Re: the disturbing sound of scratching on a blackboard

This is probably more a question about psychobiology than biophysics, but 
I'll give it a try.  I actually did some experiments using modulation of 
pure tones and their annoying qualities on myself, so this is purely 
anecdotal.  

First, I checked the frequency spectrum of a recording of "nails on 
blackboard" (NOB)and didn't find it to be different from most normal 
sounds.  What I did find is that high frequency sounds by themselves, 
although somewhat annoying, don't even approach the annoying quality of 
nails on a blackboard.  Then I played around with a stereo rendition of 
high frequency pure tones.  I found that, when I modulated an 8,000 Hz 
pure tone in one channel relative to another (so one sine wave changed by 
5-20 Hz every second or so), thus producing an audio "diffraction 
pattern," it got much more annoying.  I think that the cyclic augmentation 
and then cancellation of the tones was much more irritating.  It's 
probably similar to the effect of any "reverbrating" stimulus that we 
encounter--perceived quality is low or negative.

Jim Clack
Associate Professor of Biology
Indiana Univ -- Purdue Univ