Speed: A typical value for the speed of sound in air is 330 m/s.

The idea

This is the value to use whenever a question about sound in air supplies none, and it powers two classic calculations. For an echo, the sound travels to the wall and back, so the distance in the equation is twice the separation: an echo returning after 0.40 s means the sound covered 132 m, so the wall stands 66 m away. For thunder, light arrives effectively instantly, so the delay between the flash and the bang is the sound's travel time, and a gap of 6.0 s means the strike was about 2000 m away. Sound is roughly a million times slower than light, which is why the two are treated so differently. Keep the time in seconds and the answer comes out in metres.

What earns the marks

• Using 330 m/s as the speed of sound in air.
• The correct equation, distance = speed × time.
• Halving the total distance in an echo calculation.
• The answer in metres with the time used in seconds.

Common mistakes and how to put them right

• Forgetting that an echo travels out and back, so quoting twice the true distance.

• Using the speed of light for the sound in a thunder question.

• Leaving the time in milliseconds.

• Using 330 m/s for sound travelling through water or a solid.

Exam tips

• Sketch the path of the sound; a there-and-back arrow reminds you to halve the distance.
• State the value you are using, because the question may not give it to you.