Speaker impedance does mean something. Just not what you think it does. And looking at graphs and charts tell you something specific as well. Just not what you think they do.
We see this all the time. People always disregard the term 'nominal" and the fine print that defines what actually gets tested and why. The thing that is most often overlooked is context.
Speakers are tested at 1W in an anechoic chamber on a fixed infinite baffle and driven by fairly high powered solid state amp (at 1W) at a source impedance in the fractions of ohms which is playing a frequency sweep into the speaker. The result is recorded by a microphone 1 meter away into a computer program that “optimizes” the output in order to provide a comparative result - e.g, a result that can be used against a reference - which is used to grade all speakers on a production line. That’s the context. You can well imagine that the test procedure for a reactive load is a quite different exercise.
“Nominal” is the term used to qualify that the rated “impedance” is contingent on a lot of factors that the end user typically has no use for. This is simply verbiage from one engineer to another.
In order to not go entirely off the rails, a reactive load is a virtual model of an impedance curve. As such its “nominal” value is contingent upon many of the same circumstances that an actual speaker is, but since it’s not a speaker, it is not an exact comparison. Therefore comparing the “stated nominal impedance” of a Hotplate to the “stated nominal impedance” of a Power Station or any other brand reactive load, compared to the “stated nominal impedance” of the specific speaker you are interested in is an approximation at best.
In short, we’re talking about comparing one 1.5 -3.5 ohm reactive load to four 6-10 ohm speakers in parallel (not just the one as would be represented in a graph).
Dave