What does Nominal Impedance mean?

Nominal impedance means blah blah blah blah, meh...that said, Almarg was one of the reasons I bought a Pass XA-25. A cool guy who's missed around here. 

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?

ozzy

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.

esarhaddon1,

So, what number(s) are meaningful when it comes to speaker specs?

Is this what you mean by a Zobel Network?

RLC network to prevent unstable amplifiers from oscillating, to reduce distortion, and to act as an RF filter

ozzy

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.

impedance of a speaker is one of many design parameters that speaker designers trade off in very complicated ways

think of speaker impedance very roughly as how much work an amp needs to do.... you can put in a few simple drivers and a minimal crossover in an undamped box, and the amp will have a super easy time doing the work, but how will it sound? (if this were so easy, we would all have speakers like this...) - but easy to drive, simple lightweight drivers will distort when pushed and don’t have broad, controlled, flat frequency response (they shout, they ring, they break up...)

so the designer uses more robust drivers, and more of them, then this complicates the crossover and cabinet design, all of which makes for more work for the amp... more stuff to grip, to stop and start, to deliver current into (how the amp does the work...)

so the designer says, well, mister speaker owner, you want better super clean sound, full range response, plenty of power handling to play loud AND clean... so i have had to do all this to my speaker design -- include all these well damped drivers, put in a well tuned box, match them all, filter them all superbly with a complicated crossover -- to give you the performance you want... but hey now, you gotta use a strong amp to do the work to get that sound...

whenever i see 3 way tower speakers with 3-4-5-6 drivers, multiple woofers, think of an amp needing to grip it all (think big focals, magico, thiels)... there is alot of work to do (compared to say a single driver speaker) so you can bet the impedance will be lower... not to mention the efficiency

tradeoffs....