Patent Publication Number: US-11051097-B2

Title: Installation structure of vibrator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2019-159004 filed with the Japan Patent Office on Aug. 30, 2019, the entire content of which is hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     One aspect of the present disclosure relates to an installation structure for installing a vibrator on a listening device. 
     2. Related Art 
     In recent years, as a vibrator having a structure in which an electromechanical transducer that transduces an electric signal into mechanical vibration is accommodated in a housing, a structure having a small size and a small mass has been proposed (for example, see Japanese Patent No. 5653543). Such a vibrator that has been made smaller and lighter is suitable for use in, for example, a device that can be worn on an ear. Examples of this type of device include a wireless earphone and a listening device such as a headset connectable to a mobile phone. Then, by wearing the above-mentioned listening device, on which the vibrator is installed, on a user&#39;s ear and disposing the vibrator to contact skin near ear cartilage, sound can be transmitted through a cartilage conduction course. 
     SUMMARY 
     An installation structure of a vibrator includes elastic members formed of an elastic material, the elastic members being arranged between a housing of a listening device, and the vibrator accommodating an electromechanical transducer for transducing an electric signal into mechanical vibration. The vibrator is installed on the listening device such that a lower surface of the vibrator is disposed at a position facing an ear cartilage in a state where the listening device is worn on an ear, and a first mechanical impedance of the elastic members between the vibrator and the housing is set smaller, at a frequency of 200 Hz to 1000 Hz, than twice a second mechanical impedance, with which the vibrator is loaded, of the ear cartilage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a mechanical model according to one aspect of the present disclosure; 
         FIG. 2  illustrates an equivalent circuit corresponding to the mechanical model of  FIG. 1 ; 
         FIG. 3  schematically illustrates an example of a state where a device of  FIG. 1  is worn on a human ear; 
         FIG. 4  is a perspective view of an installation structure of a first embodiment; 
         FIG. 5  is a partial cross-sectional view of the installation structure of the first embodiment; 
         FIG. 6  is a partial cross-sectional view of the installation structure of a second embodiment; 
         FIG. 7  is a top view of the installation structure of the second embodiment; 
         FIG. 8  is a partial cross-sectional view like  FIG. 6  regarding a modification of the second embodiment; 
         FIG. 9  is a partial cross-sectional view of the installation structure of a third embodiment; 
         FIG. 10  is a top view of the installation structure of the third embodiment; 
         FIG. 11  is a perspective view of the installation structure of a fourth embodiment; 
         FIG. 12  is a partial cross-sectional view of the installation structure of the fourth embodiment; and 
         FIG. 13  is a partial cross-sectional view like  FIG. 12  regarding a modification of the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     When size and weight of a vibrator are reduced as in recent years, influence of mass of a housing itself that accommodates the vibrator and a device (listening device or the like) on which the vibrator is installed is a problem. That is, if the mass of the vibrator is relatively larger than the mass of the device, the vibrator is less likely to be affected by the device. On the other hand, as the mass of the vibrator is relatively smaller due to weight reduction, the vibrator is more strongly affected by the mass of the device. Specifically, when the vibrator is simply joined to the device, a vibration force generated by the vibrator with a small mass is unintentionally used to vibrate the device, and for example, it is difficult to efficiently transmit vibration to be transmitted to ear cartilage. As a result, when the vibrator having a relatively small mass is driven in a state where it is installed on the device, there is a possibility that sound volume and vibration level may be lower than when the vibrator alone is driven. 
     An object of the present disclosure is to provide an installation structure of a vibrator that can transmit vibration to the ear cartilage with good transmission efficiency even when the vibrator with a small mass is installed on the listening device. 
     In order to address the above-described problem, an installation structure of a vibrator (this installation structure) according to an aspect of the present disclosure includes elastic members ( 22 ) formed of an elastic material, the elastic members being arranged between a housing ( 21 ) of a listening device, and the vibrator ( 20 ) accommodating an electromechanical transducer for transducing an electric signal into mechanical vibration. The vibrator is installed on the listening device such that a lower surface of the vibrator is disposed at a position facing an ear cartilage in a state where the listening device is worn on an ear, and a first mechanical impedance (r 2 −js 2 /ω) of the elastic members between the vibrator and the housing is set smaller, at a frequency of 200 Hz to 1000 Hz, than twice a second mechanical impedance (zc), with which the vibrator is loaded, of the ear cartilage. 
     According to the present installation structure, the vibration is transmitted to the ear cartilage from the vibrator located at a position facing the ear cartilage in a state where the listening device is worn on the ear. At this time, since a first mechanical impedance of the elastic members between the housing of the listening device and the vibrator is sufficiently small, even when the mass of the vibrator is smaller than that of the listening device, energy of vibration is suppressed from being transmitted to the listening device. As a result, the vibration from the vibrator can be efficiently transmitted to the ear cartilage. 
     In the present installation structure, the elastic members may constitute a pair of elastic members ( 22 ,  23 ) including a second elastic member ( 23 ) that is disposed on an upper surface facing the lower surface of the vibrator and applies a pressing force to the vibrator, and, in this case, the pair of elastic members sandwiches and holds the vibrator. In this structure, due to the pressing force of the pair of elastic members, the vibrator slightly projects from the housing side of the listening device toward the ear cartilage. Therefore, the vibration of the vibrator is easily transmitted to the ear cartilage. 
     The present installation structure may further include a protrusion ( 24 ) provided on the lower surface of the vibrator and projecting toward the ear cartilage, and a recess ( 22   c ) provided on the elastic member and conforming to a shape of the protrusion, and the vibrator may be held by the elastic member with the protrusion being fitted in the recess. Thus, the vibrator can be stably held through the recess of the elastic member. In addition, the protrusion of the elastic member can be necessarily projected toward the ear cartilage. In this case, the recess of the elastic member can be formed such that its central axis is the same as a central axis of the columnar member as the protrusion and it has a diameter substantially the same as that of the columnar member. 
     The present installation structure may further include a flexible porous body ( 25 ) disposed on the upper surface facing the lower surface of the vibrator, and a holder ( 21   d ) provided in the housing and holding the porous body. In this case, the first mechanical impedance is a combined mechanical impedance of the elastic member and the porous body. As the flexible porous body, for example, sponge is used. Thus, the vibrator can be stably held by applying the pressing force to the porous body through the holder. In addition, the mass added to the vibrator can be suppressed 
     The installation structure may further include an inner peripheral portion ( 22   f ) provided in the elastic member and covering a side surface of the vibrator, and the vibrator may be held by the elastic member while contacting the inner peripheral portion. Thus, the side surface of the vibrator is stably held by the inner peripheral portion of the elastic member. In addition, a total number of members can be reduced, and the structure can be simplified. 
     As described above, according to the present installation structure, the first mechanical impedance of the elastic members between the vibrator and the housing of the listening device is set smaller than twice the second mechanical impedance, with which the vibrator is loaded, of the ear cartilage. Therefore, even when the vibrator with a small mass is installed on the listening device, it is possible to increase the transmission efficiency of the vibration from the vibrator to the ear cartilage. The present installation structure is a structure in which the vibrator is not directly fixed to the housing of the listening device. Therefore, it is possible to suppress unnecessary vibration applied to the housing of the listening device. 
     Hereinafter, preferred embodiments of the present disclosure will be described with reference to the accompanying drawings. However, the embodiment described below is an example of a mode to which technology of the present disclosure is applied. The technology of the present disclosure is not limited by contents of the present embodiment. Hereinafter, a mode in which the technique of the present disclosure is applied to the vibrator that is installed on the listening device and transmits the vibration (sound) using cartilage conduction will be described. 
       FIG. 1  illustrates a mechanical model for studying characteristics required for the vibrator according to an embodiment of the present disclosure.  FIG. 1  illustrates a vibrator  10  and a device  11  such as a listening device on which the vibrator  10  is installed. The vibrator  10  includes an internal vibrator body  10   a  and a vibrator case  10   b  that covers the vibrator body  10   a . The ear cartilage is illustrated at the bottom of  FIG. 1 . The ear cartilage (skin is omitted) and one end of the vibrator  10  are in contact with each other at a contact portion Ca. A human head is illustrated at the top of  FIG. 1 . The head and one end of the device  11  are in contact with each other at a contact portion Cb. 
     In the mechanical model of  FIG. 1 , the vibrator body  10   a , the vibrator case  10   b , and the device  11  respectively have masses m 1 , m 2 , and m 3 . A force F is applied between the vibrator body  10   a  and the vibrator case  10   b . The contact portions Ca and Cb, a space between the vibrator body  10   a  and the vibrator case  10   b , and a space between the vibrator case  10   b  and the device  11  can be modeled using springs and dampers. Here, the contact portion Ca, the space between the vibrator body  10   a  and the vibrator case  10   b , the space between the vibrator case  10   b  and the device  11 , and the contact portion Cb are respectively supposed to have stiffnesses of springs s 0 , s 1 , s 2 , and s 3  and have mechanical resistances r 0 , r 1 , r 2 , and r 3  of the dampers. 
     Here, vibration displacements of the masses m 1 , m 2 , and m 3  are respectively supposed to be x 1 , x 2 , and x 3 . Vibration displacement of the ear cartilage is supposed to be x 0 . The mechanical impedance of the ear cartilage, with which the vibrator  10  is loaded, is supposed to be zc. In this case, the following motion equations of formulas (1) to (4) are established for the mechanical model of  FIG. 1 . Here, ′ (dash) represents a time derivative. For example, x 1 ′ represents velocity and x 1 ″ represents acceleration.
 
[Equation 1]
 
 m   1   x   1   ″=F−r   1 ( x   1   ′−x   2 ′)− s   1 ( x   1   −x   2 )  (1)
 
 m   2   x   2   ″=−F−r   1 ( x   2   ′−x   1 ′)− s   1 ( x   2   −x   1 )− r   2 ( x   2   ′−x   3 ′)− s   2 ( x   2   −x   3 )− r   0 ( x   2   ′−x   0 ′)− s   0 ( x   2   −x   0 )  (2)
 
 m   3   x   3   ″=−r   2 ( x   3   ′−x   2 ′)− s   2 ( x   3   −x   2 )− r   3   x   3   ′−s   3   x   3   (3)
 
 z   c   x   0   ′=−r   0 ( x   0   ′−x   2 ′)− s   0 ( x   0   −x   2 )  (4)
 
     An electrical circuit equivalent to the mechanical model can be derived from the above formulas (1) to (4).  FIG. 2  illustrates an equivalent circuit corresponding to the mechanical model of  FIG. 1 . The velocities x 0 ′, x 1 ′, x 2 ′, and x 3 ′ in the formulas (1) to (4) respectively correspond to currents in the equivalent circuit. Comparing  FIG. 1  and  FIG. 2 , the force F corresponds to an electromotive force of the equivalent circuit. The masses m 1  to m 3  correspond to inductances of the equivalent circuit. The stiffnesses s 0  to s 3  correspond to capacitances of the equivalent circuit (its value is a reciprocal of stiffness value). The mechanical resistances r 0  to r 3  correspond to resistances of the equivalent circuit. The mechanical impedance zc corresponds to an impedance of the equivalent circuit. 
     An object in the mechanical model of  FIG. 1  is to apply as much vibration as possible to the ear cartilage. In this regard, since the vibrator  10  and the ear cartilage are in contact with each other at the contact portion Ca(r 0 , s 0 ), the vibration applied to the ear cartilage is increased by increasing zc·x 0 ′ illustrated in  FIG. 2  as much as possible. On the other hand, since the mass m 3  of the device  11  is large, the mechanical impedance r 2 −js 2 /ω between the vibrator  10  and the device  11  is preferably set small in  FIG. 2 . If the mechanical impedance r 2 −js 2 /ω is large, the vibration x 2 ′ is extremely small. On the other hand, it is preferred that the mechanical impedance r 0 −js 0 /ω of the contact portion Ca(r 0 , s 0 ) described above is set large. 
     Here, when the mechanical impedance of the ear cartilage was verified, it was confirmed that this impedance was approximately 5 (Ns/m) in a frequency range of 200 to 1000 Hz. An actual mechanical impedance value of the ear cartilage is supposed to change due to individual differences or the like. In the installation structure of the vibrator  10  of the present embodiment, the elastic member is placed between the vibrator  10  and the device  11  in order to achieve the above operational effects. The elastic member has a mechanical impedance (corresponding to r 2 −js 2 /ω in  FIG. 2 ) smaller than the mechanical impedance of the ear cartilage corresponding to the mechanical impedance zc. This realizes a structure for efficiently vibrating the ear cartilage. A mechanical impedance value decreases in inverse proportion to a frequency. Therefore, even when the mechanical impedance value of stiffness of the elastic member is 10 Ns/m at a frequency of 100 Hz, the mechanical impedance of the elastic member only needs to be about 10 Ns/m or less. 
       FIG. 3  schematically illustrates an example of a state where the device  11  of  FIG. 1  is actually worn on a human ear. As illustrated in  FIG. 3 , the device  11  has a shape that fits a shape of the ear. The device  11  worn on the ear is held sandwiched between a pinna and the head. At this time, the lower surface of the vibrator  10  installed on the device  11  is disposed at a position facing the ear cartilage through the skin of the ear. Thus, when the vibrator  10  is driven, the vibration of the vibrator  10  is transmitted through the ear cartilage. A wearing state on the ear illustrated in  FIG. 3  can be applied to the installation structures of the first to fourth embodiments, which will be specifically described below. 
     First Embodiment 
     Hereinafter, a first embodiment of the present disclosure will be described with reference to  FIGS. 4 and 5 . The installation structure according to the first embodiment includes a vibrator  20 , a housing  21  of the listening device on which the vibrator  20  is installed, the elastic member  22  (first elastic member) disposed between the vibrator  20  and the housing  21 , and the elastic member  23  (second elastic member) disposed on the vibrator  20 . Regarding the installation structure of the first embodiment,  FIG. 4  is a perspective view and  FIG. 5  is a partial cross-sectional view. The partial cross-sectional view of  FIG. 5  includes the side surface of the vibrator  20  of  FIG. 4 , and a cross-sectional structure of other members at a substantially central position in a Y direction of  FIG. 4 . In  FIGS. 4 and 5 , for convenience of explanation, an X direction, the Y direction, and a Z direction, which are orthogonal to each other, are indicated by arrows. Meanings of the X direction, the Y direction, and the Z direction are the same in  FIG. 6  and subsequent drawings. 
     The vibrator  20  has a structure in which an electromechanical transducer that transduces an electric signal into the vibration is accommodated therein. The upper surface of the vibrator  20  in the Z direction is defined as an upper surface, and the lower surface in the Z direction is defined as a lower surface. The electromechanical transducer constituting a main body of the vibrator  20  includes, for example, a yoke, a coil, a magnet, an armature, an electric terminal, and the like (not shown). The housing  21  is, for example, the housing of the listening device such as an earphone on which the vibrator  20  is installed. An entire listening device including the housing  21  actually includes a structure that extends upward in the Z direction as illustrated in  FIG. 3 . In  FIGS. 4 and 5 , only a bottom surface portion of an entire housing of the listening device is illustrated as the housing  21 , and the other structures of the housing are omitted. 
     The elastic member  22  is made of an elastic material having a predetermined elastic force and has a rectangular plate shape. A central portion  22   a  of the elastic member  22  at a center in the X direction is disposed on the lower surface of the vibrator  20 . Both ends  22   b  of the elastic member  22  on both sides in the X direction are fixed to an upper surface of the housing  21 . The central portion  22   a  of the elastic member  22  corresponds to the contact portion Ca in  FIG. 1 . This portion contacts the skin near the human ear cartilage. That is, the vibration of the vibrator  20  is transmitted to the ear cartilage through the elastic member  22 . An opening  21   a  is formed in the housing  21 . A pair of projecting portions  21   b  slightly projecting in the Z direction is formed on both sides of the housing  21  in the X direction. Further, the housing  21  is formed with a pair of slit portions  21   c  adjacent to both sides of the pair of projecting portions  21   b  in the X direction. The opening  21   a  is formed in a region surrounding the vibrator  20  when viewed from the Z direction. Then, the elastic member  22  has a structure in which the central portion  22   a  overlaps the region of the opening  21   a , and a portion from the central portion  22   a  to the both ends  22   b  is bent upward through the pair of slit portions  21   c  described above. 
     The elastic member  23  is made of an elastic material having a predetermined elastic force, and has a rectangular plate shape. A central portion  23   a  of the elastic member  23  at the center in the X direction is disposed on the upper surface of the vibrator  20 . Both ends  23   b  of the elastic member  23  on both sides in the X direction are fixed to upper surfaces of the both ends  22   b  of the elastic member  22  or the housing  21 . Therefore, the elastic member  22  and the elastic member  23  (the pair of elastic members) have a structure in which the vibrator  20  is sandwiched from above and below in the Z direction. The elastic member  23  extends from the central portion  23   a  to the both ends  23   b , and its cross-section is inclined. The elastic member  23  presses the vibrator  20  downward in the Z direction by its tension. Therefore, the pressing force of the elastic member  23  acts so that an lower side of the vibrator  20  and the central portion  22   a  of the elastic member  22  slightly project downward in the Z direction of the housing  21  (outside the listening device) (not illustrated in  FIG. 5 ). Therefore, the vibration of the vibrator  20  is easily transmitted to the ear cartilage. 
     In the first embodiment, as described using the mechanical model and the equivalent circuit ( FIGS. 1 and 2 ), it is characteristic that the combined mechanical impedance (hereinafter, referred to as “first mechanical impedance”) of the elastic member  22  disposed between the vibrator  20  and the housing  21 , and the elastic member  23  disposed on the upper surface of the vibrator  20  is set smaller, at a frequency of 200 Hz to 1000 Hz, than twice the mechanical impedance (hereinafter, referred to as “second mechanical impedance”), with which the vibrator  20  is loaded, of the ear cartilage. The first mechanical impedance depends on parameters such as size, thickness, and elastic modulus of the elastic members  22  and  23 . Therefore, it is preferable to adjust the mechanical impedance value to an appropriate value by appropriately setting the parameters. Adjustment of the first mechanical impedance and its effect will be described in detail below. 
     There are various methods for fixing the both ends  22   b  of the elastic member  22  and the both ends  23   b  of the elastic member  23  to the housing  21 . As this method, there can be employed, for example, a method such as adhesion or fusion, a method in which a pin provided on the housing  21  is passed through holes provided in the elastic members  22  and  23 , or the combination of these methods. Note that it is preferred that a certain amount of tension is applied to the elastic member  23  when fixed to the housing  21 . However, it is not desired to apply unnecessary tension to the elastic member  22 . 
     Here, regarding the elastic member  22  and the elastic member  23 , the above-described method for adjusting the first mechanical impedance will be described. First, in relation to the size of the elastic member  22 , the larger an area (a length) and the thinner the thickness in the Z direction, the smaller the first mechanical impedance. Further, the larger the elastic modulus of the elastic member  22 , the larger the first mechanical impedance. Therefore, in order to reduce the first mechanical impedance, the area (length) of the elastic member  22  may be increased, the thickness may be reduced, and the elastic modulus may be reduced. In an example of  FIGS. 4 and 5 , the thickness of the elastic member  22  is restricted by strength or the like. Therefore, in order to reduce the first mechanical impedance, it is preferable to secure a certain length in the X direction of the opening  21   a  overlapping the elastic member  22 . The same applies to the elastic member  23 . In the first embodiment, the combined mechanical impedance of the elastic member  22  and the elastic member  23  is the first mechanical impedance. Examples of the elastic material forming the elastic members  22  and  23  include low hardness rubber, thermoplastic elastomer, and gel, having a Shore A hardness of 40 or more and 50 or less. 
     As described above, by employing the installation structure of the first embodiment, even when the mass of the vibrator  20  is relatively smaller than the mass of the listening device, it is possible to obtain an effect of increasing the transmission efficiency of vibration from the vibrator  20  to the ear cartilage. That is, in the installation structure of the first embodiment, the first mechanical impedance obtained by combining the elastic member  22  disposed between the vibrator  20  and the housing  21  of the listening device, and the elastic member  23  disposed on the upper surface of the vibrator  20  is set smaller than twice the second mechanical impedance, with which the vibrator  20  is loaded, of the ear cartilage. Therefore, as described with reference to  FIGS. 1 and 2 , it is possible to suppress vibration energy transmitted to the listening device. As a result, the vibration energy transmitted to the ear cartilage can be sufficiently increased. The installation structure of the first embodiment is a structure in which the vibrator  20  is not directly fixed to the housing  21  of the listening device. Therefore, it is possible to obtain an effect of reducing unnecessary vibration applied to the housing  21  of the listening device. Further, the vibrator  20  can be brought into good contact with the skin near the ear cartilage. Furthermore, a waterproof effect can be obtained by the elastic member  22  and the housing  21  of the listening device. The above basic effects are common to the second to fourth embodiments described below in addition to the first embodiment. 
     Second Embodiment 
     Hereinafter, the second embodiment of the present disclosure will be described with reference to  FIGS. 6 and 7 . The installation structure according to the second embodiment includes a columnar member  24  connected to the lower surface of the vibrator  20  in addition to the vibrator  20 , the housing  21 , and the elastic member  22 . Regarding the installation structure of the second embodiment,  FIG. 6  is a partial cross-sectional view like  FIG. 5 , and  FIG. 7  is a top view seen from above in the Z direction. In the second embodiment, the structure of the vibrator  20  is the same as that of the first embodiment. The second embodiment is different from the first embodiment in that the structures of the housing  21  and the elastic member  22  are different from those of the first embodiment, and the columnar member  24  is provided without providing the elastic member  23 . The columnar member  24  is an example of a protrusion projecting toward the ear cartilage. The columnar member  24  connected to the vibrator  20  may be formed separately from the vibrator  20  and joined to the vibrator  20 . Alternatively, the columnar member  24  may be formed integrally with the vibrator  20 . 
     As illustrated in  FIG. 7 , outlines of the housing  21  and the elastic member  22  are both circular when viewed in a plan view from the Z direction. The diameter of the elastic member  22  is smaller than that of the housing  21  concentric with the elastic member  22 . The housing  21  is formed with a circular opening  21   a  ( FIG. 6 ) in a plan view. The diameter of the upper portion of the opening  21   a  matches the diameter of the elastic member  22 . The lower portion of the opening  21   a  has a diameter slightly smaller than that of the elastic member  22 . As illustrated in  FIG. 6 , a cylindrical portion  22   c  is formed in a center of the elastic member  22 . A donut-shaped outer peripheral portion  22   d  is formed around the cylindrical portion  22   c  of the elastic member  22 . The cylindrical portion  22   c  is an example of a recess provided in the elastic member  22  and conforming to a shape of the columnar member  24  that is the protrusion. The cylindrical portion  22   c  is formed such that its central axis is the same as a central axis of the columnar member  24  and it has a diameter substantially the same as that of the columnar member  24 . Therefore, in this structure, vicinity of an outer edge of the outer peripheral portion  22   d  of the elastic member  22  is held at a step of the opening  21   a  of the housing  21 . Also in the second embodiment, as in the first embodiment, a method using adhesion or fusion, a method using a pin, or the combination of these methods is employed as a method for fixing the elastic member  22  to the housing  21 . 
     On the other hand, in  FIG. 7 , the columnar member  24 , which is indicated by a broken line overlapping the vibrator  20 , is formed in a cylindrical shape having a diameter smaller than an entire diameter of the elastic member  22  concentric with the column member  24  in a plan view seen from the Z direction. An inside of the columnar member  24  is hollow for weight reduction. Then, an inner peripheral surface of the cylindrical portion  22   c  of the elastic member  22  has a shape fitting with the columnar member  24 . That is, in this structure, the columnar member  24  is covered with the cylindrical portion  22   c  of the elastic member  22 . The vibrator  20  is held by the elastic member  22  in a state where the cylindrical portion  22   c  is fitted onto the cylindrical member  24 . 
     In the installation structure of the second embodiment, the columnar member  24  connected to the vibrator  20  contacts the skin near the ear cartilage through the cylindrical portion  22   c  of the elastic member  22 . Therefore, compared to the first embodiment, the vibrator  20  can be positioned so that a narrower area faces the ear cartilage. Further, the vibrator  20  is not pressed by the elastic member  23  like the first embodiment, but the cylindrical member  24  can be held together with the vibrator  20  from its outer peripheral side by a side surface of the cylindrical portion  22   c  of the elastic member  22 . 
     The second embodiment is also similar to the first embodiment in that the first mechanical impedance of the elastic member  22  is set smaller than twice the second mechanical impedance of the ear cartilage. However, the elastic member  22  of the second embodiment has a different structure from the elastic member  22  of the first embodiment. Therefore, in the elastic member  22  of the second embodiment, parameters such as area and thickness are required to be adjusted differently from those of the first embodiment. Note that, in the second embodiment, since the basic effect obtained by setting the first mechanical impedance smaller than twice the second mechanical impedance is the same as the first embodiment, its description will be omitted. Further, in the structure of the second embodiment, the columnar member  24  and the cylindrical portion  22   c  of the elastic member  22  project in a pinpoint manner toward the contact portion Ca ( FIG. 1 ). Thus, the pressing force of the cylindrical portion  22   c  may cause pain on the skin of the ear. Therefore, in the second embodiment, a structural design is required in consideration of the force applied to the skin of the ear when the listening device is worn on the ear. 
     In the second embodiment, shapes of the columnar member  24  that is the protrusion and the cylindrical portion  22   c  of the elastic member  22  that is the recess are not respectively limited to a columnar shape and a cylindrical shape. That is, if the protrusion connected to the lower surface of the vibrator  20  and the recess of the elastic member  22  can be fitted with each other, the protrusion and the recess can be formed to have various cross-sectional shapes. 
     Next,  FIG. 8  illustrates a partial cross-sectional view like  FIG. 6  regarding a modification of the second embodiment. In the present modification, a structure of the elastic member  22  in  FIG. 6  is mainly changed. That is, as illustrated in  FIG. 8 , the elastic member  22  of the present modification has a substantially S-shaped cross-sectional shape. In this structure, a shape near the outer edge of the elastic member  22  and a shape near the opening  21   a  of the housing  21  match each other. At this portion, the elastic member  22  is held by the housing  21  directly below. In addition, vicinity of an inner edge of the elastic member  22  has a structure that surrounds and holds a side surface of the columnar member  24 . 
     By employing the structure of the modification of  FIG. 8 , it is possible to increase a path length in a cross-sectional view of the elastic member  22  from the vibrator  20  to the housing  21  and to increase a substantial area of the elastic member  22 . Thus, it is possible to realize a structure that is advantageous for reducing the first mechanical impedance without increasing an overall size of the elastic member  22 . In this case, as compared with  FIG. 6 , when the same first mechanical impedance is set, the thickness of the elastic member  22  can be set larger, for example, as the path length in a cross-sectional view of the elastic member  22  is longer. Therefore, it is possible to realize a stronger structure. 
     Third Embodiment 
     Hereinafter, a third embodiment of the present disclosure will be described with reference to  FIGS. 9 and 10 . The installation structure according to the third embodiment includes a holder  21   d  and a sponge  25  in addition to the vibrator  20 , the housing  21 , and the elastic member  22 . The holder  21   d  is provided on the housing  21  and is disposed above the vibrator  20 . The sponge  25  is a flexible porous body disposed between the vibrator  20  and the holder  21   d . The holder  21   d  holds the sponge  25 . Regarding the installation structure of the third embodiment,  FIG. 9  is a partial cross-sectional view like  FIG. 5 , and  FIG. 10  is a top view seen from above in the Z direction. In the third embodiment, the structure of the vibrator  20  is the same as in the first and second embodiments. In the third embodiment, the structures of the housing  21  and the elastic member  22  are different from those in the first and second embodiments. 
     A step portion  21   e  is formed on the housing  21 . The step portion  21   e  projects slightly upward in a range surrounding the opening  21   a . The step portion  21   e  is formed with a pair of holders  21   d  facing each other in the X direction at a predetermined height in the Z direction. The pair of holders  21   d  forms a pair of side wall portions on a YZ plane adjacent to both ends in the X direction of the opening  21   a . Further, the pair of holders  21   d  forms a pair of upper wall portions on an XY plane partially facing the vibrator  20  below by bending the uppermost portions of the side walls. Then, the sponge  25  is disposed in a space directly below the holder  21   d . The sponge  25  is applied with a certain amount of pressing force in all directions as a porous body having elasticity, and is disposed slightly deformed. Therefore, the sponge  25  can stably hold the vibrator  20  directly below. Further, since the sponge  25  is lightweight, the mass added to the vibrator  20  can be reduced. In the present embodiment, the sponge is used as the member disposed between the vibrator  20  and the holder  21   d . In this regard, the member is not limited to the sponge as long as it is lightweight and can stably hold the vibrator. 
     As illustrated in  FIG. 9 , the elastic member  22  has a flat plate shape within a range of the opening  21   a . The both ends in the X direction of the elastic member  22  are bent upward, and are fitted in groove portions directly below the step portion  21   e  of the housing  21 . In this state, the elastic member  22  is fixed to the housing  21 . As the method of fixing the elastic member  22  to the housing  21 , there can be employed a method of using adhesion or fusion, a method of using the pin as in the first and second embodiments of the combination of these methods, and further a method of fixing the elastic member  22  to the housing  21  by fitting a frame  26  corresponding to a shape of a circumference of the groove portion in the entire groove portion, and the like. The third embodiment is also similar to the first and second embodiments in that the first mechanical impedance obtained by combining the elastic member  22  and the sponge  25  is set smaller than twice the second mechanical impedance of the ear cartilage. However, the third embodiment has an overall structural difference from the first and second embodiments. Therefore, in the elastic member  22  of the third embodiment, the parameters such as area and thickness are required to be adjusted differently from those of the first and second embodiments. Note that, in the third embodiment, since the basic effect obtained by setting the first mechanical impedance smaller than twice the second mechanical impedance is the same as the first and second embodiments, the description will be omitted. 
     Fourth Embodiment 
     Hereinafter, a fourth embodiment of the present disclosure will be described with reference to  FIGS. 11 and 12 . The installation structure according to the fourth embodiment includes the vibrator  20 , the housing  21 , and the elastic member  22 , and other members are unnecessary. Therefore, the number of members can be reduced. The structure of the elastic member  22  is particularly characteristic in the fourth embodiment. Regarding the installation structure of the fourth embodiment,  FIG. 11  is a perspective view like  FIG. 4 , and  FIG. 12  is a partial cross-sectional view like  FIG. 5 . In the fourth embodiment, the structure of the vibrator  20  is the same as those of the first to third embodiments. The fourth embodiment is different from the other embodiments in that both an upper surface  20   a  and a lower surface  20   b  of the vibrator  20  are exposed. 
     An inner peripheral portion  22   f  is formed in the elastic member  22 . In a plan view seen from the Z direction, a region where the vibrator  20  is disposed is opened in the inner peripheral portion  22   f , and the inner peripheral portion  22   f  entirely covers the side surface of the vibrator  20 . That is, as illustrated in  FIG. 12 , the elastic member  22  is formed to have a T-shaped cross-section. The elastic member  22  includes the above-described inner peripheral portion  22   f  that extends in an XZ plane and the YZ plane, and an outer peripheral portion  22   g  that extends in the XY plane. The vibrator  20  is stably held by the elastic member  22  in a state where four side surfaces of the vibrator  20  are in contact with the inner peripheral portion  22   f  of the elastic member  22 . The outer peripheral portion  22   g  of the elastic member  22  is formed to have a size larger than that of the opening  21   a  in a center of the housing  21 . An outer edge of the outer peripheral portion  22   g  is fixed to the upper surface of the housing  21 . As a method of fixing the elastic member  22  to the housing  21 , the method of using adhesion or fusion, the method of using the pin, or the combination of these methods can be employed as in the first to third embodiments. 
     The fourth embodiment is also similar to the first to third embodiments in that the first mechanical impedance of the elastic member  22  is set smaller than twice the second mechanical impedance of the ear cartilage. However, in the installation structure of the fourth embodiment, the lower surface  20   b  of the vibrator  20  directly contacts the skin near the ear cartilage. Therefore, it is preferable to adjust the first mechanical impedance in consideration of the influence. In the fourth embodiment, since the basic effect obtained by setting the first mechanical impedance smaller than twice the second mechanical impedance is the same as the first to third embodiments, the description will be omitted. Further, in order to form the elastic member  22  having a T-shaped cross-section, a mold or the like having a similar cross-section may be satisfactorily used. 
     Next,  FIG. 13  illustrates a partial cross-sectional view like  FIG. 12  regarding a modification of the installation structure of the fourth embodiment. In the present modification, the structure of the elastic member  22  in  FIG. 12  is mainly changed. That is, as illustrated in  FIG. 13 , the elastic member  22  of the present modification has a structure in which the inner peripheral portion  22   f  illustrated in  FIG. 12  extends upward and covers the entire upper surface  20   a  of the vibrator  20 . By employing the structure of the modification of  FIG. 13 , an area of the vibrator  20  in contact with the elastic member  22  is increased as compared with  FIG. 12 . Therefore, the vibrator  20  can be held more stably. 
     Details of the technology of the present disclosure have been specifically described above based on the above-described embodiments. The installation structure of the vibrator  20  according to the present disclosure is not limited to the structures disclosed in the above-described embodiments, and various modifications can be made without departing from the spirit of the invention. Further, as for the material, shape, and fixing method of the elastic member  22 , various forms can be widely employed as long as they have basic characteristics described in each of the above embodiments and can obtain the same effects. Furthermore, a site where the elastic member  22  or the vibrator  20  contacts is not limited to an outside of the pinna as illustrated in  FIG. 3 , but may be another site such as an inside of the pinna. 
     The installation structure of the vibrator according to the present embodiment may be the following first installation structure of the vibrator. The first installation structure of the vibrator is the installation structure for installing the vibrator in which the vibrator accommodating the electromechanical transducer that transduces the electric signal into mechanical vibration is installed on the listening device, wherein the elastic member made of an elastic material is disposed between the housing of the listening device and the vibrator, the vibrator is disposed at a position where the lower surface of the vibrator faces the ear cartilage in the state where the listening device is worn on the ear, and the first mechanical impedance of the elastic member between the vibrator and the housing is set smaller, at the frequency of 200 Hz to 1000 Hz, than twice the second mechanical impedance, with which the vibrator is loaded, of the ear cartilage. 
     The foregoing detailed description has been presented for the purposes of illustration and description. Many modifications and variations are possible in light of the above teaching. It is not intended to be exhaustive or to limit the subject matter described herein to the precise form disclosed. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims appended hereto.