Patent Publication Number: US-8526659-B2

Title: Earphone and acoustic transducer

Description:
BACKGROUND 
     The present disclosure relates to an earphone and an acoustic transducer and, in particular, those using a balanced armature unit. 
     An example of an acoustic transducer for use in an earphone and others is a so-called balanced armature unit as described in U.S. Pat. No. 6,751,326. 
     An example of an outer view of the balanced armature unit is depicted in  FIG. 10 . In this balanced armature unit, a yoke, a coil, an armature, and a diaphragm are accommodated in an accommodation casing  100  with a predetermined arrangement. With a driving current flowing through the coil, the armature is vibrated. That vibration is transferred to the diaphragm to produce sound. 
     In this balanced armature unit, a funnel-shaped member  101  is integrally mounted on the accommodation casing  100 . This member  101  has a sound output hole  101   a.    
     The sound obtained by the inner diaphragm is outputted from the sound output hole  101   a  of the member  101  to the outside of the unit. 
     This balanced armature unit is mounted inside of an earphone. In this case, in order to guide sound to an earpiece part to be inserted in an ear hole of a user, a sound path in an earphone casing and the member  101  are coupled together with a tube. 
     SUMMARY 
     In the earphone structure using this balanced armature unit, however, the following problems arise. 
     First, although the balanced armature unit itself is suitable for decreasing the size because the driving system and the vibrating system are accommodated in one unit case, in order to couple the funnel-shaped member  101  and the sound path in the earphone casing together with the tube, a space for disposing the tube is provided, thereby restricting a decrease in the entire size of the earphone. 
     Also, obtaining the funnel-shaped member  101  by processing with a high degree of accuracy and mounting it with hermeticity to the sound path is a task at a high degree of difficulty, which is disadvantageous in manufacturing cost and manufacturing efficiency. Moreover, due to the accuracy and degree of difficulty in mounting of the member  101 , hermeticity of the sound path is less prone to being stable and as a result, acoustic performance for each product is less prone to being stable. 
     Still further, since the diameter (the sectional area) of the sound output hole  101   a  of the member  101  is not large by reason of structure, attenuation occurs in high frequencies. Therefore, a loss in sound quality occurs in the sound reaching the user&#39;s ear. 
     It is desirable to provide an earphone using a balanced armature unit, the earphone that is easy to manufacture, advantageous for decreasing the size, and stable in acoustic performance. 
     An acoustic transducer according to an embodiment of the present disclosure includes an accommodation casing having accommodated therein a yoke on which paired magnets disposed so as to face each other are mounted, a coil to which a driving current is supplied, an armature provided with a vibrating part vibrating when the driving current is supplied to the coil, the vibrating part being disposed between the paired magnets through the coil, and a diaphragm coupled to the vibrating part of the armature. A sound output hole is formed on a surface that faces a vibration surface of the diaphragm in the accommodation casing. 
     An earphone according to another embodiment of the present disclosure includes an earphone casing inside which a sound path that guides sound to a sound discharging hole is formed and the acoustic transducer according to the embodiment of the present disclosure disposed inside the earphone casing. The acoustic transducer has the sound output hole disposed in the earphone casing so that the sound output hole is acoustically coupled to the sound path. 
     For example, the acoustic transducer has the sound output hole disposed in the earphone casing so that the sound output hole faces the sound path. In this case, the sound output hole may face the sound path via a shock absorbing member. 
     Alternatively, an air chamber communicating with the sound path is formed in the earphone casing in which the acoustic transducer is disposed, and the acoustic transducer is disposed in the earphone casing so that an output sound from the sound output hole reaches the sound path via the air chamber. 
     Alternatively, the acoustic transducer is disposed in the earphone casing so that the sound output hole is positioned inside the sound path. 
     Also, the sound output hole of the acoustic transducer is formed at a position eccentric from a center on the surface that faces the vibration surface of the diaphragm. 
     In these embodiments of the present disclosure, the acoustic transducer of the embodiment of the present disclosure as a balanced armature has a sound output hole formed on one surface of the accommodation casing, that is, a surface that faces the diaphragm inside. In this structure, the funnel-shaped member as depicted in  FIG. 10  can be omitted, and the sound output hole can be increased. 
     In the earphone of the embodiment of the present disclosure, the acoustic transducer is disposed so that the sound output hole of this acoustic transducer communicates with the sound path in the earphone casing directly or via the shock absorbing member. Therefore, a member that couples the sound output hole of the acoustic transducer and the sound path in the earphone casing together can be omitted. 
     According to the embodiments of the present disclosure, the acoustic transducer can have a simple structure with a funnel-shaped member being omitted, and can prevent the occurrence of a loss in sound quality at the sound output hole. 
     Also, when the acoustic transducer is mounted inside the earphone casing, all you have to do is to dispose the sound output hole of the acoustic transducer so that the sound output hole communicates with the sound path. Therefore, a member such as a tube can be omitted, which makes mounting easy and is also advantageous in decreasing the size of the earphone. Furthermore, instability in acoustic performance depending on mounting accuracy can be avoided. 
     For these reasons, an earphone that is easy to manufacture, simple in structure, suitable for decreasing the size, and stable in acoustic characteristics can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1E  are a plan view, a side view, a bottom view, a front view, and a perspective view, respectively, of an acoustic transducer of an embodiment of the present disclosure; 
         FIG. 2  is an exploded perspective view of the acoustic transducer of the embodiment; 
         FIG. 3  is a sectional view depicting an inner structure of the acoustic transducer of the embodiment; 
         FIGS. 4A and 4B  are perspective views of an earphone of another embodiment; 
         FIGS. 5A and 5B  illustrate a first example of structure of the earphone of the other embodiment; 
         FIGS. 6A to 6C  illustrate a second example of structure of the earphone of the other embodiment; 
         FIG. 7  illustrates a third example of structure of the earphone of the other embodiment; 
         FIGS. 8A to 8C  illustrate a fourth example of structure of the earphone of the other embodiment; 
         FIGS. 9A to 9C  illustrate a fifth example of structure of the earphone of the other embodiment; and 
         FIG. 10  illustrates an outer view of a balanced armature unit. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Description of the acoustic transducer and the earphone according to embodiments of the present disclosure is made according to the following sequence. 
     1. Structure of the Acoustic Transducer 
     2. First Example of Structure of the Earphone 
     3. Second Example of Structure of the Earphone 
     4. Third Example of Structure of the Earphone 
     5. Fourth Example of Structure of the Earphone 
     6. Fifth Example of Structure of the Earphone 
     7. Modification Example 
     1. Structure of Acoustic Transducer 
     An acoustic transducer of an embodiment is first described. The acoustic transducer of the embodiment is of a type of a so-called balanced armature unit. 
       FIGS. 1A to 1E  are a plan view, a side view, a bottom view, a front view, and a perspective view, respectively, of an acoustic transducer  1  of the embodiment. 
     In the acoustic transducer  1 , an accommodation unit  4  is formed of a case body  26  and a cover body  27  depicted in  FIGS. 1A to 1E . This accommodation unit  4  serves as a casing of the acoustic transducer  1 , and has disposed therein a yoke, a coil, an armature, a diaphragm, and others, as will be described further below. 
     From the accommodation unit  4 , a circuit board  8  (a flexible board) is led. On this circuit board  8 , a circuit part for supplying a driving signal to an inner coil is formed. 
     In the present embodiment, the funnel-shaped member  101  in the balanced armature unit of the related art in  FIG. 10  is not provided. On an upper surface of the cover body  27  forming the accommodation unit  4 , a sound output hole  27   a  is formed as depicted in the drawings. This sound output hole  27   a  is formed in the accommodation unit  4  and on a surface that faces a vibration surface of an inner diaphragm unit  3 , which will be described further below. 
     Also in this example, the sound output hole  27   a  is formed at a position eccentric in a longitudinal direction from a center on the upper surface of the cover body  27 . 
       FIG. 2  is an exploded perspective view of the acoustic transducer  1 , and  FIG. 3  is a sectional view of the acoustic transducer  1 . With reference to these  FIGS. 2 and 3 , an example of an inner structure of the acoustic transducer  1  is described. 
     Note that, in the description of the structure of the acoustic transducer  1  below, a side depicted in the front view of  FIG. 1D  is taken as front, a direction in which the circuit board  8  is led is taken as rear, and representations of front, rear, above, below, left, and right are made accordingly. However, these front, rear, above, below, left, and right are merely for convenience of description. 
     As depicted in  FIGS. 2 and 3 , the acoustic transducer  1  is configured to have a driving unit  2  and the diaphragm unit  3  accommodated in the accommodation unit  4 . 
     The driving unit  2  includes a yoke  5 , paired magnets  6 , a coil  7 , a circuit board  8 , and an armature  9 . 
     The yoke  5  is formed of a first member  10  in a flat shape oriented in a vertical direction and a second member  11  in an invertedeC shape that is open upward, the first member  10  and the second member  11  coupled together. The first member  10  has both of left and right sides attached onto an inner surface side of the side surfaces of the second member  11  by bonding or the like. With this, the first member  10  and the second member  11  are coupled together to form the yoke  5  in a square rod shape that is hollow in a longitudinal direction. 
     Inside this yoke  5 , the paired magnets  6  are attached. The magnets  6  are disposed so as to be separated from each other and face each other in the vertical direction, with different polarities on facing sides. The magnet  6  positioned above is attached to a lower surface of the first member  10 , and the magnet  6  positioned below is attached to an upper surface of a bottom part of the second member  11 . 
     The coil  7  is formed in a cylindrical shape with its axial direction being set in a longitudinal direction and, for example, is formed in a long hole shape. The coil  7  is in regular winding, with its upper surface and lower surface each being formed in a flat shape. 
     The circuit board  8  is mounted on the upper surface of the coil  7 . The circuit board  8  has a length in the longitudinal direction longer than a length of the coil  7  in the longitudinal direction, and is partially mounted on the upper surface of the coil  7 . An approximately rear half of this circuit board  8  protrudes rearward from the coil  7  and, furthermore, a rear side of the circuit board  8  is led from the accommodation unit  4 . 
     Both ends of the coil  7  are connected to predetermined terminals on the circuit board  8 , thereby forming an electric circuit for applying a driving current to the coil  7 . 
     Note that because the coil  7  is in regular winding with its upper surface being formed in a flat shape, an excellent joint state between the coil  7  and the circuit board  8  can be ensured. 
     The armature  9  has parts made of a magnetic metal material and integrally formed. 
     The armature  9  has a coil mounting part  12  oriented in the vertical direction, a coupling part  13  protruding upward from a rear end of the coil mounting part  12 , a vibrating part  14  protruding forward from an upper end of the coupling part  13 , side wall parts  15  protruding upward from both left and right ends of the coil mounting part  12 , and fixed parts  16  protruding forward from front surfaces of the side wall parts  15  in their approximately front half portion. 
     The vibrating part  14  has a length in the longitudinal direction longer than a length of the coil mounting part  12  in the longitudinal direction, and has its front end positioned forward from a front end of the coil mounting part  12 . 
     At a center of a front surface of this vibrating part  14  in a horizontal direction, a coupling recessed part  14   a  that is open forward is formed. 
     Upper surfaces of the side wall parts  15  and upper surfaces of the fixed parts  16  are coplanar, and these coplanar surfaces positioned so as to be horizontally separated apart from each other are formed as fixing surfaces  17 . 
     On an upper surface of the coil mounting part  12 , the coil  7  is mounted by bonding, for example. Since the coil  7  is in regular winding and has its lower surface formed in a flat shape, an excellent joint state of the coil  7  with respect to the coil mounting part  12  can be ensured. 
     As can be seen from  FIG. 3 , in the state where the coil  7  is mounted on the coil mounting part  12 , the vibrating part  14  penetrates through the coil  7  and partially protrudes forward from the coil  7 . 
     In this acoustic transducer  1 , the coil mounting part  12  on which the coil  7  is mounted and the vibrating part  14  to penetrate through the coil  7  are both provided on the armature  9 . Therefore, the position of the vibrating part  14  with respect to the coil  7  can be ensured with a high degree of accuracy, and accuracy in position of the vibrating part  14  with respect to the coil  7  can be improved. 
     In the armature  9 , with the coil  7  mounted on the coil mounting part  12 , the fixed parts  16  are fixed by bonding or welding on outer surfaces of side surfaces of the yoke  5 . With the armature  9  fixed to the yoke  5 , upper surfaces of side surfaces of the yoke  5  are positioned slightly upward from the fixed surfaces  17  of the armature  9 . Also, the coupling recessed part  14   a  formed at the front end of the vibrating part  14  is positioned slightly forward from directly under the front ends of the magnets  6 . 
     Note that although the armature  9  having the parts integrally formed is taken as an example here, the armature can be any as long as it has at least the vibrating part, which is to be magnetized, formed of a magnetic metal material. 
     The diaphragm unit  3  includes a holding frame  20 , a resin film  21 , a diaphragm  22 , and a beam part  23 . 
     The holding frame  20  is made of a metal material and is formed in a longitudinally elongated frame shape, with its width in the horizontal direction approximately equal to the armature  9  in the horizontal direction. 
     The resin film  21  is formed so as to be equal in size to that of the outer shape of the holding frame  20 , and is affixed by bonding or the like to an upper surface of the holding frame  20  so as to enclose an opening of the holding frame  20 , for example. 
     The diaphragm  22  is made of a metal material having a thin thickness, for example, aluminum or stainless steel, and its outer shape is formed in a rectangular shape with a size slightly smaller than an inner shape of the holding frame  20 . In the diaphragm  22 , reinforcing ribs  22   a  longitudinally extending and positioned so as to be horizontally separated from one another are provided. These reinforcing ribs  22   a  are each formed so as to rise upward. 
     The diaphragm  22  is assumed to be in a state of, for example, being affixed from below to the resin film  21 . 
     The diaphragm  22  has a rear end positioned slightly forward from an inner surface of the holding frame  20  at a rear end, and a gap is formed between the rear end of the diaphragm  22  and the inner surface at the rear end of the holding frame  20 . As depicted in  FIG. 3 , an adhesive agent  24  is applied so as to fill this gap. Therefore, the diaphragm  22  and the holding frame  20  are connected via the adhesive agent  24  and the resin film  21 . 
     As the adhesive agent  24 , for example, an acrylic-base non-curable adhesive agent or an acrylic-base ultraviolet-curable adhesive agent is used. 
     Note that the adhesive agent  24  fills the gap and extends to a surface opposite to a side of the diaphragm  22  affixed to the resin film  21 . That is, while the diaphragm  22  is supported by the holding frame  20  with the resin film  21 , the adhesive agent  24  functions as a reinforcing member for reinforcing this support. 
     The beam part  23  is integrally formed with the diaphragm  22 . For example, the beam part  23  is formed by partially bending the diaphragm  22  downward. The beam part  23  is formed, for example, in a narrow plate shape extending in the vertical direction. 
     The diaphragm unit  3  is fixed to the driving unit  2  from above by boding or laser welding, for example. That is, the diaphragm unit  3  is fixed, with the lower surface of the holding frame  20  being jointed to the fixing surfaces  17  of the armature  9 . 
     Also, when the diaphragm unit  3  is fixed to the driving unit  2 , a lower end of the beam part  23  is attached to a front end of the vibrating part  14  in the armature  9  by bonding. That is, the beam part  23  is coupled to the armature  9  with an adhesive agent  25  as being inserted in the coupling recessed part  14   a  formed in the vibrating part  14 . 
     As described above, since the beam part  23  is integrally formed with the diaphragm  22 , only by attaching the lower end of the beam part  23  to the vibrating part  14 , the diaphragm  22  and the armature  9  are coupled together via the beam part  23 , and the structure in which vibrations of the vibrating part  14  of the armature  9  are transferred to the diaphragm  22  is formed. 
     Note that the shape of the beam part  23  is not meant to be restricted to the narrow plate shape. 
     As described with reference to  FIGS. 1A to 1E , the accommodation unit  4  includes the case body  26  in a box shape that is open upward and the cover body  27  in a shallow box shape that is open downward. 
     In the case body  26 , the sound output hole  27   a  is formed on the upper surface. This surface where the sound output hole  27   a  is formed is a surface facing the vibration surface of the inner diaphragm  22 . 
     As depicted in  FIG. 3 , a sound due to vibrations of the diaphragm  22  is discharged into a space  31  above the vibration surface. This sound is outputted from the sound output hole  27   a  formed at the position facing the vibration surface. 
     In the above-structured acoustic transducer  1 , a driving current based on a sound signal is applied to the coil, the vibrating part  14  of the armature  9  inserted in the coil  7  and between the magnets  6  of the yoke  5  vibrates. 
     The vibrations of the vibrating part  14  are transmitted to the diaphragm  22  via the beam part  23  to cause the diaphragm  22  to vibrate. The sound due to the vibrations of the diaphragm  22  is discharged to the space  31  above the vibration surface depicted in  FIG. 3 . This sound is outputted from the sound output hole  27   a  formed on the surface facing the vibration surface. 
     The acoustic transducer  1  of the present embodiment is different from the acoustic transducer in related art depicted in  FIG. 10  in that the sound output hole  27   a  is formed on one surface of the accommodation unit  4  and no funnel-shaped member is provided. 
     In this structure, since the funnel-shaped member can be omitted, a decrease in size of the acoustic transducer  1  can be promoted, and ease in manufacturing is also increased. 
     Furthermore, as a matter of course, an increase in accuracy of the funnel-shaped member can also be omitted. 
     Still further, the sound output hole  27   a  is formed on the flat part on the upper surface of the cover body  27 , and can have a wide hole area. Therefore, a decrease in high frequency characteristics of output sound due to passage of the sound output hole  27   a  can also be prevented. 
     Still further, since the sound output hole  27   a  is formed on the upper surface of the cover body  27 , which is a relatively wide surface, flexibility of the position for forming the sound output hole  27   a  is high. 
     2. First Example of Structure of the Earphone 
     Examples of structure of an earphone according to the embodiment having the above acoustic transducer  1  are described. 
       FIGS. 4A and 4B  are perspective view of the outer shape of an earphone  50 . The earphone  50  has a casing formed of a front casing  52  and a rear casing  51  jointed together. To the front casing  52 , an earpiece  53  is attached, which is inserted in the ear hole of the user. 
     Note that a cord for supplying a sound signal to the earphone is omitted in the drawings. 
       FIG. 5A  depicts a first example of structure of the earphone  50 .  FIG. 5B  is an enlarged view of a portion surrounded by a one-dot-chain line in  FIG. 5A . 
     As depicted in  FIG. 5A , the front casing  52  and the rear casing  51  are jointed together to form each space in the earphone casing. That is, a sound path tube  90  is formed inside on a front casing  52  side. Also, the front casing  52  and the rear casing  51  form an accommodation space  91 . Also, a cord space  92  in which the cord not shown is inserted is formed on a rear casing  51  side. 
     To the front casing  52 , the earpiece  53  made of, for example, a flexible material, is mounted as being engaged, for example. In this state, a sound discharging hole  53   a  of the earpiece  53  linearly communicates with the sound path tube  90 . 
     In this case, the acoustic transducer  1  described above is fixed and disposed in the accommodation space  91 . Here, as described above, the acoustic transducer  1  has the sound output hole  27   a  formed on the upper surface of the casing (the cover body  27 ). In  FIG. 5A , the position of the sound output hole  27   a  is represented by a broken line for description. 
     As can be seen from  FIGS. 5A and 5B , the sound output hole  27   a  of the acoustic transducer  1  is directly and acoustically coupled to the sound path tube  90 . That is, the acoustic transducer  1  is disposed in the accommodation space  91  with its sound output hole  27   a  being pressed onto an end  52   a  of the sound path tube  90 . 
     With this, the sound output hole  27   a  faces the sound path tube  90 . In this case, the structure can be said such that the diaphragm  22  in the acoustic transducer  1  is disposed at an end of the sound path tube  90  via the sound output hole  27   a.    
     The circuit board  8  is connected to each of lines (a sound signal line from either one of aneL channel and an R channel and a ground line) in the cord not shown and inserted in the cord space  92 . Through the coil  7  inside the acoustic transducer  1 , a driving current based on the sound signal of either one of theeL channel and the R channel is caused to flow. 
     With the driving current flowing, a sound is outputted from the sound output hole  27   a  by vibrations of the inner diaphragm  22 . That sound directly reaches the sound path tube  90  and also reaches the user&#39;s ear hole from the sound discharging hole  53   a  of the earpiece  53 . 
     The earphone  50  with the first example of structure has effects as described below. 
     First, the acoustic transducer  1  can have a simple structure with a funnel-shaped member being omitted. 
     Also, the sound output hole  27   a  of the acoustic transducer  1  can have a relatively large hole area, and a loss in sound quality can be prevented. 
     Furthermore, when the acoustic transducer  1  is mounted inside the casing of the earphone  50 , all you have to do is to dispose the acoustic transducer  1  so that the sound output hole  27   a  faces the sound path tube  90 . Therefore, a member such as a tube can be omitted, which makes mounting easy and makes the manufacturing process efficient. Still further, in addition to improvement in manufacturing efficiency, the structure is simple with a small number of components, and therefore advantageous in decreasing cost. 
     Still further, instability in acoustic performance due to a funnel-shaped member and depending on accuracy of mounting the tube can be eliminated. 
     Still further, since the tube does not have to be accommodated in the casing, the size of the earphone casing can also be advantageously decreased. With a small-sized casing structure, an earphone insertable in a good condition with less interference with the ear pinna can be provided. 
     Still further, since the tube does not have to be accommodated in the casing, the layout of the inner structure of the earphone casing can be simplified. 
     In the acoustic transducer  1  depicted in  FIGS. 1A to 1E ,  FIG. 2 , and  FIG. 3 , the sound output hole  27   a  is formed at a position eccentric in the longitudinal direction from the center on the upper surface of the case body  26 . 
     This is suitable when the acoustic transducer  1  is disposed in the earphone casing at a position where the sound output hole  27   a  faces the sound path tube  90 . 
     If the sound output hole  27   a  is formed near the center on the upper surface of the case body  26 , in the earphone structure of  FIGS. 5A and 5B , the space where the acoustic transducer  1  is to be disposed is widened on an upper surface side (upward in the drawing) of the earphone. As the upper surface of the earphone casing is increased (for example, when the upper surface is equivalent in height to an edge of the earpiece  53 ), that portion may abut particularly on the ear pinna of the user when inserted into the ear hole, thereby possibly degrading insertability. By contrast, when the sound output hole  27   a  is formed at a position eccentric from the center on the upper surface of the case body  26  and disposed as depicted in  FIGS. 5A and 5B , the height of the upper surface of the earphone casing can be lowered, thereby allowing excellent insertability to the user. 
     However, even when the sound output hole  27   a  is formed at the center of the upper surface of the case body  26 , an earphone with excellent insertability can be formed depending on the structure of the earphone casing. 
     3. Second Example of Structure of the Earphone 
     A second example of structure of the earphone according to the embodiment is described with reference to  FIGS. 6A to 6C . 
     Note that a basic structure of the second example of structure is similar to that of the first example of structure ( FIGS. 5A and 5B ). In descriptions of the second to fifth examples of structure below, portions identical to those of the first example of structure are provided with the same reference characters and are not described. 
     In the case of  FIGS. 6A to 6C , the acoustic transducer  1  has the sound output hole  27   a  disposed so as to face the sound path tube  90  via a shock absorbing member. 
       FIG. 6A  depicts an example of structure.  FIG. 6B  is an enlarged view of a portion surrounded by a one-dot-chain in  FIG. 6A . 
     As depicted in  FIGS. 6A and 6B , the sound output hole  27   a  of the acoustic transducer  1  faces the sound path tube  90  via a shock absorbing member  80  formed of a soft material. The shock absorbing member  80  is assumed to have a flat ring shape having a center hole  80   a  as depicted in  FIG. 6C , for example. As a soft material, for example, a material, such as elastomer or a silicon rubber base material, with a low air-flow resistance is suitable. 
     That is, as abutting around the sound output hole  27   a  on the cover body  27  of the acoustic transducer  1 , the ring-shaped shock absorbing member  80  is in a state of being pressed onto the end  52   a  of the sound path tube  90  in the front casing  52 . 
     Even in the case of the structure in which the acoustic transducer  1  has the sound output hole  27   a  disposed so as to face the sound path tube  90  via the shock absorbing member  80  (the center hole  80   a ), effects similar to those of the first example of structure described above can be obtained. 
     In addition, with the acoustic transducer  1  disposed so as to being pressed via the shock absorbing member  80 , an error in molding accuracy of the front casing  52  and the rear casing  51  is effectively absorbed to eliminate the gap between the sound output hole  27   a  and sound path tube  90  and to stably mount the acoustic transducer  1 . 
     4. Third Example of Structure of the Earphone 
     A third example of structure is described with reference to  FIG. 7 . 
     In this example, the structure is such that an air chamber  93  communicating with the sound path tube  90  is formed in the earphone casing where the acoustic transducer  1  is disposed. As can be seen from  FIG. 7 , the inner structure of the front casing  52  and the acoustic transducer  1  (the upper surface side of the cover body  27 ) form the air chamber  93  with hermeticity. 
     The sound from the sound output hole  27   a  is outputted to the air chamber  93 , and this sound reaches the sound discharging hole  53   a  of the earpiece  53  from the sound path tube  90 . 
     Also in this structure, effects similar to those of the first example of structure can be obtained. Furthermore, depending on the design of the air chamber  93 , acoustic characteristics can be adjusted. 
     5. Fourth Example of Structure of the Earphone 
     A fourth example of structure is described with reference to  FIGS. 8A ,  8 B, and  8 C.  FIG. 8A  depicts an inner structure when the earphone is viewed from above,  FIG. 8B  depicts the inner structure when the earphone is viewed from a side surface direction, and  FIG. 8C  depicts a rear view of the earphone. 
     Compared with the first to third examples of structure described above, the posture of the disposed acoustic transducer  1  is different by 90 degrees in this example. 
     The front casing  52  is configured to be a casing in an approximatelyeL shape when viewed from above as depicted in  FIG. 8A . Inside the front casing  52 , the acoustic transducer  1  is disposed so as to abut on the end  52   a  of the sound path tube  90 . Accordingly, the sound output hole  27   a  faces the sound path tube  90 . 
     Also in this structure, effects similar to those of the first example of structure can be obtained. That is, although the shape of the earphone casing is varied, the sound output hole  27   a  can be disposed so as to face the sound path tube  90 , while the posture of the disposed inner acoustic transducer  1  is changed according to the shape of the earphone casing. 
     In view of this idea, other examples can be thought such that the acoustic transducer  1  is disposed as being tilted leftward or rightward or as being tilted forward or backward in the earphone casing. 
     Note that also in the example of structure in  FIGS. 8A to 8C , as in the example of  FIGS. 6A to 6C , the sound output hole  27   a  may face the sound path tube  90  via the shock absorbing member  80 . 
     Also, the acoustic transducer  1  of the present example has the sound output hole  27   a  formed at the position eccentric from the center on the upper surface of the cover body  27 . In the structure as depicted in  FIGS. 8A to 8C , however, with the sound output hole  27   a  being at an eccentric position, the layout of the inner structure and the outer shape design of the earphone casing can be facilitated. 
     6. Fifth Example of Structure of the Earphone 
     A fifth example of structure is described with reference to  FIGS. 9A to 9C . 
       FIG. 9A  is a sectional view of an earphone structure, and  FIG. 9B  is a perspective view of a section of an inner structure.  FIG. 9C  is a perspective view of an example of an outer view of the earphone. 
     This fifth example of structure is advantageous for, by way of example, an extremely small earphone casing such that the front casing  52  and the rear casing  51  are almost entirely hidden from the earpiece  53  as depicted in  FIG. 9C . 
     As depicted in  FIGS. 9A and 9B , in the earphone casing, the acoustic transducer  1  is disposed so as to take a posture in which its front end (the portion depicted in  FIG. 1D ) is on an earpiece  53  side. 
     And, the cover body  27  having the sound output hole  27   a  is in a state of forming the side surface of the sound path tube  90 , and the sound output hole  27   a  is positioned inside the sound path tube  90 . 
     Also in this case, the sound outputted from the sound output hole  27   a  reaches the sound discharging hole  53   a  of the earpiece  53  from the sound path tube  90 , and is guided to the ear hole of the user. 
     Also in this example of structure, effects similar to those of the first example of structure can be obtained. In particular, since the structure is such that part of the acoustic transducer  1  enters the inside the sound path tube  90 , the structure is very advantageous for decreasing the size of the earphone casing. 
     7. Modification Example 
     While various examples of structure as embodiments have been described, the structure of the earphone of the embodiments of the present disclosure is not meant to be restricted to the examples described above. The outer and inner structures of the earpiece  53 , the front casing  52 , and the rear casing  51  can be variously assumed. In any event, these structures can be any as long as the sound output hole  27   a  of the acoustic transducer  1  can be disposed so as to be acoustically coupled directly to the sound path that guides sound to the user&#39;s ear hole without any coupling member such as a tube. 
     While the earpiece  53  is structured to be mounted on the front casing  52  separately from the earphone casing in the description above, the earpiece part may be integrally formed as part of the earphone casing. That is, a sound discharging hole for outputting sound to the user may be formed of the earpiece or the earphone casing. 
     Also, the structure of the acoustic transducer  1  is not meant to be restricted to the examples depicted in  FIGS. 1A to 1E ,  FIG. 2 , and  FIG. 3 . The acoustic transducer  1  of the embodiments of the present disclosure may be any as long as it has the sound output hole  27   a  formed on the surface that faces the vibration surface of the diaphragm  22  inside. 
     The shape of the sound output hole  27   a  can be variously assumed in addition to a circle, such as an oval, an ellipse, a polygon, and any other various figures. 
     Also, the hole size of the sound output hole  27   a  can be variously assumed. In particular, in consideration of a loss in high frequency, it is suitable to set a large hole area. 
     The present disclosure contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2010-229527 filed in the Japan Patent Office on Oct. 12, 2010, the entire contents of which are hereby incorporated by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.