Abstract:
A piezoelectric loudspeaker includes: a diaphragm; a first piezoelectric material provided in a first area of the diaphragm; and a second piezoelectric material provided in a second area of the diaphragm different from the first area. The second area has a sound reproduction band different from that of the first area. This piezoelectric loudspeaker has a wide reproduction frequency range.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a piezoelectric loudspeaker employing a piezoelectric material, a loudspeaker system using the loudspeaker, and an electronic device using the loudspeaker.  
       PRIOR ART  
       [0002]     A conventional piezoelectric loudspeaker employing a piezoelectric material disclosed in Japanese Laid-Open Publication No. 11-164396 includes a single diaphragm and a single piezoelectric material provided at the diaphragm.  
         [0003]     Since having the single diaphragm and piezoelectric material, the conventional piezoelectric loudspeaker has difficulty in reproducing of a sound in wide frequency range. Specifically, this loudspeaker having the piezoelectric material vibrating by deforming causes the piezoelectric material to have a high Q factor, hence having a narrow reproducing frequency range.  
       SUMMARY OF THE INVENTION  
       [0004]     A piezoelectric loudspeaker includes: a diaphragm; a first piezoelectric material provided in a first area of the diaphragm; and a second piezoelectric material provided in a second area of the diaphragm different from the first area. The second area has a sound reproduction band different from that of the first area. This piezoelectric loudspeaker has a wide reproduction frequency range. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a perspective view of a loudspeaker box according to an exemplary embodiment of the present invention.  
         [0006]      FIG. 2  is a perspective view of a tweeter according to the embodiment.  
         [0007]      FIG. 3  shows a sound pressure frequency characteristic of the tweeter according to the embodiment.  
         [0008]      FIG. 4  is a cross-sectional view of the tweeter shown in  FIG. 2  taken at line  4 - 4 .  
         [0009]      FIG. 5  is a block diagram of an electronic device according to the embodiment.  
         [0010]      FIG. 6  is a perspective view of another tweeter box according to the embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0011]      FIG. 1  is a perspective view of a loudspeaker system according to an exemplary embodiment of the present invention. A front face of loudspeaker box  1  having a rectangular shape has tweeter  2 , squawker  3 , woofer  4 , and bass-reflex port  5  provided from the upper direction to the lower direction. Tweeter  2  has a sound reproduction frequency range, such as 5 kHz to 100 kHz. Squawker  3  has a sound reproduction frequency range from 500 Hz to 5 kHz. Woofer  4  has a sound reproduction frequency range from 20 Hz to 500 Hz. Bass-reflex port  5  emphasizes a portion lower than 100 Hz in the reproduction range of woofer  4 .  
         [0012]      FIG. 2  is a perspective view of tweeter  2  according to the embodiment.  FIG. 3  shows a sound pressure frequency characteristic of tweeter  2 . Tweeter  2  includes tweeter box  6  and a single diaphragm  7  exposed at the front face of the tweeter. As shown in  FIG. 2 , diaphragm  7  includes plural areas  8 , plural areas  9  smaller than areas  8 , and plural areas  10  smaller than areas  9 . As shown in  FIG. 3 , area  8  has characteristic  108  having a sound reproduction frequency range from 5 kHz to 80 kHz. Area  9  has characteristic  109  having a sound reproduction frequency range from 10 kHz to 100 kHz. Area  10  has characteristic  110  having a sound reproduction frequency range from 40 kHz to 100 kHz. Areas  8 ,  9 , and  10  compose tweeter  2  having characteristic  102  of a range from 5 kHz to 100 kHz as the combination of the sound frequency ranges.  
         [0013]      FIG. 4  is a cross-sectional view of diaphragm  7  of tweeter  2  shown in  FIG. 2  taken at line  4 - 4 . Areas  8  to  10  are provided on diaphragm  7 . Diaphragm  7  is made of SiO 2  and has a thickness of 30000 Å. A back face of diaphragm  7  has base  11  that is made of Si and has a thickness of 500 μm. Base  11 , a frame body provided around openings  8   a  to  10   a , has openings  8   a  to  10   a  corresponding to areas  8  to  10 , respectively (opening  8   a  is not shown). Openings  8   a  to  10   a  have respective areas corresponding to areas  8  to  10  so that opening  9   a  is smaller than opening  8   a  while opening  10   a  is smaller than opening  9   a . The base as the frame body provides areas  8  to  10  with sound reproduction frequency ranges different from each other easily.  
         [0014]     Diaphragm  7  has lower electrodes  12  made of platinum thereon. Lower electrodes  12  corresponding to openings  8   a  to  10   a  has thereon piezoelectric thin film  14  via buffer layer  13 . Piezoelectric thin film  14  is ceramic of mixture of lead titanate and lead zirconate consisting of PZT. Lower electrodes  12  around piezoelectric thin films  14  have thereon insulating films  15  made of resin on which upper electrodes  16  are provided, respectively. Piezoelectric thin films  14  may be provided on diaphragm  7  at once by a piezoelectric-thin-film-forming process.  
         [0015]      FIG. 5  is a block diagram of an electronic device according to the embodiment. As shown in  FIG. 5 , piezoelectric thin films  14  corresponding to areas  8  to  10 , respectively, are fed with sound source signals via upper electrodes  16 . Sound source  17  is connected with amplifier  18  and amplifier  18  is connected to piezoelectric thin films  14  of areas  8  to  10  in parallel to each other. Piezoelectric thin films  14  of areas  8  to  10  and amplifier  18  have protection circuits  19   a  to  19   c  for preventing over-currents between thisn films and the amplifier. Phase controllers  20   a  to  20   c  control phases of signals applied to areas  8  to  10 , respectively. Gain adjustment circuits  21   a  to  21   c  adjust the amplitudes of signals applied to areas  8  to  10 , respectively. This structure provides tweeter  2  with a flat sound pressure frequency characteristic shown in characteristic  102  of  FIG. 3  in a wide and high frequency range from 5 kHz to 100 kHz.  
         [0016]     Sounds in a nature include frequency components higher than 20 kHz, which human beings cannot hear. For example, a musical instrument, such as a cymbal, emits a sound having a component higher than 20 kHz. Human beings hear a sound from 20 Hz to 20 kHz out of a combination and interference of such sounds having such high frequency components.  
         [0017]     Therefore, tweeter  2  of the embodiment reproducing a sound from 5 kHz to 100 kHz can reproduce sounds more naturally. Thus, it is recently said that sound source  17 , such as an audio device, needs to output a signal having a frequency up to 100 kHz.  
         [0018]      FIG. 6  shows another tweeter  602  according to the embodiment. Tweeter  602  is of a so-called add-on-type for emphasizing a treble added to an existing electronic device. Tweeter  602  includes therein protection circuits  19   a  to  19   c , phase controllers  20   a  to  20   c , and gain adjustment circuits  21   a  to  21   c  shown in  FIG. 5  and has at the back face side a connection terminal for the connection to amplifier  18 .  
         [0019]     Tweeter  2  as a piezoelectric loudspeaker according to the embodiment has areas  8  to  10  having sizes different from each other. The number of the areas different from each other is not limited to three and thus may be two or more, hence providing the same effects as those of the loudspeaker according to the embodiment.  
       INDUSTRIAL APPLICABILITY  
       [0020]     As described above, the piezoelectric loudspeaker according to the present invention has a wide reproduction frequency range.