Joy Division

The Twelfth Doctor compared the appearance of an audio waveform to "that Joy Division t-shirt" he presumed a podcast listener's dad might have, describing a "landscape" of mountains and valleys.

Continuing with his analogy, the Doctor described that if the listener's dad were to wear two t-shirts, one with a vertically flipped image of the original, the two shirts would cancel out, and both vanish, just as inverse waveforms would do. The Doctor noted this may make for an awkward situation, in the case of the t-shirts. (AUDIO: Dead Media)

Behind the scenes
The design the Doctor is referring to is from Joy Division's first album,. That album's iconic cover art was in fact made from an inverted image of radio waves from the pulsar.