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Waves · Subtopic 5 of 5

Sound

Sound is a longitudinal wave made by vibrating objects: a series of compressions and rarefactions that needs a medium to travel through. Louder sounds have larger amplitude; higher-pitched sounds have higher frequency. Echoes are reflected sound; using distance == speed ×\times time, and remembering the sound goes there and back, we can measure distances, including with ultrasound.

Cambridge 5054 Paper 1Cambridge 5054 Paper 2Cambridge 0625 Papers 2, 4Cambridge 0625 Papers 1, 3Edexcel 4PH1 Paper 1PEdexcel 4PH1 Paper 2P≈ 25 minRevision 5 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Describe sound as a longitudinal wave needing a medium.
  • Relate pitch to frequency and loudness to amplitude.
  • Recall the range of human hearing and what ultrasound is.
  • Use echoes to calculate speeds and distances.
Pause and think

In space films, explosions make huge booms. In real space, would you hear anything?

Section 2 of 14

Core idea

Start hereCounting after lightning

See lightning, count the seconds to the thunder, divide by 3: that's roughly the distance in km. Light arrives almost instantly; sound plods along at about 330 m/s.

The big idea

Sound: longitudinal compressions and rarefactions; needs a medium; ≈330 m/s in air. Pitch ↔ frequency; loudness ↔ amplitude. Echo: halve the total distance.

1. How sound travels

Sound: compressions and rarefactions moving through air
compressionrarefactionair particles vibrate back and forth along the direction of travel →
Where the lines bunch up the air is squashed (high pressure); where they spread out it is stretched (low pressure).
  • Faster in solids than liquids than gases.
  • Human hearing 20 Hz to 20 kHz; above: ultrasound.
  • No sound in a vacuum.

2. Echoes

d=vt2d=\frac{vt}2
where
tt
time for the sound to go there and back.
dd
distance to the reflector.
Worked exampleClap at a cliff

Echo after 1.2 s; v=330v=330 m/s.

Goal
Distance.
Strategy
Halve the round trip.
  1. 330×1.2=396 m330\times1.2=396\text{ m}
    There and back.
  2. 396÷2=198 m396\div2=198\text{ m}
    One way.
Answer198198 m.
✓ CheckAbout 0.2 km in 0.6 s each way ✓.
Worked exampleSonar

Echo after 0.4 s; water 1500 m/s.

Goal
Depth.
Strategy
Same formula.
  1. 1500×0.4=6001500\times0.4=600
    Round trip.
  2. 600÷2=300 m600\div2=300\text{ m}
    Depth.
Answer300300 m.
✓ CheckSound faster in water than air ✓.
Common misconceptionForgetting to halve The echo time covers the journey twice.

3. Oscilloscope traces

Two sounds on an oscilloscope
24681012−2−1120timesignalquiet, lowloud, high
The dashed wave has twice the amplitude (louder) and twice the frequency (higher pitch).
Pause and think

On a trace, waves get closer together. What changed?

Summary

  • Longitudinal; needs a medium.
  • Echo: halve.
Key words
Ultrasound
Sound above 20 kHz.
Compression
Region of squashed particles.
HistoryCannons and the speed of sound. In 17381738 French scientists fired cannons at night between hills near Paris 2929 km apart and timed the gap between seeing the flash and hearing the bang. They got 332332 m/s, impressively close to modern values.
Fun factBats see with sound. Bats emit ultrasound clicks up to 200200 times a second and listen for the echoes, building a sound picture of moths and obstacles in complete darkness. Some moths have evolved to jam bat sonar with their own clicks.
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