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

(N/A) Apparatus required: $A$ bell jar,an electronic bell,a cork,and a vacuum pump.
Procedure:
$1$. Take an electronic bell and suspend it inside an airtight glass bell jar.
$2$. Connect the bell to a power supply through the cork at the top of the jar.
$3$. Switch on the bell; you will hear the sound of the ringing bell.
$4$. Now,connect the bottom of the jar to a vacuum pump and start the pump to remove the air from the jar.
$5$. As the air is gradually removed,the loudness of the sound decreases.
$6$. When the air is completely removed (creating a vacuum),you will observe that no sound is heard,even though the hammer of the bell is still seen striking the gong.
Conclusion: Since the bell was working but its sound could not be heard in the absence of air,it is concluded that sound requires a material medium for its propagation.