All speakers are a design compromise, and thus all are different in order to accomplish certain goals. A speaker designer will often use the impedance curve in order to help optimize other more important driver parameters.
Three things that affect speaker impedance:
1. Voice coil's electrical impedance (resistance, inductance, stray capacitance)
2. Driver's mechanical impedance (stiffness, mass, damping)
3. Driver's acoustic radiation impedance (resistance, reactance)
Obviously, because there are so many things that affect impedance of a driver, many manufacturers use conjugate techniques to accomplish a given "nominal" impedance. "Nominal" impedance is used to define an average impedance over the driver's frequency range. It is not a term recognized by the IEC. The IEC uses a concept called "rated value", which allows any "increase" above the rated value, but limits the "decrease". The standard does not allow the impedance to fall below the 80 % of the nominal value at any frequency, including DC.
Typically, you can guesstimate the nominal impedance of a driver by measuring the DC resistance and multiplying that value by 1.3. Example: Most 8 ohm drivers (<-nominal) will measure around 6 ohms DC. Not the most accurate method, but close and it doesn't require sophisticated equipment.
Now, to define order of importance. I'm not going to make this overly complicated, mainly because the equations are available with a google search and it's beyond the scope of this tutorial.
Electrical side: The voice coil DC resistance has the biggest impact on speaker impedance. DCR dominates the resistance of a driver at nearly every frequency. Next is inductance on the electrical side. Stray capacitance is miniscule compared to the others, but still there.
Mechanical side: Keep in mind we're measuring the electrical equivalent of these parameters. Major mechanical limitations boil down to suspension compliance, cone mass, and suspension losses. Your resonance peak (Fs), is dependant upon this mechanical branch, as well as it's relation with the enclosure. And, as you may have guessed, the enclosure is a contributor to impedance, since the enclosure is effectively a part of the suspension of a driver.
Finally you have the radiation impedance (how the driver interacts with the surrounding air). You can't really approximate this with a single lumped-parameter synthesis. It's ELECTRICAL importance is low enough to not worry about (though it is a very important factor, as you could imagine).
What we really want to know is what happens at different frequencies. In other words, how do all those different driver parameters affect the impedance curve?
1. At DC, the impedance is completely dominated by the DC resistance of the voice coil
2. Up to the point of the fundamantal mechanical resonance, the reflected motional impedance begins to dominate and is inductive in nature. The total phase angle of the impedance RARELY exceeds 45 degrees and thus the resistive and reactive (inductive) parts of the impedance are just about equal.
3. At fundamental resonance, the impedance is purely resistive, its phase angle is 0, and is determined by the effective series combination of the voice coil DC resistance and the reflected mechanical losses of (primarily) the suspension
4. Above fundamental resonance, the impedancs drops, has a negative phase angle (rarely exceeding 45 degrees) and is capacitive in nature. The impedance drops until it approaches the driver's "intended" range. The midrange for midbass/midrange drivers (duh) and treble range for tweeters. You'll see this point on the impedance graph of a driver
5, In the intended range, impedance approaches the DC resistance of the voice coil, but is SLIGHTLY higher than that DC resistance for a variety of reasons, typically about 10-20%. This is the region that manufacturers (respectable ones, anyway) use to specify the nominal impedance. The impedance at these frequencies is predominantly resistive in nature and is dominated by the DC resistance of the voice coil.
6. Above this region, the inductance of the voice coil begins to influence the impedance. Note that it NEVER becomes purely inductive, or even remotely close. Over the majority of the range of operation, the voice coil resistance still dominates. Second, eddy current losses in the pole piece dominate quickly, such that the phase angle of the impedance asymtotically approaches about 45 degrees, and NEVER 90 degrees.
Other factors: Obviously, impedance is measured at low levels. The next thing to consider is the fact that as a driver increases it's excursion (output), the coil is moving further from the gap, causing a large Thiele/Small parameter shift. This changes impedance due to the changing mechanical and electrical parameters. Heat also affects impedance. Few measure at these levels, so you're SOL on a graph for that one.
The major driver design options that affect impedance are shorting "devices" (Plated polepieces, faraday rings, shorting rings, etc) and the polepieces (old style polepieces, T-shaped or yoke style, and extended polepieces are the most common). Shorting rings, faraday rings, and polepiece platings are designed to shunt out eddy currents from the voice coil, they effectively lower inductance and flux modulation. They lower distortion dramatically as well. Polepiece design affects magnetic saturation as well as determines the surface area available for cooling.
Phase plugs/heat sinks are often designed to help remove heat from the voice coil. Note that removing heat doesn't really affect measured parameters as much as it ensures that the performance stays more consistent as power and heat increases.
Now measure at different output/input and you can see how power compression affects output to some extent. (losses affect it so its hard to tell but it gives you a rough idea)
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if you look over at the Termpro website, someone tested the DDz 18 in his extreme install capable of doing over 180dB if i remember correctly, power compression was detected with around 1800 watts.. pretty damned high considering most 3" coils start around 750-800 watts, and with really high power coils around 1200 watts.
