From Mary Beth Griggs' 10-27-15 Popular Science article entitled "This Tractor Beam Uses Holograms Made of Sound to Move Objects":
"In a new paper published today in Nature Communications, researchers figured out a way to manipulate a tiny physical object using what they call 'acoustic holograms.' The researchers used a grid of tiny loudspeakers to levitate, rotate,
and otherwise manipulate a tiny ball in mid air. They did this by
programming the speakers to send out high intensity sounds that create a
kind of force field around the object."
To read Griggs' entire article, click HERE.
The Nature Communications paper, entitled "Holographic Acoustic Elements for Manipulation of Levitated Objects," can be read HERE.