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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system for attenuating noise
electronically and, in particular, to an electronic noise attenuation
system which is capable of attenuating non-steady noise occurring in
propagation passages such as duct lines or the like by exercising an
adaptive control using a computer system including a digital filter
therein.
2. Description of the Prior Art
Conventionally, there has been widely put into practical use a passive
noise attenuation apparatus which attenuates noise occurring within ducts
by use of the interference due to the duct structure or the noise
absorption due to a porous material attached to the duct. However, this
type of noise attenuation apparatus is found disadvantageous in that it is
too big in size, it involves too much loss of pressure, and so on.
On the other hand, there is also available an active noise attenuation
apparatus which has been long proposed and employs another method of the
reduction of unwanted sounds within the duct. That is, recently, special
interest has been given to an electronic noise attenuation system of such
active type in which noise propagated from a source of noise is sensed, a
cancellation sound having the same sound pressure and an opposite phase
with respect to the sensed noise is generated against the noise to provide
sound wave interference between the noise and the cancellation sound, and
thus the noise can be cancelled forcibly by the sound wave interference.
With the rapid progress of an electronic device, signal processing
technique and the like, there have been recently published various kinds
of study results on such active electronic noise attenuation method and
apparatus.
However, there are left many problems to be solved and thus such electronic
noise attenuation method or apparatus has not yet come into a stage of
seriously practical application.
A technical problem in putting into practice such electronic noise
attenuation system consists in the construction of a model which can be
used as a basis for design of a control system of the electronic noise
attenuation system. The model must be able to cope with the following
points. At first, there is necessary a filter which is capable of
cancelling noise of continuous spectra. That is, if a cancellation sound
can be generated with respect to the noise of continuous spectra such as
automotive noise, air current noise and the like as well as the noise of
discrete spectra such as transformer noise, compressor noise and the like,
the applications of the electronic noise attenuation system can then be
expanded further. To realize this, a filter is required which is able to
provide arbitrary amplitude characteristics and phase characteristics.
Secondly, it is necessary to prevent the feedback of the cancellation sound
with respect to a sensing microphone. That is, in the electronic noise
attenuation system, there is interposed the sensing microphone between a
source of noise and a source of cancellation sounds within a propagation
passage through which sound waves are propagated, and it is necessary to
create an electric signal to drive the cancellation sound source which
generates sound waves to cancel the propagated sound waves from the noise
source, in accordance with the sounds sensed by the sensing microphone and
by some proper signal generation means. In this case, the sound waves
generated from the cancellation sound source is also caught by the sensing
microphone and, as a result of this, there is produced an acoustic
feedback system between the cancellation sound source and the sensing
microphone. For this reason, it is essential to take a countermeasure to
cope with this situation. Especially, in order to make compact the
electronic noise attenuation system and to allow it to be mounted at an
arbitrary position in a pipe line such as a duct line, the sensing
microphone and the cancellation sound source must be located adjacent to
each other. Therefore, the above-mentioned acoustic feedback has a great
influence on the electronic noise attenuation system and thus the
counter-measure to cope with this problem is very important.
Thirdly, it is necessary to make it possible to correct the characteristics
of electro-accoustic transducers such as a microphone, speaker and the
like used in the electronic noise attenuation system. That is, in order to
stabilize the the control function of the electronic noise attenuation
system, it is essential that the control system of the electronic noise
attenuation system is provided with a function to correct the minute
amount of deterioration of the characteristics of the electro-acoustic
transducers. This is another problem to be solved.
According to the conventional electronic noise attenuation systems of this
kind, the above-mentioned technical problems have not been solved at all,
making it impossible to put the conventional systems into practice.
In contrast with this, we have successfully come up with models on an
electronic noise attenuation system using a monopole system as well as on
an electronic noise attenuation system using a dipole system, both of
which are able to cope with the above-mentioned technical problems as
described in detail afterwards.
Out of our models, the model on the electronic noise attenuation system of
the monopole type is able to perfectly deal with the above-mentioned first
and third technical problems for realization of the electronic noise
attenuation system. However, with regard to the second technical problem,
that is, the prevention of the feedback of the cancellation sound with
respect to the sensing microphone, since a control system for cancellation
of such feedback becomes complicated in structure, the model cannot help
employing passive means: for example, the consideration of the
directivities of the respective electro-acoustic transducers such as the
sensing microphone and the like as well as the positional relationships
therebetween; and, the attachment of a sound absorption material to the
inside of the propagation passage of sound waves extending from the
cancellation sound source to the sensing microphone.
Also, the other model of our models mentioned above, namely, the electronic
noise attenuation system of the dipole type according to the other model
is able to cope with all of the above-mentioned three technical problems.
However, it has been found too complicated in structure for practical
application, although the control system thereof is simpler in structure
when compared with that of the electronic noise attenuation system of the
monopole type in realizing the prevention of the feedback of the
cancellation sound with respect to the sensing microphone.
As described above, as the passive means to prevent the feedback of the
cancellation sound, there are known several methods: in one of them, the
directivities of mechano-electric transducing means such as a sensing
microphone or electro-mechanical transducing means such as a speaker are
improved for prevention of the cancellation sound feedback; and, in
another method, the distance between the sensing microphone and the
cancellation sound source is extended to reduce the energy of the
cancellation sound.
However, in view of the fact that in the low frequency noises that produce
a large amount of feedback the wavelengths are about several meters to
several tens of meters, in order to provide the sensing microphone with an
extreme directivity, the electronic noise attenuation system must be large
in size whether it employs a waveguide or microphone arrays. This prevents
the miniatuarization of a noise attenuator which is one of the effects
given by the electronic noise attenuation system, making the system
impractical. This is a common problem in a method in which the distance
between the sensing microphone and the cancellation sound source is
extended to prevent the feedback of the cancellation sound.
Further, in order to give a directivity to a speaker which is a typical
example of the electro-mechanical transducers, there has been proposed a
method in which three speakers are used to produce a directional sound
source which generates only progressive waves. However, it requires a
complicated control circuit but the effect thereof on the prevention of
the feedback is not so great for the complication of the control circuit,
that is, this method is disadvantageous in being not practical.
As discussed above, it is not easy to prevent the feedback of the
cancellation sound with respect to the sensing microphone. But, it is
currently requested that this problem is solved by a practical means.
SUMMARY OF THE INVENTION
The present invention aims at eliminating the drawbacks found in the
above-mentioned prior art methods of and apparatuses for the attenuation
of noise.
Accordingly, it is an object of the invention to make clear a model which
can be used as a basis for design of a control system of an electronic
noise attenuation system capable of positively restricting by means of a
simple structure the acoustic feedback of a cancellation sound from a
electro-mechanical transducer, or a source of the cancellation sound to a
mechano-electrical transducer for sensing a propagation wave from a source
of noise, as well as to provide such electronic noise attenuation system
that can accurately cancel non-steady noise occurring in a propagation
passage such as a duct line and the like in accordance with the
above-mentioned model.
In order to accomplish the above object, according to the invention, there
is provided an electronic noise attenuation system for achieving
attenuation of a propagation sound wave from a source of noise in a sound
wave propagation passage by generating a cancellation sound wave
180.degree. out of phase and of the same sound pressure with the
propagation sound wave to produce a sound wave interference between them
at a given position in the sound wave propagation passage, the electronic
noise attenuation system comprising: a first mechano-electrical
transducing means located closer to the noise source from the given
position in the propagation passage to sense the propagation sound wave
from the noise source and convert it into an electrical signal; an
electro-mechanical transducing means interposed between the position of
the first mechano-electrical transducing means and the given position in
the propagation passage to generate a sound wave for cancelling the
propagation sound wave from the noise source at the given position; a
second mechano-electrical transducing means interposed between the
position of the electro-mechanical transducing means and the given
position or located at the given position to sense the propagation sound
wave from the electro-mechanical transducing means and the noise source
and convert it into an electric signal; an operation means for obtaining a
difference between the output signals of the first and second
mechano-electrical transducing means; a drive signal generating means for
receiving the output signal of the operation means to generate a drive
signal to be given to the electro-mechanical transducing means so that the
amount of sound cancellation of the electronic noise attenuation system
can be the greatest in accordance with a given transfer function; and, a
control means for determining the transfer function to be given to the
drive signal generating means, establishing in the drive signal generating
means control parameters to specify the transfer function, and correcting
the control parameters according to the changes of the propagation
characteristics of the propagation passage and the characteristics of the
control system of the electronic noise attenuation system.
BRIEF DESCRIPTION OF THE DRAWINGS
The exact nature of this invention, as well as other objects and advantages
thereof, will be readily apparent from consideration of the following
specification relating to the accompanying drawings, in which like
reference characters designate the same or similar parts throughout the
figures thereof and wherein:
FIG. 1 is a schematic view to show the principles of an electronic noise
attenuation system with dual sensing microphones in accordance with the
present invention;
FIG. 2 is an explanatory view to illustrate a model of the electronic noise
attenuation system shown in FIG. 1 in which the propagation
characteristics of a propagation passage as well as the conversion
characteristics of electro-acoustic transducers themselves are taken into
consideration;
FIG. 3 is an explanatory view to illustrate a simplified version of the
model shown in FIG. 2;
FIG. 4 is a block view to show the concrete structure of the electronic
noise attenuation system according to the invention;
FIG. 5 is an explanatory view to illustrate the electronic noise
attenuation system of the invention when it is applied to an air
conditioning system;
FIG. 6 is a characteristic view to illustrate the noise attenuation effects
of the applied electronic noise attenuation system shown in FIG. 5;
FIG. 7 is an explanatory view to show a model for an electronic noise
attenuation system of a monopole sound source type;
FIG. 8 is a block view to show the concrete structure of the electronic
noise attenuation system of the monopole sound source type; and,
FIG. 9 is a block view to show the structure of an electronic noise
attenuation system of a dipole sound source type.
DETAILED DESCRIPTION OF THE INVENTION
Detailed description will hereunder be given of the preferred embodiments
of a system for attenuating noise according to the present invention with
reference to the accompanying drawings.
Prior to explanation of concrete embodiments of the invention, the
principles of an electronic noise attenuation system of a monopole sound
source type employing a single source of a cancellation sound will be
described in connection with FIG. 7. In FIG. 7, within a propagation
passage 1 for sound waves there are provided a sensing microphone M.sub.1
and a microphone M.sub.2 which is located downstream of the position of
the sensing microphone M.sub.1 and is used to evaluate the noise
attenuation effects. A source of a cancellation sound S is interposed
between the two microphones M.sub.1 and M.sub.2. Also, between the sensing
microphone M.sub.1 and the cancellation sound source S there is arranged a
controller 2.
In the above-mentioned structure, a propagation sound wave from a source of
noise is first sensed and converted into an electric signal by the
microphone M.sub.1 and is then input to the controller 2.
Also, to the controller 2 is input an evaluation signal 3 for evaluation of
the noise attenuation effects from the microphone M.sub.2. Controller 2
outputs to the cancellation sound source S a drive signal allowing the
output of the microphone M.sub.2 to be zero at the position of the
microphone M.sub.2 due to interference produced between a cancelling sound
wave generated from the cancellation sound source S and a sound wave
propagated from the noise source. That is, such structure is able to
cancel a sound wave generated from the noise source at the position where
the microphone M.sub.2 is located.
In order to enhance the noise cancellation effects in the thus constructed
electronic noise attenuation system, it is necessary to examine a model in
which transfer functions Gd, Gd', Gt representing the sound propagation
characteristics between the respective electro-acoustical transducers
shown in FIG. 7 as well as the conversion characteristics of the
electro-acoustical transducers themselves such as the microphones M.sub.1,
M.sub.2, cancellation sound source S and the like are taken into
consideration. Also, it is necessary that the respective elements of the
thus examined model are defined clearly.
From these viewpoints, we have already developed models which are able to
cope with the three problems discussed in the BACKGROUND OF THE INVENTION
of this specification, and also which can respectively be used as basis
for designing the respective control systems of an electronic noise
attenuation system of a monopole sound source type (FIG. 8) and an
electronic noise attenuation system of a dipole sound source type (FIG.
9), and concrete structures for realizing these models have also been made
clear. These models and structures are disclosed in detail in Japanese
patent application No. 139293 of 1985 and No. 128294 of 1985 and,
therefore, the description thereof is omitted here.
Now, the present invention provides an electronic noise attenuation system
of a dual sensing microphones system which is an improved version of a
monopole sound source system and employs two sensing microphones, and the
present electronic noise attenuation system is capable of easy restriction
of the acoustical feedback from the cancellation sound source to the
microphone M.sub.1.
Referring now to FIG. 1, there is shown a view of the principles of an
electronic noise attenuation system of a dual sensing microphones system
according to the present invention.
The electronic noise attenuation system in FIG. 1 is different in structure
from the electronic noise attenuation system of the monopole sound source
system shown in FIG. 7 in that the two sensing microphones M.sub.1,
M.sub.2 for sensing the propagated wave from the noise source are
respectively located upstream and downstream of the cancellation sound
source S in the sound wave propagation passage 1, and that the output of
the sensing microphone M.sub.2 is made 180.degree. out of phase with the
output of the sensing microphone M.sub.1, the output signals thereof are
input to an add circuit 20, and the output signal of the add circuit 20 is
input to the controller 2.
In FIG. 1, reference character He designates a transfer function which
indicates the control characteristic of the controller 2. Also, the output
terminal of the sensing microphone M.sub.1, the input terminal of the
cancellation sound source S and the output terminal of the sensing
microphone M.sub.2 are respectively given evaluation points V.sub.A,
V.sub.B, V.sub.C which can be measured electrically. In FIG. 2, there is
illustrated a model in which the propagation characteristics of the sound
wave within the the propagation passage 1 as well as the conversion
characteristics of the respective electro-acoustical transducers
themselves are taken into consideration on the basis of these evaluation
points V.sub.A, V.sub.B, V.sub.C. In FIG. 2, wider arrow lines are used to
show the directions of propagation of the sound wave, while solid arrow
lines are used to show the flows of the electric signals.
Also, reference characters P.sub.1, P.sub.2 respectively stand for the
sound pressures of the sound wave propagated from the noise source toward
the downstream direction within the propagation passage 1 at the
respective positions where the two microphones M.sub.1, M.sub.2 are
located, and V.sub.A, V.sub.B, V.sub.C, as described above, represent
voltages measured at points set for the microphone M.sub.1, a speaker S
serving as the cancellation sound source, and the microphone M.sub.2.
Further, Gd designates a transfer function which indicates the propagation
characteristic of the sound wave propagated from the microphone M.sub.1 to
the microphone M.sub.2, and H.sub.M1, H.sub.M2 respectively represent
transfer functions to indicate the sound pressure--voltage conversion
characteristics with respect to the sound wave sensed by the two
microphones M.sub.1, M.sub.2 within the propagation passage 1.
Moreover, H.sub.M1' designates a transfer function to indicate the sound
pressure--voltage conversion characteristic of the sensing microphone
M.sub.1 with respect to the sound wave propagated from the direction of
the cancellation sound speaker S; H.sub.M2' a transfer function to
indicate the sound pressure--voltage conversion characteristic of the
sensing microphone M.sub.2 with respect to the sound wave propagated from
the direction of the cancellation sound speaker S; H.sub.S a transfer
function to indicate the voltage--sound pressure conversion characteristic
of the cancellation sound speaker S toward the direction of the sensing
microphone M.sub.2 ; and, Hs' a transfer function to indicate the
voltage--sound pressure conversion characteristic of the cancellation
sound speaker S toward the direction of the sensing microphone M.sub.1.
In addition, Gd' designates a transfer function to indicate the propagation
characteristic of the sound wave propagated from the cancellation sound
speaker S to the sensing microphone M.sub.1 within the propagation
passage; and, Gt denotes a transfer function to indicate the propagation
characteristic of the sound wave propagated from the cancellation sound
speaker S to the sensing microphone M.sub.2 within the propagation
passage.
In the model shown in FIG. 2, when Hr is used to express a transfer
function indicating the propagation characteristic of the sound wave
propagated from the cancellation sound source S to the sensing microphone
M.sub.1 with the conversion characteristics of the cancellation sound
source S and the sensing microphone M.sub.1 added thereto, and Ht is used
to express a transfer function indicating the propagation characteristic
of the sound wave propagated from the cancellation sound source S to the
sensing microphone M.sub.2 with the conversion characteristics of the
cancellation sound source S and the sensing microphone M.sub.2 added
thereto, then the respective transfer functions can be expressed as:
Hr=H.sub.M1' .multidot.Gd'.multidot.Hs' (1)
Ht=Hs.multidot.Gt.multidot.H.sub.M2' (2)
As shown above, if the model shown in FIG. 2 is replaced by the transfer
functions Hr, Ht, then the model can be further simplified as shown in
FIG. 3.
In the dual sensing microphones system proposed here, the two sensing
microphones M.sub.1, M.sub.2 having the matched characteristics are
respectively located at positions with respect to the cancellation sound
source S where the two transfer functions Ht, Hr are equal to each other
(briefly, two positions equidistant from the cancellation sound source S
within the propagation passage 1); the output of the sensing microphone
M.sub.2, with the phase thereof being made 180.degree. out of phase with
that of the output of the microphone M.sub.1, is input to the add circuit
20; and, the output of the add circuit 20 is input to the controller 2.
In this structure, the propagation sound wave generated from the
cancellation sound source S and sensed by the sensing microphone M.sub.1
can be cancelled electrically by the add circuit 20 and thus the
oscilation of the control system can be suppressed.
As discussed above, the dual sensing microphones system is very
advantageous in that it is able to prevent the acoustical feedback of the
cancellation sound simply by adding to the monopole sound source system a
sensing microphone and a basic add circuit as an electric circuit.
Next, a transfer function He is derived on the basis of FIG. 3 which
indicates the control characteristic of the controller 2 that allows the
cancellation sound source S to generate the sound wave for cancelling the
sound wave propagated from the noise source.
Here, the sound pressure P.sub.2 measured at the location of the sensing
microphone M.sub.2 and the voltages V.sub.A, V.sub.B, V.sub.C at the
measured points are respectively as:
P.sub.2 =P.sub.1 .multidot.Gd (1)
V.sub.A =P.sub.1 H.sub.M1 +V.sub.B Hr (2)
V.sub.B =(V.sub.A -V.sub.C) He (3)
V.sub.C =P.sub.2 H.sub.M2 +V.sub.B Ht (4)
Also, from the equations (2), (3) V.sub.B can be shown as:
##EQU1##
Similarly, from the equations (4), (5) V.sub.C can be shown as:
##EQU2##
Also, by substituting the equation (1) the equation (6) can be expressed
as:
##EQU3##
Here, for V.sub.C =0, the following equation must be obtained from the
equation (7):
He (H.sub.M1 .multidot.Ht-Gd.multidot.H.sub.M2
.multidot.Hr)=-Gd.multidot.H.sub.M2 (8)
As a result of this, the transfer function He can be expressed as follows:
##EQU4##
As can be seen from the equation (9), in order to determine the transfer
function He, there are necessary the transfer functions
Gd.multidot.H.sub.M2 /H.sub.M1, Ht, Hr. As mentioned before, these
transfer functions can be easily identified respectively, using V.sub.A,
V.sub.B, V.sub.C as the measured points thereof.
Next, in FIG. 4, there is illustrated a concrete structure of an electronic
noise attenuation system according to the present invention constructed in
accordance with the above-mentioned model.
In FIG. 4, within the propagation passage 1 the two sensing microphones
M.sub.1, M.sub.2 are located at opposite positions with the cancellation
sound source S therebetween in which the transfer functions Hr, Ht
indicating the propagation characteristics of the sound wave generated
from the cancellation sound source are equivalent to each other, for
example, at the positions respectively equidistant from the cancellation
sound source S.
Also, in FIG. 4, reference character 28 designates an input/output
interface which comprises A/D conversion parts 24, 25 and a D/A conversion
part 26. Reference numeral 29 stands for a digital filter which generates
a drive signal to be output via the D/A conversion part 26 to the speaker
S for generating a sound to cancel the sound propagated from the noise
source.
Further, there is shown in FIG. 4 a control part which is designated by 30.
The control part 30 is adapted to receive the output signal of the add
circuit 20 to which the output of the sensing microphone M.sub.1, M.sub.2
are inputted and the output signal of the sensing microphone M.sub.2 which
also serves as a microphone for evaluation of the noise cancellation
effect, respectively through the A/D conversion parts 24, 25. In
accordance with these signals input therein, the control part 30, when
there is no noise present within the propagation passage 1, outputs test
signals to the respective circuits to derive the transfer functions that
indicate the propagation characteristics of the propagation sound wave
between the respective electro-acoustical transducers or the conversion
characteristics of the respective electro-acoustical transducers
themselves; or, when there is present noise in the propagation passage 1,
it sets up a control parameter to give a given transfer function to the
digital filter 29.
In addition, the control part 30 is capable of adaptive controls so that
the above-mentioned control parameter can be corrected according to the
changes of the propagation characteristics of the sound wave resulting
from possible disturbances within the propagation passage 1, for example,
variations of air flow and so on, and the change of the characteristics of
the control system.
In the above-mentioned structure, at first in the digital filter 29 there
is set up by the control part 30 a control parameter to give a
transmission function corresponding to the transmission function He that
is determined from the derived results of the transmission functions and
is shown in FIG. 2. In this state, if the propagation sound wave generated
from the noise source within the propagation passage 1 is sensed by the
two microphones M.sub.1, M.sub.2, then the output signals from the add
circuit 20, into which the output signals of the sensing microphones
M.sub.1 and M.sub.2, are input via the A/D conversion part 24 of the
input/output interface 28 to the digital filter 29 and the control part
30, respectively.
In the control part 30, in consideration of the changes of the propagation
characteristics within the propagation passage 1 as well as the variations
of the characteristics of the respective electro-acoustical transducers
themselves, the transfer functions that indicate these characteristics are
obtained, on the basis of the thus obtained transfer functions, a transfer
function to be given to the digital filter 29 is determined so that the
output signal of the microphone M.sub.2 sensing the noise cancellation
effect, that is, the state of interference between the sound wave
propagated from the noise source and the sound wave generated from the
speaker S can be minimized, and a control parameter to specify the thus
determined transfer function is established in the digital filter 29. As
discussed above, the control part 30 is able to correct the control
parameter as desired according to the variations of the propagation
characteristics of the propagation passage 1 as well as the
characteristics of the control system. As a result of this, the
propagation sound wave from the noise source sensed by the microphones
M.sub.1 and M.sub.2 is converted to an electric signal, the converted
electric signal is then input to the digital filter 29 via the add circuit
20 and the A/D conversion part 24 of the input/output interface 28, and
the input signal is converted into a digital signal having pretermined
amplitude and phase characteristics by the digital filter 29 on the basis
of the transfer function given from the control part 30. This digital
signal is converted from digital to analog by the | | |
