GB2194707A - Electromechanical transducer - Google Patents

Abstract

An electromechanical transducer such as a loud speaker has a diaphragm (12) to which is secured a voice coil (24) through which an electrical signal is passed, and a permanent magnet (26) to provide a magnetic field so that the diaphragm (12) vibrates of a frequency corresponding to the frequency of an electrical signal applied to the coil (24), the transducer having ventilation means comprising a chamber (40), a first passage (41,46) to permit air to pass from the chamber (40) past the coil (24) to cool the coil. and a second passage (not shown) communicating with the first passage (41) to permit the air to pass from the transducer. <IMAGE>

Description

SPECIFICATION Electromechanical transducer Description of Invention This invention relates to an electromechanical transducer and more particularly, but not exclusively, to a loud speaker.

Loud speakers usually comprise a diaphragm to which is secured a voice coil through which an electrical signal is passed. A, usually annular, permanent magnet provides a magnetic field and thus the diaphragm vibrates at a frequency corresponding to the frequency of the electrical signal applied to the voice coil.

It will be appreciated that where the loud speaker is primarily for producing low frequency sound signals, the diaphragm is necessarily large. Usually such loud speakers are of generally circular configuration.

In order to provide a magnetic field of sufficient strength to cause the diaphragm to vibrate, a large permanent magnet is required.

Permanent magnets having an outer diameter of up to 9 inches (22.86 cm) are known.

The voice coil usually hasan impedence of only a few ohms and thus a relatively high current passes through the voice coil. For optimum performance and reliability, it is necessary to provide some means to dissipate the heat produced. Conventionally, a ferrous sink is provided in the annular opening of the permanent magnet and air is free to move within a passage through the sink to cool the ferrous sink which is positioned adjacent the voice coil. Such means are however ineffective.

An object of the present invention is to provide a new or improved electromechanical transducer which overcomes ot reduces this problem.

According to one aspect of the invention we provide an electromechanical transducer comprising a vibratory part to which a coil is secured, a permanent magnet providing an opening to receive the coil, ventilation means comprising a chamber, first passage means to permit air to pass from the chamber past the coil to cool the coil, second passage means communicating with the first passage means to permit the air to pass from the transducer.

Thus more efficient cooling of the coil is provided. Preferably, the air is urged from the chamber by the movement of the vibratory part.

If desired, air may pass from one or more chambers past both sides of the coil, via first passage means, into the second passage means.

The invention is particularly applicable to loud speakers, where the vibratory part comprises a diaphragm and the coil comprises a voice coil. Conventionally in loud speakers, a frame is provided in which the diaphragm is mounted, via a suspension means which comprise a resilient ring. One chamber may be provided between the diaphragm and/or the resilient ring and the frame, the air pressure within the chamber thus changing as the diaphragm vibrates.

It will be appreciated that air will not only pass from the chamber as the volume of the chamber reduces as the diaphragm vibrates, but will also be drawn into the chamber from the outside of the transducer through the second and first passage means respectively, as the volume of the chamber increases as the diaphragm vibrates.

Such an arrangement in which air passes to and from the chamber, provides efficient cooling of the coil, because the air drawn into the chamber has to pass the coil at least twice before passing from the transducer i.e. on the way to the chamber, and from the chamber.

The frame may comprise two different diameter axially spaced annulus frame parts, with frame members extending between the frame parts, to provide a conical configuration in which the diaphragm is mounted, the permanent magnet being secured to the smaller diameter annulus frame part.

The voice coil may be wound around a hub of the diaphragm which is received in the annular opening of the smaller diameter annulus frame part, and in the opening of the magnet, and the first passage means may comprise a passage between the boundary walls of the smaller diameter annulus frame part and the opening in the magnet, and the hub.

Preferably, the magnet is also annular so that the hub on which the voice coil is wound, is received in the annular opening of the magnet.

A ferrous sink may also be provided within the annular opening of the magnet and a second chamber may be provided between the sink and the diaphragm, again from which air may pass via first passage means and second passage means from the transducer.

The frame may also comprise a base part which may comprise a ring or plate, and the magnet may be sandwiched between the base part and the smaller diameter annulus frame part, the base and smaller diameter annulus frame part and the magnet being secured together, for example with suitable fasteners.

The frame members which extend between the larger and smaller diameter annulus frame parts may be secured by fasteners, but preferably are integrally formed at least with the larger diameter annulus frame part. For example, the larger diameter annulus frame part and the frame members may be made as an integral casting.

The ends of the frame members which are secured to the smaller diameter annulus frame part, may be formed into a rim which is secured to the smaller diameter annulus frame part. The chamber may be formed between the rim, the smaller diameter annulus base part and the diaphragm.

The first passage means preferably comprises at least one passage which extends at least over part of its length axially of the transducer. If desired, the second passage means may also comprise at least one passage which extends over at least part of its length axially of the transducer, for example the second passage means may comprise one or more bored passages through the base frame part.

Preferably though the second passage means comprises at least one passage which extends over at least part of its length transverse to the axis of the transducer. For example, the second passage means may comprise one or more passages provided in the magnet, or between the magnet and the smaller diameter frame part, or the base part.

Where passages are provided in the magnet, if necessary, the passages may need to be lined, for example with a steel or other metal tube, to prevent disintegration of the sintered magnetic material. In an alternative arrangement, the second passage means may comprise one or more passages provided in the smaller diameter annulus frame part, or in the base frame part.

In a preferred embodiment, the magnet is annular, but is provided in segments, for example, four quadrants, or three segments, which may make up the annulus, the segments being separated from one another by gaps which extend transverse to the axis of the transducer, for example, radially.

The second passage means may comprise the gaps, the smaller diameter annulus frame part and the base frame part sandwiching the magnet therebetween, and sealing the gaps to provide passages, the walls of the passages comprising in part the boundary walls of the adjacent magnet segments, and in part the smaller diameter annulus frame part and in another part, the base frame part.

By providing the magnet in segments rather than as a continuous annulus, a magnet having a larger diameter than 9 inches (22.86 cm) can be produced which is thus correspondingly more powerful. This enables a larger diameter transducer to be produced, thus giving improved frequency response to low frequency signals and improved overall performance.

Such a construction is particularly suited to loud speakers which are intended primarily to be responsive to base signals, that is, signals having a frequency in the range 16 Hertz to 500 Hertz. However, such a construction may be employed to manufacture smaller diameter transducers which are responsive to frequencies above this range.

According to a second aspect of the invention we provide a permanent magnet for an electromechanical transducer, the magnet being of generally annular form and comprising a plurality of segments.

Preferably the magnet has an external diameter of greater than 9 inches (22.86 cm) when the segments are assembled together to provide an annular form.

According to a third aspect of the invention we provide an electromechanical transducer having a permanent magnet in accordance with the second aspect of the invention.

The transducer of the third aspect of the invention may have any of the features of the transducer according to the first aspect of the invention.

According to a fourth aspect of the invention we provide an electromechanical transducer having a permanent magnet of a diameter greater than nine inches (22.86 c.m).

The invention will now be described with the aid of the accompanying drawings in which: Figure 1 is a diagrammatic sectional view through a transducer embodying the invention.

Figure 2 is a plan view of part of the transducer of Fig. 1.

Referring to the drawings, an electromechanical transducer 10 comprising a loud speaker, has a frame 11 which mounts a vibratory part comprising diaphragm 12. The frame 11 comprises a base frame part 13 of annular form, a smaller diameter annulus frame part 14, and a sub frame 15 comprising a plurality of frame members comprising arms which are connected to provide an annular rim 15a, the other ends of the arms being integral with a larger diameter annulus frame part 16. In the present example, the sub frame 15 has arms cast integrally with the rim 15a and the larger diameter frame part 16.

The larger diameter frame part 16 and the arms of the sub frame 15 together provide a conical formation 18 in which the diaphragm 12 is mounted, on suspension means comprising a first resilient ring 20 between rim 15a and a hub 22 of the diaphragm 12 and a second resilient ring 21 between the larger diameter annulus part 16 and the diaphragm 12.

The hub part 22 has a voice coil wound about it.

The smaller diameter annulus frame part 14 provides an annular opening 23 which receives the hub 22 and the voice coil 24 and the hub 22 extends to an annular opening 25 of an annular permanent magnet 26 which is sandwiched between the base frame part 13 and the smaller diameter annulus frame part 14.

Also received within the openings 23 and 25 is a ferrous sink 30, a smaller diameter part 31 of which is a close fit within an opening 32 in the base frame part 13.

The base frame part 13, magnet 26, and smaller diameter annulus frame part 14, are secured together by suitable fasteners, some of which are shown at 33, to provide a rigid construction.

Leads 34 extend from the voice coil 24 to connectors 35 which are secured to the arms of the sub frame 15, to enable an electrical signal to be passed through the voice coil 24.

It will be appreciated that the permanent magnet 26 provides a magnetic field and thus the diaphragm 12 will tend to vibrate in the frame 11 by virtue of the suspension means 20 and 21, at a frequency corresponding to the frequency of the electrical signal applied to the voice coil 24. Because the impedence of the voice coil 24 is generally only of a few ohms, a significant current passes through the voice coil 24.

Between the resilient ring 20 of the suspension means and the larger diameter annular frame part 14, an annular chamber 40 is formed bounded by rim 15a. It will be appreciated that as the diaphragm 12 vibrates, the volume of the chamber 40 will change as the resilient ring 20 moves. Extending from the chamber 40 is a first passage means 41 which comprises an annular passage bounded by the boundary wall of the smaller diameter annulus frame part 14 and the hub 22, and in part by the boundary wall 43 of the magnet 26 and the hub 22. Thus air may pass to and from chamber 40 via the first passage means past the voice coil 24.

A second chamber 44 is also provided between the ferrous sink 30 and a cover 45 of a diaphragm 12, the volume of which will change as the diaphragm 12 vibrates. Air may pass from chamber 44 via a further first passage means comprising a passage 46 bounded between the ferrous sink 30 and the hub 22, again past the voice coil 24. Thus air may pass both sides of the coil, i.e. inside and outside to enhance cooling.

Referring now particularly to Fig. 2, the magnet 26 does not comprise a conventional continuous annular magnet; but a magnet made up of a plurality of segments, in the present example four quadrants 47,48,49 and 50. Between adjacent quadrants, for example between quadrants 47 and 48, a gap 51 is provided, and the boundary walls 52 and 53 of the quadrants 47 and 48 respectively, and the smaller diameter annulus frame part 54 and base frame part 13 (not clearly seen in Fig. 2) together define the walls of passages 55 provided by the gaps.

The passages 55 terminate at the inner and outer boundary walls 56,57, of the annular magnet 26.

Thus air which passes from the first passage means 41,46 may flow into the passages 55 transverse to the axis A of the transducer, from the transducer, to provide a cooling effect on the coil. It will be appreciated that as the volumes of the chambers 40 and 44 decrease, air will be expelled from the chambers 40 and thus pass into the first passages 41,46, and into the second passages 55 and from the transducer, but when the volumes of the chambers 40 and 44 increase as the diaphragm 12 vibrates, air will be drawn into the chambers 40 and 44 via the second passages 55 and first passages 41,46, respectively. Thus before air can flow from the transducer, the air must pass the voice coil 24 twice thus providing efficient cooling of the voice coil 24.

If desired, instead of providing passages 55 bounded by the boundary walls 52,53, of segments 47 to 50 and the smaller diameter annulus frame part 14 and base part 13, the second passage means may otherwise be provided. For example, if desired, passages transverse of the axis A may be bored through the permanent magnet 26 although such passages may need to be lined to prevent disintegration of the sintered magnet material. Alternatively, passages may be provided between the permanent magnet 26 and only the smaller diameter annulus frame part 14 or the base part 13. In each of these examples, the passages of the second passage means extend over at least part of their length in a direction transverse to the axis A, although if desired, could extend parallel to axis A of the transducer for example, directly through the base frame part 13.

By providing the permanent magnet 26 by a plurality of segments 47,48,49,50, a magnet having a diameter greater than 9 inches (22.96 cm), which is the present maximum diameter, can be produced, although the construction could be applied to smaller diameter magnets if required.

The transducer described is intended primarily to be responsive to electrical signals having a frequency in the range 16 Hertz to 500 Hertz, although for smaller diameter magnets, where smaller diameter diaphragms 12 are used, a transducer embodying the invention may be made primarily to be responsive to signal having a greater frequency than those in the range given.

The features disclosed in the foregoing description, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims (31)

1. An electro-mechanical transducer comprising a vibratory part to which a coil is secured, a permanent magnet providing an opening to receive the coil, ventilation means comprising a chamber, first passage means to permit air to pass from the chamber past the coil to cool the coil, second passage means communicating with the first passage means to permit the air to pass from the transducer.

2. A method according to claim 1 wherein the air is urged from the chamber by the movernent of the vibratory part.

3. A transducer according to claim 1 or claim 2 wherein air passes from one or more chambers past both sides of the coil, via first passage means, into the second passage means.

4. A transducer according to any one of the preceding claims wherein the transducer is a loud speaker, the vibratory part comprising a diaphragm and the coil comprising a voice coil, a frame being provided in which the dia phrngm is mounted, via a suspenstion means comprising a resilient ring.

5. A transducer according to claim 4 wherein a chamber is provided between the diaphragm and/or the resilient ring and the frame, the air pressure within the chamber thus changing as the diaphragm vibrates.

S. A transducer according to claim 5 wherein air is drawn into the chamber from the outside of the tranducer through the second and first passage means respectively, as the volume of the chamber increases as the diaphragm vibrates.

7.. A transducer according to any one of clair;ns 4 to 6 wherein the frame comprises twc) different diameter axially spaced annular frarne parts, with frame members extending between the frame parts, to provide a conical confiauration in which the diaphragm is mounted, the permanent magnet being secured to the smaller diameter annular frame part.

8. A transducer according to claim 7 wherein the voice coil is wound around a hub of the diaphragm which is received in an annular opening of the smaller diameter annular frame part, and the first passage means comprises a passage between the boundary walls of th1e smaller diameter annular frame part, and the hub.

9.. A transducer according to claim 8 wherein the magnet is annular and the hub on which the voice coil is wound, is received in the annular opening of the magnet.

10. A transducer according to claim 9 wherein a ferrous sink is provided within the annular opening of the magnet.

11. A transducer according to claim 10 wherein a second chamber is provided between the sink and the diaphragm, from which air may pass via first passage means and second passage means from the transducer.

12. A transducer according to any one of claims 7 to 11 wherein the frame also com prises a base part and the magnet is sandwiched between the base part and the smaller diameter annular frame part.

13. A transducer according to claim 12 wherein the base and smaller diameter annular frame part and the magnet are secured to gether.

14. A transducer according to any one of claims 7 to 13 wherein the frame members which extend between the larger and smaller diameter annular frame parts are integrally formed at least with the larger diameter annular frame part.

15. A transducer according to claim 14 wherein the ends of the frame members are formed into a rim which is secured to the smaller diameter annular frame part, the chamber being formed between the rim, the smaller diameter annular base part and the diaphragm.

16. A transducer according to any one of the preceding claims wherein the first passage means comprises at least one passage which extends at least over part of its length axially of the transducer.

17. A transducer according to any one of the preceding claims wherein the second passage means comprises at least one passage which extends over at least part of its length transverse to the axis of the transducer.

18. A transducer according to claim 17 wherein the second passage means comprises one or more passages provided in the magnet.

19. A transducer according to claim 18 wherein the magnet comprises sintered magnetic material and the passages are lined, to prevent disintegration of the sintered magnetic material.

20. A transducer according to claim 17 where appendant to claim 7 wherein the second passage means comprises one or more passages provided in the smaller diameter annular frame part, or in the base frame part, or one or more passages provided between the magnet and the smaller diameter frame part, or the base part.

21. A transducer according to any one of the preceding claims wherein the magnet is annular, but is provided in segments, which make up the annulus, the segments being separated from one another by gaps which extend transverse to the axis of the transducer.

22. A transducer according to claim 21 where appendant to claim 7 wherein the second passage means comprise the gaps, the smaller diameter annular frame part and the base frame part sandwiching the magnet therebetween, and sealing the gaps to provide passages, the walls of the passages comprising in part the boundary walls of the adjacent magnet segments, and in part the smaller diameter annular frame part and in another part, the base frame part.

23. A transducer according to any one of the preceding claims which is a loud speaker responsive to base signals, having a frequency in the range 16 Hertz to 500 Hertz.

24. A transducer substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

25. A permanent magnet for an electromechanical transducer, the magnet being of generally annular form and comprising a plurality of segments.

26. A magnet according to claim 25 wherein the magnet has an external diameter of greater than 9 inches (22.86 cm) when the segments are assembled together to provide an annular form.

27. A magnet for a transducer substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

28. An electromechanical transducer having a permanent magnet in accordance with any one of claims 25 to 27.

29. A transducer according to claim 28 having any of the features of the transducer according to any one of claims 1 to 24.

30. An electromechanical transducer having a permanent magnet of a diameter greater than nine inches (22.86 cm).

31. Any novel feature or novel combination of features as herein defined and/or shown in the accompanying drawings.