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How fast sound travels

Waves, sound and light · level 3, lesson 3 of 6

A question to start

In a thunderstorm you see the lightning flash, then hear the thunder a few seconds later. They happen at the same moment. Why does the thunder arrive late?

The idea

Sound is fast, but light is far faster. The flash reaches you almost at once. The sound takes a few seconds.

In air at about 15 °C, sound travels about 340 metres every second. That is about 1 kilometre in 3 seconds.

Speed of sound Mars Earth
In the air at the ground about 240 m/s about 340 m/s
Time to travel 1 km about 4.2 s about 2.9 s

Sound goes faster through liquids and solids: about 1,500 m/s in water, and about 5,000 m/s in steel.

1 km: about 3 secondsflash: almost at once

Worked examples

1. How far is the storm? You count 6 seconds between the flash and the thunder. Each 3 seconds is about 1 km, so the storm is about 2 km away.

2. An echo. You shout at a cliff 170 m away. The sound goes there and back, 340 m, so the echo comes after about 1 second.

3. On Mars. Sound is slower, about 240 m/s. A sound from 1 km away takes about \(1000 \div 240 \approx 4.2\) seconds.

Try it

  1. Why do you see lightning before you hear thunder?
  2. You count 9 seconds between the flash and the thunder. About how far away is the storm?
  3. An echo comes back after 2 seconds. How far away is the cliff?
  4. Does sound travel faster in air or in water?
Answers
  1. Light travels much faster than sound.
  2. About 3 km.
  3. The sound travels 680 m there and back, so the cliff is 340 m away.
  4. In water.

Common mistake

Forgetting that an echo goes there and back. The echo's time is for the trip to the cliff and back, so the cliff is only half the distance the sound travelled.

Where it leads