Patent Publication Number: US-9838799-B2

Title: Electroacoustic converter

Description:
BACKGROUND 
     Field of the Invention 
     The present invention relates to an electroacoustic converter that can be applied to earphones, headphones, mobile information terminals, etc., for example. 
     Description of the Related Art 
     Piezoelectric sounding elements are widely used as simple means for electroacoustic conversion, where popular applications include earphones, headphones, and other acoustic devices as well as speakers for mobile information terminals, etc. Piezoelectric sounding elements are typically constituted by a vibration plate and a piezoelectric element attached on one side or two sides of the plate (refer to Patent Literature 1, for example). 
     On the other hand, Patent Literature 2 describes headphones equipped with a dynamic driver and a piezoelectric driver, where these two drivers are driven in parallel to allow for wide playback bandwidths. The piezoelectric driver is provided at the center of the interior surface of a front cover that blocks off the front side of the dynamic driver and functions as a vibration plate, so that constitutionally this piezoelectric driver can function as a high-pitch sound driver. 
     BACKGROUND ART LITERATURES 
     [Patent Literature 1] Japanese Patent Laid-open No. 2013-150305 
     [Patent Literature 2] Japanese Utility Model Laid-open No. Sho 62-68400 
     SUMMARY 
     In recent years, there is a demand for greater ease of assembly and higher sound quality in the field of earphones, headphones and other acoustic devices, for example. However, the constitution of Patent Literature 2 presents a problem in that, because the dynamic driver is blocked off by the front cover, sound waves cannot be generated with desired frequency characteristics. To be specific, it is difficult to flexibly cope with the peak level adjustment in a specific frequency band, or the optimization of frequency characteristics at the cross point between the low-pitch sound characteristic curve and high-pitch sound characteristic curve, or the like. 
     In light of the aforementioned situations, an object of the present invention is to provide an electroacoustic converter capable of obtaining desired frequency characteristics easily, while providing greater ease of assembly at the same time. 
     Any discussion of problems and solutions involved in the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion were known at the time the invention was made. 
     To achieve the aforementioned object, an electroacoustic converter pertaining to an embodiment of the present invention has an enclosure, piezoelectric sounding body, electromagnetic sounding body, passage, and wiring members. 
     The piezoelectric sounding body includes a first vibration plate supported directly or indirectly on the enclosure, and a piezoelectric element placed at least on one side of the first vibration plate. In the above, “directly or indirectly” may refer to “without or with an intervening part” which is not a part of the enclosure. The piezoelectric sounding body divides the interior of the enclosure into a first space and a second space. 
     The electromagnetic sounding body has a second vibration plate and is placed in the first space. 
     The passage is provided at the piezoelectric sounding body or around the piezoelectric sounding body, to connect the first space and second space. 
     The wiring members are electrically connected to the piezoelectric element and led out toward the electromagnetic sounding body, from the piezoelectric element, through the first space or second space. 
     With the electroacoustic converter, sound waves generated by the electromagnetic sounding body are formed by composite waves having a sound wave component that propagates to the second space by vibrating the first vibration plate of the piezoelectric sounding body, and a sound wave component that propagates to the second space via the passage. Accordingly, sound waves output from the piezoelectric sounding body can be adjusted to desired frequency characteristics by optimizing the size of the passage, number of passages, etc. The electromagnetic sounding body is typically constituted so that it generates sound waves that are lower in pitch than sound waves generated by the piezoelectric sounding body. This way, frequency characteristics having a sound pressure peak in a desired low-pitch band can be obtained with ease, for example. 
     Also, because the passage is provided at the piezoelectric sounding body, the resonance frequencies of the first vibration plate (frequency characteristics of the piezoelectric sounding body) can be adjusted by the mode of the passage. This makes it easy to achieve desired frequency characteristics, such as flat composite frequencies around the cross point between the low-pitch sound characteristic curve by the electromagnetic sounding body and the high-pitch sound characteristic curve by the piezoelectric sounding body. 
     In addition, the passage functions as a low-pass filter that cuts, from among the sound waves generated by the electromagnetic sounding body, those high-frequency components of or above a specified level. This way, sound waves in a specified low-frequency band can be output without affecting the frequency characteristics of high-pitch sound waves generated by the piezoelectric sounding body. 
     And, the constitution where the wiring member electrically connected to the piezoelectric element is led out toward the electromagnetic sounding body, from the piezoelectric element, through the first or second space, allows the piezoelectric sounding body to be installed in the enclosure without losing any ease of operation. 
     As described above, according to the present invention, desired frequency characteristics can be obtained easily, while providing greater ease of assembly at the same time. 
     For purposes of summarizing aspects of the invention and the advantages achieved over the related art, certain objects and advantages of the invention are described in this disclosure. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
     Further aspects, features and advantages of this invention will become apparent from the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will now be described with reference to the drawings of preferred embodiments which are intended to illustrate and not to limit the invention. The drawings are greatly simplified for illustrative purposes and are not necessarily to scale. 
         FIG. 1  is a schematic lateral section view showing an electroacoustic converter pertaining to an embodiment of the present invention. 
         FIG. 2  is a schematic lateral section view showing the electromagnetic sounding body and piezoelectric sounding body of the electroacoustic converter in a pre-assembled state. 
         FIG. 3  is a schematic plan view of the electromagnetic sounding body. 
         FIG. 4  is a schematic perspective view showing a constitutional example of the piezoelectric element constituting the piezoelectric sounding body. 
         FIG. 5  is a schematic lateral section view of the piezoelectric element in  FIG. 4 . 
         FIG. 6  is a schematic perspective view showing another constitutional example of the piezoelectric element. 
         FIG. 7  is a schematic lateral section view of the piezoelectric element in  FIG. 6 . 
         FIG. 8  is a schematic plan view showing a constitutional example of the piezoelectric sounding body. 
         FIG. 9  is a schematic plan view showing another constitutional example of the piezoelectric sounding body. 
         FIG. 10  is a drawing showing the frequency characteristics of an electroacoustic converter pertaining to a comparative example. 
         FIG. 11  is a drawing showing the frequency characteristics of the electroacoustic converter in  FIG. 1 . 
         FIG. 12  is a schematic lateral section view showing an electroacoustic converter pertaining to another embodiment of the present invention. 
         FIG. 13  is a schematic plan view showing a constitutional example of the piezoelectric sounding body of the electroacoustic converter in  FIG. 12 . 
         FIG. 14  is a schematic plan view showing another constitutional example of the piezoelectric sounding body. 
         FIG. 15  is a schematic plan view showing yet another constitutional example of the piezoelectric sounding body. 
         FIG. 16  is a drawing showing the frequency characteristics of the electroacoustic converter in  FIG. 12 . 
         FIG. 17  is a schematic diagram showing an example of constitutional variation of the electroacoustic converter. 
         FIG. 18  is a section view showing schematically the internal structure of the electromagnetic sounding body. 
         FIG. 19  is a section view of key parts, showing an example of constitutional variation of the electroacoustic converter. 
         FIG. 20  is a schematic lateral section view showing an electroacoustic converter pertaining to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE SYMBOLS 
     
         
         
           
               10  - - - Earphone body 
               11  - - - Sound path 
               20  - - - Earpiece 
               30 ,  50 ,  70 ,  300  - - - Sounding unit 
               31  - - - Electromagnetic sounding body 
               32 ,  52 ,  72  - - - Piezoelectric sounding body 
               34 ,  54  - - - Ring-shaped member 
               35 ,  55  - - - Passage 
               41  - - - Enclosure 
               321 ,  323 ,  521  - - - Vibration plate 
               322  - - - Piezoelectric element 
             S 1  - - - First space 
             S 2  - - - Second space 
           
         
       
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention are explained below by referring to the drawings. 
     First Embodiment 
       FIG. 1  is a schematic lateral section view showing the constitution of an earphone  100  as an electroacoustic converter pertaining to an embodiment of the present invention. 
     In the figure, the X-axis, Y-axis and Z-axis represent three axial directions crossing one another at right angles. 
     Overall Constitution of Earphone 
     The earphone  100  has an earphone body  10  and earpiece  20 . The earpiece  20  is attached to a sound path  11  of the earphone body  10 , while constituted in such a way that it can be worn in the user&#39;s ear. 
     The earphone body  10  has a sounding unit  30 , and a housing  40  that houses the sounding unit  30 . 
     The sounding unit  30  has an electromagnetic sounding body  31  and piezoelectric sounding body  32 . The housing  40  has an enclosure  41  and cover  42 . 
     Enclosure 
     The enclosure  41  has the shape of a cylinder with a bottom and is typically constituted by injection-molded plastics. The enclosure  41  has an interior space in which the sounding unit  30  is housed, and at its bottom  410  the sound path  11  is provided that connects to the interior space. 
     The enclosure  41  has a support  411  that supports the periphery of the piezoelectric sounding body  32 , and a side wall  412  enclosing the sounding unit  30  all around. The support  411  and side wall  412  are both formed in a ring shape, where the support  411  is provided in such a way that it projects inward from near the bottom of the side wall  412 . The support  411  is formed by a plane running in parallel with the XY plane, and supports the periphery of the piezoelectric sounding body  32  mentioned later either directly or indirectly via other member. It should be noted that the support  411  may be constituted by multiple pillars placed in a ring pattern along the inner periphery surface of the side wall  412 . 
     Electromagnetic Sounding Body 
     The electromagnetic sounding body  31  is constituted by a speaker unit that functions as a woofer to play back low-pitch sounds. In this embodiment, it is constituted by a dynamic speaker that primarily generates sound waves of 7 kHz or below, for example, and has a mechanism  311  containing a voice coil motor (electromagnetic coil) or other vibration body, and a base  312  that vibratively supports the mechanism  311 . The base  312  is formed roughly in the shape of a disk whose outer diameter is roughly identical to the inner diameter of the side wall  412  of the enclosure  41 , and has a periphery surface  31   e  ( FIG. 2 ) that engages with the side wall  412 . 
     The constitution of the mechanism  311  of the electromagnetic sounding body  31  is not limited in any way.  FIG. 18  is a section view of key parts, showing a constitutional example of the mechanism  311 . The mechanism  311  has a vibration plate E 1  (second vibration plate) vibratively supported on the base  312 , permanent magnet E 2 , voice coil E 3 , and yoke E 4  that supports the permanent magnet E 2 . The vibration plate E 1  is supported on the base  312  by having its periphery sandwiched between the bottom of the base  312  and a ring-shaped fixture  310  assembled integrally to the bottom. 
     The voice coil E 3  is formed by a conductive wire wound around a bobbin serving as a winding core, and is joined to the center of the vibration plate E 1 . Also, the voice coil E 3  is positioned vertically to the direction of the magnetic flux of the permanent magnet E 2  (Y-axis direction in the figure). As AC current (voice signal) flows through the voice coil E 3 , electromagnetic force acts upon the voice coil E 3  and therefore the voice coil E 3  vibrates in the Z-axis direction in the figure according to the signal waveform. This vibration is transmitted to the vibration plate E 1  coupled to the voice coil E 3  and vibrates the air inside the first space S 1 , and low-pitch sound waves generate as a result. 
       FIG. 2  is a schematic lateral section view of the sounding unit  30  in a state not yet assembled into the enclosure  41 , while  FIG. 3  is a schematic plan view of the sounding unit  30 . 
     The electromagnetic sounding body  31  has the shape of a disk having a first surface  31   a  facing the piezoelectric sounding body  32  and a second surface  31   b  on the opposite side. Provided along the periphery of the first surface  31   a  is a leg  312   a  contactively facing the periphery of the piezoelectric sounding body  32 . The leg  312   a  is formed in a ring shape, but it is not limited to the foregoing and may be constituted by multiple pillars. 
     The second surface  31   b  is formed on the surface of a disk-shaped projection  31   c  provided at the center of the top surface of the base  312 . The second surface  31   b  has a circuit board  33  fixed to it that constitutes the electrical circuit of the sounding unit  30 . Provided on the surface of the circuit board  33  are multiple terminals  331 ,  332 ,  333  that connect to various wiring members, as shown in  FIG. 3 . The circuit board  33  is typically constituted by a wiring board, but any board can be used so long as it has terminals that connect to various wiring members. Also, the location of the circuit board  33  is not limited to the second surface  31   b  as in the example, and it can be provided elsewhere such as on the interior wall of the cover  42 , for example. 
     The terminals  331  to  333  are each provided as a pair. The terminal  331  connects to a wiring member C 1  that inputs playback signals sent from a playback device not illustrated here. 
     The terminal  332  connects electrically to a terminal  313  of the electromagnetic sounding body  31  via a wiring member C 2 . The terminal  333  connects electrically to terminals  324 ,  325  of the piezoelectric sounding body  32  via a wiring member C 3 . It should be noted that the wiring members C 2 , C 3  may be connected directly to the wiring member C 1  without going through the circuit board  33 . 
     Piezoelectric Sounding Body 
     The piezoelectric sounding body  32  constitutes a speaker unit that functions as a tweeter to play back high-pitch sounds. In this embodiment, its oscillation frequency is set in such a way to primarily generate sound waves of 7 kHz or above, for example. The piezoelectric sounding body  32  has a vibration plate  321  (first vibration plate) and piezoelectric element  322 . 
     The vibration plate  321  is constituted by metal (such as 42 alloy) or other conductive material, or by resin (such as liquid crystal polymer) or other insulating material, and its plane is formed roughly circular. “Roughly circular” means not only circular, but also virtually circular as described later. The outer diameter and thickness of the vibration plate  321  are not limited in any way, and can be set as deemed appropriate according to the size of the enclosure  41 , frequency band of playback sound waves, and so on. The outer diameter of the vibration plate  321  is set smaller than the outer diameter of the electromagnetic sounding body  31 , and a vibration plate of approx. 12 mm in diameter and approx. 0.2 mm in thickness is used in this embodiment. It should be noted that the vibration plate  321  is not limited to a planer shape, and it can be a three-dimensional structure having a dome shape, etc. 
     The vibration plate  321  can have a concave shape sinking in from its outer periphery toward the inner periphery, or cutouts formed as slits, etc. It should be noted that the planar shape of the vibration plate  321 , when not strictly circular due to formation of the cutouts, is considered virtually circular so long as the shape is roughly circular. 
     As shown in  FIG. 1  and  FIG. 2 , the vibration plate  321  has a periphery  321   c  supported by the enclosure  41 . The sounding unit  30  further has a ring-shaped member  34  placed between the support  411  of the enclosure  41  and the periphery  321   c  of the vibration plate  321 . The ring-shaped member  34  has a support surface  341  that supports the leg  312   a  of the electromagnetic sounding body  31 . The outer diameter of the ring-shaped member  34  is formed roughly identical to the inner diameter of the side wall  412  of the enclosure  41 . 
     It should be noted that the periphery  321   c  of the vibration plate  321  includes the periphery of one principle surface (first principle surface  32   a ) of the vibration plate  321 , periphery of the other principle surface (second principle surface  32   b ) of the vibration plate  321 , and side surfaces of the vibration plate  321 . 
     The material constituting the ring-shaped member  34  is not limited in any way, and it may be constituted by metal material, synthetic resin material, or rubber or other elastic material, for example. If the ring-shaped member  34  is constituted by rubber or other elastic material, resonance wobble of the vibration plate  321  is suppressed and therefore stable resonance action of the vibration plate  321  can be ensured. 
     The vibration plate  321  has the first principle surface  32   a  facing the sound path  11 , and the second principle surface  32   b  facing the electromagnetic sounding body  31 . In this embodiment, the piezoelectric sounding body  32  has a unimorph structure where the piezoelectric element  322  is joined only to the second principle surface  32   b  of the vibration plate  321 . 
     The piezoelectric element  322  is not limited to the foregoing and it can be joined to the first principle surface  32   a  of the vibration plate  321 . Also, the piezoelectric sounding body  32  may be constituted by a bimorph structure where a piezoelectric element is joined to both principle surfaces  32   a ,  32   b  of the vibration plate  321 , respectively. 
       FIG. 4  is a schematic perspective view showing a constitutional example of the piezoelectric element  322 , while  FIG. 5  is a schematic section view of the example. 
       FIG. 6  is a schematic perspective view showing another constitutional example of the piezoelectric element  322 , while  FIG. 7  is a schematic section view of the example. 
     The planar shape of the piezoelectric element  322  is formed polygonal, and although it is a rectangle (oblong figure) in this embodiment, the shape can be square, parallelogram, trapezoid, or other quadrangle, or any polygon other than quadrangle, or circle, oval, ellipsoid, etc. The thickness of the piezoelectric element  322  is not limited in any way, either, and can be approx. 50 μm, for example. 
     The piezoelectric element  322  is structured as a stack of alternating multiple piezoelectric layers and multiple electrode layers. 
     Typically the piezoelectric element  322  is made by sintering at a specified temperature a stack of alternating multiple ceramic sheets (piezoelectric layers) Ld, each made of lead zirconate titanate (PZT), alkali metal-containing niobium oxide, etc., and having piezoelectric characteristics on one hand, and electrode layers Le on the other. The ends of respective electrode layers are led out alternately to both longitudinal end faces of the piezoelectric layer Ld. The electrode layers Le exposed to one end face are connected to a first leader electrode layer Le 1 , while the electrode layers Le exposed to the other end face are connected to a second leader electrode layer Le 2 . The piezoelectric element  322  expands and contracts at a specified frequency when a specified AC voltage is applied between the first and second leader electrode layers Le 1 , Le 2 , while the vibration plate  321  vibrates at a specified frequency. The numbers of piezoelectric layers and electrode layers to be stacked are not limited in any way, and the respective numbers of layers are set as deemed appropriate so that the required sound pressure can be obtained. 
     In the constitutional example of the piezoelectric element  322  in  FIG. 4  and  FIG. 5 , the first leader electrode layer Le 1  is formed from one end face to the bottom surface of the piezoelectric layer Ld, while the second leader electrode layer Le 2  is formed from the other end face to the top surface of the piezoelectric layer Ld. The bottom surface of the piezoelectric element  322  is joined to the second principle surface  32   b  of the vibration plate  321  via conductive adhesive or other conductive material. In this case, the vibration plate  321  is constituted by metal material, but the second principle surface  32   b  may be constituted by insulating material covered with conductive material. 
     Accordingly in this embodiment, one wiring member C 3  (first wiring member) of the two wiring members C 3  is connected to the terminal  324  provided on the vibration plate  321 , while the other wiring member C 3  (second wiring member) is connected to the terminal  325  provided on the piezoelectric element  322 , as shown in  FIG. 2 . The one terminal  324  is provided on the second principle surface  32   b  of the vibration plate  321 , while the other terminal  325  is provided on the second leader electrode layer Le 2  on the top surface of the piezoelectric element  322 . This way, a specified drive voltage can be applied between the first and second leader electrode layers Le 1 , Le 2 . 
     On the other hand, in the constitutional example of the piezoelectric element  322  in  FIG. 6  and  FIG. 7 , the first leader electrode layer Le 1  is formed from one end face to one part of the top surface of the piezoelectric layer Ld, while the second leader electrode layer Le 2  is formed from the other end face to the other part of the top surface of the piezoelectric layer Ld. In this case, the two leader electrode layers Le 1 , Le 2  are exposed to the top surface of the piezoelectric element  322  in a manner adjacent to each other, the terminals  324 ,  325  may be provided on top of them. In this case, the vibration plate  321  may be constituted by insulating material. 
     As shown in  FIG. 1 , the piezoelectric sounding body  32  is assembled to the support  411  of the enclosure  41  with the ring-shaped member  34  installed on the periphery  321   c  of the vibration plate  321 . An adhesive layer can be provided between the ring-shaped member  34  and support  411  to join the two. The interior space of the enclosure  41  is divided into a first space S 1  and second space S 2  by the piezoelectric sounding body  32 . The first space S 1  is a space where the electromagnetic sounding body  31  is housed, formed between the electromagnetic sounding body  31  and piezoelectric sounding body  32 . The second space S 2  is a space connecting to the sound path  11 , formed between the piezoelectric sounding body  32  and the bottom of the enclosure  41 . 
     The electromagnetic sounding body  31  is assembled onto the ring-shaped member  34 . An adhesive layer is provided, as necessary, between the outer periphery of the electromagnetic sounding body  31  and the side wall  412  of the enclosure  41 . This adhesive layer also functions as a sealing layer to enhance the air-tightness of the sound field forming space (first space S 1 ) of the electromagnetic sounding body  31 . Also the close contact of the electromagnetic sounding body  31  and ring-shaped member  34  allows a specified volume to be secured for the first space S 1  in a stable manner, so that sound quality variation between products due to fluctuation of this volume can be prevented. 
     Cover 
     The cover  42  is fixed to the top edge of the side wall  412  so as to block off the interior of the enclosure  41 . The interior top surface of the cover  42  has a pressure part  421  that presses the electromagnetic sounding body  31  toward the ring-shaped member  34 . This way, the ring-shaped member  34  is sandwiched strongly between the leg  312   a  of the electromagnetic sounding body  31  and the support  411  of the enclosure  41 , to allow the periphery  321   c  of the vibration plate  321  to be connected integrally to the enclosure  41 . 
     The pressure part  421  of the cover  42  is formed as a ring, and its tip contacts a ring-shaped top surface  31   d  (refer to  FIG. 2  and  FIG. 3 ) formed around the projection  31   c  of the electromagnetic sounding body  31  via an elastic layer  422 . This way, the electromagnetic sounding body  31  is pressed with a uniform force by the entire circumference of the ring-shaped member  34 , thus making it possible to position the sounding unit  30  properly inside the enclosure  41 . It should be noted that the formation of the pressure part  421  is not limited to a ring shape, and it may be constituted by multiple pillars. 
     A feedthrough is provided at a specified position of the cover  42 , in order to lead the wiring member C 1  connected to the terminal  331  of the circuit board  33  to a playback device not illustrated here. 
     Leader Structure for Wiring Member C 3   
     The constitution of this embodiment is such that each wiring member C 3  connected to the piezoelectric sounding body  32  is led out from the second principle surface  32   b  side of the vibration plate  321 . In other words, the terminals  324 ,  325  of the piezoelectric sounding body  32  are placed facing the first space S 1 , which means a wiring path is needed to lead these wiring members C 3  to the terminal  333  on the circuit board  33 . Accordingly in this embodiment, a guide groove that can house each wiring member C 3  is provided on the side periphery surface of the base  312  of the electromagnetic sounding body  31  and also on the ring-shaped member  34 , and the wiring member C 3  is constituted in such a way that it is led out toward the electromagnetic sounding body  31 , from the piezoelectric sounding body  32 , through the first space S 1 . 
     As shown in  FIG. 2 , a first guide groove  31   f  to house the multiple wiring members C 3  wired between the first surface  31   a  and second surface  31   b  is provided on the periphery surface  31   e  and top surface  31   d  of the electromagnetic sounding body  31 . This way, the wiring members C 3  can be wired easily without risking damage between the periphery surface  31   e  of the electromagnetic sounding body  31  and the side wall  412  of the enclosure  41 , and also between the top surface  31   d  of the electromagnetic sounding body  31  and the pressure part  421  of the cover  42 . 
     The first guide groove  31   f  is formed in the diameter direction on the top surface  31   d , and in the height direction (Z-axis direction) on the periphery surface  31   e . The guide grooves  31   f  formed on the top surface  31   d  and periphery surface  31   e  are connected to each other. The first guide groove  31   f  is constituted as a square groove, but it may be constituted as a concave groove of round or other shape. The position at which the first guide groove  31   f  is formed is not limited in any way, but preferably it is provided at a position close to the terminal  333  on the circuit board  33 , as shown in  FIG. 3 . 
     It should be noted that, if the pressure part  421  of the cover  42  is constituted by multiple pillars, the wiring members C 3  can be guided between these pillars and therefore formation of guide groove  31   f  on the top surface  31   d  can be omitted. 
     On the other hand, a second guide groove  34   a  that can house multiple wiring members C 3  is provided on the support surface  341  of the ring-shaped member  34 . The second guide groove  34   a  is formed linearly in the diameter direction so as to connect the inner periphery and outer periphery of the ring-shaped member  34 . The second guide groove  34   a  is formed at a position where it connects to the first guide groove  31   f  in a condition where the sounding unit  30  is assembled into the enclosure  41 . This way, the wiring members C 3  can be wired easily without risking damage between the leg  312   a  of the electromagnetic sounding body  31  and the ring-shaped member  34 . 
     As described above, according to this embodiment, the electromagnetic sounding body  31  can be assembled to the enclosure  41  without losing any ease of operation. 
     Passage 
     When the first space S 1  is closed in an air-tight manner, low-pitch sound waves may not be generated with desired frequency characteristics. To be specific, it is difficult to flexibly cope with the peak level adjustment in a specific frequency band, or the optimization of frequency characteristics at the cross point between the low-pitch sound characteristic curve and high-pitch sound characteristic curve, or the like. 
     Accordingly in this embodiment, passages  35  that connect the first space S 1  and second space S 2  are provided in the piezoelectric sounding body  32 .  FIG. 8  is a schematic plan view showing the constitution of the piezoelectric sounding body  32 . 
     The passages  35  are provided in the thickness direction of the vibration plate  321 . In this embodiment, the passages  35  are each constituted by multiple through holes provided in the vibration plate  321 . As shown in  FIG. 8 , the passage  35  is formed at multiple locations around the piezoelectric element  322 . Since the ring-shaped member  34  is attached to a periphery  321   e  of the vibration plate  321 , the passages  35  are provided in the area between the piezoelectric element  322  and ring-shaped member  34 . In this embodiment, the piezoelectric element  322  has a rectangular planar shape, so by providing the passages  35  in the area between at least one side of the piezoelectric element  322  and the periphery  321   c  (ring-shaped member  34 ) of the vibration plate  321 , sufficient area in which to form the passages  35  can be secured without limiting the size of the piezoelectric element  322  more than necessary. 
     The passages  35  are used to pass some of the sound waves generated by the electromagnetic sounding body  31  from the first space S 1  to the second space S 2 . Accordingly, low-pitch sound frequency characteristics can be adjusted or tuned by the number of passages  35 , passage size, etc., meaning that the number of passages  35 , passage size, etc., are determined according to the desired low-pitch sound frequency characteristics. Because of this, the number of passages  35  and passage size are not limited to those in the example of  FIG. 8 , and there may be one passage  35 , for example. 
     It should be noted that the opening shape of the passage  35  is not limited to circular, either, and the number of openings may also be different from one location to another. For example, the passages  35  may include oval passages  351  as shown in  FIG. 9 . 
     Earphone Operation 
     Next, a typical operation of the earphone  100  of this embodiment as constituted above is explained. 
     With the earphone  100  of this embodiment, playback signals are input to the circuit board  33  of the sounding unit  30  via the wiring member C 1 . The playback signals are input to the electromagnetic sounding body  31  and piezoelectric sounding body  32  via the circuit board  33  and wiring members C 2 , C 3 , respectively. As a result, the electromagnetic sounding body  31  is driven to generate low-pitch sound waves primarily of 7 kHz or below. 
     With the piezoelectric sounding body  32 , on the other hand, the vibration plate  321  vibrates due to the expansion/contraction action of the piezoelectric element  322 , and high-pitch sound waves primarily of 7 kHz or above are generated. The generated sound waves in different bands are transmitted to the user&#39;s ear via the sound path  11 . This way, the earphone  100  functions as a hybrid speaker having a sounding body for low-pitch sounds and sounding body for high-pitch sounds. 
     Here, sound waves generated by the electromagnetic sounding body  31  are formed by composite waves having a sound wave component that propagates to the second space S 2  by vibrating the vibration plate  321  of the piezoelectric sounding body  32 , and a sound wave component that propagates to the second space S 2  via the passages  35 . Accordingly, low-pitch sound waves output from the piezoelectric sounding body  32  can be adjusted or tuned to frequency characteristics that give a sound pressure peak in a specified low-pitch sound band, for example, by optimizing the size of the passage  35 , number of passages, etc. 
     In this embodiment, the passages  35  are each constituted by a through hole penetrating the vibration plate  321  in its thickness direction, so the sound wave propagation path from the first space S 1  to the second space S 2  can be minimized (made the shortest). This makes it easier to set a sound pressure peak in a specified low-pitch sound range. 
     For example,  FIG. 10  is a characteristic diagram of playback sound waves where the sound wave propagation path is longer than necessary. In the figure, the horizontal axis represents frequency and the vertical axis represents sound pressure level (in arbitrary units), while F 1  indicates the frequency characteristics of low-pitch sounds played back by the electromagnetic sounding body and F 2  indicates the frequency characteristics of high-pitch sounds played back by the piezoelectric sounding body. In the example of  FIG. 10 , there is a large dip near approx. 3 kHz. When a musical piece is played, generally the 3-kHz band corresponds to the frequency band of sounds uttered by vocalists. Accordingly, a dip in this band tends to decrease the quality of vocal sound. 
     On the other hand,  FIG. 11  is a characteristic diagram similar to the one in  FIG. 10 , this time showing playback sound waves where the passage  35  is constituted by the shortest path. According to this embodiment, low-pitch sound frequency characteristics with a peak near 3 kHz can be achieved. This improves the quality of vocal sound, which in turn improves the playback quality of musical pieces. 
     Also, the passage  35  functions as a low-pass filter that cuts, from among the sound waves generated by the electromagnetic sounding body those high-frequency components of or above a specified level. This way, sound waves in a specified low-frequency band can be output without affecting the frequency characteristics of high-pitch sound waves generated by the piezoelectric sounding body  32 . 
     Furthermore, according to this embodiment, the piezoelectric sounding body  32  is constituted in a manner leading all of the multiple wiring members C 3  toward the second principle surface  32   b  side of the vibration plate  321 , which improves not only the ease of connecting the wiring members C 3  to the piezoelectric element  322 , but also the ease of assembly to the enclosure  41 , compared to when the wires are led out from the first principle surface  32   a  side of the vibration plate  321 . 
     Moreover, the sounding unit  30  allows the electromagnetic sounding body  31  and piezoelectric sounding body  32  to be assembled into the enclosure  41  at once while being connected to each other via the wiring members C 3 , which improves the ease of assembly further. Also, the first and second guide grooves  31   f ,  34   a  that can house the wiring members C 3  are provided on the periphery surface  31   e  of the electromagnetic sounding body  31  and the support surface  341  of the ring-shaped member  34 , respectively, which allows for wiring of the wiring members C 3  through proper paths without risking damage. This way, stable assembly accuracy can be ensured without requiring mastery of work. 
     Second Embodiment 
       FIG. 12  is a schematic section view of an earphone  200  pertaining to another embodiment of the present invention. Constitutions different from those of the first embodiment are primarily explained below, and the same constitutions as in the aforementioned embodiment are not explained or explained briefly using the same symbols. 
     The earphone  200  of this embodiment is different from the aforementioned first embodiment in terms of the constitution of a sounding unit  50 , especially that of a piezoelectric sounding body  52 . The piezoelectric sounding body  52  has a vibration plate  521 , and the piezoelectric element  322  joined to one principle surface (principle surface facing the first space S 1  in this example) of the vibration plate  521 . 
       FIG. 13  is a schematic plan view showing the constitution of the piezoelectric sounding body  52 . As shown in  FIG. 13 , multiple (three in the illustrated example) projecting pieces  521   g  that project radially outward in the diameter direction are provided along the periphery of the vibration plate  521 . The multiple projecting pieces  521   g  are fixed to the inner periphery of the ring-shaped member  34 . Accordingly, the vibration plate  521  is fixed to the support  411  of the enclosure  41  via the multiple projecting pieces  521   g  and ring-shaped member  34 . 
     The multiple projecting pieces  521   g  are typically formed at equal angular intervals. The multiple projecting pieces  521   g  are formed by providing multiple cutouts  521   h  along the periphery of the vibration plate  521 . How far the projecting pieces  521   g  project is adjusted by the cutout depth of the cutouts  521   h.    
     Passages  55  that connect the first space S 1  and second space S 2  are provided in the piezoelectric sounding body  52 . In this embodiment, the cutout depth of each cutout  521   h  is set so that arc-shaped openings of specified width are formed between the inner periphery surface of the ring-shaped member  34  and the multiple projecting pieces  521   g  positioned adjacent to each other. The openings form the passages  55  penetrating the vibration plate  521  in its thickness direction. 
     The number of passages  55 , opening width in the diameter direction of the vibration plate  521 , opening length in the circumferential direction of the vibration plate  521 , etc., can be set as deemed appropriate, and are determined according to the desired low-pitch sound frequency characteristics. This way, playback sound frequency characteristics with a sound pressure peak in a specified low-pitch sound range (such as 3 kHz) can be achieved just like in the first embodiment.  FIG. 14  shows a constitutional example of a vibration plate  521  having four projecting pieces  521   g , while  FIG. 15  shows a constitutional example of a vibration plate  521  having five projecting pieces  521   g.    
     In addition, the vibration plates  521  in this embodiment are each constituted to vibrate around some or all of the multiple projecting pieces  521   g  as fulcrums, which makes it possible to adjust the resonance frequency of the vibration plate  521  according to the number of projecting pieces  521   g , their shape, layout or fixing method. If the designed resonance frequency of the vibration plate  521  having four fulcrums as shown in  FIG. 14  is 10 kHz, for example, the resonance frequency of the vibration plate  521  with three fulcrums as shown in  FIG. 13  becomes lower, such as 8 kHz, while the resonance frequency of the vibration plate  521  with five fulcrums as shown in  FIG. 15  becomes higher, such as 12 kHz. Besides the above, the thickness, outer diameter, material, etc., of the vibration plate  521  can also be used to adjust the resonance frequency. 
     As described above, the resonance frequency of the vibration plate  521  can be adjusted according to the number of projecting pieces  521   g , etc., which makes it easy to achieve desired frequency characteristics, such as a flat composite frequency at the cross point between the low-pitch sound characteristic curve by the electromagnetic sounding body  31  and the high-pitch sound characteristic curve by the piezoelectric sounding body  52 . 
     A in  FIG. 16  through C in  FIG. 16  are schematic diagrams explaining the relationship between the resonance frequency of the vibration plate  521  and the playback sound frequency characteristics of the earphone  200 , where the horizontal axis represents frequency and the vertical axis represents sound pressure level. In each figure, F 1  (thin solid line) indicates the frequency characteristics of low-pitch sounds played back by the electromagnetic sounding body  31 , F 2  (broken line) indicates the frequency characteristics of high-pitch sounds played back by the piezoelectric sounding body  52 , and F 0  (thick solid line) indicates the composite characteristics of the foregoing. Furthermore, P indicates the point of intersection between the curves F 1  and F 2 , or specifically the cross point mentioned above. 
     In A through C in  FIG. 16 , the resonance frequency of the vibration plate  521  increases in the order of B, C and A. 
     In the example of A in  FIG. 16 , a dip is likely to occur in the band of the cross point P, while in the example of B in  FIG. 16 , a peak is likely to occur in the band of the cross point P. In the example of C in  FIG. 16 , on the other hand, flat characteristics are achieved in the band of the cross point P. 
     Generally with hybrid speakers, one important point in sound quality tuning is the cross point between the low-pitch sound characteristic curve and high-pitch sound characteristic curve. Typically the cross point is adjusted so that the composite frequencies of low-pitch sounds and high-pitch sounds become flat in the band of the cross point P, as shown in C in  FIG. 16 . According to this embodiment, the resonance frequency of the vibration plate  521  can be adjusted according to the number of fulcrums (projecting pieces  521   g ) of the vibration plate  521 , which makes it possible to easily achieve desired frequency characteristics, such as flat characteristics in the band of the cross point P. 
     Third Embodiment 
       FIG. 20  is a schematic section view of an earphone  400  pertaining to another embodiment of the present invention. Constitutions different from those of the first embodiment are primarily explained below, and the same constitutions as in the aforementioned embodiment are not explained or explained briefly using the same symbols. 
     The earphone  400  of this embodiment is different from the aforementioned first embodiment in terms of the constitution of a sounding unit  70 , especially that of a piezoelectric sounding body  72 . The sounding unit  70  has an electromagnetic sounding body  31  and piezoelectric sounding body  72 . The piezoelectric sounding body  72  is constituted in the same manner as the piezoelectric sounding body  32  in the first embodiment, except that the piezoelectric element  322  is joined to the second principle surface  32   a  of the vibration plate  321 . The sounding unit  70  further has a ring-shaped member  54  placed between the support  411  of the enclosure  41  and the periphery  321   c  of the vibration plate  321 . 
     The ring-shaped member  54  has a contact surface  413  that contacts the support  411 , and a second guide groove  35   a  that is provided on the contact surface  413 , connects to the first guide groove  31   f , and stores the wiring member C 3 . The contact surface  413  includes the outer periphery surface ad bottom surface of the ring-shaped member  54 . The second guide groove  35   a  is formed along the outer periphery surface and bottom surface of the ring-shaped member  54 , where it is linearly formed in the height direction (Z-axis direction) on the outer periphery surface and in the diameter direction on the bottom surface. The second guide groove  35   a  can house multiple wiring members C 3  just like the first guide groove  31   f.    
     The wiring member C 3  is electrically connected to the piezoelectric element  322  and led out toward the electromagnetic sounding body  31 , from the piezoelectric element  322 , through the second space S 2 . In other words, the terminals  324 ,  325  of the piezoelectric sounding body  72  are positioned in a manner facing the second space S 2 , and the wiring members C 3  connected to the terminals  324 ,  325  are led to the terminal  333  on the circuit board  33  via the second guide groove  35   a  and first guide groove  31   f . According to this embodiment, where the second guide groove  35   a  faces the second space S 2  and no guide groove facing the first space S 1  is provided, the first space S 1  has greater air-tightness. This way, leakage of sound pressure from the electromagnetic sounding body  31  is prevented and low-pitch sound pressures become easier to control. Also, wiring vibration from the guide grooves caused by sound pressure leakage and wiring interference may generate audible noises in the form of rattles (abnormal sounds, noises); according to this embodiment, however, such rattles can be prevented because each wiring member C 3  is positioned on the opposite side of the electromagnetic sounding body  31  with respect to the piezoelectric sounding body  72 . 
     Also, the sounding unit  70  allows the electromagnetic sounding body  31  and piezoelectric sounding body  72  to be assembled into the enclosure  41  at once while being connected to each other via the wiring members C 3 , which improves the ease of assembly. Also, the first and second guide grooves  31   f ,  35   a  that can house the wiring members C 3  are provided on the periphery surface  31   e  of the electromagnetic sounding body  31  and the contact surface  413  of the ring-shaped member  34 , respectively, which allows for wiring of the wiring members C 3  through proper paths without risking damage. This way, stable assembly accuracy can be ensured without requiring mastery of work. 
     While the piezoelectric element  322  is joined to the second principle surface  32   a  of the vibration plate  321  in this embodiment, it can also be joined to the first principle surface  32   b . In this case, each wiring member C 3  is led out from the first principle surface  32   b  side, guided through the passage  35 , and stored in the second guide groove  35   a . In other words, the wiring member C 3  is led out toward the electromagnetic sounding body, from the piezoelectric element  322 , through the first space S 1 . Such constitution can be applied to each of the aforementioned embodiments. 
     The foregoing explained embodiments of the present invention, but the present invention is not limited to the aforementioned embodiments and it goes without saying that various modifications may be added. 
     For example, in the aforementioned embodiments the passages that guide low-pitch sound waves to the sound path were provided in the piezoelectric sounding body; however, the passages are not limited to the foregoing and may be provided around the piezoelectric sounding body. In this case, the outer diameter of the piezoelectric sounding body U 2  is formed smaller than the inner diameter of the side wall of the enclosure B, as shown schematically in  FIG. 17 , for example, and passages T through which to pass low-pitch sound waves generated by the electromagnetic sounding body U are formed between the two. It should be noted that the piezoelectric sounding body U 2  is fixed to the bottom B 1  of the enclosure B via multiple support pillars R. This way sound waves passing through the passages T can be guided to the sound path B 2 . 
     Also, the aforementioned embodiments were explained using earphones  100 ,  200 ,  300  as examples of the electroacoustic converter, but the present invention is not limited to the foregoing and can also be applied to headphones, hearing aids, etc. 
     In addition, the present invention can also be applied as speaker units installed in mobile information terminals, personal computers and other electronic devices. 
     Furthermore, with the sounding units  30 ,  50 ,  70  of the respective embodiments above, the electromagnetic sounding body  31  and piezoelectric sounding body  32  ( 52 ,  72 ) were constituted as separate components; however, they may be constituted as one integral component. For example,  FIG. 19  shows a constitutional example of a sounding unit  300  constituted by the electromagnetic sounding body  31  and piezoelectric sounding body  32  joined integrally together. 
     In  FIG. 19 , a periphery  323   c  of a vibration plate  323  of the piezoelectric sounding body  32  is fixed to the base  312 , together with the periphery of the vibration plate E 1  of the electromagnetic sounding body  31 , by the ring-shaped fixture  310 . The ring-shaped fixture  310 , when assembled to the base  312 , constitutes a fixing part that commonly supports the peripheries of the two vibration plates  323 , E 1 . Also, the center area of the vibration plate  323  of the piezoelectric sounding body  32 , which is joined to the piezoelectric element  322  to constitute a vibration surface, has the shape of a shallow bowl curving from the periphery  323   c  in the direction of moving away from the vibration plate E 1  of the electromagnetic sounding body  31 . This way, the two vibration plates  323 , E 1  can vibrate independently without interfering with each other. 
     Also, the passage  35  through which the low-pitch sound waves generating at the electromagnetic sounding body  31  can pass is provided in the center area of the vibration plate  323 . The passage  35  is constituted by a through hole as in the first embodiment, but it may also be constituted by a cutout formed along the periphery  323   c  as in the second embodiment. 
     According to the sounding unit  300  of the above constitution, where the electromagnetic sounding body  31  and piezoelectric sounding body  32  are constituted as one mutually integral component, the sounding unit  300  can have a simpler and thinner constitution. The number of components can also be reduced, which improves the ease of assembly of the electroacoustic converter. 
     In the present disclosure where conditions and/or structures are not specified, a skilled artisan in the art can readily provide such conditions and/or structures, in view of the present disclosure, as a matter of routine experimentation. Also, in the present disclosure including the examples described above, any ranges applied in some embodiments may include or exclude the lower and/or upper endpoints, and any values of variables indicated may refer to precise values or approximate values and include equivalents, and may refer to average, median, representative, majority, etc. in some embodiments. Further, in this disclosure, “a” may refer to a species or a genus including multiple species, and “the invention” or “the present invention” may refer to at least one of the embodiments or aspects explicitly, necessarily, or inherently disclosed herein. The terms “constituted by” and “having” refer independently to “typically or broadly comprising”, “comprising”, “consisting essentially of”, or “consisting of” in some embodiments. In this disclosure, any defined meanings do not necessarily exclude ordinary and customary meanings in some embodiments. 
     The present application claims priority to Japanese Patent Application No. 2014-217519, filed Oct. 24, 2015 and No. 2015-090335, filed Apr. 27, 2015, each disclosure of which is incorporated herein by reference in its entirety, including any and all particular combinations of the features disclosed therein, for some embodiments. 
     It will be understood by those of skill in the art that numerous and various modifications can be made without departing from the spirit of the present invention. Therefore, it should be clearly understood that the forms of the present invention are illustrative only and are not intended to limit the scope of the present invention.