Maybe I should go into a bit more depth...

I guess I should break this up into 3 parts - intro and XBL², LMT, and Split Coil. So let's start with the intro section, and start with a primer about how a speaker works.

A 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.

The 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.

So 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).

XBL² 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:



As 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.

And 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.

The advantages of XBL² are pretty self-evident:

1. 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.

2. 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!

3. 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).

4. 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.

5. 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.

6. 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.

The disadvantages are:

1. 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.

2. Licensing. Yes, you do have to pay for it - I don't give it away for free - 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.

Dan Wiggins
Adire Audio®

Now the split coil approach. The original concept to this approach was patented in the early 1970s, and used off-and-on in a few drivers here and there. As I hinted at earlier, we're actually licensing XBL² to the owner of this patent; they've used both topologies and decided that when all things are considered, XBL² simply works better. If that's not the biggest "plug" for the superiority of XBL² over the Split Coil, I don't know what is...

Anyway, Split Coil basically operates as XBL² but in reverse; rather than having dual gaps with a single coil, you have dual coils with a single gap. Here's the basic design:



You have two voice coils - each wound in the same direction - and seperate them. You space them such that there is ~50% of the gap height between the two windings. Then you make each winding about 50% of the height of the gap as well (so the winding:separation:winding:gap ratio is 1:1:1:2).

As the voice coils - on their common former - move up, an equal amount of the upper winding leaves the gap as enters the gap from the lower winding. And it continues to move that way until the lower winding starts to leave the gap. So you get some good stroke!

The advantages of this approach are:

1. Long stroke. You can get long stroke from a total voice coil height that's not too much longer than the gap height itself.

2. Flat BL. You do get flat BL, just like XBL² and LMT.

3. Royalty free. The patent on this approach expired in the early 90s.

There are several downsides, though:

1. Higher moving mass compared to XBL² from the dual sets of windings. This is actually the biggest reason that the split coil inventor is moving to XBL². Much lower mass for a given stroke.

2. Higher inductance versus XBL² because of the multiple turns. And again, this brings with it lower bandwidth, greater flux modulation.

3. EXTREME sensitivity to winding length and winding spacing. Get the spacing off by a turn of wire, and you get a pretty significant peak in the BL curve center. Likewise, make it a shade too long in spacing, and you get a big dip. Much more sensitive than XBL² to winding errors. Winding tolerance is much harder to control versus machining tolerance, as it takes a LOT more effort to get proper stacking for the correct lengths.

4. Low efficiency. Because you're purposefully avoiding a good chunk of the gap, you trade off high flux (in the gap) for low flux (fringe). You can get high overall BL, but it's not very efficient at flux utilization, being somewhere between 25% and 50% efficient in terms of flux usage.

5. Motor size. Because the overall voice coil effective length is much longer, you simply need to have a deeper/taller magnet stack. And that brings the issues with rocking, etc. with it.

6. Shorting-ring-versus-flux tradeoff. I should have brought this up with the other motors, too... If you want to use a shorting ring, the best place to use it is in the gap (where the steel is closest to both sides of the voice coil), and then plate the pole in its entirety. However, that means you widen the gap, so you lose flux. So do you want lower inductance, or lower efficiency? That's often the tradeoff.

Look at the LMT and Split Coil - both would need a wider gap to get a shorting ring in the gap. You have to shave back the steel to put copper in there. And of course, the lower in frequency you want to operate, the thicker the ring must be, meaning the wider the gap. Want to be effective below 300 Hz or so? Get ready to shave off 5-6mm of your gap, which REALLY widens things out and loses a LOT of flux...

But look at XBL² see that rebate? Guess what - fill that with copper, and you get a great shorting ring location. Right next to the voice coil, in the heart of the gap. Right where you want it. And guess what - it doesn't lose ANY gap face - it's already accounted for. Meaning we can (and often do) make that rebate a good 5-6mm deep - so it works to low frequencies - and don't lose ANY flux in the system. And we can do both sides of the voice coil without any losses...

Dan Wiggins
Adire Audio®

Thanks Dan.  Very informative.  I was wondering about a few of the pros/cons between the different motor topologies.