Handbook for Sound Engineers

1278 Chapter 34

tion. It may be that a single number is simply not suffi-
cient to fully describe a loudspeaker’s power capacity.

34.3.5.2 Protecting against Excessive Low-Frequency
Peak Power

The cone/diaphragm excursion of a loudspeaker
increases at low frequencies. The exact amount of
increase depends partly on the enclosure or horn the
loudspeaker (or high-frequency driver) is used with.
Nevertheless, there is some frequency below which
each loudspeaker/enclosure or driver/horn should not be
used. This low-frequency limit is normally given in the
manufacturer’s specifications or, for a vented enclosure
design, may be estimated as fb (the vented box resonant
frequency). At very low input power, frequencies lower
than this limit will not cause damage. At normal to high
power inputs, however, low frequencies can cause loud-
speaker damage due to overexcursion.
The cure for this overexcursion is simply to prevent
these low frequencies from ever reaching the loudspeaker
by using some type of high-pass filter. This may be in the
form of a system crossover network, which prevents low
frequencies from reaching the high-frequency loud-
speaker, or in the form of a separate high-pass filter
(often part of a graphic equalizer or DSP), which
prevents very low frequencies from reaching the
low-frequency loudspeaker. One valuable protection
device is a series capacitor, used on the high-frequency
loudspeaker, which can reduce the effects of any low
frequencies that may pass through the power amplifier
due to such problems as turnon/turnoff transients.
Significantly, excessive power input to a loudspeaker
at frequencies above its rated frequency range can also
be dangerous. Since the loudspeaker cannot produce
sound from these frequencies, the input power is mostly
converted into heat, adding to the potential problem of
excessive average power.

34.3.5.3 Loudspeaker Protection Devices

Careful system design and operation by an experienced
operator are the best protection against loudspeaker fail-
ure. The following devices can help, however, and may
be used in almost any system design.

34.3.5.3.1 Fuses

Fuses are poor loudspeaker protection devices. Standard
fuses may be capable of protecting a loudspeaker
against excessive average power, but they are too slow

to protect a loudspeaker successfully against sudden

peaks. Fast-blow instrumentation fuses, with improved

time response, may blow on normal program peaks and

needlessly disrupt sound system operation. Slow-blow-

ing fuses, on the other hand, may not blow quickly

enough to prevent loudspeaker damage due to voice coil

overheating.

Despite these limitations, fuses are sometimes used

as loudspeaker protection devices. If fuses are used,

fuse each loudspeaker separately so that a single fuse

failure will not completely interrupt system operation.

Choose a starting fuse size from the following equation:

(34-29)

where,

F is the fuse size in amperes,

P is the rated power capacity of the loudspeaker,

Z is the rated impedance of the loudspeaker.

This equation gives a fuse size that will blow when

the input power to the loudspeaker reaches 75% of its

rated value. Fuse size may be increased if this fuse

blows frequently, but avoid fuses larger than about

twice this value since they will pass enough current to

overpower the loudspeaker.

Early direct-coupled power amplifiers, when they

failed, would often pass their full dc supply voltage to

the loudspeaker. This voltage can result in loud-

speaker/driver voice coils that are described as being

“french fried.” A fuse will help protect a loudspeaker

against this type of power amplifier failure mode. On

high-frequency loudspeakers, however, a capacitor is

probably a better protection device against this problem.

In addition, most modern power amplifiers have some

kind of internal protection (such as an output relay) that

should prevent the problem of dc at the output even

when the amplifier itself fails.

34.3.5.3.2 Capacitors

A series capacitor (connected electrically in series with

the loudspeaker’s positive input lead) can help prevent

excessive low-frequency power and can protect the

loudspeaker against dc power from a faulty power

amplifier. Capacitors can be chosen from

(34-30)

where,

C is the value of the capacitor in microfarads,

Z is the rated impedance of the loudspeaker,

F 0.75 P

Z

= ---

C 500,000

Sf Z

=-------------------