Patent Publication Number: US-2022223134-A1

Title: Noise reduction device, vehicle, and noise reduction method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims priority of Japanese Patent Application No. 2021-3086 filed on Jan. 12, 2021. 
     FIELD 
     The present disclosure relates to an active noise reduction device that actively reduces noise by interfering a cancelling sound with the noise, a vehicle that includes the active noise reduction device, and an active noise reduction method. 
     BACKGROUND 
     Conventionally, an active noise reduction device is known that actively reduces noise by outputting a cancelling sound for cancelling out the noise from a cancelling sound source by using a reference signal that has a correlation with the noise and an error signal that is based on a residual sound generated through the interference between the noise and the cancelling sound in a predetermined space (see, for example, PTL 1). The active noise reduction device generates a cancelling signal for outputting the cancelling sound by using an adaptive filter so as to minimize the sum of squares of the error signal. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: WO 2014/006846 
       
    
     SUMMARY 
     The present disclosure provides an active noise reduction device capable of improving upon the above related art. 
     An active noise reduction device according to one aspect of the present disclosure includes: a first reference signal inputter to which a first reference signal that has a correlation with first noise in a space in a vehicle is input, the first reference signal being output by a first reference signal source attached to the vehicle; a first adaptive filter that generates a first cancelling signal by applying a first adaptive filter to the first reference signal that is input to the first reference signal inputter, the first cancelling signal being used to output a first cancelling sound for reducing the first noise; a first filter coefficient updater that updates a coefficient of the first adaptive filter; and a controller that determines, based on a first parameter of the first adaptive filter, whether first noise control based on the first cancelling sound is in a stable state or an unstable state. The controller transitions the first filter coefficient updater to a restriction state in which an effect of reducing the first noise is smaller than in a normal state when it is determined that the first noise control is in the unstable state while the first filter coefficient updater is updating the coefficient of the first adaptive filter in the normal state, and transitions the first filter coefficient updater back to the normal state when it is determined that the first noise control is in the stable state while the first filter coefficient updater is in the restriction state. 
     The active noise reduction device according to one aspect of the present disclosure is capable of improving upon the above related art. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, advantages and features of the present disclosure will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the present disclosure. 
         FIG. 1  is a schematic diagram of an automobile according to Embodiment 1 as viewed from above. 
         FIG. 2  is a block diagram showing a functional configuration of an active noise reduction device according to Embodiment 1. 
         FIG. 3  is a flowchart of a normal operation performed by the active noise reduction device according to Embodiment 1. 
         FIG. 4  is a flowchart of a restriction operation performed by the active noise reduction device according to Embodiment 1. 
         FIG. 5  is a flowchart of a transition back operation of example 1 performed by the active noise reduction device according to Embodiment 1. 
         FIG. 6  is a flowchart of a transition back operation of example 2 performed by the active noise reduction device according to Embodiment 1. 
         FIG. 7  is a flowchart of processing for fixing the active noise reduction device according to Embodiment 1 in the restriction operation. 
         FIG. 8  is a diagram showing an example of a specific operation performed by the active noise reduction device according to Embodiment 1. 
         FIG. 9  is a schematic diagram of an automobile according to Embodiment 2 as viewed from above. 
         FIG. 10  is a block diagram showing a functional configuration of an active noise reduction device according to Embodiment 2. 
         FIG. 11  is a diagram showing an overall operation of a signal processor. 
         FIG. 12  is a functional block diagram of a signal processor. 
         FIG. 13  is a flowchart of a normal operation performed by the signal processor. 
         FIG. 14  is a flowchart of a restriction operation of example 1 performed by the active noise reduction device according to Embodiment 2. 
         FIG. 15  is a flowchart of a restriction operation of example 2 performed by the active noise reduction device according to Embodiment 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described specifically with reference to the drawings. The embodiments described below shows generic or specific examples of the present disclosure. The numerical values, shapes, materials, structural elements, the arrangement and connection of the structural elements, steps, the order of the steps, and the like shown in the following embodiments are merely examples, and therefore are not intended to limit the scope of the present disclosure. Also, among the structural elements described in the following embodiments, structural elements not recited in any one of the independent claims are described as arbitrary structural elements. 
     Also, the diagrams are schematic representations, and thus are not necessarily true to scale. Also, in the diagrams, structural elements that are substantially the same are given the same reference numerals, and a redundant description may be omitted or simplified. 
     Embodiment 1 
     [Configuration of Automobile] 
     Hereinafter, an automobile according to Embodiment 1 and an active noise reduction device that is mounted on the automobile will be described. First, the automobile according to Embodiment 1 will be described.  FIG. 1  is a schematic diagram of the automobile according to Embodiment 1 as viewed from above. 
     Automobile  50  is an example of a vehicle, and includes active noise reduction device  10  according to Embodiment 1, first reference signal source  51 , cancelling sound source  52 , error signal source  53 , automobile main body  54 , and automobile controller  55 . Automobile  50  is specifically a passenger car, but the present disclosure is not limited thereto. 
     First reference signal source  51  is a transducer that outputs a reference signal that has a correlation with noise in space  56  of a cabin of automobile  50 . In Embodiment 1, first reference signal source  51  is an acceleration sensor, and is provided outside space  56 . Specifically, first reference signal source  51  is attached to the subframe near the left front wheel (or, the wheelhouse of the left front wheel). 
     However, there is no particular limitation on the attachment position of first reference signal source  51 . In the case where first reference signal source  51  is an acceleration sensor, active noise reduction device  10  can reduce a roadway noise component (an example of first noise) that is included in the noise in space  56 . The roadway noise has a complex propagation path, and it is therefore advantageous to provide acceleration sensors at a plurality of locations. Here, first reference signal source  51  may be a microphone. 
     Cancelling sound source  52  outputs a first cancelling sound to space  56  by using a first cancelling signal. In Embodiment 1, cancelling sound source  52  is a speaker, but the cancelling sound may be output as a result of one (for example, sunroof or the like) of the structural bodies of automobile  50  being vibrated by a driving mechanism such as an actuator. Also, in active noise reduction device  10 , a plurality of cancelling sound sources  52  may be used, and there is no particular limitation on the attachment positions of cancelling sound sources  52 . 
     Error signal source  53  detects a residual sound generated by interference between the noise and the cancelling sound in space  56 , and outputs an error signal based on the residual sound. It is desirable that error signal source  53  is a transducer such as a microphone and is disposed in space  56  such as a headliner. Here, automobile  50  may include a plurality of error signal sources  53 . 
     Automobile main body  54  is a structural body that includes a chassis, a body, and the like of automobile  50 . Automobile main body  54  forms space  56  (the space in the automobile cabin) in which cancelling sound source  52  and error signal source  53  are disposed. 
     Automobile controller  55  controls (drives) automobile  50  based on operations and the like of the driver of automobile  50 . Also, automobile controller  55  outputs an automobile state signal that indicates the state of automobile  50  to active noise reduction device  10 . Automobile controller  55  is, for example, an ECU (Electronic Control Unit), and is specifically implemented by using a processor, a microcomputer, a dedicated circuit, or the like. 
     Automobile controller  55  may be implemented by a combination of two or more of a processor, a microprocessor, and a dedicated circuit. 
     [Configuration of Active Noise Reduction Device] 
     Next, a configuration of active noise reduction device  10  will be described.  FIG. 2  is a block diagram showing a functional configuration of active noise reduction device  10 . 
     As shown in  FIG. 2 , active noise reduction device  10  includes first reference signal input terminal  11 , cancelling signal output terminal  12 , error signal input terminal  13 , automobile state signal input terminal  14 , first adaptive filter  15 , first simulated acoustic sound transfer characteristics filter  16 , first filter coefficient updater  17 , first storage  18 , and controller  19 . First adaptive filter  15 , first simulated acoustic sound transfer characteristics filter  16 , first filter coefficient updater  17 , and controller  19  are implemented by, for example, a processor such as a DSP (Digital Signal Processor) or a microcomputer executing software. First adaptive filter  15 , first simulated acoustic sound transfer characteristics filter  16 , first filter coefficient updater  17 , and controller  19  may be implemented by using hardware such as circuits. Also, some of first adaptive filter  15 , first simulated acoustic sound transfer characteristics filter  16 , first filter coefficient updater  17 , and controller  19  may be implemented by using software, and the remaining ones may be implemented by using hardware. 
     [Normal Operation] 
     As described above, active noise reduction device  10  performs a noise reduction operation. First, a normal operation performed by active noise reduction device  10  will be described with reference to  FIGS. 2 and 3 .  FIG. 3  is a flowchart of the normal operation performed by active noise reduction device  10 . 
     First, a first reference signal that has a correlation with noise N 0  is input from first reference signal source  51  to first reference signal input terminal  11  (S 11 ). First reference signal input terminal  11  is an example of a first reference signal inputter, and is specifically a terminal made of a metal or the like. 
     The reference signal input to first reference signal input terminal  11  is output to first adaptive filter  15  and first simulated acoustic sound transfer characteristics filter  16 . First adaptive filter  15  generates a first cancelling signal by applying (convolving) a first adaptive filter to the first reference signal that is input to first reference signal input terminal  11  (S 12 ). First adaptive filter  15  is implemented by using a so-called FIR filter or IIR filter. First adaptive filter  15  outputs the generated first cancelling signal to cancelling signal output terminal  12 . The first cancelling signal is used to output first cancelling sound N 1  for reducing noise NO, and is output to cancelling signal output terminal  12  (S 13 ). 
     Cancelling signal output terminal  12  is an example of a cancelling signal outputter, and is a terminal made of a metal or the like. The first cancelling signal generated by first adaptive filter  15  is output to cancelling signal output terminal  12 . 
     Cancelling signal output terminal  12  is connected to cancelling sound source  52 . Accordingly, the first cancelling signal is output to cancelling sound source  52  via cancelling signal output terminal  12 . Cancelling sound source  52  outputs first cancelling sound N 1  based on the first cancelling signal. 
     Error signal source  53  detects a residual sound generated by interference between noise NO and first cancelling sound N 1  generated by cancelling sound source  52  to correspond to the first cancelling signal, and outputs an error signal that corresponds to the residual sound. As a result, the error signal is input to error signal input terminal  13  (S 14 ). Error signal input terminal  13  is an example of an error signal inputter, and is a terminal made of a metal or the like. 
     Next, first simulated acoustic sound transfer characteristics filter  16  generates a first filtered reference signal by correcting the first reference signal by using acoustic sound transfer characteristics obtained by simulating simulated transfer characteristics from cancelling signal output terminal  12  to error signal input terminal  13  (S 15 ). In other words, the simulated transfer characteristics are obtained by simulating acoustic sound transfer characteristics from the position of cancelling sound source  52  to the position of error signal source  53  (or in other words, the acoustic sound transfer characteristics in space  56 ). The simulated transfer characteristics are measured in, for example, space  56  and stored in first storage  18  in advance. The simulated transfer characteristics may be determined by using an algorithm that does not use predetermined values. 
     First storage  18  is a storage device that stores the simulated transfer characteristics. First storage  18  also stores the coefficient of the adaptive filter, which will be described later, and the like. First storage  18  is specifically implemented by using a semiconductor memory or the like. In the case where first adaptive filter  15 , first simulated acoustic sound transfer characteristics filter  16 , first filter coefficient updater  17 , and controller  19  are implemented by using a processor such as a DSP, a control program that is executed by the processor is also stored in first storage  18 . First storage  18  may also store other parameters that are used in signal processing operations performed by first adaptive filter  15 , first simulated acoustic sound transfer characteristics filter  16 , first filter coefficient updater  17 , and controller  19 . 
     First filter coefficient updater  17  sequentially updates first adaptive filter coefficient W based on the error signal and the generated first filtered reference signal (S 16 ). 
     Specifically, first filter coefficient updater  17  calculates first adaptive filter coefficient W by using an LMS (Least Mean Square) method so as to minimize the sum of squares of the error signal, and outputs calculated first adaptive filter coefficient W to first adaptive filter  15 . Also, first filter coefficient updater  17  sequentially updates first adaptive filter coefficient W. First adaptive filter coefficient W is expressed by Equation 1 given below, where the vector of the error signal is represented by e, and the vector of the first filtered reference signal is represented by R. Here, n is a natural number, and represents the n-th sample in sampling period Ts. μ is a scalar quantity, and is a step size parameter for determining the update amount of adaptive filter coefficient W per sample. 
       [Math. 1] 
         W ( n+ 1)= W ( n )−μ× e ( n )× R ( b )  (Equation 1)
 
     First filter coefficient updater  17  may update adaptive filter coefficient W by using a method other than the LMS method. 
     [Restriction Operation] 
     Next, a restriction operation performed by active noise reduction device  10  will be described. The acoustic sound transfer characteristics of space  56  of automobile  50  vary when a window of automobile  50  is opened or when the temperature of space  56  of automobile  50  varies. In this case, a difference occurs between the acoustic sound transfer characteristics of space  56  and the simulated transfer characteristics stored in first storage  18 , and the noise control becomes unstable, as a result of which, first cancelling sound N 1  may be transformed into an abnormal sound. 
     To address this, when it is determined that the noise control is unstable during the normal operation, controller  19  performs a restriction operation of transitioning first filter coefficient updater  17  to a restriction state in which the effect of reducing noise NO is smaller than in a normal state. Hereinafter, the restriction operation will be described with reference to  FIG. 4 .  FIG. 4  is a flowchart of the restriction operation performed by active noise reduction device  10 . 
     While active noise reduction device  10  is performing the normal operation and first filter coefficient updater  17  is updating first adaptive filter coefficient W in the normal state (S 21 ), controller  19  acquires a first parameter of first adaptive filter  15  (S 22 ). Controller  19  determines, based on the acquired first parameter, whether first noise control that is based on first cancelling sound N 1  has changed from a stable state to an unstable state (S 23 ). 
     The first parameter is, for example, first adaptive filter coefficient W, but may be absolute value |ΔW| of the update amount of first adaptive filter coefficient W. More specifically, ΔW is calculated based on a second term of Equation 1. Also, the first parameter may be the amplitude of the first cancelling signal that is output by first adaptive filter  15 . Also, controller  19  may use, as the first parameter, two or more of first adaptive filter coefficient W, absolute value |ΔW| of the update amount of the first adaptive filter coefficient, and the level of the first cancelling signal. That is, it is sufficient that the first parameter includes at least one of first adaptive filter coefficient W, absolute value |ΔW| of the update amount of first adaptive filter coefficient W, and the level of the first cancelling signal. 
     It is considered that the first parameter takes a great value when the first noise control becomes unstable. Accordingly, for example, when the first parameter continuously remains to be greater than a first threshold value for a predetermined period or more while first filter coefficient updater  17  is in the normal state, controller  19  determines that the first noise control has changed from the stable state to the unstable state. The predetermined period and the first threshold value used in this case are determined empirically or experimentally as appropriate. Also, controller  19  may determine that the first noise control has changed from the stable state to the unstable state when it is determined that the first parameter exceeds the first threshold value a predetermined number of times or more in a predetermined period. The predetermined period, the first threshold value, and the predetermined number of times used in this case are determined empirically or experimentally as appropriate. 
     If it is determined that the first noise control is continuously in the stable state (No in S 23 ), the normal state is continued (S 21 ). On the other hand, if it is determined that the first noise control has changed from the stable state to the unstable state (Yes in S 23 ), controller  19  transitions first filter coefficient updater  17  to the restriction state (S 24 ). In other words, first filter coefficient updater  17  is transitioned to the restriction state based on an instruction from controller  19 . 
     As described above, the restriction state is a state in which the effect of reducing noise NO is smaller than in the normal state (or a state in which it is estimated that the effect is reduced). For example, first filter coefficient updater  17  in the restriction state updates first adaptive filter coefficient W by using step size parameter p that is smaller than that used in the normal state. First filter coefficient updater  17  may initialize first adaptive filter coefficient W to 0, and then update first adaptive filter coefficient W by using small step size parameter μ. If the value of step size parameter μ is too large, the adaptive filter is likely to diverge. If the value is too small, first filter coefficient updater  17  cannot update adaptive filter coefficient W in time, and thus the effect of reducing noise NO decreases. 
     Also, first filter coefficient updater  17  in the restriction state may stop the update of first adaptive filter coefficient W. Specifically, first filter coefficient updater  17  in the restriction state sets step size parameter μ in Equation 1 given above to 0, and continuously outputs the same first adaptive filter coefficient W to first adaptive filter  15 . First filter coefficient updater  17  may also stop the update of first adaptive filter coefficient W by setting W(n+1)=W(n), and not rewriting W. First filter coefficient updater  17  may initialize first adaptive filter coefficient W to 0, and then stop the update of first adaptive filter coefficient W. 
     Also, first filter coefficient updater  17  in the restriction state may stop the output of the first cancelling signal from first adaptive filter  15 . For example, first filter coefficient updater  17  stops the output of first cancelling sound N 1  by fixing first adaptive filter coefficient W to 0 (or in other words, by setting the amplitude of the first cancelling signal to 0). Also, first filter coefficient updater  17  in the restriction state may initialize first adaptive filter coefficient W to 0. 
     Also, first filter coefficient updater  17  in the restriction state may multiply first adaptive filter coefficient W by leak coefficient α that is less than 1. In addition, first filter coefficient updater  17  can fade out the first cancelling signal by stopping the update of first adaptive filter coefficient W and setting, for example, W(n+1)=αW(n). 
     In the manner described above, when it is determined that the first noise control is in the unstable state while first filter coefficient updater  17  is updating first adaptive filter coefficient W in the normal state, controller  19  transitions first filter coefficient updater  17  to the restriction state in which the effect of reducing the first noise is smaller than in the normal operation. By doing so, it is possible to prevent first cancelling sound N 1  from being transformed into an abnormal sound while the first noise control is being performed. 
     [Transition Back Operation of Example 1] 
     In general, transitioning from the restriction operation back to the normal operation is performed when active noise control device  10  is reset such as when the ignition power supply of automobile  50  is turned off and again turned on. That is, an ordinary active noise reduction device is transitioned from a restriction operation back to a normal operation when the supply of power to the active noise reduction device is resumed. In contrast, in active noise reduction device  10 , transitioning back to the normal operation can be performed even when the ignition power supply of automobile  50  is not turned off. Hereinafter, a transition back operation of example 1 performed in this case will be described with reference to  FIG. 5 .  FIG. 5  is a flowchart of the transition back operation of example 1 performed by active noise reduction device  10 . 
     While active noise reduction device  10  is performing the restriction operation, and first filter coefficient updater  17  is in the restriction state (S 31 ), controller  19  acquires a first parameter of first adaptive filter  15  (S 32 ). Controller  19  determines, based on the acquired first parameter, whether the first noise control has changed from the unstable state to the stable state (S 33 ). 
     As described above, the first parameter is, for example, first adaptive filter coefficient W, but may be absolute value |ΔW| of the update amount of first adaptive filter coefficient W, or the level of the first cancelling signal. Also, controller  19  may use, as the first parameter, two or more of first adaptive filter coefficient W, absolute value |ΔW| of the update amount of the first adaptive filter coefficient, and the level of the first cancelling signal. 
     It is considered that the first parameter takes a small value when the first noise control becomes stable. Accordingly, for example, when the first parameter continuously remains to be less than a second threshold value for a predetermined period or more while first filter coefficient updater  17  is in the restriction state, controller  19  determines that the first noise control has changed from the unstable state to the stable state. The predetermined period and the second threshold value used in this case are determined empirically or experimentally as appropriate. The second threshold value may be the same as or different from the first threshold value. Also, controller  19  may determine that the first noise control has changed from the unstable state to the stable state when the first parameter reaches a value less than a second threshold value a predetermined number of times or more in a predetermined period. The predetermined period, the second threshold value, and the predetermined number of times used in this case are determined empirically or experimentally as appropriate. The second threshold value may be the same as or different from the first threshold value. 
     If it is determined that the first noise control is continuously in the unstable state (No in S 33 ), the restriction operation (restriction state) is continued (S 31 ). On the other hand, if it is determined that the first noise control has changed from the unstable state to the stable state (Yes in S 33 ), controller  19  transitions first filter coefficient updater  17  back to the normal state (S 34 ). In other words, first filter coefficient updater  17  is transitioned back to the normal state based on an instruction from controller  19 . 
     As described above, controller  19  transitions first filter coefficient updater  17  back to the normal state when it is determined that the first noise control is in the stable state while first filter coefficient updater  17  is in the restriction state. By doing so, even when the ignition power supply is not turned off and on, active noise reduction device  10  can resume the normal operation at a timing at which it is estimated that the variation in the acoustic sound transfer characteristics is improved. 
     [Transition Back Operation of Example 2] 
     Active noise reduction device  10  may perform transitioning from the restriction operation back to the normal operation based on information indicating the state of automobile  50 . Hereinafter, a transition back operation of example 2 performed in this case will be described with reference to  FIG. 6 .  FIG. 6  is a flowchart of the transition back operation of example 2 performed by active noise reduction device  10 . Active noise reduction device  10  may perform both the transition back operation of example 1 and the transition back operation of example 2, or may perform only either one of the transition back operation of example 1 or the transition back operation of example 2. 
     While active noise reduction device  10  is performing the restriction operation, and first filter coefficient updater  17  is in the restriction state (S 41 ), controller  19  acquires an automobile state signal that indicates the state of automobile  50  from automobile state signal input terminal  14  (S 42 ). The automobile state signal is input to automobile state signal input terminal  14  by automobile controller  55  of automobile  50 . Controller  19  determines, based on the acquired automobile state signal, whether the state of automobile  50  has changed to a predetermined state (S 43 ). 
     The automobile state signal is a signal that indicates, for example, whether a window of automobile  50  is open or closed, and the predetermined state is a state in which, for example, the window of automobile  50  is closed. In this case, controller  19  determines, based on the acquired automobile state signal, whether the window of automobile  50  has changed from an open state to a closed state. As described above, when the window of automobile  50  is in the open state, a difference occurs between the acoustic sound transfer characteristics in space  56  of automobile  50  and the simulated transfer characteristics stored in first storage  18 . However, when the window of automobile  50  is in the closed state, it is considered that the difference between the acoustic sound transfer characteristics and the simulated transfer characteristics decreases, and thus noise NO can be effectively reduced by performing the normal operation. 
     Alternatively, the automobile state signal may be a signal that indicates the internal temperature of automobile  50 . In this case, the predetermined state is a state in which, for example, the temperature of space  56  is within a predetermined range. In this case, controller  19  determines, based on the acquired automobile state signal, whether the temperature of space  56  has changed from outside of the predetermined range to within the predetermine range. The simulated transfer characteristics stored in first storage  18  are set assuming that the temperature of space  56  is within the predetermined range. It is considered that, as long as the temperature of space  56  is within the predetermined range, the difference between the acoustic sound transfer characteristics and the simulated transfer characteristics decreases, and thus noise NO can be effectively reduced by performing the normal operation. The predetermined range is, for example, a range of 20° C. to 25° C., or the like. 
     If it is determined that the state of automobile  50  has not changed to the predetermined state (the state of automobile  50  remains in a non-predetermined state) (No in S 43 ), the restriction operation (restriction state) is continued (S 41 ). On the other hand, if it is determined that the state of automobile  50  has changed to the predetermined state (Yes in S 43 ), controller  19  transitions first filter coefficient updater  17  back to the normal state (S 44 ). In other words, first filter coefficient updater  17  is transitioned back to the normal state based on an instruction from controller  19 . 
     In the manner described above, controller  19  transitions first filter coefficient updater  17  back to the normal state when the state of automobile  50  indicated by the acquired automobile state signal has changed to the predetermined state while first filter coefficient updater  17  is in the restriction state. By doing so, even when the ignition power supply is not turned off and on, active noise reduction device  10  can resume the normal operation at a timing at which it is estimated that the variation in the acoustic sound transfer characteristics is improved. 
     [Transition Back Operation of Example 3] 
     Active noise reduction device  10  may perform a transition back operation composed of a combination of the transition back operation of example 1 and the transition back operation of example 2. That is, controller  19  may transition first filter coefficient updater  17  back to the normal state in the following both cases: when it is determined that the first noise control is in the stable state while first filter coefficient updater  17  is in the restriction state; and when it is determined that the state of automobile  50  indicated by the acquired automobile state signal has changed to the predetermined state while first filter coefficient updater  17  is in the restriction state. 
     [Processing for Fixing to Restriction Operation] 
     Active noise reduction device  10  may be configured such that, in the case where transitioning from the unstable state to the stable state and transitioning from the stable state to the unstable state are frequently repeated, active noise reduction device  10  is fixed in the restriction operation, and does not transition back to the normal operation unless the ignition power supply is turned off and on.  FIG. 7  is a flowchart of processing for fixing active noise reduction device  10  in the restriction operation. 
     In a state in which switching between the restriction operation and the normal operation is permitted (S 51 ), controller  19  determines whether transitioning from the stable state to the unstable state and transitioning from the unstable state to the stable state have been performed a predetermined number of times or more in a predetermined period (S 52 ). The predetermined period and the predetermined number of times used in this case are determined empirically or experimentally as appropriate. 
     If it is determined that transitioning from the stable state to the unstable state and transitioning from the unstable state to the stable state have not been performed a predetermined number of times or more in a predetermined period (No in S 52 ), switching between the restriction operation and the normal operation is permitted (S 51 ). 
     On the other hand, if it is determined that transitioning from the stable state to the unstable state and transitioning from the unstable state to the stable state have been performed a predetermined number of times or more in a predetermined period (Yes in S 52 ), controller  19  fixes first filter coefficient updater  17  in the restriction state so as to fix active noise reduction device  10  in the restriction operation (S 53 ). 
     In the manner described above, controller  19  fixes first filter coefficient updater  17  in the restriction state when it is determined that transitioning from the stable state to the unstable state and transitioning from the unstable state to the stable state have been performed a predetermined number of times or more in a predetermined period. By doing so, active noise reduction device  10  continuously performs the restriction operation when the variation in the acoustic sound transfer characteristics is not improved, and it is therefore possible to prevent first cancelling sound N 1  from being transformed into an abnormal sound. 
     When active noise reduction device  10  is fixed in the restriction operation as described above, basically, active noise reduction device  10  does not transition back to the normal operation unless the ignition power supply is turned off and on. However, active noise reduction device  10  may transition back to the normal operation at a timing at which the state of automobile  50  indicated by the automobile state signal has changed to the predetermined state. 
     Specific Operation Example 
     Hereinafter, a description will be given of an example of a specific operation of switching between the normal operation and the restriction operation by plotting variation of the first parameter in a graph.  FIG. 8  is a diagram showing an example of a specific operation performed by active noise reduction device  10 . The vertical axis shown in  FIG. 8  indicates first parameter, and specifically, absolute value |ΔW| of the update amount of first adaptive filter coefficient W. The horizontal axis shown in  FIG. 8  indicates time. 
     Active noise reduction device  10  initially performs the normal operation. In the normal operation, in the case where noise NO is stationary noise, at an early stage of adaptation, |ΔW| is large because the error relative to ideal coefficient W is large. However, |ΔW| converges toward 0 as the error becomes smaller. Actually, noise NO constantly varies, and thus |ΔW| repeatedly increases and decreases within a predetermined range. When the first noise control based on first cancelling sound N 1  becomes unstable, |ΔW| increases at an accelerated pace and exceeds a threshold value. The threshold value shown in  FIG. 8  corresponds to the first threshold value and the second threshold value described above. 
     When |ΔW| exceeds the threshold value a predetermined number of times or more for predetermined period Ta, controller  19  determines that the first noise control has changed from the stable state to the unstable state, and transitions first filter coefficient updater  17  to the restriction state at timing t 1  at which the determination was made. That is, active noise reduction device  10  performs the restriction operation. In the example shown in  FIG. 8 , predetermined period Ta is a period starting from a timing at which |ΔW| once has exceeded the threshold value, and the predetermined number of times is three. 
     In the example shown in  FIG. 8 , first filter coefficient updater  17  in the restriction state stops the update of first adaptive filter coefficient W and then multiplies first adaptive filter coefficient W by a leak coefficient that is greater than 0 and smaller than 1, or fades out the cancelling sound. Accordingly, |ΔW| is set to 0. 
     When |ΔW| continuously remains to be less than the threshold value for predetermined period Tb, controller  19  determines that the first noise control has changed from the unstable state to the stable state, and transitions first filter coefficient updater  17  to the normal state at timing t 2  at which the determination was made. That is, active noise reduction device  10  performs the normal operation. In the example shown in  FIG. 8 , predetermined period Tb is a period starting from a timing at which |ΔW| reached a value less than the threshold value. 
     After that, active noise reduction device  10  transitions from the normal operation to the restriction operation at timing t 3 , transitions from the restriction operation to the normal operation at timing t 4 , and transitions from the normal operation to the restriction operation at timing t 5 . That is, controller  19  determines that transitioning has been made from the stable state to the unstable state at timings t 1 , t 3 , and t 5 , and determines that transitioning has been made from the unstable state to the stable state at timings t 2  and t 4 . 
     Controller  19  determines that transitioning from the stable state to the unstable state and transitioning from the unstable state to the stable state have been performed a predetermined number of times or more for predetermined period Tc, and fixes first filter coefficient updater  17  in the restriction state at timing t 5  at which the determination was made. That is, active noise reduction device  10  performs the restriction operation after timing t 5 . In the example shown in  FIG. 8 , predetermined period Tc is a period starting from timing t 1  at which the restriction operation was first performed (or in other words, it was first determined that the first noise control has changed to the unstable state), and the predetermined number of time is five. 
     First filter coefficient updater  17  is fixed in the restriction state at timing t 5 , and thus even when predetermined period Tb elapses after timing t 5 , first filter coefficient updater  17  is not transitioned back to the normal state. After that, at timing t 6 , controller  19  determines, based on the automobile state signal, that the window of automobile  50  has changed from the open stage to the closed state, and transitions first filter coefficient updater  17  back to the normal state. That is, active noise reduction device  10  transitions back to the normal operation. 
     Advantageous Effects, Etc 
     As described above, active noise reduction device  10  includes: first reference signal input terminal  11  to which a first reference signal that has a correlation with first noise in space  56  of automobile  50  is input, the first reference signal being output by first reference signal source  51  attached to automobile  50 ; first adaptive filter  15  that generates a first cancelling signal by applying a first adaptive filter to the first reference signal that is input to first reference signal input terminal  11 , the first cancelling signal being used to output first cancelling sound N 1  for reducing the first noise; first filter coefficient updater  17  that updates a coefficient of the first adaptive filter; and controller  19  that determines, based on the first parameter of first adaptive filter  15 , whether first noise control based on first cancelling sound N 1  is in a stable state or an unstable state. Controller  19  transitions first filter coefficient updater  17  to a restriction state in which an effect of reducing the first noise is smaller than in a normal state when it is determined that the first noise control is in the unstable state while first filter coefficient updater  17  is updating the coefficient of the first adaptive filter in the normal state, and transitions first filter coefficient updater  17  back to the normal state when it is determined that the first noise control is in the stable state while first filter coefficient updater  17  is in the restriction state. 
     With active noise reduction device  10  configured as described above, first filter coefficient updater  17  can be transitioned back to the normal state when the first noise control becomes stable while preventing first cancelling sound N 1  from being transformed into an abnormal sound as a result of first filter coefficient updater  17  being transitioned to the restriction state. 
     Also, for example, controller  19  determines that the first noise control is in the unstable state when the first parameter exceeds a first threshold value for a predetermined period or more. 
     With active noise reduction device  10  configured as described above, the requirement that the first parameter continuously exceeds the first threshold value can be used to determine that the first noise control is in the unstable state. 
     Also, for example, controller  19  determines that the first noise control is in the unstable state when the first parameter exceeds a first threshold value a predetermined number of times or more in a predetermined period. 
     With active noise reduction device  10  configured as described above, the requirement that the first parameter frequently exceeds the first threshold value can be used to determine that the first noise control is in the unstable state. 
     Also, for example, controller  19  determines that the first noise control is in the stable state when the first parameter is less than a second threshold value for a predetermined period or more. 
     With active noise reduction device  10  configured as described above, the requirement that the first parameter is continuously less than the second threshold value can be used to determine that the first noise control is in the stable state. 
     Also, for example, controller  19  determines that the first noise control is in the stable state when the first parameter reaches a value less than the second threshold value a predetermined number of times or more in a predetermined period. 
     With active noise reduction device  10  configured as described above, the requirement that the first parameter frequently reaches a value less than the second threshold value can be used to determine that the first noise control is in the stable state. 
     Also, for example, controller  19  fixes first filter coefficient updater  17  in the restriction state when it is determined that transitioning from the stable state to the unstable state and transitioning from the unstable state to the stable state have been performed a predetermined number of times or more in a predetermined period. 
     With active noise reduction device  10  configured as described above, by fixing first filter coefficient updater  17  in the restriction state when the first noise control is unstable, it is possible to prevent first cancelling sound N 1  from being transformed into an abnormal sound. 
     Also, for example, controller  19  further acquires an automobile state signal that indicates the state of automobile  50 , and transitions first filter coefficient updater  17  back to the normal state when the state of automobile  50  indicated by the acquired automobile state signal changes to a predetermined state while first filter coefficient updater  17  is in the restriction state. 
     With active noise reduction device  10  configured as described above, when the state of automobile  50  changes to a state in which the variation in the acoustic sound transfer characteristics of space  56  is improved, first filter coefficient updater  17  can be transitioned back to the normal state. 
     Also, for example, the predetermined state is a state in which a window of automobile  50  is closed. 
     With active noise reduction device  10  configured as described above, when the window of automobile  50  is closed, first filter coefficient updater  17  can be transitioned back to the normal state. 
     Also, for example, the predetermined state is a state in which space  56  has a temperature within a predetermined range. 
     With active noise reduction device  10  configured as described above, when the temperature of space  56  is in the predetermined range, first filter coefficient updater  17  can be transitioned back to the normal state. 
     Also, for example, the first parameter includes first adaptive filter coefficient W. 
     With active noise reduction device  10  configured as described above, it is possible to determine, based on first adaptive filter coefficient W, whether the first noise control is in the stable state or the unstable state. 
     Also, for example, the first parameter includes the update amount of first adaptive filter coefficient W. 
     With active noise reduction device  10  configured as described above, it is possible to determine, based on the update amount of first adaptive filter coefficient W, whether the first noise control is in the stable state or the unstable state. 
     Also, for example, the first parameter includes the amplitude of the first cancelling signal. 
     With active noise reduction device  10  configured as described above, it is possible to determine, based on the amplitude of the first cancelling signal, whether the first noise control is in the stable state or the unstable state. 
     Also, for example, first filter coefficient updater  17  in the restriction state updates first adaptive filter coefficient W by using a step size parameter smaller than a step size parameter used in the normal state. 
     With active noise reduction device  10  configured as described above, by setting the step size parameter to a small value, it is possible to prevent first cancelling sound N 1  from being transformed into an abnormal sound. 
     Also, for example, first filter coefficient updater  17  in the restriction state stops the update of first adaptive filter coefficient W. 
     With active noise reduction device  10  configured as described above, by stopping the update of first adaptive filter coefficient W, it is possible to prevent first cancelling sound N 1  from being transformed into an abnormal sound. 
     Also, for example, first filter coefficient updater  17  in the restriction state stops the output of the first cancelling signal from first adaptive filter  15 . 
     With active noise reduction device  10  configured as described above, by stopping the output of the first cancelling signal, it is possible to prevent first cancelling sound N 1  from being transformed into an abnormal sound. 
     Also, automobile  50  includes active noise reduction device  10  and first reference signal source  51 . 
     With automobile  50  configured as described above, first filter coefficient updater  17  can be transitioned back to the normal state when the first noise control becomes stable while preventing first cancelling sound N 1  from being transformed into an abnormal sound as a result of first filter coefficient updater  17  being transitioned to the restriction state. 
     Also, an active noise reduction method executed by active noise reduction device  10  includes: determining, based on a first parameter of first adaptive filter  15 , whether first noise control based on first cancelling sound N 1  is in a stable state or an unstable state; and performing control to transition first filter coefficient updater  17  to a restriction state in which an effect of reducing the first noise is smaller than in a normal state when it is determined that the first noise control is in the unstable state while first filter coefficient updater  17  is updating a coefficient of first adaptive filter  15  in the normal state, and transition first filter coefficient updater  17  back to the normal state when it is determined that the first noise control is in the stable state while first filter coefficient updater  17  is in the restriction state. 
     With the active noise reduction method as described above, it is possible to transition first filter coefficient updater  17  back to the normal state when the first noise control becomes stable while preventing first cancelling sound N 1  from being transformed into an abnormal sound as a result of first filter coefficient updater  17  being transitioned to the restriction state. 
     Embodiment 2 
     [Configuration of Automobile] 
     Hereinafter, an automobile according to Embodiment 2 and an active noise reduction device that is mounted on the automobile will be described. First, the automobile according to Embodiment 2 will be described.  FIG. 9  is a schematic diagram of the automobile according to Embodiment 2 as viewed from above. 
     Automobile  50   a  shown in  FIGS. 9 and 10  includes engine  57  and engine controller  58  in addition to the structural elements of automobile  50 . 
     Engine  57  is a driving device that serves as a power source of automobile  50   a  and a noise source that produces noise in space  56 . Engine  57  is disposed in, for example, a space that is different from space  56 . 
     Engine controller  58  controls (drives) engine  57  based on an acceleration operation and the like of the driver of automobile  50   a . Also, engine controller  58  outputs a pulse signal (engine pulse signal) according to the number of revolutions (frequency) of engine  57  as a second reference signal. Engine controller  58  is an example of a second reference signal source. The frequency of the pulse signal is proportional to, for example, the number of revolutions (frequency) of engine  57 . Specifically, the pulse signal is a so-called tachopulse or the like. The second reference signal may be in any form as long as the second reference signal has a correlation with noise. 
     [Configuration of Active Noise Reduction Device] 
     Also, automobile  50   a  includes active noise reduction device  10   a .  FIG. 10  is a block diagram showing a functional configuration of active noise reduction device  10   a.    
     Active noise reduction device  10   a  includes second reference signal input terminal  21 , signal processor  20 , and adder  30  in addition to the structural elements of active noise reduction device  10 . 
     Second reference signal input terminal  21  is an example of a second reference signal inputter, and a second reference signal output by engine controller  58  is input to second reference signal input terminal  21 . Specifically, second reference signal input terminal  21  is a terminal made of a metal or the like. 
     Signal processor  20  performs signal processing for reducing noise (hereinafter also referred to as “second noise”) based on the sound of engine  57 . The second noise is, for example, a muffled sound based on the sound of engine  57 . The second noise is instantaneously a sound close to a sine wave of a single frequency. Accordingly, signal processor  20  acquires the second reference signal that indicates the frequency of engine  57  from engine controller  58  that controls engine  57 , and outputs a second cancelling sound for cancelling out the second noise from cancelling sound source  52 . The generation of the second cancelling sound is performed using an adaptive filter, and the second cancelling sound is generated such that the residual sound acquired by error signal source  53  is small.  FIG. 11  is a diagram showing an overall operation of signal processor  20 . 
     As shown in  FIG. 11 , the transfer characteristics from the position of cancelling sound source  52  (hereinafter also referred to as “sound output position”) to the position of error signal source  53  (hereinafter also referred to as “sound recording position”) are represented by c 1 , and the second cancelling signal for outputting the second cancelling sound is represented by a sign “out”. In this case, the second cancelling sound that arrives at the position of error signal source  53  (sound recording position) is represented by a sign “c 1 *out”. Here, a sign “*” indicates a convolutional operator, c 1  indicates an impulse response of transfer characteristics, and C 1  indicates simulated transfer characteristics in a frequency domain. 
     Second noise Nm at the position of error signal source  53  is expressed by Equation 2 given below, where amplitude is represented by R, angular frequency is represented by ω, and phase is represented by θ. Also, the sign “c 1 *out” is expressed by Equation 3-1 and Equation 3-2 given below. Active noise reduction device  10   a  can output the second cancelling sound for cancelling out second noise Nm by calculating first filter coefficient A and second filter coefficient B in Equation 3-1 and Equation 3-2 based on, for example, an LMS method. 
       [Math. 2] 
         N   m   =R ×sin(ω t +θ)  (Equation 2)
 
         c   i *out= R ×sin[ω t +(θ−π)]
 
     When C 1 =1, 
         c   1 *out= R ×sin[ω t +(θ−π)]= A ×sin(ω t )+ B ×cos(ω t )
 
       Where 
         R =√{square root over ( A   2   +B   2 )},θ−π=tan −1 ( B/A )  (Equation 3-1)
 
     When C 1 ≠1, 
         c   1 *out= R ×sin[ω t +(θ−π)]= A ′×sin(ω t )+ B ′×cos(ω t )
 
         R =√{square root over ( A′   2   +B′   2 )},θ−π=tan −1 ( B′/A ′)  (Equation 3-2)
 
       Where 
         A′+jB′=C   1 (ω)( A+jB )
 
     As described above, as a result of the second cancelling sound with a phase opposite to that of second noise Nm being output, the noise heard at the position of error signal source  53  is reduced. 
     [Detailed Configuration of Signal Processor and Normal Operation of Signal Processor] 
     Next, a detailed configuration of signal processor  20  and a normal operation performed by signal processor  20  will be described with reference to  FIGS. 12 and 13  in addition to  FIGS. 9 to 11 .  FIG. 12  is a functional block diagram of signal processor  20 .  FIG. 13  is a flowchart of a normal operation performed by signal processor  20 . 
     As shown in  FIG. 12 , signal processor  20  includes reference signal generator  22 , second adaptive filter  23 , corrector  24 , second filter coefficient updater  25 , and second storage  26 . Reference signal generator  22 , second adaptive filter  23 , corrector  24  and second filter coefficient updater  25  are implemented by, for example, a processor such as a DSP or a microcomputer executing software. Hereinafter, related structural elements will be described in detail for each step of the flowchart shown in  FIG. 13 . 
     [Generation of Reference Signal] 
     First, reference signal generator  22  generates a reference signal based on a reference signal input to second reference signal input terminal  21  (S 61  shown in  FIG. 13 ). More specifically, reference signal generator  22  identifies an instantaneous frequency of the second noise based on the second reference signal input to second reference signal input terminal  21 , and generates a reference signal that has the identified frequency. Reference signal generator  22  specifically includes frequency detector  22   a , sine wave generator  22   b , and cosine wave generator  22   c.    
     Frequency detector  22   a  detects the frequency of the pulse signal, and outputs the detected frequency to sine wave generator  22   b , cosine wave generator  22   c , and corrector  24 . In other words, frequency detector  22   a  identifies the instantaneous frequency of the second noise. 
     Sine wave generator  22   b  outputs a sine wave of the frequency detected by frequency detector  22   a  as a first reference signal. The first reference signal is an example of a reference signal, and is a signal expressed by sin(2nft)=sin(ωt), where the frequency detected by frequency detector  22   a  is represented by f. That is, the first reference signal has the frequency identified by frequency detector  22   a  (the same frequency as that of the second noise). The first reference signal is output to first filter  23   a  of second adaptive filter  23  and first corrected signal generator  24   b  of corrector  24 . 
     Cosine wave generator  22   c  outputs a cosine wave of the frequency detected by frequency detector  22   a  as a second reference signal. The second reference signal is an example of a reference signal, and is a signal expressed by cos(2nft)=cos(ωt), where the frequency detected by frequency detector  22   a  is represented by f. That is, the second reference signal has the frequency identified by frequency detector  22   a  (the same frequency as that of the second noise). The second reference signal is output to second filter  23   b  of second adaptive filter  23  and second corrected signal generator  24   c  of corrector  24 . 
     [Generation of Second Cancelling Signal] 
     Second adaptive filter  23  generates a second cancelling signal by applying a coefficient of the second adaptive filter to the reference signal generated by reference signal generator  22  (by multiplying the reference signal by the coefficient) (S 62  in  FIG. 13 ). In other words, second adaptive filter  23  applies the coefficient of the second adaptive filter to the second reference signal that was input to second reference signal input terminal  21  and was converted to a reference signal. The second cancelling signal is used to output the second cancelling sound for reducing the second noise, and is output to adder  30 . Second adaptive filter  23  includes first filter  23   a , second filter  23   b , and adder  23   c . Second adaptive filter  23  is a so-called adaptive notch filter. 
     First filter  23   a  multiplies the first reference signal output from sine wave generator  22   b  by a first filter coefficient. The first filter coefficient used in the multiplication is a filter coefficient that corresponds to A in Equation 3-1 and Equation 3-2 given above, and is sequentially updated by first updater  25   a  of second filter coefficient updater  25 . The first reference signal multiplied by the first filter coefficient is output to adder  23   c.    
     Second filter  23   b  multiplies the second reference signal output from cosine wave generator  22   c  by a second filter coefficient. The second filter coefficient used in the multiplication is a filter coefficient that corresponds to B in Equation 3-1 and Equation 3-2 given above, and is sequentially updated by second updater  25   b  of second filter coefficient updater  25 . The second reference signal multiplied by the second filter coefficient is output to adder  23   c.    
     Adder  23   c  adds the first reference signal that was multiplied by the first filter coefficient and output from first filter  23   a  and the second reference signal that was multiplied by the second filter coefficient and output from second filter  23   b . Adder  23   c  outputs, to adder  30 , a second cancelling signal obtained by the addition of the first reference signal multiplied by the first filter coefficient and the second reference signal multiplied by the second filter coefficient. 
     [Correction of Reference Signal] 
     Corrector  24  generates corrected reference signals by applying the simulated transfer characteristics stored in second storage  26  to the reference signals. That is, corrector  24  generates corrected reference signals by correcting the reference signals (S 63  in  FIG. 13 ). Corrector  24  includes controller  24   a , first corrected signal generator  24   b , and second corrected signal generator  24   c.    
     Specifically, the simulated transfer characteristics include a gain and a phase (phase delay) for each frequency. The simulated transfer characteristics are measured in, for example, space  56  for each frequency, and stored in second storage  26  in advance. That is, the gain and the phase used to correct the signal of the frequency are stored in second storage  26 . 
     Controller  24   a  acquires a frequency output by frequency detector  22   a , and reads a gain and a phase that correspond to the acquired frequency from second storage  26 . Also, controller  24   a  outputs the read gain and the read phase. 
     First corrected signal generator  24   b  generates a first corrected reference signal by correcting the first reference signal based on the gain and the phase output by controller  24   a . The first corrected reference signal is an example of a corrected reference signal. The first corrected reference signal is expressed by α·sin(ωt+φα), where the gain and the phase output by controller are represented by α and φα, respectively. The generated first corrected reference signal is output to first updater  25   a  of second filter coefficient updater  25 . 
     Second corrected signal generator  24   c  generates a second corrected reference signal by correcting the second reference signal based on the gain and the phase output by controller  24   a . The second corrected reference signal is an example of a corrected reference signal. The second corrected reference signal is expressed by β·cos(ωt+φβ), where the gain and the phase output by controller  24   a  are represented by β and φβ, respectively. The generated second corrected reference signal is output to second updater  25   b  of second filter coefficient updater  25 . 
     Second storage  26  is a storage device that stores the simulated transfer characteristics. Second storage  26  also stores the coefficient of the second adaptive filter and the like. Second storage  26  is specifically implemented by using a semiconductor memory or the like. In the case where signal processor  20  is implemented by using a processor such as a DSP, a control program that is executed by the processor is also stored in second storage  26 . Second storage  26  may also store other parameters that are used in signal processing performed by signal processor  20 . 
     [Update of Filter Coefficient] 
     Second filter coefficient updater  25  sequentially updates the coefficient (including the first filter coefficient and the second filter coefficient) of the second adaptive filter based on the error signal input to error signal input terminal  13  and the generated corrected reference signal (S 64  in  FIG. 13 ). 
     Specifically, second filter coefficient updater  25  includes first updater  25   a  and second updater  25   b.    
     First updater  25   a  calculates the first filter coefficient based on the first corrected reference signal acquired from first corrected signal generator  24   b  and the error signal acquired from error signal source  53 . Specifically, first updater  25   a  calculates the first filter coefficient by using an LMS method so as to minimize the error signal, and outputs the calculated first filter coefficient to first filter  23   a . Also, first updater  25   a  sequentially updates the first filter coefficient. First filter coefficient A (corresponding to A in Equation 3-1 and Equation 3-2 given above) is expressed by Equation 4 given below, where the first corrected reference signal is represented by r 1  and the error signal is represented by e. Here, n is a natural number and is a variable that indicates how many times updating has been performed (or in other words, a variable that indicates the number of updates). That is, A(n) indicates a state at the n-th update. μ is a scalar quantity, and is a step size parameter for determining the update amount of the filter coefficient per sample. 
       [Math. 3] 
         A ( n )= A ( n− 1)−μ× r   i ( n )× e ( n )  (Equation 4)
 
     Second updater  25   b  calculates the second filter coefficient based on the second corrected reference signal acquired from second corrected signal generator  24   c  and the error signal acquired from error signal source  53 . Specifically, second updater  25   b  calculates the second filter coefficient by using an LMS method so as to minimize the error signal, and outputs the calculated second filter coefficient to second filter  23   b . Also, second updater  25   b  sequentially updates the second filter coefficient. Second filter coefficient B (corresponding to B in Equation 3-1 and Equation 3-2 given above) is expressed by Equation 5 given below, where the second corrected reference signal is represented by r 2 , and the error signal is represented by e. 
       [Math. 4] 
         B ( n )= B ( n− 1)−μ× r   2 ( n )× e ( n )  (Equation 5)
 
     [Normal Operation of Active Noise Reduction Device] 
     As described in Embodiment 1, first adaptive filter  15  generates the first cancelling signal by applying the first adaptive filter to the first reference signal input to first reference signal input terminal  11 . The first cancelling signal is a signal for outputting the first cancelling sound for reducing the first noise (roadway noise). Here, the first noise control based on the first cancelling sound is noise control based on a filtered-X LMS algorithm, and first filter coefficient updater  17  updates first adaptive filter coefficient W based on the filtered-X LMS algorithm. 
     Also, as described in Embodiment 2, second adaptive filter  23  generates the second cancelling signal by applying the coefficient of the second adaptive filter to the reference signal generated by reference signal generator  22 . The second cancelling signal is a signal for outputting the second cancelling sound for reducing the second noise (a muffled sound based on the sound of engine  57 ). 
     Here, second noise control based on the second cancelling sound is noise control based on an SAN (Single-frequency Adaptive Notch filter) algorithm, and second filter coefficient updater  25  updates the coefficient of the second adaptive filter based on the SAN algorithm. 
     First adaptive filter  15  and second adaptive filter  23  may update adaptive filter coefficients W, A, and B by using a method other than the LMS method and the SAN algorithm. 
     Adder  30  of active noise reduction device  10   a  adds the first cancelling signal output from first adaptive filter  15  and the second cancelling signal output from second adaptive filter  23 , and outputs a cancelling signal obtained as a result of the addition to cancelling signal output terminal  12 . Adder  30  is implemented by using, for example, a processor such as a DSP, but may be implemented by using an addition circuit that uses a microcomputer, an operational amplifier, or the like. 
     As described above, when the cancelling signal obtained as a result of the addition is output from adder  30  to cancelling signal output terminal  12 , a cancelling sound generated by combining the first cancelling sound and the second cancelling sound is output from cancelling sound source  52 . By doing so, active noise reduction device  10   a  can reduce both the first noise in space  56  and the second noise. 
     In active noise reduction device  10   a , only one cancelling sound source  52  is used in both the first noise control and the second noise control, but the cancelling sound source that outputs the first cancelling signal and the cancelling sound source that outputs the second cancelling signal may be different. 
     [Restriction Operation of Active Noise Reduction Device] 
     Here, as in active noise reduction device  10 , in active noise reduction device  10   a  as well, controller  19  can determine whether the first noise control based on the first cancelling sound is in the stable state or the unstable state. Also, in active noise reduction device  10   a , controller  19  can determine whether the second noise control based on the second cancelling sound of signal processor  20  is in the stable state or the unstable state. Specifically, controller  19  can determine, based on the second parameter related to second adaptive filter  23 , whether the second noise control is in the stable state or the unstable state. 
     The second parameter is, for example, the coefficient (including first filter coefficient A and second filter coefficient B described above) of the second adaptive filter, but may be the absolute value of the update amount of the coefficient of the second adaptive filter, coefficient R, or the level of the second cancelling signal output by second adaptive filter  23 . Also, controller  19  may use, as the second parameter, two or more of the coefficient of the second adaptive filter, the absolute value of the update amount of the coefficient of the second adaptive filter, coefficient R, and the level of the second cancelling signal. That is, it is sufficient that the second parameter includes at least one of the coefficient of the second adaptive filter, the absolute value of the update amount of the coefficient of the second adaptive filter, coefficient R, and the level of the second cancelling signal. The method for determining whether the second noise control is in the stable state or the unstable state based on the second parameter is the same as that used in Embodiment 1. 
     Also, controller  19  can switch the state of second filter coefficient updater  25  between the normal state and the restriction state based on the result of determination. For example, controller  19  transitions second filter coefficient updater  25  to the restriction state in which the effect of reducing the second noise is smaller than in the normal state when it is determined that the second noise control is in the unstable state while second filter coefficient updater  25  is updating the coefficient of the second adaptive filter in the normal state. 
     The restriction state is a state in which the effect of reducing the second noise is smaller than in the normal state (or a state in which it is estimated that the effect is reduced). For example, second filter coefficient updater  25  in the restriction state updates the coefficient of the second adaptive filter by using step size parameter μ that is smaller than that used in the normal state. Second filter coefficient updater  25  in the restriction state may stop the update of the coefficient of the second adaptive filter, stop the output of the second cancelling signal from second adaptive filter  23 , or initialize the second adaptive filter coefficient to 0. Also, second filter coefficient updater  25  in the restriction state may set an upper limit for coefficient R. Also, second filter coefficient updater  25  in the restriction state may multiply coefficients A and B of the second adaptive filter by a leak coefficient that is less than 1. 
     Also, controller  19  transitions second filter coefficient updater  25  back to the normal state when it is determined that the second noise control is in the stable state while second filter coefficient updater  25  is in the restriction state. Controller  19  can also transition second filter coefficient updater  25  back to the normal state when the state of automobile  50   a  indicated by the acquired automobile state signal has changed to the predetermined state while second filter coefficient updater  25  is in the restriction state. 
     Also, controller  19  can also fix second filter coefficient updater  25  in the restriction state when it is determined that transitioning from the stable state to the unstable state and transitioning from the unstable state to the stable state have been performed a predetermined number of times or more in a predetermined period. 
     [Specific Example of Restriction Operation of Active Noise Reduction Device] 
     As described above, controller  19  can separately determine whether the first noise control is in the unstable state and whether the second noise control is in the unstable state. Here, controller  19  may change the state of the filter coefficient updater that performs another noise control based on the result of determination as to one of the first noise control and the second noise control.  FIG. 14  is a flowchart of a restriction operation of example 1 performed by active noise reduction device  10   a.    
     While active noise reduction device  10   a  is performing the normal operation, first filter coefficient updater  17  is updating first adaptive filter coefficient W in the normal state, and second filter coefficient updater  25  is updating the coefficient of the second adaptive filter in the normal state (S 71 ), controller  19  acquires a first parameter of first adaptive filter  15  (S 72 ). Controller  19  determines, based on the acquired first parameter, whether the first noise control has changed from the stable state to the unstable state (S 73 ). 
     If it is determined that the first noise control remains in the stable state (No in S 73 ), the normal state is continued (S 71 ). On the other hand, if it is determined that the first noise control has changed from the stable state to the unstable state (Yes in S 73 ), controller  19  transitions first filter coefficient updater  17  to the restriction state (S 74 ), and also transitions second filter coefficient updater  25  to the restriction state (S 75 ). 
     In the manner described above, controller  19  transitions second filter coefficient updater  25  to the restriction state when it is determined that the first noise control is in the unstable state while second filter coefficient updater  25  is in the normal state. With active noise reduction device  10  configured as described above, it is possible to prevent the second cancelling sound from being transformed into an abnormal sound when the first noise control based on the first cancelling sound becomes unstable. 
       FIG. 15  is a flowchart of a restriction operation of example 2 performed by active noise reduction device  10   a.    
     While active noise reduction device  10   a  is performing the normal operation, first filter coefficient updater  17  is updating first adaptive filter coefficient W in the normal state, and second filter coefficient updater  25  is updating coefficients A and B of the second adaptive filter in the normal state (S 81 ), controller  19  acquires a second parameter of second adaptive filter  23  (S 82 ). Controller  19  determines, based on the acquired second parameter, whether the second noise control has changed from the stable state to the unstable state (S 83 ). 
     If it is determined that the second noise control remains in the stable state (No in S 83 ), the normal state is continued (S 81 ). On the other hand, if it is determined that the first noise control has changed from the stable state to the unstable state (Yes in S 83 ), controller  19  transitions second filter coefficient updater  25  to the restriction state (S 84 ), and also transitions first filter coefficient updater  17  to the restriction state (S 85 ). 
     In the manner described above, controller  19  transitions first filter coefficient updater  17  to the restriction state when it is determined that the second noise control is in the unstable state while first filter coefficient updater  17  is in the normal state. With active noise reduction device  10  configured as described above, it is possible to prevent the first cancelling sound from being transformed into an abnormal sound when the second noise control based on the second cancelling sound becomes unstable. 
     Although not shown in the diagrams, in active noise reduction device  10   a , whether first filter coefficient updater  17  in the restriction state is transitioned back to the normal state may be determined based on whether the requirement that both the first noise control and the second noise control are in the stable state is satisfied. Likewise, whether second filter coefficient updater  25  in the restriction state is transitioned back to the normal state may be determined based on whether the requirement that both the first noise control and the second noise control are in the stable state is satisfied. 
     Advantageous Effects, Etc 
     As described above, active noise reduction device  10   a  includes: second reference signal input terminal  21  to which a second reference signal that has a correlation with second noise in a space in automobile  50   a  is input, the second reference signal being output by engine controller  58  attached to automobile  50   a ; second adaptive filter  23  that generates a second cancelling signal by applying a second adaptive filter to a reference signal that has a frequency identified based on the second reference signal input to second reference signal input terminal  21 , the second cancelling signal being used to output a second cancelling sound for reducing the second noise; and second filter coefficient updater  25  that updates a coefficient of the second adaptive filter based on an SAN algorithm. 
     Controller  19  determines, based on a second parameter of second adaptive filter  23 , whether second noise control based on the second cancelling sound is in a stable state or in an unstable state. Controller  19  transitions second filter coefficient updater  25  to a restriction state in which an effect of reducing the second noise is smaller than in a normal state when it is determined that the second noise control is in the unstable state while second filter coefficient updater  25  is updating the coefficient of the second adaptive filter in the normal state, and transitions second filter coefficient updater  25  back to the normal state when it is determined that the second noise control is in the stable state while second filter coefficient updater  25  is in the restriction state. First filter coefficient updater  17  updates a coefficient of a first adaptive filter based on a filtered-X LMS algorithm. 
     With active noise reduction device  10  configured as described above, it is possible to transition second filter coefficient updater  25  back to the normal state when the second noise control becomes stable while preventing the second cancelling sound from being transformed into an abnormal sound as a result of second filter coefficient updater  25  being transitioned to the restriction state. 
     Also, controller  19  transitions first filter coefficient updater  17  to the restriction state when it is determined that the second noise control is in the unstable state while first filter coefficient updater  17  is in the normal state, and transitions second filter coefficient updater  25  to the restriction state when it is determined that the first noise control is in the unstable state while second filter coefficient updater  25  is in the normal state. 
     With active noise reduction device  10   a  configured as described above, it is possible to prevent the second cancelling sound from being transformed into an abnormal sound when the first noise control based on the first cancelling sound becomes unstable. Also, with active noise reduction device  10   a , it is possible to prevent the first cancelling sound from being transformed into an abnormal sound when the second noise control based on the second cancelling sound becomes unstable. 
     The first noise is roadway noise, and the second noise is noise based on the engine sound of automobile  50   a.    
     With active noise reduction device  10   a  configured as described above, it is possible to reduce both the roadway noise and the noise based on the engine sound. 
     OTHER EMBODIMENTS 
     Up to here, Embodiments 1 and 2 have been described. However, the present disclosure is not limited to Embodiments 1 and 2 given above. 
     For example, Embodiment 1 given above was described focusing mainly on an active noise reduction device for reducing roadway noise, and Embodiment 2 given above was described focusing mainly on an active noise reduction device for reducing roadway noise and a muffled sound. However, the invention envisaged in the present disclosure also encompasses an active noise reduction device for reducing mainly a muffled sound. The active noise reduction device for reducing mainly a muffled sound has the same configuration as that of the active noise reduction device of Embodiment 2 except that, for example, the structural elements for reducing roadway noise are excluded. 
     Also, the active noise reduction devices according to Embodiments 1 and 2 given above may be incorporated in a vehicle other than an automobile. The vehicle may be, for example, an aircraft or a vessel. Also, the present disclosure may be implemented as a vehicle other than an automobile. 
     The configurations of the active noise reduction device according to Embodiments 1 and 2 given above are merely examples. For example, the active noise reduction devices may include a structural element such as a D/A converter, a filter, a power amplifier, or an A/D converter. 
     Also, in Embodiments 1 and 2 given above, the first reference signal inputter, the error signal inputter, and the cancelling signal outputter were described as different terminals, but may be configured as a single terminal. For example, by using a digital communication standard with which devices such as the first reference signal source, the error signal source, and the cancelling sound source can be connected in a chain, the reference signal inputter, the error signal inputter, and the cancelling signal outputter can be implemented by using a single terminal. 
     Also, the processing performed by the active noise reduction devices according to Embodiments 1 and 2 given above is merely an example. For example, a portion of the digital signal processing described in the embodiments given above may be implemented by analog signal processing. 
     Also, for example, in Embodiments 1 and 2 given above, the processing performed by a specific processor may be performed by another processor. Also, the order in which a plurality of processing operations are performed may be changed, and a plurality of processing operations may be performed in parallel. 
     Also, in Embodiments 1 and 2 given above, the structural elements may be implemented by executing a software program suitable for the structural element. The structural elements may be implemented by a program executor such as a CPU or a processor reading a software program recorded in a recording medium such a hard disk or a semiconductor memory and executing the software program. 
     Also, in Embodiments 1 and 2 given above, the structural elements may be implemented by using hardware. For example, the structural element may be circuits (or integrated circuits). These circuits may constitute one circuit as a whole, or may be separate circuits. Also, each of these circuits may be a general-purpose circuit or a dedicated circuit. 
     Also, the structural elements may be circuits (or integrated circuits). These circuits may constitute one circuit as a whole, or may be separate circuits. Also, each of these circuits may be a general-purpose circuit or a dedicated circuit. 
     Also, general and specific aspects of the present disclosure may be implemented by using a system, a device, a method, an integrated circuit, a computer program, or a computer-readable non-transitory recording medium such as a CD-ROM. Alternatively, general and specific aspects of the present disclosure may be implemented by using any combination of systems, devices, methods, integrated circuits, computer programs, or computer-readable recording media. 
     For example, the present disclosure may be implemented as an active noise reduction method that is executed by an active noise reduction device (a computer or a DSP), or may be implemented as a program for causing a computer or a DSP to execute the active noise reduction method. Also, the present disclosure may be implemented as a computer-readable non-transitory recording medium in which the program is recorded. Also, the present disclosure may be implemented as a vehicle (for example, an automobile) or an active noise reduction system. The vehicle or the active noise reduction system described above includes, for example, the active noise reduction device and the first reference signal source according to the embodiments given above. 
     The present disclosure also encompasses other embodiments obtained by making various modifications that can be conceived by a person having ordinary skill in the art to the above embodiments as well as embodiments implemented by any combination of the structural elements and the functions of the above embodiments without departing from the scope of the present invention. 
     While various embodiments have been described herein above, it is to be appreciated that various changes in form and detail may be made without departing from the spirit and scope of the present disclosure as presently or hereafter claimed. 
     Further Information about Technical Background to this Application 
     The disclosure of the following patent application including specification, drawings and claims is incorporated herein by reference in its entirety: Japanese Patent Application No. 2021-3086 filed on Jan. 12, 2021. 
     INDUSTRIAL APPLICABILITY 
     The active noise reduction device according to the present disclosure is useful as a device that can reduce noise in, for example, an automobile cabin.