Abstract:
An active noise control device detects composite vibration of a vibration transmitting route to which both vibration of a rotating body caused by generation or transmission of drive force of a vehicle and vibration of a wheel generated by contact between the wheel and a road surface are transmitted. A first reference signal for defining a reference waveform of a canceling sound for canceling vibration noise in a vehicle interior is generated based on the composite vibration. The component of the canceling sound for canceling vibration noise of the rotating body is removed from the first reference signal to generate a second reference signal for defining a reference waveform of the canceling sound for canceling vibration noise of the wheel. The canceling sound is outputted based on the second reference signal.

Description:
CROSS-REFERENCED TO RELATED APPLICATION 
     This application is a National Stage entry of International Application No. PCT/JP 2009/060240, filed on Jun. 4, 2009, which claims priority to Japanese Patent Application 2008-238945, filed on Sep. 18, 2008. The disclosure of the prior application is hereby incorporated in its entirety by reference. 
     TECHNICAL FIELD 
     The present invention relates to an active noise control apparatus (device) that generates a canceling sound for a vibration noise such as a road noise to reduce the vibration noise. 
     BACKGROUND ART 
     An active noise control apparatus (referred to as an “ANC apparatus” hereinafter) is known as an apparatus for controlling sound related to vibration noise in a vehicle interior. The ANC apparatus outputs a canceling sound of a phase opposite to that of the vibration noise from a speaker inside a vehicle interior to reduce the vibration noise. An error between the vibration noise and the canceling sound is detected as a residual noise by a microphone disposed near an ear of a vehicle occupant and is used for determining the canceling sound at a later stage. The ANC apparatus reduces the vibration noise (muffled engine sound) generated in the vehicle interior in accordance with an operation (vibration) or the like of an engine installed in a vehicle, or reduces the vibration noise (road noise) generated in the vehicle interior in accordance with a contact between wheels and a road surface while the vehicle is running. Generating mechanism of the road noise is very complicated, however, a path for the road noise to reach to an ear of a vehicle occupant can be exemplarily illustrated as in  FIG. 6 . 
     In order to calculate the canceling sound for the road noise, some of the ANC apparatus that attenuate the road noise employ one or more acceleration sensors provided on a suspension for detecting the vibration of the wheel (Japanese Laid-Open Patent Publication No. 05-265471, Japanese Laid-Open Patent Publication No. 06-059688, Japanese Laid-Open Patent Publication No. 06-250672 and Japanese Laid-Open Patent Publication No. 07-028474). Such ANC apparatus apply adaptive control processing to a base signal based on detected values of the acceleration sensors and outputs the canceling sound in accordance with a control signal generated by the adaptive control processing. In the adaptive control processing, the amplitude of the base signal is adjusted so that an error between the vibration noise and the canceling sound is minimized. 
     SUMMARY OF INVENTION 
     When an acceleration sensor is provided on a suspension as disclosed in the above publications, the acceleration sensors detect not only vibrations from a wheel but also vibrations from an engine. Specifically, as shown in  FIG. 7 , a muffled engine sound is generated also by the vibration reaching a body through a knuckle, lower arm, upper arm and damper spring of the suspension. Thus, when the acceleration sensors are provided on the suspension, the values detected by the acceleration sensors contain the vibration components of the engine as well as the vibration components of the wheel. Accordingly, the adaptive control processing is performed based on the vibration components of the engine in addition to the vibration components of the wheel. Since there is no interrelationship between the vibration components of the wheel and the engine, noise-canceling performance of the road noise is lowered. 
     The present invention has been made in view of the above problem. An object of the invention is to provide an active noise control apparatus that can improve noise-canceling performance of a vibration noise. 
     An active noise control apparatus according to an aspect of the invention includes: a vibration detector that detects a composite vibration in a vibration transmission path in which both a vibration of a rotary body caused on account of generation or transmission of a drive force of a vehicle and a vibration of a wheel generated on account of a contact between the wheel and a road surface are transmitted; a first base signal generator that generates a first base signal that defines a standard waveform of a canceling sound for a vibration noise in a vehicle interior based on the composite vibration detected by the vibration detector; a second base signal generator that removes a component of the canceling sound for a vibration noise of the rotary body from the first base signal to generate a second base signal that defines a standard waveform of the canceling sound for a vibration noise of the wheel; a control signal generator that applies adaptive control processing to the second base signal for reducing an error between the vibration noise in the vehicle interior and the canceling sound to generate a control signal; a canceling sound output unit that outputs the canceling sound based on the control signal; and an error detector that detects a residual noise indicating the error between the vibration noise in the vehicle interior and the canceling sound and outputs an error signal corresponding to the residual noise. 
     According to the above aspect of the invention, the components of the canceling sound for the vibration noise of the rotary body is removed from the first base signal that is based on the composite vibration including the vibration components of the wheel and the vibration components of the rotary body, to generate the second base signal that defines the standard waveform of the canceling sound for the vibration noise of the wheel. Then, adaptive control processing for reducing the error between the vibration noise in the vehicle interior and the canceling sound is applied to the second base signal to obtain the control signal that is used to output the canceling sound. The components of the canceling sound for the vibration noise of the rotary body is removed from second base signal used in the adaptive control processing. Accordingly, a calculation for reducing the error on account of the vibration noise of the rotary body is not conducted in the adaptive control processing. Thus, the influence of the vibration components of the rotary body contained in the composite vibration can be eliminated from the arithmetic processing of the vibration components of the wheel. Accordingly, the noise-canceling performance of the active vibration control apparatus can be enhanced. 
     In the above arrangement, the second base signal generator may include: a third base signal generator that generates a third base signal that defines a standard waveform of the canceling sound for the vibration noise of the rotary body; a first adaptive filter that applies adaptive filtering using a first filter coefficient to the third base signal to output a second control signal; a subtractor that subtracts the second control signal from the first base signal to output the second base signal after removing from the first base signal the component of the canceling sound for the vibration noise of the rotary body; a delay unit that delays the second base signal; and a first filter coefficient updating unit that sequentially updates the first filter coefficient so that the second base signal is minimized. With the above arrangement, the components of the canceling sound for the vibration noise of the rotary body can be further accurately removed from the first base signal. 
     In the above arrangement, the control signal generator may include: a second adaptive filter that applies adaptive filtering to the second base signal using a second filter coefficient to output the control signal; a reference signal generator that corrects the second base signal based on a transmission characteristic from the canceling sound output unit to the error detector to generate a reference signal; and a filter coefficient updating unit that sequentially updates the second filter coefficient based on the reference signal and the error signal so that the error signal is minimized. According to the above arrangement, the error between the vibration noise in the vehicle interior and the canceling sound can be further accurately reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an illustration schematically showing a vehicle in which an active noise control apparatus according to an exemplary embodiment of the invention is installed; 
         FIG. 2  illustrates an attachment position of an acceleration sensor unit provided in the vehicle and a transmission path of vibrations from an engine and from a wheel; 
         FIG. 3  is a block diagram showing in a circuit configuration general functions of the active noise control apparatus implemented by software; 
         FIG. 4  is a flowchart for generating a canceling sound in the exemplary embodiment; 
         FIG. 5A  is a graph showing sound pressure level characteristics in a vehicle interior when the active noise control apparatus is actuated without removing muffled engine sound component, and sound pressure level characteristics in the vehicle interior when the active noise control apparatus is not actuated;  FIG. 5B  is a graph showing sound pressure level characteristics in a vehicle interior when the active noise control apparatus is actuated while removing muffled engine sound component, and sound pressure level characteristics in the vehicle interior when the active noise control apparatus is not actuated; 
         FIG. 6  illustrates a generation mechanism of a road noise; and 
         FIG. 7  illustrates a generation mechanism of a muffled engine sound. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A. Exemplary Embodiment 
     An exemplary embodiment of the invention will be described below with reference to attached drawings. 
     1. Entire and Partial Arrangements 
     (1) Entire Arrangement 
       FIG. 1  is an illustration schematically showing a vehicle  10  in which an active noise control apparatus  12  (referred to as an “ANC apparatus  12 ” hereinafter) according to the exemplary embodiment of the invention is installed. The vehicle  10  may be a gasoline vehicle, an electric vehicle, a fuel cell vehicle and the like. 
     The ANC apparatus  12  is connected to: a plurality of acceleration sensor units  16  provided on a suspension  14 ; a fuel injection control unit  18  (hereinafter referred to as an “FI ECU (Fuel Injection Electronic Control Unit)  18 ” for controlling a fuel injection of an engine E; a speaker  20 ; and a microphone  22 . An amplifier  24  is interposed between the ANC apparatus  12  and the speaker  20 . The ANC apparatus  12  generates a second composite control signal Scc 2  based on vibration accelerations Ax, Ay, Az [mm/s/s] in three orthogonal axes detected by the acceleration sensor units  16 , engine pulses Ep from the FI ECU  18  and an error signal e outputted by the microphone  22 . The second composite control signal Scc 2  is outputted to the speaker  20  after being amplified by the amplifier  24 . The speaker  20  outputs a canceling sound CS corresponding to the second composite control signal Scc 2 . 
     The vibration noise generated in the vehicle interior of the vehicle  10  is a noise (composite noise NZc) combining the vibration noise generated in accordance with the vibration of the engine E (muffled engine sound NZe) and the vibration noise (road noise NZr) generated in accordance with the vibration of the wheel  26  caused when the wheel  26  contacts a road surface R while the vehicle  10  is running. According to the ANC apparatus  12  of this exemplary embodiment, the canceling sound CS cancels the components of the road noise NZr in the composite noise NZc to obtain a noise-canceling effect. 
     Incidentally, the ANC apparatus  12  may be designed to cancel the muffled engine sound NZe in addition to the road noise NZr. In other words, the ANC apparatus  12  may be additionally provided with a typical arrangement for the muffled engine sound (such as one disclosed in Japanese Laid-Open Patent Publication No. 2004-361721). 
     Further, though not shown in  FIG. 1 , the acceleration sensor unit  16  includes four sensor units (see  FIG. 3 ), which are respectively provided corresponding to four wheels  26  (left front wheel, right front wheel, left rear wheel and right rear wheel). Further, though only one speaker  20  and one microphone  22  are illustrated in  FIGS. 1 and 3  for facilitating the understanding of the invention, a plurality of speakers  20  and a plurality of microphones  22  may be used in accordance with the usage of the ANC apparatus  12 . In this case, the number of the other components is also altered as necessary. 
     (2) Suspension  14  and Acceleration Sensor Unit  16   
     As shown in  FIG. 2 , each of the acceleration sensor units  16  is provided on a knuckle  30  of the suspension  14  connected to a wheel unit  32  of the wheel  26 . In addition to the knuckle  30 , the suspension  14  includes: an upper arm  34  connected to the knuckle  30  and a body  36  via connectors  38   a  and  38   b ; a lower arm  40  connected to the knuckle  30  and a sub frame  42  via connectors  44   a ,  44   b ; and a damper  46  connected to the body  36  via a damper spring  48  and connected to the lower arm  40  via a connector  50 . The body  36  and the sub frame  42  are connected via a connector  52 . A drive shaft  54  extending from the engine E is rotatably inserted into the knuckle  30 . The engine E and the sub frame  42  are connected via a connector  56 . 
     As shown in  FIG. 3 , each of the acceleration sensor units  16  includes three acceleration sensors  60   x ,  60   y ,  60   z  respectively for detecting vibration accelerations Ax, Ay, Az of the knuckle  30 . The vibration acceleration Ax detected by the acceleration sensor  60   x  represents the vibration acceleration [mm/s/s] of the knuckle  30  in front-back direction (X-direction in  FIG. 1 ). The vibration acceleration Ay detected by the acceleration sensor  60   y  represents the vibration acceleration [mm/s/s] of the knuckle  30  in right-left direction (Y-direction in  FIG. 2 ). The vibration acceleration Az detected by the acceleration sensor  60   z  represents the vibration acceleration [mm/s/s] of the knuckle  30  in up-down direction (Z-direction in  FIG. 1 ). 
     The respective acceleration sensor units  16  transmit the vibration accelerations Ax, Ay, Az (the signals representing the accelerations) detected on the respective knuckles  30  to the ANC apparatus  12 . 
     (3) FI ECU  18   
     The FI ECU  18  controls the fuel injection and ignition of the engine E and transmits the engine pulses Ep in accordance with the ignition to the ANC apparatus  12 . 
     (4) ANC Apparatus  12   
     (a) Entire Arrangement 
     The ANC apparatus  12  controls the output of the canceling sound CS from the speaker  20 . The ANC apparatus  12  includes a microcomputer  58 , a memory  59  ( FIG. 1 ), non-illustrated input and output circuits and the like. The microcomputer  58  is adapted to execute a function to determine the canceling sound CS (canceling sound determining function) and the like by software. 
       FIG. 3  is a block diagram showing in a circuit configuration general functions of the microcomputer  58  implemented by software. 
     As described above, the vehicle  10  has the acceleration sensor units  16  corresponding to the four wheels  26 . Each of the acceleration sensor units  16  includes the acceleration sensor  60   x  for detecting the vibration acceleration Ax, the acceleration sensor  60   y  for detecting the vibration acceleration Ay and the acceleration sensor  60   z  for detecting the vibration acceleration Az. The vibration accelerations Ax, Ay, Az detected by the acceleration sensor units  16  are outputted to the ANC apparatus  12 . Further, the engine pulses Ep from the FI FCU  18  are also outputted to the ANC apparatus  12 . 
     The ANC apparatus  12  includes: a base signal generator  62  for the muffled engine sound NZe; signal controllers  64   x ,  64   y ,  64   z  provided on each of the acceleration sensor units  16 ; a first adder  66 ; and a second adder  68 . The signal controller  64   x  is provided to each of the acceleration sensors  60   x , the signal controller  64   y  is provided to each of the acceleration sensors  60   y  and the signal controller  64   z  is provided to each of the acceleration sensors  60   z . All of the signal controllers  64   x ,  64   y ,  64   z  have similar arrangement. 
     Incidentally, an inner arrangement of the uppermost one of the plurality of signal controllers  64   x ,  64   y ,  64   z  (the signal controller  64   z ) is described in detail in  FIG. 3  and inner arrangements of the other signal controllers  64   x ,  64   y ,  64   z  are not illustrated. 
     (b) Base Signal Generator  62  for Muffled Engine Sound NZe 
     The base signal generator  62  generates a muffled engine sound base signal Sbe (sometimes referred to as “base signal Sbe” hereinafter) based on the engine pulses Ep. The engine pulses Ep are equal to the combustion cycle in the engine E. The combustion cycle is equal to a rotation period [s] of the engine E. In other words, the frequency [Hz] of the engine pulses Ep is equal to the vibration frequency of the engine E. As a result, the frequency of the engine pulses Ep interrelates to the frequency of the vibration noise (muffled engine sound NZe) originated from the engine E. 
     Since the base signal Sbe is generated in accordance with the engine pulses Ep, the frequency of the base signal Sbe is equal to respective order components (e.g. second, fourth, sixth, eighth . . . orders in case of a four-cylinder engine) of the frequency of the engine pulses Ep. In other words, the frequency [Hz] of the base signal Sbe is equal to the order components of the rotation frequency [Hz] of the engine E. Accordingly, the base signal Sbe interrelates to the frequency of the muffled engine sound NZe. 
     (c) Signal Controllers  64   x ,  64   y ,  64   z    
     Each of the signal controllers  64   x ,  64   y ,  64   z  has a base signal generator  70  for the composite noise NZc, a base signal generator  72  for the road noise NZr and a control signal generator  74 . 
     (i) Base Signal Generator  70   
     The base signal generator  70  generates a composite noise base signal Sbc (also referred to as “base signal Sbc” hereinafter) that represents a standard waveform of the canceling sound CS for the composite noise NZc based on the vibration acceleration Ax, Ay, Az detected by the acceleration sensors  60   x ,  60   y ,  60   z.    
     As shown in  FIG. 2 , the vibration (engine vibration Ve) generated in accordance with the actuation of the engine E is transmitted to the knuckle  30  in addition to the vibration (wheel vibration Vr) caused on the wheel  26  when the wheel  26  contacts the road surface R. Accordingly, the vibration accelerations Ax, Ay, Az show the acceleration [mm/s/s] of the vibration (composite vibration Vc) of the combination of the wheel vibration Vr and the engine vibration Ve. The acceleration [mm/s/s] of the wheel vibration Vr interrelates to a frequency [Hz] of the road noise NZr. The acceleration [mm/s/s] of the engine vibration Ve interrelates to a frequency [Hz] of the muffled engine sound NZe. Thus, the acceleration of the composite vibration Vc interrelates to the frequencies of the road noise NZr and the muffled engine sound NZe. Hence, the vibration accelerations Ax, Ay, Az representing the components of accelerations of the composite vibration Vc in respective axis directions interrelate to the respective frequencies of the road noise NZr and the muffled engine sound NZe. However, as described below, the road noise NZr cannot be sufficiently canceled when the composite noise NZc is to be directly canceled. 
     (ii) Base Signal Generator  72  for Road Noise NZr 
     The base signal generator  72  removes the components of the canceling sound CS for the muffled engine sound NZe from the base signal Sbc generated by the base signal generator  70  to extract the components of the canceling sound CS for the road noise NZr, and then to generate a base signal Sbr representing the standard waveform of the canceling sound CS for the road noise NZr. The base signal generator  72  includes a filter  80 , a subtractor  82 , a delay unit  84  and a filter coefficient updating unit  86 . 
     The filter  80  is a notch filter that filters the base signal Sbe from the base signal generator  62  using a filter coefficient We to output a control signal Sce that defines the waveform of the canceling sound CS for the muffled engine sound NZe. As described above, since the base signal Sbe interrelates to the frequency of the muffled engine sound NZe, the control signal Sce based on the base signal Sbe interrelates to the frequency of the muffled engine sound NZe. 
     The subtractor  82  subtracts the control signal Sce from the base signal Sbc generated by the base signal generator  70  to generate the road noise base signal Sbr (referred to as the “base signal Sbr” hereinafter) based on the difference therebetween (Sbc−Sce). Though the composite noise base signal Sbc contained the components of the road noise NZr and the muffled engine sound NZe, since the control signal Sce representing the components of the canceling sound CS for the muffled engine sound NZe has been subtracted, the road noise base signal Sbr contains only the components of the canceling sound CS for the road noise NZr. The base signal Sbr is outputted to the control signal generator  74  and the delay unit  84 . 
     After the delay unit  84  delays the base signal Sbr by one calculation cycle, the delay unit  84  outputs the base signal Sbr to the filter coefficient updating unit  86 . 
     The filter coefficient updating unit  86  sequentially calculates and updates the filter coefficient We. The filter coefficient updating unit  86  calculates the filter coefficient We using an adaptive algorithm [e.g. least square method (LMS) algorithm]. Specifically, the filter coefficient updating unit  86  calculates the filter coefficient We so that respective order components of engine speed frequency [Hz] contained in the base signal Sbr is minimized based on the muffled engine sound base signal Sbe from the base signal generator  62  and the road noise base signal Sbr from the subtractor  82 . 
     (iii) Control Signal Generator  74   
     The control signal generator  74  applies adaptive filtering to the road noise base signal Sbr to generate a control signal Scr. The control signal generator  74  includes an adaptive filter  90 , a reference signal generator  92  and a filter coefficient updating unit  94 . 
     The adaptive filter  90  is an FIR (Finite impulse response) filter that applies adaptive filtering to the base signal Sbr using the filter coefficient Wr to output the control signal Scr that represents the waveform of the canceling sound CS for reducing the road noise NZr. 
     The reference signal generator  92  applies transfer function processing to the base signal Sbr outputted by the base signal generator  72  to generate a reference signal Sr. The reference signal Sr is used for calculating the filter coefficient Wr by the filter coefficient updating unit  94 . The transfer function processing wave-filters the base signal Sbr based on the transfer function (filter coefficient) of the canceling sound CS from the speaker  20  to the microphone  22 . The transfer function used in the transfer function processing is a measurement value or a predicted value of an actual transfer function C of the canceling sound CS from the speaker  20  to the microphone  22 . 
     The filter coefficient updating unit  94  sequentially calculates and updates the filter coefficient Wr. The filter coefficient updating unit  94  calculates the filter coefficient Wr using an adaptive algorithm [e.g. least square method (LMS) algorithm]. Specifically, the filter coefficient updating unit  94  calculates the filter coefficient Wr based on the reference signal Sr from the reference signal generator  92  and an error signal e from the microphone  22  so that square (e2) of the error signal e becomes zero. 
     (d) First Adder  66   
     Respective one of the first adders  66  synthesizes the control signals Scr outputted by the three signal controllers  64   x ,  64   y ,  64   z  corresponding to the respective acceleration sensor units  16  to generate a first composite control signal Scc 1 . 
     (e) Second Adder  68   
     The second adder  68  synthesizes the first composite control signals Scc 1  outputted by the respective first adders  66  to generate the second composite control signal Scc 2 . The second composite control signal Scc 2  is outputted to the speaker  20  after being amplified by the amplifier  24 . 
     (5) Speaker  20   
     The speaker  20  outputs the canceling sound CS corresponding to the second composite control signal Scc 2  from the ANC apparatus  12  (microcomputer  58 ). Thus, the components of the road noise NZr in the composite noise NZc can be canceled. 
     (6) Microphone  22   
     The microphone  22  detects an error between the composite noise NZc containing the components of the road noise NZr and the canceling sound CS as a residual noise and outputs the error signal e representing the residual noise to the ANC apparatus  12  (microcomputer  58 ). 
     2. Generation of Canceling Sound CS 
     Next, a flow for generating the canceling sound CS in this exemplary embodiment will be described below.  FIG. 4  is a flowchart for generating the canceling sound CS. 
     In step S 1 , the acceleration sensors  60   x ,  60   y ,  60   z  of the respective acceleration sensor units  16  detect the vibration acceleration Ax in X-axis direction, the vibration acceleration Ay in Y-axis direction and the vibration acceleration Az in Z-axis direction. The vibration accelerations Ax, Ay, Az contain both the components of the vibration Vr of the wheel  26  (i.e., the components of the road noise NZr) and the components of the vibration Ve of the engine E (i.e., the components of the muffled engine sound NZe). 
     In step S 2 , the base signal generator  70  outputs the composite noise base signal Sbc based on the detected vibration accelerations Ax, Ay, Az. 
     In step S 3 , the respective base signal generators  72  output the road noise base signal Sbr corresponding to the difference between the composite noise base signal Sbc outputted by the base signal generator  70  and the control signal Sce outputted by the filter  80  (muffled engine sound components removing process). As described above, since the control signal Sce is set to be equal to the components of the canceling sound CS for the muffled engine sound NZe contained in the base signal Sbc, the base signal Sbr contains only the components of the canceling sound CS for the road noise NZr. 
     In step S 4 , the respective control signal generators  74  generate the control signal Scr by applying the adaptive filtering to the base signal Sbr based on the base signal Sbr outputted by the base signal generator  72  and the error signal e outputted by the microphone  22 . 
     In step S 5 , the first adders  66  synthesize the control signal Scr outputted from the three control signal generators  74  corresponding to the respective acceleration sensor units  16  to generate the first composite control signal Scc 1 . 
     The ANC apparatus  12  performs the above steps S 1  to S 5  for each of the four wheels  26  (i.e., for the acceleration sensor units  16 ). 
     In step S 6 , the second adder  68  synthesizes the first composite control signals Scc 1  outputted by the respective first adders  66  to generate the second composite control signal Scc 2 . In step S 7 , the amplifier  24  amplifies the second composite control signal Scc 2  at a predetermined amplification factor. In step S 8 , the speaker  20  outputs the canceling sound CS based on the amplified second composite control signal Scc 2 . 
     In step S 9 , the microphone  22  detects the difference between the composite noise NZc containing the road noise NZr and the canceling sound CS, and outputs the error signal e corresponding to the residual noise. The error signal e is used in the subsequent processing of the respective control signal generators  74 . 
     The ANC apparatus  12  repeats the above steps S 1  to S 9 . 
     3. Advantages of the Exemplary Embodiment 
     As described above, according to this exemplary embodiment, the components corresponding to the vibration Ve of the engine E (i.e. the components of the canceling sound CS for the muffled engine sound NZe) are removed from the composite vibration base signal Sbc based on the composite vibration Vc including the components of the vibration Vr of the wheel  26  and the vibration Ve of the engine E to generate the road noise base signal Sbr that defines the standard waveform of the canceling sound CS for the road noise NZr. Further, the adaptive control processing for minimizing the error (square (e2) of the error signal e) between the composite noise NZc containing the road noise NZr and the canceling sound CS is applied on the base signal Sbr to obtain the control signal Scr, which is used for outputting the canceling sound CS. The components of the canceling sound CS for the muffled engine sound NZe has been removed from the base signal Sbr used in the adaptive control processing. Accordingly, no calculation for reducing the error caused by the muffled engine sound NZe is conducted in the adaptive control processing. Thus, the influence of the components of the vibration Ve of the engine E contained in the composite vibration VC can be eliminated from the arithmetic processing of the components of the vibration Vr of the wheel  26 . Accordingly, the noise-canceling performance of the ANC apparatus  12  can be enhanced. 
       FIG. 5A  shows sound pressure level characteristics C 1  (shown in a solid line in  FIG. 5A ) of the composite noise NZc when the ANC apparatus  12  is actuated without removing the muffled engine sound in the base signal generator  72  and sound pressure level characteristics C 2  (shown in a dashed line in  FIG. 5A ) of the composite noise NZc without actuating the ANC apparatus  12 .  FIG. 5B  shows sound pressure level characteristics C 3  (shown in a solid line in  FIG. 5B ) of the composite noise NZc when the ANC apparatus  12  is actuated while removing the muffled engine sound and the same sound pressure level characteristics C 2  (shown in a dashed line in  FIG. 5B ) as in  FIG. 5A . 
     In  FIG. 5A , though the sound pressure level characteristics C 1  exhibit a certain noise-canceling effect at a peak value (around 180 Hz) of the components of the road noise NZr (150-400 Hz) as compared to the sound pressure level characteristics C 2 , no significant noise-canceling effect is obtained including the components of the muffled engine sound NZe (50-150 Hz). 
     In contrast, in  FIG. 5B , though the sound pressure level characteristics C 3  do not exhibit a noise-canceling effect for the components of the muffled engine sound NZe (50-150 Hz), the sound pressure level characteristics C 3  generally exhibit eminent noise-canceling effect over the components of the road noise NZr (150-400 Hz) as compared with the sound pressure level characteristics C 2 . 
     The base signal generator  72  includes: the base signal generator  62  for generating the muffled engine sound base signal Sbe that defines the standard waveform of the canceling sound CS for the muffled engine sound NZe; the filter  80  for applying the adaptive filtering to the base signal Sbe using the filter coefficient We for outputting the control signal Sce; the subtractor  82  that subtracts the control signal Sce from the composite noise base signal Sbc for removing the components of the canceling sound CS for the muffled engine sound NZe from the base signal Sbc to output the road noise base signal Sbr; the delay unit  84  for delaying the base signal Sbr; and the filter coefficient updating unit  86  that sequentially updates the filter coefficient We so that the respective order components of the engine speed frequency in the base signal Sbr are minimized. Thus, the components of the canceling sound CS for the muffled engine sound NZe can be further accurately removed from the composite vibration base signal Sbc. 
     In this exemplary embodiment, the respective control signal generators  74  include: the adaptive filter  90  that applies the adaptive filtering to the road noise base signal Sbr using the filter coefficient Wr for outputting the control signal Scr; the reference signal generator  92  that corrects the road noise base signal Sbr based on the transmission characteristics Ĉ to generate the reference signal Sr; and the filter coefficient updating unit  94  that sequentially updates the filter coefficient Wr based on the reference signal Sr and the error signal e so that the square e2 of the error signal is minimized. Thus, the components of the road noise NZr in the error between the canceling sound CS and the composite noise NZc containing the road noise NZr can be more accurately reduced. 
     B. Modifications of the Invention 
     It should be understood that the present invention can be embodied not only as in the above exemplary embodiment but also in various arrangement according to the disclosure of the present description. Examples of the arrangements are as follows. 
     Though the acceleration sensor unit  16  is provided on each of the four wheels  26 , the acceleration sensor unit  16  may be provided only one or some of the wheels  26 . Though the acceleration sensor units  16  respectively detect the vibration accelerations Ax, Ay, Az of the vibrations in three axis-directions (i.e. X-axis direction, Y-axis direction and Z-axis direction), the acceleration sensor unit  16  may be arranged to detect the vibration in one, two or four or more axis-directions. 
     Though the vibration accelerations Ax, Ay, Az are directly detected by the acceleration sensors  60   a ,  60   b ,  60   c  in the above exemplary embodiment, the displacement [mm] of the knuckle  30  may be detected by a displacement sensor and the vibration accelerations Ax, Ay, Az may be calculated based on the displacement. Similarly, the vibration accelerations Ax, Ay, Az may be calculated using detection values of a load sensor. 
     Though the acceleration sensor units  16  are respectively provided on the knuckle  30 , the acceleration sensor units  16  may be provided on the other part such as a hub. 
     Though the composite vibration Vc composed of the vibration Vr of the wheel  26  and the vibration Ve of the engine E is detected by the acceleration sensor unit  16  in the above exemplary embodiment, the composite vibration Vc detected by the acceleration sensor units  16  may include a vibration of a drive shaft in addition to or in place of the vibration Ve of the engine E.