Loudspeaker pole piece and loudspeaker assembly

A generally cylindrical pole piece for a moving coil loudspeaker includes a blind recess in the axial end face that faces an air-permeable dust cap. The recess is lined with a plurality of spaced ribs or fins to increase the surface area available for heat dissipation. The recess and ribs are preferably tapered, to enable the pole piece to be made as a forging.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the figures of the drawings, unless stated otherwise, identical reference symbols denote identical parts. Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a loudspeaker assembly 10 constructed according to the preferred embodiments of the present invention including a housing or chassis 12 from which a cone 14 is suspended by a front suspension 16 and a rear suspension 18 . The front suspension 16 is mounted to the housing by an annular gasket 20 . Within the cone 14 is a dust cap 22 that is porous, i.e., permeable to air. Secured by non-illustrated screws to the back of the housing 12 is a front plate 24 of the magnet assembly. Rearwardly of the front plate 24 is an annular magnet 26 , to which is fixed a pole piece or yoke 28 . The pole piece 28 has a central boss 30 that extends axially through the magnet and front plate, defining an annular voice coil gap around the pole piece. Within this gap is located a voice coil 32 carried by a cylindrical former secured to the cone 14 . As shown in FIG. 2 , the pole piece boss 30 of a first preferred embodiment of the present invention has a blind recess 34 in its axial end face that faces the dust cap 22 . The internal wall of the recess 34 is provided with a plurality of projecting ribs or fins 36 that extend the full depth of the recess. Eight ribs 36 are shown in FIG. 2 , but a greater or lesser number could be provided. The ribs 36 are preferably equispaced, as shown. In the first illustrated preferred embodiment, the mark-to-space ratio of the ribs to the grooves between them is approximately 1 to 1. The recess 34 is slightly tapered, as are the ribs 36 . The tapering enables the pole piece to be made by a forging process. Consequently, each rib 36 has a radially sloping inner surface 38 and sloping sides 40 . In horizontal cross-section, each rib 36 , therefore, has a trapezoidal shape. Although the ribs 36 and the interstices between them of the first preferred embodiment are shown in FIGS. 2 and 3 with sharp edges and corners, these edges and corners could be rounded to help the metal to flow into shape in the forging process. The radially inwardly facing surface of each rib 36 would then not be flat as shown, but slightly convex. Referring now to FIGS. 4 and 5 , a second preferred embodiment of pole piece 28 is shown. In this second embodiment, instead of having eight ribs of equal size as in the first embodiment, here eight small ribs 42 alternate with eight large ribs 44 . Each small rib 42 is set midway between the larger ribs 44 on each side thereof. The larger ribs 44 extend radially inwards towards the center of the blind recess 34 . The radial dimension of each of the larger ribs 44 is approximately twice that of the smaller ribs 42 . As can be seen from the drawings, the amount of taper in the radial direction is greater for the larger ribs 44 . As shown, it is approximately twice as great, e.g. 16° as compared with 8°. Also, in contrast to the sharp-edged ribs of the first embodiment, the ribs 42 and 44 have rounded edges. The primary purpose of providing alternate large and small ribs 44 , 42 is to increase the surface area from which heat can be dissipated. Given that a forging process will have a lower limit for the rib thickness, ribs of equal radial length all round would have to be shorter to prevent them from joining together as they near the central axis of the pole piece. Alternative long and short ribs give a greater surface area, but due to the lower limit for rib thickness this configuration is more practical for relatively large diameter pole pieces. The depth of the recess 34 is a matter of choice, although a depth equal to about half the axial length of the pole piece 28 is generally suitable. What is important is that the depth of the recess 34 should be sufficient so that it substantially spans the path of travel of the voice coil 32 . In other words, no matter in what position the voice coil 32 is located, the greater part of its axial length should be opposed, across the voice coil gap, by the ribbed wall of the pole piece. The provision of the ribs 36 , 42 , 44 dramatically increases the surface area available for heat dissipation. Air can flow in to the center of the pole piece, enabling cooling close to the coil over the whole of the path of travel of the coil. By such a construction, the creation of steep temperature gradients along the axial length of the coil is avoided. In the conventional pole piece configuration referred to above, where heat is only dissipated from the front or rear of the pole piece, i.e., some way from the coil position, in the invention, steep temperature gradients along the coil are obtained, which reduces the linearity of the cone movement. In use, hot air within the pole piece escapes by convection and by radiation through the dust cap 22 , thereby drawing in cool air to continue the cooling process. The hot air escapes into the ambient atmosphere, not into the loudspeaker cabinet. Such transfer ensures a relatively constant dissipation of heat. It is to be understood that the construction and configuration of ribs shown in the illustrated embodiments is by way of example only. The number, shape and orientation of the ribs or fins can be varied from that shown without exceeding the scope of the invention.