A electroacoustic transducer includes a housing, a piezoelectric element, a partition wall, a first tube, and a second tube. The piezoelectric element is disposed in the housing and includes a porous film and a pair of electrodes that sandwich the porous film therebetween. The partition wall divides an inner space of the housing into a first space closer to one of the pair of electrodes, and a second space closer to another of the pair of electrodes. The first tube establishes communication between a sound wave emission opening that is open to an outer space of the housing and the first space. The second tube establishes communication between the sound wave emission opening and the second space.

BACKGROUND

The following disclosure relates to an electroacoustic transducer such as a speaker, an earphone, and headphones.

An electroacoustic transducer includes a diaphragm that vibrates in accordance with an externally applied sound signal (an electric signal representing a sound waveform) to output a sound wave based on the sound signal. For instance, there is an earphone that includes an electromagnetic tweeter including a piezoelectric element as the diaphragm and a dynamic woofer. In the earphone, sounds output from the tweeter and sounds output from the woofer are output from the same sound emitting portion.

SUMMARY

There has been proposed using, as the diaphragm for the speaker, a piezoelectric element that includes a porous film and a pair of electrodes sandwiching the porous film. In such a piezoelectric element, the porous film expands or contracts in its thickness direction in accordance with a voltage applied between the electrodes, so that the piezoelectric element vibrates. In the speaker including the piezoelectric element, sound waves are emitted from both surfaces of the diaphragm depending on how the diaphragm is disposed. The conventional speakers, however, utilize only the sound wave emitted from one surface of the diaphragm.

Accordingly, one aspect of the present disclosure is directed to a technique of enabling effective utilization of sound waves respectively emitted from opposite surfaces of a diaphragm in an electroacoustic transducer in which a piezoelectric element is used as the diaphragm.

In one aspect of the present disclosure, an electroacoustic transducer includes: a housing; a piezoelectric element disposed in the housing and including a porous film and a pair of electrodes sandwiching the porous film therebetween; a partition wall dividing an inner space of the housing into a first space closer to one of the pair of electrodes and a second space closer to the other of the pair of electrodes; a first tube that establishes communication between a sound wave emission opening that is open to an outer space of the housing and the first space; and a second tube that establishes communication between the sound wave emission opening and the second space.

Other objects, features, advantages, as well as the technical and industrial significance of the present disclosure will be better understood by reading the following detailed description of embodiments, when considered in connection with the accompanying drawings, in which:

DETAILED DESCRIPTION

Referring to the drawings, there will be hereinafter described embodiments of the present disclosure.FIGS. 1-3are cross-sectional views of an earphone1A, as one example of an electroacoustic transducer, according to an embodiment of the present disclosure.FIG. 2is a cross-sectional view taken along a plane along line Z-Z′ inFIG. 1.FIG. 3is a cross-sectional view taken along a plane along line Y-Y′ inFIG. 1. As illustrated inFIGS. 1-3, the earphone1A includes a housing10, a diaphragm20, a partition wall30, and a tube50.

The housing10is a hollow cylindrical member formed of resin. A through-hole, to which the tube50is mounted, is formed in one of two circular end faces of the housing10. The tube50connects the housing10and an earpiece to be inserted into an earhole of a user. Like the housing10, the tube50is formed of resin. InFIG. 1and other drawings, illustration of the earpiece is omitted.

The diaphragm20is a piezoelectric element that vibrates in accordance with an externally applied sound signal. As illustrated inFIGS. 1 and 3, the diaphragm20is shaped like a flat disk having a diameter smaller than an inside diameter of the housing10. As illustrated inFIG. 1, the diaphragm20includes a porous film22and a pair of electrodes24-1,24-2sandwiching the porous film22therebetween. In the following description, a direction from one of the two electrodes24-1,24-2toward the other of the two electrodes24-1,24-2will be referred to as a thickness direction of the porous film22. InFIGS. 1-3, a Z direction corresponds to the thickness direction of the porous film22. The diaphragm20may have any planar shape, namely, may have any shape viewed in the Z direction, other than a circle. That is, the planar shape of the diaphragm20may be an ellipse or a polygon such as a quadrangle or a pentagon.

The porous film22is formed of a piezoelectric material. One of the electrodes24-1,24-2is grounded. To the other of the electrodes24-1,24-2, a voltage based on the sound signal is applied. The porous film22expands or contracts in the thickness direction based on the voltage applied between the electrodes24-1,24-2. Specifically, based on the voltage applied between the electrodes24-1,24-2, a portion of the porous film22sandwiched between the electrodes24-1,24-2expands in mutually opposite directions from the center of the porous film22in the thickness direction toward the respective electrodes24-1,24-2or contracts in mutually opposite directions from the respective electrodes24-1,24-2toward the center in the thickness direction. With this configuration, the diaphragm20vibrates, and sound waves are emitted to spaces located outside the respective electrodes24-1,24-2.

The piezoelectric material of which the porous film22is formed has piezoelectric characteristics given as follows. For instance, a multiplicity of flat pores are formed in polytetrafluoroethylene (PTFE), polypropylene (PP), polyethylene(PE), polyethylene terephthalate (PET) or the like, and opposed faces of the flat pores are polarized and electrified by a corona discharge or the like. A lower limit of an average thickness of the porous film22is preferably 10 μm and more preferably 50 μm. An upper limit of the average thickness of the porous film22is preferably 500 μm and more preferably 200 μm. When the average thickness of the porous film22is less than the lower limit, the strength of the porous film22may be insufficient. When the average thickness of the porous film22is greater than the upper limit, the deformation amount of the porous film22may decrease, resulting in an insufficient output sound pressure.

The electrodes24-1,24-2are laminated respectively on opposite surfaces of the porous film22. When it is not necessary to distinguish the electrode24-1and the electrode24-2from each other, each of them will be referred to as “electrode24”. The electrode24may be formed of any conductive material examples of which include: metals such as aluminum, copper, and nickel: and a carbon. An average thickness of the electrode24, which may vary depending on a laminating process, is not smaller than 0.1 μm and not greater than 30 μm, for instance. When the average thickness of the electrode24is less than the lower limit, the strength of the electrode24may be insufficient. When the average thickness of the electrode24is greater than the upper limit, the vibration of the porous film22may be inhibited. The electrodes24may be laminated on the porous film22by any suitable method such as vapor deposition of a metal, printing with a conductive carbon ink, and application and drying of a silver paste.

As illustrated inFIG. 1, the partition wall30includes a first member32, a second member34, and a third member36. As illustrated inFIG. 2, the first member32is shaped like a flat disk whose diameter is equal to the inside diameter of the housing10. As illustrated inFIG. 3, the second member34is shaped like a rectangular plate whose length in an X direction is equal to the inside diameter of the housing10. The third member36is shaped like a plate having a planar shape illustrated inFIG. 3. Like the housing10, the first member32, the second member34, and the third member36are formed of resin.

As illustrated inFIG. 2, the first member32has two elliptical cutouts320formed at its diametrically opposite ends. As illustrated inFIGS. 1-3, the second member34is bonded by an adhesive or the like to one of two generally circular surfaces of the first member32at a middle position thereof in a direction from one of the two cutouts320toward the other of the two cutouts320, i.e., in the Z direction, such that the second member34extends so as to be orthogonal to the Z direction. The third member36is bonded by an adhesive or the like to the other of the two generally circular surfaces of the first member32at a middle position thereof in the Z direction, such that the third member36extends so as to be orthogonal to the Z direction. In the present embodiment, the partition wall30is constituted by the three separate members, i.e., the first member32, the second member34, and the third member36. The partition wall30may be formed by integral molding of all of or a part of these three members.

The second member34has a through-hole to which the diaphragm20is mounted. As illustrated inFIGS. 1 and 3, the diaphragm20is mounted to the through-hole of the second member34via a ring-like elastic member40. The diaphragm20is mounted to the through-hole of the second member34via the elastic member40for preventing the vibration of the diaphragm20in the thickness direction from being inhibited. As illustrated inFIGS. 1 and 3, the diaphragm20is disposed in the housing10in a state in which the diaphragm20is attached to the partition wall30, more strictly, in a state in which the diaphragm20is attached to the second member34of the partition wall30. As illustrated inFIG. 1, the diaphragm20is disposed such that the diaphragm20and the second member34of the partition wall30are arranged in a row in a Y direction. Thus, it is noted that the diaphragm20is disposed on the same plane as the second member34of the partition wall30.

An inner space of the housing10(a space of the housing10closer to the diaphragm20) is divided into four spaces100-1,100-2,100-3,100-4by the partition wall30to which the diaphragm20is attached. The space100-2and the space100-4are in communication with each other through the one of the two cutouts320. In the following description, a space provided by the spaces100-1,100-3that are in communication with each other through the other of the two cutouts320will be referred to as a first space110-1, and a space provided by the spaces100-2,100-4that are in communication with each other through the one of the two cutouts320will be referred to as a second space110-2. In the present embodiment, the first space110-1and the second space110-2are substantially identical in shape and volume. That is, as illustrated inFIG. 1, the partition wall30divides the inner space of the housing10into the first space110-1closer to one of the two electrodes of the diaphragm20, i.e., the electrode24-1, and the second space110-2closer to the other of the two electrodes, i.e., the electrode24-2. As illustrated inFIG. 1, the diaphragm20is attached to the second member34of the partition wall30via the elastic member40. Accordingly, the diaphragm20divides, as a part of the partition wall30, the inner space of the housing10into the first space110-1and the second space110-2.

When one of opposite surfaces of the diaphragm20that is located on a side of the electrode24-1is referred to as a first surface20-1and the other of the opposite surfaces of the diaphragm20that is located on a side of the electrode24-2is referred to as a second surface20-2as illustrated inFIG. 1, the first surface20-1is exposed to the first space110-1without being exposed to the second space110-2, and the second surface20-2is exposed to the second space110-2without being exposed to the first space110-1.

As illustrated inFIG. 1, the tube50is divided, by the third member36of the partition wall30, into two tubes, i.e., a first tube50-1and a second tube50-2, that have substantially the same tube length and substantially the same cross-sectional area. The first tube50-1establishes communication between a sound wave emission opening60that is open to an outer space of the housing10and the first space110-1. The second tube50-2establishes communication between the sound wave emission opening60and the second space110-2.

In the earphone1A of the present embodiment, one of the two electrodes24-1,24-2is grounded. When a voltage based on the sound signal is applied to the other of the two electrodes24-1,24-2, the diaphragm20vibrates and sound waves in the same phase based on the sound signal are emitted respectively from the first surface20-1located on the side of the electrode24-1and the second surface20-2located on the side of the electrode24-2. The sound wave emitted from the first surface20-1of the diaphragm20located on the side of the electrode24-1is emitted through the sound wave emission opening60to the outer space of the housing10via the first space110-1and the first tube50-1. The sound wave emitted from the second surface20-2of the diaphragm20located on the side of the electrode24-2is emitted through the sound wave emission opening60to the outer space of the housing10via the second space110-2and the second tube50-2.

The sound waves respectively emitted from the first surface20-1of the diaphragm20located on the side of the electrode24-1and the second surface20-2of the diaphragm20located on the side of the electrode24-2are in the same phase, and acoustic spaces to which the respective sound waves propagate have substantially the same shape. Thus, frequency characteristics of sounds that are emitted from one of the opposite surfaces of the diaphragm20to reach the ear of the user are identical to frequency characteristics of sounds that are emitted from the other of the opposite surfaces of the diaphragm20to reach the ear of the user. For instance, if the frequency characteristics of the former are flat frequency characteristics not including peaks and dips, the frequency characteristics of the latter are also flat. In the earphone1A of the present embodiment, the sounds emitted from both surfaces of the diaphragm20are superposed on one another at the sound wave emission opening60, so that the earphone1A of the present embodiment can obtain characteristics in which the output (sound volume) is doubled, as compared with conventional earphones that utilize only sounds emitted from its one surface.

As explained above, the earphone1A of the present embodiment effectively utilize the sound waves respectively emitted from both surfaces of the diaphragm20so as to attain doubled output, as compared with the conventional earphones that utilize only the sounds emitted from its one surface.

FIGS. 4 and 5are cross-sectional views respectively illustrating an earphone1B and an earphone1C according to an embodiment of the present disclosure. The same reference signs as used inFIG. 1are used to identify the corresponding constituent elements inFIGS. 4 and 5. In each of the earphones1B,1C of the present embodiment, two acoustic spaces, to which the sound waves respectively emitted from one and the other of the opposite surfaces of the diaphragm20propagate, are different in shape. The earphone1B of the present embodiment differs from the earphone1A of the previous embodiment in this aspect.

In the earphone1B illustrated inFIG. 4, the third member36is disposed so as to be shifted in the Z direction such that the cross-sectional area of the second tube50-2is smaller than the cross-sectional area of the first tube50-1. In the earphone1C illustrated inFIG. 5, the cross-sectional area of the first tube50-1and the cross-sectional area of the second tube50-2are equal to each other. In the earphone1C, however, the second member34is disposed so as to be shifted in the Z direction such that the volume of the space100-1is smaller than the volume of the space100-2, in other words, such that the volume of the first space110-1is smaller than the volume of the second space110-2. The two acoustic spaces, to which the sound waves respectively emitted from one and the other of the opposite surfaces of the diaphragm20propagate, have mutually different shapes for the following reasons.

Some adjustment such as emphasis of high- and low-frequency ranges is often needed in the earphone depending on the sound signal based on which sounds are to be reproduced, tastes or preferences of the user, etc. In the configuration illustrated inFIG. 4, reflection of sounds in the high-frequency range is small in the first tube50-1whose cross-sectional area is enlarged, thus enabling emission of sounds in which characteristics of the high-frequency range are emphasized. In the second tube50-2whose cross-sectional area is reduced, on the other hand, reflection of sounds in the high-frequency range is strong, and sounds in the low-frequency range are relatively allowed to pass. As a result, sounds in the mid-frequency range are relatively lowered at the sound wave emission opening60of the earphone1B, as compared with the earphone1A of the previous embodiment, thus achieving characteristics in which the low-frequency range and the high-frequency range are emphasized. It is noted that the cross-sectional area of one of the first tube50-1and the second tube50-2may remain the same as the cross-sectional area thereof in the previous embodiment while the cross-sectional area of the other of the first tube50-1and the second tube50-2may be changed, whereby only the low-frequency range or only the high-frequency range may be emphasized.

In the earphone1B illustrated inFIG. 4, the high-frequency range and the low-frequency range are emphasized by adjusting the cross-sectional area of the first tube50-1and the cross-sectional area of the second tube50-2. In the earphone1C illustrated inFIG. 5, the volume of the first space110-1and the volume of the second space110-2are adjusted to adjust the sound quality similarly. The reasons are as follows.

In the earphone1A of the previous embodiment, there is generated Helmholtz resonance (hereinafter referred to as “first Helmholtz resonance”) in which the first space110-1serves as a cavity and the first tube50-1serves as a neck, and there is generated Helmholtz resonance (hereinafter referred to as “second Helmholtz resonance”) in which the second space110-2serves as a cavity and the second tube50-2serves as a neck. As described above, in the earphone1A of the previous embodiment, the volume of the first space110-1and the volume of the second space110-2are substantially equal to each other, and the cross-sectional area of the first tube50-1and the cross-sectional area of the second tube50-2are substantially equal to each other. Thus, the resonance frequency of the first Helmholtz resonance and the resonance frequency of the second Helmholtz resonance in the earphone1A of the previous embodiment are substantially equal to each other. When the volume of each of the first space110-1and the second space110-2is represented as V and the cross-sectional area of each of the first tube50-1and the second tube50-2is represented as S, the resonance frequency f0of the first Helmholtz resonance and the second Helmholtz resonance is represented by the following expression (1). In the expression (1), l represents a length of the neck, c represents a sound speed, and δ represents an open end correction value. When the diameter of the opening of the neck is d, δ is approximately equal to 0.8×d, i.e., δ≅0.8×d.

Also in the earphone1C ofFIG. 5, the first Helmholtz resonance and the second Helmholtz resonance are generated. In the earphone1C, the position at which the second member34is disposed is shifted upward in the Z direction with respect to the middle position of the first member32in the Z direction, so that the volume of the first space110-1is smaller than that of the second space110-2. As a result, the volume of the first space110-1in the earphone1C ofFIG. 5is smaller than the volume of the first space110-1in the earphone1A ofFIG. 1. Thus, the resonance frequency of the first Helmholtz resonance in the earphone1C is shifted to a higher frequency side than the resonance frequency f0in the previous embodiment. In the earphone1C ofFIG. 5, the volume of the second space110-2is larger than the volume of the second space110-2in the earphone1A. Thus, the resonance frequency of the second Helmholtz resonance in the earphone1C is shifted to a lower frequency side than the resonance frequency f0in the previous embodiment. Like the earphone1B, the earphone1C ofFIG. 5also achieves the characteristics in which the low-frequency range and the high-frequency range are emphasized.

As explained above, the present embodiment enables the sound-quality adjustment in specific frequency ranges while effectively utilizing the sound waves emitted from both surfaces of the diaphragm20.

In addition, the earphones according to the present embodiment enjoy constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies. Conventional earphones sometimes include driver units of different types provided for different frequency ranges. In this case, the vibration characteristics unique to the respective driver units are different among the driver units, causing unnaturalness in the crossover frequency range. For instance, in a case where the driver unit for the low-frequency range and the driver unit for the high-frequency range are different in material, sound reverberation in the low-frequency range and sound reverberation in the high-frequency range may not match with each other. In contrast, the earphones according to the present embodiment do not include driver units of different types used for different frequency ranges, thus achieving constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies. Further, because the earphones according to the present embodiment do not include driver units of different types used for different frequency ranges, resulting in cost and size reductions.

FIGS. 6 and 7are cross-sectional views respectively illustrating an earphone1D and an earphone1E according to an embodiment of the present disclosure. The same reference signs as used inFIG. 1are used to identify the corresponding constituent elements inFIGS. 6 and 7. As apparent from a comparison betweenFIG. 1andFIG. 6, the earphone1D illustrated inFIG. 6differs from the earphone1A of the previous embodiment in that a sound absorber70formed of a nonwoven fabric or the like is packed in the first tube50-1. Further, as apparent from a comparison betweenFIG. 7andFIG. 5, the earphone1E illustrated inFIG. 7differs from the earphone1C of the previous embodiment in that i) the cross-sectional area of the second tube50-2is smaller than the cross-sectional area of the first tube50-1and ii) the sound absorber70is packed in the second tube50-2.

Packing the sound absorber in the tube50is equivalent to reducing the cross-sectional area of the tube50. According to the present embodiment, the fine adjustment of the sound-quality in specific frequency ranges can be easily performed by packing the sound absorber in any one of the first tube50-1and the second tube50-2. Also in the present embodiment, the sound waves emitted from both surfaces of the diaphragm20can be effectively utilized as in the previous embodiment. Further, the earphones of the present embodiment do not include driver units of different types used for different frequency ranges, thus achieving constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies and resulting in cost and size reductions, as in the previous embodiment. In the present embodiment, the sound absorber70is packed in one of the first tube50-1and the second tube50-2. The sound absorber70may be packed in both the first tube50-1and the second tube50-2.

FIGS. 8-11are cross-sectional views respectively illustrating an earphone1F, an earphone1G, an earphone1H, and an earphone1I according to an embodiment of the present disclosure. The earphone1F illustrated inFIG. 8differs from the earphone1A of the previous embodiment in the following three aspects. The first different aspect is that the earphone1F includes a partition wall30′ in place of the partition wall30. As apparent from a comparison betweenFIG. 8andFIG. 5, the partition wall30′ differs from the partition wall30in that i) the partition wall30′ does not have the through-hole to which the diaphragm20is mounted and ii) the partition wall30′ has a generally L-shaped cross section. In the earphone1F of the present embodiment, the inner space of the housing10is divided by the partition wall30′ into the space100-1and the space100-2whose volume is smaller than that of the space100-1.

The second different aspect is that the diaphragm20is disposed such that one surface of the diaphragm20, namely, one surface thereof located on the side of the electrode24-1, faces the space100-1and the space100-2. An elastic member40′ inFIG. 8is a member filling a gap between the diaphragm20and one end of the partition wall30′ without inhibiting the vibration of the diaphragm20in the thickness direction. The third different aspect is that the tube50is not divided into the first tube50-1and the second tube50-2. The tube50establishes communication between the space100-1and the sound wave emission opening60and communication between the space100-2and the sound wave emission opening60.

In the earphone1F constructed as illustrated inFIG. 8, reflection of sounds in the high-frequency range is small in the space100-1, thus enabling emission of sounds in which characteristics of the high-frequency range are emphasized. In the space100-2, on the other hand, reflection of sounds in the high-frequency range is strong, and sounds in the low-frequency range are relatively allowed to pass. As a result, sounds in the mid-frequency range are relatively lowered at the sound wave emission opening60at which sounds in the low-frequency range and sounds in the high-frequency range are superposed, as compared with the earphone1A of the previous embodiment, thus achieving the characteristics in which the low-frequency range and the high-frequency range are emphasized.

Helmholtz resonance is generated also in the earphone1F of the present embodiment. In the earphone1F, the first Helmholtz resonance is generated in which the space100-1serves as a cavity and the tube50serves as a neck, and the second Helmholtz resonance is generated in which the space100-2serves as a cavity and the tube50serves as a neck. As described above, in the earphone1F, the volume of the space100-1is larger than the volume of the space100-2, and the resonance frequency of the first Helmholtz resonance is lower than the resonance frequency of the second Helmholtz resonance. Thus, like the earphone1C of the previous embodiment, the earphone1F of the present embodiment enables the sound-quality adjustment in specific frequency ranges. In addition, the earphone1F of the present embodiment does not include driver units of different types used for different frequency ranges, thus achieving constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies and resulting in cost and size reductions.

The earphone1G illustrated inFIG. 9differs from the earphone1F in that the diaphragm20is disposed in the housing10so as to be shifted in the Z direction, such that a region of the diaphragm20facing the space100-1is larger than a region thereof facing the space100-2. Like the earphone1F, the earphone1G ofFIG. 9enables the sound-quality adjustment in specific frequency ranges, achieves constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies, and enjoys cost and size reductions.

The earphone1H illustrated inFIG. 10differs from the earphone1F in that the space100-2is defined by a partition wall30″ shaped like a plate and the sound absorber70. The earphone1I illustrated inFIG. 11differs from the earphone1F in that the space100-2is defined by the partition wall30′ and the sound absorber70. The earphones1H,1I also enable the sound-quality adjustment in specific frequency ranges, achieve constant acoustic characteristics over a wide frequency range from low frequencies to high frequencies, and enjoy cost and size reductions.

While the embodiments have been described above, the embodiments may be modified as follows.

(1) In the embodiments illustrated above, the present disclosure is applied to the earphones. The electroacoustic transducer to which the present disclosure is applicable is not limited to the earphones but may be headphone speakers.

(2) The diaphragm in the previous embodiment is not limited to the piezoelectric element that includes the porous film formed of the piezoelectric material described above. The piezoelectric element may be a piezoelectric element in which lead zirconate titanate (PZT) or the like is used as the piezoelectric material, namely, a piezoelectric element capable of outputting from only one surface thereof. The diaphragm may be driven by a voice coil.

(3) In the previous embodiment, the inner space of the housing is divided into two spaces by one partition wall. The inner space of the housing may be divided into three or more spaces by two or more partition walls. That is, the electroacoustic transducer includes the housing, one or a plurality of partition walls that divide the inner space of the housing into a plurality of spaces such that at least one of the plurality of spaces has a volume different from a volume of at least one of others of the plurality of spaces except the at least one of the plurality of spaces, the diaphragm disposed in the housing such that one surface thereof faces the plurality of spaces, and a tube that establishes communication between the sound wave emission opening that is open to the outer space of the housing and the plurality of spaces. The sound quality can be adjusted in at least two different frequency ranges if at least one of the plurality of spaces has a volume different from those of other spaces.

In an earphone1J illustrated inFIG. 12, the space in the housing10is divided, by partition walls30′-1,30′-2, into three spaces, i.e., the space100-1, the space100-2, and the space100-3having mutually different volumes. An elastic member40′-1inFIG. 12is a member filling a gap between the diaphragm20and one end of the partition wall30′-1without inhibiting the vibration of the diaphragm20in the thickness direction. An elastic member40′-2is a member filling a gap between the diaphragm20and one end of the partition wall30′-2without inhibiting the vibration of the diaphragm20in the thickness direction. In the earphone1J illustrated inFIG. 12, the sound quality can be adjusted in three different frequency ranges by dividing the inner space of the housing10into the three spaces having mutually different volumes.

The diaphragm whose one surface faces the plurality of spaces is not limited to one diaphragm. That is, the earphone may include a plurality of diaphragms, as illustrated inFIG. 13. Specifically, an earphone1K ofFIG. 13includes a diaphragm20-3as a diaphragm whose one surface faces the space100-1, a diaphragm20-4as a diaphragm whose one surface faces the space100-2, and a diaphragm20-5as a diaphragm whose one surface faces the space100-3. In each of the diaphragm20-3, the diaphragm20-4, and the diaphragm20-5, one of the two electrodes, which is provided on the other surface of the diaphragm attached to the inner wall surface of the housing10, is grounded, and a voltage based on the sound signal is applied to the other of the two electrodes. In this configuration, the diaphragm20-3, the diaphragm20-4, and the diaphragm20-5respectively emit sound waves in the same phase. Similarly, in the earphones1F-1I ofFIGS. 8-11, the diaphragm facing the space100-1and the diaphragm facing the space100-2may be separate diaphragms.

(4) The earphones in the illustrated embodiments may be configured such that a ratio among the volumes of the plurality of spaces each serving as the cavity in the Helmholtz resonator and/or a ratio among the cross-sectional areas of the plurality of tubes each serving as the neck in the Helmholtz resonator may be variable. The thus configured earphone enables the user to finely adjust the sound quality in specific frequency ranges depending on the user's preferences or tastes.

In the earphone1A of the previous embodiment, for instance, by packing the sound absorber in one of the first tube50-1and the second tube50-2from an end portion of the tube50closer to the sound wave emission opening60, the cross-sectional area of the one of the first tube50-1and the second tube50-2can be adjusted. For instance, the earphone1F of the previous embodiment may be modified as illustrated inFIG. 14, such that the partition wall30′ is constituted by a plate-like first member32′ and a second member34′ provided so as to be perpendicular to the first member32′ and slidable in the Y direction inFIG. 14and such that one end of a rod-like member90protruding outside the housing10through a through-hole80formed in the housing10is connected to the second member34′ and a knob92is attached to the other end of the rod-like member90. In this configuration, the volume of the space100-2can be increased by pushing the knob92in a Y′ direction or decreased by pulling the knob92in the Y direction. Likewise, in the earphone1A of the previous embodiment, the volume of any one of the first space110-1and the second space110-2may be made variable. The second member34inFIG. 1may be configured to be movable in the Z direction by providing the rod-like member90and the knob92inFIG. 14, thus enabling a ratio between the volume of the first space110-1and the volume of the second space110-2to be variable in the configuration ofFIG. 1. Further, the third member36inFIG. 1may be configured to be movable in the Z direction by providing the rod-like member90and the knob92, thus enabling a ratio between the cross-sectional area of the first tube50-1and the cross-sectional area of the second tube50-2to be variable in the configuration ofFIG. 1. Further, the rod-like member90and the knob92may be provided for each of the second member34and the third member36, thus enabling both i) the ratio between the volume of the first space110-1and the volume of the second space110-2and ii) the ratio between the cross-sectional area of the first tube and the cross-sectional area of the second tube to be variable in the earphone1A ofFIG. 1.