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The nicest sounds included flowing water, thunder and a laughing baby, they found. Some of the most unpleasant sounds, according to the participants' ratings, included a knife on a bottle, a fork on a glass and chalk on a blackboard. Moreover, the more averse the sound, the greater the activity between these two brain regions, the researchers said. 'It's a possible distress signal from the amygdala to the auditory cortex.' 'It appears there is something very primitive kicking in,' study researcher Sukhbinder Kumar, a research fellow at Newcastle University, told Live Science in 2012. 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. Researchers used functional magnetic resonance imaging (fMRI) to examine how the participants' brains responded to the sounds. 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. 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. Related: Why do seashells sound like the ocean? Brain pickingsĪnother study, published in the Journal of Neuroscience in 2012, reveals what's happening in the brain when people hear screechy sounds.

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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. 'The only thing we can definitively say is where we found the unpleasant frequencies.' 'But this is really just speculation,' Oehler told Live Science in 2011, when the research was presented at a meeting for the Acoustical Society of America. Thus, a painfully amplified chalkboard screech is just an unfortunate side effect of this (mostly) beneficial development. Oehler pointed out that the shape of the human ear canal may have evolved to amplify frequencies that are important for communication and survival. The human ear is most sensitive to sounds that fall in this frequency range, said study researcher Michael Oehler, a professor of media and music management at Macromedia University of Applied Sciences in Germany.

The most painful frequencies were not the highest or lowest, but instead those that were between 2,000 and 4,000 Hertz.

They found that the offensive sounds changed the listeners' skin conductivity significantly, showing that they really do cause a measureable, physical stress reaction. Finally, they played back the new sounds for the participants, while monitoring certain indicators of stress, such as heart rate, blood pressure and the electrical conductivity of skin.

They told half of the listeners the true source of the sounds, and the other half that the sounds came from pieces of contemporary music. The researchers then created variations of these two sounds by modifying certain frequency ranges, removing the harmonic portions (or other concordant tones).