Whether you’re a movie buff or have spent some time around teenagers, you’ll probably have noticed that screams are unlike any other form of human communication. There’s something shockingly immediate and direct about them: A human scream can convey information and spur others into action even without the use of any actual words. We all know there’s something special about shrill shrieks and blood-curdling wails, but a new study published in Current Biology casts some light on what that something is.
A team led by David Poeppel, a neuroscience professor at New York University, sought to examine the acoustic qualities of screams, as well as the effect they have on the brains of those in earshot. First, they used a newly developed system, the modulation power spectrum, to study the frequencies at which human speech, musical instruments, and a wide variety of other sounds change in volume level (if a sound went from deafeningly loud to almost imperceptible dozens of times a second, it would change volume at a very high frequency). They found that screams occupy a specific territory on the acoustic spectrum: While the volume of normal speech fluctuates around four or five times a second (measured on the scale at 4–5 Hz), screams live between 30 and 150 Hz, meaning their volume oscillates wildly — a “perceptual attribute” known as roughness.
While roughness had been thought to be irrelevant to human communication, the authors demonstrated that it actually marks screams as separate from normal speech, helping them serve as a more effective means of conveying that there is an emergency and avoiding false alarms in more mundane conversation.
Roughness also has a telling effect on the brains of those exposed to it. Using fMRI to study participants while they listened to a variety of noises, including the recorded shrieks of other volunteers, the authors found that “acoustic roughness engages subcortical structures critical to rapidly appraise danger.” That fluctuating register activates the amygdala — part of an ancient part of the brain governing emotion — which is responsible for processing fear and engaging the fight or flight response. Screams cut the line, bypassing the brain’s normal structures for processing auditory information to enable a more immediate response from those who hear them.
Screams are not the only sounds that occupy this acoustic territory. Car alarms and emergency vehicle sirens have similar qualities and evoke similar responses, and these findings have practical applications. “You could optimise alarm sounds to make them scarier,” Poeppel told The Guardian, or fine-tune the audio for horror movie screams to really drive the audience up the wall. More than likely, though, Hollywood will continue its bizarre love affair with Wilhelm’s version.