It illustrates the same point that people are used to seeing
with basic horns, that is, the lower the frequency of control,
the larger the mouth must be.
Line Array Performance and General Geometry
The vertical profile of a line array can either be symmetrical
or asymmetrical. What is meant by that is that you can either
have a straight-line array or a curved section but symmetry
still exists about the center axis of the system. The sharpest
beam width will occur for flat or linear line arrays. The higher
the number of sources (n) the more the polar lobing errors are
minimized. This condition occurs independent of the physical
realization or design of the line array itself, and is purely related
to the number of radiating surfaces. Symmetrical curved arrays
broaden the beam width as compared to flat arrays. The more
the curve (the less the radius), the broader the beamwidth. The
third type of profile is that of an asymmetric design. This is
typically the case where there is a curved or flat section on
the top of the array and a more curved or (j) section at the
bottom. The result of this j is to further increase the included
vertical angle of the system, but also to tilt the major lobe.
This tilt is accomplished via the steering properties of the
asymmetrical portion of the array.
Figure 37
shows the vertical lobe generated from a
perfectly flat (or standard linear) line array. It can be shown
that the lobe is extremely sharp and it should always be
remembered that the major lobe emanates from the vertical
center of the system. Early applications of line arrays consisted
of aiming the systems with a laser mounted on the top of the
overall array. This is very inappropriate as can be seen from
any of the figures (
Figure 37, Figure 38
and
Figure 39
).
Regardless of the shape, whether flat, symmetrical, curved
symmetrical, or asymmetrical, the major lobe always emanates
from the physical center of the system and may be steered by
the asymmetrical portion of the array, but generally continues
to emanate from the center.
Figure 38
shows a curved array,
and again, shows symmetry about the center axis of the array.
Figure 39
is a classic J array, and examination will reveal a
lobe very similar to that of
Figure 38
with the addition of the
increase in energy toward the bottom half of the array, where
the j curve is steering the system.
Figure 40
is an idealized
representation of a flat or linear source, showing the center of
the acoustic lobe emanating from the vertical center of the
system. It also represents “old custom” of a laser mounted at
the top and assuming the top box pointed at the back of the
venue presented a major potion of the energy into that area.
As can be seen very quickly from the simple example the
response with a proper line array is very high Q and the ampli-
tude falls off very rapidly from either side of the center of the
acoustic cube. This is desirable working below the center of the
acoustic lobe, as proper aiming can, in fact, compensate for
attenuation of sound with distance and produce remarkably
even front to back coverage. That advantage becomes a
disadvantage if the upper portion of the lobe is attempted to
be used to cover the audience in the rear portion of the venue.
Figure 36
Figure 37
Figure 38
Figure 39
11
