Active silencer

Details Active silencer Active silencer

1997-12-05

2001-12-11

Nguyen, Duc (Department: 2643)

Electrical audio signal processing systems and devices

Acoustical noise or sound cancellation

C381S071500, C381S071800, C381S071110

Reexamination Certificate

active

06330336

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active silencer and particularly to an active silencer which utilizes the technique for canceling noise by generating the sound having the waveform in the same amplitude but inverse phase from the noise generated from a noise source and then causing these sounds to interfere with each other.
2. Description of the Related Art
As the technique for generating a secondary sound in the same amplitude and inverse phase from noise to cancel these sound by causing these noise and secondary sound to interfere with each other, there are examples described in the Official Gazettes, Japanese Published Unexamined Patent Application Nos. Hei 4-221965 and Hei 4-221967.
FIG. 15
is a diagram showing an example of a basic structure of a silencer of the related art. In
FIG. 15
, a noise source
1
is surrounded by a duct
2
in its peripheral space. However, the cross-sectional shape of the duct
2
must be within the range in which the sound wave radiated from the noise source
1
can be assumed as a plane wave. An input microphone
3
is provided in the vicinity of the noise source
1
to detect noise from the noise source. An output of the input microphone
3
is input to signal processing means
4
and its output is then connected to a secondary sound source speaker
5
. In this figure, K indicates the frequency response characteristic of the input microphone
3
, while G
0
indicates the frequency response characteristic of the area up to the silencing point
6
from the noise source
1
, G
1
indicates the frequency response characteristic of the area up to the silencing point
6
from the secondary sound source speaker
5
, Sp indicates the frequency response characteristic of the secondary sound source speaker
5
and C indicates the frequency response characteristic of signal processing means
4
.
Noise radiated from the noise source
1
reaches the silencing point
6
passing through the inside of duct
2
. Meanwhile, a signal detected by the input microphone
3
is input to signal processing means
4
, it is then processed as explained later and output as the secondary sound controlled from the secondary sound source speaker
5
and reaches the silencing point
6
. At the silencing point
6
, noise waveform of the noise source
1
and the secondary sound from the secondary sound source speaker
5
interfere with each other and thereby sound pressure becomes zero. The duct
2
is provided to approximate the sound wave in the duct to the plane wave propagated in the longitudinal direction of duct and the fact that the sound pressure in the silencing point
6
becomes zero is thought to suggest that the sound pressure at the cross-section including the silencing point
6
and in the space in the downstream side thereof is also zero.
Here, the signal processing method in the signal processing means
4
will be explained. First, the frequency response characteristic of the area up to the silencing point
6
from the noise source
1
including the secondary sound source speaker
5
and signal processing means
4
is expressed by the following formula.
G
0
+
K×C×Sp×G
1
  (1)
Therefore, following condition is required to perfectly reduce the noise of the noise source
1
at the silencing point
6
.
G
0
+K×C×Sp×G
1
=0  (2)
From this formula,
C=−G
0
×1/(
K×Sp×G
1
)  (3)
Therefore, the signal processing means
4
may be structured depending on each frequency response characteristic. Since the frequency response characteristic to form the signal processing means
4
is fixed, this system is hereinafter called as the fixed parameter system.
Moreover, the Official Gazettes, Japanese Published Unexamined Patent Application Nos. Hei 4-332676 and Hei 6-8581 describe a system wherein the secondary sound is combined using the method of applied algorithm. A structure of the basic system is shown in FIG.
16
.
FIG. 16
shows an example of another basic structure of the silencer of the related art. In
FIG. 16
, a noise source
1
is surrounded by a duct
2
, a detection microphone
7
is provided in the vicinity of the noise source
1
and an error detection microphone
8
is also provided at the downstream side of the duct
2
. Outputs of these detection microphone
7
and error detection microphone
8
are input to signal processing means
9
and an output of this signal processing means
9
is connected to a secondary sound source speaker
5
. The signal processing means
9
is provided with an application filter
9
a.
The application filter
9
a
updates the coefficients depending on the following formula on the basis of the noise signal x(t) caught by the detection microphone
7
and an error signal e(t) caught by the error detection microphone
8
.
H
(
i
)
new
=h
(
i
)
old
+k×e
(
t
)×
x
(
t
−1)  (4)
Where, h(i)(i=0 . . . , n) is a coefficient of the application filter
9
a,
n is the maximum order number of the application filter
9
a,
x(t−1) is a preceding noise signal, and k is a constant. The signal processed by the application filter
9
a
of which coefficient is updated is sent to the secondary sound source speaker
5
and is then radiated as the sound wave.
This algorithm is called the Filtered-X algorithm and when this applicable operation is repeated, the error signal e(t) comes close to zero to realize silencing.
In this system, it is not required to previously obtain each frequency response expressed by the formula (3) in the fixed parameter system and moreover this system has a merit that variation in the frequency response due to the environmental change can also be covered. This system is hereinafter called as the application parameter system for the convenience of the explanation.
As the other examples of the application parameter system, there are official gazettes of the Japanese Published Unexamined Patent Application Nos. Hei 2-97877 applied to a compressor noise of a home electric refrigerator, Sho 59-9699 applied to control of sound field within chamber of automobile and Sho 7-97989 applied to the duct of air conditioner. In any of these examples, the structure same as that of
FIG. 16
has been employed.
In the fixed parameter system described in the Official Gazette of Japanese Published Unexamined Patent Application No. Hei 4-221965, the characteristic 1/(K×Sp×G
1
) of the formula (3) called generally as the inverse filter is realized to cancel (K×Sp×G
1
), however, under the condition that the initial characteristic (K×Sp×G
1
) must be the minimum phase system which does not results in any delay of phase for the gain characteristic.
However, in general, many acoustic transfer systems surely allow existence of time delay until the sound wave reaches the output point from the input point. Therefore, the condition of the minimum phase system is not satisfied. Accordingly, when the signal processing means is formed depending on the formula (3) from the measured each frequency response, it operates as an unstable filter which disperses an output to an finite input. Moreover, when non-minimum phase is forcibly approximated by the minimum phase, the signal processing means will change to signal processing means under the causal relationship in which a future information which is leading as much as an amount of delay for the current input is previously required to compensate for the amount of delay. However, such signal processing means cannot be realized easily.
However, in the examples described in the Official Gazettes of the Japanese Published Unexamined Patent Application Nos. Hei 4-221965 and Hei 4-221967 explained previously, it is described that silencing can be realized only by introducing the formula (3), and such problem is not yet explained.
Meanwhile, in the application parameter system described in the Official Gazettes of the Japanese Published Unexamined Patent Application No

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