Quoth the Raven:

This is a little device that I designed for the simple purpose of being discreetly annoying. It waits for a predetermined amount of time, and then it starts emitting high-pitched beeps. I have programmed mine to take advantage of an interesting property of sound. That is, in general most people above the age of 25-30 can’t hear very high-pitched tones (say, 17 KHz for instance). This means if you were to (hypothetically of course) place it in a classroom, it would start bugging the heck out of the students while the teacher/professor will (most likely) be completely unaware of the source of the disturbance.

The Raven, high-pitched torture