What makes hearing a scream so scary?
If you want to be really frightening this Halloween, don’t waste your time dressing up as a ghost or goblin. Instead, just let loose with a bloodcurdling scream. As it turns out, the acoustics of the human scream communicates fear even more effectively than Jason or Freddy Kruger.
But why are screams so particularly startling to us when other noisy sounds like airplanes, construction noise and thunder surely boom in our ears as loudly? What is unique to the human shriek that sends a chill down our backs when we hear it?
It’s a rough world out there
Human screams, while using the same voice box as when we say “boo,” are particularly noticeable because they occupy an acoustic range not found in our everyday speech. The result is a specific sound quality, called “roughness,” that seems to register in our brains as very unpleasant or cringe-worthy, the vocal version of nails on a chalkboard.
Speech scientists are able to quantify this acoustic “roughness” using a measure known as a modulation power spectrum or MPS. MPS is basically a way of measuring how quickly a sound shifts back and forth in loudness. It seems when there are wide and fast volume changes, it is heard as harsh, jarring, or rough sounding.
Screams are characterized by such very rapid shifts (at a rate ranging from 30 to 150 Hz), while normal human speech uses a much slower modulation range (e.g. around 5 Hz). So, when comparing human screams to speech from a variety of languages, research has found that screams have higher roughness measures. In addition, screaming is typically louder and has a more variable and higher pitch than spoken sentences, but neither of these attributes are unique to screams.
Instead, it seems like it is this specific acoustic quality of roughness that humans psychologically react to when we hear someone scream. In other words, we notice a harsh and grating quality to screaming because these fast changes in volume do not seem to happen in our regular speech, while high pitch and loudness alone sometimes do. As a result, a scream’s MPS range seems to be an innate short-hand for “Danger, Will Robinson.”
The horror, the horror
Other natural and artificial signals—like animal screeches, sirens, and car horns—also have similarly quick shifts in loudness, suggesting that hearing this specific rough quality is important for alerting us to danger. And the rougher the scream, the more scary people seem to find it. Which is why no one can forget Janet Leigh’s famous shower scream in the horror movie "Psycho."
Interestingly, in a study that compared the music in climactic horror-scenes with the music playing during non-scary scenes, tracks that built up to a murderous slashing scene replicated the “rough” acoustics of screams—suggesting that it is not just the serial killer pulling back the shower curtain that gets the audience to the edge of their seat. In short, there’s a reason a haunted house or horror flick just isn’t the same without a hair-raising scream or two.
Image courtesy of Shutterstock.