EPFL researchers have developed a 100% effective, ultra-thin active noise cancelling system that uses an ionized air plasma propulsion system instead of speakers. A 17mm (0.6 inch) thick layer can block 20 Hz noise as well as a 4m (13 ft) thick wall.
Maybe I missed it, but are they suggesting this is omnidirectional? As in, do I need to be standing in exactly the right spot for it to work?