A speaker can be modeled as an inductor LS (the coil that generates the magnetic field to move the speaker diaphragm) in series with a resistor RS (resistance associated with the coil wire, as well as the energy lost to the acoustic output the speaker produces). It is possible to measure LS and RS with the following circuit. Connect the speaker in series with a known resistor Rext and apply a sinusoidal signal vin of frequency f. Measure the amplitudes of vin and the voltage vext across Rext as you increase f. Derive the amplitude ratio |vi| / |vext| as a function of f.