Abstract:
In a condenser microphone in which a microphone unit section is replaceable, an electromagnetic wave is prevented without fail from entering into an internal circuit through a coupled portion of the microphone unit section and a microphone case and generation of noises caused by the electromagnetic wave is prevented. The condenser microphone has a microphone case, a unit case which is detachably attached to the microphone case and incorporates a condenser microphone unit, a circuit substrate contained in the microphone case, and an electric path which electrically connects a rear end of the unit case with the circuit substrate in a state that the unit case is attached to the microphone case, wherein the electric path electrically connects the rear end of the unit case with the circuit substrate via an inductor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a condenser microphone in which a microphone unit section can be attached to and detached from a microphone case and, more particularly, to a condenser microphone in which a microphone unit section can be arbitrarily replaced with, for example, a nondirectional microphone unit section or a unidirectional microphone unit section and an increase in noise due to the fact that a microphone unit section is made replaceable can be suppressed. 
   2. Related Background of the Invention 
   Since an impedance of a microphone unit is high, an impedance converter composed of a field effect transistor (FET) etc. is used in a condenser microphone. Further, in order to fulfill performance necessary as a microphone or perform required functions, an output circuit, a low cut circuit, etc., are incorporated in the microphone. 
   On the other hand, there is a condenser microphone in which a microphone unit section can be replaced depending on purposes. For example, there is a condenser microphone, in which it is possible to selectively replace and attach a nondirectional microphone unit section and a unidirectional microphone unit section. A unit replacement mechanism of a condenser microphone capable of replacing its microphone unit section is generally designed into a structure in which a unit case that incorporates a microphone unit is attached to a microphone case by a screw. A sound signal to be converted into an electric signal and output in the microphone unit is transmitted to the microphone case side via an electric contact, its impedance is converted by an impedance converter contained in the microphone case, and the converted sound signal is output to the outside after being subjected to predetermined process such as low cut process. 
     FIG. 4  and  FIG. 5  show a conventional example of a condenser microphone capable of replacing its microphone unit section depending on purposes. In  FIG. 4  and  FIG. 5 , symbol  1  denotes a microphone case and symbol  3  denotes a microphone unit section. The microphone case  1  has a female screw  19  on the inner circumferential surface at the front end (the left end in  FIG. 4  and  FIG. 5 ) portion and the microphone unit section  3  has a male screw  32  on the rear end portion, and it is structured such that the microphone unit section  3  can be attached to the microphone case  1  by screwing the male screw  32  into the female screw  19 . 
   The microphone unit section  3  has a substantially cylindrical unit case  31 , and a condenser microphone unit  30  is attached near the front end of the unit case  31 . Since the condenser microphone unit  30  has a general unit configuration conventionally known, its detailed description is omitted. The front end of the unit case  31  is covered with a front cover  39  having a plurality of slit-like sound introduction holes. An inner side end plate  33  is engaged with and fixed to the rear end side of the unit case  31  and an outer side end plate  34  is engaged with and fixed to the further outside. A flanged contact  38  is inserted from inside into a hole formed at the center of the outer side end plate  34 . A rod-like terminal  36  is inserted into a hole formed at the center of the inner side end plate  33  from outside toward inside and the terminal  36  is fixed to the inner side end plate  33  in a state of protruding toward the microphone unit  30 . A compression spring  37  made of a conductor is interposed in the holes at the centers of the inner side end plate  33  and the outer side end plate  34 , the contact  38  is pressed toward the outside of the outer side end plate  34  by a repulsion of the compression spring  37 , and the rear end portion of the contact  38  which is formed into a semispherical shape protrudes from the outer side end plate  34 . To the front end portion of the rod-like terminal  36 , an output terminal plate  35  extending backward from the microphone unit  30  is screwed. Therefore, a sound signal converted by the microphone unit  30  is transmitted to the output terminal plate  35 , the terminal  36 , the compression spring  37 , and the contact  38 . 
   The microphone case  1  mainly comprises a relatively elongated cylindrical case  11 . An end plate  16  is engaged with and fixed to the inner circumferential surface of the front end portion of the cylindrical case  11 , located adjoining the female screw  19  and on the rear side thereof. A contact  20  is engaged with and fixed to the end plate  16  so as to penetrate its center in the direction of thickness. A circuit substrate  15  is attached to the inside of the cylindrical case  11  along its length direction. On the circuit substrate  15 , an impedance converter circuit composed mainly of the FET, the low cut circuit, the output circuit, and the like are incorporated. The contact  20  is connected to an input terminal of the circuit substrate  15  via a proper conductive material. A connector case  12  is engaged with and fixed to a rear end portion of the cylindrical case  11  by a screw etc. A connector base  13  having insulation properties is fixed in the connector case  12 . Three conductive pins  14  are buried in the connector base  13 , constituting a connector having a standardized three-pin configuration. An output terminal and a ground terminal of the output circuit formed on the circuit substrate  15  are electrically connected with the corresponding pins, respectively. 
     FIG. 4  shows a state in which the microphone unit section  3  is attached to the microphone case  1  by screwing the male screw  32  of the unit case  31  into the female screw  19  of the cylindrical case  11 . In this state, the contact  38  of the microphone unit section  3  comes into contact with the front end of the contact  20  on the microphone case  1  side and the contact  38  is pressed against the contact  20  by a compressive force of the compression spring  37 , thereby the contact  38  and the contact  20  are electrically connected with each other. Therefore, a sound signal converted in the microphone unit  30  and output therefrom is input to the input terminal of the circuit substrate  15  via the output terminal plate  35 , the terminal  36 , the compression spring  37 , the contact  38 , and the contact  20 . On the circuit substrate  15 , a predetermined process such as impedance conversion, low cut, and amplification is performed and an output signal is transmitted to the three pins  14  as an output terminal. One of the three pins  14  is a ground pin connected to the ground. Into the connector case  12 , a connector plug conforming to the standards is inserted, and a signal is input to an external circuit via the three pins  14  and a microphone cord. 
   According to the conventional condenser microphone described above, the microphone case  1  and the microphone unit section  3  are electrically connected with each other by screwing the male screw  32  into the female screw  19 , therefore, the microphone case  1  and the microphone unit section  3  serve as a ground and function as a shielding member against an external electromagnetic wave. 
   However, the coupled portion of the microphone case  1  and the microphone unit section  3  is a path of an electromagnetic wave that enters from the outside and also a path of a signal of a sound signal on the grounding side, therefore, its structure allows a sound signal to be subject to the electromagnetic wave. In other words, the coupled portion by screw of the microphone case  1  and the microphone unit section  3  has an impedance for a high frequency signal, therefore, an electromagnetic wave is likely to enter into an internal circuit from the coupled portion and the electromagnetic wave that has entered into the internal circuit is detected by a semiconductor constituting the internal circuit and generates noises. In particular, when a signal current flows through the coupled portion by screw, the noises increase. Recently, a mobile phone prevails widely and in the situation in which a mobile phone is used frequently everywhere, it is more likely to occur that a mobile phone is used in the vicinity of a condenser microphone and an electromagnetic wave emitted from a mobile phone enters through the coupled portion and causes noises to be output from the condenser microphone. 
   As an invention described in a patent document relating to the condenser microphone according to the present invention, there is an invention as follows. In a condenser microphone in which a microphone capsule including a microphone unit is made replaceable to a grip part that incorporates an electric circuit for sound signal output, it is made possible to easily adjust a sensitivity when replacing a capsule by providing, for example, two capacitors for pads in the microphone capsule and making it possible to selectively connecting the capacitors for pads in parallel to the effective capacitance of the microphone unit using a selection switch (for example, refer to the patent document 1). 
   The object of the invention described in the patent document 1 is to make the microphone capsule including the microphone unit replaceable to the grip part and to make it possible to easily adjust the sensitivity when replacing the capsule. Therefore, as will be described later, its object is different from that of the invention of the present application, which object is to prevent noises due to the electromagnetic wave that tries to enter through the coupled portion of the microphone unit section and the microphone case in the condenser microphone in which the microphone unit section is made replaceable. 
   As an invention described in another patent document relating to the condenser microphone according to the present invention, there is an invention, that is, a unidirectional electret condenser microphone comprising a back electrode holder made of an insulating material and which engages and fixes a back electrode plate constituting an electrode on one side and a lead wire ring which is attached internally to the back electrode holder and electrically connects the back electrode holder with a printed circuit board, wherein an IC chip including a capacitor for removing high frequency noises, an inductor, and other circuit elements is contained in a back electrode chamber formed inside the back electrode holder and attached to the printed circuit board (for example, refer to the patent document 2). 
   The invention described in the patent document 2 comprises the inductor for removing high frequency noises, however, its object is different from that of the invention of the present application, which object is to prevent noises due to the electromagnetic wave that tries to enter through the coupled portion of the microphone unit section and the microphone case in the condenser microphone in which the microphone unit section is made replaceable. 
   As an invention described in still another patent document, there is a vibration pickup microphone using a ceramic bimorph element, in which even if a mobile phone is used near the microphone, a filter composed of an inductor and a capacitor is interposed between the ceramic bimorph element and a FET as an impedance converting circuit in order to avoid a digital noise from the mobile phone (for example, refer to the patent document 3). 
   The object of the invention described in the patent document 3 is to avoid a digital noise when a mobile phone is used in the vicinity of a microphone, therefore, its object is different from that of the invention of the present application, which object is to prevent noises due to the electromagnetic wave that tries to enter through the coupled portion of the microphone unit section and the microphone case in the condenser microphone in which the microphone unit section is made replaceable. 
   Further, a configuration is also known, in which an inductor as an impedance element is inserted in series with a transmission wave signal output line of a microphone at a position close to the microphone and the impedance element is used to prevent the intrusion of the radio transmission wave signal induced in the above transmission wave signal output line into the microphone (for example, refer to the patent document 4). 
   The invention described in the patent document 4 has disclosed that the impedance element connected in series with the transmission wave signal output line of the microphone are effective to avoid noises due to the electromagnetic wave, however, its object is different from that of the invention of the present application, which object is to prevent noises due to the electromagnetic wave that tries to enter through the coupled portion of the microphone unit section and the microphone case in the condenser microphone in which the microphone unit section is made replaceable. 
   [Patent document 1] Japanese Patent Application Laid-Open No. 2004-23705 
   [Patent document 2] Japanese Patent Application Laid-Open No. 2000-232700 
   [Patent document 3] Japanese Patent Application No. Hei 10-336794 
   [Patent document 4] Japanese Patent Application Laid-Open No. Hei 09-83463 
   SUMMARY OF THE INVENTION 
   The present invention has been developed in consideration of the above-mentioned problems of the conventional condenser microphone, and an object thereof is to provide a condenser microphone in which a microphone unit section can be detachably attached to a microphone case and which is capable of preventing without fail an electromagnetic wave from entering into an internal circuit through a coupled portion of the microphone unit section and the microphone case and of preventing generation of noises due to the electromagnetic wave. 
   The present invention is characterized as the most main feature by comprising a microphone case, a unit case that can be detachably attached to the microphone case and incorporates a condenser microphone unit, a circuit substrate contained in the microphone case, and an electric path that electrically connects a rear end of the unit case with the circuit substrate in a state in which the unit case is attached to the microphone case, wherein the electric path electrically connects the rear end of the unit case with the circuit substrate via an inductor. 
   The unit case is detachably attached to the microphone case and if a plurality of kinds of unit case or microphone unit section having different specifications of the incorporated condenser microphone unit is prepared, it is possible to use the microphone as a microphone having specifications suited for the intention of a user by selecting a unit case or a microphone unit section having any specifications and attaching it to the microphone case in accordance with the use conditions and the like of the microphone. When the rear end of the unit case is electrically connected with the circuit substrate in the microphone case via the inductor in a state in which the unit case is attached to the microphone case, the unit case functions not only as a microphone case but also as an electric path on the ground side of a sound current and a high frequency current. In the microphone case, the microphone case, which is an electric path on the grounding side of the high frequency current, and the electric path on the ground side of the circuit substrate, which is an electric path on the ground side of the sound current, are separated, therefore, the electric path on the circuit substrate is isolated from the high frequency current and it is unlikely that noises caused by the high frequency current mixedly enters into the sound circuit, and a clear sound signal with less noises can be obtained. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a longitudinal sectional view showing an embodiment of a condenser microphone according to the present invention. 
       FIG. 2  is a longitudinal sectional view showing the embodiment in a state in which a microphone unit section is separated from a microphone case. 
       FIGS. 3(   a ) and  3 ( b ) show the microphone unit section in the embodiment, wherein  FIG. 3(   a ) shows an elevation view and  FIG. 3(   b ) shows a side view. 
       FIG. 4  is a longitudinal sectional view showing an example of a conventional condenser microphone. 
       FIG. 5  is a longitudinal sectional view showing the conventional example in a state in which a microphone unit section is separated from a microphone case. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of a condenser microphone according to the present invention will be described below with reference to drawings. Here, the same reference numerals are attached to the same components as the conventional components shown in  FIG. 4  and  FIG. 5 . In addition, the embodiment schematically shown shows a condenser microphone the microphone unit section of which can be arbitrarily replaced with, for example, a nondirectional microphone unit section, a unidirectional microphone unit section, or a microphone unit section with other specification, however, only one kind of microphone unit section is shown schematically. 
   In  FIG. 1  and  FIG. 2 , reference numeral  1  denotes a microphone case and reference numeral  3  denotes a microphone unit section. The microphone unit case  1  has a female screw  19  on the inner circumferential surface of the front end (the left end in  FIG. 1  and  FIG. 2 ) and the microphone unit section  3  has a male screw  32  at its rear end. It is arranged such that the microphone unit section  3  is attached to the microphone case  1  by relatively rotating the microphone case  1  and the microphone unit section  3  about their center axial line so as to screw the male screw  32  into the female screw  19  and the microphone unit section  3  is detached from the microphone case  1  by unscrewing the male screw  32  from the female screw  19 . 
   The microphone unit section  3  has a substantially cylindrical unit case  31 , and a condenser microphone unit  30  is attached at a position near the front end in the unit case  31 . The condenser microphone unit  30  may be one having a general configuration conventionally known, comprising a diaphragm the outer circumferential edge portion on one side of which is held by a holding ring, a fixed electrode arranged in opposition to the holding ring with a small gap in between with the diaphragm produced by an interposed spacer, and other adequate parts. The front end of the unit case  31  is covered with a front cover  39  having a plurality of slit-like sound introduction holes  391  as shown in  FIG. 3(   a ) and  FIG. 3(   b ). On the front side of the condenser microphone unit  30 , there is provided a front side acoustic terminal, which is an inlet of sound. In the case of a unidirectional microphone unit, there is provided a rear side acoustic terminal. 
   An inner side end plate  33  is engaged with and fixed to the rear end side of the unit case  31  and an outer side end plate  34  is engaged with and fixed to the further outside adjoiningly. A flanged contact  38  is inserted into a stepped hole formed at the center of the outer side end plate  34  and the flange of the contact  38  comes into contact with the step of the hole to prevent the contact  38  from dropping off. A rod-like terminal  36  is inserted into a hole formed in the center of the inner side end plate  33  from outside toward inside, and the terminal  36  is fixed to the inner side end plate  33  in a state of protruding toward the microphone unit  30 . A compression spring  37  made of a conductor is interposed in the holes at the centers of the inner side end plate  33  and the outer side end plate  34  and the contact  38  is pressed toward the outside of the outer side end plate  34  by a repulsion of the compression spring  37  and the rear end portion of the contact  38  which is formed into a semispherical shape protrudes outward from the outer side end plate  34 . To the front end portion of the rod-like terminal  36 , an output terminal plate  35  extending backward from the microphone unit  30  is screwed. Therefore, a sound signal converted by the microphone unit  30  is transmitted to the output terminal plate  35 , the terminal  36 , the compression spring  37 , and the contact  38 . 
   The microphone case  1  mainly comprises a relatively elongated cylindrical case  11 , which functions as a grip of the microphone. An end plate  16  is engaged with and fixed to the inner circumferential surface of the front end portion of the cylindrical case  11 , located adjoining the female screw  19  and on the rear side thereof. A contact  20  is engaged with and fixed to the end plate  16  so as to penetrate its center in the direction of thickness. A circuit substrate  15  is attached to the inside of the cylindrical case  11  along the length direction. On the circuit substrate  15 , the impedance converter circuit composed mainly of the FET, the low cut circuit, and the output circuit described above are incorporated. The contact  20  is connected with an input terminal of the circuit substrate  15  or a solder land that functions as an input terminal via a proper conductive material. A connector case  12  is engaged with and fixed to a rear end portion of the cylindrical case  11  by a screw etc. A connector base  13  having insulation properties is fixed in the connector case  12 . Three pins  14  made of a conductive material are buried in the connector base  13 , constituting a connector having a standardized three-pin configuration. An output terminal and a ground terminal of the output circuit formed on the circuit substrate  15  are electrically connected with the corresponding pins, respectively. 
     FIG. 1  shows a state in which the microphone unit section  3  is attached to the microphone case  1  by screwing the male screw  32  of the unit case  31  into the female screw  19  of the cylindrical case  11 . The contact  20  arranged in the cylindrical case  11  mainly comprising the microphone case  1  and the contact  38  arranged in the unit case  31  are arranged at the center of the relative rotation for attaching and detaching the microphone unit section  3  to and from the microphone case  1 . Further, it is arranged such that both the contacts  20  and  38  of the cylindrical case  11  and the unit case  31  are in contact with each other when the unit case  31  is attached to the cylindrical case  11 . The contacts  20  and  38  are contacts for transmitting a sound signal. In the state described above, in which the microphone unit section  3  is attached to the microphone case  1 , the contact  38  of the microphone unit section  3  comes into contact with the front end of the contact  20  on the microphone case  1  side and the contact  38  is pressed against the contact  20  by a compressive force of the compression spring  37 , thereby the contact  38  and the contact  20  are electrically connected with each other. Therefore, a sound signal converted in the microphone unit  30  and output therefrom is input to the sound signal input terminal of the circuit substrate  15  via the output terminal plate  35 , the terminal  36 , the compression spring  37 , the contact  38 , and the contact  20 . On the circuit substrate  15 , predetermined process such as impedance conversion, low cut, and amplification is performed and an output signal is transmitted to the three pins  14  as an output terminal. One of the three pins  14  is a ground pin connected with the ground. Into the connector case  12 , a connector plug conforming to the standards is inserted and a signal is input to an external circuit via the three pins  14  and a microphone cord. 
   The description so far is substantially the same as that of the configuration of the conventional condenser microphone shown in  FIG. 4  and  FIG. 5 . The configuration described below is a characteristic one of the present invention. In  FIG. 1  and  FIG. 2 , to the end plate  16  made of an insulating material attached internally near the front end of the cylindrical case  11 , a contactor  17  is attached on the outer side of the front end, that is, on the microphone unit section  3  side. The contactor  17  is formed into a ring shape and the inner circumferential edge portion of the contactor  17  is drawn and formed into a cylindrical shape toward the inside of the microphone case  1  and the cylindrical inner circumferential edge portion is engaged with the cylinder portion formed into a stepped shape in the end plate  16 . The outer circumferential edge portion of the contactor  17  is drawn and formed into a cylindrical shape in the opposite direction to that of the inner circumferential edge portion. The front end of the cylindrical outer circumferential edge portion of the contactor  17  opposes the rear end face of the unit case  31  attached to the microphone case  1  and in a state in which the microphone case  1  and the microphone unit section  3  are coupled to each other by the female screw  19  and the male screw  32  screwed to each other, it is arranged such that the front end of the cylindrical outer circumferential edge portion of the contactor  17  is pressed to and made to come into contact with the rear end face of the unit case  31 . The contactor  17  has conductivity and elasticity and in the state described above, in which the front end of the cylindrical circumferential edge portion of the contactor  17  is pressed to and made to come into contact with the rear end face of the unit case  31 , it is arranged such that the contactor  17  is warped and biased and by the bias force, the contactor  17  is pressed to and made into contact with the unit case  31  and the contactor  17  is integrally and electrically attached to the unit case  31 . It is necessary to arrange so that the contactor  17  can warp as described above. To this end, only the cylindrical portion formed on the inner circumferential side is fixed on the end plate  16  and other portions are put into a state of being floated from the end plate  16 . Incidentally, it may be also possible to form a plurality of slits in the radial direction on the portions other than the cylindrical portion of inner circumferential side in order to make it possible for the contactor  17  to warp easily. 
   To the contactor  17 , one end of an inductor  18  is connected. The other end of the inductor  18  is connected to an electrode or a ground pattern formed on the circuit substrate  15  by a proper electrical connecting method such as soldering. The one end side of the inductor  18  penetrates through the end plate  16  and is connected with the contactor  17  by a proper electrical connecting method such as soldering. The ground pattern of the circuit substrate  15  is a one of a sound signal circuit and the ground pattern is connected with the unit case  31  of the condenser microphone unit section  3  as a result via an electric path consisting of the inductor  18  and the contactor  17 . In other words, the contactor  17  and the inductor  18  constitute an electric path that electrically connects the rear end of the unit case  31  with the circuit substrate  15  in a state in which the unit case  31  is attached to the microphone case  1 . 
   On the other hand, a high frequency current due to an electromagnetic wave, which tries to enter from the outside flows to the cylindrical case  11  and the unit case  31  through the coupled portion of the cylindrical case  11  and the unit case  31  comprising the microphone case unit  1 . To the unit case  31 , the high frequency current and a current on the ground side of a sound signal flow, however, in the microphone case  1  and the coupled portion of the cylindrical case  11  and the unit case  31  at which the sound signal is subject to the high frequency current, the electric path of the high frequency current and the electric path on the ground side of the sound signal are separated by the inductor  18 , therefore, it is no longer likely that noises caused by the high frequency current enter into the sound signal and thus a condenser microphone with high sound quality can be obtained. 
   The present invention can be applied to a condenser microphone that has made it possible to detachably attach a microphone unit to a microphone case. In particular, the present invention is effective for a condenser microphone that has made it possible to select and attach a microphone unit section having different specifications to a microphone case. In such a case, it is only necessary to make it possible for the unit case to select an arbitrary one of a plurality of unit cases classified according to specification for each condenser microphone unit and attach the unit case to the microphone case. To this end, it is necessary to make common the specifications of the coupled portion of the unit case and the microphone case even if the specifications of the condenser microphone unit are different.