"Everybody screams and everybody has an intuition about what constitutes screams—that they are loud and high-pitched," says David Poeppel. "But neither turns out to be quite correct." (Credit: Allison Waffles/Flickr)

acoustics

Why nothing else sounds like a scream

There is nothing quite like the sound of a scream to make the hair on the back of the neck stand up. But what is about a scream that grabs our attention?

All screams have an “acoustic signature”—a sound we make only when we want to let others know we’re in distress.

“Everybody screams and everybody has an intuition about what constitutes screams—that they are loud and high-pitched,” says David Poeppel, a professor in New York University’s psychology department and at the Center for Neural Science.

“But neither turns out to be quite correct. In fact, screams have their own acoustic niche separate from other sounds. While, like some sounds, they may be high-pitched and loud, screams are modulated in a particular way that sets them apart from the rest.

Scream ‘roughness’

“Screams have a trait called ‘roughness,’ which refers to how fast a sound changes in loudness,” says Poeppel, who is also director of the Max Planck Institute for Empirical Aesthetics in Frankfurt.

A standard measure of sound amplitude modulation—how loudness changes in speech—is Hertz (Hz), or cycles per second. Normal speech rates are typically between four and five Hz, but for roughness, the rate is between 30 and 150 Hz—a remarkably higher rate.

For the study, published in Current Biology, researchers conducted both experiments and analyses that measured sound modulation and identified which parts of the brain were active while listening to screams and other sounds.

In one experiment, they created a bank of sounds, downloaded from the internet, containing several types of human vocalizations (screams and sentences), artificial sounds (alarms like a buzzer and instrument sounds), and sound intervals (pure tone intervals such as “a perfect fifth”).

The findings show that the recorded screams and the artificial alarm sounds and dissonant intervals, such as a “mistuned fifth,” fell into the roughness domain (30–150 Hz)—a finding that suggests alarm manufacturers have effectively captured the modulation of a human scream—while the other sounds did not.

“It’s right behind you!”

In another lab experiment, one group of subjects, which included men and women, recorded a series of sounds: screams, screamed sentences (“It’s right behind you!”), meaningless vocalizations (“aahhhhhh”), and normally spoken sentences. Both screams and screamed sentences occupied the “roughness domain” but the other sounds did not.

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In an effort to further confirm the findings, another group of subjects listened to these sounds, which included both screams and alarms as well as other sounds, and indicate which seemed “alarming.” Screams and alarm sounds were rated as more disturbing than the others depending on roughness: the rougher the sound rating, the scarier it seems.

Finally, in order to see how these sounds are processed, the researchers monitored brain activity—using functional magnetic resonance imaging (fMRI)—of the study’s subjects while they listened to these sounds. For both the screams and the alarm sounds, the subjects showed increased activity in the amygdala, which is the region of the brain used for processing and remembering fear.

“As a whole, our findings show that screams occupy a privileged acoustic niche that, because they are separated from other communication signals, ensures their biological and ultimately social efficiency—we use them only when we need them,” Poeppel says.

Researchers from the University of Geneva are coauthors of the paper, which was supported, in part, by the National Institutes of Health.

Source: NYU

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