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.