Speaker impedance patterns - how to read them?

Published loudspeaker frequency response is virtually always measured with a solid state amp which puts out increased wattage into an impedance dip, and decreased wattage into an impedance peak. Most speakers are "voiced" for this type of amplifier.

Relative to a solid state amp, a tube amp puts out more wattage into the impedance peaks, and less wattage into the impedance dips. So the speaker’s frequency response will be modified accordingly. In general the lower the speaker’s impedance, the more pronounced this effect.

Ime a relatively high and smooth impedance curve tends to work well with tube amps. The bass impedance peaks can sometimes be manipulated a bit by lowering the port tuning frequency, and/or adding damping material inside the cabinet, if necessary.

Occasionally you find a loudspeaker whose impedance curve zigs where its frequency response curve zags, and that can work well with tube amps. I recall seeing such "complementary" measurements for one or two Silverline Audio models.

I’d suggest asking the loudspeaker manufacturer whether they recommend tube amps for their speakers. Audio show reports can also be a source of information about amp/speaker pairings.

See this paper by amp designer Ralph Karsten:  

http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html  

Duke
tube amp dealer, tube-friendly speaker designer

Your drivers impedance will peak at its resonant frequency. Enclosure (size and type) and crossover (not simply the crossover point, but the design of the circuit) will both have a direct effect on the impedance curve of the speaker. Amplifiers (some) like to see an easy load. My ESL-57’s do not present an easy load, and are fussy when it comes to the type of power supplied.

Ralph sent me this link in my thread on the ESL57’s. Found it to be extremely helpful.

http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html

@audiokinesis 

Darn it, you beat me to it :) We had the same thought…

@perkri   
Your example of the ESL is an Electrostatic speaker. The op is trying to learn how to read impedance chart of a loud speaker.  Duke did give an excellent representation of how a solid state amp reacts to a speaker load vs a tubed amp. Which also shows why impedance compensation on a speaker design can be helpful. 
Your analysis of a crossover design can be true of how it effects a impedance curve, but this is normally due to how an inductance coil reacts to the EM field and natural inductance of the drivers. Depending on drivers,  it may be very beneficial to use a series or parallel design based on crossover point and what type of inductors are needed. But for the op's question and in general learning to read an impedance chart, my original response is less confusing for that purpose. 

@timlub 

Last I checked, the 57’s are indeed a loudspeaker. And they have an impedance curve. Brought them up because of the demanding impedance curve they present to an amp. They are a good point of discussion when dealing with amp/loudspeaker matching. 
The amp needs to deal with peaks and dips properly. So as not to over drive the low impedance, and under drive the high.

pretty simple.