Active noise control device

Abstract

The present invention provides an active noise control device. The active noise control device (10) generates a control signal for controlling a speaker (16) by processing a reference signal corresponding to a control object frequency (f) through a control filter (W) as an adaptive notch filter, and adaptively updates the coefficients (W0, W1) of the control filter (W) one by one, and compares the amplitude of a primary path filter (H^) with the amplitude of the control filter (W) to determine whether the state of the control filter (W) is unstable. Thus, even if the transfer characteristic changes, the noise can be reduced.

Description

Technical Field

[0001] The invention relates to an active noise control device. Background Art

[0002] An active noise control device is disclosed in Japanese Patent Publication No. 2008-239098. The active noise control device generates a control signal that causes a speaker to output a cancelling sound for cancelling noise, and the noise is transmitted from a transmission shaft to the inside of a vehicle. The control signal is generated by processing a reference signal by an adaptive filter. The reference signal is generated according to the rotation frequency of the transmission shaft. The adaptive filter is updated according to an error signal output by a microphone installed in the vehicle and a reference signal generated by correcting the reference signal by a correction value. Summary of the invention

[0003] In the active noise control device disclosed in Japanese Patent Application Publication No. 2008-239098, the transfer characteristic of the canceling sound between the speaker and the microphone is used as a correction value. This correction value is a transfer characteristic measured in advance. Therefore, if the transfer characteristic changes, the noise may not be reduced.

[0004] The purpose of the present invention is to solve the above technical problems.

[0005] The present invention provides an active noise control device, which performs active noise control for controlling a speaker according to an error signal, wherein the error signal changes according to a synthesized sound synthesized by noise transmitted from a vibration source and a canceling sound output from the speaker for canceling the noise, and the active noise control device comprises: a reference signal generating unit, which generates a reference signal corresponding to a control target frequency; a control signal generating unit, which performs signal processing on the reference signal through a control filter as an adaptive notch filter to generate a control signal for controlling the speaker; a first estimated cancellation signal generating unit, which performs signal processing on the control signal through a secondary path filter as an adaptive notch filter to generate a first estimated cancellation signal; an estimated noise signal generating unit, which performs signal processing on the reference signal through a primary path filter as an adaptive notch filter to generate an estimated noise signal; and a reference signal generating unit, which performs signal processing on the reference signal through the secondary path filter to generate an estimated noise signal. a control filter coefficient updating unit, which adaptively updates the coefficient of the secondary path filter based on the control signal and the first virtual error signal so as to minimize the magnitude of the first virtual error signal; a control filter coefficient updating unit, which adaptively updates the coefficient of the control filter based on the reference signal and the second virtual error signal so as to minimize the magnitude of the second virtual error signal; and a state determining unit, which determines whether the state of the control filter is an unstable state by comparing the amplitude of the primary path filter with at least the amplitude of the control filter.

[0006] The active noise control device of the present invention can reduce noise even if the transfer characteristics vary.

[0007] The above-mentioned objects, features and advantages will be easily understood from the following description of the embodiments with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 FIG. 2 is a diagram for explaining an overview of active noise control performed in the active noise control device.

[0009] Figure 2 This is a block diagram of an active noise control device using the method proposed by the present inventors.

[0010] Figure 3 is a block diagram of an active noise control device.

[0011] Figure 4 It is a diagram for explaining the updating of filter coefficients.

[0012] Figure 5 : is a flowchart showing the flow of the filter coefficient update process.

[0013] Figure 6 : is a flowchart showing the flow of the filter state determination process.

[0014] Figure 7 This is a block diagram of the signal processing unit.

[0015] Figure 8 This is a block diagram of the signal processing unit.

[0016] Fig. 9 : is a flowchart showing the flow of the filter state determination process.

[0017] Fig.10 is a block diagram of an active noise control device. DETAILED DESCRIPTION

[0018] [First embodiment]

[0019] Figure 1 2 is a diagram for explaining an overview of active noise control performed in active noise control device 10 .

[0020] The active noise control device 10 causes a speaker 16 provided in a vehicle cabin 14 of a vehicle 12 to output a cancelling sound. In this way, the engine roar (hereinafter referred to as noise) transmitted to the occupants in the vehicle cabin 14 due to the vibration of the engine 18 is reduced. The active noise control device 10 generates a control signal u0 based on an error signal e and an engine speed Ne. The error signal e is a signal output from a microphone 22 provided on a headrest 20a of a seat 20 in the vehicle cabin 14. A synthesized sound of the cancelling sound and the noise (hereinafter referred to as cancelling error noise) is input to the microphone 22. The engine speed Ne is detected by an engine speed sensor 24. The control signal u0 is a signal for causing the speaker 16 to output the cancelling sound.

[0021] [Regarding the active noise control device of the prior art]

[0022] In the prior art, an active noise control device is proposed. The active noise control device uses an adaptive notch filter (for example, a SAN (Single-frequency Adaptive Notch) filter) with a small amount of computational processing.

[0023] In the active noise control device of the prior art, first, a reference signal x is generated, which has the frequency of the noise to be silenced (the control target frequency). The active noise control device performs signal processing on the generated reference signal x through a control filter W as an adaptive notch filter. Based on this, a control signal u0 is generated. The active noise control device controls the speaker 16 according to the control signal u0, so that the speaker 16 outputs a cancellation sound for canceling the noise.

[0024] The filter W is updated and controlled by an adaptive algorithm (eg, LMS (Least Mean Square) algorithm) so that the error signal e output from the microphone 22 is minimized.

[0025] There is a transfer characteristic C in the transfer path of the sound from the speaker 16 to the microphone 22. Therefore, the transfer characteristic C needs to be considered in the update of the control filter W. In addition, the transfer characteristic C also includes the electronic circuit characteristics of the speaker 16 and the microphone 22. In the active noise control device of the prior art, the transfer characteristic C is identified in advance as the filter C^. The reference signal x corrected by the filter C^ is used to update the control filter W. This control system is called a Filtered-X type.

[0026] The filter C is a fixed filter identified in advance. Therefore, when the transfer characteristic C changes, the phase characteristic of the filter C may deviate greatly from the phase characteristic of the transfer characteristic C. In this case, the control filter W may diverge due to the update. Therefore, the canceling sound output from the speaker 16 may amplify the noise or generate an abnormal sound.

[0027] Therefore, the present inventors proposed a method for enabling the filter C^ to follow the change of the transfer characteristic C in active noise control. In this method, there is no need to identify the transfer characteristic C in advance. The present invention further improves the method already proposed by the present inventors. The following is a brief description of an active noise control device 100 using the method already proposed by the present inventors.

[0028] Figure 2 1 is a block diagram showing an active noise control device 100 using a method proposed by the present inventors. Hereinafter, the transmission path of sound from the engine 18 to the microphone 22 is referred to as a primary path. Also, hereafter, the transmission path of sound from the speaker 16 to the microphone 22 is referred to as a secondary path.

[0029] The active noise control device 100 includes a reference 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.

[0030] The reference signal generating unit 26 generates reference signals xc and xs according to the engine speed Ne. The reference signal generating unit 26 includes a frequency detecting circuit 26a, a cosine signal generator 26b, and a sine signal generator 26c.

[0031] The frequency detection circuit 26a detects the control target frequency f. The control target frequency f is the vibration frequency of the engine 18 detected based on the engine speed Ne. The cosine signal generator 26b generates a reference signal xc (=cos(2πft)) as a cosine signal of the control target frequency f. The sine signal generator 26c generates a reference signal xs (=sin(2πft)) as a sine signal of the control target frequency f. Here, t represents time.

[0032] The control signal generating unit 28 generates control signals u0 and u1 based on the reference signals xc and xs. The control signal generating unit 28 includes a first control filter 28a, a second control filter 28b, a third control filter 28c, a fourth control filter 28d, an adder 28e, and an adder 28f.

[0033] The control signal generation unit 28 uses a SAN filter as the control filter W. The control filter W has a filter W0 for the reference signal xc and a filter W1 for the reference signal xs. The control filter W is optimized by updating the coefficient W0 of the filter W0 and the coefficient W1 of the filter W1 in the control filter coefficient update unit 42 described later.

[0034] The first control filter 28a has a filter coefficient W0. The second control filter 28b has a filter coefficient W1. The third control filter 28c has a filter coefficient -W0. The fourth control filter 28d has a filter coefficient W1.

[0035] The reference signal xc corrected by the first control filter 28a and the reference signal xs corrected by the second control filter 28b are added in the adder 28e to generate the control signal u0. The reference signal xs corrected by the third control filter 28c and the reference signal xc corrected by the fourth control filter 28d are added in the adder 28f to generate the control signal u1.

[0036] The control signal u0 is converted into an analog signal by the digital-analog converter 17, and is output to the speaker 16. The speaker 16 is controlled according to the control signal u0, and a canceling sound is output from the speaker 16.

[0037] The first estimated cancellation signal generating unit 30 generates a first estimated cancellation signal y1^ based on the control signals u0 and u1. The first estimated cancellation signal generating unit 30 includes a first secondary path filter 30a, a second secondary path filter 30b, and an adder 30c.

[0038] In the first estimated cancellation signal generating unit 30, a SAN filter is used as the secondary path filter C^. In the secondary path filter coefficient updating unit 40 described later, the coefficient (C0^+iC1^) of the secondary path filter C^ is updated. Based on this, the secondary path transfer characteristic C is identified as the secondary path filter C^.

[0039] The first secondary path filter 30a has a filter coefficient C0^ which is a real part of the coefficient of the secondary path filter C^. The second secondary path filter 30b has a filter coefficient C1^ which is an imaginary part of the coefficient of the secondary path filter C^. The control signal u0 corrected in the first secondary path filter 30a and the control signal u1 corrected in the second secondary path filter 30b are added in the adder 30c to generate a first estimated cancellation signal y1^. The first estimated cancellation signal y1^ is an estimated signal equivalent to a signal of the cancellation sound y input to the microphone 22.

[0040] The estimated noise signal generator 32 generates an estimated noise signal d^ based on the reference signals xc and xs. The estimated noise signal generator 32 includes a first primary path filter 32a, a second primary path filter 32b, and an adder 32c.

[0041] In the estimated noise signal generating unit 32, the SAN filter is used as the primary path filter H^. In the primary path filter coefficient updating unit 38 described later, the coefficient (H0^+iH1^) of the primary path filter H^ is updated. Based on this, the transfer characteristic H of the primary path (hereinafter referred to as the primary path transfer characteristic H) is identified as the primary path filter H^.

[0042] The first primary path filter 32a has a filter coefficient H0^ which is a real part of the coefficient of the primary path filter H^. The second primary path filter 32b has a filter coefficient -H1^ obtained by inverting the polarity of the imaginary part of the coefficient of the primary path filter H^. The adder 32c adds the reference signal xc corrected in the first primary path filter 32a and the reference signal xs corrected in the second primary path filter 32b to generate an estimated noise signal d^. The estimated noise signal d^ is an estimated signal of a signal equivalent to the noise d input to the microphone 22.

[0043] The reference signal generator 34 generates reference signals r0 and r1 based on the reference signals xc and xs. The reference signal generator 34 includes a third secondary path filter 34a, a fourth secondary path filter 34b, a fifth secondary path filter 34c, a sixth secondary path filter 34d, an adder 34e, and an adder 34f.

[0044] In the reference signal generating unit 34, a SAN filter is used as the secondary path filter C^.

[0045] The third secondary path filter 34a has a filter coefficient C0^ which is a real part of the coefficient of the secondary path filter C^. The fourth secondary path filter 34b has a filter coefficient -C1^ obtained by inverting the polarity of the imaginary part of the coefficient of the secondary path filter C^. The fifth secondary path filter 34c has a filter coefficient C0^ which is a real part of the coefficient of the secondary path filter C^. The sixth secondary path filter 34d has a filter coefficient C1^ which is an imaginary part of the coefficient of the secondary path filter C^.

[0046] The reference signal xc corrected by the third secondary path filter 34a and the reference signal xs corrected by the fourth secondary path filter 34b are added in the adder 34e to generate a reference signal r0. The reference signal xs corrected by the fifth secondary path filter 34c and the reference signal xc corrected by the sixth secondary path filter 34d are added in the adder 34f to generate a reference signal r1.

[0047] The second estimated cancellation signal generating unit 36 ​​generates a second estimated cancellation signal y2^ based on the reference signals r0 and r1. The second estimated cancellation signal generating unit 36 ​​includes a fifth control filter 36a, a sixth control filter 36b, and an adder 36c.

[0048] In the second estimated cancellation signal generation unit 36, a SAN filter is used as the control filter W. The fifth control filter 36a has a filter coefficient W0. The sixth control filter 36b has a filter coefficient W1.

[0049] The adder 36c adds the reference signal r0 corrected by the fifth control filter 36a and the reference signal r1 corrected by the sixth control filter 36b to generate a second estimated cancellation signal y2̂. The second estimated cancellation signal y2̂ is an estimated signal corresponding to the signal of the cancellation sound y input to the microphone 22.

[0050] The analog-to-digital converter 44 converts the error signal e output from the microphone 22 from an analog signal to a digital signal.

[0051] The error signal e is input to the adder 46. The polarity of the estimated noise signal d^ generated by the estimated noise signal generating unit 32 is inverted by the inverter 48 and input to the adder 46. The polarity of the first estimated cancellation signal y1^ generated by the first estimated cancellation signal generating unit 30 is inverted by the inverter 50 and input to the adder 46. In the adder 46, the first virtual error signal e1 is generated. The adder 46 is equivalent to the first virtual error signal generating unit of the present invention.

[0052] The estimated noise signal d 2 generated by the estimated noise signal generating unit 32 is input to the adder 52. The second estimated cancellation signal y2 generated by the second estimated cancellation signal generating unit 36 ​​is input to the adder 52. ^ The signal is input to the adder 52. The adder 52 generates a second virtual error signal e2. The adder 52 corresponds to the second virtual error signal generating unit of the present invention.

[0053] The primary path filter coefficient updating unit 38 adaptively updates the primary path filter coefficient H in sequence based on the LMS algorithm. ＾ The coefficient of the first virtual error signal e1 is set to minimize the magnitude of the first virtual error signal e1. The primary path filter coefficient updating unit 38 includes a first primary path filter coefficient updating unit 38a and a second primary path filter coefficient updating unit 38b.

[0054] The first primary path filter coefficient updating unit 38a and the second primary path filter coefficient updating unit 38b update the filter coefficient H according to the following formula: ＾ 、H1 ＾ In the formula, n represents the number of time steps (n=0, 1, 2, ...), and μ0 and μ1 represent step parameters. The active noise control device 100 performs signal processing in a specified cycle. The time step represents the length of the cycle. The number of time steps represents the number of cycles of signal processing.

[0055] H0^ n+1 =H0^ n -μ0×e1 n ×c n

[0056] H1^ n+1=H1^ n -μ1×e1 n ×xs n

[0057] The filter coefficients H0 ^ and H1 ^ are repeatedly updated in the primary path filter coefficient updating unit 38. Based on this, the primary path transfer characteristic H is identified as the primary path filter H ^. In the active noise control device 100 using the SAN filter, the update formula of the coefficient of the primary path filter H ^ is composed of four arithmetic operations and does not include a convolution operation, so the calculation load caused by the update process of the filter coefficients H0 ^ and H1 ^ can be suppressed.

[0058] The secondary path filter coefficient updating unit 40 adaptively updates the coefficients of the secondary path filter C^ in sequence according to the LMS algorithm so as to minimize the magnitude of the first virtual error signal e1. The secondary path filter coefficient updating unit 40 includes a first secondary path filter coefficient updating unit 40a and a second secondary path filter coefficient updating unit 40b.

[0059] The first secondary path filter coefficient updating unit 40a and the second secondary path filter coefficient updating unit 40b update the filter coefficients C0 ^ and C1 ^ according to the following formula: μ2 and μ3 in the formula represent step size parameters.

[0060] C0^ n+1 =C0^ n -μ2×e1 n ×u0 n

[0061] C1^ n+1 =C1^ n -μ3×e1 n ×u1 n

[0062] In the secondary path filter coefficient updating unit 40, the filter coefficients C0^ and C1^ are repeatedly updated. Based on this, the secondary path transfer characteristic C is identified as the secondary path filter C^. In the active noise control device 100 using the SAN filter, the update formula of the filter coefficients C0^ and C1^ is composed of four arithmetic operations and does not include a convolution operation. Therefore, the calculation load caused by the update process of the filter coefficients C0^ and C1^ can be suppressed.

[0063] The control filter coefficient updating unit 42 adaptively updates the coefficients W0 and W1 of the control filter W in sequence according to the LMS algorithm so as to minimize the magnitude of the second virtual error signal e2. The control filter coefficient updating unit 42 includes a first control filter coefficient updating unit 42a and a second control filter coefficient updating unit 42b.

[0064] The first control filter coefficient updating unit 42a and the second control filter coefficient updating unit 42b update the filter coefficients W0 and W1 according to the following formula: μ4 and μ5 in the formula represent step size parameters.

[0065] W0 n+1 =W0 n -μ4×e2 n ×r0 n

[0066] W1 n+1 =W1 n -μ5×e2 n ×r1 n

[0067] In the control filter coefficient update unit 42, the filter coefficients W0 and W1 are repeatedly updated. Based on this, the control filter W is optimized. In the active noise control device 100 using the SAN filter, the update formula of the filter coefficients W0 and W1 is composed of four arithmetic operations and does not include convolution operations. Therefore, the calculation load generated by the update process of the filter coefficients W0 and W1 can be suppressed.

[0068] The noise to be silenced by the active noise control device 100 is the engine roar. The engine roar is mainly generated in the range of 40 [Hz] to 200 [Hz]. The active noise control device 100 generates a control signal u0 when the frequency (control target frequency f) detected by the frequency detection circuit 26a is within a predetermined range (for example, 40 [Hz] to 200 [Hz]), and outputs a canceling sound from the speaker 16.

[0069] [About improvements]

[0070] With respect to the present invention, the improved features of the active noise control device 100 using the above-mentioned method proposed by the present inventors will be described.

[0071] Figure 3 1 is a block diagram of the active noise control device 10 of the present embodiment. The structure of the signal processing unit 54 of the active noise control device 10 of the present embodiment is substantially the same as that of the above-mentioned active noise control device 100. 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 operation unit 62, an update value table operation unit 64, a result value table operation unit 66, an end state determination unit 68, and a filter state determination unit 69.

[0072] The active noise control device 10 has an operation processing device and a storage device not shown in the figure. The operation processing device has, for example, a processor such as a central processing unit (CPU), a microprocessor (MPU), and a memory composed of ROM, RAM, etc. The storage device is, for example, a hard disk, a flash memory, etc. The active noise control device 10 may not have a storage device. In this case, the active noise control device 10 can send and receive data with the storage device on the cloud through communication. The signal processing unit 54, the initial value table operation unit 62, the update value table operation unit 64, the result value table operation unit 66, the end state determination unit 68 and the filter state determination unit 69 are implemented by executing the program stored in the storage device by the operation processing unit.

[0073] The initial value table 56 is a table-shaped memory area provided in the ROM. The initial values ​​of the filter coefficients C0^ and C1^ of the secondary path filter C^ described later are stored in the initial value table 56. The update value table 58 is a table-shaped memory area provided in the RAM. The update values ​​of the filter coefficients C0^ and C1^ are stored in the update value table 58. The result value table 60 is a table-shaped memory area provided in the ROM. The result values ​​of the filter coefficients C0^ and C1^ are stored in the result value table 60.

[0074] The initial value table operation unit 62 performs writing of initial values ​​to the initial value table 56 , etc. The updated value table operation unit 64 performs writing of updated values ​​to the updated value table 58 , etc. The result value table operation unit 66 performs writing of result values ​​to the result value table 60 , etc.

[0075] The end state determination unit 68 determines the end cause of the active noise control. When one of the following three end causes occurs, the active noise control ends. The three end causes are the engine 18 stopping, the abnormality occurring in the active noise control, and the active noise control diverging. When the active noise control ends due to the engine stopping, the end state determination unit 68 determines that the active noise control ends normally. When the active noise control ends due to the abnormality occurring in the active noise control, the end state determination unit 68 determines that the active noise control ends abnormally. When the active noise control ends due to the divergence of the active noise control, the end state determination unit 68 determines that the active noise control ends abnormally.

[0076] The filter state determination unit 69 determines the state of the control filter W every time the filter coefficients W0 and W1 of the control filter W are updated. The filter state determination unit 69 corresponds to the state determination unit of the present invention. The determination of the state of the control filter W will be described in detail later.

[0077] The filter coefficients C0^, C1 in the secondary path filter coefficient update unit 40 of this embodiment are ^The updating process of the filter coefficient C0 in the secondary path filter coefficient updating unit 40 of the active noise control device 100 is similar to that of the filter coefficient C0 in the secondary path filter coefficient updating unit 40 of the active noise control device 100. ^ 、C1 ^ The update process is slightly different.

[0078] In the secondary path filter coefficient updating unit 40 of the active noise control device 100, the filter coefficients C0^ and C1^ are updated in the first secondary path filter coefficient updating unit 40a and the second secondary path filter coefficient updating unit 40b according to the following equation.

[0079] C0^ n+1 =C0^ n -μ2×e1 n ×u0 n

[0080] C1^ n+1 =C1^ n -μ3×e1 n ×u1 n

[0081] On the other hand, in the secondary path filter coefficient updating unit 40 of the signal processing unit 54 of the present embodiment, the first secondary path filter coefficient updating unit 40a and the second secondary path filter coefficient updating unit 40b update the filter coefficients C0^ and C1^ according to the following equation.

[0082] C0^(f) n+1 =C0^(f)_u-μ2×e1 n ×u0 n

[0083] C1^(f) n+1 =C1^(f)_u-μ3×e1 n ×u1 n

[0084] In the coefficients C0^(f)_u and C1^(f)_u in the above equation, update values ​​corresponding to the control target frequency f stored in the update value table 58 are input. Hereinafter, the first term on the right side of the update equation of the filter coefficients C0^ and C1^ may be referred to as the previous value.

[0085] In the proposed method, the filter coefficients C0^n and C1^n updated in the previous cycle (time step number n) are used as the previous values ​​of the update formula. That is, during the period from the update in the previous cycle (time step number n) to the update in the current cycle (time step number n+1), even if the control object frequency f changes, the filter coefficients C0^n and C1^n updated in the previous cycle are used as the previous values ​​of the update formula.

[0086] On the other hand, in this embodiment, the update value corresponding to the control object frequency f at the time of update in this cycle (time step number n+1) is used as the last value of the update formula. That is, when it is the control object frequency f, the filter coefficients C0^(f)_u and C1^(f)_u of the latest update time among the updated filter coefficients are used as the last value in the update formula. That is, in this embodiment, the last value is not limited to the value updated in the last time (time step number n).

[0087] The secondary path filter coefficient updater 40 copies the updated filter coefficients C0 ^ and C1 ^ to 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 generator 34 .

[0088] [Update of coefficients of secondary path filter]

[0089] Figure 4 : is a diagram for explaining the updating of filter coefficients C0^ and C1^. Figure 4 As shown, the initial value table 56 stores the initial values ​​C0^(f)_i and C1^(f)_i corresponding to the frequencies in a table form. The update value table 58 stores the update values ​​C0^(f)_u and C1^(f)_u corresponding to the frequencies in a table form. In addition, the result value table 60 stores the result values ​​C0^(f)_r and C1^(f)_r corresponding to the frequencies in a table form.

[0090] The initial value corresponding to each frequency stored in the initial value table 56 is set to any one of the following (i) to (vi).

[0091] (i) Measured value of secondary path transfer characteristic C for each frequency

[0092] (ii) Phase information of the measured value of the secondary path transfer characteristic C for each frequency

[0093] (iii) Measure the secondary path transfer characteristics C at representative frequencies and supplement the estimated values ​​of the secondary path transfer characteristics C based on the measured values

[0094] (iv) Measuring the secondary path transfer characteristic C at a representative frequency and supplementing the phase information of the estimated value of the secondary path transfer characteristic C based on the measured value

[0095] (v) The estimated value of the secondary path transfer characteristic C is estimated by the following formula:

[0096] C0^(f)=a(f)×cos(-2πfT)

[0097] C1^(f)=a(f)×sin(-2πfT)

[0098] Here, T is the time it takes for sound to reach the microphone 22 from the speaker 16, and a is the amplitude constant.

[0099] (vi) A convenient smaller value (when no specific initial value is set for the sake of system setting efficiency, etc.)

[0100] Figure 5 : is a flowchart showing the flow of the update process of the filter coefficients C0 ^, C1 ^. The update process of the filter coefficients C0 ^, C1 ^ is executed every time the active noise control is performed.

[0101] In step S1, the update value table operation unit 64 writes the initial value corresponding to each frequency in the initial value table 56 into the update value corresponding to each frequency in the update value table 58 ( Figure 4 Then, transfer to step S2.

[0102] In step S2, the frequency detection circuit 26a of the signal processing unit 54 detects the control target frequency f. Then, the process moves to step S3.

[0103] In step S3, the secondary path filter coefficient update unit 40 reads the update value corresponding to the control target frequency f as the previous value ( Figure 4 Then, transfer to step S4.

[0104] In step S4, the secondary path filter coefficient update unit 40 updates the filter coefficients C0^ and C1^. Then, the process moves to step S5.

[0105] In step S5, the update value table operation unit 64 writes the updated filter coefficients C0^ and C1^ into the update values ​​corresponding to the control target frequency f. Figure 4 (C)) Then, transfer to step S6.

[0106] In step S6, the end state determination unit 68 determines whether the active noise control has ended. If the active noise control has not ended, the process returns to step S2, and if the active noise control has ended, the process moves to step S7.

[0107] In step S7, the end state determination unit 68 determines whether the active noise control has ended normally. If it is determined that the active noise control has ended normally, the process proceeds to step S8, and if it is determined that the active noise control has ended abnormally or if it is determined that the active noise control has ended divergence, the process proceeds to step S10.

[0108] In step S8, the initial value table operation unit 62 determines whether rewriting of the initial value in the initial value table 56 is permitted. If rewriting of the initial value table 56 is permitted, the process proceeds to step S9, and if rewriting of the initial value table 56 is not permitted, the update process of the filter coefficients C0 ^ and C1 ^ is terminated.

[0109] In step S9, the initial value table operation unit 62 rewrites the initial values ​​corresponding to the frequencies in the initial value table 56 with the updated values ​​corresponding to the frequencies in the updated value table 58 ( Figure 4 After that, the update process of the filter coefficients C0^ and C1^ is terminated.

[0110] In step S10, the result value table operation unit 66 writes the update value corresponding to each frequency in the update value table 58 into the result value corresponding to each frequency in the result value table 60 ( Figure 4 After that, the update process of the filter coefficients C0 ^ and C1 ^ is terminated.

[0111] The initial value table 56 and the result value table 60 may be copied to a personal computer or the like connected to the vehicle 12. Accordingly, the update value stored in the initial value table 56 and the result value stored in the result value table 60 may be compared. Therefore, the cause of abnormality in the active noise control or the cause of divergence in the active noise control may be verified.

[0112] [Filter status determination process]

[0113] Figure 6 2 is a flowchart showing the flow of the filter state determination process executed in the filter state determination unit 69. The filter state determination process is executed every time the control filter W is updated.

[0114] In step S21, the filter state determination unit 69 calculates the amplitude A of the primary path filter H^. After that, the process moves to step S22. The amplitude A can also be called the amplitude characteristic of the primary path filter H^. The amplitude A of the primary path filter H^ can be obtained by the following formula.

[0115] A=|H^| 2 =H0^ 2 +H1^ 2

[0116] In step S22, the filter state determination unit 69 calculates the amplitude B of the filter characteristic after the secondary path filter C^ and the control filter W are connected in series, and then moves to step S23. The amplitude B represents the amplitude B of the filter characteristic after the secondary path filter C^ and the control filter W are connected in series. ^ The amplitude characteristic of the filter characteristics after being connected in series with the control filter W. The amplitude B can be obtained by the following formula.

[0117] B=|C^·W| 2 =(C0^·W0+C1^·W1) 2 +(C0^·W1-C1^·W0) 2

[0118] In addition, in the signal processing unit 54, coefficients normalized by the amplitude |C^| of the secondary path filter C^ can be used as the filter coefficients C0 and C1 of the secondary path filter C^. In this case, the amplitude B is obtained by the following equation.

[0119] B=|W| 2 =W0 2 +W1 2

[0120] In step S23, the filter state determination unit 69 determines whether the amplitude A is smaller than a predetermined value β. If the amplitude A is smaller than the predetermined value β, the filter state determination process ends, and if the amplitude A is greater than or equal to the predetermined value β, the process proceeds to step S24.

[0121] In step S24 , the filter state determination unit 69 determines whether the amplitude B is larger than the amplitude A. If the amplitude B is larger than the amplitude A, the process proceeds to step S25 , and if the amplitude B is smaller than the amplitude A, the process proceeds to step S26 .

[0122] In step S25 , the filter state determination unit 69 determines that the state of the control filter W is unstable. Thereafter, the filter state determination process ends.

[0123] In step S26 , the filter state determination unit 69 determines that the state of the control filter W is stable. Thereafter, the filter state determination process ends.

[0124] When it is determined that the state of the control filter W is unstable, the active noise control device 10 stops the active noise control.

[0125] [Effects]

[0126] The active noise control device 10 of the present embodiment is provided with an initial value table 56 and an update value table 58. According to this, the active noise control device 10 can set the initial values ​​of the filter coefficients C0^ and C1^ for each frequency. In addition, the active noise control device 10 can update the filter coefficients C0^ and C1^ for each frequency. Therefore, the active noise control device 10 can significantly improve the initial noise cancellation performance, especially after the start of active noise control. However, the secondary path filter C^ sometimes converges to a characteristic that is greatly 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 fully canceled by the canceling sound output from the speaker 16. In particular, when the phase characteristic of the secondary path filter C^ 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. In the case where the control filter W diverges, the active noise control device 10 stops the active noise control. However, before the active noise control is suspended, an abnormal sound is output from the speaker 16 .

[0127] Therefore, in the active noise control device 10 of the present embodiment, the filter state determination unit 69 compares the amplitude of the primary path filter H^ and the amplitude of the control filter W. The filter state determination unit 69 determines whether the state of the control filter W is unstable based on the comparison result. Accordingly, when it is determined that the state of the control filter W is unstable, the active noise control device 10 can stop the active noise control before the control filter W diverges. Therefore, the active noise control device 10 can suppress the abnormal sound output from the speaker 16 due to the divergence of the control filter W.

[0128] In the active noise control device 10 of the present embodiment, the filter state determination unit 69 determines that the state of the control filter W is unstable in the following case. The following case refers to the case where the amplitude A of the filter characteristic after the secondary path filter C^ and the control filter W are connected in series is larger than the amplitude B of the primary path filter H^. In addition, the amplitude A = |C^·W|. In addition, the amplitude B = |H^|. By Figure 2 As can be seen from the block diagram of FIG. 1 , when the active noise control is performed normally, H^=C^·W holds. When the amplitude A is greater than the amplitude B, a canceling sound that is more than necessary is output relative to the size of the noise. Therefore, it is determined that the state of the control filter W is unstable. Based on this, in the active noise control device 10, the filter state determination unit 69 can determine the state of the control filter W with high accuracy.

[0129] In addition, in the active noise control device 10 of the present embodiment, when the amplitude of the primary path filter H^ is less than a predetermined value, the filter state determination unit 69 does not determine the state of the control filter W. After the active noise control starts, the amplitudes of the primary path filter H^, the secondary path filter C^, and the control filter W are all small. In this state, even if the filter state determination unit 69 wants to determine the state of the control filter W, it may make an erroneous determination. Accordingly, in the active noise control device 10, the filter state determination unit 69 can suppress erroneous determination of the state of the control filter W.

[0130] [Second embodiment]

[0131] In this embodiment, when the state of control filter W becomes unstable, the volume of the canceling sound output from speaker 16 is suppressed. Two methods, method 1 and method 2, are described below as signal processing methods for suppressing the volume of the canceling sound output from speaker 16.

[0132] [Method 1]

[0133] Figure 7 : is a block diagram of the signal processing unit 54. In the signal processing unit 54 of the method 1, the signal processing unit 54 ( Figure 2 ) A stabilizing filter 70 is added. By setting the stabilizing filter 70, the size of the second estimated cancellation signal y2^ input to the adder 52 reaches (1+α) times. When it is determined that the state of the control filter W is stable, the stabilizing filter 70 is set to α=0. When it is determined that the state of the control filter W is unstable, the stabilizing filter 70 is set to a value of α that gradually increases with the passage of time. Accordingly, the second estimated cancellation signal y2^ input to the adder 52 can be increased to (1+α) times. Therefore, the second hypothetical error signal e2 generated by the adder 52 becomes larger. Accordingly, the amplitude of the control filter W can be suppressed. As a result, the size of the control signal u0 is suppressed, thereby suppressing the size of the cancellation sound output from the speaker 16.

[0134] [Method 2]

[0135] Figure 8 : is a block diagram of the signal processing unit 54. In the signal processing unit 54 of the method 2, the signal processing unit 54 ( Figure 2 ) is added with a stable signal generating unit 72. The stable signal generating unit 72 generates a stable signal αy2^. The stable signal αy2^ is generated by performing signal processing on the second estimated cancellation signal y2^ by a stable filter as an adaptive filter. In addition, in the signal processing unit 54 of the method 2, the signal processing unit 54 ( Figure 2) is added with an adder 53. The adder 53 generates a third virtual error signal e3 based on the second virtual error signal e2 and the stable signal αy2^. In addition, in the signal processing unit 54 of the method 2, the signal processing unit 54 ( Figure 2 ) A stable filter coefficient updating unit 74 is added. The stable filter coefficient updating unit 74 adaptively updates the filter coefficient α of the stable filter based on the second estimated cancellation signal y2^ and the second virtual error signal e2 in order to minimize the size of the second virtual error signal e2.

[0136] The second virtual error signal e2 generated by the adder 52 is input to the adder 53. The stable signal αy2^ generated by the stable signal generator 72 is input to the adder 53. The adder 53 generates a third virtual error signal e3. The adder 53 corresponds to the third virtual error signal generator of the present invention.

[0137] 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.

[0138] Accordingly, the second estimated cancellation signal y2^ included in the third virtual error signal e3 is increased by (1+α) times the second estimated cancellation signal y2^ included in the second virtual error signal e2. Therefore, it is possible to suppress the amplitude of the control filter W. Thus, it is possible to suppress the size of the control signal u0, thereby suppressing the size of the cancellation sound output from the speaker 16.

[0139] [Filter status determination process]

[0140] Fig. 9 2 is a flowchart showing the flow of the filter state determination process executed in the filter state determination unit 69. The filter state determination process is executed every time the control filter W is updated.

[0141] In step S31, the filter state determination unit 69 calculates the primary path filter H ^ Amplitude A. After that, transfer to step S32. Amplitude A can also be called the amplitude characteristic of the primary path filter H^. Amplitude A can be calculated by the following formula.

[0142] A=|H^| 2 =H0^ 2 +H1^ 2

[0143] In step S32, the filter state determination unit 69 calculates the amplitude B of the filter characteristic after the secondary path filter C^ and the control filter W are connected in series. Thereafter, the process moves to step S33. The amplitude B represents the amplitude characteristic of the filter characteristic after the secondary path filter C^ and the control filter W are connected in series. The amplitude B can be obtained by the following formula.

[0144] B=(1+α) 2 |C^·W| 2 =(1+α) 2 (C0^·W0+C1^·W1) 2 +(1+a) 2 (C0^·W1-C1^·W0) 2

[0145] In addition, in the signal processing unit 54, the value normalized by the amplitude |C^| of the secondary path filter C^ can be used as the filter coefficients C0^ and C1^ of the secondary path filter C^. In this case, the amplitude B is obtained by the following formula.

[0146] B=(1+α) 2 |W| 2 =(1+α) 2 W0 2 +(1+α) 2 W1 2

[0147] In step S33, the filter state determination unit 69 determines whether the amplitude A is less than the predetermined value β. If the amplitude A is less than the predetermined value β, the filter state determination process is terminated. If the amplitude A of the primary path filter H^ is greater than the predetermined value β, the process moves to step S34.

[0148] In step S34 , the filter state determination unit 69 determines whether the amplitude B is larger than the amplitude A. When the amplitude B is larger than the amplitude A, the process proceeds to step S35 . When the amplitude B is smaller than the amplitude A, the process proceeds to step S36 .

[0149] In step S35 , the filter state determination unit 69 determines that the state of the control filter W is unstable. Thereafter, the filter state determination process ends.

[0150] In step S36 , the filter state determination unit 69 determines that the state of the control filter W is stable. Thereafter, the filter state determination process ends.

[0151] In the case of the above method 1, when it is determined that the state of the control filter W is stable, the signal processing unit 54 sets the stable 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 to gradually increase with the passage of time.

[0152] [Effects]

[0153] The active noise control device 10 of the present embodiment includes a stabilizing filter 70. When the filter state determination unit 69 determines that the state of the control filter W is unstable, the stabilizing filter 70 is corrected so that the second estimated cancellation signal y2^ input to the adder 52 becomes larger. As a result, the second virtual error signal e2 generated by the adder 52 becomes larger. Therefore, the amplitude of the control filter W can be suppressed. Therefore, when the state of the control filter W is unstable, the size of the cancellation sound output from the speaker 16 can be suppressed. As a result, the amplification of noise caused by the cancellation sound and the generation of abnormal sound can be suppressed.

[0154] In addition, in the active noise control device 10 of the present embodiment, the stabilization signal generation unit 72 generates a stabilization signal αy2^. The stabilization signal αy2^ is generated by performing signal processing on the second estimated cancellation signal y2^ by the stabilization filter as an adaptive notch filter. In addition, the adder 53 generates a third virtual error signal e3 based on the second virtual error signal e2 and the stabilization signal αy2^. Moreover, the stabilization filter coefficient update unit 74 sequentially and adaptively updates the filter coefficient α of the stabilization filter based on the second estimated cancellation signal y2^ and the second virtual error signal e2 so that the magnitude of the second virtual error signal e2 is minimized. In addition, the control filter coefficient update unit 42 sequentially and adaptively updates the filter coefficients W0 and W1 of the control filter W based on the reference signals r0 and r1 and the third virtual error signal e3 so that the magnitude of the third virtual error signal e3 is minimized.

[0155] As a result, the third virtual error signal e3 generated by the adder 53 becomes larger. Therefore, the size of the control filter W can be suppressed. Therefore, when the state of the control filter W is unstable, the size of the canceling sound output from the speaker 16 can be suppressed. As a result, the amplification of noise caused by the canceling sound and the generation of abnormal sound can be suppressed.

[0156] [Third embodiment]

[0157] The signal processing unit 54 of the first embodiment and the second embodiment generates a control signal u0 and causes the speaker 16 to output a canceling sound when the following condition is satisfied. The following condition means that the control object frequency f is within a prescribed range (e.g., 40 [Hz] to 200 [Hz]). The control object frequency f is a frequency detected by the frequency detection circuit 26a. That is, when the control object frequency f is outside the prescribed range, the signal processing unit 54 of the first embodiment and the second embodiment does not generate the control signal u0. In this case, the updating of the primary path filter H^ is not performed. Therefore, even if time has passed since the start of active noise control, there is a case where the primary path filter H^ is not updated from the initial value (e.g., H0^=0, H1^=0). In this case, when the control object frequency f is within the prescribed range and the control signal u0 is started to be generated, it may take time for the control filter W to converge.

[0158] In the signal processing unit 54 of the present embodiment, even if the control target frequency f is outside the predetermined range, the generation of the control signal u0 and the updating of the primary path filter H 2 are continued.

[0159] Fig.10 FIG. 5 is a block diagram of a signal processing unit 54 used when the control target frequency f is outside a predetermined range. Figure 2 The signal processing unit 54 shown, Fig.10 The reference signal generating unit 34, the second estimated cancellation signal generating unit 36, and the adder 52 are omitted from the signal processing unit 54. In addition, the structure of the control filter coefficient updating unit 42 is different.

[0160] The control filter coefficient updating unit 42 includes a third control filter coefficient updating unit 42c and a fourth control filter coefficient updating unit 42d. The third control filter coefficient updating unit 42c performs a forgetting process on the control filter coefficient W0. The fourth control filter coefficient updating unit 42d performs a forgetting process on the control filter coefficient W1. The forgetting process is a process of decreasing the control filter coefficient W0 and the control filter coefficient W1 by multiplying the control filter coefficient W0 and the control filter coefficient W1 by a forgetting coefficient (for example, 0.999), respectively.

[0161] According to this, even when the control target frequency f is outside the prescribed range, the updating of the primary filter H^ can be continued, and the amplitude of the control filter W can be reduced. Therefore, when the control target frequency f is outside the prescribed range, the cancellation sound output from the speaker 16 can be faded out. In addition, when the control target frequency f enters the prescribed range from outside the prescribed range, the initial value of the control filter W is set to H^ / C^. According to this, the convergence of the control filter W can be accelerated, and the performance of the active noise control device 10 can be transiently improved.

[0162] [Technical invention obtained from the embodiment]

[0163] The invention that can be grasped from the above-mentioned embodiments will be described below.

[0164] The present invention provides an active noise control device (10) for performing active noise control for controlling a speaker (16) based on an error signal, wherein the error signal changes according to a synthesized sound synthesized from noise transmitted from a vibration source and a cancelling sound output from the speaker (16) for cancelling the noise. The active noise control device (10) comprises: a reference signal generating unit (26) for generating a reference signal corresponding to a control target frequency; a control signal generating unit (28) for performing signal processing on the reference signal through a control filter as an adaptive notch filter to generate a control signal for controlling the speaker; a first estimated cancelling signal generating unit (30) for performing signal processing on the control signal through a secondary path filter as an adaptive notch filter to generate a first estimated cancelling signal; an estimated noise signal generating unit (32) for performing signal processing on the reference signal through a primary path filter as an adaptive notch filter to generate an estimated noise signal; and a reference signal generating unit (34) for performing signal processing on the reference signal through the secondary path filter to generate an estimated noise signal. generating a reference signal; a second estimated cancellation signal generating unit (36) for performing signal processing on the reference signal through the control filter to generate a second estimated cancellation signal; a first hypothetical error signal generating unit (46) for generating a first hypothetical error signal based on the error signal, the first estimated cancellation signal and the estimated noise signal; a second hypothetical error signal generating unit (52) for generating a second hypothetical error signal based on the estimated noise signal and the second estimated cancellation signal; and a secondary path filter coefficient updating unit (40) for updating the coefficients of the secondary path filter based on the control filter. The invention relates to a control filter comprising: a control filter coefficient updating unit (42), which adaptively updates the coefficient of the control filter based on the reference signal and the first hypothetical error signal, so as to minimize the size of the first hypothetical error signal; a control filter coefficient updating unit (42), which adaptively updates the coefficient of the control filter based on the reference signal and the second hypothetical error signal, so as to minimize the size of the second hypothetical error signal; and a state determination unit (69), which determines whether the state of the control filter is an unstable state by comparing the amplitude of the primary path filter with at least the amplitude of the control filter.

[0165] In the active noise control device of the present invention, the state determination unit may determine that the state of the control filter is unstable when the amplitude after the control filter and the secondary path filter are connected in series is larger than the amplitude of the primary path filter.

[0166] In the active noise control device of the present invention, the state determination unit may not determine the state of the control filter at least when the amplitude of the primary path filter is smaller than a predetermined value.

[0167] The active noise control device of the present invention may include a stabilization filter (70) which, when the state determination unit determines that the state of the control filter is unstable, performs correction so as to increase the magnitude of the second estimated cancellation signal input to the second virtual error signal generation unit.

[0168] The active noise control device of the present invention may also include: a stabilization signal generating unit (72) for generating a stabilization signal by processing the second estimated cancellation signal using a stabilization filter as an adaptive notch filter; a third hypothetical error signal generating unit (53) for generating a third hypothetical error signal based on the second hypothetical error signal and the stabilization signal; and a stabilization filter coefficient updating unit (74) for adaptively updating the coefficient of the stabilization filter based on the second estimated cancellation signal and the second hypothetical error signal so as to minimize the size of the second hypothetical error signal; and a control filter coefficient updating unit for adaptively updating the coefficient of the control filter based on the reference signal and the third hypothetical error signal so as to minimize the size of the third hypothetical error signal.

[0169] The active noise control device of the present invention may also include a primary path filter coefficient updating unit (38), which adaptively updates the coefficients of the primary path filter in succession based on the reference signal and the first hypothetical error signal so as to minimize the size of the first hypothetical error signal.

[0170] The present invention is an active noise control device, comprising a primary path filter coefficient updating unit (38), wherein the primary path filter coefficient updating unit (38) adaptively updates the coefficient of the primary path filter in sequence according to the reference signal and the first hypothetical error signal so as to minimize the magnitude of the first hypothetical error signal; when the control object frequency is outside a prescribed range, the control filter coefficient updating unit gradually reduces the coefficient of the control filter.

Claims

1. An active noise control device (10) for performing active noise control on a loudspeaker according to an error signal, wherein: The error signal changes according to a synthesized sound synthesized from noise transmitted from a vibration source and a cancelling sound output from the speaker for cancelling the noise, and the active noise control device (10) is characterized in that: have: A reference signal generating unit (26) for generating a reference signal corresponding to a frequency of a control object; a control signal generating unit (28) for generating a control signal for controlling the speaker by performing signal processing on the reference signal through a control filter as an adaptive notch filter; a first estimated cancellation signal generating unit (30) for generating a first estimated cancellation signal by processing the control signal using a secondary path filter as an adaptive notch filter; an estimated noise signal generating unit (32) for generating an estimated noise signal by processing the reference signal using a primary path filter as an adaptive notch filter; a reference signal generating unit (34) for generating a reference signal by performing signal processing on the reference signal through the secondary path filter; a second estimated cancellation signal generating unit (36) for generating a second estimated cancellation signal by performing signal processing on the reference signal through the control filter; a first hypothetical error signal generating unit (46) for generating a first hypothetical error signal based on the error signal, the first estimated cancellation signal and the estimated noise signal; a second hypothetical error signal generating unit (52) for generating a second hypothetical error signal based on the estimated noise signal and the second estimated cancellation signal; A secondary path filter coefficient updating unit (40) is used to adaptively update the coefficient of the secondary path filter in sequence according to the control signal and the first hypothetical error signal so as to minimize the magnitude of the first hypothetical error signal; a control filter coefficient updating unit (42) which adaptively updates the coefficients of the control filter in sequence according to the reference signal and the second hypothetical error signal so as to minimize the magnitude of the second hypothetical error signal; and A state determination unit (69) determines whether the state of the control filter is an unstable state by comparing the amplitude of the primary path filter with at least the amplitude of the control filter.

2. The active noise control device according to claim 1, characterized in that: When the amplitude of the control filter and the secondary path filter connected in series is larger than the amplitude of the primary path filter, the state determination unit determines that the state of the control filter is unstable.

3. The active noise control device according to claim 1 or 2, characterized in that: At least when the amplitude of the primary path filter is smaller than a predetermined value, the state determination unit does not determine the state of the control filter.

4. The active noise control device according to claim 1 or 2, characterized in that: A stabilizing filter (70) is provided for correcting the second estimated cancellation signal input to the second virtual error signal generating unit so as to increase the magnitude thereof when the state determining unit determines that the state of the control filter is unstable.

5. The active noise control device according to claim 1 or 2, characterized in that: have: a stable signal generating unit (72) for generating a stable signal by performing signal processing on the second estimated cancellation signal using a stable filter as an adaptive filter; a third virtual error signal generating unit (53) for generating a third virtual error signal based on the second virtual error signal and the stable signal; and a stable filter coefficient updating unit (74) which adaptively updates the coefficient of the stable filter in sequence based on the second estimated cancellation signal and the second hypothetical error signal so as to minimize the magnitude of the second hypothetical error signal; The control filter coefficient updating unit adaptively updates the coefficients of the control filter in sequence based on the reference signal and the third virtual error signal so as to minimize the magnitude of the third virtual error signal.

6. The active noise control device according to claim 1 or 2, characterized in that: A primary path filter coefficient updating unit (38) is provided, which adaptively updates the coefficients of the primary path filter in succession based on the reference signal and the first hypothetical error signal so as to minimize the size of the first hypothetical error signal.

7. The active noise control device according to claim 1 or 2, characterized in that: A primary path filter coefficient updating unit is provided, wherein the primary path filter coefficient updating unit adaptively updates the coefficient of the primary path filter in sequence according to the reference signal and the first hypothetical error signal so as to minimize the magnitude of the first hypothetical error signal. When the control target frequency is out of a predetermined range, the control filter coefficient updating unit gradually decreases the coefficient of the control filter.

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