GB2176673A - Electro-mechanical transducer diaphragm - Google Patents

Abstract

A diaphragm for an electro- mechanical transducer such as a loud speaker comprises a first layer (30) comprising a foamed or fibrous plastics material and a second layer (32) comprising material embedded in or bonded by a synthetic resin, and preferably a third layer (31) also comprising material embedded in or bonded by a synthetic resin with the foamed or fibrous plastics material layer (30) interposed between the second (32) and third (31) layers. <IMAGE>

Description

SPECIFICATION Electro-mechen ical transducer diaphragm This invention relatesto a diaphragm foran electro-mechanical transducer such as a loud speaker and a method of manufacturing such a transducer.

However, the invention is applicableto any other electro-mechanical transducer which has a di aphragmwhich vibrates either in responseto an electrical signal to produce sound, or in response to sound to produce an electrical signal.

According to one aspect ofthe invention we provide a diaphragm for an electro-mechanical transducer, the diaphragm comprising at leasttwo superposed layers in contact, afirstlayercomprising afoamed orfibrous plastics material and second layer comprising material embedded in or bonded by a synthetic resin.

Preferably a third layer is provided comprising material embedded in or bonded by synthetic resin, with the foamed or fibrous plastics material layer interposed between the second and third layers.

Thus the foamed or fibrous plastic material layer comprises a core layer ofthe diaphragm which is flexible and resilient whilst the further layer or layers comprise a skin layer or layersforthe core layer, the skin layer or layers being substantially more rigid than the core layer.

It has been found that such a construction consider ablyenhancestheperformanceofthetransducerof which the diaphragm is a part.

The more rigid skin layer or layers, when the diaphragm vibrates, gives good response to an electrical signal or sound wave whilst the resilient core layer absorbs unwanted vibrations such as resonances.

The first core, layer may have a density of between 24 and 192 kilograms per cubic metre (approximately 1 .5to 12 pounds per cubic foot) but preferably has a density lying in the range 32 kilograms per cubic metre to 160 kilograms per cubic metre (approximately 2 to 10 pounds per cubic foot).

For example the core layer may be made of Styrofoam which is an extruded polystyrene from (as herein defined), having a density of between 32 and 48 kilograms per cubic metre (approximately 2 to 3 pounds per cubic foot).

By "foam" we mean that the material comprises closed cell bubbles, as opposed to an expanded plastics material which commonly comprises gra- nules which are adhered together. The compressive strength of a foamed material is, generally, higher than for an equivalent expanded material and is thus more suited to use in a core layer of a diaphragm in accordance with the invention.

Alternatively, the core layer may be made of Urethane having a density of about 32 kilograms per cubic metre (approximately 2 pounds per cubic foot); or PVC having a density of between 48 and 128 kilograms per cubic metre (approximately 3 to 8 pou nds per cubic foot), or a mixtu re of Urethane and polyester having a density of between 72 and 160 kilograms per cubic metre (approximately 4.5 to 10 pounds per cu bic foot) or polyu rethane having a density of between 32 and 80 kilograms per cubic metre (approximately 2 to 5 pounds per cubic foot).

The core layer is preferably at least one millimetre thick but may have a thickness upto 10 mm.The thickness ofthe core layer may vary over the diaphragm or may be constant as required.

The second, or second and third skin, layer material maycompriseglassfibres, carbon fibres, graphite fibres or Kevlar or a mixture of such fibres. Kevlar is a 49 -Aramid fibre material i.e. an organic man-made fibre as opposed to a natural fibre.

The fibres may be in the form of a woven lattice such as a mat or cloth, coated on both sides and hence embedded in the resin, or loose rovings embedded in or bonded by the synthetic resin. The density ofthe fibres may vary or be substantially constant over the diaphragm.

The resin in which the fibres are embedded or by whichthefibresare bonded bymaycomprisefor example, a polyester or epoxy resin.

The skin layer or layers may be bonded to the core layer by the synthetic resin which the material of the skin layer or layers is embedded in or bonded by. The or each skin layer preferably has a thickness between 0.5 and5 mm but preferably is between 1 mm and 2 mm thick.

The invention is particularly applicable to an electromechanical transducer comprising a loud speaker which produces sound in response to an electrical signal although may be applied to an electro-mechanical transducer such as a microphone which produces an electrical signal in response to sound.

The diaphragm may generally, be conical or a truncated cone, or any other shape as required.

According to a second aspect ofthe invention we provide an electro-mechanical transducer comprising a diaphragm in accordance with thefirst aspect ofthe invention.

Particularlywherethediaphragm isconical,ora truncated cone, the diaphragm may be made by a method including the steps of cutting a pattern from a sheetoffoamed orfibrous material, and forming the pattern between a former and a clamping means, to provide the first, core layer, and thereafter bonding to the core layer, a second, skin layer comprising a material embedded in or bonded bya synthetic resin.

Priorto forming, the pattern may comprise a substantially circulardisc with a keyhole shaped aperture provided therein comprising a pair of converging edgesextendingfromthecircumferencetoa circular portion ofthe aperture atthecentre ofthedisc.

One or both ofthe edges ofthe aperture may be coated with adhesive priortoforming, and the edges may be brought into contactthe pattern thus being formed into atruncated cone first layer.

The second layer of material embedded in or bonded bythe synthetic resin may be applied to the outwardly facing, convex, surface of the first layer, by applying a resin and fibrous mixture, or alternatively resin may be applied, followed buy a mat of material, followed by another coating of resin to embed the material in the resin.

Shaping of the second layer may be achieved with the assistance of aconical convex former.

The small diameter, end ofthetruncated cone diaphragm, may be surrounded by a cylinder of material which is adhered to the convex surface ofthe diaphragm subsequentto providing the second layer.

To assist shaping of this part, a mandrel may be used, which may be coated with a material such as a plastic sheetto preventthe mandrel becoming adhered to the resin ofthe second layer.

The method may include providing a third, skin, layer comprising a material embedded in or bonded by a synthetic resin. Such a third layer of material may be provided, by applying to the inwardly facing, concave, surface ofthe cone first layer, a resin and fibrous mixture or alternatively resin may be applied followed by a mat of material followed by another coating of resin to embed the material in the resin.

Shaping ofthe third layer may be achieved using a concave former, and a mandrel may again be inserted into the cylindrical part. The mandrel may be coated to preventthe mandrel adhering to the resin material of the third layer.

The invention will now be described with the aid of the accompanying drawings in which: FIGURE lisa diagrammatic cross sectional view of a loud speaker having a diaphragm in accordance with thefirstaspectofthe invention.

FIGURE 2 is a cross section through part ofthe diaphragm of the loud speakeroffigure 1,to an enlarged scale.

FIGURE 3 is a perspective view of the diaphragm of the loud speaker of Figure 1.

FIGURE 4 is a plan view of a pattern for use in manufacturing a firstlayerofthe diaphragm of the loud speaker shown in Figure 1.

Referring first to figure 1, a loud speaker 10 comprises a frame 11 including a circular rim 12with apertures to facilitate mounting the loud speaker 10.

The frame 11 supports a magnet assembly 16, and comprises a centre pole piece 17 with a central vent opening 17a, and a ferrite permanently magnetised ring 18, with an annularfaceplate 19aand backplate 1 9bto complete the magnetic circuit.

Instead of a continuous ring 18, the magnet could be made up of segments, or quadrants, if desired.

The loud speaker 10 further comprises a truncated conical diaphragm20tothelargerdiameterend of which an outer suspension comprising a flexible annular band 21 is secured, the band 21 also being secured to the rim 12. Atthesmallerdiameter end, the diaphragm 20 is supported by another band 23 which provides a further suspension.

The diaph ragm 20 further com prises a voice coil 22 adjacent the smaller diameter, truncated, end ofthe cone, to which a varying electrical signal is applied, and a cylindrical portion 14which is secured atthe smaller diameter end as hereinafter described.

Thus the diaphragm 20 will vibrate in response to changes in magnitude of the electrical signal.

The construction ofthe diaphragm 20 will now be described with reference to figures 2 and 3.

The diaphragm 20 comprises a core layer 30 which in the present example comprises blue Styrofoam having a density of between 32 and 48 kilograms per cubic metre (approximately 2 to 3 pounds per cubic foot) although could comprise another suitable foamed or fibrous plastics material which exhibits resilience so that the core layer absorbs unwanted vibrations and resonances.

Examples of other suitable core materials are: Urethane having a density of 32 kilograms per cubic metre (approximately2 pounds per cubic foot), or, PVC having a density of between 48 and 128 kilograms per cubic metre (approximately3to 8 pounds per cubic foot) or a Urethane/Polyester mixture having a density of between 72 and 160 kilograms per cubic metre (approximately 4.5 to 10 pounds per cubicfoot), or Polyurethane having a density of between 32 and 80 kilograms per cubic metre (approximately 2 to 5 pounds percubicfoot).

The minimum core thickness in the example shown is in the order of 1 millimetre adjacentthe smaller diameter, truncated end of cone shaped part of the diaphragm increasing to a maximum of about 10 mm adjacent the larger diameter, outer periphery of the diaphragm. Preferably the core layerthickness is not greaterthan 5 mm.

The core layer 30 is between two skin layers 31 and 32 of substantially identical construction. The skin layers 31 and 32 each comprise a glass fibre mat coated on both sides, and thus embedded in an epoxy resin, the layers 31 and 32 being bonded to the core layer30 bythe resin.

Other suitable materials for the skin layers are carbon fibres, graphite fibres, Kevlar, and these materials, may also be provided intheform of a mat or other woven lattice, orthese materials orthe glass fibre may be provided as loose rovings embedded in the synthetic resin material.

Further alternatively, a suitableskin material may be made by a mixture of such fibres.

As an alternative to the epoxy resin described polyester resin may instead be used.

The skin layers in theexample shown are preferably between 1 and 2 mm thick but could be anything between 0.5 and 5 mm thick as desired.

It can be seen from figure 3, that attached to the outer layer 32 ofthe conical shaped part of the diaphragm 20, is the cylindrical portion 14, which has a skirt of castellated form indicated at 15, which is adhered to the skin 32, the cylindrical portion 14 providing a circular opening in which the voice coil 22 (not shown in figure 3) and the centre pole piece 17 are accommodated, when the loud speaker is assembled.

Although as described, the diaphragm 20 comprises two substantially identical skin layers 31,32, between which is interposed a core layer 30, if desired only one skin layer may be provided, preferably on the outwardlyfacing surface ofthe core layer 30, ortwo skin layers of differing construction could be used. For example, the skin layers could be of different thick- ness, or of different materials, or both.

Although the invention has been described in relation to a conical diaphragm fora loud speaker, it will be appreciated that a diaphragm in accordance with the invention may be of other configurations, and may be used with any other electro-mechanical transducer as required, such as a microphone which produces a varying electric signal in response to sound waves which impinge upon the diaphragm.

Although as described the loud speaker is circular in plan view, the invention may be applied to an eliptical loud speaker or indeed a loud speaker having a diaphragm of any other shape.

Aconical truncated cone shaped diaphragm is however preferred, and a method of making the diaphragm20showninthefigures,will now be described.

Referring to figure 4, a pattern P is cut from a sheet of Styrofoam, the pattern comprising a substantially disc shape but having a keyhole shaped aperture A provided therein, the aperture A having a pair of converging edges 40 and 41 which extend from the circumference C to a central opening 0 which is of circular shape.

Adhesive is applied to both of the edges 40 and 41, and then the pattern is placed on a former and a clamping means applied to bring the edges 40 and 41 together to form the pattern Pinto a substantially truncated cone shape.

Afterthe adhesive has set, the clamping means is removed. Resin is then applied to the couter surface of the pattern P, which resin may be mixed with glass microspheres if required. The resin is spread thickly, and thereafter a pre-shaped mat of glass fibres is laid overthe pattern P on the resin and the outer surface of the mat is then coated on the outside with further resin our a resin and glass microsphere mixture.

Thus the layer 32 will beformed on the pattern with the mat embedded in the resin.

A mandrel is inserted in the smaller diameter end of the cone thus provided by the central opening O in the pattern, and a fu rther preshaped matofglassfibres has resin applied to the exterior surface ofthe mat.

Thereafter, one edge of the mat is cut to a castellated configurationto provide the skirt 15, and then the mat is wrapped around the mandrel to form the cylindrical part 14 ofthe diaphragm 20, the mat being adhered to the layer32 byvirtueofthe resin applied to the mat.

After the resin has set, which can typically take 24 hours or so, the mandrel is removed and the partly formed diaphragm is removed from theformerand placed in a further, concave, formerto allow the inner layer 31 ofthe diaphragm 20 to be applied.

The procedure described above for the application of layer 31 is repeated, that is a layer of resin which may contain glass micro spheres is applied to the concave inner surface of the pattern P, followed by a mat ofglass fibres cut to the desired shape. A further layer of resin is applied to embed the mat in the resin.

Amandrel is then inserted into cylinder part 14ofthe coneto maintaintheshapeofthepart 14.

In each case to preventthe mandrel becoming adhered to the resinous material, preferablythe mandrel is coated, for example with a plastic coating, which does not adhere to the resin.

Again, when the resin has set, the mandrel may be removed from the cone, and the cone may be removedfromtheformerandfinished by trimming to leave the diaphragm 20 substantially as shown in figure 3.

If required, the layers 31 and 32 may be coloured by mixing pigment with the resin priorto applying the resin.

Any alternative methods could of course be used to provide the diaphragm described, or alternative diaphragms in accordance with the invention.

The features disclosed in the foregoing description, inthefollowing claims, orthe accompanying drawings, expressed in their specific forms or in terms of a meansfor performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims (37)

1. A diaphragm for an electro-mechanical.transducer, the diaphragm comprising at least two superposed layers in contact, a first layer comprising a foamed orfibrous plastics material and a second layer comprising material embedded in or bonded buy a synthetic resin.

2. A diaphragm according to claim 1 wherein a third layer is provided comprising material embedded in or bonded by synthetic resin, with the foamed or fibrous plastics material layer interposed between the second and third layers.

3. A diaphragm according to claim 1 or claim 2 wherein the core layer has a density of between 24 and 1982 kilograms per cubic metre (approximately 1.5 to 12 pounds per cubic foot).

4. A diaphragm according to claim 3 wherein the core layer has a density lying in the range 32 to 160 kilograms per cubic metre (approximately 2 to 10 pounds percubicfoot).

5. A diaphragm according to any one of the preceding claims wherein the core layer is made of Styrofoam having a density of between 32 and 48 kilograms per cubic metre {approximately 2 to 3 pounds per cubic foot).

6. A diaphragm according to any one of claims 1 to 4wherein the core layeris madeof Urethane having a density of about 32 kilograms per cubic metre (approximately 2 pounds per cubic foot), or PVC having a density of between 48 and 128 kilograms per cubic metre (approximately 3 to 8 pounds per cubic foot), or a mixture of Urethane and polyester having a density of between 72 and 160 kilograms per cubic metre (approximately 4.5 to 10 pounds percubicfoot) or polyurethane having a density of between 32 and 80 kilograms per cubic metre (approximately 2 to 5 pounds per cubic foot).

7. Adiaphragm accordingtoanyoneofthe preceding claimswherein the core layer is at least one millimetre thick.

8. A diaphragm according to claim 7 wherein the core layer hasa thickness upto 10 mm.

9. A diaphragm according to any one ofthe preceding claims wherein the thickness of the core layervaries over the diaphragm.

10. Adiaphragm according to any one ofthe preceding claims wherein the second, or second and third skin layer material comprises glass fibres, carbon fibres, graphite fibres or Kevlar or a mixture of such fibres.

11. Adiaphragmaccordingtoclaim 10wherein the fibres are in the form of a woven lattice such as a mat.

12. Adiaphragm according to claim i0wherein the fibres are in the form of loose rovings embedded in or bonded bythe synthetic resin.

13. A diaphragm according to any one at claims 10 to 1 2wherein the density ofthefibres is constant over the diaphragm.

14. Adiaphragm according to any one of claims 10 to 12 wherein the resin in which the fibres are embedded or bywhich the fibres are bonded comprises a polyester or epoxy resin.

15. Adiaphragm according to any one ofthe preceding claims wherein the skin layer or layers is/are bonded to the core layer by the synthetic resin which the material ofthe skin layer or layers is embedded in or bonded by.

16. A diaphragm according to any one of claims 1 to 15 wherein the or each skin layer has a thickness between 0.5 and 5 mm.

17. A diaphragm according to claim 16 wherein the or each skin layer is between 1 mm and 2 mm thick.

18. Adiaphragm accordingto any one ofthe preceding claimsfora loud speakerwhich produces sound in responseto an electrical signal.

19. Adiaphragm accordingto any one ofthe preceding claims which is generally conical or a truncated cone shape.

20. A diaphragm substantially as herein before described with reference to and as shown in the accompanying drawings.

21. An electro-mechanical transducer comprising a diaphragm in accordance with any one of claims i to 20.

22. Atransducer substantially as hereinbefore described with referenceto and as shown in figure 1 of the accompanying drawings.

23. A method of making a diaphragm according to claim 19 including the steps of cutting a pattern from a sheet of foamed orfibrous material and forming the pattern between a former and a clamping means to providethefirst, core, layer, and thereafter bonding to the core layer a second, skin, layer comprising material embedded in or bonded by a synthetic resin.

24. A method according to claim 23 wherein prior toforming,the pattern comprises a substantially circular disc with a keyhole shaped aperture provided therein comprising a pair of converging edges extend ing fromthe circumference of the discto a circular portion of the aperture at the centre ofthe disc.

25. A method according to claim 24 wherein one or both ofthe edges ofthe aperture are coated with adhesive priorto forming and upon forming,the edges are brought into contactthe pattern thus being formed into a truncated cone first layer.

26. A method according to claim 25 wherein the second layer of material embedded in or bonded by a synthetic material is applied to the outwardly facing, convex, surface of the first layer by applying a resin and fibrous mixture, or applying resin followed by a mat of material, followed by another coating of resin to embed the material in the resin.

27. A method according to claim 26 wherein shaping ofthe second layer is assisted by a conical convex former.

28. A method according to any one of claims 23 to 27 which includes the steps of adhering to the convex surface of the diaphragm a cylinder of material which surrounds the small diameter end ofthe cone.

29. A method according to claim 28 wherein to assist shaping of the cylinder of material, a mandrel is used.

30. A method according to claim 29 wherein the mandrel is coated with a material to prevent the mandrel becoming adhered to the resin ofthe second layer.

31. A method according to anyoneofclaims23to 30 which includes providing a third, skin, layer comprising a material embedded in or bonded buy a synthetic resin.

32. A method according to claim 31 wherein the third layer of material is provided by applying to the inwardlyfacing, concave, surface ofthe cone first layer, a resin and fibrous mixture or resin followed by a mat of material followed by another coating of resin to embed the material in the resin.

33. A method according to claim 32 wherein shaping ofthe second layer is assisted by a concave former.

34. A method according to claim 22 or claim 23 wherein during shaping ofthethird layer, a mandrel is inserted into the cylindrical part.

35. A method according to claim 34 wherein the mandrel is coated to prevent the mandrel adhering to the resin material ofthethird layer.

36. A method of manufacturing a diaphragm substantially as hereinbefore described with reference to the accompanying drawings.

37. Anynovelfeatureornovelcombination of features disclosed herein and/or shown in the accompanying drawings.