Sound Waves do not travel in

Sound is a mechanical wave. It is produced by vibration and needs a material medium, because each particle passes the disturbance on to its neighbour. Solids, liquids and gases all have particles that can be compressed and set in motion, so sound travels through all three. Its speed differs: about 5000 m/s in steel, about 1500 m/s in water and about 343 m/s in air at 20 degrees C. In a vacuum there are no particles to carry the vibration, so sound cannot travel at all. This is why a ringing bell inside a jar goes silent as the air is pumped out.

Core concept: mechanical versus electromagnetic waves. Light and radio waves need no medium because they are oscillations of electric and magnetic fields, not of matter. This distinction matters in Physics and Science and Technology, and it explains why sunlight reaches Earth but sound from space does not.

Trap: Aspirants often read the question as 'in which medium is sound fastest' and pick solids. Only vacuum blocks sound completely.