Active noise control system

Abstract

An "active noise control system" is provided that is not easily affected by the displacement of a user who cancels noise. A first cancellation signal output from a first speaker (12) and a second cancellation signal output from a second speaker (14) cancel noise at a first cancellation point (41) which is a standard position of the user's right ear. In addition, a second cancellation signal output from the second speaker (14) and the first cancellation signal output from the first speaker (12) cancel noise at a second cancellation point (42) which is a standard position of the user's left ear. The first speaker (12) and the second speaker (14) are arranged on a second line segment perpendicular to the first line segment and passing through the midpoint of a first line segment connecting the first cancellation point and the second cancellation point, and the relationship between the noise, the first cancellation signal and the second cancellation signal becomes the same as the range (400) of the cancellation point.

Description

Technical Field

[0001] The present invention relates to an active noise control (ANC) technology for reducing noise by radiating noise cancellation sound for canceling noise. Background Art

[0002] As a technology for active noise control that reduces noise by radiating noise cancellation sound for canceling noise, the following technology is known: a microphone and a speaker are arranged near a noise cancellation position, and an adaptive filter is used to generate a noise cancellation sound output from a speaker by applying an adaptively set transfer function to an output signal of a noise source or a signal simulating the output signal, wherein the transfer function is adaptively set in the adaptive filter as an error signal obtained by correcting the output of the microphone using an auxiliary filter.

[0003] Here, in this technology, a transfer function that is corrected by using the pre-learned difference between the transfer function from the noise source to the noise cancellation position and the transfer function from the noise source to the microphone, and the difference between the transfer function from the speaker to the noise cancellation position and the transfer function from the speaker to the microphone is set in the auxiliary filter. By using such an auxiliary filter, noise is eliminated at a noise cancellation position that is different from the position of the microphone.

[0004] In addition, it is also known that a group of microphones, speakers, adaptive filters and auxiliary filters are set to correspond to two noise cancellation positions, respectively, and the above-mentioned technology is used to output noise cancellation sounds for eliminating noise at the corresponding noise cancellation positions in each group, thereby eliminating the noise generated by the noise source at the two noise cancellation positions respectively (for example, patent document 1).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2018-72770 Summary of the invention

[0008] With the purpose of eliminating noise audible to a user sitting in a seat, the standard positions of the left ear and the right ear of the user sitting in the seat are set as two noise elimination positions. When the noise generated from the noise source is eliminated at the two noise elimination positions respectively by the above-mentioned technology, if the left ear and the right ear of the user deviate from the noise elimination positions due to the displacement of the user caused by the movement of the seat or the activity of the user sitting in the seat, the noise audible to the user may not be eliminated well sometimes.

[0009] Therefore, the technical problem of the present invention is to provide an active noise control system that eliminates noise audible to a user and is not easily affected by the user's displacement.

[0010] In order to solve the above-mentioned technical problems, the present invention relates to an active noise control system for reducing noise, in which: a first speaker that outputs a first elimination sound; a second speaker that outputs a second elimination sound; and an elimination sound generating unit are provided, which generates a first elimination sound output from the first speaker and a second elimination sound output from the second speaker in a manner such that noise is eliminated at a predetermined first elimination point and noise is eliminated at a predetermined second elimination point, and the first speaker and the second speaker are configured to be arranged in a direction perpendicular to a line segment connecting the first elimination point and the second elimination point, and the position in the direction of the line segment becomes a position between the first elimination point and the second elimination point.

[0011] Here, in such an active noise control system, it is preferred that the first speaker and the second speaker are arranged in a direction perpendicular to a line segment connecting the first cancellation point and the second cancellation point, and the position in the direction of the line segment becomes the same position as the midpoint of the first cancellation point and the second cancellation point.

[0012] Here, in the active noise control system as described above, the first cancellation point and the second cancellation point may be set as a point where the left ear of a person sitting in a predetermined seat is located at a standard position and a point where the right ear of the user is located at a standard position.

[0013] In this case, the predetermined seat may be a seat of a car, and the first speaker and the second speaker may be arranged side by side in the front-rear direction of the car on a ceiling in front of the seat of the car.

[0014] Here, the above active noise control system may also be provided with a first microphone in the cancellation sound generating unit; a second microphone; a first adaptive filter, which inputs a noise signal representing the noise and generates the first cancellation sound; and a second adaptive filter, which inputs a noise signal representing the noise and generates the second cancellation sound. Here, the first adaptive filter and the second adaptive filter use the input sounds from the first microphone and the second microphone to adapt their own transfer functions to the first cancellation sound output from the first speaker and the second cancellation sound output from the second speaker in such a way that the noise is cancelled at the first cancellation point and the noise is cancelled at the second cancellation point.

[0015] In addition, in this case, the cancellation sound generation unit includes a first auxiliary filter and a second auxiliary filter, and the first adaptive filter and the second adaptive filter are configured to update their own transfer functions by a predetermined adaptive algorithm using a difference between an input sound from the first microphone and an output of the first auxiliary filter and a difference between an input sound from the second microphone and an output of the second auxiliary filter as errors, and when transfer functions in which noise is cancelled at a first cancellation point and a second cancellation point are set for the first adaptive filter and the second adaptive filter, the first auxiliary filter and the second auxiliary filter are set to transfer functions that are learned as transfer functions in which the difference between the input sound from the first microphone and the output of the first auxiliary filter disappears and the difference between the input sound from the second microphone and the output of the second auxiliary filter disappears.

[0016] According to the active noise control system as described above, the phase (distance) of the first cancellation sound output from the first speaker and the phase (distance) of the second cancellation sound output from the second speaker can be set to a range near the first cancellation point which is the same as the first cancellation point, and the phase of the first cancellation sound output from the first speaker and the phase of the second cancellation sound output from the second speaker can be set to a relatively wide range near the second cancellation point which is the same as the second cancellation point, thereby achieving noise cancellation that is not easily affected by the user's displacement.

[0017] Effects of the Invention

[0018] As described above, according to the present invention, it is possible to provide an active noise control system that cancels noise audible to a user and is less susceptible to the influence of the user's displacement. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a block diagram showing the configuration of an active noise control system according to an embodiment of the present invention.

[0020] Figure 2 It is a diagram showing the arrangement of speakers and microphones in the active noise control system according to the embodiment of the present invention.

[0021] Figure 3 It is a block diagram showing the structure of a signal processing module according to an embodiment of the present invention.

[0022] Figure 4 It is a diagram showing the operation of the active noise control system according to the embodiment of the present invention.

[0023] Figure 5 It is a diagram showing another configuration example of the active noise control system according to the embodiment of the present invention.

[0024] Description of Reference Numerals

[0025] 1…active noise control system, 2…noise source, 11…signal processing module, 12…first speaker, 13…first microphone, 14…second speaker, 15…second microphone, 111…first signal processing unit, 112…second signal processing unit, 1111…first system auxiliary filter, 1112…first system variable filter, 1113…first system adaptive algorithm execution unit, 1114…first system first estimation filter, 1115…first system second estimation filter, 1116…first system subtractor, 1121…second system auxiliary filter, 1122…second system variable filter, 1123…second system adaptive algorithm execution unit, 1124…second system first estimation filter, 1125…second system second estimation filter, 1126…second system subtractor. DETAILED DESCRIPTION

[0026] Hereinafter, embodiments of the present invention will be described.

[0027] Figure 1 The configuration of the active noise control system according to the present embodiment is shown.

[0028] As shown in the figure, the active noise control system 1 includes a signal processing module 11 , a first speaker 12 , a first microphone 13 , a second speaker 14 , and a second microphone 15 .

[0029] Furthermore, the active noise control system 1 is a system installed in a car, and is a system that uses the position of the right ear of a user sitting in a specified seat of the car as a first cancellation point and the position of the left ear of the user as a second cancellation point, and cancels the noise generated by the noise source 2 at the two cancellation points.

[0030] Here, if Figure 2 As shown in a1 and a2 of FIG. 1 , the first speaker 12 and the second speaker 14 are arranged on the ceiling above and in front of the noise elimination target seat, with the seat where the user who is to be eliminated in the car sits (the right front seat in the figure) as the noise elimination target seat, along the front-to-back direction of the car. In addition, the first speaker 12 and the second speaker 14 are arranged in a manner such that their positions in the left-right direction of the car coincide with the position of the center of the noise elimination target seat in the left-to-right direction. In other words, in the present embodiment, the first speaker 12 and the second speaker 14 are arranged in a direction perpendicular to the line segment connecting the first elimination point and the second elimination point (the front-to-back direction of the car), and the position of the direction of the line segment (the left-to-right direction of the car) becomes the same position as the midpoint of the first elimination point and the second elimination point.

[0031] In addition, if Figure 2As shown in a1 and a2, the first microphone 13 is arranged on the ceiling above and in front of the standard position of the right ear of the user sitting in the noise cancellation target seat, and the second microphone 15 is arranged on the ceiling above and in front of the standard position of the left ear of the user sitting in the noise cancellation target seat.

[0032] Back to Figure 1 The signal processing module 11 of the active noise control system 1 uses the noise signal x(n) representing the noise generated by the noise source 2, the voice signal picked up by the first microphone 13, namely the first microphone error signal err1(n), and the voice signal picked up by the second microphone 15, namely the second microphone error signal err2(n), to generate a first cancellation signal CA1(n) and output it from the first speaker 12, and generates a second cancellation signal CA2(n) and outputs it from the second speaker 14.

[0033] Here, the first cancellation signal CA1(n) output from the first speaker 12 cancels the noise generated by the noise source 2 at the first cancellation point together with the second cancellation signal CA2(n) output from the second speaker 14. In addition, the second cancellation signal CA2(n) output from the second speaker 14 cancels the noise generated by the noise source 2 at the second cancellation point together with the first cancellation signal CA1(n) output from the first speaker 12.

[0034] then, Figure 3 The structure of the signal processing module 11 of the active noise control system 1 is shown.

[0035] The signal processing block 11 includes a first signal processing unit 111 that mainly performs processing related to generation of a first cancellation signal CA1 (n) and a second signal processing unit 112 that mainly performs processing related to generation of a second cancellation signal CA2 (n).

[0036] As shown in the figure, the first signal processing unit 111 includes: a first system auxiliary filter 1111 for which a transfer function H1(z) is preset, a first system variable filter 1112, a first system adaptive algorithm execution unit 1113, and a first system adaptive algorithm execution unit 1114 for which a transfer function S is preset. 11 ^ The first system first estimation filter 1114 of (z) is pre-set with a transfer function S 21 ^ (z) includes a first system second estimation filter 1115 and a first system subtractor 1116.

[0037] In the configuration of the first signal processing unit 111 as described above, the input noise signal x(n) passes through the first variable filter 1112 and is output to the first speaker 12 as the first cancellation signal CA1 (n).

[0038] In addition, the input noise signal x(n) passes through the first system auxiliary filter 1111 and is sent to the first system subtractor 1116. The first system subtractor 1116 subtracts the output of the first system auxiliary filter 1111 from the first microphone error signal err1(n) picked up by the first microphone 13, and outputs it as error e1 to the first system adaptive algorithm execution unit 1113 and the second signal processing unit 112.

[0039] Next, the first system variable filter 1112, the first system adaptive algorithm execution unit 1113, the first system first estimation filter 1114 and the first system second estimation filter 1115 constitute a Multiple Error Filtered-X adaptive filter. The first system first estimation filter 1114 is pre-set with a transfer function S from the first signal processing unit 111 to the first microphone 13 calculated by actual measurement or the like. 11 The estimated transfer characteristic S of (z) 11 ^ (z), the first system first estimation filter 1114 transfers the transfer characteristic S 11 ^ (z) is convolved with the input noise signal x(n) and then input to the first system adaptive algorithm execution unit 1113. In addition, the first system second estimation filter 1115 is pre-set with a transfer characteristic S representing the transfer function from the first signal processing unit 111 to the second microphone 15 calculated by actual measurement or the like. 21 The estimated transfer characteristic S of (z) 21 ^ (z), the first system second estimation filter 1115 transfers the transfer characteristic S 21 ^ (z) is convolved with the input noise signal x(n) and then input to the first system adaptive algorithm execution unit 1113.

[0040] Then, the first system adaptive algorithm execution unit 1113 inputs the convolution of the transfer function S in the first system first estimation filter 1114. 11 ^ The noise signal x(n) of (z) is convolved with the transfer function S by the second estimation filter 1115 of the first system 21 ^ The transfer function W1(z) of the first system variable filter 1112 is updated by executing an adaptive algorithm such as NLMS in such a way that the errors e1 and e2 become zero.

[0041] The second signal processing unit 112 also has the same structure as the first signal processing unit 111. The second signal processing unit 112 includes a second system auxiliary filter 1121 with a transfer function H2(z) preset, a second system variable filter 1122, a second system adaptive algorithm execution unit 1123, and a transfer function S2(z) preset. 12 ^ The second system first estimation filter 1124 of (z) is pre-set with a transfer function S 22 ^ (z) a second system second estimation filter 1125 and a second system subtractor 1126.

[0042] In such a configuration of the second signal processing unit 112 , the input noise signal x(n) passes through the second variable filter 1122 and is output to the second speaker 14 as the second cancellation signal CA2 (n).

[0043] In addition, the input noise signal x(n) is sent to the second system subtractor 1126 after passing through the second system auxiliary filter 1121. The second system subtractor 1126 subtracts the output of the second system auxiliary filter 1121 from the second microphone error signal err2(n) picked up by the second microphone 15, and uses it as the error e2 to execute the second system adaptive algorithm unit 1123 and the first signal processing unit 111.

[0044] Next, the second system variable filter 1122, the second system adaptive algorithm execution unit 1123, the second system first estimation filter 1124 and the second system second estimation filter 1125 constitute a Multiple Error Filtered-X adaptive filter. The transfer function S from the second signal processing unit 112 to the second microphone 15 calculated by actual measurement is pre-set in the second system first estimation filter 1124. 22 The estimated transfer characteristic S of (z) 22 ^ (z), the second system first estimation filter 1124 transfers the transfer characteristic S 22 ^ (z) is convolved with the input noise signal x(n) and then input to the second system adaptive algorithm execution unit 1123. In addition, the transfer characteristic S representing the transfer function from the second signal processing unit 112 to the first microphone 13 calculated by actual measurement is preset in the second system second estimation filter 1125. 12 The estimated transfer characteristic S of (z) 12 ^ (z), the second system second estimation filter 1125 transfers the transfer characteristic S 12(z) is convolved with the input noise signal x(n) and then input to the second system adaptive algorithm execution unit 1123.

[0045] Then, the second system adaptive algorithm execution unit 1123 convolves the transfer function S 22 ^ The noise signal x(n) of (z) is convolved with the transfer function S by the second estimation filter 1125 of the second system 12 ^ The noise signal x(n) of the second system variable filter 1122, the error e2 output from the second system subtractor 1126, and the error e1 output from the first signal processing unit 111 are taken as inputs, and an adaptive algorithm such as NLMS is executed to update the transfer function W2(z) of the second system variable filter 1122 in such a way that the errors e1 and e2 become zero.

[0046] Here, the first system auxiliary filter 1111 of the first signal processing unit 111 is provided to correct the first microphone error signal err1(n) by the difference between the position of the first microphone 13 and the first elimination point, and the second system auxiliary filter 1121 of the second signal processing unit 112 is provided to correct the second microphone error signal err2(n) by the difference between the position of the second microphone 15 and the second elimination point.

[0047] In addition, the transfer function H1(z) set in the first system auxiliary filter 1111 of the first signal processing unit 111 and the transfer function H2(z) set in the second system auxiliary filter 1121 of the second signal processing unit 112 are transfer functions set after pre-learning. For the transfer functions obtained by configuring microphones for learning at the first elimination point and the second elimination point in the learning environment, in which each noise is eliminated at the first elimination point and the second elimination point, the transfer function H1(z) and the transfer function H2(z) are set so that the error e1 output by the first system subtractor 1116 and the error e2 output by the second system subtractor 1126 become 0, which are obtained when the transfer functions of the first system variable filter 1112 and the second system variable filter 1122 are fixed.

[0048] In addition, the transfer functions at which each noise is eliminated at the first elimination point and the second elimination point of the first system variable filter 1112 and the second system variable filter 1122 change from the transfer functions under the environment during learning according to changes in the environment, and the changes are reflected in the transfer functions of the first system variable filter 1112 and the second system variable filter 1122 by the above-mentioned update of the transfer functions based on adaptive algorithms such as NLMS.

[0049] In addition, here, it can be considered that the noise is propagated in the range near the first cancellation point in the same way as the first cancellation point. Therefore, at the position in the range near the first cancellation point where the phase (distance) of the first cancellation signal CA1(n) output from the first speaker 12 and the phase (distance) of the second cancellation signal CA2(n) output from the second speaker 14 are the same as those of the first cancellation point, the relationship between the noise, the first cancellation signal CA1(n) and the second cancellation signal CA2(n) becomes the same as that of the first cancellation point, and the effect of noise cancellation can be expected.

[0050] Similarly, in the range near the second cancellation point, it can be considered that noise is propagated in the same way as at the second cancellation point. Therefore, at a position in the range near the second cancellation point where the phase of the first cancellation signal CA1(n) output from the first speaker 12 and the phase of the second cancellation signal CA2(n) output from the second speaker 14 are the same as at the second cancellation point, the relationship between the noise, the first cancellation signal CA1(n) and the second cancellation signal CA2(n) becomes the same as at the second cancellation point, and a noise cancellation effect can be expected.

[0051] In addition, as described above, in the present embodiment, the first speaker 12 and the second speaker 14 are arranged in a direction perpendicular to the line segment connecting the first elimination point and the second elimination point (the front-rear direction of the vehicle), and the position of the direction of the line segment (the left-right direction of the vehicle) is arranged in the same position as the midpoint of the first elimination point and the second elimination point.

[0052] And, in the setting Figure 4 In a, 41 is a first cancellation point, 42 is a second cancellation point, and when the phase of the first cancellation signal CA1(n) is the same range between adjacent circles of the concentric circle centered on the first speaker 12, and the phase of the second cancellation signal CA2(n) is the same range between adjacent circles of the concentric circle centered on the second speaker 14, when schematically represented in two dimensions, the range in which the phase of the first cancellation signal CA1(n) output from the first speaker 12 and the phase of the second cancellation signal CA2(n) output from the second speaker 14 become the same as the first cancellation point 41, and the range in which the phase of the first cancellation signal CA1(n) output from the first speaker 12 and the phase of the second cancellation signal CA2(n) output from the second speaker 14 become the same as the second cancellation point 42 are both ranges 400 surrounded by thick lines.

[0053] Therefore, within the range 400, in the vicinity of the first elimination point 41, where noise is propagated in the same manner as the first elimination point 41, the same noise elimination effect as the first elimination point 41 can be achieved, and within the range 400, in the vicinity of the second elimination point 42, where noise is propagated in the same manner as the second elimination point 42, the same noise elimination effect as the second elimination point 42 can be achieved.

[0054] On the other hand, assuming that the first speaker 12 is arranged in the left-right direction of the car at a position in front of the right ear of the user sitting in the noise cancellation target seat as the first cancellation point, and the second speaker 14 is arranged at a position in front of the left ear of the user sitting in the noise cancellation target seat as the second cancellation point, Figure 4 As shown in FIG. 1 , the phase of the first cancellation signal CA1(n) output from the first speaker 12 and the phase of the second cancellation signal CA2(n) output from the second speaker 14 are the same as the first cancellation point 41, which is a range 412 surrounded by a thick line. The ranges 411 and 412 are both Figure 4 The range 400 is a narrow range when the first speaker 12 and the second speaker 14 are arranged as shown in a.

[0055] Therefore, as in the present embodiment, the first speaker 12 and the second speaker 14 are arranged in a direction perpendicular to the line segment connecting the first cancellation point and the second cancellation point and the position in the direction of the line segment is the same as the midpoint of the first cancellation point and the second cancellation point, thereby being able to cancel the noise audible to the user in a manner that is not easily affected by the user's displacement.

[0056] in addition, Figure 4 The distances between the first speaker 12, the second speaker 14 and the first cancellation point 41, the second cancellation point 42 in the front-rear direction of the car are set to Figure 4 The average distances among the first speaker 12, the second speaker 14, the first cancellation point 41, and the second cancellation point 42 in the front-rear direction of the automobile.

[0057] The embodiments of the present invention have been described above.

[0058] Here, in the above embodiment, the positions of the first speaker 12 and the second speaker 14 in the direction of the line segment connecting the first elimination point and the second elimination point may not necessarily be strictly set to the same position as the midpoint of the first elimination point and the second elimination point. The positions of the first speaker 12 and the second speaker 14 in the direction of the line segment may also be set to any position between the first elimination point and the second elimination point in the direction of the line segment. Even so, a certain degree of effect can be foreseen.

[0059] In addition, the above embodiment is described for the case where noise cancellation is performed for a user in one seat of a car, but it can also be performed as follows. Figure 5 As shown in a and b of FIG. 1 , a first speaker 12, a first microphone 13, a second speaker 14, and a second microphone 15 are provided for each seat of the car, and noise cancellation is performed for the user in each seat.

[0060] In addition, in the above embodiment, the noise signal x(n) input to the active noise control system 1 can also be an audio signal output by the noise source 2, a sound signal obtained by picking up the noise of the noise source by a separately arranged noise microphone, or a signal simulating the noise of the noise source generated by a separately arranged simulated sound generating device.

[0061] That is, for example, when the engine is used as the noise source 2, the engine sound picked up by a noise microphone can be used as the noise signal x(n), or the simulated sound generated by simulating the engine sound using a simulated sound generating device provided separately can be used as the noise signal x(n).

[0062] In addition, in the signal processing module 11 of the above embodiment, any signal processing different from the signal processing shown above can be performed as long as the first cancellation signal CA1(n) is generated and output from the first speaker 12, and the second cancellation signal CA2(n) is generated and output from the second speaker 14 in a manner that noise is cancelled at both the first cancellation point and the second cancellation point.

[0063] In addition, the above embodiment describes the case of performing noise cancellation for users in car seats, but it can also be applied to the case of performing noise cancellation for both ears of users in any seats of the car, and the case of performing noise cancellation at any two cancellation points.

[0064] Furthermore, the above embodiment shows the case where there is only one noise source, but the above embodiment can also be applied to the case where there are multiple noise sources by expanding the configuration of the signal processing module 11 to consider the propagation of each noise source to each cancellation point.

Claims

1. An active noise control system for reducing noise, characterized in that: have: A first speaker outputs a first cancellation sound; a second speaker, outputting a second cancellation sound; and a cancellation sound generating unit for generating a first cancellation sound output from the first speaker and a second cancellation sound output from the second speaker in such a manner that noise is cancelled at a preset first cancellation point and noise is cancelled at a preset second cancellation point, The first speaker and the second speaker are arranged in a direction perpendicular to a line segment connecting the first cancellation point and the second cancellation point, and their positions in the direction of the line segment are the same as a midpoint between the first cancellation point and the second cancellation point. The first cancellation point and the second cancellation point are a point where the left ear of a person sitting at a predetermined seat is located at a standard position and a point where the right ear of the person is located at a standard position.

2. The active noise control system according to claim 1, characterized in that: The specified seat is a seat of a car, The first speaker and the second speaker are arranged side by side in a front-rear direction of the vehicle on a ceiling in front of the seat of the vehicle.

3. The active noise control system according to claim 2, characterized in that: The cancellation sound generating unit includes: a first microphone; a second microphone; a first adaptive filter that receives a noise signal representing the noise and generates the first cancellation sound; and a second adaptive filter that receives a noise signal representing the noise and generates the second cancellation sound. The first adaptive filter and the second adaptive filter use the input sounds from the first microphone and the second microphone to adapt their own transfer functions to the first cancellation sound output from the first speaker and the second cancellation sound output from the second speaker in such a way that the noise is cancelled at the first cancellation point and the noise is cancelled at the second cancellation point.

4. The active noise control system according to claim 3, characterized in that: The cancellation sound generation unit includes a first auxiliary filter and a second auxiliary filter. The first adaptive filter and the second adaptive filter use the difference between the input sound from the first microphone and the output of the first auxiliary filter and the difference between the input sound from the second microphone and the output of the second auxiliary filter as errors, and update their own transfer functions through a predetermined adaptive algorithm. When the transfer functions in which noise is cancelled at a first cancellation point and a second cancellation point are set for the first adaptive filter and the second adaptive filter, the first auxiliary filter and the second auxiliary filter are set to the following transfer functions, which are learned as transfer functions in which the difference between the input sound from the first microphone and the output of the first auxiliary filter disappears, and the difference between the input sound from the second microphone and the output of the second auxiliary filter disappears.

5. The active noise control system according to claim 1, characterized in that: The cancellation sound generating unit includes: a first microphone; a second microphone; a first adaptive filter that receives a noise signal representing the noise and generates the first cancellation sound; and a second adaptive filter that receives a noise signal representing the noise and generates the second cancellation sound. The first adaptive filter and the second adaptive filter use the input sounds from the first microphone and the second microphone to adapt their own transfer functions to the first cancellation sound output from the first speaker and the second cancellation sound output from the second speaker in such a way that the noise is cancelled at the first cancellation point and the noise is cancelled at the second cancellation point.

6. The active noise control system according to claim 5, characterized in that: The cancellation sound generation unit includes a first auxiliary filter and a second auxiliary filter. The first adaptive filter and the second adaptive filter use the difference between the input sound from the first microphone and the output of the first auxiliary filter and the difference between the input sound from the second microphone and the output of the second auxiliary filter as errors, and update their own transfer functions through a predetermined adaptive algorithm. When the transfer functions in which noise is cancelled at a first cancellation point and a second cancellation point are set for the first adaptive filter and the second adaptive filter, the first auxiliary filter and the second auxiliary filter are set to the following transfer functions, which are learned as transfer functions in which the difference between the input sound from the first microphone and the output of the first auxiliary filter disappears, and the difference between the input sound from the second microphone and the output of the second auxiliary filter disappears.

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