Active noise reduction system

Abstract

The present invention relates to an active noise reduction system comprising: a cancellation sound output device configured to output a cancellation sound; an error microphone configured to generate an error signal; and a controller configured to control the cancellation sound output device based on the error signal. The controller includes: a control signal generation unit including a control filter configured to generate a control signal for controlling the cancellation sound output device; and a sound field learning unit including a sub-path filter representing an estimated value of a transfer function of a sub-path from the cancellation sound output device to the error microphone, the control filter and the sub-path filter being configured to adaptively update, and the controller is configured to distinguish between a timing of an adaptive update of the control filter and a timing of an adaptive update of the sub-path filter.

Description

Technical Field

[0001] The present invention relates to an active noise reduction system that reduces noise by causing a cancelling sound in an opposite phase to the noise to interfere with the noise. Background Art

[0002] In recent years, considering the vulnerable groups such as the elderly and children among traffic participants, efforts have been actively made to provide sustainable transportation systems for these people. To achieve this goal, research and development on further improving traffic safety and convenience through the development of vehicle comfort is attracting attention.

[0003] In order to improve the comfort of a vehicle, it is desirable to reduce the noise inside the vehicle. Therefore, research and development of an active noise reduction system that reduces noise by interfering with a canceling sound in a phase opposite to that of the noise is actively conducted.

[0004] For example, JP2014-6709A discloses an active noise reduction system, which includes a control filter that can be adaptively updated (see "Adaptive filter C1 for control signal" in JP2014-6709A) and a sub-path filter that can be adaptively updated (see "Adaptive filter K for sub-path" in JP2014-6709A).

[0005] In JP2014-6709A, adaptive updating of the control filter and adaptive updating of the sub-path filter are performed simultaneously (see paragraph 0046 of JP2014-6709A). Therefore, during the adaptive updating of the control filter and the sub-path filter, the computational load on the controller becomes large. Therefore, the controller needs to be composed of an expensive processor that can withstand such a large computational load, which may lead to an increase in the manufacturing cost of the active noise reduction system. Summary of the invention

[0006] In view of the above background, an object of the present invention is to provide an active noise reduction system capable of reducing the computational load on the controller during filter updating. In addition, another object of the present invention is to contribute to the development of sustainable transportation systems.

[0007] To achieve such a purpose, one aspect of the present invention provides an active noise reduction system 1, 81, 91, 101 and 111, the active noise reduction system comprising: a cancellation sound output device 21 and S1 to Sn, the cancellation sound output device is configured to output a cancellation sound for canceling noise; an error microphone 23 and M1 to Mm, the error microphone is configured to generate an error signal e1 based on the noise and the cancellation sound; and a controller 25, 83, 93 and 115, the controller is configured to control the cancellation sound output device based on the error signal, wherein the controller comprises: a control signal generating unit 31, 8 5 and 120, the control signal generating unit includes a control filter W1, the control filter is configured to generate a control signal u1 for controlling the cancellation sound output device; and a sound field learning unit 32, 86 and 121, the sound field learning unit includes a sub-path filter C^1, the sub-path filter represents an estimated value of the transfer function of the sub-path from the cancellation sound output device to the error microphone, the control filter and the sub-path filter are configured to be adaptively updated, and the controller is configured to distinguish the timing of the adaptive update of the control filter and the timing of the adaptive update of the sub-path filter.

[0008] According to this aspect, the calculation load on the controller during the update of these filters can be reduced compared to the case where the adaptive update of the control filter and the adaptive update of the sub-path filter are performed simultaneously. Therefore, the controller does not need to be composed of an expensive processor, so that an increase in the manufacturing cost of the active noise reduction system can be suppressed.

[0009] In the above aspect, preferably, the controller is configured to alternately perform adaptive updating of the control filter and adaptive updating of the secondary path filter.

[0010] According to this aspect, it is possible to prevent the adaptive update of either the control filter or the sub-path filter from being stopped for a long time.

[0011] In the above aspect, preferably, the control signal generating unit also includes an auxiliary sub-path filter C^1p, the auxiliary sub-path filter represents the estimated value of the transfer function of the sub-path, and the controller is configured to: determine whether the fluctuation of the sub-path filter based on its adaptive update has converged, and when the fluctuation of the sub-path filter based on its adaptive update has converged, update the auxiliary sub-path filter by copying the value of the sub-path filter to the auxiliary sub-path filter.

[0012] According to this aspect, the updating of the auxiliary sub-path filter is stopped until the fluctuation of the sub-path filter converges. Therefore, the auxiliary sub-path filter can be prevented from being updated based on the fluctuating sub-path filter. Therefore, when the control filter is adaptively updated based on the auxiliary sub-path filter, the control filter can converge quickly.

[0013] In the above aspect, preferably, the controller is configured to determine whether fluctuations according to which the secondary path filter is adaptively updated have converged based on changes in at least one of an amplitude of the secondary path filter and a phase of the secondary path filter.

[0014] According to the above aspect, it is possible to directly determine whether the fluctuation of the sub-path filter has converged based on the state of the sub-path filter. Therefore, compared with the case where it is indirectly determined whether the fluctuation of the sub-path filter has converged based on a signal related to the sub-path filter (e.g., an error signal), it is possible to accurately determine whether the fluctuation of the sub-path filter has converged.

[0015] In the above aspect, preferably, one of the cancellation sound output device and the error microphone is installed in the passenger seat 5 of the vehicle 3, the other of the cancellation sound output device and the error microphone is installed in a part of the vehicle other than the passenger seat, and the controller is configured to: determine whether the fluctuation of the sub-path filter according to its adaptive update has converged, stop the adaptive update of the sub-path filter when the fluctuation of the sub-path filter according to its adaptive update has converged, determine whether the transfer function of the sub-path has changed based on the state of the passenger seat when the adaptive update of the sub-path filter is stopped, and resume the adaptive update of the sub-path filter when the transfer function of the sub-path has changed.

[0016] According to this aspect, in the case where the fluctuation of the secondary path filter has converged, the computational load on the controller can be reduced by stopping the adaptive update of the secondary path filter. Therefore, the speed of the adaptive update of the control filter can be increased. In addition, even if the adaptive update of the secondary path filter is stopped (i.e., even if the fluctuation of the secondary path filter is stopped), the adaptive update of the secondary path filter can be resumed at an appropriate timing by determining whether the transfer function of the secondary path has changed based on the state of the passenger seat.

[0017] In the above aspect, preferably, the controller is configured to: obtain multiple changes in the front and rear position of the passenger seat, the height of the passenger seat and the inclination angle of the reclining portion 8 of the passenger seat, set a first threshold and a second threshold for each of the multiple changes, the second threshold is less than the first threshold, and determine that the transfer function of the secondary path has changed when at least one of the multiple changes is greater than the first threshold and when all of the multiple changes are greater than the second threshold.

[0018] The transfer function of the secondary path may change significantly not only in the case of at least one significant change among the multiple changes, but also in the case of all slight changes among the multiple changes. According to the above aspect, by considering these two cases, it is possible to accurately determine whether the transfer function of the secondary path has changed.

[0019] In the above aspect, preferably, at least one of the cancellation sound output device and the error microphone is installed in the vehicle compartment 4, and the controller is configured to: determine whether the sub-path filter has converged according to the fluctuation of its adaptive update, stop the adaptive update of the sub-path filter when the sub-path filter has converged according to the fluctuation of its adaptive update, determine whether the transfer function of the sub-path has changed based on the opening and closing state of the window 96 of the compartment when the adaptive update of the sub-path filter is stopped, and resume the adaptive update of the sub-path filter when the transfer function of the sub-path has changed.

[0020] According to this aspect, in the case where the fluctuation of the sub-path filter has converged, the computational load on the controller can be reduced by stopping the adaptive update of the sub-path filter. Therefore, the speed of the adaptive update of the control filter can be increased. In addition, even if the adaptive update of the sub-path filter is stopped (i.e., even if the fluctuation of the sub-path filter is stopped), the adaptive update of the sub-path filter can be resumed at an appropriate timing by determining whether the transfer function of the sub-path has changed based on the opening and closing state of the window.

[0021] In the above aspect, preferably, the sound field learning unit also includes a main path filter H^1, the main path filter represents an estimated value of the transfer function of the main path from the noise source to the error microphone, the main path filter is configured to be adaptively updated, and the controller is configured to use a common normalization divisor to normalize the adaptive update amount of the sub-path filter and the adaptive update amount of the main path filter.

[0022] Even if one of the sub-path filter or the main path filter is close to convergence, its convergence may be delayed due to the influence of the fluctuation of the other. According to the above aspect, a common normalization divisor is used to normalize the adaptive update amount of the sub-path filter and the adaptive update amount of the main path filter. Therefore, the change between the convergence speed of the sub-path filter and the convergence speed of the main path filter can be suppressed. Therefore, the convergence of one of the sub-path filter and the main path filter can be prevented from being delayed due to the fluctuation of the other, and the sub-path filter and the main path filter can be made to converge quickly.

[0023] In the above aspect, preferably, the main path filter is configured to be adaptively updated based on a reference signal corresponding to the noise, the secondary path filter is configured to be adaptively updated based on the control signal, and the common normalized divisor includes the norm of the signal vector of the reference signal and the norm of the signal vector of the control signal.

[0024] When the initial value of the control signal is zero, the initial value of the reference signal has a certain amplitude (that is, the initial value of the reference signal is not zero). Therefore, if the adaptive update amount of the sub-path filter is normalized based on the control signal, and the adaptive update amount of the main path filter is normalized based on the reference signal, a large difference may be caused between the adaptive update amounts of the two filters in the early stage of convergence. According to the above aspect, the adaptive update amount of the sub-path filter and the adaptive update amount of the main path filter are both normalized based on the reference signal and the control signal. Therefore, it is possible to prevent a large difference between the adaptive update amounts of the two filters in the early stage of convergence. Therefore, the sub-path filter and the main path filter can be made to converge faster.

[0025] In the above aspect, preferably, the active noise reduction system also includes a reference microphone 24 arranged separately from the error microphone, and the reference microphone is configured to generate a determination signal e2 based on at least the noise, and the controller is configured to: determine whether the sub-path filter has converged according to the fluctuation of its adaptive update, stop the adaptive update of the sub-path filter when the sub-path filter has converged according to the fluctuation of its adaptive update, determine whether the transfer function of the sub-path has changed based on the error signal and the determination signal when the adaptive update of the sub-path filter is stopped, and resume the adaptive update of the sub-path filter when the transfer function of the sub-path has changed.

[0026] According to this aspect, in the case where the fluctuation of the sub-path filter has converged, the computational load on the controller can be reduced by stopping the adaptive update of the sub-path filter. Therefore, the speed of the adaptive update of the control filter can be increased. In addition, even if the adaptive update of the sub-path filter is stopped (that is, even if the fluctuation of the sub-path filter is stopped), the adaptive update of the sub-path filter can be restored at an appropriate timing by determining whether the transfer function of the sub-path has changed based on the error signal and the determination signal. In addition, by determining whether the transfer function of the sub-path has changed based on the error signal and the determination signal, it is possible to determine whether the transfer function of the sub-path has changed without using a sensor that detects an external state of the controller (e.g., the state of the passenger seat).

[0027] In the above aspect, preferably, the controller includes a second control signal generating unit 33 provided separately from the control signal generating unit, and a microphone corresponding to the second control signal generating unit is used as the reference microphone.

[0028] According to this aspect, by using the microphone corresponding to the second control signal generating unit as the reference microphone, it is not necessary to provide a dedicated reference microphone for generating the determination signal. Therefore, it is possible to suppress an increase in the number of components.

[0029] In the above aspect, preferably, the controller is configured to determine that the transfer function of the secondary path has changed when the difference between the error signal and the determination signal is equal to or greater than a first reference value and the time variation of the error signal is equal to or greater than a second reference value.

[0030] According to this aspect, it is possible to determine whether the transfer function of the sub-path has changed not only based on the difference between the error signal and the determination signal but also based on the time variation of the error signal. Therefore, even if the transfer function of the sub-path from the cancellation sound output device to the reference microphone has changed, it is possible to avoid erroneously determining that the transfer function of the sub-path from the cancellation sound output device to the error microphone has changed. In addition, based on the difference between the error signal and the determination signal and the time variation of the error signal, it is possible to easily determine whether the transfer function of the sub-path has changed. Therefore, compared with the case where it is determined whether the transfer function of the sub-path has changed based on the similarity between the error signal and the determination signal, the computational load on the controller can be reduced.

[0031] In the above aspect, preferably, the controller is configured to determine that the transfer function of the secondary path has changed when the difference between the error signal and the determination signal is equal to or greater than the first reference value, the time variation of the error signal is equal to or greater than the second reference value, and the error signal is equal to or greater than a third reference value.

[0032] Even in a state where the transfer function of the secondary path has not changed, the error signal may change slightly. In the case where the change in the transfer function of the secondary path is determined using only the first reference value and the second reference value, it is necessary to increase the second reference value to offset the above-mentioned slight change in the error signal. However, if the second reference value is increased in this manner, the accuracy of determining the change in the transfer function of the secondary path may be reduced. According to the above aspect, by using a third reference value in addition to the first reference value and the second reference value to determine the change in the transfer function of the secondary path, the above-mentioned slight change in the error signal can be offset without increasing the second reference value. Therefore, the change in the transfer function of the secondary path can be determined with high accuracy.

[0033] In the above aspect, preferably, the controller includes multiple control channels Ch1 to ChS, each control channel includes the control signal generating unit and the sound field learning unit, and the controller is configured to distinguish the timing of adaptive update of the sub-path filter in each of the multiple control channels.

[0034] According to this aspect, the amount of calculation of the controller during the updating of these filters can be reduced compared to the case where adaptive updating of the sub-path filters in each control channel is performed simultaneously. Therefore, the controller does not need to be composed of an expensive processor, thereby suppressing an increase in the manufacturing cost of the active noise reduction system.

[0035] In order to achieve such a purpose, another aspect of the present invention provides an active noise reduction system 111, which includes: a cancellation sound output device S1 to Sn, which is configured to output a cancellation sound for canceling noise; error microphones M1 to Mm, which are configured to generate an error signal e based on the noise and the cancellation sound; and a controller 115, which is configured to control the cancellation sound output device based on the error signal, wherein the controller includes: a control signal generating unit 120, which includes a control filter W, which is configured to generate A control signal u for controlling the cancellation sound output device; and a sound field learning unit 121, the sound field learning unit including a sub-path filter C^, the sub-path filter representing an estimated value of a transfer function of a sub-path from the cancellation sound output device to the error microphone, the control filter and the sub-path filter being configured to be adaptively updated, the controller including a plurality of control channels Ch1 to ChS, each control channel including the control signal generating unit and the sound field learning unit, and the controller being configured to distinguish the timing of the adaptive update of the sub-path filter in each of the plurality of control channels.

[0036] According to this aspect, the computational load on the controller during the updating of the filters can be reduced compared to the case where adaptive updating of the sub-path filters in each control channel is performed simultaneously. Therefore, the controller does not need to be composed of an expensive processor, thereby suppressing an increase in the manufacturing cost of the active noise reduction system.

[0037] Therefore, according to the above aspects, an active noise reduction system can be provided that can reduce the computational load on the controller during filter updating. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a schematic diagram showing a vehicle to which the active noise reduction system according to the first embodiment is applied;

[0039] Figure 2 is a functional block diagram showing an active noise reduction system according to a first embodiment;

[0040] Figure 3 is a flowchart showing a sound field change determination process according to the first embodiment;

[0041] Figure 4 is a graph showing a signal level of an error signal according to the first embodiment;

[0042] Figure 5 is a flowchart showing a convergence determination process according to the first embodiment;

[0043] Figure 6 is a graph showing an impulse response of a sub-path filter according to the first embodiment;

[0044] Figure 7 is a flowchart showing an update process according to the first embodiment;

[0045] Figure 8 is a flowchart illustrating an update process according to another embodiment;

[0046] Fig. 9 is a functional block diagram showing an active noise reduction system according to a second embodiment;

[0047] Fig.10 is a schematic diagram showing a current position and a reference position of a first passenger seat according to a second embodiment;

[0048] Fig.11 is a flowchart showing a sound field change determination process according to the second embodiment;

[0049] Fig.12 is a functional block diagram showing an active noise reduction system according to a third embodiment;

[0050] Fig.13is a flowchart showing a sound field change determination process according to the third embodiment;

[0051] Fig.14 is a functional block diagram showing an active noise reduction system according to a fourth embodiment;

[0052] Fig.15 is a flowchart showing an update process according to a fourth embodiment;

[0053] Fig.16 is a functional block diagram showing an active noise reduction system according to a fifth embodiment;

[0054] Fig.17 is an update processing table according to the fifth embodiment;

[0055] Fig.18 is a flowchart showing an update process according to the fifth embodiment; and

[0056] Fig.19 is a flowchart illustrating an update process according to another embodiment. DETAILED DESCRIPTION

[0057] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that in the following description, "^" (diacritic symbol) added to various symbols indicates an identified value or an estimated value. "^" is added above each symbol in the accompanying drawings, but is added after each symbol in the specification.

[0058] (First Embodiment)

[0059] First, refer to Figures 1 to 7 A first embodiment of the present invention is described.

[0060] <Vehicle 3>

[0061] Figure 1 1 is a schematic diagram showing a vehicle 3 to which an active noise reduction system 1 according to a first embodiment (hereinafter referred to as "noise reduction system 1") is applied. The vehicle 3 is, for example, a four-wheeled automobile.

[0062] In the vehicle compartment 4 of the vehicle 3, a plurality of passenger seats 5, 6 are arranged. The plurality of passenger seats 5, 6 include a first passenger seat 5 and a second passenger seat 6. For example, the first passenger seat 5 is a passenger seat, and the second passenger seat 6 is a driver seat. In another embodiment, a seat other than a passenger seat (for example, a driver seat or a rear seat) may be the first passenger seat 5, and a seat other than a driver seat (for example, a passenger seat or a rear seat) may be the second passenger seat 6. In other words, the combination of the first passenger seat 5 and the second passenger seat 6 can be freely determined.

[0063] Each passenger seat 5, 6 (hereinafter referred to as "passenger seat 5, 6") includes a seat cushion 7 and a reclining portion 8, which is arranged above and behind the seat cushion 7 and is configured to rotate relative to the seat cushion 7. The reclining portion 8 includes a seat back 9 and a headrest 10 fixed to the upper end of the seat back 9.

[0064] The front-rear position of the passenger seats 5, 6, the height of the passenger seats 5, 6 and the inclination angle of the reclining portion 8 of the passenger seats 5, 6 are adjusted by an electric motor (not shown) according to the operation of the seat position operation unit (not shown) by the passenger. In other words, the passenger seats 5, 6 are so-called electric seats.

[0065] <Noise reduction system 1>

[0066] The noise reduction system 1 is an active noise control device (ANC device) configured to reduce noise d generated in a cabin 4 of a vehicle 3. More specifically, the noise reduction system 1 reduces the noise d by generating cancelling sounds y1, y2 in phase opposite to the noise d and causing the generated cancelling sounds y1, y2 to interfere with the noise d.

[0067] For example, the noise d to be reduced by the noise reduction system 1 is road noise caused by the vibration of the wheels due to the force from the road surface. The noise d to be reduced by the noise reduction system 1 may be noise other than the road noise mentioned above, for example, driving noise caused by the vibration of a driving source such as an internal combustion engine and an electric motor.

[0068] The noise reduction system 1 includes: multiple speakers 21, 22, which are configured to output cancellation sounds y1, y2 for canceling noise d; multiple microphones 23, 24, which are configured to generate error signals e1, e2 based on the noise d and the cancellation sounds y1, y2; and a controller 25, which is configured to control the multiple speakers 21, 22 based on the error signals e1, e2.

[0069] <Multiple Speakers 21, 22>

[0070] The plurality of speakers 21 , 22 include a first speaker 21 (an example of a cancelling sound output device) configured to output a cancelling sound y1 and a second speaker 22 (an example of a second cancelling sound output device) configured to output a cancelling sound y2 . The second speaker 22 is provided separately from the first speaker 21 .

[0071] The first speaker 21 is arranged at a position corresponding to the first passenger seat 5, but is installed in a portion other than the first passenger seat 5. For example, the first speaker 21 is installed on the floor in front of the first passenger seat 5 or in a door on a lateral side of the first passenger seat 5. In another embodiment, the first speaker 21 may be installed in the first passenger seat 5.

[0072] The second speaker 22 is arranged at a position corresponding to the second passenger seat 6, but is installed in a portion other than the second passenger seat 6. For example, the second speaker 22 is installed on the floor in front of the second passenger seat 6 or in a door on a lateral side of the second passenger seat 6. In another embodiment, the second speaker 22 may be installed in the second passenger seat 6.

[0073] <Multiple Microphones 23, 24>

[0074] The plurality of microphones 23 , 24 include a first microphone 23 (an example of an error microphone) and a second microphone 24 (an example of a reference microphone). The second microphone 24 is provided separately from the first microphone 23 .

[0075] The first microphone 23 is installed in a freely selectable portion of the first passenger seat 5. For example, the first microphone 23 is installed in the headrest 10 of the reclining portion 8 of the first passenger seat 5. In another embodiment, the first microphone 23 may be arranged at a position corresponding to the first passenger seat 5 but installed in a portion other than the first passenger seat 5.

[0076] Reference Figure 2 , the first microphone 23 is configured to generate an error signal e1 based on the cancellation sound y1, the cancellation sound y2 and the noise d1 (the noise d at the position of the first microphone 23). In another embodiment, the first microphone 23 may be configured to generate an error signal e1 based only on the cancellation sound y1 and the noise d1.

[0077] Reference Figure 1 , the second microphone 24 is installed in a freely selected portion of the second passenger seat 6. For example, the second microphone 24 is installed in the headrest 10 of the reclining portion 8 of the second passenger seat 6. In another embodiment, the second microphone 24 may be arranged at a position corresponding to the second passenger seat 6, but installed in a portion other than the second passenger seat 6.

[0078] Reference Figure 2 , the second microphone 24 is configured to generate an error signal e2 (an example of a determination signal) based on the cancelling sound y1, the cancelling sound y2, and the noise d2 (the noise d at the position of the second microphone 24). In another embodiment, the second microphone 24 may be configured to generate the error signal e2 based only on the cancelling sound y2 and the noise d2.

[0079] Figure 2 “C1” in the figure represents the transfer function of the secondary path from the first speaker 21 to the first microphone 23, and Figure 2 "H1" in represents the transfer function of the main path from the noise source to the first microphone 23. Similarly, Figure 2“C2” in the figure represents the transfer function of the secondary path from the second speaker 22 to the second microphone 24, and Figure 2 “H2” in φ represents a transfer function of a main path from a noise source to the second microphone 24. These transfer functions C1, H1, C2, and H2 correspond to the sound field in the vehicle cabin 4.

[0080] <Controller 25>

[0081] The controller 25 is constituted by a computer including an arithmetic processing unit (processor such as CPU, MPU, etc.) and a storage device (memory such as ROM, RAM, etc.) The controller 25 may be configured as one piece of hardware or may be configured as a unit including a plurality of hardware.

[0082] A reference signal r corresponding to the noise d is input to the controller 25. The reference signal r is input to the controller 25, for example, from a reference microphone (not shown) that generates the reference signal r according to the noise d. In another embodiment, the reference signal r may be input to the controller 25 from a vibration sensor (not shown) that detects vibration corresponding to the noise d, or from a component other than the reference microphone or the vibration sensor.

[0083] Reference Figure 2 The controller 25 includes a first control signal generating unit 31, a first sound field learning unit 32, a second control signal generating unit 33, a second sound field learning unit 34, a sound field change determining unit 35, a convergence determining unit 36, and an update processing unit 37 as functional components. The first control signal generating unit 31 and the first sound field learning unit 32 correspond to the first speaker 21 and the first microphone 23. The second control signal generating unit 33 and the second sound field learning unit 34 correspond to the second speaker 22 and the second microphone 24.

[0084] <First Control Signal Generating Unit 31>

[0085] The first control signal generating unit 31 of the controller 25 includes a first control filter unit 41 , a first auxiliary sub-path filter unit 42 , and a first control updating unit 43 .

[0086] The first control filter unit 41 includes a control filter W1. The control filter W1 is composed of a finite impulse response filter (FIR filter). In another embodiment, the control filter W1 can be composed of a single frequency adaptive notch filter (SAN filter) or the like.

[0087] When the control filter W1 of the first control filter unit 41 filters the reference signal r, the first control filter unit 41 generates a control signal u1 for controlling the first speaker 21. The first control filter unit 41 outputs the generated control signal u1 to the first speaker 21 and the first sound field learning unit 32. Therefore, the first speaker 21 generates the cancellation sound y1 according to the control signal u1 output from the first control filter unit 41.

[0088] The first auxiliary sub-path filter unit 42 includes an auxiliary sub-path filter C^1p. The auxiliary sub-path filter C^1p is a filter that represents an estimated value of the transfer function C1 of the sub-path. The auxiliary sub-path filter C^1p is composed of an FIR filter. In another embodiment, the auxiliary sub-path filter C^1p can be composed of a SAN filter or the like.

[0089] When the auxiliary sub-path filter C^1p of the first auxiliary sub-path filter unit 42 filters the reference signal r, the first auxiliary sub-path filter unit 42 corrects the reference signal r. The first auxiliary sub-path filter unit 42 outputs the corrected reference signal r to the first control update unit 43.

[0090] The first control update unit 43 adaptively updates the control filter W1 using an adaptive algorithm such as a least mean square algorithm (LMS algorithm). More specifically, the first control update unit 43 adaptively updates the control filter W1 so that the error signal e1 output from the first microphone 23 is minimized.

[0091] <First Sound Field Learning Unit 32>

[0092] The first sound field learning unit 32 of the controller 25 includes a first cancellation sound estimation signal generating unit 51, a first sub-path updating unit 52, a first noise estimation signal generating unit 53, a first main path updating unit 54, a first cancellation sound estimation signal inversion unit 55, a first noise estimation signal inversion unit 56 and a first virtual error signal generating unit 57.

[0093] The first cancellation sound estimation signal generating unit 51 includes a sub-path filter C^1. Similar to the auxiliary sub-path filter C^1p, the sub-path filter C^1 is a filter that represents the estimated value of the transfer function C1 of the sub-path. The sub-path filter C^1 is composed of, for example, an FIR filter. In another embodiment, the sub-path filter C^1 may be composed of a SAN filter or the like.

[0094] Since the sub-path filter C^1 of the first cancelling sound estimated signal generating unit 51 filters the control signal u1, the first cancelling sound estimated signal generating unit 51 generates a cancelling sound estimated signal y^1 indicating an estimated value of the cancelling sound y1. The first cancelling sound estimated signal generating unit 51 outputs the generated cancelling sound estimated signal y^1 to the first cancelling sound estimated signal inverting unit 55.

[0095] The first sub-path updating unit 52 adaptively updates the sub-path filter C^1 using an adaptive algorithm such as an LMS algorithm. More specifically, the first sub-path updating unit 52 adaptively updates the sub-path filter C^1 so that the virtual error signal ev1 (described later) output from the first virtual error signal generating unit 57 is minimized.

[0096] The first noise estimation signal generating unit 53 includes a main path filter H^1. The main path filter H^1 is a filter that represents an estimated value of the transfer function H1 of the main path. The main path filter H^1 is, for example, composed of an FIR filter. In another embodiment, the main path filter H^1 may be composed of a SAN filter or the like.

[0097] Since the main path filter H1 of the first noise estimation signal generating unit 53 filters the reference signal r, the first noise estimation signal generating unit 53 generates a noise estimation signal d1 indicating an estimated value of the noise d1. The first noise estimation signal generating unit 53 outputs the generated noise estimation signal d1 to the first noise estimation signal inverting unit 56.

[0098] The first main path updating unit 54 adaptively updates the main path filter H^1 using an adaptive algorithm such as an LMS algorithm. More specifically, the first main path updating unit 54 adaptively updates the main path filter H^1 so that the virtual error signal ev1 (described later) output from the first virtual error signal generating unit 57 is minimized.

[0099] First cancelling sound estimated signal inverting section 55 inverts the polarity of cancelling sound estimated signal ŷ1 output from first cancelling sound estimated signal generating section 51 . First cancelling sound estimated signal inverting section 55 outputs the cancelling sound estimated signal ŷ1 with inverted polarity to first virtual error signal generating section 57 .

[0100] The first noise estimation signal inverting unit 56 inverts the polarity of the noise estimation signal d̂1 output from the first noise estimation signal generating unit 53. The first noise estimation signal inverting unit 56 outputs the noise estimation signal d̂1 with the inverted polarity to the first virtual error signal generating unit 57.

[0101] The first virtual error signal generating unit 57 generates a virtual error signal ev1 by adding the error signal e1 output from the first microphone 23, the cancellation sound estimation signal y^1 passed through the first cancellation sound estimation signal inverting unit 55, and the noise estimation signal d^1 passed through the first noise estimation signal inverting unit 56. The first virtual error signal generating unit 57 outputs the generated virtual error signal ev1 to the first sub-path updating unit 52 and the first main path updating unit 54.

[0102] <Second Control Signal Generating Unit 33>

[0103] The second control signal generating unit 33 of the controller 25 is provided separately from the first control signal generating unit 31. The second control signal generating unit 33 includes a second control filter unit 61, a second auxiliary sub-path filter unit 62, and a second control updating unit 63.

[0104] When the control filter W2 (an example of a second control filter) of the second control filter unit 61 filters the reference signal r, the second control filter unit 61 generates a control signal u2 (an example of a second control signal) for controlling the second speaker 22. The second auxiliary sub-path filter unit 62 includes an auxiliary sub-path filter C^2p representing an estimated value of the transfer function C2 of the sub-path. The auxiliary sub-path filter C^2p corrects the reference signal r, and the second auxiliary sub-path filter unit 62 outputs the corrected reference signal r to the second control update unit 63. The second control update unit 63 adaptively updates the control filter W2 so that the error signal e2 output from the second microphone 24 is minimized.

[0105] <Second Sound Field Learning Unit 34>

[0106] The second sound field learning unit 34 of the controller 25 is provided separately from the first sound field learning unit 32. The second sound field learning unit 34 includes a second cancelling sound estimation signal generating unit 71, a second sub-path updating unit 72, a second noise estimation signal generating unit 73, a second main path updating unit 74, a second cancelling sound estimation signal inverting unit 75, a second noise estimation signal inverting unit 76 and a second virtual error signal generating unit 77.

[0107] The second cancellation sound estimation signal generating unit 71 includes a sub-path filter C^2 (an example of a second sub-path filter) representing an estimated value of the transfer function C2 of the sub-path. When the sub-path filter C^2 filters the control signal u2, the second cancellation sound estimation signal generating unit 71 generates a cancellation sound estimation signal y^2 representing an estimated value of the cancellation sound y2. The second sub-path updating unit 72 adaptively updates the sub-path filter C^2 so that the virtual error signal ev2 (described later) is minimized.

[0108] The second noise estimation signal generating unit 73 includes a main path filter H^2 representing an estimated value of a transfer function H2 of the main path. When the main path filter H^2 filters the reference signal r, the second noise estimation signal generating unit 73 generates a noise estimation signal d^2 representing an estimated value of noise d2. The second main path updating unit 74 adaptively updates the main path filter H^2 so that a virtual error signal ev2 (to be described later) is minimized.

[0109] The second cancellation sound estimation signal inversion unit 75 inverts the polarity of the cancellation sound estimation signal y^2. The second noise estimation signal inversion unit 76 inverts the polarity of the noise estimation signal d^2. The second virtual error signal generation unit 77 generates a virtual error signal ev2 by adding the error signal e2 output from the second microphone 24, the cancellation sound estimation signal y^2 passed through the second cancellation sound estimation signal inversion unit 75, and the noise estimation signal d^2 passed through the second noise estimation signal inversion unit 76.

[0110] <Sound Field Change Determination Unit 35>

[0111] The sound field change determination unit 35 of the controller 25 determines whether the transfer function C1 of the sub path has changed based on the error signal e1 and the error signal e2. The determination method of the sound field change determination unit 35 will be described later.

[0112] <Convergence Determination Unit 36>

[0113] The convergence determination unit 36 ​​of the controller 25 determines whether the sub-path filter C^1 has converged according to the fluctuations of its adaptive update based on the sub-path filter C^1. The determination method of the convergence determination unit 36 ​​will be described later.

[0114] <Update Processing Unit 37>

[0115] The update processing unit 37 of the controller 25 determines the order and timing of adaptive updating of the filter based on the determination results of the sound field change determination unit 35 and the convergence determination unit 36. The determination method of the update processing unit 37 will be described later.

[0116] <Sound Field Change Determination Processing>

[0117] Subsequently, description will be given of the sound field change determination process performed by the sound field change determination unit 35. The sound field change determination process is a process for determining whether the transfer function C1 of the sub path has changed.

[0118] See also Figure 3 , when the sound field change determination process starts, the sound field change determination unit 35 acquires the error signal e1 from the first microphone 23 and acquires the error signal e2 from the second microphone 24 (step ST1 ).

[0119] Subsequently, the sound field change determination unit 35 calculates the signal level L1 of the error signal e1 and the signal level L2 of the error signal e2 (step ST2). For example, the sound field change determination unit 35 may determine the sum of the squares of the error signal e1 within a certain period as the signal level L1 of the error signal e1, or determine the sum of the absolute values ​​of the error signal e1 within a certain period as the signal level L1 of the error signal e1. The same method may be applied to the signal level L2 of the error signal e2.

[0120] Subsequently, the sound field change determination unit 35 determines whether the absolute value of the difference between the signal level L1 of the error signal e1 and the signal level L2 of the error signal e2 is equal to or greater than the first reference value R1 (step ST3). In the case where the absolute value of the difference between the signal level L1 of the error signal e1 and the signal level L2 of the error signal e2 is less than the first reference value R1 (step ST3: No), the sound field change determination unit 35 determines that the transfer function C1 of the sub-path has not changed (step ST4).

[0121] In the case where the absolute value of the difference between the signal level L1 of the error signal e1 and the signal level L2 of the error signal e2 is equal to or greater than the first reference value R1 (step ST3: Yes), the sound field change determination unit 35 determines whether the time variation ΔL1 of the signal level L1 of the error signal e1 (for example, the difference between the current value of the signal level L1 of the error signal e1 and the previous value of the signal level L1 of the error signal e1) is equal to or greater than the second reference value R2 (step ST5). In the case where the time variation ΔL1 of the signal level L1 of the error signal e1 is less than the second reference value R2 (step ST5: No), the sound field change determination unit 35 determines that the transfer function C1 of the sub-path has not changed (step ST4).

[0122] In the case where the time variation ΔL1 of the signal level L1 of the error signal e1 is equal to or greater than the second reference value R2 (step ST5: Yes), the sound field change determination unit 35 determines whether the signal level L1 of the error signal e1 is equal to or greater than the third reference value R3 (step ST6). In the case where the signal level L1 of the error signal e1 is less than the third reference value R3 (step ST6: No), the sound field change determination unit 35 determines that the transfer function C1 of the sub-path has not changed (step ST4). In the case where the signal level L1 of the error signal e1 is equal to or greater than the third reference value R3 (step ST6: Yes), the sound field change determination unit 35 determines that the transfer function C1 of the sub-path has changed (step ST7).

[0123] like Figure 1As shown by the double-dashed line in , when the reclining portion 8 of the first passenger seat 5 is reclined, the position of the first microphone 23 installed in the reclining portion 8 of the first passenger seat 5 changes. Therefore, the transfer function C1 of the sub-path changes, and the difference between the transfer function C1 of the sub-path and the sub-path filter C^1 temporarily increases. Therefore, the control effect of the noise reduction system 1 temporarily decreases, and the signal level L1 of the error signal e1 increases.

[0124] See also Figure 4 , for example, when the reclining portion 8 of the first passenger seat 5 is reclined at time T1, the signal level L1 of the error signal e1 significantly increases relative to the signal level L2 of the error signal e2. Therefore, all determinations in steps ST3, ST5, and ST6 become "yes". Therefore, the sound field change determination unit 35 can determine that the transfer function C1 of the secondary path has changed.

[0125] In addition, the sound field change determination unit 35 can determine whether the transfer function C2 of the secondary path has changed based on the error signal e1 and the error signal e2 through a process similar to the above-mentioned sound field change determination process. For example, in the case where the absolute value of the difference between the signal level L1 of the error signal e1 and the signal level L2 of the error signal e2 is equal to or greater than the first reference value R1, the time change ΔL2 of the signal level L2 of the error signal e2 is equal to or greater than the second reference value R2, and the signal level L2 of the error signal e2 is equal to or greater than the third reference value R3, the sound field change determination unit 35 determines that the transfer function C2 of the secondary path has changed. Otherwise, the sound field change determination unit 35 determines that the transfer function C2 of the secondary path has not changed.

[0126] <Convergence determination processing>

[0127] Subsequently, a description will be given of a convergence determination process performed by the convergence determination unit 36. The convergence determination process is a process for determining whether the fluctuation according to which the sub-path filter C^1 is adaptively updated (hereinafter referred to as "fluctuation of the sub-path filter C^1") has converged.

[0128] Reference Figure 5 and Figure 6, when the convergence determination process starts, the convergence determination unit 36 ​​obtains the amplitude A and phase P of the sub-path filter C^1 (step ST11). For example, the convergence determination unit 36 ​​obtains the maximum value of the amplitude of the impulse response of the sub-path filter C^1 as the amplitude A of the sub-path filter C^1. In addition, the convergence determination unit 36 ​​obtains the delayed sampling number Sd corresponding to the maximum value of the amplitude of the impulse response of the sub-path filter C^1 as the phase P of the sub-path filter C^1. The delay time ΔT (the time from the time T0 when the first speaker 21 outputs the cancellation sound y1 to the time Tmax when the amplitude of the impulse response of the sub-path filter C^1 becomes the maximum) is calculated by multiplying the delayed sampling number Sd by the sampling time Ts. That is, the convergence determination unit 36 ​​uses the delayed sampling number Sd corresponding to the delay time ΔT as the phase P of the sub-path filter C^1.

[0129] Subsequently, the convergence determination unit 36 ​​calculates a change ΔA in the amplitude A of the secondary path filter C^1 (e.g., the difference between the current value of the amplitude A of the secondary path filter C^1 and the previous value of the amplitude A of the secondary path filter C^1). In addition, the convergence determination unit 36 ​​calculates a change ΔP in the phase P of the secondary path filter C^1 (e.g., the difference between the current value of the phase P of the secondary path filter C^1 and the previous value of the phase P of the secondary path filter C^1) (step ST12).

[0130] Subsequently, the convergence determination unit 36 ​​determines whether the change ΔA of the amplitude A of the sub-path filter C^1 is less than the amplitude threshold AT (step ST13). In the case where the change ΔA of the amplitude A of the sub-path filter C^1 is equal to or greater than the amplitude threshold AT (step ST13: No), the convergence determination unit 36 ​​determines that the fluctuation of the sub-path filter C^1 has not converged (step ST14).

[0131] In the case where the change ΔA of the amplitude A of the secondary path filter C^1 is less than the amplitude threshold AT (step ST13: Yes), the convergence determination unit 36 ​​determines whether the change ΔP of the phase P of the secondary path filter C^1 is less than the phase threshold PT (step ST15). In the case where the change ΔP of the phase P of the secondary path filter C^1 is equal to or greater than the phase threshold PT (step ST15: No), the convergence determination unit 36 ​​determines that the fluctuation of the secondary path filter C^1 has not converged (step ST14). In the case where the change ΔP of the phase P of the secondary path filter C^1 is less than the phase threshold PT (step ST15: Yes), the convergence determination unit 36 ​​determines that the fluctuation of the secondary path filter C^1 has converged (step ST16).

[0132] As described above, the convergence determination unit 36 ​​performs convergence determination processing based on the change ΔA in the amplitude A of the sub-path filter C^1 and the change ΔP in the phase P. Therefore, compared with the case where the convergence determination processing is performed based on only one of the change ΔA in the amplitude A and the change ΔP in the phase P of the sub-path filter C^1, it is possible to accurately determine whether the fluctuation of the sub-path filter C^1 has converged.

[0133] <Update Process>

[0134] Next, description will be given of an update process performed by the controller 25. The update process is a process for updating the control filter W1, the sub-path filter C^1, the main path filter H^1, and the auxiliary sub-path filter C^1p.

[0135] Reference Figure 7 When the updating process starts, the sound field change determining unit 35 performs the above-described sound field change determining process. That is, the sound field change determining unit 35 determines whether the transfer function C1 of the sub-path has changed based on the error signal e1 and the error signal e2 (step ST21).

[0136] In the case where the transfer function C1 of the secondary path has changed (step ST21: Yes), the update processing unit 37 sets the state of the secondary path filter C^1 to the update necessary state (step ST22), and proceeds to step ST23. In the case where the transfer function C1 of the secondary path has not changed (step ST21: No), the update processing unit 37 proceeds to step ST23 without performing the processing of step ST22.

[0137] Next, the update processing unit 37 updates the number of times Cnt (initial value=0) of the update processing to Cnt+1 (step ST23 ), and determines whether the number of times Cnt is an odd number (step ST24 ).

[0138] When the number Cnt is an even number (step ST24: No), the first control update unit 43 adaptively updates the control filter W1 (step ST25). Next, the first control filter unit 41 generates a control signal u1 using the adaptively updated control filter W1, and outputs the generated control signal u1 to the first speaker 21. As a result, the first speaker 21 outputs the canceling sound y1 (step ST26).

[0139] When the number of times Cnt is an odd number (step ST24: Yes), the update processing unit 37 determines whether the state of the sub-path filter C^1 is set to the update-required state (step ST27). When the state of the sub-path filter C^1 is not set to the update-required state (step ST27: No), the processing of steps ST25 and ST26 is performed in the same manner as when the number of times Cnt is an even number (step ST24: No).

[0140] When the state of the sub-path filter C^1 is set to the update-required state (step ST27: Yes), the first sub-path updating unit 52 adaptively updates the sub-path filter C^1, and the first main path updating unit 54 adaptively updates the main path filter H^1 (step ST28).

[0141] Subsequently, the convergence determination unit 36 ​​performs the above-described convergence determination process to determine whether the fluctuation of the sub-path filter C^1 has converged (step ST29).

[0142] When the fluctuation of the sub-path filter C^1 has converged (step ST29: Yes), the update processing unit 37 updates the auxiliary sub-path filter C^1p to the value of the sub-path filter C^1 by copying the value of the sub-path filter C^1 to the auxiliary sub-path filter C^1p (step ST30).

[0143] Next, the first control filter unit 41 generates a control signal u1 using the temporarily fixed control filter W1 (the control filter W1 adaptively updated in the previous update process), and outputs the generated control signal u1 to the first speaker 21. Therefore, the first speaker 21 outputs the canceling sound y1 (step ST31).

[0144] In the case where the fluctuation of the sub-path filter C^1 has not converged (step ST29: No), the update processing unit 37 updates the number Cnt to Cnt+1 without updating the auxiliary sub-path filter C^1p to the value of the sub-path filter C^1 (step ST32). Next, when the above step ST31 is executed, the first control filter unit 41 outputs the control signal u1 to the first speaker 21, and the first speaker 21 outputs the cancellation sound y1.

[0145] When step ST26 or step ST31 ends, the update process ends, and the next update process (new update process) is executed after a prescribed time. However, the value of the number of times Cnt is maintained even after the update process ends, and is used for the next update process.

[0146] In the case where the fluctuation of the sub-path filter C^1 has not converged in the current update process (step ST29: No), the update processing unit 37 sets the state of the sub-path filter C^1 to the update necessary state in the next update process (step ST22). In addition, in the case where the fluctuation of the sub-path filter C^1 has not converged in the current update process (step ST29: No), when the number of updates Cnt is twice in step ST32 of the current update process and step ST23 of the next update process, step ST24 of the next update process becomes "Yes". Therefore, in the next update process, the sub-path filter C^1 is adaptively updated (step ST28), and it is determined in the same manner as in the current update process whether the fluctuation of the sub-path filter C^1 has converged (step ST29). In this way, in the present embodiment, without performing the adaptive update of the control filter W1, the adaptive update of the sub-path filter C^1 and the determination of whether the fluctuation of the sub-path filter C^1 has converged are repeated until the fluctuation of the sub-path filter C^1 converges.

[0147] The controller 25 repeatedly performs the above-mentioned update processing at a prescribed time interval. In the update processing, the controller 25 determines whether the fluctuation of the sub-path filter C^1 has converged (step ST29). In the case where the fluctuation of the sub-path filter C^1 has converged (step ST29: yes), the controller 25 stops the adaptive update of the sub-path filter C^1. In the next update processing, the controller 25 determines whether the transfer function C1 of the sub-path has changed based on the error signal e1 and the error signal e2 in the state where the adaptive update of the sub-path filter C^1 is stopped (step ST21). In the case where the transfer function C1 of the sub-path has changed (step ST21: yes), the controller 25 resumes the adaptive update of the sub-path filter C^1 (steps ST22, ST27 and ST28).

[0148] <Adaptive Update of Sub-Path Filter C^1 and Main Path Filter H^1>

[0149] In step ST28 of the above-mentioned update process, the first sub-path update unit 52 adaptively updates the sub-path filter C^1 according to the following formula (1), and the first main path update unit 54 adaptively updates the main path filter H^1 according to the following formula (2). In the following formula (1), "μ1" represents a weighting coefficient (step parameter) for adjusting the adaptive update amount of the sub-path filter C^1. In the following formula (2), "μ2" represents a weighting coefficient (step parameter) for adjusting the adaptive update amount of the main path filter H^1. In the following formulas (1) and (2), "N" represents a normalization divisor for normalizing the adaptive update amounts of the sub-path filter C^1 and the main path filter H^1.

[0150]

[0151] It can be clearly seen from the above formula (1) that the sub-path filter C^1 is adaptively updated based on the control signal u1, and the main path filter H^1 is adaptively updated based on the reference signal r. In addition, as is clear from the above formulas (1) and (2), the adaptive update amount of the sub-path filter C^1 and the adaptive update amount of the main path filter H^1 are normalized by a common normalization divisor N. The normalized divisor N is expressed by the following formula (3). In the following formula (3), "||r(t)||" represents the norm of the signal vector of the reference signal r, and "||u1(t)||" represents the norm of the signal vector of the control signal u1. In addition, in the following formula (3), "σ" represents a constant (a small positive number) used to prevent the normalization divisor N from becoming zero.

[0152] N=||r(t)||+||u1(t)||+σ...(3)

[0153] "||r(t)||" and "||u1(t)||" in the above formula (3) are respectively expressed by the following formulas (4) and (5). In the following formula (4), "L1" represents the number of data of the signal vector of the reference signal r. In the following formula (5), "L2" represents the number of data of the signal vector of the control signal u1.

[0154]

[0155] <Effect>

[0156] In the above-mentioned update process, the controller 25 prohibits the adaptive update of the control filter W1 and the adaptive update of the sub-path filter C^1 from being performed simultaneously in one update process. In other words, the controller 25 distinguishes the timing of the adaptive update of the control filter W1 and the timing of the adaptive update of the sub-path filter C^1. Therefore, compared with the case where the adaptive update of the control filter W1 and the sub-path filter C^1 is performed simultaneously, the calculation load on the controller 25 during the update of these filters can be reduced. Therefore, the controller 25 does not need to be composed of an expensive processor, so that the increase in the manufacturing cost of the noise reduction system 1 can be suppressed.

[0157] Furthermore, by distinguishing the timing of adaptive updating of the control filter W1 from the timing of adaptive updating of the sub-path filter C^1, it is possible to prevent the control filter W1 from being adaptively updated based on the sub-path filter C^1 that fluctuates according to its adaptive update. Therefore, it is possible to suppress the convergence delay of the control filter W1 and improve the noise reduction performance of the noise reduction system 1.

[0158] In addition, in the above-mentioned update process, the control filter W1 is not adaptively updated while the sub-path filter C^1 and the main path filter H^1 are adaptively updated. Therefore, the calculation load on the controller 25 can be reduced compared to the case where the adaptive update of the control filter W1 is performed in each update process.

[0159] <Modification example>

[0160] In the first embodiment described above, the convergence determination unit 36 ​​performs convergence determination processing based on the change ΔA of the amplitude A of the secondary path filter C^1 and the change ΔP of the phase P. In another embodiment, the convergence determination unit 36 ​​may perform convergence determination processing based on only one of the change ΔA of the amplitude A of the secondary path filter C^1 and the change ΔP of the phase P.

[0161] In the first embodiment described above, the convergence determination unit 36 ​​obtains the maximum value of the amplitude of the impulse response of the sub-path filter C^1 as the amplitude A of the sub-path filter C^1 (step ST11). In another embodiment, the convergence determination unit 36 ​​can calculate the amplitude A of the sub-path filter C^1 by the following formula (6). In the following formula (6), "L" represents the total number of coefficients of the sub-path filter C^1, and "n" represents the number of coefficients of the sub-path filter C^1.

[0162]

[0163] By using the above formula (6), the amplitude A of the secondary path filter C^1 can be calculated more accurately.

[0164] In the above-mentioned first embodiment, the convergence determination unit 36 ​​determines whether the change ΔA of the amplitude A of the secondary path filter C^1 is less than the amplitude threshold AT (step ST13). In another embodiment, the convergence determination unit 36 ​​can determine whether the state in which the change ΔA of the amplitude A of the secondary path filter C^1 is less than the amplitude threshold AT lasts for a prescribed period of time. Alternatively, the convergence determination unit 36 ​​can determine whether the ratio of the current value of the amplitude A of the secondary path filter C^1 to the previous value of the amplitude A of the secondary path filter C^1 is equal to or less than a prescribed value. The same method can be applied to determine the change ΔP of the phase P of the secondary path filter C^1.

[0165] In the first embodiment described above, the update processing unit 37 copies the value of the sub-path filter C^1 to the auxiliary sub-path filter C^1p at the timing of the adaptive update of the sub-path filter C^1 (steps ST28 to ST30). In another embodiment, the update processing unit 37 may copy the value of the sub-path filter C^1 to the auxiliary sub-path filter C^1p at a timing different from the adaptive update of the sub-path filter C^1 (for example, at the timing of the adaptive update of the control filter W1).

[0166] See also Figure 7 In the first embodiment, when the fluctuation of the secondary path filter C^1 has not yet converged (step ST29: No), after the update processing unit 37 updates the number Cnt to Cnt+1, the first speaker 21 outputs the cancellation sound y1 (steps ST31 and ST32). Figure 8 In another embodiment, when the fluctuation of the sub-path filter C^1 has not converged (step ST29: No), the first speaker 21 can output the cancellation sound y1 (step ST31) without the update processing unit 37 updating the number Cnt to Cnt+1. That is, in another embodiment, the processing of step ST32 can be omitted. Therefore, in the next update processing, step ST24 becomes "No", and the control filter W1 is adaptively updated (step ST25). Therefore, in the case where the transfer function C1 of the sub-path changes, the adaptive update of the control filter W1 (step ST25) and the adaptive update of the sub-path filter C^1 (step ST28) are performed alternately.

[0167] In the first embodiment described above, the second microphone 24 is used as a reference microphone for generating a determination signal. In another embodiment, a dedicated reference microphone may be provided to generate a determination signal. In this case, the reference microphone may generate a determination signal based only on the cancellation sound y1 and the noise d. In other words, the reference microphone may generate a determination signal based at least on the noise d.

[0168] In the first embodiment described above, the controller 25 performs an update process to adaptively update the control filter W1, the sub-path filter C^1, and the main path filter H^1. In another embodiment, the controller 25 may perform an update process to adaptively update the control filter W2, the sub-path filter C^2, and the main path filter H^2. In this case, the second microphone 24 may be used as an error microphone, and the first microphone 23 may be used as a reference microphone.

[0169] (Second Embodiment)

[0170] Then, see Figures 9 to 11, an active noise reduction system 81 according to a second embodiment of the present invention (hereinafter referred to as "noise reduction system 81") will be described. Components other than the controller 83 are the same as those of the first embodiment. Therefore, the same reference numerals as those in the first embodiment are given to these components in the drawings, and their description is omitted.

[0171] <Controller 83>

[0172] Reference Fig. 9 , the controller 83 includes a control signal generating unit 85, a sound field learning unit 86, a sound field change determining unit 87, a convergence determining unit 88, and an update processing unit 89 as functional components. The configurations of the control signal generating unit 85, the sound field learning unit 86, the convergence determining unit 88, and the update processing unit 89 are respectively the same as those of the first control signal generating unit 31, the first sound field learning unit 32, the convergence determining unit 36, and the update processing unit 37 according to the first embodiment. Therefore, a description thereof will be omitted.

[0173] <Sound Field Change Determination Unit 87>

[0174] The seat state information Is is transmitted from the first passenger seat 5 to the sound field change determination unit 87 of the controller 83. The seat state information Is is information about the state of the first passenger seat 5. The seat state information Is includes information about the front-rear position of the first passenger seat 5, information about the height of the first passenger seat 5, and information about the inclination angle of the reclining portion 8 of the first passenger seat 5 (hereinafter referred to as "the inclination angle of the first passenger seat 5").

[0175] <Sound Field Change Determination Processing>

[0176] Subsequently, the sound field change determination processing performed by the sound field change determination unit 87 will be described. The sound field change determination processing is a processing for determining whether the transfer function C1 of the sub-path has changed. Hereinafter, the position of the first passenger seat 5 in the current seat state information Is will be referred to as the "current position Pc of the first passenger seat 5", and the position of the first passenger seat 5 in the previous seat state information Is will be referred to as the "reference position Pr of the first passenger seat 5".

[0177] See also Fig.10 and Fig.11 When the sound field change determination processing starts, the sound field change determination unit 87 calculates the change ΔX in the front-rear position of the first passenger seat 5, the change ΔH in the height of the first passenger seat 5, and the change Δα in the inclination angle of the first passenger seat 5 based on the current position Pc of the first passenger seat 5 and the reference position Pr of the first passenger seat 5 (step ST41).

[0178] Subsequently, the sound field change determination unit 87 determines whether at least one of the following conditions 1 to 3 is satisfied (step ST42 ).

[0179] Condition 1: The change ΔX in the front-rear position of the first passenger seat 5 is greater than a first front-rear threshold value.

[0180] Condition 2: The change ΔH in the height of the first passenger seat 5 is greater than the first height threshold.

[0181] Condition 3: The change Δα of the inclination angle of the first passenger seat 5 is greater than the first angle threshold value.

[0182] In the case where at least one of the above-mentioned conditions 1 to 3 is satisfied (step ST42 : Yes), the sound field change determination unit 87 determines that the transfer function C1 of the sub path has changed (step ST43 ).

[0183] In the case where the above conditions 1 to 3 are not satisfied (step ST42: No), the sound field change determination unit 87 determines whether all the conditions in the following conditions 4 to 6 are satisfied (step ST44). The second front-back threshold, the second height threshold, and the second angle threshold in the following conditions 4 to 6 are respectively smaller than the first front-back threshold, the first height threshold, and the first angle threshold in the above conditions 1 to 3.

[0184] Condition 4: The change ΔX in the front-rear position of the first passenger seat 5 is greater than the second front-rear threshold value.

[0185] Condition 5: The change ΔH in the height of the first passenger seat 5 is greater than the second height threshold.

[0186] Condition 6: The change Δα of the inclination angle of the first passenger seat 5 is greater than the second angle threshold.

[0187] In the case where all of the above conditions 4 to 6 are satisfied (step ST44: Yes), the sound field change determination unit 87 determines that the transfer function C1 of the sub-path has changed (step ST43). In the case where at least one of the above conditions 4 to 6 is not satisfied (step ST44: No), the sound field change determination unit 87 determines that the transfer function C1 of the sub-path has not changed (step ST45).

[0188] <Update Process>

[0189] Next, the update process according to the second embodiment and the update process according to the first embodiment (see Figure 7 ) between the two.

[0190] In step ST21 of the update process according to the first embodiment, the sound field change determination unit 35 determines whether the transfer function C1 of the sub-path has changed based on the error signal e1 and the error signal e2. In contrast, in step ST21 of the update process according to the second embodiment, the sound field change determination unit 87 determines whether the transfer function C1 of the sub-path has changed based on the seat state information Is.

[0191] In the case where the sub-path filter C^1 is adaptively updated in the update process according to the second embodiment (step ST28), it is preferable to determine whether the transfer function C1 of the sub-path has changed based on the current position Pc of the first passenger seat 5 in step ST21 of the next update process. Therefore, in the update process according to the second embodiment, when the fluctuation of the sub-path filter C^1 has converged (step ST29: Yes), the update processing unit 89 updates the reference position Pr of the first passenger seat 5 to the current position Pc of the first passenger seat 5.

[0192] <Modification example>

[0193] In the second embodiment, the sound field change determination unit 87 determines whether the transfer function C1 of the secondary path has changed based on the seat state information Is. In another embodiment, the sound field change determination unit 87 may determine whether the transfer function C1 of the secondary path has changed based on the number of occupants, the external temperature, the size of the error signal e1, the change in tire pressure, etc.

[0194] (Third Embodiment)

[0195] Then, see Fig.12 and Fig.13 , an active noise reduction system 91 according to a third embodiment of the present invention (hereinafter referred to as "noise reduction system 91") will be described. The components other than the sound field change determination unit 94 of the controller 93 are the same as those of the second embodiment. Therefore, the same reference numerals as those in the second embodiment are given to these components in the drawings, and their description is omitted.

[0196] <Sound Field Change Determination Unit 94>

[0197] See also Fig.12 , the window opening and closing information Iw is sent from the window 96 (such as a side window or a roof window) of the vehicle compartment 4 to the sound field change determination unit 94 of the controller 93. The window opening and closing information Iw is information about the opening and closing state of the window 96.

[0198] <Sound Field Change Determination Processing>

[0199] Subsequently, the sound field change determination processing performed by the sound field change determination unit 94 will be described. The sound field change determination processing is a processing for determining whether the transfer function C1 of the sub-path has changed. Hereinafter, the position of the window 96 in the current window opening and closing information Iw will be referred to as the "current position of the window 96", and the position of the window 96 in the previous window opening and closing information Iw will be referred to as the "reference position of the window 96".

[0200] Reference Fig.13 When the sound field change determination process starts, the sound field change determination unit 94 determines whether the current position of the window 96 does not match the reference position of the window 96 (step ST51). In other words, the sound field change determination unit 94 determines whether the window 96 is open or closed.

[0201] In the case where the current position of the window 96 does not match the reference position of the window 96 (step ST51: Yes), the sound field change determination unit 94 determines that the transfer function C1 of the sub-path has changed (step ST52). In other words, in the case where the window 96 is opened or closed, the sound field change determination unit 94 determines that the transfer function C1 of the sub-path has changed.

[0202] In the case where the current position of the window 96 matches the reference position of the window 96 (step ST51: No), the sound field change determination unit 94 determines that the transfer function C1 of the sub-path has not changed (step ST53). In other words, in the case where the window 96 is not opened or closed, the sound field change determination unit 94 determines that the transfer function C1 of the sub-path has not changed.

[0203] <Update Process>

[0204] Next, the update process according to the third embodiment and the update process according to the first embodiment (see Figure 7 ) between the two.

[0205] In step ST21 of the update process according to the first embodiment, the sound field change determination unit 35 determines whether the transfer function C1 of the sub-path has changed based on the error signal e1 and the error signal e2. In contrast, in step ST21 of the update process according to the third embodiment, the sound field change determination unit 94 determines whether the transfer function C1 of the sub-path has changed based on the window opening and closing information Iw.

[0206] In the case where the sub-path filter C^1 is adaptively updated in the update process according to the third embodiment (step ST28), it is preferable to determine whether the transfer function C1 of the sub-path has changed based on the current position of the window 96 in step ST21 of the next update process. Therefore, in the update process according to the third embodiment, when the fluctuation of the sub-path filter C^1 has converged (step ST29: Yes), the update processing unit 89 updates the reference position of the window 96 to the current position of the window 96.

[0207] (Fourth Embodiment)

[0208] Then, see Fig.14 and Fig.15 , an active noise reduction system 101 according to a fourth embodiment of the present invention (hereinafter referred to as “noise reduction system 101”) will be described. Fig.14 As shown, the noise reduction system 101 has a configuration similar to the noise reduction system 81 according to the second embodiment, except that the controller 103 does not include the sound field change determination unit 87 and the convergence determination unit 88. Therefore, the components of the noise reduction system 101 are given the same reference numerals as in the second embodiment in the drawings, and their description will be omitted.

[0209] <Update Process>

[0210] Next, description will be given of an update process performed by the controller 103. The update process is a process for updating the control filter W1, the sub-path filter C^1, the main path filter H^1, and the auxiliary sub-path filter C^1p.

[0211] Reference Fig.15 , when the update processing starts, the update processing unit 89 updates the number of update processing times Cnt (initial value=0) to Cnt+1 (step ST61).

[0212] Next, the update processing unit 89 determines whether the number of times Cnt is an odd number (step ST62 ). In the case where the number of times Cnt is an odd number (step ST62 : Yes), the control signal generating unit 85 adaptively updates the control filter W1 (step ST63 ).

[0213] Next, the control signal generating unit 85 generates a control signal u1 using the adaptively updated control filter W1, and outputs the generated control signal u1 to the first speaker 21. As a result, the first speaker 21 outputs the canceling sound y1 (step ST64).

[0214] When the number Cnt is an even number (step ST62: No), the sound field learning unit 86 adaptively updates the sub-path filter C^1 and the main path filter H^1 (step ST65). Next, the update processing unit 89 updates the auxiliary sub-path filter C^1p to the value of the sub-path filter C^1 by copying the value of the sub-path filter C^1 to the auxiliary sub-path filter C^1p (step ST66).

[0215] Next, the control signal generating unit 85 generates a control signal u1 using the temporarily fixed control filter W1 (the control filter W1 adaptively updated in the previous updating process), and outputs the generated control signal u1 to the first speaker 21. Thus, the first speaker 21 outputs the canceling sound y1 (step ST67).

[0216] <Modification example>

[0217] In step ST62 of the update processing in the fourth embodiment, the update processing unit 89 determines whether the number of times Cnt is an odd number. Therefore, one adaptive update of the control filter W1 and one adaptive update of the sub-path filter C^1 are performed alternately. In step ST62 of the update processing according to another embodiment, the update processing unit 89 may perform a determination different from the above determination. For example, the update processing unit 89 may determine whether the number of times Cnt is a number other than a multiple of 3. Therefore, two adaptive updates of the control filter W1 and one adaptive update of the sub-path filter C^1 are performed alternately. In this way, "the case of alternately performing the adaptive update of the control filter W1 and the adaptive update of the sub-path filter C^1" not only includes the case of alternately performing one adaptive update of the former and one adaptive update of the latter, but also includes the case of performing the adaptive update of at least one of the former and the latter in sequence.

[0218] (Fifth Embodiment)

[0219] Then, see Figures 16 to 18 , an active noise reduction system 111 according to a fifth embodiment of the present invention (hereinafter referred to as "noise reduction system 111") will be described. Descriptions overlapping with the first embodiment will be omitted.

[0220] <Noise Reduction System 111>

[0221] Reference Fig.16The noise reduction system 111 includes: n (n≧2) speakers S1 to Sn (an example of a cancellation sound output device), each speaker is configured to output a cancellation sound y for canceling the noise d; m (m≧2) microphones M1 to Mm (an example of an error microphone), each microphone is configured to generate an error signal e based on the noise d and the cancellation sound y; and a controller 115, the controller 115 is configured to control the n speakers S1 to Sn based on the error signal e.

[0222] <Controller 115>

[0223] Reference Fig.16 , the controller 115 includes S (2≦S≦n) control channels Ch1 to ChS corresponding to the n speakers S1 to Sn and an update determination unit 118.

[0224] Each control channel Ch1 to ChS includes a control signal generating unit 120 and a sound field learning unit 121. The control signal generating unit 120 includes a control filter W and an auxiliary sub-path filter C^P. The sound field learning unit 121 includes a sub-path filter C^ and a main path filter H^. The configuration of the control signal generating unit 120 and the sound field learning unit 121 is similar to the configuration of the control signal generating unit 85 and the sound field learning unit 86 according to the second embodiment. Therefore, the description thereof will be omitted.

[0225] The update determination unit 118 determines whether the transfer function C of the sub-path from one of the speakers S1 to Sn to one of the microphones M1 to Mm has changed using the same processing as the sound field change determination processing according to the first embodiment. The update determination unit 118 determines whether the fluctuation of the sub-path filter C^ according to its adaptive update (hereinafter referred to as "fluctuation of the sub-path filter C^") has converged using the same processing as the convergence determination processing according to the first embodiment. The update determination unit 118 determines the order and timing of the adaptive update of the filter.

[0226] Reference Fig.17 , the update determination unit 118 stores the update processing table T. In the update processing table T, an error microphone and a reference microphone are set for each control channel Ch1 to ChS among the m microphones M1 to Mm. The combination of the error microphone and the reference microphone for each control channel Ch1 to ChS can be set based on various parameters.

[0227] The state (update necessary state or update unnecessary state) of the sub-path filter C^ is set for each control channel Ch1 to ChS in the update processing table T. The state of the sub-path filter C^ is not a fixed value but a value that can be rewritten in the update processing to be described later.

[0228] In the update processing table T, the update flag ("1: update" or "0: not update") state of the sub-path filter C^ is set for each control channel Ch1 to ChS, where the state of the sub-path filter C^ is set to the update necessary state. In the update processing table T, one control channel Ch1 to ChS in which the update flag of the sub-path filter C^ is set to "1: update" is switched sequentially.

[0229] <Update Process>

[0230] Next, the update process performed in each control channel Ch1 to ChS will be described. The update process is a process for updating the control filter W, the auxiliary sub-path filter C^p, the sub-path filter C^, and the main path filter H^ of each control channel Ch1 to ChS. Steps ST71 and ST72 of the update process according to the fifth embodiment are similar to steps ST21 and ST22 of the update process according to the first embodiment (see Figure 7 ). Therefore, the description of these steps will be omitted.

[0231] Reference Fig.18 When step ST72 of the update process is completed, the update determination unit 118 determines whether the state of the sub-path filter C^ is set to the update-required state (step ST73). In the case where the state of the sub-path filter C^ is not set to the update-required state (step ST73: No), the control signal generation unit 120 adaptively updates the control filter W (step ST74).

[0232] Next, the control signal generating unit 120 generates a control signal u using the adaptively updated control filter W, and outputs the generated control signal u to the corresponding speakers S1 to Sn. As a result, the speakers S1 to Sn output the canceling sound y (step ST75). Thus, the updating process ends.

[0233] When the state of the sub-path filter C^ is set to the update-necessary state (step ST73: Yes), the update determination unit 118 determines whether the update flag of the sub-path filter C^ is set to “1: Update” by referring to the update processing table T (step ST76).

[0234] When the update flag of the sub-path filter C^ is set to "0: not updated" (step ST76: No), after the control filter W is adaptively updated in step ST74, the speakers S1 to Sn output the cancellation sound y in the above-mentioned step ST75. Therefore, the update process ends.

[0235] When the update flag of the sub-path filter C^ is set to “1: update” (step ST76 : Yes), the sound field learning unit 121 adaptively updates the sub-path filter C^ and the main path filter H^ (step ST77 ).

[0236] Subsequently, the update determination unit 118 determines whether the fluctuation of the sub-path filter C^ has converged (step ST78).

[0237] In the case where the fluctuation of the sub-path filter C^ has converged (step ST78: Yes), the update determination unit 118 updates the auxiliary sub-path filter C^P to the value of the sub-path filter C^ by copying the value of the sub-path filter C^P to the auxiliary sub-path filter C^P (step ST79). Subsequently, in the above-mentioned step ST75, the speakers S1 to Sn output the cancellation sound y. Therefore, the update process ends.

[0238] In the case where the fluctuation of the sub-path filter C^ has not converged (step ST78: No), the speakers S1 to Sn output the cancellation sound y in the above-mentioned step ST75 without updating the auxiliary sub-path filter C^P to the value of the sub-path filter C^ by the update determination unit 118. Therefore, the update process ends.

[0239] <Effect>

[0240] As described above, the controller 115 distinguishes the timing of the adaptive update of the sub-path filter C^ in each control channel Ch1 to ChS. Therefore, compared with the case where the adaptive update of the sub-path filter C^ in each control channel Ch1 to ChS is performed at the same time, the amount of calculation of the controller 115 during the filter update can be reduced. Therefore, the controller 115 does not need to be composed of an expensive processor, so that the increase in the manufacturing cost of the noise reduction system 111 can be suppressed.

[0241] <Modification example>

[0242] Reference Fig.18 In the updating process according to the fifth embodiment, when the sub-path filter C^ is adaptively updated (step ST77), the speakers S1 to Sn output the cancellation sound y, and the control filter W is not adaptively updated (step ST75). Fig.19 In another embodiment, when the sub-path filter C^ is adaptively updated (step ST77), after the control filter W is adaptively updated (step ST74), the speakers S1 to Sn can output the cancellation sound y (step ST75). Therefore, the adaptive update of the control filter W is performed in each update process, so that the response of the noise reduction control to the noise d can be improved.

[0243] exist Figure 8In the update process shown, adaptive update of the control filter W1 and adaptive update of the sub-path filter C^1 are performed alternately when the transfer function C1 of the sub-path has changed, and adaptive update of only the control filter W1 is performed when the transfer function C1 of the sub-path has not changed (hereinafter referred to as "update process 1"). Fig.19 In the update process shown, the adaptive update of the control filter W is performed each time, regardless of whether the sub-path filter C^ is adaptively updated (hereinafter referred to as "update process 2"). In another embodiment, update process 1 or update process 2 may be selectively performed in each control channel Ch1 to ChS. For example, update process 2 may be performed when the change of the sub-path filter C^ is greater than a prescribed value, and update process 1 may be performed when the change of the sub-path filter C^ is equal to or less than a prescribed value.

[0244] <Modification example>

[0245] In the above-described first to fifth embodiments, the noise reduction systems 1, 81, 91, 101, and 111 are applied to the cabin 4 of the vehicle 3. In another embodiment, the noise reduction systems 1, 81, 91, 101, and 111 may be applied to the interior space of a moving body other than the vehicle 3 (e.g., a ship or an airplane), or to the interior space of a fixed object (e.g., a house).

[0246] The specific embodiments of the present invention have been described above, but the present invention should not be limited to the foregoing embodiments, but various modifications and changes may be made within the scope of the present invention.

Claims

1. An active noise reduction system, the active noise reduction system comprising: a cancelling sound output device, the cancelling sound output device being configured to output a cancelling sound for cancelling noise; an error microphone configured to generate an error signal based on the noise and the canceling sound; as well as a controller configured to control the cancellation sound output device based on the error signal, Wherein, the controller comprises: a control signal generating unit, the control signal generating unit comprising a control filter, the control filter being configured to generate a control signal for controlling the canceling sound output device; and a sound field learning unit including a sub-path filter representing an estimated value of a transfer function of a sub-path from the cancellation sound output device to the error microphone, The control filter and the side path filter are configured to be adaptively updated, and The controller is configured to differentiate between a timing of an adaptive update of the control filter and a timing of an adaptive update of the secondary path filter.

2. The active noise reduction system according to claim 1, wherein: The controller is configured to alternately perform adaptive updating of the control filter and adaptive updating of the secondary path filter.

3. The active noise reduction system according to claim 1, wherein: The control signal generating unit further includes an auxiliary secondary path filter, the auxiliary secondary path filter representing the estimated value of the transfer function of the secondary path, The controller is configured to: determining whether the secondary path filter has converged according to the fluctuations of its adaptive update, and In the event that the fluctuation of the secondary path filter according to which it is adaptively updated has converged, the auxiliary secondary path filter is updated by copying the value of the secondary path filter to the auxiliary secondary path filter.

4. The active noise reduction system according to claim 3, wherein: The controller is configured to determine whether fluctuations according to which the secondary path filter is adaptively updated have converged based on changes in at least one of an amplitude of the secondary path filter and a phase of the secondary path filter.

5. The active noise reduction system according to claim 1, wherein: One of the cancellation sound output device and the error microphone is installed in a passenger seat of a vehicle, The other of the cancellation sound output device and the error microphone is installed in a portion of the vehicle other than the passenger seat, and The controller is configured to: determining whether the secondary path filter has converged according to the fluctuations of its adaptive update, stopping the adaptive update of the secondary path filter when the fluctuation of the secondary path filter according to its adaptive update has converged, determining whether the transfer function of the secondary path has changed based on the state of the passenger seat while adaptive updating of the secondary path filter is stopped, and Adaptive updating of the secondary path filter is resumed in the event that the transfer function of the secondary path has changed.

6. The active noise reduction system according to claim 5, wherein: The controller is configured to: acquiring a plurality of changes among a change in the front-rear position of the passenger seat, a change in the height of the passenger seat, and a change in the inclination angle of a reclining portion of the passenger seat, A first threshold and a second threshold are set for each of the plurality of changes, the second threshold being smaller than the first threshold, and In case at least one of the plurality of changes is greater than the first threshold and in case all of the plurality of changes are greater than the second threshold, it is determined that the transfer function of the secondary path has changed.

7. The active noise reduction system according to claim 1, wherein: At least one of the cancellation sound output device and the error microphone is installed in a vehicle compartment, and The controller is configured to: determining whether the secondary path filter has converged according to the fluctuations of its adaptive update, stopping the adaptive update of the secondary path filter when the fluctuation of the secondary path filter according to its adaptive update has converged, In a state where adaptive updating of the secondary path filter is stopped, determining whether the transfer function of the secondary path has changed based on an open or closed state of a window of the vehicle cabin, and Adaptive updating of the secondary path filter is resumed in the event that the transfer function of the secondary path has changed.

8. The active noise reduction system according to claim 1, wherein: The sound field learning unit further includes a main path filter representing an estimated value of a transfer function of a main path from a noise source to the error microphone, The main path filter is configured to adaptively update, and The controller is configured to normalize an adaptive update amount of the secondary path filter and an adaptive update amount of the primary path filter using a common normalization divisor.

9. The active noise reduction system according to claim 8, wherein: The main path filter is configured to be adaptively updated based on a reference signal corresponding to the noise, The secondary path filter is configured to be adaptively updated based on the control signal, and The common normalization divisor includes a norm of a signal vector of the reference signal and a norm of a signal vector of the control signal.

10. The active noise reduction system according to claim 1, further comprising a reference microphone disposed separately from the error microphone, The reference microphone is configured to generate a determination signal based at least on the noise, The controller is configured to: determining whether the secondary path filter has converged according to the fluctuations of its adaptive update, stopping the adaptive update of the secondary path filter when the fluctuation of the secondary path filter according to its adaptive update has converged, determining whether the transfer function of the sub-path has changed based on the error signal and the determination signal in a state where adaptive updating of the sub-path filter is stopped, and Adaptive updating of the secondary path filter is resumed in the event that the transfer function of the secondary path has changed.

11. The active noise reduction system according to claim 10, wherein: The controller includes a second control signal generating unit, the second control signal generating unit is provided separately from the control signal generating unit, and The microphone corresponding to the second control signal generating unit is used as the reference microphone.

12. The active noise reduction system according to claim 10, wherein: The controller is configured to determine that the transfer function of the sub path has changed if a difference between the error signal and the determination signal is equal to or greater than a first reference value and a time variation of the error signal is equal to or greater than a second reference value.

13. The active noise reduction system according to claim 12, wherein: The controller is configured to determine that the transfer function of the secondary path has changed when the difference between the error signal and the determination signal is equal to or greater than the first reference value, the time variation of the error signal is equal to or greater than the second reference value, and the error signal is equal to or greater than a third reference value.

14. The active noise reduction system according to any one of claims 1 to 13, wherein: The controller includes a plurality of control channels, each control channel includes the control signal generating unit and the sound field learning unit, and The controller is configured to differentiate timing of adaptive updating of the secondary path filter in each of the plurality of control channels.

15. An active noise reduction system, the active noise reduction system comprising: a cancelling sound output device, the cancelling sound output device being configured to output a cancelling sound for cancelling noise; an error microphone configured to generate an error signal based on the noise and the canceling sound; as well as a controller configured to control the cancellation sound output device based on the error signal, Wherein, the controller comprises: a control signal generating unit, the control signal generating unit comprising a control filter, the control filter being configured to generate a control signal for controlling the canceling sound output device; and a sound field learning unit including a sub-path filter representing an estimated value of a transfer function of a sub-path from the cancellation sound output device to the error microphone, The control filter and the secondary path filter are configured to be adaptively updated, The controller includes a plurality of control channels, each control channel includes the control signal generating unit and the sound field learning unit, and The controller is configured to differentiate timing of adaptive updating of the secondary path filter in each of the plurality of control channels.

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