Part - Studio Lights

KNOW-HOW ›PRO-SOUND‹ TECHNOLOGY
PA-Systems
Basically, there are two kinds of PA systems: the passive and
the active version.
1. The passive system:
This is the simplest way of frequency segmentation. The
full range signal is sent into the power amplifier and then
arrives at the separating filter, which is located between
the power amplifier and the individual loudspeakers and
consists of spools, condensers and resistors. Passive
separating filters are often directly implemented inside a
loudspeaker. The advantages to active systems are small
technical effort and compactness. However, a big disadvantage
arises from the fact that the frequency segmentation
is carried out behind the power amplifier, and therefore
the full output power runs through the separating filter.
Consequently, large analogue components are necessary,
which heat up very strongly, caused by the energy input,
so that the performance is reduced.
High-pass Hochpass filter
Assembling of loudspeakers
If loudspeaker boxes are piled on top of each other, periodically
the deletion of frequencies occurs. The offset of the speakers
is equivalent to half the wavelength of the first deleted frequency
(see fig. 4). It is therefore essential to install the
loudspeaker cabinets flush mount.
Fig. shows incorrect assembling
λ
2
Sound pressure level (SPL – dBA)
The sound pressure level is an important requirement for the
planning of a room´s acoustic irradiation. The following chart
(fig. 5) shows, which requirements are made to power amplifiers
and loudspeakers in order to implement the project
sizes realistically.
λ
Fig. 4
Low-pass Tiefpassfilter
Fig. 2
2. The active system
Here, in contrast to the passive version, the separating filter
is located in front of the power amplifier. The following
significant advantages result out of this: Smaller components
(IC) are used. No heating takes place. Several features
are possible such as level control, mute, phasing turner,
and the cut-off-frequency can be adjusted.
Line-Signal signal vom
Mischpult
from mixer
High-pass Hochpassfilter
Band-pass Bandpassfilter
Highs Höhen
Mids Mitten
Lows Bass
Decrease of the sound pressure level with
distance (outdoor). A doubling always
results in -6 dB based on 1 m.
Fig. 5
Fig. 3
Low-pass Tiefpass filter
Separating Weiche filter
Lows Bass
The rule of thumb for 3-way power segmentation is:
Approx. 10% of the power are used for the tweeter,
approx. 30% for the mids and approx. 60% for the
woofer.
The widespread opinion, that the speaker´s load rating
has to be higher than the output power sent out from the
amplifier, is wrong. It should be exactly vice versa in
order to avoid damages to the loudspeakers. The output
power of the amplifier should be a little higher than the
designation of the loudspeakers (consider headroom
and clipping). Furthermore, one should bear in mind that
the cables from the power amplifier to the speaker ought
to be as short as possible. It is therefore better to use
two amp-racks for large systems.
Sound control
The application of various kinds of mixers not only depends
on the number of channels and aux lines, but also on the
sound control. The most common mixers for disco applications
or live mixers mostly have a 2- or 3-band sound control
for individual channels: one control for lows, mids and
highs. A decrease or an increase of a certain frequency can
be generated within a frequency range predefined by the
manufacturer, with these controls.
This kind of sound control is sufficient for DJ mixers, as
mostly canned music is used.
In order to equalize e.g. a (singing) microphone and, if necessary,
to filter unwanted feedbacks or characteristics of the
voice, regarding live acoustic irradiation, a so-called “(semi-)
304