Sonic barber pole. If you never heard an auditory illusion before, here is your chance. This particular tone is called a Shepard glissando, and to your brain it seems as if it constantly descends, while in fact, it is a looped sound.
This is the same as staring to a moving barber pole. Here is the spectrograph analysis to prove it.
- vertical: frequency
- horizontal: time
- black intensity: amplitude (volume)