close

Вход

Забыли?

вход по аккаунту

?

JP2001119787

код для вставкиСкачать
Patent Translate
Powered by EPO and Google
Notice
This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate,
complete, reliable or fit for specific purposes. Critical decisions, such as commercially relevant or
financial decisions, should not be based on machine-translation output.
DESCRIPTION JP2001119787
[0001]
The present invention is a so-called active ("active") method in which the sound to be muffled is
positively canceled by interfering the sound wave of substantially the same size and opposite
phase to the sound to be muffled. The present invention relates to an active type silencer.
[0002]
2. Description of the Related Art The above-mentioned active silencer is mainly used for
efficiently silencing the sound of a relatively low frequency band of, for example, 200 Hz or less.
A conventional example of this noise suppressor is shown in FIG. As shown in the figure, this
noise reduction device is intended for noise reduction of exhaust noise discharged from the
engine 1 to the outside through the inside of the exhaust duct 2, and an arbitrary position in the
exhaust duct 2, for example, the engine 1 A reference microphone 3 for picking up the exhaust
noise is provided at a position close to it. An output signal of the reference microphone 3, socalled noise signal x (t), is input to a control unit 4 having an adaptive digital filter constituted of,
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 is a secondary sound source speaker disposed
downstream of the position where the reference microphone 3 of the exhaust duct 2 is provided
(right side in the figure). (Hereafter, it is simply called a speaker. (5) generates a mute control
signal y (t) for emitting a sound wave substantially equal in magnitude and opposite to the
exhaust sound to the speaker 5. As a result, a sound wave substantially equal in magnitude and
opposite in phase to the exhaust sound is emitted from the speaker 5 into the exhaust duct 2 to
cancel the exhaust sound.
08-05-2019
1
[0003]
Furthermore, an error microphone 6 for collecting so-called residual noise after canceling the
exhaust sound by the radiation sound of the speaker 5 on the downstream side of the position
where the speaker 5 is provided in the exhaust duct 2, It is combined. 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 coefficients of the adaptive digital filter so as to minimize the signal level
of the error signal e (t), that is, the residual noise, ie, performs an adaptive operation.
[0004]
By the way, although the residual noise is finally discharged to the outside from the outlet 2a of
the exhaust duct 2, since the acoustic impedance is suddenly changed at the outlet 2a, a part of
the residual noise is indicated by the arrow 2b in the figure. As shown, the light is reflected at the
exhaust port 2a and propagates in the exhaust duct 2 in the opposite direction (the left side of
the figure, ie, the upstream side). The so-called reflected wave reflected by the discharge port 2a
and the so-called traveling wave propagating inside the exhaust duct 2 toward the discharge port
2a (right side in the figure, ie downstream side) normally interfere with each other. As a result,
dips (where the sound pressure is reduced) occur in the frequency characteristics in the exhaust
duct 2.
[0005]
For example, it is assumed that the above dip occurs at a position where the error microphone 6
is provided in the exhaust duct 2 for a sound of a certain frequency f. In this case, the sound
pressure at the frequency f becomes small at the place where the dip occurs, that is, the position
of the error microphone 6. Therefore, the error microphone 6 can not correctly pick up the
component of the frequency f among the residual noises to be picked up, which makes it
impossible to obtain a sufficient silencing effect at the frequency f.
[0006]
08-05-2019
2
The influence of this dip also extends to the case where, for example, a commonly known
Filtered-x LMS algorithm is used as a control system of the above-mentioned adaptive digital
filter in the control unit 4. That is, when the control system of the adaptive digital filter has the
Filtered-x LMS algorithm configuration, the output unit of the control unit 4 passes through the
speaker 5, a part of the exhaust duct 2 and the error microphone 6 and the input unit of the
control unit 4 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, random noise is
intentionally radiated from the speaker 5, this is picked up by the error microphone 6, and the
secondary sound path C is identified by analyzing the picked up result. There is a way to
However, even at the time of identification of the secondary channel C, the random noise and the
reflected wave due to the random noise being reflected at the outlet 2 a interfere with each other
and cancel each other, thereby the secondary sound A dip may occur in the frequency
characteristics of path C. Thus, if a dip occurs at the position of the error microphone 6 at a
certain frequency f, the error microphone 6 can not accurately identify the secondary sound path
C at that frequency f. As a result, the convergence of the muffling operation may be delayed at
the frequency f, or the control system may diverge and can not be muffled at all.
[0007]
Here, the distance between the position of the error microphone 6 in the exhaust duct 2 and the
discharge port 2a (strictly speaking, a value obtained by applying generally known tube end
correction to this distance. It is known that the above dip occurs at the position of the error
microphone 6 at a frequency f which is an integral multiple of V / (2L), where L) and the speed of
sound in the exhaust duct 2 between them is V. This relationship is expressed by the following
equation 1 when expressed by an equation.
[0009]
Also, in the equation 1, the image of the sound pressure distribution in the exhaust duct 2
(between the error microphone 6 and the outlet 2a) when the integer n is n = 1, 2 and 3 is shown
respectively. 5 (a), (b) and (c) are shown exaggerated.
[0010]
According to Equation 1 and FIG. 5, it can be understood that the frequency f at which the dip
occurs at the position of the error microphone 6 becomes higher as the distance L from the error
microphone 6 to the discharge port 2a becomes shorter.
08-05-2019
3
Therefore, it is desirable to provide the error microphone 6 as close as possible to the outlet 2a.
In this way, 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 targeted by the muffling apparatus
for muffling operation (secondary sound path C). The effect of the above dip on the identification
of
[0011]
However, there are cases where the error microphone 6 can not 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. 6, in a multi-storey building etc., exhaust ducts 2, 2,... Provided for
each floor A, B,. It is a case where it connects and it exhausts the exhaust sound of each exhaust
duct 2, 2, ... via this collection duct 7 outside.
[0012]
That is, according to the configuration of FIG. 6, the acoustic impedance also changes
significantly at the outlet 7 a portion of the collecting duct 7. Therefore, also at the outlet 7a of
the collecting duct 7, reflection of sound (residual noise) occurs as shown by the arrow 7b in the
same drawing. Then, this reflected wave and traveling waves propagating toward the
downstream side (the outlet 7a side) in the exhaust ducts 2, 2, ... and in the collecting duct 7
mutually interfere with each other, thereby The above dips occur in the exhaust ducts 2, 2,... And
in the collecting duct 7. In order to avoid the influence of the dip, for example, by the same
method as described above, it is necessary to install an error microphone 6 in the vicinity of the
outlet 7 a of the collecting duct 7. However, in a relatively large-scale facility (plant) as shown in
the figure, each floor A, B,... Where the exhaust ducts 2, 2,. In many cases, it is not possible to
install the error microphone 6 in the vicinity of the outlet 7 a of the collecting duct 7 because the
outlets 7 a are separated from each other. In such a case, the error microphones 6 must be
installed in the individual exhaust ducts 2 (the exhaust ducts 2 of the floor A in the same
drawing), and inevitably from the error microphones 6 to the outlet 7 a of the collective duct 7
Distance L becomes considerably long. Here, for example, if the error microphone 6 is installed in
the vicinity of the outlet 2 a of the exhaust duct 2, at least the influence of the dip caused by the
reflection at the outlet 2 a of the exhaust duct 2 can be avoided. However, the effect of dip
caused by the reflection at the outlet 7a of the collecting duct 7 can not be avoided. Specifically,
the dip at the position of the error microphone 6 in the frequency band targeted by the muffling
08-05-2019
4
apparatus It occurs, and a sufficient muffling effect can not be obtained.
[0013]
As described above, according to the above-mentioned prior art, when the error microphone 6
can not be installed in the vicinity of the outlet 2a (or 7a) causing the generation of the reflected
wave, the influence of the above dip is As a result, there is a problem that a sufficient silencing
effect can not be obtained. In this problem, as shown in FIG. 6, in a series of sound propagation
paths consisting of the exhaust duct 2 and the collecting duct 7, so-called reflection portions
causing the generation of reflected waves such as the above-mentioned discharge ports 2a and
7a It becomes remarkable when there exist two or more.
[0014]
Therefore, an object of the present invention is to provide a muffling apparatus capable of
avoiding the influence of the dip even when the error microphone 6 can not be installed in the
vicinity of the sound reflection parts such as the discharge ports 2a and 7a. It is also an object of
the present invention to realize such a noise reduction device with a relatively simple
configuration.
[0015]
[Means for Solving the Problems] In order to achieve the above object, the present invention is
provided in a sound propagation path for propagating and discharging a first sound to be muted
input from one end to the other end. And the first microphone for picking up the first sound and
the other end side, that is, the downstream side of the position where the first microphone of the
sound propagation path is provided, Speaker means for emitting a corresponding second sound
into the sound propagation path to cause interference with the first sound, and further disposed
on the other end side of the position of the sound propagation path provided with the speaker
means Output signals of the second microphone and the first and second microphones for
picking up the sound propagating in the sound propagation path at the arrangement position,
and the second microphone The characteristics of the sound that are necessary to cancel the first
sound and The noise reduction control means for generating the noise reduction control signal
and supplying the noise reduction control signal to the speaker means, and the other end side
than the position where the second microphone in the sound propagation path is provided, In the
vicinity of the position where the two microphones are provided, for example, the acoustic
impedance in the sound propagation path is suddenly changed (discontinuously made to the
08-05-2019
5
same extent as the degree of change of the acoustic impedance at the other end of the sound
propagation path) Impedance sudden change means to form an impedance sudden change
section.
[0016]
According to the present invention, the second sound necessary for canceling the first sound to
be muffled, for example, the first sound, according to the output signals of the first and second
microphones, the muffling control means Sound of substantially the same size and in opposite
phase is emitted from the speaker means (strictly speaking, a mute control signal for emitting
this second sound is generated and supplied to the speaker means).
Thereby, the first sound interferes with the second sound and is canceled. The so-called residual
noise remaining without being canceled by the second sound in the first sound propagates
toward the other end in the sound propagation path and is discharged to the outside from the
other end.
[0017]
In addition, an impedance rapid change part is formed by the impedance rapid change means
between the position where the 2nd microphone in the sound propagation path is provided, and
the other end, ie, the discharge port. In this impedance sudden change portion, the acoustic
impedance in the sound propagation path is suddenly changed, for example, to the same degree
as the degree of change in the acoustic impedance in the vicinity of the outlet of the sound
propagation path. Therefore, the above-mentioned residual noise that propagates in the sound
propagation path toward the outlet side is partially reflected by the rapid change in impedance
portion before reaching the exhaust port, and the sound propagation path in the opposite
direction (sound It propagates toward one end side of the propagation path, that is, the upstream
side. That is, the sudden change in impedance exerts an acoustic action substantially similar to
that of the outlet with respect to the residual noise, and functions as a so-called pseudo outlet.
Then, the reflected wave that is reflected at this impedance sudden change portion and the
traveling wave that propagates in the sound propagation path toward the outlet normally
interfere with each other and cancel each other out, whereby the frequency characteristics in the
sound propagation path Dip occurs.
08-05-2019
6
[0018]
However, the impedance sudden change portion is formed between the second microphone and
the outlet of the sound propagation path and in the vicinity of the downstream side of the second
microphone. Therefore, the frequency at which the dip occurs at the position of the second
microphone is higher than that in the case where no sudden impedance change portion is formed
(in the case where no rapid impedance change means is provided). Therefore, the impedance
sudden change portion and the second microphone are made as close as possible, and the
frequency at which the dip occurs at the position of the second microphone is the frequency
band that the muffling apparatus of the present invention targets muffling (strictly, By setting the
frequency band higher than the upper limit value of this frequency band, it is possible to avoid
the influence of the above dip on the muffling operation.
[0019]
Among the residual noise, a component that propagates toward the outlet side in the sound
propagation path as it is without being reflected at the impedance abrupt change part is also
partially reflected at the outlet of the sound propagation path. And the said dip generate | occur |
produces in the sound propagation path also by this reflected wave. However, since the sound
pressure or energy of the reflected wave reflected at the outlet of this sound propagation path is
smaller than the sound pressure of the reflected wave reflected at the sudden change in
impedance section, the reflected wave of the outlet of this sound propagation path The dip
generated is much smaller than the dip generated by the reflected wave of the impedance sudden
change portion. Therefore, the influence of the dip generated by the reflected wave at the outlet
of the sound propagation path on the muffling operation of the muffling apparatus of the present
invention is much smaller than that of the above-mentioned prior art.
[0020]
The fact that the influence of the dip can be avoided in this way is the same as in the case of
identifying the secondary sound path C using the above-mentioned random noise.
[0021]
The above-mentioned impedance sudden change means in the present invention is, for example,
a cross section across the sound propagation path in the direction from one end of the sound
propagation path to the outlet, that is, the sound propagation direction in the sound propagation
path. It can be configured by forming a space in which the cross-sectional area is expanded along
08-05-2019
7
a predetermined direction along the above-mentioned direction while enlarging the area.
[0022]
That is, as described above, the cross-sectional area in the sound propagation path which crosses
the above direction is rapidly expanded, and the space where the cross sectional area is
expanded is formed in a predetermined section along the above direction. A state (environment)
substantially equivalent to the exit can be configured.
According to this configuration, in the portion where the cross-sectional area in the sound
propagation path is rapidly expanded, or in the slightly inward portion of the space where the
cross-sectional area is expanded relative to this portion It is formed.
As such an arrangement, there is, for example, a commonly known inflatable muffler.
[0023]
Then, it may be determined based on the relation of the above-mentioned equation 1 which
position in the sound propagation path this impedance sudden change section is to be formed.
That is, the distance between the position where the second microphone in the sound
propagation path is provided and the above-mentioned impedance sudden change portion is set
to be twice the sound speed in the sound propagation path between these two times the upper
limit value of the noise reduction target frequency. A value obtained by dividing by a value
(strictly speaking, a value obtained by applying the above-mentioned tube end correction to this
value. Make smaller than).
[0024]
In this way, the frequency at which the dip occurs at the position of the second microphone is
shifted to a higher frequency side than the frequency band targeted by the silencer of the present
invention for muffling, and driven out of the muffling frequency band. be able to.
[0025]
08-05-2019
8
Furthermore, as in the configuration of FIG. 6 described above, a plurality of sound propagation
paths are respectively connected to a common sound propagation path common to each sound
propagating through the respective sound propagation paths via the common sound propagation
path. The present invention may be applied to a plant configured to discharge from the open end
of the joint sound propagation path to the outside.
In this case, the first and second microphones may be connected to a sound propagation path
coupled to a noise source that emits particularly loud noise, such as the above-described engine
1 or the like, of at least one of the sound propagation paths. , The speaker means, the noise
reduction control means, and the impedance rapid change means, and only the sound
propagation path is configured as an active noise reduction device.
[0026]
That is, in a plant in which a plurality of sound propagation paths are connected to a common
joint sound propagation path as described above, at the open end of the joint sound propagation
path that is the final sound outlet from the entire plant. Also, the reflection of sound occurs.
Then, the reflected wave due to the reflection and the traveling wave propagating toward the
downstream side in each sound propagation path and in the joint sound propagation path
interfere with each other, and thereby, in each sound propagation path and the joint sound
propagation The above dip occurs in the path. In order to avoid the effect of this dip, it is
desirable to place a second microphone near the open end of the joint sound propagation path.
However, this is often not possible due to various causes such as the use of the silencer and the
installation conditions. Therefore, in the present invention, measures are taken to provide the
above-mentioned rapid impedance change means only for the sound propagation path that
constitutes the above-mentioned active noise reduction device and is concerned about the
influence of dip. In this way, it is possible to avoid not only the dip generated by the reflection of
the outlet of the sound transmission path, but also the influence of the dip generated by the
reflection of the opening end of the common sound transmission path.
[0027]
Thus, in the present invention, only by taking the above measures only for the sound propagation
path where the influence of the dip is concerned, both the reflection at the outlet of the sound
propagation path and the reflection at the opening end of the joint sound propagation path The
08-05-2019
9
effect of the resulting dip can be avoided. Therefore, even if the present invention is applied to a
relatively large-scale plant having a plurality of sound propagation paths and a joint sound
propagation path to which each sound propagation path is coupled as described above, the entire
plant is extremely extreme. There is no increase in size or cost.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a noise suppressor
according to the present invention will be described with reference to FIGS. 1 and 2. FIG.
[0029]
FIG. 1 is a view showing a schematic configuration of the present embodiment, and is a view
extracting and showing a portion corresponding to the floor A in FIG. 6 described above.
As shown in the figure, the noise reduction device of this embodiment is between the position
where the error microphone 6 of the exhaust duct 2 is provided and the outlet 2a of the exhaust
duct 2 in the configuration of FIG. An expansion type muffler 8 generally known is provided at a
position close to the error microphone 6. The configuration other than this is the same as that of
FIG. 6 described above, so the same reference numerals are given to the equivalent parts and the
detailed description thereof will be omitted.
[0030]
In the expansion muffler 8, the above direction in the exhaust duct 2 (propagation of the exhaust
noise over a predetermined section M in the direction from the engine 1 side of the exhaust duct
2 toward the exhaust port 2 a side, in other words, the propagation direction of exhaust noise)
The cross-sectional area S1 across the direction). For example, assuming that the exhaust duct 2
is a round pipe (so-called circular duct), the expansion muffler 8 has, for example, a hollow
cylinder having a diameter larger than that of the exhaust duct 2 and a length dimension of the
section M; The two ends of the cylinder are closed, and a cross section of the exhaust duct 2
which crosses the above direction (hereinafter, the cross section is simply expressed as a cross
section with the same interpretation unless otherwise described. And 2) hollow disks provided
with circular through holes having substantially the same shape and size as the above.
08-05-2019
10
[0031]
By providing such an expansion type muffler 8, in the middle of the exhaust duct 2, a portion
where the cross-sectional area S1 becomes discontinuous is formed. Then, the acoustic
impedance in the exhaust duct 2 suddenly changes (discontinuities) at discontinuous portions of
the cross-sectional area S1, that is, both ends of the expansion muffler 8 (each end of the inlet
and outlet sides of the exhaust noise). An abrupt change in impedance is formed.
[0032]
Therefore, the cross-sectional area S2 of the expansion muffler 8 is made sufficiently larger than
the cross-sectional area S1 of the exhaust duct 2. In this way, the sudden change in impedance, in
particular, the inlet end 8a of the expansion muffler 8 against exhaust noise (strictly speaking,
residual noise described above) propagating in the exhaust duct 2, It produces an acoustic action
substantially the same as that of the outlet 7a, and functions as a pseudo outlet.
[0033]
That is, after the exhaust sound propagating in the exhaust duct 2 is canceled by the radiation
sound of the speaker 5, the expansion type muffler 8 and the remaining exhaust duct 2
(downstream from the expansion type muffler 8) as the residual noise. , And through the
collecting duct 7, it is discharged to the outside from the outlet 7 a of the collecting duct 7.
However, at the inlet end 8a of the expansion muffler 8 where the acoustic impedance suddenly
changes first in this propagation path, as indicated by an arrow 2c in the figure, a part of the
residual noise is reflected, and an exhaust duct is produced. 2. Propagating in 2 in the opposite
direction (ie upstream). Then, the reflected wave and the traveling wave that normally
propagates in the exhaust duct 2 toward the collecting duct 7 (that is, downstream) mutually
interfere and cancel each other, whereby the frequency characteristic in the exhaust duct 2 is
reduced. The dip described above occurs.
[0034]
Therefore, if the expansion type muffler 8 is provided closer to the error microphone 6 as
described above and the entrance end 8a thereof is brought close to the error microphone 6, the
08-05-2019
11
above-mentioned relation of the number 1 leads to the above position of the error microphone 6
The frequency f at which the dip occurs can be shifted to a higher frequency side than the
frequency band targeted by the silencer of the present embodiment for muffling. The distance L
'between the position of the error microphone 6 and the inlet end 8a of the expansion muffler 8
for realizing this is, for example, according to the following equation 2 based on the equation 1.
[0036]
However, f H is the upper limit value of the frequency band that the muffling apparatus
according to the present embodiment is targeted for muffling.
[0037]
If the distance L 'from the error microphone 6 to the inlet end 8a of the expansion muffler 8
(strictly, the value obtained by applying the tube end correction to this L') is set based on the
equation 2, the error microphone The frequency f at which the dip occurs at the position 6 can
be driven out of the noise reduction target frequency band.
Thereby, the influence of the above-mentioned dip on the muffling operation of the muffling
apparatus of the present embodiment can be avoided. As described above, according to the
present embodiment, even when the error microphone 6 can not be installed in the vicinity of the
outlet 7a of the collecting duct 7, a stable and reliable muffling operation can be realized without
being affected by the dip. As a result, a sufficient silencing effect can be obtained. The same
applies to the case of identifying the secondary channel C by using the above-described random
noise.
[0038]
The residual noise is also reflected at the discharge port 7a of the collecting duct 7 as in the
above-described prior art. Then, a dip is generated in the exhaust duct 2 also by the reflected
wave reflected by the discharge port 7a. However, since the sound pressure or energy of the
reflected wave reflected at the outlet 7 a of the collecting duct 7 is smaller than the sound
pressure of the reflected wave reflected at the inlet end 8 a of the expansion muffler 8, the
collecting duct 7 is The dip generated in the exhaust duct 2 by the reflected wave of the
discharge port 7a is much smaller than the dip generated by the reflected wave at the inlet end
8a of the expansion muffler 8. Therefore, the influence of the dip generated by the reflected wave
08-05-2019
12
of the discharge port 7a of the collecting duct 7 on the muffling operation of the muffling
apparatus of the present embodiment is much smaller, for example, negligible.
[0039]
As mentioned above, although the structure which couple | bonds the exhaust duct 2 with the
accumulation | aggregation duct 7 was demonstrated in this Embodiment, it does not restrict to
this. That is, in the case where the error microphone 6 can not be installed in the vicinity of the
outlet, which is the final outlet for the exhaust noise to the outside, due to some conditions or the
like, the present invention exhibits the above-mentioned unique effect.
[0040]
However, the present invention is relatively large, as described above, including a plurality of
exhaust ducts 2, 2,... And a collective duct 7 to which these exhaust ducts 2, 2,. Demonstrate
further effectiveness in large-scale plants. That is, according to the present invention, it is not
necessary to take measures to avoid the influence of the dip on the whole plant, and the exhaust
noise of the exhaust duct 2 which is concerned about the influence of the dip, for example, the
engine 1, can be active. Both the reflection at the exhaust port 2a of the exhaust duct 2 and the
reflection at the exhaust port 7a of the collecting duct 7 only by providing the expansion muffler
8 only in the exhaust duct 2 constituting the active type silencer for muffling The effects of dips
caused by 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-sized or expensive.
[0041]
Further, in the present embodiment, the exhaust duct 2 is a round pipe, but may be another
shape such as a square pipe. And although this embodiment explained the case where exhaust
noise of engine 1 was made into the object of muffling, the present invention can be applied also
to the device which makes noise other than exhaust sound into the object of muffling.
[0042]
08-05-2019
13
Furthermore, although the expansion muffler 8 is used to rapidly change the acoustic impedance
in the exhaust duct 2, the present invention is not limited to this. That is, if the acoustic
impedance in the exhaust duct 2 can be suddenly changed to the same degree as the degree of
change of the acoustic impedance in the vicinity of the outlet 7a of the collecting duct 7, for
example, 2. A substantially conical hollow body as shown in 2 (specifically, a hollow body in
which through holes having substantially the same shape and dimensions as the cross section of
the exhaust duct 2 are provided in each portion corresponding to the apex and the bottom of the
cone) 9 may be provided. In this case, it goes without saying that using the bottom side 9a of the
hollow body 9 as the inlet side of the exhaust noise is advantageous in shortening the distance L
'. Of course, the one having the same function as the above-mentioned expansion type muffler 8
may be realized by one having a structure other than this. However, depending on the structure,
instead of the inlet end portions 8a and 9a of the mufflers (spaces whose cross-sectional area is
expanded to S2) 8 and 9, the inner portions of the mufflers 8 and 9 are more than the inlet end
portions 8a and 9a. , An impedance sudden change part may be formed. Even in such a case,
there is no problem as long as the distance L 'between the impedance sudden change portion and
the error microphone 6 can be made close enough not to be affected by the dip. On the contrary,
it may be more convenient to form the rapid change in impedance portion in the mufflers 8 and
9 as described above in terms of downsizing the entire device (shortening the exhaust duct 2).
[0043]
EXAMPLE In the configuration shown in FIG. 1 below, the cross-sectional area ratio P (= S2 / S1)
of the cross-sectional area S2 of the expansion type muffler 8 and the cross-sectional area S1 of
the exhaust duct 2 is set to It will be specifically described whether the inlet side end 8a of the
shape muffler 8 can function as a pseudo outlet. That is, the sound pressure reflectance at the
inlet end 8a of the expansion muffler 8 is, for example, as shown in FIG. 4 in the state where the
outlet 2a of the exhaust duct 2 is exposed to the outside (so-called free space) The cross-sectional
area ratio P which is equal to the reflectance at 2a (or the reflectance at the outlet 7a of the
collecting duct 7) is determined. Here, it is assumed that sound waves such as exhaust noise
propagating in the exhaust duct 2 are plane waves. In addition, the exhaust duct 2 is a round
pipe, that is, a circular duct, and the frequency band to be silenced by the active silencer (the
radiated sound of the speaker 5) is 200 Hz or less (f ≦ 200 Hz).
[0044]
First, when the sound wave of frequency f = 200 Hz or less propagates in the circular duct, the
radius r of the circular duct can be regarded as a plane wave having a wave front parallel to the
08-05-2019
14
cross section of the duct (sound wave of so-called one-dimensional mode). (Strictly, the maximum
value of this radius r is determined). The relationship between the radius r of the circular duct
and the upper limit value of the frequency f at which the sound wave propagating in the circular
duct can be regarded as a plane wave is expressed by the following formula 3.
[0046]
Therefore, the radius r of a circular duct which can be regarded as a plane wave can be
determined from the following equation 4 obtained by modifying the equation 3. However, in
consideration of the margin here, the above-mentioned frequency f (upper limit value) is set to f
= 250 Hz, that is, a sound wave having a frequency of 200 Hz or less can be considered to be a
plane wave with certainty. Also, although the speed of sound V changes depending on the
temperature T in the circular duct, here, the situation where the temperature T is T = 300 ° C. is
considered.
[0048]
From the equation (4), if the radius r in the circular duct is r = 0.56 m (56 cm) or less, the sound
wave with the frequency f = 200 Hz or less can be regarded as a plane wave with certainty.
[0049]
By the way, when the outlet (open end) of the circular duct is exposed to free space, the sound
pressure reflectance | R | at the outlet is the propagation constant k of the sound wave in free
space and the radius r of the circular duct Product (hereinafter, this is called the kr product).
It is known that the relationship is as shown by solid line X in Fig. 3 (Levine H, Schwinger J, "On
the radiation from an unflanged circular pipe, Phys. Rev., vol. 73, no. 4, pp. 383-406 (1948), see
fig. 1). Here, the propagation constant k is a constant depending on the wavelength λ of the
sound wave, and is expressed by the following equation 5.
[0051]
Then, the relationship of the above graph X can be expressed by the following approximate
expression (Davies POAL, Bento Coelho JL, Bhattcecharya M, << reflection coefficients for an
unflanged pipe with flow, J. Sound Vib, Vib ., vol. 72, no. 4, pp. 543-546 (1980) >> Refer to
08-05-2019
15
equation 3).
[0053]
However, when the reflectance | R | for the above-mentioned kr product is actually calculated
based on the equation (6) and the calculation result is represented by a graph, it becomes as
shown by a dotted line Y in FIG.
As apparent from the difference between the graph Y indicated by the dotted line and the graph
X indicated by the solid line, the approximate expression of the equation 6 is that the kr product
is approximately kr = 1.5 or less (kr ≦ 1.5). It is effective only within the range.
[0054]
Here, the above-mentioned kr product is obtained by multiplying the above equation 5 by the
radius r of the circular duct. Then, the values of f = 200 Hz and V = 480 m / s are substituted as
the frequency f and the sound velocity V in Equation 5, respectively, and r = 0.5 m (≦ 0.56 m) is
substituted as the radius r. Whether or not the value of the kr product obtained by substitution
satisfies the above condition of kr = 1.5 or less is verified by the following equation (7).
[0056]
According to the equation (7), the condition of kr ≒ 1.3 and the condition of kr = 1.5 or less is
satisfied. Therefore, the approximate expression of the above equation 6 can be applied. Then,
substituting the value of kr = 1. 3 into the above equation 6 to obtain the reflectance | R |, the
value of | R | ≒ 0.57 ≒ 0.6 is obtained. This is because, under the environment of temperature T
= 300 ° C., sound waves of frequency f = 200 Hz or less propagate in the circular duct of radius
r = 0.5 m or less and are discharged from the discharge port into free space, At the outlet of this
circular duct, it means that the reflectance of | R | = 0.6 acts on the sound wave.
[0057]
The cross-sectional area ratio P (= S 2 / S 1) for obtaining the reflectance | R | equivalent to this
at the entrance end 8a of the expansion type muffler 8 is determined by the following equation 8.
08-05-2019
16
[0059]
That is, when | R | = 0.6 is substituted in Eq. 8, a value of P = 4 or P = 1/4 is obtained as the
cross-sectional area ratio P.
That is, the exhaust duct 2 has a structure that can be regarded as a plane wave sufficiently for
sound waves of frequency f = 200 Hz or less (a circular duct of radius r = 0.5 m or less), and the
temperature T in the exhaust duct 2 is When T = 300 ° C., if the cross-sectional area ratio P is
set to P = 4 or more, at the inlet end 8a of the expansion muffler 8, the outlet 2a of the exhaust
duct 2 exposed to the free space is substantially the same. An equal or higher reflectance | R |
can be obtained.
[0060]
Incidentally, in order to satisfy the above theory, the sound wave propagating there must be
regarded as a plane wave even in the expansion type muffler 8 having a cross sectional area S2
which is four times the cross sectional area S1 in the exhaust duct 2 . Therefore, strictly
speaking, not only the exhaust duct 2 but also the expanded muffler 8 needs to have its radius
equal to or less than the maximum value of the radius r (i.e., r = 0.56). Therefore, the radius r of
the exhaust duct 2 becomes smaller than the above description, and inevitably, the kr product
also becomes smaller. Thus, when the kr product decreases, the reflectance | R | at the outlet of
the exhaust duct 2 (circular duct) also increases from the relationship shown in FIG. 3, and as a
result, the cross-sectional area ratio P increases. For example, when it is necessary to achieve | R
| = 0.8 as the above-mentioned reflectance | R |, the cross-sectional area ratio P is larger than P =
4, and P = 9 (3 in terms of radius ratio). There must be.
[0061]
As described above, it should be noted that the cross-sectional area ratio P for making the inlet
end 8a of the expansion muffler 8 be regarded as a pseudo exhaust port depends on the radius r
of the exhaust duct 2. Further, the cross-sectional area ratio P is changed not only by the radius r
but also by the frequency f of the sound wave which can be regarded as a plane wave, and the
08-05-2019
17
temperature T (i.e., the sound velocity V) in the exhaust duct 2.
[0062]
However, the above description is based on the premise that the inlet end 8a of the expansion
muffler 8 exhibits the reflectance | R | equivalent to the outlet 2a of the exhaust duct 2 exposed
to the free space until it gets tired. However, it is not necessarily the case that the specific effects
of the present embodiment can not be obtained unless this premise is satisfied. That is, the
reflectance | R | equivalent to the outlet 2a of the exhaust duct 2 exposed to the free space can
not be obtained at the inlet end 8a of the expansion muffler 8; for example, the cross-sectional
area ratio P is P = 3 Even if it is about five, the unique effect by this embodiment can fully be
expected.
[0063]
As described above, in the noise reduction device of the present invention, the rapid change in
impedance portion is formed in the vicinity of the outlet side of the position where the second
microphone in the sound propagation path is provided. Therefore, the frequency at which the dip
occurs at the position of the second microphone can be shifted to the high frequency side as
compared with the case where this impedance sudden change portion is not formed, and hence
the silencer of the present invention Can be driven out of the frequency band being Therefore, if
the second microphone can not be installed in the vicinity of the outlet of the sound propagation
path due to the use of the silencer or the installation conditions, or if the dip (reflected wave)
such as the outlet is generated in the sound propagation path. Even when there are a plurality of
sound reflection parts, a stable and reliable muffling operation can be realized without being
affected by the above-mentioned dip, and a sufficient muffling effect can be obtained. Also, it is a
major feature of the present invention that this effect can be realized by an extremely simple
configuration in which means for rapidly changing the impedance such as an expansion type
muffler is provided.
08-05-2019
18
Документ
Категория
Без категории
Просмотров
0
Размер файла
34 Кб
Теги
jp2001119787
1/--страниц
Пожаловаться на содержимое документа