What is a line array, and why would you want to hire one?

Line array has been a buzz word in the live sound and PA hire industry for a good few years now, ever since the French company L'Accoustics introduced the first concert worthy system in the early 1990's. But what is a line array?
In simple terms, a line array is a set of speakers, designed to create a coherent line source sound, normally for reproduction of live music or speach.  Well, what's so special about that then?
A 'normal' speaker system is typically a point source.  Point sources of sound can be measured to reduce by 6dB for every doubling of distance you move away from them. Therefore if you were 10m away from a speaker and you have a sound pressure of 95dB(A), then you move to 20m away, all things be equal, and in free space outside, you'll have a sound pressure level of 92dB(A). That's because a point source (at least theretically) radiates in all directions.  A vertical line source doesn't and to cut a long story short, the sound level is supposed to fall 3dB for every doubling of distance. It's because sound radiating up and down interferes and cancels.  Yes there is science behind it, but all you need to know is, the sound dispersion pattern of the speakers is more like a slice of cake and less like a football.  And the sound carries further. That's at least part of the theory, but real life factors like absorption of high frequencies by air and the restrictive length of the array it's self arguably limit some of the benefits.  Have a look here for a more detailed discussion on line array theory.
Some argue that point source speakers still sound better, but the bottom line is that the direction of the sound is often better controlled with a line array, and with much less space taken up in the truck.
Take a look at a big festival sound system, and 9 out of 10 times you'll see line arrays in use.

Martin Audio Line array