Abstract:
The present invention provides an electret condenser microphone including a diaphragm, a frame adapted to affix the diaphragm to, and a backplate positionable inside the frame and opposite the diaphragm with a space reserved therebetween.

Description:
[0001]    The present application claims priority under 35 U.S.C. §119 of Japanese Patent Application No. 2008-115255 filed on Apr. 25, 2008, the disclosure of which is expressly incorporated by reference herein in its entity. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to electret condenser microphones. 
       BACKGROUND ART 
       [0003]    A conventional electret condenser microphone has a frame affixed with a vibratory diaphragm and a backplate disposed substantially parallel to the diaphragm with a predetermined space reserved therebetween, as disclosed in Japanese Unexamined Patent Publication No. 2000-050393 and Japanese Unexamined Patent Publication No. 2004-222091. 
         [0004]    The frame and the backplate have substantially the same outer diameters and are vertically stacked one on top of the other. This structure is a bottleneck in reducing the thickness of the conventional electret condenser microphone. 
       SUMMARY OF INVENTION 
       [0005]    The present invention was made in view of the foregoing circumstances. It is an object of the invention to provide an electret condenser microphone in which the frame and the backplate can be disposed at substantially the same level for reducing the thickness of the microphone. 
         [0006]    In order to solve the above-mentioned problem, an electret condenser microphone according to the present invention includes a diaphragm, a frame adapted to affix the diaphragm to, and a backplate positionable inside the frame and opposite the diaphragm with a space reserved therebetween. 
         [0007]    In such an electret condenser microphone, the backplate is disposed inside the frame. That is, the frame and the backplate are positioned substantially level with each other, making it possible to reduce the thickness of the microphone as compared with the conventional exemplary microphones. 
         [0008]    The microphone may further include a spacer to be provided at a surface of the backplate opposite the diaphragm so as to reserve the space between the backplate and the diaphragm. In this case, it is desirable that the backplate as disposed inside the frame can press the diaphragm via the spacer. 
         [0009]    The diaphragm, pressed by the backplate via the spacer, becomes tightened to gain tension. Accordingly, this aspect of the invention enables reduction of the space between the diaphragm and the backplate and improves sensitivity of the diaphragm. In addition, increased tension of the diaphragm helps to reduce undesirable variations in tension between diaphragms produced by affixing a film to a plurality of frames. More particularly, in mass production of the microphones, a plurality of frames may be affixed to a single film, which may be cut apart to form a plurality of diaphragms affixed to the respective frames. Although such diaphragms may vary in tension, this aspect of the invention can reduce the variations because each diaphragm is pressed by the backplate via the spacer. 
         [0010]    In a case where the diaphragm is affixed to a first surface of the frame, it is desirable that the backplate be adapted to press the diaphragm via the spacer from the first surface side of the frame. 
         [0011]    In this case, simply disposing the backplate and the spacer inside the frame allows the diaphragm to be pressed by the combined thickness of the backplate and spacer. This aspect of the invention provides a further advantage in improving tension on the diaphragm. 
         [0012]    The microphone may further include a capsule for accommodating the frame. The capsule may have a protruded portion to be disposed inside the frame and to function as the backplate. In this case, the microphone having the backplate as the protruded portion of the capsule may be manufactured with reduced number of components and in less assembly man-hours, in comparison with a case of providing the backplate as a discrete component. Consequently, this aspect of the invention is advantageous in cost reduction. 
         [0013]    Further, the capsule may accommodate the backplate as well as the frame. Such capsule may have a positioning means for positioning and holding the backplate. 
         [0014]    Moreover, the capsule may accommodate the spacer in addition to the frame and the backplate. Such capsule may include a positioning means for positioning and holding the backplate as pressing the diaphragm. 
         [0015]    The positioning means of either type makes it easy to position and hold the backplate, facilitating attachment of the backplate. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a schematic cross-sectional view of an electret condenser microphone according to a first embodiment of the present invention; 
           [0017]      FIG. 2  is a schematic cross-sectional view of an electret condenser microphone according to a second embodiment of the present invention; and 
           [0018]      FIG. 3  is a schematic cross-sectional view of an electret condenser microphone according to a modification of the first embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    Embodiments of the present invention are described below. 
       First Embodiment  
       [0020]    First, an electret condenser microphone according to a first embodiment of the present invention is described with reference to  FIG. 1 . 
         [0021]    The electret condenser microphone as shown in  FIG. 1  is a back electret condenser microphone. The electret condenser microphone includes a frame  100  having a diaphragm  110  affixed thereto, a backplate  200 , a spacer  300 , a printed circuit board (PCB)  400 , a field-effect transistor (FET)  500 , an electrically conductive ring  600 , and a capsule  700  containing all the other components. Each component of the microphone will be described in detail below. 
         [0022]    The frame  100  is a circular electrically conductive ring. The frame  100  is set on an electrically isolated portion (not shown) of a bottom plate  710  (to be described below) of the capsule  700 . 
         [0023]    The diaphragm  110  is made of a well-known metal thin film. The diaphragm  110  is affixed to a lower surface  101  (a first surface) of the frame  100  with conductive adhesive. 
         [0024]    The ring  600  (gate ring) is an annular member having substantially the same outer diameter as that of the frame  100 . The ring  600  is disposed on an upper surface  102  of the frame  100  and is interposed between the PCB  400  and the frame  100 . 
         [0025]    The PCB  400  is a well-known circular circuit board and is disposed above the frame  100 . The PCB  400  has substantially the same outer diameter as that of the frame  100 . The PCB  400  is provided with first and second conductive lines (not shown). The first conductive line contacts the ring  600 , while connecting to a gate terminal of the FET  500  that is mounted at the center of a lower surface of the PCB  400 . The second conductive line contacts a swaged portion  730  (to be described below) of the capsule  700  for connection with a ground terminal (not shown). 
         [0026]    The spacer  300  is an insulative ring formed on the peripheral edge of an upper surface of the backplate  200 . As described below, the spacer  300  is interposed between the diaphragm  110  and the backplate  200  so as to reserve a predetermined space C therebetween. 
         [0027]    The backplate  200  is a circular conductive metal plate. The backplate  200  has an outer diameter that is smaller than the inner diameter of the frame  100  and a thickness that is substantially the same as the thickness of the frame  100 . On the upper surface (the surface opposing the diaphragm) of the backplate  200 , there is formed an electret layer  201 , which may be a thin film of polymer such as fluorinated ethylene propylene (FEP). 
         [0028]    The backplate  200  is disposed inside the frame  100 . In this state, the backplate  200  presses the diaphragm  110  via the spacer  300  from below (from the first surface side of the frame) by the combined thickness of the backplate  200  and spacer  300 . The space C (capacitor) having the thickness of the spacer  300  is thereby formed between the electret layer  201  on the backplate  200  and the diaphragm  110 , while tightening the diaphragm  110  to apply tension thereto. 
         [0029]    Additionally, the backplate  200  has a plurality of through holes  210  penetrating through the thicknesses of the backplate  200  and electret layer  201 . These columnar through holes  210  connect a sound receiving aperture  711  (to be described below) in the capsule  700  with the space C between the backplate  200  and the diaphragm  110 . That is, sound enters the capsule  700  from the sound receiving aperture  711 , past the through holes  210  and into the space C to cause the diaphragm  110  to vibrate. The vibrations of the diaphragm produce changes in capacitance of the capacitor. 
         [0030]    The capsule  700  is a substantially circular cup-shaped member formed by press-molding a conductive metal plate. The capsule  700  includes the bottom plate  710 , a cylindrical peripheral wall  720  upstandingly provided on the outer peripheral edge of the bottom plate  710 , and the swaged portion  730  provided at the leading end of the peripheral wall  720 . 
         [0031]    The bottom plate  710  is provided at its center with the sound receiving aperture  711  in a substantially circular shape. The bottom plate  710  further has a ring-shaped projection  712  (a positioning means) projecting upward along the periphery of the sound receiving aperture  711 . The inner diameter of the projection  712  is slightly smaller than the outer diameter of the backplate  200 . That is, the projection  712  is used to hold the backplate  200  in position on the bottom plate  710 , whereby connection is established between the backplate  200  and the second conductive line of the PCB  400  via the capsule  700 . Along the peripheral edge of the bottom plate  710  there is provided the electrically isolated portion (not shown), on top of which the frame  100  is placed. 
         [0032]    The swaged portion  730  is an inwardly bent piece member. The distance between the lower surface of the swaged portion  730  and an upper surface of the bottom plate  710  is substantially equal to the total thickness of the PCB  400 , the ring  600  and the frame  100 . That is, the stacked frame  100 , ring  600 , and PCB  400  are held between the swaged portion  730  and the bottom plate  710 . 
         [0033]    The electret condenser microphone having the above-described structure is assembled in the following steps. First, the electret layer  201  of thin film is formed over the upper surface of the backplate  200  using a well-known film-forming method such as spin coating, sputtering, and chemical vapor deposition (CVD). The spacer  300  is then printed on the outer peripheral edge of the electret layer  201 . 
         [0034]    The backplate  200  is then inserted inside the projection  712  of the capsule  700 , so that the backplate  200  is held in position by the projection  712  and is electrically connected to the capsule  700 . 
         [0035]    After that, the frame  100  with the diaphragm  110  affixed is set on the electrically isolated portion of the bottom plate  710  of the capsule  700 , with the diaphragm  110  facing downward. It should be noted here that the backplate  200  and the spacer  300  thereon are placed inside the frame  100  from below, and that the diaphragm  110  is pressed from below by the spacer  300  on the backplate  200 . As a result, the diaphragm  110  becomes tightened and gains tension. Simultaneously therewith, the space C is formed between the diaphragm  110  and the electret layer  201  on the backplate  200 . 
         [0036]    After that, the ring  600  and the PCB  400  with the FET  500  mounted thereon are stacked, in this order, on top of the frame  100 . The ring  600  thus comes into contact with the first conductive line of the PCB  400 , and the diaphragm  110  on the frame  100  is electrically connected with the gate terminal of the FET  500  by way of the ring  600  and the first conductive line of the PCB  400 . 
         [0037]    After that, the leading end of the peripheral wall  720  of the capsule  700  is bent inward. The bent portion becomes the swaged portion  730  to abut on the outer peripheral edge of the upper surface of the PCB  400 . As a result, the frame  100 , the ring  600 , and the PCB  400  are held between the swaged portion  730  and the bottom plate  710 . Simultaneously therewith, the swaged portion  730  comes into contact with the second conductive line of the PCB  400 . The electret layer  201  on the backplate  200  is thus electrically connected to the ground terminal by way of the capsule  700  and the second conductive line of the PCB  400 . 
         [0038]    The electret condenser microphone assembled in the above steps allows sound to pass from the sound receiving aperture  711  of the capsule  700 , through the through holes  210  and into the space C, thereby making the diaphragm  110  vibrate. The vibrations of the diaphragm  110  produce changes in capacitance of the capacitor. Changes in capacitance are fed as electrical signals to the FET  500 , by way of the frame  100 , the ring  600 , and the first conductive line. 
         [0039]    In such an electret condenser microphone, the backplate  200  is disposed inside the frame  100 , i.e., substantially level with the frame  100 . As such, the microphone may have an advantageously reduced thickness compared with the conventional exemplary microphones. 
         [0040]    In addition, the backplate  200  is set inside the frame  100  and presses the diaphragm  110  affixed to the lower surface of the frame  100  via the spacer  300  from below by the combined thickness of the backplate  200  and spacer  300 . By being pressed by the backplate  200  and the spacer  300 , the diaphragm  110  becomes tightened and favorably gains tension. Consequently, it becomes possible to reduce the distance between the diaphragm  110  and the backplate  200 —i.e., the height of the space C—from a conventional distance of 25-38 μm to around 10 μm, and also possible to improve sensitivity of the diaphragm  110 . Increased tension of the diaphragm  110  helps to reduce undesirable variations in tension between diaphragms produced by affixing a film to a plurality of frames. More particularly, in mass production of the microphones, a plurality of frames may be affixed to a single film, which may be cut apart to form a plurality of diaphragms affixed to the respective frames. Although such diaphragms may vary in tension, the invention can reduce the variations because each diaphragm is pressed by the backplate via the spacer. 
       Second Embodiment  
       [0041]    An electret condenser microphone according to a second embodiment of the present invention is described below with reference to  FIG. 2 .  FIG. 2  is a schematic cross-sectional view of the electret condenser microphone according to the second embodiment of the present invention. 
         [0042]    The electret condenser microphone as shown in  FIG. 2  is different from the microphone of the first embodiment in that the backplate  200  and the spacer  300  are replaced by a backplate portion  741 ′ and a spacer  742 ′ of a protruded portion  740 ′ of a capsule  700 ′. The differences will be elucidated below, and description on overlapping components will not be given to avoid redundancy. The reference numerals of the capsule and its subcomponents are distinguished from those of the first embodiment by adding the suffix “′”. 
         [0043]    The capsule  700 ′ is a substantially circular cup-shaped member formed by press-molding a conductive metal plate. The capsule  700 ′ includes a bottom plate  710 ′, a cylindrical peripheral wall  720 ′ upstandingly provided on the outer peripheral edge of the bottom plate  710 ′, a swaged portion  730 ′ provided at the leading end of the peripheral wall  720 ′, and the upwardly protruded portion  740 ′ formed at a central portion of the bottom plate  710 ′ by performing drawing press. 
         [0044]    The protruded portion  740 ′ includes the backplate portion  741 ′ in a substantially circular pedestal-like shape, and the spacer  742 ′ protruded from the outer peripheral edge of a top plate (to be is described below) of the backplate portion  741 ′. 
         [0045]    The spacer  742 ′ is a ring-shaped protrusion and its upper surface to contact a diaphragm  110  is electrically isolated. 
         [0046]    The backplate portion  741 ′ has a cylindrical portion and the top plate that closes an upper opening of the cylindrical portion. The portion other than the outer peripheral edge of the top plate has an electret layer  743 ′, which may be a thin film of polymer such as FEP. 
         [0047]    The backplate portion  741 ′ is set inside a frame  100 . In this state, the backplate portion  741 ′ presses the diaphragm  110  via the spacer  742 ′ from below (from the first surface side of the frame) by the combined thickness of the backplate portion  741 ′ and spacer  742 ′. Space C (capacitor) having the thickness of the spacer  742 ′ is thereby formed between the electret layer  743 ′ on the backplate portion  741 ′ and the diaphragm  110 , while tightening the diaphragm  110  to apply tension. 
         [0048]    The top plate of the backplate portion  741 ′ is provided with a plurality of through holes  741   a ′ penetrating through the top plate and the electret layer  743 ′. The columnar through holes  741   a ′ serve as sound receiving apertures, which connect the outside of the capsule  700 ′ with the space C between the backplate portion  741 ′ and the diaphragm  110 . The through holes  741   a ′ allow sound to enter therethrough into the space C to make the diaphragm  110  vibrate. The vibrations of the diaphragm  110  produce changes in capacitance of the capacitor. 
         [0049]    The electret condenser microphone configured as above is assembled in the following steps. First, the frame  100  affixed with the diaphragm  110  is set on an electrically isolated portion of the bottom plate  710 ′ of the capsule  700 ′ with the diaphragm  110  facing downward. It should be noted here that the backplate portion  741 ′ and the spacer  742 ′ thereon are placed inside the frame  100  from below, and that the diaphragm  110  is pressed from below by the spacer  742 ′ on the backplate portion  741 ′. As a result, the diaphragm  110  becomes tightened and gains tension. Simultaneously therewith, the space C is formed between the diaphragm  110  and the electret layer  743 ′ on the backplate portion  741 ′. 
         [0050]    After that, a ring  600  and a PCB  400  with an FET  500  mounted thereon are stacked, in this order, on top of the frame  100 . The ring  600  thus contacts a first conductive line of the PCB  400 , and the diaphragm  110  on the frame  100  is electrically connected with a gate terminal of the FET  500  by way of the ring  600  and the first conductive line of the PCB  400 . 
         [0051]    After that, the leading end of the peripheral wall  720 ′ of the capsule  700 ′ is bent inward. The bent portion becomes the swaged portion  730 ′ to abut on the outer peripheral edge of an upper surface of the PCB  400 . As a result, the frame  100 , the ring  600 , and the PCB  400  are held between the swaged portion  730 ′ and the bottom plate  710 ′. Simultaneously therewith, the swaged portion  730 ′ comes into contact with a second conductive line of the PCB  400 . The electret layer  743 ′ on the backplate portion  741 ′ is thus electrically connected with a ground terminal via the capsule  700 ′ and the second conductive line of the PCB  400 . 
         [0052]    The electret condenser microphone assembled in the above steps allows sound to pass from the through holes  741   a ′ in the capsule  700 ′ into the space C, thereby making the diaphragm  110  vibrate. The vibrations of the diaphragm  110  produce changes in capacitance of the capacitor. Changes in the capacitance are fed as electrical signals to the FET  500 , by way of the frame  100 , the ring  600 , and the first conductive line. 
         [0053]    In such an electret condenser microphone, the backplate portion  741 ′ is disposed inside the frame  100 , i.e., substantially level with the frame  100 . The microphone may have an advantageously reduced thickness compared with the conventional exemplary microphones. 
         [0054]    In addition, the backplate portion  741 ′ is set inside the frame  100  and presses the diaphragm  110  affixed to the lower surface  101  of the frame  100  from below via the spacer  742 ′ by the combined thickness of the backplate portion  741 ′ and spacer  742 ′. By being pressed by the back plate portion  741 ′ and the spacer  742 ′, the diaphragm  110  tightened and favorably gains tension. Consequently, it becomes possible to reduce the distance between the diaphragm  110  and the backplate portion  741 ′—i.e., the height of the space C—from the conventional distance of 25-38 μm to around 10 μm, and also possible to improve sensitivity of the diaphragm  110 . Increased tension of the diaphragm  110  helps to reduce undesirable variations in tension between diaphragms produced by affixing a film to a plurality of frames. More particularly, in mass production of the microphones, a plurality of frames may be affixed to a single film, which may be cut apart to form a plurality of diaphragms affixed to the respective frames. Although such diaphragms may vary in tension, the invention can reduce the variations because each diaphragm is pressed by the backplate via the spacer. 
         [0055]    Another advantageous feature of the second embodiment is that a portion of the capsule  700 ′ (the protruded portion  740 ′) forms the backplate portion  741 ′ and the spacer  742 ′. The microphone allows it possible to reduce the number of components and assembly man-hours, in comparison with a case of providing the backplate portion  741 ′ and the spacer  742 ′ as discrete components. Consequently, the microphone is advantageous in cost reduction. 
         [0056]    The above-described electret condenser microphones may be appropriately modified in design as described below, without departing from the scope of the present invention as set forth in the claims.  FIG. 3  illustrates a modified electret condenser microphone of the first embodiment of the present invention in a schematic cross-sectional view. 
         [0057]    The backplate  200  as disposed inside a frame  100  may or may not press a diaphragm  110 . For example, the backplate  200  disposed inside a frame  100  may be placed to face the diaphragm  110  affixed to an upper surface of the frame  100  with a predetermined space provided between the diaphragm and the backplate. This arrangement may be applied to the backplate portion  741 ′. In this case, a spacer  300  or  742 ′ may be or may not be provided in order to form the space. 
         [0058]    The spacer  300  of the present invention is not limited to a ring-shaped one. For example, a plurality of spacers  300  may be arranged annularly on the backplate  200 . 
         [0059]    The spacer  742 ′ is not limited to the above-described one formed by means of drawing press along the outer peripheral edge of the top plate of the backplate portion  741 ′. For example, as in the case of the spacer  300 , the spacer  742 ′ may be formed by printing an insulative layer on the outer peripheral edge of the top plate of the backplate portion  741 ′. In this case, a plurality of spacers may be arranged annularly on the backplate portion  741 ′ as described above. 
         [0060]    The protruded portion  740 ′ of the second embodiment is made by performing drawing press on the central portion of the bottom plate  710 ′ of the capsule  700 ′; however, the present invention is not limited thereto. For example, if the capsule  700 ′ is a resin molded article as is described later, the protruded portion may be formed in the course of resin molding of the capsule  700 ′. In this case, a plurality of spacers may be formed annularly on the backplate portion. 
         [0061]    The diaphragm  110  of either embodiment is provided on the lower surface of the frame  100 . However, the diaphragm may be given appropriate pressure even if it is provided on the upper surface of the frame. More particularly, the diaphragm  110  of the first embodiment can be pressed if the combined thickness of the backplate  200  and the spacer  300  disposed inside the frame  100  is larger than the thickness of the frame  100 . Similarly, the diaphragm  110  of the second microphone can be pressed if the combined thickness of the backplate portion  741 ′ and the spacer  742 ′ disposed inside the frame  100  (i.e., the height of the protruded portion  740 ′) is larger than the thickness of the frame  100 . 
         [0062]    The board  400  may be disposed above the frame  100  as in the above embodiments but may also be disposed below the frame  100 . The latter case is exemplified in a modified microphone as shown in  FIG. 3 , wherein an insulative ring  800  is interposed between the PCB  400  and the frame  100  so as to establish connection between the frame  100  and the second conductive line of the PCB  400  through the capsule  700 ″. Meanwhile, the PCB  400  is provided thereon with a ring-shaped conductive retaining portion  900  for retaining the backplate  200  so as to establish connection between the backplate  200  and the first conductive line of the PCB  400  through the conductive retaining portion  900 . It should be noted that the bottom plate  710 ″ of the capsule  700 ″ has a plurality of sound receiving apertures  711 ′. The modified microphone may have similar advantageous effects to the preferred first embodiment because the backplate  200  of the modification is also disposed inside the frame  100  and presses the diaphragm  110 . 
         [0063]    The capsule  700  of the above embodiments is made of a conductive metal but may be a molded article of insulative resin. In this case, conductive lines may be provided on the inner or outer surface of the capsule so as to establish connections as in the foregoing embodiments, particularly, between the backplate  200  and the PCB  400 , or between the electret layer  743 ′ on the backplate portion  741 ′ and the PCB  400 . As described above, the frame  100  and the PCB  400  may be connected via the conductive lines of the capsule  700 . 
         [0064]    The bottom plate  710  of the invention is not limited to the above-described one having the projection  712  as a positioning means for the backplate  200 . That is, any type of positioning means may be adopted insofar as the means can position and hold the backplate  200 . For example, the bottom plate  710  may be provided with a recess to fittingly receive the backplate  200 . 
         [0065]    The electret condenser microphones according to the foregoing embodiments are back electret condenser microphones in which the electret layer is provided on the surface of the backplate  200  or  741 ′ opposite the diaphragm. However, the invention may be applied to a foil electret condenser microphone, in which the diaphragm  110  itself is made of a polymer film for use as electret. 
         [0066]    Any modifications or changes may be made to the above-described components in shape, material and number as long as they can carry out similar functions to ones described above. Obviously, the components may not take circular (i.e., round) shapes as described above but may take polygonal (i.e., angular) shapes. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 REFERENCE SIGNS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 100 
                 Frame 
               
               
                   
                 110 
                 Diaphragm 
               
               
                   
                 200 
                 Backplate 
               
               
                   
                 300 
                 Spacer 
               
               
                   
                 700 
                 Capsule 
               
               
                   
                 712 
                 Projection (Positioning means) 
               
               
                   
                 700′ 
                 Capsule 
               
               
                   
                 740′ 
                 Protruded portion 
               
               
                   
                 741′ 
                 Backplate portion 
               
               
                   
                 742′ 
                 Spacer 
               
               
                   
                   
               
             
          
         
       
     
       Citation List 
       [0067]    Patent Literature 1: Japanese Unexamined Patent Publication No. 2000-050393 
         [0068]    Patent Literature 2: Japanese Unexamined Patent Publication No. 2004-222091