AN INTRODUCTION TO "LINE-ARRAY" SPEAKER SYSTEMS

While not intended to be a complete technical essay about line-array systems, this chapter will provide a brief summary of the basic theory of operation and of the benefits of line-array systems.
Understanding the physical principals that determinate the behavior of these systems will allow the user to optimize its application in mobile or fixed installations.
The desired result from any sound reinforcement system is high sound pressure and uniform coverage over the audience area. 
This objective has often been addressed by using more powerful speakers, or a larger number of them.
The problems encountered using this strategy are well known.
Line array systems use a different approach to solve these problems.

WHAT MAKES A LOT OF STACKED SPEAKER BOXES A "LINE-ARRAY"?

The simplest definition of a "line array" is: a number of special independent units (loudspeakers), vertically stacked and aligned, that operate as a single sound source and  which provide coherent summing, if some conditions are met.
The vertical stacking achieves a sound field that has a narrow vertical coverage, with higher directivity and sound pressure than conventional systems.
The sound waves emitted are referred as "cylindrical waves" (figure 1), and they attenuate only 3 dB for every doubling of the distance from the source, as opposed to the "spherical waves" (figure 2) emitted by conventional loudspeakers, which attenuate 6 dB for every doubling of distance.

Fig. 1 Fig. 2

This is true up to a distance from the source which is dependent upon its frequency and the height of the array, thus the longer the array is made (building it with more loudspeakers) the longer the throw of the system.
Cylindrical waves only expand in the horizontal plane, not in the vertical plane. The area doubles every time the radius (distance from the source) is doubled, which is equivalent to a loss of pressure of only 3 dB.

The loudspeakers ("elements") that make up a "line-array" must meet a certain set of conditions for the effects to be coherent and acceptable over a wide frequency range:
1 - The distance between the acoustic centers of the various elements must be equal to or lower than half the wavelength corresponding to the maximum frequency to be reproduced.
This means that an array made with small cabinets fitted with small loudspeakers may be effective to a higher frequency (it is for this reason that the PALCOPLUS RA16 loudspeaker is fitted with 8" woofers). This is true for frequencies that are higher than a critical one, that is a function of the array length. This means that, in order to correctly generate low-frequency cylindrical waves, the array must be very long. 
2 - The separation between the high frequency radiators (wave-guides) must be minimal: the wave-guides must be tightly spaced. This is the reason that the array is assembled by tightly coupling the front of the loudspeakers. The wave-guides must be of special design, because the sound waves emitted must be time-coherent: they must generate plane waves. In this way there is no disruptive interference (figure 3 - A) between radiation from the separate wave-guides: they generate plane waves that sum coherently (figure 3 - B).

Fig. 3

The theoretical line array should be a straight line, but in many cases this cannot be done, especially when the array must be flown.
A flown, straight array will not give adequate coverage throughout the audience area and, in practice, it may be necessary to curve the array in order to achieve sufficient coverage of the nearest areas.
This result in a J-shaped array, where the upper speakers (in a straight line) are used for the long-throw coverage and the lower speakers, in a curved line, are used for the short-throw coverage (figure 4).

Fig. 4

Each speaker that makes up the line array must incorporate a rigging system that allows aiming in the vertical plane.
The rigging system, usually an integral part of the units, allows for hinging at the front of the box so that the separation between the speakers stays the same, while the rear plates allow for adjusting the angle between speakers ("splay" angle).
A software program is used to determine the correct angle between the various speakers of the array. Starting from the geometry of the venue, the desired coverage and the number of speakers available, this program will give the correct splay angle between the speakers and the correct rigging point. For the PALCOPLUS system we supply the custom EASE FOCUS software.

REPRODUCTION OF LOW FREQUENCIES

For the reasons explained above, a line-array's low frequency reproduction is limited by the array's length (and by it's components). Special low frequency units are thus used.
To extend the response to the lowest octaves, the PALCOPLUS system utilizes a specially designed sub-bass unit, model RAB1815.
It's two low frequency woofers, an 18" and a 15", are acoustically loaded in different ways, while the geometry of the system and the frequency and phase correction supplied by the LM24 digital controller transform it into a directive low-frequency source, with a cardioid directivity pattern.
The RAB1815, due to it's size and weigh, is not intended to be flown.