Patent Publication Number: US-6904155-B2

Title: Electret capacitor microphone

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This present invention relates to an electret capacitor microphone, particularly to a construction for permitting its surface mounting. 
   2. Background Art 
   Generally, an electret capacitor microphone is constructed such that a cylindrical metal case accommodates an electret capacitor section in which a diaphragm and a backplate are disposed to oppose each other, an impedance conversion element for converting a change in the electrostatic capacity of the electret capacitor section into an electric impedance, and a substrate on which this impedance conversion element is mounted. 
   This electret capacitor microphone is provided with a plurality of terminal members projecting from the substrate in the form of pins which are electrically conductive with the impedance conversion element. It is therefore structurally difficult to surface mount the electret capacitor microphone on an external substrate (e.g., a printed circuit board of a portable telephone or the like). 
   Accordingly, as described in JP-A-8-237797 for example, a measure is devised for surface mounting the electret capacitor microphone on an external substrate. In this measure, the electret capacitor microphone is fitted to a holder having contact pieces for surface mounting and is surface mounted on the external substrate via the holder. 
   However, with the above-described conventional electret capacitor microphone, it is necessary to interpose the holder at the time of surface mounting it on the external substrate. Accordingly, there occur problems such that an extra part becomes required and that the overall thickness becomes fairly large when the surface mounting is performed. 
   SUMMARY OF THE INVENTION 
   The present invention has been conceived in view of the above-described circumstances, and has its objective to provide an electret capacitor microphone which can be surface mounted on an external substrate, while being made thin with a small number of component parts. 
   The invention attains the above objective by forming the conventional substrate and terminal members in anew arrangement. 
   The invention provides an electret capacitor microphone, which includes: an electret capacitor section comprising a diaphragm and a backplate positioned opposite the diaphragm; an impedance conversion element for converting a change in the electrostatic capacity of the electret capacitor section into an electric impedance; and a case for accommodating the electret capacitor section and the impedance conversion element. Part of the case comprises a synthetic resin-made base member formed integrally with a plurality of terminal members by insert molding. One end of the terminal member is exposed on an inner surface of the base member so as to form part of a conductive pattern. The other end of the terminal member is exposed on an outer surface of the base member as an external connection terminal portion. The impedance conversion element is mounted on the base member at a predetermined position in the conductive pattern. 
   The invention also provides an electret capacitor microphone, which includes: an electret capacitor section comprising a diaphragm and a backplate positioned opposite the diaphragm; an impedance conversion element for converting a change in the electrostatic capacity of the electret capacitor section into an electric impedance; and a case for accommodating the electret capacitor section and the impedance conversion element. Part of the case comprises a synthetic resin-made base member formed integrally with a plurality of terminal members by MID molding. One end of the terminal member is exposed on an inner surface of the base member so as to form part of a conductive pattern. The other end of the terminal member is exposed on an outer surface of the base member as an external connection terminal portion. The impedance conversion element is mounted on the base member at a predetermined position in the conductive pattern. 
   The above-described “electret capacitor microphone” may be a foil electret-type electret capacitor microphone in which the diaphragm is provided with the function of an electret, or may be a back electret-type electret capacitor microphone in which a backplate is provided with the function of an electret. 
   This “electret capacitor microphone” may be constructed such that only the impedance conversion element is accommodated in the case as an electronic component, or may be constructed such that another electronic component such as a capacitor may be accommodated in addition to the impedance conversion element. 
   The aforementioned “impedance conversion element” is not limited to a specific element insofar as it is capable of converting a change in the electrostatic capacity of the capacitor section into an electric impedance. For example, it is possible to adopt a field effect transistor (FET) or the like. 
   As for portions other than the aforementioned “base member” in the aforementioned “case,” their materials, shapes, and other specific arrangements are not particularly limited. 
   As for the aforementioned “conductive pattern,” its specific shape of the pattern is not particularly limited insofar as it is capable of being formed on the inner surface of the base member. 
   As for the aforementioned “external connection terminal portion,” its specific shapes, layout, and the like are not particularly limited insofar as they are exposed on the outer surfaces of the base member. 
   The aforementioned “MID molding” means a molding process for manufacturing molded interconnection devices (MID&#39;s). The “MID” referred to herein means a three-dimensional molded circuit device in which a three-dimensional circuit or pattern is formed on a resin molded device having a three-dimensional configuration. 
   According to the invention as described above, part of the case, which accommodates the electret capacitor section and the impedance conversion element, is a synthetic resin-made base member formed integrally with a plurality of terminal members by insert molding or MID molding. The terminal member has one end exposed on the inner surface of the base member so as to form part of the conductive pattern, while the other end is exposed on the outer surface of the base member as an external connection terminal portion. Further, the impedance conversion element is mounted on the base member at a predetermined position in the conductive pattern. Accordingly, while the base member is provided with the function of the conventional substrate, the shape and layout of the external connection terminal portion can be easily set arbitrarily. Thus, a suitable shape and layout of the external connection terminal portion would enable the electret capacitor microphone to be directly surface mounted on the external substrate without use of the conventional holder. 
   Therefore, according to the invention, it becomes possible to surface mount the electret capacitor microphone on the external substrate with a small number of component parts while making the electret capacitor microphone compact. 
   Moreover, with the conventional electret capacitor microphone, the case is generally made of metal and is electrically conductive with a grounding terminal. Then, it is necessary to space the case apart from the external substrate. In contrast, with the electret capacitor microphone according to the invention, since the base member is made of synthetic resin, it is unnecessary to space the base member apart from the external substrate. For this reason, the outer surface of the base member can be formed flush with the respective external connection terminal portions. Therefore, it becomes possible to further reduce the thickness when the electret capacitor microphone is surface mounted on the external substrate. 
   According to the invention, it is possible to provide post processing such as the cutting out of part of the conductive pattern after insert molding, with the result that the degree of freedom can be enhanced for the layout of the conductive pattern. 
   It should be noted that the electret capacitor microphone according to the invention is not necessarily surface mounted on the external substrate. It may be inserted into and mounted on the external substrate by forming the external connection terminal portions in the shape of pins, for example. 
   According to the invention, a through hole may be formed in a predetermined position of the base member, and the conductive pattern may be divided at the position where the through hole is formed. Then, it becomes possible to make part of the conductive pattern as an electrical island which is electrically separated from the other part of the conductive pattern, while maintaining their positional relationship. 
   On the other hand, according to the electret capacitor microphone in the invention, the plurality of terminal members may be formed integrally with the base member by MID molding, and the conductive pattern is formed by surface treatment such as plating or printing. Therefore, part of the conductive pattern can be formed in the shape of an island at the time of MID molding. 
   According to the invention, the electret capacitor section may be covered with a cylindrical metal cover. Then, it is possible to handle these members as one unit, thereby making the process of manufacturing the electret capacitor microphone simple. 
   If the case is comprised of the base member and a synthetic resin-made housing member fixed to the base member, the metal cover is covered by the housing member. In this case, even if heat is applied from outside, it is possible to make the heat difficult to be transmitted to the metal cover by virtue of the heat buffering function of the housing member made of synthetic resin. Hence, it is possible to suppress the temperature rise of the electret capacitor section. Accordingly, even in the case where the surface mounting on the external substrate is effected by reflow processing, it is possible to effectively suppress loss or decrease of the charge accumulated in the electret of the electret capacitor section due to the heat applied. 
   As for the metal cover which covers the electret capacitor section, the external shape is likely to be a substantially circular cylindrical shape. It is, however, preferable to set the external shape substantially in the shape of a rectangular parallelepiped, so that the electret capacitor microphone can be positioned easily at the time of surface mounting. At this time, if recessed spaces communicating with the internal space of the base member are formed in the respective corner portions of the housing member, it is possible to enlarge the back pressure space of the electret capacitor section by these recessed spaces, thereby making it possible to improve the sensitivity of the electret capacitor microphone. Furthermore, these recessed spaces can be utilized as thickness reducing spaces to prevent a surface sink on the housing member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side sectional view illustrating a state in which an electret capacitor microphone in accordance with an embodiment of the invention of this application is disposed upwardly; 
       FIG. 2A  is a view taken in the direction of arrow IIa in  FIG. 1 ; 
       FIG. 2B  is a view taken in the direction of arrow IIb in  FIG. 1 ; 
       FIG. 3  is an exploded side sectional view of the electret capacitor microphone; 
       FIGS. 4A  to  4 C is an exploded plan view of the electret capacitor microphone; 
       FIG. 5  is a sectional view taken along line V—V of  FIG. 3 , and illustrates a base member in detail; 
       FIGS. 6A  to  6 C is a process diagram illustrating the process of manufacturing and assembling the base member and its accessories; and 
       FIG. 7  is a diagram similar to FIG.  5  and illustrates a modification of the embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings, a description will be given of an embodiment of the present invention. 
     FIG. 1  is a side sectional view illustrating a state in which an electret capacitor microphone in accordance with the embodiment is disposed upwardly. In addition,  FIG. 2A  is a view taken in the direction of arrow IIa in  FIG. 1 , and  FIG. 2B  is a view taken in the direction of arrow IIb in FIG.  1 . Further,  FIGS. 3 and 4  are an exploded side sectional view and an exploded plan view of the electret capacitor microphone. 
   As shown in these drawings, an electret capacitor microphone  10  in accordance with this embodiment is a compact microphone which has a substantially square outer configuration whose one side is 4.5 mm or thereabouts in a plan view and which is about 1.8 mm high. Accommodated within a case  12  are an electret capacitor unit  14 , an FET  16  (impedance conversion element), two capacitors  18  and  20 , a coil spring  22 , and a contact frame  24 . 
   As for the electret capacitor unit  14 , a diaphragm subassembly  34 , an insulating ring  36 , a spacer  38 , a backplate  40 , and an insulating bush  42  are accommodated in a cylindrical metal cover  32  extending vertically and having a low height. 
   The metal cover  32  has a sound hole  32   a  formed in an upper end wall thereof. Its open lower end portion  32   b  is fixed to the insulating bush  42  by caulking. 
   The diaphragm subassembly  34  has a diaphragm  34 A stretched underneath and fixed to a lower surface of a diaphragm supporting ring  34 B. The diaphragm  34 A is so arranged that a metal vapor-deposited film of nickel or the like is formed on an upper surface of a circular film made of synthetic resin (e.g., polyphenylene sulfide (PPS)), and is electrically conductive with the diaphragm supporting ring  34 B. A vent hole  34   a  is formed in a central portion thereof. The diaphragm supporting ring  34 B is formed of a metallic ring member having an outside diameter substantially equal to the inside diameter of the metal cover  32 . 
   The insulating ring  36  is a ring member having an outside diameter substantially equal to the inside diameter of the metal cover  32 , and insulation treatment (alumite coating) is provided on an aluminum surface. 
   The spacer  38  is formed of a thin sheet ring made of synthetic resin (e.g., PPS) and having an outside diameter substantially equal to the inside diameter of the insulating ring  36 . 
   The backplate  40  is comprised of a stainless steel-made backplate body  40 A and an electret  40 B made of synthetic resin (e.g., fluorinated ethylene propylene (FEP)) thermally welded (laminated) on an upper surface of this backplate body  40 A, and a plurality of through holes  40   a  are formed therein. The electret  40 B is provided with polarization treatment to allow a predetermined surface potential (e.g., −125 V or thereabouts) to be obtained. 
   Inside the metal cover  32 , the diaphragm  34 A and the electret  40 B are opposed to each other with a predetermined very small interval with the spacer  38  disposed therebetween, thereby forming a capacitor section C. 
   The insulating bush  42  is a synthetic resin molding (e.g., a liquid crystal polymer (LCP) molding), and is formed by a ring member having an outside diameter substantially equal to the inside diameter of the insulating ring  36 . 
   As for the case  12 , a base member  52  which is upwardly open and is made of synthetic resin (e.g., LCP) and a housing member  54  which is downwardly open and is made of synthetic resin (e.g., LCP) are fixed to each other by ultrasonic welding (which will be described later). 
     FIG. 5  is a sectional view taken along line V—V of  FIG. 3 , and illustrates the base member  52  in detail.  FIG. 6  is a process diagram illustrating the process of manufacturing and assembling the base member  52  and its accessories. In  FIG. 3  (and in FIG.  1 ), the base member  52  is shown by a section taken along line III—III of FIG.  4 C. 
   As shown in these drawings as well, the base member  52  is comprised of a substantially square bottom wall portion  52 A and a peripheral wall portion  52 B extending upward from an outer peripheral edge of this bottom wall portion  52 A, and is formed integrally with four terminal members  56 A,  56 B,  56 C, and  56 D by insert molding. These four terminal members  56 A,  56 B,  56 C, and  56 D are formed as inserts by subjecting a strip-shaped conductive member to blanking and bending. 
   One end portions of these terminal members  56 A,  56 B,  56 C, and  56 D are exposed on an inner surface (upper surface) of the bottom wall portion  52 A as four land portions  56 Aa,  56 Ba,  56 Ca, and  56 Da which constitute portions of an electrically conductive pattern P. Meanwhile, the other end portions of the terminal members  56 A,  56 B,  56 C, and  56 D are exposed on an outer surface of the bottom wall portion  52 A as four external connection terminal portions  56 Ab,  56 Bb,  56 Cb, and  56 Db. These external connection terminal portions  56 Ab,  56 Bb,  56 Cb, and  56 Db are formed in L-shapes in such a manner as to extend along the lower surface of the bottom wall portion  52 A and to be bent and extend along the outer surface of the peripheral wall portion  52 B in the vicinities of the respective corners of the bottom wall portion  52 A. At that time, with respect to the bottom wall portion  52 A, the external connection terminal portions  56 Ab,  56 Bb,  56 Cb, and  56 Db are formed flush with the lower surface of the bottom wall portion  52 A by insert molding, while, with respect to the peripheral wall portion  52 B, they are formed in such a manner as to project by their thickness from the outer surface of the peripheral wall portion  52 B by cutting and bending after insert molding. 
   Of the four terminal members  56 A,  56 B,  56 C, and  56 D, the terminal member  56 A is an output terminal which is connected to a power supply through a load resistor when it is mounted on an external substrate. The terminal member  56 B is a grounding terminal, and the remaining two terminal members  56 C and  56 D are dummy terminals. 
   A plurality of cavity portions  52   a  are formed in the bottom wall portion  52 A of the base member  52  by insert support pins at the time of insert molding, and one of these cavity portions  52   a  is formed on the lower side of the electrically conductive pattern P, as shown in FIG.  6 A. Further, as a pin is inserted in this cavity portion  52   a  after insert molding in such a manner as to pierce the electrically conductive pattern P from above (or by such as the application of a laser beam), thereby forming a through hole  52   b  to divide the electrically conductive pattern P, as shown in FIG.  6 B. In consequence, another land portion  58  which is electrically separated from the land portion  56 Aa is formed on the inner surface of the bottom wall portion  52 A of the base member  52 . 
   The FET  16  and the capacitors  18  and  20  are mounted on the base member  52  at predetermined positions of the electrically conductive pattern P. 
   The FET  16  is an element for converting a change in the electrostatic capacity of the electret capacitor section C into an electric impedance, and is mounted such that its drain electrode D conducts with the land portion  56 Aa of the terminal member  56 A, its source electrode S conducts with the land portion  56 Ba of the terminal member  56 B, and its gate electrode G conducts with the land portion  58 . In addition, the capacitors  18  and  20  are two kinds of capacitors having different electrostatic capacities and provided to eliminate noise, and are mounted in parallel in such a manner as to straddle the land portion  56 Aa of the terminal member  56 A and the land portion  56 Ba of the terminal member  56 B. 
   A spring loading boss  52   c  projecting upward at the position where the land portion  58  is formed is formed on the inner surface of the bottom wall portion  52 A of the base member  52 . The coil spring  22  is loaded on this spring loading boss  52   c . This coil spring  22  is made of metal. When the electret capacitor microphone  10  is assembled, the coil spring  22  is compressively and resiliently deformed in a state in which each end portion thereof abuts against the land portion  58  or the backplate body  40 A. As a result, the gate electrode G of the FET  16  is made conductive with the backplate body  40 A through the land portion  58  and the coil spring  22 . 
   The contact frame  24  is formed by blanking a stainless steel sheet substantially into an L-shape and bending a portion thereof, and three terminal contact pieces  24   a ,  24   b , and  24   c  projecting diagonally downward are formed at three portions thereof. This contact frame  24  has an external shape which is substantially identical to the shape of inner surface of the peripheral wall portion  52 B of the base member  52 . When the contact frame  24  is fitted inside the base member  52 , its terminal contact pieces  24   a ,  24   b , and  24   c  are brought into contact with the land portions  56 Ba,  56 Ca, and  56 Da of the terminal members  56 B,  56 C, and  56 D. 
   Further, this contact frame  24  is arranged such that when the electret capacitor microphone  10  is assembled, this contact frame  24 , its terminal contact pieces  24   a ,  24   b , and  24   c  are slightly flexurally deformed by coming into contact with the metal cover  32  of the electret capacitor unit  14 . As a result, the source electrode S of the FET  16  is made conductive with the diaphragm  34 A through the land portion  56 Ba of the terminal member  56 B, the contact frame  24 , the metal cover  32 , and the diaphragm supporting ring  34 B, and is also made conductive with the land portions  56 Ca and  56 Da of the terminal members  56 C and  56 D, thereby making it possible for these terminal members  56 C and  56 D to be used as the grounding terminals. 
   A shallow circular recess  52   d  having an inside diameter substantially equal to the outside diameter of the electret capacitor unit  14  is formed in the outer surface (lower surface) of the bottom wall portion  52 A of the base member  52 . A metallic shield plate  60  which is thinner than the depth of that circular recess  52   d  is bonded and fixed to the circular recess  52   d.    
   The housing member  54  has a top wall portion  54 A whose shape is identical to that of the bottom wall portion  52 A of the base member  52 , a peripheral wall portion  54 B extending downward from an outer peripheral edge of this top wall portion  54 A, and an annular wall portion  54 C extending downward from the top wall portion  54 A in such a manner as to surround the electret capacitor unit  14 . A plurality of sound releasing holes  54   a  are formed in this housing member  54 . Recessed spaces  54   b  communicating with the internal space of the base member  52  are formed in the respective corner portions of this housing member  54  by the peripheral wall portion  54 B and the annular wall portion  54 C. 
   The ultrasonic welding of the base member  52  and the housing member  54  is performed in the following manner. 
   As shown in  FIG. 3 , the peripheral wall portion  52 B of the base member  52  has an upper end face  52   e  having the substantially pyramidical shape over the entire periphery. Meanwhile, the peripheral wall portion  54 B of the housing member  54  has a lower end face  54   c  formed flat over the entire periphery. As ultrasonic vibrations are imparted to the upper end face  52   e  of the peripheral wall portion  52 B and the lower end face  54   c  of the peripheral wall portion  54 B in a state that they are brought into surface contact with each other over their entire peripheries. Whereby, portions of the peripheral wall portion  52 B located in the vicinities of their upper end face are primarily deformed plastically. Consequently, as shown in  FIG. 1 , the upper end face  52   e  of the peripheral wall portion  52 B and the lower end face  54   c  of the peripheral wall portion  54 B are welded and fixed over their entire peripheries. 
   As described above in detail, as for the electret capacitor microphone  10  in accordance with this embodiment, part of the case  12 , which accommodates the electret capacitor section C, the FET  16 , and the capacitors  18  and  20 , comprises the synthetic resin-made base member  52  formed integrally with the plurality of terminal members  56 A,  56 B,  56 C, and  56 D by insert molding. As for the terminal members  56 A,  56 B,  56 C, and  56 D, their one end portions are exposed on the inner surface of the bottom wall portion  52 A of the base member  52  as the land portions  56 Aa,  56 Ba,  56 Ca, and  56 Da which form part of the electrically conductive pattern P, while their other end portions are exposed on the outer surface of the bottom wall portion  52 A of the base member  52  as the external connection terminal portions  56 Ab,  56 Bb,  56 Cb, and  56 Db. Further, the FET  16  and the capacitors  18  and  20  are mounted on the base member  52  at predetermined positions on the electrically conductive pattern P. Accordingly, it is readily possible to set the shapes and layout of the external connection terminal portions  56 Ab,  56 Bb,  56 Cb, and  56 Db arbitrarily, while providing the base member  52  with the function of the conventional substrate. 
   Further, since the external connection terminal portions  56 Ab,  56 Bb,  56 Cb, and  56 Db in this embodiment are formed in the shape of plates in the respective corner portions of the base member  52 , they are suitable for surface mounting on the external substrate. Specifically, the surface mounting on the external substrate can be effected stably. Consequently, it becomes possible to directly surface mount the electret capacitor microphone  10  on the external substrate without the interposition of the holder in the conventional manner. 
   Therefore, in accordance with this embodiment, it becomes possible to surface mount the electret capacitor microphone  10  on the external substrate with a small number of component parts while making the electret capacitor microphone  10  compact. 
   Particularly in this embodiment, by virtue of the presence of the contact frame  24 , not only the terminal member  56 B but the terminal members  56 C and  56 D can be used as grounding terminals, so that the surface mounting on the external substrate can be performed more easily. 
   Since the electret capacitor microphone  10  in this embodiment is provided with the base member  52  made of synthetic resin, it is unnecessary to space the base member  52  apart from the external substrate. Further, the external connection terminal portions  56 Ab,  56 Bb,  56 Cb, and  56 Db are formed flush with the lower surface of the bottom wall portion  52 A at the corner portions of the base member  52 . Therefore, it becomes possible to further reduce the thickness when the electret capacitor microphone  10  is surface mounted on the external substrate. 
   In addition, in this embodiment, since the through hole  52   b  is formed in a predetermined position of the bottom wall portion  52 A of the base member  52 , and the electrically conductive pattern P is divided at the position where the through hole  52   b  is formed, part of the electrically conductive pattern P formed integrally at the time of insert molding can be electrically separated and formed in the shape of an island while its positional relationship with the other part of the electrically conductive pattern P is maintained. Namely, in this embodiment, the land portion  58  which is made conductive with the gate electrode G of the FET  16  can be formed in the shape of an island on the inner surface of the bottom wall portion  52 A of the base member  52  while its positional relationship with the land portion  56 Aa of the terminal member  56 A, which is made conductive with the drain electrode D of the FET  16 , is maintained. 
   Further, in this embodiment, the electret capacitor section C is covered with the cylindrical metal cover  32 , and is formed as the electret capacitor unit  14 , thereby making the process of manufacturing the electret capacitor microphone  10  simple. 
   Moreover, the case  12  is comprised of the base member  52  and the synthetic resin-made housing member  54  fixed thereto. It is possible to make the heat difficult to be transmitted to the metal cover  32 , even in a case where heat is applied from outside, due to the heat buffering characteristic of the housing member  54  covering the metal cover  32 . Hence, it is possible to suppress the temperature rise of the electret capacitor section C. Accordingly, even in a case where the surface mounting on the external substrate is effected by reflow processing, it is possible to effectively suppress loss or decrease of the charge accumulated in the electret  40 B of the electret capacitor section C due to the heat applied thereto at the time of reflow processing. 
   In this embodiment, since the fixation of the base member  52  and the housing member  54  is effected by ultrasonic welding over their entire peripheries, it is possible to enhance the sealing capabilities of the two members. In addition, since this makes it unnecessary to use an adhesive agent, it eliminates the possibility of generation of gas from the adhesive agent at the time of reflow processing. It therefore eliminates the possibility of loss or decrease of the charge stored in the electret  40 B of the electret capacitor section C due to accumulated gas in the case  12 . 
   Further, the upper end face  52   e  of the peripheral wall portion  52 B of the base member  52  is formed in the shape of substantially pyramidical surface, it is possible to concentrate the energy of ultrasonic vibration on the contact surface of the peripheral wall portion  52 B and the peripheral wall portion  54 B of the housing member  54 , thereby making it possible to easily effect the ultrasonic welding with respect to the housing member  54 . Moreover, since this ultrasonic welding is effected in a state in which the upper end face  52   e  of the peripheral wall portion  52 B and the lower end face  54   c  of the peripheral wall portion  54 B are brought into surface contact with each other over their entire peripheries, it becomes possible to impart transverse ultrasonic vibrations in the direction parallel to the contact surface. Further, by adopting the transverse vibration, it is possible to suppress the effect of the vibration on the component parts (FET  16  and capacitors  18  and  20 ) mounted on the base member  52 . 
   It should be noted that the upper end face  52   e  having the shape of substantially pyramidical surface may be intermittently formed at a plurality of portions of the peripheral wall portion  52 B at predetermined intervals. In such a case as well, the base member  52  and the housing member  54  can be finally fixed by ultrasonic welding over their entire peripheries, so that it is possible to sufficiently ensure the sealing capabilities of the two members. 
   In addition, instead of forming the upper end face  52   e  of the peripheral wall portion  52 B in the base member  52  in the shape of substantially pyramidical surface, the lower end face  54   c  of the peripheral wall portion  54 B in the housing member  54  may be formed in the shape of substantially pyramidical surface. In such a case as well, it is possible to obtain operational advantages similar to those of this embodiment. 
   In this embodiment, since the external shape of the case  12  is set substantially in the shape of a rectangular parallelepiped, the electret capacitor microphone  10  can be positioned easily when it is mounted on the external substrate. Moreover, since the recessed spaces  54   b  communicating with the internal space of the base member  52  are formed in the respective corner portions of the housing member  54 , it is possible to enlarge the back pressure space of the electret capacitor section C by these recessed spaces  54   b , thereby making it possible to improve the sensitivity of the electret capacitor microphone  10 . In addition, the recessed spaces  54   b  can be utilized as thickness reducing spaces to prevent a surface sink on the housing member. 
   Next, a description will be given of a modification of the above-described embodiment. 
     FIG. 7  is a diagram similar to FIG.  5  and illustrates the modification. 
   As shown in the drawing, in this modification, four terminal members  76 A,  76 B,  76 C, and  76 D are formed integrally with a base member  72  by molded interconnection device (MID) molding. It should be noted that the other component elements of the electret capacitor microphone are constructed in the same way as in the above-described embodiment. 
   In the same way as the base member  52  of the above-described embodiment, the base member  72  is comprised of a substantially square bottom wall portion  72 A and peripheral wall portion  72 B extending upward from outer peripheral edge of this bottom wall portion  72 A. The terminal members  76 A,  76 B,  76 C, and  76 D are surface treated films of copper plating or the like. 
   One end portions of the terminal members  76 A,  76 B,  76 C, and  76 D are exposed on an inner surface (upper surface) of the bottom wall portion  72 A of the base member  72  as four land portions  76 Aa,  76 Ba,  76 Ca, and  76 Da which form portions of the electrically conductive pattern P. The other end portions thereof are exposed on an outer surface of the base member  72  as four external connection terminal portions  76 Ab,  76 Bb,  76 Cb, and  76 Db. These external connection terminal portions  76 Ab,  76 Bb,  76 Cb, and  76 Db are formed flush with the peripheral wall portion  72 B and the bottom wall portion  72 A in such a manner as to extend along the outer surfaces of the peripheral wall portion  72 B and to be bent and extend along the lower surface of the bottom wall portion  72 A. 
   To realize the construction of such terminal members  76 A,  76 B,  76 C, and  76 D, notched portions  72   a  which are notched substantially flush with the inner surface of the bottom wall portion  72 A are formed at four portions of the peripheral wall portions  72 B of the base member  72 . 
   In this modification, a land portion  78 , which is electrically separated from the land portions  76 Aa,  76 Ba,  76 Ca, and  76 Da, is also formed simultaneously on the inner surface of the base member  72 A of the base member  72  at the time of the aforementioned MID molding. 
   Also in the case where the construction of this modification is adopted, it is possible to obtain operational advantages similar to those of the above-described embodiment. Moreover, in this modification, part of the electrically conductive pattern P can be formed in the shape of an island at the time of MID molding. 
   In addition, in this modification, since the external connection terminal portions  76 Ab,  76 Bb,  76 Cb, and  76 Db of the terminal members  76 A,  76 B,  76 C, and  76 D are formed flush with the peripheral wall portion  72 B and the bottom wall portion  72 A, the electret capacitor microphone can be constructed compactly, and it is possible to reduce the occupied space when it is surface mounted on an external substrate. 
   In this modification, although the ultrasonic welding of the base member  72  and the housing member cannot be effected over the entire peripheries, it is possible to ensure the sealing capabilities of the two members if a filler material is subsequently filled in the respective notched portions  72   a.    
   Incidentally, instead of forming the terminal members  76 A,  76 B,  76 C, and  76 D in such a manner as to extend along the outer surfaces of the peripheral wall portion  72 B and to be bent and extend along the lower surface of the bottom wall portion  72 A as in this modification, an arrangement may be provided such that four through holes are formed in advance in the bottom wall portion  72 A of the base member  72 , and the other end portions of the terminal members  76 A,  76 B,  76 C, and  76 D are made to be exposed from the lower surface of the base member  72  through these through holes.