Why screams are scary (hint: It’s not because they’re loud)

You know it when you hear it -- the blood-curdling peal, the hair-raising shriek -- but for years scientists have not fully understood what makes a scream a scream, says one team of researchers who believe they have figured out the secret ingredient.

“Everyone screams and everybody has an intuition about what constitutes screams -- that they are loud and high-pitched. But neither turns out to be quite correct,” said David Poeppel, a psychology professor at New York University who was part of the effort, in a statement.

Rather, he and an international team of coauthors found, screams get their unique and terrifying power from an acoustic property known as “roughness,” which corresponds to how quickly a sound changes in loudness.

The researchers described their study Thursday in the journal Current Biology.

They conducted a variety of experiments to pinpoint why it is that screams alarm us so. First, the team analyzed sound amplitude modulations -- that’s the term for those changes in loudness -- in recordings of spoken and screamed sounds.

Screamers’ modulation rate tended to fall in a range between 30 and 150 hertz, which the authors identified as a “roughness” range; speakers, on the other hand, tended to have much slower modulations. When screams were compared with natural spoken speech and song, screams again had far more roughness.

Turning their attention to nonbiological sounds, the researchers discovered a similar effect: Musical instruments had relatively little roughness, while alarm signals such as buzzers and horns had a lot.

“The fact that roughness appears to be used in the design of artificial alarm signals in human culture, perhaps unwittingly, underlines both the perceptual salience and the ecological relevance of rough sounds,” the researchers wrote.

Next, the team tried to assess whether that roughness in sounds was associated with behavioral or neurological responses. Listeners were asked to rate how scary they found noises -- and sounds that had more roughness, whether natural or artificial, got higher fear-inducing ratings.

The scientists found that listeners could locate sounds more quickly when the noises had roughness. Using functional MRI, they also observed that hearing rough sounds activated a portion of the brain known to be involved in fear and danger response -- the amygdala.

Altogether, the researchers wrote, the evidence suggested that it’s roughness that makes a scream so terrifying, speeding the delivery of warning messages by activating the fear centers of the brain.

In future studies, the researchers plan to investigate whether babies’ screams are particularly rough, and whether animal screams share the same acoustic qualities. They suggested that security systems could be made even better if more roughness were incorporated into alarm tones.

“Screaming really works,” Poeppel said.

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