A Dukane spokesperson has previously acknowledged that the 33.331kHz pings were what could be expected. The oscillator is not Xtal controlled, being a resistive/capacitive oscillator with the low Q transducer forming part of the circuit.
Hi mm43
I suspect that the transducer will operate in a manner similar to a quartz crystal oscillator, just at much lower frequencies due to its much larger dimensions. Once internal amplifiers begin to operate non-linearly due to the low voltage, it might be possible to stimulate other oscillatory modes in the transducer. This would account for significant deviation from the base frequency. Just theorizing of course.
Detection of a pinger while it is operating in a low voltage manner would probably indicate you were almost on top of it, thus the question of what happens at very low voltage is not entirely irrelevant.