Then began the elaborations upon the theme. Phrases hummed too slow, with long pauses in between, so that the listener's mind helplessly waits and waits for the next note, the next phrase. And then, when that next phrase comes, it is so out of key, so unbelievably awfully out of key, not just out of key for the previous phrases but sung at a pitch which does not correspond to any key, that you would have to believe this person had spent hours deliberately practicing their humming just to acquire such perfect anti-pitch.

It bears the same semblance to music as the awful dead voice of a Dementor bears to human speech.

— Harry Potter and the Methods of Rationality