This demonstration consists of a bell jar mounted on a platform. Inside the bell jar a sound source is hanging from a support, and connected to a switch mounted external to the bell jar. The bell jar also has a fitting to attach a vacuum pump and a pressure release valve.

Demonstrate that sound waves require a medium to travel.

• Bell jar containing alarm and pressure gauge/valves
• Vacuum pump

1. Attach the vacuum pump hose to the connector on the demo.
2. Ensure that the valve on the vacuum pump hose is open, and that the pressure relief valve on the demo is closed.
3. Turn on the alarm with the switch to demonstrate how loud it is.
4. Turn off the alarm and turn the pump on to evacuate the air in the jar. You can also just leave the alarm on during this step.
5. Once the vacuum has evacuated most of the air, close the valve on the vacuum pump hose, turn off the vacuum pump, and turn on the alarm back on. You should notice that it's dramatically quieter.
6. Release outlet valve while alarm is on to show how it gets loud again when the air is reintroduced.

You will still hear a little noise from the alarm due to vibrations being transmitted by the supports inside the jar and the limits of the vacuum pump.

Take care when handling this demo. The glass bell jar is probably only held to the platform by the stickiness of the vacuum grease and the gasket used to make a seal.