Finger nails on chalkboard - why so disturbing

By Dr. Ananya Mandal, MDNov 7 2011

Some sounds are especially disturbing – like the sound of a person's fingernails scraping down a chalkboard. Now researchers know why.

The frequency of the screechy, scratchy sound ranges between 2000 and 4000 Hz, a frequency most sensitive to the human ear. “Some frequencies are amplified due to the anatomy of the ear canal,” write musicologists Christoph Reuter, of the University of Vienna, and Michael Oehler, of the Macromedia University for Media and Communication in Cologne, Germany.

In Reuter's and Oehler's study, they played two unpleasant sounds - fingernails on a chalkboard and squeaking chalk on a slate - but before doing so, they told half of their participants that they'd be hearing contemporary music. The other half knew the horror that was coming. The researchers then modified the recordings of fingernails and chalk, removing or attenuating various frequency ranges. They also modified the sounds by selectively extracting either the tonal, musical-pitch parts or the scraping, growling, noise like parts of the sound.

Both groups reacted physically to the sounds, which induced a galvanic skin response - a change in the skin's electrical activity that is often triggered by fright and can be measured, like a polygraph test. Those who thought they were listening to contemporary music judged the sound as slightly less unpleasant - but they still didn't like it, suggesting psychology plays a role in what makes the noise so noxious.

The researchers suspect that the shape of the human ear canal may be to blame for the pain. Previous studies have shown that the ear canal amplifies certain frequencies. A loud screech on a chalkboard could be amplified within our ears to painful effect, the researchers propose.

Combining physiological measurements of pain such as skin conductance with psychological ratings of uncomfortable sounds is new and makes an important point about perception, says Reinhard Kopiez, a musicologist at the Hanover University of Music, Drama and Media in Germany, who was not involved in the study. Kopiez says the research shows how important context is in the enjoyment of music. “The audience enjoys the performance because of the knowledge about the (artistic) origins of a sound, although the physiological response remains the same as for uncomfortable sounds,” Kopiez says.

Reuter and Oehler presented their finding this week at the Acoustical Society of America meeting.