Deep Note is a crescendo used for THX trailers.  It was created by James A. Moorer in 1983, who was now an employee of the Lucasfilm Computer Division. The sound is used on trailers for THX-certified movie theatres and video releases. It was first used in the THX trailer shown before the 1983 premiere of Return of the Jedi.

Description
The U.S. trademark registration for the sound contains this description of it:

The Deep Note had originally been from a soft to a loud pitch. Over the years it has been remixed digitally which made the Deep Note with a more abridged sound. Beginning in 1988 the Deep Note became louder and abridged. In 1993 the Deep Note was cut short to save time for Laserdisc (1995 for VHS). Most recently, however, the Deep Note has been cut short to the single note (where both sounds stay in one pitch). This is in favor of other sound effects in certain THX logos.

The sound is perceived as louder than it actually is; sound designer Gary Rydstrom explains that, "from a technical standpoint, 'Deep Note' just feels loud because it has a spectrum of frequencies that grows from small to large."

James A. Moorer has been quoted as saying, "I like to say that the THX sound is the most widely-recognized piece of computer-generated music in the world. This may or may not be true, but it sounds cool!"

Furthermore,

Previous works

Prior to the creation of Deep Note, several other works made use of similar techniques of frequency spread.

In their book Analog Days, Trevor Pinch and Frank Trocco point to the track "Spaced", from the 1970 Beaver & Krause album In a Wild Sanctuary as the source for Deep Note. They quote synthesizer builder Tom Oberheim as saying the original analog form is much richer than the "digital perfection" used in movie theatres.

Other similar predecessors include:
 Iannis Xenakis' piece Metastasis (1955), which contains a very similar effect in the first minute that begins on a single tone and slowly spreads into a quarter tone cluster.
 The Beatles' 1967 song "A Day in the Life," which concludes with "an orchestral crescendo".
 Styx's 1974 song "Krakatoa," which features the spread tone.
 Yellow Magic Orchestra's 1981 track "Loom," with an upward slow glissando to crescendo at the beginning.

In popular culture 
The perceived loudness of the Deep Note is frequently depicted as having actual destructive effects:

In movies and TV shows
 The 1992 made-for-video feature Tiny Toon Adventures: How I Spent My Vacation shows a Deep Note–like trailer blasting a theatre, concluding with the text "THUD: The Audience Is Now Deaf."
 In the 1994 Simpsons episode "Burns' Heir," a THX trailer plays before a movie, blowing the audience back, shattering eyeglasses, teeth, and even causing one person's head to explode; despite this, Grampa Simpson shouts, "Turn it up! Turn it up!"  This segment was later turned into a THX theatrical trailer.
 In the 2006 movie Over the Hedge, Deep Note is featured in a scene late in the movie, when the animals enter a house and the porcupines were trying to setup a camera with a television, but accidentally tune the TV to a movie starting with the sound. The shear force of the sound blows some porcupine quills off.
 In the Boondocks episode "...Or Die Trying", Grandpa mimics the "Deep Note" when reassuring Jazmin that the theater they are going to is a quality theater.
 In the very beginning of the 2006 movie Tenacious D and the Pick of Destiny, Deep Note is parodied in the beginning by Jack Black and Kyle Gass with cartoon versions of themselves flying via flatulence propulsion, and the words "THC: The Audience is now baked" appear on the screen.

In Video Games 

 Tex, and the THX trailer that shares his name, are featured in an easter egg on Midnight Club: Los Angeles Xbox 360 and PlayStation 3 releases and Taz: Wanted PlayStation 2, Xbox, GameCube and PC versions, the lower-pitched version of the "Deep Note" is heard.

In music
 Rapper Dr. Dre was sued in 2000 by Lucasfilm, then-owner of THX, for using an unauthorized cover of "Deep Note" on his album 2001. 
 Australian grindcore band The Berzerker use the sound in their concerts before they perform the song "Mono Grind."
 The "Mega Lo Mania Remix" of Mylène Farmer's song "California" begins with Deep Note.
 Rock band Asia used Deep Note as the opening for the song "Countdown to Zero" from their 1985 album Astra.

Homage
 A sound similar to Deep Note is included as an easter egg in the BitTorrent client µTorrent.
 A sound similar to Deep Note is used in the logo for Tokuma Japan Communications, the home video division of Tokuma Shoten.
 Radio host Tom Leykis used Deep Note to "take out" callers, "THX style," until he received a cease and desist notice. 
 The LucasArts adventure game The Curse of Monkey Island opens with a large 'CMI' logo with a Deep Note, accompanied with the caption "The Monkeys are Listening"
 Tex, and the THX trailer that shares his name, are parodied in an easter egg on The Simpsons Movie DVD and Blu-ray releases. The trailer was made without the involvement of THX. In this parody, Tex awakes Homer Simpson, who was sleeping in a hammock. He is then chased around the frame, and was smashed before he can "repair" the trailer. Besides, the logo is not shown entirely. The logo reads "TH\" and a parody of the "Deep Note" is heard. This trailer is a parody and was not approved by THX.
 The sound was nominated for a Golden Raspberry Award for Worst Movie Trend.
 Tex, and the THX trailer that shares his name, are featured in an easter egg on Duck Dodgers Starring Daffy Duck Nintendo 64 release. The trailer was made without the involvement of THX. In this parody, Tex awakes Daffy Duck, who was sleeping in a hammock. He is then chased around the frame, and was smashed before he can "repair" the trailer. Besides, the logo is shown entirely and a lower-pitched version of the "Deep Note" is heard.

References

Other websites
 Deep Note  mp3 at USPTO
 THX trailers 
Electronic music