GB2079564A - Loudspeaker diaphragms - Google Patents

Abstract

A loudspeaker diaphragm 10 includes a core member 11 formed of a hollow-centre peripheral section 11a and a centre section 11b, and having a flat front surface 11c from which a frustum-shaped side surface extends to a back end portion 11d, a cover member 15 secured to the back end portion 11d and arranged to be connected to a voice coil bobbin 16 of the loudspeaker, an outer radiation layer 12 mounted on the front surface 11c and having a relatively high resonance sharpness, and an intermediate layer 13 laminated between the front surface 11c and the outer radiation layer 12 and having a resonance sharpness lower than that of the outer radiation layer 12. <IMAGE>

Description

SPECIFICATION Loudspeaker diaphragms This invention relates to loudspeaker diaphragms, and to loudspeakers including such diaphragms.

Diaphragms for use in loudspeakers are known in which a foamed resin material, such as foamed polystyrene, is moulded into a generally frusto-conical configuration to be used for the core member of the diaphragm.

Such diaphragms typically have the same mass as conventional diaphragms of paper and the like which are also formed into a frusto-conical configuration.

Although these foamed core members provided certain advantages over conventional paper core members, for example, hindering the formation of a split vibration and improving the acoustic characteristics thereof, they disadvantageously have a low vibration or acoustic propagation speed and a relatively low natural resonance frequency, while retaining a high resonance sharpness 0. Thus, the acoustic output-frequency characteristic of the foamed diaphragms have a substantial peak dip, particularly in the high-pitched tone region, and do not exhibit a satisfactory flat response.

The foamed diaphragm usually has the back or small-diameter portion connected to a voice coil bobbin and is driven thereby. However, the distance between the back or small-diameter end portion and the centre of the flat front surface of the frustum differs from the distance between the back portion and the front surface at the periphery of the latter, resulting in a degradation of the phase characteristic of the acoustic output. Further, since the voice coil bobbin is adhesively affixed directly to the back end of the foamed core member, it is difficult to maintain the bobbin in a fixed condition with any satisfactory degree of accuracy, and the resulting movement with respect to the adhesively fixed condition affects the acoustic output-frequency characteristic in the high-pitched tone region.

Further, since the foamed resin material has a generally poor heat resistance, it is difficult to use such material in a diaphragm of a large output loudspeaker because of the large amount of heat transferred from the bobbin to the core member.

According to the present invention there is provided a loudspeaker diaphragm comprising: a frustum-shaped core member having a front surface; an outer radiation layer mounted on said front surface and having a high resonance sharpness; and an intermediate layer interposed between said front surface and said outer radiation layer, and having a resonance sharpness lower than that of said outer radiation layer; wherein said core member includes a first peripheral section having a central hollow portion and a tapered exterior surface, and a second centre section which conforms in shape to said central hollow portion, said sections being integrated with each other so as to form said front surface.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of an embodiment of loudspeaker diaphragm according to the invention; Figure 2 is an enlarged fragmentary crosssectional view of a peripheral portion of the diaphragm of Fig. 1, and showing details of the attachment of an edge member; Figure 3 is an enlarged fragmentary crosssectional view of a back end portion of the diaphragm of Fig. 1, particularly showing the connection to the voice coil bobbin of the loudspeaker; Figure 4A is a graphical representation of the acoustic output-frequency characteristic of a conventional diaphragm; Figure 4B is a graphical representation similar to that of Fig. 4A, but illustrating the acoustic output-frequency characteristic of a conventional diaphragm having an outer radiation layer;; Figure 4C is a graphical representation illustrating the acoustic output-frequency characteristic of an embodiment of diaphragm according to the invention; and Figure 4D is a graphical representation, comparing the acoustic output-frequency characteristic of a conventional diaphragm with that of a similar diaphragm having a cover member.

Referring initially to Fig. 1, a diaphragm 10 for a loudspeaker comprises a core member 11 moulded into a generally frustum or frustoconical configuration, an outer radiation layer 1 2 having a relatively high resonance sharpness Q, and an intermediate layer 1 3 interposed between the core member 11 and the outer radiation layer 1 2 and having a resonance sharpness 0 lower than that of the outer radiation layer 1 2. Preferably, the core member 11 is composed of a foamed resin material, such as foamed polystyrene or the like, the outer radiation layer 1 2 is made from a sheet of polyester, vinyl chloride or the like, having an approximate thickness of 100 mm, and the intermediate layer 1 3 is composed of foamed butyl rubber, sponge or like material, having a thickness of approximately 1 mm.

As shown in Fig. 1, the core member 11 is divided into two sections, that is, an outer or peripheral section 11 a having a central hollow portion into which there is fitted a centre section 11 b, in order that the diaphragm 10 may be more easily produced. The sections 11 a and 11 b are integrated with each other such that the centre section 11 b conforms in shape to the central hollow portion of the peripheral section 11 a and such that the sections 11 a and 11 b together form a continuous flat front surface 11 C which has an intermediate layer 1 3 and an outer radiation layer 12 laminated thereon.An edge member 14may be connected to the diaphragm 10 by sandwiching a portion thereof between the intermediate layer 1 3 and the front surface 11 c, as shown in Fig. 2.

Referring to Fig. 3, it will be seen that the peripheral section 11 a is formed, at its back end portion 11 d, with a circumferential inwardly-directed flange 11 e, and the centre section 1 b is stepped at its back end 11 fso as to form a circumferential or annular stepped or cut-out portion 11 g in communication with the central hollow portion of the peripheral section tl a.

The diaphragm 10 also includes a cover member 15 of a unitary or laminated lightweight, rigid material, for example, a punched metal or mesh material, such as aluminium, magnesium, titanium, beryllium, boron, or the like, which covers an area of the back end portion 11 d of the core member 11. The cover member 15 is shown to be comprised of a first flaring portion 1 5a adhesively secured to, and covering the peripheral tapered surface of the core member 11 and a second cylindrical portion 1 sub shaped to extend closely through the hole defined by the flange 11 e.A free edge portion 1 sic of the cylindrical portion 1 sod is outwardly swaged, as shown, to embrace the flange 11 e and thereby further secure the cover member 15 to the core member 11. As shown in Fig. 1, the distance La between an edge 1 5d of the flaring portion 1 spa and the peripheral edge of the acoustic output or radiation surface 1 Oa of the diaphragm 10, that is, of the outer radiation layer 12, is approximately equal to the distance Lb between the edge portion 1 sic of the cylindrical portion 1 sub and the front surface 11 c of the core member 11, taken along the inner surface of the peripheral section 11 a, in order that a satisfactory phase characteristic can be obtained for the diaphragm 10.

The diaphragm 10 is provided with a voice coil bobbin 1 6 wound with a voice coil 1 7 of the same type used in conventional loudspeakers, and one end of the voice coil bobbin 1 6 is inserted into a recess defined by the cylindrical portion 1 sub of the cover member 15, the inner surface of the peripheral section 11 a and the cut-out portion 11 g of the centre section 11 b, and is fixedly secured to the cover member 1 5 by a suitable adhesive 1 8.

In constructing the diaphragm 10, it is preferable and more efficient to adhere the voice coil bobbin 1 6 to the cover member 15 before the centre section 11 b is inserted into and integrated with the peripheral section 11 a of the core member 11.

As shown in Fig. 1, the loudspeaker also includes a magnetic circuit 1 9 having a magnetic gap 1 9a in which the voice coil 1 7 is disposed, whereby the diaphragm 10 is driven by electric input signals applied to the voice coil 17. The location of the voice coil bobbin 1 6 is regulated by a damper member 20, which is secured to a frame (not shown), along with the edge member 14 and the magnetic circuit 19.

In the above-described embodiment, the split vibration frequency of the primary mode of the core member 11 can be raised to a higher frequency by mounting thereon the outer radiation layer 1 2 which has a relatively high resonance sharpness Q. Moreover, the resonance sharpness 0 of the core member 11 can be reduced by mounting the intermediate layer 1 3 between the front surface 11 C and the outer radiation layer 12, with the intermediate layer 1 3 having a resonance sharpness 0 lower than that of the outer radiation layer 12 so as to provide a desired internal loss. By reason of the foregoing, a relatively flat response is obtained by reducing the peak dip of the acoustic output-frequency characteristic, particularly in the high-pitched tone region.

The acoustic output-frequency characteristic of a loudspeaker using a conventional diaphragm is shown in Fig. 4A. There is some improvement in the acoustic output-frequency characteristic of a diaphragm when only an outer radiation layer 1 2 of a vinyl chloride material is adhered to the front surface 11 C of the core member 11 (without the intermediate layer 1 3). Such improvement is realized particularly in respect of the rise in the split vibration frequency of the primary mode and a slight reduction in the peak dip of the acoustic output-frequency characteristic, as shown in Fig. 4B. However, with the addition of the outer radiation layer 1 2 alone, a sufficiently flat response is still not obtained.Moreover, alteration of the thickness and material of the outer radiation layer 1 2 may have an undesired effect on the adhesive property of the core member 11 without bringing about an effective change in the acoustic outputfrequency characteristic.

In the embodiment of the invention in which the intermediate layer 1 3 is made from a butyl rubber sheet with a resonance sharpness 0 less than one-fifth of that of the outer radiation layer 1 2 and is interposed between the core member 11 and the outer radiation i layer 12, the acoustic output-frequency characteristic (Fig. 4C) is substantially improved or flattened, particularly in the high-pitched tone region from 1 KHz to 5 KHz. It has been found that, by proper selection of the materials and properties of the outer radiation layer 1 2 and the intermediate layer 13, the acoustic output-frequency characteristic can be controlled over a wide range thereof.On the other hand, it has been found that the type of adhesive used for connecting the outer radiation layer 12, the intermediate layer 13, and the core member 11 does not have a major effect on the acoustic output-frequency characteristic of the loudspeaker.

The diaphragms which are the subject of the characteristics shown in Figs. 4A to 4C each had a cover member 1 5 secured to the core member 11. It is to be noted that, with the voice coil bobbin 1 6 adhesively fixed to the cover member 15, the accuracy of alignment and connection of the voice coil bobbin 1 6 to the diaphragm 10 is improved so as to provide a diaphragm with a more uniform acoustic output-frequency characteristic than is obtained when the voice coil bobbin 1 6 is directly adhered to the foamed resin material of the core member 11.Moreover, since heat produced in the voice coil 1 7 can be radiated from the diaphragm 10 through the cover member 1 5 adhered to the core member 11, the heat resistance is improved so as to permit production of a large acoustic output. Also, since the core member 11 is made from a foamed resin material and is driven through the cover member 15, which is made of a light and rigid material, any adverse effect on the acoustic output-frequency characteristic caused by the natural vibrations of the core member 11, can be reduced. Further, since the previously described distances La and Lb are made approximately equal, the acoustic subject of the diaphragm can be provided with a satisfactory phase characteristic.

As shown in the acoustic output-frequency graph of Fig. 4D, the use of the cover member 1 5 can effectively raise the resonance frequency in the high-pitched tone region by about 3000 Hz, while, at the same time, reducing the peak dip in the same region to provide a relatively more satisfactory acoustic output-frequency characteristic. In Fig. 4D, the broken line illustrates the characteristic of a conventional disphragm having a core member 11 of foamed polystyrene directly connected to the voice coil bobbin 16, and the solid line illustrates the characteristic of the same diaphragm merely provided with the cover member 1 5. In other words, neither diaphragm represented by Fig. 4D has the outer radiation layer 1 2 and the intermediate layer 1 3 laminated thereon. Therefore, Fig.

4D shows only the effect of the cover member 1 5.

Attention is drawn to our copending patent application no. 7928806 (serial no.

2029163) on which the present application is divided, and describing and claiming similar subject matter.

Claims (8)

1. A loudspeaker diaphragm comprising: a frustum-shaped core member having a front surface; an outer radiation layer mounted on said front surface and having a high resonance sharpness; and an intermediate layer interposed between said front surface and said outer radiation layer, and having a resonance sharpness lower than that of said outer radiation layer; wherein said core member includes a first peripheral section having a central hollow portion and a tapered exterior surface, and a second centre section which conforms in shape to said central hollow portion, said sections being integrated with each other so as to form said front surface.

2. A diaphragm according to claim 1 wherein said first peripheral section includes an inwardly directed flange and said second centre section includes a cut-out portion in communication with said central hollow portion.

3. A diaphragm according to claim 2 further comprising a cover member connected to said first peripheral section and covering said flange and a portion of said tapered exterior surface.

4. A diaphragm according to claim 3 wherein said loudspeaker includes a voice coil bobbin connected to said cover member.

5. A diaphragm according to claim 4 wherein said core member has a recess therein defined by said cover member, said central hollow portion and said out-out portion, and said bobbin is disposed within said recess.

6. A diaphragm according to claim 5 wherein said cover member has a first edge at its connection to said flange member and a second edge along said exterior tapered surface, and the distance from said first edge to said front surface, at the mating of said first peripheral section with said second centre section, is substantially equal to the distance from said second edge to a peripheral edge of said outer radiation layer.

7. A diaphragm according to claim 1 and substantially as hereinbefore described with reference to Figs. 1 to 3 of the accompanying drawings.

8. A loudspeaker comprising a diaphragm according to any one of the preceding claims and a magnetic circuit.