Estudiante

Line Arrays for Live Sound
Stephen Bunting

Line array systems are now the forerunner in large scale live sound reinforcement. This report describes why they have become so accepted and widely-used over the last two decades, explores different arrangements of arrays and why these are necessary, the benefits they have over more traditional horizontally clustered systems in both directionalityand high frequency throw and how these are achieved. It also illustrates practical advantages, disadvantages, cabinet design and techniques for setup, configuration and touring.

1 Background
In the 60s and 70s, sound reinforcement systems used in popular music concerts were often not sufficient to compete with the high levels of cheering, clapping and screaming from the audience. It wasnecessary to begin using an increasing number of speaker cabinets to provide the sound pressure level required, which were usually horizontally arrayed in clusters with a ‘point and shoot’ philosophy and stacked on the side of stage [Webb 2003, 1]. There were some inherent difficulties with this system. • There was often an irregular frequency response due to destructive interference from the closelystacked cabinets; also added to by the lack of HF coupling and the relative abundance of LF coupling. • Systems were generally short throw, again due to the lack of HF coupling, requiring the use of delay stacks at regular intervals.

Line Arrays for Live Sound
Stephen Bunting

-1-

•

Significant floor or stage space was required to stack such a large number of physically large cabinets.These systems started to be superseded by line array systems in the 80s and 90s. These feature a vertical arrangement of specially designed loudspeakers that produce a highly directional sound beam in the vertical plane. They were developed from earlier designs, also known as column loudspeakers, which comprise a tall cabinet with a number of equally spaced, identical drivers, and werehistorically installed in reverberant environments, such as churches or railway stations to aid vocal clarity for announcements. Among the advantages of line arrays are their increased directivity in the vertical plane allowing HF to be projected further, a more consistent frequency response, improved direct to reverberant ratio and the convenience of a modular package that can easily be flown above thestage [Webb 2003, 1-2] [Klepper 1963, 198].

2 The importance of distance
It is important when considering line arrays to define the difference between near field and far field, as this characteristic provides the extended HF throw [Webb 2003, 2]. The far field is identified by a sound pressure level which decreases at 6dB for every doubling of distance. In the near field the sound pressure levelundulates and decreases nominally at 3dB per doubling of distance [Ureda 2001, 6]. This is an oversimplification however. It has been suggested in the past that the array creates a cylindrical wave front in the near field, providing the 3dB drop off per doubling of distance, and transforms to a spherical one at some point. However, it is more accurate to consider the near field as an interferencefield. A point on the line array outputs a signal which will not be in phase with another point on the line, creating destructive interference. This is more prominent at high frequencies because they create more interference than low frequencies, due to the shorter wavelengths. When the far-field boundary is crossed, the interference has diminished enough for the SPL to decay as normal [Button2002, 2]. There are many different opinions of when the far field starts. One source states: “The far field typically begins at a distance about 8 or 10 times the longest dimension of the array, and the critical distance (the transition point from near field to far field) is frequency dependant” [Eargle 2000, 3]. Mark Ureda however shows it slightly more mathematically, stating “The transition...