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Sound travelling through the air into an ear A loudspeaker on the left. Four curved bands spread out from it across the middle, each one wider than the last, and they reach an ear on the right.

Exploring Sound Waves

About 7 minutes

Sound is not a thing by itself — it is a vibration passing through something else, whether that is air, water, or solid rock, and how it travels explains a lot about how you hear it.

What are Sound Waves?

Imagine dropping a pebble into a calm pond. The ripples that spread out are a good picture of a sound wave. Sound waves are vibrations that travel through a medium — air, water, or even solids — carrying energy from one place to another. The louder the sound, the bigger the wave.

The two things you can measure on a sound wave A smooth wave drawn along a dashed centre line. An upright bar runs from the centre line up to the top of the first crest and is labelled amplitude, which is how loud the sound is. Lower down, a double-headed arrow runs from one crest across to the next crest and is labelled wavelength. amplitude how loud it is wavelength one crest to the next
Amplitude is how tall the wave is, and that is the loudness. Wavelength is the distance from one crest to the next.

How Sound Travels

Sound waves need something to travel through. That "something" is called a medium. Sound travels fastest through solids, then liquids, and slowest through gases (like air). Think about it: you can hear a train coming from far away by putting your ear to the tracks.

Three things a sound can travel through. Put them in order, fastest first.

  • Air — the molecules are spread far apart
  • A steel railway track — the molecules are packed tightly together
  • Water in a swimming pool — closer together than air, further apart than steel

Sound cannot travel in a vacuum, which is a space with no air or anything else. A vacuum has nothing in it to wobble, and a sound is nothing but a wobble passing along. That's why there's no sound in outer space.

A spaceship explodes in a film and you hear a huge bang. Why could that never really happen?

What Makes Sound Different?

Sound has different qualities, like loudness (volume) and pitch (how high or low it is). Loudness depends on the amplitude (height) of the sound wave. Pitch depends on the frequency (how many waves pass a point in one second). A high frequency means a high-pitched sound, like a whistle. A low frequency means a low-pitched sound, like a tuba.

A high-pitched sound and a low-pitched sound, drawn as waves Two waves of exactly the same height. The upper one is squashed up with many crests packed close together, which is a high sound like a whistle. The lower one is stretched out with only a few wide crests, which is a low sound like a tuba. Same height means the same loudness; it is the spacing that changes the pitch. High pitch — crests packed close a whistle Low pitch — crests spread wide a tuba
Both waves are the same height, so both sounds are equally loud. Only the spacing changed, and that is what changes the pitch.

Next time you hear a whistle and a tuba side by side, you are hearing frequency itself — the whistle's wave passing far more often per second than the tuba's, even though both can be exactly as loud.