JP3287467B2 - Silencer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called active (or so-called active Active) type silencer.

[0002]

2. Description of the Related Art The above-mentioned active silencer mainly comprises, for example,
It is used to efficiently muffle sound in a relatively low frequency band below 00 Hz. FIG. 3 shows a conventional example of this silencer. As shown in FIG.
The exhaust noise exhausted from outside through the exhaust duct 2 is targeted for noise reduction. The exhaust noise is collected at an arbitrary position in the exhaust duct 2, for example, at a position close to the engine 1. A reference microphone 3 is provided.
An output signal of the reference microphone 3, that is, a noise signal x (t) is input to a control unit 4 having an adaptive digital filter constituted by, for example, a DSP (digital signal processing device) or a CPU (central processing unit). . Based on the noise signal x (t), the control unit 4 controls the secondary sound source speaker disposed downstream (to the right in FIG. 3) of the exhaust duct 2 from the position where the reference microphone 3 is provided. (Hereinafter, simply referred to as a speaker.) 5
Thus, a mute control signal y (t) for emitting a sound wave having substantially the same phase as the exhaust sound and having the opposite phase is generated and supplied to the speaker 5. As a result, sound waves substantially equal in phase and opposite to the exhaust sound are emitted from the speaker 5 into the exhaust duct 2, and the exhaust sound is canceled.

[0003] Further, an error microphone for collecting so-called residual noise after canceling the exhaust sound by the radiated sound of the speaker 5 is provided downstream of the position where the speaker 5 is provided in the exhaust duct 2. 6 are connected. The output signal of the error microphone 6, that is, the error signal e (t) is also input to the control unit 4. The control unit 4 updates the filter coefficient of the adaptive digital filter so as to minimize the signal level of the error signal e (t), that is, the residual noise, that is, performs an adaptive operation.

[0004] The residual noise is finally discharged to the outside from the outlet 2a of the exhaust duct 2, but the acoustic impedance at the outlet 2a changes suddenly.
A part of the residual noise is reflected at the outlet 2a and propagates in the exhaust duct 2 in the opposite direction (leftward in the figure, ie, upstream), as indicated by an arrow 2b in the figure. Then, a so-called backward wave propagating in the exhaust duct 2 in the opposite direction, and a so-called traveling wave propagating in the exhaust duct 2 toward the discharge port 2a side (to the right in FIG. 2, that is, downstream). Interfere with each other and cancel each other out, thereby causing a dip in the frequency characteristics in the exhaust duct 2 (where the sound pressure decreases).
Occurs.

For example, for a sound having a certain frequency f,
It is assumed that the dip occurs at a position in the exhaust duct 2 where the error microphone 6 is provided. In this case, at the place where the dip occurs, that is, at the position of the error microphone 6, the sound pressure at the frequency f becomes small. Therefore, the error microphone 6 cannot accurately pick up the component of the frequency f among the residual noises to be picked up, thereby failing to obtain a sufficient noise canceling effect at the frequency f.

The influence of the dip extends to a case where, for example, a generally known Filtered-x LMS algorithm is used as a control system of the adaptive digital filter in the control unit 4. That is, when the control system of the adaptive digital filter is configured as the Filtered-x LMS algorithm, the input unit of the control unit 4 is transmitted from the output unit of the control unit 4 through the speaker 5, a part of the exhaust duct 2, and the error microphone 6. , It is necessary to identify (estimate) a transfer function generally called a secondary sound path (error path) C. As a method of realizing this, for example, the secondary sound path C is identified by intentionally radiating random noise from the speaker 5, collecting the random noise with the error microphone 6, and analyzing the collected sound. There is a way to do that. However, even at the time of identification of the secondary sound path C, the random noise and the backward wave caused by the reflection of the random noise at the outlet 2a interfere with each other and cancel each other. A dip may occur in the frequency characteristic of C. Therefore, if this dip occurs at the position of the error microphone 6 at a certain frequency f, the error microphone 6 cannot accurately identify the secondary sound path C at that frequency f. As a result, there is a possibility that the convergence of the silencing operation is delayed at the frequency f, or that the control system diverges and the silencing cannot be performed at all.

Here, the distance between the position of the error microphone 6 in the exhaust duct 2 and the outlet 2a (strictly, a value obtained by applying a generally known pipe end correction to this distance).
Is L, and the sound velocity in the exhaust duct 2 during this time is V. It is known that the dip occurs at the position of the error microphone 6 at a frequency f that is an integral multiple of V / (2L). This relationship is represented by the following equation (1).

[0008]

F = n {V / (2L)} (n = 1, 2,
3, ...)

In this equation 1, when the integer n is, for example, n = 1, 2, and 3, images of the sound pressure distribution in the exhaust duct 1 (between the error microphone 6 and the outlet 2a) are respectively shown. 4A, 4B and 4C are exaggerated.

According to Equation 1 and FIG. 4, it can be seen that the shorter the distance L from the error microphone 6 to the outlet 2a, the higher the frequency f at which a dip occurs at the position of the error microphone 6. Therefore, it is desirable that the error microphone 6 be provided as close to the outlet 2a as possible. By doing so, the frequency f at which the dip occurs at the position of the error microphone 6 can be shifted to a higher frequency side than the frequency band to be silenced by the silencer, and the silencing operation (secondary sound path C Identification)
This is because the influence of the above-mentioned dip can be avoided.

However, there is a case where the error microphone 6 cannot be installed in the vicinity of the discharge port 2a depending on the use of the silencer and the installation conditions. For example, as shown in FIG.
In a multi-storey building or the like, the exhaust ducts 2, 2, ... provided for each floor A, B, ... are connected to a common collecting duct 7, respectively. Are exhausted to the outside through the exhaust ducts 2, 2,.

That is, according to the configuration of FIG.
The acoustic impedance also changes greatly at the discharge port 7a. Therefore, the outlet 7a of this collecting duct 7
In this case, reflection of sound (residual noise) occurs as shown by an arrow 7b in FIG. Then, the backward wave caused by the reflection and the traveling wave propagating in the exhaust ducts 2, 2,... And in the collecting duct 7 toward the downstream side (the exhaust port 7a side) interfere with each other. The above dip occurs in each of the exhaust ducts 2, 2,... And in the collecting duct 7. In order to avoid the influence of the dip by, for example, the same method as described above, it is necessary to install the error microphone 6 near the outlet 7a of the collecting duct 7. However, in a relatively large-scale facility (plant) as shown in the figure, floors A, B,... Where the exhaust ducts 2, 2,. Since the outlet 7a is separated from the outlet 7a, it is often impossible to install the error microphone 6 near the outlet 7a of the collecting duct 7. In such a case, the error microphone 6 must be installed in each of the exhaust ducts 2 (the exhaust duct 2 on the floor A in the figure). Is considerably long. Here, for example, if the error microphone 6 is installed near the outlet 2a of the exhaust duct 2, it is possible to avoid at least the effect of the dip caused by the reflection at the outlet 2a of the exhaust duct 2.
However, the influence of the dip caused by the reflection at the outlet 7a of the collecting duct 7 cannot be avoided, and specifically,
A dip occurs at the position of the error microphone 6 within the frequency band to be silenced by the silencer, and a sufficient silencing effect cannot be obtained.

[0013]

As described above, according to the above-mentioned prior art, when the error microphone 6 cannot be installed near the outlet 2a (or 7a) which causes the generation of the backward wave, the influence of the dip occurs. As a result, there is a problem that a sufficient silencing effect cannot be obtained. The problem is that, as shown in FIG. 5, in a series of sound propagation paths composed of the exhaust duct 2 and the collecting duct 7, a so-called reflective portion such as each of the outlets 2a and 7a which causes backward wave generation is formed. This is remarkable when there are a plurality of them.

Therefore, the present invention provides the above-mentioned discharge ports 2a, 7a
It is an object of the present invention to provide a noise reduction device that can avoid the influence of the dip even when an error microphone cannot be installed near a reflection part of a sound such as a sound. It is also an object of the present invention to realize such a silencer with a relatively simple configuration.

[0015]

In order to achieve the above object, the present invention provides a sound transmission path provided in a sound propagation path for transmitting a first sound input from one end to another end and discharging the first sound. A first microphone that picks up the first sound, and a second sound that is arranged on the other end side of the sound propagation path from the position where the first microphone is provided, and that transmits the second sound according to the muffling control signal. Loudspeaker means for radiating into the sound propagation path and interfering with the first sound; and locating the sound propagation path at the other end of the sound propagation path from the position where the loudspeaker means is provided. A second microphone that picks up a sound propagating in a road and output signals of the first and second microphones are input, and the characteristics of the second sound are changed according to the input signals. The mute control signal is generated so that the characteristics required to cancel And a muffling control means for supplying the sound to the speaker means, wherein the muffler is interposed between a position in the sound propagation path where the second microphone is provided and the other end; A passive silencer having an attenuation characteristic in a predetermined frequency band including a part or all of a frequency band to be silenced by the silencer is provided.

According to the present invention, the mute control means responds to each output signal of the first and second microphones to cancel the second sound, for example, the second sound required to cancel the first sound to be mute. A sound having substantially the same phase as the first sound and having a phase opposite to that of the first sound is emitted from the speaker means (strictly, a muffling control signal for emitting the second sound is generated and supplied to the speaker means). Thus, the first sound interferes with the second sound and is canceled. Then, of the first sounds, the so-called residual noise remaining without being canceled by the second sounds is discharged to the outside from the other end of the sound propagation path via the passive muffler.

By the way, at the other end of the sound propagation path, that is, at the outlet, the acoustic impedance changes suddenly. Therefore, part of the residual noise is reflected at the outlet and propagates in the sound propagation path in the opposite direction (one end of the sound propagation path, that is, upstream). The backward wave propagating in the sound propagation path in the opposite direction and the traveling wave propagating normally in the sound propagation path toward the outlet (ie, downstream) interfere with each other and cancel each other out. As a result, the above-described dip occurs in the frequency characteristics in the sound propagation path.

However, in the present invention, as described above, at least the active silencing operation is performed between the position where the second microphone is provided in the sound propagation path and the outlet. A passive silencer having an attenuation characteristic in a frequency band including a part or all of the frequency band is provided. Therefore, the residual noise is attenuated by the so-called passive silencing action of the passive silencer before reaching the outlet of the sound propagation path. Then, a part of the attenuated residual noise is reflected at the outlet, becomes the backward wave, and propagates in the sound propagation path in the opposite direction. In this way, the fact that a part of the residual noise is reflected and becomes a backward wave, in other words, it can be said that the backward wave is the residual noise attenuated by the reflection. Then, the backward wave returns to the position where the second microphone is provided, but is attenuated in the middle thereof by the silencing effect of the passive silencer again.

As described above, by providing the passive silencer, the backward muffler causing the dip is generated by the two-way reciprocating damping effect of the passive silencer and the damping effect by the reflection at the discharge port. Can be attenuated to a very low level. Therefore, the size (depth) of the dip generated by the backward wave can be suppressed, and the adverse effect of the dip on the active silencing operation can be reduced. The fact that the influence of the dip can be reduced as described above is the same in the case where the secondary sound path C is identified using the above-described random noise.

Incidentally, a plurality of passive silencers may be provided in series with respect to the sound propagation path. In this way, the influence of the dip can be reduced more reliably.

However, depending on the structure of the passive silencer, a so-called impedance suddenly changing portion where the acoustic impedance suddenly changes may be formed inside (including near the entrance and near the exit). In this case, the residual noise is a passive silencer (when there are a plurality of passive silencers,
In particular, when passing through the second microphone closest to the second microphone), a part thereof is reflected by the impedance suddenly changing portion, and a dip occurs in the sound propagation path due to a backward wave generated by this reflection. It is also possible. There is a concern that the dip may affect the active silencing operation.

In the case where the sudden impedance change portion is formed inside the passive silencer as described above, the passive silencer is provided near the second microphone, and the impedance between the sudden impedance change portion and the second microphone is reduced. Shorten the distance between them. By doing so, the frequency at which the dip occurs at the position of the second microphone can be shifted to a higher frequency side from the relationship of the above-described Expression 1, and, consequently, the active silence operation causes the silence. It can be higher than the frequency band of interest.

More specifically, from the relationship of the above equation 1, the distance between the position of the second microphone and the impedance suddenly changing part, the sound velocity in the sound propagation path between these two parts, and the sound velocity in the sound propagation path are determined by the active silencing operation. 2 of the upper limit of the frequency band to be silenced
A value obtained by dividing by a double value (strictly, a value obtained by performing the above-described pipe end correction on this value) is set smaller. With this configuration, the frequency at which the dip occurs at the position of the second microphone can be driven to a higher frequency side than the frequency band to be silenced by the active silence operation, and the dip for the active silence operation can be eliminated. Can be avoided.

When a plurality of passive silencers are provided, a passive silencer other than the one located closest to the second microphone, that is, a passive silencer located downstream with respect to the traveling direction of the traveling wave. There is a concern that the backward wave may also occur due to the sudden change portion of the impedance, and a dip may occur in the sound propagation path due to the backward wave. However, the residual noise (traveling wave) is gradually attenuated by sequentially passing through each passive silencer from the upstream one. Accordingly, the backward wave generated by the passive silencer (rapid impedance change portion) located on the downstream side has a lower sound pressure level. Then, the backward wave reflected by the passive silencer located on the downstream side and propagating toward the upstream side is further attenuated by another passive silencer located on the upstream side from the backward wave, and then the second microphone is turned off. Return to position.
Therefore, the influence of the dip caused by the backward wave of the passive silencer located on the downstream side is smaller than the influence of the dip caused by the backward wave of the passive silencer located on the most upstream side (closer to the second microphone). Very small to neglect.

Further, according to the present invention, a plurality of sound propagation paths are connected to a common common sound propagation path, respectively, and each sound propagating through each sound propagation path is connected to each other, as in the configuration of FIG. The present invention can also be applied to a plant configured to discharge outside from the open end of the common sound propagation path through the common sound propagation path. In this case, at least one of the above sound propagation paths
The first and second microphones, the speaker unit, the muffling control unit, and the passive muffler are provided in a sound propagation path coupled to a noise source that generates a particularly large noise such as the engine 1 described above. Only the sound propagation path is provided as an active silencer.

That is, in a plant in which a plurality of sound propagation paths are connected to a common common sound propagation path as described above, a common sound propagation path serving as a final sound outlet as viewed from the whole plant is formed. Sound reflection also occurs at the open end. Then, the backward wave caused by the reflection and the traveling wave propagating downstream in each sound propagation path and the common sound propagation path interfere with each other, and thereby, each sound propagation path and the common sound propagation path The dip occurs inside. In order to avoid the influence of the dip, it is desirable to dispose the second microphone near the open end of the common sound propagation path. But,
This is often not possible due to various restrictions such as the application of the silencer and the installation conditions. Therefore, in the present invention, a measure is taken to provide a passive silencer only for the sound propagation path constituting the active silencer, which is likely to be affected by the dip. By doing so, not only the dip caused by the reflection of the outlet of the sound propagation path, but also
The effect of the dip caused by the reflection at the open end of the common sound propagation path can also be avoided.

As described above, in the present invention, only the above measures are taken for the sound propagation path where the influence of the dip is concerned, and the reflection at the outlet of the sound propagation path and the reflection at the open end of the common sound propagation path are obtained. The effect of the dip caused by both can be avoided. Therefore, even if the present invention is applied to a relatively large-scale plant including a plurality of sound propagation paths and a joint sound propagation path in which the sound propagation paths are combined as described above, even if the entire plant is extremely There is no increase in size or cost.

Further, when the present invention is applied to a relatively large plant as described above, at least a cross-sectional area transverse to a sound propagation direction in a sound propagation path provided with a passive muffler and the like is required. The cross-sectional area that traverses the sound propagation direction in the common sound propagation path is configured to be different.

In this way, the acoustic impedance at each connection between each sound propagation path and the common sound propagation path can be changed, and the sound passing through each of these connection parts has an effect of attenuating the sound to some extent. Obtainable. That is,
When the sound discharged from each sound propagation path, particularly the residual noise discharged from the sound propagation path including the passive muffler, flows through the connection section to the common sound propagation path, Decay. Then, the attenuated residual noise is reflected at the outlet of the common sound propagation path to be a backward wave, and conversely, when flowing from the common sound propagation path to the sound propagation path via the connection portion, again. , At the connection. Therefore, the sound pressure level of the backward wave can be further attenuated by the attenuation effect at the connection portion, and the influence of the dip can be further reduced.

When the residual noise passes through the connection portion, a part thereof is reflected, and a dip may be generated in the sound propagation path due to a backward wave generated by the reflection. However, since the backward wave generated by reflection at the connection portion is attenuated by the passive silencer before returning to the position of the second microphone, the influence of the dip caused by the backward wave is small.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of a silencer according to the present invention will be described with reference to FIGS.

FIG. 1A is a diagram showing a schematic configuration of the present embodiment, and is a diagram showing a portion corresponding to the floor A in FIG. 5 described above. As shown in the figure,
The silencer according to the present embodiment has the structure shown in FIG.
A generally known passive silencer 8 is provided between the position of the exhaust duct 2 where the error microphone 6 is provided and the outlet 2 a of the exhaust duct 2. In addition,
Other configurations are the same as those in FIG. 5 described above, and therefore, the same parts are denoted by the same reference numerals and detailed description thereof will be omitted.

In the passive silencer 8, small through holes 8a, 8a,... Are formed in the peripheral wall of the exhaust duct 2 so as to cover the portions where the through holes 8a, 8a,. For example, a sound absorbing material 8b such as a kiln material is wound thereon, and the sound absorbing material 8b is covered with a protective material 8c. The frequency bands to be silenced by the passive silencer 8 of the passive silencer 8 correspond to the through holes 8a, 8a,
Can be adjusted by the diameter and arrangement of the... Or the material of the sound absorbing substance 8b. Here, the passive silencer 8
For example, the sound is adjusted so that a sound in a relatively high frequency band of 200 Hz or more is targeted for silencing. However, in practice,
The passive silencer 8 also has a slight attenuation characteristic in a frequency band lower than that, that is, a frequency band to be silenced in an active silence operation by the radiation sound of the speaker 5.

In the thus configured silencer,
For example, it is assumed that the exhaust sound propagating in the exhaust duct 2 is actively silenced by the radiated sound of the speaker 5 as described above. Here, of the exhaust noise, the residual noise remaining without being silenced by the radiation sound of the speaker 5 is shown in FIG.
As shown by the arrow [A], the light propagates in the exhaust duct 2 toward the side connected to the collecting duct 7. On the way, the residual noise, a so-called traveling wave [A], passes through the passive silencer 8 and is attenuated by the silencing effect of the passive silencer 8, and as shown by an arrow [A] in FIG. Then, the gas flows into the collecting duct 7 via a connecting portion (that is, the outlet of the exhaust duct 3) 2a between the exhaust duct 2 and the collecting duct 7.

Here, it is assumed that the cross-sectional area S ₂ crossing the sound propagation direction in the exhaust duct 2 is different from the cross-sectional area S ₇ crossing the sound propagation direction in the collecting duct 7. in this case,
The traveling wave [A] passing through the connecting portion 2a of these two 2 and 7 is attenuated to some extent at the connecting portion 2a. This is because each duct 2 in the connecting portion 2a
By and 7 the difference in the sectional area S ₂ and S _7, the acoustic impedance changes suddenly in this binding portion 2a, thereby, is more or less muffling effect on the traveling wave [i] passing through the coupling portion 2a Because it works. In a plant having the collecting duct 7 as shown in FIG.
In order to improve the sound propagation efficiency, it is general that the sectional area S ₇ of the collecting duct ₇ is larger than the sectional area S ₂ of the exhaust duct 2.

The traveling wave [A] attenuated by the coupling portion 2a propagates through the collecting duct 7 and reaches the discharge port 7a as shown by an arrow [C] in FIG. Is discharged. However, a part of this traveling wave [c] is reflected at the outlet 7a and flows backward in the propagation path up to that point, as shown by the arrow [d] in FIG. More specifically, the so-called backward wave [d] reflected at the outlet 7a enters the exhaust duct 2 via the joint 2a between the collecting duct 7 and the exhaust duct 2 as shown by an arrow [o] in FIG. Inflow. At that time, the backward wave [d] is attenuated to some extent at the coupling portion 2a as described above. The backward wave [e] after the attenuation is further attenuated by the passive silencer 8, and then returns to the position of the error microphone 6 as indicated by an arrow [f] in FIG. The backward wave [f] and the traveling wave (residual noise) [a] interfere with each other and cancel each other, so that the above-described dip occurs at the position of the error microphone 6.

However, as described above, the backward wave (residual noise) [f] causing the dip is attenuated twice by the passive silencer 8 and the coupling portion 2a for each of two round trips. , Returning to the position of the error microphone 6. Also, as described above, the outlet 7 of the collecting duct 7
In a, a part of the traveling wave [c] is reflected and becomes a backward wave [d], in other words, the backward wave [d].
Is a traveling wave [c] attenuated by reflection. Therefore, for example, the amount of attenuation by the passive silencer 8 is A [dB], and the amount of attenuation in the coupling portion 2a is B [dB].
Assuming that the amount of attenuation due to reflection at the outlet 7a is C [dB], the backward wave [A] is attenuated by at least 2 (A + B) + C [dB] as compared with the traveling wave [A]. Will be.

As described above, according to the present embodiment, it is possible to reduce the level of the backward wave [f] which causes a dip at the position of the error microphone 6. Therefore, the size (depth) of the dip generated by the backward wave [f] can be suppressed, and the adverse effect of the dip on the active silencing operation of the present embodiment can be reduced. The fact that the influence of the dip can be reduced as described above is the same in the case where the secondary sound path C is identified using the above-described random noise. Therefore, according to the present embodiment, a stable and reliable silencing operation can be realized and a sufficient silencing effect can be obtained as compared with the above-described conventional technology.

Depending on the structure of the passive silencer 8, the inside thereof (including the vicinity of the entrance and the exit of the sound) is included.
In some cases, an impedance sudden change portion where the acoustic impedance suddenly changes is formed. For example, as shown in FIG. 2, it is assumed that the impedance suddenly changing portion is formed at the end 8 d of the passive silencer 8 at the sound entrance side (left side in FIG. 2). In this case, when the residual noise passes through the passive silencer 8, a part of the residual noise is reflected by the impedance sudden change section 8d as shown by an arrow 8e in FIG. As a result, a dip occurs in the exhaust duct 2. Then, there is a concern that the dip may have an adverse effect on the silencing operation.

Therefore, when the impedance sudden change portion 8d is formed in the passive silencer 8 as described above, the passive silencer 8 is provided near the error microphone 6,
The impedance sudden change part 8d and the error microphone 6
Is shortened. By doing so, the frequency f at which the dip occurs at the position of the error microphone 6 is determined from the above-described relationship of Expression 1 by the frequency band in which the muffling device of the present embodiment is a target of muffling by active muffling operation. Can be shifted to a higher frequency side. Specifically, the distance L 'between the position of the error microphone 6 and the sudden impedance changing unit 8d is calculated by the following equation 2 based on the above equation 1.

[0041]

L ′ <V / (2f _H )

Here, f _H is the upper limit value of the frequency band to be silenced by the silencer of this embodiment by active silence operation.

Based on the equation (2), the distance L 'from the error microphone 6 to the sudden change portion 8a (strictly, a value obtained by correcting the pipe end of the exhaust duct 2 to this L')
Is set, the frequency f at which the dip occurs at the position of the error microphone 6 can be driven out of the frequency band to be silenced by the active silence operation.
The influence of the dip on the active silencing operation can be avoided.

Further, at the connecting portion 2a between the exhaust duct 2 and the collecting duct 7 where the acoustic impedance changes suddenly, as shown by an arrow 7c in FIG. Part of the wave [I]
Reflections are also conceivable. However, this traveling wave [a]
Is the sound that has already been attenuated by the passive silencer 8, and the backward wave reflected at the coupling portion 2 a and propagated in the opposite direction is also an error after being attenuated by the passive silencer 8. Return to the position of the microphone 6. Therefore, the influence of the dip due to this backward wave is extremely small to a negligible level.

In the present embodiment, only one passive silencer 8 is provided for the exhaust duct 2, but a plurality of passive silencers 8 may be provided in series. In this case, there is a concern about the influence of the reflection by each of the impedance sudden change parts 8d in each passive silencer 8, but the error microphone 6
A passive silencer 8 located closer to the propagation direction of the residual noise (traveling wave) may be provided near the error microphone 6. That is, the passive silencer 8 located on the downstream side (the sudden impedance changing unit 8d)
Is generated by the passive muffler 8 located upstream before returning to the position of the error microphone 6, similarly to the backward wave generated by the coupling portion 2a. This is because there is no need to worry so much about the effect of the dip that occurs.

Further, a passive silencer 8 is provided in the exhaust duct 2.
, But is not limited to this. For example, a passive silencer 8 may be provided in the middle of the collecting duct 7. However, considering the size and cost of the passive silencer 8 itself, it is more advantageous to provide the passive silencer 8 in the exhaust duct 2 than in the collecting duct 7.

In this embodiment, the collecting duct 7
Although the case where the present invention is applied to a plant having a configuration in which a plurality of exhaust ducts 2 are connected to each other has been described, the present invention is not limited to this. That is, when the error microphone 6 cannot be installed in the vicinity of a discharge port that is a final discharge port of the exhaust sound to the outside due to some limitation, the present invention exerts the above-described specific effects.

The present invention provides a plurality of exhaust ducts 2, 2,... As described above and these exhaust ducts 2, 2,.
.. Exerts further effectiveness when applied to a relatively large plant with a collecting duct 7 to which the. That is, according to the present invention, it is not necessary to take measures for avoiding the influence of the dip on the entire plant, and the exhaust sound of the exhaust duct 2, for example, the engine 1, which is concerned about the influence of the dip, can be actively reduced. Only the passive silencer 8 is provided only in the exhaust duct 2 that constitutes an active type silencer for silencing both the reflection at the outlet 2a of the exhaust duct 2 and the reflection at the outlet 7a of the collecting duct 7. The effect of the dip caused by this can be avoided. Therefore, even if the present invention is applied to a relatively large-scale plant as in the present embodiment, the entire plant does not become extremely large or costly.

The passive silencer 8 has the structure as shown in FIGS. 1 and 2, but is not limited to this. That is, the passive silencer 8 is not limited to the above configuration as long as it has a certain degree of silencing characteristics even in part or all of the frequency band to be silenced by the active silencing operation.

[0050]

As described above, the muffler of the present invention has a passive muffler between the position of the sound propagation path where the second microphone is provided and the outlet. Therefore,
A backward wave (residual noise) that causes a dip can be attenuated in the sound propagation path, particularly at the position of the second microphone, and the size (depth) of the dip can be suppressed. Therefore, depending on the use and installation conditions of the muffler of the present invention, the second microphone cannot be installed in the vicinity of the outlet of the sound propagation path that causes the generation of the backward wave, or the second microphone cannot be installed in the sound propagation path. Even when there are a plurality of parts that cause backward wave generation, etc., without being affected by the dip, a stable and reliable silencing operation can be realized,
It is possible to obtain a sufficient silencing effect. A major feature of the present invention is that this effect can be realized by an extremely simple configuration in which the passive silencer is provided.

[Brief description of the drawings]

FIGS. 1A and 1B are diagrams showing an embodiment of a muffler according to the present invention, wherein FIG. 1A is a diagram showing an overall schematic configuration, and FIG.
It is the figure which expanded a part of (a) and represented the propagation state of sound.

FIG. 2A is a diagram showing another sound propagation state.

FIG. 3 is a diagram showing a schematic configuration of a conventional silencer.

FIG. 4 is a diagram schematically showing a sound pressure distribution in an exhaust duct.

FIG. 5 is a diagram showing a problem in a conventional silencer.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine 2 Exhaust duct (sound propagation path) 3 Reference microphone (first microphone) 4 Control unit (silence control means) 5 Secondary sound source speaker (speaker means) 6 Error microphone (second microphone) 7 Collective duct (joint) Sound propagation path) 7a Outlet 8 Passive silencer

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-8-303224 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) F01N 1/00 F01N 1/04 F16L 55 / 02 G10K 11/16 G10K 11/178

Claims (6)

(57) [Claims] 1. A first microphone that is provided in a sound propagation path that propagates a first sound input from one end to another end and emits the first sound, and that collects the first sound, and the sound propagation path. A speaker that is disposed on the other end side of the position where the first microphone is provided and emits a second sound according to a muffling control signal into the sound propagation path to interfere with the first sound Means, a second microphone arranged on the other end side of the sound propagation path with respect to the position where the speaker means is provided, and a second microphone for collecting sound propagating in the sound propagation path at this arrangement position; The output signals of the first and second microphones are input, and the mute control signal is generated in response to the output signals so that the characteristics of the second sound are the characteristics necessary to cancel the first sound. And silence control means for supplying the sound to the speaker means. A silencing device provided between the position where the second microphone is provided in the sound propagation path and the other end, and at least a part or all of a frequency band to be silenced by the silencing device. A passive silencer having attenuation characteristics in a predetermined frequency band including: 2. The muffler according to claim 1, wherein a plurality of said passive mufflers are provided in series with said sound propagation path. 3. The passive silencer includes an impedance abruptly changing part in which the acoustic impedance suddenly changes, and the passive muffler is provided with the second microphone of the sound propagation path. The muffler according to claim 1, wherein the muffler is provided near the position. 4. A distance between a position of the sound propagation path where the second microphone is provided and the impedance sudden change part, a sound velocity in the sound propagation path between the two, and the sound frequency band to be silenced. 4. The noise reduction device according to claim 3, wherein the noise reduction device is smaller than a value obtained by dividing the upper limit value by approximately twice the upper limit value. 5. A state in which a plurality of sound propagation paths are respectively connected to a common common sound propagation path, and each sound propagating in each of the sound propagation paths is discharged outside through the common sound propagation path. The first and second microphones, the speaker means, the muffling control means, and the passive muffler are provided on at least one of the sound propagation paths, respectively. The sound deadening device according to claim 1. 6. A cross-sectional area transverse to a sound propagation direction in the sound propagation path provided with the first and second microphones, the speaker means, the muffling control means, and the passive muffler, and The noise reduction device according to claim 5, wherein a cross-sectional area of the sound propagation path crossing a propagation direction of the sound is different.