CHAPTER 4

CHAPTER 4: LINE ARRAYS AND SYSTEM INTEGRATION

A line array, in the most basic sense, is a group of closely spaced loudspeakers arrayed in a straight line, operating with equal amplitude and in phase. Although line arrays have been used since the 1950s, line array systems that provide full bandwidth directivity are relatively new to the sound reinforcement industry.

HOW LINE ARRAYS WORK

Line arrays achieve directivity through constructive and destructive interference. For example, consider one loudspeaker with a single 12-inch cone radiator in an enclosure. We know from experience that this loudspeaker’s directivity varies with frequency: at low frequencies it is omnidirectional; as the frequency increases (wavelength grows shorter), directivity narrows. Above about 2 kHz, it becomes too beamy for most applications, which is why practical system designs employ crossovers and multiple elements to achieve directivity across the audio band.

Stacking two of these loudspeakers one atop the other and driving both with the same signal results in a different radiation pattern. At common points on-axis, there is constructive interference, and sound pressure increases by 6 dB relative to a single unit. At other points off-axis, path length differences produce cancellation, resulting in a lower sound pressure level. In fact, if you drive both units with

a sine wave, there will be points where the cancellation is complete, which can be shown in an anechoic chamber. This is destructive interference, sometimes referred to as combing.

A typical line array comprises a line of loudspeakers carefully spaced so that constructive interference occurs on-axis of the array, and destructive interference (combing) is aimed to the sides. While combing has traditionally been considered undesirable, line arrays use combing to positive effect: Without combing, there would be no directivity.

M2D loudspeaker’s REM technology provides very narrow coverage in order to:

Minimize destructive interference between adjacent elements

Maximize coupling to throw longer distances

As more and more elements are arrayed in a vertical column, they throw mid- and high-frequency energy more effectively through coupling. The amount of energy can then be controlled using the relative splay between the elements. Gently curving a line array (no more than 7 degrees of splay between cabinets) can aid in covering a broader vertical area, while narrow angles provide a longer throw and coverage that more closely matches that of the low frequencies.

NOTE: Radically curving a line array introduces problems. While a drastic angle

can spread high frequencies over a larger area, low frequencies remain directional (the curvature change is trivial at long wavelengths), resulting in uneven coverage. In addition, a vertically narrow high- frequency pattern combined with large angles can produce hot spots and areas of poor high-frequency coverage.

Mid to Low Frequencies

For the mid to low frequencies, array elements must be coupled together to narrow their vertical coverage and throw mid and low energy to the far field. As frequencies get lower and wavelengths get longer, the splay angle between cabinets has little effect. The number of array elements, however, is important: the more M2D loudspeakers used, the narrower the vertical beamwidth becomes.

THE M2D LINE ARRAY

The M2D loudspeaker employs a unique combination of drivers to enable you to optimize both coverage and directivity in an M2D system. To achieve optimal results, it’s critical to understand how these components work together.

High Frequencies

For high frequencies, the M2D loudspeaker provides a consistent beamwidth of coverage in both the vertical and horizontal planes. In the horizontal pattern of the array, the M2D loudspeaker’s horn works just as any wave guide does to produce wide coverage; in the vertical, however, the

Adjusting Line Array Coverage

Regardless of the needs of your system design, fine-tuning coverage for a single M2D array will be dependent on three factors:

Number of Array Elements. Determining the number of elements to use is critical: Too few elements can drastically affect the uniformity of coverage of both SPL and frequency.

Vertical Splay Angles. Changing the splay angles between array elements has a significant impact on vertical coverage, with the result that narrower vertical splay angles produce a higher Q vertical beamwidth, while wider splay lowers the Q at high frequencies.

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