The specs claim this ultrasound is 40kHz.
I've tried putting drops of water on the transducer element and at that point it just stopped working (gave me a 0 cm response). Now going from transducer to air to water to air might not be a good idea as the interface between each medium would give a pretty strong reflection with little energy going through.
I guess there are two directions I can go at this:
1- test by nearly submerging the ultrasound elements in hotglue so that the sound wouldnt have an interface when coming out and would go straight to the outside world --> this will permanently mess it up if it doesnt work
2- place the transducers deeper in the bumper so that they are more protected from rain and maybe put them in a tube to protect them from most rain direction
still no response from manufacturer