Abstract:
A condenser microphone unit and a condenser microphone with enhanced sensitivity and reduced noise that can further be miniaturized by devising a structure of a vibrator and peripheral members thereof is obtained. A condenser microphone unit that causes conversion into an electric signal by capacity change of a capacitor formed with a vibrator that vibrates by receiving sound and a fixed pole disposed opposite to the vibrator. The vibrator is a cylindrical member and the fixed pole is a columnar member disposed on an inner side of the vibrator. The vibrator is disposed through a spacer disposed on an outer circumferential surface of the fixed pole to form a cylindrical space sealed with the outer circumferential surface of the fixed pole, spacer, and vibrator.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a condenser microphone unit and a condenser microphone that can enhance sensitivity even if miniaturized.  
         [0003]     2. Related Background of the Invention  
         [0004]     A condenser microphone unit used in a miniature microphone such as a tiepin microphone and a headset microphone is demanded to be as small as possible so that it is inconspicuous. Currently, a small-diameter condenser microphone unit whose diameter is approximately 2.5 mm has become commercialized to apply a polarization voltage by an electret.  
         [0005]      FIG. 3  and  FIG. 4  show a structure of a general conventional condenser microphone unit. In  FIG. 3  and  FIG. 4 , reference numeral  12  denotes a case of a condenser microphone unit, that is, a unit case. The unit case  12  has a bottomed cylindrical shape and is shown upside down with a portion corresponding to a bottom at an upper side. A plurality of pores  121  to guide a sound wave inside the unit is formed on a flat wall on the upper side corresponding to the bottom and these pores  121  are covered with a front mesh (not shown) on an inner surface side of the unit case  12 . Inside the unit case  12 , a diaphragm assembly  30  is disposed in a location nearest to the bottom of the unit case  12 . The diaphragm assembly  30  comprises a ring-shaped diaphragm supporter  24  and a diaphragm-shaped diaphragm  22  whose outer circumferential edge is fixed to an end face of the diaphragm supporter  24  by, for example, adhesion.  
         [0006]     A fixed pole  16  is disposed opposite to the diaphragm  22  with a spacer  20  interposed. The spacer  20  is a ring-shaped member made of thin resin and is in intimate contact with an outer circumferential edge of a rear end face (lower end face in  FIG. 3  and  FIG. 4 ) of the diaphragm  22 . There is a gap corresponding to a thickness of the spacer  20  between the diaphragm  22  and fixed pole  16 . The fixed pole  16  uses a metallic disc as a base material and constitutes an electret board with an electret plate  18  attached at least on one face side, for example, on a face side opposite to the diaphragm  22 . The diaphragm  22  and fixed pole  16  constitute a kind of capacitor and, as the diaphragm  22  vibrates in accordance with the sound wave guided through the front mesh, a capacitance of the capacitor changes and a change in capacitance is output as a sound signal. The diaphragm  22  changes its position based on a difference between an air pressure of a sealed space formed by the electret board  18 , spacer  20 , and inner surface of diaphragm  22 , and an atmospheric pressure applied to the opposite surface of the diaphragm  22  caused by formation of the pores  121 . When a sound wave is guided through the pores  121 , a pressure applied to an atmosphere side of the diaphragm  22  varies in accordance with the sound wave and the diaphragm  22  vibrates accordingly.  
         [0007]     Inside the unit case  12 , a cylindrical fixed pole support  14  is disposed on a rear face side of the fixed pole  16 . A circular step part  141  is formed by extending an inner circumference of a front end of the fixed pole support  14  toward the outside in a radial direction, and the fixed pole  16  is fitted into the step part  141 . A disc-like printed circuit board  10  hits on the rear end face of the fixed pole support  14  and the printed circuit board  10  is fitted into a rear end open part of the unit case  12  to be fixed to the unit case  12  by appropriate fixing means, for example, by press fitting or caulking the rear end open part of the unit case  12 . A force to press the fixed pole support  14 , fixed pole  16 , spacer  20 , and diaphragm assembly  30  toward an inner bottom surface of the unit case  12  is applied in this order to the circuit board  10 , and these members are fixed at predetermined locations.  
         [0008]     A field-effect transistor (hereinafter referred to as “FET”)  26  constituting an impedance converter is disposed in a central part on a top face of the printed circuit board  10 . Since an output impedance of a condenser microphone unit is extremely high, the impedance converter consisting principally of the FET  26  is incorporated. Part of terminals of the FET  26  is connected to a predetermined circuit pattern of a printed circuit board  8  by soldering or the like and one end of a contact  28  made of a curved elastic conductor is pressure-contacted to one of other terminals. The other end of the contact  28  is pressure-contacted to the fixed pole  16  and the contact  28  causes one of the terminals of FET  26  and the fixed pole  16  to be electrically conductive.  
         [0009]     A condenser microphone unit used in a tiepin or headset condenser microphone is as extremely small as 2.5 mm in diameter D and 2.0 mm in length (height) L, and a structure thereof has basically no difference from that shown in  FIG. 3  and  FIG. 4 . Nondirectional microphone units are used in most cases for a tiepin or headset condenser microphone unit. For example, a plastic film, as a base material, on which gold is vacuum-deposited, is used as a diaphragm  22 . The diaphragm assembly  30  is constructed by affixing the diaphragm  22  to the ring-shaped diaphragm supporter  24 .  
         [0010]     In order to enhance sensitivity, reduce noise, and improve various characteristics of a condenser microphone unit, an effective capacity should be increased. A capacity that contributes to generation of an electric signal by vibration of the diaphragm  22  is called an effective capacity and, in the case of the condenser microphone unit, in addition to the effective capacity, there is a stray capacitance that reduces sensitivity. The effective capacity depends on an area of a vibrating portion of the diaphragm  22  and a distance to the fixed pole  16  (more specifically, to the electret plate  18 ). The stray capacitance is obtained by subtracting the effective capacity from the capacitance between the diaphragm  22  and fixed pole  16 . The stray capacitance also includes an input capacitance of the FET  26  constituting the impedance converter. If the effective capacity and the stray capacitance are equal, sensitivity at a signal input end to the FET  26  would be reduced to half. Therefore, sensitivity of the condenser microphone unit can be enhanced by making the effective capacity larger and the stray capacitance smaller.  
         [0011]     When the effective capacity is small, sensitivity is reduced, as described above, and in addition, intrinsic noise resulting from a gate part of the FET  26  increases. This intrinsic noise is called 1/f noise. An equivalent sound pressure level of the intrinsic noise of a large condenser microphone unit of, for example, 21 mm in diameter of the diaphragm  22  is approximately several dBSPL, but in the case of a small condenser microphone unit of about 2.5 mm in outside diameter, the equivalent sound pressure level of the intrinsic noise is approximately 30 dBSPL. The equivalent sound pressure level of the intrinsic noise of a microphone for broadcasting service is demanded to be 30 dBSPL or below. Therefore, the outside diameter of 2.5 mm can currently be said be a limit to miniaturization of the condenser microphone unit.  
         [0012]     Miniaturization of the condenser microphone unit has also a problem in terms of manufacturing. As described above, the effective capacity should be increased to enhance sensitivity, and an area in which the diaphragm  22  functions effectively as a diaphragm should be broadened by increasing an inside diameter of the diaphragm supporter  24  to increase the effective capacity. However, a tiepin or headset microphone unit preferably has components including the diaphragm  22  and diaphragm supporter  24  that are made as small as possible and thus a condition conflicting with that of broadening an effective area of the diaphragm  22  must be satisfied. To satisfy this conflicting condition, making an adhesion margin of the diaphragm  22  with respect to the diaphragm supporter  24  as small as possible can be considered.  
         [0013]     In a nondirectional condenser microphone unit, on the other hand, tension of the diaphragm  22  must be increased. This is because sensitivity must be enhanced by raising the polarization voltage, adsorption of the diaphragm  22  by the fixed pole  16  must be prevented by a high polarization voltage, and so on. However, when a plastic film is used as a material of the diaphragm  22 , a creep may occur in the plastic material if tension is high and characteristics are degraded, and thus the tension is limited to an extent that no creep occurs in the plastic material. When, on the other hand, the adhesion margin is made small, as described above, an adhesion part cannot resist tension and the diaphragm  22  are more apt to fall off the diaphragm supporter  24 . Thus, it is difficult for a conventional structure of the condenser microphone unit to make the diameter thereof smaller, and currently, as described above, the diameter of about 2.5 mm is considered to be a limit.  
         [0014]     Next, vibration noise will be considered. When vibrations are applied to a microphone unit, the fixed pole vibrates together with the microphone unit, but the diaphragm attempts to stay on in its original location and a sound signal is emitted by a change in capacitance between the diaphragm and fixed pole. This sound signal is not caused by conversion from a sound wave into an electric signal, but vibration noise based on application of vibrations to the microphone unit. According to the structure of a conventional condenser microphone unit, the diaphragm is flat and thus the structure facilitates generation of the vibration noise.  
         [0015]     Including the example shown in  FIG. 3  and  FIG. 4 , diaphragms of conventionally known condenser microphone units are flat plate-like diaphragms (See, for example, Patent document 1, Patent document 2, Patent document 3, Patent document 4, Patent document 5, Patent document 6, Patent document 7, and Patent document 8). Therefore, there is a problem that the miniaturization described above is limited and generation of vibration noise is likely to occur.  
         [0016]     [Patent document 1] Japanese Patent Application Laid-Open No. 2005-198196  
         [0017]     [Patent document 2] Japanese Patent Application Laid-Open No. 2005-150991  
         [0018]     [Patent document 3] Japanese Patent Application Laid-Open No. 2004-343377  
         [0019]     [Patent document 4] Japanese Patent Application Laid-Open No. 2004-343368  
         [0020]     [Patent document 5] Japanese Patent Application Laid-Open No. 2004-72235  
         [0021]     [Patent document 6] Japanese Patent Application Laid-Open No. 2003-163997  
         [0022]     [Patent document 7] Japanese Patent Application Laid-Open No. 2004-516725  
         [0023]     [Patent document 8] Japanese Patent Application Laid-Open No. 2004-40584  
       SUMMARY OF THE INVENTION  
       [0024]     The present invention has been developed to resolve the above-mentioned problems of the conventional technologies and has an object to provide a condenser microphone unit and a condenser microphone with enhanced sensitivity and reduced noise that can further be miniaturized by incorporating novel ideas free from the conventional technologies with respect to a shape or structure of a vibrator and peripheral members.  
         [0025]     The present invention is characterized as the most main feature in which a condenser microphone unit that forms a capacitor with a vibrator that vibrates by receiving sound and a fixed pole disposed opposite to the vibrator and causes conversion into an electric signal by capacity change of the capacitor, wherein the vibrator is a cylindrical member and the fixed pole is a columnar member disposed on an inner side of the vibrator.  
         [0026]     The vibrator may be disposed through a spacer disposed on the outer circumferential surface of the fixed pole to form a sealed cylindrical space with the outer circumferential surface of the fixed pole, the spacer, and the vibrator.  
         [0027]     A perimeter of the columnar fixed pole is enclosed by the vibrator and a capacitor is constituted by the fixed pole and vibrator. Since the vibrator and fixed pole expand three-dimensionally, instead of conventionally expanding two-dimensionally, it becomes easy to expand an effective area of the vibrator, increase the capacitance, and also raise a ratio of effective capacitance even if the diameter of the unit is limited. To increase the capacitance, the vibrator and fixed pole can be elongated in a central axis direction and sensitivity can also be enhanced while retaining a small diameter. When a physical force such as a vibration is applied from outside, the cylindrical vibrator is translated by approaching the fixed pole on one side and moving away from the fixed pole on the other side, and thus the capacitance increases on one side and decreases on the other side, advantageously canceling out change of the capacitance as a whole to make generation of noise more difficult. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  is a longitudinal sectional view showing an embodiment of a condenser microphone unit according to the present invention.  
         [0029]      FIG. 2  is an exploded longitudinal sectional view of the embodiment.  
         [0030]      FIG. 3  is a longitudinal sectional view showing an example of a conventional condenser microphone unit.  
         [0031]      FIG. 4  is an exploded longitudinal sectional view of the conventional example. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     An embodiment of a condenser microphone unit and a condenser microphone according to the present invention will be described below with reference to  FIG. 1  and  FIG. 2 . Here, the same reference numerals are attached to substantially the same components as those used in the conventional example shown in  FIG. 3  and  FIG. 4 .  
         [0033]     In  FIG. 1  and  FIG. 2 , reference numeral  12  denotes a case of a condenser microphone unit, that is, a unit case. The unit case  12  has a bottomed cylindrical shape and is shown upside down with a portion corresponding to a bottom at an upper side. A plurality of pores  121  to guide a sound wave inside the unit is formed on a flat wall on the upper side corresponding to the bottom and these pores  121  are covered with a front mesh (not shown) on the inner surface side of the unit case  12 .  
         [0034]     Inside the unit case  12 , the disc-like circuit board  10  is fitted into the inside of a lower end open part of the unit case  12  and fixed. Any fixing means of the circuit board  10  can be selected from press fitting, adhesion, caulking the lower end edge of the unit case  12  and the like. Inside the unit case  12 , a columnar fixed pole  36 , an electret layer  38 , a spacer  42 , a vibrator  40  and an FET  26 , which is a main component of an impedance converter, are disposed.  
         [0035]     The fixed pole  36  forms a space  44  between an inner circumferential surface of the unit case  12  and a ceiling surface corresponding to the bottom of the unit case  12  and is fixed onto the circuit board  10 . A recess is formed on the bottom side of the fixed pole  36  and a circular sealed space  361  is formed by the recess and the circuit board  10 . The FET  26  is disposed on the circuit board  10  and the FET  26  is positioned inside the space  361 . The fixed pole  36  is made of a conductive material, for example, a metal.  
         [0036]     The electret layer  38  is formed on the outer circumferential surface of the fixed pole  36  and the polarization voltage is applied to the electret layer  38 . The electret layer  38  can be formed by, for example, putting a heat-shrinkable tube made of FEP (fluororesin: abbreviation of tetrafluoroethylene-hexafluoropropylene copolymer (4.6 fluorinated)) over the columnar fixed pole  36 , fixing the tube to the outer circumferential surface of the fixed pole  36  by heat-shrinking, and performing electret treatment.  
         [0037]     The cylindrical vibrator  40  is disposed on the outer circumference of the fixed pole  36 , more specifically on the circumference of the electret layer  38  with the spacer  42  interposed. The cylindrical vibrator  40  has a radius larger than that of the fixed pole  36  (more correctly, that of the electret layer  38 ) by the thickness of the spacer  42 , and there is a space  46  for the thickness of the spacer  42  between the vibrator  40  and fixed pole  36  (more correctly, the electret layer  38 ). The vibrator  40  has a length in the central axis direction approximately equal to that of the fixed pole  36 , and the spacer  42  is fixed on the inner circumferential surface at both ends in the length direction of the vibrator  40  respectively and the inner circumferential surface of these spacers  42  is fitted into the outer circumferential surface of the electret layer  38  and fixed by suitable fixing means. Therefore, the sealed space  46  is formed by the outer circumferential surface of the fixed pole  36 , upper and lower spacers  42 , and vibrator  40 . The vibrator  40  is formed by a membranous member so that it can vibrate in the radial direction by receiving a sound wave. For example, a metal film of nickel or the like is formed by electroforming, which is a kind of plating, and the formed metal film can be used as a vibrator. The thickness of the metal film is 5 to 10 μm.  
         [0038]     As shown in  FIG. 1 , by placing the fixed pole  36  (more correctly, the electret layer  38 ) and the vibrator  40  opposite to each other with the space  46  therebetween, a capacitor having these electret layer  38  and vibrator  40  as electrodes is constituted. A space  44  is formed among the outer circumferential surface of the vibrator  40 , inner circumferential surface of the unit case  12 , and ceiling surface corresponding to the bottom of the unit case  12 , and the space  44  communicate with outside air through the pores  121  formed on the unit case  12 . Therefore, when a sound wave comes into the space  44  through the pores  121 , a difference is generated between a pressure on the space  44  and a pressure on the sealed space  46 , and the vibrator  40  vibrates in the radial direction due to a pressure difference in accordance with the sound wave. Since the pressure caused by a sound wave is generated in a direction that causes the vibrator  40  to shrink or expand all around the circumference of the vibrator  40 , the capacitance of the vibrator  40  all around the circumference changes in accordance with the sound wave. The change of the capacitance is output as a sound signal.  
         [0039]     In this way, since, according to the embodiment shown in  FIG. 1  and  FIG. 2 , the capacitance increases in proportion to the area of the vibrator  40  facing the space  46  expanding in a circumferential direction, it is easy to increase the effective capacity compared with a flat-shaped diaphragm of a conventional condenser microphone unit by ensuring, to a certain extent, dimensions of the vibrator  40 , fixed pole  36 , and electret layer  38  in the central axis direction. The effective capacitance can also be increased by reducing a unit diameter D or elongating a dimension L in the central axis direction to obtain a condenser microphone unit with high sensitivity and low noise while having a small diameter.  
         [0040]     According to the present embodiment, a condenser microphone unit with low vibration noise can be obtained. That is, when an impact force or vibration is applied to a unit, the fixed pole  36  and its integral component electret layer  38  vibrate together with the circuit board  10  and unit case  12 , while the vibrator  40  attempts to stay on in its original location and is relatively displaced with respect to the fixed pole  36 . Here, the relative displacement of the vibrator  40  is, in contrast to the case in which a sound wave is received, is a movement similar to a parallel translation, and the distance between the fixed pole  36  and vibrator  40  decreases on one side in a diametral direction while the distance between the fixed pole  36  and vibrator  40  increases on the other side in the diametral direction. Therefore, the capacitance increases on one side and decreases on the other side to cancel out each other as a whole so that this structure makes a level of output signal, that is, a noise level low and generation of vibration noise difficult.  
         [0041]     In the description so far, the vibrator  40  was assumed to have a cylindrical shape, that is, a cylindrical shape with both ends open in the length direction, but may be a metal film having a bag-like shape with a bottom denoted by an imaginary line  401  in  FIG. 2 . The spacer  42  is fixed to upper and lower ends on the inner surface side of the bag-like vibrator  40 , the vibrator  40  is put together with the spacer  42  over the fixed pole  36  from above, and the vibrator  40  is fixed to the fixed pole  36 . A portion corresponding to the bottom of the bag-like vibrator  40  is brought into intimate contact with the top face of the fixed pole  36 . Or, a space may be created between the portion corresponding to the bottom of the bag-like vibrator  40  and the top face of the fixed pole  36  to constitute a capacitor between the portion corresponding to the bottom of the bag-like vibrator  40  and the top face of the fixed pole  36  to further increase the effective capacitance.  
         [0042]     The embodiment shown is an example of an electret condenser microphone unit in which the electret layer  38  is formed on the outer circumference of the fixed pole  36 , but a condenser microphone unit omitting the electret layer  38  will also do.  
         [0043]     The condenser microphone unit shown in  FIG. 1  and  FIG. 2  can be made to be a condenser microphone by incorporating the condenser microphone unit into a microphone case and further incorporating necessary circuits into the microphone case. A transmitter is incorporated into a wireless pin microphone or a headset microphone, and a connector for connecting a cable is incorporated into a cable-connected microphone.  
         [0044]     In the present invention, a diaphragm and a fixed pole are devised based on ideas completely different from those of a conventional condenser microphone or a condenser microphone unit, and a highly sensitive condenser microphone unit and condenser microphone can be obtained by increasing the effective capacitance while retaining a small diameter. Therefore, the present invention will be very likely to be widely applied to uses where miniaturization is required such as a pin microphone and a headset microphone.