Phil Lewandowski wrote on Fri, 03 December 2010 21:56 
(Actually the Growler's average impedance is closer to 10 ohms as well, like with the T39. It just so happens that one of the "standards" is to pick the standard 2, 4, 8... as the nominal impedance; whichever it is closest to. So that is why a impedance plot is so helpful.)
Take Care, Phil

You are exaclty correct. You cannot describe a complex number (such as impedacne) with a simple single number).
You would not believe how many people have no idea how impedance and wattage and voltage are related.
If a loudspeaker had a published impedance of 10 ohms (or any other number than the "standards") you would see a WHOLE bunch of questions like "My new speaker has an impedance of 10 ohmsI can't see that rating on my amps spec sheetwhat kind of amplifier drives a 10 Ohm load?".
I'm not kidding. But if the same exact loudspeaker was rated at 8 ohmsthere would be no questions
Even if there was a published impedance curve of the loudspeaker that SHOWED it to be a 10 ohm load.
Many people simply cannot get past the few couple of numbers on the front page of a spec sheet. They want simple answers (and they totally belive the simple numbers given)the truth be damned.
Coverage angle of a loudspeaker is another area in which very often the actual coverage angle is nowhwere near the published number. Yes that number may be rightat some freqbut not across the intended freq band. At some freq it is narrower and at other freq it is wider. How much? You have to look at the polar or a ballon plot or directivity plot to figure that out. And then people believe that the sound "stops" at those printed angles
Simple numbers simply don't do itit takes more data to give a real understanding of how a loudspeaker performs.
But I'll stop now.