One of the reasons for 40 khz is that the resonance of water is right at that frequency. 41.2khz, approx. IIRC.....
So a 40khz ultrasonic resonator is designed for that frequency and then to wander in that range (+/- 250hz, approx) until it finds the lowest reflections of energy in the resonator and the highest rate of transfer. So not the water resonance really, but the best condition of energy transfer through the medium of water.
This comes out of the ultrasonic sealing industry, the place the ultrasonic resonators at’ 40khz’...come from. Most times they work with water-based products like toothpaste, creames, hair shampoo, etc.
In that, I’m not sure what is accomplished with 120khz. Other than cavitation and heating at the surface of the resonator. Maybe the odd photon.
I’m ready to be wrong about all that, in the face of other arguments. We shall see, kinda thing. Not negative, just neutral.
A bigger number is not automatically better, is the deal, ....but...there may be good reasons to go to 120kz, and such hardware would likely take some tuning, tied to correct analysis...in order to get it to the right spot.