Who Put the Zip in Speaker Wire?
Well, I’ve been writing this column for almost
four years. You’d think I would run out of things to say
about wire and cable.
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What We Can Measure in Speaker Cable?
Resistance (in ohms):
This effect turns electrical flow into heat.
It affects all frequencies equally. Changing the size (gage)
of the conductor changes its resistance. Resistance usually
is specified per unit length (i.e., 100 feet, 100 meters,
1,000 feet etc.) and is additive.
Capacitance (in picofarads):
With speaker cables that are paired, or coaxial,
the two conductors with insulation in between form a capacitor.
Capacitors hold an electrical charge. The capacitance is
small and measured in picofarads. It is additive and usually
displayed per foot or per meter and must be multiplied by
the actual cable length to get the total capacitance. Capacitance
affects the signal level and is frequency-dependant. The
higher the frequency, the greater the reactance caused by
the capacitance and the greater the signal loss.
Inductance (in microhenries):
Inductance is the ability to hold a magnetic
charge. All conductors have inductance. It is also frequency-dependent
but is in series with the cable, as opposed to capacitance,
which is in parallel. The inductance of a cable is small,
and the effect is cancelled out by the capacitance. Therefore,
inductance rarely is specified in manufactured cables.
Impedance (in ohms):
The total of resistance, capacitance and inductance.
As frequencies get higher, resistance becomes less and less
of a factor. At frequencies above 10 MHz or so, only capacitance
and inductance are left, so the impedance settles to a "characteristic"
value.
Skin Effect (in inches):
As frequencies get higher, the signal tends
to travel on the outside or "skin" of a conductor.
A minor effect for analog audio, skin effect isn’t
a major factor until well into the Megahertz.
Copper Purity (in percent):
While some high-end aficionados require high
purity, such as "six nines" (i.e. 99.9999 percent),
most cable manufactured domestically uses ASTM B115 ETP
copper, which is 99.95 percent pure.
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Oh, no. There’s lots to talk about. And one
subject I have never talked about in depth is speaker cable.
Among the Internet newsgroupies I talk to, there
is no other wire and cable subject that stirs up more mud than
speaker cable. So, let me spend the next few columns trying to
calm the waters and maybe filter out the truth from the snake
oil.
Speaker cable conducts the output of an amplifier
to a speaker. What we want is a cable that conducts the electricity
to the speaker and has no effect on the signal. Well, of course,
this is impossible. There will always be some kind of effect.
The real question is, what effects are they? And,
which effects have an audible result?
Golden ears
After all, we’re talking about "hearing"
the results. And this is the crux of the problem.
While I can tell you the resistance or capacitance
of the cable (and a dozen other parameters), there is no current
way to correlate these measurable effects into what you hear.
Especially when the high-end crowd talk about ‘soundstage’
or ‘detail’ or a myriad of similar terms, there is no
known laboratory process that will tell us the electrical parameters
that cause these effects, or even if these effects exist at all.
In fact, while those high-end folks are arguing
that point, let me tell you a story about a friend of mine. He
is quite well known in the high-end audio community, but I have
omitted names here to protect both the innocent and the
guilty.
This friend was determined to see if a panel of
highly-qualified judges could tell the difference between speaker
cables. He took some 12 AWG zip cord and a number of other exotic
speaker cables, you know, the ones which cost dozens or hundreds
of dollars per foot.
He used a top-of-the-line amp, speakers and top-quality
source material. All these choices were approved by his panel,
as was the source material. He laid the cables out on the floor,
in full view of the panel, and started by hooking up the 12 AWG
zip cord. He had the panel listen for a few minutes to a number
of selections, using 12 AWG zip as the "reference."
Then he had his assistants go behind each speaker,
and behind the power amp, to change the cable. And, of course,
when they got to the expensive cables, well, the soundstage, the
detail, it was dramatic! They played the same selections through
each of the cables. While some of the panel members preferred
one cable over another, they all agreed that they left the 12
AWG in the dust.
My friend told me the names and qualifications
of the panel members, some of whom had advanced degrees. Others
had spent their lives working in the professional audio industry.
These were the tops of their craft. At the end of the test, they
thanked my friend for such an enlightening experience.
My friend said he didn’t have the heart (or
guts) to tell them what really happened. Each time the assistants
went behind the speakers and amplifier to change cables, they
had done nothing. The entire time the panel was listening to
the 12 AWG zip cord.
Subtle vs. gross
What does this mean? It means that, like life, we
hear what we want to hear.
If we think the hundred-dollar-a-foot wire is being
used, then that’s what it should sound like. It’s not
like the thousand-dollar tire on your Ferrari. If someone substitutes
a 4-for-$100 cheapie tire, one sharp curve and you’ll know
there’s something wrong.
Our ears are interpretive. They are not something
we can measure. How very few of us have "perfect pitch"
and can even tell the frequency we are hearing.
Does this mean that all wire sounds the same? No,
it means that the measurable differences in wire do not always
correlate with what we hear. That being said, I would also say
that I could easily design two cables that would sound dramatically
different. In fact, you could do such a test yourself.
For instance, if I had two speaker cables, and
one was 20 pF per foot, and the other 1,000 pF per foot, I guarantee
that you could hear the difference. If I compared 20 pF per foot
to 30, I doubt any human could hear the difference.
Likewise, if you ran down 10 feet of cable to one
speaker, and 1,000 feet to the other, I would bet you could hear
that difference too. And it would be more than just resistance
(signal level).
But these are gross differences. What we’re
talking about are subtle differences, or maybe even differences
that can’t be measured. If you change the jacket of a cable
from PVC to Teflon, does it make a difference in how it sounds?
Not according to anyone in the lab. But ask some high-end audio
folks! For every two of them, you will get three opinions.
So what’s in a speaker cable, what electrical
effects does it create and which of these effects make a difference?
Next time, we’ll start examining the basic "laboratory"
list of measurable parameters.
Steve Lampen is technology specialist, multimedia
products for Belden Electronics Division in San Francisco. His
book "Wire, Cable, and Fiber Optics for Video and Audio Engineers"
is published by McGraw-Hill. Reach him at shlampen@aol.com
