The U1 device from « musicoftheplants.com » uses two electrodes, one is placed in the soil near the roots and the other on a leaf of the plant. The device measures the electric micro-currents which traverse the plant. These electric currents contain frequencies that can easily be turned into audible sound frequencies by scale effect.
You know how to recognize a melody whether it is played on flute or base not by the pitch but the interval between each note. The "A" reference used is at 432 Hz which is more relaxing than the 440 Hz standard frequency.