Transducers

Abstract

An electro-acoustic transducer with a voice coil which is encapulated in a setting material and bonded to the diaphragm to provide an improved temperature rating.

Description

The present invention relates to voice coils for electro-acoustic transducers, and particularly to loudspeaker voice coils having an improved temperature rating.

One of the problems associated with loudspeaker coils is that of heat dissipation since the temperature of a coil rises considerably in use. In general, loudspeaker coils are either wound on a former which may be made of paper, aluminium foil or a glass-filled epoxide resin, or alternatively the coils may be "self-supporting." Self-supporting coils are formed without a former but are held together with a bonding material. At higher temperatures, say in the region of 160.degree.C - 180.degree.C, bonding materials tend to soften and, under the influence of the magnetic field of the loudspeaker, the turns of the coil tend to separate. The term "temperature rating" as used in this Specification will be understood to mean the temperature which a material is able to withstand for a given period without significant deterioration.

If the loudspeaker coil is wound on a former the drive from the coil to the diaphragm of a loudspeaker is transmitted via the former, which normally has a smaller diameter than the winding. This indirect transmission of drive from a coil via a former to a diaphragm does not provide the best possible coupling between the winding of the coil and the diaphragm of the loudspeaker.

The present invention seeks to provide a loudspeaker coil in which both of the abovementioned disadvantages can be overcome, and which is both simple to manufacture and robust in use.

According to the present invention, there is provided a voice coil for an electro-acoustic transducer, comprising a plurality of turns of insulated wire, in which the coil is encapsulated in a setting material, the encapsulation including a portion extending axially of the coil for forming a driving connection between the coil and a diaphragm of the transducer.

If the turns of the winding of the coil are held together by a bonding agent it is preferred that the encapsulating material has a higher temperature rating than the bonding material, and if the turns of the coil are supported on a former it is preferred that the encapsulating material has a temperature rating at least as high as the material of the former. Preferably, the temperature rating of the encapsulating material is at least comparable to that of the insulation material of the coil. In embodiments of the invention using self-supporting coils, difficulties due to softening of the bonding material are reduced since the encapsulation material will provide structural support for the turns of the winding even at a temperature higher than that at which the bonding material starts to soften.

The encapsulating material can be used to transmit drive directly to the diaphragm of a loudspeaker whether the coil is of the self-supporting type or wound on a former. The axial extension of the encapsulation may be shaped in such a way that it can transmit drive to the diaphragm efficiently.

Preferably the extension of the encapsulating material for transmitting drive to the diaphragm of a loudspeaker is shaped at the end to assist in forming a secure connection between the coil and the diaphragm. In this way, for conical diaphragm loudspeakers, a better coupling between the winding and the diaphragm cone can be achieved than in the previous arrangements with conventional coils wound on formers where the drive was transmitted via the former. In the case of self-supporting coils where drive could any way be transmitted directly from the windings to the diaphragm the encapsulation does not greatly improve the coupling between the coil and the diaphragm but on the other hand does overcome the problem, mentioned above, due to softening of the bonding material at higher temperatures.

In an alternative embodiment of the invention the axial extension of the encapsulation is shaped to form at least part of a diaphragm of a loudspeaker. If desired the extension could be shaped to form the whole diaphragm of a loudspeaker so that the coil of the loudspeaker would be encapsulated within a unitary structure including the diaphragm. Such an arrangement would obviously permit extremely good coupling between the coil and the diaphragm.

The encpasulation can be undertaken for coils having any number of turns arranged in any number of layers although conveniently the winding has from one to four layers of turns.

Various setting materials are suitable for use as the encapsulating material although phenolic resin and glass-filled epoxide resin are both particularly suitable. The encapsulating material may be provided with a filler to improve the thermal conductivity thereof so that heat is more effectively radiated into the magnet structure of the loudspeaker. In the case of glass-filled epoxide resin the filler for improving thermal conductivity may be used in addition to or in place of glass fibres.

If a winding is supported on a former the former may be dispensed with after encapsulation or may be originally of a minimum length to support the coil before encapsulation and retained after encapsulation. Alternatively, a former may be provided and used to transmit drive to the diaphragm of a loudspeaker if this is desired.

Normally, the encapsulated coil will be coated on the outside and the two ends, although for certain applications it may be desirable to coat only the outside and one end.

According to another aspect of the present invention there is provided a method of manufacturing an encapsulated coil for an electro-acoustic transducer comprising the steps of winding a selected number of turns of wire, providing support for the turns, placing the turns on a mandrel in a moulding cavity of an injection moulding machine, and injecting an encapsulating material into the moulding cavity to surround the coil.

If the winding is of the self-supporting type the method preferably further comprises the step of cooling the moulding cavity to keep the temperature thereof below a temperature at which the bonding agent of the self-supporting winding will be affected.

The present invention also comprehends a loudspeaker when provided with an encapsulated coil as defined above.

Two embodiments of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which

FIG. 1 is a longitudinal section of a conventional cone/coil assembly and

FIG. 2 is a longitudinal section of a first embodiment of the invention.

FIG. 3 is a longitudinal section of a second embodiment of the invention.

Referring to FIG. 1, the conventional assembly comprises a coil former 11, on which is wound a coil 12. The wire of the coil is normally coated with adhesive, which serves the purpose of bonding the turns of the coil to each other and also to the former. The layer of adhesive bonding the coil 12 to the former 11 is indicated at 13. The former 11 is inserted into the neck of the cone 14 and is held in position by a further layer of adhesive 15.

It will be seen that there is no mechanical constraint which would prevent the turns of the coil from separating under the influence of a magnetic field if the current in the coil raises the temperature to the point at which the adhesive softens. It will also be apparent that the drive between the coil and the cone is transmitted via the former, which has a smaller diameter than the coil, and the drive is to that extent indirect. In addition, two layers of adhesive are involved in the transmission of energy, and loss of energy can take place in the adhesive material.

In the embodiment of the invention shown in FIG. 2, a former 21 carries a winding 22, and although there is a layer of adhesive 23 between the coil winding 22 and the former 21 this merely facilitates the handling of the coil until it has been encapsulated. The winding 22 is enclosed in an encapsulation 24 the material of which may be a phenolic resin or a glass-filled epoxide resin or any other suitable temperature resistant setting material. The encapsulation 24 has a part 24a extending axially of the coil in driving connection with a diaphragm 25 of the loudspeaker. The function of the encapsulation 24 is first to provide a mechanical restraint for the turns of the coil 22, and secondly, via the axial extension 24a, to provide drive, at the same diameter as the coil 22, to the loudspeaker cone 25. A layer of adhesive 26 is used to bond the axial extension 24a of the encapsulating material 24 to the cone 25.

It will be seen that, should the adhesive 23 of the coil 22 become softened due, for example, to overheating during use, the encapsulating material 24, being more resistant to heat, will hold the turns of the coil 22 together so that the loudspeaker will continue to function properly. It will also be seen that, in contrast to the coil of FIG. 1, only one layer of adhesive is involved in the transmission of the drive from the coil, and that much of the adhesive transmits drive by compression or tension at the conical face of the diaphragm 25, rather than in shear, which is the case with the layers 13 and 15 of the prior art device shown in FIG. 1.

Further, it can be seen that, in the encapsulated form, the coil of FIG. 2 does not rely on the former 21 for support. If the coil 22 were of the self-supporting type, the result after encapsulation would have much the same properties as the embodiment shown in the drawing.

Referring now to the embodiment of FIG. 3, a coil 31 is initially supported on a former 32, and in this case the encapsulating material 33 has an axial extension 34a and a part 34b formed as a portion of a diaphragm. This embodiment has similar advantages to that of FIG. 2, and in addition it dispenses completely with glued joints. Again the former 32 is only used as a support in the initial stages of manufacture and could be omitted if a self-supporting coil were employed.

Any of the embodiments described above may be manufactured by injection moulding. The winding 22 or 31 is first formed and then located in the moulding cavity of an injection moulding machine on a mandrel which is preferably of the expanding type to ensure that the coil does not move during the moulding process, and also to ensure that the roundness of the coil is maintained during moulding. An encapsulating material of a thermosetting type, such as a phenolic resin is then injected into the moulding cavity which is cooled by water cooling so that the bonding material or the material of the former is not deleteriously affected by the temperature.

The use of a glass-filled epoxide resin for the encapsulating material has various advantages over phenolic resins. For example, it may be injection moulded at a lower pressure than phenolic resin, for example at 200 lbs. per square inch. Moreover glass-filled epoxide resin is less brittle than phenolic resins.

Claims

I claim:

1. A voice coil for an electro-acoustic transducer, said coil comprising a plurality of turns of insulated wire encapsulated in a setting material forming an encapsulation, said encapsulation including a portion extending axially of said coil and forming a connection between said coil and a diaphragm of an electro-acoustic transducer.

2. The coil of claim 1 wherein the turns of said winding of said coil are held together, prior to encapsulation, by a bonding agent, and said encapsulating material has a higher temperature rating than said bonding material.

3. The coil of claim 1 wherein turns of said coil are supported on a former and said encapsulating material has a temperature rating at least as high as the material of said former.

4. The coil of claim 1, wherein the temperature rating of said encapsulating material is at least comparable to that of the insulation material of said windings of said coil.

5. The coil of claim 1, wherein said axial extension of said encapsulating material is shaped to form at least part of a diaphragm of an electro-acoustic transducer.

6. The coil of claim 1, wherein said encapsulating material is a phenolic resin.

7. The coil of claim 1, wherein said encapsulating material is a glass filled epoxide resin.

8. The coil of claim 1 wherein said encapsulating material is provided with a filler to improve the thermal conductivity thereof.

9. A method of manufacturing an encapsulated coil for an electro-acoustic transducer, comprising the steps of:

winding a selected number of turns of wire,

providing support for said turns,

placing said turns on a mandrel in a moulding cavity of an injection moulding machine,

injecting an encapsulating material into said moulding cavity to surround said coil, allowing said encapsulating material to set, and removing said encapsulated coil from said mould.

10. The method of claim 9 wherein said support provided for said turns of said coil prior to encapsulation is a former of suitable material.

11. The method of claim 9 wherein the support provided for said turns of said coil prior to encapsulation is in the form of a bonding agent.

12. The method of claim 9 further comprising the step of cooling said moulding cavity to keep the temperature thereof below a temperature at which said support for said turns will be affected.

13. A loudspeaker fitted with the encapsulated coil of claim 1.