Ultrasonic bath
There's probably a pair of power transistors or MOSFETs that's
a power oscillator. If you trace around that area, it could help.
But no guarantees. But assuming it is resonant, that may be
part of what determines the frequency.
Particularly if it's an inexpensive product. It would make sense to have the
circuit oscillate at the natural frequency of the transducer.
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