- 2. The Skin
The "skin effect" is one of several frequency
dependent phenomenon. Our precious music is electronically encoded
in the form of a rapidly varying electromagnetic wave that passes
through a conductive metal (wires) and causes the displacement
of a shared surface cloud of electrons. People often speak of
the movement of electrons as the signal, which isn't quite right.
In fact, the velocity of the signal is much faster (close to
light speed) than the speed at which the electrons move. The
reason the signal does not travel at light speed is ultimately
due to reactive damping effects of the cable itself. The wave
really travels through the conductor, displacing electrons much
the way a wave, which is a non-physical entity (energy), travels
through water. However, this is still an over simplification
as the exact details of signal propagation remain an enigma with
no universally agreed upon complete explanation for the phenomenon.
The true behavior is probably best explained by an interaction
of both particle (matter) and wave (energy) properties similar
to that of light conduction.
As an electromagnetic wave (the signal) penetrates into a conductor,
it is quickly damped in amplitude such that the higher the frequency,
the shorter a distance it will travel before it is damped. This
is analogous to the way quicker temperature changes penetrate
a shorter distance into thermoconductors than slower ones per
unit of time. Moreover, the deeper the frequency travels, the
more it is damped, until it reaches an energetic equilibrium
that becomes its "ride depth" or depth of penetration.
Higher frequencies are continually pushed out from the center
of the conductor to their ride depth (the "skin" of
the wire) by a force, the changing magnetic field, which is produced
by the rapidly fluctuating AC current. This force is a result
of self- inductance which is a phenomenon resulting in the opposition
to a change in direction of a signal (AC) due to locally circulating
The "skin depth" is often decided on from a common
formula; (depth of penetration=1/sq root (frequency*pi*magnetic
permeability*conductivity) to calculate the depth to which, for
example, a 20K frequency will penetrate and hence how thick a
conductor could be used for the intended frequency. From this
formula it could be mistakenly concluded that we only need to
use a conductor whose radius is smaller than the depth of penetration
of the highest frequency in audio (20 khz). Also, from this formula
it is evident that Silver wires actually have an even shorter
depth of penetration necessitating even smaller conductors than
copper! This is because of the different conductive characteristics
What is overlooked is where this formula originally derives from.
To calculate to what depth a given frequency penetrates is a
function of to what degree the frequency is attenuated since
it is a continuously increasing effect. The above formula actually
yields the 1/e (inv log of a rather ugly formula) depth to which
a frequency penetrates before it is damped to 36% power (64%
power loss). We may calculate, if we want, the distance a 20kHz
wave would penetrate before it is 99% damped which as you might
expect, is greater. If however, we calculate the distance it
can travel before it is only 1% damped, for instance, we find
it is much shorter and well within the smallest conductor size
used in virtually any audio cable! This formula is very conservative
when applied to audio because it and others were originally derived
for application in radio communication electronics where the
skin effect is a much more serious problem due to the much higher
frequencies (mega hertz and higher) involved.
At very high radio frequencies, the signal only rides on the
very outside or the conductor and does not penetrate into the
conductor at all. The skin effect has somewhat different implications
for complex audio frequencies than for single very high RF carrier
signals, since audio band frequencies are continually cycling
through a gradient of differential resistance as they travel
through the cable. An audio signal consists of extraordinarily
complex multi-frequency bundles whose precise timing relative
to each other (phase) must remain constant in order to accurately
convey the original event. The subtle skin effect non-linearity's
produce very small phase shifts between these grouped frequencies
which is probably manifested in the form of diminished transient
accuracy and reduced or altered harmonic structure.
- The sensible choice therefore, is to use conductors that
are small as possible to keep this gradient of differential resistance
as short as possible which is why we insist on using multiple,
very small, individually insulated conductors (the popular "Litz"
concept) in place of one larger one. This is one of the two reasons
we use two or more runs of the smallest feasible gauge pure Silver
conductors in all our designs. Use of smaller conductors must
however, be compensated by using more of them to avoid increasing
simple DC resistance and producing current limiting effects.
It also requires a very precise, symmetrical design.