JP6468796B2 - Variable directivity electret condenser microphone - Google Patents

Description

  The present invention relates to a variable directional condenser microphone in which two unidirectional microphone units are arranged back to back, and particularly to a variable directional electret condenser microphone whose characteristics such as sensitivity and S / N do not change when the directivity is switched. .

As a microphone that can change the directivity, one in which two condenser microphone units are arranged back to back in front and back are known.
In one example, each of the two microphone units has a unidirectionality (cardioid characteristic). The diaphragm electrodes of the first microphone unit having the directional axis on the front side and the second microphone unit having the directional axis on the back side are connected in parallel in an AC manner by a coupling capacitor.

The conventional microphone achieves variable directivity by varying the polarity and voltage value of the polarization voltage applied to the diaphragm or fixed pole of the second microphone unit. This is disclosed in FIG.
In this case, the directivity of the microphone can be changed stepwise or continuously from omnidirectional to bidirectional, and unidirectionality can be obtained between omnidirectional and bidirectional.

When the variable directivity condenser microphone having this configuration is used as a single directivity, an audio signal from the diaphragm electrode of the second microphone unit is not used or output. However, since the diaphragm electrodes of the first and second microphone units are connected in parallel in an alternating manner, the audio signal generated on the first microphone unit side is attenuated by the capacitance on the second microphone unit side. Become.
Therefore, when the capacitances of the first and second microphone units are substantially the same, the audio signal level output from the microphone is attenuated by 6 dB, and the S / N is also deteriorated accordingly. Become.

In order to compensate for the above-described 6 dB attenuation, for example, a means for boosting the polarization voltage twice with a DC / DC converter is conceivable. However, when the value of the polarization voltage is increased, there is a problem with the withstand voltage of the element. Another problem occurs such as an increase in current consumption.
In this case, when the electret is used without using the polarization voltage source, the DC / DC converter can be made unnecessary. However, the attenuation of the audio signal due to AC connection of the front and rear diaphragm electrodes is also left as a problem.

  On the other hand, Patent Document 2 discloses a variable directional condenser microphone in which two condenser microphone units are arranged back and forth, and the outputs of the respective microphone units are coupled by a variable capacitor (variable capacitor) to continuously change the directivity. Has been.

According to this variable directivity condenser microphone, the fixed pole of the first microphone unit and the fixed pole of the second microphone unit are arranged close to each other back to back.
Moreover, the fixed pole of the first microphone unit is connected to one pole of the variable capacitor, and the fixed pole of the second microphone unit is connected to the other pole of the variable capacitor.
Then, by sequentially changing the capacitance of the variable condenser, the output level and polarity from the second microphone unit side change, and as a result, the directivity changes continuously.

According to the variable directivity condenser microphone disclosed in Patent Document 2 described above, each fixed pole of the first and second microphone units is connected to one of the poles of the variable condenser. The floating state from the reference potential point (ground), in other words, the potential of each fixed pole is made unstable.
For this reason, a phenomenon occurs in which the audio signal leaks and is always mixed due to the capacitance between the fixed poles of the first and second microphone units (stray capacitance between the fixed poles). For this reason, the variable directivity microphone of Patent Document 2 has a problem that directivity cannot be switched normally.

JP 54-133320 A Japanese Patent No. 4828375

  The present invention has been made to solve the above-described problems of the conventional variable directivity condenser microphone. Even if the directivity is switched, the sensitivity and S / N are not changed, and each microphone unit is fixed. It is an object of the present invention to provide a variable directivity electret condenser microphone that is not affected by electrostatic capacitance (stray capacitance) between electrodes.

  The variable directivity electret condenser microphone according to the present invention, which has been made to solve the above-described problems, includes a unidirectional first microphone unit and a second microphone unit in which a diaphragm and a fixed pole are arranged to face each other. Are arranged in a back-to-back manner so that the respective sound collecting axes face each other in the front-rear direction, and the first microphone unit has an electret dielectric film formed on a diaphragm. In addition, the second microphone unit has an electret dielectric film formed on a fixed pole, and each fixed pole of the first and second microphone units is DC-connected to a reference potential point of the circuit. It is characterized by that.

  In this case, preferably, a directivity selection switch that enables selection between the first directivity mode and the second directivity mode is provided, and when the first directivity mode is selected, the diaphragm of the first microphone unit. Each signal from the electrode and the diaphragm electrode of the second microphone unit is balanced and output from the diaphragm electrode of the first microphone unit when the second directivity mode is selected by the directivity selection switch. A configuration is adopted in which the signal and the signal at the reference potential point of the circuit are output in a balanced manner.

  More preferably, the signal from the diaphragm electrode of the first microphone unit is converted into one balanced output signal via the first impedance converter and the first buffer circuit connected to the first impedance converter. In addition, a signal from the diaphragm electrode of the second microphone unit selected by the directivity selection switch, or a signal at a reference potential point of the circuit via a DC cut capacitor, the second impedance converter, and the second A configuration is adopted in which the other balanced output signal is provided via a second buffer circuit connected to the impedance converter.

  In addition, a subtracting circuit that subtracts the balanced output signal is provided, and an audio signal is obtained by the subtracting circuit.

According to the electret condenser microphone having the above-described configuration according to the present invention, the first microphone unit employs a membrane electret system, and the second microphone unit employs a back electret system. As a result, signals obtained from the diaphragm electrodes of the first and second microphone units can be obtained in an opposite phase relationship.
Therefore, an omnidirectional audio signal can be obtained by using the signals obtained from the diaphragm electrodes of the first and second microphone units as balanced output signals. Further, by using the signal obtained from the diaphragm electrode of the first microphone unit and the non-signal at the reference potential point as a balanced output signal, a unidirectional audio signal can be obtained.

  In this case, when directivity is switched, as shown in Patent Document 1, one microphone unit becomes a load of a signal obtained from the diaphragm electrode of the other microphone unit, and an element that attenuates an audio signal Does not occur. Therefore, it is possible to provide a variable directivity condenser microphone that does not cause changes in sensitivity and S / N.

  Further, each fixed pole of the first and second microphone units is configured to be DC-connected to the reference potential point of the circuit. Therefore, as shown in Patent Document 2, it is possible to provide a variable directional condenser microphone that is not affected by the capacitance (stray capacitance) between the fixed poles.

It is sectional drawing of the variable directivity electret condenser microphone which concerns on this invention. It is a circuit connection example of the condenser microphone shown in FIG.

A variable directivity electret condenser microphone according to the present invention will be described based on an embodiment shown in the drawings.
FIG. 1 is a cross-sectional view showing an overall configuration in which two electret condenser microphone units are combined back to back.
1 is referred to as a composite unit 1 for convenience of explanation. The unidirectional first microphone unit disposed on the sound collecting axis side indicated by the arrow 0 deg in the composite unit 1 is referred to as a front unit Uf, and the unidirectional second microphone unit disposed on the opposite side. Is referred to as a rear unit Ub.

The composite unit 1 is provided with an insulating seat 2 formed in an annular shape at the center thereof. Retaining rings 3f and 3b are fitted and attached to the insulating seat 2 before and after.
The front unit Uf is mounted between the insulating seat 2 and the front pressing ring 3f, and the rear unit Ub is mounted between the insulating seat 2 and the rear pressing ring 3b. The composite unit 1 is configured by being mounted.

A diaphragm 12f held by the diaphragm holding ring 11f is disposed in the front unit Uf. A fixed pole 14f is disposed on the back surface of the diaphragm 12f via a ring-shaped spacer 13f. At this time, the periphery of the fixed pole 14f is in contact with the insulating seat 2.
A diaphragm electrode (not shown) is formed on the front surface of the diaphragm 12f. A part of the diaphragm electrode is electrically connected to a diaphragm electrode terminal 15f superimposed on the diaphragm holding ring 11f via a conductive diaphragm holding ring 11f.

In addition, an electret dielectric film 16f is attached to a surface of the front unit Uf facing the fixed pole 14f of the diaphragm 12f.
That is, the front unit Uf in this embodiment constitutes a membrane electret unidirectional condenser microphone unit.

The rear unit Ub is substantially the same as the configuration of the front unit Uf described above. A diaphragm 12b held by a diaphragm holding ring 11b is disposed in the rear unit Ub. A fixed pole 14b is disposed on the back surface of the diaphragm 12b via a ring-shaped spacer 13b. At this time, the peripheral edge of the fixed pole 14b is in contact with the insulating seat 2.
A diaphragm electrode (not shown) is formed on the front surface of the diaphragm 12b. A part of the diaphragm electrode is electrically connected to the diaphragm electrode terminal 15b superimposed on the diaphragm holding ring 11b via the conductive diaphragm holding ring 11b.

An electret dielectric film 16b is attached to the surface of the fixed unit 14b of the rear unit Ub facing the diaphragm 12b.
That is, the rear unit Ub in this embodiment constitutes a back electret unidirectional condenser microphone unit.

  In addition, a damper 4 that functions as an acoustic resistance is attached to the insulating seat 2 in the vicinity of the central portion of the insulating seat 2 described above. The damper 4 forms back air chambers 5f and 5b between the fixed pole 14f of the front unit Uf and the fixed pole 14b of the rear unit Ub, respectively.

FIG. 2 shows an example of circuit connection constituting a variable directivity electret condenser microphone using the composite unit 1 shown in FIG.
In FIG. 2, the main members of the composite unit 1 shown in FIG. 1 are denoted by the same reference numerals, and therefore detailed description thereof is omitted.

  M1 in FIG. 2 indicates the circuit configuration on the electret condenser microphone side including the composite unit 1 described above. A mixer M2 is connected to the electret condenser microphone M1 via a balanced shield cable Ca. On the mixer M2 side, a circuit is configured to obtain an audio signal from the electret condenser microphone M1.

As shown in FIG. 2, the fixed poles 14f and 14b of the composite unit 1 described above are connected in common and are DC-connected to a reference potential point (ground) of the circuit.
The diaphragm electrode terminal 15f of the front unit Uf in the composite unit 1 is connected to the impedance converter F1. The first impedance converter F1 includes a FET (field effect transistor), and is configured by, for example, a source follower circuit.

  The output signal from the first impedance converter F1 is supplied to the first buffer circuit E1. The first buffer circuit E1 is configured by an emitter follower circuit using, for example, a bipolar transistor, and an output signal from the first buffer circuit E1 is output as a hot-side output signal to the terminal pin P2 of the electret condenser microphone M1. Is done.

On the other hand, the diaphragm electrode terminal 15b of the rear unit Ub in the composite unit 1 is connected to one selected terminal of the directivity selection switch SW. The other selected terminal of the directivity selection switch SW is connected to a reference potential point (ground) of the circuit via a DC cut capacitor C1.
The state of the directivity selection switch SW shown in FIG. 2 indicates a state (first directivity mode) in which the diaphragm electrode of the rear unit Ub is selected. The selection terminal (common terminal) of the directivity selection switch SW is connected to the second impedance converter F2.

  The second impedance converter F2 has the same circuit configuration as the first impedance converter F1. The output signal from the second impedance converter F2 is supplied to the second buffer circuit E2. The second buffer circuit E2 has the same circuit configuration as the first buffer circuit E1. The output signal of the second buffer circuit E2 is output as a cold-side output signal to the terminal pin P3 of the electret condenser microphone M1.

  Each of the output signals provided to the terminal pins P2 and P3 is balanced output to the mixer M2 via the balanced shielded cable Ca having the terminal pin P1 as a ground line as an audio signal from the composite unit 1. Is done.

The mixer M2 that receives the balanced output signal via the balanced shielded cable Ca is equipped with a subtracting circuit OP1 using an operational amplifier. The hot side signal passing through the terminal pin P2 is supplied to the non-inverting input terminal of the subtraction circuit OP1, and the cold side signal passing through the terminal pin P3 is supplied to the inverting input terminal of the subtraction circuit OP1.
The subtraction output from the subtraction circuit OP1 is provided to the output terminal Out as an audio signal from the electret condenser microphone M1.

  In the composite unit 1 shown in FIG. 1, the front unit Uf employs a membrane electret system, and the rear unit Ub employs a back electret system. Therefore, when the directivity selection switch SW is in the first directivity mode shown in FIG. 2, signals of opposite phases are provided to the terminal pins P2 and P3. Therefore, an omnidirectional audio signal is provided to the output terminal Out of the mixer M2 as the subtraction output.

When the directivity selection switch SW is switched from the state shown in FIG. 2 to select the second directivity mode, the reference potential point (ground) of the circuit via the DC cut capacitor C1 shown in FIG. 2 is selected. Will be. In this embodiment, the terminal pin P3 is in a no-signal state.
Then, the audio signal from the diaphragm electrode of the front unit Uf is supplied to the terminal pin P2. Therefore, the audio signal from the front unit Uf is directly supplied to the output terminal Out of the mixer M2 as the subtraction output. As a result, the directivity in the second directivity mode is unidirectional.

  As described above, the variable directivity electret condenser microphone according to the above-described embodiment is switched between non-directional and unidirectional by the directivity selection switch SW. Then, when the directivity is switched, an element such as a capacitor that becomes a load of audio output from the front unit Uf or the rear unit Ub does not occur. Therefore, it is possible to provide a variable directivity condenser microphone in which the sensitivity and S / N do not change when the directivity is switched.

  Further, a configuration in which each fixed pole of the front unit Uf and the rear unit Ub is DC-connected to a reference potential point of the circuit is employed. Therefore, it is possible to provide a variable directivity condenser microphone that is not affected by the capacitance (stray capacitance) between the fixed poles, and obtains the operational effects as described in the column of the effect of the invention described above. Can do.

DESCRIPTION OF SYMBOLS 1 Composite unit 2 Insulator 3f, 3b Holding ring 12f, 12b Diaphragm 14f, 14b Fixed pole 15f, 15b Diaphragm electrode terminal 16f, 16b Electret dielectric film C1 DC cut capacitor Ca Balance shield cable E1, E2 Buffer circuit F1, F2 Impedance converter M1 Electret condenser microphone M2 Mixer OP1 Subtraction circuit P1 Terminal pin (Ground line)
P2 terminal pin (hot side output)
P3 terminal pin (cold side output)
SW Directivity selection switch Uf First microphone unit (front unit)
Ub 2nd microphone unit (rear unit)

Claims (4)

A unidirectional first microphone unit and a second microphone unit in which the diaphragm and the fixed pole are arranged to face each other are arranged back to back so that the respective sound collection axes face each other in the front-rear direction. A variable directional condenser microphone,
The first microphone unit has an electret dielectric film formed on a diaphragm, the second microphone unit has an electret dielectric film formed on a fixed pole, and the first and second microphone units. Each of the fixed poles of the variable directivity electret condenser microphone is characterized in that each of the fixed poles is DC-connected to a reference potential point of the circuit.   A directivity selection switch that enables selection of either the first directivity mode or the second directivity mode is provided, and when the first directivity mode is selected, the diaphragm electrode and the second of the first microphone unit are selected. When each signal from the diaphragm electrode of the microphone unit is balanced and the second directivity mode is selected, the signal from the diaphragm electrode of the first microphone unit and the signal at the reference potential point of the circuit are balanced. The variable directivity electret condenser microphone according to claim 1, wherein the variable directivity electret condenser microphone is output.   The signal from the diaphragm electrode of the first microphone unit is converted into one signal of a balanced output via a first impedance converter and a first buffer circuit connected to the first impedance converter, and the directivity is set. The signal from the diaphragm electrode of the second microphone unit or the signal at the reference potential point of the circuit through the DC cut capacitor selected by the selection switch is connected to the second impedance converter and the second impedance converter. 3. The variable directivity electret condenser microphone according to claim 2, wherein the other signal of the balanced output is made via a second buffer circuit. 4. The variable directivity electret condenser microphone according to claim 2, further comprising a subtracting circuit for subtracting the balanced output signal, wherein the subtracted output from the subtracting circuit is an audio signal.

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