Sound System Design 1277matter of preventing these two failure modes and
protecting it from weather, physical damage, and pests.34.3.5.1.1 Choosing Power Amplifiers to Prevent
Excessive Average PowerIt is possible to destroy a loudspeaker by using a power
amplifier that is too large (one whose power output
exceeds the power capacity of the loudspeaker by some
margin). It is also possible, under some conditions, to
destroy a loudspeaker by using a too-small amplifier.
The too-small amplifier is one that does not have
enough power output to meet the requirements of both
the needed LP at the farthest listener and the system head
room (from the electrical power required equation).
Attempting to reach the system requirements will exceed
output capabilities of the amplifier, which will cause the
amplifier to clip, turning sine waves into semisquare
waves and vastly increasing distortion levels.
This clipping causes two problems. First, the square
wave can actually draw twice the power output from the
power amplifier. That is, if the amplifier is rated at
100 W, a full-voltage square wave can cause the ampli-
fier to deliver as much as 200 W, depending on
power-supply limitations and the internal protection
circuits of the amplifier. This double power output can
be a threat to the loudspeaker all by itself. Second, the
square wave causes the loudspeaker cone/diaphragm to
move outward (or inward) and stay there for awhile,
then move in the other direction and stay there for
awhile. When the loudspeaker cone/diaphragm is not
moving (at the top and bottom of the square wave), the
energy supplied to the voice coil is being entirely
converted into heat, with obvious consequences.
Thus, one way to prevent loudspeaker damage is to
use a power amplifier that has enough output to reach the
maximum LP requirement and the system head room
requirement. Fortunately, in most sound reinforcement
systems, actual electrical power required is small, and
therefore the problem of a too-small power amplifier
shows up primarily in large sound reinforcement systems
or in popular music-oriented entertainment systems.
Loudspeaker power capacity is usually rated using
some type of noise with a specified head room factor.
For example, a loudspeaker may be rated at 100 W
continuous pink-noise, with a 10 dB crest factor from
50–1000 Hz. That crest factor is the difference between
the average and peak power in the pink-noise signal.
The crest factor concept is similar to head room in the
sound reinforcement system. Thus, in theory, this loud-
speaker can be fed 100 W of pink-noise, band limited
from 50–1000 Hz, and with peaks that reach 1000 W.
The 100 W loudspeaker is, theoretically, safe with a
1000 W amplifier if system head room is kept at 10 dB
or above. In practice, of course, it is very risky to power
a 100 W loudspeaker from a 1000 W amplifier. The
reasons are many but include the operator who will
push the system past its design limits, ignoring distor-
tion and the possibility of sustained feedback that can
draw full power from the power amplifier for an
extended period.
What then is a safe power amplifier size for a 100 W
loudspeaker? In most systems, about twice the rated
power capacity of the loudspeaker will be safe provided
other potential problems are considered, as discussed in
Section 34.3.5.1. In addition, the amplifier should, as
previously discussed, be at least capable of supplying
enough power to meet the system maximum LP and
head room needs. Full-power, sustained feedback can
still, of course, destroy the loudspeaker, but at only
twice the rated power capacity of the loudspeaker, the
system will likely sound very distorted before the loud-
speaker is in danger. This should prompt the system
operator to turn down the level, preventing damage.34.3.5.1.2 Loudspeaker Power Capacity SpecificationsBesides pink-noise power capacity, manufacturers com-
monly use several other power capacity rating methods.
Variations on the concept of program power are used in
an attempt to define the loudspeaker’s power capacity
when the source is normal program material. The inter-
pretation of normal program material, of course,
depends on the manufacturer and it is common for this
power capacity rating to be significantly higher than
other ratings.
Rms power is another common rating method. Math-
ematically, there is no such thing as rms power. rms
power is calculated from rms voltage and load resis-
tance. The correct term should be average power.
However, an rms power rating for a loudspeaker is
usually similar to a pink-noise rating.
The EIA (Electronic Industries Association) has a
loudspeaker power capacity standard using shaped
noise that is similar to a pink-noise rating with low- and
high-frequency roll-off. This standard, known as EIA
RS-426A, is a reasonably reliable indication of the
loudspeaker’s thermal power capacity and is similar to a
pink-noise power capacity.
There are other methods of rating loudspeaker power
capacity (see Chapter 17), but because of the
complexity of the subject, all require some interpreta-