Loudspeaker

Abstract

In a loudspeaker, the upper part and the lower part of a cone-shaped diaphragm with a voice coil are suspended from a frame by a ring shape suspension member, herein referred to as a first suspension means, and by a ring shape suspension member, herein referred to as a suspension means, respectively. At least one of the suspension means is formed of a mixture of elastic material such as rubber, with carbon fibers therein. The fibers are needle-like and are aligned radially in the material of at least one of the suspension means.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a dynamic, cone type loudspeaker, and more particularly to a loudspeaker with suspension means for supporting the cone from a frame.

2. Description of the Prior Art

In a typical conventional dynamic loudspeaker, there is found a circular yoke, an annular yoke, an annular magnet and a central pole fixed on the circular yoke. The vibration system comprises a cylindrical bobbin, a voice coil wound around the cylindrical bobbin, a suspension means or a damper suspending the bobbin from a cone-shaped frame and a cone-shaped diaphragm fixed adhesively on the upper end of the bobbin. An annular suspension means has its outer edge bonded to the upper end of the frame with a suitable binder and has its inner edge bonded to the upper end of the diaphragm by a suitable binder.

Generally, the suspension means provides an edge for the cone which has a curved portion with a nearly semicircular cross section projecting upward at its center, which is elastically deformable due to its own elasticity, whereby the diaphragm can be linearly vibrated in the vertical direction. The stiffness of the edge has an important effect on the lower frequency characteristic. The stiffness should be reduced for the lowering of the lower threshold frequency. For such a purpose, the edge is generally formed of urethane sponge or cotton cloth impregnated with phenol resin, or of a mixture of nitryl butadiene rubber, styrene butadiene rubber or isobutyl-isoprene rubber with carbonic fine grains.

For satisfactory vibration of the diaphragm, it is required that the edge be deformable both radially and peripherally. Moreover, it is necessary that the peripheral deformation be larger than the radial deformation.

An edge formed of cotton cloth has the following disadvantages. When the diaphragm is strongly vibrated, the edge is wrinkled and its internal loss is increased, since it is little deformed in the peripheral direction. Moreover, sounds are generated from the edge.

An edge formed of the vulcanized mixture of the above-mentioned rubber with carbonic fine grains has the following disadvantages: The thickness of the edge should be relatively large for the required stiffness. Therefore, the sound conversion efficiency is lowered with the increase of the weight of the edge.

Generally, the damper is annular and undulated in shape, and deformable in a radial direction. This prevents the voice coil from contacting the yoke during vibration. It supports the diaphragm and the voice coil.

Also, the stiffness of the damper has an important effect on the lower threshold frequency f.sub.o. With a damper having a large degree of stiffness, sounds cannot be reproduced with high fidelity, since the damper does not vibrate accompanied with the diaphragm. For that reason, the damper is manufactured in such a manner that the cotton cloth impregnated with phenol resin is formed under heat into undulation so as to obtain a lower stiffness.

However, the conventional damper cannot be deformed in a peripheral direction when the vibration system is operated. Accordingly, the linearity of the deformability is unsatisfactory under large vibrations. The size and the thickness of the damper are increased for the required stiffness and so the weight of the damper is increased. Accordingly, the internal loss of the damper is decreased. Therefore, the mechanical impedance of the loudspeaker is increased in the lower frequency range. As the result, the sharpnesses of the mechanical resonance and the electrical resonance are increased and so the quality of the reproduced sounds is worsened.

SUMMARY OF THE INVENTION

One object of this invention is to provide a loudspeaker with a good frequency response characteristic.

Another object of this invention is to provide a loudspeaker with a good sound characteristic in which the suspension means is improved for supporting the cone or the diaphragm.

Another object of this invention is to provide a loudspeaker with a light suspension means having a high degree of stiffness, in which a diaphragm is suspended by a suspension means formed of a mixture of elastic material with carbon fibers.

Another object of this invention is to provide a loudspeaker generating no radiate sound from a suspension means, in which a diaphragm is suspended by a suspension means containing short carbon fibers aligned radially, and in which the suspension means is prevented from wrinkling.

Another object of this invention is to provide a loudspeaker with a cone-type diaphragm in which the cone-type diaphragm is suspended by an improved suspension means with a large internal loss at its lower end.

Another object of this invention is to provide a loudspeaker with a cone-type diaphragm in which the cone-type diaphragm is suspended by an improved suspension means formed of a mixture of elastic material with carbon fibers and deformable in the peripheral direction, at its lower end, and therefore in which the sharpnesses of mechanical and electrical resonances are decreased.

A still further object of this invention is to provide a loudspeaker with a cone-type diaphragm in which the cone-type diaphragm is suspended by a suspension means containing carbon fibers at its upper end and lower end, whereby signals can be converted into sounds with high fidelity in a higher frequency range and vibration can be sufficiently damped in a lower frequency range.

In accordance with an aspect of this invention, a loudspeaker comprises a diaphragm with a voice coil, a frame surrounding the diaphragm, and a suspension means for suspending the diaphragm from the frame, the suspension means being formed of a mixture of elastic material with carbon fibers.

The above, and other objects, features and advantages of the invention will be apparent in the following detailed description of illustrative embodiments thereof which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a loudspeaker of one preferred embodiment of this invention;

FIG. 2 is a plan view of a first suspension means in FIG. 1;

FIG. 3 is an enlarged plan view of a part of the first suspension means shown in FIG. 2;

FIG. 4 is a cross-sectional view taken along the line IV--IV of FIG. 3;

FIG. 5 is a cross-sectional view of a metal mold employed for the manufacture of the first suspension means shown on FIG. 2;

FIG. 6 is a plan view of a second suspension means in FIG. 1;

FIG. 7 is an enlarged plan view of a part of the second suspension means shown on FIG. 6;

FIG. 8 is a cross-sectional view taken along the line VIII--VIII of FIG. 7;

FIG. 9 is a graph showing a frequency-response characteristic; and

FIG. 10 is a graph showing a frequency-impedance characteristic.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the magnetic portion of a loudspeaker 1, according to one embodiment of this invention, comprises a circular yoke 2a, an annular yoke 2b, an annular magnet 3 disposed between the circular yoke 2a and the annular yoke 2b, and a central pole 4 which extends upwardly from the center of the circular yoke 2a and is fixed with respect thereto. A vibration system comprises a bobbin 5 for a voice coil, a voice coil 6 wound around the bobbin 5 and a cone-shaped diaphragm 9 fixed on the upper end of the bobbin 5. The first annular suspension means 20 is bonded to the upper end of the frame 7 and to the upper end of the diaphragm 9, with a suitable binder. A second annular suspension means, namely a damper 28 is bonded to the lower end of the diaphragm 9 to suspend the diaphragm 9 elastically from the frame 7. A sound absorber 11 is fixed on the upper face of the bobbin 5. A cap 12 covers the surface of the sound absorber 11. A clamping ring 13 is fixed on the upper end of the edge 20 to prevent the edge 20 from peeling from the upper end of the frame 7.

The first suspension means 20 will now be described in detail with reference to FIGS. 2 to 4.

The first suspension means 20 defines a curved or ridge-like portion 20a with a nearly semicircular cross section projecting upward, an inclining portion 20b and an outer horizontal circumferential portion 20c. The first suspension means 20 is manufactured in such a manner that carbon fibers 21 with lengths of 0.3 to 0.8 mm are mixed into an elastic material such as synthetic rubber, (for example, nitryl butadiene rubber, styrene butadiene rubber and isobutyl-isoprene rubber), natural rubber or synthetic resin (for example, soft-foamed-urethane). The carbon fibers 21 are aligned radially as diagrammatically shown in FIG. 3.

The upper end of the diaphragm 9 is fixed on the inclined portion 20b of the first suspension means 20, and the upper end of the frame 7 is fixed to the horizontal circumferential portion 20c of the first suspension means 20.

The method for manufacturing the edge 20 will be described with reference to FIG. 5.

1 to 40 percent carbon fibers, with Young's modulus 20,000 kg/mm.sup.2, diameter 8.mu. and length 3 to 6 mm, and a different material, such for example, stearic acid, sulpher, carbon black, etc., are mixed into solid styrene butadiene rubber by a mixing mill, where the carbon fibers are cut needle-like into 0.3 to 0.8 mm. A resulting mixture 22 is put on the center of a metal mold 23, heated up to a required temperature, for example 150.degree.C., which defines an annular groove 24 having the same cross section as the first suspension means 20. Another metal mold 26 is provided having a mating annular projection 25 of the same cross section as the annular groove 24. The metal mold 26 is brought over the one metal mold 23, heated up to the said required temperature. The mixture 22 is put between the metal molds 23 and 26. Accordingly, as the metal mold 26 is pressed down, the mixture 22 is radially spread out from the center of the metal mold 23 in the direction shown by the arrows 27 in FIG. 5 and then led into the annular groove 24, between the metal molds 23 and 26. When the annular projection 25 is fitted into the annular groove 24, the mixture 22 is vulcanized at a temperature over 140.degree.C., between the metal molds 23 and 26. Thereafter, the metal molds 23 and 26 are cooled to solidify the mixture 22. The formed mixture 22 is drawn out from the metal mold 23. As the result, the first suspension means 20 is obtained, as shown on FIG. 2 and FIG. 4.

In the manufacturing process, since the mixture 22 is spread out directionally, the carbon fibers 21 contained in the mixture 22 are displaced directionally. Therefore, the carbon fibers 21 with the lengths of 0.3 to 0.8 mm are radially dispersed in the direction of the diameter of the first suspension means 20 (FIG. 3).

With such a dispersion of the carbon fibers 21, the deformability of the first suspension means 20 is improved on the peripheral direction. There is no possibility that the first suspension means 20 will be wrinkled. The specific weight of the first suspension means 20 is small and the stiffness of the first suspension means 20 is large, since it is manufactured in a vulcanizing process of the mixture containing carbon fibers. Consequently, the thickness of the curved portion 20a of the first suspension means 20 can be reduced to the minimum for the required stiffness and so the total weight of the first suspension means 20 can be reduced.

According to this invention, the second suspension means 28 is also formed of the mixture of elastic material with the carbon fibers.

Referring to FIG. 6 and FIG. 8, a second suspension means 28 has a plurality of concentric curved portions 28a with a semicircular cross section projecting upward. It also has curved portions 28b with a semicircular cross section projecting downward, an inner edge portion 28c with a U-shaped cross section projecting downward, and a horizontal circumferential portion 28d. The second suspension means 28 is formed of a mixture of the same elastic material as the first suspension means 20 with the carbon fibers. As shown in FIG. 7, carbon fibers 31 with lengths of 0.3 to 0.8 mm are radially aligned in the direction of the diameter of the second suspension means 28. It is preferable that the Young's modulus of the carbon fiber be more than 20,000 kg/mm.sup.2 and that the diameter of the carbon fiber is smaller than the length of the carbon fiber, for example, about 8.mu.. 1 to 40 percent carbon fibers 31 are contained in the second suspension means 28.

In the loudspeaker 1, the inner end portion 28c is bonded to the bobbin 5 and the horizontal circumferential portion 28d is bonded to the frame 7. The second suspension means 28 can be manufactured in the same manner as the first suspension means 20.

The deformability of the second suspension means 28 is higher in the peripheral direction than in the radial direction, since the carbon fibers 31 are radially dispersed in the second suspension means 28. When a mixture of styrene butadiene rubber with the carbon fibers is vulcanized for the manufacture of the second suspension means 28, a small specific weight and a large stiffness can be obtained for the second suspension means 28. Consequently, the thickness of the second suspension means 28 can be reduced to a minimum for the required stiffness and hence the total weight of the second suspension means 28 can be reduced.

FIG. 9 shows the frequency-response characteristics of cone-type dynamic speakers using the first suspension means 20 according to this invention and the conventional first suspension means formed of cotton cloth.

In a curve a for a speaker using cotton cloth, the response fluctuates in frequencies above 500 Hz, where peaks or dips occur. Therefore, the quality of tone is deteriorated. In a curbe b for the first suspension means according to this invention, no peak or dip occurs in the higher frequencies. The response lowers abruptly in the frequencies above a higher threshold frequency 1,500 Hz. Therefore, high quality of tone can be obtained. The reasons are that the suspension means according to this invention is not wrinkled in the vibration and no sound is generated from the suspension means, and that the sound conversion efficiency is improved due to the reduction of the weight.

FIG. 10 shows frequency-impedance characteristics. Curve a is for a conventional loudspeaker using a damper formed of cotton cloth impregnated with phenol resin, where the resonant frequency f.sub.o is 78 Hz, the maximum impedance Z.sub.o 100 .OMEGA. and the minimum impedance R.sub.E 6 .OMEGA.. The sharpness Q.sub.M of the mechanical resonance is represented by an equation ##EQU1## where f.sub.1 and f.sub.2 are frequencies for an impedance ##EQU2## From the curve a, f.sub.1 = 67 Hz and f.sub.2 = 88 Hz are apparent. Therefore, ##EQU3## for the curve a. The sharpness Q.sub.o of the electrical resonance is represented by an equation ##EQU4## for the curve b.

The values of Q.sub.M and Q.sub.o are large for the conventional damper, as above described and so the damping effect is low for the lower frequency range. Therefore, it will be understood that the tone quality is inferior.

The frequency characteristic is remarkably improved by using the second suspension means 28 according to this invention, as shown by a curve b on FIG. 10, where the resonant frequency f.sub.o is 84 Hz, the maximum impedance Z.sub.o 24 .OMEGA. and the minimum impedance R.sub.E 6 .OMEGA.. The frequencies f.sub.1 and f.sub.2 are 50 Hz and 143 Hz for an impedance ##EQU5## Therefore, the sharpness Q.sub.M of the mechanical resonance is ##EQU6## And the sharpness Q.sub.o of the electrical resonance is ##EQU7##

It will be understood that Q.sub.M and Q.sub.o can be remarkably lowered by the second suspension means according to this invention, compared with the conventional suspension means.

Although an illustrative embodiment of this invention has been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of this invention, as defined in the appended claims. For example, the shape and the size of the first and the second suspension means may be varied. Moreover, this invention can be applied to a double cone-type loudspeaker or a dome-type loudspeaker, too.

Claims

We claim as our invention:

1. A loudspeaker comprising a diaphragm with a voice coil, a frame surrounding said diaphragm, and a suspension means for suspending said diaphragm from said frame, said suspension means being formed of a mixture of elastic material impregnated with carbon fibers.

2. A loudspeaker according to claim 1, wherein said elastic material is rubber and said carbon fibers are needle-like and are radially aligned in said suspension means.

3. A loudspeaker according to claim 2, wherein said suspension means is disposed between the upper portion of said diaphragm and said frame and it defines an intermediate portion having an arcuate cross section.

4. A loudspeaker according to claim 2, wherein said suspension means is disposed between the lower portion of said diaphragm and said frame and it defines an intermediate portion having a wave-like cross section.

5. A loudspeaker according to claim 1, wherein said elastic material is a plastic resin.

6. A loudspeaker comprising a cone-shaped diaphragm with a voice coil, a frame surrounding said cone-shaped diaphragm, a first suspension means for suspending the upper part of said diaphragm from said frame, and a second suspension means for suspending the lower part of said diaphragm from said frame, at least one of said suspension means being formed of a mixture of elastic material impregnated with carbon fibers.

7. A loudspeaker according to claim 6, wherein said elastic material is rubber and said carbon fibers are needle-like and are radially aligned in said one of said first suspension means and said second suspension means.

8. A loudspeaker according to claim 6, wherein the other suspension means is also formed of a mixture of elastic material with carbon fibers.

9. A loudspeaker according to claim 8, wherein said elastic material is rubber and said carbon fibers are needle-like and are radially aligned in the material of said first and said second suspension means.