What Is Sound?

Sound is a form of energy that travels as waves of vibration through air, water or other materials. When something vibrates (a guitar string, a speaker cone, your vocal cords), it pushes the air around it back and forth in a rhythmic pattern. The pattern spreads outward as a sound wave, until it reaches your ears and makes your eardrums vibrate. Your brain decodes the vibration as sound. Without something to vibrate through, sound cannot travel. That is why no one can hear an explosion in the airless vacuum of space.

  • What it isVibrations travelling as wavesThrough air, water or solids
  • Speed in airAbout 343 m/sAt 20 C, sea level
  • Speed in water1,500 m/s4x faster than air
  • Speed in steel5,000 m/sEven faster
  • Human range20 Hz to 20 kHzDrops with age
  • Cannot travel throughA vacuumNo matter to vibrate

How sound starts

Sound starts with a vibration. Anything that vibrates can make sound:

  • A plucked guitar string vibrates back and forth.
  • A drum skin vibrates after you hit it.
  • Your vocal cords vibrate as air flows past them.
  • A speaker cone vibrates electrically.
  • A bell vibrates as a clapper strikes it.
  • Even a flag flapping in the wind makes a low sound.

How sound travels

When the vibrating object moves, it pushes the air molecules right next to it. Those molecules bump into the next ones, which bump into the next ones, and so on. The result is a wave of pressure changes (alternating slightly squashed and slightly stretched air) spreading outwards from the source.

Sound waves are longitudinal: the air molecules move back and forth along the direction the wave is travelling, rather than side to side (like waves on water).

Fact Sound travels much faster through dense materials. In air, sound moves at about 343 metres per second. In water, it moves over 4 times faster (1,500 m/s). In solid steel, sound moves at almost 5,000 m/s. This is why train spotters used to put their ear to the rails: they could hear an approaching train through the steel much earlier than through the air.

How you hear

Your ear is a brilliant little machine designed to catch sound waves and convert them into nerve signals:

  1. The outer ear (the bit you can see) funnels sound waves into the ear canal.
  2. Sound vibrates the eardrum, a thin membrane stretched across the canal.
  3. The eardrum shakes three tiny bones in the middle ear (the smallest bones in your body), called the hammer, anvil and stirrup.
  4. The bones pass the vibration to a coiled, fluid-filled organ called the cochlea.
  5. Tiny hair cells inside the cochlea sense the vibrations and send nerve signals to the brain.
  6. Your brain decodes the signals as sounds.

Why sound cannot travel through space

Sound needs a medium to vibrate through. In the vacuum of space, there are almost no atoms or molecules, so there is nothing for sound waves to travel through. Two astronauts standing in space could shout at each other and not hear a thing. They have to talk through radio links instead. (Radio waves are not sound: they are electromagnetic waves, which travel happily through a vacuum.)

The qualities of a sound

Three main features describe a sound:

  • Pitch: how high or low the sound is. Decided by frequency. (See frequency and pitch.)
  • Volume (loudness): how loud the sound is. Decided by amplitude. (See amplitude and volume.)
  • Timbre: the unique tone-quality. The reason a violin and a flute sound different even when playing the same note.
Did you know? Blue whales make some of the loudest natural sounds on Earth, reaching 188 decibels at the source. Their low-frequency calls can travel thousands of kilometres through the ocean. A whale singing off South Africa might be heard by another whale off Brazil. They use this long-distance signalling to find mates and stay in touch across vast ocean distances.

Sound in different materials

Sound behaves differently depending on what it is travelling through.

  • Air: 343 m/s. Easily absorbed and scattered, especially by soft materials like fabric and foam.
  • Water: 1,500 m/s. Travels well over long distances; sonar uses this.
  • Steel: 5,000 m/s. Travels extremely fast; train track signals.
  • Diamond: 12,000 m/s. The fastest known medium for sound.
  • Cork or wool: very slow and quickly absorbed. Used as soundproofing.
Try this Take a balloon and hold it close to your ear. The thin rubber membrane catches and amplifies tiny sounds from the room around you. Now blow up the balloon a little and try again. The denser air inside lets sound travel into the balloon, which then vibrates and acts as a tiny amplifier. Try tapping the inflated balloon gently with a finger and listening: you can hear it ring like a tiny drum.
Deeper dive: how bats and dolphins see with sound

Some animals have evolved an extraordinary ability called echolocation: they use sound waves to "see" the world around them, even in total darkness. Bats and dolphins are the best-known examples.

A hunting bat emits very high-pitched clicks (usually above the range of human hearing, around 20 to 200 kHz). The clicks travel out, bounce off any objects in front of the bat, and return to the bats sensitive ears. From the time delay and pitch changes of the echoes, the bats brain builds a detailed mental picture of what is in front of it: trees, buildings, prey insects, even tiny details of the insects shape.

The system is so accurate that bats can detect a single human hair stretched across their flight path, or pluck a flying moth from the air in pitch darkness. Some bats can even tell what kind of insect they are chasing by the precise pattern of returning echoes.

Dolphins use the same trick underwater, with even more sophisticated calls and a special fatty organ in their forehead (the melon) that focuses sound waves into a tight beam. Dolphins can use echolocation to find fish buried in sand on the seabed, identify objects from kilometres away, and even (some scientists think) sense the internal organs of other animals.

Humans have copied the principle in sonar (sound navigation and ranging) used by submarines, fishing boats and oceanographers. Bats and dolphins, however, were doing it 50 million years before humans even existed.

For more, see frequency and pitch and echoes.