Patent Publication Number: US-9838768-B2

Title: Microphone device

Description:
TECHNICAL FIELD 
     The present invention relates to a microphone device. 
     BACKGROUND ART 
     A typical microphone device is known which has a microphone unit distanced from the base, specifically, has a microphone unit at the tip of the rod support on the base. Examples of such a microphone device include headset microphones and goose-neck microphones. 
     A headset microphone has a flexible support pipe with a cable passing therethrough for electrical connection to the microphone unit. The flexible pipe in the headset microphone is made thin (has a small inner diameter) not to draw much attention. A thick cable, which is difficult to pass through the flexible pipe, is connected to the thin cable at or around the inlet of the flexible pipe. 
     Condenser microphones, which are typically used as microphone units in headset microphones, suffer from audible noise caused by RF current flowing through impedance converters in the condenser microphones. To prevent such noise, the audio signal wires in the microphone are protected (shielded) from static damage. Unfortunately, the junction between the thick and thin cables at or around the inlet of the flexible pipe cannot be adequately shielded. 
     For example, Japanese Unexamined Patent Application Publication No. 2006-033216 (hereinafter referred to as “the patent literature”) discloses a condenser microphone in which the inner surface of a metal pipe is in contact with an exposed shield-covered cable in the pipe for the microphone. The shield-covered cable in the technique in the patent literature is a braided cable consisting of core wires and a copper braid therearound. 
     Even in the technique in the patent literature, a cable cannot be passed through a pipe with a small inner diameter and adequate shielding is not achieved due to a gap between the inner wall of the pipe and the low-density shielded wire. 
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide a microphone device with improved shielding in and around the support. 
     Solution to Problem 
     The present invention includes a tubular support of a conductive material, a microphone unit that has a signal output terminal and is provided at and grounded to one end of the support, and a cable passing through the support. The cable includes core wires connected to the signal output terminal of the microphone unit, and a conductive coat that covers the core wires and is electrically connected to the support. 
     Advantageous Effects of Invention 
     The present invention improves shielding in and around the support. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view illustrating an embodiment of a microphone device of the present invention. 
         FIG. 2  is a partial cross-sectional view of the microphone device in  FIG. 1 . 
         FIG. 3  is a partial cross-sectional view of the microphone device in  FIG. 1 . 
         FIG. 4  is a schematic view of a cable in the microphone device in  FIG. 1 . 
         FIG. 5  is a graph showing the noise spectrum of the cable in  FIG. 4 . 
         FIG. 6  is a graph showing the noise spectrum of a cable of the related art. 
         FIG. 7  is a partial enlarged cross-sectional view of the cable in the support of the microphone device in  FIG. 1 . 
         FIG. 8  is a partial cross-sectional view of a microphone device of the related art. 
         FIG. 9  is the equivalent circuit diagram of the support and the cable of the microphone device in  FIG. 1 . 
         FIG. 10  is a diagram of an example audio signal circuit in the microphone device in  FIG. 1 . 
         FIG. 11  is a partial enlarged cross-sectional view of a cable in a support of a microphone device of the related art. 
         FIG. 12  is an equivalent circuit diagram of a support and a cable of a microphone device of the related art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now will be described an embodiment of a microphone device of the present invention with reference to the attached drawings. 
     As shown in  FIG. 1 , a microphone device  10  of this embodiment is a headset microphone which is mounted on the speaker&#39;s head. The microphone device  10  includes a microphone case  11  accommodating a microphone unit  15  to be described below and a flexible pipe  12  supporting the microphone case  11 . The microphone device  10  further includes a pad  13  to be in contact with the speaker&#39;s head and a connector  14  to connect the microphone device  10  to an external apparatus. 
     Referring to  FIGS. 2 and 3 , the microphone case  11  is a substantially cylindrical case provided at one end of the flexible pipe  12 . The microphone case  11  can accommodate the microphone unit  15 . The microphone case  11  has openings to admit external voice. 
     The microphone unit  15  includes a diaphragm, a back plate, and a circuit to operate the microphone device. The microphone unit  15  is generally a compact lightweight condenser microphone. The condenser microphone includes an impedance converter. A capacitor in the condenser microphone, which is composed of the diaphragm and the back plate, has a low capacitance. The condenser microphone thus includes, for example, a field effect transistor (FET) with high input impedance as the impedance converter. The microphone unit  15  is grounded to the flexible pipe  12 . 
     The flexible pipe  12 , one example of the support, has the microphone case  11  at one end and the pad  13  at the other end. A typical headset microphone or goose-neck microphone includes the flexible pipe  12  which allows the microphone unit  15  to be moved to and fixed in an appropriate position to the speaker&#39;s mouth. The flexible pipe  12  is a hollow cylinder with a cable  16  passing therethrough. The cable  16  has the microphone unit  15  at one end and the connector  14  at the other end and electrically connects these components to each other. 
     The pad  13  is a base supporting the microphone case  11  and the flexible pipe  12 . The pad  13  includes a hollow-cylindrical segment with the cable  16  passing therethrough and a holding segment to come into contact with the speaker&#39;s head to fix the position of the microphone unit  15 . 
     Structures of Cable and Flexible Pipe 
     The structures of the cable  16  and the flexible pipe  12  in the microphone device  10  will now be described. 
     As shown in  FIG. 4 , the cable  16  includes core wires  161  connected to the signal output terminal of the microphone unit  15  and a conductive covering material  162  covering the core wires  161 . The cable  16  further includes a shielding braid  163  covering the conductive covering material  162  and an insulating covering material  164  covering the shielding braid  163 . The conductive covering material  162  directly covers the core wires  161  without a sealant or any other material therebetween. The conductive covering material  162  has an outer surface electrically connected to the inner surface of the flexible pipe  12 . 
     A typical cable has a capacitor storing electric charge between each core wire and shielding braid. The capacitance of the capacitor varies in response to the stress applied to the cable. The cable  16  thus causes microphonic noise. 
     To reduce the noise caused by variations in the capacitance in the cable  16  of the microphone device  10 , the cable  16  includes the conductive covering material  162  (conductive tube) that is disposed between the shielding braid  163  and each core wire  161  and is composed of a resin, such as polyvinyl chloride (PVC) containing conductive particles, such as carbon black. The resistivity of the conductive covering material  162  is approximately 10-10 7  Ω·cm. 
     Comparison between the noise spectrum of the cable  16  in  FIG. 5  and that of a cable of the related art in  FIG. 6  demonstrates that the noise level of the cable  16  is much lower than that of the cable of the related art over the estimated noise frequency range on the whole. 
     As shown in  FIG. 7 , the flexible pipe  12  is composed of a first wire rod  121  and a second wire rod  122  inserted in the space in the first wire rod  121 . The first wire rod  121  is a coil spring of a steel wire or any other wire with a circular section. The second wire rod  122  is a coil spring of a brass wire or any other wire with a triangular section. The friction between the first wire rod  121  and the second wire rod  122  both made of a flexible plastic conductive material enables the flexible pipe  12  to be deformable while keeping its basic tubular shape. The flexible pipe  12  in the headset microphone is made thin (has a small inner diameter) not to draw much attention. 
     The shielding braid  163  and the insulating covering material  164  are removed from the cable  16  in the flexible pipe  12 . The shielding braid  163  and the insulating covering material  164  are removed from the cable  16  and the remaining core wires  161  and conductive covering material  162  are passed through the flexible pipe  12 . 
     The flexible pipe  12  has such an inner diameter that the cable  16  can pass therethrough while the conductive covering material  162  around the core wires  161  is in contact with the inner wall of the flexible pipe  12 . In the flexible pipe  12 , the conductive covering material  162  around the core wires  161  is in continuous or intermittent contact with the inner wall of the flexible pipe  12 . 
     In a headset microphone, a cable should be thin to pass through a flexible pipe. For this reason, as in the related art in  FIG. 8 , a conventional microphone device  20  has a cable  26  in the flexible pipe  22  and a cable  27  outside the flexible pipe  22 , which cables are connected to each other at or around the opening of the flexible pipe  22 . The cable  26  has a smaller outer diameter than the cable  27 . Since the headset microphone includes many resin components, adequate shielding from static damage is not achieved at the junction between the cable  26  in the pipe and the cable  27  out of the pipe. 
     In the microphone device  10  of this embodiment, as described above, the core wires  161  and the conductive covering material  162  are passed through the flexible pipe  12  and the conductive covering material  162  is electrically connected to the inner wall of the flexible pipe  12 . Thus, in the microphone device  10 , the resistance generated between the flexible pipe  12  and the conductive covering material  162  consumes the electric power from the RF current induced in the flexible pipe  12  by intense RF waves, thereby reducing noise. 
     In addition, the shielding braid  163  in the microphone device  10  is electrically connected to around the opening of the flexible pipe  12 , improving the shielding of the microphone device  10 . 
     With reference to  FIG. 9 , will be explained how the resistance generated between the flexible pipe  12  and the conductive covering material  162  consumes the electric power from the RF current. In the drawing, the RF current is supposed to be generated from the power supply En. In the microphone device  10 , the conductive covering material  162  and the flexible pipe  12  are electrically connected at multiple points, and every pair of connection points constitutes a loop circuit. In this circuit, the RF current is attenuated by impedance Zp 1  from the resistance of the conductive covering material  162  and impedance Zp 2  from the resistance of the flexible pipe  12 . 
       FIG. 10  is a diagram of an example audio signal circuit in the microphone device  10 . As shown in the drawing, direct current E is supplied to the microphone unit  15  including an electroacoustic transducer and the impedance converter, through a power supply line  161   a  of the core wires  161 , in the microphone device  10 . An FET constituting the impedance converter has a source connected to a capacitor C through a signal line  161   b , and a drain connected to one terminal of a power supply E. The ground pattern of a circuit substrate contained in the microphone unit  15  is connected to the conductive covering material  162  and the flexible pipe  12  for connection to the other terminal of the power supply E. Such connection constitutes a part of the audio signal circuit that contains the microphone unit  15 . 
     In a microphone device of the related art illustrated in  FIG. 11 , core wires  261  in a cable, a shielding braid  263  around the core wires  261 , an insulating covering material  264  around the shielding braid  263  are passed through the flexible pipe  22 . As shown in  FIG. 12 , such a microphone device of the related art includes a plurality of circuits, each composed of impedance Zp from the resistance of the cable  26  and a power supply En generating RF current, which are sequentially connected in series. Hence, RF current in the microphone device of the related art is not attenuated but accumulated to increase the noise level. 
     Advantageous Effects of Embodiment 
     As explained above, the microphone device of this embodiment provides the following advantageous effects. 
     In the microphone device  10 , the resistance of the conductive covering material  162  attenuates RF current, thereby reducing the noise in the audio signals. 
     Although a headset microphone requires a mechanically strong cable that is not broken even by the strenuous movement of the speaker, the microphone device of the related art may be subjected to a wire break or intrusion of water at the junction. 
     In contrast, the microphone device  10  of this embodiment does not include any junction of the cable  16  inside or outside the flexible pipe  12  as described above, improving the mechanical strength and water impermeability of the cable  16 . 
     The flexible pipe used as a support in the embodiment may be replaced with an inflexible or rigid pipe.