Abstract:
A sound-generating device comprises at least two first enclosures and a thin film. The at least two first enclosures with at least one first bendable element coupled between two neighboring first enclosures. The thin film comprising at least one electrode and at least one piezoelectric layer, the at least one electrode being coupled with a terminal of an audio signal output, wherein the at least one piezoelectric layer is configured to respond to a signal supplied by the audio signal output and to generate sound waves. The thin film and the at least two first enclosures are coupled together forming at least two first cavities between the thin film and the first enclosure, and the first bendable element is attached to the thin film.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of and claims priority from U.S. patent application Ser. No. 12/169,569, filed Jul. 8, 2008, the disclosure of which is hereby incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates to sound-generating devices, and more particularly, to flexible piezoelectric loudspeakers. 
     BACKGROUND 
     In the recent years, there have been continued developments for electronic products. One design concept has been providing lightweight, thin, portable and/or small devices. In this regard, flexible electronic technology has been increasingly used in various applications, such as thin-screen displays, LCDs, flexible circuits and flexible solar cells. Applications for flexible electronics, such as flexible speakers, may benefit from their low profile, reduced weight, and/or low manufacturing cost. 
     A loudspeaker may produce sound by converting electrical signals from an audio signal source into mechanical motions. Moving-coil speakers are widely used currently, which may produce sound from the back-and-forth motion of a cone that is attached to a coil of wire suspended in or movably coupled with a magnetic field. A current flowing through the coil may induce a varying magnetic field around the coil. The interaction of the two magnetic fields causes relative movements of the coil, thereby moving the cone back and forth, which compresses and decompresses the air, and thus generates sound waves. Due to structural limitations, moving-coil speakers are less likely to be made flexible or in a low profile. 
     Flexible piezoelectric loudspeakers, such as piezoelectric polyvinylidene fluoride speakers, may be made of flexible polymer materials. With electric polarization, the flexible polymer material may possess characteristics of permanent polarization and resistance to environmental conditions. Thus, such flexible polymers are suitable for being fabricated as loudspeakers. 
     U.S. Pat. No. 4,638,207 relates to a piezoelectric balloon speaker with a piezoelectric polymer film. The inflated balloon may provide tension for the piezoelectric polymer film. In addition, the resonance frequency may be adjustable by the pressure applied to the balloon. However, such a speaker may not be fabricated as a low-profile flexible loudspeaker. U.S. Pat. No. 6,504,289 relates to a piezoelectric transducer for transmitting acoustic energy. The transducer is enclosed in a rigid enclosure and thus is not flexible at all. U.S. Pat. No. 6,349,141 relates to a flexible audio transducer with a balloon structure. The balloon structure may result in some issues on structure strength and designs relating to resonance frequency. U.S. Pat. No. 6,717,337 relates to an acoustic actuator with a piezoelectric drive element made of piezoelectric ceramics in the lead zirconate titanate (PZT) or PZT derivatives. In response to the radial contraction and expansion of the piezoelectric drive element, an acoustic diaphragm may vibrate to generate sound waves. The piezoelectric ceramics however are vulnerable and susceptible to fragmentation. 
     SUMMARY 
     One example consistent with the invention provides a sound-generating device comprises at least two enclosures with at least one bendable element coupled between two neighboring enclosures and a thin film comprising at least one electrode and at least one piezoelectric layer. The at least one electrode is coupled with a terminal of an audio signal output. The at least one piezoelectric layer is configured to respond to a signal supplied by a signal input and to generate sound waves. The thin film and the at least two first enclosures are coupled together forming at least two first cavities between the thin film and the first enclosure, and the first bendable element is attached to the thin film. 
     One example consistent with the invention provides a sound-generating device comprises at least two first enclosures with at least one first bendable element coupled between two neighboring first enclosures; at least two second enclosures with at least one second bendable element coupled between two neighboring second enclosures; and a thin film. The thin film comprises at least one electrode and at least one piezoelectric layer, the at least one electrode is coupled with a terminal of an audio signal output. The at least one piezoelectric layer is configured to respond to a signal supplied by the audio signal output and to generate sound waves. The thin film and the at least two first enclosures are coupled together forming at least two first cavities between the thin film and the first enclosure, and the first bendable element is attached to the thin film. The thin film and the at least two second enclosures are coupled together forming at least two second cavities between the thin film and the second enclosure, and the second bendable element is attached to the thin film. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended, exemplary drawings. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
       In the drawings: 
         FIG. 1  is a sectional view of an exemplary flexible piezoelectric loudspeaker in examples consistent with the present invention; 
         FIG. 2  is a detailed sectional view of portions of an exemplary flexible piezoelectric loudspeaker in examples consistent with the present invention; 
         FIG. 3  is a sectional view of an exemplary flexible piezoelectric loudspeaker in examples consistent with the present invention; 
         FIG. 4  is a sectional view of an exemplary flexible piezoelectric loudspeaker in examples consistent with the present invention; 
         FIG. 5  is a top view of an exemplary application of an exemplary flexible piezoelectric loudspeaker in examples consistent with the present invention; 
         FIG. 6  is a top view of an exemplary application of an exemplary flexible piezoelectric loudspeaker in examples consistent with the present invention; 
         FIG. 7  is a sectional view of an exemplary piezoelectric diaphragm in examples consistent with the present invention; and 
         FIG. 8  is a sectional view of an exemplary piezoelectric diaphragm in examples consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary flexible piezoelectric loudspeaker in examples consistent with the present invention. The flexible piezoelectric loudspeaker of  FIG. 1  may include a number of enclosures  40 , a number of bendable elements  41 , a thin film  45  and a driving circuit  100  with two terminals  101  and  102 . 
       FIG. 2  shows details of the enclosures  40  and the bendable elements  41 . The enclosures  40  and bendable elements  41  may be fabricated by pressing, thermal pressing, vacuum compression, injection molding or a roll-to-roll process. The enclosures  40  may be in a circular, rectangular, or polygonal shape. As shown in  FIG. 1 , the enclosures  40  and the substrate  45  may provide a cavity  46 . The rigidity of the enclosures  40  may be substantially hard to form the enclosures. The bendable elements  41  with flexural rigidity may be provided over the substrate  45  as shown in  FIG. 1 . 
     The enclosures  40  and the bendable elements  41  may comprise a flexible layer  4  and a piezoelectric structure  3 . The flexible layer  4  may be provided over the piezoelectric structure  3  by a process, such as ultrasound pressing, thermal pressing, mechanical press, gluing or a roll-to-roll pressing process. The flexible layer  4  may be a transparent material. The flexible layer  4  may be made of plastic materials with plasticity, blended fibers or thin metal plates. The thickness of the flexible layer  4  may be in a range of 10 micrometers and 10000 micrometers. The flexible layer  4  may provide different thicknesses for the bendable element  41  and the enclosures  40 . The flexible layer  4  may be formed by a process, such as thermal molding, injection molding, pressing or a roll-to-roll molding process. The piezoelectric structure  3  may include a first electrode  31 , a second electrode  32  and a piezoelectric layer  30  sandwiched between the first and second electrodes  31  and  32 . The piezoelectric layer  30  may be a transparent material. The piezoelectric layer  30  may be made of materials in polyvinylidene difluoride (PVDF) or PVDF derivatives. In one example, the piezoelectric layer  30  may be made of poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) or poly(vinylidene fluoride/tetrafluoroethylene) (P(VDF-TeFE)). In another example, the piezoelectric layer  30  may be made of a blend of a material in PVDF or PVDF derivatives and at least one of lead zirconate titanate (PZT) fibers or particles, polymethylmethacrylate (PMMA), or poly(vinyl chloride) (PVC). These materials may be first processed by spray molding, injection molding, a roll-to-roll pressing process or thermal molding to form a processed material. A piezoelectric layer  30  may be formed by uniaxial tensile and corona discharge on the processed material. The thickness of the piezoelectric layer  30  may be in a range of 0.1 micrometers to 3000 micrometers. The electrodes  31  and  32  may be a transparent material. The electrodes  31  and  32  made of gold, silver, aluminum, copper, chromium, platinum, indium tin oxide, silver gel, copper gel or other conductive materials, may be coated on both surfaces of the piezoelectric layer  30  by sputtering, evaporation, spin-coating or screen-printing. The thickness of the electrode  31  and  32  may be in a range of 0.01 micrometers to 100 micrometers. 
     With respect to fabrication of a flexible piezoelectric loudspeaker, the enclosures  40  are provided over the thin film  45  by a roll-to-roll pressing process or a vertical pressing process so that the bendable elements  41  may be in contact with the thin film  45 . In one example, the bendable elements  41  may be affixed to the thin film  45  by thermal pressing, ultrasound pressing, or mechanical press. Alternatively, the bendable elements  41  may be affixed to the thin film  45  by an adhesive element, such as a double-sided adhesive tape, epoxy resin or instant adhesive glues. The first enclosures  40  and the bendable elements  41  on the thin film  45  may constitute one unit  42  (shown in  FIG. 5 ) of a flexible piezoelectric loudspeaker. A number of these units arranged together may constitute a flexible piezoelectric loudspeaker as shown in  FIG. 5 . 
     In operation of a flexible piezoelectric loudspeaker of  FIG. 1 , the terminal  101  of the driving circuit  100  may output an audio signal to the first electrode  31 . The second terminal  102  may be connected to ground and the second electrode  32 . According to the piezoelectric constitutive equation,
 
 S   p   =s   pq   E   T   q   +d   pj   E   j ,
 
where
 
     
       
         
           
             
               d 
               pj 
             
             = 
             
               [ 
               
                 
                   
                     0 
                   
                   
                     0 
                   
                   
                     0 
                   
                   
                     0 
                   
                   
                     
                       d 
                       15 
                       + 
                     
                   
                   
                     0 
                   
                 
                 
                   
                     0 
                   
                   
                     0 
                   
                   
                     0 
                   
                   
                     
                       d 
                       24 
                       + 
                     
                   
                   
                     0 
                   
                   
                     0 
                   
                 
                 
                   
                     
                       d 
                       31 
                       + 
                     
                   
                   
                     
                       d 
                       32 
                       + 
                     
                   
                   
                     
                       d 
                       33 
                       - 
                     
                   
                   
                     0 
                   
                   
                     0 
                   
                   
                     0 
                   
                 
               
               ] 
             
           
         
       
       
         
           
             
               and 
               ⁢ 
               
                   
               
               ⁢ 
               
                 E 
                 j 
               
             
             = 
             
               
                 [ 
                 
                   
                     
                       0 
                     
                   
                   
                     
                       0 
                     
                   
                   
                     
                       
                         E 
                         3 
                         - 
                       
                     
                   
                 
                 ] 
               
               . 
             
           
         
       
     
     According to the equation, when a voltage is applied to the electrodes, it changes thickness and length of the piezoelectric layer. The change of its thickness may be very small but the change in its length may be significant. These changes may cause contraction and expansion of the piezoelectric layer. As such, the air is compressed and decompressed to generate sound waves. 
       FIG. 3  illustrates an exemplary flexible piezoelectric loudspeaker in examples consistent with the present invention. In this example, the flexible piezoelectric loudspeaker may include a number of first enclosures  40   a , first bendable elements  41   a , second enclosures  40   b , and second bendable elements  41   b . These elements may have the same structure as the enclosures  40  and the bendable elements  41  described above in connection with  FIGS. 1 and 2 , and thus, these elements and their detailed structure will not be repeated here. 
     The enclosures  40   a  and  40   b , and the bendable elements  41   a  and  41   b  may provide a cavity  47  shown in  FIG. 3 . The first enclosures  40   a  may be provided over the second enclosures  40   b  by a roll-to-roll pressing process or a vertical pressing process. The first bendable elements  41   a  may be affixed to the second bendable elements  41   b  by, for example, thermal pressing, ultrasound pressing, or mechanical press. Alternatively, the first bendable elements  41   a  may be affixed to the second bendable elements  41   b  by an adhesive element such as a double-sided adhesive tape, epoxy resin or instant adhesive glues. 
     The driving circuit  100   a  may have a first terminal  103 , a second terminal  104  and a third terminal  105 . In operation of a flexible piezoelectric loudspeaker of  FIG. 3 , the terminal  103  may output a signal to the first electrode  31   a  of the first enclosures  40   a . The terminal  105  may output a signal having the same phase as the signal from the terminal  103  to the first electrode  31   b  of the second enclosures  40   b . The terminal  104  may connected to ground, the second electrode  32   a  of the first enclosures  40   a  and the second electrode  32   b  of the second enclosures  40   b . According to the piezoelectric constitutive equation above, when a voltage is applied to the electrodes, it changes thickness and length of the piezoelectric layer. The change of its thickness may be very small but the change in its length may be significant. These changes may cause contraction and expansion of the piezoelectric layer. As such, the air is compressed and decompressed to generate sound waves. 
       FIG. 4  illustrates a piezoelectric loudspeaker in examples consistent with the present invention. The piezoelectric loudspeakers may include a number of first enclosures  400   a , first bendable elements  410   a , second enclosures  400   b  and second bendable elements  410   b , a piezoelectric diaphragm  35  and a driving circuit  100   b . The first enclosures  400   a , the second enclosures  410   a  and the piezoelectric diaphragm  35  may provide cavities  50   a  and  50   b.    
     The first and second enclosures  400   a  and  400   b  and the first and second bendable elements  410   a  and  410   b  may be made of plastic materials with plasticity, blended fibers or thin metal plates. They may be formed by a process, such as thermal molding, injection molding, vacuum molding, pressing or a roll-to-roll molding process. The first enclosures  400   a  may comprise a number of openings, such as acoustic holes  51   a . The second enclosures  400   b  may comprise a number of acoustic holes  51   b . The first and second enclosures  400   a  and  400   b  may be in a circular, rectangular, polygonal shape. The rigidity of the first and second enclosures  400   a ; and  400   b  may be substantial hard to form the enclosures. The first and second bendable elements  410   a  and  410   b  with flexural rigidity may be provided over each side of the piezoelectric diaphragm  35 . 
       FIG. 7  shows a piezoelectric diaphragm  35  in examples consistent with the present invention. The piezoelectric diaphragm  35  may comprise a first electrode  351 , a second electrode  352  and a piezoelectric layer  350  placed between the first and second electrodes  351  and  352 . The piezoelectric layer  350  may be made of materials in polyvinylidene difluoride (PVDF) or PVDF derivatives. In one example, the piezoelectric layer  350  may be made of P(VDF-TrFE) or P(VDF-TeFE). In another example, the piezoelectric layer  350  may be made of a blend of a material in PVDF or PVDF derivatives and at least one of lead zirconate titanate (PZT) fiber or particles, polymethylmethacrylate (PMMA), or poly(vinyl chloride (PVC). These materials may be first processed by spray molding, injection molding, a roll-to-roll pressing process or thermal molding to form a processed material. A piezoelectric layer  350  may be formed by uniaxial tensile and corona discharge on the processed material. The electrodes  351  and  352  made of gold, silver, aluminum, copper, chromium, platinum, indium tin oxide, silver gel, copper gel or other conductive materials, may be coated on both surfaces of the piezoelectric layer  350  by sputtering, evaporation, spin-coating or screen-printing. 
     With respect to fabrication of a flexible piezoelectric loudspeaker of  FIG. 4 , the piezoelectric diaphragm  35  may be provided between first enclosures  400   a  and the second enclosures  400   b  by a roll-to-roll pressing process or a vertical pressing process. In one example, the bendable elements  410   a  and  410   b  may be affixed to the diaphragm  35  by thermal pressing, ultrasound pressing, and mechanical pressing. Alternatively, the bendable elements  410   a  and  410   b  may be affixed to the diaphragm  35  by an adhesive element, such as a double-sided adhesive tape, epoxy resin or instant adhesive glues. The assembly of the enclosures  400   a  and  400   b , the bendable elements  410   a  and  410   b , and the diaphragm  35  may constitute one unit  420  (shown in  FIG. 6 ) of a flexible piezoelectric loudspeaker. A number of these units arranged together may constitute a flexible piezoelectric loudspeaker as shown in  FIG. 6 . 
     The driver circuit  100   b  may include a first terminal  101   b  and a second terminal  102   b . In operation of a flexible piezoelectric loudspeaker of  FIG. 4 , the terminal  101   b  of the driving circuit  100   b  may output an audio signal to the first electrode  351 . The terminal  102   b  may be connected to ground and the second electrode  352 . According to the piezoelectric constitutive equation, when a voltage is applied to the electrodes, it may cause the piezoelectric diaphragm  35  to vibrate, thus generating sound waves. In addition, the cavities  50   a  and  50   b  may be designed in accordance with the Helmholtz equation to adjust the resonance frequency and increase the efficient of the loudspeaker. 
       FIG. 8  shows an exemplary piezoelectric diaphragm  36  in examples consistent with the present invention. The piezoelectric diaphragm  36  may have a bimorph structure. In one example, the diaphragm  36  may include a first electrode  362 , a second electrode  363 , a third electrode  364 , a first piezoelectric layer  360  and a second piezoelectric layer  361 . The polarization directions of the two piezoelectric layers  360  and  361  may be opposite to each other. An exemplary flexible piezoelectric loudspeaker may be made in the same way as the one of  FIG. 4  with a piezoelectric diaphragm  36  replacing the diaphragm  35  of  FIG. 4 . A flexible piezoelectric loudspeaker with a diaphragm in a bimorph structure may include a driving circuit  100   c  with three terminals  103   c ,  104   c  and  105   c . In operation, the terminal  103   c  may output a signal to the first electrode  362 . The terminal  105   c  may output a signal having the same phase as the signal from the terminal  103   c  to the third electrode  364 . The terminal  104   c  may be connected to ground and the second electrode  363 . According to the piezoelectric constitutive equation above, a voltage applied to the electrodes may cause the diaphragm  36  to vibrate, and thus generating sound waves. 
     It will be appreciated by those skilled in the art that changes could be made to the examples described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular examples disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.