Screams have a unique acoustic signature that activates the brain's fear centre and is used exclusively to signal danger or distress, scientists have discovered.
If asked, most people say that screams are identifiable because of the pitch or volume, but little research has been carried out to establish exactly what makes them special.
Luc Arnal, from the University of Geneva, realised there was very little research into human screams and their properties, so he carried out a series of studies to analyse the sounds.
Publishing his findings in the Cell Press journal Current Biology, Arnal and colleagues used recordings from films, volunteers and YouTube videos and plotted the sound waves in a way that reflects the firing of auditory neurons. They realised the screams activate a range of acoustic information not thought to be important for communication.
David Poeppel, the paper's senior author, explained: "We found that screams occupy a reserved chunk of the auditory spectrum, but we wanted to go through a whole bunch of sounds to verify that this area is unique to screams.
"In a series of experiments, we saw [that] this observation remained true when we compared screaming to singing and speaking, even across different languages. The only exception – and what was peculiar and cool – is that alarm signals (car alarms, house alarms, etc.) also activate the range set aside for screams."
The researchers found that screams and alarms have an acoustic property called roughness (how fast a sound changes in volume). Normal speech only has slight differences in loudness, but screams can vary from between 30 and 150 Hz.
Screams with the highest roughness were considered to be most frightening, with increases in roughness corresponding a greater activation of the fear response in the amygdala.
Poeppel said: "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.
"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."
The study also suggests engineers accidentally came across this roughness when designing alarms: "These findings suggest that the design of alarm signals can be further improved," Arnal says. "The same way a bad smell is added to natural gas to make it easily detectable; adding roughness to alarm sounds may improve and accelerate their processing."
Researchers now plan to look at infants to see if their screams are particularly rough, Poeppel said: "Screaming really works. It is one of the earliest sounds that everyone makes – it's found across cultures and ages – so we thought maybe this is a way to gain some interesting insights as to what brains have in common with respect to vocalisation."