Sensationally low - due to brains and the consistent execution!
The force factor amplitude modulates the entire signal. What does this mean?
The amplitude, i.e. the distance of the deflection of the membrane, determines the level or volume. But if now the force acting on the diaphragm varies with the distance of movement, the reproduced volume is not that of the music, but we hear the errors of the drive. So all forms of amplitude modulation, resulting from force-factor modulation, must be eliminated if we want to build a "significantly better loudspeaker."
How do we do that? The solution used here is based on two steps:
The first step is to reduce the inductance by reducing the permeability of the pole. It would be tempting to simply use a larger magnet to saturate the pole. This works, but only at low signal levels. With any realistic magnet, it doesn't take much drive current to bring a saturated iron piece back out of saturation. When that happens, the inductance shoots up. So that's not how it works! Instead, most of the pole is replaced by a piece of pre-saturated material, a permanent magnet. At this point, the inductance still rises for negative excursions, but only by a small amount.
The second step is to compensate for this with a piece of unsaturated material placed on the pole. In terms of gap geometry and DC field, the resulting design is standard. Only the alternating field shows a completely different picture, since the field lines no longer return through the air gap. The measured inductance curves confirm that force-factor modulation has now been effectively eliminated. One challenge in assembly is that the pole (a neodymium magnet) must be installed after both it and the main magnet have already been magnetized.
SERENDIPITY. Distortion values sensationally low - through brains and the consistent execution! The force factor amplitude modulates the entire signal.