Abstract:
A condenser microphone includes a cylindrical microphone case having a condenser microphone unit therein; a microphone connector having an insulating base, one ground pin and two signal pins are embedded in the microphone connector; and a cylindrical connector sleeve fitted in the microphone case and accommodating the microphone connector therein, in which the connector sleeve has a concave-convex part on the outer peripheral surface thereof, the concave-convex part is in contact with the inner peripheral surface of the microphone case at a plurality of points after the connector sleeve is fixed into the end of the microphone case by fixing means, and thereby the connector sleeve is electrically conducted with the microphone case.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a condenser microphone capable of blocking entrance of electromagnetic waves to a microphone case and noise generation. 
     2. Description of the Related Art 
     An acoustic signal converted in a condenser microphone unit is output from a condenser microphone to the exterior thereof through a balanced shielded cable. The condenser microphone is detachably connected with the balanced shielded cable via a three-pin microphone connector. The connector is generally specified in EIAJ RC-5236 “Latch Lock Type Round Connector for Audio Equipment” (See “EIAJ RC-5236 Latch Lock Type Round Connector for Audio Equipment according to the Standards of the Japan Electrical Manufacturers&#39; Association”). 
       FIG. 6  illustrates an example condenser microphone including such a specified connector (hereinafter referred to as “microphone connector”). Technical ideas of the present invention can be applied to the condenser microphone shown in  FIG. 6 . 
     An example conventional condenser microphone will be described with reference to  FIG. 6 . As shown in  FIG. 6 , a microphone unit  2  is included in an inner front end of a cylindrical microphone case  1  (the lower end in  FIG. 6 ). The microphone unit  2  outputs electrical acoustic signals in response to sound waves. 
     A circuit board  3  is built in the microphone case  1 . The acoustic signals are impedance-converted by an impedance converter that is included in the circuit board  3 . Furthermore, signal processing such as amplification processing is performed. The specified microphone connector  4  is included in the rear end of the microphone case  1  (the upper end in  FIG. 6 ). 
     The microphone connector  4  is a male connector which includes a pin insert  5  that is housed in a cylindrical connector sleeve  6  and is fixed to the connector sleeve  6  with a screw  9 . 
     The pin insert  5  penetrates a base  7  that is composed of an insulating material, such as thermoplastic resin in a thickness direction of the base  7 . The pin insert  5  includes three connector pins  11 ,  12  and  13  which are integrally mounted to the base  7 . Since the connector pins  11  and  12  are aligned in a depth direction, they are depicted as one connector pin in  FIG. 6 . 
     The base  7  has a cylindrical external shape and is fitted to the connector sleeve  6  along the inner periphery of the connector sleeve  6 . A threaded hole is formed in the base  7 . The screw  9  is screwed into the threaded hole from the outer peripheral surface of the base  7  towards the center. The screw  9  is radially projected from the outer peripheral surface of the base  7 . A shoulder of screw  9  is in contact with the inner peripheral surface of the connector sleeve  6 . The outer peripheral surface of the base  7  at the opposite side of the screw  9  presses the inner peripheral surface of the connector sleeve  6  by screwing the screw  9  into the threaded hole. Thus, the pin insert  5  is fixed in the connector sleeve  6 . 
     The connector sleeve  6  is fixed at a predetermined position in the microphone case  1  with a screw  10  screwed from the outer peripheral surface of the microphone case  1 . Details about the fixing mode of the connector sleeve  6  to the microphone case  1  will be described below. 
     Since an electronic circuit including a semiconductor device is incorporated in the circuit board  3 , penetration of electromagnetic waves into the microphone case  1  causes mixing of a noise into acoustic signals output from the microphone unit  2 . In order to prevent such penetration of electromagnetic waves into the microphone case  1 , an electromagnetic shield is effectively applied to the microphone unit  2 . At the rear end of the condenser microphone (adjacent to the microphone connector) has an opening through which the electromagnetic waves disadvantageously penetrate. Accordingly, the electromagnetic shield is effectively applied by putting a shielding material on the rear end of the microphone. 
     As shown in  FIG. 7 , however, the condenser microphone is assembled by inserting a unit ( FIG. 7A ) that is composed of the circuit board  3  and the microphone connector  4  housed in the connector sleeve  6  into the microphone case  1  ( FIG. 7B ) through the opening (upper end of the microphone case  1 ), and fixing this unit to the microphone case  1  with the screw  10  screwed from the outer peripheral surface of the microphone case  1 . Accordingly, the opening of the microphone case  1  cannot be completely sealed. 
     Furthermore, in order to insert the unit including the microphone connector  4  and the circuit board  3  into the microphone case  1 , the outer periphery of the connector sleeve  6  needs to be slightly smaller in diameter than the inner periphery of the microphone case  1 . 
     Accordingly, a gap between the connector sleeve  6  and the microphone case  1  is inevitably formed. 
     Next, the relationship between the inner peripheral surface of the microphone case  1  and the outer peripheral surface of the connector sleeve  6  will be described below with reference to  FIG. 8 , which is an enlarged cross sectional view of B-B′line shown in  FIG. 6 . A gap  8  is the difference between two diameters of the outer periphery of the connector sleeve  6  and the inner periphery of the microphone case  1 . As explained above, the dimension of the gap  8  need to have a width enough to insert the connector sleeve  6  into the microphone case  1 . The gap  8  shown in  FIG. 8  is depicted larger than its real size. 
     A threaded hole is formed on the outer peripheral surface of the connector sleeve  6 . A hole is formed on the outer peripheral surface of microphone case  1  at the position where corresponds to the threaded hole of the connector sleeve  6 . The screw  10  is inserted into the hole of the microphone case  1  and screwed into the threaded hole of the connector sleeve  6 , thereby fixing the connector sleeve  6  to the predetermined position of the microphone unit  2 . Generally, about three screws  10  are used as shown in  FIG. 8 . 
     In a part fixed with the screw  10  and a certain area surrounding the fixed part, since the microphone case  1  and connector sleeve  6  attract each other with the screw  10 , the outer peripheral surface of the connector sleeve  6  is distorted outward while the inner peripheral surface of the microphone case  1  is distorted inward. Accordingly, the gap  8  at which these distortions are created becomes smaller in width. Moreover, the connector sleeve  6  and microphone case  1  are electrically conducted through the screw  10 . 
     The narrow portion of the gap  8  and the connector sleeve  6  and microphone case  1  electrically conducted through the screw  10  have an electromagnetic shielding effect against electromagnetic waves from the exterior. Since the electromagnetic shielding effect, however, cannot be obtained in the other area, the electromagnetic waves can enter the inside of the microphone case  1  through the gap  8 . 
     In brief, the conventional microphone has an area through which the electromagnetic waves enter the microphone case  1  in the gap  8  between microphone case  1  and the connector sleeve  6 . This causes noise in the condenser microphone. 
     For the purpose of preventing the electromagnetic waves from entering the inside of the condenser microphone, condenser microphones which include a microphone connector in which a shielding case covers one end surface of the base  7  in the pin insert  5 , and the connector pin  11  for grounding is soldered to the shielding case to be electrically connected are known (disclosed, for example, in Japanese Unexamined Patent Application Publication Nos. 2005-094575 and 2005-311752). 
     Each of the condenser microphones disclosed in Japanese Unexamined Patent Application Publication Nos. 2005-094575 and 2005-311752 includes the microphone connector that prevents factors generating noise from entering the inside of the microphone case. These condenser microphones, however, do not have the shielding effect for preventing the electromagnetic waves from entering the microphone case through the gap between the connector sleeve and the microphone case. Accordingly, these condenser microphones do not have enough function of the electromagnetic shield that prevents electromagnetic waves entering from the opening of the microphone case. 
     SUMMARY OF INVENTION 
     In view of the circumstances above, an object of the present invention is to provide a condenser microphone including a connector sleeve that prevents electromagnetic waves from entering the inside of a microphone case and prevents noise caused by the electromagnetic waves. 
     According to an aspect of the present invention a condenser microphone including: a cylindrical microphone case having a condenser microphone unit therein; a microphone connector having an insulating base one ground pin and two signal pins are embedded in the microphone connector; and a cylindrical connector sleeve fitted in the microphone case and accommodating the microphone connector therein. The connector sleeve has the concave-convex part on the outer peripheral surface thereof, the concave-convex part is in contact with the inner peripheral surface of the microphone case at a plurality of points after the connector sleeve is fixed into the end of the microphone case by fixing means, and thereby the connector sleeve is electrically conducted with the microphone case. 
     According to an aspect of the present invention, a condenser microphone including: a cylindrical microphone case having a condenser microphone unit therein; a microphone connector having an insulating base in which one ground pin and two signal pins are embedded; and a cylindrical connector sleeve fitted in the microphone case and having the microphone connector therein. The microphone case has the concave-convex part on the inner peripheral surface thereof which is in contact with the outer peripheral surface of the connector sleeve at a plurality of points on fixing the connector sleeve into the end of the microphone case by fixing means, and thereby the microphone case is electrically conducted with the connector sleeve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal cross-sectional view of an exemplary condenser microphone according to an embodiment of the present invention; 
         FIG. 2  is a side view of an exemplary connector sleeve as a main part of the condenser microphone according to an embodiment of the present invention; 
         FIG. 3  is a cross sectional view taken from line A-A′ in  FIG. 1 ; 
         FIG. 4  is a partially enlarged view of an exemplary concave-convex surface according to an embodiment of the present invention; 
         FIG. 5  is a partially enlarged view of another exemplary concave-convex surface according to an embodiment of the present invention; 
         FIG. 6  is a longitudinal cross-sectional view of a typical conventional condenser microphone; and 
         FIG. 7  A is a longitudinal cross-sectional view of a unit including a circuit board and a microphone connector in a typical conventional condenser microphone; 
         FIG. 7  B is a longitudinal cross-sectional view of a microphone case in a typical conventional condenser microphone; and 
         FIG. 8  is a cross sectional view taken from line B-B′ in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A condenser microphone according to an embodiment of the present invention will be described below with reference to the accompanying drawings. In the condenser microphone according to an embodiment of the present invention, elements similar to those in the conventional condenser microphones shown in  FIGS. 6 to 8  are given the same reference numerals. 
     As shown in  FIG. 1 , the condenser microphone according to an embodiment of the present invention includes a cylindrical microphone case  1 , a microphone unit  2 , and a specified microphone connector  4 . The microphone unit  2  and the specified microphone connector  4  are respectively mounted in the front and rear ends of the microphone case  1 . The circuit board  3  is interposed between the microphone unit  2  and the microphone connector  4 . Sound waves received by the microphone unit  2  are converted into electric acoustic signals. The converted signals are amplified by an impedance converter, to be output from the microphone connector  4 . 
     The output signals are transmitted from the microphone connector  4  to the predetermined external circuit and device via a cable connector and cable both connected to the microphone connector  4 . 
     In the microphone connector  4 , a pin insert  5  is disposed inside a cylindrical connector sleeve  60  such that the pin insert  5  is fixed to the connector sleeve  60  with a screw  9 . 
     The pin insert  5  includes a base  7  composed of an insulating material such as thermoplastic resin and three connector pins  11 ,  12  and  13  which are integrally-mounted in the base  7  so as to be embedded in a thickness direction of the base  7 . The base  7  has a cylindrical external shape and is fitted to the connector sleeve  60  along with the inner periphery thereof. A threaded hole is formed in the base  7  into which the screw  9  is screwed from the outer peripheral surface of the base  7  towards the center. 
     The screw  9  is radially projected from the outer peripheral surface of the base  7 . A shoulder of the base  7  is in contact with the inner peripheral surface of the connector sleeve  60 . The screw  9  screwed into the threaded hole urges the opposite side of the outer peripheral surface of the base  7  presses the inner peripheral surface of the connector sleeve  60 . Thus the pin insert  5  is fixed in the connector sleeve  60 . 
     The connector sleeve  60  is fixed at a predetermined position in the microphone case  1  with a screw  10  screwed in the connector sleeve  60  penetrating the peripheral wall of the microphone case  1 . The connector sleeve  60  has a concave-convex part  61  on the outer peripheral surface thereof. 
       FIG. 2  illustrates an embodiment of the connector sleeve  60  that is the main feature of the condenser microphone according to the present invention. In  FIG. 2 , the left side of the connector sleeve  60  is the front end of the condenser microphone (the side on which the microphone unit  2  is mounted). As shown in  FIG. 2 , the concavo-convex part  61  is formed around the entire periphery and adjacent to the rear end of the connector sleeve  60 . 
     The concavo-convex part  61  is formed by embossing the peripheral surface of the connector sleeve  60 , like a patterned surface. For example, the concavo-convex part  61  can be formed by knurling that press a roller against the peripheral surface of the connector sleeve  60  to make convexity on the patterned surface. 
     The concavo-convex part  61  formed by the above-described processing on the periphery of the connector sleeve  60  has a larger diameter than the other part. 
     A feature of the condenser microphone including this connector sleeve  60  according to an embodiment of the present invention will be described below with reference to  FIG. 3 .  FIG. 3  is an enlarged cross sectional view taken from line A-A′ in  FIG. 1 . As shown in  FIG. 2 , the concavo-convex part  61  is formed on a part of the peripheral surface of the connector sleeve  60  and the other part thereof has the same diameter as the conventional one. The outer diameter of the part of the connector sleeve  60 , other than the concavo-convex part  61 , is slightly smaller than the inner diameter of the microphone case  1 . Accordingly, a gap for inserting the connector sleeve  60  in the microphone case  1  is formed between the inner peripheral surface of the microphone case  1  and the outer peripheral surface of the connector sleeve  60 . 
     A threaded hole is provided on the outer peripheral surface of the inserted connector sleeve  60 . A hole is provided on outer peripheral surface of microphone case  1  at the position corresponding to the threaded hole of connector sleeve  60 . The connector sleeve  60  is fixed at the predetermined position of the microphone case  1  by inserting the screw  10  into the hole of the microphone case  1  and screwing into the threaded hole of the connector sleeve  60 . In general, about three screws  10  are used to fix the connector sleeve  60  at the center of the microphone case  1  as shown in  FIG. 3 . 
     Next, the contact surface of microphone case  1  and the connector sleeve  60  will be explained below with reference to the enlarged view of the area C taken from the chain double-dashed line in  FIG. 3 . 
     As shown in  FIG. 4 , the connector sleeve  60  is in contact with the microphone case  1  via the concavo-convex part  61 . The contact portion of the connector sleeve  60  and the microphone case  1  is the convex portion of the concavo-convex part  61  (top of the convex portion). Thus, the connector sleeve  60  is electrically conducted to the microphone case  1  at multiple points. 
     The conduction of the microphone case  1  and the microphone sleeve  60  at these plural contact portions (points) generates an electromagnetic shielding effect. As described above, the concavo-convex part  61  generates the effective electromagnetic shielding between the microphone case  1  and connector sleeve  60 , thereby preventing the electromagnetic waves from entering the inside of the microphone and preventing noise generation. 
     Since the concavo-convex part  61  is formed at only one part of the rear end of the connector sleeve  60 , the circuit board  3  can be inserted through the opening at the rear end of the microphone case  1 . The circuit  3  board is not easily inserted through the concavo-convex part  61  compared with the other part. However, because the concavo-convex part  61  has not a large width, the circuit  3  board can be inserted up to the predetermined position by weak applied force. 
     Although the knurling processing is explained as an example process for forming concavo-convex part  61  in the illustrated embodiment, the present invention should not be limited to the embodiment. For example, as shown in  FIG. 5 , a concavo-convex part  61   a  is formed by broaching on the inner peripheral surface of the microphone case  1  such that the convex part of the concavo-convex part  61   a  is in contact with the outer peripheral surface of the connector sleeve  60 . Thus, the same effect as the above-illustrated embodiment can be obtained. 
     In the condenser microphone according to the present invention, the concavo-convex surfaces formed on the fitting surface of the connector sleeve and the microphone case ensure a plurality of contact points uniformly on the entire peripheral surfaces thereof. Thus, an excellent electrostatic shield can be obtained to prevent electromagnetic waves from entering the inside of the microphone case, and prevent noise generation caused by the electromagnetic waves.