My guess is that the arc through air gap is functioning as both the detector and speaker here. I suspect that the arc can only be sustained in one direction, pr

My guess is that the arc through air gap is functioning as both the detector and speaker here. I suspect that the arc can only be sustained in one direction, probably from ground to the cable, which would give us the required clipping. At the same time, I assume the arc is acting as a plasma speaker, and, as you say, the air and our ear provides the low-pass filtering.