What makes a scream a scream? New York University researchers tasked themselves with listening to people scream to find out and have published their findings in Current Biology. NYU psychology and neural science professor David Poeppel and his colleagues collected screams from YouTube, films, and 19 volunteers who screeched in a sound booth. The team measured sound properties, volume, and people's responses to the screams via brain scans, then compared the data to that of normal speech, reports Time. They found that screams, unlike regular conversation, up the activity in the amygdala, "a nucleus in the brain especially sensitive to information about fear," Poeppel says, meaning screams are inherently processed as a kind of alarm. But not all screams are alike.
While normal speech varies in loudness between 4 and 5 hertz, screams range between 30 and 150 hertz. How fast that variation occurs is referred to as a sound’s "roughness." Now here's the cool part: The faster a scream fluctuates at higher rates, the scarier the scream is perceived, Poeppel says. Researchers searched for sounds with a similar roughness, and found only alarms like ambulance or fire engine sirens came close. "This wasn't known when they were designed, but it makes good sense," Poeppel tells the New York Times. "These are sounds that are really precise, obnoxious, and attention-getting, and that's what you want." The research may actually lead to scarier movies or more startling alarms, a researcher tells Discovery. As for why humans scream in the first place, Poeppel says the behavior probably started with babies; their cries' ability to activate their parents' amygdala likely helped the infants survive, notes NPR. (If you're a screamer, avoid this hospital.)