LOUDSPEAKERS

Abstract: A tweeter has its voice coil former and diaphragm integrally formed as a single unit. The magnet, pole piece and plate are encapsulated as a complete magnetic circuit in an injection moulded casing. A robust, inexpensive and easy to manufacture product is achieved.

Description: LOUDSPEAKERS This invention relates to loudspeakers, especially small high frequency loudspeakers known as tweeters. Tweeters are manufactured in very large numbers (tens of millions per annum) for vehicle audio systems. Vehicle manufacturers insist on high performance and high reliability at minimum unit cost. Existing tweeters typically have a loudspeaker cone or diaphragm and a separate voice coil former of a different material which is adhesively bonded to the diaphragm. This bonding operation is difficult to perform consistently whether undertaken manually or automatically, leading to increased costs and an unacceptably high rejection rate. The cost of the conventional product is further increased by the need to assemble a number of small parts, notably the magnet, a pole piece or field shaper, an electrical connector and a capacitor or other cross-over circuit element. The present invention in at least its preferred embodiment seeks to overcome or at least reduce these difficulties. In one aspect the invention provides a loudspeaker diaphragm having a voice coil former integrally formed therewith in a single piece. In another aspect the invention provides a loudspeaker having a diaphragm and a voice coil former integrally formed together. The voice coil may cooperate with a magnetic circuit which is encapsulated in a moulded structure and defines an annular gap wherein the voice coil is received. The magnetic circuit may comprise a magnet, a pole piece and a member surrounding the pole piece to define the annular gap. A connector may be encapsulated in or formed integrally with the moulded structure. A receptacle for a capacitor or other components may be incorporated into the encapsulated parts. There may be means for mechanically locating the diaphragm in place on the moulded structure. The locating means may be a cover or grill which is attached to the moulded structure. The cover or grill may be acoustically transparent, or may include means (eg. reflecting surfaces) for directing the sound produced by the loudspeaker. It may also be arranged to horn-load the diaphragm. The invention also provides a method of manufacturing a loudspeaker comprising encapsulating a magnetic circuit of the loudspeaker is a moulded structure so as to define an annular gap, forming a diaphragm and a voice coil former as an integral unit, disposing a voice coil on the former, assembling the diaphragm and voice coil with the encapsulated structure so that the voice coil is in the annular gap and cooperates with the magnetic circuit, and mechanically locating the diaphragm in place on the moulded structure. The method may also include mechanically locating the diaphragm by engaging a retaining part with the moulded structure. The method may comprise also forming a dust cover integrally with the diaphragm. The diaphragm and the coil former may be hot-pressed from thermoplastic sheet material. The moulded structure may be produced by injection moulding. The method may also comprise encapsulating a connector in the moulded structure. Alternatively the connector can be formed as part of the moulding. During winding of the coil, a mandrel may be inserted into the bore thereof to support the former and retained therein by a magnetic field. A further aspect of the invention is a method of winding a coil on a former of a loudspeaker diaphragm having a bore comprising inserting a mandrel in the bore to support the former and winding the coil on the supported former, characterised by inserting and/or retaining the mandrel in the bore by a magnetic field. This aspect also provides apparatus for winding a coil on a former of a loudspeaker having a bore comprising a mandrel insertable into the bore to support the former, and means for providing a magnetic field to insert and/or retain the mandrel in the bore during winding of the coil. Of course whether the former is placed on a fixed mandrel, or a mandrel is moved to enter the bore is of the former a matter of choice; both are comprised in the term "inserting a mandrel in the bore". The means for providing a magnetic field may be a permanent magnet. There may be support structure for the diaphragm in which the permanent magnet is located. The support structure may be rotatable for winding the coil on the former. The invention now will be described merely by way of example with reference to the accompanying drawings in which FIGURE 1 shows a section through a tweeter according to one embodiment of the invention; FIGURE 2 shows part of the structure of Figure 1 ; FIGURE 3 is a perspective view of an alternative form of the tweeter of figures 1 and 2. FIGURE 4 shows another form of a tweeter diaphragm according to the invention; FIGURE 5 illustrates manufacture of the diaphragm of figure 4; FIGURE 6 shows the winding of a voice coil on to the diaphragm of figure 5; FIGURE 7 is a scrap end view of part of figure 6; and FIGURES 8 and 9 show variations on the concept of figure 6. The tweeter consists an annular cylindrical ferrite magnet 10 disposed upon a shaped or sintered pole piece 12 having an axial portion 14 extending through the bore of the magnet and a disc-shaped portion 16. An annular cover or top plate 18 preferably but not essentially similar in shape to the disc portion 16 of the pole piece is disposed on top of the magnet 10 and defines a narrow annular gap 20 with the circumference of the axial portion 14 of the pole piece. The magnet, pole piece and the cover plate together define a magnetic circuit which directs magnetic flux through the gap 20. The pole piece is recessed at 22, 24 to reduce the amount of material required; in high volume applications even small material savings are worthwhile, and the recesses also assist in concentrating the magnetic field where it is needed. The tweeter has a diaphragm or cone 26 (shown separately in figure 2) which has a cylindrical annular extension 28 integrally formed therewith. The diaphragm is thermo-formed or vacuum-formed from polycarbonate, Kapton (trade mark) or other material conventionally used for this purpose. The cylindrical portion 28 constitutes a former on which a voice coil 30 is wound and lacquered to fix it adhesively to the former. The former 28 and coil 30 are received in the annular gap 20 so that an audio signal applied to the coil causes displacement of the former and the diaphragm in a conventional manner, the outer rim of the diaphragm being restrained against axial movement by an acoustically transparent cover 32 or it may be stuck to a casing 42 of the tweeter, both described further hereafter. A dome-shaped dust cap 40 is adhesively fixed to close the former bore 28. The magnet 10, pole piece 12 and cover plate 18 are encapsulated in an injection- moulded casing 42 of polyethylene, polystyrene or other suitably tough plastics or elastomeric material. This casing locates the components relative to each other without the need for them to be assembled and fastened together mechanically in separate assembly operations. The ends of the coil 30 are routed via a channel moulded in the casing 42 extending from the joint between the casing and the cone 26 to a connector 38 having terminals 39. The connector 38, may be encapsulated during the moulding operation, or may be formed as an integral part of the moulding. A recess is provided in the external surface of the casing to accommodate a capacitor and if necessary a resistor as a cross-over network. The casing 42 has a circumferential recess or groove 43, and is tapered so that its outer portion 44 is of frusto-conical form. The cover 32 comprises an inner spider 46 and an outer decorative grille 47, both being injection mouldings. The spider has a number of angularly spaced-apart axially- extending legs 48 with radially inwardly-directed projections 50. The legs are displaceable outwardly to enable the cover to be expanded over the frustoconical surface 44 until the projections snap into the groove 43 and the cover 32 thereby is retained in place. The spider has an inner axially extending ring which contacts the top surface of the casing 42 and defines therewith a recess 52 in which the outer rim of the diaphragm 26 is received and restrained against axial movement. The cover as illustrated is acoustically neutral (transparent). If desired its grill- defining portions 33 may be angled to act as reflectors and impose some directivity and/or horn loading on the sound emitted by the diaphragm. Improved performance or conversion efficiency may result. Alternatively the part 32 may be simplified to provide just the spider 46 or other peripheral ring to retain the diaphragm in the moulded structure 42. Figure 3 shows a modified form of the tweeter of figures 1 and 2. In this embodiment the ends 34, 36 of the coil 30 are routed via a channel moulded in the casing 42 and soldered or wire-wrapped to tags 35 mounted externally on the casing 42. These tags are crimped to two wires 37 leading to the connector 38 (of different form to that of figure 1), one directly and the other via a capacitor 41. The tweeter is manufactured broadly in the following stages a) The diaphragm 26 and former 28 are vacuum-formed or thermo-formed as a single unit. The dust cap 40 is vacuum-formed or thermo-formed and glued around its edge to the inside of the diaphragm. b) The coil 30 is wound on to the former and a lacquer or other adhesive applied to fix it thereto. The coil ends 34, 36 are left of substantial length. c) The magnet 10, pole piece 12 and cover plate 18 are cast, formed or sintered in conventional fashion by a specialist subcontractor. d) The magnet, pole piece and cover plate are assembled in an injection moulding tool which serves also to jig the parts correctly. The casing 42 is then moulded around the jigged components. The top and bottom surfaces respectively of the cover plate 18 and the pole piece 12 preferably are curved as shown to facilitate the flow of the molten plastics material and to provide the effective magnetic circuit already described. e) The magnet 10 is magnetised and ferromagnetic fluid such as FERRO FLUID (trade mark) is injected to fill the gap between the magnet 10 and the pole piece 12. The fluid acts as a damping medium, and due to the fine iron particles dispersed therein improves the efficiency and power-handling capacity of the loudspeaker. f) The diaphragm is placed in position and the coil leads are led in a predetermined path to the connector block and left accessible. The ends of the coil are terminated as already described and the capacitor and other external components (if any) are fitted. g) The cover 32 is snap-fitted on to the casing and the unit is subjected to a functional test, and packed. Figure 4 shows an alternative form of the tweeter diaphragm or cone 26. The cone 26 and dust cap 40 are deep drawn by thermoforming (hot pressing) from sheet material such as that noted for the figure 1 embodiment so that a double-walled substantially cylindrical coil former section 28 is provided, the parts 26, 28 and 40 all being integrally formed with each other as a single component. The coil 30 (shown diagrammatically-it comprises many more turns than illustrated) is wound on the outer surface of the former as described hereafter. Referring to figure 5, thermoplastic sheet material 58 for the manufacture of the composite diaphragm/former/dust cap is unwound from a spool 60 and advanced by an indexing feed apparatus 62 to a pair of heater blocks 64. The blocks heat and thereby soften the material 58, which then passes to a press 66 having at least one set of dies 68. Here diaphragms are pressed or drawn at intervals in the softened sheet material which then is loosely wound on to a take-up mechanism 70 prior to the diaphragms being stamped-out from the sheet. Alternatively they may be separated from the sheet immediately downstream of the press 66. When each diaphragm is separated from the sheet a pair of small diametrically- opposite cut-outs 72 (fig. 7) are made in its periphery, for purposes described hereinafter. To wind the coil on the former 28, the diaphragm 26 is mounted on a rotatable winding head 74 (fig. 6), the end of which is shaped to conform to the front surface of the diaphragm. Pegs 76 engage the cut-outs 72 so that the diaphragm rotates with the winding head. Because the bore of the former is blind (ie. it is closed by the integral dust cap) it is not possible to retain the former on a mandrel passing through the bore. To solve this difficulty the winding head contains a powerful (eg. neodymium) magnet 78 disposed on its axis, adjacent the apex of the dust cap 40. A ferromagnetic mandrel or plug 80 is inserted in the bore of the former 28 and retained by the field created in the bore by the magnet 78, the mandrel being shaped to provide support for the tubular former whilst the coil is wound on to it. Varnish or other suitable adhesive is applied to the external surface of the former, and the coil is wound on to it by conventional equipment, the winding head being rotated for this purpose. The mandrel is then withdrawn and the diaphragm is removed from the winding head. During subsequent assembly of the tweeter, the ends of the coil are taken out through the cut-outs 72 and led to the connector 39, as already described. Magnetic force can be employed to locate the diaphragm and former on the winding head even if the bore is not blind. Thus, as shown in figure 8 a mandrel 82 can be inserted through the bore of the 84 of the former and held in place by a magnet 86 within the winding head (not shown). Similarly, in figure 9 a mandrel 98 is inserted from the other side of the former and is held in place by a magnet 100 which attracts primarily a flange portion 102 of the mandrel rather than the portion thereof in the bore of the coil former. Here the magnetic field passes primarily through the diaphragm rather than through the bore. Indeed in this example the major part of the mandrel may be of non-magnetic material, provided magnetic material is present in the circumferential portion to be attracted to the magnet. The inverse arrangement may also be utilised; the mandrel 98 may be fixed, with permanent magnetic material at least around its periphery and the diaphragm and former can be held in place by a profiled ring or annulus shaped as the part 100 in figure 9. In the specifically described embodiments, the following advantages may be obtained: - the integrally formed diaphragm and voice coil former may be lighter than a fabricated part and therefore require less force from the encapsulated magnet structure, which can therefore be smaller and therefore cheaper. - the assembly process is very much simplified and lowered in cost since the parts (diaphragm assembly and motor unit) can be clipped together instead of being glued together. - the integrally formed diaphragm and voice former may be produced to very exact tolerances, due to its manufacturing process, as can the magnet structure by its moulding process. This allows the subsequent assembly to be effected without any jigging and without the use of skilled labour the tweeter can be manufactured with a minimum of manual operations, and a robust but inexpensive product results. To summarise, a tweeter has its voice coil former and diaphragm integrally formed as a single unit. The magnet pole pieces and connector are encapsulated in an injection moulded casing. A robust, inexpensive and easy to manufacture product is achieved. Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

Claims: CLAIMS 1. A loudspeaker diaphragm having a voice coil former integrally formed therewith. 2. A loudspeaker having a diaphragm and a voice coil former integrally formed together. 3. A diaphragm or loudspeaker as claimed in claim 1 or claim 2 comprising a dust cap also formed integrally with the diaphragm and coil former. 4. A loudspeaker as claimed in claim 2 or 3 wherein the voice coil cooperates with a magnetic circuit which is encapsulated in a moulded structure and defines an annular gap wherein the voice coil is received. 5. A loudspeaker as claimed in claim 4 wherein the magnetic circuit comprises a magnet, a pole piece and a member surrounding the pole piece to define the annular gap- 6. A loudspeaker as claimed in claim 4 or 5 comprising a connector encapsulated in or formed integrally with the moulded structure. 7. A loudspeaker as claimed in claim 4, 5 or 6 comprising means for mechanically locating the diaphragm in place on the moulded structure. 8. A loudspeaker as claimed in claim 7 wherein the locating means is a cover or grill which is attached to the moulded structure. 9. A loudspeaker or diaphragm as claimed in any preceding claim, being a tweeter or a tweeter diaphragm. 10. A method of manufacturing a loudspeaker comprising encapsulating a magnetic circuit of the loudspeaker in a moulded structure so as to define an annular gap, forming a diaphragm and a voice coil former as an integral unit, disposing a voice coil on the former, assembling the diaphragm and voice coil with the encapsulated structure so that the voice coil is in the annular gap and cooperates with the magnetic circuit, and mechanically locating the diaphragm in place on the moulded structure. 11. A method as claimed in claim 10 comprising mechanically locating the diaphragm by engaging a retaining part with the moulded structure. 12. A method as claimed in claim 10 or 11 comprising also forming a dust cover integrally with the diaphragm. 13. A method as claimed in any of claims 10 to 12 wherein the diaphragm and coil former are hot-pressed from thermoplastic sheet material. 14. A method as claimed in any of claims 10 to 13 wherein the moulded structure is produced by injection moulding. 15. A method as claimed in any of claims 10 to 14 comprising encapsulating or forming a connector integrally with the said moulded structure. 16. A method as claimed in any of claims 10 to 15 wherein during winding of the coil a mandrel is inserted into the bore thereof to support the former and is retained therein by a magnetic field. 17. A method of winding a coil on a former of a loudspeaker diaphragm having a bore comprising inserting a mandrel in the bore to support the former and winding the coil on the supported former, characterised by inserting and/or retaining the mandrel in the bore by a magnetic field. 18. Apparatus for winding a coil on a former of a loudspeaker having a bore, in accordance with the method of claim 16 or 17 comprising a mandrel insertable into the bore to support the former, a d means for providing a magnetic field to insert and/or retain the mandrel in the bore during winding of the coil. 19. Apparatus as claimed in claim 18 wherein the means for providing a magnetic field is a permanent magnet. 20. Apparatus as claimed in claim 19 comprising support structure for the diaphragm in which the permanent magnet is located. 21. Apparatus as claimed in claim 20 wherein the support structure is rotatable for winding the coil on the former.

Inventors: PEAKE KEITH ROBIN, SCAIFE STEVEN, PEAKE, KEITH, ROBIN, SCAIFE, STEVEN
Applicants: GOODMANS LOUDSPEAKERS LIMITED, PEAKE KEITH ROBIN, SCAIFE STEVEN, PEAKE, KEITH, ROBIN, SCAIFE, STEVEN
Application year: 1998
Main classification: H04R9
IPC codes: H04R9, H04R31