The zapper is frequency independent because its mechanism is the positive offset. This way it will generate a current flow through the body, something the mixed offset will not do.. The square wave has many harmonic frequencies and will hit many frequencies. If you use 30kHz, then you will have harmonics at 90kHz, 150kHz, 210kHz (odd multiples) and so forth. The frequency generator produces specific frequencies. It’s a focused treatment that hits the target.