Sound Needs A Medium To Travel

Sound is a mechanical wave and needs a material medium like air, water, steel etc. For its propagation. It cannot travel through vacuum.

An electric bell is suspended in airtight glass bell jar . The electric bell is suspended inside the airtight bell jar. The bell jar is connected to a vacuum pump.

When bell jar is full of air, we hear the sound but when air is pumped out from the bell jar by vacuum pump and we ring the bell, no sound is heard. So, medium is necessary for propagation of sound.