Active noise reduction system

An active noise reduction system reduces a noise in an internal space of a mobile body. The active noise reduction system includes a controller configured to control a canceling sound output device. The controller is configured to generate a first canceling estimation signal as an estimation signal of a canceling sound at a position of an error detector based on a reference signal, generate a second canceling estimation signal as an estimation signal of the canceling sound at a head position of an occupant by adjusting a time delay and an amplitude of the first canceling estimation signal based on a reference distance, and update a control filter for controlling the canceling sound output device based on the second canceling estimation signal.

TECHNICAL FIELD

The present invention relates to an active noise reduction system that reduces a noise by causing a canceling sound in an opposite phase to the noise to interfere with the noise.

BACKGROUND ART

Conventionally, an active noise reduction system reduces a noise by causing a canceling sound in an opposite phase to the noise to interfere with the noise. For example, the active noise reduction system includes a canceling sound output device configured to output the canceling sound for canceling the noise, an error detector configured to detect an error between the noise and the canceling sound and generate an error signal corresponding to the error, and a controller configured to control the canceling sound output device based on the error signal.

For example, JP2021-162849A discloses a speaker that outputs a canceling sound, a microphone that outputs an error signal, and an active noise controller that generates a control signal for causing the speaker to output the canceling sound based on the error signal.

In the conventional active noise reduction system, an area with a high control effect (high noise reduction effect) is limited to a portion of an area around the error detector such as a microphone. Accordingly, if a head position of an occupant changes, the noise at the head position of the occupant may not be reduced sufficiently.

SUMMARY OF THE INVENTION

In view of the above background, an object of the present invention is to provide an inexpensive active noise reduction system that can effectively reduce the noise at a head position of an occupant even if the head position of the occupant changes.

To achieve such an object, one aspect of the present invention provides an active noise reduction system (11) for reducing a noise in an internal space (5) of a mobile body (1), the active noise reduction system comprising: a reference signal generator (12) configured to generate a reference signal corresponding to the noise; a canceling sound output device (13) configured to output a canceling sound for canceling the noise; an error detector (14) configured to detect an error between the noise and the canceling sound and generate an error signal corresponding to the error; a reference distance detector (15) configured to detect a reference distance that is a distance from the canceling sound output device to a head position of an occupant; and a controller (16) configured to control the canceling sound output device based on the reference signal, the error signal, and the reference distance, wherein the controller is configured to: generate a first canceling estimation signal based on the reference signal, the first canceling estimation signal being an estimation signal of the canceling sound at a position of the error detector; generate a second canceling estimation signal by adjusting a time delay and an amplitude of the first canceling estimation signal based on the reference distance, the second canceling estimation signal being an estimation signal of the canceling sound at the head position of the occupant; and update a control filter (W) based on the second canceling estimation signal, the control filter being a filter for controlling the canceling sound output device.

According to this aspect, by updating the control filter based on the second canceling estimation signal (an estimation signal of the canceling sound at the head position of the occupant), the characteristics of the control filter can be changed so as to follow the change in the head position of the occupant. Accordingly, even if the head position of the occupant changes, the noise at the head position of the occupant can be reduced effectively. Further, the second canceling estimation signal is generated by adjusting the time delay and the amplitude of the first canceling estimation signal (an estimation signal of the canceling sound at the position of the error detector). Accordingly, it is not necessary to use a filter with a high calculation load to generate the second canceling estimation signal. Accordingly, the calculation load of the controller can be reduced, and the controller can be composed of a relatively inexpensive processor.

In the above aspect, preferably, the controller is configured to: set a correction coefficient corresponding to the reference distance; and correct an update amount of the control filter by multiplying the update amount of the control filter by the correction coefficient.

According to this aspect, the update amount of the control filter can be adjusted according to the reference distance, so that the update amount of the control filter can be maintained at an appropriate value.

In the above aspect, preferably, the controller is configured to adjust the amplitude of the first canceling estimation signal by using an amplitude adjustment coefficient that decreases as the reference distance increases, and the correction coefficient is set to a reciprocal of the amplitude adjustment coefficient.

According to this aspect, the correction coefficient can be increased in a case where the amplitude adjustment coefficient decreases as the reference distance increases. Accordingly, the update amount of the control filter can be prevented from decreasing excessively, so that the update performance of the control filter can be maintained.

In the above aspect, preferably, the controller is configured to adjust the amplitude of the first canceling estimation signal by using an amplitude adjustment coefficient that decreases as the reference distance increases, and the correction coefficient is set such that a product of the amplitude adjustment coefficient and the correction coefficient is less than 1.

According to this aspect, in a case where the update accuracy of the control filter decreases as the reference distance increases, an excessive increase in the update amount of the control filter can be suppressed. Accordingly, it is possible to avoid a situation in which the performance of the control filter deteriorates due to the update of the control filter.

In the above aspect, preferably, the controller is configured to store a correction coefficient table that defines a relationship between the reference distance and the correction coefficient.

According to this aspect, since the correction coefficient can be freely set according to the reference distance, the degree of freedom in setting the correction coefficient can be enhanced.

In the above aspect, preferably, the controller is configured to: update an estimation value of transmission characteristics of the canceling sound; and generate the first canceling estimation signal by correcting the reference signal based on the updated estimation value of the transmission characteristics of the canceling sound.

According to this aspect, in a case where the transmission characteristics of the canceling sound change, the change in the transmission characteristics of the canceling sound can be learned, and the first canceling estimation signal can be generated based on the learned result thereof. Accordingly, the noise at the head position of the occupant can be reduced more effectively.

In the above aspect, preferably, the canceling sound output device and the error detector are installed in a headrest (6a) of an occupant seat (6) provided in the internal space, and the controller is configured to generate the second canceling estimation signal by adjusting only the time delay and the amplitude of the first canceling estimation signal.

According to this aspect, the canceling sound output device, the error detector, and the head of the occupant can be sufficiently close to each other. Accordingly, most of the canceling sound reaches the error detector and the head of the occupant directly from the canceling sound output device, so that the dependence of the canceling sound on the time delay and the distance attenuation can be increased. Accordingly, by adjusting only the time delay and the amplitude of the first canceling estimation signal, the second canceling estimation signal can be generated with high accuracy.

Thus, according to the above aspects, it is possible to provide an inexpensive active noise reduction system that can effectively reduce the noise at a head position of an occupant even if the head position of the occupant changes.

DETAILED DESCRIPTION OF THE INVENTION

In the following, embodiments of the present invention will be described with reference to the drawings. In this specification, “{circumflex over ( )}” (circumflexes) shown together with symbols each indicate an identification value or an estimation value. “{circumflex over ( )}” are shown above the symbols in the drawings and formulas, but are shown subsequently to the symbols in the text of the description.

The First Embodiment

FIG.1is a schematic diagram showing a vehicle1(an example of a mobile body) to which an active noise reduction system11(hereinafter abbreviated as “noise reduction system11”) according to the first embodiment is applied. When wheels2vibrate due to the force received from a road surface S and the vibration of the wheels2are transmitted to a vehicle body4via suspensions3, a road noise d is generated in a vehicle cabin5(an example of an internal space of the mobile body). The noise reduction system11according to the first embodiment is a feedback-controllable active noise control device (ANC device) for reducing such a road noise d. More specifically, the noise reduction system11reduces the road noise d by generating a canceling sound y that is in an opposite phase to the road noise d and causing the generated canceling sound y to interfere with the road noise d. In another embodiment, the noise reduction system11may reduce a noise (for example, an aerodynamic noise transmitted from an undercover attached to a lower surface of the vehicle body4) other than the road noise d generated as the vehicle1travels.

With reference toFIGS.1and2, the noise reduction system11includes a vibration sensor12(an example of a reference signal generator) configured to generate a reference signal x corresponding to the road noise d, a plurality of speakers13(an example of a canceling sound output device) configured to generate the canceling sound y for canceling the road noise d, a plurality of error microphones14(an example of an error detector) configured to detect an error (synthetic sound) between the road noise d and the canceling sound y and generate an error signal e corresponding to the detected error, a reference distance detector15configured to detect a distance (hereinafter referred to as “reference distance Lr”) from the plurality of speakers13to a head position of an occupant, and a controller16configured to control the plurality of speakers13based on the reference signal x, the error signal e, and the reference distance Lr.

A symbol HminFIG.2indicates transfer characteristics of the road noise d (transfer characteristics of a primary path) from a noise source (in the present embodiment, the road surface S) to each error microphone14. A symbol CminFIG.2indicates transfer characteristics of the canceling sound y (transfer characteristics of a secondary path) from the speaker13to each error microphone14.

With reference toFIG.1, the vibration sensor12of the noise reduction system11is installed in at least one suspension3, for example. The vibration sensor12detects the acceleration of the suspension3according to the road noise d and generates the reference signal x according to the acceleration of the suspension3. In another embodiment, the vibration sensor12may be installed in a location other than the suspension3of the vehicle1. In another embodiment, a reference microphone (not shown) may generate the reference signal x according to the road noise d.

Each speaker13of the noise reduction system11is installed, for example, in a headrest6aof an occupant seat6provided in the vehicle cabin5. In another embodiment, the speaker13may be installed in a location other than the headrest6aof the occupant seat6.

Each error microphone14of the noise reduction system11is installed, for example, in the headrest6aof the occupant seat6. In another embodiment, the error microphone14may be installed in a location other than the headrest6aof the occupant seat6.

The reference distance detector15of the noise reduction system11consists of, for example, an occupant monitoring system including an occupant camera that captures an image of the occupant. The reference distance detector15detects the reference distance Lrbased on the image of the occupant captured by the occupant camera, and outputs the detected reference distance Lrto the controller16. In another embodiment, the reference distance detector15may consist of a distance sensor that directly detects the reference distance Lr.

The controller16of the noise reduction system11consists of an electronic control unit (ECU) that includes an arithmetic processing unit (a processor such as CPU and MPU) and a storage device (memory such as ROM and RAM). The controller16may consist of one piece of hardware, or may consist of a unit composed of plural pieces of hardware.

With reference toFIG.2, the controller16includes, as functional components, a first A/D conversion unit21, a control signal output unit22, a D/A conversion unit23, a second A/D conversion unit24, an acoustic characteristics update unit25, a reference signal correction unit26, an acoustic characteristics adjustment unit27, an adjustment amount determination unit28, and a control filter update unit29. Symbols “ADA” inFIG.2indicate “adaptive”.

The first A/D conversion unit21of the controller16converts an analog reference signal x output from the vibration sensor12into a digital reference signal x, and outputs the digital reference signal x to the control signal output unit22, the acoustic characteristics update unit25, and the reference signal correction unit26. Hereinafter, “reference signal x” without explanation indicates the reference signal x that has passed through the first A/D conversion unit21.

<The Control Signal Output Unit22>

The control signal output unit22of the controller16consists of a control filter W. A finite impulse response filter (FIR filter) is used for the control filter W. In another embodiment, a single-frequency adaptive notch filter (SAN filter) may be used for the control filter W. The control signal output unit22generates a control signal u by filtering the reference signal x by using the control filter W, and outputs the generated control signal u to the D/A conversion unit23and the acoustic characteristics update unit25.

The D/A conversion unit23of the controller16converts a digital control signal u output from the control signal output unit22into an analog control signal u, and outputs the analog control signal u to the speaker13. Thus, the speaker13generates the canceling sound y according to the control signal u.

The second A/D conversion unit24of the controller16converts the error signal e output from the error microphone14from an analog signal to a digital signal, and outputs the converted error signal e to the acoustic characteristics update unit25. Hereinafter, “error signal e” without explanation indicates the error signal e that has passed through the second A/D conversion unit24.

The acoustic characteristics update unit25of the controller16updates an estimation value of the acoustic characteristics in the vehicle cabin5based on the reference signal x, the control signal u, and the error signal e. The acoustic characteristics update unit25includes a canceling estimation signal generation unit31, a noise estimation signal generation unit32, and an adder33.

The canceling estimation signal generation unit31includes a secondary path filter unit35and a secondary path update unit36.

The secondary path filter unit35consists of a secondary path filter C{circumflex over ( )}. The secondary path filter C{circumflex over ( )} is a filter corresponding to an estimation value of the transfer characteristics Cmof the canceling sound y from the speaker13to the error microphone14. An FIR filter is used for the secondary path filter C{circumflex over ( )}. In another embodiment, a SAN filter may be used for the secondary path filter C{circumflex over ( )}.

The secondary path filter unit35generates a canceling estimation signal y{circumflex over ( )}m1by filtering the control signal u using the secondary path filter C{circumflex over ( )}. The canceling estimation signal y{circumflex over ( )}m1is an estimation signal of the canceling sound y at a position of the error microphone14(hereinafter referred to as “microphone position”). The secondary path filter unit35outputs the generated canceling estimation signal y{circumflex over ( )}m1to the adder33.

The secondary path update unit36updates the secondary path filter C{circumflex over ( )} using an adaptive algorithm such as a Least Mean Square algorithm (LMS algorithm). More specifically, the secondary path update unit36updates the secondary path filter C{circumflex over ( )} such that a virtual error signal e1(that will be described later) output from the adder33is minimized.

The noise estimation signal generation unit32includes a primary path filter unit38and a primary path update unit39.

The primary path filter unit38consists of a primary path filter H{circumflex over ( )}. The primary path filter H{circumflex over ( )} is a filter corresponding to an estimation value of the transfer characteristics Hmof the road noise d from the noise source to the error microphone14. An FIR filter is used for the primary path filter H{circumflex over ( )}. In another embodiment, a SAN filter may be used for the primary path filter H{circumflex over ( )}.

The primary path filter unit38generates a noise estimation signal d{circumflex over ( )} by filtering the reference signal x using the primary path filter HA. The noise estimation signal d{circumflex over ( )}functions as both an estimation signal of a road noise dmat the microphone position and an estimation signal of a road noise deat the head position of the occupant. The primary path filter unit38outputs the generated noise estimation signal d{circumflex over ( )} to the adder33and the control filter update unit29.

The primary path update unit39updates the primary path filter H{circumflex over ( )} using an adaptive algorithm such as the LMS algorithm. More specifically, the primary path update unit39updates the primary path filter H{circumflex over ( )} such that the virtual error signal e1(that will be described later) output from the adder33is minimized.

The adder33generates the virtual error signal e1by adding together the error signal e, the canceling estimation signal y{circumflex over ( )}m1, and the noise estimation signal d{circumflex over ( )}. The adder33outputs the generated virtual error signal e1to the canceling estimation signal generation unit31and the noise estimation signal generation unit32.

The reference signal correction unit26of the controller16, like the canceling estimation signal generation unit31, consists of the secondary path filter C{circumflex over ( )}. When the secondary path filter C{circumflex over ( )} is updated in the canceling estimation signal generation unit31, the updated secondary path filter C{circumflex over ( )} is output to the reference signal correction unit26, and the secondary path filter C{circumflex over ( )} is updated in the reference signal correction unit26. That is, the secondary path filter C{circumflex over ( )}set in the reference signal correction unit26is not a fixed value but a value that is successively updated based on the signal from the canceling estimation signal generation unit31.

The reference signal correction unit26generates a canceling estimation signal y{circumflex over ( )}m2(first canceling estimation signal) by filtering the reference signal x. More specifically, the reference signal correction unit26generates the canceling estimation signal y{circumflex over ( )}m2by correcting the reference signal x based on the updated secondary path filter C{circumflex over ( )}. The canceling estimation signal y{circumflex over ( )}m2is, like the canceling estimation signal y{circumflex over ( )}m1, an estimation signal of the canceling sound y at the microphone position. The reference signal correction unit26outputs the generated canceling estimation signal y{circumflex over ( )}m2to the acoustic characteristics adjustment unit27.

The acoustic characteristics adjustment unit27of the controller16generates a canceling estimation signal y{circumflex over ( )}e1(an example of a second canceling estimation signal) by adjusting the time delay and the amplitude (distance attenuation) of the canceling estimation signal y{circumflex over ( )}m2. The canceling estimation signal y{circumflex over ( )}e1is an estimation signal of the canceling sound y at the head position of the occupant. The acoustic characteristics adjustment unit27outputs the generated canceling estimation signal y{circumflex over ( )}e1to the control filter update unit29.

The acoustic characteristics adjustment unit27includes a delay unit41and an amplitude adjustment unit42. The delay unit41adjusts the time delay of the canceling estimation signal y{circumflex over ( )}m2by using delay characteristics Z−d. More specifically, the delay unit41delays the canceling estimation signal y{circumflex over ( )}m2by d samples. The amplitude adjustment unit42adjusts the amplitude of the canceling estimation signal y{circumflex over ( )}m2by using an amplitude adjustment coefficient a. More specifically, the amplitude adjustment unit42adjusts the amplitude of the canceling estimation signal y{circumflex over ( )}m2by multiplying the canceling estimation signal y{circumflex over ( )}m2by the amplitude adjustment coefficient a.

The adjustment amount determination unit28of the controller16determines an adjustment amount of the time delay in the acoustic characteristics adjustment unit27based on the reference distance Lroutput from the reference distance detector15. More specifically, the adjustment amount determination unit28determines the delay characteristics Z−dof the delay unit41according to the following formula (1). Incidentally, “round” in the following formula (1) indicates an operation for rounding off to an integer, “c” in the following formula (1) indicates the speed of sound, and “FS” in the following formula (1) indicates a sampling frequency.

The adjustment amount determination unit28determines the adjustment amount of the amplitude in the acoustic characteristics adjustment unit27based on the reference distance Lroutput from the reference distance detector15. More specifically, the adjustment amount determination unit28determines the amplitude adjustment coefficient a of the amplitude adjustment unit42according to the following formula (2). Incidentally, “Lm” in the following formula (2) indicates the distance from one speaker13to the corresponding error microphone14, N (N=1, 2, . . . ) in the following formula (2) indicates a parameter for adjusting the amplitude, and “σ” in the following formula (2) indicates an adjustment constant (a constant of a relatively small value to prevent the denominator on the right side of the following formula (2) from becoming zero and prevent the amplitude from becoming excessively large).

As is clear from the above formula (2), the amplitude adjustment coefficient a is set to decrease as the reference distance Lrincreases.

<The Control Filter Update Unit29>

The control filter update unit29of the controller16consists of the control filter W, like the control signal output unit22. The control filter update unit29updates the control filter W based on the canceling estimation signal y{circumflex over ( )}e1output from the acoustic characteristics adjustment unit27. The control filter update unit29includes a control filter unit45, an adder46, and a control update unit47.

The control filter unit45generates a canceling estimation signal y{circumflex over ( )}e2by filtering the canceling estimation signal y{circumflex over ( )}e1by using the control filter W. The canceling estimation signal y{circumflex over ( )}e2is an estimation signal of the canceling sound y at the head position of the occupant, like the canceling estimation signal y{circumflex over ( )}e1. The canceling estimation signal y{circumflex over ( )}e2can be expressed by the following formula (3).
ŷe2=aZ−dxĈW=aZ−dŷm1(3)

The adder46generates a virtual error signal ee by adding together the canceling estimation signal y{circumflex over ( )}e2and the noise estimation signal d{circumflex over ( )}. The adder46outputs the generated virtual error signal ee to the control update unit47.

The control update unit47updates the control filter W by using an adaptive algorithm such as the LMS algorithm. More specifically, the control update unit47updates the control filter W such that the virtual error signal ee output from the adder46is minimized.

When the control filter W is updated in the control filter update unit29in this way, the updated control filter W is output to the control signal output unit22, and the control filter W is updated in the control signal output unit22. That is, the control filter W set in the control signal output unit22is not a fixed value but a value that is successively updated based on the signal from the control filter update unit29.

<The Noise Reduction Mechanism and the Precondition>

Next, a noise reduction mechanism and a precondition of the noise reduction system11will be described with reference toFIG.3. Each of the curved lines p inFIG.3indicates a wave front of the road noise d (a surface where the sound pressure of the road noise d is uniform) transmitted from the noise source.

The head position of the occupant (for example, the driver) may change significantly in the front-and-rear direction depending on the driving posture of the occupant, but is unlikely to change in the up-and-down direction. Accordingly, in a case where each speaker13and the corresponding error microphone14are installed in the headrest6aof the occupant seat6, it is estimated that the head position of the occupant and the error microphone14are located at substantially the same height. The road noise d is transmitted in the vehicle cabin5from the feet of the occupant to the head thereof. Accordingly, if the head position of the occupant and the error microphone14are located at substantially the same height, it is estimated that the road noise dmat the microphone position and the road noise deat the head position of the occupant are substantially equal. That is, the following formula (4) is satisfied with regard to the road noise d.
de≈dm(4)

Since the above formula (4) is satisfied, the noise estimation signal d{circumflex over ( )}can function as both the estimation signal of the road noise dmat the microphone position and the estimation signal of the road noise deat the head position of the occupant.

By the way, in a case where each speaker13and the corresponding error microphone14are installed in the headrest6aof the occupant seat6, the reference distance Lrmay change significantly as the head position of the occupant changes significantly in the front-and-rear direction. Accordingly, the canceling sound yeat the head position of the occupant also changes significantly due to the influence of the time delay and the distance attenuation.

As such, the controller16generates the canceling estimation signal y{circumflex over ( )}e1by adjusting the time delay and the amplitude (distance attenuation) of the canceling estimation signal y{circumflex over ( )}e2. In other words, the controller16estimates the canceling sound yeat the head position of the occupant by adjusting the time delay and the amplitude (distance attenuation) of the canceling sound ymat the microphone position. That is, the following formula (5) is satisfied with regard to the canceling sound y.
ye≈aymZ−d(5)

By adjusting the time delay and the amplitude (distance attenuation) of the canceling sound ymat the microphone position in this way, the canceling sound yeat the head position of the occupant can be estimated accurately. Accordingly, the road noise d at the head position of the occupant can be reduced effectively.

To use such a noise reduction mechanism, it is preferable that most of the canceling sound y reach directly from each speaker13to the corresponding error microphone14and the head position of the corresponding occupant so that the dependence of the canceling sound y on the time delay and the distance attenuation can be increased. That is, the precondition to use such a noise reduction mechanism is that each speaker13, the corresponding error microphone14, and the head position of the corresponding occupant are sufficiently close to each other.

The Effects of the First Embodiment

The controller16according to the first embodiment updates the primary path filter H{circumflex over ( )} and the secondary path filter C{circumflex over ( )}based on the reference signal x and the error signal e. In other words, the controller16updates the estimation value of the acoustic characteristics of the internal space based on the reference signal x and the error signal e. Accordingly, even if the acoustic characteristics of the internal space change according to the displacement of each error microphone14, the characteristics of the control filter W can be changed to follow the change in the acoustic characteristics. Accordingly, the error microphone14can be arranged on a movable portion such as the headrest6a, and thus located closer to the head position of the occupant.

By the way, the area where the control effect (sound reduction effect) of the noise reduction system11is high is limited to an area around each error microphone14(see circles A inFIG.1). Accordingly, when the head of the occupant moves away from the error microphone14due to the driving posture of the occupant, the control effect of the noise reduction system11that the occupant can feel may decrease.

As such, the controller16generates the canceling estimation signal y{circumflex over ( )}e1(the estimation signal of the canceling sound y at the head position of the occupant) by adjusting the time delay and the amplitude of the canceling estimation signal y{circumflex over ( )}e2(the estimation signal of the canceling sound y at the microphone position) based on the reference distance Lr, and updates the control filter W based on the canceling estimation signal y{circumflex over ( )}e1. Thus, the characteristics of the control filter W can be changed so as to follow the change in the head position of the occupant. Accordingly, in a case where the head of the occupant moves away from the error microphone14, it is possible to suppress the decrease in the control effect of the noise reduction system11that the occupant can feel.

By the way, when a broadband noise is to be reduced, the noise reduction system11may generate the canceling estimation signal y{circumflex over ( )}e1by filtering the canceling estimation signal y{circumflex over ( )}e2by an FIR filter. However, if the canceling estimation signal y{circumflex over ( )}e1is generated by using an FIR filter in this way, the calculation load of the controller16for generating the canceling estimation signal y{circumflex over ( )}e1becomes large.

As such, the controller16generates the canceling estimation signal y{circumflex over ( )}e1by adjusting only the delay characteristics Z−dand the amplitude adjustment coefficient a of the canceling estimation signal y{circumflex over ( )}e2. Accordingly, it is not necessary to use an FIR filter to generate the canceling estimation signal y{circumflex over ( )}e1even when a broadband noise is to be reduced. Accordingly, the calculation load of the controller16for generating the canceling estimation signal y{circumflex over ( )}e1can be greatly reduced when a broadband noise is to be reduced.

FIG.4is a graph showing the effect of reducing the road noise d at the head position of the occupant (more specifically, the ear position of the occupant). As shown inFIG.4, when the noise reduction of the present embodiment (that is, the noise reduction system11that updates the control filter W based on the head position of the occupant) is ON, the road noise d can be reduced in a wide frequency band as compared with a case where the conventional noise reduction (that is, the noise reduction system that updates the control filter W without considering the head position of the occupant) is ON and a case where the noise reduction is OFF.

The Modification of the First Embodiment

In the first embodiment, the controller16adjusts only the delay characteristics Z−dand the amplitude adjustment coefficient a of the canceling estimation signal y{circumflex over ( )}e2. In a case where the abovementioned precondition of the noise reduction mechanism (the precondition that the speaker13, the corresponding error microphone14, and the head position of the corresponding occupant are sufficiently close) is unlikely to be satisfied, the controller16may adjust not only the delay characteristics Z−dand the amplitude adjustment coefficient a of the canceling estimation signal y{circumflex over ( )}e2but also other parameters thereof.

The Second Embodiment

Next, the second embodiment of the present invention will be described with reference toFIGS.5and6. Explanations that overlap with those of the first embodiment of the present invention will be omitted as appropriate.

FIG.5is a functional block diagram showing an active noise reduction system51(hereinafter abbreviated as “noise reduction system51”) according to the second embodiment. The components of the noise reduction system51other than a control filter update unit54and an adjustment amount determination unit55of the controller53are the same as those of the noise reduction system11according to the first embodiment. Accordingly, descriptions of these components will be omitted. Symbols “ADA” inFIG.5indicate “adaptive”.

<The Control Filter Update Unit54>

The control filter update unit54of the controller53includes a control filter unit56, an adder57, an estimation signal correction unit58, and a control update unit59. The configurations of the control filter unit56and the adder57of the control filter update unit54are the same as those of the control filter unit45and the adder46of the control filter update unit29according to the first embodiment. Accordingly, descriptions of these components will be omitted.

The estimation signal correction unit58corrects the canceling estimation signal y{circumflex over ( )}e1by using a correction coefficient b. The estimation signal correction unit58outputs the corrected canceling estimation signal y{circumflex over ( )}e1to the control update unit59.

The control update unit59updates the control filter W by using an adaptive algorithm such as the LMS algorithm. More specifically, the control update unit59updates the control filter W such that the virtual error signal ee output from the adder57is minimized. For example, the control update unit59updates the control filter W according to the following formula (6). Incidentally, “μ” in the following formula (6) indicates a step size parameter.
W(n+1)=W(n)−bμêe(n)(r*aZ−dĈ)  (6)

As is clear from the above formula (6), the control update unit59corrects the update amount of the control filter W by multiplying the update amount (μe{circumflex over ( )}e(n)(r*aZ−dC{circumflex over ( )})) of the control filter W by the correction coefficient b.

The adjustment amount determination unit55of the controller53sets the correction coefficient b based on the reference distance Lroutput from the reference distance detector15. Hereinafter, setting methods of the correction coefficient b by the adjustment amount determination unit55will be described.

<The Setting Method 1 of the Correction Coefficient b>

When the amplitude adjustment coefficient a decreases as the reference distance Lrincreases, the update amount of the control filter W also decreases. If the update amount of the control filter W decreases excessively, the update performance (learning speed) of the control filter W may deteriorate.

As such, the adjustment amount determination unit55sets the correction coefficient b to a reciprocal of the amplitude adjustment coefficient a in order to reduce the dependence of the update amount of the control filter W on the amplitude adjustment coefficient a. Accordingly, the correction coefficient b can be increased in a case where the amplitude adjustment coefficient a decreases as the reference distance Lrincreases. Accordingly, the update amount of the control filter W can be prevented from decreasing excessively, so that the update performance of the control filter W can be maintained.

<The Setting Method 2 of the Correction Coefficient b>

When the reference distance Lrincreases, the abovementioned precondition of the noise reduction mechanism (the precondition that the speaker13, the corresponding error microphone14, and the head position of the corresponding occupant are sufficiently close) may not be satisfied. Accordingly, the update accuracy of the control filter W may deteriorate.

As such, the adjustment amount determination unit55sets the correction coefficient b such that the product of the amplitude adjustment coefficient a and the correction coefficient b is less than 1. Accordingly, in a case where the reference distance Lrincreases, the update amount of the control filter W can be prevented from increasing excessively. Accordingly, it is possible to avoid a situation in which the performance of the control filter W deteriorates due to the update of the control filter W.

<The Setting Method 3 of the Correction Coefficient b>

With reference toFIG.6, the adjustment amount determination unit55stores a correction coefficient table T that defines the relationship between the reference distance Lrand the correction coefficient b. The correction coefficient b is set such that the product of the amplitude adjustment coefficient a and the correction coefficient b is less than 1, for example, similarly to the setting method 2 of the correction coefficient b.

The adjustment amount determination unit55sets the correction coefficient b by referring to the correction coefficient table T based on the reference distance Lr. By using the correction coefficient table T in this way, the correction coefficient b can be freely set according to the reference distance Lr, so that the degree of freedom in setting the correction coefficient b can be increased.

The Effect of the Second Embodiment

The controller53according to the second embodiment sets the correction coefficient b corresponding to the reference distance Lr, and corrects the update amount of the control filter W by multiplying the update amount of the control filter W by the correction coefficient b. Accordingly, the update amount of the control filter W can be adjusted according to the reference distance Lr, and thus maintained at an appropriate value.

Concrete embodiments of the present invention have been described in the foregoing, but the present invention should not be limited by the foregoing embodiments and various modifications and alterations are possible within the scope of the present invention.