BL linearization is a good thing. I'm not at all trying to downplay the Stereo Integrity woofers nor was I trying to debate against someone buying them. Nick is a good guy.
There is however no perfect motor, and there are unfortunately by products that are not good in the way BL linearization is often done. An underhung driver is going to have the most linear BL of any driver over it's operating range until the coil begins to exit the gap. The problem is that you have a small coil so power handling is decreased and power compression goes up. It's a tradeoff. You're also distributing flux over a large gap plate, meaning you are nowhere near saturation. The farther from saturation your field is, the more you can modulate the flux field as the coil moves. This means more distortion unless a proper shorting ring is used to eliminate this effect at all points throughout the travel, not just at rest. A copper cap on top of the pole doesn't do a whole lot as the coil is inward and not adjacent to it.
This goes for any driver where flux is distributed over multiple gaps as well as the underhung. You get benefit from having the same core inside and around the outside of the coil which linearizes inductance, but inductance is still high as the core on both sides is steel. You get the benefit of lower distortion due to flatter BL. But at the same time you can't keep the flux from moving without using a shorting ring and you have a shorter coil so less power handling. In a multiple gap design you also have air space between the gaps. This air space has much less flux density than even the steel has. As a result the field in the air gap modulates much easier than the field in the steel even. Therefore, the tighter you can make the gap, the better you're going to be. The flux level in the air gap is higher, and there is less air gap period. Filling the space between the multiple gaps with copper helps, but it doesn't remove the steel "core" from the VC.
Regarding the transition from back to forward, a woofer is a constant acceleration device. It doesn't just stop and then change direction and start again. There is no "slow" transition from forward to rearward stroke as you mention. Again, speed is directly related to the frequency being played. If the motion was slowed down in any way, it would no longer be playing the same frequency, it would be playing a lower frequency. There are mechanical issues where say a cone could flex due to box pressure as it goes from one direction to another, but that is again another mechanical issue to look at and not part of the motor design.
Regarding the Lambda motor, the concept alone is fairly simple. Roger Russel(of McIntosh) and Scan Speak both own patents on shorting rings. Their methods have different benefits and the full copper sleeve has the benefits of both and then some. The trick is to get the coil to work like an air core instead of an iron core. The full copper sleeve shorts any eddy currents caused by the coil's motion anywhere over it's usable excursion. This means your field doesn't move no matte how far the woofer is being driven and non-linear distortion is greatly reduced. The core stays the same throughout any range of travel, meaning inductance also stays the same. Many drivers without a proper pole extension have coils that go from being nearly air core on the outward stroke to iron core on the inward stroke. This is huge in terms of non-linear distortion.
The other thing that makes the Lambda motor special is the tolerances to which they are built. Everything is assembled to within .003" tolerances. That is about the thickness of the human hair and I am not aware of anyone else who manufactures drivers to these tolerances. The gap needs to be extremely tight for the copper sleeve on the pole to fully be able to wick heat from the coil and into the pole. The steel pole alone can absorb a lot of heat, but steel doesn't absorb heat very fast and the copper does. The phase plug then acts as a large heat sink to pull heat from the pole and into the air at the front of the woofer. Without the tight tolerances and the full copper sleeve, you don't get this heat sinking benefit.
Jeff Bagby has recently used one of the Lambda drivers in a speaker for Salk Sound. For those not familiar with him, he's been around the industry for awhile and has written a few different programs for speaker modeling:
http://audio.claub.net/software/jbagby.html
You can see his comments in the following thread regarding the driver and it's low non-linear distortion:
http://www.htguide.com/forum/showthread.php4?t=29988&page=2&pp=43
I'd put up one of the Lambda drivers against any other driver available in terms of low distortion and low power compression in any third party test at any time.
John
