Patent Abstract:
A speaker system includes: a speaker cabinet; a speaker unit installed in a wall surface of said speaker cabinet; and an acoustic tube having ends, one of which is open and the other of which is closed, in which said acoustic tube is provided in said speaker cabinet such that a side wall surface of said acoustic tube crosses a direction in which standing waves propagates, the waves occurring inside said speaker cabinet.

Full Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to suppression of disturbances in sound pressure frequency characteristics due to the cabinet shape of a speaker system. 
       BACKGROUND ART 
       [0002]    In recent years, with reduction in the thickness of crystal liquid displays and practical application of organic EL, television sets have become thinner. At the same time, speaker systems for television sets have also become thinner. However, in a low-profile speaker system, the propagation direction of sound within a speaker cabinet is limited by its thinness, and effects of standing waves that occur between the opposing walls in the cabinet are larger than a conventional cuboid cabinet. This causes large peaks and troughs in sound pressure frequency characteristics of a speaker, system. 
         [0003]    The speaker system disclosed in Patent Literature 1 is a related art to solve this problem.  FIG. 13  is a cross-sectional view of the conventional speaker system disclosed in Patent Literature 1. The speaker system illustrated in  FIG. 13  includes a cuboid speaker cabinet  60 , a speaker unit  63 , first acoustic tubes  64   a  and  64   b,  and second acoustic tubes  66   a  and  66   b.    
         [0004]    The speaker cabinet  60  includes a top board  61   a,  a bottom board  61   b,  and side boards  62   a,    62   b,    62   c,  and  62   d.  Sound absorbing materials  65   a  and  65   b  are provided at the openings of the first acoustic tubs  64   a  and  64   b,  respectively. Sound absorbing materials  67   a  and  67   b  are provided at the openings of the second acoustic tubs  66   a  and  66   b,  respectively. 
         [0005]    The operations of a conventional speaker system configured as above will be described. When an electrical signal is inputted into the speaker unit  63  attached to the side board  62   b  of the speaker cabinet  60 , sound is also emitted into the speaker cabinet  60 . At this time, standing waves occur between the top board  61   a  and the bottom board  61   b  opposed to each other in the longer direction of the speaker cabinet  60 . The standing waves occur at a frequency f 1  having a wavelength that is equal to a half of the distance between the top board  61   a  and the bottom board  61   b.    
         [0006]    Here, the first acoustic tubes  64   a  and  64   b  are provided at the corner parts between the side boards  62   a  and  62   d,  and between the side boards  62   a  and  62   b  of the speaker cabinet  60 , respectively. The first acoustic tubes  64   a  and  64   b  with end parts closed are perpendicular to the bottom board  61   b,  maintain a gap X from the bottom board  61   b,  and have the absorbing materials  65   a  and  65   b  at each opening. In addition, each length of the first acoustic tubes  64   a  and  64   b  is equal to one-fourth of the wavelength of standing waves which occur at the frequency f 1 . The first acoustic tubes  64   a  and  64   b  absorb and suppress the standing waves at the frequency 
         [0007]    Likewise, standing waves occur at a frequency f 2  (twice the frequency f 1 ) having a wavelength that is equal to the distance between the top board  61   a  and the bottom board  61   b.  Standing waves at the frequency f 2  are suppressed by the second acoustic tubes  66   a  and  66   b  which are provided at the corner parts between the side boards  62   c  and  62   b,  and between the side boards  62   c  and  62   d  of the speaker cabinet  60  respectively, in the same configuration as the acoustic tubes  64   a  and  64   b  in the speaker cabinet. In this case, each length of the second acoustic tubes  66   a  and  66   b  is half length of the first acoustic tubes  64   a  and  64   b  (i.e., one eighth of the wavelength of standing waves at the frequency f 1 ). 
         [0008]    As a result, the first acoustic tubes  64   a  and  64   b  suppress standing waves having a frequency 2n−1 times the frequency f 1 . Here, n=1, 2, 3 . . . . In addition, the second acoustic tubes  66   a  and  66   b  suppress standing waves having a frequency 2(2n−1) times the frequency f 1 . This reduces disturbance in sound pressure frequency characteristics due to the standing waves of the speaker cabinet  60 . 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [PTL 1] Japanese Unexamined Patent Application Publication No. 2000-125387 
         [PTL 2] Japanese Unexamined Patent Application Publication No. 2009-55605 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0011]    However, in the speaker system disclosed in Patent Literature 1, the speaker cabinet  60  is required to have the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b  of different lengths in order to suppress standing waves at the different frequencies f 1  and f 2 . Furthermore, in terms of the narrow internal space of the speaker cabinet  60 , it is also difficult to provide the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b  of two different lengths within the low-profile speaker cabinet  60 . 
         [0012]    In addition, a bass reproduction limit frequency depends on the internal capacity of the speaker cabinet  60 . In other words, it is advantageous to have a larger capacity of the speaker cabinet  60 . In this case, the internal capacities of the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b  are also considered as a part of the capacity of the speaker cabinet  60 . However, since the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b  have the absorbing materials  65   a,    65   b,    67   a,  and  67   b  respectively at each opening, a part of sound in the bass range passes through the absorbing materials  65   a,    65   b,    67   a,  and  67   b.  Therefore, damping effect by the absorbing materials  65   a,    65   b,    67   a,  and  67   b  is apparent in the bass range and this leads to a problem that sound pressure level is lowered in the bass range. 
         [0013]    The present invention has been made in view of the above problems. Accordingly, an object of the present invention is to provide a speaker system that can suppress occurrence of standing waves without lowering sound pressure level in the bass range. 
       Solution to Problem 
       [0014]    A speaker system in accordance with an embodiment of the present invention includes a speaker cabinet; a speaker unit which is installed in a wall surface of the speaker cabinet and outputs sound; and an acoustic tube having ends, one of which is open and the other of which is closed. The acoustic tube is provided inside the speaker cabinet such that a side wall surface of the acoustic tube crosses a direction in which standing waves propagates, the waves occurring inside the speaker cabinet. 
         [0015]    The above placement of the acoustic tube can suppress standing waves at multiple frequencies which are caused by the relationship between the distance between the opposing walls within the speaker cabinet and a wavelength of sound emitted into the speaker cabinet. Moreover, in the bass range having lower frequencies than those at which standing waves occur, the capacity of the acoustic tube serves as a part of the capacity of the speaker cabinet and thus sound pressure level in the bass range is not lowered. 
         [0016]    As an example, the speaker cabinet may be a pillar-shaped speaker cabinet that is greater in height than in width or depth. The acoustic tube may be provided inside the speaker cabinet so as to reduce an apparent height of an inside of the speaker cabinet. 
         [0017]    As another example, the speaker cabinet may be a thin cuboid that is smaller in thickness than in length or breadth. The acoustic tube may be provided inside the speaker cabinet so as to reduce an apparent length in a longer direction of an inside of the speaker cabinet. 
         [0018]    Moreover, the speaker cabinet may have a bass reflex port. 
         [0019]    Moreover, a resonance frequency that is determined by an inductance component of an acoustic impedance of the acoustic tube and an acoustic compliance of the speaker cabinet may substantially be identical to a peak frequency of a sound pressure of the speaker unit which is installed in the speaker cabinet. 
         [0020]    According to the above configuration, the resonance between the acoustic tube provided in the speaker cabinet and the internal space of the speaker cabinet can suppress the sound pressure peak of a resonance frequency f o  of the speaker unit which is attached to the speaker cabinet. As a result, flat sound pressure frequency characteristics with fewer peaks and troughs can be obtained. 
         [0021]    Moreover, the speaker system may be a bass reflex speaker system. The resonance frequency may substantially be identical to the peak frequency which is higher than a lowest resonance frequency of the speaker unit which is not installed in the speaker cabinet. 
         [0022]    Moreover, the larger a band width of a sound pressure peak of the speaker unit is, the larger an ratio of an internal space capacity of the acoustic tube to an internal space capacity of the speaker cabinet may be. 
         [0023]    Moreover, the acoustic tube may be formed of an inner wall surface of the speaker cabinet and partition boards that are connected to the inner wall surface. 
         [0024]    Moreover, a sound absorbing material is provided at the closed end of said acoustic tube. 
       Advantageous Effects of Invention 
       [0025]    A speaker system according to the present invention can suppress standing waves at multiple frequencies which are caused by the relationship between the distance between the opposing walls inside the speaker cabinet and a wavelength of sound emitted into the speaker cabinet. Moreover, in the bass range having lower frequencies than those at which standing waves occur, the capacity of the acoustic tube serves as a part of the capacity of the speaker cabinet and thus sound pressure level in the bass range is not lowered. As a result, a speaker system with high sound quality which has small disturbances in the reproduction sound pressure due to the standing waves can be made without lowering the sound pressure level in the bass range. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]      FIG. 1A  is a plan view of a speaker system in accordance with the first embodiment. 
           [0027]      FIG. 1B  is a cross-sectional view of a speaker system in accordance with the first embodiment. 
           [0028]      FIG. 2  shows sound pressure frequency characteristics of a speaker system in accordance with the first embodiment. 
           [0029]      FIG. 3A  is a plan view of a speaker system in accordance with the second embodiment. 
           [0030]      FIG. 3B  is a cross-sectional view of a speaker system in accordance with the second embodiment. 
           [0031]      FIG. 4  shows sound pressure frequency characteristics of a speaker system in accordance with the second embodiment. 
           [0032]      FIG. 5A  is a plan view of a speaker system in accordance with the third embodiment. 
           [0033]      FIG. 5B  is a cross-sectional view of a speaker system in accordance with the third embodiment. 
           [0034]      FIG. 6  shows sound pressure frequency characteristics of a speaker system in accordance with the third embodiment. 
           [0035]      FIG. 7  is an equivalent circuit diagram of a speaker system in accordance with the third embodiment. 
           [0036]      FIG. 8  shows sound pressure frequency characteristics when changing the location of an absorbing material in a speaker system in accordance with the third embodiment. 
           [0037]      FIG. 9  shows sound pressure distortion frequency characteristics of a speaker system in accordance with the first embodiment. 
           [0038]      FIG. 10  is a cross-sectional view of a speaker system in accordance with the fourth embodiment. 
           [0039]      FIG. 11  shows sound pressure frequency characteristics of a conventional bass reflex speaker system. 
           [0040]      FIG. 12  shows sound pressure frequency characteristics when changing the capacity ratio of an acoustic tube of a speaker system in accordance with the fourth embodiment. 
           [0041]      FIG. 13  is a cross-sectional view of a conventional speaker system. 
           [0042]      FIG. 14  is a cross-sectional view of a conventional speaker system. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0043]    Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
       First Embodiment 
       [0044]      FIGS. 1A and 1B  show a speaker system in accordance with the first embodiment of the present invention.  FIG. 1A  is a plan view, partially cut-away, of the surface of the speaker system in accordance with the first embodiment.  FIG. 1B  is a cross-sectional view taken along the line A-B in  FIG. 1A . The speaker system shown in  FIGS. 1A and 1B  includes a cuboid and low-profile speaker cabinet  1 , partition boards  8   a  and  8   b  provided within the speaker cabinet  1 , and a speaker unit  9 . 
         [0045]    The speaker cabinet  1  includes a front board  2 , a back board  3 , side boards  4  and  5  in the longitudinal direction, and side boards  6  and  7  in the lateral direction. The speaker unit  9  is attached to the front board  2  of the speaker cabinet  1 . The partition board  8   a  is connected with the front board  2 , the back board  3 , and the side board  6  in the lateral direction of the speaker cabinet  1 . On the other hand, the partition board  8   b  is connected with the front board  2 , the back board  3 , and the side board  7  in the lateral direction of the speaker cabinet  1 . Furthermore, an acoustic tube  11  within the speaker cabinet  1  is formed of the partition boards  8   a  and  8   b,  the front board  2 , the back board  3 , and the side boards  6  and  7 . The acoustic tube  11  has one end (opening  12 ) open and the other end (end part  13 ) closed. 
         [0046]    With reference to the sound pressure frequency characteristics in  FIG. 2 , the operations of a speaker system configured as above will be described. When an electrical input is applied to the speaker unit  9  attached to the front board  2  of the speaker cabinet  1 , a diaphragm vibrates to emit sound. At the time, the sound emitted into the internal space of the speaker cabinet  1  is transmitted to the inside of the acoustic tube  11  which is formed of the partition boards  8   a  and  8   b.  Here, since the end part  13  of the acoustic tube  11  is closed, the sound in the speaker cabinet  1  is not emitted from the acoustic tube  11  into the outside of the speaker cabinet  1 . 
         [0047]    Thus, the major difference between a conventional speaker system and a speaker system in accordance with the first embodiment is that the acoustic tube  11  is provided inside the speaker cabinet  1 . Therefore, the operations of the speaker system in accordance with the first embodiment will be described in comparison with a conventional closed-type and thin-profile speaker system. 
         [0048]    Here, the measurements of the inside of the speaker cabinet  1  in accordance with the first embodiment illustrated in  FIGS. 1A and 1B  are 410 mm long, 210 mm wide and 10 mm thick. In addition, the electrodynamic speaker unit  9  has an aperture of 8 cm and a thickness of 12 mm. Furthermore, the partition boards  8   a  and  8   b  are both 180 mm long and the distance between each other is 30 mm. 
         [0049]    In other words, the speaker cabinet  1  in accordance with the first embodiment is a cuboid that has a thin thickness measurement compared to length and width measurements. In other words, the ratio of the thickness measurement to the measurement of the longer direction (longitudinal direction) is 410/10=41. It is more preferable that the acoustic cabinet  11  be provided in the speaker cabinet  1  with the ratio of 10 or more, or more preferably 20 or more as follows. 
         [0050]    The acoustic tube  11  in accordance with the first embodiment is provided so as to reduce the apparent length in the longer direction (longitudinal direction in this example) of the inside of the speaker cabinet  1 . In other words, the acoustic tube  11  is provided such that the side wall surface of the acoustic tube  11  (partition board  8   b ) and the propagation direction of standing waves which occur inside the speaker cabinet  1  (longer direction) cross each other or intersect at right angles. 
         [0051]    In the speaker system shown in  FIGS. 1A and 1B , the characteristic I in  FIG. 2  indicates the sound pressure frequency characteristic of a conventional closed-type speaker system in the absence of the acoustic tube  11 . In this case, standing waves occur between the side boards  4  and  5  opposed to each other in the longer direction of the speaker cabinet  1 . This leads to a peak and a trough in sound pressure at around 400 Hz, i.e., a large disturbance to the sound pressure frequency characteristics. 
         [0052]    Next, the operations of the speaker system when the acoustic tube  11  in accordance with the first embodiment is provided within the speaker cabinet  1  will be described. The acoustic tube  11  with one end open and the other end closed is formed of the partition boards  8   a  and  8   b.  The partition boards  8   a  and  8   b  are provided almost parallel with the side board  4  which is one side in the longer direction of the speaker cabinet  1 . In other words, the partition boards  8   a  and  8   b  are almost perpendicular to the direction of the mode of the standing waves which occur between the side boards  4  and  5  in the longer direction when the acoustic tube  11  is not provided. 
         [0053]    As a result, the inside of the speaker cabinet  1  can be acoustically divided into the space where the acoustic tube  11  is provided and a back capacity  10  of the speaker unit  9 . Note that the back capacity  10  of the speaker unit  9  means the capacity of the space which excludes the space enclosed by the partition boards  8   a  and  8   b  (i.e., acoustic tube  11 ) from the internal space of the speaker cabinet  1 . 
         [0054]    Thus, the sound from the speaker unit  9  is emitted into the back capacity  10  and then transmitted to the acoustic tube  11 . Here, since the partition boards  8   a  and  8   b  have a narrow distance of 30 mm therebetween, it is acoustically considered that the long and narrow acoustic tube  11  is attached to the back capacity  10 . More specifically, the acoustic tube  11  in accordance with the first embodiment is a sound path that is turned around by the partition boards  8   a  and  8   b  and the length is approximately 400 mm. The acoustic tube  11  is rectangular in cross section and when the tube viewed from cross section is considered as a circle, the diameter is approximately 20 mm. 
         [0055]    Thus, both the back capacity  10  and the acoustic tube  11  are located between the side boards  4  and  5  opposed to each other in the longer direction of the speaker cabinet  1 . The characteristic II in  FIG. 2  is a sound pressure frequency characteristic of the speaker system in accordance with the first embodiment. As is evident from the characteristic II, it is possible to remove the standing waves which occur at around 400 Hz when the acoustic tube  11 , as indicated by the characteristic I is not provided. On the other hand, although a resonance that occurs due to the newly provided acoustic tube  11  causes a small trough in sound pressure at around 250 Hz, this does not cause a large disturbance to the sound pressure frequency characteristics of the speaker system. 
         [0056]    Furthermore, a peak and a trough in sound pressure at around 800 Hz which is twice 400 Hz can be found from a detailed analysis of the sound pressure frequency characteristics shown in  FIG. 2 . The frequency is due to the standing waves equivalent to the frequency f 2  which is twice the frequency f 1  of 400 Hz recited in the reference 1. The characteristic II of the first embodiment shows a flat characteristic without a peak and a trough at around 800 Hz. In other words, it is clear that the acoustic tube  11  suppresses the standing waves not only at the frequency f 1 , but also at the frequency f 2.    
         [0057]    Thus, according to the first embodiment, a speaker system with high sound quality can be made, which has very small disturbances in the sound pressure frequency characteristics due to the multiple standing waves which occur in the speaker cabinet  1 . Furthermore, unlike the reference 1, a sound absorbing material is not provided at the opening  12  of the acoustic tube  11 . Therefore, the sound in the speaker cabinet  1  is not damped by the sound absorbing material, thus preventing the decline in sound pressure level, especially in the bass range. 
         [0058]    Note that as shown in  FIGS. 1A and 1B , the sound absorbing material  100  may additionally be placed on the end part  13  of the acoustic tube  11 . Accordingly, when there is a large resonance at around 250 Hz due to the acoustic tube  11 , the placement of the sound absorbing material  100  can more effectively suppress the resonance and lead to flat sound pressure frequency characteristics (For the sound pressure frequency characteristic indicated by the characteristic II in  FIG. 2 , the sound absorbing material  100  is not placed.) In this case, the sound absorbing material  100  is provided within the speaker cabinet  1 . However, since the sound absorbing material  100  is placed on the end part  13  which is the closed end of the acoustic tube  11 , only a small amount of sound passes through the end part  13 . Thus, there is only a slight decline in sound pressure level in the bass range due to the absorbing effects of the absorbing material  100 . 
         [0059]    Note that although the acoustic tube  11  is provided near the side board  4  in the longitudinal direction, another acoustic tube may also be provided nearby the side board  5  which is opposed to the side board  4 . In this case, since both of the surfaces opposed to each other in the longitudinal direction have the acoustic tubes  11 , occurrence of standing waves is suppressed more effectively than when the acoustic tube  11  is provided on only one side. 
         [0060]    Note that although the acoustic tube  11  is provided in the cuboid speaker cabinet  1  which has a thin thickness measurement compared to length and width measurements in the above example, placement of the acoustic tube  11  is not limited to a speaker cabinet of this shape. For example, an acoustic tube may be provided within a pillar-shaped speaker cabinet that has a tall height compared to width and depth measurements (the following embodiments are the same). In this case, the acoustic tube may be provided near the top or bottom board inside the speaker cabinet so as to reduce the apparent height of the inside of the speaker cabinet. 
       Second Embodiment 
       [0061]    Next,  FIGS. 3A and 3B  show a speaker system in accordance with the second embodiment of the present invention.  FIG. 3A  is a plan view, partially cut-away, of the surface of the speaker system in accordance with the second embodiment.  FIG. 3B  is a cross-sectional view taken along the line C-D in  FIG. 3A . The speaker system shown in  FIGS. 3A and 3B  includes a cuboid and low-profile speaker cabinet  20 , partition boards  27   a,    27   b,    27   c,  and  29 , an acoustic tube  28 , an acoustic port  30 , and a speaker unit  31  attached to a front board  21 . 
         [0062]    The speaker cabinet  20  includes a front board  21 , a back board  22 , side boards  23  and  24  in the longitudinal direction, and side boards  25  and  26  in the lateral direction. The partition board  29  is provided in parallel with the side board  25 . Furthermore, the acoustic port (bass reflex port)  30  is formed of the front board  21 , the back board  22 , the side board  25 , and the partition board  29 . In addition, the acoustic tube  28  with one end open and the other end closed is formed of the partition boards  27   a,    27   b,    27   c,  and  29 , the front board  21 , the back board  22 , and the side boards  23  and  26 . 
         [0063]    With reference to the sound pressure frequency characteristics in  FIG. 4 , the operations of a speaker system configured as above will be described. The difference from the first embodiment is that a type of speaker system is changed from the closed type to the bass reflex type. 
         [0064]    When an electrical input is applied to the speaker unit  31  attached to the front board  21  of the speaker cabinet  20 , a diaphragm vibrates to emit sound. At the time, the sound emitted into the internal space of the speaker cabinet  20  is transmitted to the inside of the acoustic tube  28  which is formed of the partition boards  27   a,    27   b,  and  27   c.  Here, since the end part of the acoustic tube  28  is closed, the sound in the speaker cabinet  20  is not emitted from the acoustic tube  28  into the outside of the speaker cabinet. 
         [0065]    Although the operations above are the same as the first embodiment, in the bass reflex speaker system in accordance with the second embodiment, the speaker cabinet  20  includes the acoustic port  30  by providing the partition board  29 . In other words, sound pressure level in the bass range is higher than the first embodiment due to the acoustic resonance between the acoustic port  30  and the internal capacity of the speaker cabinet  20 . 
         [0066]    In order to explain the effects of the second embodiment, sound pressure frequency characteristics of a conventional bass reflex speaker system which eliminates the acoustic tube  28  from the speaker cabinet  20  in  FIG. 3A  and  FIG. 3B  will be compared with those of a speaker system in accordance with the second embodiment. Thus, the major difference between the conventional speaker system and the speaker system in the second embodiment is that the acoustic tube  28  is provided inside the speaker cabinet  20 . Therefore, the operations of the speaker system in accordance with the second embodiment will be described in comparison with a conventional bass reflex and thin-profile speaker system. 
         [0067]    Here, the measurements of the inside of the speaker cabinet  20  in accordance with the second embodiment are 410 mm long, 210 mm wide and 10 mm thick as same as the first embodiment. In addition, the electrodynamic speaker unit  31  has an aperture of 8 cm and a thickness of 12 mm. Furthermore, each of the partition boards  27   a,    27   b,  and  27   c  is 88 mm long and the distances between each other are 30 mm. Furthermore, the acoustic port  30  is 130 mm long. 
         [0068]    In addition, the acoustic tube  28  is provided so as to reduce the apparent length in the longer direction (longitudinal direction in this example) of the inside of the speaker cabinet  28 . In other words, the acoustic tube  28  is provided such that the side wall surface of the acoustic tube  28  (partition board  27   c ) and the propagation direction of standing waves which occur inside the speaker cabinet  20  (longer direction) cross each other or intersect at right angles. 
         [0069]    The characteristic III in  FIG. 4  indicates a sound pressure frequency characteristic of the conventional bass reflex speaker system which does not include the acoustic tube  28  in the speaker system shown in  FIGS. 3A and 3B . Since a resonance of the acoustic port  30  increases the sound pressure level at around 80 Hz in the characteristic III, it is clear that the effects of the bass reflex speaker system are obtained. On the other hand, standing waves occur between the side boards  23  and  24  opposed to each other in the longer direction of the speaker cabinet  20 , leading to a peak and a trough in sound pressure at around 360 Hz. This causes a large disturbance to the sound pressure frequency characteristics. 
         [0070]    Next, the operations of the speaker system in accordance with the second embodiment, which has the acoustic tube  28  inside the speaker cabinet  20 , will be described. Each of the partition boards  27   a,    27   b,  and  27   c  is provided almost parallel with the side board  23  which is one side in the longer direction of the speaker cabinet  20 . In other words, the acoustic tube  28  with one end open and the other end closed are almost perpendicular to the direction of the mode of the standing waves which occur between the side boards  23  and  24  in the longer direction when the acoustic tube  28  is not provided. 
         [0071]    As a result, the inside of the speaker cabinet  20  can be divided into the space where the acoustic tube  28  is provided, a back capacity  32  of the speaker unit  31 , and the acoustic port  30 . Note that the back capacity  32  of the speaker unit  31  means,the capacity of the space which excludes the acoustic tube  28  and the acoustic port  30  from the internal space of the speaker cabinet  20 . Thus, the sound from the speaker unit  31  is emitted into the back capacity  32  and then transmitted to the acoustic tube  28  and the acoustic port  30 . 
         [0072]    Here, the partition boards  27   a,    27   b,  and  27   c  have a narrow distance of 30 mm therebetween as same as the first embodiment. 
         [0073]    Therefore, it is acoustically considered that the acoustic tube  28  with the end part closed and the acoustic port  30  are attached to the back capacity  32 . More specifically, the acoustic tube  28  is approximately 480 mm. When the cross-section area of the acoustic tube  28  is considered as a circle, the diameter is approximately 20 mm. Thus, both the back capacity  32  and the acoustic tube  28  are provided between the side boards  23  and  24  opposed to each other in the longer direction of the speaker cabinet  20 . 
         [0074]    The characteristic IV in  FIG. 4  is a sound pressure frequency characteristic of the speaker system in accordance with the second embodiment. The standing waves which occur at around 360 Hz when the acoustic tube  28  is not provided, as indicated by the characteristic III in  FIG. 4  can be suppressed. On the other hand, although there is a little resonance at around 270 Hz due to the newly-provided acoustic tube  28 , this does not cause a large disturbance to the sound pressure frequency characteristics of the speaker system. In other words, the speaker cabinet  20  allows for a speaker system with high sound quality. 
         [0075]    In addition, in the characteristic in the absence of the acoustic tube  28  as indicated by the characteristic III in  FIG. 4 , a trough in sound pressure occurs at the frequency f 2  of 700 Hz due to the second standing waves. The frequency f 2  is twice the frequency f 1  of 350 Hz of the first standing waves. However, as shown in the characteristic IV in accordance with the second embodiment, the sound pressure frequency characteristic at 700 Hz is flat. In other words, according to the second embodiment, multiple standing waves are suppressed by the acoustic tube  28  alone without the need of the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b  of different lengths, which are provided in the reference 1 in accordance with the first and second standing waves. 
         [0076]    Here, in order to improve sound pressure level in the bass range, the bass reflex speaker system uses an acoustic resonance of an acoustic compliance that is determined by the acoustic mass of the acoustic port  30  and the capacity of the speaker cabinet  20 . For reproduction in the lower bass range, it is necessary to increase the acoustic compliance of the speaker cabinet  20 , i.e., to increase the internal capacity of the speaker cabinet  20 . 
         [0077]    In the second embodiment, since the acoustic tube  28  is provided within the speaker cabinet  20 , the acoustic capacity seems to be reduced. However, in the band which has lower frequencies than the band which has a longer wavelength than the equivalent length of the acoustic tube  28  (for example, a wavelength of 3.4 m at 100 Hz), the space of acoustic tube  28  can be considered a part of the capacity of the speaker cabinet  20 . 
         [0078]    Therefore, the Internal capacity of the speaker cabinet  20  is the total capacity of the back capacity  32  of the speaker unit  31  and the capacity of the acoustic tube  28 . As a result, there is no difference from the capacity of the conventional bass reflex type speaker cabinet  20  in the absence of the acoustic tube  28 , and thus there are few differences in the bass range characteristics which are determined by the acoustic compliance of the speaker cabinet  20  and the resonance of the acoustic port  30 . Thus, it is possible to make a bass reflex speaker system that has fewer disturbances in sound pressure due to multiple standing waves which occur within the speaker cabinet  20  and that is able to reproduce rich bass sound. 
         [0079]    In addition, since a sound absorbing material is not provided at the opening of the acoustic tube  28  in contrast to the reference 1, the sound in the speaker cabinet  20  is not damped by the sound absorbing material. Therefore, the sound pressure level does not decrease especially in the bass range. 
         [0080]    Here, in order to provide a lower-profile speaker system, it is necessary to reduce the thickness of a speaker unit to be installed in the speaker system so as to fit a low-profile cabinet. The current mainstream speaker units are electrodynamic speaker units that obtain a driving force by gathering magnetic flux from a magnet around a voice coil. 
         [0081]    However, with reduction in the thickness of an electrodynamic speaker unit, a magnet constituting its magnetic circuit is also made thinner, thus reducing magnetic energy of the magnet. This results in a smaller driving force to be generated in the voice coil and lower sound pressure level. In addition, for electrodynamic speaker units, the Q-value of the lowest resonance frequency is damped by electromagnetic damping resistance that is caused by a counter-electromotive force generated by vibration of the voice coil. Thus, the decrease in magnetic flux due to the reduction in the thickness of the magnet lowers the electromagnetic damping force and a large peak in sound pressure occurs in sound pressure frequency characteristics at around the lowest resonance frequency f OB  of the speaker unit which is attached to a speaker cabinet. This degrades sound quality. 
         [0082]    Furthermore, another type of low-profile speaker unit is a piezoelectric speaker unit. Unlike the electrodynamic speaker unit, the piezoelectric speaker unit does not have a magnetic circuit that gathers magnetic flux from a magnet, and bends a diaphragm by the expansion and contraction of a thin piezoelectric element in the form of a board to emit sound. This allows a significant reduction in the thickness compared to the electrodynamic speaker unit. However, for the piezoelectric speaker unit, it is difficult to suppress the Q value of a resonance of the diaphragm and thus a large peak in sound pressure occurs at around the lowest resonance frequency f OB . This disturbs sound pressure frequency characteristics of the speaker system and degrades sound quality as in the case of the electrodynamic speaker system with reduced magnetic energy of a magnet. 
         [0083]    The speaker system disclosed in Patent Literature 2 is the known art to solve this problem.  FIG. 14  is a cross-sectional view of the conventional speaker system recited in Patent Literature 2. The speaker system illustrated in  FIG. 14  is a bass reflex speaker system that includes a loudspeaker cabinet  70 , an electrodynamic loudspeaker unit  71 , an acoustic resistance member  72 , and a bass reflex port  75 . 
         [0084]    The operations of a conventional speaker system configured as above will be described. The sound from the rear of the diaphragm of the speaker unit  71  is emitted into the capacity  74  of the space enclosed by the rear of the diaphragm of the speaker unit  71  and the acoustic resistance member  72  after passing through the acoustic resistance member  72  from the volume  73  of the space enclosed by the acoustic resistance member  72  and the speaker cabinet  70 . At this time, the acoustic resistance member  72  damps the sound which passes through the acoustic resistance member  72 , thus dampening the vibration of the diaphragm of the speaker unit. This damps the sound pressure of the speaker system which is emitted from the front of the speaker unit. This damping effect flattens peaks and troughs in the sound pressure frequency characteristics of the speaker system. 
         [0085]    In addition, as mentioned above, the speaker system disclosed in Patent Literature 1 has the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b,  each of which has an opening at one end in order to prevent the standing waves, which occur in the opposing faces of the wall of the speaker cabinet  60 , from disrupting movements of the diaphragm of the speaker unit  63  and disturbing the sound pressure frequency characteristics. Furthermore, the sound absorbing materials  65   a,    65   b,    67   a,  and  67   b  which seal the openings separate the internal spaces of the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b  from the internal space of the speaker cabinet  60 , respectively. Furthermore, each of the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b  has a tube length of approximately 1/(2n) times the wavelength corresponding to the lowest resonance mode of the sanding waves to be generated along an inner wall surface of the speaker cabinet  60 , and the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b  are provided such that the openings are located in the vicinity of nodal points of standing waves. Here, n is a natural logarithm of 2 or more. This suppresses the standing waves and flattens the sound pressure frequency characteristics of the speaker system. 
         [0086]    However, the speaker system disclosed in Patent Literature 2 has a damping effect on the wide bass range from around the lowest resonance frequency f OB  of the speaker unit  71  which is attached to the speaker cabinet  70  to around the resonance frequency f OP  of the bass reflex port  75 . In particular, the vicinity of the resonance frequency f OP  for the bass reflex port  75  of the speaker cabinet  70  is an important frequency band to obtain the sense of bass sound of the speaker system. The problem is a shortage of the sense of bass sound when the damping effect of the acoustic resistance member  72  suppresses into the sound pressure level around the resonance frequency f OP  which is a bass reproduction limit. 
         [0087]    In addition, in the speaker system disclosed in Patent Literature 1, the acoustic resonance of the first and second acoustic tubes  64   a,    64   b,    66   a,  and  66   b  suppresses the standing waves which occur in the speaker cabinet  60  to allow the diaphragm of the speaker unit  63  to easily move, thus flattening the trough in sound pressure. Therefore, peaks of sound pressure cannot be suppressed by controlling the movement of the speaker unit  63  at around the lowest resonance frequency f OB  of the speaker unit  63 . 
         [0088]    The third and fourth embodiments have been made in view of the above problems. Accordingly, objects of the third and fourth embodiments are to provide a speaker system which can flatten peaks of sound pressure of a speaker unit without lowering sound pressure level in the bass range. 
       Third Embodiment 
       [0089]      FIGS. 5A and 5B  show a speaker system in accordance with the third embodiment of the present invention.  FIG. 5A  is a plan view, partially cutaway, of the surface of a speaker system in accordance with the third embodiment.  FIG. 5B  is a cross-sectional view taken along the line E-F in  FIG. 5A . 
         [0090]    The speaker system shown in  FIGS. 5A and 5B  includes a speaker cabinet  41 , a piezoelectric speaker unit  44 , a drone cone  45 , an acoustic tube  46 , and a sound absorbing material  40 . The speaker cabinet  41  includes a front board  42  and a back board  43 . In addition, an acoustic tube  46  with one end (opening  48 ) open and the other end (end part  49 ) closed is formed of partition boards  47   a  and  47   b.  Furthermore, the sound absorbing material  40  is provided at the end part  49  of the acoustic tube  46 . 
         [0091]    Here, the speaker system described above is designed such that the resonance frequency which is determined by an inductance component of an acoustic impedance of the acoustic tube  46  and an acoustic compliance of the speaker cabinet  41  is substantially identical to a peak frequency of sound pressure of the speaker unit  44  which is attached to the speaker cabinet  41 . The peak frequency at the time is higher than the lowest resonance frequency of the speaker unit  44  which is not attached to the speaker cabinet  41 . In other words, the peak frequency should nearly identical to the lowest resonance frequency f OB  of the speaker unit  44  which is attached to the speaker cabinet  41 . 
         [0092]    Note that the inductance component of the acoustic impedance of the acoustic tube  46  changes according to the length of the acoustic tube  46  or the cross-sectional area of the acoustic tube  46 . More specifically, the longer the length of the acoustic tube  46 , the larger the inductance component. In addition, the acoustic compliance of the speaker cabinet  41  changes according to the capacity of the speaker cabinet  41 . More specifically, the larger the capacity of the speaker cabinet  41 , the larger the acoustic compliance. 
         [0093]    For example, the resonance frequency f 0  can be obtained from the following equation 1. Here, M denotes the inductance component of the acoustic impedance of the acoustic tube  46  and C denotes the acoustic compliance of the speaker cabinet  41 . In other words, the resonance frequency f 0  can be set to a given value by adjusting the length (or cross-section area) of the acoustic tube  46  and the capacity of the speaker cabinet  41 . 
         [0000]    
       
         
           
             
               
                 
                   
                     [ 
                     
                       Equation 
                        
                       
                           
                       
                        
                       1 
                     
                     ] 
                   
                    
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     f 
                     0 
                   
                   = 
                   
                     
                       1 
                       
                         2 
                          
                         π 
                       
                     
                      
                     
                       
                         1 
                         MC 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0094]    With reference to the sound pressure frequency characteristics in  FIG. 6  and the equivalent circuit in  FIG. 7 , the operations of a speaker system configured as above will be described. When an electrical input is applied to the speaker unit  44  attached to the front board  42  of the speaker cabinet  41 , a diaphragm vibrates to emit sound. At the time, the sound emitted into the internal space of the speaker cabinet  41  is transmitted to the drone cone  45  attached to the front board  42  of the speaker cabinet  41 . In addition, the sound from the rear of the speaker unit  44  is also transmitted to the acoustic tube  46  which is formed of the partition boards  47   a  and  47   b.  Here, since the end part  49  of the acoustic tube  46  is closed, the sound is not emitted from the acoustic tube  46  into the outside of the speaker cabinet. 
         [0095]    The major difference between a conventional drone cone speaker system and a speaker system in accordance with the third embodiment is that the acoustic tube  46  is provided inside the speaker cabinet  41 . Therefore, the operations of the speaker system in accordance with the third embodiment will be described in comparison with a conventional drone cone speaker system. 
         [0096]    Here, in the third embodiment illustrated in  FIGS. 5A and 5B , the measurements of the inside of the speaker cabinet  41  are 360 mm long, 210 mm wide and 8 mm thick. The speaker unit  44  is 90 mm long and 50 mm wide. Furthermore, the drone cone  45  has almost the same external size as the speaker unit  44 . 
         [0097]    The characteristic i in  FIG. 6  shows a sound pressure frequency characteristic of the speaker system which does not include the acoustic tube  46  in the speaker system illustrated in  FIGS. 5A and 5B , i.e., a conventional drone cone speaker system. 
         [0098]    The bass reproduction limit of the characteristic i in  FIG. 6  is extended up to around a resonance frequency f pp  of 120 Hz between the mass of the drone cone  45  and an acoustic compliance of the internal space of the speaker cabinet  41  due to a resonance of the drone cone  45 . On the other hand, the peak of sound pressure at 200 Hz is caused by a resonance of the speaker unit  44  attached to the speaker cabinet  41 . The speaker unit  44  has a high Q value of resonance due to a resonance of the diaphragm. Thus, the peak of sound pressure at 200 Hz is approximately 15 dB higher than the sound pressure level in the band around 200 Hz. If this remains the same, sound quality of the speaker system is significantly degraded. 
         [0099]    Next, the operations of the speaker system when the acoustic tube  46  in accordance with the third embodiment is provided within the speaker cabinet  41  will be described. Here, the length L of the partition boards  47   a  or  47   b  is 150 mm and the width W of the sound path is 50 mm. The acoustic tube  46  is turned around by the partition boards  47   a  and  47   b.  When sound is considered to pass through the edge of the partition board  47   a  in an arc as shown in a broken line in  FIG. 5A , the length of the sound path is approximately 410 mm. Therefore, a capacity Vb of the speaker cabinet  41  excluding a capacity Vh of 0.15 liters of the acoustic tube  46  is 0.45 liters. 
         [0100]      FIG. 7  shows an equivalent circuit of the speaker system in accordance with the third embodiment. In  FIG. 7 , F denotes a driving force. Zms denotes a machine impedance of the speaker unit  44 . Sd denotes an area of the diaphragm. Cb denotes an acoustic compliance of the capacity Vb of the speaker cabinet  41 . Zh denotes acoustic impedance when the acoustic tube  46  is viewed from the opening  48 . Cd denotes an acoustic stiffness of the drone cone. Md denotes an acoustic mass of the drone cone. 
         [0101]    When viewed from the diaphragm of the speaker unit (piezoelectric speaker)  44 , the acoustic compliance Cb of the speaker cabinet  41  and an inductance component of the acoustic impedance of the acoustic tube  46  cause a resonance at around the resonance frequency f pp . As is evident from the equivalent circuit in  FIG. 7 , this resonance is a parallel resonance. Therefore, when viewed from the diaphragm side of the speaker unit  44 , the acoustic impedance of the resonance is very high, thus significantly dampening the vibrations of the diaphragm of the speaker unit (piezoelectric speaker)  44 . 
         [0102]    The characteristic ii in  FIG. 6  is a sound pressure frequency characteristic when the acoustic tube  46  is formed of the partition boards  47   a  and  47   b  in the speaker cabinet  41 . The resonance between the acoustic compliance Cb of the speaker cabinet  41  and an inductance component of the acoustic impedance of the acoustic tube  46  significantly suppresses the peak of the sound pressure in the sound pressure frequency characteristic at around a frequency f pp  of 200 Hz, when compared to the characteristic in the absence of the acoustic tube  46 , and causes a trough of around 6 dB. 
         [0103]    Next, the characteristic iii in  FIG. 6  shows a sound pressure frequency characteristic when the absorbing material  40  is provided near the end part  49  of the acoustic tube  46 . The absorbing material  40  relaxes the Q value of the resonance between the acoustic compliance Cb of the speaker cabinet  41  and the inductance component of the acoustic impedance of the acoustic  46 , leading to almost a flat sound pressure frequency characteristic at around 200 Hz, compared to when only the acoustic tube  46  is provided. 
         [0104]    On the other hand, the acoustic tube  46  does not function as an acoustic tube in the bass range at the resonance frequency f pp  of around 120 Hz between the mass of the drone cone  45  and the acoustic compliance of the speaker cabinet  41 . Therefore, the capacity Vh of 0.15 liters and the capacity Vb of 0.45 liters of the speaker cabinet  41  are added to make a total capacity of Vh and Vb. In other words, the capacity of the acoustic tube  46  is included in a capacity of a conventional drone cone speaker cabinet. Thus, the sense of bass sound is rarely in shortage in contrast to the Patent Literature 2 in which the acoustic resistance member  72  provided at the rear of the speaker unit  73  lowers the sound pressure level to around the frequency f op , which is the bass reproduction limit. 
         [0105]    Here, the location of the absorbing material  40  in the acoustic tube  46  will be described. The case when the absorbing material  40  is provided at the end part  49  of the acoustic tube  46  as described in third embodiment will be compared with the case when the absorbing material  40  is provided at the opening  48  as disclosed in Patent Literature 2. 
         [0106]      FIG. 8  shows the measurement result of sound pressure frequency characteristics of the speaker system, in almost the same configuration as the one shown in  FIGS. 5A and 5B , (iv) when the acoustic tube  46  is not provided, (vi) when the absorbing material  40  is provided at the end part  49  of the acoustic tube  46  and (v) when the absorbing material  49  is provided at the opening  48  of the acoustic tube  46 . 
         [0107]    With reference to  FIG. 8 , in the characteristic iv in the absence of the acoustic tube  46 , a high peak of sound pressure occurs at around 200 Hz due to the resonance of the speaker unit  44 . 
         [0108]    Next, in the characteristic v when the absorbing material  49  is provided at the opening  48  of the acoustic tube  46 , the frequency at which the peak of sound pressure occurs increases to around 250 Hz. Therefore, sound pressure cannot be flattened. In contrast, in the characteristic vi when the absorbing material  40  is provided at the end part  49  of the acoustic tube  46 , the peak of sound pressure at 200 Hz is suppressed and flat sound pressure frequency characteristic is achieved. 
         [0109]    This result leads to a problem that the resonance frequency fluctuates when the absorbing material  40  is provided at the opening  48 , rather than the effects that the acoustic impedance of the acoustic tube  46  changes and suppresses the Q value of the resonance. In addition, when the absorbing material  40  is provided at the opening  48  of the acoustic tube  46 , damping effect of the absorbing. material  40  also lowers sound pressure level in the bass range at around 100 Hz. In other words, it is clear that locating the absorbing material  40  at the end part  49  of the acoustic tube  46  is an effective means of suppressing the Q value of the resonance of the speaker system in accordance with the third embodiment, but of not affecting reproduction of the bass range. 
         [0110]    In addition, the effect of decreasing harmonic distortion in accordance with the third embodiment will be described.  FIG. 9  compares a sound pressure frequency characteristic and second harmonic distortion characteristic in sound pressure as to when the acoustic tube  46  is not provided in the speaker cabinet  41 , and when the acoustic tube  46  is provided. In  FIG. 9 , the characteristic vii shows a sound pressure frequency characteristic when the acoustic tube  46  is not provided. The characteristic viii shows a second harmonic distortion when the acoustic tube  46  is not provided. The characteristic ix shows a sound pressure frequency characteristic when the acoustic tube  46  is provided. The characteristic x shows a second harmonic distortion when the acoustic tube  46  is provided. Note that as mentioned above, the acoustic tube  46  suppresses the peaks of sound pressure at around 200 Hz. 
         [0111]    Here, as to distortion characteristics, the second harmonic distortion having a peak of 45 dB at around 100 Hz occurs as indicated by the characteristic viii in absence of the acoustic tube  46 . However, by providing the acoustic tube  46 , the second harmonic distortion at around 100 Hz decreases by around 20 dB as indicated by the characteristic x. 
         [0112]    This is a secondary effect of suppressing the peak of sound pressure at 200 Hz by a resonance between the acoustic tube  46  and the capacity of the speaker cabinet  41 . This is because the resonance between the acoustic tube  46  and the capacity of the speaker cabinet  41  dampens vibrations of sound pressure components at 200 Hz included in vibration components of the diaphragm at 100 Hz, i.e., vibrations of second harmonic components. This reduces the distortion at 100 Hz which is a bass reproduction limit and a speaker system with improved sound quality can be made. 
         [0113]    Note that in the third embodiment, the acoustic tube  46  is formed by placing partition boards  47   a  and  47   b  between the front board  42  and back board  43  of the speaker cabinet  41 . However, the third embodiment is not limited to this configuration. When the separate acoustic tube  46  of any opening shape such as a round shape is provided in the speaker cabinet  41 , the same effects are obtained as the third embodiment. 
       Fourth Embodiment 
       [0114]    Next,  FIG. 10  shows a cross-sectional view of a speaker system in accordance with the fourth embodiment. The speaker system illustrated in  FIG. 10  includes a speaker cabinet  50 , an electrodynamic speaker unit  51 , a bass reflex port  52 , an acoustic tube  53 , and a sound absorbing material  56 . The acoustic tube  53  with one end (opening  54 ) open and the other end (end part  55 ) closed has the absorbing material  56  at the end part  55 . 
         [0115]    The operations of a speaker system configured as above will be described. The differences from the third embodiment are that the piezoelectric speaker unit  44  is replaced by the electrodynamic speaker unit  51 , and that the drone cone  45  is replaced by the bass reflex port  52 . 
         [0116]    The change from the drone cone  45  to the bass reflex port  52  does not dramatically change the operations of the speaker system. A resonance is caused by an acoustic compliance of an internal space  57  of the speaker cabinet  50  and the acoustic mass of the bass reflex port  52 , and a bass reproduction range is extended. This is a basic function of a bass reflex speaker system as same as the third embodiment. 
         [0117]    On the other hand, unlike the piezoelectric speaker unit  44 , the Q value of the lowest resonance frequency is suppressed by electromagnetic damping resistance in the electrodynamic speaker unit  51 . However, the electromagnetic damping resistance is inversely proportional to the square of the product of a length of a voice coil L and a magnetic flux density B, (BL) 2 . Therefore, when a magnet of a magnetic circuit constituting the electrodynamic speaker unit  51  becomes smaller, the magnetic flux density B also becomes smaller. Thus, damping of the Q value is no longer effective. 
         [0118]      FIG. 11  shows sound pressure frequency characteristics of a bass reflex speaker system that includes the 8-cm-aperture electrodynamic speaker unit  51  which is attached to the speaker cabinet  50  having an internal capacity of 1 liter. The characteristics are calculated by changing the value of BL. Here, as constants for the 8-cm-aperture speaker, the vibration mass is 4.5 g, a voice coil impedance is 8Ω, an effective radius of the diaphragm is 30 mm. 
         [0119]    In  FIG. 11 , BL=6 in the characteristic (a), BL=4 in the characteristic (b), and BL=2 in the characteristic (c). When BL=6, the electromagnetic damping resistance is large. Therefore, the sound pressure frequency characteristic at around 200 Hz which corresponds to the resonance frequency f OB  of the speaker unit  51  attached to the speaker cabinet  50  is almost flat. On the other hand, when BL=2, there is a shortage of damping of the Q value of the resonance and a sound pressure peak of around 10 dB occurs at around 200 Hz. Even though such a speaker is in shortage of damping of the Q value due to small BL, when the acoustic tube  53  is provided within the speaker cabinet  50  as described in the fourth embodiment illustrated in  FIG. 10 , the same effects as the third embodiment can be obtained. In other words, vibrations of the diaphragm of the electrodynamic speaker unit  51  can be suppressed by a resonance between an acoustic compliance of the capacity Vb of the internal space  57  of the speaker cabinet  50  which excludes the capacity Vh of the acoustic tube  53  and an inductance component of an acoustic impedance of the acoustic tube  53 . In addition, the absorbing material  56  which is provided at the end part  55  of the acoustic tube  53  can achieve flat sound pressure frequency characteristics. 
         [0120]    Here, the relationship between the capacity Vh of the acoustic tube  53  and the capacity Vb of the internal space  57  of the speaker cabinet  50  which excludes the capacity Vh of the acoustic tube  53  will be described. The peaks of sound pressure at around 200 Hz can be suppressed by a resonance between an acoustic compliance of the capacity Vb of the internal space  57  of the speaker cabinet  50  and an inductance component of an acoustic impedance of the acoustic tube  53 . The tube diameter and the tube length of the acoustic tube  53  can be set to any value. 
         [0121]    The longer the tube diameter and tube length of the acoustic tube  53 , the larger the capacity Vh of the acoustic tube  53 . This means the smaller capacity Vb of the internal space  57  of the speaker cabinet  50  which excludes the capacity Vh of the acoustic tube  53 .  FIG. 12  shows sound pressure frequency characteristics when changing the ratio Vh/Vb of the two capacities described above from 0.2 to 0.5 to 0.8. In  FIG. 12 , in order to clarify the effects of the acoustic tube  53 , the absorbing material  56  is not provided at the end part  55  of the acoustic tube  53 . 
         [0122]    In  FIG. 12  shows sound pressure frequency characteristics. A characteristic (d) shows when the acoustic tube  53  is not provided. A characteristic (e) shows when Vh/Vb=0.2. A characteristic (f) shows when Vh/Vb=0.5. A characteristic (g) shows when Vh/Vb=0.8 The larger the ratio Vh/Vb, i.e., the larger the ratio of the capacity of the acoustic tube  53  to that of the speaker cabinet  50  by increasing the tube diameter or tube length of the acoustic tube  53 , the larger the frequency band width of the trough of sound pressure. Therefore, the ratio Vh/Vb may be determined in accordance with a frequency band width of a sound pressure peak of the electrodynamic speaker unit  51 . For instance, it is preferable that the larger the band width of the sound pressure peak of the speaker unit  51 , the larger the ratio of the internal space capacity of the acoustic tube  53  to that of the speaker cabinet  50 . 
         [0123]    The embodiments described above can independently be implemented or may optionally be combined. 
         [0124]    Although the embodiments of the present invention have been described with reference to the drawings, the present invention is not limited to the above-illustrated embodiments. Various kinds of modifications and variations may be added to the illustrated embodiments within the same or equal scope of the present invention. 
       INDUSTRIAL APPLICABILITY 
       [0125]    The present invention can be used in a wide variety of applications especially as a speaker system for television sets and mobile computers which have become thinner or as a speaker system for cars and others. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1 ,  20 ,  41 ,  50 ,  60 ,  70  speaker cabinet 
           2 ,  21 ,  42  front board 
           3 ,  22 ,  43  back board 
           4 ,  5 ,  6 ,  7 ,  23 ,  24 ,  25 ,  26 ,  62   a,    62   b,    62   c,    62   d  side board 
           8   a,    8   b,    27   a,    27   b,    27   c,    29 ,  47   a,    47   b  partition board 
           9 ,  31 ,  44 ,  51 ,  63 ,  71  speaker unit 
           10 ,  32  back capacity 
           11 ,  28 ,  46 ,  53  acoustic tube 
           12 ,  48 ,  54  opening 
           13 ,  49 ,  55  end part 
           30  acoustic port 
           45  drone cone 
           61   a  top board 
           61   b  bottom board 
           64   a,    64   b  first acoustic tube 
           40 ,  56 ,  65   a,    65   b,    67   b,    100  absorbing material 
           66   a,    66   b  second acoustic tube 
           72  acoustic resistance member 
           73  volume 
           74  capacity 
           52 ,  75  bass reflex port

Technology Classification (CPC): 7