Patent Publication Number: US-7912227-B2

Title: Sound reproducing screen for an ultrasonic converting and reproducing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Korean Patent Application No. 10-2004-0108910, which was filed on Dec. 20, 2004, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Apparatuses consistent with the present invention relate to reproducing sound signals which are carried over ultrasonic waves, and in particular, to a sound reproducing screen for an ultrasonic converting and reproducing method. 
     2. Description of the Related Art 
     Among methods of transferring sound signals over a long distance, methods of transmitting sound signals carried on ultrasonic waves have been developed. Such methods were first developed for military purposes such as a submarine, and such methods have led development of speakers for home and industry in recent years. 
     The term “ultrasonic wave” means a sound wave having a frequency of 20 kHz or more, which is higher than an audible frequency. When a sound signal is carried on an ultrasonic wave and then transferred, a sound source can be obtained which has a stronger intensity and which has directivity. 
     A speaker which uses the conventional properties of the ultrasonic wave, usually employs a method of amplitude-modulating the sound wave to the ultrasonic wave. An output signal of such speaker is converted to a sound signal in an audible band which can be heard by a person during its transferring procedure by the non-linearity of a medium. 
     The conventional ultrasonic sound source can send a sound wave over a long distance, or can make a sound wave directed, to a specific point. However, only a small portion of acoustic power of the sound signal carried on the ultrasonic wave is actually transferred, so that a strong output must be used as compared to the typical speaker. Accordingly, a listener is exposed to a portion of a strong ultrasonic wave field. It is thus not suitable as a sound source for a listener as compared to the conventional speaker which only generates a sound signal in the typical audible band. 
     SUMMARY OF THE INVENTION 
     It is therefore one aspect of the present invention to provide a sound reproducing screen for an ultrasonic wave converting and reproducing method by suggesting a structure of ultrasonic converting apparatus which separates, from the ultrasonic wave, a transmitted sound signal carried on the ultrasonic wave so as to reproduce the sound signal such that the structure is simple and a conversion efficiency is maximized. 
     According to one aspect of the present invention, there is provided a sound reproducing screen for an ultrasonic converting and reproducing method, which includes: at least one cell having a predetermined volume and having an oscillation member reflecting a sound signal when an ultrasonic wave carried with the sound signal is incident on the cell; and a screen having a matrix structure in which the cells are continuously distributed. 
     Preferably, but not necessarily, a size of the screen has a wave size larger than 1, wherein the wave size is a ratio between a diameter of the screen and a wavelength, wherein the sound signal has a frequency of 10 Hz to 10 kHz. 
     Preferably, but not necessarily, the cell includes a flat and thin elastic member having an elastic property reflecting the sound signal while oscillating in response to the ultrasonic wave when the ultrasonic wave is incident; and a hard net spaced from the elastic member by a predetermined interval, having continuous holes of a network structure, and being disposed in parallel with the elastic member so as to limit a displacement of one side of the oscillation of the elastic member. Further, the elastic member may be a thin film. 
     Furthermore, a mechanical impedance which is a multiplication of a thickness, density of the thin film and an angular velocity of the ultrasonic wave, is preferably, but not necessarily, substantially equal to an impedance of an air. 
     The cell according to another exemplary embodiment may include a horn reflecting the sound signal while oscillating in response to the ultrasonic wave when the ultrasonic wave is incident; an elastic shell of a cylinder or dome structure having a displacement at an opposite direction to a direction where the oscillation is propagated, and supporting the horn; a supporting member connected to the horn and the shell and transferring an oscillation of the horn to the shell; and a hard net supporting the shell, and having continuous holes of a network structure disposed in parallel with the elastic member so as to make it possible oscillate the elastic member by the incident ultrasonic wave. 
     The horn may have a flat and circular disc shape, and the supporting member is preferably, but not necessarily, a rigid body having a straight line shape formed via a center of the disc and a center of the shell. 
     The cell according to yet another exemplary embodiment of the present invention includes a flat and thin elastic member having an elastic property reflecting the sound signal while oscillating in response to the ultrasonic wave when the ultrasonic wave is incident; a coil winding the elastic member and a predetermined space to induce a current by means of an oscillation of the elastic member so as to make the space specified where a magnetic field is generated and the elastic member is used as one surface; and a diode connected to both terminals of the coil, and limiting an oscillation of the elastic member in response to a direction where a current induced to the coil flows. 
     The elastic member is preferably, but not necessarily, an elastic membrane. 
     The cell according to another exemplary embodiment of the present invention includes a data processing section extracting an envelope of the ultrasonic wave from an electrical signal converted from the ultrasonic wave; and an oscillating section receiving the ultrasonic wave and converting it to the electrical signal, and receiving the envelope and converting it to a sound signal to be output. 
     The oscillating section may include a receiving section receiving the incident ultrasonic wave, converting it to the electrical signal, and outputting the converted signal to the data processing section; and a transmitting section converting the envelope extracted from the data processing section to a sound signal to be output. 
     Furthermore, the data processing section may include a high pass filter dividing a signal having a predetermined frequency or higher from the electrical signal; a rectifier extracting the envelope from an output of the high pass filter; and a low pass filter dividing a signal having a predetermined frequency or lower from the envelope output from the rectifier, and may further include a first amplifier amplifying the electrical signal by a predetermined gain and outputting it to the high pass filter; and a second amplifier amplifying an output of the low pass filter with a predetermined gain so as to make the output have a desired predetermined value. 
     The oscillating section includes a flat and thin first film having an elastic property reflecting the sound signal while oscillating in response to the ultrasonic wave when the ultrasonic wave is incident; first and second metallization layers adhered to inside and outside of the first film, inducing an electrical signal in response to the oscillation, and being connected to the data processing section; a flat second film adhered to the second metallization layer and allowing a resonance to occur to the oscillation of the first film; and third and fourth flat metallization layers connected to an output of the data processing section, being formed at inside and outside of a predetermined space so as to form the space at the inside of the second film, and being in parallel with the second film. 
     In this case, the first film is preferably, but not necessarily, a piezoelectric film, and the second film is preferably, but not necessarily, a polyethylene film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above aspects and/or other aspects of the present invention will become more apparent by describing certain exemplary embodiments of the present invention with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view illustrating a structure of a sound reproducing screen for an ultrasonic wave converting and reproducing method in accordance with an exemplary embodiment of the present invention; 
         FIG. 2A ,  FIG. 2B  and  FIG. 2C  are cross-sectional views of each cell of a sound reproducing screen for an ultrasonic wave converting and reproducing method in accordance with an exemplary embodiment of the present invention; 
         FIG. 3A  and  FIG. 3B  are graphs illustrating displacements  4  of an incident wave and a reflected wave of the exemplary sound reproducing screen using a polymer film of  FIG. 2 ; 
         FIG. 4A ,  FIG. 4B  and  FIG. 4C  illustrate a portion of a cross-sectional view of a sound reproducing screen for an ultrasonic wave converting and reproducing method in accordance with another exemplary embodiment of the present invention; 
         FIG. 5  is a perspective view of unit cell of a sound reproducing screen for an ultrasonic wave converting and reproducing method in accordance with yet another exemplary embodiment of the present invention; 
         FIG. 6A  and  FIG. 6B  are block views of unit cell of a sound reproducing screen for an ultrasonic wave converting and reproducing method in accordance with yet another exemplary embodiment of the present invention; and 
         FIG. 7  is a cross-sectional view illustrating another exemplary embodiment of unit cell of the sound reproducing screen of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS OF THE INVENTION 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to accompanying drawings. 
       FIG. 1  is a perspective view illustrating a structure of a sound reproducing screen for an ultrasonic wave converting and reproducing method in accordance with an exemplary embodiment of the present invention. 
     The sound reproducing screen  100  of the exemplary embodiment shown in  FIG. 1  is formed to be a screen having a matrix structure, which may be simply referred to as a speaker. The screen  100  operates with at least one set of small ultrasonic sound sources (not shown) which are positioned at different locations on a front of the screen  100  so as to radiate ultrasonic waves to the screen  100 . 
     Signals produced by a non-linear ultrasonic sound source (not shown) are signals in an ultrasonic band which the listener cannot hear, and which include sound signals in an audible band by means of amplitude modulation. The screen  100  has a matrix structure in which a plurality of unit cells  101  is continuously integrated, and each cell  101  has a predetermined width and a predetermined area. 
     The ultrasonic waves which reach the screen  100  are reflected, and are converted by a rectification operation of each cell  101  of the screen  100 . Thus, a reflected sound wave is a rectified sound wave and the reflected sound wave includes a sound signal in an audible band which the listener can hear. In this case, each cell  101  of the screen  100  operates as a rectifier. 
     The sound signal output from the screen  100  is perceived as if it is transferred from a virtual sound source (not shown) which is positioned behind the screen  100 . The reflected sound signal of the audible band maintains phase information included in the incident ultrasonic wave as it is. 
     The ultrasonic wave including the sound signal, which is reflected by the screen  100 , can be produced from at least one device (not shown) and can be directed toward the screen  100 . 
     The cells  101  are mainly classified as being passive devices or active devices. 
     The cells classified as passive devices and the cells classified as active devices convert an energy of the ultrasonic wave to an energy of the sound signal. 
     The cells classified as passive devices do not require any energy for the screen itself to operate. An advantage of these passive devices is that the cells  101  of the screen  100  are relatively simple. 
     The cells classified as active devices require a power for operation. However, power consumption at each cell  101  may be relatively small. And, an advantage of the active devices is that the listener can be prevented from being exposed to an ultrasonic wave having a high intensity. 
     In addition, the screens  100  of the present invention are classified into screens that operate independently and screens which operate as a whole based on the operation of each cell  101 . 
     Moreover, the screens  100  can be classified into mechanical screens and electromagnetic screens. Further, the screens  100  can be classified into screens that utilize the displacement of the waves for rectification and screens that utilize the velocity of the waves for rectification. 
     Hereinafter, total operations of the sound reproducing screen  100  of the exemplary embodiment shown in  FIG. 1  will be described, and to this end, description will be given assuming that each cell  101  that reflects the ultrasonic signal from an ultrasonic source (not shown) to output a sound signal is regarded as one compact speaker. 
     The screen  100  ultimately reproduces sound by making each cell  101  in charge of the high frequency band of the sound signal and making the entire screen  100 , including all of the cells  101 , in charge of the low frequency band of the sound signal. All of the cells  101  are individually operated. 
     According to the exemplary sound reproducing screen  100  shown in  FIG. 1 , a realistic sound space can be reproduced, which is generated by a virtual sound source (not shown) which is positioned behind the screen  100 . Such an effect is supported by the Huygens-Fresnel principle. 
     According to the Huygens-Fresnel principle, a virtual sound source (not shown) is present behind the screen  100 , and a secondary sound source is generated over the front surface of the screen  100  by the virtual sound source (not shown), which makes the listener hear the sound. 
     The cells  101  of the screen  100  correspond to the secondary sound source. That is, according to conventional methods, an actual sound source is positioned behind the screen  100  and the virtual secondary sound source is positioned at the front surface of the screen in accordance with the Huygens-Fresnel principle. However, in accordance with exemplary embodiments of the present invention, the virtual sound source is positioned behind the screen  100  and the actual sound source, referred to as the cells  101 , is present in the screen  100 . When an amplitude and a phase at each cell  101  are properly selected and adjusted, the effect of making the sound source reach the listener becomes the same. The listener can think that the sound signal is delivered from the virtual sound source (not shown) which is present behind the screen  100 . 
     Hereinafter, a size of each cell  101  for making the sound reproducing screen  100  operate as a continuous system will be described. According to the exemplary embodiment shown in  FIG. 1 , the screen  100  is totally covered by the cells  101 . As used in the following discussion, the size of a cell  101  means a width of each cell  101 . 
     A frequency f that the listener can hear is theoretically about 20 kHz. A wavelength corresponding to this frequency can be obtained by dividing a velocity of sound by the frequency f A wavelength w corresponding to the frequency f is about 2 cm. 
     When the size of the cell  101  is smaller than 1 cm, which is half the size of the wavelength w in accordance with an exemplary embodiment of the present invention, the arrangement of the cells  101  can be regarded as continuous, and an effect due to the individually divided property of the screen  100  can be ignored. 
     However, the size of 1 cm is substantially small. A threshold value of the substantial audible frequency may be different for different people, however, such a threshold value is extremely low. A frequency less than at least 10 kHz can be regarded as one having information. In this case, the size of the cells  101  becomes about 2 cm. 
     The screen  100  having a width of 1 cm can operate in a continuous way, and can generate a substantially three-dimensional sound field in all audible frequency bands. Further, it is sufficient to generate the three-dimensional sound when the size of the cells  101  are about 5 to 7 cm. 
     Hereinafter, operations in the low and middle frequency band of the audible sound signal will be described. 
     A major difference between generating a sound signal having a low frequency and a sound signal having a middle frequency is associated with the wave size of the speaker, which is a ratio between a diameter and a wavelength of the speaker. When the wave size of a speaker is larger than 1, a sound output of the speaker can be effective. 
     However, when the wave size of a speaker is less than 1, an efficiency of the sound output of the speaker is poor. An air load applied to the speaker is very low in this case. This is due to the very strong power applied to a very small mass, so that the amplitude of the oscillation is very large and an energy applied to the air load becomes very small. 
     From a point of view of each cell  101 , the wave size of the cell  101  is small as compared to the lower frequencies from 5 kHz to 10 kHz. However, for the measurement of whole operations of the screen  100 , the wave size of the screen  100  must be considered, and the wave size of the entire screen  100  is determined differently. When the screen  100  is sufficiently large, the wave size of the screen  100  is sufficient up to about 100 Hz, however, the wave size of the screen  100  is small for frequencies lower than 100 Hz. The value of the wave size of the screen  100  is relatively small, so that an efficiency of the screen  100  in the range from 10 Hz to 100 Hz can be made complete by simply correcting amplitudes of audio channels corresponding to respective cells  101 . 
     Hereinafter, different exemplary embodiments for each cell  101  of the sound reproducing screen  100  will be described in detail. 
       FIG. 2A ,  FIG. 2B  and  FIG. 2C  are cross-sectional views of each cell of the sound reproducing screen for an ultrasonic wave converting and reproducing method in accordance with an exemplary embodiment of the present invention. 
     Referring to  FIG. 2A ,  FIG. 2B  and  FIG. 2C , the sound reproducing screen  100  is also referred to as a film-net (FN) device, which is a passive system in which each cell  200  is discrete and operates in a mechanical way and does not depend on an external power supply. Each cell  200  does not have a separate speaker inside. Further, each cell  200  receives the external sound carried on the ultrasonic wave directed toward the front of the screen  100 , and makes only the sound signal reflected by the screen  100 .  FIG. 2A ,  FIG. 2B  and  FIG. 2C  show a cross-sectional view of the cell  200 . 
     Hereinafter, operations and properties of respective components of the exemplary embodiment shown in  FIG. 2A ,  FIG. 2B  and  FIG. 2C  will be described. Referring to  FIG. 2A , each cell  200  of the sound reproducing screen  100  includes a hard net  203  which has a thin film  201  on its front and includes continuous holes of a net structure spaced by a predetermined interval d 1 . The thin film  201  is preferably, but not necessarily, a polymer film. 
     When an ultrasonic wave as an incident wave is delivered toward the film  201  of the cell  200 , the film  201  oscillates in response to the incident wave. As shown in  FIG. 2C , when the incident wave pulls the film  201  from the net  203 , the film  201  freely moves. And, as shown in  FIG. 2B  when the incident wave pushes the film  201  toward the net  203 , the net  203  is so hard that the net  203  is not moved by the film  201 , thereby limiting the displacement ξ of the film  201  on the side of the net  203 . 
     The oscillation of the film  201  can generate a reflective wave. Obviously, the response of the film  201  is non-linear, so that the reflective wave includes a non-linear component. 
     The operation of the cell  200  including the film  201  and the net  203  is similar to a diode, so that it can rectify the displacement ξ of the incident wave. The incident wave can be regarded as a wave which has been amplitude-modulated. Graphs of the displacements ξ of the incident wave and the reflective wave are shown in  FIG. 3 . 
       FIG. 3A  and  FIG. 3B  show graphs illustrating displacements  4  of the incident wave and the reflective wave of the sound reproducing screen  100  which uses the polymer film of  FIG. 2 .  FIG. 3A  represents the displacement ξ of the incident wave, and  FIG. 3B  represents the displacement ξ of the reflective wave. The horizontal axes of  FIG. 3A  and  FIG. 3B  denote a time and the vertical axes denote respective angular displacement ξ. 
     Referring to  FIG. 3A , it can be seen that the sound signal having a low frequency is amplitude-modulated to an ultrasonic wave having a high frequency.  FIG. 3B  shows the rectified wavelength as the displacement on the side of the net  203  of the film  201  is limited. 
     The reflective wave includes a low frequency component. When the incident wave ξ in  displayed using a displacement is the same as Equation 1 below, an amplitude of the low frequency component can be A Ω =A ω /π. In this case, π is a circle ratio, and A ω  is an amplitude of the incident wave.
 
ξ in   =A   ω (sin(ω+Ω) t +sin ω   t )  Equation 1
 
     In this case, Ω is an angular velocity of the reflective wave, and ω is an angular velocity of the incident wave. Accordingly, the frequency f of the incident wave is as follows: f=ω/(2π)=40 kHz, and the frequency F of the low frequency component is as follows: F=Ω/(2π)=about 1 kHz. 
     The reflective wave of the sound reproducing screen  100  of  FIG. 2A  plays the same role as the speaker of each cell  101  of  FIG. 1 , and the sound reproducing screen  100  of  FIG. 2A  operates in the same manner as that shown in  FIG. 1 . Subsequent operations are also the same as those discussed with respect to  FIG. 1 . 
     The film  201  oscillates with an ultrasonic wave and has the same amplitude as the amplitude of the oscillation of the ultrasonic wave. To this end, the film  201  must be sufficiently light. 
     To determine whether the film  201  is light in a given circumstance, a mechanical impedance of the film  201  can be calculated for comparison with an impedance of air. When the impedance of the film  201  is close to the impedance of the air, it means that there is no damping due to the material of the film  201 . 
     When a thickness h of the film  201  is 2×10 −6 m, an impedance of the film Z f  can be calculated as Equation 2 below. 
     
       
         
           
             
               
                 
                   
                     Z 
                     f 
                   
                   = 
                   
                     
                       p 
                       u 
                     
                     = 
                     
                       
                         
                           p 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           ω 
                         
                         a 
                       
                       = 
                       
                         
                           ρ 
                           f 
                         
                         ⁢ 
                         
                           h 
                           ω 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     In this case, p denotes a pressure, u denotes a velocity, a denotes an acceleration, ω denotes an angular velocity of the incident wave, and ρ f  denotes a density of the film. When the density ρ f  of the film is, for example, 10 3  kg/m 3 , Z f  becomes an impedance of 5×10 2  Pa*s/m, which is close to an impedance of the air, and can thus be selected as a proper film. 
     The net  203  has a lattice form, which has a series of continuous holes, and the film  201  can oscillate when a size of the acoustic boundary layer is smaller than the holes of the net  203 . In contrast, when the size of the acoustic boundary layer is larger than the holes of the net  203 , the net  203  acts as a wall. 
     The acoustic boundary layer δ is calculated by Equation 2 below, and the holes of the net  203  must be larger than δ. 
     
       
         
           
             
               
                 
                   δ 
                   = 
                   
                     
                       
                         η 
                         
                           
                             ρ 
                             o 
                           
                           ⁢ 
                           ω 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
     
     In this case, co denotes an angular velocity of the incident wave, η denotes a dynamic viscosity of the air and has a value of 3×10 −5  kg/ms, and ρ 0  denotes a density of the air. Accordingly, δ becomes about 10 −5  m. 
     To sum up, the sound reproducing screen  100  of  FIG. 2  can be implemented by the hard net  203 , which has the film  201  with a thickness smaller than about 2 microns and holes larger than about 10 −5  m. An interval between the film  201  and the net  203  must be smaller than the displacement of the wave in the ultrasonic wave. 
       FIG. 4A ,  FIG. 4B , and  FIG. 4C  illustrate a portion of a cross-sectional view of a sound reproducing screen for an ultrasonic wave converting and reproducing method in accordance with another exemplary embodiment of the present invention.  FIG. 4A  shows a portion of the sound reproducing screen  400  and  FIG. 4B  shows each cell  410  of the screen  400  in accordance with the another exemplary embodiment of the present invention. 
       FIG. 4C  is a view for explaining a movement of the shell  405  included in each cell  410 , which shows that the shell  405  is moved by the displacement ξ 1  when a power resulting from the incident ultrasonic wave is delivered to a vertex of the semicircle shell  405 , which has a dome structure. 
     The sound reproducing screen  400  of  FIG. 4A  is configured such that a plurality of cells  410  is arranged in a lattice shape to form an entire screen. Each cell  410  has a circular disc-shaped horn  401  at its front, and the shell  405 , which comprises a semicircle elastic member having a dome structure, is disposed at a rear side of the horn  401 . Further, the horn  401  and the shell  405  are connected to each other by a supporting member  403  as a rigid body having a needle shape. A net  407 , which has a series of connected holes, is formed at the rear side of the shell  405 . 
     The horn  401  is for acoustic impedance matching, and can be implemented as a flat metal disc. An impedance matching is also required between the horn  401  as a non-linear element and the air, as is the same case with the impedance of the film  201 . 
     The disc-shaped horn  401  must have a size corresponding to the wavelength of the air so as to operate as an effective transmitter or receiver. The resonance frequency of the disc-shaped horn  401  must be the same as the ultrasonic wave frequency (o as the carrier. An output impedance of the disc-shaped horn  401  can be obtained by a ratio between a force applied to the supporting member  403  and an oscillation velocity, which is shown as Equation 4 below: 
     
       
         
           
             
               
                 
                   Z 
                   = 
                   
                     
                       
                         F 
                         u 
                       
                       ~ 
                       
                         Z 
                         o 
                       
                     
                     ⁢ 
                     S 
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
     
     In this case, F denotes a force applied to the supporting member  403 , u denotes an oscillation velocity, Z o  denotes an impedance of the air, and S denotes an area of the horn  401 . 
     The shell  405  forms a surface, which has a non-linear elastic property, to divide the sound signal carried on the ultrasonic signal. The displacement δ 1  of the shell  405  is a secondary function to a pressure, which gives a secondary non-linear response. The shell  405  may be a shape other than dome-shaped, and may have an elastic cylinder structure. 
     The supporting member  403  may be a straight line connecting a center of the shell  405  and the horn  401 . Moreover, the supporting member  403  may have a structure such that it is connected to at least one point of a periphery of the horn  401  and such that at least one supporting part extended from the supporting member  403  forms a triangular pyramid, and such that the supporting member  403  extended again from the vertex of the triangular pyramid is connected to the shell  405 . The straight line connected to the shell  405  of the supporting member  403  while passing through the center of the horn  401  and the vertex of the triangular pyramid is orthogonal to the net  407  toward the center of the horn  401 . 
     The sound reproducing screen of  FIG. 4A ,  FIG. 4B , and  FIG. 4C  is operated by the similar principle as the FN device of  FIG. 2 . The horn  401  is connected to the shell  405  as the non-linear element. The shell  405  gives a non-linear response between the sound pressure P and the displacement ξ 1 . By means of the non-linear property of the displacement ξ 1 , the reflective wave includes a modulated frequency Ω. 
       FIG. 5  is a perspective view of a unit cell  510  of a sound reproducing screen for an ultrasonic wave converting and reproducing method in accordance with yet another exemplary embodiment of the present invention. 
     The sound reproducing screen using the cell  510  of  FIG. 5  as a unit structure utilizes an electromagnetic induction phenomenon. 
     The cell  510  is positioned within a magnetic field, and an elastic and thin film  501  is covered on a front surface of the space  505 , which is a predetermined hexagonal shape and is filled with air. Further, the coil  503  is wound by a method of winding the space  505  and the film  501 , and both ends of the coil  503  are connected to a diode as a semiconductor (not shown). 
     The cell  510  is positioned within the magnetic field, and an electromagnetic field is induced to the coil  503 , which oscillates together when the film  501  oscillates by means of the incident ultrasonic wave. When the electromagnetic field is forward-biased for the diode (not shown), currents flow along the coil  503 , and these currents dampen the oscillation of the film  501 . When the currents flow in the opposite direction, the film  501  moves in a free manner. Accordingly, the oscillation velocity of the film  501  is rectified. In this case, the film  501  must be sufficiently light. 
     An alternative of the exemplary embodiment of  FIG. 5  may further have a film (not shown) disposed on the film  501  and spaced by a predetermined interval from the film  501 . In this case, the interval between the two films is adjusted so as to make a resonance occur by means of the two films in response to the frequency ω of the incident ultrasonic wave. 
       FIG. 6A  and  FIG. 6B  are block views of unit cell of a sound reproducing screen by an ultrasonic wave converting and reproducing method in accordance with yet another exemplary embodiment of the present invention. 
       FIG. 6A  shows the unit cell  610  in accordance with yet another exemplary embodiment of the present invention. The unit cell  610  includes a receiving section  601 , which receives the incident wave as the ultrasonic wave, a data processing section  620 , and a transmitting section  630 , which outputs the rectified sound signal. 
       FIG. 6B  shows the data processing section  620 , which includes a first amplifier  621 , a high pass filter  623 , a rectifier  625 , a low pass filter  627 , and a second amplifier  629 . 
     The high pass filter  623  and the low pass filter  627  act to prevent a positive feedback between the input and the output of the signal. The high pass filter  623  only passes signals having a predetermined frequency or higher, and preferably, but not necessarily, filters signals having about 30 kHz or higher. The low pass filter  627  filters the signals of the audible frequency band, and preferably, but not necessarily, passes frequency signals lower than about 10 kHz. 
     The rectifier  625  operates as a non-linear element and tracks and detects an envelope of the signal incident through the receiving section  601 . Accordingly, the signal incident through the receiving section  601  detects the envelope of the signal incident through the data processing section  620 , and amplifies it to a proper output level to be output to the transmitting section  630 . 
     An exemplary alternative of  FIG. 6A  and  FIG. 6B  may be implemented with an oscillating section (not shown), which has the receiving section  601  and the transmitting section  630  as one device, and an example of such an alternative is shown in  FIG. 7 . 
       FIG. 7  is a cross-sectional view illustrating another exemplary embodiment of a unit cell of the sound reproducing screen of  FIG. 6 . 
     Referring to  FIG. 7 , the receiving section  601  and the transmitting section  630  are formed as an oscillating section  700  as one structure. The oscillating section  700  has a first film  701  with a predetermined thickness, and first and second metallization layers  705  and  707 , which are formed outside and inside of the first film  701 , respectively. A second film  703  for forming an oscillation mode is disposed inside the second metallization layer  707 . The first film  701  is preferably, but not necessarily, formed of a piezoelectric film, and the second film  703  is preferably, but not necessarily, formed of a polyethylene film. 
     A space  709  which is filled with an air is formed inside the second film  703 , and an outside and an inside of the space  709  are surrounded by third and fourth metallization layers  711  and  713 , respectively. The first and second metallization layers  705  and  707 , which surround the first film  701 , are connected to the data processing section  620 , which corresponds to an output of the receiving section  601 . In addition, the third and fourth metallization layers  711  and  713 , which surround the space  709 , are connected to an output of the data processing section  620 , which corresponds to the transmitting section  630 . 
     Oscillation resulting from the ultrasonic wave incident on the first film  701  causes a voltage to the first and second metallization layers  705  and  707  of the oscillating section  700 . This voltage is processed in the data processing section  620  as a sound signal, and a voltage signal having a low frequency output from the data processing section  620  is delivered to the third and fourth metallization layers  711  and  713 . The first and second films  701  and  703 , the space  709 , and the fourth metallization layer  713  operate in the same manner as the condenser output speaker. 
     By means of the above-described method, the sound reproducing screen for the ultrasonic converting and reproducing method according to an exemplary embodiment of the present invention is operated. 
     According to exemplary embodiments of the present invention as described above, the following effects can be obtained. 
     First, a sound reproducing screen consistent with the present invention can make most of the sounds that are delivered to an arbitrary constant region among the entire space where the sound is spread. As a result, the listener can perceive a three-dimensional sound field, much like the actual sound, in front of the listener or behind the listener. 
     In addition a virtual sound field can have a very high spatial resolution, that is, by means of a sound reproducing screen consistent with the present invention, each position of each instrument of one band can be represented. 
     Second, a speaker consistent with the present invention can produce a specific sound effect such as the virtual sound source. 
     Third, consistent with the present invention, a significant property can be obtained in generating a low frequency region of the sound spectrum. 
     Fourth, a sound reproducing screen consistent with the present invention can be manufactured with a very simple structure, and can be manufactured as thin as a wall paper, so that ease of installment and management can be ensured. A sound reproducing screen consistent with the present invention can also be utilized as a video screen in response to a material of the screen. In this case, viewers can hear the sound correctly output from positions of the sound sources that they see such as an automobile and an animal. 
     The foregoing exemplary embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments of the present invention is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.