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DESCRIPTION JPH0772872

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DESCRIPTION JPH0772872
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
active noise reduction method.
[0002]
2. Description of the Related Art Explosive noise of an engine, rotational noise of a fan of a fan,
etc. are periodic noises having a repetitive waveform synchronized with the rotation. In order to
reduce the noise due to such periodic sound, active noise reduction technology for reducing
noise by utilizing the interference of sound waves has attracted attention. This is described, for
example, in Japanese Patent Application Laid-Open No. Hei 2-94900, and sends a noise signal
detected by a monitor microphone to a signal processing device, and calculates and creates a
signal that cancels a sound wave with the signal processing device. Based on this signal, a sound
wave is transmitted from the muffling speaker to interfere with the noise to reduce the noise.
This method is particularly effective when the sound wave from the sound source travels through
a tubular body such as a duct and is regarded as substantially a one-dimensional plane traveling
wave. JP-A-4-306477 discloses an example in which this active noise reduction technology is
applied to a cooling device.
[0003]
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1
SUMMARY OF THE INVENTION In the conventional active noise reduction system, one detection
microphone and one additional sound source speaker (sound reduction speaker) are usually
used. However, when the sound wave is considered to be a virtually one-dimensional plane
traveling wave, if the detection microphone or the additional sound source speaker strikes the
position of the node of the traveling wave of the frequency, it becomes unobservable or
uncontrollable. The sound reduction effect is reduced for that frequency. FIG. 6 is a diagram
showing this state, and shows an example in which the microphone 3A and the additional sound
source speaker 4A are positioned at the nodes of the plane traveling wave traveling in the duct.
[0004]
In such a case, no sound wave is detected at the position of the microphone 3A. Further, it is
impossible to cancel the plane traveling wave even if sound waves are emitted at the position of
the additional sound source speaker. Placing the microphone and the additional sound source
speaker at the outlet of the duct avoids this problem, but such a placement may not be possible
in a real plant. In addition, since the sound wave can not be regarded as a one-dimensional plane
traveling wave at the outlet of the duct, when the microphone and the additional sound source
speaker are disposed at the outlet of the duct, directivity may appear in the silencing effect. .
[0005]
The present invention has been made to solve the problems as described above, and it is an
object of the present invention to obtain a high-performance active noise reduction method that
does not reduce the sound reduction effect at a specific frequency.
[0006]
The above object is an active noise reduction method for detecting a sound wave in a onedimensional sound field with a sound wave detector and emitting a sound wave that cancels the
sound wave from a sound wave generator for muffling. (2n-1) / 4 (n is a natural number of the
wavelength of the highest frequency sound wave to be silenced using a plurality of at least one of
a sound wave detector and a sound wave generator, the sound wave detector and the sound wave
generator This is achieved by the method of being spaced apart in the traveling direction of the
sound wave by the distance corresponding to.
[0007]
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[Operation] By arranging a plurality of sound wave detectors and a sound wave generator as
described above, even if one sound wave detector or sound wave generator hits a node position
of a one-dimensional plane traveling wave at a frequency desired to be muffled, The other at
least one sound wave detector or sound wave generator will be in a non-node position.
FIG. 1 shows the principle of the present invention.
[0008]
As shown in FIG. 1, the first microphone (sound wave detector) 3A and the second microphone
3B, the first additional sound source speaker (sound wave generator) 4A and the second
additional sound source speaker 4B are ΔS (= 1 / 4λ: λ is set apart by the wavelength at the
highest frequency to be muted.
The sound wave detected by the first microphone 3A is input to a calculation device (not shown),
and the sound wave necessary to cancel this is calculated and output from the first speaker 4A.
[0009]
Similarly, the sound wave detected by the second microphone 3B is input to a calculation device
(not shown), the sound wave necessary to cancel this is calculated, and the sound wave is output
from the second speaker 4B. Since the first microphone 3A and the first additional sound source
speaker 3B are located at the nodes of the one-dimensional plane traveling wave, they are in the
unobservable, uncontrollable state, but the second microphone 3A and the second additional
sound source speaker Since 3B is 1⁄4λ away from this position, it will be located at the antinode
of the one-dimensional plane traveling wave, and these can be used for muffling.
[0010]
Although two microphones and two speakers are used in FIG. 1, for example, if it is certain that
the mounting position of the speakers will not be a node of a plane traveling wave, then only a
plurality of microphones are used. It can also be a method to use. In this case, sound waves
necessary for canceling the sound waves detected by the respective microphones may be
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respectively calculated and calculated by the calculation device, and the speaker may be driven
by an output combining these.
[0011]
Conversely, if it is certain that the mounting position of the microphone does not become a node
of the plane traveling wave, it is possible to use a plurality of loudspeakers alone. In this case, in
order to cancel the sound wave detected by the microphones, the sound waves to be transmitted
by the respective speakers may be respectively determined, and the respective speakers may be
driven by this.
[0012]
Next, as an embodiment of the present invention, an example in which the present invention is
used for muffling exhaust noise of a gas engine will be described. In this embodiment, two sound
wave detectors are used and one sound wave generator is used. The exhaust noise of a gas
engine has an engine speed as a fundamental frequency, and has a high sound pressure level in a
low frequency region of about 200 Hz or less mainly composed of integer multiples and 0.5th
order components. The rest spread to about 500 Hz while gradually reducing the sound pressure
level.
[0013]
The structure of the active silencer will be described with reference to FIGS. 2 and 3. When the
active silencer is installed in the middle of the duct 2, the additional sound source speaker 4 and
the microphone 3 must be protected from the heat and the components of the exhaust gas 8. The
temperature of the exhaust gas 8 of the gas engine 1 is about 300 ° C. or more along the
exhaust duct 2. Further, the component of the exhaust gas 8 contains water vapor. In order to
withstand such an environment and to provide a compact structure that can fit in the dead space
of piping, the structure is as shown in FIG.
[0014]
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That is, in order to prevent the high temperature exhaust gas containing water vapor from
flowing directly to the silencer side, the additional sound source speaker 4 and the microphone 3
are disposed via the cooling unit 6 in which the heat resistant film 7 is placed and many thin
pipes are embedded. It has a structure to be installed. As a silencer, two 40 cm cone speakers 5
and a condenser microphone are used, and no special speaker or microphone is used, and a
commercially available inexpensive one is used. Thus, the natural cooling alone realized a
silencer with a transparent structure that shuts off heat but acoustically. Since the engine exhaust
noise is a periodic noise, the muffling algorithm uses a synchronous waveform synthesis method.
Signal processing is performed by the logical expressions shown in Equations (1) and (2).
[0016]
In order to realize the equations (1) and (2), an active silencer as shown in FIG. 4 was
manufactured. That is, the A / D converter 11 externally samples the sound pressure signal from
the microphone 3 based on the pulse 13 synchronized with the engine rotational speed, and
transfers the direct memory access to the computer 10. The computer 10 divides the transferred
data into four blocks and performs Fourier transform for each block.
[0017]
Next, the vector calculation shown in equation (1) is performed on all the frequency components
to be silenced to obtain the optimum value, and then the inverse Fourier transform is performed.
The result is direct memory access transferred again to the D / A converter 12, and synthetic
sound is emitted from the additional sound source speaker 4. At this time, calculation of the
input from the A / D converter 11 and the next output signal by the computer 10 is
simultaneously executed. By repeating this series of cycles, input and output of data are
continuously processed.
[0018]
Explain the noise reduction effect of the active silencer. The following experiment was conducted
to investigate the noise reduction effect of the active silencer alone. The experiment target is a
gas engine with an output of 500 kW and a fundamental frequency of 16.7 Hz. During operation
of the engine, the active silencer was turned on and off, and the noise reduction effect was
measured at 1 m (C in FIG. 2) from the control point of the microphone 3A in FIG. 2 and the
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outlet of the duct 2. As a result, no sound reduction effect was obtained near 80 Hz and 180 Hz.
Then, the distance of the microphone 3 B is separated by 50 cm and taken into the A / D
converter 11. This is because the control point microphone 3A point is close to the node of the
sound pressure at both frequencies, and the input sound pressure level becomes small, and
sufficient control can not be performed. Therefore, when control was performed at two points of
the microphones 3A and 3B shown in FIG. 1, the noise was reduced at all frequency peaks, and
an effect of about 30 dB could be obtained at maximum.
[0019]
In order to investigate the directivity of the exhaust gas 8 from the outlet of the duct 2, the noise
reduction effects at the measurement points D (45 ° direction) and E (90 ° direction) in FIG. 2
were measured. As a matter of course, since noise is silenced inside the duct 2, the same noise
reduction effect is obtained in any direction, and no directivity for noise reduction appears.
[0020]
In order to confirm the durability of this active silencer, long-term tests were conducted on the
equipment currently in operation. The temporal change of the temperature inside the active
silencer is shown in FIG. The temperature saturates in about one hour, and when the temperature
of the exhaust gas 8 is 350 ° C., it is maintained at most 40 to 50 ° C. immediately before the
additional sound source speaker 4 and the microphones 3A and 3B and can sufficiently
withstand It was a temperature environment. Further, with regard to water vapor, there was no
moisture passing through the heat resistant film 7 and there was no particular problem. The
system ran smoothly for 1000 hours.
[0021]
As described above, in the present invention, a plurality of at least one of a sound wave detector
and a sound wave generator are used, and the sound wave detector and the sound wave
generator are of the highest frequency sound wave to be silenced. The sound wave is separated
by a distance corresponding to (2 n-1) / 4 (n is a natural number) of the wavelength in the
traveling direction of the sound wave, so one sound wave detector or sound wave generator
wants to mute the one-dimensional plane Even if the position of the node of the traveling wave is
hit, at least one other sound wave detector or sound wave generator is at a non-node position,
and the sound reduction effect does not decrease at a specific frequency.
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