Plastic Wiffle Ball with built-in shriek to throw past (or at) your audience.
What it shows:
Waves emitted from a moving source are Doppler shifted to higher frequencies when moving toward the observer, and shifted to lower frequencies when moving away. This audio demonstration is also a useful analog to the optical red shift and blue shift exhibited by astronomical sources moving relative to the Earth.
How it works:
A plastic Wiffle-Ball™ is filled with foam padding to protect an enclosed mini-speaker 1 and its 9V battery power supply. Choose a catcher at the back of the audience, turn on the horrible shrieking monotone, and throw.
Setting it up:
To create the Wiffle-ball shrieker the ball needs to be cut in half and held together with duct tape. It can therefore be easily opened and the speaker engaged just before throwing. Make sure the speaker itself isn't muffled by the foam padding, and that the battery is good.
The effect is obviously best when the ball is coming directly towards or away from you, so choose a couple of catchers in different parts of the hall. Good audience participation, and a chance to show off your curve-ball.
1 Archer 273-065A speaker