This varies from person to person because how we respond to sound is largely learnt. When I did an experiment into the worst sound in the world, the sound of someone vomiting came top (or should that be bottom?).
People were probably having a disgust reaction to the horrible sound. When we encounter something that might cause us harm or make us ill, like someone being sick, we have a revulsion to it to make sure we stay away. A disgust response is pretty universal, because infants are taught it by their parents to keep them safe from disease.
Other studies have found screeching and scraping sounds to be very unpleasant, with the archetypal example being fingernails scraping down a blackboard. Why we should find this sound so unpleasant seems to be down to the acoustic similarity between a scraping sound and someone screaming in distress.
If you really let rip with a scream, you get a rough, hoarse sound. Fingernails scraping down a blackboard create a similar effect, as the nails stop and start as they judder down the board.
One study played such sounds to people while monitoring their responses in a brain scanner. They found that unpleasant rough sounds of both scraping and screaming produced a large response in the amygdala. The amygdala is a collection of neurons that are known to play an important role in helping us detect and deal with threats.
Read more about the science of sound:
- Do I need to buy expensive wires for good audio quality?
- How does a soundbar simulate three-dimensional sound?
- What’s a bitrate, and what has it got to do with music quality?
- Bluetooth, AirPlay, Chromecast… Does it matter how I connect to my wireless speakers?