Active noise reduction apparatus

An apparatus for active reduction of noises transmitted from a noise source into a space. The active noise reduction apparatus comprises residual noise sensors for detecting the residual noises in the space. A reference signal is produced based upon the noise generating condition of the noise source. The reference signal is used, along with the detected residual noises, to drive control sound sources so as to reduce the noises in the space. A filter is adjusted to correspond to acoustic transfer characteristics between the control sound sources and the residual noise sensors. An identification sound is generated to correspond to the background noise level detected in the space and to the spectral distribution of the noises transmitted into the space. The coefficients of the filter are updated according to acoustic transfer characteristics between the control sound sources and the residual noise sensors. The acoustic transfer characteristics are obtained based upon the identification sound and the residual noises.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus for active reduction of noises 
transmitted into a space from noise sources by producing control sounds 
for interference with the transmitted noises. 
For example, British Patent No. 2,149,614 discloses a conventional active 
noise reduction apparatus for use in airplane passenger compartments or 
other closed spaces. The conventional active noise reduction apparatus is 
applicable to reduce noises transmitted from a single source of noises 
having a fundamental frequency f.sub.0 and its higher harmonics f.sub.1 to 
f.sub.n. The noise source is an engine or the like placed in the exterior 
of the closed space. A plurality of microphones are placed at different 
positions within the closed space for detecting the sound pressures 
applied thereon. In order to produce control sounds for interference with 
the transmitted noises, a plurality of loudspeakers are placed at 
different positions within the closed space. The loudspeakers are driven 
by drive signals having frequencies in opposite phase to the frequencies 
f.sub.0 to f.sub.n of the transmitted noises to cancel the transmitted 
noises. A "WIDROW LMS" algorithm developed for multiple channels is used 
to drive the loudspeakers. The "WIDROW LMS" algorithm is described in an 
article published 1975, in PROCEEDINGS OF THE IEEE. Vol. 63, page 1692, 
entitled "Adaptive Noise Cancellation: Principles and Applications". The 
"WIDROW LMS" algorithm developed for multiple channels is described in an 
article published 1987, in IEEE TRANS. ACOUST., SPEECH, SINGLE PROCESSING, 
VOL. ASSP-35, PP. 1423-1434 entitled "A MULTIPLE ERROR LMS ALGORITHM AND 
ITS APPLICATION TO THE ACTIVE CONTROL OF SOUND AND VIBRATION". 
The LMS (least mean square) algorithm is one of a number of appropriate 
algorithms for use in updating the filter coefficients of adaptive digital 
filters. For example, in a so-called Filtered-X LMS algorithm, all of the 
transfer function filters modeled on the transfer functions between the 
loudspeakers and the microphones are set for all of the 
loudspeaker-microphone combinations. The filter coefficient of each of the 
variable filter coefficient digital filters provided for the respective 
loudspeakers is updated in a manner to reduce the value of the performance 
function calculated based upon the residual noise levels detected by the 
respective microphones. 
The conventional active noise reduction apparatus is designed on such an 
assumption that the filter accurately represents the acoustic transfer 
characteristic between the loudspeaker and the microphone. It is, 
therefore, impossible to reduce the noises if there is a great difference 
between the acoustic transfer characteristic represents by the filter and 
the acoustic transfer characteristic of the actual physical space. 
Japanese Patent Kokai No. 3-259722 discloses another active noise reduction 
apparatus applied to a refrigerator. The active noise reduction apparatus 
employs a microphone to measure the sound pressure at a predetermined 
position within the refrigerator and a loudspeaker to produce a control 
sound so as to cancel the noises produced from the compressor used in the 
refrigerator before the noises are emitted through the duct to the 
exterior. The control sound is produced based upon the operating condition 
of the compressor. Each time the compressor comes to a stop, an 
identification sound is produced to measure an acoustic transfer 
characteristic between the loudspeaker and the microphones. The measured 
acoustic transfer characteristic is utilized to identify the filter. 
However, this conventional active noise reduction apparatus cannot be 
applied directly to a vehicle passenger compartment since the passenger 
will be put to great annoyance by the identification sound. In addition, 
the temperature and humidity change in a short time to a great extent in 
the vehicle passenger compartment. This causes a deviation of the acoustic 
transfer characteristic represented by the filter from the actual acoustic 
transfer characteristic of the actual physical space even though the 
filter coefficient is updated each time the engine comes to a stop. The 
deviation will increase with time. 
SUMMARY OF THE INVENTION 
It is a main object of the invention to provide an improved active noise 
reduction apparatus which can maintain good noise control over a long 
period of time without annoying the man or woman in the space to be 
controlled. 
There is provided, in accordance with the invention, an apparatus for 
active reduction of noises transmitted from a noise source into a space. 
The active noise reduction apparatus comprises control sound sources for 
producing control sounds in the space, residual noise detecting means for 
detecting residual noises at predetermined positions in the space, noise 
generating condition detecting means for detecting a noise generating 
condition of the noise source to generate a reference signal, signal 
processing means for filtering the reference signal corresponding to an 
acoustic transfer function between the control sound sources and the 
residual noise detecting means, active control means for driving the 
control sound sources to reduce the noises in the space based upon the 
reference signal and the residual noises, background noise level detecting 
means for detecting a background noise level in the space, identification 
sound generating means for producing an identification sound corresponding 
to the detected background noise level in the space, and updating means 
for obtaining acoustic transfer characteristics between the control sound 
sources and the residual noise detecting means based upon the 
identification sound and the residual noises to update a content of the 
signal process of the signal processing means. 
In another aspect of the invention, there is provided an apparatus for 
active reduction of noises transmitted from a noise source into a space. 
The active noise reduction apparatus comprises control sound sources for 
producing control sounds in the space, residual noise detecting means for 
detecting residual noises at predetermined positions in the space, noise 
generating condition detecting means for detecting a noise generating 
condition of the noise source to generate a reference signal, signal 
generating means for generating drive signals to drive the control sound 
sources based upon the reference signal, signal processing means for 
filtering the reference signal corresponding to an acoustic transfer 
between the control sound sources and the residual noise detecting means, 
control means for adjusting a content of the process of the signal 
generating means based upon a value resulting from the reference signal 
processed in the signal processing means to reduce the noises in the 
space, identification signal generating means for generating an 
identification signal having a spectral distribution similar to a spectral 
distribution of the noises transmitted from the noise source, background 
noise level detecting means for detecting a background noise level in the 
space, gain adjusting means for adjusting a gain of the identification 
signal based upon the detected background noise level, signal 
superimposing means for superimposing the identification signal having the 
adjusted gain on the drive signals generated from the signal generating 
means to produce signals to the control sound sources, and updating means 
for obtaining acoustic transfer characteristics between the control sound 
sources and the residual noise detecting means based upon the 
identification signal having the adjusted gain and the residual noises in 
the space when the control sound sources are driven by the signals 
produced from the signal superimposing means to update a content of the 
signal process of the signal processing means. 
In still another aspect of the invention, there is provided an apparatus 
for active reduction of noises transmitted from a noise source into a 
vehicle passenger compartment. The active noise reduction apparatus 
comprises control sound sources for producing control sounds in the 
vehicle passenger compartment, residual noise detecting means for 
detecting residual noises at predetermined positions in the vehicle 
passenger compartment, noise generating condition detecting means for 
detecting a noise generating condition of the noise source to generate a 
reference signal, signal processing means for filtering the reference 
signal corresponding to acoustic transfer characteristics between the 
control sound sources and the residual noise detecting means, active 
control means for driving the control sound sources to reduce the noises 
in the vehicle passenger compartment based upon the reference signal and 
the residual noises, background noise level detecting means for detecting 
a background noise level in the vehicle passenger compartment, 
identification sound generating means for producing an identification 
sound corresponding to the detected background noise level in the vehicle 
passenger compartment, and updating means for obtaining acoustic transfer 
characteristics between the control sound sources and the residual noise 
detecting means based upon the identification sound and the residual 
noises to update a content of the signal process of the signal processing 
means. 
In still another aspect of the invention, there is provided an apparatus 
for active reduction of noises transmitted from a noise source into a 
vehicle passenger compartment. The active noise reduction apparatus 
comprises control sound sources for producing control sounds in the 
vehicle passenger compartment where noises are transmitted from noise 
sources in the vicinity of vehicle road wheels, residual noise detecting 
means for detecting residual noises at predetermined positions in the 
vehicle passenger compartment, noise generating condition detecting means 
for detecting a noise generating condition of the noise source to generate 
a reference signal, signal generating means for generating drive signals 
to drive the control sound sources based upon the reference signal, signal 
processing means for filtering the reference signal corresponding to 
acoustic transfer characteristics between the control sound sources and 
the residual noise detecting means, control means for adjusting a content 
of the process of the signal generating means based upon a value resulting 
from the reference signal processed in the signal processing means to 
reduce the noises in the vehicle passenger compartment, identification 
signal generating means for generating an identification signal having a 
spectral distribution exhibiting a smaller level on a higher frequency 
side, background noise level detecting means for detecting a background 
noise level in the vehicle passenger compartment, gain adjusting means for 
adjusting a gain of the identification signal based upon the detected 
background noise level, signal superimposing means for superimposing the 
identification signal having the adjusted gain on the drive signals 
generated from the signal generating means to produce signals to the 
control sound sources, and updating means for obtaining acoustic transfer 
characteristics between the control sound sources and the residual noise 
detecting means based upon the identification signal having the adjusted 
gain and the residual noises in the vehicle passenger compartment when the 
control sound sources are driven by the signals produced from the signal 
superimposing means to update a content of the signal process of the 
signal processing means.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawings, and in particular to FIG. 1, there is shown 
an active noise reduction apparatus embodying the invention. The invention 
will be described in connection with an automotive vehicle 1 supported on 
front- and rear-road wheels 3 placed on a road surface 2. The automotive 
vehicle 1 has a passenger compartment (closed space) 10 into which road 
noises are transmitted from road noise sources. The active noise reduction 
apparatus includes noise generating condition detecting means in the form 
of a vibration sensor 4 positioned for sensing the road noise source 
information produced in the vicinity of the road wheel 3, a main control 
section 20 including active control means for performing adaptive controls 
based upon a reference signal x indicative of the vibrations sensed in the 
vicinity of the road wheel 3, control sound sources in the form of 
loudspeakers LS.sub.0 and LS.sub.1 placed in the vehicle passenger 
compartment 10, residual noise detecting means in the form of microphones 
MP.sub.0 and MP.sub.1, signal processing means in the form of filters 
C.sub.00 ', C.sub.10 ', C.sub.01 ' and C.sub.11 ' provided in the main 
control section 20, and identification processing section 40 of 
identifying the filters C.sub.00 ', C.sub.10 ', C.sub.01 ' and C.sub.11 '. 
The main control section 20 includes signal generating means in the form of 
adaptive digital filters W.sub.0 and W.sub.1 for generating drive signals 
y.sub.0 and y.sub.1 to drive the loudspeakers LS.sub.0 and LS.sub.1, and 
an adaptive processing section 21 for updating the filter coefficients of 
the adaptive digital filters W.sub.0 and W.sub.1 based upon the reference 
signal x produced from the vibration sensor 4 and the residual noise 
signals e.sub.0 and e.sub.1 outputted from the microphones MP.sub.0 and 
MP.sub.1. The adaptive processing section 21 is arranged to execute a 
so-called Multiple Error Filtered-X LMS algorithm. For this purpose, the 
adaptive processing section 21 includes filters C.sub.00 ', C.sub.10 ', 
C.sub.01 ' and C.sub.11 ' modeled in the form of finite impulse response 
functions on the acoustic transfer characteristics (transfer functions 
C.sub.00, C.sub.10, C.sub.01 and C.sub.11) between the loudspeakers 
LS.sub.0 and LS.sub.1 and the microphones MP.sub.0 and MP.sub.1, and 
control means in the form of filter coefficient updating sections 22A and 
22B for updating the filter coefficients of the adaptive digital filters 
W.sub.0 and W.sub.1 to minimize the noises in the vehicle passenger 
compartment 10 based upon the values r.sub.00, r.sub.10, r.sub.01 and 
r.sub.11 to which the reference signal x is processed by the filters 
C.sub.00 ', C.sub.10 ', C.sub.01 ' and C.sub.11 ' and the residual noise 
signals e.sub.0 and e.sub.1. 
Assuming now that e.sub.1 (n) is the residual noise signal detected by the 
l-th microphone (l=0, 1, . . . , L where L=1 in this embodiment), ep.sub.1 
(n) is the residual noise signal detected by the l-th microphone with no 
control sound produced from the loudspeakers, C.sub.lmj ' is the j-th 
filter coefficient (j=0, 1, 2, . . . , Ic-1 where Ic is a constant) of the 
filter C.sub.lm ' modeled in the form of a finite impulse response 
function on the transfer function C.sub.lm between the m-th loudspeaker 
(m=0, 1, . . . , M where M=1 in this embodiment) and the l-th microphone, 
x(n) is the reference signal, W.sub.mi is the i-th filter coefficient 
(i=0, 1, 2, . . . , Ik-1 where Ik is a constant) of the adaptive digital 
filter which drives the m-th loudspeaker in response to the reference 
signal x(n), the following equation is established: 
##EQU1## 
where the terms affixed to (n) indicate the values sampled at a sampling 
time n, Ic is the number of taps of the filter C.sub.lm ', and Ik is the 
number of taps of the adaptive digital filter W.sub.m. The term 
".SIGMA.W.sub.mi x(n-j-i)" on the right side of the above equation 
indicates the output y.sub.m (n) when the reference signal x(n) is 
inputted to the adaptive digital filter, the term ".SIGMA.C.sub.lmj 
'[.SIGMA.W.sub.mi x(n-j-i)]" indicates the signal arriving at the l-th 
microphone through the transfer function C.sub.lm when the m-th 
loudspeaker is driven by the drive signal y.sub.m (n) to produce a control 
sound, and the term ".SIGMA..SIGMA.C.sub.lmj '[.SIGMA.W.sub.mi x(n-j-i)]" 
indicates the sum of the control sounds arriving at the l-th microphone. 
It is now assumed that the performance function Je is given as: 
##EQU2## 
A least mean square (LMS) algorithm is used to calculate the filter 
coefficients W.sub.mi for which the performance function is at minimum. In 
more detail, the performance function Je is partially differentiated with 
respect to the filter coefficients W.sub.mi. The partially differentiated 
value is used to update the filter coefficients W.sub.mi. From Equation 
(2), 
##EQU3## 
From Equation (1), 
##EQU4## 
Replacing the right side of Equation (4) with r.sub.lm (n-i), the filter 
coefficient as updated with the weight coefficient .gamma..sub.1 is given 
as: 
##EQU5## 
where .alpha. is the convergence coefficient that takes part in the rate 
at which the filter converges in an optimum fashion and contributes to the 
stability of the optimum convergence of the filter. The filter coefficient 
updating sections 22A and 22B update the filter coefficients of the 
adaptive digital filters W.sub.0 and W.sub.1 according to Equation (5). 
The identification processing section 40 includes identification signal 
generating means in the form of an identification signal generating 
section 41 for generating an identification signal x.sub.0 used in 
producing an identification sound, background noise level detecting means 
in the form of a background noise level detecting section 42 for detecting 
the level of the background noise in the vehicle passenger compartment, 
gain adjusting means in the form of a gain adjusting section 43 for 
adjusting the gain of the identification signal x.sub.0 according to the 
result of the detection made in the background noise level detecting 
section 42 to output a gain adjusted identification signal 
a(=G.multidot.x.sub.0), variable filter coefficient filters C.sub.00 ", 
C.sub.10 ", C.sub.01 " and C.sub.11 " having the same tap length as the 
filters C.sub.00 ', C.sub.10 ', C.sub.01 ' and C.sub.11 ' set in the 
adaptive processing section 21 of the main control section 20, adaptive 
processing sections 44 and 45 for performing adaptive processing in a 
manner to bring the filters C.sub.00 ", C.sub.10 ", C.sub. 01 " and 
C.sub.11 " into conformance with the actual transfer functions C.sub.00, 
C.sub.10, C.sub.01 and C.sub.11 so as to update the filter coefficients of 
the filters C.sub.00 ", C.sub.10 ", C.sub.01 " and C.sub.11 ", and 
subtractors 46, 47, 48 and 49 for subtracting the values resulting from 
the processes of the identification signal a in the filters C.sub.00 ", 
C.sub.10 ", C.sub.01 " and C.sub.11 " from the residual noises e.sub.0 and 
e.sub.1 and applying the resulting differences to the corresponding 
adaptive processing sections 44 and 45. The identification signal a is 
also supplied to the main control section 20 where superimposing means in 
the form of adders 23 and 24 add the identification signal to the signal 
y.sub.0 and y.sub.1. The added signals are supplied to drive the 
loudspeakers LS.sub.0 and LS.sub.1. Therefore, the loudspeakers LS.sub.0 
and LS.sub.1 produce control sounds corresponding to the signals y.sub.0 
and y.sub.1 along with an identification sound corresponding to the 
identification signal a. If both of the loudspeakers LS.sub.0 and LS.sub.1 
produce identification sounds, the identification process would be made in 
order since the identification signal a is of one kind. In this 
embodiment, a switching section 50 is provided to supply the 
identification signal a to only one of the adders 23 and 24. The 
identification signal a is also supplied through the switching section 50 
to two of the filters C.sub.00 ", C.sub.10 ", C.sub.01 ", C.sub.11 " and 
to one of the adaptive processing sections 44 and 45 corresponding to the 
loudspeakers LS.sub.0 or LS.sub.1 from which the identification sound is 
produced. 
The identification signal x.sub.0 generated from the identification signal 
generating section 41 exhibits a spectral distribution similar to that of 
the road noises transmitted from the road surface 2 and the road wheel 3. 
The road noises occur when the road wheel 3 passes the uneven road surface 
2. The shape of the spectrum of the uneven road surface is substantially 
the same for general road surfaces. For this reason, it may be considered 
that the spectrum shape is dependent on the kind of the vehicle. From 
macro-standpoints, the spectrum shape exhibits a sound pressure level 
decreasing as the frequency increases, as shown in FIG. 2. Assuming now 
that the identification signal x.sub.0 produced from the identification 
signal generating section 41 exhibits such a spectral distribution that it 
is attenuated at a gradient ranging from -10 dB/octave to -15 dB/octave, 
the identification signal can be produced in a form of a signal exhibiting 
a spectral distribution similar to that of the road noises. 
The identification signal generating section 41 may be arranged as 
including an M series signal generator 41a, and a low pass filter 41b, as 
shown in FIG. 3(a). The M series signal generator 41a generates white 
noises having a spectral distribution as shown in FIG. 3(b). As can be 
seen from FIG. 3(b), the M series signal generator 14a produces an output 
signal having the same level over the entire frequency range. FIG. 3(c) 
shows the spectral distribution of the signal outputted from the low pass 
filter 41b. As can be seen from FIG. 3(c), the low pass filter 41b 
produces an identification signal x.sub.0 having a level attenuated to a 
greater degree as the frequency increases. Alternatively, the 
identification signal generating section 41 may be arranged as including a 
digital memory for storing the waveform as shown in FIG. 3(c). In this 
case, the identification signal generating section 41 produces the 
identification signal x.sub.0 based upon the waveform stored in the 
digital memory. 
The background noise level detecting section 42 may be arranged as 
including an adder 42a, a band pass filter 42b and a root mean square 
(rms) calculator 42c. The adder 42a adds the residual noises e.sub.0 and 
e.sub.1 produced from the respective microphones MP.sub.0 and MP.sub.1. 
The added signal is fed from the adder to the band pass filter 42b. The 
band pass filter 42b removes the signal component at a frequency lower or 
higher than that of the identification signal x.sub.0. This is effective 
to adjust the identification signal x.sub.0 with higher accuracy. The 
filtered signal is fed from the band pass filter 42b to the rms calculator 
42c which calculates the rms value of the filtered signal. The gain G, 
which is multiplied by the identification signal x.sub.0 in the gain 
adjusting section 43, is determined based upon the output from the rms 
calculator 42c, as shown in FIG. 4(b). The gain G is determined in such a 
manner that the sound pressure level of the identification sound is a 
predetermined value is less than the sound pressure level of the 
background noise, as shown in FIG. 5. This predetermined value is in a 
range of 5 dB to 10 dB. The adaptive processing sections 44 and 45 update 
the filter coefficients of the filters C.sub.00 ", C.sub.10 ", C.sub.01 " 
and C.sub.11 " according to the LMS algorithm substantially in the same 
manner as made in the adaptive processing section 21 of the main control 
section 20. The detailed processes are disclosed, for example, in an 
article published 1985, by B. Windrow, S. D. Stearns entitled "Adaptive 
Signal Processing", Prentice-Hall, .sctn.9. 
The operation of the active noise reduction apparatus of the invention will 
be described. The road noises produced between the road surface 2 and the 
road wheel 3 are transmitted into the vehicle passenger compartment 10. 
The reference signal x from the vibration sensor 4 is fed through an 
analog-to-digital converter (not shown) to the adaptive digital filters 
W.sub.0 and W.sub.1 and also to the filters C.sub.00 ', C.sub.10 ', 
C.sub.01 ' and C.sub.11 '. The adaptive digital filters W.sub.0 and 
W.sub.1 process the reference signal x to produce drive signal y.sub.0 and 
y.sub.1, respectively. The drive signals y.sub.0 and y.sub.1 are used to 
drive the loudspeakers LS.sub.0 and LS.sub.1 so as to produce control 
sounds in the vehicle passenger compartment 10. Just after the control is 
initiated, the filter coefficients of the adaptive digital filters W.sub.0 
and W.sub.1 would not converge to value appropriate for minimizing the 
background noises in the vehicle passenger compartment 10. The reference 
signal x is fed to the filters C.sub.00 ', C.sub.10 ', C.sub.01 ' and 
C.sub.11 ' which produce respective values r.sub.00, r.sub.10, r.sub.01 
and r.sub.11. The filter coefficient updating sections 22A and 22B receive 
the values r.sub.00, r.sub.10, r.sub.01 and r.sub.11 and also the residual 
noises e.sub.0 and e.sub.1 sensed in the vehicle passenger compartment 10 
by the microphones MP.sub.0 and MP.sub.1 and they update the filter 
coefficients of the adaptive digital filters W.sub.0 and W.sub.1 according 
to Equation (5). As a result, the filter coefficients of the adaptive 
digital filters W.sub.0 and W.sub.1 converge at a rapid rate to the 
appropriate values. Consequently, the road noises transmitted into the 
vehicle passenger compartment 10 are canceled by the control noises 
produced from the loudspeakers LS.sub.0 and LS.sub.1. 
FIG. 6 is a flow chart showing the identification process made in the 
identification processing section 40. The identification process is 
started at the point 102. At the point 104, the adder 42a of the 
background noise level detecting section 42 adds the residual noises 
e.sub.0 and e.sub.1. The added signal is filtered by the band pass filter 
42b. The rms calculator 42c calculates the rms values of the filtered 
signal. If the calculated rms value is sufficiently small, it means that 
the main control section 20 is operating in order to reduce the background 
noise to a sufficient extent and it is not required to perform the 
following process. 
At the point 106, the gain adjusting section 43 calculates a gain G based 
upon the rms value calculated at the point 104 from a relationship stored 
in a memory. This relationship specifies the gain G as a function of rms 
value, as shown in FIG. 4(b). At the point 108, the identification signal 
generating section 41 produces an identification signal x.sub.0 at the 
present time to the gain adjusting section 43. At the point 110, the gain 
adjusting section 43 adjusts the gain of the identification signal x.sub.0 
and calculates the identification signal a(=G.multidot.x.sub.0). Upon 
completion of this calculation, at the point 112, the identification 
signal a is outputted through the switching section 50 to the adder 23 or 
24. The identification signal a is superimposed on the drive signal 
y.sub.0 or y.sub.1 produced from the adaptive digital filter W.sub.0 or 
W.sub.1. The superimposed signal is used to drive the loudspeaker LS.sub.0 
or LS.sub.1. As a result, the loudspeakers LS.sub.0 and LS.sub.1 produce 
control sounds resulting from the drive signals y.sub.0 and y.sub.1 and an 
identification sound resulting from the identification signal a in the 
vehicle passenger compartment 10. The sound pressure level of the 
identification sound is adjusted to a predetermined value ranging from 5 
dB to 10 dB less than the background noise level through the gain control 
made in the gain adjusting section 43, as shown in FIG. 5. The spectral 
distribution of the identification sound is similar to that of the road 
noises which are the main components of the background noise. For this 
reason, the produced identification sound will cause such a very small 
sound pressure level increase that the passenger cannot hear it. For 
example, the sound pressure level will increase about 0.4 dB if the sound 
pressure level of the identification sound is 10 dB less than that of the 
road noises. The identification sound produced from the loudspeaker will 
cause a residual noise component superimposed on the residual noises 
e.sub.0 and e.sub.1 measured by the microphones MP.sub.0 and M.sub.1. 
Since the filter coefficient updating sections 22A and 22B receive the 
values r.sub.00, r.sub.10, r.sub.01 and r.sub.11 along with the residual 
noises e.sub.0 and e.sub.1, however, it is possible to prevent any 
deterioration of the noise control characteristics of the main control 
section 20 by performing appropriate operations according to Equation (5) 
in the filter coefficient updating sections 22A and 22B to update the 
filter coefficients of the adaptive digital filters W.sub.0 and W.sub.1 
based upon the residual noise components related to the reference signal 
x. 
At the point 114, the adaptive processing section 44 calculates a value 
.DELTA.Ci(n) by which the i-th filter coefficient of the filter C.sub.00 " 
is to be updated. When the loudspeaker LS.sub.0 is driven by the drive 
signal y.sub.0 on which the identification signal a is superimposed, it 
produces a sound including a component related to the identification 
signal a. The fact that the transfer function C.sub.00 of the vehicle 
passenger compartment 10 (physical space) agrees with the filter C.sub.00 
" means that the component related to the identification signal a agrees 
with the value a' of the identification signal processed in the filter 
C.sub.00 ", as shown in FIG. 7. The residual noise e.sub.0 includes a 
component related to the identification signal a. It is possible to 
extract this component by supplying the identification signal a to the 
adaptive processing section 44. Thus, the value .DELTA.Ci(n) by which the 
filter coefficient is to be updated can be calculated based upon the 
identification signal a and the output of the subtractor 46 from Equation 
(6) as follows: 
EQU .DELTA.Ci(n)=.beta..multidot.a(n).multidot.[e.sub.0 (n-i)-a'(n-i)](6) 
where .beta. is the convergence coefficient that takes part in the rate at 
which the filter converges in an optimum fashion and contributes to the 
stability of the optimum convergence of the filter, and n is the step 
number on the discrete time axis. It can be judged that the filter 
C.sub.00 " converges to the transfer function C.sub.00 when the value 
.DELTA.Ci(n) for any i is almost zero. At the point 116, the filter 
C.sub.00 " is updated according to the value .DELTA.Ci(n). At the point 
118, a determination is made as to whether or not the absolute value of 
the value .DELTA.Ci(n) for any i is less than a predetermined value 
.epsilon.. If the answer to this question is "yes", then it means that the 
filter C.sub.00 " has converged to the transfer function C.sub.00 and, at 
the point 120, the filter C.sub.00 ' is replaced with the filter C.sub.00 
". Otherwise, the adaptive process is advanced to the point 124. At the 
point 122, the position of the switching section 50 is changed. Following 
this, the adaptive process is advanced to the point 124 where it is 
returned to the point 104. 
Similar processes are performing for the other filters C.sub.10 ", C.sub.01 
" and C.sub.11 ". The sound pressure level of the identification sound is 
much less than that of the background noise so that the passenger cannot 
hear the identification sound. For this reason, the residual noises 
e.sub.0 and e.sub.1 contain noise components greater than the components 
required for the identification process. However, it is possible to 
perform the identification process with no trouble by elongating the 
identification time, that is, by determining the convergence of the filter 
based upon the average of the past values -.DELTA.Ci rather than the 
instant value .DELTA.Ci. Since the S/N ratio is improved according to 
n.sup.1/2, the influence of the noise component can be reduced to 1/10 
when the number of the averaged values is about 100. 
According to the invention, the identification sound used for the 
identification process has such a small sound pressure that the passenger 
cannot hear it and it causes no noise control deterioration. It is, 
therefore, possible to execute the identification process always along 
with the noise control. Since deviations between the filters C.sub.00 ', 
C.sub.10 ', C.sub.01 ' and C.sub.11 ' and the transfer functions C.sub.00, 
C.sub.10, C.sub.01 and C.sub.11 can be minimized, it is possible to 
provide good noise control even though the acoustic transfer 
characteristics changes to a great extent in a short time in the vehicle 
passenger compartment. Since the identification signal a is superimposed 
on the drive signals y.sub.0 and y.sub.1 produced from the respective 
adaptive digital filters W.sub.0 and W.sub.1 for application to the 
loudspeakers LS.sub.0 and LS.sub.1, it is possible to provide complete 
agreement between the propagation path of the identification sound and the 
propagation path of the control sound. This is effective to provide 
accurate identification process. 
The sound pressure level of the identification sound decreases as the gain 
G decreases. However, the S/N ratio is preferred to be as high as possible 
for the identification process so that a longer time is required for the 
identification process as the sound pressure level of the identification 
sound decreases. According to the invention, the background noise level is 
detected based upon the detected residual noises e.sub.0 and e.sub.1. 
Since the background noise level in the vehicle passenger compartment can 
be monitored directly, the gain G can be set at a value appropriate in 
setting the sound pressure level of the identification sound at a value 
that is a predetermined value ranging from 5 dB to 10 dB less than the 
sound pressure level of the background noise level. That is, the sound 
pressure level of the identification sound can be set at such a maximum 
possible value that the passenger cannot hear it. It is, therefore, 
possible to execute the identification process in a short time. 
The adaptive processing sections 44 and 45 of the identification processing 
section 40, the filters C.sub.00 ", C.sub.10 ", C.sub.01 " and C.sub.11 ", 
and the steps at the points 114 to 120 institute updating means. The 
adders 23 and 24, the identification signal generating section 41, the 
gain adjusting section 43, the adaptive processing sections 44 and 45, the 
subtractors 46 to 49, the switching section 50 and the loudspeakers 
LS.sub.0 and LS.sub.1 constitute the identification sound generating 
means. 
Referring to FIG. 8, there is shown a second embodiment of the active noise 
reduction apparatus of the invention. This embodiment is substantially the 
same as the first embodiment except for the arrangement of the 
identification signal generating section 41. In this embodiment, the 
identification signal generating section 41 includes a digital filter 41c, 
and a delay circuit 41d. The digital filter 41c is in the form of a finite 
impulse response type filter having a transfer characteristic (sound 
pressure during vehicle running/vibration acceleration) obtained 
experimentally. The digital filter 41c receives an input from the 
vibration sensor 4 and it performs a process corresponding to the above 
mentioned transfer characteristic. The processed signal is fed from the 
digital filter 41c to the delay circuit 41 which delays it to form the 
identification signal x.sub.0. This embodiment can eliminate the need for 
any source for generating the identification sound. Since the magnitude of 
the suspension vibration and the level of the road noise agree well with 
each other, it is possible to provide appropriate level adjustment. If 
there are provided signal detectors related to noise sources other than 
the vibration sensor 4, the outputs from these signal detectors can be 
introduced for the noise control. This is effective to provide an improved 
identification process without complicating the arrangement of the 
identification processing section 40. If the identification signal is 
taken in the form of the output of the vibration sensor 4, the control 
sounds and the identification sound produced from the loudspeakers 
LS.sub.0 and LS.sub.1 are correlated so that the noise control is 
degraded. This difficulty can be eliminated by providing the delay circuit 
in the front or rear stage of the digital filter 41c. Thus, the delay 
circuit 41c may have a delay, for example, 0.3 seconds between the time 
at which the output of the vibration sensor 4 is used for the noise 
control and the time at which the output of the vibration sensor 4 
disappears. 
Referring to FIG. 9, there is shown a modified form of the identification 
process performed in the identification processing section 40. The 
identification process is started at the point 202. At the point 204, the 
vehicle speed V(n) is read. The vehicle speed V(n) is indicated by a 
vehicle speed signal fed from vehicle speed detecting means in the form of 
a vehicle speed sensor associated with the vehicle transmission. At the 
point 206, the read vehicle speed V(n) is multiplied by a proportional 
constant k to calculate a gain G.sub.1 (n). The level of the background 
noises, which are considered to include the road noise and the wind sound, 
is directly proportional to the vehicle speed within a certain speed 
range. The proportional relationship is obtained experimentally. Since 
proportional constant k is predetermined to obtain such a gain that the 
identification sound decreases a predetermined level based upon the 
proportional relationship, as shown in FIG. 5, an appropriate gain G.sub.1 
(n) can be obtained based upon the vehicle speed V(n). Alternatively, the 
proportional constant k may be determined based upon the fact that the 
road noise increases by 6 dB when the vehicle speed doubles. 
At the point 208, the identification signal x.sub.0 is produced. At the 
following point 210, the identification signal a(n) 
(=G(n).multidot.x.sub.0 (n)) is calculated. Upon completion of this 
calculation, at the point 212, the identification signal a(n) is outputted 
through the switching section 50 to the adder 23 or 24. The identification 
signal a(n) is superimposed on the drive signal y.sub.0 or y.sub.1 
produced from the adaptive digital filter W.sub.0 or W.sub.1. The 
superimposed signal is used to drive the loudspeaker LS.sub.0 or LS.sub.1. 
As a result, the loudspeakers LS.sub.0 and LS.sub.1 produce control sounds 
resulting from the drive signals y.sub.0 and y.sub.1 and an identification 
sound resulting from the identification signal a in the vehicle passenger 
compartment 10. The sound pressure level of the identification sound is 
adjusted to a predetermined value ranging from 5 dB to 10 dB less than the 
background noise level through the gain control made in the gain adjusting 
section 43, as shown in FIG. 5. The spectral distribution of the 
identification sound is similar to that of the road noises. For this 
reason, the produced identification sound will cause such a very small 
sound pressure level increase that the passenger cannot hear it. The 
identification sound produced from the loudspeaker will cause a residual 
noise component superimposed on the residual noises e.sub.0 and e.sub.1 
measured by the microphones MP.sub.0 and MP.sub.1. Since the filter 
coefficient updating sections 22A and 22B receives the values r.sub.00, 
r.sub.10, r.sub.01 and r.sub.11 along with the residual noises e.sub.0 and 
e.sub.1, however, it is possible to prevent any deterioration of the noise 
control characteristics of the main control section 20 by making 
appropriate operations according to Equation (5) in the filter coefficient 
updating sections 22A and 22B to update the filter coefficients of the 
adaptive digital filters W.sub.0 and W.sub.1 based upon the residual noise 
components related to the reference signal x. 
At the point 214, the adaptive processing section 44 calculates a value 
.DELTA.Ci(n) by which the i-th filter coefficient of the filter C.sub.00 " 
is to be updated. When the loudspeaker LS.sub.0 is driven by the drive 
signal y.sub.0 on which the identification signal a is superimposed, it 
produces a sound including a component related to the identification 
signal a. The fact that the transfer function C.sub.00 of the vehicle 
passenger compartment 10 (physical space) agrees with the filter C.sub.00 
" means that the component related to the identification signal a agrees 
with the value a' of the identification signal processed in the filter 
C.sub.00 ", as shown in FIG. 8. The residual noise e.sub.0 includes a 
component related to the identification signal a. It is possible to 
extract this component by supplying the identification signal a to the 
adaptive processing section 44. Thus, the value .DELTA.Ci(n) by which the 
filter coefficient is to be updated can be calculated based upon the 
identification signal a and the output of the subtractor 46 from Equation 
(6). At the point 216, the filter C.sub.00 " is updated according to the 
value .DELTA.Ci(n). At the point 218, a determination is made as to 
whether or not the absolute value of the value .DELTA.Ci(n) for any i is 
less than a predetermined value .epsilon.. If the answer to this question 
is "yes", then it means that the filter C.sub.00 " has converged to the 
transfer function C.sub.00 and, at the point 220, the filter C.sub.00 ' is 
replaced with the filter C.sub.00 ". Otherwise, the adaptive process is 
advanced to the point 224. At the point 222, the position of the switching 
section 50 is changed. Following this, the adaptive process is advanced to 
the point 224 where it is returned to the point 204. 
This modification can simplify the calculations since there is no need for 
the step of calculating the rms value. 
If the background noise contains the noise transmitted from the engine into 
the vehicle passenger compartment, the gain, which is calculated at the 
point 106 of FIG. 6 or at the point 206 of FIG. 9, may be calculated with 
reference to a map stored in the identification processing section 40. 
FIG. 10 shows one example of such a map which specifies the gain as a 
function of engine speed and engine load. 
Referring to FIG. 11, there is shown a modified form of the identification 
process made in the identification processing section 40. In this 
modification, the background noise contains a number of components 
transmitted into the vehicle passenger compartment. The identification 
process is started at the point 302. At the point 304, a gain G.sub.1 (n) 
is calculated as a function of vehicle speed V(n). This calculation may be 
made as described in connection with the point 206 of FIG. 9. At the point 
306, a gain G.sub.2 (n) is calculated as a function of engine speed and 
engine load. This calculation may be made as described in connection with 
FIG. 10. At the point 308, a gain G.sub.3 (n) is calculated based on a 
determination whether the audio unit is on or off. If the audio unit is 
one, the background noise level is considered to be great. At the point 
310, the total gain G.sub.T (n) is calculated as: 
##EQU6## 
Alternatively, the total gain G.sub.T (n) may be calculated as: 
##EQU7## 
Since the total gain G.sub.T (n) would be too large with the use of 
Equation (8), it is possible to set an upper limit for each of the gains 
G.sub.k (n). The total gain G.sub.T (n) may be calculated as: 
EQU G.sub.T (n)=max[G.sub.k (n)] (9) 
where the gain G.sub.T (n) is the maximum value of each of the gains 
G.sub.k (n). In this case, the gain G.sub.T (n) does not become too large. 
At the point 312, the identification signal generating section 41 produces 
an identification signal X.sub.0 at the present time to the gain adjusting 
section 43. At the point 314, the gain adjusting section 43 adjusts the 
gain of the identification signal x.sub.0 and calculates the 
identification signal a (=G.multidot.x.sub.0). Upon completion of this 
calculation, at the point 316, the identification signal a is outputted 
through the switching section 50 to the adder 23 or 24. The identification 
signal a is superimposed on the drive signal y.sub.0 or y.sub.1 produced 
from the adaptive digital filter W.sub.0 or W.sub.1. The superimposed 
signal is used to drive the loudspeaker LS.sub.0 or LS.sub.1. As a result, 
the loudspeakers LS.sub.0 and LS.sub.1 produce control sounds resulting 
from the drive signals y.sub.0 and y.sub.1 and an identification sound 
resulting from the identification signal a in the vehicle passenger 
compartment 10. The sound pressure level of the identification sound is 
adjusted to a predetermined value ranging from 5 dB to 10 dB less than the 
background noise level through the gain control made in the gain adjusting 
section 43, as shown in FIG. 5. The spectral distribution of the 
identification sound is similar to that of the road noises which are the 
main components of the background noise. For this reason, the produced 
identification sound will cause such a very small sound pressure level 
increase that the passenger cannot hear it. The identification sound 
produced from the loudspeaker will cause a residual noise component 
superimposed on the residual noises e.sub.0 and e.sub.1 measured by the 
microphones MP.sub.0 and MP.sub.1. Since the filter coefficient updating 
sections 22A and 22B receives the values r.sub.00, r.sub.10, r.sub.01 and 
r.sub.11 along with the residual noises e.sub.0 and e.sub.1, however, it 
is possible to prevent any deterioration of the noise control 
characteristics of the main control section 20 by making appropriate 
operations according to Equation (5) in the filter coefficient updating 
sections 22A and 22B to update the filter coefficients of the adaptive 
digital filters W.sub.0 and W.sub.1 based upon the residual noise 
components related to the reference signal x. 
At the point 318, the adaptive processing section 44 calculates a value 
.DELTA.C(n) by which the filter coefficient of the filter C.sub.00 " is to 
be updated. When the loudspeaker LS.sub.0 is driven by the drive signal 
y.sub.0 on which the identification signal a is superimposed, it produces 
a sound including a component related to the identification signal a. The 
fact that the transfer function C.sub.00 of the vehicle passenger 
compartment 10 (physical space) agrees with the filter C.sub.00 " means 
that the component related to the identification signal a agrees with the 
value a' of the identification signal processed in the filter C.sub.00 ", 
as shown in FIG. 7. The residual noise e.sub.0 includes a component 
related to the identification signal a. It is possible to extract this 
component by supplying the identification signal a to the adaptive 
processing section 44. Thus, the value .DELTA.Ci(n) by which the filter 
coefficient is to be updated can be calculated based upon the 
identification signal a and the output of the subtractor 46 from Equation 
(6). It can be judged that the filter C.sub.00 " converges to the transfer 
function C.sub.00 when the value .DELTA.Ci(n) for any i is almost zero. At 
the point 320, the filter C.sub.00 " is updated according to the value 
.DELTA.Ci(n). At the point 322, a determination is made as to whether or 
not the absolute value of the value .DELTA.Ci(n) for any i is less than a 
predetermined value .epsilon.. If the answer to this question is "yes", 
then it means that the filter C.sub.00 " has converged to the transfer 
function C.sub.00 and, at the point 324, the filter C.sub.00 ' is replaced 
with the filter C.sub.00 ". Otherwise, the adaptive process is advanced to 
the point 328. At the point 326, the position of the switching section 50 
is changed. Following this, the adaptive process is advanced to the point 
328 where it is returned to the point 304. 
In this embodiment, the gain G.sub.T (n) is calculated based upon the 
background noise level derived from various factors. Since the 
identification sound level can be set at a maximum possible value. It is 
possible to simplify the calculations required for the identification 
process and reduce the time required for the identification process. When 
the gain G.sub.T (n) is calculated from Equation (7), the gain can be set 
with higher accuracy so that the identification sound level to be set at a 
greater value and the time required for the identification process can be 
reduced. When the gain G.sub.T (n) is calculated from Equation (8) or (9), 
the calculations required for the identification process can be reduced. 
Referring to FIG. 12, there is shown another modified form of the 
identification process made in the identification processing section 40. 
The identification process is started at the point 402. At the point 404, 
the adder 42a of the background noise level detecting section 42 adds the 
residual noises e.sub.0 and e.sub.1. The added signal is filtered by the 
band pass filter 42b. The rms calculator 42c calculates the rms value of 
the filtered signal. If the calculated rms value is sufficiently small, it 
means that the main control section 20 is operating in order to reduce the 
background noise to a sufficient extent and it is not required to perform 
the following process. 
At the point 406, the gain adjusting section 43 calculates a gain G based 
upon the rms value calculated at the point 404 from an relationship stored 
in a memory. This relationship specifies the gain G as a function of the 
rms value, as shown in FIG. 4(b). At the point 408, a determination is 
made as to whether or not the calculated gain G is less than a 
predetermined value G.sub.th. If the answer to this question is "yes", 
then the process is advanced to the point 426. Otherwise, at the point 
410, the identification signal generating section 41 produces an 
identification signal x.sub.0 at the present time to the gain adjusting 
section 43. At the point 412, the gain adjusting section 43 adjusts the 
gain of the identification signal x.sub.0 and calculates the 
identification signal a (=G.multidot.x.sub.0). Upon completion of this 
calculation, at the point 414, the identification signal a is outputted 
through the switching section 50 to the adder 23 or 24. The identification 
signal a is superimposed on the drive signal y.sub.0 or y.sub.1 produced 
from the adaptive digital filter W.sub.0 or W.sub.1. The superimposed 
signal is used to drive the loudspeaker LS.sub.0 or LS.sub.1. As a result, 
the loudspeakers LS.sub.0 and LS.sub.1 produce control sounds resulting 
from the drive signals y.sub.0 and y.sub.1 and an identification sound 
resulting from the identification signal a in the vehicle passenger 
compartment 10. The sound pressure level of the identification sound is 
adjusted to a predetermined value ranging from 5 dB to 10 dB less than the 
background noise level through the gain control made in the gain adjusting 
section 43, as shown in FIG. 5. The spectral distribution of the 
identification sound is similar to that of the road noises which are the 
main components of the background noise. For this reason, the produced 
identification sound will cause such a very small sound pressure level 
increase that the passenger cannot hear it. The identification sound 
produced from the loudspeaker will cause a residual noise component 
superimposed on the residual noises e.sub.0 and e.sub.1 measured by the 
microphones MP.sub.0 and MP.sub.1. Since the filter coefficient updating 
sections 22A and 22B receives the values r.sub.00, r.sub.10, r.sub.01 and 
r.sub.11 along with the residual noises e.sub.0 and e.sub.1, however, it 
is possible to prevent any deterioration of the noise control 
characteristics of the main control section 20 by making appropriate 
operations according to Equation (5) in the filter coefficient updating 
sections 22A and 22B to update the filter coefficients of the adaptive 
digital filters W.sub.0 and W.sub.1 based upon the residual noise 
components related to the reference signal x. 
At the point 416, the adaptive processing section 44 calculates a value 
.DELTA.Ci(n) by which the i-th filter coefficient of the filter C.sub.00 " 
is to be updated. When the loudspeaker LS.sub.0 is driven by the drive 
signal y.sub.0 on which the identification signal a is superimposed, it 
produces a sound including a component related to the identification 
signal a. The fact that the transfer function C.sub.00 of the vehicle 
passenger compartment 10 (physical space) agrees with the filter C.sub.00 
" means that the component related to the identification signal a agrees 
with the value a' of the identification signal processed in the filter 
C.sub.00 ", as shown in FIG. 7. The residual noise e.sub.0 includes a 
component related to the identification signal a. It is possible to 
extract this component by supplying the identification signal a to the 
adaptive processing section 44. Thus, the value .DELTA.C(n) by which the 
filter coefficient is to be updated can be calculated based upon the 
identification signal a and the output of the subtractor 46 from Equation 
(6). It can be judged that the filter C.sub.00 " converges to the transfer 
function C.sub.00 when the value .DELTA.Ci(n) is almost zero. At the point 
418, the filter C.sub.00 " is updated according to the value .DELTA.Ci(n). 
At the point 420, a determination is made as to whether or not the 
absolute value of the value .DELTA.Ci(n) for any i is less than a 
predetermined value .epsilon.. If the answer to this question is "yes", 
then it means that the filter C.sub.00 " has converged to the transfer 
function C.sub.00 and, at the point 422, the filter C.sub.00 ' is replaced 
with the filter C.sub.00 ". Otherwise, the adaptive process is advanced to 
the point 426. At the point 424, the position of the switching section 50 
is changed. Following this, the adaptive process is advanced to the point 
426 where it is returned to the point 404. 
Similar processes are made for the other filters C.sub.10 ", C.sub.01 " and 
C.sub.11 ". Since the background noise level varies in a dynamic range 
higher than 40 or 50 or dB, the A/D converter and the identification 
signal generating section connected to the microphones are required to 
have a great dynamic range. This results in an expensive apparatus. 
According to this embodiment, the identification process is inhibited when 
the gain G is small. This permits the use of components having a small 
dynamic range, resulting in an inexpensive apparatus. 
Since the noise control cannot eliminate the noises completely, the 
background noise level may be detected based upon the level of the control 
sounds produced from the loudspeakers LS.sub.0 and LS.sub.1. In this case, 
a relationship between the residual noise level and the control sound 
level is stored in a memory. The background noise level is estimated from 
this relationship. The gains of the adaptive digital filters W.sub.0 and 
W.sub.1 may be estimated from their filter coefficients. Alternatively, 
the gains may be estimated from the drive signals y.sub.0 and y.sub.1 
produced from the adaptive digital filters W.sub.0 and W.sub.1. 
Although the invention has been described in connection with reduction of 
road noses transmitted into a vehicle passenger compartment 10, it is to 
be noted that the invention is not limited in any way to this application. 
For example, the invention is applicable to reduce the noises transmitted 
from the engine into a vehicle passenger compartment. In this case, the 
reference signal is produced based upon the engine crankshaft position 
signal. Furthermore, the invention is also applicable to reduce noises 
transmitted into a closed space. 
Although the invention has been described in connection with the switching 
section 50 for selecting the component(s) to which a single identification 
signal a is fed, it is to be noted that the switching section 50 may be 
removed. In this case, different identification signals are produced for 
the respective loudspeakers LS.sub.0 and LS.sub.1. Although the invention 
has been described in connection with two microphones MP.sub.0 and 
MP.sub.1 and two loudspeakers LS.sub.0 and LS.sub.1, it is to be noted 
that the number of the microphones and the loudspeakers may be three or 
more. If the acoustic transfer characteristic changes at a small rate, the 
sound pressure level of the identification sound may be set at a value 10 
dB or more less than the sound pressure level of the background noise. 
Although the identification sound is produced based upon an identification 
signal x.sub.0 exhibiting a spectral distribution similar to the spectral 
distribution of the noises in the vehicle passenger compartment, it is to 
be noted that the white noise may be applied as the identification signal 
x.sub.0.