Patent Publication Number: US-2005117766-A1

Title: Speaker unit for seat

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a speaker unit for reproducing low-frequency sound, particularly mounted inside a seat of a vehicle or the like.  
      2. Description of Related Art  
      As a speaker unit for reproducing low-frequency sound, phase inversion type speaker units such as a bass-reflex type and a Kelton type are conventionally known.  
      For example, a bass-reflex type speaker unit includes a casing having a cavity inside, a so-called enclosure, a speaker for reproducing low-frequency sound provided on one side surface of the enclosure, and a duct provided on the same side surface. This duct is formed into a cylindrical shape, and connects an inside of the enclosure and an outside of the enclosure. At the time of reproducing sound, such a speaker unit inverts the phase of an acoustic wave emitted from a rear surface of a speaker diaphragm, and emits the acoustic wave to the outside of the enclosure via the duct. Thereby, the speaker unit intensifies the acoustic wave emitted to a front surface from the speaker diaphragm to make it possible to reproduce low-frequency sound.  
      As a speaker unit of this kind, there is proposed, for example, a speaker unit for reproducing low-frequency band sound in which distortion is reduced by making the phase characteristic flat in an actual use band and the sound image localization in a low-frequency band is improved (for example, refer to Japanese Patent Application Laid-Open under No. 2002-159083). There is also proposed a sub woofer box device which makes it possible to reproduce low heavy sound by relieving increase and decrease of pressure caused when a chamber has an enclosed structure, and further reduces the generation of abnormal sound (for example, refer to Japanese Patent Application Laid-Open under No. 9-275595).  
      There is proposed a sub woofer level adjustment device that can accurately adjust a level of a sub woofer (for example, refer to Japanese Patent Application Laid-Open under No. 2002-186087).  
      The aforementioned speaker units are frequently loaded especially on a vehicle and the like. The speaker units are usually placed in appropriate positions inside a vehicle, for example, in a vacant space in a rear part, a periphery of a seat or the like inside the vehicle. As a result, the speaker unit is installed in a position away from the ears of a seated person in the vehicle. Accordingly, such speaker unit needs to emit sufficient low-frequency sound to the ears of the seated person in the vehicle, and therefore a speaker with large output power is used.  
      However, since such a powerful speaker has a large bore diameter and a heavy magnetic circuit, the speaker unit itself has a large shape and is heavy. Therefore, there arises the problem that the speaker unit occupies a narrow space in a vehicle.  
     SUMMARY OF THE INVENTION  
      As the problems to be solved by the present invention, those described above are cited as examples. The present invention has its object to provide a compact and light speaker unit for reproducing low-frequency sound, which is capable of being mounted inside a seat.  
      According to one aspect of the present invention, there is provided a speaker unit for a seat, including a speaker device including a pair of acoustic tubes, and a casing which houses at least a part of the speaker device and is housed inside a seat, and wherein one end of each of the acoustic tubes is opened to an outside in a sound emitting surface of the casing which becomes a top surface and is exposed to the outside of the seat when the casing is housed inside the seat.  
      According to the above-described speaker unit, at least a part of the speaker device for reproducing low-frequency sound, which includes a pair of acoustic tubes capable of reproducing low-frequency sound, for example, is housed in the casing, and the casing is housed inside the seat of a vehicle or the like. In this case, the end portions of a pair of acoustic tubes are opened to the outside in the sound emitting surface of the casing, which becomes the top surface and is exposed to the outside of the seat when the casing is housed inside the seat. More preferably, each end portion of the pair of acoustic tubes is located in the vicinity of left and right ears of the seated person in the seat. Therefore, low-frequency sound can be reproduced to the area in the vicinity of both the ears of the seated person, at the time of driving the speaker unit.  
      Since the low-frequency sound can be reproduced to the area in the vicinity of both the ears of the seated person, the output sound pressure level of the speaker device is not needed so much. Therefore, a compact speaker device with a small bore diameter can be applied to the speaker unit, and the speaker unit itself can be made compact and light. Thereby, the shape or the like of the acoustic tube can be freely changed, and therefore it is possible to design a compact and light speaker unit. In this speaker unit, the lowest resonance frequency can be easily set by changing the length, the sectional area and the like of the acoustic tubes.  
      In one feature of the above-described speaker unit for the seat, the casing can house a whole of the speaker device. Thereby, the speaker unit can be completely contained inside the seat.  
      In another feature of the above-described speaker unit for the seat, the casing houses only the acoustic tubes, and the speaker device further includes external units each including an acoustic capacity portion and a drive speaker. The external units are detachably attached to the other ends of the acoustic tubes housed inside the casing at a position of an outside of the seat. Thereby, only the external unit including the acoustic capacity portion and the drive speaker device can be replaced afterwards.  
      In still another feature of the above-described speaker unit for the seat, the casing houses only the acoustic tubes, and the speaker device further includes external units each having an external acoustic tube, an acoustic capacity portion and a drive speaker. The external units are detachably attached to the other ends of the acoustic tubes housed inside the casing at a position of an outside of the seat. Thereby, only the external unit including the external acoustic tube, the acoustic capacity portion and the drive speaker can be replaced afterwards. Especially, it is possible to change the lowest resonance frequency of the speaker device and change the frequency of the reproduced sound by attaching the external acoustic tubes of different lengths.  
      In yet another feature of the above-described speaker unit for the seat, it is preferable that end portions of the pair of acoustic tubes opened to the outside are separated by a distance approximately equal to a distance between both ears of an average human being in a sound emitting surface of the casing. Thereby, the end portions of a pair of acoustic tubes can be properly located at the ears of the seated person when the speaker unit is mounted to the seat.  
      The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of a construction of a speaker unit of a first embodiment;  
       FIG. 2  is a block diagram showing a method of supplying electric power and an audio signal to the speaker unit of the present invention;  
       FIG. 3  is a diagram explaining a Helmholtz resonance frequency and a resonance frequency of an acoustic tube by the speaker unit of the present invention;  
       FIG. 4  is a view schematically showing another example of the speaker unit of the first embodiment;  
       FIG. 5  is a side view of a state in which the speaker unit is mounted inside a front seat of a vehicle;  
       FIG. 6  is a plan view of a state in which the speaker units are mounted inside the front seats of the vehicle;  
       FIG. 7  is a rear view of a state in which the speaker unit is mounted inside the front seat of the vehicle;  
       FIGS. 8A and 8B  are views schematically showing constructions of speaker units of a second embodiment of the present invention;  
       FIGS. 9A and 9B  are rear views showing a state in which the speaker units of the second embodiment are mounted inside the seats of the vehicle;  
       FIG. 10  is a side view of a state in which the speaker unit is mounted inside a backrest of a chair for a personal computer; and  
       FIG. 11  is a rear view of a state in which the speaker units are mounted inside the backrests of the chairs in a beauty parlor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The preferred embodiments of the present invention will now be described below with reference to the attached drawings.  
      The embodiments provide a compact and light speaker unit for reproducing low-frequency sound, which is capable of being mounted inside a seat.  
     1st Embodiment  
       FIG. 1  schematically shows a construction of a speaker  100  according to a first embodiment of the present invention.  FIG. 1  shows a perspective view of the speaker unit  100 . Hereinafter, the construction of the speaker  100  according to the first embodiment of the present invention will be explained with reference to  FIG. 1 .  
      The speaker  100  includes an acoustic capacity portion  1 , an acoustic tube  2 , a drive speaker  3 , wiring  5  and a casing  4  having a space inside. A speaker device for reproducing low-frequency sound is constructed by the acoustic capacity portion  1 , the acoustic tube  2  and the drive speaker  3 . The speaker  100  is mounted inside various kinds of seats (including various kinds of chairs, the same thing will apply hereinafter) and used, as will be described later.  
      The acoustic capacity portion  1  is connected to an open end  2   a  of the acoustic tube  2  and a sound emitting side of the drive speaker  3 , respectively.  
      The acoustic tube  2  has two open ends  2   a  and  2   b.  The one open end  2   a  is connected to the acoustic capacity portion  1  as described above, and the other open end  2   b  is connected to an upper surface  4   c  of the casing  4 . A distance between the two open ends  2   b  on the upper surface  4   c  of the casing  4  is desired to be about the distance between both ears of an average human being, and it can be set at about 10 to 20 cm, for example. The acoustic tube  2  is formed into the curved shape to correspond to the size of the seat which will be described later and to emit low-frequency sound in a predetermined band, but the acoustic tube  2  is not limited to such a shape. Accordingly, the acoustic tube  2  can be formed into various kinds of shapes such as a U-shape and a linear shape as necessary.  
      The drive speaker  3  is preferably a compact and light speaker suitable for reproducing low-frequency sound. The wiring  5  is a wiring for supplying electric power and audio signals to the drive speaker  3  from a sound source which will be described later, or the like. The wiring  5  is connected to a voice coil (not shown) of the drive speaker  3 .  
      The casing  4  is provided with openings  4   a.  A plurality of openings  4   a  are formed on the upper surface  4   c  of the casing  4 , and the position of each of the openings  4   a  corresponds to the position of the open end  2   b  of each of the acoustic tubes  2 .  
      Next, with reference to  FIG. 2 , a method for supplying electric power and audio signals to the speaker  100  will be briefly described.  
      The electric power supply to the speaker  100  is performed through a sound source  10  as shown by the allow S 1 , for example. More specifically, the sound source  10  outputs electric power of a predetermined magnitude to the drive speaker  3  through the wiring  5  (signal S 1 ). Thereby, electric power supply to the speaker  100  is performed. Meanwhile, supply of the audio signals to the speaker  100  is performed through the sound source  10 , a composer  11 , an LPF (Low Pass Filter)  12 , and an amplifier  13 .  
      The sound source  10  corresponds to a CD (Compact Disc) player, an MD (Mini Disc) player or the like. The sound source  10  outputs left-right dual channel audio signals L and R to the composer  11 . The composer  11  adds up the outputted L and R audio signals, and outputs the added (L+R) audio signal to the LPF  12  (signal S 2 ). The LPF  12  cuts off a predetermined frequency component. More specifically, the LPF  12  extracts only an audio signal of a predetermined low frequency from the (L+R) audio signal outputted through the composer  11 , and outputs the extracted audio signal to the amplifier  13  (signal S 3 ). The amplifier  13  amplifies the extracted audio signal by a predetermined amplification factor, and outputs the amplified audio signal to the drive speaker  3  via the wiring  5  (signal S 4 ). On receiving the supply of electric power and the audio signal through the sound source  10  or the like in this manner, the drive speaker  3  emits a low frequency-band acoustic wave to the acoustic capacity portion  1  through the diaphragm (not shown).  
      In this speaker unit  100 , the acoustic capacity portion  1  and the acoustic tube  2  constitute a resonator of Helmholtz. As a result, the speaker  100  produces resonance at two resonance frequencies, i.e., the resonance frequency as the Helmholtz resonator by the acoustic capacity portion  1  and the acoustic tube  2 , and the resonance frequency by the acoustic tube  2  itself.  
      Here,  FIG. 3  schematically shows the construction of the structure constituted of the acoustic capacity portion  1 , the acoustic tube  2  and the drive speaker  3  which are housed in the speaker unit  100 . In  FIG. 3 , the acoustic tube  2  in the curved shape is formed in the linear shape for convenience of explanation. When the length of the linearly shaped acoustic tube  2  is expressed as “L”, the sectional area of the acoustic tube  2  is expressed as “S”, the acoustic capacity of the acoustic capacity portion  1  is expressed as “B”, and the sonic speed is expressed as “C”, the Helmholtz resonance frequency “f0” and the resonance frequency “f” of the acoustic tube  2  by the speaker  100  are respectively obtained as follows.  
      Helmholtz Resonance frequency  
         f   ∘     =       C     2   ⁢   π       ⁢       S   LB             
 
      Resonance frequency of a tube with both end opened  
       f   =     C     2   ⁢   L           
 
      Resonance frequency of a tube with one end opened  
       f   =     C     4   ⁢   L           
 
      It is noted that the resonance frequency of the acoustic tube  2  differs in the case that both ends of the acoustic tube  2  are opened and in the case that one end is opened.  
      When the air inside the acoustic capacity B is vibrated by the drive speaker  3 , sound is emitted with the highest sound pressure from the open end  2   b  of the acoustic tube  2  at the Helmholtz resonance frequency, namely, the resonance frequency of the compliance of the acoustic capacity B and the air mass inside the acoustic tube  2 . Since the acoustic capacity B exists at the right side of the acoustic tube  2  as shown in  FIG. 3 , the resonance frequency of the acoustic tube  2  occurs between the frequency in the case that both the ends are opened and the frequency in the case that one end is opened. In this case, sound is emitted from the open end  2   b  of the acoustic tube  2  at a predetermined higher frequency than the above-descried Helmholtz resonance frequency.  
      As a result, a frequency with a high sound pressure level can be obtained in a predetermined low-frequency sound range, for example, in the range of the lowest resonance frequency of, for example, 300 Hz or lower by the resonance of Helmholtz by the acoustic capacity portion  1  and the acoustic tube  2  and the resonance by the acoustic tube  2  itself, and hence the speaker  100  can be utilized as the speaker device for low-frequency sound. The lowest resonance frequency can be set at a predetermined value by setting the sectional area S and the length L of the acoustic tube  2 , and the acoustic capacity B of the acoustic capacity portion  1  to an appropriate area, capacity and length, respectively as necessary.  
      In the above-described embodiment, in order to reproduce low-frequency sound of a suitable sound volume for the left and right ears of a seated person, a pair of structures each constructed by the acoustic capacity portion  1 , the acoustic tube  2  and the drive speaker  3  are housed in the casing  4  to form the speaker device for reproducing low-frequency sound. However, the speaker device of the invention is not limited to this, and it is possible to properly change the number of structures housed in the casing  4  as necessary.  
      For example, as another example of the first embodiment, a speaker unit for reproducing low-frequency sound provided with only one acoustic capacity portion and one drive speaker in the casing  4  can be constructed without changing the length, the sectional area, the shape and the placement of the acoustic tube  2 .  FIG. 4  schematically shows the construction of such a speaker unit  110 .  
      The speaker unit  110  is provided with only one drive speaker  21  inside an acoustic capacity portion  20  as shown in  FIG. 4 . When electric power and an audio signal are supplied to the drive speaker  21  from the sound source  10  through the wiring  5  in the speaker unit  110  of this example, a diaphragm (not shown) of the drive speaker  21  vibrates in a major axis direction of the acoustic capacity portion  20 , and low frequency-band acoustic waves are emitted from a front surface direction (the arrow S 10 ) and a rear surface direction (the arrow S 11 ) of the drive speaker  21  by the diaphragm, respectively. These acoustic waves are emitted to an outside through the acoustic capacity portion  20  and the open end  2   b  of each of the acoustic tubes  2 , and low-frequency sound is reproduced by the speaker unit  110 .  
      Incidentally, the speaker unit  110  is provided with only one drive speaker  21 . Therefore, there is the possibility that a low-frequency sound with desired sound pressure is not reproduced sufficiently depending on the specified output sound pressure of the drive speaker  21 . Accordingly, in order to reproduce a desired low-frequency sound sufficiently, it is preferable to apply, to the speaker unit  110 , a speaker with a larger bore diameter of the diaphragm than in the aforesaid drive speaker  3 , for example.  
      Next, a method for mounting the speaker unit to a seat will be described. The speaker unit of the present invention is used in the state in which it is mounted inside various kinds of seats of a vehicle, a personal computer, a beauty parlor and the like. Mounting of the speaker units of the present invention to the inside of various kinds of seats is basically performed by seat manufacturers or the like.  
       FIG. 5  to  FIG. 7  show the state in which the speaker units  100  of the present invention are mounted to front seats  30  of a driver&#39;s seat and a front passenger&#39;s seat of a vehicle  200 .  FIG. 5  shows a side view of the mounting state, and  FIG. 6  shows a plan view of the mounting state.  FIG. 7  shows a state in which the speaker  100  is mounted inside a backrest  32  of the front seat  30 . It is noted that, in  FIG. 5  and  FIG. 6 , a front door and a roof part of the vehicle  200  are omitted from the illustration so that the state in which the speaker units  100  are mounted in the front seats  30  can be clearly shown.  
      The front seat  30  of the vehicle  200  includes a head support part  31 , the backrest  32  and a seat part  33 , and the speaker  100  of the present invention is mounted inside the backrest  32  as shown in  FIG. 5  to  FIG. 7 . In this case, the speaker  100  is mounted inside the backrest  32  in the state in which an upper surface  4   c  of the casing  4  of the speaker unit  100 , which is provided with the open end  2   b  of each of the acoustic tubes  2 , i.e., a sound emitting surface, is exposed from a top surface of the backrest. As a result, the open end  2   b  of each of the acoustic tubes  2  provided on the sound emitting surface of the speaker unit  100 , i.e., the upper surface  4   c  of the casing  4 , is disposed at a position in the vicinity of both ears of the seated person  35 .  
      Since the open end  2   b  of each of the acoustic tubes  2  is disposed at the position in the vicinity of both ears of the seated person  35  in this way and low-frequency sound can be reproduced to the vicinity of both the ears of the seated person, the output sound pressure level of the drive speaker  3  is not needed so much. Therefore, the compact drive speaker  3  with the small bore diameter can be applied to the speaker unit  100 . Accordingly, the speaker  100  itself can be made compact and light. It is experimentally proved in the scientific essays and the like that low-frequency sound reproduction to ears does not make listeners auditorily uncomfortable.  
      The speaker  100  of the present invention is designed to be mounted inside the backrest  32  of the seat  30 . Therefore, it is not necessary to take the trouble to ensure a place for providing the speaker  100  in a periphery of the seat. Therefore, a narrow space inside the vehicle can be utilized effectively, and the speaker  100  of the present invention can be applied to a light car having a narrow in-vehicle space.  
      The speaker  100  can be selectively mounted to either one of the driver&#39;s seat or the front passenger&#39;s seat, and therefore the speaker  100  can be mounted only to the seat of the seated person  35  who prefers low-frequency sound reproduction, for example.  
      The open ends  2   b  of the acoustic tubes  2  are located in the vicinity of both ears of the seated person  35 , and therefore the output sound pressure level of the drive speaker  3  required to the speaker  100  is not so high. Accordingly, even if the speaker units  100  mounted to both the driver&#39;s seat and the front passenger&#39;s seat are used at the same time, the low-frequency sound emitted from one of the speaker units  100  is hardly heard by the seated person  35  in the adjacent seat. Accordingly, the seated person  35  in each of the seats can listen to proper low-frequency sound.  
     Second Embodiment  
      In the speaker  100  of the above-described first embodiment, the acoustic capacity portion  1  and the drive speaker  3  are housed inside the casing  4 . However, in a second embodiment, a part of the acoustic tube, the acoustic capacity portion, the drive speaker and the like are provided outside the casing as an external unit, and are made freely attachable and detachable to and from the acoustic tube housed inside the casing. Thereby, it is made possible to replace the acoustic capacity portion and the drive speaker and to change the lowest resonance frequency of the speaker unit afterwards. The sound reproduction principle, the methods of supplying electric power and audio signals of the speaker unit in the second embodiment are approximately the same as the speaker  100  of the first embodiment, and therefore the explanation will be omitted in this point. Hereinafter, a speaker unit of the second embodiment will be explained with reference to  FIG. 8  and  FIG. 9 .  
       FIGS. 8A and 8B  schematically show constructions of the speaker units each provided with a part of the acoustic tube, the acoustic capacity portion and the drive speaker outside the casing.  FIG. 8A  shows a speaker unit  120  with curved acoustic tubes  122  housed in a casing  45 . Meanwhile,  FIG. 8B  shows a speaker unit  130  with linear acoustic tubes  132  housed in the casing  45 . The speaker units  120  and  130  differ only in the shapes of the respective acoustic tubes, and the other constituents are the same in both of them.  
      First, the construction of the speaker unit  120  will be described with reference to  FIG. 8A .  
      The speaker unit  120  includes the casing  45  which houses a pair of curved acoustic tubes  122 , and driving units  70  as the external unit.  
      Plural circular openings  45   a  are formed on an upper surface  45   c  of the casing  45 . A position of each of the openings  45 a corresponds to a position of one end portion, namely, an open end  122   b  of each of the acoustic tubes  122 . As in the first embodiment, it is preferable to set a distance between the openings  45   a  at about the distance between both ears of an average human being. The driving unit  70  is detachably attached to the other end portion  122   a  of each of the acoustic tubes  122 .  
      The driving unit  70  includes a drive speaker  71 , an acoustic capacity portion  72 , a casing  74  housing the drive speaker  71  and the acoustic capacity portion  72 , and an external acoustic tube  73 .  
      The drive speaker  71  is preferably a compact and light speaker suitable for reproduction of low-frequency sound. The drive speaker  71  has wiring not shown, and the wiring is connected to a sound source and the like. Therefore, the drive speaker  71  is supplied with electric power and audio signals from the sound source and the like through the wiring and is driven. The acoustic capacity portion  72  is connected to a sound emitting side of the drive speaker  71  and one end portion  73   b  of the external acoustic tube  73 . The other end portion  73   a  of the external acoustic tube  73  is detachably attached to the other end portion  122   a  of the acoustic tube  122 .  
      Next, a construction of the speaker unit  130  will be briefly explained with reference to  FIG. 8B . The explanation of the same constituents as in the above-described speaker unit  120  will be omitted.  
      The speaker unit  130  includes the casing  45  housing a pair of linear acoustic tubes  132 , and the driving units  70  as the external unit.  
      An open end  132   b  of the acoustic tube  132  is disposed at the position of the opening  45   a  formed on the upper surface  45   c  of the casing  45 . The driving unit  70  is detachably attached to other end portion  132   a  of the acoustic tube  132 .  
      Comparing the speaker unit  120  and the speaker unit  130  here, in the former, the curved acoustic tube  122  is housed inside the casing  45 , while in the latter, the linear acoustic tube  132  is housed in the casing  45 . Therefore, the length of the acoustic tube  132  in the speaker unit  130  is shorter than the length of the acoustic tube  122  in the speaker unit  120 . By suitably changing the length of the acoustic tube in this manner, the lowest resonance frequency of the speaker unit can be freely changed.  
       FIGS. 9A and 9B  show the states in which the speaker units  120  and  130  are respectively mounted inside the seats of the vehicle as examples.  FIGS. 9A and 9B  show rear views of seats  36  of the vehicle.  
      As shown in  FIGS. 9A and 9B , the speaker units  120  and  130  are used in the state in which they are mounted inside the backrests of various kinds of seats, for example, backrests  38  of the seats  36  of the vehicle so that the upper surface  45   c  of the casing  45  on which each of the open ends  122   b  of the acoustic tube  122  and the open ends  132   b  of the acoustic tube  132  is located, i.e., the sound emitting surface, is exposed on a top surface of the backrest  38 . More specifically, the open end  122   b  of the acoustic tube  122 , and the open end  132   b  of the acoustic tube  132  are disposed on the top surface of the backrest  38 , in other words, at the position in the vicinity of the head support part  37  as shown in  FIGS. 9A and 9B . Accordingly, by driving the speaker unit  120  or  130 , low-frequency sound can be reproduced to the position in the vicinity of both ears of the seated person through the open end  122   b  or  132   b  of the acoustic tube.  
      Meanwhile, the other end portion  122   a  of the acoustic tube  122  and the other end portion  132   a  of the acoustic tube  132  are respectively disposed at the lower positions of the backrests  38 , in other words, the position in the vicinity of the seat part  39 , as shown in  FIGS. 9A and 9B . The placement of the driving unit  70  is not limited to such a position. Namely, if the acoustic tube  122  or  132  is attachable to and detachable from the driving unit  70 , the driving unit  70  may be placed at any position with respect to the backrest. As understood with reference to  FIGS. 9A and 9B , in such placement state, the driving unit  70  can be freely attached and detached to and from the acoustic tube  122  or  132 .  
      Therefore, according to the speaker units  120  and  130 , it is possible to mount a driving unit having different length of the acoustic tube to the acoustic tube  122  or  132  later as necessary. Therefore, the lowest resonance frequency of the speaker unit  120  or  130  can be freely changed afterwards. Therefore, the seated person can enjoy music or the like with output sound pressure being intensified in a desired low-frequency sound range. At the time when the drive speaker  71  inside the driving unit is out of order, it is possible to easily replace the drive speaker  71  by taking out the drive speaker  71  from the driving unit  70  or replace the driving unit  70  as a whole.  
      In the above-described example, the driving unit serving as the external unit is constructed by the drive speaker  71 , the acoustic capacity portion  72  and the external acoustic tube  73 , but the driving unit may be constructed to include only the drive speaker  71  and the acoustic capacity portion  72 , without including the external acoustic tube  73 . In this case, the frequency of the output sound can be changed by changing the capacity of the acoustic capacity portion  72 .  
      A method of mounting the speaker units  120  and  130  of the second embodiment inside the seat of the vehicle is approximately the same as the method of mounting the above-described speaker  100  inside the seat of the vehicle in principle. However, since the driving unit  70  attachable to and detachable from the acoustic tube  122  or  132  exists in the speaker unit  120  or  130  of the second embodiment, it is necessary to form an opening in the portion of the backrest of the seat, which corresponds to the position of the other end portion  122   a  or  132   a  of the acoustic tube, for example.  
     Other Application Examples of the Speaker Unit  
      Next, other application examples of the speaker unit of the present invention will be described. The speaker unit of the present invention is used in the state in which it is mounted inside various kinds of seats of a vehicle, a personal computer, a beauty parlor and the like. The examples in which the speaker  100  of the first embodiment of the present invention is applied to the chair for a personal computer, the chair of a beauty parlor and the like will be described. It is noted that the speaker units  120  and  130  of the second embodiment are also applicable to these chairs.  
       FIG. 10  shows a side view of a state in which the speaker  100  is mounted inside the backrest of a chair  51  for a personal computer  50  based on the above-described method. In this example, the open ends  2   b  of the respective acoustic tubes  2  are also reliably disposed near both ears of the seated person  35 . In this example, a surround system is constructed by a speaker attached to the personal computer  50  and the speaker  100  of the present invention. Accordingly, the seated person  35  can enjoy DVDs (Digital Video Disc) and games with the sound having realism by such a surround system. If the speaker unit  120  or  130  of the second embodiment is applied to the chair  51  for the personal computer  50 , the lowest resonance frequency can be changed afterwards, and therefore the seated person  35  can enjoy DVDs and games with a desired low-frequency sound sufficiently reproduced.  
      The speaker  100  is mounted inside the backrest of the chair  51  for the personal computer  50 , and therefore the speaker  100  itself does not become an interference, either in this example.  
      Next,  FIG. 11  shows the state in which the speaker unit  100  is mounted inside the backrest of a chair  61  of a beauty parlor  300  according to the above-described same method, as a rear view. As shown in  FIG. 11 , the seated person  35  is seated in the chair  61  facing to a dressing table  60 , and the speaker  100  is mounted inside the backrest of the chair  61 . In this case, the open ends  2   b  of the respective acoustic tubes  2  are reliably disposed near both ears of the seated person  35 . Accordingly, the speaker  100  is driven while the seated person  35  is having a haircut or having his or her hair permanently waved, whereby the seated person  35  can spend such a time comfortably.  
      The invention may be embodied on other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning an range of equivalency of the claims are therefore intended to embraced therein.  
      The entire disclosure of Japanese Patent Application No. 2003-390599 filed on Nov. 20, 2003 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety.