JP2000112480A - Electronic muffling system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electronic muffling system which improves the muffling effect by placing a reference microphone at an appropriate location. SOLUTION: The system is set up for an exhausting duct DD in which one end of the duct DD is connected to a noise source NS, the middle of the duct DD is bent and a projected section PP is provided at the bent section. The system is provided with a reference microphone RM in the section PP. Then, a secondary sound source SS is placed at the downstream side of the duct DD with respect to the microphone RM and an error microphone EM is placed at the downstream side of the duct DD with respect to the sound source SS. Waveforms received by the microphones RM and EM are processed by a processing circuit 1 and sound waves for muffling are generated from the sound source SS.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic noise reduction system in which a reference microphone is installed at an improved position.

[0002]

2. Description of the Related Art FIG. 2 schematically shows a configuration of a general electronic noise reduction system, in which a reference microphone and an error microphone are arranged along an exhaust duct with a secondary sound source for generating a sound wave for noise reduction interposed therebetween. It is arranged. This system emits sound waves in the duct in the opposite phase to the noise in the duct,
It is designed to cancel noise. In the figure,
A noise source NS is arranged at one end of the exhaust duct DD,
The reference microphone RM, the secondary sound source SS, and the error microphone EM are arranged in this order from upstream to downstream of the exhaust flow in the exhaust duct DD, and the output signals of the reference microphone RM and the error microphone EM are processed by the processing unit 1. Later, it is given to the secondary sound source SS.

In the processing unit 1, an analog signal representing a sound wave received by the reference microphone RM is converted into a digital signal, processed by the adaptive filter 11, and then converted into an analog signal. And is emitted into the exhaust duct DD as a sound wave for silencing.

A signal obtained by converting the output signal of the reference microphone RM into a digital signal is also applied to the controller 12, and the controller 12 receives a signal obtained by digitally converting an analog output signal from the error microphone EM as an error signal. The controller 12 determines the filter coefficients of the adaptive filter 11 so that the error signal is minimized.

[0005] However, it is actually difficult to install the reference microphone at an appropriate position in the exhaust duct. In the exhaust duct, a node of sound pressure is generated by a sound wave propagation mode. Since the sound pressure level is infinitely small at this sound pressure node, it is difficult to detect the frequency of the sound wave in the propagation mode even if a reference microphone is installed at the sound pressure node. A noise reduction effect cannot be obtained, and inconveniences such as unstable control occur. Therefore, a reference microphone must be installed so as to avoid nodes of sound pressure.

In addition, at the design stage, since the temperature in the exhaust duct is not constant, it is not possible to specify an appropriate installation position of the reference microphone, and the installation position of the reference microphone must be determined by actual measurement. . Actually, it is necessary to predict and set several positions, and determine the final installation position based on the result of setting the reference microphone at each predicted position.

Another problem is that it is difficult to avoid the influence of corrosive substances such as water vapor and oxides contained in the exhaust stream on the life of the reference microphone and the error microphone. Furthermore, since the reference microphone is installed between the noise source and the secondary sound source, if the reference microphone is removed while the exhaust gas is flowing through the exhaust duct, the static pressure inside the exhaust duct usually causes the high temperature. There is also a problem that corrosive exhaust gas is ejected from the mounting hole and is extremely dangerous, and such a problem has been urgently solved.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned various problems.
It is an object of the present invention to provide an electronic noise reduction system that can improve a noise reduction effect by installing a reference microphone at an appropriate location.

[0009]

In order to achieve the above object, the present invention provides an exhaust duct having one end connected to a noise source, a reference microphone, a secondary sound source and a secondary microphone in a direction of an exhaust flow from the noise source. An error microphone is installed in this order, an electronic silencer system that processes sound waves received by the reference microphone and the error microphone to generate sound waves for silencing from the secondary sound source, wherein the reference microphone includes the exhaust duct. An electronic silencing system characterized by being installed at a portion having an impedance that can be regarded as an acoustically reflective end.

In one embodiment, the electronic noise reduction system according to the present invention is installed in an exhaust duct which is connected at one end to a noise source, is bent in the middle, and is provided with a projection at the bent portion. Electronic muffler system, the reference microphone installed in the protrusion, the exhaust duct, a secondary sound source installed in a position downstream with respect to the reference microphone, and the exhaust duct, An error microphone installed at a position on the downstream side with respect to the secondary sound source, a processing circuit that processes sound waves received by the reference microphone and the error microphone, and generates sound waves for muffling from the secondary sound source, Is provided.

When the exhaust duct is made of a material that reflects sound, the protrusion may be made of the same material as that of the exhaust duct. Alternatively, the protrusion may have an impedance that can be acoustically regarded as a reflection end.

[0012]

Since the reference microphone is installed at a portion of the exhaust duct having an impedance that can be regarded as an acoustically reflective end, no sound pressure node is formed at this portion, and the reference microphone is a sound wave propagating in the exhaust duct. The frequency of is detected.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIG. In FIG. 1, the same or similar components as those shown in FIG. 2 are denoted by the same reference numerals.

In FIG. 1, an exhaust duct DD having one end connected to a noise source NS is provided for generating acoustic resistance.
It is bent at an arbitrary angle (90 ° in FIG. 1) at an intermediate position, and the first duct portion D1 is sandwiched between the bent portions.
And a second duct portion D2. The bending angle of the exhaust duct DD is 90 ° in FIG. 1 (when bent at a right angle), but a deeper angle, for example, 180 °
° (in the case of turning back) or a more gentle angle. By bending the middle of the exhaust duct DD,
Acoustic impedance (acoustic resistance) can be created at the bent portion. The present invention utilizes the property that sound reflection occurs in a place where such an acoustic impedance exists and the sound pressure increases at the reflection end, so that the reflection end does not become a node of the sound pressure.

As a result of bending the exhaust duct DD halfway,
The exhaust gas flows in the first duct section D1 in the first direction, changes the direction of the flow at the bending point, and then flows in the second duct section D1.
2 flows in the second direction. A secondary sound source SS is installed at an appropriate position in the second duct portion D2, and an error microphone EM is further provided downstream thereof. The signals output from the reference microphone RM and the error microphone EM are processed in the processing unit 1 as described above with reference to FIG.
Sound is emitted into D.

As shown in FIG. 1, a protruding portion PP is provided at a bent portion of the exhaust duct DD in a direction opposite to the direction of the second duct portion D2. When the exhaust duct DD is made of a material having a high density, which is hard, clogged, and has a sound-reflecting property (for example, iron or stainless steel), the protrusion PP is the same as the material constituting the exhaust duct DD. Can be made of material. Further, the protruding portion PP may be configured to have an acoustic impedance that can be regarded as an acoustically reflective end. As a result, the acoustic impedance in the vicinity of the wall surface on which the reference microphone RM is mounted becomes high, and does not become a node of the sound pressure due to the mode in the exhaust duct DD. Therefore, the frequency that cannot be detected by the reference microphone RM substantially disappears. Not only can the sound deadening effect be ensured, but also the control becomes stable.

Further, it is desirable that the reference microphone RM be attached to a wall surface of the protrusion PP that is parallel to the second direction and that does not directly hit the exhaust flow. As a result, the exhaust flow does not directly hit the reference microphone RM, so that the corrosion of the reference microphone RM due to water vapor or oxides contained in the exhaust flow is reduced, so that not only the life is extended, but also the exhaust flow is reduced. Exhaust air does not blow out of the mounting holes when the reference microphone is removed.

[0018]

As is apparent from the above description of one embodiment of the electronic noise reduction system according to the present invention, according to the present invention, the frequency which cannot be detected by the reference microphone is substantially eliminated, and the noise reduction effect is surely achieved. Not only is it possible to play, but also there is an extraordinary effect that the control becomes stable.

Moreover, if the reference microphone is mounted on the projecting portion, the reference microphone can be mounted on the wall surface not directly hitting the exhaust flow, so that the exhaust flow does not directly hit the reference microphone and is included in the exhaust flow. Corrosion of the reference microphone due to water vapor, oxides, etc. is reduced, extending the life of the reference microphone. In addition, even if the reference microphone is removed in the presence of the exhaust flow, the exhaust flow does not blow out from the mounting hole. You.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of one embodiment of an electronic noise reduction system according to the present invention.

FIG. 2 is a diagram schematically showing a configuration of a known electronic silencing system.

[Explanation of symbols]

RM: Reference microphone, EM: Error microphone,
SS: Secondary sound source, DD: Exhaust duct, PP: Projection

Claims (4)

[Claims] 1. A reference microphone, a secondary sound source, and an error microphone are installed in this order on an exhaust duct having one end connected to a noise source in a direction of an exhaust flow from the noise source, and receive the reference microphone and the error microphone. An electronic silencer system for processing sound waves generated from the secondary sound source to generate sound waves for silencing, wherein the reference microphone is installed in a portion of the exhaust duct having an impedance that can be regarded as an acoustically reflective end. Characteristic electronic silencing system. 2. An electronic silencing system installed in an exhaust duct having one end connected to a noise source, a halfway bent portion, and a protruding portion provided at the bent portion, wherein the electronic silencing system is installed on the protruding portion. The reference microphone, and the exhaust duct, a secondary sound source installed at a position downstream with respect to the reference microphone, and the exhaust duct, installed at a position downstream with respect to the secondary sound source An electronic silencing system comprising: an error microphone; and a processing circuit configured to process sound waves received by the reference microphone and the error microphone and generate sound waves for silencing from the secondary sound source. 3. The electronic noise reduction system according to claim 2, wherein the exhaust duct has a property of reflecting sound, and the projection is made of the same material as that of the exhaust duct. . 4. The electronic silencing system according to claim 2, wherein the protrusion has an impedance that can be acoustically regarded as a reflection end.