The sound doesn’t propagate in a vacuum because there is no physical medium in it, and the sound is a mechanical wave that only spread though a physical medium, whether liquid, gaseous or solid. You can’t hear sounds in a vacuum because in it there isn’t a medium for sound transmission, only the electromagnetic waves can be spread, because they don’t need a medium to move – for example: the light, it propagates in electromanegtic wave form.

• Example: Church Bell

When a church bell rings, it vibrates, swinging back and forth fastly. When the bell swings forward, it pushes the air particles and then these particles push others around and so on. And when the bell goes back, it pulls the air particles which automatically pull other air particles.

This pushing and pulling movement is a sound wave. The bell vibrations push and pull the air molecules around, creating a sound wave. The bell vibration is the original disturbance, and the air particles are the physical medium.