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Why do sound waves need a medium for their propagation?

Sound needs a material medium for their propagation like solid, liquid or gas to travel
because the molecules of solid, liquid and gases carry sound waves from one point
to another. Sound cannot progress through the vacuum because the vacuum has no
molecules which can vibrate and carry the sound waves.

Experiment:

Now think how the sound produced by drums is heard by us? How is this transferred
to our ears? Let us understand this with the help of an experiment.

1. Take a bell jar. Bell jar is a laboratory equipment used for creating a vacuum.
Its shape is similar to that of a bell.
2. Place an electric bell in the bell jar.
3. Start pumping out the air of the sealed bell jar. As we start pumping out the
air, the sound of an electric bell coming out of bell jar starts fading.
4. As soon as the vacuum is achieved in the bell jar, no more sound will be
audible from the bell jar. However, in the bell jar, hammer continues to hit the
gong.

This means that still the sound is being produced but now we cannot hear it. This is
because sound waves always need a medium to propagate. In the bell jar, it was,
hence sound waves cannot travel.