User Guide

Sound System Design Reference Manual
Constant Voltage Distribution Systems
(70-volt lines)
Many distribution systems in the United States
make use of the 70-volt line for powering multi-
loudspeaker paging systems. In Europe the 100-volt
line is common. In either system, the full output
power of the driving amplifier is available at a line
voltage of 70 Vrms or 100 Vrms, respectively.
In placing loads across the line, the design
engineer simply keeps a running count of the number
of watts of power drawn from the line. There is no
need to calculate the aggregate load impedance at
any point in the process. When the total number of
watts drawn from the line is equal to the power rating
of the amplifier, then the line is fully loaded and
properly matched.
Figure 7-4 shows details of a 70-volt distribution
system. The maximum load on the amplifier is
transformed so that the applied voltage will be 70
Vrms. This then will correspond to a total transformed
load impedance, Z
L
, equal to 5000/P
O
, where P
O
is
the maximum power output of the amplifier.
Individual loads are placed across the amplifier
in parallel using line-to-loudspeaker distribution
transformers that have a 70-volt primary and a
tapped secondary designated in watts. The system
designer (or installer) merely has to keep a running
tally of watts drawn from the line, and when the
number of watts equals the continuous output power
rating of the amplifier, then the system is fully loaded.
Ordinarily, no additional loads will be placed across
the line, but there is some leeway here.
The alternative to 70-volt distribution is to
laboriously keep track of combined load impedances
in parallel, a big task. Details of a 70-volt transformer
are shown in Figure 7-5.
In Europe, a 100-volt transmission system,
derived in a similar manner, is used.
Low Frequency Augmentation —
Subwoofers
Whether in the cinema or in open spaces, LF
augmentation systems are becoming popular for
special effects. For indoor applications many
acoustical engineers calculate the reverberant sound
pressure level that can be produced by a transducer,
or group of transducers, operating continuously over
an assigned low frequency band, normally from 25
Hz to about 80 Hz. The equation for determining the
reverberant level is:
7-6
Figure 7-3. Calculation of resistance in wire runs