Described by Olson in his 1957 classic text, Acoustical Engineering , line arrays are useful in applications where sound must be projected over long distances. This is because line arrays afford very directional vertical coverage and thus project sound effectively. Note also the strong vertical lobes at 500 Hz. Notice that this array maintains its directional characteristic to 1 kHz, where the strong vertical lobe appears. This illustrates the fact that directionality at high frequencies requires progressively more closely spaced elements.
Line arrays achieve directivity through constructive and destructive interference.
A simple thought experiment illustrates how this occurs. Consider a speaker comprising a single twelve-inch cone radiator in an enclosure. This is why practical system designs employ crossovers and multiple elements to achieve more or less consistent directivity across the audio band. Stacking two of these speakers one atop the other and driving both with the same signal results in a different radiation pattern. At points on-axis of the two there is constructive interference, and the sound pressure increases by 6 dB relative to a single unit.