Patent Publication Number: US-2023139520-A1

Title: Sound system

Description:
TECHNICAL FIELD 
     The present invention relates to a technology for reducing noise in an aircraft. 
     BACKGROUND ART 
     Conventionally, when users are annoyed with noise while viewing a movie, listening to music, or the like in an aircraft, the users have used earphones or headphones including a noise canceling function (see Non-Patent Literature 1). 
     CITATION LIST 
     Non-Patent Literature 
     
         
         Non-Patent Literature 1: Inflight Entertainment/JAL First Class, [online], [retrieved on Mar. 16, 2020], Internet &lt;URL: https://www.jal.co.jp/jp/ja/inter/service/first/entertain ment/index.html&gt; 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, wearing earphones or headphones makes users uncomfortable. Moreover, some users do not like wearing such devices because hairstyles become untidy. There are also some users who dislike pressure on ears caused by wearing the devices. Further, a long period of wearing earphones or headphones may make users tired of hearing in some cases. 
     Accordingly, an object of the present invention is to provide a technology that reduces noise heard when a user sits in a seat of an aircraft, without using earphones or headphones. 
     Means for Solving the Problem 
     An aspect of the present invention includes: a control system that generates a control signal for canceling noise in a place close to a head of a user using a seat of an aircraft, from a signal of the noise (hereinafter, referred to as the noise signal); and a noise-cancellation speaker system including M speaker units that emit sound based on the control signal (hereinafter, referred to as the noise-cancellation speaker units), M being an integer equal to or larger than one, the noise-cancellation speaker system being installed at the place close to the head of the user using the seat, wherein assuming that the M noise-cancellation speaker units are a first noise-cancellation speaker unit, . . . , and an M-th noise-cancellation speaker unit, and assuming that a direction in which the m-th noise-cancellation speaker unit faces the user is an m-th noise-cancellation user direction (m=1, . . . , M), the m-th noise-cancellation speaker unit (m=1, . . . , M) is disposed such that sound emitted from the m-th noise-cancellation speaker unit in the m-th noise-cancellation user direction is canceled in a place other than the place close to the head of the user using the seat, due to bending around of sound emitted from the m-th noise-cancellation speaker unit in an opposite direction to the m-th noise-cancellation user direction. 
     Effects of the Invention 
     According to the present invention, it is possible to reduce noise heard when a user sits in a seat of an aircraft. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    shows an example of a configuration of an active noise control system. 
         FIG.  2    is a view for describing directivity of sound emitted from a speaker unit. 
         FIG.  3    shows an example of a sound system installed in a seat of an aircraft. 
         FIG.  4    is a block diagram showing an example of a configuration of a sound system  500 . 
         FIG.  5    shows an example of disposition of reference microphones and error microphones. 
         FIG.  6    is a block diagram showing an example of a configuration of a sound system  501 . 
         FIG.  7    is a block diagram showing an example of a configuration of a sound system  502 . 
         FIG.  8    shows an example of a configuration of a noise-cancellation speaker system  520  to which a member  5222  is attached. 
         FIG.  9    shows an example of a sound system installed in a seat of an aircraft. 
         FIG.  10    is a block diagram showing an example of a configuration of a sound system  1000 . 
         FIG.  11    is a block diagram showing an example of a configuration of a sound system  100 . 
         FIG.  12    is a block diagram showing an example of a configuration of a sound system  102 . 
         FIG.  13    shows an example of a configuration of a speaker unit pair  122  to which a member  1222  is attached. 
         FIG.  14    is a block diagram showing an example of a configuration of a sound system  104 . 
         FIG.  15    is a block diagram showing an example of a configuration of a sound system  200 . 
         FIG.  16    is a block diagram showing an example of a configuration of a sound system  300 . 
         FIG.  17    is a block diagram showing an example of a configuration of a sound system  106 . 
         FIG.  18    shows an example of a configuration of the speaker unit pair  122  to which a member  1224  is attached. 
         FIG.  19    is a block diagram showing an example of a configuration of a sound system  108 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail. Note that components having the same functions are denoted by the same reference signs, and an overlapping description is omitted. 
     TECHNICAL BACKGROUND 
     Active noise control, which is one of noise reduction technologies, is used to reduce noise heard when a user sits in a seat of an aircraft (see Reference Non-Patent Literatures 1, 2). 
     (Reference Non-Patent Literature 1: ANC noise reduction testing system, [online], [retrieved on Mar. 16, 2020], Internet &lt;URL: https://micronet.jp/product/anc/index.html&gt;) 
     (Reference Non-Patent Literature 2: Active Noise Control (The Institute of Electronics, Information and Communication Engineers “Chishiki-no-Mori”, Group 2-Edition 6—Chapter 6), [online], [retrieved on Mar. 16, 2020], Internet &lt;URL: http://www.ieice-hbkb.org/files/02/02gun_06hen_06.pdf&gt;) 
       FIG.  1    shows an example of a configuration of an active noise control system. The active noise control system includes a microphone that picks up noise (hereinafter, referred to as reference microphone), a controller that generates, from a signal outputted from the reference microphone (hereinafter, referred to as noise signal), a control signal for canceling the noise, and a noise-cancellation speaker for emitting sound based on the control signal. Note that the active noise control system may further include a microphone that picks up sound that cannot be canceled out (hereinafter, referred to as error microphone), and may be configured such that a signal outputted from the error microphone (hereinafter, referred to as error signal) is fed back to the controller and the controller generates a control signal by using also the error signal. 
     Main components of noise in an aircraft concentrate at low frequencies. Accordingly, unless the noise-cancellation speaker has a correspondingly large size, low-frequency sound cannot sufficiently be reproduced, so that a satisfactory noise reduction effect cannot be obtained. However, the larger the noise-cancellation speaker is in size, the more difficult it is to use the noise-cancellation speaker in a seat of an aircraft. Moreover, if sound from the noise-cancellation speaker is picked up by the reference microphone, a problem arises that a signal of the sound from the noise-cancellation speaker is included in a noise signal, resulting in noise reduction performance being degraded. 
     To solve these two problems, the invention of the present application uses a speaker unit instead of using a speaker, to emit sound based on a control signal (see  FIG.  2   ). 
     Generally, a speaker includes a speaker unit and a speaker box. The speaker unit is a component including a diaphragm that converts a sound signal, which is an electrical signal, into aerial vibration (that is, generates sound waves). The speaker box is a component that houses the speaker unit. 
     When a sound signal is inputted into the speaker, the diaphragm of the speaker unit vibrates, and sound waves are emitted in both directions in which the diaphragm vibrates. Here, a sound wave emitted to the outside of the speaker box (that is, in a direction toward the front of the speaker unit) is referred to as positive sound wave, and a sound wave emitted to the inside of the speaker box (that is, in a direction toward the rear of the speaker unit) is referred to as negative sound wave. The negative sound wave is a sound wave with opposite phase to phase of the positive sound wave. When the speaker is used, while positive sound waves are emitted from the speaker in all directions, negative sound waves do not go out of the speaker box. In contrast, when only the speaker unit is used, negative sound waves are also emitted because there is no speaker box. In the latter case, since the positive sound waves and the negative sound waves have an antiphase relationship, the positive sound waves and the negative sound waves cancel each other out. However, in the vicinity of the speaker unit, positive sound waves remain because negative sound waves that bend around do not reach in time. If the remaining positive sound waves and noise have an antiphase relationship, the positive sound waves cancel the noise out, whereby a noise reduction effect can be obtained in the vicinity of the speaker unit. 
     In other words, if the speaker unit is installed in a form without using the speaker box at a place close to an ear of a user, noise reduction in an aircraft can be achieved. Moreover, since an area where the positive sound waves remain is limited to a relatively small area such as the vicinity of the speaker unit, bending around of sound to the reference microphone is restrained, and degradation in noise reduction performance can also be restrained. 
     The form in which only the speaker unit is installed has a merit that an installation space can be minimized because no speaker box is used. Moreover, in addition to such a merit, the form in which only the speaker unit is installed also has a merit that lower-frequency sound can be reproduced than in a form in which the speaker unit is installed in combination with the speaker box. The reason will be described below. Generally, when the speaker unit is housed in the speaker box, negative sound waves do not go out of the speaker box. However, when the speaker unit is housed in the speaker box, aerial vibration caused by the negative sound waves that are enclosed in the speaker box and cannot go anywhere constrains a cone of the speaker unit and hinders next vibration of the cone. As a result, low-frequency sound cannot be reproduced even if the speaker unit is housed in the speaker box. It is therefore conceivable that an inside of the speaker box is filled with a sound absorption member. However, such a method does not bring about a satisfactory effect with respect to low-frequency sound, so that the speaker box needs to be large in size to some extent in order to reproduce low-frequency sound. In other words, after all, use of the speaker box that is small enough to be installable in a seat in an aircraft cannot gain sound at sufficiently low frequencies. 
     First Embodiment 
     A system that reproduces a sound signal is referred to as a sound system. The sound system includes a speaker system in order to emit the sound signal as sound (hereinafter, such sound will be referred to as sound based on a sound signal). Here, the speaker system is a device that converts the sound signal, which is an analog signal, into sound. Moreover, the sound signal to be reproduced by the sound system is a sound signal obtained from, for example, data recorded on a CD, a DVD, or a record, data received over the Internet, or a signal received through radio broadcasting or television broadcasting. 
     Hereinafter, a description will be given of a sound system that reduces noise around a user who is present in the vicinity of a speaker system by reproducing a control signal generated from a noise signal obtained from the noise. If such a sound system is utilized for, for example, a sound system for a user using a seat of an aircraft, a system that can reduce noise around the user using the seat can be provided.  FIG.  3    shows an example of the sound system installed in a seat of an aircraft. The sound system in  FIG.  3    is disposed such that a speaker system is positioned in the vicinity of a head of a user sitting in the seat. Note that such a sound system can also be installed on vehicles other than aircrafts, such as automobiles and trains, and in chairs used in houses and commercial facilities, and can also be installed in a wearable form to be put on a shoulder or the like. Moreover, a driver unit pair including two driver units arranged side by side, which corresponds to a pair of the above-mentioned speaker units, may be installed in each of right and left units of headphones or earphones. 
     Hereinafter, a sound system  500  will be described with reference to  FIG.  4   .  FIG.  4    is a block diagram showing a configuration of the sound system  500 . As shown in  FIG.  4   , the sound system  500  includes a control system  510  and a noise-cancellation speaker system  520 . The control system  510  includes K (K is an integer equal to or larger than one) reference microphones  511 , L (L is an integer equal to or larger than zero) error microphones  512 , and a control signal generation apparatus  514 . Here, the reference microphones and the error microphones are, as described in the &lt;Technical background&gt; section, microphones that pick up noise, and microphones that pick up sound that cannot be canceled out, respectively. The error microphones are used for filter updating, and are used in many practical cases. Note that the minimum number of the microphones required for the sound system  500  is one (when K=1, L=0).  FIG.  5    shows disposition of the reference microphones and the error microphones when K=4, L=2. It is preferable that the reference microphones be disposed at places where noise arrives earlier than the noise arrives at the error microphones, as seen in  FIG.  5   . Moreover, ideally, the error microphones had better be disposed at a position of an ear of a user, and it is therefore preferable that the error microphones be disposed at places as close to the ears as possible. The noise-cancellation speaker system  520  includes one noise-cancellation speaker unit  5221  that is a speaker unit emitting sound based on a control signal. The noise-cancellation speaker system  520  is installed at a place close to a head of the user using the seat. 
     Note that a direction in which the noise-cancellation speaker unit  5221  faces the user will be referred to as a noise-cancellation user direction. The noise-cancellation speaker unit  5221  is disposed such that sound emitted from the noise-cancellation speaker unit  5221  in the noise-cancellation user direction is canceled in places other than the place close to the head of the user using the seat, due to bending around of sound emitted from the noise-cancellation speaker unit  5221  in an opposite direction to the noise-cancellation user direction. Here, the noise-cancellation user direction is a direction toward the front of the noise-cancellation speaker unit  5221 . Moreover, the opposite direction to the noise-cancellation user direction is a direction toward the rear of the noise-cancellation speaker unit  5221 . 
     Hereinafter, operation of the sound system  500  will be described, according to  FIG.  4   . 
     The control system  510  generates a control signal for canceling noise in the place close to the head of the user using the seat of the aircraft, from a signal obtained from the noise (hereinafter, referred to as noise signal), and outputs the control signal. More specifically, the reference microphones  511  pick up noise in the place close to the head of the user using the seat of the aircraft, and outputs a noise signal that is obtained by converting the noise into an electrical signal. The error microphones  512  pick up sound that cannot be canceled out in a place extremely close to the head of the user, and outputs an error signal that is obtained by converting the sound that cannot be canceled out into an electrical signal. The control signal generation apparatus  514  receives the noise signal and the error signal as inputs, generates a control signal from the noise signal by using the error signal, and outputs the control signal. The control signal may be a signal with almost the same amplitude as and opposite phase to the noise signal. 
     The noise-cancellation speaker system  520  receives the control signal outputted by the control system  510  as an input, and emits sound based on the control signal. More specifically, the noise-cancellation speaker unit  5221  receives the control signal as an input, and emits sound based on the control signal. 
     According to the embodiment of the present invention, it is possible to reduce noise heard when a user sits in a seat of an aircraft. 
     Second Embodiment 
     The sound system  500  in the first embodiment uses only one noise-cancellation speaker unit, and an area where noise is reduced is therefore small. Here, a description will be given of a sound system including two or more noise-cancellation speaker units in order that the noise-cancellation speaker units can be installed at places respectively close to both ears of a user. 
     Hereinafter, a sound system  501  will be described with reference to  FIG.  6   .  FIG.  6    is a block diagram showing a configuration of the sound system  501 . As shown in  FIG.  6   , the sound system  501  includes a control system  510  and a noise-cancellation speaker system  520 , similarly to the sound system  500 . However, the sound system  501  is different from the sound system  500  in a point that the noise-cancellation speaker system  520  includes M (M is an integer equal to or larger than two) noise-cancellation speaker units  5221 . The M noise-cancellation speaker units  5221  receive the same control signal, as respective inputs. 
     Hereinafter, the M noise-cancellation speaker units will be referred to as first noise-cancellation speaker unit, . . . , and M-th noise-cancellation speaker unit. Moreover, a direction in which an m-th noise-cancellation speaker unit faces the user will be referred to as m-th noise-cancellation user direction (m=1, . . . , M), and the m-th noise-cancellation speaker unit (m=1, . . . , M) is disposed such that sound emitted from the m-th noise-cancellation speaker unit in the m-th noise-cancellation user direction is canceled in places other than a place closed to a head of the user using the seat, due to bending around of sound emitted from the m-th noise-cancellation speaker unit in an opposite direction to the m-th noise-cancellation user direction. Here, the m-th noise-cancellation user direction is a direction toward the front of the m-th noise-cancellation speaker unit  5221 . The opposite direction to the m-th noise-cancellation user direction is a direction toward the rear of the m-th noise-cancellation speaker unit  5221 . 
     Hereinafter, operation of the noise-cancellation speaker system  520  will be described, according to  FIG.  6   . 
     The noise-cancellation speaker system  520  receives a control signal outputted by the control system  510  as an input, and emits sound based on the control signal. More specifically, the m-th noise-cancellation speaker unit  5221  (m=1, . . . , M) receives the control signal as an input, and emits sound based on the control signal. 
     Note that although M is an integer equal to or larger than two here, the configuration corresponds to the first embodiment when M=1. 
     According to the embodiment of the present invention, it is possible to reduce noise heard when a user sits in a seat of an aircraft. 
     Third Embodiment 
     According to the sound system  501  in the second embodiment, the area where noise is reduced is enlarged by increasing the number of noise-cancellation speaker units. Here, a description will be given of a sound system having a structure that enlarges the area where noise is reduced, with one noise-cancellation speaker unit. 
     Hereinafter, a sound system  502  will be described with reference to  FIG.  7   .  FIG.  7    is a block diagram showing a configuration of the sound system  502 . As shown in  FIG.  7   , the sound system  502  includes a control system  510  and a noise-cancellation speaker system  520 , similarly to the sound system  501 . However, the sound system  502  is different from the sound system  501  in a point that a member  5222  is attached to an m-th noise-cancellation speaker unit  5221  (m=1, . . . , M). 
     Hereinafter, a structure of the m-th noise-cancellation speaker unit  5221  (m=1, . . . , M) will be described, according to  FIG.  7   . 
     The member  5222  is attached to the m-th noise-cancellation speaker unit  5221  (see  FIG.  8   ). The member  5222  is for lengthening a path of sound bending around in the m-th noise-cancellation user direction, of sound emitted from the m-th noise-cancellation speaker unit  5221  in the opposite direction to the m-th noise-cancellation user direction. For example, the member  5222  may be a member such as a partition plate that prevents sound from bending around from the rear of the speaker unit. The member  5222  is attached in order to prevent interference of sound waves and to enlarge the area where noise is reduced. 
     The m-th noise-cancellation speaker unit  5221  to which the member  5222  is attached reduces noise in a larger area than the m-th noise-cancellation speaker unit  5221  in the second embodiment. 
     According to the embodiment of the present invention, it is possible to reduce noise heard when a user sits in a seat of an aircraft. 
     Fourth Embodiment 
     In the first to third embodiments, the description has been given of the sound systems (noise reduction sound systems) for reducing noise around a user using a seat of an aircraft. The noise reduction sound systems can be combined with a sound system (reproduction sound system) that reproduces sound based on a sound signal obtained from a subject to be reproduced such that the sound can be heard only by a user in the vicinity of a speaker system. Here, the subject to be reproduced is, for example, data or a signal from which the sound signal can be obtained through predetermined processing, such as data recorded on a CD, a DVD, or a record, data received over the Internet, or a signal received through radio broadcasting or television broadcasting. 
     An example of a sound system configured by combining a noise reduction sound system and a reproduction sound system is shown in  FIG.  9   .  FIG.  9    shows an example of the sound system installed in a seat of an aircraft. A speaker system of the reproduction sound system in  FIG.  9    is installed in the seat so as to interpose a head of a user sitting in the seat in between, and is disposed such that speaker unit pairs are positioned in the vicinities of the right and left ears. On the other hand, a noise-cancellation speaker system of the noise reduction sound system is installed in the seat so as to be positioned at the back of the head of the user sitting in the seat. Note that the sound system can also be installed on vehicles other than aircrafts, such as automobiles and trains, and in reclining chairs and the like, and can also be installed in a wearable form to be put on a shoulder or the like. Moreover, similarly to the noise reduction sound system, the reproduction sound system may have a configuration in which a driver unit pair including two driver units arranged side by side, which corresponds to a pair of the above-described speaker units, is installed in each of right and left units of headphones or earphones. The headphones are broadly divided into open-air type and closed type in general, and when the above-described technique is applied particularly to the open-air type, sound leakage from which is a concern, a reduction in leaking sound can be expected. 
     Hereinafter, a sound system  1000  will be described with reference to  FIG.  10   .  FIG.  10    is a block diagram showing a configuration of the sound system  1000 . The sound system  1000  includes a noise reduction sound system and a reproduction sound system. The noise reduction sound system can be the sound system  500 , the sound system  501 , or the sound system  502 . On the other hand, the reproduction sound system can be a sound system  100 , a sound system  102 , a sound system  104 , a sound system  106 , a sound system  108 , a sound system  200 , or a sound system  300 , which will be described below. 
     Hereinafter, each form of the reproduction sound system will be described. 
     &lt;&lt;Form 1: Sound System  100 &gt;&gt; 
     Hereinafter, the sound system  100  will be described with reference to  FIG.  11   .  FIG.  11    is a block diagram showing a configuration of the sound system  100 . As shown in  FIG.  11   , the sound system  100  includes a reproduction apparatus  110  and a speaker system  120 . The reproduction apparatus  110  includes N (N is an integer equal to or larger than one) reproduction units  112  (that is, a first reproduction unit  112 , . . . , an N-th reproduction unit  112 ). The speaker system  120  includes N speaker unit pairs  122  (that is, a first speaker unit pair  122 , . . . , an N-th speaker unit pair  122 ). Each speaker unit pair  122  includes two speaker units (that is, a positive speaker unit  1221  and a negative speaker unit  1221 ). The negative speaker unit  1221  receives, as an input, a sound signal with opposite phase to that of a sound signal inputted into the positive speaker unit  1221 . The speaker system  120  is installed at a place close to a head of a user using the seat. 
     Note that a direction in which an n-th speaker unit pair  122  faces the user is referred to as an n-th user direction (n=1, . . . , N). The positive speaker unit  1221  and the negative speaker unit  1221  of the n-th speaker unit pair  122  (n=1, . . . , N) are disposed such that sound emitted from the positive speaker unit  1221  in an opposite direction to the n-th user direction and sound emitted from the negative speaker unit  1221  in the opposite direction to the n-th user direction propagate in the n-th user direction due to bending around of the sound. Here, the n-th user direction is a direction toward the front of the positive speaker unit  1221  and the negative speaker unit  1221  of the n-th speaker unit pair  122 . The opposite direction to the n-th user direction is a direction toward the rear of the positive speaker unit  1221  and the negative speaker unit  1221  of the n-th speaker unit pair  122 . 
     Moreover, the positive speaker unit  1221  and the negative speaker unit  1221  of the n-th speaker unit pair  122  (n=1, . . . , N) are disposed in a positional relationship in which sound emitted from the positive speaker unit  1221  and sound emitted from the negative speaker unit  1221  cancel each other out so that the sound cannot be heard by users using other seats. 
     Hereinafter, operation of the sound system  100  will be described, according to  FIG.  11   . 
     The reproduction apparatus  110  receives, as inputs, a first sound signal, a third sound signal, . . . , and a (2N−1)-th sound signal that are sound signals obtained based on a subject to be reproduced, and outputs the first sound signal, a second sound signal, . . . , and a 2N-th sound signal. More specifically, an n-th reproduction unit  112  (n=1, . . . , N) receives a (2n−1)-th sound signal as an input, generates, from the (2n−1)-th sound signal, a 2n-th sound signal that is a sound signal with opposite phase to that of the (2n−1)-th sound signal, and outputs the (2n−1)-th sound signal and the 2n-th sound signal. The (2n−1)-th sound signal and the 2n-th sound signal are inputted into the positive speaker unit  1221  and the negative speaker unit  1221  of the n-th speaker unit pair  122 , respectively. 
     The speaker system  120  receives, as inputs, the first sound signal, the second sound signal, . . . , and the 2N-th sound signal outputted by the reproduction apparatus  110 , and emits sound based on the first sound signal, sound based on the second sound signal, . . . , and sound based on the 2N-th sound signal. More specifically, the n-th speaker unit pair  122  (n=1, . . . , N) receives the (2n−1)-th sound signal and the 2n-th sound signal as inputs, and emits sound based on the (2n−1)-th sound signal from the positive speaker unit  1221 , and emits sound based on the 2n-th sound signal from the negative speaker unit  1221 . Since the (2n−1)-th sound signal and the 2n-th sound signal are in an antiphase relationship with each other, sound is heard only in the vicinity of the seat in which the speaker system  120  is installed. For example, when N=2, and assuming that the first sound signal and the third sound signal are sound signals from a right channel and a left channel of a sound source, respectively, stereo sound can be heard only in the vicinity of the seat in which the speaker system  120  is installed. 
     Note that the sound emitted from the positive speaker unit  1221  of the n-th speaker unit pair  122  in the n-th user direction and the sound emitted from the positive speaker unit  1221  of the n-th speaker unit pair  122  in the opposite direction to the n-th user direction are in an antiphase relationship. Similarly, the sound emitted from the negative speaker unit  1221  of the n-th speaker unit pair  122  in the n-th user direction and the sound emitted from the negative speaker unit  1221  of the n-th speaker unit pair  122  in the opposite direction to the n-th user direction are in an antiphase relationship. 
     &lt;&lt;Form 2: Sound System  102 &gt;&gt; 
     The sound system  100  has a small so-called sweet spot, which is an area where emitted sound is heard. Here, a description will be given of a sound system having a structure that enlarges the sweet spot. 
     Hereinafter, the sound system  102  will be described with reference to  FIG.  12   .  FIG.  12    is a block diagram showing a configuration of the sound system  102 . As shown in  FIG.  12   , the sound system  102  includes a reproduction apparatus  110  and a speaker system  120 , similarly to the sound system  100 . However, the sound system  102  is different from the sound system  100  in a point that a member  1222  is attached to each speaker unit pair  122 . 
     Hereinafter, a structure of an n-th speaker unit pair  122  (n=1, . . . , N) will be described, according to  FIG.  12   . 
     The member  1222  is attached to the n-th speaker unit pair  122  (see  FIG.  13   ). The member  1222  is for lengthening a path of sound bending around in the n-th user direction, of sound emitted from a positive speaker unit  1221  and a negative speaker unit  1221  of the n-th speaker unit pair  122  in an opposite direction to the n-th user direction. For example, the member  1222  may be a member such as a partition plate that prevents sound from bending around from the rear of the speaker units. The member  1222  is attached, not to prevent bending around of sound, but to make a phase difference larger between the sound bending around from the rear and sound from the front, that is, to lengthen the path of the sound bending around. 
     The n-th speaker unit pair  122  to which the member  1222  is attached has a larger sweet spot than the n-th speaker unit pair  122  in the form 1. 
     &lt;&lt;Form 3: Sound System  104 &gt;&gt; 
     Since high-frequency sound has short wave lengths, phases of sound bending around from the rear and sound from the front do not easily coincide. Accordingly, high-frequency sound has a characteristic of being difficult to cancel, compared to low-frequency sound, in both the vicinity of a speaker unit and other relatively distant places. Since none of the positive speaker units  1221  and the negative speaker units  1221  of the speaker unit pairs  122  included in the sound system  100  are housed in speaker boxes, the area where high-frequency sound is heard is large due to the above-described characteristic, so that sound leakage may occur in some cases. Accordingly, here, a description will be given of a sound system having a structure that makes it difficult for high-frequency sound to leak into places other than the vicinity of a speaker system. 
     Hereinafter, the sound system  104  will be described with reference to  FIG.  14   .  FIG.  14    is a block diagram showing a configuration of the sound system  104 . As shown in  FIG.  14   , the sound system  104  includes a reproduction apparatus  110  and a speaker system  120 , similarly to the sound system  100 . However, the sound system  104  is different from the sound system  100  in a point that a tweeter  1223  is added to each of a positive speaker unit  1221  and a negative speaker unit  1221  of each speaker unit pair  122 . Here, the tweeter is a speaker unit for reproducing a signal at high frequencies. Note that the tweeters  1223  are assumed to be added to the positive speaker units  1221  and the negative speaker units  1221  in such a manner that sound from the rear does not leak, as if the tweeters  1223  were housed in speaker boxes. 
     Hereinafter, operation of the speaker system  120  will be described, according to  FIG.  14   . 
     The speaker system  120  receives, as inputs, a first sound signal, a second sound signal, . . . , and a 2N-th sound signal outputted by the reproduction apparatus  110 , and emits sound based on the first sound signal, sound based on the second sound signal, . . . , and sound based on the 2N-th sound signal. More specifically, an n-th speaker unit pair  122  (n=1, . . . , N) receives a (2n−1)-th sound signal and a 2n-th sound signal as inputs, and emits sound based on the (2n−1)-th sound signal from the positive speaker unit  1221  and the tweeter  1223  added to the positive speaker unit  1221 , and emits sound based on the 2n-th sound signal from the negative speaker unit  1221  and the tweeter  1223  added to the negative speaker unit  1221 . 
     Although higher-frequency sound has higher straightness by nature, a form is made such that sound from the rear of the tweeters  1223  does not leak, and it is therefore possible to prevent high-frequency sound emitted from the tweeters  1223  from leaking in all directions. 
     &lt;&lt;Form 4: Sound System  200 &gt;&gt; 
     The tweeter is a speaker unit for reproducing a signal at high frequencies. A configuration may be made therefore such that only signals at high frequencies are inputted into tweeters through band division processing. Accordingly, here, a description will be given of a sound system that performs band division processing. 
     Hereinafter, the sound system  200  will be described with reference to  FIG.  15   .  FIG.  15    is a block diagram showing a configuration of the sound system  200 . As shown in  FIG.  15   , the sound system  200  includes a reproduction apparatus  110 , a band division apparatus  210 , and a speaker system  120 . The band division apparatus  210  includes N band division units  212  (that is, a first band division unit  212 , . . . , an N-th band division unit  212 ). The sound system  200  is different from the sound system  104  in a point that the band division apparatus  210  is included. 
     Hereinafter, operation of the band division apparatus  210  and the speaker system  120  will be described, according to  FIG.  15   . 
     The band division apparatus  210  receives, as inputs, a first sound signal, a second sound signal, . . . , and a 2N-th sound signal outputted by the reproduction apparatus  110 , and outputs a first high-frequency signal and a first low-frequency signal that are a signal at high frequencies and a signal at low frequencies of the first sound signal, respectively, a second high-frequency signal and a second low-frequency signal that are a signal at high frequencies and a signal at low frequencies of the second sound signal, respectively, . . . , and a 2N-th high-frequency signal and a 2N-th low-frequency signal that are a signal at high frequencies and a signal at low frequencies of the 2N-th sound signal. More specifically, an n-th band division unit  212  (n=1, . . . , N) receives a (2n−1)-th sound signal and a 2n-th sound signal as inputs, generates a (2n−1)-th high-frequency signal and a (2n−1)-th low-frequency signal that are a signal at high frequencies and a signal at low frequencies of the (2n−1)-th sound signal, respectively, generates a 2n-th high-frequency signal and a 2n-th low-frequency signal that are a signal at high frequencies and a signal at low frequencies of the 2n-th sound signal, respectively, and outputs the (2n−1)-th high-frequency signal, the (2n−1)-th low-frequency signal, the 2n-th high-frequency signal, and the 2n-th low-frequency signal. 
     The speaker system  120  receives, as inputs, the first high-frequency signal, the first low-frequency signal, the second high-frequency signal, the second low-frequency signal, . . . , the 2N-th high-frequency signal, and the 2N-th low-frequency signal outputted by the band division apparatus  210 , and emits sound based on the first high-frequency signal, sound based on the first low-frequency signal, sound based on the second high-frequency signal, sound based on the second low-frequency signal, . . . , sound based on the 2N-th high-frequency signal, and sound based on the 2N-th low-frequency signal. More specifically, an n-th speaker unit pair  122  (n=1, . . . , N) receives the (2n−1)-th high-frequency signal, the (2n−1)-th low-frequency signal, the 2n-th high-frequency signal, and the 2n-th low-frequency signal as inputs, and emits sound based on the (2n−1)-th low-frequency signal and sound based on the (2n−1)-th high-frequency signal from the positive speaker unit  1221  and the tweeter  1223  added to the positive speaker unit  1221 , respectively, and emits sound based on the 2n-th low-frequency signal and sound based on the 2n-th high-frequency signal from the negative speaker unit  1221  and the tweeter  1223  added to the negative speaker unit  1221 , respectively. 
     &lt;&lt;Form 5: Sound System  300 &gt;&gt; 
     In the sound system  200 , speaker units to each of which the tweeter  1223  is added are used for the positive speaker unit  1221  and the negative speaker unit  1221 . Here, a description will be given of a sound system that uses speaker unit pairs each including two speaker units and one tweeter, instead of the speaker unit pairs each including two speaker units to which tweeters are added, respectively. 
     Hereinafter, the sound system  300  will be described with reference to  FIG.  16   .  FIG.  16    is a block diagram showing a configuration of the sound system  300 . As shown in  FIG.  16   , the sound system  300  includes a reproduction apparatus  110 , a band division apparatus  310 , and a speaker system  320 . The band division apparatus  310  includes N band division units  312  (that is, a first band division unit  312 , . . . , an N-th band division unit  312 ). The speaker system  320  includes N speaker unit pairs  322  (that is, a first speaker unit pair  322 , . . . , an N-th speaker unit pair  322 ). Each speaker unit pair  322  includes two speaker units (that is, a positive speaker unit  1221  and a negative speaker unit  1221 ) and a tweeter  3221 . The sound system  300  is different from the sound system  200  in a point that the band division apparatus  310  and the speaker system  320  are included instead of the band division apparatus  210  and the speaker system  120 . 
     It is preferable that each tweeter  3221  be housed in a speaker box such that sound from the rear does not leak. Moreover, the speaker system  320  is installed at a place close to a head of a user using the seat. 
     Note that a direction in which an n-th speaker unit pair  322  faces the user is referred to as an n-th user direction (n=1, . . . , N). The positive speaker unit  1221  and the negative speaker unit  1221  of the n-th speaker unit pair  322  (n=1, . . . , N) are disposed such that sound emitted from the positive speaker unit  1221  in an opposite direction to the n-th user direction and sound emitted from the negative speaker unit  1221  in the opposite direction to the n-th user direction propagate in the n-th user direction due to bending around of the sound. Here, the n-th user direction is a direction toward the front of the positive speaker unit  1221 , the negative speaker unit  1221 , and the tweeter  3221  of the n-th speaker unit pair  322 . The opposite direction to the n-th user direction is a direction toward the rear of the positive speaker unit  1221 , the negative speaker unit  1221 , and the tweeter  3221  of the n-th speaker unit pair  322 . 
     Moreover, the positive speaker unit  1221  and the negative speaker unit  1221  of the n-th speaker unit pair  322  (n=1, . . . , N) are disposed in a positional relationship in which sound emitted from the positive speaker unit  1221  and sound emitted from the negative speaker unit  1221  cancel each other out so that the sound is not heard by users using other seats. 
     Hereinafter, operation of the band division apparatus  310  and the speaker system  320  will be described, according to  FIG.  16   . 
     The band division apparatus  310  receives, as inputs, a first sound signal, a second sound signal, . . . , and a 2N-th sound signal outputted by the reproduction apparatus  110 , and outputs a first high-frequency signal and a first low-frequency signal that are a signal at high frequencies and a signal at low frequencies of the first sound signal, respectively, a second low-frequency signal that is a signal at low frequencies of the second sound signal, . . . , a (2N−1)-th high-frequency signal and a (2N−1)-th low-frequency signal that are a signal at high frequencies and a signal at low frequencies of the (2N−1)-th sound signal, respectively, and a 2N-th low-frequency signal that is a signal at low frequencies of the 2N-th sound signal. More specifically, an n-th band division unit  312  (n=1, . . . , N) receives a (2n−1)-th sound signal and a 2n-th sound signal as inputs, generates a (2n−1)-th high-frequency signal and a (2n−1)-th low-frequency signal that are a signal at high frequencies and a signal at low frequencies of the (2n−1)-th sound signal, respectively, generates a 2n-th low-frequency signal that is a signal at low frequencies of the 2n-th sound signal, and outputs the (2n−1)-th high-frequency signal, the (2n−1)-th low-frequency signal, and the 2n-th low-frequency signal. 
     The speaker system  320  receives, as inputs, the first high-frequency signal, the first low-frequency signal, the second low-frequency signal, . . . , the (2N−1)-th high-frequency signal, the (2N−1)-th low-frequency signal, and the 2N-th low-frequency signal outputted by the band division apparatus  310 , and emits sound based on the first high-frequency signal, sound based on the first low-frequency signal, sound based on the second low-frequency signal, . . . , sound based on the (2N−1)-th high-frequency signal, sound based on the (2N−1)-th low-frequency signal, and sound based on the 2N-th low-frequency signal. More specifically, the n-th speaker unit pair  322  (n=1, . . . , N) receives the (2n−1)-th high-frequency signal, the (2n−1)-th low-frequency signal, and the 2n-th low-frequency signal as inputs, and emits sound based on the (2n−1)-th high-frequency signal from the tweeter  3221 , emits sound based on the (2n−1)-th low-frequency signal from the positive speaker unit  1221 , and emits sound based on the 2n-th low-frequency signal from the negative speaker unit  1221 . 
     &lt;&lt;Form 6: Sound System  106 &gt;&gt; 
     The sound system  104  is a system that makes it difficult for high-frequency sound to leak by using the speaker units  1221  to which the tweeters  1223  are added, respectively. Here, a description will be given of a sound system that makes it difficult for high-frequency sound to leak by using a member having a sound absorption characteristic, instead of using the speaker units to which the tweeters are added. 
     Hereinafter, the sound system  106  will be described with reference to  FIG.  17   .  FIG.  17    is a block diagram showing a configuration of the sound system  106 . As shown in  FIG.  17   , the sound system  106  includes a reproduction apparatus  110  and a speaker system  120 , similarly to the sound system  104 . However, the sound system  106  is different from the sound system  104  in points that speaker units  1221  to which no tweeters  1223  are added are used instead of the speaker units  1221  to which the tweeters  1223  are added, and that a member  1224  is attached to each speaker unit pair  122 . 
     Hereinafter, a structure of an n-th speaker unit pair  122  (n=1, . . . , N) will be described, according to  FIG.  17   . 
     The member  1224  is attached to the n-th speaker unit pair  122  (see  FIG.  18   ). The member  1224  is for absorbing sound emitted from a positive speaker unit  1221  and a negative speaker unit  1221  of the n-th speaker unit pair  122  in an opposite direction to an n-th user direction. The member  1224  may be any member that can prevent high-frequency sound from being emitted on the rear. Note that the member  1224  may be installed so as to enclose the speaker unit pair  122  except a front face, instead of being installed only on a rear face of the speaker unit pair  122 . 
     &lt;&lt;Form 7: Sound System  108 &gt;&gt; 
     The sound system  106  is a system that makes it difficult for high-frequency sound to leak by using the speaker units  1221  to which the members  1224  are attached. Here, a description will be given of a sound system that makes it difficult for high-frequency sound to leak by housing each speaker unit of each speaker unit pair in a perforated speaker box, instead of using the speaker unit pairs to which the sound absorption members are attached. 
     Hereinafter, the sound system  108  will be described with reference to  FIG.  19   .  FIG.  19    is a block diagram showing a configuration of the sound system  108 . As shown in  FIG.  19   , the sound system  108  includes a reproduction apparatus  110  and a speaker system  120 , similarly to the sound system  106 . The sound system  108  is different from the sound system  106  in a point that speaker unit pairs  122  each including speaker units  1221  each housed in a speaker box  1225  are included, instead of the speaker unit pairs  122  to which the members  1224  are attached. 
     Hereinafter, a structure of an n-th speaker unit pair  122  (n=1, . . . , N) will be described, according to  FIG.  19   . 
     A positive speaker unit  1221  and a negative speaker unit  1221  of the n-th speaker unit pair  122  are housed in the speaker boxes  1225 , respectively. Note that each speaker box  1225  is perforated with many holes. 
     According to the embodiment of the present invention, it is possible to reduce noise heard when a user sits in a seat of an aircraft. At the same time, it is possible to reproduce sound that can be heard only in a very limited small area, that is, the vicinity of a speaker system. 
     &lt;Supplement&gt; 
     The above description of the embodiments of the present invention is provided for illustrative and descriptive purposes. The embodiments are not intended to be exhaustive, or to limit the invention to the exact forms disclosed. Modifications and variations can be made from the above-described teachings. The embodiments are selectively presented in order to provide the best illustrations of the principle of the present invention, and to allow persons skilled in the art to use the present invention in various embodiments, or with addition of various modifications, so that the invention can be adapted to contemplated actual uses. All of such modifications and variations are within the scope of the present invention specified by the accompanying claims that are interpreted according to a justifiably, legitimately, and fairly given range.