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JP2004140629

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DESCRIPTION JP2004140629
An object of the present invention is to provide a bi-directional condenser microphone suitable
for high performance and mass production. SOLUTION: Vibrating films 2 and 2 are disposed
opposite to each other on the front and back sides of a fixed electrode 1, and the fixed electrode
1 and each vibrating film 2 on the outer peripheral side of the fixed electrode 1 and vibrating
films 2 and 2, An impedance conversion circuit was arranged to convert a change in capacitance
between 2 and 3 into an electrical signal. [Selected figure] Figure 2
コンデンサマイクロホン
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
condenser microphone, and more particularly to a condenser microphone suitable for providing
bidirectionality. 2. Description of the Related Art An example of the structure of a conventional
general electret condenser microphone (hereinafter referred to as ECM) will be described with
reference to FIGS. 3 and 4. FIG. As shown in FIGS. 3 and 4, the outer portion is generally covered
with a capsule 21. This capsule 21 is made of metal such as aluminum and the like, and has a
bottomed cylindrical shape having a bottom integrally formed by pressing or the like. There are
many things. The bottom of the capsule 21 is disposed on the upper side shown in the figure, ie,
on the front side as a microphone, ie, on the sound receiving side, but in this state on the front
side (hereinafter referred to as the front plate 21a) In order to be closed, a hole for introducing a
sound (hereinafter referred to as a sound hole 21b) is provided at the center of the front plate
21a. In the capsule 21, a predetermined tension is applied to a resin film, a metal foil or the like,
and the diaphragm 2 is accommodated, the outer periphery of which is attached to the
diaphragm mounting ring 3. The diaphragm attachment ring 3 is in contact with the inner
surface of the front plate 21a. The vibrating membrane 2 is coated with an electret dielectric film
on the back side and the lower side as shown in FIG. 4, and the vibrating membrane 2 is disposed
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opposite to the fixed electrode 20 via a spacer 7 via a predetermined clearance. ing. The fixed
electrode 20 is mounted on and held by a ring-shaped fixed electrode holder 22, and the fixed
electrode holder 22 is closed behind the shield 24 via the ring cushion 23 at the back of the
capsule 21 with respect to the shield 24. The end is bent and crimped by a press or the like. As
described above, the diaphragm attachment ring 3, the diaphragm 2, the spacer 7, the fixed
electrode 20, the fixed electrode holder 22, the ring cushion 23, and the shielding plate 24 are
mechanically fixed in the capsule 21. Many of the shielding plates 24 also serve as wiring boards,
and many of them have the impedance conversion circuit 25 mounted between the fixed
electrode 20 and the shielding plates 24 as shown in the figure, and the impedance conversion
circuit 25 It is common to be composed of field effect transistors, resistors and other circuit
elements. The vibrating membrane 2 and the fixed electrode 20 are connected to the impedance
conversion circuit 25, and the capacitances of the vibrating membrane 2 and the fixed electrode
20 are impedance transformed to reach the output end. Therefore, the sound, that is, the
compressional wave of air, enters the capsule 21 through the sound hole 21b provided in the
front plate 21a, and the pressure of the compressional wave vibrates the vibrating membrane 2
mounted on the front surface of the fixed electrode 20. The change in capacitance formed
between the vibrating membrane 2 and the fixed electrode 20 is converted into an acoustic
signal.
-Generally, the ECM has the above-described structure, but since the back of the diaphragm 2 is
closed, it is common knowledge that the ECM has the characteristics of a nondirectional
microphone. ing. However, recently, in the field of speech recognition or noise cancellation, etc.,
the function of the bi-directional microphone has been reviewed, and naturally the bi-directional
function is also required in the ECM. It is known that in order to obtain a bi-directional function,
it is a condition that both the front and back surfaces of the vibrating membrane 2 have a
structure that can receive sound pressure. In the ECM having the above structure, the surface of
the vibrating membrane 2 will be described. If the front plate 21 a side is the surface of the
vibrating membrane 2, the fixed electrode 20 side is the back surface. Therefore, in order to
obtain a bi-directional function, a sound pressure receiving area similar to the surface of the
vibrating membrane 2 must be provided on the back surface. That is, an open space similar to
the front side is required on the back side. For that purpose, it is conceivable to provide sound
holes similar to the front plate 21 a in the fixed electrode 20 and the shielding plate 24, and
various sound holes have been tried, but there are the following disadvantages. , Not bidirectional. That is, although it is possible to provide the sound hole 24 a having the same area as
the front plate 21 a in the shielding plate 24, when the sound hole 20 a is provided in the fixed
electrode 20, the electrode area basically becomes small, The ability as an electrode falls.
Therefore, the sensitivity is lowered and it does not function as an ECM. It is known that if a
tolerance of sensitivity is provided as a compromise and the area of the sound hole 20a of the
fixed electrode 20 is limited, only cardioid characteristics far from bi-directionality can be
obtained. Japanese Patent Application Laid-Open No. 7-143595 proposes a variable directional
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condenser microphone structure, and this publication describes a method for obtaining bidirectionality, but the method is a unidirectional microphone. Are housed in a capsule in a backto-back state. In other words, the structure is complicated as well as the number of parts for two
microphones is increased, so that the cost increases. [Patent Document 1] Japanese Patent
Application Laid-Open No. 7-143595 (page 3-4, FIG. 1) The present invention was made to solve
the above-mentioned problems. An object of the present invention is to provide a condenser
microphone having a bi-directional function, and further to provide a condenser microphone that
is high-performance and suitable for mass production.
SUMMARY OF THE INVENTION In order to solve the problems, approximately coaxial vibration
films are mounted on both surfaces of the fixed electrode, that is, on the front surface and the
back surface, and are constituted by field effect transistors, resistors and other circuit elements.
The impedance conversion circuits and the like are arranged outside the two diaphragms and the
fixed electrode so that signals of the respective diaphragms mounted on the front and back of the
fixed electrode can be arbitrarily controlled. That is, in the condenser microphone of the present
invention, the vibrating membrane is disposed opposite to the front side and the back side of the
fixed electrode, and the fixed electrode and the vibrating membrane between the fixed electrode
and the respective vibrating membrane on the outer peripheral side. An impedance conversion
circuit is disposed to convert a change in the capacitance of the sensor into an electrical signal. In
the condenser microphone of the present invention, the fixed electrode is formed in a bottomed
cylindrical shape provided with a flange at the upper part of the cylindrical part and a bottom at
the lower part, and vibrating films are respectively formed on the upper and lower surfaces of
the bottom. Are disposed opposite to each other, a substrate is disposed around the cylindrical
portion, and the substrate is provided with an impedance conversion circuit for converting a
change in capacitance between the bottom surface of the fixed electrode and each vibrating film
into an electrical signal. It is In each of the condenser microphones, the vibrating film is formed
of an electret dielectric film. In each of the condenser microphones, signals from the respective
diaphragms are processed with arbitrary conversion characteristics. Further, in each of the
condenser microphones, the fixed electrode is provided with an acoustic cavity facing the
vibrating film. Embodiments of the present invention will be described based on the drawings.
FIG. 1 is a plan view showing an ECM which is an embodiment of the present invention, and FIG.
2 is a cross-sectional view showing the same ECM. The fixed electrode 1 shown in the figure is a
bottomed cylindrical metal plate with a thickness of 0.5 mm, and a cylindrical portion 1b with a
height of 1 mm is formed at a position of 8 mm in diameter on the outer peripheral portion of
the bottom surface 1a. The flange 1c which extends horizontally toward the is formed. The outer
diameter of the flange 1c is 10 mm. The printed circuit board 4 is formed in a ring shape having
a thickness of 2 mm and an outer diameter of 15 mm and an inner diameter of 8.1 mm, and
predetermined wiring patterns are provided on the front and back surfaces. The inner periphery
of the printed board 4 is fitted with the cylindrical portion 1 b of the fixed electrode 1. A
vibrating film 2 is placed on a front surface of the bottom surface 1 a of the fixed electrode 1 and
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a central portion of the upper surface shown in FIG.
The vibrating film 2 has a resin film or the like coated with an electret dielectric film as in the
prior art, and the outer peripheral portion thereof is attached to the vibrating film mounting ring
3 in a state where a predetermined tension is applied. The diaphragm 2 to which the diaphragm
mounting ring 3 is attached as described above is held by the clamp 5 on the bottom surface 1 a
of the fixed electrode 1 so as to sandwich the spacer 7. That is, the convex portion around the
sound hole 5 b of the press fitting 5 presses the diaphragm attachment ring 3. Further, the inner
surface of the press fitting 5 presses the flange 1 c of the fixed electrode 1 against the printed
circuit board 4 via the ring cushion 8. The holding metal fitting 5 and the holding metal fitting 6
described later have the function of the capsule 21 of the conventional example. As shown in the
figure, pin-shaped convex portions 5a, 5a are provided at predetermined positions on the outer
bottom of the metal bracket 5, in the case of the present embodiment, at 180 ° divided positions
on the central line. The printed circuit board 4 is provided with four holes 4a, 4a... Corresponding
to the convex portions 5a, 5a. That is, the holes 4a, 4a,... Are provided at positions divided by 90
degrees with respect to the center of the printed board 4 as shown in the figure. When the
projections 5 a of the presser 5 are inserted into the corresponding holes 4 a of the printed
circuit board 4 and a predetermined pressure is applied, the projections of the presser 5 contact
the diaphragm attachment ring 3, The vibrating membrane 2 and the spacer 7 are pressed
against the fixed electrode 1, and the fixed electrode 1 is also pressed against the printed circuit
board 4 through the ring cushion 8. Since the copper foil pattern for printed wiring is provided
around the holes 4a, 4a into which the convex portions 5a, 5a of the printed circuit board are
inserted, the copper foil pattern and the convex portions 5a are maintained while the pressed
state is maintained. , 5a are soldered with solder 11. That is, the vibrating membrane 2, the
spacer 7, and the fixed electrode 1 are mechanically attached to the printed circuit board 4 at a
predetermined pressure. After the assembly process described above, the integral metal fitting 5,
the diaphragm 2, the fixed electrode 1 and the printed circuit board 4 are integrated, and the
same assembling operation as described above is performed, as shown in FIG. The vibrating
membrane 2 is mounted in a state in which the vibrating membrane 2 attached to the vibrating
membrane mounting ring 3 is sandwiched between 1 and the pressing metal fitting 6. Further,
the press fitting 6 is fixed to the printed board 4 in a state in which the ring cushion 9 is
sandwiched between the printed board 4 and the press fitting 6. In the case of the present
embodiment, holes 4a, 4a... Corresponding to the convex portions 5a and 6a of the pressing
brackets 5 and 6, respectively, are provided at four places on the printed circuit board 4 to which
the pressing brackets 5 and 6 are attached. There is.
And since two holes 4 a and 4 a (180 degrees division) are used for mounting the front side
diaphragm 2, the two holes 4 a and 4 a left for fixing the backside diaphragm 2 are used. Use
Then, the convex portion 6 a of the press fitting 6 is soldered to the copper foil pattern of the
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printed circuit board 4. When the above assembling operation is completed, the vibrating
membranes 2 and 2 are coaxially mounted on the front and back surfaces of the fixed electrode
1, and naturally the sound holes 5b and 6b are also provided on the front and back of the fixed
electrode 1. In the plan view of FIG. 1, the wiring pattern of the printed board 4 is not shown. As
shown in the plan view of FIG. 1, impedance conversion circuits 10 were mounted on both sides
of the printed circuit board 4. The fixed electrode 1 and the vibrating films 2 mounted on the
front and back surfaces of the fixed electrode 1 are composed of respective field effect
transistors, resistors and other circuit elements via the wiring pattern of the printed board 4. ,
10, and signals of capacitances between the vibrating membranes 2 and 2 and the fixed
electrode 1 are impedance converted and output to respective output terminals. That is, two
circuits for front and rear are respectively configured for the front and rear vibrating membranes
2 and 2. Therefore, in this state, sound pressure is applied to both the front and rear vibrating
films 2 and 2 with the fixed electrode 1 as a boundary. When a sound is emitted from a direction
of the ECM configured in this way, the sound enters through the sound hole 5b provided in the
press fitting 5 mounted on the front surface of the fixed electrode 1, that is, fixed by the
compressional wave of air. The vibrating membrane 2 on the front surface of the electrode 1
vibrates, and the vibrating membrane 2 mounted on the back surface of the fixed electrode 1
also vibrates due to the sound entering through the sound hole 6 b provided on the back side
pressing fitting 6. That is, acoustic signals due to the capacitance change formed between the
fixed electrode 1 are divided into two lines, and the signals are respectively led to the electric
circuits of the two circuits and reach the respective output terminals, By controlling the acoustic
signal output from this output terminal, it is possible to obtain a bi-directional function. In
addition, by controlling the characteristics of the respective impedance conversion circuits 10, it
becomes possible to obtain the cardioid characteristics. In the present embodiment, the fixed
electrode 1 is not provided with a cavity, but it is also possible to provide a cavity, which makes it
possible to adjust the frequency characteristics of the respective vibrating films 2 and 2 is there.
In the case of the present embodiment, independent impedance conversion circuits are provided
for each of the two systems of electrical circuits, but if the function is limited, acoustic signals
generated from the two systems of electrical circuits have one impedance. It is also possible to
process by a conversion circuit.
According to the present invention, an ECM having a bi-directional function is provided by
arranging a vibrating film before and after a fixed electrode and controlling two acoustic signals
derived from each vibrating film. In addition, conventional ECM production methods can be
applied as they are. Further, as in the conventional example (Japanese Patent Application LaidOpen No. 7-143595), it is possible to obtain a variable directivity condenser microphone by
using only one fixed electrode without using two fixed electrodes, and to obtain a bidirectional
capacitor. It becomes possible to obtain a microphone. Therefore, it is possible to provide an
inexpensive and stable quality bi-directional ECM. In addition, by controlling two acoustic signals,
it is possible to change the characteristics of bi-directionality, and it is also possible to easily
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provide an ECM according to the design purpose. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1
is a plan view showing an electret condenser microphone according to an embodiment of the
present invention. FIG. 2 is a cross-sectional view showing the same microphone. FIG. 3 is a plan
view showing an example of a conventional electret condenser microphone. FIG. 4 is a crosssectional view showing the same microphone. [Explanation of the code] 1 fixed electrode, 1a
bottom surface, 1b cylindrical portion, 1c flange 2 vibrating membrane 3 vibrating membrane
mounting ring 4 printed circuit board 4a hole 5 pressing bracket 5a convex portion 5b sound
hole 6 pressing bracket 6a convex portion 6b sound hole 7 spacer 8 ring cushion 9 ring cushion
10 impedance conversion circuit 11 solder 20 fixed electrode, 20a sound hole 21 capsule, 21a
front plate, 21b sound hole 22 fixed electrode holder 23 ring cushion 24 shield plate 24a sound
hole 25 Impedance conversion circuit
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