Patent Publication Number: US-2018035203-A1

Title: Waterproof sound transmisson structure, and electronic device and electronic device case including same

Description:
TECHNICAL FIELD 
     The present invention relates to a waterproof sound transmission structure having both waterproofness and sound permeability and to an electronic device and electronic device case that include the structure. 
     BACKGROUND ART 
     Nowadays, it is typical for electronic devices such as mobile phones, tablet computers, digital cameras, and game consoles to have an audio function. In the housing of such an electronic device having an audio function, an audio component such as a speaker and/or a microphone is placed as an audio part (specifically a sound emitter and/or a sound receiver). The housing of the electronic device typically has a sound transmission port positioned in correspondence with the audio part. This sound transmission port allows sound to be transmitted between the outside of the electronic device and the audio part. 
     Naturally, ingress of water into the housing of an electronic device must be prevented; however, the above sound transmission port for transmission of sound may act as a passage that permits water to easily enter the housing. In particular, portable electronic devices have an increased risk of suffering from ingress of water because they are often exposed to rain or water used in daily life and because the orientation of the opening of the sound transmission port cannot be fixed at a given orientation that allows the avoidance of water (for example, a downward orientation for which rain is less likely to come into the housing). For this reason, a waterproof sound-permeable membrane permeable to sound but impervious to water is placed to cover the sound transmission port so that the housing has a waterproof sound transmission structure. The waterproof sound transmission structure allows transmission of sound between the audio part and the outside of the housing while preventing water from entering the housing from the outside through the sound transmission port. The waterproof sound transmission structure is applicable not only to a housing of an electronic device but also to any part required to ensure both sound permeability through a sound transmission port and waterproofness at the sound transmission port. 
     An exemplary waterproof sound-permeable membrane is a non-porous resin film having through holes formed to extend through the thickness of the film (see Patent Literature 1). The waterproof sound-permeable membrane of Patent Literature 1 is formed by irradiating a non-porous resin film with an ion beam and then chemically etching the irradiated film. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2012-195928 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide: a waterproof sound transmission structure having higher levels of both waterproofness and sound permeability than conventional waterproof sound transmission structures; and an electronic device and electronic device case that include the structure. 
     Solution to Problem 
     A waterproof sound transmission structure of the present disclosure is a waterproof sound transmission structure including: a wall separating a first space from a second space where water can be present, the wall being provided with a sound transmission port for transmission of sound between the first and second spaces; and a waterproof sound-permeable membrane placed to cover the sound transmission port, the waterproof sound-permeable membrane being adapted to permit transmission of sound between the first and second spaces and being further adapted to prevent ingress of water from the second space into the first space through the sound transmission port. The first space is sealed when the second space is filled with water, the sealed first space having a volume of 300 mm 3  or less. The waterproof sound-permeable membrane includes: a non-porous resin film having through holes formed to extend through the thickness of the resin film; and a liquid-repellent layer formed on a principal surface of the resin film and having openings positioned in correspondence with the through holes, the through holes having a diameter of 5.0 μm or more and 13.0 μm or less. 
     An electronic device of the present disclosure is an electronic device having an audio part, the electronic device including the above waterproof sound transmission structure of the present disclosure. In the waterproof sound transmission structure, the wall is a housing of the electronic device, the second space is a space located outside the housing, the first space is a space located inside the housing and containing the audio part, and the sound transmission port is a sound transmission port for transmission of sound from and/or to the audio part. 
     An electronic device case of the present disclosure is an electronic device case for enclosing an electronic device having an audio part, the electronic device case including the above waterproof sound transmission structure of the present disclosure. In the waterproof sound transmission structure, the wall is a housing of the case, the second space is a space located outside the housing, the first space is a space located inside the housing for enclosing the electronic device, and the sound transmission port is a sound transmission port for transmission of sound from and/or to the audio part of the electronic device enclosed in the case. 
     Advantageous Effects of Invention 
     The present invention makes it possible to attain: a waterproof sound transmission structure having higher levels of both waterproofness and sound permeability than conventional waterproof sound transmission structures; and an electronic device and electronic device case that include the structure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view schematically showing an example of the waterproof sound transmission structure of the present invention. 
         FIG. 2  is a cross-sectional view schematically showing the state of the waterproof sound-permeable membrane when the second space is filled with water in the waterproof sound transmission structure shown in  FIG. 1 . 
         FIG. 3  is a cross-sectional view schematically showing an example of the waterproof sound-permeable membrane included in the waterproof sound transmission structure of the present invention. 
         FIG. 4  is a cross-sectional view schematically showing another example of the waterproof sound-permeable membrane included in the waterproof sound transmission structure of the present invention. 
         FIG. 5  is a plan view schematically showing an example of the relationship among the through holes of the waterproof sound-permeable membrane included in the waterproof sound transmission structure of the present invention in terms of the directions in which the through holes extend. 
         FIG. 6  is a plan view schematically showing another example of the relationship among the through holes of the waterproof sound-permeable membrane included in the waterproof sound transmission structure of the present invention in terms of the directions in which the through holes extend. 
         FIG. 7  is a cross-sectional view schematically showing still another example of the relationship among the through holes of the waterproof sound-permeable membrane included in the waterproof sound transmission structure of the present invention in terms of the directions in which the through holes extend. 
         FIG. 8  is a cross-sectional view schematically showing still another example of the waterproof sound-permeable membrane included in the waterproof sound transmission structure of the present invention. 
         FIG. 9  is a cross-sectional view schematically showing still another example of the waterproof sound-permeable membrane included in the waterproof sound transmission structure of the present invention. 
         FIG. 10  is a schematic diagram for illustrating the overview of ion beam irradiation in a method for forming a resin film of the waterproof sound-permeable membrane included in the waterproof sound transmission structure of the present invention by employing the ion beam irradiation and the subsequent chemical etching. 
         FIG. 11  is a schematic diagram for illustrating an example of the ion beam irradiation in a method for forming a resin film of the waterproof sound-permeable membrane included in the waterproof sound transmission structure of the present invention by employing the ion beam irradiation and the subsequent chemical etching. 
         FIG. 12  is a perspective view schematically showing an example of the waterproof sound-permeable membrane (a waterproof sound-permeable member including a supporting member) included in the waterproof sound transmission structure of the present invention. 
         FIG. 13  is a plan view schematically showing another example of the waterproof sound-permeable membrane (a waterproof sound-permeable member including a supporting member) included in the waterproof sound transmission structure of the present invention. 
         FIG. 14A  is a perspective view schematically showing an example of the electronic device of the present invention. 
         FIG. 14B  is a cross-sectional view schematically showing an example of how the waterproof sound-permeable membrane is placed in the electronic device shown in  FIG. 14A . 
         FIG. 15A  is a perspective view schematically showing another example of the electronic device of the present invention. 
         FIG. 15B  is a cross-sectional view schematically showing an example of how the waterproof sound-permeable membrane is placed in the electronic device shown in  FIG. 15A . 
         FIG. 16A  is a perspective view schematically showing an example of the electronic device case of the present invention. 
         FIG. 16B  is a cross-sectional view schematically showing an example of how the waterproof sound-permeable membrane is placed in the electronic device case shown in  FIG. 16A . 
         FIG. 17A  is a cross-sectional view schematically showing: a simulated housing used for evaluation of sound pressure loss (insertion loss) caused by waterproof sound-permeable membranes in Examples; and how a speaker is placed in the housing. 
         FIG. 17B  is a cross-sectional view schematically showing: a member prepared for evaluation of sound pressure loss (insertion loss) caused by waterproof sound-permeable membranes in Examples; and a state where the member is attached to a simulated housing. 
         FIG. 18A  is a view for illustrating a method employed for evaluation of the waterproofness of waterproof sound transmission structures in Examples. 
         FIG. 18B  is a view for illustrating a method employed for evaluation of the waterproofness of waterproof sound transmission structures in Examples. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The first aspect of the present disclosure provides a waterproof sound transmission structure including: 
     a wall separating a first space from a second space where water can be present, the wall being provided with a sound transmission port for transmission of sound between the first and second spaces; and 
     a waterproof sound-permeable membrane placed to cover the sound transmission port, the waterproof sound-permeable membrane being adapted to permit transmission of sound between the first and second spaces and being further adapted to prevent ingress of water from the second space into the first space through the sound transmission port, wherein 
     the first space is sealed when the second space is filled with water, the sealed first space having a volume of 300 mm 3  or less, and 
     the waterproof sound-permeable membrane includes:
         a non-porous resin film having through holes formed to extend through the thickness of the resin film; and   a liquid-repellent layer formed on a principal surface of the resin film and having openings positioned in correspondence with the through holes,   the through holes having a diameter of 5.0 μm or more and 13.0 μm or less.       

     The second aspect of the present disclosure provides the waterproof sound transmission structure as set forth in the first aspect, wherein a water entry pressure of the waterproof sound transmission structure is greater than a water entry pressure of the waterproof sound-permeable membrane placed to cover the sound transmission port, the water entry pressure of the waterproof sound-permeable membrane being measured according to Method B (high hydraulic pressure method) of water penetration test specified in JIS L 1092. 
     The third aspect of the present disclosure provides the waterproof sound transmission structure as set forth in the first or second aspect, being compliant with IPX7 specified as a degree of protection against water ingress in JIS C 0920. 
     The fourth aspect of the present disclosure provides the waterproof sound transmission structure as set forth in the third aspect, wherein the water entry pressure of the waterproof sound-permeable membrane is 3.0 kPa or more and less than 9.8 kPa, and the volume is 10 mm 3  or less. 
     The fifth aspect of the present disclosure provides the waterproof sound transmission structure as set forth in the third aspect, wherein the water entry pressure of the waterproof sound-permeable membrane is 2.0 kPa or more and less than 3.0 kPa, and the volume is 5 mm 3  or less. 
     The sixth aspect of the present disclosure provides the waterproof sound transmission structure as set forth in any one of the first to fifth aspects, wherein an effective area of the waterproof sound-permeable membrane is 4.9 mm 2  or less. 
     The seventh aspect of the present disclosure provides the waterproof sound transmission structure as set forth in any one of the first to sixth aspects, wherein the waterproof sound-permeable membrane has an air permeability in a thickness direction of the waterproof sound-permeable membrane, the air permeability being 2.0 cm 3 /(cm 2 ·sec) or more and 120 cm 3 /(cm 2 ·sec) or less as expressed in terms of Frazier number measured according to JIS L 1096. 
     The eighth aspect of the present disclosure provides the waterproof sound transmission structure as set forth in any one of the first to seventh aspects, wherein an insertion loss in a frequency range of 100 Hz to 5 kHz is 2 dB or less. 
     The ninth aspect of the present disclosure provides the waterproof sound transmission structure as set forth in any one of the first to eighth aspects, wherein the waterproof sound-permeable membrane is subjected to a coloring treatment that enables the waterproof sound-permeable membrane to absorb light in a wavelength range of 380 nm to 500 nm. 
     The tenth aspect of the present disclosure provides the waterproof sound transmission structure as set forth in any one of the first to eighth aspects, wherein the waterproof sound-permeable membrane is colored black, gray, brown, or pink. 
     The eleventh aspect of the present disclosure provides an electronic device having an audio part, the electronic device including the waterproof sound transmission structure according to any one of the first to tenth aspects, wherein in the waterproof sound transmission structure, the wall is a housing of the electronic device, the second space is a space located outside the housing, the first space is a space located inside the housing and containing the audio part, and the sound transmission port is a sound transmission port for transmission of sound from and/or to the audio part. 
     The twelfth aspect of the present disclosure provides an electronic device case for enclosing an electronic device having an audio part, the electronic device case including the waterproof sound transmission structure according to any one of the first to tenth aspects, wherein in the waterproof sound transmission structure, the wall is a housing of the case, the second space is a space located outside the housing, the first space is a space located inside the housing for enclosing the electronic device, and the sound transmission port is a sound transmission port for transmission of sound from and/or to the audio part of the electronic device enclosed in the case. 
     [Waterproof Sound Transmission Structure] 
       FIG. 1  shows an example of the waterproof sound transmission structure of the present invention. The waterproof sound transmission structure  1  shown in  FIG. 1  includes: a wall  2  provided with a sound transmission port  11 ; and a waterproof sound-permeable membrane  3  placed to cover the sound transmission port  11 . The waterproof sound-permeable membrane  3  is a membrane permeable to sound but impervious to water. The wall  2  separates two spaces  12  and  13  from each other, and the sound transmission port  11  is an opening portion for transmission of sound between these two spaces. In one of the spaces, the space  12  (second space), there can be present water. Water may be present also in the other space  13  (first space); however, the space  13  is typically a space in which water is not or should not be present, such as the internal space of the housing of an electronic device. The term “water” as used herein refers to liquid water, unless otherwise specified. 
     The wall  2  prevents transfer of water between the two spaces  12  and  13  and also limits transmission of sound between the spaces  12  and  13 . In the waterproof sound transmission structure  1 , sound  21  and  22  can be transmitted between the spaces  12  and  13  through the sound transmission port  11  formed in the wall  2  and the waterproof sound-permeable membrane  3  placed over the sound transmission port  11 . The waterproof sound-permeable membrane  3  can prevent ingress of water into the first space  13  from the second space  12  where water can be present. When an audio part  4  that emits and/or receives sound is disposed in the space  13  as in the example shown in  FIG. 1  (when the first space  13  contains the audio part  4 ), the sound  21  is transmitted to the audio part  4  from the space  12 , while the sound  22  is emitted from the audio part  4  and transmitted to the space  12 . 
     The first space  13  in the waterproof sound transmission structure  1  is sealed when the second space  12  is filled with water, the sealed first space  13  having a volume of 300 mm 3  or less. When the space  12  is filled with water, a water pressure  31  is applied to the waterproof sound-permeable membrane  3  so that the membrane  3  is deformed, specifically bowed, toward the first space  13  (see  FIG. 2 ). The deformation of the membrane  3  correspondingly reduces the volume of the first space  13  which is a sealed space, resulting in an increase in the internal pressure of the space  13 . This increase in internal pressure produces a pressure  32  acting on the waterproof sound-permeable membrane  3  in the direction from the first space  13  to the second space  12 , and the pressure  32  cancels out some of the water pressure  31 . Assuming that the volume of the sealed space is varied while the water pressure  31  applied to the membrane  3 , and therefore the amount of the deformation of the membrane  3  caused by the water pressure  31 , are kept constant, the smaller the volume of the sealed space is, the greater the increase in internal pressure and the degree of cancellation of the water pressure  31  are, and thus the smaller the water pressure substantially applied to the waterproof sound-permeable membrane  3  is. The waterproof sound-permeable membrane  3  has waterproofness inherent to itself (inherent waterproofness) which depends on its configuration. Even with the use of the waterproof sound-permeable membrane  3  having inherent waterproofness lower than the waterproofness required of the waterproof sound transmission structure  1 , the structure  1  can be formed as a waterproof sound transmission structure having the required waterproofness since the water pressure substantially applied to the membrane  3  is reduced as described above. The sound permeability of a waterproof sound-permeable membrane (the properties of sound transmitted through the waterproof sound-permeable membrane) becomes poorer as the inherent waterproofness of the membrane increases. That is, there is a trade-off relationship between the waterproofness and sound permeability of the waterproof sound-permeable membrane  3 . The waterproof sound transmission structure  1 , which can achieve required waterproofness with the use of a waterproof sound-permeable membrane having inherent waterproofness lower than the required waterproofness, can have higher levels of both waterproofness and sound permeability than conventional waterproof sound transmission structures. 
     The volume of the first space  13  sealed when the second space  12  is filled with water, namely, the volume of the first space  13  in the sealed state (sealed-state volume) is 300 mm 3  or less. If this volume is more than 300 mm 3 , the degree of cancellation of the water pressure  31  applied to the waterproof sound-permeable membrane  3  is insufficient so that waterproofness and sound permeability cannot be achieved at higher levels than ever before. 
     The waterproof sound-permeable membrane  3  included in the waterproof sound transmission structure  1  includes: a non-porous resin film having through holes formed to extend through the thickness of the resin film; and a liquid-repellent layer formed on a principal surface of the resin film and having openings positioned in correspondence with the through holes. The waterproofness and sound permeability of such a waterproof sound-permeable membrane  3  can be controlled by adjusting the diameter of the through holes, which means that the control of waterproofness and sound permeability can be accomplished with high flexibility. This contributes to achieving high levels of both waterproofness and sound permeability of the waterproof sound transmission structure  1 . In addition, since the waterproof sound-permeable membrane  3  employs a non-porous resin film as its base (original material), the membrane  3  is highly uniform in mechanical properties such as strength and can be formed as a waterproof sound-permeable membrane that has through holes with uniform diameters and/or in which the through holes are uniformly distributed. Thus, penetration of water due to non-uniformity of the membrane can be prevented, and the effect of cancelling out the water pressure  31 , which is shown in  FIG. 2 , can be satisfactorily obtained. This also contributes to achieving high levels of both waterproofness and sound permeability of the waterproof sound transmission structure  1 . The waterproof sound-permeable membrane  3  having a configuration as described above intrinsically has high sound permeability. The high sound permeability is evidently reflected, in particular, in a low insertion loss in the frequency range of 100 Hz to 5 kHz. The lowness of the insertion loss in this frequency range is important for waterproof sound transmission structures. 
     The diameter of the through holes in the waterproof sound-permeable membrane  3  is 5.0 μm or more and 13.0 μm or less. If the diameter of the through holes is more than 13.0 μm, the inherent waterproofness of the membrane  3  is low, so that it is difficult to achieve high levels of both waterproofness and sound permeability of the waterproof sound transmission structure  1 . If the diameter of the through holes is less than 5 μm, the sound permeability of the waterproof sound transmission structure  1  tends to decrease, although the structure can, due to the very high inherent waterproofness of the membrane  3 , have sufficient waterproofness even when the sealed-state volume is equal to or larger than a certain value. 
     The sealed-state volume of the first space  13  is preferably 200 mm 3  or less, more preferably 150 mm 3  or less, and even more preferably 100 mm 3  or less, although this depends on the level of waterproofness required of the waterproof sound transmission structure  1 . A reduction in the sealed-state volume of the first space  13  allows an increase in the extent to which the inherent waterproofness of the waterproof sound-permeable membrane  3  to be used is lower than the waterproofness required of the waterproof sound transmission structure  1 , thereby leading to increase in the achieved levels of both waterproofness and sound permeability of the waterproof sound transmission structure  1 . The sealed-state volume of the first space  13  can be further reduced and may be, for example, 50 mm 3  or less, 30 mm 3  or less, 10 mm 3  or less, or even 5 mm 3  or less. 
     The sealed-state volume of the first space  13  refers to the volume occupied by air present in the space and communicable with the waterproof sound-permeable membrane  3  and sound transmission port  11 , and does not include the volume of any product lying in the space. This is because the volume of a product in the space is not responsible for the generation of the pressure  32  due to the deformation of the waterproof sound-permeable membrane  3 . The sealed-state volume of the first space  13  corresponds to the volume when the second space  12  is not filled with water (when the waterproof sound-permeable membrane  3  is not deformed by the water pressure  31 ). 
     A typical measure of the waterproofness is a water entry pressure. A water entry pressure of the waterproof sound transmission structure  1  can be higher than a water entry pressure (inherent water entry pressure) of the waterproof sound-permeable membrane  3  placed to cover the sound transmission port  11 , the water entry pressure of the waterproof sound-permeable membrane  3  being measured according to Method B (high hydraulic pressure method) of water penetration test specified in JIS L 1092. 
     The waterproofness of the waterproof sound transmission structure can be evaluated also by the degrees of protection against water ingress which are specified in JIS C 0920. Among these degrees of protection, the degree of protection “IPX7” is essentially relevant to the situation where the second space  12  is filled with water. When the waterproof sound transmission structure  1  is compliant with IPX7, the water entry pressure as measured for the structure  1  according to the Method B of water penetration test is estimated to be 9.8 kPa or more on the basis of the principle of the IPX7 testing which evaluates whether water enters the inside of a waterproof structure when the structure is immersed in water at a depth of 1 m. 
     The waterproof sound transmission structure  1  can be compliant with IPX7 specified as a degree of protection against water ingress in JIS C 0920. An electronic device having the waterproof sound transmission structure  1  compliant with IPX7 can avoid ingress of water into the device even when accidentally dropped into water, insofar as the water depth and the duration of submergence are at or below given limits. 
     For the waterproof sound transmission structure  1 , for example, the relationships as specified below can be established between the sealed-state volume of the first space  13  and the inherent water entry pressure of the waterproof sound-permeable membrane  3 . That is, an example of the relationships established when the waterproof sound transmission structure  1  is compliant with IPX7 is one in which the water entry pressure of the waterproof sound-permeable membrane  3  as measured according to the Method B of water penetration test is 3.0 kPa or more and less than 9.8 kPa while the sealed-state volume of the first space  13  is 10 mm 3  or less. Another exemplary relationship is one in which the water entry pressure of the waterproof sound-permeable membrane  3  as measured according to the Method B of water penetration test is 2.0 kPa or more and less than 3.0 kPa while the sealed-state volume of the first space  13  is 5 mm 3  or less. It is evident that when these relationships are established, the water entry pressure of the waterproof sound transmission structure is higher than the inherent water entry pressure of the waterproof sound-permeable membrane placed to cover the sound transmission port of the structure. 
     The design of the first space  13  is not limited, as long as the space  13  is a sealed space when the second space  12  is filled with water. The waterproof sound-permeable membrane  3  is impervious to water, so that sealing of the space  13  at the sound transmission port  11  covered by the membrane  3  is maintained when the second space  12  in contact with the membrane  3  is filled with water. An exemplary situation where the second space  12  is filled with water is when the sound transmission port  11  of the waterproof sound transmission structure  1  is entirely submerged in water. 
     The first space  13  is, for example, a space surrounded by the wall  2  having no opening portion other than the sound transmission port  11 . The first space  13  may be, for example, a space surrounded by the wall  2  having another opening portion in addition to the sound transmission port  11 . In this case, a waterproof membrane is placed to cover the other opening portion so that sealing of the space is maintained also at the other opening portion when the second space  12  is filled with water. The waterproof membrane placed to cover the other opening portion may be the waterproof sound-permeable membrane  3  or may be another waterproof membrane (such as a waterproof air-permeable membrane or waterproof sound-permeable membrane). A known membrane may be used as the other waterproof membrane. 
     In one example, the first space  13  is a space located inside the housing of an electronic device provided with the waterproof sound transmission structure  1 , and may be the internal space itself of the housing. In this example, the second space  12  is a space located outside the housing of the electronic device and opposite to the space  13  across the wall  2  and the sound transmission port  11 . In another example, the first space  13  is a space located inside the housing of an audio component such as a speaker, microphone, or transducer which is provided with the waterproof sound transmission structure  1 , and may be the internal space itself of the housing. In this case, the second space  12  is a space located outside the housing of the audio component and opposite to the space  13  across the wall  2  and the sound transmission port  11 . 
     As can be understood from the above examples, there is no limitation to the wall  2  of the waterproof sound transmission structure  1 . The waterproof sound transmission structure  1  is applicable to any location where both transmission of sound between the spaces  12  and  13  and prevention of penetration of water into the space  13  from the space  12  are required, as long as the spaces for formation and arrangement of the sound transmission port  11  and the waterproof sound-permeable membrane  3  covering the sound transmission port  11  can be obtained. Specific examples of the audio part  4  placed in the first space  13  therefore include a wide variety of audio parts. In the former of the above examples, the audio part  4  may be an audio component such as a speaker, microphone, or transducer enclosed in the housing of an electronic device. In the latter example, the audio part  4  may be an audio element such as a diaphragm enclosed in the housing of an audio component. As previously described, the sealed-state volume of the first space  13  refers to the volume occupied by air present in the space and communicable with the waterproof sound-permeable membrane  3  and sound transmission port  11 , and does not include the volume of any product lying in the space. Thus, in the former example, the volumes of the components such as the audio component placed in the space  13  are not included in the sealed-state volume, while in the latter example, the volumes of the components such as the acoustic element placed in the space  13  are not included in the sealed-state volume. 
     The waterproof sound-permeable membrane  3  will now be described. 
       FIG. 3  shows an example of the waterproof sound-permeable membrane  3 . The waterproof sound-permeable membrane  3  shown in  FIG. 3  includes a resin film  51  and a liquid-repellent layer  52  formed on the principal surfaces of the resin film  51 . The resin film  51  has through holes  53  formed to extend through the thickness of the resin film  51 . The through holes  53  extend from a first principal surface  54   a  of the resin film  51  to a second principal surface  54   b  of the resin film  51 . The liquid-repellent layer  52  has openings  55  positioned in correspondence with the through holes  53  of the resin film  51 . The resin film  51  is a non-porous resin film and has no passages that allow through-thickness air permeation other than the through holes  53 . The resin film  51  is typically an imperforate (solid) resin film having no holes other than the through holes  53 . The through holes  53  have openings at both principal surfaces of the resin film  51 . 
     The through holes  53  are straight holes having a central axis (axial line)  56  extending straight. The through holes  53  can be formed as straight holes, for example, by ion beam irradiation and subsequent chemical etching of an original film which is a resin film. With the combination of ion beam irradiation and etching, a large number of through holes  53  having more uniform diameters (opening diameters) can be formed in the resin film  51 . The resin film  51  can be a film obtained by ion beam irradiation and etching of an original film. The high uniformity in diameter of the through holes  53  in the waterproof sound-permeable membrane  3  contributes to achieving high levels of both waterproofness and sound permeability of the waterproof sound transmission structure  1 . In  FIG. 3  and the subsequent figures illustrating the structure of a waterproof sound-permeable membrane, the diameters of through holes are exaggeratedly shown to make it easy to understand the shape of the through holes. 
     In the example shown in  FIG. 3 , the direction in which the through holes  53  extend is perpendicular to the principal surfaces  54   a  and  54   b  of the resin film  51 . The direction in which the through holes  53  extend may be oblique to the direction perpendicular to the principal surfaces  54   a  and  54   b  of the resin film  51 , as long as the through holes  53  extend through the thickness of the resin film  51 . All of the through holes  53  present in the resin film  51  may extend in the same direction (that is, the directions of their central axes  56  may be identical). Alternatively, as shown in  FIG. 4 , the resin film  51  may have through holes  53  ( 53   a  to  53   g ) extending in oblique directions with respect to the direction perpendicular to the principal surfaces  54   a  and  54   b  of the film, and the oblique directions in which the through holes  53   a  to  53   g  of the resin film  51  extend may be different from each other. In the example shown in  FIG. 4 , there is a combination of through holes  53  extending (penetrating through the resin film  51 ) in oblique directions with respect to the direction perpendicular to the principal surfaces  54   a  and  54   b  of the resin film  51 , the oblique directions being different from each other. In such a case, the resin film  51  may have a combination of through holes  53  extending in the same direction (the through holes  53   a ,  53   d , and  53   g  extend in the same direction in the example shown in  FIG. 4 ). The resin film  51  may have both a through hole  53  extending in the direction perpendicular to the principal surfaces  54   a  and  54   b  of the film and a through hole  53  extending obliquely to the perpendicular direction. The term “set” may hereinafter be used instead of “combination”. The term “set” is used not only to refer to the relationship (a pair) between one through hole and another through hole but also to refer to the relationship between one or more through holes and one or more other through holes. Saying that there is a set of through holes having the same characteristics means that there are two or more through holes having the characteristics. In  FIG. 4 , the liquid-repellent layer is omitted (the same applies to  FIGS. 7 and 8 ). 
     In the waterproof sound-permeable membrane  3  as shown in  FIG. 4  which includes the resin film  51  having the through holes  53  extending in different oblique directions, the oblique angles and the proportion of the through holes  53  extending in each direction can be varied. This allows a high flexibility in control of the sound permeability and waterproofness of the waterproof sound-permeable membrane  3 . The high flexibility contributes to achieving higher levels of both waterproofness and sound permeability of the waterproof sound transmission structure  1 . 
     For the through holes  53  shown in  FIG. 4 , the angle θ 1  formed by the oblique direction D 1  (the direction of the central axis  56 ) with the direction D 2  perpendicular to the principal surfaces of the resin film  51  is preferably 45° or less and more preferably 30° or less. When the angle θ 1  falls within the preferable ranges, the flexibility in control of the sound permeability and waterproofness of the waterproof sound-permeable membrane  3  is increased. The lower limit of the angle θ 1  is not particularly defined, and the angle θ 1  is, for example, 10° or more and preferably 20° or more. If the angle  61  is excessively large, the mechanical strength of the waterproof sound-permeable membrane  1  tends to decrease. The through holes  53  shown in  FIG. 4  include a set of through holes for which the angles θ 1  are different. 
     When the waterproof sound-permeable membrane  3  as shown in  FIG. 4  which includes the resin film  51  having through holes  53  extending in different oblique directions is viewed in a direction perpendicular to a principal surface of the resin film  51 , namely when the directions in which the through holes  53  extend are projected on the principal surface, the projected directions in which the through holes  53  extend may be parallel to each other. However, it is preferable that the resin film  51  have a set of through holes  53  for which the projected directions are different from each other (through holes  53  for which the projected directions are different from each other coexist in the resin film  51 ). In the latter case, the flexibility in control of the sound permeability and waterproofness of the waterproof sound-permeable membrane  3  is increased. 
       FIG. 5  shows an example where the directions in which the through holes  53  extend when viewed in a direction perpendicular to a principal surface of the resin film  51  are parallel to each other. In the example shown in  FIG. 5 , there can be seen three through holes  53  ( 53   h ,  53   i , and  53   j ). In the view taken in a direction perpendicular to a principal surface of the resin film  51 , the directions D 3 , D 4 , and D 5  in which the three through holes  53  respectively extend (the directions from openings  58   a  of the through holes  53  at the principal surface depicted on the sheet plane toward openings  58   b  of the through holes  53  at the opposite principal surface) are parallel to each other (this means that θ 2  described later is 0°). It should be noted that the angles θ 1  formed by the through holes  53   h ,  53   i , and  53   j  are different from each other. The angle θ 1  formed by the through hole  53   j  is smallest, and the angle θ 1  formed by the through hole  53   h  is largest. Thus, the directions in which the through holes  53   h ,  53   i , and  53   j  extend are different from each other in three dimensions. 
       FIG. 6  shows an example where the directions in which the through holes  53  extend when viewed in a direction perpendicular to a principal surface of the resin film  51  are different from each other. In the example shown in  FIG. 6 , there can be seen three through holes  53  ( 53   k ,  531 , and  53   m ). In the view taken in a direction perpendicular to a principal surface of the resin film  51 , the directions D 6 , D 7 , and D 8  in which the three through holes  53  respectively extend are different from each other. When viewed in a direction perpendicular to a principal surface of the resin film  51 , the through holes  53   k  and  53   l  extend from the principal surface in different directions forming an angle θ 2  of less than 90°. In contrast, the through holes  53   k  and  53   m  extend from the principal surface of the resin film  51  in different directions forming an angle θ 2  of 90° or more when viewed in the direction perpendicular to the principal surface of the resin film  51 . The latter set of through holes is preferred; that is, the resin film  51  preferably has a set of through holes  53  that, when viewed in a direction perpendicular to a principal surface of the film, extend from the principal surface in different directions forming an angle θ 2  of 90° or more. In other words, the resin film  51 , when viewed in a direction perpendicular to a principal surface of the film, preferably has a set of the through hole  53   k  extending from the principal surface in one direction D 6  and the through hole  53   m  extending from the principal surface in another direction D 8  forming an angle θ 2  of 90° or more with the one direction D 6 . In this case, the flexibility in control of the sound permeability and waterproofness of the waterproof sound-permeable membrane  3  is further increased. The angle θ 2  is preferably 90° or more and 180° or less, and may be 180°. 
     In the waterproof sound-permeable membrane  3  as shown in  FIG. 5  which includes the resin film  51  having the through holes  53  extending in different oblique directions, two or more of the through holes  53  may cross each other at the inside of the resin film  51 . That is, the resin film  51  may have a set of through holes  53  crossing each other at the inside of the film  51 . In this case, the flexibility in control of the sound permeability and waterproofness of the waterproof sound-permeable membrane  3  is further increased. Such an example is shown in  FIG. 7 . In the example shown in  FIG. 7 , the through holes  53   p  and  53   q  cross each other at the inside of the resin film  51 . 
     The directions in which the through holes  53  extend (the directions of the central axes  56  of the through holes  53 ) in the resin film  51  (in the waterproof sound-permeable membrane  3 ) can be known, for example, by observing the principal surfaces and a cross-section of the film  51  with a scanning electron microscope (SEM). 
     The shape of the openings of the through holes  53  at the principal surfaces  54   a  and  54   b  of the resin film  51  is not limited, and is typically circular (when the direction of the central axis  56  is perpendicular to the principal surfaces  54   a  and  54   b  of the resin film  51 ) or elliptic (when the direction of the central axis  56  is oblique to the direction perpendicular to the principal surfaces  54   a  and  54   b  of the resin film  51 ). In this case, the shape of the openings of the through holes  53  need not be exactly circular or elliptic. For example, some degree of shape distortion caused by unevenness of etching performed in the production method described later is acceptable. The same applies to the shape of the cross-section of the through holes  53 . 
     In the examples shown in  FIGS. 3 to 7 , the diameter of the through holes  53  hardly varies from the first principal surface  54   a  of the resin film  51  to the second principal surface  54   b . This means that the shape of the cross-section of the through holes  53  remains almost unchanged from the principal surface  54   a  to the principal surface  54   b . As shown in  FIG. 8 , the through holes  53  of the waterproof sound-permeable membrane  3  may have a shape in which the area of a cross-section  57  perpendicular to the direction of the central axis  56  increases from the first principal surface  54   a  of the resin film  51  toward the second principal surface  54   b . This makes it possible to achieve higher levels of both waterproofness and sound permeability of the waterproof sound-permeable membrane  3  (waterproof sound transmission structure  1 ). In this case, it is preferable to place the waterproof sound-permeable membrane  3  over the sound transmission port  11  of the wall  2  in such a manner that the principal surface  54   a  at which the diameter of the through holes  53  is relatively small faces the second space  12  where water can be present while the principal surface  54   b  at which the diameter of the through holes  53  is relatively large faces the first space  13 . Each through hole  53  shown in  FIG. 8  is a through hole having a shape that is asymmetrical in the thickness direction of the waterproof sound-permeable membrane  3  and whose cross-section  57  varies in shape in the direction of the central axis  56 . 
     When the through holes  53  have a shape in which the area of the cross-section  57  perpendicular to the direction of the central axis  56  increases from the first principal surface  54   a  of the resin film  51  toward the second principal surface  54   b , the through holes  53  may have the cross-section  57  that is circular or elliptic and whose area increases continuously from the principal surface  54   a  toward the principal surface  54   b  at a constant or substantially constant rate. In this case, the shape of the through holes  53  corresponds to the entirety or a part of a circular or elliptic cone whose central line coincides with the axial line  56 . The below-described production method which employs ion beam irradiation and etching is capable of forming the waterproof sound-permeable membrane  3  including the resin film  51  having the through holes  53  whose cross-section  57  is circular or elliptic. 
     When the through holes  53  have a shape in which the area of the cross-section  57  perpendicular to the direction of the central axis  56  increases from the first principal surface  54   a  of the resin film  51  toward the second principal surface  54   b , the ratio a/b of the smaller diameter (diameter a) of the through holes  53  at the principal surface  54   a  to the larger diameter (diameter b) of the through holes at the principal surface  54   b  is, for example, 80% or less. To further increase the flexibility in control of the waterproofness and sound permeability, the ratio a/b is preferably 75% or less and more preferably 70% or less. The lower limit of the ratio a/b is not particularly defined and is, for example, 10%. 
     The area of the cross-section  57  may increase continuously from the principal surface  54   a  toward the principal surface  54   b  or may increase stepwise from the principal surface  54   a  toward the principal surface  54   b  (this means that the through holes  53  may have a region over which the area of the cross-section  57  is constant). It is preferable that the area of the cross-section  57  increase continuously from the principal surface  54   a  toward the principal surface  54   b  as shown in  FIG. 8 , and it is more preferable that the increase rate be constant or substantially constant. The below-described production method which employs ion beam irradiation and etching is capable of forming: the waterproof sound-permeable membrane  3  including the resin film  51  having the through holes  53  having the cross-section  57  the area of which increases continuously from the principal surface  54   a  toward the principal surface  54   b ; and the waterproof sound-permeable membrane  3  in which the increase rate of the area is constant or substantially constant. 
     The above characteristics of the through holes  53  of the waterproof sound-permeable membrane  3  can be freely combined. For example, the through holes  53  may have a central axis  56  whose direction is oblique to a direction perpendicular to the principal surfaces  54   a  and  54   b  of the resin film  51  and have a shape in which the area of the cross-section  57  perpendicular to the direction of the central axis  56  increases from the first principal surface  54   a  of the resin film  51  toward the second principal surface  54   b.    
     The through holes  53  have a diameter of 5.0 μm or more and 13.0 μm or less. When, as shown in  FIG. 8 , the through holes  53  have a shape in which the area of the cross-section  57  perpendicular to the direction of the central axis  56  increases from the first principal surface  54   a  of the resin film  51  toward the second principal surface  54   b , the smaller diameter (the diameter of the through holes  53  at the principal surface  54   a  in the example shown in  FIG. 8 ) is 5.0 μm or more and 13.0 μm or less. 
     The diameter (opening diameter) of a through hole  53  is determined as the diameter of a circle on the assumption that the opening of the through hole has the shape of the circle. In other words, the diameter of a through hole  53  is defined to correspond to the diameter of a circle having an area equal to the cross-sectional area (opening area) of the opening of the through hole. The diameters of the through holes  53  can be determined by observing the surfaces of the waterproof sound-permeable membrane  3  or resin film  51  with a microscope and analyzing the microscopic image. The diameters of the openings of the through holes  53  at each principal surface of the resin film  51  need not be exactly equal for all of the openings lying at the principal surface. However, it is preferable for the diameters in the effective portion of the resin film  51  (the portion that can be used in the waterproof sound-permeable membrane  3 ) to be so uniform that the diameters can be considered substantially equal (e.g., the standard deviation is 10% or less of the average). The below-described production method which employs ion beam irradiation and etching is capable of forming the waterproof sound-permeable membrane  3  in which the through holes have such uniform diameters. 
     A through hole  53  extending obliquely to the direction perpendicular to the principal surfaces  54   a  and  54   b  of the resin film  51  can have an opening of elliptic shape. Also in such a case, the cross-section  57  of the through hole  53  inside the film  51  can be considered to be in the shape of a circle, and the diameter of this circle is equal to the minor axis of the ellipse corresponding to the shape of the opening. Thus, for the through hole  53  extending obliquely and having an opening of elliptic shape, the minor axis of the ellipse can be regarded as the opening diameter of the through hole. 
     It is preferable for the waterproof sound-permeable membrane  3  to have an air permeability of 2.0 cm:/(cm 2 ·sec) or more and 120 cm:/(cm 2 ·sec) or less as expressed in terms of Frazier number measured according to JIS L 1096 in the thickness direction of the waterproof sound-permeable membrane  3 . The air permeability being in this range, coupled with the diameter of the through holes  53  which falls within the range described above, ensures improved sound permeability of the waterproof sound-permeable membrane  3  and the waterproof sound transmission structure  1  including the membrane, and allows the membrane and structure to have higher levels of both waterproofness and sound permeability. The air permeability of the waterproof sound-permeable membrane  3 , as expressed in terms of Frazier number, is preferably 10.0 cm 3 /(cm 2 ·sec) or more and 120 cm 3 /(cm 2 ·sec) or less. The lower limit of the air permeability can be 50.0 cm 3 /(cm 2 ·sec) or more or 90 cm 3 /(cm 2 ·sec) or more. When the air permeability is in such a range, the diameter of the through holes  53  is preferably within the desired range described above. 
     When, as shown in  FIG. 8 , the waterproof sound-permeable membrane  3  has the through holes  53  having the cross-section  57  the area of which increases from the first principal surface  54   a  toward the second principal surface  54   b , the air permeability of the membrane  3  in the direction from the second principal surface  54   b , at which the diameter of the through holes  53  is larger, to the first principal surface  54   a , at which the diameter of the through holes  53  is smaller, is preferably within the above range as expressed in terms of Frazier number. 
     Given that the liquid-repellent layer  52  has almost no influence on the air permeability of the waterproof sound-permeable membrane  3 , the air permeability of the resin film  51  is preferably within the range described above for the air permeability of the waterproof sound-permeable membrane  3 . 
     The density of the through holes  53  (hole density) in the waterproof sound-permeable membrane  3  (or in the resin film  51 ) is not particularly limited and is, for example, 1×10 3  holes/cm 2  or more and 1×10 9  holes/cm 2  or less. When the hole density is in this range, the waterproofness and sound permeability of the waterproof sound-permeable membrane  3  can be controlled within the preferred ranges, and higher levels of both of the properties can be achieved. The hole density is more preferably 1×10 5  holes/cm 2  or more and 1×10 8  holes/cm 2  or less. The hole density need not be exactly constant over the entirety of the waterproof sound-permeable membrane  3 . However, the hole density in the effective portion of the membrane is preferably so uniform that the maximum value of the hole density is equal to or less than 1.5 times the minimum value of the hole density. The hole density can be determined, for example, by observing the surfaces of the waterproof sound-permeable membrane  3  or resin film  51  with a microscope and analyzing the microscopic image. 
     The opening area ratio in the waterproof sound-permeable membrane  3  (or in the resin film  51 ) is preferably 50% or less, more preferably 10% or more and 45% or less, and even more preferably 20% or more and 40% or less. The opening area ratio refers to the ratio of the sum of the areas of the openings of the through holes  53  at a principal surface of the membrane or resin film to the area of the principal surface. When the opening area ratio is in such a range, the waterproofness and sound permeability of the waterproof sound-permeable membrane  3  can be controlled within the preferred ranges, and higher levels of both of the properties can be achieved. In addition, the opening area ratio being in the above range, coupled with the diameter of the through holes  53  which falls within the range described above, allows the waterproof sound-permeable membrane  3  to have good sound permeability even if, for example, the membrane has a reduced effective area. The opening area ratio can be determined, for example, by observing the surfaces of the waterproof sound-permeable membrane  3  or resin film  51  with a microscope and analyzing the microscopic image. 
     When, as shown in  FIG. 8 , the waterproof sound-permeable membrane  3  has the through holes  53  having the cross-section  57  the area of which increases from the first principal surface  54   a  toward the second principal surface  54   b , the opening area ratio is preferably in the above range for the principal surface  54   a  at which the diameter of the through holes is smaller. 
     The porosity of the waterproof sound-permeable membrane  3  (or of the resin film  51 ) is preferably 25% or more and 45% or less and more preferably 30% or more and 40% or less. When the porosity is in such a range, the waterproofness and sound permeability of the waterproof sound-permeable membrane  3  can be controlled within the preferred ranges, and higher levels of both of the properties can be achieved. When the diameter of the through holes  53  is in the range described above, the waterproof sound-permeable membrane  3  can have good sound permeability even if, for example, the membrane has a reduced effective area, and when the state where the air permeability of the waterproof sound-permeable membrane  3  is in the range described above is further established, the sound permeability can be further improved even if, for example, the membrane has a reduced effective area. When the waterproof sound-permeable membrane  3  has the through holes  53  having the cross-section  57  the area of which is constant in the resin film  51  as shown in  FIG. 3 , the opening area ratio corresponds to the porosity. When, as shown in  FIG. 8 , the waterproof sound-permeable membrane  3  has the through holes  53  having the cross-section  57  the area of which increases from the first principal surface  54   a  toward the second principal surface  54   b , the porosity can be determined, for example, by calculation based on the opening area ratios in both of the principal surfaces  54   a  and  54   b  and on the shape of the through holes  53  which is confirmed by observing a cross-section of the waterproof sound-permeable membrane  3  (or of the resin film  51 ). 
     As previously described, the waterproofness of the waterproof sound-permeable membrane  3  can be evaluated, for example, by a water entry pressure measured for the membrane  3  according to the Method B of water penetration test. When, as shown in  FIG. 8 , the waterproof sound-permeable membrane  3  has the through holes  53  having the cross-section  57  the area of which increases from the first principal surface  54   a  toward the second principal surface  54   b , the water entry pressure may be evaluated by applying a water pressure to the principal surface of the membrane  3  that is to face the second space  12  when the membrane  3  is placed in the waterproof sound transmission structure  1 . Given that the water entry pressure of the membrane  3  is greater when the principal surface  54   a , at which the diameter of the through holes  53  is smaller, is exposed to water than when the principal surface  54   b , at which the diameter of the through holes  53  is larger, is exposed to water, the membrane  3  may be placed in the waterproof sound transmission structure  1  in such a manner that the principal surface  54   a  faces the second space  12  where water can be present. 
     The sound permeability of the waterproof sound-permeable membrane  3  can be such that, for example, the insertion loss in the frequency range of 100 Hz to 5 kHz is 5 dB or less, 3 dB or less, 2 dB or less, or even 1 dB or less. The insertion loss in the frequency range of 100 Hz to 3 kHz can be 5 dB or less, 3 dB or less, 2 dB or less, or even 1 dB or less. The frequencies ranging from 100 Hz to 5 kHz are those that humans use in their usual vocalization and conversation and correspond to those that humans can perceive most sensitively when listening to played-back music etc. The small insertion loss in this frequency range enhances the market appeal of an electronic device including the waterproof sound-permeable membrane  3 . For example, the insertion loss caused by the waterproof sound-permeable membrane  3  at a frequency of 1 kHz, which is considered a median in the frequency range of human voice, can be 5 dB or less, 3 dB or less, or even 1 dB or less. 
     The waterproof sound-permeable membrane  3  has the liquid-repellent layer  52 . The liquid-repellent layer  52  can be formed, for example, by liquid-repellent treatment of the resin film  51 . In the example shown in  FIG. 3 , the liquid-repellent layer  52  is formed on both of the principal surfaces  54   a  and  54   b  of the resin film  51  and on the surfaces of the through holes  53 . The liquid-repellent layer  52  may be formed only on one principal surface of the resin film  51  or may be formed only on one principal surface and the surfaces of the through holes  53 . The liquid-repellent layer  52  is preferably formed at least on one principal surface that faces the first principal surface  12  where water can be present when the membrane  3  is attached to the waterproof sound transmission structure  1 . When, as shown in  FIG. 8 , the waterproof sound-permeable membrane  3  has the through holes  53  having the cross-section  57  the area of which increases from the first principal surface  54   a  toward the second principal surface  54   b , it is preferable to form the liquid-repellent layer  52  on the principal surface  54   a  at which the diameter of the through holes  53  is smaller, given the above-mentioned difference in water entry pressure depending on which of the principal surfaces is exposed to water. 
     The liquid-repellent layer  52  is a water-repellent layer and preferably further has oil repellency. The liquid-repellent layer  52  has openings positioned in correspondence with the through holes  53  of the resin film  51 . 
     The liquid-repellent layer  52  can be formed, for example, as follows: A treatment liquid prepared by diluting a water-repellent agent or hydrophobic oil-repellent agent with a diluent is thinly applied and dried on the resin film  51 . Examples of the water-repellent agent and hydrophobic oil-repellent agent include fluorine compounds such as perfluoroalkyl acrylate and perfluoroalkyl methacrylate. The thickness of the liquid-repellent layer  52  is preferably less than ½ of the diameter of the through holes  53 . 
     When the liquid-repellent layer  52  is formed by thinly applying a treatment liquid onto the resin film  51 , the surfaces (inner peripheral surfaces) of the through holes  53  can, depending on their diameter, be coated with the liquid-repellent layer  52  extending continuously from the principal surfaces of the resin film  51  (this is the case for the example shown in  FIG. 3 ). 
     The thickness of the resin film  51  and the thickness of the waterproof sound-permeable membrane  3  are, for example, 5 μm or more and 100 μm or less and preferably 15 μm or more and 50 μm or less. 
     The material composing the resin film  51  is, for example, a material that allows the below-described production method to form the through holes  53  in an original film which is a non-porous resin film. The resin film  51  is composed of, for example, a resin degradable by an alkaline solution, an acidic solution, or an alkaline or acidic solution to which has been added at least one selected from an oxidant, an organic solvent, and a surfactant. In this case, the formation of the through holes  53  in the original film by ion beam irradiation and chemical etching becomes easier in the below-described production method. The solutions as mentioned above are typical etchants. From another standpoint, the resin film  51  is composed of, for example, a resin that can be etched by hydrolysis or oxidative degradation. The original film used can be a commercially-available film. 
     The resin film  51  is composed of, for example, at least one resin selected from polyethylene terephthalate (PET), polycarbonate, polyimide, polyethylene naphthalate, and polyvinylidene fluoride. 
     The waterproof sound-permeable membrane  3  may include two or more resin films  51 . Such a waterproof sound-permeable membrane  3  can be formed, for example, by ion beam irradiation and chemical etching of a stack of two or more original films. 
     The waterproof sound-permeable membrane  3  may, if desired, include any member and/or layer other than the resin film  51  and liquid-repellent layer  52 . An example of the member is an air-permeable supporting layer  59  shown in  FIG. 9 . In the waterproof sound-permeable membrane  3  shown in  FIG. 9 , the air-permeable supporting layer  59  is placed on the principal surface  54   b  of the resin film  51  of the waterproof sound-permeable membrane  3  shown in  FIG. 8 . The placement of the air-permeable supporting layer  59  improves the strength and handling properties of the waterproof sound-permeable membrane  3 . The air-permeable supporting layer  59  may be placed on the principal surface  54   a  of the resin film  51  or on both of the principal surfaces  54   a  and  54   b.    
     The air-permeable supporting layer  59  has a higher air permeability in the thickness direction than the resin film  51 . The air-permeable supporting layer  59  used can be, for example, a woven fabric, non-woven fabric, net, or mesh. Examples of the material composing the air-permeable supporting layer  59  include polyester, polyethylene, and aramid resin. The liquid-repellent layer  52  may or may not be formed on the principal surface of the resin film  51  on which the air-permeable supporting layer  59  is to be placed. The shape of the air-permeable supporting layer  59  may be the same as or different from the shape of the resin film  51 . For example, the air-permeable supporting layer  59  can have a shape adapted for placement only on the peripheral portion of the resin film  51  (in particular, a ring shape adapted for placement only on the peripheral portion of the resin film that is circular). The air-permeable supporting layer  59  is placed, for example, by a technique such as thermal welding, or bonding by an adhesive, to the resin film  51 . 
     The surface density of the waterproof sound-permeable membrane  3  is preferably 5 to 100 g/m 2  and more preferably 10 to 50 g/m 2 , in terms of the strength, production yield, handling properties including attachment accuracy, and sound permeability of the membrane. 
     The waterproof sound-permeable membrane  3  may be subjected to a coloring treatment. Depending on the type of the material composing the resin film  51 , the waterproof sound-permeable membrane  3  not subjected to any coloring treatment is, for example, transparent or white. Such a waterproof sound-permeable membrane  3  may be conspicuous when the membrane  3  is placed to cover a sound transmission port of a housing. Such a conspicuous membrane may so stimulate the curiosity of a user as to induce the user to stab the waterproof sound-permeable membrane with a needle or the like, thereby impairing the function of the membrane as a waterproof sound-permeable membrane. When the waterproof sound-permeable membrane  3  has been subjected to a coloring treatment so that, for example, the membrane  3  has a color identical or similar to the color of the housing, the potential to attract the user&#39;s attention can be relatively reduced. In some cases, a colored waterproof sound-permeable membrane is required in view of the visual appearance of the housing of an electronic device or the like. Such a requirement as to visual appearance can be met by means of the coloring treatment. 
     The coloring treatment can be accomplished, for example, by dyeing the resin film  51  or by incorporating a colorant into the resin film  51 . The coloring treatment may be carried out, for example, so as to enable absorption of light in the wavelength range of 380 nm to 500 nm. That is, the waterproof sound-permeable membrane  3  may be subjected to a coloring treatment that enables the membrane  3  to absorb light in the wavelength range of 380 nm to 500 nm. To this end, for example, the resin film  51  contains a colorant having the ability to absorb light in the wavelength range of 380 nm to 500 nm or is dyed with a dye having the ability to absorb light in the wavelength range of 380 nm to 500 nm. In this case, the waterproof sound-permeable membrane  3  can be colored, for example, blue, gray, brown, pink, green, or yellow. The waterproof sound-permeable membrane  3  may be colored black, gray, brown, or pink. 
     The degree of coloring is preferably such that the whiteness W described below is in the range of 10.0 to 70.0. The whiteness W can be determined as follows: The lightness L, hue a, and chroma b of a principal surface of the waterproof sound-permeable membrane  3  are measured using a color-difference meter according to JIS L 1015 (Hunter method), and the whiteness W is calculated by the equation: W=100−sqr[(100−L) 2 +(a 2 +b 2 )]. The lower the value of the whiteness W is, the blacker the waterproof sound-permeable membrane  3  is. 
     The method for producing the waterproof sound-permeable membrane  3  is not particularly limited. For example, the waterproof sound-permeable membrane  3  can be produced by the production method described below. 
     (Method for Producing Waterproof Sound-Permeable Membrane) 
     In the production method which will be described hereinafter, the resin film  51  is formed by ion beam irradiation and the subsequent etching (chemical etching) of an original film. The resin film  51  formed by ion beam irradiation and etching can be processed into the waterproof sound-permeable membrane  3  through a step of forming the liquid-repellent layer  52  and optionally an additional step such as a step of performing a coloring treatment or stacking the air-permeable supporting layer  59 . 
     With the method which employs ion beam irradiation and the subsequent etching, it is easy, for example, to control various properties such as the diameters of the through holes  53  of the resin film  51 , the uniformity of the diameters, the direction of the central axis  56 , the hole density, the opening area ratio, and the porosity. This means that the waterproofness and sound permeability of the waterproof sound-permeable membrane  3  can easily be controlled. 
     The original film is a non-porous resin film having no passage that allows through-thickness air permeation in its portion that is processed into the waterproof sound-permeable membrane  3  through ion beam irradiation and etching. The original film may be an imperforate film. The fact that the original film is a non-porous resin film means that when the original film is irradiated with an ion beam and then chemically etched to form the through holes  53  and thus obtain the resin film  51 , the uniformity and surface smoothness of the film  51  can be made higher than, for example, those of a woven structure such as a mesh or of a non-woven fabric structure. This contributes to improvement in the waterproofness and sound permeability of the waterproof sound-permeable membrane  3  and therefore of the waterproof sound transmission structure  1 . 
     When the original film is irradiated with an ion beam, the polymer chains constituting the resin film are bombarded with and damaged by ions in those portions of the film through which the ions pass. The damaged polymer chains are more susceptible to chemical etching than the other polymer chains not bombarded with the ions. Chemical etching of the ion beam-irradiated original film thus results in a resin film having minute holes (through holes) extending along the tracks of the bombarding ions. That is, the directions of the central axes  56  of the through holes  53  coincide with the directions in which the ions have passed through the original film during the ion beam irradiation. In general, those portions of the original film through which no ions have passed have no minute holes formed therein. 
     This process for forming the resin film  51  from the original film may include the steps of: (I) irradiating the original film with an ion beam; and (II) chemically etching at least part of the ion-bombarded portions of the ion beam-irradiated original film to form through holes  53  extending along the tracks (ion tracks) of the bombarding ions in the film. This process is capable of forming the resin film  51  as shown in  FIG. 3  which has the through holes  53  having the cross-section  57  (cross-section perpendicular to the direction of the central axis  56 ) the area of which is constant or substantially constant from the first principal surface  54   a  toward the second principal surface  54   b , and is also capable of forming the resin film  51  having the through holes  53  in which the cross-sectional area increases from the first principal surface  54   a  toward the second principal surface  54   b . The resin film  51  of the former kind can be formed, for example, by chemically etching the ion-irradiated original film directly. The etching removes the portions corresponding to the ion tracks formed in the original film. Thus, the through holes  53  whose cross-section  57  has a constant or substantially constant area are formed by allowing the chemical etching to proceed over a sufficiently long time. 
     The resin film  51  of the latter kind can be formed, for example, by carrying out the chemical etching in the step (II) in such a manner that the extent of the etching of the ion-bombarded portions from one principal surface is greater than the extent of the etching of the ion-bombarded portions from the other principal surface. Specifically, for example, the resin film can be formed by performing the chemical etching with a masking layer placed on one principal surface of the ion-irradiated original film. In this chemical etching, the extent of the etching from the other principal surface is greater than the extent of the etching from the one principal surface with the masking layer placed thereon. Such non-uniform etching, in particular etching in which the rate of etching from one principal surface of the ion-irradiated original film and the rate of etching from the other principal surface are different, is capable of forming the through holes  53  having a shape in which the area of the cross-section  57  perpendicular to the direction of the central axis  56  increases from one principal surface of the resin film  51  toward the other principal surface of the resin film  51 . In the etching process for forming the resin film  51  of the former kind without the use of a masking layer, the etching of the ion beam-irradiated original film progresses uniformly from both principal surfaces of the original film. 
     Hereinafter, the steps (I) and (II) will be described in more detail. 
     [Step (I)] 
     In the step (I), an original film is irradiated with an ion beam. The ion beam is composed of accelerated ions. The irradiation with an ion beam causes the original film to be bombarded with the ions in the beam. 
       FIG. 10  illustrates irradiation of an original film with an ion beam. Ions  61  in the beam collide with an original film  62 , and the ions  61  having collided with the film  62  leave tracks (ion tracks)  63  within the film  62 . When viewed on the size scale of the original film  62  to be irradiated, the ions  61  bombard the original film  62  typically along a substantially straight line, thus forming the tracks  63  extending substantially straight in the film  62 . In general, the ions  61  penetrate through the original film  62 . 
     The method for irradiating the original film  62  with the ion beam is not limited. For example, the original film  62  is placed in a chamber, the internal pressure of the chamber is reduced (for example, a high vacuum atmosphere is created in the chamber to prevent energy attenuation of the bombarding ions  61 ), and then the ions  61  are emitted from a beamline to irradiate the original film  62 . A particular gas may be introduced into the chamber. Alternatively, ion beam irradiation of the original film  62  placed in the chamber may be carried out, for example, at atmospheric pressure without reduction in the internal pressure of the chamber. 
     It is also conceivable to prepare a roll on which the original film  62  in the form of a long sheet is wound and continuously irradiate the original film  62  with the ion beam while feeding the original film  62  from the roll. This allows efficient formation of the resin film  51 . It is also conceivable to dispose the roll (feed roll) and a take-up roll for winding up the ion beam-irradiated original film  62  in the chamber described above, create an appropriate atmosphere such as a reduced-pressure or high vacuum atmosphere in the chamber, then continuously irradiate the original film  62  in the form of a long sheet with the ion beam while feeding the film from the feed roll, and then wind the beam-irradiated original film  62  on the take-up roll. 
     The resin composing the original film  62  is identical to the resin composing the resin film  51  and is, for example, at least one selected from PET, polycarbonate, polyimide, polyethylene naphthalate, and polyvinylidene fluoride. The original film  62  composed of at least one of these resins is characterized in that chemical etching progresses smoothly in those portions of the film which have been bombarded with the ions  61 , while chemical etching progresses slowly in the rest of the film. This allows easier control of chemical etching of those portions of the original film  62  which correspond to the tracks  63 . Thus, for example, the use of such an original film  62  makes easier the control of the shape of the through holes  53  of the resin film  51 . 
     The original film  62  may be composed of two or more resins and may contain a material different from resins, as long as the resin film  51  can be formed through the steps (I) and (II). Examples of the material include: additives such as a light stabilizer and an antioxidant; oligomer components derived from resin materials; and metal oxides (such as white pigments, including alumina and titanium oxide). 
     The thickness of the original film  62  is, for example, 5 to 100 μm. In general, the thickness of the original film  62  remains unchanged before and after the ion beam irradiation in the step (I). 
     The original film  62  to be irradiated with the ion beam is, for example, an imperforate film. In this case, the resin film  51  having no holes other than the through holes  53  formed by the steps (I) and (II) can be obtained unless an additional step of forming holes in the film is performed in addition to the steps (I) and (II). When the additional step is performed, the resulting resin film  51  has the through holes  53  formed by the steps (I) and (II) and holes formed by the additional step. 
     The type of the ions  61  with which the original film  62  is irradiated and bombarded is not limited. It is preferable for the ions to include ions having a larger mass number than neon, specifically at least one species selected from argon ions, krypton ions, and xenon ions, since these ions do not readily chemically react with the resin composing the original film  62 . 
     The energy (acceleration energy) of the ions  61  is typically 100 to 1000 MeV. When the original film  62  used is a polyester film having a thickness of about 5 to 100 μm and the ions  61  are argon ions, the energy of the ions  61  is preferably 100 to 600 MeV. The energy of the ions  61  to be applied to the original film  62  can be adjusted depending on the type of the ions and on the type of the resin composing the original film  62 . 
     The ion source of the ions  61  to be applied to the original film  62  is not limited. For example, the ions  61  emitted from the ion source are accelerated by an ion accelerator, then passed through a beamline, and applied to the original film  62 . The ion accelerator is, for example, a cyclotron, a specific example of which is an AVF cyclotron. 
     The pressure in the beamline serving as a path of the ions  61  is preferably a high vacuum pressure of about 10 −5  to 10 −3  Pa, in terms of preventing the energy attenuation of the ions  61  in the beamline. When the pressure in the chamber enclosing the original film  62  to be irradiated with the ions  61  does not reach a high vacuum pressure, a partition permeable to the ions  61  may be used to maintain the pressure difference between the beamline and the chamber. The partition is made up of, for example, a titanium membrane or aluminum membrane. 
     The ions  61  are applied to the original film  62 , for example, in a direction perpendicular to the principal surfaces of the film. The irradiation in the example shown in  FIG. 10  is performed in this manner. In this case, the tracks  63  extend perpendicular to the principal surfaces of the original film  62 ; thus, the subsequent chemical etching results in the resin film  51  having through holes  53  formed to have a central axis  56  extending in the direction perpendicular to the principal surfaces of the resin film  51 . The ions  61  may be applied to the original film  62  in a direction oblique to the principal surfaces of the film. In this case, the subsequent chemical etching results in the resin film  51  having through holes  53  formed to have a central axis  56  extending in a direction oblique to the direction perpendicular to the principal surfaces of the resin film  51 . The direction of the ions  61  applied to the original film  62  can be controlled by known means. The angle θ 1  shown in  FIG. 4  can be controlled, for example, by adjusting the incident angle of the ion beam to the original film  62 . 
     The ions  61  are applied to the original film  62 , for example, in such a manner that the trajectories of the ions  61  are parallel to each other. The irradiation in the example shown in  FIG. 10  is performed in this manner. In this case, the subsequent chemical etching results in the resin film  51  having through holes  53  formed to extend parallel to each other. 
     The ions  61  may be applied to the original film  62  in such a manner that the trajectories of the ions  61  are non-parallel to each other (random with respect to each other, for example). This results in, for example, the resin film  51  as shown in any of  FIGS. 4 to 7 . Specifically, for example, a possible method for forming the resin film  51  as shown in any of  FIGS. 4 to 7  is to apply the ion beam to the original film  62  in a direction oblique to the direction perpendicular to the principal surfaces of the original film  62  while changing the oblique direction continuously or stepwise. Since the ion beam is composed of ions traveling parallel to each other, the resin film  51  typically has a set of through holes  53  extending in the same direction (there are typically two or more through holes  53  extending in the same direction in the resin film  51 ). 
       FIG. 11  shows an example of the method in which the oblique direction is changed continuously or stepwise. In the example shown in  FIG. 11 , the original film  62  in the form of a long sheet is fed from a feed roll  71 , passed through an irradiation roll  72  with a predetermined curvature, and irradiated with an ion beam  64  while moving on the roll  72 , after which the irradiated original film  62  is wound on a take-up roll  73 . During this process, the ions  61  in the ion beam  64  travel parallel to each other and reach the original film  62  successively. Thus, the angle (incident angle θ 1 ) at which the ion beam collides with the principal surface of the original film  62  varies with the movement of the original film  62  on the irradiation roll  72 . Continuous emission of the ion beam  64  allows continuous change of the oblique direction, while intermittent emission of the ion beam  64  allows stepwise change of the oblique direction. Such control can be considered to be based on ion beam emission timing. The properties (for example, angle θ 1 ) of the tracks  63  to be formed in the original film  62  can be controlled also by adjusting the cross-sectional shape of the ion beam  64  and the cross-sectional area of the beamline of the ion beam  64  formed on the area of the irradiation target surface of the original film  62 . 
     The hole density of the resin film  51  can be controlled by the conditions of the irradiation of the original film  62  with the ion beam (such as the type of the ions, the energy of the ions, and the density of the bombarding ions (irradiation density)). 
     The ions  61  may be emitted from two or more beamlines to irradiate the original film  62 . 
     The step (I) may be performed in the presence of a masking layer on a principal surface, such as the one principal surface as described above, of the original film  62 . In this case, for example, the masking layer can be used also in the step (II). 
     [Step (II)] 
     The original film  62  irradiated with the ion beam in the step (I) has portions bombarded with the ions  61  and, in the step (II), at least part of the ion-bombarded portions are chemically etched to form through holes  53  extending along the tracks  63  of the bombarding ions  61  in the film. The resin film  51  thus obtained is basically identical to the original film  62  yet to be subjected to the ion beam irradiation except for the presence of the through holes  53 , unless another step of modifying the nature of the film is performed. 
     The specific technique employed for the etching may be the same as any of known techniques. For example, the ion beam-irradiated original film  62  may be immersed in an etchant at a predetermined temperature for a predetermined time. Adjusting the etching conditions such as the etching temperature, the etching time, and the composition of the etchant allows, for example, control of the diameter of the through holes  53 . 
     The etching temperature is, for example, 40 to 150° C., and the etching time is, for example, 10 seconds to 60 minutes. 
     The etchant used in the chemical etching is not particularly limited. The etchant is, for example, an alkaline solution, an acidic solution, or an alkaline or acidic solution to which has been added at least one selected from an oxidant, an organic solvent, and a surfactant. The alkaline solution is, for example, a solution (typically an aqueous solution) containing a base such as sodium hydroxide or potassium hydroxide. The acidic solution is, for example, a solution (typically an aqueous solution) containing an acid such as nitric acid or sulfuric acid. The oxidant is, for example, potassium dichromate, potassium permanganate, or sodium hypochlorite. The organic solvent is, for example, methanol, ethanol, 2-propanol, ethylene glycol, amino alcohol, N-methylpyrrolidone, or N,N-dimethylformamide. The surfactant is, for example, an alkyl benzenesulfonic acid salt or an alkyl sulfuric acid salt. 
     In the step (II), the chemical etching is performed in the presence of a masking layer on one principal surface of the ion beam-irradiated original film  62 . In this chemical etching of those portions of the original film  62  which have been bombarded with the ions  61 , the extent of etching from the other principal surface is greater than the extent of etching from the one principal surface with the masking layer thereon. That is, the chemical etching of those portions of the original film  62  which have been bombarded with the ions  61  is performed in such a manner that the etching from one principal surface of the film and the etching from the other principal surface of the film progress in a non-uniform fashion (such etching may be referred to as “non-uniform etching”). Saying that “the extent of etching is great” specifically means, for example, that the amount of etching of the ion-bombarded portions per unit time is large, namely, that the rate of etching of the portions is high. 
     In the step (II), a masking layer more resistant to chemical etching than those portions of the original film  62  which have been bombarded with the ions  61  may be placed on one principal surface of the original film  62  to perform chemical etching in which the etching of the portions from the other principal surface of the original film  62  is allowed to progress while the etching of the portions from the one principal surface is inhibited. Such etching can be accomplished, for example, by appropriately selecting the type and thickness of the masking layer, the manner of the placement of the masking layer, and the etching conditions. 
     The type of the masking layer is preferably, but not limited to, a layer composed of a material more resistant to chemical etching than those portions of the original film  62  which have been bombarded with the ions  61 . Saying that a material is “resistant to etching” specifically means, for example, that the amount of the material etched per unit time is small, namely, that the rate at which the material is etched is low. Whether a material is resistant to chemical etching can be determined on the basis of the conditions (such as the type of the etchant, the etching temperature, and the etching time) of the non-uniform etching to be actually performed in the step (II). When, in the step (II), non-uniform etching is performed a plurality of times by changing the type of the masking layer and/or alternating the surface on which the layer is placed, whether a material is resistant to chemical etching can be determined for each etching on the basis of the etching conditions. 
     The masking layer may be more susceptible or more resistant to chemical etching than those portions of the original film  62  which have not been bombarded with the ions  61 . The masking layer is preferably more resistant to chemical etching than such portions. In this case, for example, the thickness required of the masking layer used in the non-uniform etching can be decreased. 
     When the original film  62  with a masking layer thereon is irradiated with the ion beam in the step (I), ion tracks are formed also in the masking layer. Given this, the material composing the masking layer is preferably a material having polymer chains resistant to damage by ion beam irradiation. 
     The masking layer is composed of, for example, at least one selected from polyolefin, polystyrene, polyvinyl chloride, polyvinyl alcohol, and a metal foil. These materials are resistant to chemical etching as well as being resistant to damage by ion beam irradiation. 
     When a masking layer is used to perform non-uniform etching, the masking layer should be placed on at least a portion of one principal surface of the original film  62 , the portion corresponding to the area to be subjected to the non-uniform etching. It should be understood that the masking layer can be placed over the entirety of one principal surface of the original film  62  if desired. 
     The method for placing the masking layer on a principal surface of the original film  62  is not limited as long as the masking layer is not separated from the principal surface during the non-uniform etching. The masking layer is placed on the principal surface of the original film  62 , for example, by means of an adhesive. That is, in the step (II), the chemical etching (non-uniform etching) may be performed in the presence of a masking layer bonded to the one principal surface of the original film  62  by means of an adhesive. It is relatively easy to dispose the masking layer by means of an adhesive. Appropriately selecting the type of the adhesive makes it easy to separate the masking layer from the original film  62  after the non-uniform etching. 
     When the non-uniform etching is performed in the step (II), this etching may be performed a plurality of times. Uniform etching in which etching of the tracks  63  is allowed to progress uniformly from both principal surfaces of the original film  62  may be performed in combination with the non-uniform etching. For example, the masking layer may be separated from the original film  62  in the course of the etching to switch the mode of etching from the non-uniform etching to the uniform etching. Alternatively, the masking layer may be placed on the original film  62  after the end of the uniform etching to subsequently perform the non-uniform etching. 
     When the non-uniform etching employing a masking layer is performed in the step (II), a part or the whole of the masking layer may, if desired, be allowed to remain on the resin film  51  after the etching. The masking layer remaining on the resin film  51  can be used, for example, as an indicator for differentiating between the one principal surface (the principal surface with the masking layer thereon) of the resin film  51  and the other principal surface of the resin film  51 . 
     When etching is performed a plurality of times in the step (II), the etching conditions may be changed for each time of etching. 
     The method for producing the resin film  51  may include any step other than the steps (I) and (II). 
     How to place the waterproof sound-permeable membrane  3  on the wall  2  in the waterproof sound transmission structure  1  is not particularly limited, as long as the waterproof sound-permeable membrane  3  is placed to cover the sound transmission port  11  of the wall  2 . For the placement of the waterproof sound-permeable membrane  3  on the wall  2 , techniques such as adhesion using a double-sided tape, thermal welding, high-frequency welding, and ultrasonic welding can be employed. Adhesion using a double-sided tape is preferred, because the double-sided tape can be used as a supporting member for the waterproof sound-permeable membrane  3  and because the waterproof sound-permeable membrane  3  can be attached accurately and securely. 
     The supporting member for the waterproof sound-permeable membrane  3  is a member that reinforces the membrane  3  and improves the handling properties of the membrane  3 . The supporting member is placed, for example, on a peripheral portion of the waterproof sound-permeable membrane  3 . When the waterproof sound-permeable membrane  3  is circular as viewed in the direction perpendicular to its principal surfaces, the supporting member is, for example, a ring-shaped sheet joined to the peripheral portion of the membrane  3 . The supporting member can function as a portion to be attached to the wall  2  when the waterproof sound-permeable membrane  3  is placed on the wall  2 , and this enables more accurate and secure placement of the membrane  3  on the wall  2 . 
     The shape of the supporting member is not limited. For example, as shown in  FIG. 12 , the supporting member may be a supporting member  81  which is ring-shaped sheet joined to the peripheral portion of the waterproof sound-permeable membrane  3  which is circular as viewed in the direction perpendicular to its principal surfaces. Alternatively, as shown in  FIG. 13 , the supporting member may be a supporting member  81  which is a frame-shaped sheet joined to the peripheral portion of the waterproof sound-permeable membrane  3  which is rectangular as viewed in the direction perpendicular to its principal surfaces. 
     Conforming the shape of the supporting member  81  to the shape of the peripheral portion of the waterproof sound-permeable membrane  3  as shown in  FIGS. 12 and 13  reduces the deterioration in sound permeability of the waterproof sound-permeable membrane  3  caused by the placement of the supporting member  81 . It is preferable for the supporting member  81  to be in the form of a sheet, in terms of improving the handling properties and ease of placement of the waterproof sound-permeable membrane  3 . 
     Examples of the material composing the supporting member  81  include resins, metals, and composites thereof. Examples of the resins include: polyolefins such as polyethylene and polypropylene; polyesters such as PET and polycarbonate; polyimides; and composites of these resins. Examples of the metal include metals having high corrosion resistance such as stainless steel and aluminum. 
     The thickness of the supporting member  81  is, for example, 5 to 500 μm and preferably 25 to 200 μm. In particular, in view of its function as the portion for attachment, the ring width (frame width: difference between the outer size and inner size) is suitably about 0.5 to 2 mm. A foamed material made of any of the resins mentioned above may be used as the supporting member  81 . 
     The method for joining the waterproof sound-permeable membrane  3  and the supporting member  81  together is not particularly limited. Examples of methods that can be employed include heat welding, ultrasonic welding, bonding by an adhesive, and bonding by a double-sided tape. As previously described, a double-sided tape itself can be used as the supporting member. 
     In the example shown in  FIG. 1 , the waterproof sound-permeable membrane  3  is placed on the first space  13 -side of the wall  2 . In the waterproof sound transmission structure  1 , the waterproof sound-permeable membrane  3  may be placed on the second space  12 -side of the wall  2 . The wall  2  is, for example, a housing of an electronic device, and, in this example, the first space  13  is a space located inside the housing, while the second space  12  is a space located outside the electronic device. The wall  2  is, for example, a housing of an electronic device case, and, in this example, the first space  13  is a space located inside the case enclosing an electronic device, while the second space  12  is a space located outside the electronic device case. 
     The wall  2  is composed of, for example, a resin, metal, glass, or composite thereof. 
     In the waterproof sound transmission structure  1 , the region (region a in the example shown in  FIG. 1 ) of the waterproof sound-permeable membrane  3  that overlaps an opening of the sound transmission port  11  may have no air-permeable supporting layer  59  so that the resin film  51  and/or liquid-repellent layer  52  is exposed in the region. In this case, the resin film  51  and/or liquid-repellent layer  52  may be exposed only at one principal surface of the waterproof sound-permeable membrane  3  or at both principal surfaces of the membrane  3 . This allows the waterproof sound transmission structure  1  to have higher levels of both waterproofness and sound permeability. For example, the waterproof sound-permeable membrane  3  is joined to the wall  2  at the region (region B in the example shown in  FIG. 1 ) of the waterproof sound-permeable membrane  3  that is other than the region overlapping the sound transmission port  11 . The joining to the wall  2  may be accomplished via the supporting member  81 , and an example of the supporting member  81  is a double-sided tape. 
     For the waterproof sound transmission structure  1 , the insertion loss in the frequency range of 100 Hz to 5 kHz can be, for example, 5 dB or less, 3 dB or less, 2 dB or less, or even 1 dB or less. In addition, the insertion loss in the frequency range of 100 Hz to 3 kHz can be 5 dB or less, 3 dB or less, 2 dB or less, or even 1 dB or less. For the waterproof sound transmission structure  1 , the insertion loss at a frequency of 1 kHz, which is considered a median in the frequency range of human voice, can be, for example, 5 dB or less, 3 dB or less, or even 1 dB or less. 
     The waterproof sound transmission structure  1  can be endowed with good sound permeability, for example, depending on the configuration of the waterproof sound-permeable membrane  3 , in particular depending on the diameter and shape of the through holes  53 , even when the waterproof sound-permeable membrane  3  has a reduced effective area. For example, the effective area of the waterproof sound-permeable membrane  3  in the waterproof sound transmission structure  1  may be 4.9 mm 2  or less. The advantageous feature of allowing a reduction in the effective area contributes, for example, to a reduction in the space to be occupied by the waterproof sound transmission structure  1  and to an increase in the flexibility in the visual appearance and design of an electronic device including the structure, in particular a reduction in size and/or thickness of the electronic device. The effective area of the waterproof sound-permeable membrane  3  refers to the area of a portion (effective portion) of the membrane through which, when the membrane is placed to cover a sound transmission port of a housing, sound actually enters, travels in, and exits the membrane. For example, the effective area does not include the area of a supporting member or a joining portion placed or formed on the peripheral portion of the waterproof sound-permeable membrane  3  for placement of the membrane  3 . The effective area can typically be equal to the area of the sound transmission port over which the membrane is placed. In the case of a waterproof sound-permeable member including the waterproof sound-permeable membrane and a supporting member placed on the peripheral portion of the membrane, the effective area can be equal to the area of an opening portion of the supporting member. 
     For the waterproof sound transmission structure  1 , the insertion loss as described above can be achieved when the effective area of the waterproof sound-permeable membrane  3  included in the structure  1  is, for example, 4.9 mm 2  (when the membrane is, for example, in the shape of a circle with a diameter of 2.5 mm). It should be understood that high levels of both waterproofness and sound permeability can be achieved not only when the effective area of the waterproof sound-permeable membrane  3  is small but also when the effective area is large. However, the waterproof sound transmission structure  1  is particularly advantageous when the effective area of the waterproof sound-permeable membrane  3  is or must be small. 
     When, as shown in  FIG. 8 , the waterproof sound-permeable membrane  3  is a membrane having the through holes  53  having the cross-section  57  the area of which increases from the first principal surface  54   a  toward the second principal surface  54   b , the sound permeability from the second principal surface  54   b  at which the dimeter of the through holes  53  is larger is generally better than the sound permeability from the first principal surface  54   a  at which the diameter is smaller. In this case, the sound permeability from the second principal surface  54   b , which is better than that from the first principal surface  54   a , can be such that the insertion loss as described above is achieved. 
     When, as shown in  FIG. 8 , the waterproof sound-permeable membrane  3  is a membrane having the through holes  53  having the cross-section  57  the area of which increases from the first principal surface  54   a  toward the second principal surface  54   b , the orientation of the waterproof sound-permeable membrane  3  in the waterproof sound transmission structure  1  is not limited. For example, the waterproof sound-permeable membrane  3  may be placed in such a manner that the principal surface  54   a  (principal surface  54   a  at which the diameter of the through holes  53  is smaller) of the resin film  51  faces the second space  12 . In this case, higher sound permeability from the first space  13  as well as higher waterproofness can be achieved. 
     The waterproof sound transmission structure  1  can be formed for any sound transmission port  11  of any wall  2  where both waterproofness and sound permeability should be ensured. Products that can include the waterproof sound transmission structure  1  are not limited to particular ones. 
     The waterproof sound transmission structure  12  can be used in various applications similarly to conventional waterproof sound transmission structures. 
     [Electronic Device] 
     An example of the electronic device of the present invention is shown in  FIG. 14A . The electronic device shown in  FIG. 14A  is a smartphone which is a type of mobile phone. A housing  102  of the smartphone  101  has a sound transmission port  103   a  provided in proximity to a transducer which is a type of sound emitting-receiving device, a sound transmission port  103   b  provided in proximity to a microphone which is a type of sound receiver, and a sound transmission port  103   c  provided in proximity to a speaker which is a type of sound emitter. Sound is transmitted through the sound transmission ports  103   a  to  103   c  between the outside of the smartphone  101  and the audio components (transducer, microphone, and speaker) which are placed as audio parts in the housing  102 . In the smartphone  101 , as shown in  FIG. 14B , the waterproof sound-permeable membranes  3  are attached to the inner surface of the housing  102  via the supporting members  81  to cover these sound transmission ports  103   a  to  103   c , so that the waterproof sound transmission structure  1  is formed for each sound transmission port. This makes it possible to prevent water from entering an interior  104  of the housing  102  from the outside of the smartphone  101  through the sound transmission ports while permitting transmission of sound between the outside of the smartphone  101  and the audio parts. 
     In the waterproof sound transmission structure  1  of the electronic device shown in  FIGS. 14A and 14B , the wall  2  is the housing  102  of the smartphone  101 , the second space  12  where water can be present is a space located outside the smartphone  101  (or the housing  102 ) such as a space in which the user of the smartphone  101  lives, the first space  13  is a space located in the interior  104  of the housing  102  of the smartphone  101  and containing the audio parts, and the sound transmission ports  11  are the sound transmission ports  103   a  to  103   c  for transmission of sound from and/or to the audio components (audio parts). In the example shown in  FIGS. 14A and 14B , the audio part-containing space, namely, the first space  13 , is a space located inside the housing  102  of the smartphone  101 , and the volumes occupied by components such as the audio components are excluded from the sealed-state volume of the space. 
     Another example of the electronic device of the present invention is shown in  FIGS. 15A and 15B . The electronic device shown in  FIGS. 15A and 15B  is a smartphone  111 , similarly to the electronic device shown in  FIGS. 14A and 14B . The smartphone  111  shown in  FIGS. 15A and 15B  has the same configuration as the smartphone  101  shown in  FIGS. 14A and 14B , except that audio components  112  enclosed in the interior  104  of the housing  102  are attached to the sound transmission ports  103   a  to  103   c  with the waterproof sound-permeable membranes  3  interposed therebetween. In the waterproof sound transmission structure  1  of the electronic device shown in  FIGS. 15A and 15B , the wall  2  is the housing  102  of the smartphone  11   l , the second space  12  where water can be present is a space located outside the smartphone  111  (or the housing  102 ), the first space  13  is a space located inside a housing  113  of each of the audio components  112  enclosed in the interior  104  of the housing  102  of the smartphone  111 , and the sound transmission ports  11  are the sound transmission ports  103   a  to  103   c  for transmission of sound from and/or to audio elements (audio parts) inside the housings  113 . In the example shown in  FIGS. 15A and 15B , the audio-part containing space, namely, the first space  13 , is a space located inside the housing  113  of each audio component  112 , and the volumes occupied by audio elements such as a diaphragm placed in the space are excluded from the sealed-state volume of the space. 
     With the waterproof sound transmission structure  1 , high levels of both waterproofness and sound permeability are achieved at sound transmission ports. In the structure  1 , even when, for example, the effective area of the waterproof sound-permeable membrane  3  is small, good sound permeability can be obtained depending on the configuration of the membrane  3 . The structure  1  also allows an increase in flexibility in design and visual appearance of an electronic device such as a smartphone, in particular a reduction in size and/or thickness of the electronic device. 
     In the electronic device of the present invention, the location where the waterproof sound transmission structure  1  is formed is not limited as long as sound can be transmitted between the audio part in the electronic device and the outside of the electronic device. 
     The housing  102  is composed of a resin, metal, glass, or composite thereof. The display portion (such as a surface glass layer of a liquid crystal display) of the electronic device may constitute a part of the housing  102 , as in smartphones and tablet computers. 
     The electronic device of the present invention is not limited to smartphones. Electronic devices that fall under the category of the electronic device of the present invention include all types of electronic devices that are equipped with an audio part, that have a housing provided with a sound transmission port for transmission of sound between the outside of the housing and the audio part, that require prevention of ingress of water into the housing from the outside through the sound transmission port, and that allow the waterproof sound-permeable membrane  3  to be placed to cover the sound transmission port so that the waterproof sound transmission structure  1  is formed. Examples of the electronic device of the present invention include: mobile phones such as feature phones and smartphones; mobile computers such as tablet computers, wearable devices, PDAs, game consoles, and notebook computers; electronic notebooks; digital cameras; video cameras; and electronic book readers. 
     [Electronic Device Case] 
     An example of the electronic device case of the present invention is shown in  FIG. 16A . The case  201  shown in  FIG. 16A  is provided with sound transmission ports  202   a  to  202   c  for transmission of sound between audio parts of an electronic device enclosed in the case  201  and the outside of the case  201 . The case  201  shown in  FIG. 16A  is a case for a smartphone different in type from the smartphone  101  shown in  FIG. 14A . The sound transmission port  202   a  is provided for transmission of sound to the voice receiver of the smartphone, the sound transmission port  202   b  is provided for transmission of sound to the voice transmitter of the smartphone, and the sound transmission port  202   c  is provided for transmission of sound from the speaker of the smartphone to the outside. Sound is transmitted through the sound transmission ports  202   a  to  202   c  between the outside of the case  201  and the audio parts of the smartphone enclosed in the case  201 . In the case  201 , as shown in  FIG. 16B , the waterproof sound-permeable membranes  3  are attached to the inner surface of the case  201  via the supporting members  81  to cover the sound transmission ports  202   a  to  202   c  so that the waterproof sound transmission structure  1  is formed for each sound transmission port. This makes it possible to prevent water from entering an interior  203  of the case  201 , and then the electronic device enclosed in the case  201 , from the outside of the case  201  through the sound transmission ports while permitting transmission of sound between the outside of the case  201  and the audio parts. 
     In the waterproof sound transmission structure  1  of the electronic device case shown in  FIGS. 16A and 16B , the wall  2  is the housing  204  of the case  201 , the second space  12  where water can be present is a space located outside the case  201  (or the housing  204 ) such as a space in which the user of the case  201  lives, the first space  13  is a space located in the interior  203  of the case  201  (or of the housing  204 ) for enclosing the electronic device, and the sound transmission ports  11  are the sound transmission ports  202   a  to  202   c  for transmission of sound from and/or to audio parts of the electronic device enclosed in the case  201 . In the example shown in  FIGS. 16A and 16B , the audio part-containing space, namely, the first space  13 , is a space located in the interior  203  of the housing  204  of the case  201  for enclosing an electronic device having audio parts, and the volumes occupied by products which are disposed inside the case  201 , such as the electronic device enclosed in the case  201 , are excluded from the sealed-state volume of the space. It should be noted that, when air can pass between a space that is inside of the case  201  and outside of the electronic device enclosed in the case  201 , and the interior of the housing of the device, the sealed-state volume of the first space  13  includes the volume of a portion of the space inside the housing of the electronic device, the portion communicating with the interior of the case  201 . The sealed-state volume of the first space  13  is the volume occupied by air communicable with the waterproof sound-permeable membrane  3  and sound transmission port  11 . 
     The formation of the waterproof sound transmission structure  1  allows high levels of both waterproofness and sound permeability to be achieved at the sound transmission ports. In the structure  1 , even when, for example, the effective area of the waterproof sound-permeable membrane  3  is small, good sound permeability can be obtained depending on the configuration of the membrane  3 . The structure  1  also enables the electronic device case  201  to be adapted for an electronic device that allows high flexibility in design and visual appearance and that has a reduced size and/or thickness. In addition, the opening  202   a  ( 202   b ,  202   c ) of the electronic device case  201  can have a small area in the structure  1 , which provides an increase in the flexibility in the design and visual appearance of the case  201  itself. 
     In the electronic device case of the present invention, the location where the waterproof sound transmission structure  1  is formed is not limited, as long as sound can be transmitted between audio parts of the electronic device enclosed in the case and the outside of the case. 
     The electronic device case  201  is composed of a resin, metal, glass, or composite thereof. The electronic device case  201  can have any configuration, as long as the effects of the present invention are obtained. For example, the case  201  shown in  FIG. 16A  is a case for a smartphone and includes a film  205  that enables external operation of a touch panel of the smartphone enclosed in the case. 
     EXAMPLES 
     Hereinafter, the present invention will be described in more detail by way of examples. The present invention is not limited to the examples given below. 
     First, the methods for evaluation of resin films, waterproof sound-permeable membranes, and waterproof sound transmission structures fabricated in Examples and Comparative Examples will be described. 
     [Opening Diameter of Through Holes] 
     The principal surface of each resin film was observed with a scanning electron microscope (SEM),  10  through holes were randomly selected from those captured in the SEM image, and the opening diameters of the selected through holes were determined on the basis of the image. The average of the opening diameters was determined as the opening diameter of the through holes of the resin film. 
     [Air Permeability] 
     The through-thickness air permeability of each waterproof sound-permeable membrane was determined according to JIS L 1096 (Method A of air permeability measurement: Frazier method). 
     [Water Entry Pressure] 
     The water entry pressure of each waterproof sound-permeable membrane was determined according to Method B (high hydraulic pressure method) of water penetration test specified in JIS L 1092. If a test piece of the membrane has an area specified in this standard, the membrane undergoes a significant change in shape. Thus, in order to reduce the change in shape of the membrane to some extent, a stainless steel mesh (opening diameter=2 mm) was placed on one side of the membrane opposite to its surface subjected to pressure, and in this state the measurement was performed. 
     [Whiteness] 
     The whiteness W of each waterproof sound-permeable membrane was determined by measuring the lightness L, hue a, and chroma b of the principal surface of the waterproof sound-permeable membrane using a color-difference meter (Spectrophotometer SE 6000 manufactured by NIPPON DENSHOKU INDUSTRIES CO., LTD.) complying with JIS L 1015 and then by calculating the whiteness W by the following equation using the measured lightness L, hue a, and chroma b: W=100−sqr[(100−L) 2 +(a 2 +b 2 )]. 
     [Acoustic Properties] 
     The acoustic properties (including sound pressure loss) of each fabricated waterproof sound-permeable membrane were evaluated as a measure of its sound permeability. The evaluation method was as follows. 
     First, as shown in  FIG. 17A , a simulated housing  91  (made of polystyrene and having outer dimensions of 60 mm×50 mm×28 mm) imitating a housing of a mobile phone was prepared. The simulated housing  91  was provided with: one speaker attachment hole  92  (having the shape of a circle of 2.5 mm diameter) serving as a sound transmission port for allowing sound output from a speaker to be transmitted to the outside of the housing; and one guide hole  93  for a speaker cable. The housing had no openings other than these holes. Next, a speaker  95  (SCG-16A, manufactured by STAR MICRONICS CO., LTD) was embedded in a filler  94  made of urethane sponge and having formed therein a sound transmission port having the shape of a circle of 5 mm diameter, and the filler  94  with the speaker  95  was placed inside the housing  91 . The speaker cable  96  of the speaker  95  was led to the outside of the housing  91  through the guide hole  93 , and then the guide hole  93  was filled with putty. 
     Next, a double-sided tape  97  (manufactured by NITTO DENKO CORPORATION, identified as No. 57120B, and having a thickness of 0.2 mm) made of a polyethylene foam, a PET film  98  (having a thickness of 0.1 mm), and a double-sided tape  99  (manufactured by NITTO DENKO CORPORATION, identified as No. 5603, and having a thickness of 0.03 mm) made of PET were prepared. A ring-shaped piece having an inner diameter of 2.5 mm and an outer diameter of 5.8 mm was punched from each of the prepared tapes and film. Additionally, circular pieces having a diameter of 5.8 mm were punched from waterproof sound-permeable membranes  300  fabricated in Examples and Comparative Examples. Next, the ring-shaped piece of the double-sided tape  97  having an inner diameter of 2.5 mm, the circular piece of the waterproof sound-permeable membrane  300 , the ring-shaped piece of the double-sided tape  99  having an inner diameter of 2.5 mm, and the ring-shaped piece of the PET film  98  having an inner diameter of 2.5 mm were stacked in this order in such a manner that their entire outer peripheries exactly overlapped each other. Thus, a waterproof sound-permeable member A (in which the waterproof sound-permeable membrane had an effective area of 4.9 mm 2 ) for acoustic property evaluation was prepared (see  FIG. 17B ). 
     Next, the waterproof sound-permeable member prepared was attached to the exterior of the simulated housing  91  by means of the polyethylene foam double-sided tape  97  of the member so that the waterproof sound-permeable membrane  300  fully covered the sound transmission port  92  and that a waterproof sound transmission structure  301  was formed. This was done in such a manner that no gap was formed between the waterproof sound-permeable membrane  300  and the double-sided tape  97  and between the double-sided tape  97  and the simulated housing  91 . 
     Next, the speaker cable  96  and a microphone (Spm0405Hd4H-W8 manufactured by Knowles Acoustic) were connected to an acoustic evaluation system (Multi-analyzer System 3560-B-030 manufactured by B&amp;K Sound &amp; Vibration Measurement A/S), and the microphone was placed at a distance of 21 mm from the sound transmission port  92  of the simulated housing  91 . Then, SSR analysis (test signals of 20 Hz to 10 kHz, sweep) was selected as an evaluation mode and carried out to evaluate the acoustic properties (THD and sound pressure loss) of the waterproof sound-permeable membrane  300 . The sound pressure loss was automatically determined on the basis of the signal input to the speaker  95  from the acoustic evaluation system and the signal detected through the microphone. Additionally, a blank sound pressure loss was determined in the same manner except for the absence of the waterproof sound-permeable membrane, and a value obtained by subtracting the blank sound pressure loss from the sound pressure loss determined in the presence of the waterproof sound-permeable membrane was determined as the sound pressure loss (insertion loss) to be used as a measure of the sound permeability of the waterproof sound-permeable membrane. A smaller insertion loss can be considered to indicate better maintenance of the properties of sound transmitted through the waterproof sound-permeable membrane. This evaluation was carried out on the waterproof sound-permeable membranes fabricated in Examples and Comparative Examples. Table 1 given below shows values of insertion loss at a frequency of 5 kHz. In general, the insertion loss caused by a waterproof sound-permeable membrane increases with increasing frequency in the frequency range of 20 Hz to 10 kHz. Thus, the insertion loss at a frequency of 5 kHz corresponds to the maximum value of the insertion loss in the frequency range of 10 Hz to 5 kHz. 
     [Waterproof Sound Transmission Structure] 
     Waterproof sound transmission structures were formed using the fabricated waterproof sound-permeable membranes. Whether each structure was compliant with IPX7 which is a degree of protection against water ingress was evaluated for different values of the sealed-state volume of the first space. The evaluation method was as follows. 
     (Sealed-State Volume of First Space: 300 mm 3  or More) 
     As shown in  FIG. 18A , each of the waterproof sound-permeable membranes  300  fabricated in Examples and Comparative Examples was set in jigs  302  and  303  for IPX7 testing. Specifically, the waterproof sound-permeable membrane  300  was attached, using a ring-shaped double-sided tape  306 , to a polycarbonate plate  304  provided with an opening  305  of 2.0 mm diameter so as to cover the opening. After that, the plate  304  with the membrane  300  was fixed by means of the jigs  302  and  303  and an O-ring  307 . The diameter of the opening of the double-sided tape  306  was 2.5 mm, which means that the effective area of the waterproof sound-permeable membrane  300  was 4.9 mm 2 . The space  308  bounded by the jig  302 , plate  304 , O-ring  307 , and waterproof sound-permeable membrane  300  corresponds to the first space  13 . The volume of the space was varied using various jigs  302  differing in height h and width w, and whether the waterproof sound transmission structure including the waterproof sound-permeable membrane  300  was compliant with IPX7 was evaluated using the same membrane  300  for different values of the sealed-sate volume of the space  308 . This evaluation was conducted by immersing the waterproof sound transmission structure in water at a depth of 1 m for 30 minutes according to JIS C 0920. When this immersion causes no ingress of water into the space  308 , the waterproof sound transmission structure is determined to be compliant with IPX7. The water pressure acting on the waterproof sound-permeable membrane  300  during evaluation was 9.8 kPa since the water immersion depth was 1 m. 
     (Sealed-State Volume of First Space: Less than 300 mm 3 ) 
     As shown in  FIG. 18B , each of the waterproof sound-permeable membranes  300  fabricated in Examples and Comparative Examples was set in jigs  302  and  303  for IPX7 testing. The waterproof sound-permeable membrane  300  was attached, using a ring-shaped double-sided tape  306 , to a polycarbonate plate  304  provided with an opening  305  of 2.0 mm diameter so as to cover the opening. Then, unlike the case where the sealed-state volume of the second space was 300 mm 3  or more ( FIG. 18A ), a polycarbonate plate  309  having a given recess (typically a groove) in its one principal surface was joined to that surface of the plate  304  to which the waterproof sound-permeable membrane  300  was attached. This joining was done so that the waterproof sound-permeable membrane  300  was placed inside the recess. The pair of plates  304  and  309  enclosing the waterproof sound-permeable membrane  300  was fixed by means of the jigs  302  and  303  and an O-ring  307 . The diameter of the opening of the double-sided tape  306  was 2.5 mm, which means that the effective area of the waterproof sound-permeable membrane  300  was 4.9 mm 2 . The space  308  bounded by the plates  304  and  309  and the waterproof sound-permeable membrane  300  corresponds to the first space  13 . The volume of the space was varied by changing the size of the recess of the plate  309 , and whether the waterproof sound transmission structure including the waterproof sound-permeable membrane  300  was compliant with IPX7 was evaluated using the same membrane  300  for different values of the sealed-sate volume of the space  308 . This evaluation was conducted by immersing the waterproof sound transmission structure in water at a depth of 1 m for 30 minutes according to JIS C 0920. When this immersion causes no ingress of water into the space  308 , the waterproof sound transmission structure is determined to be compliant with IPX7. 
     Example 1 
     There was prepared a commercially-available non-porous PET film (Track etched membrane manufactured by it4ip S.A. and having a thickness of 50 μm) having through holes formed to extend through the thickness of the film. The diameter of the through holes of the film was 10.6 μm, and the hole density of the film was 3.0×10 5  holes/cm 2 . 
     Next, the PET film prepared was immersed in an etchant (an aqueous solution of 20 mass % potassium hydroxide) maintained at 80° C. for 30 minutes. After the etching, the film was taken out of the etchant, immersed and washed in RO water (water filtered through a reverse osmosis membrane), and then dried by a drying oven set at 50° C. Thus, a non-porous resin film having through holes formed to extend through the thickness of the film was obtained. The diameter of the through holes of the resin film obtained was 13.0 μm, and the area of a cross-section of each through hole taken perpendicular to the direction of the central axis of the hole was constant in the thickness direction of the film. The hole density remained unchanged before and after the etching. 
     Next, the resin film thus obtained was immersed in a liquid-repellent treatment solution for 3 seconds and then left to dry at ordinary temperature for 30 minutes to form a liquid-repellent layer on the surfaces of the film and on the inner peripheral surfaces of the through holes, thus obtaining a waterproof sound-permeable membrane. The liquid-repellent treatment solution was prepared by diluting a liquid-repellent agent (X-70-029C, manufactured by Shin-Etsu Chemical Co., Ltd.) with a diluent (FS thinner, manufactured by Shin-Etsu Chemical Co., Ltd.) to a concentration of 0.7 wt %. 
     The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. 
     Example 2 
     A waterproof sound-permeable membrane was obtained in the same manner as in Example 1, except that the duration of immersion of the prepared PET film in the etchant was changed to 20 minutes. Changing the duration of immersion in the etchant causes a change in the diameter of through holes of the resulting resin film. 
     The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. 
     Example 3 
     A waterproof sound-permeable membrane was obtained in the same manner as in Example 1, except that the prepared PET film was not etched. 
     The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. 
     Example 4 
     There was prepared a commercially-available non-porous PET film (Track etched membrane manufactured by it4ip S.A. and having a thickness of 45 μm) having through holes formed to extend through the thickness of the film. The diameter of the through holes of the film was 3.0 μm, and the hole density of the film was 2.0×10 6  holes/cm 2 . 
     Next, the PET film prepared was immersed in an etchant (an aqueous solution of 20 mass % potassium hydroxide) maintained at 80° C. for 30 minutes. After the etching, the film was taken out of the etchant, immersed and washed in RO water (water filtered through a reverse osmosis membrane), and then dried by a drying oven set at 50° C. Thus, a non-porous resin film having through holes formed to extend through the thickness of the film was obtained. The diameter of the through holes of the resin film obtained was 5.9 μm, and the area of a cross-section of each through hole taken perpendicular to the direction of the central axis of the hole was constant in the thickness direction of the film. The hole density remained unchanged before and after the etching. 
     Next, the dried resin film was dyed with a disperse dye. The dyed film was black when viewed with the naked eye. 
     Next, the black film thus fabricated was immersed in a liquid-repellent treatment solution identical to that used in Example 1 for 3 seconds, and then left to dry at ordinary temperature for 30 minutes to form a liquid-repellent layer on the surfaces of the film and on the inner peripheral surfaces of the through holes, thus obtaining a waterproof sound-permeable membrane. 
     The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. 
     Comparative Example 1 
     A waterproof sound-permeable membrane was obtained in the same manner as in Example 1, except that the duration of immersion of the prepared PET film in the etchant was changed to 40 minutes. The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. 
     Comparative Example 2 
     A waterproof sound-permeable membrane was obtained in the same manner as in Example 4, except that the duration of immersion of the prepared PET film in the etchant was changed to 20 minutes. The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. 
     Comparative Example 3 
     A waterproof sound-permeable membrane was obtained in the same manner as in Example 4, except that the prepared PET film was neither etched nor dyed with the disperse dye. The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. 
     Comparative Example 4 
     A waterproof sound-permeable membrane was obtained in the same manner as in Example 2, except that the prepared PET film was a different commercially-available non-porous PET film (Track etched membrane manufactured by it4ip S.A. and having a thickness of 30 μm) having through holes formed to extend through the thickness of the film. The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. The diameter of the through holes of the prepared PET film was 1.0 μm, and the hole density of the film was 3.0×10 7  holes/cm 2 . 
     Comparative Example 5 
     A waterproof sound-permeable membrane was obtained in the same manner as in Comparative Example 4, except that the prepared PET film was not etched. The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. 
     Comparative Example 6 
     There was prepared a commercially-available mesh film (Smartmesh-P 180/460-27, manufactured by Nippon Tokushu Fabric Inc. and having a thickness of 43 μm). The diameter of the openings of the film was 40 μm. The mesh prepared was subjected to a liquid-repellent treatment identical to that in Example 1, and thus a waterproof sound-permeable membrane was obtained. The properties of the waterproof sound-permeable membrane thus obtained and the waterproof sound transmission structure formed using this membrane are shown in Table 1 below. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Comparative 
                 Example 
                 Example 
                 Example  
                 Example  
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 Example 
                 1 
                 2 
                 3 
                 4 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 5 
                 Example 6 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Whiteness W 
                 — 
                 60 
                 — 
                 — 
                 12 
                 41 
                 — 
                 — 
                 — 
                 — 
               
               
                 Water entry 
                 1.9 
                 2.4 
                 4.0 
                 6.5 
                 8.5 
                 13.8 
                 27.0 
                 40.0 
                 70.0 
                 6.0 
               
               
                 pressure 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (kPa) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Through hole 
                 15.0 
                 13.0 
                 12.0 
                 10.6 
                 5.9 
                 4.5 
                 3.0 
                 2.1 
                 1.0 
                 28.0 
               
               
                 diameter 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (μm) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Air  
                 195.3 
                 119.7 
                 92.4 
                 63.5 
                 11.8 
                 9.64 
                 2.90 
                 0.97 
                 0.77 
                 — 
               
               
                 permeability 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (cm 3 /cm 2  · sec)) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Insertion loss 
                 0.0 
                 0.2 
                 0.1 
                 0.2 
                 1.7 
                 4.6 
                 6.7 
                 10.4 
                 13.8 
                 17.8 
               
               
                 (dB at 5 kHz) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 IPX7 
                 5 
                 Poor 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                 testing/ 
                 10 
                 — 
                 Poor 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                 Volume 
                 40 
                 — 
                 — 
                 Poor 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                 of first 
                 80 
                 — 
                 — 
                 — 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Poor 
               
               
                 space 
                 200 
                 — 
                 — 
                 — 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 — 
               
               
                 (mm 3 ) 
                 300 
                 — 
                 — 
                 — 
                 Poor 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 — 
               
               
                   
                 500 
                 — 
                 — 
                 — 
                 — 
                 Poor 
                 Good 
                 Good 
                 Good 
                 Good 
                 — 
               
               
                   
                 1000 
                 — 
                 — 
                 — 
                 — 
                 — 
                 Good 
                 Good 
                 Good 
                 Good 
                 — 
               
               
                   
                 1500 
                 — 
                 — 
                 — 
                 — 
                 — 
                 Good 
                 Good 
                 Good 
                 Good 
                 — 
               
               
                   
                 4000 
                 — 
                 — 
                 — 
                 — 
                 — 
                 Good 
                 Good 
                 Good 
                 Good 
                 — 
               
               
                   
                 5000 
                 — 
                 — 
                 — 
                 — 
                 — 
                 Good 
                 Good 
                 Good 
                 Good 
                 — 
               
               
                   
               
               
                 * “—” means being unmeasured. 
               
               
                 “Good” means being compliant with IPX7. 
               
               
                 “Poor” means not being compliant with IPX7. 
               
               
                 * The through hole diameter shown for Comparative Example 6 is the diameter (μm) of the mesh openings. 
               
            
           
         
       
     
     As seen from Table 1, the waterproof sound transmission structures of Examples 1 to 4, each of which employed a waterproof sound-permeable membrane including a non-porous resin film having through holes extending through the thickness of the resin film and having a diameter of 5.0 μm or more and 13.0 μm or less, were compliant with IPX7 when the volume of the first space (space  308 ) was equal to or smaller than a given value, despite the fact that the inherent water entry pressure of the waterproof sound-permeable membrane used in each structure was lower than 9.8 kPa corresponding to a water pressure acting on the membrane during the IPX7 testing. The volume of the second space was 5 mm 3  or less in Example 1 where the inherent water entry pressure of the waterproof sound-permeable membrane was 2.4 kPa, 10 mm 3  or less in Example 2 where the inherent water entry pressure was 4.0 kPa, 200 mm 3  or less in Example 3 where the inherent water entry pressure was 6.5 kPa, and 300 mm 3  or less in Example 4 where the inherent water entry pressure was 8.5 kPa. 
     By contrast, the waterproof sound transmission structure of Comparative Example 1, which employed a waterproof sound-permeable membrane including a non-porous resin film having through holes extending through the thickness of the resin film and having a diameter of 15.0 μm, failed to be compliant with IPX7 even when the volume of the first space was 5 mm 3 . The waterproof sound transmission structures of Comparative Examples 2 to 5, each of which employed a waterproof sound-permeable membrane including a non-porous resin film having through holes extending through the thickness of the resin film and having a diameter of less than 5.0 μm, was compliant with IPX7 regardless of the volume of the first space; however, it should be noted that the inherent water entry pressure of these waterproof sound-permeable membranes was more than 9.8 kPa. The waterproof sound-permeable membranes of Comparative Examples 2 to 5 caused a much greater insertion loss than those of Examples 1 to 4. In Comparative Example 6 where a waterproof sound-permeable membrane having a mesh structure was used, the insertion loss was considerably great, namely 17.8 dB. 
     The present invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this specification are to be considered in all respects as illustrative and not limiting. The scope of the present invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein. 
     INDUSTRIAL APPLICABILITY 
     The waterproof sound transmission structure of the present invention is applicable to any product required to prevent penetration of water while maintaining sound permeability.