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JPH11346394

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DESCRIPTION JPH11346394
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
microphone for a silencer using active noise control (ANC) technology.
[0002]
2. Description of the Related Art Silencers using active noise control (ANC) technology, as is
known, are for silencing against the interior space of a sound absorption duct through which
noise propagates, and for noise cancellation that is equal in magnitude and opposite in phase A
sound wave is introduced, and a muffling effect is obtained by the interference between the noise
and the sound wave for muffling.
[0003]
FIG. 5 shows a general configuration of this type of noise reduction device, which is a sound
absorption duct 1 for introducing noise P, a noise detector 2 for detecting noise P, and a sound
wave generator for generating a sound wave for muffling And a mute deviation detector 4 for
detecting a mute deviation.
The speaker 3 is driven by the muffling signal y calculated by the muffling signal generator 5
based on the outputs x and e of the noise detector 2 and the muffling deviation detector 4. The
output x of the noise detector 2 is an adaptive filter (hereinafter abbreviated as ADF) via the
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amplifier 6 and the A / D converter 7. And 8) and the delay filter 9. The ADF 8 has N taps, whose
coefficients are updated by the output of the coefficient calculator 10. The delay filter 9 has an
acoustic transfer characteristic corresponding to a path from the speaker 3 to the muff deviation
detector 4. That is, the coefficients of the delay filter 9 are determined in consideration of this
time delay, which are measured or identified in advance. Further, the output e of the muffling
deviation detector 4 is supplied to the coefficient calculator 10 through the amplifier 11 and the
A / D converter 12. The coefficient calculator 10 is an adaptive algorithm for sequentially
calculating an optimum coefficient to be given to the ADF 8. The muffling signal y calculated by
the ADF 8 is supplied to the speaker 3 through the D / A converter 13 and the amplifier 14, and
a muffling sound wave having the same magnitude and opposite phase as the noise P is
contained in the sound absorbing duct 1. It is generated towards. Then, the muffling signal y is
adjusted so that the muffling deviation becomes zero at the position of the muffling deviation
detector 4. The silencing signal generator 5 is configured to use the known ANC technique as
described above.
[0004]
The noise detector 2 and the muffle deviation detector 4 are each configured by a small
microphone. In general, the maximum input sound pressure level of a low-cost, compact
condenser microphone for consumer use is approximately 110 dB to about 120 dB, and
distortion occurs when the sound pressure exceeds this. However, since the inside of the engine
exhaust gas pipe usually exceeds 120 dB, it is necessary to attenuate the input sound pressure
level to the microphones 2 and 4 when the microphone is applied thereto. Therefore,
conventionally, the noise detector 2 or the muffling deviation detector 4 is installed in the input
sound pressure attenuator configured as shown in FIGS. 6 and 7 so as to attenuate the input
sound pressure level of the sound wave.
[0005]
That is, in the configuration shown in FIG. 6, the microphone (the noise detector 2 or the
muffling deviation detector 4) M is disposed at the bottom 21 of the cylindrical member 20 filled
with the sound absorbing material 22 inside to attenuate the input sound pressure level. The On
the other hand, in the configuration shown in FIG. 7, the microphone M is disposed at the bottom
24 of the box-shaped container 25 in which the sound inlet 27 is formed, and the input sound
pressure level is narrowed by the sound inlet 27. The microphone M is a condenser microphone,
and lead wires La and Lb respectively connected to the internal diaphragm and the back
electrode are led to the outside.
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[0006]
However, in the conventional configuration as described above, there is a problem that the
frequency characteristic of the microphone M is disturbed in the used frequency band. That is, in
the configuration of FIG. 6, the high frequency component of the sound wave is well attenuated
but the low frequency component is hardly attenuated, and in the configuration of FIG. 7, the
volume of the internal space 26 of the box-shaped container 25, ie, the volume of the air
chamber. The resonance system formed by the acoustic mass of the sound inlet 27 has a peak at
a relatively low frequency, and the frequency characteristic of the microphone M is disturbed.
Also, there is a problem that the frequency characteristic does not become flat even if designed
to reduce the influence of the peak.
[0007]
The present invention has been made in view of the above problems, and is capable of detecting
high sound pressure using an inexpensive general small-sized microphone and for attenuating
device capable of attenuating input sound pressure without affecting the inherent frequency
characteristics of the microphone. It is an object to provide a microphone.
[0008]
SUMMARY OF THE INVENTION The above problems are addressed to a muffling signal
generator for forming a muffling signal for generating sound waves from the sound generator
that are equal in magnitude and opposite in phase to the noise from the noise source. A
microphone having a sound receiving port, a casing having a sound receiving port, and a
diaphragm extended in the casing and facing the sound receiving port; It is solved by a
microphone for a muffling device, characterized by comprising: input sound pressure attenuation
means for attenuating input sound pressure.
[0009]
That is, the present invention is characterized in that the microphone itself is provided with an
input sound pressure attenuation function.
Therefore, the configuration of claim 2 is intended to attenuate the input sound pressure level by
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covering the sound receiving port with a sound insulation member, and by forming a diaphragm
on the sound receiving port in the configuration of claim 4.
As a result, it is possible to attenuate the input sound pressure without affecting the original
frequency characteristic of the microphone while making it possible to detect the large sound
pressure even if an inexpensive general small microphone is used.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be
described below with reference to the drawings.
[0011]
FIG. 1 shows a first embodiment of the present invention.
The microphone is generally indicated 31 and points to both a noise detector for detecting noise
and a muffling deviation detector for detecting muffling deviation. A sound receiving port 32a
having an opening diameter of 2 mm to 3 mm is formed on one side surface of the metal casing
32, and the inside of the casing 32 is made of metal or polymer film so as to face the sound
receiving port 32a. The diaphragm 33 is stretched. A back electrode 34 is disposed on the side of
the diaphragm 33 opposite to the sound receiving port 32a with a predetermined gap, and the
diaphragm 33 and the back electrode 34 constitute a capacitor. In the figure, La and Lb are lead
wires respectively. The back electrode 34 is supported by the casing 32 via the insulator 35. The
sound receiving port 32a is covered with the sound insulation member 36 from the outside of
the casing 32, and this constitutes the input sound pressure attenuation means in the present
embodiment. The sound insulation member 36 is made of, for example, a polyimide film (trade
name: Kapton), a polytetrafluoroethylene (trade name: Teflon), a polymer film such as cellophane
or polyethylene, an inorganic material such as mica, or a cloth-like thing made by knitting fibers.
It is a film-like body having a thickness necessary to attenuate the input sound pressure level of
the noise sound wave below the maximum input sound pressure level (for example, 110 dB to
120 dB) of the microphone 31.
[0012]
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Then, the natural frequency determined by the effective vibration mass of the sound insulation
member 36 or the effective mass m of air of the sound receiving port 32a and the volume V1 of
the air chamber 37 formed on the front surface of the diaphragm 33 It is set to be higher than
the high frequency limit or the highest frequency of noise to be muted. In order to satisfy this
condition, it is better for both m and V1 to be smaller. m is determined by the modal mass
(mainly bending stiffness) of the sound insulating member 36. Thus, the influence of the sound
insulation member 36 is reduced. Further, the primary natural frequency due to the film
vibration of the sound insulation member 36 is made to be equal to or higher than the high
frequency limit of the frequency band to be used similarly.
[0013]
Next, the function of the microphone 31 configured as described above will be described. The
sound wave to be detected vibrates the sound insulation member 36 and is transmitted to the
inside of the microphone 31. At this time, according to the bending rigidity of the sound
insulation member 36. And is attenuated below the maximum input sound pressure level of the
microphone 31. The sound wave introduced through the sound receiving port 32 a vibrates the
diaphragm 33, and the vibration of the diaphragm 33 is detected as a change in capacitance with
the back electrode 34. Further, in the present embodiment, the primary natural frequency
(minimum resonance frequency) of the resonance system determined by the mass of air at the
sound receiving port 32a and the air stiffness of the air chamber 37, and the inherent vibration
of the film of the sound insulating member 36. Since the frequency is set higher than the highest
frequency of the noise to be muted, these two vibration systems do not adversely affect the
muffling performance.
[0014]
Therefore, according to the present embodiment, noise exceeding the input sound pressure level
of the microphone 31 can be detected without disturbing the frequency characteristic (always
with the frequency characteristic flat). Moreover, since the muffling signal for supplying to the
speaker which generate ¦ occur ¦ produces the sound wave for muffling in a muffling signal
generator can be always formed based on a correct output by this, the muffling effect can be
heightened further.
[0015]
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FIG. 2 shows a second embodiment of the present invention. In the figure, parts corresponding to
those in the first embodiment described above are given the same reference numerals, and
detailed descriptions thereof will be omitted.
[0016]
A microphone in the present embodiment is generally indicated by 41, and a spacer member 39
for partitioning the second air chamber 38 is provided integrally with the casing 32 on the front
face of the sound receiving port 32a, and the end of this spacer member 39 Further, a sound
insulating member 36 'is provided to close the front of the sound receiving port 32a. The sound
insulation member 36 'is configured in the same manner as the sound insulation member 36 in
the first embodiment. According to the present embodiment, the bending rigidity of the sound
insulating member 36 ′ can be freely adjusted according to the inner diameter of the spacer
member 39. Although the acoustic system is more complicated than the first embodiment, the
lowest resonance frequency in this embodiment is the sum V1 + V2 of the volume V1 of the air
chamber 37 and the volume V2 of the second air chamber 38, It depends on the effective
vibration mass m of the sound insulating member 36 '. The same effects as described above can
be obtained by the present embodiment.
[0017]
FIG. 3 shows a third embodiment of the present invention. In the figure, parts corresponding to
those in the first embodiment described above are given the same reference numerals, and
detailed descriptions thereof will be omitted.
[0018]
The microphone in the present embodiment is generally indicated by 51, and a diaphragm
forming member 46 having a diaphragm 46a is provided on the front surface of the sound
receiving port 32a. That is, in the present embodiment, instead of providing the sound insulation
member, the diaphragm forming member 46 is provided as input sound pressure attenuation
means, and the input sound pressure level of the sound wave to be detected is attenuated by the
diaphragm 46a. Even in this configuration, practically sufficient effects can be obtained as long
as the mass of air in the throttle 46a and the volume V1 of the air chamber 37 satisfy the same
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conditions as those in the first embodiment described above. Instead of separately providing the
diaphragm forming member 46, as shown in FIG. 4, a sound receiving port 32a 'equivalent to the
diaphragm 46a may be formed directly on the casing 32.
[0019]
As mentioned above, although each embodiment of the present invention was described, of
course, the present invention is not limited to these, and various modification is possible based
on the technical thought of the present invention.
[0020]
For example, in each of the above-mentioned embodiments, although it applied and explained to
a condenser microphone (electrostatic pressure microphone), it replaces with this and the
present invention is applicable also to a dynamic voltage force microphone which accommodated
a converter.
[0021]
As described above, according to the muffling apparatus microphone of the present invention,
high sound pressure can be detected even when using a low-cost general small-sized microphone,
and the frequency characteristic inherent to the microphone can be affected. You can attenuate
the input sound pressure without
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