JP2001117567A - Multi-duct electronic muffling system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-duct electronic muffling system in which deterioration in muffling effect caused by diffraction of secondary sound, etc., is prevented without installing a sound absorbing duct on the downstream side from the system. SOLUTION: An error microphone 14 of the electronic muffling system consists of two microphone elements 26 and 28 having the same characteristic, a delaying circuit 30, which delays the sound wave signals detected by the element 26 located on the downstream side by an amount equivalent to the propagation time of the sound waves that propagate between the elements 26 and 28, and a differential amplifier 32 which outputs the difference between the sound wave signals detected by the element 28 and the sound wave signals that are detected by the element 26 and delayed by the circuit 30.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-duct electronic noise reduction system, and more particularly, to generating a sound wave having the same sound pressure in opposite phase with respect to a sound wave propagated from a noise source, and actively suppressing the sound by sound wave interference. The present invention relates to a multi-duct electronic silencing system.

[0002]

2. Description of the Related Art As one method of reducing low-frequency noise propagating in a pipeline, an electronic noise reduction system that radiates a secondary sound having the same phase and the same amplitude as noise and suppresses the noise by a sound wave interference effect is practically used. Has been

The frequency range to which the electronic silencing system can be applied is up to the frequency at which air conditioning duct noise propagates in the duct as a plane wave sound wave, and the frequency range tends to decrease as the duct diameter increases. Is 1 m square, it can be applied to a low frequency range of about 200 Hz or less.

However, in air conditioning and ventilation facilities such as underground substations and subways having a duct diameter of about several square meters, the frequency range to which the electronic silencing system is applied is significantly reduced. The realization of a system is required.

On the other hand, Multiple error filtered-
A configuration using multiple sensors and actuators using the X LMS algorithm has been proposed. In this method, since a multi-input multi-output signal processing system is required, the amount of calculation of the signal processing system becomes enormous as the number of control target modes increases.

Further, a configuration has been proposed in which a higher-order mode component of duct propagation sound is analyzed and control is performed for each mode component.
The control system is constructed by combining multiple processing systems for modal decomposition and generation and adaptive control systems using a single-channel filtered-X LMS algorithm. This method is 1
Even for the next mode control, at least four secondary sound sources are required, and the arrangement of the secondary sound sources must be designed for each duct size. Further, the effect of the control of the second mode or higher has not been demonstrated.

Therefore, as a means for solving these problems, a multi-duct electronic noise reduction system in which a plurality of electronic noise reduction systems are configured in a cell-shaped duct shape has been proposed. This method is based on the principle that a one-dimensional sound field is locally formed in the middle of a large-diameter duct system, and a higher-order mode is changed to a lower-order mode. At the same time, the conventional electronic silencing system is applied to achieve a significant cost reduction.

[0008]

However, the multi-duct electronic silencing system has a problem that the silencing volume is greatly reduced as compared to the electronic silencing system alone. It was said that the cause was that secondary sounds circulated from each of the muffler system ducts downstream of the multi-duct electronic muffler system, amplifying the error signal of the control system.

As a remedy, a method of installing a sound-absorbing duct downstream of the multi-duct electronic silencing system has been proposed. The length of the sound-absorbing duct is at least 63 Hz.
Since 1500 mm or more corresponding to に wavelength is required, there is a problem that the multi-duct electronic noise reduction system becomes huge, and an improvement measure has been demanded.

The present invention has been made in view of such circumstances, and solves the above-mentioned disadvantages of the prior art. Therefore, without installing a sound-absorbing duct downstream of a multi-duct electronic noise reduction system, it is possible to diffract secondary sounds. It is an object of the present invention to provide a multi-duct electronic noise reduction system that can prevent a reduction in noise reduction effect due to noise.

[0011]

In order to achieve the above object, according to the first aspect of the present invention, a pipeline is divided into a plurality of cells, and each cell is propagated from an upstream side to a downstream side for each cell. In a multi-duct electronic noise reduction system that performs active noise reduction by sound wave interference using secondary sound sources of opposite phases at the same sound pressure, the noise is disposed near the downstream end of each cell to evaluate the active noise reduction of the noise. The two microphones are arranged on the upstream side and the downstream side of the cell and have two sound wave detecting means having the same characteristics, and the sound wave signal detected by the sound wave detecting means on the downstream side of the two sound wave detecting means is A delay unit that delays by a time corresponding to the propagation of a sound wave between the two sound wave detection units, a sound wave signal detected by the upstream sound wave detection unit, and a sound wave signal detected by the downstream sound wave detection unit, The delay And a difference signal output means for outputting a difference from the sound wave signal delayed by the stage, wherein residual noise of another cell propagated from the downstream side to the upstream side is prevented from being detected by the microphone. And

The invention described in claim 2 is the first invention.
In the invention described in the above, the two sound wave detecting means have directivity such that the detection sensitivity of noise propagating from a direction perpendicular to the propagation direction of the noise propagating from the upstream side to the downstream side is low. Features.

According to a third aspect of the present invention, the pipeline is divided into a plurality of cells, and the noise propagating in each cell from the upstream side to the downstream side in each cell is converted into a secondary sound having the same sound pressure and opposite phases. In a multi-duct electronic noise reduction system that performs active noise reduction by acoustic interference using a sound source, microphones installed in each cell for active noise reduction of the noise are arranged in a propagation direction of the noise that propagates from the upstream side to the downstream side. The sound wave detecting means has directivity such that the detection sensitivity of noise propagating from the vertical direction is reduced.

The invention described in claim 4 is the same as the claim 2.
Alternatively, in the invention according to claim 3, the sound wave detecting means is provided with two microphone elements having the same characteristic adjacent to each other, and outputs a difference between sound wave signals detected by the two microphone elements. I have.

According to the present invention, the sound propagating from the downstream side of the multi-duct electronic silencing system, which causes a decrease in the silencing volume, is obtained.
Since it is possible to prevent the detection of the diffracted sound of the next sound or the transmitted sound transmitted through the wall surface from another cell, it is possible to prevent a problem in which the sound deadening effect is reduced in each cell due to the secondary sound of the other cell. This makes it possible to obtain a noise reduction effect comparable to that of the electronic noise reduction system alone, without having to install a diffraction sound prevention duct on the downstream side and keeping the dimensions down.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a multi-duct electronic noise reduction system according to the present invention will be described below in detail with reference to the accompanying drawings.

First, it will be described that the multi-duct electronic noise reduction system is effective for high-order mode component sound waves in addition to plane wave sound waves. Consider the propagation of sound waves in a rectangular duct formed by a rigid wall in a state where there is no flow. The sound pressure p at an arbitrary point of the sound wave propagating in the + Z direction is expressed by the following equation.

[0018]

(Equation 1) Here, m and n are subscripts (0, 1, 2,...) Representing modes,
A _{m, n} represents the amplitude of the (m, n) mode, φ _{m, n} represents the eigenfunction of the duct, and the eigenfunction is represented by equation (2) for a rectangular duct.

[0019]

(Equation 2) K _{Z in} equation (1) is a propagation constant in the Z direction,
It is expressed by equation (3).

[0020]

(Equation 3) K _{m, n} is the cutoff frequency of the (m, n) mode, and is expressed by equation (4) assuming that the inner surface of the duct is a perfect reflection surface.

[0021]

(Equation 4) Therefore, the propagation constant in equation (3) can be expressed as in equation (5).

[0022]

(Equation 5) FIG. 2 shows the result of calculating the m and n mode frequencies and the duct diameter based on the equation (5). Here, calculation was performed for a duct having a square cross section of a = b. If the frequency of the sound wave in the duct is lower than the (1,0) or (0,1) mode, the sound wave propagates as a plane wave. As can be seen from FIG. 2, the limit frequency of the plane wave propagation decreases as the diameter of the duct increases. 343 when the duct diameter is 500mm square
Hz, and 172 Hz for 1000 mm square.

For this reason, in a large-diameter duct, it is required to perform electronic noise suppression in a region including propagation of higher-order mode components in addition to plane wave propagation.

Further, the length of the electronic silencing system needs to be as short as possible in consideration of application to actual air conditioning and ventilation equipment. Since the length between the secondary sound source and the sensor microphone is designed to satisfy the causality of the feedforward adaptive control system, it is set to be considerably wider than the distance between the secondary sound source and the error microphone. On the other hand, since the length between the secondary sound source and the error microphone is designed as a propagation distance until the wavefront of the secondary sound source is converted into a plane wave, the position of the error microphone can be set near the secondary sound source. Yes, contributing to shortening the length of the electronic silencing system.

Therefore, in order to bring the noise reduction effect of the multi-duct electronic noise reduction system closer to the noise reduction effect of the electronic noise reduction system alone, it is necessary to reduce the diffraction sound with respect to the error microphone side, and it is necessary to set a reduction target value.

FIG. 3 shows a basic configuration in which an active silencer constituted by a feedforward adaptive control system is installed in a single duct. FIG. 1A is a schematic diagram of an active silencer, in which reference numeral 1 denotes a duct, reference numeral 2 denotes a primary sound source (noise source), reference numeral 10 denotes a secondary sound source (speaker), reference numeral 12 denotes a sensor microphone, and reference numeral 14 denotes an error. A microphone 16 is a controller. FIG. 2B is a basic configuration diagram of a control system, and shows a configuration using a filtered-X LMS algorithm as an adaptive control algorithm. Further, in FIG. 2B, the output signal e (ω) of the error microphone 14 is defined as shown in Expression (6), and electric noise of the controller 16 and intrusion sound from outside the duct 1 are also detected by the error microphone 14. , Is superimposed as noise.

[0027]

(Equation 6)

[0028]

(Equation 7) However, these noises n (ω) are the signals x of the primary sound source 2
(Ω) and uncorrelated. In addition, it is assumed that there is no acoustic feedback of the secondary sound source 10. FIG. 3 (3) is a simplified version of FIG. 2 (2) displayed in the frequency domain, and is described as a one-input one-output linear system.
The authors have previously derived the theoretical silencing volume of the active silencer based on FIG. The silencing expression (8) is expressed by the ratio of the auto power spectrum before control and the auto power spectrum after control at the position of the error microphone 14.

[0029]

(Equation 8)

[0030]

(Equation 9) In equation (8), S _ee (ω), S _dd (ω), S
_xx (ω) indicates an auto power spectrum of the error signal, the reference signal, and the input signal, respectively, and S _xd (ω) indicates a cross power spectrum of the sensor microphone 12 and the error microphone 14 at the time of non-control. γ ² (ω) is the coherence between the sensor microphone 12 and the error microphone 14.
Therefore, equation (8) does not consider the design of the adaptive control system,
This is to provide an ideal silencing volume of the active silencer determined only from the correlation between the input and output signals.

FIG. 4 shows a calculation result when the expression (9) is converted into a silencing volume. The figure also shows the results of actual measurement of coherence between the sensor microphone and the error microphone of the electronic noise reduction system installed in two air conditioning units. Theoretical and measured values correspond well.

Therefore, in order to obtain a noise reduction volume of 15 dB, coherence is required to be 0.82 or more. Diffracted sound from other multiple ducts with respect to the error microphone of the multi-duct electronic noise reduction system has a combined acoustic power of at least. It is considered necessary to reduce the content to 18% or less.

As one of the countermeasures, as shown in FIG. 5, the duct 1 is divided into, for example, four cells 1A to 1D, and for each of the cells 1A to 1D, the same secondary sound source ( Speaker), a sensor microphone 12, an error microphone 14, and a controller (adaptive controller) 16 attached to a multi-duct electronic silence system 1500 mm downstream of 63 Hz, which is a quarter-wavelength of 63 Hz.
A method of installing the diffraction sound prevention duct 4 has been proposed, but shortening the length has been a major problem.

Therefore, the present invention focuses on the structure of the error microphone 14 from a completely new point of view, constructs the error microphone 14 by combining a plurality of microphones and a delay circuit, gives sharp error directivity to the error microphone 14, and generates a diffracted sound. It is characterized by taking measures to prevent detection.

Hereinafter, an embodiment of a multi-duct electronic noise reduction system according to the present invention will be described with reference to FIG. FIG. 1 shows an electric circuit configuration inside the cylindrical error microphone 14 shown in FIGS. 3A and 5 installed in each cell of the multi-duct, and an opening is provided in the duct wall 5. 2 shows a cross-sectional view when the error microphone 14 is installed on the outer wall of the duct. Reference numeral 20 denotes a sound absorbing material lined on the inner wall of the duct, and reference numeral 22 denotes a porous plate such as a wire mesh installed at the opening of the duct wall 5.

According to the present invention, the error microphone 14 detects the diffracted sound propagating from the downstream side of the muffler system, instead of reducing the diffracted sound in the duct structure and improving the muffling volume of the multi-duct electronic muffler system. It is configured not to. That is, the error microphone 14 is connected to the microphone housing 24.
It is composed of two microphone elements 26 and 28, a delay circuit 30 and a differential amplifier 32 disposed therein, giving a sharp directional characteristic to the error microphone 14 and taking measures to prevent detection of a diffracted sound. . In the figure, two microphone elements 26 and 28 mounted inside the microphone housing 24 are mounted side by side in the longitudinal direction of the duct system, and the negative and positive input terminals of each differential amplifier 32 are provided. Connected to. However, the microphone element 26 on the downstream side is connected to the differential amplifier 32 via the delay circuit 30 that delays the signal by the microphone interval L. Therefore, the diffracted sound detected by the microphone elements 26 and 28 is electrically erased by the differential amplifier 32, and the diffracted sound cannot be detected by the error microphone 14. The set time D of the delay time of the delay circuit 30 can be determined by equation (10).

[0037]

(Equation 10) In the equation (10), C is a sound speed, and L is a microphone interval. Further, the error microphone 14 is attached to the outer wall of the duct and detects noise through the sound absorbing material 20, so that the error microphone 14 is not affected by airflow noise in the duct.
The noise in the duct can be accurately detected.

FIG. 1 shows an error microphone 1 according to the present invention.
Reference numeral 4 denotes two microphone elements 26 and 28 mounted inside the microphone housing 24, a delay circuit 30, and a differential amplifier 32. The diffracted sound propagates only through the duct system and reaches. It is a condition. However, depending on the duct structure, there may be diffracted sound transmitted through the adjacent duct wall. FIG. 6 is an invention of an error microphone corresponding to this problem. Members having the same or similar functions as those shown in FIG. 1 are denoted by the same reference numerals as those in FIG. 1 (the sound absorbing material 20, the porous plate 22).
Is omitted), and the description is omitted.

In the present invention, the microphone element 2 shown in FIG.
6 and 28 instead of two microphone elements 40
A, 40B, microphone elements, 42A, 42B, and differential amplifiers 44, 46 are provided. The microphone element 40A,
Microphone element 2 composed of 40B and differential amplifier 44
6 correspond to the microphone element 40 and the microphone element 42
A part corresponding to the microphone element 28 composed of A and 42B and the differential amplifier 46 is referred to as a microphone element 42. The microphone element 40 corresponding to the microphone element 26 of FIG. 1 will be described. As shown in FIG. 7, microphone elements 40A and 40B having similar electroacoustic characteristics are attached to face each other,
The difference between these detection signals is obtained by the differential amplifier 44. Therefore, the directional characteristics of the microphone element 40 thus configured have a figure-eight shape on a two-dimensional plane due to cosine characteristics, and sound waves orthogonal to the microphone axis direction cannot be detected. . Then, as shown in FIG. 6, these two sets of microphone elements 4 opposed to each other are arranged.
The microphones 0 and 42 are attached side by side in the longitudinal direction of the duct system. Based on the principle shown in FIG. 7, the microphone elements 40 and 42 are affected by sound waves in the cross-sectional direction of the duct, that is, the sound waves transmitted from the ducts of other cells. The structure is difficult to receive. And FIG.
Similarly to the above, the microphone element 40 on the downstream side of the noise incorporates a delay circuit that delays the signal by the microphone interval L, thereby having a structure in which the diffracted sound cannot be detected.

The microphone elements 40 and 42 shown in FIG.
Can be used alone to prevent the influence of transmitted sound waves from other cells in the multi-duct. Also, the microphone elements 40 in each of the microphone elements 40 and 42
The installation direction of the microphone elements A and 40B or the microphone elements 42A and 42B may be a vertical direction (perpendicular to the paper surface) instead of the longitudinal direction of the duct system. Further, the microphone elements 40A, 40B
(Or the microphone elements 42A and 42B) may be arranged in parallel in the same direction (for example, toward the upstream side) instead of facing each other, and the two microphone elements 40A and 40B (or the microphone elements 42A and 42B). ) May be arranged so as to have the same detection sensitivity for sound waves propagating from a direction perpendicular to the duct wall surface.

According to the present invention, the coherence between the sensor microphone and the error microphone is improved to 0.82 or more, so that a silencing volume of 15 dB can be obtained.

As described above, the prevention of the detection of the diffracted sound in the error microphone 14 has been described in the above embodiment, but the above described microphone configuration can be similarly applied to the sensor microphone 12.

[0043]

As described above, according to the multi-duct electronic noise reduction system according to the present invention, the wall surface is formed from the diffracted sound of the secondary sound or another cell that propagates from the downstream side of the multi-duct electronic noise reduction system, which causes the reduction of the volume. Since it is possible to prevent the detection of a transmitted sound that is transmitted and propagated, it is possible to prevent a problem in which the sound deadening effect is reduced due to the secondary sound of another cell in each cell, and a diffraction sound prevention duct is provided downstream. Without reducing the size, a noise reduction effect comparable to that of the electronic noise reduction system alone can be obtained.

[Brief description of the drawings]

FIG. 1 is a structural diagram of an error microphone for a multi-duct electronic noise reduction system.

FIG. 2 is a diagram illustrating a relationship between a duct diameter and an eigenmode frequency.

FIG. 3 is a configuration diagram of an electronic silencing system.

FIG. 4 is a diagram illustrating a relationship between coherence and silencing volume.

FIG. 5 is a diagram showing a conventional improvement measure of a multi-duct electronic noise reduction system.

FIG. 6 is a structural diagram of an improved error microphone for a multi-duct electronic noise reduction system.

FIG. 7 is a directional characteristic diagram of a microphone element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... duct, 2 ... primary sound source (noise source), 10 ... secondary sound source (speaker), 12 ... sensor microphone, 14 ... error microphone, 16 ... controller, 20 ... sound absorbing material, 22 ... porous plate, 26, 28, 40, 40A, 40B, 42, 42
A, 42B: microphone element, 30: delay circuit, 32, 4
4, 46… Differential amplifier

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryusuke Gotoda 1-1-14 Uchikanda, Chiyoda-ku, Tokyo F-term in Futatsu Plant Construction Co., Ltd. 5D061 EE11 FF02

Claims (4)

[Claims] 1. A pipe line is divided into a plurality of cells, and noise propagating in each cell from an upstream side to a downstream side is actively silenced by sound interference using secondary sound sources having the same sound pressure and opposite phases. In the multi-duct electronic silencing system that performs the above, the microphones arranged near the downstream end of each cell and for evaluating the active silence of the noise have the same characteristics arranged on the upstream side and the downstream side of the cell. Two sound wave detecting means, and a delay for delaying a sound wave signal detected by a sound wave detecting means on the downstream side of the two sound wave detecting means by a time required for a sound wave to propagate between the two sound wave detecting means. Means for outputting a difference signal between the sound wave signal detected by the upstream sound wave detecting means and the sound wave signal detected by the downstream sound wave detecting means and delayed by the delay means. A multi-duct electronic noise reduction system comprising: a microphone configured to prevent detection of residual noise of another cell transmitted from a downstream side to an upstream side by the microphone. 2. The apparatus according to claim 1, wherein said two sound wave detecting means have directivity such that the detection sensitivity of noise propagating from a direction perpendicular to the direction of propagation of said noise propagating from said upstream side to said downstream side is low. The multi-duct electronic silencing system according to claim 1, wherein 3. A pipe line is divided into a plurality of cells, and noise propagating in each cell from upstream to downstream in each cell is actively silenced by sound wave interference using secondary sound sources having the same sound pressure and opposite phases. In the multi-duct electronic noise reduction system, a microphone installed in each cell for active noise reduction of the noise detects noise propagating from a direction perpendicular to a propagation direction of the noise propagating from the upstream side to the downstream side. A multi-duct electronic noise reduction system, characterized in that it is a sound wave detecting means having directivity such that sensitivity is reduced. 4. The sound wave detecting means according to claim 2, wherein
4. The multi-duct electronic silencing system according to claim 2, wherein two microphone elements are provided adjacent to each other, and a difference between sound wave signals detected by the two microphone elements is output.