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The origin of a **sound wave** is always some **oscillating body**. Oscillations can be produced by a tuning fork, drum, string of a guitar, bell, or vocal chords. Sound transfers in the form of a **longitudinal wave** and it requires a **material medium** for its propagation.
When we place a **source of sound** in a bell jar with some air inside, the sound can travel and be heard. Then we can remove the air from the bell jar by the vacuum pump. **When all air is removed no sound is heard**.
This shows that **sound cannot pass through vacuum**. Sound needs particles and bonds among the particles to travel. Sound is a **mechanical wave** that results from the back and forth vibration of the particles of the medium through which the sound wave is moving.
If a sound wave is moving from left to right through air, then **particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it**. *The motion of the particles is parallel to the direction of the energy transport.*