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JPH07334170

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DESCRIPTION JPH07334170
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
speaker device of an active noise control system which emits sound waves of the same phase as
the noise in the propagation path of noise in a duct to mute noise.
[0002]
2. Description of the Related Art Conventionally, passive muffling methods such as a method of
absorbing sound by a sound absorbing material internally attached to a duct have been taken as
one method of muffling noise propagating in the duct. And other problems.
[0003]
On the other hand, active noise control systems have been actively researched in which sound
waves of the same sound pressure and antiphase with respect to sound waves of noise
propagating in the duct are simultaneously radiated into the duct and muffling is performed by
interference of both sound waves.
However, many problems still remain.
[0004]
03-05-2019
1
FIG. 4 is a speaker device of a conventional active noise control system. That is, the muffling
speaker 32 is provided at the wall surface opening 33 of the duct 31 so as to emit muffling
sound waves into the duct 31. Sound waves in a low frequency region to be silenced are emitted
into the duct almost in a plane wave form.
[0005]
When the muffling speaker 32 for reproducing the muffling signal is installed alone on the wall
surface of the duct 31 as in the prior art, the wall surface of the duct 31 for installing the
muffling speaker 32 and the wall surface of the opposing duct 31 are parallel. The sound wave
34 for muffling emitted from the muffling speaker 32 repeats reflection between the wall
surfaces of the opposing duct 31. Therefore, a standing wave is generated due to the acoustic
interference between the wall surfaces of the opposing ducts 31, and the acoustic waves of a
certain frequency determined by the size of the duct 31 are large at certain places in the duct 31
and small at certain places.
[0006]
Especially when the duct size is large, the frequency at which standing waves occur is in the low
frequency region. Therefore, the frequency which produces the standing wave exists in the
frequency domain which is the target of muffling. Therefore, the frequency characteristic of the
sound field reproduced in the duct 31 by the muffling speaker 32 does not become the same as
the frequency characteristic of the muffling signal, and high-accuracy noise control can not be
performed.
[0007]
Further, in order to obtain a high output sound pressure level in the low frequency range of the
sound wave for muffling, the distance for performing the piston movement of the diaphragm of
the muffling speaker becomes long, and a large vibration is accompanied as a whole. The duct is
also accompanied by a large vibration due to the vibration of the device. Due to the influence of
these vibrations, the muffling speaker becomes unstable and a sound field faithful to the muffling
signal can not be generated in the duct.
03-05-2019
2
[0008]
SUMMARY OF THE INVENTION The present invention has been made in view of the abovementioned circumstances, and by avoiding the sound wave interference generated between the
opposing duct wall surfaces and obtaining a sound wave for muffling having a sufficient sound
power. An object of the present invention is to provide a speaker device of an active noise control
system for improving a noise reduction effect.
[0009]
In order to achieve the above object, the speaker device of the active noise control system
according to the present invention comprises simultaneously ducting sound waves of the same
sound pressure and antiphase from the muffling speaker with respect to sound waves of noise
propagating in the duct. In a speaker device of an active noise control system that mutes noise
propagating in a duct by emitting both inward and causing both sound waves to interfere with
each other, a plurality of loudspeaker devices provided opposite to the duct wall and radiating
the same silencing sound wave into the duct And a speaker for muffling, which are physically
connected by a rigid material.
[0010]
According to the present invention, by using a plurality of muffling speakers as means for
generating muffling sound waves according to the above-mentioned means, it is possible to
obtain sufficient acoustic power for muffling noise propagating in the duct, and further facing
each other. By installing on the duct wall surface, it is possible to prevent the interference of the
sound waves generated between the opposing duct wall surfaces, to form a sound field faithful to
the muffling signal in the duct, and to make the plural opposing muffling speakers rigid. By
physically combining with a certain material, the large vibration of the speaker itself which
occurs when the muffling speaker generates a sound wave in the low frequency region is offset
to generate the same sound wave at the same time to synchronize a plurality of speakers As a
result, it is possible to form a sound field in the duct that is faithful to the muting signal.
Moreover, since the vibration of the muffling speaker can be prevented from being transmitted to
the duct wall, unnecessary noise from the duct wall can be avoided.
[0011]
03-05-2019
3
Embodiments of the present invention will now be described in detail with reference to the
drawings.
FIG. 1 is a configuration explanatory view showing an example of an active noise control system
according to the present invention. That is, the first microphone (first sensor) 2 detects noise
propagating in the duct 1 and converts an acoustic signal into an analog electrical signal. The
analog electric signal obtained by the first microphone 2 is amplified by the microphone
amplifier 5 and then passes through the analog frequency cut-off filter 8 by analog signal
processing of a moderate frequency cut-off characteristic with a short time delay once to be a
target of muffling It becomes an analog electrical signal in the low frequency range. The analog
electrical signal passing through the analog frequency cutoff filter 8 is converted into a digital
electrical signal by the analog-digital converter 11, and then the digital frequency cutoff filter 14
by digital signal processing having steep frequency cutoff characteristics with a short time delay.
To become a digital electric signal in a low frequency region to be silenced. By the above
processing, the analog electrical signal based on the noise information from the first microphone
2 becomes a digital electrical signal including only the signal of the low frequency region to be
silenced.
[0012]
On the other hand, the second microphone (second sensor) 4 detects how muffled the noise in
the duct 1 is muffled by operating the system, detects the muffled state in the duct 1, and
converts the acoustic signal into an analog electric signal Do. After the analog electric signal
obtained by the second microphone 4 is amplified by the microphone amplifier 7, it passes
through the analog frequency cut-off filter 10 by analog signal processing of a moderate
frequency cut-off characteristic with a short time delay once to be a target of muffling It becomes
an analog electrical signal in the low frequency range. The analog electrical signal passing
through the analog frequency cutoff filter 10 is converted to a digital electrical signal by the
analog-to-digital converter 13, and then the digital frequency cutoff filter 16 by digital signal
processing having steep frequency cutoff characteristics with a short time delay. To become a
digital electric signal in a low frequency region to be silenced. As a result of the above
processing, the analog electrical signal according to the silencing status information from the
second microphone 4 becomes a digital electrical signal including only the signal of the low
frequency region to be silenced.
03-05-2019
4
[0013]
In the operation unit 19, a digital electric signal including only a low frequency region signal to
be muffled by noise information from the first microphone 2 and a low frequency region to be
muffled by muffling status information from the second microphone 4 The muffling speaker 3 is
driven so that the sound wave for muffling optimum at that time can be emitted to the sound
wave of noise in the duct 1 at the place where the muffling speaker 3 is installed from the digital
electric signal containing only the signal. The following operation is performed to create a noise
reduction signal.
[0014]
(A) In the active noise control system, when a sound wave propagates in the duct 1 or a signal
passes through the circuit, it is affected by its own transfer function such as time delay or change
in frequency characteristics.
An operation is performed to correct the transfer function.
[0015]
(B) From the information of the first microphone 2 and the second microphone 4, a coefficient is
calculated based on the adaptive control algorithm 18 so that the input signal from the second
microphone 4 becomes zero (that is, noise cancellation is performed). The coefficient is used as a
filter coefficient of the muffling signal generation filter 17 to convolve the digital electric signal
from the first microphone 2 to create a muffling digital electric signal.
[0016]
The noise reduction digital electrical signal generated by the operation unit 19 passes through
the digital frequency cutoff filter 15 by digital signal processing with steep frequency cutoff
characteristics with a short time delay to be targeted for noise reduction in the low frequency
domain It becomes an electrical signal. The muffling digital electrical signal that has passed
through the digital frequency cutoff filter 15 is converted into a muffling analog electrical signal
by the digital-to-analog converter 12. The analog electric signal for muffling from the digitalanalog converter 12 passes through the analog frequency cut-off filter 9 by the analog signal
03-05-2019
5
processing of the gentle frequency cut-off characteristic with a short time delay and only the
signal of the low frequency region to be muffled It becomes an included analog electrical signal
for noise reduction. The muffling analog electric signal that has passed through the analog
frequency cutoff filter 9 is amplified by the power amplifier 6 and then drives the two muffling
speakers 301 and 302. Two muffling speakers 301 and 302 are provided opposite to the wall
surface of the duct 1, convert the muffling analog electric signal from the power amplifier 6 into
an acoustic signal to generate the same muffling sound wave, and Radiate to
[0017]
As described above, the same sound pressure sound waves of the same sound pressure and
antiphase are simultaneously radiated into the duct 1 from the sound generation speakers 301
and 302 with respect to the sound waves in the duct 1 to generate noise sound waves and sound
sound waves. The noise propagating in the duct 1 is silenced by causing the interference.
[0018]
The noise propagating in the duct is a very loud sound, and a single muting speaker can not
obtain sufficient acoustic power to cancel it.
Therefore, if two muffling speakers 301 and 302 are used as in this embodiment, it can be
overcome to obtain sufficient sound power.
[0019]
However, there are also points to be considered when using two muffling speakers 301 and 302.
Although the muffling speakers 301 and 302 must emit sound waves of opposite phase to the
phase of the propagation noise in the duct 1, when performing highly accurate noise control,
phase errors are hardly tolerated. However, when two muffling speakers 301 and 302 are used,
depending on the situation where the muffling speakers 301 and 302 are placed, the same sound
wave may not be generated at all, and an error may occur. In particular, since the muffling
speakers 301 and 302 vibrate the diaphragm to emit sound waves, the fixing situation of the
portion holding the diaphragm largely affects the phase. When two muffling speakers 301 and
302 are used, the same sound wave can not be emitted unless the condition of the place where
the speaker unit is held is the same.
03-05-2019
6
[0020]
FIG. 2 is an explanatory view showing the influence of vibration given to the duct 1 by the
muffling speakers 301 and 302. When sound waves in the low frequency range are reproduced
with great power by the muffling speakers 301 and 302, the muffling speakers 301 and 302
cause large vibrations, which are transmitted to the wall of the duct 1 and unnecessary noise is
generated from the wall of the duct 1. May be generated.
[0021]
Therefore, as shown in FIG. 3, two muffling speakers 301 and 302 are physically connected. That
is, the portions of the magnets of the muffling speakers 301 and 302 are coupled using the rigid
metal member 20. 211,212はスピーカボックスである。 Therefore, since the two
muffling speakers 301 and 302 vibrate the diaphragm under the same conditions, the same
sound waves can be emitted into the duct 1 at the same time.
[0022]
Incidentally, the enclosures of the two muffling speakers or the speaker unit itself may be
physically connected with a rigid plate or rod material so that the same sound waves are
generated from the two muffling speakers at the same time. Good.
[0023]
As described above, according to the present invention, when the muffling sound waves are
radiated into the duct from the muffling speakers respectively installed on the facing duct wall
surfaces, the sound waves generated between the facing duct wall surfaces are Since the
interference can be prevented, a sound field having the same frequency characteristic as that of
the muffling signal can be generated in the duct.
Furthermore, since a plurality of muffling speakers are used, it is possible to generate a muffling
sound wave having sufficient acoustic power to muffle noise propagating in the duct.
03-05-2019
7
[0024]
In addition, since the plurality of muffling speakers are physically connected, it is possible to
offset the vibration generated when the speaker itself generates a sound wave in the low
frequency region, and the same sound wave may be canceled by a plurality of muffling speakers
at the same time. Can be emitted from As a result, a sound field having sufficient acoustic power
and the same frequency characteristics as the noise reduction signal can be formed, and active
noise control with high accuracy can be performed.
03-05-2019
8
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