Sound waves are made of areas of high and low pressure. When they travel through a medium such as air they produce a force.
Sonic tractor beams are created by a sequence of sound waves creating a region of low pressure.
Essentially this counteracts gravity, meaning that they can trap an object in midair.
If the object tries to move up or down, zones of high pressure push it back into the low-pressure zone.
The size of the low-force region depends on the size of the wavelength – the longer the wavelenth the larger the area of pressure.
Previous attempts to trap particles larger than the wavelength of sound had been unstable, causing them to uncontrollably spin.
The new approach uses rapidly fluctuating sound vortexes.
These are similar to tornadoes of sound, made of a twister-like structure.
By quickly changing the rotational direction of the mini-tornadoes, researchers found the core increased in size, allowing larger objects to levitate.