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. [Breaking the Code: Why Yuor Barin Can Raed Tihs]
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."
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. [Why Do Seashells Sound Like the Ocean?]
Another study, published in the Journal of Neuroscience in 2012, 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.
"It appears there is something very primitive kicking in," study researcher Sukhbinder Kumar, a research fellow at Newcastle University, told Live Science in 2012. "It's a possible distress signal from the amygdala to the auditory cortex."
Moreover, the more averse the sound, the greater the activity between these two brain regions, the researchers said. 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. The nicest sounds included flowing water, thunder and a laughing baby, they found. [Why Does the Sound of Water Help You Sleep?]
Frequencies between 2,000 and 5,000 Hertz were found to be unpleasant — roughly the same frequencies found by the 2011 research. "This is the frequency range where our ears are most sensitive," Kumar said. The reason for such sensitivity is not exactly understood, but this range includes the sounds of screams, which people find intrinsically unpleasant, he said.
Ig Noble Prize
A study investigating shrill sounds won a 2006 Ig Nobel Prize, awarded by the Society for Improbable Research. For the study, published in 1986 in the journal Perception & Psychophysics, scientists recorded the sound of a garden tool scraping over a chalkboard. Then the researchers fiddled with the recording, removing the high, middle and low frequencies from different recordings.
After playing the modified sounds to volunteers, the researchers found that removing the high frequencies didn't make the sounds more pleasant. Rather, eliminating the low and middle frequencies of the sound made the sounds more appealing, they learned, according to Medical Press.
In addition, the warning cry of a chimpanzee is similar to the sound of fingernails on a chalkboard, they found. Perhaps people have an unconscious reflex to this sound because of its uncanny resemblance to a warning call, the researchers told Medical Press.
Additional reporting by Live Science staff and Joseph Castro, a Live Science contributor. Original article on Live Science.