Sharon Drummond, Flickr
First thing that comes to mind? Nails on a chalkboard. This sound has always made the hair stand up on the back of witnesses necks.
Turns out that this sound, along with others in similar pitch, all affect parts of the brain responsible for making us feel displeasure. A study done at Newcastle University in England found that the top 10 worst sounds for the human ear are:
6. A female scream
7. An angle-grinder (Power tool)
8. Squealing brakes on a bicycle
9. A baby crying
10. An electric drill
Anatomography, Wikipedia
Hopefully you turned down your computer volume while listening to those. What exactly is going on here? Well, the unpleasant sound waves enter your ear, shoot through your cochlea (inner ear) to the cells responsible for registering frequencies from 2000hz - 5000hz, your auditory nerve brings this signal to your brain, and your brain's amygdala (responsible for negative emotions) freaks out a little.
Cliparts
Most sounds DON'T activate the amygdala, it just so happens that these unpleasant 2000hz - 5000hz sounds do. There is a possible primitive link here. Take a look at the top 10 list again you'll notice that the only two naturally occurring sounds are a female scream and a baby crying. These are distress calls, and it makes sense that our brains would be most sensitive to these sounds. If distress calls were in a more pleasant range of pitch, then we'd be less likely pay so much attention to them and act on them. 100,000 years ago, for example, a scream could have signaled a predator or other danger. It just so happens that modern humans have invented all sorts of things that make sounds in that same unpleasant range.
For now, stay away from chalkboards and bad brakes, tend to your screaming children, and find a babbling brook nearby if your ears need break. Or check out our article on the "Autonomous Sensory Meridian Response" for some feel good sounds.
Thumbnail Photo by: Ben Husmann, Flickr