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Motion & Forces · Question 20 of 20

Sound waves

Nothing travels from a speaker to your ear except a shove. Sound is air passing along a push, molecule to molecule, at 343 metres per second.

Plate VI — A pressure wave longitudinal · 343 m/s · λ = v/f
Watch one dot wiggle in place while the squeeze races past it.
pressure along the waveone wavelengththe squeeze travels — the molecules just wiggle in place
FIG. VI — A PRESSURE WAVE
Frequency (pitch) ~320 Hz
mid — a spoken voice
Amplitude (loudness) 70%
Wavelength in real air
1.1m
Speed in air
343m/s
Watch one dot: it just wiggles on the spot while the dark stripes — the squeezes — race to the right. That's sound: a travelling squeeze. Faster wiggling = higher pitch; bigger wiggling = louder.
The short answer

Sound is air wiggling. Something vibrates, squishes the air next to it, and that squeeze travels to your ear as a wave you hear.

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.

Try it at home See pitch with a ruler
  1. 1Press a ruler flat on a desk with 20 cm hanging off the edge, and twang the free end.
  2. 2Shorten the overhang and twang again: the buzz rises in pitch. A shorter free end vibrates faster — more squeezes per second.
  3. 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.