Line array
The term line array describes a concept of PA systems that are used at concerts or other live events. It is a loudspeaker system for the transmission of music or speech over greater distances, for example the auditorium of a large concert.
Basics
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A distinction is made between conventional loudspeaker systems , in which several loudspeakers are stacked horizontally - i.e. next to each other - in order to adequately cover a certain area, and line array systems, which supply the auditorium with sound with loudspeakers arranged on top of each other .
The basic condition of a sound reinforcement situation is to supply a spatially limited area with sound. Ideally, this should provide all points in the room with the same signal at a sufficient level with a linear frequency response and linear phase behavior without interference . In most cases , a single loudspeaker on each side of the stage ( stereo sound ) is not sufficient to achieve a sufficient level . When several loudspeakers are combined, there are overlapping areas ( interference ) which lead to cancellation in the frequency image and thus to poor sound. The reason for this is that identical sound waves are sent from at least two different points (speakers). Depending on the phase position, this leads to cancellation at certain frequencies, and to amplifications at others. The result is a distorted frequency response and thus unsatisfactory sound.
technology
To counter this problem, a different approach was taken with the line array technology: These work on the principle of a line wave of finite length. In the horizontal plane, each element has a firmly defined radiation angle of mostly 70 to 120 degrees, which is sufficient to supply the entire auditorium from two points - i.e. without speakers standing next to each other. In the vertical plane the radiation behavior is isophasic , i. This means that all frequencies to be transmitted are directed in phase in only one direction. If several such elements are grouped one on top of the other, a coherent wavefront results in the vertical that has almost no interference , as each point in the room is covered by only one loudspeaker per (stereo) side.
With this technology, it is possible without any problems to fill a complete auditorium with several thousand people from the stage, since each loudspeaker element of the line array has a significantly higher sound pressure level due to the strong bundling of the sound at a distance .
Cylindrical wave propagation (line source)
With conventional loudspeakers, be it as direct radiators or horn systems, the far field is based on a spherical form of wave propagation. In this case, measurements under free field conditions result in a frequency response curve that is independent of the measuring distance and that drops by 6 dB per doubling of the distance without changing its shape. The far field condition is already clearly fulfilled for a typical measuring distance of 4 m. The situation is different with loudspeakers that are large in size in one direction or plane. Line sources or surface radiators have an extensive near field that extends far into the usable area of the loudspeaker. A line source of finite extent first of all emits a cylindrical wave in the near field, which turns into a spherical wave front in the far field. In an idealized approach, the level for the cylindrical wave area only drops by 3 dB per doubling of the distance. Since this transition takes place at a frequency-dependent distance, the frequency response of such a system changes with the measuring distance.
Speaker systems
In order to produce loudspeaker elements with the radiation characteristics of a line wave, some technical and physical hurdles have to be overcome. Achieving a defined angle over the entire frequency spectrum in the horizontal direction of radiation is not a problem. As with conventional PA systems, this is also achieved through the use of horns with appropriate opening angles and bends for the individual paths.
The requirement for the vertical radiation behavior to generate an isophasic wave front is far more difficult to implement. There are different approaches to this in the high frequency range. What they all have in common is the use of so-called wave guides . These are horns to which a conventional high-frequency driver is connected and the sound of which emerges at the horn opening in phase.
The French company L-Acoustics is considered a pioneer in the field of modern line array technology. Their world-wide first functioning line source array, launched in 1992 with the V-DOSC system, works with a phase plug or waveguide placed inside the horn , which is used to redirect the sound of different wavelengths. This waveguide is protected by patents worldwide. The V-DOSC system has been in use practically unchanged since 1992 and is generally regarded as the industry standard in the field of line array systems. With the approach of the manufacturer JBL , this happens through a long canalization of the sound with paths of different lengths for the different wavelengths . By the time the horn opens, the differences in transit time are evened out and the sound reaches the outside as a coherent wave front. The manufacturer NEXO takes a different approach. Here an ordinary high frequency driver radiates its sound onto a parabolic reflector, which aligns the different wavelengths in phase with its curvature.
The result of all approaches is a coherent wave front for the high frequency range. In the mid-range, loudspeakers arranged one above the other have the ability to bundle their sound due to the longer waves. Line arrays make use of this by placing the midrange drivers on top of each other in each individual element. When the entire array is stacked, the drivers of all elements are ultimately in a vertical line and thus act as a line source .
In the vertical plane, this also tends to apply to the low-frequency range, but due to the narrow design of the loudspeakers, the lower frequencies are bent around the array at least in the horizontal direction and emitted in a more spherical manner.
With regard to the transmission of the sub-bass range, there is no difference between a line array and a conventional sound system. For this purpose, horn-loaded basses with large bass drivers are still used according to the bass reflex principle . These stand in front of or to the side of the stage as usual, but some manufacturers can also arrange them in a line array.
commitment
In order to fill an entire hall with sound, it is not enough to simply stack the elements of a line array straight on top of one another. Since the line-like directional behavior of the loudspeakers becomes wider from a certain distance, this would again result in overlapping areas. Concentrating the entire service in a relatively small room that can be exposed to sound would also be disadvantageous. In order to counteract these factors and to make use of them, the line array is hung in an elevated position (flown) and in a curved arrangement (curved). The individual transverse elements are fixed at their front corners, but can be angled. At the rear corners of the trapezoidal housing, connecting pieces that can be used in several grid points ensure the correct angle between the loudspeakers. If you apply this technique to eight connected elements, for example, you get a curved array of loudspeakers (“banana”).
advantages
The advantages in terms of the radiation behavior of the entire array are primarily the freedom from interference and the even distribution of sound pressure across the entire auditorium. This is achieved through targeted and controlled radiation of the sound and the precise interaction of the individual elements. However, this also has a disadvantage: In order to use the system correctly and effectively, precise planning is required. The system engineer must receive information about the geometry and nature of the room to be covered. Here, the user is provided with computer simulations , without which an exact calculation of the installation location, the correct height and angle as well as the number and type of loudspeakers would be very complicated and time-consuming. These programs are usually made available by the manufacturers of the line array systems and are tailored to them.
The long range of this type of sound reinforcement makes it easy to see that line arrays are not suitable for small rooms. Strong reflections from the rear wall would be expected here, which would counteract the avoidance of interference.
Line arrays have a major advantage in terms of handling over conventional systems. In contrast to these, a line array consists of many small elements due to its design, which make assembly and dismantling as well as transport and replacement much easier.
swell
- Volker Holtmeyer: Mystery Line Array - Fashion or Trend? March 5, 2003 (PDF file; 2 MB).
- Volker Holtmeyer: Line Arrays - The Hype Goes On. 2006 (PDF file; 550 kB).
- John Meyer: "Can Line Arrays Form Cylindrical Waves? A Line Array Theory Q&A" 2005
