Sound is a mechanical wave and needs a material medium to propagate.

It cannot travel in vacuum and can be shown by the following experiment.

• Take an electric bell and an airtight glass bell jar. The electric bell is suspended inside the airtight bell jar. Switch ‘ON’ the electric bell.
  
• Now, connect the bell jar to vacuum pump.
  
• Pump out the air from the jar, the sound becomes fainter, although the same current passes through the bell.
  
• Pump out some more air from the jar, a very feeble sound is heard.
  
• When the air is completely removed from the jar, no sound is heard.