To demonstrate the Doppler effect.
Doppler ball on string.
A ball with a 3000 Hz whistle inside is hooked to the end of a cord and whirled about the instructor's head or thrown from person to person. The Doppler effect can easily be heard throughout even a large room.
G. F. Herrenden Harker, The Doppler Effect When Both Source and Observer are in Motion, AJP 12, 175-177, (1944).