Arraying Loudspeaker Systems
- Page 3
Some people recognize the need for two loudspeakers to provide
the proper coverage but fail to adjust the level of the amplifier
channels that are driving each loudspeaker system. If the Near
Field loudspeaker enclosure is not calibrated to be 6 dB lower
than the Far Field enclosure, the Near Field enclosure will
be the one to feedback first and limit the gain of the whole
system.
I will remind you once again the concept here is to try to
get as much of the audience within the pattern of the loudspeaker
array, while trying to minimize wasted energy that is not reaching
the congregations ears directly.
What if the room is even longer still? There is a point where
trying to cover a large number of seats from a single cluster
of source becomes much more difficult or even impossible. How
far can one of these single enclosures throw with a 90 degree
by 45 degree high frequency constant directivity horn? The
answer depends on several variables, such as room acoustics,
program material, and the actual distances involved. Most of
our two-way loudspeaker enclosures designed for FOH (Front
of House) sound reinforcement can do a good job up to 50 ft.
or more. Under certain more ideal acoustical
Conditions, they may still perform well at 60 to 70 ft., but
I can't tell you that you would be satisfied with their performance
at distances beyond 80 - 100 ft., let alone at 150 ft. or even
greater distances.
In times past and present, some sound engineers have employed
what are called long-throw horns to increase the coverage with
distance. These so-called long throw devices are far from perfect.
Often, to provide the necessary sound pressure level at the
farthest distances, they are operated at levels that are so
high that they interfere with the array's ability to provide
even coverage in the first place.
No matter how you slice it, sound drops in level -30 dB at
105 ft. (32 meters). (20 log D1 / D2 = 20 log 32 / 1 = 20 x
1.505 = -30.1 dB) or (20 log 105 / 3.28 = 20 log 32 = -30.1
dB). If the main FOH system has to be so loud in level as to
cause pain to the listeners close to the front, what then are
they accomplishing? There becomes a point where the best course
of action is to use properly delayed loudspeaker systems to
cover the rear of the audience. Until recent years, the digital
delay lines were of too poor a quality to do the job effectively.
This is no longer the case, and delayed loudspeaker systems
are becoming more and more the viable solution to the problem
of proper coverage with great distances.
In order to understand how the properly delayed remote loudspeaker
works, you must know about the propagation of sound itself.
Sound travels at a speed of approximately 1130 feet per second.
In milliseconds that is 1.13 feet per millisecond (a millisecond
is 0.001 seconds). In milliseconds per foot this becomes 0.885
or roughly 0.9 milliseconds per foot. So if you want to place
a delayed remote loudspeaker system in a room and calibrate
the delay to the arrival time of the FOH system, you would
measure the distance between the main and delayed loudspeaker
system locations, and multiply this distance measurement in
feet by 0.9. As an example, let's say that the delayed loudspeaker
system is 80 feet out from the main FOH loudspeaker array;
then 0.9 x 80 = 72 milliseconds. Seventy-two milliseconds is
the time it takes sound from the main FOH system to reach the
location of the delayed loudspeaker, which means that without
any delay, the secondary loudspeaker is 72 milliseconds ahead
(in time) of the main FOH system.
It is not enough to set the delay for the signal sent to the
power amplifier driving the remote loudspeaker at 72 milliseconds,
as this will do nothing to maintain the image of the sound
as having originated from the front of the sanctuary. There
is a concept in audio that says that we pay attention most
to that sound that arrives within the first 20 milliseconds.
This is sometimes called the precedence effect or the Haas
effect. Mr. Haas did testing of human auditory perception,
and is more or less the father of Physco-acoustics.
It has been proven that if you add approximately 20 milliseconds
to the actual calculated propagation time (when setting the
delay for the remote loudspeaker), proper imaging is maintained,
and the results are such that no one will even be able to tell
that the delayed loudspeaker system is even operating.
It is not a hard and fast rule that the delay setting is set
at exactly 20 milliseconds greater, I have seen it set any
where from 16 to 26 milliseconds more delay than the calculated
direct propagation time. With live music I have observed that
certain sounds, particularly the cymbals, can appear as if
there is a phase shifter on them if the delay time is exactly
20 milliseconds beyond the calculated propagation time.
Now let us take a look at applications where the room is wider
than in these past examples.
The typical approach in wider rooms is to provide for left
and right near field coverage along with a Far Field System.
This can be best done with a three-loudspeaker array. The outside
loudspeakers are turned upside down to cover the near field
left and right seating areas, while the center loudspeaker
is mounted right side up. The center loudspeaker is still angled
downward somewhat while the outside loudspeakers are angled
down about forty degrees farther.
Below is a scanned photograph of a three-loudspeaker array
as we just described. This particular array is what we have
in our auditorium at the Peavey Dealer Training Center in Meridian,
Mississippi. There is also a complete article that covers how
this approach can be done with a single power amplifier operated
in bridge mode.
There are also some church sanctuaries where the actual longest
distances are to the left and right rear corners of the church.
These rooms are usually more octagon or even pie shaped. In
the case of an application such as this, another type of a
three-loudspeaker system array can be employed. This time,
however, the two outside speakers are to be mounted right side
up while the center loudspeaker is mounted upside-down. The
outside loudspeakers now provide for the Far Field coverage
to the left and right rear corners while the center loudspeaker
becomes our Near Field center fill enclosure. However, this
approach cannot be driven from a single amplifier operated
in bridge mode. In this application the center (Near Field)
loudspeaker needs to be operated -6 dB below the level of the
two Far Field enclosures.
What about balconies and alcoves or under balcony spaces. All
of the above are best addressed as separate acoustic spaces,
which they actually are. Any time the Free Field is truncated
(or reduced to a smaller space), the acoustics involved are
totally different. The Free Field is that portion of the direct
field not influenced by the boundaries. Anytime you introduce
a new space with smaller dimensions; it is not a good idea
to try to provide coverage from the main FOH array. The best
approach for these special requirements is to use smaller loudspeaker
enclosures with delay, as outlines above for delayed remote
loudspeaker systems.
This paper is only intended to be a general guideline of the
principles discussed, and is by no means intended as a cookbook
solution. If you are to have utmost success with a sound system
installation, you need to rely upon someone with solid experience
in the design, calibration, and operation of such systems.
To do otherwise is to risk the possibility of an expensive
short-term experiment in audio. Prayer can be a powerful tool,
but it can't fix a poor sound system design.
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