JPS644720B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a condenser microphone, and in particular, it is possible to achieve high sensitivity, high S/N ratio, and high stability by keeping the boundary conditions of the diaphragm constant. The purpose is to provide a condenser microphone.

In general, capacitor microphones detect the capacitance between a movable electrode formed on the diaphragm and a fixed electrode, so it is necessary to apply a bias voltage to the electrode. Broadly divided into DC bias method and AC bias method.

Figure 1 shows the basic circuit configuration of a DC bias type capacitor microphone. In Fig. 1, 1 is the microphone cartridge, C _M is the capacitance between the diaphragm of the microphone cartridge 1 and the fixed electrode, and E _O is the capacitance between the diaphragm of the microphone cartridge 1 and the fixed electrode from the bias supply terminal 5. The applied DC bias voltage 2 is a high input impedance head amplifier (impedance converter) using a field effect transistor or the like, to which the output of the microphone cartridge 1 is applied via a coupling capacitor 4. 3 is the output circuit, V _O
is the microphone output taken out to output terminal 6, R
is the limiting resistance. A change in capacitance C _M caused by the incidence of a sound wave is detected as a change in voltage via the impedance converter 2 by applying a DC bias voltage E _O . FIG. 2 shows a microphone cartridge used in a DC bias type condenser microphone. In FIG. 2, 20 is a diaphragm, which is fixed to a diaphragm ring 21. 22 is a fixed electrode, and a vibrating membrane 20
A DC bias voltage E _O is applied between the two.
Electrostatic force generated by applying DC bias voltage E _O
The vibrating membrane 20 is attracted to the fixed electrode 22 by F _E and is displaced by Δx to a position balanced with the mechanical force F _M due to the tension of the vibrating membrane 20 . The electrostatic force F _E at this time,
The mechanical force F _M is F _E = ε _O・S・E ² / _O / Z (x _O − Δx) ² …(1
) F _M = s _O・Δx ...(2) However, ε _O : Dielectric constant of air S : Effective area of capacitance formed by the vibrating membrane 20 and fixed electrode 22 x _O : E _O = OV Vibrating membrane 20 and fixed electrode 22
The air gap between s _O : Stiffness of the diaphragm and the displacement Δx is given by F _E = F _M , where x _O ≫ Δx, Δx≒ε _O・S・E ² / _O /2・x ² / _O 1/ (s _O −ε _O S
E ² / _O / d ³ / _O )......(3) is given. By the way, in order to improve the S/N of a DC bias type capacitor microphone, since the noise component is limited by the hand amplifier 2, it is usually necessary to increase the DC bias voltage E _O or increase the voltage between the diaphragm and the fixed electrode. Possible means to improve the sensitivity of the microphone cartridge include narrowing the air gap _xO or loosening the tension of the diaphragm. However, as shown in Equation 1, the electrostatic attractive force F _E increases rapidly by using any of the above methods, and in extreme cases F _E > F _M , and the diaphragm is attached to the fixed electrode. Adsorb. Furthermore, even if F _E <F _M in a state where no sound waves are incident, F _E >F _M may occur due to the incidence of sound waves, and adsorption may occur.
That is, in a DC bias type capacitor microphone, there is an inverse relationship between improved sensitivity and improved stability, and high sensitivity and high stability cannot be obtained. Incidentally, the sensitivity S _eos and stability M of the microphone cartridge are given by the following equations.

S _eos = x ₁ /x _O・E _O ...(4) M=s _O /s _o (usually M>10) ...(5) where, x ₁ : Effective displacement of the diaphragm with respect to the reference sound pressure s _o = ε _O・S・E ² / _O / (x _O − Δx) ³ : (Negative stiffness) (6) Figure 3 shows the basic circuit configuration of an AC bias type capacitor microphone. In FIG. 3, 1' is a microphone cartridge, 2' is a capacitance meter, and 3' is an output circuit. Capacitance meter 2',
is a high-Q high-frequency oscillator 10 such as a crystal oscillator,
It is composed of a phase shifter 11 determined by the capacitance of the microphone cartridge 1', a phase detector 12, and a low-pass filter 13. In this microphone, the high frequency generated by the high frequency oscillator 10 is divided into two parts, one of which is sent directly to the phase detector 12, and the other is phase modulated by the phase shifter 11 in accordance with the capacitance between the diaphragm and the fixed electrode. Similarly, it is sent to the phase detector 12, the phases of the two are compared, and the difference is outputted to the low-pass filter 13.
The high frequency components are waved. The greatest feature of this AC bias type capacitor microphone is that by using a high frequency oscillator with a very high Q, a microphone with an extremely high S/N ratio can be produced. Furthermore, since an output proportional to the capacitance C _M of the microphone cartridge 1' can be obtained, the output can also include a DC component. Further, unlike the DC bias method, the adhesion phenomenon between the fixed electrodes of the vibrating membrane does not occur. The disadvantage is that, because the sensitivity is high, variations in the initial value of capacitance C _M and the tension of the diaphragm greatly affect the sensitivity of the microphone.

Problems common to such conventional DC bias type and AC bias type capacitor microphones will be described below. Figure 4 shows what happens when a sound wave is incident on a completely rigid wall. In this case, the boundary conditions are as follows: P=maximum U=O, where P is the sound pressure and U is the particle velocity. In the case of a normal object, the light is not completely reflected as described above, but is partially transmitted. For something that is extremely thin and moves lightly, such as a vibrating membrane, the boundary conditions are complex and not constant. In other words, changes in the boundary constant have an adverse effect on subtle aspects such as sound quality.

The present invention solves these conventional drawbacks by applying a reference DC bias voltage to the capacitance formed by the diaphragm and the fixed electrode of the microphone cartridge, and by controlling the change in capacitance. The detected output is superimposed on the reference DC bias voltage and applied between the vibrating membrane and the fixed electrode. According to this configuration, negative feedback can be applied so that the capacitance between the diaphragm and the fixed electrode remains constant even when a sound wave is incident, so that high sensitivity can be achieved by keeping the boundary condition of the diaphragm constant. , it has the advantage of being able to achieve high S/N and high stability.

Examples of the condenser microphone of the present invention will be described below. FIG. 5 shows an embodiment of the present invention. In FIG. 5, 1" is a microphone cartridge, 2" is a capacitance meter, 3" is an output circuit, 7 is a bias control circuit, and 8 is a booster. The absolute value of the capacitance of the microphone cartridge 1'' is detected in real time by a capacitance meter 2'', and is output from an output circuit 3'' to an output terminal 6'' via a capacitor 9. It is sent to the bias control circuit 7 via the DC amplifier of the output circuit 3'', where it is
It is superimposed on the plate quasi-DC bias voltage V _REF , amplified as DC, and sent to the booster 8, where it is applied as a bias to the microphone cartridge 1''.
The structure is such that the value of the static capacitance C _M of is always constant. The microphone output may be taken out from any position in this feedback system. The bias voltage for the capacitance C _M when there is no sound wave is adjusted by the reference DC bias voltage V _REF so that it is at 1/2 the maximum voltage of the booster 8, as shown in FIG.

As described above, according to the present invention, the reference DC bias voltage is superimposed between the diaphragm and the fixed electrode constituting the microphone cartridge, and the output based on the change in capacitance between the diaphragm and the fixed electrode is generated as a DC voltage. Since it is configured to provide negative feedback to the capacitance, the DC operating point of the capacitance can be kept constant at all times, and the boundary condition of the diaphragm is kept constant to achieve high sensitivity and high S/N, as well as high stability. This has the advantage that it can be used for various purposes.

[Brief explanation of drawings]

Figure 1 is a basic configuration diagram of a DC bias type condenser microphone, Figure 2 is an enlarged sectional view of its main parts, Figure 3 is a basic configuration diagram of an AC bias type condenser microphone, and Figure 4 is an ideal diagram. Figure 5 is a configuration diagram showing an embodiment of the condenser microphone of the present invention, and Figure 6 is the relationship between sound pressure and bias voltage of the condenser microphone. FIG. 1''...Microphone cartridge, 2''...Capacitance meter, 3''...Output circuit, 7...Bias control circuit, 8...Booster, 9...Coupling capacitor, R...Bias resistor.

Claims (1)

[Claims] 1. A microphone cartridge is configured to detect changes in capacitance formed by a diaphragm and a fixed electrode, and extract it as a microphone output, and superimposes the microphone output on a reference DC bias voltage to A capacitor-type microphone configured to provide negative feedback between a fixed electrode and a fixed electrode.