Patent Publication Number: US-6985597-B2

Title: Variable directional capacitor microphone comprising elastic acoustic resisting member

Description:
FIELD OF THE INVENTION 
   The present invention relates to a variable directional microphone. More particularly, the invention relates to the variable directional microphone having two capacitor elements each of which includes a vibrating plate and a fixed electrode with the both capacitor elements combined in the microphone. 
   BACKGROUND OF THE INVENTION 
   Japanese Patent registration No. 3299829 discloses a variable directional capacitor microphone including two capacitor elements each of which has a vibrating plate and a fixed electrode. The two capacitor elements are combined as to supply a part of sound pressure impressed to a vibrating plate of the one capacitor element (a rear capacitor element) to the back side of a vibrating plate of the other capacitor element (a front capacitor element). The structure will be described referring to  FIGS. 3 and 4 .  FIG. 3  is a cross sectional view of the variable directional capacitor microphone which has been assembled and  FIG. 4  is an exploded cross sectional view of that of  FIG. 3 . 
   The variable directional capacitor microphone includes a first and a second capacitor elements  10   a ,  10   b . Each of the capacitor elements has the same structure so that the first capacitor element  10   a  will be explained hereinafter. Constitutional elements corresponding to the second capacitor element  10   b  are attached with the same reference numerals as that of constitutional elements corresponding to the first capacitor element, together with a reference symbol “b”. 
   The capacitor element  10   a  has a case  16   a  which is formed in a ring shape with electric insulating material. A vibrating plate supporting member  12   a,  a spacer ring  13   a,  a fixed electrode  14   a  and pedestal  15   a  are assembled in the case  16   a  in this order. A vibrating plate  11   a  is fixed and strained with a predetermined tension force on the vibrating plate supporting member  12   a.    
   The case  16   a  has an inner edge flange  161  latched with the circumference of the vibrating plate supporting member  12   a  and female screw threads  162  formed on an inner surface of a body of the case. The outer circumference of the pedestal  15   a  has male screw threads  151  screwed with the female screw threads  162 . Therefore, the pedestal  15   a  is screwed to the case  16   a  so that the vibrating plate  11   a  and the fixed electrode  14   a  are faced together and are securely fixed through the spacer ring  13   a.    
   An electrode rode  121  is extracted from the vibrating plate supporting member  12   a.    FIG. 3  or  4  illustrates only one through hole  141 , however, the fixed electrode  14   a  has a number of through holes  141 . The pedestal  15   a  is formed in a saucer-shape with the circumference of the pedestal  15   a  protruding such that an air chamber having a predetermined air volume is formed between the pedestal  15   a  and the fixed electrode  14   a.  The center of the bottom of the pedestal  15   a  has a through hole  152 . The both sides of the through hole  152  are covered with two sheets of acoustic resisting members  17   a  and  18   b  formed with nylon mesh or the like. 
   The first and the second capacitor elements  10   a  and  10   b  are combined through a connecting ring  22  having female screw threads  221  in the state the pedestals  15   a  and  15   b  are faced back-to-back and a gasket  21  formed in a ring shape is disposed between the both pedestals. 
   Male screw threads  151  of the pedestal  15   a  are screwed from one side of the connecting ring  22  and on the other hand, the male screw threads  151  of the pedestals  15   b  are screwed from the other side of the connecting ring  22 . Then the first and the second capacitor elements  10   a  and  10   b  are combined with each other through the connecting ring  22 . 
   A first chamber A 1  is formed in the space between the fixed electrode  14   a  and the pedestal  15   a  of the first capacitor element  10   a,  a second air chamber A 2  formed in the space between acoustic resisting members  17   a  and  17   b,  a third air chamber A 3  formed in the space of the center porting of the gasket with the both sides of the spaced surrounded by the pedestals  15   a  and  15   b,  a fourth air chamber A 4  formed between the acoustic resisting members  17   b  and  18   b  of the second capacitor element  10   b,  a fifth air chamber A 5  formed between the fixed electrode  14   b  and the pedestal  15   b.  The back sides of the vibrating plates  14   a  and  14   b  acoustically communicate through the acoustic capacities connecting in a ladder-form of the five air chambers A 1  to A 5 . 
   According to the prior art of the structure described above, each of the first and the second capacitor elements  10   a  and  10   b  can be operated respectively before the both elements are combined so that a pair of capacitor elements having similar technical performances are selected and combined to obtain a microphone having wholly stable characteristics. 
   The variable directional capacitor microphone of the prior art described above is required that the output of each of the capacitor elements  10   a ,  10   b  has a satisfactory cardioid directional characteristic in the state that each of the capacitor elements has been assembled. 
   Therefore, the capacitor elements  10   a ,  10   b  are combined, after each of the capacitor elements has been adjusted such that an acoustic resistance of each capacitor element has a predetermined value. 
   However, no expected characteristics may be obtained after the capacitor elements have been combined. In this case the acoustic resistance should be re-adjusted. In the above-described example of the prior art, the acoustic resistance cannot be adjusted in the state that the both capacitor elements have been left combined so that the connecting ring should be removed and the acoustic resistance should be re-adjusted after each of the capacitor elements has been decomposed. However, there is no guarantee that the adjustment of the acoustic resistance is completed only once. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a variable directional capacitor microphone in which two capacitor elements are combined. Each of the capacitor elements includes a vibrating plate and a fixed electrode, and an acoustic resistance can be adjusted in the state that the microphone has been assembled. 
   In order to achieve the object, the variable directional capacitor microphone includes a first and a second capacitor elements. In the first capacitor element, a first vibrating plate supporting member on which a first vibrating plate is fixed and strained, a first spacer ring, a first fixed electrode having through holes and a first pedestal having a through hole in the center thereof are integratedly assembled in this order in a first case which is formed in a ring-shape. In the second capacitor element, a second vibrating plate supporting member on which a second vibrating plate is fixed and strained, a second spacer ring, a second fixed electrode having through holes and a second pedestal having a through hole in the center thereof are integratedly assembled in this order in a second case which is formed in a ring-shape. The first and the second capacitor elements are combined through a connecting ring having female screw threads in the state that the first and the second pedestals are faced back-to-back. The variable directional capacitor microphone is characterized in that an elastic acoustic resisting member is disposed between the first and the second pedestals and the disposed position of the acoustic resisting member corresponds to the through hole bored in the center of each of the pedestals. 
   An acoustic resistance of the acoustic resisting member can be adjusted with adjusting compress volume of the resisting member. 
   In this invention, it is preferable that an elastic gasket formed with rubber material or the like is disposed around the circumference of the acoustic resisting member installed between the first and the second pedestals to prevent sound leakage through the acoustic resisting member. 
   In this case, from the viewpoint to cut the relation ship between compressed volume of the acoustic resisting member and that of the gasket and to increase degree of freedom of the compressed volume, it is preferable that a groove having a predetermined depth for receiving a part of each side of the acoustic resisting member is formed on one side of each through hole of the first and the second pedestals. 
   Further, an aspect that each through hole of the first and the second pedestals is covered with an acoustic resisting mesh member is included in this invention. 
   According to the invention, the compressed volume of the elastic acoustic resisting member is variable with the degree of fastening each of the pedestals to the connecting ring so that the acoustic resistance can be adjusted. Therefore, the acoustic resistance can be adjusted to obtain a good directional characteristic in the state that each of the capacitor elements has been left combined (assembled) through the connecting ring. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view of a variable directional capacitor microphone which has been assembled embodying the present invention; 
       FIG. 2  is an exploded cross sectional view of the variable directional capacitor microphone embodying the present invention; 
       FIG. 3  is a cross sectional view of a variable directional capacitor microphone which has been assembled of a prior art; and 
       FIG. 4  is an exploded cross sectional view of the variable directional capacitor microphone of the prior art. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 and 2 , an embodiment of the present invention will be described. The invention is not restricted to this embodiment.  FIG. 1  is a cross sectional view of a variable directional capacitor microphone which has been assembled embodying the present invention.  FIG. 1  corresponds to  FIG. 3  described before.  FIG. 2  is an exploded cross sectional view of the variable directional capacitor corresponding to  FIG. 4  described before. In the explanation of the embodiment, attached to the constituent elements which are the same or are deemed to be the same as that of a prior art are the same reference numerals and symbols as that of the prior art. 
   As the basic structure, the variable directional capacitor microphone of the invention includes a first and a second capacitor elements with the both elements combined through a connecting ring  22 . Since the first and the second capacitor elements have the same structure as described above, hereinafter, the first capacitor element will be mainly described, however, the reference numerals and symbols of the constitutional elements relating to the second capacitor element will be written in parentheses. 
   The capacitor element  10   a  ( 10   b ) has a case  16   a  ( 16   b ) which is formed with electric insulating material. A vibrating plate supporting member  12   a  ( 12   b ), a spacer ring  13   a  ( 13   b ), a fixed electrode  14   a  ( 14   b ) and pedestal  15   a  ( 15   b ) are assembled in the case  16   a  ( 16   b ) in this order. A vibrating plate  11   a  ( 11   b ) is strained with a predetermined tension force and fixed on the vibrating plate supporting member  12   a  ( 12   b ). 
   The case  16   a  ( 16   b ) has an inner edge flange  161  latched with the circumference of the vibrating plate supporting member  12   a  ( 12   b ) and female screw threads  162  formed on the inner surface of the body of the case. The outer circumference of the pedestal  15   a  ( 15   b ) has male screw threads  151  screwed with the female screw threads  162 . Therefore, the vibrating plate  11   a  ( 11   b ) and the fixed electrode  14   a  ( 14   b ) are faced through the spacer ring  13   a  ( 13   b ) and are securely fixed by screwing the insulating pedestal  15   a  ( 15   b ) to the case  16   a  ( 16   b ). 
   An electrode rode  121  is extracted from the vibrating plate supporting member  12   a  ( 12   b ), respectively.  FIG. 1  or  2  illustrates only one through hole  141 , however, the fixed electrode  14   a  ( 14   b ) has a number of through holes  141 . The pedestal  15   a  ( 15   b ) is formed in a saucer-shape with the circumference of the pedestal protruding such that an air chamber having a predetermined air volume is formed between the pedestal and the fixed electrode. The center of the bottom of the pedestal has a through hole  152 . 
   The male screw threads  151 ,  151  of the pedestal  15   a,    15   b  are screwed to the female screw threads  221  of a connecting ring  22  with the pedestals  15   a  and  15   b  facing back-to-back so that the first and the second capacitor elements  10   a,    10   b  having the structure described above are combined. Further, an acoustic resisting member  31  which has a larger diameter than that of the through hole  152  is coaxially disposed between the pedestals  15   a  and  15   b.  That is, each of the through holes  152  is covered with the acoustic resisting member  31 . 
   The acoustic resisting member  31  is formed with a spongy elastic material which has continuous air bubbles. An acoustic resistance of the spongy material changes with a compressed volume thereof. For example, product No. HR 50 of urethane sponge of Bridgestone Corporation is exemplified as the acoustic resisting member. 
   The acoustic resisting member  31  can be disposed in the whole area between the pedestals  15   a,    15   b,  however, it is preferable that an elastic gasket  32  formed with rubber material or the like is disposed around the circumference of the acoustic resisting material  31  in order to prevent a sound leakage through the acoustic resisting material  31 . 
   According to this structure, the compressed volume of the acoustic resisting member  31  or the acoustic resistance is variable with the length of thread engagement of the pedestal  15   a,    15   b  with the connecting ring  22  so that the acoustic resistance between the vibrating plates  11   a  and  11   b  can be adjusted in the state that the first and the second capacitor elements  10   a  and  10   b  have been assembled. 
   In the case that the gasket  32  is disposed around the acoustic resisting member  31 , when the gasket  32  is tightly pressed, the acoustic resisting member  31  is compressed until the resisting member  31  becomes the same thickness as that of the gasket because the gasket is harder than the acoustic resisting member so that an appropriate acoustic resistance may not be obtained. 
   In order to prevent the case described above and to increase degree of freedom of the compressed volume of the acoustic resisting member  31 , as shown in  FIG. 2 , a groove  153  having a predetermined depth for receiving a part of each side of the acoustic resisting member  31  is formed on one side of each through hole  152  of the pedestal  15   a,    15   b.  The groove  153  is also used as positioning means of the acoustic resisting member  31 . 
   An acoustic resisting member formed with nylon mesh or the like, which is not shown in  FIG. 1  or  2 , can be installed on the both sides (or one side) of the through hole  152 . This aspect is included in this invention.