A high-pitched alarm on the end of a rope is whirled about the head.
What it shows:
Doppler shift of sound emitted by an object moving in a circular orbit, with the pitch clearly changing as the object move towards, away or perpendicular to the line of the observer. Useful as an analogy to the redshift and blueshift of spectral lines from a rotating astronomical source such as a planet or binary star system.
How it works:
We have a Powerhorn™ Security System buzzer attached to a 1.5m length of nylon cord. Swing it in a horizontal circle above your head, adjusting the cord length for the desired rotation period.
Horrible noise, good demo. Any loud, self contained buzzer or single pitch alarm will do, provided it can be easily and safely swung around ones head.