If you’re like most people, you probably can’t stand the Shivers sound of fingernails scraping across a blackboard. You’re probably cringing just thinking about it. This ear-piercing noise is so universally disliked, perhaps it’s no surprise that dozens of scientists have researched why it evokes such a visceral reaction.
Overall, research shows that this ear-splitting noise has the same frequency as that of a crying baby and a human scream, indicating that these sounds are tied to survival. For instance, people attuned to these frequencies may rescue a crying infant sooner, improving the baby’s longevity.
One study has suggested that the shape of our ear canals, as well as our own perceptions, are to blame for our distaste of shrill sounds.
The study’s participants rated their discomfort to various unpleasant noises, such as a fork scraping against a plate or Styrofoam squeaking. The two sounds rated as the most unpleasant, they said, were fingernails scratching on a chalkboard and a piece of chalk running against slate.
The researchers then created variations of these two sounds by modifying certain frequency ranges, removing the harmonic portions (or other concordant tones). 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. 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 found that the offensive sounds changed the listeners’ skin conductivity significantly, showing that they really do cause a measureable, physical stress reaction.
The most painful frequencies were not the highest or lowest, but instead those that were between 2,000 and 4,000 Hertz. 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.
Oehler pointed out that the shape of the human ear canal may have evolved to amplify frequencies that are important for communication and survival. Thus, a painfully amplified chalkboard screech is just an unfortunate side effect of this (mostly) beneficial development. “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. “The only thing we can definitively say is where we found the unpleasant frequencies.”