Hello Steve,
It is based on the principle that at resonance frequency, speaker impedance is maximum and hence current thru it is minimum.
So the technique is insert a small resistor in series with the speaker. Generate a sweep say for e.g. 20 to 120 Hz for a woofer and at some discrete steps, measure the voltage across the series resistor. the freq. at which the voltage is minimum is the resonance frequency.
Hardware required is a sweep generator, Power amplifier, precision rectifier and an
ADC.
This is the basic technique. Do u have any other idea or infomration, please pass it on.
Thanks..............