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 86-88 sensitivity and a nominal impedance of 3.5 flat. Although graphs show it about 6 ohms from 20hz-500hz and at 2Khz and above about 3 ohms.

My present speakers, Focal Sopra 2 are rated as 91 sensitivity and 8-ohm 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