GB2404520A - Phase plug equalizer used as heat sink for loudspeaker - Google Patents

Abstract

A horn-loaded loudspeaker unit is provided with an acoustic restrictor 210 element which also functions as a heatsink to dissipate heat from and hence cool the voice-cool 120. The motor assembly 100 is located forwardly of the loudspeaker diaphragm 70, in thermal contact with the restrictor element, so that heat can be conducted from the voice coil via the motor assembly to the restrictor element and thence to the external air. In a first embodiment, the motor is located in the concave interior of the cone. The restrictor element is of tapered cross-section and the horn wall and the outer surface of the restrictor form a waveguide. In a second embodiment, the cone presents its convex face to the horn throat and is closed at its centre by a dustcap. Also, the restrictor element is annular (fig. 2a). In a third embodiment, the restrictor element is provided with a number of heat-sinking fins (fig. 3).

Description

LOUDSPEAKER DRIVE UNIT

This invention relates to loudspeakers.

A typical loudspeaker has a moving voice coil immersed in a stationary magnetic circuit. As current is passed through the coil, electromagnetic forces act to move the coil and attached diaphragm. This process is inefficient and a significant proportion of the input power is dissipated as heat generated within the conducting material. As the conductor increases in temperature the resistivity of the material increases, such that there is no longer a linear relationship between an increase in voltage across the loudspeaker terminals and the output of the loudspeaker. The effect is known as 'power compression'.

Another unwanted effect of this generated heat relates to neodymium magnets. The magnet material has become popular since it has a much greater capacity for magnetic charge than traditional materials, and so the whole motor system and loudspeaker can be made significantly lighter. However with modern adhesives and materials, neodymium is usually the least resistant part of the design to high temperatures and can suffer from demagnetization.

To combat these effects, thermal paths, which allow the heat to transfer to the ambient environment, need to be introduced. These are often via a bore in the pole piece, or holes through the back plate or chassis. It has become standard for neodymium magnets to have additional heat sinks attached to dissipate the heat away.

High efficiency loudspeaker systems often employ horn loading, in which the driver is placed in a small cavity and then a compression device is used in very close proximity to the front of the diaphragm to match the load impedance of the acoustic system to the mechanical system. The highpressure wave then enters the horn section, which allows a smooth impedance match to the external environment and can also control the dispersion characteristics of the emitted sound. However in a standard orientation the coil and motor system are placed in the small cavity, which inevitably compromises the ability for the heat to dissipate. This can be overcome by mounting the drivers such that the motor system is on the radiating side of the diaphragm. However traditional large magnetic circuits will obstruct the acoustic wave resulting in turbulence and audible distortion. The performance of any additional compression devices attached to the system will then be compromised by the interference of the motor system position.

A need exists for a device that can act as a heat sink for the motor system which is then placed on the sound-radiating side of the diaphragm, but also provides the compression required for the acoustic impedance matching and is designed to provide a smooth waveguide path for the emerging wave.

According to the present invention, there is provided a loudspeaker drive unit comprising: an acoustic diaphragm; a motor for causing the diaphragm to vibrate to generate sound, the motor including a voice-coil; an acoustic modification element disposed in front of one face of the diaphragm to modify the acoustic characteristics of the drive unit; wherein the acoustic modification element is thermally conductive and in thermal contact with the motor so as to act as a heatsink to dissipate heat generated by energisation of the voice-coil in operation of the drive unit.

Preferably the motor and acoustic modification element are both offset in the same direction along the axis of the voice-coil relative to the diaphragm, so that they are located to the same axial side of the diaphragm. The drive unit can then be oriented relative to an enclosure such that the motor and acoustic element are in the air in front of the diaphragm, unlike a conventional infinite baffle or ported loudspeaker where the motor is behind the loudspeaker diaphragm and located in an interior chamber of the loudspeaker.

As applied to a horn-loaded loudspeaker, the acoustic modification element may constitute a compression device to match the load of the acoustic system to the mechanical system.

The diaphragm may be a conventional loudspeaker cone type. As will become apparent from the following description, the drive unit may be constructed such that the concave face of the cone faces outwards, or such that the convex face of the cone faces outwards. The acoustic modification element may be either solid or annular, and in either case, it and the diaphragm and motor are conveniently oxisymmetric.

Other, optional features of the invention are defined in the sub-claims.

The invention also provides a loudspeaker unit incorporating a drive unit according to the invention, as well as a loudspeaker including at least one such loudspeaker unit.

The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which: Figure la is a cross-sectional view of a horn-loaded loudspeaker unit according to a first embodiment of the present invention; Figure lb is an enlarged detail of part of Figure la; Figure 2a is a cross-section view of a second embodiment of the invention, which is similar in construction and operation to that of Figure 1; Figure 2b is an enlarged details of part of Figure 2a; and Figure 3 is a cross-sectional view of a third embodiment, in which the transducer is similar to that of Figure 2, with the motor assembly forward of the convex face of the cone.

In the various Figures, like reference numerals indicate like parts.

The loudspeaker unit 10 shown in Figures la and lb comprises a cone-type electro-acoustic transducer drive unit 20 mounted between the wall 30 of a sealed rear cavity 40 and the wall 50 of the horn throat 60.

The cone diaphragm 70 is suspended around its outer periphery from the transducer chassis via a flexible outer front suspension element 80 and around its inner periphery via a flexible rear suspension element 90 relative to the stator 1 10 of the transducer motor 100.

The motor 100 comprises a voice coil 120 wound around a former 130 rigidly connected to the cone 70 and the stator 110; it will be noted that the voice coil and former are accommodated in the concave interior of the cone, so that they are in the air volume leading via the horn throat 60 to the exterior of the loudspeaker unit, rather than the sealed air volume behind the loudspeaker cone. The stator comprises a neodymium cylindrical permanent magnet 140 sandwiched between a cylindrical steel plate 150 of the same diameter and an outer steel body 160. The body 160 includes a circular face 170 in contact with one face of the magnet, a radially outwardly ending portion 180 and an annular part l 90 whose inner periphery defines, with the outer peripheries of the plate, magnet and standoff, an annular air-gap 200 which accommodates the voice coil and former. The stator forms a magnetic circuit which extends across the air-gap in the conventional manner and which interacts with the field generated by energisation of the voice coil.

An acoustic restrictor element or compression device 210 is located in the inner end of the horn throat, adjacent the motor unit. It will be seen that starting from the cone, the horn throat tapers smoothly to a minimum cross-section at 220 and then flares outwardly, continuing beyond the lower part of Figure I a. In the construction shown in Figures la and lb, the restrictor element, which along with the cone and motor is axisymmetric, is of a shape resembling an onion, in that its crosssection is rounded from rear to front but tapers to a point at 230. The rear-to-front profile of the restrictor 210, and the cross section of the narrowing part of the horn throat are contoured so that the crosssectional area of the airspace through which sound radiating forwardly from the cone diaphragm initially reduces, to increase the acoustic loading on the cone, and thereafter increases; the horn wall and outer surface of the restrictor thus constitute a waveguide for sound from the cone.

The motor and restrictor element are rigidly mounted together, and to the chassis of the loudspeaker by radial arms not shown in Figures 1 a and lb and are offset from teh cone in the same direction axially of the drive unit, i.e. they are both to the same axial side of the cone.

In order to alleviate the problems associated with heat generated by the voice coil 120, the restrictor 210 is adapted to serve the additional function of a heatsink, to dissipate heat from the voice coil. In particular, the restrictor is a body of aluminium or similar material which has good thermal conductivity but relatively low thermal capacity, and has an end wall 240 which surrounds and is in physical and thermal contact with the axial end face and the adjacent part of the periphery of the motor body part 160. This provides a heat dissipation path from the air-gap 200, where the inner and outer peripheral surfaces of the air gap are heated by the voice coil, to the outer peripheral surface of the restrietor, which being in the free air of the horn throat can then dissipate heat to the exterior by radiation and convection.

The heatsinking capabilities of the restrictor element may be enhanced by giving it a black, suitably matte, surface and also by providing it with heatsink fins (see Fig. to increase its surface area. To minimise the influence of these fins on acoustic operation, they preferably extend longitudinally of, rather than eircumferentially around, the rear to front axis of the restrictor element and horn throat.

Figures 2a and 2b show a second embodiment of the invention, which is similar in construction and operation to that of Figure 1, except for the following differences: - The cone diaphragm 70' presents its convex face to the horn throat and is closed at its centre by a rearwardly-facing dustcap 250.

- The inner suspension 90' extends radially outwardly from the junction of the inner periphery of the cone and voice coil to a cylindrical rim on the motor's magnet assembly.

- The magnet assembly of the motor is "inside out" compared with Figure 1, in that the neodymium magnet is an annulus, whose inner periphery forms part of the outer wall of the air-gap.

- The restrictor element 210' is annular.

- The wall 260 of the inner end of the horn throat, rearwardly of the forward edge of the restrictor element, is of constant, cylindrical crosssection. In order that the cross-sectional area of the acoustic path leading from the front face of the cone initially reduces in crosssectional area in the desired manner, the restrictor element increases in external diameter and decreases in internal diameter, from its rear up to a plane where the effective horn cross-section is at a minimum, and then the outer surface reduces in diameter and the inner surface increases in diameter up to the forward circular edge or rim 270 where they meet. There are thus two waveguide channels, one through the interior of the restrictor, and the other defined between the external surface of the restrictor and the internal surface of the horn wall.

Figure 3 shows a third embodiment, in which the transducer is similar to that of Figures 2a and 2b, with the motor assembly forward of the convex face of the cone.

It differs in that the restrictor element does not have a region adjacent the cone where it initially thickens in cross-sectional area to reduce the cross-sectional area of the horn throat. Instead, the horn throat cross-section is determined principally by the configuration of the horn walls, though it will be noted that as in Figure 2, the restrictor has inner and outer surfaces which increase and decrease, respectively in diameter towards the rim 270 where they meet.

In the Figure 3 embodiment the restrictor element is provided with a number of radially inwardly and outwardly-directed heatsinking fins 280. Each of these fins starts at the inner rear of the restrictor, extends forwardly over the rim 270 and then over the outer surface of the restrictor to its rear edge.

Claims (17)

1. A loudspeaker drive unit comprising: an acoustic diaphragm; a motor for causing the diaphragm to vibrate to generate sound, the motor including a voice-coil; an acoustic modification element disposed in front of one face of the diaphragm to modify the acoustic characteristics of the drive unit; wherein the acoustic modification element is thermally conductive and in thermal contact with the motor so as to act as a heatsink to dissipate heat generated by energisation of the voice-coil in operation of the drive unit.

2. A loudspeaker drive unit according to claim 1 wherein the motor and loudspeaker drive unit are both offset in the same direction along the axis of the voice-coil relative to the diaphragm.

3. A loudspeaker drive unit according to claim 1 or 2 wherein the motor has an end face directed away from the diaphragm and which is butted against and in thermal contact with an end face of the acoustic modification element.

4. A loudspeaker drive unit according to claim 1, 2 or 3 wherein the acoustic modification element has a rearwardly directed skirt which fits around and is in thermal contact with at least part of the external periphery of the motor.

5. A loudspeaker drive unit according to any one of claims I to 4 wherein the acoustic modification element is an axisymmetric body coaxial with the voice coil and having a diameter which smoothly increases and then decreases along the voice-coil axis in the forward direction away from the diaphragm.

6. A loudspeaker drive unit according to claim 5 wherein the diaphragm is a loudspeaker cone and the motor is located in the concave volume of the cone.

7. A loudspeaker drive unit according to claim 5 or 6 wherein the cone has a central aperture which is closed by a suspension element which suspends the inner periphery of the cone from the motor.

8. A loudspeaker drive unit according to any one of claims 1 to 4 wherein the acoustic modification element is an axisymmetric annular body coaxial with the voice coil which tapers in external diameter to a forward circular edge.

9. A loudspeaker drive unit according to claim 8 wherein the diaphragm is a loudspeaker cone and voice-coil of the motor is coupled to the convex face of the cone.

10. A loudspeaker drive unit according to claim 8 or 9 wherein the cone has a central aperture which is closed by dustcap which also acts as a rearward closure of the interior volume of the annular acoustic modification element

11. A loudspeaker unit comprising a loudspeaker drive unit according to any one of the preceding claims and means defining an acoustic passageway from the diaphragm of the drive unit to the air external to the loudspeaker unit, the drive unit being oriented so that the motor and acoustic modification element are located in the passageway.

12. A loudspeaker unit according to claim 11 and including an internal acoustic chamber closed by the rear face of the diaphragm.

13. A loudspeaker unit according to claim 1 1 or 12 wherein the acoustic modification element is effective to operate as an acoustic restrictor element which decreases the effective cross-sectional area of the passageway and so loads the diaphragm.

14. A loudspeaker unit according to claim 1 1, 12 or 13 wherein the passageway

15. A loudspeaker drive unit constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

16. A loudspeaker unit constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

17. A loudspeaker incorporating at least one loudspeaker unit according to any one ofclaim 11 to Wand 16.