Interesting!

i have found the following

if distance x is = to the speed times the seconds than if the gap has 2.29mm it will take around 1,5us for a sound wave to cross the gap from one electrode to the other considering the speed of sound in water if we add a time to reflect back the all period become 3us this makes the resonant frequency to be in the range of 333khz

if we consider that the bubbles interfere the speed of sound in hydrogen is still high around 1200 if its mixed with oxigen this lower it a bit...

if we consider the light speed in water

the frequency is much higher the time constant would be 20ps making a frequency of 50 ghz

if considere light speed

65,555,555,555