Abstract:
There is provided a dynamic microphone in which the output impedance does not increase and also the failure rate does not increase though using a microphone unit and a vibration detecting unit to reduce handling noise. In the dynamic microphone including a microphone unit  20  that includes a first diaphragm  24  and a first magnetic circuit  26  and delivers sound signals and a vibration detecting unit  30  that includes a second diaphragm  32  and a second magnetic circuit  33  and detects vibrations applied to a microphone case, whereby the output signal of the vibration detecting unit  30  being delivered as an opposite phase with respect to the output signal of the microphone unit  20 , a field coil  41  excited by the output signal of the vibration detecting unit  30  is provided on the first magnetic circuit  26  side of the microphone unit  20.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on, and claims priority from, Japanese Application Serial Number JP2007-325703, filed Dec. 18, 2007, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a dynamic microphone, and more particularly, to a dynamic microphone having a function of reducing vibration noise. 
     BACKGROUND ART 
     A microphone unit includes a vibrating part having a diaphragm capable of vibrating with respect to a microphone case and a fixed part including a magnetism generating circuit or a backplate supported directly on the microphone case. 
     Of various types of microphones, a hand-held microphone frequently presents a problem of vibration noise generated, for example, when the microphone case is rubbed with a finger. The vibration noise of this kind is generated by an inertial force such that the effective mass of the vibrating part of microphone unit tends to stay in the original state when vibrations are applied to the microphone case. 
     Generally, for a microphone provided with a diaphragm having the same area density, vibration noise of a low frequency component increases in the order of non-directional, unidirectional, and bidirectional. Also, comparing with a condenser microphone and a ribbon microphone, a dynamic (electrodynamic) microphone is liable to generate vibration noise because its diaphragm is heavy. 
     Therefore, for a hand-held unidirectional dynamic microphone especially used for vocals, conventionally, handling noise generated by the rubbing of microphone case with a finger has caused a problem. 
     As one method for reducing such handling noise, there is available a so-called shock mount method in which vibrations are insulated by using an elastic body such as rubber when the microphone unit is supported on the microphone case (for example, Japanese Patent Application Publication No. H01-197000). 
     However, the shock mount method has problems described below. The vibration insulating effect of the shock mount method depends on the resonance frequency and resonance sharpness of a vibration system. Therefore, the vibration noise reducing effect can be anticipated only on a frequency band on which frequencies are higher than or equal to the frequency correlated with the resonance frequency. Also, when solid-borne noise is loud, the vibration insulating effect cannot be achieved for the high frequency component thereof. 
     Accordingly, the applicant of the present invention has proposed a method for canceling handling noise by providing a vibration detecting unit having almost the same configuration as that of the microphone unit in the microphone case and by adding an output signal from the vibration detecting unit to an output signal from the microphone unit as a negative phase (Japanese Patent Application Publication No. H11-196489). 
     With this method, if vibrations are applied to a microphone case, not shown, whereas, for example, an output signal of positive phase is delivered from a microphone unit  1 , an output signal of negative phase is delivered from a vibration detecting unit  2 . Therefore, after, for example, the magnetization direction of a permanent magnet has been reversed or the winding method of a voice coil has been reversed, the microphone unit  1  and the vibration detecting unit  2  are connected to each other in series as shown in  FIG. 3 . Thereby, handling noise can be reduced satisfactorily over a relatively wide frequency range. 
     However, since the microphone unit and the vibration detecting unit are connected to each other in series, if the impedance of each unit is the same, the output impedance of microphone doubles. 
     As the output impedance of microphone decreases, the noise caused by electrostatic coupling or magnetic coupling from the outside is more difficult to be brought in. Therefore, the rise in output impedance of microphone is unfavorable in this respect. 
     Also, a main cause for failure of dynamic microphone is wire breaking. Wire breaking takes place mainly at a soldering location in the end part of voice coil. In the case where the voice coils of the microphone unit and the vibration detecting unit are connected to each other in series, if the wire breaks even at only one place, no sound is produced, so that the failure rate doubles as compared with the ordinary dynamic microphone having no vibration detecting unit. 
     Accordingly, an object of the present invention is to provide a dynamic microphone in which the output impedance does not increase and also the failure rate does not increase while handling noise is reduced by using a microphone unit and a vibration detecting unit. 
     SUMMARY OF THE INVENTION 
     To achieve the above object, the present invention provides a dynamic microphone including a microphone unit, a microphone case, and a vibration detecting unit, 
     the microphone unit including a first magnetism generating circuit and a first diaphragm, the first magnetism generating circuit having a first magnetic gap formed by a first permanent magnet, and the first diaphragm having a first voice coil arranged in the first magnetic gap; 
     the microphone case supporting the microphone unit on one end side thereof and being provided with a back air chamber having a predetermined volume communicating with the back surface side of the first diaphragm therein; and 
     the vibration detecting unit being arranged in the back air chamber in a state in which a second magnetism generating circuit and a second diaphragm are provided, the second magnetism generating circuit having a second magnetic gap formed by a second permanent magnet, and the second diaphragm having a second voice coil arranged in the second magnetic gap, whereby the output signal of the vibration detecting unit being delivered as an opposite phase with respect to the output signal of the microphone unit, wherein a field coil excited by the output signal of the second voice coil is provided on the first magnetism generating circuit side. 
     As a preferred mode, the field coil is arranged around the first permanent coil. 
     According to this dynamic microphone, the voice coils of the microphone unit and the vibration detecting unit are not connected to each other directly in series, but are connected to each other magnetically via the field coil. Therefore, the output impedance of the microphone itself does not rise, and also even if wire breaking takes place on the vibration detecting unit side, the sound output does not cease unless wire breaking takes place on the microphone unit side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing a general configuration of a unidirectional dynamic microphone in accordance with an embodiment of the present invention; 
         FIG. 2  is an enlarged sectional view of an essential portion of the dynamic microphone shown in  FIG. 1 ; and 
         FIG. 3  is a schematic view showing a state in which a microphone unit and a vibration detecting unit are connected to each other in a conventional example. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will now be described with reference to  FIGS. 1 and 2 .  FIG. 1  is a sectional view showing a general configuration of a unidirectional dynamic microphone in accordance with the embodiment of the present invention, and  FIG. 2  is an enlarged sectional view of an essential portion of the dynamic microphone. 
     As shown in  FIGS. 1 and 2 , this dynamic microphone includes a cylindrical microphone case  10  formed of a metal such as aluminum. In the microphone case  10 , a cylindrical inner cylinder  11  is held coaxially via a shock mount  12 . 
     The shock mount  12  is formed of, for example, a rubber elastic body, and on the inner peripheral surface side thereof, a holding ring  13  for the inner cylinder  11  is provided. In  FIG. 1 , at a position on the lower edge side of the shock mount  12  of the inner cylinder  11 , a stopper ring  14  is installed. At a position on the upper edge side of the shock mount  12  of the inner cylinder  11 , a reinforcing ring  15  is fitted. 
     At one end (the upper end in  FIG. 1 ) of the inner cylinder  11 , a microphone unit  20  for receiving sound waves is installed. The other end of the inner cylinder  11  is closed by a bottomed cylinder-shaped spacer cylinder  16 . Therefore, the interior of the inner cylinder  11  serves as an air chamber (acoustic volume)  18  having a predetermined volume, which communicates with the microphone unit  20 . 
     The other end of the inner cylinder  11  is supported on the lower end side of the microphone case  10  via the spacer cylinder  16 . On the lower end side of the microphone case  10 , an output connector  17  is provided. At the upper end of the microphone case  10 , a wind screen  19  for covering the microphone unit  20  is provided. 
     The microphone unit  20  is provided with a cylindrical unit case  21 . In this embodiment, the unit case  21  has a large-diameter main cylindrical part  22  and a small-diameter subsidiary cylindrical part  23  connectingly provided under the main cylindrical part  22 , and a diaphragm (first diaphragm)  24  having a voice coil  241  is provided in the opening part of the main cylindrical part  22 . Also, in the opening part of the main cylindrical part  22 , a resonator  25  having a front acoustic terminal  251  is installed so as to cover the diaphragm  24 . 
     In the main cylindrical part  22 , a magnetic circuit (first magnetic circuit)  26  is provided. The magnetic circuit  26  includes a columnar magnet  261  magnetized in the up and down direction in  FIG. 1 , a cylindrical side yoke  262  arranged concentrically around the magnet  261 , and a tail yoke  263  that connects the side yoke  262  to one pole of the magnet  261 . A magnetic gap G is formed between the magnet  261  and the side yoke  262 , and the voice coil  241  of the diaphragm  24  is arranged in this magnetic gap G. 
     In a level difference part between the main cylindrical part  22  and the subsidiary cylindrical part  23 , a rear acoustic terminal  211  is provided. The rear acoustic terminal  211  communicates with a space on the back surface side of the diaphragm  24  through an air passage  212  formed in the main cylindrical part  22 . In the air passage  212 , a predetermined acoustic resistance material  213  is provided. 
     The subsidiary cylindrical part  23  communicates with the interior of the main cylindrical part  22 , that is, the space on the back surface side of the diaphragm  24  via air holes  264  formed in the tail yoke  263 . Also, in a bottom part  230  of the subsidiary cylindrical part  23 , an air hole  231  communicating with the air chamber  18  in the microphone case  10  is formed. On the tail yoke  263  side and on the bottom part  230  side in the subsidiary cylindrical part  23 , acoustic resistance materials  232  and  233  are provided, respectively. 
     The subsidiary cylindrical part  23  is fittedly supported on one end side of the inner cylinder  11 . On the bottom part  230  side of the subsidiary cylindrical part  23 , a sleeve  234  is connectingly provided, and a vibration detecting unit  30  is attached to the sleeve  234 . 
     In this embodiment, the vibration detecting unit  30  is provided with a unit case  31  fittedly held in the sleeve  234 . In this case, the unit case  31  is fittedly held in the sleeve  234  so that a bottom part  311  of the unit case  31  faces to the bottom part  230  of the subsidiary cylindrical part  23 . Therefore, the opening part of the unit case  31  is directed downward in  FIGS. 1 and 2 . 
     In the opening part of the unit case  31 , a diaphragm (second diaphragm)  32  having a voice coil  321  is vibratably provided via a corrugation  322  formed of a plastic sheet. In this case, as the diaphragm  32 , for example, a brass sheet having a thickness of 0.8 mm and a diameter of about 10 mm is used. Thereby, the signal output caused by vibrations is secured, and a change in pressure in the air chamber  18  is produced surely. 
     In the unit case  31 , a magnetic circuit (second magnetic circuit)  33  is housed. Like the magnetic circuit  26  of the microphone unit  20 , the magnetic circuit  33  includes a columnar magnet  331  magnetized in the up and down direction, a cylindrical side yoke  332  arranged concentrically around the magnet  331 , and a tail yoke  333  that connects the side yoke  332  to one pole of the magnet  331 . 
     In this case, on the magnet  331 , an annularly-shaped center pole piece  334  is provided, and on the side yoke  332  side as well, a ring yoke  335  is provided so as to face to the center pole piece  334 , thereby forming a gap G between the center pole piece  334  and the ring yoke  335 . In this gap G, a voice coil  321  of the diaphragm  32  is arranged. 
     The tail yoke  333  is formed with a plurality of air holes  336 , and an acoustic resistance material  337  is provided in each of the air holes  336 . Also, in a part of the sleeve  234 , for example, a slit-shaped opening  235  is formed. 
     The vibration detecting unit  30  is housed in the air chamber  18  of the inner cylinder  11  in a state of being fittedly held in the sleeve  234 , and the air chamber  18  communicates with the space on the back surface side of the diaphragm  24  via an air passage including the opening  235  of the sleeve  234 , the air hole  231  of the subsidiary cylindrical part  23 , and the air holes  264  of the tail yoke  263 . 
     According to the above-described configuration, if the microphone case  10  is driven, for example, at a vibration speed of V 1  in the direction indicated by an upward arrow mark B 1  in  FIG. 1 , the diaphragm  24  of the microphone unit  20  relatively vibrates at a vibration speed of V 2  in the direction indicated by a downward arrow mark B 2  in  FIG. 2 , and also the diaphragm  32  of the vibration detecting unit  30  relatively vibrates likewise at a vibration speed of V 3  in the direction indicated by a downward arrow mark B 3  in  FIG. 2 . 
     That is to say, when the microphone case  10  is displaced upward, relatively, both of the diaphragm  24  of the microphone unit  20  and the diaphragm  32  of the vibration detecting unit  30  are displaced downward. 
     By this downward displacement of the diaphragm  32 , the pressure in the air chamber  18  is raised, and is applied to the back surface of the diaphragm  24  of the microphone unit  20  through the air passage so as to push back the diaphragm  24  downward. 
     In contrast, when the microphone case  10  is displaced in the reverse direction, that is, in the downward direction, relatively, both of the diaphragm  24  of the microphone unit  20  and the diaphragm  32  of the vibration detecting unit  30  are displaced upward. 
     By this upward displacement of the diaphragm  32 , the pressure in the air chamber  18  is lowered, and is applied to the back surface of the diaphragm  24  of the microphone unit  20  through the air passage so as to allow the diaphragm  24  that tends to be displaced upward to stay at the original position. 
     Thus, the vibrations of the diaphragm  24  of the microphone unit  20  are restrained mechanically, and thereby the vibration noise is reduced. Sound waves go to the back surface side of the diaphragm  24  through the rear acoustic terminal  211  of the microphone unit  20 , and are absorbed or significantly damped by the acoustic resistance materials  232  and  233  in the subsidiary cylindrical part  23 . Therefore, the sound waves are not picked up by the vibration detecting unit  30 . 
     Regarding the sound output, as shown in  FIG. 2 , both ends (the winding start end and the winding finish end) of the voice coil  241  of the microphone unit  20  are connected to output terminals OUT 1  and OUT 2  on the hot side and the cold side, and sound signals are taken out of the output terminals OUT 1  and OUT 2 . 
     To reduce a vibration noise signal included in the sound signal, in the present invention, a field coil  41  is provided around the magnet  261  of the microphone unit  20 , and both ends (the winding start end and the winding finish end) of the voice coil  321  of the vibration detecting unit  30  are connected to the field coil  41  via lead wires  411  and  412 . In this case, the output signal of the microphone unit  20  and the output signal of the vibration detecting unit  30  are in antiphase. 
     As described above, since both of the diaphragm  24  of the microphone unit  20  and the diaphragm  32  of the vibration detecting unit  30  tend to be displaced in the same direction by external vibrations, for example, the magnetization directions of the magnets  261  and  331  are reversed, or the winding directions of the voice coils  241  and  321  are reversed, by which both the output signals can be made in antiphase. 
     Thus, by exciting the field coil  41  by means of a voltage in antiphase with the microphone unit  20  generated by the vibration detecting unit  30 , the vibration noise signal included in the sound signal of the microphone unit  20  and the vibration noise signal produced by the vibration detecting unit  30  are offset in terms of alternating current, so that the vibration noise can be reduced electrically. 
     Thus, in the present invention, the output of the vibration detecting unit  30  is magnetically coupled with the magnetic circuit  26  of the microphone unit  20  via the field coil  41  like a transformer. Since the voice coil that is operated by the ordinary sound is used for the microphone unit  20  only, the output impedance of the microphone itself does not become high. 
     Also, since the voice coils of the microphone unit  20  and the vibration detecting unit  30  are not connected to each other directly in series, the rate of failure caused by wire breaking decreases. 
     In the above-described embodiment, as a preferred mode, the pneumatic pressure produced by the diaphragm  32  of the vibration detecting unit  30  is applied to the back surface of the diaphragm  24  of the microphone unit  20 . In some cases, however, the vibration detecting unit  30  may be arranged in a closed space.