Cite an experiment to show that sound needs a material medium for its propagation.
Take an electric bell and connect it to electrical supply. An airtight glass bell jar with a vacuum pump.
When we press the switch, we will be able to hear the bell. One can hear the sound of the ringing bell. Now, take a jar and make a small hole in the bottom and insert a small pipe though it. Connect the other end of the pipe to a vacuum cleaner. When the air in the jar is pumped out gradually, the sound becomes feeble although the same amount of current is flowing through the bell. It will be observed that the sound of the ringing bell decreases. When there is no air present inside, we will not be able to hear the sound of the bell. This shows that sound requires a medium for its propagation.
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