The ideal drive unit is one that can reproduce the entire audio spectrum, from the lowest bass notes to the highest treble. Unfortunately, such a device does not exist, as the physical demands of the two are very different. Multiple drive units are therefore accepted as a compromise, with the bass and treble parts of the signal being reproduced from different places.
The benefit of having the high frequency drive unit in the centre of the low frequency one is that the whole audio range is perceived as coming from the same point in space, like the ideal drive unit.
With Dual Concentric, both sources come from the same coincident time aligned point in space, and this phase coherence leads to constant directivity through the crossover region, producing an even off axis frequency response.
