It's actually off-the-shelf tech now. I've been in a number of supermarkets where they use this to beam ads at you while standing in line for the checkout. (All of which have since stopped doing it, for what it's worth. YMMV.)

There was some hope for a while that this might turn out to be a way to hack sound so that you could produce a flat speaker full of those tiny ultrasonic speakers, and then use non-linear interference to produce deep, booming bass sounds, without having to have the much-larger speakers you need today for that to work correctly. Unfortunately, either nobody could crack the technique, or it's simply impossible, because the real speakers that emerged are quite tinny. Here's a commercial offering, where they're only willing to promise down to 150Hz in the text: http://ultrasonic-audio.com/acouspade-technical-specificatio... "The sound is of high-quality, with low noise and great frequency response, which includes authentic reproduction of bass notes and frequencies as low as 150 Hz." Which, if accurate, is better than the speakers I've heard, but still a long ways from "thumping bass" coming out of a square that could be hung on the wall as casually as a picture frame.