It accurately Measures AC Resistance (Impedance) at either 1Khz or 120 Hz.
You have a serious misunderstanding here. Impedance is not just AC resistance. Impedance is the phasor combination of resistance and reactance.
The standard symbols for the relevant quantities are:
Z = impedance (at a particular frequency)
R = resistance (at a particular frequency)
X = reactance (at a particular frequency)
These quantities may vary with frequency, and the relationship among them is:
Z = R + jX
The AC resistance of a speaker is almost the same as the DC resistance below a few kilohertz, except for the region right around resonance. At higher frequencies the AC resistance increases due to losses in the magnetic circuit, and due to skin and proximity effect in the voice coil.
But impedance is
not just AC resistance.
And DC Resistance has Nothing to do with Impedance.
It has a lot to do with impedance. Impedance varies with frequency because the components of impedance, resistance and reactance, vary with frequency. The AC resistance of the voice coil is composed of the DC resistance plus a component that increases with frequency. If you were to remove the DC resistance of the voice coil, the AC resistance would change a lot, because the DC resistance is a substantial part of the AC resistance.
I've attached 3 images showing the AC resistance, reactance and impedance of a raw driver, with a nominal impedance of 8Ω, as the frequency varies from 100 Hz to 20 kHz.
As you can see, at low frequencies the AC resistance is essentially the same as the DC resistance (which measures 6.8Ω with my Fluke meter).
The impedance of the driver reaches 8Ω somewhere between 3 and 4kHz. This behavior is typical of all drivers; the DC resistance is just a little less than the nominal impedance rating.
The web is full of information about speaker impedance; for example:
**broken link removed**