Patent Publication Number: US-10327067-B2

Title: Three-dimensional sound reproduction method and device

Description:
TECHNICAL FIELD 
     The present invention relates to a method and apparatus for reproducing a stereophonic sound and, more specifically, to a method and apparatus for generating a virtual sound source at a predetermined location by using a reflected sound of a speaker located at a side surface. 
     BACKGROUND ART 
     Along with developments in image and sound processing techniques, pieces of content having high image quality and high sound quality have been produced. An audience requesting content having high image quality and high sound quality desires a realistic image and sound, and accordingly, research into stereoscopic imaging and stereophonic sound has been actively conducted. 
     However, recently, a speaker having a plurality of speaker units integrated in one enclosure, such as a miniaturized wireless speaker and sound bar, has been widely used, but with respect to this speaker, it is difficult to provide a wide sound stage intended in a stereo system since a distance between a left speaker and a right speaker is relatively short. 
     Therefore, when a speaker is miniaturized, an audience may not feel a sense of spaciousness or a three-dimensional (3D) effect. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
     Provided are a stereophonic sound reproduction apparatus and method for providing a three-dimensional (3D) effect and a sense of space to an audience. 
     In addition, provided is a computer-readable recording medium having recorded thereon a program for executing, in a computer, the method. The technical problems according to the present embodiments are not limited to the technical problems described above, and other technical problems may be derived from the embodiments below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a stereophonic sound reproduction environment of an audience, according to an embodiment. 
         FIG. 2A  is a block diagram of a stereophonic sound reproduction apparatus according to an embodiment. 
         FIG. 2B  is a detailed block diagram of the stereophonic sound reproduction apparatus according to an embodiment. 
         FIG. 3A  illustrates various pieces of spatial information of the stereophonic sound reproduction environment of  FIG. 1 . 
         FIG. 3B  illustrates graphs showing a magnitude of an acoustic signal output from a side speaker and transferred to an audience, which has been measured at a location of the audience over time. 
         FIG. 4A  is a detailed block diagram of the stereophonic sound reproduction apparatus according to an embodiment. 
         FIG. 4B  is a block diagram of an attenuation signal generation unit according to an embodiment. 
         FIG. 5  illustrates an example in which a left speaker and a right speaker in the stereophonic sound reproduction apparatus rotate in a horizontal or vertical direction with respect to the ground. 
         FIG. 6  illustrates a sound stage of an acoustic signal input in the stereophonic sound reproduction environment of  FIG. 1 . 
         FIG. 7  illustrates a relationship between a frequency of an acoustic signal and magnitudes of acoustic signals output from a left speaker and a right speaker, according to an embodiment. 
         FIG. 8A  illustrates various shapes of a horn-shaped side speaker. 
         FIG. 8B  illustrates a structure for rotating a horn-shaped side speaker, according to an embodiment. 
         FIG. 9  illustrates shapes of an enclosure included in the stereophonic sound reproduction apparatus, according to an embodiment. 
         FIG. 10  is a flowchart of a method by which a stereophonic sound reproduction apparatus reproduces a stereophonic sound, according to an embodiment. 
         FIG. 11  is a detailed flowchart of a method by which a stereophonic sound reproduction apparatus reproduces a stereophonic sound, according to an embodiment. 
     
    
    
     BEST MODE 
     According to an embodiment, a stereophonic sound reproduction apparatus includes: an input unit configured to receive an acoustic signal; a control unit configured to acquire an output acoustic signal for generating a virtual sound source for the received acoustic signal; and an output unit configured to output the acquired output acoustic signal by using a front speaker and a side speaker, wherein the control unit is further configured to generate an attenuation signal that is a signal for attenuating or cancelling an inflow acoustic signal to be directly transferred to an audience in the output acoustic signal output from the side speaker, and the output acoustic signal output from the front speaker includes the attenuation signal. 
     The side speaker may include a left speaker and a right speaker, the control unit may be further configured to generate at least one of a first attenuation signal for attenuating or cancelling, at a location of the audience, a left inflow acoustic signal to be directly transferred to the audience without being reflected from a left wall in an output acoustic signal output from the left speaker and a second attenuation signal for attenuating or cancelling, at the location of the audience, a right inflow acoustic signal to be directly transferred to the audience without being reflected from a right wall in an output acoustic signal output from the right speaker, and the front speaker may include at least one speaker configured to output at least one attenuation signal of the first attenuation signal and the second attenuation signal. 
     The control unit may be further configured to predict the left inflow acoustic signal and the right inflow acoustic signal arriving at the location of the audience, based on an acoustic transfer function using path information between a location of the side speaker and the location of the audience and generate the attenuation signal based on the predicted left inflow acoustic signal and right inflow acoustic signal, and on an acoustic transfer function using path information between a location of the speaker outputting the attenuation signal and the location of the audience. 
     The virtual sound source may include a first virtual sound source for a left channel signal of the received acoustic signal and a second virtual sound source for a right channel signal of the received acoustic signal, and the control unit may be further configured to acquire the output acoustic signal by controlling at least one of a magnitude, a time delay, and an output direction of the received acoustic signal, to generate the first virtual sound source and the second virtual sound source based on an acoustic signal generated when the output acoustic signal output from the side speaker is reflected from a wall and on the output acoustic signal output from the front speaker. 
     The side speaker may include a left speaker located to the left of the stereophonic sound reproduction apparatus and a right speaker located to the right thereof, and the control unit may be further configured to control at least one of a magnitude, a time delay, and an output direction of the received acoustic signal, to generate the first virtual sound source and the second virtual sound source based on an acoustic signal generated when the output acoustic signal output from the left speaker is reflected from the left wall, on an acoustic signal generated when the output acoustic signal output from the right speaker is reflected from the right wall, and on the output acoustic signal output from the front speaker. 
     The control unit may be further configured to control at least one of a magnitude, a time delay, and an output direction of the left channel signal of the received acoustic signal to generate the first virtual sound source at a first location by using an acoustic signal generated when a left channel signal of the output acoustic signal output from the left speaker is reflected from the left wall, an acoustic signal generated when a left channel signal of the output acoustic signal output from the right speaker is reflected from the right wall, and a left channel signal of the output acoustic signal output from the front speaker and control at least one of a magnitude, a time delay, and an output direction of the right channel signal of the received acoustic signal to generate the second virtual sound source at a second location by using an acoustic signal generated when a right channel signal of the output acoustic signal output from the left speaker is reflected from the left wall, an acoustic signal generated when a right channel signal of the output acoustic signal output from the right speaker is reflected from the right wall, and a right channel signal of the output acoustic signal output from the front speaker, and the first location and the second location may be respectively located to the left and the right of the audience based on a direction in which the audience looks at the stereophonic sound reproduction apparatus. 
     The control unit may be further configured to determine the first location and the second location based on spatial characteristics of a sound image provided by the received acoustic signal and control at least one of magnitude values of the left channel signal and the right channel signal of the received acoustic signal based on the determined first location and second location. 
     The control unit may be further configured to determine a distance from the side speaker to the wall and an angle between the side speaker and the wall and control a direction in which the side speaker outputs an acoustic signal as a horizontal or vertical direction with respect to the ground based on the determined distance and angle. 
     The side speaker may have a horn shape. 
     The side speaker may be included in an enclosure of a woofer inside the stereophonic sound reproduction apparatus. 
     The control unit may include a panning unit and an attenuation signal generation unit, the panning unit may be configured to control at least one of a magnitude, a time delay, and an output direction of the received acoustic signal to generate the virtual sound source based on the acoustic signal generated when the output acoustic signal output from the side speaker is reflected from the wall and on the output acoustic signal output from the front speaker, and the attenuation signal generation unit may be configured to generate the attenuation signal that is a signal for attenuating or cancelling the inflow acoustic signal to be directly transferred to the audience in the output acoustic signal output from the side speaker. 
     According to an embodiment, a stereophonic sound reproduction method includes: receiving an acoustic signal; acquiring an output acoustic signal for generating a virtual sound source for the received acoustic signal; and outputting the generated output acoustic signal by using a front speaker and a side speaker, wherein the acquiring of the output acoustic signal includes generating an attenuation signal that is a signal for attenuating or cancelling an inflow acoustic signal to be directly transferred to an audience in the output acoustic signal output from the side speaker, and the output acoustic signal output from the front speaker includes the attenuation signal. 
     The side speaker may include a left speaker and a right speaker, the generating of the output acoustic signal may include generating at least one of a first attenuation signal for attenuating or cancelling, at a location of the audience, a left inflow acoustic signal to be directly transferred to the audience without being reflected from a left wall in an output acoustic signal output from the left speaker and a second attenuation signal for attenuating or cancelling, at the location of the audience, a right inflow acoustic signal to be directly transferred to the audience without being reflected from a right wall in an output acoustic signal output from the right speaker, and the front speaker may include at least one speaker for outputting at least one attenuation signal of the first attenuation signal and the second attenuation signal. 
     The virtual sound source may include a first virtual sound source for a left channel signal of the received acoustic signal and a second virtual sound source for a right channel signal of the received acoustic signal, the generating of the output acoustic signal may include controlling at least one of a magnitude, a time delay, and an output direction of the received acoustic signal, to generate the first virtual sound source and the second virtual sound source based on an acoustic signal generated when the output acoustic signal output from the side speaker is reflected from a wall and on the output acoustic signal output from the front speaker, and the generated output acoustic signal may include the controlled acoustic signal. 
     A computer-readable recording medium has recorded thereon a program for executing, in a computer, the stereophonic sound reproduction method. 
     MODE OF THE INVENTION 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Advantages and features, and a method for achieving them will be clear with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being 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 concept of the invention to those of ordinary skill in the art, and the invention is only defined by the scope of the claims. The terms used in this specification are those general terms currently widely used in the art, but the terms may vary according to the intention of those of ordinary skill in the art, precedents, or new technology in the art. Also, specified terms may be selected by the applicant, and in this case, the detailed meaning thereof will be described in the detailed description. Thus, the terms used in the specification should be understood not as simple names but based on the meaning of the terms and the overall description. Hereinafter, embodiments will be described in detail with reference to the drawings. The embodiments disclosed in the specification and the configurations shown in the drawings are merely exemplary embodiments of the present invention and do not entirely represent the technical spirit of the present invention, and thus it should be understood that various equivalents and modifications for replacing the exemplary embodiments may exist at the filing date of the present application. 
     In addition, the term ‘ . . . unit’ or “ . . . module” used in the specification indicates a hardware component or circuit, such as a Field Programmable Gate Array (FPGA) or an Application-Specific Integrated Circuit (ASIC). 
       FIG. 1  illustrates a stereophonic sound reproduction environment of an audience, according to an embodiment. 
     A stereophonic sound reproduction environment  100  is an example of an environment in which an audience  110  listens to a sound through a stereophonic sound reproduction apparatus  150 . The stereophonic sound reproduction environment  100  is an environment for reproducing acoustic content alone or in combination with other content such as a video and may indicate a randomly open, partially closed, or completely closed region such as a room embodied by a house, a cinema, a theater, a hall, a studio, a game console, or the like. 
     According to an embodiment, the stereophonic sound reproduction environment  100  may include a left wall  170  and a right wall  175  existing around the audience  110 . The left wall  170  is a wall located to the left based on a direction in which the audience  110  looks at the stereophonic sound reproduction apparatus  150 , and the right wall  175  is a wall located to the right based on the direction in which the audience  110  looks at the stereophonic sound reproduction apparatus  150 . According to an embodiment, each of the left wall  170  and the right wall  175  may be located in parallel to the stereophonic sound reproduction apparatus  150  or obliquely with respect to the stereophonic sound reproduction apparatus  150 . Although  FIG. 1  shows that the left wall  170  and the right wall  175  are walls, the left wall  170  and the right wall  175  may include any type of object or organism capable of reflecting an acoustic signal in the stereophonic sound reproduction environment  100 . 
     The audience  110  may listen to a sound through the stereophonic sound reproduction apparatus  150 . According to an embodiment, the stereophonic sound reproduction apparatus  150  may include miniaturized wired or wireless speakers such as a sound bar, a sound ball, and a Bluetooth speaker. According to an embodiment, the stereophonic sound reproduction apparatus  150  may receive an acoustic signal from an external device such as a television, a computer, a smartphone, or a tablet personal computer (PC) through a communication path and reproduce the received acoustic signal. 
     According to an embodiment, in the inside of the stereophonic sound reproduction apparatus  150 , a side speaker (may include a left speaker  152  located to the left and a right speaker  154  located to the right) and a front speaker  156  located at the front in the direction of the audience  110  may exist. According to an embodiment, the front speaker  156  may include a tweeter speaker for outputting (or emitting) an acoustic signal of a high frequency band of a received acoustic signal and a mid-range speaker for outputting an acoustic signal of a mid-frequency band thereof. According to an embodiment, the tweeter speaker in the front speaker  156  may include a left tweeter speaker and a right tweeter speaker. According to an embodiment, the left speaker  152  and the right speaker  154  may include only a tweeter speaker or both a mid-range speaker and a tweeter speaker. 
     According to an embodiment, an output acoustic signal output from the left speaker  152  may be transferred to the audience  110  by being reflected after colliding with the left wall  170 . According to an embodiment, an output acoustic signal output from the right speaker  154  may be transferred to the audience  110  by being reflected after colliding with the right wall  175 . 
     According to an embodiment, a portion of an output acoustic signal output from the left speaker  152  may be directly transferred to the audience  110  without being reflected after colliding with the left wall  170  and is referred to as a left inflow acoustic signal. According to an embodiment, a portion of an output acoustic signal output from the right speaker  154  may be directly transferred to the audience  110  without being reflected after colliding with the right wall  175  and is referred to as a right inflow acoustic signal. According to an embodiment, as output acoustic signals output from the left speaker  152  and the right speaker  154  are in a high frequency band, directivity of the output acoustic signals may increase, and thus a left inflow acoustic signal and a right inflow acoustic signal may have a smaller magnitude than a magnitude of the total acoustic signals output from the left speaker  152  and the right speaker  154 . The left speaker  152  and the right speaker  154  may have a horn shape to improve directivity. 
     According to an embodiment, an output acoustic signal output from the front speaker  156  may be directly transferred to the audience  110  without reflection. 
     According to an embodiment, the stereophonic sound reproduction environment  100  may include a sweet spot (not shown) that is a spatial range in which an optimal stereophonic sound may be enjoyed. The stereophonic sound reproduction environment  100  may set locations of virtual ears of the audience  110  such that an optimal stereophonic sound is outputted at the sweet spot near the ears. Hereinafter, it is assumed that the stereophonic sound reproduction apparatus  150  knows the location of the sweet spot. 
     Hereinafter, the side speaker may include the left speaker  152  and/or the right speaker  154 , and the wall may include the left wall  170  and/or the right wall  175 . In addition, an output acoustic signal may include a left channel signal and a right channel signal. 
     Hereinafter, an operation of the stereophonic sound reproduction apparatus  150  will be described in detail with reference to  FIGS. 2A through 9  below. 
       FIG. 2A  is a block diagram of a stereophonic sound reproduction apparatus according to an embodiment. 
     According to an embodiment, the stereophonic sound reproduction apparatus  150  may include an input unit  210 , a control unit  230 , and an output unit  260 . 
     The input unit  210  may receive an acoustic signal (that is, an audio signal) from a device such as a digital versatile disc (DVD) player, a Blu-ray disc (BD) player, or an MP3 player. According to an embodiment, the input unit  210  may receive an acoustic signal input through various communication paths described above. For example, the input unit  210  may receive, through a communication path, an acoustic signal from an external device such as a television, a computer, a cellular phone, or a tablet PC. 
     The communication path may indicate various networks and network topologies. For example, the communication path may include wireless communication, wired communication, optics, ultrasound waves, or a combination thereof. Satellite communication, mobile communication, Bluetooth, infrared data association standard (IrDA), wireless fidelity (WiFi), and worldwide interoperability for microwave access (WiMAX) are examples of wireless communication which may be included in the communication path. Ethernet, digital subscriber line (DSL), fiber to the home (FTTH), and plain old telephone service (POTS) are examples of wired communication which may be included in the communication path. In addition, the communication path may include personal area network (PAN), local area network (LAN), metropolitan area network (MAN), wide area network (WAN), or a combination thereof. 
     The received acoustic signal may be a multi-channel acoustic signal such as a stereo signal (two channels), a 5.1-channel signal, a 7.1-channel signal, a 10.2-channel signal, or a 22.2-channel signal. The stereophonic sound reproduction apparatus  150  may control and output the received multi-channel acoustic signal so as to generate a virtual sound source with a different location. Hereinafter, for convenience of description, it is assumed that the virtual sound source is generated by using a left channel signal and a right channel signal of the received acoustic signal. According to an embodiment, the input unit  210  may convert the multi-channel acoustic signal into a stereo signal by down-mixing the multi-channel acoustic signal. 
     The control unit  230  may acquire an output acoustic signal for generating the virtual sound source for the received acoustic signal. The output acoustic signal may include acoustic signals to be output from a side speaker  151  and the front speaker  156 . 
     According to an embodiment, the virtual sound source may include a first virtual sound source existing to the left and a second virtual sound source existing to the right, based on a direction in which the audience  110  looks at the stereophonic sound reproduction apparatus  150 . According to an embodiment, the control unit  230  may acquire, from the received acoustic signal, an output acoustic signal for generating the first virtual sound source for the left channel signal of the received acoustic signal and generating the second virtual sound source for the right channel signal of the received acoustic signal. According to an embodiment, the control unit  230  may use acoustic signals reflected from the left wall  170  and the right wall  175  to generate the first virtual sound source and the second virtual sound source. 
     According to an embodiment, the control unit  230  may control at least one of a magnitude, a time delay, and an output direction of the received acoustic signal to generate the virtual sound source based on an acoustic signal generated when an output acoustic signal output from the side speaker  151  is reflected from a wall and on an output acoustic signal output from the front speaker. The acoustic signal of which at least one of the magnitude, the time delay, and the output direction has been controlled may be acquired as an output acoustic signal, and the acquired output acoustic signal may be output through the left speaker  152 , the right speaker  154 , and the front speaker  156  in the output unit  260 . According to an embodiment, the control unit  230  may determine a magnitude, a time delay, and an output direction of an output acoustic signal to be output from each speaker ( 152 ,  154 , or  156 ) by controlling at least one of the magnitude, the time delay, and the output direction of the received acoustic signal. According to an embodiment, the control unit  230  may independently control the left channel signal and the right channel signal of the received acoustic signal and independently determine a left channel signal and a right channel signal of the output acoustic signal to be output from each speaker ( 152 ,  154 , or  156 ). 
     According to an embodiment, the control unit  230  may control at least one of the magnitude, the time delay, and the output direction of the received acoustic signal to generate the virtual sound source based on an acoustic signal generated when the output acoustic signal output from the left speaker located to the left of the stereophonic sound reproduction apparatus is reflected from the left wall, on an acoustic signal generated when the output acoustic signal output from the right speaker located to the right of the stereophonic sound reproduction apparatus is reflected from the right wall, and on the output acoustic signal output from the front speaker. 
     According to an embodiment, the control unit  230  may control at least one of a magnitude, a time delay, and an output direction of the left channel signal of the received acoustic signal to generate the first virtual sound source at a first location by using an acoustic signal generated when a left channel signal of the output acoustic signal output from the left speaker  152  is reflected from the left wall  170 , an acoustic signal generated when a left channel signal of the output acoustic signal output from the right speaker  154  is reflected from the right wall  175 , and a left channel signal of the output acoustic signal output from the front speaker  156 . 
     In addition, according to an embodiment, the control unit  230  may control at least one of a magnitude, a time delay, and an output direction of the received right channel signal to generate the second virtual sound source at a second location by using an acoustic signal generated when a right channel signal of the output acoustic signal output from the left speaker  152  is reflected from the left wall  170 , an acoustic signal generated when a right channel signal of the output acoustic signal output from the right speaker  154  is reflected from the right wall  175 , and a right channel signal of the output acoustic signal output from the front speaker  156 . The first location and the second location may be respectively located to the left and the right of the audience  110  based on a direction in which the audience  110  looks at the stereophonic sound reproduction apparatus  150 . 
     According to an embodiment, the control unit  230  may determine the first location and the second location, which are locations at which the virtual sound source is to be generated, based on spatial characteristics of a sound image provided by the acoustic signal, control at least one of magnitude values of the left channel signal and the right channel signal of the received acoustic signal based on the determined first location and second location, and determine an output acoustic signal to be outputted from each of the left speaker  151  and the front speaker  156 . 
     According to an embodiment, the control unit  230  may determine a distance from the side speaker  151  to the wall and an angle between the side speaker  151  and the wall and control a direction in which the side speaker  151  outputs an acoustic signal as a horizontal or vertical direction with respect to the ground based on the determined distance and angle. An operation performed by the control unit  230  will be described in detail with reference to  FIG. 2B  later. 
     The control unit  230  may generate an attenuation signal that is a signal for attenuating or cancelling an inflow acoustic signal to be directly transferred to the audience  110  in the output acoustic signal output from the side speaker  151 . The generated attenuation signal may attenuate or cancel the inflow acoustic signal at a location of the audience  110 . 
     According to an embodiment, the control unit  230  may generate a left attenuation signal for attenuating or cancelling, at the location of the audience  110 , the left inflow acoustic signal to be directly transferred to the audience  110  without being reflected from the left wall  170  in the output acoustic signal output from the left speaker  152  of the side speaker  151  and a right attenuation signal for attenuating or cancelling, at the location of the audience  110 , the right inflow acoustic signal to be directly transferred to the audience  110  without being reflected from the right wall  175  in the output acoustic signal output from the right speaker  154  of the side speaker  151 . 
     According to an embodiment, the control unit  230  may predict an inflow acoustic signal arriving at the location of the audience  110 , based on an acoustic transfer function using path information between a location of the side speaker  171  and the location of the audience  110  and generate an attenuation signal based on the predicted inflow acoustic signal and an acoustic transfer function using path information between a location of a speaker outputting the attenuation signal and the location of the audience  110 . 
     According to an embodiment, the output acoustic signal acquired by the control unit  230  may include a control signal in which at least one of the magnitude, the time delay, and the output direction of the received acoustic signal and/or the attenuation signal for attenuating or cancelling the inflow acoustic signal. 
     The output unit  260  may output the output acoustic signal acquired by the control unit  230 , through the side speaker  151  and the front speaker  156 . The output acoustic signal may generate a virtual sound source for the received acoustic signal. An output acoustic signal output from the front speaker  156  may include an attenuation signal. According to an embodiment, each output acoustic signal output from the side speaker  151  may include a left channel signal and a right channel signal. According to an embodiment, the output acoustic signal output from the front speaker  156  may include a left channel signal, a right channel signal, and the attenuation signal. According to an embodiment, the left channel signals and the right channel signals output from the side speaker  151  and the front speaker  156  may generate a virtual sound source for the received acoustic signal, and the attenuation signal output from the front speaker  156  may attenuate or cancel the inflow acoustic signal to which the audience  110  listens. 
       FIG. 2B  is a detailed block diagram of the stereophonic sound reproduction apparatus according to an embodiment. 
     According to an embodiment, the control unit  230  of the stereophonic sound reproduction apparatus  150  may include an attenuation signal generation unit  234  and a panning unit  232 . 
     According to an embodiment, the control unit  230  may acquire, from the received acoustic signal, an output acoustic signal for generating, at the first location, the first virtual sound source for the left channel signal of the received acoustic signal and generating, at the second location, the second virtual sound source for the right channel signal of the received acoustic signal. 
     According to an embodiment, the panning unit  232  may control the received acoustic signal to generate, at a predetermined location, a left virtual sound source for the left channel signal of the acoustic signal received by the input unit  210  and to generate, at a predetermined location, a right virtual sound source for the right channel signal of the received acoustic signal. 
     According to an embodiment, the panning unit  232  may control at least one of the magnitude, the time delay, and the output direction of the received acoustic signal to generate, at the predetermined locations, the left virtual sound source and the right virtual sound source by using the acoustic signal generated when the output acoustic signal output from the left speaker  152  is reflected from the left wall  170 , the acoustic signal generated when the output acoustic signal output from the right speaker  154  is reflected from the right wall  175 , and the output acoustic signal output from the front speaker  156 . The output acoustic signal output from the front speaker  156  to be used to generate the left virtual sound source and the right virtual sound source may be a signal obtained by excluding the attenuation signal from the output acoustic signal output from the front speaker  156 . 
     According to an embodiment, the left virtual sound source is a virtual left speaker generated by sound panning of the left speaker  152 , the right speaker  154 , and the front speaker  156  and indicates a virtual sound source located to the left based on a direction in which the audience  110  looks at the stereophonic sound reproduction apparatus  150 , in an external space of the stereophonic sound reproduction apparatus  150 . According to an embodiment, the right virtual sound source is a virtual right speaker generated by sound panning of the left speaker  152 , the right speaker  154 , and the front speaker  156  and indicates a virtual sound source located to the right based on the direction in which the audience  110  looks at the stereophonic sound reproduction apparatus  150 , in the external space of the stereophonic sound reproduction apparatus  150 . 
     That is, the left speaker  152  is actually located inside the stereophonic sound reproduction apparatus  150 , but the audience  110  may feel that a sound source exists at a location of the left virtual sound source generated by the sound panning. In addition, the right speaker  154  is actually located inside the stereophonic sound reproduction apparatus  150 , but the audience  110  may feel that a sound source exists at a location of the right virtual sound source generated by the sound panning. 
     Referring to  FIG. 3A , according to an embodiment, the stereophonic sound reproduction apparatus  150  may generate a left virtual sound source  390  and a right virtual sound source  395  by using output acoustic signals output from the left speaker  152 , the right speaker  154 , and the front speaker  156 . The left virtual sound source  390  and the right virtual sound source  395  are virtual sound sources generated at respective predetermined locations. 
     In more detail, according to an embodiment, the panning unit  232  may generate the left virtual sound source  390  at a predetermined location by using the acoustic signal generated when the left channel signal output from the left speaker  152  is reflected from the left wall  170 , the acoustic signal generated when the left channel signal output from the right speaker  154  is reflected from the right wall  175 , and the left channel signal output from the front speaker  156 . According to an embodiment, the panning unit  232  may control at least one of the magnitude, the time delay, and the output direction of the left channel signal of the received acoustic signal to generate the left virtual sound source  390 . As a result, the panning unit  232  may determine at least one of the magnitude, the time delay, and the output direction of the left channel signal to be output from each of the left speaker  152 , the right speaker  154 , and the front speaker  156 . 
     In addition, according to an embodiment, the panning unit  232  may generate the right virtual sound source  395  at a predetermined location by using the acoustic signal generated when the right channel signal output from the left speaker  152  is reflected from the left wall  170 , the acoustic signal generated when the right channel signal output from the right speaker  154  is reflected from the right wall  175 , and the right channel signal output from the front speaker  156 . According to an embodiment, the panning unit  232  may control at least one of the magnitude, the time delay, and the output direction of the right channel signal of the received acoustic signal to generate the right virtual sound source  395 . As a result, the panning unit  232  may determine at least one of the magnitude, the time delay, and the output direction of the right channel signal to be output from each of the left speaker  152 , the right speaker  154 , and the front speaker  156 . 
     According to an embodiment, the attenuation signal generation unit  234  may generate an attenuation signal that is a signal for attenuating or cancelling inflow acoustic signals to be directly transferred to the audience  110  in the output acoustic signals output from the left speaker  152  and the right speaker  154 . According to an embodiment, the attenuation signal generation unit  234  may generate the left attenuation signal for attenuating or cancelling, at the location of the audience  110 , the left inflow acoustic signal and/or the right attenuation signal for attenuating or cancelling, at the location of the audience  110 , the right inflow acoustic signal. 
     Referring to  FIG. 3A , partial signals  340  and  345  of acoustic signals respectively output from the left speaker  152  and the right speaker  154  toward the left wall  170  and the right wall  175  are directly transferred to the audience  110  without being respectively reflected from the left wall  170  and the right wall  175 , and these inflow acoustic signals may make a size of a sound field recognized by the audience from the received acoustic signal be reduced and make intelligibility of an acoustic signal to which the audience  110  listens be decreased. 
     Referring to  FIG. 3B , a graph  320  shows values obtained by measuring, at the location of the audience  110  along the lapse of time, a magnitude of an acoustic signal output from the left speaker  152  or the right speaker  154  and transferred to the audience  110 . 
     For example, an output acoustic signal  322  output from the left speaker  152  may be measured by being reflected from the left wall  170 , transferred through a path  360 , and arriving at the audience  110 . However, a portion  324  of an output acoustic signal output from the left speaker  152  may be measured by being directly transferred to the audience  110  without being reflected from the left wall  170 . That is, the measured magnitude value  324  is a magnitude value of an inflow acoustic signal transferred to the audience  110 . 
     According to an embodiment, a left attenuation signal output from a speaker in the output unit  260  may be transferred to the location of the audience  110  according to a transfer function and added to a left inflow acoustic signal  340  at the location of the audience  110  so as to attenuate or cancel the left inflow acoustic signal  340 . According to an embodiment, the front speaker  156  may include at least one speaker for outputting an attenuation signal, and the attenuation signal may be simultaneously output from the same speaker as a speaker which outputs a controlled acoustic signal. Hereinafter, it is assumed that the front speaker  156  outputs an attenuation signal. 
     A graph  330  shows values obtained by measuring, at the location of the audience  110  along the lapse of time, a magnitude of an acoustic signal output from the left speaker  152  or the right speaker  154  and transferred to the audience  110  when the attenuation signal generation unit  234  generates a left attenuation signal and a right attenuation signal and the output unit  260  outputs the generated left attenuation signal and right attenuation signal. 
     For example, the left inflow acoustic signal  340  is attenuated by an attenuation signal output from the front speaker  156 , and thus a magnitude value  334  shown in the graph  330  may be less than the magnitude value  324  shown in the graph  320 . 
     According to an embodiment, the attenuation signal generation unit  234  may determine the left attenuation signal and the right attenuation signal to be output from the output unit  260 , by using a transfer function based on location information between the side speaker  152  and  154  and the audience  110  and a transfer function based on location information between the front speaker  260 , which outputs an attenuation signal, and the audience  110 . An operation of generating an attenuation signal will be described in detail with reference to  FIG. 4A . 
     According to an embodiment, the output unit  260  may output the output acoustic signal acquired by the control unit  230 , through the left speaker  152 , the right speaker  154 , and the front speaker  156 . According to an embodiment, the output acoustic signal output from the output unit  260  may generate the left virtual sound source and the right virtual sound source. According to an embodiment, the audience  110  may feel that sound sources exist at locations of the left virtual sound source  390  and the right virtual sound source  395  generated by using the left speaker  152 , the right speaker  154 , and the front speaker  156 . 
     According to an embodiment, the output unit  260  may include speakers for outputting the left attenuation signal and the right attenuation signal generated by the attenuation signal generation unit  234 . The front speaker  150  may include at least one speaker for outputting an attenuation signal. A speaker for outputting the attenuation signal may include a speaker for outputting the left attenuation signal and a speaker for outputting the right attenuation signal. The left attenuation signal and the right attenuation signal output from the attenuation signal generation unit  234  may arrive at the location of the audience  110  and respectively be added to the left inflow acoustic signal  340  and a right inflow acoustic signal  345  so as to attenuate or cancel the inflow acoustic signal. 
       FIG. 4A  is a detailed block diagram of the stereophonic sound reproduction apparatus according to an embodiment. 
     The stereophonic sound reproduction apparatus  150  of  FIG. 4A  is a detailed embodiment of the stereophonic sound reproduction apparatus  150  of  FIG. 2B . Therefore, although omitted hereinafter, the description about the stereophonic sound reproduction apparatus  150  of  FIG. 2B  is also applied to the stereophonic sound reproduction apparatus  150  of  FIG. 4A . 
     According to an embodiment, the control unit  230  of the stereophonic sound reproduction apparatus  150  may further include a band filter  410 , a spatial analysis and rotation unit  433 , an acoustic signal analysis unit  420 , a virtualizer  430 , and an amplification unit  440 . 
     According to an embodiment, the band filter  410  may divide an acoustic signal received by the input unit  210  into a high frequency band and a low frequency band. The band filter  410  may include a high pass filter and a low pass filter. The band filter  410  may be an analog circuit filter or a digital filter but is not limited thereto. The band filter  410  may output a high frequency band signal of the received acoustic signal to the panning unit  232  and output a low frequency band signal thereof to the virtualizer  530 . That is, the panning unit  232  may perform sound panning for only the high frequency band signal of the received acoustic signal. The high frequency band signal may be output to the left speaker  152 , the right speaker  154 , and the front speaker  156 , and the low frequency band signal may be output through the front speaker  156 . 
     According to an embodiment, the spatial analysis and rotation unit  433  may analyze spatial characteristics of the stereophonic sound reproduction environment  100 . Although  FIG. 4A  shows that the spatial analysis and rotation unit  433  is separated from the panning unit  232 , according to an embodiment, the spatial analysis and rotation unit  433  may be included in the panning unit  232 . 
     According to an embodiment, the spatial analysis and rotation unit  433  may determine, referring back to  FIG. 3A , a distance  370  from the left speaker  152  to the left wall  170  and an angle  375  between the left speaker  152  and the left wall  170 . In addition, the spatial analysis and rotation unit  433  may determine a distance  380  and an angle  385  between the right speaker  154  and the right wall  175 . 
     According to an embodiment, the spatial analysis and rotation unit  433  may determine the distances  370  and  380  and the angles  375  and  385  by using an audible sound wave, an inaudible sound wave (ultrasonic wave), or an electromagnetic wave. For example, the spatial analysis and rotation unit  433  may determine the distances  370  and  380  by measuring time delays until a reflected wave is detected after an acoustic signal is output to the left wall  170  and the right wall  175 . According to an embodiment, the spatial analysis and rotation unit  433  may determine the angles  375  and  385  by outputting an acoustic signal to the left wall  170  and the right wall  175  in one or more directions and measuring, by using a microphone mounted inside the stereophonic sound reproduction apparatus  150 , energy of a signal returned when the output acoustic signal is reflected from a wall. 
     According to an embodiment, the spatial analysis and rotation unit  433  may adjust an acoustic signal output direction of at least one of the left speaker  152  and the right speaker  154  to a direction horizontal or vertical with respect to the ground based on the measured distances  370  and  380  and angles  375  and  385 , to generate virtual sound sources at predetermined constant locations  390  and  395 . 
     For example, referring to  FIG. 5 , when the distances  370  and  380  to side walls are short, the spatial analysis and rotation unit  433  may adjust a horizontal direction of the side speaker  152  and  154  such that the left speaker  152  and the right speaker  154  face the audience  110 . 
     According to an embodiment, when the distances  370  and  380  to the side walls are sufficiently long, the spatial analysis and rotation unit  433  may adjust a horizontal direction of the side speaker  152  and  154  such that the left speaker  152  and the right speaker  154  respectively face the left wall  170  and the right wall  175 . 
     According to an embodiment, when the distance  370  to the left wall  170  is shorter than the distance  380  to the right wall  175 , the spatial analysis and rotation unit  433  may adjust a horizontal direction of the side speaker  152  and  154  such that the left speaker  152  faces the audience  110  and the right speaker  154  faces the right wall  175 . 
     According to an embodiment, when the angle  375  to the left wall  170  differs from the angle  385  to the right wall  175 , the spatial analysis and rotation unit  433  may adjust a horizontal direction of the side speaker  152  and  154  such that the left speaker  152  faces the opposite direction of the audience  110  and the right speaker  154  faces the right wall  175 . 
     According to an embodiment, the spatial analysis and rotation unit  433  may adjust a vertical direction such that at least one of the left speaker  152  and the right speaker  154  faces the ceiling, thereby reducing an influence of the bottom surface or making the audience  110  feel a sense of elevation. 
     According to an embodiment, the spatial analysis and rotation unit  233  may physically adjust angles in the horizontal direction and the vertical direction of the left and right speakers  152  and  154  having a horn shape. This will be described below with reference to  FIG. 7 . 
     Referring back to  FIG. 4A , according to an embodiment, the acoustic signal analysis unit  420  may analyze a sound stage provided by the acoustic signal received by the input unit  210 . The sound stage indicates a spatial distribution in which a sound image provided by the received acoustic signal is located. 
     The sound stage indicates a size of a sound field in which the received acoustic signal is reproduced, wherein a size of a sound stage of an acoustic signal of which a sound image is concentrated to the center is determined to be small, and a size of a sound stage of an acoustic signal of which a sound image is concentrated to the left and the right is determined to be large. 
     For example, when a speaker outputs an orchestra performance, a musical instrument located at the leftmost of the orchestra, a musical instrument located at the rightmost thereof, a musical instrument recognized to be the closest to an audience, and a musical instrument recognized to be the farthest from the audience in the speaker direction may determine a location and size of a sound stage. 
     Referring to  FIG. 6 , in general, a space  610  between the left speaker  152  and the right speaker  154  may be determined as a sound stage. However, according to an embodiment, the acoustic signal analysis unit  420  may analyze a received acoustic signal and determine a different sound stage suitable for the received acoustic signal. 
     For example, the acoustic signal analysis unit  420  may determine an appropriate sound stage by analyzing energy of a left channel signal and a right channel signal of the received acoustic signal. When energy of a mono signal is dominant rather than the energy of the left channel signal and the right channel signal of the received acoustic signal, the acoustic signal analysis unit  420  may locate a sound stage  670  at the center and reduce a left and right width. In addition, when the energy of the left channel signal and the right channel signal of the received acoustic signal is much greater than the energy of the mono signal, the acoustic signal analysis unit  420  may use an expanded sound stage  680  which is expanded to the left and the right. 
     In addition, according to an embodiment, the acoustic signal analysis unit  420  may analyze a correlation between the left channel signal and the right channel signal of the received acoustic signal, determine a size of a sound stage to be small when the correlation is high, and determine a size of a sound stage to be large when the correlation is low. That is, an angle  640  or  645  for determining the sound stage  670  or  680  may be determined inversely proportional to the correlation between the left channel signal and the right channel signal. 
     In addition, according to an embodiment, the acoustic signal analysis unit  420  may determine a location and a size of a sound stage by analyzing a genre of the received acoustic signal or considering a sense of reverberation. 
     According to an embodiment, the acoustic signal analysis unit  420  may deliver information about the determined sound stage to the panning unit  232  and the virtualizer  430 . For example, the acoustic signal analysis unit  420  may deliver information about a distance  650  and the angle  640  between the audience  110  and the sound stage  670  to the panning unit  232  and the virtualizer  430 . In addition, the acoustic signal analysis unit  420  may deliver information about a distance  655  and the angle  645  between the audience  110  and the sound stage  680  to the panning unit  232  and the virtualizer  430 . 
     According to an embodiment, information about a sound stage may include location information of the left virtual sound source  390  and the right virtual sound source  395 . That is, when the sound stage  680  is determined for a received acoustic signal, a location of a left virtual sound source to be generated may be determined as a location  620 , and a location of a right virtual sound source may be determined as a location  630 . In addition, when the sound stage  670  is determined for a received acoustic signal, a location of a left virtual sound source to be generated may be determined as a location  625 , and a location of a right virtual sound source may be determined as a location  635 . 
     According to an embodiment, the panning unit  232  may change at least one of a magnitude (gain) and a time delay of each of left channel signals and right channel signals output from the left speaker  152 , the right speaker  154 , and the front speaker  156  to generate a left virtual sound source and a right virtual sound source at predetermined constant locations. As described above, the locations of the left virtual sound source and the right virtual sound source may be determined from information about a sound stage, which has been received from the acoustic signal analysis unit  420 . 
     According to an embodiment, the panning unit  232  may determine magnitudes of the left channel signals and the right channel signals to be output from the left speaker  152 , the right speaker  154 , and the front speaker  156  such that the magnitudes are different, by considering directivity according to frequencies of acoustic signals output from the left speaker  152  and the right speaker  154 . 
     According to an embodiment, the panning unit  232  may form virtual sound sources at the constant locations  390  and  395  regardless of frequencies of output acoustic signals by considering that as an output acoustic signal output from the side speaker  152  and  154  has a high frequency, directivity is improved such that a sound image is generated closely to the side wall  170  and  175 , and as an output acoustic signal output from the side speaker  152  and  154  has a low frequency, directivity is reduced such that a sound image is generated closely to the side speaker  152  and  154 . 
     According to an embodiment, the panning unit  232  may simultaneously use left channel signals output from at least two speakers of the left speaker  152 , the right speaker  154 , and the front speaker  156  to generate the left virtual sound source  390  at a constant location regardless of frequency. The front speaker  156  used to generate the left virtual sound source  390  at a constant location may be a tweeter speaker located to the left of the front speaker  156 . 
     According to an embodiment, the panning unit  232  may increase a magnitude of a left channel signal to be output from the right speaker  154  by considering that directivity of a left channel signal output from the left speaker  152  increases as a frequency of a left channel signal of a received acoustic signal is high. In addition, according to an embodiment, the panning unit  232  may increase a magnitude of a right channel signal to be output from the left speaker  152  by considering that directivity of a right acoustic signal output from the right speaker  154  increases as a frequency of a right channel signal of a received acoustic signal is high. 
     For example, referring to  FIG. 7 , lines  730  and  710  may indicate a magnitude of a left channel signal output from any one speaker of the left speaker  152  and the right speaker  154  according to frequency. For example, when the line  730  indicates a left channel signal output from the left speaker  152 , the line  710  may indicate a left channel signal output from the right speaker  154 , and a line  720  may indicate a left acoustic signal output from a tweeter speaker located to the left of the front speaker  156 . 
     If the line  730  indicates a right channel signal output from the right speaker  154 , the line  710  may indicate a right channel signal output from the left speaker  152 , and the line  720  may indicate a right channel signal output from a tweeter speaker located to the right of the front speaker  156 . Hereinafter, for convenience of description, it is assumed that the line  730  indicates a left channel signal output from the left speaker  152 . 
     According to an embodiment, a sum of left channel signals output from the left speaker  152 , the right speaker  154 , and the left tweeter speaker of the front speaker  156  is a constant value  740 . 
     Since directivity of a left channel signal output from the left speaker  152  increases as a frequency is high, a virtual sound source generated by the left speaker  152  is generated closely to the left wall  170  when only the left speaker  152  is used, and thus it is needed to move the virtual sound source in the right direction to generate a left virtual sound source at a desired location  390 . 
     Therefore, according to an embodiment, the panning unit  232  may increase a magnitude of a left channel signal to be output from at least one speaker of the front speaker  156  and the right speaker  154  as a frequency of a left channel signal output from the left speaker  152  is high. On the contrary, according to an embodiment, the panning unit  232  may decrease a magnitude of a left channel signal to be output from at least one speaker of the front speaker  156  and the right speaker  154  as a frequency of a left channel signal output from the left speaker  152  is low. 
     According to an embodiment, the control unit  230  may determine a time delay of output acoustic signals output from the left speaker  152  and the right speaker  154  such that an output acoustic signal output from the side speaker  152  and  154 , reflected from the side wall  170  and  175 , and arriving at the audience  110  and an output acoustic signal output from the front speaker  156  and directly transferred to the audience  110  arrive at the audience  110  at the same time. 
     Referring back to  FIG. 3A , according to an embodiment, the panning unit  232  may determine a length  360  of a path along which an output acoustic signal output from the left speaker  152  arrives at the audience  110  after being reflected from the left wall  170 . In addition, according to an embodiment, the control unit  230  may determine a length  350  of a path along which an output acoustic signal output from the front speaker  156  is directly transferred to the audience  110 . According to an embodiment, the panning unit  232  may delay a time of an output acoustic signal to be output from the left speaker  152  by (the length  360 −the length  350 )/C 0  than an output acoustic signal to be output from the front speaker  156 , to maintain articulation by making the output acoustic signal output from the left speaker  152  and the output acoustic signal output from the front speaker  156  arrive at the audience  110  at the same time. 
     In addition, according to an embodiment, the panning unit  232  may determine a length  365  of a path along which an output acoustic signal output from the right speaker  154  arrives at the audience  110  after being reflected from the right wall  175 . In addition, according to an embodiment, the control unit  230  may determine a length  355  of a path along which an output acoustic signal output from the front speaker  156  is directly transferred to the audience  110 . According to an embodiment, the panning unit  232  may delay a time of an output acoustic signal to be output from the right speaker  154  by (the length  365 −the length  355 )/C 0  than an output acoustic signal to be output from the front speaker  156 , to maintain articulation by making the output acoustic signal output from the right speaker  154  and the output acoustic signal output from the front speaker  156  arrive at the audience  110  at the same time. 
     According to an embodiment, the panning unit  232  may compare the length  360  and the length  365  to determine a magnitude of an output acoustic signal to be out from a speaker having a longer length such that the magnitude is greater than the other. 
     According to an embodiment, when the panning unit  232  determines a left channel signal and a right channel signal to be output from the left speaker  152  and the right speaker  154 , the attenuation signal generation unit  234  may predict the left inflow acoustic signal  340  and the right inflow acoustic signal  355  based on the determined acoustic signals and generate a left attenuation signal and a right attenuation signal for respectively attenuating or cancelling the predicted inflow acoustic signals. 
       FIG. 4B  is a block diagram of an attenuation signal generation unit according to an embodiment. 
     According to an embodiment, the attenuation signal generation unit  234  may include a prediction unit  470  and a determination unit  480 . 
     According to an embodiment, the prediction unit  470  may predict a left inflow acoustic signal or a right inflow acoustic signal which arrives at the audience  110  by being directly transferred thereto without being reflected from the left wall  170  or the right wall  175  in an output acoustic signal output from the left speaker  152  or the right speaker  154 . According to an embodiment, the prediction unit  470  may receive, from the panning unit  232 , information about an output acoustic signal to be output from the left speaker  152  or the right speaker  154 . The left inflow acoustic signal or the right inflow acoustic signal indicate an inflow acoustic signal generated from the output acoustic signal output from the left speaker  152  and an inflow acoustic signal generated from the output acoustic signal output from the right speaker  154 , respectively. 
     According to an embodiment, the prediction unit  470  may predict a left inflow acoustic signal arriving at the location of the audience  110  as H L,side ·X L ″  475  by applying an acoustic transfer function H L,side  based on path information between the left speaker  152  and the audience  110  to an output acoustic signal X L ″  460  to be output from the left speaker  152 . That is, the left inflow acoustic signal measurable at the location of the audience  110  may be predicted as H L,side ·X L ″  475 . 
     According to an embodiment, the determination unit  480  may determine an attenuation signal for attenuating or cancelling, at the location of the audience  110 , the inflow acoustic signal predicted by the prediction unit  470 . According to an embodiment, the determination unit  480  may determine, as −H L,side ·X L ″ (that is, a left attenuation signal arriving at the location of the audience  110 ), a left attenuation signal for attenuating or cancelling, at the location of the audience  110 , the left inflow acoustic signal H L,side ·X L ″  475  predicted by the prediction unit  470 . In addition, the determination unit  480  may determine, as −H L,side ·X L ″/H L,front    485 , a left attenuation signal to be output from the front speaker  156  by applying a transfer function H L,front  to the left attenuation signal −H L,side ·X L ″ at the location of the audience  110 . H L,front  is an acoustic transfer function based on path information between a location of a speaker which outputs a left attenuation signal and the audience  110 . That is, the determination unit  480  may determine a left attenuation signal to be output from the front speaker  156  by inversely applying an acoustic transfer function based on path information between a location of a speaker which outputs an attenuation signal and the audience  110  to an attenuation signal to be transferred to the location of the audience  110 . 
     According to an embodiment, the left attenuation signal −H L,side ·X L ″/H L,front    485  output from the front speaker  156  is transferred to the location of the audience  110  through the acoustic transfer function H L,front , and thus an attenuation signal arriving at the location of the audience  110  becomes −H L,side ·X L ″ and may cancel the left inflow acoustic signal H L,side ·X L ″  475  arriving at the location of the audience  110 . An acoustic transfer function may be information previously given based on characteristics of the stereophonic sound reproduction environment  100 , and the characteristics of the stereophonic sound reproduction environment  100  may include information about a distance between speaker units, an output angle, and the like. 
     Referring back to  FIG. 4A , the virtualizer  430  may perform rendering for localizing a virtual sound source at a predetermined location with respect to a low frequency band signal in a received acoustic signal. For example, the virtualizer  430  may acquire an acoustic signal of the front speaker, which corresponds to the low frequency band signal, by processing the received acoustic signal through a head related transfer function rendering algorithm, a beam-forming rendering algorithm, or a focused source rendering algorithm. 
     For example, the virtualizer  430  may make the low frequency band signal pass through a predetermined head related transfer filter (HRTF). The HRTF includes path information from a spatial location of a sound source to both ears of the audience  110 , i.e., a frequency transfer characteristic. The HRTF enables a stereophonic sound to be recognized by not only simple path differences such as an inter-aural level difference (ILD) and an inter-aural time difference (ITD) but also a phenomenon that complex path characteristics such as diffraction on the head surface and reflection from an auricle change according to a sound arrival direction. Since the HRTF has a unique characteristic in each spatial direction, when this characteristic is used, a stereophonic sound may be generated. That is, the virtualizer  430  may expand a sound state by using a predetermined head related transfer function to localize the low frequency band signal at a predetermined location. 
     According to an embodiment, the amplification unit  440  may amplify (or attenuate) a received acoustic signal according to a gain value determined by the panning unit  232  and the virtualizer  430 . 
     For example, the amplification unit  440  may amplify, according to a first gain value, a left channel signal to be output to the left speaker  152  and amplify, according to a second gain value, a left channel signal to be output to the right speaker  154 . In addition, the amplification unit  440  may amplify, according to the first gain value, a right channel signal to be output to the right speaker  154  and amplify, according to the second gain value, a right channel signal to be output to the left speaker  152 . 
     In addition, according to an embodiment, the amplification unit  440  may amplify, according to a third gain value and a fourth gain value, a right channel signal and a left channel signal to be output to the front speaker  156 , respectively. According to an embodiment, the amplification unit  440  may differently determine gain values of output acoustic signals to be output to a left tweeter speaker, a right tweeter speaker, a left mid-range speaker, and a right mid-range speaker of the front speaker  156 , respectively. 
     According to an embodiment, the amplification unit  540  may include an equalizer (not shown). The equalizer may process and adjust a general frequency characteristic of a received acoustic signal so as to maintain an appropriate pitch. The equalizer may be coupled to the virtualizer  430  to correct the received acoustic signal such that a tone is not changed regardless of a frequency. In addition, the equalizer may maintain a frequency response according to signal processing of the panning unit  232  to be constant at the location of the audience  110 . 
       FIG. 8  illustrates various shapes of a horn-shaped side speaker. 
     As described above, according to an embodiment, the side speaker  152  and  154  may have a horn shape such that an acoustic signal output in a direction of the side wall  170  and  175 . The horn may include a horn tube-shaped frame including a neck part and an opening part. 
     According to an embodiment, a horn  810  of the left speaker  152  and the right speaker  154  may be inclined by an angle α in a direction of the audience  110  within an enclosure  820  such that a reflected wave from the side wall  170  and  175  moves to the audience  110 . The enclosure  820  may be a speaker enclosure included in the stereophonic sound reproduction apparatus  100 . 
     According to an embodiment, a horn  830  of the left speaker  152  and the right speaker  154  may be inclined upward by an angle β within the enclosure  820  so as to reduce an influence of reflection due to the bottom surface. 
     According to an embodiment, a horn of the left speaker  152  and the right speaker  154  may be inclined by an angle γ in a horizontal direction with respect to the ground and by an angle δ in a vertical direction with respect to the ground within the enclosure  820 . When the horn of the left speaker  152  and the right speaker  154  is inclined by the angle δ in the vertical direction, a virtual sound source is located at a predetermined elevation such that the audience  110  may feel a sense of elevation. 
     According to an embodiment, a horn  840  of the left speaker  152  and the right speaker  154  may have a helical shape within the enclosure  820 . As a length of a horn is long in an output direction of an acoustic signal, and as a size of an entrance through which the acoustic signal is output is large, the acoustic signal has high directivity in a specific frequency band. 
     That is, as a length of a horn of the left speaker  152  and the right speaker  154  is long, directivity increases, but a speaker having a long horn is long and has a shape in which a cross-sectional area thereof is wider in a direction to the left and the right based on a neck part, and thus a volume is expanded, thereby making it difficult to produce, install, and carry the speaker. In addition, since a horn also influences a size and an outer appearance of an enclosure of a speaker, as the size of the enclosure is small, a physical limited distance of the horn may be short. 
     Therefore, according to an embodiment, a horn  850  of the left speaker  152  and the right speaker  154  may have a helical shape instead of a straight shape to have high directivity with a small volume. 
     According to an embodiment, a shape of an opening part of a horn  870  of the left speaker  152  and the right speaker  154  may be changed according to a shape of the enclosure  820 . 
     As described above, since a horn of the left speaker  152  and the right speaker  154  may be inclined in the horizontal or vertical direction with respect to the ground within the enclosure  820 , an inclined horn  865  may be not matched with a shape of the enclosure  820  formed with straight lines and planar surfaces. For example, the horn  865  may be inclined by the angle α in the horizontal direction with respect to the ground within the enclosure, such that the horn  865  is not matched with the shape of the enclosure  820 . Therefore, the horn  870  of the left speaker  152  and the right speaker  154  may have a changed shape of an opening part so as to be fit to the shape of the enclosure  820 . That is, the opening part of the horn  870  may be cut obliquely in the horizontal or vertical direction with respect to the ground so as to be fit to the shape of the enclosure  820 . In this case, an output pattern of an acoustic signal of the horn  870  may be maintained. 
     According to an embodiment, a steering plug  883  by which an output direction of a horn  880  of the left speaker  152  and the right speaker  154  is easily adjustable through rotation may be located inside the horn  880 . 
       FIG. 9  illustrates shapes of an enclosure included in the stereophonic sound reproduction apparatus, according to an embodiment. 
     As described above, as a horn is long, matching with air is good, and thus efficiency is improved, but a speaker having a long horn is long and has a shape in which a cross-sectional area thereof is wider in a direction to the left and the right based on a neck part, and thus a volume is expanded, thereby making a total volume of the stereophonic sound reproduction apparatus  150  expanded. 
     According to an embodiment, the stereophonic sound reproduction apparatus  150  may include the side speaker  152  and  154  inside a woofer enclosure in the stereophonic sound reproduction apparatus  150  so as to be miniaturized. 
     In more detail, according to an embodiment, the stereophonic sound reproduction apparatus  150  may include a horn of the side speaker  152  and  154  in a duct that is a low frequency band acoustic signal discharge hole. For example, ducts  920  and  960  inside a vented enclosure  810  and a bandpass enclosure  850  may include horns  930  and  970 , respectively. 
     Therefore, according to an embodiment, a high frequency band output from a horn of the side speaker  152  and  154  and a low frequency band output from a woofer may be simultaneously output from the duct  920  or  960 . Even though the horn  930  or  970  exists together inside the duct  920  or  960 , since a low frequency band acoustic signal output from a woofer and a high frequency band acoustic signal output from the horn  930  or  970  have different frequency bands, an interference phenomenon such as constructive interference or destructive interference of an acoustic signal does not occur. 
       FIG. 10  is a flowchart of a method by which a stereophonic sound reproduction apparatus reproduces a stereophonic sound, according to an embodiment. 
     In operation  1020 , the stereophonic sound reproduction apparatus  150  may receive an acoustic signal. According to an embodiment, the stereophonic sound reproduction apparatus  150  may receive an acoustic signal from an external device such as a television, a computer, a smartphone, or a tablet PC through a communication path. 
     In operation  1040 , an output acoustic signal for generating a virtual sound source for the received acoustic signal may be acquired from the received acoustic signal. According to an embodiment, the stereophonic sound reproduction apparatus  150  may control the received acoustic signal to generate a left virtual sound source and a right virtual sound source for the received acoustic signal. Operation  1040  may include operation  1042  of generating an attenuation signal for attenuating or cancelling an inflow acoustic signal. 
     In operation  1042 , the stereophonic sound reproduction apparatus  150  according to an embodiment may generate an attenuation signal for attenuating or cancelling an inflow acoustic signal to be directly transferred to an audience in an output acoustic signal to be output from the side speaker  151 . 
     According to an embodiment, the stereophonic sound reproduction apparatus  150  may generate a left attenuation signal for attenuating or cancelling, at the location of the audience  110 , a left inflow acoustic signal to be directly transferred to the audience  110  without being reflected from the left wall  170  in an output acoustic signal output toward the left wall  170  from the left speaker  152  and a right attenuation signal for attenuating or cancelling, at the location of the audience  110 , a right inflow acoustic signal to be directly transferred to the audience  110  without being reflected from the right wall  175  in an output acoustic signal output toward the right wall  175  from the right speaker  154 . 
     According to an embodiment, the stereophonic sound reproduction apparatus  150  may predict a left inflow acoustic signal to be transferred to the location of the audience  110  by applying an acoustic transfer function based on path information between a location of the left speaker  152  and the location of the audience  110  to the output acoustic signal to be output toward the left wall  170  from the left speaker  152  and predict a right inflow acoustic signal to be transferred to the location of the audience  110  by applying an acoustic transfer function based on path information between a location of the right speaker  154  and the location of the audience  110  to the output acoustic signal to be output toward the right wall  175  from the right speaker  154 , to generate the attenuation signal. 
     According to an embodiment, the stereophonic sound reproduction apparatus  150  may determine a left attenuation signal to be output from a speaker by inversely applying an acoustic transfer function based on path information between a location of the speaker which outputs the left attenuation signal and the location of the audience  110  to an acoustic signal for attenuating or cancelling, at the location of the audience  110 , the predicted left inflow acoustic signal. In addition, the stereophonic sound reproduction apparatus  150  may determine a right attenuation signal to be output from a speaker by inversely applying an acoustic transfer function based on path information between a location of the speaker which outputs the right attenuation signal and the location of the audience  110  to an acoustic signal for attenuating or cancelling, at the location of the audience  110 , the predicted right inflow acoustic signal. 
     In operation  1060 , the stereophonic sound reproduction apparatus  150  may output the output acoustic signal acquired in operation  1040  by using the side speaker  151  and the front speaker  156 . The output acoustic signal output through the side speaker  151  and the front speaker  156  may generate a virtual sound source. The output acoustic signal output through the front speaker  156  may include the attenuation signal generated in operation  1042 . 
       FIG. 11  is a detailed flowchart of a method by which a stereophonic sound reproduction apparatus reproduces a stereophonic sound, according to an embodiment. 
     Operations  1120 ,  1140 ,  1144 , and  1160  correspond to operations  1020 ,  1040 ,  1042 , and  1060  of  FIG. 10 , and thus a detailed description thereof will be omitted herein. 
     Operation  1140  may include operation  1142  of controlling at least one of a magnitude, a time delay, and an output direction of an acoustic signal received in operation  1020 . 
     In operation  1142 , the stereophonic sound reproduction apparatus  150  according to an embodiment may acquire an output acoustic signal for generating a virtual sound source by controlling at least one of the magnitude, the time delay, and the output direction of the received acoustic signal. 
     According to an embodiment, the stereophonic sound reproduction apparatus  150  may control at least one of the magnitude, the time delay, and the output direction of the received acoustic signal to generate a left virtual sound source at a predetermined location by using an acoustic signal generated when a left channel signal output from the left speaker  152  is reflected from the left wall  170 , an acoustic signal generated when a left channel signal output from the right speaker  154  is reflected from the right wall  175 , and a left channel signal output from the front speaker  156 . 
     In addition, according to an embodiment, the stereophonic sound reproduction apparatus  150  may control at least one of the magnitude, the time delay, and the output direction of the received acoustic signal to generate a right virtual sound source at a predetermined location by using an acoustic signal generated when a right channel signal output from the left speaker  152  is reflected from the left wall  170 , an acoustic signal generated when a right channel signal output from the right speaker  154  is reflected from the right wall  175 , and a right channel signal output from the front speaker  156 . 
     According to an embodiment, the predetermined location at which the left virtual sound source is located may be located to the left based on a direction in which an audience looks at the stereophonic sound reproduction apparatus in a space outside the stereophonic sound reproduction apparatus  150 , and the predetermined location at which the right virtual sound source is located may be located to the right based on the direction in which the audience looks at the stereophonic sound reproduction apparatus in the space outside the stereophonic sound reproduction apparatus  150 . 
     According to an embodiment, the stereophonic sound reproduction apparatus  150  may determine locations of the left virtual sound source and the right virtual sound source by analyzing a sound stage provided by the acoustic signal received in operation  1020  and control at least one of gains, time delays, and output directions of left channel signals and right channel signals to be output from the left speaker  152 , the right speaker  154 , and the front speaker  156  so as to localize the left virtual sound source and the right virtual sound source at the determined locations. 
     According to an embodiment, the stereophonic sound reproduction apparatus  150  may determine a distance and an angle between the left speaker  152  and the left wall  170  and a distance and an angle between the right speaker  154  and the right wall  175  and change at least one of gains and delay values of the left channel signals and the right channel signals to be output from the left speaker  152 , the right speaker  154 , and the front speaker  156 , based on the determined distances and angles. In addition, the stereophonic sound reproduction apparatus  150  may adjust output directions of the left speaker  152 , the right speaker  154 , and the front speaker  156  in a horizontal or vertical direction based on the determined distances and angles. 
     In operation  1142 , when at least one of the magnitude, the time delay, and the output direction of the received acoustic signal is controlled, magnitudes, time delays, and output directions of an output acoustic signal to be output from the left speaker  152 , an output acoustic signal to be output from the right speaker  154 , and an output acoustic signal to be output from the front speaker  156  may be determined. Each of the output acoustic signals output from the left speaker, the right speaker, and the front speaker may include a left channel signal and a right channel signal. 
     In operation  1144 , the stereophonic sound reproduction apparatus  150  according to an embodiment may predict an inflow acoustic signal to be listened to by the audience  110 , based on the acoustic signal to be output from each speaker, which has been determined in operation  1142 , and generate an attenuation signal for attenuating or cancelling the predicted inflow acoustic signal. According to an embodiment, the stereophonic sound reproduction apparatus  150  may predict a left inflow acoustic signal to be transferred to the audience  110 , based on an acoustic signal to be output from the left speaker  152 , and determine a left attenuation signal for attenuating or cancelling the predicted left inflow acoustic signal. In addition, according to an embodiment, the stereophonic sound reproduction apparatus  150  may predict a right inflow acoustic signal to be transferred to the audience  110 , based on an acoustic signal to be output from the right speaker  154 , and determine a right attenuation signal for attenuating or cancelling the predicted right inflow acoustic signal. 
     In operation  1160 , the stereophonic sound reproduction apparatus  150  may output the output acoustic signal generated in operation  1140 , through the side speaker  151  and the front speaker  156 . The output acoustic signal may include the received acoustic signal generated in operation  1142 , of which at least one of a magnitude, a time delay, and an output direction has been controlled, and the attenuation signal generated in operation  1144 . The attenuation signal may be output through the front speaker  156 . 
     The above-described stereophonic sound reproduction method may be implemented as computer-readable code on a computer-readable recording medium. The computer-readable recording medium may include any data storage device that can store data that can thereafter be read by a computer system. Examples of the computer-readable recording medium include read-only memories (ROMs), random access memories (RAMs), compact disc read-only memories (CD-ROMs), magnetic tapes, floppy disks, and optical data storage devices, and also include implementation in the form of carrier waves such as transmission through the Internet. In addition, the computer-readable recording medium can also be distributed over network coupled computer systems so that the process-readable code is stored and executed in a distributed fashion. 
     Methods, processes, apparatuses, products and/or systems according to the present invention are simple, expense-effective, not complex, and very diverse and accurate. In addition, by applying known components to the processes, the apparatuses, the products and the systems according to the present invention, immediately usable, efficient, and economical production, application and utilization can be implemented. Another important aspect of the present invention is to meet a current trend of requiring expense reduction, system simplification, and performance enhancement. As a result, the useful aspects according to the embodiments of the present invention may at least increase a level of the current technology. 
     While the present invention has been described with reference to exemplary embodiments, the inventions derived by applying replacements, modifications, and updates to the present invention would be obvious to those of ordinary skill in the art in the light of the above description. That is, the claims are analyzed so as to include all the replaced, modified, and updated inventions. Therefore, all the contents described in the specification and the drawings should be analyzed as illustrative and non-restrictive meaning.