Doppler Ball
Doppler shift - red shift - expanding universe
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 tapetot. 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.
Comments:
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. Rating ***
1 Archer 273-065A speaker