There is an impedance peak at resonance, that is true. If the driver is mounted on an open baffle, it will have an impedance peak at fs. If it is put in a sealed box, then the resonant frequency will rise to fo, but the system will still exhibit a single peak. If used in a vented system, then there will be two impedance peaks at fl and fh. And if used in a horn or transmission line, there will be several impedance peaks from the cutoff frequency up.

Wayne