There was a great Radiolab episode about the lower-frequency stuff. IIRC, the hypothesis there is that you can stimulate certain types of brainwaves that tend to run on similar frequencies, so that seems like it's probably working via a separate mechanism from the proposed effects of the ultrasound treatment, since ultrasound is way too high-frequency to correspond to any known brainwave frequencies. Perhaps both treatments could be complementary, though, assuming they both pan out.