Cite an experiment to show that sound needs a material for its propagation.

Explanation

Take an electric bell and an air tight glass bell jar. The electric bell is suspended inside an air tight glass jar which is connected to a vacuum pump.

Working : (i) When we press the switch, we hear the sound of the bell.
(ii) When the air in the jar is pumped out gradually, the sound becomes feeble although current is flowing through the bell. We see the hammer of bell vibrating but do not hear sound.
(iii) When the air is removed completely, we can not hear the sound of the bell.
Conclusion : This experiment shows that sound requires a medium for its propagation.