Electroacoustic Transducer Having A Preformed Non-magnetic Gentering Sleeve

Abstract

A microphone or loudspeaker having a magnet system which comprises an annular permanent magnet and pole plates which are arranged one on each side of the magnet and the first of which is rigidly connected to a centrally arranged soft-iron core, which together with the second pole plate forms an effective air-gap. The casing serves as a centering sleeve and hence is provided with abutment members, preferably abutments studs, which are arranged on different levels and one of which may be in the form of a fitting edge. The various component parts are stacked in the centering sleeve, which is a simple operation in mass manufacture.

Description

This invention relates to a device for converting acoustic vibrations into electric oscillations and conversely. In particular, the invention relates to an electro-acoustic transducer provided with a magnet system comprising an annular permanent magnet and pole plates arranged one on either side of the magnet, the first pole plate being rigidly secured to a soft-iron core which is arranged centrally relative to the two pole plates and together with the second pole plate defines an annular effective air gap, and a centering member made of a non-magnetic material centering the core relative to the second pole plate.

Such a device is known from German Patent Specification No. 953,411. The device described in this patent is in the form of a microphone and has a magnet system in which an annular centering member is disposed between the core and the other pole plate. This centering member has the function of centering the core with respect to the upper plate, i.e. of adjusting the effective annular air gap entirely within the permissible tolerances. A magnet system as described in the said patent specification is widely used in microphone technology and is fixed in a known manner by gluing, soldering or clamping.

Owing to the provision of a separate centering ring the assembly of such microphones is comparatively laborious and hence complicated.

The invention is characterized in that the centering member forms part of a casing which encloses the magnet system as a centering sleeve on the inner surface of which are provided at least two abutment members which, viewed in the axial direction, are disposed on different levels and the disposition of which corresponds to that of the pole plates.

This provides the advantage that in assembling the device the various pole plates of the device can simply be successively inserted in the respective centering members in the centering sleeve. Thus, the pole plates are centered and the effective airgap is accurately dimensioned.

Preferably the centering sleeve is provided with an abutment member for the magnet ring which is located between the abutments for the pole plates.

Although an eccentric arrangement of the annular magnet made of an oxydic ceramic material, for example, "Ferroxdure," scarcely influences the effective flux in the effective air-gap, accurate adjustment of the magnet ring is desirable to provide space beneath it for the coil located in the air-gap. Obviously, in this adjustment tolerances greater than those of the pole plate are permissible. The easiest manner of stacking the components is provided by locating the abutments on different diameters. In order of succession the pole plate having the smaller diameter is first inserted in the associated abutment member in the centering sleeve, then the annular permanent magnet and finally the pole plate having the larger diameter are placed in position.

The abutments may take the form of an assembly of at least three projections arranged in a circle of a fitting rim, the diameter of both forms corresponding with the diameter of the component of the magnet system to be centered.

Stacking is made even easier by using conical abutments. This is also of advantage in manufacturing the centering sleeve from a synthetic material. Manufacture is effected by injection moulding, the conical design being highly desirable for release from the mould.

In microphone and loudspeaker practice, when a flat annular magnet of large coercive force is used a large effective magnetic flux is generally produced in the effective air-gap by making the outer diameter of each of the two pole plates smaller than that of the annular magnet.

For this purpose, in a device according to the invention, the pole plate having the larger diameter, which, however, is smaller than that of the magnet, is provided with projections on a diameter corresponding to that of the member. If the abutment comprises a plurality of studs, the number and the positions of the projections correspond to those of the studs.

In assembling a device by means of the stacking system according to the invention, the core may simply be made in one piece with the adjacent pole plate.

A magnet system centered by stacking elements in the centering sleeve must be fixed. The fixing may be effected not only by the aforementioned known methods, but also by means of the centering sleeve itself.

For this purpose, in one embodiment of the invention, in order to fix the magnet system in the centering sleeve an axial abutment is provided at the side of the smaller pole plate, while at the side of the greater pole plate the centering sleeve is provided with a plurality of radially arranged inwardly bent lugs which press against the larger pole plate. In assembly, these lugs are thrust inwards by pressing and simultaneous heating.

In another embodiment of the invention, in order to fix the magnet system there is provided in the centering sleeve at the side of the smaller pole plate an axial abutment and at the side of the larger pole plate a retaining ring made of a non-magnetic material, the centering sleeve being further provided with a fitting rim which corresponds with the circumference of the retaining rim.

The retaining rim is secured to the centering sleeve by means of an adhesive.

In these two embodiments the magnet system is fixed between the axial abutment and the lugs or the retaining rim, respectively. In order to improve the acoustic properties of the device, in a device according to the invention, the core together with the adjacent pole plate is provided with a central continuous duct. The pole plate, which together with the core forms the annular effective air-gap, carries a diaphragm. When the device is used as a microphone, this diaphragm substantially covers the entire pole plate. Through the said duct the space immediately behind the diaphragm communicates with the space behind the magnet system. This influences the acoustic properties of the microphone and especially its directional characteristic.

When a retaining ring is used, it is suitably formed with a hole which corresponds to and is aligned with the channel. The hole may be closed by an acoustic resistance in known manner. In order to produce an air volume in the centering sleeve between the magnet system and the retaining ring, spacers are interposed between the retaining ring and the adjacent pole plate.

Another embodiment of a device according to the invention is characterized in that the duct contains a partition which extends in the axial direction and divides the duct into two partial ducts, one of which opens into a closed space in the casing behind the retaining ring and the other opening into the space between the retaining ring and the magnet system. The rear surface of the diaphragm then communicates with two independent spaces.

In a modification of this embodiment of the invention the partition is tubular so as to form an annular second duct.

By forming at least one aperture between the spacers -- which may partly take the form of rings -- the aperture or each aperture being aligned with a corresponding aperture in the centering sleeve, which in this embodiment forms the outer envelope of the device, a communication path is established between the annular chamber (between the retaining ring, the centering sleeve, the pole plate and the tubular partition) and the ambient atmosphere. Thus, the space behind the diaphragm communicates with the ambient atmosphere through the annular channel and, as the case may be, through the acoustic resistances so that the device (rod microphone) has a cardioid directional characteristic.

The device according to the invention may take the form of a box microphone. In this case the centering sleeve forms part of the top cover of a box-shaped case, and a tube integral with the bottom cover of the microphone casing is concentrically arranged in the duct provided in the core so as to form an annular second duct. The tube communicates with the ambient atmosphere and the annular channel communicates with the box-shaped space.

From a manufacturing point of view it is advantageous for the said retaining ring, the tube and, as the case may be, the spacers to be made in one piece from a synthetic material.

When a microphone is to be mounted in a centering sleeve according to the stacking method, it has proved of advantage for the pole plate adjacent the effective airgap to be provided with a diaphragm beforehand. The diaphragm is cemented to the pole plate. In microphones the thickness of a diaphragm is negligible for all practical purposes so that the axial abutment need not be modified. However, if the diaphragm has a reinforced edge, the axial abutment should be dimensioned to take this into account. It is possible not to cement the diaphragm having a reinforced edge to the associated pole plate, but to place it in a specially provided fitting edge in the centering sleeve. Thus, the diaphragm is not only centered but also fixed in this fitting edge.

It should be noted that the stacking of the device according to the invention may be started at either end.

Alternatively, the larger pole plate may be provided with a diaphragm and introduced as the last component into the centering sleeve, the assembly then being fixed in the manner described.

The centering sleeve may take the form of a casing of a microphone capsule adapted to be accommodated in the microphone casing. The centering sleeve may alternatively be designed as an internally threaded ring which can be screwed onto a standardized handset. When the device according to the invention is used in a headset the centering sleeve forms a part of the earpiece. The device then is designed as a telephone receiver.

The invention will now be described more fully with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a device in the form of a microphone provided with a retaining ring according to the invention,

FIG. 2 shows a similar rod microphone having a cardioid directional characteristic;

FIG. 3 is an exploded view of this microphone;

FIG. 4 is a cross-sectional view of a modified embodiment of this microphone, and

FIGS. 5 and 6 are cross-sectional views of box-shaped microphones employing different techniques for fixing the magnet system.

In FIG. 1, the magnet system of a microphone is accommodated in a casing 1. The casing is made of a synthetic material by means of injection moulding. The casing is designed as a centering sleeve for centering and adjusting the component parts of the magnet system. This magnet system comprises a magnet ring 2 made of an oxydic ceramic material, for example, Ferroxdure. The upper and lower surfaces of this ring have been ground flat to ensure good magnetic engagement with pole plates 3 and 4. A core 5 is integral with the pole plate 4 and forms an annular effective air gap 6 together with the pole plate 3.

The pole plates 3 and 4 and the core 5 are made of soft iron. A coil 7, secured to the diaphragm 8, is arranged in the air-gap 6. The diaphragm 8 has a flat edge 9 and a diameter equal to that of the pole plate 3. The edge 9 is cemented to the pole plate 3.

Assembly is effected as follows. The casing 1 is turned upside down. The pole plate 3 -- provided with the diaphragm 8 -- is placed against an abutment 10 in a centering member 11. This centering member may comprise either a circular fitting edge or at least three abutment studs arranged in a circle. The magnet ring 2 is then placed in a fitting edge 12. Subsequently the pole plate 4 -- with the core 5 -- is placed in a centering member 13 which has a form similar to that of the centering member 11. However, the diameter of the member 13 is larger than that of the member 11. The diameter of the fitting edge 12 is intermediate those of the centering members 11 and 13.

By the centering of the two pole plates 3 and 4 the air-gap 6 is automatically set to the desired size.

A concentric arrangement of the magnet ring 2 is not very critical since the desired magnetic flux in the effective air-gap 6 is scarcely influenced thereby.

However, the fitting edge 12 is provided to prevent the inner surface of the magnet ring from extending so far towards the axis as to interfere with the axial movement of the coil 7.

In order to fix the elements of the magnet system in place, a retaining ring 15 is pressed against the lower surface of the pole plate 4. During the stacking operation this retaining ring, which is made of a synthetic material, is arranged in a fitting edge 16 in the casing. The ring 15 is secured in the casing 1 by means of an adhesive in known manner.

In order to connect a space 17 beneath the diaphragm with a space 18 beneath the magnet, the core 5 is formed with a central duct 19 and the retaining ring 15 is formed with a corresponding aperture 20 in which an acoustic resistance is inserted.

FIG. 2 is a cross-sectional view of a rod microphone, the magnet system of which has been assembled in a manner similar to that used in the microphone of FIG. 1.

However, the retaining ring 15 has a different form. It is provided with a tube 21 arranged centrally in the core 5 so that a second annular duct 22 is produced. The retaining ring 15 has a protruding rim 23 at its outer surface so that an annular chamber 24 is formed between the polate plate 4 and the retaining ring 15.

In order to impart a cardioid directional characteristic to the microphone, the protruding rim 23 is formed with apertures 25 which register with apertures 26 in the casing 1. The upper apertures are closed in the usual manner by acoustic resistances. Thus, the space 17 beneath the diaphragm communicates through the central duct 19 with the large-volume space 18 in the stem of the microphone. The space 17 also communicates through the annular duct 22 with the annular space 24 which communicates with the ambient atmosphere through the apertures 25 and 26. In this rod-shaped microphone the casing is divided into two parts. The lower part 27 forms the stem of the microphone.

FIG. 3 is an exploded view of this microphone with the exception of the stem 27. It should be noted that the pole plate 4 is provided with three projections 28 which lie on a circle corresponding to that of the centering member 13. As a result, the diameter proper of the pole plate 4 is reduced and it also is smaller than that of the magnet ring 2, which is magnetically desirable.

A microphone having a closed chamber 24 -- a pressure microphone -- is shown in FIG. 4, and this chamber communicates with the space 17 beneath the diaphragm 18 through an acoustic resistor 29.

FIGS. 5 and 6 show box-shaped microphones. The component parts of the magnet system 30 are stacked in the same manner as has been described hereinbefore. However, the stacking takes place in a central portion 31 of an upper cover 32, which portion acts as the centering sleeve.

The centrally arranged tube 21 is not connected to a retaining ring, but to a lower cover 35.

In the embodiment shown in FIG. 6, the retaining ring is dispensed with. Fixing is effected by means of lugs 33 formed at the lower end of the centering sleeve 31. These lugs are bent inwards by heating same and exerting pressure thereon.

Claims

What is claimed is:

1. An electroacoustic transducer device comprising a magnet system that includes an annular permanent magnet ring and first and second pole plates arranged one on each side thereof, the first of said pole plates being rigidly secured to a soft-iron core which is centrally arranged with respect to the two pole plates and together with the second pole plate forms an annular effective air-gap, a preformed casing that includes a centering sleeve made of a non-magnetic material for centering the core relative to the second pole plate, said centering sleeve enclosing the magnet system and having an inner surface provided with at least two axially spaced abutment members which correspond respectively to the locations of the first and second pole plates.

2. A device as claimed in claim 1 wherein the centering sleeve further comprises an abutment member for the magnet ring interposed between the abutment members for the pole plates.

3. A device as claimed in claim 1 wherein the diameter of the abutment member adjacent the second pole plate is smaller than the diameter of the abutment member adjacent the first pole plate.

4. A device as claimed in claim 3 wherein the diameter of the second pole plate is smaller than the diameter of the first pole plate and that in order to fix the magnet system in place, the device further comprises an axial abutment provided in the centering sleeve at the side of the smaller diameter pole plate, whereas at the side of the larger diameter pole plate the centering sleeve is further provided with a plurality of radially arranged inwardly bent lugs which press against the larger pole plate.

5. A device as claimed in claim 1 wherein at least one abutment member comprises at least three abutment studs arranged in a circle having a diameter which corresponds to the diameter of the component part of the magnet system to be centered.

6. A device as claimed in claim 5, characterized in that the component part to be centered comprises a pole plate provided with projections the number and positions of which correspond to those of the abutment studs.

7. A device as claimed in claim 1 wherein at least one abutment member takes the form of a fitting edge in the centering sleeve with a diameter which corresponds to the diameter of the component part of the magnet system to be centered.

8. A device as claimed in claim 1 wherein at least one of the abutment members is conically shaped.

9. A device as claimed in claim 1 wherein the core and the first pole plate form an integral unit.

10. A device as claimed in claim 1 wherein the second pole plate is smaller in diameter than the first pole plate and that in order to fix the magnet system in place, the device further comprises an axial abutment provided in the centering sleeve at the side of the smaller diameter pole plate and a retaining ring made of a non-magnetic material is provided at the side of the larger diameter pole plate, the centering sleeve having a fitting edge which corresponds with the circumference of the retaining ring.

11. A device as claimed in claim 10, characterized in that the core is provided with a central duct.

12. A device as claimed in claim 11, characterized in that the retaining ring is formed with a hole which registers with the duct.

13. A device as claimed in claim 12 further comprising spacers interposed between the retaining ring and the adjacent pole plate.

14. A device as claimed in claim 13 characterized in that the spacers form part of a ring engaging the centering sleeve.

15. A device as claimed in claim 14, characterized in that between the spacers at least one aperture is formed which registers with a corresponding aperture in the centering sleeve, said sleeve constituting the outer envelope of the device.

16. A device as claimed in claim 13 wherein said retaining ring includes an axial tube extending into said duct and wherein the retaining ring, the tube and the spacers form an integral unit.

17. A device as claimed in claim 12 characterized in that a partition which extends in the axial direction is accommodated in the duct and divides the duct into two part-ducts, the first of which opens into a closed chamber in the casing beneath the retaining ring and the second of which opens into a space formed between the retaining ring and the magnet system.

18. A device as claimed in claim 17, characterized in that the partition is tubular so as to form an annular second duct.

19. A device as claimed in claim 1 wherein said preformed casing comprises upper and lower mating covers that together form a box-shaped casing, and wherein said core comprises an axially extending tube with a duct therein and the centering sleeve forms part of the upper cover of the box-shaped casing, the lower cover of the casing including a second axial tube which is arranged concentrically in the core duct so as to form an annular second duct.

20. A device as claimed in claim 1 further comprising a diaphragm fixed to said second pole plate and wherein said core comprises a first axially extending tube with a duct that communicates with a space beneath the diaphragm, and a retaining ring adjacent the first pole plate and comprising a second axially extending tube coaxially arranged within said first tube to form a second annular duct therewith.