JP2002333886A - Active noise controller - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a feedback-type active noise controller, through which occurrence of howling is fully prevented and as a result, proper noise reduction effect is obtained over a wide band. SOLUTION: The controller is provided with an acceleration sensor 3, which detects the acceleration of the vibration plate of a noise-control loudspeaker 2 and a differencing device 7 which generates the difference between a signal, based on the output of a noise-detecting microphone 1 and a signal based on the output of the sensor 3; an inverse phase circuit 8 drives the loudspeaker 2, based on the reversed output phase signals of the device 7 instead of a signal that is obtained, by making output phase of the microphone 1 reversed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a noise detection microphone for detecting noise, a noise control speaker disposed near the noise detection microphone, and a signal obtained by inverting the output phase of the noise detection microphone. The present invention relates to a feedback-type active noise control device having negative phase means for driving a noise control speaker based on the feedback signal, and reducing the noise by negatively feeding back the noise signal to the noise control speaker.

[0002]

2. Description of the Related Art There are a feedforward type and a feedback type in an ANC (active noise control) system. In addition, the feedforward type and the feedback type may be used together.

Many active noise control devices employing a feedforward type ANC perform digital signal processing for the purpose of reducing one-dimensional duct noise. In order to effectively reduce noise in space, there is also a configuration in which a noise control speaker is arranged close to a low-frequency periodic noise source.

The feedback type ANC is also called a tightly coupled monopole (TCM). By disposing a noise control speaker and a noise detection microphone close to each other, an analog signal processing circuit is relatively analog. Because it can be simplified, it is widely used. For example, Japanese Patent Application Laid-Open No. 9-506189 discloses an example in which the present invention is applied to headphones with a noise reduction function.

FIG. 10 is a schematic configuration diagram of a conventional feedback type active noise control device. In this conventional active noise control device, a noise detection microphone 52 is arranged near a noise source 51, and a noise control speaker 53 is arranged near the noise detection microphone 52. Is amplified by the amplifier 54 and the amplifier 5
4 is inverted by an anti-phase circuit 55, and the output characteristic of the anti-phase circuit 55 is appropriately corrected by an equalizer 56.
The output of the equalizer 56 is amplified by the amplifier 57, and the noise control speaker 53 is driven by the output of the amplifier 57.

In the TCM, the output terminal of the noise detection microphone 52 passes from the input terminal of the amplifier 54 to the output terminal of the amplifier 54, the negative phase circuit 55, the equalizer 56, the amplifier 57, the noise control speaker 53, and the noise detection microphone 52. The transfer function up to is generally called open loop characteristic. As an example of this open loop characteristic, FIG. 11 shows a gain frequency characteristic and FIG. 12 shows a phase frequency characteristic.

The open loop characteristics mainly include the phase characteristics of the noise control speaker 53 and the noise control speaker 53.
Due to the delay of the spatial distance between the noise and the noise detection microphone 52, the phase characteristic has a phase rotation as shown in FIG. The smaller the distance between the noise control speaker 53 and the noise detection microphone 52 is, the smaller the phase rotation is.
For example, it is preferably about 10 cm or less. However, in the case of TCM, it may be about 50 cm or more for structural reasons. Thus, the noise control speaker 53
The distance between the microphone and the noise detection microphone 52 is large.
When there is a delay time of about 5 msec, the phase rotation increases.

In the TCM, the noise can be reduced at the position of the noise detection microphone 52 according to a feedback gain of 0 dB or more near the frequency having a phase of 180 degrees, that is, negative feedback. In order to provide negative feedback, a configuration in which the polarity is inverted when the noise control speaker 53 is connected can be adopted in addition to the configuration in which the signal is inverted using the reverse phase circuit 55 as shown in FIG. Therefore, by disposing the noise detection microphone 52 and the noise control speaker 53 near the noise source 51, it is possible to obtain a noise reduction effect in a three-dimensional space, that is, at the position of the listener. FIG. 13 shows an example of this noise reduction effect.

[0009]

However, when the open-loop characteristic has a frequency near the phase of 0 degree, the feedback becomes positive and conversely increases the noise. This causes a howling phenomenon, which sufficiently reduces the feedback gain. It will not be possible to obtain. In reality, the above-described phase rotation cannot be avoided. Therefore, it is necessary to limit the band by the equalizer 56 and adjust the feedback gain so as not to increase too much. Since the phase rotation occurs in the filter for band limitation, that is, the equalizer 56, the noise reduction effect can be obtained only in a narrow frequency band.

In the case of an analog circuit, a signal processing circuit such as an anti-phase circuit 55 and an equalizer 56 has no time delay except for a phase characteristic of filter processing, but in the case of a digital circuit, an A / D converter is used. Due to the disadvantage that the phase rotation is further increased due to the time delay caused by the conversion and the D / A conversion, it is not practical to use it for TCM.

A method of canceling howling using a digital signal processing technique has been proposed. However, when such a technique is adopted in TCM, the noise between the noise detection microphone 52 and the noise control speaker 53 is reduced. , The howling cannot be prevented if the delay time is shorter than the delay time due to digital signal processing.

The present invention has been conceived under such circumstances, and it is possible to sufficiently prevent howling, and as a result, to provide a feedback type active device capable of obtaining a good noise reduction effect over a wide band. It is an object to provide a noise control device.

[0013]

DISCLOSURE OF THE INVENTION In order to solve the above-mentioned problems, the present invention takes the following technical means.

According to the first aspect of the present invention, a noise detection microphone for detecting noise, a noise control speaker disposed near the noise detection microphone, and an output phase of the noise detection microphone are inverted. A negative-phase means for driving a noise control speaker based on the received signal, and a feedback type active noise control device for reducing noise by negatively feeding back the noise signal to the noise control speaker. An acceleration detection unit that outputs a signal corresponding to the acceleration of the diaphragm of the speaker, and a difference generation unit that generates a difference between a signal based on the output of the noise detection microphone and a signal based on the output of the acceleration detection unit. The noise is detected based on the signal obtained by inverting the output phase of the difference generating means instead of the signal obtained by inverting the output phase of the noise detection microphone. Characterized by being configured to drive the patronage speaker, active noise control apparatus is provided.

According to a preferred embodiment, the acceleration detecting means has an acceleration sensor attached to the diaphragm or voice coil of the noise control speaker.

According to another preferred embodiment, the acceleration detecting means has a speed detecting means for detecting the speed of the diaphragm of the noise control speaker, and a differentiating means for differentiating the output of the speed detecting means.

According to another preferred embodiment, the speed detecting means has a detecting coil attached to a diaphragm or a voice coil of a noise control speaker, and a magnetic circuit for supplying a magnetic flux to the detecting coil.

According to another preferred embodiment, the reverse phase means has a reverse phase circuit for inverting the phase of the output signal of the difference generation means.

According to another preferred embodiment, the difference generation means includes a filter circuit having a filter characteristic simulating a space propagation characteristic between the noise control speaker and the noise detection microphone, and a filter circuit of the noise detection microphone. A difference generation circuit that generates a difference between the signal based on the output and the output signal of the filter circuit.

According to the present invention, the acceleration detecting means for outputting a signal corresponding to the acceleration of the diaphragm of the noise control speaker, and the signal based on the output of the noise detecting microphone and the signal based on the output of the acceleration detecting means. And a difference generating means for generating a difference, and the anti-phase means is configured to drive the noise control speaker based on a signal obtained by inverting the output phase of the difference generating means, so that howling can be sufficiently prevented. As a result, a good noise reduction effect can be obtained over a wide band.

That is, since the difference between the signal based on the output of the noise detecting microphone and the signal based on the output of the acceleration detecting means is negatively fed back to the noise controlling speaker, the output of the noise controlling speaker is relative to the noise. This results in a negative feedback resulting in a howling, so that howling can be sufficiently prevented. Of course, the output of the acceleration detecting means corresponds to the output of the speaker for noise control.

Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be specifically described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an active noise control device according to an embodiment of the present invention. This active noise control device includes a noise detection microphone 1, a noise control speaker 2, an acceleration sensor 3, an amplifier 4, a filter circuit 5, an amplifier 6, a differentiator 7, an antiphase circuit 8, an equalizer 9, and an amplifier 10. ing. Microphone for noise detection 1
Are arranged near the noise source N. The noise control speaker 2 is arranged near the noise detection microphone 1.

FIG. 2 is an enlarged sectional view of a main part of the speaker 2 for noise control. The noise control speaker 2 includes a magnetic circuit 11 and a diaphragm 12. Magnetic circuit 11
Has a magnet 13. The diaphragm 12 includes a cone portion 14 and a cylindrical portion 15, and the voice coil 16 and the acceleration sensor 3 are mounted on the cylindrical portion 15.

FIG. 3 is a functional block diagram of the filter circuit 5. The filter circuit 5 includes constant multipliers 21 to 2
6, adders 28 to 30, band pass filter 32, equalizers 33 and 34, and high pass filter 35
It has.

The noise detection microphone 1 converts the noise from the noise source N and the control sound output from the noise control speaker 2 into an electric signal, and supplies the electric signal to the amplifier 6.

The noise control speaker 2 converts the output of the amplifier 10 into a control sound and outputs it.

The acceleration sensor 3 supplies a detection signal corresponding to the acceleration of the diaphragm 12 of the noise control speaker 2 to the amplifier 4. The acceleration sensor 3 is, for example, a piezoelectric acceleration sensor, and a + power supply and a −power supply are supplied by a power supply circuit (not shown).

The amplifier 4 amplifies the output of the acceleration sensor 3 and supplies it to the filter circuit 5.

The filter circuit 5 performs a predetermined filtering process on the output of the amplifier 4 and supplies the output to a differentiator 7.

The amplifier 6 amplifies the output of the microphone 1 for noise detection and supplies it to the differentiator 7.

The difference unit 7 supplies the difference between the output of the amplifier 6 and the output of the filter circuit 5 to the reverse phase circuit 8.

The reverse phase circuit 8 inverts the phase of the output of the differentiator 7 and supplies it to the equalizer 9.

The equalizer 9 performs a predetermined correction process on the output of the amplifier 4 and supplies the output to the amplifier 10. This equalizer 9 includes a filter having a predetermined characteristic. The equalizer 9 may be provided as needed, and is not necessarily provided.

The amplifier 10 amplifies the output of the equalizer 9 and supplies it to the voice coil 16 of the noise control speaker 2.

The constant multiplier 21 multiplies the output of the amplifier 4 by a constant 3 and supplies the result to the bandpass filter 32.

The constant multiplier 22 multiplies the output of the band-pass filter 32 by a constant 0.5 and supplies the result to the adder 28.

The constant multiplier 23 multiplies the output of the adder 28 by a constant 0.7 and supplies the result to the adder 29.

The constant multiplier 24 multiplies the output of the band-pass filter 32 by a constant 0.7 and supplies the result to the equalizer 33.

The constant multiplier 25 multiplies the output of the adder 28 by a constant (−0.3) and supplies the result to the equalizer 34.

The constant multiplication section 26 multiplies the output of the addition section 29 by a constant (−3) and supplies the result to the high-pass filter section 35.

The adder 28 adds the output of the constant multiplier 22 and the output of the equalizer 33 and supplies the result to the constant multiplier 23 and the constant multiplier 25.

The adder 29 adds the output of the constant multiplier 23 and the output of the equalizer 34 and supplies the result to the adder 30 and the constant multiplier 26.

The adder 30 adds the output of the adder 29 and the output of the high-pass filter 35 and supplies the result to the differentiator 7.

The band-pass filter section 32 has a filter characteristic having a center frequency of 80 Hz, a gain of 0 dB, and a Q of 0.2.

The equalizer 33 has a center frequency of 300
It has a filter characteristic of Hz, gain of 0 dB, and Q of 0.7.

The equalizer 34 has a center frequency of 500
Hz, a gain of +3 dB, and a Q of 2.0.

The high-pass filter section 35 has a filter characteristic having a center frequency of 1.25 kHz, a gain of 0 dB, and a Q of 2.0.

The present embodiment is an example showing a case where the distance between the noise control speaker 2 and the noise detection microphone 1 is about 10 cm. Is also possible.

Next, the operation will be described. The noise from the noise source N and the control sound from the noise control speaker 2 are converted into electric signals by the noise detection microphone 1 at the position of the noise detection microphone 1 and amplified by the amplifier 6 at a predetermined amplification degree. Then, the signal is supplied to one input terminal of the differentiator 7. On the other hand, the acceleration of the diaphragm 12 of the noise control speaker 2 that outputs a control sound is converted into an electric signal by the acceleration sensor 3, corrected by the filter circuit 5, and supplied to the other input terminal of the differentiator 7. .

The differentiator 7 generates a signal obtained by subtracting the output of the filter circuit 5 from the output of the amplifier 6. This signal is
The phase is inverted by the anti-phase circuit 8, corrected by the equalizer 9, amplified by the amplifier 10, and supplied to the voice coil 16 of the noise control speaker 2. As a result, the diaphragm 12 vibrates, a control sound is output from the noise control speaker 2, and the diaphragm 12
Is detected by the acceleration sensor 3.

Since the output of the acceleration sensor 3 is used to prevent howling caused by the control sound output from the noise control speaker 2, the sound of the control sound input to the noise detection microphone 1 is used. Although it is preferable that the noise signal coincides with the pressure signal, since the noise control speaker 2 and the noise detection microphone 1 are arranged at a predetermined distance, the sound pressure signal of the control sound input to the noise detection microphone 1 And the output signal of the acceleration sensor 3 do not completely match. However, since the propagation characteristic of the spatial distance between the noise control speaker 2 and the noise detection microphone 1 is approximated by a simple delay, it can be simulated by a relatively flat filter characteristic with a slight phase rotation. . Therefore, by providing the filter circuit 5 and correcting the output signal of the acceleration sensor 3, the sound pressure signal of the control sound input to the noise detection microphone 1 is simulated. When a piezoelectric acceleration sensor is used as the acceleration sensor 3 as in the present embodiment, since the acceleration sensor has a self-resonant frequency, the characteristics of the filter circuit 5 are determined so as to cancel the characteristics due to the self-resonant frequency. Is preferred. Alternatively, these unnecessary frequency bands may be cut by providing a low-pass filter or the like in a feedback loop in consideration of phase characteristics.

The transfer function (P / E) between the noise control speaker 2 and the noise detection microphone 1 is such that the input voltage to the voice coil 16 of the noise control speaker 2 is E and the noise detection microphone 1 is input to the noise detection microphone 1. Assuming that the sound pressure of the control sound to be performed is P and the acceleration of the diaphragm 12 is A, it is expressed by the following equation 1. FIG. 4 shows gain frequency characteristics, and FIG. 5 shows phase frequency characteristics.

[0055]

(Equation 1)

From these relationships, the sound pressure P of the control sound input to the noise detection microphone 1 and the acceleration A of the diaphragm 12 of the noise control speaker 2 have a high correlation, and C = ( P / A) is almost flat and has a phase delay of time delay. Therefore, the air propagation characteristic C
Can be simulated by the filter circuit 5 composed of a relatively simple analog filter. 6 and 7 show a comparison between the air propagation characteristic C and the propagation characteristic Cf of the filter circuit 5. FIG. FIG. 6 shows the frequency characteristic of the gain, and FIG. 7 shows the frequency characteristic of the phase. Howling cancellation is realized by subtracting the acceleration signal that has passed through the filter circuit 5 from the signal detected by the noise detection microphone 1. FIG. 8 shows a residual characteristic when howling cancellation is performed using the air propagation characteristic C, and FIG. 9 shows a residual characteristic when howling cancellation is performed using the propagation characteristic Cf of the filter circuit 5. As is clear from FIGS. 8 and 9, howling cancellation can be satisfactorily realized even when the propagation characteristic Cf simulating the air propagation characteristic C by the filter circuit 5 is used.

As a result, the noise from the noise source N and the control sound from the noise control speaker 2 are superimposed on the noise detection microphone 1, but the output of the amplifier 6 and the output of the filter circuit 5 are output by the differentiator 7. , The control sound is relatively weakened more than the noise. Therefore, since the open loop gain can be set high, the effect of noise reduction at the listener's position can also be enhanced. That is, the amplifier 10 in the feedback loop
Since the external signal, that is, the noise is relatively strengthened, the signal amplified by the signal is reproduced as the opposite phase, thereby producing the effect of noise reduction.

As described above, the acceleration sensor 3 and the filter circuit 5 are newly provided in the TCM system, and the signal detected by the noise detection microphone 1 is used to convert the sound wave emitted from the noise control speaker 2 and transmitted to the noise detection microphone 1. By subtracting the corresponding signal to cancel howling and negatively feedback only the external noise signal, it is possible to obtain a large feedback gain in a wider frequency band than the conventional TCM system, thus further improving the noise reduction effect. Can be improved.

Although the acceleration sensor 3 is attached to the diaphragm 12 in the above embodiment, the acceleration sensor 3 may be attached to the voice coil 16.

Although the acceleration sensor 3 is used as the acceleration detecting means in the above embodiment, a speed sensor and a differentiation circuit may be used instead of the acceleration sensor 3. For example, the diaphragm 12 of the noise control speaker 2
Alternatively, a speed detection coil is attached to the voice coil 16 and a magnetic circuit dedicated to the coil is provided, and a differentiating circuit for differentiating a speed signal obtained from the coil is provided at a stage preceding the filter circuit 5 so that the output of the differentiating circuit An acceleration signal is obtained. In this case, if the speed detecting coil and the voice coil 16 are arranged close to each other, the influence of the mutual induction may not be negligible at frequencies higher than the resonance frequency f0 of the noise control speaker 2 to some extent. Alternatively, only the frequency band near f0 in the acceleration signal may be used for howling cancellation.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an active noise control device according to an embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of a main part of the noise control speaker.

FIG. 3 is a functional block diagram of a filter circuit.

FIG. 4 is an explanatory diagram of frequency characteristics of a gain among transfer function characteristics between a noise control speaker and a noise detection microphone.

FIG. 5 is an explanatory diagram of phase frequency characteristics of transfer function characteristics between a noise control speaker and a noise detection microphone.

FIG. 6 is an explanatory diagram of a frequency characteristic of a gain among an air propagation characteristic between a noise control speaker and a noise detection microphone and a propagation characteristic of a filter circuit.

FIG. 7 is an explanatory diagram of a phase frequency characteristic among an air propagation characteristic between a noise control speaker and a noise detection microphone and a propagation characteristic of a filter circuit.

FIG. 8 is an explanatory diagram of residual characteristics when howling cancellation is performed using air propagation characteristics between a noise control speaker and a noise detection microphone.

FIG. 9 is an explanatory diagram of residual characteristics when howling cancellation is performed using the propagation characteristics of a filter circuit.

FIG. 10 is a schematic configuration diagram of a conventional feedback-type active noise control device.

FIG. 11 is a gain frequency characteristic diagram of an open loop characteristic of the TCM.

FIG. 12 is a phase frequency characteristic diagram of an open loop characteristic of the TCM.

FIG. 13 is an explanatory diagram of a noise reduction effect by the TCM.

[Explanation of symbols]

 Reference Signs List 1 microphone for noise detection 2 speaker for noise control 3 acceleration sensor 5 filter circuit 7 differentiator 8 reverse phase circuit 9 equalizer 11 magnetic circuit 12 diaphragm 13 magnet 16 voice coil

Claims (6)

[Claims] 1. A noise detection microphone for detecting noise, a noise control speaker arranged near the noise detection microphone, and the noise based on a signal obtained by inverting an output phase of the noise detection microphone. A feedback-type active noise control device having negative-phase means for driving a control speaker, wherein the noise signal is negatively fed back to the noise control speaker to reduce noise. An acceleration detection unit that outputs a signal corresponding to the acceleration of the plate; and a difference generation unit that generates a difference between a signal based on the output of the noise detection microphone and a signal based on the output of the acceleration detection unit. The phase means outputs a signal obtained by inverting the output phase of the difference generation means instead of the signal obtained by inverting the output phase of the noise detection microphone. An active noise control device characterized in that the noise control speaker is driven based on a signal. 2. The active noise control device according to claim 1, wherein said acceleration detection means includes an acceleration sensor attached to a diaphragm or a voice coil of said noise control speaker. 3. The acceleration detecting device according to claim 1, wherein the acceleration detecting device includes: a speed detecting device that detects a speed of a diaphragm of the noise control speaker; and a differentiating device that differentiates an output of the speed detecting device. Active noise control device. 4. The speed detection means includes: a detection coil attached to a diaphragm or a voice coil of the noise control speaker; and a magnetic circuit that supplies a magnetic flux to the detection coil.
The active noise control device according to claim 3. 5. The anti-phase means has an anti-phase circuit for inverting the phase of an output signal of the difference generation means.
5. The active noise control device according to any one of claims 1 to 4. 6. A filter circuit having a filter characteristic simulating a space propagation characteristic between the noise control speaker and the noise detection microphone,
The active noise control device according to any one of claims 1 to 5, further comprising a difference generation circuit configured to generate a difference between a signal based on an output of the noise detection microphone and an output signal of the filter circuit.