Measuring distance with sound

What the Activity is for

A widespread activity is to make a rough measurement of the speed of sound by timing the gap between making a sound and hearing an echo from a wall at a measured distance.

This activity takes things the other way around by assuming the speed of sound (340 metre / second), measuring the time the sound takes and thereby finding the distance that the sound travelled.

What to Prepare

• A sonic ruler (These are available from many hardware stores. When working it should emit an audible click and then report the distance to the object you pointed it at.)
• An outside wall, about 150 m away from where the pupils can stand
• A stopwatch, accurate to 0.01 s

What Happens During this Activity

Start with the sonic ruler and ask if anybody knows how it works. Point out that it seems to be able to measure how far away an object is, simply by bouncing a sound off it. Ask how this is possible. The ruler actually measures the time that a sound pulse takes to travel to and from the object, then finds the distance by multiplying half the round-trip time (out and back) by the speed that it has it has stored in its memory.

If necessary, revise the relationship between speed, distance and time with simple examples.

The next activity takes the class outside. The challenge is for the pupils to find their distance from the wall using the known speed of sound (340 metre / second) and half the round trip time that they measure. Different groups of pupils will need varying amounts of help in carrying out this activity:

• Stand about 150 m in front of the wall.
• Make a loud sound (maybe clapping two blocks of wood together).
• Time how long it takes to hear the echo from the wall.
• Calculate the distance.

It's good fun to run this activity as a competition. It is also a good place to think about the possible uncertainties in this experiment – beware of those who get suspiciously close to the correct result. How reliable are they as witnesses? This can lead to lively discussion!

This activity offers the opportunity to make connections to the technique of remote sensing. For example, radar mapping of a planet is done in exactly the same way. Times are measured for radar signals to travel to and from the planet, then distances of the surface from the satellite are calculated, thereby giving the height profile of the surface. A number of impressive images, developed by remote sensing, are available on the internet. (See, for example, the NASA site.)