Patent Publication Number: US-8126171-B2

Title: Electrostatic ultrasonic transducer and ultrasonic speaker

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to an electrostatic ultrasonic transducer, a method of manufacturing an electrostatic ultrasonic transducer, an ultrasonic speaker, a method of reproducing a sound signal, an super-directivity sound system, and a display device that are capable of improving an output sound pressure by increasing an effective membrane displacement of a vibrating membrane and increasing an opening ratio of radiating holes (through-holes) radiating a sound wave. 
     2. Related Art 
     An ultrasonic transducer outputs a modulated wave obtained by modulating a carrier wave in an ultrasonic wave band with a sound signal in an audible band, and reproduces a sound having sharp directivity. 
       FIGS. 15A and 15B  are diagrams illustrating an example of a structure of an ultrasonic transducer according to the related art. Most of ultrasonic transducers according to the relate art are resonance-type ultrasonic transducers using piezoelectric ceramic. A piezoelectric ultrasonic transducer shown in  FIG. 15A  performs a conversion from an electrical signal to an ultrasonic wave and a conversion from the ultrasonic wave to the electrical signal (transmission and reception of an ultrasonic wave) by using piezoelectric ceramic as a vibration element. 
     A bimorph-type ultrasonic transducer shown in  FIG. 15A  includes two sheets of piezoelectric ceramics  61  and  62 , a cone  63 , a case  64 , leads  65  and  66 , and a screen  67 . The piezoelectric ceramics  61  and  62  are bonded to each other, and the leads  65  and  66  are connected to the surfaces of the piezoelectric ceramics  61  and  62  opposite to the bonding surface between the piezoelectric ceramics  61  and  62 . 
     Since the piezoelectric ultrasonic transducer utilizes a resonance phenomenon of the piezoelectric ceramics, ultrasonic wave transmitting and receiving characteristics become superior in a relatively narrow frequency band near a resonance frequency thereof. However, since the piezoelectric transducer utilizes a sharp resonance characteristic of an element, a high sound pressure is obtained, but a frequency band is extraordinarily narrow. For this reason, a reproducible frequency band is narrow in an ultrasonic speaker that uses the piezoelectric transducer, and a reproducing sound quality becomes deteriorated, as compared with a loud speaker. 
     Different from the above-described piezoelectric transducer, the electrostatic ultrasonic transducer has been generally known as a wide range oscillation type ultrasonic transducer that can reproduce a high sound pressure over a high frequency band.  FIG. 15B  shows an example of a structure of a wide band oscillation type electrostatic ultrasonic transducer. The electrostatic ultrasonic transducer is referred to as a pull type, because a vibrating membrane only moves in a direction toward a electrode side. 
     The electrostatic ultrasonic transducer that is shown in  FIG. 15B  uses as a vibrator (vibrating membrane), a dielectric  131  (insulator), such as a PET (polyethyleneterephthalate) resin, which has a thickness in a range of about 3 to 10 μm. In regards to the dielectric  131 , an upper electrode  132  formed by using a metal foil such as aluminum is integrally formed on a top surface of the dielectric  131  by means of a deposition process or the like, and a lower electrode  133  formed of brass is provided to come into contact with the bottom surface of the dielectric  131 . A lead  152  is connected to the lower electrode  133 , and the lower electrode  133  is fixed on a base plate  135  that is made of Bakelite. 
     Further, the upper electrode  132  is connected to a lead  153 , and the lead  153  is connected to a direct current bias power supply  150 . By means of the direct current bias power supply  150 , the upper electrode  132  is always applied with a direct current bias voltage for upper electrode absorption in a range of about 50 to 150 V, and the upper electrode  132  is attracted to the side of the lower electrode  133 . Reference numeral  151  indicates a signal source. 
     The dielectric  131 , the upper electrode  132 , and the base plate  135  are caulked by a case  130  together with metal rings  136 ,  137 , and  138 , and a mesh  139 . 
     A plurality of minute grooves (unevenness portions), which have a size in a range of about several tens to several hundreds of micrometers and an irregular shape, are formed on a surface of the lower electrode  133  at the dielectric  131  side. Since the minute grooves form gaps between the lower electrode  133  and the dielectric  131 , the distribution of the capacitance between the upper electrode  132  and the lower electrode  133  is minutely varied. The random minute grooves are formed by manually polishing the surface of the lower electrode  133 . In the electrostatic ultrasonic transducer, a plurality of capacitors where sizes or depths of gaps are different are formed, which achieves a wide band of a frequency characteristic (JP-A-2000-50387 and JP-A-2000-50392). 
     As described above, the electrostatic ultrasonic transducer that is shown in  FIG. 15B  has been generally known as a wide band ultrasonic transducer (pull type) that can generate a relatively high sound pressure over a wide frequency band. 
     However, a maximum output sound pressure of the electrostatic ultrasonic transducer is slightly low, it is difficult to obtain an ultrasonic sound pressure necessary when obtaining a parametric array effect, and a ceramic piezoelectric element such as PZT or a high molecular piezoelectric element such as PVDF is used as an ultrasonic generator. However, the piezoelectric element has a sharp resonance point without depending on a type of material thereof, is driven with a resonance frequency, and is put to practical use as an ultrasonic speaker. Therefore, a frequency region capable of ensuring a high sound pressure is very narrow. That is, it has a frequency region of a narrow band. 
     In order to solve this problem, the applications have suggested the static ultrasonic transducer, as shown in  FIGS. 16A and 16B  (Japanese Patent Application No. 2004-173946). This structure is generally referred to a push-pull type, and is capable of simultaneously satisfying a wide band and a high sound pressure, as compared with the pull-type electrostatic ultrasonic transducer. 
       FIGS. 16A and 16B  are diagrams illustrating an example of a structure of a push-pull type electrostatic ultrasonic transducer. Specifically,  FIG. 16A  is a diagram illustrating a sectional structure of a push-pull type electrostatic ultrasonic transducer, and  FIG. 16B  is a plan view illustrating a electrode when viewed from a vibrating membrane side.  FIG. 16A  is a cross-sectional view taken along the line X-Y of  FIG. 16B . 
     In  FIGS. 16A and 16B , the push-pull type electrostatic ultrasonic transducer includes a pair of electrodes  20  and  21  each having a conductive member formed of a conductive material functioning as an electrode, a vibrating membrane  22  that is interposed between the pair of electrodes  20  and  21  and has a conductive layer (vibrating membrane electrode)  221 , and a supporting member  25  that holds the pair of electrodes  20  and  21  and the vibrating membrane  22 . 
     The vibrating membrane  22  has insulating layers  220 , and a conductive layer  221  that is formed of a conductive material, and the conductive layer  221  is applied with a direct current bias voltage having a single polarity (both a positive polarity voltage and a negative polarity voltage are possible) by means of a direct current bias power supply  27 . 
     Further, the pair of electrodes  20  and  21  have the same number and a plurality of through-holes  24  at locations facing each other with the vibrating membrane  22  interposed therebetween. Between the conductive members of the pair of electrodes  20  and  21 , an alternating current signal is applied by means of the signal sources  28 A and  28 B. Between the electrode  20  and the conductive layer  221 , and between the electrode  21  and the conductive  221 , capacitors are respectively formed. 
     According to this configuration, in the electrostatic ultrasonic transducer, the conductive layer  221  of the vibrating membrane  22  is applied with the direct current bias voltage having a single polarity (in this example, positive polarity) by mans of the direct current bias power supply  27 . Meanwhile, the pair of electrodes  20  and  21  are applied with the alternating current signal by mans of the signal sources  28 A and  28 B. As a result, since the positive voltage is applied to the electrode  20  during a positive half cycle of the alternating current signal output from the signal sources  28 A and  28 B, an electrostatic repulsive force acts in a surface portion  23 A of the vibrating membrane  22  that is not interposed between the electrodes of the vibrating membrane  22 , and the surface portion  23 A extends downward in  FIG. 16A . At this time, since the negative voltage is applied to the electrode  21  that faces the electrode  20 , an electrostatic absorption force acts in a rear surface portion  23 B at the rear surface side of the vibrating membrane  22 , and the rear surface portion  23 B extends downward in  FIG. 16 . 
     Accordingly, a membrane portion of the vibrating membrane  22  that is not interposed between the pair of electrodes  20  and  21  is applied with an electrostatic repulsive force and electrostatic repulsion in the same direction. In the same manner with respect to the negative half cycle of the alternating current signal that is output from the signal sources  28 A and  28 B, in  FIG. 16A , the electrostatic absorption force acts upward in the surface portion  23 A of the vibration membrane  22 , and the electrostatic repulsive force acts upward in the rear surface portion  23 B in  FIG. 16A . A membrane portion of the vibrating membrane  22  that is not interposed between the pair of electrodes  20  and  21  is applied with an electrostatic repulsive force and electrostatic repulsion in the same direction. In this way, while the vibrating membrane  22  is applied with the electrostatic repulsive force and the electrostatic repulsion in the same direction as the polarity of the alternating current signal is varied, a direction where the electrostatic force alternately acts is varied. Therefore, it is possible to generate a sound signal having a sufficient sound pressure level that is necessary when obtaining a strong membrane vibration, that is, the parametric array effect. 
     As such, the ultrasonic transducer that is shown in  FIGS. 16A and 16B  is referred to as a push-pull type because the vibrating membrane  22  receives a force from the pair of electrodes  20  and  21  and vibrates. The push-pull type electrostatic ultrasonic transducer has a capability that is capable of simultaneously achieving a wide band and a high sound pressure, as compared with the pull type electrostatic ultrasonic transducer where the electrostatic absorption force is only applied to the vibrating membrane. 
     As described above, in the push-pull type electrostatic ultrasonic transducer, a high direct current bias voltage is applied to the vibrating membrane and the alternating current voltage is applied to the electrodes, and thus the membrane portion vibrates due to an electrostatic force (attraction or repulsion) that is applied to the electrode and the vibrating membrane. In this case, in order to achieve the vibration in the ultrasonic wave band, the diameter of the hole of the vibrating portion needs to be several mm or less. For example, as shown in  FIG. 17 , it is required to form a transducer having a high following characteristic and a high output characteristic by providing the plurality of through-holes (vibration holes)  24  on the electrode  20 . 
       FIGS. 18A to 18C  are diagrams illustrating a structure of a electrode that is used in a push-pull type electrostatic ultrasonic transducer shown in  FIGS. 16A and 16B  and a process for manufacturing the electrode. 
     As described above, the electrode needs to be provided with the through-holes for radiating the sound wave, and through-holes of 1000 or more may be formed. The mechanical processing is suitable in terms of processing precision, but since instead of the mechanical processing, the etching is used because of the problem of the cost. However, there is a restriction between the diameter of the through-hole formed by the etching and the thickness. For example, it is difficult to manufacture with the etching process, the electrodes that have the through-hole diameter of 0.75 mm and the thickness of 1.5 mm and satisfy the predetermined processing precision. 
     Accordingly, as shown in  FIG. 18A , a mask member  201  for forming the predetermined through-holes  203  is coated on a conductor (it is generally metal, and copper or stainless can be used as the conductor)  202  that has the thickness sufficiently smaller than the diameter of the through-hole, for example, the thickness of 0.25 mm, which is then subjected to the etching process. In this way, the plurality of conductors  202  are prepared in which the through-holes  203  are formed. 
     In addition, as shown in  FIG. 18B , when the total thickness of the conductors is 1.5 mm, six sheets of conductors  202  are laminated in a state where all of the through-holes  203  are aligned. As shown in  FIG. 18C , in a state where the laminated conductors  202  are pressed from both sides, the laminated conductors  202  are subjected to a thermal compressing process or a dispersion bonding process. As a result, it is possible to form an integral (metal-coupled) electrode having the thickness of 1.5 mm.  FIGS. 18A and 18B  show an example where square electrodes are manufactured. However, when the circular electrodes are manufactured, the circular conductor  202  is used. 
     Meanwhile, as described above, a plurality of through-holes for radiating the sound wave need to be formed in the electrodes of the electrostatic ultrasonic transducer. In this case, as shown in  FIG. 16B , around the through-holes  24 , the counter electrode portions  26  are disposed to make the electrostatic force applied to the vibrating membrane, and the electrostatic force is applied between the counter electrode portion  26  and the vibration region of the vibrating membrane  22  (portion of the vibrating membrane that is not interposed between the electrodes). 
     In this case, the diameter D 1  of the through-hole  24  is set to half the diameter D 2  of the electrode that forms the counter electrode portion  26 . This relationship is set such that the relationship between the radiating efficiency of the sound wave and the membrane vibration amplitude becomes most excellent. For example, if the diameter of the through-hole becomes smaller (that is, if the area of the counter electrode portion  26  becomes larger), the electrostatic force becomes stronger, which increases the membrane vibration amplitude. However, the radiating area of the sound wave is decreased, which lowers the radiating sound pressure. Meanwhile, if the diameter of the through-hole becomes larger (that is, if the area of the counter electrode portion  26  becomes smaller), the radiating area of the sound wave is increased. However, since the electrostatic force becomes weaker, the membrane vibration amplitude is decreased, which lowers the radiating sound pressure. 
     The transducer is constructed according to the above-described relationships. However, in the structure according to the related art shown in  FIG. 16B , the electrostatic force that is applied to the vibrating membrane is only applied to the outer circumferential portion of the vibration region, and it is difficult to generate the membrane vibration with high efficiency. 
     As described above, in the push-pull type electrostatic ultrasonic transducer according to the related art, the electrostatic force that is applied to the vibrating membrane is only applied to the outer circumferential portion of the vibration region, and it is difficult to generate the membrane vibration with high efficiency. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides an electrostatic ultrasonic transducer, a method of manufacturing an electrostatic ultrasonic transducer, an ultrasonic speaker, a method of reproducing a sound signal, and super-directivity sound system, and a display device that are capable of improving an output sound pressure by increasing an effective membrane displacement of a vibration membrane and increasing an opening ratio of radiating holes (through-holes) radiating an ultrasonic wave. 
     According to a first aspect of the invention, an electrostatic ultrasonic transducer includes a first electrode that has through-holes, a second electrode that has through-holes, and a vibrating membrane that is disposed such that the through-holes of the first electrode and the through-holes of the second electrode form a pair, interposed between a pair of electrodes composed of the first electrode and the second electrode, and having a conductive layer applied with a direct current bias voltage. The first electrode and the second electrode each have counter electrode portions that are formed in the through-holes to face the vibrating membrane, and a modulated wave, which is obtained by modulating a carrier wave in an ultrasonic frequency band with a signal wave in an audible frequency band, is applied between the pair of electrodes. 
     According to this configuration, the counter electrode portions are disposed in the through-holes to face the vibration region of the vibrating membrane (portion of the vibrating membrane that faces the through-holes). 
     Therefore, a membrane vibration amplitude of the vibration region of the vibrating membrane can be increased. Further, the counter electrode portions are formed in the through-holes, and thus the diameter of the through-hole can be increased. As a result, an opening ratio can be increased and an output sound pressure can be improved. 
     Preferably, the counter electrode portions have a bridge structure that builds a bridge between an outer circumferential portion and an inner portion of the through-hole. 
     According to this structure, the counter electrode portion is constructed to have the bridge structure that passes through the center of the through-hole and crosses the through-hole. In addition, the bridge is set to have a small width, and the counter electrode portion is constructed not to hinder the sound wave radiation from the vibrating membrane. 
     Therefore, the membrane vibration amplitude of the vibration region of the vibrating membrane can be increased. The counter electrode portion is constructed not to hinder the sound wave radiation, and thus an opening ratio can be increased. 
     Preferably, the bridge structure is a cross-shaped structure. 
     Accordingly, the membrane vibration amplitude of the vibration region of the vibrating membrane can be increased. Further, the counter electrode portion is constructed not to hinder the sound wave radiation, and thus an opening ratio can be increased. 
     Preferably, the bridge structure is a cross-shaped structure, and a central counter electrode portion, which is wider than the bridge structure, is provided at a central portion of the cross-shaped structure. 
     According to this configuration, the counter electrode portion has the cross-shaped bridge structure, and the central counter electrode portion, which is wider than the bridge, is disposed at the central portion of the cross-shaped structure. 
     Therefore, the membrane vibration amplitude of the vibration region of the vibrating membrane can be further increased. 
     Preferably, the bridge structure is a Y-shaped structure. 
     Therefore, the membrane vibration amplitude of the vibration region of the vibrating membrane can be increased and an opening ratio can be further increased. 
     According to a second aspect of the invention, there is provided a method of manufacturing an electrostatic ultrasonic transducer which includes a first electrode that has through-holes, a second electrode that has through-holes, and a vibrating membrane that is disposed such that the through-holes of the first electrode and the through-holes of the second electrode form a pair, interposed between a pair of electrodes composed of the first electrode and the second electrode, and having a conductive layer applied with a direct current bias voltage, the first electrode and the second electrode each having counter electrode portions that are formed in the through-holes to face the vibrating membrane, a modulated wave obtained by modulating a carrier wave in an ultrasonic frequency band with a signal wave in an audible frequency band being applied between the pair of electrodes. The method includes manufacturing a conductive electrode base material where the counter electrode portions facing a vibration region of the vibrating membrane are formed in the through-holes, disposing a mask member masking regions of the through-holes of the conductive electrode base material and neighboring regions of the through-holes on one surface of the conductive electrode base material, setting a counter electrode forming material for forming an insulating counter electrode forming body to the one surface of the conductive electrode base material where the mask member is disposed, and coating the counter electrode forming material on a portion of the one surface of the conductive electrode base material which is not masked by the mask member, and separating the mask member after the counter electrode forming material is completely coated, and drying the counter electrode forming body formed on the one surface of the conductive electrode base material. 
     According to this configuration, the conductive electrode base material is prepared in which the counter electrode portions are formed in the through-holes. In addition, the screen for masking the region of the through-hole and the peripheral regions of the through-holes (mask member for forming a counter electrode forming body to be an insulator on a surface of the conductive electrode base material) is set to one surface of the conductive electrode base material, and by moving a squeegee, the counter electrode forming material to be the insulator is coated to a portion of the conductive electrode base material which is not masked by the mask member. The counter electrode forming material is a material that can be permanently constructed as the counter electrode forming body, and has a non-conductive property. For example, the counter electrode forming material is a masking ink that is used as liquid solder resist for packaging or resist for sand blast generally used in a circuit board. In addition, the screen for forming the counter electrode is separated after the coating process is completed, the counter electrode forming body is dried, and a desired electrode is formed. 
     As a result, when manufacturing the electrodes of the electrostatic ultrasonic transducer, the counter electrode forming body can be easily formed on the surface of the conductive electrode base material. Accordingly, it is possible to reduce the manufacture cost of the electrostatic ultrasonic transducer. 
     Preferably, the conductive electrode base material is constructed by laminating flat conductive materials each having a plurality of through-holes and counter electrode portions formed in the plurality of through-holes. 
     According to this configuration, when the conductive electrode base material is manufactured, the flat conductor materials having the through-holes and the counter electrode portions formed in the thorough-holes are formed by an etching process. The flat conductor materials are laminated, thereby manufacturing a conductive electrode base material. 
     Therefore, it is possible to easily manufacture the conductive electrode base material that has a predetermined thickness. 
     According to a third aspect of the invention, an ultrasonic speaker includes an electrostatic ultrasonic transducer that includes a first electrode that has through-holes, a second electrode that has through-holes, and a vibrating membrane that is disposed such that the through-holes of the first electrode and the through-holes of the second electrode form a pair, interposed between a pair of electrodes composed of the first electrode and the second electrode, and having a conductive layer applied with a direct current bias voltage, the first electrode and the second electrode each having counter electrode portions that are formed in the through-holes to face the vibrating membrane, a modulated wave obtained by modulating a carrier wave in an ultrasonic frequency band with a signal wave in an audible frequency band being applied between the pair of electrodes, a signal source that generates a signal wave in an audible frequency band, a carrier wave supply unit that generates a carrier wave in an ultrasonic frequency band and outputs the carrier wave, and a modulating unit that modulates the carrier wave with the signal wave in the audible frequency band output by the signal source. The electrostatic ultrasonic transducer is driven by a modulated signal that is applied between the pair of electrodes and an electrode layer of the vibrating membrane and is output from the modulating unit. 
     Therefore, it is possible to improve the output sound pressure of the ultrasonic speaker. 
     According to a fourth aspect of the invention, there is provided a method of reproducing a sound signal using an electrostatic ultrasonic transducer which includes a first electrode that has through-holes, a second electrode that has through-holes, and a vibrating membrane that is disposed such that the through-holes of the first electrode and the through-holes of the second electrode form a pair, interposed between a pair of electrodes composed of the first electrode and the second electrode, and having a conductive layer applied with a direct current bias voltage, the first electrode and the second electrode each having counter electrode portions formed in the through-holes to face the vibrating membrane, a modulated wave obtained by modulating a carrier wave in an ultrasonic frequency band with a signal wave in an audible frequency band being applied between the pair of electrodes. The method includes causing a signal source to generate a signal wave in an audible frequency band, causing a carrier wave supply unit to generate a carrier wave in an ultrasonic frequency band and output the carrier wave, generating a modulated signal by causing a modulating unit to modulate the carrier wave with the signal wave in the audible frequency band, and driving the electrostatic ultrasonic transducer by applying the modulated signal between the electrodes and an electrode layer of the vibrating membrane. 
     According to the method of reproducing a sound signal using an electrostatic ultrasonic transducer according to the fourth aspect of the invention including the above-described processes, the signal source generates the signal wave in the audible frequency band, and the carrier wave supply source generates the carrier wave in the ultrasonic frequency band and outputs it. In addition, the modulating unit modulates the carrier wave with the signal wave in the audible frequency band, the modulated signal is applied between the electrodes and the electrode layer of the vibrating membrane, and the electrostatic ultrasonic transducer is driven. 
     Therefore, when using the electrostatic ultrasonic transducer having the above-described structure, a low voltage can be applied between the electrodes, the membrane vibration can be increased, it is possible to output the sound signal having a sufficiently high sound pressure level in obtaining a parametric array effect over a wide frequency band, and the sound signal can be reproduced. 
     Further, the method of reproducing the sound signal using the electrostatic ultrasonic transducer according to the fourth aspect of the invention uses the electrostatic ultrasonic transducer that is constructed such that the pair of electrodes have the counter electrode portions formed in the through-holes to face the vibrating membrane, that is, the counter electrode portions are disposed in the through-holes to face the vibration region of the vibrating membrane (portion of the vibrating membrane that faces the through-hole), it is possible to increase the membrane vibration amplitude of the vibration region of the vibrating membrane. Further, when the counter electrode portions are formed in the through-holes, the diameter of the through-hole can be increased. Therefore, the opening ratio can be increased, and the output sound pressure can be improved. 
     According to a fifth aspect of the invention, an super-directivity sound system includes an ultrasonic speaker that is constructed by using an electrostatic ultrasonic transducer and reproduces a sound signal of a first sound area among sound signals supplied from a sound source, the electrostatic ultrasonic transducer including a first electrode that has through-holes, a second electrode that has through-holes, and a vibrating membrane that is disposed such that the through-holes of the first electrode and the through-holes of the second electrode form a pair, interposed between a pair of electrodes composed of the first electrode and the second electrode, and a having conductive layer applied with a direct current bias voltage, the first electrode and the second electrode each having counter electrode portions formed in the through-holes to face the vibrating membrane, a modulated wave obtained by modulating a carrier wave in an ultrasonic frequency band with a signal wave in an audible frequency band being applied between the pair of electrodes, and a reproducing speaker that reproduces a sound signal of a second sound area among the sound signals supplied from the sound source. The ultrasonic speaker reproduces the sound signals supplied from the sound source, and a virtual sound source is formed in the vicinity of a sound wave reflecting surface, such as a screen. 
     The super-directivity sound system according to the fifth aspect of the invention uses the ultrasonic speaker that includes an electrostatic ultrasonic transducer, the electrostatic ultrasonic transducer including a first electrode that has through-holes, a second electrode that has through-holes, and a vibrating membrane that is disposed such that the through-holes of the first electrode and the through-holes of the second electrode form a pair, is interposed between a pair of electrodes composed of the first electrode and the second electrode, and a conductive layer applied with a direct current bias voltage, the first electrode and the second electrode each having counter electrode portions formed in the through-holes in a state where the counter electrode portions face the vibrating membrane, and a modulated wave obtained by modulating a carrier wave in an ultrasonic frequency band with a signal wave in an audible frequency band being applied between the pair of electrodes. In addition, the ultrasonic speaker reproduces the sound signals of the intermediate and high sound areas among the sound signals supplied from the sound source. The sound signal of the low sound area among the sound signals that are supplied from the sound source is reproduced by the low sound reproducing speaker. 
     Accordingly, while the sound of the first sound area (intermediate and high sound) has the sufficient sound pressure and the wide band characteristic in a state where the low voltage is applied between the electrodes of the electrostatic ultrasonic transducer and the sound pressure characteristic is improved, the sound can be reproduced such that the it is generated from the virtual sound source formed in the vicinity of the sound wave reflecting surface, such as the screen. Further, since the sound in the second sound area (low sound area) is directly output from the reproducing speaker included in the super-directivity sound system, the low sound area can be reinforced, and a realistic sound field environment can be constructed. 
     Further, the super-directivity sound system according to the fifth aspect of the invention uses the electrostatic ultrasonic transducer constructed such that the pair of electrodes have the counter electrode portions formed in the through-holes to face the vibrating membrane, that is, the counter electrode portions are disposed in the through-holes to face the vibration region of the vibrating membrane (portion of the vibrating membrane that faces the through-holes). Therefore, it is possible to increase the membrane vibration amplitude of the vibration region of the vibrating membrane. Further, when the counter electrode portions are formed in the through-holes, the diameter of the through-hole can be increased. As a result, the opening ratio can be increased, and the output sound pressure can be improved. 
     According to a sixth aspect of the invention, a display device includes an ultrasonic speaker that includes an electrostatic ultrasonic transducer and reproduces a signal sound of an audible frequency band from sound signals supplied by a sound source, the electrostatic ultrasonic transducer including a first electrode that has through-holes, a second electrode that has through-holes, and a vibrating membrane that is disposed such that the through-holes of the first electrode and the through-holes of the second electrode form a pair, interposed between a pair of electrodes composed of the first electrode and the second electrode, and having a conductive layer applied with a direct current bias voltage, the first electrode and the second electrode each having counter electrode portions formed in the through-holes to face the vibrating membrane, a modulated wave obtained by modulating a carrier wave in an ultrasonic frequency band with a signal wave in an audible frequency band being applied between the pair of electrodes, and a projection optical system that projects an image onto a projection surface. 
     The display device according to the sixth aspect of the invention that has the above-described structure uses an ultrasonic speaker that includes an electrostatic ultrasonic transducer, the electrostatic ultrasonic transducer including a first electrode that has through-holes, a second electrode that has through-holes, and a vibrating membrane that is disposed such that the through-holes of the first electrode and the through-holes of the second electrode form a pair, interposed between a pair of electrodes composed of the first electrode and the second electrode, and a conductive layer applied with a direct current bias voltage, the first electrode and the second electrode each having counter electrode portions formed in the through-holes in a state where the counter electrode portions face the vibrating membrane, a modulated wave obtained by modulating a carrier wave in an ultrasonic frequency band with a signal wave in an audible frequency band being applied between the pair of electrodes, and a projection optical system that projects an image onto a projection surface. In addition, the ultrasonic speaker reproduces the sound signal that is supplied from the sound source. 
     As a result, while the sound signal has the sufficient sound pressure and the wide band characteristic in a state where the sound pressure characteristic is improved, the sound signal can be reproduced such that the it is generated from the virtual sound source formed in the vicinity of the sound wave reflecting surface, such as the screen. Therefore, the reproducing range of the sound signal can be easily controlled. Further, it is possible to control the directivity of the sound that is radiated from the ultrasonic speaker. 
     Further, the super-directivity sound system according to the sixth aspect of the invention uses the electrostatic ultrasonic transducer constructed such that the pair of electrodes have the counter electrode portions formed in the through-holes to face the vibrating membrane, that is, the counter electrode portions are disposed in the through-holes to face the vibration region of the vibrating membrane (portion of the vibrating membrane that faces the through-holes). Therefore, it is possible to increase the membrane vibration amplitude of the vibration region of the vibrating membrane. Further, when the counter electrode portions are formed in the through-holes, the diameter of the through-hole can be increased. As a result, the opening ratio can be increased, and the output sound pressure can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, where like numbers reference like elements. 
         FIGS. 1A and 1B  are diagrams illustrating an example of a counter electrode portion of a cross-shaped bridge structure according to an embodiment of the invention. 
         FIG. 2  is a diagram illustrating an example of a counter electrode portion of a Y-shaped bridge structure according to an embodiment of the invention. 
         FIGS. 3A and 3B  are diagrams illustrating an example of a electrode that has a counter electrode portion of a cross-shaped bridge structure. 
         FIG. 4  is a diagram illustrating an example of a electrode having a counter electrode portion of a Y-shaped bridge structure. 
         FIGS. 5A and 5B  are diagrams illustrating a sectional structure of a electrode  1  shown in  FIG. 3A . 
         FIG. 6  is a diagram illustrating an assembled state of an ultrasonic transducer according to an embodiment of the invention. 
         FIGS. 7A to 7C  are diagrams illustrating a process for manufacturing a electrode of an ultrasonic transducer according to an embodiment of the invention. 
         FIGS. 8A and 8B  are diagrams illustrating specifications and evaluation conditions of a counter electrode portion. 
         FIGS. 9A and 9B  are diagrams illustrating a variation of a membrane displacement ratio of a counter electrode portion in each evaluation device. 
         FIG. 10  is a diagram illustrating an ultrasonic speaker using an ultrasonic transducer according to an embodiment of the invention. 
         FIG. 11  is a diagram illustrating a used state of a projector according to an embodiment of the invention. 
         FIGS. 12A and 12B  are diagrams illustrating an external structure of a projector shown in  FIG. 11 . 
         FIG. 13  is a diagram illustrating an electrical structure of a projector shown in  FIG. 11 . 
         FIG. 14  is a diagram illustrating a reproducing state of a reproducing signal by an ultrasonic transducer. 
         FIGS. 15A and 15B  are diagrams illustrating an example of a structure of an ultrasonic transducer according to the prior art. 
         FIGS. 16A and 16B  are diagrams illustrating an example of a push-pull type electrostatic ultrasonic transducer according to the prior art. 
         FIG. 17  is a diagram illustrating an aspect of arrangement of through-holes in a electrode. 
         FIGS. 18A to 18C  are diagrams illustrating a structure of a electrode and a process for manufacturing the electrode. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, the preferred embodiments of the invention will be described with reference to the accompanying drawings. 
       FIGS. 1A ,  1 B, and  2  are diagrams illustrating an example of a structure of a counter electrode portion of a electrode of an electrostatic ultrasonic transducer according to an embodiment of the invention, and are perspective views of a electrode base material that is a conductive member for manufacturing the electrode. 
       FIG. 1A  is a diagram illustrating a counter electrode portion according to a first embodiment of the invention, and illustrates an example of when counter electrode portions  12  having a cross-shaped bridge structure are disposed in through-holes  11  of a electrode base material  1 A that is formed of a conductor  10 . In this case, the bridge structure means a structure in which the counter electrode portions  12  build bridges between outer circumferential portions and inner portions of the through-holes  11 , when the electrodes and the vibrating membrane are assembled so as to form the ultrasonic transducer, as apparent from  FIG. 6  that shows an assembled state of the ultrasonic transducer. 
       FIG. 1B  is a diagram illustrating a counter electrode portion according to a second embodiment of the invention, and illustrates an example of when counter electrode portions  13 , which form a cross-shaped bridge structure and have a structure where a central portion thereof (central counter electrode portion  13   a ) is wider than a bridge, are disposed in through-holes  11  of a electrode base material  2 A that is formed of a conductor  10 . 
       FIG. 2  is a diagram illustrating a counter electrode portion according to a third embodiment of the invention, and illustrates an example of when counter electrode portions  14  each having a Y-shaped bridge structure are disposed in through-holes  11  of a electrode base material  3 A. 
     Since it is very difficult to form the bridge structure through mechanical processing, it is preferable that an etching process be applied to a case where the bridge structure is formed. Further, there is a restriction in the relationships between the diameter of the through-hole formed by etching and the thickness of the base material to be processed. When forming a electrode that has the thickness larger than the diameter of the through-hole, a general method is used, as shown in  FIGS. 18A to 18C . 
       FIGS. 18A to 18C  show an example of a case where a through-hole has a circular shape. However, in order to form the counter electrode portion of the electrode according to the embodiment of the invention, the bridge structure may be formed in the through-holes. This configuration is achieved through the following processes. 
     In a first process, a conductor (copper or stainless) that has the thickness smaller than the diameter of the through-hole is coated with a mask member for forming desired counter electrode portions (that is, bridge structures), and an etching process is then performed thereon. Then, a plurality of conductors having been subjected to the above-described process are prepared. 
     In a second process, the counter electrode portions of the conductors are aligned and are then laminated to have a predetermined electrode thickness. 
     In a third process, the laminator is pressed from both sides, and is subjected to a thermal compressing process or a dispersion bonding process. Then, an integral electrode is completed. 
     The above example is constructed such that the thicknesses of the counter electrode portions  12 ,  13 , and  14  are the same as the thicknesses of the electrode base materials  1 A,  2 A, and  3 A. The invention is not limited to the above-described structure. For example, the counter electrode portions having the small thicknesses may be only formed on the one surface (for example, surface facing the vibrating membrane) of the electrode base materials  1 A,  2 A, and  3 A. 
       FIGS. 3A ,  3 B, and  4  are perspective views illustrating a final form of a electrode that forms an ultrasonic transducer according to an embodiment of the invention. 
     A electrode  1  that is shown in  FIG. 3A  is obtained by forming a counter electrode forming body  15  as an insulator on one surface of the electrode base material  1 A shown in  FIG. 1A . 
     A electrode  2  that is shown in  FIG. 3B  is obtained by forming a counter electrode forming body  15  as an insulator on one surface of the electrode base material  2 A shown in  FIG. 1B . 
     A electrode  3  that is shown in  FIG. 4  is obtained by forming a counter electrode forming body  15  as an insulator on one surface of the electrode base material  3 A shown in  FIG. 2 . 
       FIGS. 5A and 5B  are diagrams illustrating a sectional structure of a electrode  1  shown in  FIG. 3A . The sectional structure diagram shown in  FIG. 5B  is a cross-sectional view taken along the line X-Y in  FIG. 5A . 
     As shown in  FIG. 5B , the counter electrode forming body  15  as the insulator is formed on an entire region of the remaining portion of the conductor  10 , except for the bridge structure that forms the counter electrode portion  12 , and thus a electrode  1  having a predetermined structure is formed. 
       FIG. 6  is a diagram illustrating an assembled state of an ultrasonic transducer according to an embodiment of the invention. In the state shown in  FIG. 6 , two electrodes  1  are disposed opposite to each other in a vertical direction, and a vibrating membrane (vibration electrode membrane)  22  is interposed between the two electrodes  1 . In this case, since the vibrating membrane  22  needs to be insulated from the electrode  1 , the vibrating membrane  22  has a metal deposition layer for a conductive layer  221  formed at a central portion of the vibrating membrane  22 , and the conductive layer  221  forms a sandwich structure in which both sides of the conductive layer  221  are coated with insulating layers  220  that are high molecular films having excellent insulating resistance. The vibrating membrane  22  is only interposed between the facing electrode forming bodies  15  of the electrodes  1  facing each other, and in the region where the counter electrode portions  12  exist, a predetermined gap is formed between the electrode and the vibrating membrane  22 . 
     As such, the ultrasonic transducer (electrostatic ultrasonic transducer) according to the embodiment of the invention includes a electrode (first electrode)  1  that has through-holes  11 , another electrode (second electrode)  1  that has through-holes  11 , and a vibrating membrane  22  that is disposed such that the through-hole  11  of the first electrode  1  and the through-hole  11  of the second electrode  1  form a pair, is interposed between a pair of electrodes composed of the first electrode  1  and the second electrode  1 , and has a conductive layer  221  applied with a direct current bias voltage, as shown in  FIG. 6 . Each of the first electrode  1  and the second electrode  1  has counter electrode portions  12  that are formed in the through-holes  11  to face the vibrating membrane  22 , and a modulated wave, which obtained by modulating a carrier wave in an ultrasonic frequency band with a signal wave in an audible frequency band, is applied between the pair of electrodes  1  and  1 . 
     By using this structure, in the through-holes, the count electrode portions are disposed to face the vibration region of the vibrating membrane (portion of the vibrating membrane that faces the through-holes). 
     As a result, it is possible to increase the membrane vibration amplitude of the vibration region of the vibrating membrane. Further, when the counter electrode portions are formed in the through-holes, the diameter of the through-hole can be increased. Therefore, the opening ratio can be increased, and the output sound pressure can be improved. 
       FIGS. 7A to 7C  are diagrams illustrating a process for manufacturing a electrode of an ultrasonic transducer according to an embodiment of the invention. Hereinafter, a process for manufacturing the electrode will be described with reference to  FIGS. 7A to 7C . 
     First, as shown in  FIG. 7A , the conductor  10  is prepared which is formed by an etching process and a bonding process and becomes a electrode base material. 
     Then, as shown in  FIG. 7B , a mask member  16  becoming a screen to form a counter electrode forming body and a liquid counter electrode forming material  15   a  are set to the conductor  10 , and the counter electrode forming material  15   a  is coated on an entire surface of the mask while the squeegee  17  is moved. 
     In this case, the effective counter electrode forming material  15   a  is a material that can be permanently constructed as the counter electrode forming body  15  and has a non-conductive property. For example, the effective counter electrode forming material  15   a  is a masking ink that is used as liquid solder resist for packaging or resist for sand blast that is generally used in a circuit board. 
     In particular, since solder resist for a flexible printed circuit board is relatively flexible (has hardness in a range of HB to 3H like a pencil), the solder resist is superior in the adhesion strength with various conductors (conductive resin or the like) including a metal, and is very effective in an interposed property of a vibration electrode membrane made of a high molecular film. 
     Then, as shown in  FIG. 7C , if the mask member  16  serving as the mask plate for forming the counter electrode is separated after the counter electrode forming material  15   a  is completely coated, a non-conductive layer (that is, counter electrode forming body  15 ) remains on the other portions excluding the counter electrode portion  12 . By drying the remaining non-conductive layer, a desired electrode is formed. 
     Next, examples of specifications of the counter electrode portion according to the embodiment of the invention, and wavelengths and effects in the specifications will be described. In theory, when the sound pressure characteristic is evaluated on the assumption that the membrane vibration is a piston vibration, if the membrane vibration amplitude is increased twice, a work volume is increased twice. As a result, the sound pressure is increased by about 6 dB. 
     In all of the counter electrode portions according to the embodiment of the invention, the maximum displacement of the membrane vibration is increased, as compared with the case according to the related art (see  FIGS. 16A and 16B ). However, since the counter electrode portion may hinder the sound wave radiation, the maximum displacement cannot be evaluated. Accordingly, with respect to a specification of each counter electrode portion, a work volume by the membrane vibration is evaluated (calculated) by the exclusion air volume (that is, exclusion volume). 
       FIGS. 8A and 8B  are diagrams illustrating an example of a specification of a counter electrode portion and an evaluation condition. Specifically, a specification of each counter electrode portion is shown in  FIG. 8A , and an evaluation condition is shown in  FIG. 8B . Further, it is assumed that an electrostatic force only acts in a region of the vibrating membrane that faces a counter electrode portion. 
     In the specifications shown in  FIG. 8A , the general specification is associated with the specification of the electrode of the shape shown in  FIG. 16B , the diameter D 2  of the counter electrode portion  26  satisfies the condition ‘f=1.5 mm’, and the diameter D 1  of the hole (through-hole  24 ) satisfies the condition ‘f=0.75 mm’. Further, in the general specification, the counter electrode portion does not exist at the central portion of the hole. 
     The first form of the invention is associated with the electrode  1  of the shape shown in  FIG. 3A . In the first form, the counter electrode portions  12  having a cross-shaped bridge structure are disposed in the holes (through-holes  11 ). In the first form, the diameter of the counter electrode (length of the bridge crossing the through-hole  11 ) is the same as the diameter (that is, f=1.5 mm) of the external shape of the holes. 
     The second form of the invention is associated with the electrode  2  of the shape shown in  FIG. 3B . In the second form, the counter electrode portions  13 , which have a cross-shaped bridge structure and have the central portions where the central counter electrode portions  13   a  having the diameter (f=0.5 mm) are formed, are disposed in the holes (through-holes  11 ). In the second form, the diameter of the counter electrode (length of the bridge crossing the through-hole  11 ) is the same as the diameter (that is, f=1.5 mm) of the external shape of the hole. 
     The third form of the invention is associated with the electrode  3  of the shape shown in  FIG. 4 . In the third form, the counter electrode portions  14  having a Y-shaped bridge structure are disposed in the holes (through-holes  11 ). In the third form, the diameter of the counter electrode (length of the bridge crossing the through-hole  11 ) is the same as the diameter (that is, f=1.5 mm) of the external shape of the hole. 
     Further, as shown in  FIG. 8B , as evaluation conditions, the thickness of the vibrating membrane, a physical property of the vibrating membrane (young&#39;s modulus, Poisson&#39;s ratio, and density), and an electrostatic force applied to the vibrating membrane are determined. 
       FIGS. 9A and 9B  are diagrams illustrating a variation of a membrane displacement ratio at each evaluation location of a counter electrode portion. Specifically, In  FIG. 9A , the distance 0 mm in the horizontal axis corresponds to a central portion (that is, the center of the vibrating membrane) of the counter electrode portion, and the distance 0.75 mm corresponds to an outer circumferential edge of the counter electrode portion. Further, the film displacement ratio indicates a ratio based on a value (=1) in the general specification (see  FIGS. 16A and 16B ). 
     Further, the evaluated result is shown in  FIG. 9B . In regards to the vibrating membrane displacement,  FIG. 9B  shows a maximum vibrating membrane displacement at each of the central portion of the vibrating membrane and the hole of the electrode. In this case, the membrane displacement of the electrode hole indicates a membrane displacement in a region close to a substantially central portion as the portion where the counter electrode portion does not hinder the sound wave radiation. Further, similar to  FIG. 9A , the vibrating membrane displacement and the exclusion volume are shown by a ratio based on the value (=1) in the general specification. 
     According to the general specification, since the central portion of the vibrating membrane corresponds to the central portion of the hole, the absolute displacement of the vibrating membrane is the same. Meanwhile, in the first, second, and third forms of the invention, the counter electrode portion exists at the central portion of the vibrating membrane. As can be understood from  FIG. 9A , the maximum amplitude of the vibrating membrane is increased, as compared with the general specification (see  FIGS. 16A and 16B ), and it is natural that the vibration amplitude is increased in the second form of the invention (see  FIG. 3B ) in which the area of the central counter electrode is large. However, since the counter electrode portion hinders the sound wave radiation, it is appropriate to compare the vibration amplitudes at the holes of the electrodes. 
     From the graph shown in  FIG. 9A , it can be understood that a value obtained by calculating the maximum displacement in the region where the counter electrode portion does not hinder the sound wave radiation is a value of an electrode hole shown in  FIG. 9B . However, even if the absolute displacement of the electrode hole is small as compared with that of the central portion of the vibrating membrane, in the case of the cross-shaped bridge, the maximum displacement is large by 10 to 20%, as compared with the general specification. Further, the maximum displacement in the Y-shaped bridge structure is the same as that of the general specification. 
     If comparing the exclusion volumes that are calculated from the membrane displacement of the electrode hole shown in  FIG. 9A  in each evaluating device and discharged from the electrode hole, in the first to third forms, the exclusion volume is increased, as compared with the general specification. In particular, in the cross-shaped bridge structure, the membrane displacement is increased twice or more in the first form of the invention (see  FIG. 3A ). As a result, the sound pressure can be increased by 6 dB or more. The reason why the sound pressure is increased is because the sound wave can be more effectively discharged by the fact that the absolute displacement of the electrode opening is large by 20% as compared with the general specification and the opening ratio is increased by about 68% (about 2.7 times) with respect to 25% in the general specification. 
     Further, an opening ratio is defined as ‘opening ratio=(through-hole area)/(counter electrode portion area+through-hole area’). For example, the opening ratio in the electrode  20  according to the related art shown in  FIG. 16B  corresponds to a ratio of an area of the through-hole  24  shown in void at a strip black part in the drawing with respect to an entire area of the inner side of an outline C of the counter electrode portion  26 . That is, the opening ratio is 25% in the example shown in  FIG. 16B . In the bridge structure according to the embodiment of the invention, for example, in each small hole in  FIG. 3A , the opening ratio in the electrode  20  corresponds to a ratio of an area of the through-hole  11  with respect to an entire area of the inner side of the outline C of the through-hole  11  (divided into four regions) shown in void at a strip black part in the drawing. 
     Further, in the third form of the invention that has the Y-shaped bridge structure (see  FIG. 4 ), the maximum displacement is increased by 2.9 times as compared with that in the general specification, and the sound pressure can be increased by about 9 dB. In this case, the maximum factor that causes the sound pressure to increase even though the membrane displacement is almost the same as that in the general specification is an opening ratio (about 78%). Specifically, this is because the opening ratio is increased to three times the opening ratio in the general specification and is increased by 10% as compared with the opening ratio in the cross-shaped bridge structure, and the sound wave is more effectively radiated. 
     From the above-described result, if the counter electrode portion has the bridge structure and the minimally required counter electrode portions are disposed in the center portion of the vibrating membrane, the effective membrane displacement can be increased, and the loss of the sound wave radiation can be minimally suppressed. As a result, it is possible to increase the exclusion volume of the air discharged due to the membrane vibration twice or more, which increases the sound pressure by 6 dB or more. 
       FIG. 10  is a diagram illustrating an example of a structure of an ultrasonic speaker that uses an electrostatic ultrasonic transducer according to an embodiment of the invention. The ultrasonic speaker subjects the ultrasonic wave referred to as a carrier wave to AM modulation by using a sound signal (audible frequency signal) and discharges the ultrasonic wave to the air, the original sound signal is reproduced in the air due to the non-linearity of the air. That is, since the sound wave is a compression wave that propagates using the air as the medium, while the modulated ultrasonic wave propagates, the dense portion and the sparse portion of the air are distinguished, and the sound speed is high in the dense portion and the sound speed is low in the sparse portion. As a result, the distortion occurs in the modulated wave, the sound wave is separated into the carrier wave (ultrasonic wave) and the audible wave (original sound signal), and a person can only hear an audible sound (original sound signal) of 20 KHz or less. This is generally referred to as a parametric array effect. 
     The ultrasonic speaker  30  shown in  FIG. 10  includes an audible frequency signal source (sound signal source)  31  that generates a signal wave of an audible frequency band, a carrier wave signal source  32  that generates a carrier wave of an ultrasonic frequency band and outputs it, a modulator  33 , a power amplifier  34 , and an electrostatic ultrasonic transducer  35  according to an embodiment of the invention. 
     In this case, in this specification, the ‘audible frequency band’ means a frequency band of less 20 KHz, and the ‘ultrasonic frequency band means a frequency band of 20 KHz or more. 
     In this structure, by the sound signal wave output from the audible frequency signal source  31 , the carrier wave of the ultrasonic frequency band output from the carrier wave signal source  32  is modulated by the modulator  33 , and the electrostatic ultrasonic transducer  35  is driven by the modulated signal amplified by the power amplifier  34 . As a result, the modulated signal is converted into a sound wave of a finite vibration amplitude level by the electrostatic ultrasonic transducer  35 , the sound wave is radiated through the medium (air), and the original audible frequency band signal sound is reproduced by the non-linearity of the medium (air). 
     In the ultrasonic speaker  30 , the electrostatic ultrasonic transducer  35  according to the embodiment of the invention is used, and it is possible to radiate the ultrasonic wave of the high sound pressure, as compared with the ultrasonic speaker according to the related art. 
     As described above, in the electrostatic ultrasonic transducer according to the embodiment of the invention, the counter electrode portion has the bridge structure, and the counter electrode portion is disposed on the central portion of the vibration region of the vibrating membrane. Further, the bridge structure is set to the cross-shaped structure or the Y-shaped structure. As a result, the counter electrode potion is disposed on the central portion of the vibration region to increase the membrane vibration amplitude, and the counter electrode portion does not hinder the ultrasonic wave radiation. In this way, the opening ratio can be increased, which improves the sound pressure. 
     Example of Structure of Super-directivity Sound System According to Embodiment of the Invention 
     Next, an electrostatic ultrasonic transducer according to an embodiment of the invention, that is, a super-directivity sound system will be described in which a speaker including a push-pull type electrostatic ultrasonic transducer is used. The ultrasonic directivity sound system includes a first electrode that has through-holes, a second electrode that has through-holes, and a vibrating membrane that is disposed such that the through-hole of the first electrode and the through-hole of the second electrode form a pair, interposed between a pair of electrodes composed of the first electrode and the second electrode, and having a conductive layer applied with a direct current bias voltage. Each of the first electrode and the second electrode has counter electrode portions that is disposed in the through-holes in a direction facing the vibrating membrane, and a modulated wave c, which is by modulating a carrier wave in an ultrasonic frequency band by a signal wave in an audible frequency band, is applied between the pair of electrodes. 
     Hereinafter, a projector that is an example of an ultrasonic directivity sound system according to an embodiment of the invention will be described. Further, the super-directivity sound system according to the embodiment of the invention is not limited to the projector, but can be widely applied to a display device that is capable of reproducing a sound and an image. 
       FIG. 11  is a diagram illustrating a used state of a projector according to an embodiment of the invention. As shown in  FIG. 11 , the projector  301  is disposed in the back of the viewer  303 , and projects an image onto a screen  302  disposed in front of the viewer  303 . The projector  301  forms a virtual sound source on a projection surface of the screen  302  by the ultra speaker mounted on the projector  301 , and reproduces the sound source. 
       FIGS. 12A and 12B  are diagrams an external structure of the projector  301 . As shown in  FIGS. 12A and 12B , the projector  301  includes a projector main body  320  that includes a projection optical system that projects an image onto a projection surface, such as a screen, ultrasonic transducers  324 A and  324 B that are capable of oscillating a sound wave in an ultrasonic frequency band, and an ultrasonic speaker that reproduces a signal in an audible frequency band from a sound signal supplied from the sound source, which are integrally constructed. In this embodiment, in order to reproduce the stereo sound signal, ultrasonic transducers  324 A and  324 B are mounted on the projector main body. The ultrasonic transducers  324 A and  324 B interpose a projector lens  331  forming a projection optical system and form ultrasonic speakers at the left and right sides. 
     Furthermore, a low sound reproducing speaker  323  is provided on the bottom surface of the projector main body  320 . Reference numeral  325  indicates a height adjusting screw that adjusts the height of a projector main body  320 , and reference numeral  326  indicates an outlet for a cooling fan. 
     Further, the projector  301  uses a push-pull type electrostatic ultrasonic transducer according to an embodiment of the invention that serves as an ultrasonic transducer forming an ultrasonic speaker, and can oscillate a sound signal in an ultrasonic wave frequency band (sound wave in an ultrasonic frequency band) with a high sound pressure. For this reason, it is possible to achieve a sound effect that is obtained in a stereo surround system or a 5.1 ch surround system by increasing a spatial reproducing range of a reproducing signal in an audible frequency band by changing a carrier frequency without requiring a large-sized sound system that is generally needed, and it is possible to achieve a projector having superior portability. 
       FIG. 13  shows an electrical structure of a projector  301 . The projector  301  includes an ultrasonic speaker that includes an operation input unit  310 , a reproducing range setting unit  312 , a reproducing range control processing unit  313 , a sound/image signal reproducing unit  314 , a carrier wave oscillating source  316 , modulators  318 A and  318   b , power amplifiers  322 A and  322 B, and electrostatic ultrasonic transducers  324 A and  324 B, bypass filters  317 A and  317 B, a low pass filter  319 , an adder  321 , a power amplifier  322 C, a low sound reproducing speaker  323 , and a projector main body  320 . Further, the electrostatic ultrasonic transducers  324 A and  324 B are the push-pull type electrostatic ultrasonic transducers according to the embodiment of the invention. 
     The projector  320  includes an image generating unit  332  that generates video, and a projection optical system  333  that projects the generated image onto a projection surface. The projector  301  includes an ultrasonic speaker and a low sound reproducing speaker  323 , and a projector main body  320 , which are integrally provided. 
     The operation input unit  310  has various functional keypads that include a numerical keypad, a numbered keypad, and a power supply keypad for turning on and off a power supply. The reproducing range setting unit  312  is constructed such that data designating a reproducing range of a reproducing signal (signal source) can be input by operating the keypads in the operation input unit  310 . In the reproducing range setting unit  312 , if the data is input, a frequency of a carrier wave defining the reproducing range of the reproducing signal is set, and is held. The reproducing range of the reproducing signal is set by designating the distance until the reproducing signal reaches from the sound wave radiating surfaces of the ultrasonic transducers  324 A and  324 B in a radial axis direction. 
     Further, the reproducing range setting unit  312  is constructed such that a carrier frequency can be set by a control signal output from the sound/image signal reproducing unit  314  according to the display contents. 
     Further, the reproducing range control processing unit  313  has a function of referring the image contents from the reproducing range setting unit  312  and controlling the carrier wave oscillating source  316  to change the carrier frequency generated by the carrier wave oscillating source  316  to become the set reproducing range. 
     For example, when the carrier frequency is set to 50 KHz as internal information of the reproducing range setting unit  312 , the reproducing range control processing unit  313  controls the carrier wave oscillating unit  316  such that the carrier wave oscillating unit  316  oscillates at a frequency of 50 KHz. 
     The reproducing range control processing unit  313  has a storage unit that stores a table indicating a relationship between the distance until the reproducing signal reaches from the sound wave radiating surfaces of the ultrasonic transducers  324 A and  324 B defining the reproducing range in a radial axis direction, and the carrier frequency. The data in the table is obtained by measuring the relationship between the carrier frequency and the reproducing signal reaching distance. 
     The reproducing range control processing unit  313  calculates a carrier wave according to the distance information set by referring to the table on the basis of the set contents of the reproducing range setting unit  312 , and controls the carrier wave oscillating source  316  so as to become the corresponding frequency. 
     The sound/image signal reproducing unit  314  is a DVD player that uses a DVD as a recording medium. The sound signal of an R channel among the reproduced sound signals is output to the modulator  318 A through the bias filter  317 A, the sound signal of the L channel is output to the modulator  318 A through the bypass filter  317 B, and the image signal is output to the image reproducing unit  332  of the projector main body  320 . 
     Further, the sound signal of the R channel and the sound signal of the L channel that are output from the sound/image signal reproducing unit  314  are synthesized by an adder  321 , and are input to the power amplifier  322 C through the low pass filter  319 . The sound/image signal reproducing unit  314  corresponds to a sound source. 
     The bypass filters  317 A and  317 B have characteristics of making the frequency components of the sound signals of the R channel and the L channel only pass through the bypass filters  317 A and  317 B. Further, the low pass filter has characteristics of making the frequency components of the sound signals of the R channel and the L channel only pass through the low pass filter. 
     Accordingly, the sound signals in the intermediate and high sound area among the sound signals of the R channel and the L channel are reproduced by the ultrasonic transducers  324 A and  324 B, and signals in the low sound area among the sound signals of the R channel and the L channel are reproduced by the low sound reproducing speaker  323 . 
     Further, the sound/image signal reproducing unit  314  is not limited to the DVD player, but may be a reproducing device that reproduces a sound signal input from the outside. Further, the sound/image signal reproducing unit  314  has a function of outputting a control signal indicating a reproducing range to a reproducing range setting unit  312 , in order to dynamically change the reproducing range of the reproducing sound such that the sound effect according to the reproduced image scene can be achieved. 
     The carrier wave oscillating source  316  has a function of generating a carrier wave of a frequency in an ultrasonic frequency band that is instructed by the reproducing range setting unit  312  and outputting it to the modulators  318 A and  318 B. 
     The modulators  318 A and  318 B have a function of subjecting the carrier wave supplied from the carrier wave oscillating unit  316  to AM modulation by using a sound signal of the audible frequency band output from the sound/image signal reproducing unit  314 , and outputting the modulated signals to the power amplifiers  322 A and  322 B, respectively. 
     The ultrasonic transducers  324 A and  324 B are driven by the modulated signals output from the modulators  318 A and  318 B through the power amplifiers  322 A and  322 B, and have a function of converting the modulated signals into sound waves having finite amplitude levels, radiating the sound waves in the medium, and reproducing the signal sources in the audible frequency band (reproducing signals). 
     The image creating unit  332  has a display such as a liquid crystal display or a plasma display panel (PDP), and a driving circuit that drives the display on the basis of the image signal output from the sound/image signal reproducing signal  314 , and creates the image that is obtained from the image signal output from the sound/image signal reproducing unit  314 . 
     The projection optical system  333  has a function of projecting the image displayed on the display onto the projection surface, such as the screen, which is disposed in front of the projector main body  320 . 
     Next, an operation of the projector  301  that has the above-described structure will be described. First, the data (distance information) that indicates the reproducing range of the reproducing signal form the operation input unit  310  when the user operates the keypad is set in the reproducing range setting unit  312 , and the reproducing instruction is given to the sound/image signal reproducing unit  314 . 
     As a result, in the reproducing range setting unit  312 , distance information defining the reproducing range is set. The reproducing range setting unit  313  receives the distance information set to the reproducing range setting unit  312 , refers to the data stored in the storage unit incorporated in the reproducing range setting unit  312 , calculates the carrier wave corresponding to the set distance information, controls the carrier wave oscillating unit  316  to generate the carrier wave of the corresponding frequency. 
     As a result, carrier wave oscillating source  316  generates a plurality of carrier waves corresponding to the distance information set to the reproducing range setting unit  312 , and outputs them to the modulators  318 A and  318 B. 
     Meanwhile, the sound/image signal generating unit  314  outputs the sound signal of the R channel among the reproduced sound signal to the modulator  318 A through the bypass filter  317 A, outputs the sound signal of the L channel to the modulator  318 B through the bypass filter  317 B, outputs the sound signal of the R channel and the sound signal of the L channel to the adder  321 , and outputs the image signal to the image creating unit  332  of the projector main body  320 . 
     Accordingly, the sound signals of the intermediate and high sound areas among the sound signals of the R channel are input to the modulator  318  by the bypass filter  317 A, and the sound signals of the intermediate and high sound areas among the sound signals of the L channel are input to the modulator  318 B by the bypass filter  317 B. 
     Further, the sound signal of the R channel and the sound signal of the L channel are synthesized by the adder  321 , and the sound signals of the low sound area among the sound signal of the R channel and the sound signal of the L channel are input to the power amplifier  322 C by the low pass filter  319 . 
     The image creating unit  332  drives the display on the basis of the input image signal, and creates and displays the video. The image that is displayed on the display is projected onto the projection surface, that is, the screen  302  shown in  FIG. 11 , by means of the projection optical system  333 . 
     Meanwhile, the modulator  318 A subjects the carrier wave output from the carrier wave oscillating source  316  to AM modulation by using the sound signal of the intermediate and high sound area in the sound signals of the R channel output from the bypass filter  317 A, and outputs it to the power amplifier  322 A. 
     Further, the modulator  318 B subjects the carrier wave output from the carrier wave oscillating source  316  to AM modulation by using the sound signal of the intermediate and high sound area in the sound signals of the L channel output from the bypass filter  317 B, and outputs it to the power amplifier  322 B. 
     The modulated signals that are amplified by the power amplifiers  322 A and  322 B are applied between the upper electrode  10 A and the lower electrode  10 B (see  FIGS. 1A and 1B ) in each of the ultrasonic transducers  324 A and  324 B. As a result, the modulated signals are converted into the sound waves having finite amplitude levels (sound signals), and are radiated to the medium (air). From the ultrasonic transducer  324 A, the sound signals of the intermediate and high sound area in the sound signals of the R channel are reproduced, and from the ultrasonic transducer  324 B, the sound signals of the intermediate and high sound area in the sound signals of the L channel are reproduced. 
     Further, the sound signals of the low sound area in the R channel and the L channel amplified by the power amplifier  322 C are reproduced by the low sound reproducing speaker  323 . 
     As described above, when propagating the ultrasonic wave that is radiated into the medium (air) by the ultrasonic transducer, as the ultrasonic wave propagates, a sound speed is high in the portion having the high sound pressure, and the sound speed is delayed in the low sound pressure. As a result, the distortion is generated in the waveform. 
     When the radiated signal of the ultrasonic wave band (carrier wave) is converted into the signal of the audible frequency band (AM modulation), according to the result of the waveform distortion, the signal wave of the audible frequency band used at the time of modulation is formed in the form where is separated from the carrier wave of the ultrasonic frequency band and is then demodulated. At this time, the expansion of the reproducing signal becomes a beam shape due to the characteristic of the ultrasonic wave, and the sound is generated in a specific direction that is completely different from the general speaker. 
     The reproducing signal that is output from the ultrasonic transducer  324  forming the ultrasonic speaker and has a beam shape is radiated toward the projection surface (screen) onto which the image is projected by the projection optical system  333 , is reflected on the projection surface, and is propagated. At this time, in accordance with the frequency of the carrier wave that is set to the reproducing range setting unit  312 , the distance until the reproducing signal is separated from the carrier wave from the sound wave radiating surface of the ultrasonic transducer  324  in a radial axis direction (normal direction), and the beam width (expansion angle of the beam) of the carrier wave are different. Therefore, the reproducing range is varied. 
       FIG. 14  shows a state at the time of reproducing a reproducing signal by an ultrasonic speaker that is constructed by including ultrasonic transducers  324 A and  324 B in a projector  301 . In the projector  301 , when the ultrasonic transducer is driven by a modulated signal obtained by modulating the carrier wave by the sound signal, in the case where the carrier frequency set by the reproducing range setting unit  312  is low, the distance until the reproducing signal is separated from the carrier wave from the sound wave radiating surface of the ultrasonic transducer  324  in the radial axis direction (normal direction of the ultrasonic radiating surface), that is, the distance to the reproducing point is reduced. 
     Accordingly, the beam of the reproducing signal of the audible frequency band that has been reproduced reaches the projection surface (screen)  302  without being diffused, and in this state, it is reflected on the projection surface  302 . Therefore, the reproducing range becomes an audible range A shown by a dot-line arrow in  FIG. 14 , and the reproducing sound (reproducing signal) can only hear in a narrow range that is relatively distant from the projection surface  302 . 
     Meanwhile, when the carrier frequency set by the reproducing range setting unit  312  is higher than the carrier frequency in the above-described case, the sound wave radiated from the ultrasonic radiating surface of the ultrasonic transducer  324  is narrowed more than the case where the carrier frequency is low. However, the distance until the reproducing signal is separated from the carrier wave from the sound wave radiating surface of the ultrasonic transducer  324  in a radial axis direction (normal direction of the sound wave radiating surface), that is, the distance to the reproducing point is reduced. 
     Accordingly, the beam of the reproducing signal of the audible frequency band that has been reproduced reaches the projection surface  302  with diffused before reaching the projection surface  302 , and in this state, it is reflected on the projection surface  302 . Therefore, the reproducing range becomes an audible range B shown by a solid-line arrow in  FIG. 14 , and the reproducing sound (reproducing signal) can only hear in a wide range that is relatively close from the projection surface  302 . 
     As described above, the projector according to the embodiment of the invention uses the ultrasonic speaker using the push-pull type or pull type electrostatic ultrasonic transducer. In the projector, the sound signal has the sufficient sound pressure and the wide band characteristic, and can be reproduced such that it is emitted from the virtual sound source formed in the vicinity of the sound wave reflecting surface, such as the screen. 
     For this reason, the reproducing range can be easily controlled. Further, the electrostatic ultrasonic transducer is controlled such that the vibration region of the vibrating membrane is divided into a plurality of blocks, and a phase of the alternating current signal applied between the electrode layer of the vibrating membrane and each block of the electrode pattern for vibration is allowed to have a predetermined phase difference between neighboring blocks. In this way, it is possible to control the directivity of the sound that is radiated from the ultrasonic speaker. 
     Further, the projector according to the embodiment of the invention uses the push-pull type electrostatic ultrasonic transducer constructed such that the pair of electrodes have the counter electrode portions with respect to the central portion of the vibration region of the vibrating membrane or the peripheral portion of the central portion, that is, the counter electrode portions are disposed in the through-holes to face the vibration region of the vibrating membrane (portion of the vibrating membrane facing the through-holes). Therefore, it is possible to increase the membrane vibration amplitude of the vibration region of the vibrating membrane. Further, the counter electrode portions are formed in the through-holes, and thus the diameter of the through-hole can be increased. As a result, the opening ratio can be increased, and the output sound pressure can be improved. Accordingly, a strong ultrasonic wave can be generated over the wide frequency band, and the sound quality of the reproducing sound can be improved. 
     The preferred embodiment of the invention has been described. However, the electrostatic ultrasonic transducer and the ultrasonic speaker according to the embodiment of the invention are not limited to the above-described example, and various changes and modifications can be made without departing from the spirit and scope of the invention. 
     The preferred embodiment of the invention has been described. However, the electrostatic ultrasonic transducer according to the embodiment of the invention is not limited to the above-described example, and various changes and modifications can be made without departing from the spirit and scope of the invention. 
     The entire disclosure of Japanese Patent Application Nos: 2006-043484, filed Feb. 21, 2006 and 2007-018182, filed Jan. 29, 2007 are expressly incorporated by reference herein.