High frequ. noise from the cpu that would be very interesting to me..
althought it might not help the war agaist computer intrusion but being able to find out what sombody is typing just by the sound of the keys on a keyboard now that is high tech. I recored the keystroks of this post with a mic conected to my machine. I looked at the freq on a graph. I found only some keys make diff sounds. The biggest problem is that some keys are hit harder or softer than others making it very diffacult to try to figure out what sounds the diffrent keys make. If you hit the keys with the same amount of force then you could seperate the keys into diff sounds. But the human factor makes noise freq. keylogging unreliable...