Co-incident Drivers

It naturally follows that, to minimise the time difference between sounds arriving from both units, the tweeter should be located inside the lf unit's voice coil. The drivers would then be said to be not just "co-axial", but "co-incident". The practical constraints of the tweeter magnet made this very difficult. If the magnet size of the hf unit could be reduced, then a truly co-incident driver could be realised.

Typical mf/lf units have voice coils which range from 25 mm to 50 mm in diameter. Tweeter voice coils are generally in the range 12 mm to 33 mm diameter. Unless a very insensitive unit can be tolerated, this means that only pot type magnet designs where the magnet diameter is smaller than the tweeter voice coil can be used. This constraint precludes the use of ferrite magnets because of their low remanence and high coercivity. Alnico magnets with their high remanence, could achieve the high flux density required in a small diameter but their low coercivity would mean that the magnet length needed, to produce a stable design, would extend too far back into the volume available for the lf/mf unit pole. This would lead to compromises in the woofer magnet design.

next>>