Looped knocking

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Quick musical(?) experiment. I recorded knocking on the desk, repeated it a hundred times, and combined it with two speeds: the original one and slower by 1%. This way the slower knocking shifts with respect to the original.

It’s simultaneously obvious and magical that a rhythm layered on itself with some shift gives a new rhythm that is sometimes pleasing to the ear (or at least acceptable). Every couple dozen of seconds one can hear something that could be “composed” by a human: around 0:20, 0:44, 1:20, 2:05, 2:25, 2:40… It’s great that these new rhythms appear unexpectedly and after a few seconds they dissolve into noise.

I recorded the knocking using QuickTime. I processed it on the command line using ffmpeg and sox:

ffmpeg -i source.{aifc,wav}
sox source.wav repeated.wav repeat 100
sox repeated{,-slower}.wav tempo 0.99
sox repeated{,-slower}.wav --combine merge combined.wav
play combined.wav

I combined the audio with an image using ffmpeg:

ffmpeg -loop 1 -i knocking.png -i combined.wav -c:v libx264 -tune stillimage -c:a aac -b:a 192k -pix_fmt yuv420p -shortest out.mp4

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