GB2114855A

GB2114855A - Moving coil transducer

Info

Publication number: GB2114855A
Application number: GB08203650A
Authority: GB
Inventors: Frank Taylor; Norman William Tester; Trevor George Young; Trevor Burton; Malcolm Roger King; Leslie Eugene Basil D Bradshaw
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-02-09
Filing date: 1982-02-09
Publication date: 1983-08-24
Also published as: GB2114855B; ZA829558B; AU1097883A; AU560342B2; ES270258U; SG42087G; NZ203080A; ES270258Y; HK75987A

Abstract

A moving coil transducer has its diaphragm, which is of thin aluminium, made in two parts, one of which is a central circular part (14) while the other is an annular outer part (11). The central part carries the coil (18), on a cylindrical portion (16), and the outer part has another cylindrical portion (12). When the diaphragm is assembled the two cylindrical portions (11, 12) together enclose the coil. The magnetic circuit has a central hollow portion (3) with a flat disc at one end which supports an annular magnet (5). There is another disc (6) on the other end of the magnet (5), which disc's inner edge defines with the end of the pole piece portion (3) the air gap for the coil. In another version the outer pole is a flat disc with a circumferential wall carrying an annular pole-piece whose inner rim is the outside of the air gap. A cylindrical magnet is secured to the middle of the disc, and its upper end carries a flat disc pole piece whose outer rim is the inner side of the air gap. <IMAGE>

Description

SPECIFICATION Moving coil transducer This invention relates to electro-acoustic transducers of the moving coil type, especially but not solely for use in telephone subscriber's apparatus.

Moving coil transducers are well known, and have been much used, especially for loudspeakers.

With the advent of the so-called "electronic" telephone, i.e. one in which the subscriber's set includes electronic circuitry, it is convenient to use such transducers in telephones. Although the fact that they need amplifiers may be an apparent disadvantage, this is not so since the presence of electronic circuitry for other reasons means that the amplifier needed can be readily provided.

Further the sound quality characteristics of moving coil transducers are better than for such existing devices as the carbon granules tranducer.

The existing transducers as currently used in telephones are relative cheap so it is desirable to provide moving coil transducers which are compact and inexpensive.

According to the invention there is provided an electro-acoustic transducer of the moving coil type, which includes a diaphragm unit including the diaphragm and a coil, and a magnet unit including a magnet and a plurality of pole pieces so located as to define an annular air gap in which the coil is located, wherein the diaphragm is made in two parts one of which is a substantially circular central part whose outer periphery carries the coil, and the other of which is a substantially annular outer portion attached to the coil-carrying portion of the central part of the diaphragm.

According to the invention there is also provided an electro-acoustic transducer of the moving coil type, which includes a diaphragm unit including the diaphragm and a coil, and a magnet unit including a magnet and a plurality of pole pieces so located as to define an annular air gap in which the coil is located, wherein the diaphragm is made in two parts one of which is a substantially circular central part the outer edge of which supports the coil, wherein the other part of the diaphragm is a substantially annular portion which is attached to the coil when the transducer is assembled, wherein one of the pole pieces is a cylindrical member of a ferromagnet material with a flat disc-like portion at one of its ends, which disc-like portion supports an annular magnet, and wherein the other pole piece is a flat annular member of a ferromagnet material attached to the other end of the magnet, its inner circular periphery with the outer periphery of the other end of the cylindrical portion of the first mentioned pole piece defining the air gap in which said coil is located.

According to the invention there is further provided an electro-acoustic transducer of the moving coil type, which includes a diaphragm unit including the diaphragm and a coil, and a magnet unit including a magnet and a plurality of pole pieces so located as to define an annular air gap in which the coil is located, wherein the diaphragm is made in two parts one of which is a substantially circular central part the outer edge of which supports the coil, wherein the other part of the diaphragm is a substantially annular portion which is attached to the coil when the transducer is assembled, wherein one of the pole pieces is a disc of a ferromagnet material with an upstanding wall at its outer rim, wherein an annulus of a ferromagnetic material is mounted with its outer rim in magnetic contact with said wall, wherein a cylindrical magnet is supported at the centre of the disc, and wherein the other pole piece is a flat disc of a ferromagnet material attached to the other end of the magnet, its outer circular periphery with the inner periphery of the annulus on said wall defining the air gap in which said coil is located.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. 1, 2 and 3 each shows a cross-section of a moving coil transducer embodying the invention.

The transducers described herein are so designed that fully automated assembly can be achieved, and the assembly technique used ensures that the speech coil is accurately positioned in the air gap. The speech 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 diaphragm is formed in situ on the mounting ring, such that a nest into which the speech coil locates is accurately positioned with respect to the datum diameter on the mounting ring for the diaphragm.

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 diaphragm, and also provides protection for the coil itself.

The transducers described herein each have a two part diaphragm, which includes an aluminium 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 diaphragm.

The first transducer to be described, see Fig. 1, has a rear cover 1 which is generally circular and has three upstanding pillars such as 2 in its middle which locate the inner pole 3, which is of magnetic material. This pole has a central hole to fit over the pillars 2 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 one of the modern magnetic materials which can have high magnetic strength. To the upper face of the magnet there is secured a disc 6 of magnetic material, whose inner rim provides the outer pole.

As can be seen from Fig. 1, 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) which has a mounting ring 8 integral with its upper end. This mounting ring is so dimensioned as to provide an annular face which locates on the upper face of the magnet. It also has an inner datum face which rests against the outer cylindrical rim of the pole piece 6. The mounting ring 8 has an annular region 9 for supporting the outer ring of the diaphragm.

Mounted between the magnet 5 and the inner pole 3 there is a metallic'ring 10 which forms a short-circuited turn, which minimises hum pick up when the transducer is in use. Between the pole 3 and the rear cover 1 there is an acoustic damper 11 of a fibrous material.

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

We now consider the diaphragm unit. This includes a two portion diaphragm, 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. The inner part of the diaphragm portion 11 is a bent down cylindrical part 1 2 with an annular portion 13 at its lower end. The inner portion 1 4 of the diaphragm is of aluminium, and is convex, except for its central region which is concave, to provide adequate clearance within the transducer's front chamber.This inner portion also has a cylindrical portion 1 6 bent down to and integral with an annular portion 1 7. The coil 18 is carried by the portions 1 6 and 1 7 of the inner portion of the diaphragm. Thus the inner portion of the diaphragm with the coil fits accurately into the nest formed by the portions 12-1 3 of the diaphragm's outer portion.

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, i.e. the side which faces the user, of the transducer, has so-called spit guard 22, which is a thin flexible membrane, e.g. of poiyethylene terephthalate fifteen microns thick.

Outside of this there is the front cover 23, which has a ring, for instance of twelve holes such as 24, for the ingress or egress of sound, dependent on whether the transducer (in its telephone application) is used as a transmitter or as a receiver. Behind these holes there is a mesh membrane 25, which may be of nylon, and which imparts acoustic damping to the transducer.

During the assembly of the transducer, the coil is wound on the cylindrical portion of the inner part of the diaphragm, and the result is then ,assembled to the magnet structure and casing in situ. The outer part of the diaphragm is, as already indicated attached to the raised part 9 of the mounting ring 8, and the recess 31 in that part 11 of the diaphragm is located by the diaphragm forming tool relative to the datum surface referred to above. Hence the coil is accurately located relative to that datum and therefore 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 accuate location of the coil in the gap.Thus the necessity for the use of an adhesive bonding of the coil to the diaphragm, as in many known transducers is avoided.

In certain cases, the short-circuited turn 10, and/or the spit guard 11, and possibly also the membrane 25, may not be needed, in which case whichever is not needed is not provided.

The transducer shown in Fig. 2 is similar in most respects to that shown in Fig. 1, so the description is confined to the points of difference.

Whereas in Fig. 1 the damping arrangement include a disc of fibrous material at the rear of the inner pole, this is not so in Fig. 2. In this arrangement, the inside of the inner pole 40 has a stepped recess 41 into which is secured an annular damper 42 of a fibrous material. The upper edge of this recess is formed over as shown at 43 to secure the damper in place. Thus in this case we have a through-flow damper mounted within the inner pole.

The transducer shown in Fig. 3, while it uses the same method of constructing the diagram used in Figs. 1 and 2, differs from the other two transducers in a number of respects, as will be seen. Thus the magnet is a cylindrical magnet 50 located at the centre of an outer pole formed by a flat disc 51 and an outer wall 52. This outer pole is located at the rear cover 53 and the support ring 54 substantially as for the other two transducers.

Mounted on the outer wall 52 of the outer pole is a disc 55 whose inner cylindrical rim defines the outside of the air gap. The inner side of the air gap is defined by the outer cylindrical rim of a disc shaped pole piece 56 mounted on the upper end of the magnet.

In this transducer it will be noted that the only damping arrangement shown is the mesh 57 on the inside of the front cover 58. However, further damping arrangements, e.g. as in Fig. 1 or 2, can be added if these are needed.

The magnets used in the transducers described herein can be either ceramic magnets or metal magnets.

Claims

1. An electro-acoustic transducer of the moving coil type, which includes a diaphragm unit including the diaphragm and a coil, and a magnet unit including a magnet and a plurality of pole pieces so located as to define an annular air gap in which the coil is located, wherein the diaphragm is made in two parts one of which is a substantially circular central part whose outer periphery carries tne coil, and the other of which is a substantially annular outer portion attached to the coil-carrying portion of the central part of the diaphragm.

2. An electro-acoustic transducer of the moving coil type, which includes a diaphragm unit including the diaphragm and a coil, and a magnet unit including a magnet and a plurality of pole pieces so located as to define an annular air gap in which the coil is located, wherein.the diaphragm is made in two parts one of which is a substantially circular central part the outer edge of which supports the coil, wherein the other par of the diaphragm is a substantially annular portion which is attached to the coil when the transducer is assembled, wherein one of the pole pieces is a cylindrical member of a ferromagnet material with a flat disc-like portion at one of its ends, which disc-like portion supports an annular magnet, and wherein the other pole piece is a flat annular member of a ferromagnel material;;attached to the other end of the magnet, its inner circular periphery with the outer periphery of the other end of the cylindrical portion of the first mentioned pole piece defining the air gap in which said coil is located.

3. A transducer as claimed in claim 2, wherein each of the two parts of the diaphragm has a cylindrical portion which extends into the air gap so that the coil is between the two cylindrical portions, and wherein each of the cylindrical portions of the two parts of the diaphragm has an annular portion so located that the two annular portions are contiguous when the two parts of the diaphragm are assembled together.

4. A transducer as claimed in claim 2 or 3, wherein an acoustic damper formed by an annular damper is located between the rear face of the first mentioned pole piece and the rear cover portion of the transducer, the hole of the annulus being substantially concentric with the inside of the cylindrical member of that pole piece.

5. A transducer as claimed in claim 2 or 3, wherein the inside of the cylindrical member of the first-mentioned pole piece has a stepped recess on its inside at its front end, and wherein an annular damping ring is located in the recess.

6. A transducer as claimed in claim 5, and wherein the annular damper is held in place by a formed-over rim from the pole piece.

7. A transducer as claimed in claim 4, 5 or 6, and wherein the damper is a ring of a fibrous material.

8. A transducer as claimed in claim 2, 3, 4, 5, 6 or 7, and wherein a generally cylindrical member of an eiectrically conductive material is located about the cylindrical member of the first pole piece, between that member and the magnet, such that that member forms a short-circuited turn.

9. A transducer as claimed in claim 2 or in any claim appendent thereto, and wherein the cylindrical member of the front pole piece is hollow and fits over one or more upstanding pillars on the rear cover of the transducer.

10. An electro-acoustic transducer of the moving coil type, which includes a diaphragm unit including the diaphragm and a coil, and a magnet unit including a magnet and a plurality of pole pieces so located as to define an annular air gap in which the coil is located, wherein the diaphragm is made in two parts one of which is a substantially circular central part the outer edge of which supports the coil, wherein the other part of the diaphragm is a substantially annular portion which is attached to the coil when the transducer is assembled wherein one of the pole pieces is a disc of a ferromagnet material with an upstanding wall at its outer rim, wherein an annulus of a ferromagnetic material is mounted with its outer rim in magnetic contact with said wall, wherein a cylindrical magnet is supported at the centre of the disc, and wherein the other pole piece is a flat disc of a ferromagnet material attached to the other end of the magnet, its outer circular periphery with the inner periphery of the annul us on said wall defining the air gap in which said coil is located.

11. A transducer as claimed in claim 10, wherein each of the two parts of the diaphragm has a cylindrical portion which extends into the air gap so that the coil is between the two cylindrical portions, and wherein each of the cylindrical portions has an annular portion at its lower end so located that the two annular portions are contiguous when the two parts of the diaphragm are assembled together.

1 2. A transducer as claimed in any one of the preceding claims, wherein the central circular part of the diaphragm is of a highly conductive material such as aluminium.

13. A transducer as claimed in claim 12, wherein the central part is domed outward with a shallow saucer-like recess at its middle.

14. A transducer as claimed in claim 12 or 13, wherein the outer annular part of the diaphragm is also made of a highly conductive material such as aluminium.

15. An electro-acoustic transducer of the moving coil type, substantially as described with reference to Fig. 1, Fig. 2 or Flg. 3 of the accompanying drawings.

New claims filed on 13th May 1982 Superseded claims None New claims:

1 6. An electro-acoustic transducer of the moving coil type, which includes a diaphragm unit including the diaphragm and a coil, and a magnet unit including a magnet and a plurality of pole pieces so located as to define an annular air gap in which the coil is located, wherein the diaphragm is made in two parts one of which is a substantially circular central part whose outer periphery carries the coil, and the other of which is a substantially annular outer portion attached to the coil-carrying portion of the central part of the diaphragm, and wherein each of the two parts of the diaphragm has a cylindrical portion which extends into the air gap so that the coil is between the two cylindrical portions.

1 7. An electro-acoustic transducer of the moving coil type, which includes a diaphragm unit including the diaphragm and a coil, and a magnet unit including a magnet and a plurality of pole pieces, so located as to define an annular air gap in -.

which the coil is located, wherein the diaphragm is made in two parts one of which is a substantially circular central part the outer edge of which supports the coil, wherein the other part of the diaphragm is a substantially annular portion which is attached to the coil when the transducer is assembled, wherein each of the two parts of the diaphragm has a cylindrical portion which extends into the air gap so that the coil is between the two cylindrical portions, wherein each of the cylindrical portions of the two partsof the diaphragm has an annular portion so located that the two annular portions are contiguous when the two paits of the diaphragm are assembled together, wherein one of the pole pieces is a cylindrical member of a ferromagnetic material with a flat disc-like portion at one of its ends, which disc-like portion supports an annular magnet, and wherein the other pole piece is a flat annular member of a ferromagnet material attached to the other end of the magnet its inner circular periphery with the outer periphery of the other end of the cylindrical portion of the first mentioned pole piece defining the air gap in which said coil is located.

18. A transducer as claimed in claim 17, wherein an annular acoustic damper is located between the rear face of the first mentioned pole piece and the rear cover portion of the transducer, the hole of the annulus being substantially concentric with the inside of the cylindrical member of that pole piece.

19. A transducer as claimed in claim 1 7, wherein the inside of the cylindrical member of the first-mentioned pole piece has a stepped recess on its inside at its front end, and wherein an annular damping ring is located in the recess.

20. A transducer as claimed in claim 19, and wherein the annular damper is held in place by a formed-over rim from the pole piece.

21. A transducer as claimed in claim 18, 19 or 20, and wherein the damper is a ring of a fibrous material.

22. A transducer as claimed in claim 17, 18, 19, 20 or 21, and wherein a generally cylindrical member of an electrically conductive material is located about the cylindrical member of the first pole piece, between that member and the magnet, such that that member forms a short-circuited turn.

23. A transducer as claimed in claim 1 7 or in any claim appendent thereto, and wherein the cylindrical member of the front pole piece is hollow and fits over one or more upstanding pillars on the rear cover of the transducer.

24. An electro-acoustictransducerofthe moving coil type, which includes a diaphragm unit including the diaphragm and a coil, and a magnet pnit including a magnet and a plurality of pole pieces so located as to define an annular air gap in which the coil is located, wherein the diaphragm is made in two parts one of which is a substantially circular central part the outer edge of which supports the coil, wherein the other part of the diaphragm is a substantially annular portion which is attached to the coil when the transducer is assembled, wherein each of the two parts of the diaphragm has a cylindrical portion which extends into the air gap so that the coil is between the two 'cylindrical portions, wherein each of the cylindrical portions has an annular portion at its lower end so located that the two annular portions are contiguous when the two parts of the diaphragm are assembled together, wherein one of the pole pieces is a disc of a ferromagnet material with an upstanding wall at its outer rim, wherein an annulus of a ferromagnetic material is mounted with its outer rim in magnetic contact with said wall, wherein a cylindrical magnet is supported at the centre of the disc, and wherein the other pole piece is a flat disc of a ferromagnet material attached to the other end of the magnet, its outer circular periphery with the inner periphery of the annulus on said wall defining the air gap in which said coil is located.

25. A transducer as claimed in any one of claims 1 6 to 24, wherein the central circular part of the diaphragm is of a highly conductive material such as aluminium.

26. A transducer as claimed in claim 25, wherein the central part is domed outward with a shallow saucer-like recess at its middle.

27. A transducer as claimed in claim 25 or 26, wherein the outer annular part of the diaphragm is also made of a highly conductive material such as aluminium.