Thought some of you may find this interesting, as discussed with Dan Wiggins (from Adire Audio):\r\n=================================================\r\n\r\nXBL²\r\nA speaker is essentially two magnets, pushing and pulling on each other. There's a static magnet and an electromagnet. The static magnet is the big stack you're used to seeing - the permanent magnets in the motor. The electromagnet is the voice coil. When you put a current through a coil, it sets up a magnetic field; the direction and strength of that magnetic field depends upon the direction and magnitude of the current. And the more windings that the current flows through, the stronger that magnetic field is.\r\n\r\nThe actual force - BL - is the integral of the flux over the voice coil. Basically add up all the flux that is passing through the voice coil and you get the BL. It's not a "flux in the gap times the length of wire" type approach, because flux is everywhere - in the gap AND outside the gap. It is weaker (and falls to essentially zero) outside the gap, but is still quite potent. And in fact, you can have up to 50-60% of the total flux in the system reside not in the gap, but outside the gap, or what is called the fringe field.\r\n\r\nSo we see that the actual motor works by the dynamic magnetic field set up by the voice coil interacting with the static magnetic field set up by the permanent magnet, and that the total field is the field in the gap and the fringe field. And the fringe field can be a significant contributor of the total flux in the system, so much so that you cannot ignore this field; the way the fringe field is integrated by the voice coil is critical, especially when looking to make this total integral constant over multiple locations (flat BL curve).\r\n\r\nXBL² works by splitting the static field into two parts. Then you have a voice coil designed so that equal amounts of the voice coil sit - at rest - in each of the fields. Like this drawing:\r\n\r\nAs the voice coil moves up, some of it leaves the lower field, but an equal amount enters the upper field, meaning that you have a net zero loss\/gain in flux over the voice coil, so the total flux integrated by the voice coil is constant. We also optimize the length of the voice coil and\/or the depth of the rebates so that the fringe field between the gaps (inside the rebates) is roughly equal to that outside the gaps. This makes the total integration of the voice coil constant.\r\n\r\nAnd in fact, it will be constant until the voice coil starts to leave both gaps. In the image shown, imagine the voice coil moving up. Some leaves the lower gap, and an equal amount enters the upper gap. Keep moving up, and the voice coil ends above the top plate and in the rebate, and completely covers the upper gap. Once the lower end of the voice coil starts to enter the upper gap do we start to lose BL - the integration is no longer constant. This is how you get such long stroke - you have to completely leave one gap, and start leaving the next before you start to lose motor force.\r\n\r\nThe advantages of XBL² are pretty self-evident:\r\n\r\n1. Short voice coil, meaning low moving mass. This is not always required, but it's always easier to add mass to a driver than to remove it! Having a lower starting point in terms of moving mass is a huge benefit, because mass is your biggest enemy in terms of efficiency.\r\n\r\n2. The short voice coil also means fewer turns, which means lower inductance. Inductance is the prime limiter in terms of extension on the top end of a speaker. You may not need extension to 10 kHz, but it's a lot easier to get there if you have low inductance!\r\n\r\n3. Low inductance also means low flux modulation. Remember about the strength of the magnetic field relying on the number of turns? Well, the number of turns dictates the inductance as well. The lower the inductance, the fewer the turns. And for a given BL, if you can lower the number of turns, that means you're using more of the static B to generate the force. Which means the total force changes less with power applied (since the total force is a combination of the static field AND the voice coil's dynamic field).\r\n\r\n4. Tolerance to production errors. The crucial dimensions in an XBL² motor are the sizes and position of the gap. Since these are either cut or forged in to the steel, it's pretty simple to get them accurate and repeatable. Errors in the critical dimensions are naturally pushed to the operations that are the easiest to control to tight tolerance - machining and forging.\r\n\r\n5. Lower production cost. It should be obvious that the voice coil is cheaper - it's short. Less copper means lower price. But also with short voice coils come easier times with gap widths. You don't need as wide a gap to accomodate the voice coil. Why? Rocking. For a given angular deflection (rock),there's less radial (in and out to the side) deflection of the end of the voice coil.\r\n\r\n6. Overall motor size. We see that since the voice coil is ENTIRELY within the height of the top plate, we can use a short stack of magnets. No need for huge stack heights, unless you want them from a cosmetic standpoint. And because we're always integrating more than 50% of the total motor flux (one full gap plus fringe around it; usually we integrate 70% of the total flux in the system),we don't need a huge diameter magnet, either.\r\n\r\nThe disadvantages are:\r\n\r\n1. Short voice coils mean lower power handling. If you're looking to handle multiple kilowatts of power, this may not be the best approach. Sure, it works, but is not optimal.\r\n\r\n2. Licensing. Yes, you do have to pay for it - but it's quite reasonable, typically being a percent or two of the retail price of a speaker. But you still have to deal with licensing.