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.
The father of the THX Deep Note was James A. Moorer, an employee of Lucasfilm's Computer Division, a pioneering skunkworks that eventually sprouted not just THX but Pixar as well. THX was Lucasfilm’s new audio certification standard, a project that sprang from George Lucas's interest in ensuring that The Return of the Jedi played in theaters at the highest possible quality. As Jedi approached release, the THX team was busy putting together a mark for THX to show before the film, and they wanted a singular sound to play with it.
The spec was vague. THX creator Tom Holman asked Moorer, who was head of the computer division's audio group at the time, to create a sound that "comes out of nowhere and gets really, really big." Not much to go by, but Moorer got to work on a massive computer mainframe that Lucasfilm had just built: the ASP, or Audio Signal Processor, which was used to mix sounds for Jedi, Indiana Jones and the Temple of Doom, and other '80s Lucasfilm productions.
By modern standards, the ASP's ability to process digital sounds in real time was incredibly limited: The average laptop today can handle audio processing thousands of times more effectively than Lucasfilm's bulky old mainframe. But for the task of creating the Deep Note, the ASP was up for the job. Manipulating the sound of a cello as a base, Moorer wrote a "score" that consisted of 20,000 lines of computer code, which then randomly spit out a new sound every time the program was run.
"Every time I ran the C-program, it produced a new 'performance' of the piece," explained Moorer back in 2005. "The one we chose had that conspicuous descending tone that everybody liked."
As it turns out, although the THX Deep Note has been widely parodied for its incredibly loud volume—The Simpsons famously showed it exploding heads and shattering teeth—this effect is actually something of an aural illusion. Academy Award–winning sound designer Gary Rydstrom has explained that the Deep Note "just feels loud" because of the spectrum of frequencies it uses.
Curiously, the THX Deep Note that everyone recognizes today was almost replaced with another sound. Some months after the Deep Note's debut before Return of the Jedi, Lucasfilm lost the original recording of the sound. Asked to re-create the noise, Moorer quickly ran into difficulties. Because a computer program randomly generated the original Deep Note, there was no way to reproduce it exactly. "They kept complaining it didn't sound the same," Moorer said.
Luckily, the THX Deep Note's original recording eventually turned back up, and it's a good thing it did: It has since gone on to become one of the world's most recognizable audio logos. "I like to say that the THX sound is the most widely recognized piece of computer-generated music in the world," Moorer said in a 2005 email. "This may or may not be true, but it sounds cool!"