Nominal impedance is NOT the average impedance. Generally speaking, listing the average impedance would be totally useless. The nominal impedance is supposed to be what you can safely pretend the overall impedance is. As such, it tends to be near the minimum impedance of the speaker. If the speakers reached your hypothetical 1 ohm, it would probably be an extremely difficult to drive speaker and absolutely ought not be rated at 6 ohms nominally. Of course, some companies are run by worthless liars who incorrectly rate their speakers, but good companies rate them properly so that one can use the nominal rating to judge whether a particular amplifier can deal with the speaker or not.
Originally Posted by FirstReflection
(I don't know what their current policy is, but McIntosh used to have a policy that their nominal impedance ratings for their speakers would be such that they never had a minimum impedance lower than 10% less than the nominal rating, or in other words, no lower than 7.2 ohms for a nominally 8 ohm speaker. That is the sort of policy that a good company will have, but evil lying bastards abound in business, so you should try to find a professional review in which they measured the speaker to find out the minimum impedance of whatever speaker you are considering [some manufacturers also state the minimum impedance of a speaker, and then you need not find a review for this information]. And pay attention to which companies are liars about this sort of thing for future use, in case you can't find a review of the particular model in question, to have some idea of which companies are not generally trustworthy.)
In any case, the nominal impedance is not determined by the average of the impedance curve.
A fool and his money are soon parted.
Too many people seem to spend too much time reading between the lines for what is not there instead of paying attention to what is there.