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Generally, whenever the word 'impedance' is used, it includes variability (which is why when quoted as a spec you will often see the word 'nominal' in front of it; though these days that is often left out due to being implied/known). This is because impedance is resistance + additional reactive resistance (due to either/both capacitive or inductive properties of the part i question). A speaker's electrical element is the voice coil.... coil.... like an inductor/choke (which is why you find VC Inductance on most speaker spec sheets). This is also the cause of high frequency loss in long guitar cable runs (an in line capacitor = 1st order crossover, and instrument level signal is in the right impedance range for cable capacitance to affect the very top of the audio spectrum).

So an 8 ohm nominal speaker is 8 ohms only by rounded weighted average within the speaker's intended operating range.

This is why post-amp DI boxes (vs instrument in DI boxes) have speaker sims (or require the speaker to be connected) - they simulate the reactive load of a speaker because otherwise the amp behaves differently. That difference may be subjectively better or worse, depending on one's preferences, but different none the less.

The graph linked to is typical of just about any given speaker (same shape). The main differences are the amplitude of the 2 peaks (upper and lower) as well as the exact frequency at which they occur (see the Fs T/S parameter - that is the middle of the low end impedance spike, which is the free air resonant frequency of the speaker and indicator of the lowest useful frequency the unit can be expected to reproduce effectively, depending on the enclosure type used). Also the minimum impedance may hit the nominal rating or be just above or below it - always in the midrange for a woofer/fullrange type unit (tweeters - the whole thing moves up a few octaves obviously).

This is also why the DC R (resistance) of a speaker, when measured, is not the same as the impedance, but just close (always less or equal to, because the reactance needs to be added in on top of that).
 

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The graph linked to is typical of just about any given speaker (same shape). The main differences are the amplitude of the 2 peaks (upper and lower) as well as the exact frequency at which they occur (see the Fs T/S parameter....
@Granny Gremlin Thanks very much for taking the time to write this.
Much appreciated.

The above quote is the only part I can't understand.
 
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