Ultrasonic Transducer driver
Recently, I fixed an ultrasonic cleaning device (f&nbsp;=&nbsp;40&nbsp;kHz). The circuit consists of two high voltage bipolar transistor feeding a resonant converter. The piezo actuator is connected to the secondary of the converter's transformer. I have strong reason to believe that the circuit is of the Royer resonant converter type because it has two bipolar transistors, see this answer for more info.

There's another self-oscillating schematic with just one transistor in Sam Goldwasser's Sci.Electronics.Repair FAQ 

Some clever techniques allow you to build the ciruit in a way that it catches onto the resonant frequency of the piezo acutuator, maximizing the output amplitude just right.

For your circuit, it would be a good starting point to check whether the triac has the job to turn the rest of the circuit on or off or if it is used to modify the power using a phase angle control technique. The rest of the circuit around the two bipolar transistors will very likely be some sort of resonant oscillator or converter stage.

Use caution. The entire thing is likely connected to the mains supply, featuring high and dangerous voltages with reference to earth ground (=&nbsp;your body).

Related, with a schematic: Replacement Transistor for Ultrasound Cleaner