EP0072991A2

EP0072991A2 - Speaker system

Info

Publication number: EP0072991A2
Application number: EP82107390A
Authority: EP
Inventors: Shinichiro Kawamura
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1981-08-28
Filing date: 1982-08-13
Publication date: 1983-03-02
Also published as: JPS5838097A; DE3264559D1; EP0072991B1; EP0072991A3

Abstract

In a speaker system having a cone type vibration plate (12), a part of the cone type vibration plate is made to protrude toward a space beyond a buffle surface (16') to which the sound wave is radiated from the vibration plate (12) while a dome type vibration plate (22) having a bump effect is fixed to the cone type vibration plate (12) so that the cone type vibration plate (12) may have both dip and bump effects simultaneously, whereby using a complementary characteristic for these dip and bump effects required sound pressure-frequency characteristic and sound pressure phase-frequency characteristic can be obtained.

Description

This invention relates to a speaker system, and more particularly to a speaker system using a cone type in which the dip effect of the cone is decreased.
In principle, a buffle with an infinite size is suitable for the buffle surface of the cone type speaker. Usually, however, a buffle with a finite plane surface has been applied. Occasionally, cylindrical surface, spherical surface and other types are used. As the cone surface is more depressed than the buffle surface, a depression in the cone surface causes a later-mentioned so-called "dip effect" in the voice pressure-frequency characteristic and the voice pressure phase-frequency characteristic. This disturbs reproduction with high fidelity.
In Fig. 1, the cross-section of a cone type speaker is shown to explain the "dip effect". In this figure, 1 and 1' denote cone type vibration plates with semi-vertical angles a and β, respectively.
Fig. 2 shows frequency characteristics (described in IECE of Japan Vol. 77 No. 128 KGEA 77-26, Oie et al) of the cone type speaker of Fig. 1 with different semi-vertical angles. It is assumed that vibration plates 1 and 1' of cone type are making a piston-like vibration in the horizontal direction. Now, defining k as the wave number (k = 2 π/λ, λ is wave length), a as the actual working radius of the vibration plate and c as the sound speed, it can be seen in Fig. 2 that for a deeper cone and a smaller semi-vertical angle the bump at ka = 1 - 2 becomes larger while the dip at ka = 3 - 4 becomes smaller.
Recently, a plane speaker has been used in order to suppress such a dip effect. However, the plane type speaker has usually a heavier vibration plate and hence the electric-acoustic conversion efficiency is worse than that of cone type. If the magnetic energy is increased in order to enhance this efficiency, the cost required therefor could be above that of the vibration plate itself. This will raise the cost of the speaker. This has been the above-mentioned drawback of the plane speaker.
The object of this invention is to eliminate the above-mentioned defect of the prior art and to provide a speaker system of cone type with the dip effect reduced. In order to attain this object, according to this invention, a part of the cone type vibration plate is made to protrude above the buffle surface, or in the direction of a space to which the sound wave is radiated from the vibration plate, so that the dip and bump effects appear simultaneously in the center and outer peripheral parts respectively. Alternatively, a dome type vibration plate with a bump effect is fixed to the cone type vibration plate so that the bump and dip effects appear in the center and outer peripheral parts respectively. With use of such a cone type speaker with a complementary characteristic for dip and bump effects, this invention realizes voice pressure-frequency and voice pressure phase-frequency characteristics which are similar to those of a plane type speaker.
It is needless to say that the complementary characteristic for the dip and bump effects is not perfect. However, if the frequency range of both the cone type and dome type vibration plates is limited in such a range where the plates may be assumed to be a point sound source on an infinite size buffle (ka < √2), a fair complementary characteristic can be obtained under the following condition: the protruded and depressed parts of the vibration plate with respect to a plane containing the buffle surface have substantially the same volume. This invention makes use of the complementary characteristic in this region.
Preferred embodiments of this invention will be described hereinafter in conjunction with the accompanying drawings, in which:

Fig. 1 is the cross-sectional view of a cone type speaker for the explanation of the dip effect.
Fig. 2 is a graph showing the frequency characteristic of the cone type speaker of Fig. 1 for different semi-vertical angles.
Fig. 3 is the cross-sectional view of a dome type vibration plate for the explanation of the bump . effect as applied by this invention.
Fig. 4 is a diagram of the frequency characteristic of the dome type vibration plate (refer to "Acoustic Vibration Theory" Hayasaka et al published by Maruzen in 1974) shown in Fig. 3.
Fig. 5 is the cross-sectional view showing a first embodiment of the speaker system according to this invention.
Fig. 6 is a planar view of Fig. 5.
Fig. 7 is the cross-sectional view showing a second embodiment of the speaker system according to this invention.
Fig. 8 is the cross-sectional view showing a third embodiment of the speaker system according to this invention.

Embodiments of the speaker system of this invention will be explained hereinafter with reference to the drawings.
Fig. 3 is the cross-sectional view of a dome type vibration plate for the explanation of the bump effect as applied by this invention.
Fig. 4 is a diagram of the frequency characteristic of the dome type vibration plate shown in Fig. 3.
In Fig. 3, according as the distance H is decreased from R of a semi-sphere ⓓ to © , ⓑ and ⓐ, the fractional variation D of the velocity potential against ka varies due to the bump effect, as shown in Fig. 4. It is seen further that as the distance H increases from @ to @ in Fig. 3 the dip in the frequency characteristic at ka = 1 - 2 increases. According to this invention, another vibration plate having a bump effect is fixed to a cone type vibration plate having a dip effect as mentioned above. Through utilization of a complementary characteristic for the dip and bump effects, the invention obtains a characteristic similar to that of a plane speaker.
In Fig. 5 showing the first embodiment of the speaker system according to this invention and the cross-section of the peripheral portions thereof, a reference numeral 11 denotes a speaker, 12 a cone type vibration plate (hereinafter referred to as a cone simply) and 12' a flexible edge-suspension. The edge-suspension 12' may be of the same material as that of the cone 12, or of a different material therefrom. In the latter case, a part of the edge-suspension is adhered to the cone 12. 12" denotes a margin part of adhering the edge-suspension 12'. 13 denotes a center cap which is used to avoid the generation of noise due to invasion of dust and iron powder, etc. into the speaker 11. The center cap 13 is also useful to avoid the bending vibration of the cone type vibration plate 12 and is called as a dust cap. 14 denotes a frame, 14' a frame raising part, 15 a magnetic circuit, 16 a buffle plate, 16' a front surface of the buffle plate 16 (hereinafter referred to as a buffle surface simply), and 16" a virtual plane which contains the buffle surface 16' and crosses the speaker 11. Even when the buffle surface 16' is a curves surface, the surface to which the speaker 11 is fixed may be a plane. The virtual surface 16" divides a group of surfaces consisting of the front surface of the cone 12, the front surface of the edge-suspension 12' and the front surface of the center cap 13 into front and rear parts. The front part B contains the edge-suspension 12' while the rear part C contains the center cap 13.
Now, let us consider the band width where the cone 12 performs such a piston vibration as shown by an arrow A of Fig. 5. The inner periphery of the edge-suspension 12' moves with the cone 12, but the outer periphery thereof is fixed to the frame 14. The effective radius a of the edge-suspension 12' in this case is obtained by the following equation (1)
where a₁ is a maximum radius of that part of the edge-suspension in which the velocity of vibration is the same as that of the cone 12 and a₂ is the radius of the outermost part of the edge-suspension which stands still. In another word, it may be assumed that the peripheral region with a radius a is making a piston vibration and that the outer part is standing still. We call the surface of the region Ⓓ with a piston vibration as an equivalent piston vibration surface.
Now, consider a front part of the equivalent piston vibration surface @ above the virtual plane 16", and define a shaded area 17 formed by projecting the front part of the equivalent piston vibration surface onto a plane vertical to the virtual plane 16". Next, consider a rear part of the equivalent piston vibration surface @ below the virtual plane 16", and define a shaded area 18 formed by projecting the rear part of the equivalent piston vibration surface onto a plane vertical to the virtual plane 16". If no center cap 13 is formed, the shaded part inside the bobbin 19 in Fig. 5 disappears. Hence, the shaded volume below the virtual plane 16" is decreased by a corresponding fraction. Furthermore, when the center cap 13 is formed by either a mesh or an apertured plate, the volume expressed by a shade is decreased by a fraction corresponding to the apertured ratio. In any case, it is preferable to determine the position of the virtical plane 16" with respect to the equivalent piston vibration surface in such a manner that the above-mentioned volumes 17 and 18 become substantially the same. The height of the frame raising part 14' should be determined along this line. Here, it is preferable to make as small as possible the volume defined by a virtual cylindrical surface 20 which lies on the periphery of the edge-suspension 12' with a radius a vertically to the virtual plane 16", a surface part of the edge-suspension 12' lying outside the virtual cylindrical surface 20, the surface of the margin part 12", the outer surface of the frame raising part 14' and the virtual surface 16". The reason is as follows. The existence of the above-mentioned volume has equivalently an effect that a part of the buffle surface 16' protrudes toward the edge-suspension 12' and the margin part 12". Therefore, the effect of protruding the group of surfaces above the virtual plane 16" is to some extent cancelled. If the diameter of the frame raising part 14' is made infinitely large, not only the group of surfaces but also the buffle surface 16' protrude. This means that the group of surfaces do not protrude above the buffle surface 16' at all. In order to recover the cancelled part, the group of surfaces should be further protruded. If the frame 14 does not have such a frame raising part 14', an annular body similar to 14' may be separately formed and combined.
Fig. 7 shows the second embodiment of a speaker system according to this invention and the cross-section of the peripheral part of it. In this figure, by a combination of a cone type vibration plate and a vibration plate with a concave surface, e.g. a dome type vibration plate, the aforementioned object can be attained without protruding the frame 14 above the virtual plane 16".
In Fig. 7, 11' denotes a speaker, 21 a frame, and 22 a dome type vibration plate (hereinafter referred to as a dome simply). The dome 22 is fixed to the cone 12. Define a volume @ which consists of a surface part of the dome 22 below the virtual plane 16" and a surface part of the cone 12 that is not covered with the dome 12 and lies below the virtual plane 16". The volume Ⓔ projected to a plane vertical to the virtual plane 16" is shown by a shade. Further, define a volume Ⓕ which consists of a surface part of the dome above the virtual plane 16" and is shown by another shade. If the size and the shape of the dome 22 are determined in such a manner that the volumes Ⓔ and Ⓕ become substantially equal to each other, the afore-mentioned object can be attained without protruding the frame 21 above the virtual plane 16". However, the mass of the dome 22 according to this method is liable to become large. Moreover, in order to support the dome 22, the strength of the cone 12 should be considerably large. A drawback of this method, therefore, is that the mass of the cone becomes easily large.
Fig. 8 is a diagram showing the third embodiment of a speaker system according to this invention and the cross-section of the peripheral part of it. In this embodiment, the methods of the first and second embodiments shown in Figs. 5 and 7 are commonly applied.
In Fig. 8 as in Fig. 5, a part of the cone type vibration plate 12 is protruded above the buffle surface 16' in the direction of a space to which the sound wave is radiated from the cone type vibration plate 12, thereby to obtain a dip effect. At the same time, as in Fig. 7, a vibration plate with a concave surface, e.g. a dome type vibration plate 22, is combined with the cone type vibration plate, thereby to obtain a bump effect. As a result, in Fig. 8, a complementary characteristic of dip and bump effects is attained by setting
Thus, with use of a cone type speaker with a low cost, sound pressure-frequency and sound pressure phase-frequency characteristics similar to those of a plane speaker may be obtained.
As described above, according to this invention, since the dip effect of a vibration plate can be decreased without using any planar vibration plate, sound pressure-frequency and sound pressure phase-frequency characteristics which can match those of a speaker system with use of a planar vibration plate can be obtained. Moreover, this invention has an effect of making the cost as low as or equal to that of a prior art speaker system using a cone type vibration plate.

Claims

1. A speaker system comprising a speaker (11) having therein a cone type vibration plate (12), a buffle plate (16) to which said speaker (11) is fixed, and a magnetic circuit (15) provided at the back of said speaker (11); characterized by constituting such that said cone type vibration plate (12) is divided into front and rear parts Ⓑ( and Ⓒ) by a virtual plane (16") containing the front surface of said buffle plate (16) and that volumes in front of said cone type vibration plate (12) represented by shaded areas when projected vertically to said virtual plane (16") become substantially equal to each other.

2. A speaker system according to Claim 1, characterized in that, by protruding a part of said cone type vibration plate (12) above said buffle surface (16') in the direction of a space to which the sound wave is radiated from said cone type vibration plate (12), the central and peripheral parts of said cone type vibration plate (12) are made to have dip and bump effects respectively so that the complementary characteristic of said dip and bump effects may be utilized.

3. A speaker system according to Claim 1, characterized in that a dome type vibration plate (22) having a concave surface is provided on the front surface of said cone type vibration plate (12) and that, by protruding the vertex of said dome type vibration plate (22) above said buffle surface (16') in the direction of a space to which the sound wave is radiated from said dome type vibration plate (22), the center and peripheral parts of said dome type vibration plate (22) are made to have bump and dip effects respectively so that a complementary characteristic of both effects may be utilized.

4. A speaker system according to Claim 1, characterized in that a dome type vibration plate (22) having a concave surface is provided on the front surface of said cone type vibration plate (12), and that, by protruding the vertex of said dome type vibration plate (22) above said buffle surface (16') in the direction of a space to which the sound wave is radiated from said dome type vibration plate (22), the center part of said dome type vibration plate (22) is made to have a bump effect, and the peripheral part of said dome type vibration plate (22) and the mid part of said cone type vibration plate (12) are made to have a dip effect, and the peripyheral part of said cone type vibration plate (12) is made to have a bump effect so that a complementary characteristic of both effects may be utilized.