Experiment: Take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar. The bell jar is connected to a vacuum pump as shown in figure. If we press the switch we will be able to hear the bell. Now start the vacuum pump. When the air in the jar is pumped out gradually, the sound becomes painter, although the same current is passing through the bell. After some time when less air is left inside the bell jar we will near a feeble sound. If the air is removed completely we can’t hear the bell.
From this experiment we conclude that sound needs a material medium for its propagation.
