It is probably the most scary scene in cinematic history. The shower curtain is drawn back and actress Janet Leigh lets out a spine-chilling scream that warps into a frenzied cacophony of staccato music as she confronts an unseen, dagger-wielding madman.
When Alfred Hitchcock put the soundtrack to his 1960 masterpiece Psycho he was almost certainly unaware that the discordant musical notes he was adding to the disturbing shower scene were in fact based on the sort of non-harmonic sounds used in the distress calls of wild animals.
Scientists have found that many of the emotionally-evocative moments in some of the most popular films are enhanced with a sound score that exploits the human brain's natural aversion to the "non-linear" sounds widely used in the animal kingdom to express fear and distress.
Sounds are classed as non-linear when they become too loud for the normal musical range of an instrument or an animal's vocal chords. Alternatively they can be produced by the sudden frequency changes of acoustic instruments, like those that accompanied Leigh's primal scream.
Scientists who normally study the non-linear alarm calls of marmots – an American ground squirrel – have found that the use of similar, non-linear sounds in the musical scores of films is widespread as a way of enhancing the most emotionally evocative moments of a cinematic story.
Their study of more than 100 film soundtracks has found that film makers appear to exploit our natural aversion to non-linear sounds in order to get the most out of a moment of drama, whether it is the sad scene in the film Forrest Gump as the eponymous hero sits on a park bench, or the menacing pathos of a Corleone family funeral in Godfather II.
"We all know that things like tempo and volume are used by musicians to create tension and elicit particular emotions. We know that certain chords also do that," said Professor Daniel Blumstein, an evolutionary ecologist at the University of California at Los Angeles.
"We looked for things that would have been non-linear had they been naturally produced in film soundtracks. We call these nonlinear analogues.
"What is novel about this study is that we specifically looked for these non-linear analogues and found that indeed they're present in evocative scenes and that different sorts of emotions are associated with different types of non-linearities," Professor Blumstein said.
Sounds become non-linear when the volume increases beyond a certain point, when the sound becomes "raspy" and jumps around, indicating that it is beyond the normal, linear range of the instrument or the vocal chords, Professor Blumstein said.
"Imagine a horn. You blow it gently and a nice sound comes out. You blow it a little louder and a nice but louder sound comes out. At some point, when you blow it too hard, the sound gets unpredictable, distorted and noisy.
"You've hit the non-linear zone of that horn. The same thing happens in your vocal tract. Indeed you can imagine that if you're really scared, you'll really yell, and the yell or scream will contain [non-linear] noise," Professor Blumstein explained. The study, published in the journal Biology Letters, involved a detailed acoustic analysis of 30-second clips chosen from the most iconographic moments of a film, such as the shower scene in Psycho or the execution scene in The Green Mile.
"Soundtracks contain more than simply music and sound engineers can create sounds that would be impossible for an individual to produce," the scientists write in their paper. As well as male and female screams, the scientists analysed the non-linear noises in the sound effects, the ambient background noise and any sudden changes in sound frequency.
They looked at four broad genres of film: adventure, horror, drama and war. It was only in horror and drama that the scientists found a significant use of non-linear sound to amplify an iconic scene's emotional content, whether it is a scary moment in a horror film or a tearful moment in a drama.
"Our results suggest that film makers manipulate sounds to create nonlinear analogues in order to manipulate emotional responses," the scientists conclude.
the sounds of suspense
King Kong, the 1933 classic horror movie, saw the first use of recorded animal sounds that were subsequently manipulated to produce non-linear sounds, the scientists said. The pitch and timbre of the animal calls were changed by the manipulation of the play back medium. This idea has been used many times in certain films depicting prehistoric, alien or otherwise monstrous characters. The natural sounds may be difficult to synthesise, which is why film makers resort to real recordings that they manipulate to make them sound more non-linear.
A notable early exception was in Alfred Hitchcock's 1963 film The Birds. Here, the director used an electronic instrument, the trautonium, to create a horrifying avian language rather than use recorded bird calls.
Musical composers also use non-linear sound to emphasise evocative emotions. The use of the music of 20th Century composer Krysztof Penderecki in the The Exorcist and The Shining "inspired the use of noise techniques as a style marker of horror genre films", the researchers said.