Abstract:
A loudspeaker is provided, which includes a frame, a voice coil bobbin, and a diaphragm connected to the frame and the voice coil bobbin, wherein the diaphragm is connected to the voice coil bobbin in such a manner that a cone angle between the diaphragm and the voice coil bobbin becomes small enough to obtain a preferable treble reproduction threshold frequency, the diaphragm ha a first connected portion connected to the voice coil bobbin and a second connected portion connected to the frame, and the diaphragm has an arch-shaped cross section so as to have a top portion formed outward from both the first connected portion and the second connected portion.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a loudspeaker in which a treble reproduction threshold frequency used in acoustic reproduction equipment is increased. 
     2. Description of the Related Art 
     In the past, as a method for increasing a treble reproduction threshold frequency in loudspeakers, a method described on pages 157-158 of “Loudspeaker System”, 1st Vol., T. Yamamoto (Radio Gijutsusha) has been known. More specifically, according to this method, it is effective. 
     1. to render a diaphragm system lighter, 
     2. to permit use of a diaphragm having high stiffness, and 
     3. to decrease a cone angle at a connected portion between a diaphragm and a voice coil bobbin. 
     However, a conventional method has the following problems. 
     First, if a diaphragm system is rendered light, the strength of a diaphragm is decreased. Therefore, nodal resonances of the diaphragm becomes likely to be generated, and a number of peak dips occur in sound pressure—frequency characteristics. Furthermore, resistance to input cannot be increased. 
     Furthermore, a material for a diaphragm having high stiffness is expensive. Generally, a material having high stiffness has a small internal loss, which causes a large peak dip in sound pressure—frequency characteristics. 
     Furthermore, if a cone angle at a connected portion between a diaphragm and a voice coil bobbin is decreased, a mild and large peak dip occurs in sound pressure—frequency characteristics due to a cavity effect of the diaphragm. 
     The cavity effect occurs when a cavity is formed due to a recess on a front surface of a diaphragm of a loudspeaker and this cavity generates resonance in a particular band, causing a peak dip in sound pressure—frequency characteristics. 
     SUMMARY OF THE INVENTION 
     A loudspeaker of the present invention includes a frame, a voice coil bobbin, and a diaphragm connected to the frame and the voice coil bobbin, wherein the diaphragm is connected to the voice coil bobbin in such a manner that a cone angle between the diaphragm and the voice coil bobbin becomes small enough to obtain a preferable treble reproduction threshold frequency, the diaphragm has a first connected portion connection to the voice coil bobbin and a second connected portion connected to the frame, and the diaphragm has an arch-shaped cross section so as to have a top portion formed outward from both the first connected portion and the second connected portion. 
     In one embodiment of the present invention, the diaphragm is connected to the voice coil bobbin in such a manner that the cone angle become substantially 45° or less. 
     In another embodiment of the present invention, the voice coil bobbin has a substantially cylindrical shape, and the first connected portion is connected to an end of the voice coil bobbin. 
     In another embodiment of the present invention, the first connected portion is connected to an outer diameter portion of the voice coil bobbin. 
     In another embodiment of the present invention, the loudspeaker further includes a connecting section for connecting the diaphragm to the frame and the voice coil bobbin, and the connecting section contains a rubber adhesive. 
     In another embodiment of the present invention, the first connected portion includes a tapered portion with a tapered shape, and the tapered portion is connected to an inner diameter portion of the voice coil bobbin. 
     In another embodiment of the present invention, the diaphragm is connected to the voice coil bobbin in such a manner that the tapered portion directly comes into contact with the inner diameter portion. 
     In another embodiment of the present invention, the frame seals a space on a rear side of the diaphragm, and the diaphragm has a air flow portion which allows air to flow therethrough. 
     In another embodiment of the present invention, the air flow portion is disposed in the vicinity of an outer periphery of the diaphragm. 
     In another embodiment of the present invention, the air flow portion is made of a member having a density lower than a member for forming a portion other than the air flow portion. 
     In another embodiment of the present invention, the air flow portion has a plurality of small holes for allowing air to flow therethrough. 
     A loudspeaker of the present invention has a structure in which a diaphragm has an arch-shaped cross-section with a top portion curved outward from inner and outer attachment portions, and a cone angle at a connected portion between the diaphragm and a voice coil bobbin is about 45° or less. Because of this structure, the cavity effect can be suppressed, and the strength of the diaphragm is improved. Furthermore, a treble reproduction threshold frequency is easily increased by using an inexpensive material while sound pressure—frequency characteristics are flattened. 
     According to an aspect of the present invention, while a cone angle at a connected portion between a diaphragm and a voice coil bobbin is decreased, the height of the diaphragm can be rendered smaller due to its arch-shaped cross section. This suppresses the cavity effect. Furthermore, because of the arch-shaped cross section, the strength of the diaphragm is improved, and nodal resonances become unlikely to be generated. 
     According to another aspect of the present invention, a diaphragm can be directly connected to a voice coil bobbin without any adhesive layer therebetween. 
     According to still another aspect of the present invention, air can flow through a diaphragm, and stiffness of the air in a space on a rear side of the diaphragm can be decreased. Since base resonance is proportional to a square root of stiffness, bass resonance can be decreased, i.e., bass characteristics can be flattened. 
     Thus, the invention described herein makes possible the advantage of providing an inexpensive loudspeaker in which flat sound pressure—frequency characteristics are obtained while a treble reproduction threshold frequency is increased. 
     This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a loudspeaker in Embodiment 1 of the present invention. 
     FIG. 2 is a graph showing sound pressure—frequency characteristics of the loudspeaker in Embodiment 1 of the present invention and a conventional loudspeaker. 
     FIG. 3 is a cross-sectional view of a loudspeaker in Embodiment 2 of the present invention. 
     FIGS. 4A and 4B show an enlarged view of a connected portion between a diaphragm and a voice coil bobbin of a loudspeaker, respectively. 
     FIG. 5 is a graph showing sound pressure—frequency characteristics of the loudspeakers in Embodiments 1 and 2 of the present invention. 
     FIG. 6 is a front view of a loudspeaker in Embodiment 3 of the present invention. 
     FIG. 7 is a graph showing sound pressure—frequency characteristics of the loudspeakers in Embodiments 1 and 3 of the present invention. 
     FIG. 8 is a front view of a modified loudspeaker in Embodiment 3 of the present invention. 
     FIG. 9 is a cross-sectional view of a conventional loudspeaker. 
     FIG. 10 is a diagram illustrating the relationship between a treble reproduction threshold frequency and a cone angle. 
     FIG. 11 is an equation representing the relationship between a treble reproduction threshold frequency and a cone angle. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinabove, the present invention will be described by way of illustrative embodiments of the present invention. 
     EMBODIMENT 1 
     FIG. 1 is a cross-sectional view showing a structure of a loudspeaker  100  in Embodiment 1. The loudspeaker  100  is a tweeter with a bore of about 6 cm. In FIG. 1, a diaphragm  1  is a made of a pulp. The diaphragm  1  has an arch-shaped cross section with a top portion  1 C. The top portion  1 C is positioned outward (in a direction represented by an arrow A 1 ) from an inner attachment portion  1 A (D 1 =about 7 mm) and from an outer attachment portion  1 B (D 2 =about 2.9 mm). The diaphragm  1  is composed of a curved surface  1 D (from the inner attachment portion  1 A to an edge of a region with a diameter φ1 (about 19.2 mm)), a curved surface  1 E (from the edge of the region with the diameter φ1 (about 19.2 mm) to an edge of a region with a diameter φ2 (about 38.6 mm)), and a curved surface  1 F (from the edge of the region with the diameter φ2 (about 38.6 mm) to the outer attachment portion  1 B, i.e., an edge of a region with a diameter φ3 (about 30 mm)). The curved surfaces  1 D,  1 E, and  1 F have a radius of curvature of about 10 mm, about 15 mm, and about 7 mm, respectively. Furthermore, the edge of the region with the diameter φ2 (about 38.6 mm) is positioned on the top portion  1 C of the diaphragm  1 . A cone angle φ1 between a voice coil bobbin  3  made of kraft paper (thickness: about 0.05 mm) and an inner diameter portion of the diaphragm  1  is about 27.5°. Furthermore, a cone angle θ 2  at the outer attachment portion  1 B is about 40°. 
     An outer peripheral portion of the diaphragm  1  is fixed to a frame  5  (thickness: about 0.6 mm, a total height: about 13.5 mm) with a rubber adhesive. The frame  5  has no holes, and a rear surface  1 G (a space S 1  on the frame  5  side) of the diaphragm  1  is sealed. An inner peripheral portion of the diaphragm  1  is fixed to the voice coil bobbin  3  with an adhesive. An outer peripheral portion of the voice coil bobbin  3  is held by a cotton cloth damper  4  fixed to the frame  5  so that the voice coil bobbin  3  is positioned at the center. Furthermore, a center cap  2  made of a pulp is fixed to a front surface of the diaphragm  1 . The center cap  2  has an outermost diameter of about 17.5 mm, an outer diameter of a curved surface of about 15.5 mm, and a radius of curvature of about 8 mm. 
     Under the frame  5  is disposed a field portion  6  composed of a ferrite magnet, a top plate made of iron, and a yoke. The field portion  6  has an outer diameter of about 45 mm and a thickness of about 8 mm. Furthermore, a voice coil  3 A wound around a lower portion of the voice coil bobbin  3  is inserted in a magnetic gap portion composed of the top plate and the yoke. 
     Hereinafter, the operation of the loudspeaker having the above-mentioned structure will be described. 
     In the loudspeaker  100  in Embodiment 1, the cone angle θ 1  at the connected portion between the voice coil bobbin  3  and the inner diameter portion of the diaphragm  1  is about 27.5°. The loudspeaker  100  includes the diaphragm  1  having an arch-shaped cross section having the top portion  1 C outward (in the direction represented by the arrow A 1 ) from the inner attachment portion  1 A (D 1 =about 7 mm) and from the outer attachment portion  1 B (D 2 =about 2.9 mm). 
     The relationship between the treble reproduction threshold frequency fh and the cone angle θ 1  is as represented by an equation (5-14) and FIG.  5 ′  19  on pages 157-158 of “Loudspeaker System”, 1st Vol., T. Yamamoto. FIGS. 10 and 11 show this relationship. More specifically, it is effective to decrease the cone angle θ 1  in order to increase the treble reproduction threshold frequency fh. 
     However, in general, when the cone angle θ 1  is decreased, the total height of the diaphragm  1  is increased. Therefore, a cavity formed of a recess on the front surface of the diaphragm  1  of the loudspeaker  100  becomes large. Due to the cavity effect, a mild and large peak dip occurs in sound pressure—frequency characteristics. Thus, the cone angle θ 1  is conventionally set at about 45° or more. 
     In contrast, in Embodiment 1, since the diaphragm  1  has an arch-shaped cross section having the top portion  1 C outward, even if the cone angle θ 1  is sufficiently small (i.e., about 27.5°), a cavity C 1  formed of a recess on the front surface of the diaphragm  1  can be made smaller. Therefore, a peak dip caused by the cavity effect can be prevented from occurring in sound pressure—frequency characteristics. Furthermore, because of the curved shape of the diaphragm  1 , the strength of the diaphragm  1  can be improved. As a result, nodal resonances can be prevented from occurring in the diaphragm  1 . 
     Furthermore, regarding the cone angle θ 1  at the inner diameter portion of the diaphragm  1  having the above-mentioned shape, the following can be confirmed by FEM simulation and measurement: in the case where the cone angle θ 1  is prescribed to be larger than about 45°, it is difficult to increase the treble reproduction threshold frequency fh exceeding an audible band (about 20 kHz), and a large peak dip occurs in sound pressure—frequency characteristics. In Embodiment 1, this problem is solved. That is, while the treble reproduction threshold frequency fh is sufficiently increased, flat sound pressure—frequency characteristics without any peak dip due to the cavity effect can be obtained. 
     Hereinafter, the effect in Embodiment 1 will be described with reference to measured characteristics. 
     FIG. 2 shows a comparison in measured sound pressure—frequency characteristics between the loudspeaker  100  in Embodiment 1 and a conventional loudspeaker  900  in which the cone angle θ 1  and weight of a diaphragm  11  are prescribed to be the same as those of the diaphragm  1  of the loudspeaker  100  in Embodiment 1 for the purpose of increasing the treble reproduction threshold frequency. 
     In the conventional loudspeaker  900  shown in FIG. 9, peak dips BP 1  occur due to the cavity effect of a cavity C 2  at a frequency in the vicinity of about 3 to 12 kHz, as represented by characteristics B. Furthermore, because of the decrease in the strength of the diaphragm  11 , a number of large peak dips BP 2  occur due to the nodal resonances of the diaphragm  11  at a frequency of about 15 kHz or more. On the other hand, as represented by characteristics A, the following can be confirmed in the loudspeaker  100  in Embodiment 1. The treble reproduction threshold frequency fh (frequency at which a sound pressure of about 10 dB is decreased from an average sound pressure (average of 3, 4, 6, and 8 kHz; in this case, the average sound pressure is about 92 dB)) is about 40 kHz. Thus, the treble reproduction threshold frequency fh can be sufficiently increased to such a degree as to exceed an audible band. In addition, since the diaphragm  1  has an arch-shaped cross section having the top portion  1 C outward, the cavity C 1  becomes smaller than the cavity C 2 . Therefore, a peak dip caused by the cavity effect and nodal resonances of the diaphragm  1 , can be remarkably decreased. 
     In Embodiment 1, the diaphragm  1  is made of a pulp. However, even if a material such as a film is used, the same effect can be obtained. Furthermore, in Embodiment 1, the outer attachment portion  1 B of the diaphragm  1  is attached with a rubber adhesive. However, by using any other highly viscoelastic material such as butyl rubber, the same effect can be obtained. 
     EMBODIMENT 2 
     A loudspeaker in Embodiment 2 of the present invention will be described with reference to the drawings. FIG. 3 is a cross-sectional view showing a structure of a loudspeaker  200  in Embodiment 2. Embodiment 2 is different from Embodiment 1 in a shape of a connected portion between a diaphragm  7  and a voice coil bobbin  3 . Embodiment 2 is also different from Embodiment 1 in that the diaphragm  7  is integrally formed with a center cap  7 A having a top portion downward (in a direction represented by an arrow A 2 ). More specifically, the diaphragm  7  has a peripheral tapered portion  7 B from an edge of a region having a diameter φ4 (about 13.5 mm) to an edge of a region having a diameter φ5 (about 13 mm) (FIG.  4 B). The edge of a region having a diameter φ4 (about 13.5 mm) has an innermost radius of curvature of about 10 mm, and positioned outside of the voice coil bobbin  3  of the diaphragm  7 . The diaphragm  7  is integrally formed with the center cap  7 A which has a radius of curvature of about 8 mm and a top portion  7 C in the downward direction. The voice coil bobbin  3  is connected to the tapered portion  7 B. 
     The operation of the loudspeaker  200  having the above-mentioned structure will be described. FIG. 4A is an enlarged view of a connected portion between the diaphragm  1  and the voice coil bobbin  3  in Embodiment 1. FIG. 4B is an enlarged view of a connected portion between the diaphragm  7  and the voice coil bobbin  3  in Embodiment 2. As shown in FIG. 4B, the loudspeaker  200  in Embodiment 2 has a structure in which the tapered portion  7 B of the diaphragm  7  is inserted into an inner diameter portion  3 B of the voice coil bobbin  3  in a direction represented by an arrow A 3 , for connecting the diaphragm  7  to the voice coil bobbin  3 . Since the tapered portion  7 B is inserted while an adhesive  8  is removed in a direction represented by an arrow A 4 , the diaphragm  7  can be directly connected to the voice coil bobbin  3 . 
     On the other hand, the loudspeaker  100  in Embodiment 1 has a conventional general connected structure. More specifically, as shown in FIG. 4A, the loudspeaker  100  has a structure in which the inner diameter portion (i.e., the diaphragm attachment portion  1 A) of the diaphragm  1  is connected to an outer diameter portion  3 C of the voice coil bobbin  3 . It is required in terms of production operation that a predetermined clearance D 3  is provided between the outer diameter portion  3 C of the voice coil bobbin  3  and the inner diameter portion of the diaphragm  1 , and an adhesive layer  8 A of the adhesive  8  is inevitably formed in this clearance. 
     Hereinafter, the effect in Embodiment 2 will be described with reference to measured characteristics. FIG. 5 shows a comparison in measured sound pressure—frequency characteristics of the loudspeaker  200  in Embodiment 2 and the loudspeaker  100  in Embodiment 1. 
     In FIG. 5, A represents the characteristics of the loudspeaker  100  in Embodiment 1, and C represents the characteristics of the loudspeaker  200  in Embodiment 2. As described above with reference to FIG. 4B, in the loudspeaker  200  in Embodiment 2, an adhesive layer through which a sound is transmitted at a low speed is not formed at the connected portion between the diaphragm  7  and the voice coil bobbin  3 . Furthermore, the diaphragm  7  can be directly connected to the voice coil bobbin  3 , so that a loss of transmission of a sound is not caused. 
     As shown in FIG. 5, the treble reproduction threshold frequency can be further increased, compared with the characteristics A in Embodiment 1. More specifically, the treble reproduction threshold frequency is about 40 kHz in the characteristics A in Embodiment 1, and the treble reproduction threshold frequency is about 50 kHz in the characteristics C in Embodiment 2 at an average sound pressure of about 92 dB in both Embodiments 1 and 2. Thus, the treble reproduction threshold frequency can be increased by about 10 kHz, compared with Embodiment 1. 
     In Embodiment 2, the integrally formed center cap  7 A is provided with the top portion  7 C downward. However, even if the center cap  7 A is provided with the top portion  7 C upward, the same effect can be obtained. Furthermore, in Embodiment 2, the center cap  7 A is integrally formed with the diaphragm  7 . However, even if the center cap  7 A is separately formed so as to be attached to the diaphragm  7  in the same way as in Embodiment 1, the same effect can be obtained. 
     EMBODIMENT 3 
     Hereinafter, a loudspeaker in Embodiment 3 of the present invention will be described with reference to the drawings. FIG. 6 is a front view showing a structure of a loudspeaker  300  in Embodiment 3. Embodiment 3 is different from Embodiment 1 in that a low density portion  9 A, which has a remarkably low material density and allows air to flow therethrough, is provided on a vibration surface of a diaphragm  9 . More specifically, a region  9 B having a diameter of about 40 mm has a density of about 600 kg/m 2 , whereas a hatched region  9 A has a density of about 200 kg/m 2 , which allows air to flow therethrough. 
     The operation of the loudspeaker  300  having the above-mentioned structure will be described. In the loudspeaker  300  in Embodiment 3, a space on a rear side of the diaphragm  9  is sealed with a frame  5 . However, since the diaphragm  9  is provided with the low density region  9 A, air can flow through the low density region  9 A. 
     Generally, in the case where the rear surface of a diaphragm is sealed in an area of a small volume, stiffness of the air in the sealed space increases to increase bass resonance (the size of bass resonance is proportional to a square root of stiffness of air), and a large peak occurs in bass characteristics in sound pressure—frequency characteristics. However, the structure shown in Embodiment 3 allows air to flow through the diaphragm  9 , and decreases stiffness of air, resulting in a decrease in bass resonance. 
     Hereinafter, the effect in Embodiment 3 will be described with reference to measured characteristics. FIG. 7 shows a comparison in measured sound pressure—frequency characteristics of the loudspeaker  300  in Embodiment 3 and the loudspeaker  100  in Embodiment 1. Herein, as described with reference to FIG. 1, the rear surface of the diaphragm  1  in Embodiment 1 is sealed with the frame  5 . In FIG. 7, A represents characteristics of the loudspeaker  100  in Embodiment 1, and D represents characteristics of the loudspeaker  300  in Embodiment 3. In the loudspeaker  300  in Embodiment 3, the flow of air through the low density portion  9 A of the diaphragm  9  decreases stiffness of the air in a space on the rear side of the diaphragm  9 . As a result, a peak dip caused by bass resonance can be reduced in the characteristics D, compared with the characteristics A. 
     In Embodiment 3, the diaphragm  9  is provided with the low density portion  9 A. However, as shown in FIG. 8, even if holes  10 A each having a diameter of about 1 mm are formed on a diaphragm  10 , the same effect can be obtained. 
     As described above, according to the present invention, while a cone angle at a connected portion between a diaphragm and a voice coil bobbin is decreased, the height of the diaphragm can be rendered smaller due to its arch-shaped cross section. Therefore, the cavity effect can be suppressed. Furthermore, because of the arch-shaped cross section, the strength of the diaphragm is improved, and nodal resonances become unlikely to occur. As a result, even when a general pulp is used in place of an expensive highly rigid material, a treble reproduction threshold frequency can be easily increased, and sound pressure—frequency characteristics can be flattened. 
     Furthermore, according to the present invention, a diaphragm can be directly connected to a voice coil bobbin without any adhesive layer therebetween, and a treble reproduction threshold frequency can be increased. 
     Furthermore, according to the present invention, air can flow through a diaphragm, and stiffness of the air in a space on a rear side of the diaphragm can be decreased. Since bass resonance is proportional to a square root of stiffness, bass resonance can be decreased. As a result, bass characteristics can be easily flattened. 
     Various other modifications will be apparent to and can be readily made by those killed in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.