"Just as in the case of the D5x family of drivers,
each driver in the D9x family has its own special
attractive points. Within this product group, in
terms of cost vs. performance ratio, the D9nf (965g AlNiCo 5-7 magnet) stands out; it has truly impressive performance.
The magnetic flux density of the D9nf is about 16,000 Gauss (actual measurement).
The D9 is probably the king of this family; with the D9 you can hear information that you can't hear even with the D9nf. This is to be expected since the AlNico 5-7 magnet weighs 2600g and the voice coil gap is surrounded by Permendur pole pieces -- the magnetic flux density of the D9 is about 20,000 Gauss (actualmeasurement).

 

"With the field coil drivers, changing the power supply can affect the sound just as much as changing the amplifier. Varying the voltage applied to the field coil can also have a huge effect on the sound by changing the damping of the driver, allowing the user to achieve a wide range of effects with every type of enclosure from horn to open baffle. The user can obtain a huge degree of freedom in creating his own sound. In the case of all the field coil drivers, the extremely high permeability of the magnetic circuit means back electromotive force forms in the voice coil, for powerful electromagnetic braking and a significant reduction of excess motion in the diaphragm. As a result, transient response is excellent. The Type 1 field coil driver uses a one-piece (monococque) yoke and pole piece made entirely of iron, and an iron face plate, for a magnetic permeability of 4,000 mu and a magnetic flux density of 16,000 Gauss. The Type 2 field coil driver uses a yoke made of iron with a precision interference fit Permendur pole piece and Permendur face plate,  for a magnetic permeability of 10,000 mu and a magnetic flux density of 20,600 Gauss. The expressive (resolving) powers of the Type 2 driver are better than the Type 1 driver. Finally, the Type 3 driver uses a construction identical to that of the Type 1 driver, but uses all Permendur instead of all iron. It represents the ultimate in luxuriousness, the ultimate in resolution, the ultimate in expressive power.