Loudspeakers

Hi-fi enthusiasts of a certain age (we weren't 'audiophiles' then) always recall KEF's B139 bass unit. For two reasons, principally: first, it wasn't round – often carelessly called elliptical, the diaphragm was actually shaped like an athletics track, with two straight and two semi-circular edges. Second, because it had a flat diaphragm behind which was a tapered foamed plastic structure, to which thin skins were applied, which joined it to the voice coil. It was an early example of a sandwich diaphragm which, because of its flat front surface, avoided the diffraction effects caused by the cavity of a large conventional cone. The bass drivers in today's KEF R Series may be circular, have a dished diaphragm and not use plastic foam but they are nonetheless spiritual successors to the B139. Connection of the front face of the diaphragm to the voice coil is by a paper cone structure behind, but the dishing of the surface is shallow enough to avoid cavity effects while increasing stiffness. Attaching the rear cone at a modal circle further enhances diaphragm resonance performance. KH