BRIDGE MODE APPLICATION
OF THREE LOUDSPEAKER CLUSTER
By Marty McCann
Copyright 1995
If you have read my previous article on the operation of power
amplifiers in bridge mode, or if you already understand how
bridge mode works, I will present an application to you that
employs a CS-X amplifier in bridge mode to drive three loudspeakers
in a single cluster or array.
In many rooms such as churches and auditoriums where a single
source of sound is desired, a common approach is to employ
an array of loudspeakers flown directly over the front edge
of the stage or sanctuary platform. Some rooms have an aspect
ratio where they are twice as long as they are wide, and perhaps
a single loudspeaker system will not have adequate horizontal
coverage to include all the seats to the near left and right
in the pattern of the horn.
Modern loudspeaker systems employ constant directivity high
frequency horns that exhibit uniform frequency response within
their included angles of coverage. This constant directivity
horns, called CD horns, are the best tools for accurate reproduction
of high frequency information. However, these CD horns should
not be placed side by side where they can overlap in their
coverage angles. If two of these CD horns are allowed to overlap
a great deal, they will have diminished output directly on
axis between the two horns at some frequencies. Part of the
rationale for trapezoid boxes is that they are supposed to
minimize the overlapping of these horn patterns. However, it's
a fact that many trapezoid boxes have trapezoidal angles that
are smaller than the angles of coverage of their high frequency
constant directivity horns.
Therefore with many boxes, if you pack the trapezoidal enclosures
tightly, you still have severe overlapping of the high frequency
output directly on-axis. Often the boxes must be splayed farther
apart to minimize the overlap of the horns.
In many auditorium applications where you need coverage for
both the near and far field seats, you can address the room
with two sets of loudspeakers. The near seats are in the near
field and the far seats, of course, are in the far field. You
can solve the problem of excessive overlapping, and still address
the near and far field requirements by employing an array of
three loudspeakers. In my example I am going to use three HDH-244T
enclosures. The three speakers would be flown in the center,
directly above the front edge of the stage or sanctuary platform.
The center HDH-244T would be flown right side up, with the
horn on top, and it would have some downward angle to it. The
two outside HDH-244T loudspeakers would then be flown upside
down, with their horns on the bottom, and they would have more
downward angle. With most of out Peavey enclosures, this downward
angle would be twenty to twenty-five degrees greater than that
of the center loudspeaker.
Now the loudspeakers would be "Arrayed" properly, but we must
do something to allow the center speaker to truly take care
of the Far Field seats. The inverse square law says that the
sound coming from a loudspeaker is reduced in level directly
proportional to the inverse of the square of the distance away
from the source. The sound coming directly from the loudspeaker
is called the direct field. The inverse square law can be simplified
if we relate it to the decibel scale. The direct field emanating
from a loudspeaker drops in level or diminishes, at a rate
of -6 dB, every time you double the distance away from the
loudspeaker. Therefore, in our application, in order for the
Far Field loudspeaker to provide adequate coverage, the Near
Field loudspeaker system must be turned down in level by -6
dB, so that the Far Field speakers can have a chance to provide
the proper level to the farthest seats.
Also, if the Near Field speakers are not reduced in level,
the gain of the entire system, before feedback occurs, will
be limited by the Near Field speakers themselves. This is because
the Near Field speakers will react with the open microphones,
causing the system to feedback before the Far Field speakers
can reach the level necessary to provide the desired SPL to
the farthest seats.
In the past, if you wanted to use one power amplifier for this
application, the two Near Field loudspeakers would be put on
one channel of a power amplifier, and the gain or level control
of that amp would be turned down -6 dB. The single Far Field
speaker would be on its own channel and that channels level
control would be set wide open. We would now have a situation
where our gain structures would be set properly for the Near
and Far Field components of the array. If we were to use a
CS-800X in this situation, when the Far Field loudspeaker received
200 Watts from the power amp, each of the Near Field speakers
would receive 50 Watts (-6 dB equals ΒΌ Power). I am basing
this exercise on the condition that each individual loudspeaker
has an impedance of eight ohms. Two hundred Watts may or may
not be adequate power, depending on the room and the type of
music to be reproduced. Also, I would worry that some individual,
who did not understand the need for limiting the level of the
Near Field coverage loudspeakers by -6 dB would come along
and turn the amps' Near Field channel up all the way. This
would completely change the performance capability of the system
as a whole.
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