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A few rules
Public Address Systems
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Foreword
When a room is designed from the acoustic point of view, it is advisa-
ble to consider, first and foremost, the speaker units. Following this,
the power and the model of the amplifier have to be defined, and lastly
the sources of sound and the most suitable system for connecting the
loudspeakers have to be chosen.
It is possible to identify 6 basic steps to be followed:
1)
Determining the functions required of the system starting out from
the user’s needs (microphone messages, background music, emer-
gency announcements, etc.)
2)
Analysing the environmental characteristics of the areas in which
to broadcast sound: if they are outdoor areas, it is important to mea-
sure the existing noise and also to determine the highest level that the
sound-broadcasting system can reach without disturbing neighbou-
ring activities; if indoors, it is necessary to investigate the acoustic
criticality of the rooms, e.g. by measuring reverberation.
3)
Choosing the speaker units on the basis of the type and size of the
room involved, of the type of information to be broadcast (speech and/
or music), of the noise level in the room and of its acoustic characte-
ristics. The type of speaker unit will differ depending on whether they
are ceiling-mounted or wall-mounted: the sound level they must be
able to reproduce must be a function of the ambient noise. In addition,
the power output required of the amplifier in order to achieve a level
able to ensure proper intelligibility of the message will depend on its
efficiency. Attention also has to be paid to the directivity of the speaker
units, which becomes particularly important if there is any reverbera-
tion in the rooms.
4)
Opting for a suitable amplifier for the speaker units as a whole, with
a sufficient number of inputs to cater for all the sources of sound; if
the system has to include emergency services, it will be necessary to
use a system meeting the specifications indicated in the applicable
standards according to the law.
5)
Defining the sources of sound such as the microphones, wireless
microphones, microphone consoles, wireless tuners, CD players, MP3
players, and which of them must cater for emergency services.
6)
Evaluating the manner of connecting the speaker units, depending
on whether they have a constant impedance (4, 8, 16 Ω) or a constant
voltage (100, 70, 50 V) and determining the cross-section and type of
the cables. The great majority of installations require many loudspe-
akers and therefore constant-voltage systems offer undisputable
advantages in terms of the cross-section of the leads (and therefore of
purchasing and laying them) and of simplicity of installation.
Speaker units
The environment is a fundamental link in the amplification chain, and
it is therefore extremely important to investigate it in depth. The perfor-
mance of the chosen speaker units, their number and their positions
must be based on the environment in which they are to be installed,
above all in order to ensure intelligibility of any announcements so as
not to miss important instructions, e.g. in emergencies. To broadcast
sound out-of-doors, it is necessary to use solely exponential horns
or watertight sound projectors, while indoors it is possible to install
box-type speakers, ceiling-mounted speakers and/or sound columns.
Any architectural , functional or purely aesthetic constraints of the en-
vironment must be taken into consideration. In industrial areas or very
noisy places, it can be advantageous for announcements or voice calls
to use small exponential horns with directional characteristics, so as
to concentrate the sound in certain points of the room. Following are
some detailed suggestions referred to the way in which the loudspea-
kers should be positioned.
Multi-point broadcasting
This solution enables the best possible intelligibility of music/an-
nouncements as it ensures that the sound is broadcast more evenly,
and is achieved using a suitable number of sound-output points, that
must be correctly distributed (best if ceiling-mounted) and driven by
lower power levels. In this way, even if the environment is reverbe-
rating, working with single low volume levels is the best way to avoid
causing annoying acoustic resonance effects.
Broadcasting with ceiling-mounted fixtures
It is a known fact that ceiling-mounted loudspeakers (sound ceiling)
are the best way of broadcasting sound for announcements and
background music. In some circumstances this may not be feasible,
due for example, to the height or particular structure of a ceiling. This
type of system is more expensive than broadcasting via only a few
points, however broadcasting from above has many advantages, and
this type of broadcasting should always be considered before any
other alternatives.
Once it has been decided what type of ceiling-mounted speaker units
to install, and the angle of coverage of the single speakers units is
known, calculating the number of speaker units to use, starting out
from the height of the ceiling and the surface area of the room, is a
simple matter.
The distance
D
between the centre of one loudspeaker and the next
(see Fig.1), is obtained using the following formula:
D = 2*(H-h)*tan(
α
/2)
where:
H
= height of ceiling;
h
= height of listening point;
α
= angle of
dispersion of the speaker unit
The number of speaker units is obtained by dividing the surface area
of the room in m2 by that of the single speaker unit at the listening
point
(D²)
.
Fig. 1