Patent Publication Number: US-8526656-B2

Title: Microphone unit

Description:
This application is based on Japanese Patent Application No. 2008-310502 filed on Dec. 5, 2008 in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microphone unit which transduces an input voice into an electric signal, and more particularly, to a structure of a microphone unit which is so formed as to allow a sound pressure to act on both surfaces (front and rear surfaces) of a diaphragm and generates an electric signal by using a vibration of the diaphragm based on a sound pressure difference. 
     2. Description of Related Art 
     Conventionally, a microphone unit is used in, for example, voice communication devices such as a mobile phone, a transceiver and the like, or in information process systems such as a voice identification system and the like which use a technology for analyzing an input voice, or in a recording device and the like. For over-the-telephone talking, voice recognition and voice recording, it is preferable that only a target voice (user&#39;s voice) is collected. For this purpose, a microphone unit which accurately extracts a target voice and removes noise (background noise and the like) other than the target voice is being developed. 
     As a technology which in a use environment where noise is present, removes noise and collects a target voice only, there is a technology for providing a microphone unit with directivity. As an example of a microphone unit which has directivity, a microphone unit which is so formed as to allow a sound pressure to act on both surfaces of a diaphragm and generates an electric signal by a vibration of the diaphragm based on a sound pressure difference is conventionally known (e.g., see patent documents 1 and 2). 
     Incidentally, conventionally, a microphone unit is equipped with an electric circuit portion that processes (e.g., amplification process and the like) an electric signal which is generated based on a vibration of a diaphragm. And, conventionally, this electric circuit portion is disposed outside a sound guide space which extends from a sound hole to the diaphragm (e.g., see FIG. 2 of the patent document 2). 
     [Patent document 1] JP-A-1992-217199 
     [Patent document 2] JP-A-2005-295278 
     SUMMARY OF THE INVENTION 
     In recent years, miniaturization of a microphone unit is important. Because of this, in a microphone unit which is so formed as to allow a sound pressure to act on both surfaces of the above diaphragm, disposing the electric circuit portion in a sound guide space which extends from a sound hole to the diaphragm has been studied and it is found out that an excellent directional characteristic is not obtained especially in a high-frequency band. In other words, it is found out that in the case where the electric circuit portion is disposed in the sound guide space only for miniaturization, the performance of the microphone unit drops. 
     Accordingly, it is an object of the present invention to provide a microphone unit which is capable of being miniaturized and has high performance. 
     To achieve the above object, a microphone unit according to the present invention includes: a housing; a diaphragm which is disposed in the inside of the housing; and an electric circuit portion which processes an electric signal that is generated based on a vibration of the diaphragm. And, in the housing, a first sound guide space which guides a sound outside the housing to a first surface of the diaphragm via a first sound hole and a second sound guide space which guides a sound outside the housing to a second surface, that is, an opposite surface of the first surface of the diaphragm via a second sound hole are formed; the electric circuit portion is disposed in either one of the first sound guide space and the second sound guide space; and an acoustic resistance portion which adjusts at least one of a frequency characteristic of the first sound guide space and a frequency characteristic of the second sound guide space is formed. 
     According to this structure, a structure is employed, in which the electric circuit portion which performs an amplification process of a signal and the like is disposed in either one of the first sound guide space and the second sound guide space. Accordingly, it is possible to miniaturize the microphone unit compared with the case where the electric circuit portion is disposed outside the sound guide space like the conventional one. 
     If the electric circuit portion is disposed in the sound guide space, the shapes of the two sound guide spaces (the first sound guide space and the second sound guide space) become imbalanced and the like, which causes generation of a difference between the frequency characteristics of the two sound guide spaces. Specifically, for example, a frequency-characteristic difference occurs in a high-frequency band and excellent noise prevention performance is not obtained in the high-frequency side. In this point, because the present structure has a structure in which the frequency characteristics of the sound guide spaces are adjusted by forming the acoustic resistance portion, it is possible to obtain excellent noise prevention performance in the high-frequency side. In other words, according to the present structure, it is possible to obtain a less-noise and high-quality voice signal (electric signal) which is output from the microphone unit. 
     In the microphone unit having the above structure, it is preferable that the acoustic resistance portion is so formed as to selectively act on a sound in a specific high-frequency band. The above frequency-characteristic difference between the two sound guide spaces which is generated by disposing the electric circuit portion in the sound guide space is hardly detected in a low-frequency band, for example, and detected in the high-frequency band. Accordingly, by employing the present structure in which the acoustic resistance portion selectively acts on a specific frequency band (e.g., the high-frequency band), it is easy to reduce the frequency-characteristic difference between the two sound guide spaces. 
     Besides, in the microphone unit having the above structure, the acoustic resistance portion may be formed by mounting an acoustic resistance member on the housing. 
     As a specific structure which uses the acoustic resistance member, the acoustic resistance member may be so disposed as to block at least part of a route that extends from the first sound hole to the first surface or at least part of a route that extends from the second sound hole to the second surface. 
     Besides, as another specific structure which uses the acoustic resistance member, the acoustic resistance member may be so disposed as to block at least part of a route that extends from the first sound hole to the first surface and at least part of a route that extends from the second sound hole to the second surface. And, in this case, the acoustic resistance member may include a first acoustic resistance member and a second acoustic resistance member that are separately mounted on the housing. 
     In the microphone unit having the above structure, at least one of the first sound hole and the second sound hole includes a plurality of through-holes and may double as the acoustic resistance portion. 
     According to the present invention, it is possible to miniaturize the microphone unit. And, because it is possible to prevent “deterioration in noise prevention performance” which can occur in a case where the miniaturization is achieved, a high-quality voice signal is obtained. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view showing a structure of a microphone unit according to an embodiment. 
         FIG. 2  is a schematic sectional view taken along an A-A position of  FIG. 1 . 
         FIG. 3  is a schematic sectional view showing a structure of a MEMS chip which a microphone unit according to an embodiment includes. 
         FIG. 4  is a view for describing a circuit structure of an ASIC which a microphone unit according to an embodiment includes. 
         FIG. 5A  is a view for describing a directional characteristic which is required for a microphone unit according to an embodiment. 
         FIG. 5B  is a view for describing a directional characteristic which is required for a microphone unit according to an embodiment. 
         FIG. 6  is a graph for describing a problem in a case of a structure where an acoustic resistance portion is not formed in a microphone unit according to an embodiment. 
         FIG. 7  is a view for describing a characteristic of an acoustic resistance portion which a microphone unit according to an embodiment includes. 
         FIG. 8  is a view for describing an effect in a case where an acoustic resistance member is so disposed as to block a sound guide space. 
         FIG. 9  is a view for describing a modification of a microphone unit according to an embodiment. 
         FIG. 10  is a view for describing a modification of a microphone unit according to an embodiment. 
         FIG. 11  is a view for describing a modification of a microphone unit according to an embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of a microphone unit to which the present invention is applied are described in detail with reference to the drawings. 
       FIG. 1  is a schematic perspective view showing a structure of a microphone unit according to an embodiment.  FIG. 2  is a schematic sectional view taken along an A-A position of  FIG. 1 . As shown in  FIGS. 1 and 2 , a microphone unit  1  includes: a housing  11 ; a MEMS (Micro Electro Mechanical System) chip  12 ; an ASIC (Application Specific Integrated Circuit)  13 ; a circuit board  14 ; and an acoustic resistance portion  15 . 
     The housing  11  is formed into substantially a rectangular-parallelopiped shape and houses in the inside thereof: the MEMS chip  12  which includes a vibration membrane (diaphragm)  122 ; the ASIC  13 ; and the circuit board  14 . Here, the outward form of the housing  11  is not limited to the shape in the present embodiment and may be a cube, for example, nor limited to hexahedrons such as a rectangular parallelopiped and a cube, and may be a polyhedral structure other than hexahedrons or may be a structure (e.g., a spherical structure, a semi-spherical structure or the like) other than polyhedrons. 
     In the inside of the housing  11 , as shown in  FIGS. 1 and 2 , a first sound guide space  113  and a second sound guide space  114  are formed. The first sound guide space  113  and the second sound guide space  114  are divided by the vibration membrane  122  that the MEMS chip  12  described in detail later includes. In other words, the first sound guide space  113  is in contact with an upper surface (first surface)  122   a  side of the vibration membrane  122  and the second sound guide space  114  is in contact with a lower surface (second surface)  122   b  side of the vibration membrane  122 . 
     Besides, through the upper surface  11   a  of the housing  11 , a first sound hole  111  and a second sound hole  112  each of which has substantially a circular shape when seen in a planar fashion are formed. The first sound hole  111  communicates with the first sound guide space  113 , and thus the first sound guide space  113  is in a state to communicate with the space outside the housing  11 . In other words, a sound outside the housing  11  enters the first sound guide space  113  via the first sound hole  111  and is guided to the upper surface  122   a  of the vibration membrane  122  by the first sound guide space  113 . Here, in the present embodiment, although the acoustic resistance portion  15  is formed over the first sound hole  111 , a sound wave passes through the acoustic resistance portion  15  and enters the first sound guide space  113  from the space outside the housing  11 . 
     Besides, the second sound hole  112  communicates with the second sound guide space  114 , and thus the second sound guide space  114  is in a state to communicate with the space outside the housing  11 . In other words, a sound outside the housing  11  enters the second sound guide space  114  via the second sound hole  112  and is guided to the lower surface  122   b  of the vibration membrane  122  by the second sound guide space  114 . Here, it is preferable that the distance between the first sound hole  111  and the second sound hole  112  is in a range of about 4 mm to about 6 mm for a purpose of improving the S/N (Signal to Noise) ratio of a voice output from the microphone unit  1  and the like. 
     In addition, in the present embodiment, although the first sound hole  111  and the second sound hole  112  each have substantially a circular shape when seen in a planar fashion, this is not a limitation, and the shape may be a shape other than the circular shape, or may be a rectangular shape or the like. Further, in the present embodiment, although the number of first sound hole  111  is one and the number of second sound hole  112  is also one, this is not a limitation, that is, a plurality of the first sound holes  111  and a plurality of the second sound holes  112  may be used. 
     Besides, in the present embodiment, although the first sound hole  111  and the second sound hole  112  are formed on the same plane of the housing  11 , this structure is not a limitation, and these sound holes may be formed on different planes, that is, may be formed , for example, on adjacent planes or on planes opposite to each other. Nevertheless, it is preferable that the two sound holes  111 ,  112  are formed on the same plane of the housing  11 , because a sound path in a voice input apparatus (e.g., a mobile phone and the like) which incorporates the microphone unit  1  according to the present embodiment does not become complicated. 
       FIG. 3  is a schematic sectional view showing a structure of the MEMS chip  12  which the microphone unit  1  according to the present embodiment includes. As shown in  FIG. 3 , the 
     MEMS chip  12  includes: an insulation base substrate  121 ; the vibration membrane  122 ; an insulation membrane  123 ; and a fixed electrode  124 , and constitutes a capacitor type microphone. Here, the MEMS chip  12  is fabricated by using a semiconductor technology. 
     For example, an opening  121   a  which has substantially a circular shape when seen in a planar fashion is formed through the base substrate  121 , and thus a sound wave which comes from a lower-portion side of the vibration membrane  122  reaches the vibration membrane  122 . The vibration membrane  122  formed on the base substrate  121  is a thin film which is vibrated (vibrated in a vertical direction) by a sound wave, has electric conductivity and constitutes one end of an electrode. 
     The fixed electrode  124  is so disposed as to face the vibration membrane  122  with the insulation membrane  123  interposed therebetween. Thus, the vibration membrane  122  and the fixed electrode  124  form a capacitor. Here, the fixed electrode  124  is provided with a plurality of sound holes  124   a , so that a sound wave which comes from an upper-portion side of the vibration membrane  122  reaches the vibration membrane  122 . 
     In such MEMS chip  12 , when a sound wave enters the MEMS chip  12 , a sound pressure pf acts on the upper surface  122   a  of the vibration membrane  122  and a sound pressure pb acts on the lower surface  122   b  of the vibration membrane  122 . As a result of this, the vibration membrane  122  vibrates depending on a difference between the sound pressure pf and the sound pressure pb; a gap Gp between the vibration membrane  122  and the fixed electrode  124  changes, so that the electrostatic capacity between the vibration membrane  122  and the fixed electrode  124  changes. In other words, the entering sound wave is drawn out as an electric signal by the MEMS chip  12  which functions as the capacitor type microphone. 
     Here, in the present embodiment, although the vibration membrane  122  is under the fixed electrode  124 , these may be disposed into an inverse relationship (the vibration membrane is over the fixed electrode). 
       FIG. 4  is a view for describing a circuit structure of the ASIC  13  which the microphone unit  1  according to the present embodiment includes. The ASIC  13  is an embodiment of an electric circuit portion in the present invention and is an integrated circuit which performs an amplification process with a signal amplification circuit  133  to amplify an electric signal that is generated based on a change in the electrostatic capacity of the MEMS chip  12 . In the present embodiment, to accurately capture a change in the electrostatic capacity in the MEMS chip  12 , a charge pump circuit  131  and an operational amplifier  132  are included. Besides, a gain adjustment circuit  134  is included, so that it is possible to adjust the amplification factor (gain) of the signal amplification circuit  133 . An electric signal amplified by the ASIC  13  is output, for example, to a sound process portion, not shown, on a mount board on which the microphone unit  1  is mounted and processed. 
     Back to  FIG. 2 , the circuit board  14  is a board on which the MEMS chip  12  and the ASIC  13  are mounted. In the present embodiment, both MEMS chip  12  and ASIC  13  are mounted by flip-chip bonding and electrically connected to each other by a wiring pattern formed on the circuit board  14 . Here, in the present embodiment, although the MEMS chip  12  and the ASIC  13  are mounted by flip-chip bonding, this is not a limitation, and they may be mounted by using wire bonding, for example. 
     The acoustic resistance portion  15  is formed over the first sound hole  111 . In the present embodiment, the acoustic resistance portion  15  is composed of a sheet-shape acoustic resistance member which is formed into substantially a circular shape when seen in a planar fashion and is so disposed as to block the first sound hole  111 . As the acoustic resistance member, for example, a mesh member formed of a resin such as polyester, nylon or the like, or a stainless steel or the like is used. The opening of the mesh member is in a range of about 20 μm to about 100 μm, for example, and its thickness is about 0.1 mm, for example. However, these are merely examples, and the opening, the mesh number, the thickness and the like of the mesh member which is used as the acoustic resistance member are suitably changed according to a purpose, and are not limited to the above values. Here, the mesh number refers to the number of meshes that are present per inch (25.4 mm) Besides, the opening refers to a value which is obtained by the following formula in a case where the diameter of a line that constitutes a mesh is defined as a line diameter: the opening (μm)=(25400÷the mesh number)−the line diameter 
     Here, in the present embodiment, the acoustic resistance member which constitutes the acoustic resistance portion  15  is formed into substantially a circular shape when seen in a planar fashion. However, this is not a limitation, and the shape may be suitably changed, that is, may be formed into substantially a rectangular shape or the like, for example, when seen in a planar fashion. 
     The acoustic resistance portion  15  is so formed as to adjust the frequency characteristic of the first sound guide space  113 . This is for reducing a difference between the frequency characteristics of the first sound guide space  113  and the frequency characteristic of the second sound guide space  114 . Hereinafter, reasons for why such acoustic resistance portion  15  is formed are described in detail. 
     First, with reference to  FIGS. 5A and 5B , a directional characteristic which is required for the microphone unit  1  according to the present embodiment is described. Here, as shown in  FIG. 5A , a direction which connects the first sound hole  111  and the second sound hole  112  with each other is formed of 0° and 180° directions. Besides, the middle point between the first sound hole  111  and the second sound hole  112  is defined as M. 
     In this case, as shown in  FIG. 5B , assuming that the distance between a sound source and the middle point M is constant, the microphone unit  1  is so required as to allow the sound pressure (pf-pb) acting on the vibration membrane  122  to reach the maximum when the sound source is present in the 0° direction or in the 180° direction. On the other hand, it is required that the sound pressure (pf-pb) acting on the vibration membrane  122  to reach the minimum (0) when the sound source is present in the 90° direction or in the 270° direction. In other words, the microphone unit  1  according to the present embodiment is desired to have a feature (bidirectional characteristic) that the microphone unit  1  easily receives a sound wave which is carried from the 0° and 180° directions and does not easily receive a sound wave which is carried from the 90° and 270° directions. And, symmetry of the directional characteristic shown in  FIG. 5B  is related to background noise prevention performance and the microphone unit  1  is desired to have a directional characteristic that has excellent symmetry in the entire service frequency range. 
       FIG. 6  is a graph for describing a problem in a case of a structure where the acoustic resistance portion  15  is not formed in the microphone unit  1  according to the present embodiment. In  FIG. 6 , the horizontal axis (logarithmic axis) is the frequency, and the vertical axis is the output from the microphone. Besides, in  FIG. 6 , a graph (a) represented by a solid line indicates a frequency characteristic in a case where the acoustic resistance portion  15  is not formed in the microphone unit  1  and a sound wave is inhibited from entering through the second sound hole  112 . In addition, in  FIG. 6 , a graph (b) represented by a broken line indicates a frequency characteristic in a case where the acoustic resistance portion  15  is not formed in the microphone unit  1  and a sound wave is inhibited from entering though the first sound hole  111 . 
     Here, to obtain the data in  FIG. 6 , the sound source is set at a constant position in a direction which is deviated from the 90° and 270° directions (see  FIG. 5A ). Besides, the amplitudes (sound pressures) of the sound waves are the same in obtaining the data for each frequency. 
     Here, assuming that the microphone unit  1  is required to have the bidirectional characteristic shown in  FIG. 5B  for all the frequencies in the entire service frequency range (e.g, 100 Hz to 10 KHz). In this case, it is required that in the case where a sound wave is carried from the sound source set at a position in the direction deviated from the 90° and 270° directions into the microphone unit  1 , a constant output difference is maintained between the graph (a) and the graph (b) in  FIG. 6  in the service frequency range even if the frequency changes. Here, the constant output difference is a value which is decided based on a difference between the distance from the sound source to the first sound hole  111  and the distance from the sound source to the second sound hole  112 . With regard to this point, in the experimental result shown in  FIG. 6 , the graph (a) and the graph (b) maintain the constant output difference in a range of about 100 Hz to about 6 kHz. However, the above constant output difference is not maintained in a high-frequency band which exceeds about 6 kHz, and an inverse relationship in the magnitudes of output values between the graph (a) and the graph (b) is also seen. 
     As a cause of the above tendency in the high-frequency band, there is a cause that the frequency characteristic of the first sound guide space  113  and the frequency characteristic of the first sound guide space  114  are different from each other. In other words, in the microphone unit  1  according to the present embodiment, the ASIC  13  is disposed in the first sound guide space  113  for an aim of miniaturizing the apparatus. Because of this, it is suspected that an imbalance becomes great between the volume of the sound guide space  113  and the volume of the sound guide space  114 , so that a difference between the frequency characteristic of the first sound guide space  113  and the frequency characteristic of the second sound guide space  114  occurs. And, it is suspected that the difference between the frequency characteristics is a cause which brings the result shown in  FIG. 6 . Accordingly, in the microphone unit  1  according to the present embodiment, the frequency characteristic of the first sound guide space  113  is adjusted by forming the acoustic resistance portion  15 , so that the difference between the frequency characteristic of the first sound guide space  113  and the frequency characteristic of the second sound guide space  114  is reduced. 
     As understood from the result shown in  FIG. 6 , in the microphone unit  1  according to the present embodiment, if the acoustic resistance portion  15  is not formed, a desired bidirectional characteristic (the characteristic shown in  FIG. 5B ) is obtained in a low-frequency side (a range of frequencies lower than about 6 kHz) while a desired bidirectional characteristic is not obtained in a high-frequency side (a range of frequencies higher than about 6 kHz). To avoid this, it is possible to dispose the acoustic resistance portion  15  which has a function to provide a microphone output represented by a broken line in  FIG. 7  in the microphone unit  1 . In other words, it is possible to form the acoustic resistance portion  15  which hardly acts on a sound in the low-frequency side and selectively acts on (drops the output in the high-frequency side) a sound in the high-frequency side (e.g, frequencies between 6 kHz and 20 kHz). 
     Here,  FIG. 7  is a view for describing the characteristic of the acoustic resistance portion  15  that the microphone unit  1  according to the present embodiment includes. In  FIG. 7 , the horizontal axis is a logarithmic axis. 
       FIG. 8  is a view for describing an effect in a case where an acoustic resistance member is so disposed as to block the sound guide space. In  FIG. 8 , the horizontal axis (logarithmic axis) is the frequency and the vertical axis is the output from the microphone unit. Besides, in  FIG. 8 , a graph (a) is a result in a case where an acoustic resistance member is not disposed; a graph (b) is a result in a case where an acoustic resistance member a is disposed; and a graph (c) is a result in a case where an acoustic resistance member b which has a characteristic different from that of the acoustic resistance member a is disposed. Here, although  FIG. 8  shows the results in a case where a microphone unit which has a structure different from the structure of the microphone unit  1  is used, the tendency obtained here is also true of the microphone unit  1  according to the present embodiment. 
     As shown in  FIG. 8 , it is understood that by disposing the acoustic resistance members a and b to block the sound guide spaces, the microphone output is able to be selectively attenuated in the high-frequency band side without hardly changing the microphone output in the low-frequency band side. Besides, it is also understood that by changing the characteristics of the acoustic resistance members, the attenuation amount of the microphone output for each frequency is able to be changed. Accordingly, it is understood that by so forming the acoustic resistance portion  15  as to block the first sound guide space  113  as in the microphone unit  1  according to the present embodiment, the difference between the frequency characteristic of the first sound guide space  113  and the frequency characteristic of the second sound guide space  114  is able to be reduced. 
     Here, the main determinants of the characteristic of an acoustic resistance member which is formed of a sheet-shape mesh member are the mesh number (which corresponds to the density of holes formed through the mesh member), the opening (which corresponds to the size of a hole of the mesh member) of the mesh, and the thickness. Accordingly, by adjusting these determinants, it is possible to obtain an acoustic characteristic member which has a desired characteristic. 
     Here, effects in the case where the microphone unit  1  according to the present embodiment is used are described. 
     For example, in a case where the microphone unit  1  according to the present embodiment is applied to a close-talking type voice input apparatus, a use&#39;s voice is generated from the vicinities of the first sound holes  111  and the second sound hole  112 . The user&#39;s voice which is thus generated in the vicinity of the vibration membrane  122  has a large sound pressure difference depending on a difference in the distance which extends to the vibration membrane  122 . Accordingly, a sound pressure difference is generated by the user&#39;s voice between the upper surface  122   a  of the vibration membrane  122  and the lower surface  122   b  of the vibration membrane  122  of the microphone unit  1 , so that the vibration membrane  122  vibrates. 
     On the other hand, as for noise such as background noise and the like, a sound wave appears at a position away from the first sound hole  111  and the second sound hole  112  compared with a user&#39;s voice. The noise which thus appears at the position away from the vibration membrane  122  hardly generates a sound pressure difference even if there is a difference in the distance which extends to the vibration membrane  122 . Because of this, the sound pressure difference depending on the noise is cancelled by the vibration membrane  122 . 
     Accordingly, in the microphone unit  1 , it is possible to consider that the vibration membrane  122  is vibrated by a user&#39;s voice only which is near the vibration membrane  122 . Because of this, it is possible to consider an electric signal output from the microphone unit  1  as a signal which indicates the user&#39;s voice only with the noise removed. In other words, according to the microphone unit  1  in the present embodiment, it is possible to obtain the user&#39;s voice with the noise removed. Here, it is preferable that the distance between the first sound hole  111  and the second sound hole  112  is 5 mm or less. As the applicants disclose in JP-A-2008-258904, a ratio of the intensity based on a phase difference component between two sound waves which respectively enter from the first sound hole  111  and the second sound hole  112  and reach the vibration membrane  122  to the intensity of a sound wave which enters from the first sound hole  111  and reaches the vibration membrane  122  or of a sound wave which enters from the second sound hole  112  and reaches the vibration membrane  122  is able to be adjusted to 0 dB or less in an employed frequency band of 100 Hz to 10 kHz, so that it is possible to achieve an excellent background noise suppression function. 
     Besides, in the microphone unit  1  according to the present embodiment, because the ASIC  13  which processes an electric signal that is generated based on the vibration of the vibration membrane  122  is disposed in the first sound guide space  113 , miniaturization of the microphone unit  1  is possible. If the distance between the first sound hole  111  and the second sound hole  112  decreases to 5 mm or less, absolute volumes of the first sound guide space  113  and the second sound guide space  114  also decrease. In such a case, if the ASIC  13  is disposed in one of the sound guide spaces  113  and  114 , an imbalance between the volumes occurs, so that a phenomenon easily takes place, in which a difference between the frequency characteristic of the first sound guide space  113  and the frequency characteristic of the second sound guide space  114  occurs. 
     When the ASIC  13  is disposed in the first sound guide space  113 , because of the imbalance between the volume of the first sound guide space  113  and the volume of the second sound guide space  114 , the desired bidirectional characteristic is not obtained especially in the high-frequency band, so that excellent noise prevention performance is not obtained. However, in the microphone unit  1  according to the present embodiment, because a difference in the frequency characteristics between the first sound guide space  113  and the second sound guide space  114  is able to be reduced by forming the acoustic resistance portion  15 , it is possible to obtain excellent noise prevention performance in the high-frequency side. In other words, it is possible to say that the microphone unit  1  according to the present embodiment is a small-size and high-performance microphone unit. 
     The above-described embodiments are examples and the microphone unit according to the present invention is not limited to the structures of the above-described embodiments. Various modifications may be made within the scope which does not depart from the object of the present invention. 
     For example, in the above-described embodiments, the acoustic resistance portion  15  is formed by disposing the acoustic resistance member over the first sound hole  111 . However, the acoustic resistance member (the acoustic resistance portion) may be formed at a position through which a sound wave that propagates from the first sound hole  111  to the vibration membrane  122  via the first sound guide space  113  passes. In other words, the acoustic resistance member may be so disposed as to block at least part of the route which extends from the first sound hole  111  to the upper surface  122   a  of the vibration membrane  122 . Here, in the present embodiment, the acoustic resistance member blocks all the portions of the route which extends from the first sound hole  111  to the upper surface  122   a  of the vibration membrane  122 . 
     Besides, in the above-described embodiments, the acoustic resistance portion  15  is formed by mounting the acoustic resistance member on the housing  11 . However, the structure of the acoustic resistance portion  15  is not limited to this, and for example, it may be formed by machining the housing  11 . Specifically, for example, as shown in  FIG. 9 , a microphone unit  21  may have a structure in which the first sound hole  111  is an aggregate of a plurality of small through-holes and the first sound hole  111  doubles the acoustic resistance portion  15 . 
     In addition, in the above-described embodiments, the acoustic resistance portion  15  is formed on only the first sound hole  111  side. However, this is not a limitation, and the acoustic resistance portion may be formed on the second sound hole  112  side as well besides the first sound hole  111  side. In this structure, the acoustic resistance portion is formed, both frequency characteristics of the first sound guide space  113  and the second sound guide space  114  are adjusted, and both frequency characteristics are matched with each other. 
     As a specific example of the structure in which the acoustic resistance portion is formed on the second sound hole  112  side as well besides the first sound hole  111  side, for example, as shown in  FIG. 10 , a structure (microphone unit  31 ) may be employed, in which two acoustic resistance members which have different characteristics are prepared and two acoustic resistance portions  15 ,  16  are formed. The two acoustic resistance members having different characteristics may be formed of different materials, for example, or may be formed of the same material, with parameters such as a thickness and the like changed. 
     As another specific example, as shown in  FIG. 11 , a structure (microphone unit  41 ) may be employed, in which the first sound hole  111  and the second sound hole  112  are blocked by only one acoustic resistance member (single member), for example. In this structure, for example, as shown in  FIG. 11 , a structure may be employed, in which by forming a step portion  17   a , an acoustic resistance portion  17  is so formed as to have different thicknesses at the first sound hole  111  side and the second sound hole  112  side. Thus, it is possible to reduce a difference between both frequency characteristics by adjusting both frequency characteristics of the first sound guide space  113  and the second sound guide space  114 . 
     Besides, in the above-described embodiments, although the acoustic resistance portion  15  is formed on only the first sound hole  111  side, the acoustic resistance portion  15  may be formed on only the second sound hole  112  side. For example, unlike the present embodiments, if the frequency characteristic of the second sound guide space  114  side is adjusted by changing the space shape of the microphone unit  1 , a difference between the frequency characteristic of the first sound guide space  113  and the frequency characteristic of the second sound guide space  114  can be reduced. 
     In addition, in the above-described embodiments, the structure is employed, in which the vibration membrane  122  (diaphragm) is disposed in parallel with a plane  11   a  through which the sound holes  111 ,  112  of the housing  11  are formed. However, this structure is not a limitation, and a structure may be employed, in which the diaphragm is not parallel with the plane through which the sound holes of the housing are formed. 
     Further, in the above-described microphone unit  1 , as a structure of the microphone (which corresponds to the MEMS chip  12 ) which includes the diaphragm, the structure is employed, in which the capacitor type microphone is disposed. However, of course, the present invention is applicable to a microphone unit which includes a microphone other than the capacitor type microphone. As structures other than the capacitor type microphone which includes the diaphragm, there are microphones such as a moving conductor microphone (dynamic type), an electromagnetic microphone (magnetic type), a piezoelectric microphone and the like. 
     The microphone unit according to the present invention is applicable to voice communication devices such as a mobile phone, a transceiver and the like, voice process systems such as a voice identification system and the like which employ a technology for analyzing an input voice, recording devices and the like.