Patent Publication Number: US-8111856-B2

Title: Variable directional microphone unit and variable directional microphone

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a variable directional microphone unit in which a physical structure thereof and a structure of a circuit for electrical switching used therewith can be made simple, and to a variable directional microphone. 
     2. Description of the Related Art 
     As a microphone with variable directionality, a microphone is known that has a microphone unit composed of two capacitor microphone units connected back-to-back (see, for example, Japanese Patent Application Laid-open No. H7-143595 and No. 2008-67286). Both microphone units have cardioid characteristic. The variable directionality is achieved through adjusting their outputs or, as described in Japanese Patent Application Laid-open No. H7-143595, through adjusting polarization voltages applied to each element. 
     An example of a conventional variable directional microphone unit similar to those of Japanese Patent Application Laid-open No. H7-143595 and No. 2008-67286 is shown in FIG. 11. In FIGS. 11 to 14, a variable directional capacitor microphone unit is composed of two individually formed capacitor microphone units  21  and  41  connected back-to-back. A diaphragm-like vibrating plate  22  has its outer peripheral portion fixed to one side of a vibrating plate holding ring  23  to compose a vibrating plate assembly therewith. The vibrating plate holding ring  23  is made of a conductive material and an electrode plate  24  having a plurality of acoustic terminal holes  241  is disposed thereon. An electrode  25  electrically conducted to the vibrating plate  22  is fixed to the electrode plate  24 . The vibrating plate  22  integrally held by the vibrating plate holding ring  23  is placed on a disk-shaped fixed electrode  26  with a ring-shaped spacer  27  made of an extremely thin insulating material in between. Thus, the vibrating plate  22  faces an upper surface of the fixed electrode  26  with a slight gap with a size corresponding to a thickness of the spacer  27  in between. The spacer  27  is sandwiched by the vibrating plate  22  and the fixed electrode  26  at the position near their outer peripheries. 
     The fixed electrode  26  is placed on an insulative base  30  with a receiving ring  28  in between. The base  30  has circular flanges  31  and  32  formed along peripheries of an upper and a lower surface thereof, respectively. The receiving ring  28  and the fixed electrode  26  are dropped into a space surrounded by the flange  31 . Both upper and lower surfaces of the base  30  gradually inclined towards the center and a vertically through hole is formed at the center. An acoustic resisting member  34  is fit into the hole. A upper surface of the fixed electrode  26  protrudes above that of the flange  31 . The spacer  27 , the vibrating plate assembly formed of the vibrating plate holding ring  23  and the vibrating plate  22 , and the electrode plate  24  are stacked on the fixed electrode  26  in this order. A holding ring  29  is fit around the outer peripheries of the electrode plate  24  and the vibrating plate assembly. The holding ring  29  is also fit around an outer periphery of the flange  31  of the base  30  to be fixed thereto with any appropriate fixing methods. An upper edge of the holding ring  29  is formed to be an inner extending edge  291 . As the inner extending edge  291  pushes down the electrode plate  24 , the units described above are secured to the base  30  by being urged thereto. The microphone unit  21  is thus formed. 
     The acoustic terminal holes  241  of the electrode plate  24  serve as a front acoustic terminal of the microphone unit  21 . The fixed electrode  26  has a plurality of holes as well. Through the acoustic terminal holes  241  of the electrode plate  24  and the holes formed on the fixed electrode  26 , a space behind the vibrating plate  22  is communicated with: a space formed by the upper surface of the base  30  being gradually inclined towards the center; and, via the acoustic resisting member  34 , a space formed by the lower surface of the base  30  being gradually inclined towards the center. 
     The other microphone unit  41  has a similar structure as the above described microphone unit  21  connected back-to-back therewith. The microphone unit  41  includes: a vibrating plate  42 ; a vibrating plate holding ring  43 ; an electrode plate  44 ; an electrode  45 ; a fixed electrode  46 ; a spacer  47 ; a receiving ring  48 ; a holding ring  49 ; a front acoustic terminal  441  formed of a plurality of holes; and an inner extending edge  491  of the holding ring  49 . Above elements have similar structures as that of the corresponding elements of the microphone unit  21 . At the lower surface side of the base  30 , the microphone unit  41  is formed as the counterpart of the microphone unit  21 . Polarization voltages are individually applied to the vibrating plates  22  and  42  of the microphone units  21  and  41 . 
       FIG. 15  depicts an equivalent circuit of the above described microphone unit. In the figure, the two microphone units are connected to each other via acoustic resistance r 1  of the acoustic resisting member  34 . The microphone unit  21  is at the left of the acoustic resistance r 1  while the microphone unit  41  is at the right thereof. In the figure, the microphone unit  21  includes: sound pressure P 1 ; mass m OA , stiffness s OA , and acoustic resistance r OA  of a front air chamber; and stiffness S 1A  of the hole formed in the fixed electrode and a rear air chamber in communication therewith. Similarly, the microphone unit  41  includes: sound pressure P 2 ; mass m OB , stiffness s OB , and acoustic resistance r OB  of a front air chamber; and stiffness S 1B  of the hole formed in the fixed electrode and a rear air chamber in communication therewith. 
       FIG. 16  depicts an example of a directionality switching circuit that can be applied to the conventional variable directional microphone unit. Constant polarization voltage is applied to the vibrating plate of one of the microphone units  21  and  41  and a level of polarization voltage applied to the vibrating plate of the other microphone unit is switched. Thus, the directionality of the microphone unit can be switched. The exemplary circuit shown in  FIG. 16  has DC power sources of +60V and −60V, and +60V is constantly applied to the vibrating plate of one of the microphone units. Voltages of both power sources of +60V and −60V are divided into two levels (for example, into +60V and +30V, and −60V and −30V). Thus, five levels (including 0V) of voltages are generated. The voltage to be applied to the vibrating plate of the other microphone unit is selected from the five levels by means of a switch. 0V (no voltage) is applied to the fixed electrode (also referred to as “a back plate”) included in both microphone units. 
       FIG. 17  depicts examples of directionalities of the microphone unit obtained through switching between the polarization voltages of different levels by means of the switch. Under a condition in which a contact  1  is selected with a switch shown in  FIG. 16 , the polarization voltage of +60V is applied to one of the microphone unit whereas the polarization voltage of −60V is applied to the other microphone unit. Here, the microphone unit has bidirectional characteristic as shown in “ 1 ” of  FIG. 17  in which an output from the front microphone unit is subtracted by an output from the rear microphone unit. Under a condition in which a contact  2  is selected with the switch, the polarization voltage applied to the other microphone unit becomes higher than −60V and lower than 0V (for example −30V). Here, the microphone unit has hypercardioid characteristic as shown in “ 2 ” of  FIG. 17 . Under a condition in which a contact  3  is selected with the switch, polarization voltage of 0V (no polarization voltage) is applied to the other microphone unit. Here, the microphone unit has cardioid characteristic as shown in “ 3 ” of  FIG. 17  in which only the front microphone unit performs the output. Under a condition in which a contact  4  is selected with the switch, the polarization voltage applied to the other microphone unit becomes higher than 0V and lower than 60V (for example 30V). Here, the microphone unit has wide cardioid characteristic as shown in “ 4 ” of  FIG. 17 . Under a condition in which a contact  5  is selected with the switch, the polarization voltage applied to the other microphone unit is +60V. Here, the microphone unit has omnidirectional characteristic as shown in “ 5 ” of  FIG. 17  in which the output of the rear microphone unit is added to the output of the front microphone unit. 
     The directionality of the above described conventional variable directional microphone is variable by connecting two microphone units back-to-back and by making the polarization voltage applied to one of the microphone units variable or, as described above, by making the output level of each of the microphone units variable. However, this method of achieving variable directionality requires a complex circuit structure. 
     The directionalities the above described variable directional microphone can generally have are cardioid, bidirectional, and omnidirectional. An intermediate of the directionalities can be obtained through further providing alternatives for the mixing ratio of the outputs from the pair of microphone units or the level of the applied polarization voltages. However, this requires even more complex circuit structure. 
     With the exemplary circuit of  FIG. 16 , the directionality can be switched between several different levels. Here, the circuit requires the power supplies capable of generating voltages at different levels and the voltage must be selectable. Thus, the circuit structure is complex. 
     Directionality of handheld microphones widely used on stages and the like is cardioid or hypercardioid. A microphone having which directionality is to be used is chosen according to the sound the user prefers, in terms of preventing acoustic feedback, or the like. The variable directional microphone unit described above may be used but incorporating the switching circuit having such a complex structure as described above in a handheld microphone is difficult. 
     Therefore, a microphone is called for that has a simple circuit structure and enables the user to arbitrarily select the directionality from cardioid, hypercardioid, and supercardioid. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the above problems in related art. Thus, the object of the present invention is to provide a variable directional microphone unit and a variable directional microphone having a circuit structure for switching directionality simple enough to be incorporated in a microphone. 
     In the present invention, a variable directional microphone unit includes two capacitor units. Each of the two capacitor elements has: a back plate formed on one side of an insulating plate to be insulated from a back plate of the other capacitor element; and a vibrating plate disposed to face the back plate with a certain amount of space therebetween, in which a polarization voltage is applied between each of the back plates and the vibrating plates so that an electroacoustically transduced signal is obtainable from each of the back plates. The variable directional microphone unit is characterized in that the two vibrating plates of the two capacitor elements are acoustically connected in series as a plurality of holes are formed on both of the back plates. 
     EFFECT OF THE INVENTION 
     A gap composed of a plurality of holes of the vibrating plate and the back plate of one of the capacitor elements serves as an acoustic resistance for the other capacitor element. Thus, directionality of the other capacitor element becomes closer to bidirectional, e.g., becomes hypercardioid. The directionality is variable through selecting or mixing outputs from the capacitor elements. Thus, the structure of the circuit and the physical structure of the microphone unit can be simplified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a rear view of a variable directional microphone unit of an embodiment according to the present invention; 
         FIG. 2  is a vertical cross sectional view of the variable directional microphone unit; 
         FIG. 3  is a front view of the variable directional microphone unit; 
         FIG. 4  is an exploded vertical cross sectional view of the variable directional microphone unit; 
         FIG. 5  is a diagram of a circuit acoustically equivalent to the variable directional microphone unit; 
         FIG. 6  is a circuit diagram of a variable directional microphone to which the variable directional microphone unit can be applied; 
         FIG. 7  is a diagram depicting a directionality of a front capacitor element of the variable directional microphone unit; 
         FIG. 8  is a graph depicting frequency responses of the front capacitor element; 
         FIG. 9  is a diagram depicting a directionality of a rear capacitor element of the variable directional microphone unit; 
         FIG. 10  is a graph depicting frequency responses of the rear variable directional microphone unit; 
         FIG. 11  is a rear view of an example of a conventional variable directional microphone unit; 
         FIG. 12  is a vertical cross sectional view of the conventional variable directional microphone unit; 
         FIG. 13  is a front view of the conventional variable directional microphone unit; 
         FIG. 14  is an exploded vertical cross sectional view of the conventional variable directional microphone unit; 
         FIG. 15  is a diagram of a, acoustical equivalent circuit of the conventional variable directional microphone unit; 
         FIG. 16  is a circuit diagram of a variable directional microphone to which the conventional variable directional microphone unit can be applied. 
         FIG. 17  is a graph depicting examples of directionalities of the microphone unit obtained through switching between polarization voltages. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a variable directional microphone unit and a variable directional microphone according to the present invention is described below with reference to the accompanying drawings. 
     In  FIGS. 1 to 4 , an insulating plate  60  has back plates  56  and  76  insulated from each other on the upper surface and the lower surface thereof, respectively. Vibrating plates  52  and  72  are respectively disposed on the back plates  56  and  76  with certain amount of spaces therebetween. Thus, two electrically independent capacitor elements  51  and  71  are formed on the upper and the lower surfaces of the insulating plate  60 , respectively. The insulating plate  60  and the back plates  56  and  76  can be fabricated with, for example, a method similar to that for fabricating a printed circuit board. Specifically, the back plates  56  and  76  are conductive circuit patterns formed on both sides of the substrate made of an insulating material. The vibrating plates  52  and  72  are diaphragm-like members made of thin resin films. The vibrating plates  52  and  72  are fixed to one side of vibrating plate holding ring  53  and one side of vibrating plate holding ring  73 , respectively to form vibrating plate assemblies. 
     Instead of being formed on both sides of a single insulating plate  60 , each of the two back plates  56  and  76  may be formed on one of the surfaces of two different insulating plates in contact with each other at sides not having the back plates. 
     The vibrating plate assemblies have the vibrating plates  52  and  72  respectively disposed at sides closer to back plates  56  and  76 , respectively. The ring-shaped spacers  57  and  77  are disposed in between the vibrating plate  52  and the back plate  56 , and in between the vibrating plate  72  and the back plate  76 . With the spacers  57  and  77 , slight gaps having sizes corresponding to the thickness of the spacers  57  and  77  are respectively formed between: the vibrating plate  52  and the back plate  56 ; and the vibrating plate  72  and the back plate  76 . The capacitor elements  51  and  71  electrically independent from each other are thus respectively formed with: the vibrating plate  52  and the back plate  56 ; and the vibrating plate  72  and the back plate  76 . Electroacoustically transduced signals can be obtained from the back plates  56  and  76  by applying polarization voltages between: the vibrating plate  52  and the back plate  56 ; and the vibrating plate  72  and the back plate  76 . 
     The insulating plate  60  has a plurality of through holes  601  penetrating in the thickness direction thereof (vertically as viewed in  FIGS. 2 and 4 ). Though it may not be clearly shown in the figures, the back plates  56  and  76  also have a plurality of holes each having the size and the disposed position corresponding to the holes  601 . Thus, the two vibrating plates  52  and  72  are acoustically connected in series through the holes formed on the back plates  56  and  76  and the holes  601  formed on the insulating plate  60 . 
     The capacitor elements  51  and  71  are fit in a circular recess  542  formed on one side (a lower side as viewed in  FIGS. 2 and 4 ) of a base  54 . More specifically, the vibrating plate assembly formed of the vibrating plate  52  and the vibrating plate holding ring  53 , the spacer  57 , the insulating plate  60  having the back plates  56  and  76 , the spacer  77 , and the vibrating plate assembly formed of the vibrating plate  72  and the vibrating plate holding ring  73  are fit in the recess  542  of the base  54  in this order. A holding plate  78  is disposed over the elements and one end face (at the lower side as viewed in  FIGS. 2 and 4 ) of the base  54 . As the holding plate  78  is screwed to the base  54  at a plurality of appropriate positions at the peripheral portion thereof, the elements are secured to the base  54  as being urged thereto. The holding plate  78  has a plurality of holes into which sound waves are guided. A part of the insulating plate  60  extends outward in a radial direction. Terminal patterns  561  and  761  are respectively formed on an upper and a lower surfaces of the extended portion to electrically connect the back plates  56  and  76  with outside. A part of the circular flange surrounding the recess  542  of the base  54  is cut out and the extending portion of the insulating plate  60  extends outward therefrom. 
     The base  54  has a plurality of through holes  541  penetrating in the thickness direction thereof that serves as acoustic terminals disposed in front of the vibrating plate  52  of the capacitor element  51 . At the surface of the base  54  at which the recess  542  is not formed, a shallow recess  543  is formed into which a plate shaped acoustic resisting member  55  that covers the holes  541  is fit. The acoustic resisting member  55  and a holding plate  58  are fixed to the base  54  as: the holding plate  58  is disposed to cover the upper surface as viewed in  FIGS. 2 and 4  of the acoustic resisting member  55  and the base  54 ; the holding plate  58  is screwed to the base  54  at a plurality of appropriate position at the periphery thereof; and the center of the holding plate  58  is screwed to the base  54  with a screw  59 . The holding plate  58  has a plurality of holes into which sound waves are guided. 
     As described above, the acoustic resisting member  55  is disposed behind the vibrating plate  52  of the capacitor element  51 . Here, the side at which the acoustic resisting member  55  is disposed, i.e., the upper side as viewed in  FIGS. 2 and 4 , is the rear side that is at an angle of 180° with respect to the direction to which the sound enters, whereas the side at which the acoustic resisting member  55  is not disposed is the front side that is at an angle of 0° with respect to the direction into which the sound enters. With the configuration of the present embodiment, the directionality of the capacitor element  51  at the rear side is closer to bidirectional compared with that of the capacitor element  71  at the front side. This is because the acoustic resisting member  55  is disposed in front of the rear capacitor element  71  but not in front of the front capacitor element  51 . The front capacitor element  71  has no acoustic resisting member  55  at the front side thereof and has, at the rear side thereof: acoustic resistance r OB  of the space between the vibrating plate  52  of the rear capacitor element  51 , and the back plate  56  and the insulating plate  60 ; and acoustic resistance r 1  of the acoustic resisting member  55 . Meanwhile, the rear capacitor element  51  has: at the front side thereof, acoustic resistance r OA  (=r OB ) of the space between the vibrating plate  72  of the front capacitor element  71 , and the back plate  76  and the insulating plate  60 ; and at the rear side thereof, the acoustic resistance r 1  of the acoustic resisting member  55 . Thus, the acoustic resistance ratio between the front side and the rear side differs between the front capacitor element  71  and the rear capacitor element  51 . The rear capacitor element  51  has an airspace (a thin airspace between the vibrating plate  72  of the front capacitor element  71  and the back plate  76 ) at the front as viewed thereform. The airspace serves as front acoustic resistance making the directionality of the rear capacitor element  51  closer to bidirectional compared with that of the front capacitor element  71 . 
     Thus, upon adjusting the acoustic resistance of the acoustic resisting member  55  so that the directionality of the output from front capacitor element  71  becomes cardioid, the directionality of output from the rear capacitor element  51  becomes hypercadioid. This is because the gap formed between the vibrating plate  72  of the front capacitor element  71 , and the back plate  76  and the insulating plate  60  serves as the front acoustic member for the rear capacitor element  51  to make the directionality thereof closer to bidirectional. 
     In the embodiment shown with the figures, the directionality of the microphone unit can easily be switched by selectively switching the output of the front and rear capacitor elements  51  and  71  or mixing the outputs therefrom. Moreover, as described above, the directionality of the output from the rear capacitor element  51  is closer to bidirectional compared with that of the output from that of the front capacitor element  71 . Therefore, the directionality can be switched between wide cardioid and cardioid, between cardioid and hypercardioid, or the like by adjusting the resistance of the acoustic resistance member  55 . 
       FIG. 5  is an equivalent circuit of the above described embodiment. In the figure, the two microphone elements  51  and  71  and the acoustic resistance r 1  of the acoustic resisting member  55  are acoustically connected in series. In the figure, the microphone element  71  includes: sound pressure P 1 ; mass m OA , stiffness s OA , and acoustic resistance r OA  of a front air chamber; and stiffness S b  of the hole formed in the fixed electrode and a rear air chamber in communication therewith. Similarly, the microphone element  51  includes: sound pressure P 2 ; mass M OB , stiffness s OB , and acoustic resistance r OB  of a front air chamber; and stiffness S 1  of the hole formed in the fixed electrode and a rear air chamber in communication therewith. 
       FIG. 6  depicts an example of a circuit of a capacitor microphone with which the directionality is switchable that can be applied to the embodiment described above. The directionality of the output from the microphone unit can be switched by selecting between the front microphone element  71  and the rear microphone element  51  by means of a switch  80 . The circuit is advantageous compared with the circuit applied in the conventional variable directional microphone unit shown in  FIG. 16  in that the structure thereof can be made simple as no power sources capable of providing different levels of voltages are required. Portions other than the directionality switching circuit are not directly related to the present invention and thus, the description thereof is omitted. 
       FIGS. 7 to 10  depict directionalities and frequency responses that can be obtained with the embodiment.  FIGS. 7 and 8  respectively depict directionality and frequency responses of the front capacitor element  71 . Here, the directionality is cardioid. The frequency responses shown in  FIG. 8  are measured at 0°, 90°, and 180° with respect to the direction into which the sound waves enter. 
       FIGS. 9 and 10  respectively depict directionality and frequency responses of the rear capacitor element  51 . The directionality of the rear capacitor element  51  is closer to bidirectional compared to that of the front capacitor element  71  because the acoustic resistance r OA  of the front capacitor element  71  serves as the front acoustic resistance of the rear capacitor element  51 . Therefore, as shown in  FIG. 9 , the directionality is hypercardioid. The frequency responses shown in  FIG. 10  are measured at 0°, 90°, and 135° with respect to the direction into which the sound waves enter. 
     A variable directional microphone can be formed by incorporating the above described microphone unit as well as the circuit as shown in  FIG. 6  in a microphone casing. The circuit includes the switch with which the outputs from the two capacitor elements composing the variable directional microphone unit can be mixed or either one of the outputs can be selected. With the microphone casing having the switch, the user can arbitrarily select the preferable directionality. 
     INDUSTRIAL APPLICABILITY 
     With the variable directional microphone unit and the variable directional microphone according to the present invention, the directionality can be switched according to the use. Upon use, the directional characteristics are switched according to the sound the user prefers, in terms of preventing acoustic feedback, or the like.