Noise-cancelling headphones

Noise-cancelling headphones were headphones which actively cancelled out the waveforms of undesired external noise by feeding in its inverse. Because an audio waveform could be cancelled out by a duplicated waveform with an inverted phrase, played back from a recording, noise-cancelling headphones could effectively make both sounds perceptibly "vanish".

Petra compared the Twelfth Doctor's plan, along a similar vein, for dealing with a portal to other dimensions to "turning the world into a pair of noise-cancelling headphones". (AUDIO: Dead Media)