Patent Publication Number: US-2022230621-A1

Title: Active noise control device and vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-007158 filed on Jan. 20, 2021, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an active noise control device and a vehicle. 
     Description of the Related Art 
     JP H05-265471 A discloses a vehicle compartment interior noise reduction device including a control block, a plurality of speakers, and a plurality of microphones. The control block generates a cancellation signal based on vibrations detected by the plurality of vibration sensors. The plurality of speakers generate cancellation vibrations based on the cancellation signals. The microphones detect cancellation errors between the cancellation vibrations and the vibrations from the vibration sources. 
     SUMMARY OF THE INVENTION 
     However, the active noise control device disclosed in JP H05-265471 A requires complicated control because it handles inputs from a plurality of vibration sensors, outputs from a plurality of speakers, and inputs from a plurality of microphones. Therefore, the design cost of such an active noise control device increases. In addition, such an active noise control device requires an expensive processor due to a large amount of computation, resulting in an increase in component cost. 
     An object of the present invention is to provide an active noise control device and a vehicle that can suitably reduce noise and realize cost reduction. 
     An active noise control device according to an aspect of the present invention causes an actuator to output a canceling sound based on a control signal in order to reduce noise in a vehicle compartment of a vehicle, and includes a first adaptive filter configured to generate the control signal by performing a filtering process on a reference signal corresponding to the noise, and a first filter coefficient updating unit configured to update a filter coefficient of the first adaptive filter, based on the reference signal and an added error signal that is acquired by adding a first error signal acquired by detecting residual noise due to interference between the noise and the canceling sound by a first microphone disposed on one side in the vehicle compartment with respect to a center line of the vehicle in a front-rear direction of the vehicle, and a second error signal acquired by detecting the residual noise by a second microphone disposed on another side in the vehicle compartment with respect to the center line of the vehicle. 
     A vehicle according to another aspect of the present invention includes the active noise control device as described above. 
     According to the present invention, it is possible to provide an active noise control device and a vehicle that can suitably reduce noise and realize cost reduction. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an outline of active noise control; 
         FIG. 2  is a graph illustrating a relationship between the frequency of noise and the magnitude of noise; 
         FIG. 3  is a plan view illustrating noise measurement points; 
         FIG. 4  is a block diagram illustrating a part of a vehicle equipped with an active noise control device according to an embodiment; 
         FIG. 5  is a plan view illustrating an example of a vehicle equipped with an active noise control device according to an embodiment; 
         FIG. 6  is a graph illustrating an example of an added error signal; and 
         FIG. 7  is a flowchart illustrating an example of operations of the active noise control device according to the present embodiment. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Preferred embodiments of an active noise control device and a vehicle according to the present invention will be described in detail below with reference to the accompanying drawings. 
     Embodiment 
     An active noise control device and a vehicle according to an embodiment will be described with reference to  FIGS. 1 to 6 .  FIG. 1  is a diagram illustrating an outline of active noise control. 
     An active noise control device  10  causes an actuator  16  to output a canceling sound for reducing noise (vibration noise) in a vehicle compartment  14  of a vehicle  12 . 
     The noise in the vehicle compartment  14  may include, for example, road noise. Road noise is noise that is transmitted to an occupant in the vehicle compartment  14  when a wheel vibrates due to force received from the road surface and the vibration of the wheel is transmitted to the vehicle body via a suspension. 
     The vehicle  12  is provided with a vibration sensor  18  that detects vibration of the vehicle  12 , which is specifically an acceleration sensor. The signal r detected by the vibration sensor  18 , i.e., a signal indicative of vibration, is supplied to the active noise control device  10 . That is, a signal indicating vibration is supplied to the active noise control device  10 . 
     A microphone  20  is further provided in the vehicle compartment  14 . The microphone  20  detects residual noise (cancellation error noise) due to interference between the noise and the canceling sound output from the actuator  16 . The residual noise detected by the microphone  20  is supplied to the active noise control device  10 . That is, an error signal e detected by the microphone  20  is supplied to the active noise control device  10 . 
     The active noise control device  10  generates a control signal u for outputting a canceling sound from the actuator  16 , based on the signal r detected by the vibration sensor  18  and the error signal e detected by the microphone  20 . More specifically, the active noise control device  10  generates the control signal u such that the error signal e detected by the microphone  20  is minimized. Since the actuator  16  outputs the canceling sound based on the control signal u that minimizes the error signal e detected by the microphone  20 , the noise in the vehicle compartment  14  can be suitably canceled out by the canceling sound. In this way, the active noise control device  10  can reduce noise transmitted to an occupant in the vehicle compartment  14 . 
     Incidentally, the distribution of noise in the vehicle compartment  14  varies depending on the frequency of the noise.  FIG. 2  is a graph illustrating a relationship between the frequency of noise and the magnitude of noise.  FIG. 3  is a plan view illustrating noise measurement points. A point A (measurement point A) is located on the right side in the vehicle compartment  14 , with respect to a center line CL of the vehicle  12  in the front-rear direction of the vehicle  12 . A point B (measurement point B) is located on the center line CL of the vehicle  12  in the front-rear direction of the vehicle  12 . A point C (measurement point C) is located on the left side in the vehicle compartment  14 , with respect to the center line CL of the vehicle  12  in the front-rear direction of the vehicle  12 . The solid line in  FIG. 2  shows the noise characteristic at the measurement point B. A dotted line in  FIG. 2  shows the noise characteristic at the measurement point A. A one-dot-dashed line in  FIG. 2  shows noise characteristic at the measurement point C. 
     As can be seen from  FIG. 2 , in the vicinity of 80 Hz and in the vicinity of 160 Hz, the magnitude of noise at measurement points A and C is significantly different from that at measurement point B. This significant difference in noise magnitude occurs because resonance at these frequencies occurs in the vehicle compartment  14 . It is conceivable to detect the noise on the left side, on the right side and in the center of the vehicle compartment  14  in order to exactly cancel the noise having different distribution of magnitude in the vehicle compartment  14 . However, in a case where the microphones  20  are provided at all of the left side, the right side, and the center of the vehicle compartment  14 , complicated control is required, which leads to an increase in design cost. In addition, when the microphones  20  are provided on all of the left side, the right side, and the center of the vehicle compartment  14 , the computation amount increases, and thus an expensive processor is required, which leads to an increase in component cost. As a result of intensive studies, the inventors of the present application have conceived the active noise control device  10  as described below. 
       FIG. 4  is a block diagram showing a part of a vehicle equipped with the active noise control device according to the present embodiment.  FIG. 5  is a plan view illustrating an example of a vehicle equipped with an active noise control device according to an embodiment. 
     As shown in  FIG. 4 , the active noise control device  10  includes a determination unit  26 , a control unit  28 , a storage unit  30 , filter units  34 A to  34 C, and a computation unit  44 . The reference character  34  is used when describing the filter unit in general. The reference characters  34 A to  34 C are used when describing the individual filter units. 
     The active noise control device  10  includes a computation device (computational processing device) (not shown). The computation device may be configured by a processor such as a CPU (Central Processing Unit), a DSP (Digital Signal Processor), or the like. However, the present invention is not limited to this feature. A DDS (Direct Digital Synthesizer), a DCO (Digitally Controlled Oscillator), or the like can be included in the computation device. In addition, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or the like can be included in the computation device. 
     As described above, the active noise control device  10  includes the storage unit  30 . The storage unit  30  may be configured by a volatile memory (not illustrated) and a nonvolatile memory (not illustrated). Examples of the volatile memory include, for example, a RAM or the like. Examples of the nonvolatile memory include, for example, a ROM, a flash memory, or the like. Data or the like may be stored, for example, in the volatile memory. Programs, tables, maps, and the like may be stored, for example, in the nonvolatile memory. 
     The determination unit  26 , the control unit  28 , the filter unit  34 , and the computation unit  44  can be realized by programs, which are stored in the storage unit  30 , being executed by the computation device. 
     As shown in  FIG. 5 , the vehicle  12  may be provided with a vibration sensor  18 . As the vibration sensor  18 , for example, an acceleration sensor may be used, but the vibration sensor  18  is not limited thereto. As the acceleration sensor, for example, a three axis acceleration sensor can be used, but the acceleration sensor is not limited thereto. Although one vibration sensor  18  is illustrated in  FIG. 5 , the number of vibration sensors  18  is not limited to one. The vibration detected by the vibration sensor  18  is supplied to the active noise control device  10  as a reference signal r. 
     As shown in  FIG. 5 , microphones  20 L and  20 R for detecting residual noise caused by interference between noise and a canceling sound are provided in the vehicle compartment  14 . That is, microphones  20 L and  20 R for detecting error signals e are provided in the vehicle compartment  14 . The reference character  20  is used when describing the microphone in general. The reference characters  20 L and  20 R are used when describing individual microphones. The microphone  20 L is provided on one side (left side) in the vehicle compartment  14  with respect to a center line CL (see  FIG. 3 ) of the vehicle  12  in the front-rear direction of the vehicle  12 . The microphone  20 R is provided on the other side (right side) in the vehicle compartment  14  with respect to the center line CL of the vehicle  12  in the front-rear direction of the vehicle  12 . 
     As shown in  FIG. 5 , actuators  16 L,  16 R, and  16 C that output canceling sounds based on the control signals u are provided in the vehicle compartment  14 . The reference numeral  16  will be used when describing the actuator in general, whereas the reference numerals  16 L,  16 R, and  16 C will be used when describing the individual actuators. The actuator  16  may be, for example, a speaker. The actuator (one-side actuator)  16 L is provided on one side (left side) in the vehicle compartment  14  with respect to the center line CL of the vehicle  12  in the front-rear direction of the vehicle  12 . The actuator (other-side actuator)  16 R is provided on the other side (right side) in the vehicle compartment  14  with respect to the center line CL of the vehicle  12  in the front-rear direction of the vehicle  12 . The actuator  16 C is provided on the center line CL of the vehicle  12  in the front-rear direction of the vehicle  12 . That is, the actuator  16 C is provided at the center in the vehicle width direction. The distance between the actuator  16 C and the microphone  20 L and the distance between the actuator  16 C and the microphone  20 R are equal to each other. 
     As shown in  FIG. 4 , the filter unit  34 A includes an adaptive filter  36 A, acoustic characteristic filters  38 A 1  and  38 A 2 , and filter coefficient updating units  40 A 1  and  40 A 2 . The filter unit  34 B includes an adaptive filter  36 B, acoustic characteristic filters  38 B 1  and  38 B 2 , and filter coefficient updating units  40 B 1  and  40 B 2 . The filter unit  34 C includes an adaptive filter  36 C, an acoustic characteristic filter  38 C, and a filter coefficient updating unit  40 C. The reference character  36  is used when describing the adaptive filter in general. The reference characters  36 A,  36 B, and  36 C are used when describing the individual adaptive filters. The reference character  38  is used when describing the acoustic characteristic filter in general. The reference characters  38 A 1 ,  38 A 2 ,  38 B 1 ,  38 B 2 , and  38 C are used when describing the individual acoustic characteristic filters. The reference character  40  is used when describing the filter coefficient updating unit in general. The reference characters  40 A 1 ,  40 A 2 ,  40 B 1 ,  40 B 2 , and  40 C are used when describing each of the filter coefficient updating units. 
     The adaptive filter (second adaptive filter)  36 A generates a control signal uL by performing a filtering process on the reference signal r. The adaptive filter (third adaptive filter)  36 B generates a control signal uR by performing a filtering process on the reference signal r. The adaptive filter (first adaptive filter)  36 C generates a control signal uC by performing a filtering process on the reference signal r. The reference character u is used when describing a control signal in general. The reference characters uL, uR, and uC are used when describing the individual control signals. As the adaptive filter  36 , for example, an FIR (Finite Impulse Response) filter or the like can be used, but the present invention is not limited to this feature. The FIR filter can generate the control signal u by performing a convolution operation on the reference signal r. 
     A filter coefficient W 0  of the adaptive filter  36 A is updated by the filter coefficient updating units  40 A 1  and  40 A 2  as described later. A filter coefficient W 1  of the adaptive filter  36 B is updated by the filter coefficient updating units  40 B 1  and  40 B 2  as described later. A filter coefficient W 2  of the adaptive filter  36 C is updated by the filter coefficient updating unit  40 C as described later. The filter coefficient W 0  of the adaptive filter  36 A, the filter coefficient W 1  of the adaptive filter  36 B, and the filter coefficient W 2  of the adaptive filter  36 C are different from each other. Therefore, the filtering process performed by the adaptive filter  36 A, the filtering process performed by the adaptive filter  36 B, and the filtering process performed by the adaptive filter  36 C are different from each other. 
     The acoustic characteristic filter  38 A 1  corrects the reference signal r by performing a filtering process on the reference signal r according to an acoustic characteristic (transfer characteristic) from the actuator  16 L to the microphone  20 L. The acoustic characteristic from the actuator  16 L to the microphone  20 L is obtained in advance. That is, a transfer characteristic Ĉ 00  from the actuator  16 L to the microphone  20 L is obtained in advance. The acoustic characteristic filter  38 A 2  corrects the reference signal r by performing a filtering process on the reference signal r according to an acoustic characteristic from the actuator  16 L to the microphone  20 R. The acoustic characteristic from the actuator  16 L to the microphone  20 R is obtained in advance. That is, a transfer characteristic Ĉ 01  from the actuator  16 L to the microphone  20 R is obtained in advance. 
     The acoustic characteristic filter  38 B 1  corrects the reference signal r by performing a filtering process on the reference signal r according to an acoustic characteristic from the actuator  16 R to the microphone  20 L. The acoustic characteristic from the actuator  16 R to the microphone  20 L is obtained in advance. That is, a transfer characteristic Ĉ 10  from the actuator  16 R to the microphone  20 L is obtained in advance. The acoustic characteristic filter  38 B 2  corrects the reference signals r by performing a filtering process on the reference signals r according to an acoustic characteristic from the actuator  16 R to the microphone  20 R. The acoustic characteristic from the actuator  16 R to the microphone  20 R is obtained in advance. That is, a transfer characteristic Ĉ 11  from the actuator  16 R to the microphone  20 R is obtained in advance. 
     The acoustic characteristic filter  38 C corrects the reference signal r by performing a filtering process on the reference signal r in accordance with an acoustic characteristic from the actuator  16 C to the microphone  20 L and an acoustic characteristic from the actuator  16 C to the microphone  20 R. That is, the acoustic characteristic filter  38 C corrects the reference signal r by performing a filtering process on the reference signal r in accordance with the acoustic characteristics from the actuator  16 C to a pair of microphones  20 L and  20 R. The acoustic characteristic from the actuator  16 C to the microphone  20 L is obtained in advance. In other words, a transfer characteristic Ĉ 20  from the actuator  16 C to the microphone  20 L is obtained in advance. The acoustic characteristic from the actuator  16 C to the microphone  20 R is obtained in advance. That is, a transfer characteristic Ĉ 21  from the actuator  16 C to the microphones  20 R is obtained in advance. The acoustic characteristics from the actuator  16 C to the pair of microphones  20 L and  20 R can be expressed by expression (1) as follows. That is, a transfer characteristic Ĉ 22  from the actuator  16 C to the pair of microphones  20 L and  20 R can be expressed by expression (1) as follows. 
         Ĉ 22= Ĉ 20+ Ĉ 21  (1)
 
     That is, the acoustic characteristic from the actuator  16 C to the pair of the microphones  20 L and  20 R can be obtained based on an added acoustic characteristic obtained by adding the acoustic characteristic from the actuator  16 C to the microphone  20 L and the acoustic characteristic from the actuator  16 C to the microphone  20 R. 
     The filter coefficient updating unit  40 A 1  updates the filter coefficient W 0  of the adaptive filter  36 A based on an error signal eL acquired by detecting the residual noise by the microphone  20 L and the reference signal r corrected by the acoustic characteristic filter  38 A 1 . More specifically, the filter coefficient updating unit  40 A 1  updates the filter coefficient W 0  of the adaptive filter  36 A such that the error signal eL acquired by detecting the residual noise by the microphone  20 L is minimized. 
     The filter coefficient updating unit  40 A 2  updates the filter coefficient W 0  of the adaptive filter  36 A based on an error signal eR acquired by detecting the residual noise by the microphone  20 R and the reference signal r corrected by the acoustic characteristic filter  38 A 2 . More specifically, the filter coefficient updating unit  40 A 2  updates the filter coefficient W 0  of the adaptive filter  36 A such that the error signal eR acquired by detecting the residual noise by the microphone  20 R is minimized. 
     The reference character e is used when describing the error signal in general, whereas the reference characters eL, eR, and eC are used when describing the individual error signals. The reference character W is used when describing the filter coefficient in general. The reference characters W 0 , W 1 , and W 2  are used when describing the individual filter coefficients. When the filter coefficient W is updated, for example, a filtered-X LMS algorithm can be used, but the present invention is not limited to this feature. 
     The filter coefficient updating unit  40 B 1  updates the filter coefficient W 1  of the adaptive filter  36 B based on the error signal eL acquired by detecting the residual noise by the microphone  20 L and the reference signal r corrected by the acoustic characteristic filter  38 B 1 . More specifically, the filter coefficient updating unit  40 B 1  updates the filter coefficient W 1  of the adaptive filter  36 B such that the error signal eL acquired by detecting the residual noise by the microphone  20 L is minimized. 
     The filter coefficient updating unit  40 B 2  updates the filter coefficient W 1  of the adaptive filter  36 B based on the error signal eR acquired by detecting the residual noise by the microphone  20 R and the reference signal r corrected by the acoustic characteristic filter  38 B 2 . More specifically, the filter coefficient updating unit  40 B 2  updates the filter coefficient W 1  of the adaptive filter  36 B such that the error signal eR acquired by detecting the residual noise by the microphone  20 R is minimized. 
     The computation unit  44  adds the error signal eL acquired by detecting the residual noise by the microphone  20 L and the error signal eR acquired by detecting the residual noise by the microphone  20 R. The computation unit (adder)  44  supplies the added error signal eC acquired by adding the error signal eL and the error signal eR, to the filter coefficient updating unit  40 C. 
       FIG. 6  is a graph illustrating an example of an added error signal. The horizontal axis in  FIG. 6  indicates frequency, and the vertical axis in  FIG. 6  indicates magnitude of the signal. A solid line in  FIG. 6  is obtained by dividing by two, an addition value that is obtained by adding the error signal eL acquired by detecting the residual noise by the microphone  20 L and the error signal eR acquired by detecting the residual noise by the microphone  20 R. That is, the solid line in  FIG. 6  is an average value of the error signal eL acquired by detecting the residual noise by the microphone  20 L and the error signal eR acquired by detecting the residual noise by the microphone  20 R. A dotted line in  FIG. 6  illustrates residual noise detected by a microphone when the microphone is disposed at the center in the vehicle width direction. As can be seen from  FIG. 6 , the difference between them is very small. Therefore, even if the added error signal eC acquired by adding the error signal eL detected by the microphone  20 L and the error signal eR detected by the microphone  20 R are used, there is no particular problem with the accuracy of noise suppression. 
     The filter coefficient updating unit  40 C updates the filter coefficient W 2  of the adaptive filter  36 C, based on the added error signal eC supplied from the computation unit  44  and the reference signal r corrected by the acoustic characteristic filter  38 C. More specifically, the filter coefficient updating unit  40 C updates the filter coefficient W 2  of the adaptive filter  36 C such that the error signal eC is minimized. 
     The control signal uL output from the adaptive filter  36 A is supplied to the actuator  16 L via a power amplifier  15 L. That is, the control signal uL output from the filter unit  34 A is supplied to the actuator  16 L via the power amplifier  15 L. The control signal uR output from the adaptive filter  36 B is supplied to the actuator  16 R via a power amplifier  15 R. That is, the control signal uR output from the filter unit  34 B is supplied to the actuator  16 R via the power amplifier  15 R. The control signal uC output from the adaptive filter  36 C is supplied to the actuator  16 C via a power amplifier  15 C. That is, the control signal uC output from the filter unit  34 B is supplied to the actuator  16 C via the power amplifier  15 C. 
     The determination unit (abnormality determination unit)  26  can determine whether or not an abnormality has occurred in either of the error signal eL acquired by the microphone  20 L or the error signal eR acquired by the microphone  20 R. As examples of such an abnormality, there may be cited a disconnection of a wire between the microphone  20  and the active noise control device  10 , failure of microphone  20 , and the like but are not limited to these examples. When the magnitude of the error signal eL is larger than or equal to a volume threshold value VTH, the magnitude of the error signal eR is smaller than the threshold value VTH, and such a state continues for time threshold value TTH or longer, the determination unit  26  performs the following determination. That is, in such a case, the determination unit  26  determines that an abnormality has occurred in the error signal eR acquired by the microphone  20 R. When the magnitude of the error signal eR is equal to or larger than a volume threshold value VTH, the magnitude of the error signal eL is smaller than the volume threshold value VTH, and such a state continues for a time threshold value TTH or longer, the determination unit  26  performs the following determination. That is, in such a case, the determination unit  26  determines that an abnormality has occurred in the error signal eL acquired by the microphones  20 L. The determination result by the determination unit  26  is supplied to the control unit  28 . 
     When the determination unit  26  determines that an abnormality occurs in one of the error signal eL or the error signal eR, the control unit  28  can perform the following control. That is, in such a case, the control unit  28  causes the filter coefficient updating unit  40 C to update the filter coefficient W 2  of the adaptive filter  36 C, based on the other of the error signals eL and eR and the reference signal r corrected by the acoustic characteristic filter  38 C. 
     Next, an example of operations of the active noise control device according to the present embodiment will be described with reference to  FIG. 7 .  FIG. 7  is a flowchart illustrating an example of operations of the active noise control device according to the present embodiment. 
     First, in step S 1 , the determination unit  26  determines whether or not an abnormality has occurred in the error signal eL. When an abnormality has occurred in the error signal eL (YES in step S 1 ), the process transitions to step S 3 . If no abnormality has occurred in the error signal eL (NO in step S 1 ), the process transitions to step S 2 . 
     In step S 2 , the determination unit  26  determines whether or not an abnormality has occurred in the error signal eR. When an abnormality has occurred in the error signal eR (YES in step S 2 ), the process transitions to step S 4 . When no abnormality has occurred in the error signal eR (NO in step S 2 ), the process illustrated in  FIG. 7  is completed. 
     In step S 3 , the control unit  28  causes the filter coefficient updating unit  40 C to update the filter coefficient W 2  of the adaptive filter  36 C, based on the reference signal r corrected by the acoustic characteristic filter  38 C and the error signal eR. 
     In step S 4 , the control unit  28  causes the filter coefficient updating unit  40 C to update the filter coefficient W 2  of the adaptive filter  36 C, based on the reference signal r corrected by the acoustic characteristic filter  38 C and the error signal eL. 
     In this manner, the process shown in  FIG. 7  is brought to an end. 
     As described above, in the present embodiment, the error signal eL detected by the microphone  20 L arranged on one side and the error signal eR detected by the microphone  20 R arranged on the other side are added to acquire the added error signal eC. The added error signal eC corresponds to the residual noise at the center in the vehicle width direction. Then, the filter coefficient W 2  of the adaptive filter  36 C is updated such that the added error signal eC acquired in this way is minimized. According to the present embodiment, it is possible to accurately reduce residual noise at the center in the vehicle width direction without separately providing a microphone at the center in the vehicle width direction. According to the present embodiment, the complexity of control can be suppressed, and thus the design cost can be suppressed. In addition, according to the present embodiment, since an increase in the amount of computation can be suppressed, an expensive processor is not required, and component cost can be suppressed. Therefore, according to the present embodiment, it is possible to provide the active noise control device  10  that is capable of suitably reducing noise and realizing cost reduction. 
     Modified Embodiment 
     Although preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made thereto without departing from the essence and gist of the present invention. 
     For example, in the above-described embodiment, although an exemplary case has been described in which the central actuator  16 C is provided, the central actuator  16 C may be dispensed with. In this case, the identical control signal may be supplied to the one-side actuator  16 L and the other-side actuator  16 R. That is, the central actuator  16 C need not necessarily be provided if a monaural sound is output by the pair of actuators  16 L and  16 R. However, from the viewpoint of more effectively reducing noise, it is preferable for the central actuator  16 C to be provided. 
     The above-described embodiments can be summarized in the following manner. 
     The active noise control device ( 10 ) causes the actuator ( 16 C) to output a canceling sound based on the control signal (uC) in order to reduce noise in the vehicle compartment ( 14 ) of the vehicle ( 12 ), and includes the first adaptive filter ( 34 C) configured to generate the control signal by performing a filtering process on the reference signal (r) corresponding to the noise, and the first filter coefficient updating unit ( 40 C) configured to update the filter coefficient (W 2 ) of the first adaptive filter, based on the reference signal and the added error signal (eC) that is acquired by adding the first error signal (eL) acquired by detecting residual noise due to interference between the noise and the canceling sound by the first microphone ( 20 L) disposed on one side in the vehicle compartment with respect to the center line (CL) of the vehicle in a front-rear direction of the vehicle, and the second error signal (eR) acquired by detecting the residual noise by the second microphone ( 20 R) disposed on another side in the vehicle compartment with respect to the center line of the vehicle. According to such a configuration, by adding the first error signal detected by the first microphone disposed on one side and the second error signal detected by the second microphone disposed on the other side, the added error signal is acquired, corresponding to the residual noise at the center in the vehicle width direction. Then, the filter coefficient of the first adaptive filter is updated so as to minimize the added error signal acquired in such a way. According to such a configuration, the residual noise at the center in the vehicle width direction can be accurately reduced without separately providing a microphone at the center in the vehicle width direction. According to such a configuration, the complexity of control can be suppressed, and thus the design cost can be suppressed. In addition, according to such a configuration, since an increase in the amount of computation can be suppressed, an expensive processor is not required, and the component cost can be suppressed. Therefore, according to such a configuration, it is possible to provide an active noise control device that is capable of suitably reducing noise and realizing cost reduction. 
     The active noise control device may further include the first acoustic characteristic filter ( 38 C) configured to correct the reference signal by performing a filtering process on the reference signal according to the acoustic characteristic (Ĉ 20 ) from the actuator to the first microphone and an acoustic characteristic (Ĉ 21 ) from the actuator to the second microphone, wherein the first filter coefficient updating unit may be configured to update the filter coefficient of the first adaptive filter, based on the added error signal and the reference signal corrected by the first acoustic characteristic filter. According to such a configuration, since the filtering process according to the acoustic characteristics from the actuator to the pair of microphones is performed by the first acoustic characteristic filter, it is possible to effectively suppress noise while suppressing the amount of computation. 
     The active noise control device may further include the determination unit ( 26 ) configured to determine whether an abnormality has occurred in either of the first error signal or the second error signal, and the control unit ( 28 ) configured to cause the first filter coefficient updating unit to update the filter coefficient of the first adaptive filter, when the determination unit determines that the abnormality has occurred in one of the first error signal or the second error signal, the filter coefficient of the first adaptive filter being updated based on the reference signal corrected by the first acoustic characteristic filter and another of the first error signal or the second error signal. According to such a configuration, even when an abnormality occurs in either one of the pair of microphones, it is possible to suitably reduce noise while suppressing an adverse effect of the microphone in which the abnormality occurs. 
     The actuator may be provided at the center in the vehicle width direction. According to such a configuration, it is possible to provide an active noise control device that is capable of suitably reducing noise in a vehicle compartment. 
     The active noise control device may further include the second adaptive filter ( 36 A) configured to perform a filtering process that is different from the filtering process performed by the first adaptive filter, on the reference signal to generate the control signal (uL) to be supplied to the one-side actuator ( 16 L) disposed on the one side in the vehicle compartment with respect to the center line of the vehicle, the second acoustic characteristic filter ( 38 A 1 ) configured to correct the reference signal by performing a filtering process on the reference signal according to the acoustic characteristic (Ĉ 00 ) from the one-side actuator to the first microphone, the second filter coefficient updating unit ( 40 A 1 ) configured to update the filter coefficient (W 0 ) of the second adaptive filter, based on the first error signal and the reference signal corrected by the second acoustic characteristic filter, the third acoustic characteristic filter ( 38 A 2 ) configured to correct the reference signal by performing a filtering process on the reference signal according to the acoustic characteristic (Ĉ 01 ) from the one-side actuator to the second microphone, the third filter coefficient updating unit ( 40 A 2 ) configured to update the filter coefficient of the second adaptive filter, based on the second error signal and the reference signal corrected by the third acoustic characteristic filter, the third adaptive filter ( 36 B) configured to perform a filtering process different from both the filtering process performed by the first adaptive filter and the filtering process performed by the second adaptive filter, on the reference signal to generate the control signal (uR) to be supplied to the other-side actuator ( 16 R) disposed on the another side in the vehicle compartment with respect to the center line of the vehicle, the fourth acoustic characteristic filter ( 38 B 1 ) configured to correct the reference signal by performing a filtering process on the reference signal according to the acoustic characteristic (Ĉ 10 ) from the other-side actuator to the first microphone, the fourth filter coefficient updating unit ( 40 B 1 ) configured to update a filter coefficient (W 1 ) of the third adaptive filter based on the first error signal and the reference signal corrected by the fourth acoustic characteristic filter, the fifth acoustic characteristic filter ( 38 B 2 ) configured to correct the reference signal by performing a filtering process on the reference signal according to the acoustic characteristic (Ĉ 11 ) from the other-side actuator to the second microphone, and the fifth filter coefficient updating unit ( 40 B 2 ) configured to update the filter coefficient of the third adaptive filter based on the second error signal and the reference signal corrected by the fifth acoustic characteristic filter. According to such a configuration, it is possible to provide an active noise control device that is capable of suitably reducing noise. 
     The actuator may include the one-side actuator disposed on the one side in the vehicle compartment with respect to the center line of the vehicle and the other-side actuator disposed on the another side in the vehicle compartment with respect to the center line of the vehicle, and the identical control signal may be supplied to the one-side actuator and the other-side actuator. According to such a configuration, since it is not necessary to provide an actuator at the center in the vehicle width direction, it is possible to contribute to cost reduction. 
     The vehicle includes the active noise control device as described above.