Patent Publication Number: US-11043202-B2

Title: Active noise control system, setting method of active noise control system, and audio system

Description:
RELATED APPLICATIONS 
     The present application claims priority to Japanese Patent Appln. No. 2018-243647, filed Dec. 26, 2018, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure relates to active noise control (ANC) technology that reduces noise by emitting noise-canceling sound to cancel out noise. 
     Description of the Related Art 
     One known technology for active noise control that reduces noise by emitting noise-canceling sound to cancel out noise is provided with a microphone disposed near a noise cancellation position, a speaker disposed near the noise cancellation position, and an adaptive filter that performs a transfer function set to a noise signal that expresses noise and generates noise-canceling sound to be output from the speaker. In the adaptive filter, the transfer function is set adaptively by using a signal obtained by correcting the output of the microphone using an auxiliary filter as an error signal (for example, JP 2018-72770 A). 
     With this technology, a transfer function learned in advance that corrects a difference between the transfer function from the noise source to the noise cancellation position and the transfer function from the noise source to the output of the microphone, and a difference between the transfer function from the speaker to the noise cancellation position and the transfer function from the speaker to the output of the microphone, is set in the auxiliary filter. By using such an auxiliary filter, it is possible to cancel noise at a noise cancellation position that is different from a position of the microphone. 
     Another known technology is provided with sets of a microphone, a speaker, an adaptive filter, and an auxiliary filter corresponding to each of a plurality of noise cancellation positions. By using the technology described above to output noise-canceling sound that cancels noise at the corresponding noise cancellation position in each set, noise is canceled at each of the plurality of noise cancellation positions (JP 2018-72770 A). 
     The technologies described above anticipate only the case of a single noise source. In cases where a plurality of noise sources exists, the noise from each noise source cannot be canceled appropriately at each noise cancellation position. 
     SUMMARY 
     The present disclosure deals with a case where a plurality of noise sources exists, and addresses the issue of canceling noise from each noise source appropriately at each of a plurality of noise cancellation positions. 
     In order to address the issues described above, the present disclosure provides an active noise control system that reduces noise. In one form, an active noise control system includes: n (where n≥2) subsystems respectively provided in correspondence with each of n noise cancellation positions, wherein each subsystem includes a microphone and a speaker disposed near the corresponding noise cancellation position, a canceling sound-generating adder, an error-computing adder, m (where m≥2) adaptive filters, respectively provided in correspondence with each of m noises, that accept the corresponding noise as input, and m auxiliary filters, respectively provided in correspondence with each of the m noises, that accept the corresponding noise as input. Here, the canceling sound-generating adder of each subsystem adds together outputs from the m adaptive filters of the subsystem, and outputs a result to the speaker of the subsystem, the error-computing adder of each subsystem adds together and outputs an output from the microphone of the subsystem and outputs from the m auxiliary filters of the subsystem, and an adaptive filter of each subsystem updates a transfer function of the adaptive filter by executing a predetermined adaptive algorithm that treats the output from the error-computing adder of each subsystem as an error. Then, a transfer function is set in each auxiliary filter such that each error computed by the error-computing adder of each subsystem becomes zero (0) when a transfer function in which each noise is canceled at each cancellation position in a predetermined standard acoustic environment is set in each adaptive filter. 
     Further, in order to address the issues described above, the present disclosure provides an active noise control system that reduces noise, including: two subsystems respectively provided in correspondence with each of two noise cancellation positions, wherein each subsystem includes a microphone and a speaker disposed near the noise corresponding cancellation position, a canceling sound-generating adder, an error-computing adder, two adaptive filters, respectively provided in correspondence with each of two noises, that accept the corresponding noise as input, and two auxiliary filters, respectively provided in correspondence with each of the two noises, that accept the corresponding noise as input. Here, the canceling sound-generating adder of each subsystem adds together outputs from the two adaptive filters of the subsystem, and outputs a result to the speaker of the subsystem, the error-computing adder of each subsystem adds together and outputs an output from the microphone of the subsystem and outputs from the two auxiliary filters of the subsystem, and an adaptive filter of each subsystem updates a transfer function of the adaptive filter by executing a predetermined adaptive algorithm that treats the output from the error-computing adder of each subsystem as an error. Provided that P jk  is the transfer function of the jth noise to the output from the microphone of the kth subsystem, S Pjk  is the transfer function from the speaker of the jth subsystem to the output from the microphone of the kth subsystem, V jk  is the transfer function of the jth noise to the kth cancellation position, S Vjk  is the transfer function from the speaker of the jth subsystem to the kth cancellation position, and H jk  is the transfer function of the auxiliary filter corresponding to the jth noise of the kth subsystem,
 
 H   11 ( z )=−[ P   11 ( z )+{ V   12 ( z ) S   V21 ( z )− V   11 ( z ) S   V22 ( z )} S   P11 ( z )+{ V   11 ( z ) S   V12 ( z )− V   12 ( z )S V11 ( z )} S   P21 ( z )]/[ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )]
 
 H   12 ( z )=−[ P   12 ( z )+{ V   12 ( z ) S   V21 ( z )− V   11 ( z ) S   V22 ( z )} S   P12 ( z )+{ V   11 ( z ) S   V12 ( z )− V   12 ( z )S V11 ( z )} S   P22 ( z )]/[ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )]
 
 H   21 ( z )=−[ P   21 ( x )+{ V   22 ( z ) S   V21 ( z )− V   21 ( z ) S   V22 ( z )} S   P11 ( z )+{ V   21 ( z ) S   V12 ( z )− V   22 ( z )S V11 ( z )} S   P21 ( z )]/[ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )]
 
 H   22 ( z )=−[ P   22 ( x )+{ V   22 ( z ) S   V21 ( z )− V   21 ( z ) S   V22 ( z )} S   P12 ( z )+{ V   21 ( z ) S   V12 ( z )− V   22 ( z )S V11 ( z )} S   P22 ( z )]/[ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )].
 
     Further, in order to achieve the issues described above, the present disclosure provides a setting method of an active noise control system that reduces noise. Here, the active noise control system includes two subsystems respectively provided in correspondence with each of two noise cancellation positions, in which each subsystem includes a microphone and a speaker disposed near the corresponding noise cancellation position, a canceling sound-generating adder, an error-computing adder, two adaptive filters, respectively provided in correspondence with each of two noises, that accept the corresponding noise as input, and two auxiliary filters, respectively provided in correspondence with each of the two noises, that accept the corresponding noise as input. Further, the canceling sound-generating adder of each subsystem adds together outputs from the two adaptive filters of the subsystem, and outputs a result to the speaker of the subsystem, the error-computing adder of each subsystem adds together and outputs an output from the microphone of the subsystem and outputs from the two auxiliary filters of the subsystem, and an adaptive filter of each subsystem updates a transfer function of the adaptive filter by executing a predetermined adaptive algorithm that treats the output from the error-computing adder of each subsystem as an error. 
     One form of a setting method is a method of setting the transfer function of each auxiliary filter, including: executing a first step of learning the transfer function of each adaptive filter that converges in a configuration obtained by respectively disposing two setting microphones at each of two noise cancellation positions, and changing a configuration of the active noise control system such that each adaptive filter executes a predetermined adaptive algorithm treating an output from each setting microphone as error to update the transfer function of the adaptive filter, and executing a second step of learning the transfer function of each adaptive filter replacing each auxiliary filter as the transfer function to set in the auxiliary filter replaced by the adaptive filter that converges in a configuration of the active noise control system obtained by fixing the transfer function of each adaptive filter to the transfer function learned in the first step and replacing each auxiliary filter with an adaptive filter that treats the output from the error-computing adder of the same subsystem as the subsystem of the auxiliary filter as error to execute a predetermined adaptive algorithm and update the transfer function of the adaptive filter. 
     According to forms of the active noise control system and the setting method of the active noise control system as above, a transfer function is set in each auxiliary filter such that each error computed by the error-computing adder in each subsystem becomes zero (0) when a transfer function in which each noise is canceled at each cancellation position in a predetermined standard acoustic environment is set in each adaptive filter. Consequently, even in the case where a plurality of noises exists, in the standard state, noise from each noise source may be canceled appropriately at each of the plurality of noise cancellation positions, while in addition, even in the case where a variation from the standard acoustic environment occurs in the acoustic environment, each noise may be canceled appropriately at each of the plurality of noise cancellation positions by the adaptive operation of the adaptive filters. 
     Here, the present disclosure also provides an audio system onboard an automobile provided with the active noise control system described above, including: an audio device for a user seated in a first seat of the automobile, that emits audio inside the automobile. Here, in this audio system, the two noises may be left-channel audio and right-channel audio emitted by the audio device, and the two noise cancellation positions may be a position of a left ear and a position of a right ear of a user seated in a second seat of the automobile. 
     As above, according to the present disclosure, even in the case where a plurality of noise sources exists, it is possible to cancel noise from each noise source appropriately at each of a plurality of noise cancellation positions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating one form of a configuration of an active noise control system; 
         FIGS. 2A, 2B, and 2C  are diagrams illustrating an application example of the active noise control system; 
         FIG. 3  is a block diagram illustrating one form of a configuration of a signal processing block; 
         FIG. 4  is a block diagram illustrating one form of a configuration of a first learning block; 
         FIGS. 5A and 5B  are diagrams illustrating an example of the placement of a dummy microphone; and 
         FIG. 6  is a block diagram illustrating one form of a configuration of a second learning block. 
     
    
    
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates one form of a configuration of the active noise control system. 
     As illustrated in the diagram, an active noise control system  1  is provided with a signal processing block  11 , a first microphone  12 , a first speaker  13 , a second microphone  14 , and a second speaker  15 . 
     The active noise control system  1  is a system that cancels noise produced by a first noise source  21  and noise produced by a second noise source  22  at each of two points, namely a first cancellation point and a second cancellation point. 
     The first microphone  12  and the first speaker  13  are disposed near the first cancellation point, while the second microphone  14  and the second speaker  15  are disposed near the second cancellation point. 
     Additionally, the signal processing block  11  uses a first noise signal x 1 (n) expressing noise produced by the first noise source  21 , a second noise signal x 2 (n) expressing noise produced by the second noise source  22 , a first microphone error signal err p1 (n), which is a sound signal picked up by the first microphone  12 , and a second microphone error signal err p2 (n), which is a sound signal picked up by the second microphone  14 , to generate and output from the first speaker  13  a first canceling signal CA 1 (n) that cancels the noise produced by the first noise source  21  and the noise produced by the second noise source  22  at the first cancellation point, and to generate and output from the second speaker  15  a second canceling signal CA 2 (n) that cancels the noise produced by the first noise source  21  and the noise produced by the second noise source  22  at the second cancellation point. 
     Herein, such an active noise control system  1  may be applied to an audio system installed in an automobile, for example. 
     In other words, for example, as illustrated in  FIG. 2A , for an in-vehicle audio system  3  provided with a left rear speaker  31  disposed on the left side of the rear seats of an automobile, a right rear speaker  32  disposed on the right side of the rear seats of the automobile, and an audio source  33  that outputs audio content for users in the rear seats from the left rear speaker  31  and the right rear speaker  32 , the active noise control system  1  may applied by treating a left-channel audio signal output to the left rear speaker  31  by the audio source  33  as the first noise signal x 1 (n), treating a right-channel audio signal output to the right rear speaker  32  by the audio source  33  as the second noise signal x 2 (n), treating the position of the left ear of the user sitting in the driver&#39;s seat as the first cancellation point, and treating the position of the right ear of the user sitting in the driver&#39;s seat as the second cancellation point. In this way, the sound of the audio content for users in the rear seats output by the audio system  3  may be canceled for the user sitting in the driver&#39;s seat. 
     Note that in this case, the audio source  33  corresponds to the first noise source  21  and the second noise source  22 . 
     Also, in this case, as illustrated in  FIGS. 2B and 2C , the first microphone  12  and the first speaker  13  are disposed at positions in the headrest of the driver&#39;s seat near the position of the left ear of the user sitting in the driver&#39;s seat, while the second microphone  14  and the second speaker  15  are disposed at positions in the headrest of the driver&#39;s seat near the position of the right ear of the user sitting in the driver&#39;s seat. 
     Next,  FIG. 3  illustrates a configuration of the signal processing block  11  of the active noise control system  1 . 
     Note that the active noise control system  1  is divided into Sections 1 and 2, in which Section 1 is a subsystem that mainly performs processing related to the first cancellation point and Section 2 is a subsystem that mainly performs processing related to the second cancellation point. The first microphone  12 , the first speaker  13 , and regions of the signal processing block  11  labeled “Section 1” hereinafter form Section 1, while the second microphone  14 , the second speaker  15 , and regions of the signal processing block  11  labeled “Section 2” hereinafter form Section 2. 
     Additionally, as illustrated in the diagram, the signal processing block  11  is provided with a Section 1 first auxiliary filter  1111  in which a transfer function H 11 (z) is preset, a Section 2 first auxiliary filter  1112  in which a transfer function H 12 (z) is preset, a Section 1 first variable filter  1113 , a Section 1 first adaptive algorithm execution unit  1114 , a Section 2 first variable filter  1115 , a Section 2 first adaptive algorithm execution unit  1116 , a Section 1 error-correcting adder  1117 , and a Section 1 canceling sound-generating adder  1118 . 
     The Section 1 first variable filter  1113  and the Section 1 first adaptive algorithm execution unit  1114  form an adaptive filter, in which the Section 1 first adaptive algorithm execution unit  1114  updates a transfer function W 11 (z) of the Section 1 first variable filter  1113  according to a multiple error filtered X least mean squares (MEFX LMS) algorithm. Also, the Section 2 first variable filter  1115  and the Section 2 first adaptive algorithm execution unit  1116  form an adaptive filter, in which the Section 2 first adaptive algorithm execution unit  1116  updates a transfer function W 12 (z) of the Section 2 first variable filter  1115  according to a MEFX LMS algorithm. 
     In addition, the signal processing block  11  is provided with a Section 1 second auxiliary filter  1121  in which a transfer function H 21 (z) is preset, a Section 2 second auxiliary filter  1122  in which a transfer function H 22 (z) is preset, a Section 1 second variable filter  1123 , a Section 1 second adaptive algorithm execution unit  1124 , a Section 2 second variable filter  1125 , a Section 2 second adaptive algorithm execution unit  1126 , a Section 2 error-correcting adder  1127 , and a Section 2 canceling sound-generating adder  1128 . 
     Then, the Section 1 second variable filter  1123  and the Section 1 second adaptive algorithm execution unit  1124  form an adaptive filter, in which the Section 1 second adaptive algorithm execution unit  1124  updates a transfer function W 21 (z) of the Section 1 second variable filter  1123  according to a MEFX LMS algorithm. Also, the Section 2 second variable filter  1125  and the Section 2 second adaptive algorithm execution unit  1126  form an adaptive filter, in which the Section 2 second adaptive algorithm execution unit  1126  updates a transfer function W 22 (z) of the Section 2 second variable filter  1125  according to a MEFX LMS algorithm. 
     In such a configuration, the first noise signal x 1 (n) input into the active noise control system  1  is sent to the Section 1 first auxiliary filter  1111 , the Section 2 first auxiliary filter  1112 , the Section 1 first variable filter  1113 , and the Section 2 first variable filter  1115 . 
     Also, the first microphone error signal err p1 (n) input from the first microphone  12  is sent to the Section 1 error-correcting adder  1117 , while the second microphone error signal err p2 (n) is sent to the Section 2 error-correcting adder  1127 . 
     Additionally, the output of the Section 1 first auxiliary filter  1111  is sent to the Section 1 error-correcting adder  1117 , the output of the Section 2 first auxiliary filter  1112  is sent to the Section 2 error-correcting adder  1127 , the output of the Section 1 first variable filter  1113  is sent to the Section 1 canceling sound-generating adder  1118 , and the output of the Section 2 first variable filter  1115  is sent to the Section 2 canceling sound-generating adder  1128 . 
     In addition, the first noise signal x 1 (n) input into the active noise control system  1  is sent to the Section 1 second auxiliary filter  1121 , the Section 2 second auxiliary filter  1122 , the Section 1 second variable filter  1123 , and the Section 2 second variable filter  1125 . 
     Additionally, the output of the Section 1 second auxiliary filter  1121  is sent to the Section 1 error-correcting adder  1117 , the output of the Section 2 second auxiliary filter  1122  is sent to the Section 2 error-correcting adder  1127 , the output of the Section 1 second variable filter 1123 is sent to the Section 1 canceling sound-generating adder  1118 , and the output of the Section 2 second variable filter  1125  is sent to the Section 2 canceling sound-generating adder  1128 . 
     The Section 1 error-correcting adder  1117  adds together the output of the Section 1 first auxiliary filter  1111 , the output of the Section 1 second auxiliary filter  1121 , and the first microphone error signal err p1 (n) to generate a first error signal err h1 (n), while the Section 2 error-correcting adder  1127  adds together the output of the Section 2 first auxiliary filter  1112 , the output of the Section 2 second auxiliary filter  1122 , and the second microphone error signal err p2 (n) to generate a second error signal err h2 (n). Subsequently, the first error signal err h1 (n) and the second error signal err h2 (n) are output as multi-error to the Section 1 first adaptive algorithm execution unit  1114 , the Section 2 first adaptive algorithm execution unit  1116 , the Section 1 second adaptive algorithm execution unit  1124 , and the Section 2 second adaptive algorithm execution unit  1126 . 
     Also, the Section 1 canceling sound-generating adder  1118  adds together the output of the Section 1 first variable filter  1113  and the output of the Section 1 second variable filter  1123  to generate the first canceling signal CA 1 (n) to be output from the first speaker  13 , while the Section 2 canceling sound-generating adder  1128  adds together the output of the Section 2 first variable filter  1115  and the Section 2 second variable filter  1125  to generate the second canceling signal CA 2 (n) to be output from the second speaker  15 . 
     Additionally, the Section 1 first adaptive algorithm execution unit  1114  updates the transfer function W 11 (z) of the Section 1 first variable filter  1113  according to a MEFX LMS algorithm such that the first error signal err h1 (n) and the second error signal err h2 (n) input as the multi-error become 0. The Section 2 first adaptive algorithm execution unit  1116  updates the transfer function W 12 (z) of the Section 2 first variable filter  1115  according to a MEFX LMS algorithm such that the first error signal err h1 (n) and the second error signal err h2 (n) input as the multi-error become 0. The Section 1 second adaptive algorithm execution unit  1124  updates the transfer function W 21 (z) of the Section 1 second variable filter  1123  according to a MEFX LMS algorithm such that the first error signal err h1 (n) and the second error signal err h2 (n) input as the multi-error become 0. The Section 2 second adaptive algorithm execution unit  1126  updates the transfer function W 22 (z) of the Section 2 second variable filter  1125  according to a MEFX LMS algorithm such that the first error signal err h1 (n) and the second error signal err h2 (n) input as the multi-error become 0. 
     Next, in the active noise control system  1  as above, the transfer function H 11 (z) of the Section 1 first auxiliary filter  1111 , the transfer function H 12 (z) of the Section 2 first auxiliary filter  1112 , the transfer function H 21 (z) of the Section 1 second auxiliary filter  1121 , and the transfer function H 22 (z) of the Section 2 second auxiliary filter  1122  of the signal processing block  11  are preset by a learning process indicated below. 
     The learning process is performed in a standard acoustic environment, which is a normal acoustic environment to which the active noise control system  1  is applied. 
     Also, the learning process includes a first-stage learning process and a second-stage learning process. 
     As illustrated in  FIG. 4 , the first-stage learning process is performed in a configuration in which the signal processing block  11  of the active noise control system  1  has been replaced with a first learning block  40 . Herein, as illustrated in  FIG. 4 , the first learning block  40  is provided with a configuration in which the Section 1 first auxiliary filter  1111 , the Section 2 first auxiliary filter  1112 , the Section 1 second auxiliary filter  1121 , the Section 2 second auxiliary filter  1122 , the Section 1 error-correcting adder  1117 , and the Section 2 error-correcting adder  1127  have been removed from the signal processing block  11  illustrated in  FIG. 3 . 
     Also, the first-stage learning process is performed by connecting a first dummy microphone  41  disposed at the first cancellation point and a second dummy microphone  42  disposed at the second cancellation point to a first learning block  40 . 
     Also, in the first learning block  40 , a sound signal err v1 (n) output by the first dummy microphone  41  and a sound signal err v2 (n) output by the second dummy microphone  42  are configured to be used as the multi-error of the Section 1 first adaptive algorithm execution unit  1114 , the Section 2 first adaptive algorithm execution unit  1116 , the Section 1 second adaptive algorithm execution unit  1124 , and the Section 2 second adaptive algorithm execution unit  1126 . 
     Note that in such a first learning block  40 , the Section 1 first adaptive algorithm execution unit  1114  updates the transfer function W 11 (z) of the Section 1 first variable filter  1113  according to a MEFX LMS algorithm such that err v1 (n) and err v2 (n) input as the multi-error become 0. The Section 2 first adaptive algorithm execution unit  1116  updates the transfer function W 12 (z) of the Section 2 first variable filter  1115  according to a MEFX LMS algorithm such that err v1 (n) and err v2 (n) input as the multi-error become 0. The Section 1 second adaptive algorithm execution unit  1124  updates the transfer function W 12 (z) of the Section 1 second variable filter  1123  according to a MEFX LMS algorithm such that err v1 (n) and err v2 (n) input as the multi-error become 0. The Section 2 second adaptive algorithm execution unit  1126  updates the transfer function W 22 (z) of the Section 2 second variable filter  1125  according to a MEFX LMS algorithm such that err v1 (n) and err v2 (n) input as the multi-error become 0. 
     Herein, in the case of applying the active noise control system  1  to the in-vehicle audio system  3  as illustrated in  FIGS. 2A to 2C , the placement of the first dummy microphone  41  at the first cancellation point and the placement of the second dummy microphone  42  at the second cancellation point are achieved by, for example, disposing the first dummy microphone  41  at the position of the left ear of a dummy  figure 51  seated in the driver&#39;s seat and disposing the second dummy microphone  42  at the position of the right ear of the dummy  figure 51  seated in the driver&#39;s seat, as illustrated in  FIGS. 5A and 5B . 
     Next, in the first-stage learning process using such a first learning block  40 , the first noise signal x 1 (n) and the second noise signal x 2 (n) are input into the first learning block  40 , and if the transfer function W 11 (z) of the Section 1 first variable filter  1113 , the transfer function W 12 (z) of the Section 2 first variable filter  1115 , the transfer function W 21 (z) of the Section 1 second variable filter  1123 , and the transfer function W 22 (z) of the Section 2 second variable filter  1125  have convergence and converge, each of the transfer functions W 11 (z), W 12 (z), W 21 (z), and W 22 (z) is acquired. 
     Herein, as illustrated in  FIG. 4 , provided that V 11 (z) is a transfer function of the first noise signal x 1 (n) to the output of the first dummy microphone  41 , V 12 (z) is a transfer function of the first noise signal x 1 (n) to the output of the second dummy microphone  42 , V 21 (z) is a transfer function of the second noise signal x 2 (n) to the output of the first dummy microphone  41 , V 22 (z) is a transfer function of the second noise signal x 2 (n) to the output of the second dummy microphone  42 , S V11 (z) is a transfer function of the first canceling signal CA 1 (n) to the output of the first dummy microphone  41 , S V12 (z) is a transfer function of the first canceling signal CA 1 (n) to the output of the second dummy microphone  42 , S V21 (z) is a transfer function of the second canceling signal CA 2 (n) to the output of the first dummy microphone  41 , S V22 (z) is a transfer function of the second canceling signal CA 2 (n) to the output of the second dummy microphone  42 , x i (z) is the Z-transform of x i (n), and err vi (z) is the Z-transform of err vi (n), err v1 (z) output by the first dummy microphone  41  becomes 
     
       
         
           
             
                 
             
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                               21 
                             
                             ⁡ 
                             
                               ( 
                               x 
                               ) 
                             
                           
                           + 
                           
                             
                               
                                 W 
                                 21 
                               
                               ⁡ 
                               
                                 ( 
                                 x 
                                 ) 
                               
                             
                             ⁢ 
                             
                               
                                 S 
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   11 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 z 
                                 ) 
                               
                             
                           
                           + 
                           
                             
                               
                                 W 
                                 22 
                               
                               ⁡ 
                               
                                 ( 
                                 z 
                                 ) 
                               
                             
                             ⁢ 
                             
                               
                                 S 
                                 
                                   V 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   21 
                                 
                               
                               ⁡ 
                               
                                 ( 
                                 z 
                                 ) 
                               
                             
                           
                         
                         } 
                       
                     
                   
                 
               
               , 
             
           
         
       
     
     and 
     err v2 (z) output by the second dummy microphone  42  similarly becomes
 
err v2 ( z )= x   1 ( z ){ V   12 ( z )+ W   11 ( z ) S   V12 ( z )+ W   12 ( z ) S   V22 ( z )}+ x   2 ( z ){ V   22 ( x )+ W   21 ( x ) S   V12 ( z )+ W   22 ( z ) S   V22 ( z )}.
 
     Because x 1 (z)≠0 and x 2 (z)≠0, err v1 (z)=0 and err v2 (z)=0 hold when
 
{ V   11 ( z )+ W   11 ( z ) S   V11 ( z )+ W   12 ( z ) S   V21 ( z )}=0
 
{ V   21 ( x )+ W   21 ( x ) S   V11 ( z )+ W   22 ( z ) S   V21 ( z )}=0
 
{ V   12 ( z )+ W   11 ( z ) S   V12 ( z )+ W   12 ( z ) S   V22 ( z )}=0
 
{ V   22 ( x )+ W   21 ( x ) S   V12 ( z )+ W   22 ( z ) S   V22 ( z )}=0,
 
     solving the system of simultaneous equations for W 11 , W 12 , W 21 , and W 22  gives
 
 W   11   ={V   12 ( z ) S   V21 ( z )− V   11 ( z ) S   V22 ( z )}/{ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )}
 
 W   12   ={V   11 ( z ) S   V12 ( z )− V   12 ( z ) S   V11 ( z )}/{ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )}
 
 W   21   ={V   22 ( z ) S   V21 ( z )− V   21 ( z ) S   V22 ( z )}/{ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )}
 
 W   22   ={V   21 ( z ) S   V12 ( z )− V   22 ( z ) S   V11 ( z )}/{ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )}.
 
     In the first learning block  40 , the transfer functions W 11 (z), W 12 (z), W 21 (z), and W 22 (z) converge on these values. 
     Also, the values of the converged transfer functions W 11 , W 12 , W 21 , and W 22  cancel the noise produced by the first noise source  21  and the noise produced by the second noise source  22  at the first cancellation point and the second cancellation point. 
     Next, if such transfer functions W 11 (z), W 12 (z), W 21 (z), and W 22 (z) converged by the first-stage learning process using the first learning block  40  are acquired, the first-stage learning process ends, and a second-stage learning process is performed. 
     As illustrated in  FIG. 6 , the second-stage learning process is performed in a configuration in which the signal processing block  11  of the active noise control system  1  has been replaced with a second learning block  60 . Herein, as illustrated in  FIG. 6 , the second learning block  60  is provided with a configuration obtained by omitting the Section 1 first adaptive algorithm execution unit  1114 , the Section 2 first adaptive algorithm execution unit  1116 , the Section 1 second adaptive algorithm execution unit  1124 , and the Section 2 second adaptive algorithm execution unit  1126  from the signal processing block  11  illustrated in  FIG. 3 , replacing the Section 1 first variable filter  1113  with a Section 1 first fixed filter  61  in which the transfer function is fixed to the transfer function W 11 (z) acquired by the first learning process, replacing the Section 2 first variable filter  1115  with a Section 2 first fixed filter  62  in which the transfer function is fixed to the transfer function W 12 (z) acquired by the first learning process, replacing the Section 1 second variable filter  1123  with a Section 1 second fixed filter  63  in which the transfer function is fixed to the transfer function W 21 (z) acquired by the first learning process, and replacing the Section 2 second variable filter  1125  with a Section 2 second fixed filter which the transfer function is fixed to the transfer function W 22 (z) acquired by the first learning process. 
     Also, as illustrated in  FIG. 6 , the second learning block  60  is provided with a configuration in which, in the signal processing block  11  illustrated in  FIG. 3 , the Section 1 first auxiliary filter  1111  has been replaced by a Section 1 first variable auxiliary filter  71  and a Section 1 learning first adaptive algorithm execution unit  81  that updates the transfer function H 11 (z) of the Section 1 first variable auxiliary filter  71  according to an FXLMS algorithm has been provided, the Section 2 first auxiliary filter  1112  has been replaced by a Section 2 first variable auxiliary filter  72  and a Section 2 learning first adaptive algorithm execution unit  82  that updates the transfer function H 12 (z) of the Section 2 first variable auxiliary filter  72  according to an FXLMS algorithm has been provided, the Section 1 second auxiliary filter  1121  has been replaced by a Section 1 second variable auxiliary filter  73  and a Section 1 learning second adaptive algorithm execution unit  83  that updates the transfer function H 21 (z) of the Section 1 second variable auxiliary filter  73  according to an FXLMS algorithm has been provided, and the Section 2 second auxiliary filter  1122  has been replaced by a Section 2 second variable auxiliary filter  74  and a Section 2 learning second adaptive algorithm execution unit  84  that updates the transfer function H 22 (z) of the Section 2 second variable auxiliary filter  74  according to an FXLMS algorithm has been provided. 
     Also, the second learning block  60  is configured such that the first error signal err h1 (n) output by the Section 1 error-correcting adder  1117  is output to the Section 1 learning first adaptive algorithm execution unit  81  and the Section 1 learning second adaptive algorithm execution unit  83  as error, while the second error signal err h2 (n) output by the Section 2 error-correcting adder  1127  is output to the Section 2 learning first adaptive algorithm execution unit  82  and the Section 2 learning second adaptive algorithm execution unit  84  as error. 
     Additionally, the Section 1 learning first adaptive algorithm execution unit  81  updates the transfer function H 11 (z) of the Section 1 first variable auxiliary filter  71  according to a FXLMS algorithm such that the first error signal err h1 (n) input as the error become zero (0). The Section 2 learning first adaptive algorithm execution unit  82  updates the transfer function H 12 (z) of the Section 2 first variable auxiliary filter  72  according to a FXLMS algorithm such that the second error signal err h2 (n) input as the error becomes zero (0). The Section 1 learning second adaptive algorithm execution unit  83  updates the transfer function H 21 (z) of the Section 1 second variable auxiliary filter  73  according to a FXLMS algorithm such that the first error signal err h1 (n) input as the error becomes zero (0). The Section 2 learning second adaptive algorithm execution unit  84  updates the transfer function H 22 (z) of the Section 2 second variable auxiliary filter  74  according to a FXLMS algorithm such that the second error signal err h2 (n) input as the error becomes zero (0). 
     Next, in the second-stage learning process using such a second learning block  60 , the first noise signal x 1 (n) and the second noise signal x 2 (n) are input into the first learning block  40 , and if the transfer function H 11 (z) of the Section 1 first variable auxiliary filter  71 , the transfer function H 12 (z) of the Section 2 first variable auxiliary filter  72 , the H 21 (z) of the Section 1 second variable auxiliary filter  73 , and the transfer function H 22 (z) of the Section 2 second variable auxiliary filter  74  have convergence and converge, each of the transfer functions H 11 (z), H 12 (z), H 21 (z), and H 22 (z) is acquired. 
     Herein, as illustrated in  FIG. 6 , provided that P 11 (z) is a transfer function of the first noise signal x 1 (n) to the output of the first microphone  12 , P 12 (z) is a transfer function of the first noise signal x 1 (n) to the output of the second microphone  14 , P 21 (Z) is a transfer function of the second noise signal x 2 (n) to the output of the first microphone  12 , P 22 (z) is a transfer function of the second noise signal x 2 (n) to the output of the second microphone  14 , S P11 (z) is a transfer function of the first canceling signal CA 1 (n) to the output of the first microphone  12 , S P12  is a transfer function of the first canceling signal CA 1 (n) to the output of the second microphone  14 , S P21  is a transfer function of the second canceling signal CA 2 (n) to the output of the first microphone  12 , S P22  is a transfer function of the second canceling signal CA 2 (n) to the output of the second microphone  14 , err pi (z) is the Z-transform of err pi (n), and err hi (z) is the Z-transform of err hi (n), err p1 (z) output by the first microphone  12  becomes 
     
       
         
           
             
               
                 err 
                 
                   p 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
               ⁡ 
               
                 ( 
                 z 
                 ) 
               
             
             = 
             
               
                 
                   
                     
                       x 
                       1 
                     
                     ⁡ 
                     
                       ( 
                       z 
                       ) 
                     
                   
                   ⁢ 
                   
                     
                       P 
                       11 
                     
                     ⁡ 
                     
                       ( 
                       z 
                       ) 
                     
                   
                 
                 + 
                 
                   
                     { 
                     
                       
                         
                           
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                           ⁡ 
                           
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                             11 
                           
                           ⁡ 
                           
                             ( 
                             z 
                             ) 
                           
                         
                       
                       + 
                       
                         
                           
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                           ⁡ 
                           
                             ( 
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                             ) 
                           
                         
                         ⁢ 
                         
                           
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                             21 
                           
                           ⁡ 
                           
                             ( 
                             x 
                             ) 
                           
                         
                       
                     
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                   ⁢ 
                   
                     
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                         ⁢ 
                         
                             
                         
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                     ⁡ 
                     
                       ( 
                       z 
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                             ( 
                             z 
                             ) 
                           
                         
                       
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                             ( 
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                             ( 
                             z 
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                   ⁢ 
                   
                     
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                       ( 
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                               ⁢ 
                               
                                   
                               
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                             ( 
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                               ⁢ 
                               
                                   
                               
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                       z 
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                           21 
                         
                         ⁡ 
                         
                           ( 
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                             x 
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                               ⁢ 
                               
                                   
                               
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                             ( 
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                               ⁢ 
                               
                                   
                               
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                             ( 
                             z 
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     and err p2 (z) output by the second microphone  14  similarly becomes
 
err p2 ( z )= x   1 ( z ){ P   12 ( z )+ W   11 ( z ) S   P12 ( z )+ W   12 ( z ) S   P22 ( z )}+ x   2 ( z ){ P   22 ( x )+ W   21 ( x ) S   P12 ( z )+ W   22 ( z ) S   P22 ( z )}.
 
     Consequently, when the first error signal err h1 (n) output by the Section 1 error-correcting adder  1117  becomes zero (0), 
     
       
         
           
             
               
                 err 
                 
                   h 
                   ⁢ 
                   
                       
                   
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                   1 
                 
               
               ⁡ 
               
                 ( 
                 z 
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             = 
             
               
                 
                   
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                       ⁢ 
                       
                           
                       
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                       ⁡ 
                       
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                                 ⁢ 
                                 
                                     
                                 
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                         21 
                       
                       ⁡ 
                       
                         ( 
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                 = 
                 0. 
               
             
           
         
       
     
     Further, similarly, when the second error signal err h2 (n) becomes zero (0), 
     
       
         
           
             
               
                 err 
                 
                   h 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
               ⁡ 
               
                 ( 
                 z 
                 ) 
               
             
             = 
             
               
                 
                   
                     err 
                     
                       p 
                       ⁢ 
                       
                           
                       
                       ⁢ 
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                     ( 
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                       2 
                     
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                       ( 
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                       22 
                     
                     ⁡ 
                     
                       ( 
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                       ) 
                     
                   
                 
               
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                         x 
                         1 
                       
                       ⁡ 
                       
                         ( 
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                         ) 
                       
                     
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                             ( 
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                               11 
                             
                             ⁡ 
                             
                               ( 
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                               ) 
                             
                           
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                                 P 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 12 
                               
                             
                             ⁡ 
                             
                               ( 
                               z 
                               ) 
                             
                           
                         
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                               12 
                             
                             ⁡ 
                             
                               ( 
                               z 
                               ) 
                             
                           
                           ⁢ 
                           
                             
                               S 
                               
                                 P 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 22 
                               
                             
                             ⁡ 
                             
                               ( 
                               z 
                               ) 
                             
                           
                         
                       
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                         2 
                       
                       ⁡ 
                       
                         ( 
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                     ⁢ 
                     
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                             22 
                           
                           ⁡ 
                           
                             ( 
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                             ) 
                           
                         
                         + 
                         
                           
                             
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                               21 
                             
                             ⁡ 
                             
                               ( 
                               x 
                               ) 
                             
                           
                           ⁢ 
                           
                             
                               S 
                               
                                 P 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 12 
                               
                             
                             ⁡ 
                             
                               ( 
                               z 
                               ) 
                             
                           
                         
                         + 
                         
                           
                             
                               W 
                               22 
                             
                             ⁡ 
                             
                               ( 
                               z 
                               ) 
                             
                           
                           ⁢ 
                           
                             
                               S 
                               
                                 P 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 22 
                               
                             
                             ⁡ 
                             
                               ( 
                               z 
                               ) 
                             
                           
                         
                       
                       } 
                     
                   
                   + 
                   
                     
                       
                         x 
                         1 
                       
                       ⁡ 
                       
                         ( 
                         z 
                         ) 
                       
                     
                     ⁢ 
                     
                       
                         H 
                         12 
                       
                       ⁡ 
                       
                         ( 
                         z 
                         ) 
                       
                     
                   
                   + 
                   
                     
                       
                         x 
                         2 
                       
                       ⁡ 
                       
                         ( 
                         z 
                         ) 
                       
                     
                     ⁢ 
                     
                       
                         H 
                         22 
                       
                       ⁡ 
                       
                         ( 
                         z 
                         ) 
                       
                     
                   
                 
                 = 
                 0. 
               
             
           
         
       
     
     Consequently, because x 1 (z)≠0 and x 2 (z)≠0, err h1 (z)=0 and err h2 (z)=0 hold when
 
 H   11 ( z )=−{ P   11 ( z )+ W   11 ( z ) S   P11 ( z )+ W   12 ( z ) S   P21 ( z )}
 
 H   12 ( z )=−{ P   12 ( z )+ W   11 ( z ) S   P12 ( z )+ W   12 ( z ) S   P22 ( z )}
 
 H   21 ( z )=−{ P   21 ( x )+ W   21 ( x ) S   P11 ( z )+ W   22 ( z ) S   P21 ( z )}
 
 H   22 ( z )=−{ P   22 ( x )+ W   21 ( x ) S   P12 ( z )+ W   22 ( z ) S   P22 ( z )},
 
substituting the above into the transfer functions W 11 (z), W 12 (z), W 21 (z), and W 22 (z) acquired by the first learning process and set in the Section 1 first fixed filter  61 , the Section 2 first fixed filter  62 , the Section 1 second fixed filter  63 , and the Section 2 second fixed filter  64  gives
 
 H   11 ( z )=−[ P   11 ( z )+{ V   12 ( z ) S   V21 ( z )− V   11 ( z ) S   V22 ( z )} S   P11 ( z )+{ V   11 ( z ) S   V12 ( z )− V   12 ( z ) S   V11 ( z )} S   P21 ( z )]/[ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )]
 
 H   12 ( z )=−[ P   12 ( z )+{ V   12 ( z ) S   V21 ( z )− V   11 ( z ) S   V22 ( z )} S   P12 ( z )+{ V   11 ( z ) S   V12 ( z )− V   12 ( z ) S   V11 ( z )} S   P22 ( z )]/[ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )]
 
 H   21 ( z )=−[ P   21 ( x )+{ V   22 ( z ) S   V21 ( z )− V   21 ( z ) S   V22 ( z )} S   P11 ( z )+{ V   21 ( z ) S   V12 ( z )− V   22 ( z ) S   V11 ( z )} S   P21 ( z )]/[ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )]
 
 H   22 ( z )=−[ P   22 ( x )+{ V   22 ( z ) S   V21 ( z )− V   21 ( z ) S   V22 ( z )} S   P12 ( z )+{ V   21 ( z ) S   V12 ( z )− V   22 ( z ) S   V11 ( z )} S   P22 ( z )]/[ S   V11 ( z ) S   V22 ( z )− S   V12 ( z ) S   V21 ( z )].
 
     In the second learning block  60 , the transfer functions H 11 (z), H 12 (z), H 21 (z), and H 22 (z) converge on these values. 
     Next, if such transfer functions H 11 (z), H 12 (z), H 21 (z), and H 22 (z) converged by the second-stage learning process using the second learning block  60  are acquired, the second-stage learning process ends. 
     At this point, the transfer functions H 11 (z) and H 21 (z) acquired in this way correct the difference in the transfer functions of each of the noise signals x 1 (n) and x 2 (n) and each of the canceling signals CA 1 (n) and CA 2 (n) to the first cancellation point and the position of the first microphone  12 , while the transfer functions H 12 (z) and H 22 (z) acquired in this way correct the difference in the transfer functions of each of the noise signals x 1 (n) and x 2 (n) and each of the canceling signals CA 1 (n) and CA 2 (n) to the second cancellation point and the position of the second microphone  14 . 
     Subsequently, the transfer function H 11 (z) of the Section 1 first variable auxiliary filter  71  acquired by the second-stage learning process in this way is set as the transfer function of the Section 1 first auxiliary filter  1111  of the signal processing block  11  in  FIG. 3 , the acquired transfer function H 12 (z) of the Section 2 first variable auxiliary filter  72  is set as the transfer function of the Section 2 first auxiliary filter  1112  of the signal processing block  11  in  FIG. 3 , the acquired transfer function H 21 (z) of the Section 1 second variable auxiliary filter  73  is set as the transfer function of the Section 1 second auxiliary filter  1121  of the signal processing block  11  in  FIG. 3 , the acquired transfer function H 22 (z) of the Section 2 second variable auxiliary filter  74  is set as the transfer function of the Section 2 second auxiliary filter  1122  of the signal processing block  11  in  FIG. 3 , and the learning process ends. 
     The above describes the learning process in the signal processing block  11  that sets the transfer function H 11 (z) of the Section 1 first auxiliary filter  1111 , the transfer function H 12 (z) of the Section 2 first auxiliary filter  1112 , the transfer function H 21 (z) of the Section 1 second auxiliary filter  1121 , and the transfer function H 22 (z) of the Section 2 second auxiliary filter  1122 . 
     In this way, in the signal processing block  11  of  FIG. 3  in which H 11 (z), H 12 (z), H 21 (z), and H 22 (z) are set, similarly to the second learning block  60 , the first error signal err h1 (n) output by the Section 1 error-correcting adder  1117  becomes
 
err h1 ( z )=err p1 ( z )+ x   1 ( z ) H   11 ( z )+ x   2 ( z ) H   21 ( z ),
 
     and 
     the second error signal err h2 (n) becomes
 
err h2 ( z )=err p2 ( z )+ x   1 ( z ) H   12 ( z )+ x   2 ( z ) H   22 ( z ).
 
     At this point, H 11 (z), H 12 (z), H 21 (z), and H 22 (z) are the values learned according to the second-stage learning process using the second learning block  60  such that err h1 (z) and err h2 (z) become zero (0) when the transfer functions W 11 , W 12 , W 21 , and W 22  are the values acquired by the first-stage learning process using the first learning block  40 . Consequently, in the same standard acoustic environment as the first-stage learning process and the second-stage learning process, by updating the transfer functions W 11 , W 12 , W 21 , and W 22  of the Section 1 first variable filter  1113 , the Section 2 first variable filter  1115 , the Section 1 second variable filter  1123 , and the Section 2 second variable filter  1125  in the signal processing block  11  such that err h1 (z) and err h2 (z) become zero (0), the transfer functions W 11 , W 12 , W 21 , and W 22  of the Section 1 first variable filter  1113 , the Section 2 first variable filter  1115 , the Section 1 second variable filter  1123 , and the Section 2 second variable filter  1125  converge on the values acquired by the first-stage learning process using the first learning block  40 . 
     In other words, when the transfer functions W 11 , W 12 , W 21 , and W 22  of the Section 1 first variable filter  1113 , the Section 2 first variable filter  1115 , the Section 1 second variable filter  1123 , and the Section 2 second variable filter  1125  are the values acquired by the first-stage learning process using the first learning block  40 , 
     because, as described earlier, 
     
       
         
           
             
                 
             
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                 = 
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     hold. 
     Additionally, the transfer functions W 11 , W 12 , W 21 , and W 22  acquired by the first-stage learning process using the first learning block  40  are values that cancel the noise produced by the first noise source  21  and the noise produced by the second noise source  22  at the first cancellation point and the second cancellation point. Consequently, in the same standard acoustic environment as the acoustic environment in which the first-stage learning process and the second-stage learning process are performed, the active noise control system  1  provided with the signal processing block  11  of  FIG. 3  is capable of canceling the noise produced by the first noise source  21  and the noise produced by the second noise source  22  at the first cancellation point and the second cancellation point away from the first microphone  12  and the second microphone  14 . 
     Also, with respect to variations of the acoustic environment from the same acoustic environment as the first-stage learning process and the second-stage learning process, by updating the transfer functions W 11 , W 12 , W 21 , and W 22  of the Section 1 first variable filter  1113 , the Section 2 first variable filter  1115 , the Section 1 second variable filter  1123 , and the Section 2 second variable filter  1125  according to the MEFX LMS of the transfer functions W 11 , W 12 , W 21 , and W 22  such that the first error signal err h1 (n) and the second error signal err h2 (n) become 0, the noise produced by the first noise source  21  and the noise produced by the second noise source  22  may be canceled adaptively at the first cancellation point and the second cancellation point. 
     The foregoing describes embodiments and implementations of the present disclosure. 
     Note that embodiments and implementations may be configured such that the functions for performing the learning process described above are included in the signal processing block  11 , and the learning process is executed in the signal processing block  11 . 
     Also, in the foregoing embodiments and implementations, the first noise signal x 1 (n) and the second noise signal x 2 (n) that are input into the active noise control system  1  may be sound signals from separately-provided noise microphones that pick up the noise from each noise source, or signals that simulate the noise from each noise source generated by separately-provided sound simulation devices. 
     In other words, for example, in the case of treating the engine as the first noise source  21 , engine noise picked up by a separate noise microphone may be taken to be the first noise signal x 1 (n), or simulated sound that simulates engine noise generated by a separately-provided sound simulation device may be taken to be the first noise signal x 1 (n). 
     Also, the active noise control system  1  according to the foregoing embodiments and implementations may be applied by expanding the configuration to canceling noise from three or more noise sources. 
     It is intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this disclosure.