Echo & the speed of sound

You shout into a valley and hear your own voice come back. In a thunderstorm you see the lightning first and hear the thunder only seconds later. Both show the same thing: sound is fast, but not instant. And you can use exactly this delay to measure distances, just like bats and ships do.

Sound takes time

Sound travels through air at about 343 m/s, which is over 1,200 km/h. That value holds for air at around 20 degrees Celsius, in cold air sound is a little slower. That sounds fast, but light is almost a million times faster. That is why you see the lightning immediately while the thunder still has to travel to you. From speed and time you can work out the distance: distance equals speed times time. So in 2 seconds, sound covers about 686 m.

Echo: there and back

When sound hits a rock face or a large wall, it is thrown back: that is the echo. Careful, here comes the most popular trap of this chapter: the sound travels the distance twice, once there and once back. If you hear your echo after 2 seconds, the sound has travelled 686 m, but the wall is only 343 m away. So: divide the total distance by 2! For thunderstorms there is a handy rule of thumb: count the seconds between lightning and thunder and divide by 3 to get the distance in kilometres.

Ultrasound: measuring with echoes

Bats call at pitches so high we cannot hear them and listen for the echo. From the travel time their brain instantly works out how far away an obstacle or an insect is. Doctors use the same principle with an ultrasound machine: it sends sound into the body and measures the echoes from organs. And ships use sonar to find out how deep the water is beneath them. Careful with such calculations: in water, sound travels more than four times as fast as in air, about 1,500 m/s. So a sonar works with that number, not with 343 m/s.