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There’s a scene in the 1951 Western Distant Drums when a group of soldiers begins to wade through a swamp in the Florida Everglades. The camera cuts to an alligator, lurking patiently underneath the water, and then—water splashes, arms flail, a man screams.

The same man screams again, later, as a Storm Trooper in Star Wars, as an orc in Lord of the Rings, and as Buzz Lightyear in Toy Story. He screams in Spiderman and Family Guy, too, and Anchorman and Reservoir Dogs and Titanic and Cars and Cars 2. He screams a lot.

The prolific sound effect, used in hundreds of movies and TV shows since its Distant Drums debut, is known as the Wilhelm scream, named after a character who uses it in the 1986 Western The Charge at Feather River. It’s not totally clear whose voice is the source of the scream, but many believe it was recorded by the actor Sheb Wooley, better known as the musician behind the song Purple People Eater— the point being that some people offer more than their fair share of gifts to the world, and also that screams, whether they’re in Florida or space or Middle Earth, all sound a lot alike.

“Screams are the one uncontroversially universal vocalization,” said David Poeppel, a professor of psychology and neural science at New York University. “This is the most obvious response to signal ‘Go away, run away.’ [Screams] in an evolutionary context far precede other vocalizations.” The ability to scream is innate; after drawing a breath, it’s one of the very first things a brand-new human will do upon entering the world. A scream can signal pleasure (like on a roller coaster) or enthusiasm (like at a hockey game), but more often, it signals a need for help: Screams of pain, screams of fear.

“If you ask someone, ‘What is a scream really like?’ the typical answer might be, ‘Well, it’s louder and it’s higher pitched and maybe kind of shrill,’” Poeppel said. “But there’s lots of stuff that’s loud, including lots of vocalizations, and there’s lots of stuff that’s high-pitched … The real question is, what’s a unique signature that’s not part of other vocalizations?”

In a study published today in the journal Current Biology, Poeppel and colleagues from NYU and the University of Geneva identify exactly what it is that separates a scream of distress from all the other sounds a person can make. Analyzing a collection of fear screams recorded by volunteers, the researchers found that they contained much higher levels of roughness, or the rate at which the volume of a sound changes, than normal speech does.

“A regular conversation like you and I are having, the sound changes in loudness about four to five times a second,” Poeppel explained; for a single scream, by contrast, that number may be anywhere between 30 and 150. So a scream's volume oscillates too quickly to be perceived by a listener, but that's what makes it “rough.” “If you make it go faster and faster, it starts sounding rough,” he said. “And the more a sound has that acoustic feature, the more bizarre and scary it sounds.”

To test that theory, the researchers manipulated the recordings to make some rougher than others, and asked the volunteers to rank to the level of fear in each one. Unsurprisingly, the screams were ranked as more fearful than the spoken words; within each category, though, the rougher the sound, the more fearful it was considered. When the researchers played the recordings on either side of the volunteers, the screams were the ones that they could locate the most quickly and accurately. And in MRI scans, the screams, unlike the more neutral sounds, activated the amygdala, a part of the brain that plays a role in fear conditioning.

The study focused only on screams meant to signal fear, Poeppel noted, adding that other screams would likely have their own acoustic and behavioral signatures. A 2013 study examining the screaming voices of heavy-metal singers, for example, found that they had a high degree of jitter, a vocal feature that makes the speaker’s pitch sound uneven, while a small 2012 study found that baby screams were processed more quickly in the brain than other sounds of distress, like adults crying or animals in pain.

The only other sounds that affected the study participants the same way as fear screams, in fact, were alarms like buzzers or car horns, a group of rough noises that the researchers called “synthetic screams.”

“You can actually demonstrate that the more roughness modulation a sound has, the more the amygdala is activated,” Poeppel said. “The rougher it is, the more you know that it’s an alarm signal that’s scary and needs immediate action.” (Especially, probably, when your alarm signal is the Wilhelm scream itself.)

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