Hi Johny,
On a technical point, the Beck Zapper does not produce a AC signal. A couple of marketers have used this piece of mis-imformation to claim superiority for their design, but all they actually proved is that their most sophisticated test instrument was a digital Multimeter, and that they didn't know how to work it.
Beck Zappers output a Bi-Phasic DC signal. The two terminals switch between positive and ground four times a second. Digital meters reference off the negative terminal, if the reference is switching between positive and ground, the meter will read positive or negative depending on where the ground is at the time. Half the time the meter is connected the wrong way around. At no time does the output voltage actually swing below zero.
I did raise this with the marketer, but it was like talking to a brick wall. Science has no place in marketing.