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.
I’d love to see how well this cancels out some subwoofer loving neighbors I’ve had in previous apartments. I’m skeptical considering the level of wall shaking vibrations that are generated from some systems, but I’d pay a decent price if it actually worked.