"Line Source" or "Line Array" are terms to describe a hybrid speaker system, which works with sound-wave cancellations for the low freq, and very narrow dispersion ( vertical ) for the high freq, the exit of the high freq module ( disspersion device ) has to project planar sound waves,
So, You have extreme control in the vertical disspersion, in almost all freq. with power, and crystal clear sound.
I´ve worked with those systems in very big shows (stadiums, arenas ) and let me tell You that this is the most amazing sound system there are for big rooms, where You need long-throw, with no extra energy in the ceiling.
Check this tho links from 2 of the most important Line Array Manufacturers in the world. ( Pro Audio )
http://www.meyersound.com/support/papers/line_array_theory.htm
http://www.l-acoustics.com/anglais/produitsgb.htm
In L-Acoustics, download the V-DOSC Manual.