What makes a scream so alarming?
A scream's 'roughness' - the rate at which its volume varies - is what makes it so alarming, rather than the volume or pitch, new research has shown.
Screams are some of the most ubiquitous human noises - basic instincts that we're all programmed to follow from the moment we're born, no matter what culture we're part of.
Whether you're at the cinema or walking through a forest late at night, a scream always has the same effect on you, causing goosebumps on your skin and a cold sweat to run down your neck. Yet until now, very little research had been done into just what makes screams so alarming.
Conventional wisdom may suggest the volume or the pitch of a scream as its distinguishing feature. However, upon analysing a wide range of different screams, David Poeppel and his team at the University of New York noticed that it was an entirely separate feature that sets them apart: their roughness.
Writing in Current Biology, they found that the volume of normal human conversation goes up and down at a frequency of about 4 or 5Hz (times per second) - a rate that, interestingly, is universal across all languages. Screams, however, have roughness levels between 30 and 150Hz, meaning that their volume varies at a much faster rate than normal speech.
Digitally adding this roughness to every-day sentences immediately made them sound scarier to listeners. The converse was also true: by removing the volume modulation from a scream, it lost its spine-tingling scariness and was interpreted more as normal sounds are.
Using an fMRI scanner to observe people's brains whilst playing a range of different sounds, the team saw a strong correlation between levels of activity in the amygdala - the emotional hub of the brain - and the roughness in the sound being played: the more roughness, the more activity there was and the greater the reported scariness of the sound.
Furthermore, volunteers also were able to determine which direction a rougher sound was coming from much more quickly. This ensures that a scream elicits a very specific response, alerting us that there is danger nearby, but without the risk of it being confused with other noises.
Intriguingly, though, artificial sounds such as car alarms and police sirens were found also to have a very high level of roughness, meaning that our brains process them in much the same way they do a scream. Over the years, then, it appears that sound designers have managed to stumble upon roughness in their attempts to grab our attention.
In theory, the identification of roughness as a key component of a scream will allow the design of more effective alarm signals, with a lower risk of going unnoticed. There is also potential to make sound effects in films even scarier than they currently are by artificially applying roughness to really get us leaping behind the sofa.