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think about a magnet reacting to another magnet. The most strength will naturally occur when the magnets are closest together, and become weaker as they move further away. Similarly, in the case of a speaker, you have a fixed magnet and the voice coil, in which the coil is an electromagnet. They will either repel or attract each other with current applied. With a typical speaker, the center point of the motor (permanent magnet) is where the strength is focused, and at rest, the voice coil is centered in that. As current is applied, forces either repel or attract this, moving the coil up or down. As you can imagine, as it moves, less and less of the voice coil is retained within the gap, so magnetic control over the entire voice coil is reduced. The idea with these motor designs is that the longer the coil, the more there is to apply strength to, so you'll get more excursion with a longer coil. XBL^2, from a functional standpoint, has two or more points that focus strength over a narrower gap, just think of it as two magnets, and as the coil leaves one gap, it will enter another, which allows a higher level of control and motor strength than if it were one. As the coil moves further from one gap, it moves closer to the other, so you will have more control over the coil since you are always within a strong point of flux, and you will get more motor strength over the excursion limitations.
