The "ch ch ch ah ah ah" whisper effect is an iconic part of Friday The 13th's score, but here's the secret meaning behind it. Friday The 13th started life as a cheap horror movie designed to cash in on the surprise popularity of Halloween. Despite these origins, the film contained enough shocks and scares of its own to become a classic. It also confirmed that slasher movies were going nowhere, with the '80s seeing a slew of similar stalk and slash horrors like Sleepaway Camp.
Friday The 13th itself also launched a long-running franchise, receiving a sequel every year in the '80s apart from 1987. By now, it's common knowledge among audiences that Jason Voorhees only turned up as the killer from Friday The 13th Part 2 onwards, and it was in fact his mother Pamela Voorhees (who made several franchise appearances) who carried out the killing in the original. Pamela's decapitation at the end of the original would have made sequels with her difficult, so Jason was brought back and became the killer in (nearly) every subsequent entry.
Just like Halloween was aided by John Carpenter's chilling soundtrack, Friday The 13th had a memorable score by composer Harry Manfredini. One of the creepiest motifs from the score is the "ch ch ch ah ah ah" whisper effect that often plays when the killer is present, and in addition to being an unnerving piece of music, it also has a secret meaning. Manfredini explained in the documentary Crystal Lake Memories: The Complete History of Friday the 13th that he was inspired by this effect by a close-up of Pamela actor Besty Palmer saying the line "Kill her mommy," and reduced this to two syllables for the "ch ch ch ah ah ah" sound.
It was Friday The 13th's (which almost had a lame alternate ending) Manfredini himself who recorded the sound and ran it through a device dubbed an Echoplex to get that echoing effect. The "ch ch ch ah ah ah" also conveys the idea that Pamela is actually hearing Jason's voice in her head as she's killing victims. Like the shark theme in Jaws, this whispering let audiences know that Friday The 13th's killer is actually around, even though they can't be seen.
Manfredini's "ch ch ch ah ah ah" sound effect in Friday The 13th was an inspired touch and helped add extra layers of tension to what may have been other dull shots of a camera looking through some trees. It also makes Jason a part of the story, even if he's not directly killing any characters in the movie.