I was just reading a review of the Rockport Orion speakers in The Absolute Sound by Robert Harley.
They weigh about 350lbs! Cost over $133,000 and it is still a 4-ohm speaker!
So, I guess I understand that now, nominal impedance is just a number. Highly questionable, I guess that is so with sensitivity ratings. Different frequencies produce different amounts of energy, so that could be all over the map in reality.
But if 8 ohms is optimum then why are there so many 4-ohm speakers? I put together a DIY speaker a while back and I was able to use all 8-ohm speakers. So, the drivers are out there.
And companies like Rockport, Wilson, etc. claim to make their own drivers, but they design them with lower ohms. Why?
Loosely translated into Layman's language it is the impedance at any given frequency. And of course Impedance is roughly (improperly) equivalent to resistance. Most speakers , as the sound frequency approaches its resonant frequency, the impedance starts to spike. I have had many people ask why I use what is called a 'ZOBEL' network in my crossovers, and this is the reason. A Zobel network tries to compensate for that spike and lets the speaker operate more uniformly at lower frequencies. And be careful when Ozzy says it is just a number. It is but it is a very important number when designing a crossover.