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.
After reading, I was somewhat disappointed that it’s a clever tiny speaker mechanism. Much more fun to picture Blofeld walking down the hallway in his lair and pushing the noise cancel button outside a raucous conference room resulting in immediate silence and a distressing burning smell.