What audioguru says is true... the port adds an extra reactive element (a mass or inductance type reactance), giving the cabinet a distinct resonance of its own. Cascaded with the resonance of the driver, this results in the steeper roll-off below resonance.
I've dug out that piece of university work and had a brief look over it (for the first time in years). Bear in mind that it's an assignment rather than a tutorial, but I think it gives a pretty good walk-through of how to derive an electrical speaker model. Note that in this treatment I think everything ends up in acoustical units - you might need to start from "the other end" of the circuit and change everything into electrical units. Also we ignore the voice coil impedance - you may want to leave this in if you're interested in modeling crossovers.
This kind of analysis is called a "lumped parameter" model (at least, that's what we called it). You can model any kind of cabinet by simply changing the rear load of the driver (and the front load too, for a bandpass box!).
Hope this is useful.