How to control the driving frequency when working with high power ultrasonic transducers
An idea once I had, but never tried, is to treat an ultrasonic transducer like a high-power crystal oscillator.
Your typical crystal oscillator circuit looks like this:  







simulate this circuit &ndash; Schematic created using CircuitLab

Simplistically, the crystal (along with the 2 capacitors) provides a 180 degree phase shift at its resonant frequency and this determines the output frequency of the oscillator.  

So why not try something similar with your transducer?
You would of course need to use something significantly more powerful than a little logic inverter, and you would probably need an additional band-pass filter to make sure you don't end up with one of the transducer's harmonics, but I imagine it would look something like this:  







simulate this circuit

You may need to introduce some method of kick-starting it if there isn't enough 'natural' noise in the system to get it going, and the filter may need to have some gain to compensate for the low voltage across the sense resistor.