JP3340139B2 - Active noise control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active noise control apparatus for actively reducing noise in a passenger compartment of an automobile, a passenger compartment of an aircraft, and the like.

[0002]

2. Description of the Related Art As a conventional active noise control device of this type, for example, there is one shown in FIG. 7 described in Japanese Patent Application Laid-Open No. 60-151150. This conventional device is used, for example, to reduce noise in a vehicle cabin, in particular, "muffled sound".
And a microphone 103, and the loudspeaker 10
1 generates a secondary sound (control sound) that interferes with the “muffled sound (noise)”, and detects an evaluation point for noise reduction by the microphone 103, for example, a residual signal (residual noise) at the occupant's ear position. It is supposed to.

That is, the signal from the microphone 103 passes through a preamplifier 105 and a bandpass filter 107,
The signal is converted into a rectangular wave by the saturation amplifier 109, and
At 1, the phase is delayed.

Further, the duty ratio of the PLL circuit 113 is 5
After being adjusted to a rectangular wave of 0%, it is converted into a sine wave by the converter 115, amplified by the amplifiers 117 and 119, and output as a secondary sound from the loudspeaker 101. The control of the phase of the secondary sound received by the phase shifter 111 is performed by a control circuit 125 including a frequency-voltage converter 121 and a weighting circuit 123.

Further, a voltage corresponding to the level of the "muffled sound" is supplied by a voltage level detection circuit 131 comprising a detection circuit 127 and a smoothing circuit 129, thereby controlling the voltage control amplifier 117 to thereby produce a secondary sound. The amplitude is determined. Therefore, such feedback control can reduce the muffled noise in the vehicle interior.

[0006]

However, such a control circuit having a feedback system as in the conventional active noise control apparatus has a problem that the secondary sound output by itself when the wrong control is entered is "muffled." Despite worsening the "sound", there is a risk that the secondary sound will continue to be louder, judging that a larger secondary sound is needed, and eventually become unbearable. This phenomenon is hereinafter referred to as divergence.

Here, since the magnitude of the "muffled sound" to be controlled in an automobile or the like is generally known, the level of the secondary sound is detected before or after the power amplifier 119 or the level of the input to the voltage control amplifier 117 is determined. Monitoring, if the level of the secondary sound has become greater than a predetermined value exceeding the "muffled sound", or the number of times the level of the secondary sound exceeds the predetermined value is a predetermined number of times A method of stopping the control of the device when it exceeds the limit (hereinafter, referred to as “divergence stop”) may be considered. As a result, the divergence of the device can be prevented (Japanese Patent Application No. 3-171054).

However, this method is not always perfect in some cases. That is, it may not be possible to distinguish between a case where the sound pressure level of the frequency of the "muffled sound" to be controlled due to the influence of a car stereo or outside noise becomes high and a case where the noise control device malfunctions. It is.
In the conventional noise control device, a DC voltage proportional to the sound pressure level of the “muffled sound” detected by the microphone 103 is output by the smoothing circuit 129, appropriately amplified, and output as a secondary sound from the loudspeaker 101. For example, even if the loudspeaker 101 outputs an appropriate secondary sound corresponding to the “muffled sound” in the vehicle cabin detected by the microphone 103, the microphone may be output due to erroneous operation of the car stereo or a temporary increase in noise outside the vehicle. When the sound pressure level of the noise detected at 103 is temporarily increased, the loudspeaker 101 outputs a secondary sound having a large level corresponding thereto, and the apparatus diverges the state to the secondary sound. Cannot be distinguished from the case where
There is a risk that the divergence may be stopped even though the device performs appropriate noise control.

Accordingly, an object of the present invention is to provide an active noise control device capable of more accurately performing noise control.

[0010]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention provides a control sound source for generating a control sound that interferes with noise to reduce noise at an evaluation point, and a control sound source that remains at a predetermined position after the interference. Means for detecting noise, means for detecting a signal relating to the noise generation state of the noise source, and a signal for driving the control sound source based on an output signal of the residual noise detection means and an output signal of the noise generation state detection means. An active noise control device comprising a control means for outputting, a secondary sound level determination unit that determines whether a secondary sound level from the control sound source exceeds a threshold value as a first predetermined value, Means for detecting a rise in sound pressure at the evaluation point, and a secondary sound output if the secondary sound level does not exceed a threshold value as a first predetermined value as determined by the secondary sound level determination unit. Sound level as the first predetermined value If the threshold value is exceeded, it is determined whether or not the sound pressure level of the evaluation point exceeds the threshold value as the second predetermined value by the determination of the means for detecting an increase in the sound pressure of the evaluation point, and the threshold value as the second predetermined value is determined. If it does not exceed, it is determined to be a divergence and the secondary sound output is set to 0, and if it exceeds the threshold value as the second predetermined value, it is determined that a loud volume due to causes other than divergence is affecting and the secondary sound output is maintained And a cancel control means for performing the operation.

[0011]

The control means outputs a signal for driving the control sound source based on the output signal of the residual noise detection means and the output signal of the noise generation state detection means. This allows the control sound source to
A control sound that interferes with noise is generated to reduce the noise at the evaluation points.

If the secondary sound level determination section determines that the secondary sound level does not exceed the threshold value as the first predetermined value, the secondary sound output is performed, and the secondary sound level becomes the threshold value as the first predetermined value. If the sound pressure level exceeds the threshold value as the second predetermined value, it is determined whether the sound pressure level at the evaluation point exceeds the threshold value as the second predetermined value. If not, it is determined to be a divergence and the secondary sound output is set to 0, and if it exceeds a threshold value as a second predetermined value, it is determined that a large volume due to a cause other than the divergence is affecting and the secondary sound output is maintained. .

[0013]

Embodiments of the present invention will be described below. In addition,
The description will be made taking the interior space of the vehicle as an example.

FIG. 1 is a block diagram of an active noise control apparatus according to one embodiment of the present invention, in which loudspeakers 1a and 1b as control sound sources in a closed space and a microphone as residual noise detection means. The loudspeakers 1a and 1b generate control sounds that interfere with the noise, and the microphones 3a and 3b attempt to reduce noise. For example, a residual signal at the ear position of the occupant (residual noise) ) Is to be measured. These loudspeakers 1a and 1b are connected to a power amplifier 5
a, 5b are connected to the signal processor 7, and the microphones 3a, 3b are also connected to the signal processor 7.

The noise in the vehicle interior is, for example, from a power plant, which includes an engine and a transmission as a power transmission device.
The differential gear is housed integrally. For example, a crank angle signal sensor 8 is used as the noise generation state detecting means, and the detected crank angle signal is output to a digital filter in the signal processor 7. The noise generation state detection means only needs to be able to detect a reference signal relating to the noise generation state of the noise source. Examples of the signal include an output signal of a vibration sensor provided on the outer surface of the engine, an ignition pulse signal of the engine,
A rotation speed signal or the like obtained by detecting the rotation speed of the crankshaft with a rotation speed sensor can also be used.

On the other hand, in the embodiment of the present invention, a divergence control circuit 9 and a cancel circuit 11 are provided.

The divergence restricting circuit 9 detects the increase in the sound pressure at the evaluation point, and determines whether the detected increase in the sound pressure exceeds a first predetermined value higher than the sound pressure level of the noise to be controlled. Means for operating to regulate the increase in the sound pressure.

When functioning as sound pressure rise detecting means,
The configuration is such that it is always determined whether or not the movement of the signal voltage to the power amplifiers 5a and 5b exceeds a first predetermined value. That is, the noise to be controlled is, for example, the "muffled sound" in the vehicle interior, and since the sound pressure level of the "muffled sound" is known in advance, a first predetermined value higher than this sound pressure level is set. In addition, when the value exceeds the first predetermined value, the apparatus is determined to be in the divergent state. In this determination, it can be determined that the device is in the divergent state when the movement of the signal voltage exceeds the first predetermined value a plurality of times, for example, three times.

When the divergence restricting circuit 9 functions as a sound pressure rise restricting means, a signal is sent to the switch 13 when it is determined that the divergence state is established. The switch 13 is an ON-OFF switch of the apparatus. When a signal is sent from the divergence control circuit 9, the signal is turned off to stop the apparatus, and the outputs from the loudspeakers 1a and 1b are set to zero and the output at the evaluation point Regulates sound pressure rise. The canceling circuit 11 constitutes canceling means in this embodiment, and comprises a band-pass filter, an adder, and the like, and a second predetermined value at which the sound pressure rise at the evaluation point is higher than the first predetermined value. When it is determined that the value exceeds the limit, the operation of the divergence restriction circuit 9 as the restriction means is canceled. That is, the cancel circuit 11 always determines whether the movement of the signal voltage from the microphones 3a and 3b exceeds the second predetermined value. The second predetermined value is set to be higher than the first predetermined value of the divergence level, and outputs a signal to the divergence control circuit 9 when it is determined that the movement of the signal voltage exceeds the second predetermined value. Then, the output of the divergence control circuit 9 to the switch 13 is canceled.

FIG. 2 is a functional block diagram of the apparatus shown in FIG. That is, the muffled sound detection unit corresponds to the microphones 3a and 3b, the secondary sound generation unit 7 corresponds to the signal processor 7, and the secondary sound output unit corresponds to the loudspeakers 1a and 1b. The in-vehicle noise level determination unit corresponds to the cancel circuit 11, and the secondary sound level determination unit corresponds to the divergence control circuit 9. Therefore, the respective reference numerals in FIG. 1 are assigned to the corresponding blocks in FIG.

The muffled sound detectors 3a and 3b detect "muffled sound" in the vehicle interior, and the muffled sound detectors 3a and 3b detect the muffled sound.
The secondary sound generation unit 7 adjusts the phase and the amplitude according to the signal input from a3b, and outputs the adjusted secondary sound to the secondary sound output unit 1a1b. The secondary sounds are output from the secondary sound output units 1a and 1b in correspondence with the sound pressure level of the "muffled sound" and in a reverse phase, so that the muffled sound in the vehicle cabin can be reduced.

The secondary sound level judging section 9 constantly monitors the secondary sound level. If the secondary sound level exceeds a first predetermined value, the secondary sound output sections 1a, 1a, 1
A signal is issued to stop the output from b (divergence stop signal). That is, the output of the secondary sound is lost.

When the in-vehicle noise level determining section 11 determines that the value exceeds a second predetermined value higher than the first predetermined value, it outputs a cancel signal (divergence stop cancel signal) to the secondary sound level determining section 9. Then, the secondary sound level determination section 9 outputs a signal for releasing the output stop from the secondary sound output sections 1a and 1b (divergence stop release signal). That is, if the secondary sound level determination section 9 determines that the secondary sound level does not exceed the threshold value as the first predetermined value, the secondary sound output is performed, and the secondary sound level becomes the threshold value as the first predetermined value. If the sound pressure level exceeds the threshold value as the second predetermined value, it is determined whether the sound pressure level at the evaluation point exceeds the threshold value as the second predetermined value. If not, it is determined to be a divergence and the secondary sound output is set to 0, and if it exceeds a threshold value as a second predetermined value, it is determined that a large volume due to a cause other than the divergence is affecting and the secondary sound output is maintained. The configuration includes a cancel control unit.

FIG. 3 shows a control flowchart.

[0025] First, the microphone 3 in step S ₁
a, 3b uptake detected microphone noise (residual noise "booming noise") performed, in step S ₂ is generated in the secondary sound generator 7 by secondary sound is performed. In step S ₃ Subsequently, secondary sound level is determined in the secondary sound level determination section 9 is determined whether above a threshold as a first predetermined value, in step S ₄ to be exceeded secondary sound output unit Secondary sound output by 1a and 1b is performed.

[0026] In step S _3, if exceeds the threshold secondary sound level, determined by the determination of the cabin noise level determining unit 11 in step S _5, whether the vehicle interior noise level exceeds the threshold value as a second predetermined value is, the increase in the secondary sound it is determined in step S ₃ if exceeded, determines that is based on the divergence of the system, in step S ₆ is a secondary sound = 0, secondary sound in step S ₄ outputs Is not performed.

Further despite if the vehicle interior noise level exceeds the threshold value control normal secondary sound has been performed in step S _5, it is determined that the loud erroneous operation or the like of the car stereo is affected, secondary sound in step S ₄ output is continued as it is.

Therefore, under such control, when the secondary sound generator 7 attempts to generate an excessive secondary sound due to, for example, an accidental increase in the volume of the car stereo, the secondary The sound level determination unit 9 attempts to send a divergence stop signal to the secondary sound output units 1a and 1b in order to stop the output of the secondary sound having the first predetermined value or more. On the other hand, the in-vehicle noise level determination unit 11 detects that the in-vehicle noise is already larger than the first predetermined value, and sends a divergence stop cancel signal to the secondary sound level determination unit 9, so that the divergence stop is released and the normal divergence stop is canceled. The muffled sound control is continued.

Here, let us consider a case where the control is actually erroneously made into a divergent state, not the influence of the car stereo or the noise outside the vehicle.

Since the noise level in the vehicle generating the “muffled sound” is not originally high, the divergence stop cancel signal is not output from the vehicle interior noise level determination unit 11. On the other hand, the secondary sound generation unit 7 outputs a signal corresponding to the secondary sound larger than the first predetermined value, and at the same time, sends a signal of that level to the secondary sound level determination unit 9. The secondary sound level determination unit 9 outputs a divergence stop signal based on this signal, and stops the secondary sound output in the secondary sound output units 1a and 1b (steps S ₁ → S ₂ → S ₃ → S ₅ → S ₆ → S ₄ ). Therefore, an increase in sound pressure due to divergence at the occupant's ear position can be prevented. At this time, the in-vehicle noise level determination unit 11 determines a determination level (second level) at which a divergence stop cancel signal is issued.
If the judgment level (first predetermined value) at which the divergence stop signal is output by the secondary sound level judgment unit 9 is set lower than the divergence stop function, the divergence stop function is more effective than the divergence stop cancel function. Is set to be easy to operate, the divergence stop function once worked will not be inadvertently released by the wrong secondary sound emitted by itself. Also, in the situation where the divergence stop is activated due to malfunction of the control, even if the divergence stop cancel signal is issued due to the noise outside the vehicle and the divergence stop is canceled, the divergence stop is canceled while the noise outside the vehicle continues. The loud secondary sound is also masked by the noise outside the vehicle and cannot be heard by the occupants. If the noise outside the vehicle disappears, the divergence stop cancel signal is not issued, so that the divergence stop works without any problem.

FIG. 4 shows the relationship between the change in the sound pressure level at the occupant's ear position and the first and second predetermined values. The curve in the figure shows a process in which the sound pressure level temporarily decreases due to the secondary sound, but the secondary sound diverges due to incorrect control, and the sound pressure level gradually increases. When the sound pressure level reaches a first predetermined value due to the divergence, the control is stopped, and the inside of the vehicle interior becomes the muffled sound level when not controlled. Further, a second predetermined value is provided at a value higher than the first predetermined value, and diverges when the sound pressure level exceeds the second predetermined value due to an erroneous operation of the car stereo or noise outside the vehicle. Therefore, it is determined that the sound pressure does not increase due to the noise, the stop of the device is canceled, and the noise control is continued.

Next, the control method will be described.

Now, the noise signals detected by the l-th microphones 3, 3b are represented by e _l (n) and the loudspeakers 1, 1
l-th microphone 3 when there is no control sound from b
The noise signal detected by a and 3b is represented by e _pl (n), and the j-th (j) of a transfer function FIR (finite impulse response) function between the m-th loudspeaker 1a and 1b and the l-th evaluation point.
= 0, 1, 2,..., I _c -1) by a digital filter, the filter coefficient is C _lmj , the reference signal, ie, the sound source information signal is x _p (n), and the reference signal x _p (n) And the i-th (i = 0, 1, 2,..., I _k -1) of the adaptive filter for driving the m-th loudspeakers 1a and 1b
Let W _mi be the coefficient of

[0034]

(Equation 1)

The following holds. Here, the term with (n) is
Each is a sample value at the sampling time n, and L is the number of microphones 3 and 3b (2 in this embodiment).
, M is the number of loudspeakers 1 and 1b (two in this embodiment), IC is the number of taps (filter order) of the transfer function Clm expressed by the FIR digital filter, and IK is the number of taps of the adaptive filter (filter). Degree).

In the above equation (1), “Σ W _mi · x” on the right side
_{The term “p} (n−ji)” (= y _m ) is applied to the filter (coefficient W _m ) for each output channel in the adaptive filter to obtain the signal x.
represents the output when _p is input, and “Σ C _lmj · ｛Σ
W _mi · (n−j−i)} ”is the m-th speaker 1
The signal energy input to the loudspeakers 1a, 1b
1b is output as an acoustic energy from, l th microphone 3a through a transfer function Clm the cabin, represents the signal when it reaches the 3b, further _"Σ Σ C lmj · {ΣW
_The entire right side of _mi · w _p (n−j−i｝) is obtained by adding the signals reaching the l-th microphones 3a and 3b for all the speakers, and thus the secondary sound reaching the l-th evaluation point. Represents the sum of

Next, an evaluation function (variable to be minimized) J
e

[0038]

(Equation 2)

Here,

[0040] Then, for determining the filter coefficients W _m of the evaluation function Je minimized, in the present embodiment employs the LMS algorithm. That is, the filter coefficient W _mi is obtained by partially differentiating the evaluation function Je with respect to each filter coefficient W _mi.
To update.

[0041]

(Equation 3)

From the equation (1),

[0043]

(Equation 4)

Thus, the right side of the equation (4) is represented by r
_{If lm} ( _ni ) is used, the rewriting equation of the filter coefficient is obtained by the following equation (5).

[0045]

(Equation 5)

Here, α is a convergence coefficient, which is related to the speed at which the filter converges optimally and the stability at that time.

By performing such control, when "muffled sound" is generated, it is controlled to minimize the muffled sound, and the "muffled sound" can be effectively silenced. .

FIG. 5 shows another embodiment.

In this embodiment, a vehicle condition determining unit 15 is used instead of the in-vehicle noise level determining unit 11 of FIG.

That is, in this embodiment, if the vehicle condition determination unit 15 does not exceed the threshold value as the second predetermined value, the vehicle condition determination unit 15 determines that the radiation is divergent, sets the secondary sound output to 0, and sets the threshold value as the second predetermined value. If it exceeds, it constitutes a cancel control means for judging that a large volume due to causes other than divergence is affecting and maintaining the secondary sound output. The vehicle condition determining unit 15 does not directly determine whether the increase in sound pressure at the occupant's ear position exceeds the second predetermined value, but indirectly determines the vehicle condition. The vehicle condition determination unit 15 determines whether or not the current vehicle state matches the condition for outputting the divergence stop cancellation signal, based on various information regarding the vehicle state. Vehicle conditions to consider include:
For example, the open / closed state of the window and the vehicle speed can be considered. That is, when the window is opened during high-speed running, the sound pressure level in the vehicle interior increases due to the wind noise, and the output of the divergence stop signal to the secondary sound output units 1a and 1b is canceled by the increase in the sound pressure level. . As another example of the vehicle state, the volume of a car teleo, a horn switch, a suspension vibration level, an engine vibration level, and the like can be considered.

More specifically, a logical product of "one of the windows is open" and "the vehicle speed is equal to or higher than a predetermined value" is
If the logical sum of "the horn switch is pressed" and "the suspension vibration level is equal to or higher than a predetermined value" is true, a divergence stop cancel signal is output.

The reason for considering the suspension vibration level here is to avoid outputting a divergence stop signal to the secondary sound output units 1a and 1b due to excessively large road noise which is not considered as a control target in this embodiment. That's why. In the case of the muffled sound control, it is also effective to confirm that the engine vibration which is the noise source does not exceed the assumed vibration level. That is, when the engine vibration level is equal to or higher than the predetermined value, the "muffled sound" in the vehicle may exceed the predetermined value. In this case, the divergence stop cancel signal is not output.

In this embodiment, the same operation and effect as those of the above-described embodiment can be obtained. In addition, the erroneous operation of the divergence stop for the noise which continues to be generated as long as the constant vehicle conditions such as the wind noise at the time of high-speed running is prevented. It is effective for

FIG. 6 shows still another embodiment.

In this embodiment, an external noise level determining section 17 and an external noise detecting section 19 are provided in place of the internal noise level determining section 11 of the embodiment of FIG. The outside noise level determination unit 17 and the outside noise detection unit 19 determine that the sound is divergent if the value does not exceed the threshold value as the second predetermined value, set the secondary sound output to 0, and set the threshold value as the second predetermined value. If it exceeds, it constitutes a cancel control means for judging that a large volume due to causes other than divergence is affecting and maintaining the secondary sound output. The outside noise detection unit 19 is composed of a microphone, a preamplifier, etc., and its output is output from the outside noise level determination unit 1.
7 is input. The outside noise level determination section 17 constitutes a canceling means, and indirectly determines that the increase in sound pressure at the occupant's ear position exceeds a second predetermined value by determining the detected outside noise. Is what it is.

Therefore, the outside noise level determining section 17 outputs a divergence stop cancel signal to the secondary sound level determining section 9 when the outside noise exceeds a predetermined value corresponding to the second predetermined value, and outputs an excessive outside vehicle level. Malfunction of the divergence stop function due to noise can be prevented.

This embodiment has the same functions and effects as those of the above-described embodiment, and is also effective when used in an environment that is often exposed to a high level of vehicle exterior noise.

In the above embodiments, the in-vehicle noise level judging section 11, the vehicle condition judging section 15, and the out-of-vehicle noise level judging section 17 are provided separately, but they may be used in any combination. In each of the above embodiments, the "muffled sound" of a vehicle has been described as a control target. However, control of road noise and wind noise may be performed, and the present invention can be applied to not only noise but also vibration control. Further, the present invention can be applied not only to automobiles but also to aircraft, ships and the like.
When the device diverges, the LMS algorithm can be modified as appropriate without stopping the device, so as to operate so as to regulate the increase in sound pressure. The residual noise detection means is not limited to detecting residual noise at evaluation points such as the occupant's ear position, but detecting residual noise at a spatially separated position,
The transfer function ratio between the noise source and the evaluation point and the transfer function between the control sound source and the evaluation point can be measured in advance and used.

[0059]

As is clear from the above, according to the structure of the present invention, whether the device causes an increase in sound pressure due to erroneous control, or the sound is caused by other noise factors despite the normal control. It is possible to distinguish whether a pressure rise is caused, and when the device is causing a sound pressure rise due to an erroneous control, the control can be stopped or the like to regulate the sound pressure rise. When a rise in sound pressure is caused by a factor, proper control of the device can be continued by canceling the control stop, and accurate active noise control can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram according to an embodiment of the present invention.

FIG. 2 is a functional block diagram according to one embodiment.

FIG. 3 is a flowchart according to one embodiment.

FIG. 4 is a graph showing a change in sound pressure level.

FIG. 5 is a functional block diagram according to another embodiment.

FIG. 6 is a functional block diagram according to still another embodiment.

FIG. 7 is a block diagram according to a conventional example.

[Explanation of symbols]

1a, 1b Loudspeaker (control sound source) 3a, 3b Microphone (residual noise detection means) 7 Signal processor (control means) 9 Divergence restriction circuit (sound pressure rise detection means and sound pressure rise restriction means) 11 Cancel circuit (cancel means) 15) Vehicle condition determination unit (cancellation means) 17 Outside vehicle noise level determination unit (cancellation means)

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-282696 (JP, A) JP-A-3-70490 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G10K 11/178 B60R 11/02 B64C 1/40

Claims (1)

(57) [Claims] 1. A control sound source for generating a control sound that interferes with noise to reduce noise at an evaluation point, means for detecting residual noise at a predetermined position after the interference, and a signal relating to a noise generation state of a noise source An active noise control device comprising: a control unit that outputs a signal for driving the control sound source based on an output signal of the residual noise detection unit and an output signal of the noise generation state detection unit. The secondary sound level from the control sound source is set as a first predetermined value.
Secondary sound level judgment unit to judge whether it exceeds the threshold of
When means for detecting a sound pressure increase of the evaluation point, the determination in the second sound level determination section secondary sound level
Secondary sound output if the value does not exceed the threshold value as the first predetermined value
And if the secondary sound level exceeds the threshold value as the first predetermined value,
The evaluation point is determined by the means of detecting the increase in sound pressure at the evaluation point.
Whether the sound pressure level exceeds the threshold value as the second predetermined value
And if it does not exceed the threshold value as the second predetermined value, it is determined that divergence
The secondary sound output to 0, and if it exceeds the threshold value as the second predetermined value,
The secondary sound output
An active noise control device, comprising: a cancel control unit for maintaining the active noise control unit.