Richard A Lovett
When people hear a scream, English fantasy writer and humorist Terry Pratchett once wrote, they don’t necessarily come running.
“That’s not how humans work,” he said. “Humans look at other humans and say, ‘Did you hear a scream?’ because the first scream might have been you screaming inside your head, or a horse backfiring.”
Pratchett, who died in 2015, was famous for peppering his often-humorous novels with such pearls of folk wisdom.
But in this case, scientists say, he might not be fully correct. Because it turns out that humans are pretty good at recognising screams for what they are — even though there are plenty of opportunities for confusion.
When animals scream, says bioacoustics researcher Jay Schwartz, a graduate student in neuroscience and animal behaviour at Emory University in Atlanta, Georgia, US, it’s probably for a fairly specific reason, such as trying to startle an attacking predator in order to escape, or to recruit help when in trouble.
But humans scream for a great many other reasons: terror, anger, pain, excitement, surprise, psychological distress, or even the delicious fear of roaring downhill on a roller-coaster.
And however much attention screams may have drawn from horror film-makers, or artists such as Edvard Munch, scientists have largely ignored them.
“Animal screams have been studied in depth, [but] human screams have received little attention,” Schwartz recently told a meeting of the Acoustical Society of America in Louisville, Kentucky.
To remedy this, his team asked 181 volunteers, mostly undergraduate students, to listen to 75 non-verbal human vocal sounds — sounds that represented both a broad acoustical range and an array of emotional contexts. In each case, the study participants were asked to classify them as screams or non-screams.
Despite the wide range of vocal options, the study participants were in “substantial agreement” over whether any given one was or wasn’t a scream. In fact, the researchers reported, for 26 of the sounds, more than 90% of the participants agreed that they were screams, with the consensus sometimes reaching as high as 99%. {%recommended 4841%}
Schwartz and colleagues then turned to physics to look at the acoustical structures of the sounds.
Screams, they found, tended to be relatively high in pitch and to have a sweeping, arcing range in pitch as they progressed.
They also tended to score highly on a parameter called roughness, which, Schwartz explains, is perceived by listeners as a harsh buzzing or gravelly sound, as opposed to a more “pure” tone.
“Sounds that were more rough were classified as screams,” he says. “Even though [they represented] different emotional contexts, people agreed they were screams.”
Nor did the gender of the recorded voice appear to play a role.
“We found that a vocalisation was no more or less likely to be classified as a scream if it was female or male,” Schwartz says.
But the agreement wasn’t always perfect. Some of the recorded sounds produced mixed responses, Schwartz reported. And one that fooled 71% of the study participants was actually a whistle, not a scream.
Interestingly, though, that may be the exception that proves the rule.
“This made sense when we looked into the acoustic qualities of the whistle,” Schwartz says. “It exhibited many of the traits we found to be associated with screams.”
The next step, he adds, is to look into the acoustical variations among sounds recognised as screams — versions that might relate to differences between the roller-coaster yell of faux fear and a shriek of pure horror.
“It is possible that some of the acoustical variation might map onto different emotions,” he says.
Schwartz’s team leader, psychologist Harold Gouzoules, adds that even higher-tech methods may also be brought into play.
“In the future we plan to incorporate functional magnetic resonance imaging [brain scans] to examine the neurobiological underpinnings of people’s reactions to screams,” he says.
The results, the two researchers note, may not be of interest just to acoustics researchers.
To start with, the research may help scientists understand the evolution of humans’ ability to communicate emotions.
And, Gouzoules adds, “there are also potential health applications, as there are multiple psychiatric disorders that involve screaming.”