Hi mitsuko,
such devices also exist for cleaning your third dents. Perhaps you will find
one in a big storefront.
Building such a device by your own is a little bit more complex. You have
to drive your transducer exactly at the resonance-frequency because they
have a high Q. Driving them will require a high voltage (over 100V) to get
enough power. The resonant frequency changes with temperature, the
amount of water and, and, and...
The best design I saw up to now was with a DSP measuring the current
and generating a sinewave. Easier design with a self-adjusting oscillator
will work only for small power (<20W). But for a 'cup of tea' it could be
enough.
Bye