Getting a scare out of horror movie fans takes a lot more than a guy in a hockey mask murdering hippies in the middle of the woods. To get a boost from the soundtrack, sound designers use infrasound, a low-frequency burst of consciously inaudible noise that creates unease in people who don’t even know they’re hearing it. 

What Is Infrasound Actually?

Even before today’s sophisticated horror moviegoers got used to the sight of teenagers taking an ax to the forehead, it took more than a little blood to get an emotional response. Called the “fear frequency,” infrasound is so low that the human ear can’t actually detect it, but it can still have a horrifying effect on the psyche. That’s why movies like Friday the 13th can still actually scare us, despite being the template for every bad horror movie joke there is. And while it’s not the “ki-ki-ki ma-ma-ma” found in every Friday the 13th movie, the use of infrasound is good sound design.

Sound design is an integral part of any film, and it is the mix that triggers our emotional responses. The sounds of creaking doors, slow-moving footsteps, and harsh violin chords are just a part of the disturbing scene. But what the audience doesn’t hear can be just as powerful as what they do hear; thus, infrasound. 

In horror movies, creating fear is as much a science as it is an art. Maybe it’s a slight, droning hum in an otherwise empty scene with little action that makes the viewer feel uneasy. Maybe it’s just a feeling of dread that leaves the audience knowing that something terrible is going to happen. Or, like the end of Friday the 13th, it could be the music ramping up as an unseen figure systematically eliminates New Jersey’s most annoying campers. It could also just be 20 hertz of slow terror. Either way, you know things are about to go bad.

It’s Science Silly

It turns out they were on to something at Camp Crystal Lake. In 2003, British scientists showed that extreme bass used in film scores can produce deeply emotional effects in humans. This includes the anxiety and chilling sensation often found in horror movie scores. These scientists tested the effect on audiences at a London concert. Some of the music contained 17-hertz infrasound while other pieces did not. Those who experienced the infrasound reported emotions of unease, sorrow, and nervousness. Scientists also believe this inaudible hum creates the unease associated with purported ghost sightings.

Beyond infrasound, however, a lot of forethought and artistic planning goes into sound design. Designers use visual cues in films while simultaneously placing certain sounds, ones sure to create anxiety in the viewer. Researchers even know the exact minor chords that evoke distress in people, such as the so-called “Devil’s Chord.”

Friday the 13th is a perfect example of the power of sound design. The movie makes minimal use of sound, but when it does, the effect is powerful. Heavy breathing, heavy rains, and heavy petting are just part of the mix. Along with disturbing ambient noise, this movie uses the screech of violins, harsh notes in minor keys, and, of course, infrasound. You don’t have to feel silly watching this 40-year-old movie if you still feel overwhelmed by anxiety and fear when the old woman rolls up in a Jeep. It’s not your fault, it’s science.

Meanwhile, people in real life don’t need an undetectable noise to be anxious while walking through the dark woods alone. Your subconscious fears work just fine for that. Here’s a short video on how Infrasound works.