Just the thought of nails raking across a chalkboard is enough to send shivers down many people's spines — but the reasoning behind this reaction has remained a mystery for decades. Scientists have attributed it to everything from the visual stimuli associated with the sound to an inborn reaction that helps preserve one's hearing — there's even been an Ig Nobel Prize awarded for nail-on-chalkboard research.
Now, two European researchers think they've uncovered some new clues to the fingernail-chalkboard mystery. The first is physiological; the second, psychological.
Scientists have known for a number of years that many of the sounds that are most unpleasant to the human ear occur between two and four kilohertz, the range spanned roughly by the highest octave on a piano.
Incidentally, when researchers Michael Oehler and Christoph Reuter varied the sound of various unpleasant noises played for volunteers (noises including forks scraping against plates, fingernails raking across a chalkboard, and the squeaking of rubbing styrofoam), they found that the most "painful" frequencies (as measured by indicators of stress like heart rate and blood pressure) also occurred in this range.
This discovery, in and of itself, isn't completely new. NYU neuroscientist Josh McDermott drew a similar conclusion back in 2009, also pointing out that most noise-induced hearing loss occurs between 2kHz and 4kHz, and that it's "conceivable that the aversive reaction partly reflects the ear's vulnerability."
What Oehler and Reuter point out is that many of the acoustic features of human speech also fall within this frequency band, and suggest that the shape of our ear canals may have in fact evolved to amplify frequencies in this range specifically, referring to this amplification as "open ear gain."
For the second (and I think more interesting) half of Oehler and Robert's experiment, the researchers played what they had previously determined to be the two most unpleasant sounds (fingernails raking against a chalkboard and chalk squeaking against slate) for their volunteers. This time, however, they informed half of their listeners that they would be listening to a piece of contemporary music, and the other half that they would be listening to chalkboard screeches.
Listeners who were told they would be subjected to the chalkboard sounds were more likely to rate the noises as unpleasant than those who were told they would be listening to music. Even more interesting, however, was the observation that participants' stress indicators changed consistently, regardless of what they were told or how they rated the sound.
So does this mean the book is closed on why we find the sound of nails on a chalkboard so awful? Not by a long shot.
The researchers speculate that the amplification of frequencies in the 2kHz—4kHz range could have been important for human survival early in our evolutionary history — allowing us to respond to a baby's cry for help, or heed the warning call of another individual. This hypothesis is one that has been explored before in monkeys, and the one species that was examined did not demonstrate the same aversion to these sounds as humans.
For now, however, Oehler and Reuter intend to explore whether or not their findings could help engineers make the annoying sounds of day to day living (think whining vacuum cleaners) more bearable.
"Our findings might be useful for the sound design of everyday life," Oehler said.