Curve steepness and width important too. Possibly of interest: https://audiojudgement.com/speakerimpedancecurveexplained/
What does Nominal Impedance mean?
What does Nominal Impedance mean?
I’m trying to decide on some new speakers (Clarisys Minute). They are rated at 8688 sensitivity and a nominal impedance of 3.5 flat. Although graphs show it about 6 ohms from 20hz500hz and at 2Khz and above about 3 ohms.
My present speakers, Focal Sopra 2 are rated as 91 sensitivity and 8ohm nominal impedance, but minimum is 3 ohms.
So, I am presently using Bob Carver 350 amps (rated as 350 Amps/channel 8 ohm and 400 watts /channel 4 ohm) which are tube mono blocks. and I like to crank it up at times!
Can someone explain about Nominal Impedance and if my amp(s) will have a problem driving the Clarisys Minute speakers?
ozzy
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I take it to mean the impedance where most of it lies across the frequency range. Case in point: my speakers are 6 ohm nominal, with a minimum of 4.4 ohm @175Hz. Because of that, they state 4 ohm on the speaker. Not all speaker makers do that. Some just use the average while others state the lowest point. That may add to the confusion. All the best, 
The Carver should have no problem with the new loudspeakers. The above 6 ohm impedance in the bass frequencies is good and the 3 ohm readings are at frequencies that demand less current. You might have to turn the volume setting up to match previous volume levels. There could be rolled off high frequencies if the Carver has a high output impedance in the 3 ohm range. 
Jim Clark from Carver just replied to my email saying that the Carver amps would have no problem driving the speaker based on the specs in this posting. But I wish he had provided more detail. Here is his reply, "The nominal impedance is an average of the impedance curve. Many speakers vary between 2 and 20 ohms at various frequencies. Your 350's should have no problem driving them. Enjoy! Thanks, Jim" ozzy 
+1 @milpai, To say he had a way with words is an understatement. Being a copyright lawyer (among other things) he was concise, succinct and a joy to read. All the best, 
michaellent, randym860, Well, that’s what I think, but sometimes you just never know how different components interact. Still, it is so confusing trying to figure out the importance of speaker sensitivity and impedance when it comes to speakers. Wonder why companies just don’t make speakers that are 8+ ohm and with high sensitivity? But it seems to be just the opposite, that is; many of the bigname speaker manufacturers produce speakers that are 4ohm and low sensitivity. Wonder why that is? ozzy 
Nominal means "in name only" and has little to do with the actual impedance of the speaker. That’s why it’s confusing. The speaker impedance is measured across the frequency band and someone at the speaker company looks at it and says hmmm... that looks like about 6 ohmsish on average so we’ll call these 6ohm speakers. This is one area where published specs are important. I look at the impedance curve of my speakers and they are a flat 2.5 to 3 ohms from 100hz to 10kHz which tells me I need a hefty solid state amplifier for the low impedance but a tube amp with a 3 ohm output tap would work great also since the impedance is flat. 
“Nominal impedance in electrical engineeringand audio engineering refers to the approximate designed impedance of an electrical circuit or device.” 
@gs5556 How right you are! 'Nominal impedence' is generally concocted by a speaker manufacturer to hide elements of his speaker's impedence curve that he would rather hide. It has no basis whatsoever in physics. Responsible manufacturers show the full curve for impedence vs. frequency and the phase angle at audio frequencies, because bad combinations of impedence and phase angle can often occur making a speaker tougher to drive than could be apprehended from the impedance curve alone. Incidentally I have never seen a speaker whose impedence measures a flat 2.53 ohms over 10010k Hz. Have you? This poor manufacturer behaviour is mirrored by not quoting a dB range when specifying low frequency extension, and by overstating sensitivity, which occurs very often. (Proper) measurements matter. 
@ozzy Wrote:
Maybe Hofmann's iron law is one of the reason. Article Mike 
Thank you @dweller! 
I know this is a complicated subject & not as simple as this but the primary way to get decent, extended, deep bass that can play relatively loudly from smaller woofers ( less than 8” or so) is to have 2 or more of them usually wired in parallel which lowers the effective impedance ( the reciprocal of the sum of the reciprocals). The taller, narrow baffle designs popular now for the sake of theoretically better imaging lends itself to smaller woofers if the cabinet cost is to remain reasonable & not “sculpted” in some way. many new speakers today have pretty low impedance & thus their actual effective sensitivity is lower than stated. 
@ozzy it is rare that a large cabinet speaker is also not sensitive. The reason for the large cabinet is to increase its efficiency and thereby sensitivity. Small speakers are typically more difficult to drive and require greater power to overcome their lack of sensitivity and low impedance curves. Ohm’s law explains these electrical properties but I suspect it will be a little over your head unless you intend to do a deep dive into the subject. 
I have the Thiel 3.6's. From Stereophile:

Impedance is a combination of linear R (resistance) and nonlinear X (reactance) A Reactance is a combination of Inductive and capacitive components. In the world of complex algebra the impedance Z is represented by = R + jX. In case of the DC, X component is null so the Impedance in this case is linear resistance R. Inductive component impedance is known as wL where w is frequency and L is Inductance. Capacitive component impedance is known as 1/wC where C is capacitance Overall Z = R + j(1/wC + wL) where j is directional vector Notably, with increase of frequency Inductive component increases while capacitive component decreases. that means that on DC capacitor will have an infinite impedance and Inductor will act like short circuit. 
@clearthinker Wrote:
I agree 110%! See the manual below of my speakers, it's the most comprehensive speaker manual of specifications I known of. They even explain how they arrive at their numbers. See last page specifications: Mike https://www.lansingheritage.org/html/jbl/specs/prospeakers/1984443035.htm 
@fritzspeaks Wrote:
JBL’s impedance graph is from 20Hz to 2KHz conforming to international standard IEC. International Electrotechnical Commission  Wikipedia Mike 
no, depends on speaker sensitivity and more broadly, i believe nominal impedance is basically a useless measure speaker makers don't use the same standards and pretty much write anything into their marketing literature  always always looks at the whole impedance curve 
@ozzyWrote:
As I understand, only in 8 Ohm load does 2.83 volts produce 1 watt. In a 4 Ohm load 2.83 volts produces 2 watts. 😎 Mike 
@ozzy Wrote:
Both! Also the lower the impedance the amp operates in, the higher the distortion produced by the amp. Mike 
Turns out you don't need to know what Nominal Impedance is in this case as your speaker has essentially flat response of 3.5 to 4.2 ohm (based on youtube measurement of it). Normal speakers have wildly varying impedance with frequency so no one number ever represents its impedance. Your speaker is then is an easy load for most amplifiers. Better yet, I have tested your Carver 350 amplifier with 4 ohm load. Here is its frequency response: The amplifier has a peaking response with this type of load so may sound a bit bright. You do have a ton of power available: And even more for short periods: So you can crank it up good, assuming the copious amount of distortion is not bothersome to you. In that regard, you are going to hear more of that distortion than your current, more sensitive speakers. If you like, I can measure your new speakers. Start a conversation with me and we can take it from there. Good luck. 
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 4ohm speakers? I put together a DIY speaker a while back and I was able to use all 8ohm 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. 