L-Acoustics V-DOSC/dV-DOSC line array at a concert

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A line array Download older version of keynote for ios. is a loudspeaker system that is made up of a number of usually identical loudspeaker elements mounted in a line and fed in phase, to create a near-line source of sound. The distance between adjacent drivers is close enough that they constructively interfere with each other to send sound waves farther than traditional horn-loaded loudspeakers, and with a more evenly distributed sound output pattern.

Line arrays can be oriented in any direction, but their primary use in public address is in vertical arrays which provide a very narrow vertical output pattern useful for focusing sound at audiences without wasting output energy on ceilings or empty air above the audience. A vertical line array displays a normally wide horizontal pattern useful for supplying sound to the majority of a concert audience. Horizontal line arrays, by contrast, have a very narrow horizontal output pattern and a tall vertical pattern. A row of subwoofers along the front edge of a concert stage can behave as a horizontal line array unless the signal supplied to them is adjusted (delayed, polarized, equalized) to shape the pattern otherwise. Loudspeakers can be designed to be arrayed horizontally without behaving as a horizontal line source.^[1]

Modern line arrays use separate drivers for high-, mid- and low-frequency passbands. For the line source to work, the drivers in each passband need to be in a line. Therefore, each enclosure must be designed to rig together closely to form columns composed of high-, mid- and low-frequency speaker drivers. Increasing the number of drivers in each enclosure increases the frequency range and maximum sound pressure level, while adding additional boxes to the array will also lower the frequency in which the array achieves a directional dispersion pattern.

The large format line array has become the standard for large concert venues and outdoor festivals, where such systems can be flown (rigged, suspended) from a structural beam, ground support tower^[2] or off a tall A-frame truss tower.^[3] Safari 5.1.7. Since the enclosures rig together and hang from a single point, they are more convenient to assemble and cable than other methods of arraying loudspeakers. The lower portion of the line array is generally curved backward to increase dispersion at the bottom of the array and allow sound to reach more audience members. Typically, cabinets used in line arrays are trapezoidal, connected by specialized rigging hardware.^[4]

History[edit]

Four-driver column loudspeaker polar patterns, taken at six frequencies

The line array effect of the narrowing of the beam with increasing frequency was first demonstrated by acoustical pioneer Harry Olson.^[5] He published his findings in his 1957 text, Acoustical Engineering.^[6] Olson used line array concepts to develop the column speaker in which vertically aligned drivers in a single enclosure produced mid-range output in a wide horizontal and narrow vertical pattern. Line arrays have been around for over half a century but until recently most were voice range only. The application for these were for highly reverberant spaces where a narrow vertical design kept from exciting the reverberant field.^[7]

A multi-band line array elements in a horizontally oriented enclosure was suggested by Joseph D'Appolito in 1983.^[8] However, it was L-Acoustics' V-DOSC line array in the mid-1990s that would show the concert world that a more level and smoother frequency response can come from fewer boxes in a line array. As soon as people realized that there was no destructive interference in the horizontal plane and waves combine mostly in phase in the vertical plane, the race was on for loudspeaker manufacturers.^[7]

Theory[edit]

Pure line array theory is based on pure geometry and the thought experiment of the 'free field' where sound is free to propagate free of environmental factors such as room reflections or temperature refraction.

In the free field, sound which has its origin at a point (a point source) will be propagated equally in all directions as a sphere. Since the surface area of a sphere = 4π r² where r is the radius, every doubling of the radius results in a four-fold increase in the sphere's surface area. The result of this is that the sound intensity quarters for every doubling of distance from the point source. Sound intensity is the acoustic power per unit area, and it decreases as the surface area increases since the acoustic power is spread over a greater area. The ratio between two acoustic pressures in deciBels is expressed by the equation dB = 20log(p1/p2), so for every doubling of distance from the point source p1 = 1 and p2 = 2, thus there is a sound pressure decrease of approximately 6 dB.

A line source is a hypothetical one-dimensional source of sound, as opposed to the dimensionless point source. As a line source propagates sound equally in all directions in the free field, the sound propagates in the shape of a cylinder rather than a sphere. Since the surface area of the curved surface of a cylinder = 2π r h, where r is the radius and h is the height, every doubling of the radius results in a doubling of the surface area, thus the sound intensity halves with each doubling of distance from the line source. Since p1 = 1 and p2 = 4 for every distance doubled, this results in a sound pressure decrease of approximately 3 dB.^[9]^[10]

In reality, dimensionless point sources and one-dimensional line sources cannot exist; however, calculations can be made based on these theoretical models for simplicity. Thus there is only a certain distance where a line source of a finite length will produce a sound pressure higher than an equally loud point source. Marvinsketch download mac software.

Interference pattern is the term applied to the dispersion pattern of a line array. It means that when you stack a number of loudspeakers vertically, the vertical dispersion angle decreases because the individual drivers are out of phase with each other at listening positions off-axis in the vertical plane. The taller the stack is, the narrower the vertical dispersion will be and the higher the sensitivity will be on-axis. A vertical array of drivers will have the same horizontal polar pattern as a single driver.

Other than the narrowing vertical coverage, the length of the array also plays a role in what wavelengths will be affected by this narrowing of dispersion. The longer the array, the lower frequency the pattern will control.^[7] At frequencies below 100 Hz (wavelength of 11.3 ft) the line array which is less than approximately 3 meter long will start to become omnidirectional, so the system will not conform to line array theory across all frequencies.^[11] Above about 400 Hz the driver cones themselves become directional, again violating the theory’s assumptions, and at high frequencies, many practical systems use directional waveguides whose behavior cannot be described using classical line array theory. In short, the geometry of real-world audio line arrays as used in public address systems can only be modeled approximately by line array theory, and only in the 100–400 Hz range.^[11]

High frequencies[edit]

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Practical line array systems act as line sources only in the low- and mid- frequencies. For the high frequencies, some other method must be employed to attain directional characteristics that match those of the lows and mids. The most practical method for rei