Won,

(This is kinda long)

The cool deal with a perfect line source (infinite length, single transducer) is that it launchs a hemicylindrical wavefront. This pressure wave is different from a point-source hemispherical wavefront in that its width spreads as it travels outward, but its height remains the same (theoretically infinite). A point-source wavefront spreads in width and hight and its pressure decays at 6dB per doubling of distance, but a perfect line source wavefront, spreading in only one dimension (width), decays at only 3dB per doubling of distance.

This effect is a definite bonus if you're trying to illuminate long distances as evenly as possible, such as a crowd at a large concert, ergo the popularity of large line array PA systems.

There is no infinitely tall loudspeaker, so there is no such thing as a perfect self-contained line source. However, if you physically limit the height dimension with a floor and ceiling and illuminate that space with a floor-to-ceiling line source, you still end up with a fine hemicylindrical wavefront and all the joy that it brings.

Speakers that are not infinitely tall or loaded by a ceiling also launch a hemicylindrical wavefront, but that wave will begin spreading vertically. Over distance, it will become more hemispherical and tend toward the corresponding 6dB decay rate.

I don't remember exactly, but I think the wavefront from a finite line souce in free space loses much of its line source characteristic after traveling 2x the line height. So if you want to hear line source effect from 4' tall array in free space, you'll probably have to keep your head no more than 4' from the middle of it.

Incidentally, line arrays (multiple drivers) in free space only approximate a line source at wavelengths longer than total array length. At higher frequencies, the beams begin to decouple and develop interference effects--lobing & comb filtering. (See my previous post about line array comb filtering.) Also, in your MMMTTMMM situation, the Ms will behave more like a line source than the Ts will, so they will have different decay rates (highs will drop off faster as you move away).

All this is to say that, strictly speaking, a line source isn't a line source unless A) it is loaded by a floor and ceiling or B) you sit within its shadow.

And none of this is to say that a 4' tall MMMTTMMM quasi-not-really line source couldn't sound glorious

Bill