| There are a few
misunderstandings about some of the more basic terms used on car
audio, Resistance and Impedance. They both regard the
same concept: The opposition to the flow of electrical current. That
is in fact the definition of Resistance. Impedance, however, is a
little more complex. Impedance is the resistance of a component at a
given frequency. That difference is an important one, as we will see
later.
For now, we will look at resistance. Resistance is quite simple to
understand. Everything in the universe has electrical resistance. It
so happens, that even wood, rubber, plastic, and glass do in fact
conduct electricity just like copper wires do. Their resistance,
however, is so very high, that we use them to "insulate" wires, or
keep them protected from other things (namely, other wires). They are
called "insulators", for obvious reasons.
There are materials, whose resistance is not high enough to be
insulators, but is too high to be a conductor. These have a practical
application as "resistors", or components whose purpose it is to
resist. The amount of resistance these devices have (or the resistance
of anything, for that matter) is measured in a unit called "Ohm"
(pronounce "O - m"), and is represented by a symbol called the Omega.
Although a resistors value (in Ohms) changes slightly with temperature
and with age, for practical purposes, it is the same always, no matter
what the electricity is doing.
Impedance, however, is dependant on a property called reactance, and
frequency. Impedance only exists where there is AC, or fluctuating DC
(AC with a DC bias).
In a coil or capacitor, the reactance
changes due to the way the component works with electricity. A
capacitor, will increase its resistance as the frequency gets lower. A
coil on the other hand, will increase it's resistance as the frequency
gets higher. They will both continue like this until their resistance
is so very high, that no useable current will pass, at which point
they are said to be "saturated". In a normal resistor, at any given
frequency, we can say that it's resistance is still the same. When
graphed, the value of a resistor will be a straight line across the
graph. In a reactive component, however, the line will gently slope
upward, or downward, with frequency.
This is reactance. Reactance is necessary for crossovers to do their
job. Whenever you look inside a crossover (passive crossover, at
least), you will see usually nothing more than a few coils and
capacitors, and occasionally the odd sand block resistor. Remember,
coils resist high frequencies, and capacitors resist low frequencies.
When the two are combined, they form crossover networks. A crossover
network typically uses a capacitor to keep low frequencies from going
to a tweeter, and a coil to keep high frequencies from going to a
woofer.
Another component that has reactance, is the speaker itself. A
speaker's voice coil behaves electrically just like a coil in a
crossover network. Because of this, speaker designers face special
problems when designing midranges and tweeters based on voice coil
drivers. Also, due to it's reactance, a speaker is almost never at
it's rated "Impedance" (a word often used incorrectly by speaker
manufacturers). When a speaker is measured at 4 ohms, it is measured
using a device that puts out DC current to do the measuring. The only
other time the subwoofer will have anything near 4 ohms is when it is
at resonance. The rest of the time, the voice coils impedance is very
high. My 10" subwoofers reach a peak of 45 ohms, and they have a 6 ohm
voice coils.
So now we've seen the difference and similarity of Impedance and
Resistance. Resistance is in everything, Impedance only exists when
you have an AC or fluctuating DC current. Impedance of a reactive
component.
|
DC and
AC themselves are often misunderstood, but for more technical reasons.
DC, or Direct Current, is when current flows in one direction, from -
to +. AC, however, means Alternating current. This is defined as
"Current whose direction of flow changes periodically". While most
people view this as the typical sine wave, AC current does not have to
go below 0 and actually reverse it's direction of flow. Fluctuating DC
is also AC. In fluctuating DC, you actually have an AC signal, which
also has current added to it, so that it never actually goes below 0.
This DC amount must be at least equal to the inverse of the peak
voltage point on the AC wave. Whenever voltage is added to another
voltage, it is called bias. In a 10 volt AC wave, you need to add at
least 10 volts of DC to bias it and make it fluctuating DC. Any less,
and the wave would actually go below 0, and you would have true AC.
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