Demos: 4A-03 Sound Production in Bell Jar


A loud battery-operated tone generator is placed inside a bell (vacuum) jar and turned on. The sound can be heard through the glass. When the vacuum pump is turned on an the air evacuated, the sound disappears, indicating the need for a material medium to conduct sound.

Directions: Turn on the switch to the tone generator and place the mechanism on the foam rubber pad inside the bell jar (this is to minimize sound conduction through the solid parts of the vacuum apparatus). If necessary place the microphone from your lapel mike close to the jar so that the students can hear the tone generator. Then turn on the vacuum pump and hold the mike close to the jar. The sound will diminish to an inaudible amplitude.

Suggestions for Presentation: Ask how the students are able to hear you in the lecture hall. Focus on the compressional nature of sound waves and that air is the medium carrying the sound disturbances. What would happen if the air disappeared? Since you can’t do that in the room(!), illustrate what happens in the bell jar.

Applications: Ask the students if they have ever seen a science fiction film in which there was an explosion in space accompanied by the sound of the explosion!

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Last Updated: May 9, 2016 11:44 AM