Originally Posted by Mark_V
Please forgive me, but I don't understand most of what you said or what the linked article says. If you will, keep it simple for me.
Say one unhooks a speaker from a receiver and measures the ohm level of that speaker just sitting there. Should it be 8 (assuming it's an 8 ohm speaker) exactly? Or +- a little? If +-, how much +-?
Unfortunately, the DC resistance of a speaker (what you measure with an Ohm meter) is irrelevant and unpredictable without a careful study of the specific crossover designs used in a particular speaker.
"Impedance" (measured in units of Ohms) is the generic term describing how much "push back" you get when a certain amount of current (Amps) flows through a circuit. "Resistance" is the specific term for how much "push back" you get when you apply a constant
amount of pressure (Voltage) while pushing current through the circuit. (A constantly flowing current is also called DC, or Direct Current.) Ohm meters use a battery to apply a constant amount of voltage to a circuit and measure the amount of current that results: Resistance = Voltage / Amps
However, in general, and in speakers in particular, impedance varies depending on the frequency (how often, measured in units of Hertz or cycles per second) at which that pressure fluctuates. (Current which flows back and forth at some frequency is called AC or Alternating Current.) To measure impedance, you have to apply a fluctuating voltage (using an oscillator) and see how much fluctuating current flows.
Crossover circuits, for example, are designed using capacitors (which block DC but pass AC, passing current flowing at higher frequencies better than they do current flowing at lower frequencies), inductors (also called chokes, which pass DC and lower frequencies better than they do higher frequencies) and resistors (which pass a constant amount of current no matter what the frequency). The coils of wire forming the electromagnet of a speaker driver also are an inductor. As a result, a speaker's resistance varies depending on the frequency of the voltage applied to it. In general, speakers let lower frequencies through more easily than they do higher frequencies. This is why bass speakers use more power than tweeters, but that's a gross oversimplification.
The graph in the speaker review that I linked to shows exactly how the impedance of that particular speaker design varies as the frequency of the applied voltage changes. It's very complicated and almost unpredictable if you don't know the exact design of the speaker.