Alarm Clock in Vacuum - Bell Jar

A ringing bell inside a bell jar becomes quieter as air is removed with a vacuum pump, eventually falling silent when no air remains. When air is let back in, the sound returns.

1. Place a small ringing bell or electronic buzzer inside a transparent bell jar.
2. Seal the bell jar and connect it to a vacuum pump.
3. Switch on the bell so that it is clearly heard through the jar.
4. Gradually operate the vacuum pump to remove air from the jar.
5. Observe that the bell still vibrates, but its sound becomes weaker until it cannot be heard in a vacuum.
6. Turn off the pump and allow air to slowly re-enter the jar.
7. Notice that the sound becomes audible again as the air pressure returns to normal.

Alarm clock experiment - Leybold:

What Happens to an Alarm Clock in a Vacuum? - Jason Gibson:

• Use a ringing bell instead of an alarm clock.
• Connect the demonstration to space exploration, where sound cannot travel in the vacuum of space.

• Handle the bell jar carefully—it is made of glass and fragile under pressure changes.
• Do not exceed recommended vacuum limits of the equipment.
• Ensure students do not touch the pump or jar during operation.
• Always release the vacuum slowly to avoid sudden pressure changes.

• Why does the bell become silent when air is removed? (Because sound waves need air molecules to carry vibrations, and in a vacuum there are no molecules to transmit sound.)
• Why does the sound return when air is let back in? (Because the returning air provides particles that can once again carry sound vibrations to our ears.)
• What does this experiment tell us about sound in space? (It shows that sound cannot travel in the vacuum of space, so astronauts must use radios to communicate.)
• How does reducing the amount of air (but not all of it) affect sound? (The sound becomes quieter because fewer particles are available to transmit vibrations.)