Patent Publication Number: US-11399230-B2

Title: Electroacoustic transducer

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application of International Application No. PCT/JP2019/044724, filed on Nov. 14, 2019, which claims priority to Japanese Patent Application No. 2018-223178, which was filed on Nov. 29, 2018. The contents of these applications are incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The following disclosure relates to an electroacoustic transducer such as a speaker, an earphone, and headphones. 
     An electroacoustic transducer includes a diaphragm that vibrates in accordance with an externally applied sound signal (an electric signal representing a sound waveform) to output a sound wave based on the sound signal. For instance, there is an earphone that includes an electromagnetic tweeter including a piezoelectric element as the diaphragm and a dynamic woofer. In the earphone, sounds output from the tweeter and sounds output from the woofer are output from the same sound emitting portion. 
     SUMMARY 
     There has been proposed using, as the diaphragm for the speaker, a piezoelectric element that includes a porous film and a pair of electrodes sandwiching the porous film. In such a piezoelectric element, the porous film expands or contracts in its thickness direction in accordance with a voltage applied between the electrodes, so that the piezoelectric element vibrates. In the speaker including the piezoelectric element, sound waves are emitted from both surfaces of the diaphragm depending on how the diaphragm is disposed. The conventional speakers, however, utilize only the sound wave emitted from one surface of the diaphragm. 
     Accordingly, one aspect of the present disclosure is directed to a technique of enabling effective utilization of sound waves respectively emitted from opposite surfaces of a diaphragm in an electroacoustic transducer in which a piezoelectric element is used as the diaphragm. 
     In one aspect of the present disclosure, an electroacoustic transducer includes: a housing; a piezoelectric element disposed in the housing and including a porous film and a pair of electrodes sandwiching the porous film therebetween; a partition wall dividing an inner space of the housing into a first space closer to one of the pair of electrodes and a second space closer to the other of the pair of electrodes; a first tube that establishes communication between a sound wave emission opening that is open to an outer space of the housing and the first space; and a second tube that establishes communication between the sound wave emission opening and the second space. 
     Other objects, features, advantages, as well as the technical and industrial significance of the present disclosure will be better understood by reading the following detailed description of embodiments, when considered in connection with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an inventive earphone; 
         FIG. 2  is a cross-sectional view of the earphone of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the earphone of  FIG. 1 ; 
         FIG. 4  is a cross-sectional view of an inventive earphone; 
         FIG. 5  is a cross-sectional view of an inventive earphone; 
         FIG. 6  is a cross-sectional view of an inventive earphone; 
         FIG. 7  is a cross-sectional view of an inventive earphone; 
         FIG. 8  is a cross-sectional view of an inventive earphone; 
         FIG. 9  is a cross-sectional view of an inventive earphone; 
         FIG. 10  is a cross-sectional view of an inventive earphone; 
         FIG. 11  is a cross-sectional view of an inventive earphone; 
         FIG. 12  is a cross-sectional view of an inventive earphone; 
         FIG. 13  is a cross-sectional view of an inventive earphone; and 
         FIG. 14  is a cross-sectional view of an inventive earphone. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, there will be hereinafter described embodiments of the present disclosure.  FIGS. 1-3  are cross-sectional views of an earphone  1 A, as one example of an electroacoustic transducer, according to an embodiment of the present disclosure.  FIG. 2  is a cross-sectional view taken along a plane along line Z-Z′ in  FIG. 1 .  FIG. 3  is a cross-sectional view taken along a plane along line Y-Y′ in  FIG. 1 . As illustrated in  FIGS. 1-3 , the earphone  1 A includes a housing  10 , a diaphragm  20 , a partition wall  30 , and a tube  50 . 
     The housing  10  is a hollow cylindrical member formed of resin. A through-hole, to which the tube  50  is mounted, is formed in one of two circular end faces of the housing  10 . The tube  50  connects the housing  10  and an earpiece to be inserted into an earhole of a user. Like the housing  10 , the tube  50  is formed of resin. In  FIG. 1  and other drawings, illustration of the earpiece is omitted. 
     The diaphragm  20  is a piezoelectric element that vibrates in accordance with an externally applied sound signal. As illustrated in  FIGS. 1 and 3 , the diaphragm  20  is shaped like a flat disk having a diameter smaller than an inside diameter of the housing  10 . As illustrated in  FIG. 1 , the diaphragm  20  includes a porous film  22  and a pair of electrodes  24 - 1 ,  24 - 2  sandwiching the porous film  22  therebetween. In the following description, a direction from one of the two electrodes  24 - 1 ,  24 - 2  toward the other of the two electrodes  24 - 1 ,  24 - 2  will be referred to as a thickness direction of the porous film  22 . In  FIGS. 1-3 , a Z direction corresponds to the thickness direction of the porous film  22 . The diaphragm  20  may have any planar shape, namely, may have any shape viewed in the Z direction, other than a circle. That is, the planar shape of the diaphragm  20  may be an ellipse or a polygon such as a quadrangle or a pentagon. 
     The porous film  22  is formed of a piezoelectric material. One of the electrodes  24 - 1 ,  24 - 2  is grounded. To the other of the electrodes  24 - 1 ,  24 - 2 , a voltage based on the sound signal is applied. The porous film  22  expands or contracts in the thickness direction based on the voltage applied between the electrodes  24 - 1 ,  24 - 2 . Specifically, based on the voltage applied between the electrodes  24 - 1 ,  24 - 2 , a portion of the porous film  22  sandwiched between the electrodes  24 - 1 ,  24 - 2  expands in mutually opposite directions from the center of the porous film  22  in the thickness direction toward the respective electrodes  24 - 1 ,  24 - 2  or contracts in mutually opposite directions from the respective electrodes  24 - 1 ,  24 - 2  toward the center in the thickness direction. With this configuration, the diaphragm  20  vibrates, and sound waves are emitted to spaces located outside the respective electrodes  24 - 1 ,  24 - 2 . 
     The piezoelectric material of which the porous film  22  is formed has piezoelectric characteristics given as follows. For instance, a multiplicity of flat pores are formed in polytetrafluoroethylene (PTFE), polypropylene (PP), polyethylene(PE), polyethylene terephthalate (PET) or the like, and opposed faces of the flat pores are polarized and electrified by a corona discharge or the like. A lower limit of an average thickness of the porous film  22  is preferably 10 μm and more preferably 50 μm. An upper limit of the average thickness of the porous film  22  is preferably 500 μm and more preferably 200 μm. When the average thickness of the porous film  22  is less than the lower limit, the strength of the porous film  22  may be insufficient. When the average thickness of the porous film  22  is greater than the upper limit, the deformation amount of the porous film  22  may decrease, resulting in an insufficient output sound pressure. 
     The electrodes  24 - 1 ,  24 - 2  are laminated respectively on opposite surfaces of the porous film  22 . When it is not necessary to distinguish the electrode  24 - 1  and the electrode  24 - 2  from each other, each of them will be referred to as “electrode  24 ”. The electrode  24  may be formed of any conductive material examples of which include: metals such as aluminum, copper, and nickel: and a carbon. An average thickness of the electrode  24 , which may vary depending on a laminating process, is not smaller than 0.1 μm and not greater than 30 μm, for instance. When the average thickness of the electrode  24  is less than the lower limit, the strength of the electrode  24  may be insufficient. When the average thickness of the electrode  24  is greater than the upper limit, the vibration of the porous film  22  may be inhibited. The electrodes  24  may be laminated on the porous film  22  by any suitable method such as vapor deposition of a metal, printing with a conductive carbon ink, and application and drying of a silver paste. 
     As illustrated in  FIG. 1 , the partition wall  30  includes a first member  32 , a second member  34 , and a third member  36 . As illustrated in  FIG. 2 , the first member  32  is shaped like a flat disk whose diameter is equal to the inside diameter of the housing  10 . As illustrated in  FIG. 3 , the second member  34  is shaped like a rectangular plate whose length in an X direction is equal to the inside diameter of the housing  10 . The third member  36  is shaped like a plate having a planar shape illustrated in  FIG. 3 . Like the housing  10 , the first member  32 , the second member  34 , and the third member  36  are formed of resin. 
     As illustrated in  FIG. 2 , the first member  32  has two elliptical cutouts  320  formed at its diametrically opposite ends. As illustrated in  FIGS. 1-3 , the second member  34  is bonded by an adhesive or the like to one of two generally circular surfaces of the first member  32  at a middle position thereof in a direction from one of the two cutouts  320  toward the other of the two cutouts  320 , i.e., in the Z direction, such that the second member  34  extends so as to be orthogonal to the Z direction. The third member  36  is bonded by an adhesive or the like to the other of the two generally circular surfaces of the first member  32  at a middle position thereof in the Z direction, such that the third member  36  extends so as to be orthogonal to the Z direction. In the present embodiment, the partition wall  30  is constituted by the three separate members, i.e., the first member  32 , the second member  34 , and the third member  36 . The partition wall  30  may be formed by integral molding of all of or a part of these three members. 
     The second member  34  has a through-hole to which the diaphragm  20  is mounted. As illustrated in  FIGS. 1 and 3 , the diaphragm  20  is mounted to the through-hole of the second member  34  via a ring-like elastic member  40 . The diaphragm  20  is mounted to the through-hole of the second member  34  via the elastic member  40  for preventing the vibration of the diaphragm  20  in the thickness direction from being inhibited. As illustrated in  FIGS. 1 and 3 , the diaphragm  20  is disposed in the housing  10  in a state in which the diaphragm  20  is attached to the partition wall  30 , more strictly, in a state in which the diaphragm  20  is attached to the second member  34  of the partition wall  30 . As illustrated in  FIG. 1 , the diaphragm  20  is disposed such that the diaphragm  20  and the second member  34  of the partition wall  30  are arranged in a row in a Y direction. Thus, it is noted that the diaphragm  20  is disposed on the same plane as the second member  34  of the partition wall  30 . 
     An inner space of the housing  10  (a space of the housing  10  closer to the diaphragm  20 ) is divided into four spaces  100 - 1 ,  100 - 2 ,  100 - 3 ,  100 - 4  by the partition wall  30  to which the diaphragm  20  is attached. The space  100 - 2  and the space  100 - 4  are in communication with each other through the one of the two cutouts  320 . In the following description, a space provided by the spaces  100 - 1 ,  100 - 3  that are in communication with each other through the other of the two cutouts  320  will be referred to as a first space  110 - 1 , and a space provided by the spaces  100 - 2 ,  100 - 4  that are in communication with each other through the one of the two cutouts  320  will be referred to as a second space  110 - 2 . In the present embodiment, the first space  110 - 1  and the second space  110 - 2  are substantially identical in shape and volume. That is, as illustrated in  FIG. 1 , the partition wall  30  divides the inner space of the housing  10  into the first space  110 - 1  closer to one of the two electrodes of the diaphragm  20 , i.e., the electrode  24 - 1 , and the second space  110 - 2  closer to the other of the two electrodes, i.e., the electrode  24 - 2 . As illustrated in  FIG. 1 , the diaphragm  20  is attached to the second member  34  of the partition wall  30  via the elastic member  40 . Accordingly, the diaphragm  20  divides, as a part of the partition wall  30 , the inner space of the housing  10  into the first space  110 - 1  and the second space  110 - 2 . 
     When one of opposite surfaces of the diaphragm  20  that is located on a side of the electrode  24 - 1  is referred to as a first surface  20 - 1  and the other of the opposite surfaces of the diaphragm  20  that is located on a side of the electrode  24 - 2  is referred to as a second surface  20 - 2  as illustrated in  FIG. 1 , the first surface  20 - 1  is exposed to the first space  110 - 1  without being exposed to the second space  110 - 2 , and the second surface  20 - 2  is exposed to the second space  110 - 2  without being exposed to the first space  110 - 1 . 
     As illustrated in  FIG. 1 , the tube  50  is divided, by the third member  36  of the partition wall  30 , into two tubes, i.e., a first tube  50 - 1  and a second tube  50 - 2 , that have substantially the same tube length and substantially the same cross-sectional area. The first tube  50 - 1  establishes communication between a sound wave emission opening  60  that is open to an outer space of the housing  10  and the first space  110 - 1 . The second tube  50 - 2  establishes communication between the sound wave emission opening  60  and the second space  110 - 2 . 
     In the earphone  1 A of the present embodiment, one of the two electrodes  24 - 1 ,  24 - 2  is grounded. When a voltage based on the sound signal is applied to the other of the two electrodes  24 - 1 ,  24 - 2 , the diaphragm  20  vibrates and sound waves in the same phase based on the sound signal are emitted respectively from the first surface  20 - 1  located on the side of the electrode  24 - 1  and the second surface  20 - 2  located on the side of the electrode  24 - 2 . The sound wave emitted from the first surface  20 - 1  of the diaphragm  20  located on the side of the electrode  24 - 1  is emitted through the sound wave emission opening  60  to the outer space of the housing  10  via the first space  110 - 1  and the first tube  50 - 1 . The sound wave emitted from the second surface  20 - 2  of the diaphragm  20  located on the side of the electrode  24 - 2  is emitted through the sound wave emission opening  60  to the outer space of the housing  10  via the second space  110 - 2  and the second tube  50 - 2 . 
     The sound waves respectively emitted from the first surface  20 - 1  of the diaphragm  20  located on the side of the electrode  24 - 1  and the second surface  20 - 2  of the diaphragm  20  located on the side of the electrode  24 - 2  are in the same phase, and acoustic spaces to which the respective sound waves propagate have substantially the same shape. Thus, frequency characteristics of sounds that are emitted from one of the opposite surfaces of the diaphragm  20  to reach the ear of the user are identical to frequency characteristics of sounds that are emitted from the other of the opposite surfaces of the diaphragm  20  to reach the ear of the user. For instance, if the frequency characteristics of the former are flat frequency characteristics not including peaks and dips, the frequency characteristics of the latter are also flat. In the earphone  1 A of the present embodiment, the sounds emitted from both surfaces of the diaphragm  20  are superposed on one another at the sound wave emission opening  60 , so that the earphone  1 A of the present embodiment can obtain characteristics in which the output (sound volume) is doubled, as compared with conventional earphones that utilize only sounds emitted from its one surface. 
     As explained above, the earphone  1 A of the present embodiment effectively utilize the sound waves respectively emitted from both surfaces of the diaphragm  20  so as to attain doubled output, as compared with the conventional earphones that utilize only the sounds emitted from its one surface. 
       FIGS. 4 and 5  are cross-sectional views respectively illustrating an earphone  1 B and an earphone  1 C according to an embodiment of the present disclosure. The same reference signs as used in  FIG. 1  are used to identify the corresponding constituent elements in  FIGS. 4 and 5 . In each of the earphones  1 B,  1 C of the present embodiment, two acoustic spaces, to which the sound waves respectively emitted from one and the other of the opposite surfaces of the diaphragm  20  propagate, are different in shape. The earphone  1 B of the present embodiment differs from the earphone  1 A of the previous embodiment in this aspect. 
     In the earphone  1 B illustrated in  FIG. 4 , the third member  36  is disposed so as to be shifted in the Z direction such that the cross-sectional area of the second tube  50 - 2  is smaller than the cross-sectional area of the first tube  50 - 1 . In the earphone  1 C illustrated in  FIG. 5 , the cross-sectional area of the first tube  50 - 1  and the cross-sectional area of the second tube  50 - 2  are equal to each other. In the earphone  1 C, however, the second member  34  is disposed so as to be shifted in the Z direction such that the volume of the space  100 - 1  is smaller than the volume of the space  100 - 2 , in other words, such that the volume of the first space  110 - 1  is smaller than the volume of the second space  110 - 2 . The two acoustic spaces, to which the sound waves respectively emitted from one and the other of the opposite surfaces of the diaphragm  20  propagate, have mutually different shapes for the following reasons. 
     Some adjustment such as emphasis of high- and low-frequency ranges is often needed in the earphone depending on the sound signal based on which sounds are to be reproduced, tastes or preferences of the user, etc. In the configuration illustrated in  FIG. 4 , reflection of sounds in the high-frequency range is small in the first tube  50 - 1  whose cross-sectional area is enlarged, thus enabling emission of sounds in which characteristics of the high-frequency range are emphasized. In the second tube  50 - 2  whose cross-sectional area is reduced, on the other hand, reflection of sounds in the high-frequency range is strong, and sounds in the low-frequency range are relatively allowed to pass. As a result, sounds in the mid-frequency range are relatively lowered at the sound wave emission opening  60  of the earphone  1 B, as compared with the earphone  1 A of the previous embodiment, thus achieving characteristics in which the low-frequency range and the high-frequency range are emphasized. It is noted that the cross-sectional area of one of the first tube  50 - 1  and the second tube  50 - 2  may remain the same as the cross-sectional area thereof in the previous embodiment while the cross-sectional area of the other of the first tube  50 - 1  and the second tube  50 - 2  may be changed, whereby only the low-frequency range or only the high-frequency range may be emphasized. 
     In the earphone  1 B illustrated in  FIG. 4 , the high-frequency range and the low-frequency range are emphasized by adjusting the cross-sectional area of the first tube  50 - 1  and the cross-sectional area of the second tube  50 - 2 . In the earphone  1 C illustrated in  FIG. 5 , the volume of the first space  110 - 1  and the volume of the second space  110 - 2  are adjusted to adjust the sound quality similarly. The reasons are as follows. 
     In the earphone  1 A of the previous embodiment, there is generated Helmholtz resonance (hereinafter referred to as “first Helmholtz resonance”) in which the first space  110 - 1  serves as a cavity and the first tube  50 - 1  serves as a neck, and there is generated Helmholtz resonance (hereinafter referred to as “second Helmholtz resonance”) in which the second space  110 - 2  serves as a cavity and the second tube  50 - 2  serves as a neck. As described above, in the earphone  1 A of the previous embodiment, the volume of the first space  110 - 1  and the volume of the second space  110 - 2  are substantially equal to each other, and the cross-sectional area of the first tube  50 - 1  and the cross-sectional area of the second tube  50 - 2  are substantially equal to each other. Thus, the resonance frequency of the first Helmholtz resonance and the resonance frequency of the second Helmholtz resonance in the earphone  1 A of the previous embodiment are substantially equal to each other. When the volume of each of the first space  110 - 1  and the second space  110 - 2  is represented as V and the cross-sectional area of each of the first tube  50 - 1  and the second tube  50 - 2  is represented as S, the resonance frequency f 0  of the first Helmholtz resonance and the second Helmholtz resonance is represented by the following expression (1). In the expression (1), l represents a length of the neck, c represents a sound speed, and δ represents an open end correction value. When the diameter of the opening of the neck is d, δ is approximately equal to 0.8×d, i.e., δ≅0.8×d. 
     
       
         
           
             
               
                 
                   
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                         s 
                         
                           V 
                           ⁡ 
                           
                             ( 
                             
                               l 
                               + 
                               δ 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
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     Also in the earphone  1 C of  FIG. 5 , the first Helmholtz resonance and the second Helmholtz resonance are generated. In the earphone  1 C, the position at which the second member  34  is disposed is shifted upward in the Z direction with respect to the middle position of the first member  32  in the Z direction, so that the volume of the first space  110 - 1  is smaller than that of the second space  110 - 2 . As a result, the volume of the first space  110 - 1  in the earphone  1 C of  FIG. 5  is smaller than the volume of the first space  110 - 1  in the earphone  1 A of  FIG. 1 . Thus, the resonance frequency of the first Helmholtz resonance in the earphone  1 C is shifted to a higher frequency side than the resonance frequency f 0  in the previous embodiment. In the earphone  1 C of  FIG. 5 , the volume of the second space  110 - 2  is larger than the volume of the second space  110 - 2  in the earphone  1 A. Thus, the resonance frequency of the second Helmholtz resonance in the earphone  1 C is shifted to a lower frequency side than the resonance frequency f 0  in the previous embodiment. Like the earphone  1 B, the earphone  1 C of  FIG. 5  also achieves the characteristics in which the low-frequency range and the high-frequency range are emphasized. 
     As explained above, the present embodiment enables the sound-quality adjustment in specific frequency ranges while effectively utilizing the sound waves emitted from both surfaces of the diaphragm  20 . 
     In addition, the earphones according to the present embodiment enjoy constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies. Conventional earphones sometimes include driver units of different types provided for different frequency ranges. In this case, the vibration characteristics unique to the respective driver units are different among the driver units, causing unnaturalness in the crossover frequency range. For instance, in a case where the driver unit for the low-frequency range and the driver unit for the high-frequency range are different in material, sound reverberation in the low-frequency range and sound reverberation in the high-frequency range may not match with each other. In contrast, the earphones according to the present embodiment do not include driver units of different types used for different frequency ranges, thus achieving constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies. Further, because the earphones according to the present embodiment do not include driver units of different types used for different frequency ranges, resulting in cost and size reductions. 
       FIGS. 6 and 7  are cross-sectional views respectively illustrating an earphone  1 D and an earphone  1 E according to an embodiment of the present disclosure. The same reference signs as used in  FIG. 1  are used to identify the corresponding constituent elements in  FIGS. 6 and 7 . As apparent from a comparison between  FIG. 1  and  FIG. 6 , the earphone  1 D illustrated in  FIG. 6  differs from the earphone  1 A of the previous embodiment in that a sound absorber  70  formed of a nonwoven fabric or the like is packed in the first tube  50 - 1 . Further, as apparent from a comparison between  FIG. 7  and  FIG. 5 , the earphone  1 E illustrated in  FIG. 7  differs from the earphone  1 C of the previous embodiment in that i) the cross-sectional area of the second tube  50 - 2  is smaller than the cross-sectional area of the first tube  50 - 1  and ii) the sound absorber  70  is packed in the second tube  50 - 2 . 
     Packing the sound absorber in the tube  50  is equivalent to reducing the cross-sectional area of the tube  50 . According to the present embodiment, the fine adjustment of the sound-quality in specific frequency ranges can be easily performed by packing the sound absorber in any one of the first tube  50 - 1  and the second tube  50 - 2 . Also in the present embodiment, the sound waves emitted from both surfaces of the diaphragm  20  can be effectively utilized as in the previous embodiment. Further, the earphones of the present embodiment do not include driver units of different types used for different frequency ranges, thus achieving constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies and resulting in cost and size reductions, as in the previous embodiment. In the present embodiment, the sound absorber  70  is packed in one of the first tube  50 - 1  and the second tube  50 - 2 . The sound absorber  70  may be packed in both the first tube  50 - 1  and the second tube  50 - 2 . 
       FIGS. 8-11  are cross-sectional views respectively illustrating an earphone  1 F, an earphone  1 G, an earphone  1 H, and an earphone  1 I according to an embodiment of the present disclosure. The earphone  1 F illustrated in  FIG. 8  differs from the earphone  1 A of the previous embodiment in the following three aspects. The first different aspect is that the earphone  1 F includes a partition wall  30 ′ in place of the partition wall  30 . As apparent from a comparison between  FIG. 8  and  FIG. 5 , the partition wall  30 ′ differs from the partition wall  30  in that i) the partition wall  30 ′ does not have the through-hole to which the diaphragm  20  is mounted and ii) the partition wall  30 ′ has a generally L-shaped cross section. In the earphone  1 F of the present embodiment, the inner space of the housing  10  is divided by the partition wall  30 ′ into the space  100 - 1  and the space  100 - 2  whose volume is smaller than that of the space  100 - 1 . 
     The second different aspect is that the diaphragm  20  is disposed such that one surface of the diaphragm  20 , namely, one surface thereof located on the side of the electrode  24 - 1 , faces the space  100 - 1  and the space  100 - 2 . An elastic member  40 ′ in  FIG. 8  is a member filling a gap between the diaphragm  20  and one end of the partition wall  30 ′ without inhibiting the vibration of the diaphragm  20  in the thickness direction. The third different aspect is that the tube  50  is not divided into the first tube  50 - 1  and the second tube  50 - 2 . The tube  50  establishes communication between the space  100 - 1  and the sound wave emission opening  60  and communication between the space  100 - 2  and the sound wave emission opening  60 . 
     In the earphone  1 F constructed as illustrated in  FIG. 8 , reflection of sounds in the high-frequency range is small in the space  100 - 1 , thus enabling emission of sounds in which characteristics of the high-frequency range are emphasized. In the space  100 - 2 , on the other hand, reflection of sounds in the high-frequency range is strong, and sounds in the low-frequency range are relatively allowed to pass. As a result, sounds in the mid-frequency range are relatively lowered at the sound wave emission opening  60  at which sounds in the low-frequency range and sounds in the high-frequency range are superposed, as compared with the earphone  1 A of the previous embodiment, thus achieving the characteristics in which the low-frequency range and the high-frequency range are emphasized. 
     Helmholtz resonance is generated also in the earphone  1 F of the present embodiment. In the earphone  1 F, the first Helmholtz resonance is generated in which the space  100 - 1  serves as a cavity and the tube  50  serves as a neck, and the second Helmholtz resonance is generated in which the space  100 - 2  serves as a cavity and the tube  50  serves as a neck. As described above, in the earphone  1 F, the volume of the space  100 - 1  is larger than the volume of the space  100 - 2 , and the resonance frequency of the first Helmholtz resonance is lower than the resonance frequency of the second Helmholtz resonance. Thus, like the earphone  1 C of the previous embodiment, the earphone  1 F of the present embodiment enables the sound-quality adjustment in specific frequency ranges. In addition, the earphone  1 F of the present embodiment does not include driver units of different types used for different frequency ranges, thus achieving constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies and resulting in cost and size reductions. 
     The earphone  1 G illustrated in  FIG. 9  differs from the earphone  1 F in that the diaphragm  20  is disposed in the housing  10  so as to be shifted in the Z direction, such that a region of the diaphragm  20  facing the space  100 - 1  is larger than a region thereof facing the space  100 - 2 . Like the earphone  1 F, the earphone  1 G of  FIG. 9  enables the sound-quality adjustment in specific frequency ranges, achieves constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies, and enjoys cost and size reductions. 
     The earphone  1 H illustrated in  FIG. 10  differs from the earphone  1 F in that the space  100 - 2  is defined by a partition wall  30 ″ shaped like a plate and the sound absorber  70 . The earphone  1 I illustrated in  FIG. 11  differs from the earphone  1 F in that the space  100 - 2  is defined by the partition wall  30 ′ and the sound absorber  70 . The earphones  1 H,  1 I also enable the sound-quality adjustment in specific frequency ranges, achieve constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies, and enjoy cost and size reductions. 
     While the embodiments have been described above, the embodiments may be modified as follows. 
     (1) In the embodiments illustrated above, the present disclosure is applied to the earphones. The electroacoustic transducer to which the present disclosure is applicable is not limited to the earphones but may be headphone speakers. 
     (2) The diaphragm in the previous embodiment is not limited to the piezoelectric element that includes the porous film formed of the piezoelectric material described above. The piezoelectric element may be a piezoelectric element in which lead zirconate titanate (PZT) or the like is used as the piezoelectric material, namely, a piezoelectric element capable of outputting from only one surface thereof. The diaphragm may be driven by a voice coil. 
     (3) In the previous embodiment, the inner space of the housing is divided into two spaces by one partition wall. The inner space of the housing may be divided into three or more spaces by two or more partition walls. That is, the electroacoustic transducer includes the housing, one or a plurality of partition walls that divide the inner space of the housing into a plurality of spaces such that at least one of the plurality of spaces has a volume different from a volume of at least one of others of the plurality of spaces except the at least one of the plurality of spaces, the diaphragm disposed in the housing such that one surface thereof faces the plurality of spaces, and a tube that establishes communication between the sound wave emission opening that is open to the outer space of the housing and the plurality of spaces. The sound quality can be adjusted in at least two different frequency ranges if at least one of the plurality of spaces has a volume different from those of other spaces. 
     In an earphone  1 J illustrated in  FIG. 12 , the space in the housing  10  is divided, by partition walls  30 ′- 1 ,  30 ′- 2 , into three spaces, i.e., the space  100 - 1 , the space  100 - 2 , and the space  100 - 3  having mutually different volumes. An elastic member  40 ′- 1  in  FIG. 12  is a member filling a gap between the diaphragm  20  and one end of the partition wall  30 ′- 1  without inhibiting the vibration of the diaphragm  20  in the thickness direction. An elastic member  40 ′- 2  is a member filling a gap between the diaphragm  20  and one end of the partition wall  30 ′- 2  without inhibiting the vibration of the diaphragm  20  in the thickness direction. In the earphone  1 J illustrated in  FIG. 12 , the sound quality can be adjusted in three different frequency ranges by dividing the inner space of the housing  10  into the three spaces having mutually different volumes. 
     The diaphragm whose one surface faces the plurality of spaces is not limited to one diaphragm. That is, the earphone may include a plurality of diaphragms, as illustrated in  FIG. 13 . Specifically, an earphone  1 K of  FIG. 13  includes a diaphragm  20 - 3  as a diaphragm whose one surface faces the space  100 - 1 , a diaphragm  20 - 4  as a diaphragm whose one surface faces the space  100 - 2 , and a diaphragm  20 - 5  as a diaphragm whose one surface faces the space  100 - 3 . In each of the diaphragm  20 - 3 , the diaphragm  20 - 4 , and the diaphragm  20 - 5 , one of the two electrodes, which is provided on the other surface of the diaphragm attached to the inner wall surface of the housing  10 , is grounded, and a voltage based on the sound signal is applied to the other of the two electrodes. In this configuration, the diaphragm  20 - 3 , the diaphragm  20 - 4 , and the diaphragm  20 - 5  respectively emit sound waves in the same phase. Similarly, in the earphones  1 F- 1 I of  FIGS. 8-11 , the diaphragm facing the space  100 - 1  and the diaphragm facing the space  100 - 2  may be separate diaphragms. 
     (4) The earphones in the illustrated embodiments may be configured such that a ratio among the volumes of the plurality of spaces each serving as the cavity in the Helmholtz resonator and/or a ratio among the cross-sectional areas of the plurality of tubes each serving as the neck in the Helmholtz resonator may be variable. The thus configured earphone enables the user to finely adjust the sound quality in specific frequency ranges depending on the user&#39;s preferences or tastes. 
     In the earphone  1 A of the previous embodiment, for instance, by packing the sound absorber in one of the first tube  50 - 1  and the second tube  50 - 2  from an end portion of the tube  50  closer to the sound wave emission opening  60 , the cross-sectional area of the one of the first tube  50 - 1  and the second tube  50 - 2  can be adjusted. For instance, the earphone  1 F of the previous embodiment may be modified as illustrated in  FIG. 14 , such that the partition wall  30 ′ is constituted by a plate-like first member  32 ′ and a second member  34 ′ provided so as to be perpendicular to the first member  32 ′ and slidable in the Y direction in  FIG. 14  and such that one end of a rod-like member  90  protruding outside the housing  10  through a through-hole  80  formed in the housing  10  is connected to the second member  34 ′ and a knob  92  is attached to the other end of the rod-like member  90 . In this configuration, the volume of the space  100 - 2  can be increased by pushing the knob  92  in a Y′ direction or decreased by pulling the knob  92  in the Y direction. Likewise, in the earphone  1 A of the previous embodiment, the volume of any one of the first space  110 - 1  and the second space  110 - 2  may be made variable. The second member  34  in  FIG. 1  may be configured to be movable in the Z direction by providing the rod-like member  90  and the knob  92  in  FIG. 14 , thus enabling a ratio between the volume of the first space  110 - 1  and the volume of the second space  110 - 2  to be variable in the configuration of  FIG. 1 . Further, the third member  36  in  FIG. 1  may be configured to be movable in the Z direction by providing the rod-like member  90  and the knob  92 , thus enabling a ratio between the cross-sectional area of the first tube  50 - 1  and the cross-sectional area of the second tube  50 - 2  to be variable in the configuration of  FIG. 1 . Further, the rod-like member  90  and the knob  92  may be provided for each of the second member  34  and the third member  36 , thus enabling both i) the ratio between the volume of the first space  110 - 1  and the volume of the second space  110 - 2  and ii) the ratio between the cross-sectional area of the first tube and the cross-sectional area of the second tube to be variable in the earphone  1 A of  FIG. 1 .