Eighth octave C
The musical note C8 (European c''''') is the C two full octaves above soprano high C. The note is one octave above the top of common musical keyboards, but the highest note of an 88-key piano. The pitch vibrates at 4186.009 Hz (the A note above middle C vibrates at 440 Hz in comparison).
For a vocalist to hit a note of this pitch, one's vocal cords must vibrate at over 4,000 times per second. This is nearly impossible because of the muscular control required in adducting the vocal cords that high. Adam Lopez and Georgia Brown have reached, and surpassed C8. Every octave jump doubles the rate of vibration (i.e. C5 vibrates twice as fast as middle C (C4) and soprano C (C6) two octaves higher than middle C vibrates four times as fast). The Guinness record for highest note sung by a male is now held by Adam Lopez, hitting E♭8 live on the Australian Television Broadcast of Guinness World Records.