User Guide

Sound System Design Reference Manual
Using Directivity Information
A knowledge of the coverage angles of an HF
horn is essential if the device is to be oriented
properly with respect to an audience area. If polar
plots or isobars are available, then the sound
contractor can make calculations such as those
indicated in Figure 3-9. The horn used in this
example is the JBL 2360 Bi-Radial. We note from the
isobars for this horn that the -3 dB angle off the
vertical is 14°. The -6 dB and -9 dB angles are 23°
and 30° respectively. This data is for the octave band
centered at 2 kHz. The horn is aimed so that its
major axis is pointed at the farthest seats. This will
ensure maximum reach, or “throw,” to those seats.
We now look at the -3 dB angle of the horn and
compare the reduction in the horn’s output along that
angle with the inverse square advantage at the
closer-in seats covered along that axis. Ideally, we
would like for the inverse square advantage to
exactly match the horn’s off-axis fall-off, but this is
not always possible. We similarly look at the
response along the -6 and -9 dB axes of the horn,
comparing them with the inverse square advantages
afforded by the closer-in seats. When the designer
has flexibility in choosing the horn’s location, a good
compromise, such as that shown in this figure, will be
possible. Beyond the -9 dB angle, the horn’s output
falls off so rapidly that additional devices, driven at
much lower levels, would be needed to cover the
front seats (often called “front fill” loudspeakers).
Aiming a horn as shown here may result in a
good bit of power being radiated toward the back
wall. Ideally, that surface should be fairly absorptive
so that reflections from it do not become a problem.
Directional Characteristics of Combined
Radiators
While manufacturers routinely provide data on
their individual items of hardware, most provide little,
if any, data on how they interact with each other. The
data presented here for combinations of HF horns is
of course highly wavelength, and thus size,
dependent. Appropriate scaling must be done if this
data is to be applied to larger or smaller horns.
In general, at high frequencies, horns will act
independently of each other. If a pair of horns are
properly splayed so that their -6 dB angles just
overlap, then the response along that common axis
should be smooth, and the effect will be nearly that of
a single horn with increased coverage in the plane of
overlap. Thus, two horns with 60° coverage in the
horizontal plane can be splayed to give 120°
horizontal coverage. Likewise, dissimilar horns can
be splayed, with a resulting angle being the sum of
the two coverage angles in the plane of the splay.
Splaying may be done in the vertical plane with
similar results. Figure 3-10 presents an example of
horn splaying in the horizontal plane.
Figure 3-9. Off-axis and inverse square calculations
Figure 3-10. Horn splaying for wider coverage
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