Thanks for the clarification. I guess I did confuse/conflate resistance with impedance. I will cogitate on this.
Resistance (in Ohms) is tied to DC, Impedance (also in Ohms) to AC.
Resistance is constant (barring possible fluctuations with heat, etc.), Impedance variable.
Music is a collection of oscillating signals of varying frequencies... represented electrically, this would be alternating current (of varying frequencies)... impedance would vary with the frequency. If you take a sine wave and reduce the frequency until you reach zero Hz, you pass from AC to DC (non-oscillating current), then, correct? Resistance seems, then, a sort of "frozen" Impedance... it seems that there must be some relationship between the two, though I'll grant that it might not be direct or intuitively grasped.
Linguistically, it's confusing, as "to impede" and "to resist" mean much the same thing in normal parlance. I think I might actually manage to keep them straight now, thanks.
Is a resistor in an AC circuit still a resistor, then? Does this variation in impedance only apply to loudspeakers and other electrically similar components -- whatever they might be? Is this what people are referring to when they speak of "reactive loads" -- a load with impedance that varies with the frequency of the applied signal? That would seem to jive with folks saying that a tube amp reacts differently to a simple dummy load/power-soak than it does to a real speaker...
Just trying to wrap my head around all this.
Okay, so when folks refer to a "high" or "low" 8 Ohm cab, are they talking about the DC resistance of the voice coils, or the nominal impedance (measured at X Hertz) of the speaker? Is there a relationship between the two? Is there a standard for how nominal impedance is determined -- is it always measured at a certain frequency, for instance? -- and is nominal impedance always some "neat" number (4 Ohms, 8 Ohms) or are there speakers with 5.3 Ohms nominal impedance, 6.7 Ohms nominal impedance, etc.?
That's a nice piece of cogitation.
Impedance gets even more complicated.
Resistance and impedance both oppose (resist) the flow of current.
Resistance is defined as opposing the flow of Direct Current.
Impedance is the opposition to Alternating Current flow.
There are related electrical properties called Reactances and they come in two flavors, Capacitive Reactance and Inductive Reactance. Together they make up Impedance.
If you have an inductance such as a voice coil, and apply an alternating current it will have Inductive Reactance.
Likewise, a capacitor will have it's own capacitive reactance.
They both and oppose the flow of current in values relative to frequency.
The two work in opposition to each other.
A cool feature is that when both reactances are equal and the circuit is said to be resonant, and magic stuff happens at resonance. Almost all things in electronics depends on the reactance relationships in the circuits.
Radio and TV and a lot of other things would not work if it were not for the resonance effect.
The impedance of a circuit changes with frequency because the capacitive and inductive reactances change with frequency.
A resistor is a pure resistance at DC. But a resistor in an AC circuit will become more rreactive as you move higher in frequency.
So an impedance usually has at least some degree of the two reactances as well as resistance.
Your eight Ohm driver is a special balance of the two reactances and the resistance.
Your 4 Ohm speaker is just built a little differently to balance how all the reactances and resistances work together to make the impedance.
At radio frequencies a resistor might not act very much like a resistor.
Often, in radio circuits you have to use capacitors and inductors together to do the work that a resistor does at DC or very low frequencies.
In dummy loads or power soaks, those are often made to be mostly resistive.
Those resistors might be built in special ways to minimize their reactance.
Thus it will present an equal load across the audio spectrum to an amp under test, or being used without soeakers.
They do this for radio as well, building special non-inductive resistors for transmitter dummy loads.
Some of the math involved with impedances involves what the math whizzes call Imaginary Numbers. That's the point where I jump ship 'cause it's way over my head to think about, let alone try to explain.
