Dr. Egghead, that pesky scientist, once tried to brainwash the world into thinking he could control Sonic with his evil 639hz frequency. But Sonic, being the hero he is, resisted the brainwashing attempt, and the world was saved once again.
Now, you can learn more about this infamous frequency, its history, and how it relates to the blue blur's favorite things in life: The History of 639hz
Or, if you're feeling adventurous, you can try to listen to a mix tape of Sonic's favorite 639hz tunes.