It needs something to travel through

Knock on a long steel fence. A friend at the far end hears the knock twice.

In the figure, the pieces are the tiny particles of the medium.

The first knock comes through the steel. The second comes through the air. Steel carries sound faster than air.

Rule

Sound needs a medium to travel through. It is usually fastest in solids, slower in liquids, slowest in gases. In a vacuum, no sound arrives.

• Medium — the solid, liquid or gas that carries the sound.
• Compression — a patch where the pieces are closer than usual.
• Rarefaction — a patch where the pieces are further apart than usual.
• Vacuum — a space with no matter in it.

A squeeze, as in the figure, is a compression.

Time = distance ÷ speed. Over 680 m, steel takes 680 ÷ 5000 = 0.136 s. Air takes 680 ÷ 340 = 2 s. So the knocks arrive 2 − 0.136 = 1.864 s apart.

Two sounds 0.1 s or more apart are heard as two. A short fence gives a gap under 0.1 s. So you hear one knock.

An electric bell rings inside a sealed jar. Pump the air out. You still see the bell shake, but the sound fades away. With no air, nothing carries the sound to you.