About 5 years ago, I showed one of my kids a clip by Physics Girl (awesome channel BTW) of an ultrasonic levitator:
He immediately wanted to build one so we bought the kit and we spent most of an afternoon soldering it together. He was pretty proud of himself and he decided to make a science fair project on Sound – he even went to the local science fair! I highly recommend you watch the Physics Girl clip as she gives a great explanation of how it works but essentially there are two opposing sets of ultrasonic transducers emitting waves of 40 kHz ultrasound 180 degrees out of phase with each other. That creates standing waves of low and high pressure where a light object can hang out. The bowl shapes that the transducers sit in helps focus the sound.
Fast forward about 4 years; I ran across an interesting paper in the journal Nature of a group at the university of Sussex who took this concept further, way further. Essentially, they used a phased array of ultrasonic transducers to move a Styrofoam ball through the air so quickly, they were able to draw floating images in mid-air! http://sro.sussex.ac.uk/id/eprint/86930/
Of course, I had to build one! What was even cooler was that not only was it possible to display 3D images, it was possible to also modulate the ultrasonic waves in such a way that you could feel the waves focused in 3D space - a 3D haptic device! And you could further modulate the ultrasonic signals to interfere with each other in such a way that they could produce sound in the human audio range! A display, a haptic device and a speaker, oh my!