Abstract:
A condenser microphone includes a housing and a microphone unit arranged within the housing. The microphone unit includes an annular retainer oriented in confronting relation to the front end wall of the housing, a movable diaphragm connected to the annular retainer, an annular spacer connected to one side of the movable diaphragm opposite the annular retainer, a substrate including a back electrode oriented in confronting relation to the movable diaphragm and having an electret thereon, and a printed circuit board having electronic components. The back electrode is operatively associated with the movable diaphragm to constitute a condenser. The condenser has a variable electric capacitance. The electronic components of the printed circuit board develop an electric signal in response to a change in the electrical capacitance between the movable diaphragm and the back electrode. A conductive adhesive is employed to sealingly hold the annular retainer against the front end wall of the housing.

Description:
This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2004-362673 filed Dec. 15, 2004, the entire content of which is hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a condenser microphone with improved waterproof capability and improved electric shielding effect and a method for manufacturing the same. 
   BACKGROUND OF THE INVENTION 
   An electret condenser microphone has a wide variety of applications owing to its structural simplicity, compactness and reasonable fabrication cost. The electret condenser microphone has recently been used in a cellular phone and other portable audio devices. There is thus an increasing need to further reduce the size of and enhance the performance and reliability of the electret condenser microphone. To meet this need, Japanese patent application publication No. 11-088992 proposes a condenser microphone wherein a semiconductor chip, a conductive layer or a back electrode, an electret, a spacer and a movable diaphragm are stacked in that order. 
   Referring specifically to  FIG. 5 , there is shown, in section, a condenser microphone generally designated as at  30 . The condenser microphone  30  includes a semiconductor chip  31  with a FET as an impedance converter and an amplifier. A conductive layer  32  is vacuum deposited on the semiconductor chip  31 . A dielectric layer or electret  33  is formed on the conductive layer  32 . A spacer  34  is printed on the outer peripheral edge of the electret  33 . A movable diaphragm  35  is attached to the spacer  34  and cooperates with the electret  33  to form a working gap or air chamber  36 . 
   The semiconductor chip  31 , the conductive layer  32 , the electret  33 , the spacer  34  and the movable diaphragm  35  collectively form a microphone unit  37 . A housing  38  is made of ceramic and encloses the microphone unit  37 . The housing  38  has a front end wall, a rear end wall and a peripheral wall extending between the front and rear end walls. A plurality of sound inlet ports  38   a  are formed in the front end wall of the housing  38 . A fabric or cloth  39  is attached to the front end wall of the housing. The semiconductor chip  31  includes two terminals or leads  31   a ,  31   b . The two leads  31   a ,  31   b  extend through the rear end wall of the housing  38  and are soldered thereto. 
   As described above, the semiconductor chip  31  and all the other main components are integrated into a small unit. This arrangement enables the condenser microphone  30  to be economically manufactured on a mass production basis. 
   Japanese patent application publication No. 2003-230195 discloses a condenser microphone wherein a housing serves as an electric shield to inhibit entry of electric noise into the housing within which a microphone unit is contained. Referring specifically to  FIG. 6 , there is shown, in section, a condenser microphone  40  which includes a metallic housing  41 . The housing  41  has a front end wall  43  and a cylindrical side wall  44  extending from the front end wall  43 . A plurality of sound inlet ports  42  extend through the front end wall  43  of the housing  41 . The front end wall  43  of the housing  41  acts as a fixed electrode. The housing  41  is formed on its inner surface with an electret  45 . Disposed within the housing  41  are an annular electrically insulative spacer  46 , an electrically conductive movable diaphragm  48  supported on a support ring  47  and acting as a movable electrode, and a cylindrical conductive ring  49 . 
   The housing  41  has an open rear end in which a printed circuit board  51  is arranged. A plurality of electronic components  50  such as a FET are surface mounted onto the inner surface of the printed circuit board  51 . The printed circuit board  51  is provided on its outer periphery with a conductive layer or ground section  51   a . The lower end of the side wall  44  is radially inwardly curved to form a bent end  44   a . The bent end  44   a  makes contact with the ground section  51   a  to provide an electrical connection between the housing  41  and the ground section  51   a . This arrangement electrically isolates the interior of the housing  41  from the outside of the housing  41 . A filter  52  is attached to the outer surface of the front end wall  43  of the housing  41  and is made of a non-woven fabric, a cloth and other materials. The front end wall  43  of the housing  41  and the movable diaphragm  48  collectively constitute a condenser. With this arrangement, a change in electrical capacitance between the front end wall  43  of the housing  41  and the movable diaphragm  48  occurs when the diaphragm  48  is vibrated or deflected in response to an incident sound pressure wave through the sound inlet ports  42 . Such a capacitive change is converted to an impedance by means of the electronic components  50  and then output as an electrical signal from a terminal  51   b . The terminal  51   b  is formed on the outer surface of the printed circuit board  51 . 
   Again, the ground section  51   a  and the bent end  44   a  of the metallic housing  41  are connected to electrically isolate the interior of the microphone unit. The condenser microphone  40  is thus capable of preventing entry of electric noise into the interior of the microphone and providing a relatively low signal to noise ratio. 
   A problem with the condenser microphone  30  shown in Japanese patent application publication No. 11-088992 is that water may enter the interior of the housing  38  if clearances are left between the side wall and the rear end wall of the housing and between the through holes in the rear end wall of the housing and the corresponding leads  31   a ,  31   b , as shown by the imaginary arrows in  FIG. 5 . The water, when entered, can oxidize the surface of the movable diaphragm  35 . Obviously, such oxidization adversely affects the sensitivity and the frequency characteristics of the movable diaphragm  35 . This problem becomes more serious particularly in case that the movable diaphragm is formed with a plurality of perforations (not shown). In such a case, the water may even flow over the rear side of the movable diaphragm  35 . This further deteriorates the sensitivity and the frequency characteristics of the movable diaphragm. The clearances also create another problem. Sound pressure waves normally move into the housing  38  through the sound inlet ports  38   a  and cause the movable diaphragm  35  to vibrate or deflect. If the clearances are formed in the housing  38  as shown in  FIG. 5 , the sound pressure waves can enter the interior of the housing  38  through the clearances. This alters the directionality of the microphone and adversely affects the frequency characteristics of the microphone. 
   There is also a drawback to the condenser microphone  40  shown in Japanese patent application publication No. 2003-230195. The electrical connection between the bent end  44   a  of the housing  41  and the ground section  51   a  of the printed circuit board  51  may be damaged if dust or water droplets are attached thereto. If this occurs, the housing  41  and the ground section  51   a  of the printed circuit board  51  will have a resultant high electrical resistance, and the housing  41  will no longer act as an electric shield. As a consequence, electric noise (or burst noise) is free to enter the housing and significantly lowers the performance of the microphone. 
   It is, therefore, an object of the present invention to overcome the foregoing drawbacks and provides a reliable condenser microphone which can prevent entry of water into the interior of the microphone and also, entry of sound pressure waves into the housing of the microphone through a portion of the microphone other than predetermined sound inlet ports, and which can maintain the sensitivity, the frequency characteristics and the directionality of the microphone. It is another object of the present invention to provide a high performance condenser microphone which can exhibit a high level of electrical shielding effect. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, there is provided a condenser microphone comprising a microphone unit and a housing shaped to enclose the microphone unit to mechanically protect the microphone unit, wherein the housing has a front end wall and a peripheral wall extending from the front end wall, wherein the microphone unit includes an annular retainer having a front face oriented to face against the front end wall of the housing and a rear face, a movable diaphragm having a front face, a rear face and a peripheral edge attached to the rear face of the annular retainer, the movable diaphragm being placed in confronting relation to the front end wall of the housing and deflected in response to an incident sound pressure wave through the front end wall of the housing, an annular spacer having a front face attached to the rear face of the movable diaphragm adjacent to the peripheral edge of the movable diaphragm and a rear face, a substrate connected to the rear face of the annular spacer, the substrate including a back electrode having an electret thereon and oriented in confronting relation to the movable diaphragm, and the back electrode being operatively associated with the movable diaphragm to provide a condenser having a variable electrical capacitance, and a printed circuit board connected to the substrate and including an electronic component for generating an electric signal in response to the variable electrical capacitance, and wherein the microphone unit is mounted to the housing so that the annular retainer is sealingly held against the front end wall of the housing. 
   As opposed to the conventional condenser microphones, the condenser microphone of the present invention can prevent entry of dust and water into the microphone unit through the rear end of the microphone which would, otherwise, deteriorate the performance of the movable diaphragm and also, substantially prevent entry of incident sound pressure waves through the rear end of the microphone. 
   In one embodiment, the printed circuit board includes a ground terminal, and the microphone unit further includes an electrically conductive joining member through which the annular retainer is sealingly held against the front end wall of the housing, and an electrically conducive adhesive for interconnecting the annular retainer, the movable diaphragm and the annular spacer. The annular retainer and the annular spacer are electrically conductive, and the housing is made of a metallic material and electrically connected to the ground terminal of the printed circuit board through the electrically conductive joining member, the annular retainer, the movable diaphragm and the annular spacer. 
   This arrangement effectively prevents entry of dust into the microphone unit. 
   Preferably, the electrically conductive joining member is made from an electrically conductive adhesive. It is also preferred that the front end wall of the housing includes at least one sound inlet port, and the condenser microphone further includes a water repellent dust filter attached to one side of the front end wall of the housing opposite the microphone unit and positioned to cover the sound inlet port. 
   According to another aspect of the present invention, there is provided a method for manufacturing a condenser microphone which comprises preparing a housing having a front end wall and a peripheral wall having a front end connected to the front end wall and an open rear end, preparing a microphone unit by stacking an annular retainer, a movable diaphragm, an annular spacer, a substrate and a printed circuit board one above the other, the substrate including a back electrode oriented in confronting relation to the movable diaphragm, having an electret thereon and operatively associated with the movable diaphragm to provide a condenser having a variable electrical capacitance, the printed circuit board including an electronic component for producing an electric signal in response to the variable electrical capacitance, and inserting the electronic unit into the housing through the open rear end of the peripheral wall of the housing while keeping said annular retainer in the front of the microphone unit and sealingly joining the annular retainer to the front end wall of the housing by a joining member. 
   In one embodiment, the joining member is placed on the annular retainer before the microphone unit is inserted into the housing. Preferably, the joining member is made of a thermosetting material and heated after the joining member is pressed against the front end wall of the housing. 
   ADVANTAGES OF THE INVENTION 
   As described above, the present invention is capable of preventing entry of water into the microphone and entry of sound pressure waves through a portion of the microphone other than the sound inlet ports. The present invention thus provides a reliable condenser microphone which prevents deterioration in the sensitivity and the frequency characteristics of the microphone. Also, the electrically conductive member provides a secure electrical connection between the housing and the ground of the microphone unit. The present invention thus provides a high performance condenser microphone which can effectively prevent entry of electric noise into the microphone. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is sectional view of a condenser microphone according to a preferred embodiment of the present invention; 
       FIG. 2  is a perspective disassembled view of the microphone unit shown in  FIG. 1 ; 
       FIG. 3  is a perspective view showing the manner in which the microphone unit is assembled into a housing; 
       FIG. 4  is a schematic diagram showing one example of a circuitry used in the condenser microphone; 
       FIG. 5  is a sectional view of a conventional condenser microphone; and 
       FIG. 6  is a sectional view of another conventional condenser microphone. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described with reference to the accompanying drawings.  FIG. 1  is sectional view of a condenser microphone according to a preferred embodiment of the present invention.  FIG. 2  is a perspective disassembled view of the condenser microphone.  FIG. 3  is a perspective view showing the manner in which a microphone unit is assembled into a housing.  FIG. 4  is a schematic diagram showing one example of a circuitry used in the condenser microphone. 
     FIG. 1  shows a condenser microphone constructed in accordance with the present invention and generally designated as at  1 . The condenser microphone  1  includes a metallic housing  2  of a generally rectangular parallelepiped shape. The housing  2  has a front end wall  2   a  and an open rear end  2   b . A plurality of sound inlet ports  3  are defined in the front end wall  2   a  of the housing  2  to allow sound pressure waves to move into the housing  2 . A microphone unit is inserted into the housing  2  through the open rear end  2   b  of the housing, as will later be described. A movable diaphragm  4  vibrates or deflects in response to an incident sound pressure wave. The movable diaphragm  4  is in the form of a thin membrane made of polyphenylene sulfide, polyethylene naphthalate, polyimide and similar resinous materials. A conductive layer is vacuum deposited on the movable diaphragm  4 . A retainer  5  rests on the upper surface of the movable diaphragm  4 . 
   A substrate  6  is made of glass epoxy and similar materials and includes a back electrode  6   a . The back electrode  6   a  is in the form of a copper film placed on the front surface of the substrate  6 . A dielectric layer or electret  6   b  is arranged on the back electrode  6   a . A spacer  7  is arranged below the movable diaphragm  4  and extends along the outer periphery of the movable diaphragm  4 . The spacer  7  cooperates with the retainer  5  to hold the movable diaphragm  4  in place. The spacer  7  separates the movable diaphragm  4  from the back electrode  6   a  by a predetermined distance. The back electrode  6   a  and the movable diaphragm  4  collectively form a condenser. The retainer  5  and the spacer  7  are preferably made of an electrically conductive material. In the illustrated embodiment, the spacer  7  and the substrate  6  are discrete members. The present invention is not limited to this embodiment. For example, the spacer  7  may be integrally formed with the substrate  6 . 
   A printed circuit board  8  is made of glass epoxy and similar materials. A FET as an impedance converter and other electronic components  9  are surface mounted to the printed circuit board  8 . Formed on the rear side of the printed circuit board  8  are a ground terminal  8   a  and an output terminal  8   b  of the electronic components  9 . The ground terminal  8   a  and the output terminal  8   b  are in the form of electrically conductive layers made of copper. The electronic components  9  have a circuitry, as will later be described. The substrate  6  has a cavity  6   c  within which the electronic components  9  are located. The present invention is not limited to this arrangement. As an alternative, the substrate  6  may be in the form of a backplate, and the printed circuit board  8  may have a concave portion to receive the electronic components  9 . As thus far described, the retainer  5 , the movable diaphragm  4 , the spacer  7 , the substrate  6  and the printed circuit board  8  are stacked one above the other so as to form a microphone unit  10 . 
   The microphone unit  10  is inserted into the interior of the housing  2  through the open rear end  2   b  of the housing  2 . A conductive layer or member  11  rests on the retainer  5 . The conductive member  11  is sandwiched between the retainer  5  and an inner surface  2   c  of the front end wall of the housing  2  so as to hold the microphone unit  10  in place within the housing  2 . The housing  2  encloses the microphone unit  10  and serves to mechanically protect the microphone unit  10 . The conductive member  11  is preferably made from a suitable material such as a conductive paste, an anisotropic conductive film and a conductive washer. A water repellent dust plate  12  is attached to the front end wall  2   a  of the housing  2  to cover the sound inlet ports  3 . 
   As pointed out earlier, water or dust, if entered into the housing  2  possibly through the sound inlet ports  3  or a clearance  13  between the rear end  2   b  of the housing  2  and the printed circuit board  8  and then, attached to the surface of the diaphragm  4 , can deteriorate the sensitivity and the frequency characteristics of the condenser microphone  1 . According to the present invention, the dust plate  12  prevents entry of water or dust into the microphone unit  10  through the sound inlet ports  3 . Also, the conductive member  11  prevents entry of water or dust into the housing through the clearance  13 . More specifically, water and dust, after entered into the condenser microphone  1  through the clearance  13 , move between an inner surface  2   d  of the side wall of the housing  2  and a side surface  6   d  of the substrate  6 . The water and dust then reach between the inner surface  2   c  of the front end wall of the housing  2  and the retainer  5 . 
   The water and dust can not, however, enter the microphone unit  10  as the conductive member  11  securely holds the retainer  5  in intimate contact with the inner surface  2   c  of the front end of the housing  2 . The present invention can therefore provide a highly reliable condenser microphone. A sound pressure wave could enter the housing  2  through the clearance  13  when the condenser microphone  1  is mounted in a particular orientation within an electronic device. In such a case, the conductive member  11  effectively prevents such an incident sound pressure wave from reaching the movable diaphragm  4 . The condenser microphone  1  is thus capable of preventing deterioration of its directionality and frequency characteristics regardless of the orientation of the microphone within electronic devices. Advantageously, when the retainer  5  and the spacer  7  are both made from an electrically conductive material, the housing  2  can be electrically connected to the ground terminal  8   a  of the printed circuit board  8  through the conductive member  11 . In this way, the housing  2  can electrically isolate the microphone unit  10  from the outside of the housing  2 . The electric shielding effect and the circuitry of the condenser microphone will later be described with reference to  FIG. 4 . 
     FIG. 2  illustrates detailed structure of the microphone unit  10  and the manner in which the microphone is assembled. As shown, the microphone unit  10  is constructed in such a manner that the retainer  5 , the movable diaphragm  4 , the spacer  7 , the substrate  6  and the printed circuit board  8  are stacked one above the other. The retainer  5  has an opening  5   a  through which the underlying movable diaphragm  4  is exposed to the outside of the microphone unit  10 . The conductive member  11  surrounds the opening  5   a  of the retainer  5  when the conductive member  11  rests on the upper surface of the retainer  5 . Also, the conductive member  11  provides a tight seal between the inner surface  2   c  of the front end wall of the housing  2  and the retainer  5  when the microphone unit  1  is mounted within the housing  2 . In the illustrated embodiment, the conductive member  11  is annular in shape. Alternatively, the conductive member  11  may extend over the entire surface of the retainer  5  except where the opening  5   a  is defined. In this way, the condenser microphone enjoys improved dustproof and waterproof capabilities. 
   The spacer  7  is located below the movable diaphragm  4  and cooperates with the retainer  5  to securely hold the movable diaphragm  4  in place. The spacer  7  has a central opening  7   a  so that the movable diaphragm  4  faces with the back electrode  6   a  of the substrate  6 . The movable diaphragm and the fixed back electrodes collectively constitute a condenser. The back electrode  6   a  and the electret  6   b  are preferably circular in shape although they may take any other shapes. The printed circuit board  8  is located at the lowermost part of the microphone unit  10 . The printed circuit board  8  is assembled to the substrate  6  after all the electronic components  9  are surface mounted thereto. In the illustrated embodiment, the condenser microphone unit  10  has a generally rectangular parallelepiped shape. The present invention is not limited thereto. For example, the condenser microphone unit  10  may have a cylindrical shape. It is to be understood that assembly of the microphone unit  10  requires mechanical connection as well as electrical connection. It is, therefore, preferred that an electrically conductive adhesive or a similar agent be used to stack and secure the microphone components. 
     FIG. 3  schematically shows the manner in which the microphone unit  10  is mounted within the housing  2 . As described above, the retainer  5 , the movable diaphragm  4 , the spacer  7 , the substrate  6  and the printed circuit board  8  collectively form the microphone unit  10 . The conductive member  11  may be placed on the retainer  5  after or before assembly of the microphone unit  10  is completed. 
   The microphone unit  10 , after completely assembled, is inserted into the housing  2  through its rear end  2   b . It is desirable that in order to place the conductive member  11  into intimate contact with the inner surface  2   c  of the front end wall of the housing  2 , a tool (not shown) be employed to apply a force in such a direction as to move the microphone unit  10  toward the front end wall of the housing  2 . It is also desirable that where the conductive member  11  is made of a thermosetting material, the conductive member  11  be heated to a suitable temperature. Assembly of the condenser microphone  1  is completed when the microphone unit  10  is brought into close contact with the housing  2 . 
   To more tightly secure the microphone unit  10  to the housing  2 , the rear end  2   b  of the housing  2  may be inwardly bent. As an alternative, the clearance  13  (see  FIG. 1 ) may be filled with a suitable molding material. It is to be noted that the dust plate  12  may be securely held against the front end wall  2   a  of the housing  2  before or after the microphone unit  10  is mounted within the housing  2 . 
     FIG. 4  illustrates one example of the circuitry of the condenser microphone  1 . A FET (Field Effect Transistor)  9   a  forms part of the electronic components  9 . The FET  9   a  includes a source terminal S coupled to ground (shown as “GND” in  FIG. 4 ) and a drain terminal D coupled to the output terminal  8   b  of the printed circuit board  8 . The ground terminal  8   b  of the printed circuit board  8  is coupled to ground. The ground terminal  8   b  and the output terminal  8   b  provide microphone outputs. A resistor  9   b  forms part of the electronic components  9 . The FET  9  also includes a gate terminal G. The resistor  9   b  provides an electrical connection between a gate terminal G and ground. A condenser is designated as at  14  and composed of the movable diaphragm  4  and the back electrode  6   a . The back electrode  6   a  has a conductive layer (not show) and is coupled to the gate terminal G of the FET  9  through the conductive layer. The other electrode or movable diaphragm  4  is electrically coupled through the conductive spacer  7  and the conductive layer of the back electrode  6  to the printed circuit board  9  and also, to ground. 
   The retainer  5  (see  FIG. 1 ) is tightly held against the movable diaphragm  4 , as described earlier and thus, is connected to ground. As a result of this connection, the housing  2  is also connected to ground through the conductive member  11 . It should be noted that the conductive member  11  not only prevents entry of water and dust into the microphone unit  10  by providing a mechanical connection between the microphone unit  10  and the housing  2 , but also electrically shields the microphone unit  10  by electrically connecting the housing  2  to ground. 
   Operation of the microphone circuitry will now be described in detail with reference again to  FIG. 4 . The movable diaphragm  4  deflects when a sound pressure wave is transmitted through the sound inlet ports  3 . This results in a change in electrical capacitance between the movable diaphragm  4  and the back electrode  6   a . This capacitive change is transmitted to the gate terminal G as a change in electrical potential. The FET  9   a  amplifies the differential electrical potential and provides an electrical signal through the drain terminal D. The electrical signal is then outputted from the output terminal  8   b . Again, the metallic housing  2  is connected to ground through the conductive member  11  so that the microphone unit  10  is electrically shielded by the housing  2 . The present invention is thus capable of providing a high performance condenser microphone with improved electrical shielding effect and lower signal to noise ratio. 
   As thus far described, the conductive member  11  holds the microphone unit  10  in intimate contact with the inner surface  2   c  of the housing  2 . This arrangement prevents entry of water and dust into the microphone unit  10  and entry of sound pressure waves into the diaphragm through portions of the housing other than the sound inlet ports. The present invention thus provides a reliable condenser microphone which prevents deterioration in the sensitivity and the frequency characteristics of the microphone and also, prohibits a change in the directionality of the microphone. The conductive member  11  also provides a secure electrical connection between the housing  2  and the ground of the microphone so that the condenser microphone  1  is electrically shielded by the housing  2 . The present invention thus provides a high performance condenser microphone which prevents entry of electric noise into the microphone unit. It should be noted that the circuitry of the condenser microphone is not limited to the one shown in  FIG. 4 , but may take any other forms. 
   Although the present invention has been described in terms of specific embodiments, it is anticipated that alternations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alternations and modifications as fall within the true sprit and scope of the invention.