There’s something about hearing a person scream that makes the hairs on the back of my neck stand up. As it turns out, this visceral reaction may be unique to screaming itself.
Scientists from New York University and the University of Geneva (paywall) in Switzerland have discovered that screams—while rather unpleasant to listen to—have a distinctive sound pattern that target the fear center in our brain.
According to Luc Arnal, a neuroscientist at the University of Geneva and lead author of the paper, the key to making a scream effective is roughness. This is a measure of the frequency, combined with the speed at which we hear syllables.
In normal speech, we hear syllables at a rate of 4 Hz (one every 200 milliseconds). But in a scream, the syllable rate is between 30 to 150 Hz. It’s what Arnal calls a higher intensity modulation rate. “It has nothing to do with the pitch of the voice,” Arnal told Quartz, “You have the same carrier…but it’s going to be much faster.”
When study participants were asked to rank how frightening screams were, they found that the rougher the screams, the more frightening they were perceived to be. “Roughness is a good predictor at how scary a sound is,” he told Quartz. When Arnal and his team filtered out the roughness from screams, subjects were less alarmed. These smoother screams sound like shrieking or yelling, similar to joyful screams. Conversely, when they added roughness to normal speaking voices, participants found the sounds to be more alarming.
Screams alerted participants to a level of danger much faster than regular sounds. The researchers found that screams produced a response in the brain’s fear center, the amygdala, whereas ordinary speech or music does not. Arnal explained that he thinks that this is because when we scream, we’re both signaling for help and alerting others that danger is present.
These screams are the kind that hurt your throat: “It has to be a little bit painful, so you don’t produce it all the time,” he told Quartz.