JP4873475B2 - Condenser microphone unit and condenser microphone - Google Patents

Description

  The present invention relates to a condenser microphone unit capable of continuously changing an attenuation amount of an input attenuator for preventing signal distortion due to excessive sound pressure input, and a microphone using the unit.

  Capacitor microphones that use a capacitance (capacitor) as an element that converts an audio input signal into an electrical signal are widely used from commercial use to consumer use because they can be reduced in size and weight. In a normal condenser microphone, a DC voltage is applied between electrodes of a microphone unit via a high resistance, and a change in voltage generated at both ends of the high resistance due to a change in unit capacitance is taken out as an electric signal. In addition, an electret condenser microphone in which the fixed pole or diaphragm of the microphone unit is formed of a thin film having an electret phenomenon instead of a normal capacitor is connected to the electrode on the side from which the signal is extracted with a high impedance, and the capacitance of the unit The change is taken out as an electrical signal. In general, such a microphone unit has a capacitance of several tens of pF and a high resistance of several hundred MΩ or more, but the output impedance of the microphone needs to be a low impedance of several hundreds of Ω. Therefore, an impedance conversion circuit composed of an FET, a vacuum tube, or the like is provided to convert a high input impedance into a low output impedance.

An example in which an impedance conversion circuit is used in the electret condenser microphone unit is shown in FIG. In FIG. 5, one of the fixed pole and the diaphragm of the microphone unit 1 is connected to a resistor 13 and an impedance converting FET 14 for taking out the capacitance change as a voltage change. The drain of the FET 14 is connected to the power supply terminal 16 via another FET 15. The FET 15 improves the attenuation of the signal due to the input capacitance of the FET 14 being substantially zero, and improves the distortion rate by making the voltage between the drain and source of the FET 14 almost unchanged with respect to the input signal. Is to do. The source of the FET 14 is connected to the ground terminal 18 through the FET 17 constituting the constant current circuit. Like the FET 15, the FET 17 improves the distortion and attenuation of the signal due to its high internal impedance. The source of the FET 14 is connected to the output terminal 19, and the output terminal 19 is also connected to the gate of the FET 15, one end of the resistor 13, and the drain of the FET 17. The other end of the microphone unit 1 is connected to the ground terminal 18. Device 13 described above, device 14, device 15, and the circuit is configured by the element 17 is the impedance conversion circuit 30.

  As described above, the microphone unit 1 using the electret requires the impedance conversion circuit 30. However, when an excessive sound pressure is input to the microphone unit 1, that is, when a signal input from the microphone unit 1 to the impedance conversion unit is too large, the signal output from the output terminal 19 through the impedance conversion circuit 30 is changed. Distortion occurs. In order to prevent such signal distortion, an attenuator (PAD) comprising a capacitor 12 and a switch 11 of several tens to several hundreds pF between the microphone unit 1 and the impedance conversion circuit 30 as shown in FIG. The unit 20 is interposed. The attenuator unit 20 closes the switch 11 (ON) and connects the capacitor 12 in series with the microphone unit 1 when the audio input signal is predicted to increase in advance. By doing so, it becomes equivalent to an increase in stray capacity, and an audio input signal input from the microphone unit 1 to the impedance conversion unit 30 can be attenuated. Further, when it is predicted in advance that the level of the audio input signal is low, the switch 11 is opened to release the operation of the attenuator unit 20 and the capacitor 12 is turned off to attenuate more than necessary. You can avoid that.

However, in the attenuator unit 20 as described above, when the switch 11 is closed and the capacitor 12 is connected in parallel to the microphone unit 1, the S / N ratio of the audio signal is deteriorated, so that it is not necessary to operate the attenuation unit 20. A switch 11 is provided to prevent the capacitor 12 from operating at times. However, there is a risk that the switch 11 picks up external noise. In order to prevent this, a strict shield for protecting the switch 11 may be provided. However, the structure of the shield is complicated, and the microphone case itself is enlarged.

  Therefore, an invention relating to a condenser microphone having a phase inverting circuit that phase-converts the output of the impedance conversion unit and feeds it back to the converter unit in order to attenuate the audio input signal without degrading the S / N ratio is known ( (See Patent Document 1).

Japanese Patent No. 3148348

Attenuation by the attenuator unit 20 to change the capacitance of the capacitor 1 2 is changed. Therefore, when it is desired to vary the amount of attenuation by the attenuator unit 20, it can be realized by using a plurality of capacitors corresponding to the capacitor 12 and using a plurality of switches 11 corresponding to each capacitor. Further, by using the variable capacitance (variable capacitor) to the capacitor 1 2, it is possible to vary the attenuation amount in the same way.

However, when using a plurality of capacitors corresponding to capacitors 1 2, it is necessary to pay the microphone case together with switch 1 2, microphone becomes large. In the case of using a variable capacitor in place of the condenser 1 2, and the vibrating electrode plate constituting the variable capacitor is by ultrasonic or mechanical vibration, it becomes a cause of noise is mixed into the audio input signal.

Furthermore, by using a variable capacitance diode instead of the capacitor 1 2, attenuators but the attenuation of the unit 20 can be varied, the distortion of the audio signal by the variable capacitance diodes the signal from the microphone unit 1 increases occurred Adversely affects the quality of the microphone output.

  The present invention has been made in view of the above problems, and is configured so that the capacitance of an attenuator used in a condenser microphone unit can be varied, and the amount of attenuation can be varied, and a microphone using the unit. The purpose is to provide.

The present invention is a condenser microphone unit comprising a diaphragm, a diaphragm holder to which the diaphragm is fixed, and a fixed pole that is disposed opposite to the diaphragm with a gap therebetween and constitutes a capacitor. there are, capacitor unit for use in attenuator connected in parallel with the condenser microphone unit is a voltage variable capacitance using a piezoelectric element, the voltage for driving the piezoelectric element, the output signal of the capacitor microphone unit and a smoothing circuit The main feature is that the voltage is DC converted by a limiter circuit .

  Further, the present invention is characterized in that the capacitor unit is a voltage variable capacitance using a piezoelectric element.

  According to the present invention, when an excessive sound pressure is input to the microphone unit, the attenuation amount of the PAD can be increased by the voltage due to the excessive sound pressure, so that a microphone unit having an attenuator linked to the input sound pressure is obtained. be able to.

  Embodiments of a condenser microphone unit according to the present invention will be described below with reference to the drawings. FIG. 1 is an example of a circuit diagram of a condenser microphone unit according to the present application, which includes a microphone unit 1, an attenuator unit 20 a, and an impedance conversion unit 30. As is well known, the microphone unit 1 has a film-like diaphragm and a fixed pole arranged to face the diaphragm with a predetermined gap, and vibration caused by vibration of the diaphragm upon receiving sound waves. A change in capacitance between the plate and the fixed pole is output as a change in voltage. In FIG. 1, either the fixed pole or the diaphragm of the microphone unit 1 is connected to a resistor 13 and an impedance conversion FET 14 for extracting the capacitance change as a voltage change. The drain of the FET 14 is connected to the power supply terminal 16 via another FET 15. The FET 15 improves the attenuation of the signal due to the input capacitance of the FET 14 being substantially zero, and improves the distortion rate by making the voltage between the drain and source of the FET 14 almost unchanged with respect to the input signal. Is to do. The source of the FET 14 is connected to the ground terminal 18 through the FET 17 constituting the constant current circuit. Like the FET 15, the FET 17 improves the distortion and attenuation of the signal due to its high internal impedance. The source of the FET 14 is connected to the output terminal 19, and the output terminal 19 is also connected to the gate of the FET 15, one end of the resistor 13, and the drain of the FET 17. The other end of the microphone unit 1 is connected to the ground terminal 18. A circuit constituted by the elements 13 to 15 described above is an impedance conversion circuit 30.

  In the attenuator unit 20a, the side of the microphone unit 1 connected to the FET 14 is also connected to one terminal of the switch 11. The other terminal of the switch 11 is connected to the fixed pole of the voltage variable capacitor 2 formed by a piezoelectric element. The movable pole of the voltage variable capacitor 2 is connected to the ground terminal 18. The positive electrode of the power supply 27 for driving the voltage variable capacitor 2 is connected to the piezoelectric element, and the negative electrode is connected to the ground terminal 18. The structure of the voltage variable capacitor 2 will be described in detail below.

  Next, the structure and operation of the voltage variable capacitor 2 will be described with reference to FIG. As shown in FIG. 2A, the voltage variable capacitor 2 is a movable pole that is a piezoelectric element 21, a capacitor pole that is in contact with the longitudinal plane of the piezoelectric element 21, and is bent downward by the bending of the piezoelectric element 21. 22, a fixed pole 24 made of a copper metal foil that forms a capacitor opposite to the movable pole 22, a substrate 25 that fixes the fixed pole 24, and a spacer 23 that provides a gap between the movable pole 22 and the fixed pole 24. It is formed integrally. FIG. 2B is a cross-sectional view of the voltage variable capacitor 2 formed by stacking the above-described constituent members.

  Here, the outline of the piezoelectric element 21 will be described with reference to FIG. In FIG. 6, a piezoelectric element 21 is formed by joining two piezoelectric elements 211 and 212 that expand and contract in the length direction and applying a voltage between the two piezoelectric elements so that when one piezoelectric element extends, the other piezoelectric element is expanded. The device is configured to shrink. In any piezoelectric element, the polarization direction is the thickness direction. By connecting a voltage source between the electrodes outside the two piezoelectric elements 211 and 212 that overlap each other and applying a voltage to the two piezoelectric elements 211 and 212 in series, one piezoelectric element extends and the other piezoelectric element extends. As the element contracts, the piezoelectric element 21 is bent upward or downward in FIG.

  Returning to FIG. 2, the movable pole 22 is in contact with one of the longitudinal outer surfaces of the piezoelectric element 21, and the drive power supply 26 is connected to the other surface. As shown in FIG. 2C, when the piezoelectric element 21 is bent downward as viewed in the drawing by the driving power source 26, the movable pole 22 in contact with the piezoelectric element 21 is also bent downward. FIG. 2D shows this state. When the gap between the movable pole 22 and the fixed pole 24 is narrowed as shown in FIG. 2D, the capacitance increases as compared with the state shown in FIG. Since the amount of bending of the piezoelectric element 21 is determined by the voltage of the drive power supply 26, the capacitance of the voltage variable capacitor 2 can be varied by varying the voltage of the drive power supply 26.

  Returning to FIG. 1, the voltage variable capacitor 2 having the above-described structure is bent so that the piezoelectric element 21 pushes the movable pole 22 toward the fixed pole 24 when a voltage is applied from the power supply 27. . In the example shown in FIG. 1, the power source 27 is a variable power source. Therefore, when the capacitance of the voltage variable capacitor 2 is varied, the output voltage of the power supply 27 is changed. As in the example shown in FIG. 1, the power source 27 may be a variable power source using a built-in battery, but may be a variable power source using a DC power source that drives the microphone unit itself. Moreover, you may use together the DC power source which drives a battery and a microphone unit. If the sound pressure input to the microphone unit 1 is expected to be low, the PAD unit 2 does not need to be operated. Therefore, the voltage variable capacitor 2 may be disconnected from the circuit by opening the switch 11.

In the above embodiment, when the sound pressure is input to the microphone unit 1 and the output voltage is generated in the impedance conversion unit 30, if the switch 11 is closed, the capacitor built in the microphone unit 1 and the voltage variable Capacitance capacitor 2 is connected in parallel . In the variable capacitor 2, the piezoelectric element 21 bends in accordance with the voltage applied by the power supply 27, thereby narrowing the gap between the movable pole 22 and the fixed pole 24, so that an arbitrary capacitance can be obtained. The capacitance of the variable capacitor 2 is connected to the microphone unit 1 in parallel , which is equivalent to a state in which the stray capacitance is increased, and the signal input to the impedance conversion unit 30 can be attenuated. In this way, it is possible to set an optimum attenuation amount for the excessive sound pressure input.

  Next, another embodiment of the condenser microphone unit according to the present invention will be described. In FIG. 3, the power source for driving the piezoelectric element 21 of the voltage variable capacitor 2 converts the audio signal output from the output terminal 19 into a DC voltage by the detection smoothing unit 28 provided at the subsequent stage of the impedance conversion unit 30. The converted DC voltage is used. When the sound pressure input from the microphone unit 1 increases, the sound signal output from the output terminal 19 also increases. Therefore, the DC voltage by the detection smoothing unit 28 is also increased. As already described, the piezoelectric element 2 used for the voltage variable capacitor 2 has a large amount of deflection depending on the applied voltage. For this reason, when the input audio signal increases, the capacitance of the voltage variable capacitor increases and the attenuation in the attenuator unit 20b increases, and the attenuation can be varied according to the excessive input. . The other circuit configuration is the same as that of the embodiment shown in FIG.

Next, another embodiment of the condenser microphone unit according to the present invention will be described. In FIG. 4, the power source for driving the piezoelectric element 21 of the voltage variable capacitor 2 converts the audio signal output from the output terminal 19 into a DC voltage by the detection smoothing unit 29 provided at the subsequent stage of the impedance conversion unit 30. The converted DC voltage is given a linear characteristic by a limiter circuit. Since the magnitude of the audio signal inputted to the capacitor unit 1 is changed from time to time, the voltage also changes I accompanied to it at the output terminal 19. If the output voltage has a non-linear characteristic, the capacitance by the voltage variable capacitor 2 is also non-linear, and the attenuation by the attenuator unit is not proportional to the input sound pressure. In order to prevent this, a limiter circuit is used in the detection smoothing unit 29. The other circuit configuration is the same as that of the embodiment shown in FIG. By configuring in this way, it is possible to obtain a condenser microphone capable of linearly varying the attenuation while eliminating the influence of the magnitude of the input audio signal.

  The condenser microphone unit according to the embodiment described above can be configured as a variable directivity condenser microphone by incorporating it into a microphone case.

It is a circuit diagram which shows the Example of the capacitor | condenser microphone unit which concerns on this invention. It is sectional drawing which shows the structure of the voltage variable capacitor used for the capacitor | condenser microphone unit which concerns on this invention. It is a circuit diagram which shows another Example of the capacitor | condenser microphone unit which concerns on this invention. It is a circuit diagram which shows another Example of the capacitor | condenser microphone unit which concerns on this invention. It is a circuit diagram which shows the example of the conventional capacitor | condenser microphone unit. It is explanatory drawing which shows the structure of the piezoelectric element applicable to this invention.

Explanation of symbols

1 Microphone unit 2 Voltage variable capacitor 13 Resistance 14 FET
15 FET
16 Power supply terminal 17 FET
DESCRIPTION OF SYMBOLS 18 Ground terminal 19 Output terminal 20a Attenuator unit 20b Attenuator unit 20c Attenuator unit 22 Movable pole 23 Spacer 24 Fixed pole 25 Substrate 26 Drive power supply 27 Power supply 28 Detection smoothing unit 29 Detection smoothing unit 30 Impedance conversion unit

Claims (3)

A condenser microphone unit comprising a diaphragm, a diaphragm holder to which the diaphragm is fixed, and a fixed pole that is disposed to face the diaphragm with a gap therebetween and constitutes a capacitor. ,
The capacitor unit used for the attenuator connected in parallel with the capacitor microphone unit is a voltage variable capacitance using a piezoelectric element,
The voltage for driving the piezoelectric element is a voltage obtained by direct- converting the output signal of the capacitor microphone unit by a smoothing circuit and a limiter circuit . 2. The condenser microphone unit according to claim 1, further comprising a switch for disconnecting the attenuator from the condenser microphone unit. Condenser microphone unit is a condenser microphone comprising integrated into the microphone case, the capacitor microphone capacitor microphone unit is a capacitor microphone unit according to claim 1 or 2.