Additional Info

• ID Code: H1-01
• Purpose: Demonstrates sound wave requirement for a medium
• Description:

  An alarm-style electric bell is mounted inside a large glass bell jar, with external switches to control both the bell and the pump. This enables the instructor to compare the propagation of sound and light.

  Start the bell, then pump the air out of the jar. Air pressure in the jar is read by the large gauge. As the air is removed, the sound intensity decreases, ultimately to nearly zero. Turn off the vacuum pump when the jar is evacuated and crack the valve open, allowing air to re-enter the jar. As the pressure increases the sound of the bell comes back, but without the noise of the pump.

  Engagement Suggestion

  • Consider asking the students to make predictions before each step - how will removing the air change what they hear? What they see? What will happen as the air returns?
  • Compare this to videos the see of people working in the vacuum of space, in real life and in the movies. What do you see and hear in real life? How is this presented in fiction, and why?

  Background

  There are subtleties to this effect. The pump is not creating a true vacuum within the chamber. The vast majority of the air has been removed, reducing the environment’s ability to transmit sound; but the other (perhaps more important) effect in play is the difference in density between the interior of the chamber and the glass and the external atmosphere; this creates a major change in impedance, causing what little sound can be transmitted within the chamber to reflect back. Also, off course, the bell is not floating in free space, and some vibrations can always be transmitted through the supports and wires.

  For small groups, also consider H1-04, a more portable version of this demonstration.

• Availability: Available
• Loc codes: FS1