Another source of high intensity focused ultrasound uses peizoelectrics, either arranged over the inside of a section of a sphere or as a plane source which is then focused by an acoustic lens. To play with ultrasound from peizoelectrics, I ordered a ultrasound transducer and a driving board (driving peizoelectric crystals is supposedly nontrivial, so it will be nice to have a working driver in front of me for inspiration if I go futher down this path). Other people have done interesting things with this sort of transducer: https://www.youtube.com/watch?v=pFeek0a8s7Q
Just to test if it works, I glued the transducer to a metal tea tin and did the test of putting a piece of aluminum foil in the bath. The foil started disintegrating. This setup has already come in handy as an ultrasonic cleaner.
This transducer is driven at 24kHz, and given the speed of sound in water of 1.5km/s, this gives a wavelength of around 5cm. Having a number this large is both good and bad. It’s good because it implies that the surface quality on a lens or mirror will not need to be very precise. It’s bad because optics generally assumes that lenses are much larger than the wavelength, so there are thin film effects to worry about. I tried to make an acrylic lens by turning a thick block on a lathe, but ultimately failed at mounting it. I’m looking into other ways to make one currently.