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. If you press the switch, you will be able to hear the bell. Now start the vacuum pump. Then the air in the jar is pumped out gradually, the sound becomes fainter, although the same current is passing through the bell. After some time when less air is left inside the bell jar you will hear a very- feeble sound. When all air is removed no sound is heard. This shows that sound can not pass through vacuum.