Line Arrays

The line array loudspeaker system has become the dominant speaker system in the touring sound industry. Line arrays offer significant benefits over horizontally arrayed clusters. They have a more consistent frequency response over the entire area and increased high frequency throw for long distances. Line arrays increase directivity in the vertical plane dramatically producing a narrow vertical beam whilst the horizontal coverage remains the same as for a single unit. They minimize also floor and ceiling reflections. Another advantage is that set-up times are significantly shorter than with conventional speaker systems.
See also JBL's white paper.

Theoretically a line array is a group of radiating elements arrayed in a straight line and closely spaced. The line array principle was already described by Harry F. Olson in his ‘Elements of Acoustical Engineering’, published in 1940 and 1957. Olsen showed that the directivity of the line array increases with array length (the distance between the individual drivers should be smaller than the wavelength that is being produced by them).

The resulting principle, that a simple vertical array of acoustic radiators produced increased directivity in the vertical plane, has been around for a long time. The units have been smaller and (as a result) were not able to transmit lower frequencies. Most were voice-range only. They were much smaller than horn-loaded boxes. These systems were used often for reverberant environments like churches and railway stations.
In addition to the narrowing vertical coverage angles, the array length determines what wavelengths will be affected by narrowing of dispersion. The longer the array vertically is, the lower is the frequency there a pattern control is possible.

But in the early 90s the French company L-Acoustics came out with the first practical system marketed as a line array. The system was designed by Dr. Christian Heil.

The line array is not able to create a real cylindrical sound wave, as is often announced. It is theoretically possible to construct an audio line array that follows the line array theory at low frequencies, but it requires a very lange number of single units. This line array would be several hundreds of meters long ...
Different effects have to be considered over the audio bandwidth. In the very low frequency range, under 100Hz, the length of the array is too small compared to the sound wave length. Line arrays in a size that can be handled don't work for very low frequencies.
In the lo-mid and mid range the effect is closest to the line array theory but the practical effect is still more based on constructive interference that occurs on-axis of the array and destructive interference (combing) off-axis.
At high frequencies the HF-drivers are spaced too far apart and the line array theory cannot work at this frequency range. As the sound wavelength decreases, more and smaller drivers are required to provide the required directivity. To compensate, all practical line array systems on the market use directional wave guides and horns for the high frequency range to match the sound propagation of the mid and lo-mid range. These horns achieve the required directivity not by the line array theory but by reflecting sound into a specified coverage pattern. Horns for extreme narrow vertical coverage and a wide horizontal coverage are chosen.

In the near field a line array will produce sound waves that drop nearly 3 dB per doubling of the distance. But the extent of the near field varies by frequency and the array length. At the far field line arrays react like any other sound source (no magic involved ...).

The nearly 'cylindrical wave' in the mid and lo-mid range can only be created by using a certain number of systems. Just a few cabinets will not work in the desired way because the line array theory is based on these large number of chassis working together in a vertical system.

Curving line arrays is used to change the vertical coverage angle. But because of the differences in the wave length at lo, mid and high frequencies the line array reacts different in these frequency ranges. A gentle curving of a line array can aid in covering a broader coverage area.
For lo frequencies the curving angle is very small compared to the own wave length and don't make a significant difference. For mid frequencies the splaying works best, as the line array theory works best for these frequency range. For high frequencies horn drivers are used and any change in the distribution angles will be perceivable in the high frequency sound coverage. A too large curving angle will produce high-frequency hot spots and areas of poorer coverage.