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DESCRIPTION JP2010136133

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DESCRIPTION JP2010136133
The present invention provides a microphone unit which is formed to apply sound pressure to
both surfaces of a diaphragm and generates an electric signal by vibration of the diaphragm
based on a sound pressure difference, and provides a high performance microphone unit capable
of securing high SNR. . A microphone unit (1) includes a housing (11), a diaphragm (122)
disposed inside the housing (11), and an electric circuit unit (13) that processes an electrical
signal generated based on the vibration of the diaphragm (122). Prepare. The housing 11
includes a first sound introducing space 113 for guiding the sound outside the housing to the
first surface 122 a of the diaphragm 122 via the first sound hole 111, and a housing via the
second sound hole 112. A second sound introducing space 114 for guiding an external sound to
a second surface 122 b which is a back surface of the first surface 122 a of the diaphragm 122 is
provided. The resonance frequency of the diaphragm 122 is set within a range of ± 4 kHz with
reference to the resonance frequency of the first sound introducing space 113 or the second
sound introducing space 114. [Selected figure] Figure 2
マイクロホンユニット
[0001]
The present invention relates to a microphone unit that converts an input voice into an electrical
signal, and in particular, it is formed such that sound pressure is applied to both surfaces (front
and back surfaces) of the diaphragm, and the electric signal is converted by vibration of the
diaphragm based on the sound pressure difference. It relates to the configuration of the
microphone unit to be generated.
[0002]
18-04-2019
1
Conventionally, for example, a microphone unit is provided in an information processing system
using an audio communication device such as a cellular phone or a transceiver, or a technology
for analyzing input voice such as a voice authentication system, or a recording device.
It is preferable to pick up only the target voice (user's voice) at the time of a telephone call or the
like, voice recognition and voice recording. For this reason, development of a microphone unit
that accurately extracts a target voice and removes noise (background noise etc.) other than the
target voice is in progress.
[0003]
As a technique for removing noise and collecting only a target voice in a use environment in
which noise is present, it is possible to give directivity to a microphone unit. As an example of a
microphone unit having directivity, there is conventionally known a microphone unit that is
formed so that sound pressure is applied to both surfaces of a diaphragm (diaphragm) and
generates an electrical signal by vibration of the diaphragm based on the sound pressure
difference ( See, for example, Patent Document 1). JP-A-4-217199
[0004]
By the way, the microphone unit which is formed so that sound pressure is applied to both
surfaces of the diaphragm and generates an electric signal by the vibration of the diaphragm
based on the sound pressure difference applies the sound pressure only to one side of the
diaphragm to vibrate the diaphragm. The displacement due to the vibration of the diaphragm is
smaller than that of the microphone unit. For this reason, the microphone unit formed so that
sound pressure may be applied to both surfaces of the above-mentioned diaphragm may not be
able to obtain a desired SNR (Signal to Noise Ratio), and may be improved to secure high SNR. It
was being asked.
[0005]
Therefore, an object of the present invention is a microphone unit that is formed so that sound
pressure is applied to both surfaces of a diaphragm, and generates an electrical signal by
vibration of the diaphragm based on a sound pressure difference, and is a high performance that
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2
can ensure high SNR. It is providing a microphone unit.
[0006]
In order to achieve the above object, the present invention comprises a housing, a diaphragm
disposed inside the housing, and an electric circuit unit that processes an electrical signal
generated based on the vibration of the diaphragm. A microphone unit, wherein the housing
includes a first sound introducing space for guiding the sound of the outside of the housing to
the first surface of the diaphragm via the first sound hole, and a second sound hole. A second
sound introducing space for guiding the sound of the outside of the housing to a second surface
which is the back surface of the first surface of the diaphragm; It is characterized in that it is set
within a range of ± 4 kHz with reference to the resonance frequency of the sound introduction
space or the second sound introduction space.
[0007]
In the microphone unit of this configuration, the sound pressure difference between the sound
pressure exerted by the sound wave from the first sound hole on the diaphragm and the sound
pressure exerted by the sound wave from the second sound hole on the diaphragm in
consideration of the improvement of the SNR. Needs to be increased.
In this case, the space between the first sound hole and the second sound hole must be increased
to increase the volume of the first sound introducing space and the second sound introducing
space, and the first sound introducing space and the second sound introducing space Can not be
high enough.
That is, in the working frequency band of the microphone unit, it is inevitable that the resonance
of the sound introducing space influences the frequency characteristic of the microphone unit. In
this configuration, utilizing the fact that the resonance of the sound introducing space can not
affect the frequency characteristics of the microphone unit, the resonance frequency of the
diaphragm is lowered by the concept of reverse to the conventional one, It is configured to be
close to the resonance frequency. For this reason, according to this configuration, the stiffness of
the diaphragm can be lowered to increase the sensitivity, and a high performance microphone
unit capable of securing a high SNR can be provided.
[0008]
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3
In the microphone unit of the above configuration, it is preferable that a center-to-center distance
between the first sound hole and the second sound hole is 4 mm or more and 6 mm or less. With
such a configuration, it is possible to provide a microphone unit that can ensure the abovementioned sound pressure difference sufficiently and can also suppress the influence of phase
distortion to ensure a high SNR.
[0009]
Further, in the microphone unit having the above configuration, it is preferable that a resonance
frequency of the first sound introducing space or the second sound introducing space is 10 kHz
or more and 12 kHz or less. According to this configuration, the adverse effect on the frequency
characteristics of the microphone unit due to the resonance of the sound introducing space can
be suppressed as much as possible, which is preferable.
[0010]
Further, in the microphone unit having the above configuration, the resonance frequency of the
diaphragm may be set to be substantially the same as the resonance frequency of the first sound
introducing space or the second sound introducing space.
[0011]
According to the present invention, a high-performance microphone unit is formed by securing a
high SNR for a microphone unit that is formed so that sound pressure is applied to both surfaces
of the diaphragm and generates an electrical signal by vibration of the diaphragm based on the
sound pressure difference. Can provide
[0012]
Hereinafter, embodiments of a microphone unit to which the present invention is applied will be
described in detail with reference to the drawings.
[0013]
FIG. 1 is a schematic perspective view showing the configuration of the microphone unit of the
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4
present embodiment.
FIG. 2 is a schematic cross-sectional view at a position A-A in FIG.
As shown in FIGS. 1 and 2, the microphone unit 1 of the present embodiment includes a housing
11, a micro electro mechanical system (MEMS) chip 12, an application specific integrated circuit
(ASIC) 13, and a circuit board 14. Equipped with
[0014]
The housing 11 is formed in a substantially rectangular parallelepiped shape, and accommodates
the MEMS chip 12 including the diaphragm (diaphragm) 122, the ASIC 13, and the circuit board
14 therein.
The outer shape of the housing 11 is not limited to the shape of this embodiment, and may be,
for example, a cube, or is not limited to a hexahedron such as a rectangular parallelepiped or a
cube, and is other than a polyhedron structure or polyhedron other than hexahedron (For
example, a spherical structure, a hemispherical structure, etc.).
[0015]
As shown in FIGS. 1 and 2, the housing 11 is formed therein with a first sound introducing space
113 and a second sound introducing space 114. The first sound introducing space 113 and the
second sound introducing space 114 are divided by a vibrating film 122 included in the MEMS
chip 12 which will be described in detail later. That is, the first sound introducing space 113 is in
contact with the upper surface (first surface) 122 a side of the vibrating film 122 and the second
sound introducing space 114 is in contact with the lower surface (second surface) 122 b of the
vibrating film 122. It has become.
[0016]
Further, in the upper surface 11 a of the housing 11, a first sound hole 111 and a second sound
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5
hole 112 which are substantially circular in plan view are formed. The first sound hole 111 is
connected to the first sound introducing space 113, whereby the first sound introducing space
113 and the external space of the housing 11 are connected. That is, the sound outside the
housing 11 is guided to the upper surface 122 a of the vibrating membrane 122 by the first
sound introducing space 113 through the first sound hole 111.
[0017]
In addition, the second sound hole 112 is connected to the second sound introducing space 114,
whereby the second sound introducing space 114 and the external space of the housing 11 are
connected. That is, the sound outside the housing 11 is guided to the lower surface 122 b of the
vibrating membrane 122 by the second sound introducing space 114 through the second sound
hole 112. The distance from the first sound hole 111 through the first sound introducing space
113 to the diaphragm 122 and the distance from the second sound hole 112 through the second
sound introducing space 114 to the diaphragm 122 are equal to each other There is.
[0018]
The center-to-center distance between the first sound hole 111 and the second sound hole 112 is
preferably about 4 to 6 mm, and more preferably about 5 mm.
[0019]
Further, in the present embodiment, the first sound hole 111 and the second sound hole 112 are
substantially circular in a plan view, but the present invention is not limited to this. The shape
may be other than circular, for example, rectangular Or the like.
Further, in the present embodiment, one first sound hole 111 and one second sound hole 112
are provided, but the present invention is not limited to this configuration, and the number of
each may be plural.
[0020]
Further, in the present embodiment, the first sound hole 111 and the second sound hole 112 are
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formed on the same surface of the housing 11. However, the present invention is not limited to
this configuration. For example, they may be formed on adjacent surfaces or opposed surfaces.
However, when the two sound holes 111 and 112 are formed in the same surface of the housing
11 as in the present embodiment, the sound path in the voice input device (for example, a mobile
phone etc.) mounting the microphone unit 1 of the present embodiment. Is preferred in that it is
not complicated.
[0021]
FIG. 3 is a schematic cross-sectional view showing the configuration of the MEMS chip 12
provided in the microphone unit 1 of the present embodiment. As shown in FIG. 3, the MEMS
chip 12 has an insulating base substrate 121, a vibrating film 122, an insulating film 123, and a
fixed electrode 124, and forms a capacitor type microphone. The MEMS chip 12 is manufactured
using a semiconductor manufacturing technology.
[0022]
For example, an opening 121a having a substantially circular shape in plan view is formed in the
base substrate 121, whereby sound waves coming from the lower side of the vibrating film 122
reach the vibrating film 122. The vibrating film 122 formed on the base substrate 121 is a thin
film that vibrates (vibrates in the vertical direction) by receiving a sound wave, has conductivity,
and forms one end of an electrode.
[0023]
The fixed electrode 124 is disposed to face the vibrating film 122 with the insulating film 123
interposed therebetween. Thereby, the vibrating membrane 122 and the fixed electrode 124
form a capacitance. A plurality of sound holes 124 a are formed in the fixed electrode 124 so
that sound waves can pass therethrough, and sound waves coming from the upper side of the
vibrating film 122 reach the vibrating film 122.
[0024]
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In such a MEMS chip 12, when a sound wave is incident on the MEMS chip 12, the sound
pressure pf is applied to the upper surface 122a of the vibrating film 122, and the sound
pressure pb is applied to the lower surface 122b. As a result, the vibrating membrane 122
vibrates according to the difference between the sound pressure pf and the sound pressure pb,
and the distance Gp between the vibrating membrane 122 and the fixed electrode 124 changes,
and the static charge between the vibrating membrane 122 and the fixed electrode 124 The
capacitance changes. That is, the MEMS chip 12 functioning as a condenser type microphone can
extract the incident sound wave as an electric signal.
[0025]
In the present embodiment, the vibrating membrane 122 is lower than the fixed electrode 124,
but it is configured such that the opposite relationship to this (the vibrating membrane is upper
and the fixed electrode is lower) It does not matter.
[0026]
As shown in FIG. 2, in the microphone unit 1, the ASIC 13 is disposed in the first sound
introducing space 113.
FIG. 4 is a diagram for explaining the circuit configuration of the ASIC 13 provided in the
microphone unit 1 of the present embodiment. The ASIC 13 is an embodiment of the electric
circuit unit of the present invention, and is an integrated circuit that amplifies an electric signal
generated based on a change in capacitance in the MEMS chip 12 by the signal amplification
circuit 133. In the present embodiment, the charge pump circuit 131 and the operational
amplifier 132 are configured so as to accurately obtain the change in capacitance in the MEMS
chip 12. In addition, a gain adjustment circuit 134 is included so that the amplification factor
(gain) of the signal amplification circuit 133 can be adjusted. The electrical signal amplified by
the ASIC 13 is output to, for example, an audio processing unit of a mounting substrate (not
shown) on which the microphone unit 1 is mounted.
[0027]
Referring to FIG. 2, circuit board 14 is a substrate on which MEMS chip 12 and ASIC 13 are
mounted. In the present embodiment, the MEMS chip 12 and the ASIC 13 are both flip-chip
mounted, and both are electrically connected by the wiring pattern formed on the circuit board
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14. In the present embodiment, the MEMS chip 12 and the ASIC 13 are flip-chip mounted.
However, the present invention is not limited to this configuration. For example, the MEMS chip
12 and the ASIC 13 may be mounted using wire bonding.
[0028]
Next, the operation of the microphone unit 1 will be described.
[0029]
Prior to describing the operation, the nature of the sound wave will be described with reference
to FIG.
As shown in FIG. 5, the sound pressure of the sound wave (amplitude of the sound wave) is
inversely proportional to the distance from the sound source. Then, the sound pressure
attenuates sharply at a position close to the sound source, and gently attenuates as it gets away
from the sound source.
[0030]
For example, when the microphone unit 1 is applied to a close-talking type voice input device,
the user's voice is generated in the vicinity of the microphone unit 1. Therefore, the user's voice
is greatly attenuated between the first sound hole 111 and the second sound hole 112, and the
sound pressure incident on the upper surface 122a of the vibrating membrane 122 and the
sound pressure incident on the lower surface 122b of the vibrating membrane 122 There is a big
difference between
[0031]
On the other hand, a noise component such as background noise is present at a position where
the sound source is farther from the microphone unit 1 than the user's voice. Therefore, the
sound pressure of noise hardly attenuates between the first sound hole 111 and the second
sound hole 112, and the sound pressure incident on the upper surface 122a of the diaphragm
122 and the sound pressure incident on the lower surface 122b of the diaphragm 122 There is
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9
almost no difference between the sound pressure and the sound pressure.
[0032]
The vibrating film 122 of the microphone unit 1 vibrates due to the sound pressure difference of
the sound waves simultaneously incident on the first sound hole 111 and the second sound hole
112. As described above, since the difference in sound pressure of noise incident on the upper
surface 122 a and the lower surface 122 b of the vibrating membrane 122 from a distance is
very small, the noise is canceled by the vibrating membrane 122. On the other hand, since the
difference in sound pressure of the user's voice incident on the upper surface 122 a and the
lower surface 122 b of the diaphragm 122 from the close position is large, the user's voice
vibrates the diaphragm 122 without being canceled by the diaphragm 122.
[0033]
From this, according to the microphone unit 1, the vibrating membrane 122 can be regarded as
vibrating only by the user's voice. Therefore, the electrical signal output from the ASIC 13 of the
microphone unit 1 can be regarded as a signal indicating only user voice from which noise
(background noise and the like) has been removed. That is, according to the microphone unit 1 of
the present embodiment, it is possible to obtain an electric signal indicating only the user's voice
from which noise is removed with a simple configuration.
[0034]
By the way, when the microphone unit 1 is configured as in the present embodiment, the sound
pressure applied to the diaphragm 122 is the difference between the sound pressures input from
the two sound holes 111 and 112. For this reason, the sound pressure for vibrating the vibrating
membrane 122 is small, and the SNR of the electric signal to be taken out tends to be poor. In
this regard, the microphone unit 1 of the present embodiment is devised to improve the SNR.
This will be described below.
[0035]
18-04-2019
10
FIG. 6 is a diagram for describing a method of designing a diaphragm in a conventional
microphone unit. As shown in FIG. 6, the resonance frequency of the diaphragm provided in the
microphone unit changes with the stiffness of the diaphragm, and when designed so as to reduce
the stiffness, the resonance frequency of the diaphragm decreases. Conversely, if the stiffness is
designed to be high, the resonant frequency of the vibrating membrane will be high.
[0036]
Conventionally, when designing a microphone unit, the diaphragm has been designed so that the
resonance of the diaphragm does not affect the frequency band (the used frequency band) in
which the microphone unit is used. Specifically, with regard to the frequency characteristics of
the diaphragm, as shown in FIG. 6, the stiffness of the diaphragm is set so that the change in gain
with respect to the frequency change hardly occurs in the working frequency band of the
microphone unit (flat band). It was For example, when the operating frequency band is 100 Hz to
10 kHz, the stiffness of the vibrating membrane is set large so that the resonant frequency of the
vibrating membrane is about 20 kHz.
[0037]
When the stiffness of the vibrating membrane is set to be high so that the resonance frequency
of the vibrating membrane becomes high as described above, the sensitivity of the microphone
decreases. For this reason, there is a problem that the SNR tends to be deteriorated for the
microphone unit 1 configured to vibrate the vibrating film 122 by the sound pressure difference
between the upper surface 122a and the lower surface 122b of the vibrating film 122 as in this
embodiment.
[0038]
By the way, in the microphone unit 1, when the distance between the first sound hole 111 and
the second sound hole 112 is narrow, the differential pressure in the diaphragm 122 is small
(see Δp1 and Δp2 in FIG. 5), so the SNR of the microphone is improved. For this purpose, it is
necessary to increase the distance between the two sound holes 111 and 112 to some extent.
[0039]
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11
On the other hand, it has been found from the previous researches of the present inventors that
if the distance between the first sound hole 111 and the second sound hole 112 is too large, the
SNR of the microphone decreases due to the influence of the phase difference of sound waves.
(See, for example, Japanese Patent Application No. 2007-98486).
Because of this, the present inventors desirably set the center-to-center distance between the first
sound hole 111 and the second sound hole 112 to 4 mm or more and 6 mm or less, and further
to set it to about 5 mm. It is concluded that it is more desirable. With such a configuration, a
microphone unit capable of securing a high SNR (for example, 50 dB or more) can be obtained.
[0040]
In the microphone unit 1, in order to suppress the deterioration of the acoustic characteristics, it
is necessary to secure a cross-sectional area of the sound path of at least a certain size (for
example, the area of a circle of about φ 0.5 mm). Then, considering that the distance between
the first sound hole 111 and the second sound hole 112 is set to about 4 mm to 6 mm as
described above, the volume of the first sound introducing space 113 and the second sound
introducing space 114 Is a big thing.
[0041]
FIG. 7 is a diagram for explaining the frequency characteristics of the sound introducing space.
As shown in FIG. 7, the resonance frequency of the sound introducing space becomes lower as its
volume becomes larger, and becomes higher as its volume becomes smaller. As described above,
in the microphone unit of this embodiment, the volume of the sound guiding spaces 113 and 114
tends to be large, and the resonance frequency of the sound guiding spaces 113 and 114 tends
to be lower than that of the conventional microphone unit. Specifically, for example, the
resonance frequency of the sound introduction spaces 113 and 114 appears at about 10 kHz.
The frequency characteristics of the first sound introducing space 113 and the second sound
introducing space 114 are designed to be substantially the same.
[0042]
FIG. 8 is a diagram for explaining the frequency characteristic of the microphone unit. In FIG. 8,
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12
(a) is a graph showing the frequency characteristic of the vibrating membrane, (b) is the
frequency characteristic of the sound introducing space, and (c) is a graph showing the
frequency characteristic of the microphone unit. As shown in FIG. 8, the frequency characteristic
of the microphone unit exhibits the same frequency characteristic as the frequency characteristic
obtained by combining the frequency characteristic of the vibrating membrane and the
frequency characteristic of the sound introducing space.
[0043]
In the microphone unit 1 of the present embodiment, as described above, the volumes of the
sound introducing spaces 113 and 114 have to be increased to some extent. Therefore, it is
difficult to set the resonance frequencies of the sound introducing spaces 113 and 114 to be
high so that the resonances of the sound introducing spaces 113 and 114 do not affect the
above-mentioned use frequency band. When this point is taken into consideration, it makes sense
to set the resonance frequency of the vibrating membrane 122 to a high frequency (for example,
20 kHz) so that the resonance of the vibrating membrane does not affect the above-mentioned
use frequency band. Rather, improving the sensitivity of the vibrating membrane 122 by bringing
the resonant frequency of the vibrating membrane 122 close to the resonant frequency of the
sound introducing spaces 113 and 114 may be advantageous for improving the SNR of the
microphone unit 1.
[0044]
In the microphone unit 1 of the present embodiment, the resonance frequency fd of the
diaphragm 122 is set within a range of ± 4 kHz from the resonance frequency f1 of the first
sound introducing space 113 or the resonance frequency f2 of the second sound introducing
space 114. And as a result of examination, it turns out that SNR becomes good. Hereinafter, this
will be described with reference to FIG. 9, FIG. 10 and FIG. As described above, in the microphone
unit 1, the resonance frequency f1 of the first sound introducing space 113 and the resonance
frequency f2 of the second sound introducing space 114 are configured to be substantially the
same. Therefore, in the following, the resonance frequency f1 of the first sound introducing
space 113 will be described as a representative, if not particularly required.
[0045]
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13
FIG. 9 is a diagram showing frequency characteristics when the resonance frequency fd of the
vibrating membrane 122 is set to be approximately 4 kHz higher than the resonance frequency
f1 of the first sound introducing space 113 in the microphone unit 1 of the present embodiment.
FIG. 10 is a diagram showing frequency characteristics when the resonance frequency fd of the
vibrating membrane 122 is set to be substantially the same as the resonance frequency f1 of the
first sound introducing space 113 in the microphone unit 1 of the present embodiment. FIG. 11
is a diagram showing frequency characteristics when the resonance frequency fd of the vibrating
membrane 122 is set to be approximately 4 kHz lower than the resonance frequency f1 of the
first sound introducing space 113 in the microphone unit 1 of the present embodiment. In FIGS.
9 to 11, (a) shows the frequency characteristic of the diaphragm 122, (b) shows the frequency
characteristic of the first sound introducing space 113, and (c) shows the frequency
characteristic of the microphone unit 1.
[0046]
In order to increase the SNR of the microphone unit 1, it is desirable that the resonance
frequency f1 of the first sound introducing space 113 be as high as possible. Taking this point
into consideration, in FIGS. 9 to 11, the resonance frequency of the sound introduction spaces
113 and 114 of the microphone unit 1 is set to be near 11 kHz (10 Hz or more and 12 Hz or
less).
[0047]
As shown in FIG. 9, the peak derived from the resonant frequency fd of the vibrating membrane
122 is sharp, and the peak derived from the resonant frequency f1 of the first sound introducing
space 113 is broad. For this reason, even if the resonance frequency fd of the vibrating
membrane 122 approaches from the resonance frequency f1 of the first sound introducing space
113 to a frequency approximately 4 kHz higher, the frequency characteristic of the microphone
unit 1 on the low frequency side is hardly affected.
[0048]
Specifically, in FIG. 9, although the resonance frequency fd of the vibrating membrane 122 is
lowered to improve the sensitivity, it can be seen that the frequency characteristic of the
microphone unit 1 hardly changes around 10 kHz. That is, for example, when the upper limit of
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the high frequency side of the operating frequency band in the microphone unit 1 is 10 kHz, the
sensitivity of the diaphragm 122 is improved compared to the conventional one while
maintaining the characteristics of the microphone unit 1 in the operating frequency band. it can.
[0049]
As described above, in the microphone unit 1, since the resonance frequency of the sound
introducing spaces 113 and 114 can not be increased, it is not necessary to set the resonance
frequency of the vibrating membrane 122 high. Therefore, the stiffness is lowered (meaning that
the resonance frequency is lowered), and the sensitivity of the vibrating membrane 122 is
increased to improve the SNR. In order to increase the sensitivity of the vibrating membrane 122
to improve the SNR, the resonant frequency fd of the vibrating membrane 122 has never been
lower. However, if the resonant frequency fd of the vibrating membrane 122 is lowered too
much, the above-mentioned flat band (see, for example, FIG. 6) may be narrowed, and the SNR
may be lowered. That is, there is a lower limit to lowering the resonant frequency fd of the
vibrating membrane 122.
[0050]
Referring to FIG. 10, assuming that the resonance frequency fd of the vibrating membrane 122
and the resonance frequency f1 of the first sound introducing space 113 are substantially the
same, the frequency characteristic of the microphone unit 1 exceeds the resonance of 7 kHz and
the resonance of the vibrating membrane 122 The effect of lowering the frequency fd starts to
appear. When the upper limit of the operating frequency band of the microphone unit 1 is 10
kHz, there is some influence in the vicinity of 10 kHz, but such a design is also possible in
balance with the SNR improvement effect by raising the sensitivity of the vibrating membrane
122 is there.
[0051]
Also, the upper limit of the current voice band of the mobile phone is 3.4 kHz. In this case, when
the resonant frequency fd of the vibrating membrane 122 and the resonant frequency f1 of the
first sound introducing space 113 are substantially the same, the vibrating membrane 122 is
maintained as compared to the prior art while maintaining the characteristics of the microphone
unit 1 in the operating frequency band. It can be said that the sensitivity of can be improved.
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[0052]
Then, the result of further study on how much the resonance frequency fd of the vibrating
membrane 122 is lowered in consideration of the voice band of the present portable telephone is
a result shown in FIG. Considering the current cellular phone, the frequency characteristic of 3.4
KHz, which is the upper limit of the voice band used, is required to be within ± 3 dB with respect
to the 1 kHz output. In this respect, it has been found that even if the resonance frequency fd of
the vibrating membrane 122 is lowered to about 4 kHz than the resonance frequency f1 of the
first sound introducing space 113, the above-mentioned requirement is satisfied. In this case, the
resonance frequency fd of the vibrating membrane 122 can be lowered to about 7 kHz, and an
improvement in SNR due to the improvement of the sensitivity of the vibrating membrane 122
can be expected.
[0053]
As described above, in the microphone unit 1 of the present embodiment, the resonance
frequency fd of the vibrating membrane 122 is ± 4 kHz from the resonance frequency f1 of the
first sound introducing space 113 (or the resonance frequency f2 of the second sound
introducing space 114). If within the range, it can be said that the SNR can be improved when the
microphone unit 1 is applied to a voice input device.
[0054]
The vibrating film 122 of the microphone unit 1 of the present embodiment can be formed of, for
example, silicon.
However, the material for forming the vibrating film 122 is not limited to silicon. Desirable
design conditions in the case where the vibrating film 122 is formed of silicon will be described.
Note that, in deriving the setting conditions, the vibrating film 122 is modeled as shown in FIG.
[0055]
The resonance frequency fd (Hz) of the vibrating membrane 122 is expressed by the following
equation (1), where the stiffness of the vibrating membrane 122 is Sm (N / m) and the mass of
the vibrating membrane 122 is Mm (kg). .
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[0056]
Further, the stiffness Sm of the vibrating membrane 122 and the mass Mm of the vibrating
membrane 122 are expressed as the following formulas (2) and (3), respectively (see Non-Patent
Document 1).
Here, E: Young's modulus (Pa) of vibrating membrane 122, ρ: density of vibrating membrane
122 (kg / m <3>), ν: Poisson's ratio of vibrating membrane 122, a: radius of vibrating membrane
(m), t: thickness (m) of the vibrating membrane 122
[0057]
Jen-Yi Chen, Yu-Chun Hsu1, Tamal Mukherjee, Gray K. Fedder, "MODELING AND SIMULATION
OF A CONDENSER MICROPHONE", Proc. Transducers' 07, LYON, FRANCE, vol. 1, pp. 1299-1302,
2007.
[0058]
By substituting the equations (2) and (3) into the equation (1), the resonance frequency fd of the
vibrating membrane 122 is expressed as the following equation (4).
[0059]
As described above, it is desirable that the resonant frequency fd of the vibrating membrane 122
be ± 4 kHz from the resonant frequency f1 of the first sound introducing space 113.
Then, assuming that the desired resonant frequency f1 of the first sound introducing space 113
is 11 kHz, it is desirable that the resonant frequency fd of the vibrating membrane 122 satisfy
the following equation (5).
[0060]
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[0061]
Substituting E = 190 (Gpa), ν = 0.27, = 2 = 2330 (kg / m <3>) as the material characteristics of
silicon into the equation (5), the following equation (6) is obtained.
[0062]
That is, in the microphone unit 1 of the present embodiment, when silicon is selected as the
material of the vibrating film 122, high SNR can be obtained by setting the radius a and the
thickness t of the vibrating film 122 so as to satisfy the equation (6). A high performance
microphone unit 1 can be obtained.
[0063]
The embodiment shown above is an example, and the microphone unit of the present invention is
not limited to the composition of the embodiment shown above.
Various changes may be made to the configuration of the embodiment described above without
departing from the object of the present invention.
[0064]
For example, in the embodiment described above, the diaphragm 122 (diaphragm) is disposed in
parallel to the surface 11 a of the housing 11 in which the sound holes 111 and 112 are formed.
However, the present invention is not limited to this configuration, and the diaphragm may be
configured not to be parallel to the surface on which the sound hole of the housing is formed.
[0065]
Further, in the microphone unit 1 described above, a so-called condenser type microphone is
adopted as the configuration of the microphone (corresponding to the MEMS chip 12) having the
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diaphragm.
However, the present invention can also be applied to a microphone unit adopting a
configuration other than a capacitor type microphone as a configuration of a microphone having
a diaphragm. As a configuration other than the capacitor type microphone having a diaphragm,
for example, a dynamic type (dynamic type), an electromagnetic type (magnetic type), a
piezoelectric type microphone or the like can be mentioned.
[0066]
The microphone unit of the present invention is suitable for use in, for example, voice
communication devices such as mobile phones and transceivers, information processing systems
using techniques for analyzing input voice such as voice authentication systems, and recording
devices.
[0067]
These are schematic perspective views which show the structure of the microphone unit of this
embodiment.
FIG. 2 is a schematic cross-sectional view at a position A-A in FIG. These are schematic sectional
drawings which show the structure of the MEMS chip with which the microphone unit of this
embodiment is provided. These are figures for demonstrating the circuit structure of ASIC with
which the microphone unit of this embodiment is provided. These are figures for demonstrating
the attenuation | damping property of a sound wave. These are figures for demonstrating the
design method of the diaphragm in the conventional microphone unit. These are figures for
demonstrating the frequency characteristic of sound introduction space. These are figures for
demonstrating the frequency characteristic of a microphone unit. These are figures which show
the frequency characteristic at the time of setting the resonance frequency fd of a vibrating film
higher about 4 kHz than the resonance frequency f1 of 1st sound introduction space in the
microphone unit of this embodiment. These are figures which show the frequency characteristic
at the time of setting the resonance frequency fd of a diaphragm to the resonance frequency f1
of 1st sound introduction space substantially the same in the microphone unit of this
embodiment. These are figures which show the frequency characteristic at the time of setting the
resonant frequency fd of a vibrating film lower about 4 kHz than the resonant frequency f1 of
1st sound introduction space in the microphone unit of this embodiment. These are the figures
for demonstrating the model used in order to derive the conditions in the case of forming a
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vibrating film with silicon in the microphone unit of this embodiment.
Explanation of sign
[0068]
DESCRIPTION OF SYMBOLS 1 microphone unit 11 case 12 MEMS chip 13 ASIC (electric circuit
part) 111 1st sound hole 112 2nd sound hole 113 1st sound-conduction space 114 2nd soundconduction space 122 vibrating membrane (diaphragm) 122a upper surface of vibrating
membrane (First surface of diaphragm) 122b Lower surface of diaphragm (second surface of
diaphragm)
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