Why Fingernails On a Chalkboard Sound Painful

Fingernails on a chalkboard is bad, but I can't stand fingernails over nylon at all - makes me shudder and the hairs on my neck and arms stand right up every time! What bothers you most?

http://news.sciencemag.org/sciencenow/2 ... tml?ref=hp
Cover Your Ears!
by Kim Krieger on 28 October 2011

Credit: Fotosearch

Some sounds are excruciating. Take fingernails squeaking on a chalkboard. The noise makes many people shudder, but researchers never knew exactly why. A new study finds that there are two factors at work: the knowledge of where the sound is coming from and the unfortunate design of our ear canals.

Fingernails raking against a chalkboard and chalk squeaking against slate were the most unpleasant sounds from a family of recordings, which also included sounds such as Styrofoam squeaks and scraping a plate with a fork.

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, noiselike parts of the sound. Some listeners were told the true source of the sounds, whereas others were told that the sounds were part of contemporary musical compositions. The same listeners then rated the pleasantness or unpleasantness of the sounds while the researchers measured physical indicators of distress: the listeners’ heart rate, blood pressure, and the electrical conductivity of their skin.

As they will report next week at the Acoustical Society of America conference in San Diego, California, Oehler and Reuter found that a listener’s skin conductivity changed significantly when the person heard a sound he or she later reported as unpleasant, showing that disturbing sounds do cause a measurable physical reaction. The researchers suspect that the shape of the human ear canal may be to blame for the pain.