Sound is air wiggling. Something vibrates, squishes the air next to it, and that squeeze travels to your ear as a wave you hear.
Most people think molecules of air travel all the way from a speaker to your ear. In fact no molecule makes the journey; each just nudges its neighbour and springs back, like a stadium crowd doing the wave, so only the pattern of pressure travels.
What's actually happening
Pluck a guitar string and it shoves the air beside it, squeezing molecules together. That compressed pocket shoves the next layer of air and springs back, leaving a slight emptiness behind; the next layer does the same, and a ripple of squeeze-and-stretch races outward at 343 metres per second. Crucially, no molecule makes the journey — each one just nudges its neighbour and returns, like a stadium crowd doing the wave. What travels is the pattern.
Everything you hear is encoded in two numbers of that pattern. How many squeezes arrive per second is frequency: your ear reads it as pitch, from the 27.5 Hz growl of a piano's lowest A to a child's-hearing limit near 20,000 Hz. How hard each squeeze presses is amplitude: that is loudness. The eardrum is a drumhead beaten by these arriving pressure pulses; three tiny bones lever the beats inward, and a spiral organ unpacks them by frequency before the brain reassembles music, speech, and the door you just heard creak.
Because sound needs neighbours to nudge, it has no way across a vacuum — space's explosions are genuinely silent, whatever cinema says. And its travel speed, brisk but a million times slower than light, leaks into everyday life: you see the lightning now and hear it later. Count the seconds and divide by three for kilometres. The thunder is the news arriving on foot.
Sound is a push passed along, molecule to molecule, at about 343 m/s — frequency sets the pitch, amplitude sets the loudness, and a vacuum carries nothing.
- 1Press a ruler flat on a desk with 20 cm hanging off the edge, and twang the free end.
- 2Shorten the overhang and twang again: the buzz rises in pitch. A shorter free end vibrates faster — more squeezes per second.
- 3Now press your ear to the desk and twang. The wood delivers the sound louder and crisper than the air did: denser materials pass the nudge along far better.
Common questions
How many squeezes arrive per second is the frequency, which your ear reads as pitch. How hard each squeeze presses is the amplitude, which you hear as loudness.
Sound needs neighbouring molecules to nudge along, so it cannot cross a vacuum. Space's explosions are genuinely silent, whatever films suggest.
Sound moves nearly three times faster in helium, shifting your vocal tract's resonances upward. Your vocal cords vibrate the same; it is the room inside your mouth that changes, not the singer.