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 filter configured to generate a control signal for controlling the cancellation sound output device; and a sub-path filter that represents an estimated value of a transfer function of a sub-path from the cancellation sound output device to the error microphone. The active noise reduction system also includes a reference microphone disposed separately from the error microphone, the reference microphone configured to generate a determination signal based at least on the noise, and the controller configured to determine whether a transfer function of the secondary path has changed based on the error signal and the determination signal.

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, JPH7-28474A discloses an active noise reduction system including a side path filter (see "filter 14c for generating a filtered X signal" in JPH7-28474A) which represents an estimated value of a transfer function of a side path.

[0005] In such an active noise reduction system, when the transfer function of the secondary path changes and the difference between the transfer function of the secondary path and the secondary path filter becomes larger, the noise reduction effect may be reduced or the noise may be amplified. Therefore, in the case where the transfer function of the secondary path has changed, there is a known technology for adaptively updating the secondary path filter. In the case of adopting such technology, it is necessary to accurately determine whether the transfer function of the secondary path has changed to adaptively update the secondary path filter at an appropriate timing. 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 accurately determining whether the transfer function of the secondary path has changed. In addition, another object of the present invention is to contribute to the development of sustainable transportation systems.

[0007] In order to achieve such a purpose, one aspect of the present invention provides an active noise reduction system 1, which includes: a cancellation sound output device 21, which is configured to output a cancellation sound for canceling noise; an error microphone 23, which is configured to generate an error signal e1 based on the noise and the cancellation sound; and a controller 25, which is configured to control the cancellation sound output device based on the error signal, wherein the controller includes: a control filter W1, which is configured to generate a control signal u1 for controlling the cancellation sound output device; and a sub-path filter C^1, which represents an estimated value of a transfer function of a sub-path from the cancellation sound output device to the error microphone, and the active noise reduction system also includes a reference microphone 24 arranged separately from the error microphone, which is configured to generate a determination signal e2 based on at least the noise, and the controller is configured to determine whether the transfer function of the sub-path has changed based on the error signal and the determination signal.

[0008] According to this aspect, whether the transfer function of the sub-path has changed can be accurately determined based on the error signal and the determination signal. Therefore, the sub-path filter can be adaptively updated at appropriate timing.

[0009] In the above aspect, preferably, the active noise reduction system also includes a second cancellation sound output device 22 arranged separately from the cancellation sound output device, wherein the controller includes a second control filter W2, the second control filter is configured to generate a second control signal u2 for controlling the second cancellation sound output device, and the second control filter is configured to be adaptively updated based on the determination signal.

[0010] According to this aspect, the second control filter can be adaptively updated using the determination signal generated by the reference microphone. Therefore, the number of elements can be reduced compared to the case where a microphone for generating the determination signal and a microphone for generating a signal for adaptive updating of the second control filter are separately provided.

[0011] In the above aspect, preferably, the controller includes a second sub-path filter C^2, which represents an estimated value of the transfer function of the second sub-path from the second cancellation sound output device to the reference microphone, and the controller is configured to determine whether the transfer function of the second sub-path has changed based on the error signal and the determination signal.

[0012] According to this aspect, not only whether the transfer function of the secondary path has changed can be determined based on the error signal and the determination signal, but also whether the transfer function of the second secondary path has changed can be determined based on the error signal and the determination signal. Therefore, compared with the case where the signal for determining the change of the transfer function of the secondary path and the signal for determining the change of the transfer function of the second secondary path are generated by separate components, the number of components can be reduced.

[0013] In the above aspect, preferably, the controller is configured to: determine that the transfer function of the sub-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, and determine that the transfer function of the second sub-path has changed when the difference between the error signal and the determination signal is equal to or greater than the first reference value and the time variation of the determination signal is equal to or greater than the second reference value.

[0014] According to this aspect, it is possible to easily determine whether the transfer function of the sub-path has changed and whether the transfer function of the second sub-path has changed based on the error signal and the determination signal. Therefore, the calculation load on the controller can be reduced.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] In the above aspect, preferably, the active noise reduction system also includes a second cancellation sound output device 22 arranged separately from the cancellation sound output device, wherein the controller includes a second sub-path filter C^2, and the second sub-path filter represents an estimated value of the transfer function of the second sub-path from the second cancellation sound output device to the reference microphone.

[0020] According to this aspect, the second cancelling sound output device can be controlled using the reference microphone. Therefore, the number of components can be reduced compared to the case where the second cancelling sound output device is controlled using a microphone other than the reference microphone.

[0021] Therefore, according to the above aspects, it is possible to provide an active noise reduction system capable of accurately determining whether the transfer function of the sub-path has changed. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a schematic diagram showing a vehicle to which an active noise reduction system according to an embodiment is applied;

[0023] Figure 2 is a functional block diagram showing an active noise reduction system according to an embodiment;

[0024] Figure 3 is a flowchart showing a sound field change determination process according to an embodiment;

[0025] Figure 4 is a graph showing a signal level of an error signal according to an embodiment;

[0026] Figure 5 is a flow chart illustrating a convergence determination process according to an embodiment;

[0027] Figure 6 is a graph showing an impulse response of a secondary path filter according to an embodiment;

[0028] Figure 7 is a flowchart illustrating an update process according to an embodiment; and

[0029] Figure 8 is a flowchart illustrating an update process according to another embodiment. DETAILED DESCRIPTION

[0030] 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.

[0031] <Vehicle 3>

[0032] Figure 1 1 is a schematic diagram showing a vehicle 3 to which an active noise reduction system 1 according to the embodiment (hereinafter referred to as "noise reduction system 1") is applied. The vehicle 3 is, for example, a four-wheeled automobile.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] <Noise reduction system 1>

[0037] 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.

[0038] 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.

[0039] 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.

[0040] <Multiple Speakers 21, 22>

[0041] 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 .

[0042] 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.

[0043] 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.

[0044] <Multiple Microphones 23, 24>

[0045] 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 .

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] <Controller 25>

[0052] 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.

[0053] 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.

[0054] 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.

[0055] <First Control Signal Generating Unit 31>

[0056] 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 .

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] <First Sound Field Learning Unit 32>

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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 .

[0071] 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.

[0072] 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.

[0073] <Second Control Signal Generating Unit 33>

[0074] 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.

[0075] 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.

[0076] <Second Sound Field Learning Unit 34>

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] <Sound Field Change Determination Unit 35>

[0082] 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.

[0083] <Convergence Determination Unit 36>

[0084] 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.

[0085] <Update Processing Unit 37>

[0086] 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.

[0087] <Sound Field Change Determination Processing>

[0088] 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.

[0089] 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 ).

[0090] 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.

[0091] 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).

[0092] 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).

[0093] 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).

[0094] like Figure 1 As 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.

[0095] 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.

[0096] 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.

[0097] <Convergence determination processing>

[0098] 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.

[0099] 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 T when the cancellation sound y1 is output from the first speaker 21) is calculated by multiplying the delayed sampling number Sd by the sampling time Ts. 0 The convergence determination unit 36 ​​uses the delayed sample number Sd corresponding to the delay time ΔT as the phase P of the sub-path filter C^1.

[0100] 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).

[0101] 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).

[0102] 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).

[0103] 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.

[0104] <Update Process>

[0105] 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.

[0106] 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).

[0107] 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.

[0108] 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 ).

[0109] 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).

[0110] 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).

[0111] 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).

[0112] 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).

[0113] 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).

[0114] 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).

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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).

[0119] <Effect>

[0120] The controller 25 can determine whether the transfer function C1 of the sub-path has changed based on external information (for example, information about the position of the first passenger seat 5). However, if this determination method is applied, it is impossible to determine whether the transfer function C1 of the sub-path has changed without receiving external information. If it is impossible to determine whether the transfer function C1 of the sub-path has changed, the controller 25 can always adaptively update the sub-path filter C^1 regardless of whether the transfer function C1 of the sub-path has changed. However, if this update method is applied, the computational load on the controller 25 becomes large. Therefore, the controller 25 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 noise reduction system 1.

[0121] In view of this problem, the controller 25 determines whether the transfer function C1 of the secondary path has changed based on the error signal e1 and the error signal e2. Therefore, even if external information cannot be received, it is possible to determine whether the transfer function C1 of the secondary path has changed. In addition, by determining whether the transfer function C1 of the secondary path has changed, the secondary path filter C^1 can be adaptively updated only when the transfer function C1 of the secondary path has changed. Therefore, compared with the case where the secondary path filter C^1 is always adaptively updated, the calculation load on the controller 25 can be reduced.

[0122] The controller 25 can determine whether the transfer function C1 of the secondary path has changed based only on the error signal e1 (e.g., based only on the magnitude of the error signal e1). However, if such a determination method is applied, there is a risk of erroneously determining that the transfer function C1 of the secondary path has changed even though the transfer function C2 of the secondary path has changed.

[0123] In view of such a problem, 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. Therefore, it can be accurately determined that not the transfer function C2 of the sub-path but the transfer function C1 of the sub-path has changed.

[0124] <Modification example>

[0125] In the above embodiment, the convergence determination unit 36 ​​performs the convergence determination process 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 the convergence determination process 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.

[0126] In the above embodiment, 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 (1). In the following formula (1), "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.

[0127]

[0128] By using the above formula (1), the amplitude A of the secondary path filter C^1 can be calculated more accurately.

[0129] In the above 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 continues 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.

[0130] In the above embodiment, 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).

[0131] See also Figure 7 In the above embodiment, when the fluctuation of the secondary path filter C^1 has not 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 8In 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.

[0132] In the above embodiment, 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.

[0133] In the above embodiment, 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.

[0134] <Modification example>

[0135] In the above embodiment, the noise reduction system 1 is applied to the cabin 4 of the vehicle 3. In another embodiment, the noise reduction system 1 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).

[0136] 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 filter configured to generate a control signal for controlling the canceling sound output device; and a secondary path filter, the secondary path filter representing an estimated value of a transfer function of a secondary path from the cancellation sound output device to the error microphone, The active noise reduction system further includes 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, and The controller is configured to determine whether the transfer function of the secondary path has changed based on the error signal and the determination signal.

2. The active noise reduction system according to claim 1, further comprising a second cancelling sound output device provided separately from the cancelling sound output device, in, The controller includes a second control filter configured to generate a second control signal for controlling the second canceling sound output device, and The second control filter is configured to be adaptively updated based on the determination signal.

3. The active noise reduction system according to claim 2, wherein: The controller includes a second side path filter representing an estimate of a transfer function of a second side path from the second cancellation sound output device to the reference microphone, and The controller is configured to determine whether the transfer function of the second secondary path has changed based on the error signal and the determination signal.

4. The active noise reduction system according to claim 3, wherein: The controller is configured to: In the case where 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, it is determined that the transfer function of the sub-path has changed, and In the case where the difference between the error signal and the determination signal is equal to or greater than the first reference value and the time variation of the determination signal is equal to or greater than the second reference value, it is determined that the transfer function of the second sub path has changed.

5. The active noise reduction system according to claim 1, 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.

6. The active noise reduction system according to claim 5, 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.

7. The active noise reduction system according to claim 1, further comprising a second cancelling sound output device provided separately from the cancelling sound output device, in, The controller includes a second side path filter representing an estimated value of a transfer function of a second side path from the second canceling sound output device to the reference microphone.

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