Electro-Acoustic Transducer

This invention relates to an electro-acoustic transducer of the moving coil type, which is suitable for use as a tone caller for a telephone subscriber's instrument, or as a loudspeaker, e.g. in a loudspeaking telephone.

According to the invention there is provided an electro-acoustic transducer of the moving coil type, which includes a driven unit including a drive coil, and a magnet unit including a magnet and pole-piece so located as to define an annular air gap in which the coil is located, wherein the outer periphery of the driven unit is secured to part of the structure of the transducer and wherein the driven unit also has a cone whose inner periphery is secured to the coil-carrying part of the driven unit.

According to the invention there is further provided an electro-acoustic transducer of the moving coil type, which includes a driven unit including a drive coil, and a magnet unit including a magnet and pole pieces located as to define an annular air gap in which the coil is located, wherein the driven unit is made in a plurality of parts one of which is a substantially circular central part whose outer periphery carries the coil, a second of which is substantially annular support member the inner perphery of which is secured to the coil carrying part of the central part and whose outer periphery is secured to part of the structure of the transducer, and a third part of which is a cone whose inner perphery is also secured to the coil carrying part of the central part and whose outer periphery is secured to part of the structure of the transducer.

Such a transducer may be a development of the transducer described and claimed in our Application No. 8203650 (F. Taylor et al 7-7-3-1-1-25), (which corresponds to U.S. patent application Ser. No. 464,827 filed Feb. 8, 1983 and abandoned June 6, 1985, for which a continuation application, Ser. No. 719,936 was filed on Apr. 4, 1985, and is now pending) and the embodiments to be described herein may use many of the piece parts of that transducer. This means that cost of production is substantially reduced as compared with other tone callers or loudspeakers.

Embodiments of the invention will now be described with reference to the accompanying drawing, in which FIG. 1 is a cross-section of a transducer embodying the invention, and FIGS. 2 and 3 are perspective views of parts of two versions of transducers according to FIG. 1.

The transducers described herein are so designed that fully automated assembly can be achieved, and the assembly technique used ensures that the driving coil is accurately positioned in the air gap. This coil is wound on an aluminium former, which ensures accuracy of shape, and the aluminium also acts as an electrical damper. The outer part of the driven unit is formed in situ on a mounting ring, such that a nest into which the speech coil locates is accurately positioned with respect to the datum diameter on that mounting ring. The same datum also positions the outer pole piece of the magnetic assembly, which reduces the build up of tolerances associated with conventional transducers. The formation of a nest as just mentioned provides a secure way to attach a speech coil to the driven unit and also protects the coil.

The transducers described herein have a plural part driven unit, which includes the coil former, so that automatic coil winding and termination of the lead out wires can be achieved. It also permits the use of an assembly technique, which ensures accurate alignment of the coil to the pole gap (air gap) and gives a positive means of attaching the coil to the driven unit.

The transducer, FIG. 1, has a circular rear cover 1, on which is located the inner pole 3, which is of magnetic material. This pole has a central hole and a flat portion parallel to the rear cover. Secured to this flat portion is an annular magnet 5 so magnetised that its end faces are the poles. This magnet is made from a modern magnetic material having high magnetic strength. The the upper face of the magnet there is secured a disc 6 of magnetic material, whose inner rim provides the outer pole. The poles are so shaped as to provide an annular gap for the reception of the coil.

Also mounted on the rear cover, and ultrasonically welded to it, there is a support ring 7 of a non-magnetic material (like the rear cover) with a mounting ring 8 integral with its upper end. This ring is dimensioned to provide an annular face which locates on the upper face of the magnet. The mounting ring 8 has an end face 9 for supporting an outer ring of the driven unit.

With the method of assembly indicated above it is found that the tolerances accumulate at the welded joint, which facilitates assembly with a low reject rate.

We now consider the driven unit. This includes a two part arrangement, the outer portion of which is an annulus 11 of a suitable stiff insulating material or aluminium whose outer rim is secured to the annular region 9 of the support ring 7. This annulus is slotted with radial members left in situ, as in FIG. 2, or is formed as a spider, as in FIG. 3. The inner part is a bent down cylindrical part with an annular portion at its lower end. The inner part 14 of this unit is of aluminium, and is convex, and it has a cylindrical portion bent down to and integral with an annular portion. The coil 18 is carried by the cylindrical portions of the inner part of the driven unit. Thus this inner part, with the coil, fits accurately into the nest formed by the portions just mentioned of the units outer portion. These parts are not shown in detail; their specific arrangement can be seen by reference to our above-mentioned application.

The leads from the winding, one of which is shown at 20, go from the ends of the winding of the coil 18 to two terminals one of which is shown at 21. Each of these terminals is fixed into a tunnel in the wall of the support ring 7.

The outer side of the transducer carries a cone 22 which is attached at its centre to the coil 18.

This cone is of a stiff material, and has a corrugated outer surround which is fastened to a circular frame 23 whose inner rim seats onto the support ring 8. During the assembly of the transducer, the coil is wound on the cylindrical portion of the inner part of the driven unit, and the result is then assembled to the magnet structure and casing in situ. The outer part of the driven unit is attached to the raised part 9 of the mounting ring 8. Thus the coil is accurately located relative to that datum and so to the magnetic circuit. The magnetic circuit structure is also located by the outer diameter of the outer pole piece 6 relative to the datum surface, thus ensuring the accurate location of the coil in the gap. Thus we do not need adhesive bonding of the coil to the diaphragm, as in many known transducers.

The transducers just described are, in effect, modifications of the transducer of the above-mentioned application, to make them suitable for use as tone callers for so-called electronic telephones, or for loudspeakers as used in loudspeaking telephones. The adaptations consist in replacing the outer part of the diaphragm by a support spider or the like, attaching a cone to the speech coil, central dome, and leaving out the dampers included in the transducer of the above application.

The structure shown in FIG. 1 is, when in use, mounted in the case of the instrument behind a perforated or slotted region, sealed by, for instance a thin membrane to keep dust and moisture out.