What makes a sound?

Every sound starts with something that vibrates: a tuning fork, the paper cone of a loudspeaker, a guitar string, your own vocal folds. Touch your throat while you hum and you can feel it.

Here is what the vibrating object does to the air next to it. When the cone of a speaker moves forward it pushes the nearby air particles together. That patch is a compression: the pressure is a little higher than normal. When the cone moves back the particles spread out. That patch is a rarefaction: the pressure is a little lower than normal. The cone keeps going back and forth, and a train of compressions and rarefactions spreads out through the air. That train is a sound wave.

It reaches your ear, where the changing pressure moves the eardrum back and forth in the same rhythm, and your brain turns that motion into a sound.

A longitudinal wave. In a sound wave each air particle moves back and forth along the direction the wave travels. This is called a longitudinal wave. Compare it with a wave on a rope or on a pond, where the material moves across the direction of travel (a transverse wave). You can picture a sound wave as a slinky that you push and pull along its length.

The air does not travel to your ear. This is the part most people get wrong. The particles stay close to where they were and only wiggle. What travels is the pattern, and with it energy. Think of a crowd doing a stadium wave: nobody runs round the stadium, yet the wave does. The wiggles are also tiny. For a 1,000 Hz tone at the level of normal speech (about 60 dB) the air moves back and forth by only about 8 nanometres, thousands of times thinner than a hair.

Sound needs a medium. A medium is any material that can be squeezed and can spring back: a gas, a liquid or a solid. With nothing to squeeze there is nothing to carry the wave. In the 1660s the scientist Robert Boyle rang a bell inside a glass jar and pumped the air out with an early vacuum pump. The sight of the bell stayed the same, but the sound faded away as the air was removed.

That is why space is silent. Between the stars there is almost nothing to carry a pressure wave. Astronauts outside a spacecraft talk by radio, which is light-like waves that do not need a medium, and inside the spacecraft they hear each other because the cabin is full of air.

How fast? Sound travels at different speeds in different media (rounded values):

• Air at 20 °C: about 343 m/s (about 1,235 km/h).
• Water: about 1,480 m/s, more than four times faster than in air.
• Steel: about 5,000 m/s (a little more or less depending on the steel and the shape).

Solids and liquids are far harder to squeeze than a gas. They spring back stiffly and fast, and that makes the wave move faster even though they are denser. Put your ear on a long metal railing while a friend taps the far end: the sound arrives through the metal before it arrives through the air.