therefore, in a configuration using the common diaphragm or in a configuration using separate first and second diaphragms, a loud sound output is enabled, the rolling motion of the diaphragm is prevented, and a smooth piston motion can be obtained. As a result, excellent transient response, low distortion rate, and wide frequency band can be obtained. 
     According to the second and third configurations of the electroacoustic transducer of the present invention, the both-end lead wire from the first voice coil is fastened to the recessed portion formed in the central region of the diaphragm and led out to the outside, and the both-end lead wire from the second voice coil is fastened to the recessed portion formed in the central region of the diaphragm and led out to the outside. Therefore, in a configuration using the separate first and second diaphragms, a loud sound output is enabled, it is easy to hold an adhesive agent or the like in the central region, a fastening work of the both-end lead wire is simplified, and a manufacturing efficiency is further improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view showing an embodiment of a first configuration of an electroacoustic transducer of the present invention. 
         FIGS. 2A and 2B  are perspective views of a first configuration of a diaphragm of the electroacoustic transducer of  FIG. 1 , and  FIG. 2C  is a schematic cross-sectional view of the first configuration of the diaphragm of the electroacoustic transducer of  FIG. 1 . 
         FIG. 3  is a total harmonic distortion characteristic view of the first configuration of the electroacoustic transducer of  FIG. 1  and a conventional electroacoustic transducer. 
         FIG. 4  is a transient response waveform view of a first configuration of the electroacoustic transducer of  FIG. 1 . 
         FIG. 5  is a transient response waveform view of a conventional electroacoustic transducer. 
         FIG. 6  is a vertical cross-sectional view showing a modified example of the first configuration of the electroacoustic transducer of the present invention. 
         FIG. 7  is a vertical cross-sectional view showing an embodiment of a second configuration of the electroacoustic transducer of the present invention. 
         FIG. 8  is a vertical cross-sectional view showing an embodiment of a third configuration of the electroacoustic transducer of the present invention. 
         FIG. 9  is a cross-sectional view showing a conventional electroacoustic transducer together with use examples. 
         FIGS. 10A through 10C  each is a schematic view for explaining an operation of a conventional electroacoustic transducer and the electroacoustic transducer of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of an electroacoustic transducer according to the present invention will be described hereafter, with reference to the drawings, using a speaker device (for example, an earphone device) as an example. 
       FIG. 1  is a vertical cross-sectional view showing an embodiment of a first configuration of an electroacoustic transducer of the present invention. 
     In  FIG. 1 , a yoke  29  is formed by molding a magnetic plate material into a cylindrical shape, having a ring-shaped flange portion  29   a  formed by bending an open-end side (upper side in  FIG. 1 ) outward, in which an outer circumferential portion of the flange portion  29   a  is bent so as to rise slightly upward, and is fitted into a case body  31 . 
     On the outer circumference of the yoke  29  body, a cylindrical drive magnet  33  is placed so as to overlap on the flange portion  29   a , with a space provided between the yoke  29  and the drive magnet  33 . 
     In  FIG. 1 , a ring-shaped pole piece  35  made of a magnetic material is placed on the lower surface of the drive magnet  33  so as to overlap thereon. In  FIG. 1 , the inner circumference of the pole piece  35  and the downward tip end portion side of the yoke  29  are circumferentially faced with each other at almost the same level position, with a ring-shaped space provided therebetween. 
     The pole piece  35  is made of a member for concentrating magnetic flux to thereby obtain high sound quality (hereinafter the same). Such a pole piece  35  can be identified with the drive magnet  33 . 
     The yoke  29 , the drive magnet  33 , and the pole piece  35  are coaxially fixed and integrally formed using an adhesive agent or the like (not shown), and are fixed to the flange portion  29   a  from below. 
     A magnetic circuit is formed by the magnetic flux from the yoke  29 , between the inner circumference of the pole piece  35  and the tip end portion of the yoke  29  facing the pole piece  35  with a small space provided therebetween. 
     As shown in  FIG. 2 , the diaphragm  37  is formed into a disc shape and made of a conventionally known thin insulated film material, and has a central region  37   a  which is formed by expanding a relatively wide central part into a slightly a dome-shape, and an outer circumferential region  37   b  surrounding the central region  37   a  in a concentric circular or ring-shaped shape with a narrow width.  FIGS. 2A and 2B  show the diaphragm  37  being turned upside down respectively. 
     In the central region  37   a  of the diaphragm  37 , a bent portion is formed, which is obtained by bending a film material by giving thereto a rib of a folded shape, or a radial or concentric circular rib, or the like, and the central region  37   a  has rigidity so as to be less flexible. At the center of the central region  37   a , the recessed portion  37   c  protruding downward in  FIG. 1  is formed, and this recessed portion  37   c  also serves as a bent portion to improve the rigidity of the central region  37   a.    
     In the outer circumferential region  37   b , a radial fine concavo-convex strip called a corrugation edge is formed, but the outer circumferential region  37   b  is easily flexible compared with the central region  35   a . Therefore, flexibility to vibration is increased. 
     That is, the central region  37   a  has a great rigidity and is less flexible compared with the outer circumferential region  37   b . It is preferable that the central region  37   a  has a rigidity of about 2 to 5 times greater than the outer circumferential region  37   b . Nevertheless, it is important that a rigidity range is within a range in which good vibration of the diaphragm  37  including the central region  37   a  is ensured. 
     The diaphragm  37  may be configured as follows: a bending portion that suppresses flexing of the central region  37   a  is formed on the diaphragm  37  itself, and in addition, as shown in  FIG. 2C , a reinforcing layer  37   d  such as a separate film board, a coating layer of diamond, ruby or the like, a metal vapor deposition layer of aluminum or titanium or a sputtering layer, is formed on the central region  37   a  so as to overlap thereon. With this configuration as well, it is possible to make the central region  37   a  have a great rigidity so as to be less flexible compared with the outer circumferential region  37   b.    
     As shown in  FIG. 1 , the diaphragm  37  covers the open-end surface (tip end portion) side of the yoke  29  and the pole piece  35  with a space provided therebetween, and an entire outer edge of the outer circumferential region  37   b  is fixed to an outer circumferential edge of the pole piece  35  and supported by the pole piece  35 . 
     A circuit board  39  is fixed to an outer surface (upper surface side in  FIG. 1 ) of the flange portion  29   a  of the yoke  29 . A voice signal is supplied to the circuit board  39  from an electronic device (not shown) by a cable (not shown) (see  FIG. 9 ). 
     Reference numeral  41  in  FIG. 1  denotes a cover substrate, and a small hole  41   a , which is a braking hole for sound quality adjustment, is penetrated through a cover board  41 . 
     On one surface side (upper surface side in  FIG. 1 ) of the diaphragm  37 , one end surface side of the cylindrical voice coil  43  is fixed to a ring-shaped boundary between the central region  37   a  and the outer circumferential region  37   b  so as to surround the central region  37   a.    
     The voice coil  43  is an integral piece of thin, insulated conductor wire wrapped in a circular cylinder, wherein the both-end lead wire  43   a  is led out to the recessed portion  37   c  formed in the central region  37   a , and is directly fastened to the diaphragm  37  using an adhesive agent (not shown) applied to the recessed portion  37   c.    
     The both-end lead wire  43   a  is partially fastened to the diaphragm  37  in such a manner as slightly touching on the diaphragm  37  at a plurality of locations from a root of the voice coil  43  to the recessed portion  37   c , for example, at three locations, in the central region  37   a.    
     The voice coil  43  is inserted between an outer circumference of the yoke  29 , and inner circumferences of the drive magnet  33  and the pole piece  35 , with a small space provided between the yoke  29 , and the drive magnet  33  and the pole piece  35 , and circumferentially faces the outer circumference of the yoke  29  and the inner circumference of the drive magnet  33  with a small space provided therebetween. 
     The both-end lead wire  43   a  from the voice coil  43  is led out from the recessed portion  37   c  of the diaphragm  37  to the outside, and is connected to the circuit board  39 . The both-end lead wire  43   a  is wired in the air with a margin to prevent the stress from being added by the vibration of the diaphragm  37  described later. 
     The voice coil  43  is displaced by the application of the voice signal to the voice coil  43  via the both-end lead wire  43   a , so that the diaphragm  37  is vibrated and driven. 
     That is, a driving unit  45  for vibrating and driving the diaphragm  37  is formed by the yoke  29 , the drive magnet  33 , and the voice coil  43 , to thereby constitute an external magnetic type speaker. 
     The abovementioned case main body  31  has a large-diameter tubular portion  31   a  into which the yoke  29 , the drive magnet  33  and the pole piece  35  are fitted, and a small-diameter tubular portion  31   b  having almost the same size as the yoke  29  continuously from the large-diameter tubular portion  31   a . The large-diameter tubular portion  31   a  and the small-diameter tubular portion  31   b  are integrally molded from an insulating synthetic resin or the like. 
     The yoke  29 , the drive magnet  33 , and the pole piece  35  are integrally fitted into the large-diameter tubular portion  31   a , and are fixed to the inside of the large-diameter tubular portion  31   a  and supported thereby. 
     In the case main body  31 , a portion which is changed from the large-diameter tubular portion  31   a  to the small-diameter tubular portion  31   b  covers the diaphragm  37  with a space provided therebetween, and a control magnet  47  is fixed to this portion so as to face the yoke  29  interposing the diaphragm  37 , and so as to cover the small-diameter tubular portion  31   b.    
     The control magnet  47  has a ring plate shape having an outer diameter size substantially equal to that of the yoke  29 , and is placed in parallel with the drive magnet  33  or the pole piece  35 , and the surface side facing the yoke  29  is magnetized to have the same polarity as that of the yoke  29 . 
     The control magnet  47  suppresses a diffusion of the leak flux from the yoke  29  to the pole piece  35  and compresses the magnetic flux, and has a function of reducing a driving loss of the diaphragm  37  through an increase of a magnetic flux density. 
     The speaker device having the abovementioned configuration is commercialized as an earphone device, in which a cover or the like (not shown) is put on the outside of the large-diameter tubular portion  31   a  in the case body  31 , and a flexible ear tip  49  is fitted to the outer circumference of the small-diameter tubular portion  31   b . The ear tip is not shown in the second and later configurations described later. 
     In such a speaker device, the driving unit  45  causes the diaphragm  37  to vibrate and output sound by applying the voice signal to the voice coil  43 , and a vibration sound is propagated to the outside through the small-diameter tubular portion  31   b  which serves as a sound tube. 
     Then, as shown in  FIG. 9  described above, the speaker device of the present invention is used by wearing it in such way that the case body  31  is housed in the ear conchal cavity  25  surrounded by the tragus  19 , the antitragus  21 , and the concha  23 , and the tip end ear tip  49  is inserted into the ear canal  27 . 
     As shown in  FIG. 10 , in the first configuration of such a speaker device of the present invention, a lead-out position P 5  of the both-end lead wire  43   a  from the voice coil  43 , that is, a load position, is the center of the central region  37   a  of the diaphragm  37 , and therefore an entire diaphragm  37  is uniformly displaced in a piston mode. Therefore, it is possible to reproduce high-quality sound in a wide frequency band, with excellent transient response waveform, particularly in a low frequency range. Specifically, it is easy to distinguish reproduced sounds of musical instruments and the like. 
     Further, an operation of the speaker device will be described in detail hereafter. 
     In the abovementioned speaker device of the present invention, the both-end lead wire  43   a  is led out from the root of the voice coil  43  to the central region  37   a  of the diaphragm  37 , and is fixed to the recessed portion  37   c  using an adhesive agent or the like, and is led out from the recessed portion  37   c  by aerial wiring. Therefore, when the diaphragm  37  is in operation, a mechanical load is added on the diaphragm  37  due to an influence of the both-end lead wire  43   a  of the central region  37   a , for example, due to the mass and stiffness of the both-end lead wire  43   a.    
     However, the central region  37   a  inside of the voice coil  43  is vertically vibrated simultaneously with the voice coil  43 , but as shown in  FIG. 1 , a bent portion such as a folded-back portion or a recessed portion  37   c  is formed in the central region  37   a  so as to be strengthened and have rigidity, and the central region  37   a  itself becomes an inflexible region that is not flexible. 
     Therefore, the load by the both-end lead wire  43   a  of the voice coil  43  is hardly added on the diaphragm  7 , by receiving the load at the central part of the central region  37   a  which is an inflexible region. The diaphragm  37  is uniformly displaced as a whole without performing rolling motion like a conventional case, thus contributing to improving the frequency characteristics, transient response characteristics, and the like. 
     In addition to the reinforcing function, the recessed portion  37   c  also serves as a center marker of the diaphragm  37 . 
     On the other hand, in the conventional speaker device, in order to improve a low frequency response, a vibration amplitude of the diaphragm  37  is increased, and therefore increase of distortion, occurrence of chattering noise, and the like are likely to cause abnormal vibration. 
     In the conventional speaker device, as shown in  FIG. 10A , the diaphragm  7  is liable to perform an operation called unbalanced rolling due to the load of the both-end lead wire  9   a . The diaphragm  7  is designed on the premise of this matter. Then, in order to absorb the distortion to some extent at the time of vibration of the diaphragm  7 , it has been common that the portion corresponding to the central region  7   a  is formed into a spherical shape so that it is vibrated while flexing to some extent. 
     Then, in a conventional configuration, when the rigidity of the diaphragm  43  is increased as in the present invention, further distortion occurs in the outer circumferential region  37   b  which is vibrated while flexing, and an abnormal vibration sound etc. is likely to occur. 
     In the configuration of the present invention, the rolling motion hardly occurs on the diaphragm  37 , and therefore distortion such as twisting hardly occurs in the outer circumferential region  37   b , and the degree of freedom in designing a corrugation edge portion can be much improved compared with the conventional configuration. In addition, since distortion, chattering noise, and the like are less likely to occur, the response in the low frequency range can be controlled relatively freely. 
     Then, as shown in a full harmonic distortion characteristic shown by the solid line in  FIG. 3 , the first configuration of the speaker device of the present invention shows the characteristic with less distortion particularly in the low frequency range, and it is found that high-quality sound can be reproduced in a wide frequency band. 
     On the other hand, as shown in  FIG. 10A , in the conventional configuration in which the both-end lead wire  9   a  is led out from the root of the voice coil  9 , the total harmonic distortion characteristic in the low frequency range is likely to deteriorate as shown by the frequency characteristic shown by the broken line in  FIG. 3 . 
     As shown in  FIG. 10B , the characteristic such as the configuration in which the both-end lead wire  9   a  of the voice coil  9  is led out to the outer circumferential edge of the diaphragm  7 , is also the same as the characteristic shown by the configuration of  FIG. 10A . 
     Further, in the first configuration of the speaker device of the present invention, a transient response waveform shown by this configuration is shown in  FIG. 4 , and it is found that the output signal waveform is less disturbed and the transient response waveform is good when a tone burst signal of 200 Hz and 1000 Hz is applied as the voice signal. 
     On the other hand, in the conventional configuration of  FIG. 10A , as shown in  FIG. 5 , a comparatively large disturbance occurs in the output signal waveform when a 200 Hz and 1000 Hz tone burst signal is applied, and it is found that the transient response waveform of the speaker device of the present invention is improved. 
     As described above, in the first configuration of the speaker device of the present invention, the both-end lead wire  43   a  from the voice coil  43  is directly fastened to the diaphragm  37  in the central region  37   a  using an adhesive agent or the like. Therefore, the both-end lead wire  43   a  is prevented from rubbing against the diaphragm  37 , so that the both-end lead wire  43   a  is less likely to be damaged and the characteristics become stable. 
     Further, the both-end lead wire  43   a  is led out to the outside through the recessed portion  37   c  formed in the central region  37   a  of the diaphragm  37 . Therefore, it is easy to hold the adhesive agent or the like in the central region  37   a , and a work of fastening the both-end lead wire  43   a  becomes easy. Further, in addition to an advantage that the recessed portion  37   c  functions as a reinforcement and as a marker at the central part as described above, there is also a secondary advantage that manufacturing efficiency is improved. 
     In the first configuration of the speaker device of the present invention, an object of the present invention can be achieved by fastening the both-end lead wire  43   a  from the voice coil  43  to the diaphragm  37  at least in the central region  37   a , and it is not absolutely necessary that the both-end lead wire  43   a  is fixed to the diaphragm  37  between the voice coil  43  and the central area  37   a.    
     The abovementioned speaker device is configured so that the both-end lead wire  43   a  of the voice coil  43  is led out to the outside from the central part of the central region  37   a . However, it is not necessary that the lead-out position is strictly located at the center of the central region  37   a , and for example, as indicated by the symbol “ϕ” in  FIG. 10C , the both-end lead wire  43   a  may be led out within a width of the recessed portion  37   c . Specifically, an object of the present invention can be achieved by leading out the both-end lead wire  43   a  from the center of the central region  37   a  within a range of 10% or less of the outer diameter of a speaker. 
     A modified example of the first configuration of the speaker device of the present invention will be described next. 
       FIG. 6  is a vertical cross-sectional view showing the modified example of the first configuration. 
     In  FIG. 6 , in the speaker configuration of  FIG. 1 , which includes the yoke  29 , the drive magnet  33 , the pole piece  35 , the diaphragm  37 , and the voice coil  43 , the diaphragm  37  is directed not to the small-diameter tubular portion  31   b  on the front side but to a back open side of the large-diameter tubular portion  31   a , and the flange portion  29   a  of the yoke  29  is directed toward the small-diameter tubular portion  31   b , when the small-diameter tubular portion  31   b  of the case main body  31  is set as the front side (lower side in  FIG. 6 ). 
     Therefore, arrangements of the main components such as the yoke  29 , the case body  31 , the drive magnet  33 , the pole piece  35 , the diaphragm  37 , and the voice coil  43 , are the reverse of those shown in  FIG. 1 , but almost the same as those shown in  FIG. 1 . In the configuration shown in  FIG. 6 , the same reference numerals are given to portions in common with those of  FIG. 1 . 
     In the diaphragm  37  of  FIG. 6 , the central region  37   a  has a great rigidity so as to be less flexible compared with the outer circumferential region  37   b . This point is the same as the abovementioned configuration, but the diaphragm  37  of  FIG. 6  is shown slightly differently from  FIG. 1 . 
     In the configuration of  FIG. 6 , The circumferential edge portion of the flange portion  29   a  of the yoke  29  is fitted into the case body  31 , in contact with a flat portion extending to the small-diameter tubular portion  31   b  from the large-diameter tubular portion  31   a , and the diaphragm  37  is placed to face the open-end side of the large-diameter tubular portion  31   a.    
     The inside of the side wall of the cup-shaped support base  51  is integrally fitted into the outer circumference of the yoke  29 , drive magnet  33 , pole piece  35 , and diaphragm  37  which are integrally formed, so as to cover the diaphragm  37  with a space provided therebetween. The support base  51  is fitted and fixed so that the outside of the side wall is in contact with the inner wall of the large-diameter tubular portion  31   a  of the case main body  31 . 
     In the same way as the abovementioned control magnet  47 , the control magnet  53 , which is magnetized to have the same polarity as the yoke  29  on the side facing the yoke  29 , is fixed to a middle space of the support base  51  so as to face the yoke  29  interposing the diaphragm  37 . 
     A small hole for controlling a sound quality is opened on the support base  51  or the control magnet  53 . 
     On the support base  51 , a circuit board  55  similar to the circuit board  39  is fixed to a position near the control magnet  53 . The both-end lead wire  43   a  of the voice coil  43  is led out to the side of the support base  51  through the small hole  37   e  formed on the recessed portion  37   c  of the diaphragm  37  and is connected to the circuit board  55 . 
     The other configuration and operation in the modified example of the first configuration shown in  FIG. 6  are the same as the abovementioned first configuration shown in  FIG. 1 , and the obtained characteristics are also similar. 
     Further, in the modified example of the first configuration, the both-end lead wire  43   a  is led out to the outside through the small hole  37   e  formed on the recessed portion  37   c  of the diaphragm  37 . Therefore the both-end lead wire  43   a  can be led out not only from the placement side of the voice coil  43 , but also from the opposite side thereto on the diaphragm  37 , and the lead-out position is liable to be constant. Therefore, various configurations are acceptable while maintaining a stable wide frequency characteristic. 
     An embodiment of a second configuration of the speaker device of the present invention will be described next. 
       FIG. 7  is a vertical cross-sectional view showing the second configuration of the speaker device of the present invention, in which external magnetic type first speaker unit A and second speaker unit B are coaxially placed so as to face each other with the diaphragm in common to each other. 
     The first speaker unit A has a configuration similar to the speaker unit of  FIG. 6 , and is formed having a first yoke  57 , a first drive magnet  59 , a first pole piece  61 , a common diaphragm  63 , and a first voice coil  65 , and is fixed in the case main body  67 . 
     The first yoke  57  is formed by molding a magnetic plate material into a cylindrical shape, having a ring-shaped first flange portion  57   a  formed by bending an open-end side (lower side in  FIG. 7 ) outward, and the outer circumferential portion of the first flange portion  57   a  is slightly bent downward. 
     On the outer circumference of the yoke  57  body, a cylindrical first drive magnet  59  is placed so as to overlap on the first flange portion  57   a , with a space provided between the yoke  57  and the drive magnet  59 . 
     On the upper end surface of the first drive magnet  59  in  FIG. 7 , a ring-shaped first pole piece  61  made of a magnetic material is disposed so as to overlap thereon. In  FIG. 7 , the inner circumference of the first pole piece  61  and the upward tip end portion side of the first yoke  57  are circumferentially faced with each other at almost the same level position, with a ring-shaped space provided therebetween. 
     Such a first pole piece  61  can also be identified with the first drive magnet  59 . 
     The first yoke  57 , the first drive magnet  59 , and the first pole piece  61  are coaxially fixed using an adhesive agent or the like not shown, and are integrally fixed to the first flange portion  57   a.    
     A first circuit board  69  is fixed to an outer surface (a lower surface in  FIG. 7 ) of the first flange portion  57   a  of the first yoke  57 . A voice signal is supplied to the first circuit board  69  from an electronic device (not shown) by a cable (not shown) (see  FIG. 9 ). 
     The common diaphragm  63  is formed in the same manner as the abovementioned diaphragm  37 , and has a central region  63   a  similar to the central region  37   a  and a ring-shaped outer circumferential region  63   b  surrounding the central region  63   a  in the same manner as the outer circumferential region  37   b . The common diaphragm  63  is differently shown in the figure from the abovementioned diaphragm  37 . 
     In the common diaphragm  63 , the central region  63   a  has a great rigidity so as to be less flexible compared with the outer circumferential region  63   b . This point is the same as the abovementioned first configuration. 
     The common diaphragm  63  covers the first pole piece  61  and the tip end portion side of the first yoke  57  with a space provided therebetween, and the entire outer edge of the first outer circumferential region  63   b  is fixed to the outer circumferential edge of the first pole piece  61 . 
     On one surface side (lower surface side in  FIG. 7 ) of the common diaphragm  63 , one end surface side of the cylindrical first voice coil  65  similar to the voice coil  43 , is fixed at a ring-shaped boundary between the first central region  63   a  and the first outer circumferential region  63   b , so as to surround the first central region  63   a.    
     The first both-end lead wire  65   a  of the first voice coil  65  is led out from the root of the first voice coil  65  to the central region  63   a , and is fastened thereto using an adhesive agent (not shown). 
     The first both-end lead wire  65   a  is fastened to the diaphragm  63  using an adhesive agent (not shown), for example, at three places extending to the central region  63   a  on the common diaphragm  63 . 
     The first voice coil  65  is inserted between the outer circumference of the first yoke  57  and the inner circumference of the first drive magnet  59 , with a small space provided between the first yoke  57  and the first drive magnet  59 , and the outer circumference of the first yoke  57  and the inner circumference of the first drive magnet  59  are faced with each other with a small space provided therebetween, interposing the first voice coil  65 . 
     The first both-end lead wire  65   a  from the first voice coil  65 , is led out so as to rise from the center of the central region  63   a  of the common diaphragm  63 , and is connected to the first circuit board  69  through the space of the first circuit board  69 . 
     The first voice coil  65  is displaced by the application of the voice signal to the first voice coil  65  via the first both-end lead wire  65   a , and the common diaphragm  63  is caused to vibrate. 
     That is, a first driving unit  71  for vibrating the common diaphragm  63  is formed by the first yoke  57 , the first drive magnet  59 , and the first voice coil  65 , to thereby constitute an external magnetic type first speaker unit A. 
     The second speaker unit B is formed having the second yoke  73 , the second drive magnet  75 , the second pole piece  77 , the second voice coil  79 , and the common diaphragm  63  described above, and is integrally overlapped on the first speaker unit A, and is fixed in the case main body  67 . 
     The second yoke  73  is formed by molding a magnetic plate material into a cylindrical shape, having a ring-shaped second flange portion  73   a  formed by bending an open-end side (upper side in  FIG. 7 ) outward, and an outer circumferential portion of the second flange portion  73   a  is bent so as to rise slightly upward. 
     On the outer circumference of the second yoke  73 , a cylindrical second drive magnet  75  is placed so as to overlap on the second flange portion  73   a , with a space provided between the second yoke  73  and the second drive magnet  75 . 
     A ring-shaped second pole piece  77  made of a magnetic material is placed on the lower surface of the second drive magnet  75  in  FIG. 7  so as to overlap thereon. In  FIG. 7 , the inner circumference of the pole piece  77  and the downward tip end portion side of the second yoke  73  are circumferentially faced with each other at almost the same level position, with a ring-shaped space provided therebetween. 
     The second pole piece  77  can also be identified with the second drive magnet  75 . 
     The second yoke  73 , the second drive magnet  75 , and the second pole piece  77  are coaxially fixed using an adhesive agent or the like not shown, and are integrally fixed to the second flange portion  73   a.    
     Further, a magnetic circuit is formed by the magnetic flux from the second yoke  73 , between the inner circumference of the second pole piece  77  and the tip end portion of the second yoke  73  facing the first pole piece  77  with a small space provided therebetween. 
     A second circuit board  81  is fixed to the outer surface (upper surface side in  FIG. 7 ) of the second flange portion  73   a  of the second yoke  73 , and a voice signal is supplied to the second circuit board  81  from an electronic device (not shown) by a cable (not shown) (see  FIG. 9 ). 
     The common diaphragm  63  covers the tip end portion side of the second pole piece  77  and the second yoke  73  with a space provided therebetween, and the entire circumferential edge portion of the outer circumference region  63   b  is supported by the outer circumferential edges of the first and second pole pieces  61  and  77  so as to be interposed therebetween. 
     On the other surface side (upper surface side in  FIG. 7 ) of the common diaphragm  63 , the abovementioned second voice coil  79  is fixed so as to surround the central region  63   a.    
     The second both-end lead wire  79   a  of second voice coil  79  is extended from the root of the second voice coil  79  to the first central region  63   a , and is fastened thereto using an adhesive agent (not shown). 
     The second both-end lead wire  79   a  is fastened to the common diaphragm  63  using an adhesive agent (not shown), for example, at three places extending to the central region  63   a  on the common diaphragm  63 . 
     The second voice coil  79  is inserted between the outer circumference of the second yoke  73  and the inner circumference of the second drive magnet  75 , with a small space provided between the second yoke  73  and the second drive magnet  75 , and the outer circumference of the second yoke  73  and the inner circumference of the second drive magnet  75  are faced with each other with a small space provided therebetween, interposing the second voice coil  79 . 
     The second both-end lead wire  79   a  from the second voice coil  79 , is led out so as to rise from the center of the central region  63   a  of the common diaphragm  63 , and is connected to the second circuit board  81 . 
     The voice signal which has a phase opposite to that of the first voice coil  65 , is transmitted to the second voice coil  79  via the second both-end lead wire  79   a , the second voice coil  79  is displaced in the same direction as the first voice coil  65 , and the common diaphragm  63  is vibrated. 
     Wiring (not shown) of the voice signal to the first speaker unit A is led out to an upper part of  FIG. 7  through a notch portion which is formed by vertically cutting a part of an inside of the large-diameter tubular portion  67   a.    
     A second driving unit  83  for vibrating the common diaphragm  63  is formed by the second yoke  73 , the second driving magnet  75  and the second voice coil  79 , to thereby constitute an external magnetic type second speaker unit B. 
     The first and second speaker units A and B are integrally fitted into the abovementioned cylindrical holding tube  85 , and are fixed in the case main body  67 . 
     The other configuration and operation of the second configuration of the speaker unit of the present invention are the same as the abovementioned configurations of  FIG. 1 or 6 . 
     In the second configuration, the external magnetic type first and second speaker units A and B having the common diaphragm  63  are integrally formed. Therefore, due to the abovementioned favorable operation of the common diaphragm  63  in the first and second speaker units A and B, it is possible to reproduce high-quality sound in a wide frequency band, particularly in a low frequency range, while making good use of the same frequency characteristics. 
     In addition, in the second configuration, the first and second voice coils  65  and  79  are displaced in the same direction to thereby largely displace the common diaphragm  63 . Therefore, a loud sound can be reproduced. 
     An embodiment of a third configuration of the speaker device of the present invention will be described next. 
       FIG. 8  is a vertical cross-sectional view showing a third configuration of the speaker device of the present invention, having the first speaker unit A having the second configuration of the speaker device of  FIG. 7  described above, and a second speaker unit C which is similar to the second speaker unit B, in which the first speaker unit A and the second speaker unit C have first and second diaphragms  87  and  89  respectively. 
     That is, the first diaphragm  87  has a first central region  87   a  and a first outer circumferential region  87   b  in the same manner as the common diaphragm  63 , and an entire circumferential edge portion is fixed to an upper open-end of the first pole piece  61  in  FIG. 8 . The abovementioned first voice coil  65  is fixed to the first diaphragm  87 . 
     The second diaphragm  89  has a second central region  89   a  and a second outer circumferential region  89   b  in the same manner as the common diaphragm  61  and the first diaphragm  87 , and the entire circumferential edge portion of the second diaphragm  89  is fixed to a lower open-end of the second pole piece  77  in  FIG. 8 , with a small space provided between the first and second diaphragm  87  and  89 . The abovementioned second voice coil  79  is fixed to the second diaphragm  89  at a position overlapping on the first voice coil  65 . 
     The first and second central regions  87   a  and  89   a  of the first and second diaphragms  87  and  89  are formed in the same manner as the abovementioned central region  37   a  of the diaphragm  37 , and the first and second outer circumferential regions  87   b  and  89   b  are also formed in the same manner as the abovementioned outer circumferential region  37   b  of the diaphragm  37 . 
     The space between the first and second diaphragms  87  and  89  is kept acoustically airtight, but it is not necessary that the diaphragm  87  is completely airtight. 
     Reference numeral  91  in  FIG. 8  is a holding tube for connecting and holding the first and second pole pieces  61  and  77  by fitting them, to thereby connect the external magnetic type first speaker unit A and second speaker unit B. 
     The holding tube  91  is fitted into the large-diameter tubular portion  67   a  of the case main body  67 , and the first and second speaker units A and C are held in the case main body  67 . 
     The other configuration and operation of the first and second speaker units A and C are almost the same as the second configuration of the speaker device of  FIG. 7 . 
     In the third configuration, favorable operations of the first and second diaphragms  87  and  89  in the first and second speaker units A and C are secured, and in addition, the space between the first and second diaphragms  87  and  89  is maintained in an airtight state, and the first and second diaphragms  87  and  89  are displaced in the same direction in such a manner as in the configuration of  FIG. 7 . Therefore, with this configuration, a loud sound output can be obtained in a state of satisfactorily maintaining the frequency characteristic, by two first and second speaker units A and C which are capable of reproducing high-quality sound in a wide frequency band, particularly in a low frequency range. 
     In addition, when the first speaker unit A or the second speaker unit C is placed as a simple body, and when a high input signal is transmitted, the operations of the first and second diaphragms  87 ,  89  do not show symmetrical vibrations in an upper (protruding) direction and in a lower (recessing) direction due to the configuration of the speaker and the shape of the diaphragm, and generally, distortion easily occurs from one side of the vibration and it is difficult for the vibrations of the first and second diaphragms  87 ,  89  to become symmetrical. 
     However, in the third configuration of the present invention, the first and second diaphragms  87  and  89  of the first and second speaker units A and C placed to face each other in a reverse direction are vibrated at the same time in the same direction with the same vibration width as described above. Therefore, the distortion is compensated for each other, the vibrations of the first and second diaphragms  87  and  89  are likely to be symmetrical, and the distortion is reduced. 
     Further, in the abovementioned second and third configurations of the present invention, the following configurations are also acceptable: the both-end lead wires  65   a  and  79   a  from the first and second voice coils  65  and  79  are fastened at least to the common diaphragm  63  or the central regions  63   a ,  87   a ,  89   a  of the first and second diaphragms  87  and  89 ; or the both-end lead wires  65   a  and  79   a  are led out to the outside through the recessed portion (not indicated by signs and numerals in  FIG. 7  and  FIG. 8 ) formed in the central regions  63   a .  87   a , and  89   a . With these configurations as well, the same effect as the first configuration and the like can be obtained. 
     In the speaker device of the present invention, when two speaker units A to C are placed, the both-end lead wire  65   a  from the first voice coil  65  is directly fastened to the central regions  63   a  and  87   a  of the diaphragms  63  and  87  to which at least the first voice coil  65  is fixed, and the both-end lead wire  79   a  from the second voice coil  79  is directly fastened to the central regions  63   a  and  89   a  of the diaphragms  63  and  89  to which at least the second voice coil  79  is fixed. 
     Further, in the speaker device of the present invention, the first and second speaker units A to C are not necessarily required to be placed in the same shape or coaxially. However, by forming them in the same shape or placing them coaxially, the common speaker unit can be used for the first and second speaker units A to C, then cost can be reduced, miniaturization becomes easy, and desired characteristics can be easily obtained. 
     Further, in the speaker device of the present invention, both the yoke and the drive magnet can have either an external magnetic type or an internal magnetic type configuration located on the inner side or the outer side, and it may be configured so that the drive magnet is placed coaxially with the yoke so as to circumferentially face the yoke, and the magnetic circuit is formed between the cylindrical tip end portion of the yoke and the drive magnet. 
     Further, in the speaker device of the present invention, the sound tube formed by the small-diameter tubular portion  67   b  of the case main bodies  31  and  67  protrudes along the central axes of the case main bodies (small-diameter tubular portions)  31  and  67 . In addition to this configuration, it is also acceptable that the sound tube is protruded obliquely with respect to the central axes. 
     The electroacoustic transducer of the present invention can be widely used as a speaker device such as a headphone, a large-sized speaker, and further, a microphone. 
     DESCRIPTION OF SIGNS AND NUMERALS 
     
         
           1 ,  31 ,  67  case main body 
           1   a  base portion 
           1   b  front cover 
           3  yoke 
           5 ,  33  drive magnet 
           7 ,  37  diaphragm 
           9 ,  43  voice coil 
           11  sound tube 
           13 ,  49  ear tip (earpat, earpiece) 
           15  cable 
           15   a  knot 
           17 ,  45  driving unit 
           19  tragus 
           21  antitragus 
           23  concha 
           25  ear conchal cavity 
           27  ear canal 
           29  yoke 
           29   a  flange portion 
           31   a  large-diameter tubular portion 
           31   b  small-diameter tubular portion 
           35  pole piece 
           37   a ,  63   a  central region 
           37   b ,  63   b  outer circumferential region 
           37   c  recessed portion (bent portion) 
           37   d  reinforcing layer 
           37   e  small hole 
           39 ,  55  circuit board 
           41  cover board 
           41   a  small hole 
           43   a  both-end lead wire 
           47 ,  53  control magnet 
           51  support base 
           27  first yoke 
           57   a  first flange portion 
           59  first drive magnet 
           61  first pole piece 
           63  common diaphragm 
           65  first voice coil 
           65   a  first both-end lead wire 
           69  first circuit board 
           71  first driving unit 
           73  second yoke 
           73   a  second flange portion 
           75  second drive magnet 
           77  second pole piece 
           79  second voice coil 
           81  second circuit board 
           83  second driving unit 
           85 ,  91  holding tube 
           87  first diaphragm 
           87   a  first central region 
           87   b  first outer circumferential region 
           89  second diaphragm 
           89   a  second central region 
           89   b  second outer circumferential region 
         A first speaker unit 
         B, C second speaker unit 
         P 1 , P 2 , P 3 , P 4 , P 5  lead-out position