Speaker cables can
terminate in bare wire or in a number of different connector types, including
¼-inch and XLR connectors, banana plugs and spade lugs. Obviously,
two of these types—¼-inch and XLR—are also used on line
and mic cables, so care must be taken not to substitute mic or line cables
for speaker cables with identical connectors. The cables may look the
same on the outside, but they are quite different on the inside. For example,
line cables use the shield as a second conductor. However, a speaker connection
requires two conductors of equal electrical performance to work properly.
Because the shield in a line cable is not designed for this application,
using one in place of a proper speaker cable would result in poor performance
from both the amplifier and the speaker. In extreme cases, this can result
in equipment damage.
Transmission
Problems
A cable’s job is to transmit an audio signal with little or no signal
loss. As it turns out, though, the cable itself is a major contributor
to problems in signal transmission. The conductors, shield, and insulators
that make up a cable all contribute resistance, capacitance, and inductance,
which affect the signal. This is a complicated subject, beyond the scope
of this article, but here are some basic issues to think about.
All cables exhibit a certain amount of impedance, which is measured in
ohms (represented by ž, the Greek letter omega). Impedance is a measure
of how much the cable impedes the flow of electrical current, and it’s
actually a combination of three electrical properties: resistance, capacitive
reactance, and inductive reactance. Let’s take a closer look at all
three, starting with resistance, which is the tendency to resist the flow
of a direct current. (Of course, audio signals manifest as an alternating
current, but resistance is part of a cable’s total impedance, so
it must be considered.)
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A handy rule of thumb is that 500 feet of 16-gauge wire has a resistance of about 4 ohms. Every change of three gauges doubles or halves the resistance, because such a change also doubles or halves the cross-sectional area of the cable. This means that 500 feet of 13-gauge cable has a resistance of about 2 ohms and 19-gauge of about 8 ohms.
In line-level and mic-level connections, cable resistance isn’t much of a concern; with speaker cables, however, it becomes more of an issue. Because speakers exhibit input impedances in the range of 2 to 8 ohms, the resistance of the cable can add significantly to the overall load. For example, if a 4-ohm speaker is connected to an amplifier with a cable that exhibits a 4-ohm resistance, the cable will dissipate half of the amp’s power before it even gets to the speaker!
Most good-quality speaker cable is of sufficient gauge (12 to 18 AWG) to work at moderate lengths (less than 100 feet); beyond that, however, the cable resistance becomes an important consideration. At 100 feet, for example, 18-gauge cable has an impedance of 1.3 ohms, which could mean a significant power loss. Consider this issue carefully when selecting cable for long cable runs.
Reprinted with
permission from 
Magazine, January, 2001
© 2001, Intertec Publishing, A Primedia Company All Rights Reserved
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