Screams tailor-made to frighten, study reveals
Alfred Hitchock must have had an inkling of what scientists have now discovered - that nature designed screams to be spine chilling.
Unlike any other spoken or sung sound, they possess acoustic properties tailor-made to frighten, tests have shown.
Tellingly, other non-human sounds sharing the same characteristic include shrieking car and house alarms.
Modifying an artificial sound to make it more scream-like had the same effect on volunteers and activated fear responses in their brains.
Lead researcher Professor David Poeppel, from New York University in the US, said: "Everybody screams and everybody has an intuition about what constitutes screams - that they are loud and high-pitched.
"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."
The unique thing about a scream turns out to be a property the scientists call "roughness" which relates to how rapidly the loudness of a sound changes.
This is measured in Hertz (Hz), or numbers of cycles per second.
Normal speech varies between being louder and softer at a rate of around four or five cycles a second.
But screams are much faster - between 30 and 150 Hz.
When volunteers were asked to judge the scariness of different sounds, those with the greatest roughness were rated as the most terrifying.
During the study, participants listened to the different sounds of human vocalisations, alarm buzzers, musical instruments, and synthesised "pure" tones.
Recorded screams and alarm sounds both shared the same "roughness" range of around 30 to 150 Hz, while other noises did not.
In a further experiment, volunteers were given functional magnetic resonance imaging (fMRI) brain scans while they listened to the sounds.
Both screams and alarms triggered increased activity in the amygdala, a region of the brain linked to fear processing.
The findings, published in the journal Current Biology, indicated that alarm design had tapped into a fundamental aspect of human biology just by trial and error.
But perhaps increasing the fear factor could make the devices more effective, scientists said.
Co-author Dr Luc Arnal, from the University of Geneva in Switzerland, said: "These findings suggest that the design of alarm signals can be further improved.
"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."
The scientists plan to continue investigating screams, including those of infants which are said to be especially "rough".
:: Although not tested in the study, the famous incidental music from the shower scene in Alfred Hitchock's film Psycho almost certainly falls into the "scream" category.
Composed by Bernard Hermann, the high-pitched screeching violin heard as Marion is stabbed to death by deranged motel owner Norman Bates in the 1960 thriller was voted the scariest movie theme tune of all time in a survey of film goers.