"[...] They set up two sets of parallel speakers, pointing at each other — a
sound geyser from north, south, east and west. The ultrasonic waves are
opposite in phase as they pass each other — this creates a little
pocket inside the waves. The force generated by the speakers cancel, and
after accounting for gravity, keeps the object suspended.
The theory behind acoustic levitation has been around for decades but only recently has it started to come to life (in some cases, literally).
Suspending an object in the air is one trick, but moving it in three
dimensions is another. Not to mention that for most of the practical R/D
purposes, such as combining chemicals, tight control over the object’s
position is a must. [...]"
http://blog.sfgate.com/techchron/2014/01/02/sound-waves-suspension/
http://www.disclose.tv/news/Scientists_Use_Sound_Waves_To_Levitate_And_Manipulate_Matter/102074
