Oscillations and Waves > Waves > Transfer of Energy
DCS# 3B30.30
BELL IN A VACUUM
APPARATUS
vacuum pump
002
bell jar with wire contacts
101-04-D
amplifier
201A
function generator
202-08-
hanging speaker
202-07-
wireless microphone
201A
DESCRIPTION
This demonstrates that sound waves
require a medium for propagation. Evacuate the bell jar with the
speaker suspended inside. Turn off the vacuum pump and open the
valve so that the sound intensity can be heard to increase as air
enters the chamber. Sound is faint, so clip
lavalier microphone just outside or inside of bell jar.
NOTE
Using the battery-powered
doppler
ball buzzer eliminates sound transmitted through the wires.