Abstract:
It provides a condenser microphone such as a tiepin type including a small microphone unit, which is capable of effectively suppressing noise generation due to an electromagnetic wave. It covers the entirety from a microphone unit  12  to a connecting cable  22  with an integrally formed cover member  32  and connects the cover member  32  to a shielded wire  25  so as to electromagnetically shield the entirety.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is based on, and claims priority of, Japanese Patent Application NO. JP2004-335317, filed Nov. 19, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety. 
   TECHNICAL FIELD 
   The present invention relates to a small condenser microphone, and in particular, to the condenser microphone connected to be capable of effectively suppressing noise generation due to an electromagnetic wave. 
   BACKGROUND ART 
   As shown in Patent Document 1 (Japanese Utility Model Registration No. 3027420) for instance, a connection between a microphone unit and a microphone cord of a condenser microphone is normally made by electrically connecting two cores consisting of a signal wire and a power wire of a connecting cord to a predetermined position on an impedance converter side respectively and soldering a shielded wire on a predetermined ground side. 
   Tiepin-type and headset-type condenser microphones are required to be rather unnoticeable in use and lightweight, and so they are designed so that an outside diameter of the microphone unit becomes 5 mm or less for instance. In the case of such a small condenser microphone, the microphone unit contains a condenser portion and an impedance converter, and an inputted sound signal is connected to a power module separately placed via a dedicated connecting cable. The power module contains main circuits such as a low-cut circuit and an output circuit for driving the microphone therein. As for the connecting cable, a microphone cable consisting of a two-core shielded covered wire is used. 
     FIG. 2  is a relevant part enlarged view showing a conventional example of a coupling structure of this small microphone unit and the connecting cable. In this example, a connecting cable  2  uses a power wire  4  for supplying power to a microphone unit  1 , a signal wire  5  for sending a sound signal outputted from the impedance converter not shown to the power module side and a shielded wire  6  for electrostatically shielding and grounding the power wire  4  and signal wire  5 , which are covered by a sheath  3 . 
   The connecting cable  2  has the power wire  4 , signal wire  5  and shielded wire  6  projected on its one end  2   a  side electrically connected by soldering them to the predetermined positions on the microphone unit  1  side respectively. A joint between the microphone unit  1  and the connecting cable  2  is protected by covering and solidifying the portion between a back end side of the microphone unit  1  and the one end  2   a  side of the connecting cable  2  with an appropriate synthetic resin material  7  such as polypropylene to couple them integrally for protection. 
   In the case of the small condenser microphone  1  shown in  FIG. 2 , however, the sound signal in the connecting cable  2  is apt to be transmitted in an unbalanced state and so it is weak against noise from outside unlike a large condenser microphone. Therefore, there is a problem that if a strong electromagnetic wave is applied to the connecting cable  2  from a cellular phone for instance, the electromagnetic wave goes inside the microphone unit  1  and the power module so as to be detected by a semiconductor and generate the noise. 
   In the case of the coupling structure shown in  FIG. 2  in particular, the power wire  4  and signal wire  5  are not covered by the shielded wire  6  but are connected to the microphone unit  1  side in an exposed state. Besides, the joint is covered by the synthetic resin material  7  having no shielding effect so that, if the electromagnetic wave goes into the power wire  4  and signal wire  5 , there is a possibility that a high-frequency current may go into the shielded wire  6  and induce the noise. 
   SUMMARY OF THE INVENTION 
   Thus, the present invention has been made to solve the above-mentioned problem, and an object thereof is to provide a condenser microphone such as a tiepin type including a small microphone unit, which is capable of effectively suppressing noise generation due to an electromagnetic wave. 
   To achieve the above-mentioned object, the present invention is a condenser microphone comprising: a microphone unit including a condenser portion and an impedance converter; a connecting cable connected to the microphone unit; and a metallic cover member for supporting a part of the microphone unit and covering a joint between the microphone unit and the connecting cable, wherein: the connecting cable has a shielded wire covering a periphery of cores connected to the microphone unit and a sheath portion covering the cores and the shielded wire; and the shielded wire is partially folded back on the sheath portion of the connecting cable and fixed in contact with the cover member. 
   According to this, it is possible, by covering the entire area between the microphone unit and the connecting cable with the shielded cover member, to exert an excellent shielding effect and securely prevent penetration of a high-frequency current generated due to the electromagnetic wave so as to block noise generation. 
   As a preferred embodiment, it is desirable to provide projections for increasing a contact pressure on a supporting surface of the cover member for supporting the microphone unit. 
   According to this, it is possible, by providing the projections on an internal surface of the cover member facing the microphone unit, to increase the contact pressure of the cover member against the microphone unit so as to further ensure a mutual conducting state. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a relevant part enlarged view showing an example of a coupling structure between a microphone unit and a connecting cable according to an embodiment of the present invention; and 
       FIG. 2  is a relevant part enlarged view showing a conventional example of the coupling structure of the connecting cable to a small microphone unit. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a relevant part enlarged view showing an example of a coupling structure between a microphone unit and a connecting cable of a condenser microphone according to the present invention. The present invention is not limited thereto. 
   As shown in  FIG. 1 , a condenser microphone  11  comprises a microphone unit  12  consisting of a cylindrical metallic case  13 , a connecting cable  22  having its one end  22   a  electrically connected to the microphone unit  12 , and a cover member  32  for covering a joint between the microphone unit  12  and the connecting cable  22  by having a front acoustic terminal opening surface  14  side opened. 
   The microphone unit  12  comprises a condenser portion having a diaphragm and a back plate placed face-to-face via a spacer for instance and an impedance converter for converting an electrical signal of high impedance obtained from the condenser portion to low impedance (neither is shown). The microphone unit  12  has the cylindrical metallic case  13  consisting of a conductive metallic material such as aluminum, and has the front acoustic terminal opening surface  14  formed on its apical surface. 
   The connecting cable  22  comprises a power wire  27  for supplying power to the microphone unit  12  side, a signal wire  28  as a signal wire for sending a sound signal outputted from the impedance converter to a power module side and a shielded wire  25  formed to cover a periphery of the power wire  27  and signal wire  28 , which are covered by a sheath  23 . The shielded wire  25  is partially folded back by a required length from an end face  24  side of the sheath  23 , and is formed as a shielding layer  26  on a peripheral surface  23   a  thereof. 
   The power wire  27  and signal wire  28  drawn forth from the one end  22   a  side of the connecting cable  22  are soldered to predetermined positions on the microphone unit  12  respectively. 
   The cover member  32  consists of a conductive metal plate such as aluminum which is stamped, and integrally comprises a first cover portion  34  for housing the microphone unit  12 , a second cover portion  35  for covering over a surrounding space  29  from a back end (lower end in  FIG. 1 ) of the microphone unit  12  to the end of the connecting cable  22 , and a third cover portion  36  for covering over the shielding layer  26  from the end of the connecting cable  22 . 
   The first cover portion  34  has an opening  33  for inserting the microphone unit  12  provided at its end (upper end in  FIG. 1 ). The second cover portion  35  is formed with a little smaller diameter than the first cover portion  34 . The third cover portion  36  has a diameter even smaller than the second cover portion  35 , and has an inside diameter slightly smaller than an outside diameter of the connecting cable  22  so as to put its inner face forcibly in contact with the shielding layer  26  of the connecting cable  22 . 
   As shown in the partial enlarged view of  FIG. 1 , it is desirable to have projections  40  projected toward the microphone unit  12  on an internal surface  34   a  of the first cover portion  34  facing a peripheral surface  13   a  of the microphone unit  12 . According to this, it is possible to increase a contact pressure of mutual supporting surfaces between the peripheral surface  13   a  of the metallic case  13  of the microphone unit  12  and the cover member  32 . In this example, the multiple projections  40  are projected hemispherically, where a point-like form or a linear form is suitable used. It is desirable to have more than one projection  40  provided. The form and the number of the projections  40  can be set arbitrarily according to the specification. 
   The microphone unit  12  is supported as if propped up by having a bottom surface  15  and its margin  15   a  put in contact with a step portion  35   a  formed between the first cover portion  34  and the second cover portion  35 . 
   The third cover portion  36  has at least a length to cover the shielding layer  26  formed on the one end  22   a  on the connecting cable  22  side. In this example, the third cover portion  36  is formed to have a smaller diameter than the connecting cable  22  in order to obtain secure conduction with the shielding layer  26 . It is also possible, however, to form the third cover portion  36  with the same diameter as the second cover portion  35  side so as to caulk an area facing the shielding layer  26 , reduce the diameter thereof and perform the conduction. It is also possible to provide the above-mentioned projections on the internal surface of the third cover portion  36  so as to increase contact area. 
   According to this, even if the power wire  27  and signal wire  28  are connected to the microphone unit  12  side in a state of being exposed from the shielded wire  25 , it is also possible to shield the surroundings securely by covering over the entirety from the microphone unit  12  to the connecting cable  22  with the cover member  32  and conducting the cover member  32  to the shielding layer  26 . 
   Therefore, even in the case of using a device generating an electromagnetic wave such as a cellular phone in proximity to the condenser microphone  11 , it is possible to interrupt the electromagnetic wave generated from the cellular phone and effectively prevent it from going inside the cover member  32  so as to suppress noise generation. 
   Furthermore, in the case where the projections are provided on the internal surface of the cover member facing the microphone unit, it is possible to increase the contact pressure of the cover member against the microphone unit side so as to further ensure a mutual conducting state and thereby further enhance a shielding effect. 
   The above described the present invention based on the examples shown in the drawings, and a concrete configuration thereof is not limited thereto. For instance, as for the cover member, its concrete appearance and form may be adopted as appropriate only if capable of securely covering the entire area from the microphone unit to the shielding layer located on the one end side of the connecting cable in a conductive state. As for the connecting cable  22 , this example shows the one having a two-wire structure. However, it may have either the structure of one wire or the structure of two or more wires. Furthermore, as for the conduction between the shielding layer  26  and the third cover portion  36 , it is also possible, without pressure-welding them by using press fitting, caulking means or the like, to place them as if wrapping them around more softly with a conductive resin or the like.