Patent Publication Number: US-10313801-B2

Title: Sound output device comprises a dual speaker including a dynamic speaker and a piezoelectric speaker

Description:
BACKGROUND 
     The present disclosure relates to a sound output apparatus, and more particularly, to a sound output apparatus capable of enhancing output characteristics in audible frequency bands including low-frequency bands and high-frequency bands. 
     In general, a piezoelectric device refers to a device having a characteristic capable of mutually changing electrical energy to/from mechanical energy. That is, an electric voltage is generated when a pressure is applied on the piezoelectric device (the piezoelectric effect), and an increase or decrease in volume or length due to an internal pressure change occurs when the electric voltage is applied thereto (the inverse piezoelectric effect). The piezoelectric device is configured with a piezoelectric layer and an electrode provided thereon, and a pressure thereof changes according to the electrical voltage applied to the piezoelectric layer through the electrode. 
     Various components such as a piezoelectric speaker, a vibration apparatus, or the like may be manufactured using the piezoelectric device. Among these, the piezoelectric speaker is a component which acoustically converts mechanical movements of the piezoelectric device into a sound in desired frequency bands using a vibration plate. It is advantageous that the piezoelectric speaker is thinner and lighter, and smaller in power consumption than an existing dynamic speaker, and thus the piezoelectric speaker can be used in electronic apparatuses such as smart phones which are required to be small, thin, and light-weighted. However, the piezoelectric speaker is disadvantageous in that the piezoelectric speaker delivers strong high-pitched sound and weak low-pitched sound, which causes a user to be unable to listen to music for a long time. 
     Meanwhile, dynamic speakers are widely used for music playback. The dynamic speakers use the principle that when an audio signal current is applied to a voice coil placed within the magnetic field of a magnet, a mechanical force acts on the voice coil according to the intensity of the current, and thus a movement is caused. However, the dynamic speakers are suitable for implementing low-frequency sound, but are relatively poor for implementing high-frequency sound, and thus the dynamic speakers have limitations in providing high sound quality. 
     RELATED ART DOCUMENTS 
     Korea Patent Application Laid-Open Publication No. 2014-0083860 
     Korea Patent No. 10-1212705 
     SUMMARY 
     The present disclosure provides a sound output apparatus capable of having both the advantages of a piezoelectric speaker and the advantages of a dynamic speaker. 
     The present disclosure also provides a sound output apparatus capable of improving both low-frequency sound characteristics and high-frequency sound characteristics. 
     In accordance with an exemplary embodiment, a sound output apparatus may include: a housing having a housing space therein; a first sound output unit provided inside the housing; and a second sound output unit provided inside the housing spaced apart by a predetermined distance from the first sound output. 
     The sound output apparatus may include at least one emission hole formed in a predetermined region of the housing. 
     The first sound output unit includes a dynamic speaker, and the second sound output unit includes a piezoelectric speaker. 
     A separation space may be provided between the first and second sound output units, and the emission hole is formed such that at least a portion of the emission hole corresponds to the separation space. 
     The housing may include a first member, and a second member provided to surround the first member, and the emission hole may be formed in a predetermined region of the second member. 
     The first member may be provided in a plate shape having a predetermined thickness, and may separate the first and second sound output units from each other. 
     A stepped portion may be formed on at least one region of one surface of the first member. 
     The second sound output unit may be provided to contact the stepped portion of the first member, and the first sound output unit may be provided spaced from the other surface opposite to the one surface. 
     The sound output apparatus may further include at least one protrusion protruding from the inner portion of the second member. 
     The first member may be provided on an upper portion of the protrusion, and the first sound output unit may be in contact with a lower portion of the protrusion. 
     The emission hole may be formed to correspond to a space between the first member and the first sound output unit. 
     The emission hole may be formed to have an area of about 5% to about 90% of the to surface area of the first sound output unit. 
     The sound output apparatus may further include a coating layer formed on at least a portion of at least one of the first and second sound output units, or the housing. 
     The first and second sound output units may be driven at the same time by a signal having the same level. 
     The first and second output units may be driven in a voltage range of about 0.1V to about 5.0V. 
     The second sound output unit may include a vibration device having an opening, and a piezoelectric device provided on at least one surface over the opening of the vibration device. 
     A sound output from the second sound output unit may be output through the first sound output unit, and a sound emitted through the emission hole may be mixed outside the housing with the sound output from the first sound output unit. 
     In accordance with another exemplary embodiment, a sound output apparatus may include: a housing having a housing space therein; a first sound output unit provided inside the housing; a second sound output unit provided inside the housing spaced a predetermined space from the first sound output unit; and a separation member provided between the first and second sound output units inside the housing and to separate the first and second sound output units from each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments may be understood in more detail from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an exploded perspective view illustrating a sound output apparatus in accordance with an exemplary embodiment of the present disclosure; 
         FIG. 2  is a combined perspective view illustrating the sound output apparatus in accordance with the exemplary embodiment of the present disclosure; 
         FIG. 3  is a combined cross-sectional view illustrating the sound output apparatus in accordance with the exemplary embodiment of the present disclosure; 
         FIG. 4  is an exploded perspective view illustrating a sound output apparatus in accordance with another exemplary embodiment of the present disclosure; 
         FIG. 5  is a combined perspective view illustrating a sound output apparatus in accordance with another exemplary embodiment and yet another exemplary embodiment of the present disclosure; 
         FIG. 6  is an exploded perspective view illustrating the sound output apparatus in accordance with yet another exemplary embodiment of the present disclosure; 
         FIG. 7  is a combined cross-sectional view illustrating the sound output apparatus in accordance with yet another exemplary embodiment of the present disclosure; and 
         FIG. 8  is a graph showing characteristics of a dynamic speaker, a piezoelectric speaker, and a sound output apparatus in accordance with exemplary embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. 
       FIG. 1  is an exploded view illustrating a sound output apparatus in accordance with an exemplary embodiment of the present disclosure;  FIG. 2  is a combined perspective view of the sound output apparatus; and  FIG. 3  is a combined cross-sectional view of the sound output apparatus. 
     Referring to  FIGS. 1, 2, and 3 , the sound output apparatus in accordance with an exemplary embodiment of the present disclosure may include a first sound output unit  100 , a second sound output unit  200  provided on the first sound output unit  100 , and a housing  300  for housing the first and second sound output units  100  and  200 . That is, the first and second sound output units  100  and  200  may be provided inside the housing  300  such that the first and second sound output units  100  and  200  are spaced apart by a predetermined distance from each other. The first sound output unit  100  may include a dynamic speaker which includes a voice coil  140  and a vibration plate  150 , and outputs sound by vibrating the vibration plate  150  using vibration caused by current change in the voice coil  140 . Also, the second sound output unit  200  may include a piezoelectric speaker which includes a piezoelectric device  210  and a vibration device  220  and acoustically converts a mechanical movement of the piezoelectric device  210  into a sound by the vibration device  220 . 
     1. First Sound Output Unit 
     The first sound output unit  100  may be provided in a substantially circular shape having a predetermined thickness. The first sound output unit  100  may include: a yoke  110  and a frame  115  which define a housing space therein; a magnet  120  provided in the housing space inside the yoke  110 ; a plate  130  provided on the magnet  120 ; a voice coil  140  spaced from the inner portion of the frame  115  and disposed between the yoke  110  and the magnet  120 ; and a vibration plate  150  which is provided above the plate  130 , an edge of which is fixed to the frame  115 , and to which the voice coil  140  is fixed. 
     The yoke  110  is provided in a substantially cylindrical shape having a predetermined height, and the frame  115  is provided to an upper section of the yoke  110  and has a substantially cylindrical shape having a predetermined height. The height of the frame  115  may be greater than that of the yoke  110 , and the width of the frame  115  may be greater than that of the yoke  110 . The height of the frame  115  may be equal to or lower than the height of the yoke  110 . An upper edge of the frame  115  is in contact with at least a portion of the housing  300 , and may be housed in the housing  300 . Also, the magnet  120  and the plate  130  are housed inside the yoke  110 , the voice coil  140  is housed inside the frame  115 , and the vibration plate  150  may be provided on the frame  115  so as to cover the same. The yoke  110  and the frame  115  induce a magnetic field generated by the magnet  120  towards the plate  130  and apply the magnetic force by the magnet  120  to the voice coil  140  to the maximum. 
     The magnet  120  is fixed to a bottom surface of the yoke  110 . That is, the undersurface of the magnet  120  is in contact with and fixed to the bottom surface of the yoke  110 . The magnet  120  may be provided in a shape corresponding to the internal shape of the yoke  110 . For example, the internal shape of the yoke  110  has a substantially cylindrical bucket shape, and the magnet  120  has a substantially cylindrical shape. The height of the magnet  120  may be the lower than or equal to the height of the yoke  110 . Also, the diameter of the magnet  120  may be smaller than the inner diameter of the yoke  110 . Thus, the magnet  120  may be provided inside the yoke  110  such that the magnet  120  is spaced apart by a predetermined distance from the inner sidewall of the yoke  110 . 
     The plate  130  is provided on the top surface of the magnet  120 . The plate  130  may be provided in a shape similar to the planar shape of the magnet  120 . That is, the plate  130  may be provided in a circular plate shape having a predetermined thickness. The plate  130  has a smaller diameter than the inner diameter of the yoke  110 , and may have a diameter equal to or larger than the diameter of the magnet  120 . Thus, an outer portion of the magnet  120  may be spaced part by a predetermined distance from the inner side surface of the yoke  110 . Also, the total height of the magnet  120  and the plate  130  provided on the magnet  120  may be the same as the height of the yoke  110 . That is, top portions of the plate  130  and the yoke may be coplanar with each other. The plate  130  allows lines of magnetic force generated by the magnet  120  to be focused towards the voice coil  140 . 
     The voice coil  140  is attached to the undersurface of the vibration plate  150 , and may be spaced from the frame  115  and provided between the yoke  110  and the magnet  120 . For example, the voice coil  140  is provided between the yoke  110  and the magnet  120  so as to surround the plate  130  and a portion of the magnet  120  with a predetermined height, and has an upper portion attached to the undersurface of the vibration plate  150 . The voice coil  140  forms a magnetic field which consistently changes by an electrical signal that is input while consistently changing, and therefore the voice coil  140  vibrates by an interaction due to interference between the magnetic field and another magnetic field formed by the magnet  120 . 
     An edge of the vibration plate  150  is fixed to an inner portion of the frame  115  such that the vibration plate  150  is provided to cover an upper portion of the frame  115 . In the vibration plate  150 , at least a portion thereof may be convexly provided. For example, the vibration plate  150  may be provided in such a shape that the vibration plate  150  has the highest portion corresponding to a central portion of the plate  115 , and a peripheral portion gradually lowered from the center portion to the outside. Also, the voice coil  140  may be fixed to the undersurface the lowest portion of the vibration plate  150 . 
     The first sound output unit  100  constitutes a closed circuit in which the magnetic filed generated from the magnet  120  moves to the yoke  110  positioned at a lower side through the plate  130  provided on the magnet  120 , and moves back to the magnet  120 . The magnetic field moving into a space between the plate  130  and the yoke  110  thereunder pushes or pulls the voice coil  140  according to a magnetic polarity of the voice coil  140  when the voice coil  140  is magnetized by a current applied thereto. That is, the voice coil  140  is pushed out by a mutual repulsion and moved forward when the voice coil  140  has the same magnetic polarity as the plate  130  and the yoke  110  thereunder, and the voice coil  140  is attracted and pulled back when the voice coil  140  has a different magnetic polarity from the plate  130  and the yoke  110  thereunder. In this way, when the voice coil  140  moves, the vibration plate  150  to which the voice coil  140  is fixed reciprocates and vibrates air, and generates a sound. 
     2. Second Sound Output Unit 
     The second sound output unit  200  may include a piezoelectric device  210  and a vibration device  220 . The piezoelectric device  210  may be provided, for example, in a circular plate shape having a predetermined thickness. Alternatively, the piezoelectric device  210  may also be provided in various shapes such as a square, a rectangular, an oval, a polygonal shape or the like as well as a circular shape. The piezoelectric device  210  may include a substrate and a piezoelectric layer on which the substrate is formed on at least one surface thereof. For example, the piezoelectric device  210  may be formed as a bimorph type device in which the piezoelectric layers are formed on both sides of the substrate, or formed as a unimorph type device in which the piezoelectric layer is formed on one surface of the substrate. At least one piezoelectric layer may be laminated, and a plurality of piezoelectric layers may be preferably laminated. Also, electrodes may be respectively formed over and below the piezoelectric layer. That is, a plurality of piezoelectric layers and a plurality of electrodes may be alternately laminated to form the piezoelectric device  210 . The piezoelectric layer may be formed using for example, PZT (Pb, Zr, Ti), NKN (Na, K, Nb), BNT (Bi, Na, Ti), or a polymer-based piezoelectric material. Also, the piezoelectric layers may be laminated such that the piezoelectric layers are polarized in different or same orientation. That is, when a plurality of piezoelectric layers is formed on the same surface of a substrate, the piezoelectric layers may have polarizations that are alternated in a different or same orientation. Meanwhile, the substrate may use a material having a characteristic that vibration may occur while maintaining a laminated structure of the piezoelectric layers, for example, may be formed of metal, plastics and the like. However, the piezoelectric device  210  may not use the piezoelectric layer or the substrate, for example, the piezoelectric device  210  may be formed in such a configuration that an unpolarized piezoelectric layer is provided in the central portion thereof, and a plurality of piezoelectric layers polarized in different orientations are laminated over and below the unpolarized piezoelectric layer. Meanwhile, an electrode pattern (not illustrated) to which a driving signal is applied may be formed on one surface of the piezoelectric device  210 . At least two electrode patterns may be formed spaced apart from each other, and connected with connection terminals (not illustrated), thereby receiving a sound signal from electronic apparatuses, such as auxiliary mobile apparatuses. 
     The vibration device  220  is provided in a substantially circular shape, and may be bigger than the piezoelectric device  210 . Also, the vibration device  220  may have an opening formed in center portion thereof, and the piezoelectric device  210  may be provided over the opening. The piezoelectric device  210  may be bonded to the top surface of the vibration device  220  by adhesives. The vibration device  220  may use a polymer- or pulp-based material. For example, the vibration device  220  may use a resin film such as an ethylene propylene rubber-based material, a styrene butadiene rubber-based material, and the like having a high loss factor, in which Young&#39;s modulus is in a range from about 1 MPa to 10 GPa. Also, a lower edge of the vibration device  220  may contact an inner surface of the housing  300 . That is, the vibration device  220  and the piezoelectric device  210  bonded to the center portion of the vibration device  220  may be provided inside the housing  300 . The second sound output unit  200  is driven by a predetermined signal, and may output a sound having excellent high-frequency sound characteristics. 
     Meanwhile, a coating layer (not illustrated) may be further formed on at least a portion of the second sound output unit  200 . The coating layer may be formed using waterproof materials such as parylene. The parylene may be formed on the upper and side surfaces of the piezoelectric device  210  and the upper and side surfaces of the vibration device  220  exposed by the piezoelectric device  210 , in a state in which the piezoelectric device  210  is bonded onto the vibration device  220 . That is, the parylene may be formed on the upper and side surfaces of the piezoelectric device  210  and the vibration device  220 . Also, the parylene may be formed on the upper and side surfaces of the piezoelectric device  210  and the top, side, and bottom surfaces of the vibration device  220 , in a state in which the piezoelectric device  210  is bonded onto the vibration device  220 . That is, the parylene may be formed on the top, side, and bottom surfaces of the piezoelectric device  210  and the vibration device  220 . When the piezoelectric device  210  is provided over the opening formed in the center portion of the of the vibration device  220 , the parylene is formed on the top and side surfaces, and the bottom surface exposed by the opening, and may also be formed on the top, side, and bottom surfaces of the vibration device  220 . As such, since the parylene is formed on at least one surface of each of the piezoelectric device  210  and the vibration device  220 , moisture penetration into the second sound output unit  200  and a oxidation phenomenon may be prevented. Furthermore, a horizontal vibration caused by use of the vibration device  220  made of a thin material such as polymer may be mitigated, a response speed is enhanced by an increase in hardness of the vibration device  220 , and thus an in-depth acoustic characteristic is mitigated, and a high-frequency band sound may be stabilized. Since a resonant frequency may be adjusted in accordance with the coating thickness of the parylene, a sound pressure improvement point may is possibly adjusted. While the parylene may also be coated on the piezoelectric device  210  only, the parylene may be coated on the top, side, and bottom surfaces of the piezoelectric device  210 , and may also be coated on an FPCB which is connected to the piezoelectric device  210  for supplying power to the piezoelectric device  210 . As the parylene is coated on the piezoelectric device  210 , the moisture penetration into the piezoelectric device and the oxidation may be prevented. Also, the resonant frequency may be adjusted by adjusting a forming thickness. Meanwhile, when the parylene is formed on the FPCB, a noise generated by the FPCB and solder, and a device connecting part may also be mitigated. The parylene may be coated in different thicknesses in accordance with materials and characteristics of the piezoelectric device  210  or the vibration device  220 . The parylene may be formed thinner than the piezoelectric device  210  or the vibration device  220 , and may be, for example, formed in a thickness of about 0.1 μm to about 10 μm. For example, the parylene may be coated on at least one surface of the second sound output unit  200  by vaporizing parylene through a primary heating in a vaporizer to be turned into a dimmer state, then pyrolyzing the resultant into a monomer state through a secondary heating, and converting the resultant into a polymer state from the monomer state by cooling the parylene. Meanwhile, a waterproof layer such as the parylene or the like may also be coated on at least a portion of the first sound output unit  100  and the housing  200  as well as at least on a portion of the second sound output unit  200 . 
     3. Housing 
     The housing  300  is provided in a substantially cylindrical shape in which at least a portion of a side surface thereof is removed. That is, the housing  300  is provided in a tubular shape vertically opened, and may be provided in the shape in which at least a portion of the side surface is removed. For example, the housing  300  may also be provided as a vertical penetration type one, and also has a shape in which a predetermined inner region is closed and the upper and bottom portions thereof are opened therefrom. The vertical penetration type housing  300  may include a first member  310  having a substantially ring shape having a predetermined thickness, and at least one second member  320  provided in upward and downward direction from a predetermined region of the first member  310 . That is, the second member  320  may be provided to surround the ring-shaped first member  310 . Alternatively, when the first member  310  is provided in a circular plate shape, the housing  300  may be implemented in a shape in which the upper and lower portions thereof are opened from the first member  310 . Also, the second member  320  may be provided outside the first member  310  such that the second member  320  is spaced apart by a predetermined distance from the first member  310 . For example, the second member  320  having a predetermined width may be provided in four and the four second members  320  are spaced apart by a predetermined distance from each other. The distance between the second members  320  may be the same as or smaller than the width of the second member  320 . For example, the distance between the second members  320  may be about 10% to about 100% of the width of the second member  320 . In this exemplary embodiment, the second member  320  is illustrated to have the same thickness as the distance between the second members  320 . That is, the second member  320  having a predetermined width may be provided in plurality by being spaced apart by the same distance as the width thereof. Meanwhile, a protrusion  321  may be provided inside the second member  320 . That is, the protrusion  321  may be provided to inwardly protrude from an inner wall of the second member  320 . Also, the first member  310  may be seated on the protrusion  321 . The first and second members  310  and  320  may be manufactured separately and thereafter the first member  310  is seated on the protrusion  321  of the second member  320 . Alternatively, the first and second members  310  and  320  may be manufactured integrally. Also, the first and second members  310  and  320  may be manufactured without providing the protrusion  321  such that an outer portion of the first member  310  is attached to be in contact with an inner portion of the second member  320 , or manufactured integrally. In the housing  300 , the second sound output unit  200 , i.e., the vibration device  220  of the piezoelectric speaker, may contact the top surface of the first member  310 , and the first sound output unit  100 , i.e., a dynamic speaker, may contact a lower portion of the protrusion  321  of the second member  320 . That is, the first and second sound output units  100  and  200  may be provided spaced apart by a predetermined distance from each other with the first member  310  and the protrusion  321  disposed therebetween. When the first member  310  is in contact with the inner wall of the second member  320  without the protrusion  321  provided to the inner portion of the second member  320 , the vibration device  220  may be in contact with the top surface of the ring-shaped first member  310 , the first sound output unit  100  may be in contact with the bottom surface of the first member  310 . That is, the first and second sound output units  100  and  200  may be opposed to each other by being spaced apart by the thickness of the first member  210  from each other. Also, the first and second sound output units  100  and  200  are spaced apart by a predetermined distance from each other, and at least a portion of a region between the second members  320  may function as an emission hole  330 . That is, the first sound output unit  100  and the first member  310  are spaced apart by a predetermined distance from each other, and the emission hole  330  may be provided to correspond to the space therebetween. The emission hole  330  may be formed in a size of about 5% to about 90% of the top surface area of the first sound output unit  100 . That is, the emission hole  330  may be provided by at least one or more, and the whole area of the emission holes  330  may be formed in a size of about 5% to about 90% of the top surface area of the first sound output unit  100 , i.e. the area of the vibration plate  150 . The size of the emission holes  330  may be preferably formed in a size of about 10% to 60%, and more preferably in a size of about 15% to 30% of the top surface area of the first sound output unit  100 . A sound from the first sound output unit  100  is emitted through the emission hole  330 . Thus, the sound from the second sound output unit  200  is directly emitted to the outside, and the sound from the first sound output unit  100  is emitted through the emission hole  330  of the housing  300 , and thus the two sounds are mixed outside the housing  300  thereby further improving a sound quality. Meanwhile, the sound output apparatus may be manufactured for a speaker for vehicle speakers, in-house speakers, or the like, or as amplifiers and earphones. When the sound output apparatus is manufactured as the earphone such as a kernel-type earphone, the housing  300  may be manufactured in a substantial size that may be inserted in an ear. The earphone may be inserted in an ear from the second output unit  200 . Thus, the sound from the second sound output unit  200  is firstly output and the sound from the first sound output unit  100  is later output, so that the two sounds are then mixed inside the ear. Alternatively, the first sound output unit  100  may be firstly inserted in the ear, and the two sounds will also be then mixed inside the ear. Also, according to exemplary embodiments of the present disclosure, the sound output apparatus may be manufactured by inserting the first and second sound output units  100  and  200  in the housing so as to be spaced from each other, or may also be manufactured by combining a portion of the housing  300  in which the first sound output unit  100  is inserted, with another portion of the housing  300  in which the second sound output unit  200  is inserted. For example, the sound output apparatus may be manufactured such that the thickness of the first member  310  is divided in half, the first sound output unit  100  is then inserted inside a first housing with a portion of the second member  320  formed such that the emission hole  330  is formed to surround a first-thickness lower portion of the first member  310 , the second sound output unit  200  is then inserted inside a second housing with a portion of the second member  320  formed to surround a second-thickness upper portion of the first member  310 , and thereafter the first and second housings are combined. Meanwhile, the sound output apparatus in accordance with the present disclosure can be driven in a low voltage range of about 0.1V to 5.0V, preferably of about 0.1V to about 2.0V, and more preferably of about 0.1V to about 0.5V. Particularly, when the same is applied to the earphones, the sound output apparatus can be driven in a low voltage range of about 0.1V to about 0.2V, and preferably of about 0.1V to about 0.18V. That is, in the piezoelectric device  210  of the second sound output unit  200 , a plurality of piezoelectric layers is laminated, and internal electrodes are formed between the respective piezoelectric layers. Here, since the piezoelectric layer is formed in a thickness ranging from about 5 μm to about 20 μm, the second sound output unit  200  can be driven in a low voltage range. While a driving voltage of a typical piezoelectric speaker is 5V or more, the second sound output unit  200  according to the present disclosure can be driven in a low voltage range of about 0.1V to about 0.5V without using an amplifier for piezoelectric speaker, and can thus be driven in a low voltage range in combination with the dynamic speaker. Also, in the sound output apparatus according to the present disclosure, the first and second sound output units  100  and  200  may be driven at the same time by the same signal applied thereto. That is, a signal supplied from a signal source is directly applied to the first sound output unit  100 , and applied to the second sound output unit  200  after passing through a high band path filter, and thus low- and high-frequency band signals may be applied to the first and second sound output units  100  and  200  respectively. However, in the present disclosure, a signal having the same level may be applied to the first and second sound output units  100  and  200  at the same time. 
       FIG. 4  is an exploded perspective view illustrating a sound output apparatus in accordance with another exemplary embodiment of the present disclosure,  FIG. 5  is a combined perspective view illustrating a sound output apparatus in accordance with another exemplary embodiment and yet another exemplary embodiment of the present disclosure,  FIG. 6  is an exploded perspective view illustrating the sound output apparatus in accordance with yet another exemplary embodiment, and  FIG. 7  is a combined cross-sectional view illustrating the sound output apparatus in accordance with yet another exemplary embodiment of the present disclosure. 
     Referring to  FIGS. 4 to 7 , the sound output apparatus in accordance with another exemplary embodiment and yet another exemplary embodiment of the present disclosure may include: a first sound output unit  100  including a voice coil  140  and a vibration plate  150 ; a second sound output unit  200  provided on the first sound output unit  100  and including a piezoelectric device  210  and a vibration device  220 ; and a housing  300  for housing the first and second sound output units  100  and  200 . In accordance with another exemplary embodiment and yet another exemplary embodiment of the present disclosure, the second sound output unit  200 , i.e. a piezoelectric speaker, may be implemented such that the piezoelectric device  210  is provided under the vibration device  220 . Also, an emission hole  330  is formed in a predetermined region of a second member  320  of the housing  300 . As illustrated in  FIG. 4 , a first member  310  may be provided in a ring shape, and as illustrated in  FIGS. 6 and 7 , a first member  310   a  may be provided in a plate shape. Also, in the plate-shaped first member  310   a , a height difference may be formed in a predetermined region. For example, a stepped portion may be formed along an upper edge of the first member  310   a , and thus the height difference may be formed. That is, the first member  310   a  may be formed in a plate shape in which a first region having a predetermined width is formed relatively higher on an edge thereof, and a second region is formed relatively lower than the first region at inner portion of the edge. The first region may be formed in a ring shape along the edge, or may be provided in at least two sub-regions spaced apart from each other. Also, the second region of the first member  310   a  may be provided larger than the piezoelectric device  210 . That is, the piezoelectric device  210  may be provided smaller than the vibration device  220  and smaller than the second region such that the piezoelectric device  210  is not in contact with the first region. The vibration device  220  of the second sound output unit  200  is in contact with the stepped portion of the plate-shaped first member  310   a , i.e., the first region, and a resonant space is provided between the second region of the first member  310   a  and the vibration device  220 , so that the piezoelectric device  210  may be disposed therein. Also, since the first member  310   a  is provided in a plate shape, the first member  310   a  separates the first and second sound output units  100  and  200  from each other. That is, the plate-shaped first member  310   a  may serve as a separation member for separating the first and second sound output units  100  and  200 . As the first and second sound output units  100  and  200  are separated by the separation member, i.e. the plate-shaped first member  310   a , sounds output from the first and second sound output units  100  and  200  are output without being mixed together inside the housing  300 . Meanwhile, at least one second member  320  in which an emission hole  330  is formed may be provided such that the second member  320  surrounds the ring-shaped first member  310  or the plate-shaped first member  310   a . For example, the second member  320  is provided singly, and a predetermined region of the second member  320  is cut away in a vertical direction. A signal line for supplying a signal to the second sound output unit  200  may be provided to the cut-away region. The width of the second member  320  in the cut-away region, i.e., the distance between both ends of the second member  320  may be about 1% to about 5% of the width of the second member  320 . Also, at least one emission hole  330  is formed in upper portion of the second member  320 . The emission holes  330  are provided in, for example four, and the four emission holes  330  may be spaced apart by a predetermined distance from each other at a predetermined height on a lower portion of the first members  310  and  310   a , and may be formed to the height of the second region of the first sound output unit  100 . That is, at least a portion of the second region of the first sound output unit  100  may be exposed by the emission holes  330 . Also, the width of the emission hole  330  may be for example, about 20% to about 80%, and preferably about 50% of the width of the second member  320 . The area of the emission hole  330  may be about 5% to about 90% of the top space area of the first sound output unit  100 , i.e., the area of the vibration plate  150 . The size of the emission hole  330  is preferably about 10% to about 60%, and more preferably about 15% to 30% of the top surface area of the first sound output unit  100 . Meanwhile, when the first member  310  is formed in a plate shape, the first and second sound output units  100  and  200  may be spatially separated, and the sounds respectively output from the first and second sound output units  100  and  200  may be prevented from being mixed inside the housing  300 . 
     As described above, in the sound output apparatus in accordance with exemplary embodiments of the present disclosure, the first and second sound output units  100  and  200  may be provided inside the housing  300 , and output characteristics of both low-frequency and high-frequency sounds may be improved. That is, the first sound output unit  100  having excellent bass characteristics, that is the dynamic speaker, and the second sound output unit  200  having excellent treble characteristics, that is the piezoelectric speaker are provided inside the housing  300 , and thus acoustic characteristics in the audible frequency bands may be enhanced. Also, a predetermined separation space is provided between the first and second sound output units  100  and  200  inside the housing  300 , and the emission hole  330  is formed in at least a region of the housing  300 , and thus the sound output to the separation space from the first sound output unit  100  may be emitted to the outside. That is, since a sound is firstly output from the second sound output unit  200  and a sound is then output from the first sound output unit  100  through the emission hole  330 , the two sounds are mixed outside the housing  300 . Sound quality may therefore be enhanced because the two sounds are mixed outside the housing  300  when compared to the case in which the two sounds are mixed inside the housing  300 . 
       FIG. 8  is a graph showing characteristics of a dynamic speaker, a piezoelectric speaker, and a sound output apparatus in accordance with the present disclosure. A refers to characteristics graph of the dynamic speaker, B refers to characteristics graph of the piezoelectric speaker, and C refers to characteristics graph of the sound output apparatus in accordance with an exemplary embodiment of the present disclosure in which the dynamic speaker and the piezoelectric speaker are combined inside a housing. As illustrated in  FIG. 8 , the dynamic speaker (A) has excellent characteristics up to about 7 kHz of audio frequency range, that is, excellent the low-frequency sound characteristics, but exhibits a phenomenon in which an attenuation of about 20 dB to 30 dB occurs at audio frequency range higher than 7 kHz when compared with the piezoelectric speaker (B). Thus, the dynamic speaker is proven to be poor in high-frequency sound characteristics. Also, the piezoelectric speaker (B) has excellent characteristics in a mid- and high-frequency band higher than 8 kHz, but exhibits a phenomenon in which the attenuation of about 30 dB or higher occurs at audio frequency range of 3 kHz or lower, which demonstrates that the piezoelectric speaker is poor in low-frequency sound characteristics. However, the sound output apparatus (C) in accordance with an exemplary embodiment of the present disclosure may be figured out that overall acoustic characteristics in the audible frequency band up to 20 kHz are excellent. That is, the sound output apparatus in accordance with the present disclosure may be proven to have similar acoustic characteristics to the dynamic speaker in an audio frequency band up to 1.5 kHz, and better acoustic characteristics than the piezoelectric speaker in the audio frequency band higher than 1.5 kHz. In an audio output apparatus in accordance with exemplary embodiments of the present disclosure, a dynamic speaker and a piezoelectric speaker are spaced a predetermined distance from each other inside a housing. Thus, acoustic characteristics in audible frequency band can be enhanced by disposing, inside a single housing, the dynamic speaker having excellent low-frequency sound characteristics and the piezoelectric speaker having excellent high-frequency sound characteristics. Also, sound quality can be further enhanced by separating the dynamic speaker and the piezoelectric speaker from each other using a separation member provided inside the housing such that the sounds separately output from the dynamic speaker and the piezoelectric speaker are not mixed inside the housing but mixed outside of the housing. Meanwhile, the sound output apparatus in accordance with exemplary embodiments of the present disclosure may be implemented as speakers, earphones, or the like. 
     Meanwhile, the technical idea of the present invention has been specifically described with respect to the exemplary embodiments, but it should be noted that the foregoing embodiments are provided only for illustration while not limiting the present invention. Further, it will be readily understood by those skilled in the art that various modifications and changes can be made thereto without departing from the spirit and scope of the present invention.