Patent Publication Number: US-7212648-B2

Title: Loudspeaker system in which a diaphragm panel is driven by an electromechanical acoustic converter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a loudspeaker system and, more specifically, to a loudspeaker system in which a diaphragm panel is driven by an electromechanical acoustic transducer. 
     2. Description of the Background Art 
     Loudspeaker systems in which a diaphragm panel is driven by an electromechanical acoustic transducer have been suggested. One exemplary loudspeaker system employs a scheme in which an electromechanical transducer is directly attached to a diaphragm panel. In another example, a scheme is employed in which a diaphragm panel is acoustically vibrated by an electromechanical acoustic transducer via a space (such a scheme is hereinafter referred to as a sound-driving scheme). Here, the scheme in which the electromechanical transducer is directly attached to the diaphragm panel has several drawbacks. For example, in order to achieve required acoustic characteristics, there is a limitation of the location of the diaphragm panel to which the electromechanical transducer is attached. Therefore, in view of design flexibility of the loudspeaker system, the sound-driving scheme is more advantageous. 
       FIG. 14  is an illustration showing a basic configuration of a conventional loudspeaker system using the sound-driving scheme. In  FIG. 14 ,  100  denotes a plate-like diaphragm panel.  101  denotes a suspension for supporting the outer rim of the diaphragm panel  100 .  102  denotes a frame for fixing the outer rim of the suspension.  103  denotes an acoustic aperture provided on the bottom of the frame  102 .  104  denotes an electromechanical acoustic transducer such as to cover the acoustic aperture  103 .  105  denotes an enclosed space formed between the diaphragm panel  100  and the electromechanical acoustic transducer  104 . In this loudspeaker system, the suspension  101  for supporting the outer rim of the diaphragm panel  100  causes the entire diaphragm panel  100  to perform a piston action for emitting sound. That is, sound emitted from the electromechanical acoustic transducer  104  is led to the enclosed space  105 , where air is pressurized to cause the diaphragm panel  100  to vibrate, thereby emitting sound. 
     It is assumed herein that the diaphragm panel  100  performs a piston action in any frequency band. Under this assumption, an equivalent circuit of the loudspeaker system illustrated in  FIG. 14  can be presented as illustrated in  FIG. 15 . In the equivalent circuit illustrated in  FIG. 15 , F denotes a driving force of the electromechanical acoustic transducer  104  (driver). Rme denotes a magnetic damping resistance. Cms denotes a compliance of components that support vibrating components of the driver. Mms denotes a mass of the vibrating components in the driver. Rms denotes a mechanical resistance associated with the supporting of the driver. Sd denotes an effective area of a diaphragm of the driver. Furthermore, Cab denotes an acoustic compliance of the enclosed space  105 . Rab denotes an acoustic resistance of the enclosed space  105 . Cmp denotes a compliance of the suspension  101 . Rmp denotes a mechanical resistance of the suspension  101 . Mmp denotes a mass of the diaphragm panel  100 . Sp denotes an effective vibration area of a diaphragm portion composed of the diaphragm panel  100  and the suspension  101 . 
     As can be known from the equivalent circuit illustrated in  FIG. 15 , an acoustic transformer is structured based on an area ratio of the effective area Sd of the diaphragm of the electromechanical acoustic transducer  104  with respect to the effective vibration area Sp of the diaphragm portion (Sd/Sp). Therefore, at the time of the operation of the loudspeaker system, an equivalent mass of the diaphragm portion with respect to the electromechanical acoustic transducer  104  is proportional to the square of the area ratio (Sd/Sp). Therefore, if an electromechanical acoustic transducer having a diaphragm area smaller than the diaphragm panel  100  is used, the equivalent mass of the diaphragm panel  100  is small. In this case, even if the diaphragm panel  100  having a large mass is used, the efficiency of the loudspeaker system itself is not degraded. 
     In the loudspeaker system illustrated in  FIG. 14 , if a height Tg of the enclosed space  105  is lowered, a reproduction limit frequency in the treble range can be increased. Here, the reproduction-limit frequency in the treble range is defined by the mass Mmp of the diaphragm panel  100  and the acoustic compliance Cab of the enclosed space  105 . Also, the acoustic compliance Cab is defined by the capacity and height Tg of the enclosed space  105 . Therefore, in order to increase the reproduction-limit frequency in the treble range, the height Tg is lowered, thereby decreasing the acoustic compliance Cab. 
       FIG. 16  is a graph showing sound pressure frequency characteristics predicted by the equivalent circuit illustrated in  FIG. 15 . In  FIG. 16 , the illustrated characteristics can be predicted when the height Tg of the enclosed space  105  is 0.2 (mm), 0.4 (mm), or 0.8 (mm). Conditions for the above prediction are as follows. That is, an electrodynamic loudspeaker whose effective diaphragm area is approximately φ16 (mm) in diameter is used as the electromechanical acoustic transducer  104 . Also, a plate of 72 (mm) in height×51 (mm) in width×1 (mm) in thickness made of polycarbonate is used as the diaphragm panel  100 . The suspension  101  for use is made of SBR (styrene-butadiene rubber) of 5 (mm) in width×50 (μm) in thickness. As evident from  FIG. 16 , since the reproduction limit frequency in the treble range is defined by the height Tg of the enclosed space  105 , the height Tg has to be lowered in order to increase the reproduction limit frequency in the treble range. 
     In the above-mentioned conventional loudspeaker system using the sound-driving scheme, a suspension for supporting the outer rim of the diaphragm panel is required. This requirement makes the configuration of the loudspeaker system complicated. Furthermore, the complicated configuration makes it difficult to reduce the size of the loudspeaker system. Therefore, it is difficult to use the conventional loudspeaker system in devices such as portable terminals, which require downsizing and space-savings. 
     Furthermore, in the conventional loudspeaker system using the sound-driving scheme, it is difficult to improve acoustic characteristics in the bass and treble ranges simultaneously. That is, in the conventional scheme of driving the diaphragm panel by a piston action, the diaphragm panel is required to be high in stiffness and light in weight. However, there is a limitation in order to simultaneously satisfy both of high stiffness and light weight for achieving improvements in the acoustic characteristics. Details are described below. 
     Descriptions are made below to the fact that lowering the stiffness of the diaphragm panel reduces the sound pressure level.  FIGS. 17 and 18  are illustrations showing the results obtained by measuring the characteristics of the loudspeaker system under the same conditions as those of  FIG. 16 .  FIG. 17  is an illustration showing a vibration mode of the diaphragm panel of the conventional loudspeaker system at a frequency of 500 (Hz).  FIG. 18  is a graph showing sound pressure frequency characteristics of the conventional loudspeaker system. In  FIG. 17 , the height Tg of the enclosed space  105  is 0.2 (mm). 
       FIG. 17  illustrates a vibration mode of the suspension  101  on which the outer rim of the diaphragm panel  100  is mounted. Here, white portions represent a large vibration. As evident from  FIG. 17 , most of the suspension  101  is greatly vibrated. On the other hand, the diaphragm panel  100  has the outer rim portion being greatly vibrated, and a center portion being slightly vibrated. Therefore, in a bass range at a frequency of 500 (Hz), a separated resonance occurs, that is, the outer rim of the diaphragm panel  100  is greatly vibrated. In other words, in  FIG. 17 , the diaphragm panel  100  does not perform a piston action, that is, the diaphragm panel is vibrated not as a whole. This is because the stiffness of the diaphragm panel  100  is low. This also means that the equivalent circuit illustrated in  FIG. 15  is not applicable. As illustrated in  FIG. 18 , in practice, the separated resonance in the bass range occurring at the diaphragm panel  100  causes an increase of an acoustic impedance, that is, an acoustic load applied to the diaphragm. As a result, the velocity of the diaphragm is decreased, and the sound pressure level is also decreased. In  FIG. 18 , a solid line denotes actual measured values of the sound pressure frequency characteristics, while a dotted line denotes predicted values obtained by the equivalent circuit illustrated in  FIG. 15 . In  FIG. 18 , the sound pressure level of the measured values is lower than that of the values obtained by the equivalent circuit by approximately 10 (dB). 
     As described above, when the stiffness of the diaphragm panel is low, the sound pressure level in the bass range is also reduced. In order to solve this problem, the diaphragm panel requires a stiffness to some extent. One way to increase the stiffness of the diaphragm panel is, for example, to configure the diaphragm panel  100  so as to have a sandwich structure, that is, a structure with a core material sandwiched between surface materials attached thereto. Such a sandwich structure of the diaphragm panel  100 , however, has several drawbacks. Particularly, the use of the surface materials increases the mass of the diaphragm panel  100 , thereby disadvantageously lowering the sound pressure level in the treble range. Furthermore, the sandwich structure of the diaphragm panel  100  is rather a complicated structure, and also increases the thickness of the diaphragm panel  100 . 
     As such, in the conventional sound-driving scheme of causing the entire diaphragm panel  100  to perform a piston action, the stiffness of the diaphragm panel  100  has to be increased in order to improve the sound pressure level in the bass range. In order to improve the treble sound pressure level, on the other hand, the weight of the diaphragm panel  100  has to be reduced. In practice, however, in view of the structure and material of the diaphragm panel, there is a limitation to simultaneous achievement of high stiffness and light weight. Therefore, in the conventional sound-driving scheme, it is difficult to simultaneously achieve improvement in the acoustic characteristics in both the bass and treble ranges. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a sound-driving loudspeaker system achievable with a simple configuration. 
     Another object of the present invention is to provide a sound-driving loudspeaker system capable of easily improving acoustic characteristics. 
     The present invention has the following features to attain the objects mentioned above. That is, the loudspeaker system according to the present invention includes a board, an electromechanical acoustic transducer, and a diaphragm panel. The board forms a space for sound emission. The electromechanical acoustic transducer is connected to the board for emitting sound into the space for sound emission. The diaphragm panel has an outer rim portion fixed to the board in a manner to form the space with the board and has a stiffness lower than a stiffness of the board. Also, the diaphragm panel is flexed to be vibrated by energy of the sound emitted from the electromechanical acoustic transducer into the space to externally output the sound. 
     According to the above, the stiffness of the diaphragm panel is lower than that of the board. Therefore, when sound is emitted into the space, the diaphragm panel is flexed to be vibrated, thereby emitting sound. As such, when the diaphragm panel is vibrated by flex, the diaphragm panel can be directly attached to the board without a suspension, for example, for supporting the rim of the diaphragm panel. Thus, the configuration of the loudspeaker system can be simplified. With this, it is possible to achieve a small-sized, space-saving loudspeaker system. 
     Furthermore, according to the above, the entire diaphragm panel is vibrated not by a piston action but by flex. In this flex vibration scheme, for the purpose of improving a sound pressure level, the diaphragm panel is made to have a low stiffness and a light weight. Therefore, the sound pressure level in the bass range can be easily improved. That is, with the configuration of the loudspeaker system according to the present invention, the sound pressure level in the bass range can be easily improved. 
     Still further, the diaphragm panel may be made of a transparent material. Also, the board may be made of a transparent material. With this, the diaphragm panel can be made visually unobtrusive. Especially, the loudspeaker system according to the present invention can be achieved with a simple configuration without requiring a suspension. Therefore, with the diaphragm panel and the board being made transparent, a visually unobtrusive loudspeaker system can be easily achieved. 
     Still further, the loudspeaker system further includes light-emitting means. The light-emitting means is mounted onto the board and/or the diaphragm panel, for emitting light in response to an input signal supplied to the electromechanical acoustic transducer. The light-emitting means is implemented by a light-emitting diode, for example, but can be any light emitting device as long as it emits light in response to an electrical signal. With this, a loudspeaker system that can provide visual enjoyment to users can be achieved. 
     Still further, the diaphragm panel has an outer rim portion fixed to the board via a spacer. Here, the board may be a member dedicated to the loudspeaker system, or may be the entire or part of a structural component different from that of the loudspeaker system. That is, the board may serve as a structural component other than that of the loudspeaker system. The structural component is a concept including, for example, a wall of a building, a glass surface of a show window, a vehicle body, etc. If a poster pasted on a wall is used as the diaphragm panel, for example, it is possible to achieve a loudspeaker system that emits sound from the poster on the wall. Also, if a picture is pasted on a wall and a transparent diaphragm panel is placed on the picture, it is possible to achieve a loudspeaker system capable of providing users with a feeling as if the picture on the wall itself emits sound. Furthermore, with the use of a transparent diaphragm panel and a glass window as the board, for example, it is possible to achieve a loudspeaker system allowing users to see an outside view through the board and the transparent panel. 
     Still further, the board may have an acoustic aperture. In this case, the electromechanical acoustic transducer is positioned opposed to the diaphragm panel to allow sound to be emitted from the acoustic aperture into the space. This can achieve a configuration in which sound emitted from the electromechanical acoustic transducer is led to the space at the back of the diaphragm panel. 
     Still further, the loudspeaker system may further include an acoustic pipe for connecting the board and the electromechanical acoustic transducer together. In this case, the board has an acoustic aperture at a portion connected to the acoustic pipe. Also, the electromechanical acoustic transducer emits sound from the acoustic aperture through the acoustic pipe into the space. With this, the electromechanical acoustic transducer can be freely placed separately from the board and the diaphragm panel. Since the electromechanical acoustic transducer can be placed anywhere, design flexibility of the loudspeaker system is increased. It is particularly advantageous to place the electromechanical acoustic transducer, which is very difficult to be made transparent, separately from the diaphragm panel and the board both made transparent, thereby achieving a loudspeaker system with visually unobtrusive diaphragm panel and board. 
     Still further, the loudspeaker system further includes a cabinet for forming an enclosed space at the back of the electromechanical acoustic transducer. With this, sound of opposite phase from the back of the electromechanical acoustic transducer can be shielded. Therefore, a loudspeaker system excellent in reproduction of sound in the bass range can be achieved. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are illustrations each showing the configuration of a loudspeaker system according to Embodiment 1; 
         FIG. 2  is a section view of an electrodynamic loudspeaker, which is one example of an electromechanical acoustic transducer  12  illustrated in  FIG. 1B ; 
         FIG. 3  is an illustration showing a vibration mode of a diaphragm panel of the loudspeaker system according to Embodiment 1; 
         FIG. 4  is an illustration showing sound pressure frequency characteristics of the loudspeaker system according to Embodiment 1; 
         FIG. 5  is an illustration showing a board having a plurality of acoustic apertures; 
         FIGS. 6A ,  6 B, and  6 C are illustrations showing sound pressure frequency characteristics of the loudspeaker system observed when the acoustic apertures are provided at different locations; 
         FIGS. 7A and 7B  are illustrations each showing the configuration of a loudspeaker system according to Embodiment 2 of the present invention; 
         FIG. 8  is an illustration showing an exemplary case in which a loudspeaker system according to Embodiment 3 is mounted inside a vehicle; 
         FIG. 9  is a section view of a state in which a loudspeaker system  40  illustrated in  FIG. 8  is mounted onto a vehicle body; 
         FIG. 10  is an illustration showing the configuration of a loudspeaker system according to Embodiment 4 of the present invention; 
         FIG. 11  is a section view of a piezoelectric loudspeaker, which is one example of an electromechanical acoustic transducer  63  illustrated in  FIG. 10 ; 
         FIGS. 12A and 12B  are illustrations each showing the configuration of a loudspeaker system according to Embodiment 5 of the present invention; 
         FIG. 13  is an illustration showing an exemplary modification of a board used in the loudspeaker according to the present invention; 
         FIG. 14  is an illustration showing a basic configuration of a conventional loudspeaker system using the sound-driving scheme; 
         FIG. 15  is an illustration showing an equivalent circuit of the loudspeaker system illustrated in  FIG. 14 ; 
         FIG. 16  is a graph showing sound pressure frequency characteristics predicted by the equivalent circuit illustrated in  FIG. 15 ; 
         FIG. 17  is an illustration showing a vibration mode of a diaphragm panel of a conventional loudspeaker system at a frequency of 500 (Hz); and 
         FIG. 18  is a graph showing sound pressure frequency characteristics of the conventional loudspeaker system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     The configuration of a loudspeaker system according to Embodiment 1 of the present invention is now described by using  FIGS. 1A ,  1 B, and  2 .  FIGS. 1A and 1B  are illustrations each showing the configuration of the loudspeaker system according to Embodiment 1. Here,  FIG. 1A  is a front view of the loudspeaker system.  FIG. 1B  is a view the loudspeaker denoted by a line A–B in  FIG. 1A . In  FIG. 1A ,  10  denotes a board.  11  denotes a rectangular acoustic aperture provided onto the board  10 .  12  denotes an electromechanical acoustic transducer attached to the board  10  so as to cover the acoustic aperture  11 .  13  denotes a spacer provided on the rim of the board  10 .  14  denotes a diaphragm panel whose rim is attached to the spacer  13 .  15  is a base for supporting the board  10 . 
     In Embodiment 1, the board  10  and the diaphragm panel  14  are made of a transparent material. The board  10  and the spacer  13  are also made of a transparent material such as glass, polycarbonate, or acrylic. The diaphragm panel  14  is made of a transparent material such as PET (polyethylene terephthalate). Here, the diaphragm panel  14  is selected so as to have a stiffness lower than that of the board  10 . In Embodiment 1, the diaphragm panel  14  is configured to have a film shape. Also, the spacer  13  serves as a joint for jointing the board  10  and an outer rim portion of the diaphragm panel  14  together. As such, with the board  10  and the outer rim portion of the diaphragm panel  14  fixed together via the spacer  13 , a space  16  is formed between the board  10  and a center portion of the diaphragm panel  14 . Into the space  16 , sound is emitted from the electromechanical acoustic transducer  12 . The space  16  is preferably an enclosed space, but this is not meant to be restrictive. 
     As described above, the loudspeaker system illustrated in  FIGS. 1A and 1B  has a structure in which a suspension conventionally used for vibrating the diaphragm panel  14  is not used. Therefore, the structure of the conventional loudspeaker system can be simplified. 
       FIG. 2  is a section view of an electrodynamic loudspeaker, which is one example of the electromechanical acoustic transducer  12  illustrated in  FIG. 1B . In  FIG. 2 ,  20  denotes a vase-shaped yoke.  21  denotes a magnet provided at the center of the yoke  20 .  22  denotes a plate attached to the upper surface of the magnet  21 .  23  denotes a magnetic space formed between the inner rim of the yoke  23  and the outer rim of the plate  22 .  26  denotes a loudspeaker frame whose center portion is attached with the outer rim of a bottom surface of the yoke  20 .  25  denotes a diaphragm whose outer rim is attached to the loud speaker frame  26 .  24  denotes a voice coil  24  jointed to the center portion of the diaphragm  25  so as to be located in the magnetic space  23 . Also, the loudspeaker frame  26  is attached to the board  10  so that the electromechanical acoustic transducer  12  covers the acoustic aperture  11 . The electromechanical acoustic transducer  12  is positioned opposed to the diaphragm panel  14  with respect to the board  10 . In Embodiment 1, the electromechanical acoustic transducer  12  is connected directly to the board  10 . Alternatively, the electromechanical acoustic transducer  12  can be connected to the frame  26  via an acoustic pipe, which is described further below. 
     The operation of the above-structured loudspeaker system is described below. An electrical signal is applied to the voice coil  24  placed within the magnetic space  23  of the electromechanical acoustic transducer  12  to drive the voice coil  24 . This causes the diaphragm  25  to vibrate, thereby producing sound. The electromechanical acoustic transducer  12  emits the produced sound into the space  16 . Specifically, the sound emitted from the diaphragm  25  is propagated from the acoustic aperture  11  to the space  16 . Of the board  10  and the diaphragm panel  14  that form the space  16 , it is the diaphragm panel  14  that has a lower stiffness. Therefore, it is the diaphragm panel  14  that vibrates by energy (sound pressure) of the sound emitted from the electromechanical acoustic transducer  12  to the space  16 . That is, the diaphragm panel  14  is acoustically driven by the electromechanical acoustic transducer  12  to vibrate. Since the outer rim portion of the diaphragm panel  14  is fixed to the board  10  with the spacer  13 , the structural strength of the outer rim portion of the diaphragm panel  14  is higher than the structural strength of the center portion thereof. Therefore, the center portion of the diaphragm panel  14  vibrates to produce sound. With this vibration, the loudspeaker system emits sound outside for sound reproduction. 
     The characteristics of the loudspeaker system according to Embodiment 1 are described below with reference to  FIGS. 3 and 4 .  FIG. 3  is an illustration showing a vibration mode of the diaphragm panel  14  of the loudspeaker system according to Embodiment 1. Here, the vibration mode illustrated in  FIG. 3  is at a frequency of 500 (Hz). The vibration mode illustrated in  FIG. 3  is complicated compared with the vibration mode illustrated in  FIG. 17 , with the sheet-like diaphragm panel  14  being flexed like a wave. As such, in the present invention, the diaphragm panel  14  is flexed to vibrate, unlike a case in which the entire diaphragm panel  14  vibrates like a piston movement. The diaphragm panel  14  is preferably bendable, and is therefore preferably light in weight and low in stiffness. Furthermore, for vibration, the diaphragm panel  14  should be lower in stiffness than the board  10 . 
       FIG. 4  is an illustration showing sound pressure frequency characteristics of the loudspeaker system according to Embodiment 1. In  FIG. 4 , a solid line represents characteristics of a sound pressure frequency of the loudspeaker system according to Embodiment 1, while a dotted line represents predicted values by the equivalent circuit illustrated in  FIG. 15  (the same as the dotted line illustrated in  FIG. 18 ). Also, in  FIG. 4 , the dimension of the diaphragm panel  14  is similar to that illustrated in  FIG. 18 , that is, 72 (mm) in height×51 (mm) in width. Also, in the present invention, the diaphragm panel  14  is preferably bendable, and therefore is 125 (μm) in thickness. As illustrated in  FIG. 4 , it can be observed that the characteristics of the loudspeaker system according to Embodiment 1 are such that a sound pressure level in the bass range is higher, compared with those of the conventional loudspeaker system. Therefore, the loudspeaker system according to the present invention can easily improve the sound pressure level in the bass range by selecting the diaphragm panel  14  to have a low stiffness. 
     The relationship between the location of the acoustic aperture  11  on the board  10  and the sound pressure characteristics is now described with reference to  FIGS. 5 and 6A  through  6 C.  FIG. 5  is an illustration showing a board having a plurality of acoustic apertures. A board  17  illustrated in  FIG. 5  is provided with acoustic apertures  11   a  to  11   e . Only one of these acoustic apertures  11   a  to  11   e  is provided with the electromechanical acoustic transducer  12 , while the others are closed and not in use.  FIGS. 6A ,  6 B, and  6 C are illustrations showing sound pressure frequency characteristics of the loudspeaker system measured along with changes of the acoustic aperture to be provided with the electromechanical acoustic transducer  12 . In the descriptions of  FIGS. 5 , and  6 A through  6 C, an electrodynamic loudspeaker having a diameter of φ16 (mm) is exemplarily used as the electromechanical acoustic transducer  12 . Also, as the diaphragm panel  14 , a transparent PET material is exemplarily used having 87 (mm) in height×66 (mm) in width×0.188 (mm) in thickness. Furthermore, every acoustic aperture  11  is a rectangle having 3 (mm) in height×12 (mm) in width.  FIGS. 6A through 6C  each illustrate the measurement results of the sound pressure frequency characteristics obtained by placing a microphone at a location 0.1 (m) away from the center of the diaphragm panel  14 , and applying a power input of 0.1 (W) to the electromechanical acoustic transducer  12 . 
       FIG. 6A  is an illustration showing the sound pressure frequency characteristics measured when the acoustic aperture  11   a  is provided with the electromechanical acoustic transducer  12  while the other acoustic apertures are closed. Similarly,  FIG. 6B  is an illustration showing the sound pressure frequency characteristics measured when the acoustic aperture  11   b  is provided with the electromechanical acoustic transducer  12  while the others are closed.  FIG. 6C  is an illustration showing the sound pressure frequency characteristics measured when the acoustic aperture  11   e  is provided with the electromechanical acoustic transducer  12  while the others are closed. As evident from  FIGS. 6A through 6C , the sound pressure frequency characteristics are little influenced depending on which acoustic aperture is provided with the electromechanical acoustic transducer  12 . The same goes for a case, although not shown, in which the acoustic aperture  11   c  or  11   d  is used. As such, in the sound-driving scheme as in the present invention, the sound pressure is used for acoustically driving the diaphragm panel  14 . Therefore, whichever the acoustic aperture on the board  10  is used, the diaphragm panel  14  can be similarly driven. On the other hand, in the driving scheme with a transducer directly mounted on a diaphragm panel, the sound frequency characteristics are greatly varied depending on where the transducer is mounted. This disadvantageously limits the mounting location of the transducer. Unlike this, in the loudspeaker system according to the present invention, the electromechanical acoustic transducer  12  can be mounted anywhere on the board  10  so as to cover an acoustic aperture. This increases design flexibility and versatility of the loudspeaker system. 
     Furthermore, according to Embodiment 1, the diaphragm panel  14  and the board  10  are made of a transparent material. Therefore, the diaphragm panel  14  and board  10  do not interfere with a background of the loudspeaker system. Such a visually unobtrusive loudspeaker system can increase its versatility of usage. Specific application examples of the unobtrusive loudspeaker system are described further below in Embodiments 3 and 4. 
     Still further, the loudspeaker system, such as the conventional one, having the frame (board) and the diaphragm panel joined together by a suspension is highly complicated in configuration. Therefore, it is very difficult to make the loudspeaker system transparent. More specifically, since a plurality of materials have to be jointed together by an adhesive, it is difficult to make the rims of the board and the diaphragm panel transparent. By contrast, in the present invention, the configuration of the loudspeaker system can be simplified without the use of a suspension. Thus, a visually unobtrusive loudspeaker system can be easily achieved. 
     Embodiment 2 
     A loudspeaker system according to Embodiment 2 is described below with reference to  FIGS. 7A and 7B .  FIGS. 7A and 7B  are illustrations each showing the configuration of the loudspeaker system according to Embodiment 2 of the present invention. Here,  FIG. 7A  is a rear view of the loudspeaker system.  FIG. 7B  is a view of the loudspeaker system denoted by line C–D in  FIG. 7A . In  FIGS. 7A and 7B ,  30  denotes a board.  31  denotes an acoustic aperture provided on the board  30 .  32  denotes an electromechanical acoustic transducer attached to the board  30  so as to cover the acoustic aperture  31 .  33  denotes a spacer provided on the outer rim of the board  30 .  34  is a diaphragm panel attached to the spacer  33 .  35  is a base that supports the board  30 .  36  denotes a cabinet provided on the back of the electromechanical acoustic transducer  32 . 
     In the loudspeaker system according to Embodiment 2, a difference in configuration from the loudspeaker system according to Embodiment 1 is that the board  30  and the diaphragm panel  34  have a circular shape, and that the cabinet  36  is further provided. The cabinet  36  forms an enclosed space  37  on the back of the electromechanical acoustic transducer  32  (opposed to the acoustic aperture  31 ). Other than the above difference, the loudspeaker system according to Embodiment 2 is similar in configuration to that according to Embodiment 1. Therefore, also in Embodiment 2, the loudspeaker system can be simplified in configuration compared with the conventional loudspeaker system. 
     In Embodiment 2, as with Embodiment 1, an electrical signal is applied to the electromechanical acoustic transducer  32  to cause the diaphragm panel  34  to vibrate. In Embodiment 2, the circular shapes of the board  30  and the diaphragm panel  34  do not have any influence on the above operation. In the present invention, the shapes of the board  30  and the diaphragm panel  34  may be any. That is, the present invention discloses a scheme for driving the diaphragm panel  34  by sound pressure emitted from the electromechanical acoustic transducer  32 . Therefore, any arbitrary shape, such as semicircles, ellipses, or polygons, will suffice for the board  30  and the diaphragm panel  34  to perform audio reproduction. This increases design flexibility of the loudspeaker system compared with the scheme of directly driving the diaphragm panel by the transducer. 
     In the loudspeaker system according to Embodiment 2, a difference from the loudspeaker system according to Embodiment 1 lies in the cabinet  36 . Sound produced from the back of the electromechanical acoustic transducer  32  is emitted into the space  37  formed by the cabinet  36 . Therefore, the sound from the back of the electromechanical acoustic transducer  32  does not go out of the space  37 . With this, it is possible to prevent cancellation of the sound from the diaphragm panel  34  and the opposite-phase sound from the back of the electromechanical acoustic transducer  32 . Thus, the sound pressure level in the bass range can be particularly improved. 
     Note that, in Embodiment 2, the cabinet  36  is not necessarily required. Also, such a cabinet can be provided to the loudspeaker systems according to Embodiments 1 and 5, which will be described further below. 
     Embodiment 3 
     A loudspeaker system according to Embodiment 3 is described below with reference to  FIGS. 8 and 9 .  FIG. 8  is an illustration showing an exemplary case in which the loudspeaker system according to Embodiment 3 is mounted inside a vehicle. In  FIG. 8 ,  40  denotes the loudspeaker system according to Embodiment 3.  41  denotes a vehicle body.  42  denotes a dashboard.  43  denotes a windshield.  44  denotes a steering wheel. The configuration of the loudspeaker system according to Embodiment 3 is now described below. 
       FIG. 9  is a section view of a state in which the loudspeaker system  40  illustrated in  FIG. 8  is mounted onto the vehicle body. In  FIG. 9 ,  45  denotes aboard  46  denotes an acoustic aperture provided on the board  45 .  47  denotes an acoustic pipe attached to the board  45  so as to cover the acoustic aperture  46 .  48  denotes an electromechanical acoustic transducer on which the acoustic pipe  47  is mounted.  49  denotes a spacer provided on the outer rim of the board  45 .  50  denotes a diaphragm panel attached to the spacer  49 . 
     In Embodiment 3, the loudspeaker system  40  is different from that according to Embodiment 1 in that the acoustic pipe  47  is further provided for connecting the acoustic aperture  46  on the board  45  and the electromechanical acoustic transducer  48  together. That is, with the acoustic pipe  47  connecting the board  45  and the electromechanical acoustic transducer  48  together so as to cover the acoustic aperture  46 , the electromechanical acoustic transducer  48  is placed separately from the board  45  and the diaphragm panel  50 . Other than the above difference, the loudspeaker system  40  is similar to that according to Embodiment 1. Furthermore, in the loudspeaker system  40 , the acoustic pipe  47  is penetratingly mounted on the dashboard  42 . In the above-structured loudspeaker system  40 , sound from the electromechanical acoustic transducer  48  is led via the acoustic pipe  47  to the acoustic aperture  46 , and is then transferred to a space  51  formed by the board  45 , the diaphragm panel  50 , and the spacer  49 . Note that the operation of the loudspeaker system according to Embodiment 3 is similar to that according to Embodiment 1, except for the above, that is, the electromechanical acoustic transducer  48  emits sound via the acoustic pipe  47  to the acoustic aperture  46  and then to the space  51 . 
     As described above, according to Embodiment 3, the electromechanical acoustic transducer  48 , which is difficult to be made transparent, can be hidden inside the vehicle body. Furthermore, as with Embodiment 1, the board  45 , the spacer  49 , and the diaphragm panel  50  are made of a transparent material. Therefore, if the acoustic pipe  47  is also made of a transparent material, such as polycarbonate or acrylic, it is possible to achieve a loudspeaker system which is almost transparent to a user&#39;s eyes and therefore is not obtrusive to the user&#39;s view. Such a transparent loudspeaker system is particularly suitable for vehicles in view of driver&#39;s safety, since the loudspeaker system mounted on the dashboard or the like does not obstruct a view ahead of the vehicle. 
     In Embodiment 3, only a single loudspeaker system  40  is mounted at the center of the upper surface of the dashboard  42 . Alternatively, a plurality of loudspeaker systems  40  can be further mounted on right and left portions thereof for multi-channel reproduction such as stereo reproduction, together with the loudspeaker system  40  at the center being used as a center channel. Furthermore, the mounting location of the loudspeaker system  40  is not restricted to the dashboard  42 , but can be anywhere on the vehicle so as to achieve the effects of Embodiment 3. 
     Embodiment 4 
     The configuration of a loudspeaker system according to Embodiment 4 is described below with reference to  FIGS. 10 and 11 .  FIG. 10  is an illustration showing the configuration of the loudspeaker system according to Embodiment 4. In  FIG. 10 ,  60  denotes a wall (serving as a board of the loudspeaker system) that composes a building.  61  denotes an acoustic aperture provided on the wall  60 .  62  denotes an acoustic pipe penetratingly attached to the wall  60  so as to cover the acoustic aperture  61 .  63  denotes an electromechanical acoustic transducer.  64  denotes a spacer mounted on the wall  60 .  65  denotes a diaphragm panel attached to the spacer  64 . 
     In the loudspeaker system according to Embodiment 4, a difference in configuration from the loudspeaker system according to Embodiment 1 is that the wall  60  of a room of the building serves as a board of the loudspeaker system. That is, the board of the loudspeaker system according to Embodiment 4 also serves as a structural component of the building. Note that the spacer  64  and the diaphragm panel  65  are similar to those in Embodiment 1. Furthermore, as with the other embodiments described above, the wall  60  has to have a stiffness higher than that of the diaphragm panel  65 . 
       FIG. 11  is a section view of a piezoelectric loudspeaker, which is one example of the electromechanical acoustic transducer  63  illustrated in  FIG. 10 . In  FIG. 11 ,  70  and  71  denote piezoelectric elements.  72  denotes an intermediate electrode having the piezoelectric elements attached on both sides.  73  denotes a lead connected to the intermediate electrode  72  for receiving electrical input.  74  denotes a lead connected to the piezoelectric element  71 .  75  is a lead connected to the piezoelectric element  70 .  78  denotes a loudspeaker frame attached to the outer rim of the intermediate electrode  72 . The intermediate electrode  72  is made of a conductive material, such as phosphor bronze or stainless steel. The lead is connected to an input terminal  77 , while the leads  74  and  75  are connected to an input terminal  76 . The loudspeaker frame  78  is jointed to the acoustic pipe  62 . 
     In the loudspeaker system according Embodiment 4, a difference in operation from the loudspeaker system according to Embodiment 3 lies in the operation of the piezoelectric-type electromechanical acoustic transducer  63 . In the electromechanical acoustic transducer  63 , when electrical signals are applied to the input terminals  76  and  77 , the piezoelectric elements  70  and  71  attached to both sides of the intermediate electrode  72  are flexed to be vibrated. With this, the intermediate electrode  72  and the piezoelectric elements  70  and  71  emit sound. Other than the above operation, the operation of the loudspeaker system according to Embodiment 4 is similar to that according to Embodiment 3. 
     As described above, according to Embodiment 4, the wall  60 , which is a structural component, is used as a board of the loudspeaker system, and the electromechanical acoustic transducer  63  is placed outside the wall  60 . With this, the electromechanical acoustic transducer  63  is hidden from the surface of the wall  60 . Furthermore, as described in Embodiment 1, the spacer  64  and the diaphragm panel  65  are made of a transparent material. Therefore, according to Embodiment 4, it is possible to achieve a loudspeaker system that is visually unobtrusive to users. 
     Application examples of the loudspeaker system according to Embodiment 4 are as follows. For example, the loudspeaker system can be mounted on a wall of a room for use as a loudspeaker for DVD multi-channel reproduction. Also, the wall on the back of the transparent diaphragm panel  65  is attached with a poster or picture, thereby giving users a feeling as if sound is coming from the poster or the picture. Such a loudspeaker system is suitable not only for home use but also for exhibition use. Furthermore, the loudspeaker system according to Embodiment 4 can use a glass surface of a show window, a vehicle body, furniture, an electrical appliance, etc., as the board of the loudspeaker system. 
     Embodiment 5 
     A loudspeaker system according to Embodiment 5 is described below with reference to  FIGS. 12A and 12B .  FIGS. 12A and 12B  are illustrations each showing the configuration of the loudspeaker system according to Embodiment 5 of the present invention. Here,  FIG. 12A  is a front view of the loudspeaker system.  FIG. 12B  is a view the loudspeaker denoted by line E–F in  FIG. 12A . In  FIGS. 12A and 12B ,  80  denotes a board.  81  denotes an acoustic aperture provided on the board  81 .  82  denotes an electromechanical acoustic transducer attached to the board  81  so as to cover the acoustic aperture  81 .  83  denotes a spacer provided to the outer rim of the board  80 .  84  denotes a diaphragm panel attached to the spacer  83 .  85 ,  86 ,  87 , and  88  denote light-emitting diodes provided at the four corners of the board  80 .  89  denotes a CD player.  90  denotes an amplifier connected to the CD player  89  and the electromechanical acoustic transducer  82 .  91  denotes a signal controller connected to the CD player  89  and the light-emitting diodes  85  through  88 . 
     The operation of the above-structured loudspeaker system is described below. A music signal reproduced by the CD player  89  is amplified by the amplifier  90 , and is then applied to the electromechanical acoustic transducer  82 . Based on the applied music signal, the electromechanical acoustic transducer  82  emits sound, which acoustically drives the diaphragm panel  84  to produce sound. This operation is similar to that in Embodiment 1. 
     The loudspeaker system according to Embodiment 5 is different from that according to Embodiment 1 in that the light-emitting diodes  85  through  88 , which are merely an example of light emitting means, and the signal controller  91  are further provided. Supplied with a music signal by the CD player  89 , the signal controller  91  applies a signal corresponding to the music signal to the light-emitting diodes  85  through  88 . With this, it is possible to achieve a loudspeaker system that emits light in accordance with the music signal. Such a loudspeaker system can provide users with visual enjoyment. Light-emitting patterns and brightness of the light-emitting diodes  85  through  88  may be varied in accordance with the magnitude and/or frequency of the music signal. Also, the signal controller  91  may apply different signals to the light-emitting diodes  85  through  88 . This can achieve a loudspeaker system with light-emitting diodes illuminating with different brightness levels in accordance with the music signal. 
     The diaphragm panel  84  may be translucent. If the diaphragm panel  84  is transparent, rays of light emitted from the light-emitting diodes  85  through  88  merely pass through the diaphragm panel  84 . If the diaphragm panel  84  is translucent, however, the rays of light are diffused by the diaphragm panel  84 . With this, attractive lighting effects can be expected. Furthermore, rays of light emitted from the light-emitting diodes do not necessarily have a single color, but may have different colors. Still further, an arbitrary number of light-emitting diodes can be placed on arbitrary locations of the board  80 . For example, the light-emitting diodes can be located within the board  80  to achieve an effect that the board  80  itself seems to illuminate. 
     As described in the foregoing, according to the present invention, no suspension is required. Therefore, it is possible to achieve a sound-driving loudspeaker system with a simple configuration. Moreover, the diaphragm panel is vibrated not by a piston action but by flexion. With this, it is possible to easily achieve a loudspeaker system with an improved sound pressure level in the bass range. 
     The electromechanical acoustic transducer  12  is exemplarily implemented by an electrodynamic loudspeaker in Embodiment 1 and by a piezoelectric loudspeaker in Embodiment 4. Here, in Embodiments 1 through 5, the electromechanical acoustic transducer may be any as long as it causes the diaphragm panel to emit sound. Also, the conversional scheme used in the electromechanical acoustic transducer  12  may be any, such as of an electromagnetic type, piezoelectric type, or electrostatic type. 
     In Embodiments 1 through 5, the board and the outer rim portion of the diaphragm panel are fixed together via the spacer to form a space (the space  16  illustrated in  FIG. 1B , for example) for acoustically driving the diaphragm panel. Alternatively, the board can have any structure as long as the board and the diaphragm panel form the above-mentioned space. One example of the structure of the board is illustrated in  FIG. 13 .  FIG. 13  is an illustration showing an exemplary modification of the board used in the loudspeaker according to the present invention. Note that, in  FIG. 13 , components similar in structure to those in  FIG. 1B  are provided with the same reference numerals. In  FIG. 13 , a plate-like board  18  having its center portion bowed inward is used, with the diaphragm panel  14  directly jointed to the outer rim of the board  18 . As such, the board and the diaphragm panel can be directly fixed together without a spacer. In this case, the bowed center portion forms a space  19  for acoustically driving the diaphragm panel  14 . Moreover, the space can be formed by a bonding layer for bonding a flat board and a flat diaphragm panel. 
     Still further, in Embodiments 1 through 5, the board and the diaphragm panel both have a flat surface, but both can have a curved surface. Even in this case, the diaphragm panel can be vibrated as long as the board and the diaphragm panel form a space. The same goes for a case in which either one of the board and the diaphragm panel has a curved surface. Similarly, the loudspeaker system according to the present invention can be achieved even if the board has a complex shape. 
     Still further, in Embodiments 1 through 5, the diaphragm panel is implemented by a PET film. This is not meant to be restrictive. The diaphragm panel can be made of any material that has a stiffness lower than that of the board. For example, the diaphragm panel can be made of paper. This is particularly suitable for Embodiment 4. With a paper poster or photograph being used as the diaphragm panel, it is possible to achieve a loudspeaker system in which sound is emitted from the poster or photograph itself. In this case, if such a diaphragm panel is configured to be removable from the board, the user can change the poster or photograph used as the diaphragm panel according to his or her preferences. Conversely, the diaphragm panel may be fixed to the board with a predetermined tension. 
     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.