Patent Publication Number: US-7720239-B2

Title: Capacitor microphone with acoustic resistor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-339,946 filed on or around Nov. 25, 2004; the entire contents of which are incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a capacitor microphone, and more particularly relates to a capacitor microphone which can efficiently prevent noises caused by outside electromagnetic waves and wind noises. 
     2. Description of the Related Art 
     Generally, a microphone unit for a capacitor microphone has high impedance, and includes an impedance transformer in order to reduce impedance of voice signals. The impedance-reduced voice signals are then transmitted to a low-cut circuit and an output circuit. In order to make a tiepin or headset type microphone less visible, a microphone unit usually houses only an impedance transformer while a low-cut circuit and an output circuit are housed in a circuit housing. The microphone unit and the circuit housing are connected by a dedicated cable. 
     The microphone unit is provided with an acoustic terminal via which sound waves are received. A nondirectional microphone needs one acoustic terminal while a unidirectional microphone of a primary sound pressure gradient type needs a pair of acoustic terminals at its front and rear ends. (Refer to Japanese Patent Laid-Open Publications No. 2000-232,700 and No. Hei 05-007,398). 
       FIG. 3  of the accompanying drawings shows a microphone unit  4  of a capacitor microphone of the related art. The microphone unit  4  is housed in a cylindrical case  2 , and includes an acoustic terminal  10  at its front end. An acoustic resistor  8  made of a urethane sponge is housed in a front end of the cylindrical case  2 , and has its one side in close contact with the acoustic terminal  10 . A wire netting  6  is fixedly attached to the front end of the cylindrical case  2  in order to assure static shielding. The wire netting  6  is pressed in the shape of a cup having a flat bottom, and is fitted into the cylindrical case  2  with its straight sides  61  extending over the inner surface of the acoustic resistor  8 . Referring to  FIG. 4 , the straight sides  61  of the wire netting  6  and the inner surface of the case  2  are brought into contact with one another at several points, thereby forming electric contacts  12 . A rear peripheral edge of the cylindrical case  2  is crimped or is subject to the drawing compound in order to prevent the microphone unit  4  from dropping off. A cord (not shown) extends out of the rear end of the cylindrical case  2 . 
     The acoustic terminal  10  of the microphone unit  4  is statically shielded by the wire netting  6  as shown in  FIG. 3  and  FIG. 4 . However, the wire netting  6  is attached to the cylindrical case  2  using a rubber adhesive or the like, and is electrically connected to the cylindrical case  2  only via a plurality of points. This means that the electric connection is unreliable between the wire netting  6  and the cylindrical case  2 , and that the static shielding also becomes unreliable. If outside electromagnetic waves arrive at an area where static shielding is unreliable, a high frequency current will flow through an internal circuit, be detected by a semiconductor element, and cause noises. Specifically, as cellular phones become popular and are more frequently used near a microphone, the microphone often suffers from noises caused by electric waves of the cellular phones. 
     Further, when airflows caused by winds or voices, especially airflows caused by plosive sounds, strike on the acoustic terminal  10  of the microphone unit  4 , wind noises or pops will be produced. Usually, the acoustic resistor  8  is attached to the acoustic terminal  10 , and is made of a fabric or a sponge, which has acoustic resistance but is non-conductive. If such an acoustic resistor  8  is used, the foregoing unreliable electric connection between the wire netting  6  and the cylindrical case  2  becomes further unstable, which will make the static shielding less reliable. This will cause more noises resulting from electromagnetic waves. 
     SUMMARY OF THE INVENTION 
     The invention is contemplated in order to overcome the foregoing problems of the related art. According to the invention, a shield structure is devised in order to electrically connect a microphone case and a microphone unit at a plurality of points. This shield structure is effective in reliably preventing not only electromagnetic waves from getting into the microphone unit but also wind noises. Further, the shield structure has an acoustic resistance, which is effective in preventing pops as well as wind noises. 
     According to the invention, there is provided a capacitor microphone which includes a microphone unit having one acoustic terminal or more than one acoustic terminals at one end or opposite ends thereof; an acoustic resistor provided near the acoustic terminal or near one of the acoustic terminals; and a case housing the microphone unit. The acoustic resistor is constituted by a conductive elastic material such as a woven or nonwoven fabric of metal threads, and is electrically connected to the microphone unit and the case. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal section of a capacitor microphone according to the invention; 
         FIG. 2  is a perspective view of an acoustic resistor used in the capacitor microphone; 
         FIG. 3  is a longitudinal section of a capacitor microphone of the related art; and 
         FIG. 4  is a cross section showing how the capacitor microphone of  FIG. 3  is in contact with a microphone case. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be described with reference to an embodiment shown in  FIG. 1  and  FIG. 2 . For convenience sake, the invention is described to be applied to an omnidirectional tiepin microphone. Needless to say, the invention is also applicable to a unidirectional microphone of primary sound pressure gradient type which includes a pair of acoustic terminals at opposite ends thereof. 
     Referring to  FIG. 1 , a capacitor microphone unit  24  is housed in a cylindrical case  22 , and includes an acoustic terminal  30  at a front end thereof An acoustic resistor  26  is housed in a front end  28  of the cylindrical case  22 , one end of which is in contact with the acoustic terminal  30 . 
     As shown in  FIG. 2 , the acoustic resistor  26  is a disc made of conductive steel threads, for example. Preferably, the acoustic resistor  26  is made of stainless steel threads which are free from rusts or stains even with lapse of time. The conductive steel wires are woven into the shape of a fabric. The conductive steel fabric is pressed in the shape of a cup. The acoustic resistor  26  made of such woven conductive steel fabric is flexible and porous, and prevents generation of noises caused by airflows. A conductive fabric “SUI-78-5010T” manufactured by Taiyou Mesh Co., Ltd. is available, for example. 
     The acoustic resistor  26  is fitted into the front end  28  of the cylindrical case  22 , and has its rear end brought into contact with the acoustic terminal  30 . The acoustic resistor  26  is thicker than the cylindrical case  22 , is compressed and fitted into the front end  28  of the cylindrical case  22 , expands therein, and comes into close contact with the inner surface of the case  22 . The acoustic resistor  26  is electrically connected to the cylindrical case  22  and the microphone unit  24 . One or more adhesive spots  32  are provided between the acoustic resistor  26  and the microphone unit  24  in order to keep them in close contact with each other. The acoustic resistor  26  and the microphone unit  24  are preferably in contact with each other via a large area, so that the adhesive spots  32  are made as few or small as possible. 
     An appropriate fastener or ring may be provided in order to prevent the acoustic resistor  26  from dropping off from the cylindrical case  22 . Alternatively, the acoustic resistor  26  is partially fastened to the cylindrical case  22  using an adhesive. According to the invention, the rear peripheral edge of the cylindrical case  22  is crimped for the foregoing purpose. Further, a cord (not shown) extends out of the rear end of the cylindrical case  22 . 
     As described above, the acoustic resistor  26  is present at the front end of the acoustic terminal  30  of the microphone unit  24 , suppresses airflows caused by wind or pops, and prevents generation of wind noises, pops and so on. 
     The acoustic resistor  26  is made of the conductive material such as woven or non-woven metal threads, and is electrically connected to the cylindrical case  22 , so that the acoustic resistor  26  also functions as the static shield to block outside electromagnetic waves coming from a nearby cellular phone and so on. This is effective in preventing generation of noises caused by the electromagnetic waves. Further, the capacitor microphone has a simple structure by reducing the number of components thereof.