Just the idea of long, sharp fingernails running slowly down a dry blackboard can make some people wince. But why is that so? Thanks to a group of scientists from Germany, the mystery might finally be solved.

The researchers subjected a number of participants to that very sound over and over again. All vitals were then measured and they were asked to describe how the sound made them feel The results were surprisingly divided: half found in tolerable while the other half found it excruciating. Some actually thought they were listening to music.

The only time both groups found it unbearable was when the sound's pitch fell somewhere between 2,000 and 4,000 hertz. Think of it as a sweet-spot for humans, because the sound tends to get amplified inside the ear canal. The scientists will be presenting their findings at the Acoustical Society of America annual meeting in San Diego.