Active noise reduction system

Abstract

The present invention relates to an active noise reduction system including a cancellation sound generator configured to generate a cancellation sound for cancelling noise, an error detector configured to detect an error between the noise and the cancellation sound and generate an error signal corresponding to the error, and a controller configured to control the cancellation sound generator based on the error signal, wherein the controller is configured to extract noise components at a plurality of frequencies based on the error signal, determine a control target frequency among the plurality of frequencies based on the noise components at the plurality of frequencies, select a value of a prescribed control parameter based on the control target frequency, and generate a control signal to control the cancellation sound generator based on the selected value of the control parameter.

Description

Technical Field

[0001] The present invention relates to an active noise reduction system that reduces noise by canceling out sound interference noise that is out of phase with the noise. Background Technology

[0002] In a typical vehicle, the wheels vibrate due to forces received from the road surface. When this vibration is transmitted to the vehicle body via the suspension, it generates road noise inside the passenger compartment. In particular, narrow-band road noise excited by the acoustic resonance characteristics of enclosed spaces such as the passenger compartment (more specifically, road noise with a peak around 40 to 50 Hz and maintaining a constant bandwidth) is called "drum noise." Drum noise reaches the occupants' ears as a bulging, muffled sound, and therefore easily makes them feel uncomfortable.

[0003] JP2007-25527A discloses an active noise cancellation system for reducing this drumming noise. This active noise cancellation system uses the noise signal detected by a microphone at a control point as a control input and generates a control signal by adjusting the amplitude and phase of the noise signal.

[0004] For more details, please refer to JP2007-25527A. Figure 1 The processing circuit 101 extracts the f0 component of the noise signal detected by the microphone. The f0 component is the component at the control target frequency f0 (in... Figure 1 In the above, ω0 = 2πf0). The adjustment circuit 108 generates a control signal by adjusting the amplitude and phase of the f0 component of the noise signal extracted by the processing circuit 101.

[0005] Processing circuit 101 includes a single-frequency adaptive notch filter (SAN filter) with coefficients A and B, and a generator for generating reference signals (sine and cosine waves). The frequency of the reference signal is set to a control target frequency f0. The coefficients A and B of the SAN filter are updated using an adaptive algorithm to minimize the error signal e1 (e1 = e + Vout1) generated by the noise signal e detected by the microphone and the output Vout1 of the SAN filter. Therefore, "Vout1 = -e" is satisfied. More specifically, the output Vout1 of the SAN filter is a narrowband signal centered at the control target frequency f0. Therefore, "Vout1 = -e" is satisfied at the control target frequency f0. That is, the f0 component of the noise signal is extracted. JP2007-25527A Figure 5 The characteristics of the processing circuit 101 are shown.

[0006] The adjustment circuit 108 corrects the acoustic characteristics C (including the characteristics of the interior space and electronic equipment) from the speaker to the microphone, thereby generating a control signal. Refer to JP2007-25527A. Figure 8The adjustment circuit 108 includes a SAN filter and a notch filter for noise extraction. The SAN filter has coefficients A and B. The notch filter has coefficients Sa and Sb and indicates the characteristics of the adjustment circuit 108. As an example setup, the acoustic characteristic C is pre-measured as C^, and the notch filter is set to control the reciprocal of C^ at the target frequency f0, 1 / C^. At the microphone position, the following equation (1) is satisfied. Incidentally, “e” in the following equation (1) represents the sound pressure level of the noise signal after control, and “d” in the following equation (1) represents the sound pressure level of the noise signal before control.

[0007]

[0008] Assuming C^=C, the sound pressure level of the noise signal after control is half of the sound pressure level d of the noise signal before control. Therefore, the noise can be reduced by approximately 6dB.

[0009] Incidentally, for the narrow-band road noise described above (hereinafter referred to as "noise"), the sound pressure level of the noise inside the vehicle cabin is determined by the product of the noise input conditions (wheel vibration caused by forces received from the road surface) and the noise transmission characteristics (vehicle characteristics, cabin acoustic characteristics, etc.). The resonant frequency of the noise transmission characteristics does not change according to the vehicle's driving conditions (road conditions, vehicle speed, etc.). On the other hand, the noise input conditions change according to the vehicle's driving conditions, and the peak frequency of the noise can also change accordingly by a few Hz. Traditional active noise cancellation systems only reduce noise whose peak frequency is a preset fixed frequency. Therefore, traditional active noise cancellation systems cannot follow changes in the peak frequency of the noise, and thus the noise can remain at the peak frequency. Summary of the Invention

[0010] In view of the above background, the object of the present invention is to provide an active noise reduction system that effectively reduces noise at the peak frequency by following the change in the peak frequency of noise caused by changes in input conditions.

[0011] To achieve this objective, one aspect of the present invention provides an active noise cancellation system 11, the active noise cancellation system comprising: a cancelling sound generator (speaker 13) configured to generate a cancelling sound for cancelling noise; an error detector (error microphone 14) configured to detect an error between the noise and the cancelling sound, and generate an error signal corresponding to the error; and a controller 15 configured to control the cancelling sound generator based on the error signal, wherein the controller is configured to: extract noise components at a plurality of frequencies based on the error signal; determine a control target frequency among the plurality of frequencies based on the noise components at the plurality of frequencies; select a value of a predetermined control parameter based on the control target frequency; and generate a control signal based on the selected value of the control parameter to control the cancelling sound generator.

[0012] Based on this, by determining the target control frequency among multiple frequencies, the target control frequency can be made to follow the change in the peak frequency of noise caused by changes in input conditions. Therefore, noise at the peak frequency can be effectively reduced.

[0013] In the foregoing, preferably, the controller is further configured to: calculate the absolute value of the noise component at the plurality of frequencies (step ST1); calculate the correction value of the noise component at the plurality of frequencies by correcting the absolute value of the noise component at the plurality of frequencies (steps ST2, ST3); identify the maximum value among the correction values ​​of the noise component at the plurality of frequencies by comparing the correction values ​​of the noise component at the plurality of frequencies (step ST4); and determine the corresponding frequency as the control target frequency, the corresponding frequency corresponding to the maximum value among the correction values ​​of the noise component (step ST5).

[0014] Based on this, the target control frequency can be appropriately determined by correcting and then comparing the absolute values ​​of the noise components at multiple frequencies. Therefore, the tracking accuracy of the target control frequency to changes in the peak frequency of the noise can be enhanced.

[0015] In the foregoing, preferably, the controller is further configured to correct the absolute values ​​of the noise components at the plurality of frequencies based on a correction table that defines a correction coefficient for each of the plurality of frequencies according to human hearing characteristics (step ST3).

[0016] Based on this, users of active noise cancellation systems (e.g., vehicle occupants) can easily achieve noise reduction effects.

[0017] In the foregoing, preferably, the controller is further configured to correct the absolute value of the noise components at the plurality of frequencies based on the target noise reduction at each of the plurality of frequencies (step ST2).

[0018] Based on this aspect, by targeting noise reduction at each of multiple frequencies, the absolute value of the noise component after noise reduction can be converted to the absolute value of the noise component before noise reduction. Therefore, the target control frequency can be determined more appropriately.

[0019] In the foregoing, preferably, the controller is further configured to: extract the noise components at the plurality of frequencies during a predetermined sampling period, and calculate the absolute value of the noise components at the plurality of frequencies; and calculate the current value of the absolute value of the noise components at each of the plurality of frequencies based on a previous value of the absolute value of the noise components at each of the plurality of frequencies and the current value of the noise components at each of the plurality of frequencies (step ST1).

[0020] Based on this, frequent switching of the control target frequency can be suppressed due to noise and other factors included in the noise components at each of the multiple frequencies.

[0021] In the foregoing, preferably, the controller is further configured to: store a control parameter table (T1, T3), the control parameter table defining the value of the control parameter at each of the plurality of frequencies; and select the value of the control parameter corresponding to the control target frequency by referring to the control parameter table based on the control target frequency.

[0022] Based on this, the control signal can be generated using the optimal value of the control parameters corresponding to the target control frequency.

[0023] Therefore, based on the above aspects, an active noise reduction system can be provided that effectively reduces noise at the peak frequency by following the change in the peak frequency of noise caused by changes in input conditions. Attached Figure Description

[0024] Figure 1 This is a schematic diagram showing a vehicle to which the active noise cancellation system according to the first embodiment is applied;

[0025] Figure 2 This is a functional block diagram showing the active noise cancellation system according to the first embodiment;

[0026] Figure 3 A control parameter table according to the first embodiment is shown;

[0027] Figure 4This is a functional block diagram showing the control signal output unit according to the first embodiment;

[0028] Figure 5 This is a flowchart illustrating the control target frequency determination process according to the first embodiment;

[0029] Figure 6 A calibration table according to the first embodiment is shown;

[0030] Figures 7A to 7C Each graph shows the effect of reducing drum noise.

[0031] Figure 8 A control parameter table according to the second embodiment is shown;

[0032] Figure 9 This is a functional block diagram showing the control signal output unit according to the second embodiment; and

[0033] Figure 10 This is a functional block diagram showing the control target signal generation unit according to the second embodiment. Detailed Implementation

[0034] In the following description, embodiments of the invention will be described with reference to the accompanying drawings. In this specification, the “^” (swirl) shown together with the symbols each represents an identified value or an estimated value. The “^” is shown above the symbols in the drawings and formulas, but after the symbols in the text of the specification.

[0035] (First Implementation)

[0036] First, refer to Figures 1 to 7C The first embodiment of the present invention is described.

[0037] <Active Noise Cancellation System 11>

[0038] Figure 1 This is a schematic diagram showing a vehicle 1 incorporating an active noise reduction system 11 (hereinafter referred to as "noise reduction system 11") according to the first embodiment. When the wheel 2 vibrates due to the force received from the road surface S and the vibration of the wheel 2 is transmitted to the vehicle body 4 via the suspension 3, a drum noise d (an example of noise) is generated in the passenger compartment 5. The drum noise d is a narrow-band road noise with a peak value of around 40-50 Hz.

[0039] The noise reduction system 11 is a feedback-controlled active noise control device (ANC device) for reducing this drum noise d. More specifically, the noise reduction system 11 reduces the drum noise d by generating a canceling sound y that is out of phase with the drum noise d and causing the generated canceling sound y to interfere with the drum noise d.

[0040] Reference Figure 1 and Figure 2 The noise reduction system 11 includes: a plurality of speakers 13 (example of a canceling sound generator) configured to generate a canceling sound y to cancel the drum noise d; a plurality of error microphones 14 (example of an error detector) configured to detect an error (synthetic sound) between the drum noise d and the canceling sound y and generate an error signal e corresponding to the detected error; and a controller 15 configured to control the plurality of speakers 13 based on the error signal e. Figure 2 The symbol C in the figure represents the transmission characteristics of the secondary path from each speaker 13 to the corresponding error microphone 14.

[0041] <Speaker 13>

[0042] See Figure 1 Each speaker 13 of the noise reduction system 11 forms part of, for example, the audio system of vehicle 1 and is installed in the door of vehicle 1. In another embodiment, the speaker 13 may be separately located from the audio system of vehicle 1, or may be installed in a location other than the door of vehicle 1 (e.g., the speaker 13 may be installed in the headrest 6a of the passenger seat 6 or on the floor below the passenger seat 6).

[0043] <Error Microphone 14>

[0044] Each error microphone 14 of the noise reduction system 11 is installed, for example, in the headrest 6a of the passenger seat 6. In another embodiment, the error microphone 14 may be installed in a location other than the passenger seat 6 of the vehicle 1 (e.g., the error microphone 14 may be installed on the ceiling above the passenger seat 6).

[0045] <Controller 15>

[0046] The controller 15 of the noise reduction system 11 is composed of an electronic control unit (ECU) that includes a processing unit (CPU, MPU, etc.) and a storage device (ROM, RAM, etc.). The controller 15 can be composed of a single piece of hardware or a unit composed of multiple pieces of hardware.

[0047] Reference Figure 2 The controller 15 includes an A / D conversion unit 21, a multiple noise component extraction unit 22, a control target frequency determination unit 23, a parameter selection unit 24, a control signal output unit 25, and a D / A conversion unit 26 as functional components. Figure 2 The symbol "ADA" in the text stands for "Adaptive".

[0048]

[0049] ​The A / D conversion unit 21 of the controller 15 converts the analog error signal e output from the error microphone 14 into a digital error signal e, and outputs the digital error signal e to the multiple noise component extraction unit 22. In the following text, "error signal e" unless otherwise specified refers to the error signal e that has passed through the A / D conversion unit 21.

[0050] <Noise Component Extraction Unit 22>

[0051] Each noise component extraction unit 22 of the controller 15 extracts noise components Ak0 and Ak1 at a specified extraction frequency fk (k = 1, 2, ...) based on the error signal e during a specified sampling period. More specifically, the noise component extraction unit 22 extracts the noise components Ak0 and Ak1 as complex signals with real and imaginary parts at the extraction frequency fk. The noise component extraction unit 22 outputs the extracted noise components Ak0 and Ak1 together with the extraction frequency fk to the control target frequency determination unit 23.

[0052] For each noise component extraction unit 22, the extraction frequency fk is set to a different value. The extraction frequency fk is set to a frequency that can be the peak frequency of the drum noise d (around 40-50Hz). The number k of extraction frequencies fk (i.e., the number of noise component extraction units 22) is set to any integer equal to or greater than 2.

[0053] Each noise component extraction unit 22 includes a cosine wave generation circuit 31, a sine wave generation circuit 32, an extracted signal generation unit 33, and an adder 34.

[0054] The cosine wave generation circuit 31 generates an extracted cosine wave signal xck based on the extraction frequency fk, and outputs the generated extracted cosine wave signal xck to the extraction signal generation unit 33. The sine wave generation circuit 32 generates an extracted sine wave signal xsk based on the extraction frequency fk, and outputs the generated extracted sine wave signal xsk to the extraction signal generation unit 33.

[0055] The extracted signal generation unit 33 includes an extraction filter Ak. A single-frequency adaptive notch filter (SAN filter) is used for the extraction filter Ak. The extracted signal generation unit 33 includes a first extraction filter unit 35, a second extraction filter unit 36, an adder 37, a first extraction update unit 38, and a second extraction update unit 39.

[0056] The first extraction filter unit 35 has extraction filter coefficients Ak0. The extraction filter coefficients Ak0 form the real part of the coefficients of the extraction filter Ak, and also form the real part of the noise component (complex signal) extracted by the noise component extraction unit 22. The first extraction filter unit 35 filters the extracted cosine wave signal xck output from the cosine wave generation circuit 31.

[0057] The second extraction filter unit 36 ​​has extraction filter coefficients Ak1. The extraction filter coefficients Ak1 form the imaginary part of the coefficients of the extraction filter Ak, and also form the imaginary part of the noise component (complex signal) extracted by the noise component extraction unit 22. The second extraction filter unit 36 ​​filters the extracted sine wave signal xsk output from the sine wave generation circuit 32.

[0058] Adder 37 generates the extracted signal ak by adding the extracted cosine signal xck, which has passed through the first extraction filter unit 35, and the extracted sine signal xsk, which has passed through the second extraction filter unit 36. Adder 37 outputs the generated extracted signal ak to adder 34.

[0059] The first extraction update unit 38 updates the extraction filter coefficients Ak0 during the sampling period using an adaptive algorithm such as the Least Mean Square (LMS) algorithm. More specifically, the first extraction update unit 38 updates the extraction filter coefficients Ak0 such that the virtual error signal ek (described later) output from the adder 34 is minimized.

[0060] The second extraction update unit 39 updates the extraction filter coefficients Ak1 during the sampling period using an adaptive algorithm such as the LMS algorithm. More specifically, the second extraction update unit 39 updates the extraction filter coefficients Ak1 to minimize the virtual error signal ek output from the adder 34.

[0061] Adder 34 generates a virtual error signal ek by adding the extracted signal ak and the error signal e output from the extracted signal generation unit 33. Adder 34 outputs the generated virtual error signal ek back to the extracted signal generation unit 33.

[0062] <Target Frequency Determination Unit 23>

[0063] The control target frequency determination unit 23 of controller 15 determines the control target frequency fc among multiple extraction frequencies fk based on the extraction frequency fk and the noise components Ak0 and Ak1 (extraction filter coefficients) output from each noise component extraction unit 22. The control target frequency determination unit 23 outputs the determined control target frequency fc to the parameter selection unit 24, and also outputs the determined control target frequency fc and the corresponding noise components Ac0 and Ac1 to the control signal output unit 25. The method by which the control target frequency determination unit 23 determines the control target frequency fc will be described later.

[0064] <Parameter Selection Unit 24>

[0065] Reference Figure 3The parameter selection unit 24 of the controller 15 stores the control parameter table T1. The control parameter table T1 defines the values ​​of the control parameters at each frequency. In this embodiment, the control parameters include feedback gain (FB gain), feedback phase (FB phase), target noise reduction, etc.

[0066] The parameter selection unit 24 selects the value of each control parameter corresponding to the control target frequency fc based on the control target frequency fc output from the control target frequency determination unit 23 and by referring to the control parameter table T1. The parameter selection unit 24 outputs the selected value of each control parameter to the control signal output unit 25.

[0067] <Control signal output unit 25>

[0068] Reference Figure 4 The control signal output unit 25 of the controller 15 generates a control signal u for controlling the speaker 13 based on the control target frequency fc and noise components Ac0 and Ac1 output from the control target frequency determination unit 23 and the value of each control parameter output from the parameter selection unit 24. The control signal output unit 25 outputs the generated control signal u to the D / A conversion unit 26.

[0069] The control signal output unit 25 includes a SAN filter. The control signal output unit 25 includes a cosine wave generation unit 41, a sine wave generation unit 42, a first control filter unit 43, a second control filter unit 44, an adder 45, and a gain adjustment unit 46.

[0070] The cosine wave generation unit 41 generates a control cosine wave signal uc = cos(ωt + φd) based on the control target frequency fc output from the control target frequency determination unit 23 and the value of the FB phase (one of the control parameters) output from the parameter selection unit 24. More specifically, the cosine wave generation unit 41 generates the control cosine wave signal uc by shifting the phase of the reference cosine wave cos(ωt) corresponding to the control target frequency fc by an angle φd corresponding to the FB phase. The cosine wave generation unit 41 outputs the generated control cosine wave signal uc to the first control filter unit 43.

[0071] The sine wave generation unit 42 generates a control sine wave signal us = sin(ωt + φd) based on the control target frequency fc output from the control target frequency determination unit 23 and the value of the FB phase (one of the control parameters) output from the parameter selection unit 24. More specifically, the sine wave generation unit 42 generates the control sine wave signal us by shifting the phase of the reference sine wave sin(ωt) corresponding to the control target frequency fc by an angle φd corresponding to the FB phase. The sine wave generation unit 42 outputs the generated control sine wave signal us to the second control filter unit 44.

[0072] The first control filter unit 43 has a control filter coefficient A. The first control filter unit 43 filters the control cosine wave signal uc output from the cosine wave generation unit 41. The control filter coefficient A is continuously updated using the noise component Ac0 output from the control target frequency determination unit 23.

[0073] The second control filter unit 44 has a control filter coefficient B. The second control filter unit 44 filters the control sine wave signal us output from the sine wave generation unit 42. The control filter coefficient B is continuously updated using the noise component Ac1 output from the control target frequency determination unit 23.

[0074] The adder 45 generates a control signal u by adding the control cosine wave signal uc that has passed through the first control filter unit 43 and the control sine wave signal us that has passed through the second control filter unit 44. The adder 45 outputs the generated control signal u to the gain adjustment unit 46.

[0075] The gain adjustment unit 46 adjusts the gain of the control signal u output from the adder 45 based on the FB gain (one of the control parameters) output from the parameter selection unit 24. The gain adjustment unit 46 outputs the control signal u with the adjusted gain to the D / A conversion unit 26.

[0076] <D / A conversion unit 26>

[0077] Refer to Figure 2 , the D / A conversion unit 26 of the controller 15 converts the digital control signal u output from the control signal output unit 25 into an analog control signal u. The D / A conversion unit 26 outputs the analog control signal u to the speaker 13. Therefore, the speaker 13 generates a cancellation sound y corresponding to the control signal u.

[0078] <Control target frequency determination process>

[0079] The control target frequency determination unit 23 of the controller 15 performs the control target frequency determination process in the sampling period. In the control target frequency determination process, the control target frequency determination unit 23 determines the control target frequency fc based on the extraction frequency fk and the noise components Ak0 and Ak1 output from each noise component extraction unit 22. Hereinafter, in the description of the control target frequency determination process, the “(n)” shown together with each symbol represents the current value extracted or calculated at the current sampling time. On the other hand, the “(n−1)” shown together with each symbol represents the previous value extracted or calculated at the previous sampling time.

[0080] Refer to Figure 5When the control target frequency determination process starts, the control target frequency determination unit 23 uses the following formula (2) to calculate the current value of the absolute value of the noise components Ak0 and Ak1 at each extraction frequency fk, |Ak(n)| (step ST1).

[0081]

[0082] Incidentally, in equation (2) above, the current value of the absolute value of the noise components Ak0 and Ak1, |Ak(n)|, is calculated only based on the current values ​​of the noise components Ak0 and Ak1. If this calculation method is used, the target frequency fc may switch frequently due to noise contained in the noise components Ak0 and Ak1.

[0083] In this way, the target frequency determination unit 23 can calculate the current value |Ak(n)| of the absolute values ​​of noise components Ak0 and Ak1, and the previous value |Ak(n-1)| of the absolute values ​​of noise components Ak0 and Ak1, based on the current values ​​of the noise components Ak0 and Ak1. That is, the target frequency determination unit 23 can use the time average of the absolute values ​​of noise components Ak0 and Ak1 within a specified period as the current value |Ak(n)| of the absolute values ​​of noise components Ak0 and Ak1. For example, the target frequency determination unit 23 can use the following equation (3) instead of the above equation (2) to calculate the current value |Ak(n)|.

[0084]

[0085] In the following text, the current value |Ak(n)| of the absolute values ​​of the noise components Ak0 and Ak1 calculated in step ST1 will be referred to as the absolute value |Ak(n)| of the noise components Ak0 and Ak1.

[0086] Subsequently, the target frequency determination unit 23 corrects the absolute values ​​|Ak(n)| of the noise components Ak0 and Ak1 based on the control effect (the effect of reducing the drum noise d). More specifically, the target frequency determination unit 23 calculates the first correction value |Axk(n)| of the absolute values ​​|Ak(n)| of the noise components Ak0 and Ak1 by correcting the absolute values ​​|Ak(n)| of the noise components Ak0 and Ak1 based on the target noise reduction (one of the control parameters) at each frequency (step ST2). For example, the target frequency determination unit 23 uses the following equation (4) to calculate the first correction value |Axk(n)|. Incidentally, “TR” in the following equation (4) represents the target noise reduction at each frequency.

[0087] |Axk(n)|=β*|Ak(n)|, β=10 TR / 20 (4)

[0088] The absolute values ​​|Ak(n)| of the noise components Ak0 and Ak1 calculated in step ST1 are the values ​​corresponding to the noise components Ak0 and Ak1 after noise reduction (after control). Conversely, the noise d to be canceled by the canceling sound y from the speaker 13 is not the drum noise d after noise reduction (after control), but the drum noise d before noise reduction (before control). In this way, the target frequency determination unit 23 corrects the absolute values ​​|Ak(n)| of the noise components Ak0 and Ak1 by using the above equation (4), converting the values ​​corresponding to the noise components Ak0 and Ak1 after noise reduction (after control) into the values ​​corresponding to the noise components Ak0 and Ak1 before noise reduction (before control). For example, when the target noise reduction is 6dB, the coefficient β in the above equation (4) is approximately "2".

[0089] Subsequently, the target frequency determination unit 23 corrects the first correction value |Axk(n)| based on the evaluation criteria (human auditory characteristics). More specifically, the target frequency determination unit 23 calculates the second correction value |Ayk(n)| of the absolute values ​​|Ak(n)| of the noise components Ak0 and Ak1 by correcting the first correction value |Axk(n)| based on the correction table T2 (step ST3).

[0090] refer to Figure 6 The calibration table T2 is a table that defines the calibration coefficients for each frequency based on human auditory characteristics. In this embodiment, calibration table T2 defines the calibration coefficients for each frequency based on the so-called "A characteristic". In another embodiment, calibration table T2 may define the calibration coefficients for each frequency based on evaluation criteria other than human auditory characteristics.

[0091] For example, the target frequency determination unit 23 calculates the second correction value |Ayk(n)| by correcting the first correction value |Axk(n)| using the following equation (5). Incidentally, “α” in the following equation (5) represents the correction coefficient set based on the correction table T2.

[0092] |Ayk(n)|=α*|Axk(n)| (5)

[0093] Next, see Figure 5 The target frequency determination unit 23 identifies the maximum value of the second correction value |Ayk(n)| by comparing the second correction values ​​(|Ay1(n)|, ..., |Ayk(n)|) at all extracted frequencies fk (step ST4).

[0094] Subsequently, the target frequency determination unit 23 determines the corresponding extraction frequency fk (the extraction frequency fk corresponding to the maximum value of the second correction value |Ayk(n)|) as the target frequency fc (step ST5). Therefore, the target frequency determination process ends.

[0095] <Effect>

[0096] In the first embodiment, the controller 15 extracts noise components Ak0 and Ak1 at multiple extraction frequencies fk based on the error signal e, determines a control target frequency fc among the multiple extraction frequencies fk based on the noise components Ak0 and Ak1 at the multiple extraction frequencies fk, selects a value for a predetermined control parameter based on the control target frequency fc, and generates a control signal u for controlling the speaker 13 based on the selected value of the control parameter. By determining the control target frequency fc among the multiple extraction frequencies fk in this way, the control target frequency fc can follow the change in the peak frequency of the drum noise caused by the change in the driving conditions of the vehicle 1. Therefore, the drum noise d at the peak frequency can be effectively reduced.

[0097] Next, we will refer to Figures 7A to 7C The effect of reducing drum noise d as described above is further described. Figure 7A The diagram shows the state before the peak frequency of the drum noise d changes in both a conventional noise reduction system and the noise reduction system 11 according to this embodiment (e.g., the peak frequency of the drum noise d exists at around 46 Hz). Figure 7B and Figure 7C The states after the peak frequency of the drum noise d has changed are shown in both the conventional noise reduction system and the noise reduction system 11 according to this embodiment (for example, the peak frequency of the drum noise d is around 53 Hz).

[0098] Reference Figure 7A and Figure 7B In traditional noise reduction systems, the target control frequency fc is constant. Therefore, even if the peak frequency of the drum noise d changes, the target control frequency fc remains unchanged. Consequently, the target control frequency fc cannot follow changes in the peak frequency of the drum noise d, and the drum noise d remains at its peak frequency (see...). Figure 7B (Ellipse Z1 in the middle).

[0099] On the other hand, see Figure 7A and Figure 7C In the noise reduction system 11 according to this embodiment, the target frequency fc is variable, and therefore automatically changes according to the change in the peak frequency of the drum noise d. Thus, the target frequency fc can follow the change in the peak frequency of the drum noise d, thereby effectively reducing the drum noise d at the peak frequency (see [link to documentation]). Figure 7C (Ellipse Z2 in the middle).

[0100] (Second Implementation)

[0101] Next, we will refer to Figures 8 to 10 The second embodiment of the present invention is described below. Descriptions identical to those of the first embodiment of the present invention are omitted as appropriate. Figure 9 The symbol "ADA" in the text stands for "Adaptive".

[0102] <Parameter Selection Unit 24>

[0103] Reference Figure 8 The parameter selection unit 24 of the controller 15 stores the control parameter table T3. The control parameter table T3 is a table that defines the values ​​of the control parameters for each frequency. In this embodiment, the control parameters include step size parameters μ1, μ2, target noise reduction, feedback gain upper limit (FB gain upper limit), etc.

[0104] The step size parameters μ1 and μ2 are used to adjust the update amount of the SAN filter used in the control signal output unit 25 (described later). When the step size parameters μ1 and μ2 increase, the update amount of the SAN filter also increases. When the values ​​of the step size parameters μ1 and μ2 decrease, the update amount of the SAN filter also decreases.

[0105] The parameter selection unit 24 selects the value of the control parameter corresponding to the control target frequency fc based on the control target frequency fc output from the control target frequency determination unit 23 by referring to the control parameter table T3. The parameter selection unit 24 outputs the selected value of the control parameter to the control signal output unit 25.

[0106] <Control Target Signal Generation Unit 27>

[0107] Reference Figure 9 and Figure 10 In addition to the components of the controller 15 according to the first embodiment, the controller 15 according to the second embodiment also includes a control target signal generation unit 27. The control target signal generation unit 27 includes a cosine wave generation circuit 27A, a sine wave generation circuit 27B, a first filter unit 27C, a second filter unit 27D, a first adder 27E, a third filter unit 27F, a fourth filter unit 27G, and a second adder 27H.

[0108] Cosine wave generation circuit 27A generates a cosine wave signal rc based on a reference frequency f0 corresponding to the control target frequency fc. Sine wave generation circuit 27B generates a sine wave signal rs based on the reference frequency f0.

[0109] The first filter unit 27C has a first filter coefficient A0 corresponding to the noise component Ac0. The first filter unit 27C filters the cosine wave signal rc. The second filter unit 27D has a second filter coefficient A1 corresponding to the noise component Ac1. The second filter unit 27D filters the sine wave signal rs. The first adder 27E generates the control target signal efr by adding the cosine wave signal rc that has passed through the first filter unit 27C and the sine wave signal rs that has passed through the second filter unit 27D.

[0110] The third filter unit 27F has a first filter coefficient A0. The third filter unit 27F filters the sine wave signal rs. The fourth filter unit 27G has coefficients obtained by reversing the polarity of the second filter coefficient A1. The fourth filter unit 27G filters the cosine wave signal rc. The second adder 27H generates the control target signal efi by adding the sine wave signal rs that has passed through the third filter unit 27F and the cosine wave signal rc that has passed through the fourth filter unit 27G.

[0111] <Control signal output unit 25>

[0112] Reference Figure 9 The control signal output unit 25 of the controller 15 includes a control signal generation unit 51, a cancellation estimation signal generation unit 52, a noise estimation signal generation unit 53, a reference signal generation unit 54, a control filter update unit 55, and a virtual error signal generation unit 56.

[0113] The control signal generation unit 51 includes a control filter W. A SAN filter is used to control the filter W. The control signal generation unit 51 includes a first control filter unit 61, a second control filter unit 62, a first adder 63, a third control filter unit 64, a fourth control filter unit 65, and a second adder 66.

[0114] The first control filter unit 61 has control filter coefficients W0. The control filter coefficients W0 form the real parts of the coefficients of the control filter W. The first control filter unit 61 filters the control target signal efr output from the control target signal generation unit 27.

[0115] The second control filter unit 62 has control filter coefficients W1. The control filter coefficients W1 form the imaginary part of the coefficients of the control filter W. The second control filter unit 62 filters the control target signal efi output from the control target signal generation unit 27.

[0116] The first adder 63 generates a control signal u0 by adding the control target signal efr, which has passed through the first control filter unit 61, and the control target signal efi, which has passed through the second control filter unit 62. The first adder 63 outputs the generated control signal u0 to the D / A conversion unit 26 and the cancellation estimation signal generation unit 52.

[0117] The third control filter unit 64 has coefficients obtained by reversing the polarity of the control filter coefficients W0. The third control filter unit 64 filters the control target signal efi output from the control target signal generation unit 27.

[0118] The fourth control filter unit 65 has control filter coefficients W1. The fourth control filter unit 65 filters the control target signal efr output from the control target signal generation unit 27.

[0119] The second adder 66 generates a control signal u1 by adding the control target signal efi, which has already passed through the third control filter unit 64, and the control target signal efr, which has already passed through the fourth control filter unit 65. The second adder 66 outputs the generated control signal u1 to the cancellation estimation signal generation unit 52.

[0120] The cancellation estimation signal generation unit 52 includes a secondary path filter C^. The secondary path filter C^ is a filter corresponding to the estimated value of the transfer characteristic C of the secondary path from the speaker 13 to the error microphone 14. A SAN filter is used for the secondary path filter C^. The cancellation estimation signal generation unit 52 includes a first-stage path filter unit 71, a second-stage path filter unit 72, an adder 73, a first-stage path update unit 74, and a second-stage path update unit 75.

[0121] The first-stage path filter unit 71 has secondary path filter coefficients C^0. The secondary path filter coefficients C^0 form the real part of the coefficients of the secondary path filter C^. The first-stage path filter unit 71 filters the control signal u0 output from the control signal generation unit 51.

[0122] The second-stage path filter unit 72 has secondary path filter coefficients C^1. The secondary path filter coefficients C^1 form the imaginary part of the coefficients of the secondary path filter C^. The second-stage path filter unit 72 filters the control signal u1 output from the control signal generation unit 51.

[0123] Adder 73 generates a first cancellation estimate signal y^1 by adding the control signal u0 that has passed through the first-stage path filter unit 71 and the control signal u1 that has passed through the second-stage path filter unit 72. The first cancellation estimate signal y^1 is a signal corresponding to the estimated value of the cancellation sound y. Adder 73 outputs the generated first cancellation estimate signal y^1 to virtual error signal generation unit 56.

[0124] The first-stage path update unit 74 updates the coefficients C^0 of the secondary path filter at a specified sampling period using an adaptive algorithm such as the LMS algorithm. More specifically, the first-stage path update unit 74 updates the coefficients C^0 of the secondary path filter to minimize the virtual error signal ex (described later) output from the virtual error signal generation unit 56.

[0125] The second-stage path update unit 75 updates the secondary path filter coefficients C^1 during the sampling period using an adaptive algorithm such as the LMS algorithm. More specifically, the second-stage path update unit 75 updates the secondary path filter coefficients C^1 to minimize the virtual error signal ex output from the virtual error signal generation unit 56.

[0126] The noise estimation signal generation unit 53 includes a primary path filter H^. The primary path filter H^ is a filter corresponding to the estimated value of the transfer characteristic H of the path (primary path) from the noise source (road surface S in this embodiment) to the error microphone 14. A SAN filter is used for the primary path filter H^. The noise estimation signal generation unit 53 includes a first primary path filter unit 81, a second primary path filter unit 82, an adder 83, a first primary path update unit 84, and a second primary path update unit 85.

[0127] The first primary path filter unit 81 has primary path filter coefficients H^0. The primary path filter coefficients H^0 form the real part of the coefficients of the primary path filter H^. The first primary path filter unit 81 filters the control target signal efr output from the control target signal generation unit 27.

[0128] The second primary path filter unit 82 has coefficients obtained by reversing the polarity of the primary path filter coefficients H^1. The primary path filter coefficients H^1 form the imaginary part of the coefficients of the primary path filter H^. The second primary path filter unit 82 filters the control target signal efi output from the control target signal generation unit 27.

[0129] Adder 83 generates a noise estimation signal d^ by adding the control target signal efr that has passed through the first primary path filter unit 81 and the control target signal efi that has passed through the second primary path filter unit 82. The noise estimation signal d^ is a signal corresponding to the estimated value of the drum noise d. Adder 83 outputs the generated noise estimation signal d^ to virtual error signal generation unit 56.

[0130] The first primary path update unit 84 updates the primary path filter coefficients H^0 during the sampling period using an adaptive algorithm such as the LMS algorithm. More specifically, the first primary path update unit 84 updates the primary path filter coefficients H^0 to minimize the virtual error signal ex output from the virtual error signal generation unit 56.

[0131] The second primary path update unit 85 updates the primary path filter coefficients H^1 during the sampling period using an adaptive algorithm such as the LMS algorithm. More specifically, the second primary path update unit 85 updates the primary path filter coefficients H^1 to minimize the virtual error signal ex output from the virtual error signal generation unit 56.

[0132] Similar to the cancellation estimation signal generation unit 52, the reference signal generation unit 54 includes a secondary path filter C^. When the coefficients (C^0, C^1) of the secondary path filter C^ are updated in the cancellation estimation signal generation unit 52, the updated coefficients of the secondary path filter C^ are output to the reference signal generation unit 54, and the coefficients of the secondary path filter C^ are updated in the reference signal generation unit 54. That is, the coefficients of the secondary path filter C^ set in the reference signal generation unit 54 are not fixed values, but are continuously updated values ​​based on the signal from the cancellation estimation signal generation unit 52.

[0133] The reference signal generation unit 54 includes a third-stage path filter unit 91, a fourth-stage path filter unit 92, a first adder 93, a fifth-stage path filter unit 94, a sixth-stage path filter unit 95, and a second adder 96.

[0134] The third-stage path filter unit 91 has secondary path filter coefficients C^0. The third-stage path filter unit 91 filters the control target signal efr output from the control target signal generation unit 27.

[0135] The fourth secondary path filter unit 92 has coefficients obtained by inverting the polarity of the coefficients C^1 of the secondary path filter. The fourth secondary path filter unit 92 filters the control target signal efi output from the control target signal generation unit 27.

[0136] The first adder 93 generates a reference signal r0 by adding the control target signal efr, which has passed through the third-stage path filter unit 91, and the control target signal efi, which has passed through the fourth-stage path filter unit 92. The first adder 93 outputs the generated reference signal r0 to the control filter update unit 55.

[0137] The fifth secondary path filter unit 94 has secondary path filter coefficients C^0. The fifth secondary path filter unit 94 filters the control target signal efi output from the control target signal generation unit 27.

[0138] The sixth secondary path filter unit 95 has secondary path filter coefficients C^1. The sixth secondary path filter unit 95 filters the control target signal efr output from the control target signal generation unit 27.

[0139] The second adder 96 generates a reference signal r1 by adding the control target signal efi, which has passed through the fifth secondary path filter unit 94, and the control target signal efr, which has passed through the sixth secondary path filter unit 95. The second adder 96 outputs the generated reference signal r1 to the control filter update unit 55.

[0140] Similar to the control signal generation unit 51, the control filter update unit 55 includes a control filter W. The control filter update unit 55 includes a fifth control filter unit 101, a sixth control filter unit 102, an adder 103, a first control update unit 104, and a second control update unit 105.

[0141] The fifth control filter unit 101 has control filter coefficients W0. The fifth control filter unit 101 filters the reference signal r0 output from the reference signal generation unit 54.

[0142] The sixth control filter unit 102 has control filter coefficients W1. The sixth control filter unit 102 filters the reference signal r1 output from the reference signal generation unit 54.

[0143] Adder 103 generates a second cancellation estimate signal y^2 by adding the reference signal r0 that has passed through the fifth control filter unit 101 and the reference signal r1 that has passed through the sixth control filter unit 102. The second cancellation estimate signal y^2 is a signal corresponding to the estimated value of the cancellation sound y. Adder 103 outputs the generated second cancellation estimate signal y^2 to virtual error signal generation unit 56.

[0144] The first control update unit 104 updates the control filter coefficients W0 during the sampling period using an adaptive algorithm such as the LMS algorithm. More specifically, the first control update unit 104 updates the control filter coefficients W0 such that the virtual error signal ey (described later) output from the virtual error signal generation unit 56 is minimized.

[0145] The second control update unit 105 updates the control filter coefficients W1 during the sampling period using an adaptive algorithm such as the LMS algorithm. More specifically, the second control update unit 105 updates the control filter coefficients W1 to minimize the virtual error signal ey output from the virtual error signal generation unit 56.

[0146] When the coefficients W0 and W1 of the control filter W are updated in the control filter update unit 55, the updated coefficients of the control filter W are output to the control signal generation unit 51, and the coefficients of the control filter W are updated in the control signal generation unit 51. That is, the coefficients of the control filter W set in the control signal generation unit 51 are not fixed values, but are values ​​that are continuously updated based on the signals from the control filter update unit 55.

[0147] The virtual error signal generation unit 56 includes a first polarity inversion circuit 111, a second polarity inversion circuit 112, a first adder 113, and a second adder 114.

[0148] The first polarity inversion circuit 111 inverts the polarity of the first cancellation estimation signal y^1 output from the cancellation estimation signal generation unit 52. The second polarity inversion circuit 112 inverts the polarity of the noise estimation signal d^ output from the noise estimation signal generation unit 53.

[0149] The first adder 113 generates a virtual error signal ex by adding the error signal e, the first cancellation estimation signal y^1 that has passed through the first polarity inversion circuit 111, and the noise estimation signal d^ that has passed through the second polarity inversion circuit 112. The first adder 113 outputs the generated virtual error signal ex to the cancellation estimation signal generation unit 52 and the noise estimation signal generation unit 53.

[0150] The second adder 114 generates a virtual error signal ey by adding the noise estimation signal d^2 output from the noise estimation signal generation unit 53 and the second cancellation estimation signal y^2 output from the control filter update unit 55. The second adder 114 outputs the generated virtual error signal ey to the control filter update unit 55.

[0151] <Effect>

[0152] In the second embodiment, the controller 15 uses an adaptive algorithm to update the control filter W, the primary path filter H^, and the secondary path filter C^. Therefore, it is possible to learn the acoustic characteristics of the carriage 5 during feedback control and to enhance the reduction of drum noise d.

[0153] <Modified Implementation Method>

[0154] In the second embodiment described above, the controller 15 uses an adaptive algorithm to update all control filters W, primary path filters H^, and secondary path filters C^. On the other hand, in another embodiment, the controller 15 can use an adaptive algorithm to update only some of the control filters W, primary path filters H^, and secondary path filters C^. For example, the controller 15 can use an adaptive algorithm to update the primary path filters H^ and secondary path filters C^, and use the updated values ​​of the primary path filters H^ and secondary path filters C^ to calculate the control filter W using a formula.

[0155] In the first and second embodiments described above, the noise reduction system 11 is applied to the vehicle 1 to reduce the drum noise d. On the other hand, in another embodiment, the noise reduction system 11 can be applied to the vehicle 1 to reduce noise other than the drum noise d (e.g., noise from drive sources such as internal combustion engines and electric motors), or it can be applied to a moving body other than the vehicle 1 (e.g., an aircraft).

[0156] The specific embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments, and various modifications and changes can be made within the scope of the present invention.

Claims

1. An active noise cancellation system, the active noise cancellation system comprising: A cancellation sound generator, the cancellation sound generator being configured to generate cancellation sounds for canceling noise; An error detector is configured to detect the error between the noise and the canceled sound, and to generate an error signal corresponding to the error; as well as A controller configured to control the cancelling sound generator based on the error signal. The controller is configured as follows: Based on the error signal, noise components at multiple frequencies are extracted; The control target frequency is determined based on the noise components at the plurality of frequencies; The values ​​of the specified control parameters are selected based on the target control frequency; and A control signal is generated based on the selected value of the control parameter to control the canceling sound generator. The cancelling sound generator is configured to generate the cancelling sound corresponding to the control signal. The controller is further configured as follows: Calculate the absolute values ​​of the noise components at the plurality of frequencies; The correction value of the noise component at the plurality of frequencies is calculated by correcting the absolute value of the noise component at the plurality of frequencies; The maximum value among the correction values ​​of the noise components at the plurality of frequencies is identified by comparing the correction values ​​of the noise components at the plurality of frequencies; and The corresponding frequency is determined as the control target frequency, and the corresponding frequency corresponds to the maximum value among the correction values ​​of the noise component.

2. The active noise cancellation system according to claim 1, wherein, The controller is further configured to correct the absolute values ​​of the noise components at the plurality of frequencies based on a correction table that defines a correction coefficient for each of the plurality of frequencies according to human hearing characteristics.

3. The active noise cancellation system according to claim 1 or 2, wherein, The controller is further configured to correct the absolute value of the noise component at each of the plurality of frequencies based on the target noise reduction at each of the plurality of frequencies.

4. The active noise cancellation system according to claim 1 or 2, wherein, The controller is further configured to: During a specified sampling period, the noise components at the plurality of frequencies are extracted, and the absolute values ​​of the noise components at the plurality of frequencies are calculated. and The current value of the absolute value of the noise component at each of the plurality of frequencies is calculated based on the previous value of the absolute value of the noise component at each of the plurality of frequencies and the current value of the noise component at each of the plurality of frequencies.

5. The active noise cancellation system according to claim 1 or 2, wherein, The controller is further configured to: Store a control parameter table, the control parameter table defining the value of the control parameter at each of the plurality of frequencies; and The value of the control parameter corresponding to the control target frequency is selected by referring to the control parameter table based on the control target frequency.

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