Speed of travel

Echoes, imaging and measurement

Clap your hands not too far from a big wall and you can hear both the original clap and, shortly afterwards, its echo. One path is straight from your clapping hands to your ear—a short distance. The second path is from your hands, reflecting off the wall and then back to your ears—a long distance.

In fact, these are just two of the many paths followed by the sound as it travels out in all directions through the surrounding air.

Measure the total distance travelled from your hands to the wall and back. Time between the original clap and hearing the echo. Divide the distance by the time to get a rough value of the speed of sound in air.

You are likely to find a value of about 340 metre / second.

Under normal conditions, this speed stays about the same. However, since the motion of the sound depends on the movement of air particles, the speed of sound does change with temperature because this determines how fast the particles are moving.

You can turn this around. Knowling the speed, timing echoes measures distances. This is used in depth sounders on boats and ultrasound scans in hospitals.