GB1604933A - Voice coils for loudspeakers - Google Patents

Description

(54) IMPROVEMENTS IN VOICE COILS FOR LOUDSPEAKERS (71) We, MORDAUNT-SHORT LIMITED, a British company, of Durford Mill, Petersfield, Hampshire, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to voice coils for loudspeakers and to methods for producing such coils.

The most common type of loudspeaker transducer is the moving coil type having a cone or diaphragm which is held in position by a surround which secures its circumference to a chassis. A cylindrical voice coil is attached to the centre of the cone or diaphragm and suspended co-axially in an annular permanent magnetic field by a concentric surrounding suspension ring.

When an audio frequency sigal from an amplifier is fed through the coil, the coil and cone oscillate along the axis of the coil in response to the input signal according to Fleming's Rule.

A conventional moving coil loudspeaker will now be described with reference to the first two figures of the accompanying drawings in which: Figure 1 is an axial section through a conventional moving coil loudspeaker; and Figure 2 is a part-sectional side elevation of a conventional loudspeaker voice coil, the diameter of the wire being increased, and the number of windings reduced, for the sake of clarity.

As shown in Fig. 1 a conventional moving coil speaker has a permanent magnet assembly 10 comprising a magnet ring 11, a central pole piece 12 and an annular pole ring 13, the last two members together defining an annular gap 14 through which extends a radial permanent magnetic field.

Mounted on the ring 13 is a generally coincal chassis 15 within the mouth of which is affixed the outer edge of an external roll surround 16 to the inner edge of which the outer edge of a speaker cone 17 is in turn secured. The apex of the cone 17 is truncated to form a central circular aperture in which is secured one end of a coil former 18 which is suspended by a corrugated suspension ring 19. The open end of the former 18 is protected by a dust cover 20 extending across the inner end portion of the cone 17.

The other end of the former 18 is wound with an external coil 21. When e.g. an audio frequency signal is fed through the coil 21 the former 18 and associated cone 17 are caused to oscillate axially in the directions indicated by the double-headed arrow 22.

The conventional voice coil 21 is shown in more detail in Fig. 2 and consists of an inner layer 23 of insulated conducting e.g. copper wire closely wound on the former 18 with an outer layer 24 wound over the inner layer 23, such that the coils of the outer layer 24 are seated in the grooves formed by adjacent windings of the inner layer 23. Leadout wires 25, 26 extend along the outer surface of the former in an axial direction.

The majority of voice coils consist of two such above-described layers of insulated copper wire closely wound on the outside surface of a cylindrical former. The output capability of a transducer is dependent on its efficiency: the lower the efficiency, the greater the power input required to drive the transducer, and so the greater the heating effect on the coil of the current passed through it. The efficiency of a transducer depends on many factors but in order to attain optimum efficiency the voice coil must be kept as light and as thin as possible to move freely within the smallest possible air gap between the concentric poles of the magnet assembly. As the power input to the transducer is increased, the heating effect in the coil increases and the temperature Of the voice coil is raised. With current adhesives and materials technology it is possible virtually to eliminate mechanical failure of the voice coil or former, or both, caused by high operating temperatures. The sole remaining cause of failure is the electrical breakdown of the coil itself at high power input levels. This usually takes the form of partial or complete short circuiting of the coil due to the breakdown of the insulation between the two layers of the coil (at a point where temperature rise and voltage difference produce the most severe conditions). followed rapidly by burn-out, which may possibly also damage the driving amplifier.

Occurrence of this type of failure can be reduced, but not eliminated, by either: (a) using wire having a higher grade of insulation, or (b) interposing an interleaved insulating layer between the two winding layers.

However, each of these methods has its disadvantages: (i) Both methods increase the thickness and mass of the coil, thus sacrificing efficiency; (ii) Method (b), which involves thermally insulating the inner layer of the coil, results in higher temperatures than would otherwise be experienced in that layer, with consequent premature breakdown or mechanical deformation.

According to the present invention there is provided a voice coil for a loudspeaker, the coil comprising an inner coil winding and an outer coil winding both rigidly bounded to an interposed formed having a longitudinal slit through which passes a length of wire connecting the inner and outer windings.

The construction according to the invention confers the following benefits:- (a) It improves the electrical breakdown characteristics of the coil so that higher powers can be handled and consequently high operating temperatures reached; indeed, the characteristics are improved to such an extent that mechanical failure of the coil former material and/or bonding agent can be expected to occur before electrical breakdown (partial short circuit) of the coil.

(b) It gives improved immunity to "thermal shock". During operation, temperature rises in the voice coil can be extremely rapid and severe; with conventional voice coils this can result in deformation of the coil formed and/or the winding before the insulation has time to deteriorate significantly, thus hindering free movement of the coil within the air gap between the magnet poles.

(c) It engenders similartemperaturerises in both layers of the coil, so reducing the risk of "hot-spots" which cause localised breakdown.

(d) It imparts to the voice coil and former a greater mechanical rigidity due to the sandwich-type construction.

(e) It enables the coil former to perform the additional function of an interleaving layer without significantly increasing the thickness of the coil.

(f) It enables both layers of winding to radiate directly to air and to be well ventilated.

These benefits are conferred at the expense of only a minimal increase in mass due to the very slightly greater volume of bonding agent required; and only a minimal increase in coil thickness due to the second layer no longer being able to sit between adjacent turns of the first layer.

From whatever materials the former is made, and whatever grade of insulation wire is used, a sandwich-type voice coil according to the invention will have an improved performance when compared with a conventionally-wound voice coil constructed from the same materials. In specific terms, a sandwich-type voice coil can accept a higher input power level than a similar conventionally constructed voice coil, as it can operate at higher temperatures without undue electrical or mechanical stress.

The coil according to the present invention operates equally well with or without an inner core magnet or pole piece.

According to the present invention there is also provided a method of producing a voice coil for a loudspeaker. the coil having an inner layer of coil winding, an outer layer of coil winding and a former interposed between the two layers, which method comprises winding the inner layer on a mandrel, placing a coil former having a longitudinal slit over the inner layer such that a connecting length of wire between the inner and outer layers passes through the slit, and winding the outer layer on the former, bonding agent being applied to bond the layers rigidly to the former.

The voice coil may be wound, using conventional coil winding techniques, by winding the inner layer, placing a prefabricated C-section former (i.e. a former having a circularly cylindrical cross-section with a longitudinal slit in its wall) over it, and then winding the outer layer. The connection between the two layers passes through the narrow slit in the C-section former. To facilitate removal of the finished coil assembly from the mandrel, a non-stick temporary former may be placed on the mandrel before winding the first layer, and discarded after extraction of the mandrel. Alternatively, a collapsible mandrel may be used with or without a disposable temporary former. In all cases the adhesive used must allow an initial temporary bond between the wire and the coil former to become effected to permit extraction of the mandrel without disturbing the coil windings. The mandrel preferably has a conventional PTFE coating.

An embodiment of the present invention will now be described, by way of example only, with reference to Figure 3 of the accompanying drawings which is an elevation, similar to that of Fig. 2, of a coil according to the present invention.

The voice coil 30 shown in Fig. 3 comprises an inner wound layer 31 of conventional insulated copper wire bonded to the inner cylindrical surface of a C-section former 32 having a longitudinal slit 33 and a similar co-extensive outer layer 34 wound on the outer surface of the former 32. The layers 31 and 34 are continuous, the connecting length of wire 35 between the two layers passing through the slit 33 as does an integral lead-out wire 36 extending from the other, upper end of the inner layer 31. An integral lead-out wire 37 extends from the upper end of the outer layer 34.

WHAT WE CLAIM IS: 1. A voice coil for a loudspeaker, the coil comprising an inner coil winding and an outer coil winding both rigidly bonded to an interposed former having a longitudinal slit through which passes a length of wire connecting the inner and outer windings.

2. A coil as claimed in Claim 1, in which a lead-out wire from the inner coil winding also passes through said slit.

3. A voice coil as claimed in Claim 1 and substantially as herein described.

4. A voice coil for a loudspeaker, substantially as herein described with reference to Figure 3 of the accompanying drawings.

5. A method of producing a voice coil for a loudspeaker, the coil having an inner layer of coil winding, an outer layer of coil winding and a former interposed between the two layers, which method comprises winding the inner layer on a mandrel, placing a coil former having a longitudinal slit over the inner layer such that a connecting length of wire between the inner and outer layers passes through the slit, and winding the outer layer on the former, bonding agent being applied to bond the layers rigidly to the former.

6. A method as claimed in Claim 5, wherein the mandrel is collapsible to facilitate the removal of the voice coil therefrom.

7. A method as claimed in Claim 5 or 6, in which a temporary former is placed over the mandrel on to which the inner layer is wound and which is subsequently removed.

8. A method as claimed in Claim 5 and substantially as herein described.

9. A voice coil produced by a method as claimed in any one of Claims 5 to 8.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. similar co-extensive outer layer 34 wound on the outer surface of the former 32. The layers 31 and 34 are continuous, the connecting length of wire 35 between the two layers passing through the slit 33 as does an integral lead-out wire 36 extending from the other, upper end of the inner layer 31. An integral lead-out wire 37 extends from the upper end of the outer layer 34. WHAT WE CLAIM IS:

1. A voice coil for a loudspeaker, the coil comprising an inner coil winding and an outer coil winding both rigidly bonded to an interposed former having a longitudinal slit through which passes a length of wire connecting the inner and outer windings.

2. A coil as claimed in Claim 1, in which a lead-out wire from the inner coil winding also passes through said slit.

3. A voice coil as claimed in Claim 1 and substantially as herein described.

4. A voice coil for a loudspeaker, substantially as herein described with reference to Figure 3 of the accompanying drawings.

5. A method of producing a voice coil for a loudspeaker, the coil having an inner layer of coil winding, an outer layer of coil winding and a former interposed between the two layers, which method comprises winding the inner layer on a mandrel, placing a coil former having a longitudinal slit over the inner layer such that a connecting length of wire between the inner and outer layers passes through the slit, and winding the outer layer on the former, bonding agent being applied to bond the layers rigidly to the former.

6. A method as claimed in Claim 5, wherein the mandrel is collapsible to facilitate the removal of the voice coil therefrom.

7. A method as claimed in Claim 5 or 6, in which a temporary former is placed over the mandrel on to which the inner layer is wound and which is subsequently removed.

8. A method as claimed in Claim 5 and substantially as herein described.

9. A voice coil produced by a method as claimed in any one of Claims 5 to 8.