Patent Publication Number: US-7724914-B2

Title: Dynamic microphone

Description:
TECHNICAL FIELD 
   The present invention relates to a dynamic microphone and more specifically relates to a dynamic microphone which makes it possible to effectively prevent the lead of a voice coil from breaking when a diaphragm undergoes an excessive displacement. 
   BACKGROUND ART 
     FIG. 3  is a sectional view showing a main part of an example of a conventional dynamic microphone. The basic configuration of the microphone comprises a vibration part  10  vibrating with sound waves and a magnetism generating circuit  20 . The vibration part  10  comprises a diaphragm  11  and a voice coil (generator coil)  12 . In this case, the diaphragm  11  has a center dome  11   a  and a sub dome  11   b  provided around the center dome  11   a . The voice coil  12  is joined to a boundary between the center dome  11   a  and the sub dome  11   b  with an adhesive. 
   The magnetism generating circuit  20  comprises a permanent magnet  21 , a pole piece  22  provided on one pole of the permanent magnet  21 , and a ring yoke  24  magnetically connected to the other pole of the permanent magnet  21  via a cup-like yoke  23 . The voice coil  12  is supported in a magnetic gap formed between the pole piece  22  and the ring yoke  24  so as to vibrate through the diaphragm  11 . A lead  12   a  of the voice coil  12  is wired along the inner surface of the sub dome  11   b  and routed to a signal output circuit (not shown). 
   Incidentally, for picking up sound of musical instruments, dynamic microphones enabling preferred sound quality are used frequently. Particularly when a bass drum is used as a musical instrument, a dynamic microphone is placed in the bass drum to pick up the sound. 
   Dynamic microphones for base drums pick up heavy bass, and thus in general designs, diaphragms have low stiffness (soft diaphragms) and voice coils are heavy. The bass resonance frequency of dynamic microphones for bass drums is set low, and thus an amplitude is extremely large around a resonance frequency in actual sound pickup. For example, the amplitude sometimes reaches about ±0.5 mm. 
   When a base drum is strongly hit, an internal pressure rapidly increases, an air current caused by the increased pressure instantly flows from an opening of the drum to the outside, and then the air current strongly presses the diaphragm  11  to the magnetism generating circuit  20  as shown in  FIG. 4 . Since an amplitude is extremely large at this point, the lead  12   a  of the voice coil  12  may strongly strike a corner of the ring yoke  24  and break thereon. The lead  12   a  may be broken also by a drop impact and the like. 
   As a method for preventing this problem, patent document 1 (Japanese Patent Application Publication No. 2003-1191) discloses a method of elastically supporting a magnetism generating circuit through a suspension. 
   However, the technique described in patent document 1 requires the suspension and a structure becomes complicated accordingly, resulting in high cost. As shown in  FIG. 3 , the magnetism generating circuit  20  has an extremely narrow gap, and thus when the magnetism generating circuit is elastically supported through a suspension as in the technique disclosed in patent document 1, the voice coil  12  of  FIG. 3  may come into contact with the magnetism generating circuit  20 . 
   SUMMARY OF THE INVENTION 
   In view of the problem of the conventional technique, an object of the present invention is to provide a dynamic microphone which makes it possible to minimize cost and the complication of a structure, and prevent, with a simple configuration, the lead of a voice coil from breaking even when a strong sound pressure or drop impact presses a diaphragm to a magnetism generating circuit. 
   The present invention is devised to attain the object. A dynamic microphone including a diaphragm having a voice coil attached to a boundary between a center dome and a sub dome, and a magnetism generating circuit having a magnetic gap formed between a pole piece provided on one pole of a permanent magnet and a ring yoke provided on the other pole of the permanent magnet, the voice coil being supported in the magnetic gap so as to vibrate through the diaphragm, the voice coil having a lead wired along the inner surface of the sub dome facing the ring yoke, wherein the sub dome has an elastic layer made of an elastic adhesive on the inner surface, and the lead has a lead end at least a part of which is elastically held on the sub dome through the elastic layer. 
   In this case, it is preferable that the elastic layer be formed over the inner surface of the sub dome. Further, it is desirable that the elastic layer be formed by applying an adhesive solidifying with elasticity. 
   With this configuration, the lead end of the lead of the voice coil is elastically held through the elastic layer provided on the inner surface of the sub dome. Thus, even when the diaphragm is strongly pressed to the magnetism generating circuit, the cushion effect of the elastic layer can positively prevent a break. 
   Moreover, the elastic layer is formed, including the lead end of the lead, over the inner surface of the sub dome adjacent to the voice coil, thereby effectively preventing abnormal resonance of the sub dome. The abnormal resonance causes a large peak and dip in directional frequency response at a resonance frequency. 
   Further, the elastic layer is formed using an elastic adhesive which is a rubber adhesive including a silicon resin adhesive, thereby obtaining more preferable elasticity. It is desirable that the elastic adhesive have a hardness not seriously interfering with the vibration of the diaphragm when hardened. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view showing a main part of a dynamic microphone according to the present invention; 
       FIG. 2  is a sectional view showing the maximum displacement of a diaphragm of the dynamic microphone according to the present invention; 
       FIG. 3  is a sectional view showing a main part of a conventional dynamic microphone; and 
       FIG. 4  is a sectional view showing the maximum displacement of a diaphragm of the conventional dynamic microphone. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 and 2 , an embodiment of the present invention will be discussed below. The present invention is not limited to this embodiment.  FIG. 1  is a sectional view showing a main part of a dynamic microphone according to the present invention.  FIG. 1  corresponds to  FIG. 3 , and constituent elements not to be changed from those of the conventional example are indicated by the same reference numerals.  FIG. 2  is a sectional view which shows the maximum displacement of a diaphragm in a similar manner to  FIG. 4 . 
   As shown in  FIG. 1 , the dynamic microphone comprises, as a basic configuration, a vibration part  10  vibrating with sound waves and a magnetism generating circuit  20 . The vibration part  10  includes a diaphragm  11  and a voice coil  12 . 
   The diaphragm  11  has a center dome  11   a  and a sub dome  11   b  coaxially provided around the center dome  11   a . The center dome  11   a  and the sub dome  11   b  are integrally formed of an extremely thin (e.g., about 9 μm) synthetic resin film. The center dome  11   a  and the sub dome  11   b  are formed by heating and molding a single synthetic resin film. 
   The voice coil  12  is composed of a copper wire, which has an insulating coating, with a wire diameter of, e.g., about 25 μm. The voice coil  12  is joined as a generator coil to a boundary between the center dome  11   a  and the sub dome  11   b  with an adhesive while being wound a predetermined number of turns. 
   The magnetism generating circuit  20  comprises a permanent magnet  21 , a pole piece  22  provided on one pole of the permanent magnet  21 , and a ring yoke  24  magnetically connected to the other pole of the permanent magnet  21  via a cup-like yoke  23 . 
   In this example, rear acoustic terminal holes  23   a  for an operation of a unidirectional microphone are bored through the cup-like yoke  23 . The dynamic microphone of the present invention may not have the rear acoustic terminal holes  23   a , in other words, the dynamic microphone may be omnidirectional. 
   The ring yoke  24  is disposed around the pole piece  22  with a predetermined magnetic gap. In the magnetic gap, the voice coil  12  is supported so as to vibrate through the diaphragm  11 . The edge of the sub dome  11   b  is fixed to a housing (not shown). 
   The voice coil  12  has a lead  12   a  wired along the inner surface of the sub dome  11   b  (a surface facing the ring yoke  24 ) and the other end of the voice coil  12  is routed to a signal output circuit (not shown). 
   An elastic layer  30  made of an elastic adhesive is provided between the voice coil  12  and the inner surface of the adjacent sub dome  11   b . The lead  12   a  has a lead end  12   b  elastically held on the sub dome  11   b  through the elastic layer  30 . With this configuration, for example, when the dynamic microphone is used for a bass drum and a strong sound pressure or drop impact causes an excessive displacement (the diaphragm  11  is pressed to the magnetism generating circuit  20 ) on the diaphragm  11 , it is possible to prevent the lead  12   a  of the voice coil  12  from directly coming into contact with a corner of the ring yoke  24 , thereby positively preventing a break. 
   Additionally, the elastic layer  30  is formed over the inner surface of the sub dome  11   b  adjacent to the voice coil  12 , thereby effectively preventing abnormal resonance of the sub dome  11   b . The abnormal resonance causes a large peak and dip in directional frequency response at a resonance frequency. 
   For the elastic layer  30 , it is possible to use an elastic adhesive which can exhibit adhesion while keeping proper elasticity not interfering with the vibration of the diaphragm. For example, in addition to a rubber adhesive including a silicon resin adhesive, an ultraviolet curing adhesive and an acrylic adhesive are also available. 
   The above explanation described the present invention in accordance with the illustrated example. The specific configuration is not limited to this example. For example, the elastic layer  30  may be formed only on the inner surface of the sub dome  11   b , which faces the lead end  12   b  of the lead  12   a , to hold the lead end  12   b . Further, the elastic layer  30  may be applied so as to hold the lead end  12   b  of the lead  12   a  and formed so as to be dotted in the circumferential direction instead of over the inner surface of the sub dome  11   b  adjacent to the voice coil  12 . 
   The present application is based on, and claims priority from, Japanese Application Serial Number JP2004-192658, filed Jun. 30, 2004, the disclosure of which is hereby incorporated by reference herein in its entirety.