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Chapter 6 Communications and Lighting | Up
Construction Electrician Basic - Electricity handbook for electricians | Next
Cable |
The output speaker voltage will be either 25 or 70.7
volts. The speaker will have a microphone precedence
over other input singles and four output terminals with
circuit protection.
Speaker installation is an important element
whenever you install a P.A. system. No matter how
good the amplifier is, if the speaker installation is not
done properly, the sound produced will be inadequate.
There are a number of factors you must consider when
you install speakers. The placement and connection of
speakers is the most important step. For indoor systems,
two types of placement can be used. The speakers may
be placed flat against a wall and the speaker turned so
that it will radiate sound at an angle from the wall. The
other type of placement is to mount the speakers in the
corners of a room; for example, alcoves, balconies,
booths, and dividing walls. A variation of these two
methods mentioned may be considered for installation.
For outdoor systems, the main considerations are
the area to be covered and the direction of sound.
Highly directive trumpet speakers are normally used for
an outdoor area.
When connecting speakers together, you must
consider impedance matching and phase relations.
Mismatching the impedance of a speaker to an amplifier
output in either an upward or a downward manner will
produce different effects. Mismatching upward
(connecting an 8-ohm speaker to the 4-ohm output) will
affect the power delivered to the speaker. Power loss
will be about proportional to the upward impedance
mismatch; in this case, about 50 percent. As a general
rule, no serious frequency response deficiency will be
noted and cannot damage a well-designed amplifier.
Mismatching downward (connecting a 4-ohm speaker
to an 8-ohm output) should always be avoided. It will
reduce the amplifier power output and cause an
overload on the output side with possible damage to the
amplifier.
Figure 6-2 shows an example of two speakers
connected in series. Add the individual speaker
impedances together to obtain the total matching
impedance. The formula for this where Z = impedance
is Zt = Z1 + Z2.
For parallel connection (fig. 6-3) add the reciprocal
of the individual speaker impedances together to obtain
the reciprocal of the total matching impedance. The
formula for parallel connections is as follows:
For series/parallel connections, combine the two
formulas as the speaker connections indicate; for
example, see figure 6-4, and apply the series formula for
Figure 6-2.—Two speakers connected in series.
A and B, then for C and D. Take the results of this and
apply the parallel formula to obtain the final matching
impedance.
Figure 6-3.—Matching two speakers connected in parallel.
6-2
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