In the frequency range of 2000 to 5000 Hz, which our ears are the most sensitive to, lurks a universally hated sound: Nails scraping against a chalkboard. Which isn't, it turns out, quite the worst sound in the world. Researchers from Newcastle University recently rounded up 13 volunteers, stuck them in MRI machines, and studied their brains while exposing them to a total of 74 sounds. Nails on a blackboard took fifth place behind knives, forks and rulers scraping against glass and chalk itself scraping against a blackboard.
You can give 'em a listen yourself if you're feeling adventurous (this is some seriously shrill chalk on a blackboard). According to the study, the amygdala, the part of the brain that processes emotion, "modulates the activity of the auditory part of the brain so that our perception of a highly unpleasant sound, such as a knife on a bottle, is heightened as compared to a soothing sound, such as bubbling water."
In other words, when you hear something nasty, your amygdala lights up like a flare. We still don't know exactly why our ears are especially sensitive to that range of sound. It could be because screams fall within the 2000 - 5000 Hz range, and we evolved to react strongly to those sounds in the name of survival. Or maybe our ears are simply shaped in such a way that they amplify those sounds and cause us physical pain, provoking a strong unpleasant reaction.
The researchers hope the study can help broaden our understanding of medical conditions like misophonia (hatred of sound) and tinnitus, as the amygdala obvious plays a part in how we deal with noise.