Patent Publication Number: US-10313774-B2

Title: Earphone

Description:
TECHNICAL FIELD 
     The present invention relates to an earphone. 
     BACKGROUND ART 
     Some electroacoustic transducers of earphones and headphones are condenser type electroacoustic transducers that have excellent frequency response and are suitable for high fidelity (Hi-Fi) playback. In general, condenser type electroacoustic transducers are of a push-pull scheme that can reduce harmonic distortion. A condenser type electroacoustic transducer includes a diaphragm and a pair of fixed electrodes facing the respective faces of the diaphragm. 
     With respect to earphones including condenser type electroacoustic transducers, closed type earphones (for example, refer to PTL 1) and open type earphones (refer to NPL 1) have been proposed. 
     PATENT LITERATURE (PTL) 
     
         
         [PTL 1] 
         Japanese Translation of PCT International Application No. JP-T-2015-531557 
       
    
     NON PATENT LITERATURE (NPL) 
     
         
         [NPL 1] 
         “SRS-002 User&#39;s Manual,” STAX Ltd. [online], [searched on Dec. 15, 2015] &lt;URL: http://www.stax.co.jp/information/usermanual/SRS002.pdf&gt; 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In general, closed type earphones have low sound leakage and are suitable for use in public areas and recording sites. However, the movement of the diaphragms of the closed type earphones are readily restricted, and sounds are readily muffled. 
     Open type earphones can readily produce natural auditory sounds without muffling of sound, compared to closed type earphones. Thus, many users prefer open type earphones. However, open type earphones have high sound leakage and are not suitable for use in public areas or recording sites. 
     As described above, closed type earphones and open type earphones both have advantages and disadvantages. Thus, the user of earphones should appropriately select closed type earphones or open type earphones depending on the usage and/or preference. 
     In a condenser type electroacoustic transducer, the movement of the diaphragm is restricted by the stiffness of the air in the front and back of the diaphragm. That is, the volumes of the air in the front and back of the diaphragm affect the characteristics (frequency characteristics, for example) of the condenser type electroacoustic transducer. Thus, appropriate acoustic designs have been applied to condenser type closed type earphones and condenser type open type earphones. 
     An object of the present invention is to solve the problems described above and to provide an earphone including condenser type electroacoustic transducers switchable between a closed type earphone mode and an open type earphone mode and which achieves satisfactory acoustic characteristics in either mode. 
     Solution to Problem 
     An earphone according to the present invention includes a diaphragm, at least one fixed electrode constituting a capacitor with the diaphragm, a housing accommodating the diaphragm and the at least one fixed electrode, the housing having an interior and an exterior, a sound conduit protruding forward from the housing, at least one communication hole establishing communication between the exterior and the interior of the housing, and an opening and closing mechanism to open and close the at least one communication hole. The sum of the volume of the interior space of the sound conduit and the volume of the space in a front of the diaphragm in communication with the interior space of the sound conduit in the interior space of the housing is smaller than the volume of the space in a rear of the diaphragm in the interior space of the housing. 
     According to the present invention, an earphone including condenser type electroacoustic transducer can switch between a closed type earphone mode and an open type earphone mode and achieve satisfactory acoustic characteristics in either mode. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an external view illustrating an embodiment of an earphone according to the present invention. 
         FIG. 2  is a view of a left unit of the earphone viewed along the arrow A in  FIG. 1 . 
         FIG. 3  is an exploded view of the left unit in  FIG. 2 . 
         FIG. 4  is a view of a left unit body of the left unit viewed along the arrow C in  FIG. 3 . 
         FIG. 5  is a cross-sectional view of the left unit taken along line B-B of  FIG. 2 . 
         FIG. 6  is an exploded perspective view of an electroacoustic transducer of the left unit in  FIG. 2 . 
         FIG. 7  illustrates the electrical charges in an electret of the electroacoustic transducer in  FIG. 6 . 
         FIG. 8  is an external view of the left unit body in  FIG. 4  from which a rear housing half is detached. 
         FIG. 9  is a cross-sectional view of the left unit in  FIG. 2  placed in the left ear of the user. 
         FIG. 10  is an equivalent circuit diagram of  FIG. 9 . 
         FIG. 11  is a cross-sectional view of a left unit body of the left unit in  FIG. 2  from which a left earphone cover is detached, and placed in the left ear of the user. 
         FIG. 12  is an equivalent circuit diagram of  FIG. 11 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Earphone 
     Embodiments of an earphone according to the present invention will now be described with reference to the attached drawings. 
     Configuration of Earphone 
       FIG. 1  is an external view illustrating an embodiment of an earphone E according to the present invention. 
     The earphone E outputs acoustic waves corresponding to audio signals from a sound source, such as a portable music player (not shown), to the eardrums of a user of the earphone E. The earphone E includes a left earphone unit (hereinafter, referred to as “left unit”) LE and a right earphone unit (hereinafter, referred to as “right unit”) RE. The left unit LE and the right unit RE are placed in the ears of the user. The earphone E is a condenser type earphone. The left unit LE and the right unit RE respectively include a condenser type electroacoustic transducer described below. 
     The configuration of the left unit LE and the right unit RE are identical except for being symmetrical. Thus, the configuration of the left unit LE will now be described as an example below. 
       FIG. 2  is a view of the left unit LE, viewed along the arrow A in  FIG. 1 . 
     In the description below, the front side is the side (the lower side in  FIG. 2 ) proximate to the head of the user of the left unit LE when the left unit LE is placed in the left ear of the user (hereinafter, referred to as “placed state”). The upper side is the side (the upper side in  FIG. 1 ) proximate to the vertex of the head of the user. The left side is the side (the left side in  FIG. 2 ) facing forward with the face of the user. 
     The left unit LE is placed in the left ear of the user and outputs acoustic waves corresponding to audio signals from the sound source to the left eardrum of the user. The left unit LE includes a left unit body (hereinafter, referred to as “body”) LEB and a left unit cover member (hereinafter, referred to as “cover member”) LEC serving as an opening and closing mechanism. 
       FIG. 3  is an exploded view of the left unit LE. 
       FIG. 4  is a view of the body LEB viewed along the arrow C in  FIG. 3 . 
       FIG. 5  is a cross-sectional view of the left unit LE taken along line B-B of  FIG. 2 . 
     The body LEB includes a housing  1 , a hanger  2 , a sound conduit  3 , an earpiece  4 , an electroacoustic transducer  5 , a fixing member  6 , protective members  7 , a cord bush  8 , and a cord  9 . 
     The housing  1  accommodates the electroacoustic transducer  5 . The housing  1  is composed of metal, such as aluminum. As shown in  FIG. 3 , the housing  1  includes a front housing half  10 , a rear housing half  11 , and a sealing member  12  (see  FIG. 5 ). The front housing half  10  and the rear housing half  11  are fixed together with a screw (not shown). The housing  1  has a shape of a substantially rectangular box flattened in the front-rear direction (the vertical direction in  FIG. 3 ). That is, as shown in  FIG. 5 , the housing  1  is a hollow body having a front wall  10   a , a rear wall  11   a , and a circumferential wall. 
     The material of the housing may be metal or any other material having rigidity that does not cause vibration of the housing in response to the acoustic waves generated by the electroacoustic transducer. That is, the housing may be composed of synthetic resin, such as plastic. 
     The housing  1  has an inner wall  10   b . The inner wall  10   b  protrudes from the front wall  10   a  of the housing  1  toward the rear wall  11   a . The inner wall  10   b  surrounds the rear face of the front wall  10   a  in a shape of a rectangle (see  FIG. 8 ). 
     As shown in  FIGS. 5 and 8 , the front wall  10   a  has a depression  10   c  having a shape of a rectangle, a fitting depression  10   d  having a shape of a circle, and a front sound-emitting hole  10   h  having a shape of a circle. The depression  10   c  is disposed in an area on the rear face of the front wall  10   a  surrounded by the inner wall  10   b , other than the peripheral area. The fitting depression  10   d  is disposed in the lower half of the front face of the front wall  10   a  opposite from the depression  10   c . The front sound-emitting hole  10   h  guides acoustic waves from the electroacoustic transducer  5  to the sound conduit  3 . The front sound-emitting hole  10   h  is disposed in the central area of the fitting depression  10   d.    
     As shown in  FIG. 5 , the rear wall  11   a  has a depression  11   b  having a shape of a rectangle and multiple rear holes  11   h . The depression  11   b  is disposed in the front face of the rear wall  11   a  in an area facing the front wall  10   a  surrounded by the inner wall  10   b . The rear holes  11   h  establish communication between the rear space in the electroacoustic transducer  5  in the housing  1  and the exterior of the housing  1  when the rear holes  11   h  are opened as described below. The rear holes  11   h  are disposed at an equal pitch in the depression  11   b  of the rear wall  11   a . The sealing member  12  is disposed between the rear end face of the inner wall  10   b  and the front face of the rear wall  11   a  surrounding the depression  11   b.    
     The circumferential wall of the housing  1  has a fitting hole  1   h . The fitting hole  1   h  is disposed in the lower face (the left face in  FIG. 5 ) of the circumferential wall of the housing  1 . The fitting hole  1   h  is fit with the cord bush  8 . 
     The interior space (the space surrounded by the inner wall  10   b ) of the inner wall  10   b  inside the housing  1  is an accommodating room R 1  accommodating the electroacoustic transducer  5 . The exterior space (the space between the inner wall  10   b  and the circumferential wall) of the inner wall  10   b  is a wiring room R 2  in which a part of the cord  9  is disposed. The accommodating room R 1  is airtightly sealed to the wiring room R 2  by the sealing member  12 . 
     The hanger  2  is hooked around the auricle of the left ear of the user and maintain the earpiece  4 , attached to the sound conduit  3 , inserted inside the left ear canal of the user. The hanger  2  includes a connection member  20 , an arm support member  21 , a hanger arm  22 , and a biasing member  23 . 
     The connection member  20  connects the arm support member  21  to the housing  1 . The arm support member  21  supports the hanger arm  22 . The arm support member  21  has a substantially columnar shape. As shown in  FIG. 4 , the arm support member  21  is disposed in the left side (the left side in  FIG. 4 ) of the housing  1 . That is, the arm support member  21  is disposed parallel to the housing  1  such that the longitudinal direction of the arm support member  21  matches the vertical direction in  FIG. 4 . The lower end portion of the arm support member  21  is connected to the housing  1  with the connection member  20 . 
     The hanger arm  22  is placed on the auricle of the user when the earphone E is used by the user. The hanger arm  22  is swingably connected to the upper end of the arm support member  21 . The hanger arm  22  has a crescentic (arcuate) shape. The hanger arm  22  surrounds the upper side and the right side of the housing  1  in a crescentic shape in  FIG. 4  (viewed along the arrow C in  FIG. 3 ). As shown in  FIG. 3 , the hanger arm  22  is disposed in the front side (the lower side in  FIG. 3 ) of the housing  1  in the front-back direction (the vertical direction in  FIG. 3 ). 
     The biasing member  23  biases the hanger arm  22  toward the housing  1 . The biasing member  23  is attached to the connecting part of the arm support member  21  and the hanger arm  22 . 
     The sound conduit  3  guides acoustic waves from the electroacoustic transducer  5  to the ear canal of the user when the earphone E is used by the user. The sound conduit  3  is composed of metal, such as aluminum. As shown in  FIG. 5 , the sound conduit  3  has a substantially columnar shape. The sound conduit  3  is tapered from the rear end toward the front end. The rear end of the sound conduit  3  is fit with the fitting depression  10   d  of the housing  1 . The front end of the sound conduit  3  is directed diagonally forward in the lower left side of the front wall  10   a  of the housing  1 . That is, the sound conduit  3  is disposed on the front wall  10   a  of the housing  1  and protrudes diagonally forward from the housing  1 . 
     The material of the sound conduit may be metal or any other material having rigidity that does not cause vibration of the sound conduit in response to acoustic waves generated by the electroacoustic transducer. For example, the sound conduit may be composed of synthetic resin, such as plastic. 
     The earpiece  4  is in tight contact with the inner face of the ear canal of the user when the earphone E is used by the user. The earpiece  4  is composed of elastic material, such as silicone rubber. The earpiece  4  is turned up in U-shape in cross-sectional view at the front end portion and has a shape of a substantially double cylinder. The earpiece  4  includes an outer cylinder  40  and an inner cylinder  41 . The outer cylinder  40  has a shape of a barrel. The inner cylinder  41  has a shape of a cylinder. The outer cylinder  40  has a thickness smaller than the thickness of the inner cylinder  41 . Thus, the outer cylinder  40  is readily deformable. The front end portion of the sound conduit  3  is placed inside the rear half portion of the inner cylinder  41  of the earpiece  4 . In other words, the earpiece  4  is attached detachably to the front end of the sound conduit  3 . 
       FIG. 6  is an exploded perspective view of the electroacoustic transducer  5 . 
     The electroacoustic transducer  5  generates acoustic waves corresponding to audio signals from the sound source. The transduction scheme of the electroacoustic transducer  5  is of a condenser type. The electroacoustic transducer  5  includes a first transducer unit  5 A and a second transducer unit  5 B. The first transducer unit  5 A is disposed in the front of the second transducer unit  5 B. 
     The first transducer unit  5 A includes a diaphragm  50 , a first diaphragm frame  51 A, a first spacer  52 A, a first electret board  53 A, a first electrode  54 A, and a first insulator  55 A. 
     The diaphragm  50  is configured to vibrate in response to audio signals from the sound source. The diaphragm  50  is composed of synthetic resin, such as PPS (polyphenylenesulfide). The diaphragm  50  is a rectangular thin film. A metal film, such as a gold film, is deposited on one side of the diaphragm  50 . The diaphragm  50  is shared by the first transducer unit  5 A and the second transducer unit  5 B. The diaphragm  50  is stretched on the first diaphragm frame  51 A and a second diaphragm frame  51 B described below with predetermined tension, for example. The diaphragm  50  may be fixed by bonding to one of the first diaphragm frame  51 A or the second diaphragm frame  51 B. The case when the diaphragm  50  is fixed to the first diaphragm frame  51 A is described as an example below. 
     The first diaphragm frame  51 A holds the diaphragm  50 . The first diaphragm frame  51 A is composed of conductive metal, such as copper alloy. The first diaphragm frame  51 A is a rectangular frame. The first diaphragm frame  51 A includes a first protruding electrode  51 Aa. The first protruding electrode  51 Aa is connected to a first signal line  90  of the cord  9  described below. The first protruding electrode  51 Aa protrudes leftward from the central area in the longitudinal direction of the left side of the first diaphragm frame  51 A. 
     The first spacer  52 A insulates the first diaphragm frame  51 A and the first electret board  53 A. The first spacer  52 A is composed of insulating synthetic resin, such as PET (polyethylene terephthalate). The first spacer  52 A is a rectangular frame. The first spacer  52 A is disposed in the front of the first diaphragm frame  51 A. The rear face of the first spacer  52 A is in contact with the front face of the first diaphragm frame  51 A. 
     The first electret board  53 A retains electrical charges and generates a difference in potential across the first electrode  54 A and the diaphragm  50 . The first electret board  53 A is composed of a brass plate and a resin film, such as an electrically charged fluorinated ethylene polypropylene (FEP) (e.g., a copolymer of tetrafluoroethylene and hexafluoropropylene) film, bonded to the brass plate. The electrical charges in the first electret board  53 A will be described below. The first electret board  53 A is a rectangular plate. The first electret board  53 A has multiple sound holes  53 Ah. The sound holes  53 Ah are holes through which acoustic waves from the diaphragm  50  pass. The first electret board  53 A is disposed in the front of the first spacer  52 A. The periphery of the rear face of the first electret board  53 A is in contact with the front face of the first spacer  52 A. 
     The first electrode  54 A and the first electret board  53 A constitute a fixed electrode (back electrode)  56 A of the first transducer unit  5 A. The fixed electrode  56 A and the diaphragm  50  constitute a first capacitor. The first electrode  54 A is composed of conductive metal, such as copper alloy. The first electrode  54 A is a rectangular frame. The first electrode  54 A includes a second protruding electrode  54 Aa. The second protruding electrode  54 Aa is connected to a second signal line  91  of the cord  9  described below. The second protruding electrode  54 Aa protrudes leftward from the lower half in the longitudinal direction of the left side of the first electrode  54 A. The first electrode  54 A is disposed in the front of the first electret board  53 A. The rear face of the first electrode  54 A is in contact with the periphery of the front face of the first electret board  53 A. 
     The first insulator  55 A insulates the fixed electrode  56 A from the housing  1 . The first insulator  55 A is composed of insulating synthetic resin, such as PC (polycarbonate). The first insulator  55 A is a rectangular frame. The first insulator  55 A has a step portion  55 Aa and a notched groove (not shown). The step portion  55 Aa fits with the fixed electrode  56 A. The step portion  55 Aa is disposed on the inner circumferential edge of the front face of the first insulator  55 A (see  FIG. 5 ). The notched groove fits with the second protruding electrode  54 Aa. The notched groove is disposed in the lower half of the left side of the first insulator  55 A. 
     In the first transducer unit  5 A, a gap (hereinafter, referred to as a “first thin air layer”) S 1  having a width corresponding to the thickness of the first diaphragm frame  51 A and the thickness of the first spacer  52 A is formed between the diaphragm  50  and the first electret board  53 A. 
     The second transducer unit  5 B includes the diaphragm  50 , a second diaphragm frame  51 B, a second spacer  52 B, a second electret board  53 B, a second electrode  54 B, and a second insulator  55 B. As described above, the diaphragm  50  is shared by the first transducer unit  5 A and the second transducer unit  5 B. 
     The configuration of the second diaphragm frame  51 B is identical to that of the first diaphragm frame  51 A. The second diaphragm frame  51 B includes a first protruding electrode  51 Ba. The first protruding electrode  51 Ba is in contact with the first protruding electrode  51 Aa of the first diaphragm frame  51 A. 
     The configuration of the second spacer  52 B is identical to that of the first spacer  52 A. The second spacer  52 B is disposed in the rear of the second diaphragm frame  51 B. The front face of the second spacer  52 B is in contact with the rear face of the second diaphragm frame  51 B. 
     The configuration of the second electret board  53 B is identical to that of the first electret board  53 A, except for the polarity of the electrical charges retained in the second electret board  53 B. The second electret board  53 B has multiple sound holes  53 Bh. The electrical charges in the second electret board  53 B will be described below. The second electret board  53 B is disposed in the rear of the second spacer  52 B. The periphery of the front face of the second electret board  53 B is in contact with the rear face of the second spacer  52 B. 
     The configuration of the second electrode  54 B is identical to that of the first electrode  54 A, except that the second protruding electrode  54 Aa is replaced with a third protruding electrode  54 Ba. The third protruding electrode  54 Ba is connected to a third signal line  92  of the cord  9  described below. The third protruding electrode  54 Ba protrudes leftward from the upper half in the longitudinal direction of the left side of the second electrode  54 B. The second electrode  54 B is disposed in the rear of the second electret board  53 B. The front face of the second electrode  54 B is in contact with the periphery of the rear face of the second electret board  53 B. The second electrode  54 B and the second electret board  53 B constitute a fixed electrode  56 B of the second transducer unit  5 B. The fixed electrode  56 B and the diaphragm  50  constitute a second capacitor. 
     The configuration of the second insulator  55 B is identical to that of the first insulator  55 A, except for the position of the notched groove. The second insulator  55 B has a notched groove  55 Bb. The notched groove  55 Bb fits with the third protruding electrode  54 Ba. The notched groove  55 Bb is disposed on the upper half of the left side of the second insulator  55 B. 
     In the second transducer unit  5 B, a gap (hereinafter, referred to as a “second thin air layer”) S 2  having a width corresponding to the thickness of the second diaphragm frame  51 B and thickness of the second spacer  52 B is formed between the diaphragm  50  and the second electret board  53 B. 
     As described above, the electroacoustic transducer  5  is constituted by the fixed electrode  56 A of the first transducer unit  5 A and the fixed electrode  56 B of the second transducer unit  5 B. The diaphragm  50  is disposed between the fixed electrode  56 A and the fixed electrode  56 B. Thus, the diaphragm  50  is driven in a push-pull mode. 
     As shown in  FIG. 5 , the electroacoustic transducer  5  is accommodated in the accommodating room R 1  of the housing  1 . The front face of the first insulator  55 A is in contact with the rear face of the front wall  10   a  surrounded by the inner wall  10   b  of the housing  1 . The outer circumferential surface of the first diaphragm frame  51 A, the outer circumferential surface of the first insulator  55 A, the outer circumferential surface of the second diaphragm frame  51 B, and the outer circumferential surface of the second insulator  55 B are in contact with the inner circumferential surface of the inner wall  10   b  of the housing  1 . 
       FIG. 7  illustrates the electrical charges retained by the first electret board  53 A and the second electret board  53 B. 
     The dotted lines in  FIG. 7  indicate the boundary between the brass plate and the FEP film of the first electret board  53 A and the boundary between the brass plate and the FEP film of the second electret board  53 B. 
     The FEP film of the first electret board  53 A is positively charged. The FEP film of the second electret board  53 B is negatively charged. The diaphragm  50  is disposed between the positively charged first electret board  53 A and the negatively charged second electret board  53 B. Thus, the electroacoustic transducer  5  is a complementary-back-electret condenser unit. 
     Alternatively, the FEP film of the first electret board may be negatively charged, and the FEP film of the second electret board may be positively charged, for example. 
       FIG. 8  is an external view of the body LEB in  FIG. 4  from which the rear housing half  11  is detached. The inside of the wiring room R 2  is not illustrated in  FIG. 8 . 
     The fixing member  6  fixes the electroacoustic transducer  5  in the accommodating room R 1  of the housing  1 . The fixing member  6  is composed of metal, such as aluminum. The fixing member  6  is a rectangular frame. The fixing member  6  includes multiple fixing portions  60 . The fixing portions  60  protrude from the four corners of the outer periphery of the fixing member  6  in the right-left direction (the horizontal direction in  FIG. 8 ). The fixing portions  60  have screw insertion holes  60   h . The screw insertion hole  60   h  is a hole through which a fixing screw (not shown) is to be inserted. The fixing member  6  is disposed in the rear (the forward of  FIG. 8 ) of the electroacoustic transducer  5 . The front face of the fixing member  6  is in contact with the rear face of the second insulator  55 B (see  FIG. 5 ). 
     The fixing member  6  is fixed to the housing  1  with the fixing screws. As a result, the electroacoustic transducer  5  is fixed to the housing  1 . The first protruding electrode  51 Aa, the first protruding electrode  51 Ba, the second protruding electrode  54 Aa, and the third protruding electrode  54 Ba are disposed in the wiring room R 2  in the housing  1 . 
     Referring now back to  FIG. 5 , the front face of the electroacoustic transducer  5  faces the depression  10   c  of the front wall  10   a  of the housing  1 . As a result, the first thin air layer S 1  and a first space S 3  are disposed in the front of the diaphragm  50  inside the housing  1 . The first space S 3  is a space surrounded by the first electrode  54 A, the first insulator  55 A, the depression  10   c , and the front sound-emitting hole  10   h.    
     The rear face of the electroacoustic transducer  5  faces the depression  11   b  of the rear wall  11   a  of the housing  1 . As a result, the second thin air layer S 2 , a second space S 4 , a second protective member  71  described below, and the rear holes  11   h  are disposed in the rear of the diaphragm  50  inside the housing  1 . The second space S 4  is a space surrounded by the second electrode  54 B, the second insulator  55 B, and the fixing member  6 . 
     The protective members  7  prevent intrusion of foreign objects and sweat from the user of the earphone E into the housing  1 . The protective members  7  consist of a first protective member  70  and a second protective member  71 . 
     The first protective member  70  is composed of water-repellent metal fiber. The first protective member  70  has a shape of a disc. The first protective member  70  is attached to the front end of the sound conduit  3 . The first protective member  70  covers the opening at the front end of the sound conduit  3 . As a result, the sound conduit  3  accommodates a sound-conduit space S 5 . The sound-conduit space S 5  is surrounded by the sound conduit  3  and the first protective member  70 . The sound-conduit space S 5  is in communication with the first space S 3 . The first protective member  70  also functions as an acoustic resistor that adjusts the frequency characteristics of the earphone E, for example. 
     The second protective member  71  is composed of a water-repellent metal mesh. The second protective member  71  is a rectangular plate. The second protective member  71  is attached to the front face of the rear wall  11   a  of the housing  1 . The second protective member  71  covers the rear holes  11   h  from the inside of the housing  1 . The second protective member  71  may also function as an acoustic resistor. 
     The second protective member may be composed of the same material as the first protective member. 
     The first space S 3  and the sound-conduit space S 5  constitute a front air chamber R 3 . The front air chamber R 3  is in communication with the space in the front (the exterior) of the earpiece  4  through the first protective member  70  and a space (hereinafter, referred to as “cylindrical interior space”) S 6  in the front half of the inner cylinder  41  of the earpiece  4 . The front air chamber R 3  is in communication with the first thin air layer S 1  through the sound holes  53 Ah of the first electret board  53 A. 
     The second space S 4  constitutes a rear air chamber R 4 . The rear air chamber R 4  is in communication with the space in the rear (the exterior) of the housing  1  through the second protective member  71  and the rear holes  11   h . That is, the rear holes  11   h  serve as communication holes establishing communication between the interior space of the housing  1  and the exterior space of the housing  1 . The rear air chamber R 4  is in communication with the second thin air layer S 2  through the sound holes  53 Bh of the second electret board  53 B. 
     The volume of the front air chamber R 3  is smaller than that of the rear air chamber R 4 . The volume of the first thin air layer S 1  is identical to that of the second thin air layer S 2 . That is, the sum of the volume of the space in the front of the diaphragm  50  inside the housing  1  and the volume of the sound-conduit space S 5  is smaller than the volume of the space in the rear of the diaphragm  50  inside the housing  1 . 
     Referring now back to  FIG. 8 , the cord bush  8  protects the cord  9  from bending and breaking. The cord bush  8  is composed of flexible synthetic resin, such as rubber. The cord bush  8  has a substantially cylindrical shape. The cord bush  8  has a groove  80  having a shape of a ring. The groove  80  is fit with the fitting hole  1   h  of the housing  1 . The groove  80  is disposed on the outer circumferential surface at one end of the cord bush  8 . 
     The cord  9  transmits audio signals from the external sound source of the earphone E to the electroacoustic transducer  5 . The cord  9  is a three-core cord including a first signal line (reference-potential line)  90 , a second signal line  91 , and a third signal line  92  (see  FIG. 7 ), for example. One end of the cord  9  is inserted into the cord bush  8  and accommodated inside the wiring room R 2  of the housing  1 . The other end of the cord  9  is attached to a stereo plug (not shown), for example. 
     The first signal line  90  is connected to the first protruding electrode  51 Aa and the first protruding electrode  51 Ba. The second signal line  91  is connected to the second protruding electrode  54 Aa. The third signal line  92  is connected to the third protruding electrode  54 Ba. 
     Referring now back to  FIGS. 3 and 5 , the cover LEC covers the rear holes  11   h  from the rear of the housing  1 . The cover LEC is an example opening and closing mechanism provided in the earphone set according to the present invention. The cover LEC is composed of elastic synthetic resin, such as silicone rubber. The cover LEC has a shape of a rectangular dish having a front opening. The cover LEC has a rectangular ceiling and a rectangular circumferential wall surrounding the ceiling. The inner circumference of the circumferential wall of the cover LEC is slightly smaller than that of the circumferential wall of the housing  1 . 
     The circumferential wall of the cover LEC has a first cutout LEC 1  and a second cutout (not shown). The first cutout LEC 1  prevents interference of the circumferential wall of the cover LEC with the cord bush  8 . The first cutout LEC 1  has a shape of an inverted U. The first cutout LEC 1  is disposed in the lower face of the circumferential wall of the cover LEC. The second cutout prevents interference of the circumferential wall of the cover LEC with the connection member  20  of the hanger  2 . The second cutout is disposed in the area of the circumferential wall of the cover LEC facing the left side. 
     The cover LEC covers the housing  1  from the rear side. The cover LEC is attachable to and detachable from the housing  1 . The rear holes  11   h  of the housing  1  are covered by the cover LEC from the outside of the housing  1 . That is, the cover LEC opens and closes the rear holes  11   h . In other words, the rear holes  11   h  are closed when the cover LEC is attached to the housing  1 . At this time, the rear air chamber R 4  is sealed. The sealed rear air chamber R 4  is separated from the exterior space of the housing  1 . The rear holes  11   h  are opened when the cover LEC is removed from the housing  1 . When the rear holes  11   h  are opened, the rear air chamber R 4  is open to the exterior space of the housing  1 . 
     The cover LEC covers the rear wall  11   a  and the circumferential wall of the housing  1 . At this time, the circumferential wall of the cover LEC is expanded by the housing  1 . Thus, a high frictional force is generated between the inner face of the cover LEC and the external face of the housing  1 . As a result, the cover LEC is fixed to the housing  1 . 
     The earphone E is connected to an external booster unit  100  (see  FIG. 7 ) of the earphone E with the stereo plug. The booster unit  100  boosts audio signals from the sound source. The booster unit  100  accommodates a booster transformer T. The booster unit  100  connects the cord  9  and the external sound source. The booster transformer T boosts the audio signal from the sound source and transmits the boosted audio signals to the cord  9 . 
     The earphone E is of a back-electret type so that does not require a power source for generating a high voltage DC bias. 
     Operation of Earphone 
     The operation of the earphone E will now be described. 
     The operation of the earphone E having the cover LEC attached to the body LEB will now be described. 
       FIG. 9  is a cross-sectional view of the body LEB to which the cover LEC is attached is placed in the left ear of the user.  FIG. 9  illustrates a simplified ear canal Ec.  FIG. 9  does not illustrate some of the lines so as to clarify the front air chamber R 3  and the rear air chamber R 4 . 
     When the body LEB is placed in the left ear of the user of the earphone E, the earpiece  4  is placed inside the ear canal Ec of the user. The earpiece  4  deforms and tightly attaches to the inner face of the ear canal Ec. Thus, in the inside of the ear canal Ec, the earpiece  4 , the ear canal Ec, and the eardrum (not shown) define a space (hereinafter referred to as “ear-canal space”) S 7 . The ear-canal space S 7  is sealed by the earpiece  4 . 
     When audio signals are transmitted from the sound source to the electroacoustic transducer  5 , the diaphragm  50  is driven in a push-pull mode and generates acoustic pressure in response to the audio signals. The acoustic pressure from the diaphragm  50  propagates through the air near the diaphragm  50  and travels forward from the diaphragm  50  as acoustic waves. 
     The acoustic waves traveling forward from the diaphragm  50  pass through the first thin air layer S 1 , the sound holes  53 Ah of the first electret board  53 A, the front air chamber R 3 , the first protective member  70 , the cylindrical interior space S 6 , and the ear-canal space S 7 , and reach the eardrum of the user. 
     The acoustic pressure generated in the rear of the diaphragm  50  passes through the second thin air layer S 2  and the sound holes  53 Bh of the second electret board  53 B, and reaches the rear air chamber R 4 . As described above, the rear air chamber R 4  is sealed. The sealed rear air chamber R 4  functions as an acoustic impedance controlling the acoustic pressure that reaches the rear air chamber R 4 . Thus, the earphone E to which the cover LEC is attached functions as closed type earphones. 
     The equivalent circuit of the earphone E functioning as closed type earphones will now be described. 
       FIG. 10  is an equivalent circuit diagram of the earphone E functioning as closed type earphones. 
     The reference signs in  FIG. 10  are defined as follows: Reference sign Fe represents the acoustic pressure of the diaphragm  50 . Reference sign s 0  represents the stiffness of the diaphragm  50 . Reference sign s 1  represents the stiffness of the first thin air layer S 1 . Reference sign s 2  represents the stiffness of the front air chamber R 3 . Reference sign s 3  represents the stiffness of the second thin air layer S 2 . Reference sign s 4  represents the stiffness of the rear air chamber R 4 . Reference sign m 0  represents the mass of the diaphragm  50 . Reference sign m 1  represents the mass of the air inside the sound holes  53 Ah in the first electret board  53 A. Reference sign m 2  represents the mass of the air inside the sound holes  53 Bh of the second electret board  53 B. Reference sign r 0  represents the acoustic resistance of the air inside the sound holes  53 Ah of the first electret board  53 A. Reference sign r 1  represents the acoustic resistance of the first protective member  70 . Reference sign r 2  represents the mass of the air inside the sound holes  53 Bh of the second electret board  53 B. Reference sign ZE represents the load impedance inside the ear canal Ec. 
     In the earphone E functioning as closed type earphones, the rear holes  11   h  are closed by the cover LEC. That is, the rear air chamber R 4  is closed by the cover LEC. At this time, the vibration of the diaphragm  50  is damped by the stiffness s 1  of the first thin air layer S 1 , the stiffness s 2  of the front air chamber R 3 , the stiffness s 3  of the second thin air layer S 2 , and the stiffness s 4  of the rear air chamber R 4 . 
     In general, the stiffness of air in a space is inversely proportional to the volume of the air. As described above, the volume of the front air chamber R 3  is smaller than that of the rear air chamber R 4 . Thus, the stiffness s 2  of the front air chamber R 3  is greater than the stiffness s 4  of the rear air chamber R 4 . The front air chamber R 3  and the ear-canal space S 7  are sealed by the earpiece  4 . Thus, the volume of the space in the front air chamber R 3  is invariable (the stiffness s 2  is invariable). Since the volume of the space in the rear air chamber R 4  is larger than that in the front air chamber R 3 , the stiffness s 4  of the rear air chamber R 4  does not significantly affect the stiffness s 2  of the front air chamber R 3 . 
     The diaphragm  50  is disposed between the first thin air layer S 1  and the second thin air layer S 2 . The stiffness s 1  of the first thin air layer S 1  is greater than each of the stiffness s 2  of the front air chamber R 3  and the stiffness s 4  of the rear air chamber R 4 . The stiffness s 3  of the second thin air layer S 2  is larger than each of the stiffness s 2  of the front air chamber R 3  and the stiffness s 4  of the rear air chamber R 4 . Thus, the vibration of the diaphragm  50  is dominantly affected by the stiffness s 1  of the first thin air layer S 1  and the stiffness s 3  of the second thin air layer S 2 . As a result, in the earphone E functioning as closed type earphone, the vibration of the diaphragm  50  is damped by the stiffness s 4  of the rear air chamber R 4 . At this time, the effect to the diaphragm  50  from damping by the stiffness s 4  of the rear air chamber R 4  is smaller than the effect to the diaphragm  50  from damping by the stiffness s 1  of the first thin air layer S 1  and the stiffness s 3  of the second thin air layer S 2 . That is, the frequency characteristics of the earphone E are substantially unaffected by the stiffness s 4  of the rear air chamber R 4 . 
     The operation of the earphone E including the body LEB from which the cover LEC is detached will now be described. 
       FIG. 11  is a cross-sectional view of the earphone E placed in the left ear of the user, the earphone E including the body LEB from which the cover LEC is detached.  FIG. 11  illustrates a simplified ear canal Ec of the user.  FIG. 11  does not illustrate some of the lines to clarify the front air chamber R 3  and the rear air chamber R 4 . 
     The acoustic pressure generated in the front of the diaphragm  50  reaches the eardrum of the user in a manner similar to that in the earphone E including the cover LEC attached to the body LEB. 
     The acoustic pressure generated in the rear of the diaphragm  50  passes through the second thin air layer S 2  and the sound holes  53 Bh of the second electret board  53 B, and reaches the rear air chamber R 4 . The rear air chamber R 4  is open by the rear holes  11   h . That is, the rear air chamber R 4  is in communication with the exterior space of the housing  1  through the rear holes  11   h . Thus, the stiffness s 4  of the rear air chamber R 4  is small. Thus, the acoustic pressure that reaches the rear air chamber R 4  is not reduced by the stiffness s 4  of the rear air chamber R 4  and is output to the exterior space of the housing  1  through the rear holes  11   h  as acoustic waves. That is, the earphone E from which the cover LEC is removed functions as open type earphones. 
     The equivalent circuit of the earphone E functioning as open type earphones will now be described. 
       FIG. 12  is an equivalent circuit diagram of the earphone E functioning as open type earphones. 
     The reference signs in  FIG. 12  are the same as those in  FIG. 11 . 
     In the earphone E functioning as open type earphones, the rear air chamber R 4  is in communication with the exterior space of the housing  1  through the rear holes  11   h . The second protective member  71 , which is composed of metal mesh, allows air to pass through more readily than the first protective member  70 . Thus, the rear air chamber R 4  can be regarded as part of the exterior space of the housing  1 . Thus, the stiffness s 4  of the rear air chamber R 4  is small enough to be ignored in the equivalent circuit of the earphone E. That is, the vibration of the diaphragm  50  is not damped by the stiffness s 4  of the rear air chamber R 4 . At this time, the vibration of the diaphragm  50  is damped by the stiffness s 1  of the first thin air layer S 1 , the stiffness s 2  of the front air chamber R 3 , and the stiffness s 3  of the second thin air layer S 2 . 
     As described above, the stiffness s 4  of the rear air chamber R 4  is small enough to be ignored in the equivalent circuit of the earphone E. The front air chamber R 3  and the ear-canal space S 7  are sealed by the earpiece  4 . That is, in also the earphone E functioning as open type earphones, the stiffness s 2  of the front air chamber R 3  is greater than the stiffness s 4  of the rear air chamber R 4 . Thus, the stiffness s 4  of the rear air chamber R 4  does not significantly affect the stiffness s 2  of the front air chamber R 3 . In other words, the balance among the stiffnesses of the spaces in the earphone E functioning as open type earphones do not significantly vary compared to that of the earphone E functioning as closed type earphones. 
     As described above, the vibration of the diaphragm  50  is dominantly affected by the stiffness s 1  of the first thin air layer S 1  and the stiffness s 3  of the second thin air layer S 2 . As a result, in the earphone E functioning as open type earphones, the vibration of the diaphragm  50  is not damped by the stiffness s 4  of the rear air chamber R 4 . That is, the frequency characteristics of the earphone E are substantially unaffected by the variation in the degrees of stiffness of the rear air chamber R 4  in the closed type earphone mode and the open type earphone mode. Accordingly, the frequency characteristics of the earphone E are not degraded in either the closed type earphone mode or the open type earphone mode. That is, the user of the earphone E can select the closed type earphone mode or the open type earphone mode depending on the preference of the user. In other words, the earphone E according to the present embodiment is switchable between the closed type earphone mode and the open type earphone mode and achieves satisfactory acoustic characteristics in either mode. 
     As described above, the frequency characteristics of the earphone E functioning as open type earphones do not significantly vary from those of the earphone E functioning as closed type earphones. In other words, the earphone E is switchable between the open type earphone mode and the closed type earphone mode by detaching or attaching the cover LEC from or to the body LEB. 
     The earphone E is of a canal type (inner-ear type) including the sound conduit  3  and the earpiece  4 . Thus, when the earphone E is placed in the ears of the user, the distance between each diaphragm  50  and the corresponding eardrum is shorter than that in over-ear headphones (hereinafter, referred to as “headphones”), for example. The volumes of the spaces between the diaphragms  50  and the respective eardrums are smaller than those in the headphones. Thus, the diaphragms  50  of the earphone E can have dimensions smaller than those of the diaphragms of the headphones. As a result, the stiffness s 0  of the diaphragms  50  of the earphone E is greater than the stiffnesses of the diaphragms of the headphones. 
     CONCLUSION 
     In the earphone E according to the embodiments described above, the volume of the front air chamber R 3  in communication with the ear-canal space S 7  is smaller than the volume of the rear air chamber R 4  in communication with the exterior of the housing  1  through the rear holes  11   h . The earphone E includes the cover LEC covering the rear holes  11   h . The cover LEC is attachable to and detachable from the earphone E. Thus, the earphone E is switchable between an open type earphone mode and a closed type earphone mode by detaching or attaching the cover LEC to or from the body LEB. As a result, the frequency characteristics of the earphone E do not significantly vary between the operation in the open type earphone mode and the operation in the closed type earphone mode. In other words, the earphone E includes the condenser type electroacoustic transducers  5  and exhibits compatibility between the functions of the open type earphone mode and the functions of the closed type earphone mode. 
     In the embodiments described above, the cover LEC as “opening and closing mechanism” covers all of the rear holes  11   h . Alternatively, the opening and closing mechanism of the earphones according to the present invention may close only parts of the rear holes  11   h . The number of closed rear holes  11   h  influences the quality of auditory sound output from the earphone E. Thus, when the number of rear holes  11   h  closed by the opening and closing mechanism is variable, the user can select an acoustic setting in accordance with preference. 
     The configuration of the opening and closing mechanism is not limited to the present embodiment. That is, the housing may alternatively include a sliding plate having openings having the similar shape to that of the rear holes and a sliding mechanism configured to slide the sliding plate, for example. The sliding plate is an example of a closure part of the opening and closing mechanism of the earphone according to the present invention. The sliding mechanism is an example of a moving part of the opening and closing mechanism of the earphone according to the present invention. A part of the sliding mechanism may be exposed to the exterior space of the housing. In such a case, the opening and sealing of the rear air chamber relative to the exterior space of the housing can be appropriately selected through operation of the exposed portion of the sliding mechanism by a finger of the user. 
     The configuration of the opening and closing mechanism may be a structure of an openable and closeable door, for example. 
     The earphone E according to this embodiment includes the cord bush  8  and the cord  9 . Alternatively, the earphone according to the present invention may include connectors, such as jacks, in place of the cord bush and the cord.