GB2492360A

GB2492360A - Permanent rare-earth magnet assembly for a loudspeaker

Info

Publication number: GB2492360A
Application number: GB201111026A
Authority: GB
Inventors: Paul Victor Cork
Original assignee: GP Acoustics International Ltd
Current assignee: GP Acoustics International Ltd
Priority date: 2011-06-29
Filing date: 2011-06-29
Publication date: 2013-01-02
Also published as: GB201111026D0

Abstract

The assembly comprises a core body 10 with a plurality of sides and a projecting pole piece 18, a plurality of permanent rare-earth magnets 22, 24, 26, 28 each having a flat inner side of like polarity abutting against a different face of the core body, a front member 38 of magnetically-susceptible material having a hole into which the pole piece extends, defining an air gap, and one or more outer elements 30, 32, 34, 36 of magnetically-susceptible material which contact both the outer sides of the magnets and the front member to complete a magnetic circuit that includes the air gap. A cover 42 can be attached to the front member, enclosing the diaphragm. A compression driver loudspeaker, incorporating this permanent magnet assembly is disclosed. A number of such compression driver loudspeakers can then be arranged in a line array. Employing a box magnet arrangement with rare-earth magnets, a small-diameter permanent magnet assembly suitable for a compression driver can be produced.

Description

Loudspeakers, and magnet assemblies for them

FIELD OF THE INVENTION

The present invention relates to loudspeakers, and to magnet assemblies for them.

BACKGROUND ART

In many situations, a loudspeaker driver is needed which has a high sound output but compact dimensions. One such situation is a "line array", i.e. a loudspeaker system that is made up of a number of loudspeaker elements coupled together in a line segment to create a near-line source of sound. The distance between adjacent drivers is close enough that they constructively interfere with each other to send sound waves farther than traditional horn-loaded loudspeakers, and with a more evenly distributed sound output pattern. In a public address system, a line array allows a very narrow vertical output pattern focussed at an audiences without wasting output energy on ceilings or empty air above the audience.

Modern line arrays use separate drivers for high-, mid-and low-frequency passbands. For the line source to work, the drivers in each passband need to be in a line.

Therefore, each enclosure must be designed to rig together closely to form columns composed of high-, mid-and low-frequency speaker drivers. Increasing the number of drivers in each enclosure increases the frequency range and maximum sound pressure level, whilst adding additional boxes to the array will also lower the frequency in which the array achieves a directional dispersion pattern.

Line Array PA systems therefore call for a compression driver that is as small as possible, so that all the drivers' outputs better sum to make a coherent wave front.

However, conventional compression drivers require a magnet structure that is large in diameter, which results in a compression driver which has a very much larger diameter than its diaphragm. A more compact compression driver would be useful for line arrays and for other situations.

SUMMARY OF THE INVENTION

If a way could be found to reduce the overall diameter of a compression driver without sacrificing other parameters, then the performance of a line array could be improved. We have found that by employing a box' magnet arrangement, a compression driver can be produced which has an overall diameter not much bigger than the diaphragm diameter. Box structures for driver magnets were first patented by Parnell in 1968 (US-A- 3,478,289) but were not widely taken up. By using rare-earth magnets in a Parnell-style box structure, we have managed to produce a small-diameter permanent magnet assembly that can be used to construct a useful loudspeaker driver. This can be incorporated into a compression driver to produce a similarly low-diameter driver.

Thus, the present invention provides a permanent magnet assembly comprising a core body with a plurality of sides and a pole piece projecting forwardly therefrom, a plurality of permanent rare-earth magnets each having a flat inner side of like polarity abutting against a different face of the core body, and an outer side of opposite polarity, a front member of magnetically-susceptible material having a hole into which the pole piece extends thereby to define an air gap therebetween, and one or more outer elements of magnetically-susceptible material which contact both the outer sides of the magnets and the front member to complete a magnetic circuit that includes the air gap.

The permanent rare-earth magnets are ideally Neodymium-or Samarium-based.

Neodymium-Iron magnets such as Nd2Fe14B have been available since about 1982, and are both widely available and also have the necessary magnetic properties. Samarium-Cobalt magnets such as Sm5Co, 5m2Co17, and alloys of these, are slightly less strong than Neodymium-Iron magnets but are still sufficiently strong for this application.

The core body ideally has flat sides, which make it straightforward to achieve a substantially gap-free contact with the inner sides of the magnets. Each side of the core body can have an associated magnet. Further, the magnets can have fiat outer sides, such that the one or more outer elements form a case comprising flat plates arranged around the magnets.

The core body ideally cross-section that is a polygon, with the pole piece projecting forwardly from a front surface of the core body within the space bounded by the planes of the sides of the core body. The polygon is preferably regular, such as a square or a hexagon. Thus, with "n" permanent magnets disposed against the "n" sides of the core body and covering the latter, the magnets can then have like first poles in flat abutting contact with the sides and opposite second poles facing outwardly away from the core body.

The outer elements can then form a case, with their inner side surfaces disposed against the second poles and extending forwardly beyond the core body in a spaced relation with the pole piece, with the front member abutting against the forward portions of the side plates and the hole defining an annular air gap around the pole piece.

The front member and the one or more outer elements can be formed as separate items for ease of manufacture. The same applies to the pole piece and the core body.

The forward edge of the magnets is preferably spaced behind the front member, to allow a clearance gap between the magnets and the front member. The outer elements can then extend beyond the gap to the front member.

The magnets can be bonded by a suitable adhesive to the core body, and/or the one or more outer elements bonded to the magnets and to the front plate.

The magnets are preferably identical in size and shape in order to maintain a symmetrical unit.

The invention also provides a compression driver loudspeaker, incorporating a permanent magnet assembly as defined above. This will usually also include a voice coil located in the air gap, a diaphragm driven by the voice coil, and a conduit through the core body through which sound vibrations created by the diaphragm exit the loudspeaker. A cover can be attached to the front member, enclosing the diaphragm. Compression drivers commonly also include a phase plug located in the conduit, which often ends with a flared horn.

A plurality of such compression driver loudspeakers can then be arranged in a line array.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; Figure 1 shows the box magnet structure of a compression driver according to the present invention; Figure 2 shows an exploded view of the compression driver from the front; Figure 3 shows an exploded view of the compression driver from the rear; and Figure 4 shows a line array of compression drivers according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 shows the box magnet structure according to the present invention. A core body 10 consists of a generally square outline having four faces 12, 14 to its exterior and a central through-bore 16. A circular pole piece 18 projects forwardly of the remainder of the core body 10, and defines a circular ring around the central bore 16. A taper 20 provides a smooth transition from the internal diameter of the pole piece 18 to the central bore 16.

Figure 1 shows a design employing a square core. An alternative is to employ a hexagonal core, in which case the number, sizes and locations of the remaining parts (to be described) will have to be adjusted accordingly.

A set of four neodymium iron magnets 22, 24, 26, 28, each of Nd2Fe14B alloy, are attached to each of the four outer-faces 12, 14 of the core body 10. Each magnet is sized so that it substantially matches (or is slightly smaller than) the outer-face of the core body and is attached thereto with a suitabte adhesive such as a structural acwlic adhesive or an epoxy resin. Each magnet is oriented such that one pole (for example, north) is directed inwardly toward the core body 10 with the opposite pole (for example, south) directed outwardly, i.e. radially away from the core body 10.

A set of outer elements 30, 32, 34, 36 are provided, one for each of the magnets 22, 24, 26, 28. Each outer element is adhesively attached to the outer side of a magnet and is sized so that it substantially matches (or is slightly smaller than) the dimensions of the magnet, save that each overlaps the forward edge of the magnet (i.e. in the direction of the pole piece 18).

Finally, to complete the magnetic circuit, a front member 38 is provided. This is generally square in outline, and abuts to the forward edge of the outer elements 30, 32, 34, 36. It has a central through-bore 40, shghtly larger in internal diameter than the outside diameter of the pole piece 18. The outer element 30, 32, 34, 36 extends sufficiently forward beyond the magnets 22, 24, 26, 28 so as to locate the front member 38 roughly alongside the pole piece 18, with the pole piece 18 fitting within central bore 40 of the front member 38. Thus, the external diameter of the pole piece 18 and the internal diameter of the central bore 40 of the front member 38 define a small air gap between each other.

In this way, a magnetic circuit is formed for each magnet, comprising (in sequence) the relevant outer member, the front member, across the air gap, via the pole piece 18, into the core body 10 and then back into the opposing face of the magnet. A conventional diaphragm and voice coil can be located such that the voice coil lies in this air gap, and the pressure wave generated by the diaphragm will then pass through the central bore 16 of the core body 10 to leave the loud speaker in an axial direction, i.e. downwards as illustrated in figure 1.

Figure 2 shows the parts of figure 1, together with a rear cover 42 with suitable undulations 44 to improve its rigidity. This fits over the front member 38 and reduces the amount of sound emitted in this direction. The voice coil and diaphragm (not shown) fit between the front member 38 and the cover 42, and sound from the diaphragm is directed through the bores 40, 16 which thus define a conduit, to the exterior of the loud speaker via a flared horn member 46. This has a square external section to match the remainder of the thus-defined compression driver together with a central flared aperture 48. A phase plug can be provided, normally located within the central bore 16 but possibly extending into the outlet 48, as required.

Figure 4 shows a set of three such compression drivers arranged in a linear array, to achieve the effects discussed above. A horn (not shown) can then be fitted to each outlet 48.

It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention.

Claims

<claim-text>CLAIMS1. A permanent magnet assembly comprising; a core body with a plurality of sides and a pole piece projecting forwardly therefrom, a plurality of permanent rare-earth magnets each having a inner side of like polarity abutting against a different face of the core body, and an outer side of opposite polarity, a front member of magnetically-susceptible material having a hole into which the pole piece extends thereby to define an air gap therebetween, and one or more outer elements of magnetically-susceptible material which contact both the outer sides of the magnets and the front member to complete a magnetic circuit that includes the air gap.</claim-text> <claim-text>2. A permanent magnet assembly according to claim 1 in which the permanent rare-earth magnets are Neodymium-based magnets.</claim-text> <claim-text>3. A permanent magnet assembly according to claim 1 in which the permanent rare-earth magnets are Neodymium-Iron magnets.</claim-text> <claim-text>4. A permanent magnet assembly according to claim 1 in which the permanent rare-earth magnets are Nd2Fe14B magnets.</claim-text> <claim-text>5. A permanent magnet assembly according to claim 1 in which the permanent rare-earth magnets are Samarium-based magnets.</claim-text> <claim-text>6. A permanent magnet assembly according to claim 1 in which the permanent rare-earth magnets are Samarium-Cobalt magnets.</claim-text> <claim-text>7. A permanent magnet assembly according to claim 1 in which the permanent rare-earth magnets are one of Sm5Co, Sm2Co17, and an alloy thereof.</claim-text> <claim-text>8. A permanent magnet assembly according to any one of the preceding claims in which the core body has flat sides in substantially gap-free contact with the inner sides of the magnets.</claim-text> <claim-text>9. A permanent magnet assembly according to any one of the preceding claims in which each side of the core body has an associated magnet.</claim-text> <claim-text>10. A permanent magnet assembly according to any one of the preceding claims in which the magnets have flat outer sides and the one or more outer elements form a case comprising flat plates arranged around the magnets.</claim-text> <claim-text>11. A permanent magnet assembly according to any one of the preceding claims, wherein the core body is of polygonal cross-section having "n" sides and the pole piece projects forwardly from a front surface of the core body within the space bounded by the planes of the sides of the core body, the plurality of permanent magnets comprising "n" permanent magnets disposed against the "n" sides of the core body and covering the latter, the magnets having like first poles in fiat abutting contact with the sides and opposite second poles facing outwardly away from the core body.</claim-text> <claim-text>12. A permanent magnet assembly according to claim 11 in which a plurality of outer elements form a case comprising "n" side plates having inner side surfaces disposed against the second poles and extending forwardly beyond the core body in spaced relation with the pole piece, and wherein the front member abuts against the forward portions of the side plates and the hole defines an annular air gap around the pole piece.</claim-text> <claim-text>13. A permanent magnet assembly according to claim 11, wherein the core body is of hexagonal cross-section.</claim-text> <claim-text>14. A permanent magnet assembly according to claim 11, wherein the core body is of square cross-section.</claim-text> <claim-text>15. A permanent magnet assembly according to any one of the preceding claims, wherein the front member and the one or more outer elements are formed as separate items.</claim-text> <claim-text>16. A permanent magnet assembly according to any one of the preceding claims in which a forward edge of the magnets is spaced behind the front member to leave a clearance gap between the magnets and the front member, and the outer elements extend beyond the gap to the front member.</claim-text> <claim-text>17. A permanent magnet assembly according to any one of the preceding claims in which the magnets are bonded by adhesive to the core body.</claim-text> <claim-text>18. A permanent magnet assembly according to any one of the preceding claims in which the one or more outer elements are bonded by adhesive to the magnets and to the front plate.</claim-text> <claim-text>19. A permanent magnet assembly according to any one of the preceding claims in which the pole piece and the core body are separate pieces bonded together by adhesive.</claim-text> <claim-text>20. A permanent magnet assembly according to any one of the preceding claims in which the magnets are identical in size and shape.</claim-text> <claim-text>21. A compression driver loudspeaker incorporating a permanent magnet assembly according to any one of the preceding claims.</claim-text> <claim-text>22. A compression driver loudspeaker according to claim 21 further comprising a voice coil located in the air gap, a diaphragm driven by the voice coil, and a conduit through the core body through which sound vibrations created by the diaphragm exit the loudspeaker.</claim-text> <claim-text>23. A compression driver loudspeaker according to claim 22 further comprising a cover attached to the front member and enclosing the diaphragm.</claim-text> <claim-text>24. A compression driver loudspeaker according to claim 22 or claim 23 further comprising a phase plug located in the conduit.</claim-text> <claim-text>25. A compression driver loudspeaker according to any one of claims 22 to 24 in which the conduit ends with a flared horn.</claim-text> <claim-text>26. A plurality of compression driver loudspeakers according to any one of claims 21 to 25, arranged in a line array.</claim-text> <claim-text>27. A permanent magnet assembly substantially as herein described with reference to and/or as illustrated in the accompanying figures.</claim-text>