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.