MEDUSA (Mob Excess Deterrent Using Silent Audio)
is a device that uses the microwave audio effect to produce recognizable sounds right inside a person's head.
Lev Sadovnik of the Sierra Nevada Corporation in the US is working on the system; he says the device will work thanks to a new reconfigurable antenna developed by colleague Vladimir Manasson. It steers the beam electronically, making it possible to flip from a broad to a narrow beam, or aim at multiple targets simultaneously.
See article at this link:
Hambling, David. "Microwave Ray Gun Controls Crowd with Noise" New Scientist, 03 July 2008 http://www.newscientist.com/article/dn14250