Abstract:
When a diaphragm formed in the shape of an oval is held by a holding member, an electrostatic electroacoustic transducer which allows stable tension without a diaphragm flexing is provided. In the electrostatic electroacoustic transducer, a diaphragm  11  and a diaphragm holding member  12  are formed in the shape of an oval, and is provided with a corrector member  21  which presses inwardly from outside both side ends on a major axis of the oval diaphragm holding member, and opens and deforms outwardly a pair of opposed long sides  12   a  and  12   b  on a minor axis side of the diaphragm holding member.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an electrostatic electroacoustic transducer and a method of manufacturing the same, and more particularly to an electrostatic electroacoustic transducer and a method of manufacturing the same applicable to a condenser-type microphone and headphone which use a change in capacitance between a diaphragm and a fixed electrode plate wherein the above-mentioned diaphragm can be attached without being flexed with respect to a holding member. 
         [0003]    2. Description of the Related Art 
         [0004]    In recent years, electronic devices including a mobile phone are reduced in size and their interior spaces are also small. Many of the above-mentioned small electronic devices include electrostatic electroacoustic transducers, such as a condenser microphone and a capacitor loudspeaker. Therefore, there is an inevitable need for a miniaturized microphone unit or loudspeaker unit. 
         [0005]    However, in the above-mentioned electrostatic electroacoustic transducer, the shape of the diaphragm arranged to face a fixed electrode plate is generally designed to have a precise circle. Therefore, when it is installed in the interior space of the above-mentioned electronic device whose cross or vertical section is rectangular, the above-mentioned precise circular shape of the diaphragm is not one that can utilize the above-mentioned interior space efficiently. 
         [0006]    In view of such problems, Japanese Patent Application Publication No. 2005-86831 discloses a variable capacitance microphone in which a planform of a microphone unit including a diaphragm is in the shape of an oval (like athletic field track), for example. Further, it also discloses such a variable capacitance microphone which efficiently uses the interior space of the small electronic device as described above. According to the structure of the microphone as disclosed in Patent Document 1, a cross-sectional area of the microphone unit is greater than that of the conventional one which is completely circular. Therefore, as to the interior space of the small electronic device like a mobile phone, the minimum wasted space is occupied in the interior space. Further, since the effective area of the diaphragm is large, it is possible to obtain the effects of increasing the sensitivity of the microphone unit. 
         [0007]    Incidentally, as described in Patent Document 1, the diaphragm is adhered to and held by a frame-like holding member (diaphragm plate) in the microphone unit. 
         [0008]    However, when the diaphragm is in the shape of an oval, the long sides (which face each other on the minor axis side) of the holding member formed into the shape of an oval according to the diaphragm are deformed or bent towards the diaphragm by the tension of the diaphragm. As a result, the diaphragm is flexed between the long sides, and the tension of the diaphragm is reduced. Therefore, there is a problem that it is difficult to manufacture the transducer having the diaphragm with stable tension. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention has been made in view of the above-mentioned problems and aims to provide an electrostatic electroacoustic transducer and a method of manufacturing it, wherein when a diaphragm formed in the shape of an oval is held by a holding member, stable tension can be obtained without a diaphragm flexing. 
         [0010]    In order to solve the above-mentioned problems, the electrostatic electroacoustic transducer in accordance with the present invention is characterized by having a fixed electrode plate, a diaphragm arranged to face the above-mentioned fixed electrode plate and formed in the shape of an oval, a diaphragm holding member formed in the shape of a frame and in the shape of an oval for holding the periphery of the above-mentioned diaphragm, and a corrector member which presses inwardly from outside both side ends on a major axis of the above-mentioned oval diaphragm holding member, and stretches and deforms outwardly a pair of opposed long sides on a minor axis side of the above-mentioned diaphragm holding member. 
         [0011]    In addition, it is desirable that the above-mentioned corrector member has a support frame arranged so as to surround the periphery of the above-mentioned diaphragm holding member, and pressing members provided to project from the support frame side toward both the side ends on the major axis of the above-mentioned diaphragm holding member and inwardly press both the side ends on the major axis. 
         [0012]    Further, it is desirable that the above-mentioned pressing members are screws threadedly engaged with the above-mentioned support frame so as to be moveable in relation to both the side ends on the major axis of the above-mentioned diaphragm holding member. 
         [0013]    According to such a structure, when the diaphragm formed in the shape of an oval is held by the diaphragm holding member formed of an oval-shaped frame, both the side ends on the major axis of the above-mentioned diaphragm holding member are pressed inwardly. 
         [0014]    Thus, the diaphragm is in a stable state where it is not bent or flexed, since the pair of long sides which face each other on the minor axis side of the diaphragm holding member are deformed in the direction to stretch outwardly. 
         [0015]    That is to say, according to the electrostatic electroacoustic transducer in accordance with the present invention, it is easy to manufacture the oval-shaped diaphragm in a situation where its tension is stable. Further, since the diaphragm is in the shape of an oval, it is possible to efficiently use the interior space of a small electronic device like a mobile phone. Furthermore, since the effective area of the diaphragm is large, it is possible to obtain the effect of increasing the sensitivity. 
         [0016]    Still further, in order to solve the above-mentioned problems, the method for manufacturing the electrostatic electroacoustic transducer in accordance with the present invention includes the steps of holding a mother film of a diaphragm to which tension is applied uniformly, attaching a diaphragm holding member formed of an oval-shaped frame to the above-mentioned mother film through an adhesive, releasing tension applied to the above-mentioned mother film, cutting off a portion of the above-mentioned mother film at the periphery of the above-mentioned diaphragm holding member, and pushing inwardly from outside both side ends on a major axis of the above-mentioned diaphragm holding member. 
         [0017]    In addition, it is desirable that the step of pressing inwardly from outside both the side ends on the major axis of the above-mentioned diaphragm holding member includes the steps of arranging a support frame so as to surround the periphery of the above-mentioned diaphragm holding member, and providing the pressing members to project from the support frame side toward both the side ends on the major axis of the above-mentioned diaphragm holding member and inwardly press both the side ends on the major axis. 
         [0018]    According to such a manufacture method, it is possible to easily manufacture the electrostatic electroacoustic transducer in a situation where the tension of the oval-like diaphragm is stabilized. Further, since the diaphragm of the thus manufactured electrostatic electroacoustic transducer is in the shape of an oval, it is possible to efficiently use the interior space of a small electronic device like a mobile phone. Furthermore, since the effective area of the diaphragm is large, it is possible to obtain the effect of increasing the sensitivity. 
         [0019]    According to the present invention, when the diaphragm formed in the shape of an oval is held by the holding member in the electrostatic electroacoustic transducer, it is possible to obtain stable tension while keeping the diaphragm free of flexing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0020]      FIG. 1  is a plan view of an electrostatic electroacoustic transducer in accordance with the present invention. 
           [0021]      FIG. 2  is a sectional view of the electrostatic electroacoustic transducer of  FIG. 1  taken along the line A-A. 
           [0022]      FIG. 3A  is a plan view for explaining a process of manufacturing a diaphragm assembly. 
           [0023]      FIG. 3B  is a sectional view for explaining the process of manufacturing the diaphragm assembly. 
           [0024]      FIG. 4A  is a plan view for explaining the process following  FIG. 3 . 
           [0025]      FIG. 4B  is a sectional view for explaining the process following  FIG. 3 . 
           [0026]      FIG. 5A  is a plan view for explaining the process following  FIG. 4 . 
           [0027]      FIG. 5B  is a sectional view for explaining the process following  FIG. 4 . 
           [0028]      FIG. 6A  is a plan view for explaining the process following  FIG. 5 . 
           [0029]      FIG. 6B  is a sectional view for explaining the process following  FIG. 5 . 
           [0030]      FIG. 7  is a plan view for explaining the process following  FIG. 6 . 
           [0031]      FIG. 8  is a plan view for explaining the process following  FIG. 7 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Hereinafter, the preferred embodiments of the present invention will be described with reference to the drawings. 
         [0033]      FIG. 1  is a plan view of an electrostatic electroacoustic transducer in accordance with the present invention.  FIG. 2  is a sectional view of the electrostatic electroacoustic transducer of  FIG. 1  taken along the line A-A. 
         [0034]    It should be noted that a case where the electrostatic electroacoustic transducer in accordance with the present invention is applied to a condenser microphone will be described by way of example in the following description. However, the same structure can be applied to a condenser-type headphone or loudspeaker. 
         [0035]    As shown in  FIGS. 1 and 2 , the condenser microphone in accordance with this preferred embodiment is provided with a microphone unit  10  in which a diaphragm has an oval-like shape (shape of an athletic field track) in plan view. 
         [0036]    The above-mentioned microphone unit  10  is constituted by a diaphragm  11  tensioned by a diaphragm ring  12  and a fixed electrode plate  13  supported by an insulation seat  14  which are arranged to face each other via a separator ring  15 . In the present invention, the diaphragm ring  12  is formed as a frame in the shape of an oval, analogous or conformal to the diaphragm  11 . 
         [0037]    That is to say, the diaphragm ring  12  has a pair of elongated side frame portions  12   a  and  12   a  which face each other on the minor axis side and are parallel with each other, and a pair of arcuate frame portions  12   b  and  12   b  which face each other on the major axis side. The diaphragm ring  12  is made of a metal, and may be made of an aluminum material, a brass alloy, etc, for example. 
         [0038]    Further, a thin synthetic resin film which has a metal deposition film on one side is used for the diaphragm  11 . As for diaphragm  11 , the metal deposition film side is adhered to the diaphragm ring  12  through an adhesive etc. to give a diaphragm assembly. 
         [0039]    Furthermore, the separator ring  15  as well as the diaphragm ring  12  is formed as an oval-shaped frame. The separator ring  15  formed of a synthetic resin film which is electrically insulative. 
         [0040]    Still further, the fixed electrode plate  13  is similarly formed in the shape of an oval since the diaphragm  11  is oval-shaped. For example, an aluminum plate is used for the fixted electrode plate  13 . In this preferred embodiment, the fixed electrode plate  13  is constituted by the aluminum plate in which a plurality of sound holes  13   a  are formed. It should be noted that an electret film may be adhered to the fixed electrode plate  13  on the side facing the diaphragm  11  to give a back electret-type condenser microphone. 
         [0041]    Yet further, the insulation seat  14  is made of a synthetic resin material which is electrically insulative. In this preferred embodiment, the insulation seat  14  is provided with a recess  14   a  for forming an air room  16  having a predetermined volume of space in the back of the fixed electrode plate  13 . An acoustic resistor material (not shown), such as nonwoven fabric etc., may be provided in the air room  16 . Further, the insulation seat  14  may be of a ceramic material. 
         [0042]    The microphone unit  10  is arranged in a unit cover (not shown) in a situation where it stands substantially perpendicularly on a microphone base (not shown) via amount member including a support frame  20  which surrounds the diaphragm ring  12 . 
         [0043]    In this preferred embodiment, since the diaphragm ring  12  is in the shape of an oval, the support frame  20  is also in the shape of an oval and has a pair of elongated side frame portions  20   a  and  20   a  which face each other on the minor axis side and are parallel with each other, and a pair of arcuate frame portions  20   b  and  20   b  which face each other on the major axis side. 
         [0044]    Further, the support frame  20  has a corrector member  21  which presses, from the outside, the arcuate frame portions  12   b  and  12   b  which face each other on the major axis side of the diaphragm ring  12 . The pressing force of this corrector member  21  deforms the elongated side frame portions  12   a  and  12   a  which face each other on the minor axis side and adjusts tension so that the diaphragm  11  may not be flexed or bent. 
         [0045]    As illustrated, this corrector member  21  has screw members  21   a  and  21   a  (pressing members) which are provided to project from the support frame  20  side toward both the side ends on the major axis of the diaphragm ring  12  and press both the above-mentioned side ends on the major axis inwardly of the diaphragm ring  12 , for example. More specifically, the above-mentioned screw members  21   a  and  21   a  are threadedly engaged with the above-mentioned support frame  20  so as to be moveable in relation to both the side ends on the major axis of the above-mentioned diaphragm ring  12 . Further, the support frame  20  may be made of either a metal or a synthetic resin, but it is desirable to have rigidity greater than that of the diaphragm ring  12 . 
         [0046]    In order to obtain the diaphragm assembly of the microphone unit  10  as shown in  FIGS. 1 and 2 , a mother film  1 M of the diaphragm  11  is first arranged on a cylindrical base  6  as shown in plan view in  FIG. 3A  and in section in  FIG. 3B . The mother film  1 M is in the shape of a precise circle. 
         [0047]    Next, as shown in plan view in  FIG. 4A  and in section in  FIG. 4B , a weight  7  is provided for the whole periphery of the mother film  1 M to uniformly tension the mother film  1 M. 
         [0048]    Then, in this situation, the oval-shaped diaphragm ring  12  is attached to the mother film  1 M via an adhesive as shown in plan view in  FIG. 5A  and in section in  FIG. 5B . The weight  7  is removed and the tension applied to the mother film  1 M is released. In this situation, the tension of the mother film  1 M which serves as the diaphragm  11  arranged inside the diaphragm ring  12  particularly causes the elongated side frame portions  12   a  and  12   a  of the diaphragm ring  12  to inwardly flex easily (towards the diaphragm  11 ) as shown in plan view in  FIG. 6A  in section in  FIG. 6B . Thus, the tension of the mother film  1 M inside the diaphragm ring  12  which serves as the diaphragm  11  is partially reduced to below the initial tension applied to the above-mentioned mother film  1 M. 
         [0049]    Subsequently, as shown in  FIG. 7  in plan view, the excess mother film  1 M is cut off with a cutting knife etc. at the periphery portion of the frame of the diaphragm ring  12 . 
         [0050]    Now, the diaphragm assembly in which the diaphragm  11  is tensioned by the diaphragm ring  12  is obtained. However, the elongated side frame portions  12   a  and  12   a  of the diaphragm ring  12  are inwardly bent or flexed as described above. Therefore, the support frame  20  is arranged so as to surround the diaphragm assembly constituted by this diaphragm  11  and the diaphragm ring  12 , and both the side ends on the major axis of the diaphragm ring  12  are pressed inwardly by the corrector member  21  provided at both the side ends on the major axis. 
         [0051]    More particularly, the screw members  21   a  and  21   a  are threadedly attached to both the side ends on the major axis of the support frame  20 , and tips of the screw members  21   a  and  21   a  are caused to project towards both the side ends on the major axis of the diaphragm ring  12 . 
         [0052]    Thereby, the screw members  21   a  and  21   a  press both the side ends on the major axis of the above-mentioned diaphragm ring  12  (arcuate frame portions  12   b  and  12   b ) by pressing force F 1 , as shown in the plan view of  FIG. 8 . Then, the displacement produced by the above-mentioned pressing force F 1  may deform and outwardly (direction F 2  in  FIG. 8 ) stretch the elongated side frame portions  12   a  and  12   a  which face each other on the minor axis side of the diaphragm ring  12 . 
         [0053]    As a result, the diaphragm  11  tensioned by the diaphragm ring  12  is pulled in the direction to increase the size of the minor axis, thus obtaining the stable tension. 
         [0054]    As such, according to the preferred embodiment in accordance with the present invention, both the side ends on the major axis (arcuate frame portions  12   b  and  12   b ) of the diaphragm ring  12  are pressed inwardly when the diaphragm  11  formed in the shape of an oval is held by the diaphragm ring  12  of the oval-shaped frame. 
         [0055]    Thus, the pair of elongated side frame portions  12   a  and  12   a  which face each other on the minor axis side of the diaphragm ring  12  are deformed in the direction to stretch outwardly, and the diaphragm  11  is in a stable state where it is not bent or flexed. 
         [0056]    That is to say, according to the electrostatic electroacoustic transducer in accordance with the present invention, it is easy to manufacture the oval-shaped diaphragm whose tension is stable. Further, since the diaphragm is in the shape of an oval, it is possible to efficiently use an interior space of a small electronic device, such as a mobile phone. Furthermore, since the effective area of the diaphragm is large, it is possible to obtain the effect of increasing its sensitivity. 
         [0057]    It should be noted that, the preferred embodiments above have been described with reference to the case where the electrostatic electroacoustic transducer in accordance with the present invention is applied to a condenser microphone. However, as described above, the similar structure can be applied to not only a microphone but also a condenser-type headphone and a loudspeaker. 
         [0058]    Further, in the preferred embodiments above, the screw members  21   a  and  21   a  are used as the pressing members in the corrector member  21 . However, the present invention is not limited to the structure. For example, it can be arranged such that a bar-like pin component is fitted into a hole on the support frame  20  side to cause a tip thereof to project towards the diaphragm ring  12 .