Active noise reduction device, mobile device, and active noise reduction method

Abstract

The present invention provides an active noise reduction device that can suppress the perception of other noises being louder by reducing one type of noise. [Solution] The active noise reduction device 10 is a device that reduces noise at the location of an error signal source by outputting a secondary sound from a secondary sound source 52 in the space inside the vehicle 50, and comprises a filter coefficient update unit 13 that updates the coefficients of an adaptive filter used to generate a control signal for outputting a secondary sound, and a gain adjustment unit 15 that adjusts the gain multiplied by the control signal, and when the amplitude of the reference signal or the amplitude of the error signal is below a threshold, (a) the filter coefficient update unit 13 stops updating the coefficients of the adaptive filter, and (b) the gain adjustment unit 15 reduces the gain to a negative target value.

Description

Technical Field

[0001] The present disclosure relates to an active noise reduction device that actively reduces noise by interfering with secondary sound to the noise.

Background Art

[0002] Conventionally, an active noise reduction device that actively reduces noise has been known by outputting secondary sound for canceling noise from a secondary sound source using a reference signal correlated with the noise and an error signal based on residual sound obtained by interfering noise and secondary sound in a predetermined space (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present disclosure provides an active noise reduction device that can suppress a situation where other noise becomes significantly felt by reducing a certain noise.

Means for Solving the Problems

[0005] An active noise reduction device according to one aspect of the present disclosure is an active noise reduction device that reduces noise at the location of an error signal source by outputting a secondary sound from a secondary sound source in the space within a mobile device, and comprises: an adaptive filter unit that generates a control signal used for outputting the secondary sound by applying an adaptive filter to a reference signal correlated with the noise, which is output by a reference signal source attached to the mobile device; a simulated acoustic transmission characteristic filter unit that generates a filtered reference signal by correcting the reference signal with simulated acoustic transmission characteristics that simulate the acoustic transmission characteristics from the location of the secondary sound source to the location of the error signal source; a filter coefficient update unit that updates the coefficients of the adaptive filter using the generated filtered reference signal and an error signal output by the error signal source, which corresponds to residual sound due to interference between the secondary sound and the noise; and a gain adjustment unit that adjusts the gain multiplied by the control signal, wherein when the amplitude of the reference signal or the amplitude of the error signal is less than or equal to a threshold, (a) the filter coefficient update unit stops updating the coefficients of the adaptive filter, and (b) the gain adjustment unit reduces the gain to a negative target value. [Effects of the Invention]

[0006] An active noise reduction device according to one aspect of this disclosure can suppress the perception that other noises are louder by reducing one type of noise. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 shows an example configuration 1 of an active noise reduction device according to an embodiment. [Figure 2] Figure 2 is a flowchart showing the basic operation of the active noise reduction device according to the embodiment. [Figure 3] Figure 3 is a flowchart of Example 1 of the gain adjustment operation of the active noise reduction device according to the embodiment. [Figure 4] Figure 4 shows an example of gain adjustment 1. [Figure 5] Figure 5 shows example 2 of gain adjustment. [Figure 6]Figure 6 shows example 3 of gain adjustment. [Figure 7] Figure 7 shows an example of configuration 2 of an active noise reduction device according to an embodiment. [Figure 8] Figure 8 is a flowchart of Example 2 of the gain adjustment operation of the active noise reduction device according to the embodiment. [Figure 9] Figure 9 shows example 4 of gain adjustment. [Figure 10] Figure 10 shows example 5 of gain adjustment. [Modes for carrying out the invention]

[0008] The embodiments will be described in detail below with reference to the drawings. Note that the embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, arrangement and connection configurations of components, steps, and the order of steps shown in the following embodiments are examples only and are not intended to limit the disclosure. Furthermore, components in the following embodiments that are not described in an independent claim will be described as optional components.

[0009] Furthermore, each figure is a schematic diagram and not necessarily a strictly accurate representation. Note that in each figure, substantially identical components are denoted by the same reference numerals, and redundant explanations may be omitted or simplified.

[0010] (Embodiment) [Configuration Example 1] The following describes an example configuration of an active noise reduction device according to an embodiment. Figure 1 is a diagram showing an example configuration of an active noise reduction device according to an embodiment. As shown in Figure 1, the vehicle 50 is equipped with a reference signal source 51, a secondary sound source 52, an error signal source 53, and an active noise reduction device 10.

[0011] The reference signal source 51 is a transducer that outputs a reference signal correlated with the noise in the space inside the vehicle 50. The reference signal source 51 is, for example, an acceleration sensor and is located outside the space inside the vehicle 50. Specifically, the reference signal source 51 is mounted on a subframe or a wheel well, etc. The mounting position of the reference signal source 51 is not particularly limited. When the reference signal source 51 is an acceleration sensor, the active noise reduction device 10 can reduce the road noise component included in the noise in the space inside the vehicle 50. Since road noise has a complex propagation path, a configuration in which acceleration sensors are placed in multiple locations is useful. The reference signal source 51 may also be a microphone.

[0012] The secondary sound source 52 outputs secondary sound into the space inside the vehicle 50 using a control signal. In this embodiment, the secondary sound source 52 is a speaker, but secondary sound may also be output when a part of the vehicle 50 structure (for example, a sunroof) is vibrated by a drive mechanism such as an actuator. Furthermore, multiple secondary sound sources 52 may be installed inside the vehicle 50. The mounting position of the secondary sound source 52 is not particularly limited.

[0013] The error signal source 53 detects residual sound resulting from the interference of noise and secondary sound within the space inside the vehicle 50 and outputs an error signal based on the residual sound. The error signal source 53 is a transducer such as a microphone and is preferably installed in the space inside the vehicle 50, such as in the headliner.

[0014] The active noise reduction device 10 generates a control signal for outputting a secondary sound from the secondary sound source 52 by performing signal processing on a reference signal acquired from the reference signal source 51. The secondary sound is a sound that reduces noise in the space inside the vehicle 50.

[0015] The active noise reduction device 10 specifically includes an adaptive filter section 11, an acoustic transfer characteristic simulation filter section 12, a filter coefficient update section 13, a determination section 14, a gain adjustment section 15, and a storage section 16. The adaptive filter section 11, the acoustic transfer characteristic simulation filter section 12, the filter coefficient update section 13, the determination section 14, and the gain adjustment section 15 are realized, for example, by a processor such as a DSP (Digital Signal Processor) or a microcomputer (hardware) executing a computer program (software) stored in the storage section 16. The storage section 16 is realized by a semiconductor memory built in or externally attached to the DSP.

[0016] [Basic Operation] As described above, the active noise reduction device 10 performs a noise reduction operation. First, the basic operation of the active noise reduction device 10 will be described while referring to FIG. 2. FIG. 2 is a flowchart of the basic operation of the active noise reduction device 10. In the following description, mainly the case where there is one error signal source 53 will be described, and the case where there are multiple error signal sources 53 will be supplementarily described.

[0017] First, a reference signal having a correlation with noise is input from the reference signal source 51 to the active noise reduction device 10 (S11).

[0018] The reference signal input to the active noise reduction device 10 is output to the adaptive filter section 11 and the acoustic transfer characteristic simulation filter section 12.

[0019] The adaptive filter section 11 generates a control signal by applying (convolving) an adaptive filter to the reference signal (S12). The adaptive filter section 11 is realized by a so-called FIR (Finite Impulse Response) filter or an IIR (Infinite Impulse Response) filter. A gain adjustable by the gain adjustment section 15 is multiplied by the control signal generated by the adaptive filter section 11, and the control signal after gain multiplication is output to the secondary sound source 52 (S13). The secondary sound source 52 outputs a secondary sound based on the control signal.

[0020] The error signal source 53 detects residual noise resulting from the interference between the secondary sound output from the secondary sound source 52 and the noise, and outputs an error signal corresponding to the residual noise. In other words, the error signal is a signal indicating the noise state in the space inside the vehicle 50 when the secondary sound is being output. As a result, the error signal is input to the active noise reduction device 10 (S14). The error signal input to the active noise reduction device 10 is output to the filter coefficient update unit 13.

[0021] The simulated acoustic transmission characteristic filter unit 12 generates a filtered reference signal by correcting the reference signal with a simulated acoustic transmission characteristic that simulates the acoustic transmission characteristics from the position of the secondary sound source 52 to the position of the error signal source 53 (i.e., the acoustic transmission characteristics in space) (S15). The simulated acoustic transmission characteristic is measured in advance in the space inside the vehicle 50 and stored in the memory unit 16. Note that the simulated acoustic transmission characteristic may be determined by an algorithm that does not use predetermined values.

[0022] The filter coefficient update unit 13 sequentially updates the coefficients W of the adaptive filter using the error signal and the generated filtered reference signal (S16). Specifically, the filter coefficient update unit 13 uses the LMS (Least Mean Square) method to calculate the coefficients of the adaptive filter so that the sum of squares of the error signal is minimized, and outputs the calculated coefficients of the adaptive filter to the adaptive filter unit 11. If the error signal is represented as e and the filtered reference signal vector as R, the coefficients W of the adaptive filter are expressed by the following equation (Equation 1). Here, n is a natural number and represents the nth sample in the sampling period Ts. μ is a scalar quantity and is a step size parameter that determines the amount of update of the adaptive filter coefficients W per sample.

[0023]

number

[0024] As described above, the active noise reduction device 10 can generate a control signal by applying an adaptive filter, whose coefficients are updated based on an error signal, to a reference signal.

[0025] [Example of gain adjustment operation 1] According to the basic operation described above, noise caused by road noise inside the vehicle 50 is reduced. However, the noise heard inside the vehicle 50 includes not only noise caused by road noise, but also noise caused by wind noise and noise caused by engine noise, and there is a problem that if the noise caused by road noise is reduced too much, the other noises will become more noticeable.

[0026] Therefore, when the active noise reduction device 10 estimates that the road noise level is low, it stops its noise reduction function and outputs a secondary sound that slightly amplifies the noise caused by the road noise, thereby maintaining the noise caused by the road noise at a certain level without reducing it too much. As a result, other noises are masked by the noise caused by the road noise, and a certain level of noise is maintained inside the vehicle 50 (the fluctuation in noise level becomes small), thus making it more difficult for the user inside the vehicle 50 to perceive the noise.

[0027] The following describes Example 1 of the gain adjustment operation of such an active noise reduction device 10. Figure 3 is a flowchart of Example 1 of the gain adjustment operation.

[0028] While the above basic operation is being performed (S21), the determination unit 14 acquires a reference signal from the reference signal source 51 and determines whether the amplitude of the acquired reference signal is below a threshold. In other words, it determines whether the noise (e.g., road noise) is relatively small. This determination is performed continuously thereafter. The threshold is predetermined empirically or experimentally by the designer of the active noise reduction device 10.

[0029] If the determination unit 14 determines that the amplitude of the reference signal is below a threshold (S22), the filter coefficient update unit 13 stops updating the coefficients of the adaptive filter (S23), and the gain adjustment unit 15 adjusts the gain multiplied by the control signal (S24). The order of processing in steps S23 and S24 is not limited. For example, if the determination unit 14 determines that the amplitude of the reference signal is below a threshold, the gain adjustment unit 15 may adjust the gain multiplied by the control signal, and the filter coefficient update unit 13 may stop updating the coefficients of the adaptive filter if the gain of the gain adjustment unit is below a threshold.

[0030] Figure 4 shows an example of gain adjustment by the gain adjustment unit 15 (an example of gain change over time). As shown in Figure 4, during basic operation, the gain is 1 (or a value close to it). At timing t1, when it is determined that the amplitude of the reference signal is below a threshold, the gain adjustment unit 15 reduces the gain multiplied by the control signal from 1 to a target value over a predetermined period of time. The predetermined time is, for example, about 0.1 s, but is not particularly limited.

[0031] The target value is determined, for example, based on the amplitude of the reference signal at a certain point in time, with the smaller the amplitude of the reference signal, the smaller the value (i.e., the lower value on the vertical axis in Figure 4). The target value may also be predetermined empirically or experimentally by the designer of the active noise reduction device 10. When the gain reaches the target value, the gain adjustment unit 15 maintains the gain at a constant level at the target value.

[0032] Subsequently, when the determination unit 14 determines that the amplitude of the reference signal exceeds the threshold (S25), the basic operation is resumed (S21). Specifically, the gain adjustment unit 15 increases the gain from the target value to 1 over a predetermined period of time (see timing t2 onwards in Figure 4), and the filter coefficient update unit 13 resumes updating the coefficients of the adaptive filter. Note that when the basic operation is resumed, the gain may be increased immediately from the target value to 1.

[0033] The target value may be negative, and in example 1 of the gain adjustment operation, the gain multiplied by the control signal in step S24 is undershot to a negative value. When the gain becomes negative, the secondary sound source 52 outputs a secondary sound with the same phase as the road noise, and the noise caused by the road noise heard by the user inside the vehicle 50 is amplified. As a result, other noises are masked by the noise caused by the road noise, and a certain level of noise is maintained inside the vehicle 50 (the fluctuation in noise level becomes smaller), so the effect is obtained in which the user inside the vehicle 50 is less likely to perceive the noise. In addition, since the active noise reduction device 10 uses its own functions to mask other noises, it can suppress user discomfort compared to configurations that generate white noise and pink noise to mask other noises.

[0034] In step S24, the gain may be adjusted as shown in Figure 5 or Figure 6. Figure 5 shows an example 2 of gain adjustment by the gain adjustment unit 15.

[0035] In the gain adjustment example 2 shown in Figure 5, the gain adjustment unit 15 reduces the gain multiplied by the control signal from 1 to a negative target value over a predetermined period of time. The target value is determined, for example, based on the amplitude of the reference signal at a certain point in time, but may also be predetermined empirically or experimentally by the designer of the active noise reduction device 10.

[0036] The gain adjustment unit 15 maintains the gain at a constant negative value once the gain reaches the target value, and after a certain period T has elapsed, increases the gain from a negative value to 0 over a predetermined period of time. After that, the gain adjustment unit 15 maintains the gain at 0. The length of the certain period T is predetermined empirically or experimentally by the designer of the active noise reduction device 10 or the like. The length of T may be 0 or greater, or it may be 0.

[0037] This method of adjusting the gain ultimately results in a constant gain of 0, which suppresses any discomfort for the user and ultimately creates a relatively quiet environment.

[0038] Figure 6 shows an example of gain adjustment 3 by the gain adjustment unit 15. In gain adjustment example 3 shown in Figure 6, the target value is not fixed, but is changed (in other words, fluctuates) according to the amplitude of the reference signal at that time. That is, in gain adjustment example 3, the target value is updated (fluctuates) moment by moment, and the gain is adjusted toward the changed target value each time the target value is updated. For example, the target value is updated to a smaller value the smaller the amplitude of the reference signal. Gain adjustment example 3 shows an example where the target value is updated continuously, but multiple target values ​​corresponding to thresholds of multiple reference signals may be set. In this case, the target value is changed when the set threshold is exceeded, and the gain is adjusted toward the updated target value over a predetermined period of time. Hysteresis may be introduced in the determination of the threshold.

[0039] This method of adjusting the gain makes it possible to bring the level of noise caused by road noise closer to a constant level.

[0040] In the above example of gain adjustment operation 1, it was determined whether the amplitude of the reference signal was below a threshold, but it is also possible to determine whether the amplitude of the error signal is below a threshold. In the explanation of example of gain adjustment operation 1, the reference signal may be replaced with the error signal.

[0041] [Configuration Example 2] Next, a configuration example 2 of the active noise reduction device 10 will be described. Figure 7 is a diagram showing a configuration example 2 of the active noise reduction device 10. The vehicle 50 in configuration example 2 further has an operation reception unit 54.

[0042] The operation reception unit 54 receives operations from the user to switch the noise reduction function of the active noise reduction device 10 in Configuration Example 2 on and off (on operation and off operation). In other words, the enablement and disablement of the noise reduction function of the active noise reduction device 10 in Configuration Example 2 is switched by user operations on the operation reception unit 54. The operation reception unit 54 can be implemented, for example, by hardware buttons, but may also be implemented by a touch panel.

[0043] [Example of gain adjustment operation 2] In example 1 of the gain adjustment operation, the gain was adjusted based on the amplitude of the reference signal or the amplitude of the error signal. However, a similar gain adjustment may also be performed when the user switches the noise reduction function of the active noise reduction device 10 from enabled to disabled.

[0044] The following describes Example 2 of the gain adjustment operation performed by the active noise reduction device 10 in Configuration Example 2. Figure 8 is a flowchart of Example 2 of the gain adjustment operation.

[0045] While the above basic operations are being performed (S31), the user performs an off operation to switch the noise reduction function of the active noise reduction device 10 from enabled to disabled by using an operation reception unit 54 such as a push button or touch panel provided on the vehicle 50. When the determination unit 14 determines that the off operation has been received (S32), the filter coefficient update unit 13 stops updating the coefficients of the adaptive filter (S33), and the gain adjustment unit 15 adjusts the gain multiplied by the control signal (S34). The order of processing in steps S33 and S34 is not limited. For example, if the determination unit 14 determines that the amplitude of the reference signal is below a threshold, the gain adjustment unit 15 may adjust the gain multiplied by the control signal, and the filter coefficient update unit 13 may stop updating the coefficients of the adaptive filter if the gain of the gain adjustment unit is below a threshold.

[0046] Figure 9 shows an example of gain adjustment 4 by the gain adjustment unit 15. As shown in Figure 9, when it is determined that an off operation has been accepted at timing t3, the gain adjustment unit 15 reduces the gain multiplied by the control signal from 1 to a negative set value over a first time T1. The predetermined time is, for example, about 0.1s, but is not particularly limited. The negative set value is determined in advance empirically or experimentally by the designer of the active noise reduction device 10.

[0047] When the gain reaches a negative setpoint, the gain adjustment unit 15 maintains the gain at that negative setpoint, and after a certain period T has elapsed, it increases the gain from the negative setpoint to 0 over a second period T2. After that, the gain adjustment unit 15 maintains the gain at 0. The length of the certain period T is predetermined empirically or experimentally by the designer of the active noise reduction device 10. The length of T may be 0 or greater, or it may be 0.

[0048] Subsequently, the user performs an ON operation to the operation reception unit 54 to switch the noise reduction function of the active noise reduction device 10 from disabled to enabled. When the determination unit 14 determines that the ON operation has been received (S35), the basic operation is resumed (S31). Specifically, the gain adjustment unit 15 increases the gain from a negative set value to 1 over a predetermined period of time (see timing t4 onwards in Figure 9), and the filter coefficient update unit 13 resumes updating the coefficients of the adaptive filter. Note that when the basic operation is resumed, the gain may be increased immediately from a negative set value to 1.

[0049] In the example 2 of the gain adjustment operation described above, in step S34, the gain multiplied by the control signal is undershot to a negative value, and the first time T1 is set to be shorter than the second time T2. In other words, the length of the second time T2 is set to be longer than the first time T1.

[0050] As a result, immediately after the road noise is reduced by the basic operation, a control signal with inverted phase is output, increasing the noise caused by road noise. This makes it easier for the user to experience the noise reduction effect during the basic operation.

[0051] Furthermore, when the active noise reduction device 10 switches its noise reduction function from enabled to disabled as a fail-safe process (when the noise reduction function is disabled by the active noise reduction device 10 itself, not by the user), it is not necessary to output a control signal with inverted phase. In other words, it is not necessary to cause the gain to undershoot to a negative value. In this case, the gain adjustment unit 15 reduces the gain from 1 to 0 over a predetermined period of time, without making it a negative value. Figure 10 shows an example 5 of gain adjustment by the gain adjustment unit 15 in this manner.

[0052] As a result, the active noise reduction device 10 can suppress the unnecessary increase of noise caused by road noise.

[0053] [Differentiation] The above embodiment described an example in which noise caused by road noise is used to mask other noises. However, the active noise reduction device 10 is a device that reduces noise caused by engine noise, and may also mask other noises caused by engine noise. In other words, the active noise reduction device 10 is a device that reduces a first noise, and may also mask a second noise that has a different cause from the first noise. In this case, there is no particular limit to what kind of noise the first noise is.

[0054] Furthermore, in the above embodiment, the gain adjustment unit 15 changed the gain linearly when adjusting the gain, but it may also change the gain nonlinearly.

[0055] [Effects, etc.] The following describes examples of technologies that can be obtained from the disclosures in this specification, and explains the effects and other benefits that can be obtained from such technologies.

[0056] Technology 1 is an active noise reduction device 10 that reduces noise at the location of an error signal source 53 by outputting a secondary sound from a secondary sound source 52 in the space inside a vehicle 50. The active noise reduction device 10 comprises: an adaptive filter unit 11 that generates a control signal used for outputting a secondary sound by applying an adaptive filter to a reference signal correlated with noise, which is output by a reference signal source 51 attached to the vehicle 50; a simulated acoustic transmission characteristic filter unit 12 that generates a filtered reference signal by correcting the reference signal with simulated acoustic transmission characteristics that simulate the acoustic transmission characteristics from the location of the secondary sound source 52 to the location of the error signal source 53; a filter coefficient update unit 13 that updates the coefficients of the adaptive filter using the generated filtered reference signal and an error signal output by the error signal source 53 that corresponds to residual sound due to interference between the secondary sound and noise; and a gain adjustment unit 15 that adjusts the gain multiplied by the control signal. When the amplitude of the reference signal or the amplitude of the error signal is below a threshold, (a) the filter coefficient update unit 13 stops updating the coefficients of the adaptive filter, and (b) the gain adjustment unit 15 reduces the gain to a negative target value. Vehicle 50 is an example of a mobile device.

[0057] Such an active noise reduction device 10 can reduce the gain multiplied by the control signal to a negative target value, thereby preventing the noise heard inside the vehicle 50 from becoming too quiet and causing other noises with different origins to be perceived as louder.

[0058] Technology 2 is an active noise reduction device 10 of Technology 1, in which the gain adjustment unit 15 keeps the gain constant at a negative target value after the gain has reached a negative target value.

[0059] Such an active noise reduction device 10 can suppress sudden changes in noise levels by keeping the gain constant at a negative target value. In other words, the active noise reduction device 10 can prevent users from perceiving noise as louder due to sudden changes in noise levels.

[0060] Technology 3 is an active noise reduction device 10 of Technology 1, wherein the gain adjustment unit 15 updates the negative target value based on the amplitude of the reference signal or the amplitude of the error signal while the gain is being reduced to a negative target value.

[0061] Such an active noise reduction device 10 can bring the noise level closer to a constant level.

[0062] Technology 4 is an active noise reduction device 10 of Technology 1, wherein the gain adjustment unit 15 increases the gain to 0 after the gain reaches a negative target value.

[0063] Such an active noise reduction device 10 can ultimately achieve a relatively quiet environment by increasing the gain to 0.

[0064] Technology 5 is an active noise reduction device 10 according to any of Technologies 1 to 4, wherein when the user performs an operation to disable the function of the active noise reduction device 10, (a) the filter coefficient update unit 13 stops updating the coefficients of the adaptive filter, and (b) the gain adjustment unit 15 reduces the gain to a negative set value over a first time T1, and then increases the gain to 0 over a second time T2 which is longer than the first time T1.

[0065] Such an active noise reduction device 10 can help the user experience the noise reduction effect when the function of the active noise reduction device 10 is enabled.

[0066] Technology 6 is the active noise reduction device 10 of Technology 5, in which, when the function of the active noise reduction device 10 is disabled, (a) the filter coefficient update unit 13 stops updating the coefficients of the adaptive filter, and (b) the gain adjustment unit 15 reduces the gain to 0 and does not allow it to be a negative value.

[0067] Such an active noise reduction device 10 can suppress the unnecessary increase of noise.

[0068] Technology 7 is a vehicle 50 equipped with an active noise reduction device 10 according to any of Technologies 1 to 6, a reference signal source 51, a secondary sound source 52, and an error signal source 53. The vehicle 50 is an example of a mobile device.

[0069] In such a vehicle 50, by reducing the gain multiplied by the control signal to a negative target value, it is possible to prevent the noise heard inside the vehicle 50 from becoming too quiet, which would cause other noises with different origins to be perceived as louder.

[0070] Technology 8 is an active noise reduction method that reduces noise at the location of an error signal source 53 by outputting a secondary sound from a secondary sound source 52 in the space inside a vehicle 50, and includes the steps of: generating a control signal used for outputting a secondary sound by applying an adaptive filter to a reference signal correlated with noise, which is output by a reference signal source 51 attached to the vehicle 50; generating a filtered reference signal by correcting the reference signal with simulated acoustic transmission characteristics that simulate the acoustic transmission characteristics from the location of the secondary sound source 52 to the location of the error signal source 53; updating the coefficients of the adaptive filter using the generated filtered reference signal and an error signal output by the error signal source that corresponds to residual sound due to interference between the secondary sound and noise; and stopping the updating of the coefficients of the adaptive filter and reducing the gain to a negative target value when the amplitude of the reference signal or the amplitude of the error signal is below a threshold. The vehicle 50 is an example of a mobile device.

[0071] This active noise reduction method reduces the gain multiplied by the control signal to a negative target value, thereby preventing the noise heard inside the vehicle 50 from becoming too quiet and causing other noises with different origins to be perceived as louder.

[0072] (Other embodiments) Although embodiments have been described above, this disclosure is not limited to the embodiments described above.

[0073] For example, in the above embodiment, the active noise reduction device was a device that performed noise control based on the Filtered-X LMS algorithm, but it may also be implemented as a device that performs noise control using the SAN (Single-frequency Adaptive Notch filter) algorithm or the SAN Filtered-x LMS algorithm.

[0074] Furthermore, the active noise reduction device according to the above embodiment may be mounted on a mobile device other than a vehicle. The mobile device may be, for example, an aircraft or a ship. Moreover, this disclosure may be implemented as such a mobile device other than a vehicle.

[0075] Furthermore, the configuration of the active noise reduction device according to the above embodiment is merely an example. For instance, the active noise reduction device may include components such as a DA converter, a filter, a power amplifier, or an AD converter.

[0076] Furthermore, the processing performed by the active noise reduction device according to the above embodiment is merely an example. For instance, some of the digital signal processing described in the above embodiment may be implemented by analog signal processing.

[0077] Furthermore, for example, in the above embodiment, a process executed by a specific processing unit may be executed by another processing unit. Also, the order of multiple processes may be changed, or multiple processes may be executed in parallel.

[0078] Furthermore, in the above embodiment, each component may be realized by executing a software program suitable for each component. Each component may also be realized by a program execution unit such as a CPU or processor reading and executing a software program recorded on a recording medium such as a hard disk or semiconductor memory.

[0079] Furthermore, in the above embodiment, each component may be implemented by hardware. For example, each component may be a circuit (or integrated circuit). These circuits may form a single circuit as a whole, or they may be separate circuits. Also, each of these circuits may be a general-purpose circuit or a dedicated circuit.

[0080] Furthermore, each component may be a circuit (or integrated circuit). These circuits may form a single circuit as a whole, or they may be separate circuits. Also, each of these circuits may be a general-purpose circuit or a dedicated circuit.

[0081] Furthermore, the general or specific embodiments of this disclosure may be implemented in systems, apparatus, methods, integrated circuits, computer programs, or non-temporary recording media such as computer-readable CD-ROMs. They may also be implemented in any combination of systems, apparatus, methods, integrated circuits, computer programs, and computer-readable non-temporary recording media.

[0082] For example, this disclosure may be implemented as an active noise reduction method executed by an active noise reduction device (such as a computer DSP), or as a program to cause the active noise reduction device to execute the above active noise reduction method. Furthermore, this disclosure may be implemented as an application program installed on a user interface device. Furthermore, this disclosure may be implemented as a computer-readable, non-temporary recording medium on which these programs are recorded.

[0083] Furthermore, this disclosure also includes forms obtained by applying various modifications to each embodiment that a person skilled in the art could conceive, or forms realized by arbitrarily combining the components and functions of each embodiment without departing from the spirit of this disclosure. [Industrial applicability]

[0084] The active noise reduction device of this disclosure is useful, for example, as a device that can reduce noise inside a vehicle cabin. [Explanation of Symbols]

[0085] 10 Active noise reduction device 11 Adaptive filter section 12 Simulated Acoustic Transmission Characteristics Filter Section 13. Filter coefficient update section 14 Judgment section 15 Gain adjustment section 16 Memory section 50 Vehicles (Mobile Devices) 51 Reference signal source 52 Secondary sound sources 53 Error signal source 54 Operation reception unit

Claims

1. An active noise reduction device that reduces noise at the location of an error signal source by outputting a secondary sound from a secondary sound source within the space of a mobile device, An adaptive filter unit generates a control signal used for outputting the secondary sound by applying an adaptive filter to a reference signal correlated with the noise, which is output by a reference signal source attached to the mobile device, A simulated acoustic transmission characteristic filter unit generates a filtered reference signal by correcting the reference signal with a simulated acoustic transmission characteristic that simulates the acoustic transmission characteristics from the position of the secondary sound source to the position of the error signal source, A filter coefficient update unit updates the coefficients of the adaptive filter using the generated filtered reference signal and the error signal output by the error signal source, which corresponds to the residual sound resulting from the interference between the secondary sound and the noise. The system includes a gain adjustment unit that adjusts the gain multiplied by the control signal, If the amplitude of the reference signal or the amplitude of the error signal is below a threshold, (a) the filter coefficient update unit stops updating the coefficients of the adaptive filter, and (b) the gain adjustment unit reduces the gain to a negative target value. Active noise reduction device.

2. The gain adjustment unit maintains the gain at the negative target value after the gain has reached the negative target value. The active noise reduction device according to claim 1.

3. The gain adjustment unit updates the negative target value based on the amplitude of the reference signal or the amplitude of the error signal while it is reducing the gain to the negative target value. The active noise reduction device according to claim 1.

4. The gain adjustment unit increases the gain to 0 after the gain has reached the negative target value. The active noise reduction device according to claim 1.

5. If the user disables the function of the active noise reduction device, (a) the filter coefficient update unit stops updating the coefficients of the adaptive filter, and (b) the gain adjustment unit reduces the gain to a negative set value over a first period of time, and then increases the gain to 0 over a second period of time longer than the first period. The active noise reduction device according to claim 1.

6. When the function of the active noise reduction device is disabled from enabled to disabled, (a) the filter coefficient update unit stops updating the coefficient of the adaptive filter, and (b) the gain adjustment unit reduces the gain to 0 and does not allow it to become a negative value. The active noise reduction device according to claim 5.

7. An active noise reduction device according to any one of claims 1 to 6, The aforementioned reference signal source, The aforementioned secondary sound source, The error signal source is provided Mobile device.

8. An active noise reduction method that reduces noise at the location of an error signal source by outputting a secondary sound from a secondary sound source within the space inside a mobile device, The steps include: generating a control signal used for outputting the secondary sound by applying an adaptive filter to a reference signal correlated with the noise, which is output by a reference signal source attached to the mobile device; The steps include generating a filtered reference signal by correcting the reference signal with a simulated acoustic transmission characteristic that simulates the acoustic transmission characteristics from the position of the secondary sound source to the position of the error signal source, A step of updating the coefficients of the adaptive filter using the generated filtered reference signal and the error signal output by the error signal source, which corresponds to the residual sound resulting from the interference between the secondary sound and the noise; The procedure includes the step of stopping the updating of the coefficients of the adaptive filter and reducing the gain multiplied by the control signal to a negative target value if the amplitude of the reference signal or the amplitude of the error signal is below a threshold. Active noise reduction method.

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