Bell In Vacuum

Purpose

To demonstrate that sound waves require a medium for propagation.

Equipment

Jar with electric bell, vacuum pump and gauge, pre-assembled as photographed.

Suggestions

This is actually not as simple as it seems. Although the conclusion is correct, it is not strictly speaking demonstrated using this setup. Rather, this demonstrates an impedance mismatch between the very low pressure air in the jar and the glass, which ca

Images

h1-01

Description

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.

References

R. B Lindsay, Transmission of Sound Through Air at Low Pressure, AJP 16, 371-377, (1948).