What frequency is the scratching of finger nails on a blackboard? This is the frequency/intensity that sets my teeth on edge. 
Does anybody know what frequency (roughly) it is? I am guessing it is near the top of normal human hearing, 20kHZ, but I'm not sure if that's why it affects me.
I am sure the same frequency is played on some of the music I listen to, but somehow, it does not make me wince.
There is a related question here, with no answer Scratching on a Blackboard, but I just want a frequency value.
 A: 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.
A: In addition to the other answer by Cort Ammon, I have heard of other psychophysical/evolutionary explanation:
The frequency distribution of that sound  closely corresponds to the frequency of a crying baby, which has been shown to drive people crazy when exposed to it for a short amount of time (we are genetically predisposed to get distressed by such a call). In this case the specific frequency distribution of nails in a chalkboard is better as stimulating those neural pathways than the stimulation produced by the original sound.
