Why is nails on a chalkboard

Oehler and Reuter found that there was no significant difference in physiological reactions between the two groups during the study. The difference was in the ratings, which proved harsher if the subject knew he was hearing the sound of nails on a chalkboard. Removing frequencies between to Hz still makes an offending noise a little more bearable.

He adds that the two most aversive aspects of a sound are sharpness—caused by high frequencies—and roughness—caused by fluctuation of frequency intensity.

What they did was take some recordings of a three-pronged garden tool scraping over a chalkboard, and then mess with them to see what part of that sound we hate so much. By removing the high, middle and low frequencies one at a time from the sound, and playing them back, they tried to find what part of the sound makes everyone cringe. It was removing the middle frequencies that made the sound bearable.

Those are the same frequencies found in a primate warning call — which led the researchers to conclude that our aversion to the sound comes from recognizing it as a warning call. Later research, however, has refuted that idea. One study played sounds to tamarin monkeys : One was a high pitch scraping sound like nails on a chalkboard, and the other was white noise of the same volume. The monkeys reacted the same way to both. Humans, on the other hand, strongly prefer the white noise.

Another theory has to do with hearing loss. High frequency sounds can damage the structures of our ears, causing short and long-term loss of hearing. Listeners in the study, Oehler said, rated a sound as more pleasant if they thought it was pulled from a musical composition. Though this didn't fool their bodies, as participants in both study groups expressed the same changes in skin conductivity.

The implication, then, is that chalkboard screeches may not irk people so much if they didn't already think the sound was incredibly annoying. Related: Why do seashells sound like the ocean? Another study, published in the Journal of Neuroscience in , reveals what's happening in the brain when people hear screechy sounds. The findings suggest that the fingernail-chalkboard sound triggers an uptick in communication between a region of the brain involved in hearing and another region of the brain involved in emotions.

In the study, 13 participants listened to 74 sounds, including nails on a chalkboard and the whine of power tools, and rated them according to their pleasantness. Researchers used functional magnetic resonance imaging fMRI to examine how the participants' brains responded to the sounds. When the participants heard an unpleasant sound, there was an interaction between the auditory cortex, which processes sound , and the amygdala, which processes negative emotions.

Moreover, the more averse the sound, the greater the activity between these two brain regions, the researchers said. But perhaps the most surprising thing to come out of this study is that the subjects rated nails on a chalkboard the fifth most irritating sound—knife on a bottle, fork on a glass, chalk on a blackboard and ruler on a bottle all come before it.

BY Erin McCarthy. Big Questions Medicine science.