Hello, Dax702 :
Welcome to “zapper’s frequency confusion”!
Your quotation of Dr Clark is still true.
That’s all you need to start killing parasites in your body!
In Dr Clark’s original design, frequency used was around 30kHz.
Quoting again Dr Clark, from the same source (TCFAD_pp27) about Zapper’s frequency : “It should read between 20 and 40kHz”
Dr Clark, to the best of my acknowledge, is still working with this frequency, because it has an excellent transfer of energy from the zapper, to inside of the body.
There is other uses of frequencies, and everyone out there is trying to make its zapper different and better by using other frequencies, with many different reasons, including financial ones (15Hz sells good!)