Patent Publication Number: US-8989422-B2

Title: Microphone unit and voice input device comprising same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national stage entry under 35 U.S.C. §371 of PCT International Application No. PCT/JP2011/050631, filed on Jan. 17, 2011, and claims priority to Japanese Application No. JP 2010-015199, filed on Jan. 27, 2010, the contents of which are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a microphone unit comprising a function for converting inputted sounds to electrical signals and outputting the electrical signals. The present invention also relates to a voice input device comprising such a microphone unit. 
     BACKGROUND ART 
     In conventional practice, microphone units comprising a function for converting inputted sounds to electrical signals and outputting the signals have been applied to various types of voice input devices (for example, see Patent Literature 1, 2, etc.). A voice input device is a device for converting inputted voices to electrical signals and processing the signals, and examples thereof include mobile telephones, transceivers, and other voice communication devices; voice recognition systems and other information processing systems that use techniques for analyzing inputted voices; audio recording devices; and the like. 
     In Patent Literature 2, for example, the applicants have disclosed a microphone unit that has a function for suppressing background noise and picking up only proximal sounds and that is suitable for a close-talking voice input device (e.g., a mobile telephone or the like). The microphone unit of Patent Literature 2 is configured as a bidirectional differential microphone unit, thereby achieving the function of suppressing background noise and picking up only proximal sounds. 
     LIST OF CITATIONS 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent No. 3279040 
         Patent Literature 2: Japanese Laid-open Patent Application No. 2008-258904 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     When a bidirectional microphone such as the one disclosed in Patent Literature 2 is installed in a mobile telephone, for example, the direction of satisfactory microphone sensitivity is limited, and there is therefore a limit on where the microphone unit is disposed in the mobile telephone. Such limits compress the configurational degree of freedom in the manufacture of a mobile telephone or another voice input device, and it is therefore desirable that such limits be reduced as much as possible. 
     In recent years, voice input devices have often been formed so as to be multifunctional. For example, among mobile telephones, one example of a voice input device, there are those provided with a function (hands-free function) for making a call while driving an automobile without holding the telephone in hand, in addition to the function for simply making a call with the telephone in hand. There are also recent mobile telephones provided with a function for recording video. 
     When making a call with the mobile telephone in hand, the user uses the telephone with their mouth near a microphone portion. Therefore, there is demand for the microphone unit provided to the mobile telephone to have a function for suppressing background noise and picking up only proximal sounds (a function as a close-talking microphone). When the hands-free function is used, there is demand for the microphone unit to be capable of widely picking up sounds from a forward direction. When video is recorded, there is demand for good forward-directional sensitivity so that voices from the direction of the recorded subject can be picked up. 
     To adapt to such situations, one considered possibility is to prepare a plurality of microphone units (microphone packages) having different characteristics and to install these units in a voice input device. However, in this case, a need arises to increase the surface area of the mounting substrate on which the microphone unit is mounted in the voice input device. In recent years, it has become a common requirement for mobile telephones and other voice input devices to be compact, and it is not desirable to adapt as described above to the need to enlarge the surface area of the mounting substrate on which the microphone unit is mounted. Specifically, there is demand for a compact microphone unit which is readily adapted to imparting multifunctional capability to a voice input device as a single microphone unit. 
     In view of the matters described above, an object of the present invention is to provide a high-performance microphone unit which is readily adapted to the diversity (e.g., diversity in terms of design and diversity in terms of function) of a voice input device. Another object of the present invention is to provide a high-quality voice input device comprising such a microphone unit. 
     Solution to the Problem 
     To achieve the objects described above, a microphone unit of the present invention comprises a first vibrating part for converting a sound signal to an electrical signal on the basis of vibration of a first diaphragm, a second vibrating part for converting a sound signal to an electrical signal on the basis of vibration of a second diaphragm, and a housing for accommodating the first vibrating part and the second vibrating part, the housing being provided with a first sound hole, a second sound hole, and a third sound hole; and the housing is provided with a first sound path for transmitting sound pressure inputted from the first sound hole to one surface of the first diaphragm and to one surface of the second diaphragm, a second sound path for transmitting sound pressure inputted from the second sound hole to the other surface of the first diaphragm, and a third sound path for transmitting sound pressure inputted from the third sound hole to the other surface of the second diaphragm. 
     According to the present configuration, a small microphone unit can be achieved which comprises two bidirectional differential microphones having mutually different primary axial directions of directivity (the axial directions at which sensitivity is the highest). Such a microphone unit can function as a bidirectional microphone whose primary axial direction of directivity can be controlled, due to signals outputted from two differential microphones being combined and subjected to computation processing. Therefore, the microphone unit of the present configuration has less restriction on its incorporated position when it is incorporated into a voice input device, and the microphone unit is readily adapted to the diversity of the voice input device. Since the microphone unit of the present configuration is configured comprising the bidirectional differential microphones, the microphone unit has excellent distant noise (background noise) suppression performance. 
     According to the microphone unit of the present configuration, as is described hereinafter, it is possible to provide a microphone unit comprising both a function as a bidirectional differential microphone having excellent distant noise suppression performance and a function as a unidirectional microphone having excellent sensitivity in the front surface direction, due to the use of an acoustic resistance member. 
     In the microphone unit of the configuration described above, the first sound hole and the third sound hole are formed in the same surface of the housing, and the second sound hole is formed in an opposing surface that opposes the surface in which the first sound hole and the third sound hole of the housing are formed. According to the present configuration, the two bidirectional differential microphones provided to the microphone unit can have a relationship of different primary axial directions of directivity (a relationship in which they are offset by 90°, for example). 
     The microphone unit of the configuration described above may be designed such that the housing comprises an installation part for installing the first vibrating part and the second vibrating part, and a cover for forming, together with the installation part, an accommodating space for accommodating the first vibrating part and the second vibrating part, the cover being placed over the installation part; there are formed in the installation part a first open part, a second open part, a hollow space for communicating the first open part and the second open part, and a sound hole constituting the second sound hole passing through from an installation surface on which the first vibrating part and the second vibrating part are installed to a rear surface thereof; there are formed in the cover the first sound hole, the third sound hole, and a concave space communicating with the first sound hole and forming the accommodating space; the first vibrating part is disposed in the installation part so as to obscure the second sound hole; the second vibrating part is disposed in the installation part so as to obscure the first open part; the first sound path is formed using the first sound hole and the accommodating space; the second sound path is formed using the second sound hole; and the third sound path is formed using the third sound hole, the second open part, the hollow space, and the first open part. 
     According to the present configuration, in a microphone unit readily adapted to the diversity of a voice input device, a configuration in which the housing is composed of numerous components can be avoided, and the microphone unit is easily made smaller and thinner. 
     The microphone unit of the configuration described above may be configured so that the installation part comprises a base provided with a groove part and a base open part, and a microphone substrate stacked on the base, the first vibrating part and the second vibrating part being mounted on the opposite surface of the surface that faces the base; wherein there are formed in the microphone substrate a first substrate open part constituting the first open part, a second substrate open part constituting the second open part, and a third substrate open part which together with the base open part forms the second sound hole; and the hollow space is formed using the groove part and the surface of the microphone substrate that opposes the base. By a configuration of the installation part according to the present configuration, the hollow space in the installation part can be readily formed. 
     The microphone unit of the configuration described above may further comprise an electrical circuit part for processing electrical signals obtained from the first vibrating part and the second vibrating part, the electrical circuit part being accommodated within the housing. 
     In the microphone unit of the configuration described above, the electrical circuit part is preferably disposed so as to be present between the first vibrating part and the second vibrating part. According to the present configuration, both of the two vibrating parts can be disposed in close proximity to the electrical circuit part. Therefore, according to the microphone unit of the present configuration, the effects of electromagnetic noise are suppressed and a satisfactory signal to noise ratio (SNR) is easily ensured. 
     In the microphone unit of the configuration described above, the electrical circuit part preferably separately outputs signals corresponding to the first vibrating part and signals corresponding to the second vibrating part. With a configuration in which both signals are outputted separately as in the present configuration, computation processing using both signals can be performed and the primary axial direction of directivity can be controlled in the voice input device in which the microphone unit is applied. 
     In the microphone unit of the configuration described above, an acoustic resistance member may be disposed so as to block the second sound hole. According to the present configuration, a microphone unit can be provided which comprises both a function as a bidirectional differential microphone having excellent distant noise suppression performance and a function as a unidirectional microphone having excellent sensitivity in the front surface direction, as described above. Therefore, the microphone unit is readily adapted to the diversity (multifunctionality) of the voice input device (a mobile telephone or the like, for example) to which the microphone unit is applied. To give a specific example, a method of use is possible in which the function as a bidirectional differential microphone is used in the close-talking mode of the mobile telephone, and the function as a unidirectional microphone is used in the hands-free mode or video record mode, for example. Since the microphone unit of the present configuration comprises both these two functions, there is no need to separately install two microphone units, and a size increase of the voice input device is readily suppressed. 
     In the microphone unit configured having the above-described acoustic resistance member, the first sound hole and the third sound hole may be formed in the same surface of the housing, and the second sound hole may be formed in a surface of the housing that is opposite to the surface in which the first sound hole and the third sound hole of the housing are formed. 
     The microphone unit configured having the above-described acoustic resistance member may be designed such that the housing comprises an installation part for installing the first vibrating part and the second vibrating part, and a cover for forming, together with the installation part, an accommodating space for accommodating the first vibrating part and the second vibrating part, the cover being placed over the installation part; there are formed in the installation part a first open part, a second open part, a hollow space for communicating the first open part and the second open part, and a sound hole constituting the second sound hole passing through from an installation surface on which the first vibrating part and the second vibrating part are installed to the rear surface thereof; there are formed in the cover the first sound hole, the third sound hole, and a concave space communicating with the first sound hole and forming the accommodating space; the first vibrating part is disposed in the installation part so as to obscure the second sound hole; the second vibrating part is disposed in the installation part so as to obscure the first open part; the first sound path is formed using the first sound hole and the accommodating space; the second sound path is formed using the second sound hole; and the third sound path is formed using the third sound hole, the second open part, the hollow space, and the first open part. 
     The microphone unit configured having the above-described acoustic resistance member may be designed such that the installation part comprises a base provided with a groove part and a base open part, and a microphone substrate stacked on the base, the first vibrating part and the second vibrating part being mounted on a surface of the microphone substrate that is opposite the surface that faces the base; wherein there are formed in the microphone substrate a first substrate open part constituting the first open part, a second substrate open part constituting the second open part, and a third substrate open part which together with the base open part forms the second sound hole; and the hollow space is formed using the groove part and the surface of the microphone substrate that opposes the base. 
     The microphone unit configured having the above-described acoustic resistance member may further comprise an electrical circuit part for processing electrical signals obtained from the first vibrating part and the second vibrating part, the electrical circuit part being accommodated within the housing. 
     The microphone unit configured having the above-described acoustic resistance member may be designed such that there is provided a switching electrode for inputting a switch signal from the exterior, and the electrical circuit part includes a switch circuit for performing a switching action on the basis of the switch signal. According to the present configuration, either a signal corresponding to the first vibrating part or a signal corresponding to the second vibrating part can be selectively outputted, and both can be outputted with their outputting positions switched. 
     The microphone unit configured having the above-described acoustic resistance member may be designed such that the switch circuit performs the switching action based on the switch signal so as to output to the exterior either the signal corresponding to the first vibrating part or the signal corresponding to the second vibrating part. According to the present configuration, a switch circuit for selecting which of the two signals to use need not be provided to the voice input device to which the microphone unit is applied. 
     The microphone unit configured having the above-described acoustic resistance member may be designed such that the electrical circuit part separately outputs a signal corresponding to the first vibrating part and a signal corresponding to the second vibrating part. When a configuration is used in which the two signals are outputted separately as in the present configuration, switching control of the directional characteristics can be performed in the voice input device to which the microphone unit is applied. 
     To achieve the objects described above, the present invention is characterized in being a voice input device comprising the microphone unit of the configuration described above. 
     According to the present configuration, since the configuration comprises a microphone unit that is readily adapted to the diversity of the voice input device, there is a higher degree of freedom in the design (configuration) of the voice input device, and a high-quality voice input device is easily provided. 
     In the voice input device of the configuration described above, the microphone unit may be provided so as to separately output signals corresponding to the first vibrating part and signals corresponding to the second vibrating part, and the voice input device may further comprise a voice signal processor for combining and performing computation processing on signals corresponding to the first vibrating part and signals corresponding to the second vibrating part, which are outputted from the microphone unit. It is thereby possible to provide a voice input device which controls the primary axial direction of directivity of a close-talking microphone having the effect of suppressing background noise so as to face a close-talking speaker, for example. Specifically, it is possible to provide a voice input device which can with good sensitivity acquire the voice of the speaker. 
     Advantageous Effects of the Invention 
     As described above, according to the present invention, a high-performance and compact microphone unit can be provided which is readily adapted to the diversity (e.g., the diversity of the design or the diversity of functions) of a voice input device. Also according to the present invention, a high-quality voice input device can be provided which comprises such a microphone unit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  A schematic perspective view showing the outer configuration of the microphone unit of the first embodiment. 
         FIG. 2  An exploded perspective view showing the configuration of the microphone unit of the first embodiment. 
         FIG. 3A  A schematic plan view of a cover constituting the microphone unit of the first embodiment as seen from above. 
         FIG. 3B  A schematic plan view of a microphone substrate constituting the microphone unit of the first embodiment as seen from above, on which MEMS chips and an ASIC are installed. 
         FIG. 3C  A schematic plan view of a base constituting the microphone unit of the first embodiment as seen from above. 
         FIG. 4  A schematic cross-sectional view in the position A-A of  FIG. 1 . 
         FIG. 5  A schematic cross-sectional view showing the configuration of a MEMS chip provided to the microphone unit of the first embodiment. 
         FIG. 6  A block diagram showing the configuration of the microphone unit of the first embodiment. 
         FIG. 7  A graph showing the relationship between sound pressure P and the distance R from the sound source. 
         FIG. 8  A drawing for describing the directional characteristics of a differential microphone configured from a first MEMS chip, and the directional characteristics of a differential microphone configured from a second MEMS chip. 
         FIG. 9  A block diagram showing the configuration of a voice input device comprising the microphone unit of the first embodiment. 
         FIG. 10  A drawing showing the manner in which varying the variable (k) of the computation process performed by the voice signal processor causes fluctuation of the primary axial direction of directivity of the microphone unit functioning as a bidirectional microphone. 
         FIG. 11  A drawing showing the schematic configuration of an embodiment of a mobile telephone to which the microphone unit of the first embodiment is applied. 
         FIG. 12  A schematic cross-sectional view in position B-B of  FIG. 11 . 
         FIG. 13  A schematic cross-sectional view showing the configuration of the microphone unit of the second embodiment. 
         FIG. 14  A block diagram showing the configuration of the microphone unit of the second embodiment. 
         FIG. 15  A schematic plan view of the microphone substrate provided to the microphone unit of the second embodiment as seen from above. 
         FIG. 16A  A drawing for describing the directional characteristics of the microphone unit of the second embodiment. 
         FIG. 16B  A drawing for describing the directional characteristics of the microphone unit of the second embodiment. 
         FIG. 17  A block diagram for describing a modification of the microphone unit of the second embodiment. 
         FIG. 18  A drawing for describing a modification of the microphone unit of the second embodiment; a schematic plan view of the microphone substrate as seen from above. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the microphone unit and a voice input device to which the present invention is applied are described hereinbelow in detail with reference to the drawings. 
     First Embodiment 
     First, the first embodiment of the microphone unit and the voice input device to which the present invention is applied will be described. 
     (Microphone Unit of First Embodiment) 
       FIG. 1  is a schematic perspective view showing the outer configuration of the microphone unit of the first embodiment.  FIG. 2  is an exploded perspective view showing the configuration of the microphone unit of the first embodiment.  FIG. 3A  is a schematic plan view of a cover constituting the microphone unit of the first embodiment as seen from above.  FIG. 3B  is a schematic plan view of a microphone substrate on which are installed a micro-electro-mechanical system (MEMS) chip and an application-specific integrated circuit (ASIC) constituting the microphone unit of the first embodiment as seen from above.  FIG. 3C  is a schematic plan view of a base constituting the microphone unit of the first embodiment as seen from above.  FIG. 4  is a schematic cross-sectional view in the position A-A of  FIG. 1 .  FIG. 5  is a schematic cross-sectional view showing the configuration of the MEMS chip provided to the microphone unit of the first embodiment.  FIG. 6  is a block diagram showing the configuration of the microphone unit of the first embodiment. The configuration of a microphone unit  1  of the first embodiment shall be described with reference to these drawings. 
     The microphone unit  1  of the first embodiment as shown in  FIGS. 1 through 4  has in general a configuration comprising a base  11 , a microphone substrate  12  stacked on the base  11 , and a cover  13  placed over the top surface  12   a  (the surface opposite the surface facing the base  11 ) side of the microphone substrate  12 . 
     The base  11  is composed of a plate-shaped member having a substantially rectangular shape in plan view as shown in  FIGS. 2 and 3C , for example. A groove part  111  having a substantial T shape in plan view is formed near one end in the longitudinal direction of the base  11 , in the top surface  11   a  side thereof. A base open part  112  composed of a through hole having a substantially circular shape in plan view is formed in a position offset from the middle of the base  11  toward the other end in the longitudinal direction. The base  11  may be formed using FR-4, a BT resin, or another glass epoxy-based substrate material, for example, and may be obtained by resin molding using a liquid crystal polymer (LCP), polyphenylene sulfide (PPS), or another resin, for example. In cases in which the base  11  is formed from FR-4 or another substrate material, the groove part  111  and the base open part  112  are preferably formed by mechanical working using a router or drill, for example. 
     The base  11  may be formed in two layers, one layer being formed as a substrate in which only a hole constituting the base open part  112  is formed, the other layer being formed as a substrate in which holes constituting the base open part  112  and the groove part  111  are formed, and the base  11  being configured by attaching the two layers together. In this case, since both layers are configured having through holes, the holes can be formed by hole perforation working by punching, and manufacturing efficiency can be greatly improved. 
     The microphone substrate  12  is formed into a substantially rectangular shape in plan view as shown in  FIGS. 2 and 3B , for example, and the size of the plate-shaped surface thereof (the top surface  12   a ) is substantially the same as the size of the plate-shaped surface (the top surface  11   a ) of the base  11 . Three substrate open parts  121 ,  122 ,  123  aligned in the longitudinal direction are formed in the microphone substrate  12  by mechanical working, for example, as shown in  FIG. 2 . 
     The first substrate open part  121 , which is formed in a position offset from the middle of the microphone substrate  12  toward one end in the longitudinal direction (toward the left in  FIG. 3B ), is composed of a through hole having a substantially circular shape in plan view. When the microphone substrate  12  is stacked on the base  11 , the position of the first substrate open part  121  is set so as to overlap part of the groove part  111  formed in the base  11  (to be more accurate, a part of the portion that extends parallel to the longitudinal direction of the base  11 ). The second substrate open part  122 , which is formed near one end of the microphone substrate  12  in the longitudinal direction (the left end in  FIG. 3B ), is composed of a through-hole having a substantially rectangular shape in plan view, whose longitudinal direction is the transverse direction of the microphone substrate  12  (the up-down direction in  FIG. 3B ). The position of the second substrate open part  122  is set so as to overlap with the transverse direction-extending portion of the groove part  111  formed in the base  11 . The third substrate open part  123 , which is formed in a position offset from the middle of the microphone substrate  12  toward the other end in the longitudinal direction (the right end in  FIG. 3B ), is composed of a through hole having a substantially circular shape in plan view. The position of this third substrate open part  123  is set so as to overlap with the base open part  112  formed in the base  11  when the microphone substrate  12  is stacked on the base  11 . 
     The material constituting the microphone substrate  12  is not particularly limited, but a conventionally known material is preferably used as the substrate material, e.g., FR-4, a ceramic, a polyimide film, or the like is used. 
     Installed on the top surface  12   a  of the microphone substrate  12  are a first MEMS chip  14 , a second MEMS chip  15 , and an ASIC  16 , as shown in  FIGS. 3B and 4 . The configurations of the MEMS chips  14 ,  15  and the ASIC  16  installed on the microphone substrate  12  are described herein. 
     The first MEMS chip  14  and the second MEMS chip  15  are both composed of silicon chips and both have the same configuration. Therefore, the configuration of the MEMS chips is described using the first MEMS chip  14  as an example. In  FIG. 5 , the symbols in parentheses are symbols corresponding to the second MEMS chip  15 . 
     The first MEMS chip  14  is configured by stacking an insulating first base substrate  141 , a first diaphragm  142 , a first insulating layer  143 , and a first fixed electrode  144 , as shown in  FIG. 5 . An opening  141   a  having a substantially circular shape in plan view is formed in the first base substrate  141 . The first diaphragm  142  provided on top of the first base substrate  141  is a thin film which vibrates in response to sound pressure (vibrates in the up-down direction in  FIG. 5 ), and is electrically conductive. 
     The first insulating layer  143  is provided so as to be disposed creating a gap Gp between the first diaphragm  142  and the first fixed electrode  144 , and a through-hole  143   a  having a substantially circular shape in plan view is formed in the middle thereof. The first fixed electrode  144  disposed on top of the first insulating layer  143  is disposed facing the first diaphragm  142  while being substantially parallel to the first diaphragm  142 , and capacitor capacitance is formed between the first diaphragm  142  and the first fixed electrode  144 . A plurality of through-holes  144   a  are formed in the first fixed electrode  144  so that acoustic waves can pass through, and acoustic waves coming from the top side of the first diaphragm  142  reach the top surface  142   a  of the first diaphragm  142 . 
     Thus, in the first MEMS chip  14  configured as a capacitor-type microphone, when the first diaphragm  142  is made to vibrate by the arrival of acoustic waves, the electrostatic capacitance between the first diaphragm  142  and the first fixed electrode  144  changes. As a result, the acoustic waves (acoustic signals) incident on the first MEMS chip  14  are extracted as electrical signals. Similarly, the second MEMS chip  15  comprises a second base substrate  151 , a second diaphragm  152 , a second insulating layer  153 , and a second fixed electrode  154 , and acoustic waves (acoustic signals) incident on the second MEMS chip  15  are extracted as electrical signals as well. Specifically, the first MEMS chip  14  and the second MEMS chip  15  have the function of converting acoustic signals to electrical signals. 
     The configuration of the MEMS chips  14 ,  15  is not limited to the configuration of the present embodiment. For example, in the present embodiment, the diaphragms  142 ,  152  are lower than the fixed electrodes  144 ,  154 , but a configuration in which the relationship is reversed (a relationship in which the diaphragms are above and the fixed electrodes are below) may also be used. 
     The ASIC  16  is an integrated circuit for amplifying electrical signals extracted based on the changes in electrostatic capacitance of the first MEMS chip  14  (originating in the vibration of the first diaphragm  142 ), and electrical signals extracted based on the changes in electrostatic capacitance of the second MEMS chip  15  (originating in the vibration of the second diaphragm  152 ). 
     The ASIC  16  comprises a charge pump circuit  161  for applying bypass voltage to the first MEMS chip  14  and the second MEMS chip  15 , as shown in  FIG. 6 . The charge pump circuit  161  increases a power source voltage (from about 1.5 to 3 V, to about 6 to 10 V, for example) and applies the bypass voltage to the first MEMS chip  14  and the second MEMS chip  15 . The ASIC  16  also comprises a first amplifier circuit  162  for detecting changes in electrostatic capacitance in the first MEMS chip  14 , and a second amplifier circuit  163  for detecting changes in electrostatic capacitance in the second MEMS chip  15 . The electrical signals amplified by the first amplifier circuit  162  and the second amplifier circuit  163  are outputted independently from the ASIC  16 . 
     The charge pump circuit  161  has a configuration in which a shared bypass voltage is applied to the first MEMS chip  14  and the second MEMS chip  15 . Commonly a large capacitor capacitance is required in order to configure the charge pump circuit  161 , and a large semiconductor chip surface area is consumed. By having a bypass shared between the first MEMS chip  14  and the second MEMS chip  15  and supplying the bypass from a single charge pump power source, the semiconductor chip surface area is reduced and the size of the ASIC  16  is reduced. As a result, it is possible to make the microphone unit  1  compact in size. 
     The present embodiment has a configuration in which a shared bypass voltage is applied to the first MEMS chip  14  and the second MEMS chip  15 , but the present embodiment is not limited to this configuration. For example, two charge pump circuits  161  may be provided and may apply bypass voltages separately to the first MEMS chip  14  and the second MEMS chip  15 . With such a configuration, the possibility of crosstalk occurring between the first MEMS chip  14  and the second MEMS chip  15  can be reduced. 
     In the microphone unit  1 , the two MEMS chips  14 ,  15  are installed on the microphone substrate  12  with the diaphragms  142 ,  152  in an orientation of being nearly parallel to the top surface  12   a  of the microphone substrate  12 , as shown in  FIG. 4 . In the microphone unit  1 , the MEMS chips  14 ,  15  and the ASIC  16  are installed so as to be aligned in a row in the longitudinal direction of the top surface  12   a  of the microphone substrate  12  (the left-right direction in  FIGS. 3B and 4 ). The alignment order is, starting from the right referring to  FIGS. 3B and 4 , the first MEMS chip  14 , the ASIC  16 , and the second MEMS chip  15 . 
     The first MEMS chip  14  is installed on the top surface  12   a  of the microphone substrate  12  so that the first diaphragm  142  covers the third substrate open part  123  formed in the microphone substrate  12 , as can be seen by referring to  FIGS. 3B and 4 . The third substrate open part  123  is obscured by the first MEMS chip  14 . The second MEMS chip  15  is also installed on the top surface  12   a  of the microphone substrate  12  so that the second diaphragm  152  covers the first substrate open part  121  formed in the microphone substrate  12 , as can be seen by referring to  FIGS. 3B and 4 . The first substrate open part  121  is obscured by the second MEMS chip  15 . 
     In the present embodiment, the MEMS chips  14 ,  15  obscuring the substrate open parts  121 ,  123  are installed on the microphone substrate  12  so that the diaphragms  142 ,  152  cover the entire substrate open parts  121 ,  123 . However, the configuration is not limited to this example, and the MEMS chips  14 ,  15  obscuring the substrate open parts  121 ,  123  may be installed on the microphone substrate  12  so that the diaphragms  142 ,  152  partially cover the substrate open parts  121 ,  123 . 
     The two MEMS chips  14 ,  15  and the ASIC  16  are mounted on the microphone substrate  12  by die bonding and wire bonding. Specifically, the entire bottom surfaces of the first MEMS chip  14  and the second MEMS chip  15  that face the top surface  12   a  of the microphone substrate  12  are bonded without any gaps by a die bond material not shown (e.g., an epoxy resin-based or silicone resin-based adhesive or the like). Bonding in this manner ensures that there will be no situations in which sounds leak out from gaps formed between the top surface  12   a  of the microphone substrate  12  and the bottom surface of the MEMS chips  14 ,  15 . The two MEMS chips  14 ,  15  are both electrically connected to the ASIC  16  by wires  17 , as shown in  FIG. 3B . 
     The bottom surface of the ASIC  16  that faces the top surface  12   a  of the microphone substrate  12  is bonded thereto using a die bond material not shown. The ASIC  16  is also electrically connected by the wires  17  to each of a plurality of electrode terminals  18   a ,  18   b ,  18   c ,  18   d  formed on the top surface  12   a  of the microphone substrate  12 , as shown in  FIG. 3B . The plurality of electrode terminals  18   a  to  18   d  formed in the microphone substrate  12  are composed of a power source terminal  18   a  for inputting power source voltage (VDD), a first output terminal  18   b  for outputting electrical signals amplified by the first amplifier circuit  162  of the ASIC  16 , a second output terminal  18   c  for outputting electrical signals amplified by the second amplifier circuit  163  of the ASIC  16 , and a GND terminal  18   d  for a ground connection. 
     Each of the plurality of electrode terminals  18   a  to  18   d  provided to the top surface  12   a  of the microphone substrate  12  is electrically connected to external connecting electrodes  19  (specifically, a power source electrode  19   a , a first output electrode  19   b , a second output electrode  19   c , and a GND electrode  19   d  (see  FIG. 6 )) formed on the bottom surface  11   b  (see  FIG. 4 ) of the base  11 , via wiring (including through wiring) not shown formed on the microphone substrate  12  and the base  11 . The external connecting electrodes  19  are used in order to connect to connection terminals formed on the mounting substrate on which the microphone unit  1  is mounted. 
     The above description relates to a configuration in which the two MEMS chips  14 , and the ASIC  16  are mounted by wire bonding, but the configuration is not limited to this example, and the two MEMS chips  14 ,  15  and the ASIC  16  may also of course be flip-chip mounted. 
     The outer shape of the cover  13  has a substantially rectangular parallelepiped shape, and a substantially rectangular parallelepiped-shaped concave space  131  is formed therein, as shown in  FIGS. 1 through 4 . The concave space  131  has a configuration which extends to the proximity of one end side in the longitudinal direction of the cover  13  (the right side in  FIG. 4 ), but does not extend to the proximity of the other end side (the left side in  FIG. 4 ). The cover  13  is placed over the microphone substrate  12  with the concave space  131  and the microphone substrate  12  oriented facing each other so that an accommodating space for accommodating the two MEMS chips  14 ,  15  and the ASIC  16  is formed between the concave space  131  and the microphone substrate  12 . 
     The lengths of the cover  13  in the longitudinal direction (the left-right direction of  FIG. 3A ) and the transverse direction (the up-down direction of  FIG. 3A ) are provided to be substantially equal to the size of the top surface  12   a  of the microphone substrate  12 . Consequently, side surface parts are substantially flush in the microphone unit  1  in which the microphone substrate  12  and the cover  13  are stacked on the base  11 . 
     In one end side in the longitudinal direction of a cover top surface  13   a  (the right side in  FIG. 3A ) is formed a first cover open part  132  having a substantially elliptical shape in plan view, whose major axis direction is the transverse direction of the cover  13 . The first cover open part  132  is in communication with the concave space  131  of the cover  13 , as shown in  FIG. 4 , for example. In the other end side in the longitudinal direction of the cover top surface  13   a  (the left side in  FIG. 3A ) is formed a second cover open part  133  having a substantially elliptical shape in plan view, whose major axis direction is the transverse direction of the cover  13 . The second cover open part  133  is a through-hole passing through from the top surface  13   a  of the cover  13  to a bottom surface  13   b , as shown in  FIG. 4 , for example. 
     The position of the second cover open part  133  is adjusted so that when the cover  13  is covering the microphone substrate  12 , the second cover open part  133  is communicated with the second substrate open part  122  formed in the microphone substrate  12 . 
     The cover  13  may be formed using the same substrate material as the microphone substrate  12 : FR-4, a BT resin, or another glass epoxy-based substrate material, for example, and may be obtained by resin molding using an LCP, PPS, or another resin, for example. In cases in which the cover  13  is formed from FR-4 or another substrate material, the concave space  131 , the first cover open part  132 , and the second cover open part  133  are preferably formed by mechanical working using a router or drill, for example. 
     The cover  13  may be formed in two layers, one layer being formed as a substrate in which holes constituting the first cover open part  132  and the second cover open part  133  are formed, the other layer being formed as a substrate in which holes constituting the concave space  131  and the second cover open part  133  are formed, and the cover  13  being configured by attaching the two layers together. In this case, since both layers are configured having through holes, the holes can be formed by hole perforation working by punching, and manufacturing efficiency can be greatly improved. 
     The base  11 , microphone substrate  12  (on which the two MEMS chips  14 ,  15  and the ASIC  16  are mounted), and cover  13  are stacked sequentially in the stated order from the bottom and attached using an adhesive, or the like, between the members, for example. A microphone unit  1  such as the one shown in  FIG. 1  is thus obtained. In the microphone unit  1 , acoustic waves inputted from the exterior via the first cover open part  132  pass through the accommodating space (the space formed between the concave space  131  of the cover  13  and the top surface  12   a  of the microphone substrate  12 ) and reach the top surface  142   a  of the first diaphragm  142  and the top surface  152   a  of the second diaphragm  152 , as shown in  FIG. 4 . Acoustic waves inputted from the exterior via the base open part  112  and the third substrate open part  123  reach the bottom surface  142   b  of the first diaphragm  142 . Acoustic waves inputted from the exterior via the second cover open part  133  pass through the second substrate open part  122 , a hollow space (the space formed using the groove part  111  of the base  11  and a bottom surface  12   b  of the microphone substrate  12 ), and the first substrate open part  121 , and reach the bottom surface  152   b  of the second diaphragm  152 . 
     In other words, the microphone unit  1  is provided with a first sound path  41  for transmitting sound pressure inputted from the first cover open part  132  functioning as a first sound hole to one surface (the top surface  142   a ) of the first diaphragm  142  and also to one surface (the top surface  152   a ) of the second diaphragm  152 ; a second sound path  42  for transmitting sound pressure inputted from the base open part  112  and third substrate open part  123  functioning as a second sound hole to the other surface (the bottom surface  142   b ) of the first diaphragm  142 ; and a third sound path  43  for transmitting sound pressure inputted from the second cover open part  133  functioning as a third sound hole to the other surface (the bottom surface  152   b ) of the second diaphragm  152 . 
     Hereinbelow, the first cover open part  132  is sometimes referred to as the first sound hole  132 , and the second cover open part  133  is sometimes referred to as the third sound hole  133 . The sound hole formed by the base open part  112  and the third substrate open part  123  is sometimes referred to as the second sound hole  101 . 
     The first MEMS chip  14  is an embodiment of the first vibrating part of the present invention. The second MEMS chip  15  is an embodiment of the second vibrating part of the present invention. The ASIC  16  is an embodiment of the electrical circuit part of the present invention. The base  11 , the microphone substrate  12 , and the cover  13  combined together are an embodiment of the housing of the present invention. The base  11  and the microphone substrate  12  combined together are an embodiment of the installation part of the present invention. An embodiment of the hollow space of the present invention (this space communicates the first substrate open part  121  and the second substrate open part  122 ) is obtained using the groove part  111  of the base  11  and the bottom surface  12   b  of the microphone substrate  12 . 
     In the microphone unit  1  of the present embodiment, the base  11 , microphone substrate  12 , and cover  13  constituting the housing  20  are all made of the substrate material FR-4. Thus, when the material constituting the housing  20  is all the same material, it is possible to suppress the occurrence of warping in the microphone substrate  12  caused by a difference in the expansion coefficients of the materials constituting the housing, and situations are avoided in which unnecessary stress is added to the MEMS chips  14 ,  15  installed on the microphone substrate  12 , in cases in which the microphone unit  1  is reflow-mounted to the mount substrate. Specifically, degradation of the characteristics of the microphone unit  1  is avoided. 
     In the present embodiment, the base  11  constituting the installation part  10  is a flat plate, but is not limited to this shape. Specifically, the shape of the base, for example, may be a box shape or the like having an accommodating concavity for accommodating the microphone substrate  12  and the cover  13 . Such a configuration makes it possible to simplify the positional alignment of the base  11 , microphone substrate  12 , and cover  13 , and assembling the microphone unit  1  is also simplified. 
     In the present embodiment, the shape of the groove part  111  formed in the base  11  is a substantial T shape in plan view, but is not limited to this configuration. Specifically, the shape may be substantially rectangular in plan view (the configuration shown by the dashed lines in  FIG. 3C ), for example. By using a configuration such as the present embodiment, the cross-sectional area of the spaces that serve as sound paths can be ensured to a certain extent, and the surface area in which the microphone substrate  12  is supported by the base  11  can be increased. It is thereby easy to avoid situations in which bending of the microphone substrate  12  causes a decrease in the cross-sectional area of the hollow space that is formed using the bottom surface  12   b  of the microphone substrate  12  and the groove part  111  of the base  11 . 
     In the present embodiment, the two sound holes  132 ,  133  formed in the cover  13  are in the shapes of long holes, but are not limited to this configuration, and may be sound holes or the like having substantially circular shapes in plan view, for example. Long hole shapes as in the present configuration are preferred because increases in the length in the longitudinal direction of the microphone unit  1  (equivalent to the left-right direction of  FIG. 4 ) can be suppressed, and the cross-sectional area of the sound holes can be increased. 
     For the same reasons, the second substrate open part  122  provided to the microphone substrate  12  is also in the shape of a long hole, but this shape can also be suitably modified. In the present embodiment, the second substrate open part  122 , which is a passage for acoustic waves inputted from the third sound hole  133  (the second cover open part  133 ), is formed by one large through-hole (the second substrate open part  122 ). However, the configuration is not limited to such, and a plurality of small (smaller than the size of the second substrate open part  122  of the present embodiment) through-holes aligned along the transverse direction of the microphone substrate  12  (the up-down direction of  FIG. 3B ) may be used as passages for acoustic waves inputted from the third sound hole  133 , for example. Such a configuration makes it easy to form a through-hole provided to the microphone substrate  12  in order to ensure a passage for acoustic waves inputted from the third sound hole  133 . The reason for having a plurality of through-holes is to increase the cross-sectional area of the passage. The shapes of these through-holes are not particularly limited, but the shapes can be round holes (substantially circular shapes in plan view). Round holes can be formed in a simple manner by hole perforation using a drill, and manufacturing efficiency can be improved. The individual maximum hole sizes also decrease, and the effect of preventing waste from entering is therefore also achieved. 
     In the present embodiment, the ASIC  16  is configured as being disposed so as to be present between the two MEMS chips  14 ,  15 , but such a configuration is not necessarily provided by way of limitation. In the case that the ASIC  16  is configured so as to be present between the two MEMS chips  14 ,  15 , as in the present embodiment, the MEMS chips  14 ,  15  and the ASIC  16  can be readily electrically connected by the wires  17 . Since the distances between the MEMS chips  14 ,  15  and the ASIC  16  are shorter, signals outputted from the microphone unit  1  are less affected by electromagnetic noise and a satisfactory SNR is easily ensured. 
     Next, the operational effects of the microphone unit  1  of the first embodiment are described. 
     When a sound occurs in the exterior of the microphone unit  1 , acoustic waves inputted from the first sound hole  132  reach the top surface  142   a  of the first diaphragm  142  by way of the first sound path  41 , and acoustic waves inputted from the second sound hole  101  reach the bottom surface  142   b  of the first diaphragm  142  by way of the second sound path  42 . Therefore, the first diaphragm  142  vibrates due to the difference between the sound pressure applied to the top surface  142   a  and the sound pressure applied to the bottom surface  142   b . A change in electrostatic capacitance thereby occurs in the first MEMS chip  14 . An electrical signal extracted based on the change in electrostatic capacitance of the first MEMS chip  14  is amplified by the first amplifier circuit  162  and outputted from the first output electrode  19   b  (see  FIGS. 4 and 6 ). 
     When a sound occurs in the exterior of the microphone unit  1 , acoustic waves inputted from the first sound hole  132  reach the top surface  152   a  of the second diaphragm  152  by way of the first sound path  41 , and acoustic waves inputted from the third sound hole  133  reach the bottom surface  152   b  of the second diaphragm  152  by way of the third sound path  43 . Therefore, the second diaphragm  152  vibrates due to the sound pressure difference between the sound pressure added to the top surface  152   a  and the sound pressure added to the bottom surface  152   b . A change in electrostatic capacitance thereby occurs in the second MEMS chip  15 . An electrical signal extracted based on the change in electrostatic capacitance of the second MEMS chip  15  is amplified by the second amplifier circuit  163  and outputted from the second output electrode  19   c  (see  FIGS. 4 and 6 ). 
     Thus, in the microphone unit  1 , signals obtained using the first MEMS chip  14  and signals obtained using the second MEMS chip  15  are outputted to the exterior separately. The first MEMS chip  14  and the second MEMS chip  15  in the microphone unit  1  both exhibit the function of a bidirectional differential microphone. The characteristics of the microphone unit  1  configured in this manner are described hereinbelow with reference to  FIGS. 7 and 8 . 
       FIG. 7  is a graph showing the relationship between sound pressure P and distance R from the sound source.  FIG. 8  is a drawing for describing the directional characteristics of a differential microphone configured from a first MEMS chip (dashed lines), and the directional characteristics of a differential microphone configured from a second MEMS chip (solid lines). In  FIG. 8 , the orientation of the microphone unit  1  is assumed to be the same as the orientation shown in  FIG. 4 . 
     Acoustic waves attenuate as they travel through air or another medium, and the sound pressure (the strength/amplitude of the acoustic waves) decreases, as shown in  FIG. 7 . The sound pressure is inversely proportional to the distance from the sound source, and the relationship between the sound pressure P and the distance R is expressed by the following formula (1). The letter k in formula (1) represents a proportionality constant.
 
 P=k/R   (1)
 
     As is clear from  FIG. 7  and formula (1), the sound pressure rapidly attenuates at a position near the sound source (the left side of the graph), and attenuates at a slower rate the farther from the sound source (the right side of the graph). Specifically, the sound pressures transmitted to two positions whose distances from the sound source differ by an amount Δd (R 1  and R 2 , and R 3  and R 4 ) attenuate greatly (P 1 -P 2 ) from R 1  to R 2  whose distances from the sound source are small, but do not attenuate by much (P 3 -P 4 ) from R 3  to R 4  whose distances from the sound source are great. 
     In this case, it is assumed that the distance from the sound source of the target sound to be picked up by the microphone unit  1  differs between the first sound hole  132  and the second sound hole  101 . In this case, the sound pressure of the target sound generated in the proximity of the microphone unit  1  differs greatly between the top surface  142   a  and the bottom surface  142   b  of the first diaphragm  142 . The sound pressure of background noise (distant noise) has virtually no difference between the top surface  142   a  and the bottom surface  142   b  of the first diaphragm  142  because the sound source is in a farther position than the target sound. 
     Since the sound pressure difference of the background noise received by the first diaphragm  142  is small, sound pressure of background noise is substantially negated in the first diaphragm  142 . Since the sound pressure difference of the target sound received by the first diaphragm  142  is large, the sound pressure of the target sound is not negated in the first diaphragm  142 . Therefore, a signal obtained by the vibration of the first diaphragm  142  is regarded as a signal of the target sound from which background noise has been removed. Therefore, a differential microphone configured from the first MEMS chip  14  has excellent distant noise suppression performance. Similarly, a differential microphone configured from the second MEMS chip  15  also has excellent distant noise suppression performance. 
     As described above, the differential microphone configured by the first MEMS chip  14  and the differential microphone configured by the second MEMS chip  15  both display bidirectivity, but the primary axial directions of these directivities are offset by approximately 90°, as shown in  FIG. 8 . 
     With the differential microphone configured by the first MEMS chip  14 , if the distance from the sound source to the first diaphragm  142  remains constant, the sound pressure added to the first diaphragm  142  reaches a maximum when the sound source is in the 90° or 270° direction. This is because there is the greatest difference between the distance for acoustic waves to reach the top surface  142   a  of the first diaphragm  142  from the first sound hole  132 , and the distance for acoustic waves to reach the bottom surface  142   b  of the first diaphragm  142  from the second sound hole  101 . On the other hand, the sound pressure added to the first diaphragm  142  reaches a minimum when the sound source is in the 0° or 180° direction. This is because there is virtually no difference between the distance for acoustic waves to reach the top surface  142   a  of the first diaphragm  142  from the first sound hole  132 , and the distance for acoustic waves to reach the bottom surface  142   b  of the first diaphragm  142  from the second sound hole  101 . Specifically, the differential microphone configured by the first MEMS chip  14  displays the properties of readily receiving acoustic waves incident from the 90° and 270° directions and not readily receiving acoustic waves incident from the 0° and 180° directions. 
     With the differential microphone configured by the second MEMS chip  15 , if the distance from the sound source to the second diaphragm  152  remains constant, the sound pressure added to the second diaphragm  152  reaches a maximum when the sound source is the 0° or 180° direction. This is because there is the greatest difference between the distance for acoustic waves to reach the top surface  152   a  of the second diaphragm  152  from the first sound hole  132 , and the distance for acoustic waves to reach the bottom surface  152   b  of the second diaphragm  152  from the third sound hole  133 . On the other hand, the sound pressure added to the second diaphragm  152  reaches a minimum when the sound source is in the 90° or 270° direction. This is because there is virtually no difference between the distance for acoustic waves to reach the top surface  152   a  of the second diaphragm  152  from the first sound hole  132 , and the distance for acoustic waves to reach the bottom surface  152   b  of the second diaphragm  152  from the third sound hole  133 . Specifically, the differential microphone configured by the second MEMS chip  15  displays the properties of readily receiving acoustic waves incident from the 0° and 180° directions and not readily receiving acoustic waves incident from the 90° and 270° directions. 
     Thus, the microphone unit  1  is configured comprising two bidirectional differential microphones having different primary axial directions of directivity. In the microphone unit  1  as described above, signals extracted from the first MEMS chip  14  and signals extracted from the second MEMS chip  15  are separately processed (amplified) and outputted to the exterior. In this case, by combining the two separately outputted signals and performing a predetermined computation process, the microphone unit  1  can be made to function as a bidirectional microphone in which the primary axial direction of directivity can be controlled. This is described with reference to  FIGS. 9 and 10 . 
     (Voice Input Device Comprising Microphone Unit of First Embodiment) 
       FIG. 9  is a block diagram showing the configuration of the voice input device comprising the microphone unit of the first embodiment. The voice input device  5  of the first embodiment comprises the microphone unit  1  and a voice signal processor  6  for combining two signals outputted from the microphone unit  1  and performing a predetermined computation process. 
     In the present embodiment, the voice signal processor  6  executes the computation process shown in the following formula (2), for example. In formula (2), OUT 1  is a signal output corresponding to the first MEMS chip  14  (the output from the first output electrode  19   b ), and OUT 2  is a signal output corresponding to the second MEMS chip  15  (the output from the second output electrode  19   c ). In formula (2), k is a variable for weighting.
 
(1 −|k |)×OUT2 −ks ×OUT1  (2)
 
       FIG. 10  is a drawing showing the manner in which varying the variable (k) of the computation process performed by the voice signal processor causes fluctuation of the primary axial direction of directivity of the microphone unit functioning as a bidirectional microphone. As shown in  FIG. 10 , the primary axial direction of the microphone unit  1  is capable of being rotatably controlled in a direction encircling the Z axis, which is orthogonal to the X direction which is the longitudinal direction of the microphone unit  1  and the Y direction which is the thickness direction of the microphone unit  1 , by the selection of the value of k in formula (2). 
     When k=−1 or when k=1, for example, the primary axial direction of directivity of the microphone unit  1  is parallel to the Y direction which is the thickness direction of the microphone unit  1 , and when k=0, the primary axial direction of directivity of the microphone unit  1  is parallel to the X direction which is the longitudinal direction of the microphone unit  1 . 
     When the voice input device  5  is configured in this manner, the primary axial direction of directivity can be controlled by varying the value of the variable k in formula (2), and the voice of a close speaker can be acquired with high sensitivity by appropriately setting the value of the variable k, even if the installed position of the microphone unit  1  in the voice input device  5  is varied according to design convenience. When the voice input device is used, it is also possible to change the variable k and control the primary axial direction of directivity in accordance with the position of the close speaker, and to acquire the voice of the speaker with high sensitivity. 
     Herein is a description, made with reference to  FIGS. 11 and 12 , of a configurational example of a case in which the microphone unit is applied to a mobile phone (an example of a voice input device) comprising the function of a voice input device.  FIG. 11  is a drawing showing the schematic configuration of an embodiment of a mobile telephone to which the microphone unit of the first embodiment is applied.  FIG. 12  is a schematic cross-sectional view in position B-B of  FIG. 11 . 
     Two sound holes  511 ,  512  are provided in the bottom part side of a surface  51   a  of a housing  51  of the mobile telephone  5  as shown in  FIGS. 11 and 12 . One sound hole  513  is provided to a rear surface  51   b  of the housing  51  of the mobile telephone  5 , as shown in  FIG. 12 . The user&#39;s voice is inputted via these three sound holes  511 ,  512 ,  513  to the microphone unit  1  which is disposed inside the housing  51 . 
     The microphone unit  1  is installed in the mobile telephone  5  in a state of being mounted on a mounting substrate  52  provided inside the housing  51  of the mobile telephone  5 , as shown in  FIG. 12 . The mounting substrate  52  is provided with the voice signal processor  6  described above (not shown in  FIG. 12 ). The mounting substrate  52  is also provided with a plurality of electrode pads electrically connected with the plurality of external connecting electrodes  19  of the microphone unit  1 , and the microphone unit  1  is mounted to the mounting substrate  52  using soldering or the like, for example. Thereby, a power source voltage is provided to the microphone unit  1 , and electrical signals outputted from the microphone unit  1  are sent to the voice signal processor  6 . 
     The microphone unit  1  is disposed so that the first sound hole  132  overlaps the sound hole  511  formed in the housing  51  of the mobile telephone  5 , the second sound hole  101  overlaps the substrate through-hole  521  provided to the mounting substrate  52  and the sound hole  513  formed in the housing  51  of the mobile telephone  5 , and the third sound hole  133  overlaps the sound hole  512  formed in the housing  51  of the mobile telephone  5 . 
     Therefore, a voice occurring outside of the housing  51  of the mobile telephone  5  passes through the first sound path  41  of the microphone unit  1  to reach the top surface  142   a  of the first diaphragm  142  of the first MEMS chip  14 , and passes through the second sound path  42  to reach the bottom surface  142   b  of the first diaphragm  142  of the first MEMS chip  14 . The voice occurring outside of the housing  51  of the mobile telephone  5  also passes through the first sound path  41  of the microphone unit  1  to reach the top surface  152   a  of the second diaphragm  152  of the second MEMS chip  15 , and passes through the third sound path  43  to reach the bottom surface  152   b  of the second diaphragm  152  of the second MEMS chip  15 . 
     In the mobile telephone  5  of the present embodiment, an elastic body (a gasket)  53  is disposed between the housing  51  and the microphone unit  1 . Openings  531 ,  532  are formed in the elastic body  53  so that voices occurring outside of the housing  51  are inputted independently and efficiently corresponding to the two sound paths  41 ,  43  provided to the microphone unit  1 . The elastic body  53  is provided so as to ensure airtightness without any acoustic leaks. The material of the elastic body  53  is preferably butyl rubber, silicone rubber, or the like, for example. 
     For ensuring airtightness without any acoustic leaks, an airtight part  54  is also provided between the microphone unit  1  and the mounting substrate  52  so as to enclose the second sound hole  101  and the substrate through-hole  521  provided to the mounting substrate  52 . This airtight part  54  is obtained by bonding together the airtightness terminal provided to the microphone unit  1  and the airtightness terminal provided to the mounting substrate  52  by soldering or the like, for example. Another precaution taken to ensure airtightness without any acoustic leaks is that an elastic body (a gasket)  55  is disposed between the mounting substrate  52  and the housing  51  so as to enclose the substrate through-hole  521  of the mounting substrate  52  and the sound hole  513  of the housing  51 . 
     The present example is a configuration in which the microphone unit  1  is disposed in the bottom side of the mobile telephone  5  (stated with  FIG. 11  in mind), but it is possible to control the primary axial direction of directivity of the microphone unit  1  functioning as a bidirectional microphone as described above. Therefore, the placement of the microphone unit  1  is not limited to the bottom side of the mobile telephone  5  and is easily varied. 
     (Summary and Remarks of First Embodiment) 
     As described above, the microphone unit  1  of the first embodiment comprises two bidirectional differential microphones having excellent distant noise suppression performance, and the primary axial directions of directivity of these two differential microphones are mutually different directions (offset by 90° in the present example, but not necessarily limited to 90°). The microphone unit  1  can be made to function as a single microphone by using the signals outputted from two differential microphones to perform a predetermined computation process, and the primary axial directions of directivity can be controlled by suitably varying the variables during the computation process. Consequently, the microphone unit  1  of the present embodiment is readily adapted to diversity in the design of a voice input device. 
     The microphone unit  1  of the first embodiment has a configuration in which the first sound path  41 , the second sound path  42 , and the third sound path  43  are formed by three members: the base  11 , the microphone substrate  12 , and the cover  13 , and the configuration is easily assembled in a simple manner and is also easily made smaller and thinner. 
     In the above description, an example was presented of a case in which the microphone unit  1  is used as a close-talking microphone of a mobile telephone, but since the primary axial direction of directivity can be controlled, the microphone unit  1  is also readily applied to a device or the like for estimating sound sources, for example. 
     The first embodiment has a configuration in which a voice signal processor for controlling the primary axial direction of directivity is provided to the exterior of the microphone unit  1 , but this signal processor may also be provided to the interior of the ASIC  16  of the microphone unit  1 . In this case, it is possible to control the primary axial direction of directivity by inputting a control signal from the exterior to the microphone unit  1 , the control signal being equivalent to a weighted coefficient (k in formula (2)) which is used for adding the two differential microphone outputs, and switching the mode of the computation process in the interior of the ASIC  16 . 
     Second Embodiment 
     The following is a description of the second embodiment of the microphone unit and voice input device to which the present invention is applied. 
     (Microphone Unit of Second Embodiment) 
     A large portion of the configuration of the microphone unit of the second embodiment is identical to that of the microphone unit  1  of the first embodiment. Only the portion that is different is described hereinbelow. Portions duplicated from the microphone unit  1  of the first embodiment are described using the same symbols. 
       FIG. 13  is a schematic cross-sectional view showing the configuration of the microphone unit of the second embodiment. The microphone unit  2  of the second embodiment differs from the microphone unit  1  of the first embodiment in that an acoustic resistance member  21  is provided so as to block the second sound hole  101 , as shown in  FIG. 13 . The acoustic resistance member  21  is formed from felt or the like, for example, and the phase of the acoustic waves inputted from the second sound hole  101  is delayed. In the microphone unit  2  of the second embodiment, the configuration of the acoustic resistance member  21  is adjusted so that the first MEMS chip  14  functions as a unidirectional microphone. 
       FIG. 14  is a block diagram showing the configuration of the microphone unit of the second embodiment. In the microphone unit  2  of the second embodiment, a switching electrode  19   e  is provided for inputting switch signals from the exterior (the voice input device in which the microphone unit  2  is mounted), and this microphone unit differs from the microphone unit  1  of the first embodiment in that a switch circuit  164  provided to the ASIC  16  is actuated by a switch signal sent via the switching electrode  19   e , as shown in  FIG. 14 . 
     Since the configuration is provided with the switching electrode  19   e , a switching terminal  18   e  is provided to the top surface  12   a  of the microphone substrate  12 , as shown in  FIG. 15 . 
     The switch circuit  164  is a circuit for switching between externally outputting the signal outputted from the first amplifier circuit  162 , and externally outputting the signal outputted from the second amplifier circuit  163 , as shown in  FIG. 14 . Specifically, in the microphone unit  2  of the second embodiment, the signal outputted from the microphone unit  2  is either only the signal extracted from the first MEMS chip  14  or only the signal extracted from the second MEMS chip  15 . 
     Consequently, unlike the microphone unit  1  of the first embodiment, in the microphone unit  2  of the second embodiment, a single output electrode (the first output electrode  19   b ) is included in the external connecting electrodes  19  provided to the bottom surface  11   b  of the base  11 . In connection with this, only the first output terminal  18   b  is provided to the top surface  12   a  of the microphone substrate  12  as shown in  FIG. 15 , and the second output terminal  18   c  is omitted (see also  FIG. 3B ). 
     The switching action of the switch circuit  164  according to the switch signal is preferably configured to use the signals H (high level) and L (low level), for example. 
     The operational effects of the microphone unit  2  of the second embodiment thus configured are described. 
       FIGS. 16A and 16B  are drawings for describing the directional characteristics of the microphone unit of the second embodiment. In  FIGS. 16A and 16B , the orientation of the microphone unit  2  is assumed to be the same as the orientation shown in  FIG. 13 . 
     In the microphone unit  2  of the second embodiment, the first MEMS chip  14  functions as a differential microphone, but because of the presence of the acoustic resistance member  21 , the microphone unit  2  also exhibits the function of a unidirectional microphone as shown in  FIG. 16A . Specifically, the microphone unit  1  has good sensitivity with respect to sounds having a sound source to the side of one surface (the top surface side in  FIG. 13 ), and extremely low sensitivity with respect to sounds having a sound source to the side of the other surface (the bottom surface side in  FIG. 13 ). The second MEMS chip  15  configured as a differential microphone is not affected by the acoustic resistance member  21 , and therefore exhibits the function of a bidirectional differential microphone having excellent distant noise suppression performance, similar to the microphone unit  1  of the first embodiment (see  FIG. 16B ). The primary axis of directivity of a bidirectional microphone using the second MEMS chip  15  is the longitudinal direction of the microphone unit  2  (the left-right direction of  FIG. 13 ). 
     As described above, in the microphone unit  2  of the second embodiment, electrical signals extracted based on the changes in electrostatic capacitance of the first MEMS chip  14  and electrical signals extracted based on the changes in electrostatic capacitance of the second MEMS chip  15  can be selectively outputted by the switch circuit  164 . Specifically, the microphone unit  2  can be used while switching between the function of a unidirectional microphone using the first MEMS chip  14  and the function of a bidirectional microphone using the second MEMS chip  15 . Therefore, the microphone unit  2  of the second embodiment is readily adapted to the multifunctionality of the voice input device. 
     (Voice Input Device Comprising Microphone Unit of Second Embodiment) 
     The microphone unit of the second embodiment is applied to a mobile telephone, for example (one example of a voice input device). The configuration of the case of the microphone unit  2  of the second embodiment applied to a mobile telephone can be the same configuration as in the case of the first embodiment, for example (the same configuration as the one shown in  FIGS. 11 and 12 ), and a detailed description thereof is omitted. 
     The mobile telephone to which the microphone unit  2  is applied is configured to be multifunctional; for example, the mobile telephone comprises a hands-free function and a video record function. A controller (not shown) of the mobile telephone, upon perceiving that any of the functions of a close-talking mode, a hands-free mode, and a video record mode is being used, inputs a corresponding switch signal to the microphone unit  2 . According to this switch signal, the switch circuit  164  then performs a switching action so that either signals corresponding to the first MEMS chip  14  or signals corresponding to the second MEMS chip  15  can be outputted. 
     Specifically, when the mobile telephone is used in the close-talking mode, signals corresponding to the second MEMS chip  15  are outputted from the microphone unit  2  by the workings of the switch circuit  164 , and the voice signal processor of the mobile telephone (whose workings are different from the voice signal processor  6  of the first embodiment) performs a process using the signals corresponding to the second MEMS chip  15 . As described above, high-quality signals suited to close-talking are obtained in order to yield excellent distant noise suppression performance when the second MEMS chip  15  is used. 
     When the mobile telephone is used in the hands-free mode or the video record mode, due to the workings of the switch circuit  164 , a signal corresponding to the first MEMS chip  14  is outputted from the microphone unit  2 , and the voice signal processor of the mobile telephone performs processing using the signal corresponding to the first MEMS chip  14 . As described above, when the first MEMS chip  14  is used, voice pickup focused on a voice in the intended pickup direction is possible, in order to yield excellent sensitivity in the surface side (the front surface side) where the first sound hole  132  and the third sound hole  133  are provided. Specifically, the preferred signal processing is performed in each mode. 
     (Summary and Remarks of Second Embodiment) 
     As described above, the microphone unit  2  of the second embodiment is configured comprising both the function of a bidirectional differential microphone having excellent distant noise suppression performance, and the function of a unidirectional microphone having excellent front-surface-side pickup sensitivity. Therefore, the microphone unit of the present embodiment is readily adapted to the multifunctionality of the voice input device in which the microphone unit is applied. Since the microphone unit  2  of the present embodiment comprises two functions, there is no need to separately install two microphone units as in conventional practice, and a size increase of the voice input device is readily suppressed. 
     The microphone unit  2  of the present embodiment is configured having the two MEMS chips  14 ,  15 , and this configuration is obtained by adding a MEMS chip to the space originally provided with a bidirectional differential microphone having excellent distant noise suppression performance (the microphone unit previously developed by the inventors) and providing a sound hole (blocked by the acoustic resistance member  21 ) to the bottom side of the added MEMS chip. Therefore, there is no size increase in the microphone unit previously developed by the inventors. This is described below. 
     In the microphone unit  2  of the present embodiment, when the first MEMS chip  14 , the second sound hole  101 , and the acoustic resistance member  21  are taken out, a bidirectional differential microphone unit having excellent distant noise suppression performance is obtained. In this microphone unit, the distance between the centers of the two sound holes  132 ,  133  is preferably about 5 mm. This is due to the following reasons. 
     When the distance between the two sound holes  132 ,  133  is too small, the difference between sound pressures added to the top surface  152   a  and bottom surface  152   b  of the second diaphragm  152  is small, the amplitude of the second diaphragm  152  is small, and the electrical signals outputted from the ASIC  16  have a poor SNR. Therefore, it is preferable that the distance between the two sound holes  132 ,  133  be increased to a certain extent. On the other hand, when the distance between the centers of the two sound holes  132 ,  133  is too great, there is a large time difference, i.e., phase difference for acoustic waves produced from the sound source to pass through the two sound holes  132 ,  133  and reach the second diaphragm  152 , and noise removal performance decreases. Therefore, the distance between the centers of the two sound holes  132 ,  133  is preferably 4 mm or greater and 6 mm or less, and more preferably about 5 mm. 
     The lengths of the MEMS chips  14 ,  15  (lengths in a direction parallel to a line joining the centers of the two sound holes  132 ,  133 , lengths in the left-right direction in  FIG. 13 ) used in the microphone unit  2  of the present embodiment are about 1 mm, for example, and the length of the ASIC  16  in the same direction is about 0.7 mm, for example. When the microphone unit is made to function as a differential microphone, it is preferably configured so that the time for acoustic waves to reach the top surface  152   a  of the second diaphragm  152  from the first sound hole  132  and the time for acoustic waves to reach the bottom surface  152   b  of the second diaphragm  152  from the third sound hole  133  are substantially the same. Therefore, the second MEMS chip  15  is disposed in the accommodating space (the space formed between the concave space  131  of the cover  13  and the top surface  12   a  of the microphone substrate  12 ) in a position separated from the first sound hole  132  (a position near the left of the accommodating space in  FIG. 13 ). 
     Therefore, a space in which the first MEMS chip  14  can be disposed is originally present in the accommodating space of the bidirectional differential microphone unit having excellent distant noise suppression performance. Consequently, the microphone unit  2  of the present embodiment, wherein the function as a unidirectional microphone having excellent pickup sensitivity in the front surface side is added to the function as a bidirectional differential microphone having excellent distant noise suppression performance, can be made into a small microphone unit without the size being increased by the addition of a MEMS chip. 
     The present embodiment has a configuration in which the switch circuit  164  is provided after the amplifier circuits  162 ,  163 , and signals corresponding to the first MEMS chip  14  and signals corresponding to the second MEMS chip  15  are switched and outputted. This is intended to make it possible to switch and output signals corresponding to the first MEMS chip  14  and signals corresponding to the second MEMS chip  15  to the exterior, but another configuration can be employed in achieving such an object. Specifically, the configuration may have one amplifier circuit, and a switch circuit for performing a switching action according to a switch signal may be disposed between the amplifier circuit and the two MEMS chips  14 ,  15 , for example. 
     In cases in which two amplifier circuits  162 ,  163  are provided as in the present embodiment, the amplifier gains of the two amplifier circuits  162 ,  163  may be set to different gains. 
     The present embodiment has a configuration in which a shared bypass voltage is applied to the first MEMS chip  14  and the second MEMS chip  15 , but the embodiment is not limited to such and may have another configuration. Specifically, the switch signal and the switch circuit may be used to switch which of the first MEMS chip  14  and the second MEMS chip  15  is electrically connected with the charge pump circuit  161 , for example. This allows the possibility of crosstalk occurring between the first MEMS chip  14  and the second MEMS chip  15  to be reduced. 
     The microphone unit  2  of the present embodiment is configured so that either signals corresponding to the first MEMS chip  14  or signals corresponding to the second MEMS chip  15  are selectively outputted to the exterior. However, the microphone unit is not limited to this configuration. Specifically, similar to the case of the microphone unit  1  of the first embodiment (see  FIG. 6 ), the microphone unit may be configured so that both signals are outputted separately and independently to the exterior (Modification A of the microphone unit  2  of the second embodiment). In this case, the configuration is preferably such that of the two signals, which signal will be used is selected in the voice input device comprising the microphone unit. As another aspect (Modification B of the microphone unit  2  of the second embodiment), the configuration shown in  FIGS. 17 and 18  may be used. 
     In the microphone unit of Modification B, a switching electrode  19   e  is provided for inputting switch signals from the exterior (the voice input device in which the microphone unit is mounted), and a switch circuit  164  provided to the ASIC  16  is actuated by a switch signal sent via the switching electrode  19   e , as shown in  FIG. 17 . Because the switching electrode  19   e  is provided in this configuration, a switching terminal  18   e  is provided to the top surface  12   a  of the microphone substrate  12  as shown in  FIG. 18 . 
     The switch circuit  164  has a configuration for switching between which of the two output electrodes  19   b ,  19   c  (some of the external connecting electrodes  19 ) will output the signal outputted from the first amplifier circuit  162  and the signal outputted from the second amplifier circuit  163  (this function is different from the switch circuit of the microphone unit  2  of the second embodiment described above). 
     Specifically, when the switch circuit  164  is in a first mode according to the switch signal inputted from the switching electrode  19   e , a signal corresponding to the first MEMS chip  14  is outputted from the first output electrode  19   b , and a signal corresponding to the second MEMS chip  15  is outputted from the second output electrode  19   c . When the switch circuit  164  is in a second mode according to the switch signal, a signal corresponding to the second MEMS chip  15  is outputted from the first output electrode  19   b , and a signal corresponding to the first MEMS chip  14  is outputted from the second output electrode  19   c.    
     The switching action of the switch circuit  164  according to the switch signal is preferably configured to use the signals H (high level) and L (low level), for example. 
     In cases in which the microphone unit and the voice input device are manufactured by different manufacturers, the following types of manufacturers are presumed to be among the manufacturers who manufacture the voice input device: 
     (A) Those who would prefer that between the signal corresponding to the first MEMS chip  14  and the signal corresponding to the second MEMS chip  15 , which is outputted from the microphone unit be determined by switching according to the switch signal, as in the microphone unit  2  of the second embodiment and (B) Those who would prefer that both the signal corresponding to the first MEMS chip  14  and the signal corresponding to the second MEMS chip  15  are outputted separately and independently from the microphone unit, as in Modification A of the microphone unit  2  of the second embodiment. 
     Modification B of the microphone unit  2  of the second embodiment is advantageous in this respect because it can be adapted to both types of manufacturers in the above (A) and (B). 
     The second embodiment is also configured so that the signal corresponding to the first MEMS chip  14  and the signal corresponding to the second MEMS chip  15  are used independently. However, the embodiment is not limited to this configuration, and may be configured so that both signals are combined and subjected to the computation process (addition, subtraction, and so forth) by the voice signal processor. Performing such processing enables control for switching the directional characteristics of the microphone unit  2  among various types. 
     (Other) 
     The embodiments shown above are examples of the configuration to which the present invention is applied, and the applicable range of the present invention is not limited to the embodiments shown above. Specifically, various modifications may be made to the embodiments described above within a range that does not deviate from the objects of the present invention. 
     For example, in the embodiments shown above, the first vibrating part and second vibrating part of the present invention were configured as MEMS chips  14 ,  15  formed using semiconductor manufacturing techniques, but are not limited to such a configuration. For example, the first vibrating part and/or the second vibrating part may be a capacitor microphone or the like that uses an electret film. 
     In the embodiments described above, so-called capacitor type microphones were employed as the configurations of the first vibrating part and second vibrating part of the present invention. However, the present invention can also be applied to a microphone unit that employs a configuration other than that of a capacitor type microphone. For example, the present invention can also be applied to a microphone unit in which an electromotive (dynamic), electromagnetic (magnetic), piezoelectric, or other type of microphone or the like is employed. 
     In the above embodiments, the ASIC  16  (electrical circuit part) was configured as being included inside the microphone units  1 ,  2 , but the electrical circuit parts may be disposed outside of the microphone units. In the embodiments shown above, the MEMS chips  14 ,  15  and the ASIC  16  are configured from separate chips, but the integrated circuit installed on the ASIC may be formed as a monolithic integrated circuit on the silicon substrate where the MEMS chips are formed. 
     In addition, the shape of the microphone unit is not limited to the shape of the present embodiment, and can of course be modified to various other shapes. 
     INDUSTRIAL APPLICABILITY 
     The microphone unit of the present invention can be applied to a variety of voice input devices which input voices and perform processing; for example, the microphone unit is suitable for a mobile telephone or the like. 
     LIST OF REFERENCE SIGNS 
     
         
           1 ,  2  Microphone unit 
           5  Mobile telephone (voice input device) 
           6  Voice signal processor 
           10  Installation part 
           11  Base (part of housing, part of installation part) 
           11   b  Bottom surface of base (rear surface of installation surface of installation part) 
           12  Microphone substrate (part of housing, part of installation part) 
           12   a  Top surface of microphone substrate (installation surface of installation part) 
           13  Cover 
           14  First MEMS chip (first vibrating part) 
           15  Second MEMS chip (second vibrating part) 
           16  ASIC (electrical circuit part) 
           19   e  Switching electrode 
           20  Housing 
           41  First sound path 
           42  Second sound path 
           43  Third sound path 
           101  Second sound hole 
           111  Groove part (configurational element of hollow space) 
           112  Base open part (configurational element of second sound hole) 
           121  First substrate open part 
           122  Second substrate open part 
           123  Third substrate open part (configurational element of second sound hole) 
           131  Concave space (configurational element of accommodating space) 
           132  First cover open part (first sound hole) 
           133  Second cover open part (third sound hole) 
           142  First diaphragm 
           142   a  Top surface (one surface) of first diaphragm 
           142   b  Bottom surface (other surface) of first diaphragm 
           152  Second diaphragm 
           152   a  Top surface (one surface) of second diaphragm 
           152   b  Bottom surface (other surface) of second diaphragm 
           164  Switch circuit