Typically, a speaker is used to output an acoustic signal as a sound so that the listener can listen to the acoustic signal. A speaker has a function of converting an acoustic signal (electrical signal) into air vibration (physical vibration). A speaker called cone type is known as a commonly used speaker.
FIG. 5 is a side sectional view showing an example of a cone speaker. A cone speaker 100 includes a first yoke 101, a ring magnet 102, and a second yoke 103. The first yoke 101 includes a bottom 101a and a cylinder 101b. The bottom 101a is in the shape of a disc. The cylinder 101b is formed integrally with the central portion of the bottom 101a. The ring magnet 102 and second yoke 103 have annular shapes. The ring magnet 102 has, in the center thereof, an opening 102a having a larger diameter than the cylinder 101b of the first yoke 101. The second yoke 103 has, in the center thereof, an opening 103a having a diameter smaller than the opening 102a of the ring magnet 102 and larger than the cylinder 101b of the first yoke 101.
The cone speaker 100 also includes a frame 104, a voice coil bobbin 106, a cone 108, and a dome 109. The frame 104 is mounted on the second yoke 103. The voice coil bobbin 106 has a cylindrical shape. The voice coil bobbin 106 is mounted to the frame 104 through a damper 105. The cone 108 is mounted to one end, 106a, of the voice coil bobbin 106. The cone 108 is also mounted to the frame 104 through an edge 107. The dome 109 is disposed so as to cover an opening at the end 106a of the voice coil bobbin 106.
As shown in FIG. 5, the ring magnet 102 is stacked on the first yoke 101. The second yoke 103 is stacked on the ring magnet 102. The first yoke 101, ring magnet 102, and second yoke 103 are coaxially arranged. The ring magnet 102 has the opening 102a in the center thereof. The second yoke 103 also has the opening 103a in the center thereof. The first yoke 101 has the cylinder 101b in the center thereof. The cylinder 101b is disposed so as to penetrate through the opening 102a of the ring magnet 102 and the opening 103a of the second yoke 103. An edge 104a of the frame 104 is mounted on an open surface (a surface opposite to a surface in contact with the ring magnet 102) 103b of the second yoke 103. The frame 104 is approximately in the shape of a mortar and is increased in diameter as it extends from the portion thereof (the edge 104a) mounted on the open surface 103b of the second yoke 103 upward in FIG. 5. The frame 104 also has a large opening 104b at another edge thereof.
The frame 104 has, in the opening 104b, an edge 107 for mounting the cone 108 on the frame 104 without significantly reducing the vibration of the cone 108. The cone 108 is supported by the frame 104 through the edge 107 mounted to the outer edge of the cone 108.
The cone 108 consists of bowl-shaped cone paper formed of pulp or the like. Due to the vibration of the cone paper, an acoustic signal can be converted into air vibration. The cone 108 has the cylindrical voice coil bobbin 106 mounted on the bowl-shaped bottom of the cone 108 in such a manner that the voice coil bobbin 106 contains the cylinder 101b of the first yoke 101. Note that a gap is provided between the inner surface of the voice coil bobbin 106 and the outer surface of the cylinder 101b. Gaps are also provided between the outer surface of the voice coil bobbin 106 and the inner surface of the second yoke 103 (the inner circumferential surface of the opening 103a) and between the outer surface of the voice coil bobbin 106 and the inner surface of the ring magnet 102 (the inner circumferential surface of the opening 102a).
The outer surface of the other edge, 106b, of the voice coil bobbin 106 facing the inner surface of the second yoke 103 is provided with a voice coil 110 for passing an acoustic signal. The voice coil bobbin 106 also has the damper 105 mounted thereto. The voice coil bobbin 106 is mounted to the frame 104 through the damper 105. Thus, vibration generated by the voice coil 110 can be transmitted to the cone 108 without significantly reducing the vibration of the voice coil bobbin 106.
The dome 109 is disposed in the opening at the end 106a of the voice coil bobbin 106. The dome 109 consists of cone paper, silk, or the like. Due to the vibration of the dome 109, an acoustic signal can be converted into air vibration.
An acoustic signal is inputted to the voice coil 110 of the cone speaker 100 thus configured. Due to the input of the acoustic signal, the voice coil bobbin 106 vibrates so as to vertically move toward the opening of the frame 104 (so as to reciprocate in the direction in which the voice coil bobbin 106 extends). The vibration of the voice coil bobbin 106 is transmitted to the cone 108 and dome 109 so as to vibrate the cone 108 and dome 109. Due to the vibration of the cone 108 and dome 109, the acoustic signal is converted into air vibration, and a sound is outputted forward of the cone speaker 100.
On the other hand, a speaker called exciter is known (for example, see Patent Literature 1). An exciter outputs a sound by vibrating a flat diaphragm or the like rather than vibrating the cone 108 and dome 109.
FIG. 6 is a side sectional view showing an example of an exciter 200 installed on a flat plate 220. The exciter 200 includes a first yoke 201, a disc-shaped magnet 202, and a second yoke 203. The first yoke 201 is in the shape of a cylinder having a bottom and serves as the bottom of the exciter 200. The disc-shaped magnet 202 is stacked in the center inside the first yoke 201. The second yoke 203 is in the shape of a disc and is stacked on the disc-shaped magnet 202. The diameters of the disc-shaped magnet 202 and second yoke 203 are smaller than the inner diameter of the cylindrical first yoke 201. Gaps are formed between the inner surface 201a of the first yoke 201, and the outer surfaces of the disc-shaped magnet 202 and second yoke 203 facing the inner surface 201a. 
A cylindrical frame 205 having a ceiling is disposed above the first yoke 201 in such a manner that a ceiling surface 205a covers an open end of the cylindrical first yoke 201. An open lower end 205b of the cylindrical frame 205 is provided with a damper 207 for mounting the first yoke 201 to the frame 205. As shown in FIG. 7, the damper 207 consists of multiple legs 207c approximately in the shape of S. One end, 207a, of each leg 207c approximately in the shape of S is connected to the lower end 205b of the cylindrical frame 205. The other end, 207b, of each leg 207c is connected to the side of the first yoke 201. The damper 207 is formed of an elastic plate member. The use of the legs 207c approximately in the shape of S allows the vibration of the first yoke 201, disc-shaped magnet 202, and second yoke 203 to be transmitted to the frame 205 while preventing a reduction in the vibration as much as possible.
One end, 210a, of a cylindrical voice coil bobbin 210 is fixed to a portion of the ceiling surface 205a of the frame 205. The other end, 210b, of the voice coil bobbin 210 extends into the gaps between the inner surface of the first yoke 201 and the outer surfaces of the disc-shaped magnet 202 and second yoke 203. The outer circumferential surface of the extending other end, 210b, facing the inner surface 201a of the first yoke 201 is provided with a voice coil 211.
An acoustic signal is inputted to the voice coil 211 of the exciter 200 thus configured. Due to the input of the acoustic signal, the first yoke 201, disc-shaped magnet 202, and second yoke 203 vibrate. By transmitting the vibration of the first yoke 201 and the others to the frame 205 through the damper 207, it is possible to vibrate the flat plate 220 and the like and to output a sound.
The use of the exciter 200 allows the entire transmission medium in contact with the exciter 200, such as the flat plate 220, to emit a sound. Thus, it is possible to increase the spread of a sound and to improve sound quality. Further, by selecting the material or shape of the transmission medium for outputting a sound, it is possible to output deep bass, which is difficult to reproduce with only the cone speaker 100, without having to use a low-frequency speaker (woofer) or the like.