Tuesday, January 5, 2010

Line Array Speakers

Have you ever been to a concert, theater, or conference where near the front of the audience the sound was present and clear, but as you walked further away to go to the restroom, the sound became less present and became muffled almost. This is a common situation with many events. An inexperienced sound technician may attempt to fix this situation by simply turning up the sound system. This is only irritating to the people up front, as they are being deafened, and the people in the back are hearing a lot more of that unclear, muffled sound.

In recent years this issue has been remedied by Line Arrays.

Capsicum Pro Audio & Visual uses the JBL Vertec Line Array, an industry standard used by many of the nations top concert tour providers and event centers. When JBL decided to start the Vertec Training Program, Capsicum was invited to assist in the programs early development.

When line arrays are improperly deployed, they can sound horrible! They require an understanding of acoustic physics and training to set-up properly. When set-up properly, they can provide even coverage, and consistent sound to all parts of the venue. Simply put, someone in the back seats can have the same listening experience as any one in the front seats.

A line array is a loudspeaker system that is made up of a number of loudspeaker elements coupled together in a line segment 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.

The coupling between adjacent drivers results in a frequency-dependent reduction in dispersion along the axis of the line segment. A vertical line array has a narrow vertical dispersion. This results in less loss in sound pressure level over a given distance. Typically sound pressures is lost at 6dB per doubling of distance but in true line sources, it is only lost at 3dB per doubling of distance. The horizontal dispersion is unchanged.

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, whilst 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 or off a tall A-frame tower. 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 together by specialized rigging hardware.


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