Why Large Sound Systems
Need to Go Vertical
by Marty McCann
 In
this article I will address the issue of vertically arrayed
sound systems, explaining why arraying the larger sound system
vertically can immensely improve upon the performance of the
same number of components spread out in the horizontal plane.
I have been both a student and practitioner of concert-level
sound-reinforcement applications for more than twenty years
now. I have closely observed major-sound company systems and
conducted numerous experiments of my own. One problem, as I
see it, is that many smaller sound companies don't array their
loudspeakers in a tall-enough vertical array. They tend to
spread the system out in the horizontal plane, which increases
the overlap of the constant-directivity horns and results in
less-than-ideal polar patterns.
The best system (by another manufacturer) for large venues
that I have ever heard is the ShowCo Prism system, which is
arrayed four or five times taller than it is wide. Granted,
it is more work to stack the loudspeaker system in the vertical
plane. In most cases you must encase the speaker array in scaffolding,
and/or band it together with nylon webbing to insure that the
system will not tumble down on the audience. Some of the larger
touring sound companies have a pneumatic loudspeaker tower
system that enables them to attach each row of speakers beneath
each other while they lift the entire array. The ShowCo Prism
system consists of several rows of splayed (arrayed) loudspeakers
attached to a circular steel tower.
Today, just about every major touring sound company employs
its loudspeakers in an array that is at least four to five
times taller than it is wide. This is the optimal method of
providing large-scale sound. When the system is arrayed vertically,
it becomes one large line radiator that does a much better
job of projecting a correlated sound field to the farthest
seats. When the loudspeaker array is raised vertically, as
opposed to horizontally, you can obtain the proper coverage
by providing individual far, mid, and near-field segments of
the array. I have seen many small companies attempt to supply
outdoor-festival sound over the years, and typically their
loudspeaker stack is four or five times wider than it is tall.
This is not the best way to get the most performance with a
given set of loudspeakers.
To array a system vertically, it requires considerable scaffolding
to surround and secure the stability of the vertical array.
ShowCo solved this problem by engineering a safe and secure
means of quickly hoisting its vertical array based on a custom
designed pneumatic lifting tower, which doesn't require any
scaffolding. The top of the tower consists of a large "wagon
wheel"looking circular metal frame, which is attached to a
center metal pole that is telescoped (raised). The loudspeakers
are rigged in layers to the bottom of one another as the tower
is raised. Today, most of us use constant-directivity high-frequency
horns in our touring systems, which are unsurpassed for propagating
high frequencies. Before the introduction of constant directivity
(CD) horns in the late seventies, every exponential horn had
the same problem, i.e., the higher frequencies in about the
last two octaves came exclusively out of the throat of the
exponential high-frequency horns. As a result, the high frequency
information was very concentrated, or "beamy," i.e., the higher
frequencies beamed like light from a flashlight and could not
cover as wide a horizontal angle as the lower frequencies.
One mistake many sound engineers make, with systems large and
small, is allowing their constant-directivity high-frequency
horns to overlap too much in the horizontal plane. The medium
of sound is the air itself. Sound propagation actually entails
increases and decreases in the barometric pressure. When the
loudspeaker moves forward, it increases the air pressure in
front of the speaker. When the speaker moves backwards, it
acts a bit like a vacuum pump, causing the air in front of
the loudspeaker to become rarefied, or less dense. Each audio
cycle that can be reproduced consists of a compression of the
air molecules followed by a rarefaction of the air. Imagine
two billiard balls that are rolling down a pool table at an
angle to one another. Let's imagine that they are going to
collide and that their paths form an angle of forty-five degrees.
Once they collide, they will move in two different directions,
forming another forty-five-degree angle away from the point
of collision. In acoustics, we say that the angle of incidence
(arrival) is equal to the angle of coincidence (departure).
If two billiard balls traveling toward each other collide and
move apart, forming the same angle as when they hit together,
what makes anyone think that colliding air molecules would
perform any differently?
Another important factor often overlooked by the less-informed
soundperson is the sensitivity rating of the components within
a multi-way sound system. With more efficient loudspeaker enclosures,
fewer speakers and less power are required. The Peavey HDH(TM)
series enclosures are still the best tools that we offer for
large-scale sound reinforcement. Just because an enclosure
is horn-loaded, it does not necessarily mean that it is high
in efficiency. Let's use the mid-frequency horn of the HDH-1
or HDH-4 as an example: with one watt of power applied, the
MB-3 horn in the HDH product will produce 109 dB of SPL at
1 meter away from the enclosure. We and other manufacturers
also have mid-frequency horns that produce 101 dB of SPL (1
watt @ 1 meter). The MB-3 horn is rated power-wise at 700 watts
of program material. At 700 watts of power the MB-3 can produce
137 dB of SPL (10 log 700/1). A mid-frequency enclosure that
exhibited a 1 watt @ 1 meter sensitivity rating of 101 dB would
require 4,500 watts of power to produce 137 dB of SPL (10 log
4,500/700 = 8 dB) to make up for the decibel difference in
sensitivity. I don't know of any single loudspeaker enclosure
with a maximum program rating of 4,500 watts.
Now that I have provided this preliminary information, let
me show you an example of a Peavey large-stadium sound-reinforcement
system. I went to Dakar, Senegal a couple of years back and
spent a week in a 100,000-seat soccer stadium, working with
the components of a large Peavey HDH sound system. We set this
system up employing several different configurations of the
individual components. We started out with the system configured
in a typical horizontally spread arrangement. Actually, the
customer's own sound engineers had already configured and used
the system this way before my arrival. We set the left side
of the stadium up in the manner outlined in the following section
and compared the two different arrangements, i.e., we conducted
an "A/B" listening test. I had drawn up six different approaches
for the number of enclosures that we had to work with. We then
proceeded to try each of the other five versions of a vertical
array on the right side and compared it to the left side. Some
of the versions sounded better than others, but none of them
could match the performance of the first vertical array on
the left side.
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