Patent Publication Number: US-2022238093-A1

Title: Active noise control device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-012292 filed on Jan. 28, 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. 
     Description of the Related Art 
     JP 2008-239098 A discloses an active noise control device. This active noise control device generates a control signal for causing a speaker to output a canceling sound to cancel noise transmitted from a propeller shaft to the inside of a vehicle. The control signal is generated by performing signal processing on a basic signal using an adaptive filter. The basic signal is generated based on the rotational frequency of the propeller shaft. The adaptive filter is updated based on an error signal output by a microphone provided in the vehicle and a reference signal generated by correcting a basic signal with a correction value. 
     SUMMARY OF THE INVENTION 
     In the active noise control device disclosed in JP 2008-239098 A, a transfer characteristic of a canceling sound between the speaker and the microphone is used as the correction value. This correction value is a transfer characteristic measured in advance. Therefore, there is a possibility that the noise cannot be reduced when the transfer characteristic changes. 
     An object of the present invention is to solve the aforementioned problem. 
     An active noise control device according to one aspect of the present invention performs active noise control for controlling a speaker based on an error signal that changes in accordance with a synthetic sound of noise transmitted from a vibration source and a canceling sound output from the speaker to cancel the noise, and includes a basic signal generating unit configured to generate a basic signal corresponding to a control target frequency, a control signal generating unit configured to perform signal processing on the basic signal by a control filter, which is an adaptive notch filter, to generate a control signal that controls the speaker, a first estimated cancellation signal generating unit configured to perform signal processing on the control signal by a secondary path filter, which is an adaptive notch filter, to generate a first estimated cancellation signal, an estimated noise signal generating unit configured to perform signal processing on the basic signal by a primary path filter, which is an adaptive notch filter, to generate an estimated noise signal, a reference signal generating unit configured to perform signal processing on the basic signal by the secondary path filter to generate a reference signal, a second estimated cancellation signal generating unit configured to perform signal processing on the reference signal by the control filter to generate a second estimated cancellation signal, a first virtual error signal generating unit configured to generate a first virtual error signal from the error signal, the first estimated cancellation signal, and the estimated noise signal, a second virtual error signal generating unit configured to generate a second virtual error signal from the estimated noise signal and the second estimated cancellation signal, a secondary path filter coefficient updating unit configured to sequentially and adaptively update a coefficient of the secondary path filter based on the control signal and the first virtual error signal in a manner that a magnitude of the first virtual error signal is minimized, a control filter coefficient updating unit configured to sequentially and adaptively update a coefficient of the control filter based on the reference signal and the second virtual error signal in a manner that a magnitude of the second virtual error signal is minimized, and a state determination unit configured to compare a magnitude of the primary path filter with a magnitude of at least the control filter to determine whether a state of the control filter is unstable. 
     The active noise control device of the present invention can reduce noise even if the transfer characteristic changes. 
     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 executed by an active noise control device; 
         FIG. 2  is a block diagram of an active noise control device using a method that was proposed by the present inventors and the like; 
         FIG. 3  is a block diagram of an active noise control device; 
         FIG. 4  is a diagram illustrating updating of a filter coefficient. 
         FIG. 5  is a flowchart illustrating a flow of a filter coefficient update process; 
         FIG. 6  is a flowchart illustration a flow of a filter state determination process; 
         FIG. 7  is a block diagram of a signal processing unit; 
         FIG. 8  is a block diagram of a signal processing unit; 
         FIG. 9  is a flowchart illustrating the flow of a filter state determination process; and 
         FIG. 10  is a block diagram of an active noise control device. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     First Embodiment 
       FIG. 1  is a diagram illustrating an outline of active noise control executed by an active noise control device  10 . 
     The active noise control device  10  causes a speaker  16  provided in a vehicle compartment  14  of a vehicle  12  to output a canceling sound. This reduces a muffled sound of an engine  18  (hereinafter referred to as noise) that is transmitted to a vehicle occupant in the vehicle compartment  14  due to vibration of the engine  18 . The active noise control device  10  generates a control signal u0 based on the error signal e and an engine rotational speed Ne. The error signal e is a signal output from a microphone  22  provided on a headrest  20   a  of a seat  20  provided in the vehicle compartment  14 . A synthetic sound (hereinafter, referred to as canceling error noise) of the canceling sound and the noise is input to the microphone  22 . The engine rotational speed Ne is detected by an engine rotational speed sensor  24 . The control signal u0 is a signal for causing the speaker  16  to output the canceling sound. 
     [Conventional Active Noise Control Device] 
     Conventionally, an active noise control device using an adaptive notch filter (for example, a single-frequency adaptive notch (SAN) filter) having a small amount of computational processing has been proposed. 
     In the conventional active noise control device, first, a basic signal x having a frequency (control target frequency) of noise to be canceled is generated. The active noise control device performs signal processing on the generated basic signal x by a control filter W, which is an adaptive notch filter. Thus, a control signal u0 is generated. The active noise control device controls the speaker  16  by the control signal u0 to output a canceling sound for canceling the noise from the speaker  16 . 
     The control filter W is updated by an adaptive algorithm (for example, an LMS (Least Mean Square) algorithm) such that the error signal e output from the microphone  22  is minimized. 
     A transfer characteristic C is present in a sound transfer path from the speaker  16  to the microphone  22 . Therefore, it is necessary to consider this transfer characteristic C for updating the control filter W. The transfer characteristic C includes electronic circuit characteristics of the speaker  16  and the microphone  22 . The conventional active noise control device identifies the transfer characteristic C as a filter C{circumflex over ( )} in advance. The basic signal x corrected by the filter C{circumflex over ( )} is used to update the control filter W. Such a control system is called a filtered-x type. 
     The filter C{circumflex over ( )} is a fixed filter identified in advance. Thus, when the transfer characteristic C has been changed, the phase characteristic of the filter C{circumflex over ( )} and the phase characteristic of the transfer characteristic C may be significantly deviated from each other. In this case, there is concern that when the control filter W is updated, the control filter W may diverge. Therefore, there is also concern that noise may be amplified by the canceling sound output from the speaker  16 , or that an abnormal sound may be generated. 
     Therefore, the present inventors have proposed a method in which the filter C{circumflex over ( )} can follow a change in the transfer characteristic C during active noise control. In this method, it is not necessary to identify the transfer characteristic C in advance. The present invention is a further improvement of the method that was already proposed by the present inventors. An active noise control device  100  using the method already proposed by the present inventors will be schematically described below. 
       FIG. 2  is a block diagram of the active noise control device  100  using the method proposed by the present inventors. The transfer path of the sound from the engine  18  to the microphone  22  is hereinafter referred to as a primary path. Further, the transfer path of the sound from the speaker  16  to the microphone  22  is hereinafter referred to as a secondary path. 
     The active noise control device  100  includes a basic signal generating unit  26 , a control signal generating unit  28 , a first estimated cancellation signal generating unit  30 , an estimated noise signal generating unit  32 , a reference signal generating unit  34 , a second estimated cancellation signal generating unit  36 , a primary path filter coefficient updating unit  38 , a secondary path filter coefficient updating unit  40 , and a control filter coefficient updating unit  42 . 
     The basic signal generating unit  26  generates basic signals xc and xs based on the engine rotational speed Ne. The basic signal generating unit  26  includes a frequency detecting circuit  26   a , a cosine signal generator  26   b , and a sine signal generator  26   c.    
     The frequency detecting circuit  26   a  detects a control target frequency f. The control target frequency f is a vibration frequency of the engine  18  detected based on the engine rotational speed Ne. The cosine signal generator  26   b  generates the basic signal xc (=cos(2πft)) which is a cosine signal of the control target frequency f. The sine signal generator  26   c  generates the basic signal xs (=sin(2πft)) which is a sine signal of the control target frequency f. Here, t indicates time. 
     The control signal generating unit  28  generates control signals u0 and u1 based on the basic signals xc and xs. The control signal generating unit  28  includes a first control filter  28   a , a second control filter  28   b , a third control filter  28   c , a fourth control filter  28   d , an adder  28   e , and an adder  28   f.    
     In the control signal generating unit  28 , a SAN filter is used as a control filter W. The control filter W has a filter W0 for the basic signal xc and a filter W1 for the basic signal xs. The control filter W is optimized by updating a coefficient W0 of the filter W0 and a coefficient W1 of the filter W1 in the control filter coefficient updating unit  42  described later. 
     The first control filter  28   a  has the filter coefficient W0. The second control filter  28   b  has the filter coefficient W1. The third control filter  28   c  has a filter coefficient −W0. The fourth control filter  28   d  has a filter coefficient W1. 
     The basic signal xc corrected by the first control filter  28   a  and the basic signal xs corrected by the second control filter  28   b  are added by the adder  28   e  to generate the control signal u0. The basic signal xs corrected by the third control filter  28   c  and the basic signal xc corrected by the fourth control filter  28   d  are added by the adder  28   f  to generate the control signal u1. 
     The control signal u0 is converted into an analog signal by a digital-to-analog converter  17  and output to the speaker  16 . The speaker  16  is controlled based on the control signal u0, and the canceling sound is output from the speaker  16 . 
     The first estimated cancellation signal generating unit  30  generates a first estimated cancellation signal y1{circumflex over ( )} based on the control signals u0 and u1. The first estimated cancellation signal generating unit  30  includes a first secondary path filter  30   a , a second secondary path filter  30   b , and an adder  30   c.    
     In the first estimated cancellation signal generating unit  30 , a SAN filter is used as a secondary path filter C{circumflex over ( )}. The secondary path filter coefficient updating unit  40 , which will be described later, updates a coefficient (C0{circumflex over ( )}+iC1{circumflex over ( )}) of the secondary path filter C{circumflex over ( )}. Thus, a secondary path transfer characteristic C is identified as the secondary path filter C{circumflex over ( )}. 
     The first secondary path filter  30   a  has a filter coefficient C0{circumflex over ( )} which is a real part of a coefficient of the secondary path filter C{circumflex over ( )}. The second secondary path filter  30   b  has a filter coefficient C1{circumflex over ( )} which is an imaginary part of the coefficient of the secondary path filter C{circumflex over ( )}. The control signal u0 corrected by the first secondary path filter  30   a  and the control signal u1 corrected by the second secondary path filter  30   b  are added by the adder  30   c  to generate the first estimated cancellation signal y1{circumflex over ( )}. The first estimated cancellation signal y1{circumflex over ( )} is an estimation signal of a signal corresponding to a canceling sound y input to the microphone  22 . 
     The estimated noise signal generating unit  32  generates an estimated noise signal d{circumflex over ( )} based on the basic signals xc and xs. The estimated noise signal generating unit  32  includes a first primary path filter  32   a , a second primary path filter  32   b , and an adder  32   c.    
     In the estimated noise signal generating unit  32 , a SAN filter is used as a primary path filter H{circumflex over ( )}. The primary path filter coefficient updating unit  38 , which will be described later, updates a coefficient (H0{circumflex over ( )}+iH1{circumflex over ( )}) of the primary path filter H{circumflex over ( )}. Accordingly, a transfer characteristic H of the primary path (hereinafter, referred to as a primary path transfer characteristic H) is identified as a primary path filter H{circumflex over ( )}. 
     The first primary path filter  32   a  has a filter coefficient H0{circumflex over ( )} that is a real part of the coefficient of the primary path filter H{circumflex over ( )}. The second primary path filter  32   b  has a filter coefficient −H1{circumflex over ( )} obtained by inverting the polarity of the imaginary part of the coefficient of the primary path filter H{circumflex over ( )}. The basic signal xc corrected by the first primary path filter  32   a  and the basic signal xs corrected by the second primary path filter  32   b  are added by the adder  32   c  to generate the estimated noise signal d{circumflex over ( )}. The estimated noise signal d{circumflex over ( )} is an estimated signal of a signal corresponding to the noise d input to the microphone  22 . 
     The reference signal generating unit  34  generates reference signals r0 and r1 based on the basic signals xc and xs. The reference signal generating unit  34  includes a third secondary path filter  34   a , a fourth secondary path filter  34   b , a fifth secondary path filter  34   c , a sixth secondary path filter  34   d , an adder  34   e , and an adder  34   f.    
     In the reference signal generating unit  34 , a SAN filter is used as the secondary path filter C{circumflex over ( )}. 
     The third secondary path filter  34   a  has a filter coefficient C0{circumflex over ( )} which is a real part of a coefficient of the secondary path filter C{circumflex over ( )}. The fourth secondary path filter  34   b  has a filter coefficient −C1{circumflex over ( )} obtained by inverting the polarity of the imaginary part of the coefficient of the secondary path filter C{circumflex over ( )}. The fifth secondary path filter  34   c  has a filter coefficient C0{circumflex over ( )} which is a real part of a coefficient of the secondary path filter C{circumflex over ( )}. The sixth secondary path filter  34   d  has a filter coefficient C1{circumflex over ( )} which is an imaginary part of the coefficient of the secondary path filter C{circumflex over ( )}. 
     The basic signal xc corrected by the third secondary path filter  34   a  and the basic signal xs corrected by the fourth secondary path filter  34   b  are added by the adder  34   e  to generate the reference signal r0. The basic signal xs corrected by the fifth secondary path filter  34   c  and the basic signal xc corrected by the sixth secondary path filter  34   d  are added by the adder  34   f  to generate the reference signal r1. 
     The second estimated cancellation signal generating unit  36  generates a second estimated cancellation signal y2{circumflex over ( )} based on the reference signals r0 and r1. The second estimated cancellation signal generating unit  36  includes a fifth control filter  36   a , a sixth control filter  36   b , and an adder  36   c.    
     In the second estimated cancellation signal generating unit  36 , a SAN filter is used as the control filter W. The fifth control filter  36   a  has a filter coefficient W0. The sixth control filter  36   b  has a filter coefficient W1. 
     The reference signal r0 on which signal processing has been performed by the fifth control filter  36   a  and the reference signal r1 on which signal processing has been performed by the sixth control filter  36   b  are added by the adder  36   c  to generate the second estimated cancellation signal y2{circumflex over ( )}. The second estimated cancellation signal y2{circumflex over ( )} is an estimation signal of a signal corresponding to a canceling sound y input to the microphone  22 . 
     The analog-to-digital converter  44  converts the error signal e output from the microphone  22  from an analog signal to a digital signal. 
     The error signal e is input to an adder  46 . The polarity of the estimated noise signal d{circumflex over ( )} generated by the estimated noise signal generating unit  32  is inverted by an inverter  48 , and the estimated noise signal d{circumflex over ( )} is input to the adder  46 . The polarity of the first estimated cancellation signal y1{circumflex over ( )} generated by the first estimated cancellation signal generating unit  30  is inverted by an inverter  50 , and the first estimated cancellation signal y1{circumflex over ( )} is input to the adder  46 . In the adder  46 , a first virtual error signal e1 is generated. The adder  46  corresponds to a first virtual error signal generating unit of the present invention. 
     The estimated noise signal d{circumflex over ( )} generated by the estimated noise signal generating unit  32  is input to an adder  52 . The second estimated cancellation signal y2{circumflex over ( )} generated by the second estimated cancellation signal generating unit  36  is input to the adder  52 . In the adder  52 , a second virtual error signal e2 is generated. The adder  52  corresponds to a second virtual error signal generating unit of the present invention. 
     The primary path filter coefficient updating unit  38  sequentially and adaptively updates the coefficient of the primary path filter H{circumflex over ( )} based on the LMS algorithm such that the magnitude of the first virtual error signal e1 is minimized. The primary path filter coefficient updating unit  38  includes a first primary path filter coefficient updating unit  38   a  and a second primary path filter coefficient updating unit  38   b.    
     The first primary path filter coefficient updating unit  38   a  and the second primary path filter coefficient updating unit  38   b  update the filter coefficients H0{circumflex over ( )} and H1{circumflex over ( )} based on the following expressions. In the expressions, n denotes the number of time steps (time step number, n=0, 1, 2, . . . ) and μ0 and μ1 denote the step size parameters. The active noise control device  100  performs signal processing at predetermined periods. The time step indicates the length of each period. The time step number indicates how many periods (times) the signal processing is performed. 
         H 0{circumflex over ( )} n+1   =H 0{circumflex over ( )} n −μ0× e 1 n   ×xc   n  
 
         H 1{circumflex over ( )} n+1   =H 1{circumflex over ( )} n −μ1× e 1 n   ×xs   n  
 
     In the primary path filter coefficient updating unit  38 , the filter coefficients H0{circumflex over ( )} and H1{circumflex over ( )} are repeatedly updated. Thus, the primary path transfer characteristic H is identified as a primary path filter H{circumflex over ( )}. In the active noise control device  100  using the SAN filter, the update expression for the coefficient of primary path filter H{circumflex over ( )} is configured by four arithmetic operations and does not include a convolution operation. Therefore, it is possible to suppress a computation load due to update processing of the filter coefficients H0{circumflex over ( )} and H1{circumflex over ( )}. 
     The secondary path filter coefficient updating unit  40  sequentially and adaptively updates the coefficient of the secondary path filter C{circumflex over ( )} based on the LMS algorithm such that the magnitude of the first virtual error signal e1 is minimized. The secondary path filter coefficient updating unit  40  includes a first secondary path filter coefficient updating unit  40   a  and a second secondary path filter coefficient updating unit  40   b.    
     The first secondary path filter coefficient updating unit  40   a  and the second secondary path filter coefficient updating unit  40   b  update the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} based on the following expressions. In the expression, μ2 and μ3 indicate step size parameters. 
         C 0{circumflex over ( )} n+1   =C 0{circumflex over ( )} n -μ2× e 1 n ×μ0 n  
 
         C 1{circumflex over ( )} n+1   =C 1{circumflex over ( )} n −μ3× e 1 n ×μ1 n  
 
     In the secondary path filter coefficient updating unit  40 , the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} are repeatedly updated. Thus, a secondary path transfer characteristic C is identified as the secondary path filter C{circumflex over ( )}. In the active noise control device  100  using the SAN filter, the update expressions for the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} are configured by four arithmetic operations and do not include a convolution operation. Therefore, it is possible to suppress the computation load due to the update processing of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )}. 
     The control filter coefficient updating unit  42  sequentially and adaptively updates the coefficients W0 and W1 of the control filter W based on the LMS algorithm such that the magnitude of the second virtual error signal e2 is minimized. The control filter coefficient updating unit  42  includes a first control filter coefficient updating unit  42   a  and a second control filter coefficient updating unit  42   b.    
     The first control filter coefficient updating unit  42   a  and the second control filter coefficient updating unit  42   b  update the filter coefficients W0 and W1 based on the following expressions. In the expressions, μ4 and μ5 denote the step size parameters. 
         W 0 n+1   =W 0 n −μ4× e 2 n   ×r 0 n  
 
         W 1 n+1   =W 1 n −μ5× e 2 n   ×r 1 n  
 
     In the control filter coefficient updating unit  42 , the filter coefficients W0 and W1 are repeatedly updated. Thus, the control filter W is optimized. In the active noise control device  100  using the SAN filter, the update expressions for the filter coefficients W0 and W1 are configured by four arithmetic operations and do not include a convolution operation. Therefore, it is possible to suppress the computation load due to the update processing of the filter coefficients W0 and W1. 
     The noise to be canceled by the active noise control device  100  is a muffled sound of the engine. The muffled sound of the engine is mainly generated in a range of 40 [Hz] to 200 [Hz]. When the frequencies (control target frequencies f) detected by the frequency detecting circuit  26   a  are within a defined range (for example, 40 [Hz] to 200 [Hz]), the active noise control device  100  generates the control signal u0 and causes the speaker  16  to output the canceling sound. 
     [Improvement Points] 
     Improvements made in the present invention will be described, with respect to the active noise control device  100  using the technique that was already proposed by the present inventors. 
       FIG. 3  is a block diagram of the active noise control device  10  according to the present embodiment. The configuration of a signal processing unit  54  of the active noise control device  10  according to the present embodiment, is substantially the same as the configuration of the active noise control device  100  described above. The active noise control device  10  further includes an initial value table  56 , an update value table  58 , a result value table  60 , an initial value table operating unit  62 , an update value table operating unit  64 , a result value table operating unit  66 , a termination state determination unit  68  and a filter state determination unit  69 . 
     The active noise control device  10  includes an operational processing device and a storage unit (not shown). The operational processing device includes, for example, a processor such as a central processing unit (CPU) or a microprocessing unit (MPU), and a memory such as a ROM or a RAM. The storage unit is, for example, a hard disk, a flash memory, or the like. The active noise control device  10  need not necessarily have a storage unit. In this case, data may be transmitted and received via communications between the active noise control device  10  and the storage space on the cloud. The signal processing unit  54 , the initial value table operating unit  62 , the update value table operating unit  64 , the result value table operating unit  66 , the termination state determination unit  68 , and the filter state determination unit  69  are realized by the operational processing unit executing a program stored in the storage unit. 
     The initial value table  56  is a memory area in table form provided in the ROM. In the initial value table  56 , initial values of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} of a secondary path filter C{circumflex over ( )}, which will be described later, are stored. The update value table  58  is a memory area in table form provided in the RAM. In the update value table  58 , the update values of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} are stored. The result value table  60  is a memory area in table format provided in the ROM. In the result value table  60 , the result values of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} are stored. 
     The initial value table operating unit  62  writes initial values in the initial value table  56 , or performs other operations. The update value table operating unit  64  writes update values in the update value table  58 , or performs other operations. The result value table operating unit  66  writes result values in the result value table  60 , or performs other operations. 
     The termination state determination unit  68  determines a cause for termination of active noise control. When one of the following three termination causes occurs, the active noise control is terminated. The three causes for termination are stopping of the engine  18 , occurrence of an abnormality in active noise control, and divergence of the active noise control. When the active noise control is ended due to the stop of the engine, the termination state determination unit  68  determines that the active noise control is normally ended. When the active noise control is ended due to the occurrence of an abnormality in the active noise control, the termination state determination unit  68  determines that the active noise control ends abnormally. When the active noise control is ended due to the divergence of the active noise control, the termination state determination unit  68  determines that the active noise control ends abnormally. 
     The filter state determination unit  69  determines the state of the control filter W each time the filter coefficients W0 and W1 of the control filter W are updated. The filter state determination unit  69  corresponds to a state determination unit of the present invention. The determination of the state of the control filter W will be described later in detail. 
     The update processing of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} by the secondary path filter coefficient updating unit  40  of the present embodiment is partially different from the update processing of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} by the secondary path filter coefficient updating unit  40  of the above-described active noise control device  100 . 
     In the secondary path filter coefficient updating unit  40  of the active noise control device  100 , the first secondary path filter coefficient updating unit  40   a  and the second secondary path filter coefficient updating unit  40   b  respectively update the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} based on the following expressions. 
         C 0{circumflex over ( )} n+1   =C 0{circumflex over ( )} n −μ2× e 1 n   ×u 0 n  
 
         C 1{circumflex over ( )} n+1   =C 1{circumflex over ( )} n −μ3× e 1 n   ×u 1 n  
 
     On the other hand, in the secondary path filter coefficient updating unit  40  of the signal processing unit  54  according to the present embodiment, the first secondary path filter coefficient updating unit  40   a  and the second secondary path filter coefficient updating unit  40   b  respectively update the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} based on the following expressions. 
         C 0{circumflex over ( )}( f ) n+1   =C 0{circumflex over ( )}( f )_ u−μ 2× e 1 n   ×u 0 n  
 
         C 1{circumflex over ( )}( f ) n+1   =C 1{circumflex over ( )}( f )_ u−μ 3× e 1 n   ×u 1 n  
 
     Update values corresponding to the control target frequency f stored in the update value table  58  are input to the coefficients C0{circumflex over ( )} (f)_u and C1{circumflex over ( )} (f)_u in the above expressions. Hereinafter, the first terms on the right side of the update expressions of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} may be referred to as previous values. 
     In the method that was already proposed, the filter coefficients C0{circumflex over ( )} n and C1{circumflex over ( )} n updated in the previous period (time step number n) are used as previous values of the update expressions. That is, even if the control target frequency f has changed between the updating in the previous period (time step number n) and the update in the current period (time step number n+1), the filter coefficients C0{circumflex over ( )} n and C1{circumflex over ( )} n updated in the previous period are used as previous values of the update expressions. 
     On the other hand, in the present embodiment, an update value corresponding to the control target frequency f at the time of updating in the current period (time step number n+1) is used as the previous value of the update expression. That is, in the case of the control target frequency f, the filter coefficients C0{circumflex over ( )} (f)_u and C1{circumflex over ( )} (f)_u having the latest updating timing among the updated filter coefficients are used as the previous values of the update expressions. In other words, in the present embodiment, the previous value is not limited to a value updated last time (time step number n). 
     The secondary path filter coefficient updating unit  40  copies the updated filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} in the third secondary path filter  34   a , the fourth secondary path filter  34   b , the fifth secondary path filter  34   c , and the sixth secondary path filter  34   d  of the reference signal generating unit  34 . 
     [Update of Secondary Path Filter] 
       FIG. 4  is a diagram illustrating the updating of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )}. As shown in  FIG. 4 , the initial value table  56  stores initial values C0{circumflex over ( )} (f)_i and C1{circumflex over ( )} (f)_i in table form in association with frequencies. The update value table  58  stores the update values C0{circumflex over ( )} (f)_u and C1{circumflex over ( )} (f)_u in table form in association with frequencies. Further, the result value table  60  stores the result values C0{circumflex over ( )} (f)_r and C1{circumflex over ( )} (f)_r in table form in association with frequencies. 
     The initial values stored in the initial value table  56  in association with frequencies are set based on any of the following (i) to (vi).
         (i) A measured value of the secondary path transfer characteristic C at each frequency;   (ii) Phase information of a measured value of the secondary path transfer characteristic C at each frequency;   (iii) An estimated value of the secondary path transfer characteristic C complemented based on the measured values of the secondary path transfer characteristics C at representative frequencies;   (iv) Phase information of an estimated value of the secondary path transfer characteristic C complemented based on measured values of the secondary path transfer characteristics C at representative frequencies;   (v) An estimated value of the secondary path transfer characteristic C estimated by the following expressions:       

         C 0{circumflex over ( )}( f )=α( f )×cos(−2π fT )
 
         C 1{circumflex over ( )}( f )=α( f )×sin(−2π fT )
 
     Here, T is the time until the sound reaches the microphone  22  from the speaker  16 , and a is an amplitude constant; and
         (vi) A convenient small value (in a case where an initial value is not particularly set for convenience such as efficiency of system setting).       

       FIG. 5  is a flowchart showing a flow of update processing of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )}. The process of updating the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} is executed each time active noise control is performed. 
     In step S 1 , the update value table operating unit  64  rewrites the initial values corresponding to the respective frequencies of the initial value table  56  with the update values corresponding to the respective frequencies of the update value table  58  ((A) in  FIG. 4 ). Thereafter, the process proceeds to step S 2 . 
     In step S 2 , the frequency detecting circuit  26   a  provided in the signal processing unit  54  detects the control target frequency f. Thereafter, the process proceeds to step S 3 . 
     In step S 3 , the secondary path filter coefficient updating unit  40  reads update values corresponding to the control target frequency f as previous values ((B) in  FIG. 4 ). Thereafter, the process proceeds to step S 4 . 
     In step S 4 , the secondary path filter coefficient updating unit  40  updates the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )}. Thereafter, the process proceeds to step S 5 . 
     In step S 5 , the update value table operating unit  64  writes the updated filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} to the update values corresponding to the control target frequency f ((C) in  FIG. 4 ). Thereafter, the process proceeds to step S 6 . 
     In step S 6 , the termination state determination unit  68  determines whether or not the active noise control has ended. If the active noise control has not terminated, the process returns to step S 2 , and if the active noise control has terminated, the process proceeds to step S 7 . 
     In step S 7 , the termination state determination unit  68  determines whether or not the active noise control has ended normally. When it is determined that the active noise control has ended normally, the process proceeds to step S 8 . When it is determined that the active noise control has ended abnormally, or when it is determined that the active noise control has ended in divergence, the process proceeds to step S 10 . 
     In step S 8 , the initial value table operating unit  62  determines whether or not rewriting of the initial values of the initial value table  56  is permitted. If the rewriting of the initial value table  56  is permitted, the process proceeds to step S 9 , and otherwise if rewriting of the initial value table  56  is not permitted, the update processing of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} is terminated. 
     In step S 9 , the initial value table operating unit  62  rewrites the initial values corresponding to the respective frequencies of the initial value table  56  with the update values corresponding to the respective frequencies of the update value table  58  ((D) in  FIG. 4 ). Thereafter, the update processing of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} is terminated. 
     In step S 10 , the result value table operating unit  66  writes the update values corresponding to the respective frequencies of the update value table  58  in the result values corresponding to the respective frequencies of the result value table  60  ((E) in  FIG. 4 ). Thereafter, the update processing of the filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} is terminated. 
     The initial value table  56  and the result value table  60  can be copied to a personal computer or the like connected to the vehicle  12 . This makes it possible to compare the update values stored in the initial value table  56  with the result values stored in the result value table  60 . Therefore, it is possible to verify the cause for the abnormality in the active noise control or the cause for the divergence of the active noise control. 
     [Filter State Determination Process] 
       FIG. 6  is a flowchart illustrating the flow of a filter state determination process executed by the filter state determination unit  69 . The filter state determination process is executed each time the control filter W is updated. 
     In step S 21 , the filter state determination unit  69  calculates a magnitude A of the primary path filter H{circumflex over ( )}. Thereafter, the process proceeds to step S 22 . The magnitude A can also be referred to as an amplitude characteristic of the primary path filter H{circumflex over ( )}. The magnitude A of the primary path filter H{circumflex over ( )} can be obtained by the following expression. 
         A=|H{circumflex over ( )}|   2   =H 0{circumflex over ( )} 2   +H 1{circumflex over ( )} 2  
 
     In step S 22 , the filter state determination unit  69  calculates a magnitude B of the filter characteristic obtained by coupling the secondary path filter C{circumflex over ( )} and the control filter W in series, and proceeds to step S 23 . The magnitude B indicates an amplitude characteristic among filter characteristics in which the secondary path filter C{circumflex over ( )} and the control filter W are coupled in series. The magnitude B can be obtained by the following expression. 
         B=|C{circumflex over ( )}·W|   2 =( C 0{circumflex over ( )}· W 0+ C 1{circumflex over ( )}· W 1) 2 +( C 0{circumflex over ( )}· W 1− C 1{circumflex over ( )}· W 0) 2  
 
     Note that the signal processing unit  54  may use, as the filter coefficients C0 and C1 of the secondary path filter C{circumflex over ( )}, those normalized by the magnitude |C{circumflex over ( )}| of the secondary path filter C{circumflex over ( )}. In this case, the magnitude B is obtained by the following expression. 
         B=|W|   2   =W 0 2   +W 1 2    
     In step S 23 , the filter state determination unit  69  determines whether or not the magnitude A is smaller than a predetermined value β. When the magnitude A is smaller than the predetermined value β, the filter state determination process is terminated, and when the magnitude A is equal to or larger than the predetermined value β, the process proceeds to step S 24 . 
     In step S 24 , the filter state determination unit  69  determines whether or not the magnitude B is larger than the magnitude A. When the magnitude B is larger than the magnitude A, the process proceeds to step S 25 , and when the magnitude B is equal to or smaller than the magnitude A, the process proceeds to step S 26 . 
     In step S 25 , the filter state determination unit  69  determines that the state of the control filter W is unstable. Thereafter, the filter state determination process is terminated. 
     In step S 26 , the filter state determination unit  69  determines that the state of the control filter W is stable. Thereafter, the filter state determination process is terminated. 
     When it is determined that the state of control filter W is unstable, the active noise control device  10  stops active noise control. 
     [Operational Effects] 
     The active noise control device  10  of the present embodiment is provided with the initial value table  56  and the update value table  58 . Accordingly, the active noise control device  10  can set initial values of filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} for each of frequencies. Further, the active noise control device  10  can update filter coefficients C0{circumflex over ( )} and C1{circumflex over ( )} for each of frequencies. Therefore, the active noise control device  10  can significantly improve the initial silencing performance, particularly after the start of active noise control. However, the secondary path filter C{circumflex over ( )} may converge on a characteristic that is significantly different from the actual secondary path transfer characteristic C. In this case, the active noise control device  10  cannot generate a control signal u0 corresponding to the secondary path transfer characteristic C. Therefore, the noise cannot be sufficiently canceled by the canceling sound output from the speaker  16 . In particular, when the phase characteristic of the secondary path filter C{circumflex over ( )} has a phase difference of 90° or more with respect to the phase characteristic of the actual secondary path transfer characteristic C, the control filter W diverges. When the control filter W diverges, the active noise control device  10  stops the active noise control. However, immediately before the active noise control is stopped, an abnormal sound is output from the speaker  16  undesirably. 
     Therefore, in the active noise control device  10  of the present embodiment, the filter state determination unit  69  compares the magnitude of the primary path filter H{circumflex over ( )} with the magnitude of the control filter W. The filter state determination unit  69  determines whether or not the state of the control filter W is unstable based on the comparison result. Thus, when it is determined that the state of control filter W is unstable, the active noise control device  10  can stop active noise control before the control filter W diverges. Therefore, the active noise control device  10  can suppress an abnormal sound from being output from the speaker  16  due to divergence of control filter W. 
     Further, in the active noise control device  10  of the present embodiment, the filter state determination unit  69  determines that the state of control filter W is unstable in the following cases. The following case is a case where the magnitude A of the filter characteristic obtained by coupling the secondary path filter C{circumflex over ( )} and the control filter W in series is larger than the magnitude B of the primary path filter H{circumflex over ( )}. The magnitude A=|C{circumflex over ( )}·W|. Further, the magnitude B=|H{circumflex over ( )}|. As can be seen from the block diagram of  FIG. 2 , when the active noise control is normally performed, H{circumflex over ( )}=C{circumflex over ( )}·W is established. When the magnitude A is larger than the magnitude B, the canceling sound is output more than necessary with respect to the magnitude of the noise. Therefore, it is determined that the state of the control filter W is unstable. Thus, in the active noise control device  10 , the filter state determination unit  69  can accurately determine the state of the control filter W. 
     Further, in the active noise control device  10  of the present embodiment, the filter state determination unit  69  does not determine the state of control filter W when the magnitude of the primary path filter H{circumflex over ( )} is less than the predetermined value. Immediately after the active noise control starts, the magnitudes of the primary path filter H{circumflex over ( )}, the secondary path filter C{circumflex over ( )}, and the control filter W are all small. In this state, even if the filter state determination unit  69  attempts to determine the state of the control filter W, there is a risk of erroneous determination. Thus, in the active noise control device  10 , the filter state determination unit  69  can suppress erroneous determination of the state of control filter W. 
     Second Embodiment 
     In the present embodiment, when the state of the control filter W becomes unstable, the magnitude of the canceling sound output from the speaker  16  is suppressed. Two methods 1 and 2 will be described below as signal processing methods for suppressing the magnitude of the canceling sound output from the speaker  16 . 
     [Method 1] 
       FIG. 7  is a block diagram of the signal processing unit  54 . In the signal processing unit  54  of the method 1, a stabilization filter  70  is added to the signal processing unit  54  ( FIG. 2 ) of the first embodiment. By providing the stabilization filter  70 , the magnitude of the second estimated cancellation signal y2{circumflex over ( )} input to the adder  52  is multiplied by (1+α). The stabilization filter  70  is set to α=0 when it is determined that the state of the control filter W is stable. When it is determined that the state of the control filter W is unstable, the stabilization filter  70  is set such that the value of a gradually increases as time elapses. As a result, the second estimated cancellation signals y2{circumflex over ( )} input to the adder  52  can be increased by (1+α) times. Therefore, the second virtual error signals e2 generated by the adder  52  become large. This makes it possible to reduce the size of the control filter W. As a result, the magnitude of the control signal u0 is suppressed, and the magnitude of the canceling sound output from the speaker  16  can be suppressed. 
     [Method 2] 
       FIG. 8  is a block diagram of the signal processing unit  54 . In the signal processing unit  54  of the method 2, a stabilization signal generating unit  72  is added to the signal processing unit  54  ( FIG. 2 ) of the first embodiment. The stabilization signal generating unit  72  generates a stabilization signal αy2{circumflex over ( )}. The stabilization signal αy2{circumflex over ( )} is generated by performing signal processing on the second estimated cancellation signal y2{circumflex over ( )} with a stabilization filter, which is an adaptive filter. Further, in the signal processing unit  54  of the method 2, an adder  53  is added to the signal processing unit  54  ( FIG. 2 ) of the first embodiment. The adder  53  generates a third virtual error signal e3 from the second virtual error signal e2 and the stabilization signal αy2{circumflex over ( )}. Further, in the signal processing unit  54  of the method 2, a stabilization filter coefficient updating unit  74  is added to the signal processing unit  54  ( FIG. 2 ) of the first embodiment. The stabilization filter coefficient updating unit  74  sequentially and adaptively updates the filter coefficient α of the stabilization filter based on the second estimated cancellation signal y2{circumflex over ( )} and the second virtual error signal e2 such that the magnitude of the second virtual error signal e2 is minimized. 
     The second virtual error signal e2 generated by the adder  52  are input to the adder  53 . The stabilization signal αy2{circumflex over ( )} generated by the stabilization signal generating unit  72  is input to the adder  53 . In the adder  53 , a third virtual error signal e3 is generated. The adder  53  corresponds to a third virtual error signal generating unit of the present invention. 
     The control filter coefficient updating unit  42  updates the filter coefficients W0 and W1 based on the reference signals r0 and r1 and the third virtual error signal e3. 
     As a result, the second estimated cancellation signal y2{circumflex over ( )} included in the third virtual error signal e3 increases by (1+α) times the second estimated cancellation signal y2{circumflex over ( )} included in the second virtual error signal e2. Therefore, the size of the control filter W can be suppressed. As a result, the magnitude of the control signal u0 is suppressed, and the magnitude of the canceling sound output from the speaker  16  can be suppressed. 
     [Filter State Determination Process] 
       FIG. 9  is a flowchart illustrating the flow of the filter state determination process executed by the filter state determination unit  69 . The filter state determination process is executed each time the control filter W is updated. 
     In step S 31 , the filter state determination unit  69  calculates a magnitude A of the primary path filter H{circumflex over ( )}. Thereafter, the process proceeds to step S 32 . The magnitude A can also be referred to as an amplitude characteristic of the primary path filter H{circumflex over ( )}. The magnitude A can be obtained by the following expression. 
         A=|H{circumflex over ( )}|   2   =H 0{circumflex over ( )} 2   +H 1{circumflex over ( )} 2  
 
     In step S 32 , the filter state determination unit  69  calculates a magnitude B of the filter characteristic obtained by coupling the secondary path filter C{circumflex over ( )} and the control filter W in series. Thereafter, the process proceeds to step S 33 . The magnitude B indicates an amplitude characteristic among filter characteristics in which the secondary path filter C{circumflex over ( )} and the control filter W are coupled in series. The magnitude B can be obtained by the following expression. 
         B =(1+α) 2   |C{circumflex over ( )}·W|   2 =(1+α) 2 ( C 0{circumflex over ( )}· W 0+ C 1{circumflex over ( )}· W 1) 2 +(1+α) 2 ( C 0{circumflex over ( )}· W 1− C 1{circumflex over ( )}· W 0) 2  
 
     Note that the signal processing unit  54  may use, as the filter coefficients COA and C1{circumflex over ( )} of the secondary path filter C{circumflex over ( )}, those normalized by the magnitude |C{circumflex over ( )}| of the secondary path filter C{circumflex over ( )}. In this case, the magnitude B is obtained by the following expression. 
         B =(1+α) 2   |W|   2 =(1+α) 2   W 0 2 +(1+α) 2   W 1 2  
 
     In step S 33 , the filter state determination unit  69  determines whether or not the magnitude A is smaller than a predetermined value β. When the magnitude A is smaller than the predetermined value β, the filter state determination process is terminated. When the magnitude A of the primary path filter H{circumflex over ( )} is equal to or larger than the predetermined value β, the process proceeds to step S 34 . 
     In step S 34 , the filter state determination unit  69  determines whether or not the magnitude B is larger than the magnitude A. When the magnitude B is larger than the magnitude A, the process proceeds to step S 35 . When the magnitude B is equal to or smaller than the magnitude A, the process proceeds to step S 36 . 
     In step S 35 , the filter state determination unit  69  determines that the state of the control filter W is unstable. Thereafter, the filter state determination process is terminated. 
     In step S 36 , the filter state determination unit  69  determines that the state of the control filter W is stable. Thereafter, the filter state determination process is terminated. 
     In the case of the above-described method 1, when it is determined that the state of the control filter W is stable, the signal processing unit  54  sets the stabilization filter coefficient α=0. When it is determined that the state of the control filter W is unstable, the value of the filter coefficient α is set so as to gradually increase as time elapses. 
     [Operational Effects] 
     The active noise control device  10  of the present embodiment has the stabilization filter  70 . When the filter state determination unit  69  determines that the state of the control filter W is unstable, the stabilization filter  70  corrects the second estimated cancellation signal y2{circumflex over ( )} input to the adder  52  so as to increase. As a result, the second virtual error signal e2 generated by the adder  52  increases. Therefore, the size of the control filter W can be suppressed. Therefore, when the state of the control filter W is unstable, the magnitude of the canceling sound output from the speaker  16  can be suppressed. As a result, it is possible to suppress amplification of noise and generation of abnormal sound due to the canceling sound. 
     Further, in the active noise control device  10  of the present embodiment, the stabilization signal generating unit  72  generates the stabilization signal αy2{circumflex over ( )}. The stabilization signal αy2{circumflex over ( )} is generated by performing signal processing on the second estimated cancellation signal y2{circumflex over ( )} with a stabilization filter, which is an adaptive notch filter. Further, the adder  53  generates the third virtual error signal e3 from the second virtual error signal e2 and the stabilization signal αy2{circumflex over ( )}. Further, the stabilization filter coefficient updating unit  74  sequentially and adaptively updates the filter coefficient α of the stabilization filter, based on the second estimated cancellation signal y2{circumflex over ( )} and the second virtual error signal e2 such that the magnitude of the second virtual error signal e2 is minimized. Further, based on the reference signals r0 and r1 and the third virtual error signal e3, the control filter coefficient updating unit  42  sequentially and adaptively updates the filter coefficients W0 and W1 of the control filter W such that the magnitude of the third virtual error signal e3 is minimized. 
     As a result, the third virtual error signal e3 generated by the adder  53  increases. Therefore, the magnitude of the control filter W can be suppressed. Therefore, when the state of the control filter W is unstable, the magnitude of the canceling sound output from the speaker  16  can be suppressed. As a result, it is possible to suppress amplification of noise and generation of abnormal sound due to the canceling sound. 
     Third Embodiment 
     When the following condition is satisfied, the signal processing unit  54  of the first embodiment and the second embodiment generates the control signal u0 and causes the speaker  16  to output the canceling sound. The condition is that the control target frequency f is within a defined range (for example, 40 [Hz] to 200 [Hz]). The control target frequency f is a frequency detected by the frequency detecting circuit  26   a . That is, when the control target frequency f is outside the defined range, the signal processing unit  54  according to the first and second embodiments does not generate the control signal u0. In this case, no updating of the primary path filter H{circumflex over ( )} takes place. Therefore, even after time elapses from the start of the active noise control, there may be a case where the primary path filter H{circumflex over ( )} is not updated from an initial value (for example, H0{circumflex over ( )}=0, H1{circumflex over ( )}=0). In this case, when the control target frequency f falls within the defined range and the generation of the control signal u0 is started, it may undesirably take time for the control filter W to converge. 
     The signal processing unit  54  according to the present embodiment continues the generation of the control signal u0 and the updating of the primary path filter H{circumflex over ( )} even when the control target frequency f is outside the defined range. 
       FIG. 10  is a block diagram of the signal processing unit  54  used when the control target frequency f is outside the defined range. In the signal processing unit  54  shown in  FIG. 10 , the reference signal generating unit  34 , the second estimated cancellation signal generating unit  36 , and the adder  52  are deleted from the signal processing unit  54  shown in  FIG. 2 . Further, the configuration of the control filter coefficient updating unit  42  is different. 
     The control filter coefficient updating unit  42  includes a third control filter coefficient updating unit  42   c  and a fourth control filter coefficient updating unit  42   d . The third control filter coefficient updating unit  42   c  performs forgetting process on the control filter coefficient W0. The fourth control filter coefficient updating unit  42   d  performs forgetting process on the control filter coefficient W1. The forgetting process is a process of gradually decreasing the control filter coefficient W0 and the control filter coefficient W1 by multiplying each of the control filter coefficient W0 and the control filter coefficient W1 by a forgetting coefficient (for example, 0.999). 
     This makes it possible to reduce the magnitude of the control filter W while continuing the updating of the primary filter H{circumflex over ( )} even when the control target frequency f is outside the defined range. Therefore, when the control target frequency f is out of the defined range, the canceling sound output from the speaker  16  can be faded out. Further, when the control target frequency f falls within the defined range from outside the defined range, the initial value of the control filter W is set to H{circumflex over ( )}/C{circumflex over ( )}. As a result, convergence of the control filter W can be accelerated, and performance of the active noise control device  10  can be transiently improved. 
     [Technical Invention Obtained from Embodiments] 
     The invention that can be grasped from the above embodiments will be described below. 
     The active noise control device ( 10 ) according to the present invention perform active noise control for controlling a speaker ( 16 ) based on an error signal that changes in accordance with a synthetic sound of noise transmitted from a vibration source and a canceling sound output from the speaker to cancel the noise, and includes the basic signal generating unit ( 26 ) configured to generate a basic signal corresponding to a control target frequency, the control signal generating unit ( 28 ) configured to perform signal processing on the basic signal by a control filter, which is an adaptive notch filter, to generate a control signal that controls the speaker, the first estimated cancellation signal generating unit ( 30 ) configured to perform signal processing on the control signal by a secondary path filter, which is an adaptive notch filter, to generate a first estimated cancellation signal, the estimated noise signal generating unit ( 32 ) configured to perform signal processing on the basic signal by a primary path filter, which is an adaptive notch filter, to generate an estimated noise signal, the reference signal generating unit ( 34 ) configured to perform signal processing on the basic signal by the secondary path filter to generate a reference signal, the second estimated cancellation signal generating unit ( 36 ) configured to perform signal processing on the reference signal by the control filter to generate a second estimated cancellation signal, the first virtual error signal generating unit ( 46 ) configured to generate a first virtual error signal from the error signal, the first estimated cancellation signal, and the estimated noise signal, the second virtual error signal generating unit ( 52 ) configured to generate a second virtual error signal from the estimated noise signal and the second estimated cancellation signal, the secondary path filter coefficient updating unit ( 40 ) configured to sequentially and adaptively update a coefficient of the secondary path filter based on the control signal and the first virtual error signal in a manner that a magnitude of the first virtual error signal is minimized, the control filter coefficient updating unit ( 42 ) configured to sequentially and adaptively update a coefficient of the control filter based on the reference signal and the second virtual error signal in a manner that a magnitude of the second virtual error signal is minimized, and the state determination unit ( 69 ) configured to compare a magnitude of the primary path filter with a magnitude of at least the control filter to determine whether a state of the control filter is unstable. 
     In the active noise control device according to the present invention, the state determination unit may be configured to determine that the state of the control filter is unstable if a magnitude of a filter in which the control filter and the secondary path filter are coupled in series is larger than a magnitude of the primary path filter. 
     In the active noise control device according to the present invention, the state determination unit need not necessarily determine the state of the control filter if the magnitude of at least the primary path filter is less than a predetermined value. 
     The active noise control device according to the present invention may further include the stabilization filter ( 70 ) configured to correct a magnitude of the second estimated cancellation signal input to the second virtual error signal generating unit so as to be increased if the state determination unit determines that the state of the control filter is unstable. 
     The active noise control device according to the present invention may further include the stabilization signal generating unit ( 72 ) configured to perform signal processing on the second estimated cancellation signal by a stabilization filter, which is an adaptive filter, to generate a stabilization signal, the third virtual error signal generating unit ( 53 ) configured to generate a third virtual error signal from the second virtual error signal and the stabilization signal, and the stabilization filter coefficient updating unit ( 74 ) configured to sequentially and adaptively update a coefficient of the stabilization filter based on the second estimated cancellation signal and the second virtual error signal in a manner that the magnitude of the second virtual error signal is minimized, wherein the control filter coefficient updating unit is configured to sequentially and adaptively update the coefficient of the control filter based on the reference signal and the third virtual error signal in a manner that a magnitude of the third virtual error signal is minimized. 
     The active noise control device according to the present invention may further include the primary path filter coefficient updating unit ( 38 ) configured to sequentially and adaptively update a coefficient of the primary path filter based on the basic signal and the first virtual error signal in a manner that the magnitude of the first virtual error signal is minimized. 
     The active noise control device according to the present invention may further include the primary path filter coefficient updating unit configured to sequentially and adaptively update a coefficient of the primary path filter based on the basic signal and the first virtual error signal in a manner that the magnitude of the first virtual error signal is minimized, wherein if the control target frequency is outside a predetermined range, the control filter coefficient updating unit may be configured to gradually decrease the coefficient of the control filter. 
     The present invention is not particularly limited to the embodiments described above, and various modifications are possible without departing from the essence and gist of the present invention.