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DESCRIPTION JP2005277489

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DESCRIPTION JP2005277489
PROBLEM TO BE SOLVED: To realize a secondary sound pressure gradient type electret
condenser microphone by providing two microphone units as an electret condenser microphone
for supplying power and extracting a signal by a two-wire system. SOLUTION: The first and
second condenser microphone units 10 and 20 have their diaphragms formed of the same
electret material, and an electret board made of the same electret material is provided on each
fixed electrode side, and the first condenser The microphone unit 10 charges the negative charge
only on the diaphragm side to make a film electret with positive phase signal output, and the
second condenser microphone unit 20 charges the negative charge only on the electret board
side, the signal output is reverse phase. It is assumed that there is a back electret, and the
positive phase signal output and the negative phase signal output are connected in an alternating
manner. [Selected figure] Figure 1
Secondary pressure gradient type electret condenser microphone
[0001]
The present invention relates to a secondary sound pressure gradient type electret condenser
microphone, and more specifically, a secondary sound pressure gradient type electret condenser
microphone capable of supplying a power and extracting a signal in a two-wire system, which is
particularly suitable for a close talk microphone. It is about
[0002]
As described in Patent Document 1, the close-talking microphone utilizes the proximity action of
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a spherical wave, and is low in sensitivity for sound waves coming from a distance, but has an
appropriate output level for near sound sources. It is designed to be obtained.
Directivity is included in the category of unidirectionality, and is used particularly for the
purpose of clearly picking up a speaker's voice in an environment where the surrounding noise is
large.
[0003]
Usually, an electret condenser microphone is used as the close-talking microphone, but a method
of supplying power and extracting a signal in a two-wire system is widely used for this kind of
electret condenser microphone. This method is referred to as plug-in power by some
manufacturers.
[0004]
An example of this will be described with reference to FIG. 3. The electret condenser microphone
for close-talking microphones usually includes one condenser microphone unit 1, and the
condenser microphone unit 1 incorporates an FET (field effect transistor) 2 as an impedance
converter. There is. When using the microphone, it is connected to an external DC power supply
E via a two-wire cable 3.
[0005]
That is, the drain 2 d of the FET 2 is connected to the core 3 a of the two-wire cable 3, and a
predetermined current is supplied from the external DC power supply E via the load resistance R
thereof. Further, a signal appearing at the drain 2d of the FET 2 is outputted to the outside
through the core wire 3a. Note that one pole of the condenser microphone unit 1 is connected to
the gate 2g of the FET 2, and the other pole and the source 2s of the FET 2 are both grounded
via the shield wire 3b of the two-wire cable 3.
[0006]
Unexamined-Japanese-Patent No. 2003-9275
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[0007]
The specifications of the voltage of the external DC power supply E and the resistance value of
the load resistance R used in the above two-wire power supply and signal extraction method for
the electret condenser microphone are determined for each manufacturer. Are not unified.
At present, for example, the power supply voltage is appropriately determined in the range of
about 1.5 to 20 V and the load resistance is in the range of about 680 to 5.6 kΩ.
[0008]
Generally, commercially available electret condenser microphones including close-talking
microphones have one condenser microphone unit and one FET incorporated accordingly, so the
voltage and load resistance supplied from an external DC power supply Even if it is different, it is
possible if it is within the above range.
[0009]
It is known that in order to narrow the directivity of the close-talking microphone, it is sufficient
to use a secondary sound pressure gradient type by using two unidirectional condenser
microphone units of positive phase and negative phase.
However, in this case, two FETs will be used, resulting in the following problems.
[0010]
For example, when the externally supplied power supply voltage is as low as 1.5 V, the two FETs
can not be operated in series. In parallel, two FETs can be operated even at a low voltage of about
1.5 V. In this case, the signals of the microphone units can be added by connecting the drains of
the FETs in an alternating manner. it can.
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[0011]
However, if the two microphone units are both membrane electret condenser microphone units
or back electret condenser microphone units, adding the respective signals as described above
results in no directivity and the secondary sound It does not become a pressure gradient type.
For this reason, at present, the secondary sound pressure gradient type has not been
manufactured for a two-wire electret condenser microphone.
[0012]
Therefore, an object of the present invention is to provide a two-wire electret condenser
microphone for supplying power and extracting a signal, which includes two microphone units to
realize a secondary sound pressure gradient type electret condenser microphone. is there.
[0013]
In order to solve the above-mentioned problems, the present invention provides first and second
two capacitors respectively including a diaphragm and a fixed pole disposed opposite to each
other, and an FET (field effect transistor) as an impedance converter. In a secondary sound
pressure gradient type electret condenser microphone including a microphone unit, an electret
board is provided on each of the fixed poles of the first and second condenser microphone units,
and the above-mentioned first and second condenser microphone units Each diaphragm and each
electret board are made of the same electret material, and in the first condenser microphone unit,
only the diaphragm side is charged with negative charge, while in the second condenser
microphone unit Negative on fixed side only Load is charged, the first, the drain of each FET of
the second condenser microphone unit is AC coupled, its respective drain signal is characterized
in that it is outputted is added.
[0014]
Since the IDSS (current when the gate voltage is 0) of the FETs varies among the FETs, the drains
of the respective FETs are connected via a variable resistor for adjusting the IDSS in order to
balance the DCs of the FETs. Preferably, they are connected.
[0015]
In the present invention, the drains of the respective FETs are drawn out through the core of the
two-wire cable in a state where the drains of the respective FETs are connected alternately, and
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the sources of the respective FETs are connected to the shield wire of the two-wire cable.
Also, the present invention includes an aspect of a close talk microphone.
[0016]
According to the present invention, the first condenser microphone unit is a membrane electret
condenser microphone unit and the drain signal is positive.
On the other hand, the second condenser microphone unit is a back electret condenser
microphone unit and its drain signal is in reverse phase.
Therefore, the secondary sound pressure gradient type electret condenser microphone can be
obtained by adding these drain signals.
[0017]
Further, since the FET included in the first capacitor microphone unit and the FET included in the
second capacitor microphone unit are connected in parallel, they can operate even at a low
voltage of about 1.5V. Therefore, it can cope with the existing two-wire system.
[0018]
In addition, since the same electret material is used for the diaphragm and the fixed pole of each
of the condenser microphone units described above, it is to manufacture a microphone unit
having the same directional frequency response although having different polarities in positive
and negative phases. Can. This makes it possible to provide a close-talking microphone that is
sensitive to only nearby sound sources without collecting noise from a distance over a wide band.
[0019]
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Next, an embodiment of the present invention will be described with reference to FIGS. 1 and 2,
but the present invention is not limited thereto. FIG. 1 is a schematic view showing a secondary
sound pressure gradient type electret condenser microphone according to the present invention,
and FIGS. 2 (a) and 2 (b) are exploded sectional views showing two condenser microphone units
included in the present invention.
[0020]
As shown in FIG. 1, since this electret condenser microphone is a secondary sound pressure
gradient type, it is provided with first and second condenser microphone units 10 and 20, each
of which is an impedance converter. FETs (field effect transistors) 11 and 21 are connected to
each other.
[0021]
Each of the condenser microphone units 10 and 20 is connected to an external DC power supply
E via the two-wire cable 3 described in FIG. 3 and a signal is taken out via the two-wire cable 3.
In the practice of the present invention, the two-wire cable 3, the external DC power supply E and
its load resistance R can be used as they are without any change.
[0022]
As shown in FIG. 2A, in the first condenser microphone unit 10, the diaphragm 110 and the fixed
electrode 120 stretched on the support ring 111 are disposed to face each other via the spacer
ring 130. As shown in FIG. 2B, the second condenser microphone unit 20 also has the diaphragm
210 and the fixed pole 220 stretched on the support ring 211 disposed to face each other with
the spacer ring 230 in between.
[0023]
The diaphragms 110 and 210 are both made of the same electret material. As an example of this
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diaphragm, one obtained by vapor-depositing gold on one side of a film of fluoroethylene
polymer (FEP) with a thickness of 12 μm can be mentioned.
[0024]
The electret boards 121 and 221 are also provided on the fixed poles 120 and 220, respectively.
Although the same electret material is used for the electret boards 121 and 221, it is preferable
that all of the diaphragms 110 and 210 and the electret boards 121 and 221 are the same
electret material (for example, FEP). The fixed electrodes 120 and 220 themselves may be
aluminum, copper alloy or the like.
[0025]
What is important in the present invention is that one of the two condenser microphone units 10
and 20 is a membrane electret condenser microphone unit, and the other is a back electret
condenser microphone unit.
[0026]
In this example, since the first condenser microphone unit 10 is a film electret condenser
microphone unit, only the diaphragm 110 is charged (negatively charged) with a negative
charge, and no charge is given to the electret board 121 on the fixed electrode 120 side.
[0027]
On the other hand, in order to make the second condenser microphone unit 20 a back electret
condenser microphone unit, negative charge is charged only to the electret board 221 on the
fixed electrode 220 side without giving electric charge to the diaphragm 210.
[0028]
The FET 11 of the first condenser microphone unit 10 and the FET 21 of the second condenser
microphone unit 20 are connected in parallel to the two-wire cable 3.
That is, the drains 11 d and 21 d of the FETs 11 and 21 are connected in parallel to the core wire
3 a of the two-wire cable 3.
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[0029]
Further, one pole of each condenser microphone unit 10, 20 is connected to the gates 11g, 21g
of the FETs 11, 21, respectively, and the other pole is connected to the sources 11s, 21s of the
FETs 11, 21 with the shield wire 3b of the two-wire cable 3 Connected to ground through.
[0030]
When the drains 11d and 21d of the FETs 11 and 21 are connected in parallel to the core 3a of
the two-wire cable 3, the drains 11d and 21d are connected in an alternating current via
coupling capacitors C1 and C2.
[0031]
In addition, although current is supplied to the drains 11d and 21d of the FETs 11 and 21 from
the external DC power supply E through the core wire 3a of the two-wire cable 3, the FET often
often has its own IDSS (gate There is variation in the current when the voltage is zero.
[0032]
Therefore, when the drains 11d and 21d of the FETs 11 and 21 are connected in parallel to the
core 3a of the two-wire cable 3, variable resistance (half It is preferable to put in fixed resistance
R1.
[0033]
Since the first condenser microphone unit 10 is a membrane electret condenser microphone unit,
the signal output of the drain 11 d is positive phase.
On the other hand, since the second condenser microphone unit 20 is a back electret condenser
microphone unit, the signal output of the drain 21 d is in reverse phase.
[0034]
In the present invention, since the positive phase signal output of the drain 11 d and the negative
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phase signal output of the drain 21 d are added (ie, subtracted) and output from the core 3 a of
the two-wire cable 3, secondary sound pressure gradient type Act as.
At that time, more accurate subtraction can be performed by balancing the DCs of the FETs 11
and 21 with the variable resistor R1.
[0035]
Further, by using the same material for the diaphragms 110 and 210 and the electret boards of
the electret boards 121 and 221, the two condenser microphone units 10 and 20 may be units
having the same directional frequency response although having opposite polarities. Can.
[0036]
FIG. 2 is a schematic view showing a secondary sound pressure gradient type electret condenser
microphone according to the present invention.
FIG. 2 is an exploded cross-sectional view of two condenser microphone units included in the
present invention.
The schematic diagram which shows the prior art example of a 2 wire type electret condenser
microphone.
Explanation of sign
[0037]
Reference Signs List 3 two-wire cable 3a core 3b shielded wire 10, 20 capacitor microphone unit
11, 21 FET 110, 210 diaphragm 120, 220 fixed pole 121, 221 electret board E DC power source
R load resistance R1 variable resistance C1, C2 coupling capacitor
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