Lambda 001 Motor Design

1. Linear magnetic flux levels across the entire VC movement range.
2. The flux should be fixed permanently and not move, this is however not the case in 99% of speakers.
3. High heat absorption properties as close to the VC as possible. The VC if allowed to heat significantly will lower the speaker output Spl very quickly.
4. Linear inductance as the VC moves through its entire range if the driver has to contribute ANY output above the impedance minima above the Fs of the driver. This is not just at the crossover point, this is any output that is not -48dB down in our opinion. The only way to accomplish this is to keep the CORE of the VC the same at all excursions. The VC is after all an inductor, however this is the only inductor in the whole audio system that varies it's value with low bass excursion. The low bass creates massive excursions that at the same time changes the parameters of the driver at higher frequencies with every deep in and out stroke.

Examining the traditional options

To accomplish the above only one motor design comes close immediately, the Underhung. The Underhung uses a tall gap plate with a short coil. The flux is quite evenly distributed throughout the gap leading to a very flat BL curve while the coil is operating within it's linear range.

1. The flux level is the same as long as the VC stays in the gap
2. There is a lot of steel to absorb the heat from the coil.
3. No matter where the VC is in the gap the steel core is the same, forcing its inductance to stay linear with excursion.

However, has 3 problems to consider as well

1. For any significant power handling the VC diameter must increase. This increases overall inductance and significantly increases cost.
2. It needs a much larger motor structure vs an overhung as only a portion of the flux in the gap is being used at any given time. Again this leads to more cost.
3. As the coil leaves the gap the Bl drops very quickly and the driver immediately compresses the output. For this reason you need to keep the driver operating within its excursion limits
4. And most importantly, WE STILL CAN NOT KEEP THE FLUX FROM MOVING IN THE GAP.

Fixing the Flux in the Gap

This issue with flux moving is a severe distortion problem in loudspeaker motors. The flux stays fixed only when the VC is stationary. The more power applied and the farther the coil moves, the more the flux moves and the greater the distortion. There are 2 basic methods for lowering this effect.

1. Lower the electrical conductivity of the pole piece. This typically means using extremely expensive materials or sacrificing more Bl. One group of materials of interest would be the iron/cobalt alloys such as Hiperco 27 from Carpenter. The problem is the extremely high expense where the raw material for a single pole piece would be upwards of $500 on a large driver. Another option would be a powdered iron. While this is electrically non-conductive, it is also not highly magnetically permeable. You could expect to lose 20% of the flux in the gap due to the lower permeability.
2. Permanently mount a highly conductive layer that is not magnetic between the VC and the pole. This cannot be a moving piece like an aluminum VC former. It needs to be a rigidly mounted layer, much thicker than the typical VC former.

Based on these two options and our goal to make low distortion, yet affordable drivers, we have chosen the second option. We had a custom size copper tube drawn that is a .025" thick. As you will see this successfully meets our goals for fixing the flux and also has additional benefits.

The Lambda Motor is Born

In the picture you will find a full color cutaway of the Lambda motor design. Notice the highly extended pole with the full length copper Faraday. This is an extremely large piece of copper for in a loudspeaker motor. It serves all of the following functions.

1. Lowers the gap flux movement no matter where the VC is located
2. Quickly absorbs the heat from the relatively smaller VC and directs it into the large steel pole over a much larger area than the VC could have done on its own.
3. For wide bandwidth use, the VC is always surrounding the same CORE no matter where the excursion has taken it. This is very important to understand as the VC is in fact an inductor, hence it is very sensitive to what the core material is at any given location.
4. The copper effectively short circuits the inductance of the VC to an incredibly low level. This further lowers the influence of the inductance variance on the high frequency response of the driver. Also it forces a flatter phase curve for the driver. The main difference between electrostatics, ribbons, and VC driven drivers has always been the inductance. Here we can finally get a woofer to mate with these other drivers as closely as possible.