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Join Date: Oct 2003
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interestingly, this inductance effect is used for positive purpose with crossover inductors (big ones of which are used for low pass filters to midrange drivers). you'll notice that there are two broad categories of inductors: iron core and air core.
with an iron core inside the windings this back emf effect increases the inductance and so not as many windings of copper are required for a given amount of inductance. copper costs more than iron, so these inductors cost less.
so why even both with air core inductors? because putting that slug of iron in there causes problems. again folks will debate that. enhanced perceived quality while upgrading to air core inductors may be a placebo effect to some, but to others it is worth the cost in audible improvement. so what would be the best case? remove the inductor and its inductance effects altogether with an active system. by using a digital crossover before amplifying the signal, there is no inductance effect. so how does it sound? ask the guy who designed the jbl Everest. when he switched his model to full active, the bass response became more punchy. i.e. it improved in the time domain which is exactly what one would have expected.
so how does this relate back to speakers? the coil around the pole piece is like a crossover inductor with an iron core. there are ways to shield, if you will, the iron core from the voice coil and cause it to behave more like an air core crossover inductor. but as you probably would guess, that is tricky and requires additional work and materials. the td18h approach is to put a copper sleeve over the pole piece. 2242h sticks a copper ring inside the gap between the voice coil and the pole piece. not as good, but still really good. the 18lw2400 has three rings, but i'm not sure where exactly they are located or what materials are used. tough to say, but probably works about as well +/- as the 2242 approach. the 18ps76 has nothing, but it still only uses dual layer voice coil and big magnets so that amount of material in the gap is minimal. not as good as the other approaches. the Dayton selects a smaller magnet to save cost, employs no inductance control technology, and worse, doubles up the number of windings in the gap to bring bl up to where it needs to be. so, there is no free lunch on this one or at least i'm not aware of it.
but, again, this is with respect to only one dimension of the driver and there are others to consider.