What frequency is the scratching of finger nails on a blackboard?

From http://www.livescience.com/16967-fingernails-chalkboard-painful.html:

Interestingly, the most painful frequencies were not the highest or lowest, but instead were those that were between 2,000 and 4,000 Hz. The human ear is most sensitive to sounds that fall in this frequency range, said Michael Oehler, professor of media and music management at the University of Cologne in Germany, who was one of the researchers in the study.

No one knows all of the reasons why that sound is so painful to listen to, but some theorize that we evolved ear canals to amplify human speech as much as possible, and that sounds like this happen to have large portions of their energy in that frequency band.