Verified answer

...actually, there's an interesting story behind the now-famous and iconic sound effect, which signals the presence of Jason; apparently, composer Harry Manfredini had meant for the sound to resemble Mrs. Voohees, as she was speaking in Jason's voice, saying "...kill, kill, kill, mom, mom, mom"; for a more eerie effect, he shortened the 'kill' and the 'mom' sounds, and the resulting syllables were whispers into a microphone, on a time delay, so that it reverberated, hence the resulting sound effect was actually "...ki...ki...ki...ma...ma...ma..."...

...the story it told much more clearly, over in the "Friday the 13th" trivia section, at IMDB, where they say....

"Composer Harry Manfredini has said that contrary to popular belief, the famous "chi chi chi, ha ha ha" in the film's score is actually "ki ki ki, ma ma ma". It is meant to resemble Jason's voice saying "kill kill kill, mom mom mom" in Mrs. Voorhees' mind. It was inspired by the scene in which Mrs. Voorhees seems to be possessed by Jason and chants "Get her mommy....kill her!" Manfredini created the effect by speaking the syllables "Ki" and "Ma" into a microphone running through a delay effect."

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RE:

ch ch ch ch... ah ah ah ah...?

I wondered this forever, in the jason movies why does it always go Ch ch ch ch... Ah ah ah ah... when he is around and can the people hear it for fictional or just the movie veiwers in real life?

i dont know but my daddy does it to me when im sleeping and it freak me out a lot. I think its because we know jason is mentally ill and usually ppl like tht dont have a mind of their own and its evn more creepy he just kills ppl and never says anything so they jus make tht sound

nah its just the soundtrack .....its supposed to help build the suspense ...its like the music they use in halloween [no not mr sandman the little piano music that lets us know michael is around].....its for atmosphere .....good films though ...

they can hear it and so can the movie viewers