Unlike tweeters, where the movement is small and the coil stays in the gap, bass driver cones move a long way. Making the coil overhang the gap ensures that, even when the coil is well away from its rest position, the same length of winding overlaps the top plate, so the force remains more or less constant. Nice in theory, but in practice the magnetic energy is not constrained to the gap itself, but extends each side. This fringe field plays an important part in how distortion is created and making it symmetrical through the use of a dual magnet system keeps distortion low.
In a standard, single magnet motor system there is built-in asymmetry in the way the metalwork is constructed. As a result, the fringe field is also asymmetric and the force on the coil is different moving forwards compared to moving backwards. This is a recipe for distortion, which gets progressively worse as the music gets louder.
In the dual magnet system, both magnets face the top plate, but from opposite sides and are magnetised in opposition to one another. It’s more difficult to manage in production because you can’t magnetise the whole assembly in one go, but the result is not only to make the fringe field symmetric, but also to reduce the amount of fringing altogether, with more of the energy forced into the gap proper.
The reduction in distortion compared to a standard motor configuration gets more pronounced the louder you go and that leads to a more natural and consistent bass character, with proper dynamics retained to much higher playback levels.
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