Curious as to what was going on I downloaded the YouTube video, extracted the audio and ran off a spectrogram with the open source sox audio tool. I had done this before last year, so I was familiar with the process.
1. Download the YouTube video:
youtube-dl -o shepard_tone.mp4 http://www.youtube.com/watch?v=DfJa3IC1txI
2. Extract the audio from the video
ffmpeg -i shepard_tone.mp4 -f mp3 shepard_tone.mp3
3. Convert to mono (the YouTube video has stereo audio)
sox shepard_tone.mp3 shepard_tone_mono.mp3 remix 1,2
4. Create the spectrogramsox shepard_tone_mono.mp3 -n spectrogram -o spectrum.png
One small problem: most of the action is down at the lower frequencies. So I resampled the audio and regenerated the spectrogram so that the interesting bits were more visible.
5. sox shepard_tone_mono.mp3 -r 4k -o shepard_tone_4k.wav
6. sox shepard_tone_4k.wav -n spectrogram -o spectrum.png
Tools used :