Something between a black MIDI glissando and a brown note, the THX "Deep Note" is one of the world's most recognizable audio logos, signaling the highest quality audio standard in films. Parodied by The Simpsons and sampled by Dr. Dre (which got him sued), at peak popularity the THX Deep Note was played in front of 4,000 movie theater audiences a day, or around once every 20 seconds. Yet despite its distinctive crescendo, the THX Deep Note wasn't actually composed so much as it was programmed, which makes it a fascinating success story of early computer audio design.
Such a cool sound! I love it when it plays on my home theater setup.