Patent Publication Number: US-9432773-B2

Title: Speaker and audio-visual system

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This Application claims priority to Japanese Patent Application No. 2013-271209, filed on Dec. 27, 2013, the contents of which are hereby incorporated by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a speaker and an audio-visual system. 
     2. Description of the Related Art 
     In recent years, wideband speakers (hereinafter referred to as magnetic fluid speakers) that use divided suspensions and magnetic fluid and that are capable of reproducing low-pitched sound though being small-sized have been developed. 
       FIG. 10A  is a diagram illustrating a cross-section of a speaker  400  that is a conventional speaker disclosed in International Publication No. 2009/066415 and that makes use of magnetic fluid and divided suspensions.  FIG. 10B  is a cross sectional view, taken along line XB-XB, of the speaker of  FIG. 10A . The speaker illustrated in  FIGS. 10A and 10B  includes a yoke  20 , a magnet  21 , a plate  22 , a diaphragm  30 , suspensions  24   a  and  24   b , a voice coil  26 , a sound conduit tube H 2 , ribs L 1 , and magnetic fluid  27 . 
     In the above configuration, the suspensions  24   a  and  24   b  that support the diaphragm  30  in a vibratable manner are provided at different positions on the periphery of the diaphragm  30 . In the case where the speaker is reduced in size, accordingly, stiffness can be reduced and thus a minimum resonant frequency of the speaker can be lowered by adjusting widths and/or thicknesses of the suspensions  24   a  and  24   b . With the sealed-in magnetic fluid  27 , additionally, interference between sound waves and rolling that occur on surfaces of the diaphragm  30  can be reduced. By use of the speaker that makes use of the magnetic fluid  27  and the divided suspensions  24   a  and  24   b , as described above, the wideband speaker though being small-sized that is capable of reproducing low-pitched sound can be provided. 
     Japanese Unexamined Patent Application Publication No. 2008-160644 describes an example of a hearing aid in which a damper is provided in a sound conduit tube. 
     SUMMARY 
     A speaker according to the disclosure includes a diaphragm and a magnetic circuit, a plurality of acoustic paths that provide connection between a space formed on a side including the magnetic circuit with respect to the diaphragm and a space exterior to the speaker are formed in the magnetic circuit, and the plurality of acoustic paths include a first acoustic path and a second acoustic path that differs in acoustic impedance from the first acoustic path. 
     According to the speaker of the disclosure, reduction in a peak due to resonance in sound pressure characteristics and holding of the sound pressure characteristics for a minimum resonant frequency or lower frequencies can be both attained, and sound waves that are excellent in reproduction of low-pitched sound and that have flat frequency characteristics can be emitted from sound conduit tubes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross sectional view illustrating an example of a speaker according to embodiment 1 of the disclosure; 
         FIG. 1B  is a cross sectional view illustrating the example of the speaker according to embodiment 1 of the disclosure; 
         FIG. 2  is a diagram illustrating an example of a result of comparison in sound pressure frequency characteristics between the configuration of a speaker in which no damper is provided on sound conduit tubes and the configuration of a speaker in which dampers are provided on all of sound conduit tubes; 
         FIG. 3  is a diagram illustrating an example of sound pressure frequency characteristics relating to dampers and sound conduit tubes in embodiment 1 of the disclosure; 
         FIG. 4A  is a diagram illustrating an example of an equivalent circuit corresponding to a speaker of the disclosure; 
         FIG. 4B  is a diagram illustrating an example of an equivalent circuit corresponding to the speaker of the disclosure; 
         FIG. 5  is a diagram illustrating a real part of acoustic impedance characteristics (B=10 1  to 10 4 ) relating to embodiment 1 of the disclosure; 
         FIG. 6  is a diagram illustrating a real part of acoustic impedance characteristics in a speaker in which dampers are provided on all of sound conduit tubes and a real part of acoustic impedance characteristics corresponding to the speaker of the disclosure; 
         FIG. 7  is a diagram illustrating a real part of acoustic impedance characteristics (B=10 4  to 10 7 ) relating to the speaker of the disclosure; 
         FIG. 8  is a front external view illustrating an example of a flat-screen television in which the speakers according to embodiment 1 of the disclosure are installed; 
         FIG. 9A  is a cross sectional view illustrating an example of a speaker according to embodiment 2 of the disclosure; 
         FIG. 9B  is a cross sectional view illustrating the example of the speaker according to embodiment 2 of the disclosure; 
         FIG. 10A  is a cross sectional view of a conventional speaker; 
         FIG. 10B  is a cross sectional view of the conventional speaker; and 
         FIG. 11  is a cross sectional view of a conventional sound conduit tube. 
     
    
    
     DETAILED DESCRIPTION 
     Description will be given on matters the inventors examined for devising embodiments according to the disclosure. (Knowledge Underlying the Disclosure) 
     According to the conventional speaker disclosed in International Publication No. 2009/066415, a wideband speaker that is capable of reproducing low-pitched sound though being small-sized can be provided by use of the speaker that makes use of the magnetic fluid  27  and the divided suspensions  24   a  and  24   b.    
     A characteristic of a system of the conventional speaker that is illustrated in  FIGS. 10A and 10B  and that makes use of the magnetic fluid is that sound waves are emitted through the sound conduit tube H 2  to exterior space. In this system, the sound waves are emitted from the sound conduit tube H 2  positioned on the back side of the diaphragm  30  and thus the diaphragm  30  can be placed inside of a device that is out of sight of a user. Accordingly, there is an advantage in that the sound waves can be reached to the user without the user noticing that the device includes the speaker. 
     In common speakers, a back side capacity  411  and the sound conduit tube H 2  that are formed between the diaphragm  30  and the plate  22  function as a low-pass filter and it is thus difficult to reproduce a high frequency range. In magnetic fluid speakers, on the other hand, the back side capacity  411  formed between the diaphragm  30  and the plate  22  is substantially decreased by the magnetic fluid. Thus, a cutoff frequency of the low-pass filter can be increased and there is an advantage in that a higher frequency range can be reproduced in comparison with common speakers. 
     On condition that sound waves are emitted via the sound conduit tube H 2  provided in a magnetic circuit as in the conventional speakers that make use of magnetic fluid, there is caused a problem in that occurrence of Helmholtz resonance that may be caused by the back side capacity  411  and the sound conduit tube H 2  or standing wave resonance that may occur in the sound conduit tube causes formation of great peaks, degradation in sound quality, and the like. In the conventional speakers that make use of the magnetic fluid, only one sound conduit tube H 2  is provided in the magnetic circuit at the center position of the speaker, and thus sound waves emitted from the center and ends of the diaphragm  30  interfere with one another because of path difference. 
     It is conceivable that an acoustic tube (not illustrated) is provided so as to prevent resonance from occurring on the front face (opposed to the side where the magnetic circuit is provided) of the diaphragm  30  of the speaker, for instance. In this method, however, a capacity formed between the diaphragm  30  and the acoustic tube provided on the front face of the diaphragm  30  is increased in comparison with that in a model in which the sound conduit tube H 2  is provided in the magnetic circuit. Thus, the cutoff frequency of the acoustic low-pass filter formed of the space and the acoustic tube is lowered, so that a reproduction band in a high frequency range is narrowed. In addition, additional members may need to be arranged for forming a space on the front face (side opposite to the magnetic circuit) of the diaphragm  30  of the speaker, which causes increase in costs for components. For the speaker that is illustrated in  FIGS. 10A and 10B  and that is provided with the magnetic fluid, in particular, therefore, a structure that has a sound conduit tube provided in the magnetic circuit and that reduces the resonance and standing wave resonance that have been mentioned above is desired. 
     As measures against this problem, a system is conceivable in which the peak in the high frequency range is curbed by damping effect of a damper provided in the sound conduit tube as in Japanese Unexamined Patent Application Publication No. 2008-160644.  FIG. 11  is a diagram illustrating a cross-section of the sound conduit tube  500  that is the conventional sound conduit tube of Japanese Unexamined Patent Application Publication No. 2008-160644 and that curbs the peak in the high frequency range by the damper. The sound conduit tube  500  includes a tube  501 , a path  502 , a joint  503 , the damper  504 , a seal member  505 , and an earplug  506 . In the above configuration, only by provision of the damper  504  in the joint  503 , an effect is obtained in which the peak in the high frequency range that is produced in frequency characteristics of a path formed by coupling the tube  501  and the joint  503  is curbed by the damping effect of the damper  504 . 
     Japanese Unexamined Patent Application Publication No. 2008-160644 assumes a sealed type hearing aid, an earphone, or the like that is used with an external auditory canal plugged with an earplug or the like. In the sealed type hearing aid or earphone, sound pressure characteristics for minimum resonant frequency or lower are made uniform independently of the damping effect of the damper  504 , and thus provision of the damper  504  exerts no influence on low frequency characteristics. 
     However, even if the damper  504  illustrated in  FIG. 11  is provided in the sound conduit tube H 2  of the speaker illustrated in  FIGS. 10A and 10B , because the speaker illustrated in  FIGS. 10A and 10B  is an open type speaker that emits sound waves to free space, uniform sound pressure characteristics for the minimum resonant frequency or lower are not obtained, and there is a problem in that the damping effect is also exerted on the sound pressure characteristics for the minimum resonant frequency or lower due to the provision of the damper  504 , which decreases not only the peak in the high frequency range but also the sound pressure characteristics for the minimum resonant frequency or lower and degrades the sound pressure characteristics. 
     The speaker of the disclosure provides a speaker in which reduction in the peak due to the resonance in the sound pressure characteristics and holding of the sound pressure characteristics for the minimum resonant frequency or lower are both attained. 
     The speaker of the disclosure includes a diaphragm and a magnetic circuit, a plurality of acoustic paths that provide connection between a space formed on a side including the magnetic circuit with respect to the diaphragm and a space exterior to the speaker are formed in the magnetic circuit, and the plurality of acoustic paths include a first acoustic path and a second acoustic path that differs in acoustic impedance from the first acoustic path. 
     Thus, the reduction in the peak due to the resonance in the sound pressure characteristics and the holding of the sound pressure characteristics for the minimum resonant frequency or lower can be both attained, and sound waves that are excellent in the reproduction of low-pitched sound and that have flat frequency characteristics can be emitted from sound conduit tubes. 
     In the speaker of the disclosure, the magnetic circuit may be composed of a magnet, a plate, and a yoke, and the acoustic paths may be formed of at least one sound conduit tube that is provided in the magnet, the plate, and the yoke. 
     In such a configuration, which makes the sound conduit tube communicate with a space exterior to the speaker, pressure in a space formed between the diaphragm and the magnetic circuit does not change even when the diaphragm vibrates vertically. 
     In the speaker of the disclosure, a plurality of the sound conduit tubes may be provided, the speaker may include one or more dampers that cover at least one of the plurality of the sound conduit tube, the plurality of sound conduit tubes may include a first sound conduit tube that forms the first acoustic path and a second sound conduit tube that forms the second acoustic path, one of the one or more dampers may be provided on the first sound conduit tube, and any of the one or more dampers may be not provided on the second sound conduit tube. 
     By such a configuration, the peak due to the resonance can be curbed without the degradation in the low frequency characteristics of the speaker. 
     In the speaker of the disclosure, the plurality of sound conduit tubes may further include a third sound conduit tube that forms the first acoustic path and one of the one or more dampers that covers the third sound conduit tube may be provided on the third sound conduit tube. 
     By such a configuration, the peak due to the resonance can be curbed without the degradation in the low frequency characteristics of the speaker. 
     In the speaker of the disclosure, the second sound conduit tube may be placed at a substantially center position in the magnet, the plate, or the yoke, and the first sound conduit tube and the third sound conduit tube may be placed at positions symmetrical to each other with respect to the second sound conduit tube placed at the substantially center position. 
     By such a configuration, pressures that are exerted on the diaphragm when the speaker is activated are made symmetrical so that occurrence of unstable vibrations can be curbed. 
     In the speaker of the disclosure, the speaker may be in the shape of a rectangle in a top plan view, and the sound conduit tubes may be placed in a row along a direction of long sides of the speaker. 
     By such a configuration, sound waves emitted from the center and ends of the diaphragm can be prevented from interfering with one another because of path difference. Thus, the sound waves can be emitted without impairment in characteristics in a high frequency range that are prone to be influenced by the interference due to the path difference even though the speaker has an elongated shape. 
     In the speaker of the disclosure, a plurality of the sound conduit tubes may be provided, the plurality of sound conduit tubes may include the first sound conduit tube that forms the first acoustic path and the second sound conduit tube that forms the second acoustic path, and the first sound conduit tube and the second sound conduit tube may differ in radius. 
     By such a configuration, resistance component for the first sound conduit tube and resistance component for the second sound conduit tube can be adjusted. 
     In the speaker of the disclosure, a resistance component ratio for the first acoustic path and a resistance component ratio for the second acoustic path may be set to be 10 2  or higher and 10 5  or lower. 
     By such a configuration, the reduction in the peak due to the resonance in the sound pressure characteristics and the holding of the sound pressure characteristics for the minimum resonant frequency or lower are both attained. 
     In the speaker of the disclosure, only one sound conduit tube may be provided as the sound conduit tubes, and the first acoustic path and the second acoustic path may be formed in the sound conduit tube. 
     By such a configuration, the reduction in the peak due to the resonance in the sound pressure characteristics and the holding of the sound pressure characteristics for the minimum resonant frequency or lower can be both attained, and sound waves that are excellent in the reproduction of low-pitched sound and that have flat frequency characteristics can be emitted from the sound conduit tube, without providing a plurality of sound conduit tubes in the magnetic circuit. 
     In the speaker of the disclosure, a damper that partially covers the sound conduit tube may be provided on the sound conduit tube, sound waves emitted from the diaphragm pass through the sound conduit tube, the sound waves split into sound waves that pass thorough the damper and sound waves that pass through a portion, on the sound conduit tube, which the damper does not cover. 
     In such a configuration, the first acoustic path and the second acoustic path can be formed in the one sound conduit tube provided in the magnetic circuit. 
     In the speaker of the disclosure, magnetic fluid may be provided in a portion of the magnetic circuit. 
     An audio-visual system according to one aspect of the disclosure includes a television, a cellular phone, a smartphone, a tablet terminal, an earphone, a hearing aid, or a vehicle having the speaker. 
     Hereinbelow, embodiments of the disclosure will be described with reference to the drawings. Each embodiment that will be described below designates a preferable specific example of the disclosure. Numerical values, shapes, components, arrangement positions and connection configurations of the components, and the like that will be set forth for the embodiments below each represent an example and are not intended to limit the disclosure. The disclosure is limited by only the claims. Therefore, components that are not mentioned in the independent claims designating the most generic concept of the disclosure among the components in the following embodiments are not necessarily be demanded for resolution of the problems of the disclosure and will be described as components that configure preferred forms. The same components are provided with the same reference symbols and description thereof may be omitted. Any contents of all the embodiments may be combined. 
     (Embodiment 1) 
       FIG. 1A  is a cross sectional view of a speaker  100  in the present embodiment.  FIG. 1B  is a cross sectional view of the speaker  100  of  FIG. 1A , taken along line IB-IB. The speaker  100  includes a yoke  101 , a magnet  102 , a plate  103 , a diaphragm  104 , suspensions  105   a  and  105   b , a voice coil  106 , sound conduit tubes  108   a ,  108   b ,  108   c , dampers  109 , and magnetic fluid  110 . 
     A magnetic circuit that defines a magnetic gap  107  is composed of the yoke  101 , the magnet  102 , and the plate  103 . A back side capacity  111  (space on a side including the magnetic circuit with respect to the diaphragm  104 ) is defined by the plate  103 , the diaphragm  104 , the voice coil  106 , and the magnetic fluid  110 . 
     The sound conduit tubes  108   a ,  108   b , and  108   c  are spaces that link the space on the side including the magnetic circuit with respect to the diaphragm  104  and a space exterior to the speaker  100 . 
     The voice coil  106  and the magnetic fluid  110  are placed in the magnetic gap  107 . Though  FIGS. 1A and 1B  illustrate an example in which ribs  112  are provided on the diaphragm  104 , the ribs  112  are not essential components. Hereinbelow, components of the embodiment will be described. 
     The yoke  101  is shaped like a box having a top face opened and is shaped like a rectangle in a top plan view. The yoke  101  has openings on the bottom face, the openings forming portions of the sound conduit tubes  108 . An open surface inside the yoke  101  has long sides in a linear shape and short sides in a curved (oval) shape. In addition, the yoke  101  includes extended parts that extend outward from the inside open surface, and the extended parts support the suspensions  105   a  and  105   b  (this will be described later). The yoke  101  is made of magnetic materials. 
     The magnet  102  has an oval shape in a horizontal section. That is, the oval shape is substantially the same as and smaller than the shape of the open surface inside the yoke  101 . The magnet  102  has openings inside thereof and the openings form portions of the sound conduit tubes  108   a ,  108   b , and  108   c . The shape of the openings of the magnet  102  is the same as the shape of the openings provided on the yoke  101 . The magnet  102  is bonded onto the inside bottom surface of the yoke  101  so that the openings of the magnet  102  are in alignment with the openings of the yoke  101 . The magnet  102  is magnetized so that a magnetizing direction for the magnet  102  is matched with a vibration direction of the diaphragm  104 . 
     As illustrated in  FIG. 1B , the plate  103  has an oval shape in a horizontal section. That is, the oval shape is substantially the same as and smaller than the shape of the open surface inside the yoke  101 . Like the yoke  101  and the magnet  102 , the plate  103  has openings inside thereof and the openings form portions of the sound conduit tubes  108   a ,  108   b , and  108   c . The shape of the openings of the plate  103  is also the same as the shape of the openings provided on the yoke  101 . The plate  103  is bonded onto the top surface of the magnet  102  so that the openings of the plate  103  are in alignment with the openings of the magnet  102 . The magnetic fluid  110  is in contact with outer circumference of the plate  103 . The plate  103  is made of magnetic materials. 
     The diaphragm  104  has an oval shape in a horizontal section. That is, the diaphragm  104  has long sides in a linear shape and short sides in a curved shape. That is, the oval shape is substantially the same as the shape of the open surface inside the yoke  101 . There are no particular limitations on relative sizes of the horizontal section of the diaphragm  104  and the open surface inside the yoke  101 . The diaphragm  104  is made of the same material as the suspensions  105   a  and  105   b  and is bonded with curved portions thereof formed integrally with the suspensions  105   a  and  105   b . The diaphragm  104  does not have to be formed integrally with the suspensions  105   a  and  105   b  and does not have to be made of the same material of the suspensions  105   a  and  105   b . The voice coil  106  is bonded to the periphery of the bottom surface of the diaphragm  104 . As illustrated in  FIG. 1A , a plurality of ribs  112  may be formed in parallel with the short sides of the diaphragm  104 . Resonance in an audible band can be curbed by the ribs  112 . 
     The suspensions  105   a  and  105   b  are bonded to the diaphragm  104  and the yoke  101 . Sides of the suspensions  105   a  and  105   b  that are bonded to the diaphragm  104  have a curved shape. Sides of the suspensions  105   a  and  105   b  that are bonded to the yoke  101  (the extended parts thereof) have a linear shape. The suspensions  105   a  and  105   b  are integrally referred to as divided suspensions because the plurality of suspensions are bonded to only parts (short sides, curved parts) of the periphery of the diaphragm  104  without covering the entire periphery. Vertical sections of the suspensions  105   a  and  105   b  have a nonlinear shape as illustrated in  FIG. 1A . The diaphragm  104  is held in a vibratable manner by the shape. The shape of the vertical sections of the suspensions  105   a  and  105   b  may be convexly downward with respect to the vibration direction as illustrated in  FIG. 1A  or may be convexly upward. The shape of the suspensions  105   a  and  105   b  is not limited to the above. For instance, the sides that are bonded to the yoke  101  may be in a curved shape. In this configuration, the sides of the yoke  101  that are bonded to the suspensions  105   a  and  105   b  have a curved shape, as a matter of course. 
     The voice coil  106  has an oval shape in a horizontal section. That is, the oval shape is substantially the same as and smaller than the shape of the open surface inside the yoke  101 . The voice coil  106  is cylindrical in a three-dimensional shape. As illustrated in  FIG. 1A , the upper vertical end of the voice coil  106  is bonded to the periphery of the bottom surface of the diaphragm  104 . The lower vertical end of the voice coil  106  is placed in the magnetic gap  107  as illustrated in  FIG. 1A . The magnetic fluid  110  is in contact with the inner circumference of the lower vertical end of the voice coil  106 . 
     The sound conduit tubes  108   a ,  108   b , and  108   c  are formed of the openings that are provided on the yoke  101 , the magnet  102 , and the plate  103  and that are in the same shape. The sound conduit tubes  108  are shaped like cylinders as illustrated in  FIG. 1B . As illustrated in  FIGS. 1A and 1B , the sound conduit tube  108   b  is preferably provided at a middle position (or center position) in the speaker  100 , for instance. The middle position of the speaker  100  represents a middle position of the magnet  102 . The middle position of the speaker  100  also represents a middle position of the plate  103 . The middle position of the speaker  100  also represents a middle position of the yoke  101 . 
     The sound conduit tube  108   b  is not necessarily required to be placed at the middle position in the speaker  100 , for instance. For instance, the sound conduit tube  108   b  has only to be placed at a position (substantially middle position or substantially center position) that can be regarded as the center position of the speaker  100 , the magnet  102 , the plate  103 , or the yoke  101 , for instance. 
     The sound conduit tubes  108   a  and  108   c  are preferably placed at positions symmetrical to each other with respect to the sound conduit tube  108   b  (or the middle position of the speaker  100 ) placed at the center.  FIG. 1B  illustrates the example in which the sound conduit tubes  108   a ,  108   b , and  108   c  are placed along the direction of the long sides and in which the sound conduit tubes  108   a  and  108   c  are placed at the positions symmetrical with respect to the sound conduit tube  108   b  (or the middle position of the speaker  100 ) along the direction of the long sides. Arrangement positions of the sound conduit tubes  108   a ,  108   b , and  108   c , however, are not limited to those positions. 
     The dampers  109  are placed in positions at which the dampers  109  cover the lower apertures of the sound conduit tubes  108   a  and  108   c . In the speaker of  FIGS. 1A and 1B , the dampers  109  are not provided on the lower aperture of the sound conduit tube  108   b . The dampers  109  may be provided on the upper apertures of the sound conduit tubes  108   a  and  108   c . The dampers  109  may be provided inside the sound conduit tubes  108   a  and  108   c . The dampers  109  may be provided in such a position that the lower aperture of the sound conduit tube  108   b  is covered, instead of being provided on the lower apertures of the sound conduit tubes  108   a  and  108   c . That is, there has only to be a configuration in which the damper  109  is provided for at least one of the plurality of sound conduit tubes  108   a ,  108   b , and  108   c  and is not provided on the remainder. 
     A space on outer circumference of the plate  103  and on inner circumference of the voice coil  106  is filled with the magnetic fluid  110  so as not to include gaps. 
     Operations of the speaker  100  configured as described above will be described below. When electric signals are inputted into the voice coil  106 , the voice coil  106  vibrates in accordance with Fleming&#39;s left-hand rule. Then, sound waves are emitted from the diaphragm  104  because the voice coil  106  is bonded to the diaphragm  104 . The sound waves emitted from the diaphragm  104  pass through the sound conduit tube  108   a  and the dampers placed on or in the sound conduit tube  108   a  and are outputted to the outside of the speaker  100 . On the other hand, the sound waves emitted from the diaphragm  104  pass through the sound conduit tube  108   c  and the dampers placed on or in the sound conduit tube  108   c  and are outputted to the outside of the speaker  100 . Further, the sound waves emitted from the diaphragm  104  pass through the sound conduit tube  108   b  and are outputted to the outside of the speaker  100 . 
     The suspensions  105   a  and  105   b  are partially bonded to the diaphragm  104  without covering the entire periphery of the diaphragm  104 , and thus stiffness of the suspensions  105   a  and  105   b  is sufficiently lower than stiffness of a common suspension surrounding the entire periphery of a diaphragm. Accordingly, the minimum resonant frequency can be lowered and reduction in a reproduction bandwidth can be curbed. By the sound conduit tubes  108   a ,  108   b , and  108   c , pressure in the back side capacity  111  is kept constant even when the diaphragm  104  vibrates and thus increase in the minimum resonant frequency can be curbed. 
     In the speaker  100  of the embodiment, having the sound conduit tubes  108  provided in the magnetic circuit, the back side capacity  111  can be made smaller than that in a configuration in which sound conduit tubes are placed above the diaphragm  104 , and the cutoff frequency of the low-pass filter is thereby shifted toward a higher frequency range, so that wideband sound waves can be emitted. 
     Since the sound conduit tubes  108  communicate with the exterior space, additionally, the pressure in the back side capacity  111  does not change even when the diaphragm  104  vibrates vertically and thus scattering of the magnetic fluid that may be caused by variation in the pressure in the back side capacity  111  can be curbed. 
     In the case where great impact on the speaker  100  makes the magnetic fluid  110  scatter in the speaker  100  and reach the sound conduit tubes  108  under capillary action, outflow of the magnetic fluid  110  to the exterior space can be curbed by absorption of the magnetic fluid  110  by the dampers  109 , providing that an oil-absorbing material such as cloth is used for the dampers  109 . 
     In the embodiment, the three sound conduit tubes  108   a ,  108   b , and  108   c  are provided along the direction of the long sides of the rectangular speaker  100 . Therefore, the sound waves emitted from the center and ends of the diaphragm  104  can be prevented from interfering with one another due to the path difference. Thus, the sound waves can be emitted without the impairment in the characteristics in the high frequency range that are prone to be influenced by the interference due to the path difference, even though the speaker  100  has the elongated shape. 
     In the speaker  100  of the embodiment, additionally, the dampers  109  are provided only on the sound conduit tubes  108   a  and  108   c . That is, the dampers  109  are provided on the sound conduit tubes  108   a  and  108   c  that are placed at the positions symmetrical with respect to the sound conduit tube  108   b  (or the middle position of the speaker  100 ) along the direction of the long sides of the diaphragm  104 . Thus, the pressures that are exerted on the diaphragm  104  when the speaker  100  is activated are made symmetrical so that the occurrence of unstable vibrations can be curbed. 
     In the speaker  100  of the embodiment, furthermore, the peak due to the resonance is curbed without the degradation in the sound pressure characteristics for the minimum resonant frequency or lower by design (placement) of the dampers  109  with an appropriate acoustic impedance. As described above, the dampers  109  are provided in the positions at which the dampers  109  cover the lower apertures of the sound conduit tubes  108   a  and  108   c  and are not provided on the lower aperture of the sound conduit tube  108   b . Hereinbelow, effects of this configuration will be described. 
     A configuration of the speaker  100  illustrated in  FIGS. 1A and 1B  in which the dampers  109  are not mounted on the sound conduit tubes  108   a  and  108   c  will be referred to as configuration of the speaker without dampers. 
     A configuration of the speaker  100  illustrated in  FIGS. 1A and 1B  in which the dampers  109  are mounted also on the sound conduit tube  108   b  (configuration in which the dampers  109  are mounted on all of the sound conduit tubes  108   a ,  108   b , and  108   c ) will be referred to as configuration of the speaker with dampers (at three sites). 
       FIG. 2  illustrates a result of comparison in the sound pressure characteristics between the configuration of the speaker without dampers and the configuration of the speaker with dampers. 
     In  FIG. 2 , horizontal axis designates frequency and vertical axis designates sound pressure level. In the sound pressure characteristics of the configuration of the speaker without dampers, a peak is formed in vicinity of 8 kHz by influence of Helmholtz resonance that is caused by the back side capacity  111  and the sound conduit tubes  108 . In the configuration of the speaker with dampers (at three sites), by contrast, the peak in vicinity of 8 kHz is curbed by the damping effect of the dampers  109 . Concurrently, however, the sound pressure characteristics for the minimum resonant frequency or lower (1 kHz or lower) are also lowered by the damping effect of the dampers  109 . 
       FIG. 3  illustrates a result of comparison in the sound pressure characteristics between the configuration of the speaker without dampers and the configuration of the speaker  100  illustrated in  FIGS. 1A and 1B  (the dampers  109  are placed in the positions at which the dampers  109  cover the lower apertures of the sound conduit tubes  108   a  and  108   c ). In  FIG. 3 , horizontal axis designates frequency and vertical axis designates sound pressure level. In the sound pressure characteristics in the configuration of the speaker with dampers (at three sites) in  FIG. 2 , the peak in vicinity of 8 kHz and the sound pressure characteristics for the minimum resonant frequency or lower are both decreased by influence of the dampers  109 . In the speaker  100  of the embodiment, by contrast, only the peak in vicinity of 8 kHz is decreased and the sound pressure characteristics for the minimum resonant frequency or lower are equivalent to the sound pressure characteristics of the configuration of the speaker without dampers. That is, it can be observed in the sound pressure characteristics of the speaker  100  of the embodiment that the peak due to the resonance is curbed without the degradation in the low frequency characteristics. 
     Description will be given on reasons why the peak due to the resonance is curbed without the degradation in the sound pressure characteristics for the minimum resonant frequency or lower by provision of the two dampers  109  on two (the sound conduit tubes  108   a  and  108   c ) out of the three sound conduit tubes  108   a ,  108   b , and  108   c , as set forth for the embodiment. 
       FIG. 4A  is a diagram illustrating an example of an equivalent circuit in which the sound conduit tubes  108  and the dampers  109  in the configuration of the speaker with dampers (at three sites) are conceived as an acoustic impedance.  FIG. 4B  is a diagram illustrating an example of an equivalent circuit in which the sound conduit tubes  108  and the dampers  109  in the configuration of the speaker  100  of the embodiment are conceived as an acoustic impedance. 
     As an analytical approach to phenomena on occasions when the sound waves produced by the vibrations of the diaphragm  104  pass through the sound conduit tubes  108   a ,  108   b , and  108   c , an analytical approach to phenomena that occur on occasions when a current flows through a coil and a resistance connected in series can be applied. That is because there is a resemblance between the phenomena that occur on occasions when sound waves pass through a sound conduit tube and the phenomena that occur on occasions when a current flows through a coil and a resistance connected in series. In this relation, the sound waves may be conceived as the current and each of the sound conduit tubes  108   a ,  108   b , and  108   c  may be conceived as the coil and the resistance connected in series. 
     As an analytical approach to phenomena on occasions when the sound waves pass through the dampers, an analytical approach to phenomena that occur on occasions when a current flows through a resistances can be applied. That is because there is a resemblance between the phenomena that occur on occasions when sound waves pass through a damper and the phenomena that occur on occasions when a current flows through a resistance. In this relation, the sound waves may be conceived as the current and the damper may be conceived as the resistance. Therefore, the sound conduit tubes and the dampers that are located between the space (diaphragm side) on the side including the magnetic circuit with respect to the diaphragm  104  and the space (exterior space side) exterior to the speaker can be represented as such equivalent circuits as illustrated in  FIGS. 4A and 4B . 
     In the speaker  100  illustrated in  FIGS. 1A and 1B , the sound conduit tubes  108   a ,  108   b , and  108   c  have the same radius and the same length. In addition, the same materials are used for the dampers placed on or in the sound conduit tubes  108  and the dampers have the same thickness. 
     In the speaker  100  illustrated in  FIGS. 1A and 1B , the sound conduit tubes  108   a ,  108   b , and  108   c  have the same radius and the same length, and thus the acoustic impedances corresponding to the sound conduit tubes  108   a ,  108   b , and  108   c  can be regarded as the same. In addition, the same materials are used for the dampers placed on or in the sound conduit tubes  108  and the dampers have the same thickness, so that the acoustic impedances of the dampers  109  placed on the sound conduit tubes  108   a ,  108   b , and  108   c  in the speaker with dampers (at three sites) can be regarded as the same. In the speaker with dampers (at three sites), as illustrated in  FIG. 4A , a resistance component R 2  of the damper  109  is added to each of the sound conduit tubes  108   a ,  108   b , and  108   c , and thus a real part of an acoustic impedance Z 1  provided by the sound conduit tube and the damper that are located between the diaphragm side and the space exterior to the speaker has a constant value independently of value of the frequency as represented by following equation (1). 
                     Re   ⁡     (     Z   1     )       =         R   1     +     R   2       3             (   1   )               
Strength of the damping effect is proportional to the real part of the acoustic impedance and thus concurrent reduction in the peak due to the resonance and in the sound pressure characteristics for the minimum resonant frequency or lower is caused in the configuration in which the dampers  109  are provided on all of the sound conduit tubes  108   a ,  108   b , and  108   c.  
 
     In the configuration in which the dampers are provided on some of the plurality of sound conduit tubes (in the positions at which the dampers cover the lower apertures of the sound conduit tubes  108   a  and  108   c ) and are not provided on the remaining sound conduit tube (sound conduit tube  108   b ), such as the speaker  100  of the embodiment illustrated in  FIG. 4B , a real part of an acoustic impedance Z 2  between the diaphragm side and the space exterior to the speaker depends on frequency f as represented by following equation (2). 
                     Re   ⁡     (     Z   2     )       =                 {         (       R   1     +     R   2       )     ⁢     R   1       -         (     2   ⁢   π   ⁢           ⁢   f     )     2     ⁢     M   2         }     ⁢     (       3   ⁢     R   1       +     R   2       )       +               3   ⁢     (       2   ⁢     R   1       +     R   2       )     ⁢       (     2   ⁢   π   ⁢           ⁢   f     )     2     ⁢     M   2                   (       3   ⁢     R   1       +     R   2       )     2     +     9   ⁢       (     2   ⁢   π   ⁢           ⁢   f     )     2     ⁢     M     2   ⁢                             (   2   )               
Provided that a ratio of a resistance component R 1  of the sound conduit tube itself to a resistance component R 1 +R 2  of the sound conduit tube with addition of the damper is B, B is represented by following equation (3).
 
                   B   =         R   1     +     R   2         R   1               (   3   )                 FIG. 5  illustrates frequency characteristics of the real part of the acoustic impedance Z 2  under conditions of the ratios B of the resistance components of 10 1  (R 1 =2.81e+04, R 2 =2.53e+05), 10 2  (R 1 =2.81e+04, R 2 =2.78e+06), 10 3  (R 1 =2.81e+04, R 2 =2.81e+07), and 10 4  (R 1 =2.81e+04, R 2 =2.81e+08). In calculation of the real part of the acoustic impedance Z 2 , M in equation (2) is 6.68e+02.
 
     In  FIG. 5 , horizontal axis designates the frequency and vertical axis designates the value of the real part of the acoustic impedance Z 2 . 
     It is observed in  FIG. 5  that increase in the ratio B of the resistance components results in increase in the value of the real part of the acoustic impedance Z 2 , chiefly, for 1 kHz or higher. 
       FIG. 6  illustrates a comparison between frequency characteristics of the real part of the acoustic impedance Z 1  between the diaphragm side and the space exterior to the speaker in the speaker with dampers (at three sites) and frequency characteristics of the real part of the acoustic impedance Z 2  of the speaker  100  of the embodiment under the condition of B=10 4  (R 1 =2.81e+04, R 2 =2.81e+08). 
     In the calculation of the real part of the acoustic impedance Z 2 , M in equation (2) is 6.68e+02. 
     In  FIG. 6 , horizontal axis designates the frequency and vertical axis designates the value of the real part of the acoustic impedance. 
     In  FIG. 6 , the values of the real part of the acoustic impedance Z 1  in the configuration of the speaker with dampers (at three sites) are constant independently of the frequency, whereas the values of the real part of the acoustic impedance Z 2  for 1 kHz or lower in the configuration of the speaker  100  of the embodiment can be made one-fortieth or smaller of the value of the real part of the acoustic impedance at 8 kHz by setting of the ratio B of the resistance components at 10 4 . 
     In the speaker  100  in which the dampers are provided on at least one of the plurality of sound conduit tubes and are not provided on the remaining sound conduit tubes as in the embodiment, the peak due to the resonance thus can be curbed without the degradation in the low frequency characteristics. 
       FIG. 7  illustrates frequency characteristics of the real part of the acoustic impedance Z 2  under conditions of the ratios B of the resistance components of 10 4  (R 1 =2.81e+04, R 2 =2.81e+08), 10 5  (2.81e+04, R 2 =2.81e+09), 10 6  (2.81e+04, R 2 =2.81e+10), and 10 7  (2.81e+04, R 2 =2.81e+11). 
     In the calculation of the real part of the acoustic impedance Z 2 , M in equation (2) is 6.68e+02. 
     In  FIG. 7 , horizontal axis designates the frequency and vertical axis designates the value of the real part of the acoustic impedance Z 2 . It is observed in the drawing that the ratio B of the resistance components of 10 4  or higher results in decrease in the value of the real part of the acoustic impedance Z 2  for the high frequency range. Therefore, the ratio B of the resistance components is preferably designed to be 10 2  or higher and to be 10 5  or lower, in order that the effects of the embodiment may be obtained. 
     The dampers  109  may take any arrangement positions and any shapes as long as a configuration by which paths in the sound conduit tubes  108  are blocked is provided. 
     Though the sound conduit tubes  108   a ,  108   b , and  108   c  of the speaker  100  illustrated in  FIGS. 1A and 1B  have the same radius, there is no limitation to this configuration. For instance, the radius of the sound conduit tube  108   b  may differ from the radius of the sound conduit tubes  108   a  and  108   c . The resistance component (R 1 ) and the ratio B of the resistance components may be adjusted by change in the radii of the sound conduit tubes  108 . Decrease in the radii of the sound conduit tubes results in decrease in the ratio B of the resistance components. When the decrease in the ratio B of the resistance components is desired, accordingly, the sound conduit tubes are preferably designed to be small in the radii. 
     The sound conduit tube  108   b  that is illustrated in  FIGS. 1A and 1B  and that is not provided with the damper  109 , however, chiefly conducts sounds at low frequencies. Accordingly, wind noises in the sound conduit tube  108   b  can more effectively be prevented if the radius of the sound conduit tube  108   b  is set larger than the radius of the sound conduit tubes  108   a  and  108   c.    
     Providing that the dampers  109  are not used to adjust the ratio B of the resistance components, the ratio B of the resistance components can be increased by setting of the radius (or bore diameter) of the sound conduit tubes  108   a  and  108   c  smaller than the radius of the sound conduit tube  108   b . Provided that a ratio of the radius of the sound conduit tubes  108   a  and  108   c  to the radius of the sound conduit tube  108   b  is C, C is represented by following equation (5) with use of the real part of the acoustic impedance that is represented by equation (4). In consideration of the condition of the ratio B of the resistance components, the ratio C of the radii is preferably set so as to be 10 −1.25  or higher and so as to be 10 −0.5  or lower. 
                     Z   A     =       1       π   ⁢           ⁢     R   2       ⁢               ⁢     (         8   ⁢   μ       R   2       +       4   3     ⁢   j   ⁢           ⁢   ω   ⁢           ⁢   ρ       )               (   4   )                 C   =       1     B   ⁢               4       ⁢     
     ⁢   wherein   ⁢     
     ⁢     B   =               real   ⁢           ⁢   part   ⁢           ⁢   of   ⁢           ⁢   impedance               of   ⁢           ⁢   sound   ⁢           ⁢   conduit   ⁢           ⁢   tube   ⁢           ⁢   108   ⁢   a                   real   ⁢           ⁢   part   ⁢           ⁢   of   ⁢           ⁢   impedance               of   ⁢           ⁢   sound   ⁢           ⁢   conduit   ⁢           ⁢   tube   ⁢           ⁢   108   ⁢   b             =           8   ⁢   μ       π   ⁢           ⁢     R   3   4             8   ⁢   μ       π   ⁢           ⁢     R   1   4           =         R   1   4       R     3   ⁢             4       =       (     1   C     )     4                     (   5   )               
wherein μ is viscosity coefficient (1.86×10 −5  for air), and ρis density (1.18 kg/m 3  for air).
 
     Subsequently, an example in which the speaker  100  of embodiment 1 of the disclosure is installed in a flat-screen television will be described.  FIG. 8  is a front external view illustrating the example of the flat-screen television in which the speaker  100  of embodiment 1 of the disclosure is installed. In  FIG. 8 , reference numeral  201  denotes a housing of a set, numeral  202  denotes a display part such as liquid crystal and organic EL, and numeral  203  denotes the speakers. The speakers  203  are provided on both sides of the display part  202  in the housing of the set. 
     Operations in the flat-screen television configured as described above will be described below. Though not illustrated, acoustic signals processed in a signal processing unit are inputted into the speakers  203  at left and right and sounds are thereby reproduced from the speakers  203 . Through agency of the damping effect of the dampers of the embodiment, sound waves emitted from the speakers  203  allow the peak due to the resonance to be curbed without the degradation in the low frequency characteristics, and sounds that are excellent in the reproduction of low-pitched sound and that have flat frequency characteristics can be reproduced. 
     By emission from the sound conduit tubes, the sounds can be provided to users without making the users feel presence of the speakers  100 . 
     Though the speakers  100  are provided on both ends of the display part in the embodiment, number and arrangement positions of the speakers are not limited thereto. 
     Installation of the speakers  100  of the embodiment is not necessarily limited to such installation in flat-screen television as in the example of  FIG. 8 . The speaker may be employed in various devices that include a speaker, such as portable terminals, tablet terminals, personal computers (PC), earphones, hearing aids, and vehicles. 
     (Embodiment 2) 
     Hereinbelow, a speaker  300  of embodiment 2 will be described. For the embodiment, description on configurations similar to embodiment 1 is partially omitted. 
       FIG. 9A  is a cross sectional view illustrating an example of the speaker of the embodiment of the disclosure.  FIG. 9B  is a cross sectional view illustrating the example of the speaker of the embodiment of the disclosure and, specifically, is the cross sectional view of the speaker of  FIG. 9A , taken along line IXB-IXB. 
     The speaker  300  includes a yoke  301 , a magnet  302 , a plate  303 , a diaphragm  304 , suspensions  305   a  and  305   b , a voice coil  306 , a sound conduit tube  308 , a damper  309 , and magnetic fluid  310 . A magnetic gap  307  is defined by the yoke  301 , the magnet  302 , and the plate  303 . A back side capacity  311  is defined by the plate  303 , the diaphragm  304 , the voice coil  306 , and the magnetic fluid  310 . The voice coil  306  and the magnetic fluid  310  are placed in the magnetic gap  307 . Ribs  312  may be provided on the diaphragm  304  as in embodiment 1. 
     Configurations in operations of the speaker  300  that are different from those of the speaker  100  of embodiment 1 will be described below. 
     One of the configurations that are different from embodiment 1 is provision of the single sound conduit tube  308 . The one sound conduit tube  308  is provided at a center part of the speaker. 
     Another of the configurations that are different from embodiment 1 is incomplete closing of a section of the sound conduit tube by the damper  309 . As illustrated in  FIGS. 9A and 9B , a bore is provided at a center of the damper  309  (the damper  309  that is annular is used). 
     This embodiment is the same as embodiment 1 in that the voice coil  306  vibrates and in that sound waves are thereby generated from the diaphragm  304 . A difference from embodiment 1 is addition of the annular damper  309  to the one sound conduit tube  308 , as described above. 
     Sound waves emitted from the diaphragm  304  pass through the sound conduit tube  308 . The sound waves split into sound waves that pass through the damper  309  and sound waves that pass through the center of the damper  309  (a bore provided at a middle of the damper  309 ) and that undergo no damping effect. Thus, an acoustic path (first acoustic path) that extends through the damper  309  and an acoustic path (second acoustic path) that does not extend through the damper  309  (or that extends through a space not covered by the damper  309 , that is, the bore in the damper  309 ) are provided in the one sound conduit tube  308 . By provision of the two acoustic paths in the one sound conduit tube  308 , the peak due to the resonance can be curbed without the degradation in the low frequency characteristics as in embodiment 1. Accordingly, it is not necessary to provide a plurality of sound conduit tubes as in embodiment 1 and the effects can be attained with use of the same shape as a conventional sound conduit tube. 
     The damper  309  may take any arrangement position and any shape as long as a configuration by which the paths in the sound conduit tubes  308  are partially blocked is provided. A configuration in which the acoustic path that extends through the damper and the acoustic path that does not extend through the damper are formed with the damper  309  offset with respect to and bonded onto the sound conduit tube  308 , for instance, and the like are conceivable. 
     There may be any number of sound conduit tubes and any number of sound conduit tubes to which the dampers are added, as long as desired acoustic characteristics are attained. 
     The sound conduit tube may have a shape, such as an oval shape, other than a circular shape, as long as the shape allows connection between the back side capacity  311  and the exterior space. 
     The annular damper  309  can be applied to the speaker that is illustrated in  FIGS. 1A and 1B  and that has the plurality of sound conduit tubes. When signals including low-frequency components in large quantities are reproduced, use of the damper  309  causes reduction in pressures exerted on the sound conduit tubes to which no dampers are added and thus curbs wind noises. 
     Installation of the speaker of the embodiment is not limited to installation in a flat-screen television, as is the case with embodiment 1. The speaker may be employed in various devices that include a speaker, such as portable terminals, tablet terminals, personal computers (PC), earphones, hearing aids, and vehicles. 
     According to the disclosure, as described above, the peak due to the resonance can be curbed without the degradation in the low frequency characteristics, and thus the speakers that are excellent in the reproduction of low-pitched sound and that have the flat frequency characteristics can be provided for televisions, tablet terminals, and smartphones for which narrowing of frame has progressed and which emit sound waves to the outside via sound conduit tubes, and for earphones and hearing aids which are of open type and which include an earplug or the like provided with vent holes.