Patent Publication Number: US-8116498-B2

Title: Condenser microphone

Description:
TECHNICAL FIELD 
     The present invention relates to a condenser microphone and, more particularly, to a technique for preventing the generation of noise caused by disturbance electromagnetic waves. 
     BACKGROUND ART 
       FIG. 2  is a sectional view showing a general configuration example of a microphone unit that a conventional condenser microphone has. This microphone unit  10 B is unidirectional, and in this example, there is shown a microphone unit that is attachable to and detachable from a microphone body, not shown, (exchangeable), which microphone unit is applied to tie clip microphones, gooseneck microphones, and the like. 
     The microphone unit  10 B includes a cylindrical unit case  20  formed of, for example, a brass alloy. In the unit case  20 , a diaphragm and a backplate are arranged opposedly via a spacer ring (all of the three elements are not shown) as well known, and an electrostatic acoustoelectric converter  30  for converting coming sound waves into electrical signals is housed. 
     Since the microphone unit  10 B is unidirectional, the unit case  20  is provided with a rear audio terminal (rear sound wave introduction port)  22 , which takes in velocity components, on the side surface side thereof in addition to a front audio terminal (front sound wave introduction port directed to a sound source)  21  provided on the front surface thereof. 
     Usually, in the unit case  20 , a metal mesh  40  for covering the rear audio terminal  22  from the inside is provided. This metal mesh  40  is provided to inhibit foreign matters from intruding into the unit case  20  from the rear audio terminal  22 . The metal mesh  40  is brought into contact with the inner wall surface of the unit case  20  by the spring property of the metal mesh  40  itself, and is fixed by using an adhesive to prevent the metal mesh  40  from coming off due to vibrations etc. For example, Patent Document 1 (Japanese Patent Application Publication No. S55-105492) or Patent Document 2 (Japanese Patent Application Publication No. S56-43985) should be referred to. 
     On the inner surface side of the front audio terminal  21  as well, a metal mesh for inhibiting the intrusion of foreign matters is provided in the same way, but the illustration of this metal mesh is omitted. 
     From the acoustoelectric converter  30 , a signal draw-out electrode  31  connected to the backplate is drawn out. Along with the connection of the microphone unit  10 B to the microphone body, the signal draw-out electrode  31  is connected to a sound output circuit, not shown, in the microphone body. Since the acoustoelectric converter  30  has a very high impedance, an impedance converter is provided on the input side of the sound output circuit. 
     A vacuum tube is used as the impedance converter on rare occasions. In most cases, however, a field effect transistor (FET) is used as the impedance converter. In this case, the signal draw-out electrode  31  is connected to the gate electrode of the FET, and the sound output circuit is connected to between the source and the drain of the FET. 
     Since the impedance converter of this type also acts as a wave detecting device, if a high-frequency current caused, for example, by electromagnetic waves is applied to the microphone unit  10 B as disturbance, the current is detected by the impedance converter, and thereby noise of audio frequency is generated. The noise of this kind is scarcely generated in the case where the electromagnetic shield of the microphone unit  10 B is ensured. 
     At the front audio terminal  21 , the diaphragm is arranged so as to be opposed to the front audio terminal  21 , and a metallic layer formed in the diaphragm is connected to the unit case  20 , which provides grounding, via a metallic support ring (diaphragm ring), so that the electromagnetic waves intruding from the front audio terminal  21  rarely pose a problem. 
     However, regarding to the rear audio terminal  22 , the contact between the metal mesh  40  covering the rear audio terminal  22  from the inside and the inner wall surface of the unit case  20  depends on only the spring property of the metal mesh  40  itself as described above, so that the contact point area is small, and therefore the shield property is not necessarily sufficient. 
     In recent years, cellular phones have come into wide use. The cellular phone emits considerably strong electromagnetic waves (for example, within the range of about several centimeters to several tens centimeters, a field intensity reaching tens of thousands times of field intensity produced in the city by commercial electric waves). 
     Therefore, if a cellular phone is used near the microphone, since the contact between the metal mesh  40  and the unit case  20  is insufficient, the contact portion has a high impedance in terms of high frequency, so that a high-frequency current caused by the high impedance intrudes into the microphone body, which may generate loud noise. 
     Also, since the contact state differs from microphone unit to microphone unit, the degree of generation of noise caused by high-frequency current varies. Also, if the opening of the rear audio terminal  22  is made large to improve the acoustic characteristics, the high-frequency current intrudes more easily. 
     Accordingly, an object of the present invention is to ensure the electromagnetic shield at a rear audio terminal covered by a metal mesh in a unidirectional microphone unit. 
     SUMMARY OF THE INVENTION 
     To achieve the above object, the present invention provides a condenser microphone including a unidirectional microphone unit incorporating an acoustoelectric converter, in which a diaphragm and a backplate are arranged opposedly via a spacer member, in a cylindrical metallic unit case having a front audio terminal on the front surface thereof and a rear audio terminal on the side surface thereof, in which a metal mesh covering the rear audio terminal from the inside is provided in the unit case, wherein a coil spring which presses the metal mesh against the inner wall surface of the unit case is provided in the unit case. 
     According to this configuration, the metal mesh covering the rear audio terminal from the inside is pressed against the inner wall surface of the unit case by the coil spring, so that the metal mesh comes into contact with the unit case at many points, and thereby a reliable electromagnetic shield can be provided. Also, the need for fixing the metal mesh by using an adhesive is eliminated. 
     As a further preferable mode, the coil spring is plated with gold. 
     According to this configuration, since the coil spring is plated with gold, the contact resistance between the coil spring and the metal mesh is decreased extremely, and the contact portion has no impedance in terms of high frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a microphone unit that a condenser microphone in accordance with the present invention has; and 
         FIG. 2  is a sectional view of a conventional microphone unit. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will now be described by reference to  FIG. 1 . The present invention is not limited to this embodiment.  FIG. 1  is a sectional view of a microphone unit that a condenser microphone in accordance with the present invention has. In the explanation of this embodiment, the same reference numerals are applied to elements that are the same as those in the conventional example explained by reference to  FIG. 2 . 
     As shown in  FIG. 1 , in this microphone unit  10 A as well, as in the conventional example explained by reference to  FIG. 2 , a cylindrical unit case  20  formed of, for example, a brass alloy is provided. In the unit case  20 , a diaphragm and a backplate are arranged opposedly via a spacer ring (all of the three elements are not shown) as well known, and an electrostatic acoustoelectric converter  30  for converting coming sound waves into electrical signals is housed. 
     Since the microphone unit  10 A is unidirectional, the unit case  20  is provided with a front audio terminal (front sound wave introduction port directed to a sound source)  21  provided on the front surface thereof and a rear audio terminal (rear sound wave introduction port)  22 , which takes in velocity components, provided on the side surface side thereof. The sound waves coming from the rear audio terminal  22  pass through a predetermined path in the electrostatic acoustoelectric converter  30  and act on the back surface side of the diaphragm. 
     The microphone unit  10 A is attachable to and detachable from a microphone body, not shown, (exchangeable). A signal draw-out electrode  31  drawn out of the acoustoelectric converter  30  is connected to the gate terminal of an FET, serving as an impedance converter, provided on the microphone body side. 
     In the unit case  20 , a metal mesh  40  for covering the rear audio terminal  22  from the inside is provided to inhibit foreign matters from intruding into the unit case  20  from the rear audio terminal  22 . On the inner surface side of the front audio terminal  21  as well, a metal mesh for inhibiting the intrusion of foreign matters is provided in the same way, but the illustration of this metal mesh is omitted. 
     The metal mesh  40  is cut out of a net base material, not shown, into a rectangular shape having a length corresponding the inner circumference length of the unit case  20 , and is inserted into the unit case  20  in a state of being rounded into a cylindrical shape. The metal mesh  40  is brought into contact with the inner wall surface of the unit case  20  by the spring property of the metal mesh  40  itself. However, the contact point area is small, and therefore the shield property is not necessarily sufficient. 
     Accordingly, in the present invention, a coil spring  50  is pushed into the unit case  20 , and is pushed strongly against the inner wall surface of the unit case  20 , by which the metal mesh  40  is brought into contact with the unit case  20  at many points to improve the electromagnetic shield property. 
     As the coil spring  50 , a compression coil spring having an outside diameter slightly larger than the inside diameter of the unit case  20  is used. The compression coil spring is pressed (forcedly inserted) coaxially into the unit case  20  preferably while being turned. 
     According to this configuration, the metal mesh  40  is pushed strongly against the unit case  20 , and comes into contact with the unit case  20  at many points, so that the electromagnetic shield is ensured. Also, since the stress of the coil spring  50  is steady, there is no difference between microphone units. Also, an adhesive for fixing the metal mesh  40  is not needed. 
     Preferably, the coil spring  50  is plated with gold. Thereby, the contact resistance between the coil spring and the metal mesh is decreased extremely, and the contact portion has no impedance in terms of high frequency. Therefore, the generation of noise caused by electromagnetic waves can be prevented more effectively. 
     In the above-described embodiment, the microphone unit  10 A is attachable to and detachable from the microphone body. However, the microphone unit  10 A may be integrated with the microphone body. 
     The present application is based on, and claims priority from, Japanese Application Serial Number JP2006-351190, filed Dec. 27, 2006, the disclosure of which is hereby incorporated by reference herein in its entirety.