Loud speaker with minimized magnetic leakage

Claim 1

In a loudspeaker, which includes a diaphragm, a voice coil attached to the diaphragm, a basket for supporting the diaphragm, an annular permanent magnet, an annular front plate between the magnet and the basket, a back plate on the side of the magnet opposite the front plate, and a pole-piece extending through the annular magnet and forming a magnetic gap with the front plate, the improvement wherein: said back plate and pole piece are integral, said back plate being annular and having a thickness which smoothly tapers radially from a first inner thickness to a smaller circumferential thickness, said pole piece having a toroidal portion with a first surface convex to a central opening and a second surface concave to the exterior of said pole piece, an upper surface of said back plate merging smoothly with said second surface and a lower surface of said back plate merging smoothly with said first surface, said pole piece including a flat annular portion merging smoothly with the toroidal portion and extending parallel to said upper surface and terminating in a cylindrical surface forming one side of said gap, whereby the magnetic field of said magnet is concentrated into said gap and leakage of the magnetic field is minimized.