The classic constant current method uses a large (200 to 1000 ohm) resistance in series with the driver and it is assumed that variations in loudspeaker impedance will have little effect on the current through the loudspeaker.
The classic constant current method uses a large (200 to 1000 ohm) resistance in series with the driver and a signal generator is used to vary the excitation frequency. It is assumed that variations in loudspeaker impedance will have little effect on the current through the loudspeaker.