JPH01197000A - Condenser microphone - Google Patents

Abstract

PURPOSE:To reduce the vibration noise without giving any effect to the characteristic of a microphone onto a sound wave by cancelling electrically the vibration noise not prevented by the mechanical vibration isolation mechanism. CONSTITUTION:The vibration is prevented from being transmitted to the condenser microphone unit 2 by viscoelastic bodies 5, 6 constituting the vibration isolation mechanism and the vibration noise is reduced mechanically. Moreover, the output signal SN of the piezoelectric vibration pickup 3 is given to an amplitude phase characteristic correction circuit 12 and a level correction circuit 13, a signal similar to the vibration noise signal included in an output signal SM of the unit 2 not prevented by the viscoelastic bodies 5, 6 is obtained. Thus, the signal noise included in the output signal SM of the unit 2 is cancelled from the differential circuit 9 and the vibration noise is reduced electrically. Thus, the vibration noise is reduced effectively.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a condenser microphone, and more particularly to prevention of vibration noise in a directional microphone.

"Prior Art" Mechanical vibration of the microphone body generates so-called vibration noise. The generation of this vibration noise differs significantly depending on the directionality, and in bidirectional and unidirectional, extremely large vibration noise is generated in the middle and low frequencies compared to omnidirectional.

FIG. 7 shows how the microphone sensitivity to such mechanical vibrations differs depending on the directivity, with reference to omnidirectional microphones.

In the figure, a indicates omnidirectionality, b indicates bidirectionality, C indicates unidirectionality, and d indicates vibration sensitivity of unidirectionality rather than omnidirectionality. Further, fL is a low limit frequency at which the plane wave sound field sensitivity of the microphone is reduced by 3 dB from the sensitivity of a mid-range frequency having a flat characteristic, and e is an effective distance between the sound wave introduction ports before and after the diaphragm.

As is clear from Fig. 7, the vibration sensitivity of the directional microphone at the mid-range frequency is the highest in bidirectional microphones, and when the low frequency fL is 100 Hz, the vibration sensitivity at 50 Hz is lower than that of omnidirectional microphones. 31 dB bidirectional,
It has extremely high sensitivity of 25dB with unidirectionality.

The vibration sensitivity in this low frequency range is inversely proportional to the low frequency limit frequency fL, and increases as the low frequency limit frequency fL decreases.

Furthermore, if the excitation force is constant, the lighter the microphone body, the greater the vibration of the body, and therefore the greater the noise generated.

Therefore, prevention of vibration noise in a directional microphone is considered to be one of the important issues that affects the secondary performance of the microphone, as it is related to the lower limit of the frequency band and the weight of the main body.

Therefore, in the past, measures have been taken to prevent such vibration noise.For example, in dynamic microphones,
Mechanical vibration isolation and electrical cancellation methods have been used, but only mechanical vibration isolation is used for condenser microphones.

In order to mechanically isolate vibrations, for example, a method is used in which the microphone main body or the microphone unit (capsule portion) is supported by a viscoelastic material such as rubber to provide mechanical insulation.

The vibration damping effect of such a mechanism generally has characteristics as shown in FIG. In the figure, the frequency axis, which is the horizontal axis, is
The figure is normalized to the low resonance angular frequency ω0 of the vibration isolation mechanism.

In the figure, a has a resonance sharpness QO of 2, and b has a resonance sharpness QO of 1. c indicates a characteristic in which the resonance sharpness QO is 1/, /'.

As is clear from Fig. 8, the vibration isolation effect can be expected above the intersection frequency fc, and in the frequency range above the intersection frequency fc, the higher the resonance sharpness QO, the greater the vibration isolation effect. , at frequencies below the intersection frequency fc, no vibration damping effect can be expected, and the resonance sharpness Q
If O is high, vibrations will be further promoted at frequencies near the low resonance frequency fO.

Therefore, there is an appropriate value for the resonance sharpness QO, but in any case, it is necessary to set the intersection frequency fc to a low frequency in order to achieve vibration isolation down to as low a frequency as possible.

However, if the set value of the intersection frequency fc is too low, the support will become unsteady, which often causes practical problems.

"Problem to be Solved by the Invention" Conventionally, it has been possible to set the intersection frequency fc to a practically appropriate value and electrically remove noise at low frequencies that cannot be damped out using a bypass filter. However, it is undesirable because it removes the original signal component as well as the noise, which ultimately limits the low frequency limit of the signal.

In addition, in dynamic microphones, it has been put into practical use to cancel and remove noise at low frequencies that cannot be completely damped by such a mechanical vibration isolation mechanism by using an electrical cancellation method. However, this electrical cancellation method cannot be applied to condenser microphones.

The present invention takes these points into consideration and aims to provide a condenser microphone that can effectively reduce vibration noise.

"Means for Solving the Problems" The present invention provides a vibration isolating mechanism that integrally isolates vibrations between a capacitor microphone unit, a piezoelectric vibration pickup, and a capacitor microphone unit and piezoelectric vibration pickup within a microphone housing. An amplitude/phase characteristic correction circuit and a level correction circuit that correct the magnitude and phase of the output signal of the pickup to be equal to the magnitude and phase of the output signal of the condenser microphone unit, and a condenser microphone unit output signal and level compensation circuit. It consists of a differential circuit to which an output signal is supplied and outputs the differential output signal.

"Function" In the above-described configuration, the vibration isolating mechanism prevents vibrations from being transmitted to the condenser microphone unit 2, thereby mechanically reducing vibration noise.

In addition, the output signal of the piezoelectric vibration pickup 3 is passed through the amplitude/phase characteristic correction circuit 12 and the level correction circuit 13, so that vibration noise signals included in the output signal of the condenser microphone unit 2 that cannot be completely suppressed by the vibration isolation mechanism. A similar signal is obtained.

Therefore, in the differential circuit 9, the signal noise signal included in the output signal of the condenser microphone unit 2 is canceled out and removed, and vibration noise is electrically reduced.

"Example" Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 shows the structure of a condenser microphone. In the figure, 1 is a microphone housing, 2 is a condenser microphone unit, and 3 is a piezoelectric vibration pickup.

A piezoelectric element 4 constituting the vibration pickup 3 is arranged so as to be in contact with the microphone unit 2, so that the vibration of the microphone unit 2 can be detected by the vibration pickup 3. Further, 5 and 6 are viscoelastic bodies that constitute a vibration isolation mechanism, and the vibration pickup 3 is supported by the microphone housing 1 by the viscoelastic body 6, and the microphone unit 2 is connected to the microphone housing 1 by the viscoelastic body 6. Supported. Vibrations in the middle and high frequencies of the microphone unit 2 are damped by these viscoelastic bodies 5 and 6.

Further, FIG. 2 shows the configuration of the electric circuit of the condenser microphone of this example.

In the figure, 7 is the output signal S of the microphone unit 2.
M is the terminal to which the signal is supplied. The output signal S supplied to this terminal 7 is supplied to a differential circuit 9 via an impedance conversion circuit 8. Further, lO is a piezoelectric element, and therefore a terminal to which the output signal SN of the signal pickup 3 is supplied. The output signal SN supplied to this terminal 10 is supplied to a series circuit of an amplitude/phase characteristic correction circuit 12 and a level correction circuit 13 via an impedance conversion circuit 11, and an output signal of the correction circuit 13 is supplied to a differential circuit 9. be done.

Then, an audio signal output terminal 14 is led out from the 20 differential circuit 9. In this case, the correction circuits 12 and 13
The magnitude and phase of the output signal SN of the vibration pickup 3 are corrected to be equal to the magnitude and phase of the output signal SN of the microphone unit 2.

That is, in the vibration pickup 3, the viscoelastic bodies 5 and 6
Since the vibration of the microphone unit 2 which cannot be completely vibration-proofed is detected, a signal similar to the vibration noise signal included in the output signal SM of the microphone unit 2 is obtained as the output signal of the correction circuit 13.

Therefore, in the differential circuit 9, the vibration noise signal contained in the output signal SM of the microphone unit 2 is canceled out and removed, and an audio signal from which the vibration noise signal is almost completely removed is obtained at the output terminal 14.

Next, conditions for effectively implementing the above-mentioned mechanical vibration isolation and electrical cancellation will be described.

In Figure 1, the mass of the microphone unit 2 is ml
, the mass of the vibration pickup 3 is ml, the stiffness of the piezoelectric element 4 is 83, the stiffness and mechanical resistance of the viscoelastic body 5 are S1 and rl, respectively, and the stiffness and mechanical resistance of the viscoelastic body 6 are S2 and rl, respectively. The equivalent circuit of the mechanical vibration system is shown in FIG.

Here, vO is the vibration speed of the microphone housing 1, vl is the vibration speed of the microphone unit 2, and 3 is the vibration speed that contributes to the power generation of the vibration pickup 3.

Further, a basic equivalent circuit of the microphone mechanical vibration system when the microphone unit 2 vibrates is shown in FIG.

Here, 'Vl is the vibration velocity of the microphone unit 2, mO is the equivalent mass that becomes the source of the excitation force that contributes to power generation, and S
O is the equivalent stiffness of the diaphragm, rO is the equivalent resistance for damping high-frequency resonance, and sb and rb are the equivalent stiffness and equivalent resistance for providing directivity, respectively.In Fig. 4, v2 is the microphone unit 2
This provides a shooting speed that contributes to power generation.

[1] Microphone vibration sensitivity From the equivalent circuits in Figures 3 and 4, the microphone housing l
Find the display formula for the output voltage of microphone unit 2 when it is vibrated, and from this find the vibration sensitivity E1/G at the middle and low frequencies per vibration acceleration IG. As θ, the following (1)
It is expressed by the formula.

:AICθSθ・ni・k ・・・・・・・・・・・・・・・
(7) Here, AI is expressed by equation (2), where Kl is the mechanical-electrical conversion coefficient.

Furthermore, when S in equation (2) is expressed using the equivalent circuit constants shown in FIG. 4, it is given by the following equations (3) and (4).

5=(rosu)"6)'B f 5o (1-Bru・-
-=・ (3nj' = )'#t Bf (Sa f
5b) (1-8)-・・・・・・・・・'4)(3)
The formula is for directivity where the value of B is 0.2≦B≦l, and (4)
The formula shows that the value of B is 0≦B≦0. It is practical to apply this method to the second directivity.

In the above equation, B is a coefficient determined by the directivity, and ranges from 0 to l
The values are 0 for omnidirectional, 172 for unidirectional (cardioid), and 1 for bidirectional. d is the effective distance between the sound wave introduction ports before and after the diaphragm, and C is the speed of sound.

ωL is a low limit angular frequency at which the plane sound field sensitivity (sensitivity to sound waves) decreases to 3 dB from the sensitivity at the mid-range frequency where the flat characteristic occurs, and is expressed by the following equation (5).

u)t=-A-8・・・・・・・・・・・・(5)e
t From this equation (5), if the directivity is omnidirectional, B=0 and 3
Therefore, ωL=O.

In addition, in Yll of equation (1), ω0 is the low-frequency resonance inner chamber wave number of the vibration isolation mechanism, and QO gives the sharpness of the resonance, and is expressed by the following equations (8) and (7), respectively.

Qo = 5 ni ゑ--・・・・・・・・・・・・・・・
・(7) u) (, (rlfrx) In the above equation (1), AI has an angular frequency ω of ω< 4
．． ) represents the vibration sensitivity at low frequencies, which is L, and takes significantly different values depending on the directivity. Also, Yll
represents the difference in frequency characteristics due to the directivity of vibration sensitivity.

Figure 7 shows IAI-Yllll at θ; 0'', standardized by omnidirectional sensitivity. Also, Yll represents vibration isolation characteristics, and Figure 8 shows IY121. These characteristics are as explained in the prior art section.

[2] Vibration sensitivity of the vibration pickup Also, from the equivalent circuit shown in Figure 4, find the display formula for the output voltage of the vibration pickup 3 when the microphone housing 1 is excited, and use this to determine the middle and low range per vibration acceleration IG. When determining the vibration sensitivity E 2 /G in terms of frequency, it is expressed by the following equation (8), where θ is the vibration direction with respect to the main axis of the microphone.

=Az Cosθ・Yzz・・・・・・・・・・・・
・・・・・・・・・・・・・・・(8) In this equation (8), A2 is expressed as 2tm−”−・・, where the mechanical φ electrical conversion coefficient is 2, and Ql is 2tm−”−・・・・・・・・・・・・・・・・・・・・
... (10) It is expressed as ωOr1.

In equation (8), A2 is the vibration sensitivity of the vibration pickup 3, and at medium and low frequencies below the high resonance frequency formed by ml, m2, and 83, it is expressed by equation (9), and the vibration sensitivity is It has a constant vibration sensitivity proportional to acceleration.

Further, Y22 corresponds to Yll in equation (1), and represents the vibration damping characteristic of the vibration pickup 3.

[3] Conditions for vibration isolation and electrical cancellation (a) Conditions for excavation prevention characteristics In order to effectively perform electrical cancellation, first, the vibration isolation characteristics of the microphone unit 2 and the vibration pickup 3 must be adjusted. need to be equal. YI2 in equation (1) and (8
), they are clearly different. Therefore, if QO=Q1, (7), (
From equation 10), the following equation (11) is derived.

r2/rl=s2/sl ・Fist Fist (11
) By satisfying the condition of equation (11), Y22 becomes the same type as Y12. Also, equation (11) is as follows (12
) is transformed as follows.

r2/52=rl/sl Naka Naka・(12) This (12
) means that the resonance acuity Q of each of the viscoelastic bodies 5 and 6 shown in FIG. 1 is made equal.

In other words, a necessary and sufficient condition for making the vibration isolation characteristics of the microphone unit 2 and the fan pickup 3 the same at medium and low frequencies is to set the resonance acuity Q of each of the viscoelastic bodies 5 and °6 to the same value. That's true.

Therefore, in the structure shown in FIG.
The resonance acuity Q of the and 6-9 Y゛° bills are made to have the same value.

(b) Condition 8 for electrical cancellation is expressed by the following equation (13) by satisfying the condition of equation (11) above.

”Az cosθ・Ytz ・・・・・・・・・・・・
・・・・・・・・・ (I3) First, when the directivity is omnidirectional, Yll in equation (1) becomes ], (1)
The equation is isomorphic to equation (13). Therefore, in the case of omnidirectionality, it is possible to cancel the difference by correcting the difference in sensitivity between AI and A2, that is, the level, and adding both signals with opposite polarities.

However, in general, when the vibration pickup 3 has directivity such as unidirectivity or bidirectionality, it is necessary to correct the output of the vibration pickup 3 through an electric circuit having a transmission frequency Yll.

The characteristics of Yll can be easily realized by the electric circuit shown in FIG.

From Figure 5, EO/Ei is calculated as follows: (I4) Formula % Formula % The conditions for making formula (14) isomorphic with Yll are given by, and the conditions of formulas (15) and (16) are By satisfying the following, equation (14) becomes isomorphic to Yll.

In other words, the conditions of equations (15) and (1B) are necessary and sufficient conditions for electrical cancellation.

Therefore, in the electric circuit shown in FIG. 2, as the correction circuit 12, an electric circuit as shown in FIG. 5 that satisfies the conditions of equations (15) and (16) is used, and A1. The difference in sensitivity of A2 is corrected by the correction circuit 13.

As a result, as described above, the operating circuit 9 cancels out and removes the imaging noise signals of medium and low frequencies very effectively.

Note that when performing such electrical cancellation, it is necessary to minimize the increase in circuit noise caused by the cancellation. The conditions for reducing the increase in noise are AI in equation (1) and (8)
It is desirable to set A2 in the equation to the condition that A2 > AI. The reason is A2<
This is because if it is At, the level will be corrected in the direction of amplifying the output signal from the vibration pickup 3 side, and circuit noise accompanying this amplification will become a problem.

In any case, it is advantageous for the vibration sensitivity of the vibration pickup 3 to be as high as possible.

For this purpose, it goes without saying that a highly sensitive piezoelectric element is used for A2, but as is clear from equation (9), it depends on the setting conditions of m 2 / ml and s 2 / s 1. Sensitivity decreases under certain conditions. In order to reduce this sensitivity decrease, it is desirable to set the following conditions. Since this condition is related to the weight balance of the support mechanism, there may also be actual structural constraints.

In this way, according to the condenser microphone of this example,
The viscoelastic bodies 5 and 6 provide mechanical vibration isolation of the microphone unit 2, and as a result, the vibration noise signal contained in the output signal of the microphone unit 2 is electrically canceled out and removed. , vibration noise can be effectively reduced. Furthermore, according to the electrical cancellation method as in this example, when applied to a directional condenser microphone such as bidirectional or unidirectional, the correction circuit 12 in FIG. It has high-frequency attenuation characteristics depending on the frequency.

Therefore, the middle Φ high frequency component of the noise generated by the impedance conversion circuit 11 of the imaging pickup is significantly attenuated by passing through the correction circuit 12, and the increase in noise at the output of the operating circuit 9 is advantageously small. be.

FIG. 6 shows the results of applying the present invention to a unidirectional condenser microphone and actually measuring the prevention effect.

For this measurement, a sine wave was used, and the microphone was vibrated by a vibration exciter in the maximum sensitivity direction of θ=0° with constant vibration acceleration controlled, and the output was recorded by a level recorder.

In the same figure, the solid line a shows the case where neither mechanical vibration isolation nor electrical cancellation is used, the - dotted line shows only the mechanical vibration isolation, and the broken line C shows the case where neither mechanical vibration isolation nor electrical cancellation is used. This shows the case where

The axis of the figure shows the relative level normalized by the output level at a low frequency, which is a flat characteristic when no anti-vibration cancellation is performed.

The low-frequency resonant frequency fO of the vibration isolation mechanism is 3.0 Hz, but by using vibration isolation cancellation together, a prevention effect of 30 dB or more is obtained at frequencies below IKHz.

"Effects of the Invention" As explained above, the present invention electrically cancels out vibration noise that cannot be damped by a mechanical vibration isolation mechanism, so it has no effect on the characteristics of the microphone against sound waves. It is possible to effectively reduce only the vibration noise without adding any vibration noise.

In addition, since the noise is electrically canceled out, vibration noise can be reduced even if the weight of the microphone body is light, and there is an advantage that the weight of the microphone body can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of an embodiment of this invention, FIG. 2 is a diagram showing an electric circuit of an embodiment of this invention, and FIGS.
The figure is a diagram for explaining the same, and FIGS. 7 and 8 are diagrams for explaining a conventional example. 1 fist φ - Microphone housing 2, Φ and condenser microphone unit 3... Piezoelectric vibration pickup 4... Piezoelectric element 5.6... Viscoelastic body 8.11... Φ impedance conversion circuit 9... Differential circuit 12...Standing/phase characteristic correction circuit 13...Level correction circuit Patent applicant Iwa Co., Ltd. Figure 1 Figure 2 Figure 1! 9 @ 1 word (Fig. 3)! , 14f1 slime - 1 apple sieve, 1 tonne thread boat 3
Fig. 4 Fig. 5 R + Hango $ / phase special cumulation (positive turn) Fig. 6 Boke / offset/) Effect (Rhythm A 1) Fig. 7 S Imperial sumihiki Kantako Island master 1; It's time to kidnap.

Claims (1)

[Scope of Claims] A capacitor microphone unit, a piezoelectric vibration pickup, a vibration isolation mechanism for integrally vibration-isolating the capacitor microphone unit and the piezoelectric vibration pickup within a microphone housing, and the magnitude of an output signal of the vibration pickup. and an amplitude/phase characteristic correction circuit and a level correction circuit for correcting the output signal of the condenser microphone unit and the output signal of the level correction circuit to be equal to the magnitude and phase of the output signal of the condenser microphone unit. and a differential circuit that outputs a differential output signal.