● Take an electric bell and an airtight bell jar.

● Suspend the bell inside the bell jar.

● Connect a vacuum pump to the bell jar as shown in the figure.

● Press the switch to hear the bell.

● Now, start the vacuum pump.

● When the air in the jar is pumped out gradually, the sound becomes fainter even though the same current is passing through the bell.

● As the amount of air decreases inside the bell jar, the sound becomes fainter.

● When there is no air inside the jar, there is no sound of the bell.

This shows that sound requires a medium to propagate.