Patent Publication Number: US-11664001-B2

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. 2021-4545 filed on Jan. 14, 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, Patent Literature (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 
     However, the active noise reduction device according to PTL 1 described above can be improved upon. 
     In view of this, 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 reference signal inputter to which a reference signal that has a correlation with noise in a space in a vehicle is input, the reference signal being output by a reference signal source attached to the vehicle; an adaptive filter that generates a cancelling signal by applying an adaptive filter to the reference signal that is input to the reference signal inputter, the cancelling signal being used to output a cancelling sound for reducing the noise; and a filter coefficient updater that calculates a coefficient of the adaptive filter based on a predetermined update equation. At a first timing at which the output of the cancelling sound is started, the filter coefficient updater uses a first coefficient as an initial value of the update equation, the first coefficient being the coefficient of the adaptive filter calculated by the filter coefficient updater at a second timing that is prior to the first timing. 
     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 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 an embodiment as viewed from above. 
         FIG.  2    is a block diagram showing a functional configuration of an active noise reduction device according to the embodiment. 
         FIG.  3    is a flowchart of a basic operation performed by the active noise reduction device according to the embodiment. 
         FIG.  4    is a flowchart of an operation for achieving early adaptation of a cancelling sound. 
         FIG.  5    is a flowchart of a correction coefficient determining operation of example 1. 
         FIG.  6    is a flowchart of a correction coefficient determining operation of example 2. 
         FIG.  7    is a flowchart of an operation for selecting a coefficient according to an operation mode. 
         FIG.  8    is a flowchart of an operation for selecting whether to store a first coefficient in a nonvolatile storage area. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, an embodiment will be described specifically with reference to the drawings. The embodiment described below shows a generic or specific example 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 embodiment 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 embodiment, 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. 
     The term “timing” used in the embodiment given below is not used in a strict sense. For example, the expression “the timing at which the output of the cancelling sound is started” refers to a concept that has a certain time width and in which it is considered that the output of the cancelling sound is substantially started, and thus it does not mean a certain moment. The same applies to the timing at which the power source of the automobile is turned off, the timing at which the automobile stops moving, the timing at which the automobile resumes moving, and the like. 
     Embodiment 
     [Configuration of Automobile] 
     Hereinafter, an automobile according to an embodiment, and an active noise reduction device that is mounted on the automobile will be described. First, the automobile according to the embodiment will be described.  FIG.  1    is a schematic diagram of the automobile according to the embodiment as viewed from above. 
     Automobile  50  is an example of a vehicle, and includes active noise reduction device  10  according to the embodiment, 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. 
     Reference signal source  51  is a transducer that outputs a reference signal that has a correlation with noise in space  56  of the cabin of automobile  50 . In the present embodiment, reference signal source  51  is an acceleration sensor, and is provided outside space  56 . Specifically, 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 reference signal source  51 . In the case where reference signal source  51  is an acceleration sensor, active noise reduction device  10  can reduce a roadway noise component 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, reference signal source  51  may be a microphone. 
     Cancelling sound source  52  outputs a cancelling sound to space  56  by using a cancelling signal. In the present embodiment, 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 reference signal input terminal  11 , cancelling signal output terminal  12 , error signal input terminal  13 , automobile state signal input terminal  14 , adaptive filter  15 , simulated acoustic sound transfer characteristics filter  16 , filter coefficient updater  17 , and storage  18 . Adaptive filter  15 , simulated acoustic sound transfer characteristics filter  16 , and filter coefficient updater  17  are implemented by, for example, a processor such as a DSP (Digital Signal Processor) or a microcomputer executing software. Adaptive filter  15 , simulated acoustic sound transfer characteristics filter  16 , and filter coefficient updater  17  may be implemented by using hardware such as circuits. Also, some of adaptive filter  15 , simulated acoustic sound transfer characteristics filter  16 , and filter coefficient updater  17  may be implemented by using software, and the remaining ones may be implemented by using hardware. 
     [Basic Operation] 
     As described above, active noise reduction device  10  performs a noise reduction operation. First, a basic 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 basic operation performed by active noise reduction device  10 . 
     First, a reference signal that has a correlation with noise NO is input from reference signal source  51  to reference signal input terminal  11  (S 11 ). Reference signal input terminal  11  is an example of a reference signal inputter, and is specifically a terminal made of a metal or the like. 
     The reference signal input to reference signal input terminal  11  is output to adaptive filter  15  and simulated acoustic sound transfer characteristics filter  16 . Adaptive filter  15  generates a cancelling signal by applying (convolving) an adaptive filter to the reference signal input to reference signal input terminal  11  (S 12 ). Adaptive filter  15  is implemented by using a so-called FIR filter or IIR filter. Adaptive filter  15  outputs the generated cancelling signal to cancelling signal output terminal  12 . The cancelling signal is used to output 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 cancelling signal generated by 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 cancelling signal is output to cancelling sound source  52  via cancelling signal output terminal  12 . Cancelling sound source  52  outputs cancelling sound N 1  based on the cancelling signal. 
     Error signal source  53  detects a residual sound generated by interference between noise NO and cancelling sound N 1  generated by cancelling sound source  52  to correspond to the 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, simulated acoustic sound transfer characteristics filter  16  generates a first filtered reference signal by correcting the reference signal by using simulated transfer characteristics obtained by simulating acoustic sound 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, in space  56  and stored in storage  18  in advance. The simulated transfer characteristics may be determined by using an algorithm that does not use predetermined values. 
     Storage  18  is a storage device that stores the simulated transfer characteristics. Storage  18  is specifically implemented by using a semiconductor memory or the like. In the case where adaptive filter  15 , simulated acoustic sound transfer characteristics filter  16 , and filter coefficient updater  17  are implemented by using a processor such as a DSP, a control program that is executed by the processor is also stored in storage  18 . Storage  18  may also store other parameters that are used in signal processing operations performed by adaptive filter  15 , simulated acoustic sound transfer characteristics filter  16 , and filter coefficient updater  17 . Storage  18  includes volatile storage area  18   a  and nonvolatile storage area  18   b  as the areas in which adaptive filter coefficients, which will be described later, are stored. 
     Filter coefficient updater  17  sequentially updates adaptive filter coefficient W based on the error signal and the generated first filtered reference signal (S 16 ). 
     Specifically, filter coefficient updater  17  calculates 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 the calculated adaptive filter coefficient to adaptive filter  15 . Also, filter coefficient updater  17  sequentially updates the adaptive filter coefficient. 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 ( n )  (Equation 1)
 
     Filter coefficient updater  17  may update adaptive filter coefficient W by using a method other than the LMS method. 
     [Operation for Achieving Early Adaptation of Cancelling Sound] 
     In an active noise reduction device, there is a need to shorten the time required from when the output of a cancelling sound is started to when the effect of reducing noise is obtained. In general, in active noise reduction device  10 , at a timing (hereinafter also referred to as “first timing”) at which the output of cancelling sound N 1  is started for the first time, the initial value (or in other words, W(0)) of adaptive filter coefficient W is set to 0, and the calculation (update) of adaptive filter coefficient W based on a predetermined update equation (Equation 1 given above) is started. In this case, it takes time to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     To address this, filter coefficient updater  17  achieves an early adaptation of cancelling sound N 1  to noise NO by storing adaptive filter coefficient W that was calculated by filter coefficient updater  17  in nonvolatile storage area  18   b , and using adaptive filter coefficient W as the initial value.  FIG.  4    is a flowchart of an operation for achieving the early adaptation of cancelling sound N 1 . 
     While automobile  50  is driving (or in other words, moving), filter coefficient updater  17  calculates adaptive filter coefficient W based on the update equation (S 21 ). At this time, adaptive filter coefficient W of the previous sampling is stored in volatile storage area  18   a.    
     When active noise control device  10  is turned off such as when the ignition power supply of automobile  50  is turned off, filter coefficient updater  17  acquires a stop signal from automobile controller  55  via automobile state signal input terminal  14  (S 22 ). The stop signal is, in other words, a signal for notifying that automobile  50  has stopped. 
     In response to acquiring the stop signal, filter coefficient updater  17  stores adaptive filter coefficient W calculated by filter coefficient updater  17  at a most recent timing (hereinafter also referred to as “second timing”) in nonvolatile storage area  18   b  as a first coefficient (S 23 ). More specifically, filter coefficient updater  17  reads adaptive filter coefficient W calculated at the most recent timing and stored in volatile storage area  18   a , and stores adaptive filter coefficient W in nonvolatile storage area  18   b  as a first coefficient. 
     As used herein, the term “adaptive filter coefficient W calculated at the most recent timing” is not used in a strict sense, and may be, for example, a substantially final value of adaptive filter coefficient W. Adaptive filter coefficient W calculated at the most recent timing may be the average value, median value, maximum value, or the like of adaptive filter coefficient W calculated in the most recent predetermined period. After step S 23 , the supply of power from the battery of automobile  50  to active noise reduction device  10  is turned off, and thus active noise reduction device  10  is transitioned to a power off state (S 24 ). 
     After that, when the engine of automobile  50  is turned on, active noise reduction device  10  receives a supply of power from the battery of automobile  50  and is activated (S 25 ), and then starts outputting cancelling sound N 1 . At this time, filter coefficient updater  17  reads the first coefficient that was stored in nonvolatile storage area  18   b  in step S 23  (S 26 ), and calculates adaptive filter coefficient W based on the update equation by using the read first coefficient as the initial value (S 27 ). 
     As described above, filter coefficient updater  17  uses the first coefficient as the initial value of the updated equation at a first timing before the output of cancelling sound N 1  is started, the first coefficient being adaptive filter coefficient W calculated by filter coefficient updater  17  at a second timing that is prior to the first timing. By doing so, active noise reduction device  10  can shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Also, in the example shown in  FIG.  4   , the second timing is a timing at which automobile  50  stops driving, and the first timing is, for example, a timing that is immediately after automobile  50  stops driving and at which automobile  50  starts driving again. In the case where active noise reduction device  10  is designed to reduce roadway noise, it is highly likely that automobile  50  is driving the same type of roadway when automobile  50  stops driving and then resumes driving. That is, it is highly likely that adaptive filter coefficient W obtained when automobile  50  stops driving is also effective when automobile  50  resumes driving. Accordingly, the configuration of the operation of example 1 is considered to be particularly effective in active noise reduction device  10  that is designed to reduce roadway noise. 
     In the example shown in  FIG.  4   , filter coefficient updater  17  stores the first coefficient in nonvolatile storage area  18   b  in response to acquiring a stop signal (or in other words, when automobile  50  stops driving). However, this configuration is not a requirement. For example, filter coefficient updater  17  may regularly store the first coefficient in nonvolatile storage area  18   b  while automobile  50  is driving, irrespective of the stop signal. Also, the first coefficient may be stored in nonvolatile storage area  18   b  in response to a signal other than the stop signal being acquired. It is sufficient that the first coefficient is stored at the second timing that is prior to the first timing. 
     [Correction of First Coefficient] 
     In the case where the state of automobile  50  at the second timing at which the first coefficient is stored and the state of automobile  50  at the first timing at which the first coefficient is read are significantly different, when the first coefficient is used as the initial value, the first coefficient may not be appropriate for noise NO that is the current noise, and thus cancelling sound N 1  output at the first timing may be transformed into an abnormal sound. 
     To address this, at the first timing, filter coefficient updater  17  may use a corrected first coefficient as the initial value of the update equation instead of the first coefficient, the corrected first coefficient being obtained by multiplying the first coefficient by a correction coefficient that is greater than 0 and less than 1. By doing so, active noise reduction device  10  can shorten the time required to obtain the effect of reducing noise N 0  by using cancelling sound N 1  while preventing cancelling sound N 1  from being transformed into an abnormal sound. 
     The correction coefficient is, for example, a fixed value determined in advance, but may be determined according to the length of the period from the second timing to the first timing.  FIG.  5    is a flowchart of a correction coefficient determining operation of example 1 performed in this case. 
     Filter coefficient updater  17  stores the second timing in storage  18 , and determines, at the first timing, whether the length of the period from the second timing to the first timing is greater than or equal to a threshold value (S 31 ). 
     If it is determined that the length of the period (elapsed time) from the second timing to the first timing is greater than or equal to the threshold value (Yes in S 31 ), filter coefficient updater  17  corrects the first coefficient by using a first correction coefficient (S 32 ). On the other hand, if it is determined that the length of the period from the second timing to the first timing is less than the threshold value (No in S 31 ), filter coefficient updater  17  corrects the first coefficient by using a second correction coefficient that is greater than the first correction coefficient (S 33 ). The first correction coefficient and the second correction coefficient are both values that are greater than 0 and less than 1. 
     As described above, filter coefficient updater  17  determines the correction coefficient to a smaller value as the length of the period from the second timing to the first timing is longer. In general, as the length of the period from the second timing to the first timing is longer, it is more likely that the state of automobile  50  varies. With active noise reduction device  10 , by setting the correction coefficient to be a smaller value as the length of the period from the second timing to the first timing is longer, it is possible to prevent cancelling sound N 1  from being transformed into an abnormal sound. 
     Also, the correction coefficient may be determined according to the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing.  FIG.  6    is a flowchart of a correction coefficient determining operation of example 2 performed in this case. 
     Filter coefficient updater  17  stores the temperature of space  56  at the second timing in storage  18 , and determines, at the first timing, whether the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing (temperature difference) is greater than or equal to a threshold value (S 41 ). Filter coefficient updater  17  can monitor the temperature of space  56  by, for example, acquiring a signal that indicates the temperature of space  56  output by automobile controller  55  as an automobile state signal via automobile state signal input terminal  14 . 
     If it is determined that the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing is greater than or equal to the threshold value (Yes in S 41 ), filter coefficient updater  17  corrects the first coefficient by using a first correction coefficient (S 42 ). On the other hand, if it is determined that the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing is less than the threshold value (No in S 41 ), filter coefficient updater  17  corrects the first coefficient by using a second correction coefficient that is greater than the first correction coefficient (S 43 ). The first correction coefficient and the second correction coefficient are both values that are greater than 0 and less than 1. 
     As described above, filter coefficient updater  17  determines the correction coefficient to a smaller value as the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing is larger. The fact that the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing is large means that the state of automobile  50  varies significantly. Accordingly, with active noise reduction device  10 , by determining the correction coefficient to a smaller value as the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing is larger, it is possible to prevent cancelling sound N 1  from being transformed into an abnormal sound. 
     In the examples shown in  FIGS.  5  and  6   , filter coefficient updater  17  changes the value of the correction coefficient in two stages, but the value of the correction coefficient may be finely changed in three or more stages by using a plurality of different threshold values. For example, by storing, in storage  18 , a calculation equation that indicates the relationship between elapsed time and correction coefficient or table information that indicates the relationship between elapsed time and correction coefficient, filter coefficient updater  17  can finely determine the correction coefficient by using the calculation equation or the table information stored in storage  18 . Likewise, for example, by storing, in storage  18 , a calculation equation that indicates the relationship between temperature difference and correction coefficient or table information that indicates the relationship between temperature difference and correction coefficient, filter coefficient updater  17  can finely determine the correction coefficient by using the calculation equation or the table information stored in storage  18 . 
     Also, the correction coefficient may be determined according to the difference between a reference temperature (for example, a temperature of 20° C. or more and 25° C. or less) and the temperature of space  56  at the first timing. In this case, the correction value (correction amount) per unit temperature (for example, 1° C.) may be changed based on whether the temperature of space  56  at the first timing is lower or higher than the reference temperature. The reference temperature is, for example, the temperature of space  56  when measuring the acoustic sound transfer function. In general, when the temperature changes on the minus side relative to the reference temperature, the acoustic sound transfer characteristics vary more significantly than when the temperature changes on the plus side relative to reference temperature. When the temperature of space  56  at the first timing is lower than the reference temperature, by setting the correction amount to be larger than that when the temperature of space  56  at the first timing is higher than the reference temperature, it is possible to implement a correction according to the variation of the acoustic sound transfer characteristics. 
     Also, filter coefficient updater  17  may determine whether to continuously use the first coefficient or use the corrected first coefficient according to the length of the period from the second timing to the first timing. Likewise, filter coefficient updater  17  may determine whether to continuously use the first coefficient or use the corrected first coefficient according to the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing. 
     [Operation Mode] 
     In the case where active noise reduction device  10  is configured to be operable in a plurality of operation modes, filter coefficient updater  17  may store coefficient W that is used as the initial value for each operation mode. The plurality of operation modes include: for example, an operation mode in which the noise near one seat in space  56  is reduced; an operation mode in which the noise near each of four seats in space  56  is reduced; and the like. In the case where the noise near each of a plurality of seats in space  56  is reduced, error signal source  53  is provided for each of the plurality of seats. The following description will be given by defining two operation modes out of the plurality of operation modes as a first operation mode and a second operation mode. 
     For example, in response to acquiring a stop signal while active noise reduction device  10  is operating in the first operation mode, filter coefficient updater  17  stores adaptive filter coefficient W in nonvolatile storage area  18   b  as a first coefficient, adaptive filter coefficient W being calculated by filter coefficient updater  17  at the most recent timing (or in other words, the timing at which active noise reduction device  10  was operating in the first operation mode). On the other hand, in response to acquiring a stop signal while active noise reduction device  10  is operating in the second operation mode, filter coefficient updater  17  stores adaptive filter coefficient W in nonvolatile storage area  18   b  as a second coefficient separately from the first coefficient, adaptive filter coefficient W being calculated by filter coefficient updater  17  at the most recent timing (or in other words, the timing at which active noise reduction device  10  was operating in the second operation mode). 
     In the manner described above, as long as coefficient W that is used as the initial value is stored for each operation mode, by selecting coefficient W according to the current operation mode, active noise reduction device  10  can prevent cancelling sound N 1  from being transformed into an abnormal sound.  FIG.  7    is a flowchart of an operation for selecting coefficient W according to the operation mode. 
     Filter coefficient updater  17  determines whether the current operation mode is in the first operation mode or in the second operation mode when the output of cancelling sound N 1  is started (S 51 ). The current operation mode can be determined by using, for example, a flag or the like stored in storage  18 . 
     If it is determined that the current operation mode is the first operation mode (first operation mode in S 51 ), filter coefficient updater  17  reads the first coefficient from nonvolatile storage area  18   b  (S 52 ), and calculates adaptive filter coefficient W based on the update equation in which the read first coefficient is used as the initial value (S 53 ). 
     On the other hand, if it is determined that the current operation mode is the second operation mode (second operation mode in S 51 ), filter coefficient updater  17  reads the second coefficient from nonvolatile storage area  18   b  (S 54 ), and calculates adaptive filter coefficient W based on the update equation in which the read second coefficient is used as the initial value (S 55 ). 
     In the manner described above, filter coefficient updater  17  uses coefficient W that corresponds to the current operation mode as the initial value of the update equation. By doing so, active noise reduction device  10  can shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1  while preventing cancelling sound N 1  from being transformed into an abnormal sound. 
     [Variation] 
     In the embodiment given above, active noise reduction device  10  is configured such that, in response to acquiring a stop signal from automobile controller  55 , active noise reduction device  10  is transitioned to a power off state (shut down state), and the supply of power to storage  18  is interrupted while active noise reduction device  10  is in the power off state. In step S 23  shown in the flowchart in  FIG.  4    described above, on the assumption that the supply of power to storage  18  is to be interrupted, adaptive filter coefficient W that was calculated at the most recent timing and stored in volatile storage area  18   a  is read, and stored in nonvolatile storage area  18   b  as the first coefficient. 
     However, active noise reduction device  10  may be transitioned to a sleep state instead of being transitioned to the power off state in response to acquiring the stop signal. While active noise reduction device  10  is in the sleep state, the supply of power to storage  18  is maintained, and thus adaptive filter coefficient W that was calculated at the most recent timing and stored in volatile storage area  18   a  can be used as the first coefficient. That is, the first coefficient may be stored in volatile storage area  18   a.    
     Active noise reduction device  10  configured as described above may omit the processing of reading adaptive filter coefficient W stored in volatile storage area  18   a  and storing adaptive filter coefficient W in nonvolatile storage area  18   b . As a result, active noise reduction device  10  can reduce the storage capacity of storage  18 , and shorten the end time and the activation time. 
     Also, a case may be considered where the sleep state cannot be maintained as a result of the supply of power being forcibly interrupted, such as when replacing the battery of automobile  50 . In this case, by automobile controller  55  selectively outputting two different types of stop signals (a first stop signal and a second stop signal) that indicate whether the supply of power is to be interrupted, filter coefficient updater  17  can select whether to store (move) the first coefficient in (to) nonvolatile storage area  18   b .  FIG.  8    is a flowchart of an operation for selecting whether to store (move) the first coefficient in (to) nonvolatile storage area  18   b.    
     Filter coefficient updater  17  acquires a stop signal from automobile controller  55  via automobile state signal input terminal  14  (S 61 ). Filter coefficient updater  17  determines whether the acquired stop signal is a first stop signal that indicates the supply of power to active noise reduction device  10  is to be interrupted or a second stop signal that indicates the supply of power to active noise reduction device  10  is to be maintained (S 62 ). 
     If it is determined that the stop signal acquired in step S 61  is the first stop signal (first stop signal in S 62 ), filter coefficient updater  17  reads adaptive filter coefficient W that was calculated at the most recent timing and stored in volatile storage area  18   a , and stores adaptive filter coefficient W in nonvolatile storage area  18   b  as a first coefficient (S 63 ). After that, active noise reduction device  10  is transitioned to a power off state (S 64 ). 
     On the other hand, if it is determined that the stop signal acquired in step S 61  is the second stop signal (second stop signal in S 62 ), filter coefficient updater  17  does not store, in nonvolatile storage area  18   b , adaptive filter coefficient W that was calculated at the most recent timing and stored in volatile storage area  18   a  (S 65 ). After that, active noise reduction device  10  is transitioned to a sleep state (S 66 ). In this case, adaptive filter coefficient W that was calculated at the most recent timing and stored in volatile storage area  18   a  is continuously used as the first coefficient. 
     As described above, basically, when filter coefficient updater  17  receives a notification of interruption of the supply of power to active noise reduction device  10  (or in other words, when filter coefficient updater  17  acquires a first stop signal) while storing the first coefficient in nonvolatile storage area  18   b  at the second timing, filter coefficient updater  17  stores the first coefficient in volatile storage area  18   a  at the second timing. By doing so, even if the first coefficient stored in volatile storage area  18   a  is deleted due to the interruption of the supply of power, active noise reduction device  10  can shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Filter coefficient updater  17  may be configured to autonomously detect an interruption of the supply of power to active noise reduction device  10  and store the first coefficient in nonvolatile storage area  18   b  at the second timing. As the method for detecting an interruption of the supply of power, any known method may be used. 
     [Other Variations] 
     In the embodiment given above, active noise reduction device  10  uses filter coefficient W calculated by active noise reduction device  10  in the past as the initial value of the update equation, but the initial value may be a value other than 0 determined in advance by a designer or the like. 
     For example, the initial value may be a value calculated in advance by a designer or the like through a simulation. Alternatively, filter coefficients W of a plurality of automobiles  50  may be calculated by actually driving the plurality of automobiles  50 , and the average value of the plurality of calculated filter coefficients may be stored in storage  18  in advance as the initial value. When the initial value is determined through a simulation or tuning in the manner as described above, by adapting adaptive filter  15  under conditions where the roadway condition, the automobile speed, and the like are different, various bands are excited, and thus the noise reduction performance can be improved under various driving conditions. 
     Also, in the embodiment given above, adaptive filter coefficient W is stored in storage  18  at the timing at which the engine is turned off, but the present disclosure is not limited to this timing. For example, adaptive filter coefficient W may be updated at the time of shipping inspection performed when automobile  50  is shipped, and coefficient W at this time may be stored as the initial value. At the time of shipping inspection, by driving automobile  50  and adapting adaptive filter  15  under conditions (moving conditions) where the roadway condition, the automobile speed, and the like are different, coefficient W in which various components are excited can be stored in storage  18 . 
     Also, adaptive filter coefficient W may be updated at the time of transportation for delivery of automobile  50 , and coefficient W at this time may be stored in storage  18  as the initial value. The original initial value before adaptive filter coefficient W is updated is 0, but may be a value determined through a simulation or tuning as described above. By updating adaptive filter coefficient W in the manner described above before automobile  50  is delivered to its user, individual variations between automobiles can be adjusted, and the noise reduction performance can be improved. 
     In the case where automobile  50  is driven under an unordinary condition at the time of shipping inspection or the like (for example, in the case where automobile  50  is driven on a chassis dynamometer, or the like), when adaptive filter coefficient W calculated at the shipping inspection or the like is stored as the initial value, coefficient W is no longer a general value. To prevent this, a configuration may be used in which coefficient W calculated at the time of shipping inspection or the like is not stored in storage  18 . For example, when active noise reduction device  10  is transitioned to an inspection mode, coefficient W may not be stored in storage  18  as the initial value. That is, active noise reduction device  10  may have a function of prohibiting the processing of storing coefficient W in storage  18  as the initial value. The original initial value at this time is 0, but may be a value determined through a simulation or tuning in the manner as described above. 
     Advantageous Effects, Etc. 
     As described above, active noise reduction device  10  includes: reference signal input terminal  11  to which a reference signal that has a correlation with noise NO in space  56  of automobile  50  is input, the reference signal being output by reference signal source  51  attached to automobile  50 ; adaptive filter  15  that generates a cancelling signal by applying an adaptive filter to the reference signal that is input to reference signal input terminal  11 , the cancelling signal being used to output cancelling sound N 1  for reducing noise NO; and filter coefficient updater  17  that calculates adaptive filter coefficient W based on a predetermined update equation. At a first timing at which the output of cancelling sound N 1  is started, filter coefficient updater  17  uses a first coefficient as an initial value of the update equation, the first coefficient being adaptive filter coefficient W calculated by filter coefficient updater  17  at a second timing that is prior to the first timing. 
     Active noise reduction device  10  configured as described above can shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Also, for example, the second timing is before active noise reduction device  10  is powered off such as when the ignition power supply of automobile  50  is turned off, and the first timing is immediately after the second timing, and is a timing at which active noise reduction device  10  of automobile  50  is turned on. 
     With active noise reduction device  10  configured as described above, at the time when automobile  50  resumes moving, by using adaptive filter coefficient W used until automobile  50  resumes moving as the initial value of the update equation, it is possible to shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Also, for example, the second timing is before active noise reduction device  10  is shipped. 
     With active noise reduction device  10  configured as described above, by using adaptive filter coefficient W tuned before shipping as the initial value of the update equation, it is possible to shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Also, for example, the second timing is immediately after automobile  50  is driven (moved) under a plurality of different driving conditions (moving conditions). 
     With active noise reduction device  10  configured as described above, by using adaptive filter coefficient W in which various components are excited before shipping as the initial value of the update equation, it is possible to shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Also, for example, active noise reduction device  10  further includes storage  18  that stores the first coefficient. Active noise reduction device  10  has a function of prohibiting the processing of storing the first coefficient in storage  18 . 
     With active noise reduction device  10  configured as described above, it is possible to prevent adaptive filter coefficient W calculated when automobile  50  is driven under an unordinary condition from being stored as the initial value. 
     Also, for example, active noise reduction device  10  selectively operates in either one of a first operation mode or a second operation mode. At a first timing at which the output of cancelling sound N 1  in the first operation mode is started, filter coefficient updater  17  uses a first coefficient as the initial value of the update equation. Also, at a timing at which the output of cancelling sound N 1  in the second operation mode is started, filter coefficient updater  17  uses a second coefficient that is different from the first coefficient and is adaptive filter coefficient W calculated by filter coefficient updater  17  prior to the timing at which the output of cancelling sound N 1  in the second operation mode is started, as the initial value of the update equation. 
     With active noise reduction device  10  configured as described above, by using coefficient W that corresponds to the current operation mode as the initial value of the update equation, it is possible to shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Also, for example, at the first timing, filter coefficient updater  17  uses, instead of the first coefficient, a corrected first coefficient obtained by multiplying the first coefficient by a correction coefficient that is greater than 0 and is less than 1 as the initial value of the update equation. 
     With active noise reduction device  10  configured as described above, it is possible to shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 , while preventing cancelling sound N 1  from being transformed into an abnormal sound. 
     Also, for example, the correction coefficient takes a smaller value as the length of the period from the second timing to the first timing is longer. 
     With active noise reduction device  10  configured as described above, it is possible to prevent cancelling sound N 1  from being transformed into an abnormal sound based on the length of the period from the second timing to the first timing. 
     Also, for example, the correction coefficient takes a smaller value as the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing is larger. 
     With active noise reduction device  10  configured as described above, it is possible to prevent cancelling sound N 1  from being transformed into an abnormal sound based on the difference between the temperature of space  56  at the second timing and the temperature of space  56  at the first timing. 
     Also, for example, filter coefficient updater  17  stores the first coefficient in nonvolatile storage area  18   b  at the second timing, reads the first coefficient stored in nonvolatile storage area  18   b  at the first timing, and uses the first coefficient as the initial value of the update equation. Nonvolatile storage area  18   b  is an example of a nonvolatile storage. 
     With active noise reduction device  10  configured as described above, even when the supply of power to active noise reduction device  10  is interrupted at a timing between the second timing and the first timing, it is possible to shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Also, for example, filter coefficient updater  17  stores the first coefficient in volatile storage area  18   a  at the second timing, reads the first coefficient stored in volatile storage area  18   a  at the first timing, and uses the first coefficient as the initial value of the update equation. Volatile storage area  18   a  is an example of a volatile storage. 
     With active noise reduction device  10  configured as described above, it is possible to omit the processing of reading adaptive filter coefficient W stored in volatile storage area  18   a  and storing adaptive filter coefficient W in nonvolatile storage area  18   b.    
     Also, filter coefficient updater  17  stores the first coefficient in nonvolatile storage area  18   b  at the second timing, for example, when a notification of interruption of the supply of power to active noise reduction device  10  is received, or when an interruption of the supply of power is detected. 
     With active noise reduction device  10  configured as described above, even when the first coefficient stored in volatile storage area  18   a  is deleted due to the interruption of the supply of power, it is possible to shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Also, automobile  50  includes active noise reduction device  10  and reference signal source  51 . 
     With automobile  50  configured as described above, it is possible to shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     An active noise reduction method executed by active noise reduction device  10  includes: generating a cancelling signal by applying an adaptive filter to a reference signal that has a correlation with noise NO in space  56  of automobile  50  and is output by reference signal source  51  attached to automobile  50 , the cancelling signal being used to output cancelling sound N 1  for reducing noise NO; and calculating a coefficient of the adaptive filter based on a predetermined update equation. The calculating includes, at a first timing at which the output of cancelling sound N 1  is started, using a first coefficient as an initial value of the update equation, the first coefficient being the coefficient of the adaptive filter calculated by active noise reduction device  10  at a second timing that is prior to the first timing. 
     With the active noise reduction method as described above, it is possible to shorten the time required to obtain the effect of reducing noise NO by using cancelling sound N 1 . 
     Other Embodiments 
     The embodiment has been described above, but the present disclosure is not limited to the above-described embodiment. 
     For example, the active noise reduction device according to the embodiment 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 as described above. 
     Also, the configuration of the active noise reduction device according to the embodiment given above is an example. For example, the active noise reduction device may include a structural element such as a D/A converter, a filter, a power amplifier, or an A/D converter. 
     Also, in the embodiment given above, the reference signal inputter, the error signal inputter, and the cancelling signal outputter are 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 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 device according to the embodiment given above is an example. For example, a portion of the digital signal processing described in the embodiment given above may be implemented by using analog signal processing. 
     Also, for example, in the embodiment 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 the embodiment given above, the structural elements may be implemented by executing a software program suitable for the structural elements. 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 the embodiment given above, the structural elements may be implemented by using hardware. For example, 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, 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 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, and 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 reference signal source according to the embodiment 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. 2021-4545 filed on Jan. 14, 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.