This Halloween we take a look at the science of scary music. What makes us shiver, gasp and our hairs stand on end?
Next time you are watching a horror movie, try watching a key scene with the sound off. Your innocent, good looking and light-switch-averse protagonist is fast asleep. On screen, a ghostly entity is making their way slowly through the house getting closer and closer slowly turning the door handle to their bedroom door before bursting in. With the sound off, all the action is safely taking place in the spooky but separate realm of your TV. You will probably be able to watch in relative comfort stifling a smirk at the preposterous nature of the scene.
Try it again, this time with the sound on. When the intruder enters, the sound of quiet but high-pitched violins seep into your living room, accompanying the squeak of a floorboard and a slow drum beat thuds into life. As they draw closer, the violins swell, bows thrashing in an increasingly disorganized and frenetic manner while the pace of the drums quicken. As they turn the door handle, the frantic cacophony floods your sitting room before cutting to silence just after they burst through the door. Suddenly – heart in your mouth, hairs on end – you are right there with your protagonist, who just couldn’t resist the invite to a remote cabin built on an old burial ground on a late October weekend.
But why are these experiences so different? How do movie score composers add so successfully to this effect and how can it be explained?
The terror of timbre
Firstly, there is the choice of instrument and the timbre that the composer is trying to achieve with each one selected. String instruments, such as the cello or violin, are often considered to produce sounds closest to that of the human voice . The stream of air passing over vocal cords has even been described to act “in the same way as the violin bow,” an image all too familiar with fans of the horror TV show ‘Hannibal’ .
The sound produced by instruments to create music is primarily detected by regions in the temporal lobes and inferior frontal cortices  disparate to the regions that typically detect human-created sounds, such as speech, although these can overlap . However, playing certain instruments in a particular manner can lead to the sound being processed in the areas of the brain associated with human sounds, essentially tricking the brain into responding to music as if it were a human voice.
By playing violins at a high pitch with full, harsh bow strokes, the sound of a scream – hastily gasped air being expelled over taught, stretched vocal cords – is mimicked. Equally, the low rumble of a cello or double base can evoke the growl of a predator. We have evolved to respond to both of these sounds with fear and anxiety, triggering a fight or flight response to allow us to deal with the approaching danger.
Just as we have evolved to fear specific sounds that indicate danger, so we have evolved to spot the change in a sound, from pitch and volume to rhythm, as an indication of a threat. The prefrontal cortex of the brain houses a highly sophisticated “change detector” that can monitor changes in light and sound .
Changes in sound can be important, footsteps growing louder or changing in tempo may indicate a threat approaching or changing from a passive to aggressive action. Just so, the ultimate change, from loud cluttered music to sudden silence can indicate that the threat has arrived at its destination. It is the sonic harbinger to the protagonist hiding under the bed that the intruder is stood at its foot…
Hence the prevalence of the established horror score trope of a violent crescendo, using a combination of rhythmic and melodic instruments, followed by an abrupt and haunting silence. All the changes needed to indicate the approach and arrival of a threat are present, stimulating your senses.
Dread the dissonance
Much of a scary movie, however, cannot be filled with the constant ups and downs needed to create horror, and instead needs to get by creating an unsettling sense of unease and uncertainty. This can be achieved by using unresolved chords, such as the tritone – banned during the renaissance and referred to as the Devil’s interval  – to create an ambiguity in the direction of the music. These disruptions to the composition of music, deprive the human brain of something it craves – structure. One region of the brain, named Broadman Area 47  (agonizingly close to Area 51), is associated with prediction and uses patterns in music to set expectations of what is coming next. If this region is unable to predict what will come next, an eerie feeling of uncertainty can be created. So, next time you are watching a movie and marveling at the visual effects, spare a thought for the composer scoring it, who has compiled a host of tricks to play with your mind and stimulate your most fundamental evolutionary reflexes.