Patent Publication Number: US-8526664-B2

Title: Capacitor microphone unit and capacitor microphone

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a capacitor microphone unit and a capacitor microphone of which acoustic resistance can be readily controlled. 
     2. Description of the Related Art 
     Capacitor microphones include a capacitor microphone unit in which a diaphragm that vibrates upon receiving sound waves and a fixed electrode (also referred to as a “back electrode”) are placed opposite to each other with a spacer provided therebetween to form a capacitor. Capacitance of the capacitor changes when the diaphragm vibrates. Such a capacitor microphone unit is built inside a unit casing. 
     In such a capacitor microphone unit, acoustic resistance is provided to limit or control sound waves guided to the diaphragm to obtain a desired directionality. For example, acoustic resistance is provided by providing an acoustic resistance material that covers a sound communication hole on an insulating base as described in Japanese Utility Model Laid-open No. H07-29996. 
     The basic structure of a capacitor microphone unit that is incorporated in a capacitor microphone and includes an acoustic resistance material is as illustrated in  FIGS. 6  and  7 . In the capacitor microphone unit  1  as illustrated in  FIG. 6 , sound waves entering a rear acoustic terminal A pass through an air chamber provided with an acoustic resistance material  18  formed of a sponge or the like and a second acoustic resistance material  17  formed of a nylon mesh or the like to be applied to the rear side of the diaphragm. Thus, the capacitor microphone unit  1  has unidirectionality. 
     In  FIG. 6 , the capacitor microphone unit  1  is formed by installing the elements described below inside a unit casing  11  having a cylindrical shape. A side on which a bottom  11   a  is provided is the front side of the unit casing  11 . The bottom  11   a  is provided with multiple holes  22  through which sound is guided, inside the capacitor microphone unit  1 . A ring-shaped diaphragm holder  12  and a diaphragm  13  having the peripheral portion attached to one end surface of the diaphragm holder  12  are disposed at the position closest to the bottom  11   a  in the unit casing  11 . 
     A fixed electrode  15  is arranged opposite to the diaphragm  13  with a ring-shaped spacer  14  provided therebetween. Naturally, a space defined by the thickness of the spacer  14  is provided between the fixed electrode  15  and the diaphragm  13 . Thus, a capacitor is formed by the fixed electrode  15  and the diaphragm  13 . The capacitance of the capacitor changes as the diaphragm  13  vibrates in accordance with the sound entering through the holes  22 . The change in capacitance is output as a sound signal. 
     In the unit casing  11 , an insulating base  16  that serves as a fixed electrode supporting member is disposed on the rear side of the fixed electrode  15 . The insulating base  16  is formed of, for example, synthetic resin, and has a circular recessed portion on the front side (upper side as viewed in  FIG. 6 ). The fixed electrode  15  is fitted in the recessed portion. A ring-shaped, further recessed portion is formed on an inner peripheral portion of the recessed portion. A ring-shaped, second sound resistance material  17  and an elastic acoustic resistance material  18  are fitted in the ring-shaped recessed portion. The insulating base  16  is provided with a hole extending in the thickness direction in the central portion. A terminal member  19  electrically connected to a field-effect transistor (FET) (not illustrated) that forms an impedance converter is fitted in the hole. 
     In the conventional capacitor microphone unit  1  as illustrated in  FIGS. 6 and 7 , an air layer that is the space between the fixed electrode  15  and the diaphragm  13  provides an acoustic resistance for controlling the vibration of the diaphragm  13 . The acoustic resistance is adjusted by adjusting the number and the size of holes B penetrating through the fixed electrode  15  and the size of the space formed between the diaphragm  13  and the fixed electrode  15 , for example. 
     However, in the case where the fixed electrode  15  is press molded for example, the size and the number of holes B cannot be readily changed. 
     The freedom of adjusting the acoustic resistance, by adjusting the space between the fixed electrode  15  and the diaphragm  13  through changing the thickness of the spacer  14 , is limited because the spacer  14  is often formed by a commercially available plastic film and thus the choice of material is limited. 
     SUMMARY OF THE INVENTION 
     The present invention is provided in view of the above problems and an object of the present invention is to provide a capacitor microphone unit and a capacitor microphone of which acoustic resistance can be readily controlled by forming a skin layer provided on a surface of an acoustic resistance material formed of, for example, a sponge and adjusting the skin layer. 
     A capacitor microphone unit according to an aspect of the present invention includes a diaphragm, a fixed electrode that is arranged opposite to the diaphragm with a space provided between the diaphragm and the fixed electrode so that a capacitor is formed between the diaphragm and the fixed electrode, an insulating base that is disposed on a rear side of the fixed electrode and supports the fixed electrode, and a ring-shaped acoustic resistance material that is disposed on a front side of the insulating base, all of which are incorporated in a unit casing. A skin layer is formed on at least one of a front side and a rear side of the acoustic resistance material. The skin layer has a higher density than that inside the acoustic resistance material. 
     A capacitor microphone according to another aspect of the present invention includes the above-described capacitor microphone unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an embodiment of a capacitor microphone unit according to the present invention; 
         FIG. 2  is a cross-sectional enlarged view of an acoustic resistance material used in the capacitor microphone unit illustrated in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of another embodiment of a capacitor microphone unit according to the present invention; 
         FIG. 4  is a cross-sectional enlarged view of an acoustic resistance material and a second acoustic resistance material used in the capacitor microphone unit illustrated in  FIG. 3 , the acoustic resistance material and the second acoustic resistance material being illustrated in a separated state; 
         FIGS. 5A to 5D  are cross-sectional views sequentially illustrating a manufacturing process of the acoustic resistance material illustrated in  FIG. 2 ; 
         FIG. 6  is a cross-sectional view of an example of a conventional capacitor microphone unit; and 
         FIG. 7  is a cross-sectional enlarged view of an acoustic resistance material and a second acoustic resistance material used in the capacitor microphone unit illustrated in  FIG. 6 , the acoustic resistance material and the second acoustic resistance material being illustrated in a separated state. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of a capacitor microphone unit and an embodiment of a capacitor microphone according to the present invention are described with reference to some of the accompanying drawings. Elements similar to those in the above-described conventional microphone unit are given the same reference numerals. 
     Capacitor Microphone Unit 
     A capacitor microphone unit according to an embodiment illustrated in  FIGS. 1 and 2  is different from the conventional capacitor microphone unit only in the structure of an acoustic resistance material. 
     Similar to the conventional capacitor microphone unit as illustrated in  FIG. 6 , in this capacitor microphone unit  1  illustrated in  FIG. 1 , sound waves entering from a rear acoustic terminal A pass through an air chamber provided with an acoustic resistance material  18  to be applied to the rear side of a diaphragm. Thus, the capacitor microphone unit  1  has unidirectionality. 
     The capacitor microphone unit  1  is formed by installing the elements described below inside a unit casing  11  having a cylindrical shape. A side on which a bottom  11   a  is provided is the front side of the unit casing  11 . The bottom  11   a  is provided with multiple holes  22  through which sound is guided inside the capacitor microphone unit  1 . A ring-shaped diaphragm holder  12  and a diaphragm  13  having the peripheral portion attached to one end surface of the diaphragm holder  12  are disposed at the portion closest to the bottom  11   a  in the unit casing  11 . 
     A fixed electrode  15  is arranged opposite to the diaphragm  13  with a ring-shaped spacer  14  provided therebetween. Thus, a space defined by the thickness of the spacer  14  is provided between the fixed electrode  15  and the diaphragm  13 . Thus, a capacitor is formed by the fixed electrode  15  and the diaphragm  13 . The capacitance of the capacitor changes as the diaphragm  13  vibrates in accordance with sound entering through the holes  22 . The capacitance change is output as a sound signal. 
     In the unit casing  11 , an insulating base  16  that serves as a fixed electrode supporting member is disposed on the rear side of the fixed electrode  15 . The insulating base  16  is formed of, for example, synthetic resin, and has a circular recessed portion on the front side (upper side as viewed in  FIG. 1 ). The fixed electrode  15  is fitted in the recessed portion. A ring-shaped, further recessed portion is formed on an inner peripheral portion of the recessed portion. An elastic and ring-shaped acoustic resistance material  18  is fitted in the ring-shaped recessed portion. The insulating base  16  is provided with a hole extending in the thickness direction in the central portion. A terminal member  19  electrically connected to a FET (not illustrated) that forms an impedance converter is fitted in the hole. 
     The acoustic resistance material  18  in the capacitor microphone unit  1  according to the present embodiment is formed of a sponge. More specifically, the acoustic resistance material  18  is formed of a sponge made of porous urethane resin material. Skin layers  18   a  and  18   d  of high density are respectively formed on the front and the rear surfaces of the acoustic resistance material  18  formed of a sponge. A low density layer  18   b  that has lower density than that of the skin layers  18   a  and  18   d  is formed on the inner side of the skin layers  18   a  and  18   d.    
     The skin layers  18   a  and  18   d  are formed by heating the surfaces of a sponge material  18   c  (see  FIG. 5A ) according to a method of manufacturing an acoustic resistance material as described later. The acoustic resistance of the acoustic resistance material  18  can be increased by forming the skin layers  18   a  and  18   d  of higher density. The damping of the diaphragm  13  can be controlled with the skin layer  18   a , which is in contact with the fixed electrode  15 , adjusting the air flow in the air chamber on the rear side of the diaphragm  13 . Directionality of the capacitor microphone unit  1  is adjusted with the skin layer  18   d , which is in contact with the insulating base  16 , providing an acoustic resistance to the rear acoustic terminal A. 
     The low density layer  18   b , a portion of the sponge material  18   c , is not thermally modified by the heat applied for manufacturing the acoustic resistance material  18 . In other words, a portion of the sponge material  18   c  with no change in density is the low density layer  18   b . The low density layer  18   b , which has a low density, serves as an air chamber in the capacitor microphone unit  1 . 
     The acoustic resistance of the above-described capacitor microphone unit  1  according to the present embodiment can be readily adjusted as desired by appropriately changing the thickness and the density of the skin layers  18   a  and  18   d  of the acoustic resistance material  18 . 
     A capacitor microphone unit according to another embodiment of the present invention is described with reference to  FIGS. 3 and 4 . The capacitor microphone unit according to this embodiment is different from the capacitor microphone unit of the previously described embodiment in the structure of an acoustic resistance material. Thus, the structure of the acoustic resistance material is mainly described below. 
     This capacitor microphone unit  1  according to this embodiment is a unidirectional capacitor microphone unit and an acoustic resistance material  18  is formed of a sponge as in the previously described embodiment. Skin layers  18   a  and  18   d  of high density are respectively formed on the front and the rear surfaces of the acoustic resistance material  18  formed of a sponge. A low density layer  18   b  that has lower density than that of the skin layers  18   a  and  18   d  is formed on the inner side of the skin layers  18   a  and  18   d.    
     A nylon mesh serving as a second acoustic resistance material  17  is attached to the insulating base  16  side surface of the acoustic resistance material  18 . The second acoustic resistance material  17  has a planer shape same as that of the acoustic resistance material  18 . 
     The acoustic resistance of the above-described capacitor microphone unit  1  according to the present embodiment can be readily adjusted as desired by appropriately changing the thickness and the density of the skin layers  18   a  and  18   d  of the acoustic resistance material  18 . In addition, the acoustic resistance material  18  can be more strongly pressed towards the opening of the rear acoustic terminal A of the insulating base  16 , due to the elastic force of the second acoustic resistance material  17  applied in the thickness direction. Thus, acoustic resistance can be provided more stably. 
     In the above-described two embodiments, the two skin layers  18   a  and  18   d  are respectively formed on the front and the rear surfaces of the acoustic resistance material  18 . The present invention is not limited thereto, and only one of the skin layers  18   a  and  18   d  may be formed. If the skin layer  18   a  is formed on the fixed electrode  15  side surface, the skin layer  18   a  provides acoustic resistance and the damping of the diaphragm  13  can be adjusted as in the above-described embodiments. If the skin layer  18   d  is formed on the insulating base  16  side surface, the skin layer  18   d  provides acoustic resistance and the directionality of the capacitor microphone unit  1  can be adjusted by adjusting the acoustic resistance as in the above-described embodiments. 
     Method for Manufacturing Acoustic Resistor 
     A method for manufacturing a sponge serving as the acoustic resistance material  18  in the above-described embodiments is described with reference to  FIGS. 5A to 5D . 
     First, the sponge material  18   c , which is the material of the acoustic resistance material  18 , made of porous urethane resin material is provided in a pressing apparatus  30  as illustrated in  FIG. 5A . The pressing apparatus  30  is an apparatus for pressing and heating the sponge material  18   c  sandwiched therein to melt the surfaces of the sponge material  18   c , thereby forming the skin layers  18   a  and  18   d.    
     Next, as illustrated in  FIG. 5B , the sponge material  18   c  is pressed and heated by the pressing apparatus  30 . Thus, the surfaces of the sponge material  18   c  that come in contact with the pressing apparatus  30  are melted and the skin layers  18   a  and  18   d  are formed. The thickness and the density of the skin layers  18   a  and  18   d  to be formed can be adjusted as required by appropriately setting the pressing force, heating temperature, and heating and pressing time of an upper mold  30   a  and a lower mold  30   b  of the pressing apparatus  30 . Thus, the acoustic resistance of the acoustic resistance material  18  can be adjusted as desired. The lower density layer  18   b  is an inner portion of the sponge material  18   c  that remains unmelted. 
     Then, as illustrated in  FIG. 5C , the acoustic resistance material  18  on which the skin layers  18   a  and  18   d  are formed is taken out from the pressing apparatus  30 . 
     Finally, as illustrated in  FIG. 5D , the acoustic resistance material  18  is taken out of the pressing apparatus  30  and is provided with the center hole by drilling and its outer peripheral portion is cutoff so as to have a ring shape so that it can incorporated in the unit casing  11 . 
     Capacitor Microphone 
     A capacitor microphone according to an embodiment of the present invention is formed by incorporating the capacitor microphone unit described in any of the above-described embodiments in a microphone casing. If required, the microphone casing may be provided with a connector to which a microphone cable is connected. 
     The acoustic resistance of the capacitor microphone according to the present embodiment can be readily controlled by forming a skin layer on a surface of the acoustic resistance material formed of a sponge and adjusting the density and the thickness of the skin layer. 
     In the above-described embodiments, the acoustic resistance material  18  is formed of a sponge made of porous urethane resin material. The present invention is not limited thereto and various modifications can be made. 
     For example, other porous resin material can be used instead of the porous urethane resin material. Alternatively, a nonwoven cloth made of resin can be used instead of a porous material. 
     Furthermore, the second acoustic resistance material  17  in the above-described second embodiment can be nonwoven cloth made of cotton or resin instead of the nylon mesh.