Sunday, 17 July 2011

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PURPOSE: To demonstrate that sound waves require a medium for propagation.
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.
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 cannot be moved sufficiently to transmit sound under these conditions. I would use it anyway to make the physics point.
It is interesting to note that the "modern" centrifugal vacuum pump was invented by Joseph Boyle in the early sixteenth century to perform this experiment and determine whether sound propagated as waves (and would not pass through a vacuum) or particles (and would pass through a vacuum).
REFERENCES: Available. (PIRA 3B30.30)
EQUIPMENT: Jar with electric bell, vacuum pump and gauge, pre-assembled as photographed.
SETUP TIME: 10 minutes.

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