Loudspeakers

For each of these Radialstrahlers there is not one but a vertical array of three 'melons' built around a hollow spine and separated by a series of cylindrical magnet assemblies (the black discs). Each magnet drives the melon segments (petals) directly above it via a conventional voice-coil arrangement, rather like a traditional moving-coil motor but turned through 90o, pointing upwards not forwards. The top of each melon is glued into an anchor so the elements are squeezed upwards and outwards on the positive stroke and pulled downwards and inwards on the negative-going stroke, producing a 360° pulse of sound for each cycle. Only the large bass melon is supported with something like a traditional spider/suspension – the mid and treble drivers have no spider or suspension and the voice-coil is left free-floating in the air gap. For these two smaller drivers, only the petals are moving, so MBL was able to more accurately control the uniformity of their output.

The petals in the main bass melon are fashioned from an annealed aluminium/magnesium/silicon alloy. While this material is both light and rigid, in practice the segments do not deform uniformly when squeezed – they have both primary and secondary bending modes. So each alloy petal is braced by a pair of copper rods whose own behaviour smooths the transition between these modes. The gaps between the petals are air-sealed with a flexible polymer similar to that used in some soft midrange domes.