Flat impedance curve?


How big impedance intervals can speakers have and still be considered as "flat"? I have a pair of 4 ohm speakers (Isophon Galileo) that has a maximum impedance of 10 ohm at a certain frequency and dips to 2 ohm at another. Are these speakers flat?

Can I use these speakers together with an 4 ohm tube amp or is that not a wise choice?

You might find it that it might be a bit more complicated than that. It might be more beneficial to see a graph of the impedance, to see how and where the impedance varies. The phase angle can be just as important in considering amps for speakers. IMHO, what your proposing doesn't appear to be ideal.
2 ohms at 10000 Hz is not much of problem because there's not much amp draw there. 2 ohms at 100 Hz will be a problem for most amps.

I would be surprised if the maximum is 10 ohms for any dynamic driver getting near Fs.
Ngjockey, FWIW at Fs the impedance could be much higher than that; 20-40 ohms is not that weird if there is no port.
When building speakers/crossovers, we can regulate the impedance down only, meaning we can stop big peaks, but we cannot stop big dips. The big deal is to have stable impedance around the crossover points so that they are accurate. Rises elsewhere typically don't bother most amps tubed or ss, but as Unsound mentioned, impedance phase angles can deal tubes a fit, especially in lower frequencies at lower impedances. Most designers choose drivers that don't have big impedance dips then stableize rises, then overall, you end up with a well behaved speaker.
Sorry Jwal, I realized that I did not directly answer your question, you will need to post your tubed amp here and a couple tubed amp guru's can chime in... the answer is some tubed amps can handle impedance variences better than others.