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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.
THE MILO CURVILINEAR ARRAY
The MILO loudspeaker employs a unique combination
of drivers to enable you to optimize both coverage and
directivity in a MILO line array system. To achieve optimal
results, it's critical to understand how these components
work together.
High Frequencies
For high frequencies, MILO uses very precise
Constant Q horns, developed using Meyer Sound's own
anechoic chamber, which provide a consistent beamwidth
of coverage in both the vertical and horizontal planes.
In the horizontal pattern of the array, these horns work just
as any wave guide does to produce wide coverage; in the
vertical, however, MILO'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 project 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 five degrees of
splay between cabinets) can aid in covering a broader
vertical area, while narrow angles provide a longer throw
and coverage which 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 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, line arrays must be coupled
together to narrow their vertical coverage and project 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 MILO loudspeakers used, the narrower
the vertical beamwidth becomes, as illustrated by Figure
4.1.
CHAPTER 4
15
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